041_sillitoe.pdf

©2005 Society of Economic Geologists, Inc. Economic Geology 100th Anniversary Volume pp. 723–768

Supergene Oxidized and Enriched Porphyry Copper and Related Deposits RICHARD H. SILLITOE† 27 West Hill Park, Highgate Village, London N6 6ND, England

Abstract Supergene leaching, oxidation, and chalcocite enrichment in porphyry and related Cu deposits take place in the weathering environment to depths of several hundred meters. The fundamental chemical principles of supergene processes were elucidated during the early decades of the twentieth century, mainly from studies in the western United States. The products of oxidation and enrichment continue to have a major economic impact on Cu mining in the central Andes and southwestern North America, currently accounting for >50 percent of world-mined Cu, and have sustained these two premier Cu provinces for the past 100 years. Enriched grades may attain 1.5 to >2 percent Cu, commonly two or three times the hypogene tenor. Deep oxidation also transforms low-grade refractory Au mineralization into bulk-mineable ore. The mechanisms of oxidative weathering are well understood because of studies in support of heap leaching of supergene Cu ores and amelioration of acid mine drainage. Sulfide oxidation takes place above the water table as an electrochemical process mediated by acidophilic, Fe- and S-oxidizing bacteria. Where acidic conditions prevail, Cu is efficiently leached and transferred downward to the reduced environment, beneath the water table, where sulfide enrichment takes place. Enrichment appears to be mainly the product of abiotic cation-exchange reactions involving substitution of Cu for more electronegative metals. Much of the required S is inherited from the replaced sulfide minerals, although sulfate-reducing bacteria may generate a minor proportion. Bacteria may also contribute to the enrichment process by facilitating metal adsorption. Where hydraulic conditions permit lateral flow of Cu-charged solutions from porphyry Cu deposits into contiguous drainage channels, exotic oxide Cu deposits form at the base of, or within, coevally accumulating piedmont gravel sequences. Several local and regional controls optimize supergene profile development. Orebodies should be vertically extensive and contain a well-developed array of steep faults and fractures, elevated pyrite/Cu-bearing sulfide ratios to maximize acidity of the supergene solutions, and nonreactive advanced argillic and sericitic alteration assemblages to minimize neutralization of the acidity. Porphyry Cu deposits pass through a natural supergene cycle in which leaching and mature enrichment in advanced argillic and sericitic zones give way during eventual exposure of deeper potassic zones to in situ oxidation without attendant enrichment. Mature oxidation and enrichment are promoted by the following: uninterrupted supergene activity for at least 0.5 m.y. but typically minima of 3 to 9 m.y.; tectonically or isostatically induced surface uplift responsible for depression of water tables and exposure of sulfides to oxidative weathering; and hot, semiarid to pluvial climates so long as erosion rates remain in balance with rather than outpacing supergene processes. Subplanar erosion surfaces, such as pediplains, are not considered to be a requirement for efficient supergene activity. Leached cappings are traditionally subdivided on the basis of their dominant limonite component into hematitic above mature enrichment, goethitic above hypogene ore or protore where Cu leaching is limited, and jarositic above pyrite-rich mineralization. Major oxidized Cu orebodies are developed either in situ where pyrite contents and leaching are minimal or as exotic accumulations located lateral to enriched zones. Oxidized ore comprises Cu minerals and mineraloids of both green and black color, with the latter, such as Cu wad, Cu pitch, and neotocite, being poorly characterized and tending to typify low-grade rock volumes. Among the green Cu species, chrysocolla dominates most high-grade ores of both in situ and exotic origin, hydrated sulfates and hydroxysulfates typify oxidation of pyrite-bearing enriched zones, and prevalence of hydroxychlorides in northern Chile testifies to exceptionally arid supergene conditions. Enrichment, by factors of three or even more, generates chalcocite and other Cu-rich sulfides in proximity to the overlying water table but lower grade covellite mineralization at depth. Gold and Mo do not normally undergo significant enrichment. Oxidized and enriched zones undergo pervasive supergene argillic alteration, with kaolinite, accompanied under arid to semiarid conditions by alunite, dominating leached and enriched deposits; smectite is more common in zones of in situ sulfide oxidation. Dissolution of hypogene anhydrite typifies the supergene profiles of porphyry Cu deposits and extends deeper than all significant enrichment. Geologic context and mesoscopic textural criteria facilitate distinction between hypogene and supergene Cu sulfides, limonite, clay, and alunite. Supergene oxidation and enrichment are active throughout much of the world, although supergene profiles are commonly immature because historical denudation rates are high. In parts of the North and South American Cordillera and elsewhere, however, fossil supergene profiles exist as a result of lower erosion rates combined with intermittent concealment beneath volcanic or sedimentary sequences or, as in northern Chile, of climatic desiccation commencing at ~14 Ma. Oxidation and enrichment of Paleozoic and Mesozoic age are preserved locally, although the major enriched zones of the central Andes and southwestern North America date back to only ~40 Ma. Copper enrichment in the central Andes coincided with several major periods of contractional deformation, crustal shortening, surface uplift, and erosional exhumation, which gave rise to descent of paleowater tables and thick piedmont gravel accumulations. The uplift that stimulated supergene activity in † E-mail,

[email protected]

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RICHARD H. SILLITOE

southwestern North America, however, was largely an accompaniment to extreme crustal extension. Variability in supergene profiles is fundamentally attributed to differential uplift of fault blocks. Supergene transformations of the upper parts of many Cu and related Au deposits are beneficial because the ores are either upgraded or can be more easily and cheaply processed, commonly using heap-leaching technology. Nevertheless, heap-leaching efficiency is highly dependent on the oxide and sulfide mineralogy. An exception to this general rule is provided by porphyry Cu-Au deposits in which supergene oxidation may introduce metallurgical complexity. Moreover, some supergene products, in particular clays, may negatively affect processing. Exploration of exposed porphyry Cu systems remains highly dependent on leached-capping appraisal for interpretation of the nature and grade of subjacent sulfide mineralization. In contrast, the search for concealed supergene profiles, which may be stunted where piedmont gravel sequences are excessively thick, relies heavily on geologic and geochemical vectoring and geophysical techniques. Exotic oxide Cu accumulations constitute targets in their own right besides acting as guides to potentially undiscovered porphyry Cu sources, in which the corresponding enriched zones may or may not be preserved.

Introduction SULFIDE OXIDATION and leaching of ore deposits in the weathering environment, and any attendant secondary sulfide enrichment, are supergene processes (Ransome, 1912), implying that they are products of descending aqueous solutions of surface derivation. Metal cations and sulfate anions are released through oxidative weathering of sulfide minerals in the aerated or vadose zone above the water table. These cations migrate in response to predominantly downward solution flow and may be deposited on reaction with a variety of anions. Copper, in particular, undergoes this redistribution and, under reduced conditions, is precipitated in the saturated zone beneath the water table by hypogene sulfides to form replacive sulfides richer in Cu. The surest sign of this supergene activity is a zone of Cu sulfide mineralization overlain by an oxidized and/or leached zone and underlain by lower grade hypogene Cu ore or protore, the whole constituting a diagnostic supergene profile. Websky (1852), describing veins in Silesia (present-day Poland), appears to have been the first to appreciate the existence of supergene Cu enrichment, followed by Whitney (1855), Hunt (1875), and Olcott (1875), all of whom documented Cu enrichment beneath zones of leaching in volcanogenic massive sulfide deposits of the southern Appalachians, eastern United States. Current understanding of supergene oxidation and enrichment of Cu deposits is the result of more than 100 years of observation, analysis, experimentation, and modeling, much of which was carried out in the western United States during the first 30 years of the twentieth century. Penrose (1894) was the first to formally address the results of supergene oxidation, while Emmons (1900), Van Hise (1900), and Weed (1900) set out the basic principles of enrichment. Ransome (1910) and Tolman (1913) presented criteria for the recognition of supergene enrichment in sulfide deposits, and experimental studies by Zeis et al. (1916) further clarified the chemical reactions involved. Landmark treatises by Emmons (1913, 1917) and Schneiderhöhn (1924) documented oxidation and enrichment mechanisms with many examples. Detailed descriptions of supergene profiles and their mineralogy were prepared as parts of classic geologic studies by the U.S. Geological Survey of several porphyry Cu districts in the western United States that were economically important at the time, most notably Bisbee, Arizona (Ransome, 1904), Bingham, Utah (Boutwell, 1905), Morenci, Arizona (Lindgren, 1905), Ely, Nevada (Spencer, 1917), Ray-Miami, Arizona (Ransome, 1919), and Tyrone, New Mexico (Paige, 1922). 0361-0128/98/000/000-00 $6.00

Many of the major early advances in the understanding of oxidation and enrichment processes were the direct result of multidisciplinary research—the Secondary Enrichment Investigation (Graton, 1913). The program was extended to study the textures of leached outcrops (leached cappings) as guides to the nature of underlying sulfide mineralization (Morse and Locke, 1924; White, 1924; Locke, 1926; Blanchard and Boswell, 1928). This long-term project, involving United States-based Cu companies, Harvard and other universities, the U.S. Geological Survey, the Geophysical Laboratory of the Carnegie Institution in Washington, D.C., and consultants, was the first and arguably most successful collaborative academia-industry research ever undertaken in the field of economic geology. In the 1960s, Bear Creek Mining Company, Kennecott Copper Corporation’s exploration arm, embarked on further major investigations of porphyry Cu leached cappings, which led to improved prediction of subjacent sulfide species and Cu tenors (Anderson, 1982). During the last half-century, a number of other notable contributions to the understanding of supergene oxidation and enrichment deserve brief mention. Smirnov (1954) adopted the first rigorous geochemical treatment of supergene alteration, which was followed by Garrels’ (1954) subdivision of oxidized and enriched zones in terms of oxidation potential (Eh) and pH represented on Pourbaix diagrams. In the 1950s, it was widely realized that sulfide oxidation was not simply a chemical process but also involved naturally occurring microorganisms in the form of Fe- and S-oxidizing bacteria (e.g., Bryner et al., 1954). The oxidative role of bacteria is now well documented as a result of extensive research in support of commercial heap and dump leaching of porphyry Cu and other sulfide ores and amelioration of acid mine drainage (e.g., Rossi, 1990). Microorganisms were first isolated from an active supergene profile at Morenci, Arizona (Enders et al., 1998). Brimhall et al. (1985) analyzed supergene ore-forming processes using mass-balance principles, and Ague and Brimhall (1989), Lichtner and Biino (1992), and Lichtner (1994) employed numerical simulation to model supergene enrichment. Atwood (1916) was the first to relate supergene processes to physiographic evolution with studies in the Butte, Montana, and Bingham, Utah, porphyry Cu districts. Nearly half a century later, Richard and Courtright (1958) and Segerstrom (1963) pursued a similar approach by linking supergene oxidation and enrichment in southern Peru and northern Chile, respectively, to regionally extensive, geomorphically mature land surfaces. Hollingworth (1964) raised the possibility of

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

correlating supergene profiles in the central Andes with specific erosion surfaces, a concept that led directly to interrelated geomorphic and supergene chronologies, first in northern Chile (Clark et al., 1967a, b; Sillitoe et al., 1968; Mortimer, 1973) and more recently in southern Peru (Clark et al., 1990; Quang et al., 2005). Early proposals that some supergene profiles in southwestern North America (e.g., Lindgren, 1905; Atwood, 1916; Ransome, 1919; Paige, 1922; Gilluly, 1946; Schwartz, 1949) and the central Andes (e.g., March, 1935; Taylor, 1935; Bandy, 1938; Jarrell, 1944) are of mid-Tertiary age and, in effect, fossilized were confirmed first by radiometric dating of overlying volcanic rocks (Clark et al., 1967b; Livingston et al., 1968; Sillitoe et al., 1968; Clark et al., 1990) and then by direct dating of supergene alunite and other K-bearing minerals from the supergene profiles themselves (Alpers and Brimhall, 1988; Cook, 1994; Sillitoe and McKee, 1996). Kilometer-scale lateral transport of Cu from porphyry Cu sources to form exotic oxide deposits was first reported from the Ray porphyry Cu deposit, Arizona (Tenney, 1935) but much later in northern Chile (Newberg, 1967). Incremental increases in the understanding of supergene oxidation and enrichment, in particular the mineralogy and chemistry of the processes involved, were charted over the last century in a series of reviews by Emmons (1918, chap. 15; 1933), Lindgren (1933, chap. 32), McKinstry (1948, chaps. 9–10), Bateman (1950, chap. 5.8), Anderson (1955), Schwartz (1966), Ney et al. (1976), Brimhall and Crerar (1987), and Titley and Marozas (1995). Economic significance From the 1850s through 1870s, Chile first became the world’s leading Cu producer due to exploitation of major intrusion-related veins, many of which contained high-grade oxidized and enriched ores. These veins were handsorted to produce direct-smelting ore assaying 5 to >20 percent Cu (Miller and Singewald, 1919; Little, 1926; Ruiz et al., 1965). Since the advent of bulk mining of porphyry Cu deposits in 1904, the world’s premier porphyry Cu provinces throughout much of the last century—southwestern North America and northern Chile-southern Peru—have depended in large measure for their economic viability on the existence of well-developed supergene oxidized and/or enriched zones. Indeed, Parsons (1933) even considered supergene enrichment to be one of the defining characteristics of porphyry Cu deposits.

Several major supergene Cu concentrations were company makers (e.g., Phelps Dodge at Morenci, Arizona, and Anaconda at Chuquicamata, Chile). Although average mined Cu grades in both these premier porphyry Cu provinces have fallen dramatically during the last several decades and continue to do so, few deposits would be exploitable even today without the existence of these supergene ores, which probably account for >50 percent of current world mined Cu production. Average supergene grades of bulk-mined porphyry Cu ore during the early decades of the twentieth century were normally between 1.4 and 2.2 percent Cu (Emmons, 1918, chap. 15; Parsons, 1933; Table 1). Today, however, most such ores range in grade from <0.4 to 1.0 percent, with only a few relatively recently discovered deposits containing higher Cu contents (e.g., Escondida, Chile; mill grade of 1.49 % Cu in first quarter, 2003). Nevertheless, these grade decreases have been compensated to some degree by technologic innovations, particularly the processing of oxidized and enriched ores by means of heap leaching and solvent extraction-electrowinning (SX-EW), including bacterial pretreatment of some of the latter (e.g., Brierley, 2000). Leaching of dump or run-of-mine material with total Cu contents as low as 0.1 percent has also been common practice for the last decade or so, and in situ leaching is practiced locally (e.g., San Manuel, Arizona) Gold concentrations in or around porphyry Cu deposits do not generally undergo significant supergene Au enrichment, although they are subjected to variable degrees and depths of oxidative weathering. This sulfide oxidation is an important economic factor in many large-tonnage, low-grade (<1–2 g/t) Au deposits, especially those of high-sulfidation epithermal type. Most such deposits in the unoxidized state prove metallurgically refractory and, hence, uneconomic unless hypogene Au contents are especially high (e.g., El Indio, Chile; Siddeley and Araneda, 1986). Aim This review of oxidized and enriched Cu deposits is based mainly on porphyry Cu deposits in the central Andes because these are the deposit type and the metallogenic province that are currently of prime global economic importance (Fig. 1; Table 2). Reference is also made to the southwestern North American porphyry Cu province because of its huge historical significance and continued economic viability. Other Cu

TABLE 1. Resources of Principal Oxidized and Enriched Porphyry Cu Deposits in Production during the Early 20th Century1 Deposit Bingham Morenci Ely (Robinson) Ray Chino (Santa Rita) Inspiration Potrerillos Chuquicamata El Teniente 1 Taken

Year of estimate

Tonnage (million short tons)

Grade (% Cu)

1917 1930 1929 1929 1929 1926 1924 1922 1929

338 379.4 70.0 85.1 125 96 137.4 684.3 234.8

1.50 1.02 1.48 1.65 1.40 1.40 1.51 2.12 2.18

from Parsons (1933), except figures for Bingham, which are taken from Emmons (1918)

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Supergene zone Weakly enriched Enriched (Clay orebody) Enriched Enriched Enriched Oxidized and enriched Oxidized and enriched Oxidized and enriched Enriched

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RICHARD H. SILLITOE

70°W

PERU Arequipa Cerro VerdeSanta Rosa

porphyry Cu deposits besides occurring as the principal metal in several deposit types that are genetically related to porphyry Cu deposits. No reference is made to Au enrichment during lateritization because the topic is dealt with elsewhere in this volume (Freyssinet et al., 2005). This practical, field-oriented review summarizes current understanding of the processes, local and regional controls, and mineral products of supergene oxidation and enrichment preparatory to addressing the physiographic, climatic, and tectonic controls and age and status (active vs. inactive) of supergene-profile development, with particular reference to the central Andes. A consideration of selected economic factors and some exploration guidelines concludes the review.

65°W

LA PAZ

Cuajone

Quellaveco Toquepala

BOLIVIA

20°

Cerro Colorado

Sagasca

Huinquintipa Rosario Ujina Radomiro Tomic El Abra Chuquicamata Mina Sur MM Quetena Toki El Tesoro Vicky Lomas Bayas Escondida Norte-Zaldívar Escondida

Quebrada Blanca

Spence Gaby Mantos Blancos ANTOFAGAS TA

Oxidation and Enrichment Processes

25°

Damiana

El Salvador Potrerillos

Mantoverde

Agua Rica Candelaria

Metallogenic Belt Middle-late Jurassic

CHILE

Bajo de la Alumbrera

30° Andacollo

ARGENTINA

Los Pelambres El Pachón

Early Cretaceous Paleocene-early Eocene Middle Eocene-early Oligocene Miocene-early Pliocene

Deposit Type Porphyry copper Exotic Iron oxide-coppergold Manto-type copper

Supergene Type El Soldado MENDOZA

Río Blanco Los Bronces

SANTIAGO

El Teniente 35°

0

km

Oxidized (in situ) Enriched Oxidized+enriched Exotic Minor supergene 500

FIG. 1. Distribution and types of supergene Cu deposits in the central Andes. Principal Cu deposits, deposit types, and metallogenic belts and epochs are shown (modified from Sillitoe, 1988, 1990).

deposit types—strata-bound sediment-hosted, skarn, carbonatereplacement, Fe oxide-Cu-Au, Chilean manto-type, volcanogenic massive sulfide, and high-sulfidation enargite bearing— are also mentioned en passant. Oxidation and enrichment of Au deposits comprise a subsidiary review topic, which is covered because Au is a by- or coproduct in a number of the 0361-0128/98/000/000-00 $6.00

Sulfide oxidation Sulfide oxidation is an electrochemical process (e.g., Sato, 1992) catalyzed by bacteria (e.g., Singer and Stumm, 1970; McIntosh et al., 1997). The process is active in the vadose zone, to the top of the capillary fringe (Fig. 2), but may also penetrate beneath the water table in zones of enhanced ground-water flow (e.g., Locke, 1926). Sulfide oxidation depends on the joint availability in fractures and pore spaces of atmospheric oxygen and water (e.g., Spencer, 1917; Lovering, 1948), the latter attracted to sulfide mineral surfaces by capillary forces. Mass flux during bioassisted leaching and other weathering reactions in the vadose zone results in friable, fragmental textures, abundant solution cavities and, hence, enhanced porosities and permeabilities that facilitate infiltration and passage of air and water. Magnesium, Ca, and Na are particularly mobile elements during oxidative weathering and tend to exit supergene systems (e.g., Williams, 1990). Much of the vadose water flow is channeled by faults and fracture networks, although the nearly complete destruction of even finely dispersed sulfide grains in most oxidized rocks implies that air and water access minute cavities and pores, including recently documented nanometer-scale openings (nanopores; Wang et al., 2003). Nevertheless, diffusion through massive gangue minerals and local clogging of fractures, cavities, and pores by precipitation of limonite, gypsum, and clays slow the leaching process. As stated by Weed (1900), Gottschalk and Buehler (1912), and many subsequent investigators, pyrite oxidizes abiotically + to aqueous Fe(II), SO2– 4 , and H ions if dissolved O2 is the oxidant, whereas chalcopyrite yields Fe(II), Cu, and SO4 ions, but no H+ (Table 3). The Fe(II), in turn, oxidizes to Fe(III), which, with a rise in pH (>2.8 at 35ºC; Hackl, 1997), hydrolyzes to precipitate Fe3+ oxide, hydroxide, and sulfate-bearing solids, all components of limonite. The Fe(III) itself is an extremely aggressive oxidizing agent for Fe, Cu-Fe, and Cu sulfides under acidic conditions and is far more effective in this role than O2 alone (e.g., Dutrizac and Macdonald, 1974; Table 3). Copper ions remain in solution at pH <5.5 (Bloom, 1966), a condition enabling descent of most of the Cu to the water table to generate enrichment (path 1, Fig. 2). Acidic surges, producing especially effective downward Cu transport, result from dissolution of ephemeral accumulations of soluble secondary sulfate minerals in the oxidized zone during seasonal rainstorms or snowmelt (e.g., Seal and Hammarstrom,

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS 3+

CHALCANTHITE

Fe

ANTLERITE

ria

BROCHANTITE

Cu2+ Atm

os

H2 SO4

ia

MALACHITE

2+

Fe

Ba ct er

Cu2+

te Bac

1.0

ph

2

eri c

0.5

pO

H2O

O2

S0

2

1

H+

10-1 TENORITE

FeS 2

CO

VE

LL

0.0

CH A

ITE

LC

OC IT E

CU

PR

ITE

10-40

BO

RN

ITE

Lo

,C

Cu0

HA

we

LC

O mi PYR I to f w TE, C ate H r s ALC O t

r li

-0.5

pO2 (atm)

Eh

10-55

ab

ility CITE

Vadose zone Capillary fringe Saturated zone -1.0 0

2

e e

CuFeS 2

4

pO2 contours Stability boundaries of Cu minerals 6

8

10

12

10-83

14

pH FIG. 2. Eh-pH diagram showing the stability fields of some common supergene Cu sulfide and oxide minerals in the system Cu-S-H2O (after Anderson, 1982). Arrow 1 approximates path followed by solutions descending from oxidized zone, generating enrichment, and exiting into hypogene ore or protore. Arrow 2 approximates path followed by solutions responsible for progressive oxidation of chalcocite-group minerals in the presence of pyrite. Vadose and saturated zones and intervening capillary fringe are approximated. Conditions assumed: 25ºC, p = 1 atm, CO3 = 10–3, and S = 10–1.

2003). At higher pH, however, the Cu is at least partly precipitated within the oxidized zone by one or more of several competing anions (path 2, Fig. 2) omnipresent in vadose water (hydroxyl, carbonate, chloride) or produced abundantly by sulfide oxidation (sulfate, arsenate) or acid attack of rock-forming and alteration minerals (silicate, phosphate). Therefore where pyrite contents, and hence acidities, are low, little transport of Cu occurs, Cu-bearing sulfides oxidize largely in place, and underlying Cu sulfide enrichment is minimal (e.g., Anderson, 1982; Williams, 1990; Chávez, 2000). Many sulfide minerals are electrically conductive so two of them together, pyrite and chalcopyrite for example, create a galvanic cell and constitute a corrosion system (Fig. 3). The more noble sulfide, in this case pyrite, acts as a cathode and the less noble one, chalcopyrite, is an anode and decomposes (e.g., Natarajan, 1990; Rossi, 1990). Sulfide minerals may be arranged as an electrochemical series based on their rest potentials, a list that provides an indication of their proclivity to oxidize (Gottschalk and Buehler, 1912; Table 4). Nevertheless, 0361-0128/98/000/000-00 $6.00

FIG. 3. Schema for the galvanic oxidation mechanism of chalcopyrite in contact with pyrite. As shown by arrows, electrons released from the chalcopyrite surface travel to the surface of the pyrite grain where oxygen is reduced and water formed; native S along with Fe(II) and Cu cations are liberated at the chalcopyrite surface; and bacteria mediate the oxidation of the S and Fe(II) to H2SO4 and Fe(III), respectively (from Hackl, 1997, and McIntosh et al., 1997).

the position of a sulfide mineral in the series may also be influenced by textural and impurity differences and by the nature of the oxidizing solution (e.g., Natarajan, 1990; Rossi, 1990). Consortia of Fe- and S-oxidizing bacteria, most of them acidophilic (growing between pH 0.5–3) and obligately aerobic (requiring O2), play a role in sulfide oxidation (e.g., Hackl, 1997; Mills, 1999; Nordstrom and Alpers, 1999), although Nordstrom and Southam (1997) hypothesized that only relatively low density populations exist in the natural state prior to mining. Nevertheless, both fossilized and living bacteria were recognized in leached capping at Morenci, Arizona (Enders et al., 1998; Enders, 2000). Acidithiobacillus ferrooxidans (Kelly and Wood, 2000), generally one of the most abundant, is a mesophile, implying that it flourishes between 20º and 40ºC, a temperature range typically attained and even exceeded during active exothermic oxidation of sulfide minerals (e.g., Melchiorre et al., 2000; Melchiorre and Enders, 2003). The microorganisms derive their metabolic energy by catalyzing the oxidation in solution of Fe(II) to Fe(III) and/or reduced S compounds to sulfate (Fig. 3) and as a cellular C source utilize atmospheric CO2 (e.g., Hackl, 1997; McIntosh et al., 1997; Nordstrom and Southam, 1997; Mills, 1999; Nordstrom and Alpers, 1999). The most important catalytic reaction, the oxidation of Fe(II), occurs in the cell envelope of the bacterium, and is estimated to result in >105-times rate increases over the purely abiotic chemical reaction (Singer and Stumm, 1970). Iron-rich precipitates accumulate at the outer surfaces of the bacterial cell envelopes. The resulting Fe(III) then acts as an abiotic oxidant of pyrite and Cu-bearing sulfides, as noted above. During the initial stages of supergene leaching and under near-neutral pH conditions extant during flushing caused by rainstorms, the bacteria are presumed to attach to mineral surfaces where they can

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Absent

Minor

Hematitic, goethiticjarositic 20-250 Minor

Goethitichematitic (structural control) Hematitic 5-90 Absent

El Abra (P)

Radomiro Tomic (P)

Chuquicamata (P), Mina Sur (E)

Toki (P)

Mantos Blancos (M)

Spence (P)

Quetena (P)

Ujina (P)

Goethitichematitic 20–250 Hematitic 20–150

Hematitic av. 100

Absent5

Hematitic 50–200

Hematitic and jarositic 5–60 Hematitic and jarositic 30–350

Hematitic Few–120

Hematitic 0–150

Leachedcapping type and thickness (m)

Rosario (P), Huinquintipa (E)

Quebrada Blanca (P)

Chile: Cerro Colorado (P) Sagasca (E)

Toquepala (P)

Quellaveco (P)

Peru: Cerro VerdeSanta Rosa (P) Cuajone (P)

Deposit name (E = exotic, I = Fe ox-Cu-Au, M = manto type, P = porphyry)

154 @ 1.04 (CuS)

79 @ 1.18

365 @ 0.46 (0.3)

237 @ 0.44 (0.3)

506 @ 1.56 (0.2)

850 @ 0.62 (0.2)

836 @ 0.5 (0.26)

39 @ 1.22 (0.3)

23 @ 1.26 (0.4)

4.5 @ 1.08

228 @ 1.0 (including enrichment) Unknown

Absent

Minor

85 @ 1.1 (409 @ 0.63) (0.1 CuS) 24 @ 1.3

Oxidized zone tonnage1 (Mt) and grade (%) CuT2 (cutoff, % CuT)

Broch, atac, chrys up to 70 Atac, chrys (mal, ant, cup, Cu) 50–200

Chrys, cup, mal, pitch, wad 5–10 Chrys, pseudomal, clay, ten (lib, broch, wad, tur) 90–300 Atac, chrys, clay, wad (mal, broch, pseudomal, samp, lib, az, cup) 150–200 Ant, broch, atac, chrys, pitch, chenev, chalc, kröhn, natro 250 Mal, chrys (clay, conichalc, wad, cup, Cu) 50–170 Mal, chrys, atac, wad 100–160

Chrys, broch 0–100

Broch, chrys (cup, Cu)

Broch, paratac, chrys, mal, neot 0–200

Chrys, mal, chalc, broch, cup, Cu, ten 0–45 Chrys, mal, pitch, broch, cup, Cu

Broch, pitch 0–45

Oxidized zone minerals,3 and thickness (m)

Minor

230 @ 1.14

205 @ 0.45 (0.3)

30 @ 0.6

2,229 @ 1.41 (0.2)

180 @ 0.93 (0.2)

Minor

150 @ 1.71 (0.45)

166 @ 1.36 (0.5 CuS) + 211 @ 0.44 (0.1 CuS) 52 @ 1.33 (0.45)

228 @ 1.0 (including oxide) Unknown

465 @ 1.07

232 @ 0.92

75 @ 1.5

270 @ 0.8

Enriched zone tonnage1 (Mt) and grade (% CuT) (cutoff, % CuT)

50–200 (including mixtos) ~1 up to 150 2 Mainly on faults

15–30 1.4

50–750 3

20–150 1.7

10–20 (mixtos)

10–100 2.4

0–50 1.3

10–200 2.4

0–200 2

0–150 1.3

Few-100 1.9

0–80 1.7

0–150 1.6

Enriched zone thickness (m) and enrichment factor4

Absent

Minor

Minor

Fairly minor @ 0.5

409 @ 1.22 (0.2)

4 @ 0.96

Wad

Chrys, pitch, atac, wad (lib, pseudomal, broch) 8 Wad

Atac, chrys, wad <0.5

Absent

30–180 (G)

100–200 (G)

70–140 (G)

Absent (G: 120–200 at Mina Sur)

30–150 (G)

Absent

Absent6

Few (G)

0–150 (G)

0–400 (G + I)

0–80 (G) 0–100 (I)

Few (G)

0-60 (G), 0–300 (I)

0–150 (mainly I)

None

Thickness of gravel (G) or ignimbrite (I) cover (m)

20–100 (I) 0–20 (G)

Chrys, wad 6

Chrys, pitch, wad (atac) >5 Chrys, wad (Cu)

Chrys 3

Chrys

Exotic Cu minerals3 and lateral transport distance from source (km)

Minor

29 @ 1.07 (0.7)

44.5 @ 1.72 (0.5) Minor

Minor

Absent

Minor

Absent

Absent

Exotic Cu tonnage (Mt) and grade (% CuT)

TABLE 2. Selected Attributes of Principal Supergene Cu Deposits in the Central Andes

Chávez (1983), Ramírez (1996)

Tapia (2003)

Rivera et al. (2003b), Rivera and Pardo (2004)

Rivera et al. (2003a)

Taylor (1935), Jarrell (1944), Flores (1985), Ossandón et al. (2001)

Cuadra and Camus (1998), Cuadra and Rojas (2001), Lorca et al. (2003)

Ambrus (1977), Dean et al. (1996), Gerwe et al. (2003)

Bisso et al. (1998)

Bisso et al. (1998)

Fam (1979), N. Saric, unpub. data., 2002 Hunt et al. (1983)

Bouzari and Clark (2002)

Richard and Courtright (1958), Mattos and Valle (1999)

Manrique and Plazolles (1975), Satchwell (1983), Concha and Valle (1999) Estrada (1975), Kihien (1995)

Quang et al. (2003)

Reference(s)

728 RICHARD H. SILLITOE

0361-0128/98/000/000-00 $6.00

729

Hematitic 5–400

Hematitic 10–500

Hematitic, jarositic 300-500

Hematitic <15 (upper), <50 (lower)

Absent

Hematitic 30 Goethiticjarositic 0–110 Jarositic 30–300 Jarositic 30–300

Escondida NorteZaldívar (P)

Escondida (P)

El Salvador (P), Damiana (E), Turquesa (E)

PotrerillosSan Antonio (P)

Mantoverde (I)

Andacollo (P)

Relatively minor

Minor

Minor

Absent

212 @ 0.63

184 @ 0.91

41 @ 0.62

219 @ 0.63

330 @ 0.77

615 @ 0.31

Ant, broch, chalc (pitch, neot, chrys) Chrys, mal, broch, ant, cup (Cu, az, ten, oliv, chalcoph)

Broch, pitch, neot, wad, chrys, pseudomal, clay (az, cup, Cu, lib) 30–200 Broch, chrys, mal, atac 80–300

Chrys, atac, neot, pitch (mal, pseudomal, broch, cup, Cu) 60–250 Broch, ant, chalc, atac, chrys, mal, cup, neot, cred, pseudomal 300 Broch, ant, chrys, neot, ten, chalc, cup, Cu, turq, lib 30–200 Broch, ant (pseudomal, lib, chrys, turq, ten, cup, Cu, mal) 50–150 Broch, chrys

Oxidized zone minerals,3 and thickness (m)

170 @ 0.96 (0.4) 956 @ 1.68

560 @ 0.93

25 @ 0.8

Minor

126 @ 1.1

340 @ 1.5

1,670 @ 1.59 (0.7)

1,280 @ 1.24

Minor

Minor

Minor

Enriched zone tonnage1 (Mt) and grade (% CuT) (cutoff, % CuT)

100–400 1.4 100–500 2.4

40 1.8 100–200 1.5

10–60 2

Few–300 ~3

10–400 3

0-300 ~5

Few

Few

Enriched zone thickness (m) and enrichment factor4

Absent

Absent

Minor

Absent

Absent

Minor

389 @ 0.34, 52 @ 0.27

Minor

Absent

Minor

256 @ 0.87 (0.45) 180 @ 0.45

Exotic Cu tonnage (Mt) and grade (% CuT)

Wad, chrys (mal, az, cup, Cu, turq, lib) 8 Chrys

Chrys

Chrys, wad

Atac, paratac, chrys, wad Wad, pitch, chrys

Exotic Cu minerals3 and lateral transport distance from source (km)

Absent

Absent

0–10 (G)

0–10 (G)

Absent

0–25 (G)

Absent

Minor (G)

0–125 (G)

0–45 (G)

40-50 (G)

0–300 (G)

Thickness of gravel (G) or ignimbrite (I) cover (m)

Warnaars (1983), Warnaars et al. (1985) Lindgren and Bastin (1922), Howell and Molloy (1960), Camus (1975), Cuadra (1986)

Atkinson et al. (1996)

Llaumett et al. (1975)

Vila et al. (1996)

March (1935), Oyarzún and Cuadra (2003)

Gustafson and Hunt (1975), Rojas and Müller (1994)

Ojeda (1990), Padilla et al. (2001), Véliz and Camacho (2003)

Maturana and Saric (1991), Monroy (2000), Williams (2003)

Boric et al. (1990), V. Carrasco (pers. commun., 2004)

Camus (2001), Aguilar et al. (2003)

Mora et al. (2004)

Reference(s)

+ reserves = total Cu; if acid-soluble Cu, CuS added approximate order of abundance (minor minerals): ant = antlerite, atac = atacamite, az = azurite, broch = brochantite, chalc = chalcanthite, chalcoph = chalcophyllite, chenev = chenevixite, conichalc = conichalcite, chrys = chrysocolla, clay = Cu clays, Cu = native Cu, cup = cuprite, kröhn = kröhnkite, lib = libethenite, mal = malachite, natro = natrochalcite, neot = neotocite, oliv = olivenite, paratac = paratacamite, pitch = Cu pitch, pseudomal = pseudomalachite, samp = sampleite, ten = tenorite, turq = turquoise, wad = Cu wad 4 Degree of enrichment only with respect to underlying hypogene ore/protore; therefore, enrichment factor may be incorrect 5 Except as clasts in gravels 6 Exotic Cu mineralization at Ichuno and Cerro Turquesa not believed to be derived from El Abra

3 In

2 CuT

1 Production

El Teniente (P)

Los Bronces (P)

Los Pelambres (P)

Jarositic, goethitic 0-10

Lomas Bayas (P)

890 @ 0.4 (0.2)

5

Absent

Low grade

Absent5

El Tesoro (E)

Gaby (P), Vicky (E)

Oxidized zone tonnage1 (Mt) and grade (%) CuT2 (cutoff, % CuT)

Leachedcapping type and thickness (m)

Deposit name (E = exotic, I = Fe ox-Cu-Au, M = manto type, P = porphyry)

TABLE 2. (Cont.)

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

729

730

RICHARD H. SILLITOE TABLE 3. Some Common Sulfide Oxidation and Enrichment Reactions1

Mineral

Reaction

Abiotic (a)/biotic (b)

+ FeS2 + 3.5O2 + H2O = Fe2+ + 2SO2– 4 + 2H FeS2 + 3.75O2 + 0.5H2O = Fe3+ + H+ + 2SO2– 4 + FeS2 + 14Fe3+ + 8H2O = 15Fe2+ + 2SO2– 4 + 16H Fe2+ + 0.25O2 + H+ = Fe3+ + 0.5H2O

a, b a, b a b

Chalcopyrite

CuFeS2 + 4O2 = Cu2+ + Fe2+ + 2SO2– 4 + CuFeS2 + 16Fe3+ + 8H2O = Cu2+ + 17Fe2+ + 2SO2– 4 + 16H

a, b a

Chalcocite

Cu2S + 2.5O2 + 2H+ = 2Cu2+ + SO2– 4 + H2O Cu2S + 2Fe3+ = CuS + Cu2+ + 2Fe2+

a, b a

Covellite

CuS + 2O2 = Cu2+ + SO2– 4 + CuS + 8Fe3+ + 4H2O = Cu2+ + 8Fe2+ + SO2– 4 + 8H

a, b a

Enargite

2– + Cu3AsS4 + 8.75O2 + 2.5H2O = 3Cu2+ + HAsO2– 4 + 4SO4 + 4H 2– + Cu3AsS4 + 35Fe3+ + 20H2O = 3Cu2+ + 35Fe2+ + HAsO2– 4 + 4SO4 + 39H

a, b a

2+ + 5FeS2 + 14Cu2+ + 14SO2– + 17SO2– 4 + 12H2O = 7Cu2S + 5Fe 4 + 24H

a

2+ + SO2– CuFeS2 + Cu2+ + SO2– 4 = 2CuS + Fe 4 2– + 5CuS + 3Cu2+ + 3SO2– 4 + 4H2O = 4Cu2S + 4SO4 +8H

a

Oxidation Pyrite

Enrichment Pyrite Chalcopyrite

1 Taken

from Lindgren (1933), Nickel and Daniels (1985), Alpers and Brimhall (1989), Sikka et al. (1991), and Plumlee (1999)

directly catalyze the oxidation of pyrite (Mielke et al., 2003), chalcopyrite, covellite, and other sulfides, which notwithstanding the fact that As is toxic to many microorganisms, may include enargite and arsenopyrite (Ehrlich, 1995). Oxidation of the released arsenite to arsenate is similarly mediated by a variety of specialized bacteria, followed by rapid sorption of much of the resultant arsenate onto limonite (Nordstrom, 2003, and references therein). A third role for bacteria involves electron transfer from sulfides that are adopting the electrochemical role of cathodes (Ehrlich, 1995). Furthermore, Melchiorre and Enders (2003) proposed, based on δ13C isotope determinations, that CO2 derived by oxidation of dead bacteria near the paleowater table played a significant role in azurite generation at Morenci, Arizona. Copper sulfide enrichment Copper sulfide enrichment takes place in the saturated zone below the water table, where air is largely excluded and TABLE 4. Electrochemical Series Based on Rest Potentials1 High, cathodic, noble, reducing, electropositive

↓ Low, anodic, reactive, oxidizing, electronegative 1 Taken

from Sikka et al. (1991)

0361-0128/98/000/000-00 $6.00

Pyrite Marcasite Chalcopyrite Sphalerite Covellite Bornite Galena Argentite Stibnite Molybdenite Cobaltite Pyrrhotite Arsenopyrite

water movement far more sluggish than in the overlying vadose zone because rock permeabilities are lower (Figs. 2, 4). The commonly small size of disseminated grains of supergene Cu sulfide minerals again shows that water is capable of accessing exceedingly minute rock pores (Ransome, 1910). Precipitation of the Cu that descends in acidic solutions from above the water table is generally considered to take place by substitution for Fe and other relatively electronegative metals in hypogene sulfide minerals by means of cation-exchange reactions (e.g., Emmons, 1917). Both δ34S values for most supergene Cu sulfides (e.g., Field and Gustafson, 1976) and theoretical modeling (Ague and Brimhall, 1989) suggest direct inheritance of S and would seem to rule out major external S sources, for example by sulfate reduction. Schürmann (1888) arranged the common metals into an electromotive series (Hg-Ag-Cu-Bi-Cd-SbSn-Pb-Zn-Ni-Co-Fe-As-Tl-Mn) whereby the sulfide of any one of the metals (e.g., chalcocite) will be precipitated at the expense of any sulfides of metals lower in the series (e.g., sphalerite and pyrite). The Fe displaced from the hypogene sulfides is precipitated as one of the limonite compounds, as noted above. The other metals, such as Zn or Pb, are largely lost from the enriched zones, although rare examples of possible supergene zinc (sphalerite) enrichment are reported at Chuquicamata (Aracena et al., 1997; Ossandón et al., 2001) and elsewhere (e.g., Teal and Branham, 1997; Norby and Orobona, 2002; Bawden et al., 2003). The Cu content of the descending solutions depletes progressively below the water table as a result of precipitation to form Cu sulfides, thereby resulting in marked mineralogic changes and decreases in grade of the resulting supergene enrichment blankets from their tops downward. The pH of the descending solutions, still necessarily <5.5 for Cu transport, also increases progressively downward as acid-generating chalcocite enrichment

730

731

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS Lithocap remnant

10-Ma ignimbrite

Exotic chalcocite enrichment

Porphyry stock

Andesitic

Paleowater table

14-Ma paleo surface volcanic

rocks

Gravel-bedrock contact

Outer limit of pyrite halo 500 m

Hypogene Alteration Advanced argillic

Supergene Profile

Post-mineralization Units

Hematitic Leached Jarositic capping

Ignimbrite/airfall tuff

Sericitic Potassic

Oxidized zone

Propylitic

Enriched zone

Exotic copper mineralization in gravel and bedrock

Piedmont gravel 0

500 m

Anhydrite front

FIG. 4. Classic mature supergene profile developed during the Oligocene and early Miocene in northern Chile in systems where ratios of pyrite to Cu-bearing sulfides are sufficiently high to permit significant Cu mobilization. Note control of leached capping and enriched zone by roots of advanced argillic lithocap and underlying sericitic alteration, both unreactive alteration types. Oxidized ore develops at and beyond margins of porphyry stock in potassic and innermost propylitic alteration zones where solution pH is higher and Cu is fixed. The position of the anhydrite or sulfate front is well beneath the main supergene profile. Exotic Cu sulfide enrichment is transitional to exotic oxide Cu mineralization controlled by the water table centered near the contact between bedrock (andesitic in this case) and an overlying piedmont gravel sequence. The transition from exotic Cu sulfide to oxide mineralization is controlled by the outer limit of the pyrite halo. The exotic Cu mineralization tends to diverge from the bedrock-gravel unconformity at its distal extremity (modified from Sillitoe, 1995b).

(Table 3) becomes weaker and the acid-consuming capacity of wall rocks likely increases. Notwithstanding the cation-exhange mechanism generally favored for Cu sulfide enrichment, some theoretical models predict sulfide provision by sulfate reduction (Lichtner and Biino, 1992), a reaction that would have to be microbial rather than chemical at supergene temperatures (Trudinger et al., 1985). Moreover, microbial sulfate reduction has been documented in sulfidic tailings impoundments, where sulfate and organic C for bacterial growth are both supplied by acidophilic bacterial activity in the overlying oxic zone (Fortin et al., 1995). Using scanning electron microscopy, Sillitoe et al. (1996a) showed that Cu sulfides from several major porphyry Cu enriched zones in northern Chile reveal bacterioform, spherical to beanlike bodies, up to 0.2 µm in size, especially at replacement fronts with remanent grains of pyrite and chalcopyrite. The size of these mineralized objects is considered to be too small for them to have been microorganisms (Southam and Donald, 1999); however, the fossilization of bacterial components (Southam and Donald, 1999) or enzymatic breakdown of organic materials (Schieber and Arnott, 2003), i.e., less-competetive bacteria, may generate these bacterioform bodies rather than the pseudomorphed bacteria themselves. Enders (2000) documented viable and fossilized dissimilatory sulfate-reducing bacteria at the top of the Morenci chalcocite enriched zone and suggested that they 0361-0128/98/000/000-00 $6.00

played a subordinate role in the enrichment process. Sulfatereducing bacteria were also involved in the probable supergene Zn enrichment at the Mike Au deposit, Nevada (Bawden et al., 2003). Since Cu ions are known to bind to bacterial cell walls in large quantities (e.g., Beveridge and Murray, 1976; Mendelson, 1992), collection and concentration of Cu ions could be a function of bacteria in the enrichment process (Sillitoe et al., 1996a). Nevertheless, the precise mechanism by which metal substitution occurs during enrichment and the quantitative importance of bacterially generated versus preexisting sulfide S remains to be clarified. Supergene enrichment is one of the few geochemical processes in economic geology that can be effectively modeled using mass-balance principles. Brimhall et al. (1985) and Alpers and Brimhall (1989) formulated Cu assay, bulk-rock density, and volume mass-balance equations to describe and link source zones, transport pathways, and depositional sinks of Cu at a deposit scale. The main methodologic uncertainty is the former hypogene Cu distribution in the leached-capping source, the upper parts of which have usually already been eroded (Mote et al., 2001a). Determination of relict sulfide mineralogy allows approximation of former Cu grade in the remanent leached capping but only upward-grade projection can be applied to the eroded part. Nevetheless, these inherent assumptions were apparently reasonable in the modeling of the El Salvador porphyry Cu system, Chile, because

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RICHARD H. SILLITOE

Mote et al. (2001a) were able to define Cu-deficient parts of the enriched zone and predict the magnitude and direction of corresponding lateral Cu fluxes responsible for exotic Cu formation. Exotic Cu formation During active supergene-profile development, a portion of the acidic cupriferous solutions generated in the vadose zone may, upon reaching the water table, follow the hydraulic gradient rather than percolating steeply downward into the saturated zone below the water table. Commonly, but not always, such diverted solutions follow the contact between the top of bedrock and unconformably overlying sequences of variably consolidated piedmont gravels; such stratigraphic boundaries act as zones of enhanced hydraulic conductivity (Fig. 4). The gravels invariably contain clasts of leached capping, rarely containing supergene Cu minerals, derived from the porphyry Cu source (Newberg, 1967; Mora et al., 2004). Lateral movement of Cu-charged solutions attained observed distances of >8 km during supergene- profile development at some of the porphyry Cu deposits in northern Chile (e.g., Münchmeyer, 1996; Table 2). Solution percolation tends to be focused by paleochannels, hundreds of meters wide and several tens of meters deep, incised into bedrock surface (e.g., Mortimer et al., 1977; Fam, 1979) but may also overtop the channels to form sheetlike Cu concentrations (e.g., Damiana, Chile; Rojas and Müller, 1994). The solutions also drain downward into the underlying bedrock, chiefly along fractures where, exceptionally, they attain depths of 400 m (e.g., Vicky, Chile). Solution seepage and Cu precipitation are believed to overlap temporally with aggradation of the host gravel sequences (e.g., Fam, 1979) and to take place at, and up to a few meters beneath, the paleosurface. This proposal gains support from rarely observed clasts of chrysocolla-cemented gravel within exotic Cu deposits (Münchmeyer, 1996), a relationship requiring synmineralization erosion within some paleochannels. The laterally migrating solutions are still capable of Cu sulfide enrichment if suitable hypogene sulfides are available (e.g., Langton, 1973; Saegart et al., 1974), but once lateral flow carries them beyond the outer limits of the pyrite halo, oxide Cu minerals alone are precipitated once solution pH exceeds ~5.5. Progressive solution neutralization results from reaction with gravel clasts, which are typically highly reactive because of their largely unaltered nature. Silica gel and chloride anions act as the principal precipitants of the Cu, with the former liberated during acid attack and consequent argillization of rock-forming minerals, principally feldspars, in gravel clasts (Newberg, 1967). No evidence has yet been obtained to suggest bacterial involvement in exotic Cu formation but it may exist. Physical factors favoring efficient exotic Cu mineralization by maximizing chemical reaction time include stable hydrologic conditions, slow flow rates imposed by shallow (say, <3º) hydraulic gradients, and solution ponding caused by either marked flattening of paleochannels or presence of rock barriers, including active fault scarps (e.g., Sagasca, Chile; Roethe, 1975; Fam, 1979). Argillization of gravel and subjacent bedrock also tends to impede solution flow but may also occlude solution access. Storm- or snowmelt-induced surges of 0361-0128/98/000/000-00 $6.00

highly acidic, Cu-charged solutions may also account for especially effective Cu concentration events. Copper tenor in gravels is controlled by porosity and permeability, with silty interbeds being weakly mineralized or barren (Fam, 1979); however, since most of the Cu minerals occur as gravel cement, extremely coarse grained gravel tends to be low grade because clast/matrix ratios are high. Theoretically, exotic Cu deposits may grow quasicontinuously at both their proximal and distal extremities as a result of progressive upstream gravel aggradation and downstream loss of neutralization potential through argillization, respectively. Oxidation and Enrichment Controls Local controls Orebody geometry: Orebody geometry is a key supergene control because hypogene Cu mineralization must show continuity over a substantial vertical interval if cumulative enrichment is to be effective. For example, if 300 m of leached capping has to be developed to create a 100-m-thick enriched zone, the Cu deposit must have a minimum vertical extent of 400 m. Hence, porphyry deposits and vertically extensive breccia pipes and veins are ideally suited to development of mature supergene profiles (Fig. 4), whereas deposit types that tend to be vertically restricted, like undeformed volcanogenic massive sulfide deposits, skarns, and other stratabound ores, are less suitable. Such geometric forms may undergo complete oxidation where they lie above the water table, if not shielded by impermeable caprocks (e.g., Little, 1926) but, at best, are likely to contain only immature enriched zones. Structural features: Steep, throughgoing faults in association with well-developed veinlet stockworks provide much of the permeability required for effective solution descent and resulting supergene oxidation and enrichment. This fact is particularly evident at the base of leached cappings, where downward-projecting prongs of oxidized rock controlled by faults can give rise to great irregularity (Schwartz, 1966). Where structural permeability is particularly high, the water table tends to undergo depression resulting in deep oxidation and especially deep penetration of chalcocite enrichment, as observed at the Butte (McClave, 1973) and Chuquicamata porphyry Cu deposits in which continuous, closely spaced fault veins and faults facilitated descent of Cu-charged ground water and chalcocite enrichment, to a depth of at least 750 m at the latter locality (Alvarez et al., 1980; Flores, 1985). Nevertheless, even single throughgoing faults can channel oxidation or enrichment to depths of several hundred meters (e.g., Rosario, Chile; Bisso et al., 1998; Fig. 5). Partial sulfide oxidation to depths of ~750 m is also particularly commonplace throughout the Zambian Copperbelt (Mendelsohn, 1961). The concept of hydrologic sumps, whereby descending and laterally moving supergene solutions are funneled into fault-defined trap sites, has been advanced to explain some mature enriched zones. Grabens formed after hypogene mineralization are considered to exert this effect and control the highest grade enrichment at Escondida (Ojeda, 1990) and Morenci (Enders, 2000). Although faults commonly enhance overall rock permeability and corresponding hydraulic conductivity, some posthypogene

732

733

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

NE

SW

m asl

4500 FA

T UL

T OM BOT

CK K RO JAC

FA U

LT

0

250 m

Leached capping Oxidized/mixed zone Strong enriched Weak zone Pyritic Hypogene Principal fault 4000

FIG. 5. Cross section of the supergene profile in the Rosario porphyry Cu deposit, Collahuasi district, northern Chile, where oxidized and enriched ore are rather poorly developed. Note the stunted supergene profile in the northeast, probably caused by a shallow paleowater table in a structurally depressed block during the main supergene activity, and the 600-m penetration of weak Cu sulfide enrichment down Jack Rock fault. The oxidized/mixed category includes enriched ore that underwent partial oxidation besides completely oxidized material, and the pyritic enriched zone category is enrichment that affects pyrite mineralization rather than pyrite plus Cu-bearing sulfides (from Bisso et al., 1998).

mineralization structures lined with gouge may act as permeability barriers and lead to deeper oxidation and/or enrichment on single sides of the faults, typically the hanging walls. Such gouge-induced permeability reduction is confirmed by local presence of steep bands of sulfidic fault gouge perched above the base of oxidation (e.g., Escondida; Ojeda, 1990). Sulfide mineralogy: Emmons (1917), Locke (1926), Blanchard (1968), Anderson (1982), Titley and Marozas (1995), and Chávez (2000), among others, stressed the fundamental importance of pyrite (and, to a lesser degree, pyrrhotite) in the generation of supergene acidic solutions for the solubilization of Cu and Cu-Fe sulfides. Bioassisted pyrite oxidation generates aqueous Fe(III) (Fig. 3; see above), which when acting as the oxidant for other sulfide minerals leads to generation of more acid more quickly than if O2 were the sole oxidant (e.g., Nordstrom and Alpers, 1999). High pyrite/chalcopyrite or pyrite/chalcocite ratios, leading to excess acid production, tend to favor chalcocite enrichment because some loss of acid through wall-rock reaction does not greatly affect the efficiency of the process. Pyrite/chalcopyrite or pyrite/chalcocite ratios of roughly 4 to 5:1 are optimal, with higher ratios tending to suffer from a deficiency of available Cu. Hence, alteration types characterized by high pyrite/Cubearing sulfide ratios, namely sericitic and advanced argillic assemblages, are ideal protoliths for leached cappings over enriched zones. The hypogene Cu-bearing sulfide species may range from chalcopyrite to high-sulfidation assemblages containing one or more of bornite, chalcocite, digenite, covellite, and enargite (Table 3) but, whatever the mineralogy, there is a general tendency for the highest grade enrichment to occur above the highest grade hypogene zones of porphyry Cu deposits. Alteration mineralogy: The alteration mineralogy and zoning, in addition to pyrite content, of porphyry Cu and other deposit types influences the degree and extent of supergene changes because of the controls it exerts on acid-buffering capacity and permeability. 0361-0128/98/000/000-00 $6.00

Reactive alteration types, including calcic, sodic, potassic, and propylitic assemblages, are those that readily neutralize acids by consuming hydrogen ions in hydrolysis reactions. The specific acid-neutralizers are carbonate and, to lesser degrees, mafic minerals plus feldspars (e.g., Emmons, 1917; Anderson, 1982; Lichtner, 1994; Chávez, 2000; Jambor et al., 2002). All are inimical to supergene enrichment because the near-neutral pH conditions that attend oxidative weathering of alteration types rich in these minerals result in fixation of all or much of the available Cu before it reaches the water table. Therefore potassic alteration is host to most major oxidized porphyry Cu deposits, and skarn, Fe oxide-Cu-Au, and Chilean manto-type Cu deposits, all dominated by calcic, sodic, and/or potassic assemblages, also undergo oxidation largely in situ (Fig. 1; Table 2). In contrast, rocks pervasively altered to sericitic or advanced argillic assemblages have little, if any, acid-consuming capacity because minerals like quartz, sericite (fine-grained muscovite), pyrophyllite, alunite, and dickite are stable under acidic supergene conditions, having been generated in similarly acidic hypogene environments (e.g., Meyer and Hemley, 1967). Hence, mature chalcocite enrichment blankets and major exotic Cu accumulations are only generated in association with well-developed zones of feldspar-destructive alteration. Most enriched zones in southwestern North America occur in porphyry Cu deposits characterized by well-developed sericitic alteration (e.g., Titley and Marozas, 1995). While sericitic alteration containing pyrite and chalcopyrite is also an effective host for enrichment in the central Andes (e.g., Quebrada Blanca and Ujina; Hunt et al., 1983; Bisso et al., 1998), major enriched zones in Chile, like Cerro Colorado, Chuquicamata, Escondida Norte-Zaldívar, and Escondida, are localized in the downward telescoped root zones of lithocaps where high-sulfidation hypogene sulfide assemblages span the sericitic-advanced argillic transition (Sillitoe, 1995a; Figs. 4, 6). In addition to their low neutralization potentials and elevated pyrite contents, the advanced argillic portions of lithocaps tend to be more siliceous than sericitic alteration, m asl

W

E

3000

2600

2200

0

Leached capping Oxidized zone Strong enriched zone Weak Hypogene

500 m

Base of advanced argillic lithocap Base of sericitic alteration (chlorite-sericite and biotite alteration below)

FIG. 6. Cross section of the Escondida porphyry Cu deposit, illustrating the close relationships between Cu sulfide enrichment and advanced argillic and sericitic alteration zones. The advanced argillic and sericitic zones are considered as the roots of a formerly much thicker lithocap. Note that the high pyrite content and low-neutralization potential of the lithocap roots were highly favorable for Cu leaching to generate the mature enriched zone. Consequently, oxidized ore is minimal in the section (from sections prepared from drill holes and pit benches by Véliz and Camacho, 2003).

733

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RICHARD H. SILLITOE

Perelló, 2005), gives rise to a natural supergene cycle as porphyry Cu systems are progressively degraded during supergene activity. Oxidation of pyrite likely commences at or near the paleosurface and continues throughout the upper parts of lithocaps, but supergene enrichment is only initiated when hypogene Cu values are first subjected to oxidative weathering, commonly within the root zones of lithocaps. Nevertheless, the shallow parts of some lithocaps may also contain appreciable hypogene Cu and even early-stage enriched zones (e.g., Yanacocha; Harvey et al., 1999). Cumulative enrichment beneath hematitic leached cappings proceeds as erosion exposes the sericitic zone (Fig. 8a) but eventually slows and

a.

S

S

PS 1 PWT 1

S

S K

K

K K K

b. K

S

S

S

PS 2 PWT 2

K

K

K

K

K

K

K K

c. K

K

K K

Clay-rich alteration

K K

K

K

PS 3 PWT 3

500m

Base of sulfide oxidation

K

Immature enriched zone

K

500m

0

Perched sulfidic body

S

Silicification + quartz-alunite alteration

K

b.

S S

S

Silicification + vuggy quartz + hydrothermal breccia

a.

S

S S

Progressive degradation of paleosurface

thereby resisting erosion, remaining in place for longer, and, consequently, undergoing more thorough Cu leaching (e.g., Escondida). Rock and alteration types have variable porosities and permeabilities, with the low values typical of some argillized and silicified rocks leading to almost total inhibition of sulfide oxidation and attendant enrichment. Some of the most dramatic examples of the impermeability of clay-rich rocks occur in high-sulfidation epithermal Au-Ag deposits (Plumlee, 1999; Plumlee et al., 1999; Sillitoe, 1999). At Yanacocha, Peru, oxidation to depths of ~300 m in centrally located, brecciated, vuggy quartz and quartz-alunite alteration zones (Harvey et al., 1999) gives way abruptly to intermediate argillic halos containing totally unoxidized pyrite to within only 2 or 3 m of the surface (Fig. 7a). In contrast, deep leached zones underlie pyrite-rich, siliceous, but less-permeable lithocap remnants preserved atop steep ridges in the porphyry systems at Marte (Vila et al., 1991) and Río Hurtado (R. H. Sillitoe, pers. observation, 2002), northern Chile. Such inverted supergene profiles result from ingress of supergene solutions through the less-siliceous rocks beneath the ridge flanks (Fig. 7b). Large volumes of silicified rock devoid of oxidation are not commonplace in porphyry Cu deposits, although perched sulfide zones in leached cappings commonly coincide with noticeably more siliceous (e.g., Escondida) or clay-rich patches. The observed vertical alteration zoning of porphyry Cu deposits, from advanced argillic lithocaps through their sericitic roots to deeper potassic assemblages (e.g., Sillitoe, 2000), especially commonplace in the central Andes (Sillitoe and

Sericitic alteration zone Potassic alteration zone

Hematitic leached capping Oxidized zone Enriched zone

100 m 0

Intermediate argillic alteration

100 m

Base of sulfide oxidation

100 m Immature enriched zone

FIG. 7. Atypical supergene profiles caused by marked contrasts in rock permeability. a. Deep sulfide oxidation in the core of a high-sulfidation Au(Cu) deposit surrounded by a virtual absence of oxidation in contiguous clayrich halo (e.g., Yanacocha and Summitville). b. Perched sulfide body encapsulated in siliceous advanced argillic lithocap underlain by deep oxidation in highly fractured intermediate argillic or other alteration type as a result of lateral ingress of vadose water (e.g., Marte and Río Hurtado). 0361-0128/98/000/000-00 $6.00

FIG. 8. Schematic summary of the supergene cycle in a classical porphyry Cu deposit. a. Sericitic alteration is exposed at the paleosurface (PS 1) and undergoes cumulative chalcocite enrichment beneath the paleowater table (PWT 1), which is overlain by hematitic leached capping. b. Erosion progresses during ongoing supergene activity to the point (PS 2) where only the roots of the sericitic zone remain, and the enriched zone beneath the paleowater table (PWT 2) spans the boundary between the sericitic and underlying potassic zone. c. Further degradation of the paleosurface (PS 3) results in complete removal of the sericitic zone, the near cessation of enrichment as a result of the deficiency of pyrite in the potassic zone, and consequent in situ oxidation of the contained hypogene Cu mineralization to generate a goethite-bearing oxidized zone above the new paleowater table (PWT 3). Only traces of enrichment are present. The potassic alteration zone will continue to oxidize in situ during further descent of the paleosurface and paleowater table, so long as erosion and oxidation rates remain broadly balanced, until the low-grade roots of the system are eventually exposed.

734

735

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

finally halts as reactive potassic alteration deficient in pyrite is exposed to supergene effects (Fig. 8b). The enriched zones then oxidize in situ for lack of pyrite-generated acid, and during continued surface degradation the underlying hypogene mineralization is directly transformed to in situ oxidized zones (Fig. 8c). Therefore, enrichment versus in situ oxidation of porphyry Cu deposits is ultimately attributable to erosion level. Regional controls Time factor: In principle, and assuming that physical and chemical conditions are optimized, time is clearly an important factor in the development of mature supergene profiles. Old deposits may be expected to display more mature supergene profiles than young deposits (Emmons, 1917). Extrapolation of recent Cu leaching and enrichment rates in Chilean sulfidic tailings impoundments to natural systems suggests that enriched zones could form in <15,000 yr, given the unlikely assumption that permeabilities and, hence, hydraulic conductivities of tailings and supergene profiles are similar (Dold, 2003). Numerical simulations by Lichtner and Biino (1992) suggested that several tens of thousands of years are needed for enrichment beneath deep water tables. Oxidation of all pyrite in the preserved leached capping at San Manuel, Arizona, took 40,000 yr on the basis of the measured premine geothermal gradient (Lovering, 1948). Titley (1978) suggested that 225,000 yr were required to form the thin, immature enriched zone at the Plesyumi porphyry Cu prospect, Papua New Guinea, based on the measured ground-water Cu concentration and current high rainfall. Assuming a balance between erosion and enrichment, Graybeal (1982) used a hypothetical erosion rate to calculate that the enriched zone at Silver Bell, Arizona, formed in 2 m.y. The most realistic, albeit still minimum estimates of the actual time taken for deep oxidation and/or mature chalcocite enrichment are likely to be provided by oxidized and/or enriched deposits for which tight groupings of supergene alunite ages are available. Based on such data, deep oxidation in Nevada was completed in 4.1 to 8.8 m.y. and

mature enrichment in the central Andes and southwestern North America in 3.3 to 6.5 m.y. (Table 5). The absolute minimum time needed for deep oxidation and/or enrichment may be approximated from the age of young deposits that underwent major supergene modification. Perhaps the best example is provided by Ok Tedi, Papua New Guinea, which was leached and enriched since emplacement below sea level only 1.1 to 1.2 m.y. ago (Chivas et al., 1984; Page, 1975). During uplift to the premine elevation of ~2,000 m asl, the deposit was unroofed and underwent enrichment by a factor of approximately 1.5 to 2, implying that the supergene event cannot have lasted for more than a few hundred thousand years. Broadly similar, albeit perhaps somewhat longer intervals of enrichment at El Teniente, Chile, and Sungai Mak, Indonesia, along with deep oxidation at Boyongan, Philippines, are also indicated, given that these deposits concluded their hypogene histories ~4 to 2.5 m.y. ago (Perelló, 1994; Maksaev et al., 2004; Waters, 2004). Therefore, on balance, it may be concluded that well-developed supergene profiles may form in <1 m.y., perhaps as little as 0.5 m.y., but that most major leached cappings and mature enriched zones represent at least 3 to 9 m.y. of largely uninterrupted supergene activity. Tectonic events: In active orogenic belts, where most supergene Cu deposits occur, surface uplift is commonplace, typically in response to contractional tectonic events and consequent crustal shortening and thickening. Surface uplift, equal to tectonic uplift less the effects of erosion (England and Molnar, 1990), is commonly accompanied by descent of water tables, a process that is fundamental to promotion of supergene processes because it places chalcocite-enriched sulfides above the water table, enabling their oxidation and the generation of new enrichment beneath the deeper, revised water table (e.g., Emmons, 1917; Locke, 1926; Brimhall et al., 1985). Block faulting under extensional conditions also adjusts base levels, with the depressed water tables within isostatically uplifted footwall zones of large-displacement normal faults (King and Ellis, 1990) triggering oxidation and enrichment in the same manner. In contrast, however, water table rise within downthrown blocks is likely to cause flooding

TABLE 5. Estimated Minimum Durations of Deep Oxidation and Mature Enrichment Based on Clustered Supergene Alunite Ages Deposit

Supergene zone

Number of ages

Age range (Ma)

Duration (m.y.)

Gold Quarry

Au-bearing leached capping

9

30.0–25.9

4.1

Arehart et al. (1992)

Alligator Ridge

Au-bearing leached capping

8

12.4–3.6

8.8

Sillitoe and Bonham (1990), Heitt (1992), Arehart and O’Neil (1993)

Morenci

Leached capping and enriched zone

11

11.0–7.0

4.0

Cook (1994), Enders (2000)

Round Mountain

Au-bearing leached capping

6

16.1–9.5

6.6

Tingley and Berger (1985), Sander (1988)

Chuquicamata

Leached capping, oxidized and enriched zones

7

19.0–15.2

4.8

Sillitoe and McKee (1996)

Escondida

Leached capping and enriched zone

4

18.0–14.7

3.3

Alpers and Brimhall (1988)

El Salvador

Leached capping and enriched zone

9

19.4–12.9

6.5

Mote et al. (2001b)

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Reference(s)

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RICHARD H. SILLITOE

of the basal parts of oxidized zones and the consequent cessation of both oxidation and enrichment (e.g., Brimhall et al., 1985). Accumulation of sedimentary or volcanic sequences is also likely to result in water table ascent, producing the same inhibiting effect (Emmons, 1917). In marked contrast, passive continental margins and stable cratons tend to undergo only slow and relatively minor uplift and, hence, very low denudation rates, as documented, for example, from the West African craton (Gunnell, 2003). In such places, where topography is typically subdued and water tables shallow, descent of water tables is exceedingly slow. Consequently, only extremely limited volumes of sulfide-bearing rock are exposed to the effects of oxidative weathering giving rise to thin supergene profiles (maximum of a few tens of meters) and limited Cu sulfide enrichment. Under tropical and subtropical weathering conditions, such thin supergene profiles are essentially equivalent to laterite profiles (see Freyssinet et al., 2005). Climatic influence: Climate, by means of its control of precipitation and temperature, is also an important influence on supergene processes (e.g., Emmons, 1917). However, separation of the effects of climate and tectonics is difficult, in part because of the positive feedback between precipitation and uplift (e.g., northern vs. central Andes; Montgomery et al., 2001; Lamb and Davis, 2003). Hence, for example, it is not easy to decide if a drop in paleowater table is due to tectonic uplift, decreased recharge because of lower rainfall, or a combination of both. Water is an essential ingredient for supergene activity so all climatic regimes with the exception of hyperarid deserts and glacial terrains, including associated permafrost development, are able to sustain appreciable oxidative weathering and attendant enrichment. Even under ice-dominated conditions, any seasonal melting results in immediate short-term resumption of supergene processes. Hot climates are likely to be more conducive to efficient oxidative (and silicate) weathering than temperate or frigid conditions because rates of chemical reactions are accelerated and mesophilic bacteria will proliferate. Drying and freezing greatly reduce Acidithiobacilli populations, although they survive periodic flooding (Hackl, 1997; Nordstrom and Southam, 1997). Indeed, surficial microbial activity is largely absent in the hyperarid parts of northern Chile (Navarro-González et al., 2003). Furthermore, lower pH and higher dissolved Cu concentrations, both of which should favor supergene processes, are likely in dry rather than wet climates (Plumlee et al., 1999; Seal and Hammarstrom, 2003). Indeed, the greater abundance of large, high-grade exotic Cu deposits in the central Andes than elsewhere may reflect the especially arid conditions and, hence, limited dilution of supergene solutions at the time of formation; however, minor exotic Cu accumulation is possible even in the tropics (e.g., Khanong, Sepon district, Laos; R. H. Sillitoe, pers. observation, 1998; Loader, 1999). Many investigators have concluded that arid to semiarid climatic regimes are the most conducive to supergene oxidation and enrichment (e.g., Emmons, 1917; Blanchard, 1968; Alpers and Brimhall, 1988; Bouzari and Clark, 2002), probably because many of the best-observed supergene profiles are preserved under such present-day conditions (e.g., central 0361-0128/98/000/000-00 $6.00

Andes, southwestern North America). It is often argued that alternation of wet and dry seasons characteristic of arid to semiarid regions assists supergene processes because of the beneficial effects of intermittent flushing and removal of accumulated soluble oxidation products. Nevertheless, most climatic regimes, from arid through temperate to tropical, provide essentially similar wet to dry alternations. Therefore, with adequate provision of water, supergene profiles should develop effectively under all climatic extremes, but a tropical climate is likely to be the most favorable for rapid supergene upgrading, given the special circumstance of a relatively deep water table, because of the coincidence of copious, intermittent rainfall with ~30ºC temperatures. Certainly, pyrite decomposes more rapidly under humid conditions (Borek, 1994). Inhibition of supergene profile development in tropical regions is generally due to nonclimatic reasons, as noted below, but the existence of youthful, deeply developed supergene profiles in the tropics, at Ok Tedi and Boyongan (see above), is believed to confirm climatic favorability. Although water is essential for promotion of supergene processes, aridity favors the preservation of the supergene products. Alpers and Brimhall (1988), Sillitoe and McKee (1996), and Bouzari and Clark (2002) emphasized intensification of aridity as an effective means of preserving the midTertiary enriched zones in northern Chile. The region exemplifies the intimate linkage between tectonic uplift and intensifying aridity, with the Andean range creating a topographic barrier to westward ingress of moisture from the Amazon basin besides cooling the prevailing onshore winds from the Pacific Ocean (e.g., Abele, 1989). Erosion rate: Erosional efficiency depends chiefly on rainfall and slope steepness, an increase in one or the other usually leading to enhanced runoff and mechanical removal of sediment (e.g., Willgoose et al., 1991). From the standpoint of supergene processes, the average erosion rate must be in overall balance with the rate of water table descent so that most of the contained Cu can pass into solution and be transferred downward before any particular rock volume is removed by erosion (e.g., Emmons, 1917; Ransome, 1919; Locke, 1926; Bateman, 1950, chap. 5.8; Brimhall et al., 1985). Arid to semiarid regions appear to provide an approximate balance between surface degradation and supergene processes, thereby optimizing Cu sulfide enrichment. A pluvial climate typically results in high runoff and, as a consequence, fairly high erosion rates. In mountainous regions subjected to high rainfall, catastrophic storms account for most erosion (Kirchner et al., 2001), and rockfalls and landslides are principal contributors to denudation because of the oversteepening of mountainsides caused by pronounced valley incision. Erosion rates under such conditions may outpace supergene processes, resulting in rock being removed mechanically before its sulfide content can undergo significant oxidation. Prominent mountain masses isolated from drainage incision, such as Mount Fubilan at Ok Tedi (Bamford, 1972), appear to be a requirement for development of deep supergene profiles in the mountainous tropics, hence their scarcity. Irrespective of climatic conditions, however, hypogene sulfides typically crop out on deeply incised valley floors and lie at only relatively shallow depths beneath their lower slopes. Perhaps counterintuitively, it is even argued that

736

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

valley incision and relief production are maximized under arid rather than pluvial conditions (Molnar, 2001; Gabet et al., 2004). Continental and alpine glaciation is also an extremely effective erosive agent, as confirmed by deeply incised cirques and U-shaped valleys (Fig. 9e), associated moraines, and other glacial landforms. The highest erosion rates, perhaps exceeding 10 km/m.y., seem likely to occur during advance of large, temperate valley glaciers (Hallet et al., 1996), although even the Canadian Shield was lowered by an average of 120 m during Quaternary glaciation (Bell and Laine, 1985). Hence, it has long been appreciated that any pre-Quaternary supergene profiles are largely eroded at high latitudes (Emmons, 1917) as well as in high mountain ranges that were subjected to the effects of Quaternary glaciation (e.g., southern Andes, Alps, Himalayas). Geomorphologic conditions: Geomorphology combines tectonic, climatic, and erosional factors and, hence, has been largely covered by the preceding comments. Erosion surfaces deserve mention, however, because supergene profiles have been related to specific land surfaces, especially pediplains (e.g., Clark et al., 1967a, b; Sillitoe et al., 1968; Mortimer, 1973; Clark et al., 1990; Quang et al., 2005). Pediplains or coalescent pediments are subplanar erosional landforms generated especially under arid to semiarid climatic conditions, apparently where relative tectonic quiescence prevails (e.g., Dohrenwend, 1994). Bouzari and Clark (2002) suggested that the subplanar nature of pediplains was conducive to supergene oxidation and enrichment beneath them, although it might be argued that topographic prominences within or around pediplains could prove equally if not more favorable because of potentially deeper water table conditions. The importance of pediplains and other erosion surfaces is that they help to understand the physiographic evolution of a region, which, in turn, can be used to underpin supergene chronology. By the same token, however, supergene history can be tied to the stratigraphy of continental sedimentation in forearc, intramontane, and foreland basin settings; a response to enhanced erosion rates triggered by tectonic and/or climatic factors. However, only low-resolution chronostratigraphy based on dated volcanic intercalations is generally available for these subaerial sedimentary packages, as charted below for the central Andes and southwestern North America. Oxidation and Enrichment Products Leached cappings Leached cappings, typified by the presence of limonite and absence of appreciable amounts of oxidized Cu minerals, overlie zones of Cu sulfide enrichment (Figs. 4, 9a-e). Limonite comprises goethite, lepidocrocite, hematite, and jarosite, along with admixed quartz (Posnjak and Merwin, 1919; Blanchard, 1968; Table 6). The leached cappings in the central Andes are typically a few tens of meters to 200 m thick but attain 500 m at Escondida (Ojeda, 1990) and El Salvador (Gustafson and Hunt, 1975). Layers or lenses of oxidized Cu minerals may be isolated within the leached cappings or, more commonly, separate them from underlying enriched zones (Fig. 4; Table 2). Leached cappings are developed from rock containing <20 volume percent sulfides, whereas gossan 0361-0128/98/000/000-00 $6.00

737

is the term applied to oxidation products of more highly sulfidic material (Locke, 1926). Lithocaps—advanced argillic alteration zones developed in the upper parts of porphyry Cu systems—may constitute leached capping but should not be confused with it terminologically (Sillitoe, 1995a). Total Cu contents of leached cappings are typically only several hundred parts per million but may exceed 1,000 ppm locally (Chávez, 2000), some of it sorbed by limonite (Smith, 1999). Schwertmannite [Fe8O8(OH)6SO4· nH2O] and ferrihydrite (nominally 5Fe2O3·9H2O) are additional products of Fe3+ 2 (SO4)3 hydrolysis in acid mine drainage but appear to be scarce in leached cappings because of their metastability (Bigham, 1994; Bigham and Nordstrom, 2000). Blanchard (1968) classified limonite on the basis of boxwork morphology and position relative to precursor sulfide sites, with indigenous varieties signifying low ratios of pyrite to Cu-bearing sulfides and exotic or transported products forming where pyrite contents are high. Anderson (1982) adopted a more mineralogic approach to limonite classification and determined the proportion of goethite, hematite, and jarosite (Table 6), which is the most practical means of subdividing leached cappings (Fig. 9a-e). Lepidocrocite is uncommon in porphyry Cu leached cappings (Blanchard, 1968) but is reported locally (e.g., Butte; McClave, 1973). Goethitic cappings are characterized by only minor leaching and the presence of oxide Cu minerals and, in the central Andes, occur at El Abra (Fig. 9a), Radomiro Tomic, Quetena, and Gaby (Table 2). Hematitic leached cappings occur above mature enriched zones (pyrite/chalcocite = ~5:1; Figs. 4, 9bc, d; Table 2) and are clearly observable at Toquepala (Richard and Courtright, 1958), Cerro Colorado (Bouzari and Clark, 2002), Quebrada Blanca (Hunt et al., 1983), Ujina (Bisso et al., 1998), Chuquicamata (Taylor, 1935), Spence (Tapia, 2003), Escondida Norte-Zaldívar (Maturana and Saric, 1991), Escondida (Ojeda, 1990), and Andacollo (Llaumett et al., 1975). Jarositic leached cappings typically occur around the peripheries of goethitic and hematitic leached cappings, where they denote the pyrite halos of systems (Figs. 4, 9e), as well as occurring below some hematitic leached cappings, where they develop from pyritic protore beneath enriched zones during abrupt drops in paleowater table levels (e.g., Quellaveco; R.H. Sillitoe, pers. observation, 1996; Toquepala; Richard and Courtright, 1958; Anderson, 1982). Leached cappings above immature Cu sulfide enrichment, as observed at Los Pelambres (Atkinson et al., 1996; Fig. 9e), Los Bronces (Warnaars, 1983), and El Teniente (Lindgren and Bastin, 1922; Howell and Molloy, 1960), contain variable proportions of jarosite and goethite (Table 2). Oxidized Cu ore Types: Zones of oxide Cu minerals tend to be transitional upward as well as laterally to leached cappings of various types but, where formed in situ, are commonly in either abrupt or gradational contact with underlying enriched zones (Fig. 4). Although not under- or overlain by enriched zones, exotic oxide Cu deposits may give way to limonite-cemented gravel (ferricrete) at their proximal extremities (e.g., Huinquintipa, Tesoro, Damiana; Fig. 1) as well as lying alongside leached cappings (Fig. 4), from which they may have become physically separated by faulting and consequent erosion.

737

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RICHARD H. SILLITOE

a

b

c

d ig lc ez

e f

FIG. 9. Views of leached cappings and related supergene oxide Cu mineralization. a. The goethitic leached capping (lower half of photograph) over the El Abra porphyry Cu deposit, northern Chile, with surficial chrysocolla-bearing rocks (pale green) visible where the original surface has been disturbed by access roads and shallow mine workings. Buildings at lower center for scale. October 1970. b. The hematitic leached capping over the Escondida porphyry Cu deposit, northern Chile, is exposed on the two low reddish hills flanked by piedmont gravel-filled depressions, with the early open pit on the southern side of the nearer one. The contact between the leached capping (reddish brown) and enriched zone (grayish white) is visible (dashed line) near the base of the high wall in the pit. February 1992. c. The irregular contact between the hematitic leached capping (red) and underlying enriched zone (whitish) in the high wall of the Cananea porphyry Cu deposit, Sonora, Mexico, showing the marked structural control on the base of leaching. June 1975. d. Postmineral ignimbrite (brown, ig) underlain by hematitic leached capping (red, lc), and enriched zone (white due to supergene kaolinite, ez) in the high wall of the Ujina porphyry Cu deposit, Collahuasi district, northern Chile. Electric shovel at lower right for scale. June 1996. e. The jarositic leached capping (yellowish brown) developed over the pyrite-rich sericitic halo to the Los Pelambres porphyry Cu deposit, central Chile. The ore-bearing potassic zone underlies the U-shaped glacial valley, down which a Cu-charged stream flowed for much of the year. January 1970. f. Piedmont gravel cemented by chrysocolla (green) at the Huinquintipa exotic Cu deposit, Collahuasi district, northern Chile. Vehicle parked on mid-Miocene pediment surface. Hammer for scale. October 1971. 0361-0128/98/000/000-00 $6.00

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TABLE 6. Principal Supergene Minerals in Oxidized Porphyry Cu Deposits, Northern Chile Mineral(oid)/mineral mixture

Formula

Abundance

Characteristics

Limonite Hematite Goethite Jarosite

γ-Fe2O3 α-FeO(OH) (K,Na,H3O)FeIII3(SO4)2(OH)6

Abundant Abundant Ubiquitous

Red to dark crimson powdery aggregates and coatings Coffee-brown powdery aggregates and coatings Straw-yellow powdery to granular aggregates and coatings

Oxide copper species Chrysocolla1

Cu2H2Si2O5(OH)4

Ubiquitous

Malachite

Cu2CO3(OH)2

Abundant

Azurite Antlerite Brochantite Chalcanthite Kröhnkite Atacamite/paratacamite/clinoatacamite Pseudomalachite Libethenite Sampleite Turquoise Conichalcite Chenevixite Cuprite Tenorite Paramelaconite Pitch limonite

Cu3(CO3)2(OH)2 Cu3(SO)4(OH)4 Cu4(SO)4(OH)6 CuSO4.5H2O Na2Cu(SO4)2.2H2O Cu2Cl(OH)3 Cu5(PO4)2(OH)4 Cu2PO4(OH) (Na,Ca,Cu)5(PO4)4Cl.5H2O CuAl6(PO4)4(OH)8.4H2O CuCaAsO4OH Cu2Fe2(AsO4)2(OH)4.H2O Cu2O CuO Cu4O3 α-(Fe,Cu)O(OH)

Uncommon Common Abundant Uncommon Very uncommon Abundant Uncommon Very uncommon Very uncommon Uncommon Very uncommon Very uncommon Uncommon Uncommon Uncommon Common

Crednerite Copper wad2 Copper pitch (black chrysocolla)1, 2

CuMnO2 Cu-bearing Mn oxyhydroxides + other phases (Cu,Fe,Mn)2H2Si2O5(OH)4

Uncommon Abundant in exotic deposits Common

Pale-green to sky-blue, amorphous, botryoidal veinlets and masses Dark-green crystals, locally mamillary, radiating, and fibrous Deep-blue crystals and crusts Dark-green, typically in cross-fiber veinlets Dark-green, commonly stubby prismatic crystals Pale-blue crystals and cross-fiber veins, astringent Sky to vitreous blue cross-fiber veins and crusts Dark-green, commonly acicular crystals Emerald green, botryoidal, fibrous Olive green, striated crystals common Turquoise blue, flattish crystals Pale-blue, hard to chalky masses Grass-green botryoidal crusts with radiating structure Yellowish-green, earthy masses and crusts Deep-red, massive to crystalline aggregates Dull-black, massive bands Black, massive, metallic luster, conchoidal fracture Dark-brown, resinous, pale-brown streak, black conchoidal fracture showing ruby-red internal reflections Blackish earthy coatings, metallic luster Dull-black, earthy, dark-brown streak

Neotocite1, 2

(Cu,Mn,Fe)SiO3.H2O

Copper clays2

Smectite or kaolinite with sorbed Cu or intergrown chrysocolla

Abundant in in situ ore Abundant

Lustrous black, botryoidal veinlets and masses, commonly interbanded with chrysocolla Dull black, typically as rounded, raised patches, dark-brown waxy streak Yellowish-green clay (smectite); pale-blue clay (kaolinite)

1 Mineraloid 2 Mineral

mixture

Three types of oxidized Cu zones are recognizable: those underlain mainly by hypogene ore, those underlain by enriched zones, and those of exotic origin. In the central Andes, discontinuous mixed zones (mixtos), from a few to ~100 m thick and composed of Cu oxide and sulfide minerals, commonly intervene between the oxidized and hypogene or enriched zones (Fig. 10) and clearly represent arrested, disequilibrium conditions. In porphyry Cu deposits characterized by low ratios of pyrite to chalcopyrite ± bornite in former hypogene ore or protore and consequently lacking important enrichment, oxidized zones typically span the entire interval from the top of bedrock to the top of sulfide-bearing rock (Fig. 10) but are commonly transitional laterally to peripheral jarositic leached cappings developed at the expense of pyrite halos. Malachiterich oxidized ore of this type was first described from the Ajo porphyry Cu deposit, Arizona (Joralemon, 1914). The >800Mt oxide Cu orebodies at El Abra (Ambrus, 1977; Dean et al., 1996; Gerwe et al., 2003), Radomiro Tomic (Cuadra and Camus 1998; Cuadra and Rojas, 2001; Lorca et al., 2003), and 0361-0128/98/000/000-00 $6.00

Gaby (Camus, 2001; Aguilar et al., 2003), with maximum thicknesses of 200 to 300 m (Table 2), are the largest examples in northern Chile (Fig. 1). Closely similar supergene profiles characterize other typically pyrite-deficient Cu deposit types, such as Fe oxide-Cu-Au and manto-type Cu, as represented in northern Chile by Manto Verde (Vila et al., 1996) and Mantos Blancos (Chávez, 1983; Ramírez, 1996), respectively (Fig. 1; Table 2). In marked contrast, where ratios of pyrite to Cu-bearing sulfides were appreciably higher and enrichment moderately to well developed, as at Cerro Verde-Santa Rosa (Quang et al., 2003), Cerro Colorado (Bouzari and Clark, 2002), Ujina (Bisso et al., 1998), Chuquicamata (Flores, 1985; Ossandón et al., 2001), Toki (Rivera et al., 2003b; Rivera and Pardo, 2004), Spence (Tapia, 2003), Escondida Norte-Zaldívar (Maturana and Saric, 1991; Monroy, 2000; Williams, 2003), Escondida (Ojeda, 1990; Padilla et al., 2001; Véliz and Camacho, 2003), and Potrerillos-San Antonio (March, 1935; Oyarzún and Cuadra, 2003), oxidized ore tends to be more restricted in volume and extent and to occur as isolated lenses within and,

739

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RICHARD H. SILLITOE

SW

NE

a

m asl 4000

3800

P

P

P

P P 3400

C

C

3600

C

C

P

CB

C C

0

CB C CB

C

CB

CB C

C

CB C

C

C

CB

C

C

500 m

Oxidized zone Mixed oxide-chalcocitezone Strong enriched zone Weak enriched zone

CB CB

CB CB C CB CB C P

C C

CB P

CB C

P

P

P P

P

P

Hypogene chalcopyrite-bornite Hypogene chalcopyrite Hypogene pyrite Fault

FIG. 10. Cross section of the supergene profile at the El Abra porphyry Cu deposit, northern Chile. Note the well-developed, chrysocolla-dominated oxidized zone and only limited weak Cu sulfide enrichment consequent upon low ratios of pyrite to Cu-bearing hypogene sulfides and reactive intrusive host rocks. The northwest-striking faults and numerous smaller parallel fractures are marked by pyrite-rich sericitic alteration (D veins) and, where oxidized, by jarositic zones, neither of which can be depicted at this scale. From Dean et al. (1996).

particularly, as semicontinuous horizons at the base of dominantly hematitic leached cappings (Figs. 4, 6; Table 2). In a few deposits, however, persistent horizons or more restricted lenses of leached capping intervene between the basal oxide Cu mineralization and the underlying enrichment zone, as documented at Cerro Colorado (Bouzari and Clark, 2002), Chuquicamata (Taylor, 1935; Perry, 1952), and Potrerillos (March, 1935). Oxidized ore typically totals <100 Mt and does not exceed 200 m thick, although in giant deposits the volume and thickness may be greater, as at Chuquicamata (506 Mt; Ossandón et al., 2001; Table 2). In some deposits, notably Escondida Norte-Zaldívar and Escondida, peripherally located andesitic volcanic rocks altered to biotite and chloritesericite, and hence possessing relatively high neutralization potentials, localized much of the oxidized ore (Maturana and Saric, 1991; Padilla et al., 2001). In some enriched porphyry Cu deposits, however, Cu leaching was very efficient because of a deficiency of host-rock neutralization potential, leading to development of only relatively minor volumes of oxidized Cu ore, as reported at Toquepala (Richard and Courtright, 1958), Quebrada Blanca (Hunt et al., 1983), and El Salvador (Gustafson and Hunt, 1975; Fig. 1, Table 2). These three types of oxidized Cu ore possess distinctive mineralogic features (Table 2). In oxidized ore formed under near-neutral to alkaline conditions, and lacking appreciable attendant enrichment, chrysocolla, atacamite, and malachite, any one of which may predominate, are accompanied by Cubearing limonite, neotocite, and Cu clays, typically minor amounts of tenorite, paramelaconite, crednerite, pitch limonite, Cu wad, and Cu pitch, and in places one or more subordinate Cu-bearing phosphate and arsenate minerals (Tables 2, 6). Pitch limonite is an in situ replacement product (Anderson, 1982), whereas the Cu in the other minerals is variably transported. The black oxide Cu minerals (Table 6) typically occur peripheral to the green oxide minerals in zones with lower Cu contents. The Cu hydroxysulfates— antlerite and brochantite (Fig. 11a, b; Table 6)—are notably 0361-0128/98/000/000-00 $6.00

b

c

FIG. 11. Supergene oxide minerals in porphyry Cu deposits, northern Chile. a. Brochantite (green) replacing massive supergene chalcocite (gray), Quebrada M pit, El Salvador. Fingers for scale. b. Antlerite veinlets (green) cutting kaolinized porphyry (white) partially stained by hematite (red), Chuquicamata pit. Hand lens for scale. c. Neotocite (black) coating kaolinized porphyry (white), El Abra pit. Hammer for scale.

absent or greatly subordinate because precursor pyrite and, hence, derivative sulfate contents are low. In oxidized zones generated under more acidic conditions and, hence, underlain by enrichment, the Cu hydroxysulfates (Fig. 12b) are important and commonly the principal mineral

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

a.

0

AZURITE

Log Pco

2

-1

-2

MALACHITE -3

PARATACAMITE -4

TENORITE -5 -26

-22

-18

-14

-10

Log aH+ aCl -

b.

0

AZURITE

CHALCANTHITE

-2

ANTLERITE

Log Pco2

-1

MALACHITE -3

-4

BROCHANTITE TENORITE -20

-16

-12

-8

Log aH2+ aSO= 4

FIG. 12. Stability fields for hydroxysalts of Cu under supergene conditions (25ºC and p = 1 atm). a. Hydroxycarbonates and hydroxychloride. b. Hydroxycarbonates and hydroxysulfates. Note that most surface waters, other than those from arid regions, would plot in the malachite field (from Woods and Garrels, 1986).

components, although chrysocolla, atacamite, malachite, Cu pitch, neotocite (Fig. 11c), and other minor species are generally also present (Table 2). Under still lower pH conditions in northern and central Chile, the hydroxysulfates are accompanied by water-soluble hydrated Cu sulfates, such as chalcanthite and kröhnkite (Fig. 12b; Table 6). The presence of abundant Cu hydroxysulfates may be taken as a good indicator that pyrite-bearing Cu sulfide enrichment has been oxidized, at least partly in situ (Chávez, 2000). Exotic Cu ore (Figs. 4, 9f) typically contains fewer oxidized mineral species than its in situ counterparts. Chrysocolla, atacamite, copper wad, and copper pitch, accompanied by gypsum gangue (e.g., Fam, 1979), are the main minerals present, among which any one of the first three may predominate. Chrysocolla is commonly coated by opal—the 0361-0128/98/000/000-00 $6.00

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final precipitation product from copper-depleted solutions. At El Tesoro and possibly elsewhere, however, atacamite is reportedly accompanied by appreciable amounts of the polymorph paratacamite (Mora et al., 2004) or, possibly, clinoatacamite (see Jambor et al., 1996), and at Mina Sur and Damiana minor amounts of Cu-bearing phosphate minerals are reported (Mortimer et al., 1977; Mote et al., 2001b; Pincheira et al., 2003). Black oxide Cu minerals occur in proximal, distal, and/or basal parts of exotic Cu deposits but, as in the case of in situ oxidized ore, they appear to be the dominant Cubearing species in either low-grade deposits (e.g., Damiana; Rojas and Müller, 1994) or low-grade parts of richer deposits rather than displaying any systematic lateral and vertical zoning pattern (e.g., Münchmeyer, 1996). Mineralogic relationships: Irrespective of whether the oxidized ore is developed essentially in situ or exotic, the minerals noted above and in Tables 2 and 6 mainly coat or fill fractures and cavities as well as impregnate and partly replace rock-forming and supergene alteration minerals. Chrysocolla and Cu pitch display colloform texture, whereas the Cu hydroxysulfates, hydroxychlorides, and hydroxycarbonates typically occur as crystalline aggregates (Fig. 11a, b; Table 6). Even oxidized ore formed essentially in situ is made up mainly of transported minerals, although transport distances for the contained Cu typically range from only centimeters to meters, with maxima of perhaps a few tens of meters, in marked contrast to the kilometer-scale migration of the Cu in exotic deposits. Locally, however, Cu-Fe sulfides are directly replaced by pitch limonite, and supergene Cu sulfides by cuprite or brochantite (Fig. 11a). Paragenetic relationships are varied and complex due to the constantly evolving chemical conditions during downward progression of the oxidative weathering process. Moreover, metastable, disequilibrium assemblages are commonplace, resulting from slow reaction rates under low-temperature conditions (e.g., Sato, 1992). Notwithstanding the inherent complexity, chrysocolla tends to be the last Cu-bearing mineral to form in many in situ oxidized zones (e.g., Schwartz, 1934), where it is particularly abundant in the first few meters to tens of meters below surface. Some of the chrysocolla occurs as a prominent filling to minor faults and fractures (Chávez, 2000). These shallow chrysocolla enrichments, particularly evident at El Abra (Ambrus, 1977) and Gaby (Aguilar et al., 2003), reflect elevated silica concentrations in neutral to alkaline, near-surface solutions, presumably resulting from more complete breakdown of rock-foming silicate minerals than at greater depths. Chrysocolla commonly occurs as a direct replacement of atacamite (including its polymorphs) or malachite, although the former mineral is likewise restricted to near-surface horizons in a few deposits (e.g., Chuquicamata; Lindgren, 1917; Taylor, 1935). Evaporation-driven ascent and concentration of solutions by capillary forces induced by the arid climate, as documented from tailings impoundments in the region (Dold and Fontboté, 2001), may have played a role in development of surface concentrations of chrysocolla and/or atacamite as well as nitrate minerals (e.g., Jarrell, 1944). Atacamite and malachite, in turn, are commonly replacement products of brochantite and antlerite, transformations that reflect lower sulfate activities and higher pH once pyrite oxidation approaches completion (path 2, Fig. 2; Fig. 12).

741

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RICHARD H. SILLITOE

PE

15º S

70º W

LE

-E

RU

O

Arequipa

L BO IV I A

20º S

Antofagasta 25º S 200 km

LE-EO

AXIS OF MIDDLE EOCENE-EARLY OLIGOCENE PORPHYR Y Cu BELT

NTINA

AXIS OF PALEOCENE-EARLY EOCENE PORPHYRY Cu BELT

LE-EO

P-EO

ARGE

0

CHILE

0361-0128/98/000/000-00 $6.00

(Cameron et al., 2002) and dissolution of Mesozoic marine evaporites (Arcuri and Brimhall, 2003), fail to explain the regionally ubiquitous presence of atacamite, even in the smallest of the many thousands of Cu occurrences throughout the hyperarid region (e.g., Little, 1926; Boric et al., 1990; Fig. 13). Marine evaporites, for example, are absent throughout much of coastal northern Chile (e.g., Boric et al., 1990). Of the major porphyry Cu deposits in the hyperarid parts of northern Chile, only Toki is reported to have more malachite than atacamite (Rivera et al., 2003b; Rivera and Pardo, 2004), possibly because of bicarbonate ions released by oxidation of its appreciable hypogene siderite content (S. Rivera, pers. commun., 2003). Mid-Tertiary aridity is also invoked to explain the local occurrence of water-soluble sulfates, including chalcanthite and kröhnkite (Table 6), as additions to the hydroxysulfates, in some oxidized ore developed at the expense of Cu sulfide enrichment zones (Table 2). At Chuquicamata, for example, chalcanthite is reported to have occurred commonly between remanent masses of pyrite rimmed by Cu sulfides and replacive antlerite, thereby confirming it as the first supergene

P-EO

The deepest parts of in situ oxidized zones, contiguous with the upper limits of subjacent sulfide zones, commonly contain relatively minor amounts of cuprite and lesser native Cu (Fig. 2; Table 6). Cuprite typically replaces supergene Cu sulfides at the tops of enriched zones (e.g., Ransome, 1904; Lindgren, 1905; Schwartz, 1934, 1949; Anderson, 1982) but tends to line fractures and open spaces as the transported dendritic variety, chalcotrichite, where chalcopyrite ± bornite undergoes direct oxidation in the absence of enrichment (e.g., El Abra). Native Cu and cuprite are stabilized by relatively low Eh conditions, transitional between values for the main oxidized and enriched zones (Fig. 2). Elsewhere in the world, cuprite and native Cu are locally more widespread, notably in the porphyry Cu-Au deposits at Afton in British Columbia and Boyongan, both of which are unusual in being oxidized to depths as great as 600 m. At Afton, 13 percent of recovered Cu was in the native form (Carr and Reed, 1976), whereas at Boyongan chalcotrichite predominates over native Cu (R. H. Sillitoe, pers. observation, 2000). At both localities, the native Cu and cuprite are locally transported minerals developed over several hundred meters vertically and, at least at Boyongan where enrichment is absent, they were precipitated from neutral to alkaline solutions (Fig. 2). Clearly, redox conditions appropriate for cuprite and native Cu formation existed more extensively than usual during oxidation at both Afton and Boyongan, possibly consequent upon the buffering effect imposed by the abundant hydrothermal magnetite. In most porphyry and all other types of Cu deposits in the currently hyperarid parts of northern Chile, north of approximately latitude 26ºS and at elevations <~3,000 m, atacamite (and its polymorphs)—with or without the Cu hydroxysulfate minerals—greatly predominates over the Cu hydroxycarbonates, chief of which is malachite (Table 2). Since stabilization of the Cu hydroxychlorides, hydroxysulfates, and hydroxycarbonates is a direct reflection of solution chemistry (Woods and Garrels, 1986; Fig. 12), the ubiquity of atacamite is attributed to the arid conditions that prevailed during mid-Tertiary weathering, whereby evaporative concentration resulted in vadose solutions containing elevated chloride contents. Therefore supergene profiles in northern Chile clearly developed under more arid conditions than those in other desertic Cu provinces, including southwestern North America (Emmons, 1917) and Iran (Bazin and Hübner, 1969; Waterman and Hamilton, 1975), where atacamite is a rather rare supergene mineral. A notable exception is the deeply buried Santa Cruz deposit in Arizona (Kreis, 1995), which may have been influenced by unusually saline ground water resulting from halite dissolution (e.g., Faulds et al., 1997). The ultimate source of the chloride (and minor accompanying bromide and iodide) contents of oxidized Cu deposits in northern Chile is believed to be marine aerosols transported inland from the Pacific Ocean by the prevailing onshore winds (e.g., Schemenauer and Cereceda, 1992). Some of the sea salt was probably also contributed during sporadic winter rains of westerly provenance and by burning off of fog banks (camanchaca) that exceptionally roll inland for nearly 100 km, assuming, of course, that fog formation was prevalent in the mid-Tertiary. Alternative proposed sources for the chloride contents of specific atacamite-rich deposits in northern Chile, including earthquake-induced pumping of bedrock brine to the surface

EASTERN LIMIT OF ATACAMITE AND PARATACAMITE IN PRE-14 Ma OXIDIZED ZONES CURRENT HYPERARID CLIMATIC ZONE

FIG. 13. Eastern limit of atacamite and its polymorphs, the hydroxychlorides, in Cu deposits and prospects of the central Andes (from Table 2 and pers. observations, 1965–2005). The currently hyperarid parts of the central Andes are also shown (after Hartley and Chong, 2002). Note the correspondence between mid-Tertiary Cu hydroxychloride distribution, indicative of chloride-rich vadose water and, hence, arid conditions, and the currently hyperarid region. Paleocene to early Eocene and middle Eocene to early Oligocene porphyry Cu belts, abstracted from Figure 1, are marked for reference.

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

oxidation product generated under the most acidic conditions (Jarrell, 1944; Figs. 2, 12b), albeit “frozen in” simply because of the especially arid conditions. The aridity also offers an explanation for the associated preservation of a wide variety of water-soluble Fe2+ and Fe3+ sulfates (e.g., copiapite, coquimbite, voltaite) as well as the nitrates (Bandy, 1938; Jarrell, 1944). Throughout the central Andes (Table 2), and in most other parts of the world (e.g., Emmons, 1917; Schwartz, 1934) where supergene profiles contain Cu hydroxycarbonates, azurite tends to be greatly subordinate to malachite; indeed, it is commonly absent altogether. One of the few exceptions is the deep oxidized zone developed at the Boyongan porphyry CuAu deposit, mentioned above, in which azurite and malachite are present in roughly equal proportions in addition to the cuprite and native Cu. Slightly higher CO2 pressure or carbonate activity or slightly lower water activity or pH may be invoked to explain the relative abundance of azurite (Woods and Garrels, 1986; Vink, 1986; Fig. 12a). In view of the high rainfall, higher carbonate and lower pH seem most likely to be responsible at Boyongan, perhaps due to the presence of bacterially derived organic carbon during sulfide oxidation (e.g., Melchiorre et al., 2000; Melchiorre and Enders, 2003). Complex Cu-bearing species: An appreciable percentage of Cu in both in situ and exotic oxide ores in northern Chile and southern Peru, as well as elsewhere, occurs as mineraloids and mineral mixtures with little-studied, poorly defined, and extremely variable compositions, even at a microscopic scale. The compounds involved include Cu-bearing limonite, pitch limonite, the black Cu-bearing phases (tenorite, paramelaconite, crednerite, neotocite, Cu pitch, and Cu wad), and Cu clays (Table 6). Cupriferous limonite is extremely poorly characterized but is assumed to contain coprecipitated or sorbed Cu (e.g., Smith, 1999) or (sub)microscopic intergrowths of Cu-bearing minerals. Pitch limonite, synonymous with glassy limonite, is a characteristic form of cupriferous goethite generated as pseudomorphs after chalcopyrite or bornite in the absence of acid (Anderson, 1982; Table 6). Neotocite (Fig. 11c), Cu pitch, and Cu wad are amorphous, noncrystalline, transported phases with the characteristics given in Table 6. Traditionally, neotocite is considered as Cu-Mn-Fe silicate containing up to 21 percent Cu (Anderson, 1982), Cu pitch as Fe- and Mnbearing varieties of chrysocolla (hence the alternative name of black chrysocolla; Throop and Buseck, 1971), and Cu wad as cupriferous Mn ± Fe oxides. However, results of recent detailed mineralogic studies using a variety of techniques challenge some of these assumptions and suggest greater mineralogic complexity. Pincheira et al. (2003) concluded that Cu pitch and Cu wad from the Mina Sur exotic deposit (Table 2) are chemically indistinguishable, notwithstanding their clear textural differences (Table 6), with both mineraloids containing Si, Cu, Mn, Fe, and Al as major elements. Mn- and Fe-rich subtypes were recognized, with Cu values varying from 1 to 54 percent (Pincheira et al., 2003). Mote et al. (2001b), Cuadra and Rojas (2001), and Oyarzún and Cuadra (2003) concluded that Cu wad from the Damiana exotic deposit and Radomiro Tomic and San Antonio (Potrerillos) porphyry Cu deposits is a finegrained mineral mixture, which may include one or more of 0361-0128/98/000/000-00 $6.00

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crednerite (Table 6) and several Cu-bearing Mn oxyhydrates, such as birnessite, cryptomelane, and todorokite. Copper contents commonly approximate 20 percent. Presumed neotocite from the El Abra porphyry Cu deposit reportedly comprises mainly tenorite and paramelaconite (Chávez, 2000), minerals that are determined elsewhere as generally minor components of the black oxide Cu phases. Copper clays are a complex group of minerals and mineral mixtures, commonly belonging to the smectite group, but also including kaolinite and sericite. Copper may be present as microinclusions of chrysocolla or atacamite or, in some smectites, in structurally bound form accounting for as little as 1 to >20 percent Cu (Brimhall et al., 2001; Cuadra and Rojas, 2001; Oyarzún and Cuadra, 2003). In northern Chile and southern Peru, the most widespread minor components of oxidized porphyry Cu ore are cupriferous arsenates and phosphates (Table 2). The former, most commonly conichalcite and chenevixite (Table 6), derive their As contents mainly from enargite, a characteristic component of late-stage high-sulfidation overprints to the deposits (e.g., Sillitoe and Perelló, 2005). In such deposits, as exemplified by Chuquicamata, Cu sulfide enrichment of the enargite causes marked depletion of As by dispersal into the ground-water system (Lindgren, 1917; Ossandón et al., 2001). Hence, during subsequent oxidation of the enriched ore, the amount of As available for incorporation into arsenate minerals is substantially less than that present originally. The commonest phosphates, including pseudomalachite, libethenite, sampleite, and turquoise (Table 6), are suspected to derive their P contents by apatite destruction during supergene weathering. One or more of the first three of these minerals are reported from the zones of in situ oxidation at El Abra (Gerwe et al., 2003), Radomiro Tomic (Cuadra and Rojas, 2001), and Gaby (Aguilar et al., 2003) and, hence, are stable under neutral to alkaline conditions, contrary to their predicted stabilities (Williams, 1990). In marked contrast, turquoise is commonly the only oxidized Cu mineral to persist in leached cappings (e.g., Escondida Norte-Zaldívar; Monroy, 2000; El Salvador; Gustafson and Hunt, 1975), and hence must be tolerant of acidic conditions. Chalcocite-enriched ores Porphyry Cu deposits containing appreciable pyrite develop Cu sulfide-enriched zones beneath leached cappings, with or without varied amounts of associated oxidized Cu ore (Fig. 4). The mature, cumulatively enriched zones in the central Andes attain maximum thicknesses that generally range from 60 to 300 m, and in the case of Chuquicamata 750 m, but tend to thin laterally to only a few or few tens of meters (Table 2). The thicknesses of the mature enriched zones listed in Table 2 chart the main upper parts in which even pyrite besides Cu-bearing sulfides are extensively replaced (e.g., Titley, 1982) and enrichment factors (based on average subjacent hypogene Cu contents) range from 1.5 to 3. At deeper levels, weak or nascent (Lichtner and Biino, 1992) enrichment grades imperceptibly into underlying hypogene ore or protore. In some deposits, chalcocite dominates the main enriched zone, whereas covellite increases in abundance downward where enrichment is weaker (e.g., Hunt et al., 1983; Sillitoe et al., 1984; Alpers and Brimhall, 1989; Chávez, 2000).

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RICHARD H. SILLITOE

The main enriched zones typically comprise massive, steely chalcocite, with pulverulent, sooty chalcocite tending to predominate at deeper levels, especially in proximity to recent water tables. Transformation of sooty to steely chalcocite has been hypothesized to be a diagenetic or aging phenomenon (Sillitoe, 1990). Massive and sooty chalcocite typically occurs as a volume-for-volume replacement of chalcopyrite, bornite, pyrite, enargite, or other hypogene sulfide grains along external contacts, internal fractures, and cleavage planes. Chalcocite typically contains angular remnants of the precursor sulfides (e.g., Emmons, 1917), although total replacement by the massive variety is observed rarely in throughgoing veins near the tops of mature enriched zones (Fig. 11a). The term chalcocite is widely used shorthand for a transient series of chalcocite-group minerals with progressively lower Cu contents that span the compositional gap between chalcocite (Cu2S) and covellite (CuS): djurleite (Cu1.97S), digenite (Cu1.8S), anilite (Cu1.75S), geerite (Cu1.6S), spionkopite (Cu1.4S), and yarrowite (Cu1.12S). Where detailed studies of central Andean enriched ore have been carried out, djurleite, digenite, and anilite have been identified besides stoichiometric chalcocite (Gustafson and Hunt, 1975; Flores, 1985; Alpers and Brimhall, 1989; Monroy, 2000; Williams, 2003). A theoretical downward progression from chalcocite to djurleite, digenite, anilite, and eventual covellite, consequent upon progressively lesser amounts of Cu in solution, would be predicted and has been roughly charted at Escondida (Alpers and Brimhall, 1989) and elsewhere (Sikka et al., 1991; Enders, 2000). However, at least some of the admixed covellite and intermediate Cu sulfide phases could also be initial metastable products of supergene chalcocite oxidation near the tops of enriched zones rather than true enrichment phases (Peterson et al., 1951; Sillitoe and Clark, 1969; Sikka et al., 1991; Enders, 2000). Under oxidative attack of chalcocite by either H2SO4 or Fe(III), the full spectrum of Cu sulfides, from djurleite through covellite, may be generated (Whiteside and Goble, 1986; Scott, 1991). Gold distribution During the development of leached cappings and oxidized and enriched zones in porphyry Cu-Au deposits, as well as during weathering of some associated epithermal Au deposits, there is little apparent leaching and enrichment of Au. There is, however, local coarsening of Au grain size and even increases in fineness during oxidative destruction of the host sulfide grains (e.g., Sillitoe, 1999, 2000). In most bulk-tonnage deposits, the coarsening rarely results in Au grains bigger than micron size, in keeping with the overall paucity of fluvial placers, although very coarse, near-surface Au is generated locally (e.g., Summitville, Colorado; Stoffregen, 1986). Nevertheless, average Au tenors may increase slightly as a result of reductions in rock density consequent upon supergene silicate alteration, carbonate removal, and sulfide oxidation but appreciably where semimassive sulfides occur in some skarn, carbonate-replacement, and high-sulfidation epithermal deposits. Local mechanical concentration of Au may also occur near surface, especially in vein deposits (Emmons, 1917). However, most significant Au enrichment takes place in the lateritic environment (Freyssinet et al., 2005). 0361-0128/98/000/000-00 $6.00

Gold dissolution and transport in the supergene environment may be in colloidal form or as halide, organic, sulfide, or thiosulfate complexes (e.g., Stoffregen, 1986; Webster, 1986; Butt, 1989; Bowell et al., 1993; Saunders, 1993; Cohen and White, 2004). In the central Andes and other arid regions, Au remobilization during oxidation of pyritic deposits is probably via chloride complexes given the saline, acidic, and oxidized conditions (e.g., Kori Kollo, Bolivia; Darke et al., 1997) but thiosulfate is favored where acidity and/or salinity are lower (e.g., Summitville; Stoffregen, 1986). The general absence of appreciable Au enrichment is attributable to short-term stability of the Au complexes. This may be due to sorption of the complexed Au onto limonite (Machesky et al., 1991; Greffié et al., 1996), to solution neutralization in the case of Au-chloride complexes (Saunders, 1993), and to the innate transience of thiosulfate ions during sulfide oxidation (Williams, 1990). Molybdenum distribution Molybdenum displays variable mobility during the oxidation of porphyry Cu deposits but under acidic conditions is less mobile than Cu (e.g., Bloom, 1966). Hypogene molybdenite is commonly transformed to ferrimolybdite [Fe3+ 2 (MoO4)3·8H2O], commonly intimately admixed with limonite where pH is <8, although lindgrenite [Cu3(MoO4)2(OH)2] is reported locally (e.g., Chuquicamata; Jarrell, 1944; Escondida Norte-Zaldívar; C. Monroy, pers commun., 2004). Appreciable sulfide enrichment of Mo is unknown. Supergene argillic alteration Widespread and pervasive supergene argillic alteration is a ubiquitous accompaniment to sulfide oxidation and enrichment (e.g., Anderson, 1982; Titley and Marozas, 1995) as well as to exotic Cu mineralization (e.g., Münchmeyer, 1996). The intensity and mineralogy of the alteration broadly correlate with the former pyrite content of the leached and/or oxidized zones. The main supergene alteration minerals are kaolinite and smectite, the latter typically a member of the montmorillonite subgroup. Where sulfide oxidation is accompanied by chalcocite enrichment as a result of the acidic conditions consequent upon destruction of appreciable pyrite, kaolinite is the dominant clay mineral. It is pervasive throughout the leached cappings and any associated horizons or lenses of oxidized Cu mineralization, from where it extends for several tens or even hundreds of meters into the underlying chalcocite enrichment zone and for several kilometers into the exotic Cu environment. Kaolinization, like chalcocite enrichment, becomes less intense downward and may give way to smectite because of progressive neutralization of the descendant solutions. Similarly, kaolinite dominates throughgoing fluid conduits in exotic Cu deposits, where it also tends to give way to smectite where solution pH increases. The pervasive kaolinization observed in jarositic and hematitic leached cappings forms incrementally as kaolinization accompanying pyrite oxidation at the redox front is added to that formed previously during chalcocite enrichment beneath former redox fronts (Table 3). The kaolinite replaces most of the rock-forming feldspar and mafic minerals, besides any hydrothermal illite and chlorite. Calcic plagioclase is affected preferentially over sodic plagioclase and K-feldspar, in keeping with experimental results

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

(Casey et al., 1991; Lasaga and Berner, 1998). Sericite and advanced argillic assemblages appear typically to be stable during supergene kaolinization (e.g., Gustafson and Hunt, 1975), although the former may undergo partial replacement by kaolinite (Enders, 2000). Where stockworks of sericite-bordered D-type veinlets overprint potassic alteration, the former tend to be preserved whereas the intervening potassic alteration remnants are pervasively kaolinized. Vermiculite and chlorite are intermediate stages in the transformation of biotite phenocrysts to a poorly characterized, white to strawcolored phyllosilicate. Kaolinite and/or halloysite, a hydrated member of the kaolinite group (Giese, 1988), also occur widely in monomineralic veinlets, especially in the shallower parts of these supergene profiles. In oxidized zones developed under less-acidic conditions from pyrite-poor protoliths, commonly within potassic alteration zones, kaolinite is typically less abundant and yellowishgreen smectite predominates. The smectite normally replaces plagioclase phenocrysts as well as lining prominent fractures in the rock. The more intensely green colored smectite contains Cu, as noted above. Smectite is also generated during supergene oxidation of Cu-bearing calcic skarn assemblages, which are also typically efficient acid neutralizers besides being poor in pyrite. Supergene alunite Supergene alunite and natroalunite [(K,Na)Al3(SO4)2 (OH)6] were poorly appreciated during early studies of supergene oxidation and enrichment (e.g., Emmons, 1917; Locke, 1926; Blanchard, 1968), although Lindgren (1917) correctly identified the former in both oxidized and enriched zones at the Chuquicamata porphyry Cu deposit. During the last 30 years, however, supergene alunite has been progressively more widely recognized, in part because of its suitability for radiometric dating by the K-Ar and 40Ar/39Ar methods (e.g., Gustafson and Hunt, 1975; Alpers and Brimhall, 1988; Sillitoe and McKee, 1996). The low pH conditions and elevated sulfate contents required for supergene alunite (and natroalunite) formation (Hemley et al., 1969) normally exist only during and immediately following pyrite oxidation (Bladh, 1982), so the resulting radiometric ages approximate the timing of oxidation and any attendant enrichment. Alunite is observed to be widespread in the central Andean and southwestern North American porphyry Cu deposits where pyrite contents were high and host rocks retained some acid-neutralizing capacity. Rocks that underwent complete and pervasive hypogene sericitic or advanced argillic alteration are poor hosts for supergene alunite formation, even where highly pyritic, because they have already been subject to acid attack and, hence, are essentially nonreactive to low-temperature supergene solutions. Consequently, supergene alunite is uncommon in most high-sulfidation Au deposits. Supergene alunite is prominent as millimetric to centimetric veinlets of massive, fine-grained, porcelaneous material that may be white, cream, yellow (jarosite admixture), red (hematite admixture), or dirty green (structurally bound Cu; Sillitoe and McKee, 1996). The veinlets, in common with those composed of kaolinite (see above), typically constitute the youngest veinlet generation and are devoid of sulfide boxworks. Alunite also occurs 0361-0128/98/000/000-00 $6.00

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as open-space fillings and, uncommonly, patchy replacements of rock-forming plagioclase feldspar. The alunite veinlets occur in both oxidized and, less abundantly, sulfide-enriched zones. The alunite is believed to form originally during active pyrite oxidation at redox interfaces and, in progressively lesser amounts, downward through the underlying chalcocite-enriched zones, although Alpers and Brimhall (1988) favored all alunite precipitation in the latter manner. Some alunite in leached cappings is probably inherited from former enriched zones during downward progression of oxidative weathering, a likelihood that urges caution when alunite ages are used to deduce the temporal evolution of individual supergene profiles (e.g., Bouzari and Clark, 2002). For example, alunite veinlets separated by 200 vertical meters in a leached capping may have formed in a preexisting enriched zone anytime from several million years apart to synchronously. Supergene alunite is notably rare in supergene profiles developed under pluvial conditions, as in the southwestern Pacific region, an observation that may suggest that evaporative concentration of supergene solutions may be a prerequisite for the solubility product of alunite to be exceeded under most supergene conditions. Anhydrite dissolution Anhydrite is a widespread and abundant hypogene mineral in many alteration types characteristic of porphyry Cu deposits (e.g., Sillitoe, 2000; Seedorff et al., 2005), where it occurs as veinlets with or without other minerals, disseminated grains, and hydrothermal breccia cement. Anhydrite is readily hydrated to gypsum (Lindgren, 1910), which, in turn, dissolves, irrespective of the acidity of the supergene solution involved. Hence, anhydrite is either removed or, at least, partially hydrated to gypsum in all rock that has seen the passage of ground water. The thickness of the transitional zone in which anhydrite is wholly or partially transformed to gypsum varies from a few to several hundred meters. The gypsum occurs in two main forms: massive aggregates as pseudomorphs after anhydrite and cross-fibre veins where transported in origin (Sillitoe and Gappe, 1984). Ground-water penetration and anhydrite leaching normally extend downward for several hundred meters, locally to >1,000 m, beneath the present surface, commonly appreciably below even the roots of any weakly developed Cu sulfide enrichment zones (Sillitoe, 1973; Gustafson and Hunt, 1975). Even where supergene profiles are thinly and incipiently developed, as in the southwestern Pacific region, hydration and leaching of anhydrite penetrate at least 200 m beneath surface (Sillitoe and Gappe, 1984) and, exceptionally, reach 1,000 m (e.g., Batu Hijau, Indonesia; J.H. Dilles, pers. commun., 2004). Rocks between the enrichment zone and the anhydrite front undergo only weakly developed supergene argillic alteration (e.g., Barter and Kelly, 1982). Anhydrite removal is controlled by rock permeability so there is a distinct tendency for the top of preserved anhydrite or sulfate, the anhydrite or sulfate front, to be centered on the most highly fractured cores of some deposits and to approximate bowlshaped forms (e.g., Sillitoe, 1973; Gustafson and Hunt, 1975; Sillitoe and Gappe, 1984; Brimhall et al., 1985; Clark, 1990; Fig. 4). Relatively impermeable rock volumes may resist anhydrite removal and form pinnacles extending upward from

745

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RICHARD H. SILLITOE

the anhydrite or sulfate fronts, as seen in the case of the Anhydrite Breccia in the Río Blanco-Los Bronces porphyry Cu deposit, central Chile (Warnaars, 1983; Warnaars et al., 1985). At the El Teniente porphyry Cu deposit, deep anhydrite removal from highly fractured and mineralized wall rocks along the margins of the relatively impermeable, late-mineral Braden breccia resulted in a sulfate front with the form of a curvilinear trough. Present-day ground-water flows along the trough before debouching into the deeply incised Río Teniente valley (Cuadra, 1986; Camus, 2003; Fig. 14). Rocks containing all their original anhydrite have extremely low porosities and permeabilities and act as effective barriers to ground-water movement, but on anhydrite dissolution there is enhancement of permeability and decreases in both rock competency and specific gravity (Gustafson and Hunt, 1975; Sillitoe, 2000). Anhydrite removal must therefore presage sulfide oxidation and enrichment, which, upon completion, may give rise to renewed

22

22

2346 2284

EN NI

2577

BRADEN BRECCIA

RI O

7 262

TO

TE

N

2527 262 2577 7

247 4

22

84

2346

2284

2346 2401

84

22

02 21 5 6 21

TE

R

IO

TE

01 24

IE NT 500 m

Axis of trough in sulfate front, indicating direction of groundwater flow 247 4

Contours (m asl) on sulfate front

24

27

74

E 26 0

252 7

Surface drainage Zone of mixing between subsurface flow and surface infiltration

FIG. 14. Contoured plan of the supergene sulfate front in the El Teniente porphyry Cu deposit, central Chile. Rock permeability differences have resulted in the sulfate front having >500 m of relief, which defines an annular trough around the perimeter of the relatively impermeable Braden breccia (shown at 2,284-m elevation). The trough channels ground water into the south-flowing Río Teniente drainage system via northern and southern routes (from Cuadra, 1986). 0361-0128/98/000/000-00 $6.00

increases in rock competency where kaolinite or smectite alteration is intense as well as further reductions in specific gravity where sulfide minerals are destroyed. Distinctions from hypogene equivalents Several supergene products, in particular Cu sulfide (chalcocite group) minerals, limonite (hematite and jarosite), clays (kaolinite and smectite), sericite, and alunite, are either generally accepted or at least claimed by some investigators to possess hypogene equivalents. Where both hypogene and supergene origins are undisputed, their distinction is generally relatively straightforward using field techniques, to which results of laboratory studies may be effectively added in some cases (e.g., Sheppard et al., 1969; Rye et al., 1992). Copper sulfides: A century ago, the existence of hypogene as well as supergene chalcocite and covellite was hotly debated, but the observations by Sales (1910, 1914) and Rogers (1913) at Butte confirmed the existence of both minerals as components of hypogene ores. Recent exploration of porphyry Cu deposits little affected by supergene processes has resulted in further development of the concept of hypogene Cu sulfide enrichment, which, like its supergene counterpart, upgrades preexisting Cu-bearing sulfide assemblages (Sillitoe, 1999, 2000). Hypogene enrichment accompanies overprinting of the apical parts of porphyry stocks by the basal parts of advanced argillic lithocaps during telescoping of porphyry Cu systems. The porphyry stocks and their immediately adjoining wall rocks first acted as hosts to potassic alteration and quartz-veinlet stockworks associated with chalcopyrite ± bornite mineralization. As a result of the overprinting, the potassic alteration is transformed to sericite ± dickite ± pyrophyllite assemblages, the chalcopyrite ± bornite in both veinlet and disseminated form is stripped, and disseminated pyrite plus covellite ± chalcocite ± bornite ± enargite grains are introduced (cf. Brimhall, 1979). These high-sulfidation sulfide assemblages tend to occupy the altered rock between the quartz veinlets, which themselves are essentially barren. The Cu sulfides tend to coat pyrite grains, which lack clear evidence of the replacement textures typical of supergene enrichment (see above). In deposits where both hypogene and supergene enrichment has taken place within the same overall rock volume, distinction of the two products is particularly difficult (Sales, 1914). This is the case at the Chuquicamata porphyry Cu deposit, where the deeply penetrating, structurally controlled Cu sulfide zone owes its high grade to a combination of hypogene and supergene processes. Pyrite, coarse-grained covellite, and associated digenite constitute a clearcut hypogene assemblage, with fine-grained covellite and sooty chalcocite thought to be supergene (Bandy, 1938; Ossandón et al., 2001). Much of the deep chalcocite, however, is of uncertain origin, and the precise lower limit of chalcocite enrichment is difficult to decipher (Ossandón et al., 2001). Chalcocite-group minerals of hypogene origin are also commonplace as parts of low-sulfidation sulfide assemblages, which in contradistinction to their high-sulfidation counterparts, lack pyrite (Meyer and Hemley, 1967; Einaudi et al., 2003). Such low-sulfidation assemblages, comprising chalcocite, digenite, djurleite, and bornite, characterize the potassic cores of some porphyry Cu deposits (Einaudi et al., 2003)

746

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

and other deposit types, including Chilean manto-type Cu deposits (e.g., Chávez, 1983). Confusion of these hypogene assemblages with supergene Cu sulfides is not an issue because the lack of pyrite promotes oxidation in situ, with only minor, if any, supergene enrichment. Limonite: Hematite is the only limonite component that occurs widely as a hypogene mineral. Nevertheless, in its specular form, hematite is exclusively hypogene in origin and breaks down to goethite when subjected to intense weathering under acidic conditions (Blanchard, 1968). Jarosite is largely supergene in origin, although a few coarse-grained occurrences have been considered to be hypogene (Stoffregen et al., 2000). A sample from the Paradise Peak high-sulfidation epithermal Au-Ag deposit, Nevada reportedly formed at 150ºC based on 18O fractionation data (Stoffregen et al., 1994). The jarosite in the shallowest parts of the leached capping above the El Salvador porphyry Cu deposit has been interpreted as a product of hybrid hot spring-supergene processes active during the inferred transition from hypogene to supergene conditions (Gustafson and Hunt, 1975; Gustafson et al., 2001). However, it is difficult to imagine how this putative early supergene leaching could penetrate as deeply as 1 to 2 km, the likely emplacement depth of the shallowest preserved parts of the deposit currently represented by the leached capping, given that normal supergene oxidation processes during the subsequent 25 m.y. managed to penetrate only 200 to 300 m below the shallow jarosite zone (Gustafson and Hunt, 1975). Furthermore, a sample of this shallow jarosite has δD and δ18O values indicative of a supergene origin (Watanabe and Hedenquist, 2001). Another current debate surrounds the supergene versus hypogene origin of oxidized Au-Ag ore in some high-sulfidation epithermal deposits. Although the leached cappings and underlying Cu sulfide enrichment in such deposits are generally considered to be of conventional supergene origin (e.g., Sillitoe and Lorson, 1994; Harvey et al., 1999), some workers prefer a hypogene oxidation mechanism (e.g., John et al., 1991). For example, hypogene sulfide oxidation along with accompanying covellite enrichment and Au introduction were proposed for the largely oxidized Pierina high-sulfidation deposit, Peru, based on the existence of centimeterto meter-sized patches of nonoxidized rock above the main base of oxidation, and the presence of covellite rims to these patches (Noble et al., 1997; Volkert et al., 1998). However, such marginally enriched patches of marooned sulfides are commonplace in the deeper parts of leached cappings (see above) and a normal outcome of differential supergene oxidation controlled by permeability contrasts. The covellite or, elsewhere, chalcocite rims are generated because the edges of the sulfidic patches act as redox fronts exactly like the main underlying oxide-sulfide interfaces (Sillitoe, 1999). Recent isotopic studies appear to provide further support for supergene origin of the oxidation at Pierina (Rainbow et al., 2005). Hypogene oxidation is considered to be an unlikely general mechanism for sulfide destruction in high-sulfidation systems. If the process actually operates and accounts for some deep oxidation zones in high-sulfidation deposits, then it is difficult to explain why its supposed effects are observed only in deposits located in the arid and semiarid arc terranes along 0361-0128/98/000/000-00 $6.00

747

the eastern side of the Pacific Ocean and not in the western Pacific region. Furthermore, any putative hypogene fluid capable of sulfide oxidation would have to be chloride and bromide rich, like supergene solutions in arid regions, rather than dilute as observed in primary fluid inclusions (Arribas, 1995), in order to account for the Ag halide minerals (e.g., cerargyrite, embolite) that are such widespread accompaniments to Au in deeply developed oxidized zones at several deposits, including Paradise Peak (Sillitoe and Lorson, 1994) and La Coipa, Chile (Oviedo et al., 1991). Clay minerals: Although kaolinite was once considered to be exclusively supergene in origin (e.g., Ransome, 1910), it is now widely accepted to be hypogene as well, particularly in high-sulfidation epithermal Au-Ag deposits. There it commonly occupies a distal position in zoning patterns, typically external to quartz-alunite and internal to intermediate argillic assemblages (e.g., Steven and Ratté, 1960). Distinction between supergene and hypogene kaolinite in high-sulfidation deposits is not straightforward because the acidic solutions generated during pyrite oxidation leave the hypogene kaolinite intact while causing kaolinization of the contiguous intermediate argillic zones, thereby considerably expanding the total widths of kaolinized rock (Sillitoe, 1999). Typically, however, hypogene kaolinite is accompanied by at least weak silicification of the rock groundmass, whereas the supergene product is not intergrown with neoformed quartz. Hypogene kaolinite is also commonly better crystallized than its supergene equivalent, as observed in X-ray diffraction patterns and PIMA spectra, as well as possessing distinctive δD and δ18O values (Sheppard et al., 1969; Watanabe and Hedenquist, 2001). Sericite: Although a number of investigators refer to supergene sericite and even pyrophyllite (e.g., Titley, 1982; Titley and Marozas, 1995), it is my observation that most sericite is of hydrothermal origin and largely stable under supergene conditions, a fact emphasized by detailed studies of the El Salvador and Morenci porphyry Cu deposits (Gustafson and Hunt, 1975; Enders, 2000; Watanabe and Hedenquist, 2001). Nevertheless, sericite may undergo local kaolinization (e.g., Ransome, 1910; Enders, 2000) and, especially if the pale-colored phyllosilicate pseudomorphs after biotite are included, even be supergene in origin. Texturally destructive sericitic (i.e., quartz–fine-grained muscovite) alteration is, however, an exclusively hypogene phenomenon. Theoretically, illite may form as a supergene mineral, but the high K/H conditions that would be necessary (Watanabe and Hedenquist, 2001, Fig. 9) seem to be only rarely, if ever, attained. Alunite: The massive porcelaneous habit of supergene alunite (and natroalunite) veinlets clearly distinguishes it in hand sample from the crystalline to powdery hypogene alunite generated as a component of either high-sulfidation epithermal deposits or acid-leached zones in the steam-heated environment above paleowater tables atop all types of epithermal deposits (Sillitoe, 1993). Supergene alunite also lacks the cores of Al phosphate-sulfate minerals so characteristic of alunite from high-sulfidation systems (Stoffregen and Alpers, 1987). The four stable isotope sites (δD, δ18OSO4, δ18OOH, and δ34S) that exist in alunite (and jarosite) may also be utilized in the distinction of supergene from hypogene material (Rye et al., 1992).

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RICHARD H. SILLITOE

Age of Oxidation and Enrichment Active supergene profiles The majority of supergene profiles developed over porphyry Cu and related mineralization types throughout the semiarid, temperate, and tropical orogenic belts of the world are immature and actively forming, because historical denudation rates are too high for mature supergene profiles to be developed and preserved and current precipitation is adequate to sustain supergene processes (Figs. 15, 16). Furthermore, Plio-Pleistocene glaciation truncated or removed any preexisting profiles in high-latitude regions, including most of British Columbia (Ney et al., 1976) and the central and southern Andes. In the western Americas (Fig. 15), active immature profiles dominate in British Columbia and the Yukon (Ney et al., 1976; McMillan and Panteleyev, 1995), Central America, and the Andes (Figs. 15, 16) of Colombia (Sillitoe et al., 1982), Ecuador (e.g., Gendall et al., 2000), northern and central Peru (e.g., Schwartz, 1982; Braun et al., 1999), and central and southern Chile (Serrano et al., 1996; Sillitoe and

McKee, 1996). Radiometric ages for supergene products have not been obtained from these regions, in part because of the general deficiency of supergene alunite (see above). The active supergene profiles under these climatic and physiographic conditions typically attain maximum thicknesses under ridges but are essentially absent beneath valley floors. Recent supergene activity in the central Chile porphyry Cu belt (Figs. 9e, 16), as in other mountainous regions of the American Cordillera and elsewhere, is functional only during the summer months and depends mainly on snowmelt rather than direct precipitation. Annual precipitation at the Los Bronces deposit, for example, averages 780 mm, essentially all as snow (1980–2002; Compañía Minera Disputada Limitada, unpub. data, 2002). Copper-rich ground water is associated with all the central Chilean deposits, with a Cu content of 4 mg/L at pH 4.8 recorded in a premine stream (Fig. 9e) draining the Los Pelambres deposit (Maranzana, 1972). Incipient exotic Cu mineralization is also taking place in parts of the Los Bronces district and, premining, at Los Pelambres (Sillitoe, 1973). 105

?

?

Late Cretaceous

217-183

British Columbia (49-60º N)

~200

SW North America (30-32º N)

EG SHG

Nevada (37-42º N)

~108

?

(Bisbee) 131-166

Colombia (1-9º N)

150-158 S Ecuador-N Peru (3-10º N) Southern Peru (14-18º S) Northern Chile (20-27º S) Central Chile (32-35º S) 0

10

20

30

40

50

60

Duration of hypogene mineralization Duration of supergene activity Possible supergene activity

70

Ma Fig. 15. Ages of porphyry Cu deposits and, in Nevada, Au deposits and their corresponding supergene chronologies for selected latitudinal intervals along western North and South America. Note active, youthful supergene activity in temperate and tropical regions, contrasting with preservation of late Eocene through Pliocene supergene profiles in arid to semiarid regions. Supergene chronologies based on alunite (and a few jarosite) ages in the case of northern Chile (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Mote et al., 2001b; Rowland and Clark, 2001; Bouzari and Clark, 2002), southern Peru (R.H. Sillitoe and S. Redwood, unpub. data, 1997; Quang et al., 2003, 2005; Perelló et al., 2004b), southwestern North America (Cook, 1994; Enders, 2000), and Nevada (Ashley and Silberman, 1976; Tingley and Berger, 1985; Sander, 1988; Ilchik, 1990; Sillitoe and Bonham,1990; Bloomstein et al., 1991; Arehart et al., 1992; Heitt, 1992; Arehart and O’Neil, 1993; Sillitoe and Lorson, 1994; Teal and Branham, 1997). Several older supergene ages from El Salvador in the middle Eocene to early Oligocene belt of northern Chile (Gustafson and Hunt, 1975; Mote et al., 2001b) are discounted on geologic grounds (see text and discussions by Clark et al., 1990, and Sillitoe and McKee, 1996). Supergene chronologies elsewhere are undated and inferred on the basis of geologic and geomorphologic considerations and constraints imposed by hypogene emplacement ages. Hypogene ages taken from summaries by McMillan and Panteleyev (1995), Arehart et al. (2003), Sillitoe and Perelló (2005), and articles listed above. EG = epithermal Au, SHG = sediment-hosted Au. 0361-0128/98/000/000-00 $6.00

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

7000

Andean Crest

m asl

7000

6000

6000

5000

5000

7

4000

4 3000

2

2000

8

5

11 9 12

14

6

3

4000

15

3000

16 2000

1

10 13

1000

0 10ºN

0º Guayaquil

<10

10-100

10º Lima

100-200

1000

30º Valparaiso

20º

200-400

400-1000

0 40º

1000-2000

50ºS

2000-4000

>4000

Precipitation, mm FIG. 16. Schematic plot of the average annual precipitation vs. elevation along the Andes and their western slopes between latitudes 10ºN and 57ºS, simplified from Abele (1989), showing approximate locations of selected Cu deposits. Deposits in arid and hyperarid zones with precipitation of <100 mm/yr, and probably also <200 mm/yr, are judged to have fossilized supergene profiles, whereas those in tropical and temperate zones with >200 mm/yr retain active supergene activity. Deposits: 1 = Mocoa, southern Colombia; 2 = San Carlos and nearby prospects, Pangui belt, southern Ecuador; 3 = La Granja; 4 = Michiquillay; 5 = Cuajone and Toquepala; 6 = Cerro Colorado; 7 = Collahuasi; 8 = El Abra; 9 = Chuquicamata; 10 = Mantos Blancos; 11 = Escondida; 12 = El Salvador; 13 = Andacollo; 14 = Los Pelambres; 15 = Río Blanco-Los Bronces; 16 = El Teniente.

Porphyry Cu and related mineralization types in the tropical island-arc terranes of the southwestern Pacific and Southeast Asian regions possess similar active and immature supergene profiles to those described above from the western Americas (e.g., Titley, 1975; Taylor and van Leeuwen, 1980; Sillitoe and Gappe, 1984). Supergene oxidation and enrichment are of little economic prominence unless a youthful supergene profile develops beneath a deep water table controlled by isolated, erosionally stable mountain massifs, as occurred at Ok Tedi (Bamford, 1972) and Kinking, eastern Mindanao, Philippines (Sillitoe and Gappe, 1984). Fossil supergene profiles Tertiary supergene profiles dating back 30 to 40 m.y. are widely preserved in northern Chile (e.g., Sillitoe and McKee, 1996), southern Peru (e.g., Clark et al., 1990), southwestern North America (e.g., Cook, 1994), and Nevada (e.g., Arehart et al., 1992), as compiled in Figure 15 (full references in caption). Fossilization and preservation are attributed either to paleowater table ascent due to concealment beneath sedimentary and/or volcanic formations or to arid or semiarid climatic conditions in which erosion rates and supergene processes are grossly in balance. Climatic desiccation is an especially effective means of supergene profile fossilization (Alpers and Brimhall, 1988). Preservation beneath later rock units tends to be a deposit-scale or, at most, district-wide phenomenon, whereas climatic mechanisms tend to be more regional in extent. The oldest supergene profiles tend to be preserved as a result of burial beneath postmineralization sequences. Enrichment of the Paleoproterozoic United Verde Extension 0361-0128/98/000/000-00 $6.00

volcanogenic massive sulfide deposit at Jerome, Arizona, was complete before burial by early Paleozoic sedimentary rocks (Lindgren, 1926). At the Early Jurassic Bisbee porphyry Cu deposit, Arizona, the nearby Late Jurassic to Early Cretaceous Glance Conglomerate, up to several thousand meters of coarse clastic debris shed from uplifting fault blocks (Bilodeau et al., 1987), contains clasts of gossan and chalcocite near its base, which roughly parallels the top of supergene enrichment in the deposit (Bonillas et al., 1917; Emmons, 1917; Bryant and Metz, 1966; Cook, 1994; Fig. 15); however, an Early Cretaceous age of 107.6 ± 2.6 Ma for supergene alunite is too young to record the pre-Glance supergene event (Cook, 1994). Cretaceous supergene alunite ages of 117 ± 1 and 93 ± 1 Ma are reported for a restricted enriched zone at the Devonian Oyu Tolgoi porphyry Cu-Au deposit in the Gobi Desert of southern Mongolia, where preservation was likely due to concealment beneath a lacustrine sedimentary sequence during the Late Cretaceous (Perelló et al., 2001). The Late Jurassic Kemess South porphyry Cu deposit, British Columbia, contains a native Curich supergene zone that formed under arid conditions before being preserved beneath the Late Cretaceous Sustut molasse basin (Rebagliati et al., 1995; Fig. 15). Carr and Reed (1976) and Ney et al. (1976) proposed that the deep oxidation zone at Afton is Paleogene (Fig. 15) and became fossilized during sediment burial in a late Eocene basin. The best-developed supergene profile in the Canadian Cordillera, at the Casino porphyry Cu deposit in the Yukon, may be of the same general age as that at Afton (Godwin, 1976; Bower et al., 1995) although, in the absence of any obvious preservation mechanism other than lack of Quaternary

749

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RICHARD H. SILLITOE

glaciation, more recent formation should also be contemplated (Ney et al., 1976). Tens to hundreds of meters of Oligocene to middle Miocene piedmont gravel capped and preserved supergene profiles at several early to mid-Tertiary porphyry Cu deposits in northern Chile, most notably Radomiro Tomic, Quetena, Toki, Spence, and Gaby, as well as at all the exotic Cu deposits (Table 2). Piedmont gravel also concealed Tertiary supergene profiles in southwestern North America, for example at Ajo (Gilluly, 1946) and Resolution, Superior district (Manske and Paul, 2002). At the unusual Mike prospect in the Carlin Au trend, Nevada, up to 240 m of lacustrine sedimentary rocks assigned to the Miocene Carlin Formation cap oxidation and Cu and Zn enrichment that extends to a depth of 450 m (Teal and Branham, 1997; Norby and Orobona, 2002). Pliocene porphyry Cu deposits at Hale and Boyongan in the Philippines are overlain by well-developed supergene profiles preserved beneath clastic basins of late Pliocene to Pleistocene age (Sillitoe and Gappe, 1984; R. H. Sillitoe, pers. observation, 2000). In the case of Boyongan, the deep oxidized zone (see above) developed when the deposit cropped out on a steep-sided mountain massif, which rapidly subsided into a pull-apart basin on the Philippine fault zone to become fossilized and preserved beneath lacustrine sediments. Voluminous ignimbrite flows derived from distant calderas played a similar role in protecting supergene profiles in several northern Chile porphyry and exotic Cu deposits, especially at Cerro Colorado, Sagasca, and Ujina (Table 2; Fig. 9d). Blanketing of the Paleocene to early Eocene porphyry Cu belt of southern Peru by easterly sourced ignimbrite flows took place several times since 25 Ma, resulting in episodic cessation of supergene activity at Cuajone, Quellaveco, and perhaps elsewhere (Clark et al., 1990; Quang et al., 2005). Much of the southwestern North American porphyry Cu province was also buried under a thick volcanic pile containing abundant ignimbrite during the Oligocene, causing cessation of supergene processes and preservation of extant oxidation and enrichment profiles at a number of deposits (Livingston et al., 1968; Cook, 1994). Climatic desiccation appears to have stopped supergene oxidation and enrichment only in the currently hyperarid parts of northern Chile (latitudes 18º–26ºS), where water appears to have become too scarce to promote appreciable supergene, including bacterial, activity at ~14 Ma (Fig. 15; Alpers and Brimhall, 1988; Sillitoe and McKee, 1996); however, such dryness may also have played a role in the Gobi Desert (Perelló et al., 2001). Major Supergene Cu-(Au) Provinces Central Andes Porphyry and associated Cu-(Au) deposits in northern Chile and southern Peru were emplaced during four discrete metallogenic epochs, each corresponding to a markedly linear metallogenic belt (e.g., Sillitoe, 1988; Sillitoe and Perelló, 2005). The epochs and belts young systematically eastward from Middle Jurassic to Early Cretaceous along the Pacific coast, through Paleocene to early Eocene and middle Eocene to early Oligocene, to Miocene and early Pliocene near and east of the Chile-Argentina frontier (Fig. 1). Deposits and 0361-0128/98/000/000-00 $6.00

prospects in all four belts underwent supergene oxidation and enrichment, although the youngest easternmost belt displays the least mature profiles. Segments of the three younger belts containing the major porphyry Cu deposits were generated under contractional conditions that gave rise to tectonically thickened crust, although the Mesozoic coastal belt was emplaced through attenuated crust in extensional and transtensional regimes (e.g., Sillitoe and Perelló, 2005). Supergene oxidation and enrichment in the Paleocene to early Eocene belt was active between at least 40 and 14 Ma in northern Chile, albeit probably through to the present day in southern Peru (Clark et al., 1990; Sillitoe and McKee, 1996; Bouzari and Clark, 2002; Quang et al., 2003; Figs. 15-18). Sparse data suggest that supergene activity was probably also taking place during the same time interval in the coastal Mesozoic belt (Sillitoe and McKee, 1996; Quang et al., 2005). Porphyry Cu deposits in the middle Eocene to early Oligocene metallogenic belt of northern Chile lack conclusive evidence for supergene modification until ~23 Ma, also continuing to ~14 Ma (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Mote et al., 2001b; Figs. 15, 17). A 35 Ma supergene age reported by Mote et al. (2001b) for Cu wad from the Damiana exotic Cu deposit at El Salvador is discounted because the Atacama Gravels, host to part of the exotic Cu mineralization, are no older than Miocene in age (Clark et al., 1967b; Mortimer, 1973). These data, if fully representative, suggest at least 12 m.y. were required to unroof the Paleogene porphyry Cu systems (Figs. 17, 18), a time interval that implies average exhumation rates of perhaps 150 to 250 m/m.y. (if paleoemplacement depths of 2–3 km are assumed). Much of the surface uplift that gave rise to the Andean mountain chain took place between 40 Ma and the present day. During this interval, all morphostructural provinces (see Sillitoe and Perelló, 2005, Fig. 1) were uplifted as a direct response to contractional tectonism and crustal thickening, with the Western Cordillera beginning its principal phase of ascent somewhat earlier than the Altiplano-Puna plateau, which, in turn, rose before the Eastern Cordillera and, farther south, the Sierras Pampeanas (Jordan et al., 1997; Lamb et al., 1997; Gregory-Wodzicki, 2000; Kennan, 2000; Hartley, 2003). The Western Cordillera, and parts of the morphostructural provinces farther east, underwent initial uplift as part of the Incaic contractional orogeny in the Eocene (Noble et al., 1979; Maksaev and Zentilli, 1988; Benavides-Cáceres, 1999; Coutand et al., 2001; Camus, 2003; Fig. 17). Uplift rates estimated from fission-track data for the Incaic event in northern Chile are 100 to 200 m/m.y. (Maksaev and Zentilli, 1999), in keeping with the independent estimate given above. This rapid uplift event was followed from the late Oligocene (~26 Ma) onward by more subdued surface ascent, 50 m/m.y. according to Maksaev and Zentilli (1999), which accounted for 50 to 75 percent of present-day elevation in the Western Cordillera (Jordan et al., 1997; Gregory-Wodzicki, 2000; Kennan, 2000). Orogen-parallel, high- and low-angle reverse faults, of both west and east vergence and thin- and thick-skinned style, account for shortening of the upper crust in the central Andes since ~40 Ma (e.g., Lamb et al., 1997; Coutand et al., 2001; Skarmeta et al., 2003). Strike-slip motion is currently judged to be relatively modest by a number of investigators (e.g.,

750

751

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

Altos de Pica Formation

20

>50% of Western Cordillera uplift

El Diablo Formation, Atacama Gravels

Coastal Cordillera uplift

ONSET OF HYPERARIDITY

El Salvador

Telégrafo

Escondida

MM

Ujina

Chuquicamata

Lomas Bayas Inca de Oro

Cerro Colorado

10

Spence Sierra Gorda

Ma 0

Incaic orogeny and uplift

40

Dubious ages

11 ages

Upper Calama, Cerro Casado, Sichal Formations

Middle Eocene-early Oligocene porphyry copper formation

30

Lower Calama Formation

Pre-Eocene basement 60

Paleocene-early Eocene porphyry copper formation

50

K-Ar

age for

Ar/ Ar alunite or jarosite Conglomerate/gravel, minor sandstone

40

39

Ignimbrite flows Dacite domes Andesitic volcanic rocks

70

FIG. 17. Ages of supergene activity for selected porphyry Cu deposits in the Paleocene to early Eocene and middle Eocene to early Oligocene belts (hypogene age ranges represented by heights of boxes), northern Chile in the context of the chronology of sedimentary and volcanic formations (summarized from Blanco et al., 2003; Farias et al., 2003; Perelló et al., 2004a), the Incaic tectonic event (Maksaev and Zentilli, 1988; Blanco et al., 2003), west-vergent thrusting along the western edge of the Altiplano (Muñoz and Charrier, 1996), and Coastal Cordillera uplift (Allmendinger et al., 2005). Alunite and jarosite ages suggest onset of hyperaridity at ~14 Ma (cf. Alpers and Brimhall, 1988; Sillitoe and McKee, 1996). Note apparent continuity of supergene activity for a minimum of 26 m.y. Supergene ages from Gustafson and Hunt (1975), Alpers and Brimhall (1988), Sillitoe and McKee (1996), Gustafson et al. (2001), Mote et al. (2001b), Rowland and Clark (2001), Bouzari and Clark (2002), and age ranges of hypogene porphyry Cu formation after Sillitoe (1988). As discussed by Sillitoe and McKee (1996) and Bouzari and Clark (2002), the two oldest ages for El Salvador are considered of dubious validity (as is a 35 Ma age for Cu wad from the contiguous Damiana exotic Cu deposit; see text).

Camus, 2003; Skarmeta et al., 2003), although others prefer tens of kilometers of transcurrent offset (e.g., Tomlinson and Blanco, 1997). The intra-arc Domeyko fault system was active within the middle Eocene to early Oligocene Cu belt during the Incaic orogenic event (e.g., Maksaev and Zentilli, 1988; Camus, 2003) but has undergone appreciable continued motion through to the present. Reverse faults controlled 0361-0128/98/000/000-00 $6.00

development of Oligocene through middle Miocene depositional basins containing several hundred meters of piedmont gravel in places throughout the belt (e.g., alongside the MM deposit; Sillitoe et al., 1996b), and, at the Chuquicamata deposit, postmineralization vertical displacement on the West fault is calculated to be 600 ± 100 m (McInnes et al., 1999). Elsewhere, along the eastern side of the Western Cordillera,

751

752

Ma 0

Capillune Fm Sencca Formation

10

ll

Chuntacala Formation

l

Huaylillas Formation

Quechuan tectonic events

lll

Barroso Group

Toquepala

Cerro VerdeSanta Rosa Cuajone Quellaveco

RICHARD H. SILLITOE

lV

20

lll

Upper Moquegua Formation

40

ll

Lower Moquegua Formation

Eocene and older basement

Incaic tectonic events

30

60

l

Paleocene-early Eocene porphyry copper formation

50

FIG. 18. Ages of supergene activity for selected Paleocene to early Eocene porphyry Cu deposits (hypogene age range represented by height of box), southern Peru in the context of the chronology of sedimentary and volcanic formations (simplified from Quang et al., 2005) and tectonic events (Benavides-Cáceres, 1999). Note that the alunite ages may be interpreted in terms of episodic supergene activity or a single 40-m.y. event. Supergene ages from R.H. Sillitoe and S. D. Redwood (unpub. data, 1997) and Quang et al. (2003). Legend as in Figure 17, except for stipple, which represents fine-grained sedimentary rocks.

a west-vergent reverse fault zone and related monoclinal flexure controlled late Oligocene to present-day uplift of the Altiplano plateau, with uplift rates decreasing from ~150 to 60 m/m.y. in the middle Miocene (Muñoz and Charrier, 1996; Farias et al., 2003). Thin- and thick-skinned reverse faulting is also documented over the last 40 m.y. along the western margin of and within the Salar de Atacama intermontane basin (Muñoz et al., 2002). Even the Coastal Cordillera, magmatically inactive since the Mesozoic, probably underwent 0361-0128/98/000/000-00 $6.00

~1,000 m of surface uplift since the early Miocene to generate a steep coastal escarpment (González et al., 2003). Variability in supergene development in the central Andes is believed to be largely due to differences in uplift histories between adjoining structural domains. The most profound result of this control is exposure of potassic alteration subjected to in situ oxidation where protracted uplift was most severe (e.g., El Abra), in contrast to more shallow, pyritic alteration zones with enrichment blankets where exhumation was less (e.g., Conchi Viejo prospect 10 km southeast of El Abra). At a district scale, differential fault-block uplift within the Chuquicamata cluster was controlled by the West fault. Chuquicamata and Radomiro Tomic on the eastern side of the fault have deeply developed oxidation and/or enrichment, whereas to the west the MM deposit has a stunted supergene profile despite its propitious structural, alteration, and mineralization characteristics (Sillitoe et al., 1996b). At a deposit scale, the Jack Rock and associated reverse faults that cut the Rosario porphyry Cu deposit resulted in only shallow penetration of supergene effects within the structurally depressed block that hosts most of the Cu orebody (Bisso et al., 1998; Fig. 5). A spectacular Plio-Pleistocene example of this differential uplift control is provided by two porphyry Cu deposits in the Miocene to early Pliocene belt, which lie on opposite sides of a high-angle reverse fault in the Sierras Pampeanas morphostructural province of northwestern Argentina. Bajo de la Alumbrera, on the downthrown side, is almost devoid of supergene mineralization whereas Agua Rica (Fig. 1), on the upthrown side and 500 m higher in elevation, possesses an immature but deeply penetrating enriched zone, which developed during the main uplift event (Ramos et al., 2002) and only ~1 m.y. after completion of deposit emplacement at 4.9 Ma (Perelló et al., 1998; Fig. 19). Porphyry Cu centers of the Famatina district (Fig. 19), within another reverse faultbounded block in the Sierras Pampeanas, underwent such rapid post-4 Ma (Taylor et al., 1997) surface uplift and exhumation that supergene effects are negligible. Farther south, in the Miocene to early Pliocene belt of central Chile, Kurtz et al. (1997) calculated uplift rates as high as even 3 km/m.y. during exhumation of the El Teniente and neighboring Cu deposits (Fig. 1), but lower rates prevailed since, during the still-active oxidation and enrichment. Uplift history dictates whether paleowater table descent progresses steadily through an enriched zone, as seems to have been commonplace (e.g., Escondida; Alpers and Brimhall, 1989), or is more dramatic leading to its reestablishment in underlying hypogene ore beneath a stranded enriched zone. This latter situation eventually gives rise to the previously described leached zones beneath oxidized ore (e.g., Jarrell, 1944; Bouzari and Clark, 2002) and jarositic beneath hematitic leached cappings (e.g., Anderson, 1982). The uplift and concomitant exhumation of the main central Andean Cu belts since 40 Ma gave rise to widespread piedmont gravel deposition. The upper Calama, Sichal, and Cerro Casado Formation gravels are directly linked to the Eocene Incaic orogeny, while the younger, more extensive gravel accumulations west, within, and east of the Western Cordillera record continued contractional deformation from the late Oligocene to middle Miocene (e.g., Naranjo and Paskoff,

752

SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

EO

RU

70º W LE

PE

15º S

Cotabambas 3.3

Cerro Verde 4.9

Quellaveco 3.2 Kori Kollo 3.7

Ma

km

200

Inca de Oro 14.2

P-EO

AXIS OF PALEOCENE-EARL Y EOCENE PORPHYR Y Cu BELT

LE-EO

AXIS OF MIDDLE EOCENE-EARL Y OLIGOCENE PORPHYR Y Cu BELT

10 Ma

SUPERGENE ISOCHRON

9.1

MINIMUM AGE DETERMINED FOR SUPERGENE MINERAL

MM 20.4 Telégrafo 20.0

LE-EO

0

Conchi Viejo 13.1 Chuquicamata 15.2

CHILE

25º S

IA LIV a BO5 M

Ujina 15.2 Puntillas 21.1 Spence 20.9 Sierra Gorda 14.1 Lomas Bayas 20.8 Angelina 31.0 El Peñón 17.7

10

Ma

O

14

P-E

Cerro Colorado 14.6

20º S

La Coipa 14.5

Escondida 14.7 Taca Taca Bajo 11.6 Diablillos 7.1 El Salvador 12.9 Cerro Sílica 9.1 Agua Rica 3.9 Farallón Negro 2.7

ARGENTINA Sierra de Famatina <3

FIG. 19. Compilation of youngest supergene mineral ages determined by K-Ar and 40Ar/39Ar methods for individual Cu and Au deposits and prospects in the central Andes. Although there is no guarantee that the youngest supergene activity is everywhere recorded, there is always a higher probability of the youngest alunites and jarosites being preserved and therefore dated. Note the marked eastward and northward younging of final supergene activity caused by the progressive onset of conditions that were too arid. Supergene activity ceased at ~14 Ma in northern Chile, north of about latitude 28ºS, while it probably continues today in parts of northwestern Argentina, western Bolivia, and southern Peru. Paleocene to early Eocene and middle Eocene to early Oligocene porphyry Cu belts, abstracted from Figure 1, are marked for reference. Alunite ages compiled from Alpers and Brimhall (1988), Sillitoe and McKee (1996), Darke et al. (1997), Marsh et al. (1997), R.H. Sillitoe (unpub. data, 1997), R.H. Sillitoe and E.H. McKee (unpub. data, 1997), R.H. Sillitoe and S.D. Redwood (unpub. data, 1997), Perelló et al. (1998, 2004b), Stein et al. (2000), Rowland and Clark (2001), Mote et al. (2001b), Bouzari and Clark (2002), Perelló (2003), Quang et al. (2003), and I. Warren (writ. commun., 2004). A single cryptomelane age (from Farallón Negro) is taken from Sasso and Clark (1998).

1981, 1985; Sáez et al., 1999; Mpodozis et al., 2000; Blanco et al., 2003; Mpodozis and Perelló, 2003; Perelló et al., 2004a). Middle Miocene to Quaternary gravels occur farther east in the Miocene to early Pliocene Cu belt where uplift commenced in the middle to late Miocene (e.g., Allmendinger, 1986; Ramos et al., 2002). These gravel sequences are broadly contemporaneous with the supergene activity and, where they accumulated on top of supergene profiles, paleowater tables must have risen to eventually extinguish supergene activity. Nevertheless, several porphyry Cu deposits became concealed beneath gravels only after mature supergene profiles had developed (e.g., Radomiro Tomic, Quetena, Toki, Spence, Gaby; Table 2; Fig. 17), although not in the case of 0361-0128/98/000/000-00 $6.00

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the MM deposit, as noted above. Contemporaneity of piedmont gravel accumulation and supergene activity provides strong evidence that paleoclimate was semiarid to arid since at least 40 Ma (cf., Galli-Oliver, 1967; Mortimer, 1973; Mortimer and Saricˇ, 1975; Alpers and Brimhall, 1988; Quang et al., 2005). The climatic conditions were probably similar to those sustaining oxidation and enrichment in the Miocene to early Pliocene porphyry Cu deposits of the Sierras Pampeanas and central Chile today (see above). The large volumes of easterly sourced ignimbrites, noted above, are intercalated in the gravel sequences north of about latitude 22ºS, where they are also locally responsible for premature termination of supergene activity (Clark et al., 1990; Bouzari and Clark, 2002; Quang et al., 2005; Table 2; Figs. 17, 18). Exotic Cu accumulation was another economically important facet of the late Oligocene to middle Miocene supergene event but only where piedmont gravels were accumulating alongside actively oxidizing, shallow pyritic parts of porphyry Cu deposits (Figs. 4, 9f). Exotic Cu deposition was synchronous with the early stages of gravel accumulation in most but not all deposits because it is contained in paleochannels incised directly into bedrock and overlain by up to 250 m of barren, postmineralization gravel. Therefore exotic Cu mineralization is taken to accompany the main enrichment episode in its source deposits rather than typically being a late facet of supergene activity as proposed by Bouzari and Clark (2002). Two tectonic settings for exotic Cu mineralization are recognized: porphyry Cu source and exotic deposit in the same structural block, and source and exotic site separated by a high-angle reverse fault (Fig. 20). In the first case, both the enriched zone and exotic Cu deposit are likely to show transitional relationships and to be at least partially preserved (e.g., Huinquintipa, Mina Sur, Damiana; Fig. 20a). In the second case, however, it is possible for the porphyry Cu source to be uplifted to the point that pyrite-poor potassic alteration is exposed, thereby deactivating both the enrichment process and exotic Cu formation (Fig. 20b). Hence, the enriched zone that corresponds to an observed exotic Cu deposit may have been lost to erosion (e.g., El Tesoro; Fig. 20b). In northernmost Chile and southern Peru, correlations have been made between supergene profiles and mapped or extrapolated pediplain surfaces of regional extent that developed during the late Oligocene or early Miocene: the Altos de Camilaca (Tosdal et al., 1984; Clark et al., 1990) and Choja surfaces (Galli-Oliver, 1967; Mortimer and Saricˇ, 1975), respectively. Between approximately latitudes 26º and 28ºS, the extensive Atacama pediplain, probably middle Miocene in age, variably truncated existing supergene profiles; however, because of its late timing vis à vis intensifying aridity, it failed to initiate another major supergene cycle (Clark et al., 1967a, b; Sillitoe et al., 1968; Mortimer, 1973), although sooty chalcocite did develop beneath it locally (e.g., Andacollo; Fig. 1). Rounded, pediment-flanked hills and mountains grading to gravel-filled depressions typify the most prolifically mineralized part of northern Chile, between latitudes 23º and 26ºS (Fig. 9b). To date, however, regionally extensive pediplains have proved difficult to define, and any correlation between planation surfaces and supergene chronology remains elusive. Although gravel and ignimbrite deposition interrupted supergene activity in several porphyry Cu deposits in the

753

754

RICHARD H. SILLITOE

LATE OLIGOCENEEAR LY MIOCENE

MID-MIOCENE- PRESEN T

a. PALEOSURF ACE K

K

BEDROCK

K

K

K K K

K

K K PORPHY RY COPPER DEPOSIT

b.

K K

K

K

K

K

K

K

1km

K K K

1km

0

Hematitic leached capping Oxidized zone

Upper gravel sequence

Basal gravel sequence Exotic copper mineralization spanning Reverse Enriched zone basal gravel-bedrock contact fault K Potassic alteration zone FIG. 20. Two end-member situations for exotic Cu deposit in northern Chile. a. Hematitic leached capping and underlying enriched zone in the porphyry Cu source are preserved alongside the exotic Cu deposit, although the actual connection between the two may have been removed by erosion in some cases. b. Hematitic leached capping and underlying enriched zone to which the exotic Cu deposit is directly linked have been eroded due to steady or staged uplift on an intervening reverse fault, leading to exposure of a deeper potassic alteration zone and its oxidation in situ. Once potassic alteration is subjected to oxidation, lateral Cu transport ceases because of lack of pyrite-generated acidity. In (a) the exotic Cu deposit is typically preserved beneath only thin postmineral gravel (±ignimbrite) cover, whereas in (b) a thin postmineral gravel sequence may conceal the porphyry Cu source and substantial thicknesses of postmineral gravel (±ignimbrite) typically accumulated on the downthrown side of the fault.

central Andes, the apparent cessation of all significant supergene activity in northern Chile is attributable to deficiency of vadose water caused by climatic desiccation from ~14 Ma onward (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Bouzari and Clark, 2002). Widespread preservation of mixed oxide-sulfide zones (mixtos) is interpreted as a direct result of this fossilization process. Some combination of two main factors is generally accepted as the mechanism for the increased aridity (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; and references therein): (1) intensification and/or cooling of a north-flowing ancestral Humboldt Current along the Pacific coast of South America, perhaps linked to expansion of the Antarctic icecap, leading to upwelling of colder water; and (2) Andean uplift to create a topographic barrier to moisture from the tropical Amazon lowlands, producing a rainshadow along the Pacific coast. Support for this scenario is provided by: (1) prolonged cooling of surface coastal waters off South America between 15 and 12 Ma based on studies of planktonic foraminifera (Tsuchi, 2002); and (2) an average elevation of approximately 2,500 m a.s.l. for the Western Cordillera by 14 Ma (Hartley, 2003). At Chuquicamata, where antlerite is later than most of the chalcanthite (Jarrell, 1944), supergene alunite from an antlerite veinlet was dated at 18.1 Ma 0361-0128/98/000/000-00 $6.00

(Sillitoe and McKee, 1996), a relationship clearly demonstrating that highly arid conditions appropriate for both formation and preservation of water-soluble minerals existed in the early Miocene just prior to shutdown of supergene activity. Compilation of the youngest supergene alunite and jarosite ages available for Cu and Au-Ag deposits and prospects in the central Andes shows that the cessation of supergene activity migrated systematically eastward from a zone delimited by the 14 Ma isochron (Fig. 19). This ≥14 Ma zone coincides almost precisely with both the extent of present-day hyperaridity and the zone characterized by Cu hydroxychlorides (Figs. 13, 19) that, like the soluble sulfates, must also be pre 14 Ma in age. As described previously, supergene activity probably remains active north, south, and east of the approximately positioned 5 Ma isochron (Figs. 16, 19). Limited supergene activity may also occur intermittently farther west (e.g., Rosario) where precipitation is enhanced (>~200 mm/yr) by high elevation (>4,000 m). A contrary argument, based on limited stratigraphic evidence, purports to show that supergene activity in northern Chile did not cease at ~14 Ma for reasons of hyperaridity (Hartley and Chong, 2002) but is considered implausible because it cannot account satisfactorily for the observations

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SUPERGENE PORPHYRY COPPER AND RELATED DEPOSITS

summarized above. Hartley and Chong (2002) tried to explain the lack of supergene alunite ages younger than 14 Ma in northern Chile as a result of fault-related uplift, a mechanism that should have accelerated not deactivated supergene activity if water infiltration had been adequate. Therefore, while the climate in northern Chile may have become drier about 3 to 4 m.y. ago (Hartley and Chong, 2002; Allmendinger et al., 2005), this desiccation event was not responsible for shutting down supergene activity but merely perpetuated its inactivity. Indeed, even Quaternary climate in the Atacama Desert, not to mention that during the last 14 m.y., was marked by appreciable precipitation fluctuations (e.g., Geyh et al., 1999; Bobst et al., 2001; Latorre et al., 2002) but was still too dry to permit widespread reactivation of supergene processes. Incision of deep, east-west–oriented canyons in northern Chile and southern Peru took place since at least the end of the Miocene during uplift of the Altiplano-Puna block to the east (Clark et al., 1967b; Mortimer, 1973, 1980; Mortimer and Saricˇ, 1975; Tosdal et al., 1984). Supergene profiles over deposits cut by these transverse valleys (e.g., Mocha) were largely destroyed, as were parts of some exotic Cu deposits (e.g., Sagasca, Huinquintipa; Fam, 1979). The water for canyon formation came from the high Andes and does not imply substantially increased precipitation on the arid western slope of the Andes (Mortimer and Saricˇ, 1975; Mortimer, 1980). Canyon incision between latitudes 18º and 22ºS may be the result of sapping by easterly derived ground water that emerged on the arid Andean western slope (Hoke et al., 2004). These canyons abruptly truncate a relict, parallel-patterned drainage network older than at least 10 Ma that formed when precipitation was higher on the western slope (Hoke et al., 2004). A series of thin felsic ash-flow and fall horizons ranging from ~10 to 1 Ma in age are found within 1 to 3 m of the present surface over a wide area between latitudes 23º and 26ºS (Marinovic and Lahsen, 1984). These ash layers have been radiometrically dated at numerous localities, including Radomiro Tomic (9.7 ± 0.7 Ma; Cuadra et al., 1997), Toki (9.6 Ma; Rivera et al., 2003a), Esperanza (10.6 ± 1.2 Ma; Perelló et al., 2004a), and Escondida (8.7 ± 0.4, 6.5 ± 0.2, 4.2 ± 0.2 Ma; Alpers and Brimhall, 1988). Their preservation bears testimony to extremely slow erosion rates, estimated as <10 m/m.y. (Scholl et al., 1970; Alpers and Brimhall, 1988), beyond the main transverse valleys. Such slow erosion since at least 10 Ma further refutes the claim that highly arid conditions have only prevailed for the last 3 to 4 m.y. (Hartley and Chong, 2002). Southwestern North America The history of supergene oxidation and enrichment in southwestern North America spans broadly the same time interval as that in the central Andes (Fig. 15) but is inherently more complicated because of the region’s distinctive tectonic development. The main episode of porphyry Cu emplacement in southwestern North America, essentially southern Arizona, western New Mexico, and northern Sonora, coincides with an interval of subduction-related contractional tectonism during the Laramide orogeny (75–52 Ma; e.g., Heidrick and Titley, 1982). The compression caused crustal thickening (50–60 km) and enhanced surface uplift (2,500–3,000 m asl; Coney and Harms, 1984; Wolfe et al., 0361-0128/98/000/000-00 $6.00

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1997; Dilek and Moores, 1999; Davis et al., 2004), conditions that led to erosional unroofing of many of the porphyry Cu deposits and the initiation of supergene oxidation and enrichment. As in the central Andes, supergene activity broadly correlates with periods of accumulation of major subaerial sedimentary sequences, which were responses to regional-scale erosional events triggered by tectonism and concomitant adjustments in base level. Scarborough (1989) defined three main fluviolacustrine sedimentary assemblages in which piedmont gravels (fanglomerates) are a particularly prominent component. In southern Arizona, the Whitetail assemblage is late Eocene to early Oligocene in age, the San Manual assemblage is early to middle Miocene in age, and the Gila assemblage is late Miocene to Pliocene in age (Fig. 21). The Whitetail and San Manuel assemblages are separated by a thick ignimbrite pile of predominantly Oligocene age (Fig. 21; Scarborough, 1989), which concealed most supergene profiles active at the time (Livingston et al., 1968; Cook, 1994). The Whitetail was a product of dissection of the Laramide orogen. The Oligocene volcanic rocks and San Manuel assemblage accumulated during extreme crustal extension (orogenic collapse), which resulted in low-angle detachment faulting and exhumation of metamorphic core complexes (30–12 Ma; Dickinson, 2002). The Gila assemblage accompanied high-angle Basin and Range style extensional block faulting, deep basin subsidence, and sporadic basaltic volcanism. Pedimentation was widespread during mid-Gila times (9–4 Ma), signifying an interval of relative tectonic quiescence (Scarborough, 1989). Although the San Manuel and Gila assemblages accompanied large-scale (~100%) extension rather than compression, erosion and consequent sedimentation were triggered by localized flexural isostatic uplift of the footwalls of large-displacement normal faults (King and Ellis, 1990; Davis et al., 2004). During both the detachment and Basin and Range extensional faulting, base levels must have changed in a complex manner, especially during the former when some fault blocks were variably tilted, extended, and detached for distances as great as several tens of kilometers. At least 40 percent of porphyry Cu deposits in southwestern North America became tectonically reoriented, and a few dismembered, during the mid-Tertiary (Wilkins and Heidrick, 1995). Supergene profiles were clearly developed in southwestern North America during the late Eocene and early Oligocene, both before and during Whitetail sedimentation (Livingston et al., 1968; Cook, 1994). Supergene alunite from surface at the Sacaton deposit was dated at 41.0 ± 1.1 Ma (Cook, 1994; Fig. 21), and supergene profiles are concealed beneath Whitetail assemblage rocks at Resolution (Manske and Paul, 2002) and, although profoundly tilted, also at Ajo (Gilluly, 1946). Clasts of leached capping or chalcocite-enriched material were also observed in fanglomerate beneath Oligocene volcanic rocks at Chino (Cook, 1994), Silver Bell (Richard and Courtright, 1966), La Caridad (Saegart et al., 1974), and elsewhere. Oligocene volcanic sequences, as old as 33.4 Ma at Chino (Cook, 1994), cap broadly Whitetail-age supergene profiles (Fig. 21). Existence of appreciable thicknesses and volumes of exotic native Cu mineralization in Whitetail fanglomerate in the Superior East area (Sell, 1995), 3.5 km

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0

Gila assemblage 10

Core complexes+ detachment faulting

Sa ass n Ma em nue bla l ge 20

Vo lc roc anic ks

Crustal extension

Ma

Basin and Range faulting

Ajo Sacaton Silver Bell San Xavier North Red Mountain Bisbee Safford Morenci Chino Tyrone

northeast of Resolution, confirms overlap of supergene activity and early Whitetail sedimentation. Alunite and fewer jarosite ages falling into the latest Oligocene to early Miocene (25.4–16.2 Ma; 4 deposits) and middle Miocene to Pliocene (13–2.6 Ma; 7 deposits) intervals were obtained by Cook (1994) and, for Morenci, Enders (2000), thereby showing that supergene activity overlapped with Oligocene to Miocene volcanism and San Manuel and Gila sedimentation so long as the deposits concerned did not become permanently and deeply buried beneath these postmineral products (Fig. 21). This 40-m.y. span of alunite ages from the southwestern North American porphyry Cu province may be interpreted in terms of at least three discrete supergene events (Cook, 1994) or, perhaps more probably, as a semicontinuous record of supergene modification that was periodically revitalized by profound base-level changes and interrupted (and preserved) during concealment beneath volcanic or sedimentary units (Fig. 15). Interestingly, whichever interpretation is preferred, the Eocene and early Oligocene supergene activity took place under warm, humid, subtropical conditions, which gave way to a cooler, semiarid climate from the Oligocene onward (Scarborough, 1989). Cook (1994) and others proposed that drainage incision during the last 2 m.y. or so was not accompanied by appreciable chalcocite enrichment, although the occurrence of supergene alunite dated at <1 Ma (Enders, 2000), an exotic Cu deposit <7,000 years old at Ray (Phillips et al., 1971), and widespread ephemeral chalcanthite efflorescence on pit walls throughout the region suggest otherwise (cf., Enders, 2000).

30

W ass hite em tail bla ge

40

Elsewhere in western North America The timing of supergene oxidation and enrichment in southwestern North America overlaps the age of deep supergene oxidation in Nevada (Fig. 15 and references therein). This is based on the numerous supergene alunite ages for sediment-hosted Au deposits in the Carlin trend (30.0–8.6 Ma), Getchell trend (23.0–11.3 Ma), and at Alligator Ridge (12.4–3.6 Ma), and on those for epithermal Au deposits at Goldfield (11.6–9.2 Ma), Round Mountain (16.1–9.5 Ma), and Paradise Peak (10.0 Ma). As noted above, isolated supergene profiles in British Columbia and the Yukon escaped glacial erosion and may have developed during the Late Cretaceous and early Tertiary.

Mogollon highlands

50

Basement rocks affected by Laramide orogeny 60

Laramide porphyry copper formation

Economic Factors 70

FIG. 21. Ages of supergene activity at selected porphyry Cu deposits of Laramide age (defined by height of box) in southern Arizona and southwestern New Mexico in the context of fluviolacustrine sedimentary assemblages and volcanic activity defined by Scarborough (1989) and tectonic styles synthesized by Dickinson (2002). Note three supergene events (Cook, 1994) or a single 40-m.y. event, depending on preferred interpretation. Supergene activity predates and spans mid- to late Tertiary extension. The KAr ages are taken from Cook (1994), 40Ar/39Ar ages from Enders (2000), and approximate volcanic ages from text. Legend as in Figure 17, except for arrows, which represent minimum supergene ages based on dating of overlying volcanic rocks. 0361-0128/98/000/000-00 $6.00

Oxidized and enriched porphyry Cu deposits The effects of supergene oxidation and enrichment are generally beneficial from a commercial standpoint because they may generate Cu ores amenable to heap leaching and SX-EW processing. Furthermore, in the case of enriched zones and derivative oxidized zones, Cu grades may be increased by factors of 2 to 3 or even more. Higher Cu grades normally translate into higher concentrate grades, thereby rendering the mill product more readily saleable. Moreover, supergene Cu ore is typically softer than its hypogene equivalent and so has lower work indices and requires less crushing and grinding, which may permit increased mill throughputs. Supergene Cu ore is also somewhat easier to mine, not only because of its reduced hardness but also because of the

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fact that anhydrite and gypsum have been leached. Hypogene ore from beneath the sulfate front, unlike that in oxidized and enriched zones, is sealed by anhydrite and gypsum, making it more costly to break and extract from open pits and more difficult to block cave underground (e.g., Kvapil et al., 1989). Any calcite and other carbonate minerals are also likely to have been leached by acidic supergene solutions during mature oxidation and enrichment, thereby minimizing acid consumption at the heap-leaching stage. Oxidation and enrichment may, however, prove disadvantageous if flotation is the chosen beneficiation route, especially where supergene profiles are immaturely developed. Thin rinds or fracture coatings of replacive chalcocite on pyrite grains may be incompletely released during grinding, and hence during ensuing flotation report along with the pyrite to the tails, with a concomitant loss of Cu (e.g., Ney et al., 1976; Satchwell, 1983). Reduction in flotation recovery may also result from small amounts of nonrecoverable oxide Cu minerals occurring as a partial overprint to enriched ore. This problem is exacerbated in the case of mixed oxide-sulfide ores (mixtos), which may be treated by either flotation or acid leaching, depending on which mineralogic type predominates but nearly always with a negative impact on Cu recoveries. While Cu extraction by sulfuric acid or ferric sulfate leaching followed by the SX-EW process is the cheapest means of producing high-grade Cu metal, recoveries are highly dependent on oxide or sulfide mineralogy. Ores rich in hydroxysulfates can leach twice as fast as a chrysocolla-dominated ore and with only half the acid consumption, especially if acidgenerating hydrous sulfates (e.g., chalcanthite) are also present. Hydroxycarbonate-rich ores obviously consume a little more acid than those composed mainly of hydroxysulfates or hydroxychlorides, although the Cl content of the latter must be removed to facilitate efficient electrowinning. Nitrate minerals are also problematic for the SX-EW process. Commonly, the black oxide Cu minerals, especially Cu wad, Cu pitch, and neotocite (Table 6), along with the Cu clays, have much less favorable leach kinetics and Cu recoveries (e.g., Brimhall et al., 2001; Aguilar et al., 2003). Locally, these ores result in Cu recoveries of considerably <50 percent unless grades are high enough for agitation leaching to be employed. Chrysocolla is characterized by variable leaching rates, depending on its degree of fracturing. Other supergene Cu oxide minerals, like native Cu and cuprite, are almost entirely insoluble (refractory) under heap-leaching conditions, although the chalcotrichite variety of cuprite is a notable exception. Native Cu also smears during grinding but can be readily recovered in a gravity circuit (e.g., Afton; Carr and Reed, 1976). On the basis of these observations, it is clear that the difference between total and acid-soluble Cu assays of oxide Cu ores is not only indicative of remanent chalcopyrite and bornite contents. Although chalcocite replacements of pyrite grains may cause problems during flotation, such material is optimal for heap leaching. Where other sulfides, such as chalcopyrite, constitute the cores of chalcocite grains, the hypogene Cu component is poorly recovered by leaching, leading to low overall Cu recoveries. Such material may be treated by flotation if Cu grades are high enough. The reduction in the work indices of oxidized and enriched ores is accompanied by development of clays, which may 0361-0128/98/000/000-00 $6.00

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impede the flotation of chalcocite ores besides occluding permeability and reducing hydraulic conductivity in heaps (stockpiles) during leaching. Smectite in abundance is particularly deleterious because of its swelling nature, which not only results in clogging of flow paths in the heaps but also increases acid consumption (e.g., Aceituno, 2000). Smectite, along with biotite and retrograde chlorite, are commonplace in potassic alteration zones and may also display preg-robbing or preg-borrowing characteristics by sorbing up to several percent Cu from leach solutions (Baum, 1996). Kaolinite is typically less problematic, although where abundant it may lead to generation of fines capable of compromising heap permeability. The obvious importance of the mineralogy of oxidized and enriched ores from the standpoints of both mining and metallurgy implies that detailed mineralogic (geometallurgical) mapping is a fundamental part of all fairly advanced exploration and delineation programs. In the case of oxidized zones, in particular, subdivision into mineralogically based ore types (e.g., green oxides, black oxides, Cu clays, Cu-bearing limonites), followed by column testing to determine the leachability of each individually, is essential. Degree of enrichment, the dominant supergene sulfide mineral (e.g., chalcocite group vs. covellite), and the nature of remanent hypogene sulfide minerals are key factors for classification of enriched ores. Gold-rich porphyry Cu deposits While it is clear that the overall economic potential of Aurich porphyry Cu deposits is based on the combined value of the two metals, it may also be profoundly influenced by the interplay between alteration type and degree of supergene oxidation. Where oxidation is well developed, as is commonly the case in the central Andes and western United States, pyrite-poor hypogene sulfide assemblages in potassic alteration zones tend to oxidize in situ. The resulting oxide Cu minerals may be recovered by sulfuric acid leaching, but the Au would normally be lost. Conversely, the oxide Cu content would preclude effective Au recovery by cyanidation. Therefore, where Au-rich porphyry Cu deposits are dominated by potassic alteration, as they normally are (e.g., Sillitoe, 2000), limited oxidation is advantageous, implying that systems in tropical environments (southwestern Pacific region, Southeast Asia, northern Andes, Central America) and glaciated regions (British Columbia, Alaska, southern Andes) may be economically the most attractive. Nevertheless, even there, problems may result because of admixture of oxide Cu minerals and Au in shallow ore zones to be mined first. Earlystage exploitation of Au from the leached capping at Ok Tedi was less than successful because remanent oxide Cu minerals caused serious problems during cyanidation (Rush and Seegers, 1990). In the event that pyrite-rich sericitic or advanced argillic alteration is widely developed, however, oxidation may induce nearly total Cu leaching, and if Au contents are high enough (say, >0.8 g/t), result in Au-only ore suitable for cyanidation. The leached Cu would normally accumulate as an enriched zone, from which both Cu and Au could be extracted by flotation, as at Ok Tedi (Rush and Seegers, 1990).

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Epithermal Au deposits Most high-sulfidation epithermal Au deposits, as well as a minority of intermediate- and low-sulfidation deposits (e.g., Round Moutain, Kori Kollo), are to some degree refractory to cyanidation when in their nonoxidized states. The Au is encapsulated in enargite or pyrite to produce ores that need roasting, autoclaving, or bioleaching to give acceptable Au recoveries. Thus, where Au contents are too low (say, <2 g/t) for these alternative treatment methods to be commercially viable, as in many bulk-tonnage deposits in the central Andes and elsewhere, supergene oxidation is mandatory (e.g., Sillitoe, 1999). The oxidation transforms the sulfide minerals to limonite, within which much of the Au then resides in easily leachable form. The oxidation process also augments rock permeability, an aid to heap leaching, by breaking down semimassive sulfides in high-sulfidation deposits and leaching any carbonate in the intermediate- and low-sulfidation deposits. A corollary of the importance of supergene oxidation of these Au deposit types is that exploration for them should be focused on regions where supergene profiles are well developed. Clearly, the central Andes and western North America are the prime targets, whereas the western Pacific region in general is less promising unless high-grade deposits are sought. Exploration Guidelines Outcropping mineralization Appraisal of leached cappings has been the principal means of exploring porphyry Cu prospects for enrichment blankets since the early decades of the last century (e.g., Locke, 1926). Lowell (1991) proposed that capillary action caused surficial removal or superleaching of limonite from sulfide boxworks in the hyperarid parts of northern Chile, thereby complicating leached-capping interpretation, but the process and its inferred effects lack solid supporting evidence. The three dominant limonite components define three end-member leached capping types (Locke, 1926; Anderson, 1982): 1. Goethite-rich limonite is typically indigenous and forms in low-acid leached cappings in which widespread Cu fixation takes place (Fig. 9a). Goethitic cappings retain much of the original hypogene Cu content in oxidized form and are unlikely to be underlain by appreciable chalcocite enrichment. 2. Indigenous hematite-rich limonite, with a characteristic deep-maroon color and fluffy appearance (live or relief limonite), implies the former presence of mature chalcocite enrichment containing some residual pyrite (Fig. 9b-d). Hence, beneath the leached capping, a mature enriched zone or, economically even more attractive, an oxidized zone rich in Cu hydroxysulfates and water-soluble Cu sulfates, may be anticipated. Confusion with either transported hematite produced by dehydration of other limonite minerals, either by prolonged solar exposure under low-humidity conditions or blanketing by ignimbrite (e.g., possibly at Cuajone) or in situ hematite generated by magnetite oxidation needs to be avoided. 3. Jarosite-rich limonite, most of it transported, is indicative of pyrite-rich protoliths (Fig. 9e), even where present in epithermal systems. The limonite distribution (indigenous vs. 0361-0128/98/000/000-00 $6.00

transported), texture, and composition (determined in the field by means of streak color since the mineral color alone is an unreliable indicator) provide a valuable guide to the nature of underlying sulfide mineralization, when combined with supergene and hypogene alteration features, rock geochemistry, fracture and veinlet intensities, and relict-sulfide determinations. Anderson (1982) emphasized two leached-capping situations that might not be easily appreciated. First, goethitic leached cappings in which much of the Cu occurs as black oxide Cu minerals, especially neotocite, which might be underestimated because of failure to recognize the true nature of the black mineral(s); they might be mistaken for Cu-free Mn oxides, for example. Second, jarositic leached cappings need not necessarily cap pyrite-only zones but may be derived from mineralization containing appreciable chalcopyrite, albeit with high pyrite/chalcopyrite ratios. If this is the case, then economically interesting chalcocite enrichment may underlie the leached capping. The latter situation is more commonly encountered in systems other than those of porphyry Cu type, in particular high-sulfidation epithermal deposits, as exemplified by Yanacocha, Peru (Harvey et al., 1999). A variant on the latter situation is development of jarositic leached capping at the expense of pyrite-chalcopyrite mineralization during abrupt descent of the paleowater table, combined with complete erosion of formerly overlying hematitic leached capping developed from a preexisting enriched zone. In such situations, the contribution made to the new enriched zone developed beneath the jarositic leached capping by oxidation of the older, higher one may go unsuspected (Anderson, 1982). Study of relict sulfides, which escaped both enrichment and oxidation because of encapsulation by veinlet quartz, assists with prediction of former hypogene Cu contents by showing both the abundance and nature of the sulfides (e.g., Gustafson and Hunt, 1975; Hunt et al., 1983). If sulfides are shown to be relatively abundant and to include appreciable pyrite plus chalcopyrite, bornite, enargite, or higher sulfidation species, preferably with an optimal pyrite/chalcopyrite ratio of approximately 5:1, significant enrichment may be anticipated beneath the leached capping. It needs to be stressed, however, that relict sulfides may preferentially record early relatively low-sulfidation states rather than late high-sulfidation hypogene assemblages (see above), because the latter tend to be less fully encapsulated and, hence, more readily leached (e.g., Alpers and Brimhall, 1989). Nevertheless, high-sulfidation assemblages, important because they commonly localize high-grade enrichment, were detected during relict-sulfide studies at Escondida and elsewhere (J. Perelló, pers. commun., 2004). The geochemical response of a leached capping correlates closely with the dominant limonite type. Goethitic leached cappings, with their contained oxide Cu minerals, may report geochemical Cu values approaching those of underlying hypogene mineralization, although in the central Andes surficial leaching typically lowers outcropping Cu contents substantially and may even remove most visible Cu minerals to depths of 1 or 2 m. Hematitic and jarositic leached cappings are characterized by efficient Cu removal and even the

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former may have as little as 200 ppm Cu, albeit appreciable retained Mo (>25 ppm) in outcrop (e.g., Lowell, 1991). Concealed deposits Search for concealed oxidized and enriched porphyry Cu deposits dominates most green- and brownfields exploration programs in the central Andes and southwestern North America, with the cover mainly comprising postmineral piedmont gravels and/or volcanic rocks, especially ignimbrite flows. The best targeting technique is believed to be geologic and geochemical vectoring from outcropping bedrock in proximity to the covered areas (Sillitoe and Perelló, 2005), although selected geophysical methods, especially magnetics and induced polarization, may also add value once subsurface data have been furnished by initial drilling. Detection of subtle supergene geochemical anomalies propagated upward through the cover sequences from buried mineralization using partial extraction geochemistry has been claimed (e.g., Cameron et al., 2002; Kelley et al., 2003), but so far exploration results have been disappointing. However, in southwestern North America, where vegetation is widespread, biogeochemistry, especially using twigs from deep-rooted phreatophytes, has proved effective where the cover is relatively shallow (<45 m; e.g., Bloom, 1966). Existence of supergene profiles beneath postmineral cover cannot be taken for granted and, indeed, in parts of northern Chile only a few meters of oxidation underlie some gravel sequences that are >200 m thick. This situation may be anticipated because supergene activity broadly spanned the protracted interval of gravel accumulation (Sillitoe and McKee, 1996; see above). If thicker gravel sequences are taken as proxies for longer intervals of gravel accumulation, a not unreasonable assumption, then the time available for supergene oxidation and enrichment to take place becomes proportionately less as gravels become thicker. Clearly, efforts to determine gravel thicknesses are an important component of supergene Cu exploration in the central Andes, bearing in mind that depths to bedrock in some structurally depressed fault blocks exceed 1,000 m. In view of the greater postmineral structural complexity in southwestern North America, the search for gravel-concealed supergene ore is likely to be correspondingly more difficult, as exemplified by the discovery of ‘blind’ hypogene mineralization at Superior East (Sell, 1995) compared to a 600-m supergene profile at Santa Cruz (Kreis, 1995). Deposition of ignimbrite or lava flows is a geologically instantaneous process so the thickness of volcanic cover does not have the same implications as gravel cover, although concealment beneath ignimbrite is of course equally effective in curtailing supergene activity. Role of exotic Cu deposits During supergene Cu exploration in the central Andes, exotic deposits may be used as pathfinders to concealed porphyry Cu enrichment blankets or may be considered as oxide Cu targets in their own right. Several of the best-known exotic deposits in northern Chile (Sagasca, El Tesoro) still lack clearcut or, at least, publicized hypogene sources for their contained Cu and thus present exploration challenges. In contrast, other exotic deposits are known to lie alongside their enriched porphyry Cu sources, either partially outcropping 0361-0128/98/000/000-00 $6.00

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(Huinquintipa at Rosario) or entirely concealed by younger piedmont gravels (e.g., Mina Sur at Chuquicamata, Vicky at Gaby, Damiana at El Salvador; Fig. 1; Table 2)). These concealed deposits were discovered either serendipitously (Mortimer et al., 1977) or conceptually (Sillitoe, 1995b): basically the same options available when trying to locate the unknown porphyry Cu sources for exotic Cu mineralization. As explained previously, however, the enriched zone responsible for provision of the exotic Cu is not necessarily preserved although its potassic roots, potentially containing oxide Cu mineralization, are likely to remain (Fig. 20). Wherever piedmont gravel accumulations flank outcrops of porphyry Cu systems in northern Chile (and potentially elsewhere), current drainage configurations and hydrologic gradients provide valuable pointers to the physiographic settings extant during pre-middle Miocene supergene activity and, hence, may help to predict areas with potential for exotic Cu mineralization. Peripheral parts of enriched zones characterized by Cu sulfide replacement of pyrite alone (exotic chalcocite) denote a lateral component of Cu transport and may also help to predict downslope sites favorable for exotic Cu mineralization. Similarly, ferricrete accumulations record zones where acidic supergene solutions seeped from porphyry Cu systems preparatory to exotic Cu precipitation in more distal downstream positions (e.g., Münchmeyer, 1996; Mote et al., 2001b). Mass-balance modeling of supergene Cu systems is able to define parts of supergene profiles from which Cu has been lost, potentially laterally to generate exotic Cu mineralization (see above). However, the entire supergene profile must be extremely well documented in terms of Cu content and bulk-rock density, in addition to relict sulfide studies of the leached capping, if meaningful models are to be constructed (e.g., El Salvador; Mote et al., 2001a). Exploration for exotic Cu in southwestern North America might be assigned somewhat lower priority than in the central Andes given the lack of known economic examples. Nevertheless, apparently large tonnages of fairly high grade exotic native Cu mineralization accumulated in a thick package of piedmont gravel in the vicinity of the Superior East porphyry Cu center (Sell, 1995) and elsewhere. Moreover, even minor exotic Cu occurrences may prove valuable in the search for enriched porphyry Cu deposits under cover if the structural and stratigraphic framework can be properly deciphered. Conclusions This review affirms that supergene oxidation and enrichment are economically pivotal in the creation of the world’s two greatest Cu provinces, the central Andes and southwestern North America. The supergene histories of both provinces span 40 m.y., and except in the hyperarid parts of northern Chile, are ongoing. Clearly, this protracted event merits metallogenic status (Bouzari and Clark, 2002). The physicochemical processes and mechanisms responsible for the vertical and lateral flux of Cu and other elements in supergene systems are now well understood as a result of intensive research during the early decades of the last century and, during the last 10 to 15 years, as an adjunct to heap and dump leaching of supergene Cu ores and efforts to ameliorate acid mine drainage. The mediating role of bacteria in sulfide oxidation is the most novel addition to understanding of

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supergene processes, although much additional work is required to elucidate the nature of bacterial involvement in chalcocite enrichment and, possibly, even in exotic Cu deposition. Arguably the greatest recent advance in the understanding of supergene activity is the broader appreciation of its close linkage with the tectonic and climatic evolution of orogenic belts. Supergene processes are viable across a broad spectrum of climatic regimes, as presaged by Titley (1975), so long as surface uplift is active. Nevertheless, when denudation rate is taken into account, mature oxidation and cumulative enrichment are generally confined to relatively arid environments. The complex local and regional controls exerted by contractional tectonism on supergene profile development, including exotic Cu accumulation, are only now being recognized in the central Andes and must be equally applicable to the evolving extensional regime that characterized much of the supergene history of southwestern North America. In these regions, marked differences in uplift rates and timing between adjoining fault-bounded blocks appear to be a fundamental control on the type and maturity of supergene profiles. This linkage between surface uplift and concomitant exhumation and supergene activity emphasizes the overall synchroneity between profile development and piedmont gravel accumulation. A direct corollary of the tectonoclimatic controls of supergene processes is the provision of information of direct relevance to the Cenozoic tectonic and climatic evolution of Cu provinces. Differently developed supergene profiles in adjoining structural blocks may help to constrain the sense and timing of fault displacement, particularly if supergene activity responsible for both profiles can be dated using alunite, jarosite, or K-bearing Mn oxyhydrates. The existence of widespread and abundant hydroxychlorides throughout the oxidized zones of Cu deposits in northern Chile, accompanied in places by water-soluble hydrated sulfates, demonstrates that exceptionally arid conditions prevailed during at least the final stages of supergene activity, for at least a few million years prior to fossilization of profiles at about 14 Ma. The fact that the zone that underwent cessation of supergene activity at 14 Ma is virtually coincident with the extent of current hyperaridity provides strong evidence that the overall distribution and fundamental controls of climatic zones in the central Andes have remained essentially unchanged for at least the last 14 m.y. and probably longer. Further work on supergene profiles in the central Andes and elsewhere holds the promise of more tectonic and climatic revelations as well as further insights relevant to exploration. Acknowledgments The pioneers who formulated the basic tenets of supergene oxidation and enrichment nearly a century ago are widely cited in this review and deserve special recognition. My own introduction to supergene Cu deposits in northern Chile during the mid-1960s owes much to the late Sydney Hollingworth of University College London, England, and benefited greatly from cooperation at that time with colleagues Alan Clark, Ron Cooke, and Cedric Mortimer. Subsequent prolonged exposure to supergene processes and their products worldwide has broadened and changed my perspective, not 0361-0128/98/000/000-00 $6.00

least by sharpening an appreciation of the economic issues involved. Enrichment, however, is still awaited! Numerous company geologists have shared their knowledge of individual supergene Cu deposits for which I am deeply grateful. Recent fieldwork and discussion with Pepe Perelló proved especially illuminating. Provision of tonnage-grade data by Alejandro Contreras, Vicente Irarrazaval, Graeme Lyall, Mario Orrego, Cristián Monroy, Joseph Salas, Juan Carlos Toro, and Bill Williams is gratefully acknowledged. Comments on the manuscript by Pepe Perelló and Gordon Southam, reviewers John Dilles and Steve Enders, and editors Jeff Hedenquist and John Thompson improved the final product, although its content is my responsibility alone. REFERENCES Abele, G., 1989, The interdependence of elevation, relief, and climate on the western slope of the central Andes: Zentralblatt für Geologie und Paläontologie, v. 5/6, p. 1127–1139. Aceituno, J., 2000, Caracterización geológica de la mineralización del yacimiento Zaldívar y su implicancia en el proceso metalúrgico: Congreso Geológico Chileno, 9th, Puerto Varas, 2000, Actas, v. 1, p. 142–146. Ague, J.J., and Brimhall, G.H, 1989, Geochemical modeling of steady state fluid flow and chemical reaction during supergene enrichment of porphyry copper deposits: ECONOMIC GEOLOGY, v. 84, p. 506–528. Aguilar, A., Gomez, M., and Pérez, P., 2003, Discovery and geology of the undeveloped Codelco’s Gaby copper deposit [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, CD-ROM, 1 p. Allmendinger, R.W., 1986, Tectonic development, southeastern border of the Puna plateau, northwest Argentine Andes: Geological Society of America Bulletin, v. 97, p. 1070–1082. Allmendinger, R.W., González, G., Yu, J., Hoke, G., and Isacks, B., 2005, Trench-parallel shortening in the northern Chilean forearc: Tectonic and climatic implications: Geological Society of America Bulletin, v. 117, p. 89–104. Alpers, C.N., and Brimhall, G.H, 1988, Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene mineralization at La Escondida: Geological Society of America Bulletin, v. 100, p. 1640–1656. –––1989, Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile: ECONOMIC GEOLOGY, v. 84, p. 229–255. Alvarez, O., Miranda, J., and Guzman, P., 1980, Geología del complejo Chuqicamata, in Minería de cobres porfídicos, v. 2: Santiago, Instituto de Ingenieros de Minas de Chile, Congreso Cincuentenario 1930–1980, p. 314–363. Ambrus, J., 1977, Geology of the El Abra porphyry copper deposit, Chile: ECONOMIC GEOLOGY, v. 72, p. 1062–1085. Anderson, C.A., 1955, Oxidation of copper sulfides and secondary sulfide enrichment: ECONOMIC GEOLOGY 50TH ANNIVERSARY VOLUME, pt. 1, p. 324–340. Anderson, J.A., 1982, Characteristics of leached capping and techniques of appraisal, in Titley, S.R., ed., Advances in geology of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona Press, p. 275–295. Aracena, I., Ossandón, G., and Zentilli, M., 1997, Mineralogía y distribución de zinc en Chuquicamata: Enriquecimiento supergénico del zinc?: Congreso Geológico Chileno, 8th, Antofagasta, Actas, v. 3, p. 1908–1912. Arcuri, T., and Brimhall, G., 2003, The chloride source for atacamite mineralization at the Radomiro Tomic porphyry copper deposit, northern Chile: ECONOMIC GEOLOGY, v. 98, p. 1667–1681. Arehart, G.B., and O’Neil, J.R., 1993, D/H ratios of supergene alunite as an indicator of paleoclimate in continental settings: American Geophysical Union, Geophysical Monograph 78, p. 277–284. Arehart, G.B., Kesler, S.E., O’Neil, J.R., and Foland, K.A., 1992, Evidence for the supergene origin of alunite in sediment-hosted micron gold deposits, Nevada: ECONOMIC GEOLOGY, v. 87, p. 263–270. Arehart, G.B., Chakurian, A.M., Tretbar, D.R., Christensen, J.N., McInnes, B.A., and Donelick, R.A., 2003, Evaluation of radioisotope dating of Carlin-type deposits in the Great Basin, western North America, and implications for deposit genesis: ECONOMIC GEOLOGY, v. 98, p. 235–248.

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