the veladero high-sulfidation epithermal au-ag deposit, argentina

231 Pages·2012·5.04 MB·English

THE VELADERO HIGH-SULFIDATION EPITHERMAL AU-AG DEPOSIT,
ARGENTINA: VOLCANIC STRATIGRAPHY, ALTERATION,
MINERALIZATION, AND QUARTZ PARAGENESIS

by
Elizabeth A. Holley
A thesis submitted to the Faculty and the Board of Trustees of the Colorado
School of Mines in partial fulfillment of the requirements for the degree of Doctor of
Philosophy (Geology).

Golden, Colorado
Date _________________

Signed:___________________________
Elizabeth A. Holley

Signed:___________________________
Dr. Thomas Monecke
Thesis Advisor

Golden, Colorado
Date _________________

Signed:___________________________
Dr. J. D. Humphrey
Associate Professor and Head
Department of Geology and Geological Engineering

ii
ABSTRACT

The Veladero Au-Ag high-sulfidation epithermal deposit is located in the El
Indio-Pasuca belt in Argentina. Veladero is an oxidized deposit that contained reserves of
12.2 Moz of Au and 226.2 Moz of Ag at the end of 2008. Ore is primarily hosted in
silicified breccias. The volcanic package at Veladero is a coalescing complex of domes,
diatremes, and hydrothermal breccia bodies, mantled by a thick apron of volcaniclastic
deposits. These units are inferred to be of Cerro de las Tórtolas age (16.0 ± 0.2 to 14.9 ±
0.7 Ma) at Amable in the southern part of the Veladero area, and Vacas Heladas age
(12.7 ± 0.9 to 11.0 ± 0.2 Ma) at Filo Federico in the northern part of Veladero.
Emplacement of hydrothermal breccia units was accompanied or shortly followed
by multiple pulses of magmatic-hydrothermal and alternating jarosite alteration from
about 15.4 to 8.9 Ma. Alunite and jarosite 40Ar-39Ar ages and U-Pb zircon ages of
crosscutting dikes restrict mineralization at Amable to the period from 15.4 to 12.14 
0.11 Ma, probably commencing closer to 12.7 Ma. At Filo Federico, Au mineralization
commenced after the beginning of alunite alteration at 11.05  0.12 Ma. The new data
suggest that the maximum age of mineralization in the belt is slightly higher than
previously recognized.
The bulk of Veladero Ag is hosted in Ag-bearing jarosite in the jarosite-
argentojarosite solid solution. Some silver occurs as < 0.1 m iodargyrite and aggregates
of <0.1 m particles with variable Ag, I, Br, Cl, Hg, Se, S, and Fe concentrations. Rare
Ag-Fe sulfides were identified. Veladero Au exists as >800 fineness grains of native Au.
A phase of euhedral, fracture-controlled quartz with growth zone parallel iron
oxide inclusions has been identified at Veladero, post-dating leaching of the host rock to
vuggy quartz and pre-dating Au mineralization. The quartz is evidence that the fluids that
transport the precious metals are distinct from the magmatic vapors that interact with
meteoric water to produce alteration in high-sulfidation disseminated deposits, and that at
least the first stage of metals are transported by a low-temperature fluid also capable of
precipitating quartz.
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TABLE OF CONTENTS

ABSTRACT ...................................................................................................................... iii
LIST OF FIGURES ........................................................................................................ viii
LIST OF TABLES ..............................................................................................................x
LIST OF ABBREVIATIONS ........................................................................................... xi
ACKNOWLEDGMENTS ............................................................................................... xii
DEDICATION ................................................................................................................ xiii
CHAPTER 1 INTRODUCTION ........................................................................................1
1.1 The Magmatic-Hydrothermal System ...............................................................2
1.2 Characteristics of High-Sulfidation Epithermal Deposits .................................3
1.3 The Veladero Deposit ......................................................................................4
1.4 Study Overview ...............................................................................................7
1.4.1 Volcanic Stratigraphy at Veladero ..................................................7
1.4.2 Veladero Alteration .........................................................................8
1.4.3 Veladero Mineralization .................................................................8
1.4.4 Quartz Paragenesis at Veladero ......................................................9
CHAPTER 2 VOLCANIC STRATIGRAPHY ................................................................10
2.1 Introduction .....................................................................................................10
2.2 Regional Stratigraphy .....................................................................................10
2.3 Local Structural Setting ..................................................................................15
2.4 The Veladero Deposit .....................................................................................15
2.5 Methods ...........................................................................................................18
2.6 Breccia Nomenclature .....................................................................................21
2.7 Major Facies ....................................................................................................23
2.7.1 Facies 1 ............................................................................................24
2.7.2 Facies 2 ............................................................................................27
2.7.3 Facies 3 ............................................................................................28
2.7.4 Facies 4 ............................................................................................28
2.8 Minor Facies ...................................................................................................32
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2.8.1 Facies 5 ............................................................................................32
2.8.2 Facies 6 ............................................................................................35
2.8.3 Facies 7 ............................................................................................35
2.8.4 Facies 8 ............................................................................................38
2.8.5 Facies 9 ............................................................................................38
2.8.6 Facies 10 ..........................................................................................39
2.9 Alteration ........................................................................................................41
2.10 Mineralization ...............................................................................................43
2.11 Genetic Model ................................................................................................47
2.12 Discussion .....................................................................................................50
2.13 Conclusions ...................................................................................................53
CHAPTER 3 ALUNITE AND JAROSITE ALTERATION ............................................55
3.1 Introduction .....................................................................................................55
3.2 Geology of the El Indio-Pascua Belt ..............................................................57
3.3 The Veladero Deposit .....................................................................................59
3.3.1 Mineralization ..................................................................................60
3.3.2 Deposit-Scale Alteration ..................................................................60
3.4 Methods ...........................................................................................................62
3.5 Quartz-Alunite Alteration Zones ....................................................................63
3.6 Jarosite Alteration Zones ................................................................................64
3.7 Relative Timing of Alteration Minerals ..........................................................67
3.8 Alteration Mineral Chemistry .........................................................................67
3.9 Sulfate Mineral Stable Isotopes ......................................................................71
3.9.1 Stable Isotope Results ......................................................................72
3.9.2 Discussion of Sulfate Mineral Origins .............................................73
3.9.3 Estimates of Temperature and Elevation .........................................78
3.10 Geochronology ..............................................................................................79
3.10.1 Alteration Ages ..............................................................................79
3.10.2 Discussion of Alteration Ages .......................................................81
3.11 Constraints on Timing of Mineralization ......................................................85
3.12 Conclusions ...................................................................................................87
v
CHAPTER 4 SILVER MINERALOGY ..........................................................................91
4.1 Introduction .....................................................................................................91
4.2 Deposit Geology .............................................................................................91
4.3 Mineralized Centers ........................................................................................92
4.4 Methods ...........................................................................................................95
4.5 Veladero Ore Minerals ....................................................................................97
4.6 Ag Sulfides ......................................................................................................98
4.7 Ag Iodides .......................................................................................................98
4.8 Ag-bearing Sulfates ........................................................................................99
4.9 Trace Element Compositions of Jarosite ......................................................101
4.10 Quantitative Analysis of Ag-bearing Jarosite .............................................103
4.11 Discussion ...................................................................................................106
4.12 Conclusions .................................................................................................110
CHAPTER 5 QUARTZ PARAGENESIS ......................................................................114
5.1 Introduction ...................................................................................................114
5.2 The Veladero Deposit ...................................................................................116
5.3 Methods .........................................................................................................120
5.4 Veladero Ore-hosting Breccias .....................................................................121
5.5 Veladero Ore .................................................................................................122
5.6 Optical Cathodoluminescence of Quartz ......................................................125
5.7 Fluid Inclusions in Quartz .............................................................................125
5.8 Trace Element Characterization of Quartz ...................................................128
5.9 Discussion: Fluid Inclusions .........................................................................128
5.10 Discussion: Fracture-Controlled Quartz .....................................................131
5.11 Conclusions .................................................................................................132
CHAPTER 6 CONCLUSIONS ......................................................................................134
6.1 Veladero Geology .........................................................................................134
6.2 Veladero Exploration Implications ...............................................................136
6.3 Silver Recovery in High-Sulfidation Deposits .............................................138
6.4 Improving the High-Sulfidation Epithermal Deposit Model ........................139
REFERENCES CITED ...................................................................................................141
vi
APPENDIX A ANALYTICAL TECHNIQUES ............................................................157
APPENDIX B WHOLE ROCK ANALYTICAL RESULTS ........................................164
APPENDIX C QUANTITATIVE XRD .........................................................................168
APPENDIX D SAMPLE LOCATIONS AND ANALYSES .........................................170
APPENDIX E ELECTRON MICROPROBE DATA ....................................................186
APPENDIX F LA-ICP-MS DATA ................................................................................215
vii
LIST OF FIGURES

Figure 1.1 Location of the Veladero high-sulfidation deposit ............................................5
Figure 2.1 Location of the Veladero high-sulfidation deposit ..........................................11
Figure 2.2 Veladero area geology and stratigraphy ..........................................................13
Figure 2.3 Plan view maps of grade contours in the Veladero ore body ..........................16
Figure 2.4 North-south longitudinal sections of grade contours .......................................17
Figure 2.5 Map showing deposit-scale lithology ..............................................................19
Figure 2.6 Volumetrically significant facies in Amable and Filo Federico ......................26
Figure 2.7 Quartz-cemented, medium bedded, silicified-clast pebble breccia .................31
Figure 2.8 Feldspar porphyry intrusions and zircon U-Pb TIMS ages .............................34
Figure 2.9 Plots showing zircon U-Pb TIMS ages of feldspar porphyry ..........................35
Figure 2.10 Minor Veladero facies ...................................................................................37
Figure 2.11 Non-stratified, matrix-supported, polymict cobble breccia ...........................40
Figure 2.12 Map of deposit-scale alteration at Veladero ..................................................42
Figure 2.13 Amable 4145 m bench lithology and blast face Au, Ag ................................44
Figure 2.14 Drill core logs from Filo Federico and Amable ........................................... 45
Figure 2.15 Gold and silver grade for individual Veladero facies ....................................46
Figure 2.16 Schematic cross section .................................................................................48
Figure 3.1 Location of the Veladero high-sulfidation deposit ..........................................56
Figure 3.2 Veladero area geology and stratigraphy ..........................................................61
Figure 3.3 Deposit-scale alteration at Veladero with sample locations ............................62
Figure 3.4 Photomicrographs of Veladero alunite and jarosite ........................................65
Figure 3.5 BSE images of Veladero alunite and jarosite ..................................................66
Figure 3.6 Zoning in jarosite and alunite ..........................................................................69
Figure 3.7 EMP analyses of alunite and jarosite in apfu ..................................................70
Figure 3.8 Jarosite, alunite, and barite δ34S and δ18O ...................................................75
SO4
Figure 3.9 Veladero jarosite and alunite δ18O δ18O , and δD ....................................75
SO4, OH
Figure 3.10 Ar-Ar plateau ages .........................................................................................82
Figure 3.11 Ar-Ar plateau ages .........................................................................................83
viii
Figure 4.1 Location of the Veladero high-sulfidation deposit ..........................................93
Figure 4.2 Plan view maps of grade contours in the Veladero ore body ..........................94
Figure 4.3 North-south longitudinal sections of grade contours .......................................95
Figure 4.4 BSE images of Veladero iodide minerals ......................................................100
Figure 4.5 Jarosite textures .............................................................................................102
Figure 4.6 Jarosite samples analyzed by LA-ICP-MS ....................................................104
Figure 4.7 Jarosite samples analyzed by LA-ICP-MS ....................................................105
Figure 4.8 Whole rock Ag (ppm) and LA-ICP-MS Ag in jarosite (ppm) ......................106
Figure 4.9 Histogram showing wt % jarosite in whole rock samples .............................107
Figure 5.1 P-T-X phase diagram of the H O-NaCl system .............................................116
2
Figure 5.2 Location of the Veladero high-sulfidation deposit ........................................118
Figure 5.3 Plan view maps of grade contours in the Veladero ore body ........................119
Figure 5.4 Map showing alteration at Veladero ..............................................................120
Figure 5.5 Hand sample and thin section images of Veladero quartz ............................123
Figure 5.6 Photomicrographs of euhedral, fracture-controlled quartz ............................124
Figure 5.7 PPL and optical cathodoluminescence images of Veladero quartz ...............126
Figure 5.8 Fluid inclusions in residual quartz and a quartz vein fragment .....................127
Figure 5.9 Trace element maps .......................................................................................129

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LIST OF TABLES

Table 1.1 Major deposits and prospects of the El Indio-Pascua belt ..................................6
Table 2.1 U-Pb geochronology of zircons from Facies 5 .................................................33
Table 3.1 Sample locations, textures, ages, Au and Ag ....................................................89
Table 3.2 Summary of stable isotope results ....................................................................90
Table 4.1 Whole rock Au and Ag content, Ag in jarosite ...............................................112
Table 4.2 Ag-bearing minerals and modal % .................................................................113
x