Interpretation Leached Outcrops Blanchard

ROLAND BLANCHARD 1890-1966

NEVADA BUREAU OF MINES Vernon E. Scheid, Director

Bulletin 66

INTERPRETATION OF LEACHED OUTCROPS By

ROLAND BLANCHARD

MACKAY

SCHOOL

OF

MINES

UNIVERSITY OF NEVADA

1968

CONTENTS PAGE

Foreword_____________________________________ ________________________________________ ______________________________ xiii Preface ____ ---______ ___ __________ _______ ___ ______ __ _____ ______ ___ __ __ ____ ____ ____ ____ __ _____ _______ ________ ___ ________ xv

Part 1 Introduction to Part L ___________________________________________________________________ ---- ____________ _ Chapter I-Introduction____________________________________________________________ _____________________ Origin and history of the investigation_______________________________________________________ Basis of leached outcrop interpretation_______________________________________________________ Importance of chemistry to the leached outcrop technique_________________________ Scope and method of presentation_______________________________________________________________

3 3 4 5 6

Chapter 2-The term "limonite" defined_________________________________________________ Hydrous ferric oxides and other iron-bearing compounds___________________________ Goethite_______________________ _______________________________________________________________________ Lepidocrocite___________________ ___________________________________________________________________ Other natural compounds____________________________________________________________________ Hematite______________________________________________________________________________________ Specularite_ ________________________ __ ____________ __________________________ __ ____ _______ ____ Sulfates and carbonates_______________________________________________________________ Impurities present in limonite______________________________________________________________________ Silica_ ____ ____ _________ ___ ___________ ___ _____ ____ ____ ____ ____ ________ ____________ ______ _________ ____ ____ Carbonates__________________________________________________________________________________________ Manganese__________________________________________________________________________________________ Gypsum_______________________________________________________________________________________________ Other minor impurities________________________________________________________________________ Sunllnary_____________________________________________________________________________________________________

7 7 7 7 8 8 8 8 8 8 8 9 9 9 9

Chapter 3-Indigenous, fringing, and exotic limonites_______________________________________ Indigenous limonite_ __ ____ _____________ _____ ___ ____________ ______ ___________________________ ___ _________ Fringing limonite_________________________________________________________________________________________ Exotic limonite____________________________________________________________________________________________ Difficulties in classification_____________________ ____ _________ ___________________ _____________________

11 11 11 12 13

Chapter 4-Formation of limonitic jaspeL________________________________________________________ True solutions and colloidal solutions distinguished____________________________________ The solution, transportation, and precipitation of silica and iron________________ Silica___________________________________________________________________________________________________ Iron _____ ______ __ ____ ____ _____ ____ ____ __________ __________________________ __________ ___________ _____ ___ __ Mutual precipitation of silica and ferric oxide hydrosols in jaspilites__ Modification of silica and iron reactions in vicinity of oxide sulfide bodies_ __________ ____________ _______ _________ ________ ____ ______ ________ ____ ___ __ Silica reactions_____ ________ ___________ ___________________________________ _________________ Iron reactions__________ __________ ____ _________ ___________________ _________________ _________ Precipitation of silica and iron____________________________________________________

15 15 16 16 16 17

Chapter 5-Types of limonitic jasper: I. Cellular pseudomorphs_______________________ The pseudomorphic replacement process____________________________________________________ Formation of cellular pseudomorphs___________________________________________________________ Pseudomorphic replacement of sulfides_______________________________________________ Effect of mineral cleavage___________________________________________________________________ Types of cellular structure_____ ____________ ____ ________ _____________________________ _________ Cellular boxwork_________________________________________________________________________ Cellular sponge______________________ __________________________________________________ Webwork_____________________________________________________________________________________ Siliceous nature of cellular pseudomorphs___________________________________________________ Resistance of jasper to chemical attack_______________________________________________ Differences between earlier and later leaching products______________________ Summary ______________________________________________________________________ .... __ .... __ ........... ______ ..

21 21 22 22 22 23 23 23 23 23 25 26 27

18 18 18 18

v

CONTENTs-Continued PAGE

Chapter 6-Types of limonitic jasper: II. Massive jasper.................................. 29 Two general varieties.................................................................................. 29 Open space precipitates and far-traveling varieties.............................. 29 Replacement jasper.............................................................................. 30 Replacements of kaolinite-montmorillonite clays........................ 30 Nature and occurrence of clay "soap"................................. 31 Impregnation and replacement of "soap"............................ 32 Ragged-edged jaspcr........................................................... 33 Replacement of nontronite.................................................. 34 Replacement of opaL................................................................ 35 Other less common kinds of massive jasper and pseudo-jasper................... 35 Siliceous-irony knobs or caps.............................................................. 35 Jasper covers over dolomite................................................................ 36 Australian "Billy"............................................................................... 37 Pseudo-jaspers...................... .............................................................. 38 Jasperoid..................................................................................... 38 Silica breccias.............................................................................. 38 Composites containing pseudo-jasper.......................................... 39 Sumnlary..................................................................................................... 40 Chapter 7-Extent of limonite precipitation above and below the water table.... Oxidation above the water table.................................................................. Oxidation below the water table..................................................................

41 41 43

Chapter 8-Need for excess sulfur to provide free sulfuric acid.......................... Degree of solution dependent on amount of acid generated.. .................. ..... Oxidation by air-water processes................................................................. Oxidation of pyrite.............................................................................. Oxidation of pyrrhotite................... ......................... ........................... Oxidation of chalcopyrite......... ..................... ...................................... Oxidation of chalcocite....................................................................... Oxidation of bornite............................................................................ Oxidation of tetrahedrite..................................................................... Summary......................................................................................................

45 45 45 45 47 47 48 49 49 50

Chapter 9-Limonite precipitation through dilution of iron-bearing solutions.... The isothermal equilibrium diagrams of the system Fe"O,-SO,,-H"O.......... Goethite and the compound 3Fe"Oj.4S0,.9H"O........................................ Oxidation products of pyrite and chalcopyrite............................................. The transition minerals copiapite and coquimbite....................................... Formation of jarosite in presence of potassium ion..................................... Summary.....................................................................................................

51 51 52 53 54 54 54

Chapter la-Limonite precipitation related to oxidation of iron-free sulfides.... Oxidation of sphalerite and galena by air-water processes in an inert environment............................................................... Oxidation of sphalerite and galena by air-water processes in presence of pyrite....................................................................... Oxidation of molybdenite by air-water processes in the presence of pyrite.. Summary.....................................................................................................

57

Chapter II-Leaching and limonite precipitation in the zone of saturation....... Reaction of ferric sulfate with pyrite and chalcopyrite................................ Oxidation by acid solutions of cupric sulfate during supergene enrichment........................................................................... Precipitation or absence of precipitation of oxidized iron minerals during reactions in the zone of saturation.................... Precipitation of siderite............................................................................... Summary.....................................................................................................

61 61

vi

57 58 59 60

61 62 63 64

CONTENTs-Continued PAGE

65

Chapter 12-Limonite precipitation by reaction with neutralizing gangues________ Precipitation by gangues of moderate neutralizing poweL_____________________ Precipitation by gangues of strong neutralizing power (limestone and other carbonate rocks) _______________________ ______________ Typcs of limonite precipitated____________________________________________________ Fluffy limonite_____________________________________________________________________________ Massive jaspeL___________________________________________________________________________ Limonite "dice" ______________________________________________________________________ Summary________ ____________________________________________________________________________________________

66 66 66 67 68 68

Chapter 13-Some examples of the products of the overall oxidation, leaching and enrichment processes____________________________ Great Cobar mine, New South Wales__________________________________________________________ Home of Bullion mine, Northern Territory _____________________________________________ Mount Oxide mine, Queensland__________ _________________________________________________ Mount Isa mine, Queensland____________________________________________________________________ Mount Stewart mine, New South Wales_____________________________________________________ C.S.A. mine, New South Wales_________________________________________________________________ Mount Cuthbert mine, Queensland_____________________________________________________________ Bisbee, Arizona___________________________________________________________________________________________ Ely, Nevada________________________________________________________________________________________________ Summary_____________________________________________________________________________________________________

69 69 70 72 72 74 76 77 78 78 78

Chapter 14-Influence of the sulfur-iron ratio and the host rock on the character of leaching products_____________________________________ Disseminated sulfide deposits______________________________________________________________________ MiamL_________________________________________________________________________________________________ Tyrone_______________________________________________ _________________________________________________ The differences explained__ ___________ _____ ____ ________ _____ ____ _______ ___ ____ ____ ___ _______ _ Estimating grade of ore prior to leaching____________________________________________ Difficulties encountered_ _____ _____________ ________ ___ ____ _____ __ ____ ____ ____ ____ ___ ____ ___ ___ Effect of the sulfur-iron ratio______________________________________________________________ Ajo______________________________________________________________________________________________________ Influence of the neutralizing gangue____________________________________________ Massive sulfide deposits______________________________________________________________________________ Deposits in shale or feldspar-rich rocks_______________________________________________ Deposits in limestone__________________________________________________________________________ Deposits in quartz-rich rocks______________________________________________________________ Width of outcrops over massive sulfide bodies____________________________________

81 81 81 81 81 82 83 84 84 84 85 85 86 87 87

Chapter IS-Limonite color_______________________________________________________________________________ The color analyzed______________________________________________________________________________________ Early investigations of the significance of limonite coloL___________________________ Limitations on limonite color as a prospecting guide__________________________________ Summary_____________________________________________________________________________________________________

89 89 89 90 91

Chapter 16-Standard types of leaching products______________________________________________ Mainly indigenous types ___________________ ,__________________________________________________________ Cellular pseudomorphs______________________________________________________________________ Cellular boxworks __________________________________________________ . __ ......... __ ._..... Hypogene boxworks ___ ... ______________________ .. _.... ____________ .. _.. ____ .... Supergene boxworks ______________ .. ______. _______________ ......... __ . __ .... Cellular sponge ____________________ . __ . __________________________ . __ ... _.. _________ .... ___ Hypogene sponge ____ ._. ____________________________ . __ ._ ... _... _._. _______ ...... Supergene sponge ___________ . __________________ ._ ... ____________________ ._. _____ . Flaky crusts ____________________________________________ . __________________________________ ...... ____ Rosette limoni te _____ . ____ .. _____ . __ .. _.. _.... _____________________ -_--.- -.. _....... -.- --. Granular limonite._ ...... ____________________________________________________________ .. _-....

93 93 93 93 93 93 94 94 94 96 97 97

65

Vll

CONTENTs-Continued

PAGE

Fluffy limonite _______________________________ ._____________________________________________________ 98 Hard pseudomorphs ___________________________________ .________________________________________ 98 Partly indigenous and partly fringing types_________________________________________________ 98 Relief limonite____________________________________________________________________________________ 98 Craggy limonite___________________________________________________________________________ 98 Radiating fibrous crusts_____________________________________________________________ 99 Arborescent limonites_________________________ ________________________________________ 99 Derivatives of arsenopyrite-pyrite mixtures_________________________ 99 Partially sintered crusts_ ____________________________ ___ ___________ _____________________ ______ 101 Pyramidal boxwork ___________________________________________________________________________ 101 Surface coalescences_____ ___________ _____ _____________ ____ __ _____ ___ ____ ___ _____ ____ ____ _______ 101 Exotic types ________________________________________________________________________________________________ 102 Granular and coagulated limonites ______________________________________________________ 102 Flat crusts_____________________________________________________ _____ ___ ____ ____ _______ ___ ____ ____ ____ _ 103 Smeary-crusted limonites ___________________________________________________________________ 103 Thick-walled limonites____ _________ ___ ___________ __ ____ ___ ____ ___ __ __ _____ ___ ____ ____ ___ ___ 104 Iridescent limonite crusts_________________________ __ ____ ____ _______________ __ _____ _______ ___ __ 104 Columnar limonites_____________________________________________________________________ 104 Caked crusts _______________________________________________________________________________________ 105 Shrinkage structures_______________________________________________________________ 105 Surface coalescences __________________________________________________________________________ 105 Desert varnish _____________________________________________________________________________________ 105 Limonite-soaked earth _____________________________________________________________________ 106 Summary _____________________________________________________________________________________________________ 106 Chapter 17-Examples indicating the value of leached outcrop interpretation ___ Classification of leached outcrops over disseminated deposits _____________________ Blind leached zones ____________________________________________________________________________________ Deposit of sub-ore grade ____________________________________________________________________________ Absence of widespread leaching-Ely, Nevada ___________________________________________ Non-sulfide gossan-Lawn Hill, Queensland ______________________________________________ The massive iron-oxide outcrop at Mount Isa, Queensland _________________________

109 109 109 110 111 111 111

Part 2 Introduction to Part 2 _________________________________________________________________________________________ 113 Chapter 18-Pyrite _____________________ -___________ -_ __________ ____ ________ ___ ___________ _______ ____ ___ ____ ___ 115 Chapter 19-Pyrrhotite ______________________________________________________________________________________ 122 Chapter 20-Arsenopyrite _________________________________________________________________________________ 126 Chapter 21-Chalcopyrite _________________________________________________________________________________ 132 Chapter 22-Chalcocite _____________________________________________________________________________________ 135 Chapter 23-Bornite __________________________________________________________________________________________ 138 Chapter 24-Tetrahedrite _________________________________________________________________________________ 141 Chapter 25-0xidized copper minerals ______________________________________________________________ 144 Chapter 26-Galena and cerussite _____________________________________________________________________ 145 Chapter 27-Sphalerite ______________________________________________________________________________________ 154 Chapter 28-Molybdenite __________________________________________________________________________________ 160 Chapter 29-Chromite _____________________________________________________________________________________ 161 Chapter 30-Hematite and magnetite ________________________________________________________________ 164 Chapter 31-Manganite and pyrolusite ______________________________________________________________ 165 Chapter 32-Calcite ___________________________________________________________________________________________ 166 Chapter 33-Siderite _________________________________________________________________________________________ 167 Chapter 34-Fluorite _________________________________________________________________________________________ 169 Chapter 35-Salite_________________________________ ____ ____ ________ __ ____ ___________ _______ ______________ _____ 171 Chapter 36-Supergene silica _____________________________________________________________________________ 172 Appendix A-Usage of two terms basic in the investigation of leached outcrops_ __ ____ ____________ ________ __________ ____ ___ _____ ____ __ ____ ________ __ ____ _____ 173 Appendix B-Replacement of amphibolite gangue by massive jasper at the Ninety-mile mine, Queensland _______________________________________ 174

viii

CONTENTS-C ontinued

Appendix C-Leaching and redeposition of copper minerals at Mount PAGE Oxide, Queensland_______________________________________________ 180 Appendix D-The limited role of organic acids in limonite precipitation __________ 185 References_________________________________________ __ __ ______________________________________ 186 Index ___________________________________________________________________________________________________________________ 189

ILLUSTRA TIONS (All plates appear in a 12-pagc color section following pagc 112.) Platc

I. 2. 3. 4. 5. 6. 7. 8. 9. 10.

II. 12.

l3. 14. 15. 16. 17. 18. 19. 20. 21. 22. Figure 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

II. 12. 13. 14. 15.

Columnar limonite derived from massive pyrite. Oxidation products of an arsenopyrite-pyrite mixturc. Boxwork derived from chalcopyrite. Coarse and fine cellular boxwork derived from chalcopyrite. Finely cellular boxwork derived from chalcopyrite. Goethite boxwork derived from chalcopyrite. Limonite formed by weathering of a chalcopyrite-pyrite vein. Oxidation products of a disseminated chalcocite-pyrite mixture. Cellular limonite formed from a mixture of chalcocite and pyrite. Cellular and fluffy limonites from Bagdad, Ariz. Fluffy limonite derived from cuprite. Cleavage boxwork formed from oxidized galena. Oxidation products of galena in limestone ganguc. Relief limonite derived from galena. Hieroglyphic boxwork derived from sphalerite. Cellular boxwork derived from sphalerite. Limonite formed by leaching of smithsonite. Leaching products of pyrite-chalcopyrite-sphalerite mineralization. Chromite-derived honeycomb boxwork and cellular sponge. Alteration of magnetite to hematite and then to goethite. Weathering products of manganite and pyrolusite. Leaching products of crystalline fluorite in a galena-marmatite orebody. Sketches showing typical relations of indigenous, fringing, and PAGE exotic limonites ________ . ________ . ___ ... _______________________ . __________ .. _________ ._ 12 A, B. Typical specimens of Australian "Billy" ___ .. __ . __ . _____ ._. ______ .. _ 37 The Posnjak-Merwin 50° C. (122° F.) isothermal equilibrium diagram ___________ . ___ . __________ . ___ . ___ . ____________ . _______ . ______ . ___________ . ______ ._ 52 Enlarged corner of the Posnjak-Merwin 50° C. isothermal equilibrium diagram shown in Fig. 3 ________ . __ . ___ . _____________________ .________ 53 Longitudinal section of Great Cobar mine, New South Wales ______ .. _ 70 Sections of workings at Mount Isa mine, Queensland _____ .. _______________ 73 Sections of workings at Mount Stewart mine, New South Wales_____ 75 Sketch showing cellular pseudomorphs and other limonite products formed at the Republic mine, Ariz. ____ . _______________ ._____________ 86 Map showing a method commonly used in classifying leached outcrops over disseminated copper deposits __________ :. __ .. ______ . _____ 110 Section showing how leached material, found only underground, led to discovery of an important orebody __ .. ___ . __ . ___ . __________ .______ 111 Sketch showing precipitation of pyrite-derived limonite at limestone contact____________ ______________________________________________________________ 115 Sketches of typical oxidation products of pryrite in three types of gangue _________________________________________________________________________________ 116 Characteristic oxidation product of pyrite in siliceous shalc ____________ 116 Oxidation products of pyrite in slightly and well kaolinized quartz monzonite ___________________________________________________________________ 117 Flat limonite derived from massive pyrite in quartzite _____________________ 117 ix

ILLUSTRA TIONS-C ontinued

Figure 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

x

Botryoidal limonite crusts formed by weathering of pyrite in PAGE limcstone ganguc ................................................................... 118 Coarse and fine cellular sponge resulting from oxidation of pyrite in granodiorite.............................................................. 118 Replacement jaspcr formed by oxidation of pyrite above the water table.... .... ... ..... ........... .... .... ............................ ... ............. 119 Stalactitic and columnar limonite of exotic origin............................ 119 Columnar limonite derived from massive pyrite ............................. 120 Columnar limonite derived from massive pyrite.............................. 120 Fluffy limonite in cavities formed by leaching of pyrite ................... 121 Indigenous thin-walled cellular limonite derived from massive pyrite ....................................................................................... 121 Limonite "dice" after pyrite ............................................................ 121 Radiating fibrous limonites derived from pyrite and from pyrrhotite................................................................................. 122 Flaky limonite crusts formed by leaching of pyrrhotite ................... 123 Characteristic structures and relationships of pyrrhotite derivatives ......................................................................................... 123 Comparison of pyrite- and pyrrhotite-derived leaching products .... 124 Boxwork and flaky limonite formed by pyrrhotite oxidation ........... 124 Pyrrhotite-derived cellular sponge ................................................... 124 Oxidized and weathered pyrrhotite sponge ...................................... 125 Oval cells in sponge derived from pyrrhotite and pyrite .................. 125 Typical oxidized derivative of massive arsenopyrite-pyrite in shale ........................................................................................ 126 Leached derivatives of arsenopyrite and pyrite in slate and mica schist. ....................................................................................... 127 Cellular boxwork emerging from a scorodite-limonite matrix ......... 127 Sketches of boxwork and other leached derivatives of chalcopyrite ....................................................................................... 132 Additional sketches of limonite boxwork derivatives of chalcopyrite........................ .............................................................. 133 A. Sketch of oxidation products of essentially pure disseminated chalcopyrite. B. Sketch of oxidation products of a disseminated chalcopyritepyrite mixture .......................................................................... 133 Cellular boxwork formed by leaching of chalcopyrite in quartz gangue..................................................................................... 134 Cellular boxwork formed by leaching of chalcopyrite in feldspathic gangue........................................................................ 134 Sketches of typical oxidation products of disseminated mixtures of chalcopyrite and pyrite ........................................................ 135 Cellular boxwork derivative of a 2 to 1 chalcocite-pyrite mixture in monzonite ............................................................................ 135 Cellular boxwork derivative of finely disseminated chalcocitepyrite mixture in feldspathic gangue ....................................... 136 Limonite derivative of disseminated chalcocite-pyrite in kaolinized granite porphyry .............................................................. 136 Craggy limonite derived from a chalcocite-pyrite mixture .............. 137 Sketches of limonite boxworks derived from bornite....................... 138 Sawed surface of triangular boxwork formed by leaching of bornite ..................................................................................... 139 Limonite sponge derived from bornite............................................. 139 Bornite-derived boxwork and partially sintered crusts .................... 140 Sketches of contour boxworks derived from tetrahedrite ................. 141 Boxwork and coagulated limonite derived from tetrahedrite .......... 141 Incrustations of antimony oxides on tetrahedrite-derived contour boxworks ......................................................................... 141

ILLUSTRA TroNs-Continued

Figure 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105.

Thin section showing kaolinized amphibolite being replaced by PAGE limonite_____________________ _____ ___ _________ ____ _____ ___ ________ ___ ____ __ ________ ____ _____ 174 Electron micrograph of "soap" being replaced by limonitic jasper____________________________ _____________________________ ___ ____ ___ ____ ______ ____ ____ _ 175 Thin section of kaolinized amphibolite being replaced by quartz and limonite _____________________________________________________________________________ 175 Thin section of progressive replacement of amphibolite by quartz and limonite; polarized lighL _______________________________________ 176 Enlarged portion of Fig. 97 to illustrate replacement process ________ 176 Selective replacement of alunite by limonite ____________________________________ 177 Enlarged view of quartz residual shown in Fig. 99 __________________________ 177 Pscudo-cellular boxwork in amphibolite _____________________________________ 178 Pseudo-cellular boxwork in amphibolite ______________________________________ 179 Map of key bed and specularite-hematite outcrops at the Mount Oxide mine, Queensland_____________________ _ __________________ 180 Cross scction through main workings of the Mount Oxide mine _____ 181 Cross section through high-grade oreshoot of the Mount Oxide minc _____________________________________________________________________________________ _ 182

TABLES Table 1. 2. 3. 4.

5. 6.

7.

8.

xii

Mineralogical constitution and chemical composition of 20 PAGE Iimonites from various localities_____________________________________________ 9 Silica contents of mine waters_____________________________________________________ 19 Solution of SiO" and Fe"O, from norite and diabase______________________ 25 Solution of Si0 2 and Fe"Ol from limonitic jasper, with MgH 2 (COJ"-CaH 2 (CO')2 as solvenL_______ ___________________________________ 26 Progressive formation of limonitic jasper at Ninety-mile copper mine, Queensland__________________ _____________________________________________ 34 Typical jasper outcrop specimens, showing variable silica-ferric oxide content. Hampden and Great Australia mines, Queensland__ ______________________________________________________________ 39 Analyses and probable mineralogical compositions of galenaarsenopyrite sulfide ores and their derived gossans ________________ 130 Analyses and mineralogical composition of massive chalcocite orc on various levels of the Mount Oxide mine, Queensland ___ 183

FOREWORD Many of the significant contributions to the literature of the geological sciences have been concerned with the description and classification of rocks, minerals, and geological processes. They have served to summarize knowledge in their subject areas at the time they were written, and have in many cases served as bases from wbich further investigations could logically proceed. Several of the more significant treatises, such as Lindgren's classic work, have been concerned with all phases of the formation and occurrence of economic mineral deposits, while others, such as that of Locke, have been restricted to one or another of the many approaches that can be taken to an overall understanding of the nature of the mineralizing process. The present book, "Interpretation of Leached Outcrops" by the late Roland Blanchard, is published by the Nevada Bureau of Mines with the hope that it will be accepted by the mining profession in general, and exploration geologists in particular, as one of the more significant works within its area of interest. Roland Blanchard, newly graduated from the University of Minnesota in 1919, was sent west by W. H. Emmons to work with Augustus Locke; who, while directing a search for copper deposits in western North America for Calumet and Hecla Consolidated Copper Co., had originated and was then concurrently directing an investigation into the significance of the differences in structure, texture, and color that had been observed in leached outcrops over known ore bodies. Alone, or together with other geologists in this group, Blanchard spent the next three years in investigations of promising districts in the southwestern United States; including Tyrone, N. Mex., and Silverbell, Bisbee, and Morenci, Ariz. The first of a succession of published reports describing oxidation products of various sulfide ore minerals, many with Paul F. Boswell as co-author, was published in 1925 while Blanchard was investigating the ore deposits at Bisbee. In 1926 Blanchard left the group working under Locke's direction, and in the ensuing two years made studies for other interests of deposits at Kimberly, Nev., at Carlisle and Vanadium, N. Mex., and at Patagonia, Ariz. In 1928 the author was engaged by Julius Kruttschnitt to undertake an interpretation of surface evidences of the disseminated copper deposits at Silverbell, and thus entered a relationship with the American Smelting and Refining Co. that was to continue with only one minor break for the remainder of his career. When Kruttschnitt was informed by Roland Blanchard that he had been offered a position by Mining Trust, Ltd., of London to examine silver-lead deposits at Lawn Hill, Queensland, Kruttschnitt advised Blanchard to accept the offer, and in November 1929 he and several assistants sailed from San Francisco for Australia. The investigation at Lawn Hill ended in 1931 and

Blanchard then went south some 200 miles to Mount Isa, where he accepted the position of chief geologist, and organized the geological staff that in the following decades was to play so vital a role in the development of what would one day become one of the Commonwealth's most productive mines. Julius Kruttschnitt had in the meantime become general manager at Mount Isa as a result of acquisition of a controlling interest in the property by American Smelting and Refining Co. It was during the long fruitful association of the two men that Blanchard developed the system of close day-to-day contact of the geologists with mining operations that would be followed later by other mining companies throughout Australia. At Mount Isa during a period of more than a dozen years, he continued his detailed study of all evidences of the oxidation and leaching phenomena, and it was there that the classification scheme for leaching products was evolved. During the same period Roland made investigations for his company of numerous other deposits in Australia; reference to the pages of this book will serve to show that he travelled widely, and as always, continued to note and study all aspects of each investigation that were pertinent to the leaching problem. In 1939 a summary of the technique of leached outcrop interpretation as it then existed Was published under the same title as the present book by the Chemical, Metallurgical, and Mining Society of South Africa; for that Blanchard received the Society's Gold Medal. Roland Blanchard suffered a severe heart attack while at Mount Isa in 1945, and returned to the United States to recuperate. Although he recovered sufficiently to return to Australia in 1948, in less than a year he was again hospitalized, this time by a near-fatal cerebral hemorrhage. In 1949 he returned to America for the last time, where he was to reside in retirement at Sierra Madre, California until his death. The long years following his illness gave Roland Bl<mchard an opportunity to examine the mass of data accumulated during his earlier research, and with characteristic determination he began to set down on paper not only descriptions of all of the leaching products he had encountered, but also explanations of the oxidation and leaching processes that had produced them. It was now that an association formed in earlier, more active years became significant in Blanchard's efforts. George Tunell, who in the early 1920's had worked with him under Locke's direction in New Mexico, Arizona, and Utah on outcrops of porphyry copper deposits and had subsequently worked for several years at the Geophysical Laboratory of the Carnegie Institution of Washington on the physico-chemical problems of the oxidation of iron and copper sulfides, was then Professor of Geology at the University of California at xiii

Los Angeles. Aware that Blanchard was at work on the manuscript, Tunell, although busily engaged not only in teaching but in his own research activities, reviewed and criticized it as chapter by chapter it took form. Blanchard's manuscript, which he completed in early 1960, had been brought to the attention of the Nevada Bureau of Mines, and in May of that year the Bureau accepted it for publication. Because of delays occasioned by the death of its editor and the appointment of his successor, no serious attempts by the Bureau to edit the manuscript took place for more than a year. Then in the period from 1961 through 1965, as time permitted, the manuscript was edited by Dr. Tunell and Ira Lutsey, Technical Editor for the Bureau. It became apparent to both that the manuscript could be further improved by consolidation and rearrangement of some of the chapters in Part 1 in order to present a more logical development of the section dealing with the chemistry of leaching. These changes were made with the author's approval.

xiv

All substantive changes had been made, and the manuscript was in essentially its final form when on January 15, 1966, Roland Blanchard passed away at his home in Sierra Madre. It is unfortunate that Blanchard did not live to see his work in print, but it is hoped that this book will serve as his most permanent monument among the geological profession. The Nevada Bureau of Mines is pleased to publish "Interpretation of Leached Outcrops" as a service to the mineral industry in general, but most particularly to that small segment of the geological profession upon whom rests the responsibility for the continuing discovery of new ore deposits so vital to the economy of the modern world. VERNON E. SCHEID, Director Nevada Bureau of Mines February 1968 Mackay School of Mines University of Nevada

PREFACE For over 40 years Locke's "Leached Outcrops as Guides to Copper Ore" has stood as a landmark for the Quidance of £:eolo2,ists in the technique of leached outcrop interpretation. It has furnished a background against which the technique became comprehensible, and defined most of the underlying principles which at the time were recognized and had been assimilated. It described the more common derivatives of the dominant copper sulfides, and set forth data whereby those derivatives might be distinguished from one another, and from the more usual derivatives of pyrite. Although restricted mainly to the disseminated copper deposits, the discussion went beyond them, and felt its way cautiously toward other types. Most of the principles as enunciated by Locke have withstood the test of time. As the pioneer effort in providing a perspective for leached outcrop interpretation the book is destined to endure. Since its publication much new information has come to light. Additional sulfides not necessarily confined to the copper family, as well as other iron-yielding minerals which upon leaching leave behind distinctive limonite derivatives, have been added to the list, and much has been learned about outcrops yielded by massive sulfides. Furthermore, a broader comprehension has been gained of the processes governing formation of the various limonite products encountercd in nature, and the basis upon which the technique rests has been amplified through refinements in interpretation not recognized in the earlier work. Some of this later information has been published in the technical journals, at irregular intervals, over a period of more than 30 years. Some was withheld because, unless fitted into a much fuller discussion of the subject, its publication did not seem appropriate. For the technique to attain its maximum usefulness a need thus would seem to exist for a new compilation of the data, which incorporates the more recent findings, and presents a unified and comprehensive approach to the subject. That the need has been recognized within the mining profession is attested by the fact that in recent years there have come from examining engineers and geologists, from mine operators and prospectors, numerous requests for an up-to-date presentation of the subject in a single volume, together with descriptions, sketches, and photographs in each case, which would facilitate a more rapid field identification of the different limonite products encountered. The present volume is offered in response to those requests. In some quarters the opinion has been voiced that the day for leached outcrop interpretation has passed; that the orebodies which cropped out, either directly as ore or indirectly as its leached equivalents, have been found and in large part already mined; that orebodies of the future therefore must be detected not by the

time-worn and simple expedient of searching beneath the earth's surface for ore as such, but by employing highly specialized personnel traincd to project geologic structure to favorable loci hidden beneath surface exposures which themselves are barren; and that the subject matter herein dealt with consequently becomes one chiefly of historical interest. No one who has spent many years searching for orebodies in different parts of the earth has failed to be impressed by the small number of deposits remaining untested that boldly advertise their presence at the surface, nor by the fact that geologic methods, able to penetrate far below the earth's largely opaque exterior, not only must be improved, but new ones devised and brought into being, if the flow of metals upon which our industrial civilization depends is to be maintained. The author has played a part in the discovery of several major ore bodies by employing such methods of long range structural projection, and by predicting favorable loci in ground where the surface was marked neither by ore nor by its oxidized equivalents. He thus is in a position to evaluate, in a measure, the significance of these methods. Although these new methods are important when employed with discretion and restraint, and although they are certain to improve, the author remains un convinced that a revolutionary change in the science of finding ore is about to overwhelm us, that the older methods of searching for ore are to be at once outmoded or supplanted, or that the finding of valuable orebodies within or subjacent to the currently somewhat discredited oxidized zone is a thing of the past. The author's experience has failed to confirm it, as has that of various other geologists engaged in the search for ore with whom the author has maintained close contact in recent years. Not every observer is ready to concede that the earth's surface has been combed over sufficiently by those qualified to appraise leached outcrops, to assure that all of the Utah Copper's or Morenci's or Toquepala's have been discovered. The more obvious ones admittedly have been taken in hand; but many others, inherently of lower grade, possibly still may be found; and the ability to accurately interpret the outcrops of such lower grade material should assist measurably in reducing the exploratory costs. This is especially true for the prospectors who do not live in the United States. Even where recourse is had to the newer procedure of searching for orebodies entirely hidden beneath the earth's surface by long range structural projection, the advantage manifestly is with the person able to translate into terms of the parent minerals the leached material of the overlying or projected outcrops involved, even though no actual ore minerals be present in the outcrops. xv

The fact that there are reserves of ore in districts which already have substantial ore production to their credit is encouraging to the person inexperienced in leached outcrop interpretation who desires to gain reasonable proficiency in it; for in such districts he is usually able to trace a given limonite product down through the mine workings directly to its source. In the long run his proficiency in interpretation will be proportional to his own experience and success in observing the limonitic products grading into the parent mineral, either directly, or through successive stages which he himself has traced. The information presented here can merely assist and guide him in developing a technique which to an important extent must be his own creation. Aerial surveying has provided a partial answer to the initial assessment of the resources of large and previously unexplored areas. 1 But as pointed out in the Bulletin of the Alaskan Territorial Department of Mines (1955), "All the fancy gear in existence will not fully take the place of a good prospector patiently examining the country on foot. On the other hand, a prospector will have greater chances of success when working in an area where aerial work has disclosed above-normal radioactivity." Not all prospectors are familiar with radioactivity in its various phases; and, except in special cases, aerial surveying is only one of the tools for the geologist, not a substitute for geology itself. Although the new technique of geochemistry has shown that in many places soils, leaves, and roots have their stories to tell, these geochemical processes are not considered here in detail, as various books on the subject have been published recently. In setting forth the subject matter of leached outcrop interpretation the alternatives have been: 1) presentation in language strictly technical, permitting greater condensation but restricting the volume's usefulness mainly to the trained geologist; 2) presentation in language comprehensible to interested mine operators and prospectors. The author feels that unless the information is set forth in language that may be grasped readily both by the mine operator (who often must serve as the prospecting authority at mines not large enough to justify employing a full-time geologist), and by the more progressive "The instruments used in aerial surveying are: the airborne magnetometer, which records the variations in the earth's magnetic field, and is valuable in prospecting for iron, nickel, titanium, asbestos and oil; the scintillometer and Geiger counter, which detect the presence of radioactive materials in the earth's crust; and the electromagnetic detector, which sometimes makes it possible to locate sulfide orebodies which contain copper, lead, zinc or nickel. A recent design development has made possible the use of all three electronic systems in a single aircraft.

xvi

prospector, the purpose for which the volume has been written-namely, to increase the amount of ore found and reduce the cost of finding it-may be in large part, at least, defeated. In the following pages there consequently will be found discussed certain fundamentals pertaining to chemistry and physics which to the trained geologist may seem elementary and superftous. They are limited to portions of the two subjects that in 40 years of field contacts the author has found it necessary in most cases to explain in detail to the prospector, to the mine operator, and in numerous instances to geologists themselves, before those individuals were able to grasp the principles intelligently. If to the reader of more advanced training the method of presentation in places appears over-simplified, his indulgence and tolerance are asked for the sake of those less fortunate." The author is indebted to Augustus Locke, for his book has been used freely. In a series of published papers Paul Frederick Boswell and the author have made some changes in the technique, and these are indicated in this volume. The author is indebted to Boswell, who, during the period 1923-1931, was the mainspring in the development of the chemistry and physics of the leached outcrops in the field; we miss him greatly. George Tunell also very carefully criticized the work, and gave the author many helpful suggestions in chemistry and physics. E. N. Pennebaker, Kenyon R. Richard, and Graham Hall, who have made leached outcrops a lifetime study, have contributed valuable information. William C. Kelly (1958) brought the information concerning lead-zinc deposits up-todate. The officials of various companies in different parts of the world were very helpful; the author is especially grateful to the Calumet & Hecla Consolidated Copper Co., the Phelps Dodge Corp., and the American Smelting & Refining Co., and particularly to Mount Isa Mines Ltd., which company for 17 years allowed the author the widest latitude in pursuing his investigation, consistent only with his regular duties. The author, however, accepts sole responsibility for all statements made. The present volume is in two parts. Part 1: Chapters 1 to 17 explain the general geology of leached outcrops. Part 2: In Chapters 18 to 36, nineteen sulfides or other minerals, and their oxidized or weathered equivalents are explained and illustrated. Some of the material on the oxidized equivalents of ore minerals in this book was included in H. E. McKinstry's "Mining Geology" on pages 261-276, First Edition, 1948, as a contribution by the present author. 2To the beginner in elementary mineralogy a discussion of the six systems of crystals would be helpful, but such a discussion is not included in this volume.

PART 1 Part 1 of the book is chiefly concerned with descriptions of the various processes and substances involved in the creation of leached outcrops. Following the introduction, which constitutes chapter I, the term "limonite," is defined in chapter 2. Chapter 3 describes three types of limonite texture that are significant in the interpretation of leached outcrops. Chapter 4 discusses the processes and products of the precipitation of iron and silica compounds from solution. Chapter 5 is concerned with the nature and significance of one type of limonitic jasper, the cellular pseudomorphs, and chapter 6 is concerned with the nature and significance of a second type of limonitic jasper, massive jasper. Chapters 7 through 14 are concerned with the chemical reactions that take place in various types of oxidizing sulfide and non-sulfide ore deposits. The availability of sulfur to form sulfuric acid, and the oxidation reactions of the common sulfides are first dis-

cussed. The common reactions by which limonite is precipitated are then described, followed in the last three of these chapters by illustrations of the chemical processes actually taking place at various mines with differing types of host rocks and sulfur-iron ratios. Chapter 15 treats the uses and limitations of leached outcrop color as a guide in ore exploration. Chapter 16 in many ways represents the heart of Part 1, in that the essential facts ot the composition and chemistry of leached materials given in earlier chapters, are applied to a systematic discussion of the types or forms of the final products encountered. Here are defined all of the specific terms by which particular leaching products are described in Part 2. Chapter 17 lists several of the ways in which the leached outcrop technique has been applied in actual exploration situations. They are based on the author's personal knowledge and experience, and serve to indicate in a general way the usefulness of the technique.

Chapter 1 INTRODUCTION ORIGIN AND HISTORY OF THE INVESTIGATION It is well known that some of the old-time prospectors were notably successful in selecting untested outcrops under which ore was likely to be found. In some instances, as when they were searching for gold ore, they may have been guided by the type of fracturing in the quartz of the lode. Whatever constituted their bases for judgment, it is well established that their success varied in direct proportion to the care and accuracy with which they analyzed the particular details of an outcrop, rather than in trusting to its general appearance. One of the most successful of such prospectors for disseminated copper ores was Clyde Wardlow, of Plumas County, Calif., with whom we came in contact shortly after the close of World War I. Clyde had worked out for himself certain elementary distinctions between the limonites left by pyrite and those left by bornite in the rocks of that district. Although unable to define the distinctions for the benefit of others, his own success in differentiating the good from the poor outcrops, based mainly upon the so-called "dead" look of the pyrite derivative and the "live" look of the bornite derivative, was so marked that an investigation of the distinctions on a much broader scale seemed warranted. That investigation was originated by, and for a number of years was prosecuted under the direction of, Augustus Locke, as a side issue in connection with a scouting campaign for copper deposits which he was conducting at the time in the western half of North America for Calumet & Hecla Consolidated Copper Co., and during the first year or two under the supervision of Charles Henry White. Before the work had proceeded far, it was observed that a pyrite or a bornite that had oxidized in Plumas County did not necessarily leave the same type of limonite as did one that had oxidized in the moist climate of British Columbia and the Alaskan Coast, or in the desert regions of Arizona and Mexico. It was observed further that, even in the same district, when any given sulfide, such as chalcopyrite, oxidized in a quartz gangue it did not necessarily leave the same type of limonite as when it oxidized in a feldspar gangue or in a limestone gangue. It thus became evident that although the gathering of field facts relating to the various limonite products was fundamental, and was a necessary preliminary to classifying the different limonite types, there was needed, in addition, a knowledge of the physics and

chemistry of the processes involved in the oxidation of the respective sulfides. Investigation had shown that several of the chemical "reactions" listed in the textbooks, and supposed to take place during oxidation of the various sulfides, rested upon assumptions that were not borne out in the products actually yielded by nature. It therefore became necessary to attack the problem from the beginning, and to obtain trustworthy information as to the sulfur, iron, water and oxygen available for and entering into the reactions in any given case; and to take into account, further, the amount and character of neutralizer in the gangue, which tended to obscure or obliterate the products that would have been formed under normal air-water oxidation processes alone. Thus, just as the idea of secondary enrichment sixtyodd years ago ena.bJed geologists safely to forget reams of field details relating to the various ore occurrences, because they could connect up those details into an idea of the processes of secondary enrichment, so was it recognized that a correct knowledge of the processes of limonite formation in their many varying phases would simplify the interpretation of leached outcrops, and would, in fact, be essential before the leached outcrops data could become either intelligible or universally applicable. Locke thereupon enlisted the cooperation in the field of such physical chemists as H. W. Morse, P. F. Boswell, and George Tunell, and in the laboratory of such men as Tunell, E. Posnjak, H. E. Merwin, E. G. Zies, E. T. Allen and others of the Geophysical Laboratory staff at Washington, D.C. These men took the mass of field evidence that had been collected up to that time, brought their specialized knowledge of physical chemistry to bear upon the problems involved, performed a great deal of closely controlled laboratory investigation and experimentation upon the outcrop specimens, pointed out why under a given set of conditions certain products were obtained while under other conditions different products were obtained, gave the field workers a more definite idea of what to expect under conditions not yet encountered, and, in general, furnished a perspective that permitted the work to proceed along intelligent lines, with specific objectives. In addition, such men as F. L. Ransome, B. S. Butler, Waldemar Lingren, L. C. Graton, G. F. Loughlin, W. H. Emmons and other active or former members of the U. S. Geological Survey cooperated fully in the work. While these men did not, for the most part, engage in the field investigations directed specifically toward this objective, they contributed freely of their

4

INTERPRETATION OF LEACHED OUTCROPS

previous field experiences, and at all times accord.ed the

usually differ in minute physical characteristic? from

venture their whole-hearted support. The same IS true of a number of commercial geologists in different parts of the world. such as E. H. Wisser, H. M. Kingsbury. and others. It would be unfair not to pay acknowledgment also to the various mining companies and to their officials in different parts of the world, but especially in western North America, for their generosity in permitting the investigation to be carried on in their properties, particularly to the Calumet & Hecla Consolidated Copper Co., the Phelps Dodge Corp., and the American Smelting & Refining Co., for much direct encouragement, and to some extent for financial assistance, during early stages of the work. The credit for enlisting the interest and cooperation of the various individuals and companies belongs largely to Augustus Locke. It was he who originated the idea and gave the work its first semblance of organization' it was he who created interest and secured the suppor~ of those whose support was needed to make the venture a success; and, to an important extent, it was he who furnished the stimulus to keep the work going, during the early years especially, when without active stimulation it probably would have died a rapid death, as so often is the case with newly-born ideas of that sort. Chronogically it may be stated that the idea of utilizing leached outcrop products in the search for orebodies was conceived in the summer of 1919. The first intelligent basis for applying the technique upon more than a very local scale was developed in the Silverbell district of Arizona late in 1922, and in the Bisbee and Morenci districts of Arizona, and at Tyrone, New Mexico, during the spring and summer of 1923. The first paper dealing with direct application of the technique was published in 1924, and the first one which described particular types of limonite that had been found dependable in relating the leached products to specific sulfides was published late in 1925. The following year appeared the first comprehensive publication in book form (Locke, 1926), which discussed many of the underlying principles, and described a ,number of the more common limonite derivatives of several of the copper sulfides. This was followed in 1927 by descriptions of the more important derivatives of the lead and zinc minerals. Since that time, at irregular intervals, additional sulfides and other iron-yielding minerals have been added to the list, and the basis upon which the technique rests has been amplified by refinements in interpretation that were not recognized in the earlier work.

corresponding products derived from other mmerals, such as sphalerite or galena. The differences involve cellular structure, texture, pulverulency, size and arrangement of the limonite grains, and other properties. Of the various distinguishing features, the cellular structure is the most outstanding and the easiest to recognize and identify. It derives its shape or form from the cleavage or fracture pattern of the sulfide or other mineral undergoing oxidation, and results from a webwork of limonite or limonitic jasper "eating" its way along cleavage or fracture planes during oxidation. Thus galena not infrequently yields limonite with a cubic boxwork patterned after its cubic cleavage. The differences are usually marked between limonite derivatives of sulfide minerals as a group on one hand, and derivatives of non-sulfide gangue minerals as a group on the other. Thus it rarely happens that limonite yielded by bornite or chalcopyrite will be confused with that yielded by garnet or epidote. Differences in physical characteristics among the various limonite products usually are also pronounced where the products represent sulfide derivatives. Limonite of chalcocite derivation, for example, rarely if ever resembles in detail that derived from sphalerite. Limonite of pyrite derivation rarely resembles in detail that derived from tetrahedrite. On the other hand, limonite products derived from nonsulfide gangue minerals only, often closely resemble one another. One reason is that many of such products consist mainly of granular or pulverulent limonite. Exceptions exist; a few of the non-sulfide gangue minerals, such as calcite, siderite, and fluorite, under restricted conditions yield some of the most distinctive limonite structures found in nature. Likewise, several of the sulfides under certain conditions, and all of them to some extent, decompose into limonite products which seem lacking in character. In the main, however, the derivatives of sulfides possess marked individuality of shape, pattern, relief, and other physical features, whereas those of non-sulfide gangue minerals on the whole are deficient in or devoid of features that attract the eye readily. A person versed in leached outcrop interpretation may successfully differentiate the limonitic derivative of epidote from that of chlorite or garnet. The beginner almost certainly cannot do so; to him the respective products are likely to exhibit only unenlightening uniformity. That, however, need not discourage him, for usually the observer is not interested in non-sulfide gangue minerals for their own sake; he is interested chiefly in the ore minerals, together with their associated gangue sulfides such as pyrite or pyrrhotite. So long as he can distinguish the leached products derived from individual members of that group, and can distinguish them in turn from leached products derived from non-sulfide gangue minerals as a whole, he stands on reasonably firm ground. The shape of the derived cellular structures is caused in part by stresses set up within the primary ore body

BASIS OF LEACHED OUTCROP INTERPRETATION Interpretation of leached outcrops is based upon the observed fact that limonite products derived by the oxidation of any given mineral, such as chalcopyrite,

INTRODUCTION

during cooling (Blanchard and Boswell, 1934, p. 688; Kelly and others, 1958), and by the presence of joint and fracture intersections in either sulfides or other minerals, as with supergene siderite or the ferromagnesian rocks.' The experienced observer generally has little trouble in this, though the beginner sometimes has trouble in determining what is cellular and what is not cellular. However, with practice the beginner soon begins to understand what is and what is not the true cellular structure. The stresses during cooling and the joints and fracture intersections in the cellular structures, are not important, however, except in certain districts. In some instances the recognition of leached derivatives of non-sulfide gangue may be fundamental to the interpretation. The Whipstick (New South Wales) garnet pipes in granite (Andrews, 1916, p. 150-166) furnish an example. Many of the Whipstick pipes carry concentrations of native bismuth and molybdenite that proved highly profitable to mine. Within 40 feet or less of the surface those two minerals are oxidized almost wholly to bismuth ocher and molybdic ocher respectively. Since there has been little or no leaching of the metallic constituents-merely the alteration, more or less in place, of native bismuth and molybdenite to their oxidized metallic equivalents-no real problem of outcrop interpretation would arise, were it not for the fact that the garnet is the particularly vulnerable manganese variety, spessartite (Stillwell, 1943). The garnet has been oxidized so thoroughly in many of the pipes as to yield overwhelming masses of hydrous manganese dioxide almost pure enough to be shipped as manganese ore, and the smaller amounts of bismuth and molybdic ocher have been almost entirely masked by the brownish-black manganiferous gossan. Gossan derived from non-sulfide gangue in this case thus constitutes the best overall guide to the ore itself, rather than any readily detectable amounts of oxidized derivatives of either native bismuth or molybdenite. Although examples of this sort occur, which the observer from time to time will encounter and must be prepared to interpret, the non-sulfide gangue minerals usually do not disintegrate and become converted into limonite as readily or completely as do sulfides. Even though sulfides be completely leached from an outcrop, remnants, at least, of the unleached non-sulfide gangue in most cases survive conspicuously to serve directly in identifying whatever limonitic products may have been derived from them. 2 This greatly simplifies the task of interpretation, since in most cases it establishes the parentage of those products that are most likely to confuse the beginner. Although features that characterize these limonite types are often not readily distinguishable in the hand specimen, they usually are plainly visible under the hand lens of ordinary magnification, such as X 5 or 'See Chapters 4, 11, 33, and Appendix E, figs. 101, 102. 'The statement does not necessarily apply to humid tropical regions. For a noteworthy exception under other conditions, see chapter 34, Fluorite.

5

X 10. Where greater magnification is required, such as X 16 or X 20, this is indicated in the detailed descriptions which follow. The scout or prospector therefore requires no high-powered microscope or other cumbersome field apparatus for identifying the various limonite products. The ordinary hand lens, a good eye for detail, and a thorough acquaintance with the features which characterize the different limonite types, are all that is ordinarily needed for the identification of most of the leached products he encounters. Despite the fact that sulfides and other minerals (such as chromite) each yield several distinct limonite types, each sulfide or other mineral ordinarily yields one type that predominates; one whose shape or structure differs markedly from the predominating structure in limonites of other origin, and which is discernible to a greater or lesser extent throughout all limonite products yielded by that particular mineral. The predominating and identifying pattern is then called the "key" structure for limonites derived from that mineral. It is ordinarily, but not always, a characteristic variety of cellular boxwork or cellular sponge, except in the case of pyrite. Usually the "key" structure is partly obscured by fine flakes, crusts, rosettes, and grains of limonite that coat the cellular boxwork or sponge pattern. Few cases have been encountered in which the "key" cannot be identified under the hand lens, if careful search for it be made. As a rule the best limonite specimens do not occur directly at the surface, because weathering tends to "whip out" the softer limonite particles. To procure a representative specimen it usually is necessary to chip off the outcrop so as to secure a fresh exposure several inches below the surface. In soft gossans it may be necessary to dig down several feet or more, but generally no elaborate, expensive procedure is necessary. In most cases a scout or prospector may obtain the specimens needed by vigorous use of the prospector's pick. In exceptional cases the shovel, or a few "pop" shots, may be required. Granting that limonite products of different origin possess individually distinguishable characteristics which in one manner or another may be established through field observation, the task for the observer is so to familiarize himself with those characteristics that, irrespective of the degree of oxidation and leaching involved, he may recognize them with assurance under a wide variety of conditions, even though they may be mixed with, become partly obscured by, or grade insensibly over short distances into other products which they superficially resemble; in other words, that he be able to recognize his acquaintances unerringly, even amidst the crowd, in a fog.

IMPORTANCE OF CHEMISTRY TO THE LEACHED OUTCROP TECHNIQUE The outcrops produced by the oxidation and leaching of massive-sulfide deposits are composed mostly of limonite and limonitic jasper. The leached outcrops of

6

INTERPRETATION OF LEACHED OUTCROPS

disseminated sulfide deposits include, in addition, relatively large amounts of argillized and sericitized country rock. Understanding of the basis of the technique is dependent upon a knowledge of two types of chemical reaction essential for leached outcrop formation. First, the oxidation and solution of the sulfide minerals must take place; second, cellular pseudomorphs or other types of limonite must form as a result of the precipitation within the outcrop of ferric oxide or ferric oxide hydrate. The essential chemical reactions can take place only if sufficient amounts of certain reactants are present. Unless sufficient sulfur is present the sulfide minerals cannot be completely oxidized and dissolved. Even if the sulfides are dissolved, limonite cannot form at or near the same location unless sufficient iron is present in solution, and is later precipitated from it. The presence of pyrite is important to both reactions because it yields both sulfur and iron in large amounts. Its oxidation adjacent to and contemporaneously with any of the sulfide ore minerals not only hastens their oxidation, but provides a source of iron· for the formation of limonite. It should be noted that, since a high ratio of pyrite to other sulfide minerals causes high acidity in the oxidation solutions, and since high acidity favors exportation of the iron from the point where the sulfides are oxidizing, it follows that the higher the ratio of pyrite to other sulfides, the less likely is limonite to be precipitated at the point of oxidation. Chapters 7 to 14 describe the important chemical reactions that involve the sulfide minerals of oxidizing deposits. Several of these chapters are primarily concerned with the effect of reactive host rocks on sulfide leaching and limonite precipitation.

SCOPE AND METHOD OF PRESENTATION The interpretation of leached outcrops is dependent upon three essential bases: 1) a knowledge of the composition and distinguishing characteristics of the particular leached specimen or area of leached outcrop under investigation; 2) a knowledge of the physical and chemical processes needed to produce that specimen or outcrop; and 3) a knowledge of all the variations in occurrence of the more common ore minerals that yield leached outcrops. This book is an attempt to present to geologists, mine operators, and prospectors the essential facts thus far determined in the first two of these areas. The third requirement can never be fully attained, but may be approached through a sufficiently broad range of field experience. In this book the author's observations over a period of nearly 40 years of the occurrences of ore minerals and their oxidized counterparts are set down and illustrated in numerous cases by color photographs. The current practice of the U. S. Geological Survey and the Geological Society of America in citing published reports referred to in the text, is followed in this book. The method, placing the publication date in parenthesis after the cited author, for example: Smith ( 1938), has the advantage of simplifying chronological descriptions of investigations. Also it indicates to the reader the age of the reference relative to others that are cited in the same discussion. Four appendixes are included at the back of the volume.

Chapter 2 THE TERM "LIMONITE" DEFINED At this point it seems well to define specifically the term limonite, for no name in mineralogy is used to describe a wider range of substances. The gossanous material derived from leaching of sulfides or other strongly iron-yielding minerals; the rust of iron machinery exposed to the elements; the brownish, slimy, flocculent material which frequently encrusts the insides of pipelines and the mouths of faucets, or forms along drains from mine workings; the brownish or reddish crusts and stains that coat rock fractures in regions of abundant rainfall; the glistening reddish-black "desert varnish" of pebbles and boulders exposed to the action of wind and sun in the arid tropics; the pigment which imparts the reddish, yellowish, or brownish color to soil-in fact all the reddish, yellowish, brownish or brownish-black deposits formed by decomposing ironyielding minerals or substances of nature, regardless of their origin, are commonly grouped under the term "limonite." This chapter is devoted to an explanation of the use of this indispensable term.

HYDROUS FERRIC OXIDES AND OTHER IRON-BEARING COMPOUNDS No mineral corresponding to the formula for limonite given in the older textbooks-2Fe"O"o3H~O-actually exists. As the outcome of laboratory investigations connected with the Secondary Enrichment Investigation, Posnjak and Merwin (1919) established that among previously assumed hydrous ferric oxide compounds ' there actually exists only one, namely, the monohydrate, FeeO"oHeO. Two minerals found in nature, goethite and lepidocrocite, have this composition. Additional water found in some analyses of natural materials represents adsorbed and capillary water. By means of X-ray diffraction photographs of a crystal or of the powdered crystalline material if no crystal is available or preferably of both when possible to obtain both-the mineralogist can reconstruct the cell pattern of any mineral or compound possessing crystalline structure. Since each mineral or compound has its own specific cell size and shape, which is as characteristic of it as is the fingerprint of a human being, he readily can tell whether he is dealing with similar or different minerals whose chemical analyses may be identical or closely 'In the older literature a number of other supposed hydrated ferric oxides were listed as distinct minerals (turgite, xanthosiderite, hydrogoethite), but these are now considered impure varieties of hematite or goethite or lepidocrocite.

similar. Thus it was established that although goethite and lepidocrocite both contain the same amounts of ferric oxide and water," and both belong to the orthorhombic crystal system, nevertheless the two differ in their axial ratios and atomic arrangements.

Goethite Goethite crystals are blackish-brown. Earthy varieties of goethite commonly have an orange-ochreous to yellow or yellowish-brown color. The massive goethite has a yellow to dark-brown color. Goethite is the sole ferric oxide hydrate thus far identified, which, as the pigment, imparts a yellowish to brownish color to the soil.

Lepidocrocite Lepidocrocite has a more characteristically ruby-red to reddish-brown color, but may be orange-red to yellowish.:: Although an orange-ochreous color usually is strong presumptive evidence of the mineral being goethite, the two minerals are not readily distinguished by the unaided eye, and not always with certainty under the microscope. Often one can be sure of the specific mineral only after making X-ray diffraction photographs: but lepidocrocite is extremely rare in leached outcrops. 'In recent years some mineralogists have adopted the formula HFeO, to designate goethite, and the formula FeO(OH) to designate lepidocrocite. The reader should familiarize himself with both so that he will recognize them when they are encountered. It will be noted that 2HFeO, equals Fe,03 0 H,O and likewise 2FeO(OH). In adopting the formula FeO(OH) for lepidocrocite these mineralogists have attempted to make the formula express something of the structure of the crystal. The matter is not readily explained in simple language, but they have not succeeded very well in their purpose (see Bragg, 1937, p. 1l0-113), and the person who has not specialized in crystallography is in danger of becoming misled by this formula into supposing that the iron in lepidocrocite is in the ferrous condition. "Its more typically reddish color suggests that lepidocrocite may constitute the pigment of red soil. Such does not appear to be the case, however; lepidocrocite has not been thus far specifically identified in red soil. The pigment which imparts color to red soil, so far as ascertained, is either hematite or both hematite and goethite. 'Kulp and Trites (1951, p. 35-42) concluded from differential thermal analyses that lepidocrocite occurs commonly in natural hydrous ferric oxides. This conclusion was shown to be erroneous by Kelly (1956, p. 353-355; 1957, p. 536-545), who found that samples of poorly crystallized goethite yielded differential thermal analysis curves that were indistinguishable from standard curves for lepidocrocite.

8

INTERPRETATION OF LEACHED OUTCROPS

Other Natural Compounds Hematite (Fe"03) occurs in many outcrops as a supergene mineral. It is especially common in capping over disseminated copper deposits, where it is often present as the principal or exclusive derivative filling or fringing the cavities vacated by the sulfides. As filling it occurs mainly as finely fibro~s hematite, or ~s a reddish-black crust whose surface IS made up of tillY, mutually encroaching or overlapping nodules or spherules. Supergene hematite fringes surrounding the cavities may comprise either the tiny spherules, or granular and pulverulent products. Supergene hematite also forms narrow seams and veinlets in capping of this sort, and often stains surrounding rock or soil a prominent red. A feature of all such hematite is its generally porous nature, as contrasted with the dense variety .of hematite possessing metallic luster. Supergene hematite as capping over disseminated copper deposits may be the sole constituent of an individual cavity or over an area of several square inches, but generally is associated with goethite and often is intergrown with it. It should be noted that although hematite is ferric oxide, unlike goethite it is not a hydrate, as it conta!ns no water of crystallization. It is an anhydrous oXide (Greek-without water). Specularite (FeeO,,), the hypogene variety of he~a­ tite, is less common than fibrous and earthy hematite; in many districts it is absent. It occurs either with hematite, or as specularite exclusively. Specularite has a metallic luster and often has a foliated or micaceous structure. Supergene magnetite (FeO.Fe"O:J, is found occasionally (Brown, 1943, p. 137-143), but in some places (for example, Kimberly, Nev.) it is sporadically plentiful. Sulfates and Carbonates. A supergene derivative not infrequently encountered in gossans is jarosite, K"O.3Fe e0:1 ·4S0".6H2 0. It is not a ferric oxide or ferric oxide hydrate, but a basic sulfate of iron and potassium. In the vicinity of potash-bearing rocks it forms under much the same conditions, however, as do supergene goethite and hematite, and often it is closely intergrown or mixed with them. Frequently it may be distinguished in the hand specimen by its yellowish to clove-brown glistening crystals or incrustations, but in other cases it can be identified only under the microscope. In certain districts it is abundant, as for example at Utah Copper; but usually it is less stable than the ferric oxide hydrates, and in gossans of long standing it usually alters' to goethite or hematite. Because of frequent difficulty in differentiating it from goethite in the hand specimen, it usually is included under the field term, "limonite." In addition to the siderite already discussed as a component of supergene gangue carbonate co~plexes, supergene siderite may form under some condlhons as almost the exclusive iron derivative of sulfides. At the Gardner mine, Bisbee, Ariz., it is virtually the only direct iron derivative from the leaching of one major

massive pyrite-chalcopyrite-chalcocite body in the altered limestone halo which adjoins the Sacramento Hill granite-porphyry stock. At that mine it forms coarse cellular boxworks, maple-brown in color, which in outline coincide with the fracture pattern of the adjoining limestone (see ch. 33, figs. 85, 86). Supergene siderite of this type is prominent also at Cananea, Sonora, Mexico, and elsewhere. In most districts the product constitutes at best only a small perc~nt~ge ~f limonitic derivatives, and in a great many dlstncts It has not been observed. Siderite is ferrous carbonate (FeCO,,). Like jarosite, it is less stable under chemical attack than are the ferric oxide minerals, and in time it usually alters to goethite. Because of that fact, and because both as a boxwork and in other supergene forms it yields more readily to the physical attack of weathering it is not frequent. It is included in the discussion here, however, because of its often close resemblance to goethite, especially in the partly weathered state, and its not infrequent association with it.

IMPURITIES PRESENT IN "LIMONITE" Silica Virtually all limonite contains at least a small silica admixture. In limonitic jasper the silica may be present in more than one form. In cellular pseudomorphs it most often occurs as an admixed co-precipitate with ferric oxide hydrate, both usually on a microscopic scale, or as an amorphous form. In massive jasper however, silica may occur also as a component of no~tro­ nite (although usually given as 6Fe 2 0,.AI 2 0".22S10 2 • 6H.,O (Na 2 0,CaO), the composition is not fixed), a gre~nish yellow to light brownish-yellow kaoli~ic-op~­ line substance often called chloropal, descnbed III chapter 6.

Carbonates Silica, however, is only one of numerous such admixed or intergrown impurities. Even among cellular pseudomorphs a complex, made up of various intimately intergrown, fine-grained, supergene" gangue carbonate minerals, not infrequently takes the place of "Supergene denotes formation, deposition, or enrichment by waters, initially cold, descending from the earth's surf~ce. Hypogene denotes formation or deposition by water ascendmg from the earth's warmer interior. The terms were first proposed by Ransome (1912, p. 152-153) who sought, in discussing the processes of ore deposition, and especially pro~esses of sec?~d­ ary enrichment, to do away with the confUSIOn often ansmg from use of the largely but not entirely analogous terms secolldary and primary. For example, the oolitic iron or~ at Wabana Newfoundland described by Hayes (1915), IS a bedded ~rimary ore deposit because the. iron cont~nt was present in the sediments at the time the senes was laid ~own. But the materials comprising the sediments, including the Iron, ~ere carried there by supergene agencies-ground water . (nver water) emptying into the sea. Unlike secondary and pn.mary, the terms supergene and hypogene are mutually exclUSive m their meanings under all conditions.

THE TERM LIMONITE DEFINED

limonitic jasper, either forming directly as boxwork or sponge, or, more often, building or replacing the limonitic jasper subsequent to the latter's formation. Where a neutralizer is abundant in ground water, as in areas of limestone and certain other rocks, this complex product is common. At Broken Hill, New South Wales, such supergene gangue carbonate matter, made up of a complex comprising fine intergrowths of calcite (CaCO,,), magnesite (MgCO:J, rhodochrosite (Mn COJ, and siderite (FeCO,) in varying proportions, may be present locally in amounts exceeding limonitic jasper of the cellular pseudomorphs (Garretty and Blanchard, 1942). Upon weathering it yields sufficient pulverulent hydrated ferric oxide from the decomposing siderite to make the thoroughly weathered product virtually indistinguishable from normal "limonite" gossan in the hand specimen.

Manganese Again, manganese quite commonly is present as an admixed impurity of limonite. By this is not meant a condition such as that at Whipstick, New South Wales, where supergene manganese minerals form the dominant gossan constituent, but rather one in which the manganese occurs as a minor and often inconspicuous component. As such it may be present both as a replacement constituent of limonitic jasper and as a free precipitate. Usually when present as a free precipitate the manganese occurs as the hydrous dioxide, called wad. It also may occur, however, as tiny specks or granules of manganite (MnzO".HzO), pyrolusite (MnO z with up to several percent H 2 0), braunite (3Mn z0 3 ·MnO. SiO z ), and other manganese dioxide forms. If, as frequently is the case with wad, its color is dark brown instead of black, or if, in any form, the particles are finely dispersed, the manganese minerals may be almost impossible to distinguish in the same specimen from the ferric oxide matter in which they are embedded. When manganese occurs in crystalline "limonite," it is usually as an isomorphous substitution for part of the iron, and the presence of manganese is not visually detectable under the microscope.

Gypsum In some districts gypsum (CaSO 4 .2H zO), acquired the same brownish color as the limonite and is thus not easily differentiated from the latter even under the hand lens. In some cases gypsum takes the place of silica as the "gluing" or binding agent in holding the particles together to form granular limonitic aggregates. Gypsum has been known to constitute more than 25 percent of certain gossans, as over the mine fire area at United Verde Mine, Ariz.

Other Minor Impurities But disregarding cellular pseudomorphs and granular aggregates, and considering only the loosely granular or pulverulent limonitic material, many impuri-

9

ties still are present. In numerous districts the fine-grained basic sulfate jarosite (K zO.3Fe 20 3 .4S0 3 • 6H zO) is not separately distinguished, and is commonly included under the name limonite. In districts where arsenopyrite is present, scorodite (FeAs0 4 .2HzO) as well as other arsenates is common, often effectively hidden by the more abundant limonitic matter, especially if the percentage of scorodite or other arsenate is low and the mineral is well dispersed. If lead minerals are present, mimetite (3Pb 3 As 2 0s.PbCl z) frequently forms, and not uncommonly weathers in part to massicot (PbO). Both mimetite and massicot, thus derived, are widespread in Australia, and often so minutely dispersed through the ferric oxide hydrate as to be difficult to detect in the hand specimen. In humid climates organic salts of iron often form conspicuous coatings along rock fractures. Most organic salts are unstable and eventually alter to one form or other of ferric oxide hydrate, but while they exist they readily may be mistaken for "limonite." To furnish an idea of the character and extent of impurities, table 1 is presented. It sets forth analyses and calculated probable mineralogical constitutions of twenty limonitic products of diverse origin and distribution, all of which in hand specimens had the appearance of and commonly were referred to as "limonite" in the districts where they occurred.

SUMMARY From the above discussion the following conclusions can be drawn: 1. Much of the so-called limonite of nature, and most of that making up the cellular pseudomorphs, consists not of pure goethite or pure hematite, but of limonitic jasper. Limonitic jasper is of very widespread occurrence, and to a greater or lesser extent is present wherever limonitic products exist. It has no fixed composition, its content of iron and silica being highly variable. 2. The silica which forms limonitic jasper is only one of numerous admixed impurities. Complexes of supergene gangue carbonate minerals may take the place of silica, either by direct precipitation or as replacements of the silica. Various other minerals, such as gypsum, may act as a binding agent for ferric oxide hydrate particles in forming limonitic granular aggregates. When stained the same general color as ferric oxide hydrate they may be indistinguishable from the latter in the hand specimen or under the hand lens. 3. Numerous minerals, such as manganese dioxide and various arsenates, which in quantity would be readily detected and identified, may constitute up to several percent of a given limonitic product, and yet be indistinguishable in the hand specimen or under the hand lens provided they are fine grained and well dispersed through the limonitic material. 4. Uncontaminated ferric oxide hydrate exists in a

10

INTERPRETATION OF LEACHED OUTCROPS

stable form 6 only as ferric oxide monohydrate (Fe~03. H 2 0), in the form of the minerals, goethite and lepidocrocite. Lepidocrocite is extremely rare. S. Hematite exists as a supergene constituent of leached outcrops, being particularly common in the capping over some of the disseminated copper dep~sits. In such cappings it occurs mainly as the fibrous vanety, as blackish-brown crusts made up of tiny nodules or spherules, or as granular and pulverulent material, always mixed or intergrown with goethite whether found over areas of more than a few square inches, or over areas of only a few square millimeters. 6. Jarosite, siderite and various basic organic salts of iron often occur as decomposition products of ironyielding minerals in nature. All are far more subject to decomposition under attack by physical and chemical agencies of weathering than are goethite or limonitic jasper. . With the mineral "limonite" which was III the past assigned the composition 2Fe 2 0".3H 2 0 proved to be non-existent, but represented under varying conditions by perhaps a dozen or more and mostly similar-looking supergene products all containing substantial amounts 'Stable under normal conditions at or near the earth's surface.

of iron oxide, the question arises as to the degree of refinement that should be employed in designating the derivatives variously grouped under the general term of "limonite," particularly in view of the difficulty in readily identifying many of them in the field. For certain purposes, specific designation is desirable. An example is the notation of limonitic jasper as the chief and normal constituent in cellular boxwork or sponge, and certain other of the limonites, because it explains the exceptional resistance of most cellular pseudomorphs to weathering attack. In laboratory research, likewise, designation of specific minerals or intergrown mixtures is necessary. But in field work fine distinctions are not always practicable, and the term limonite is retained to denote the undifferentiated ferric oxide precipitates as a group. By common consent the word has become accepted as a collective term designating all of the reddish, yellowish, brownish, and blackish-brown supergene ferric oxide or ferric oxide hydrate precipitates derived from decomposing iron-yielding substances in nature which have not been more specifically identified (Tunell and Posnjak, 1931, p. 337, 897). As such the term serves a convenient and highly useful purpose, especially in field work.

Chapter 3 INDIGENOUS, FRINGING, AND EXOTIC LIMONITES Limonite, depending mainly upon the place of precipitation relative to the origin of the iron which enters into its composition, is classed as indigenous, fringing, or exotic. Inasmuch as these terms will appear many times in the remainder of the text, this chapter is devoted to a detailed explanation of their meanings and usages.

for all other limonite types the precipitate is classed as indigenous only if its iron was derived from and precipitated within the space formerly occupied by the mineral that has been leached.

INDIGENOUS LIMONITE

Fringing limonite is that precipitated from ironbearing solutions outside the limits of the mineral which was their source, yet sufficiently close to those limits for the source to be known beyond doubt. In most cases this means the limonite will need to have been deposited as a fringe directly adjoining the cavity or space formerly occupied by the parent. Thus, if in a disseminated copper deposit a portion of the limonite nodules or spherules of the cavity, as in figure 1, "overflows" into the gangue so as to form a halo of limonite about the cavity, the portion forming such a fringe as a halo is referred to as fringing limonite.! Fringing limonite need not, of course, be nodular; more often it is granular, and it may comprise any of various limonite types, including pulverulent. The criteria are that: 1) it has been precipitated outside the volume representing the source of the iron, but 2) still close enough so that its parentage and path of migration may be traced back indisputably. In disseminated deposits, where each small sulfide speck or bleb usually is surrounded by a large area of gangue neutralizer, so that the iron does not travel more than a few milimeters before precipitation, all limonite deposited outside of and adjoining the cavity is classed as fringing. But should the sulfide specks or blebs bc

Indigenous (Greek- native, within, at home) limonite is that precipitated from iron-bearing solutions within the cavity or space formerly occupied by the sulfide or other mineral from which the iron was derived. Thus, if in a disseminated copper deposit the limonite both originates and is precipitated within the rock cavity which formerly carried the sulfide speck or bleb, it is indigenous, irrespective of whether the limonite is cellular, composed of fine nodules or spherules coating the cavity walls, comprises granular cell filling, or embodies some other of the various limonite types. The criterion is that the iron was derived from a mineral or minerals formerly occupying the space now represented by the partly or completely filled cavity. Cellular pseudomorphs that form as replicas of the cleavage or fracture pattern of mineral nodules or masses likewise are classed as indigenous, even when they are composed of limonitic jasper a part of whose iron and silica may have been imported by ground water. Except in rare instances it manifestly is not possible, in a thoroughly leached outcrop, to determine with assurance the proportion of imported silica, and particularly that of iron, which enters into the composition of the limonitic jasper in a cellular pseudomorph, measuring, say, several inches across. Furthermore, in some cases such a pseudomorph, whether composed of limonitic jasper or some other material, indubitably has had all of its constituents derived indigenously. As a practical consideration in interpretation, and to avoid confusion, the criteria in this therefore are held to be that: 1) the precipitation took place within the space formerly occupied by the leached mineral; 2) through its structural pattern the pseudomorph serves specifically to identify that mineral; 3) the pseudomorph's composition is such that under normal conditions at least a part of its components generally have been, or could have been, derived from the leached mineral, though in exceptional instances, as with limonitic jasper patterned after the cleavage of calcite, all of the components may have been imported. These criteria apply to cellular pseudomorphs alone;

FRINGING LIMONITE

'In the early leached outcrop work this class of limonite was called transported. Locke, (1926, p. 102), changed the name to contiguous, and amplified the term transported to embrace both his contiguous and exotic products. Although in general his use of the term transported is correct, insofar as the limonite components have been transported beyond the mineral's borders, it is equally true that at least part of the components of most indigenous limonite of the cellular pseudomorphs usually have been transported; that is, imported from an outside source. In recent years the term transported therefore has become discredited as a class name. Locke's term contiguous (Latintouching, or in actual contact) has much to commend it, but in the author's experience it has not had a favorable reception among prospectors and non-geologists in the mining profession. The term fringing limonite (first suggested by E. H. Wisser) has the same meaning, and to prospectors and non-geologists it appears to convey this meaning more readily than does the term contiguous. The preference of terms will vary, however, with the individual. Whichever term is used must have its meaning and application clearly defined.

12

INTERPRETATION OF LEACHED OUTCROPS

spaced closely enough so that limonite originating in one speck becomes intermixed during precipitation with that originating in some other nearby speck, the limonite within the overlapping area usually can not be related back to either sulfide source with assurance; and the product should be classed, not as fringing limonite, but as the exotic product described below. The condition is made clear in the sketch of figure 1. Here, for ease of presentation, the picture of the precipitate is granular or pulverulent limonite instead of the finely nodular crusts or spherules. The relationship applies, of course, irrespective of the limonite type involved.

EXOTIC LIMONITE Exotic (Greek-introduced or brought from outside) limonite is that precipitated from iron-bearing solutions which have moved so far from their source that the source no longer can be identified specifically. To illustrate: The blackish-brown crusts made up of -the mutually encroaching and overlapping tiny limonite nodules or spherules previously discussed are frequent precipitates from strongly acid iron-bearing solutions. Pyrite is the most common source of such solutions. It also is one of the last sulfides to decompose under normal conditions. If, therefore, an occurrence of pyrite lies above, and decomposes subsequent to the formation of a cellular pseudomorphic mass of other parentage (for example, galena or sphalerite), if the oxidation solutions derived from the pyrite migrate over the cellular mass, and if conditions during migration are favorable to precipitation of that particular kind of limonite of pyrite origin, the cellular mass will become coated in varying degree with these tiny nodules or

spherules. Should the oxidation of pyrite furnish sufficient iron oxide, the cellular product in extreme cases even may become coated by exotic limonite. It is not difficult, for instance, to imagine a mass of coarse cellular boxwork or sponge several inches across derived from galena, being not merely coated, but largely submerged and possibly obliterated by nodular crusts, such as those represented by figure 16, chapter 18. But even though one might feel certain, from independent field evidence, that in a given instance the iron-bearing solutions which yielded the finely nodular crusts had been derived solely from pyrite; yet after the last of the pyrite in the outcrop had been oxidized it would not be possible, ordinarily, to determine whether the pyrite had lain directly above the cellular mass, in contact with it, or whether the solutions had originated from another eroded oxidizing pyrite occurrence which had lain many feet above; and that the solutions had merely followed a channel which led them by direct or devious route to the cellular mass, with precipitation occurring at the end of the journey. Thus for exotic limonite the criterion is not only: 1) that the limonite be precipitated at a place other than the iron's source, but in addition 2), that the precipitation occur far enough from that source so that the limonite does not have direct contact with it. The distance traveled by the iron before its precipitation is variable. In some cases it may be only a few millimeters; in other cases many hundreds of feet. In most occurrences of exotic limonite at least some doubt will exist also as to nature of the parent mineral or minerals, and in many cases both the nature of the parent and distance traveled in solution by the iron before precipitation will remain pure speculation. ":.'

~---

Indigenous

Indigenous Fringing

Fringing

Fringing

Exotic

.

FIGURE 1.

Sketches showing granular or pulverulent limonites at xlO magnification. The limonites were derived from disseminated sulfides and precipitated as indigenous, fringing, and exotic products.

INDIGENOUS, FRINGING, AND EXOTIC LIMONITES

DIFFICUL TIES IN CLASSIFICATION It has been shown that limonite nodules or spherules such as those under discussion, may be indigenous, fringing, or exotic; furthermore, that no reason exists why those of the exotic class may not be superimposed upon either of the other typ~. Frequently they ar:. T~at merely is another way of saymg that th.e typ.e of llll~on~te does not, in itself, determine its classlficatlOn as mdlgenous, fringing, or exotic. Even cellular boxwork, that standby of the indigenous class, is no exception. The coarse boxwork composed of supergene siderite in the Gardner mine at Bisbee, Ariz., for example, cannot be, under the definition, classed as indigenous (see fig. 85, ch. 33); for although the limonite in this case has been deposited along fracture planes which it r~~roduces faithfully, not only do those planes represent Jomts and fractures of the country rock (limestone) instead of cleavage or fracture planes inherent to a specific mineral but the boxwork structure itself fails to identify a par;icular mineraJ.2 For that reason. the ~~pe~gene siderite boxwork at Bisbee, as also limomtIc Jasper and "It is true that at the Gardner mine the iron's source could be traced directly to an overlying pyrite-chalcopyrite-chalcocite body. But the iron might just as well have b~en derived from a pyrite body free of copper, so far as formatIOn of the su~er­ gene siderite boxwork is concerned. F.urthermo.re, t~e relatIOnship to the pyrite-chalcopyrite-chalcocite bo~y In this. case was established through manways and other mIne workIngs only after that body had been in large part delimited, and probably could not have been established positively without such workings. To be of value an ore guide must become known and intelligible before, not after, the ore to which it points has been found.

13

other boxwork which forms along joints and fracture planes in country rock not of homogenous composition. always is classed as exotic. Indigenous limonite is the one whose history may be read most accurately. Its interpretation seldom gives trouble to the observer experienced with derivatives of the parent mineral involved. Fringing limonite is more difficult to interpret. Its appraisal is easiest when it has been derived from a small speck of sulfide or other iron-bearing mineral surrounded by a large area of non-ferrous or essentially non-ferrous gangue possessed of moderate neutralizing power (feldspar or normal shale), as in most disseminated copper deposits. The difficulties increase as the parent mineral grades into massive sulfide, especially into a body of mixed massive sulfides. Exotic limonite is the most difficult to interpret, because of frequent uncertainty both as to the iron's origin and to the path of migration followed by it. It often is deposited upon and through the indigenous cellular type, subsequent to the latter's formation, as iron-bearing solutions from an extraneous source flow over the indigenous product and drop a part of their iron load. It also may be superimposed upon other limonite types of both the indigenous and fringing classes. The experienced observer generally is able to differentiate such exotic coatings, at least where indigenous limonite is involved; but the coatings nonetheless are troublesome because they tend to obscure the characteristics of both indigenous and fringing limonite-upon whose distinctive features the observer relies for interpretation.

Chapter 4 FORMATION OF LIMONITIC JASPER Limonitic jasper manifests itself in many types of supergene iron oxide compounos, but is preeminent only in two: in cellular pseudomorphs, and as massive jasper product. For the present, pyrite will be the principal sulfide which will be involved in the discussion of limonitic jasper, though other sulfides will be mentioned incidentally. A variety of massive limonitic jasper common in Precambrian rocks is jaspilite, in which layers of jasper alternate with layers of chert. Jasper is commonly reddish, but may be yellowish or brownish. Consideration of jaspilites will be necessary in order to obtain an understanding of cellular pseudomorphs. In many cases silica has been precipitated as a gel, with iron oxide dispersed through it in particles so small as to make the product appear amorphous. In other cases both silica and the limonitic particles crystallized out distinctly, but so minutely that their crystal forms are recognizable only under high magnification (see table 1, ch. 2). Because of the minute grain size of the iron oxide particles within their glassy matrix, the iron content in the hand specimen frequently has been overestimated, just as the percentage of coloring matter in glass, or the percentage of ink in a glass of water, usually is overestimated by the uninitiated. This probably explains why the cellular product was not differentiated for so long a time from non-cellular limonite. In addition, the minute size of the particles may make difficult the determination of its correct content of iron by routine laboratory methods. The particles frequently are less than 1 micron across. Ordinary pulverizing of the sample therefore may not free or expose all the iron oxide particles from the silica which surrounds them, and such particles may fail to go into solution. More than one analyst has registered complaints upon that score. Among published papers, that by Moore and Maynard (1929), although written for another purpose, has done probably the most to clarify our conception of what limonitic jasper is, and how it is formed as a supergene mineral. Their experiments amplify and carry farther, in several important respects, the earlier work of Lovering and other investigators. The experiments by Moore and Maynard were conducted to show the manner of solution, transportation, and precipitation of silica and iron in ground water for areas of moderate to extensive organic decay, essentially free of oxidizing sulfides. Such water in many respects contrasts sharply with that originating in mining districts of arid and semi-arid regions, where gossans normally are best developed; but much of the informa-

tion derived from the experiments is applicable to arid and semi-arid mining regions because many of the conditions which govern solution, transportation, and precipitation of silica and iron have universal application.

TRUE SOLUTIONS AND COLLOIDAL SOLUTIONS DISTINGUISHED Before considering the results of the Moore-Maynard experiments in detail, it is important to understand the manner in which the essential components of limonitic jasper~silica and ferric oxide or ferric oxide hydratemay exist in ground water. They exist in water either in true (molecular or ionic) solution, or as colloids. Colloidal particles represent aggregates of a limited number of molecules. Though they are so small that they will be kept in suspension indefinitely by the Brownian movement, provided further coalescence and the resulting agglomeration are prevented, nonetheless they are large enough to be caught and retained py an animal membrane, parchment paper, or unglazed porcelain filter when the liquid containing them is passed through such a filter. The liquid, together with the colloidal particles dispersed in it, is called a colloidal solution, or sol. The words hydrosol, alcosal, etc. are used to indicate that the dispersing liquid is water, alcohol, etc. From the above descriptions it might be inferred that the only differences between a true solution and a colloidal solution of similar mineral composition is a matter of aggregation or group-molecule size. Some chemists believe, for example, that all dissolved silica (generally known as silicic acid) when freshly derived from a silicate, is in true or molecular solution, but that by aggregation of the molecules colloidal silica results (Freundlich, 1926, p. 421)." Because both true solutions and colloidal solutions of silica and ferric oxide "The degree to which silica in natural waters is transported in colloidal form is a matter upon which geologists and chemists are not well agreed. For at least half a century prevailing geological opinion has been that most (some writers maintain all) of the silica in solution must be colloidal. Roy (1945) has pointed out that the impression is based largely upon inferences drawn by Kahlenberg and Lincoln (1898) from laboratory experiments performed by them which were not concerned primarily with the status of silica in natural waters; and that the conclusions of those men were accepted at the time without independent check by most geologists, and have been handed down from one writer to another, until they have grown into a geological conviction which rests upon very insecure experimental and investigational data. Harman (1927) in 1925-1927 made what probably are the

16

INTERPRETATION OF LEACHED OUTCROPS

minerals are involved in the formation of limonitic jasper, persons engaged in leached outcrop interpretation should grasp firmly the distinctions between the two. A final point should be noted: although particles which pass through or fail to pass through filters of the types specified usually are spoken of, respectively, as being in true or in colloidal solution, the filters conceivably may have tiny holes in them too small to be detected but nevertheless large enough to permit fine colloids to pass through. On the other hand certain substances have single molecules many times larger than those of inorganic origin. Under some conditions filters thus might fail to pass even a single molecule of organic substances with giant molecules. The procedure of filtration as above outlined thus can not always be relied upon to furnish a sharp separation between what at present are referred to as true solutions and colloidal solutions. It does however provide a crude basis for such separation.

THE SOLUTION, TRANSPORTATION, AND PRECIPITATION OF SILICA AND IRON Reverting now to the experimental work performed by Moore and Maynard, partial results of the experiments, showing the solution of silica and iron from rock silicates (norite and diabase-igneous rocks) by distilled and several types of ground water, are set forth in table 3, chapter 5. The experiments and other data supplied by them lead to the following conclusions regarding the solution, transportation and precipitation of silica and iron oxide.

Silica 1. The average silica content of the earth's river waters is approximately 10 parts per million. The amount tends to diminish as the rivers approach the sea. On the other hand, close to the source a content of 20 to 30 parts per million is not uncommon. 2 2. Under conditions embraced by the experimentsmost complete investigations thus far carried out, checking his results by practicaIly every known method. In the last of a series of seven papers, which summarizes the principal results he discussed what he believes were fundamental errors in the work by Kahlenberg and Lincoln, and showed that the reason the previous investigators found only or mainly colloids is because they permitted the crystalloid silica to diffuse through the membrane and discarded it, so that only the colloidal silica remained. He concluded from his own work that in extremely dilute solutions of sodium silicate all of the silica is a true solute, and that only as the solutions become more concentrated and the Na,O : SiO, ratio increases beyond 1 : 2 are colloids developed significantly. Despite Harmon's findings not all geologists and chemists regard the issue as settled (see, for example, Hitchen, 1945). 'The silica content of streams depends upon the terrain over which they flow. Invariably, the streams draining granite and other igneous areas are higher in silica than those draining sedimentary and glacial areas (Tarr, 1917, p. 427).

that is, in thc absence of important sulfide oxidation to yield mineral acids-calcium and magnesium bicarbonates, and the alkali bicarbonates, followed by peat (representative of decaying organic matter) solutions, are the most effective solvents of silica. 3. When in true (molecular or ionic) solution, the silica in natural waters is thought to be carried usually as silicates of the alkalies, notably as sodium silicate. But in time the silica particles gradually coagulate, and colloidal solutions result." 4. Silica in true solution as sodium silicate, according to Moore-Maynard, when not exceeding 25 parts per million, is precipitated most effectively by sea water and calcium bicarbonate, followed much less effectively by potassium sulfate and sodium chloride. The silica comes down as a gel. The effectiveness of all electrolytes increases with concentration of silica in solution, but at ordinary concentrations in natural water, precipitation by any of them is a slow process, and rarely is complete. 5. Silica hydrosols in dilute solution are precipitated most effectively by sodium chloride at the concentration of sea water. Calcium and magnesium bicarbonate, and other electrolytes are on the whole inefficient. 6. At a concentration not exceeding 25 parts per million of silica, magnesium sulfate acts as a stabilizer for silica in true solution and as hydrosols. Above that concentration, and especially when the silica content reaches several hundred parts per million, excess magnesium sulfate precipitates the silica rapidly from solution though not from hydrosols. The silica in this case comes down not as a gel but as magnesium silicate, probably as MgO.Si02' with excess Si0 2. In regions such as those to which the Moore-Maynard experiments are applicable, silica in ground water is thus present mainly in small amounts. When fresh, the silica generally is in true solution; but, given time, it usually agglomerates, and is carried as a hydrosol. From a hydrosol it is precipitated effectively only by sea water or sodium chloride. Under all except very special conditions the silica precipitates from true or colloidal solution as a gel. The process is a slow one, extending over many months, and rarely goes to completion.

Iron 1. The average iron content of the earth's river waters probably does not greatly exceed 1 or 2 parts per million, but individual streams carry far more. The Amazon and its tributaries, for example, carry 2 to 7 'The Moore and Maynard experiments (1929, p. 401) showed that addition of sea salt in the proportion of 34,400 parts per million to solution containing respectively 25, 49, 98, 781, 1562, 3125 parts per million of silica produced no precipitate in 4 days. At the end of 19 days a precipitate of colloidal silica was clearly evident in the solution containing originally 3125 parts per million of silica and there was a slight precipitate in the silica in the solution originally containing 1562 parts per million of silica. At the end of 200 days precipitates were evident in all the solutions.

FORMATION OF LIMONITIC JASPER

parts per million of ferric oxide; 8 to 10 parts per million frequently are present in regions where bog iron deposits or swamps do not exist, and greater amounts are found in swamps where bog iron deposits are being formed. In a broad way the iron content, in regions other than in the vicinity of oxidizing sulfide bodies, may be accepted as about one-third that of the silica content, though that ratio may not hold for individual streams. 2. In regions of moderate to moderately extensive organic decay (provided sulfides and the resulting mineral acids are not important), carbonated water, followed by peat solution, are the most effective solvents of iron from rocks. This is well shown in table 3, chapter S. Under some conditions organic acids also are effective solvents. 3. In ground water of such regions iron in dilute concentration probably is present mainly as ferric oxide and/or ferric oxide monohydrate hydrosols, stabilized by organic colloids. The amount of hydrosol varies with the content of organic matter in the water, and with other factors. In general from 2 to 3 parts ferric oxide as hydrosol will be stabilized by 1 part of organic matter. Since the average content of organic matter in stream water is approximately 12 parts per million, it follows that such water may stabilize up to 30 or more parts per million of ferric oxide as hydrosol. When the content exceeds 30 or 40 parts per million, available organic matter in natural waters may be inadequate to stabilize all of it as colloids. In that case the excess iron probably exists in solution as bicarbonate. 4. Most of the electrolytes, when brought in contact with ferric oxide hydrosols at a concentration of ] 0 parts per million ferric oxide, almost immediately bring about the complete precipitation of the hydrosols, but are less effective when a large amount of organic matter is present to stabilize the hydrosols. Most electrolytes likewise are effective precipitants of iron carried in true solution. Unlike the case with silica, the electrolytes, as a wrole, thus are highly efficient precipitants of iron from hydrosols and of iron from true solution, except to the extent that the hydrosols may be stabilized by the organic matter present. S. Certain bacteria are effective in precipitating iron from bicarbonate solutions, from hydrosols, and from various salts of organic acids, but in general such precipitation probably is only incidental to the iron in ground water. Iron, like silica, thus may be and commonly is carried as a hydrosol. Some of it is likely to be carried also in true solution, in the main probably as bicarbonate. In either case it is precipitated rapidly and effectively by most electrolytes, calcium and magnesium salts being especially effective in precipitating it from true solution.

Mutual Precipitation of Silica and Ferric Oxide in Jaspilites In the absence of electrolytes, as shown above, silica and ferric oxide hydrosols in dilute concentration are

17

separately quite stable in stream water, and the silica on the whole remains stable even in the presence of most electrolytes. But a vital fact brought out by the Moore-Maynard experiments is that, when in dilute concentration in the presence of each other, mutual precipitation of silica and ferric oxide takes place from hydrosols. In the proportion of 30 parts per million of silica and 10 parts per million of ferric oxide, the iron precipitates rapidly. The silica is precipitated more slowly, but over a period of months most of it likewise precipitates. I t follows that under moderately stable stream or ground water conditions there thus may be precipitated a layer relatively high in iron and low in silica, followed by a layer high in silica and either low in iron or virtually lacking in it. Where seasonal replenishment of silica and iron takes place in a body of water, alternating layers of this type may consequently be deposited. Moore and Maynard regard this as a possible explanation for the formation of banded siliceous iron deposits generally referred to as jaspilites, which are so conspicuous a feature among Precambrian rocks of almost every continent, and with which many of the earth's most important iron orebodies are associated. Jaspilites are confined to Precambrian rocks, they have not been seen in process of formation, and geological opinion is not agreed upon their origin beyond the fact that the rhythmic banding is sedimentary: Moore and Maynard's contribution to the problem lies in establishing 'Smyth (Clements and Smyth, 1899, p. 329-487) was evidently the first person to study in detail the silica and jasper in the Lake Superior district. He conceived of jaspilites as being developed by cold waters during the weathering of basic rocks. Van Hise and Leith (1911, p. 513, 516), in an attempt to explain the tremendous amounts of silica and iron involved in the Lake Superior deposits, suggested that the silica and iron-bearing material was derived partly from the weathering of basic rocks, probably, however, predominantly from magmatic springs which poured out their silica and iron content on the sea floor. They were led to that conclusion chiefly because they could not visualize natural ground water supplying either silica or iron in sufficient amount. This view was later reconfirmed by Leith and others (1935, p. 21-23). But this explanation did not satisfy a number of other geologists. They pointed out that if a magmatic origin had been involved, the deposits should exhibit the criteria of metasomatism and metamorphism associated with the numerous magmatic deposits known elsewhere, which for the most part is not the case with the jaspilites. The view that natural ground waters could supply the necessary silica and iron, and that the precipitation could thus take place rhythmically, was first introduced by Moore and Maynard (1929, p. 272-303, 365-402, 506-527), who set forth the full mechanism clearly, with every step in the process backed by experimental evidence. They have not proved that all or any of the jaspilites have been thus formed, but have shown that cold water activity, given enough time, may be adequate to accomplish the phenomenon. Geologists of Western Australia formerly regarded jaspilites as shear zones in greenstone, silicified by magmatic waters or by surface hardening; but more recently they have concluded that Western Australian occurrences are sedimentary deposits interbedded with volcanic flows. (For a summary of their views see McKinstry, 1939, p. 51-65). Among those who have championed the idea of sole sedimentary deposition is Gruner who, with varying degrees of clarity and through a series of papers (1922, 1930, 1937) with

18

INTERPRETATION OF LEACHED OUTCROPS

that, given time, natural cold ground-water solutions may be competent to supply the necessary silica and iron, and to precipitate them in their banded pattern. Whether or not some or all of the jaspilites were formed in that manner is of no concern in this connection. What is of direct concern is that, with slight modifications to fit conditions existing in areas of oxidizing sulfide bodies, the mechanism set forth by Moore and Maynard is competent to explain formation of the limonitic jasper of cellular pseudomorphs.

Modifications of Silica and Iron Reactions in Vicinity of Oxidizing Sulfide Bodies In the arid and semi-arid regions where gossans usually are most conspicuously developed, decaying organic matter is less abundant, carbonated waters are less common (except in limestone), and the inorganic salts of minerals in ground and stream waters more often are carried in solution as sulfate and chloride. As a consequence electrolytes are likely to be more abundant in stream and other ground waters of such regions. Silica Reactions. To whatever extent they may be present and dominant, the solvents of silica noted by Moore and Maynard are equally effective in arid and semi-arid regions. (Strong organic solutions rarely gradually shifting focus, has tried to bridge the gap between the views of Van Hise and those of Moore by suggesting that hot waters, emanating from igneous intrusions at depth, have raised the temperature of natural ground waters locally so as to bring them within the low-to-moderate hydrothermal range. The discussions of Tunell and Posnjak (1931) and Dunn (1937) are also contributory to an understanding of the problem. Like Van Hise and Leith, Gruner believes that heated waters are necessary to account for the large amounts of silica and iron taken into solution, but instead of invoking the action of hypogene waters, he thinks they may comprise supergene waters heated in the same manner as those involved in geyser and hot spring activity as suggested by Allen and Day (1935, p. 164-231), by Fenner (1936, p. 310-315), and by Day (1939, p. 334). Dunn (1935) had previously proposed the idea to explain the formation of certain jaspilites in India; he concluded (1941) that the Indian jaspilites have been derived both from iron-bearing sediments and from bedded iron-bearing tuffs and their derived chloritic phyllites. Moore (1953) criticized the ideas of Dunn. Alexandrov (1955) suggested that the intermittent banding of silica and hematite in Precambrian formations was due to selective weathering processes whereby the variation of the pH of leaching solutions caused the alternate deposition of silica in warm seasons and iron oxide in cool seasons. Obviously, much remains unknown regarding the formation of jaspilites. The fact that in numerous instances the iron oxide minerals occur as bands of crystalline magnetite, hematite, or specularite extending for many miles, makes it difficult for the student of leached outcrops to conceive of such jaspilites, in their present form, as having been produced without deep burial or the occurrence of metamorphic action. The majority of geologists now favor Smyth's suggestion of their being developed by cold waters, though they often differ in details. The above mentioned problem however, does not concern the limonitic jaspers involved in the interpretation of leached outcrops; for all geologists agree that the mixed silica-ferric oxide hydrate herein referred to as limonitic jasper, may form under strictly supergene conditions.

would exist there.) But in areas of oxidizing sulfide bodies, especially pyritic ones, free mineral acid normally predominates; and, as shown by Lovering (1923, p. 525), its solvent action upon rock silicates may significantly increase the content of silica in ground water of such areas. This is brought out more emphatically by table 2, which presents a portion of the extensive data collected by Emmons, Jarrell, Boswell, and others, with regard to mine waters. Many factors other than the acidity of solutions determine the content of silica carried by mine or ground waters, and no fixed ratio of acidity to silica is shown in table 2. In some cases apparent contradictions exist. Nevertheless the correspondence between high acidity and higher than normal silica content is far too marked to be explained as coincidence. From the experiments of both Lovering and Moore and Maynard it might be expected, further, that where silica content of the water is high, especially where free mineral acid is present, a variable proportion of silica could exist in true solution. Few statistics have been published upon that phase of the subject, because normally only the total silica content is determined in stream or mine water analyses. Irrespective of whether the silica occurs as hydrosol or in true solution, the data from Emmons, Boswell, and others presented above, leave little doubt that in arid or semi-arid regions which contain oxidizing sulfide bodies, the total content of silica in ground water is likely to be well above the average. To state it another way: conditions which normally bring about the formation of gossans in sulfide areas usually also yield an adequate supply of silica in ground water for formation of limonitic jasper. Iron Reactions. Carbonated waters, and especially waters containing free mineral acid, are noteworthy for their activity in attacking sulfides as well as numerous other iron-yielding minerals. Their effectiveness in dissolving iron from rocks is well shown in table 2, and in table 3, chapter 5. Where free mineral acid is present, it is generally considered that the dissolved iron is carried in solution rather than as hydrosols, mainly as ferrous or ferric sulfate (Dole, 1909, p. 33). But even in areas of oxidizing sulfides, tests often disclose ferric oxide hydrosols, except at the immediate point of sulfide oxidation. Presumably the hydrosols are stabilized by the small amount of organic matter, or whatever other special stabilizing agent may be present. Precipitation of Silica and Iron. To whatever extent the silica and ferric oxide hydrosols are both present, no reason is apparent why the two products should not in this case experience mutual precipitation in the presence of each other, just as they did in the experiments of Moore and Maynard. The presence of sodium chloride in appreciable amount in ground water-a not uncommon condition in arid regions-might accentuate still further the precipitation of the silica hydrosols.

19

FORMATION OF LIMONITIC JASPER

To whatever extent the silica and iron salts are present in true solution, their precipitation presumably would be governed by available electrolytes. For the silica, as shown by the experiments of Moore and Maynard, calcium bicarbonate would be by far the most

effective, though by no means the only precipitant. It is a common constituent in ground water of many arid and semi-arid regions. The same electrolytes likewise are highly effective precipitants of iron from ferrous and ferric sulfate.

TABLE 2 Silica Content of Mine Waters PARTS PER M I L L J O N - - - - - - - -

No.

1 2 3

4 5 6

7

8 9 10

11 12 13 14 15 16 17 18 19 20

21 22 23

24 25 26

Source of Water

H,SO, ---_.--------

No. 20 shaft (bottom), Ducktown, Tenn.' ....................... . No. 20 shaft (top), Ducktown, Tenn.' ........................... . Callaway Shaft, Ducktown, Tenn. At water level' ............... . Callaway Shaft, Ducktown, Tenn. 37 ft. below water level' ............... . Burra Burra mine, Ducktown, Tenn. First level below black copper workingsl .. East Tennessee mine, Ducktown, Tenn. 180-ft. leveJ.1. ................. . ..... ........... Green Mountain mine, Butte, Mont. 2200-ft. level, fissure in granite remote from known veins'. Anaconda mine, Butte, Mont. 800-ft. west l ...................................... . Rothschonberger Stolln, Freiberg, Germany' ... Vulcan mine, Menominee district, Mich. 15th Level'. ............................ . Victor mine, Joplin district, Mo.' ........................... . Alabama Coon mine, Joplin district, Mo.'. Bachelor mines, Creede, Colo.' ............... . Stanley mine, Idaho Springs, Colo. I ...... . Ruth mine, Ely, Nev. Bottom incline shaft' ...................... . Gould and Curry mine, Comstock Lode, Nev. 1700-fl. level' ............................. . C & C shaft, Comstock Lode, Nev. 2250-ft. level l ............................... . Central Tunnel, Comstock Lode, Nev. ' Vadose water' ....................... . Capote mine, Cananea, Mexico 400-ft. level ' .......................... . Capote mine, Cananea, Mexico 300-ft. level l ........................................... . Panzio shaft, Chuquicamata, Chili," 30/1/30 ..................................... . Hole 151, Chuquicamata, Chili,' 30/1/30 .............................................. . Hole 70, Chuquicamata, Chili, ' 30.7 m. from collar, 22/3/30. Copper Creek, Bagdad, Ariz." Vadose water, 2124 ........... . Winze, 300-ft. level, Mount Oxide mine, Queensland. Water not disturbed for more than a year. pH 2.623/10/43' ........................................ . Main shaft sump, 300-ft. level, Mount Oxide mine, Queensland. Water pumped regularly. pH 3.3 17/5/44'........ .

SiO"

Fe

Cu

115.1 108.2

19.1 20.6

31.3 29.9

12.0 12.8

210.2

37.0

91.7

28.1

97.5

49.9

145.1

11.0

129.6

55.6

406.5

78.9

187.6

(Alk.)

23.2

1.8

(Alk.)

36.1 2.1

0.9 4.7

5.8 23.2 107.6 32.3 43.8

tr 142.8 474.6 1.5 164.8

94.8

78.4

37.9

(r

251.7 CAlk.)

133.4 2575

616

2178

312.1 40.8 tr

3.7

*

77.1

:!:

6.3 5025

147.5

nil

28.0

76

970.0

76.0

305

1659.0

40

55

794

348

40

44

443

472

60

114

101

353

337

189

193

1341

74.7

299.8

1003

38.0

472.3

135

*

'Quoted from Emmons (1917, pp. 87-89, 103. 106. For further information, such as dates of samples taken, analysts, etc., see that reference. 'Records of Chile Copper Co. Quoted from JarrelJ (1944, p. 268). "Sample of creek water directly above mouth of long Giroux Adit through limy gravels where copper silicate was being profusely precipitated Feb. 1924. Analyst, P. F. Boswell. 'Analyses by laboratory of Mount Isa Mines, Ltd. ::'Vadose water (French vadum,-a shallow, or ford) is that which, following precipitation as rain or snow, circulates from the surface downward through relatively porous soil and rock to the water table. UsualJy it is more highly charged with dissolved oxygen or carbon dioxide than is the water of deeper circulation, and thus likely to be a highly active solvent of rock constituents. Table 1 suggests the oxygenated water has little greater solvent action upon rock silicates than has distilled water. For an example of its activating effects, see Rogers and Shellshear (1937, pp. 160-166).

Chapter 5 TYPES OF LIMONITIC JASPER: 1. CELLULAR PSEUDOMORPHS Cellular pseudomorphs have been mentioned in chapters 2, 3, and 4 incidentally, and in this chapter they are described fully.

THE PSEUDOMORPHIC REPLACEMENT PROCESS If, in a normal climate, a rail or other article composed of iron be exposed to the elements for a long time it slowly disintegrates into iron rust. The process is hastened if the rail be exposed to circulating ground water, or subjected intermittently to wetting such as accompanies and follows precipitation of rain or snow. If the rail be left undisturbed-that is, if it be protected from forces which cause erosion of the surrounding earth or other environment-the rust not infrequently forms in long strips or crusts that may constitute crude replicas of the portion of the rail affected. More commonly, and always if sufficient time be allowed, the rusty product crumbles into scaly or pulverulent particles, some of which at least are carried away by ground water; and eventually nothing remains to mark the former presence of the rail except a long narrow streak of rusty smudge merging indistinctly into the enclosing mud or soil. Granted sufficient time and in extreme cases, even this disappears, leaving no record behind to mark the rail's former presence. Under usual conditions with the rail lying upon the ground, disintegration of the iron into rust is brought about solely by the action of air and rain or ground water upon iLl Its oxidation and disintegration is said to have been accomplished by air-water oxidation processes. On the other hand, if the rail or other article composed of iron such as a "tin" can, be thrown into a creek in a copper-bearing district, so that water draining from the copper area flows persistently over it, the rail will not disintegrate. Surface water which flows over and through the copper deposit dissolves some of the copper and takes it into solution as copper sulfate. Such a solution not only attacks the iron far more vigorously than would ordinary ground water, but the copper in it gradually replaces the iron as the latter in turn is dissolved by the acid water and carried away. If the process continues long enough most or all of the iron 'In arid regions dew may be the chief or only source of moisture.

disappears, just as in the previous case where the far slower air-water oxidation processes operated; but there now is left behind a replica, of essentially the same size and shape as the rail, consisting of native copper. Most prospectors in copper mining districts are familiar with the manner in which discarded "tin" cans or other iron substances thrown into the creek, have been thus converted into "copper." This phenomenon of iron being replaced by copper is employed in the precipitation of copper from solution in large-scale copper leaching operations. It is also commonly used in copper mines where water circulates freely underground through areas containing oxidized copper minerals-partly to reclaim for profit as much of the copper in the form of metal as possible, partly to lower acidity of the water so as to reduce corrosion of the pumping equipment. In such leaching operations the replaced iron usually is removed, and fresh scrap iron introduced, when replacement by copper is 30 to 50 percent complete. The reason is that, since replacement of iron proceeds from the surface inward, a coating or armor of copper soon is formed at the surface. This prevents ready access of the acid solution to greater depth, retarding the reaction more and more as the thickness of the armor increases, and in time bringing the process of replacement virtually to a halt. But in many copper mines nails, bolts, or iron brackets, in shaft sets or other position which remain in contact with acid solutions for many years, become so completely replaced that their analyses yield from 95 to 98 percent copper. In some instances even wood of the mine timber becomes replaced by copper metal sufficiently to yield from 5 to 10 percent copper. 2 Replacement of the iron of the rail by copper in this case is in most respects similar to replacement which occurs in the formation of petrified wood except that, in petrified wood, replacement of wood usually is affected by silica which is carried more or less generally in all circulating ground water. So perfect is the replacement 'At the Mount Oxide mine, Queensland, it is a common condition. Carbeen wood and sandalwood shaft sets. exposed to the strongly acid mine water for 30 years when seen by the author, showed in cross section up to three or four scattered small flecks of native copper per square inch. Harvey Mudd (written communication, 1946) stated that at the Mavronouni mine, Cyprus, pine timbers placed there by the Romans, probab~y early in the Christian era, are completely preserved by the aCid water, and may have twenty or more such small flecks of copper per square inch.

22

INTERPRETATION OF LEACHED OUTCROPS

in such petrifaction that in many instances every cell of the wood structure may be found recognizably preserved, often in minute detail, in the final product. Although wood usually is thought of as undergoing petrifaction through replacement by silica, that mineral is not essential. Instances occur in which petrifaction or replacement, with similarly detailed retention of the wood's cellular structure, has been effected by barite. At the Senorita workings of the Sierra Nacimiento, New Mexico, replacement or petrifaction of wood not only by silica but also by chalcocite is so perfect that to the unaided eye no detail of the cellular structure has been los1;3 Replacement by pyrite, although less perfect, is good enough so that the cell structure in some instances is conspicuously reproduced. An example has been reported from Louisiana in which the petrifaction has been by dark-brown to brilliant-red iron oxide hydrate, with the wood-grain structure likewise well preserved; but with replacement occurring directly, not as a subsequent replacement of previously silicified wood (Roberts, 1940). When replacement of a piece of scrap iron, a natural mineral, or an organic substance such as wood either is complete, or is sufficiently advanced so that the former structure is preserved in its essential details-irrespective of whether the replacing mineral is native copper, silica, barite, chalcocite, pyrite, ferric oxide hydrate, or some other mineral-the replacement is said to have been pseudomorphic; and the final product is said to be a pseudomorph after the scrap iron, natural mineral, or piece of wood that has been replaced. It thus is seen that, in nature, substances may undergo decomposition with distinctly different end results. In one case the iron rail may disintegrate into pulverulent rust, and even the rust be finally dissolved or eroded, and carried away so that no record remains of the rail having been there. In the other case the rail may disappear completely so far as its iron content is concerned, but in its place there remains a replica or pseudomorph which has preserved the original pattern or shape with varying degrees of fidelity, depending upon numerous factors.

THE FORMATION OF CELLULAR PSEUDOMORPHS Pseudomorphic Replacement of Sulfides When, in contrast to the previous examples, the oxidation of a buried sulfide nodule such as chalcopyrite takes place, its destruction usually does not proceed by means of decomposition inward along a continuous wave front as in the case of the iron rail either decomposing into rust or undergoing pseudomorphic replacement by copper, but by a somewhat different method. Its first conspicuous sign of attack, beyond an oxidation tarnish, generally consists of minutely thin webs of 'Observed by the author in 1928.

limonitic matter penetrating along cleavage or fracture planes. In their initial stage the webs may be less than 0.005 to 0.01 mm thick, scarcely distinguishable to the unaided eye. As more of the nodule goes into solution the webs extend in length and become thicker; interlacing or connecting cross-webs of various patterns develop, and gradually the structure "eats" its way into the nodule until a limonitic honeycomb is formed which may penetrate with an approach to uniformity through the entire nodule. If the nodule is more than several inches across, the honeycomb, as a well-knit pattern, often develops through only a portion of it. The process is a slow one which usually extends over many years and often over many centuries, depending upon the rate of decomposition within the nodule. But as time goes on and oxidation continues, even the residual sulfide particles which have become isolated within the honeycomb structure are leached out. When that stage is reached, little may remain to mark the former presence of the sulfide nodule beyond the skeleton honeycomb structure itself.

Effect of Mineral Cleavage If the mineral possesses good cleavage the honeycomb or cellular structure usually is sharply angular, with cells exhibiting distinct parallelism of orientation, so that cell walls in part are continuous in a straight line past several cells. Such continuity may persist for lengths of from a few millimeters to several centimeters or more. The more pronounced the cleavage, the more sharply angular is the cellular pattern likely to be. Where cleavage is strongly developed in more than one direction, as in some of the minerals belonging to the rhombohedral crystal subsystem, an angular grid of marked persistence may result. Such is the case with the non-sulfide gangue mineral calcite. Calcite yields cellular structure only under exceptional conditions; but when it does, the product is characterized by one of the most firmly-knit grid structures found in nature, and one with extraordinary regularity of cell size and uniformity of cell wall thickness (fig. 82, ch. 32). Sulfides rarely yield so firmly-knit and persistent a grid structure as does calcite, or one with such uniform cell size and cell thickness, because, with the exception of galena, they do not possess comparable cleavages. With many of them cleavage is far more strongly developed in one direction than in the others, with resulting corresponding greater persistence in length and greater thickness of the limonite web or rib in this direction. In the final cellular product these ribs become the dominant feature. They are called longitudinal ribs to distinguish them from the transverse or cross ribs which take out at varying angles from the longitudinal ones. The cross ribs generally are thinner, less firmly joined, and less persistent along a given course, as would be expected from the fact that they have had to "eat" their way along less well defined cleavage channels. Certain sulfides, such as chalcopyrite, are either devoid of, or possess only very inferior, cleavage; yet

CELLULAR PSEUDOMORPHS

they are characterized by a pronounced and consistent tendency toward fracture along one or more planes. These fracture planes are seldom well disclosed in sulfide hand specimens, but oxidation seeks them out and pentrates along them almost as unerringly as if welldefined cleavage existed. The result is that these sulfides frequently yield cellular products as sharply and persistently angular in pattern, and as rigid in structure, as do sulfides noted for their cleavages (see fig. 40, ch. 21).

Types of Cellular Structure Since most sulfides and some non-sulfide minerals possess distinctive cleavage or fracture patterns, a variety of honeycombs or cellular shapes thus develops in nature. It is primarily this feature which lends distinguishing character, or individuality, to the cellular derivatives of such minerals. Correlation of a specific cellular shape or pattern with a particular sulfide normally marks the first step of the beginner in the art of translating leached outcrops into terms of ore. The cellular structures of sharply angular pattern, though common, are by no means universal. Often the cells of the leached derivative are rigid and well joined to one another, but no parallelism of cell orientation is perceptible, and no longitudinal or other dominant ribs exist. Cell wall outlines are more rounded, and the cell structure appears to anastomose aimlessly through the sulfide mass, with no particular portion standing out in contrast to any other. The pattern of the cellular derivative in this case more nearly resembles sponge structure, if account be taken of the frequent lack of uniformity in cell size or specific shape (see fig. 74, ch.27). Cellular Boxwork. When the cellular pattern is distinctly angular, and its cell walls are rigid and well joined to one another, the structure is called a cellular boxwork. If the predominating cells are upward of several millimeters in diameter, the product usually is referred to as a coarse cellular boxwork; if less than l llz mm in diameter, as a fine cellular boxwork. Cellular Sponge. When the cellular pattern is lacking in angularity, and the individual cells are more distinctly rounded, and without marked contrast except, possibly, as regards individual cell size, the structure is called a cellular sponge. If the predominating cells are upward of ll1z mm in diameter, the product usually is referred to as a coarse cellular sponge; if less than 1 V2 mm, as a fine cellular sponge. The sponge pattern develops under a variety of conditions, but most often where the parent nodule was conspicuously and coarsely granular in texture rather than densely massive. Its formation is nearly identical with that of the angular types, the only difference being that in this case the cellular structure has "eaten" its way around irregular mineral grains instead of along strongly defined and persistent cleavages, or along through-going fractures inherent in the sulfide mass. Webwork. The term webwork applies to both cellu-

23

lar boxwork and cellular sponge. A webwork is, however, only the beginning product of oxidation. The term webwork refers to the minutely thin fragile threads or wisps of limonite that form the vanguard for the advancing cellular boxwork or sponge in the initial stage of formation, but which have not thickened sufficiently to become rigidly self-sustaining members in the sulfide. If a cellular boxwork containing it is struck or jarred heavily with the pick, the webwork, or a part of it, usually crumbles. Webwork, in the form of minutely thin fragile threads or wisps of limonite, grades insensibly into boxwork or sponge as the oxidation proceeds; and at some point the distinction between the two becomes arbitrary. But the extremes are readily recognized, and, as explained in subsequent chapt~rs, the separate designations serve a useful purpose m field work. Because cellular structure is patterned after the cleavage, fracture, or grain outline of the nodule undergoin.g decomposition, it is apparent that the structure constItutes a replica, not of the mineral itself in most cases, but of the cleavage, fracture, or grain pattern inherent in the mineral of whieh the particular nodule is composed. To the person versed in leached outcrop interpretation it constitutes as truly a reprodUction, or replica, of the mineral involved as does the final copper product into which the "tin" can or iron rail was transformed by the copper-bearing creek water mentioned at the beginning of the chapter. l Although most sulfides and some non-sulfide minerals yield such pseudomorphs, the product varies greatly in character and amount. Chalcopyrite, for example, generally yields a strongly defined boxwork, and yields it abundantly. Its yield of cellular sponge is more restricted. Pyrite on the other hand yields a boxwork only rarely, under special conditions. Its yield of a peculiarly distinctive type of flat or crusted sponge is far more abundant. Even though a mineral yields cellular pseudomorphs under some conditions-as for example, chalcopyriteit may fail to yield them under others. But when cellular pseudomorphs are not formed, in most cases some other identifiable limonite feature (granular texture, relief, etc.) can be found, to serve in establishing the parent mineral.

SILICEOUS NATURE OF THE CELLULAR PSEUDOMORPHS If the cellular derivative is composed of limonite, the question arises as to why it persists, as obviously it often docs for many centuries, more or less unchanged through subsequent processes of oxidation and weathering which affect the outcrop; and why it is not gradually dissolved and removed by ground water circulating over 'Other limonites are grain-for-grain, or solid, pseudomorphs. The hard pseudomorphs, however, constitute only an infinitesimal fraction of the total percentage of limonites. See chapters 12 and 16.

24

INTERPRETATION OF LEACHED OUTCROPS

it as in the case of the iron rail decomposing into rust, particularly in an environment of oxidizing sulfides where the ground water may be presumed to be at least in part acid. The answer is partly that much of the "limonite," and nearly all of that which constitutes cellular pseudomorphs, is not composed solely of iron oxide and water, but for the most part is limonitic jasper. Few persons other than those who have made a special study of the matter arc aware of the extent to which, in the limonites of nature, admixed impurities are present. Posnjak and Merwin (1919, p. 316) furnish analyses of eighteen specimens especially selected by them for laboratory investigation, all representing crystalline hydrated ferric oxides taken from museum or private collections, all examined microscopically to reduce the probability of their containing admixed substances, and therefore constituting much purer "limonites" than those usually encountered. But even these selected crystalline specimens contained, among other minor impurities, from 0.36 to 4.92 percent of SiO". Limonites exposed in the average outcrop contain much greater percentages of such admixtures, especially of silica. Most geologists are familiar with the manner in which coarse limonitic granules in outcrops often are "glued" by silica derived from ground water, into porous, clinkery aggregates which render them both rigid and highly resistant to disintegration through weathering. But limonitic jasper is no such mere "glued" porous aggregation of limonite and silica granules. It is a co-deposited intermixture of the two substances so intimate and so homogeneous in composition, and usually precipitated on so minute a scale, that the respective components often cannot be distinguished separately even under 100 X or greater magnification. Its silica content normally ranges from 20 to 50 percent, but in some cases exceeds 70 percent. With an increase in silica content the product acquires a vitreous or glassy luster not far below that of quartz. Usually, in the formation of cellular pseudomorphs, no distinct banding of precipitated silica and iron takes place; the two products are usually precipitated intimately, and are minutely intermingled with each other. When conditions favor the precipitation of abundant iron, limonitic jasper with high iron content may form; when they especially favor precipitation of silica and the supply of iron is low, limonitic jasper with low iron and high silica content is likely to result. The extensive range of SiO e and FeeO, present in both limonitic jasper of the various cellular pseudomorphs (see table 1, ch. 2), and also often within an individual deposit, thus is a natural and logical outcome. Because precipitation of silica under natural conditions is a slow process, and is rarely if ever complete, a greater or lesser supply of silica in ground water is available at all times to form the siliceous webwork along fractures or cleavage planes as oxidation encroaches farther and farther upon the sulfide or other iron-yielding body. Because cellular pseudomorphs form for the

most part either at or below the water table,5 and because at that level ground water in general tends toward alkalinity, any existing high concentration of mineral acid descending from above would be to some extent neutralized, and the concentration thereof reduced. In consequence of this reduction of acidity, as well as the reduction caused by reaction with gangue minerals, goethite is precipitated. Formation of cellular pseudomorphs of limonitic jasper along the advancing front of oxidation, especially in sulfide areas, therefore become readily understandable as the inevitable outcome of conditions induced by the oxidations going on immediately above and around them. Few occurrences have been observed in which the field evidence suggests a water table which is stationary long enough for the complete oxidation of a sulfide mass to have been accomplished at any given horizon (but see ch. 13, for a description of the C. S. A. mine). With rare exceptions the evidence points convincingly to the water table having receded before even the major boxwork or sponge structure was complete. Oxidation of sulfide residuals left marooned within the cellular pseudomorphs therefore must be completed largely after the water table has dropped below the affected area. Under such conditions free, continuous circulation of ground water through the sulfide mass would coincide mainly with heavy rainfall, and usually would be limited to periods of hours, or at most days. Water consumed in the oxidation at other times would be limited mainly to that drawn upward by capillary attraction, as noted by Locke (1926, p. 57). Under such conditions oxidation proceeding within the residual would depend more closely upon indigenous factors; and whatever limonitic cell-filling were precipitated and retained within the boxwork or sponge structure would almost certainly contain less silica, either as limonitic jasper or in other forms (for example, granular or pulverulent), than was comprised in the cellular pseudomorphs. The process of cellular pseudomorph formation in all its phases thus is the natural outcome of conditions resulting from sulfide or related mineral oxidation. The chief points to be kept in mind in connection therewith are: 1. The cellular pseudomorphs do not constitute (except occasionally) grain-for-grain replacements; they represent open-space fillings along cleavage or fracture planes, or along grain boundaries, with limonitic jasper formed from silica and iron carried in ground water that circulates over or through the affected area. Consequently they are siliceous replicas only of the cleavage, "They may form locally well below the water table where downward pull upon the solutions exists; as along faults, fissure or fractures zones, or other permeable channels. Likewise, they may form locally in an area of unleached minerals marooned within the oxidized zone provided the area lies within a natural drainage channel. Both conditions, especially the latter, are abnormal, and the formation in either case is likely to be interrupted, with ragged, poorly developed pseudomorphs.

25

CELLULAR PSEUDOMORPHS

fracture, and grain patterns of the mineral undergoing decomposition. 2. As more of the decomposing sulfide or related mineral passes into solution, the width of the open spaces increases, until under favorable conditions a well-knit boxwork or sponge results. 3. The process is a slow one, and rarely goes to completion as an unbroken unit at anyone place. When the water table drops, the general circulation of ground water over and through the area ceases, and continuous and persistent development of the cellular pseudomorphs likewise almost ceases, accounting for usually irregular development of cellular structure, even though massive bodies of a single sulfide, such as chalcopyrite, may be involved. 4. With these various factors in mind, it now seems easier to visualize the formation of cellular boxwork and sponge structures in their different stages. Granting that the drop of the water table does not necessarily wait until the cellular structure has ramified completely through the cleavage or fracture system of a specific sulfide nodule or mass, a satisfactory explanation is at hand why, in some instances, a well-knit boxwork or sponge exists, and why even well-developed fine siliceous webwork extends outward into the cellular space in some instances; whereas in other instances, in which the water table remains for a briefer period at a given horizon, even the coarser boxwork or sponge structure is ragged and incomplete, although the same parent sulfide and gangue may have been involved in both cases. Uneven development of the structure is further accentuated by the usual irregular permeability of the rock mass in a lateral plane, as unfractured rock. 5. Because development of the celluar pseudomorphs after pyrite calls for special conditions which are seldom met with, the cellular pseudomorphs are characteristic chiefly of minerals other than pyrite.

Resistance of Jasper to Chemical Attack Lovering (1923), carrying out experiments many years ago on the solubility of silica contained in various natural rocks, showed jasper to be nearly as resistant as vein quartz to solution by ground water. To some extent his results are inconsistent and even contradictory, and later experiments, such those by Moore and Maynard (1929, p. 297), made after laboratory glassware had been improved and had become better standardized, do not reveal such contradictions (see table 3). Lovering'S work nonetheless yields convincing evidence that the jasper used in his experiments is approximately equal to quartz in its resistance to attack by acid, bicarbonate, or other common types of ground water, and that both the jasper and quartz are much less vulnerable to such attack than are most of the natural silicates which occur in rock form. A weakness of Lovering's work, from the standpoint of solubility of the cellular pseudomorphs, is that he neither describes the nature of the jasper used in his experiments, nor furnishes its Fe 2 0 3 and SiO z contents. J aspers, of course, vary widely in their content of iron and silica; and it would be expected that, with conditions otherwise equal, a jasper of relatively high ironlow silica content would be more vulnerable to decomposition by ground water than one of relatively low iron-high silica content. Recognizing this weakness, Boswell in 1928 carried out solubility tests on jasper, using a suite of specimens with a broad range of iron-silica content, and including jaspers of both massive and cellular pseudomorph types. He furthermore differentiated between jasper in which crystalline structure was clearly distinguishable at 20 to 30 magnifications, and that which was "amorphous" (indistinguishable or poorly distinguishable at 200 magnification). Results of the tests are furnished in table 4.

TABLE 3 Solution of SiO, and Fe,OJ From Norite and Diabase" PARTS PER MILLION OF SILICA AND FERRIC OXIDE DISSOLVED ~-------:SI02-------_ ~-NORITE~--_

Solvent

Distilled Water70 days ................... 287 days ................... ...... Distilled Water and Peat70 days ................. 287 days ............... ... Oxygenated Water70 days ................... 287 days ............. Carbonated Water70 days .............. 287 days ..................

10

48

Mesh

Mesh

5.4 12.5

~-DIABASE-_

48

~-------FE20,,-------~

,----NORITE-_

,---DJABASE-_

150

10

48

48

150

Mesh

Mesh

Mesh

Mesh

Mesh

Mesh

7.8 18.5

4.4 14.1

8.6 13.3

0.4 0.4

1.4 1.2

0.2 0.8

1.0 0.5

52.8 49.1

64.4 50.1

39.2 43.3

45.8 45.3

5.0 4.6

8.6 7.2

7.0 7.6

11.8 12.4

4.0 8.1

4.6 11.7

3.8 7.9

5.6 12.3

0.4 0.8

0.6 1.1

0.9 1.6

1.6 1.0

64.0 70.5

72.2 87.3

45.2 52.3

49.8 48.1

11.8 26.8

36.2 45.4

7.4 9.0

11.0 11.4

'Adapted from table by Moore and Maynard (1929) after deducting blank losses. Blank losses for all solutions stated as: Si0 2-70 days, 1.6 ppm; 287 days, 1.9 ppm. Fe,O,,-70 days, 0.2 ppm; 287 days, 0.4 ppm. 'The norite carries sulfide in greater amount than does the diabase, which may account in part for more active attack upon the rock silicates. Sulfide is not a major constituent in either rock.

26

INTERPRETATION OF LEACHED OUTCROPS

The tests show that with bicarbonate solution the

It thus is not difficult to understand why the cellular

loss of both silica and iron is appreciable in material of low silica-high iron content; but that as silica content approaches 50 percent the loss of both diminishes notably, and when it exceeds 50 percent the loss in silica is little greater than that from vein quartz, except in jasper of opal variety. Special vulnerability of the latter to normal ground water attack, with probable reasons therefore, is discussed in chapter 8. The cellular pseudomorphs appear to be slightly more vulnerable to attack than massive jasper (except for the opal variety); but the difference on the whole is not great, and by no means clean-cut. The coarser material, in which crystalline structure is plainly visible at 20 to 30 magnification, likewise shows slightly greater vulnerability than the very fine grained material (again with the opal variety excepted); but here, too, the condition is not invariable, and the differences, in any event, are not large. The important facts brought out by Boswell's tests, when considered in conjunction with the work done by Lovering and by Moore and Maynard, are: 1) that the limonitic jasper of cellular pseudomorphs offers substantially the same resistance to ground water attack as do corresponding compositions of normal massive jasper; 2) that with the compositions in which silica content exceeds 50 percent vulnerability to attack in either case is little greater than that upon vein quartz.

pseudomorphs survive so well in leached outcrops. They survive the physical processes of weathering because the limonitic jasper of which most of them are composed, being siliceous, is not readily "whipped out" by weathering, or planed down by minor forces of erosion. They survive the chemical attack which dissolves the sulfides and decomposes many other minerals, because the limonitic jasper, except within the high iron-low silica ranges, offers a resistance to attack by ground water,-whether of acid, bicarbonate, or other composition,-almost as stubborn as that of vein quartz itself.

Differences Between Earlier and Later Leaching Products But two other questions arise: 1) why do not the residual sulfide particles, after becoming surrounded by the invading and enveloping boxwork or sponge, decompose into closely-knit cellular pseudomorphs of boxwork, sponge, or webwork of reduced size, but of similar composition and persistence, and, as a related question, 2) why is the limonitic matter, which often is left in varying amount as partial cell filling when the residual sulfide particle leaches out, so frequently not only non-cellular but lower in silica content than the boxwork or sponge structure?

TABLE 4 Solution of SiO, and Fe,D: From Limonitic Jasper, With MgH, (COo),-CaH, (CO.'), as Solvent! PARTS PER MILLION Number

Origin

Jasper Type

Crystalline Structure"

~ANALYSES (PERCENT)~

SiO,

Fe,O"

SiO,

Fe,O.:

54.9 49.0 48.9 51.4 31.2

63.1 (x) 6.3 57.4 (x) 5.2 67.5 43.2 29.7 26.3 19.1

29.8 1.2 22.9 1.2 34.2 27.6 16.4 13.1 8.7

56.3

35.5

17.6

11.3

72.9 71.8 76.4 72.3

16.3 17.4 12.8 16.7

14.4 14.0 11.6 16.1

6.5 4.1 5.4 6.7

Massive, opaline

47.3

39.7

Massive, opaline Massive, opaline Vein Quartz

76.0 90.6 99.1

17.8 2.4 tr

73.4 (x) 15.8 39.3 22.5 16.7 (x) 2.4

28.4 2.2 12.8 3.3 0.1

Chalcopyrite derivative, Gardner mine, Bisbee, Ariz .. .... _---------- Cellular

c

16.5

69.1

2

Outcrop. Coronado vein, Morenci, Ariz ..

Massive

c

23.1

64.4

3 4 5 6 7 8

Bornite derivative, Engels mine, Calif .. --------------Galena derivative, Ruby Hill, Eureka, Nev ........ Outcrop, Copper Flat, Hanover, N. MeL ..................... Outcrop, Carlisle mine, N. Mex. ---------- ... _--------Sphalerite derivative, Golconda mine, Ariz ................................. Chalcopyrite derivative, Rocher de Boule mine, British Columbia ............................................. Tetrahedrite derivative, World's Fair mine, Patagonia, Ariz ............................................... Sphalerite derivative, Spruce Mountain, Nev ........ Outcrop, Duquesne, Ariz .......................................... Outcrop, Los Aliados prospect, Las Palmas, Sonora. Outcrop, No.2 lode, Arroyo du Purgatorio, Santa Rosalia, Lower Calif...... --------------------- ----------------------

Cellular Cellular Massive Massive Cellular

c f c f c

35.2 38.8 42.6 41.2 59.6

Cellular

f

Cellular Cellular Massive Massive

c f c f

I500-ft. level, Lowell mine, Bisbee, Ariz ......................... Bonanza fracture zone, Alpha mine, Kimberly, Nev ... Mogollon, N. Mex ...............................................................

9 10 11 12 13 14 IS 16

-.---~-

-.-----~~.-

----------.-._------_._---

- - - _. . .

DISSOLVED ~AFTER 90 DAYs~

----~-------

----_._------------

'From unpublished data of P. F. Boswell. Quantities shown are those remaining after deduction of blank losses. Boswell used only bicarbonate solu· tion, except for four comparative (marked (x)) tests made with distilled water. The calcium and magnesium bicarbonate solutions were made up by passing carbon dioxide through water in contact with the respective carbonates for 72 hours, and combining equal parts of the two solutions. The jasper was crushed to minus 10, plus 14. mesh. 20 grams of jasper were placed in I liter of solution, agitated daily for 90 days, then analyzed by standard methods -hydrofluorizing the silica, titrating the iron with postassium dichromite_ Blanks were run for silica but not iron. Blank loss with bicarbonate solution was 2.3 ppm; with distilled water, 0.9 ppm. Comparative amounts of rock material (grams of rock used per liter of solution) used in the tests of tables 3 and 4 are: Lovering, 2; Boswell, 20; Moore and Maynard, 50. 'c=Crystalline structure clearly distinguishable at 20 to 30 magnification. f=Crystalline structure indistinguishable or poorly distinguishable at 200 mag· nifications.

27

CELLULAR PSEUDOMORPHS

The following considerations serve to answer both questions. ] . Silica is present in all normal ground water. The data set forth in chapter 3 show that in ground water of sulfide mining districts or other areas of oxidizing sulfide bodies, not only is it present in an amount more than adequate for the slow process of cellular pseudomorph formation as limonitic jasper, but that, it penetrates effectively to every crevice entered by the water. 2. Observations extending over many years and including many deposits, suggest strongly that the process of limonitic jasper "eating" its way into the sulfide nodule or mass begins at or in the immediate vicinity of ground water level, and that most cellular boxwork or sponge structure is formed there. Not only is this assumption supported by much direct observation, but where field evidence points to the water table having fluctuated markedly during the oxidation of a given sulfide mass, the boxwork or sponge structure rarely discloses as continuous and well-joined cellular pattern as where placid conditions prevailed over a long period. Moreover, where field evidence points to the oxidation having occurred wholly or mainly within a sulfide nodule or mass marooned in the oxidized zone well above the water table, there rarely have been observed more than ghosts of boxwork or sponge. Those that are present are confused in pattern, and quite generally lack the sharp outline of the product formed near a constant ground water supply. 3. Once the water table drops to a lower level the sulfide residuals enclosed by the cellular structure thus tend to be less readily and less consistently contacted by circulating ground water; and oxidation dependent more closely upon indigenous factors generally is likely to proceed within the residual. 4. In the decomposition of the residual the tendency thus is less for production of further cellular structure, even of diminished size; and more for production of other limonite types which, because of the reduced and intermittent silica supply, are similarly more likely to express themselves in shapes and patterns lacking firm structural rigidity; for example, granular or pulverulent

limonites. If, however, the decomposing residual yields oxidation solutions of sufficiently high acidity to retain in solution all the dissolved iron, the iron will be exported, no limonite will form, and the cells themselves will be empty. Depending mainly upon the type of sulfide. or other mineral involved, the cellular pseudomorph thus may either become completely vacated of its contents, or may retain as a partial cell filling the limonitic products of various non-cellular types. Where cell filling remains, its physical properties may serve to identify the leached sulfide or other parent mineral in cases where the cellular product itself is inconclusive, or may serve to substantiate more emphatically the testimony of the cellular product.

SUMMARY The products formed as a result of the pseudomorphic replacement process operating on the various ore and gangue minerals will be described in detail in chapter 16. At this stage the reader need remember only the following points: 1. The dominant, but not exclusive, product of the sulfide leaching process is the cellular pseudomorph. 2. Most sulfides and some non-sulfide minerals, yield cellular pseudomorphs with characteristics sufficiently distinctive to serve as "key" limonites in identifying the parent material. 3. Cellular pseudomorphs are either angUlar, therefore of the boxwork type; or irregularly rounded and shapeless, and of the sponge type. 4. Most cellular pseudomorphs are siliceous in nature, being composed of a mixture of iron oxides and silica known as limonitic jasper. S. Because of their siliceous nature they tend to preserve their identity in spite of both the physical and chemical attacks of weathering. 6. Because of their preservation they in turn provide clues to their origin, and usually permit direct interpretation to be made of the parent mineral from which they were derived.

Chapter 6 TYPES OF LIMONITIC JASPER: II. MASSIVE JASPER TWO GENERAL VARIETIES Among the massive jaspers derived from highly pyritic materials there exists a much broader range in the manner of formation, in variety of configurations and distribution, and in linear dimensions, than among the cellular pseudomorphs. The massive jaspers may, however, be grouped in two general varieties, based upon their manner of formation or origin: 1) openspace precipitates and far-traveling varieties; 2) replacement jaspers, of which those replacing "soap" and nontronite are the main varieties. Because the massive jasper products are exotic in nature, and are never recognizably pseudomorphic after specific minerals, they are seldom of direct value in determining the form and composition of their parent materials. The ability to recognize them is, however, important in the search for associated limonitic jasper types, mainly the cellular pseudomorphs, which may be of greater significance. Dimensions of the more compact varieties range from nodules of fractional-inch diameters through "puddingstone" lenses up to 20 or 30 feet in length, to sprawling, hulk-like masses which may spread in width unevenly across 25 or 50 feet, and persist along the strike for hundreds of feet. In addition to the compact nodular or lenticular products, massive jasper occurs also as ragged-edged seams and patches which range from mere driblets a few millimeters in length and fractions of a millimeter in thickness, to shapeless splotches a foot or more across; both often emerging indefinitely from a cellular or other gossan, and merging again as insensibly into it or into the country rock beyond. The larger, sprawling masses occur mainly as surface or near-surface crusts that die out at depths of 2 to 20 feet. The nodules, "puddingstone" lenses, and the ragged-edged seams and patches, characteristically persist throughout the oxidized zone, and often are as abundant close to the water table at depths of 100 to 200 feet as nearer the surface. The ferric oxide or ferric oxide hydrate content and the resulting color of massive jasper usually are much more variable than those of the cellular pseudomorphs. The content is most uniform in the ragged-edged seams and patches, which generally have a deep brown to black color and a glossy luster. The content is likely to be far more patchy in the compact nodules and lenses, producing an "ebb and flow" of various colors, with the normal deep brown or chocolate in one case, in another

merging imperceptibly within the space of an inch or two into a yellowish-brown or straw-colored, highly siliceous matrix. In the compact nodules and lenses the luster likewise is more variable, with the glossy sheen often giving way to dull surfaces. Irrespective of its composition, size, shape, luster or color, massive jasper in the hand specimen generally appears to be very fineand even-grained throughout, whether chocolate or straw-colored. Although involving many varieties with different origins, massive limonitic jasper is classed as an exotic product because not only is the point of origin and distance of travel of its iron usually uncertain, but the structure is never recognizably pseudomorphic after a specific mineral. Only rarely can the history of the open-space precipitates be ferreted out accurately enough to serve usefully in leached outcrop interpretation. Nevertheless, under favorable conditions certain occurrences of replacement jasper, even though exotic, may be translated with some degree of assurance into terms of the source material.

Open-space Precipitates and Far-traveling Varieties A prerequisite to the formation of open-space precipitates is an open space or channel large enough to permit the jasper to build up in massive form without replacing the adjacent rock. The surface and underground openings along faults, stratigraphic partings, and unconformities, especially where the latter structures are horizontally disposed, are the most favorable loci; but the necessary space may become available also along less permeable channels provided the dissolution of the rock keeps pace with the growth of the jasper mass. As an example, the open-space lype sometimes forms as bulges or swellings along ribs of cellular pseudomorphs, increasing the ribs' thickness locally from four to ten times or more over that prevailing elsewhere within the cellular mass. The phenomenon calls for special conditions which locally permit a much more than normal rate of decomposition of the host mineral without upsetting the balance within the ground water required for steady and persistent, concomitant precipitation of silica and iron. Such delicate balance is not likely to be maintained for long within the solutions where local decomposition of the host mineral greatly exceeds the surrounding average. Marked local bulges

30

INTERPRETATION OF LEACHED OUTCROPS

or swellings of cellular pseudomorph ribs accordingly are rare, and represent only an insignificant proportion of the massive jasper formed in nature. At the opposite extreme are the deposits whose silica and iron have been transported in solution many feet, often many miles, from the parent source or sources, before precipitation. These are called the far-traveling varieties. The larger of such masses most often are found well above the water table, at or not far below the ground surface; unconformities of flat attitude near the earth's surface are especially favorable to their formation. The ready circulation and entrapment of ground water along recessions in the irregular top, and the frequently different chemical compositions of the formations on opposite sides of an unconformity, are doubtless conducive to precipitation and growth in size of the jasper masses in such environment. From field studies it is believed, also, that intermittent floodings, followed by free air circulation and at least partial evaporation of solutions before replenishment, promote growth of larger jasper masses than does more constant submergence. The large sprawling masses often form, however, in part at least, directly at the surface. The smaller ones form in almost any open space above the water table where conditions favor co-precipitation of silica and iron. Formation of the open-space precipitate, regardless of size, may have either taken place at a slow rate and in small amounts cumulatively over many thousands of years; or may have taken place copiously and abruptly within a much shorter period. At many places these two conditions possibly prevailed at different periods, each period merging into the succeeding period. Although few direct quantitative measurements of jasper growth appear to have been made, more than one observer, upon revisiting certain localities after an absence of 15 or 20 years, has noted an increase in the size of the occurrence which lay in the path of solution flow. Long periods usually seem to have been involved for the precipitation of the larger masses, during which not only seasonal but often climatic changes took place. An almost inevitable result would be marked irregularity in deposition of the silica and ferric oxide. This harmonizes with the character of massive jasper found in the larger deposits of the far-traveling variety, in which the distribution of ferric oxide is notoriously variable and which contains substantial areas of siliceous precipitate that often have only feeble limonitic discoloration. The formation and character of only the extremes among open-space precipitates of massive jasper have been described; namely, the small swellings sometimes observed along the ribs of cellular pseudomorphs, and the large sprawling masses of the far-traveling variety, often observed on old erosion surfaces. The person engaged in leached outcrop interpretation will encounter, between those extremes, innumerable variations in size, shape, composition, and place of deposit. The

descriptions furnished should enable him, by interpolation, to visualize the origin and grasp the significance of any of the variations he encounters.

Replacement Jasper Replacement jasper comprises mainly the products yielded through the microscopic impregnation and replacement of silica-alumina substances such as clay "soap," and nontronite, by chalcedony and other forms of supergene quartz and ferric oxide. "Soap," from which probably 90 percent of replacement jasper is derived, is a smooth, white- to ivorycolored, microscopically fine-grained, clay-like product which, when wet, superficially has the appearance, and possesses the slippery feel, of wet soap; hence the name. It represents the residuum of aluminum-silicate rocks, such as feldspar or shale, or of shaly limestone or dolomite, from which vigorous leaching has removed the other, more soluble constituents. To understand the composition of "soap," and the manner in which it becomes impregnated and replaced to form massive jasper, a knowledge of the individual clay minerals is needed. Replacement of Kaolinite and Montmorillonite Clays.

In nature two broad groups of clay minerals occur: the kaolinite group (AI 2 0 3 02SiO"o2H"O) and the montmorillonite group. Both are end products of soil weathering. Each group is produced by a different set of conditions, but each usually is found in nature intermixed in varying degree with the other. Minerals of both groups may be produced by hypogene processes, but as a rule the hypogene deposits occur only as scattered and inconspicuous small seams and veinlets. The larger segregated masses, which as nodules and lenses become the host for most replacement jasper, are predominantly, and in most cases probably wholly, supergene. Ross and Hendricks (1945) 1 have furnished a concise summary and appraisal of the more important published information relating to the montmorillonite group, and have added much that is new. They point out that the formation of minerals of the kaolinite group is favored by surface weathering of rocks containing alkali feldspar and muscovite. Rhyolite and granite are common parent sources. Formation of minerals of the montmorillonite group, in contrast, is favored by surface weathering of rocks containing essential quantities of lime feldspars and the ferro-magnesian minerals. Andesite and basalt among volcanic rocks, and diorite, diabase, and gabbro among the coarsergrained rocks, are common parent sources. The presence of magnesia in the parent rock is especially favorable to formation of montmorillonite. But the parent rock does not alone determine the type of clay produced; climatic factors also playa part. Formation of minerals of the kaolinite group is promoted actively by warm, wet climates where weathering 'See also Brindley and Rustom (1958), and Ames and Sand (1958).

MASSIVE JASPER

and oxidation are rapid, as in the tropics. Their formation is promoted also by an iron-rich environment. Under such conditions they form to some extent from rocks which otherwise would not yield them. Similarly, formation of minerals of the montmorillonite group is promoted by dry soil conditions which prevail much of the year in semi-arid regions and in cold climates which retard weathering and oxidation. Under such conditions they, too, form to some extent from rocks which otherwise would not yield them. Montmorillonite (and beidellite) often are the dominant clay minerals found in the temperate zone, and in many arid regions, without strict regard to the parent rocks. Although in a broad sense the formation of minerals of the kaolinite group is promoted by an acid weathering system, and the formation of minerals of the montmorillonite group is promoted by alkaline weathering systems, the presence of pyrite or other acid-yielding sulfide in the parent rock as well as its rate of oxidation, constitute modifying factors which may locally override in importance both the nature of the parent rock and the climatic condition. This is well illustrated by the weathering of limestone and dolomite, which may yield clay-like minerals of either group, depending upon the nature and quantity of non-carbonate constituents, quantity and concentration of pyritic mineralization, and the rate of weathering. The interplay of those three factors determines the acidity or alkalinity, locally, in a weathering system that on the whole is overwhelmingly alkaline. Ross and Hendricks (1945) give the following endmember compositions for the montmorillonite group, represented as oxides, insofar as it has been possible to deduce the formulas:

31

The complex nature of, and the intimate interrelationships existing among, the minerals of the montmorillonite group is evident from these formulas. It should be pointed out that kaolinite does not have an unassailably fixed composition either. In areas of deep and protracted weathering, especially in the humid tropics, and with a granite parent, kaolinite with higher than normal AI"o" content commonly forms. Deviation of kaolinite's composition from that of the end-member formula, however, is the exception, not the rule, as it is with the montmorillonite group of minerals. The complex and variable compositions of members of the montmorillonite group suggest instability of those minerals under weathering. In the leached outcrop investigation this has been corroborated to the extent that members of that group have been observed altering

to jasper more commonly than have members of the kaolinite group under the same weathering conditions. Members of neither group, however, are broken down readily by ordinary weathering. Ross and Hendricks state that under long-continued leaching in neutral waters under oxidizing conditions, members of the montmorillonite group tend to some extent to form kaolinite; but kaolinite seldom if ever is converted to montmorillonite. Nature and Occurrence of Clay "Soap." As has been already indicated, kaolinization 3 is widespread in nature. Most of it is the product of generar decay. Where the weathering has been severe and erosion has not kept pace, beds or blanket-like bodies of kaolinitic material may extend laterally for hundreds or thousands of feet without much variation in appearance or composition. The material is made up dominantly of kaolinite and/or montmorillonite minerals but, except in beds overlain by coal seams, from which the roots of vegetation often have leached out cleanly all other constituents, kaolinized masses of this sort generally contain many impurities, and do not have a strongly bleached appearance. The product also is characteristically soft and earthy. "Soap," by contrast, never persists continuously over a large area. It may extend through the rock for hundreds of feet, but does so only in the form of seams, nodules or lenses which individually rarely attain lengths exceeding 20 or 30 feet; usually they are smaller. The product is so strongly leached that it is virtually free of all minerals other than those of the kaolinite and/or montmorillonite groups. It always occurs in a matrix of much less altered rock into which it may grade locally, but from which on the whole it emerges conspicuously by virtue of its much whiter, bleached appearance. Furthermore, instead of being characteristically soft and earthy, it invariably has smooth or slickensided surfaces, imparted to it by friction, which give it the appearance of having undergone squeezing. Even when dry, its squeezed or "polished" surface, though duller in luster, is readily distinguishable. The manner in which the product emerges as "soap" may be clarified by a comparison. If sufficient water be added to fill all voids of a mass of sand that has not assumed maximum packing, no free water is left standing. But if the sand be shaken or disturbed, the sand assumes maximum packing, and the excess water is squeezed out. A common example is the shaking of a bucket of such sand; free water then appears at the surface. In an oxidizing orebody, water or other mobile substance tends similarly to be squeezed out of the mass as the solid particles rearrange themselves and become settled. Any gelatinous substance, or other colloidal solution, would be such a mobile substance. Hydrous

'The ALO:: may be added up to make 18ALO:; but one part of the aluminum ions is octahedral and the other part is in tetrahedral coordination. In the chemical analysis though, as oxides, the two kinds of aluminum ions give a total of 18AL03.

'The term "kaoIinization" is used in this volume to denote alteration of rock into any of the generaIIy whitish, fine-grained clay minerals, and is not restricted specifically to the formation of the mineral kaolinite.

Montmorillonite..... Beidellite................ Nontronite (aluminian) ........ Nontronite ..............

5ALO:.2MgO.24Si O,.6H,O (Na20,CaO) 13ALO,.5AI,O::.38SiO,.12H,O(Na,O,CaO)2 13Fe,O::.5ALO::.38SiO,.12H,O(Na,O,CaO) 6Fe,O:.Al,O:.22SiO,.6H,O(Na,O,CaO)

32

INTERPRETATION OF LEACHED OUTCROPS

alumina, hydrous silica, or their intermediate compositions, when precipitated cold, arc typical gelatinous precipitates. In the slumping or settling of an oxidized orebody or other rock mass which is undergoing contraction in volume as the result of leaching, the colloidal clays therefore would be squeezed out of, or segregated from, the more solid, less vigorously leached, portions. Since colloidal alumina or colloidal silica may vary from the anhydrous (AI 2 0,J form up to the most highly hydrous form that can be obtained, any of the hydrous alumina or silica minerals, or any mineralogical combination or mixture of them, may become segregated in this way.4 "Soap" extruded as a seam along a fault or stratigraphic parting at points where major rock settling has taken place, often consists of only a single mineral. At Bisbee, Ariz., Kimberly, Nev., and other places, seams up to half an inch in thickness, of essentially pure gibbsite (AI 2 0:l.3H 2 0), as well as various clays, have been observed extruded along such fractures. Most "soap," however, develops indigenously within areas of more general rock slump that contain networks of intersecting fractures. Manifestly an area of this sort, regardless of the amount or character of clayey products developed within it, has not been extruded as a body. As contraction in volume occurs through removal of the more soluble rock constituents by leaching, the slumping and gliding of the kaolinized rock fragments upon one another is facilitated by the fracture-mesh; and curved, polished, and slickensided surfaces develop freely. But because pressures arising from the slumping dissipate themselves within the fractured mass, rather than exerting a pronounced weight at any given place, only a small proportion of the clayey material is squeezed out cleanly. With an intricately intersecting fracture system and sufficiently intense kaolinization, all of the material may become so strongly leached and altered that nothing but a clayey residuum remains. In this manner lenses of uncontaminated, sharply segregated, strongly slickensided, milky white "soap," 30 or more feet in length, often are developed. But such lenses, as also most of the smaller ones of like origin, are nearly always made up of a variable intermixture of minerals of both kaolinite and montmorillonite groups. Whether extruded under heavy pressure or derived in the manner above described, clay "soap" possesses a smooth, curved, well-polished or slickensided surface
-either as a single nodule or scam, or as an aggregate of pillowy masses of variable dimensions, making up the larger lenses. Even when dry, the surfaces show unmistakable evidence of having undergone frictional polish. "Soap" thus is distinguished from the more widespread kaolinized product derived from the general rock decay of nature in that: 1) it occurs as localized, strongly bleached seams, nodules or lenses of erratic distribution, which emerge emphatically, by virtue of their much whiter, bleached appearance, from the less altered enclosing rock matrix; 2) because of the intense alteration and leaching involved, it is virtually free from constituents other than minerals of the kaolinite and/or montmorillonite groups; 3) its texture is not earthy; is invariably characterized by smooth, curved surfaces that show evidence of frictional polish. Impregnation and Replacement of "Soap." Since jasper consists of an intergrowth of ferric oxide hydrate and free silica as quartz, "soap" can be transformed into jasper only through its subsequent more or less complete impregnation and replacement by those two substances. The transformation of an aluminum-silicate rock to jasper proceeds by two separate though usually related stages: 1) kaolinization of the rock to "soap"; 2) impregnation and replacement of the "soap" by quartz and ferric oxide hydrate. Under either acid or alkaline attack the more readily soluble rock constituents such as lime, magnesia, and the alkalies, may be effectively leached and carried away by ground water. The less soluble constituents, such as the silica and alumina of minerals such as feldspar and mica, arc altered to the clay-like products. But although alkaline solutions are competent to alter aluminum silicates to clayey products, usually only the vigorous attack by concentrated acid solutions suffices to effect the thorough leaching and the intense kaolinization involved in the localized production of "soap." Few decomposing minerals in nature other than rapidly oxidizing pyrite, yield, relative to their volumes, the necessary amount and strength of acid. As might be expected, "soap" thus occurs most often and in greatest volume where aluminum-silicate rocks contain seams, patches, lenses, or massive bodies of rapidly decomposing pyrite, which either merge into or adjoin the local, strongly bleached and slickensided areas, or are so disposed that solutions derived from their oxidation flood those areas. Even in limestone and dolomite "soap" usually develops in appreciable amount only i~ the vicinity of bodies of oxidizing semi-massive or massive pyrite. Granted sufficient acidity, the attack does not cease merely with production of the clayey products. Those products themselves are attacked and broken down, and leached of their alumina. But continued attack by the solutions necessarily weakens their acidity, and compels the dropping of a part of the load of the less soluble ingredients, which in this case are iron and silica. The small amount of iron derived from the decomposing aluminum-silicate

MASSIVE JASPER

rock is supplemented by significant amounts derived from any oxidizing pyrite in the vicinity. Much of the silica in solution is, however, obtained from the breakdown of the clay as the alumina is leached. But in the more siliceous jaspers, silica must be imported to some extent, because its content often exceeds that present in the intermediate clay minerals. The excess may come from decomposition of other silica-bearing minerals in the adjoining country rock, or may be imported from a greater distance. It was shown in chapter 4 that in limonitic jasper of cellular pseudomorphs, precipitation of the silica and iron are on the whole contemporaneous, though either may dominate for a brief time as the result of seasonal rainfall and other factors. In limonitic jasper of the replacement type, their co-precipitation also takes place much of the time. But in many occurrences deposition of silica precedes that of iron, often to the extent of virtually complete impregnation of the host by silica before that of iron begins. The precipitated silica is mostly in the form of fine granular quartz and chalcedony. "Amorphous" silica also is precipitated, but much less so than in the open-space precipitate type of jasper. rIn the formation of massive jasper through replacement of clay "soap," prior impregnation by quartz generally advances along a blunt-nosed, rather than a serrated, front, in the unfractured rock. It may precede the ferric oxide hydrate front by less than a millimeter; or may effect almost complete replacement of "soap" over distances of many inches before that by ferric oxide begins. The available amount of silica and iron in the ground water, the reactivity of the gangue or of other ground water encountered, the influence of protective colloids as the silica and iron content in solution decreases, and other factors, determine the result. Much of the time a delicate balance probably exists in the solutions. For example, it is not uncommon in a specimen several inches across to observe only impregnation by quartz in one portion, co-precipitation of quartz and ferric oxide in another portion of the same specimen, and replacement of the quartz by ferric oxide in a third portion. Such observations, of course, must be made under the microscope; the process is on too minute a scale to be observed by the unaided eye. Where impregnation and replacement by quartz is well advanced before that by ferric oxide begins, the product usually is ivory-colored, and its surface is hard and glossy even when dry; less complete impregnation and replacement by quartz may be difficult to detect except under the microscope. For that reason the term "soap," by common consent, is extended to include any of the quartz-impregnated products up to the phase in which brownish discoloration through precipitation of ferric oxide becomes evident to the unaided eye. "Dominance of "amorphous," or crystalline, quartz in a specimen is not, however, by itself a reliable criterion as to whether the jasper is an open-space precipitate or a replacement product.

33

Although the kaolinite and/or montmorillonite minerals are highly resistant to weathering, as noted by Ross and Hendricks (1945), they are transformed into massive jasper by their breakdown, loss of alumina, and impregnation and replacement by silica and ferric oxide or ferric oxide hydrate, in local dominantly-acid environments created by the rapid oxidation of nearby pyrite. Colloidal matter, upon drying, develops shrinkage cracks, and colloidal clays are peculiarly susceptible to their development. Often the cracks are submicroscopic, much less than a micron in length; but they pervade the mass without much regard to the number or type of clay minerals present, and afford ready ingress for ground water by capillary pull. Appendix B, fig. 95, shows typical shrinkage cracks. Development of these tiny shrinkage cracks in clayey matter accounts, at least in part, for the thorough and relatively rapid jasperization which so often occurs in areas traversed freely by silica-bearing ground waters, and which at the same time are subject to seasonal or intermittent, partial drying-out periods. Table 5 presents chemical analyses of a suite of specimens taken from the Ninety-mile Copper mine, Queensland, selected to show the transition from fresh amphibolite schist (largely hornblende) to massive jasper containing nearly 40 percent ferric oxide. The table discloses the initial loss through leaching of all the iron and virtually all the lime and magnesia, appreciable loss of silica, and recrystallization of remaining silica with residual alumina to produce a clay "soap" that corresponds closely, chemically and mineralogically, to kaolinite. Thereafter the kaolinite itself is attacked and broken down rapidly, with loss of alumina, until an irreducible minimum of about 1.5 percent is reached. Silica content in the meanwhile has increased to a maximum of 90 percent mainly through impregnation of the clayey mass by fine granular quartz and chalcedony (table 5). At this stage the product consists of a hard, glossy, ivory-colored "soap," with as yet only incipient scattered deposition of ferric oxide. Impregnation by ferric oxide to produce massive jasper thereafter takes place irregularly, until a hard, dense, dark reddish-brown jasper containing 38.1 percent ferric oxide is produced. The kaolinization and replacement process, as it occurred in the Ninety-mile Copper mine, is further illustrated by a series of photographs and photomicrographs given in Appendix B. Ragged-edged Jasper. Ragged-edged seams and patches of limonitic jasper characteristically persist throughout the oxidized zones of massive sulfide bodies, and often are as abundant close to the water table as they are near to the ground surface. Most such occurrences, especially those with the sharpest outlines, show unmistakable evidence of belonging to the replacement type. They are the most abundant form of ragged-edged jasper, and are intermediate between the less common

INTERPRETATION OF LEACHED OUTCROPS

34

TABLE 5 Progressive Formation of Massive Limonitic Jasper at Ninety-Mile Copper Mine, Queensland (Analyses by Mount Isa Mines Limited-Expressed in Percent) Description

No.

Fresh amphibolite ..... --------------- .. Do, weathered........................ - --------------Do, thoroughly leached to kaolinic product which forms "soap" when wet. Copper present as microscopic adsorbed particles of copper carbonate and silicate............. 4 Do, opaline replacement about 35% complete. Opaline matrix cream-colored, with patches of light tan ........................... 5 Do, opaline replacement about 80% complete. Opaline matrix tan, with minor patches of light brown .. 6 Do, opaline replacement thorough. Matrix as in No. 5, but patches of light brown more abundant, locally forming bands ............. --------------7 Do, half of matrix various tints of brown with a third of it dark ........... 8 Do, patches and bands of dark brown dominant throughout matrix. Minor copper carbonate along fracture ............ 9 Do, typical dense, massive dark reddish-brown jasper. Copper carbonate precipitations along fractures ............

1 2 3

____ 0 _____ "

•••••

Ignition Loss

MgO

Fe,O"

Ka,O+ Na,O

13.0 0.2

8.9 1.8

'8.7 10.1

nil nil

0.6 0.2

1.1 15.4

36.7

0.2

0.8

nil

nil

nil

12.8

62.8

23.1

0.2

0.6

1.4

nil

nil

I 1.7

83.2

6.8

0.1

0.6

2.3

nil

nil

5.4

90.4

1.2

0.2

0.6

3.6

nd

nil

3.8

86.2

1.5

0.2

0.4

9.0

nd

nil

2.3

77.6

1.7

nd

nd

16.4

nd

nil

2.7

49.3

1.5

0.2

0.2

38.1

nd

0.2

5.6

SiO,

Al,O"

CaO

49.4 46.4

17.7 25.3

48.3

._-----------------

MnO

'Present as FeO.

open space precipitates, and those that replace clay "soap." It has been shown that in the formation of jasper by the replacement of "soap," the flooding by fine granular quartz and chalcedony, and by ferric oxide, advances in most cases along a blunt-nosed, rather than a serrated, front. But if the aluminum-silicate host is isolated in occurrence, is itself jagged in outline, undergoes complete alteration to "soap," and subsequently becomes jasperized, the jasper outline will be, for the most part, equally sharp and jagged. Conditions suitable for the expression of such phenomena occur repeatedly in massive sulfide deposits where irregular, ragged-edged ribs and small islands of erratic shape remain umeplaced. If the umeplaced remnants consist of normal shale or pieces of a feldspathic rock, and if the decomposing sulfides yield strongly acid, iron-bearing solutions for the attack, the development of ragged-edged massive jasper at such places is almost inevitable. Although most ragged-edged jasper is the result of replacements of other rock as explained above, some is known to have been formed by open-space precipitation. If, in the formation of an open-space precipitate, the solutions are guided by closely-spaced intersecting fractures of diverse trend, or by other rock openings of irregular aspect, the jasper formed will to some extent be patterned after such openings. Replacement of Nontronite. Though closely related in mode of occurrence and manner of origin to other members of the montmorillonite group of clay minerals,

nontronite G is discussed separately because: 1) it is a ferric silicate rather than an aluminum silicate (in its purer phases being nearly free of alumina); 2) like alunite, it often is deposited as seams or veinlets that fill open fractures, without evidence of extrusion; 3) also like alunite, it commonly undergoes jasperization preferentially over its more aluminous silicate relatives. The product forms extensively in many districts where iron-bearing sulfides are oxidizing, and is especially common in semi-arid and arid regions. When not present as seams or veiniets, it occurs in the form of the customary "soap" -like alteration of aluminumsilicate rocks as irregular, small dabs or nodulettes 4 to 5 mm in diameter, embedded within the gangue or country rock, often away from fractures. The largest occurrence observed by the author was the size of a man's fist. Usually nontronite is soft and clayey, but may be fibrous or micaceous. Its color is straw-yellow to faintly greenish, but under weathering it acquires a dirty, ironstained appearance, and by the prospector the product often is called "iron-kaolin" (Locke, 1926, p. 63). Like aluminous "soap," it tends to polish under friction. "The term "chloropal," which has slight priority (Ross and Hendricks, 1945, p. 27-28), at one time was used interchangeably with nontronite, at other times restricted to nontronite which through partial replacement by ferric oxide had acquired more of the milk-chocolate color. In recent years the term has fallen largely into disuse, and the product formerly represented by the term now usually is spoken of either as nontronite, or as limonitic jasper, depending upon its degree of replacement by ferric oxide or by ferric oxide hydrate.

MASSIVE ,JASPER

Under high magnification nontronite exhibits the tiny shrinkage cracks that characterize gelatinous colloidal products upon drying. Along them fine granular quartz and chalcedony, and ferric oxide, invade the mass, and through replacement there is produced the same compact, brownish jasper as where aluminum-silicates are involved. In its final form, jasper thus derived cannot be distinguished from that derived from aluminum silicates. Peripheries of jasper bodies derived from either veinlcts or massive replacements of nontronite often arc marc jagged in outline than is jasper derived from the marc aluminous varieties of "soap;" but a jagged periphery is not in itsclf a criterion of derivation from nontronite; assurance of such derivation may be had only through tracing the jasper gradationally into the nontronite as host. It is not certain that all jasper derivatives of nontronite are formed by the direct quartz-limonite replacement process described in the foregoing pages. Nels Peterson, (written communication, 1947) for example, has observed that in the Globe-Miami district of Arizona, nontronite, canbyite (Fe"O,.2SiO".4H"O), and a hard, jaspery mineral or mixture of minerals, occur in a manner to suggest that each successive mineral after the first is a product of the others, and that one or more ferric-silicates other than nontronite may be formed as intermediates in the process. Much remains to be determined regarding the complete story of the transformation of nontronite into jasper. The point of interest here is that in leached outcrops the transformation does take place, and under many circumstances the jasperization is accomplished more readily than it is among the highly aluminous "soaps." Replacement of Opal. Jasper that owes its origin to replacement of opal is quantitatively insignificant compared to that representing replacement of the aluminous varieties of clay "soap," or even of nontronite. But an acquaintance with its der;vation and physical characteristics is needed if the observer is to understand such occurrences when he enounters them in an outcrop. By most persons other than geologists and mineralogists, opal is thought of only as a gem. Ordinary opal, most often met within weathered outcrops, is a more commonplace variety. It is a sub-translucent to whitish, opaque product, dominantly siliceous, that usually contains small amounts of impurities such as alumina, lime, magnesia, and clayey matter. It grades into various siliceous substances, including both nontronite and the more aluminous varieties of "soap." Essentially uncontaminated opal occurs in outcrops as veinlets up to 1 mm thick, and as nodules and irregular masses upward of 15 cm or more across. These arc the occurrences of interest, because opal is even more susceptible to replacement by ferric oxide hydrate than is nontronite, and not infrequently is transformed into seams or nodules of massive jasper with nearly complete obliteration of its former identity, during the weathering of an outcrop. Whether of gem or non-gem quality, opal possesses

35

a distinctive structure, first studied and described in detail by Behrens (1871).' As described by him and later observers, it is an amorphous colloidal hydrate of silica belonging to the group of mineral gels. In the formation of opal, films of colloidal silica, often only a millimicron in thickness, become superimposed one upon another. As a gel-film dries following its deposition, shrinkage necessarily occurs, and the film becomes riddled with innumerable sub-microscopic cracks. Solid particles between the cracks slip slightly upon the underlying firmer layers during drying; uncracked surfaces of the newly formed film tend to become bent or curved in the process; and later gel which subsequently fills the cracks quite often has a different water content than that of the earlier film (see ch. 15). This riddling by minute cracks is immeasurably more involved and extensive than the system of shrinkage cracks that develop in clayey matter, because of the successive very thin films superimposed one on the other. It imparts to the opal a vulnerability to infiltrating solutions not known to be matched elsewhere either in clayey-like or in other hard, siliceous substances. Being itself composed dominantly of quartz, the opal does not often become impregnated by the fine granular varieties of quartz or chalcedony; but few hard outcrop substances undergo more ready and uniform replacement by ferric oxide hydrate. 8 The jasper in this case represents an intimate intergrowth of opal and ferric oxide hydrate, with ferric oxide content ranging up to 50 percent or more. Replacements, with ferric oxide content exceeding 30 percent have been observed under conditions which did not suggest strong acid attack.

OTHER LESS COMMON KINDS OF MASSIVE JASPER AND PSEUDO-JASPER The special types of limonitic jasper product to be described in the following pages are usually isolated in occurrence, and may not exist at all in some districts, but they are certain to be encountered from time to time in some districts, and may prove confusing unless the mechanism of their formation is understood. They fall into three general types as to origin: siliceous-irony knobs or caps, jasper caps over dolomite, and Australian "Billy." In addition the pseudo-jaspers (jasperoid, silica-breccias, and composites involving pseudo-jasper) are discussed.

Siliceous-irony Knobs or Caps Many siliceous-irony knobs or caps are derived from semi-massive pyrite occurrences characterized by the presence of minute grains of quartz which were precipitated in variable amounts interstitially with pyrite. 'See also Ladoo and Myers (1951, p. 371-372). 'The ready vulnerability of some jasper specimens (see table 2, ch. 4, nos. 13, 14 and 15) represents merely the same vulnerability, in reverse, when jasper incorporating such riddled structure is attacked by solvents.

36

INTERPRETATION OF T.EACHED OUTCROPS

The interstitial quartz represents, either wholly or in large measure roek siliea that was nrst dissolved in conjunction with the oxidation of the pyrite and then redeposited essentially in place in finely granular form as supergene silica. As the oxidation of such material progresses, the aluminum-silicate rock immediately adjoining a pyrite grain or speck often becomes replaced by jasper. When the oxidation of pyrite yields an acid solution much of the adjoining hypogene quartz may become corroded, or converted into chalcedony or other supergene forms under acid attack-all upon an almost microscopic scale. However, when reduced acidity prevails, fine particles of ferric oxide or ferric oxide hydrate tend to be precipitated in the tiny open spaces provided by the leaching of pyrite or other soluble rock constituents. The final product thus evolves into a composite of: I) replacement jasper, 2) strongly-corroded minute grains of hypogene quartz, often re-worked into chalcedony or finely granular supergene quartz, and 3) irregularly disposed, fine, open-space precipitates of ferric oxide or ferric oxide hydrate. These three products may be dispersed uniformly or unevenly through the composite; usually they are intimately and nnely interdeposited. As the material is subjected to the protracted, slow denudation of semi-arid and arid regions, the exposed granules of ferric oxide or ferric oxide hydrate in the portion lying within a few feet of the surface, tend to become whipped out to some extent by mechanical forces of weathering. The product, naturally, evolves into a hard, siliceous-irony residual, with variable silica and iron content, which strongly resists further weathering. Its siliceous character at the immediate surface is usually emphasized even more by the accretion of additional tiny particles of silica and iron, derived from overlying eroded material, which become "sintered" to the composite through the sun's heat. Because these siliceous-irony knobs or caps characteristically grade downward within 2 to 6 feet into a generally much less siliceous and less jaspery parent rock, some observers have explained their formation as surface accumulations of silica and iron derived through capillary "pumping;" much as the caliche of certain regions is precipitated as surface crusts through evaporation of lime-bearing solutions. It is well known that ground moisture drawn through rocks to the surface by capillarity may yield supergene silica, which becomes marooned there through the water's evaporation. Such solutions effect replacement coatings, or they may deposit thin hard crusts, with the rock's surface undergoing "case hardening" in the process. The phenomenon is rather common on rock surfaces which in hot semi-arid or arid regions experience long-continued, direct exposure to the sun. Jron from solution frequently becomes concentratcd in a similar manner, and is a factor in the production of "desert varnish" (Engel and Sharp, 1958, p. 514-515), discussed in chapter 16. Although it would appear that this process is capable

of producing these knobs or caps, it probably is not sufficiently appreciated that some of the silica and iron marooned at the surface of rocks through the evaporation of transporting solutions, is re-exported mechanically or in solution at the next rainfall; and that "case hardened" siliceous crusts or "desert varnish" of the sort described-whose origin can be traced specifically to the evaporation of capillary moisture--usually possesses a thickness measured not in feet, but in millimeters. The author has observed none due solely or primarily to such action whose thickness exceeded 3 mm, or which was continuous on an outcrop over a length of several feet. This explanation therefore appears to be inadequate to explain the formation of the much thicker knobs or caps under consideration. In the leached outcrop investigation many siliceousirony knobs or caps of the type described have been followed downward, and traced back step by step to the hypogene source. Virtually without exception they have merged into lean or moderate quartz-sulfide intergrowths with depth. An essential condition seems to be an environment which provides a moderate, rather than an overwhelming, acid attack upon a rock of mildly effective reactivity, which nonetheless contains particles of fine-grained quartz well dispersed through it. Several sulfide and gangue or country rock combinations may provide the environment; but it is most often furnished by either semi-massive or lean to moderate pyrite occurrences of the sort described. On the whole, the accretion of additional particles of silica and iron, derived from the overlying eroded material, is the principal source of siliceous-irony knobs or caps. Capillary "pumping," as an agent in the formation of these siliceous-irony knobs and eaps, must be looked upon, at best, merely as incidental.

Jasper Caps Over Dolomite Jasper caps closely resembling those described above, but characteristically more siliceous and usually more uniform in composition throughout, often form along outcrops of dolomite beds which themselves have undergone no sulfide mineralization. They are more common in dolomitic shale than in dolomitic limestone, but prevail to some extent in both rock types. Most dolomite is impure, and although much of it carries iron in small amount, in much there is no sulfide content. The iron occurs as an isomorphous substitution for magnesium, or as an intergrowth with dolomite as siderite or ankerite or various other ferruginous carbonates. Upon oxidation, the iron in the dolomite alters, more or less in place, to supergene ferric oxide hydrate, causing the weathered dolomite bed to stand out, because of the more resistant silica and iron compounds, as a band of yellowish or light brown color. Weathered dolomite beds often may be identified readily at the surfaee through this peculiarity. Many such oxidized bands remain moderately soft and pulverulent through long periods of weathering. Quite commonly however, in semi-arid to arid regions,

37

MASSIVE JASPER

they undergo intermittent silicification to form jasper lenses, some times hundreds of feet in length, but usually not more than IOta 20 feet long, and a few inches to a few feet wide. For the most part the silicification consists merely of interstitial deposits of supergene quartz or amorphous silica between and around ferric oxide and dolomite grains, but in some districts extensive replacement by silica of the more strictly dolomitic material takes place. The mechanism which effects the silicification is not weil understood, for although the phenomenon has been noted mainly in districts mineralized by sulfides, the occurrences usually are prominent and often predominant well away from the sulfide areas, out of the path of migrating acid solution. Their formation consequently does not seem to depend upon the presence of iron-bearing acid solutions. Residual concentration of original silica in the bed, during protracted weathering, probably plays a part, but necessarily a minor one. The mutual precipitational attraction which silica and ferric oxide hydrosols so often display for each other in circulating ground water may not be unrelated to the phenomenon (ch. 4). This is suggested by the fact that the more iron-rich portions of weathered beds usually experience the strongest silicification. Other factors not yet detected may possibly playa part. Though in the hand specimen the material usually can not be differentiated from various other varieties of massive jasper, it rarely causes confusion in the field, as simply following a given outcrop of it along the strike or down the dip soon reveals it grading into the strictly dolomitic host, minus a sulfide content. It is to be noted that although replacement commonly takes part in the formation of this type of material, the replacement is mainly or wholly that of dolomitic rock by silica, rather than replacement of the silica by iron derived from pyrite.

to explain the frequent abundance and persistence of supergene siliceous deposits at its base. Although "billy" is primarily either a milky-white supergene quartz deposit or a whitish quartzitic grit or conglomerate, loose rock particles of whatever sort existing along the unconformity at the time may be included in it during its formation. The product thus often varies greatly in purity from place to place. Some occurrences contain conspicuous local admixtures of ferric oxide hydrate as included jaspery pebbles, as infiltration along microscopic partings between matrix and pebbles, or as irregular local replacements of both matrix and pebbles. Such replacement never is sufficient to obscure the inherent nature of "billy" over distances of more than few feet. In the field the product thus need cause no confusion, however difficult may be the interpretation of an individual specimen. The last two types of composites here described (jasper caps over dolomite and Australian "billy") are

A

B

Australian "Billy" The third type of composite is of local occurrence and rarely has a direct relationship to either sulfides or dolomitic rocks. The product, called "billy" in Australia where it is somewhat abundant, is essentially pure massive quartz of milky whiteness, often cementing grit or conglomeratic material of the same composition. As crusts, from a fraction of an inch up to occasionally 8 or 10 feet in thickness, it is precipitated from ground waters along certain unconformities. C. C. Morton, who has made a special study of the material in Australia, where it is abundant in some districts, stated (oral communication, 1944) that he never has observed it except at the base of basalt or other basic volcanic flow, or in a position for which reconstruction of the topography points to such a flow having been removed by erosion as the directly overlying rock. Material of the same general physical and chemical composition occurs at the base of several basalt flows in Arizona and New Mexico. Silica leaches readily from most basalt exposed to weathering; which tends

FIGURE 2. Australian "billy." A. Normal supergene milky-white grit (center and left side) with ferric-oxide hydrate stains on the parting-seams (top and bottom). B. Another specimen of milky white supergene grit, showing onion structures of ferric-oxide hydrate at left center and lower left side. Some of the grit shows onion structure only under the microscope and some not at all, while others. as well as conglomerates. show it clearly. The ferricoxide hydrate is usually moderately heavy.

38

INTERPRETATION OF LEACHED OUTCROPS

far removed, both in origin and significance, from the typical massive jasper derivative of semi-massive or massive sulfides. Tn the hand specimen, however, even the experienced observer often may be unable to distinguish them from such derivatives. They serve to emphasize the need of observing in place, and noting closely the field relationships, not only of these types, but of any massive jasper occurrences, before their ongm may be determined and their significance appraised. Tn this respect all types of massive jasper are contrasted with cellular pseudomorphs, which, in the hand specimen, always yield useful clues regarding their derivation.

Pseudo-Jaspers Pseudo-jasper, by definition, is a hypogene quartz mass that has weathered to resemble massive jasper. It rarely causes confusion where the field relationships may be observed. Quartz veins, stained or replaced locally and sparingly along fractures by ferric oxidederived either from oxidation of indigenous sulfides or other iron-yielding minerals, or imported by groundwaters-are so obviously hypogene that no one is likely to be misled into regarding them as jasper. The two forms of hypogene quartz described in the following sections, however, do weather into products which are more deceptive than the pseudo-jasper just described. Jasperoid. In hypogene mineralization large volumes of country rock adjoining, capping, or dispersed through the sulfide bodies frequently have become overwhelmed, through "flooding" replacement, by hypogene silica. However, the country rock is not wholly obliterated, and usually hypogene sulfides in restricted amount are dispersed through the mass. Siliceous bodies of this type are known as jasperoid. Upon weathering, the residual rock silicates within the jasperoid mass are attacked and kaolinized by the solutions yielded by the oxidizing sulfides. In many cases the hypogene quartz itself seems to be more readily altered to supergene form than does vein quartz. Impregnation of such material and subsequent replacement by ferric oxide or ferric oxide hydrate, though likely to be patchy and lean, may yield an over-all product difficult to distinguish from jasper of moderately low ferric oxide content, because during the weathering process the whole siliceous body may acquire the turbid, "dirty" look that so often characterizes the more strongly siliceous portions of certain strictly supergene massive jasper types. This is most likely to be the case where residual rock particles and sulfide both were dispersed finely and uniformly through the jasperoid. Features which help to differentiate strongly weathered jasperoid from jasper are: I) areas usually may be found in which the turbid quartz either grades unmistakably into glassy, hypogene quartz, or into

unfractured silicifications in which country rock residuals or hypogene sulfide have remained immune to the weathering; 2) well-defined and well-preserved cases of leached sulfides may survive. Even if no such identification is possible in a given exposure, the same type of material often may be found elsewhere in the district, grading into indisputable jasperoid which has been less exposed to the ravages of weathering. Though at times difficult to distinguish in a given small exposure from some varieties of massive jasper, jasperoid rarely need be mistaken for replacement jasper of clay "soap" derivation, because it lacks the sustained, even-grained, fine texture and generally uniform composition of the latter, and rarely occurs as isolated small lenses with sharp boundaries. Silica-breccia. Though weathered jasperoid is the most deceptive of the pseudo-jaspers, another type, known as silica-breccia, sometimes may be misleading. In certain districts such as Bisbee, Ariz., orebodies in the unaltered limestone quite generally occur beneath surface or near surface areas in which outcrops of hypogene brecciated quartz, hematite (much of it as specularite), and braunite (3Mn"D::.MnO.SiOJ have been precipitated, either jointly or in close proximity to one another. The quartz shows evidence of having undergone at least local brecciation, the brecciated material having been re-cemented by later quartz, usually of a slightly darker color. In the districts in which they occur, outcrops of this sort commonly are referred to as silica-breccias. Crests of the orebodies in some cases may lie hundreds of feet below the outcrops, but the surface occurrences of breccia-hematite-braunite usually serve as dependable long range guides in directing the prospecting of copper orebodies. In this case it is the hypogene, not the supergene, silica-iron-manganese outcrop which the geologist or the prospector translates into terms of ore. n "In 1923, the author advanced the hypothesis that these hematite-specularite bodies and the frequently associated silica and manganese oxide bodies, constitute a modified form of contact metamorphic zones. The latter are ferric zones; that is, the iron which goes into composition of garnet and epidote of the contact metamorphic zones is ferric iron (Fe,O,,) instead of ferrous iron (FeO). In many cases the garnet is manganesebearing, as at Bisbee, Ariz. In high temperature conditions prevailing, the ferric iron, the manganese if present, and the silica, usually combine with other elements to produce garnet, epidote, and related minerals. Under conditions of temperature too low for the formation of garnet-epidote zones, the ferric oxide, the manganese oxide (and dioxide if present), and the silica, are precipitated as separate masses. Because of their different solubilities, and for other reasons, their respective deposition may not be immediately adjacent; but the bodies usually occur in close proximity to one another. The hematite-specularite masses, with their usual accompaniments of quartz or of extensive silicifications, and with their frequent associations of manganese oxides, therefore express, under conditions of moderate or low temperature, the same mineralization phenomena that, under conditions of higher temperature, are expressed as the garnet-epidote zones in contact metamorphism. Butler (1923) has noted that hematite deposits of this type

39

MASSIVE JASPER

At Bisbee, erosion has been moderately rapid, so that most of the hematite and much of the braunite remains at the surface in the unaltered, hypogene state, and is readily recognizable as such. With the more protracted erosion of certain other districts the hematite may hydrate to some extent at the surface, and its iron may spread locally through the silica as a yellowish, brownish, or reddish limonite stain or replacement, especially if the quartz is well fractured. If braunite is present, it in turn may also decompose in part, and its limonite may stain or impregnate the quartz in like manner. Affected portions of such outcrops may sometimes be highly deceptive in appearance because of their close superficial resemblance to jasper of the replacement type. Some of the outcrops both at Copper Flat near occur in limestone districts, even though the particular deposition may not occur within limestone. He has advanced the hypothesis that as the mineralizing solutions dissolve their way through the country rock, a large amount of CO, is released as CaO goes into solution. Under the conditions of high temperature and pressure prevailing, the CO, becomes dissociated into carbon monoxide and oxygen. The free oxygen ~erves to oxidize the iron (and manganese, if present), to form hematite, specularite, the ferric minerals of the garnet-epidote group, and, under conditions of a less stable oxidizing environment, the frequently found magnetite (FeO.Fe,O,,). Butler's hypothesis has been attacked upon theoretical grounds (Lasky, 1926), but thus far no wholly satisfying alternative explanation has been forthcoming. Irrespective of the oxygen's source, the fact of its existence under the conditions named is conceded. Its presence in quantity suffices to produce ferric iron during the (usually early) stages of the mineralization period with which the designated type of copper or iron orebody is associated, and explains the frequent hematite-specularite association with such copper or iron orebodies.

Bayard, N. Mex., and at Kimberly, Nev., arc of this sort. At the latter location hypogene magnetite also is undergoing extensive hydration at the surface, and conditions are complicated still further by the presence, locally, of iron-impregnated, weathered, yellowishbrown jasperoid. Few of the silica-breccias have undergone sufficient decomposition, however, not to have retained, studded through them, prominent residuals of unweathered specularite or other crystalline hematite in massive form, intergrown with the quartz in a manner to leave no doubt as to the hypogene origin of the entire mass. Composites Containing Pseudo-jasper. Only rarely have any of the silica-breccias been found associated intimately and extensively with limonitic jasper of either the cellular or the massive types, because they do not often have appreciable amounts of pyrite or other hypogene sulfides precipitated within their boundaries. But jasperoid, being more commonly localized either as a cap directly overlying or as a body adjoining or dispersed through, portions of the rock mineralized with hypogene sulfides, often weathers jointly with those sulfides. In some districts both weathered jasperoid and silica-breccia crop out, and both products merge almost insensibly into each other. In the Cloncurry district, Queensland, the Great Australia, Hampden, Kalkadoon and Ivena oxidizedcopper orebodies all either lay beneath or adjoined strongly weathered outcrops of such composite origin. The largest and most sustained occurrence of the massive jasper and the one with the ferric oxide or ferric oxide hydrate most uniformly and densely dispersed through it was at the Hampden mine, where, as might have been inferred, the ore was the most pyritic of the

TABLE 6 Typical Jasper Outcrop Specimens Showing Variable Silica-Ferric Oxide Content-Hampden and Great Australia Mines, Queensland (Analyses by Mount Isa Mines Limited-Expressed in Percent) Description

No.

SiO,

Fe,O"

CaO

MgO

KaO+ Na,O

Ignition Loss

HAMPDEN-

1 2

3 4 5

Black shale county rock, feebly kaolinized, no jasper............................................................................... Kaolinized shale, finely impregnated with jasper, color dark tan to brown ................................................ Do, less siliceous, color increasingly dark brown ........ Do, color dark brown to chocolate.............................. Do, lean silica content, color dark brown to brownish-black.. ............................................................

65.6

20.2

]3.1

0.6

1.4

1.4

5.2

61.2 43.8 42.6

16.3 11.8 0.4

11.7 32.6 48.9

0.1 nd nd

0.2 nd nd

tr nd nd

5.9 6.1 6.6

10.2

0.3

79.7

nd

nd

nd

8.8

77.4

5.8

9.7

0.2

0.2

0.3

1.9

67.3

1.7

22.8

tr

0.1

nil

4.1

41.2 15.0

0.1 0.2

51.4 76.0

nd nd

nd nd

nd nd

6.3 8.2

GREAT AUSTRALIA-

1 Diorite-derived greenstone, so thoroughly impregnated with vitreous jasper as virtually to extinguish the greenstone characteristics. Color tan to light brown ..................................................................... 2 Do, various shades of brown, with patches of dark brown conspicuous ......................................................... 3 Do, dominantly dark brown, with patches of brownish-black............................................................... 4 Do, dark reddish-brown to brownish-black.. ................ 1Probably present in the rock as FeO.

INTERPRETATION OF LEACHED OUTCROPS

40

four, though not necessarily the lowest in copper content. H ) The outcrops at Kimberly, Nev., and certain occurrences with the Czar-Southwest portion of the Bisbee, Ariz., district, less emphatically exhibit such merging of sulfide-derived limonitic jasper, not only with jasperoid, but also, to a very small extent, with associated silica-breccias. Occurrences of pseudo-jasper thus merit close investigation, not alone as long-range guides to possible hypogene orebodies beneath, but because they occasionally merge with cellular or massive jasper, as shorter range guides to possible associated orebodies, both supergene and hypogene.

SUMMARY 1. The second main type of limonitic jaspermassive jasper derived from pyrite-has been described in this chapter, including the principal varieties commonly found in semi-arid and arid regions. 2. An attempt has been made to define the factors involved in the formation of limonitic jasper under varying conditions, so that the observer may understand why each type occurs in its particular environment; and, through such understanding, be able to grasp its significance, and in some degree translate it into terms of the source material. Descriptions also have been given of the two principal types of pseudo-jasper, and criteria adduced for their recognition and interpretation, whether they occur alone or merged with true jasper. lOAt the north, or Hampden, end of the line of lode, lenses of massive jasper of the impregnated kaolin or clay "soap" type-which at many places merged almost imperceptibly into jasperoid-constituted 5 to 25 percent of the outcrops over the more favorable ore-bearing portions. At the south, or Hampden Consols end, where they constituted 20 to 55 percent of the outcrop, the ore was so massively pyritic that fire broke out from spontaneous combustion in the upper sulfides when development was undertaken, and continued burning so vigorously that the upper levels had to be sealed off permanently.

3. Most districts disclose an almost infinite number of variations from the cellular, massive, and pseudojaspers herein described. From the descriptions furnished, and with knowledge of the factors involved in formation of limonitic jasper under different conditions, the observer should be able, by interpolation, to classify and appraise any of the variations he encounters, provided he observes in the field the conditions under which they occur. 4. Among the massive jasper products of the type derived from pyrite, usually only that derived by replacement of clay "soap" may be translated usefully into terms of the source material. Because such jasper points to local, strong acid attack with an accompanying plentiful supply of iron, the earlier presence of semimassive or massive pyrite, decomposing under moderately rapid oxidizing conditions, is strongly indicated as the source material. Its composition and grain size generally are more uniform than those of the other jaspers, and its boundaries are, at least in part, often sharply defined. Its scattered, lens-like occurrence through a usually much-less-altered country rock matrix, often assists additionally in its identification in the presence of other massive jasper occurrences. 5. The experienced observer usually has little difficulty in recognizing pseudo-jasper. Often its relationship to ore is too remote to have useful meaning, but when cellular pseudomorphs, or massive jasper of the clay "soap" replacement type, merge into it locally, the leaching of associated sulfides is indicated. Also, pseudo-jasper itself, under favorable conditions in a particular district, may constitute a long range guide to hypogene ore, especially in the silica-breccias. 6. Open-space precipitates and far-traveling varieties of massive jasper are of little value for direct interpretation in terms of the parent mineral, as the source of their iron seldom can be determined in more than very general terms. The ability to recognize them is of value in the search for associated jasper types which may be significant.

Chapter 7 EXTENT OF LIMONITE PRECIPITATION ABOVE AND BELOW THE WATER TABLE Above the water table is the zone of aeration and below the water table is the zone of saturation. To understand the extent to which limonite precipitation may take place above as well as below the water table, it is necessary to visualize the conditions which effect oxidation of sulfides at those places.

OXIDATION ABOVE THE WATER TABLE A vivid, concise picture showing the manner in which sulfide oxidation proceeds above the water table was furnished by Locke (1926, p. 56-63). The following description is largely a summary of the process as presented by him. Above the water table the ground on the whole is porous. Interconnected open cracks, comprising fractures and joint planes of varying patterns, extend more or less generally through the rocks at intervals of several inches to several feet or more, commonly decreasing in width with depth, but nevertheless permitting both the free circulation of air and the free percolation of rain or other surface water downward to the water table at all times. Blocks of rock between the openspaced fractures contain a multiplicity of minute fractures, often amounting to a mosiac-like structure. The latter often have the thinness of a sheet of tissue paper; yet they permit seams of moisture to penetrate by seepage along them. They retain moisture for a long period of time, and water continues to drip from them after it has ceased to circulate in the more open channels. The rock residuals, bounded in turn by the minute fracture seams, contain innumerable capillary pores on the surfaces of which a thin film of moisture is adsorbed and retained through the drying-out process which accompanies periods of drought. Within the oxidized zone the smaller rock fractures and the capillary pores thus seldom if ever become depleted of most of their moisture, however arid the condition along the open fractures may be. In fact the essential difference in moisture content, under normal conditions, between the oxidized zone above the water table and the zone of saturation below it, is merely that in the former water is absent from the open spaces most of the time. The more clayey the rock or soil, the less do the smaller openings yield their moisture; and for such material

there often may be no difference whatever in moisture content, per given volume, between the two zones. Air circulation along the open fractures is stimulated by the bellows effect whenever rain or other surface water percolates down through the fractures. If the rainfall is 'heavy and rapid, the lower air becomes trapped and rushes about in an effort to escape, some of it being dissolved in the percolating water, most of it eventually bubbling upward through the water to the surface. Between rainfalls the free air of the oxidized zone is kept moving less vigorously through changes in velocity and direction of wind pressure, and by means of pressure and temperature changes at the earth's surface, as well as by the heat induced within the rock by oxidation of the sulfides themselves. These causes with their evaporative effects, quite independent of water circulation along the open fractures, cause air within the interconnected open spaces to expand and contract, producing a distinct, though mild, breathing motion. Free air movement is of course precluded along the minute fractures and capillary pores, sealed off as they are by their moisture films; but air diffuses readily through such films, and the supply of air becomes subject to renewal at any point almost as rapidly as oxidation depletes it. Air circulation throughout the zone of oxidation thus is almost constant, and is certain to reach all parts adequately, even though moisture circulation be retarded or temporarily stopped. Since by the nature of hypogene deposition sulfides occur in greatest amount along rock fractures, they are particularly vulnerable to attack by the air-water oxidation processes. Even along the open fractures in semiarid and arid regions where during the dry season moisture rarely is visible to the eye, thin films, possibly only of molecular thickness, adhere to the surfaces of surviving sulfides. Oxidation thus never wholly ceases, even along the open fractures during the dry season, so long as unoxidized products vulnerable to attack remain. The abrupt changes brought about by alternate wet and dry spells, together with the never-ceasing circulation of air through the rock, favors a wide variation in the concentration of solutions derived from the oxidation. If moderately warm atmospheric temperatures also prevail, as they do during much of the year in most semi-arid and arid regions, the oxidation proceeds

42

INTERPRETATION OF LEACHED OUTCROPS

with exceptional rapidity.l With each influx of rain, the more soluble products of sulfide oxidation such as copper sulfate and zinc sulfate, are washed away, leaving fresh surfaces exposed to attack. The oxidation of the ferrous to ferric sulfate, and the bringing in of neutralizing ingredients in solution, cause the precipitation of the dissolved iron as limonite. This does not mean that leaching above the water table always is accompanied by limonite precipitation in the immediate vicinity, because if high acidity is maintained thc iron must reman in solution, and may be transported over appreciable distances (see fig. 11, ch. 18). In the semi-arid and arid regions, however, such conditions are local. In most cases marked variations in concentration of solutions over short distances, or others of the interfering factors previously discussed, come into play, so that at least a substantial part of the iron usually is precipitated before it has undergone important travel. Barring an environment almost wholly devoid of neutralizer, the distances traveled rarely exceed a few centimeters, and often not more than a few millimeters. The semi-arid to arid regions thus are the ones in which iron obtained directly or indirectly from the oxidation of sulfides is most likely to be deposited at or close to the sulfide source as indigenous limonite, or as limonite transported not more than a millimeter or two, especially in feldspar and normal shale gangues. Those regions consequently are the ones in which leached outcrop interpretation is likely to attain its highest degree of accuracy, and in which the interpretations are likely to find their most fertile field of application (see ch. 9). Although sulfides are vulnerable to attack wherever exposed within the zone of oxidation, the most persistent and widespread attack upon them normally takes place within the layer of rock 3 or 4 feet thick directly above the water table. Two reasons are noted for this phenomena. In the first place, an adequate constant supply of moisture is present, since the horizon lies within the range in which capillary attraction unfailingly draws moisture upward from the ,water table. At the same time the rock, already affected by incipient oxidation, is sufficiently porous to assure an adequate movement of air through it by circulation or by diffusion. 2 In the second place, assuming a reasonable vertical or near vertical uniformity of mineralization, the horizon directly above the water table contains the largest volume of unleached sulfide, since it has been 'Chemical activity within the ordinary range of temperature is roughly doubled with each 10' C increase in temperature. With conditions otherwise equal, sulfides thus would oxidize approximately twice as fast at 30' C (86' F) as at 20' C (68' F), and four times as fast at 30' C (86'F) as at 10' C (50' F). "To quote Locke (1926, pp, 56-57), "With 5 percent of intercommunicating voids, an air travel of 10 feet per day would supply 100 times the oxygen needed for the oxidation of fresh disseminated sulfide ore exposed each year by the sinking of the water level a distance of one inch."

exposed to attack for a shorter period than has the ground above it. As the attack upon this horizon proceeds the rock becomes increasingly porous, the water table drops correspondingly, and the process of oxidation "eats" its way ever deeper. Thus are explained the severe weathering effects which sulfides experience in the Great Basin regions of the southwestern United States and northern Mexico, in most portions of Australia except the more rainy coastal belts, and in other semi-arid to arid regions of the earth. Explained also in large part is the frequently great depth-commonly upward of 200 feet or moreat which the water table lies in such regions, In the above discussion a flat water table in the affected area has been assumed. Such a water table develops where there exist: 1) a homogeneous country rock with a reasonably uniform fracture pattern through it, and 2) erosion which has nearly but not quite kept pace with the lowering of the water table during oxidation. Those conditions obtain in many of the porphyry copper and lead-zinc districts, If erosion is more rapid than oxidation, much sulfide is likely to remain marooned within the zone of oxidation well above the water table. Some of it even may persist to the surface, If erosion is much slower than oxidation, other conditions being equal, the water table will descend to an abnormally deep level, as at the C. S. A. mine in Australia (see ch, 13). In contrast, if the rock is not homogeneous-as where rocks of different composition are involved, such as normal shale and quartzite, or porphyry and limestone-and if, as is more probable in such a case, an essential uniformity of fracture pattern does not prevail throughout the horizon under attack, then oxidation will progress downward in a highly irregular manner. In such cases its floor may lie anywhere from a few feet to several hundred feet or more lower in one type of rock than in the adjoining type, and the decomposition of sulfides may be nearly complete to within a foot or two of water level in the more porous rock, while abundant sulfides may remain un attacked well up toward the surface in the less porous one-especially if the latter be clayey. Even within a rock which is homogeneous in composition, in character, and degree of fracturing, such as prevails in many of the porphyry copper deposits, portions of the sulfide body involving blocks of ground up to a hundred or more feet across not infrequently become isolated by gouges along fault planes, so that neither water nor air can circulate freely within them, In such blocks of ground, oxidation locally may lag behind that for the body as a whole. These abnormalities however, as also the uneven water tables resulting from differences in composition and/or permeability of adjacent layers or masses of country rock, constitute local deviations from or irregularities within the general picture. The overall principle of sulfide oxidation above the water table as above outlined, does not alter much.

LIMONITE PRECIPITATIOl
OXIDATION BELOW THE WATER TABLE Conditions are markedly different below the water table from those above it. Free circulation of air has ceased, sudden inrushes of rainwater do not occur, and the temperature remains largely constant from year to year. As surface water percolates downward toward the base of the zone of aeration and into the region of gradually lessening permeability, its rate of travel decreases greatly, except locally where cracks or other openings of abnormal size exist, or where mine workings exist. Its temperature, by the time it reaches the water table, usually closely approximates that of the country rock through which it is passing. In the vertical range within which oxidation effects manifest. th.emselves below the water table the temperature vanatlOns thus are reduced largely to changes initiated through heat generation or absorption from chemical activity." Likewise since chemical activity within the zone of saturation must take place within a "bath" of water, the solutions involved in any of the reactions, except at the immediate point of chemical attack, generally are weaker than those within the zone of aeration, and rarely experience the latter's extreme and abrupt changes in concentrations of salts. Ground waters below the water table generally do not exceed 0.2 percent of dissolved matter. The reader however, must guard against a misconception which prevails among some geologists, namely, that an acid solution percolating down from above and reaching the water table, immediately spreads out laterally, and through dilution or diffusion 4 dissipates itself rapidly to the point of impotence. Water within the zone of saturation, for the most part, is present only as fillings along extremely narrow fractures, supplemented by whatever amount is held within the capillary openings. The fractures have not become widened as yet through large-scale solution of the adjoining wall rock as they have in the weathered zone of aeration above the water table, and the permeability of the rock below the water table therefore usually is much less than in the rock above. Porosity often amounts to only a few percent of the rock volume; only rarely does it approach 10 percent, though in sandstone (or other porous rock) it may approach 20 percent. Thus, except possibly along well-defined solution channels, as in faults or in highly fissured zones, and in many instances even within them, water within the zone of saturation circulates slowly. In the absence of artificial drainage, it often remains virtually stationary for weeks or months at a time. This is particularly true of semi-arid regions, where often over a 6- to 8-month period almost no replenishment of the surface water supply occurs. "Regions of recent volcanic activity are not considered here. 'Locke, (1926, p. 50) stated: "Oxygen introduced [below the water table] into one end of a 100-foot water column would, by diffusion, reach measurable concentration at the other end only after thousands of years."

43

The leached outcrop investigation presented many opportunities for observing the retention of the acidity of solutions as they penetrate the zone of saturation beneath the water table in a nearly inert gangue." The acid waters in these cases have maintained their stability when fresh cuts of shafts, winzes, drifts and crosscuts have dissected the ground beneath the water tablefar too rapidly for post-mine leaching effects to have come recognizably into play. Comparisons were made with results at nearby shafts or winzes in the same mine and in the same type of rock, but where no sulfide concentration existed. In some cases the solutions were collected from advancing drifts or crosscuts well off the bottom of shafts or winzes, to assure that the liquids had percolated through undisturbed rock. In quite a number of instances it was possible to obtain samples simultaneously at different levels or sub-levels in adjoining blocks of ground, thus building up a complete picture showing results at progressive mine workings below the water table. The solutions were tested for their degree of acidity, mostly by the use of litmus paper and supplemented with distilled water where required. In many cases complete laboratory analyses were made. The observations showed that where the normal fracturing has a general vertical continuity, with gangues of weak neutralizing power such as quartz-sericite schist, siliceous shale, or veins with dominantly quartz gangues, the solutions derived from sulfides directly below the water table may descend more than 300 or 400 feet with their acidity only slightly decreased, especially in pyrrhotite or sphalerite ores which are easy to oxidize. In similar ground distant laterally, but not containing sulfides in more than occasional minute amounts, however, the solutions under similar conditions either remain very weakly acid or slightly alkaline throughout in any given ore body. In siliceous gangues and generally also in feldsparrich or normal shale gangues, in areas where fissure zones are not influential, the sulfide minerals are attacked by cupric sulfate solutions in the following decreasing order of intensity: covellite, bornite, chalcopyrite, sphalerite, pyrrhotite and galena. They are readily replaced by supergene chalcocite in the interval from less than 2 feet to 200 feet or more below the water table, without the attendant deposition of limonite. Pyrite is not readily attacked until the previously named sulfides are partially oxidized; the whole sequence is described in chapters 8, 10, and 11. In the case of a limestone gangue, with the above conditions prevailing, limonite is generally precipitated within an interval of 5 to 50 feet because of the greater neutralizing power of the gangue. The limonite is precipitated in long tapering stringers having ragged lower boundaries. When cupric sulfate solutions are not present and galena is attacked by sulfuric acid, it is generally altered to anglesite, but galena oxidizes very slowly and oxidation generally does not penetrate it very 'The same effect does not apply when solutions traverse feldspar-rich rocks, shale, and especially limestone.

44

INTERPRETATION OF LEACHED OUTCROPS

deeply because of the formation of an insoluble sulfate coating. In time the anglesite tends to be converted to the still less soluble cerussite. In a nearly inert gangue, where the water has stood for a long time at any given position, much of the iron originally present in the sulfides may escape in solution as ferrous sulfate, as was the case at Kyshtim, Russia (Stickney, 1915). Some limonite was present in that deposit, though in small amounts, except in the oxidized hard limonite portions, about 40 feet beneath the surface. Where the water level has remained the same for a long time, pyrrhotite and iron-rich sphalerite are par-

ticularly susceptible to attack by acid solutions in a nearly inert gangue, and ferrous sulfate solutions derived from pyrrhotite and sphalerite may extend to 300 or 400 feet or even more below the water table in areas of normal fracturing without leaving a trace of limonite, as is well known by people who have studied lead-zinc deposits. The pyrrhotite and iron-rich sphalerite may thus largely disappear, leaving a honeycombed or sandy gangue (Locke, 1926, p. 54); though this is unusual. Generally the pyrrhotite and sphalerite corrode in the siliceous gangue to some extent, leaving from 90 to 99 percent of the pyrrhotite and sphalerite below the water table unaltered.

Chapter 8 THE NEED FOR EXCESS SULFUR TO PROVIDE FREE SULFURIC ACID The sulfide minerals of oxidizing orebodies are most often dissolved either by sulfuric acid, or by ferric sulfate solution. The sulfuric acid or ferric sulfate is mostly derived from within the orebody itself, but may in some cases have been introduced from an external source, usually by means of circulating groundwater. The degree to which the sulfide minerals are dissolved is dependent upon the amount of sulfuric acid or ferric sulfate available. The amount available depends in turn mainly upon the ratio of sulfur ion to total metal present. It is that ratio that determines whether the acid created is sufficient to entirely dissolve the sulfide minerals present or not. Also of importance, because of pyrite's large excess of acid-forming sulfur and its almost universal presence in sulfide orebodies, is the ratio of pyrite to total metal sulfides. The chapter describes the processes involved, and the degree of solution that may be expected to take place under assumed conditions, during the oxidation of some of the common sulfide minerals.

DEGREE OF SOLUTION DEPENDENT ON AMOUNT OF ACID GENERATED

the creation of acid sufficient for their complete solution, and upon oxidizing they yield free acid. Pyrite, pyrrhotite, and chalcopyrite are common examples. Pyrite contains a large excess of sulfur. Some of the effects produced by oxidation of pyrite have been already described in connection with formation of the impregnated-kaolin type of limonitic jasper (ch. 6). Because of its extensive generation of free acid, pyrite, when associated with chalcocite, bornite, or tetrahedrite effectively promotes their oxidation and facilitates their solution. Pyrrhotite and chalcopyrite contain less of an excess of sulfur than pyrite does, and they consequently generate less free acid. Although in general they serve to promote the oxidation and facilitate the solution of &ssociated chalcocite, bornite, and tetrahedrite, their influence is not comparable in extent to that of pyrite. Arsenopyrite also generates strongly acid solutions, but it is not often encountered in quantity in association with chalcocite or bornite, and usually is encountered only sporadically with most of the other ore sulfides here discussed. Its effects upon associated sulfides are treated separately in chapter 20. Disregarding arsenopyrite for the time being, the only common sulfides to be considered as yielding excess acid thus are pyrite, pyrrhotite, and chalcopyrite. Pyrite is so general in its distribution in nature and in association with other sulfides, the yield of excess acid is so large relative to that yielded by pyrrhotite or chalcopyrite, and the uniformity of its yield under normal conditions of oxidation is so dependable, that it serves as the standard for visualizing and calculating the effects of imported acid upon the decomposition of most other sulfides, and upon the formation, dissolution, and redistribution of their leached equivalents. Throughout the remainder of this volume it will be used as the yardstick by which to measure such affects.

Certain sulfides are deficient in sulfur. They may oxidize and partly dissolve, but will not dissolve completely unless acid from an external source becomes available to them. Chalcocite, bornite, and tetrahedrite are examples. All of them yield to oxidation, but none has been observed in the field to have dissolved completely when in the pure state, or in association only with one or both of the others. Certain other sulfides contain just enough sulfur to create acid sufficient for their complete solution. They require no acid from an external source to bring about their complete oxidation and solution, neither do they generate free acid. Covellite, sphalerite (containing no iron), and molybdenite are examples. Since they yield no free acid, their association with chalcocite, bornite, or tetrahedrite neither promotes oxidation nor facilitates solution of those three minerals. 1 A few sulfides contain more than enough sulfur for

Oxidation of Pyrite

'Possible accelerating affects in oxidation brought about by differences in electrical conductivity of the respective sulfides are not considered here. For a discussion of that subject see Wells (1914).

In the oxidation of pyrite by air-water processes, various rearrangements of the constituents may take place. When pyrite decomposes and goes into solution its iron and sulfur atoms ionize. Oxidation of pyrite by

OXIDATION BY AIR-WATER PROCESSES Oxidation of sulfides is brought about to begin with by simple interaction with air and water.

46

INTERPRETATION OF LEACHED OUTCROPS

oxygen in the presence of water yields sulfurous acid and ferrous sulfate (FeSOJ. Under natural conditions the sulfurous acid is very largely oxidized to sulfuric acid (HcSO j)' and some of the ferrous ion is oxidized to the ferric ion. ]n solutions derived from the air-water oxidation of pyrite, ferrous ions (Fe++), ferric ions (Fe+ + +), and sulfate (SO ,- -) ions thus arc present in varying amounts. The initial oxidation of pyrite (FeSJ usually is expressed by the equation 4FeS/ +- 140/ +- 4HP = 4FeSO. +- 4H cSO,.

4FeS2+-150c+-2HcO=2Fe2(SO.),+-2H2S0..

(3)

(1)

According to equation (1) only ferrous ions are involved, and 1 mole of pyrite yields I mole of sulfuric acid.:: The iron, if in solution as ferrous sulfate, would be carried away or leached from the point of oxidation. But in nature rarely if ever does the iron derived from pyrite remain wholly in the ferrous form. For example, where pyrite oxidizes below the water table in an inert gangue, such as quartz; or, possibly, in a saturated similar environment above the water table where the pyrite is massive and is oxidizing rapidly, ferrous sulfate is incompletely oxidized to ferric sulfate, [Fe c (SO,)J. McBain (1901, p. 623), and later Morse (in Locke, 1926, p. 38-40), showed by means of laboratory experiments that after blowing air through an acidified FeSO, solution at room temperature for several days, the concentration of ferric iron was so low as barely to permit measurement. The same series of experiments showed, however, that the rate of oxidation increased notably with increase in concentration of the ferrous sulfate as well as that of oxygen. Under natural conditions a variable portion of the ferrous iron thus may be expected to oxidize to the ferric state. Such oxidation is expressed by the equation 4FeSO.+-2HcSO.+-Oc=2Fec(SOJ:-f-2HcO.

cially in a dry atmosphere, some of the sulfur dioxide (SOJ gas may escape into the air instead of going into solution. Furthermore, the supply of oxygen and water may not be constant, or may not reach all parts of the oxidizing sulfide equally. In either case a portion of the potential sulfate radical (SO.- -) supply available for acid generation is lost. The formation of ferric sulfate from oxidizing pyrite may be expressed by the equation

(2)

In this case the iron may be exported, but as ferric instead of ferrous sulfate. It will be noted that the oxidation consumes half of the free acid generated in equation (1). Small amounts of certain other constituents may promote the change from ferrous to ferric sulfate. Posnjak ( 1926) has shown that small amounts of cupric sulfate in the solution may increase many times the rate of oxidation of ferrous to ferric sulfate. So effective is the process that it may operate extensively even where pyrite is oxidizing in a quartz gangue below the water table. Under some conditions still other factors come into play. For example, when pyrite oxidizes above the water table with a free flow of air surrounding it, espe'The underlined formulas indicate solids; the overlined formulas indicate gases; formulas not underlined or overlined indicate liquids. "To facilitate comparisons and calculations the respective quantities of the components involved in chemical equations usually are referred to in moles. A mole is the molecular weight of a substance in grams. For example, 1 mole of sulfuric acid contains 98.08 grams of the acid. 1 mole of pyrite contains 119.96 grams of the sulfide.

This is an abridged way of setting forth the same end resuIts in terms of ferric sulfate and sulfuric acid as is indicated by the combination of equations (1) and (2). As stated previously, probably under no conditions in nature does oxidizing pyrite yield either ferrous or ferric ions exclusively; the solutions always comprise a mixture, variable as to the proportions, of Fe++, Fe+++, and SO,,--, and SO.--ions present. Below the water table, or in a thoroughly saturated inert environment, ferrous ions usually predominate. In the zone of aeration above the water table, especially in a comparatively dry atmosphere and in the reactive substances within the gangue or in the ground water, ferric ions greatly predominate. Even under the most favorable oxidation conditions, however, a small proportion of ferrous ions always remains. The persistence and activity of some ferrous ions under virtually all conditions of oxidation is best shown, perhaps, by the presence of siderite intergrown in variable, though usually minute, amounts in practically all freshly-formed limonitic deri vatives. Iron is not a readily soluble substance; the proof is its almost universal persistence in an outcrop after most other ore constituents have been leached. In highly dilute solutions the ferric sulfate is hydrolized to form hematite (Fe 2o,,) or goethite (Fe"Oa·H 20), as indicated by the following equations: Fe 2 (SO.) ,,+-3H 2 0=Fe 2 O,,+-3H cS0 4 ,

(4)

or Fe 2(SOJ "+-4H 2 0=Fe 2 0".H 2 0+-3H 2SO..

(4a)

Hydrolysis is only important in producing goethite or hematite when solutions are greatly diluted, however, and goethite and/or hematite will redissolve partially or completely if more pyrite oxidized and the solutions become more concentrated again. Because the final product observed in gossans and cappings is mainly that derived from ferric rather than from ferrous solutions, the oxidation of pyrite, as applied to the leached outcrop interpretation, may be summarized as follows: 1. By oxidation in an inert environment pyrite yields one-half its equivalent in moles of free acid, exports all of its iron, and leaves an empty cavity. Complete leaching takes place. 2. By oxidation and hydrolysis in dilute solutions pyrite may yield twice its equivalent in moles, of f~ee acid. In this case all of its iron remains in the caVIty as indigenous limonite; none is leached.

47

NEED FOR EXCESS SULFUR

These two points should be grasped firmly by the reader, for upon their application and only through it, will he be able to visualize clearly the chemistry of leached outcrop interpretation as applied not alone to pyrite, but also to the other sulfides, and to many of the non-sulfide minerals. Hydrolysis is not the only process that will bring about precipitation of iron as limonite. Any substance which neutralizes the excess sulfuric acid will accOmplish the same result. Probably the most common factor contributing in a large way to weakening of acidity is the partial or complete neutralization of the solutions by reactive gangue minerals adjoining the oxidizing pyrite grains, seamlets or nodules, or by the presence of calcium or magnesium salts in ground water (ch. 4 and 6). In chapter 12 is described in detail the manner in which limestone, the most common strong neutralizer, brings about that result. The reader unacquainted with chemical reactions should be advised that formation of the ferrous and ferric sulfate, the sulfuric acid, and the hematite and/or goethite of equations (1), (2), (3) and (4) probably does not proceed as directly and simply as the equations might suggest. Intermediate steps in the decomposition and reunion of the different constituents may occur in the reactions represented by any of the equations, but the end results are correctly indicated by the overall equations. The point will become more clear to the reader after he has familiarized himself with the subject matter in this chapter. At present, however, the principal idea for him to grasp is that both the chemical equations listed in this volume, and those he encounters elsewhere, in general represent only the initial reagents and end products of the reactions involved and do not necessarily indicate all intermediate steps. This is true because, unless some interference by other agents (such as calcium or magnesium carbonate gangues) occurs during the oxidation of the pyrite, the amount of free acid generated at the point of oxidation will more often approach one-half the number of moles of pyrite originally present, than it will twice the number of moles of pyrite which could be formed if no oxidation of ferrous to ferric sulfate took place.

Oxidation of Pyrrhotite Pyrrhotite and chalcopyrite contain more than enough sulfur to form acid sufficient for their complete solution, and upon oxidation and hydrolysis they yield an excess of free acid. The oxidation of pyrrhotite may be expressed as FeS"+20 2=FeSO j.

(5)

'pyrrhotite is more accurately described by the general formula Pel-XS, while troilite is the pure monosulfide, PeS. Pyrrhotite has a little more sulfur than has troilite on the average, but not nearly so much as pyrite. Troilite is extremely rare, while pyrrhotite is a common iron-sulfur mineral. Pyrrhotite varies in proportions of iron and sulfur even in the same orebody; see chapter 19, for details.

But above the water table, unless it is assumed that massive pyrrhotite is rapidly oxidizing in a saturated environment, ferrous sulfate is usually not present in quantity, especially in disseminated sulfide areas of semi-arid and arid regions. Therefore the reaction may be expressed as:

or 12FeS+2702+2H 2 0= 4Fe 2(SOj),,+2(Fe 2 0 3.H2 0).

(6a)

The oxidation of pyrrhotite thus yields no free acid, but by means of the hydrolysis of the ferric sulfate formed by the oxidation of the pyrrhotite and precipitation of the iron as hematite and/or goethite, free acid may subsequently be formed. In this instance the equation would be expressed as: Fe2(S04)3+3H20=Fe203+3H2S0.,

(7)

or Fe2(SO.)3+4Hp=Fe20:l.HP+3H2S0j.

(7a)

Provided that hydrolysis of the ferric sulfate goes to completion, all of the iron of the pyrrhotite is precipitated as indigenous hematite and/or goethite within the cavity, and pyrrhotite yields its own equivalent in free acid, or one-half as much as pyrite yields under similar conditions. But to dissolve and export in solution the 2 moles of ferric oxide left over in the reaction of equation (6), 6 moles of sulfuric acid are needed, as follows: 2Fep3+6H2S0j=2Fe2(SO.)3+6HP

(8)

According to equation (1), this requires 6 moles of pyrite. The following therefore summarizes the oxidation of pyrrhotite: 1. The oxidation of pyrrhotite usually results in the removal of two-thirds of its iron in solution as ferric sulfate, leaves one-third of its iron as indigenous limonite, but generates no free acid. 2. The oxidation and hydrolysis of pyrrhotite in dilute solutions may yield its own equivalent in free acid, but in that case all of its iron remains in the cavity as indigenous limonite. 3. One mole of pyrrhotite and 1 mole of pyrite, oxidizing together, cause the solution and export of all of the iron of both. Complete leaching of both sulfides takes place. Pyrrhotite therefore is less effective and dependable in augmenting the decomposition of chalcocite, bornite and tetrahedrite than is pyrite, because of its relative deficiency of sulfur in comparison to pyrite.

Oxidation of Chalcopyrite Similarly, the oxidation of chalcopyrite (CuFeS 2 or CuS.FeS) may be expressed by the following equation:

48

INTERPRETATION OF LEACHED OUTCROPS

Where the air-water oxidation processes operate above the water table-especially in semi-arid and arid regions-the equations may be expressed as: 12CuS.FeS+510 2 = 12CuSO.+4Fe 2 (S04L+2Fe 2 0".

(10)

The copper, being far more soluble than the iron, is wholly exported in solution as cupric sulfate. The iron is disposed of in the same manner as it is in the oxidation of pyrrhotite, and free acid is formed in the same manner and in the same amount as if pyrrhotite alone were involved. On that basis no free acid can be formed by the direct oxidation of chalcopyrite. Chalcopyrite, however, rarely if ever is found uncontaminated in nature. For many years members of the Geophysical Laboratory of the Carnegie Institution of Washington, as well as others," have conducted without success a world wide search for pure chalcopyrite. Invariably small amounts of pyrite, and often other impurities, have been detected. The pyrite occurs in particles of varying size, often minute, rather than in solid solution with chalcopyrite. As such it oxidizes as in equations (1), (2), and (4), independently, although more or less contemporaneously with the chalcopyrite. But so far as the mineral chalcopyrite itself is concerned, its behavior from the standpoint of generation of free acid, and in its yield of limonite, is similar to that of pyrrhotite (FeS). The following therefore summarizes the oxidation reactions of chalcopyrite: 1. The oxidation of chalcopyrite causes the exportation of all of its copper and two-thirds of its iron in solution, leaves one-third of its iron as indigenous limonite, and generates no free acid. 2. The oxidation and hydrolysis of chalcopyrite in dilute solutions may yield its own equivalent in free acid. In so doing all of its copper is exported in solution but all of its iron remains as indigenous limonite. 3. One mole of chalcopyrite and 1 mole of pyrite, oxidizing together, dissolve and export all of the copper of the chalcopyrite, and all of the iron of the chalcopyrite and pyrite. Complete leaching of both sulfides takes place.

Oxidation of Chalcocite The oxidation of chalcocite (Cu 2 S) is expressed by the equation 2Cu 2 S+50 2 =2CuSO. +2CuO,

(11)

and the solution of cupric oxide is expressed by the equation

'Stillwell and Edwards (1943) found Mount Isa chalcopyrite to be exceptionally pure.

From these equations it is seen that chalcocite contains sulfur sufficient only for the production of enough acid to dissolve half of its copper. But wherever uncontaminated chalcocite has been observed oxidizing in nature, either brochantite (CuSO •• 3CuO.3H 2 0) or antlerite (CuS0 4 .2CuO.2H2 0) usually is a conspicuous decomposition product, well preserved in arid climates. G Since 1 mole of H 2 S0 4 is required to dissolve and remove 1 mole of CuO, and 1 mole of pyrite yields onehalf mole of H 2S0 4 , two moles of pyrite are needed to put into solution and export the remaining mole of CuO left by chalcocite. In this case all of the iron of the pyrite as well as all of the copper of the chalcocite is exported in solution. If, however, hydrolysis of the ferric sulfate goes to completion in dilute solutions, and all of the iron of the pyrite is precipitated as indigenous hematite as in equation (4), only one-half mole of the pyrite is needed to put into solution and export all of the copper from the 1 mole of CuO. The oxidation of chalcocite thus may be summarized: 1. Without an external acid supply, chalcocite cannot generate sufficient acid to dissolve itself; theoretically only one-half of the copper can be exported, or leached. 2. Two moles of chalcocite and 1 mole of pyrite, oxidizing together, provided hydrolysis of the ferric sulfate goes to completion, can cause the removal of all of the copper of the chalcocite in solution, but leaves all of the iron of the pyrite as indigenous limonite. Only the copper is leached. 3. One mole of chalcocite and 2 moles of pyrite, oxidizing together, export all of the copper and all of the iron, leaving an empty cavity. Complete leaching of both sulfides occurs. 7 In the early days of the leached outcrop investigation, these principles were utilized in the field work of Morse and Boswell, to place the technique upon a sound basis for interpreting the leached outcrops found over some of the disseminated porphyry copper deposits. As a class the porphyry coppers at best yield only a restricted amount of the cellular pseudomorphs, and often none at all. Although equation (11) expresses what is generally believed to occur in the oxidation of chalcocitenamely, decomposition of chalcocite into one mole of cupric sulfate and an additional one of cupric oxidefield evidence does not support the assumption that the GAt Chuquicamata, Chile, in a very arid climate, antlerite which veins and coats the rock in every direction, is almost the only copper mineral in the main oxidized orebody (Jarrell, 1944; also oral communication, Charles Meyer, February. 1953). Originally antlerite came from the Antler mine, Hualpai Mountains, Ariz., and elsewhere in the southwestern United States; but it was very scarce there, compared with Chuquicamata. 'Tunell has shown that for the cavity to be completely vacated of iron the mole ratio of pyrite to chalcocite must actually be greater than 2: 1. The latter ratio, however, yields a cavity essentially free of iron (see fig. 41, ch. 22).

49

NEED FOR EXCESS SULFUR

cupric oxide necessarily exists in the form of the black copper oxide, tenorite (CuO). This only occurs when the solutions of cupric sulfate are very dilute; in more concentrated solutions brochantite or antlerite is formed. Some writers explain the absence or paucity of copper oxide minerals in the oxide zones of copper ore deposits by asserting that through reaction with carbon dioxide from the air or ground water the cupric oxide is transformed into copper carbonate. Such transformation does actually take place to some extent in nature. However, almost numberless instances have been observed in the field, in which nodules or seams of essentially pure chalcocite have become isolated during their oxidation from surrounding sulfides that might supply free acid. In many cases field conditions point to their continued exposure to weathering during hundreds of thousands of years after the surrounding sulfides had been thoroughly leached. To the author's knowledge, in no such instance has oxidation of the chalcocite been observed to go to completions. Perhaps the most illuminating example because of its size and its known prolonged exposure to the severe weathering which characterizes semi-arid regions, is afforded by the Mount Oxide deposit of northwestern Queensland. The Mount Oxide chalcocite body is a supergene deposit that persists from the surface to more than 300-foot depth. The lens of high grade ore from which most of the production has come (to end of 1938, 26,892 long tons of ore averaging 29.41 percent Cu had been shipped, some of it on camels' backs, through 76 miles of difficult traveling conditions to the railhead), and which consists of more or less massive chalcocite except in its upper portions, has a maximum stoping length of 295 feet, with an irregular normal stoping width of 8 to 10 feet, and a maximum stoping width of 40 feet." Beneath the 300-foot level, which is 30 to 40 feet below the present water table, the chalcocite gradually frays out within the next 70 feet, along fissures and joint planes in massive pyrite whose hypogene copper content is limited to very sparse chalcopyrite. The deposit clearly represents the residual weathering remains of a former hypogene copper sulfide body now mostly eroded. (For a brief description of the Mount Oxide deposit, see Appendix C.) From the evidence yielded by the Mount Oxide deposit, and by the small occurrences of chalcocite in an essentially pure state elsewhere, it thus seems necessary to conclude that in nature chalcocite shows itself incompetent to bring about its own complete oxidation and leaching without an external acid supply. In this, as in many other instances the chemist knows the ultimate end products of the reaction but still is in the dark regarding some of the intermediate steps involved, and 'Chalcocite oxidizes faster than bornite, pyrite, covellite, sphalerite and chalcopyrite in the presence of free acid; otherwise slower. (E. T. Allen in Locke, 1926, p. 94.) 'The latter along a roll in the beds. The 300-foot level marks the approximate bottom of the rich lenses of massive chalcocite that yielded most of the mine's output.

has gathered little information concerning the relative reaction velocities.

Oxidation of Bornite The oxidation of bornite (Cu.,FeS. or 2Cu 2 S.CuS. PeS) is expressed by the equation 4 (2Cu"S.CuS.PeS) +370 2 = 16CuS0 4 +4CuO+2Fe 2 0

3,

(13)

and the solution of the cupric oxide is expressed by. the equation CuO+H 2 S0 4 =CuSO.+H 2 0. (12) Without an external acid supply, bornite can dissolve and export in solution only four-fifths of its copper as shown in equation (13). One-fifth remains, together with all of its iron as hematite (or goethite). Since according to equation (12) 1 mole of H 2 S0 4 is required to dissolve and remove 1 mole of CuO, and 1 mole of pyrite yields one-half a mole of H 2 S0 4 , 2 moles of pyrite are needed to put into solution and export the remaining mole of CuO. In the process all of the iron of the pyrite is exported in solution but the iron of the bornite (or its equivalent) remains as indigenous limonite. If, however, hydrolysis of the ferric sulfate goes to completion in dilute solution, and all of the iron of the pyrite is precipitated as limonite, half a mole of pyrite suffices to export all of the copper; and in that case the iron of both the pyrite and bornite is left as indigenous limonite. To dissolve and export in solution 1 mole of hematite or goethite requires 3 moles of free acid, according to equation (8). Thus it is clear that: 1. The simultaneous oxidation of 2 moles of bornite and 1 mole of pyrite results in the solution and removal of all the copper, and the retention of all of the iron derived from both minerals as indigenous limonite if complete hydrolysis takes place. 2. The oxidation and hydrolysis in dilute solution of 1 mole of bornite and 2 moles of pyrite, may cause the solution and removal of all the copper and all of the pyrite-derived iron, but allows the precipitation of the bornite-derived ferric oxide as indigenous limonite. 3. The simultaneous oxidation of 1 mole of bornite and 5 moles of pyrite results in the solution and removal of all of the copper and all of the iron. Complete leaching of both sulfides occurs. When pure bornite oxidizes, or when 2 moles of bornite and 1 mole of pyrite oxidize together, the resulting limonite, when cellular, has a velvety texture, orange to orange-ochreous in color-quite striking. This velvety texture is only produced by those bornite-pyrite combinations in which bornite predominates. With more pyrite, the limonite gradually fades to yellow or ochreous color (see ch. 23 for details).

Oxidation of Tetrahedrite Tetrahedrite, (Cu,Fe,Zn,Ag) 12 (Sb,As) ,SI3' does not possess the fixed composition of most sulfides. Its

50

INTERPRETATION OF LEACHED OUTCROPS

behavior and reactions during oxidation and leaching therefore are less predictable than most other sulfides. A theoretical tetrahedrite end-member which contains no iron, zinc, silver, or arsenic, may be expressed by the formula shown in the following equation: 2 (5Cu zS.2CuS.2Sb 2S3 ) + 5702+ 2H 20= 24CuS04+4Sb203+2H2S04'

(14)

That limonite does form during the oxidation of tetrahedrite, and remains conspicuously as an indigenous product in most instances, is attested by abundant field evidence. Valentinite, the oxide of antimony, is almost insoluble, and it usually stays in or near the place where the limonite is formed. With the copper content of any specimen probably variable between 5Cu 2S.2 (Cu,Fe,Zn,Ag) S.2 (Sb,As) ZS3 and 5Cu zS.2CuS.2Sb"S:l> the pyrite requirements for complete oxidation and thorough leaching of tetrahedrite at best are only approximations until the specimen is analyzed. The main point to note is that, based upon field observations in numerous districts, tetrahedrite, as copper and antimony sulfides, is too deficient in sulfur to complete its own oxidation, or to effect leaching of all its components (see ch. 24).

SUMMARY The above discussion has emphasized repeatedly two extreme cases of oxidation and leaching: 1) exportation of copper only, leaving all the iron as indigenous

limonite; 2) exportation of both copper and iron, leaving an empty cavity. Those two extremes give one the basis for judging leached outcrops in which cellular pseudomorphs do not form readily, as with chalcocite or bornite, and especially with most of the disseminated porphyry copper deposits. Even where cellular pseudomorphs develop, their interpretation is greatly facilitated through ability to recognize the source of the indigenous limonite deposited within the cells and to recognize the reason for absence of such limonite where it is absent. The necessary recognition can come only through knowledge of the oxidation and leaching effects produced by the individual sulfides or combinations of sulfides, as set forth above. Another point brought out in the discussion is that, where air-water oxidation processes operate, ferric sulfate is the principal oxidizing agent of the copper minerals, and supplies much of the iron of the limonite that is precipitated. The importance of ferric sulfate in the leaching process, and in yielding iron for the production of gossans and cappings, therefore cannot be too greatly stressed in the zone of aeration. Even with sulfides such as galena and sphalerite which contain enough sulfur to effect their own oxidation and leaching, limonite, as explained in subsequent chapters (especially in ch. 10), can form mainly only to the extent that ferric sulfate acts as an intermediary in dissolving and carrying away in solution the oxidized salts of such sulfides; and, in the process, dropping a part of its iron in their places.

Chapter 9 LIMONITE PRECIPITATION THROUGH DILUTION OF IRON-BEARING SOLUTIONS Most limonite is precipitated as the result of one of three common types of reactions. It may be precipitated because of the introduction of other types of metal ions into the iron-bearing solution, it may be precipitated as the result of reactions with limestone and other types of reactive wall rock, or it may be precipitated because of the dilution and consequent changes in acidity of aqueous solutions containing only iron and sulfate ions. This chapter is devoted mainly to an explanation of limonite precipitation due to the last named cause, and to descriptions of several minerals, encountered at times in some deposits, transitional to the more stable goethite or hematite.

THE ISOTHERMAL EQUILIBRIUM DIAGRAM OF THE SYSTEM, FeeO,oSO"oH20 The most effective method for visualizing these reactions is by means of the isothermal equilibrium diagrams determined by E. W. Posnjak and H. E. Merwin in the course of their investigation (1922) of the system ferric oxide-sulfur trioxide-water. The diagrams consist of triangular charts upon which have been plotted the results of many laboratory determinations. The curves in these diagrams show the percentages of the components, Fe 2 0,l' SO"' and H 2 0, in saturated solutions coexisting in equilibrium with the various crystallized compounds, ferric oxide, ferric oxide hydrate, and ferric sulfates at temperatures from 50° to 200° centigrade. Posnjak and Merwin determined, through extensive closely-controlled laboratory experiments, the abovenoted Fe 2 0 11 -SO,,-H 2 0 relationships at 200°, 140°, 110°, 75° and 50° Centigrade. For present purposes only the 50° C (122° F) isotherm need be considered, as the other temperatures greatly exceed those ordinarily encountered in natural sulfide oxidation. Even 50° C is reached only under exceptional circumstances. Extrapolation of the laboratory data of Posnjak and Merwin to ordinary outdoor temperatures show, however, that the relationships existing at 50° C persist in large part down to ordinary temperatures. For practical purposes the 50° C isotherm therefore may be considered to apply under ordinary conditions of oxidation. In figure 3 the three components, Fe 2 0", SO"' and H 20 are represented by the three apexes of the triangle. Points along the sides of the triangle represent mixtures of two of the components. A point midway along

the line between the Fe 2 0, and SO, apexes represents a mixture of equal parts (by weight) of those two components. A point lying along the line between the apexes Fe 2 0" and H 2 0 at 89.86 percent Fe 2 0" represents the composition of ferric oxide monohydrate, goethite (fig. 3). Points inside the triangle represent mixtures of the three components, the relative amounts (by weight) of each at any given point being indicated by its position within the triangle. Inside the triangle for example, all constituents within area 1 are in unsaturated, homogeneous, liquid solution. Within area 2, all are in the form of solid compounds. Points along the curved line from the H 2 0 corner to A, to B, to C, to D, to E, to F, to G represent the compositions of the various saturated solutions which may exist in equilibrium with the different solid phases. The triangle areas 4, 6, 8, 10, 12 and 14 represent three-phase fields. The areas between the threephase fields, denoted upon the diagram by the numbers 3,5, 7, 9, 11 and 13, are two-phase fields. Compositions represented by points in area 5-that is, the area bounded by the curve AB and straight lines from A and B to the point representing the compound 3Fe 20,,o4S0 3 09H 20-are mixtures of saturated solution and one solid compound, 3Fe 20 3 04S0,,o9H 20.1 Similarly, compositions represented by points in area la-that is, the area bounded by lines connecting D, the point representing 2Fe 20,,o5S0 3 017H 20 (copiapite) and the point representing Fe20303S030 7H 20 (kornelite) -are mixtures of the saturated solution D and the solid compounds corresponding to the minerals copiapite and kornelite; that is, mixtures of crystals of two solid compounds with one liquid solution (see fig. 3). Starting with the point Fe 2 0 , o3S0"o 7HP which represents crystals of korneIite, and adding water, some of the kornelite is decomposed; and one has, at first, a mixture of kornelite, copiapite, and solution D. By the time dilution reaches the point upon the diagram vertically beneath copiapite, a substantial part of the kornelite has dissolved, and a large number of copiapite crystals have formed, also more of the saturated solution D. On further dilution the mixture will reach area 'Posnjak and Merwin (1922, p. 1977) believed the compound 3Fe20304S0,o9H,O to be borgstromite, although they had seen only an artificial mineral of this composition. Borgstromite has now been discredited as a distinct mineral species (Moss, 1957), and was shown to be merely a mineral closely related to jarosite.

INTERPRETATION OF LEACHED OUTCROPS

52

~

~///;%0

AREA

40%'-',0,

AREA 3 -

.0

~~REA 2 ~~

4

y/

1/

AREA 6

Basic Salts

//~

l'l))/~//~:/~~

..

/2Fe 0 .5S0 .17H 0 (coplapitel / : .c'.c'~' Fe O'.3S0'.7H 0 (kornelitel

" , 'f / /

, .-Fe'OASO.9HO

A. 12

FIGURE 3.

Fe 0 .2S0 .5H 0

"(rho~bo~la5.)

Normal Salts

Fe0.4S0.3HO\l

n

/

,

'/).

.

(0\0 ACid Salts

a

The 50°C (l22°F) isothermal equilibrium diagram of the system, Fe,O,.SO,.H,O of Posnjak and Merwin. Adapted from Locke (1926, p. 39, with subsequent deletions and additions by George Tunell).

9, all of the kornelite having disappeared at this point. While traversing area 9 one has a mixture of copiapite crystals and saturated solution the composition of which is represented by a point on the saturation curve between D and C. On reaching the curve DC one has only saturated solution, all of the copiapite crystals having dissolved. On further dilution one has an unsaturated solution in area 1.

GOETHITE AND THE COMPOUND 3Fe20" ·4S03 .9H"0 The compound 3Fe"O".4S0".9H"O is closely related to jarosite (K"O.3Fe 2 0".4S0".6H 2 0) in both chemical composition and physical structure. From the diagram it will be observed that the precipitation of 3Fe 2 0". 4S0 e .9H 2 0 may occur, within area 5, over the entire dilution range from the 3Fe 2 0".4S0 3.9H2 0 locus of

composition 49.84 percent Fe 2 0 3 , 33.30 percent S03' 16.86 percent H 2 0, to a minimum concentration, at point A, of composition 1.44 percent Fe 2 0 3 , 2.30 percent SO"' 96.26 percent H 2 0. What is of greater interest, however, is that where dilution represents the sole variant, the composition of the total mixture moves along the line between the point 3Fep".4S0".9H"O and the H 2 0 apex (fig. 3). This line does not cut the saturation curve of the compound 3Fe"O".4S0".9H 2 0, but passes into area 4, and traverses that area. With increasing quantity of H 2 0 the proportion of saturated solution A and solid compound Fe"O".H 2 0 (goethite) increases, while more and more of the solid compound 3Fe 2 0".4S0 3 .9H 2 0 undergoes decomposition. When all of the solid 3Fe 2 0 3 • 4S0".9H"O has been consumed and more water added, the solid Fe 2 0,.H"O (goethite) co-exists in equilibrium with solution, the solution changing gradually in composition from A to H 20.

LIMONITE PRECIPITATION THROUGH DILUTION OF SOLUTIONS

Goethite will be the sole precipitate within area 3 (fig. 3), and it will continue to form until most of the Fe 20" has dropped out of solution. If dilution is now continued still further goethite should, theoretically, re-dissolve, but the reaction would be very slow, and equilibrium would not be attained in many cases even in geologic lengths of time.

OXIDATION PRODUCTS OF PYRITE AND CHALCOPYRITE When a small amount of pyrite contained in an inert gangue oxidizes in contact with air and water, the process may be represented by the line from the H 2 0 corner through the point in the diagram of figure 3, designated Fe 2 0 3 .4S0,.9H 2 0, along which the molecular ratio of SO" to Fe20:J is 4 to 1 (ratio by weight, 2 to 1), and which has been called the dilution line of solution containing the oxidized constituents of pyrite. As the pyrite begins to oxidize, the point representing the composition of the adjacent system of Fe20:J-SO::H 2 0 travels along the dilution line from the H 2 0 corner toward the point Fe 2 0".4S0".9H 2 0. At first a small amount of goethite will be precipitated, but when the point Z (98.80 percent H 2 0, fig. 4) is reached all the

goethite will be re-dissolved. Oxidation of more pyrite will result in the formation of a more concentrated solution without precipitating any solid until the saturation curve of the normal salt kornelite, (Fe 2 0,. 3S0:. 7H 2 0) is reached (fig. 3). Oxidation of still more pyrite in the same amount of water will carry the system across the two phase field, kornelite and solution into the three phase field, kornelite, rhombocIasc (Fe 2 0 3 .4S0".9H 2 0), solution. In the natural oxidation of disseminated copper ores the solutions rarely if ever become sufficiently concentrated to precipitate kornelite or rhombocIase; but in the natural oxidation of massive sulfide deposits under humid conditions, kornelite, rhombocIase and other salts have been formed. Under mine oxidation conditions such salts have been observed to form in abundance. If after a certain amount of pyrite has been oxidized there is an increase in the amount of water as a result of rainfall, the system will travel back down the dilution line toward the H 2 0 corner; and when the point Z is reached goethite will begin to precipitate. If the dilution should become sufficiently great so that the point X (fig. 4) were reached, almost all the iron in solution would be precipitated as goethite. Some of the goethite formed in veins of massive ore probably was produced in this manner. Most of the goethite formed in the

x &

&o

z

o

o

o

1.-

~O

53

1.-

~O

PIGURE 4. Enlarged corner of the 50°C (122°P) isothermal equilibrium diagram of the system Pe 2 03.S03.H20, showing the dilution line of solution containing the oxidized constituents of pyrite. Adapted from Locke (1926, p. 40) with subsequent additions by George Tunell and Roland Blanchard.

54

INTERPRETATION OF LEACHED OUTCROPS

oxidation of disseminated copper ores, however, appears to be the result of neutralization rather than the result of dilution. Still greater dilution would cause goethite to re-dissolve if equilibrium were maintained, but in such dilute solutions the rate of solution would be extremely slow and goethite might persist for geologic intervals of time. Chalcopyrite contains a higher ratio of metal to sulfur than does pyrite, and consequently the solution derived from its oxidation is less acid, and thus it yields limonite more readily. These examples emphasize the statement made in chapter 8, to the effect that chemical equations as usually set forth represent only approximations of the actual reactions.

THE TRANSITION MINERALS COPIAPITE AND COQUIMBITE Many geologists are familiar with the formation of golden-yellow copiapite in abundance as a transitional mineral between pyrite and limonite where seams of semi-massive or massive pyrite oxidize in a gangue of moderate neutralizing power. For example, in the Mount Isa, Queensland, silver-lead-zinc orebodies narrow replacement bands of pyrite and pyrite-pyrrhotite alternate with, and to an important extent are replaced by galena and sphalerite. The sulfide bands of all types alternate indiscriminately with barren shale seams of equal or greater thicknesses. Enough of the dolomitic shale bands of syngenetic origin are present so that, in conjunction with introduced dolomite veins and seamlets which cut the sulfide bodies, the ore bodies as mined contain 16 percent of neutralizing material as gangue carbonates. Those pyritic exposures which have existed for more than a year are densely coated with copiapite efflorescences. In the abandoned, poorly ventilated sections of the mine, where wall rock temperatures exceed 32°C (90°F)-and especially in the porous breccia zones close to portions in which oxidation is proceeding rapidly and wall rock temperatures may approach or exceed 66°C (l50°F)-coquimbite (Fe 2 0,.3S0". 9H 2 0) with its faint rose-pink to tan-yellow color is a common mineral, transitional between pyrite and copiapite. It was observed to be even more abundant in the R 62 sealed-off sulfide "fire" stope, when the latter was opened temporarily for inspection in 1934 somewhat more than a year after the stope had been sealed to check the fire. Depending upon the temperature, degree of ventilation, and humidity, the coquimbite at any given place may be visible from a few hours to several weeks or more before altering to copiapite; the copiapite, from several days to several years or more before altering to limonite. Where it is not visible there consequently is reason for thinking that in many instances acid sulfate hydrate may have existed in the

transitional state, even if not crystallizing out as a distinct mineral,2 Although various factors probably are involved in formation of coquimbite and copiapite at Mount Isa, Queensland, as above set forth, a major factor would seem to be the neutralizing effect of magnesium and calcium bicarbonate in ground water, which uses up some of the H 2 SO" derived from the oxidation of pyrite, and thereby moves the system into the copiapitecoquimbite range.

FORMATION OF JAROSITE IN THE PRESENCE OF POTASSIUM ION From the above considerations it will be seen that if in the oxidation of either pyrite or chalcopyrite, enough of the H 2 SO, is used up through neutralization or in other manner to bring the solutions into the 3Fe 2 n,.4S0".9H 2 0 range, and if a molecule of K 2 0 should become available and be substituted chemically for three of the water molecules, jarosite (K 2 0.3Fe 2 0". 4S0,.6HP), a more stable mineral, might readily form. It suggests that, provided K 2 0 were available, gangues of moderately strong or strong neutralizing power would favor the formation of jarosite. That is consistent with the not infrequent occurrence of jarosite as large, well-preserved crystals in the limestone gangues of semi-arid regions; though jarosite is by no means confined to gangues of semi-arid regions of such strong neutralizing power, nor is its amount necessarily in any sense proportional to either the strength or volume of such neutralizer in the country rock. Because of the ease with which, over a broad range in variation of the components Fe"O" and SO"' the compound 3Fe2 n 1.4S0".9H 2 0 may alter to goethite merely through dilution, it likewise becomes more understandable why in nature jarosite so frequently and readily alters to limonite."

SUMMARY One of the methods by which limonite is formed, is the precipitation of iron oxide and iron oxide hydrate from aqueous solutions by dilution. The isothermal equilibrium diagrams of Posnjak and Merwin aid in 2The author has seen rhomboclase, voltaite and roemerite in the "fire" section of the United Verde mine, Ariz., and he has seen rhomboclase, voltaite, roe me rite, and kornelite at the Copper Queen mine, Ariz., and Mount Isa mine, Queensland, in the fire country, with heavy pyrite, where moist conditions exist. Rhomboclase is white, gray or pale-yellow in color; voltaite is oil-green to brown in color; roemerite is chestnut-brown in color; kornelite has a delicate violet color, with silky tufts or crusts with radial-fibrous structure; but they alter generally to faint rose-pink or tan-yellow, powdery coquimbite or copiapite, and eventually to limonite with exposure to dry air. "Hematite probably has a stability field in the system Fe,O,CuO-K,O-S03-H,O at outdoor temperatures, but definitely does not have a stability field in the system Fe,O,,-CuO-SO:-H,O in this temperature range, according to George Tunell and E. W. Posnjak (unpublished work).

LIMONITE PRECIPITATION THROUGH DILUTION OF SOLUTIONS

understanding the reactions that take place in dilution processes, and indicate the relative concentrations of Fe"O" SO:" and H"O, at which goethite and the basic, normal and acid sulfates are formed.

55

The dilution process is important only when it takes place in quartz rocks or other types of inert gangue. In arid and semi-arid regions it is especially important after heavy rains.

Chapter 10 LIMONITE PRECIPITATION RELATED TO OXIDATION OF IRON-FREE SULFIDES Before limonite can form in any case where the oxidation of a sulfide or other mineral is involved, iron must be present in the solutions. It follows that sulfides which do not carry iron, such as chalcocite, covellite, molybdenite, sphalerite, or galena; cannot yield limonite unless iron from an outside source is introduced during their oxidation. This chapter describes the reactions that can be expected to take place during the oxidation of sphalerite, galena, and molybdenite bodies in the absence of significant amounts of pyrite; and of sphalerite and galena bodies in situations in which iron-yielding pyrite is present. It will be remembered that pyrite need not be present in order to cause complete solution of those sulfides, because they contain just enough sulfur to create sulfuric acid in amounts adequate for their complete dissolution. Pyrite is only considered here because it commonly furnishes the iron of limonite in leached outcrops. Limonite may be absent from a leached outcrop in two common situations: 1) it may be absent because iron was not present in either the oxidizing sulfide minerals or the reacting solutions, or 2) it may be absent because of the total leaching of pyrite under certain conditions, as explained in chapter 8. Many examples have been observed in the field in which chalcocite, sphalerite (containing no iron), galena and other sulfides have been leached without leaving behind them a record of either indigenous or transported limonite. Equations (11) and (12) of chapter 8 show that for chalcocite such is the natural outcome of oxidation, if no iron is present. Here need be noted only that limonite-free outcrops may have diverse origins, and that they do not necessarily point to the former presence of pyrite. But though the foregoing undeniably is true, and the observer must be on his guard in interpreting such outcrops, conditions under which limonite outcrops are derived entirely from minerals other than pyrite are rare. Much field observation has shown that sulfides other than pyrite generally do yield limonitic derivatives in one form or another, though usually in small amount; either as indigenous products, or as limonites transported only a fraction of a millimeter from the cavity. Naturally, where the cellular pseudomorphs alone are involved, and the boxwork penetrates along cleavage or fracture planes, iron within the decomposing mineral is not necessary; for, along with the required silica, it may be extracted from circulating ground water in adequate amount, in most instances, to meet the

requirements for the formation of limonitic jasper, and in some instances it may have been derived from sources a substantial distance from the oxidizing sulfide body, such as ferromagnesian rocks, chlorite, epidote, etc.

OXIDATION OF SPHALERITE AND GALENA BY AIR-WATER PROCESSES IN AN INERT ENVIRONMENT The chemistry of oxidation of sphalerite and galena in an inert environment is simpler than that involved in the oxidation of copper or copper-iron sulfides. Both sphalerite and galena contain enough sulfur to cause complete conversion of the zinc and the lead to their sulfates; and since neither, in its pure state, contains iron, and neither requires attack by iron-bearing solutions for its oxidation, it follows that no limonite need result. Equations (15) and (16) represent the simplest reactions in the oxidation of sphalerite (ZnS) and galena (PbS). According to these equations, no reactive agent except oxygen of the air is needed to oxidize either of the sulfides to its sulfate equivalent, though other reagents may catalyze the reactions. and

If water circulates through the area, any sulfate present may in time be removed. At ordinary temperatures and pressures, zinc sulfate is soluble to the extent of 430 grams per liter of water, and a mole could be removed when 2/5 liter of water had passed over the containing area. Lead sulfate is soluble to the extent of 0.042 gram per liter of water, and a mole of it could be removed when 7,214 liters of water 1 had passed over the containing area. Being nearly insoluble, the lead sulfate would remain in the cropping much longer than the zinc sulfate. Because of the above facts concerning their solubilities, it would be expected that even in a wholly inert 'The paper of Boswell and Blanchard (1927, pp. 444-445) indicated an incorrect quantity of water required. The same source also gave erroneous solubilities for the zinc and lead carbonates.

58

INTERPRETATION OF LEACHED OUTCROPS

environment, oxidation of both sphalerite and galena in

any time necessarily involved in the oxidation and

the absence of pyrite would procede by air-water or air oxidation; that most or all the zinc sulfate would be removed in solution and leave no trace behind, provided that waters had circulated through the oxidized area. Lead sulfate, under the same conditions, would be removed very slowly, and usually would remain in appreciable amounts in the cropping, especially if the oxidation of the galena had been recent. In neither case would limonite eventually be left in the cropping to mark the place of the former sulfide. With incoming CO 2 in ground water solution, both the zinc and lead sulfate would in time be converted into carbonates in an inert environment. At ordinary temperatures and pressures zinc carbonate is soluble in water to the extent of 0.01 gram per liter, lead carbonate to the extent of 0.001 gram per liter of water. If the water is saturated with CO 2 , the solubilities in both cases are increased about five fold. Hence it is evident that in an inert environment traversed only by water that contains CO 2 in solution, both the zinc and lead sulfates would in time be converted to carbonates, that the waters might readily dissolve and carry off the zinc carbonate in solution, that they would less readily dissolve and carry off the lead carbonates in solution, but, given time, might remove it also, but that in neither case would limonite be left to mark the place of the former sulfide in an inert environment. 2 To summarize, in an inert environment traversed only by surface waters that carried no iron-bearing compounds and no other reactive agents except CO z , sphalerite would be expected to oxidize and be removed without leaving behind evidence in the form of supergene minerals. Galena would be expected to oxidize more slowly, and ordinarily to leave behind much of its lead in the cropping either as sulfate or carbonate, unless leaching had been vigorous and had been in progress a very much longer time than in the case of sphalerite. But in neither case would limonite be left in the place of the sulfide or its oxidation products. These conclusions arc in accord with conditions actually found in the field in an inert environment, and explain why galena usually is not wholly leached from an inert environment while zinc minerals usually are. It likewise explains why limonite usually is absent in such croppings, since no iron-bearing solutions are at

removal of zinc and lead minerals. A further condition to note is that galena first oxidizes to sulfate; that this nearly insoluble mineral ordinarily coats and encloses the oxidizing galena from which it was derived; and that this nearly insoluble coating tends to keep further oxygen from reaching the galena. This explains why so frequently unoxidized galena residuals are found in a lead ore cropping that has been undergoing oxidation for a long time.

OXIDATION OF SPHALERITE AND GALENA BY AIR-WATER PROCESSES IN THE PRESENCE OF PYRITE When sphalerite or galena is found mixed with pyrite in an orebody, oxidation conditions become more complex, especially when reactive (feldspar, shale, or limestone) rather than inert gangues are present. In those instances sphalerite usually precedes galena and pyrite in oxidation. Where pyrite is admixed with sphalerite or galena inter-action of the respective oxidation solutions occurs. The ferric sulfate (Fe 2 (SO+L) derived from oxidizing pyrite probably would react with sphalerite or galena as shown in equations (17) and (18). 2Fe 2 (SO.) 3+2ZnS+302= 2ZnS0 4+4FeS0 4+2HzSO+,

(17)

and 2Fe 2(S04) 3+2PbS+302= 2PbS0 4+4FeSO.+2H 2 SO..

(18)

Again, even though pyrite is admixed with sphalerite or galena, either of the latter sulfides may oxidize to the sulfate state by ordinary air-water oxidation. The lead sulfate and the solution containing zinc sulfate, are normally converted to carbonates if CO 2 is present in circulating ground water. Excess ferrous or ferric sulfate left over from the oxidation of pyrite might then react with resulting zinc or lead carbonates, as indicated in equations (19) and (20). 3ZnCO,+Fe 2 (SO+) 3=3ZnSO.+Fe 2 0,,+3CO z ,

(19)

and "Iron-free sphalerite is not plentiful. In early stages of work on leached outcrop interpretation in the southwestern United States, P. F. Boswell and the author noted an occurrence of iron-free sphalerite in a quartz pipe that extended at least 800 feet below the surface. The sphalerite in the upper hundred feet was oxidized. No iron was found in the quartz, but some iron, presumably derived from pyrite, was concentrated in limestone around the circumference of the pipe. Sphalerite at greater depths was also iron free, and no ferromagnesian minerals were seen. Since that time, iron-free sphalerites have been seen and analyzed at several places in the southwestern United States, in Mexico. and some places in Australia, including baritized and kaolinized outcrops. Iron-free sphalerite usually is cryptocrystalline. and is seen in limestone, dolomite, or other carbonate material.

3PbCO,+FezCSO+) ,,=3PbSO++Fe2 0,,+3C0 2 •

(20)

Smithsonite (ZnCOJ is of erratic distribution above the water table in ore bodies in which feldspar or shale comprise the gangues, and it would be partly leached to limonitic jasper in the semi-arid regions. Cerussite (PbCOJ, although resistent for a long time, would in large part eventually be converted to limonitic jasper, as proved at the Mount Stewart and the C. S. A. mines in Australia (ch. 13), and other lesser occurrences in semi-arid regions. Where a strong neutralizer existed the processes would become even more complicated. Excellent

LIMONITE PRECIPITATION BY OXIDATION OF IRON-FREE SULFIDES

examples of limonitic jasper have been observed in Australia, notably at the Watson and Blacksnake lodes at Lawn Hill, Queensland, where massive sphalerite occurs in shale gangue. At both locations the cellular boxwork patterned after sphalerite inside of the massive sphalerite lenses may contain up to 65 to 70 percent Si0 2 and less than 15 to 20 percent Fe 2 0, (see ch. 2, table 1, no. 16), whereas 2 or 3 feet distant either laterally or vertically from the edges of the sphaleritepyrite areas, the ferric content may exceed that of the silica, with intervening material exhibiting gradual transition from one composition to the other. Concerning sphalerite, there seems little doubt that much of the iron that went into the composition of the boxwork inside of the cellular mass may have been derived entirely or mostly from a source other than the pyrite; but as decomposition proceeded, the oxidation of the sphalerite-pyrite areas slowly followed that of the sphalerite, the iron content of the ground water was increased by iron of pyrite derivation, with marked increase of Fe 20 3 content of the boxwork then being formed. This conclusion is corroborated by the post-mine leaching products seen at the 2,000-foot level of the North mine at Broken Hill, New South Wales (Garretty and Blanchard, 1942), where sillimanite gneiss, with carbonate distributed sparsely through it, is the country rock. Cellular boxwork, derived from sphalerite that is noticeably free from pyrite-pyrrhotite admixture, similarly yields high silica-low ferric oxide compositions; and nearly all of the boxwork there is characteristically low in Fe 20 3 content, in an orebody whose average pyrite-pyrrhotite content does not exceed 3 percent as against 20 percent sphalerite and an equal amount of galena (see figs. 66-69, 73, 74, pIs. 15, 16, ch.27). Most sphalerite, of course, contains iron, as the Watson and Blacksnake lodes show. The Broken Hill sphalerite contains 15 percent iron [making the mineral, strictly speaking, marmatite (10 percent or more Fe)]. Release of this iron during oxidation unquestionably constitutes one source of ferric oxide necessary for the production of limonite, even though a portion of the iron in the sphalerite may possibly escape during the decomposition of the sphalerite. Galena likewise undergoes incipient oxidation in advance of the pyrite, and cellular pseudomorphs of siliceous limonitic jasper not infrequently form along its cleavage planes. Much of the cubic box work of galena is formed in such a manner (see figs. 55-58, pI. 13, ch. 26). In the case of galena, however, cellular pseudomorphs never are formed as extensively as with sphalerite. Galena oxidizes so slowly that in the average life span of a human being very little of it undergoes postmine oxidation, except in hot, humid regions. Moreover, lead sulfate is very insoluble in ground water solutions. If the ratio of galena to pyrite in shale or feldspar gangues is 20 : ], there is not much change. Pyrite, however, is known to hasten the solution of the

59

oxidized lead products. With mixtures of 1 mole of galena and 2, 5, or 10 moles of pyrite, iron goes into solution moderately rapidly; with even more pyrite, the iron goes into solution still more rapidly. With 1 mole of galena and 5 moles of pyrite, the pyrite crusts contained in the oxidation products almost completely mask the galena oxidation products, although some of the cerussite "relief" product from galena shows intermittently. But it is essentially an exotic limonite, with smeary crusts formed from iron derived from the pyrite. Decomposition of mixtures of 2 moles of galena and 3 moles of pyrite, or of 1 mole of galena and 1 mole of pyrite, yields a shapeless indigenous aggregate of minute cerussite globules and grains (see fig. 60, ch. 26). The cerussite in the oxidation product is very much in evidence. Whether in this form or as cellular structure, cerussite-pyrite mixtures often are attacked subsequently by ferric sulfate or other acid, and the lead taken into solution and exported. In the process the cerussite commonly undergoes an almost grain-for-grain replacement by limonite precipitated from the ferric sulfate solution. Thus, although the galena itself may be leached without subsequent formation of cellular boxwork or other limonitic types, the cerussite aggregate often leaves behind it a characteristic limonite replica which is readily identified by the experienced observer, and which serves equally well for purpose of the original sulfide identification. If pyrite occurs with sphalerite or galena in shale, feldspar, or limestone gangues, air-water processes alone will cause precipitation of limonite. When the mixture of galena and pyrite is massive or disseminated, the siliceous limonite will be light as a rule. Instead, cerussite (PbCO,,) will be formed, and this in turn may give rise to indigenous limonite, unless pyrite is moderately heavy. Although, because of its widespread occurrence in nature and its general association with most other sulfides, pyrite must be regarded as the principal source of iron for limonite production, it is not the only source. The role played by iron occurring in other minerals has been already emphasized. Iron silicates in the gangue, especially the ferro-magnesian minerals which are so vulnerable to attack by the sulfuric acid released in the oxidation of pyrite and several other sulfides, constitute an important source of iron, as do to a much lesser degree chlorite, siderite, ferruginous dolomite, gamet, and even to some extent hematite and magnetite; but principally in the reactive gangues, (shale, feldspar, limestone), not in the inert gangues.

OXIDATION OF MOLYBDENITE BY AIR-WATER PROCESSES IN THE PRESENCE OF PYRITE Molybdenite (MoSJ is not markedly affected by sulfuric acid, nor does hydrochloric acid (HCl) affect it. Only nitric acid (HNOJ, or aqua regia (one part of

60

INTERPRETATION OF LEACHED OUTCROPS

nitric acid to three parts of hydrochloric acid) markedly affects molybdenite. Its slow oxidation, its lack of mobility, and the fact that MoS" cannot easily be taken into solution to form supergene enrichment zones, makes molybdenite, though soft, one of the most difficult minerals to oxidize. Therefore oxidized relics of molybdenite are seldom formed. In the oxidized zone of the Climax, Colo., deposit (the largest known deposit of molybdenite ore), only 10 to 20 percent of the molybdenite is oxidized, and generally only to a depth of a few feet; though in the fractured zones the oxidation characteristically extends to a depth of 100 feet or more. In the oxidized zone, molybdite (MoOJ, an unstable mineral, usually turns to ferrimolybdite (Fe"(MoO.L.8H e O, with variable water). The ferrimolybdite is a fine grained canaryyellow to straw-yellow earthy to fibrous mineral, though reddish colors occur in places. In the oxidized zone jarosite is common as ocher-yellow incrustations along fractures near the surface. This deposit contains only 0.6 to 0.7 percent MoS e, while in the ore zone the pyrite content is estimated to average 2 percent over large areas. The deposit is in granite, with a core of quartz, and is young topographically (elevation 11 ,000 to 13,600 feet), with 17 inches precipitation per year. Glaciation has planed off the outcrop. The Hall molybdenite property, 25 miles north of Tonopah, Nev., is in a desert region where the oxidation is usually thorough (5.5 inches precipitation per year; 5,900 feet in elevation at the shaft). No glaciation is evident, and 30 to 40 percent of the molybdenite in the ore is still present in the oxide zone as sulfide. There is very little supergene enrichment passing from the oxide to the sulfide zone, although pyrite is oxidized to a depth of 200 feet or more. About four or more parts of pyrite to one part of molybdenite (0.34 percent of MoSJ is the general rule (Michell, 1945) in this deposit. 3 Oxidation products derived from molybdenite have been observed by the excessive amount of limonite derived from pyrite, and the outcrop is characterized by abundant ochreous limonite, which make the small amount of ferrimolybdite present difficult to observe. The deposit lies in schist, along the southern margin of an alaskite intrusion, but it is in mature topography.' 'For additional references to the composition of molybdenite ores see also Schaller (1907); Hess (1924, pp. 1-34); Doerner (1926, pp. 1-13); Stillwell (1943); and Jacobson (1951, pp. 651-694 ).

'Wulfenite (PbMoO,), with its orange-yellow to wax-yellow color, is the most common oxidation product of molybdenite in lead ores. In Europe, the northwestern part of Africa, and in Broken Hill, Australia, it forms occasionally; but in the southwestern United States and Mexico it is common in some lead districts.

In a few disseminated deposits the grains of molybdenite are foliated, with comparatively rounded and smooth forms. The oxidation products of the molybdenite are maroon to orange to tan in color, with only minor ferrimolybdite. The limonite grains are greasy and granular. Occasionally they are up to a quarter inch in diameter, but usually they are microscopic (see fig. 77, ch. 28). They break up because they are so thin (cell flakes 0.005 to 0.03 mm thick), but in protected places they may be seen. In the Santo Nino deposit near Nogales, Ariz., excellent examples in feldspar occur. At Mineral Park, Ariz., a few examples occur in monzonite. In the Hodgkinson district (shale), and the Bamford district (massive porphyritic lava and tuff), Queensland, they occur also. But they are not common. Molybdenite does not readily oxidize, as stated, because only nitric acid or aqua regia markedly affect it, not ferric sulfate. Usually feldspar or shale is present in the gangue, and this has the effect of moderately slow neutralization. In these circumstances pyrite should oxidize and does, but much of the molybdenite persists in the outcrops of most of the deposits. Molybdenite is unique among the common sulfides.

SUMMARY 1. Unlike the copper-iron sulfides, sphalerite (containing no iron) and galena may oxidize and dissolve without generation of or attack by iron-bearing solutions, and commonly do so. In such cases they leave no limonite. 2. If limonite remains, it signifies that galena and iron-free varieties of sphalerite carried admixed pyrite, or other iron-bearing minerals, or that iron was introduced by ground water solutions. 3. In an inert environment, sphalerite and galena usually form little limonite even if admixed pyrite is present. However galena, which is nearly insoluble, may leave lead sulfate (secondary) and lead carbonate (tertiary), provided not much pyrite is admixed. With iron-free sphalerite no limonite is left. 4. In environments of moderately slow to rapid neutralization (shale, feldspar, limestone), in which pyrite or other iron-bearing minerals are present, sphalerite and galena in addition to copper or copperiron sulfides, usually leave distinctive limonite products. 5. Iron silicates, shale, feldspar-rich, or limestone gangues when present play a part in the production of limonitic jasper. 6. Molybdenite is affected by nitric acid, but not markedly by sulfuric acid or by ferric sulfate. Molybdenite therefore is oxidized with difficulty under natural conditions.

Chapter 11 LEACHING AND LIMONITE PRECIPITATION IN THE ZONE OF SATURATION REACTION OF FERRIC SULFATE WITH PYRITE AND CHALCOPYRITE Ferric sulfate, existing within or percolating downward through the zone of aeration, is competent to oxidize sulfides, independent of the air-water processes discussed in chapter 8. In this connection only its effects upon the two resistant sulfides, pyrite and chalcopyrite, will be considered. The reactions are expressed by the equations FeS 2 + 7Fe 2 (S04L+8Hp=15FeS0 4+8H 2 SO.,

(21)

and CuS.FeS+8Fe 2 (SO.)3+ 8H 2 0 = CuS0 4 +17FeS0 4 +8H 2 S0 4 •

(22)

Such oxidation takes places both above the water table and below it. Since dissolved oxygen is absent from the ground water, however, it is evident from the equations that, by this process, 14 moles of FeS 2 must oxidize above the zone of saturation in the zone of aeration to furnish sufficient ferric sulfate to oxidize 1 mole of pyrite; and 16 moles of FeS 2 must oxidize somewhere else to furnish sufficient ferric sulfate to oxidize 1 mole of chalcopyrite (see description of the Kyshtim deposit, ch. 7). In that connection it is well to bear in mind that the water in the zone of saturation does not exist as an underground lake; for the most part it exists only in fillings along fractures, often so minute that the water is present as little more than films along tiny cracks, or as capillary fillings. Except possibly along the larger fractures, and in many instances even there, the water thus circulates very slowly; and except for slight oscillations through a narrow vertical range resulting from changes in temperature or barometric pressure at the earth's surface, often remains virtually stationary for weeks or months at a time, especially in the semi-arid or arid regions. A given volume of acid solution percolating down through the zone of aeration thus may become marooned for an extended period along narrow fractures, and within a small area, so that its concentration does not become rapidly reduced. Since by the nature of hypogene deposition, sulfides tend to occur disproportionately along fractures, the conditions thus are favorable to important and often concentrated attack upon the sulfides by the downward-percolating ferric sulfate and sulfuric acid.

Even where larger openings exist, as in solution cavities within the more soluble rocks (limestone in particular), acid solutions reaching the water table do not spread out like oil upon water. The specific gravity of such solutions is greater than that of ordinary ground water, and drops of such solutions tend strongly to sink downward at first almost as entities, attaining appreciable depth before being wholly dissipated. Depending upon the neutralizing power of the rock and upon the degree of circulation beneath the water table, acid solutions of ferric sulfate thus frequently are able to effect oxidation and/or leaching of sulfides to appreciable depths within the zone of saturation. Pyrrhotite and iron-rich sphalerite are particularly susceptible to attack by acid solutions, and the ferrous sulfate derived from them often yields no limonite for 300 to 400 feet below the water table.

OXIDATION BY ACID SOLUTIONS OF CUPRIC SULFATE DURING SUPERGENE ENRICHMENT The oxidation of sulfides by acid solutions in the process of supergene enrichment is illustrated by the attack of cupric sulfate (CuSO,) solution upon sphalerite (a process which takes place when covellite replaces sphalerite in the secondary enrichment process), as shown by equation (23). ZnS+CuS0 4 =CuS+ZnS0 4 •

(23)

In this case cupric sulfate plays the role of an oxidizing agent for the sulfur of sphalerite. Since no atmospheric oxygen is involved, the reaction may proceed as readily below the water table as within the zone of aeration. The attack of cupric sulfate upon galena yields coveJlite, as shown in equation (24). PbS+CuS0 4 =CuS+PbSO..

(24)

In this instance both products are precipitated from solution. In the phenomena associated with supergene sulfide enrichment it is to be noted that chalcocite enrichment proceeds more rapidly in the presence of even a small amount of cuprous sulfate (CU 2 S0 4 ) than in the presence of cupric sulfate alone, as was stated in the discussion of chalcocite and covellite reactions (ch. 8). Further attack of cupric sulfate upon the covellite alters

62

INTERPRETATION OF LEACHED OUTCROPS

it to chalcocite, (Zies and others, 1916, p. 429) as shown by equation (25) and (26). CuS+ 7CuSO,+4H:!O=4Cu"SO,+4H"SO.,

(25)

and The attack of cupric sulfate upon chalcopyrite similarly may yield covellite initially, the reaction probably proceeding as shown by equation (27). CuS.FeS+CuS0 4 =2CuS+FeS0 4 •

7CuS+4FeSO.+4H2 S0 4 ,

(29)

and 5FeS 2 +14CuS0 4 +12H 2 0=

(27)

rn the attack upon bornite several reactions are possible, one of which is shown by equation (28). 2Cu"S.CuS.FeS+CuS0 4 =2Cu"S+2CuS+FeSO

Cupric sulphate also attacks pyrite as shown in equations (29) and (30), with the end results, as set forth by Zies and others, being those of equation (30). They consider it probable that, in this case too, covellite may be formed as an intermediate product, as in equation (29). 4FeS 2 + 7CuS0 4 +4H 2 0=

j'

(28)

Equations (23) to (27) show that the reaction of cupric sulfate solution with sphalerite, galena, and chalcopyrite causes the formation of covellite, which may subsequently alter to chalcocite if the reaction continues. The attack upon bornite (equation 28), however, yields much chalcocite directly. Whatever the intermediate reactions may be, therefore, all of the other sulfides named will ultimately yield chalcocite if enough reactants are present to allow the reactions to go to completion. Because no atmospheric oxygen is required, these reactions may proceed below as well as above the water table. In the deeper supergene bornitechalcocite bodies, bornite is enriched far below the depth at which other sulfides are affected (Graton and others, mimeographed summary of oral presentation to New York meeting of AIME, Feb., 1924). Admittedly, much oxidation and leaching that is effected by cupric sulfate occurs within the zone of aeration, because such sulfate, percolating downward through the zone and encountering any of the five sulfides named, will attack and replace them wherever met with, and chalcocite usually will be formed. Likewise, as no iron is involved in the case of sphalerite (iron-free variety), and galena, the reactions of equations (23) to (26) illustrate one of the instances, discussed in chapter 10, in which the non-ferrous sulfides may leach without leaving a limonitic record of their former presence. And because the iron derived from decomposition of chalcopyrite and bornite is in the form of ferrous sulfate, neither can limonite be precipitated in cases involving those minerals, unless the ferrous sulfate is oxidized by oxygen of the air. This, of course, applies irrespective of whether the reactions take place within the zone of saturation or the zone of aeration. Under conditions as strongly acid as prevail when supergene copper sulfide replaces any of the four sulfides, sphalerite, galena, chalcopyrite, or bornite, even the cellular pseudomorphs would have little chance of forming. Pyrrhotite similarly yields to attack by cupric sulfate, but Zies and others (19 I 6, p. 454-462) obtained inconsistent results with it. They found it underwent replacement first by chalcopyrite, and probably by bornite. These later altered to covellite and/or chalcocite.

7Cu 2 S+5FeSO.+12H2 S0 4 •

(30)

In the zone below the water table, pyrite is much less readily enriched by cupric sulfate and cuprous sulfate than covellite, bornite,' chalcopyrite, sphalerite, pyrrhotite, and galena.

PRECIPITATION OR ABSENCE OF PRECIPITATION OF OXIDIZED IRON MINERALS DURING REACTIONS IN THE ZONE OF SATURATION It should be borne in mind in this connection that insofar as equations (23) to (30) are concerned, reaction in the case of each leached sulfide is limited to the extent of that sulfide's replacement by chalcocite or other supergene copper sulfide. Galena, though rapidly decomposable through attack by either cupric sulfate or sulfuric acid, often becomes in part immunized through deposition upon its surface of a coating of the highly insoluble sulfate, anglesite, as was stated in chapter 10. Whatever may be the outcome with respect to galena, sulfuric acid readily attacks iron-free sphalerite and pyrrhotite with no compensating replacement by chalcocite or other mineral and, with high acidity prevailing, no deposition of limonite can occur. If the reactions take place within the zone of aeration, or if the area involved later enters that zone through recession of the water table, the chalcocite itself becomes subject to leaching by air-water processes. Under certain conditions, as shown in equations (11) and (12) and discussed more fully in chapter 8, chalcocite may be leached within that zone without leaving behind it a limonitic record. Cupric sulfate and sulfuric acid thus are competent to effect leaching and/or oxidation, within the zone of saturation, of anyone or more of at least six common sulfides (exclusive of galena) in a manner that leaves behind no limonitic record of any kind. Furthermore, if sufficient acidity is maintained within the zone of aeration locally in unfractured, wet places (although the oxidized zone in general is porous and relatively dry), 'See Zies and others (1916, p. 475-486), and Locke (1926, p. 96). Bornite follows chalcopyrite in its precipitating power; but bornite follows galena in the volume of the mineral that is altered.

LEACHING AND PRECIPITATION IN ZONE OF SATURATION

they may effect such oxidation and leaching to some extent above the water table. Areas leached in this manner constitute special problems in interpretation which are dealt with more fully in subsequent chapters. Because pyrrhotite is highly vulnerable to attack by sulfuric acid and often strongly vulnerable to attack by cupric sulfate also, the almost total resistance to attack of chalcopyrite (CuS.FeS) by sulfuric acid is not wholly clear. Throughout all reactions involving decomposition of pyrite and chalcopyrite, however, the tendency is for the over-all acidity of the replacing solutions to be increased, so that the dissolved iron tends to be exported without precipitation of limonite. The best evidence for this lies in the fact that limonite is absent in unoxidized portions of supergene disseminated or massive chalcocite deposits, as was illustrated by the occurrence at Mount Oxide (see Appendix C). From these various considerations it is clear that acid leaching, under any conditions, tends to proceed without leaving behind it a limonitic record. In chapter 8, however, it was stated that, independent of the hydrolysis of ferric sulfate, any substance which consumes the free sulfuric acid may serve as a precipitant of the dissolved iron as limonite; and that strong gangue neutralizer such as limestone, or its solution products, is especially effective for the purpose. Neutralization of the acid by limestone takes place below the water table, within the zone of saturation, just as it does above it within the zone of aeration. Thus, in a limestone gangue, or wherever strong neutralizer becomes available, limonite may be precipitated readily from iron-bearing solutions both above and below the water table. As long as part of the iron remains in the ferrous condition, reaction with limestone will precipitate supergene siderite. When exposed to weathering such siderite eventually alters to one form or other of the ferric oxide minerals, and when thoroughly weathered it consequently may become indistinguishable from them. Furthermore, all extensive supergene (but not hypogene) siderite occurrences observed in the leached outcrop investigation were formed beneath or adjoining the body of the leached parent; they never lay above it. Thus, even if preserved and exposed at the surface, the supergene siderite masses presumably would point to an orebody (or whatever sulfide body had constituted the parent) already removed, in large part, by erosion. Supergene siderite masses most frequently are encountered near the base of oxidation in mine workings, in some instances pointing upward to leaching in a section of ground that may not have been prospected.

PRECIPITATION OF SIDERITE The supergene siderite of principal interest here is that which forms conspicuous, usually coarsely cellular, boxworks that persist laterally more or less continuously across many feet, either co-extensive with or beneath the ore or other sulfide body from which it originated.

63

It should be understood in this connection that the supergene siderite here discussed does not include the sparse, often sub-microscopic particles referred to in chapter 5, which form in minute amounts in association with most ferric oxide limonite as a consequence of ferrous ions not becoming wholly oxidized. Such siderite is usually indistinguishable to the eye and is too inconsequential in volume to have significancc in outcrop interpretation. Another type of supergene siderite, discussed in chapter 2, forms a (usually minor) part of the cellular pseudomorphs composed in part or in whole of a complex intermixture of various supergene gangue-carbonate minerals. This type of supergene siderite is not usually identifiable among the other intergrown ganguecarbonate associated minerals except under high magnification, and furthermore it normally overlies the ore instead of underlying it. Extensive development of siderite boxwork has been observed only where solutions derived from the oxidation of sulfides rich in pyrite have penetrated adjoining or underlying limestone. The occurrences in the Gardner-Lowell area of the Copper Queen mine at Bisbee, Ariz., constitute a representative and conspicuous example, as discussed partly in chapter 2. In this ore the horizon of maximum siderite boxwork development lies slightly below the pre-mine oxidation base (1550foot level), suggesting that the boxwork formed directly below the water table. The boxwork is associated with a partly oxidized body of semi-massive to massive pyrite which contains chalcopyrite and irregular chalcocite enrichments of minable grade. It fringes the base of the sulfide body and penetrates beneath it into the limestone to a depth of possibly 50 feet, with its most extensive development beneath rather than flanking the sulfide body. It extends persistently over an area of one hundred feet square or more, occupying in a patchy manner possibly 60 percent of the area, and it rarely is wholly absent anywhere over distances as much as 3 to 5 feet (Trischka and others, 1929). In most occurrences supergene siderite exists as coarsely cellular masses, which may be observed in all stages of formation from initial isolated webs or seamlets penetrating along major joint or fracture planes in the limestone, to uninterrupted boxwork skeletons, 10 to 20 feet across, from which the limestone residuals have been wholly leached. The boxworks thus are pseudomorphs formed in the same manner as are the pseudomorphs of limonitic jasper in oxidizing sulfides; but in this case they are pseudomorphic after the joint or fracture planes of the limestone, and cell diameters usually are measurable in inches rather than in millimeters or centimeters. Few cells measure less than onefourth of an inch, and many measure as much as 2 to 4 inches across. Cell wall thicknesses normally measure one-fiftieth to one-eighth of an inch. Supergene siderite does not crystallize out with the smooth rhombohedral surfaces that characterize most hypogene siderite. It clothes the initial web penetration with successive minutely nodular, semi-resinous coatings

64

INTERPRETATION OF LEACHED OUTCROPS

or layers as the adjoining limestone goes into solution. Thus it does not constitute grain-for-grain replacement of the invaded rock. A cell wall one-fiftieth of an inch thick may contain twenty or more layers; each, after the first, superimposed upon the one beneath. The color ranges from maple to turbid brown. Such supergene siderite clearly was formed through reaction of ferrous sulfate with the limestone, the reaction being represented by the equation FeS04+CaCO;1=FeCO;l+CaS04. (31) Most of the calcium sulfate is exported in solution; but gypsum, often in beautiful selenite crystals (CaSO." 2H 2 0), has been observed as a residue. Additional, less extensive development of boxwork exists beneath other oxidizing massive sulfide bodies in the Gardner-Lowell area, and similar occurrences have been observed at Cananea, Sonora, Mexico, and elsewhere. The zinc carbonate boxwork at Leadville, Colo., has a precisely similar origin except that there the solutions were derived originally from oxidizing zinc sulfide instead of from copper-iron sulfides. Some of the product contained zinc that was mined as ore. Supergene siderite boxwork in quantity probably always is restricted to special environments such as those described above. (See figs. 85, 86, ch. 33).

SUMMARY 1. Sulfides present in the saturation zone may be attacked and dissolved by sulfuric acid and ferric sulfate. 2. Sulfides in the saturation zone are also attacked by cupric sulfate solutions. Pyrite is most resistant in cupric sulfate solution, and is on the whole feebly attacked. Galena, pyrrhotite, sphalerite, chalcopyrite, bornite and covellite are strongly attacked, and are readily replaced by chalcocite. Molybdenite is insoluble in sulfuric acid, hydrochloric acid, or ferric sulfate; and cupric sulfate does not attack it directly. 3. Any substance that robs the solution of its free sulfuric acid serves as a precipitant of the dissolved iron as limonite, for example, limestone or its solution products, even though they occur in the zone of saturation. The matter is discussed in chapter 10. 4. As long as part of the dissolved iron remains in the ferrous condition, reaction of the solution with limestone may precipitate supergene siderite. In field interpretation siderite is regarded as one of the "limonites." Such siderite clearly was formed through reaction of ferrous sulfate with limestone or its solution products. Usually supergene siderite boxwork in quantity is restricted to such an environment in the zone of saturation.

Chapter 12 LIMONITE PRECIPITATION BY REACTION WITH NEUTRALIZING GANGUES Gangues vary greatly in their neutralizing power. It has been found, however, that they may be grouped into three broad classes: I) Gangues of practically no neutralizing power, such as quartz, barite, or highly kaolinized or sericitized rock (inert gangues). 2) Gangues of moderate neutralizing power, best represented by the feldspar-rich rocks such as granite, monzonite, etc., and by normal shale. 3) Gangues of strong neutralizing power, such as the carbonate rocks, of which limestone, dolomite, and limy shale are the most common and outstanding examples. The purpose of this chapter is to explain the nature of the reactions of moderately strongly neutralizing gangues and strongly neutralizing gangues with ferric sulfate solutions, and to describe some of the more common types of products so that they may be recognized when encountered in the field. To simplify the understanding of limonite precipitation in gangues of different neutralizing powers the only sulfide whose oxidation will be considered in detail in this chapter is pyrite. The limitation has been imposed purposely because: I) thc solutions formed by oxidation of pyrite are more strongly acid than are those derivcd by thc oxidation of the other sulfides, their reaction with gangue neutralizer is the most vigorous, and the effects easiest to obscrve; and 2) uniformity of acidity is assured in the oxidation solutions, so that the only variable involved is that of the neutralizing power of the different gangues. At a later stage it will be necessary to consider the products derived for other sulfides, both singly and in combination with each other and with pyrite. To do this here would lead only to confusion.

PRECIPITATION BY GANGUES OF MODERATE NEUTRALIZING POWER Assuming the iron-bearing solution to be strongly acid, as it is when derived from rapidly oxidizing pyrite, no special limonite type characterizes that precipitated by moderately strong gangue neutralizers. Often the product is loosely to compactly granular, reflecting reaction of single granules of the gangue, as was said in chapter 4. But granular limonite is not necessarily characteristic, and many other types (always excluding the fluffy) may form; such as crusted, caked, spherulitic, and others.

The gangues of moderate neutralizing power such as granite, monzonite, normal shale, etc., cover a wide range because even among the feldspars the degree of neutralizing power varies greatly. Sericitized fcldspars have much less neutralizing power than have their unsericitized equivalents, and with conditions otherwise equal, the potash feldspar, orthoclase, and the soda feldspar, albite, have less neutralizing power than have the lime feldspar, anorthite, and the potash-soda feldspar, anorthoclase. The ferromagnesian mineralshornblende, augite, biotite, etc.-which occur scattered through most rocks as specks, blebs, or well formed crystals, decompose somewhat more readily than do the feldspars, but, except in a few rock types, they do not constitute more than a small percentage of the total volume. Assuming that pyrite is the oxidizing sulfide, with acidity of the solution well maintained so that the iron tends to be exported in solution, indigenous limonite therefore could constitute at best only a very small percentage of the total product precipitated within gangues of moderate neutralizing power. In many cases none at all is formed. Similarly, because rapid and vigorous neutralization is precluded, fluffy limonite does not form in such an environment. But, likewise, neither does the iron travel an indefinite distance before precipitation, as it may do and usually does in the non-reactive gangue. Instead it is set down mostly as a fringing product, because usually the gangue proves sufficiently reactive to effect the iron's precipitation close to the latter's source-generally within a few millimeters-the distance varying inversely with the degree of the neutralizing power of the gangue in any given case. In addition to the gangue's reactivity there must be taken into account in all cases the speed of travel of the iron-bearing solution. Obviously if a gangue is only moderately reactive and the acid is traveling through it rapidly, limonite will not be deposited as densely, per unit area, as if the acid were traveling more slowly. For that reason it is necessary to consider both the gangue's reactivity and the rate at which the iron-bearing solution travels through it. The effective neutralizing power of a gangue or rock is the result of the two factors, distinguishing it from the theoretically total neutralizing power which the gangue or rock would possess if the acid remained in contact with a given unit area long enough for the gangue to react fully. Dependent upon the degree of reactivity in any given

66

IN'rERPRETATION OF LEACHED OUTCROPS

case, limonite, when of pyrite derivation, thus may be deposited in an environment of moderate neutralizing power: I) very subordinately as an indigenous product, but mainly as a fringing or transported one immediately about the outer edges of the cavity, often but not necessarily as densely-packed limonite granules; 2) as a fringing or transported limonite forming a well-defined "halo" surrounding the cavity, but with less crowding of the granules or particles than in (1); 3) as mere iron staining or limonite "fog," mainly where the feldspar has undergone a least moderate alteration to sericite or kaolin. If the alteration is strong the zone of staining or "fog" may not begin for a millimeter or two. The pattern or arrangement of the limonite particles derived from disseminated pyrite in three contrasting gangues, all falling within the fairly broad classification of gangues possessing moderate neutralizing power, is shown in figures 11 and 13, (ch. 18). In these illustrations it will be noted that gangue of moderate neutralizing power is by no means limited to the feldspar rocks. Highly siliccous shale may have only low neutralizing power; but other varieties, as the shale at Mount 1sa, contain bands of sandy dolomite, calcite, or other gangue carbonate, or seams or flecks thereof, which arc readily soluble, and impart to the rock a neutralizing power much higher than might be suggested by comparative analysis with the more slowly decomposing feldspar rocks or siliceous shale. Sandstone may be wholly free of neutralizer; but much of it carries calcium carbonate as an inter-granular tilling or cement. In some varieties the content is high enough to justify classifying the rock as one of strong neutralizing power. Quartzite also often has little more neutralizing power than has quartz; but some varieties contain enough calcium carbonate to precipitate copper carbonate conspicuously. Since copper carbonate is soluble in ferric sulfate, and therefore will not precipitate so long as any appreciable amount of ferric iron remains in solution, it follows that such varieties of quartzite must be competent to precipitate limonite also. Several varieties of schist likewise carry gangue neutralizer, soluble in varying proportions, but rarely in amount to justify classifying any of them as gangues of strong neutralizing power. The rocks classed as possessing moderate neutralizing power thus constitute a large group, with a wide range in degree of neutralizing power. Because of the wide variations in content and nature of the neutralizing bases in this class of gangue, and because the bases, when present, do not necessarily yield readily salts proportional to their content as gangue constituents, it is easy to be led astray by too strict a reliance upon the content of neutralizing bases shown by chemical analysis of a rock. This is one case where no satisfactory substitute exists for diversified field experience in obtaining a first-hand knowledge of what actually is formed in nature under a given set of conditions.

PRECIPITATION BY GANGUES OF STRONG NEUTRALIZING POWER (LIMESTONE AND OTHER CARBONATE ROCKS) In the preceding chapters, although numerous references were made to the precipitation of limonite by neutralizing gangues, the only example for which the chemical reaction was shown was that of equation (31), chapter 11, in which it was shown that calcium carbonate (CaC0 1 ), the principal constituent of purer limestone, can react with ferrous sulfate to form supergene siderite. Calcium carbonate, along with other neutralizers, also has the power of precipitating limonite from ferric sulfate (Fe"(S04L) solution. In this case the iron, being already in the ferric state, is precipitated as one of the ferric sulfates or ferric oxide hydrate, or ferric oxide, not as siderite. The reaction shown in equation (32) is far more common above the water table than that with ferrous sulfate solution; Fe"(S04) ,,+3CaCO,= 3CaSOAFe"O,+3CO".

(32)

Although calcium carbonate is shown as a directly reactive compound, in solution it may be chiefly in the form of bicarbonate (CaH" (CO,) J; consequently we may write the reaction Fe"(SO;),+3CaH"(COJ"= 3CaS0 4+Fe"O,+3H"O+6CO".

(33)

Similarly with magnesium bicarbonate, we may write the reaction Fe 2 (SO,),,+3MgH 2 (C0 3 ) 2 = 3MgS0 4+Fe 2 0,,+3H 2 0+6C0 2 •

(34)

The net effect of the reaction of the bicarbonate rather than the carbonate, is to introduce three molecules of both water and carbon dioxide into the reaction; no other new products are formed. Despite the strong reaction of calcium carbonate with acid, it will be noted that three molecules of it none the less are required to precipitate all the iron yielded by one mole of pyrite.

Types of Limonite Precipitated Fluffy Limonite. The velocity of the reaction of ferric sulfate with gangue neutralizer has much to do with the physical properties of the type of limonite produced, especially in the zone of aeration. When an adequate supply of calcium or magnesium carbonate or bicarbonate is available to react rapidly with the ferric sulfate, the precipitated limonite particles tend to "fluff" up during their formation, rather than to become crusted, caked, or compactly granular products, as in the case with many limonite types produced by the

PRECIPITATION OF LIMONITE BY NEUTRALIZING GANGUES

slower processes in nature; for example, limonite derived from the feldspars. The fluffiness of the limonite is most noticeable perhaps, when a disseminated speck of iron-yielding sulfide oxidizes directly within a mass of calcite. The limonite particles in that case usually are large enough to be individually discernible to the unaided eye, and are among the lightest and most porous limonite particles formed in nature. The fluffy type is formed under conditions of: 1) free though not necessarily rapid oxidation; 2) porosity sufficient, during the oxidation, to permit fluffing of the limonite particles; 3) low content, or at least small precipitation of silica from the ground water. Fluffy limonites are found universally in such gangues or environments, and are restricted to them. Where the reaction is slower, as it is in shaly limestone, or in an ankerite or rhodochrosite gangue, the individual limonite particles or grains, when formed under otherwise similar conditions of precipitation, usually are smaller than those described above, and the precipitate may have a more finely velvety texture rather than a distinctly fluffy one, but if examined under the lens its particles are observed to possess the same fluffiness or pulverulency, resembling powdered sugar sprinkled over a surface. The fluffy type of limonite also may be produced in a gangue of moderate neutralizing power, or even in quartz, provided ground water which carries strong neutralizer in solution flows over the sulfide speck or nodule during the latter's oxidation. The Bagdad, Ariz., disseminated copper deposit furnished an illuminating example during an early stage of the leached outcrop investigation (1924), but one which was puzzling until the conditions were understood. Well-formed pyrite cubes were oxidizing along a subsidiary fault in strongly sericitized quartz monzonite. By digging into the wall rock and observing the oxidation through all of its gradational stages, it was found that no copper was present, and assurance was obtained that at the place in question pyrite constituted the sole sulfide. Since strongly sericitized quartz monzonite is nearly devoid of neutralizing power, the iron therefore should have been exported in solution as ferrous sulfate, and the cavities left free of limonite. Nevertheless they were filled completely with it. Closer observation revealed that the ground water flowing down the fault plane originated in the overlying limy gravels mentioned in chapter 6, and illustrated in figure 22 (ch. 18) and plate 10. The near-saturation of the ground water with calcium bicarbonate was responsible for limonite of the fluffy type being precipitated indigenously within the pyrite cavities. In this case although the gangue in which the pyrite occurred was practically devoid of neutralizing power, the "environment" in which oxidation of the pyrite took place was one of strong neutralizing power. In leached outcrop interpretation it often becomes necessary to distinguish between the two, and to bear in mind that the type of gangue does not necessarily determine the degree of neutraliza-

67

tion in all circumstances, even though it usually serves as a dependable guide. If fluffy limonite were always indigenous, as in the case cited at Bagdad, and if its source invariably were pyrite, its interpretation would be simple. But it is neither necessarily nor characteristically indigenous. The strongly acid pyrite-derived solution, if originating within a gangue of low neutralizing power, sometimes travels many feet before effective neutralization occurs. An example is the solution derived from oxidizing pyrite in a broad quartz vein, percolating downward toward a limestone footwall, illustrated in figure 11 (ch. 18). Fluffy limonite therefore may be a fringing or an exotic, as well as an indigenous, product. Nor is fluffy limonite necessarily derived from pyrite; for, unfortunately, it has the same texture and physical appearance in all cases, irrespective of whether the iron that went into its formation was derived from pyrite, chalcopyrite, bornite, siderite, magnetite, garnet, the ferro-magnesians, or any other iron-yielding mineral. By itself it signifies nothing more than that an ironyielding solution from an indefinite source has come in contact with strong neutralizer under conditions which have brought about the vigorous precipitation of the iron, as was said in chapters 1 and 10. For that reason the attempt to ferret out the source of the iron entering into the composition of fluffy limonite might seem to be nearly hopeless except in simple cases, such as the one in Bagdad, and it must be admitted that in many instances correct determination of the iron's parentage constitutes a full challenge to the interpreter's observational and analytical ability. Some cases are clearly beyond the scope of the present technique, especially where the product belongs to the fringing or exotic classes. But where the product is indigenous and its parent was a sulfide the oxidizing solutions of which were less highly acid than those derived from pyrite, it happens not infrequently that, embedded within or emerging from the fluffy mass, some remnant of cellular structure or other preserved distinctive feature characteristic of the parent mineral may be detected to assist in tracking down the limonite's source. The matter is discussed more appropriately in subsequent chapters. But even though such a clue may be preserved in some degree, fluffy limonite tends in every case to modify, obscure, or obliterate the structure that would have been formed from the parent mineral under normal air-water oxidation processes alone, and increases greatly the difficulty of correct interpretation, as was stated in chapter 1. Massive Jasper. The free sulfuric acid formed by either the initial oxidation of the pyrite (equation 1), or by the subsequent formation and hydrolysis of ferric sulfate (equation 4), may in some cases not be entirely consumed in the neutralization caused by the gangue. Rapidly oxidizing pyrite that occurs as large nodules, thick seams, or in any other massive form-especially if the gangue be an impure, shaly limestone-may yield

68

INTERPRETATION OF LEACHED OUTCROPS

excess acid that attacks and often kaolinizes extensively the gangue over widths varying from a few millimeters to several inches or more, directly adjacent to the decomposing pyrite mass, before being itself consum~d. If at the same time infiltrating ground water carnes sufficient silica, as the acidity weakens, the ferric oxide and silica are precipitated, but the resultant product will not be fluffy limonite, but may be massive jasper of the impregnated kaolin type described in chapter 6. The impregnated kaolin type of massive jasper is restricted mainly to areas which formerly were occupied by or adjoined semi-massive to massive pyrite under conditions in which excess acid persisted and kaolinized the immediately adjacent country rock (ch. 6); the kaolinization being followed in turn by contemporaneous precipitation of or replacement by both limonite and silica. Limonite "Dice." A third type of limonite characteristically yielded by oxidation of pyrite in the gangue of strong neutralizer is the hard pseudomorphs, cubes, or limonite "dice," which constitute compact, essentially grain-for-grain replacements of the pyrite parent found within the gangue. With few exceptions the limonite "dice" are found in limestone, and have not been observed thus far formed elsewhere than well above the water table, usually within 10 or 20 feet of the surface in semi-arid to arid regions. Their formation is not fully understood, but it is believed that they represent cases in which oxidation proceeds mainly through the invisible, very thin moisture films which adhere to sulfide surfaces even under arid conditions. At most times the films probably are too saturated with calcium bicarbonate for oxidation of the pyrite to proceed. When the rare incoming fresh solution does arrive, as is most likely following a soaking rain, the tiny particles of pyrite which undergo oxidation become overwhelmed by the neutralizer quickly and virtually in place. The iron consequently has little opportunity of traveling in solution,

and is converted from ferrous sulfide to ferric oxide or ferric oxide hydrate without measurable migration. 1

SUMMARY The type of limonite product that develops within any gangue with moderately strong or strong neutralizing power during the oxidation of pyrite, depends upon the local abundance and amount of sulfide involved, upon the rapidity of its oxidation, and upon the vigor with which the acid is neutralized. 1. In a gangue of moderate neutralizing power the iron is likely to be precipitated in the general vicinity of the sulfide parent, but not as fluffy limonite, and either not at all, or only to a minor degree, as indigenous product. 2. In the broad group of rocks classed as possessing moderate neutralizing power, variations both in type of limonite precipitated, and in distance traveled by the iron before its precipitation, are far greater than in the group of rocks classed as possessing strong neutralizing power. 3. Insofar as gangue neutralizer alone is involved, the distance traveled before precipitation is inversely proportional to the "effective" neutralizing power possessed by the rock. 4. In a gangue of strong neutralizing power the iron is almost certain to be precipitated, mainly as fluffy limonite, and often indigenously. Subordinate precipitation occurs as the impregnated kaolin type of massive jasper (ch. 6); and an almost negligible amount is precipitated as hard pseudomorphs. 'The hard pseudomorphs are not limited to cubic shapes; they are formed also after irregularly shaped blebs or small nodules. But the cubic shapes are more readily detected both within the gangue and when freed by erosion. They likewise seem to abound more than the other types; suggesting that the cubic crystal form of pyrite is especially favorable to formation of the hard pseudomorphs (see fig. 24, ch. 18).

Chapter 13 SOME EXAMPLES OF THE PRODUCTS OF THE OVERALL PROCESSES OF OXIDATION, LEACHING, AND SUPERGENE ENRICHMENT In this chapter are discussed specific examples of the products of the oxidation and leaching of sulfide minerals, and the often interrelated processes of supergene enrichment, and limonite precipitation both above and below the water table. The gangues present in the mines described here, consist of moderately reactive rocks such as feldspar-rich rocks, dolomitic and graphitic shale, slate, and various types of schist, but not limestone.

GREAT COBAR MINE The Great Cobar mine, New South Wales, an original pyrrhotite-chalcopyrite deposit that contains some pyrite in the upper levels, is an example of supergene chalcocite deposition in slaty and schistose gangue. The Great Cobar mine (including the New Cobar, Chesney, and Peak mines) produced about 113,780 tons of copper from 4,135,000 long tons of ore in the period from 1871 to 1919, mostly during the period between 1884 and 1914. The average copper content of the ore was therefore about 2.7 percent, with diminishing grade throughout the years. The property also produced 306,060 ounces of gold and 1,546,746 ounces of silver. The mine closed down before the author's arrival in Australia, but Andrews' report (1911) upon the district,l supplemented by Department of Mines annual reports, and by statements to the author by former mine officials and others, permit a reasonably accurate reconstruction of the conditions. The deposit occurs in steeply-dipping Silurian slate. The nearest known intrusive consists of two orthoclase porphyry "pipes" about 750 feet by 225 feet in cross section, lying 11 miles to the southeast. The slate at Great Cobar has been subjected to intense compression, and is folded, sheared and cleaved, and metamorphosed almost to a schist. The oreshoot outcrops comprised three small gossans along a 1200-foot lode length, with siliceous slate between the lenses (see fig. 5). At the surface the central gossan was copper-stained; the other two gossans were barren of copper. With depth both the "Also see Thomson (1953, pp. 863-875).

gossans and the succeeding orebodies into which they passed almost vertically beneath, expanded in size until at 525 feet or greater depth the ore lenses reached maximum dimensions of 375 by 90 feet, 425 by 80 feet, and 150 by 50 feet. Because of the marked tapering upward of the lenses toward the surface, it must be concluded that hypogene sulfide deposition had reached only a moderate distance above the gossan outcrops. Sulfides were almost wholly leached in the gossan down to 200 feet; but the gossan carried reticulating seams and expansive masses of brilliant malachite in mamillary, concentric, and mossy forms; also large radiating aggregates of azurite. At a depth of 213 feet, chalcocite, associated with cuprite, appeared in rich patches and increased in amount with depth, reaching its broadest lateral spread near and especially directly beneath the water table, which was encountered at 280 feet. Rich patches of primary chalcopyrite, carrying up to 17 percent copper, also had appeared locally in the north lens at 250-foot depth, but chalcocite still dominated for the most part as the ore mineral to the 320-foot depth. From available information, the total copper in the form of chalcocite does not appear to have exceeded that in the form of chalcopyrite, on the average, by more than 50 percent. The chalcopyrite was associated with pyrrhotite, magnetite, and the iron-silicate, ekmannite,2 and with substantial admixture of slate and quartz as gangue. Pyrite was usually absent and at no place conspicuous. 3 Below the 320-foot depth the chalcocite continued as irregular prongs of diminishing size, threading out "A long bladed, fibrous and shining blackish-green mineral in a setting of white quartz and siliceous slate. Andrews gives the composition of ekmannite as Si0 2 AI2 0, FeO Fe,O,

44.20% 5.66% 31.23% 5.10%

CaO MgO Na,O K,O

0.48% 1.96% 0.46% 2.05%

3Galena and sphalerite, deposited in association with pyrite and chalcopyrite, were present in the Great Cobar lode, but restricted to its western or footwall portion, mainly below the 525-foot level. Andrews states that these sulfides were introduced along a zone of weakness formed in the late stages by a slip of the orebody along its footwall, where they formed a mere thread, to a maximum, in one place, of 11 feet in width.

70

INTERPRETATION OF LEACHED OUTCROPS

entirely not far below the 420-foot level. In the supergene ore chalcocite replaced both pyrrhotite and chalcopyrite. 4 Beginning at a depth of about 400 feet, the rich chalcopyrite ore likewise gave way to leaner occurrences beneath. Most of the ore stoped below the 421-foot level, where the lenses attained their maximum spread, averaged less than 2.5 percent copper as chalcopyrite," intergrown with pyrrhotite, magnetite, and ekmannite. 'The chalcocite may be visualized. with essential accuracy as to the overall picture, as the cluster of icicles similar to those which commonly form on the slopes of south-facing roofs in northern climates during the February thaws. At the roof's edge (water table) the ice forms a more or less continuous fringe along the eaves, often projecting up as gripping fingers several inches in length onto the shingle; but in their downward projection the icicles gradually grow narrower, and distance between them increases. until finally they tail out altogether or with only an occasional small spine reaching to the ground. 'Because the richer chalcopyrite concentrations coincided in a broad way with the chalcocite occurrences, Andrews (1911) suggested that they might to a great extent represent bodies of supergene origin; though it is evident from the statements in his report that he was not fully convinced. At that time the world-wide search for positive evidence of supergene chalcopyrite-conducted in connection with the Secondary Enrichment Investigation sponsored by the world's larger copper companies (which failed to find satisfactory evidence of supergene chalcopyrite except as tarnished, thin crusts, pinpointed aggregarions, and occasional bacterial precipitates in swamps)-had

Few copper deposits have been exposed to active weathering for a longer period than have those on the desert plain of interior Australia, along the fringe of which the Cobar deposit occurs. Andrews has estimated, for example, that since Paleozoic time not much more than 200 feet of denudation has taken place at Cobar, an average of only 1 foot denudation every million years. Because no good evidence exists for an oscillating water table at Cobar, and because the gangue (slate containing pyrrhotite, magnetite, ekmannite, and quartz) is without special neutralizing power, it therefore might be reasonably expected that leaching of copper carbonate, as well as chalcocite and chalcopyrite, would have been far more thorough down to the water table. The explanation for the lack of thorough leaching lies in the fact that the only hypogene minerals that not been completed (also see ch. 10). Actually, however, such concentrated deposition as a hypogene mineral in the upper portion of a sulfide lens-in the vicinity of and especially just above the greatest horizontal spread of the lens-is in no sense uncommon, either in the case of primary chalcopyrite or in the cases of other primary sulfides. For example, between the 8th and 9th levels of the Mount Isa copper deposit, a particular cube of ore about 10 x lOx 10 feet, when broken assayed 27.13 percent copper as chalcopyrite, although that body as a whole contained only 4.0 percent copper as chalcopyrite; the cube, moreover, was not in a fault zone thclIgh nearby to one.

GREAT CO BAR MINE

after E, C, Andrews (1911)

LONGITUDINAL SECTION PLAN 525' LEVEl NORTHERN LENS

t CENTRAl LENS

STOPES SOUTHERN LENS o

II FIGURE 5.

L=~_~~

200 400 __'~~~~! FEET

Longitudinal section and plan of the 525-foot level, Great Cobar mine, New South Wales.

,/:'0.

EXAMPLES OF OXIDATION, LEACHING, AND ENRICHMENT PROCESSES

yield free sulfuric acid are the chalcopyrite and pyrrhotite (with some pyrite in the upper levels), and neither chalcopyrite nor pyrrhotite yields excess acid. The gossan comprises not only limonite derived from the decomposition of pyrrhotite and the copper minerals, but also from extensive and for the most part complete decomposition of such ekmannite as was present within the ore lenses, as welI as from the decomposition of some magnetite. Since acid may be presumed to have played a part in the extensive decomposition of ekmannite, and probably was instrumental also in accelerating the exceptionally deep and persistent oxidation of magnetite which has taken place, no supply of acid was probably available to effect wholesale leaching of either the copper carbonate or the chalcocite; or to accelerate the naturally dilatory leaching of chalcopyrite. Despite these handicaps, solutions of cupric sulfate penetrated more than 140 feet into the zone of saturation at the Great Cobar mine, producing enrichment by chalcocite of both chalcopyrite and pyrrhotite.

HOME OF BULLION MINE The Home of Bullion mine 22 miles southeast of Barrow Creek, in the Northern Territory, Australia, affords an example in which the demarcation between gossan and the chalcocite zone is far more sharp than at the Great Cobar mine. The mine is situated almost centralIy within the desert interior of Australia, where denudation must have been fully as slow, if not slower, than at Great Cobar. Present average rainfall is 12 inches; but occasionalIy no precipitation occurs in a year, while in other years there are occasional floods in the summer. The deposit occurs as an irregular lens in one of the belts of Precambrian sericite schist common in the Northern Territory. It is localized along a well-defined zone of fissuring which parallels or nearly parallels the schistosity, with a discordant contact of puckered, overturned folds. The main lode strikes N. 20° W., with a northeasterly dip.6 The total length of the main lode is 540 feet. The 485-foot portion wide enough for stoping averages 10.5 feet in thickness, with lenticular swells up to 20 feet in thickness. Milky quartz was noted along the lode, but is not conspicuous. The nearest intrusive is quartz monzonite, which crops out 10 miles away. The deposit had not been fully developed when visited by the author (1936), but was penetrated at enough representative points both above and below the water table to furnish what is believed to be a generally reliable picture. The pre-mine water table was encountered at 93 feet. Within a few feet of that depth, wherever tested, the lode yielded an abundant porous gossan, averaging between 3 and 4 percent copper, with minor occurrences of lead. Copper within the gossan is marooned mainly as seams and patches of malachite "About 600 feet to the south, quartz-mica schists, fine-

grained sandstones, and a little amphibolite make up the lode, but the copper mineralization is weak. Alluvium covers nearly all of the rocks to the south.

71

which so characteristically lingers irregularly within gossans derived from copper ore in semi-arid regions, but which in this case is not comparable in amount or in size of individual masses to that at Great Cobar. Associated with it there is much less azurite and very minor cuprite. Chalcocite is present as isolated kernels and scattered larger nodules and patches. The larger occurrences coincide with more or less gossan-free portions of the lode, suggesting that their survival is due to local acid deficiency. In the gossan as a whole, little chalcocite was observed above the water table. Lead is present throughout the gossan, though less abundant than copper and far more erratically distributed, as supergene cerussite and minor supergene anglesite. Between the depths of 93 and 110 feet the material changes rapidly to a limonite-free semi-massive chalcocite, which replaced both pyrite and schist, but principally pyrite. Covellite and bornite are conspicuous locally as tarnishes, and are occasionally present as seams and small nodules. The chalcocite is sooty rather than massive. At the time of the author's visit the lode had been penetrated to greater depth only by a vertical shaft, together with a crosscut at the 145-foot level, and a crosscut and small stope at the 196-foot depth-all located well within the broader portion of the lode below the water table. At a depth 196 feet in the stope, a location which afforded the largest single exposure for observation, the hypogene sulfides comprised mostly pyrite, largely as narrow replacement seams along the schistosity planes, but also as frequent nodules of varying size, with occasional patches up to 3 feet across. Intermixed with it was chalcopyrite in nodules up to 4 inches across. Judged by the limited exposures, the primary ore averaged about 3 percent copper in the form of chalcopyrite and about 6 percent copper in the form of supergene chalcocite, making a total copper content in the ore of 9 percent. No pyrrhotite or sphalerite was observed in the workings at the 196-foot depth, and only a few sparse blebs of galena. The hypogene sulfides were much seamed and fractured. Where chalcopyrite was present, chalcocite replaced it preferentially. Schist adjoining the larger chalcocite replacements invariably was welI kaolinized. The copper content of the sulfide zone ranged from 15 to 9 percent, being lowest in the lower workings. Through close sorting the grade of ore for shipping was raised to 35 to 40 percent. 7 'For accounts after the author's visit, see Hossfeld (1936); and Sullivan (1953). Three drillholes intersected the lode at vertical depth of 300 feet, and a fourth cut it at 375 feet below the surface in 1953. The primary ore, in the drillholes, contained 3 to 5 percent copper, 1 to 6 percent lead, and up to 15 percent zinc. At 300 feet the lode consists, in order of abundance, of pyrite, sphalerite, bornite, chalcopyrite, galena and small quantities of chalcocite. From the main shaft, extending to a depth of 210 feet vertically, drifts had been driven along the lode for 240 feet at the 140-foot level, 170 feet at the 180foot level, and 60 feet at the 200-foot level, with about the same results in percentages as above the 200-foot level. Fiftyfive hundred long tons of ore containing 22.5 or more percent copper were shipped in 1953.

72

INTERPRETATION OF LEACHED OUTCROPS

A point of particular interest is that the chalcocite replacement pattern found beneath the water table is mirrored closely by the indigenous limonite pattern at the surface, and within the gossan elsewhere above the water table. With pyrite not greatly exceeding chalcocite in the upper portions of the chalcocite zone, precipitation of limonite, in general, indigenous after the chalcocite, was favored. This means that the chalcocite zone, in essentially its present form as regards distribution and amount of copper content, had earlier existed throughout the whole of the gossan now exposed; and that its copper content had been carried downward, to be re-precipitated at successive depths in step with recession of the water table. Building up of copper content occurred only slowly through accretion from the fresh, much lower grade hypogene ore encountered. The Home of Bullion therefore represents a deposit in which an extensive supergene chalcocite zone was built up in a gangue of moderately weak neutralizing power (sericite schist), with copper content probably not less than three or four times that present originally in the hypogene form. This deposit thus contrasts with the Great Cobar deposit where, on a.verage, copper within the horizon of chalcocite precipitation probably did not exceed by much more than 50 percent the hypogene copper content. The increased supergene enrichment at the Home of Bullion is ascribed to: 1) much deeper denudation of the hypogene orebody, thus making available sufficient copper to build up a far more strongly enriched chalcocite zone; and 2) a more acid environment, because of greater abundance of pyrite, favoring more thorough leaching of both hypogene and supergene minerals above the water table, with necessarily poor development of copper carbonate in a gangue possessing only weak neutralizing power; and favoring also sharp demarcation at the water table between gossan and sulfides in the chalcocite zone.

MOUNT OXIDE MINE The Mount Oxide mine in Queensland (described in detail in Appendix C) furnishes an example of still another type. Massive supergene chalcocite constitutes the principal copper mineral, and almost the only sulfide encountered down to the 300-foot level, in a gangue of moderately weak neutralizing power. Below the 300foot level, massive pyrite occurred, with very scattered chalcocite going down an additional 70 feet.

MOUNT ISA MINE At Mount Isa in northwestern Queensland are located copper and silver-lead-zinc orebodies of huge proportions. The first of the lead-zinc orebodies was discovered in 1923, and 7 years later the first substantial quantities of copper ore were found. Since then

many other lead-zinc orebodies and several additional copper deposits have been located. An extensive and continuous exploratory drilling program carried out since 1951 has greatly increased known reserves of both types of are. Known reserves as of June 1963 totaled 29.5 million tons of 3.5 percent copper ore, and 26 million tons of ore containing an average of 7.8 percent lead, 5.9 percent zinc, and 5.6 ounces per ton of silver. Total production to 1963 includes nearly 18 million tons of silver-lead-zinc are which was slightly higher in all values than that of the known reserves, and about 14 million tons of ore containing an average of somewhat more than 3 percent copper. An expansion program begun in 1956 scheduled an increase in ore production to 14,400 tons per day (Foots, 1961, p. 3) by the end of 1965, slightly more than half of which would be copper ore. The Mount Isa deposits occur in folded and faulted Precambrian metamorphic rocks within a 25,000square-mile portion of the Australian Precambrian Shield described by Carter and others (1961). The area has been subjected to at least two major orogenic compressive phases during Precambrian time; fold axes trend roughly north-south. The area has been extensively faulted. Most faults belong to a conjugate strikeslip system, but there are normal faults, and high angle reverse faults occur west of Mount Isa. Sulfide are zones are concentrated in bands that are concordant with the enclosing wallrock. Copper and silver-lead-zinc orebodies are spatially separate, but have similar attitudes (see fig. 6).8 The copper mineralization, chiefly chalcopyrite, is concentrated mainly in the silica-dolomite sequence, and the sphalerite and galena in the carbonaceous shales that are lower in silica. D The copper orebodies lie in the hangingwall, are lenticular, more than 8,000 feet long and at least 2,500 feet deep. They do not crop out for the most part, because the surface in most places is covered by alluvium. The sulfide apexes are about 725 feet below the surface except in the secondary ore. The silver-lead-zinc orebodies are lenticular too, about 8,000 feet long or longer, and at least 2,500 feet deep, and most of them crop out except the Racecourse ore body (fig. 6), north of the Black Rock orebody. The Black Star is the principal silver-lead-zinc lode. 'In addition to the hypogene sulfide orebodies, geologists as early as 1957 had found a partial1y oxidized supergene chalcocite orebody, 150 feet or more in width, and 400 to 500 feet or more below the surface, between the central1y situated silverlead-zinc Black Rock orebody and the hangingwal1 chalcopyrite-pyrrhotite orebodies. It was mostly covered by al1uvium and consisted of an upper zone of copper oxides, with chalcocite beneath. In June 1963 the new, partially oxidized chalcocite orebody was reported to average 3.8 percent for 2,000,000 long tons of proved ore. The chalcocitic part of the new orebody is about 300 feet deeper than the oxidation in general. '[Editor's note: Excellent recent summaries of the geology and mineralogy of the Mount Isa mine were published in the Australasian Institute of Mining and Metalturgy Proceedings, no. 197, March 1961.]

73

EXAMPLES OF OXIDATION, LEACHING, AND ENRICHMENT PROCESSES

Its largest horizontal section is about 2,000 feet by 250 feet. The country rock adjoining the Black Star orebody is shale that contains about 16 percent dolomite. In the ore galena replaced chiefly dolomite, though in some places it replaced siliceous gangue. Sphalerite on the other hand in some places replaced siliceous gangue, in other places it replaced dolomite. Galena and sphalerite were also deposited between the shale layers to a certain extent. Oxidized Portion. In the upper part of the oxidiz~d zone the galena has been converted almost entirely into cerussite, which has undergone almost no leaching. For 8 to 10 feet below the surface, cerussite is considerably enriched along intersecting fractures of the porous rock. The only sulfide remaining above the water table, (located at about 162 feet), comprised small scattered nodules of galena as residuals encased in anglesite in the bottom of the oxidized zone. These galena residuals did not constitute more than a fraction of 1 percent of the lead-principally in the dolomite or other carbonate rocks. Silver has been leached strongly near the surface, but has been largely reprecipitated as native metal, with

some cerargyrite, between the 80-foot and 162-foot level. Zinc has been leached even more strongly near the surface than has silver, with erratic reprecipitation above the water table of somewhat less than half the original zinc content as smithsonite. The moderate neutralizing power of the gangue, caused by its dolomite content, probably accounts for the incomplete zinc leaching, also for the incomplete leaching of the silver. Oxidation of ore carrying 12.4 percent pyrite and 3.1 percent pyrrhotite (1945 figures), has yielded fringing and some indigenous limonite, forming a persistent and generally strong gossan down to the 162-foot level. Below the water table lies a zone of transition are, about 50 feet thick, which was nearly limonite-free when first encountered at the water table, although streaks of limonite persisted intermittently for about 10 feet. Galena was converted partially to anglesite for 1 to 3 feet below the water table, but from 5 to 10 feet below the water table it was only tarnished, and the tarnish diminished rapidly with depth. Supergene pyrargyrite and polybasite and supergene sphalerite are estimated to have enriched the ore around 15 percent

w

o o

g

25 E Dec. No. 10 LEVEL

1436'

No. 12 LEVEL 1817'

W 23 'C'

No. 15 2389

No. 15 LEVEL

2389'

1

'

LEGEND IiiIIlead ore _

High grade copper ore

E3 low grade copper ore E3 Silica ·dolomite ~ Greenstone

E3foult

FIGURE 6.

Cross sections of the Mount Isa mine, Queensland.

MOUNT ISA MINES LIMITED SECTION LOOKING NORTH ALONG CO-ORDINATE 7200 N

74

INTERPRETATION OF LEACHED OUTCROPS

in silver and 20 percent in zinc. Chalcopyrite, which in the silver-lead-zinc ore accounted for an average of 0.10 to 0.15 percent copper (1945), is erratic in its distribution. Both galena and sphalerite in the transition zone were replaced, chiefly by supergene chalcocite, with some supergene covellite and bornite in the lower portions, for about 45 feet below the water table. In both transition and hypogene ore there was moderate corrosion of pyrrhotite and sphalerite, because acidic ferrous sulfate solutions were strong in placesprincipally in siliceous gangues. 10 Pyrrhotite and sphalerite showed from 6 to 0.1 percent corrosion, gradually diminishing with depth. Sphalerite showed corrosion for a distance of 295 feet below the water table in the nearly unfractured rock. Pyrrhotite showed corrosion down to No. 5 level in the nearly unfractured rock, 546 feet below the surface and about 385 feet below the water table (ch. 7). Pyrrhotite masses in quartz at this depth had serrated edges, and the pyrrhotite was about onefifth corrodedY At Mount Isa the gossan cuts off sharply at the water table in the silver-lead-zinc bodies in shale as previously described; but leaching is much deeper within the jasper or silica-breccia zones, which at numerous places constitute the foot or hanging wall of the silver-lead-zinc orebodies, and in which dolomite comprises up to 25 percent or more of the total content in the silica-breccia zones. Acid derived from associated or adjacent oxidizing pyrite has attacked the dolomite of the silica-breccia masses so that locally to depth of more than 200 feet below the water table both pyrite and dolomite are largely leached, with porosity of the resulting siliceous breccia in some cases rising to 40 percent. Heavy underground water flows almost invariably are encountered when the silica-breccia is cut upon new levels (1948), and there can be no question that the silica-breccia zones constitute solution channels below the water table, in which descent of the water is far more rapid than elsewhere. In Mount [sa, to more than 200 feet depth below the water table the leached silica-breccia nearly everywhere carries abundant limonite, often across entire widths of the breccia lenses. At one place, along a fault within the breccia zone in the Black Rock mine, cerussite and native copper in arborescent, blade, and wire form, were spectacularly developed within a gossanous mass over a 3- to 12-foot width on either lOPyrite was corroded in the siliceous gangue by acidic ferrous sulfate solutions; but 50 percent or more of the pyrite is present as a very fine mixture of pyrite and pyrrhotite, so it is difficult to tell the pyrite and pyrrhotite, except in the coarser crystals. The pyrite in the coarser crystals of the siliceous gangue, was very little corroded. 11M ore pyrrhotite was present in the Black Star, No. 1 lode, than at any other place in the silver-lead-zinc orebodies. The grain size was determined largely by the texture of the replaced host, and sometimes increased to 5 mm. In one case pyrrhotite grains about 100 mm in diameter occurred.

side of the fault nearly 400 feet below the water table. 12 The explanation is that not only does the neutralizing power of dolomite or calcite diminish the acidity of any ferrous sulfate solution that comes in contact with it, but also along the strongly defined channels oxygen has been carried down by ground water in solution, and has brought about oxidation of ferrous to ferric sulfate, permitting precipitation of limonite almost as freely within the zone of saturation as normally occurs above the water table.

MOUNT STEWART MINE Mount Stewart, New South Wales, furnishes another example of the oxidation of lead and zinc orebodies. The silver-lead-zinc orebodies were not large, but the lead mineralization had been pretty well leached near the surface, in contrast to the lead ore in the Mount Isa mine. The Mount Stewart mine was opened in 1888 as a silver-lead mine (Edwards, 1953, p. 926). Production to 1893 amounted to about 15,000 tons of oxidized ore, which yielded 1,539 tons of lead and 300,000 ounces of silver. Production since then has been minor, amounting to about 1,200 tons, but about 70,000 tons of primary pyritic ores were mined between 1931 and 1937 for the manufacture of sulfuric acid. The orebodies in the Leadville district crop out over an area 2,400 feet long and 1,050 feet wide. They occur within folded, faulted, crushed, and fractured masses of slate and shale of Silurian (?) age, usually touching quartz porphyry and other acid intrusives at some point along their peripheries, as well as in limestone lenses that had been converted largely into epidote, wollastonite, and minor garnet in connection with pre-ore intrusive invasions. The Mount Stewart workings are the most northerly of the group, and the surface gossan covers a lenticular area of about 630 by 90 feet. These workings were served by several shafts (fig. 7). Strongly defined, the gossan persists to the water table at the 157 -foot level. Between the depths of 157 and 172 feet there is a transition zone from oxidized to sulfide ore. The lenses appear to represent both hypogene and supergene zinc deposits (with some lead) around the main cluster of oxidized silver-lead oreshoots. In the principal lens in the oxidized ore, 165 by 40 feet in horizontal section (Engine shaft and No. 2 shaft), extensive leaching of lead had occurred near the surface, as evidenced by well-formed, indigenous limonite pseudomorphs after cerussite. The main portion of the cerussite ore (165 by 40 feet in horizontal l!lChalcopyrite in the oxidized portions at Mount Isa in the silver-lead-zinc orebodies is erratic in distribution. In the third level of the Black Rock carbonate zone, just above the high grade cerussite, east of the O'Doughty (Black Rock) shaft, thick crystals of beautiful arborescent malachite for 30 feet along the strike and 8 to 15 feet in width, were abundant in a fold, not a fault, in 1934.

EXAMPLES OF OXIDATION, LEACHING, AND ENRICHMENT PROCESSES

75

MOUNT STEWART MINE

LOOKING

N.55° E. SCALE

O'-iiiiiiil"'ll",!"6~0i;;;;;;iiiiiiiiiiiiiiiiii';j20~",!",,,!,,~I80

SOUTHERN SHAFT

~:~~ ••

G••

~

ENGINE

AlluvIUm and Waste

•

Gossen

~

Limonlte-stolned

SHAFT

.. ~

LOOKING N. 26 1/2 0 W. Projection olong Porphyry Hang,"gwoll

Limonitic Jasper and 511 iceous semimossive

a

Massive Pyrite

~±~~j Gronophyre

Pyrite

Undifferentiated SUlfides;jr not much lead

III

Silver-Lead ore

~

Zinc ore

Adapted from Willan

FIGURE 7.

feet

',.,1 J Porphyry

~:=:=E~

Undifft'rentioted sediments

~Limestone ~

Shale-going to Slate

(I92S)with minor additions and certain reclassifications by R. B.

Cross section and longitudinal section of the Mount Stewart mine, New South Wales.

76

INTERPRETATION OF LEACHED OUTCROPS

were mixed with the pyrite, so the acidic ferrous sulfate solutions were not necessarily derived from pyrite alone; however, the pyrite showed sandy corrosion. The author did not see pyrrhotite in his brief 2-day visit in 1936, some of the workings not being accessible. Chalcopyrite was sparingly present. The author took 20 samples on the 261-foot level and in the 100-foot interval above, but he found only traces of copper. In 1918-1920, below the transition zone, analyses showed 0.07 to 0.2 percent copper. Cupric sulfate solutions were therefore of little significance. H Whether the acidic ferrous sulfate solutions below the water table (for the whole 653-foot by 90-foot area) proceeded downward or laterally outward, the author has no opinion. Because of the moderately tight fault, lateral movement was a possibilityY

section) occurred from the 73-foot level to just below the 100-foot level, with maximum concentration upon and directly below the 100-foot level; and gradually decreasing until the 157-foot level was reached. There was not much zinc ore in this portion; but some of the sphalerite present was replaced by cellular limonite both near the surface, and on the 100-foot and 157-foot levels. Most of the zinc was mined with the lead, but there is no record of zinc recovery. Cross sections of the Mount Stewart mine are shown in figure 7. In the oxidized zone (Engine shaft and No.2 shaft) the ore carried about 30 ounces of silver and 23.8 percent of lead, mostly as cerussite. The author has little information concerning the silver content, because most of it was mined in 1892-1893, and it was not reported separately. Presumably it lay beneath the surface gossan, but above the 157-foot level; however, it may in part have been present in the transition zone, between the 157 -foot level and the 172foot level. Below the water table, in the hypogene zone (below 172 feet) the main lens gave place to massive pyrite, which continued to the 261-foot level, where drifting stopped. But two winzes were put down, one an additional 32 feet and the other an additional 34 feet in massive pyrite, where the pyrite continued. The author did not see the pyrite between the 157-foot level and the 230-foot level; it had been mined out; but from the 230-foot level to the 261-foot level the author saw massive pyrite much corroded probably by acidic ferrous sulfate solutions, the corrosion running from 3 to 8 percent. 13 As in the Mount Oxide mine, the silver-lead-zinc ore rests, presumably, upon the pyrite roots of the original oreshoot. All of the other lenses in the oxidized ore of the area 630 feet by 90 feet (No.3 shaft, No.4 shaft, fig. 7) at Mount Stewart contain less cerussite, but much more zinc. The transition and hypogene zones (between the 157foot level and the 261-foot level) contained 10 to 25 percent iron-rich sphalerite, 1 to 2 percent galena, a few ounces in silver; the rest consisted of semi-massive pyrite with silica. In the 261-foot level, and above for 100 feet, about one-fifth of the sphalerite was supergene, of dark color, filling cracks, precipitated upon the more silvery hypogene sphalerite. The sphalerite below the water table was honeycombed as a result of corrosion in all other lenses except the principal lens (165 feet by 40 feet in horizontal section). Hypogene and supergene sphalerite

The C. S. A. (Cornish-Scottish-Australian) mine is located about 7 miles north-northwest of Cobar, in north-central New South Wales. Although the gossan was discovered in 1871 (Thomson, 1953, p. 886), it was not until 1905 that lead carbonate ore was encountered at 457 feet, the level of the water table. The mine was closed in 1920 following an underground fire, and had to that time produced 108,496 tons of ore that yielded 4,161 tons of copper, 3,156 tons of lead, 94,955 ounces of silver, and 1,335 ounces of gold. Godfrey (1916), from 252 samples collected on four levels, had outlined two pyritic (unoxidized) orebodies averaging 0.4 percent copper, 5.1 percent lead, and 15.75 percent zinc. The deposit occurs in a group of sediments of probable Silurian age. The gossans are surrounded by aureoles of silicification and staining by iron and manganese oxides in a claystone wallrock, and occupy a lenticular area 1,150 feet by 300 feet at the crest of a 200-foot hill on the Cobar plain. As in the Great Cobar mine, the wall rock is shattered, folded and cleaved; but there was less strain than at Great Cobar. In the oxidized zone the lead was present as a trace to several percent of cerussite in narrow bands (mostly pseudomorphs), with erratic narrow, scant seams of malachite. No zinc is mentioned in the reports, but the ratio of zinc to lead in the croppings was about 3 to 2, greater in certain places. No zinc, lead, or copper was mined above the 457 -foot level (top of the water table) . From the 435- to the 457-foot level the principal lens of lead ore was 165 by 40 feet in horizontal section, and yielded 9,207 tons averaging 35 percent lead, of

"Although in the 261-foot level the fault passes between the quartz porphyry hanging wall and the slate and shale footwall along the drift for 575 feet, the total amount of water seepage along the fault is less than 2,500 gallons per day; so there is no appreciable water. Some of the limonite reached the 261-foot level along the fault, but it was scattered, and extended less than 1 foot from the fault. At many places no limonite was present along the fault.

"But in another lens. 1,500 feet south-southwest of Mount Stewart (The Copper Lode), oxidized ore samples assayed 4 percent copper, 1 percent lead, and 0.75 percent zinc for the 700 tons shipped. In the Grosvenor lode, about 1,200 feet SW of Mount Stewart, only oxidized silver-lead-zinc are is mentioned in the uppermost 100 feet. The orebodies were small. 15For additional information see Kenny (1923); Willan (1925); and Edwards (1953).

c. S. A.

MINE

EXAMPLES OF OXIDATION, LEACHING, AND ENRICHMENT PROCESSES

which 15.3 tons assayed 1.3 ounces of gold and 451 ounces of silver per ton. There were other ore lenses in the oxidized zone, but none as rich as this one. Below the water table in the secondary enrichment zone, lay eight lenses of chalcocite-enriched ore which had a total area 400 by 200 feet. The largest lens, 200 by 25--65 feet in area and about 25 feet in thickness, lay immediately below the lead stope. The chalcocite was of irregular distribution, shown by the fact that 2,447 tons of the stoped ore averaged 14.1 percent copper. Pyrite was intermixed with all sulfides below the water table, and pyrite and sphalerite increased with depth. Pyrrhotite was sparse in the vicinity of the mine, according to reports."r. Development was carried to 660 feet. Beneath the 25-foot-thick chalcocite zone was another zone about 18 feet thick in which supergene chalcocite occurred with sphalerite and probably with chalcopyrite. This zone contained bunches of rich chalcocite ore. The chalocite was not consistent, due to the fracturing. The limited occurrences of chalcocite in this 18-foot transition ore zone, and the general paucity of copper carbonate in a gossan of more than 450 feet thickness beneath the surface, shows that cupric sulfate solutions must have existed in only a moderate volume compared to zinc sulfate solutions. In the hypogene zone, there seems no reason for thinking that corrosion by acidic ferrous sulfate solutions terminated at lesser depth below the water table than it did at Mount Isa and Mount Stewart; though this is not certain, of course. Sphalerite, at depths of 500 to 600 feet, appears to be more dominant (15.72 percent) than in the transition zone (11.9 percent). The gangue is slate and shale (identified as claystone by Thomson), which is a nearly inert gangue, unlike either the intermingled layers of dolomite and siliceous gangue seen at Mount Isa, or the slate and shale intermixed with limestone lenses with epidote and other alteration minerals seen at Mount Stewart. The gangue in the C. S. A. mine consists only of slate and shale, very much fractured, which makes for deep leaching."' In the latter part of the 1950s and early in the 1960s, South Broken Hill Limited, drilled deeper at the C. S. A and Chesney mines. At the C. S. A 20 holes drilled to depths of 600 to 3,100 feet in the western and eastern zones indicated 3.50 percent copper (mainly as chalcopyrite) for an average width of 31.5 feet. At the Chesney mine, about 9 miles southeast of the C. S. A, 10 holes drilled to depths of from 1,400 to 3,000 feet, indicated 2.72 percent copper for 31 feet average width. lOExcept at the Tinto mine. about 1,000 feet to the south. The Tinto mine was similarly leached in the oxidized portions. but below the water table part of the gangue consisted of pyrrhotite and magnetite, just as at the Great Cobar mine. The C. S. A. and Tinto mines were combined in 1913, and a smelter was erected. "See Andrews (1911); Godfrey (1916, p. 99-102); brief references in N.S.W. Dept. of Mines Annual Rept. for 1913, 1914, 1915, 1920; also Thomson (1953, p. 886-895).

77

Estimated reserves of the C. S. A. and Chesney mines together were 18 million long tons.

MOUNT CUTHBERT MINE The Mount Cuthbert, Queensland, copper deposit is situated about 120 miles northeast of Mount Isa, in the Cloncurry district, the same mineral district as the Mount Isa and Mount Oxide mines. The water table there is at a depth of 180 feet, and is nearly stationary. The deposit occurs within a northerly-trending zone of moderate shearing in Archeozoic sericite schist. The shear zone dips to the cast at a high angle. The oreshoots occur mainly along the hanging wall side of the shear zone. Mount Cuthbert produced 60,680 tons of ore averaging 7.1 percent copper content; the mine closed in 1920 (Carter and others, 1961, p. 213). The main shaft reached a depth of 484 feet, and the are shoots had a maximum width of 30 feet and were worked over a maximum length of 650 feet. Numerous fine searnlets and narrow tongues of rhyolite porphyry are intruded along schistosity planes (a quarter of a mile to the west is a large body of Precambrian granite), and the whole is impregnated with enough dispersed calcite and dolomite in irregular form to constitute a self-fluxing ore. In the hypogene ore, chalcopyrite and pyrite are the sulfide minerals. The hypogene ore contains 3 to 4 percent copper, and infrequently as much as 6 percent copper over narrow widths. Pyrite is irregularly distributed, but for the main portion as stoped, it was not observed to have exceeded chalcopyrite in amount. Oxidized Ore. Much copper carbonate and cuprite, and some chalcocite were present from the surface 'to a depth of 75 feet, increasing the grade about two to three times over that of the hypogene ore. From the 75-foot level to the 100-foot level chalcocite increased, and copper carbonate and cuprite decreased rapidly. Chalcocite and chalcopyrite were present in about equal amounts, especially near the 100-foot level, and the tenor of the ore was about 7 percent copper. Chalcocite was of the sooty kind. Limonite precipitation throughout the 100-foot interval was sufficient to form a semimassive gossan in most places. In the interval from 75 to 100 feet the chalcocite was present almost exclusively in irregular calcite and dolomite gangue; while the sericite schist, without the carbonate gangue, contained largely chalcopyrite and pyrite-especially near and at the 100-foot level and the 180-foot level. Transition Ore. The mine and the smelter closed down in 1920, and did not open again; and the author did not see the ore in the transition zone except in the shaft 180 feet down. However, mine officials whom the author met in 1931-1938 stated that chalcocite persisted below the water table to a depth of 200 feet in the carbonate gangue. According to the mine officials limonite also extended below the water table, on the whole co-extensive with diminishing chalcocite to the 250-foot level or below.

78

INTERPRETATION OF LEACHED OUTCROPS

BISBEE At Bisbee, Ariz., pre-mine deposition of limonite within the granite porphyry barely reached 800 feet as an extreme depth; but nearby, in the strongly-defined Junction fracture zone of the adjoining limestone in ore, it was abundant along a strong solution channel to a depth of 1,600 feet. At Bisbee precipitation of limonite likewise was common in the less altered limestone to depths of 40 feet or more below the water table, outside the defined channels.

ELY At Ely, Nev., hypogene chalcopyrite-pyrite ore at some places in the monzonite is almost untouched by oxidation near the surface. Monzonite in the ore area at Ely may have been capped locally, and in part protected from extensive oxidation by a post-ore rhyolite flow, (Kimberly, west of Ely). At Ely, therefore, ore in the monzonite depends upon supergene chalcocite enrichment far less generally than in most of the porphyry coppers, and much of the ore's copper content exists as chalcopyrite. Yet in the Alpha mine, in limestone, along and adjoining the strongly-defined Alpha fault, a heavy gossan of ferric oxide hydrate persists well below the 700-foot level in an area 1,000 feet by 1,000 feet. The list of examples could be greatly extended; although in many districts, where the rocks are less soluble than limestone or dolomite, with water circulation consequently less free, limonite precipitation extends to much lesser depths within the zone of saturation. In view of the ease with which limonite may be precipitated below the water table, as described at Mount Cuthbert, it would seem that the burden of proof rests heavily upon those who contend that the water table has risen wherever they find limonite at some distance beneath it. Until there has been eliminated all possibility of the limonite having been deposited below the water table-whether in copper or in silver-lead-zinc or in other deposits-the existence of a limonite zone below the water table cannot be adduced as evidence to prove the case (see also Locke, 1926, p. 51, 55).

SUMMARY This chapter has cited examples of the relation of the processes of sulfide oxidation and leaching to that of secondary enrichment at massive sulfide deposits in which the gangue is of low to moderate reactivity. All of these deposits lie in the arid and semi-arid areas of either Australia or the United States, and have been exposed to much longer than normal periods of weathering, on the order of 200 million years or more. All in Australia are located in areas that have water tables that are known to be seasonally stable and which are

assumed also to have existed at essentially the same level for millions of years. Three examples cited in this chapter illustrate the effects of cupric sulfate solutions in copper mines. At these mines the enrichment by cupric sulfate has penetrated to depths of from 110 to more than 200 feet in the zone of saturation (i.e. below the water table), but in other respects the extent and distribution of leaching and enrichment effects were quite different. In one (Home of Bullion), where pyrite exceeded somewhat the total copper minerals in the vicinity of the water table, leaching was strong and reasonably thorough, and there was left behind a conspicuous gossan which rests with fairly sharp contact upon the chalcocite zone below. In another (Great Cobar), little or no pyrite was present. Acid supply for the leaching was dependent almost wholly upon oxidation of chalcopyrite and pyrrhotite. This was adequate to affect deep oxidation and to leave behind a strong gossan, but was incapable of leaching the copper sufficiently to prevent much of it remaining marooned in the gossan as rich carbonate ore. Furthermore, the bottom of the gossan was serrated, and notable prongs of unleached chalcocite persisted for 50 feet, and of chalcopyrite for 30 feet, upward into it above the water table. By the same token, as a result of imperfect copper leaching of overlying material, the chalcocite ore below the water table was not outstandingly richer than the hypogene chalcopyrite ore, which it in part replaced. In the third (Mount Oxide), pyrite or other excess acid-yielding sulfide was so sparse above the water table (with reference to the oreshoot as it exists today) that only a small proportion of either the massive chalcocite or of its oxidized near-surface carbonate equivalent has been leached. The rich steely chalcocite body, built up through slow cumulative reprecipitation during denudation, consequently has been left behind unimpaired through a range of 260 feet above the water table, except for conversion of much of it above the 65-foot level to copper carbonate; and with both the outcrop and copper carbonate zone almost devoid of limonite, except for scattered thin coatings and stains. Three other examples indicate the effects of acid solutions of lead, zinc, and ferrous sulfate in other basemetal mines in Australia. All these deposits have been oxidized to form moderate to heavy gossans, and oxidation has extended to depths of from 157 to 457 feet. In all cases there were sharp cut-offs between oxidized and non-oxidized orcs, but in other respects the leaching and supergene enrichment effects again were different. In one (Mount Isa minc) the gangue is siliceous shale, with about 16 percent of intermingled dolomite. In the oxidized zone (about 162 feet deep), lead was stationary, while silver and zinc were very much leached near the surface. In the 50-foot transition zone below the water table, supergene pyrargyrite, supergene polybasite, and possibly supergene sphalerite made up 15 to 20 percent of the ore minerals. In the hypogene

EXAMPLES OF OXIDATION, LEACHING, AND ENRICHMENT PROCESSES

zone corrosion by acidic ferrous sulfate solutions, amounting to 6 to 0.1 percent of sphalerite and pyrrhotite, went down to about 385 feet below the water table, gradually diminishing until no signs of corrosion by acidic ferrous sulfate solutions were left. The pyrite and pyrrhotite exceeded all other sulfides by a ratio of more than 5 to 4. In another example (Mount Stewart mine) the gangue is slate and shale, with limestone lenses altered to epidote and other minerals. The lenses appear as a peripheral zone of hypogene and supergene zinc are around the main cluster of oxidized silver-lead-zinc oreshoots. In the main portion of the oxidized zone, lead was pretty well leached for 73 feet, then very rich ore occurred for another 30 feet, gradually diminishing until the water table was reached at the 157 -foot level. Below the water table there was a transition zone about 15 feet thick of mixed galena and pyrite; and in the hypogene zone, there was massive pyrite only. The other lenses at Mount Stewart carried hypogene and some supergene zinc that went 10 to 25 percent without much galena, and the pyrite was semi-massive with silica. The acidic ferrous sulfate solution in the transition and hypogene zones must have been derived from sphalerite and pyrite. In the hypogene zone, the sulfur of pyrite exceeds that of all other sulfides by a ratio of 3 to 1. In stilI another example (c. S. A. mine) the gangue is slate and shale only, very shattered, permitting thorough leaching. In the oxidized zone there was no mining until a depth of 435 feet was reached. From the 435foot level to the 457 -foot level, where the water level occurred, cerussite was heavy in places. Below the water table, in the transition zone, there were about 25 feet of chalcocite-enriched ore, averaging 7.13 percent

79

copper, and for about 18 feet below that there was a very moderate amount of supergene chalcocite. In the hypogene zone there must have been some acidic ferrous sulfate solutions acting on sphalerite and possibly on pyrite. In the hypogene zone the sulfur of pyrite exceeds that of all the other sulfides by a ratio of 2.8 to 1. The total sulfides in the hypogene zone made up 89 percent of the ore. The C. S. A. deposit consisted chiefly of pyrite, with some sphalerite, less galena, and still less copper sulfide. (Exploratory drilling by South Broken Hill Limited in the 1950s and early 1960s, indicated a body of chalcopyrite from 600 to 3,100 feet below the surface.) In none of the six examples cited above, has limonite been precipitated below the water table, if the normal fluctuations of the latter resulting from seasonal or cyclical variations in rainfall be disregarded (except at Mount Oxide and Mount Stewart; and there only slightly, along faults below the water table). Limonite has been precipitated within the zone of saturation (below the water table) at some localities however, as exemplified both by the occurrence at Mount Cuthbert, and by the conditions in areas of silica-breccia gangue (as opposed to the shale gangue) at the Mount Isa mine. In the former, limonite was precipitated as much as 250 feet below the water table, while in the latter it was precipitated to a depth of about 200 feet in normal non-fractured areas. It must be emphasized, however, that irrespective of the abundance or spectacular nature of some occurrences, limonite deposited below the water table constitutes only a very minor percentage of the total limonite in nature. In most districts, when present, it is insignificant compared to the amount deposited above the water table.

Chapter 14 INFLUENCE OF THE SULFUR-IRON RATIO AND THE HOST ROCK ON THE CHARACTER OF LEACHING PRODUCTS DISSEMINATED SULFIDE DEPOSITS Miami Leached outcrops over the disseminated copper orebodies at Miami, Ariz., principally in quartz-sericite schist, show a characteristic limonite product with thinwalled cellular structure of sharply defined angular pattern, similar to that seen in figure 41, chapter 22. The cellular structure may be coated in varying degree with minutely nodular "pin point" limonite crusts, or may grade insensibly into a mass of craggy, piled-up limonite particles more or less submerged within such cruststhe "relief" type of limonite. Irrespective of the product involved, the limonite is precipitated largely or wholly within the cavities from which the sulfides have been leached. At Miami the ratio of chalcocite to pyrite in the ore is approximately 2 moles to 1 mole. Limonite of similar type, precipitated mainly within the cavities that formerly contained sulfide minerals, is found in most quartz-sericite schist and monzonite gangues where that ratio of chalcocite to pyrite prevailed.

Tyrone Leached outcrops over the disseminated copper orebodies in monzonite at Tyrone, N. Mex., contain quite a different limonite product. At Tyrone the ratio of pyrite to chalcocite in the ore is approximately 2 moles to 1 mole, or just the reverse of that existing at Miami. Instead of occurring within the cavities as at Miami, the limonite is precipitated almost entirely about their edges as a replacement of gangue in the form of spherulites, very thin fibrous plates in mica cleavages, and irregular fibrous aggregates. Furthermore, the pseudocellular structure around the inner edges of the cavities represents mostly limonite crusts, rather than cellular boxwork formed along fracture or cleavage planes of the sulfide particle.

The Differences Explained The explanation of the differences in leaching products seen at Miami and at Tyrone, is that the total acidity present in a given outcrop is controlled to a great degree by the ratio of the various sulfide minerals present, and the type and location of the products precipitated is in turn governed by this total acidity.

Where the ratio is 2 moles of chalcocite to 1 mole of pyrite, as at Miami, the sulfur content yields a total acidity just high enough for all the copper and none of the iron to go into solution. Because of its ready solubility, all of the copper is exported in solution as cupric sulfate; but because of the comparatively weak acidity of the total solution, and the low solubility of iron, little or none of the iron is exported. Under these conditions the iron is precipitated within the cavity as indigenous limonite. Where the ratio is 2 moles of pyrite to 1 mole of chalcocite on the other hand, as at Tyrone, the large excess of sulfur maintains in the oxidation solution an acidity so high that hydrolysis is precluded. Consequently, not only is all the copper exported, but all or most of the iron as well. Iron normally is carried outward until the solution comes in contact with the feldspar neutralizer in the gangue, and there the neutralizer and the free acid react, and the iron is precipitated as limonite. But since weakening of the solution does not occur in this case, at least not to a great degree, until the solution has not only contacted but usually penetrated into the feldspar gangue, the precipitation necessarily occurs outside the cavity; which makes it a fringing product. Sericitization of the feldspar in the Tyrone monzonite on the whole is only moderate in certain areas. Because the sulfide occurs as small disseminated specks, each usually surrounded by a volume of gangue neutralizer many times greater than itself; and because decomposition of the sulfide occurs under semi-arid conditions with the oxidation necessarily proceeding slowly; the volume of acid produced at any given time is too small to enable it to percolate either far or rapidly. Its slow and restricted travel enables a gangue of only moderate reactivity to neutralize more or less completely the ironbearing solution, and to effect precipitation of all, or nearly all, of the iron as limonite usually within a fraction of or a few millimeters of the sulfide parent, as fringing limonite. However, since the neutralization in this case is neither rapid nor vigorous, fluffy limonite does not form. The ability of a mixture of 2 moles of pyrite to I mole of chalcocite upon oxidation to export all of the iron in solution thus expresses a theoretical ideal that is rarely if ever attained, even in a gangue of negligible neutralizing power such as quartz. Tunell has shown,

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INTERPRETATION OF LEACHED OUTCROPS

in fact, that for complete exportation of the iron to be effected, the pyrite-chalcocite ratio must be somewhat above 2 to 1 if the ferrous sulfate is all oxidized to ferric sulfate (see ch. 8); and under the conditions present at Tyrone, a sulfide mixture in which the ratio of pyrite to chalcocite is 2 to 1 yields a small amount of limonite crust, comprising finely nodular limonitic encrustations deposited inside the cavity's periphery, as in figure 41B, chapter 22; or occasionally a very little indigenous limonite. The volume of indigenous limonite, however, is almost negligible compared to the total limonite precipitated. Furthermore, although at Tyrone the ratio of pyrite to chalcocite approximates 2 to 1, only in rare instances do any of the sulfide specks yield precisely those proportions; in most cases the amount of pyrite is either slightly more or slightly less than the average. Some of the cavities thus may be wholly devoid of limonite while others may contain a greater amount than shown in figure 46B. But the number of cavities devoid of limonite is in reasonable balance with those containing indigenous crusts, and the amount of indigenous limonite, considered for the outcrops as a whole, is almost negligible compared to the amount of transported limonite in the gangue immediately about the cavities' edges. For purposes of interpretation, the Tyrone outcrops thus may be accepted as representative of a deposit in which, as a whole, a very close approximation to the ratio of 2 moles of pyrite to 1 mole of chalcocite exists. Similarly, because of the theoretical considerations discussed above, and as noted in chapter 8, pyrite in the Miami deposit must have exceeded slightly 2 moles of chalcocite to I mole of pyrite ratio, or a very small amount of chalcocite would have remained undecomposed. Since leaching of chalcocite at Miami is general and thorough, it follows that the pyrite does exceed the proportions shown. Repeated analysis of Miami mill heads during treatment of the supergene ore, however, showed the excess of pyrite over the theoretical 2 moles of chalcocite to 1 mole of pyrite to be small. Consequently, retention of the limonite in the cavities, under conditions present at Miami, may be accepted as representative of the ratio 2 moles of chalcocite to 1 mole of pyrite. In other words if, in dealing with gangue of moderate neutralizing power, the observer assumes a ratio of 2 moles of pyrite to 1 mole of chalcocite when he finds the limonite clustered closely about the cavities' edges with the' cavities themselves essentially free of them; or if he assumes a ratio of 2 moles of chalcocite to 1 mole of pyrite when he finds the limonite precipitated inside the cavities with the surrounding gangue essentially free of them; he will not err seriously in his interpretation. The reader must not get the impression, however, that every specimen picked up over the Miami or Tyrone orebodies shows the characteristics described. Both the permeability of the rock and the mineralization are remarkably uniform in the orebodies of the

two districts, as they are in many other porphyry copper deposits. Examples of pyrite may nevertheless be found only incipiently replaced by chalcocite in the sulfide ore at Miami; they are especially common around peripheries of the oreshoots, but may be found to some extent well within the ore bodies also. In the sulfide at Tyrone, on the other hand, examples may be found in which chalcocite exceeds the pyrite; and in some ores in this district the ratio may attain the 2 moles of chalcocite to 1 mole of pyrite ratio, or even exceed it locally. Those features necessarily are reflected in the outcrops from which the sulfides have been leached. Nevertheless, the patterns of limonite distribution as set forth for the respective districts are conspicuous and characteristic ones, and are so overwhelmingly predominant when applied to the deposits as a whole, that for purposes of interpretation they may be relied upon as representing closely the stated ratios for the respective districts. Since with the proportions of 2 moles of chalcocite to 1 mole of pyrite precipitation of the limonite proceeds indigenously without reaction of the solutions with the gangue, such precipitation ought to occur also when the oxidation takes place in a gangue of no neutralizing power. That it does so is attested by field evidence, not only in quartz veins of various districts, but in gangue such as the quartzite of the Utah Copper deposit (also see pIs. 8, 10; figs. 43, 44, ch. 22). On the other hand, with the proportions 2 moles of pyrite to 1 mole of chalcocite, precipitation of the iron as a fringing limonite depends so intimately upon the reaction of the oxidation solutions with the surrounding gangue neutralizer, that in a quartz gangue with little or no neutralizing power, the Tyrone pattern of limonite distribution with respect to the cavities is not duplicated. The iron in that case is carried well beyond the cavities' edges; and, when eventually precipitated at some indefinite distance from them, is set down as an exotic limonite product. The matter is discussed in greater detail in chapter 22.

Estimating Grade of the Ore Prior to Leaching If only gangues of moderate neutralizing power such as exist at Miami and Tyrone are considered, it is evident that by determining the percentage of voids or cavities in a rock left by leaching of the sulfides, and knowing from the type of limonite produced and the pattern of its distribution with respect to the cavities that in one case the ratio of the ore is 2 moles of chalcocite to 1 mole of pyrite, and in the other 2 moles of pyrite to 1 mole of chalcocite, the grade of the are represented by the leached outcrops may, in either case be calculated readily-quite as readily as if no leaching had taken place and an experienced observer were making a careful estimate of grade of the sulfide ore. The latter practice, estimating grade of the sulfide or other are visually, is common in most mines. As the

INFLUENCE OF SULFUR-IRON RATIO AND HOST ROCK

mine foreman or shift boss inspects the heading or stope face after a blast; he makes a visual estimate of the grade, especially if the ore be marginal; for upon that estimate usually rests his decision whether the material is to be sent as ore or waste. At many mines the geologist goes farther, and maps his estimate of grade as soon as the heading becomes exposed, checking it subsequently against the sampling results. The lower the economic cut-off for stoping a particular orebody, the more closely are grade limits usually defined in such estimates. In the porphyry copper deposits, estimates to within one quarter of one percent copper are common. The only difference, therefore, between the person interpreting a leached outcrop, and the geologist or mine foreman making his visual day-to-day estimate of the sulfide ore in the stope or pit, is that in the former case the outcrop first must be reconstructed to permit visualizing the limonitic products in terms of the parent minerals. Greater care usually is exercised in arriving at an estimate of leached material, because samplers cannot be sent in to verify the estimate, nor can a check be obtained when desired from the mill heads. As often a substantial outlay of capital may be called for to explore the deposit, it is customary to grade and map the copper or other ore content of the leached material a number of times before constructing the final map; the grading and mapping in each case purposely being carried out without reference to the grading previously done. In some of the early work of this type Boswell and the author classified and mapped the outcrops independently, later comparing results, and reclassifying jointly areas of discordant grading. When the classification is done by one geologist only, it is even more essential that a check be made upon all phases of the work to assure that no available evidence has been overlooked by him; and to assure, further, consistency in his classification of any section of outcrop made upon different days or different weeks. The matter resolves itself, however, into nothing more than reconstructing with the highest accuracy attainable the leached material in terms of the parent minerals; thereafter grading the outcrop for the soughtafter ore minerals as carefully as a visual estimate, with suitable checks for consistency, permits. It will be noted that any such estimate will be indicative only of the ore content as it existed at the horizon of the outcrop prior to leaching. Whether or not increase in grade may be expected through further accumulation of metal content derived from the leached zone lying between the outcrop and the horizon of supergene ore concentration beneath, is a matter that calls for consideration of other geological factors that must be determined and appraised in each case. For example, if erosion of the hypogene copperbearing porphyry through a vertical range of 1,000 feet has yielded at the horizon of the present surface supergene ore containing only 1 percent copper, the leaching of an additional 100 feet of such material below the

83

present outcrop manifestly will not enhance the grade materially. In many of the porphyry copper deposits the grade denoted by the leached outcrop may be accepted as virtually equivalent to the grade of the enriched chalcocite zone beneath. But obviously that does not apply in alI cases, and each deposit must be judged on its individual merit.

Difficulties Encountered Because of the sharply contrasting patterns of the distribution of limonite precipitates with respect to the cavities that formerly contained sulfides in the Miami and Tyrone districts, and because of the persistence and general uniformity with which the respective patterns are exhibited throughout the outcrops overlying the orebodies of the two districts, the reader may have gained the impression that estimating the grade of the ore leached from a given outcrop is a relatively simple matter. To dispel that illusion, a few complications may be cited. Suppose that, instead of the ratio being 2 moles of pyrite to 1 mole of chalcocite as at Tyrone, the ratio is 3 or 4 or 5 moles of pyrite to 1 mole of chalcocite. 1 In that case all the iron would be exported and an empty cavity would remain, the same effect as is seen when the ratio is only 2 to 1. Moreover, if sufficient active neutralizer were present in the gangue, all the iron likewise would be precipitated as limonite immediately about the outer edges of the cavities, the same effect as is seen when the ratio is only 2 moles of pyrite to 1 mole of chalcocite. But if both of these conditions are present, a much smaller amount of copper would be represented by a given percentage of voids in the rock than where the ratio is only 2 moles of pyrite to 1 mole of chalcocite, even though the arrangement or pattern of limonite distribution in both cases is nearly the same. This is especially true when oxidation of copper-bearing sulfides takes place in rocks rich in lime feldspars such as anorthite, the soda-lime feldspar, and the potash-soda feldspar, anorthoclase (ch. 12). It will be seen, from the considerations mentioned above, that the arrangement or pattern of limonite distribution with respect to the cavity does not by itself tell the whole story. Assume again, that instead of a pyrite-chalcocite mixture, a pyrite-chalcopyrite mixture is under consideration, and is present in the ratio of 1 to 1. In that case, under theoretically ideal conditions, all the iron would be exported, and an empty cavity would remain. The reason is that in the case of chalcopyrite there JIn the oxidation of a deposit of pyrite and galena in shale or feldspar-rich rock (ch. 10) in which the ratio is, for example, 5 moles of pyrite to 1 mole of galena, the oxidation of the sulfides yields an exotic smeary-crusted limonite derived from the pyrite. The oxidation product of galena is nearly insoluble (anglesite is secondary and cerussite is tertiary). The oxidation products of pyrite and chalcocite under the same conditions, on the other hand, are highly soluble, and copper is transported downward where it may bring about supergene enrichment.

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INTERPRETATION OF LEACHED OUTCROPS

exists a higher ratio of sulphur to copper than in the case of chalcocite (ch. 8); consequently less admixed pyrite or other sulfide is needed as an cxternal source of sulfur, in order to produce the same degree of iron exportation as is produced in the case of chalcocite.

Effect of the Sulfur-Iron Ratio It thus becomes evident that the arrangement or pattern of limonite distribution with respect to the cavity is a function primarily, not of the particular sulfides involved in the oxidation, but of the sulfur-iron ratio and the sulfur-copper ratio in the solutions derived from oxidation of the sulfides. In other words, if a eovellitearsenopyrite-pyrite mixture (an unusual combination of minerals that occurs at one place in Baja California, Mexico), upon oxidation yielded the same relative proportions of sulfur and iron available for the limonite-making reactions as was yielded by a chalcocite-pyrite mixture, or a chalcopyrite-bornite mixture, or a sphalerite-chalcopyrite mixture, or any other combinations of sulfides in the presence of the same type of rock, then essentially the same pattern of limonite distribution would be formed irrespective of the sulfides involved. Obviously, therefore, the conditions may become far more complicated than was the case at Miami or Tyrone. In order that the arrangement or pattern of limonite distribution with respect to the cavity may be translated intelligently into ore, it becomes necessary, in addition, that the nature and extent of neutralizer in the gangue or ground water be ascertained, its effect upon the oxidation solutions be fully assessed, and that identity of the sulfides or other iron-yielding minerals leached from the outcrop be satisfactorily established. Where leaching of the sulfides has been reasonably thorough, the limonite's source usually may be firmly established through recognition of the particular types of limonite produced, especially the indigenous types. It is for this reason that so much space is devoted to descriptions of individual limonite products, and why the whole of Part 2 has been set aside for defining the detailed physical characteristics that may assist in their recognition. The ideal case, of course, is one similar to that at Miami, where the ratio is 2 moles of chalcocite to 1 mole of pyrite, and in which: 1) all of the limonite produced is the type most outstandingly characteristic and the most easily recognized as being derived from chalcocite, 2) all, or essentially all, of the limonite is precipitated within the cavities, 3) each small sulfide speck is surrounded by a large area of gangue neutralizer, 4) just enough neutralizer is present in the gangue to precipitate as limonite immediately about the outer edges of the cavities any iron that may be exported where locally an excess of pyrite occurs, but still not enough to precipitate appreciable copper carbonate, copper sulfate, or copper silicate; so that all dissolved copper becomes available to be carried downward to the zone of supergene chalcocite enrichment below.

Under such ideal conditions, estimating the grade of ore represented by a leached outcrop is a comparatively simple matter. And when dealing with that type of deposit it has been possible to make close estimates of the grade of ore to be expected beneath the outcropsestimates that in numerous instances have been checked by later drilling, and by ordinary underground exploration, and found to be correct to 0.2 percent. It is only fair to state that most leached outcrops are more difficult to interpret than are those at Miami and Tyrone. The reason is that in most of them more factors are involved, both in the oxidation of the sulfides and in the formation of their Iimonites. Consequently more experience and greater skill are required in the interpretation. In spite of these factors however, it should be stated that even the complicated cases generally yield to persistent and determined investigation.

Ajo Influence of the Neutralizing Gangue. The influence of neutralizing solutions in the interpretation of leached outcrops may be important beyond their effects upon precipitation of limonite. At Miami and Tyrone the amounts of neutralizer in the gangue are so low that no appreciable dissolved copper was precipitated either as copper carbonate or as other oxidized copper minerals. Virtually all of the dissolved copper became available to be carried downward to the zone of supergene chalcocite enrichment below. The gangue at the copper deposit at Ajo, Ariz. is quartz monzonite, and no limestone or similarly reactive rock is present (see Gilluly, 1946). The quartz monzonite, however, contains so much neutralizing material that not only was all of the dissolved iron precipitated either as indigenous or fringing limonite, but all of the dissolved copper likewise was precipitated as copper carbonate or other oxidized minerals at, or immediately adjacent to, the point where the sulfide particle oxidized. This explains why the grade of oxidized ore at Ajo is the same as the grade of the primary ore; supergene enrichment did not take place. There has been a change from hypogene sulfide copper to supergene oxidized copper, without addition or subtraction of copper content. With the combination of Ajo's gangue and Miami's primary sulfide deposit, the secondary chalcocite orebodies at Miami could not have formed; and the Miami district, as a major copper producer, probably would not have become known to the world as a chalcocite district. With the combination of Miami's gangue and Ajo's primary deposit, and with an amount of erosion equal to that at Miami, thcre probably would have been produced a chalcocite orebody nearly rivaling that of the United Verde Extension (Arizona). From these considerations it is clear that although the ability to recognize the particular types of limonite produced by a given mineral is fundamental, the establishment of the character and amount of neutralizer in the gangue or in the ground water, and full assessment

INFLUENCE OF SULFUR-IRON RATIO AND HOST ROCK

of its effect upon the oxidation solutions, must receive at least equal attention before a given leached outcrop may be interpreted intelligently.

MASSIVE SULFIDE DEPOSITS Complex mixtures of various sulfides sometimes occur in oxidized massive deposits, causing greater difficulties in interpreting leached outcrops than was the case over the disseminated deposits where the sulfide speck or nodule is fully surrounded by gangue. Incidental descriptions of certain of the limonites derived from massive sulfide deposits have been already given in the preceding chapters; but the whole story has not been told. This section therefore, furnishes additional descriptions of the limonite products derived from massive sulfide deposits, and attempts in addition, to explain them in terms of the sulfur-iron ratio of the original sulfide minerals present.

Deposits in Shale and Feldspar-rich Rocks From what has been said regarding the feldspar or shale in the mixed massive deposits, it might be expected that in a mixture of, say, sphalerite-galenapyrite, or bornite-chalcopyrite-pyrite, or chalcocitesphalerite-pyrrhotite-pyrite, or some other mixture of sulfides, the solutions derived from oxidation of the different sulfides would mix to such an extent that none of the resulting limonite products characteristic of the individual sulfides would be preserved. To a limited extent that is true. It is frequently, though not invariably, true where the sulfides are finegrained and intimately intergrown-especially when pyrite is abundant. But field evidence shows that in massive sulfide deposits in which limonite derived from pyrite is about equal to that derived from the ore minerals, and in which the individual sulfide nodules are larger than 1/5-inch, in some cases down to 1/30-inch in diameter, oxidation within a given nodule commonly proceeds more or less independently of oxidation within surrounding nodules of different composition. If such a limonite mass is broken into and examined, portions of it often will be found to contain in place the limonite products characteristic of the particular sulfide nodules that have been leached. This is by no means an invariable rule, but holds sufficiently welI so as to serve as a general guide in field work. For example, in the North mine at Broken Hill, New South Wales, the pyritepyrrhotite content averages only 3 percent, whereas the ore contains about 20 percent each of galena and marmatite, all in an intimate mixture of grains that vary from 1/30 to 1/5 of an inch in diameter. In spite of this intermixing, however, the oxidized zone contains discrete cellular pseudomorphs characteristic of the sulfides from which they were formed (see fig. 65, ch. 26; fig. 75, ch. 27). Another example, but one in which more pyrite is present, is the orebody at Mount Isa, Queensland. The

85

ore in 1945 contained approximately 15.5 percent pyrite-pyrrhotite, 9.5 percent galena, and 15.5 percent sphalerite. The sulfide grains are about 1/20 to 1/300 of an inch in diameter and generally occur in separate bands from 1/10 to 1/50 of an inch in width, yet even here the oxidized portions of the orebody retain the characteristics of the various sulfide oxidation products. although minor mixing does occur along the borders of the bands. 2 A third example is the high-grade orebody at Silver Ridge, Queensland, where the sulfide ore contains 39.7 percent arsenopyrite, 8.3 percent galena, and 4.8 percent sphalerite, and 6.2 percent pyrite-pyrrhotite (sec table 7, ch. 20). The arsenopyrite generalIy yields very acid solutions, supplementing the effect of the pyritepyrrhotite. The oxidized minerals range from 1/ I 00 to 1/5 of an inch in diameter. Generally they were in roughly parallel streaks. Nevertheless the oxidation product of arsenopyrite in the banded structures is grass green to light green, while the oxidation products of other sulfides in the banded structures, especially those derived from galena, show an ochreous color where the relief limonite is conspicuous. Here also, there is some mixing of the various oxidation products along the borders. When the ratio of pyrite to all other sulfides is, say, 3,5, or 10 moles to 1 mole, the smeary crusts and other limonite types characteristic of oxidation under strongly acid conditions would tend to obscure or obliterate the types derived from the other less dominant sulfides to such an extent that the latter would not be present, or would not be visible in the outcrops in their true proportions. That is true especially for a ratio of 5 to 10 moles of pyrite to I mole of galena, 1 mole of chalcopyrite, or 1 mole of chalcocite, as explained previously. Nevertheless, in the leached outcrops that formerly overlay the massive chalcopyrite-chalcocite-bornite sulfide deposits of Sacramento Hill at Bisbee, Ariz., the limonites characteristic of these minerals were by no means obliterated, even though the ratio of pyrite to combined copper sulfides was, in certain places, 10 to 1. The same is true of the United Verde deposit at Jerome, Ariz., an area in which the gangue is mostly schistose quartz porphyry and black chloritic schist, with a diorite hangingwall. There the ratio of pyrite to combined chalcopyrite-chalcocite in some cases was greater than 10 to 1. Although no accurate estimate of the grade of copper ore leached from such outcrops would have been possible, enough evidence of chalcopyrite and chalcocite, and at Sacramento Hill enough evidence of bornite, was present in the leached outcrops to have demanded exploration on an extensive scale. Even in the Omeo Tin prospect at Herberton, Queensland (see fig. 23, ch. 18), in places cellular boxworks derived from oxidized chalcopyrite were clearly evident even though surrounded by oxidation products of pyrite. Of course, the chalcopyrite content "The gangue at Mount Isa contains 16 percent dolomitic minerals (ch. 13).

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INTERPRETATION OF LEACHED OUTCROPS

was moderately large in the occurrence at the Omeo Tin

been impregnated by pyrite (see pI. 18). The limonite

prospect, while the combined copper sulfides were small compared to pyrite at Sacramento Hill and at the United Verde. But although the distinguishing characteristics may be largely obliterated, few instances have been found in the field in which they could not be to some extent distinguished, under the hand lens, if a really careful search for them was made.

derived from the oxidation of pyrite also fills several fractures emanating from the pyrite. Figure 8 is a sketch showing the configuration of these products as they have been formed at the Republic mine. Seldom in the field work would any trouble be encountered in dealing with the cellular pseudomorphs, provided the individual sulfide nodule is upward of 1/5 of an inch (often upward of l/30-inch) in diameter, and provided the investigator takes into account the type of gangue, and provided he takes time to look

Deposits in Limestone Where limestone or other types of strong neutralizing substances are present, various types of limonite are formed, the type dependent mostly upon the vigor with which the acid is neutralized." Thus, fluffy limonite may be developed on the one hand or "soap" limonite on the other, but with a greater possibility of the development of types intermediate between these extremes. The deposition of limonite "soap" depends upon the flooding of the product by ground waters carrying silica and ferric oxide; the process takes a long time. Fluffy limonite depends upon free and rapid oxidation, and precipitation of ferric oxide hydrate with low content of silica from ground waters. Fluffy limonite has the same porous texture and physical appearance in all cases, irrespective of whether the iron that went into its formation was derived from pyrite, chalcopyrite, chalcocite, siderite, magnetite, garnet, or any other mineral that contained iron, as was said in chapters 1, 10, and 12. By itself therefore, fluffy limonite signifies nothing more than that an ironbearing solution of indefinite source came in contact with strong neutralizer. This fact would seem to make nearly hopeless the attempt to interpret limonites precipitated in porous limestone gangue in the mixed massive deposits. But the situation is actually not hopeless. Often there is preserved within the porous fluffy mass some structure of the original mineral that serves as a specific clue. Tn the case of siderite or of other iron carbonates the rhombohedral structure frequently is dimly or strongly preserved (see fig. 84, ch. 33). Often too, where in the mixed massive deposits the sulfide nodule is moderately large, the neutralizing effects of the lime manifest themselves only about the outer edges of the nodules, where the sulfide came in contact with the gangue. If the entire mass be broken open, the central portion frequently will be found to contain the limonite products characteristic of the particular sulfide that has been leached-just as so often is the case in massive sulfides found in feldspar or shale gangues. For example, in the Republic mine, Johnson, Ariz., are seen cellular pseudomorphs derived from the leaching of adjoining nodules of chalcopyrite and sphalerite in shaly limestone, together with adjoining massive jasper of the replacement type occupying the rock portion which had "Fluffy limonite, as a rule, would not be present to any great extent in the oxidation products of sphalerite or galena, because such oxidation products are tertiary instead of secondary products (ch. 10).

FIGURE 8. Sketch showing cellular pseudomorphs and other limonite products formed at the Republic mine. 1. Coarse cellular boxwork of chalcopyrite derivation. 2. Coarse cellular boxwork of sphalerite derivation. 3. Massive jasper of the replacement type, derived through oxidation of finely dispersed pyrite with which much of the affected area originally was impregnated to the extent of about 60 percent pyrite. 4. Post-sulfide open fracture, filled at least in part by nontronite, which has been largely replaced by jasper. 5. Nontronite residual. (The color is not white. Nontronite is a soft, clay mineral, sometimes fibrous or micaceous, straw to brown-yellow to faintly greenish, too dark for kaolinite.) 6. Ragged-edged massive jasper, presumably representing replacement of nontronite that formed through acid attack upon unmineralized shale and limestone. 7. U nmineralized shaly limestone. A small amount of pyrite well distributed through the chalcopyrite served to export sufficient of the latter's iron so that very little granular or pulverulent limonite was precipitated as cell filling (see pI. 18). Although the limonite derived from oxidized chalcopyrite is generally not as large as shown in the sketch, the oxidized sphalerite has yielded more flaky limonite in the specimen than ordinarily.

INFLUENCE OF SULFUR-IRON RATIO AND HOST ROCK

carefully at the limonites. For the investigator who does not know leached outcrop interpretation thoroughly, it might take a few days, or if the deposit were large a few weeks, to get most of the facts. Though perhaps taxing the investigator's ingenuity, he nevertheless would become conscious of the possibility of intelligent diagnosis. Even where the ratio of limonite derived from pyrite to all other ore limonites is 10 or 20 to 1 there are usually clues, despite the effects of strong neutralizing substances, if the ore minerals were formerly present in grains 1/5 of an inch or even 1/10 of an inch in diameter.

Deposits in Quartz-rich Rocks In areas of quartz (or barite) gangue, the lead sulfate or zinc sulfate derived from galena or iron-free sphalerite generally would be exported in solutions, whether or not pyrite was present, and irrespective of whether mixed massive or disseminated sulfides prevailed, provided that: 1) the ground water flowed continually over the ore minerals, 2) the sulfides were finely and intimately mixed, and 3) no copper was present. Galena would be very slow to oxidize in this case however, as described in chapter 10. The same lead sulfate and zinc sulfate would be exported in solution whether or not arsenopyrite were present; but if arsenopyrite were present and were oxidized, a faint greenish stain generally would be present, because arsenic compounds resist chemical changes. 4 Where massive sulfide deposits have been formed in highly kaolinized or highly sericitized gangues, the oxidation products of galena and iron-free sphalerite would be transported a certain distance from the source, depending upon the degree of kaolinization or sericitization, whether or not pyrite or arsenopyrite were involved, but there would generally be some precipitation of limonite within the adjoining gangue within several millimeters, leaving behind only smeary, thin, exotic crusts, without form or pattern. Earlier in this chapter (see also ch. 8) were described the oxidation products that would be expected in the oxidation of sulfide bodies consisting of various molar proportions of pyrite to chalcopyrite, chalcocite, and bornite. We 'In a rock of moderate or strong neutralizing power, ferric arsenate, copper arsenate, or other arsenates generally are stable (see ch. 20).

87

have observed many times in the field that these expected results have been realized, especially when the oxidized sulfides are fine-grained and intimately mixed in quartz-rich gangue. But when the oxidized sulfides are in large grains-say, 1/10 to 1/5 of an inch iIi diameter-the acid effects of pyrite, as a rule, would show only on the outer edges of the cellular box works characteristic of the other sulfides. Although there are some difficulties in interpreting the limonite produced in both quartz gangues and in gangues of great neutralizing power, those formed in normal shale or in feldspar-rich gangues are comparatively easy to interpret. In the oxidized portions of mixed sulfide deposits only patience is required, as a rule, to correctly determine the limonites present.

Width of Leached Outcrops Over Massive Sulfide Bodies Experience in examining a great many leached outcrops of the massive sulfide type has shown that such outcrops commonly are narrower at the surface than are the unleached sulfide bodies below. This is more especially true of the narrower lode deposits than of the wider and more irregular replacement type deposits; but the rule applies to some extent in most cases. Massive oxidized sulfide bodies frequently are lenticular; and it may be properly assumed that in some instances the accident of erosion has exposed the upper narrow portion of the lens. But field evidence shows that after lenticularity has been taken fully into account in the case of developed orebodies, where shape and position have been determined; and particularly in the narrower lode deposits which are not notably lenticular, the leached outcrops none the less show a contraction in width that often amounts to one-fourth to one-third (in case of the narrower lodes) the width of the sulfide body below. Several hypotheses have been suggested to explain the phenomenon, one being that lateral pressure existed in the leached outcrop, pinching it together at the surface. Whatever may be the proper or complete explanation the field evidence itself is well established. Altho~gh no rigid rule may be laid down, a fair probability therefore exists that the sulfide body below a leached outcrop of this type may be wider than the surface exposure of the leached outcrop.

Chapter 15 LIMONITE COLOR Probably no one factor has been responsible for more loose thinking and confusion in the search for ore than the attempt to use limonite color as a guide to prospecting. At the one extreme are instances of prospectors, attracted by the familiar color of the outcrop, who have tested the exposure to find important orebodies beneath. At the other extreme are the enthusiasts who talk glibly about selecting surface areas for the reddish color. If the matter were as simple as that, most of the Cambrian-age Bolsa Formation of southeastern Arizona would become favorable prospecting ground, for weathering of its ferric oxide content has stained the rock's surface a prominent and generally uniform red, even though usually only unmineralized quartzite is involved. Or, again, there would be a stampede of mining men to the red soil patches near Brisbane, Queensland, even though the latter represents only decomposing remnants of a largely-eroded basalt capping-choice land for horticulture, but barren of minerals that conceivably might constitute ore. The more practical prospector, with successes to his credit, often encounters reversals. In his particular district a given limonite color may have proved a reliablc guide to ore, and may have been found equally reliable in an adjoining district. When he goes farther afield and applies it to a third district, he may be disappointed. Even in the original district he is likely to become disconcerted when he finds the particular color that has served him so unerringly, failing him as he seeks to use it in the outlying areas. This raises questions concerning: 1) to what extent color is a significant factor, and 2) to what extent, unconsciously, the successful prospector may have been guided by other less conspicuous physical characteristics of the outcrop,-such as a specific limonite type,even though not consciously recognized by him.

THE COLOR ANALYZED When all colors are reflected back fully an object appears white. When a large portion of the light is absorbed, without anyone color being absorbed more markedly than another, the object appears gray. When all colors are fully absorbed, so that no light is reflected back, the object appears black. A blue object absorbs red, orange and yellow, and scatters blue together with some green, indigo and violet. A yeIIow object absorbs blue, indigo and violet, and usually throws back some green, orange and red; etc. White light penetrates only slightly into an object, as the result of internal reflection and refraction due to

the irregularities, it emerges again. The best method of iIIustrating this phenomenon is to take a piece of brilliantly colored glass and crush it to a fine powder; the powder appears white. The crushing creates a large amount of new surface area, at each face of which a certain amount of surface reflection takes place, so that the light is no longer able to penetrate to a sufficient depth in the substance for marked absorption to take place. If the powder is wetted with water, or better, with an oil of the same refractive index, the color is nearly or wholly restored. Adsorbed and capiIIary water makes various minerals light and fluffy. In 1940, at Wollorerang, Northern Territory, Australia, a prospector was shipping copper ore, chrysocolla (CuSi0 3 .2H2 0). The author took some samples, believing it carried 4 or 5 percent copper because of the faded light-blue appearance. When the specimens were analyzed, they were found to contain 18.7 to 22 percent copper. The adsorbed and capillary water had reflected about 35 percent of the light, giving the chrysocolla nearly the same appearance as the brilliant colored glass crushed to a fine powder. Many minerals are iridescent, which means that light falling on them is reflected in rainbow-like arrays of colors. Of these the ferric oxide hydrates often found as crusts in sulfide areas are of importance in connection with the evaluation of leached outcrops. Iridescent limonite crusts constitute a special class of the exotic dark, smeary nodular limonites. They have been correlated mostly with semi-massive to massive pyrite. IndividuaIIy the nodules rarely exceed 1 or 2 mm in diameter, and always occur in clusters, which in turn rarely exceed 3 or 4 inches in maximum area. The surfaces of the nodules usuaIIy are thickly coated with minute pin-point precipitates. The iridescent colors range from the various green and blue shades to distinctive reddish, orange, and golden tints. The darker shades predominate. The iridescence arises from the interference of light rays, which penetrate exceptionally thin layers, amounting to mere films-only about 0.001 millimeter in thickness-of essentially pure ferric oxide or ferric oxide hydrate precipitate.

EARLY INVESTIGATIONS OF THE SIGNIFICANCE OF LIMONITE COLORS Early in the leached outcrop investigation it was noticed that outcrops over most of the disseminated chalcocite-pyrite orebodies of southwestern United

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INTERPRETATION OF LEACHED OUTCROPS

States possessed a distinctive deep maroon to seal-brown color. The proportions varied: in one district sealbrown equaled or exceeded maroon; in another, maroon was predominant. The seal-brown color always was prcsent, but often had to be looked for closely, and in some instances was so obscure that when viewed at a distance of 10 or 20 feet only the maroon color was discernible in the outcrop. About this time Tunell made field investigations of the leached outcrops at Tyrone, N. Mex. (1922), Morenci, Ariz. (1923), and Bingham Canyon, Utah ( 1924); and during the next few years carried out extended and detailed microscope and laboratory investigations of the specimens collected. He concluded (early 1926) that color varied according to whether the product was indigenous or transported. He expressed the opinion (see Locke, 1926, p. 115) that correlations of colors in the hand specimens were as follows: Mainly indigenous hematite _________________________________ maroon Mainly transported hematite _____________________________ brick red Mainly indigenous goethite ___________________________ deep brown Mainly transported goethite ___________________ yellowish brown Mainly transported jarosite _________________________________ yellow These color correlations had been foreshadowed by somewhat vague and in different districts apparently opposite observations, partly recorded in the literature and partly reported locally. The ores and cappings referred to were in numerous cases reexamined in the field and laboratory by Tunell in the light of the mineralogic knowledge yielded by the microscope, and it appears probable that the observations were correct, while the apparently opposite conclusions arrived at were due to the statements of the observations in terms of two color designations where three colors were really present. Thus Ransome (1919, p. 165) stated that at Ray and Miami, Ariz., " ... a deep and conspicuous redness of the surface is less propitious than a rather subdued tint of rustiness." Blanchard (in Locke, 1926, p. 114) from studies in Silverbell, Ariz.; Santa Rita and Tyrone, N. Mex.; Morenci, Ariz.; Plumas County, Calif.; and various places in Sonora and British Columbia, concluded that in the field the capping over known disseminated ore usually has a deep maroon to sealbrown color and that the bright red to brick-red cappings have nowhere been observed except over pyritic areas where copper content of the enriched zone, as a whole, averaged much less than 0.5 percent copper. He concluded further that not all of the low grade pyritic areas are overlain by brick-red cappings, but the brickred capping has nowhere been found over actual ore ground. E. M. Sawyer (private communication) found that at Tyrone, N. Mex., reddish-brown cap pings are underlain by ore in more cases than yellowish-brown cappings, and Paige (1922, p. 39) made the same statement for this locality. Tunell's observations are that capping containing hematite as a replacement of gangue has a brick-red color and is correlated with pyritic waste; capping containing hematite and goethite in the

cavities left by the sulfides has a maroon to seal-brown color and is correlated with ore or at least with protore in which the proportion of copper sulfide to iron sulfide is high; capping containing goethite and jarosite as replacements of gangue has a yellowish-brown color and is correlated with pyritic waste. In a given district the unfavorable capping is for the most part all of the brick-red type or all of the yellowish-brown type. The confusion appears to have been caused by the fact that single observers did not happen to sev both types of cappings over waste.

LIMITATIONS ON LIMONITE COLOR AS A PROSPECTING GUIDE The conclusions reached by Tunell marked an advance in the understanding of limonite color, as regards cappings over the disseminated chalcocite-pyrite deposits not only in the three districts named, but in various others of the porphyry copper districts. But when extended beyond those districts the conclusions proved inadequate in a number of respects, and various inconsistencies appeared in other districts. Among observed conditions which indicate that these conclusions should not be regarded as handy, foolproof guides for general application are the following: 1. In one district of disseminated deposits in the southwestern United States, although the marooncolored limonite with its undertone of seal brown has proved to be a dependable guide to enriched chalcocite ore in alaskite porphyry, in the adjoining coarse-grained but otherwise mineralogically almost identical alaskite it was found to be the characteristic limonite color over very low grade pyrite areas. 2. In at least three of the porphyry copper districts, molybdenite has been found to yield a fine cellular boxwork identical in color with the dominant maroon which characterizes the capping over most of the disseminated chalcocite-pyrite orebodies; but in general ferrimolybdite (see ch. 28) is canary-yellow to straw-yellow, though maroon or red in places in other districts. 3. Goethite, both in massive and disseminated deposits, ranges from vivid orange-yellow to dull ochreous and chocolate brown; especially in either indigenous or transported limonites derived from galena, molybdenite, and sphalerite bodies. 4. Galena, molybdenite, and sphalerite sometimes yield oxidation products with maroon or reddish colors, indicating supergene hematite; though usually they tend to orange-yellow to dull ochreous and chocolate brown. 5. Fresh indigenous limonite of bornite derivation, if cellular, commonly occurs as goethite of a vivid orange-yellow to orange-ochreous color, which, once observed, often makes the spotting of bornite easy. But fresh indigenous limonite derived from galena quite as commonly is precipitated as goethite, with nearly the identical vivid orange-yellow to orange-ochreous color.

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LIMONITE COLOR

Only the distinguishing characteristics of their cellular boxworks makes it possible to differentiate between the two; and the cellular boxwork of galena is not easy to identify because the cubic cleavage pattern is uncommon. 6. Brick-red limonite, though usually thought of as a pyrite derivative of low grade portions in disseminated chalcocite-pyrite districts, has been found, both in porphyry and in limestone, to be a direct derivative of copper carbonate. Yet in the same district and in the same gangues the copper carbonate derivative may range from vivid orange-yellow to a product resembling maple sugar in both texture and color. 7. Chalcopyrite derivatives, both as cellular boxwork and granular cell filling, commonly have an ochreous color. Yet in some districts the color of such products ranges from orange to deep Indian red to purple-red; and that, too, in districts where chalcopyrite is exceptionally free from admixed pyrite. 8. In at least one of the disseminated porphyry copper districts (Silver Bell, Ariz.) jarosite has been observed as an indigenous derivative from chalcopyrite which, under the microscope, disclosed no pyrite admixture. It has been observed along the low grade fringes of other chalcopyrite deposits under conditions in which the field relationships pointed convincingly to its derivation from copper minerals rather than from pyrite. It also has been observed in a number of lead districts in close association with other limonite derivatives of lead minerals, though not to date under conditions which exclude positively the interference by solutions derived from oxidizing pyrite. 9. Jarosite, although generally yellowish in color, ranges from ochreous to dark brown in some transported limonites. 1 'However, Tunell has observed massive jarosite altering to hematite. B. S. Butler has found jarosite (yellow) abundantly in limestone country in southwestern United States. D. F. Hewett found yellow "cauliflowers" of jarosite 10 to 20 feet across, outcropping in the midst of alum-bearing, chalky, decomposed rock 25 miles southeast of Las Vegas, Nev., which he considers surface concentration by evaporation (Locke, 1926, p. 107). It would appear that jarosite, granting an available supply of potassium in the ground water, might form more readily from decomposition of chalcopyrite than from decomposition of pyrite. The fact that the field relationships more often point to pyrite as the parent, suggests that the presence of copper in the solutions may be to some extent a deterrent, even though not a complete inhibitor to the formation of jarosite. In Ajo, Ariz., where orthoclase (potassium-rich) feldspar occurs, chalcopyrite veinlets, about 1 inch thick, with no or very little pyrite, on oxidation yield indigenous, not cellular, limonite, which is very red in color, and no jarosite.

SUMMARY Color in limonite is determined largely by: 1) mineralogy, 2) size of the limonite grains, 3) state of aggregation of the limonite particles, and 4) amount of adsorbed and capillary water present in the limonite. A red limonite ground sufficiently fine becomes ochreous; ground still finer, it becomes yellow. A great many other conditions, such as acidity of the solutions, rapidity of neutralization, or adsorption (in the case of well kaolinized gangues), affect grain size (as the preceding chapters have indicated), and govern the amount of water that enters into the composition of limonite. For these reasons constancy of limonite color, either in the same or in different districts, is not to be expected. The oxidation products of chalcopyrite and sphalerite, and to some extent galena and molybdenite and possibly other sulfides, are partly brown and ochreous and to some extent red in the disseminated deposits. However, in the arid climate they have a way of turning purplered in massive deposits, provided the limonite is surface coalescent limonite (see ch. 16. Also see pis. 5 and 15); but this is unusual. The conclusions which must be drawn are: 1. No limonite color may be depended upon as being representative of a specific supergene ferric oxide or ferric oxide hydrate mineral, although of the limonites observed in nature reddish limonites most commonly point to hematite; the orange-yellow-buff-ochreous combinations usually point to goethite, and to some extent jarosite. The chocolate and dark-brown colors usually point to goethite, especially when not fresh. The yellow limonite is usually jarosite, but not always. 2. In a given district (and to some extent in several districts containing the same general type of deposit) a particular limonite color may be characteristic of a specific sulfide; or particular blends of colors, such as the maroon with seal-brown undertone, may be characteristic of specific sulfide combinations. But caution is needed in the use of color even when such a relationship has been established, because the same color or blends of colors may not represent identical parentage throughout a given district (as in alaskite porphyry and alaskite in one district). 3. Even in a single district or deposit a given mineral may yield limonites of widely contrasting colors. 4. In the restricted circumstances under which color becomes useful in establishing the limonite's parentage, it remains at best subordinate to other physical characteristics which serve in the identification.

Chapter 16 STANDARD TYPES OF LEACHING PRODUCTS This chapter contains general descriptions of the more common types of limonite products that have been found both so widespread in occurrence and so distinctive in form and pattern that they may be looked upon as standard types. The limonite derivatives that characterize the individual minerals considered in Part 2 are described in terms of these types. Although the modes of occurrence of limonite have been described as either indigenous, fringing, or exotic, according to the system established in chapter 3, it must be re-emphasized that limonites derived from particular minerals are, at certain times, found in other than the normal mode of occurrence, and that the subtype of limonite does not, in itself, determine its classification as indigenous, fringing or exotic; limonite derived from a particular mineral is at times found in more than one mode of occurrence, even within the same outcrop. Moreover, in some cases, the same subtype may be either indigenous, fringing, or exotic, although usually the subtypes fall under the types in the manner indicated in the outline in the Contents. The author's chief concern is that the ultimate significance of each limonite type and subtype be understood, the three broad types-indigenous, fringing, and exotic-serve mainly as a flexible framework of classification that the reader may adapt to his particular situation; it is not intended as a rigidly systematic presentation of all leaching products thus far discovered. No two geologists visualize the various limonite products of an outcrop in precisely the same manner. Each therefore will wish to modify, and if desirable, amplify the following descriptions so as best to serve his individual need or preference. The main consideration is that he possess a mental image of some underlying framework that will make the individual leaching products individually distinguishable, yet permit adaption of the framework to meet local conditions and the addition of as yet undescribed types.

MAINLY INDIGENOUS TYPES Cellular Pseudomorphs The essential characteristics of the cellular pseudomorphs, and of the cellular boxworks and cellular sponges, were given in chapter 5. The following pages describe in further detail the products produced by the pseudomorphic replacement process. Cellular Boxworks Hypogene Boxworks. Boxworks formed within sulfide bodies sometimes are hypogene in origin. In such

cases, they consist usually of milky or glassy quartz, but may be composed of calcite or other mineral. Often such hypogene invasion conforms to the cleavage or fracture pattern which characterizes the sulfide or other mineral that subsequently undergoes leaching (see for example fig. 39, ch. 21; also figs. 72, 73, ch. 27); but in other cases they are guided by fractures that were formed through stresses set up during cooling of the deposit, and which do not necessarily coincide with the particular mineral's cleavage or its inherent fracture system (see for example fig. 71, ch. 27). Although this type of boxwork generally survives leaching when composed of quartz, it is less reliable for identification purposes than the supergene product because of the tendency for the boxwork pattern to follow any type of fracturing system present in the rock. When hypogene and supergene patterns are essentially identical in major outline, and when one forms a continuation of the other, the parentage of the hypogene product may be regarded as having been sufficiently well established to serve as a criterion in leached outcrop interpretation; and usually only then. Boxworks of hypogene origin rarely are other than coarsely cellular; though sometimes they may be moderately fine (see fig. 72, ch. 27; also fig. 39, ch. 21). Boxworks of supergene origin, on the other hand, range all the way from coarse to fine. Similarities and contrasts between hypogene and supergene boxworks have been emphasized in order that the significance of the hypogene product may be grasped when the product is encountered. In volume, however, hypogene boxworks are quite negligible. When present, the hypogene type nearly always represents the frayed-out extensions of larger veinlets or splashes of quartz or other mineral, and rarely persists outward from them for more than a few inches into the sulfide mass. In many districts it does not exist at all. Supergene Boxworks. The supergene product thus is the one upon which the observer must rely almost wholly for his interpretation; and the discussions bclow have reference to it alone. In the formation of supergene products, chalcopyrite boxwork occurs very frequently-the most frequently of all the cellular limonites; pyrite, which yields much acid, gives rise to little of the cellular boxworks, as was stated in chapter 2. Considerable variation is seen between the cellular limonites derived from chalcocite, pyrrhotite, galena, and other sulfides. Supergene boxworks may be coarsely or finely granular. The classification is arbitrary, arranged for convenience in field use. As defined in chapter 5, in

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INTERPRETATION OF LEACHED OUTCROPS

coarsely cellular boxwork the cells are more than 1.5 or 2 mm in length and width; in finely cellular boxwork the cells are less than 1.5 mm in diameter. The cell walls usually become thinner and more fragile with decrease in cell size, frequently ending up as tiny webs or tendrils feeling their way uncertainly into space. The reason is readily apparent when it is recalled that supergene boxwork or webwork normally continues to form until the last residual fragment of the parent mineral has been consumed through oxidation. The thickness of cell walls in supergene boxworks varies also with the parent mineral. With certain sulfides, such as chalcopyrite, it is invariably thicker than with others such as galena, granting similar cell size and composition. The thickness also may vary with the nature of the country rock; box work derivatives of chalcopyrite, for example, normally have greater cell wall thickness where the gangue is limestone than when it comprises siliceous shale or schist. For any given mineral, the larger cells generally have thicker walls than have the smaller ones; and generally, too, the grid formed by the longitudinal and cross ribs becomes less firmly knit with decrease in cell size. But this latter rule is not absolute, and does not apply in some of the derivatives of sphalerite (see figs. 66, 71, ch. 27). In the calcite derivatives, a rigid grid sometimes prevails irrespective of cell size (see fig. 82, ch. 32). Certain minerals-mainly those with a strong tendency toward cleavage-quite consistently yield boxworks of a given pattern that prove invaluable for purposes of interpretation. Without exception the boxwork derived from gangue carbonate preserves the rhombohedral (or scalenohedral) form. Cubic boxes found in the oxidation products of lead deposits usually point to galena, more rarely to pyrite, and, to an even lesser degree, to magnetite. The boxworks derived from some minerals do not adhere rigidly to a specific angle, but the angle nearly always is obtuse; for example this is the case with chalcopyrite. For still others both obtuse and acute angles may be present; yet, combined with other criteria, the angle formed may serve in establishing the parent. In a few cases the angular pattern is so lacking in regularity that it resembles hieroglyphics. The hieroglyphic pattern, when supported by confirmatory criteria, itself often may be used in identifying the mineral that has been leached (see pI. 15). In a few cases abnormal shapes are linked closely with specific minerals. Contour boxwork, within which in varying degree the cell walls are cross-connected, but for which the overall appearance nonetheless yields the impression of a contour map of steeply mountainous country, has been observed only as a derivative of tetrahedrite (see fig. 50, ch. 24). Foliated boxwork, lastly, has been observed only as a derivative of molybdenite (see fig. 77, ch. 28). Cellular Sponge Hypogene Sponge.

Hypogene sponge usually invades the sulfide for only a fraction of an inch or

millimeter, if at all. Occasionally hypogene sponge has been observed in sphalerite (see fig. 71, ch. 27). Supergene Sponge. Supergene sponge may form in two varieties: 1) thin-walled sponge, in which the thickness of the cell wall is less than the diameter of the cell cavity; 2) thick-walled sponge, in which the thickness of the cell wall is greater than the diameter of the cell cavity. The thin-walled variety of cellular sponge is formed in a manner similar to that of the supergene cellular boxwork; the only differences being that: 1) the parent is granular in texture, and usually of medium to coarse grain size; and 2) the cellular sponge forms by "eating" its way irregularly around borders of the grains instead of along the more nearly straight lines of cleavage or fracture planes. Because of the grains' irregular borders, the cell walls, especially the thinner ones, often have a more crinkly structure than those of the boxworks; and for derivatives of certain minerals the crinkly structure is more marked than for that of others. But straight cell walls in sponge structures are by no means unknown, just as crinkled walls are present to some extent in boxworks. Because the cellular structure "eats" its way irregularly around granular material, however, the cellular sponges are not formed, as in the boxworks, in longitudinal ribs that pursue their way in a straight line past several cells. Cellular sponge is classified as coarse or fine, using the same limiting dimensions as in the case of boxworks. Sponge, however, rarely forms after grains less than I mm or more than 4 to 5 mm across, except from bornite and pyrrhotite; the general range lies between 1 to 3 mm, so that cell size for most of the thin-walled sponge structures lies within or close to the intermediate range. In some cases cell size is essentially uniform over a distance of many inches in a specimen (see fig. 74, ch. 27). More often however, change in cell size is abrupt over distances of only a centimeter or two. The reason may be either: 1) uneven supply of silica and ferric oxide hydrate for the formation of limonitic jasper over the full period of decomposition represented by a given specimen, or 2) unequal resistance to decomposition in different parts of the sulfide mass in question, resulting in portions thereof oxidizing and crumbling too rapidly for the leisurely process of cellular pseudomorph formation to manifest itself throughout. Unfortunately, grain shape usually does not vary sufficiently among the medium to coarse minerals for any of such minerals consistently to yield a distinguishing pseudomorphic cellular shape. In a few districts individual sulfides have proved sufficiently distinctive in grain pattern or crystal form for the purpose; but only rarely so. In various districts, too, the larger sponge cells have been found to carry within them fine cellular boxworks that proved helpful in the identification. At Broken Hill, New South Wales, the sponge type of cell wall-formed along the boundaries of large grains of decomposing galena-became filled in some instances

STANDARD TYPES OF LEACHING PRODUCTS

with crystalline cerussite during oxidation of the residual galena kernels. As the cerussite weathered, its structure in turn was preserved pseudomorphously as limonite. If and when present, such structures are of much value in identifying the parent mineral. But as they are not of general occurrence in any given deposit, their field of usefulness in interpretation is limited. In most cases where the thin-walled cellular sponge is found, identification of the parent mineral depends upon other criteria. Coalescing intergrowths of cellular boxwork and cellular sponge often are derived from leaching of mixed sulfides, and only rarely is one sulfide either exclusively crystalline or exclusively granular, so that one yields boxwork exclusively and the other sponge exclusively, as was said in the discussion of leaching products of massive sulfide deposits as described in chapter 14. This is true especially in massive sulfides. Even if the crystalline and granular forms are mutually exclusive as to sulfides, the crystalline form does not necessarily yield cellular boxwork only. Criteria other than the cellular patterns thus become important for correct interpretation. Discussion of such criteria is given more appropriately and fully in Part 2. The points to be emphasized here are: 1) because of general lack of individuality in pattern for the parent mineral when it occurs in granular form, interpretation of thin-walled cellular sponge is more difficult than the interpretation of cellular boxwork; 2) notwithstanding that difficulty, other identifying characteristics frequently exist which aid in the interpretation; 3) most of the cellular sponge structures thus may be interpreted with reasonable accuracy by an experienced observer. The second, or thick-walled variety of cellular sponge-that in which cell wall thickness exceeds diameter of the cell cavity-is more readily related to its source; for in process of formation it has been observed only as a derivative of loosely granular pyrite undergoing rapid oxidation. Under such conditions a pseudomorphic cellular pattern would have little chance of forming, because the sulfide would oxidize and crumble away too rapidly. But, in addition, this type of sponge has been observed to form only in an environment where the ground water possessed moderate neutralizing power. The dissolved iron consequently is not exported from the deposit, as in the exotic limonites, even though the parent is massive or semi-massive pyrite. It may be transported varying distances, depending upon the rate of attack, variation in vulnerability to decomposition of different portions of the pyrite mass, and rapidity of neutralization of the oxidizing solutions. But in most cases the iron is precipitated within a few millimeters, or at most centimeters, of its sulfide source, as a fringing limonite. The fact that the neutralizing power of the ground water is only moderate usually precludes formation of fluffy limonite. On the other hand, since neutralizer is present in substantial amounts, neither are there formed the glossy and smeary crusts which so often characterize limonite precipitated from strongly acid solutions. The

95

final product thus becomes a limonite somewhat resembling in lightness and porosity exploded popcorn, with its texture, however, much less uniform (see fig. 17, ch. 18). Cellularity in the thick-walled sponge derives from the fact that decomposition of loosely granular pyrite does not proceed uniformly. Irregular channelways for ground water circulation become established, and anastomose through the mass. As the dissolved iron is precipitated along these channel ways as a result of neutralization of the sulfuric acid, a limonitic aggregate, of variable thickness from place to place and lacking all semblance of a formal pattern, gradually is built up, punctuated throughout by gaping, irregularly round holes or "cells" of unsymmetrical size and distribution, which correspond to positions of the last oxidizing pyrite residuals. Because thick-walled cellular sponge is formed from decomposition of massive, loosely-granular pyrite in which oxidation proceeds too rapidly to permit the iron being precipitated as a direct pseudomorph, yet with enough neutralizer in the ground water to overcome the acidity and compel the iron's precipitation, usually before the iron has traveled more than a few millimeters from its source, the limonitic mass, in the broader sense, remains indigenous to the pyrite body that gave it birth, even though all of the individual limonite particles constituting it may be, and usually are, themselves slightly transported products. The process of limonite formation in this case might be visualized as a mass of loosely granular pyrite disintegrating and crumbling through rapid oxidation, much as a crowded city of brick and stone, built along the sandy bank of a river, might crumble as flood waters encroached to disintegrate its sandy basement. Just as the brick and stone of the buildings would crumble into largely shapeless heaps of rubble which nonetheless would correspond in some measure to positions in the landscape occupied by them when they stood as buildings, so does the iron of the distintegrating pyrite mass remain as shapeless limonite aggregations or accretions, precipitated for any given area in the general vicinity of its sulfide source, and with the overall wreckage of distintegration remaining on the whole indigenous to the area originally occupied by the parent sulfide body. The analogy is far from perfect, but may assist the reader in visualizing how, from a shapeless limonitic ruin of this type, wholly lacking in pseudomorphic pattern, there nonetheless may be reconstructed a fairly reliable picture of conditions prevailing prior to oxidation, provided the processes governing its formation are understood. Although generally classed as an indigenous product, thick-walled cellular sponge, or more exactly, a product practically indistinguishable from it, has been observed as an exotic precipitate, deposited several hundred feet from the oxidizing pyrite body at a point where strongly acid solutions entered shaly limestone country rock. It is obvious that in this case the pattern is not governed by solution channelways anastomosing

96

INTERPRETATION OF LEACHED OUTCROPS

through the granular pyrite occurrence, but by the manner in which small particles or "dabs" of limonite become aggregated into porous masses during their precipitation. Their aggregation into even a crudely pseudo-cellular structure, if it occurs, is wholly a matter of chance; and in general the product more nearly approaches, and is more appropriately classed as, an exotic limonite. In rare instances, however, the two products have been found so similar in appearance that, without the field association as a guide, they could not be differentiated from each other. In most cases such an occurrence, of course, may be readily identified in the field either as indigenous or exotic. The small amount of silica that normally is carried by ground water of moderate neutralizing power, and which, when present, nearly always is precipitated to some extent contemporaneously with precipitation of ferric oxide hydrates, in either case imparts to the final product a rigidity it otherwise would not possess, and enables it often to withstand severe weathering attack. As in the case of the impregnated kaolin type of limonitic jasper discussed in chapter 6, thick-walled cellular sponge could be conceived as forming under a specialized set of conditions in which the strongly ironbearing acid solution was not necessarily derived from pyrite. For example, arsenopyrite would yield such a product under some conditions. But thus far it has not been observed forming under conditions other than those noted for pyrite; and acceptance of its derivation from any other source, especially when it occurs in quantity, therefore would seem to call for rather conclusive field evidence. Among the cellular limonite sponge structures, the thick-walled variety, ironically enough, is the only one that ordinarily may be related directly to its source without additional confirmatory evidence; and this despite the fact that no cleavage, crystal form, characteristic fracture pattern, or other pseudomorphic outline has been preserved within it.

Flaky Crusts Flaky crusts are the most common form of cell filling in the cellular pseudomorphs, although they are not pictured in the limonites (except in fig. 26, ch. 19), because they are too complex. They may be composed of nearly pure ferric oxide hydrate, or may be siliceous enough to constitute limonitic jasper, but generally consist of colloidal gelatinous limonitic material. In the siliceous varieties, the silica content is substantially lower than that of the cell walls (ch. 2, table 1 nos. 6, 9, 11 and 15). The flaky crusts may be observed to best advantage in the zone which is sometimes saturated in wet seasons and at other times partially dried out. In this zone there is usually an abundance of sulfide. An underlying assumption is that the sulfide is one that yields iron, such as pyrrhotite, chalcopyrite, or bornite, yet is low enough in sulfur to permit retention of at least a portion of its iron indigenously. Pyrite obviously is ruled

out because all of its iron would be exported in solution, except when moderately strong neutralizer is present. As oxidation penetrates more deeply into the sulfide residual, and the gelatinous material in the plastic stage emerges more definitely from the zone of sustained saturation into one of open air circulation, it gradually dries out and shrinks. Because of their thinness (usually 0.005 to 0.1 mm) the entire crusts tend to curl up into flaky shape; hence the name flaky crusts. As result of contraction the surfaces of the flakes often become shriveled, and assume an appearance like that of cornflakes. During dehydration, the gelatinous material still is in the plastic state, and contraction may be from center toward the wall of the cell. In that event the flakes, lacking specific form or pattern, may attach or "glue" themselves at random to the cell wall. Or the contraction may be away from the wall. In that case either individual flakes settle indiscriminately at the bottom of the cell, or several flakes may merge, before their complete dehydration, into a loosely-joined, airy structureless assemblage that remains unanchored within the cell cavity. It will be observed that only rarely do attached flakes join end to end. Usually the attachment, either to the cell wall or to each other, occurs along the broad curved surfaces of the flakes (see fig. 26, ch. 19). This almost feathery attachment imparts to them a loose and structurelcss airyness, and makes them readily distinguishable from the more rigidly connected and more siliceous cellular boxworks or cellular sponges.' Even among them the thin, serrated, up-curved edges tend to crumble readily, so that only the thicker central portions are likely to be preserved permanently. Probably in few cases, even where sheltered from open weathering, is there preserved in flaky form much more than half of the material that evolved as such originally; the 'Locke, (1926, p. 125), noting the occasional mergence of fine cellular boxwork from the solidifying gelatinous limonitic material, was of opinion thaI the boxwork somehow might have evolved from the latter's dehydration, though he arrived at no final conclusion in the matter. Such a boxwork, however, even though of paper thinness, preserves its angularity, and the cell walls persist unmistakably as extensions along a formal pattern instead of constituting a loosely-connected, heterogeneous assemblage of curved flaky surfaces. Even in the case of cellular sponge pesudomorphs, the latter's close and persistent interconnecting walls serve to differentiate it readily, in most cases, from the flaky assemblages. Both cellular boxwork and the cellular sponge pseudomorphs thus remain quile distinct in appearance fram the flaky crusts derived from gelatinous limonite; and no confusion between the two broad classes of limonite need arise if their respective manner of formation, and the criteria for mutually distinguishing them, are kept in mind. The two often evolve nearly side by side; in some cases the boxwork may precede formation of the flaky crusts by less than a millimeter. Wherever the finely cellular boxworks or sponges of pseudomorphic pattern exist, they serve as additional anchorages or receptacles for preserving as indigenous limonite any of the flaky crusts that have evolved in their vicinity, and thereby assist greatly in rendering the interpretation more complete and accurate.

STANDARD TYPES OF LEACHING PRODUCTS

rest is floated away by ground waters over a long period of time. The siliceous cells thus are the only ones that normally have a long geologic life. An important point to keep in mind is that both the process of sulfide decomposition and the dehydration of the gelatinous mass into flaky crusts-and subsequent distintegration of many of the latter, other than the more siliceous ones, into tiny loose angular fragments or limonite granules-may proceed simultaneously within any given cell. Flaky crusts may be attaching themselves to cell walls, or forming small unanchored assemblages at the top of the cell, while the sulfide remains undecomposed at the center or near the base of the cell only a milimeter or two below. Furthermore, because of the long period normally involved in decomposition of a given residual of sulfide in the semi-arid or arid regions, and because of the tendency toward fluctuations in composition of the ground water during that period, both the siliceous variety and the purer ferric oxide hydrate varieties of flaky crusts arc likely to form within a single cell, and to grade into one another. It is, in fact, the frequent presence of the flaky crusts in minute amounts that accentuates the tendency, in time, of all except the more siliceous varieties to crumble. All the sulfides but pyrite form the flaky or shriveled limonite; however, there is some variation among the different sulfides. Pyrrhotite forms especially good flaky limonites (sec fig. 26, ch. 19). Rosette Limonite. The cellular pseudomorphs sometimes contain groups of flakes in a rude rosette pattern. A flake in the process of emergence, up-curved about its edges, may be still semi-gelatinous at its center. That portion may become "glued" to a freshly-formed, up-curving flake during dehydration of the gelatinous horizon next below; and so on. It is this condition which from time to time enables the flaky particles to precipitate successively one upon another to form tiny, crudely shaped rosettes that adhere to the cell wall and project therefrom into the interior of the cell. A number of sulfides yield rosettes, especially sphalerite; but the rosettes usually do not show unless fresh surfaces appear, and even then they can easily be brushed or shaken loose unless the limonite is very carefully handled (see figs. 66, 67, eh. 27).

Granular Limonite Limonite granules derived from sulfides may emerge into their final state as unattached unit grains from a pin point to I ml11 across; or a number of grains that are unsymmetrical in form may be "glued together" to make up the final limonite product." Often they coat the surfaces of cell walls, as in the oxidation products of bornite, chalcopyrite, sphalerite, and other sulfides. But they do not have the struetureless airiness of the flaky crusts. They may range from hard, sandy granules, 'Granular Iimonites may be exported; but usually the fringing or exotic limonites are radiating and fibrous in form, and are not true granular products.

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often contammg co-precipitated silica capable of scratching soft glass, down to fine earthy material. The earthy varieties generally settle to the bottom of the cell as a pulverulent mass. Granular limonites may be resinous, submetallic, dull, or earthy. The submetallie, dull, or earthy varieties are common, they need not be described. Resinous luster, on the other hand, is observed to some extent in aggregations of limonite particles, glued together with silica, gypsum, and other substances. The more pronounced type of resinous luster, shown in figure I R, chapter I R, is far more glistening, with dehydration during denudation, bringing the limonite to the surface with interstitial aluminum-silicate rock in conjunction with oxidized pyrite from overlying material, "sintered" through the sun's heat, whipped out by mechanical forces in arid regions (see eh. 6). Not only docs the "gluing" or coating of the surface-hardened particles of the resinous limonite close up the cellular spaces, but the resinous limonite is baked in a thin film as a semiglazed product, almost as desert varnish. Certain minerals decompose indigenously in part or in whole into compacted masses (sec figs. 53, 54, eh. 24). But interpretation of such compacted masses is limited strictly to cellular pseudomorphs, and even then usually calls for corroborative field evidence. The granular Iimonites of non-sulfide origin are important too, though not as important as the sulfides (see for example pI. 19; fig. 80, ch. 29; fig. 82, ch. 32; figs. 83-85, ch. 33; figs. 88, 89, ch. 34; fig. 90, ch. 35). The granular limonites arc generally eroded away in the course of time and are hence less useful as guides than boxworks or sponges. The author recalls that in surface workings in the tropics, the limonitic pseudomorphs of pyrrhotite and sphalerite were distinctive enough to rule out a prospect from an economic standpoint. But the limonite derived from a certain dark-brown, non-sulfide mineral was troublesome, because the author had not seen limonite derived from this mineral previously. By digging down 12 feet, however, he found granular limonite replacing garnet as the cellular pseudomorph of the granular variety. When the non-sulfide cellular pseudomorphs prevail, the granular limonites of epidote, wollastonite, garnet, chlorite, and ferro-magnesians or other minerals have their characteristic limonites, often distincti ve. The ferro-magnesians, with their dark-brown limonite in place, yield a distinctive limonite that contrasts very much with limonites derived from copper or lead or zinc sulfide in the disseminated areas, because it has a non-sulfide, dead look about it (see figs. 101, 102, App. B). Sometimes limonites derived from garnet, spodumene (Li"O.Al cO ..• 4SiOJ, or other lithium minerals, or other nonsulfide gangues have a special appeal for the mineral producer. But usually the nonmetallic miner in temperate zones only has to go down a few inches or a few feet until he finds what he is looking for.

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INTERPRETATION OF LEACHED OUTCROPS

Fluffy Limonite

PARTLY INDIGENOUS AND PARTLY

Fluffy limonite derived from pyrite is fully described in chapter 12. Here only the indigenous types will be described, not the fringing or the exotic limonites. In the case of fluffy limonite derived from limestone and dolomite (see fig. 22, ch. 18), the limestone and dolomite usually make up the gangue in which sulfides occur. The rapidity and vigor with which calcium carbonate or other strong neutralizer reacts with acid determines the nature of the limonite formed. Pyrite, since it yields much free acid, reacts rapidly. When sulfides of elements such as copper, lead, and zinc are present, they tend to react less rapidly. Fluffy limonite is formed, as was said in chapter 12, under conditions of: 1) free though not necessarily rapid oxidation; 2) porosity sufficient, during the oxidation, to permit fluffing of the limonite particles; and 3) low silica content, or at least low precipitation of silica from the ground water. Fluffy limonite is the lightest and most porous limonite found in nature. The distance traveled before precipitation is directly proportional to the effective neutralizing power possessed by the rock (see ch. 12). When neutralization is slower, the precipitates may have a more fine velvety texture than the more porous limonite formed from pyrite. When base metals are present some remnant of "key" cellular structure or other distinctive characteristic of the parent mineral may be preserved, either embedded within or emerging from the fluffy mass, to indicate and assist in tracking down the limonite source. But fluffy limonite tends to modify the cellular, fringing, and exotic structures that would have formed from the parent mineral under normal air-water oxidation processes alone, and increases the difficulty of correct interpretation.

FRINGING LIMONITES

Hard Pseudomorphs Other mainly indigenous types of limonite yielded by pyrite in gangues of strong neutralizing power are the hard pseudomorphs, cubes, or limonite "dice."" In this product there is little evidence that a single granule of limonite has shifted even a fraction of a millimeter from the point at which it was derived from the sulfide. Replacement of the sulfide is by limonite particles so minute, and so densely compacted together, that the resulting pseudomorphs are almost as firm and solid as the original sulfide itself. With few exceptions they are found in limestones or limy shales, and have not yet been observed elsewhere than well above the water table; usually within 10 or 20 feet of the surface in semi-arid and arid regions. The hard pseudomorphs constitute only an infinitesimal fraction of all cellular pseudomorphs. Chapter 12 describes the "hard pseudomorph" limonites. Figure 24, chapter 18, includes a picture of pseudo-cubic limonites. 3The hard pseudomorphs are not limited to cubic shapes of pyrite; but the cubic shape is particularly common (see ch. 12).

Limonites that are in part indigenous and in part fringing are usually found with gangues of moderate neutralizing power such as shale, schist, and feldsparrich rocks, seldom with gangues of great neutralizing power such as limestone.

Relief Limonite The term "relief" limonite is usually applied only to limonite formed by oxidation of disseminated sulfides. It may be applied to some oxidized products of massive sulfides when one or two oxidized sulfides are involved; but when limonites are derived from massive mixed sulfides-from chalcopyrite, sphalerite, and pyrite, for example-they tend to form the fringing or exotic types, and generally lose their individual characteristics. Relief limonite in disseminated bodies is a fibrous product, although the fibers generally are only visible under magnification. The term was coined originally to distinguish in the field between: 1) limonite products which, in the hand specimen or under the ordinary hand lens, stood out in bold relief, like objects in a stereoscopic slide, though not as fluffy as in limestone; and 2) those which had a flatter or dead appearance (see ch. 1). Examples of relief limonite are: 1) craggy particles in disseminated districts, derived from chalcocite-pyrite mixtures of low acidity, which stand out within the cavity to a greater or lesser degree as the "heaped-up" velvety aggregates discussed below; and 2) fine nodular radiating fibrous crusts and spongy interiors, generally with glossy lusterous surfaces. Both types are compacted to the walls of oxidized disseminated sulfides; but they are compacted on one side only, leaving the other side porous. The radiating fibrous limonites are more porous than the craggy particles. In general, in disseminated deposits the oxidation of chalcocite and pyrite proceeds side by side-the decomposing pyrite furnishing the surplus acid needed to put the chalcocite wholly into solution. But different mixtures of the sulfide constituents yield distinctly different limonitic end products. In both the craggy limonites and the radiating fibrous limonites, all gradations-from unoxidized sulfide to its limonitic residue left behind after leaching-have been observed, so that the evolution could be followed closely. Relief limonite is by no means restricted to chalcocite-pyrite derivatives. It forms from various sulfides, including bornite, chalcopyrite, or other sulfides possessing the proper sulfur-iron ratios. But low pyrite is essential; otherwise, when much free acid is generated, fringing or exotic limonite products result. Craggy Limonite. An example of small craggy particles of limonite derived from a mixture of chalcocite and pyrite in approximately the ratio of 2 moles of chalcocite to 1 mole of pyrite is shown in figure 45, chapter 22. The end product has an irregular, craggy,

STANDARD TYPES OF LEACHING PRODUCTS

porous, "heaped-up," velvety texture, and does not occur as rounded, solid, compacted grains. In the Warwick Castle mine in the Cloncurry district, Queensland, where approximately 2 moles of chalcocite occur to 1 mole of pyrite, and where the oxidation products of the sulfides occur in up to 20-foot widths in the quartz, the "heaped-up" velvety texture still is retained. The "heaped-up" limonite particles are not necessarily craggy. They may be made up of waferthin webwork, intermittent and faltering in outline, often emerging as mere hair-breadth wisps or threads of jasper that form the vanguard for the advancing cellular or sponge limonites, then again imperceptibly merge into the broader and more heterogeneous craggy limonites. The boxworks derived from mixtures of chalcocitepyrite as shown in figures 41 and 42, chapter 22, are an extreme type, whereas the craggy particles shown in figure 45 are of the opposite type. Both represent the products of leaching of ore that contained approximately 2 moles of chalcocite to 1 mole of pyrite. Generally the "ghost" structure is distinct enough to reproduce, in a discontinuous and wavering manner, the typical pattern of longitudinal and cross ribs characteristic of chalcocite that has been leached in the cellular pseudomorphic structure. Nevertheless in the craggy limonite many breaks occur in continuity of the wisps or threads of jasper, and the final limonitic residue is at best a poor skeleton of the structure in the chalcocite. Radiating Fibrous Crusts. The second type of relief limonite-the less compacted nodular precipitate with frequently porous texture and spongy interiors-is encountered to a greater or lesser degree in nearly all semi-arid to arid regions where sulfide bodies have oxidized. It forms along courses or channels traversed by acid solutions, and thus points to oxidizing pyrite as its source. It crumbles in part under a light blow from the pick, or under firm thumb pressure. Limonite masses of this type rarely attain diameters exceeding half an inch (see fig. 16, ch. 18). The smaller nodules, from a pin point to 1 or 2 mm across, are of much more general distribution than the larger nodules. They, too, frequently are exotic; but when they occur in minutely nodular coatings as pseudomorphs, or as cavity linings in cappings over the disseminated chalcocite-pyrite ore (or ore containing other sulfides with low excess sulfur), they are classed as indigenous because the iron has not been exported beyond limits of the cavity formerly occupied by the sulfide or sulfides (see pI. 8). Usually both the craggy particles and the radiating fibrous crusts are found together in the cavities, because they were derived from chalcocite-pyrite mixtures of approximately the same ratios, 2 moles of chalcocite to 1 mole of pyrite. The craggy particles are tiny, and each is set down individually as a unit. Craggy limonites such as this take a long time to precipitate in quantity. The radiating fibrous crusts on the other hand apparently formed more rapidly. Thus in some cases the

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craggy particles were overrun by and incorporated into a finely nodular radial fibrous crust. Also in some cases the two evolved concurrently, with the craggy particle engulfed, off-center, as an undigested inclusion in the radial structure. In either case the nodule's structure at such place lacks firm compaction, and proves vulnerable to disintegration under weathering attack, often with simultaneous development of a small, irregularly hollow or honeycomb core or segment. It may become so porous and spongy in extreme cases and after prolonged weathering, that only a few of the outer layers remain solid, and the crust, as such, may constitute little more than a limonitic "blister." When the ratio is 1 mole of chalcocite to 1 mole of pyrite, the stray craggy particles precipitate as tiny nodules upon whatever exists in the immediate vicinity; but the overwhelming radiating fibrous crusts inundate them, often two or three deep. In many cases the underlying "heaped-up" aggregates of stray craggy particles, though retaining their original shape, become completely obliterated by the finely radiating fibrous crusts. By breaking open the nodular mass, the obliterated craggy particles become visible. In such cases the iron, to some extent, moves out of the cavity to produce spherulitic and fibrous limonite around the cavity as replacement of gangue-but not too far, being a haphazard limonite. When the pyrite-chalcocite ratio increases to 2 to 1, a limonite of the type shown in figure 41 B, chapter 22, is formed. The radiating fibrous crusts still show somewhat inside the cavity, but the limonite is very spongy. Outside of the cavity the limonite, replacing gangue, forms spherulites, very thin plates in mica cleavages, and when the sericite is highly altered, irregular fibrous aggregates. This is the fringing limonite. Mixtures of chalcocite and pyrite were used as examples in this instance, but bornite-pyrite mixtures (ch. 8) or mixtures of other sulfides possessing the proper sulfur-iron ratios would do as well. Arborescent Limonites have commonly formed from arsenopyrite and chromite. The arborescent limonites constitute a special class of relief limonites. Limonite derived from chromite is described in chapter 29. Derivatives of Arsenopyrite-Pyrite Mixtures. Arborescent limonites, derived from mixtures of arsenopyrite with pyrite have a dry, crinkly appearance, but as a rule they are not cellular. They consist of porous, crinkly masses made up of loosely-aggregated but firmly-joined granules, with characteristic blunt orthorhombic crystal form and corroded edges. In the decomposition of arsenopyrite-pyrite mixtures both granular limonite and scorodite (Fe203oAszO"o4H20) are formed, and both become inextricably intermixed in deposition, although some tendency exists for scorodite to segregate. The mixtures are resinous to subresinous in luster, and are built up largely of small branching clusters or knob-like projections. The projections occur in unsymmetrical shapes 2 to 3 mm in height, their thickness being usually less than their height. The projections are disposed indiscriminately

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INTERPRETATION OF LEACHED OUTCROPS

toward one another with total disregard for pattern or orientation, giving rise to highly porous, shapeless aggregates. The masses are bound together by minute intergrowths of limonitic jasper, which makes the products clinker-like, and gives them high rigidity (see fig. 33, ch. 20). In rare instances the projections of scorodite take the form of slender fragile excrescences, with one grain after another perched precariously upon the preceding, the microscopic equivalent of a column of blocks in a child's nursery; but in general the more stubby type of projection prevails." It is noteworthy that in oxidized material exposed for a long period to weathering, if a knob or other excrescence composed largely of scorodite granules is attached to a limonitic jasper base, it tends to corrode or perforate the jasper by "eating" holes into it at the point of contact. Attachment of the arsenate granule rarely is so firm in its "gluing" to surrounding granules as is that of the ferric oxide grains to each other. This not only increases the porosity of the granular mass as a whole, but often causes it to collapse. Some of the collapsed projections are carried away by mechanical erosion; others tend to "glue" themselves discordantly to the underlying mass. Although scorodite persists to some extent in the leached derivative under the most severe weathering, it is slowly replaced by "limonite" nonetheless. Under a magnification of 20 to 30 diameters the initial attack upon an individual scorodite granule appears spotty, somewhat resembling the skin of a boy with large and excessive freckles; thereafter the limonite laboriously spreads over most or all of the scorodite grain. When an entire cluster or knob-like projection becomes thus affected, the shriveling may bring about compaction until the original granule stands out like a tiny stalagmite (see pI. 2; fig. 34, ch. 20). With continued weathering most of the scorodite-Iimonite mixture alters finally to a brownish-black product, with occasional Indian-red to copper-red limonitic patches, and occasional apple-green ferric arsenate stains resembling the malachite-green coatings of weathered native copper. In granular fretwork, more of the oddly-shaped, slender, airy, fragile scorodite excrescences are present, compared with the stubby clinker-like patterns. Presumably, this is because oxidation products of arsenopyrite exceed those of pyrite. These oddly-shaped, fragile structures commonly encrust themselves upon the walls of the limonite, and often cause the more delicate projections to collapse even more than in the case of the stubborn projections. Others have narrow 'A variation is the frequent growth. at irregular intervals through the mass, of isolated, minute, hollow, green to pale olive hemispheres. Less frequently the shape is semi-elliptical to bluntly concave. The diameter of the hemispheres rarely exceeds 1 or 2 mm, and under XIO to X20 magnification the growths resemble tiny scooped-out green half-oranges, with walls or shells composed of micro-granules of scorodite free from limonitic admixtures. The hemisphere structure crumbles more readily upon exposure to weathering than the underlying intergrown scorodite-limonitic material. This tends to accentuate the arborescent pattern of the total mass, but at most constitutes only a subsidiary effect.

bases, expanding in cross section in their central or upper portions, somewhat suggestive of an inverted pagoda, though generally in granular fretwork these are not very important. Frequently their granular arborescent projections adjoin each other so closely, and merge with one another at enough points of contact, so as to give to the mass the appearance of a cellular sponge, Closer inspection never fails to reveal the framework made up of granule assemblages in arborescent form. Many of the slender, fragile excrescences are so small that they become distinctly visible only under 10 diameters or greater magnification. Sometimes the cellular pseudomorphs develop; but rarely. Although always unsymmetrical in shape, the oddlyshaped, porous, fragile limonites rival in delicacy and beauty of outline the most fantastic of snowflake crystals. When the delicate scorodite growths become limonitized by weathering, the snowflake structure contracts notably, and becomes much more compacted and globular, as snowflakes do when contracting into neve. Despite the changes above noted in the feldspar or shale gangues, the process of limonitization has never been complete for specimens approaching hand size; for no instance has been noted in which the arsenic, present as scorodite, has been wholly leached, or otherwise completely removed by weathering, from an outcrop whose parent sulfide carried several percent arsenopyrite. In quartz-rich gangues, usually the scorodite is empty of all sulfides; but the faded apple-green ferric arsenate shows, to some extent. Disseminated Sulfide Type. If the total amount of two sulfides in the outcrop is well below 10 percent, and either they are disseminated or occur as narrow seamlets so that a large area of neutralizer in the rock is adjacent to each small area of sulfide, the leached derivatives still largely retain their identifying characteristics; but the limonitic jasper may give way to a more pulverulent or fluffy type. This applies particularly if the gangue is rich in feldspar or is a limy shale. Such mixtures of leached derivatives, possessing little or no limonitic jasper to act as binder, disintegrate more readily under weathering processes, and in outcrops subjected to long erosion the derivatives may lose most of their identifying characteristics. An ore body of Wiluna, in the East Murchison goldfield of Western Australia, affords an instructive example. Prior to exhaustion of this and associated ore bodies in 1947, the operation had yielded 1,871,000 ounces of gold (Edwards, 1953, p. 215). At Wiluna the run-of-mine ore carried approximately 2 moles of arsenopyrite to 3 moles of pyrite. Although the sulfides are concentrated or aggregated within the oreshoots in varying degree, much sulfide also occurred minutely disseminated through a finely brecciated calc-schist gangue of effective neutralizing power. The gold and sulfides occurred in chalcedony and carbonates that cemented and partially replaced the breccia. Thus at the surface it was often difficult to distinguish, either with the unaided eye or under the ordinary hand lens, which of the leached products has been derived from

STANDARD TYPES OF LEACHING PRODUCTS

arsenopyrite and which from pyrite. The matter was of economic significance, because at Wiluna the gold was associated almost wholly with the arsenopyrite," and the areas of dominantly pyritic material have little or no prospecting appeal. An external feature, however, assists in the identification; many arsenopyrite disseminations are bluntly acicular, whereas those of the pyrite are granular or to some extent cubic. In Wiluna, the acicular disseminations are favorable for gold.

Partially Sintered Crusts Galena and bornite are the only sulfides that on oxi-dation yield partially sintered crusts in any important amounts. Galena especially, gives rise to the partially sintered crusts in many cases, as mentioned in chapter 10. Bornite yields moderate amounts of such crusts, depending upon the boxwork structure. Both are principally indigenous, but the fringing variety sometimes extends a millimeter to two beyond the boxwork. These crusts usually are found in feldspar-rich rocks, shale, and sometimes in quartzose rocks. Most of the oxidation products of galena consist of pseudomorphic replacements of cerussite (more rarely anglesite; see ch. 10) by limonite. Partially sintered crusts usually are made up of coalesced limonite in surface outcrops. They resemble in appearance the surfaces of dead-burned magnesia bricks but are slightly rougher (see ch. 10). They occur in clusters which give a haphazard appearance, and which subsequently may be sintered by weathering processes (see figs. 60-62, ch. 26). Galena is slow to oxidize because of the formation of a coating of the highly insoluble sulfate, anglesite. 6 Bornite, in contrast to galena, is deficient in sulfur, and the triangular boxwork or sponge is the most conspicuous oxidation product derived from it. The boxwork or sponge grades off from triangular structure, through the velvety, relief limonite, into small, compact, haphazard aggregates or dabs of partially sintered limonite, within which the cellular structure may be wholly oblit"H. H. Carroll, general manager of Wiluna Gold Mines Ltd., at the time (1941), has kindly made available the following figures, which are of general interest wholly aside from their relationship to the leached outcrop studies: Approximate Gold-Arsenic Ratios in Wiluna Ore Ratio of the Pennyweight of Gold per Ton of Ore to the Percentage of Arsenic

290-foot level... 450-foot level.. 625-foot leveL .. SOO-foot level lOOO-foot level 1200-foot level... 1400-foot level.. 1600-foot level ISOO-foot level.. 2400-foot leveL 2700-foot leveL

8.47

7.53 5.17 3.17 2.94 2.12 2.94 2.35 2.47

2.23 1.30

6Below the water table galena, sphalerite, and bornite are easy to oxidize, especially when copper is involved (see ch. 11 and 13).

101

erated. But the haphazard aggregates are more solid than the partially sintered crusts derived from galena when fresh. Oxidized pyrite generally is involved when haphazard aggregates prevail. Perhaps a crude idea of the appearance of this type of aggregate may be obtained if the reader imagines himself looking through an inverted field glass at the side of a swallow's nest, and assumes that the nest is made up of small aggregates of partially sintered granules instead of small dabs of mud. That of course represents the extreme type of partially sintered crusts. Usually more or less webwork, or fine cellular structure, emerges through the crusts, as projecting corners and edges of boxes protrude from a tailings pond in which the boxes are becoming submerged. These projecting cells generally reveal some form of the triangular boxwork or sponge patterns, just as the protruding cellular structure in the corresponding product of galena reveals the characteristic cubic cleavage boxwork or sponge derived from it. Determination of the origin of partially sintered crusts in either case is ordinarily not difficult for the experienced observer (see fig. 49, ch. 23). But to avoid confusion the following points are emphasized: 1) The partially sintered crusts derived from galena commonly occur as coatings of cell walls, are loosely aggregated as a rule, and rarely extend halfway across individual cells when fresh. 2) The crusts derived from bornite often obliterate nearly all of the cellular structure, and the granules are closely aggregated, one cluster encroaching upon another, especially when some oxidized pyrite is involved.

Pyramidal Boxwork All pyramidal limonite observed has been found to be pseudomorphic after galena and related to the cleavage. The structure is cubic in outline, partly preserved with a matrix coated with partially sintered crusts derived from cerussite itself derived from galena. Erosion has removed most of the pyramid apexes, but the "mica plate" structure is preserved in part. It has been observed only in outcrops exposed to weathering and partial erosion under sheltered conditions, not, to date, in exposures underground. Limonite plates in this boxwork are rigidly parallel to the "mica plates," and are firmly held together and give rigidity to the structure (see figs. 63, 64, ch. 26). But partially sintered crusts derived from galena are sometimes found adjacent to the pyramidal boxwork, and these are the fringing limonite of the oxidized galena. The boxwork has been observed at Tintic, Utah; Eureka, Nev.; Aravaipa, Ariz.; in the San Javier region of Sonora, Mexico; Broken Hill New South Wales; Lawn Hill, Queensland; and in' numerous other lead districts.

Surface Coalescences A form of limonite sometimes encountered is the semi-glazed product formed through coalescence of limonite particles exposed to the action of the sun, wind, and rain at the earth's surface. In these products

102

INTERPRETATION OF LEACHED OUTCROPS

of prolonged exposure to the elements the limonite particles tend to fuse together. The layers rarely attain a thickness exceeding a quarter of an inch, and may be detected through chipping the gossan or capping to obtain, at a few inches or less of depth, fresh material protected from direct exposure to sun and wind. They usually occur in semi-arid to arid regions. All types of limonite, except massive jasper, are subject to such coalescence; but, because of the effect of climatic conditions on the surface, and because of the differences in resistance of iron oxides, silica, and other constituents that make up the limonites, some types are more easily altered to it than others. The coalesced product, though often decidedly crusted in appearance, should not be confused with true limonite crusts. The latter term is reserved for limonite precipitated as crusts at the time of their formation (the crusted structure being inherent in the precipitate), and which, in any given case, retain their individuality as specific limonite types whether occurring at the surface or elsewhere. Subsequent coalescence or compaction of some portion or all of the product through exposure to the elements, if it occurs, merely alters superficially the appearance of the product without changing it fundamentally or producing a new limonite type. It is well known that under the influence of heat and pressure, solids coalesce below their heat of fusion. A common example is the sintering process,7 in which substances coalesce much below their melting point, with weight due to gravity as the pressure. Day and Allen (1905, p. 31-59) showed that sintering of powdered glass occurs several hundred degrees below the melting point of crystals of the same composition. The process occurs earlier with increasing fineness of the powder and with more gradual application of heat. This suggests that the finer grained limonites exposed to the elements, especially to the heat of the sun and to the wind, in time may become hardened or glazed over by coalescence of the solid particles. It suggests further that such surface-hardened limonite should be developed most extensively in hot, desert regions; which in fact it is. The coalescence itself drives off much of the adsorbed and capillary water. Impurities, common in limonite, presumably assist the process, since impurities usually lower the sintering or fusion points. Although fine-grained limonite particles tend most readily to fuse in surface coalescence to a semi-glaze, the more rigid forms of massive cellular pseudomorphs likewise do so under conditions such as those prevailing in the southwestern United States and the interior desert plain of Australia. Plate 5 shows a rigid limonite which is semi-glazed; while plate 6 shows a limonite in which the cellular pseudomorphic structure is preserved. The 'Partially sintered crusts (see galena and bornite) are different. They are porous, they have a haphazard appearance, and they go down to the bottom of oxidation, instead of coalescing to a semi-glazed limonite at the surface. But the oxidation products of galena and bornite too might be coalesced near the surface.

experienced observer usually recognizes, in the hand specimen, ghosts of the cellular structure preserved in even the most thoroughly fused of such material. He would in any event chip, or if necessary trench, the rock so as to expose fresh material beneath the surface.

EXOTIC TYPES The exotic Iimonites are those precipitated from ironbearing solutions that have moved so far from their source of iron that the source no longer can be identified specifically. In extreme cases the iron travels very far, whether it be of sulfide or non-sulfide origin; for example, such far-traveling varieties as limonites along drainage ditches, and other man-made passages (see ch. 3 and 6). On the other hand, the replacement jasper, of which "soap" makes up a considerable mass, even though exotic, may be translated with some degree of assurance, under favorable conditions, into terms of the source material. Replacement jasper is not discussed here, because "soap" was fully described in chapter 6. Ordinarily the limonites of the exotic types are found near to their source, especially those derived from sulfides, but far enough distant to make the parent of the limonite wholly unidentifiable. In general, the history of iron relationships of exotic limonites can not be satisfactorily appraised; and the product can be related back to its source only vaguely, through a chain of assumptions whose validity decreases with the length of the chain.

Granular and Coagulated Limonites Most granular precipitates, formed through reaction of acid solution with the country rock, are exotic limonites. They are widespread in nature, and are not necessarily related to decomposition of sulfide minerals. Only as an indigenous product, such as cell filling of the cellular pseudomorphs, can such granular precipitates usually be related back to their source with confidence. Usually, in the exotic limonites, the oxidation of sulfides supplies more iron than the oxidation of nonsulfides, the reason being that non-sulfides generally do not contain as much iron; although ferromagnesian minerals, siderite, and a few other types are in a special class because of their unusual iron content. Sometimes the granules may "glue" themselves together either at or following the time of formation, as aggregates of limonite particles, because of the binding force of silica, gypsum, or other minerals. (See granular limonite, this chapter.) This takes place far more frequently in the exotic types than in the cellular pseudomorphs or the fringing limonites. When very firmly "glued" together, the limonite may appear merged into an indefinite, dully resinous, clinker-like material. This is more likely when the coagulated product is derived from semi-massive or massive pyrite, but non-sulfides also have been known to leave coagulated products when silica and iron were present. The binding

STANDARD TYPES OF LEACHING PRODUCTS

force of silica and other minerals therefore is the only condition necessary for formation of the granular aggregates or coagulated limonites. Pulverulent material is generally unintelligible in terms of parent material. This applies both to oxidized sulfides and to non-sulfides.

Flat Crusts The flat crusts are characterized by an essentially smooth surface and dull to sub-metallic or pitchy luster. These crusts usually tend toward curved rather than flat surfaces when more than a few millimeters in length, and any individual layer may vary greatly in thickness within distances of a centimeter or two. They vary in thickness from less than 1/10 to more than 5 mm, the usual range is Y2 to 3 mm. With few exceptions they are made up of successive coats of paint or kalsomine upon a surface; and, like the latter, were precipitated from solution along horizontal, inclined, or vertical surfaces. In some cases the individual layers are thick enough to be observed readily by the unaided eye, whereas in others they are so thin and closely adherent that the separate layers can be detected only under high magnification. Usually, under the microscope, radiating fibrous structures or spongy interiors can be seen, but under the hand lens they are smooth and essentially flat. Though spoken of as flat crusts, the surfaces of these limonites should not be thought of as necessarily comparable in smoothness to that of a planed board or plate of metal. This is readily understood when it is remembered that precipitation of any given layer takes place from a film of acid iron-bearing solution, and that capillary attraction enables the film to "creep" over minor obstructions or into and through minor depressions without break in continuity. Aside from gossans, the flat crusts are most often observed as coatings along joints or fracture planes of rocks, or along open fissures or fault planes down which iron-bearing solutions course. They may adhere as coatings upon rock surfaces even in limestone (see fig. 15, ch. 18). Sometimes this variety of crust presents unbroken surfaces over several square feet. Usually a given occurrence does not exceed 4 to 6 inches in length and width and a quarter of an inch in thickness. The color varies mostly from yellow-brown, through brown to chocolate; in some cases it is reddish. Iron humates are common constituents in humid regions with much decaying organic matter. As noted in chapter 10, these generally alter before long into ferric oxide or ferric oxide hydrate minerals. Transition from such intermediate forms, added to fluctuations in the composition of the ground water flowing over the surface at different seasons, militates against uniformity of product or evenness in thickness and distribution of the layers. Such crusts consequently often are pitted or otherwise characterized by blunt irregularities. Most of the flat-crusted limonite formed either as irregular mounds, or as a series of overlapping, tiny

103

saucerlike terraces (reversed) at orifices of springs, underground drill-holes, or other places fall under this classification. They are formed as the escaping iron sulfate-or carbonate-bearing waters become aerated and hydrolysis takes place. Sometimes a single crust extends unbrokenly for many feet, with successive layers building up to a thickness exceeding 1 inch. The dark, flat crusts are known to have been precipitated many hundreds of feet beyond the limits of an oxidizing iron-bearing body; but the more remotely situated crusts more commonly are of the type found along rock joints or open fissure walls. This is especcially true if non-sulfide minerals are present. When sulfides appear, pyrite in particular, the exotic limonites tend to go from flat crusts to the smeary limonites; and, usually, they die out gradually rather than stop abruptly.

Smeary-Crusted Limonites Smeary-crusted limonites are a succession of dark (nearly black) thin layers; but instead of being precipitated over essentially flat surfaces, they form nodules of varying size. Their diameters range from a mere pin point to an inch or more. The nodular shape varies from gentle arcs to full hemispheres. Single nodules may form, but usually they occur in aggregates or clusters. These nodular precipitates embrace two broad classes: 1) radiating fibrous masses, in general solid throughout so far as the unaided eye can see, and 2) similar shapes but with the layers less compacted, and with frequent porous texture, and radiating fibrous structure and spongy interiors or cores. Generally it is possible to see the radiating fibrous structure without a hand lens. The solid type comprises the well known goethite or hematite "kidney" ore so common in museum collections. The frequently porous, less compacted nodular precipitate to which the term crust more properly belongs, is encountered to a greater or lesser degree in nearly all semi-arid to arid regions where sulfide bodies have oxidized. Such crusts form characteristically along courses or channels traversed by strongly acid solutions, and thus point to oxidizing pyrite, or at least to very acid solutions, as their source. Their less compact and variably porous texture renders them more fragile, and more readily subject to disintegration, than the craggy type of "relief" limonite. Rupture surfaces of the larger crusts likewise usually reveal to the unaided eye the individual crusted layers, suggestive of an onion structure (see fig. 15, ch. 18). They rarely attain diameters exceeding half an inch. When approaching that size they nearly always occur in aggregates from 2 to 6 or more inches in length or breadth. Sometimes they are glossy; but their luster varies. A feature of the nodular crusts not always noted at first glance is that the surfaces of the larger ones quite generally are peppered with innumerable more minute crusts ranging from pin point to fish-roe in size. These in turn, when examined under the microscope, usually reveal upon their surfaces the same phenomenon

104

INTERPRETATION OF LEACHED OUTCROPS

repeated upon a still more minute scale. No necessary relationship exists, however, between the diameter of the major nodule and the size of the more minute ones peppered over its surface; a nodule half an inch in diameter may disclose only pin points, so that its surface appears essentially smooth to the unaided eye; a nodule a quarter inch in diameter, on the other hand, may have its surface coated consistently with fish-roe crusts, ranging from lis to 1;4 mm in diameter. In the larger exotic nodules, fine clayey particles or other extraneous particles not infrequently have been found incorporated in the various layers. The presence of impurities in the crusts thus presumably predisposes them to at least partial disintegration during weathering at the points affected. Shrinkage resulting from loss of adsorbed and capillary water also occurs during weathering. It should be noted here that ferric oxide or ferric oxide hydrate is not the only supergene substance to be precipitated as dark, nodular crusts with radial structures; hydrous manganese dioxide frequently does so along faults or open fracture planes, as do likewise intermixtures of the two substances. The pure manganese product, however, has a more dark bluish color and duller luster; and the two products are not likely to be confused by the observer familiar with both.

Thick-walled Limonites In the first part of this chapter the thick-walled limoniles were described as a type of cellular sponge, an indigenous product. They have cells highly unsymmetrical in size and distribution, varying abruptly, mostly rounded. The solutions involved usually were derived from pyrite or other acid minerals with excess sulfur (see fig. 17, ch. 18). In the thick-walled limonites of exotic origin the pattern is not governed by channelways anastomosing through the mass. They are made up of small particles of limonite aggregated into porous masses during their precipitation; and their aggregation or accretion into crudely pseudo-cellular structures, if it occurs, is wholly a matter of chance. In the exotic precipitate there have been instances where the thick-walled type was deposited several hundreds of feet from the oxidizing body, where the strongly acid solutions entered shalylimestone rock, but usually this type grades off into flat-crusted limonites. Both the indigenous and exotic thick-walled limonites are formed where neutralizer is present in the ground water in sufficient quantity to overcome the acidity of the iron-bearing solutions moderately slowly, not rapidly enough to yield fluffy limonite. Usually there is partial collapse and much local transfer of iron during oxidation of the mass involved (see fig. 17, ch. 18). The cellular sponge of the thick-walled limonites generally grades to the smeary-crusted type where very acid conditions exist, whereas the exotic type generally grades to the flat-crusted type where there is not quite so much acid. Both of these limonite types are able to withstand severe weathering attack.

Iridescent Limonite Crusts Iridescent limonite crusts constitute a special class of the exotic dark, smeary nodular limonites; they were described in chapter 15. Field occurrences of iridescent ferric oxide hydrate crusts have been correlated mostly with the oxidation of semi-massive to massive pyrite.

Columnar Limonites Columnar limonites are associated closely with oxidizing massive pyrite, although other minerals (for example, manganese minerals) may be present in part. They are an exotic type, brownish-black to black in color and submetallic to dull in luster, precipitated as long slender columns, mostly with minute billowy surfaces. The lengths of the columns may be up to 6 or more inches, but usually they do not exceed 2 or 3 inches. The normal diameter is 112 to 2 mm, occasionally up to 4 or 5 mm. Long columns are not necessarily of large diameter. Columnar limonites form much as do stalactites and stalagmites of any mineral, namely, through deposition of salts as dripping or seepage solutions become exposed to dry air currents and their partial or complete evaporation takes place. In the oxidation of pyrite they seem to form to best advantage (or at least to be preserved against weathering) where roof and floor are only a few inches apart. The columns are built up through repeated vertical deposition of minute gobs of limonite only a fraction of a millimeter across. Locally, sharp projections extend outward horizontally (see fig. 20, ch. 18), but these are incidental. In general the product retains a crudely rod-like form, but with a surface of minute billowy irregularities. It somewhat resembles nodular crusts in shape, but not in internal structure; nodular crusts, when present, are extraneous or accidental, and in no way characteristic of columnar structure. Columns of large diameter quite generally are made up of many slender individual rods, all more or less coalesced into a major unit of fluted appearance. Although exotic, most columnar limonites of ferric oxide hydrate are deposited within or closely adjoining the gossan of the sulfide body which gave them birth. Though always reflecting leaching from higher, usually eroded, material, they reveal so accurately the high acidity of the oxidation solutions and the course pursued by them or leading from the gossan that, in conjunction with other evidences of pyrite leaching, they often permit rough estimation of the amount and distribution of that mineral within the former sulfide mass. At Mount Morgan, Queensland, columnar limonite formed the dominant gossan overlying the rich goldbearing, more heavily pyritic portions of the orebody, itself often carrying free gold as fine specks and constituting rich ore. To some extent columnar limonites are present in nearly all districts in which massive pyrite oxidizes without the solutions becoming weakened (fig. 20, 21, ch. 18), early in their travel, through contact with neutralizer in the gangue or ground water. Locally they

105

STANDARD TYPES OF LEACHING PRODUCTS

form conspicuously along isolated solution channels even in limestone, most often where there is long-continued and steady dripping or seepage, rather than along open channels.

Caked Crusts The most conspicuous examples of caked crusts occur where semi-massive to massive sulfides persist to the surface, and where the iron derived from their decomposition is exported in solution to adjacent topographic depressions. There, under favorable conditions, the iron accumulates as colloidal gelatinous material in small puddles. As the iron-bearing limonite solution traverses the surface toward the pools, fine clayey matter almost inevitably is taken up in suspension, and becomes incorporated in the gelatinous material; clayey material may also be brought in and added subsequently by storm waters. The clayey matter may comprise up to 50 percent of such material, but is usually less. Colloidal substances always shrink during dehydration, and minute shrinkage cracks manifest themselves at the surfaces of the pools, especially in clayey matter, because the limonite dries out very quickly in the semiarid regions. The clayey limonite develops roughly polygonal patterns; the cracks increase in width and breadth until the mass through a vertical depth of 1 or 2 mm becomes transformed into small blocky up-curved, rough-surfaced crusts, in miniatur~ (except in color) resembling the parched surfaces of sun-baked mud-flats. The length or breadth of the blocks rarely exceed a quarter of an inch; more often it is less than an eighth of an inch. The thickness of individual crusts decreases with lowered content of clayey matter; but even with clay content as low as 5 percent the mudflat structure often is recognizable. As the top layer of the pool dries out, the dehydration process penetrates more deeply, though necessarily slowly, into the gelatinous mass. In time the whole pool is affected, and it thus is not possible to trace back the iron's source, because it is limonite mud. Furthermore, the caked crusts prove especially vulnerable to the mechanical forces of weathering, and they distintegrate sooner or later into granular or pulverulent limonites. Sometimes the caked crusts are not exotic. Where there exists free air and ground water circulation below the surface, fine clayey particles often are imported into and become mixed with limonitic matter derived from sulfide residuals undergoing decomposition in conjunction with cellular pseudomorph formation. In most cases pollution of that sort is not important; but near open channels, up to 30 percent contamination by clayey particles has been noted. Though slower in evolving than at the surface, eventual dehydration of the silica-limonite gelatinous material produces here, also, caked crusts in every respect resembling those of corresponding clay content formed as exotic products at the surface. But in this case the caked crusts are indigenous and remain within the cell, and either lie discordantly as loose aggregates at its bottom, or disintegrate in part or in whole into pulverulent limonite and/ or clay particles as cell filling.

The caked crusts, therefore, may be either indigenous or exotic.

Shrinkage Structures Shrinkage structures of colloidal limonites of the cellular pseudomorphs have been already described in the first part of this chapter under flaky crusts. Shrinkage structures in exotic limonites are important also in connection with the dehydration of the clay components. During dehydration, whether the sulfide was pyrite or other mineral, the gelatinous material in the plastic stage emerges more definitely from the water table into one of open air circulation; it gradually dries out and shrinks where weathering and oxidation are rapid in the arid regions for 6 or 8 months. In such cases individual flakes "glue" themselves lightly into the walls, or settle indiscriminately at the bottom of the cell into a feathery attachment that imparts to them a loose and structureless airiness, and only rarely do attached flakes join end to end, although the thicker central siliceous portions are likely to be preserved. All exotic limonites shrink moderately during dehydration, especially when the rock is shale or other clayey material (see fig. 18, ch. 18 for extremes).

Surface Coalescences Surface coalescences that are in part indigenous and in part fringing were described earlier in this chapter as "glazed over" limonites, which through prolonged coalescence of the particles exposed to the action of the sun, wind, and rain at the earth's surface fused ' more or less together. ~ll types of limonites, except massive jasper, are subject to such coalescence on the surface in gossans ?r cappings in semi-arid to arid regions, though in varymg degree according to the sulfide involved. They bec~me h~rdened or "glazed over," and they rarely attam a thickness of one-quarter inch. But in the fringing and cellular types "ghosts" are preserved, even in the most thoroughly fused material. In the exotic surface coalescenses there are no "ghosts." The sulfide for the most part is pyrite, which generally yields a softer limonite than the cellular limonites. Furthermore, impurities, common to limonite, are usually more plentiful in the oxidation products of pyrite than with other sulfides, because of the high acidity; and impurities usually lower the sintering or fusion point. The exotic limonites grade from semiglazed limonites to the flat and smeary types; but thc flat and smeary limonites predominate rather than the semi-glazed.

Desert Varnish A distinctive type of fine limonite is desert varnish. This I!monite forms a highly polished coating, with browmsh-black to black color. It is commonly observed upon the surfaces of boulders or pebbles that have lain exposed and undisturbed upon the ground in semi-arid and arid regions. The coating ranges from specimens so thin that the rock texture and color are readily perceived through them, to occasional samples up to onefifth of a millimeter thick. The thicker types are not so

106

INTERPRETATION OF LEACHED OUTCROPS

polished, and with these types the underlying rock is nearly always a porous one which grades insensibly into desert varnish. Source of the desert varnish is usually ascribed to iron which remains in the form of a residual coating as the more soluble rock constituents are weathered away. As such it is a laterite, developed on a small scale, and highly localized. The fact that it is developed most strongly upon portions of the boulder or pebble exposed to the sun's rays, suggests further that the varnish represents mostly the iron left through evaporation of the iron-bearing solution. The high, often brilliant, polish reflects scour and buffing by wind-blown sand and clay particles (often of microscopic size), in addition to the glaze effected by fusion of the limonite particles. That the iron is derived mainly from the boulder or pebblc itself, is attested by the fact that basalt and gabbro pebbles carry markedly thicker coatings than do those composed of latite or monzonite, assuming equal size and length of exposure. Manganese and potassium, in small amounts, are usually important also in the formation of desert varnish; for the manganese and potassium compounds take a brilliant polish in desert varnish. Although all rocks do not have the manganese-potassium type of desert varnish, they are usually seen on igneous rocks, graywacke, and shale that have been exposed to the sun for a long time. 8 Desert varnish has no meaning in terms of ore. It is listed and described merely so that it may be recognized and understood when encountered.

SUMMARY

When iron-bearing solutions rapidly traverse rock or soil of moderately low neutralizing power, little reaction between solution and gangue takes place; but still enough usually occurs so that, if the rock is strongly kaolinized or the soil is clayey, the fine pulverulent particles become stained conspicuously red or ochreous. Because ferric oxide or ferric oxide hydrate exists mostly either as thin coatings or adsorbed as microscopic specks upon surfaces of pulverulent material rather than as actual replacements of it, the iron content is deceptively low in these cases. If uniformly dispersed through the kaolinitic or clayey material, this material may appear to consist of closely-packed, minute limonite grains even though ferric oxide or ferric oxide hydrate may not exceed 5 percent. Bulk occurrences of such limonite-soaked earth are not often deceptive. But when the material consists of clay- or gouge-filled seamlets coursing thickly through a mineralized area, it may cause a highly erroneous conception of the amount of iron originally present as sulfide if its true character is not recognized. Examination under the hand lens usually suffices to reveal the true composition of the soaked material.

Indigenous Limonites. All sulfides whose oxidation products have been studied comprehensively, and a limited number of non-sulfide ore and gangue minerals, yield cellular boxworks. Most sulfides yield cellular sponges also, but the proportions differ widely. Pyrite, when massive, yields boxwork only sporadically; it yields cellular sponge far more freely; but most of the limonite derived from pyrite is of the exotic variety. Pyrrhotite, by itself, yields both cellular boxwork and cellular sponge. Inside the pyrrhotite boxwork and sponge a flaky limonite is often developed. Other cellular pseudomorphs of sulfide minerals (except pyrite) may contain flaky limonite also, but usually not as much as in the case of pyrrhotite. Arsenopyrite yields boxwork more commonly than does pyrite, but not prolifically. It has not been found to yield cellular sponge, but usually decomposes freely into arborescent granular assemblages which superficially have a spongy appearance. These arborescent granular assemblages are of the fringing type. Chalcopyrite yields boxwork more freely than any other sulfide, and of a pattern that is distinctive and always conspicuous. Its yield of cellular sponge is far more restricted. Only rarely is the limonite from chalcopyrite obscured to any marked extent by subsequent precipitates, except those derived from chalcocite. Chalcocite yields boxwork less frequently than that of most sulfides; its boxwork quite generally is coated with and largely obscured by other indigenous limonite products. 9 Galena yields both cellular boxwork and cellular sponge, but neither in quantity as a rule, because most limonite products of lead origin are derivatives of anglesite or cerussite. Sphalerite yields both boxwork and sponge more plentifully than does galena. Moreover sphalerite generally yields both in approximately equal amounts, though in a given district either may predominate almost to the exclusion of the other. Calcite does not often yield boxwork; but when it does, the boxwork is a firmly-knit, outstanding type. It does not yield cellular sponge, so far as the author has observed. Siderite and fluorite also yield boxwork, but only sporadically. Fringing Limonites. In the fringing type, although cellular pseudomorphs occur, the products travel only a millimeter, more or less, before they are precipitated as limonite. The principal fringing limonites are relief, arborescent, and partially sintered crusts, followed by granular and caked crusts. The relief limonites are not peculiar to any sulfide, nor are they necessarily derived from sulfides; but, among the sulfides, the chalcocitepyrite mixtures in disseminated areas, and galena in

'Engle and Sharp, (1958, p. 487-518), give a complete description of desert varnish, including chemical analyses.

'Except in the case of disseminated deposits where craggy and radiating fibrous coatings are found.

Limonite-soaked Earth

STANDARD TYPES OF LEACHING PRODUCTS

massive deposits, are the principal sources of "relief" products. The arborescent Iimonites are principally from arsenopyrite and chromite. Galena and bornite are the only sulfides that yield coalesced, partially sintered crusts in quantity. Exotic Limonites. In the exotic type the source of iron no longer can be identified specifically. Sometimes the "soap," the jasper caps over dolomite, and other

107

forms as well, may be identified as to source to a certain extent. But usually the iron of the exotic limonites has traveled too far for its source to be determinable. The flat, smeary, columnar limonites, desert varnish, and limonite-soaked earth are mostly exotic. All limonites, including the cellular pseudomorphs and fringing limonites, eventually are transformed into pulverulent products.

Chapter 17 EXAMPLES INDICATING THE VALUE OF LEACHED OUTCROP INTERPRETATION The technique of leached outcrop interpretation has been successfully applied to many exploration and mining problems. A study indicates that it also might have been applied in still other investigations, and that in several instances it could have prevented further unproductive efforts by indicating that the sulfide bodies being sought were of sub-ore grade. A few representative examples indicating its value, both in exploration and in actual mining operations, arc given in the following pages.

CLASSIFICA TION OF LEACHED OUTCROPS OVER DISSEMINATED DEPOSITS Figure 9 is an example of the method commonly employed in classifying leached outcrops over disseminated copper deposits. It will be noted that all outcrops have been mapped and classified, the blank areas representing alluvium. The map was constructed during a 4-day inspection of an operating property in New Mexico, its purpose being to apprise the management as to whether or not a given piece of ground merited further exploration. Secondary chalcocite ore containing approximately 3.5 percent copper had been found, and had been developed by underground workings, structurally beneath the area marked I X upon the map, but crosscuts put out in all directions had failed to yield further ore. Mapping of the surface showed another "A" rated area, 2X, north of the one already developed, of almost equal promise as to grade. Although, on strength of the surface mapping, this clearly was connected to the 1X area by a narrow neck or "isthmus" or ore, that "isthmus" had been missed by underground crosscuts put out at systematic intervals. Ore in this district commonly does not lie vertically beneath the parent outcrops; but conformity in shape and size of the underground orebody at 1X was so close, structurally, to the surface outline of favorable cropping to the southwest, that it became possible to layout the connecting crosscut to the 2X area so as to keep all underground exploration within ore. Development of the 2X area resulted in proving an ore body of approximately the size and shape shown upon the map, and of a grade only slightly below that of the I X area.

Encouraged by results then being obtained in 2X ground, the management proceeded to drill the 1Y area some distance to the northeast, notwithstanding the fact that it had been given only a "B" rating, and that 2 percent copper represented the commercial cut-off for ore of the district of that size. Two holes were put down. The first showed 30 feet of 1.62 percent copper; the second, 20 feet of 1.80 percent copper. Because the grade of ore found in this drill hole tended to confirm a direct correlation between ore grade at the surface and that beneath the surface, the 2Y area, rated "B" from the surface indications, consequently was not drilled. A point to note is that these outcrop classifications were put down in black and white upon the map in advance of exploration (save for the IX area), and the map would have left us no alibi had those classifications been wrong.

BLIND LEACHED ZONES At another mine in the southwestern United States a secondary chalcocite orebody had been located by churn drilling, and had been developed by underground workings between the 300-foot and 600-foot levels, as indicated in figure 10. Scout drillholes put out around the area had uniformly failed to yield additional ore, and the limits of ore for that section of ground apparently had been reached. But upon the SOO-foot level a crosscut disclosed a small mass of leached material, and the leached material carried appreciable limonite of unmistakable chalcocite origin. A crosscut vertically beneath upon the 600-foot level disclosed limonitic material of even greater chalcocite promise. It was recommended that the 600-foot level crosscut be extended through the leached material. When this was done, the leached material proved to be equal in width to the adjacent sulfide ore body already developed above that level. After further driving and crosscutting to prove the leached material's horizontal extent, the decision was reached to test beneath it by underground churn drilling. That drilling disclosed another chalcocite orebody nearly equal in size to the first one. The leached material upon the 500- and 600-foot levels proved to be the leached top of a copper ore lens which never had reached the horizon of the present surface,

110

INTERPRETATION OF LEACHED OUTCROPS

MAP OF A DISSEMINATED COPPER AREA

c?~ ~ (;.:;:7

()

o

i~ oI

ScALE OF TEET SOO !

!

LEGEND ~v

Gpping _ ;>;:'00% CII

"gvOppirlg_ /-i'S% -;:'00%Cu 'C'C7ppi/7g- O·SO%-/-c5% W

'if GJppil7g - ,fad 8.r.renli,;>11y b,;>rre/7 OTW

8/.7nk Are,;>s-A//uv/um Churn fJri//;'ole

FIGURE 9.

Map showing a method commonly used in classifying leached outcrops over disseminated copper deposits.

and whose existence therefore could not have been suspected from surface outcropS."

DEPOSIT OF SUB-ORE GRADE At a certain mine in Nevada a 1,000-foot-square area of heavy gossan derived from massive sulfide had disclosed upon the 700-foot level marooned patches of 'Two of the scout drillholes had penetrated the leached limonitic material, and one of them had reached within 30 feet of the orebody, but the significance of the leached material, as capping over a chalcocite orebody at greater depth, had not been recognized by the drillers, or by the owners of the property.

oxidized copper ore, and high hopes were held out by the management that at greater depth this entire area might be underlain by a rich body of secondary chalcocite ore. Examination of the leached material disclosed the fact that a 600- by 1,000-foot portion thereof was of sole pyrite derivation, and could be ruled out as desirable prospecting ground; and that because the remaining 400-foot by 1,OOO-foot portion, which carried oxidized copper bodies, showed little limonite of copper origin, and occurred in an environment of rapid neutralizer, the probability was strong that most of the copper originally present within the gossan area was represented by the small bodies of oxidized copper ore

111

EXAMPLES INDICATING THE VALUE OF INTERPRETATION

already found. Although this imposed a discouraging outlook, it seemed best to face the situation frankly, and subsequent exploration at greater depth proved the correctness of the interpretation.

ABSENCE OF WIDESPREAD LEACHING -ELY, NEVADA In the Ely, Nev., district, secondary chalcocite occurs in the disseminated copper ore. For years it was believed that the ore there represented the copper concentration from weathering of many hundreds of feet of eroded material, as at Miami and Tyrone. Investigation of the leached products, and more careful inspection of the ore itself, showed that in much of the copper-bearing ground secondary enrichment was less important than had been previously supposed; and that a number of the orebodies were primary oreshoots of chalcopyritepyrite whose tops had scarcely been reached by either oxidation or erosion, as stated in chapter 13. This at once suggested that determination of the detailed structural features within the monzonite body that had served to localize the known primary oreshoots, and search for duplication of those structures elsewhere even though little or no surface mineralization was in evidence, might yield additional orebodies. Search along those lines has been conducted since that time by the geologists of the district, and has resulted in the finding of many million tons of additional copper ore at Kimberly, Nev. in 1929-1930, mostly primary and for the most part previously unsuspected.

NON-SULFIDE GOSSAN-LA WN HILL, QUEENSLAND The Lawn Hill silver-lead district in extreme northwestern Queensland was discovered in 1887, and although several thousand tons of ore containing more

than 35 percent lead have been produced, the isolation and consequent transportation handicap have served to restrict exploration and development. The more promising deposits occur in a central 4- by II-mile area, in faulted and folded quartzites, slates, and limestones of Cambrian and possible Late Precambrian age. At one place in the district a bold and impressive gossan had been looked upon with favor by a number of engineers, and further exploration beneath had been warmly recommended by them. Investigation of the outcrop in 1930, while the district was being extensively tested by diamond drilling, showed that the lode, prior to oxidation, had contained practically no lead; that the only important sulfides present had been pyrite and sphalerite; and that even they were of minor significance; the gossan had been derived almost wholly from complete decomposition of a large body of manganiferous siderite. That statement was put into writing in a report, but for various reasons the management decided, none the less, to test beneath the outcrop. Two drillholes were put down to cut the sulfide zone. They proved in its entirety the correctness of the interpretation that had been given to the outcrop.

THE MASSIVE IRON-OXIDE OUTCROP AT MOUNT OXIDE, QUEENSLAND At Mount Oxide, Queensland, in an isolated, difficultly accessible region 76 miles by road from railhead, occurs an exceptionally rich body of massive chalcocite ore that has persisted continuously through a 300-foot vertical depth. Although most of the ore has come from above the 208-foot level, even down to the 300-foot level chalcocite continues as the dominant and almost exclusive ore mineral, with no clear evidence as yet as to character of the primary copper sulfide. Maximum length of the high grade oreshoot that has yielded the rich chalcocite is 295 feet. A more detailed description

Le.7ched OulcropsbOO 'Level.

LEGEND g.7rren Ou/cropsLe.7ched LimonilicJI.7ler/jl or Ch07/coClle ';1/70' ;yak ~riv07/ion SeCOnc7.7ry Ch07!coc;!e Ore

bOO'LEVEL ~=*==f==f==~~~~~~~~~~P=== I II

'-----'--_v_----<

II

II II

/lnde;pround If'orhngs - Old Underground WorKings -New H.o.H

28'8-,98

II II

800~EVEL:~========================~~===========

,[CALE or ,FEET

o FIGURE 10.

soo

Section showing how leached material, found only underground, led to discovery of an important orebody.

112

INTERPRETATION OF LEACHED OUTCROPS

of the deposit and of some unusual sulfide leaching and redepositional features, including several illustrations, is given in Appendix C. The oreshoot occurs within a zone of crushed shale, dipping southeastward at an average angle of and in part overlain, as the hangingwall, by a body of massive iron oxide more than 100 feet thick in many places. The iron oxide body dips similarly to the southeast. The following discussion is a condensation of a previous more detailed report by the author (see Blanchard, 1939b). In the past, various interpretations have been given by engineers and geologists to the massive iron oxide hangingwall mass. Since the chalcocite oreshoot, in large part at least, represents the "built-up," secondary accumulation of overlying or eroded, leached copperbearing material, but itself carries little limonite as a capping within the zone of crushing, a not infrequent interpretation has been that the massive iron oxide body represents the residual of a former massive pyriteprimary-copper-sulfide body from which the rich chalcocite ore has been derived by leaching. The matter is of more than academic interest, because the known high grade oreshoot occurs ncar one end of the iron oxide outcrop, which persists in a curve, without diminution in thickness, for an additional 1,000 feet, and with an 800-foot length thereof unexplored. Application of the leached outcrops technique showed that: I. Not only is derivation of the chalcocite from the massive iron oxide body illogical as to the necessary

sr,

processes, but is contrary, at every important step, to the facts of chemistry. 2. The iron oxide body is not the type, nor does it possess the characteristics, of a pyrite or mixed pyritecopper sulfide derivative. It is a partly hydrated body of primary hematite-specu1arite, or iron ore, deposited as such adjacent, but not otherwise directly related, to the (as yet undetermined) primary copper sulfides within the zone of crushing from which any secondary chalcocite must have been derived. 3. Absence of a conspicuous limonitic capping over the chalcocite body within the zone of crushing is explained by fact that the chalcocite is almost wholly free of associated pyrite or other direct iron-yielding mineral; and that iron consequently has not been available, except in minute amounts, to form a limonitic product as the chalcocite passed into its partial solution. 2 Here again, although a discouraging outlook has been imposed because the large iron oxide mass has been ruled out as the source of other possible rich secondary chalcocitc orcshoots beneath its untested portions, the ultimate saving in fruitless exploration undcr necessarily costly conditions must be counted a financial gain. 'The deposit is a conspicuous example in which essentially pure chalcocite, exposed to normal oxidation processes. has failed to go completely into solution. The normal end-product of such chalcocite decomposition.-copper carbonatc,-shows little penetration below IOO-foot depth. whereas seams of chalcocite up to 30 inches thick persist within the zone of crushin" almost unaffected to the surface. even though the chalcocit~ body clearly has been exposed to the processes of weathering in an arid climate through an extensive period of geologic time.

PLATE I. Columnar type of limonite derived from massive pyrite. Such limonite forms only above the water ta ble, usually along solution channels that drain directly from an oxidizing pyr ite body. Specimen from Mount Morgan collection, Queensland. N atural size.

PLATE 2. Oxidation products of a sulfide mixture, arsenopyrite and pyrite in a ratio of about 3 to 2, in slate and mica-schist country rock. The rock originally contained about 5 percent lead and 4 percent zinc. With increased weathering the scorodite-Iimonitic jasper has become corroded and pitted at points of attachment of slender scorodite excrescences, so that the original ribs of the boxwork now have become a series of interrupted chains of arborescent projections. Loosely aggregated but firmly joined granules have, however, given it high rigidity. An unusual green type of scorodite. Generally scorodite is pale leek green, apple green, occasionally bluish or violet. Earthy varieties are generally liver brown. Mount Bonnie, near Grove Hill, Northern Territory, Australia. Enlargement 2x.

PLA TE 5. Finely cellular, weathered boxwork , derived from chalcopyrite exposed along a fracture in an outcrop in the desert plain of interior Australia. At a dozen different places in the specimen are seen " key" structures consisting of webs of hematite and very minor goethite. The webs pass numerous cells and cross structures, and intersect at angles of approximately 100 degrees. The country rock is garnetiferous schist that has locally been 90 percent silicified in these specimens. From an outcrop 4 Y2 feet above average ground level, Killeen's Copper lode, lervois Range, Northern Territory, Australia. Natural size.

PLATE 3. Coarse and fine cellular boxwork derived from chalcopyrite in granodiorite(?) gangue. Fresh surface. Note also the sharply defined angular pattern of thin-walled cells, and frequent distinct parallelism of cell orientation. Orphan mine, Dobbyn, Queensland , Australia. Enlargement 1.25x. (After Blanchard, Australasian [nst. Mining Metall. Proc., 1939, follow p. 50.) PLATE 4. Coarse and fine cellular box work derived from chalcopyrite in granite-agglomerate country rock. Fresh surface. Longamundi prospect, Cloncurry district, Queensland, Australia. Natural size.

PLA TE 6. Finely cellul ar boxwork of goethite with very minor hematite, derived from chalcopyrite. Fresh surface, about l/ S to '/
PLATE 8. Oxidation products derived from a disseminated mixture of chalcocite and pyrite in moderately kaolinized, unfractured quartz porphyry gangue. Silica content of the rock is about 5 percent, uniformly distributed. Crusador property, Cloncurry district, Queensland , Australia. Natural size. Left: Dark reddish cellular hematite derived from a mixture of chalcocite and pyrite in a ratio of approximately 2 to I. Brownish grains are goethite. Oxide minerals are shown on a fresh, unweathered surface. Ore beneath such an outcrop (60 feet down in this instance) will analyze about 25 percent copper in the form of sooty chalcocite. Right: Less hematite and almost complete absence of goethite. Ore beneath such an outcrop (60 feet down, as in A above) will analyze 10 to 11 percent copper as chalcocite.

PLATE 7. Limonite derived from a sulfide vein in partially garnetized limestone, surrounded by quartz monzonite. The middle zone is massive to semi-massive brownish-black to black limonite derived from pyrite. The enclosing brown or tan fine-grained relief limonite was derived from chalcopyrite; " key" cellular psuedomorphs occur in both upper a nd lower zones. Duquesne mine, Ariz. Enlargement 1.25 x.

Oxidized pyrite

Oxidi zed chalcopyrite

Malachite

PLATE 9. Cellular limonite derived from a mixture contammg approximately 3 parts chalcocite to 2 parts pyrite, oxidizing together, in a silky sericite gangue. The limonite follows the schistosity closely in strike and dip. Two limonite types are involved : (I) a fresh, indigenous limonite, and (2) a smeary, acid-washed limonite which stains the sericite out of all proportion to the amount of sulfide originally present. Ore beneath such an outcrop (100 feet down) will analyze about 7 to 8 percent copper as chalcocite. Home of Bullion, Northern Territory, Australia. Enlargement 1.25x.

PLATE 10. Left: Indigenous cellular limonite derived from a disseminated ore in which the ratio of chalcocite to pyrite was approximately 2 to \. Quartz monzonite, well kaolinized. Opencut, Bagdad, Ariz. Enlargement 3x. Right: Fluffy limonite derived from pyrite, filling the cavities from which pyrite has been leached. Sericitized quartz monzonite gangue. (See also, fig. 14.) West of opencut, Bagdad, Ariz. Enlargement 3.0x.

PLATE 11. Fluffy limonite derived from cuprite. Malachite is present on the left and upper left of specimen; remainder of specimen is limonite. In the lower center are shown ghosts of limonite from cuprite. Just enough calcite and dolomite was present to permit formation of the fluffy type of Limonite. Mount Cuthbert, Queensland, Australia.

PLATE 12. Cleavage box work derived from oxidized galena, loosely coated with masses of partially sintered crusts. Weathered surface. Note the thinness and rigid parallelism of box work walls. Lawn Hill, Queensland, Australia. Enlargement 1.25x. (After Blanchard and Boswell, Ecoll . Geology , 1934, p. 673 .)

PLATE 13. Galena and cerussite in a partially epidotized white, impure limestone. Bluish-grey areas are galena; whitish-gray rims around galena grains are cerussite. Brown areas are relief limonite derived from cerussite, except the area to the left, which mainly consists of partially sintered crusts. Engine shaft, I 57-foot level, Mount Stewart, New South Wales, Australia. Enlargement 1.25x.

PLATE 14 . ReI ief limonite derived from ga len a and cerussite, mostly microscopic, in shale. Black Star orebody, Mount Tsa , Queensland , Australia. Enlargement 1.5x.

PLATE 15 . Hieroglyphic boxwork derived from sphalerite in ordinary shal e gangue . The hieroglyphic boxwork contains about 55 to 65 percent silica. From a fracture about half a foot below the surface. Mount Isa, Queensland, Austra lia. Enlargement 1.25x .

PLATE 16. Cellular box work derived from sphalerite in a sillimanite-gneiss country rock. A detailed description of a very similar specimen is given under Figure 67, B. Postmine oxidation, 2,OOO-foot level, North Broken Hill, New South Wales, Australia. Enlargement 2.5x.

PLATE 17. Limonite derived from smithsonite deposited in a shale gangue. About half is supergene silica, the remainder is ferric oxide hydrate. Small, indefinitely formed and loosely adhering aggregates and projections are visible in the walls of the cellular boxwork when much silica is present, (sharp, angular forms in the center and upper left side; fine boxwork in the upper middle and right side). About I foot below the surface, Rio Grande mine, Mount Isa, Queensland, Australia. Enlargement 1.25x .

Oxidized pyrite

PLATE 18.

Leached outcrop derived from pyrite (bluish-gray, upper right), chalcopyrite

(reddish-brown, upper left), and sphalerite (brown, center and bluish-gray, bottom). The oxidized products of these sulfides do not mix except along their common boundaries. Weathered specimen from dump, Republic mine, Johnson, Ariz. Enlargement 1.25x. See figure 8, chapter 14, for a description of a somewhat similar occurrence.

PLATE 19. Leached equivalent of the chromite ore of figure 94. The honeycomb is perfect in some places, and contains 50 to 70 percent silica. Ghost boxworks are shown in many places, but they are indistinct. Minute cellular sponge (not boxwork) can be seen, both in honeycomb and in the crumbling boxwork. Under erosive processes the coarser cellular boxwork becomes largely vacated of its sponge filling, but a thin coating invariabl y adheres to the box work walls. Although to the unaided eye the material may appear to be finely pulverulent, magnification of 30x or more reveals it to be tiny arborescent excrescences, usually loosely joined together to produce an airy and tenuous sponge structure. Only arsenopyrite and chromite exhibit the spectacularly bold, fragile arborescent outline of limonitic product to a marked degree; and the chromite-derived limonite pattern is very much finer than the pattern of arsenopyritederived limonite. Tiebaghi mine, New Caledonia. Natural size. (After Blanchard, ECOfl. Geology, 1942 p . 618.) ANALYSES Percent

Ch romite __ .... ________________________________ .. ___ __ _

4.83

Supergene ga ngue carbonateSiderite and magnesite __________ .. ___ ___ __ Limonitic jasperFerric oxide hydrate ______ _ Hydrous manganese dioxide ________ _ Silica __________________ ______________ .. __ Alumina _______ ___ ______ ________ ________________ _

Percent

0.52 12.65 3.80 68.40 5.60 90.45

Miscellaneous (adsorbed and capillary water, traces of nickel and arsenic, and probably organic matter) ___ ________________ __ ____________ _

4.20 100.00

PLATE 20. Fractured magnetite being replaced by loose scabs of hematite, and subsequently by goethite. From a surface specimen of a low-silica limestone gangue. Some sulfides are present. Semi-arid conditions. Veteran mi ne, Kimberly, Nev. Enlargement 1.5x.

PLATE 21 . Specimen from weathered surface of a manganese orebody. Dark areas are manganite and pyrolusite, light brown areas are goethite. Low silica content is characteristic of this type deposit. Cleavage planes are visible in the manganite, but no indications of the manganite cleavage are preserved in areas that have altered to goethite. Cell holes are inconspicuous, are mostly 0.5 to 2 mm in diameter, and comprise 5 to 10 percent of the area. Holes are distributed fairly uni formly, especially in the specimen to the right, and usually result in a pock-marked appearance rather than a definite cell structure. Lawn Hill, Queensland, Australia. Enlargement 1.25x. PLATE 22. Nodules of crystalline fluorite completely leached within a massive, coarsely granular matrix. The galena-marmatite mixture on the left barely shows the beginnings of oxidation. White areas are reflections from cleavage faces of some sulfide grains, which strongly reflect light during photographic exposure. Visually estimated composition of the sulfide mass is 52 to 54 percent marmatite, 45 to 48 percent galena, and about 1 percent chalcopyrite. From 2,000-foot level of Southern orebody, North mine, Broken Hill, New South Wales, Australia. Enlargement 1.25x. (After Garretty and Blanchard, Australasian /nst. Mining M etall Proc., /942 , preced. p. 165; Econ. G eology, 1942, p. 396.)

PART 2 In Part 2, descriptions are given of the oxidation products of 19 sulfide and non-sulfide minerals, together with line drawings and black and white and color photographs of examples of their various oxidation products. The oxidation and weathering products of pyrite and chalcopyrite are well known; those of chalcocite, galena and cerussite, sphalerite and smithsonite, pyrrhotite, arsenopyrite, bornite and tetrahedrite are also rather well known, but it is possible that not all the types produced by the oxidation and weathering of each of these minerals have yet been observed. The oxidation and weathering products of molybdenite, chromite, malachite, and azurite are fairly well known, but additional types may exist. Hematite, magnetite, siderite, manganese minerals, fluorite, calcite, salite, and supergene silica yield distinctive cellular or fring-

ing limonites in special cases; for the most part, however, these minerals give rise to pulverulent exotic limonites. In general, the limonites derived from nonsulfide minerals are not as important as those derived from sulfide minerals in leached outcrop work. Many of the oxidation products of any given mineral described in Part 2 are also mentioned at one or more places in Part 1; the most important of these references are noted at the appropriate places in Part 2 in chapters 18 through 36. The Index lists these and any additional references that are made concerning any specific mineral or its leached derivatives. The figures and plates contained in Part 2 are not listed in the Index save in exceptional cases; for the sketches and photographs show the localities of the specimens.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

I

Chapter 18 PYRITE In chapter 18 only the porous limonites derived from pyrite will be discussed. Massive jasper, including "soap" of pyrite derivation, was discussed fully in chapter 6. Pyrite, upon oxidizing in contact with a gangue of no neutralizing power (fig. 12a, 13), yields ferrous and ferric sulfate and sulfuric acid which are carried off in solution. Upon oxidation in the presence of moderate neutralizer, pyrite produces fringing limonite (figs. 12b, c; 14). Finally, upon oxidation in the presence of strongly neutralizing solution, pyrite tends to yield limonite fluffed up to some extent. In extreme cases the limonites would be the "fluffy" type (see fig. 22). The degrees of fluffing up, depends on: 1) the pyrite content, and 2) upon the reactivity of the country rock. Limestone or dolomite or rocks containing alkali are the most reactive (see fig. 11).

FIGURE 11. Sketch showing how iron derived from leaching pyrite in the Bisbee silica-breccia is precipitated as limonite at limestone contacts. A. solution channel. B. area of leached pyrite; no limonite present. C. heavy precipitation of exotic limonite at limestone contact. D. silica-breccia mass (quartz) in limestone. E. area of unoxidized pyrite. (After Blanchard and Boswell, Eng. Milling Jour., 1928, p. 282)

Following are descriptions of the principal types of pyrite-derived limonites. 1. Smeary crusts, including flat crusts, are one of the principal pyrite products. They have a deep brown or black color and sometimes they are iridescent, espe-

cially the smeary crusts. They are moderately flat (fig. 15) or finely nodular (fig. 16). They are mostly exotic, but sometimes they may be fringing or indigenous (figs. 23 and 24); especially where enough admixed base is present to reduce the high acidity, as with the disseminated chalcocite-pyrite specks and blebs of some porphyry copper deposits (see pI. 8). 2. Indigenous thick-walled cellular sponge (fig. 17) contains cells highly irregular in size and shape. The structure is characteristically rounded, and cell wall thickness varies abruptly and usually exceeds cell diameters. Limonite of this type is formed where sufficient neutralizer is present in ground water to overcome the acidity of the iron-bearing solutions only slowly, not rapidly enough to yield fluffy limonite. It may be exotic, however, with partial collapse, and usually much local transfer of iron during oxidation of the pyrite; and often coated in part with smeary crusts, into which it grades. 3. Columnar limonite is a characteristic stalactitestalagmite precipitate (figs. 19-21). It is formed when acid solutions, strongly charged with iron, drop a portion of the load as partial evaporation of dripping solutions takes place. It is closely related to smeary limonites and is always exotic. Plate 1 shows a columnar limonite that was formed in an oxidizing massive pyrite. 4. Caked crusts represent clayey-iron accumulations at or near the surface, as colloidal gelatinous material in pools several inches across. Generally, storm waters bring additional clay particles. On the surface, the limonite dries out, leaving small, blocky, up-curved crusts. 5. "Glazed over" limonites sometimes are encountered, effected through coalescence of particles exposed to action of sun, wind, and rain at the earth's surface, fused more or less together. Coalescence itself drives off much adsorbed and capillary water. "Ghosts" are preserved in cellular and fringing types (pI. 5), but are absent in exotic limonites; mostly the exotic limonites are pyrite derivatives. Included is desert varnish, a thin, highly polished coating observed as pebble surfaces which have lain exposed undisturbed in semidesert regions. "Glazed over" limonites and desert varnish are not very porous. 6. Thin-walled, cellular boxwork or sponge represents a webwork of limonite "eating" its way into massive pyrite during incipient oxidation. Although the thickness of the cell walls exceed the cell diameters in

116

INTERPRETATION OF LEACHED OUTCROPS

3=t:J . 2

.,'::",'.;'. '.

I

. " ;:; " ::

:.:. .

A

8

c

FIGURE 12. Sketch showing characteristic oxidation products of pyrite in kaolinized alaskite porphyry. 1. Outline of cavitv. 2. Inner b~eached zone. 3. Limonite halo. 4. Outer zone of limonite "fog." All iron has' been exported from the cavity, and has been carried a substantial distance beyond its borders before the low neutralizing power of the gangue, due to strong kaolinization, could bring about the iron's precipitation as limonite. The inner bleached zone is an area within which the highly acid solutions, derived from oxidation of the pyrite, traveled outwards before their acidity was overcome by the gangue's neutralizing power. Weak neutralizing power of the gangue, relative to acidity of the solutions, is attested further by: 1) failure of the gangue to precipitate the limonite more densely within the halo, and 2) persistence of the zone of limonite "fog," or feeble limonite precipitation. If only one of the disseminated sulfides is involved, and there are no other minerals to form limonite (siderite, garnet, etc.), it is a matter of opinion whether the limonite is exotic or fringing. Usually the fringing limonite is very close to the sulfide (see B); and limonite farther out (A) is generally known as exotic, because of the distance traveled. Silver Bell, Ariz. (After Blanchard and Boswell, Eng. Milling Jour. 1928, p. 374) B. Sketch showing characteristic oxidation products of pyrite in a gangue of moderate neutralizing power, such as slightly sericitized quartz monzonite. 1. Outline of cavity. 2. Inner bleached zone. 3. Limonite halo. 4. Outer zone of limonite "fog." All iron has been exported from the cavity, as in A; but because of higher neutralizing power of the gangue, the bleached zone is imperfectly developed and is more concentrated about the cavity, and the outer zone of limonite "fog" is but feebly developed. This is the fringing limonite. The fringing limonite usual1y is close to the sulfide cavity; as in feldspar (not sericitized) gangue. Cactus mine, Utah. (After Blanchard, Chem. Metall. Mining Soc. South Africa Jour., 1939, p. 361) C. Sketch showing characteristic oxidation products of pyrite in a gangue intermediate between moderate and strong neutralizing power, such as limy shale. 1. Outline of cavity. 2. Pulverulent limonite precipitated within the cavity (this is indigenous limonite). 3. Pulverulent limonite precipitated immediately outside the cavity (this is fringing limonite). Dense precipitation of limonite in a narrow zone about the cavity's border, and with precipitation of pulverulent limonite within the cavity itself, attests to moderately strong neutralizing power of the gangue. Mutton Gully diggings, near Ukalunda, Queensland, Australia. (After Blanchard, Chem. Metall. Mining Soc. South Africa Jour., 1939, p. 361) A.

many places, usually it is thin-walled limonite that contains sponge of heterogeneous pattern (fig. 23). It is highly siliceous, as acidity precludes much iron being

retained. This type usually is scarce, and is not seen in many districts. 7. Pyrite commonly yields limonite in the form of hard pseUdomorphs. They are mostly cubic, hard and compact, and the shape of the original pyrite is faithfully preserved (fig. 24). With few exceptions they are found only in impure limestone in semi-arid to arid regions. They are not very porous. 8. Some "relief" limonite may possibly be derived from pyrite, but this is not probable.

FIGURE 13. Characteristic oxidation product of pyrite in siliceolls shale. The shale is about 90 percent silica; the remainder is aluminum silicate. All of the iron has been exported from the cavity, though in the upper center and upper left side there are some "fog" areas. Surface specimen, Mount Isa, Queensland, Australia. Enlargement 1.25x.

117

PYRITE

FIGURE 14. Left. Oxidation products of pyrite in slightly kaolinized quartz monzonite. Pyrite residuals still remain. Niagara Gulch, Bagdad, Ariz. Two-thirds natural size. Right. Oxidation products of pyrite in well-kaolinized quartz monzonite. Nearly the entire groundmass here is a "fog" area. Pyrite residuals remain. Alum Creek, Bagdad, Ariz. Two thirds natural size. (After Blanchard and Boswell, Ecoll. Geology, 1925, p. 620)

FIGURE 15. Flat limonite derived from massive pyrite along a fracture in quartzite which was traversed during oxidation by solutions from nearby limestone. Contrast these thick-walled, rounded cells with the thin-walled, sharply angular cells derived from chalcocite that has oxidized in a similar manner, as shown in figure 42, chapter 22. Daly-Judge tunnel, Park City, Utah. Enlargement 2.0x. (After Blanchard and Boswell, Ellg. Mining Jour., 1928, p. 280)

118

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 16. Botryoidal or smeary limonite crusts derived from massive pyrite that oxidized in a limestone gangue. Finely nodular "pin point"' crusts and films coat the more smoothly surfaced botryoidal structure. This is one of the "key" types fOl: recognition of pyrite-derived limonite. Specimen taken from base of an oxidized massive body more than 50 feet in diameter. Gardner mine, Bisbee, Ariz. Enlargement 1.5x. (After Blanchard and Boswell, Eng. Mining Jour., 1928, p. 373)

FIGURE 17. Indigenous coarse and fine cellular sponge derived from massive pyrite that has oxidized along a quartzose fissure in granodiorite, which in turn was traversed during the oxidation by waters from a limestone area. Note characteristic rounded cellular structure, and thickness of cell wall commonly exceeding cell diameter. Hanover Mountain, Hanover, N. Mex. Two-thirds natural size. (After Blanchard and Boswell, Econ. Geology, 1925, p. 620)

PYRITE

FIGURE 18. A product of semi-massive pyrite oxidized directly above the water table. The clayey particles (alumina) were replaced by iron and silica when denudation brought the limonite to the surface, and the "soap" variety of replacement jasper was formed. Extensive dehydration resulted in pronounced development of shrinkage cracks. The final product contains about 40 percent ferric oxide hydrate, the remainder is mostly silica. Black Star orebody, Mount Isa, Queensland, Australia. Enlargement 1.25x. (After Blanchard, Australasian Inst. Mining Metall. Proc. 1939, follow p. 50)

FIGURE 19. Stalactitic to columnar limonite of exotic origin in the center of the picture, surrounded by limonite replacing dolomitic shale. Black Rock ore body, Mount Isa, Queensland, Australia. Natural size. (After Blanchard, Australasian Inst. Milling Metall. Proc., 1939, follow p. 50)

119

120

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 20. Columnar limonite derived from massive pyrite. About two-thirds of the distance below the top are numerous sharp horizontal projections, presumably deposited at the surface of a water table that remained stationary for some time. Lawn Hill, Queensland, Australia. Enlargement 2x.

FIGURE 21. A section of columnar limonite derived from massive pyrite. A botryoidal pattern is developed in the lower right hand corner. Mount Isa, Queensland, Australia. Enlargement 15x. (After Blanchard, Chem. Metall. Mining Soc. South Africa Proc., 1939, p. 370)

FIGURE 22. Indigenous fluffy limonite filling cavities from which pyrite has been leached. Sericitized quartz monzonite gangue. Surface waters carrying calcium bicarbonate flowed over this rock during the oxidation of the pyrite, and precipitated the iron as limonite within the cavities. In this case oxidation took place slowly, sufficient neutralizer was present at all times to convert the iron immediately into limonite at

the point of sulfide oxidation, and thus prevented any of the iron from being exported. No copper is indicated beneath thi~ cropping, although the specimen comes from a disseminated copper area. 'West of opencut, Bagdad, Ariz. Enlargement 2x. (After Blanchard and Boswell, Eng. Mining Jour., 1928, p. 284.) See also plate 10.

121

PYRITE

Oxidized Chalcopyrite

FIGURE 23. Coarse and fine thin-walled cellular indigenous limonite, derived from massive pyrite. The high acidity prevented retention of much iron. Silica content is 75 to 80 percent, iron content 20 to 25 percent. In the lower right center is an area of limonite boxwork derived from chalcopyrite, all else is limonitic jasper derived from pyrite. Omeo Tin prospect, Herberton, Queensland, Australia. Enlargement 2x.

FIGURE 24. Pyrite characteristically yields limonite in the form of hard pseudomorphs, such as these cubes or limonite "dice," in the presence of a strongly neutralizing gangue. These pseudomorphs constitute compact, essentially grain-for-grain replacements of the pyrite parent. Surface specimen, Bisbee, Ariz. Enlargement 1.25x.

Chapter 19 PYRRHOTITE The oxidation reactions of pyrrhotite were given in chapter 8. Although pyrrhotite is represented in the reactions in that chapter by the formula FeS for simplicity of calculation, it is more accurate to state it as Fe1_,S, in which X varies between about 0.05 and 0.2. In other words, iron and sulfur are present in variable proportions in pyrrhotite, the proportions varying from one occurrence to another. Pyrrhotite associated with any two orebodies, therefore, will usually differ slightly in percentage of sulfur contained, and hence will yield slightly different proportions of end products.

Troilite is the pure monosulfide, with the exact formula FeS. As stated in chapter 8, it is extremely rare. It occurs in massive form with magnetite in serpentine near Crescent City, Calif., and has been reported in a few other occurrences, including meteorites. Troilite, oxidizing in an inert gangue, leaves behind one-third of its iron in the form of indigenous limonite as ferric oxide or ferric oxide hydrate. Two-thirds is exported (ch. 8). In contrast pyrite (FeS 2 ) , oxidizing in an inert gangue, usually leaves no indigenous limonite. A mixture of 1 mole of troilite (or 1 mole of chalcopyrite) to 1 mole of pyrite, oxidizing in an inert gangue as ferric oxide or ferric oxide hydrate, usually leaves no indigenous limonite. Pyrrhotite, in contrast to troilite, has a little more sulfur (the standard is Fe,Ss, though it varies considerably, chapter 8). The theoretical compositions are as follows: Troilite (FeS) ................................... Pyrrhotite (Usually Fe$,)............... Chalcopyrite (CuS.FeS).................. Pyrite (FeS,).....................................

FIGURE 25. Radiating fibrous structure of limonites derived from pyrite and pyrrhotite, with a succession of superimposed layers. Usually limonite derived from pyrite is comparatively flat, while limonite derived from pyrrhotite is somewhat semi-spherical. A. Limonite from pyrite. Composite spherulitic growths, made up of mutually interfering repetitions of the unit pattern. The "pin point" limonite shows clearly when magnified. The spherulitic growths, or spongy interior, inside the growth shows clearly also, though the spongy interiors are not usually as regular as indicated. Limonite from pyrite, as a rule, is smeary on the exterior, and the nodules have a run-together appearance. B. Limonite from pyrrhotite (troilite). Composite semi-sphcrulitic growths, showing unit forms. Usually limonite from pyrrhotite grades insensibly into the limonite derived from pyrite. (After Bryan, Royal Soc. Queensland Proc., 1941)

Fe%

s%

63.5 60.4 30.5 46.6

36.5 39.6 35.0 53.4

Cu%

34.5

In nearly all of the chalcopyrite there is admixed pyrite. Although we cannot see it with the unaided eye (see ch. 8), high magnification usually shows it. Therefore, either pyrrhotite or chalcopyrite, oxidizing in an inert gangue, leaves about one-quarter to nearly onethird of its iron behind as ferric oxide or ferric oxide hydrate. Three-quarters to two-thirds would be exported. Pyrrhotite (using the formula Fe,Ss), upon oxidizing by reaction in the presence of a rock of moderate neutralizing power, usually would leave one-quarter to nearly one-third of its iron as cellular boxwork or sponge, but two-thirds or more would be fringing limonite. Presumably, with a strong neutralizer more of the iron would be left as boxwork sponge, or fringing limonite. The author has not seen many occurrences of pyrrhotite in gangues of great neutralizing power. Cellular boxwork derived from pyrrhotite generally is hexagonal in shape (fig. 27); but in some cases long, slender, trough-like forms emerge erratically. Cell waIls nearly always are coated with thin, flaky or shriveled limonite crusts when derived from essentially pyritefree pyrrhotite (figs. 26, 28b, 29, and 30). Cellular sponge derived from pyrrhotite has more of the oval pattern, with an underlying hexagonal shape usually detectable. Generally, cellular boxwork and sponge are found together in the same specimen, grading into each other. The oval structure is more pronounced in the sponge; the larger oval patterns have

123

PYRRHOTITE

some smaller oval patterns in them; and in the oval structure more of the radiating fibers are observable, with "pin point" limonite creeping over the minor obstructions in the structure (figs. 27b, c, 31 and 32). Varieties of fine relief lirrrnnite have been correlated with pyrrhotite in certain dinicts; for example, the deposits at Herbcrton, QucLu::;land (chloritized quartz-

ite), and Ducktown, Tenn. (schists and graywackes), contain relief limonite in moderate abundance, with cellular boxwork or sponge in between. But relief limonite derived from pyrrhotite is not, to the author's knowledge, very common. Figure 25 contrasts the radiating fibrous limonite derived from pyrrhotite with that derived from pyrite.

...r - - - - - - - l

'-------z FIGURE 26. Flaky or shriveled limonite crusts. 1. The outline of a siliceous limonitic jasper pseudomorph derived from pyrrhotite. About 15 percent is silica; the remainder is essentially ferric oxide hydrate. 2. Flaky or shriveled limonite crusts. About 2 percent is silica; the remainder is ferric oxide hydrate. More generally, the siliceous limonitic jasper outlines contain about 5 to 50 percent silica; flaky or shriveled crusts about 1 to 15 percent silica. The flaky or shriveled limonitic crusts are delicate as a rule. They are fragile, and crumble when shaken, or when lightly tapped with a pick. The flaky or shriveled limonitic crusts may be observed to best advantage directly above the water table because ofthe continuity of saturation and abundance of sulfide prevailing there. As oxidation penetrates more deeply into the sulfide residual, and the gelatinous material in any given case emerges more definitely from the zone of sustained saturation into one of open air circulation, the gelatinous composition gradually dries out. Because of its thinness, the entire crust tends to curl into a flaky shape; whence the name. As result of contraction the flake's surface often becomes shriveled, as in "corn flake" structure (see Flaky Crusts, ch. 16). All the sulfides but pyrite yield flaky or shriveled limonite at one time or another; but the flaky, shriveled limonites vary in the cases of the different sulfides. Only the flaky, shriveled limonite from pyrrhotite is illustrated. Two Treys mine, Herberton, Queensland, Australia. Enlargement 15x.

11111---2

-------2

A

B

c

FIGURE 27. Sketches showing characteristic structures and structural relationships of pyrrhotite derivatives. A. Polished surface of pyrrhotite. 1. Hexagonal shape. 2. Sponge structure. The contacts are emphasized. Zinc Corporation, Broken Hill, New South Wales, Australia. 16th level. Enlargement 2x. B. Characteristic coarse and fine boxwork and cellular sponge derived from pyrrhotite. Outcrop. The circular or semi-hexagonal, fine cellular "pin point" limonite of pyrrhotite has more body than the pyrite "pin point" limonite. 1. Coarse cellular box work. 2. Fine nodular "pin point" limonite. Burra mine, Ducktown, Tenn. Enlargement 2x. C. Characteristic coarse and fine cellular sponge derived from pyrrhotite, with a little (about 10 percent) finely divided pyrite. Weathered outcrop. 1. Cellular sponge. 2. Fine cellular "pin point" limonite. The presence of the pyrite caused this "pin point" limonite to be much flatter in cross section than the "pin point" limonite derived from the pyrrhotite in B. Boyd mine, Ducktown, Tenn. Enlargement 2x.

124

INTERPRETATION OF LEACHED OUTCROPS

B

A

FIGURE 28. Comparison of leaching products derived from pyrite and from pyrrhotite. Pyrite. Characteristic botryoidal limonite crusts from an outcrop at Mount Isa, Queensland, Australia. Natural size. B. Pyrrhotite. Characteristic cellular sponge derived from pyrrhotite in silica breccias. The cellular sponge is comparatively thin, but the flakes curl up when the limonite dries out, hence the thickness of flaky limonites. Black Rock, 3rd level, Mount Isa, Queensland, Australia. Natural size. A.

FIGURE 29. Oxidation of pyrrhotite in chloritized quartzite. Compare this specimen with figure 17, chapter 18. Boxwork is thin, with many distinct individual cells; but long, slender, trough-like boxwork emerges erratically. The flaky limonites, when they dry out, partially hide the trough-like boxworks. Two Treys mine, Herberton, Queensland, Australia. Natural size.

FIGURE 30. Fresh cellular sponge derived from pyrrhotite along a fracture in chloritized quartzite. The flaky limonites are fine, not as thick as those shown in figures 28B and 29. Two Treys mine, Herberton, Queensland, Australia. Enlargement 2x.

PYRRHOTITE

FIGURE 31. Oxidized and weathered pyrrhotite sponge. Here pyrrhotite made up 80 percent and pyrite made up 20 percent of the mixture. The "pin point" limonite has been flattened, but the pyrrhotitepyrite mixture was fine grained, so that the cellular sponge shows distinctly. Boyd mine, Ducktown, Tenn. Natural size. Selected by W. W. Simmons, formerly Chief Geologist of Ducktown, and the author.

FIGURE 32. Sponge with oval cells, with many tiny "pin point" limonitic globules showing. In this material pyrrhotite made up about 90 percent and pyrite about 10 percent. Burra mine, Ducktown, Tenn. Natural size. Selected by W. W. Simmons, formerly Chief Geologist at Ducktown, and the author.

125

Chapter 20 ARSENOPYRITE Arsenopyrite may resist oxidation longer than pyrite, but when it once starts to oxidize it may proceed rapidly through catalytic action. In all cases observed by the author, arsenopyrite was accompanied by some pyrite or pyrrhotite. Where the gangue was either feldspar-rich rock or shale, mixtures of 75 to 90 percent arsenopyrite with 25 to 10 percent pyrite or pyrrhotite generally have yielded limonite in quantity, and scorodite (Fe 2 0 3 o As 2 0"o4H 2 0), or other arsenical oxidation products, mimetite, massicot, etc., may make up about half of the weathering residue. Usually a large proportion of the arsenic is retained as scorodite within the leached outcrop, when fresh; but scorodite very slowly goes to limonite (ch. 16). Even in strongly leached mixtures of arsenopyrite with pyrite or pyrrhotite, which field evidence suggests may have been exposed for thousands

or hundreds of thousands of years, there is no instance, so far as the author knows, in which the arsenic, as scorodite, has been wholly removed from an outcrop whose parent sulfide carried several percent arsenopyrite. In quartz-rich gangue, usually the scorodite is free of all oxidized sulfides; but the faded apple-green ferric arsenate shows to some extent in the limonite. When mixtures of arsenopyrite with either pyrite or pyrrhotite occur in strongly neutralizing gangue, the resulting limonite is not strongly bound together, and fluffy limonite results. When arsenopyrite is oxidized, any ferric arsenate or copper arsenate once precipitated is likely to be stable. Some As,OG (arsenolite-arsenic trioxide) could be co-precipitated with limonite, and remain in non-crystalline, unrecognized form.

FIGURE 33. A typical oxidized derivative of gold-bearing massive, intimately mixed arsenopyritepyrite mixture in which the ratio is approximately 1 mole of arsenopyrite to 2 moles of pyrite. Shale gangue. The whole mass is bound together by minute intergrowths of limonitic jasper, which makes the product clinker-like, and gives it high rigidity. The stubby projecting clusters of tiny granules are not fully discernible without the use of a hand lens. Enterprise property, Hodginson Goldfield, Queensland, Australia. Enlargement 3x. (After Blanchard, Econ. Geology, 1942, p. 600)

127

ARSENOPYRITE

Arborescent limonite derived from arsenopyrite consists of a porous clinkery mass made up of looselyaggregated but firmly-joined granules, with characteristic blunt orthorhombic crystal form and corroded edges. It has a resinous to sub-resinous luster. The whole mass is bound together by minute intergrowths of limonitic jasper which makes the product clinkerlike, and gives it high rigidity. The granules are built up largely of small branching clusters or knob-like projections that occur in unsymmetrical shapes 2 to 3 mm in height, and which usually exceed in height by several times their habitually variable thicknesses. They are disposed indiscriminately toward one another with

FIGURE 34. Leached derivatives of a mixture of arsenopyrite and pyrite in slate and mica-schist country rock. Sulfides were originally present in a 3 to 2 ratio of arsenopyrite and pyrite, and the tenor was about 5 percent lead and 4 percent zinc. With increased weathering the scorodite-limonitic jasper has become corroded and pitted at points of attachment of slender scorodite excrescences, so that the original ribs of the boxwork now have become a series of interrupted chains of arborescent projections; loosely-aggregated but firmly-joined granules have given it high rigidity. Mount Bonnie, near Grove Hill, Northern Territory, Australia. Enlargement 1.5x. (After Blanchard, Econ. Geology, 1942, p. 608)

total disregard for orientation, giving rise to a highly porous, shapeless precipitate. Granular fretwork. If exposed to a long period of weathering, especially with moderate silica, the scorodite tends to corrode and perforate the jasper by "eating" holes into it at the points of attachment. This not only increases porosity of the granular mass as a whole, but often causes the more delicate projections to collapse. Some are carried away by mechanical erosion; others tend to "glue" themselves discordantly to the underlying mass, accentuating further the loosely-aggregated arborescent clinker-like structure (fig. 33). Although some scorodite persists in leached derivatives under the most severe weathering conditions, a slow replacement of the mineral by limonite nonetheless takes place. Under magnification of X 20 or X 30, the initial attack upon the individual granule appears spotty, somewhat resembling the skin of a boy with large and excessive freckles (ch. 16); thereafter the limonite laboriously spreads over most of the scorodite grain. When an entire cluster or knob-like projection becomes thus affected, the shriveling may bring about compaction until the original granular knob stands out like a tiny stalagmite (see fig. 34 or pI. 2) Cellular boxwork. Distinct cellula! 1J0xVv'UJ K >hequently emerges from the poro"~: :.' ranular ma~" 'f arborescent and granular limonite, with an indiviul occurrence seldom exceeding J or 2 em in length (s,', fig. 35). The main ribs consist of limonitic jasper. will scorodite increasing toward the edges. Cross rib~. ,'f '.he

FIGURE 35. Cellular boxwork emerging from a granular scorodite-limonitic matrix. The parent material consisted of an arsenopyrite band VB to ~ inch thick, with a small amount of admixed pyrrhotite (about 19 arsenopyrite to 1 pyrite). The stubby, knoblike, c1inkery structure is similar in appearance to that of the specimen shown in figure 33, but contains only about 15 percent silica. In some places the delicate, interconnected limonite has produced a distinctly fragile fretwork structure. Specimen from near the Moore shaft, Conrad mine, New South Wales, Australia. Natural size. (After Blanchard, Econ. Geology, 1942, p. 604)

128

INTERPRETATION OF LEACHED OUTCROPS

boxwork usually are poorly defined, and are feebly joined to the main longitudinal ribs. As in the case of true boxwork, the cross ribs form initially along the cleavage or fracture planes of the decomposing arsenopyrite. Their limonitic jasper content invariably is low, however, and they grade from cellular boxwork so gradually into the fragile, slender excrescences of the surrounding granular fretwork, that a clear-cut distinction between the two products is often difficult. Although distinct cellular boxwork does not develop prolifically, the hand lens discloses throughout the more granular portions many incipient ribs which parallel in a general way the main boxwork structure. These incipient ribs give the impression of having started out as true boxworks; but, through inability to incorporate into their compositions sufficient limonitic jasper (ch. 16), not only have they been unable to project boldly forward, but usually have been unable to maintain, against the surrounding arborescent encroachments, their modest rib-like beginnings. Disseminated sulfide type. Leached derivatives of disseminated arsenopyrite-pyrite and arsenopyrite-pyrrhotite mixtures usually retain their identifying characteristics if total sulfide content is well below 10 percent; but the limonitic jasper may give way to a more pulverulent or fluffy type-especially if the gangue possesses moderately strong neutralizing power, as limy shale. Such a derivative of arsenopyrite-pyrite mixtures possesses little or no limonitic jasper to act as a binder, and disintegrates more rapidly under weathering conditions. If the outcrop is subjected to a long period of erosion, the derivatives may lose most of its identifying characteristics. The former presence of arsenopyrite is indicated by the acicular shape of the cavities as contrasted with granular or cubic casts of pyrite and the oval pattern of pyrrhotite. In the case of ores containing galena with arsenopyrite, some of the oxidized minerals are difficult to recognize and interpret, especially mimetite (3Pb 3 As 2 Os.PbCI 2 ) . The mimetite is scattered through the leached derivatives, often as minute single grains, rarely in blebs or aggregates up to 1 mm across. Its color blends into faded olive drab of scorodite, commonly

the mineral particles are globular rather than crystalline, and where crystal form is developed the edges often are corroded even more than are those of scorodites, making recognition of the hexagonal-dipyramidal crystal form difficult. Added to these factors is the mineral's more ready replacement by limonite, which makes it difficult to distinguish from decomposing scorodite except in freshly oxidized specimens. 1 Mimetite usually is present in much smaller amount than the scorodite; but in exceptional cases the arsenic in the mimetite has constituted 10 to 16 percent of the total arsenic in the oxidized product, as in specimens from the Conrad and Mount Bonnie mines Australia, shown in table 7. Its chief significance as an intergrowth with the scorodite-limonitic granules is that the usually smaller grain size and greater susceptibility to decomposition, renders the granular excrescences still more fragile, causes them to break down irregularly under weathering, and thereby accentuates the arborescence of the final leached product. A noteworthy feature is the presence of earthy dullyellow massicot (PbO) as a further decomposition product of the mimetite in many of the strongly leached specimens. Usually the amount is small; but Yz to 1 percent is not uncommon. The mineral is by no means rare in Australia (see table 7). 2 'Familiarity with the mineral in oxidized derivatives is best acquired by microscope study, after which its identification under the hand lens becomes easier. 2In small amount, massicot is associated with mimetite in outcrops of the silver-lead-zinc orebodies at Mount Isa. At the Longamundi property, 56 miles northeast of Mount Isa, massicot constitutes 95 percent or more of lead minerals along a 15to 50-foot wide lode which is intermittently exposed for more than 5,000 feet. From three trenches along a 1,650-foot length of the lode, each 3 to 10 feet deep, the author obtained by channel sampling an average 30-foot width yielding 2.94 percent lead (3.74 oz. silver), of which one 5-foot cut yielded 9.1 percent lead (8.4 oz. silver). A selected specimen yielded 19 percent lead. Subsequent trenching and sampling by the Aerial Geological & Geophysical Survey of Northern Australia, yielded up to a 50-foot width of 8.7 percent lead (5.22 oz. silver) per ton. At Longamundi the massicot occurs mainly as joint- and fracture-plane fillings, from a knife edge to 1 inch thick, in an irregular stockwork in graphitic shale.

...... IoU

0

TABLE 7 Analyses and Probable Mineralogical Compositions of Galena-Arsenopyrite Sulfide Ores and Their Derived Gossans STATE, PROPERTY, AND SPECIMEN NUMBER

NEW SOUTH WALES ,-COMMONWEAL TH ___

QUEENSLAND

1

Specimen No.

Type of

Ore

2

2a

'"S;"

"'"S;

0

'1.

'1

"c:

0.

0.

><

;:;; ~.

3

'"S;" 0. ~,

3a

4

0

en

;:;;

;;

;;

".""

".

><

N'

"c:

~MOUNT EMU_

SILVER RIDGE

,--CONRAD

5

'" 0.

~----NORTH AUSTRALIA ~MOUNT BONNIE_

~-IRON B L O W - -

Sa

5b

6a

7

0

I"'

0

en

if 0.

c: "c:

><

c: N' "c:

""n

><

~.

;; " 0.

':.

7a

7b

Sa

0

I"'

0

c: N' "c:

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3.7 If 1.2 .35 tf nd 3.7 37.5 nil .14 .035 4.3 19.7

4.1 tr .7 tr tr nd 1.8 42.4 nil .45 tr

><

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c: N' "c:

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1.62

1.16 tf 5.4 tr nd nd 6.2 25.8 nil 1.8 .085 7.0 23.4

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-

Analysis: Au Dwt... ...................... tr.-l 20.05 Ag oz ........... Pb% ............................. 8.42 Zn% .............................. 2.10 Cu% .............................. 1.39 Sn% ............................... 1.06 As% ............................. ,. 6.00 Fe% ............................... 6.35 S% ................................. 7.16 Mn% ............................. nd Cl% ............................... Al,03% .......................... Si02% ............................ Insol. % .......................... CaO% ........................... MgO% .......................... C02% ............................ Additional 0" ............... Probable Mineralogical Composition: Sphalerite...................... 3.15 Galena........................... 9.7 Stannite......................... 3.85 .7 Chalcopyrite ................. Arsenopyrite ................. 13.0 Pyrrhotite ...................... 1.3 .7 Pyrite ............................. .075 Tennantite (est.) .......... Smithsonite.................... Cerussite ........................ Mimetite ........................ Massicot. ....................... Malachite 's.................... Scorodite18•••••..••••....••.•.• C;.lcite equivalent... ...... C('C.i",:rv r·x:k (est.) ...... LiIllOflliiC

1.1 37.6 13.5 3.3 2.7 2.1 14.4 12.8 13.0 nd

4.43 5.92 3.55 12.05 .75 nd 6.70 27.10 35.6 nd

9.5

4.1 tr .65 tr .25 nd 3.85 39.4 tr .3 .02

.9 3.2 .75 1.6 nd nd 3.0 6.6 6.0 nd

18.1 tr .02 .15 25.48

68.4

5.7 13.0 7.2 3.2 nd nd 18.3 17.1 13.25 nd

32.6

3.8 .72 5.4 tr nd nd 13.5 25.7 tr nil .16

4.1 nil 1.05 nil nd nd 4.4 34.9 nil nil .005

nd nd 11.2 .55 nd nd 3.9 38.5 nil tr .12

21.6 nd nd tr 26.08

24.3 nd nd nil 23.035

13.0 nd nd 1.7

4.2 13.6 3.0 5.4 .43 nd 4.9 30.25 35.0 nd nil nd nd nd nil .14 tr

24.19

nil tr .21 23.775

20.3 nil nil nil 24.94

1.1

14.8 1.05 nd nd 11.15 16.1 tr 1.6 .20 9.2 21.0

Z

-l

m

;;<:I -0 ;;<:I

m

-l

;.. -l

25

Z 0

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r

.4 tr 2.5 19.50

.1 tr .2 20.575

m

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:I:

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0

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-l

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4.93 15.58 7.63 1.25 31.25 1.08 .10

19.0 4.1

2.39 .87

4.78 8.31

8.06 3.46

2.15 14.55 .25 45.65 .025

6.52 3.26 4.91

39.74 3.37 2.85

1.24 10.64 2.35 53.24

1.70 4.25 15.635 .63

ji-:':·J'f'r18 • UI•. __ ••.

Miscellaneous" 32.475

1.0 2.2 15.9 1.1 tr Ir 11.1 19.1 tr tr .375 2.8 17.8 nd nil tr 1.3 19.67

61.82

tr .86

6.67 .40

.21 .89

0

-0

UJ

.86 8.57 5.00 .19

.54 .13 1.45

1.60 10.13 8.40

tr .70

26.92

.44 11.45

38.48

13.47

9.67

10.71

5.54

40.01 10.855 100.000

75.39 11.86 100.00

46.89 7.56 100.00

73.12 12.31 100.00

68.79 6.92 100.00

78.08 9.09 100.00

84.35 9.41 100.00

85.725

17.95

59.05

78.99

30.43 .71 18.00 28.22 2.51 100.00

tr .73 3.55 2.13 17.43 .18 15.00 51.54 9.44 100.00

lFor Nos. 2a, 3a, 5a, 5b, 6a, 7a, 7b, 8a and 8b, Pb and Zn figures of the analyses are for PbO and ZnO respectively. 2Average composition of sulfide are along 46oo-ft. continuously stoped section of Conrad lode. Average depth of sampled sections 300 to 400 feet. Country rock, granite. Conrad mine near InvereU, New South Wales. "Representative high grade are, free from banded sulfides, at NW end of Conrad lode, near King shaft.
~ [JJ

t"rl

Z

o

'"><

~

=i

1"1

w

Chapter 21 CHALCOPYRITE pyrite and pyrite. Complete leaching would take place (ch. 8). Chalcopyrite, upon oxidizing in a moderately strong neutralizer, usually would leave one-third of its iron behind as cellular boxwork or sponge; but two-thirds of its iron would be deposited as fringing limoniteessentially granular or pulverulent. Chalcopyrite, upon oxidizing in a strong neutralizer, would leave one-third of its iron behind as cellular boxwork or sponge. Two-thirds of its iron would be granular or pulverulent, fringing or fluffy limonite. But secondary or tertiary minerals-malachite, azurite, or oxides of copper-would play a part; the malachite or azurite might be indigenous, fringing, or exotic. 2 Rapidity of oxidation would be the determining factor in these cases. Sometimes storm waters would carry the fluffy limonite away in granular or pulverulent form, especially when there is low silica; the extent to which this

Massive chalcopyrite and hypogene silica were the first minerals studied under the microscope to find out what was happening in the formation of the supergene products. It was observed that the limonite developed at first along the cracks in which the hypogene silica had been deposited. Boxwork formation in other sulfides subsequently was investigated. Chalcopyrite and sphalerite, because they contained more hypogene silica veinlets than the other sulfides, showed especially well how the boxworks had developed. Theoretically, when chalcopyrite oxidizes in an inert gangue, all of its copper and two-thirds of its iron should be exported in solution, leaving one-third of its iron as indigenous limonite, mostly in the form of cellular boxwork or sponge (see fig. 39)." Oxidation of chalcopyrite alone generates no free acid. In contrast, 1 to 1 mixtures of chalcopyrite and pyrite oxidizing together in an inert gangue would dissolve and export all of the copper and all of the iron of both the chalco-

"Malachite and azurite sometimes are seen also where chalcopyrite has oxidized in gangues of moderate neutralizing power, perhaps even in inert gangues although this is not so likely in semi-arid regions. Limonite in such cases would be mostly fringing or exotic.

'But see chapter 19, in which it is explained why these theoretical conditions are seldom attained in oxidizing chalcopyrite bodies.

~-----2

oE

2

3

3 3

A

--2

B

C

FIGURE 36. Sketches of characteristic boxworks and other leaching products derived from chalcopyrite. A. Characteristic coarse boxwork. 1. Coarse cellular boxwork. 2. Generally broken webwork. 3. Granular and pulverulent limonite. Note the poorly defined cross structure and generally broken character of the webwork. Also note that the granular and pulverulent limonite (slightly coalesced in surface exposures) not only coats walls of cells but frequently fills part of the intercellular space. Absence of pulverulent limonite in some cells is probably due to admixed pyrite at such places in the sulfide ore. This is a "key" structure for chalcopyrite. Rocher de Boule mine, British Columbia. Enlargement 5x. (After Blanchard, Chelll. Metall. Mining Soc. South Africa Jour., 1939, p. 346.) B. Characteristic parallel, quadrangular cell pattern of limonite boxwork. 1. Conspicuous parallelism of cell walls. 2. Thin-walled, sharply angular webwork between larger parallel cell walls, rarely suggestive of triangular pattern. 3. Granular and pulverulent limonite. Duquesne, Ariz. Enlargement 5x. (After Blanchard and Boswell, Econ. Geology, 1930, p. 560) C. Characteristic cell pattern of limonite derived from chalcopyrite. Average type. 1. Coarse cellular boxwork. 2. Generally broken webwork. 3. Granular and pulverulent limonite. Creston Verde property, near Choix, Sinaloa, Mexico. Enlargement 5x. (After Blanchard and Boswell, Econ. Geology, 1930, p. 560)

133

CHALCOPYRITE

occurred would depend on the age of the outcrops in semi-arid regions. But one-third of its iron would be present as indigenous limonite in any event, whether or not malachite or azurite were present (ch. 25). Cellular boxwork derived from chalcopyrite is always angular, whether coarse or fine (fig. 36). Limonite derived from well fractured chalcopyrite has a tendency to concentrate along ribs that continue in straight lines past numerous cells. Fractures are uneven because of

'"

the inherent fracture pattern of chalcopyrite itself, but mostly they are microscopic. The angles between ribs are generally obtuse, from 95° to 110° (the angle of 110° is rare; but see pI. 4, where in places more acute angles are visible). The webwork of cellular boxwork is discontinuous and fragile, decreasing with increasing amount of admixed fine-grained oxidized pyrite (see fig. 38). When almost pure, chalcopyrite leaves a flaky crust, but with increasing content of admixed pyrite the

2

.

I

-c

2

3

3

B

A

FIGURE 37. Sketches showing characteristic limonite boxwork derived from chalcopyrite. 1. Large scale box work. 2. Fine cellular boxwork, or webwork. 3. Granular and pulverulent limonite. A. The longitudinal and the cross structures are well shown. The original material contained 33 percent copper and was almost pure chalcopyrite. Orphan mine, Dobbyn, Queensland, Australia. Three times natural size. B. The original material from which this boxwork was formed was almost pure chalcopyrite. The specimen was collected in the Marshall shaft, 88 feet deep, 4Y2 miles northeast of the point at which the specimens shown in plates 5 and 6 were obtained. lervois Range, Northern Territory, Australia. Five times natural size.

~iimii· A

B

FIGURE 38. A. Sketch showing characteristic pattern of oxidation of essentially pure, disseminated chalcopyrite in a gangue of moderate neutralizing power, such as an orthoclase feldspar rock. 1. Outline of cavity. 2. Coarse cellular boxwork, or webwork. 3. Granular and pulverulent limonite. The cellular structure is composed of limonitic jasper that has "eaten" its way along fracture planes during incipient oxidation. The granular and pulverulent limonite represents iron precipitated from the oxidation solutions during leaching of the intercellular sulfide residuals; the limonite outside of the cavity has been precipitated largely through the agency of gangue neutralizer. That the neutralizer is moderately efficient as a precipitant is proved by absence of limonite "fog." Longamundi, Queensland, Australia. Englargement lOx. B. Sketch showing characteristic oxidation of a disseminated mixture of chalcopyrite and pyrite in a ratio of about 3 to 2. 1. Outline of cavity. 2. Cellular boxwork, or webwork. 3. Granular and pulverulent limonite. Note that the cellular structure is less angular and more rounded than that of A, denoting corroding and dissolving action by acid solutions, which in this case have been derived from oxidizing pyrite subsequent to the formation of the cellular structure. Note also that the central portion of the cavity has been more thoroughly cleaned of its iron than in A; pointing to probable concentration of the pyrite in this portion of the nodule. Concentration of pyrite there explains absence of cellular limonite in central portion of the nodule; the cellular structure has been formed and is preserved only in the portions that were more dominantly composed of chalcopyrite. Denser precipitation of limonite immediately surrounding the cavity than seen in A, points to stronger neutralizing power of the gangue. The gangue in this case is monzonite. Nevada Consolidated Copper Company pit, Ruth, Nev. Enlargement lOx. (Both after Blanchard, Chern. Metall. Mining Soc. South Africa Jour., 1939, p. 354)

134

INTERPRETATION OF LEACHED OUTCROPS

coatings of cell walls grade into finely granular or pulverulent limonite (fig. 37, 38b). Plates 3 to 6, and figures 39 and 40 are illustrations of cellular boxworks derived from the leaching of chalcopyrite in several types of gangue. Cellular sponge, lcss abundant than boxwork, nevertheless is common. It "eats" its way irregularly around borders of sulfide grains, but oxidizes and crumbles so rapidly that it cannot form masses that continue past individual cell walls, as do the boxwork structures that are formed by a more leisurely process. Often the sponge has a more crinkly structure than the boxwork, probably because the original chalcopyrite in such areas contained more fine-grained, uniformly dispersed pyrite

FIGURE 39. Left side and upper part: a fresh surface of coarse and fine cellular boxwork derived from chalcopyrite in quartz gangue. Note limonite boxwork that continues past numerous cells in parallel lines with broken structure between veinlets. Lower part, quartz. The exported iron from oxidized pyrite is not precipitated around outer edges of cavities, because the quartz carries no neutralizer; the iron has been carried by oxidation solutions entirely outside the cavities, and these are leached. Pyrite residuals are still visible in several cavities. Los Aliodos property, Sonora, Mexico. Natural size. (After Blanchard and Boswell, Eng. Mining Jour .• 1928, p. 281)

than did the chalcopyrite in places where boxwork was produced. Relief limonite in some districts has been traced back gradationally to its parent chalcopyrite usually but not always intermixed with bornite; and ghosts of fine cellular boxwork have emerged dimly through the porous granules. But low acidity is required for the formation of relief limonite. With more pyrite and higher acidity, fringing and exotic limonite types result. Fine-grained relief limonite derived from chalcopyrite is shown in plate 7. Glassy, compact type. Chalcopyrite yields the most spectacular examples of glassy, compact limonite, but many varieties of this type also are produced by the oxidation of other sulfides. Glassy, compact limonite in time tends to alter to pulverulent limonite. Chalcopyrite varnish is composed of a "pile" of thin films of ochreous to brown limonite, superimposed one on top of another. The "pile" rarely exceeds 0.25 mm in thickness. Generally, varnish occurs along fractures within nodules of massive chalcopyrite, or as coatings in disseminated specks. Chalcopyrite pitch usually coats sulfides undergoing oxidation, and spreads or flows out over their edges to coat and replace surrounding specks. Both varnish and pitch are distinguished from desert varnish by their glassy surfaces of ochreous or brown rather than the brownish-black to black color.

FIGURE 40. Coarse and fine cellular boxwork derived from chalcopyrite in a feldspar-rich gangue. Two prominent chalcopyrite residuals (indicated by arrows), in part attacked, remain in the left center and top of the picture, serving to remove any possible doubt as to the limonite product's origin. Tres Hermanos property, southwestern Chihuahua, Mexico. Enlargement 2x. (After Blanchard, Chem. Metall. Mining Soc. South Africa Jour., 1939, p. 347)

Chapter 22 CHALCOCITE Theoretically chalcocite (Cu~S) contains only half the sulfur necessary to put all of its copper into solution as copper sulfate (ch. 8). In the Kennecott orebody, Alaska, and in a few other places not so large, chalcocite was mostly hypogene; but usually chalcocite is supergene. When supergene chalcocite is formed in chalcopyritepyrite orebodies, it replaces chalcopyrite much more extensively than it does pyrite. Thus in the upper parts of disseminated copper deposits in the semi-arid regions the copper is chiefly or entirely in the form of chalcocite. When the ore is subjected to oxidation in the presence of sufficient pyrite, the pyrite will supply the necessary sulfuric acid and ferric sulfate to dissolve all of the copper as copper sulfate. Otherwise, part of the copper remains in the capping as malachite, azurite, chrysocolla, or oxides of copper. If pyrite and chalcocite are present in an inert gangue in the proportion 2 to 1, their oxidation together would leave an empty cavity. However in the same type of

2~ I :'. ,' 3-~'.·

A

gangue, when the same minerals are present in the reverse ratio, and oxidize together, the copper is all dissolved, but the iron remains-generally as sooty or slightly glossy limonite (fig. 41a). In gangues of moderately strong neutralizing power, predominantly feldspar-rich rocks, when 2 to 1 mixtures of pyrite and chalcocite oxidize together, an essentially empty cavity is left; both copper and the greater part of the iron are leached (see fig. 41b). In this case the iron is deposited either as fringing or halo limonite-essentially granular or pulverulent. When mixtures in the ratio of 2 moles of chalcocite to 1 mole of pyrite oxidize together, the copper is leached; but the iron remains, either as boxwork, or, more likely, as relief limonite. Examples of limonites derived from such a mixture of disseminated sulfides are shown in figure 44 and plate 10. When chalcocite-pyrite mixtures oxidize together in strongly neutralizing gangue, considerable amounts of

3;O'·:~~.".~. 2

I

.... , ,

B

FIGURE 41. Sketches showing characteristic oxidation products of chalcocite and pyrite present in different ratios in two disseminated deposits. A. Leaching products of an original 2 to 1 mixture of chalcocite and pyrite in a quartz-sericite schist of weak neutralizing power. 1. Outline of cavity. 2. Cellular boxwork of sharply defined, angular pattern. 3. Granular or pulverulent limonite; about 18 percent is silica, the remainder is ferric oxide. The structure is intermittent and faltering in outline, and fragile throughout. Precipitation of the locally exported iron as limonite grains immediately outside the cavity in such gangues, corroborates other testimony that chalcocite predominated markedly over pyrite in the sulfide specimen. Miami, Ariz. Enlargement lOx. B. Characterisitc oxidation products of a disseminated 2 to 1 mixture of pyrite and chalcocite in a slightly kaolinized monzonite gangue. 1. Outline of cavity. 2. Limonite crusts. 3. Granular or pulverulent limonite. The pseudo-cellular structure around the inner edge of the cavity represents mostly limonitic crusts rather than cellular boxwork. Microscopic "pin point" limonite crusts, although present in the specimen, do not show in the sketch. Tyrone, N. Mex. Enlargement lOx. (Both after Blanchard, Chell!. Metall. Mining Soc. South Africa Jour., 1939, p. 355)

FIGURE 42. Cellular limonite boxwork derived from a mixture of chalcocite and pyrite in about a 2 to 1 ratio, that has oxidized along a fracture in monzonite. Both longitudinal and cross veins are shown. Silica about 20 percent; the rest is principally supergene hematite. Clay orebody, Morenci, Ariz. Enlargement 1.25x. (Modified from Blanchard and Boswell, Eng. Mining Jour., 1928, p. 280)

136

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 43. Cellular limonite derived from a finely disseminated chalcocite-pyrite mixture in a feldspar gangue. Weathered surface. Disseminated cavities are small, but the darker ones are almost filled with indigenous limonite. Ore beneath such cropping will analyze between 2 and 2112 percent copper as chalcocite. Outcrop over the high-grade portion of Racket orebody, Tyrone, N. Mex. Enlargement 2x. (After Blanchard and Boswell, Eng. Mining Jour., 1928, p. 283)

FIGURE 44. Limonite derived from disseminated chalcocite and pyrite present in the proportion 2 to 1 in well kaolinized granite porphyry. Ore beneath such an outcrop (120 feet down) will analyze 7 to 8 percent copper as chalcocite. Fresh surface. Outcrop of Sacramento Hill, Bisbee, Ariz. Enlargement 2x.

CHALCOCITE

malachite or other oxidized copper minerals are formed; but a certain amount of fluffy limonite is left, depending upon the iron-copper ratio, and depending on the degree of neutralization. Oxidation of chalcocite, as a rule, would leave more malachite than oxidation of chalcopyrite. (This is true also when the oxidation products of chalcocite are compared to those of bornite and tetrahedrite.) Eventually, in semi-arid regions, the malachite or other oxidized copper minerals tend to be replaced by ferric oxide hydrates in the course of geologic time. The color of limonite derived from chalcocite is generally maroon to seal brown-the maroon color being largely a chalcocite derivative (ch. 15). In gangue of great neutralizing power the color is apt to be a shade of medium brown. Relief limonite. By far the greater part of the chalcocite-derived limonites is of the relief type. Where 2 to 1 mixtures of chalcocite and pyrite oxidize together in a strongly neutralizing gangue, two distinct limonites are formed though they grade into each other to a certain degree. One type is characterized by high relief, with sooty, irregular, "heaped-up" texture, and porous, craggy and cavernous openings, with sponge rising above the general level (see fig. 45); the other type by a radial fibrous, spongy appearance, with some fine nodular "pinpoint" crusts and film coatings (see pI. 8) and a slightly glossy luster caused by the fact that silica is low. The process may take many hundreds or thousands of years to complete in the semi-arid regions. In the silky, sericitized gangues there is more fringing limonite, though indigenous limonite would still predominate. When 2 to 1 mixtures of chalcocite and pyrite oxidize together in a weakly neutralizing gangue such as quartz and sericite, only one type of limonite, the botryoidal and craggy crusts, results, even though both hematite and goethite are usually present. When 3 to 2 mixtures of chalcocite and pyrite oxidize together, the smeary and acid-washed limonite develops to a certain extent (see pI. 9).

137

Cellular boxwork has been observed only as a derivative of chalcocite-pyrite seams along fractures (see fig. 42). The planar cell walls continuing past several cells, such as were observed in the case of chalcocite-pyrite and bornite, are not found in the cellular boxworks derived from chalcocite; the structure of limonite boxwork from metallic supergene chalcocite is more wavering and fragile in its outline, with mere wisps or threads of limonite that lose themselves imperceptibly in the embedded sooty limonite of the relief type. Still, it is boxwork, not sponge. Only a few districts are well represented by wisps or threads of limonite derived from supergene chalcocite-for example, Morenci, Bagdad, and Miami, Ariz.; and Chino, N. Mex. Usually these districts have far more of the sooty limonite.

FIGURE 45. Craggy particles, derived from a mixture of chalcocite and pyrite in about a 2 to 1 mixture, in disseminated ore. Each little particle is set down individually as limonite. The end product comprises an irregular, craggy, porous, "heaped-up" limonite. Enlargement 8x. (After Locke, 1926, Plate XV)

Chapter 23 BORNITE Limonite derived from bornite (2Cu 2 S.CuS.FeS) is similar to that derived from chalcocite, whether it occurs in inert, moderately strongly, or strongly neutralizing gangues. When a mixture of 1 mole of bornite and 5 moles of pyrite oxidizes, all of the copper of the bornite and all of the iron of the pyrite and the bornite are dissolved and exported. However, when a 2 to 1 mixture of bornite and pyrite oxidizes, all of the copper is dissolved and exported, but the iron of both the pyrite and the bornite remains as indigenous limonite. Bornite, if not mixed with much pyrite, yields more indigenous

limonite than many of the other copper minerals, especially chalcopyrite and chalcocite (see ch. 8). Triangular or trapezoidal boxworks are the identifying or "key" structures of the limonite derived from bornite. Usually they are present unmistakably to a greater or lesser degree, except when pyrite and bornite are present in the ratio of 5 to 1 or greater. Cellular boxwork derived from bornite is characteristically triangular, sometimes trapezoidal in shape, though in some instances elliptical or eye-shapes prevail (see fig. 46). Larger structures may contain fine

a a

---"0.

c

A

B

FIGURE 46. Sketches showing cell patterns and features of limonite boxworks derived from bornite. Limonite crusts, flakes, grains and pulverulent particles that ordinarily coat cell walls, and often nearly fill individual cells, are not shown. A. An extreme pattern of curved spherical triangular cells. Characteristic feaures include: (a) conspicuous spherical triangular or trapezoidal pattern, (b) occasional rounded or eyeshaped cell pattern, (c) triangular pattern, and (d) incomplete webwork within larger cell. Gabbro country rock. Engels mine, Calif. Enlargement 5x. B. An average bornite-derived limonite boxwork. Quartzite country rock. Lookout prospect, Black Mountain, N. Mex. Enlargement 5x. Analyses of Limonite Derived from Bornite at the Engels mine, California Triangular Cell Pattern (Percent)

Cell FiIIing (Percent)

----~-----------------------.---------

Malachite .................................................................................... .. Supergene gangue carbonates: Calcite ........................................................................................ . Magnesite ................................................................................... . Siderite....................................................................................... . Jarosite ........................................................................................... . Limonitic jasper: Ferric oxide monohydrate ........................................................ . Silica and alumina.................................................................... .. Miscellaneous ' ............................................................................... .

3.55

2.74

1.07 .21

.36

.42

1.60

.13 2.67

76.92

90.74

15.30 .93

1.80

100.00

100.00

1.56

(After Blanchard and Boswell, Eeon. Geology, 1930 p. 560; Chemical analyses from Blanchard, 1944, following p. 112, nos. 8 and 9) 'Consists mostly of adsorbed and capillary water.

BORNITE

cellular boxworks that are essentially similar in shape to the major structures; but in the finer boxworks there is not much silica, and a slight jar would disintegrate the limonite (see fig. 47). Cells usually are filled entirely or in part with partially sintered or cake crusts (on the surface) . Cellular sponge derived from bornite is characteristically rounded rather than sharply angular. Large cells

FIGURE 47. A sawed surface of a spherical triangular boxwork derived from bornite. The webwork and the partially sintered and caked crusts do not show; a slight jar had disintegrated the pulverulent limonite. Ruby mine, Plumas County, Calif. Enlargement 5x. (After Blanchard and Boswell, Ecoll. Geology, 1930, p. 559)

139

are commonly 20 to 30 times as large as adjoining small ones. Cellular sponge grades into boxwork, which nearly always emerges sporadically through the sponge mass (see fig. 48). Partially sintered crusts, except when fresh, coalesce to resemble in appearance the surface of dead-burned magnesite brick, one cluster encroaching upon another like mud dabs in a swallow's nest. Partially sintered crusts usually have a haphazard appearance, and make up 50 percent or more of the limonite. Fine cellular structure generally emerges through the partially sintered crusts, and may be detected by a careful search. Color on fresh surfaces is usually orange or orangeyellow. Figure 49 shows a boxwork that has been obliterated to some extent by a partially sintered crust. Partially sintered crusts are probably more abundant than boxwork and sponge. Caked crusts are kaolinic masses thoroughly impregnated with limonite. On the surface they usually show numerous polygonal shrinkage cracks, with clayey masses curled up so as to resemble in miniature the surface of a dried mud fiat. The clayey masses are generally composed of individual superimposed layers from 0.05 to 0.3 mm thick. Small isolated remnants of triangular boxwork are often visible, projecting through and around the caked masses. Ordinarily the product carries 30 to 40 percent iron oxide hydrate. Generally the color of the caked crusts derived from bornite is orange-red to Indian-red.

FIGURE 48. Limonite sponge derived from bornite. Note the gradation into triangular boxwork with frequent "wishbone" structure, caused by raised spherical triangular ribs. Cell sizes frequently are varied, cell walls are thin when compared with cell dimensions, and the webwork is incomplete. LaSal Mountains, Utah. Natural size. (After Blanchard and Boswell, Ecoll. Geology, 1930, p. 562)

140

INTERPRETATION OF LEACHED OUTCROPS

Relief limonite derived from bornite ranges from haphazard aggregates of partially sintered crusts to typical porous craggy relief limonite. It is especially prominent in kaolinized gangues, such as feldspar-rich rocks that have been attacked by acids, but on the whole relief limonite derived from bornite is less abun-

dant than that derived from chalcocite. Minute projections often stand out from the pulverulent matrix, giving to the whole mass a soft, velvety appearance. Discontinuous remnants of triangular boxwork or sponge are often discernible. Color on fresh surfaces is bright orange.

FIGURE 49. Boxwork derived from bornite. In upper left portion of picture the boxwork is completely obliterated by partly sintered crusts, elsewhere it emerges sufficiently to be readily distinguishable as having been derived from bornite. Las Palomas property, Jalisco, Mexico. Enlargement 3.5x. (After Blanchard and Boswell, Econ. Geology, 1930, p. 566)

Chapter 24 TETRAHEDRITE Tetrahedrite (Cu,Fe,Zn,Ag)'2(Sb,AsLS13 though in general less important than chalcopyrite, chalcocite, or bornite as an ore mineral of copper, so frequently carries high silver values ' that its identification in leached outcrops has commercial significance. Frequently it carries up to 18 percent silver, though usually it has 5 percent or less (Palache, Berman, and Frondel, 1946, p. 379)-the variety rich in silver being called 'At the Sunshine silver mine, Coeur d'Alene district, Idaho, argentiferous tetrahedrite is the chief ore mineral.

A

B

FIGURE 50. Sketches emphasizing characteristic contour boxwork derived from tetrahedrite. Omitted in the sketches are the sandy and dully resinous granules that coat cell walls and partly fill cell cavities. With cellular structure alone showing, the resemblance to a contour map of steeply mountainous country is striking. A. Gillespie property, Hachita, N. Mex. Enlargement 5x. B. World's Fair mine, Patagonia, Ariz. Enlargement 5x. (After Blanchard and Boswell, Econ. Geology, 1930, p. 571)

freibergite in silver and lead deposits. Frequently, too, variable amounts of arsenic are present, as there is a continuous series of solid solutions from the pure antimony end member to the pure arsenic end member. e X-ray study shows that the atomic structure of tetrahedrite is closely related to that of sphalerite. Most of the tetrahedrite studied in the field and laboratory in this investigation carried silver. It has not been observed that either high or low silver content affects the type or pattern of the limonite produced. Contour boxwork derived from tetrahedrite is the most distinctive boxwork yet encountered; it bears a close resemblance to a contour map of a steeply mountainous region. The limonite is generally hieroglyphic (see fig. 50). Cells are long, narrow, and approximately twice as deep as in other limonite types, curve abruptly, and have sharp angles. Cell structure is remarkably rigid, the limonite containing silica and being hard, with closely-joined cell walls. The characteristic color is chocolate to deep brown, the latter predominating. Typical contour boxworks derived from tetrahedrite are shown in figures 51 and 52. 2In an earlier description (Blanchard and Boswell, 1930, p. 596), tetrahedrite was given as 4Cu,S.Sb,S3, but it is now known that the correct formula is (Cu,Fe,Zn,Ag)12(Sb,As),S13.

FIGURE 51. Contour boxwork derived from tetrahedrite. Right edge of specimen shows coagulated type of limonite (illustrated further in figs. 53 and 54). Hachita, N. Mex. Enlargement 3x. (A/ter Blanchard and Boswell, Econ. Geology, 1930,

FIGURE 52. Contour boxwork derived from tetrahedrite. White incrustations of antimony oxides show plainly on the limonite boxwork. Elkhorn, Mont. Enlargement 4x. (A/ter Blanchard and Boswell, Eco/!. Geology, 1930,

p.570)

p.572)

142

INTERPRETATION OF LEACHED OUTCROPS

In cross section, the cells arc from 0.2 to 10 mm in length, the width is from 0.05 to 1.5 mm, and cell lengths are two to ten times the widths. Firmly coated on the cell walls are thin, continuous mats of finegrained, sandy, dully resinous granules that glisten faintly in sunlight-somewhat resembling roughened surfaces of sand paper, but granules more irregularly sized. Limonite frequently is encrusted in part by antimony oxides, which occur as colorless, white, or yellow scabs like barnacles, and locally incrust the cellular mass. 3 (See figs. 53 and 54). Coagulated type of limonite. This type is chiefly derived from tetrahedrite that is accompanied by some

FIGURE 53. Coagulated type of limonite derived from tetrahedrite. On the right contour boxwork is poorly defined; the structure comprises rather indefinite cellular aggregates of fine grained, sandy, dully resinous granules of limonite. White incrustations over the b<1xwork in left half of picture are antimony oxides. The low solubility of Sb,03 explains its ready deposition in outcrops close to leached tetrahedrite. Lake City, Colo. Enlargement 3.5x. (After Blanchard, Chern. Metall. Mining Soc. South Africa Jour, 1939, p. 353)

3A. S. Walker, who conducted extensive laboratory tests of antimony oxides at New Mexico School of Mines, stated (private communication, 1930) that the various oxides such as valentinite (Sb203), senarmontite (Sb,03; dimorphous with valentinite), cervantite (Sb,O, ?), and stibiconite (Sb,Or.(OH) ?), almost invariably are intergrown with one another in antimony oxide precipitates of the southwestern United States and Mexico; that definite and complete crystal forms are rarely developed; and that even where distinct crystal form points to such minerals as valentinite or senarmonite, tests generally show a 25 to 50 percent admixture of impurities such as silica and alumina. Where oxides develop in limestone gangue a greater or lesser admixture of secondary lime products occurs. Despite the common absence of individual crystal form, and the general presence of such impurities as alumina, silica, and lime products, no difficulty is ordinarily experienced in establishing the yellow-white incrustations as antimony oxides.

FIGURE 54. Coagulated type of limonite derived from tetrahedrite in the presence of abundant pyrite. Note, in upper half of picture, how the contour boxwork is coated over and locally largely obliterated by the coagulated limonite particles. White incrustations in lower part of figure consist of antimony oxides. Lake City, Colo. Enlargement 5x. (After Blanchard and Boswell, Econ. Geology, 1930, p. 574)

TETRAHEDRITE

pyrite. Where tetrahedrite greatly predominates, contour boxwork is prominent, and the coagulated particles merely form a thick coating on walls, and in places tend to fill in the cellular space. Where pyrite predominates, contour boxwork is partly obliterated, and the picture is rather that of scattered bunches of hieroglyphics embedded in the coagulated mass (see fig. 54, upper left).

143

The coagulated type may be distinguished from contour boxwork by: 1) sandy texture with many dully resinous but distinctly formed limonite particles derived from pyrite, in contrast with the "run-together" appearance of smeary limonite of pyrite origin only, 2) scabs or fracture fillings of antimony oxides in most, but not all, products derived from tetrahedrite.

Chapter 25 OXIDIZED COPPER MINERALS The oxidized copper minerals are malachite, azurite, cuprite, tenorite, and chrysocolla; or in dry climates, antlerite, brochantite, or atacamite (CuCI 2 .3CuO. 3H"O) in feldspar and limestone gangues. Malachite and azurite are easy to recognize, though in geologic old-age they tend to be partially or wholly replaced by limonite. In schists or in kaolinized or sericitized gangues in semi-arid regions, most or all of the oxidized copper minerals are replaced by limonite made up of crinkly flakes that fail to join with each other

to produce a rigid structure; usually very little limonite is left. Limonite flakes derived from the copper carbonates are much less effective in building up a structure than the flakes derived from pyrrhotite (see fig. 26, ch. 19). When cuprite (Cu 2 0) weathers, it generally goes to moderate or fine granular limonite, especially in feldspar, normal shale, or schist gangues. Plate 11 illustrates fluffy limonite derived from cuprite that was leached in the presence of calcite and dolomite.

Chapter 26 GALENA AND CERUSSITE Galena contains just enough sulfur to form lead sulfate; it does not generate free acid (ch. 8). The most common reactions involving galena are given in chapter 10. Galena first oxidizes to lead sulfate (anglesite), which is a nearly insoluble mineral that coats and encloses the galena and tends to prevent more oxygen from reaching it (ch. 10). Therefore galena does not oxidize readily, unless the oxidation condition is extreme (see descriptions of the Mount Stewart and C.S.A. mines, ch. 13). When present below the water table, however, galena is easy to oxidize in an inert gangue, more so than most sulfides. But galena generally is replaced by stable pseudomorphs of anglesite or cerussite, which protect it, and oxidation does not go far, as a rule, below the water table. When galena is present mixed with pyrite above the water table, however-especially when the gangue is feldspar, shale, or limestone-different conditions exist. When a 10 to 1 mixture of galena and pyrite oxidizes in feldspar or shale there is not much change; but when a 1 to 5 mixture of galena and pyrite oxidizes in such gangues, the iron goes into solution moderately rapidly and some of the lead is also dissolved. In strongly neuFIGURE 55. Sketch showing cleavage boxwork derived from galena, as it appears when divested of partially sintered crusts. The long, minutely thin parallel ribs and cubic boxes are replicas of galena's cleavage, proof that limonitic jasper penetrated along the galena's cleavage planes during incipient oxidation before the sulfide residuals had been convertA ed into anglesite and cerussite from which the partially sintered crusts later were de.IA)I~~ rived. A characteristic feature is the thinness ~~ of the cellular walls, often not more than 0.005 mm. thick, and the main reason that the boxwork is seldom well preserved in the outcrops. Lawn HiIl, Queensland, Australia. Enlargement 3x. B. Sketch showing diamond-mesh boxwork. Note the irregular diamond-mesh B structure, with only occasional cubic boxes preserved. Note more particularly the pairs of closely spaced, rigidly parallel, minutely thin webs that constitute the structure's major ribs. UsuaIly the boxwork is coated with partiaIly sintered crusts, which in part obscure the diamond-mesh pattern (see fig. 56). Close conformity in pattern of the structure to certain crystallizations of cerussite suggests the pattern may be derived in some instances from cerussite rather than chiefly directly from galena. From an outcrop at Ruby HilI, Eureka, Nev. Enlargement 3x. (After Blanchard and Boswell, Econ. Geology, 1934, p. 675) A.

'~ w;o

tralizing gangues pyrite is also known to hasten the solution of lead products. The principal outcrop indications of the former presence of galena are partially sintered crusts, cleavage boxworks, including diamond-mesh boxworks, and relief limonites. Partially sintered limonite crusts derived from galena differ from those derived from bornite, though they resemble each other closely. Both are indigenous, and both have the haphazard appearance; but the sintered crusts derived from galena are more porous than are those from bornite. The limonite crusts derived from galena are in large part replacements of cerussite that resulted from the oxidation of galena. The limonite is more open and loosely aggregated, and extends out into the cell sufficiently to reduce cell diameter by one-half. Usually some further coalescence develops after formation of the limonite granules, especially in surface outcrops, but grains of unleached cerussite often remain. The color of the limonite is distinctly brownish, especially when weathered, though fresh material is orangeyellow or occasionally orange-red. Partially sintered crusts make up 50 to 90 percent of the lead-derived limonites of most outcrops. Cleavage boxwork derived from galena consists of a series of straight, parallel very thin ribs (0.005 to 0.05 mm thick) of limonitic jasper (see figs. 55a, 56). Frequently, other parallel cross walls run at right angles. Where this occurs, a cubic boxwork forms. This is a "key" structure, present to some extent in nearly all of lead-derived indigenous limonite, even though it is not common. Cell walls are nearly always coated and merged with partially sintered crusts to fill up half of the cell space. All gradations are known, from partially sintered crusts to fragile wisps of cellular boxwork. A typical cleavage boxwork, coated with masses of partially sintered crusts, is shown in plate 12. A common variation of cleavage boxwork is the diamond-mesh structure (fig. 55b), which gives an imperfect, unsymmetrical pattern. The main ribs, which have an apparent thickness of 0.2 to 0.5 mm, are not solid; they are composed of two or more closely-spaced, rigidly parallel, minutely thin webs, plainly visible under a hand lens. As with cleavage boxwork, the diamondmesh product is coated with partially sintered crusts. Figure 57 shows a cellular boxwork derived from galena in a carbonate gangue. Figures 61 and 62 portray unusually well developed specimens of the angular pattern indicative of derivation from cerussite.

146

INTERPRETATION OF LEACHED OUTCROPS

Relief limonite is more dull and earthy, with an intermixture of globular limonite intimately intergrown with fine crystalline anglesite or cerussite. Emerging faintly from the pulverulent mass, relief limonite generally grades into partially sintered crusts. Occasionally one or more remnants of cleavage or diamond-mesh boxwork may be detected (fig. 58). Plates 13 and 14 and figure 60 show relief limonites formed by the oxidation of cerussite and galena. Cellular sponge is derived from granular galena that has replaced sedimentary beds other than limestone. The sponge under high magnification shows the product to be composed in part of minute, interlocking cubic boxes of limonitic jasper, but so imperfectly knit together that the pattern gives the impression of being a sponge-like cluster of small limonite cells. Cellular sponge grades into partially sintered crusts.

Pyramidal boxwork is similar to cleavage boxwork, but much less common. It is patterned after the cubic cleavage, but with step-pyramid effects. The plates are more or less coated with partially sintered crusts, which bind the plates together, and preserve rigidity of the structure. A remnant of pyramidal boxwork that has been preserved within an area of relief limonite is shown in figure 63. Other boxworks of this type, still visible despite partial obliteration by other, later types of leaching products, are shown in figure 64. Ragged cellular type consists of honeycomb, with cells less regularly parallel and more ragged in appearance. This type is seen where galena carries an important admixture of pyrite. With predominance of pyrite the color generally changes to brownish red. Figures 64a and 65 show the typical products derived from mixtures of sphalerite and galena.

FIGURE 56. Cleavage boxwork derived from galena. Weathered surface. Note the rigid parallelism of cell walls and the thick coatings of partially sintered crusts. Ruby Hill, Eureka, Nev. Enlargement 3x. (After Boswell and Blanchard, Econ. Geology, 1927, p. 433)

GALENA AND CERUSSITE

FIGURE 57. Fine cellular boxwork derived from galena in carbonate gangue. There are no partially sintered crusts. Boxwork is stained by hydrous manganese dioxide, but the boxwork pattern has been preserved and stands out prominently from beneath supergene gangue carbonate and manganese coatings. In carbonate gangues about 18 to 40 percent of the limonite is derived from lead carbonate, explaining in large part why gossans over the Broken Hill ore bodies are on the whole incommensurate with total sulfides known to have been leached from the horizon of the present surface. From the outcrop of the Junction orebody, Broken Hill, New South Wales, Australia. Reduced one-third. (After Garretty and Blanchard, Australasian Illst. Milling Metall. Proc., 1942, p. 153; Econ. Geology, 1942, p. 385)

FIGURE 58.

Cellular boxwork grading into very fine relief limonite. From a weathered surface at the Lilydale prospect, Lawn Hill, Queensland, Australia. Enlargement 1.25x.

147

148

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 59. Finely and coarsely banded material is anglesite, surrounding a galena residual. From Precambrian schist, Forsayth district, Queensland, Australia. Enlargement 2x.

149

GALENA AND CERUSSITE

FIGURE 60. Limonite cellular pseudomorph derived from cerussite. Weathered surface. Relief limonite particularly evident, but very little partially sintered crust is seen. Following is an analysis of the specimen. Percent

Cerussite __ Mimetite __ _ Scorodite __ _ Supergene gangue carbonates: Calcite Magnesite Siderite ___ _ Limonitic jasper: Ferric oxide monohydrate __ Hydrous manganese dioxide __ Silica and alumina _______ _ Miscellaneous' _______ _

0.54 0.21 0.21 0.71 0.42 0.74 _______ 63.45 0.23 ___________ 29.00 4.49 100.00

From the Crystal lease, Mount Isa. Queensland, Australia. Natural size. lConsists mainly of adsorbed and capillary water.

IS

150

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 61. Limonite derived from cerussite. The specimen was obtained about 10 feet below the surface, in an impure, partly epidotized limestone. The angular pattern is indicative of replacement of cerussite. From the Engine shaft, Mount Stewart mine, Leadville, New South Wales, Australia. Enlargement 2.5x.

GALENA AND CERUSSITE

FIGURE 62.

Cerussite from Broken Hill, Australia. The honeycomb structure resulted from multiple twinning. Geological Survey of Queensland collection.

151

152

INTERPRETATION OF LEACHED OUTCROPS

A

B

FIGURE 63. Pyramidal limonite boxwork, preserved within a matrix of an intermixed relief limonite and gangue. Note particularly the step-pyramid effect in the upper left and lower middle portion of the picture. Partially sintered crusts bind the plates firmly together and maintain the rigidity of the structure. Pyramidal boxwork has been observed only in surface outcrops, under partial or complete shelter from weathering attacks. Aravaipa, Ariz. Enlargement 1.5x. (After Blanchard and Boswell, Ecoll. Geology, 1934, p.676)

FIGURE 64. A. Pyramidal limonite boxwork (upper center), partly preserved within a matrix of cellular sponge and relief limonite of mixed galena-sphalerite derivation. From outcrop in Block 14, Broken Hill, New South Wales, Australia. Enlargement 1.25x. B. Pyramidal limonite boxwork, preserved within a matrix of partially sintered crusts of galena derivation. Unnamed prospect near Tepic, Nayarit, Mexico. (After Blanchard and Boswell, Ecoll. Geology, 1934, p. 681)

GALENA AND CERUSSITE

FIGURE 65. Mixed limonite cellular pseudomorph of sphalerite and galena. Original sulfides estimated at 28 percent galena and 58 percent sphalerite. Most of the specimen consists of cellular sponge of sphalerite derivation, characterized by its crinkly cell wall structure and the dry, empty appearance of cells. Top of the specimen, particularly at right and center, shows extensive development of partially sintered crusts of globular cerussite origin. That portion consisted originally of a nodule of nearly pure galena. Post-mine oxidation, 2,OOO-foot level, sill, section 28/P-Q, Northern orebody, Broken Hill, New South Wales, Australia. Enlargement 1.5x. (After Garretty and Blanchard, Australasiall Illst. Millillg Mctall. Proc., 1942, follow p. 160; Ecoll. Geology, 1942, p. 381)

153

Chapter 27 SPHALERITE Sphalerite, if it contains no iron, has just enough sulfur to go completely into solution; it does not generate free acid (ch. 8). The resulting zinc sulfate is so soluble that oxidized zinc minerals rarely appear on

ence of a gangue of moderate neutralizing power results in the erratic precipitation of limonite. When this oxidizing sulfide mixture is moderately rich, however, a fairly uniform deposit of limonite takes place, often

the surface of the ground in semi-arid regions. Usually

with accompanying deposition of smithsonite or meager

only smithsonite (ZnCO,J and the silicates, principally hemimorphite (2ZnO.Si0 2 .H"O), persist-especially in feldspar, shale, and limestone gangues. In an inert gangue above the water table, zinc sulfate leaches away rather rapidly (ch. 10). Even iron-rich sphalerite may leach away to a depth of 10 to 50 feet or more in semiarid regions, often without a trace. Iron-rich sphalerite, if it occurs in an inert gangue below the water table, may dissolve away down to depths of 250 feet or more. The resulting acid solutions at the same time corrode the siliceous gangue, as explained in chapters 7 and 19. The oxidation of sphalerite and pyrite in the pres-

amounts of hemimorphite. When a heavy mixture of these two sulfides oxidizes above the water table in the same type of gangue, fluffy limonite often develops. More smithsonite and hemimorphite form in strongly neutralizing gangues, on the whole, than in moderately neutralizing gangues. All the limonites derived from sphalerite shown in figures 66 to 76, are in moderately neutralizing gangues. Plate 16 shows a cellular boxwork formed by the weathering of sphalerite in sillimanite gneiss. Cellular boxwork derived from sphalerite has a welljoined structure; the finer cells especially are firmly joined. Their crinkly, thin, flaky, shriveled but crisp

1+---2

...."';-----2

~---3

~---3

1J1-4r---4

--......;.----4

A

B

FIGURE 66. Sketches showing characteristic limonite boxworks derived from sphalerite. 1. Coarse cellular boxwork. 2. Well connected fine boxwork. 3. Sandy limonite grains, slightly coating cell walls. 4. Limonite rosettes. A. The cell walls in this specimen are well connected and the cells themselves almost empty of material. The boxwork usually has a sharply angular form, with intersecting angles of 30° to 55°. These features constitute the "key" structure. From a shaly limestone, Spruce Mountain, Nev. Enlargement 3x. B. In this specimen the boxwork is well connected, and is of sharply angular pattern. The angles of intersection are often about 30°, but vary somewhat. Lawn Hill, Queensland, Australia. Enlargement 3x.

155

SPHALERITE

,..-------2

.-----2

N-.----3 •

~-----3

L..:-~----4

8

A

FIGURE 67. A. Sketch showing characteristic boxwork and webwork derived from sphalerite in andesite gangue. I. Coarse cellular boxwork. 2. Well connected fine cellular boxwork. 3. Sandy limonite grains, slightly coating cell walls. 4. Limonite rosettes. Carlisle mine, N. Mex. Enlargement 5x. (After Blanchard, Chem. Metall. Mining Soc. South Africa Jour., 1939, p. 346) B. Sketch showing formation of cellular sponge derived from sphalerite. I. Pairs of closely spaced, rigidly parallel, minutely thin webs that make up the structure. This feature is rare. 2. Well connected cellular sponge. 3. Sandy limonite grains, slightly.coating cell walls, but scarce. The angular boxwork shows, but sphalerite cellular sponge is not a good place for it, because cells occur uniformly throughout the mass instead of erratically. The boxwork or webwork possesses a dry, crisp appearance. This is a complex type of limonitic jasper, comprising minute intergrowths of calcite, magnesite, siderite, and rhodochrosite in varying proportions, intimately dispersed with limonitic jasper, building upon or replacing limonitic jasper subsequent to the latter's formation to make the product indistinguishable to the unaided eye. The specimen has the following composition: Percent

Calcite, magnesite, siderite, rhodochrosite.............. 25 .................................. 65 Silica and alumina....... Ferric oxide hydrate .............................................. 10 Sillimanite gneiss country rock. Post-mine oxidation, 2000-foot level (about 1550 feet down in the sulfide zone), North Broken Hill mine, Broken Hill, New South Wales, Australia. Enlargement 2x.

A

8

FIGURE 68. Sketches showing hieroglyphic limonite boxwork derived from sphalerite. A. Hieroglyphic structure, as shown here, is joined to other cells. Occasionally the boxwork has a sharp angular form. The sandy limonite grains and rosettes are present, but are scarce, and are not shown. Most sphaleritederived limonite does not show the hieroglyphic pattern. Taken from a fracture. Gangue is ordinary shale. Mount Isa, Queensland, Australia. Enlargement 5x. B. The hieroglyphics in this case are not as prominent. The sharp angular form, however, is noticeable, and cell walls are firmly joined. Sandy limonite grains and rosettes are not shown. Golconda mine, Chloride, Ariz. Enlargement 5x.

156

INTERPRETATION OF LEACHED OUTCROPS

,

FIGURE 69. Sketch showing limonitic products formed by the leaching of sphalerite and pyrite. The host rock was a shale that contained dolomitic bands. When the rock was mineralized, mediumgrained sphalerite sc1ecti veiy replaced the dolomitic bands, finely disseminated pyrite replaced about 25 peicent of the more siliceous shale bands. When leaching later took place, the sphalerite was replaced at an early stage by cellular pseudomorphs characterisitc of that I mineral, the pyrite was later replaced by massive jasper. Each oxidized pyrite grain was neutralized by one or two dolomite particles. \' -"'---5-~ greatly retarding the exportation of ~ iron in solution, and thereby aiding the kaolinization and jasperization and confining it to the pyritized bands. The only iron visibly exported is that precipitated as sparse, finely nodular crusts or "blisters" on the surfaces of the jasper bands. 1. Coarse cellular boxwork derived from sphalerite. 2. Fine cellular boxwork derived from sphalerite. 3. Massive jasper derived from the oxidation of pyrite. 4. Sandy limonite grains, thinly coating the cell walls of sphalerite-derived boxwork. 5. Limonite rosettes. 6. Nodular limonite crusts. These are the usual types of products resulting from oxidation and leaching of sphalerite-pyrite mixtures in dolomitic shale at Mount Isa, Queensland, Australia. Enlargement 3x.

4

'"

FIGURE 70. Limonite developing along cleavage planes in sphalerite. As seen on most surfaces, the angle of intersection of the sphalerite limonite boxwork is about 55°; this is the "key" structure. Wolfram, Queensland, Australia. Enlargement 1.2x.

FIGURE 71. Sphalerite showing beginning of oxidation. Dark portions are sphalerite. Broad white bands are hypogene quartz veinlets. Thin, irregular white stringers cutting sulfide lenses are limonitic jasper veinlets "eating" their way into the sulfide mass around individual sulfide grains, especially on the right side. Weathered specimen from dump. Golconda mine, Chloride, Ariz. Enlargement 1.5x. (After Boswell and Blanchard, Ecoll. Geology, 1927, p. 425)

SPHALERITE

157

appearance (best illustrated by sponge), and their

shows a prominent acute angle of from 30 0 to 55 0

scattered minute sandy grains and occasional aggregates of small rosettes projecting from the cell wall surface, constitute the "key" features which are outstanding and readily recognized among all the limonite types (see figs. 66, 67, 72). The boxwork generally

(figs. 70 and 73). The webs of limonite may extend continuously past several cells for as much as 14 inches or more. Occasional hieroglyphic patterns occur (fig. 68, pI. 15). Rosettes are fragile; they precipitate successively one upon another in a semi-gelatinous state

FIGURE 72. Coarse, medium, and fine cellular boxwork derived from sphalerite. Weathered specimen from dump. White bands are hypogene quartz veinlets. Dark cellular structure is limonitic jasper, with well joined cel! walls. The limonitic jasper contains 35 to 65 percent silica. Note particularly the "key" structure of coarse and medium cellular boxwork. The rosettes do not show because they were too frail and they crumbled. Golconda mine, Chloride, Ariz. Enlargement 1.5x. (After Boswell and Blanchard, Ecoll. Geology, 1927, p. 426)

FIGURE 73. Coarse and fine cellular boxwork derived from sphalerite. Weathered specimen from dump. White bands are hypogene quartz veinlets. Dark cellular structure is limonitic jasper, with generally well joined cell walls in the finer cells. The limonitic jasper contains about 35 percent silica. Shale gangue. Watson's lode, Lawn Hill, Queensland, Australia, Enlargement 1.25x.

158

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 74. Weathered cellular sponge derived from sphalerite. The sponge walls are characteristically thicker than the boxwork walls. The cells are firmly, not erratically, joined. In texture and general appearance the mass resembles closely a rubber bath sponge. Its most distinguishing characteristic is the crinkly "com flake" structure of the cell wall, with its closely adhering sandy grains and occasional limonite rosettes. The limonite in this specimen contains about 55 to 65 percent silica. Shaly limestone country rock. Empire Zinc mine, Hanover, N. Mex. Enlargement 1.5x. (After Boswell and Blanchard, Econ. Geology, 1927, p. 427)

FIGURE 75. Limonitic sponge derived from an intimate mixture of about 70 percent sphalerite (marmatite) and 17 percent galena. The limonite consists of: Percent

Supergene gangue carbonate.............................. 38 Limonitic jasper ................................................... 40 (Mainly) adsorbed and capillary water............ 12 Specimen represents post-mine oxidation on the 2000-foot level, sill, section 28/P-Q, Northern orebody, Broken Hill, New South Wales, Australia. Natural size. (After Garretty and Blanchard, Australasian 1nst. Mining Metall. Proc., 1942, p. 160; Econ. Geology, 1942, p. 383)

159

SPHALERITE

(see ch. 16). The boxwork is hard, containing up to

grating through the shock of a light blow from the pick

65 percent supergene silica. Sometimes sphalerite yields pairs of closely-spaced, rigidly parallel ribs (but oxidation of galena produces them far more often). Usually limonite derived from sphalerite is ochreous in color, but in dry climates on the surface sometimes it is reddish. Figure 71 shows a mass of sphalerite in the initial stage of oxidation, in which thin veinlets of limonitic jasper are only beginning to "eat" their way into the sulfide mass. Typical products that result from the oxidation and leaching of mixtures of sphalerite and pyrite are sketched in figure 69. Cellular sponge resembles in appearance a rubber bath sponge. It has no sharply angular boxwork pattern, but possesses thicker and slightly more rounded cell walls, which do not disintegrate easily (see pI. 15, figs. 74, 75). These walls are usually 0.2 to 2.0 mm in thickness. The cells vary in size, but for the same size cell the cell wall usually is from two to five times as thick as in the boxwork types. Rosettes are occasionally formed, but are rare. The cell walls are often coated with minute crystals of smithsonite and hemimorphite. Limonite moss consists of long, loosely interconnected flakes and shreds of limonite, about 0.5 mm in cross section, that fill and overrun the cellular structure much as Spanish moss overruns and locally masks the limbs and foliage of trees. The shreds anastomose to some extent; under the hand lens the structure suggests a stalactitic origin, and, when in place, invariably points steeply downward, with characteristic shriveled appearance. Limonite moss occurs in a number of places, although on the whole it is rather rare. In the author's experience (1928), some of the best examples were seen at Duquesne, Ariz. Although smithsonite (ZnS0 4 ) is often formed as the initial product of the leaching of sphalerite (see fig. 76), limonite derived from subsequent replacement of the smithsonite has not been found to yield distinctive products, except locally and in small amounts. However in certain districts characteristic limonite derivatives grade into smithsonite. The limonite consists of small, indefirutely formed and loosely adhering aggregates of grains and irregular fine angular projections as coatings on cell walls in sphalerite-derived cellular structures; they are so fragile that, unless supported by cellular boxwork, they rarely are preserved, disinte-

(see pI. 17), except when much silica is present. Persistent search in many localities for a "key" limonite of exclusive smithsonite derivation has been disappointing. Plate 18 shows a typical leached derivative of a pyrite-chalcopyrite-sphalerite mixture. In many cases the leaching products can easily be distinguished one from the other.

FIGURE 76. Characteristic specimen of columnar stalagmite smithsonite, formed when acid solutions, strongly charged with zinc sulfate, dropped a portion of the "load" as partial evaporation of dripping solutions took place. Tunnel level, Tally-Ho property, about 30 miles west of Mackay, Queensland, Australia. Enlargement 4x.

Chapter 28 MOLYBDENITE Molybdenite does not oxidize readily, and when it does oxidize in an inert gangue, it usually leaves no limonite. Not much has been written about limonite in molybdenite deposits. Mixtures of molybdenite and pyrite, upon oxidizing with moderate neutralizer generally leave ferrimolybdite. The author has not seen

~

---\11'----A

3

~---~

"-';=cl~

B

FIGURE 77. Foliated boxworks of molybdenite derivation. 1. Outline of cavity. 2. Foliated limonite flakes. 3. Hypogene quartz veinlets. A. Specimen from the Santo Nino property, near Nogales, Ariz. From a weathered surface. Feldsparrich gangue. Enlargement 20x. B. Specimen from Hodgkinson district, Queensland, Australia. From a weathered surface. Shale gangue. Enlargement 20x. Foliated boxworks such as these are the usual mode of occurrence in both districts, and most of the boxworks are microscopic in size. Sometimes semi-greasy petals of granular limonite are embedded in the foliated boxworks. (A/ler Blanchard and Boswell, Ecoll. Geology, 1935, p. 314)

many deposits that contain oxidation products of molybdenite in a strongly neutralizing gangue, though molybdenite in limestone is common in certain parts of the Far East. The oxidation of molybdenite in the presence of pyrite above the water table was discussed in chapter 10. Ferrimolybdite (Fe 2 (MoO") :;.SH 2 0, with variable water) is a fine-grained, canary-yellow to straw-yellow earthy to fibrous mineral, sometimes reddish in places. At Climax, Colo., jarosite in the form of ocher-yellow incrustations commonly occurs with the ferrimolybdite. When much pyrite is present with molybdenite, ochreous limonite is formed. In a few disseminated molybdenite deposits the grains of limonite derived from the molybdenite are foliated (see fig. 77) with comparatively rounded and smooth forms, maroon to orange to tan in color, but usually with only minor ferrimolybdite. The grains are sometimes as much as a quarter of an inch across, but are usually microscopic. The foliated boxwork breaks up easily because the walls are commonly only 0.005 to 0.03 mm thick, but in protected places some boxwork can be seen. In certain districts granular limonite forms when molybdenite is oxidized, and often may be seen as semi-clayey, semi-greasy petals embedded in the foliated boxwork. Wulfenite (PbO.MoO:J, a secondary mineral, is common in certain lead districts, with its orange-yelJow to wax-yellow color (see ch. 10).

Chapter 29 CHROMITE Chromite deposits very often are found in serpentine (3MgO.2SiO".2H"O) and in ultrabasic rocks, sometimes in schist. Pure chromite (FeO.Cr 20:;) has been found in exceptional specimens, but usually it contains varying amounts of magnesium and aluminum, in which case its formula is (Fe,Mg)O~(Fe,Al,CrLO::. When chromite weathers, the magnesium is slowly carried away in solution, and a mixture of silica, alumina, and ferric oxide remains, forming limonitic jasper in tropical regions. Chromite has not been encountered in strongly neutralizing gangues such as limestone in the author's experience. Limonite derived from chromite is not common, because chromite does not oxidize readily, especially in temperate zones. It generally survives unaltered to the surface or nearly so. However some well preserved limonitic cellular boxworks derived from chromite have been observed in Queensland, Australia, and in occurrences in California, Montana, and New Jersey. The only example of weathering of a chromite deposit personally observed by the author is that at the Paagoumene mine in Tiebaghi, New Caledonia, where an area

FIGURE 78. Polished section of massive chromite, showing the inherent fracture or parting pattern. Tiebaghi mine, New Calendonia. Natural size. (After Blanchard, Econ. Geology, 1942, p. 618)

of irregular pipes 100 by 200 feet has been proved to a depth of 1,300 feet or more. Chromite at that deposit generally is converted into a limonite boxwork to a depth of about 30 feet, and some weathering could be seen to depths of 100 feet or more in fractured zones in the serpentine domes. An iron capping extends over practically the entire area, with no pronounced preference for the known deposits of chromite. The iron capping occurs as nodules, boulders, and masses containing chromite remnants. The serpentine host rock is somewhat nickeliferous, and farther eastward in the island nickel is produced from garnierite, hydrated nickel silicate, together with hydrated magnesium silicate. At Fantoche, near Nehoue Bay, another chromite deposit has been developed to a depth of about 1,300 feet. Limonites derived from chromite take the following forms: 1) massive honeycomb boxwork, and 2) disseminated limonitic sponge studded throughout a serpentine matrix. 1. Massive honeycomb boxwork. The weathering of massive chromite deposits at Tiebaghi and Fantoche, New Caledonia,' yields a conspicuous coarsely cellular honeycomb boxwork. Cell diameters are usually 3 to 10 mm, but occasionally are as much as an inch. The boxwork is clearly patterned after the fracture system, which in turn appears to be guided in part by chromite's octahedral parting (see fig. 78, pI. 19), but the relationship usually is poorly defined. The cross ribs are well joined, and the resulting structure is rigid and coherent. The thickness of the box work walls commonly varies from less than 0.01 to 0.02 mm, with 0.5 mm maximum. In the New Caledonia deposit the limonitic jasper is composed of 50 to 70 percent silica, 10 to 20 percent Fe"Oa, with 5 to 10 percent alumina. The color is drab-ochreous. The boxwork, if not exposed to severe weathering, is filled with an indigenous minute cellular sponge so fine that in some cases it appears almost pulverulent to the unaided eye. The sponge, as a mass, tends to shrink away from the boxwork walls, and resembles small blobs of dried-out bran mash. Continued weathering crumbles the shrunken portions, and much of it is carried away, as in granular fretwork of limonite derived 'Maxwell (1949) has made a thorough study of chromite deposits on this island. He believes that the chromite was carried upward as blocks, fragments, and disseminated crystals during emplacement of ultrabasic host rocks in serpentine. See also Udy (1956, p. 1-32).

162

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 79. Polished section of nodular chromite disseminated in a serpentine matrix. The octahedral parting can readily be seen in some places. There is a thin margin of yellowish-green serpentine around each chromite nodule which grades outward into the darker green of the country rock. Tiebaghi mine, New Caledonia. Enlargement 1.5x. (After Blanchard, Econ. Geology, 1942, p. 621)

FIGURE 80. Leached equivalent of specimen generally similar to that shown in figure 79. The serpentine has been largely recrystallized into smoky-brown to chocolate-colored limonitic jasper, with discordantly arranged, small smoky-brown quartz-jasper crystals projecting from it. Casts left by decomposing chromite nodules thus have become distorted in shape. Most cavities or casts retain as an indigenous product some of the minute cellular sponge, as evidence that chromite constituted the parent nodules. Tiebaghi mine, New Caledonia. Enlargement 2.67x. (After Blanchard, Econ. Geology, 1942, p. 623)

CHROMITE

from arsenopyrite; but usually a thin coating of sponge adheres tenaciously to the boxwork walls. The fine honeycomb sponge, like the coarse honeycomb boxwork, is made up of limonitic jasper, but its silica content is lower (20 to 40 percent), and its ferric oxide content much higher (25 to 45 percent). The minute size of the sponge cells may be judged from the fact that on a 5 millimeter length of line across one of the coarser boxwork structures, the author counted, under the microscope, 263 individual cells of sponge resting end to end somewhere along the line, making the average diameter 0.019 mm. As the cells are highly variable in size, and the diameters of some of them are much less than 0.019 mm, cells of 0.005 mm diameter have been observed. Since the thickness of the cell walls normally is only a third to a fifth the cell diameter, and may be only a tenth, that of the sponge structure lies between 0.006 and 0.002 mm. Therefore it is not surprising that without magnification much of the cellular sponge appears pulverulent. In addition to the minute size of the cells, this sponge differs in several respects from an ordinary limonitic cellular product. There are no through-going ribs or webs patterned closely after the parent mineral's fracture or cleavage planes so far as the author's experience goes, and no orthodox cell wall pattern exists. The sponge is made up of innumerable flaky excrescences that join locally and at random into an airy and tenuous, arborescent fretwork. The main difference is that the sponge particles do not comprise crystals or granules to the unaided eye, but consist mostly of crinkled limonitic crusts or flakes in miniature. Though usually

163

requIrIng a magnification of at least 30X to be seen readily, the limonitic sponge of chromite derivation yields one of the most outstanding arborescent patterns among the leached outcrop products. 2. Disseminated chromite studded through a serpentine matrix. The serpentine is found in all stages of decomposition. In well-weathered material, as at the Tiebaghi and Fantoche deposits, little remains of the chromite derivatives except an amorphous-looking, greenish-yellow or smoky chocolate-colored jaspery mass with somewhat corroded crystals up to one millimeter long and a half millimeter wide (usually smaller), which project at random into space. A well-defined cast after chromite does not remain, because much of the serpentine has altered into supergene limonitic jasper masses, and the cavity becomes grossly distorted in shape (see figs. 79 and 80). Much of the limonitic honeycomb and sponge is lost. Decomposition of the chromite generally lags slightly behind that of the serpentine at any given place, however, and remnants of the sponge may be preserved within the re-formed cavity to serve as a clue. It may be possible to determine more closely the percentage of chromite originally present by crushing and examining adjacent material that was less exposed to weathering, thus arriving at a more accurate estimate for commercial exploitation. Although the residual limonitic derivatives of disseminated nodular chromite do not furnish as accurate an index for appraisal of the leached area for prospecting as do the more homogeneous and better preserved derivatives after massive chromite, a clue nonetheless is present which often may be intelligently followed.

Chapter 30 HEMATITE AND MAGNETITE Hematite and magnetite do not weather readily in quartz, feldspar, or shale gangues unless sulfides are present, and then only in part, or when organic acids are present, and then usually only near the surface.' Magnetite, especially, oxidizes with difficulty, but under certain conditions it will ultimately break down (see pI. 20). In the fracture zones, hematite and magnetite are replaced by goethite sporadically-the alteration 'In the tropics, especially in humid regions, hematite and magnetite may be weathered as deeply as 30 feet or more, and goethite takes their place on the surface. But in comparatively dry areas, hematite or magnetite are ordinarily resistant to weathering unless the rock is highly fractured, or sulfides are present.

occurring to depths of hundreds of feet in places. Specularite (Fe20:l) is more susceptible to hydration under certain conditions, because of its platiness. Figure 81 illustrates the replacement of specularite by goethite in a moderately reactive gangue in the semi-arid environment of the Cloncurry district, Queensland, Australia. Even in a highly reactive gangue such as limestone, hematite and magnetite usually are not affected unless there is fracturing and unless acid is present, except in the humid tropical regions. In semi-arid regions they are seldom affected, and they nearly always show in the croppings.

FIGURE 81. Ochreous goethite replacing specularite along a nearly vertical contact in a schist and limestone gangue. No sulfide minerals are present. An indefinite cell structure has been almost completely masked by an earthy or very fine, granular groundmass. A rude parallelism of platy specularite structure is preserved, however, especially at the left of the specimen. Marimo, Cloncurry district, Queensland, Australia. Enlargement 1.5x. (After Blanchard and Boswell, Econ. Geology, 1935, p.318)

Chapter 31 MANGANITE AND PYROLUSITE Manganite (MneO:.HeO) and pyrolusite (MnO" with up to several percent of adsorbed HeO), have been observed to alter to goethite on the surface in quite a few places. In the manganese orebody at Lawn

Hill, Queensland, Australia, these manganese minerals have been observed weathering to a light-brown, cellular goethite (see pI. 21).

Chapter 32 CALCITE Calcite fortunately does not often yield boxwork, and when it does the boxwork is so distinctive that it is not likely to be confused with boxwork types derived from sulfide minerals. The product is characterized by a more-firmly knit and persistent grid structure than that yielded by most sulfides, because, with the possible

exception of galena, the sulfides do not possess comparable cleavages. Figure 82 illustrates the uniformly thick and evenly spaced pattern of longitudinal and cross ribs so characteristic of boxwork derived from calcite. Calcite does not yield cellular sponge, so far as the author has observed.

FIGURE 82. Limonite cellular boxwork derived from calcite. Note: 1) well-knit, sharply angular boxwork which reproduces faithfully the rhombohedral pattern of the calcite cleavage; 2) essential uniformity in thickness of longitudinal and cross ribs. At left center and bottom the boxwork is in process of "eating" its way into the rock. Specimen is from a lens of recrystallized limestone 170 feet long by 20 feet wide, bordered by fine-grained limestone. An analysis of the boxwork gave the following result: Percent

Calcite ............... . Silica.. Ferric oxide hydrate... .. ................................ .. Miscellaneous (mainly adsorbed and capillary water) ........

22 72

4 1

99 Juenburra, Cloncurry district, Queensland, Australia. Natural size.

Chapter 33 SIDERITE Siderite (FeCOJ weathers to ferric oxide monohydrate and jarosite (K 2 0,,oFe 2 n,04S0 3 o6H 2 0) well above the water table in arid and semi-arid regions, as a result of normal air-water decomposition processes. However, the resulting material usually contains from 0.25 to 2.0 percent or more of the iron in the form of undecomposed siderite. In the more highly reactive rocks such as limestone and limy shale, siderite is, as a rule, altered to fluffy limonite. This product is usually yellowish, but may be grayish brown, brown, or reddish brown. Unless the host rock contains much silica little jasper develops with the limonite, and supergene gangue carbonates may make up 25 to 50 percent of the rock; consequently the limonite is loosely cemented, and disintegrates readily. Ghost rhombohedral fragments sometimes are present, but will not be noticed unless sought. In feldspathic rocks, lime-free shales, and schists, jasper may develop in the weathered product of siderite

FIGURE 83. Yellowish limonite boxwork derived from siderite. Note the well preserved pseudomorphic rhombohedral cleavage pattern. Fluffy limonite, seen in the bottom quarter and at the right side of the specimen, was deposited because of the presence of strongly neutralizing solution. About 5 percent of the boxwork is silica, the remainder is mostly goethite and jarosite (about 30%). Jubilee property, Wenden, Ariz. Enlargement 2x. (After Blanchard and Boswell, Eng. Mining Jour., 1928, p. 375)

to a greater extent than do supergene carbonates, and this seems especially to be the case when disseminated sulfides are present, and a well developed boxwork may form. Some fluffy limonite is formed, but not as much as with limestone gangue. When siderite is massive, sometimes almost perfect limonite boxworks from 1 to 2 inches wide are formed, because of the high silica content derived from the host rock (see figs. 83, 84). In general siderite decomposes readily, leaving occasional silica-rich rhombohedral boxworks. Supergene siderite has sometimes formed below the water table when iron-rich solutions derived from overlying pyrite-copper or pyrite-zinc orebodies came into contact with limestone beneath (see Trischka, Rove, and Barringer, 1929). This type of supergene siderite and its oxidation products are seen not only at Bisbee, Ariz., but at Leadville, Colo.; Ophir, Utah; and many other places in southwestern United States and Mexico. This siderite later weathered when the water table subsequently dropped beneath it, and limonite boxworks were formed (figs. 85, 86). As pointed out by Locke (1926, p. 109), this type of occurrence may indicate mineralized ground above the siderite or the limonite derived from it, not beneath it.

FIGURE 84. Limonite boxwork derived from siderite at the contact of quartzite with amphibolite. Color of weathered surface of specimen is brown to brownish-yellow. About 8 to 10 percent is silica (jasper); about 18 to 20 percent is jarosite; the remainder is goethite. Mount Cobalt Mines, Ltd., 18 miles south of Selwyn, Cloncurry district, Queensland, Australia. Natural size.

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INTERPRETATION OF LEACHED OUTCROPS

FIGURE 86. A branching variety of siderite from below the water table at the 1550-foot level of the Gardner mine, Bisbee, Ariz. Formed in the same manner as the specimen shown in figure 85. Photographed by Keith Coke in 1952. One-fourth reduction. FIGURE 85. Limonite boxwork derived from supergene siderite. The siderite was previously formed below the water table in limestone by oxidation of a pyritic orebody containing a small amount of copper. The reactions necessary to produce this product are given in chapters 5 and II. Gardner mine, I 550-foot level, Bisbee, Ariz. (After Locke, 1926, pl. IX)

Chapter 34 FLUORITE When mixtures of fluorite (CaFJ and sulfide minerals are subjected to leaching, fluorite usually leaves boxworks made up of supergene carbonate minerals and containing little silica. These boxworks are not very stable and easily crumble away under weathering conditions.

At the North mine, Broken Hill, New South Wales, fluorite boxworks were studied extensively on the 2000foot level (Garretty and Blanchard, 1942, p. 404). At the North mine, fluorite occurs as: 1) a coarsely crystalline variety with well developed cleavage planes in masses up to 6 inches across; 2) a less common granular variety with texture resembling that of loaf sugar, also in masses up to 6 inches across, and 3) disseminated grains 1 to 3 mm across. Galena and marmatite (iron-rich sphalerite) are the chief ore minerals; the orebody there contained about 20 percent of each; the

FIGURE 87. Cellular boxwork forming from decomposition of massive crystalline fluorite. The boxwork forms as minutely thin webs of limonitic jasper or supergene gangue carbonate minerals penetrating along cleavage planes, as in the central portion of this specimen. As decomposition proceeds, the webwork or box work thickens-usually by stubby crystals or granules of supergene gangue carbonate encrusting themselves upon the webwork of cellular walls. The dominance of supergene carbonate matter derived from the gangue is shown in the following chemical analysis. Percent

Fluorite ___________________________________________ Supergene gangue carbonates: Calcite _____________ _ Magnesite ________ _ Rhodochrosite ___ _ Siderite ______________ _ Silica ____________________ _ Miscellaneous ___ _

27.81 27.86 .63 24.48 18.39 .70 .13

100.00 2000-foot level, Southern orebody, North mine, Broken HiII, New South Wales, Australia. Enlargement 1.3x. (After Garretty and Blanchard, A llstralasian Inst. Mining Metall. Proc., 1942, p. 161; Econ. Geology, 1942, p. 392)

FIGURE 88. More advanced stage in the decomposition of massive crystalline fluorite. Upper left portion consists of a galena nodule (note prominent cubic cleavage cracks). Although most of the fluorite has been leached, the galena has undergone almost no alteration except for the formation of very thin boxwork limonite. 2000-foot level, Southern orebody, North mine, Broken HiII, New South Wales, Australia. Enlargement 1.3x. (After Garretty and Blanchard, Australasian IllSI. Milling Metall, Proc., 1942, p. 164; Econ. Geology, 1942, p. 394)

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INTERPRETATION OF LEACHED OUTCROPS

fluorite is found in or near the sulfide ore bodies. Extended underground observations indicated that the fluorite is the first mineral to undergo leaching. Once started, its leaching proceeds rapidly, and it is not unusual to find the mineral completely decomposed before the surrounding sulfides have been noticeably affected (see pI. 22).1 Boxwork derived from fluorite. Crystalline fluorite yields an angular boxwork with a low siliceous jasper content. In decomposition of the coarsely crystalline variety, the initial thin webwork which eats its way along cleavage and fracture planes usually consists of limonitic jasper. As more fluorite goes into solution the webwork or boxwork grows and increases in thickness, partly through replacement of the limonitic ribs by gangue carbonate, but mainly by coating the ribs with aggregates of additional supergene gangue carbonate crystals or granules. Only rarely does limonitic jasper 'The zone of oxidation extends to a depth of about 450 feet; the sulfide zone was present below that depth. In the Broken Hill deposit, galena is leached in preference to marmatite, the reverse of the usual order. An entirely satisfying explanation has not been reached.

constitute an important ingredient of boxwork after its initial penetration into the fluorite. Final cell wall thickness when all fluorite has been leached may be 0.5 mm, but the usual thickness is 0.1 to 0.25 mm. To the unaided eye, the product bears a superficial resemblance to galena's cleavage boxwork when it is lightly covered with partially sintered limonite crusts, but that derived from fluorite is distinguished by the fact that its longitudinal ribs rarely are parallel, and its cross ribs make obtuse angles with them. Characteristic types of boxworks derived from fluorite are illustrated in figures 87 and 88. In granular fluorite the boxwork structure is much coarser and more loosely-knit with cell diameters frequently exceeding half an inch, and the boxwork walls are more likely to be loosely coated with stubby crystals of supergene gangue carbonate, or with shapeless granules of such carbonates (see fig. 89). The coarser boxwork structure develops because, in the absence of well-defined crystal faces, the ribs or webs tend to follow the cleavage pattern which appears to exist within the granular mass, even though it is not visible in the fresh rock.

FIGURE 89. Cellular boxwork derived from finely granular fluorite. Despite granularity of some of the Broken Hill fluorite, which gives it a loaf sugar texture, a fracture pattern commonly develops during decomposition similar to that within the massive crystalline nodules. 2000-foot level, Southern orebody, North mine, Broken Hill, New South Wales, Australia. Enlargement 1.6x. (After Garretty and Blanchard, A lIstmlasiall Illst. Millillg Metall. Proc., 1942, p. 164; Ecoll. Geology, 1942, p. 398)

Chapter 35 SALITE Pseudomorphs of salite result from extremely slow oxidation in limestone gangues, during which the neutralizer from ground waters remains in continual contact with the oxidizing salite. Usually there is little exportation of liberated iron, and grain-for-grain replacement by limonite occurs, resulting in the preservation of the fibrous structure. A comparison of a partial analysis of salite from Hanover, N. Mex., with partial analyses of diopside and hedenbergite is given below:

Fe (percent)

Diopsidc....... 2.9 . ........................ 17.5 Salite ' ................. Hedenbergite ................................. ........ 26.3

Mg (percent)

16.4 6.9 1.7

1Analysis of salite from Schmitt (1939).

When mixtures of salite and sphalerite such as those in the deposits at Hanover, N. Mex., are oxidized, cellular limonite is formed, and the zinc migrates until neutralized by the surrounding limestone. A pseudomorphic product derived from the grain-for-grain replacement of salite by limonite is shown in figure 90.

FIGURE 90. Limonite derived from salite in limestone gangue. Thorough limonitization has occurred almost to the base of specimen, and the limonite has remained as a pseudomorph, or grain-for-grain replacement of salite, so that the latter's radiating, fibrous, acicular structure has been perfectly preserved. Even in the upper, thoroughly weathered portion of the specimen the acicular structure remains well defined. Surface specimen from the south side of the granite batholith at Hanover, N. Mex. Enlargement 1.5x. (After Blanchard, Chem. Metall. Mining Soc. South Africa Jour., 1939, p. 363)

Chapter 36 SUPERGENE SILICA Supergene silica is very common. The far-traveling type and the "soap" replacements were fully described in chapter 6. Other types of supergene silica may be formed at depths as great as 200 to 400 feet or more

in semi-arid regions, especially when sulfides are involved. They usually are widespread in the semi-arid regions, from the flat limonite and the smeary types derived from pyrite to those derived from disseminated types of pyrite and other sulfides (fig. 18, ch. 18). Supergene silica also is formed in the non-sulfide types in the weathering of ferromagnesian silicate minerals. 1 Two unusual types are shown in figures 91 and 92. 'In dry weather, the supergene silica comes down slowly over a long period, even at the surface; for the groundwater dissolves the rock very slowly. Sometimes, in dry weather, it takes years to dissolve a few molecules of silica in the nearsurface zone (see Appendix B).

FIGURE 91. Supergene columnar silica (not jasper), from the interior of Australia, where the climate is dry and 1 foot denudation per million years is common, and where the most fantastic shapes are possible. Specimen was collected at a garnetiferous schist-silica outcrop, where it was broken off, with the columnar silica still showing in the outcrop, 4Yz feet above the general ground level. Killeen Copper lode, Jervois Range, Northern Territory, Australia. Reduction O.7x.

FIGURE 92. Surface specimen of supergene silica (chalcedony with possible opal) with the calcite completely leached. The rhombohedral structure of the calcite cleavage is preserved as a mold in the supergene silica after leaching. Agate Creek, Percyville, Queensland, Australia. Enlargement 1.5x.

Appendix A USAGE OF TWO TERMS, GOSSAN AND CAPPING, THAT ARE BASIC IN THE INVESTIGATION OF LEACHED OUTCROPS GOSSAN Gossan is a moderately heavy accumulation of limonitic material, derived in the main from massive sulfide ore minerals or from their iron-yielding gangue associates, which have been leached more or less in place, and which normally overlie an ore zone beneath. Material leached from the gossan area, when redeposited below as secondary minerals, is responsible for the creation of many ore-grade deposits. In general, the term gossan is not applicable unless: cellular or other porous limonitic material has been developed over an appreciable area; the limonitic material is at least in large part indigenous or of the fringing variety; a substantial proportion of the limonite-yielding materials have been leached; and, the gossanous material extends integrally

into the unweathered rock and is at least several inches, usually several feet or more, thick.

CAPPING Capping is the leached upper part of a body of rock that contains disseminated sulfide minerals. Isolated small specks of limonite in the capping are the result of the leaching of the small sulfide grains originally present in the unweathered rock, and the redeposition of part or all of their iron in situ or as fringing product. Locke (1926, p. 7) distinguished capping from gossan on the basis of sulfide mineral content of the unaltered rock: gossan is the leached product of rocks containing 20 percent or more sulfides, capping is the product of those containing less than that percentage of sulfides.

Appendix B REPLACEMENT OF AMPHIBOLITE GANGUE BY MASSIVE JASPER AT THE NINETY-MILE MINE, QUEENSLAND The replacement of kaolinized wall rock by massive jasper is illustrated by a series of photomicrographs (figs. 93-100) made from samples collected during an inspection of the Ninety-mile Copper mine by Mr. C. C. Morton and the author for the Queensland Geological Survey in 1942. The samples were taken in a 25-foot vertical interval above the ISO-foot level, the interval in which the entire process of kaolinization and replacement takes place. The ISO-foot level coincides with the bottom of the zone of oxidation, and also with the top of the water table during the dry season. The ore at the mine consists of the 150 feet of oxidized material, mainly small lenses of copper carbonate with minor chalcocite and cuprite, and abundant pyrite. It is contained in a strongly fissured fault zone in amphibolite, along the steeply dipping contact with a body of sericite schist. The amphibolite is the metamorphosed product of an andesite or possibly more basic rock. Throughout the ore channel the amphibolite, and

FIGURE 93. Fresh amphibolite, made up of fine crystalline amphibole minerals, predominately hornblende. Area of specimen represented by the figure measures 0.56 x 0.44mm. Enlargement 200x.

occasional fragments of sericite schist which were plucked from the hangingwall during faulting, have become irregularly kaolinized through acid attack by the leaching and supergene copper-enriching solutions, and probably in part by alkaline solutions derived from the weathering of the amphibolite. Scattered erratically but persistently through the ore channel, including the outcrop, are the compact, "puddingstone" lenses of massive jasper in all sizes up to 15 or 20 feet in length. Some grade insensibly into ore, but many have sharp boundaries. Much of the remaining ore channel is free from even pulverulent limonite.

FIGURE 94. Kaolinized amphibolite undergoing progressive replacement by limonite (black). Replacement is about one-third complete. Note the irregular, "moth eaten" replacement front, common where replacement of clay directly by limonite takes place. The serrated edges result from irregular infiltration of the replacement solutions along sub-microscopic shrinkage cracks, with attack proceeding simultaneously along many scattered fronts. Partial impregnation by minutely granular quartz may be observed in portions of the clayey mass. The vaguely striated areas represent amphibole crystals not completely kaolinized. Enlargement 200x.

REPLACEMENT OF AMPHIBOLITE BY JASPER AT NINETY-MILE MINE

175

Possibly half of the limonite within the ore channel is

channel by means of their puggy nature and their pro-

in the form of compact, massive jasper lenses, with copper carbonate and minor amounts of chalcocite and cuprite. The lenses persist without much variation in number, size, or composition downward to within 25 feet above the water table. Between that point and the floor of the ISO-foot level the entire process of alteration of amphibolite to "soap," impregnation of the latter to form the compact, massive jasper lenses, takes place. Figures 93 to 100, made from samples collected within and adjacent to this zone, show various stages of the alteration process. From the kaolinized ore channel of this 25-foot vertical section of the mine workings, lenses of "soap" 1 to 15 feet long emerge irregularly, mainly from areas cut by many intersecting fractures. In size, and in their distribution through the ore channel, they correspond closely with the lenses of massive jasper lying above. At floor level (lSD-foot level) they consist mainly of soft, clayey matter, readily distinguishable from the more general and less intense kaolinization of the ore

nounced bleaching to porcelain or china-white. Between the floor and a point 6 feet above, most of them show progressively increasing impregnation proceeding upward with fine granular quartz and chalcedony, to produce the harder, ivory-colored "soap." Within the 25-foot zone of jasper production, but mainly in the lower 10- to IS-foot portion directly above the ISO-foot level, impregnation and replacement by quartz of the strongly bleached and highly kaolinized "soap" lenses advances along blunt-nosed fronts; the material, as viewed in the hand specimen, either is soft, puggy clay, or hard, ivory-colored, glossy "soap," with little gradation visible to the unaided eye. In contrast, most of the limonite observable in the lower 4 or 5 feet above the floor is a brown to brownish-black, pulverulent material, patchy but prominent and widespread in occurrence, often fringing or enveloping the "soap" lenses. At water level the limonite material passes into a brown, mobile sludge. But beginning about 3 feet above the floor and persisting upward through a vertical range rarely exceeding 20 feet, the content of pulverulent limonite decreases progressively, and in proportion to the amount of "soap" that becomes

FIGURE 95. Electron micrograph of a silver-silica replica of clay "soap" undergoing replacement by limonitic jasper. Shrinkage cracks resulting from the drying out of gelatinous clayey mass are well defined. Many of them are less than a micron in length. Replacement of the clay may be observed in process both at top and bottom of large jasper bleb (left upper side with another smaller bleb to the right). The crack above it presumably is the one alortg which the infiltrating replacement solutions entered. Area of specimen represented by micrograph measures O.01l5mm x O.013mm. Dimensions of the larger bleb of jasper thus are a little less than 5 by 2 microns. Micrographed by the research laboratory of American Cyanimid Company. Enlargement lO,OOOx.

FIGURE 96. Thoroughly kaolinized amphibolite, uniformly replaced to the extent of about 85 percent by fine (average 1I300mm) granular quartz, both undergoing replacement by limonite (dark areas). In this specimen the clayey material had been mostly converted into the hard, ivory-colored quartz "soap" before replacement by limonite began. Without substantial impregnation of the clayey mass by quartz, and the retention of much of this through subsequent stages of alteration, the product would not be jasper, because quartz is an essential component of jasper. Enlargement 200x.

176

INTERPRETATION OF LEACHED OUTCROPS

jasperized into "puddingstone" lenses. Probably 90 percent of the jasperization occurs within 10 feet of the floor. At 20 to 25 feet above the level the process is so complete that the lenses become indistinguishable from those scattered through the ore channel nearer the surface. Impregnation and replacement of the "soap" by ferric oxide hydrate, is gradual, and observable in all stages. To some extent it forms rudely banded patterns, presumably reflecting the incipient schistosity of amphibolite. Whether banded or not, in the process of impregnation the ivory color changes by degrees to cream, straw, light tan, dark tan, brown, and finally dark brown to slightly reddish, each color beginning in an area of the preceding as isolated ragged specks or patches, and spreading slowly until the various patches join to effect rough color uniformity over areas an inch or two across. In turn these smaller nodular areas expand until they join with one another to merge into lenses of solid jasper from 1 to 12 or 15 feet in length, and embracing wholly the silica-flooded exposures at any given place. As the dark brown shade is approached at any given point, the product loses its slippery, soapy feel

FIGURE 97. Replacement as seen in polarized light. The quartz grains average 1I150mm in length. This gives an idea of the minute scale upon which replacement, both of clayey minerals by granular quartz, and of the latter by limonite, takes place in the usual formation of limonitic jasper of the replacement type. The limonite in these replacements comprises both hematite and goethite. The proportions vary greatly from specimen to specimen, and from area to area within a single specimen. For the specimens represented by figures 121 to 124, inclusive, the hematite goethite ratio averages about 15 to 85. Enlargement 165x.

and glossy luster, and takes on the duller appearance of ordinary jasper. The entire process however, from production of soft, clayey "soap" on the floor and 1 to 3 feet up from the floor, to the brown or reddish-brown, completely jasperized "puddingstone" lenses, takes place within a 20- to 25-foot vertical range above the 150-foot level, and most of it within 10 feet of the level. Annual rainfall at the mine averages 40 inches. Most of it falls during the 4 summer months. As a consequence, during the year the water table oscillates within the ore channel through a vertical range of 8 to 10 feet. With that fact in mind, and from corroborative evidence obtained in numerous other localities where formation of massive jasper has been observed in the making, it is concluded that the lenses of massive jasper derived from the replacement of clay "soap," originate, and that their formation goes through to completion, within the heavily saturated zone of rock involved through a vertical range of only a few feet above the water table; and that once the water table drops permanently to a lower level, the jasper lenses become marooned within the generally drier rock mass above, and the jasper undergoes only minor additional changes in composition. Formation of the jasper lenses so close to the water table explains also why nodules and "puddingstone" lenses of massive jasper of the replacement type persist

FIGURE 98. Greatly enlarged to emphasize the fine replacement lines followed by the encroaching limonite along borders of the quartz grains. The complete replacement by limonite throughout most of the picture shows how thorough such replacement may be. Only rarely is replacement by limonite of the quartz-clay groundmass this complete in an area as large as that represented by a hand specimen. Area of specimen embraced in this figure measures 0.14 x 0.11 mm. Enlargement 1400x.

REPLACEMENT OF AMPHIBOLITE BY JASPER AT NINETY-MILE MINE

throughout the oxidized zone; and often are as abundant close to the water table at depths of 100 or 200 feet as nearer to the surface. 1

DEVELOPMENT OF PSEUDO-CELLULAR BOXWORK IN ADJACENT NONSULFIDE AREA Although iron-rich amphibolite and other ferromagnesian rocks occur also along the southward extension of the fault-fissure zone seen in the Ninety-mile 'In dry weather silica is precipitated continuously, even at the surface, because the solution slowly dissolves the rock even in "puddingstone" areas of slight moisture content. The moisture may consist of a film only one or two molecules thick, just enough to slowly dissolve the rock even in dry weather. In arid regions, in contrast to the climate at the Ninety-mile Copper mine, the rock is dry most of the year, and a period of years is sometimes required for the dissolution of a layer of silica a few molecules thick, if it be located in the upper zone of air circulation. In these regions moisture quickly evaporates following a rainfall, and the rock is nearly always dry except in the 5 to 25 feet immediately above the water table.

FIGURE 99. An illustration of the selective replacement of alunite by limonite. Clay "soap" was completely replaced by fine granular quartz to form a hard, glossy, ivory-colored soap. The soap developed tiny (1112 to 1I60mm wide) fractures during weathering, and these cracks became filled with alunite (K,O.3Al,03.4S0 3 .6H,O). Limonite has selectively replaced the alunite vein-filling while leaving the quartz relatively unaltered. Enlargement 200x.

177

Copper mine, sulfide minerals are not present (fig. 101, 102). The zone is lenticular, about three-fourths of a mile in length and 10 to 100 feet in width. The iron occurs as pseudo-cellular limonitic boxworks at places where numerous joints and fractures intersect, and from which the amphibolite has been partially or entirely leached. The boxworks do not constitute a gossan however, because they are not indicators of useful ore. Presumably, most of the iron contained in the limonite, was precipitated from saturated solutions as they approached the water table, and deposited along open cracks formed by the intersection of numerous joints and fractures. The fault in this area was not open, but tigpt and through-going. The boxwork has a fiat, dead appearance, with no formal pattern, and no "soap" is involved here as at the "puddingstone" area at the Ninety-mile mine. The presence of iron here would be difficult for the beginner to interpret, but it is a somewhat common feature of highly jointed and fractured zones in ferro-magnesian rocks found in the tropics.

FIGURE 100. Enlarged view of the quartz residual within the limonite-filled fracture in the upper left hand corner of figure 99. Crossed nicols. The fibrous structure of the limonite that replaced alunite is well brought out in this view. The alunite was originally deposited on both walls of the vein before the quartz filled the medial portion. Contrast the fibrous limonite that replaced alunite with the granular limonite that replaced the fine granular quartz moss beneath, and into which it merges. Enlargement 800x.

178

INTERPRETATION OF LEACHED OUTCROPS

FIGURE 10l. Pseudo-cellular boxwork developed in amphibolite rock taken from the same fault found in the Ninety-mile Copper mine, but to the south. The amphibolite consists of either common hornblende or actinolite. The familiar pyroxene, augite, is also seen at places. l%x.

RFPLACEMFNT Of AMPHIBOLITE BY .TASPER AT NINETY-MILE MINE

FIGURE 102. An isolated specimen of a pseudo-cellular limonite boxwork of irregular pattern. The specimen was taken from an outcrop with especially prominent joint and fracture intersections.

179

Appendix C LEACHING AND REDEPOSITION OF COPPER MINERALS AT MOUNT OXIDE, QUEENSLAND GEOLOGIC SETTING The Mount Oxide copper deposit, situated about 45 miles west-northwest of Dobbyn, Queensland, as the crow flies, is a massive supergene chalcocite ore body formed by an unusual cycle of leaching and redeposition. The deposit is located near the center of a northwest-trending area of Precambrian rocks, some 25,000 square miles in extent, in extreme northwestern Queensland. The rocks in this Precambrian shield complex range in age from probable Archaean to late Upper Proterozoic, and include granitic intrusives, metamorphics, and little-altered sediments and lavas (Carter and others, 1961). Mineralization occurs in contorted sandy shale beds of the slightly metamorphosed Gunpowder Creek Formation of Lower Proterozoic age (Carter and others, 1961). Synclinal and anticlinal axes as well as major faults, trend northeast and northwest.

No intrusive rock has been noted at the deposit; the nearest known granitic rocks lie 15 miles to the east. The deposit occurs along a northeast-trending zone of slippage and crushing about 500 feet southeast of a major irregular fault that trends N 30° E (fig. 103), and joins a second fault 2Vz miles to the northeast. Much of the following material, including illustrations, is adapted from an earlier report by the author on a related feature seen at this mine (see Blanchard, 1939b).

DESCRIPTION OF THE OREBODY The sympathetic fault, or zone of crushing, is sharply localized between two steeply-northeast-pitching flexures or corrugations that occur about 500 feet apart along the strike of the beds, near the nose of a southwardpitching syncline (figs. 103 and 105). The high-grade

PLAN OF

MOUNT OXIDE MINE--+----+-T QUEENSLAND

LEGEND Modified after Blanchard (1939 b)

FIGURE 103.

Hematite-Specularite Outcrops _ Quartzitic Key bed. . . . . ~ Fault . . . . . . _ _

o I

FEET I

1000 I

Map of key bed and specularite-hematite outcrops at the Mount Oxide mine, Queensland.

181

LEACHING AND REDEPOSITION OF COPPER MINERALS AT MOUNT OXIDE

chalcocite ore shoot is contained in a concretionary

The copper carbonate, into which most of the chalco-

sandy shale bed 17 to 20 feet in width, with a welldefined hangingwall slip that dips irregularly, and averages 57 0 SE (fig. 104). Numerous minor slips and slickensides persist in abundance through the sandy bed, which has been locally widened by crenulation and minor folding to a body of horizontal width of more than 40 feet. Thereafter the copper dies out rapidly as an irregular and gradually diminishing zone of crushing and slickensiding in a graphitic shale through the next 150 feet. Beds immediately overlying the zone of crushing in the vicinity of the ore shoot are irregularly, and in places completely, replaced by massive hematitespecularite in variable thicknesses up to 100 feet or more. The high grade ore shoot averaged more than 21 percent copper. It has a maximum length of 295 feet in longitudinal section (fig. 105), and averages 8 to 10 feet in width. Ore containing as much as 35 to 40 percent copper normally is confined to the 3- to 6-foot section directly below the hangingwall slip, thereafter by general diminution of grade toward the footwall. At and near the surface most of the ore occurred as carbonate, mainly as malachite, with lesser atacamite and azurite. Those minerals were mixed with chalcocite which persisted to the surface as streaks and residuals. A small amount of both red and black copper oxide also was present at deeper levels.

cite has altered at and immediately below the surface, is roughly equivalent in copper content to the chalcocite lying directly below. Copper carbonate ceased largely at a depth of 65 feet, and almost entirely at 114 feet, giving place to brochantite and locally to cuprite and minor tenorite (Edwards, 1940). The copper oxide, chiefly cuprite, ranges in thickness from a layer detectable only under the microscope to layers (rarely) 4 to 5 inches in vertical thickness; but such alterations of the massive chalcocite are incomplete, and copper oxide occurrences are highly discontinuous. At the time of the author's principal inspection (Oct.-Nov. 1938), chalcocite could be observed persisting upward to the surface (especially on the 65-foot level and above) in irregular residuals as isolated nodules, and as seamlets up to 30 inches wide, throughout most of the length of the ore shoot. An estimate made at the time, mainly from pillars left after most of the ore in the upper sections had been stoped, indicated that not less than 15 percent of copper in the carbonate-oxide zone consisted of such unevenly distributed chalcocite residuals, The combined copper oxide minerals, mainly cuprite, were estimated at only a fraction of 1 percent. Below the 114-foot level the carbonate and other oxides passed rapidly into chalcocite that occurred as lenses of massive, almost pure glance that ranged from almond size to bodies 35 by 25 by 5 feet in maximum

MOUNT OXIDE MINE Section looking N. 40 0 E,

Nvv Mod ified after Blanchard (1939 b)

SE \

\

Crushed zone

208' Level III

III

.::1 oI

Section below

100

Sca Ie of Feet 50 0

_

208' level

100

projected

125'· 150' S.

I

~Original Water level II

---

II

'4~~~F---_~ P~esenf ~Q~

l.: . .-=-'- __

i:: " II II

II

Alluvium.

.

Graphitic Shale.

High Grode Cu Ore

Sandy bed

Hematite -specu/orite.

Quartzite.

Quartzitic "key" bed, ".11111.1.1 portly replaced by hematite . ~IIIII~

Fault (zone of crushing)

.

Concretionary sandy shale, portly replaced by copper ore.

.

FIGURE 104.

.

.

.

.

.

.

Cross section through main workings of the Mount Oxide mine,

182

INTERPRETATION OF LEACHED OUTCROPS

dimensions, dispersed erratically through the crushed and slickensided concretionary shale bed, and as narrow, more persistent seams throughout the zone of crushing. As the 300-foot level was approached, bornite became distinguishable to the unaided eye at numerous places, though chalcocite was by far predominant. The nature of the primary copper sulfide is unknown,l but below the 300-foot level lies a body of massive pyrite practically devoid of copper mineralization.

cambrian orogenesis. The Mount Oxide deposit is probably related to one of two orogenic periods, the later of which occurred at the close of the Lower Proterozoic (Carter and others, 1961, p. 202).

EVIDENCE OF LEACHING AND REDEPOSITION The deposit has probably been exposed to weathering for many millions of years, and field relationships point to the present chalcocite lenses having been subjected to the prevailing desert or semi-arid type of oxidation for hundreds of thousands of years at least. Since the oxidation of the ore shoot began, the supergene copper has been cumulatively re-precipitated at greater depths until, down to somewhat below present water table, it has replaced almost completely as massive chalcocite the associated and underlying pyrite which constituted the lower portion of the original ore shoot. What was of much greater interest, however, was that even under the microscope pyrite could only rarely be detected in the chalcocite residuals of the carbonateoxide zone; whereas in those portions of the orebody where copper carbonate and the interspersed copper oxide minerals persisted most deeply into the underlying chalcocite, the chalcocite adjoining and lying directly beneath the downward projections invariably contained sufficient unreplaced pyrite to be readily detected under the hand lens, and often by the unaided eye. In other words, wherever the chalcocite was altering extensively to copper carbonate and to the occasional copper oxide minerals, pyrite in varying though usually small amount, had been available to yield acid. On the other hand, wherever pyrite-free chalcocite could be identified, it

AGE OF MINERALIZATION Four lead-thorium analyses of well-preserved monazite crystals in quartz pegmatites from Mica Creek, about 5 miles southwest of Mount Isa, were made in 1941 by F. E. Connah, Chief Chemist, Queensland Geological Survey. The analyses indicated an age of 1 to 1.19 billion years, ± 25 percent (Nier and others, 1941). Northwestern Queensland, including the areas of both the Mount Oxide and Mount Isa deposits, was subsequently mapped and the formations studied and correlated (Carter and others, 1961). The Gunpowder Creek and Paradise Creek formations of the Mount Oxide area were shown to be of Lower Proterozoic age, but well up from the base of the sequence. The mineralization at Mount Oxide, indeed, all of the important mineralization of this Precambrian shield area, is believed also to be of Precambrian age. No evidence has been found to indicate that mineral deposits were formed later than the igneous activity which accompanied the several known periods of Pre'E. W. Berry, in 1919, noted a small amount of chalcopyrite associated with pyrite in a winze below the 300·foot level.

MOUNT OXIDE MINE QUEENSLAND

20% CUi Ore of lower grade not shown

\

r- Surface

I

2"

'I

II II

II

",\

0\

II

~\

:::;-;=1i:::::::=J'I~;:===-I~1 154' level

\ \

\

,, ,

Original Water ~le_v--,-e_1-"J'---------tl~-----~~W;;t_-_n~-~ (Apprax.) 300' level

'.:;:0 - - - - -

L====:;--;:::==;--;==~~~ ~===::jii:J:i:i\~ ~ ~ =====

Foreshortened Verticolly and Horizontally

Workings off lode:

II II II II

II FIGURE 105.

,

I

,, ,, o

FEET

500 I

I

Modified after Blanchard (1939 bl.

Longitudinal section through high-grade ore shoot of the Mount Oxide mine, looking N 57" W.

183

LEACHING AND REDEPOSITION OF COPPER MINERALS AT MOUNT OXIDE

had resisted more than partial decomposition even at the surface, though exposed to the ravages of desert or semi-desert weathering over possibly hundreds of thousands or millions of years. 2 The condition is well brought out by the sampling results set forth in table 8, which shows that 1) the massive chalcocite which persists to the surface is essentially free of admixed pyrite; 2) massive chalcocite 5 feet beneath the deepest prong of copper carbonate penetration contains (fig. 104) a higher percentage of pyrite than probably exists as the average content upon the 208-foot or 300-foot levels, both of which lie beneath the current weathering effects. 3 Below the 300-foot level lies a body of massive pyrite, practically devoid of hypogene copper mineralization, interpreted as representing the pyrite dregs of the original hypogene ore shoot. Within it four winzes, attaining depths to 70 feet below the 300-foot level, reveal chalcocite replacement feathering out irregularly and rapidly in all directions along fracture planes within the massive pyrite. In one winze, a crosscut at 40-foot depth contains a 14-foot width that averages 10 percent copper as a sooty chalcocite replacement of the pyrite. At greater depth here, and in all other winzes generally, replacement ceases at 60- to 70-foot depth. Unreplaced pyrite adjoining the sooty chalcocite (in contrast to the metallic chalcocite glance on the 300foot level and above), shows limited corrosion for distances up to several inches adjoining the larger chalcocite seams. It is not possible to determine whether such corrosion was effected wholly by sulfuric acid, or in some degree by downward-filtering ferric sulfate (see "It is worth noting that brochantite was more common at contacts of such chalcocite residuals than it was where minute pyrite grains were visible. "The small amount of iron left over from the calculations for the last three samples (0.07, 0.06 and 0.12 percent respectively) is believed to exist mainly as FeO in the rock silicate. Very minor, sporadic amounts of covellite are present in Mount Oxide chalcocite. Covellite contains twice as much sulfur as does chalcocite for a given amount of copper. If, therefore, the amount of covellite could be determined quantitatively in each case and taken into account, more excess iron (presumably as FeO in rock silicate) and less total pyrite would exist than shown in table 8. The effect, however, upon the chalcocitepyrite proportions shown would be almost negligible.

chapter 7 for details of the deposit at Kyshtim, Russia). The water table is flat, and has been stationary over an interval of geologic time believed to correspond closely to that for other portions of the Australian interior desert plains (since Precambrian times). 4

CONCLUSIONS REGARDING CHARACTER OF MINERALIZATION Mount Oxide thus possessed an important supergene chalcocite zone which was much richer than in earlier geologic times. (To the end of 1958, 48,249 long tons of 21 percent copper ore were mined and shipped.) Down to the present water table, it has been subjected for many millions of years to weathering characteristic of semi-arid regions. Because of the paucity, in the portion of the deposit above the water table, of pyrite or other sulfide to yield an external sulfur supply, the 'Because of fairly sharp demarcation at the 300-foot level between the massive metallic chalcocite lenses above (containing an estimated average pyrite content of only about 7 percent, table 8) and the pronounced feathering out of the sooty chalcocite everywhere along fracture planes in massive pyrite beneath, the opinion has been expressed that the water table at Mount Oxide formerly lay at the existing 300-foot level horizon, but has risen 40 feet since. That interpretation fails to take into account three factors: 1) that all known massive chalcocite lenses, in a deposit that for its size has been prospected and developed to a much greater extent than is usual (as regards the north corrugation, see figs. 104 and 105), have been found thus far in a sandy shale stratum lying between a strike fault with a prominent clay gouge along it as hangingwall, and a thick stratum of graphitic shale as footwall; 2) that coinciding closely with both the 300-foot level and the sandy shale stratum, dipping at a slightly flatter angle than does the fault, the chalcocite ore wedges out against the latter; 3) that nowhere, either above or below the water table, has there been found other than feathery, relatively low grade replacement by chalcocite along fracture planes within the graphitic shale stratum as stated in chapter 8, which seemingly is far from hospitable to chalcocite replacement at this mine. Furthermore, there exists no other evidence to support such a rise in the water table at Mount Oxide, or elsewhere in the Cloncurry district (including Mount Isa). On available evidence it certainly cannot be regarded as manifest that cessation of the rich massive chalcocite lenses in the vicinity of the 300-foot level has not been determined wholly by stratigraphic and structural controls, without regard to the water table.

TABLE 8 Analyses and Mineralogical Composition of Massive Chalcocite Ore on Various Levels of the Mount Oxide Mine, Queensland. Sample believed to represent average massive chalcocite ore at depth stated.

Surface, 30-inch width .... ___________________ 5 feet below lowest observed tip of copper carbonate prongs; 10 feet below 114-ft. level, 30-inch __________ 208-ft. leveF_______________________________________ 300-ft. leveE ______________________________________

CHEMICAL ANALYSESl (PERCENT)

PROBABLE MINERALOGICAL COMPOSITION (EXPRESSED IN PERCENT) Probable CuO, form Limonite Brochantite Chalcocite undetermined Pyrite

Cu

Fe

~

78.90

0.3

19.25

0.42

2.37

95.60

1.53

70.65 74.65 71.25

3.95 2.45 3.70

22.10 21.55 22.05

0.13 tr tr

0.73 tr tr

87.63 93.46 89.21

0.33 ? ?

SO,

nil 8.33 5.13 7.68

0.5 nil nil nil

'Analyses by laboratory of Mount Isa Mines, Ltd., 1939. Cu, Fe, and S determined to nearest 0.05 percent; S03 determined to second decimal. "Samples from 208 and 300-foot levels are composites.

Country Rock

nil 2.98 1.41 3.11

184

INTERPRETATION OF LEACHED OUTCROPS

supergene chalcocite at Mount Oxide stands today (prior to mining, of course), as a body of massive glance, stark and almost unattacked for 146 feet above the water table; and with no noticeable leaching of the copper content through the orebody's full 260-foot range above the water table, even though its upper portion has been substantially altered to copper carbonate. The same paucity of pyrite, or other iron-yielding sulfide, is responsible for the complete absence of a gossan, or for the precipitation of more than incidental limonite anywhere above the water table. The small amount of associated pyrite within the massive chalcocite (ratio of pyrite to chalcocite about 1 to 12.8, table 8) has been sufficient, however, to supply enough excess sulfur to have carried supergene metallic chalcocite replacement, over the long period of weathering, to 40 feet below the water table as an essentially massive chalcocite; and to have carried it through an additional 50- to 70-feet as sooty replacements that feather out rapidly along fracture planes of the pyrite-a total penetration into the zone of saturation of 110 feet. If pyrite had been equal to or slightly in excess of

the chalcocite in the upper portions of the chalcocite zone: 1) the rich copper carbonate existing near the surface would have been largely leached; 2) the chalcocite, during its long history of weathering under semiarid conditions, similarly would have been leached almost completely down to the water table; 3) a heavy gossan would have existed, and likewise would have extended unbrokenly down to the water table; 4) oxidation and replacement by chalcocite, through attack by cupric sulfate upon the underlying massive pyrite, presumably would have extended much below the present 110-foot depth beneath the water table. Because of the relatively minor amount of pyrite present, however, Mount Oxide does not exhibit these features to the 300-foot level. The products existing at Mount Oxide, although out of line with those commonly associated with oxidizing copper bodies (see chapter 13 for details of the Great Cobar, Home of Bullion, and Mount Oxide mines), thus represent a logical sequence once the conditions surrounding their origin, and their more immediate derivation, are understood (Blanchard, 1939b).

Appendix D THE LIMITED ROLE OF ORGANIC ACIDS IN LIMONITE PRECIPITATION Organic acids, derived largely from decaying vegetation, also are important solvents of the iron in rocks, but are seldom important in the production of limonite that is useful in leached outcrop interpretation. The reader should, however, be familiar with the facts regarding the bacterial iron precipitates of nature in order that they may be recognized clearly as such when encountered in quantity. One need only note along road cuttings the manner in which granite or other rocks are kaolinized and bleached under a layer of soil and roots, as compared with the red, brown, or yellow iron-staining of the same rocks nearby where no roots penetrate, to confirm the role of organic acids in dissolving the iron of rocks. The organic iron salts are the tartrate, citrate, formate, malate, acetate, and some others. The experienced observer often can recognize them by their sable to dark-brown color, and a certain flat crustiness (where exposed to weathering), not shared by their inorganic counterparts. In the tropics they are abundant. In cooler climates they are observed less often; but in some of the cooler humid areas, as along the rainy coastal belt of British Columbia and Alaska, they frequently form conspicuous coatings along rock fractures. Organic salts on the whole are unstable, and sooner or later the iron usually alters to ferrous bicarbonate or to one or another of the ferric oxide minerals. But in many regions the organic salts constitute an important initial form of iron in the ground water. Certain thread bacteria have been found effective under a wide range of conditions in precipitating the ferric oxides from ferrous bicarbonate solutions, from hydrosols, and from various salts of the organic acids. The same is true of manganese, copper, and other metals. 1 'See Harder (1919, pp. 5-84); Thiel (1925, pp. 301-310); (1930, pp. 242-250); Lovering (1927, pp. 45-52); and other sources. Harder has described in detail the work of the so-called iron bacteria. Certain species of the thread bacteria feed exclusively upon iron-bicarbonate, and require it to sustain life. By attacking and breaking down the CO 2 radicle, they derive the carbon, oxygen, and the available energy of the reaction, needed for their life processes. The freed iron is precipitated upon their bodies as a hydrous ferric oxide. Certain other species of the thread bacteria feed upon either or both iron and manganese bicarbonate, and bring about precipitation of the freed iron or manganese in a similar manner. Still other species of the bacteria attack organic iron salts, which eventually alter to ferric hydroxide and thence to limonite. Other species, again, do not require the iron compounds for their life processes but cause precipitation of ferric oxide when either inorganic or organic salts of iron are present. As any of these (non-toxic) thread bacteria die, the precipitated ferric oxide (or manganese

Insofar as the bacteria serve to precipitate the iron they do not assist in the production of limonite that is useful in the leached outcrop interpretation. At best such limonite occurs merely as an extraneous precipitate upon or intermixed with the useful product, and probably never occurs under circumstances or in sufficient amount to interfere with interpretation of normal sulfide derivatives except in swampy bog iron (or bog manganese and copper) deposits. The biochemical mechanism by which iron bacteria convert the bicarbonate or organic salt into ferric oxide is not fully understood. There are reasons for thinking that certain bacteria may play an important part in altering the organic salts of iron to forms more readily convertible into inorganic salts, and in so doing they may not necessarily utilize all of the iron themselves; and that thereby they may increase the total amount of iron dissolved in the circulating ground water 2 which becomes available for limonitic formation as understood in the leached outcrop interpretation. dioxide) sheaths on their bodies or skeletons remain in the water, and are carried in suspension with the current. Variable amounts of iron of such origin are present in most ground waters. Epidemics of the bacterial precipitates often coincide with excessive rainfall, as in the disastrous Breslau, Germany flood of 1906, when such impurities in the city's drinking water supply increased to 441 parts per million of iron and 231 parts per million of manganese, rendering the water utterly unfit for human consumption. Ground water percolating through glacial drift into wells on the Cuyuna Iron Range of Minnesota has carried up to 146 parts per million of manganese (Zappfe, 1931, p. 806). The Mount Isa, Queensland, domestic water supply, derived from the Rifle Creek reservoir 20 miles distant until 1932, when new facilities were installed, had yielded up to 555 parts per million of iron during such bacterial epidemics. Since Mount Isa has a semi-arid climate, and the epidemics invariably coincide with the driest portion of the year when water level at the reservoir is approaching the low point, it is obvious that the epidemics have no necessary relation to flood waters. Harder also has shown that even among mine waters where mineral acids predominate at least much of the time, skeletons of the iron bacteria have been found in the iron rust blisters upon mine rails. The iron bacteria (and to some extent manganese bacteria, copper bacteria, etc.) thus are of widespread occurrence, as they exist under a varied range of conditions, and their contribution to the ferric oxide content of ground water is substantial, especially in the swampy regions. Nevertheless in most cases not involving the sporadic epidemics, their contribution of iron to that of the ground water supply as a whole must be looked upon as merely incidental to the contribution from the other sources named. 21t is particularly true of those iron bacteria mentioned by Harder (1919) which do not require the iron compounds for their life processes.

186

INTERPRETATION OF LEACHED OUTCROPS

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Merwin, H. E., and Posnjak, E., 1937, Sulfate incrustations in the Copper Queen mine, Bisbee, Arizona: Am. Mineralogist, v. 22, no. 5, p. 567-571. Michell, W. D., 1945, Oxidation in a molybdenite deposit, Nye County, Nevada: Econ. Geology, v. 40, no. 2, p. 99-114. Moore, E. S., 1953, Banded iron formations: Econ. Geology, v. 48, no. 4, p. 312. Moore, E. S., and Maynard, J. E., 1929, Solution, transportation, and precipitation of iron and silica: Econ. Geology, v. 24, no. 3, p. 272-303; no. 4, p. 365-402; no. 5, p. 506527. Moss, A. A., 1957, The nature of carphosiderite and allied basic sulfates of iron: Mineralog. Mag., v. 31, no. 236, p. 407-412. Nier, A. 0., Thompson, R. W., and Murphey, B. F., 1941, The isotopic constitution of lead and the measurement of geologic time: Phys. Rev., v. 60, p. 112-116. Paige, Sidney, 1922, Copper deposits of the Tyrone district, New Mexico: U. S. GeoL Survey Prof. Paper 122. Pal ache, Charles, Berman, Harry, and Frondel, Clifford, 1944, The system of mineralogy of James Dwight Dana and Edward Salisbury Dana, Volume 1, Elements, sulfides, sulfosalts, oxides. 7th ed.: New York, John Wiley and Sons. Posnjak, E., 1926, Acceleration of rate of oxidation of ferrous iron in presence of copper and its application to "heap leaching" process [abs.]: Mining and MetalL, v. 7, no. 240, p. 541, Dec. 1926. Posnjak, E, and Merwin, H. E, 1919, The hydrated ferric oxides: Am. Jour. Sci., 4th Ser., v. 47, p. 311-348. .. ...................... , 1922 System Fe,O,-SO.-H,O: Am. Chem. Soc . Jour., v. 44, p. 1965-1994. Posnjak, E, and Tunell, George, 1928, Studies of weathering and sedimentation: Carnegie Inst. Washington, Yearbook no. 27, 1927-1928, p. 71-77. Ransome, F. L., 1912, Copper deposits near Superior, Arizona: U. S. Geol. Survey Bull. 540-D, p. 139-158. .. .............. , 1919, The copper deposits of Ray and Miami, Arizona: U. S. Geol. Survey Prof. Paper 115. Roberts, L. B., 1940, Petrified wood composed of iron oxide: Jour. Geology, v. 48, no. 2, p. 212-213. Rogers, W. F., and Shellshear, W. A., 1937, Corrosion of steel by oil well waste waters: Ind. Eng. Chern., v. 29, p. 160167. Ross, C. S., and Hendricks, S. B., 1945, Minerals of the montmorillonite group, their origin and relation to soils and clays: U. S. Geol. Survey Prof. Paper 205-B, p. 23-79. Roy, C. 1., 1945, Silica in natural waters: Am. Jour. Sci., v. 243, no. 7, p. 393-403; no. 10, p. 582. Schaller, W. T., 1907, The chemical composition of molybdic ocher: Am. Jour. Sci., 4th Ser., v. 23, p. 297-303. Schmitt, H. A., 1939, Outcrops of ore shoots: Econ. Geology, v. 34, no. 6, p. 654-673. Schouten, c., 1946, Some notes on micro-pseudomorphism: Econ. Geology, v. 41, no. 4, p. 348-382 . Stickney, A. W., 1915, The pyritic copper deposits of Kyshtim, Russia: Econ. Geology, v. 10, no. 7, p. 593-633. Stillwell, F. L., 1943, Molybdenite ores from Whipstick, N.S. W.: Australia (Commonwealth) Sci. Ind. Res. Org., Mineragraphic Rept. 274. Stillwell, F. L., and Edwards, A. B., 1943, Mineral Investigations, Council of Scientific Industrial Resources, Australia . .. .................... , 1945, The mineral composition of the Black Star Copper orebody, Mount Isa, Queensland: Australasian Inst. Mining Metal!., Proc., v. 139, p. 149-159.

188

INTERPRETATION OF LEACHED OUTCROPS

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INDEX (All page numbers appear in lightface (00) type and all figure and plate numbers appear in boldface (00) type.) acid, organic, role of in limonite precipitation, 185 Ajo, Ariz., influence of neutralizing gangue, 84-85 Alexandrov, E. A., 18 Allen, E. T., 3, 18, 102 American Cyanamid Co., 175,95 American Smelting & Refining Co., xiii, xv, 4 Ames, L. L., 30 amphibolite, 33 replacement of, by massive jasper, 34 (table 5), 174-177 Andrews, E. C., 5, 69 anglesite formed in oxidation of galena, 57-59, 61, 62 Mount Isa, Queensland, 73, 74 in partially sintered crusts, 101 surrounding galena residual, 148, 59 tertiary lead mineral, 83 (footnote) antimony oxidation of tetrahedrite, 49-50 tetrahedrite, leaching and oxidation products, 141-143 Antler Mine, Ariz., 48 (footnote) antlerite, 144 Antler mine, Ariz., 48 (footnote) Chuquicamata, Chile, 48 (footnote) Aravaipa, Ariz. galena oxidation products, 152, 63 pyramidal boxworks, 101 arborescent limonite, see limonite, arborescent arsenate arsenopyrite oxidation products, 126-128 arsenopyrite-pyrite derivatives, 99-101 arsenopyrite arborescent limonite derivatives, 99 leaching and oxidation products Commonwealth mine, N.S.W., 130 (table 7) Conrad mine, N.S.W., 127, 130 (table 7),35 Hodginson Goldfield, Queensland, 126, 33 Iron Blow, N. Terr., 130 (table 7) Mount Bonnie, N. Terr., 127, 130 (table 7), 34, pl. 2 Mount Emu, Queensland, 130 (table 7) Silver Ridge, Queensland, 130 (table 7) limonites derived from, 126-131, 33-35, pl. 2 ore and gossan, analyses and mineralogical composition, 130 (table 7) atacamite, 144 Australia, presence of "billy", 37 azurite, 132, 135, 144 Great Cobar; N.S.W., 69 Home of Bullion, N. Terr., 71 bacteria role in organic acid formation, 185 role in precipitation of iron, 17 Bagdad, Ariz. chalcocite oxidation products, pI. 10 Copper Creek, water analyses, 19 (table 2) fluffy limonite present, 67 pyrite oxidation products, 117, 120-121, 14,22, pI. 10 Bamford, Queensland, 60 Behrens, H., 35 beidellite, formula, 31 "billy", definition, 37 Bingham Canyon, Utah, see also Utah Copper limonite color, 90 Bisbee, Ariz., xiii, 78 chalcocite oxidation products, 136, 44 characteristic cellular pseudomorphs evident, 85

ARIZ.-Continued limonite analysis, 9 (table 1) limonite derived from pyrite, 115, 11 presence of clay "soap", 32 pseudo-jasper, 40 pyrite oxidation products, 118, 16,24 siderite oxidation products, 168, 85, 86 silica-breccia, 38, 39 solution of limonitic jasper at, 26 (table 4) supergene siderite, 8, 10, 63 bismuth, ocher, 5 Bolsa formation, 89 borgstromite, disallowed, 51 (footnote) bornite "key" structure, 138 leaching and oxidation products Engel mine. Calif., 138, 46a La Sal Mountains, Utah, 139,48 Las Palomas property, Jalisco, Mex., 140,49 Lookout prospect, Black Mountain, N. Mex., 138, 46b Ruby mine, Plumas Co., Calif., 139,47 limonites derived from, 138-140, 46-49 oxidation of, 49 Boswell, P. F., xiii, xv, 3, 19 (table 2), 25, 26 (table 4), 58 boxwork, see also limonite, cellular pseudomorph pseudocellular, Ninety-mile mine, Queensland, 178, 101, 102 Bragg, W. L., 7 Brindley, G. W., 30 (footnote) brochantite,48, 144 Broken Hill, N.S.W. carbonate gangue, 9 characteristic cellular pseudomorphs, 85 fluorite oxidation products, 169-170, 87-89, pI. 22 galena oxidation products, 147, 151, 152, 153, 57, 62, 64a, 65, pl. 22 limonite analysis, 9 (table 1) post-mine leaching products, 59 pyramidal boxworks, 101 pyrrhotite oxidation products, 123, 27a sphalerite oxidation products, 155, 158, 67b, 75, pI. 16 Brown, J. S., 8 Brownian movement, 15 Bryan, W. H., 122, 25 Butler, B. S., 3, 38, 39 Butte, Mont., mine water analyses, 19 (table 2) Cactus mine, Utah, pyrite oxidation products, 116, 12b calcite aid to formation of fluffy limonite, pI. 11 leaching and oxidation products Cloncurry district, Queensland, 166, 82 limonite derived from, 166, 82 limonite precipitation by, 65-68 Mount Cuthbert, Queensland, 77 reaction with ferric sulfate solutions, 66 reaction with ferrous sulfate solutions, 64 Calumet & Hecla Cons. Copper Co., xiii, xv, 3, 4 Cananea, Sonora, Mex. mine water analyses, 19 (table 2) supergene siderite, 8, 64 canbyite, Globe-Miami district, Ariz., 35 capping definition, 173 over dolomite, 36 siliceous-irony, 35

BISBEE,

190

INDEX

carbonate Broken Hill, N.S.w., 9 copper Mount Cuthbert, Queensland, 77 Ninety-mile mine, Queensland, 174 in gossans, 8 impurity in limonite, 8 Mount Oxide, Queensland, 181 Carlisle Mine, N. Mex., xiii dissolution of limonitic jasper, 26 (table 4) sphalerite oxidation products, 155, 67a Carroll, H. H., 101 Carter, E. K. and other, 72, 77, 180, 182 cellular boxwork, 93-94 see also limonite, cellular pseudomorph definition, 23 hypogene, 93 supergene, 93 cellular pseudomorph, 21-27, 93-96 see also limonite limonite of, classed as in digeneous, 11 pseudomorphic replacement of sulfides, 22 siliceous nature of, 23 cellular sponge, 94-96 see also limonite chromite derived, pI. 19 definition, 23 hypogene, 94 supergene, 94 cellular structure cellular boxwork, 23 cellular sponge, 23 development of, 23 size of, 23 types of, 23 webwork,23 cerargyrite (horn silver), Mount Isa, Queensland, 73 cerussite see also galena leaching and oxidation products, 145-153 Mount Isa, Queensland, 149, 150, 151, pI. 14 Mount Stewart, N.S.W., pI. 13 pseudomorphic replacements of, 101 cervantite, 142 chalcocite chemical and mineralogical composition, Mount Oxide, Queensland, 183 (table 8) Great Cobar mine, N.S.W., 69-70 Home of Bullion mine, 71-72 leaching and oxidation products Bagdad, Ariz., pI. 10 Bisbee, Ariz., 136, 44 Cloncurry district, Queensland, pl. 8 Home of Bullion, N. Terr., pI. 9 Miami, Ariz., 81-82, 135, 41a Morenci, Ariz., 135, 42 Tyrone, N. Mex., 81-82, 135, 136, 41b, 43 limonite derived from, 135-137,41-45, pl. 8-10 Mount Cuthbert, Queensland, 77 Mount Isa, Queensland, 74 Mount Oxide mine, 72, 180-184 oxidation of, 48-49 -pyrite ratio, influence on limonite products, 81-84 unusual leaching and redeposition, 180-184 chalcopyrite boxworks, 23 Ely, Nev., 78 Great Cobar mine, 69-71 Home of Bullion, N. Terr., 71 "key" structure, 132 leaching and oxidation products Creston Verde mine, Sinaloa, Mex., 132, 36c Duquesne, Ariz., 132, 36b, pI. 7 Great Cobar mine, N.S.W., 69-71

CHALCOPYRITE-Continued lervois Range, N. Terr., 133, 37b, pl. 5, 6 Longamundi, Queensland, 133, 38a, pI. 4 Los Aliodos, Sonora, Mex., 134, 39 Orphan mine, Dobbyn, Queensland, 133, 37a, pl. 3 Republic mine, Johnson, Ariz., pl. 18 Rocher de Boule mine, B.C., 132, 36a Ruth, Nev., 133, 38b Tres Hermanos property, Chihuahua, Mex., 134, 40 limonites derived from, 132-134, 36-40, pI. 3-7, 18 Mount Cuthbert mine, 77 Mount Isa, Queensland, 72-74 oxidation of, 47-48 oxidation products, process of formation, 53-54 chemical equations limonite precipitation by strong neutralizers, 66 oxidation of sulfides, air-water processes bornite, 49 chalcocite, 48 chalcopyrite, 48 galena, 57-58 pyrite, 46 pyrrhotite, 47 sphalerite, 57-58 tetrahedrite, 50 oxidation of sulfides, cupric sulfate solutions bornite, 62 chalcocite, 62 chalcopyrite, 62 galena, 61 pyrite, 62 sphalerite, 61 oxidation of sulfides, ferric sulfate solutions chalcopyrite, 61 pyrite, 61 precipitation of siderite, 64 chemistry, importance to leached outcrop technique, 5-6 Chino (Santa Rita), N. Mex., limonite derived from supergene chalcocite, 137 chloropal, 34 (footnote) chromite analysis of oxidized ore, pl. 19 leaching and oxidation products Tiebaghi mine, New Caledonia, 161-162, 78-80, pI. 19 limonites derived from, 161-163,78--80, pI. 19 chrysocolla, 84, 89, 135 Chuquicamata, Chile antierite mineralization, 48 (footnote) mine water analyses, 19 (table 2) cleavage effect on development of cellular structure, 23 influence of, information of cellular pseudomorphs, 22 Clements, J., and Smyth, H. L., 17 Climax, Colo. jarosite, 160 oxidized zone, 60 Cloncurry district Queensland, see also Longamundi prospect calcite oxidation products, 166, 82 chalcopyrite boxworks, pI. 4 craggy limonite, 99 hematite oxidation products, 164, 81 limonite analysis, 9 (table 1) oxidation products of chalcocite-pyrite ore, pl. 8 pseudo-jasper, 39 siderite oxidation products, 167, 84 variation in silica and ferric oxide in outcrops, 39, (table 6)

INDEX

colloidal solution definition, 15 importance in formation of limonitic jasper, 15-19 precipitation of jasper from, 29-30 role in formation of clay "soap", 31-35 color of limonite early investigations, 89 effect of particle size, 89 iridescence, 89 limitations as prospecting guide, 90-91 columnar limonite, see limonite, columnar Commonwealth mine, N.S.W., analysis of galena-arsenopyrite ore and gossan, 130 (table 7) Comstock Lode, Nev., mine water analyses, 19 (table 2) Connah, F. E., 182 Conrad mine, N.S.W. analysis and mineral composition of galena-arsenopyrite ore and gossan, 130 (table 7) arsenopyrite oxidation products, 127, 35 contour boxwork, see limonite copiapite stability areas shown on isothermal equilibrium diagrams, 51, 52, 54, 3 transition mineral, 54 copper, see also chalcocite, chalcopyrite minerals, leaching and redeposition, Mount Oxide, 180-184 pseudomorphic replacement by, 21 copper carbonate, see malachite, azurite Copper Flat, Bayard, N. Mex., silica-breccia, 39 Copper Queen mine, Ariz., transition minerals present, 54 (footnote) coquimbite, transition mineral, 54 covellite attack by cupric sulfate, 43, 61, 62 Baja, Calif., 84 ease of dissolution, 45 Home of Bullion mine, N. Terr., 71 oxidation of, 49 (footnote), 57 craggy limonite, see limonite, craggy Creston Verde, near Choix, Sinaloa, Mex., chalcopyrite oxidation products, 132, 36c C.S.A. mine, N.S.W., 58, 76-77 cuprite, 144 Great Cobar mine, N.S.W., 69 Mount Cuthbert, Queensland, 77, pl. 11 Ninety-mile mine, Queensland, 174 Cuyuna Iron Range, Minn., 185 Day, A. L., 18, 102 desert varnish, 36 diabase, solution of silica and iron oxide from, 25 (table 3) "dice", see limonite Dole, R. B., 18 dolomite aid to formation of fluffy limonite, pl. 11 jasper caps over, 36 limonite precipitation by, 66 Ducktown, Tenn. mine water analyses, 19 (table 2) pyrrhotite oxidation products, 123, 125, 27 b, c, 31, 32 Dunn, J. A., 18 Duquesne, Ariz. chalcopyrite oxidation products, 132, 36b, pl. 7 dissolution of limonitic jasper, 26 (table 4) Edwards, A. B., 48 (footnote), 74, 100, 180 ekmannite, 69, 70 Elkhorn, Mont., tetrahedrite oxidation products, 141,52 Ely, Nev., 78 absence of widespread leaching, 111 Ruth mine, 19 (table 2) Emmons, W. H., xiii, 3, 19 (table 2)

191 enargite (copper arsenate), 126 Engel, C. G., and Sharp, R. P., 36 Engel mine, Calif. bornite oxidation products, 138, 46a dissolution of limonitic jasper, 26 (table 4) limonite analysis, 9 (table 1) Eureka, Nev. cleavage boxwork, 146, 56 pyramidal boxworks, 10 1 exotic limonite, see limonite, exotic feldspar gangue, precipitation by, 65-66 -rich rocks, deposits in, 85, 86 sericitization of, 81, 82 Fenner, C. N., 18 ferrimolybdite, 60, 160 ferric sulfate reactions with pyrite and chalcopyrite, 61 solutions, precipitation of limonite from, 66 ferromagnesium minerals, decomposition during oxidation, 65 flaky crusts, 96-97, see also limonite, flaky crusts fluffy limonite, see also limonite, fluffy precipitation by strong neutralizers, 66-67 fluorite leaching and oxidation products, Broken Hill, N .S.W., 169, 170, 87, 88, 89, pl. 22 limonites derived from, 169-170, 87-89, pl. 22 foliated limonite, see limonite, foliated Foots, J. W., 72 Forsyath, Queensland, galena oxidation products, 148, 59 frei bergite, 141 Freundlich, E., 15 fringing limonite, see limonite, fringing galena C.S.A. mine, N.S.W., 76-77 "key" structure, 145 leaching and oxidation products Aravaipa, Ariz., 152, 63 Broken Hill, N.S.W., 147, 151, 152, 153,57,62, 64a, 65 Commonwealth mine, N .S.W., 130 (table 7) Conrad mine, N.S.W., 130 (table 7) Forsayth district, Queensland, 148, 59 Iron Blow, N. Terr., 130 (table 7) Lawn Hill, Queensland, 145, 147, 55a, 58, pl. 12 Mount Bonnie, N. Terr., 130 (table 7) Mount Emu, Queensland, 130 (table 7) Mount Isa, Queensland, 149, 60, pI. 14 Mount Stewart mine, Leadville, N.S.W., 150, 61, pl. 13 Ruby Hill, Eureka, Nev., 145, 146, 55b, 56 Silver Ridge, Queensland, 130 (table 7) Tepic, Nayarit, Mex., 152, 64b limonites derived from, 145-153, 55-65, pI. 13, 14, pl. 12 Mount Isa, Queensland, 72-74 Mount Stewart, N.S.W., 74-76 ore and gossan, analyses and mineralogical composition, 130 (table 7) oxidation by air-water processes, 57-59 garnet, pipes, Whipstick, N.S.W., 5 Geophys. Lab., Washington, D.C., xiii, 3 chalcopyrite investigation, 48 Garretty, M. D., 9, 59, 147, 153 Gilluly, James, 84 Globe-Miami, Ariz., presence of nontronite, 35 Godfrey, J. R., 76 goethite definition, 7 stability in system, Fe203~S03.H20, 51-53

192

INDEX

Golconda Mine, Chloride, Ariz. dissolution of limonitic jasper, 26 (table 4) limonite analysis, 9 (table 1) sphalerite oxidation products, 155, 156, 157, 68b, 71, 72

gossan, see also limonite, limonitic jasper definition, 17 3 galena-arsenopyrite deposits, chemical and mineralogical content, 130 (table 7) garnet derived, 5 massive iron-oxide, Mount Oxide, Queensland, 111112 non-sulfide, Lawn HilI, Queensland, 111 granite, limonite precipitation by reaction with, 65-66 granular limonite, see limonite, granular Graton, L. C., 3, 62 Great Australia mine, Queensland, variation in composition of jasper outcrops, 39 (table 6) Great Cobar, N.S.W., 69-71 Gruner, J. W., 17 gypsum, impurity in limonite, 9 Hachita, N. Mex., tetrahedrite oxidation prod ucts, 141, 50a,51 Hall, Graham, xv Hall molybdenite, Tonopah, Nev., 60 Hampden mine, Queensland, variation in composition of jasper outcrops, 39 (table 6) Hanover, N. Mex. dissolution of limonitic jasper, 26 (table 4) pyrite oxidation products, 118, 17 salite oxidation products, 171, 90 sphalerite oxidation products, 158,74 Harder, E. C., 185 hard pseudomorphs, see limonite, hard pseudomorphs Harman, R. W., 15 Hayes, A. 0., 8 hedenbergite, 171 hematite, 8 leaching and oxidation products, Cloncurry district, Queensland, 164, 81 limonite derived from, 164, 81 outcrops, Mount Oxide mine, Queensland, 111-112, 180, 103 hematite-specularite bodies, as contact metamorphic zones, 38 Mount Oxide, Queensland, 181 hemimorphite, 154, 159 Herberton, Queensland, see Omeo Tin prospect Hess, F. L., 60 hieroglyphic boxworks, see limonite, hieroglyphic boxworks Hitchen, C., 16 (footnote) Hodgkinson district, Queensland, 60 arsenopyrite oxidation products, 126, 33 molybdenite oxidation products, 160, 77b Home of Bullion, Nor. Terr., Australia, 71-72 cellular limonite derived from chalcocite-pyrite, pl. 9 horn silver, see cerargyrite Hossfeld, P. S., 71 host rock influence on character of leaching products disseminated deposits, 81-82, 84 massive deposits, 85-87 hydrogoethite, disallowed, 7 hydrosol, role of silica in formation of jasper, 15-19 hypogene definition, 8 (footnote) relation to primary, 8 (footnote) indigenous limonite, see limonite iridescence of ferric oxide hydrates, 89 iridescent crusts, 104

iron carbonates, 8 content of river waters, 16 in natural compounds, 7-8 oxides goethite, 7 hematite, 8 lepidocrocite, 7 magnetite, 8 specularite, 8 pseudomorphic replacement of, 21 reactions of, in vicinity of oxidizing sulfides, 18 solution, transportation, and precipitation of, 16-19 sulfates, 8 Iron Blow, Nor. Terr., Australia, analysis of galena-arsenopyrite ore and gossan, 130 (table 7) iron oxide, see also limonite, hematite solution from limonitic jasper, 26 (table 4) solution from norite and diabase, 25 (table 3) isothermal equilibrium diagrams, 51-54 Jacobson, C. A, 60 jarosite, 52 formation of, 54 in gossans, 8, 9 at Utah Copper, 8 Jarrell, O. W., 18, 19,48 jasper, limonitic, see limonitic jasper jaspilite, 15, 17 mutual precipitation of silica and ferric oxides in, 17 theories of origin, 17-18 Jervois Range, Nor. Terr., Australia chalcopyrite oxidation products, 133, 37b, pI. 5, 6 supergene silica oxidation products, 172, 91 Johnson, Ariz., Republic mine, pseudomorphs and other limonite products, 86-87 Joplin district, Mo., mine water analyses, 19 (table 2) Jubilee mine, Wenden, Ariz., siderite oxidation products, 167, 83 Kahlenberg, L. and Lincoln, AT., 15 kaolinite, replacement by limonitic jasper, 30-35 kaolinization of amphibolite, Ninety-mile mine, Queensland, 174177 definition, 31 (footnote) Kelly, W. C., xv, 5, 7 Kenny, E. J., 76 "key" structure bornite, 138 chalcopyrite, 132 definition, 5 galena, 145 pyrite, 118 sphalerite, 156, 157 tetrahedrite, 141 Kimberly, Nev., xiii hematite and goethite replacing magnetite, pl. 20 limonite analysis, 9 (table 1) magnetite at, 8 presence of clay "soap", 32 pseudo-jasper, 40 silica-breccia, 39 solution of limonitic jasper, 26 (table 4) Kingsbury, H. M., 4 kornelite, stability in system Fe,n,oSO,oH,O, 51-53 Kruttschnitt, Julius, xiii Kulp, J. L. and Trites, A F., 7 Kyshtim, Ural Mts., Russia, 44 Ladoo, R. B. and Myers, W. M., 35 Lake City, Colo., tetrahedrite oxidation products, 142, 53,54 La Sal Mts., Utah, bornite oxidation products, 139, 48 Lasky, S. G., 39

INDEX

Las Palomas property, lalisco, Mex., bornite oxidation products, 140, 49 Lawn Hill, Queensland, xiii galena oxidation products, 145, 147, 55a, 58, pl. 12 limonite analysis, 9 (table 1) manganite and pyrolusite oxidation products, pl. 21 non-sulfide gossan, 111 oxidation of sphalerite, 59 pyramidal boxworks, 10 1 pyrite oxidation products, 120, 20 sphalerite oxidation products, 154, 157, 66b, 73 leached outcrops absence of, Ely, Nev., 111 basis for interpretation, 4 blind, 109-110 Mount Oxide, iron-oxide, significance, 111 non-sulfide, 111 over disseminated deposits, classification of, 109 sub-ore grade, 110-111 lead, carbonate ore, at C.S.A. mine, 76 Leadville, Colo. origin of zinc carbonate boxworks, 64 supergene siderite boxworks, 167 Leith, C. K. and others, 17 lepidocrosite, 7 limestone, precipitation of limonite by, 66-68 limonite, see also limonitic jasper absence of gossan, Mount Oxide, Queensland, 180-184 arborescent, 99-101, 127, pl. 2 arsenopyrite derivation, 126-131,33-35, pl. 2 bornite derivation, 138-140, 46-49 botryoidal, 118, 120, 124 calcite derivation, 166, 82 cellular pseudomorphs, 93-97, 121 boxworks, 93-94, 115, 122, 123, 127, 132-134, 135, 137, 138, 141, 145, 146, 154, 155, 156, 157, 161, 166, 167, 169, 170, 27,29, 31, 35, 36, 37, 39, 40, 42, 46, 47, 48, 49, 50-54, 5558, 61-64, 66-68, 72, 73, 82-85, 87-89, pI. 3-6, 9, 16, 17 sponge, 94-95, 115, 118, 122-125, 134, 139, 146, 153, 155, 158, 159, 162, 17, 30-32, 48, 64a, 65, 67b, 74, 75, 80, pl. 19 chalcocite derivation, 135-137,41-45, pl. 8, 9, 10 chalcopyrite derivation, 132-134, 36-40, pl. 3, 4, 5, 6, 7,18 chromite derivation, 161-163,78-80, pl. 19 cleavage boxworks, "key" galena structure, 145, 146, 55, 56 coagulated, 142, 53, 54 color, 89-91, see also color columnar, 115, 119, 120, 159, 19-21, 76, pI. 1 content of cellular pseudomorphs, 24 contiguous, 11 contour boxworks, 141, 51-53 "corn flake" structure, 158 craggy, 98-99, 137,45 cuprite derivation, pl. 11 defined, 7 derived from mixed sulfides, 33-35, 38b, 41-45, 54, 64a, 65, pI. 2, 7-10, 18, 22 desert varnish, 105, 115 diamond-mesh structure, 145, 55b dice, 68, 121, 24 difficulties in classification, 13 effect of pyrite on character of, 81 exotic, definition, 12 fibrous crusts, radiating, 99, 122, 25 flaky crusts, 96-97, 123, 26 fluffy, precipitation of, 66-67, 98,115,120,167, pl. 10, 11

193 LiMONITE-Continued fluorite derivation, 169-170, 87-89 foliated boxworks, 160, 77 fretwork, 127 fringing, definition, 11 galena and cerussite derivation, 145-153, 55-65, pl. 12, 13, 14, 22 goethite, relation to, 7 granular, 97, 127, 132, 133, 36-38 hard pseudomorphs, 98, 116, 121,24 hieroglyphics boxworks, 155, 68, pI. 15 hematite and magnetite derivation, 164, 81, pI. 20 honeycomb boxwork, 151, 161, 163, 62, pl. 19 importance in leached outcrop interpretation, 4 indigenous, definition, 11 "key" structures, 5,118,132,138,141,145,156-157 lepidocrocite, relation to, 7 manganite and pyrolusite derivation, 165, pI. 21 mineralogical and chemical composition, 9 (table 1) minor impurities in, 8 molybdenite derivation, 160,77 partially sintered crusts, 101, 139, 140, 145, 152, 153, 49, 63, 64b, 65, pI. 12, 13 precipitation above water table, 41-42 below water table, 43-44, 61-64 by reaction with neutralizing gangues, 65-68 related to oxidation of iron-free sulfides, 57-60 through solution of iron-bearing solutions, 51-55 precipitation by organic acids, 185 processes of formation, 3 pulverulent, 132, 36 pyramidal boxwork, 101, 146, 152,63,64 pyrite derivation, 115-121, 11-24, pI. 1, 2, 7, 9, 10, 18 pyrrhotite derivation, 122-125, 25-32 relief, 98-101, 116, 134, 137, 140, 146, 147, 149, 152, 45,58, 60, 63, pl. 7, 13, 14 rosettes 97, 155, 156,68,69 salite derivation, 171, 90 siderite derivation, 167-168, 83-86 silica derivation, supergene, 172,91,92 smeary crusts, 115, 15, 16, 23, 24 sphalerite derivation, 154-159,66-75, pI. 15, 16, 17, 18 surface coalescences, 10 1 tetrahedrite derivation, 141-143,50-54 triangular boxworks, "key" bornite structure, 138-139, 46-48 type, difficulty in classification, 12 webwork, 132, 139, 155, 36, 67 limonitic jasper in cellular pseudomorphs, 21-27 dissolution of, 26 (table 4) formation of, 15-19 importance of silica in formation of, 16-19 jaspilite, 15 massive, 29-40 "billy", 37 formation in presence of strong neutralizer, 67 formation of, at Ninety-mile mine, Queensland, 34 (table 5) general varieties, 29 jasperoid, 38 open-space precipitates, 29-30 "puddingstone", 29, 174 ragged-edged, 33 replacement jasper, 30-35 replacement of kaolinite and montmorillonite, 30 replacement of opal, 35 silica-breccia, 38 in oxidized chrome ore, pI. 19 precipitation of iron during formation of, 16-19

194

INDEX

JASPER-Continued resistance to chemical attack, 25-26 "soap", 31-33, 174-177 variation in silica and ferric oxide content, 39 (table 6) Lincoln, A. T., 15 Lindgren, Waldmar, 3 Locke, Augustus, xiii, xv, 3, 11, 24, 34, 41, 44, 46, 52, 53, 90, 137, 168, 3,4 Longamundi, Queensland chalcopyrite oxidation products, 133, 38a, pI. 4 massicot occurrence, 128 (footnote) Lookout Prospect, Black Mountain, N. Mex., bornite oxidation products, 138, 46b Los Aliados Property, Sonora, Mex. chalcopyrite oxidation products, 134, 39 dissolution of limonitic jasper, 26 (tahle 4) Loughlin, G. F., 3 Lovering, T. S., 15, 18,25,26, 185 McBain, J., 46 McKinstry, H. E., xv, 17 magnesite, impurity in limonite, 9 magnetite, 8 limonite derived from, 164, pl. 1 replaced by hematite and goethite, pl. 20 malachite in chalcocite ore, Mount Oxide, Queensland, 181 in chalcopyrite ore, Duquesne mine, Ariz., pI. 7 Great Cobar mine, N.S.W., 69 Home of Bullion mine, N. Terr., 71 manganese manganite and pyrolusite, 9, 165, pl. 21 minerals, impurities in limonite, 9 manganite, see also manganese impurities in limonite, 9 marmatite, oxidation, galena-marmatite mixture, Broken Hill, N.S.w., pI. 22 massicot in arsenopyrite oxidation products, 12H in gossans, 9 massive jasper, see limonitic jasper Maxwell, J. C., 161 Mavronouni, Cypress, 21 Maynard, J. E., see Moore, E. S., and Maynard, 1. E. Merwin, H. E., 3, 7, 24, 51-55 Meyer, Charles, 48 (footnote) Miami, Ariz., 81 chalcocite oxidation products, 135, 41a effect of pyrite-chalcocite ratio, 81-82 Michell, W. D., 60 mimetite in gossans, 9 in oxidized arsenopyrite, 128 Mineral Park, Ariz., 60 mole, definition, 46 (footnote) molecular weight, 46 molybdenite leaching and oxidation products Hodgkinson district, Queensland, 160, 77b Santa Nino mine, Ariz., 160, 77a limonites derived from, 160, 77 oxidation by air-water processes, 59 montmorillonite formula, 31 replacement by limonitic jasper, 30-35 monzonite, see quartz monzonite Moore, E. S. and Maynard, J. E., 15-19, 25, 26 analyses of limonitic jasper, 15 experiments in solution, transportation and precipitation of silica, 15-19 Morenci, Ariz., xiii chalcocite oxidation products, 135, 42 LiMONITIC

dissolution of limonitic jasper, 26 (table 4) limonite analysis, 9 (table 1) limonite color, 90 Morse, H. W., 3, 46 Morton, C. C., 37, 174 Moss, A. A., 51 Mount Bonnie, Nor. Terr., Aust. analysis of galena-arsenopyrite ore and gossan, 130 (table 7) arsenopyrite oxidation products, 127,34 limonite analysis, 9 (table 1) oxidation products of arsenopyrite-pyrite ore, pl. 2 Mount Cobalt Mines Limited, Selwyn, Queensland, 167,84 Mount Cuthbert, Queensland, 77 fluffy limonite, pl. 11 Mount Emu, Queensland, analysis of galena-arsenopyrite ore and gossan, 130 (table 7) Mount Isa Mines Limited, Queensland, xiii, xv, 72-74 chalcopyrite, 48 (footnote) characteristic cellular pseudomorphs, 85 galena oxidation products. 149, 60, pI. 14 limonite analysis, 9 (table 1) mimetite and massicot, 128 (footnote) presence of coquimbite and copiapite, 54 pyrite oxidation products, 116, 119, 120, 13, 18,19,21 pyrrhotite oxidation products, 124, 28 sphalerite oxidation products, 155, J 56, 68a, 69, pI. 15, 17 water supply, bacterial epidemics, 185 Mount Morgan, Queensland, columnar limonite derived from pyrite, pl. 1 Mount Oxide, Queensland, 72 absence of limonite in unoxidized chalcocite deposits, 63 chalcocite ore, chemical and mineralogical composition, 183 (table 8) chalcocite oxidation, 49 leaching and redeposition of copper minerals, 180184, 103-105 massive iron-oxide outcrop, 111-112 mine water analyses, 19 (table 2) pseudomorphic replacement by copper, 21 Mount Stewart, Leadville, N.S.W., 58, 74-76 galena oxidation products, 150, 61, pl. 13 Mudd, Harvey, 21 (footnote) Nevada Cons. Copper Co., Ruth, Nev., chalcopyrite oxidation products, 133, 38b Nier, A. 0., and others, 182 Ninety-mile copper mine, Einasleigh, Queensland, 33 formation of limonitic jasper at, 34 (table 5) pseudo-cellular boxwork, 177-179, 101, 102 replacement of amphibolite by massive jasper, J 74177, 93-100 nontronite formula, 31 replacement of, by massive jasper, 34 norite, s,?lution of silica and iron oxide from, 25 (table 3) Omeo Till prospect, Herberton, Queensland characteristic cellular pseudomorphs present, 85 pyrite oxidation products, 121,23 opal: replacement of, by massive jasper, 35 Ophir, Utah, 167 ore effect of ratio of pyrite to other sulfides, 81 estimating grade prior to leaching, 82-83 influence of neutralizing gangue on, 84 width of, beneath leached outcrops, 87 Orphan mine, Dobbyn, Queensland, chalcopyrite oxidation products, 133, 37a, pI. 3 oxidation above water table, 41

r

r

195

INDEX

OXIDATION-Continued

pyrolusite, see a/so manganese

air-water, importance in pseudomorphic replacement, 21-22 air-water processes, 45-50 below water table, 43, 62-63 of bornite, 49, 62 of chalcocite, 48, 62 of chalcopyrite, 47,53,61,62 illustrated by isothermal equilibrium diagrams, 51-54 of iron-free sulfides, 57-60 galena, 57-59, 61 molybdenite, 59-60 sphalerite, 57-59, 61 of pyrite, 45, 53, 61, 62 of pyrrhotite, 47 of tetrahedrite, 49 Park City, Utah, pyrite oxidation products, 117, 15 partially sintered crusts, see limonite Patagonia, Ariz., xiii dissolution of limonitic jasper, 26 (table 4) tetrahedrite oxidation products, World's Fair mine, 141, SOb Pennebaker, E. N., xv Percyville, Queensland, supergene silica oxidation products, 172, 92 Peterson, N. P., 35 Phelps Dodge Corp, xv, 4 Posnjak, Eugen, 3, 7, 10, 18,24,51-55 primary ore deposit, definition, 8 (footnote) pseudomorphic replacement air-water oxidation processes, importance of, 21-22 description of process, 2 I effect of mineral cleavage, 22 pseudo-jasper, 38-40 Cloncurry district, Queensland, 39 (table 6) jasperoid, 38 silica breccia, at Bisbee, Ariz., 38 pyrite, 115-121 absence of, Mount Oxide, Queensland, 180 arborescent limonite derivatives, 99 boxworks a rarity, 23 C.S.A. mine, N.S.W., 76-77 -chalcocite ratio, influence on limonite products, 81-84 Ely, Nev., 78 Home of Bullion, N. Terr., 71-72 "key" structures, 118 leaching and oxidation products Bagdad, Ariz., 117, 120-121, 14,22, pl. 10 Bisbee, Ariz., 115, 118, 121, 11,16,24 Cactus mine, Utah, 116, 12b Cloncurry district, Queensland, pI. 8 Duquesne mine, Ariz:, pl. 7 Hanover, N. Mex., 118, 17 Home of Bullion, N. Terr., pl. 9 Lawn Hill, Queensland, 120,20 Mount Bonnie, N. Terr., pl. 2 Mount Isa, Queensland, 116, 119, 120, 18, 19, 21,113 Mount Morgan, Queensland, pl. 1 Omeo Tin prospect, Herberton, Queensland, 121, 23 Park City, Utah, 117,15 Republic mine, Johnson, Ariz., pl. 18 Silver Bell, Ariz., 116, 12a Ukalunda, Queensland, 116, 12c limonite derived from, 115-121, 11-24, pI. 1, 2, 7, 8, 9,10,18 Mount Cuthbert mine, 77 Mount Isa, Queensland, 72-74 Mount Stewart, 74-76 oxidation of, 45-47 oxidation products, process of formation, 53-54

impurities in limonite, 9 pyrrhotite Great Cobar mine, 69-71 leaching and oxidation products Broken Hill, N.S.W., 123, 27a Ducktown, Tenn., 123, 125, 27b, c, 31, 32 Mount Isa, Queensland, 124, 28 Two Treys mine, Herberton, Queensland, 123, 124, 26, 29, 30 limonites derived from, 122-125,25-32 "'lount Isa, Queensland, 72-74 oxidation of, 47 quartz, see silica quartz monzonite limonite precipitation by reaction with, 65-66 Tyrone, N. Mex., 81 Ransome, F. L., 3, 8, 90 relief limonite, see limonite, relief rhodochrosite, impurity in limonite, 9 rhomboclase, 53, 54 Richard, K. E., xv Roberts, L. B., 22 roe me rite, 54 Ross, C. S. and Hendricks, S. B., 30 Rocher de Boule, B. C. chalcopyrite oxidation products, 132, 36a dissolution of limonitic jasper, 26 (table 4) rosette limonite, see limonite, rosettes Roy, C. J., 15 Ruby Hill, Eureka, Nev. dissolution of limonitic jasper, 26 (table 4) galena oxidation products, 145, 146, 55b, 56 Ruby mine, Plumas Co., Calif., bornite oxidation products, 139,47 Ruth, Nev., see Nevada Consolidated Copper Co. Rustom, Mahmoud, 30 (footnote) Sand, L. B., 30 (footnote) salite leaching and oxidation products, Hanover, N. Mex., 171, 90 limonite derived from, 171, 90 San Javier, Sonora, Mex., pyramidal boxworks, 101 Santa Rita, N. Mex., limonite color, 90 Santo Nino, near Nogales, Ariz., 60 molybdenite oxidation products, 160, 77a Schaller, W. T., 60 (footnote) schist limonite precipitation by reaction with, 65-66 Miami, Ariz., 81 Schmitt, H. A., 171 scorodite in gossans, 9 oxidation product, arsenopyrite-pyrite ore, pl. 2 secondary, enrichment, 3 secondary enrichment investigation, 7 secondary ore deposit, definition, 8 (footnote) shale, limonite precipitation by reaction with, 65-66 Sharp, R. P., 36 siderite at Bisbee, Ariz., 8, 63 leaching and oxidation products Cloncurry district, Queensland, 167, 84 Gardner mine, Bisbee, Ariz., 168,85,86 Jubilee mine, Wenden, Ariz., 167,83 limonite derived from, 167-168, 83-86 precipitation of, 63-64 stability under chemical attack, 8 supergene, derived from sulfides, 8 Sierra Nacimiento, N. Mex., replacement of wood by silica, 22

196

INDEX

silica content of cellular pseudomorphs, 23-25 content of mine waters, table 2 content of river waters, 16 importance of formation of limonitic jasper, 16-19 impurity in limonite, 8 inground water, 16 limonite derived from, 172, 91-92 precipitation of, 16 reactions of, in vicinity of oxidizing sulfides, 18 silica-breccia, 38 at Kimberly, Nev., 39 at Bisbee, Ariz., 38, 39 solution from limonitic jasper, 26 (table 4) solution from norite and diabase, 25 (table 3) solution. transportation, and precipitation of, 15-19 supergene, leaching and oxidation products Agate Creek, Percyville, Queensland, 172, 92 Killeen Copper lode, Jervois Range, N. Terr., 172,91 Silverbell, Ariz., xiii limonite color, 90 pyrite oxidation products, 116, 12a Silver Ridge, Cloncurry district, Queensland, 85 analysis of galena-arsenopyrite ore and gossan, 130 (table 7) Simmons, W. W., 125 smeary crusts, see limonite, smeary crusts smithsonite, 154, pl. 17 columnar, 159, 76 reaction with ferric sulfate solution, 58 Smyth, H. L., 17, 18 "soap", 31-33 formation and replacement at Ninety-mile mine, Queensland, 33, 174-177 nature and occurrence, 31 replacement by limonitic jasper, 32 sol, defined, 15 solutions, true and colloidal, distinguished, 15 specular iron, 8 sphalerite C.S.A. mine, N.S.W., 76-77 "key" structure, 156-157 leaching and oxidation products Broken Hill, N.SW., 155, 158, 67b, 75, pI. 16 Carlisle, N. Mex., 155, 67a Golconda mine, Chloride, Ariz., 155, 156, 157, 68b, 71, 72 Hanover, N. Mex., Empire mine, 158,74 Lawn Hill, Queensland, 154, 157, 66b, 73 Mount Isa, Queensland, 155, 156, 68a, 69, pI. 15, 17 Republic mine, Johnson, Ariz., pI. 18 Spruce Mountain, Nev., 154, 66a Tally Ho mine, Queensland, 159, 76 Wolfram, Queensland, 156,70 limonites derived from, 154--159, 66~76, pl. 15-18 Mount Isa, Queensland, 72-74 Mount Stewart, N.S.W., 74-76 oxidation by air-water processes, 57-59 Spruce Mountain, Nev. dissolution of limonitic jasper, 26 (table 4) sphalerite oxidation products, 154, 66a Stickney, A. W., 44 Stillwell, F. H., 5, 48 (footnote) sulfate, in gossans, 8 Sullivan, C. L., 71 Sunshine mine, Idaho, 141 supergene definition, 8 (footnote) enrichment, oxidation by cupric sulfate solutions during, 61-62 relation to secondary, 8 (footnote)

surface coalescences see limonite, surface coalescences Tally-Ho mine, west of Mackay, Queensland, sphalerite oxidation products, 159, 76 Tarr, W. A., 16 tenorite (CuO), 48, 144 Tepic, Nayarit, Mex., galena oxidation products, 152, 64b tetrahedrite contour boxworks, "key" structure, 141 leaching and oxidation products Elkhorn, Mont., 141,52 Hachita, N. Mex., 141, 50a, 51 Lake City, Colo., 142, 53,54 World's Fair mine, Patagonia, Ariz., 141, SOb limonites derived from, 141-143,50-54 oxidation of, 49-50 Thiel, G. A., 185 Thomson, B. P., 76 Tiebaghi, New Caledonia chemical analysis, oxidized chromite ore, pI. 19 chromite oxidation products, 161-162,78-80, pI. 19 limonite analysis, 9 (table 1) massive honeycomb boxworks, 161 Tintic, Utah, pyramidal boxworks, 101 Tinto mine, N.S.W., 77 Toquepala, Peru, xv Tres Hermanos property, Chihuahua, Mex., chalcopyrite exidation products, 134, 40 Trischka, Carl, and others, 63 Trites, A. F., 7 troilite, 47, 122 Tunell, George, xiii, xv, 3, 10, 18,52,53,90,91,3,4 turgite, disallowed, 7 Two Treys, Herberton, Queensland limonite analysis 9 (table 1) pyrrhotite oxidation products, 123, 124, 26, 29, 30 Tyrone, N. Mex., xiii, 81, 90 chalcocite oxidation products, 135, 136, 41b, 43 effect of pyrite-chalcocite ratio, 81-82 Udy, M. J., 161 United Verde, Ariz. characteristic cellular pseudomorphs present, 85 gypsum in gossan, 9 limonite analysis, 9 (table 1) transition minerals present, 54 (footnote) Ukalunda, Queensland, pyrite oxidation products, 116, I2c Utah Copper Co., Utah, jarosite present, 8 vadose, water, definition, 19 (table 2, footnote) Van Hise, C. R. and Leith, C. E., 17 voltaite, 54 Wabana, Newfoundland, iron ore, primary nature of, 8 Wardlow, Clyde, 3 water table effect on development of cellular pseudomorphs, 27 influence on redeposition of chalcocite, Mount Oxide, Queensland, 183-184 oxidation, above the, 41-42 oxidation, below the, 43-44 webwork, definition, 23 Wells, R. C., 45 Whipstick, N.S.W., 5, 9 White, C. H., 3 Willan, T. A., 76 (footnote) Wiluna, West Australia, 100, 101 Wisser, E. H., 4, 11 Wolfram, Queensland, sphalerite oxidation products, 156, 70 Wollororang, Nor. Terr., Australia, 89 Wulfenite, 60, 160 xanthosiderite, disallowed, 7 x-ray diffraction, 7 Zappfe, Carl, 185 Zies, E. G., 3, 62