Structural and stratigraphic controls on mineralization at ...387957/Murphy_2004... · Paul Whelan...

Structural and stratigraphic controls

on mineralization at the

George Fisher Zn-Pb-Ag Deposit,

Northwest Queensland, Australia

Thesis submitted by

Travis E. Murphy

B.App.Sc. (Hons) University of Technology, Sydney

in October, 2004,

for the degree of Doctor of Philosophy

in the School of Earth Sciences,

James Cook University

Travis Murphy, 2004 _____

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Statement of Access

I, the undersigned, author of this thesis, understand that the following restriction placed

by me on this thesis will not extend beyond the date specified.

“I wish to place restriction on access to this thesis for a period of two years as per the

confidentiality agreement with MIM Ltd (now Xstrata)”

After this period has elapsed I understand that James Cook University will make this

thesis available for use within the University library and make this thesis available via the

Australian Digital These network, for use elsewhere. All users consulting this thesis will

have to sign the following statement:

“In consulting this thesis I agree not to copy or closely paraphrase it in whole or in part

without written consent of the author, and to make proper written acknowledgement for

any assistance which I obtained from it”

_____________________________ __________________

Travis E. Murphy Date


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Statement of Sources

Declaration

I declare that this thesis is my own work and has not been submitted in any form for

another degree or diploma at any university or other institution of tertiary education.

Information derived from the published or unpublished work of others has been

acknowledged in the text and a list of referees is given.

_____________________________ __________________



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Electronic Copy

I, the undersigned, author of this work, declare that the electronic copy of this thesis

provided to the James Cook University Library is an accurate copy of the print thesis

submitted, within the limits of the technology available.

_____________________________ __________________



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Acknowledgements

This project has been possible thanks to financial support from MIM/Xstrata who have

provided access to required data, drill-core, facilities, and the mine at George Fisher.

The initial concept of this study was developed by Alice Clark and Tim Bell. Andrew

Allibone, Tom Blenkinsop, and Tim Bell are thanked for their supervision of respective

parts of this thesis. Andrew Allibone, in particular, is thanked for guiding this research

and for enthusiastic editing of several drafts of the thesis.

Myles Johnston, Ann Mawdesley, Alison Morley, Brad Cox and other geologists from

both the Mt Isa and George Fisher offices are thanked for providing logistical support

during my stays in Mt Isa on fieldwork. Communication with Allen Shaw proved

invaluable due to his intimate knowledge of the stratigraphy at George Fisher.

Paul Whelan (Metech) kindly provided the MineSight software package for use in this

study. Andrew Allibone, Roger Taylor, Kylie Prendergast, and Gavin Clark donated

some of the samples used in Part C of this study and are thanked for discussions related to

the same. Alan Chappell, Gordon Warria, and Kevin Blake provided supervision and

support of various stages of laboratory work in the Advanced Analytical Centre at JCU.

Rhenium-Osmium isotopic analysis was possible thanks to funding and support from the

PMD-CRC and the efforts of Lucy Chapman were integral in making this happen. Reid

Keays, Bruce Schaeffer, Rachelle Hubregtse and other VIEPS personnel conducted this

analytical work, and Reid Keays is thanked for his role in this collaborative work.

Finally, I would like to thank Rebecca, my wife, for encouragement to undertake this PhD

study, support during my candidature, and patience while I came to blows with this

computer whilst producing the thesis.


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ABSTRACT

Sediment-hosted stratiform Zn-Pb-Ag deposits have been interpreted as forming during

exhalative hydrothermal activity synchronous with the deposition of host rocks and

during either syn-diagenetic or syn-metamorphic veining and replacement of the host-

rocks later in their orogenic history. This study analyzes the distribution metal and ore-

types at the George Fisher deposit and investigates spatial and temporal relationships

between high-grade mineralization and structures at all scales throughout the deformation

history. The study aims to determine whether remobilization and upgrading of syn-

sedimentary and/or syn-diagenetic proto-ore or primary syntectonic mineralization are

involved in the formation of economic mineralization. Analysis of the kinematic controls

on any Zn-Pb-Ag mineralization which is structurally controlled may enable other

prospective structures in the near-mine region to be identified.

The George Fisher Zn-Pb-Ag deposit is located 22km north of Mt Isa in Queensland,

Australia; and is hosted by Proterozoic sedimentary rocks of similar age and lithology to

the Mt Isa and adjacent Hilton Zn-Pb-Ag deposits. The host-rocks at George Fisher

preserve a structural history comprising four distinct ductile deformations with

concomitant faulting and younger brittle faults which cut the ore-bearing sequence. The

first, D1 is manifest as an open fold (F1) with an approximately east-west striking axial

plane and was overprinted by F2 folding and a pervasive slaty/solution cleavage (S2)

during the regionally extensive D2 episode. An episode of sub-vertical shortening/sub-

horizontal extension (D3) followed this main phase of sub-horizontal shortening which

formed folds with sub-horizontal axial planes and gently-dipping crenulations of the S2

cleavage and bedding. D3 features are overprinted by sub-vertical crenulations (S4) and

minor folds (F4) with sub-vertical axial planes.

Ore shoots that comprise high-grade and thicker mineralization plunge parallel to the F1

fold axes and are largely confined to the short-limb of this fold. Subsidiary ore shoots are

coincident with areas of more intense F2 folding, and trend parallel to both F2 and F4 fold

axes in longitudinal projection.

Vein-hosted sphalerite and medium-grained galena breccia are the main sources of Zn

and Pb metal in the deposit, respectively. Logged widths of each form shoots of thicker

mineralization which are broadly coincident with high-grade shoots defined by the assay


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data. Sulphide textural studies and vein - host-rock fabric cross-cutting relationships

suggest that both postdate D2.

Empirical relationships between ore shoot geometry and the structural framework of the

deposit imply a D4 control on metal distribution. This is supported by the interpretation

of a dominantly syn-D4 relative timing of the vein-hosted sphalerite and medium-grained

galena breccia based on development of these mineralization-types in unique structural

settings and the apparent lack of deformation of their constituent sulphides. Potential pre-

F2 mineralization types include some disseminated sphalerite and fine-grained sphalerite-

galena breccias which do not currently constitute economic mineralization and account

for ca. 10% of the Zn+Pb in the deposit. Remobilization of proto-mineralization is

supported by the Zn assay data which indicates that more than one population of Zn

grades exists and that a higher grade population is unique to the economic ore-horizons.

However, this qualitative observation does not discriminate between upgrading of a pre-

F2 or syn-/post-F2 sulphide accumulation during D4.

Re-Os isotopic analysis of sphalerite and galena at George Fisher define an isochron

whose slope indicates an age of 1423±130Ma indicating closure of the Re-Os system

postdates host-rock deposition by ~100-360Ma.

A mantle source of Pb and Zn is interpreted from the Re-Os isotopic analysis based on the

initial 187Os/188Os ratio of 0.077±0.071. This differs from previous studies of Proterozoic

Zn-Pb-Ag deposits which infer scavenging of metal from within the sedimentary basin or

from the basement rocks immediately underlying the sedimentary basin. Proximity to a

regional fault zone such as the Mount Isa-Paroo Fault system, interpreted to be part of a

fault-network linked to a major mid-crustal shear zone, is considered necessary to bring

metal-bearing fluids from depth into contact with prospective host-lithologies at George

Fisher.

At the George Fisher deposit, it is inferred that the F1 fold focussed diagenetic

hydrothermal activity and alteration and also acted as a heterogeneity focussing dilation

and final sites of mineralization later in the deformation history. The F1 fold may be the

upper level expression of a reactivated basement fault thereby accounting for the

longevity of the hydrothermal system responsible for alteration and mineralization at

George Fisher. It is possible that diagenetic processes prepared the host-rock for later

mineralization at George Fisher.


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TABLE OF CONTENTS

Statement of Access i

Statement of Sources ii

Statement of Submission of Electronic Copy iii

Acknowledgements iv

Abstract v

Table of Contents vii

List of Tables xiii

List of Figures xiii

INTRODUCTION 1

Thesis Structure 4

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PART A. STRUCTURAL DEVELOPMENT OF HOST ROCKS TO ZN-PB-AG

MINERALIZATION AT THE GEORGE FISHER MINE, NORTHWEST QUEENSLAND,

AUSTRALIA.

Abstract 7

1. Introduction 8

2. Geological Setting 9

2.1 Regional Geology 9

2.2 Local Geology 16

3. Primary Layering – Bedding 18

4. Deformation History 22

4.1 Description and interpretation of D1 structures 22


4.2.1 Thermal and stress history of the D2 deformation 37




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4.5 Description and interpretation of Post-D4 minor fabrics 69

4.6 Description and interpretation of late kink folding 71

4.7 Description and interpretation of brittle faulting 71

4.7.1 North-South reverse faults with oblique slip 75

4.7.2 East-West striking faults 75

4.7.3 North-northeast striking sub-vertical reverse faults 77

4.7.4 North-northwest striking, west-dipping sinistral faults 77

4.7.5 Northeast striking, dextral-normal faults 80

4.7.6 Bedding-parallel faults 80

5. Discussion 81

5.1 Comparison with previous structural interpretations 81

5.2 Discrete episodic foliation development vs conjugate crenulations 89

5.3 Origin and significance of D3 structures 90

6. Conclusions 94

References 96

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PART B. DISTRIBUTION OF ZN-PB-AG MINERALIZATION AND ITS

RELATIONSHIP TO STRUCTURAL DOMAINS AT THE GEORGE FISHER MINE,

NORTHWEST QUEENSLAND, AUSTRALIA.

Abstract 102

1. Introduction 103

2. Metal Distribution 105

2.1 Methods for analyzing metal grade distributions 105

2.2 Deposit-wide correlations between metal grades and

structure in plan and section 109

2.3 Distribution of metal ratios 117

2.4 Mineralization and grade distribution in C ore-horizon 122

2.5 Mineralization and grade distribution in D ore-horizon 124

2.6 Mineralization and grade distribution in G ore-horizon 126

3. Deposit-scale sulphide distribution 127


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3.1 C ore-horizon 129

3.2 D ore-horizon 131

3.3 G ore-horizon 134

3.4 Combined ‘C+D’ and ‘C+D+G’ data 135

4. Discussion 135

4.1 Correlations between the geometry, orientation, and

location of ore shoots and structural features 135

4.2 Comparison with the structural setting of ore shoots

within the Mt Isa and Hilton Zn-Pb-Ag deposits 141

4.3 Kinematics controlling ore-sulphide concentration : Implications

for extensional and near-mine exploration 145

5. Conclusions 153

References 155

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PART C. THE RELATIONSHIP BETWEEN THE FRACTAL DIMENSION OF

SPHALERITE GRAIN BOUNDARIES AND SULPHIDE DEFORMATION: APPLICATION

OF THE COASTLINE METHOD IN MICROSTRUCTURAL STUDIES.

Abstract 157

1. Introduction 158

2. Method 160

2.1 Methods of grain boundary characterization 160

2.2 Sample Preparation 162

2.3 Procedure 162

3. Samples Studied 166

4. Sulphide Microstructures 166

4.1 Class A 169

4.2 Class B 169

4.3 Class C 171

4.4 Class D 171

4.5 Class E 173


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5. Results of fractal analysis of sphalerite grain boundaries 173

6. Discussion 179

6.1 Sulphide microstructural studies 179

6.2 Implications for ore deposit analysis 186

7. Conclusions 186

References 188

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PART D. ORE CHARACTERISTICS, CROSS-CUTTING RELATIONSHIPS, AND

TIMING RELATIVE TO DEFORMATION AT THE GEORGE FISHER DEPOSIT,

NORTHWEST QUEENSLAND, AUSTRALIA.

Abstract 193

1. Introduction 194

2. Description and interpretation of sphalerite-dominant ore-types 199

2.1 Vein-hosted sphalerite 199

2.1.1 Evidence for replacive sphalerite 204

2.1.2 Sphalerite occurring as breccia-infill and replacement

of pre-existing veins 212

2.1.3 Sphalerite occurring synchronous with vein development 215

2.1.4 Implications of cross-cutting relationships and

palaeostress estimation 225

2.1.5 Summary and interpretation 237

2.2 Massive sphalerite 238

2.3 Sphalerite breccia 240

2.4 Disseminated sphalerite 247

2.5 Fine-grained sphalerite-galena breccia 251

2.6 Some microstructural evidence for sphalerite mobility during D4 259

3. Description and interpretation of galena-dominant ore-types 262

3.1 Fine-grained galena-sphalerite breccia 262

3.2 Medium-grained galena breccia 264

3.2.1 Relative timing of medium-grained galena breccia


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formation 269

3.3 Coarse-grained galena breccia 280

4. Discrimination of remobilized vs syntectonic mineralization based on

analysis of grade populations 288

5. Discussion 292

5.1 Relative timing of ore-types – integration with the structural history 292

5.2 Pre-/early-D2 mineralization at George Fisher?: Remobilization vs

syntectonic orebody formation 302

6. Conclusions 306

References 308

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PART E. RHENIUM-OSMIUM ISOTOPIC ANALYSIS OF MINERALIZATION AT THE

GEORGE FISHER DEPOSIT, NORTHWEST QUEENSLAND, AUSTRALIA:

IMPLICATIONS FOR THE ABSOLUTE AGE OF MINERALIZATION AND THE SOURCE

OF METALS.

Abstract 314

1. Introduction 315

2. Methodology 318

3. Results 321

4. Discussion 326

4.1 Implications for the timing of mineralization at George Fisher 326

4.2 Source of metal at the George Fisher Zn-Pb-Ag deposit:

comparison with previous models of Zn-Pb-Ag metallogeny in

the Western Fold Belt of the Mt Isa Inlier 330

5. Conclusions 332

References 334


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PART F. TECTONOSTRATIGRAPHIC SETTING AND CONTROLS ON STRATIFORM

BASE METAL MINERALIZATION IN THE WESTERN FOLD BELT OF THE MT ISA

INLIER, AUSTRALIA: COMPARISON OF THE MT ISA, GEORGE FISHER - HILTON,

LADY LORETTA, AND CENTURY ZN-PB-AG DEPOSITS.

Abstract 341

1. Introduction 342

2. Geological Evolution of the Western Fold Belt 342

3. Characteristics of the setting of the Mt Isa, George Fisher – Hilton,

Lady Loretta, and Century Zn-Pb-Ag deposits 350

3.1 Mt Isa 350

3.2 George Fisher – Hilton 353

3.3 Lady Loretta 358

3.4 Century 361

4. Discussion 364

4.1 Common and contrasting features in the setting of the Zn-Pb-Ag

deposits of the Western Fold Belt and implications for exploration 364

4.2 Significance of D1 (approximately east-west oriented) structures

and implications for Zn-Pb-Ag metallogenesis in the Western Fold

Belt of the Mt Isa Inlier 369

5. Conclusions 371

References 372

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CONCLUSIONS 382

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APPENDICES

APPENDIX I. SAMPLE CATALOGUE 388

APPENDIX II. STRUCTURAL DATA 392

APPENDIX III. ELECTRON MICROPROBE ANALYSES 398


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APPENDIX IV. (A) FRACTAL ANALYSIS METHOD 400

(B) DETAILED DATA FROM FRACTAL ANALYSIS 401

LIST OF TABLES PART A. Table 1. Fabric catalogue of recognized ductile deformation features 21 Table 2. Previous structural interpretations in the George Fisher - Lake Moondarra – Mt Isa area 82 PART C. Table 1. Sample locations and type for fractal analysis 168 Table 2. Descriptive statistics for the results of the fractal analysis 174 PART D. Table 1. Paragenetic sequence of mineralization and alteration 197 PART E. Table 1. Re-Os isotopic data and sample information 322

LIST OF FIGURES

PART A. Figure 1. Map of the Proterozoic McArthur and Mt Isa Inliers 10 Figure 2. Geological map of the George Fisher – Lake Moondarra area 12 Figure 3. Map of the Sybella Batholith and surrounding faults 13 Figure 4. Simplified geological map of the area southwest from George Fisher and west of the Mt Isa fault 15 Figure 5. Surface geological map above the George Fisher Mine 17 Figure 6. Stratigraphic units/ore-horizons in plan and sectional view 19 Figure 7. Bedding measurements and photomicrographs of the early stylolitic compaction related fabric 21 Figure 8. Photomicrographs and line drawing illustrating the preservation of earlier fabrics adjacent to carbonate nodules 23 Figure 9. Bedding form-line maps illustrating the s-shaped F1 flexure 24 Figure 10. Photographs and mapping indicating that the flexure is unrelated to late brittle faulting 26 Figure 11. Photomicrographs of the S2 foliation 30 Figure 12. S2 and F2 data displayed on stereonets and projected onto 12 Level of the George Fisher Mine 31 Figure 13. Variation in the intensity of the S2 foliation 32


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Figure 14. Mapping of 739 cross-cut on level 12C of the George Fisher Mine indicating the change in the sense of shear on the S2 cleavage around folds thereby interpreted as F2. 34 Figure 15. Photograph and line-drawing of rotated F2 folds in D-orebody 35 Figure 16. Mapping of F2 folds in the northern part of level 12C 37 Figure 17. Graph illustrating the relationship of the fractal dimension of quartz grain boundaries with deformation temperature and a photomicrograph of an analyzed grain boundary from the Lake Moondarra area 40 Figure 18. Determination of deformation temperature from calcite twin geometry 41 Figure 19. Image of sample with cross-cutting vein relationships used for palaeostress estimation 43 Figure 19 Electron microprobe analyses of vein and matrix carbonates 44 (cont’d). Figure 20. Palaeostress estimation graphs 45 Figure 21. Photomicrographs and line-drawing of S3 crenulations 47 Figure 22. S3 and F3 data displayed on stereonets and projected onto 12 Level of the George Fisher Mine 49 Figure 23. Variation in the intensity of the S3 foliation 50 Figure 24. Method of calculation of the extension direction in the plane of the S3 foliation 52 Figure 25. Photomicrographs and line-drawing of subtle D3 reactivation of bedding and S2. 53 Figure 26. Fold vergence relationships observed at George Fisher 54 Figure 27. Syn-D3 quartz-carbonate fibre growth in S2 parallel veins 56 Figure 28. Photomicrographs and line-drawing of S4 crenulations 57 Figure 29. Photograph of foliation-bedding intersection lineations 58 Figure 30. Photomicrograph and line-drawing of an S3 crenulation decrenulated at the intersection with a zone of D4 strain 60 Figure 31. S4 and F4 data displayed on stereonets and projected onto 12 Level of the George Fisher Mine 61 Figure 32. Variation in the intensity of the S4 foliation 62 Figure 33. Contours of the angle of intersection between S0 and S4 63 Figure 34. Method of calculation of the extension direction in the plane of the S4 foliation 64 Figure 35. Overprinting fold relationships in cross-cut mapping of D-orebody 66 Figure 36. Photomicrograph and line-drawing of foliation relationships in barren siltstone 68 Figure 37. Photomicrograph and line-drawing of the foliations S2-S6 70 Figure 38. Mapping of east-west trending kink-like folds in the hangingwall of C orebody 72 Figure 39. Map of main fault sets at George Fisher 73 Figure 40. Cataclasite from the Paroo Fault 74 Figure 41. Mapping of east-west striking south-dipping faults 76 Figure 42. East-dipping faults with minor east-side-up displacement 78 Figure 43. Orientation data for northwest and northeast striking faults 79


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Figure 44. Line-drawing of a bedding-parallel fault and associated folding and veining 79 Figure 45. District-scale map displaying form-lines of S2/F2 and S4/F4 84 Figure 46. Implications of S2/S4 asymmetry swap between Mt Isa and George Fisher 87 Figure 47. Possible explanations for observed D3 strain at George Fisher 91 PART B. Figure 1. Broader stratigraphic control on mineralization 104 Figure 2. Explanation of longitudinal projection and block-model construction 106 Figure 3. Simplified stratigraphic column 108 Figure 4. Stacked sections displaying Zn grade 110 Figure 5. Stacked sections displaying Pb grade 111 Figure 6. Stacked sections displaying Ag grade 112 Figure 7. Zn grade distribution in plan view 114 Figure 8. Pb grade distribution in plan view 115 Figure 9. Ag grade distribution in plan view 116 Figure 10. Zn vs Pb graphs of assay data 118 Figure 11. Distinction of high-Zn and high-Pb populations 120 Figure 12. Zn vs Pb graphs for the respective ore-horizons at George Fisher 121 Figure 13. C ore-horizon metal grades in longitudinal projection 123 Figure 14. D ore-horizon metal grades in longitudinal projection 125 Figure 15. G ore-horizon metal grades in longitudinal projection 128 Figure 16. C ore-horizon ore-sulphide thickness in longitudinal projection 130 Figure 17. D ore-horizon ore-sulphide thickness in longitudinal projection 132 Figure 18. G ore-horizon ore-sulphide thickness in longitudinal projection 133 Figure 19. C+D and C+D+G sulphide thickness in longitudinal projection 136 Figure 20. Generalized ore shoot geometry and relationship to structural orientations 138 Figure 21. Comparison of geology and mineralization between the near surface and 12 Level of the George Fisher Mine 140 Figure 22. Ore-fluid migration parallel to fold axes 143 Figure 23. Isometric block diagrams illustrating kinematics during the successive deformations and possible sites for dilation 146 Figure 24. Illustration of dilation in the F1 short-limb during D4 reactivation of bedding 148 Figure 25. Illustration of potential net-dilation occurring during D4 reactivation of bedding 150 Figure 26. Maps illustrating the spatial relationship between more intense S3 and S4 development and higher Pb grades 151 PART C. Figure 1. Graphical representation of the algorithm used in the fractal analysis 161 Figure 2. Photomicrographs illustrating etched vs non-etched polished


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sections 163 Figure 3. Analysis of the sensitivity of the fractal method to digitizing error 165 Figure 4. Explanation of the step-effect observed in plots of Log P vs Log L 167 Figure 5. Photomicrographs of representative microstructures in sphalerite 170 Figure 6. Photomicrograph and line-drawing of sphalerite grains interpreted as having recrystallized and subsequently deformed 172 Figure 7. Examples of representative grain boundary outlines used in the fractal analysis 175 Figure 8. Graphs illustrating the lines of best fit to data used in the fractal analysis 177 Figure 9. Frequency histogram displaying results from analysis of undeformed, deformed, and recrystallized sphalerite 178 Figure 10. Data for sphalerite grain boundaries plotted onto a graph of fractal dimension vs log-grain diameter 180 Figure 11. Potential deformation/recrystallization paths indicated by trends in the data on the D vs grain diameter plot 183 Figure 12. Data grouped according to deformation temperature 184 PART D. Figure 1. Hand-specimens of vein-hosted sphalerite mineralization 201 Figure 2. Photomicrographs of vein-hosted sphalerite mineralization 203 Figure 3. Vein-hosted sphalerite grain boundary data plotted onto a graph of fractal dimension vs grain diameter 205 Figure 4. Photomicrographs of sphalerite replacing gangue minerals in vein-hosted sphalerite 207 Figure 5. Photomicrograph and line drawing illustrating brecciation and replacement of host rock during vein-hosted sphalerite mineralization 208 Figure 6. Vein-hosted mineralization terminating on a cross-cutting vein 210 Figure 7. Vein-hosted mineralization terminating on a cross-cutting vein 211 Figure 8. Photomicrographs of subhorizontal quartz-carbonate fibres associated with vein-hosted sphalerite mineralization 214 Figure 9. Photomicrographs illustrating relict deformed quartz clasts in vein-hosted sphalerite 216 Figure 10. Photomicrograph and line-drawing of vein-hosted sphalerite occurring in veins opening due to competency contrast 217 Figure 11. Hand-specimen and photomicrographs of vein-hosted sphalerite heterogeneously distributed around a mesoscale fold 219 Figure 12. Photograph and line drawings of unfolding of an early fold creating bedding-parallel sphalerite+quartz+carbonate veins 221 Figure 13. Heterogeneous development of vein-hosted sphalerite mineralization around an F2 fold 222 Figure 14. Photomicrographs of an isoclinal microfold in a vein-hosted sphalerite layer 224


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Figure 15. Implications of palaeostress estimates on vein paragenesis 226 F3 microfolds and associated veining in vein-hosted sphalerite 227 Figure 16. Graph of strength vs temperature for sphalerite and carbonate rock types 231 Figure 17. Comparison of sphalerite textures in planar and folded strata 233 Figure 18. Fractal analysis of grains from samples in Figure 17 234 Figure 19. Distribution of vein-hosted sphalerite thickness in longitudinal projection 236 Figure 20. Hand-specimen and photomicrographs of massive sphalerite 239 Figure 21. Distribution of massive sphalerite thickness in longitudinal projection 241 Figure 22. Hand-specimen and photomicrographs of sphalerite breccia 242 Figure 23. Hand-specimen and photomicrographs of sphalerite breccia 243 Figure 24. Distribution of sphalerite breccia thickness in longitudinal projection 246 Figure 25. Photomicrographs of disseminated sphalerite 248 Figure 26. Photomicrograph of disseminated sphalerite concentrated in fold hinges 250 Figure 27. Fine-grained sphalerite breccia in hand-specimen and photomicrograph 252 Figure 28. Fine-grained sphalerite breccia in hand-specimen and photomicrograph 253 Figure 29. Fractal analysis of relict deformed grains from fine-grained breccia 255 Figure 30. Distribution of fine-grained sphalerite+galena breccia in longitudinal projection 257 Figure 31. Photomicrographs illustrating the S4 and host-rock control on sphalerite deposition 260 Figure 32. Photomicrographs illustrating D4 reactivation of bedding causing dilation and minor sphalerite precipitation in S3 crenulations 261 Figure 33. Photomicrographs of fine-grained galena 263 Figure 34. Photographs of medium-grained galena breccia in hand-specimen 265 Figure 35. Photomicrographs of medium-grained galena breccia 266 Figure 36. Photomicrographs of galena tension gashes 268 Figure 37. Fold overprinting relationships and their implications on medium-grained galena breccia timing 270 Figure 38. Cross-cut mapping illustrating post-D3 timing for medium-grained galena breccia 271 Figure 39. Mapping of a large galena-dominant bedding-discordant tension vein 273 Figure 40. Mapping of a cross-cut illustrating the timing of galena breccia development relative to folding episodes 275 Figure 41. Mapping, hand-specimen photograph, and photomicrograph of thickening and deformation of galena in a fold 276 Figure 42. Medium-grained galena breccia hosted by a boudin-neck 277 Figure 43. Distribution of medium-grained galena breccia thickness in longitudinal projection 279 Figure 44. Coarse-grained galena breccia in hand-specimen and


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photomicrograph 281 Figure 45. Coarse-grained galena breccia in hand-specimen and photomicrograph 282 Figure 46. Photograph and line drawing illustrating post-D3 timing of coarse-grained galena breccia 284 Figure 47. Photograph of a relict fold hinge in a coarse-grained galena breccia 285 Figure 48. Coarse-grained galena breccia in an apparent boudin-neck formed during bedding parallel extension 285 Figure 49. Distribution of coarse-grained galena breccia thickness in longitudinal projection 287 Figure 50. Frequency and cumulative frequency histogram for D ore-horizon data 289 Figure 51. Histograms and normal-probability plots for George Fisher data 291 Figure 52. Summary of the relative timing of ore-types 294 Figure 53. Graph of Pb isotope data and interpretations for the major Pb deposits in Australia 305 PART E. Figure 1. Samples used in the Re-Os isotopic analysis 320 Figure 2. Results plotted on a187Os/188Os vs 187Re/188Os graph 324 Figure 3. Explanation of resetting of host-rock Re-Os signature 328 PART F. Figure 1. Tectonostratigraphic divisions of the Mt Isa Inlier and simplified geology of the Western Fold Belt 344 Figure 2. Stratigraphic column for the Western Fold Belt 345 Figure 3. Schematic illustration of reactivation of basement faults 348 Figure 4. Resource information for the major Zn-Pb-Ag deposits in the Mt Isa Inlier 348 Figure 5. Illustrations of the location and structural control on Zn-Pb-Ag orebodies at Mt Isa 351 Figure 6. Illustrations of the geology and orebody locations at the George Fisher and Hilton deposits 354 Figure 7. Controls on ore shoots at the George Fisher deposit 356 Figure 8. Distribution of calcite alteration at George Fisher and relationship to the F1 short-limb 357 Figure 9. Geological map and section of the Lady Loretta deposit 359 Figure 10. Geological map of the setting of the Century deposit 363


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INTRODUCTION

The George Fisher Zn-Pb-Ag deposit is located 22 kilometres north of Mt Isa in the

north-western region of Queensland, Australia. Mineralization at George Fisher is hosted

by Proterozoic sedimentary rocks within the Mt Isa Inlier and shows many similarities to

the adjacent Hilton (2 kilometres south from George Fisher) and nearby Mt Isa base metal

deposits. Mineralization is dominantly stratiform and stratabound (Valenta, 1994;

Chapman, 2004) and is hosted exclusively by the Urquhart Shale unit which comprises

pyritic siltstones and carbonaceous siltstones interbedded with dolomitic mudstones

(Chapman, 2004). Some galena has been remobilized during deformation (Chapman,

2004). George Fisher has subtle differences from the Hilton and Mt Isa deposits such as

its relatively Zn-rich, Ag-poor resource and the absence of significant copper

mineralization. The Mt Isa deposit is relatively Pb-rich and has lower Zn grades than

George Fisher. Significant copper mineralization is located adjacent to the Pb-Zn

orebodies at Mt Isa. The George Fisher deposit therefore represents a Zn-rich, Cu and

Ag-poor end-member of the three world-class lead-zinc-silver deposits in the Mt Isa

district (Chapman, 2004).

World class base metal deposits can be divided into two categories: giant and super-giant

(Singer, 1995). Giant Zn deposits (largest 10% of deposits) comprise >1.7 Mt of Zn and

super-giant (largest 1% of deposits) >12 Mt of contained metal (Singer, 1995). Of the

eight super-giant Zn deposits in the world, five are in Australia; and include George

Fisher and the adjacent Hilton deposits (Large et al., 2002). The George Fisher deposit

had a pre-mining resource of 108 Mt grading 11.1% Zn, 5.4% Pb, and 93g/t Ag (MIM

Ltd : Report to shareholders – 1998) and qualifies as a giant deposit (Singer, 1995) for

both Pb and Ag.

Stratiform lead-zinc deposits are, by definition, conformable with their bedded

sedimentary host-rocks. At the broader scale this can be interpreted as indicating


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concurrent sedimentation and processes of mineralization, however, textural analysis of

the mineralization-types often reveals that replacement and veining are important

mechanisms of sulphide deposition, therefore indicating mineralization subsequent to

deposition of the host-rock sequence. This may occur during diagenesis or later in the

deformation history of the rocks. Determination of the timing of mineralization with

respect to host-rock formation, diagenesis, and episodes of subsequent deformation has

significant implications on the types of exploration models employed in order to discover

further Zn-Pb-Ag resources.

Several genetic models have been developed for the sedimentary-hosted Zn-Pb-Ag

deposits of the Mt Isa Inlier. The syn-sedimentary or SEDEX genetic model involves

hydrothermal fluids carrying metals in solution being expelled from a vent, which are

typically inferred to be faults active during rift-associated extension, and sulphides

deposited on the seafloor contemporaneous with sediment deposition (e.g. Russell et al.,

1981; Sawkins, 1984; Hancock and Purvis, 1990; Cooke et al., 2000). The conformity of

mineralization within the sedimentary sequence and stacked geometry of orebodies has

been interpreted as a result of episodic expulsion of ore-fluid and precipitation punctuated

by periods of sedimentation during which little or no sulphides were formed (Sawkins,

1984; Valenta, 1994). Zn-Pb-Ag mineralization at Mt Isa and Hilton/George Fisher has

also been interpreted as having formed during diagenesis (Neudert and Russell, 1981;

Valenta, 1988, 1994, Chapman, 1999, 2004). This differs from SEDEX models in that

mineralization occurs below the seafloor and occurs as replacement and cavity infill of

specific sedimentary sequences. Chapman (1999) interpreted chemically distinct phases

of carbonate alteration associated with Zn-Pb-Ag mineralization as diagenetic in origin.

Precipitation of sulphides during syntectonic replacement of lithified and deformed

sediments has been inferred by Blanchard and Hall (1942), Perkins (1997, 1998), and

Perkins and Bell (1998). The spatial correlation and parallelism of high-grade

mineralization and fold axes at Mt Isa have been interpreted as supporting the syntectonic


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genetic model for Zn-Pb-Ag mineralization (Wilkinson, 1995; Perkins, 1997; Davis,

2004).

Previous interpretations of the George Fisher/Hilton deposit infer that the Zn-Pb-Ag

mineralization records the same deformation history as the host-rocks (Valenta 1988,

1994) and initial mineralization has been interpreted as predating much of the

deformation history. Chapman (1999, 2004) inferred that the George Fisher Zn-Pb-Ag

deposit formed during diagenesis with remobilization of galena in the later stages of the

deformation history. The key observations linking mineralization with diagenesis

include:

• low temperature bitumens interpreted to be of diagenetic origin and cogenetic

with sphalerite,

• spatial coincidence of the Zn-Pb-Ag deposit within a carbonate alteration

system interpreted as pre-stylolitization and diagenetic, and

• isotopic evidence that the interpreted fluids responsible for Zn-Pb-Ag

mineralization are distinct from the Cu (D4) mineralizing fluids.

This study benefits from the wealth of studies into the stratigraphy, structure, alteration

and mineralization processes in the Mt Isa – George Fisher – Lake Moondarra area.

Fewer studies have been undertaken on the George Fisher deposit itself, principally due to

the lack of mine development pre-1998. The key studies of the George Fisher deposit and

environs include comprehensive analysis of the mineralization and alteration paragenesis

(Chapman, 1999) and structural analysis of the adjacent Hilton deposit (Valenta, 1988).

The purpose of this study is to make detailed structural observations of the George Fisher

deposit host-rocks and mineralization from thin-section to deposit-scale and assess

whether areas of higher-grade and thicker mineralization are systematically related to

deformational features or a unique structural setting. This requires analysis of grade and

sulfide distribution and interpretation of the relative timing of textural varieties of

mineralization within the framework of the structural history of the deposit. The results


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of this analysis are likely to be significant guides for within-mine definition and near-

mine/extensional exploration for Zn-Pb-Ag mineralization at George Fisher. Absolute

ages from rhenium-osmium isotopic data collected from the sulfide samples will assist in

refining the timing of mineralization relative to deformation episodes and the isotopic

signature can indicate the source of metals which comprise the deposit. Comparison of

the interpreted controls on the setting of the George Fisher Zn-Pb-Ag deposit with other

significant deposits of the Western Fold Belt in the latter part of this study aims to add to

the knowledge base on metallogenic processes in the Mt Isa Inlier and is intended as an

aid to exploration for further Zn-Pb-Ag mineralization.

Thesis Structure

This thesis is presented as six sections (A-F), each written in journal article format and

summarized below:

Part A. The structural features observed from mine-scale through to micro-scale at the

George Fisher deposit are described and their overprinting relationships demonstrated.

Emphasis is placed on micro-scale observations as the foliations are not mappable at the

exposure-scale. Areas which have unique geometric relationships of overprinting

structures and more intense foliation and/or fold development are indicated.

Part B. Grade distribution of constituent metals and the cumulative thickness of

mineralization is evaluated enabling definition of ore-shoot orientations and geometry.

Ore-shoot locations and orientations correlate with identified structures and domains in

Part A and is suggestive of some mine-scale remobilization/syntectonic mineralization.

Part C. Criteria for determining the deformation history of sphalerite through fractal

analysis of grain boundary geometry is established. Undeformed, deformed, and

recrystallized sphalerite can be distinguished using these criteria and conventional

microscopic analysis.


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Part D. Distinction and relative timing of mineralization-types is established through

mine-scale distribution, mapping at exposure to hand-specimen scale, and microtextural

analysis. Interpretation of the range of mineralization types includes both pre- and post-

deformation sulphide deposition.

Part E. Re-Os isotopic analysis of the ore-sulphides and some host-rocks suggests that

Zn and Pb have a common source from a mantle-derived fluid. An age estimate is

obtained but does not indicate a specific deformation episode controlling later

mineralization as the uncertainty is large.

Part F. Utilizing interpretations from Parts A-E, comparison is made with other large

Zn-Pb-Ag deposits in the Western Fold Belt to determine whether there are consistent

controls on the location of these deposits.

References

Blanchard, R. and Hall, G., 1942. Mt Isa ore deposition: Economic Geology, v. 32, p.

1042-1057.

Chapman, L.H., 1999. Geology and genesis of the George Fisher Zn-Pb-Ag deposit,

Mount Isa, Australia. (Unpublished PhD thesis – James Cook University,

Townsville).

Chapman, L.H., 2004. Geology and mineralization styles of the George Fisher Zn-Pb-Ag

deposit, Mount Isa, Australia: Economic Geology, v. 99, p. 233-255.

Cooke, D.R., Bull, S.W., Large, R.R., and McGoldrick, P.J., 2000. The importance of

oxidized brines for the formation of Australian proterozoic stratiform sediment-

hosted Pb-Zn (Sedex) deposits: Economic Geology, v. 95, p. 1-18.

Davis, T.P., 2004. Mine-scale structural controls on the Mount Isa Zn-Pb-Ag and Cu

orebodies: Economic Geology, v. 99, p. 543-559.


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Hancock, M.C. and Purvis, A.H., 1990. Lady Loretta Silver-Lead-Zinc Deposit, In:

Hughes, F.E. (Ed.), Geology of the mineral deposits of Australia and Papua New

Guinea. p. 943-948, (The Australasian Institute of Mining and Metallurgy:

Melbourne).

Large, R., Cooke, D., and Davidson, G., 2002, What is a world class deposit?: Abstracts

of the 16th Australian Geological Convention, p. 266.

Neudert, M.K. and Russel, R.E., 1981. Shallow water and hypersaline features from the

middle Proterozoic Mount Isa sequence: Nature, v. 293, p. 284-286.

Perkins, W.G., 1997. Mount Isa lead-zinc orebodies: Replacement lodes in a zoned

syndeformational copper-lead-zinc system?: Ore Geology Reviews, v. 12, p. 61-

110.

Perkins, W.G., 1998. Timing of formation of Proterozoic stratiform fine-grained pyrite:

Post-diagenetic cleavage replacement at Mount Isa?: Economic Geology, v. 93, p.

1153-1164.

Russell, M.J., Solomon, M., and Walshe, J.C., 1981. The genesis of sediment-hosted,

exhalative zinc and lead deposits: Mineralium Deposita, v. 16, p. 113-127.

Sawkins, F.J., 1984. Ore genesis by episodic dewatering of sedimentary basins:

Application to giant Proterozoic lead-zinc deposits: Geology, v. 12, p. 451-454.

Singer, D.H., 1995, World Class Base and Precious Metal Deposits – A Quantitative

Analysis: Economic Geology, v. 90, p. 88-104.

Valenta, R.K., 1988. Deformation, fluid flow and mineralization in the Hilton area, Mt

Isa, Australia. (Unpublished PhD thesis – Monash University, Melbourne).

Valenta, R.K., 1994, Deformation in the host rocks and stratiform mineralization in the

Hilton Mine area, Mt Isa: Australian Journal of Earth Sciences, v. 41, p. 429-443.

Wilkinson, D.L., 1995. Structural termination of 8 orebody and its implications for the

genesis of lead-zinc-silver mineralisation, Mount Isa, Queensland: Unpublished

Hons thesis, James Cook University, Townsville, Australia.

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