HYDROTHERMAL ALTERATION OF CARBONACEOUS MUDSTONES HOSTING THE ESKAY

195 Pages·2015·5.91 MB·English

by  Andrew Ritts

HYDROTHERMAL ALTERATION OF CARBONACEOUS MUDSTONES HOSTING
THE ESKAY CREEK AU DEPOSIT, BRITISH COLUMBIA

by
Tom Meuzelaar
A thesis submitted to the Faculty and 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: ________________________
Tom Meuzelaar

Signed: ________________________
Dr. Thomas Monecke
Thesis Advisor

Golden, Colorado

Date _____________________

Signed: ________________________
Dr. Paul Santi
Professor and Head
Department of Geology and Geological Engineering
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ABSTRACT

The Jurassic Eskay Creek deposit in northwestern British Columbia represents an
unusual volcanic-hosted massive sulfide deposit that is characterized by a high precious
metal content, enrichment of the epithermal-style suite of elements, a complex sulfide
and sulfosalt ore mineralogy, and a relatively low temperature of ore deposition. The
stratiform ore lenses of the deposit are hosted by carbonaceous mudstones that only show
cryptic alteration. An integrated approach consisting of mineralogical and geochemical
analysis, multivariate statistical data reduction, mass transfer analysis, and equilibrium
geochemical modeling was adopted to identify alteration vectors to ore that can be used
to identify synvolcanic precious and base metal deposits hosted by fine-grained,
carbonaceous mud-stones. Despite the fairly extensive previous research carried out on
the Eskay Creek deposit, the nature of hydrothermal alteration of the carbonaceous
mudstone host has not been previously investigated. The present thesis addresses this
critical knowledge gap. The research provides new critical insights into the nature and
evolution of the hydrothermal fluids involved in formation of the Eskay Creek deposit
and the development of the alteration halo surrounding the deposit. The results indicate
that the integration of field observations, detailed micro-analysis, multivariate data
reduction and geochemical modeling is an effective, integrated and innovative approach
for studying petrographically challenging geologic materials.

Textural evidence suggests that the carbonaceous mudstone is a complex rock
type. The mineralogical composition of this fine-grained rock can be related to primary
processes including deposition, diagenetic modification, hydrothermal alteration, and
low-grade metamorphic recrystallization. Hydrothermal alteration patterns in the
mudstones include a silicified core with peripheral chlorite and white mica formation and
albite destruction, as well as extensive carbonate alteration. Ankerite represents the most
common hydrothermal carbonate mineral and is frequently associated with kaolinite.
Locally, potassium feldspar alteration of the mudstone is strongly developed. Major
element mass transfer can be related to the changes in mudstone mineralogy. Additional
vectors to ore include increases in the iron, magnesium, and manganese contents in
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carbonate minerals proximal to ore. Iron enrichment in chlorite occurs distally in the
hanging-wall, while proximal chlorite is enriched in magnesium. Hydrothermally formed
pyrite is arsenic enriched. Base metal enrichments in proximal samples are associated
with sulfide minerals, while distal samples may contain anomalous base metal contents
related to the presence of organic material.

Mineral stability constraints suggest that the observed alteration of the host
mudstone must have occurred from slightly acidic to alkaline fluids that have been highly
equilibrated with the host rocks. Modeling suggests that the base metal sulfides, precious
metal-bearing phases, and hydrothermal clay and carbonate minerals likely do not
represent co-precipitates, but must have formed at different physicochemical conditions
during the evolution of the hydrothermal system. The primary controls on the distribution
of mineral phases in the deposit are CO fugacity (which controls the acidity and ionic
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strength of solutions), temperature, and protolith wall-rock chemistry. Seawater must
have contributed the magnesium to proximal carbonates and chlorite alteration, while the
wall rock, in particular feldspars and detrital clays, represent the likely source for the
calcium required for carbonate formation. The results of the present study demonstrate
that low-temperature (<200ºC) hydrothermal alteration, diagenesis, and a low-grade
metamorphic overprint resulted in the formation of broadly comparable mineral
assemblages. This finding has important implications to mineral exploration as minerals
of the dolomite-ankerite solid solution represent the only rock-forming minerals directly
indicative for a hydrothermal overprint of the carbonaceous mudstone that can be
identified tens to hundreds of meters from ore.

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TABLE OF CONTENTS

ABSTRACT ....................................................................................................................... iii
TABLE OF CONTENTS .....................................................................................................v
LIST OF FIGURES ......................................................................................................... viii
LIST OF TABLES ...............................................................................................................x
LIST OF ABBREVIATIONS ............................................................................................ xi
ACKNOWLEDGEMENTS ............................................................................................. xvi
CHAPTER1: INTRODUCTION .........................................................................................1
1.1 Exploration in Geologically Complex Environments ...........................................1
1.2 Eskay Creek Sulfide and Sulfosalt Deposit ..........................................................2
1.3 Previous Research .................................................................................................4
1.4 Thesis Organization ..............................................................................................5
1.5 References .............................................................................................................8
CHAPTER2: MINERALOGY AND GEOCHEMISTRY ................................................13
2.1 Abstract ...............................................................................................................13
2.2 Introduction .........................................................................................................15
2.3 Geological Setting ...............................................................................................16
2.3.1 Regional Geology .....................................................................................16
2.3.2 Stratigraphy of the Mine Succession ........................................................17
2.3.3 Ore Zones ..................................................................................................21
2.4 Materials and Methods ........................................................................................22
2.5 Analytical Results ...............................................................................................26
2.5.1 Mineralogical Composition of Carbonaceous Mudstone .........................26
2.5.2 Major Element Composition of Carbonaceous Mudstone ........................36
2.5.3 Trace Element Composition of Carbonaceous Mudstone ........................38
2.5.4 Rare Element Geochemistry of Carbonaceous Mudstone ........................44
2.6 Statistical Analysis ..............................................................................................49
2.6.1 Principal Component Analysis .................................................................49
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2.6.2 Pearce Element Ratios ..............................................................................51
2.6.3 PCA Factor Groups and Loadings Scores ................................................52
2.7 Discussion ...........................................................................................................62
2.7.1 Mudstone Compositional Variations ........................................................62
2.7.2 Styles of Hydrothermal Alteration ............................................................64
2.7.3 Alteration Halo Model ..............................................................................68
2.7.4 Implications to Gold Enrichment in Submarine Hydrothermal Systems..69
2.7.5 Implications to Exploration.......................................................................71
2.8 Conclusions .........................................................................................................73
2.9 Acknowledgements .............................................................................................74
2.10 References ...........................................................................................................75
CHAPTER3: CORRELATIVE MICROSCOPY ..............................................................83
3.1 Abstract ...............................................................................................................83
3.2 Introduction .........................................................................................................84
3.3 Geological Setting ...............................................................................................86
3.4 Materials and Methods ........................................................................................91
3.5 Results .................................................................................................................92
3.5.1 Mudstone Petrography ..............................................................................92
3.5.2 Optical Cathodoluminescence Microscopy ..............................................96
3.5.3 Scanning Electron Microscopy .................................................................98
3.5.4 Electron Microprobe Analysis of Carbonate Minerals ...........................100
3.5.5 Electron Microprobe Analysis of Illite and Chlorite ..............................100
3.5.6 Transmission Electron Microscopy of Illite ...........................................101
3.6 Discussion .........................................................................................................112
3.6.1 Primary Mudstone Composition .............................................................112
3.6.2 Devitrification of Volcanic Glass ...........................................................112
3.6.3 Feldspar Alteration..................................................................................114
3.6.4 Carbonate Alteration ...............................................................................116
3.6.5 Silicification ............................................................................................117
3.6.6 Processes of Sulfide Mineral Formation .................................................117
3.6.7 Diagenetic and Low-Grade Metamorphic Overprint ..............................119
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3.7 Conclusions .......................................................................................................121
3.7 Acknowledgements ...........................................................................................122
3.8 References .........................................................................................................124
CHAPTER4: GEOCHEMICAL MODELING ................................................................129
4.1 Abstract .............................................................................................................129
4.2 Introduction .......................................................................................................130
4.3 Geological Background ....................................................................................132
4.4 Methods.............................................................................................................135
4.4.1 H S Solubility .........................................................................................136
2
4.4.2 CO Solubility .........................................................................................138
2
4.4.3 The Role of Boiling ................................................................................142
4.4.4 Equilibrium Assumption .........................................................................143
4.4.5 Thermodynamic Models for Carbonate Minerals ...................................144
4.5 Results ...............................................................................................................144
4.5.1 Reaction with Mudstone of Rhyolitic Provenance .................................144
4.5.2 Reaction with Mudstone of Bimodal Provenance ..................................147
4.5.3 Mixing with Seawater .............................................................................153
4.6 Discussion .........................................................................................................157
4.6.1 Modern Day Vent Analogues .................................................................157
4.6.2 Comparison of Model Results to Observed Alteration Mineralogy .......159
4.6.3 Eskay Creek Alteration Model................................................................161
4.7 Conclusions .......................................................................................................164
4.8 Acknowledgements ...........................................................................................165
4.9 References .........................................................................................................166
CHAPTER5: CONCLUSIONS .......................................................................................171
APPENDIX: SUPPLEMENTARY ELECTRONIC FILES ............................................177

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LIST OF FIGURES

Figure 1-1 Bedrock terrane map of British Columbia ......................................................3
Figure 2-1 Geological map of Iskut River area and Eskay Creek deposit ......................18
Figure 2-2 Geological map of Eskay Creek anticline .....................................................19
Figure 2-3 East-west cross-section of western limb of the Eskay Creek anticline .........20
Figure 2-4 Plan view of the spatial distribution of ore zones .........................................23
Figure 2-5 Histograms depicting occurrence of rock-forming minerals ........................27
Figure 2-6 Geological section of 21C zone: quartz, plagioclase, and microcline ..........32
Figure 2-7 Geological section of 21C zone: illite and chlorite .......................................33
Figure 2-8 Geological section of 21C zone: carbonates and pyrite ................................35
Figure 2-9 Harker diagrams: mudstone major element content .....................................37
Figure 2-10 Mudstone epithermal element concentrations ..............................................40
Figure 2-11 Mudstone base metal concentrations ............................................................41
Figure 2-12 Mudstone organophile element concentrations ............................................43
Figure 2-13 Chondrite-normalized REE plots: least and weakly altered samples ...........45
Figure 2-14 Chondrite-normalized REE plots: variable altered samples .........................46
Figure 2-15 Log ratios of Al O and TiO over Au ..........................................................52
2 3 2
Figure 2-16 Component and mineral mass loss and gains ...............................................53
Figure 2-17 Trace element scatter plots............................................................................57
Figure 2-18 Whole rock and mineral abundance scatter plots .........................................59
Figure 3-1 Geological map of Iskut River area and Eskay Creek deposit ......................87
Figure 3-2 Geological map of Eskay Creek anticline .....................................................88
Figure 3-3 Plan view of the spatial distribution of ore zones .........................................90
Figure 3-4 Microphotographs of carbonaceous mudstones ............................................94
Figure 3-5 Optical catholuminescence images of carbonaceous mudstones ..................97
Figure 3-6 Back-scatter electron microscope images of carbonaceous mudstones ........99
Figure 3-7 Electron microprobe analyses of carbonate minerals .................................101
Figure 3-8 Compositional variations of chlorite as function of distance to ore ...........106
Figure 3-9 Low magnification TEM image showing large, defect-free illite...............107
Figure 3-10 Low magnification TEM image show thin, elongated illite .......................108
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Figure 3-11 Low-magnification TEM image showing cross-cutting illite crystals ........109
Figure 3-12 Compositional variations in illite ................................................................110
Figure 4-1 Geological map of Iskut River area and Eskay Creek deposit ....................133
Figure 4-2 East-west cross-section of western limb of Eskay Creek anticline .............135
Figure 4-3 Comparison of published CO solubility data ............................................141
2
Figure 4-4 Effects of changing fluid salinity on CO solubility ...................................142
2
Figure 4-5 Equilibration with mudstones of rhyolitic provenance- fluid pH ...............147
Figure 4-6 Equilibration with mudstones of rhyolitic provenance- mineral stability ..148
Figure 4-7 Equilibration with mudstones of bimodal provenance- mineral stability ...151
Figure 4-8 Seawater mixing - mineral stability ............................................................155
Figure 4-9 Compositions of modern seafloor hydrothermal vent fluids ......................158

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LIST OF TABLES

Table 2-1 Composition of representative carbonaceous mudstone samples .................28
Table 2-2 PCA Results ..................................................................................................55
Table 3-1 Representative electron microprobe analyses of carbonate minerals .........102
Table 3-2 Representative electron microprobe analyses of illite ................................103
Table 3-3 Representative electron microprobe analyses of chlorite ...........................104
Table 3-4 Representative analytical electron microscopy analyses of illite 2M .........111
Table 3-5 Representative analytical electron microscopy analyses of illite 1M .........111
Table 4-1 Model parameters for equilibration with rhyolitic mudstone .....................146
Table 4-2 Model parameters for equilibration with bimodal mudstone ......................150
Table 4-3 Model parameters for seawater mixing models ..........................................154

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