MAPPING AND STRUCTURAL GEOLOGY IN MINERAL EXPLORATION Where theory hits the fan
Rodney J Holcombe Principal Consultant, HCOV Global Adjunct Professor of Structural Geology The University of Queensland
Copyright © 2016 by Rodney J Holcombe
[email protected]
All Rights Reserved No major parts of this book, including Parts, chapters, and entire sections, may be reproduced or transmitted in any form or by any means, electronic or mechanical, including any information storage and retrieval system, without the express written permission of the publisher except for the use of brief quotations in a book review or scholarly journal ISBN 978-0-9954429-0-0 (Low resolution pdf ) ISBN 978-0-9954429-1-7 (High resolution pdf ) First Edition: 2016 HCOV Global PO Box 1618 Mudgeeraba, Queensland, 4213, Australia www.hcovglobal.com
Book reference: Holcombe, R.J., 2016. Mapping and structural geology in mineral exploration: where theory hits the fan. HCOV Global, 233p Copies of this book can be obtained from: www.holcombe.net.au/book/
Frontispiece: Top: Exposed fold hinge, Cape Liptrap, Victoria, Australia. Bottom: Array of 3D form surfaces to bedding constructed from dill core data from a gold prospect in the Cobar region, Australia
Contents
39 40 Declination gradient 41 Declination rate of change 41 Structural Measurement�������������������������������������� 42 Directions/bearings/trends 43 2-D data (bearings) 43 3-D data measurement 43 Measurement of planes 44 Indirect measurement of lines within a plane: 45 Why use pitch to define a line?45
Acknowledgementsviii
Locations and grids The Idea of North
Prefaceix Chapter 1
Review of Maps and Mapping
11
Introduction ��������������������������������������������������������� 11 What is a ‘good’ interpretation map? ������������������� 12 Mapping Campaigns������������������������������������������������� 14 Mapping scales 14 Map types 15
15 16 Where to start 16 Time allocation 16 How much to measure 16 Data confidence 18 Mapping precision 18 Contact interpretation 18 Types of contact 18 Discordant contacts in sedimentary rocks 19 Concordant contacts in sedimentary rocks 20 Concordant igneous contacts 21 Folded granitoid sills21 Reading Outcrops – the thought process���������� 22 Model vs Facts 22 Importance of map fidelity
Mapping process
Conversions of line pitches (rakes) to plunge and direction45
Map symbols and annotations 45 Notebook Entries 46 Storing Data: Databases����������������������������������������� 47 Spreadsheets as databases 47
47 48 Database design 49 Reading Stereonets������������������������������������������������ 49 Types of spherical projection and stereo nets 50 Geometric Calculations 51 Statistical Analysis 53 Classified Stereonets 53 Alphanumeric classified plots 53 Numerically-classified plots 54 Design problems
So what is a relational database?
Reading Outcrops and model synthesis – an example from the Isla Cristalina, Uruguay 24
25 29 Field ‘fact’ map 30 Compiled fact map 30 Observed Data Tables 31 Observed_unit_contacts table31 Observed_map_data: 32 Structural data file(s) tables: 32 Completed Fact Map (Outcrop Map 32 Compiled interpretation map 33 Structure of Interpretation tables 33 Example of a ‘good’ compiled fact map 34 Underground mine mapping ��������������������������������� 35 Open Pit Mapping��������������������������������������������������� 37 Creek Traverse
Chapter 3
Putting it all together: synthesizing a model
Chapter 2
Technical Basics
Bedding and Younging
57
Introduction���������������������������������������������������������� 57 Sedimentary structures and metamorphism�������� 57 Younging Criteria ������������������������������������������������� 58 Cross-lamination (cross-bedding) 59 Density flow (turbidite) structures 60
60 Sole markings 61 Load cast, flame, ball-and-pillow structures61 Tool markings, grooves, flute casts62 Convolute ripples and convolute bedding 62 Scours 62 Mudcracks and syneresis cracks 63 Other sedimentary younging structures 64 Younging structures in volcanics 64 Pillows 64 Syn-sedimentary structures���������������������������������� 65 Foundered and collapsed bedding 65 Graded beds and Bouma sequences
39
Introduction���������������������������������������������������������� 39 Geodetic Basics������������������������������������������������������� 39 v
CONTENTS
Debris flow (mass flow) folds Compaction structures
Chapter 4
Basic Structures
65 66
113 Final statement����������������������������������������������������� 116
Chapter 6
The Third Dimension
67
Introduction�������������������������������������������������������� 117 Contacts and Topography����������������������������������� 117 Projecting planar contacts across topography 117 Determination of the orientation of a plane from its topographic contact trace 119 Constructing structure contours in GIS packages 119 Folded contacts and topography 121 Pseudo-structure contour construction 121 Two dimensional views of 3D folds������������������� 123 Fold axial traces in maps 123 Fold Profiles 124 Cross-sections������������������������������������������������������ 125 Elementary cross-sections 125 Orientation of cross-sections. 126
Introduction���������������������������������������������������������� 67 Faults ����������������������������������������������������������������������� 67 Linked faults, relays, and duplexes 69 Fault displacement 70 Fault zones 70 Fault surface structures 71 Folds������������������������������������������������������������������������ 72 General terminology 72 3D shape of folds 73 Orientation-based nomenclature 74 Fold profile plane 75 Fold profile shape 75
75 75 Fold Symmetry 77 Fold Vergence 77 Fold Stacking: harmonic vs disharmonic 77 Axial Plane Foliation��������������������������������������������� 78 Types of foliation 78 Lineation����������������������������������������������������������������� 82 Basic structural fabric terminology 84 Structural Fabrics������������������������������������������������� 84 Fabric vs structural fabric 84 Deformation event vs generation of structures 87 Recognising multiple generations of structures 87 Fractures: Joints and veins,����������������������������������� 89 Joints 89 Structural Compatibility�������������������������������������� 91 Kinematically compatible linked structures 91 Local accommodation structures 92 Hinge-limb angularity
Layer shape around fold
Chapter 5
Working with breccia
117
126 127 129 130 Section construction by the kink method 131 Section construction by tangent-arc method 131 Constraining parallel folds in section 133 Freehand section construction in folded areas 133 Vertical exaggeration in cross sections 135 Faults and dykes in cross-sections 135 Shape of plutons in cross-sections 136 Drill sections������������������������������������������������������� 136 Example: drill sections and steep plunges 137 Drill sections and folds 139 Three dimensional modelling����������������������������� 140 General Comments 140 1. Direct triangulation modelling. 140 2. Serial section extrapolation 141 2. Implicit modelling. 141 3D structure form surfaces 142 Reclined folds: the extreme case
Projecting field data into the plane of section Down-plunge projection of outcrop data Cross-sections in horizontal folds
95
Introduction���������������������������������������������������������� 95 Breccia Classification�������������������������������������������� 96 Hydraulic Brecciation������������������������������������������� 98 Hydraulic fracturing 98 Hydraulic brecciation modes 99 Hydraulic diatremes 99 Hydraulic fluids 100 Hydraulic-tectonic breccia 100 Fault ‘mega-brecciation’��������������������������������������� 102 Case Histories������������������������������������������������������� 102 1) Olympic Dam: model IOCG breccia 103 2) Ernest Henry and regional breccias 103 3) Prominent Hill, Australia: OD wannabe 105 4) Drazhnje, Kosovo: karst-hosted breccia 110 5) Amulsar, Armenia: volcano-sedimentary breccia
Chapter 7
Working with folds and cleavage
145
Introduction�������������������������������������������������������� 145 Disharmonic folding������������������������������������������� 145 The general mapping problem 146 Bedding in strongly folded outcrops 147 Transposition and mapping 148 Congruency of minor folds 149 Vergence 149 Cross-sections and disharmonic folding 149 Areas lacking mappable contacts������������������������ 150 Folded or cleaved outcrops�������������������������������� 151 Down-plunge viewing 151 vi
CONTENTS
151 The advantage of vergence 151 Vergence in areas of multiple fold generations152 Location in the larger fold structure 153 Sense of structural overturning 153 Using younging (bed facing) 153 Extracting all the information 154 Using Fold Facing 155 Case History: Northern Quadrilátero Ferrífero 156
Interpolation of drillhole surveys�������������������� 190 QA/QC: Error detection and control�������������� 190 Potential errors at the drilling stage 191
What to measure
Manually ‘inserted’ Bottom Marks
Potential errors at the mark-up stage Orientation Confidence Scores
Errors at the measurement stage Errors of precision. Errors of measurement Bias
Complex multiple deformation: when to call in the experts������������������������������������������������������������� 158
Chapter 8
Working in Shear Zones
Chapter 10
Theory: Deformation and Structures
161
163 What is the nature of the fabric in the zone? 164 What is the symmetry of the shear zone fabrics? 165 What is the sense-of-shear? 165 Foliation deflection asymmetry 166 Asymmetric porphyroclast structures 166 S-C fabrics and shear bands 168 Is it a shear band or an overprinting crenulation? 170 A digression: putting mylonites in perspective 170 Recognising mylonite protoliths 172 What is the displacement likely to be? 173 Progressive deformation and shear zones��������� 175 Shear zones and Fluids���������������������������������������� 176 Gash Veins 178 Summary 178 Encoding mylonite rocks in databases��������������� 178 Oblique folds and sheath folds
Oriented Drillcore
197
Introduction�������������������������������������������������������� 197 Deformation��������������������������������������������������������� 197 Types of flow causing deformation 198 Instantaneous strain 199 Finite strain 200 Strain and structures 200 Progressive Deformation 202 Strain, Structures, and Fabrics�������������������������� 203 How structures and grain fabrics evolve 203
Introduction�������������������������������������������������������� 161 Questions to think about:���������������������������������� 162 Is it really a high strain zone? 162
Chapter 9
192 193 193 193 193 194 194
205 205 1. Grain-shape fabrics 205 2. Axial planar discontinuities 206 3. Crenulation fabrics. 207 4. Axial planar compositional segregation 207 5. Transposition and attenuation fabrics 208 Stretching lineations and strain fringes 209 Structural fabric suites 210 3D Strain and Fabric Types 211 Crustal Deformation Modes ����������������������������� 212 Local prolate (L) and oblate (S) fabrics 213 Deformation partitioning����������������������������������� 215 Shear Zones: ductile partitioning 216 Ductile shear zone to brittle fault 216 Brittle flow 217 Brittle Response to deformation����������������������� 218 Stress: the key to brittle behaviour 218 Stress Field 219 Mohr circles 219 Brittle Failure 220 Sliding failure on existing fractures 221 Fluids and hydraulic fracturing 221 Veining 222 Crack-seal and crystal growth habits in veins 223 Fold shape changes
Foliation microstructure
179
Drillcore orientation types�������������������������������� 179 Unoriented drillcore 179 Oriented drillcore 179 Partially oriented drillcore 179 Drillcore name and angle conventions 180 Orienting Core����������������������������������������������������� 181 Measurements in Oriented Core����������������������� 183 Measurement of alpha angle 183 Measurement of beta angle 184 Measurement of surfaces parallel to the core 185 Measurement of lines in core 186 Measurements in Partially Oriented core�������� 186 Geometrical relationships���������������������������������� 187 Oriented core 187 Partially oriented core 187 Stereonet Calculations��������������������������������������� 188 Procedures 188
References and Resources
225
Index227 About the Author vii
233
Acknowledgements I owe a great deal to a large number of people who have been influential in my development as a structural geologist. Among these I hold Bruce Hobbs, Paul Williams, Ron Vernon, and Tim Hopwood in high regard for lighting the fire in my University years; Nick Oliver, Paul Pearson, Terry Harbort, Renate Sliwa, Rick Gordon, Mike Williams, Tim Coughlin, Chris Stephens, and (particularly) Timothy Little for stoking the coals during my academic and teaching career; and Dave Horton, Mike Schmulian, Philip Newton, Nick Fox, Cees Swaager, Cameron Cairns, George Schroer, Stuart Smith, and all the other exploration people who, over the past twenty years or so, have given me the luxury of seeing more rocks and structures in more exotic locations than I had ever dreamed I would see. In particular, Stuart Smith was, perhaps unknowingly, instrumental in starting me to think about writing this text, initially as a manual for an in-house mapping course in Thailand. As always, I am indebted to Nick Oliver for trying to curb my enthusiasm with his enthusiasm, and particularly helping me to not be one-eyed about breccia. And to Rick Valenta for improving the cover design.
viii
Preface The best geologist is the one who has seen the most rocks Herbert Harold Read ... and ain’t it the truth!
This manual is aimed at young practising professional geologists in the minerals industry, particularly those working in deformed and folded terranes. The assumption is made that the reader already has a geological background and this manual is intended as a way to focus and revise that background. It has been arranged somewhat in reverse order to a conventional text in that the early sections explain how and when to apply particular techniques in the context of fieldwork and mapping, and later sections explain the theory behind those techniques. It is also a highly personal, and perhaps idiosyncratic, manual. It is based on my own thoughts and philosophy about what it takes to be a top tier professional geologist. Thoughts such as: • You never stop learning. In a reflection of the H.H. Read statement shown above, I believe that at least one third of the training of a geologist is experience, thus putting young professionals at a distinct early-career disadvantage. The initial drive for writing the manual was from repeatedly seeing the same mistakes made by young professional geologists, while I knew that in thirty years’ time they would probably not make that same mistake. This manual is heavily based on my own experience (be that a good or a bad thing) with the thought that if I can articulate my experience, it may speed up the acquisition of experience by the reader. • Orebodies are anomalies, and the structures that control them are commonly also anomalous. The regional context may be a systematic textbook structure, but the local mineral occurrence is more often than not controlled by non-systematic structures that rarely fit any textbook models. I use the term accommodation structures for anomalous structures that have allowed a deformation to proceed by departing from the dominant mode of behaviour - brittle or ductile. Some accommodation structures, such as gash veins, which are brittle structures accommodating a ductile shear, are well understood and systematic, but many other accommodation structures depart markedly from textbook behaviour. In particular, mineralisation in orogenic belts is commonly associated with non-systematic faults and fractures associated with ductile folding. Such structures can appear to violate many of the characteristics normally associated with faults (such as strong curvature and rapid changes in slip and slip sense) and it is important to recognise them for what they are and understand how to deal with them. • Theory is as important to a professional geologist as it is to an academic. A good geologist should be able to interpret previously unseen or anomalous structures on the basis of a good theoretical background. This viewpoint is strongly influenced on my own experience in which more than thirty years of academic teaching and applied
and theoretical research preceded a full-time structural consulting career. So I put great store in the belief that every outcrop should be understood at both a practical and a theoretical level in order to achieve a high level of confidence in any interpretation. Thus throughout this manual I have tried to integrates the description and treatment of structures with the underlying theory. • A field geologist should assess every outcrop or core sample he or she sees within the context of a continuously developing broad-scale model. Does this outcrop fit the context of the last outcrop; what do I expect at the next outcrop; and what do I see here that challenges my view of the broad-scale model? Associated with this is my belief that the concept of multiple working hypotheses is a fine ideal, rarely practiced. It is not what most geologists do. Most geologists have a dominant prevailing model (working hypothesis) – and that is no bad thing, provided it is approached with an open mind and a willingness to modify or change a model based on new observations. Awareness and continuous model testing is the key activity that avoids missing that critical outcrop that changes everything. The downside is that in some cultures (both national and corporate) junior geologists feel unable to question prevailing models devised by senior geologists. Nonetheless, for them to develop into a top class geologist they must make the effort to continually test the model in their own day-to-day observations. • Finally, I liken a geologist to a GP doctor. Both look at surface symptoms and, prior to undertaking invasive actions, have to come to a diagnosis that may be critical. The range of solutions is enormous, the symptoms for any solution can vary widely; and knowledge of those symptoms and that solution may only have occupied a few hours during the years of university training. Most geologists’ training is considerably shorter than that of a GP. No wonder then that most professional industry geologists are poorly equipped to recognise and respond to unusual geology, and it is also understandable that they may have forgotten some very basic aspects that may have been covered only in their first few weeks of a Geology degree. It is one reason why some chapters of this manual dwell at such length on some basic aspects. For example, I have run into enough examples of geologists being unaware of the relationship between topography and outcrop pattern to realise the need to cover this most basic of topics in some detail. This manual does not replace an introductory structural geology textbook. Nor does it cover all aspects of structural geology to the same level. It dwells on those aspects with which I have most experience in (such as ductile fabrics, folds, and shear zones), at the expense of other aspects (such as some faults and other semi-brittle structures such as most veins). Even then, it concentrates on those aspects ix
PREFACE
of structure in which I have seen mistakes made by exploration and mining geologists working in specific types of terranes, or in which techniques can be applied that are not in the usual armoury of an industry geologist. It also does not proceed into advanced structural topics such as the geometrical analysis of terranes that have undergone complex superposed folding . Although these are mentioned, it is such terranes that really does require bringing a specialist structural geologist into the exploration team. This manual arose out of a series of in-house mapping courses that I delivered in Thailand some years ago. The main purpose of the course was two-fold: 1. to encourage exploration geologists (some quite senior) doing regional 1:50000 scale reconnaissance mapping to get beyond the simple data acquisition stage of just recording rock-types and measuring bedding, etc., but to develop that data into a regional geological and structural model that could be used as a basis for exploration. That is, to push them beyond their corporate comfort zone into the somewhat risky realm of interpretation. 2. to give highly inexperienced young geologists with poor background training in outcrop procedures, and pit mapping. Consequently the first chapter deals with aspects of mapping at all scales. Likewise, the second chapter deals with the purely technical aspects of field, pit, and mine geology. I apologise to more experienced geologists that some of the sections in this chapter are very elementary.
Chapter 3 is mainly a basic primer on the criteria for determining stratigraphic facing (younging) in the field. Chapter 4 provides a basic overview of the structures found in rocks. Its main purpose is to allow a fast revision of the gamut of basic structures as well as to provide an outline of the terminology and concepts that I use within the rest of the text. Chapter 5 deals with the problem of recognising and interpreting breccia bodies, and includes a number of specific case histories. Chapter 6 includes all aspects of structures in 3D, starting with contacts and topography before moving into 2D cross-sections ,and finally aspects of 3D modelling. Chapter 7 describes the problematic aspects of folded and cleaved rocks, and how to extract maximum map an structural information from an outcrop. Chapter 8 deals with shear zones from a field perspective, ranging from low strain shear zones to mylonite. (Theoretical and more advanced aspects of shear zones are dealt with in Chapter 10). Chapter 9 deals with extracting structural data from oriented drillcore and providing quality control on the data. This chapter is extracted from my on-line manual on Oriented Core Procedures at: http://www.hcovglobal. com/#!downloads/lf947. Chapter 10 provides the theoretical background for many aspects of deformation underpinning the material discussed in the manual. It includes an outline of the principles of deformation and rock flow, folding, shear zones, and fracturing. Rod Holcombe Mudgeeraba, Queensland Australia November 2016 (
[email protected])
x
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