Hydrothermal Alteration of Carbonaceous Mudstones Hosting the Eskay Creek Au Deposit, British Columbia

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

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 CO2 fugacity (which controls the acidity and ionic 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.

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

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

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 H2S Solubility ...... 136

4.4.2 CO2 Solubility ...... 138 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 Al2O3 and TiO2 over Au ...... 52 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 CO2 solubility data ...... 141

Figure 4-4 Effects of changing fluid salinity on CO2 solubility ...... 142 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

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

LIST OF ABBREVIATIONS General Abbreviations: AEM Analytical electron microscopy BSE Back-scattered electron CCD Charge-coupled device CL Cathodoluminescence EDX Energy dispersive x-ray spectroscopy ICP-MS Inductively coupled plasma-mass spectrometry ICP-OES Inductively coupled plasma-optical emission spectrometry EMP Electron microprobe GWB The Geochemist’s Workbench LLNL Lawrence Livermore National Laboratory ODP Ocean drilling program PCA Principal component analysis REE Rare earth element SEM Scanning electron microscopy TEM Transmission electron microscopy USGS United States Geologic Survey VHMS Volcanic hosted massive sulfide XRD X-ray diffraction

Units: 2Θ 2-theta Å Angstrom [species] Concentration º Degrees ºC Degrees Celsius µg/kg Microgram per kilogram µm Micrometer % Percent ‰ Permil

34S Sulfur-34 isotope fraction a Activity apfu Atomic per formula unit atm Atmosphere f fugacity g Gram g/t Grams per ton k Equilibrium constant kg Kilogram keV Kilo electron volt kV Kilovolts km Kilometer m Meter mA Microangstrom mg/kg Milligram per kilogram ml Milliliter mm Millimeter mmol Millimole Mt Metric tons n Number nA Nanoangstrom nm Nanometer

P-T-XNaCl Pressure-temperature-salinity pCO2 Pressure of carbon dioxide in atmospheres ppb Parts per billion ppm Parts per million s Second wt. % Weight percent

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Chemical Abbreviations: Ag Silver Al Aluminum

Al2O3 Aluminum oxide As Arsenic Au Gold Ba Barium Be Beryllium Bi Bismuth Ca Calcium CaO Calcium oxide Cd Cadmium Ce Cerium Cl Chlorine Co Cobalt

CO2 Carbon dioxide Cr Chromium Cs Cesium Cu Copper Eu Europium F Fluorine Fe Iron FeO Iron oxide (reduced)

Fe2O3 Iron oxide (oxidized) Ga Gallium Gd Gadolinium H+ Proton

H2O Water - HCO3 Bicarbonate HS- Hydrogen sulfide (ionic)

H2S Hydrogen sulfide

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Hf Hafnium Ho Holmium Hg Mercury In Indium K Potassium

K2O Potassium oxide La Lanthanum Lu Lutetium Mg Magnesium MgO Magnesium oxide Mn Manganese MnO Manganese oxide Mo Molybdenum Na Sodium

Na2O Sodium oxide NaCl Sodium chloride Nb Niobium Nd Neodymium Ni Nickel Pb Lead Pd Palladium Pt Platinum Rb Rubidium S Sulfur Sb Antimony Sc Scandium Se Selenium Si Silicon

SiO2 Silicon dioxide Sm Samarium

SO2 Sulfur dioxide

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Sr Strontium SrO Strontium oxide Ta Tantalum Te Tellurium Th Thorium Ti Titanium

TiO2 Titanium oxide Tl Thallium U Uranium Y Yttrium Yb Ytterbium V Vanadium Zn Zinc

ACKNOWLEDGEMENTS

I am truly indebted and thankful to Thomas Monecke, my advisor, for his many hours of guidance, especially his willingness to share his comprehensive knowledge of ore deposit formation processes and high temperature hydrothermal fluids. I am grateful to him for his very significant contributions to the analytical and petrographic portions of this study and his extensive and invaluable background project knowledge. Finally, I thank him for his endless patience in working with a non-traditional, professional PhD student. I also wish to thank Nigel Kelly for his thoughtful review and discussions on high temperature-pressure fluid systems. I thank Elizabeth Holley for her helpful review, especially her feedback on the petrography, and Alexander Gysi for his review and helpful perspectives on CO2 behavior and geochemical modeling.

It is a great pleasure to thank my father, Henk Meuzelaar at the University of Utah, Emeritus, for countless hours of guidance and discussion on the multivariate statistical evaluation of the mudstones. I also express my deepest gratitude to Geoff Thyne at ESal, Wyoming, for his years of professional mentorship and guidance in geochemical modeling and deep fluid-rock interactions. I thank Stuart Simmons at the Earth and Geoscience Institute, University of Utah, for sharing his experience in fluid-mineral equilibria and phase separation. Excellent teaching by John Curtis in fossil fuels geochemistry and John Spear in geomicrobiology also provided inspiration for this research.

I owe a debt of gratitude to Heather Lowers for numerous sessions on the electron microprobe at the U.S. Geological Survey in Denver. The qualitative and quantitative XRD investigations would not have been possible without the help of Reinhard Kleeberg, TU Bergakademie Freiberg, Germany. The transmission electron microscopy was conducted by Giovanna Giorgetti at the University of Siena, Italy. Special thanks go to Dieter and Sebastian Dettmar at Bochum, Germany, for the timely and expert preparation of numerous polished thin sections.

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The research summarized in this thesis would not have been possible without the logistical support by Barrick Gold. Francois Robert and Tina Roth are thanked for facilitating initial research and sample collection at Eskay Creek. Dave Gale provided help in the field. Initial research on the mudstones by Thomas Monecke was supported by the Michael-Jürgen-Leisler-Kiep Foundation and the Emmy Noether Program of the German Research Foundation and a discovery grant by the Natural Science and Engineering Research Council of Canada awarded to Mark Hannington. Subsequent research was conducted under the Canadian Mining Industry Research Organization Project 08E04. Geoscience BC provided additional financial support. I also benefited from financial support by the Stewart R. Wallace Endowment in Economic Geology at Colorado School of Mines, RockWare, and Golder Associates. A research grant by the Society for Economic Geologists Canada Foundation is gratefully acknowledged.

Last, but not least, I owe my deepest gratitude to my wife and best friend Melissa, and her sustained support through many long days and nights of endeavor. I also thank my great community of family and friends for keeping me focused on the things that matter.

xvii CHAPTER 1 INTRODUCTION

This chapter outlines the objectives of this study, provides a brief project background, describes previous research, and details the organization of the remaining thesis.

1.1 Exploration in Geologically Complex Environments

Over the course of the last century, as near surface ore deposit discovery rates have diminished, the search for new mineral resources has to be conducted in increasingly complex geologic environments at increasing depth (e.g., Williams, 2013). To meet future global mineral resource de- mands, it is critical that comprehensive, integrated, multi-disciplinary targeting tools for vectoring towards ore at local and regional scales are developed. Furthermore, the proliferation of data that often results from such efforts requires an effective strategy for data management, reduction, and interpretation.

The present thesis focuses on the development of new alteration vectors that can be used to identify synvolcanic precious and base metal deposits hosted by fine-grained, carbonaceous mudstones. Carbonaceous mudstone horizons are a minor but ubiquitous feature of volcanic successions formed in submarine environments. The metallic and carbonaceous components of these sedimentary rocks are, in large part, a product of hydrothermal and microbial processes active at a regional scale during the development of the host volcanic complexes. Although these sedimentary rocks constitute major electromagnetic anomalies that are readily delineated and commonly drilled as potential targets for volcanic-hosted massive sulfide deposits, reliable tools for evaluating the metallogenic significance of these sedimentary rocks and for vectoring towards ore are presently largely lacking. This is primarily due to the lack of macroscopically identifiable alteration textures in these fine-grained rocks, their almost opaque nature under thin section, and significant primary compositional variations unrelated to hydrothermal processes.

Some of the most significant volcanic-hosted massive sulfide deposits worldwide are hosted by, or occur stratigraphically along, fine-grained carbonaceous mudstone horizons as these sedimentary rocks deposit form by suspension sedimentation at the ancient seafloor. The Eskay Creek depos-

- 1 - it, located 80 kilometers north of Stewart in the Iskut River area at the western margin of the allochthonous Stikine terrane of the northern Canadian Cordillera (Fig. 1-1), represents an unusually precious-metal-rich volcanic-hosted massive sulfide deposit (cf.Roth et al., 1999; Mercier-Langevin et al., 2011) that is hosted almost entirely within carbonaceous mudstone. It is the aim of this thesis to provide new critical information on the hydrothermal alteration developed within this host rock and to develop new approaches to studying these extremely fine-grained rocks.

1.2 Eskay Creek Sulfide and Sulfosalt Deposit

The Jurassic Eskay Creek deposit in the Iskut River area of British Columbia represents a world-class gold-rich volcanic-hosted massive sulfide deposit (cf. Mercier-Langevin et al., 2011). With a total past production of 2.25 million metric tons (Mt) of ore at average grades of 48.9 g/t Au and 2,334 g/t Ag (unpubl. data T. Monecke), Eskay Creek had the highest precious metal grades of all known volcanic-hosted massive sulfide deposits (cf. Franklin et al., 2005; Mercier-Langevin et al., 2011).

In addition to the high grades, the deposit displayed other characteristics unusual for this deposit class. In particular, Eskay Creek was typified by a pronounced enrichment of the epithermal suite of elements (As, Bi, Hg, and Sb; Macdonald et al., 1996; Roth et al., 1999). The bulk of the ore occurred as beds of clastic sulfide and sulfosalt minerals that are intercalated with carbonaceous mudstone. The clastic ores were composed primarily of sphalerite, tetrahedrite, lead sulfosalt minerals, galena, pyrite, and electrum (Roth et al., 1999). The sulfide mineral association and fluid inclusion evidence suggested that deposit formation occurred from a low temperature fluid (<200°C) having a relatively high gas content (Sherlock et al., 1999). Fluid-rock interaction resulted in widespread secondary potassium feldspar alteration in the felsic volcanic rocks forming the footwall to the deposit (Barrett and Sherlock, 1996), while the host mudstone and the hanging-wall to the deposit were characterized by carbonate alteration (Meuzelaar and Monecke, 2011).

Eskay Creek represents a prime example of how difficult exploration for small high-grade targets is in geologically complex terrains. The Eskay Creek property was first staked in 1932 by prospectors that were attracted to the site due to the occurrence of a line of gossanous bluffs that extended for a distance of seven kilometers (Britton et al., 1990). Despite continued exploration for the

- 2 -

FIG. 1-1: Bedrock terrane map of British Columbia (Colpron and Nelson, 2011). The Eskay Creek deposit is located north of Stewart in the Iskut River area at the western margin of the allochthonous Stikine terrane of the northern Canadian Cordillera.

following decades and abundant indications for the occurrence of high precious metal ores at the property, the Eskay Creek deposit was only discovered through blind drilling in 1989. The main ore lens of the deposit measured only 900 by 60 meters in plan (Roth et al., 1999). Discovery of this ore body represented one of the most significant technical exploration successes in Canadian mining history. Mining at Eskay Creek was conducted from 1995 to 2008. Despite the fact that the deposit is mined out today, significant exploration for similar deposits is still ongoing within the region.

- 3 - 1.3 Previous Research

Following discovery of the Eskay Creek deposit, a number of research projects were initiated to help frame the volcanotectonic setting of this world-class sulfide and sulfosalt deposit, understand the overall deposit genesis, and delineate the processes resulting in the unusual precious metal enrichment. Early accounts of the deposit geology include those by Blackwell (1990), Britton et al. (1990) and Idziszek et al. (1990). Initial research at Eskay Creek focusing on the mineralization and alteration pattern of the main ore lens was presented by Robinson (1991).

Substantial work in the Iskut River area and at Eskay Creek was conducted by the Mineral De- posit Research Unit at the University of British Columbia between 1989 and 1993. Major outcomes of this research project were published in a series of papers, including those focusing on the lithogeochemistry and alteration of the footwall rhyolite by Barrett and Sherlock (1996); the U-Pb geochronology as well as Nd and Pb isotope characteristics of the deposit by Childe (1996); the regional geological setting of Eskay Creek by Macdonald et al. (1996); and fluid inclusion and stable isotopic evidence by Sherlock et al. (1999). In addition, a number of field reports, including those by Bartsch (1992, 1993a), Ettlinger (1992), Roth and Godwin (1992), Roth (1993), and Sherlock et al. (1994) were released. The most detailed accounts of the geology of the Eskay Creek deposit and its setting are given in the graduate theses by Bartsch (1993b), Nadaraju (1993), Roth (1995), Childe (1997), and Roth (2002). A summary of the key findings was provided by Roth et al. (1999). A study focusing on the sulfur isotope and textural zoning of pyrite contained in mudstone from Eskay Creek was summarized by Roth and Taylor (2000). Maps at a 1:50,000 scale compiling the work conducted in the Iskut River area between 1989 and 1993 were published by Lewis (2013) and Lewis et al. (2013).

Despite extensive exploration drilling and continued underground development, relatively little research has been carried out at Eskay Creek during the final years of operation. A major regional mapping project in the Iskut River area was initiated in 2003 by the British Columbia Geological Survey and the Geological Survey of Canada. The results of the mapping were summarized in a number of reports, including Simpson and Nelson (2004), Alldrick et al. (2005), Barresi and Dostal (2005), Barresi et al. (2005), and Alldrick (2006). A 1:50,000 map of the Eskay Creek area was published by Alldrick et al. (2006a) and a 1:100,000 geological map of the Iskut River area is given by

- 4 - Alldrick et al. (2006b). Volcanological research on exploration diamond drill core from Eskay Creek is described by Monecke et al. (2005). In addition, publications by Grammatikopoulos and Roth (2002) as well as Grammatikopoulos et al. (2005, 2006) summarize the outcomes of a series of mineral chemical investigations on the ores from Eskay Creek. A new regional stratigraphic framework for the Jurassic volcanic rocks hosting Eskay Creek was established by Gagnon et al. (2012).

1.4 Thesis Organization

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 will address this critical knowledge gap.

Following this introductory chapter, this thesis comprises three main chapters, to be submitted as separate papers to Economic Geology. The three papers will form part of a series of papers that will contribute to a special issue that is devoted to the geology and alteration of the volcanic succession hosting the Eskay Creek sulfide and sulfosalt deposit. Contributions by other authors will focus on the regional setting of the deposit and the volcanological setting of Eskay Creek. The special issue will also include a paper that describes the alteration of the rhyolite occurring in the footwall of the stratiform ore lenses. In addition, previous research on the trace element geochemistry and isotopic composition of pyrite in the carbonaceous mudstone will be summarized as a separate paper.

Chapter 2 details the comprehensive evaluation of whole-rock mineralogical and bulk geochemical relationships within the mudstone hosting the stratiform ore lenses. A large compositional dataset for 180 mudstone samples was evaluated using multivariate statistical data reduction techniques to identify and characterize the hydrothermal processes associated with the formation of the deposit. Each sample was characterized by quantitative X-ray diffraction analysis to determine the abundances of minerals present in the mudstone and analyzed for its major and trace element concentrations. Principal component analysis (PCA) was used as the data reduction technique to establish higher order structure in the large dataset and to facilitate further data exploration. Combined with Pearce element analysis, PCA helped with the identification and interpretation of trends and vectors to ore related to alteration and mineralization within the fine-grained carbonaceous mudstones.

- 5 - Chapter 3 identifies mineralogical and petrographic characteristics of the variably altered carbonaceous mudstones hosting the stratiform ore lenses at Eskay Creek using results from detailed correlative microscopy, which included optical microscopy, optical cathodoluminescence microscopy, scanning electron microscopy, and electron microprobe analysis. Although the fine-grained and carbonaceous nature of the mudstones makes them difficult to study, the results validate the findings of the multivariate data evaluation, mass transfer analysis, and fluid-rock equilibria modeling. The study demonstrates that the petrographic characteristics of the mudstone and the effects of hydrothermal alteration can be effectively studied use this integrated microscopy approach.

Chapter 4 summarizes the results of equilibrium thermodynamic reaction path modeling which was used to provide a conceptual geochemical framework for ore genesis at Eskay Creek and to validate the metasomatic trends and target vectors to ore established by the multivariate data reduction and mass transfer analysis and by micro-analytical petrographic observations. The relatively low temperatures (<200ºC) and salinities (<10 wt. % NaCl; Sherlock et al., 1999) interpreted for the Eskay Creek ore forming fluids facilitated the use of fluid-rock equilibria modeling as a tool to evaluate critical hydrothermal alteration and mineralization processes because the thermodynamic databases commonly employed in such modeling (e.g., Delaney and Lundeen, 1990) are parameterized for temperatures up to 300ºC and salinities to 3 molal (15 wt. % NaCl), provided that NaCl is the predominant solute (Bethke, 2008) and that correct gas solubility data are used, particularly for CO2 and H2S (Duan et al., 2006, 2007). Furthermore, it can be assumed that the hydrothermal fluids that formed Eskay Creek were highly evolved, reflecting near-equilibrium between the fluids and the igneous and sedimentary wall rocks through which the fluids circulated. As such, it is relatively straight forward to derive bulk hydrothermal fluid compositions, used as input in the modeling ef- fort, by way of saturation limits of the rock-forming minerals comprising the host. These fluids can be further constrained using the composition of modern seafloor hydrothermal vents (cf. Hannington et al., 2005) measured in similar tectonic environments. Finally, the sensitivity of alteration and mineralization to variable ocean chemistry can be broadly evaluated using data from modern and paleo- ocean studies (e.g., Elderfield et al., 2006).

The appendix of the thesis comprises two short papers that have been published as progress reports during the course of study. In addition, abstracts, posters and presentations published at four

- 6 - different conferences are included. The appendix also contains data tables for the 180 analyzed carbonaceous mudstone samples.

The research presented in the three chapters 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.

- 7 - 1.5 References

Alldrick, D.J., 2006, Eskay Rift Project (NTS 103O, P, 104A, B, G, H), northwestern British Co- lumbia: British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 2005, Paper 2006-1, p. 1–4. Alldrick, D.J., Nelson, J.L., and Barresi, T., 2005, Geology and mineral occurrences of the Upper Iskut River area: Tracking the Eskay Rift through northern British Columbia (Telegraph Creek NTS 104G/1, 2; Iskut River NTS 104B/9, 10, 15, 16): British Columbia Geological Survey, Geological Fieldwork 2004, Paper 2005-1, p. 1–30. Alldrick, D.J., Nelson, J.L., Barresi, T., Hewett, T., and Tan, S.H., 2006a, NTS 104B/9, 10, 15, 16; 104G/1,2: British Columbia Geological Survey, Open File 2006-6, 1:50,000 map. Alldrick, D.J., Nelson, J.L., Barresi, T., Stewart, M.L., and Simpson K., 2006b, Geology of the Up- per Iskut River area, British Columbia: British Columbia Geological Survey, Open File 2006- 2, 1:100,000 map. Barresi, T., and Dostal, J., 2005, Geochemistry and petrography of Upper Hazelton Group volcanics: VHMS-favourable stratigraphy in the Iskut River and Telegraph Creek map areas, northwestern British Columbia: British Columbia Geological Survey, Geological Fieldwork 2004, Paper 2005-1, p. 39–48. Barresi, T., Nelson, J.L., Alldrick, D.J., and Dostal, J., 2005, Pillow Basalt Ridge facies: Detailed mapping of Eskay Creek–equivalent stratigraphy in northwestern British Columbia: British Columbia Geological Survey, Geological Fieldwork 2004, Paper 2005-1, p. 31–38. Barrett, T.J., and Sherlock, R.L., 1996, Geology, lithogeochemistry and volcanic setting of the Eskay Creek Au-Ag-Cu-Zn deposit, northwestern British Columbia: Exploration and Mining Geolo- gy, v. 5, p. 339–368. Bartsch, R.D., 1992, Eskay Creek area, stratigraphy update (104B/9, 10): British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 1991, Paper 1992-1, p. 517–520. ______, 1993a, A rhyolite flow dome in the upper Hazelton Group, Eskay Creek area (104B/9,10): British Columbia Ministry of Energy, Mines and Petroleum Resources, Geologi- cal field work 1992, Paper 1993-1, p. 331–334.

- 8 - ______, 1993b, Volcanic stratigraphy and lithochemistry of the Lower Jurassic Hazelton Group, host to the Eskay Creek precious and base metal volcanogenic deposit: Unpublished M.Sc. thesis, Vancouver, Canada, University of British Columbia, 178 p. Bethke, C.M., 2008, Geochemical and Biogeochemical Reaction Modeling: New York, Cambridge University Press, 543 p. Blackwell, J., 1990, Geology of the Eskay Creek #21 deposits: The Gangue, The Newsletter for Mineral Deposits Division, Geological Association of Canada, v. 31, p. 1–4. Britton, J.M., Blackwell, J.D., and Schroeter, T.G., 1990, #21 zone deposit, Eskay Creek, northwestern British Columbia: British Columbia Ministry of Energy, Mines and Petroleum Resources, Exploration in British Columbia 1989, p. 197–223. Childe, F., 1996, U-Pb geochronology and Nd and Pb isotope characteristics of the Au-Ag-rich

Eskay Creek volcanogenic massive sulfide deposit, British Columbia: ECONOMIC GEOLOGY, v. 91, p. 1209–1224. ______, 1997, Timing and tectonic setting of volcanogenic massive sulphide deposits in British Columbia: Constraints from U-Pb geochronology, radiogenic isotopes, and geochemistry: Un- published Ph.D. thesis, Vancouver, Canada, University of British Columbia, 298 p. Colpron, M., and Nelson, J.L., 2011, A digital atlas of terranes for the northern Cordillera: British Columbia Ministry of Energy and Mines, GeoFile 2011-11. Delaney, J.M., and Lundeen, S.R.,1990, The LLNL thermochemical database, Lawrence Livermore National Laboratory Report URCL-21658, 150 p.

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H2S solubility in pure water and brines: Energy and Fuels, v. 21, p. 2056–2065. Elderfield, H., Holland, H.D., and Turekian, K.K., 2006, The Oceans and Marine Geochemistry: Treatise on Geochemistry, Volume 6/Edition 1: Amsterdam, Elsevier, 664 p. Ettlinger, A.D., 1992, Hydrothermal alteration and brecciation underlying the Eskay Creek polymetallic massive sulphide deposit (104B/9W): British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 1991, Paper 1992-1, p. 535–541. Franklin, J.M., Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic massive sulfide

deposits: ECONOMIC GEOLOGY 100TH ANNIVERSARY VOLUME, p. 523–560.

- 9 - Gagnon, J.F., Barresi, T., Waldron, J.W.F., Nelson, J.L., Poulton, T.P., and Cordey, F., 2012, Stra- tigraphy of the upper Hazelton Group and the Jurassic evolution of the Stikine terrane, British Columbia: Canadian Journal of Earth Sciences, v. 49, p. 1027–1052. Grammatikopoulos, T.A., and Roth, T., 2002, Mineralogical characterization and Hg deportment in field samples from the polymetallic Eskay Creek deposit, British Columbia, Canada: Interna- tional Journal of Surface Mining, Reclamation and Environment, v. 16, p. 180–195. Grammatikopoulos, T.A., Roth, T., and Valeyev, O., 2005, Compositional variation in Hg–Ag-rich tetrahedrite from the polymetallic Eskay Creek deposit, British Columbia, Canada Neues Jahr- buch für Mineralogie - Abhandlungen, v. 181, p. 281–292. Grammatikopoulos, T.A., Valeyev, O., and Roth, T., 2006, Compositional variation in Hg-bearing sphalerite from the polymetallic Eskay Creek deposit, British Columbia, Canada: Chemie der Erde, v. 66, p. 307–314. Hannington, M.D., de Ronde, C.E.J., and Petersen, S., 2005, Sea-floor tectonics and submarine hy-

drothermal systems, ECONOMIC GEOLOGY 100TH ANNIVERSARY VOLUME, p. 111–141. Idziszek, C., Blackwell, J., Fenlon, R., MacArthur, G., and Mallo, D., 1990, The Eskay Creek discovery: Mining Magazine, March 1990, p. 172–173. Lewis, P.D., 2013, Geology and compilation, Iskut River Area Geology, Northwest BC (104B/08, 09, 10 and part of 104B/01, 07, 11), Geoscience BC, Report 2013-05: 1:50,000 maps. Lewis, P.D., Hart, C.J.R., and Simpson, K.A., 2013, Re-release of the Mineral Deposit Research Unit’s Iskut River area maps (1989-1993), northwestern British Columbia (NTS 104B/08, /09, /10, parts of 104B/01, /07, /11): Geoscience BC, Report 2013-1, p. 33–36. Macdonald, A.J., Lewis, P.D., Thompson, J.F.H., Nadaraju, G., Bartsch, R.D., Bridge, D.J., Rhys, D.A., Roth, T., Kaip, A., Godwin, C.I., and Sinclair, A.J., 1996, Metallogeny of an Early to

Middle Jurassic Arc, Iskut River Area, northwestern British Columbia: ECONOMIC GEOLOGY, v. 91, p. 1098–1114. Mercier-Langevin, P., Hannington, M.D., Dube, B., and Becu, V., 2011, The gold content of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 509–539. Meuzelaar, T., and Monecke, T., 2011, Carbonaceous mudstone hosting the Eskay Creek deposit, northwestern British Columbia (NTS104B/09, /10): Multivariate statistical analysis of compositional trends: Geoscience BC Summary of Activities 2010, Geoscience BC, Report 2011-1, p. 45–56.

- 10 - Monecke, T., Gale, D., Roth, T., and Hannington, M.D., 2005, The submarine volcanic succession hosting the massive sulfide and sulfosalt Eskay Creek deposit, Canada, in Mao, Y. and Bier- lein, F.P., eds., Mineral deposit research: Meeting the global challenge, Proceedings of the 8th biennial SGA meeting, Beijing, China, 2003: Berlin, Springer, p. 655–658. Nadaraju, G.T., 1993, Triassic-Jurassic biochronology of the eastern Iskut River map area, northwestern British Columbia: Unpublished M.Sc. thesis, Vancouver, Canada, University of Brit- ish Columbia, 268 p. Robinson, H.M., 1991, Mineralisation and alteration patterns of the Central Lens (21B Zone), Eskay Creek, British Columbia, Canada: Unpublished M.Sc. thesis, London, U.K., Imperial College of Science, Technology and Medicine, University of London, 129 p. Roth, T., 1993, Surface geology of the 21A Zone, Eskay, Creek, British Columbia. British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 1992, Paper 1993- 1, p. 325–330. ______, 1995, Geology, alteration and mineralization in the 21A Zone, Eskay Creek, northwestern British Columbia, Canada: Unpublished M.Sc. thesis, Vancouver, Canada, University of British Columbia, 230 p. ______, 2002, Physical and chemical constraints on mineralization in the Eskay Creek deposit, northwestern British Columbia: Evidence from petrography, mineral chemistry, and sulfur isotopes: Unpublished Ph.D. thesis, Vancouver, Canada, University of British Columbia, 401 p. Roth, T., and Godwin, C.I., 1992, Preliminary geology of the 21A Zone, Eskay Creek, British Co- lumbia (104B/9W): British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 1991, Paper 1992-1, p. 529–534. Roth, T., and Taylor, B.E., 2000, Sulfur isotope and textural zoning of pyrite in mudstone about the polymetallic Eskay Creek deposit, northwestern British Columbia, Canada, in Gemmell, J.B. and Pongratz, J., eds., Volcanic environments and massive sulfide deposits: Hobart, CODES, University of Tasmania, Australia, p. 177–180. Roth, T., Thompson, J.F.H., and Barrett, T.J., 1999, The precious metal-rich Eskay Creek deposit,

northwestern British Columbia: REVIEWS IN ECONOMIC GEOLOGY, v. 8, p. 357–373. Sherlock, R.L., Barrett, T.J., Roth, T., Childe, F., Thompson, J.F.H., Kuran, D., Marsden, H., and Allen, R., 1994, Geological investigations of the 21B deposit, Eskay Creek, northwestern Brit- ish Columbia: British Columbia Ministry of Energy, Mines and Petroleum Resources, Geolog- ical field work 1993, Paper 1994-1, p. 357–364.

- 11 - Sherlock, R.L., Roth, T., Spooner, E.T.C., and Bray, C.J., 1999, Origin of the Eskay Creek precious metal-rich volcanogenic massive sulfide deposit: Fluid inclusion and stable isotope evidence:

ECONOMIC GEOLOGY, v. 94, p. 803–824. Simpson, K.A., and Nelson, J.L., 2004, Preliminary interpretations of mid-Jurassic volcanic and sedimentary facies in the East Telegraph Creek map area: Geological Survey of Canada, Current Research 2004-A1, p. 1–8. Williams, N., 2013, Mineral exploration under deep cover [abs.]: Geological Society of America 125th Anniversary Annual Meeting, Denver, Colorado, USA, 2013, Abstracts and Program, 271–7.

- 12 - CHAPTER 2 MINERALOGY AND GEOCHEMISTRY OF THE CARBONACEOUS MUDSTONE HOSTING THE ESKAY CREEK SULFIDE AND SULFOSALT DEPOSIT, BRITISH COLUMBIA: IDENTIFICATION OF ALTERATION VECTORS THROUGH PRINCIPAL COMPONENT ANALYSIS

Chapter 2 details the comprehensive evaluation of whole-rock mineralogical and bulk geochemical relationships within the mudstone hosting the stratiform ore lenses. A large compositional dataset for 180 mudstone samples was evaluated using multivariate statistical data reduction techniques to identify and characterize the hydrothermal processes associated with the formation of the deposit. Each sample was characterized by quantitative X-ray diffraction analysis to determine the abundances of minerals present in the mudstone and analyzed for its major and trace element concentrations. Principal component analysis (PCA) was used as the data reduction technique to establish higher order structure in the large dataset and to facilitate further data exploration. Combined with Pearce element analysis, PCA helped with the identification and interpretation of trends and vectors to ore related to alteration and mineralization within the fine-grained carbonaceous mud-stones.

2.1 Abstract

The Jurassic Eskay Creek deposit in the Iskut area of northwestern British Columbia represents an unusual, precious metal-rich polymetallic volcanic hosted massive sulfide and sulfosalt deposit. Economic concentrations of precious metals at Eskay Creek are mainly confined to stratiform ore lenses hosted by a thick interval of carbonaceous mudstone between a footwall rhyolite and overlying interval of basalt. Hydrothermal alteration of the mudstone host is cryptic as the fine-grained nature and highly carbonaceous nature of the mudstone hamper identification of alteration styles and intensities in hand specimen.

To identify the alteration characteristics of the ore host at Eskay Creek, samples of carbonaceous mudstone have been collected at variable distances to ore to characterize systematic changes in mineralogy, major element and trace element geochemistry. The resulting

- 13 - dataset was analyzed using multivariate statistical data reduction techniques. Principal component analysis was utilized to establish higher order structure in the dataset and to facilitate data exploration. Combined with Pearce element analysis, principal component analysis allowed the identification and interpretation of alteration-induced mineralogical and geochemical gradients and alteration vectors within the carbonaceous mudstone.

The research demonstrates that mudstone represents a lithology susceptible to hydrothermal alteration. Major alteration styles recognized include clay alteration of the volcaniclastic component of the mudstone. Illite and chlorite present in the rock most likely formed during diagenesis and low-grade metamorphism of smectite which resulted from destruction of primary mudstone volcanic glass and feldspars. Intensely altered mudstone is characterized by Na2O depletion and MgO enrichment. Extensive carbonate alteration is an important feature at Eskay Creek. Carbonate minerals belonging to the dolomite-ankerite and magnesite-siderite solid solutions are most common proximal to ore, causing whole-rock enrichment patterns in FeO, MgO, and MnO. Samples affected by hydrothermal carbonate alteration contain trace amounts of kaolinite. Calcite is present both in hydrothermally altered samples and in least-altered mudstone which was only affected by greenschist facies metamorphism. Additional styles of alteration recognized in some samples include silicification, K-feldspar alteration, and sulfide impregnation. Pyrite is the dominant sulfide mineral although trace amounts of sphalerite, galena, and chalcopyrite also occur.

The observed alteration patterns are consistent with Eskay Creek being a subaqueous hot spring deposit that formed at comparably low temperatures (<200ºC). The hydrothermal fluids were reduced, ranging from mildly acidic to moderately alkaline, and possessed a low sulfidation state. The hydrothermal fluids contained elevated CO2 contents. Alteration of the carbonaceous mudstone was broadly synchronous with background sedimentation and is most strongly developed proximal to fluid upflow zones as marked by the occurrence of discordant zones of mineralization and alteration in the footwall rhyolite.

- 14 - 2.2 Introduction

Eskay Creek is an unusual, precious metal-rich, polymetallic volcanic-hosted sulfide and sulfosalt deposit located in northwestern British Columbia, Canada. Mined from 1995 to 2008, the deposit produced 2.25 million metric tons of ore at average grades of 48.9 g/t Au and 2,334 g/t Ag, making it the volcanic-hosted massive sulfide deposit (VHMS) with the highest precious metal grades (Franklin et al., 2005). In addition to the extraordinary gold and silver grades, the Eskay Creek deposit is characterized by the abundance of clastic ores. The bedded clastic ores are typified by complex ore mineralogy, with sulfides and sulfosalt minerals occurring in low temperature assemblages not commonly encountered in VHMS deposits (Britton et al., 1990; Idziszek et al., 1990; Macdonald et al., 1996; Roth et al., 1999; Sherlock et al., 1999).

Economic concentrations of precious and base metals at Eskay Creek are mainly confined to stratiform clastic ore lenses hosted by a thick carbonaceous mudstone interval at the contact between felsic volcanic rocks and overlying basalt (Britton et al., 1990; Ettlinger, 1992; Barrett and Sherlock, 1996; Macdonald et al., 1996). Exploration for this resource was challenging because the high-grade ore zones are laterally discontinuous and commonly interlayered with low-grade or barren mudstone intervals. Although the mineralizing hydrothermal system was active over an extensive area, as evidenced by the occurrence of sulfide veins and disseminations throughout the footwall rhyolite near the deposit and up to at least two kilometers along strike away from the mine, it is currently not well established whether mineralogical gradients within the mudstone hosting the stratiform clastic sulfide and sulfosalt mineralization can be used for target vectoring. Due to the absence of readily recognizable alteration features in these fine- grained rocks and the highly carbonaceous nature of the mudstone, previous research has largely focused on the hydrothermal alteration pattern of the footwall rhyolite (Robinson, 1991; Barrett and Sherlock, 1996).

The present paper presents the results of the first comprehensive mineralogical and geochemical study on the carbonaceous mudstone. X-ray diffraction (XRD) analysis using the Rietveld method was conducted to quantitatively determine the mineralogical composition of a large number of mudstone samples collected throughout the deposit area. Bulk geochemical

- 15 - major and trace element analysis was conducted on the same samples. Principal component analysis was used as a data reduction technique to establish higher order structure in the large mineralogical and geochemical dataset and to facilitate data exploration. Combined with Pearce element analysis, this method allowed the identification and interpretation of alteration-induced compositional changes. The present study shows that the carbonaceous mudstone hosting the Eskay Creek deposit represents a host-rock lithology that was highly susceptible to hydrothermal alteration, even if alteration cannot be easily recognized macroscopically. Mineralogical and geochemical compositional trends were identified that allow target vectoring within the thick mudstone interval hosting the stratiform ores.

2.3 Geological Setting

In this section, the regional geology, the local mine succession stratigraphy and the ore zones of Eskay Creek are described.

2.3.1 Regional Geology

The Eskay Creek deposit is located in the Iskut River area at the western margin of the allochthonous Stikinia terrane of the northern Canadian Cordillera in British Columbia (Fig. 2- 1). Lower to Middle Jurassic volcanic and sedimentary rocks in the Iskut River area are assigned to the Hazelton Group, which records a stage of intense, extensional, continental margin arc volcanism (Barrett and Sherlock, 1996; Macdonald et al., 1996).

The host rocks of the Eskay Creek deposit belong to the Upper Hazelton Group. In the deposit area, the rocks of the Upper Hazelton Group are folded into a shallowly north plunging, north-northeast trending, upright, open anticline (Fig. 2-1, 2-2). The stratiform ore lenses at Eskay Creek occur on the western limb of the fold, near the fold closure, and dip gently 30 to 45º to the west. North-northwest to north-northeast trending normal faults disrupt the deposit. The metamorphic grade in the mine area is lower greenschist (Britton et al., 1990; Roth et al., 1999).

- 16 - 2.3.2 Stratigraphy of the Mine Succession

The stratigraphic footwall to the stratiform ore lenses of the Eskay Creek deposits is composed of rhyolite (Fig. 2-2, 2-3). In the mine area, the footwall rhyolite has a maximum apparent thickness of approximately 100 m (Britton et al., 1990). The footwall rhyolite is composed of multiple rhyolite generations, including both intrusive and extrusive units (Allen, 1993; Monecke et al., 2005). Coherent rhyolite ranges from massive to flow-banded. The coherent rhyolite is in contact with, and locally grades into, non-stratified and poorly sorted, blocky to slabby rhyolite breccia that is interpreted to have formed through a combination of autobrecciation and quench fragmentation (Allen, 1993; Barrett and Sherlock, 1996; Monecke et al., 2005).

The central part of the Eskay Creek deposit is underlain by a texturally distinct stratified rhyolite sandstone and breccia facies. This facies is moderately to well sorted and diffusely stratified, with individual beds ranging from massive to internally graded (Allen, 1993; Roth et al., 1999; Monecke et al., 2005). The stratified rhyolite sandstone and breccia facies is interpreted to have formed by phreatic-hydrothermal explosions (K. Rye, unpub. report for International Corona Corp., 1992, 125 p.; Allen, 1993; Monecke et al., 2005).

Hydrothermal alteration is widespread throughout the footwall rhyolite (Robinson, 1991; Barrett and Sherlock, 1996). Peripheral to the stratiform ore and in the deeper parts of the footwall, hydrothermally altered rhyolite contains abundant secondary K-feldspar and shows moderate silicification. Intense and texturally destructive hydrothermal alteration is largely restricted to the upper contact of the footwall rhyolite, immediately underlying the stratiform ores (Ettlinger, 1992; Roth et al., 1999).

The footwall rhyolite is overlain by black, carbonaceous mudstone, which represents the direct host rock to the stratiform sulfide and sulfosalt ores at Eskay Creek. The mudstone unit ranges from less than 1 m to more than 60 m in thickness (Britton et al., 1990; K. Rye, unpub. report for International Corona Corp., 1992, 125 p.). The mudstone is laminated, thinly bedded, or massive and contains abundant intercalated, tan-colored beds of fine-grained volcaniclastic

- 17 -

FIG. 2-1: Geological map of the Iskut River area and location of the Eskay Creek deposit. The inset shows the distribution of the Stikinia terrane in British Columbia (modified from MacDonald et al., 1996). material (Britton et al., 1990; Monecke et al., 2005). Locally, rhyolite sills were emplaced along the contact between the still wet and unconsolidated stratified rhyolite sandstone and breccia and the overlying carbonaceous mudstone, suggesting that mineralization at Eskay Creek is temporally and spatially closely related to the rhyolitic volcanism.

Further up stratigraphy, the carbonaceous mudstone unit hosts abundant basalt sills and dikes (Fig. 2-3). The occurrence of mudstone-matrix basalt breccia along the bottom and top margins of coherent basalt intervals indicates that the mafic lava intruded the still wet and unconsolidated mudstone (Roth et al., 1999; Monecke et al., 2005). Hanging-wall mudstone

- 18 -

FIG. 2-2: Geological map of the Eskay Creek anticline. The map also shows the surface projection of the ore bodies (modified from Roth, 2002). intervals between these basalt sills commonly contain radiating prehnite porphyroblasts (Roth et al., 1999). Basalt units in the lower part of the carbonaceous mudstone unit are typically affected by weak hydrothermal alteration as evidenced by extensive bleaching of the rocks (Britton et al., 1990).

During operation of Eskay Creek, mine geologists used the first occurrence of basalt in the mine succession to divide the mudstone overlying the footwall rhyolite into two units, referred to as the contact and the hanging-wall mudstone, respectively. The contact mudstone unit is defined as the mudstone between the upper surface of the footwall rhyolite and the lowest basalt unit in the hanging-wall. Mudstone occurring further up stratigraphy in the mine

- 19 -

FIG. 2-3: East-west cross-section of the western limb of the Eskay Creek anticline, showing the location of stratiform and discordant zones hosted by the carbonaceous mudstone and the footwall rhyolite, respectively (modified from Roth, 2002). succession is collectively referred to as the hanging-wall mudstone (Britton et al., 1990). Although the lowest basalt unit in the mine succession is commonly intrusive in nature and therefore not a reliable stratigraphic marker (Monecke et al., 2005), mine terminology is followed in the present contribution to allow first-order distinction between mudstone samples collected at different locations within the mine succession.

The relative proportion of basalt increases in the upper part of the mine succession. The hanging-wall basalt locally exceeds 150 meters in thickness in the mine area, but thins southward away from the deposit (Britton et al., 1990; K. Rye, unpub. report for International Corona Corp., 1992, 125 p.). The mafic rocks are intercalated with variably thick intervals of carbonaceous hanging-wall mudstone. These mudstone intervals are similar to the contact mudstone hosting the stratiform ores and also contain intercalations of fine-grained volcaniclastic material as well as abundant pyrite laminations.

The basalt in the upper part of the mine succession occurs as both intrusive and extrusive phases. Sills are abundant near the base and the relative proportion of flows increases towards the top of the succession where the basalt forms intervals exceeding tens of meters in thickness.

- 20 -

Mudstone-matrix basalt breccias are common at the base of individual basalt flows, whereas their top margins are typified by the occurrence of non-stratified monomict basalt breccia that formed through autobrecciation and quench fragmentation. In addition to the basaltic flows, well-preserved pillow basalts and polymict talus breccia occur (Britton et al., 1990; Barrett and Sherlock, 1996; Roth et al., 1999; Monecke et al., 2005).

2.3.3 Ore Zones

Economic concentrations of precious and base metals at Eskay Creek are contained in a number of ore zones that are distinguishable based on mineralogy, texture, precious metal grade, metallurgical characteristics, as well as stratigraphic and spatial distribution (Roth et al., 1999). Known stratiform ore zones occur over a strike length of approximately 1,500 m and a maximum down dip extent of 350 m (Fig. 2-4). The ore zones range from 5 to 45 m in true thickness.

The main stratiform ores occur in the 21A, 21B, and 21C zones within the carbonaceous contact mudstone at or near the contact with the footwall rhyolite. The 21A zone at the southern end of the deposit is composed of two massive to semi-massive sulfide lenses that mainly consist of stibnite, realgar, cinnabar, and arsenopyrite (Britton et al., 1990; Idziszek et al., 1990; Roth et al., 1999). The stratiform ores of this zone averaged approximately 10 m in thickness (Roth et al., 1999). The largest and most precious-metal rich ore zone at Eskay Creek, the 21B zone, is located approximately 200 m to the north of the 21A lens. It is composed of clastic sulfide and sulfosalt beds. The ores form a tabular body that is about 650 m long, 60 to 150 m wide, and locally in excess of 20 m thick. In the central portion of the zone, the beds are formed by pebble- to cobble-sized sulfide and sulfosalt clasts (Britton et al., 1990; Idziszek et al., 1990; Roth et al., 1999). The beds grade laterally into thinner, finer grained, clastic beds, and laminations. The clast size and bed thicknesses also decrease stratigraphically upward, progressively thinning to fine laminations and disseminated sulfide and sulfosalt minerals (Roth et al., 1999). The 21C zone consists mainly of beds of massive to bladed barite and calcite containing abundant, very fine-grained disseminated sulfide minerals. The zone occurs 100 m downdip from the 21B zone and is subparallel to it (Britton et al., 1990; Roth et al., 1999).

- 21 - Several additional ore zones hosted by the contact mudstone have been recognized (Fig. 2- 4). Near the north end of the 21B zone, the precious metal-rich ores extend over the top of the anticline into the complexly folded and faulted East Block. Ores in this area occur in a steeply dipping, fault-bounded slab of mudstone that is located immediately east of a major north-south fault. The bulk of the ore is composed of fine-grained massive to locally clastic sulfosalt minerals (Roth et al., 1999). The NEX zone lies north of the 21B lens. It comprises mostly massive sulfides with clastic ores being restricted to the south end of the ore zone. Abundant chalcopyrite stringers overprinting the clastic ores throughout the NEX zone probably formed in relation to the stratigraphically overlying HW zone (Roth et al., 1999). The HW zone is present in the contact mudstone stratigraphically above the NEX zone. The ore zone is disrupted by numerous faults that are associated with the fold closure. The HW zone consists of semi-massive to massive sulfides. The upper contact of the lens is marked by massive barite and calcite (Britton et al., 1990; Roth et al., 1999). The HW zone has much higher copper and lower precious metal contents than the clastic sulfide-sulfosalt ore of the 21B zone (Roth et al., 1999).

Discordant zones of sulfide mineral bearing veins and disseminations are hosted by the footwall rhyolite below and adjacent to the 21B zone in the Pumphouse and Pathfinder zones, which may represent a single feeder system offset along the north-south trending Pumphouse fault zone. Additional discordant ores occur in the 109 zone that underlies the northern part of the 21B zone and the HW zone. This area is typified by distinctive, very precious metal-rich crustiform quartz veins with coarse-grained sphalerite, galena, pyrite, chalcopyrite, sulfosalt minerals, and electrum (Roth et al., 1999).

2.4 Materials and Methods

A total of 180 mudstone samples were selected from exploration drill core as well as surface and underground exposures (half core samples averaging 0.5 m in length and bulk samples of approximately 2 kg weight). The samples were collected at various distances from ore, ranging from the immediate ore zones to a maximum distance of approximately 4.4 km from

- 22 -

FIG. 2-4: Plan view of the spatial distribution of ore zones at Eskay Creek (modified from Roth, 2002). the deposit. Using a three dimensional GOCAD model of the deposit area, the distance between each sample location and the contact between the footwall rhyolite and the carbonaceous mudstone was determined. As a measure of proximity to ore, the radial distance between the sample location and the nearest known ore body was determined for each sample.

The preparation of the samples for mineralogical and geochemical analysis initially involved the crushing of the sampled material to a grain size below 0.5 mm using a jaw crusher.

- 23 - After crushing, the samples were split with a riffle splitter. Approximately 100 g of the split material was further crushed to a grain size of approximately 200 µm using a hydraulic crusher. Ten grams of material were removed for XRD analysis. The remainder of the sample was milled to analytical fineness using a ring mill.

The material for XRD analysis was further split using a riffle splitter. Approximately two grams were then milled to a grain size below 10 µm under ethanol in a vibratory McCrone micronizing mill. After drying, the fine powders were homogenized in a vibratory mixer mill. The homogenized powders were subsequently filled into conventional top-loading sample holders. Step-scan XRD data (5 to 80° 2Θ, 0.03° 2Θ step width, 8 s/step) on the randomly oriented powder mounts were collected with an URD 6 (Seifert-FPM, Germany) diffractometer equipped with a diffracted-beam graphite monochromator and a variable divergence slit. A Co tube was used and operated at 40 kV and 30 mA. Qualitative phase analysis of the raw diffraction patterns was carried out by conventional search/match procedures. Subsequent quantitative phase analysis was performed by the Rietveld method using the fundamental- parameter program AutoQuan (Bergmann et al., 1998). Details on the applied method of quantification are described by Bergmann et al. (2001), Monecke et al. (2001), and Kleeberg and Bergmann (2002). Detection limits of the Rietveld method are phase- and sample-dependent, but are 0.5 wt.% for most phases, except for sheet silicates that can only be detected at concentrations exceeding approximately 2 to 5 wt.%.

The obtained pulps were used for bulk geochemical analysis at the Geological Survey of Canada in Ottawa, Canada. Initially, 0.5 g of the powdered sample was fused with a mixed lithium metaborate-tetraborate flux. The fusion melt was dissolved in 6 M nitric acid and then made up to a volume of 250 ml with ultrapure H2O. The concentrations of the major elements were then determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a Perkin-Elmer OPTIMA 3000. The detection limits of the whole-rock major element analyses were 0.01 wt.%. The CO2 and S contents of the samples were determined using combustion followed by infrared spectrometry. The proportion of CO2 contained in carbonate minerals was measured by acid evolution followed by infrared spectrometry. The total non-

- 24 - carbonate carbon was then calculated. The loss on ignition of the mudstone samples was determined by gravimetry at 900°C.

The concentration of Ba in the samples was determined on the same solutions used for major element analysis at a detection limit of 2 ppm. For the analysis of all other trace elements, a sample solution was obtained by total dissolution of 0.5 g of powder in a mixed-acid digestion of nitric, hydrochloric, perchloric, and hydrofluoric acids followed by a lithium metaborate fusion of any residual material. The dissolved sample and fusion melt were combined and diluted to a final volume of 100 ml in 4 % nitric acid and subsequently measured by ICP-OSE using a Perkin-Elmer OPTIMA 3000. The concentrations of Cr, Cu, and Ni were determined at a detection limit of 10 ppm; Co, Sr, V, and Zn at 5 ppm; and Be and Sc at 0.5 ppm. In addition to the ICP-OSE analyses, a range of trace elements were determined by inductively coupled plasma-mass spectrometry (ICP-MS) utilizing an externally calibrated Perkin-Elmer SCIEX ELAN 6100 quadrupole instrument. This included the analysis of Pb at a detection limit of 1 ppm; Sn and Zr at 0.5 ppm; Bi, Cd, Mo, and Sb at 0.2 ppm; Ag and Ga at 0.1 ppm; Hf, In, Nb, Rb, and Ta at 0.05 ppm; as well as Cs, Th, Tl, U, and Y at 0.02 ppm. The rare earth elements (REEs) were analyzed by the same method. La, Ce, and Nd were measured at detection limits of 0.1 ppm while all other REEs had a detection limit of 0.02 ppm. Fluorine analyses were conducted by pyrohydrolysis followed by ion chromatography applying a detection limit of 50 ppm.

A split of the sample powders was submitted to Geoscience Laboratories in Sudbury, Ontario, for additional trace element analysis. The As concentrations of the samples were measured by pressing 10 g of sample into pressed pellet. Subsequent X-ray fluorescence analysis was performed at a detection limit of 1 ppm. The Se and Te concentrations were determined by a modified aqua regia leach method followed by dynamic reaction cell ICP-MS. The detection limit of the Se determination was 0.05 ppm while Te was detectable to concentrations of 0.02 ppm. The concentrations of Au, Pt, and Pd were measured by ICP-MS following pre- concentration into silver bead by the fire-assay method. The detection limits were 6 ppb for Au, 0.4 ppb for Pt, and 1.3 ppb for Pd. The Hg concentrations of the samples were determined by Actlabs in Ancaster, Ontario. A split of 0.5 g sample powder was digested with aqua regia at

- 25 - 90°C. Following reduction, the Hg content was determined using a flow injection technique on a Perkin Elmer 100 cold vapor Hg analyzer. The detection limit was 5 ppb.

The analytical procedures were validated by repeated independent preparation and analysis of samples and the use of international georeference materials. Relative standard deviations determined this way are typically below 1 % for major elements and below 5 % for elements occurring in trace concentrations. Agreement of the observed element concentrations and certified values indicates that the accuracy of the element determinations is typically much better than ± 5 %.

2.5 Analytical Results

This section details the mineralogical, major element and trace element geochemistry of the carbonaceous mudstones, as well as their rare earth element geochemistry.

2.5.1 Mineralogical Composition of Carbonaceous Mudstone

Quantitative phase analysis by the Rietveld method revealed that mudstone from the Eskay Creek host rock succession has a highly variable mineralogical composition (Fig. 2-5). The mineralogical compositions of representative samples are listed in Table 2-1.

Mudstone samples from Eskay Creek contain abundant quartz, plagioclase, and microcline. Samples from the contact mudstone are, in general, typified by higher relative quartz contents than rocks collected from the hanging-wall. A large number of rocks from the contact mudstone have quartz concentrations between 50 and 80 wt % (average of 45.5 wt.%, n= 99), whereas the quartz content of the hanging-wall samples only rarely exceeds 60 wt % (average of 32.5 wt.%). Quartz concentrations exceeding approximately 50 wt.% only occur less than 250 m from known ore lenses. This together with the observation that none of the mudstone samples collected outside the deposit area show elevated quartz contents implies that high quartz contents in carbonaceous mudstone can be related to silicification of the mudstone host. A geological cross-

- 26 -

FIG. 2-5: Histograms depicting the occurrence of rock-forming minerals in the contact and hanging-wall mudstones of the Eskay Creek deposit. Contact mudstone (purple histogram bars) is typified by a higher quartz concentration and commonly contains members of the dolomite- ankerite and magnesite-siderite solid solutions. Samples collected from the stratigraphically higher hanging-wall mudstone (lime green histogram bars) are characterized by elevated chlorite and feldspar contents, and contain abundant prehnite. section constructed through the 21C lens in the western part of the deposit, where sample coverage is highest, further illustrates that silicification is most pronounced in samples collected

- 27 - TABLE 2-1: Composition of representative carbonaceous mudstone samples from Eskay Creek.

C97854- C97830- C99980- C99986- C98892- C99964- 96.3 65.9 412.4 66.2 124.5 151.0 Alteration Least Least Least Least Clay Clay Location H (49.3) H (155.2) C (142.5) C (26.6) C (36.1) C (11.9)

Ab 16.4±1.2 9.7±1.1 20.3±0.9 - 4.6±0.6 - Ant - 0.7±0.2 0.4±0.2 0.4±0.2 0.9±0.2 0.4±0.3 Cal 13.5±0.6 6.7±0.5 0.7±0.2 1.2±0.3 - - Chl 25.1±1.2 11.9±1.0 3.1±0.7 5.5±0.8 3.7±0.8 20.5±0.9 Dol/Ank - - 0.8±0.3 - 0.7±0.5 0.6±0.3 Ilt - 10.9±1.7 12.0±1.8 26.7±2.0 38.8±2.6 17.1±2.1 Kln ------Mc 18.7±1.1 11.2±0.9 - 11.6±0.8 15.5±1.0 - Prh 9.6±0.8 - - - - - Py 5.3±0.3 3.9±0.3 10.0±0.3 4.7±0.3 8.9±0.4 8.6±0.7 Qz 11.4±0.9 45.0±1.3 52.7±1.0 49.9±1.1 26.9±0.9 52.5±1.0 Sd ------Sp - - - - - 0.3±0.1

SiO2 46.69 62.50 70.80 68.80 58.30 61.4 TiO2 0.83 0.62 0.62 0.75 0.89 0.55 Al2O3 14.34 10.30 8.90 12.30 15.40 9.20 Fe2O3T 9.13 6.00 5.80 3.90 6.00 6.10 MnO 0.14 0.04 0.04 0.03 0.03 0.02 MgO 4.51 1.90 1.25 2.03 2.56 8.01 CaO 9.9 5.26 0.90 0.72 0.86 0.54 Na2O 1.82 1.10 2.20 0.10 0.50 nd K2O 3.34 3.44 1.43 4.44 5.52 1.56 P2O5 0.14 0.09 0.18 0.18 0.29 0.12 LOI 7.12 6.50 6.00 4.20 7.00 10.00 Total 97.69 97.75 98.12 97.45 97.35 97.50 S 2.81 2.08 4.39 2.44 4.34 4.98 Au (ppb) 16 10 217 157 82 129 Ag 1.25 0.6 80.0 9.5 4.5 9.3 As 41 164 192 341 708 2,038 Ba 1,540 1,300 760 1,800 5,300 2,500 Cd 8.8 7.4 4.1 2.4 1.2 14.0 Hg (ppb) 678 1160 15,500 2,660 3,800 30,000 Pb 11 22 98 45 29 42 Sb 46 72 110 150 130 150 Tl 5.7 5.2 6.2 9.5 18.0 43.0 Zn 868 780 460 284 164 1,330

Notes: Least = least-altered; clay = clay-altered; C = contact mudstone; H = hanging-wall mudstone. Number in brackets is distance to ore. Only selected elements are shown.

- 28 - TABLE 2-1 (cont.).

C99979- C96804- C98899- C99957- CA90-427- C97857- 687.5 48.7 156.25 182.4 156.1 119.0 Alteration Carb Carb Carb Carb Carb Sil Location C (463.1) H (44.4) C (ore) H (522.4) C (14.5) C (24.1)

Ab 8.2±0.6 5.2±0.6 - 9.3±0.9 - - Ant - - 0.3±0.2 0.6±0.2 - - Cal - - - - 50.2±0.8 - Chl - - - - 8.3±0.8 11.9±0.7 Dol/Ank 15.6±0.8 13.0±0.6 14.1±0.7 7.6±0.7 25.3±0.8 0.5±0.1 Ilt 13.6±1.8 16.8±1.6 21.9±2.3 49.1±2.8 - - Kln - 0.5±0.3 - - - - Mc ------Prh ------Py 4.9±0.2 5.3±0.3 4.6±0.3 3.6±0.3 1.4±0.3 1.0±0.1 Qz 57.7±1.0 58.9±1.0 58.7±1.1 23.7±0.9 14.8±0.7 86.6±0.7 Sd - - - 6.1±0.7 - - Sp - 0.3±0.1 0.4±0.1 - - -

SiO2 65.60 66.00 54.60 52.80 16.20 85.30 TiO2 0.35 0.42 0.48 0.91 0.14 0.25 Al2O3 7.10 7.10 8.90 17.00 2.30 3.40 Fe2O3T 4.30 4.80 4.00 7.00 1.10 1.70 MnO 0.08 0.08 0.05 0.04 0.20 0.02 MgO 2.87 2.41 5.05 3.04 7.19 4.34 CaO 5.34 4.38 6.27 2.52 33.15 0.38 Na2O 0.90 0.50 Nd 1.00 nd nd K2O 1.55 1.77 2.60 4.70 0.30 0.14 P2O5 0.11 0.12 0.08 0.10 0.15 0.08 LOI 9.80 9.80 13.60 9.40 33.30 3.00 Total 98.00 97.38 95.63 98.51 94.03 98.61 S 2.57 2.90 3.10 1.50 0.77 0.55 Au (ppb) 802 25 2,969 38 55 248 Ag 17.0 1.1 12.0 1.4 1.0 1.5 As 61 73 342 44 134 81 Ba 490 960 2,000 970 340 120 Cd 6.6 16.0 15.0 1.6 3.2 1.6 Hg (ppb) 12,600 6,550 8,020 477 2,440 451 Pb 94 30 683 25 6 201 Sb 35 91 160 20 18 34 Tl 9.0 7.6 15.0 3.1 4.9 2.0 Zn 719 1,730 2,550 468 184 329

Notes: Carb = carbonate-altered; sil = silicified; C = contact mudstone; H = hanging-wall mudstone Number in brackets is distance to ore. Only selected elements are shown.

- 29 - TABLE 2-1 (cont.).

C99974- C99953- C99953- C97862- C98878- C98889- 118.0 127.1 97.8 137.6 105.3 125.5 Alteration Sil Ksp Ksp Sulf Sulf Sulf Location C (38.9) H (ore) H (30.0) C (42.1) C (53.2) H (33.0)

Ab - 4.1±0.8 11.5±1.1 - 4.3±0.7 19.5±1.2 Ap - - - - - 6.4±0.5 Ant 0.3±0.1 0.3±0.2 - - - - Cal - 1.0±0.3 6.4±0.5 - 3.4±0.4 0.5±0.3 Chl - 7.7±0.9 8.0±1.0 3.6±0.7 - 4.0±0.9 Dol/Ank - 4.6±0.5 - 1.1±0.2 1.9±0.6 - Ilt 17.0±3.2 11.5±2.1 15.8±2.6 14.0±1.7 12.4±1.o 11.6±2.2 Kln ------Mc - 47.8±1.1 30.6±1.1 - 2.3±0.6 8.5±1.0 Py 4.2±0.2 7.3±0.3 4.9±0.3 32.6±1.6 47.3±4.0 40.4±1.0 Qz 78.5±1.1 15.7±0.7 22.8±0.9 48.7±0.8 28.4±0.7 9.1±0.6 Sd ------Sp ------

SiO2 81.80 54.70 56.80 58.80 40.30 38.30 TiO2 0.37 0.71 0.65 0.20 0.26 0.52 Al2O3 6.20 14.80 13.50 6.30 5.50 10.30 Fe2O3T 3.56 6.40 6.30 16.70 28.50 23.1 MnO 0.01 0.03 0.02 0.01 0.03 0.03 MgO 1.04 2.78 1.51 1.90 0.79 1.51 CaO 0.17 2.40 4.30 0.65 3.49 4.92 Na2O nd 0.60 1.20 0.07 0.50 2.52 K2O 1.87 7.06 5.96 2.01 1.62 2.44 P2O5 0.07 0.11 0.14 0.17 0.39 3.24 LOI 3.60 5.90 6.50 10.70 16.80 13.20 Total 98.66 95.49 96.88 97.51 98.18 100.08 S 2.74 3.86 3.55 12.50 21.20 17.20 Au (ppb) 167 1,031 Nd 1,342 99 270 Ag 1.9 8.7 0.7 61 7. 13.0 As 273 111 92 15,024 7,399 133 Ba 850 19,000 1,900 1,100 584 1,970 Cd 8.2 3.6 4.6 2.1 6.5 4.5 Hg (ppb) 2,450 10,100 1,010 >100,000 8,160 3,830 Pb 481 12 18 304 101 138 Sb 67 170 31 900 2,400 84 Tl 6.1 19.0 6.6 240 480 8.8 Zn 1,710 412 450 212 758 416

Notes: Sil = silicified; Ksp = K-feldspar altered; Sulf = sulfide; C = contact mudstone; H = hanging-wall mudstone. Bracketed number = distance to ore. Only selected elements are shown.

- 30 - close to the contact with the footwall rhyolite and in areas characterized by the occurrence of discordant zones of disseminated mineralization and alteration in the footwall rhyolite (Fig. 2-6).

Contact mudstone samples are typified by lower feldspar content than the rocks collected from the hanging-wall. Plagioclase concentrations in rocks of both sample groups rarely exceeds 30 wt. %. On average, contact mudstone contains 7.3 wt. % plagioclase while hanging-wall mudstone contains 8.6 wt. % (Fig. 2-5). Plagioclase was not detected in many mudstone samples collected close to discordant zones of alteration and mineralization in the footwall rhyolite (Fig. 2-6), which may suggest that hydrothermal alteration resulted in plagioclase destruction. Microcline forms between <2 and 35 wt. % of the contact and hanging-wall mudstone. A larger number of samples from the hanging-wall show elevated microcline concentrations when compared to the contact mudstone (Fig. 2-5). In general, samples collected close to the contact with the footwall rhyolite contain no or low (<15 wt. %) microcline concentrations while high concentrations can be observed further up stratigraphy, away from discordant zones of alteration and mineralization in the footwall rhyolite (Fig. 2-6).

The principal sheet silicates detected in the mudstone samples are illite and chlorite (Fig. 2- 5). Kaolinite was recognized in the whole-rock XRD patterns of only a few samples that also contain elevated concentrations of minerals belonging to the dolomite-ankerite solid solution. The relative abundance of illite in the contact and hanging-wall mudstones ranges from <5 to 50 wt. %. Illite concentrations exceeding 30 wt. % were only encountered in some samples collected close to the contact with the footwall rhyolite (Fig. 2-7). The contact mudstone tends to have a slightly higher illite content (average of 20.9 wt. %) than the hanging-wall mudstone (average 15.4 wt. %). This may suggest that hydrothermal alteration of the carbonaceous mudstone resulted in an increase in the illite concentrations. In contrast to illite, chlorite is less abundant in the contact mudstone than in the hanging-wall samples (Fig. 2-5). Chlorite occurs in concentrations below 5 wt. % in over half of the samples taken from the contact mudstone. Chlorite concentrations only rarely exceed 15 wt. % (Fig. 2-7). The hanging-wall mudstone has a chlorite content that ranges from <5 wt. % to approximately 40 wt. %. Half of the samples of this group contain between 5 and 15 wt. % chlorite.

- 31 -

FIG. 2-6: Geological section through the 21C zone, depicting the distribution of quartz, plagioclase, and microcline in the contact mudstone samples (diamonds) and hanging-wall mudstone samples (triangles). Stratiform mineralization hosted by the mudstone is omitted for clarity. Sample positions and locations of zones of discordant mineralization were projected on the section using an envelope of ±100 m. See Fig. 2-4 for location of section.

- 32 -

FIG. 2-7: Geological section through the 21C zone, depicting the distribution of illite and chlorite in the contact mudstone samples (diamonds) and hanging-wall mudstone samples (triangles). Stratiform mineralization hosted by the mudstone is omitted for clarity. Sample positions and locations of zones of discordant mineralization were projected on the section using an envelope of ±100 m. See Fig. 2-4 for location of section.

- 33 - Carbonate minerals are an important component of the mudstone from the Eskay Creek mine succession (Fig. 2-5). The principal carbonate minerals recognized are calcite and members of the dolomite-ankerite and magnesite-siderite solid solutions. Calcite has only been identified in about half of the samples from the contact mudstone and rarely exceeds 10 wt % in abundance (average of 3.4 wt. %). In contrast, calcite concentrations range from <0.5 to approximately 20.0 wt % in the hanging-wall mudstone (average of 6.9 wt. %). Calcite concentrations above 10 wt. % do not typically occur in mudstone samples collected directly next to ore (Fig. 2-8). Minerals belonging to the dolomite-ankerite solid solution are most common in contact mudstone, accounting for about 5.0 wt. % of the fine-grained rocks. Concentrations of up to 30.0 wt. % have been detected in some samples. The distribution of samples containing minerals of the dolomite-ankerite solid solution suggests that this style of carbonate alteration in the carbonaceous mudstone interval at Eskay Creek is spatially associated with zones of discordant alteration and mineralization in the footwall rhyolite (Fig. 2-8). Minerals of the magnesite- siderite solid solution have only been observed in some of the mudstone samples.

Pyrite is abundant in the majority of mudstone samples investigated from the mine succession. Both the contact and the hanging-wall mudstones contain between <1 and 20 wt. % pyrite (Fig. 2-5). The average values are 8.2 and 8.0 wt. % for the contact and hanging-wall mudstone, respectively. No clear spatial trends can be identified that could be used for vectoring towards ore (Fig. 2-8).Full-pattern refinement of the whole-rock XRD patterns further revealed that the lattice parameter of pyrite is highly variable, presumably due to the occurrence of As substitution into the pyrite structure. In some samples from the contact mudstone, refinement of the XRD patterns could only be successfully performed assuming the presence of an As-rich pyrite having a large lattice parameter. In addition to pyrite, minor amounts of sphalerite were detected in many samples of both contact and hanging-wall mudstone. Trace amounts of galena and chalcopyrite were only observed in samples collected proximal to known orebodies.

- 34 -

FIG. 2-8: Geological section through the 21C zone, depicting the distribution of minerals of the dolomite-ankerite solid solution, calcite, and pyrite in the contact mudstone samples (diamonds) and hanging-wall mudstone samples (triangles). Stratiform mineralization hosted by the mudstone is omitted for clarity. Sample positions and locations of zones of discordant mineralization were projected on the section using an envelope of ±100 m. See Fig. 2-4 for location of section.

- 35 - The abundance of prehnite in a number of samples from the mine succession reaches a maximum of 45.0 wt. % (Fig. 2-5). Prehnite-bearing mudstone samples contain abundant chlorite (4.0-48 wt. %) and calcite (1.0-10.0 wt. %) but only rarely contain illite. The occurrence of prehnite is largely restricted to samples from the hanging-wall. Pyrrhotite was recognized in several mudstone samples containing prehnite.

Full-pattern refinement of the whole-rock XRD patterns allowed the identification of anatase in most mudstone samples from the mine succession. This mineral typically occurs in concentrations below 1 wt. %. Several mudstone samples also contained detectable amounts of apatite. Rare minerals identified by XRD included barite and armenite.

2.5.2 Major Element Composition of Carbonaceous Mudstone

The whole-rock geochemical analysis also revealed that the contact and hanging-wall mudstone samples are compositionally heterogeneous. The major and trace element contents of representative samples are given in Table 2-1.

The carbonaceous mudstone samples contain fairly high SiO2 contents (Fig. 2-9). On average, the SiO2 concentrations are higher in the contact mudstones (average 62.2 wt. %) than in the hanging-wall mudstones (average 54.0 wt. %). A total of 18 mudstone samples have concentrations exceeding 75% wt. % SiO2 (Fig. 2-9), 17 of these samples have been collected from the contact mudstones at an average of 11 meters above the contact with the footwall rhyolite (average of the entire sample population is 32 meters).

The TiO2 content of the mudstone samples is typically below 1.0 wt. %. The Al2O3 concentrations correlate inversely with SiO2 (Fig. 2-9). On average, Al2O3 concentrations are lower in the contact mudstones (average of 10.4 wt. %) than the hanging-wall mudstone samples

(average of 11.9 wt. %). Fe2O3 values are generally lower in the contact mudstone samples (average of 6.3 wt. %) when compared to the hanging-wall (average 8.7 wt. %). The MnO concentrations are overall low, ranging from below the detection limit to 0.1 wt. %. Magnesium

- 36 -

FIG. 2-9: Harker diagrams for the major element content of the contact and hanging-wall mudstone samples. High SiO2 concentrations are probably caused by the silicification of the mudstone. Low SiO2 concentrations correlate with anomalously high Fe2O3, MgO, and CaO concentrations. Samples from the contact mudstone are represented by diamonds while hanging- wall samples are shown as triangles.

- 37 - concentrations range from the detection limit to 24.7 wt. % and are on average approximately 2.8 wt. %.

The CaO concentrations in mudstone are notably higher in the hanging-wall mudstone samples (average of 6.0 wt. %) relative to the contact mudstone (average of 3.1 wt. %). The carbonaceous mudstone at Eskay Creek possesses K2O concentrations that range from below the detection limit to 8.0 wt. % (Fig. 2-9), with an average of approximately 2.9 wt. %. The Na2O concentrations of the samples are equally variable with concentrations ranging from below detection limit to 6.1 wt. % (Fig. 2-9), with an average of approximately 0.9 wt. %. Samples collected within 20 meters of known ore bodies have a significantly lower average Na2O concentration of only 0.4 wt. % Na2O. The P2O5 concentrations of the mudstone samples are typically below 0.5 wt. %.

The contact mudstone samples have a slightly higher organic carbon content (average 1.9 wt. %) than the hanging-wall mudstone samples (average 1.9 wt. %). Average total sulfur concentrations are similar in both mudstone types, ranging from 0.1 to 21.2 wt. %. Contact mudstone samples, on average, contain 4.3 wt. % total sulfur while hanging-wall mudstones have 4.1 wt. % sulfur.

2.5.3 Trace Element Composition of Carbonaceous Mudstone

The precious and base metal contents of the mudstone samples are quite variable and generally decrease in concentration with increasing distance from known ore zones (Fig. 2-10). The maximum Au concentration observed is 87,700 ppb for a sample from the contact mudstone and 8,420 ppb for a sample from the hanging-wall. On average, the contact mudstone samples contain 2,191 ppb Au, while the stratigraphically higher hanging-wall mudstone has an average Au content of 391 ppb. Similarly, the contact mudstone has a higher average Ag concentration of 29 ppm when compared to the hanging-wall mudstone samples which average 6 ppm Ag (Fig. 2- 10).

- 38 - Many of the mudstone samples show enrichment in As, Hg, and Sb (Fig. 2-10). The distribution of these elements broadly mimics that of Au and Ag. In general, the concentrations of these elements decrease away from known ore zones (Fig. 2-10). The average As content of samples from the contact mudstone is 896 ppm while the hanging-wall samples only contain 151 ppm on average. A significant number of samples possess in excess of 1,000 ppm As (Fig. 2-10). The Hg concentrations in the contact mudstone are also significantly higher than in the hanging wall samples. Concentrations exceeding 100,000 ppb Hg have been measured in a number of contact mudstone samples. The average Sb concentration of the contact mudstone samples is 217 ppm while hanging-wall samples average 84 ppm Sb. The Tl concentrations of the mudstone at Eskay Creek provide another excellent vector to ore. Concentrations exceeding 10 ppm are only observed in samples collected less than 100 m from ore (Fig. 2-10). In proximity to ore, Tl concentrations of up to 100 ppm occur in the carbonaceous mudstone. The Bi concentrations of the samples are generally below detection limit.

The carbonaceous mudstone samples from Eskay Creek contain variable concentrations of the base metals Cu, Zn, and Pb (Fig. 2-11). The Cu content of the samples varies from 16 to 10,900 ppm and is on average slightly higher in the contact mudstone (average of 245 ppm) than the hanging-wall samples (average of 100 ppm). Although high Cu concentrations are related to the occurrence of chalcopyrite in the samples, there is no clear relationship between Cu content and the distance to known ore zones (Fig. 2-11). The same lack of relationship can be observed for Zn.

The concentration of this element in carbonaceous mudstone at Eskay can range to over 28,000 ppm. The contact mudstone averages 1389 ppm Zn while the hanging-wall mudstone has an average Zn content of 961 ppm. The highest Pb content observed in the samples is 19,944 ppm. The contact mudstone samples have an average of 541 ppm Pb, while the contact mudstone samples average 218 ppm.

Other metals such as Ga, Cd, In, and Sn do not show clear trends with respect to ore. Ga concentrations in the carbonaceous mudstone samples typically range from 10 to 20 ppm, with an average of 16.5 ppm. The Cd content in the samples ranges from the detection limit to over

- 39 -

FIG. 2-10: Concentrations of the Au, Ag, As, Hg, Sb, and Tl in carbonaceous mudstone from the Eskay Creek deposit as function of the radial distance to known ore bodies or resource blocks. Samples from the contact mudstone are represented by diamonds while hanging-wall samples are shown as triangles.

- 40 -

FIG. 2-11: Concentrations of the Cu, Zn, Pb, and Ba in carbonaceous mudstone from the Eskay Creek deposit as function of the radial distance to known ore bodies or resource blocks. Samples from the contact mudstone are represented by diamonds while hanging-wall samples are shown as triangles.

100 ppm. The Cd concentration in the samples correlates broadly with the Zn content of the carbonaceous mudstone. The samples contain low In concentrations (typically <0.1 ppm). The Sn concentrations in the mudstone samples typically range from 1 to 10 ppm, with an average of 2.1 ppm.

The Rb concentrations of the mudstone samples do not show a clear spatial trend with respect to known ore lenses. However, the concentrations of this element correlate broadly with the whole-rock K2O content. The whole-rock Rb content is typically below 150 ppm. The carbonaceous mudstone samples contain low Cs concentrations (typically <10 ppm). There is no clear correlation between the whole-rock Cs and K2O concentrations. Similarly, no correlation is

- 41 - observed between F, which ranges from 100 to about 5,000 ppm, and the whole-rock K2O content.

The Be content of carbonaceous mudstone at Eskay Creek is typically below 5 ppm. The Sr concentrations of the carbonaceous mudstone samples range from less than 10 ppm to over 300 ppm. There is no strong correlation between Sr and the whole-rock CaO content. At the same time, the Sr concentrations do not vary systematically with distance to known ore zones. In contrast to Sr, the Ba content of the carbonaceous mudstone samples increase significantly with decreasing distance to know ore. Samples collected less than 100 meters from known ore bodies can show Ba concentrations exceeding 200 ppm and values of over 5,000 ppm have been recorded in some samples collected close to ore. However, on average the Ba concentrations are comparable in the contact mudstone (average of 2051 ppm) and the hanging-wall mudstone (average of 2065 ppm).

Other trace elements in the carbonaceous mudstone samples do not show clear trends that could be used as vectors to ore, but vary in concentration as function of the organic carbon content. In particular, this is the case for V, Ni, and Mo (Fig. 2-12). Elevated concentrations of Pt and U can also be observed in mudstone samples having high organic carbon content (Fig. 2-12) while such a trend is less clearly for Se and Te. The Pd concentrations of the carbonaceous mudstone samples were generally below the detection limit.

The concentrations of the high-field strength elements in the mudstone samples are probably largely related to the heavy mineral fraction of the rock. The Y/Ho ratio of the mudstone samples is on average 32. This falls into the range of typical crustal Y/Ho ratios of 24 to 34 and is comparable to the chondrite value of 28 (Bau, 1996). Similarly, the Zr/Hf ratio of the carbonaceous mudstone samples is on average 36.1, compared to typical crustal values of 26 to 46 and a chondrite value of 38 ppm (Bau, 1996). The concentrations of Nb and Ta are correlated in most of the samples, implying that both elements reside in the same heavy minerals. The Sc content of the rock broad correlates with the whole-rock TiO2 content, implying that this trace element is largely contained in Ti-bearing accessory phases. The Ti/Zr ratio of the samples is on average of 38.9. Individual mudstone samples have Ti/Zr ratios varying from 3 to 100. There is

- 42 -

FIG. 2-12: Concentrations of the elements V, Ni, Se, Mo, Pt, and U in carbonaceous mudstone from Eskay Creek as function of the whole-rock organic carbon content. Samples from the contact mudstone are represented by diamonds while hanging-wall samples are shown as triangles.

- 43 - no obvious correlation with the whole-rock SiO2 content, which may be related to differential settling of heavy minerals during suspension sedimentation of the mudstone and alteration- induced changes in SiO2 content.

2.5.4 Rare Earth Element Geochemistry of Carbonaceous Mudstone

The total rare earth element (REE) content of the mudstone samples is typically <200 ppm, with the exception of a few samples that show anomalously high concentrations of these elements, ranging up to about 4000 ppm. Chondrite-normalization of the REE patterns allowed the distinction of several groups of mudstone samples with deviating REE patterns (Figs. 2-13 and 2-14).

The first group of samples is characterized by chondrite-normalized REE patterns that show an enrichment of the light REEs over the heavy REEs (Fig. 2-13a), with Lan/Ybn ratios ranging from 1.0 to 5.4. The samples show a fairly steady decline of the normalized light REE concentrations, with Lan/Smn ratios ranging from 1.0 to 3.3. The REE patterns from Gd to Lu range from flat to slightly kinked (Gdn/Ybn ratios of 0.8 to 1.5). The patterns typically have small negative Ce anomalies (Ce/Ce* ratios ranging from 0.6 to 0.9) as well as small negative Eu anomalies (Eu/Eu* ratios ranging from 0.80 to 1.1). Mudstone samples with this REE signature are dominantly from the hanging-wall, have a comparably low average SiO2 content of 52.1 wt. %, and frequently contain prehnite and high chlorite contents (>15 wt. %). This suggests that mudstone of this group contains a significant proportion of volcaniclastic material derived from basaltic to andesitic sources.

The second group of mudstone samples has comparable REE patterns (Fig. 2-13b), but shows more pronounced negative Eu anomalies (Eu/Eu* ratios of 0.5 to 0.8). The normalized

REE patterns show an enrichment of the light REEs over the heavy REEs, with Lan/Ybn ratios between 1.3 and 4.2. The decline of the normalized light REE concentrations is indicated by

Lan/Smn ratios of 1.2 to 3.0. The patterns of the heavy REE again range from flat to slightly kinked. The Gdn/Ybn ratios are between 0.8 and 1.6. The carbonaceous mudstone samples show notable negative Ce anomalies, with Ce/Ce* ratios ranging from 0.5 to 1.0. The carbonaceous

- 44 -

FIG. 2-13: Chondrite-normalized REE plots of least- and weakly-altered carbonaceous mudstone samples from Eskay Creek. The REE patterns are interpreted to be largely unmodified by hydrothermal alteration processes and probably largely reflect the REE contents of the volcaniclastic component of the rocks. Shaded areas indicate the range of concentrations.

- 45 -

FIG. 2-14: Chondrite-normalized REE plots of carbonaceous mudstone samples from Eskay Creek that are variably altered. Shaded areas indicate the range of concentrations.

- 46 - mudstones of this group originate from both the contact and hanging-wall mudstone. The average SiO2 content of the group of samples is 57.7 wt. %. Only few samples contain prehnite or chlorite concentrations exceeding 15 wt. %. This suggests that the samples contain a higher proportion of volcaniclastic material derived from rhyolitic sources.

A number of mudstone samples collected from Eskay Creek have even larger negative Eu anomalies (Fig. 2-13c), with Eu/Eu* ratios of 0.2 to 0.5. The samples show negative Ce anomalies with Ce/Ce* ratios between 0.7 and 1.0. The shape of the normalized REE patterns is very close to the groups of samples described earlier. The Lan/Ybn ratios range from 1.6 to 4.2.

The normalized concentrations of the REEs decrease from La through to Sm, with Lan/Smn ratios between 1.3 and 2.6. The patterns of the heavy REEs are comparably flat or slightly increase from Gd to Lu, with Gd/Yb ratios of 0.8 to 1.7. Most samples of this group were collected from the contact mudstone. The average SiO2 content of this group of samples is 63.6 wt. %. The REE patterns of this sample group can probably be related to a high content of felsic volcaniclastic material in the mudstone.

A small group of samples has even more pronounced negative Eu anomalies with Eu/Eu* ratios varying from <0.1 to 0.2 (Fig. 2-13d). The negative Ce anomalies are very weakly developed with Ce/Ce* ratios of 0.9 to 1.0. The shape of the normalized REE patterns is similar to the groups of mudstone samples previously discussed. The patterns are enriched in the light

REE (Lan/Ybn ratios of 1.8 to 5.3). A steady decrease of the normalized REE concentrations can be observed for the light REEs (Lan/Smn ratios of 1.4 to 3.6) while the heavy REEs show flat or slight kinked patterns (Gdn/Ybn ratios of 1.0 to 1.5). All samples of this group were collected from the contact mudstone. The average SiO2 content of the samples of this group is 59.9 wt. %. The volcaniclastic component of these mudstone samples may also have been derived primarily from felsic volcanic sources.

A number of samples show deviating REE patterns from those described above. The deviating shapes of these patterns are likely related to the hydrothermal alteration of these samples, implying that REEs were not immobile during fluid-rock interaction at Eskay Creek.

- 47 - Several samples show normalized REE patterns that steadily decline from La to Lu (Fig. 2-

14a). The Lan/Ybn ratio of these samples ranges from 3.1 to 8.1. The samples are characterized by small negative Ce anomalies with Ce/Ce* ratios between 0.83 and 1.00. The Eu anomalies of these patterns are variable with Eu/Eu* ratios of 0.02 to 0.55. Samples of this group were all collected from the contact mudstone and represent samples that were affected by variable alteration as suggested by the fact that many of these samples have SiO2 contents exceeding 75 wt %.

A second group of hydrothermally altered samples is characterized by REE patterns that are enriched in the light REEs over the heavy REEs (Fig. 2-14b). The normalized concentrations of the REEs decrease steadily from La to Sm (Lan/Smn ratios of 1.2 to 2.6). The samples have variable negative Ce anomalies with Ce/Ce* ratios ranging from 0.8 to 1.0. Pronounced negative Eu anomalies can be observed (Eu/Eu* of <0.1 to 0.3). The patterns of the heavy REEs show a strong curvature. The normalized concentrations of the heavy REEs increase from Gd to Ho and decrease from Ho to Lu. Some samples of this group have anomalously high REE concentrations and high Y values. This may suggest that these samples contain REE-bearing hydrothermal phases enriched in heavy REEs. All samples of this group were collected from the contact mudstone.

An additional group of samples can be identified based on strong curvature of the normalized patterns of the light REEs (Fig. 2-14c). The normalized concentrations increase from La to Pr or Nd. The samples show small Ce anomalies with Ce/Ce* ratios of 0.8 to 1.3. Pronounced negative Eu anomalies can be observed in many samples, with Eu/Eu* ratios ranging from 0.2 to 0.9. The patterns of the heavy REEs range from flat to slightly kinked. In some samples, the normalized concentrations of the REEs increase from Gd to Lu. Samples of this group originate from both the contact and hanging-wall mudstone. Many of the samples show high Au, Ba, and Hg contents, which suggests that the anomalous REE patterns may be related to hydrothermal alteration processes.

Only two carbonaceous mudstone samples are typified by positive Eu anomalies, with Eu/Eu* ratios of 2.2 and 1.3 (Fig. 2-14d). Both samples are intensely altered. One sample

- 48 - contains >75 wt. % carbonate minerals, including members of the dolomite-ankerite solid solution and calcite while the other sample consists of >40 wt. % pyrite. The REE patterns of the two samples decrease steadily from La to Lu, with Lan/Ybn ratios of 2.8 and 3.8, respectively. The samples show small negative Ce anomalies.

2.6 Statistical Analysis

This section provides an introduction to principal component analysis, as well as the results and interpretation of both the multivariate statistical analysis and supporting mass transfer analysis.

2.6.1 Principal Component Analysis

To facilitate data exploration and to identify relationships between the whole-rock mineralogical composition and the bulk major and trace element concentrations as well as the spatial variables, multivariate principal component analysis (PCA) was performed for the mudstone data set. PCA uses an orthogonal transformation to convert a sample population of significantly correlated variables into a smaller number of linearly uncorrelated variable groups, referred to as principal components or factors. This minimized set of factors represents the intrinsic dimensionality of the dataset. The factors are sorted in order of decreasing overall variance. Quantitatively, a loading value is assigned to each of the variables within a factor that represents the strength of correlation with that factor. Subsequently, these loading values are sorted into categories defined in terms of the standard error of the correlation coefficient calculations. By focusing primarily on variables with loadings higher than several times the standard error, the risk of over-interpreting correlations between variables and factors that are simply due to random chance can be minimized. In addition, PCA assigns factor scores to each sample for every factor which allows evaluation of whether outliers explain most of the variance within a factor, or whether the variance is more or less normally distributed among the entire sample population.

- 49 - In the present study, PCA was conducted primarily to reduce the large mudstone dataset, which is composed of 88 analytical variables (22 identified mineral types, 12 major elements, 54 trace elements, and five spatial variables) on 180 samples, into a simpler dataset composed of groups of similarly scaled factors that are meaningfully correlated. By first reducing the mudstone dataset into factor groups, the relationships among variables within each group or between groups can be further explored using simple bivariate plots. An additional benefit of PCA is that the analysis is conducted using a scale-invariant correlation matrix, which allows for ready comparison of correlation among variables with highly different unit types (e.g., comparison of whole rock composition data that are normalized on a percentage basis to trace element data in parts per million or billion).

To allow statistical analysis, concentrations below the respective limit of detection had to be censored in the mudstone dataset by substituting them with a constant value. In the case of the XRD data, values below detection limit were set to 0.1 wt. %. Major element data of the mudstone samples at concentrations below detection limit were set to zero. The respective detection limits were used for all trace element analyses.

Statistical analysis of the dataset modified this way was conducted using the NCSS data analysis program. The data was auto-normalized (using the correlation matrix setting) to account for variable numeric ranges caused by the existence of different variable units. A maximum of 20 factors was specified (the variance of dataset can generally be explained by a much smaller number of factors). Initially, the analysis was carried out using a normal orthogonal projection as this projection captures the maximized variance of each factor. The analysis was then repeated using the Varimax orthogonal rotation, which maximizes the contrast among factor scores, to confirm the primary factor groups and identify additional variables within each factor. Following analysis, the relevant factors, explaining the dataset variance, were identified using scree plots of eigenvector values. In addition, the relevant factors and member variables were explored in bivariate plots.

For this study, a loading score for a variable within a factor group was considered statistically meaningful if it represented at least three standard deviations from the mean. This

- 50 - value implies a greater than 99.7% probability that the strength of the relationship of that variable with the factor group cannot be explained by random chance. Once the loadings were determined and evaluated, the reduced dataset was evaluated and interpreted.

2.6.2 Pearce Element Ratios

The PCA results consist of statistical correlations among mineralogical and geochemical data that are scaled to unit mass or volume. Relationships between such variables are typically plotted on simple bivariate scatterplots which are generally not suitable for identifying and interpreting hydrothermal alteration processes involving mass transfer. However, diagrams employing Pearce element ratios preserve mass transfer (Pearce, 1968). An element interpreted to be immobile during hydrothermal alteration is hereby used as the ratio denominator.

Akella (1966) observed that the weight percent ratio of two conserved components remains constant during any mass transfer process. DePangher (1989) used this principle to develop a technique for identifying conserved element pairs quantitatively. The technique relies on the observation that the log ratio of one conserved element (X) over a mobile element (Z) will be linearly correlated with the log ratio of a second conserved element (Y) over the same mobile element (Z).

Pearce element ratios were constructed to facilitate interpretation of the PCA results. The PCA results were useful for identifying conserved elements, with emphasis placed on elements not affected by mass transfer processes and those that are likely distributed in multiple mineral phases to avoid issues with congruent mobility (cf. DePangher, 1989). Gold was used as the mobile element for testing conserved component pairs. As the TiO2/Al2O3 pair produced high collinearity (Fig. 2-15), Pearce element ratios were subsequently constructed by dividing all mineral variables and element compositions by the TiO2 concentration of the given sample. A standard score (or Z-Score) was calculated for each Pearce element ratio to gauge mass change in terms of standard deviations from the sample population mean. The standard deviations are summarized relative to distance from mineralization and distance to the footwall rhyolite (Fig. 2- 16).

- 51 -

FIG. 2-15: Plot of log ratios between the conserved components Al2O3 and TiO2 and the mobile component Au. The log ratios show a high degree of collinearity.

Absolute mass gain and loss could not be quantified for the carbonaceous mudstone as this would require the identification of a protolith composition, which is not possible due to the heterogeneity of the precursor mudstone. Thus, the results should be considered semi- quantitative, useful primarily for identifying prominent mass transfer trends.

2.6.3 PCA Factor Groups and Loadings Scores

To facilitate data reduction and exploration of relationships, the dataset was subdivided into separate mineral, major element, and trace element subsets, and combinations thereof. PCA results for the entire data set (with the exception of the REEs, as discussed below), are given in Table 2-2. Variables within a factor group that share the same sign (positive or negative) indicate values that correlate directly, while those with opposite sign correlate inversely for a given rotation. However, two variables sharing a high loading score within the same factor group do not necessarily exhibit collinearity (although sometimes they do). The relationships between variables are often more complex and multidimensional in nature. Varimax rotation generally results in factor groups with fewer variables, but stronger correlations. This is a reflection of the

- 52 -

FIG. 2-16: Component and mineral mass loss and gains, relative to the population mean, plotted as a function of distance to ore (left) and distance to the footwall rhyolite contact (right).

- 53 - fact that Varimax rotation presents the maximized variable contrast instead of the maximized variance. Depending on the size of the data subset analyzed, anywhere from 10 to 25 factor groups are identified in the results. Generally, the first six to eight factor groups reflect roughly 60 to 80% of the variance of the entire dataset. These factor groups, arranged by decreasing level of dataset variance that the group accounts for, are summarized and interpreted for each data subset (the factor group numbers reflect the default orthogonal projection results, not the analysis with Varimax rotation).

PCA results for a subset consisting only of mineral data confirm some of the trends previously discussed. Factor group 1 highlights the inverse relationship between the abundance of quartz and other rock-forming minerals. Factor group 2 shows that the occurrence of galena, sphalerite, and chalcopyrite correlates with each other and to some extend with chlorite, which highlights the fact that altered mudstone samples contain elevated chlorite contents. Factor group 3 reflects the contrast between the rock-forming minerals in the contact and hanging-wall mudstones. Calcite, chlorite, microcline, prehnite, and pyrrhotite are more abundant in the stratigraphically higher mudstones while minerals of the dolomite-ankerite solid solution, kaolinite, and illite are more common in contact mudstone. Factor group 4 indicates the contrast between quartz-rich contact mudstone samples and carbonaceous mudstone containing high pyrite or calcite concentrations. Factor group 5 reflects the difference between pyritic contact mudstones and mudstone rich in carbonate minerals including calcite and minerals of the dolomite-ankerite solid solution. Factor group 6 highlights the association between prehnite and pyrrhotite as well as the distance to the contact with the footwall rhyolite. This reflects the observation that mudstone with a higher relative abundance of volcaniclastic material derived from mafic to intermediate sources is more abundant further up stratigraphy. The mafic precursor composition allowed the formation of prehnite during metamorphism. Factor group 7 indicates a genetic relationship between ankerite and kaolinite.

Factor loadings for the data subset consisting of major elements confirmed the trends inferred from the whole-rock mineralogy. Factor group 1 indicates a strong relationship between chemical components related to the occurrence of carbonate minerals and chlorite in the mudstone (CaO, MnO, MgO, and CO2), and an inverse relationship with chemical components

- 54 - TABLE 2-2: Results of PCA analysis conducted on the mineralogical and geochemical data from Eskay Creek (excluding the REE concentrations; see text for explanation). The table gives the loadings for the most important factor groups.

Factor Correlation Factor loadings Group

F1 Strong Zn (-0.83); Sp (-0.80); Pb (-0.79); Gn (-0.76); Ga (-0.75); As (- 0.73); Cd (-0.73); Cu (-0.73); Sb (-0.66); Ccp (-0.65); Ag (-0.63); Te (-0.62) Moderate F (-0.56); U (-0.48); Se (-0.43) F2 Strong Zr (0.66); Th (0.66); Ta (0.67); Hf (0.73) Moderate Pb (-0.47); Gn (-0.46); Zn (-0.46); LOI (-0.45); MnO (-0.44); CaO (-0.43); MgO (-0.43); distance to ore (0.44); Ilt (0.46); Al2O3 (0.53); Be (0.59); K2O (0.60); Nb (0.61); Rb (0.61) F3 Strong Qz (-0.85); SiO2 (-0.81); TiO2 (0.64); Sc (0.72) Moderate Fe2O3 (0.43); CaO (0.44); Cr (0.45); distance to footwall rhyolite (0.45); MnO (0.46); LOI (0.47); Co (0.54); Al2O3 (0.57) F4 Strong Tl (-0.80); Y (-0.80); As-bearing py (-0.79); Py (-0.71); S (-0.67); Au (-0.66) Moderate LOI (-0.49); Fe2O3 (-0.48) F5 Strong CO2 (-0.68) Moderate Cal (-0.56); CaO (-0.50); Ank (-0.50), Sr (-0.48); Th (-0.44); Mn (- 0.43); Fe2O3 (0.42) F6 Strong Illite (-0.65) Moderate Cs (-0.57); Rb (-0.46); K2O (-0.36); distance to ore (0.26); Ab (0.45); Na2O (0.45) F7 Moderate Pd (-0.58); V (-0.49); Pt (-0.49); Corg (-0.42); Ni (-0.32); Chl (0.51); MgO (0.50)

that form quartz, feldspars, and illite (SiO2, K2O, and Na2O). Factor group 2 represents the contrast between SiO2 and Al2O3 abundance of the mudstones, which is related to the fact that mudstone affected by silicification shows low abundances of other rock-forming silicate minerals. Factor group 3 suggests a correlation between Fe2O3 and TiO2 contents in mudstones as a function of distance to ore. The factor group also highlights the inverse relationship of these chemical components and the concentration of chemical components related to carbonate alteration. Factor group 4 indicates a strong relationship between TiO2, Al2O3, and K2O. This factor group was used to identify conserved elements for the Pearce element evaluation. Factor group 5 indicates that stratigraphically higher mudstones have lower organic carbon contents.

Finally, factor group 6 suggests that Na2O and MgO concentrations are inversely correlated,

- 55 - reflecting the findings of the Pearce evaluation which indicate Na2O depletion and MgO enrichment proximal to mineralization.

The groups and loadings for the trace element data also identified a range of important trends. Factor groups 1 and 2 indicate that base metals Cu, Pb, and Zn correlate strongly with each other and Ga (Fig. 2-17). A weaker correlation is observed with other metals, including As, Sb and Ag, as well as Cd and Te. This metal suite shows a close relationship with distance to mineralization. Elements representing the heavy mineral fraction of the mudstone (Hf, Nb, Ta, Th, and Zr) are co-associated and essentially independent from the base and precious metals. When these elements are evaluated as Pearce element ratios, they appear to behave conservatively. Factor group 3 confirms the co-association of a large number of elements (Co, Cr, Ni, Pd, Pt, Sc, Se and V) with the organic fraction in the mudstones, and indicates an inverse relationship between these elements and Au and Tl. Factor group 4 suggests that Au behaves independently from the other base and precious metals. However, there is a notable correlation between Au and Tl.

To develop a comprehensive understanding of the relationships between all mineralogical, geochemical, and spatial variables, PCA was performed on the entire dataset as a last step. Initial PCA indicated that a single factor group, consisting predominantly of the REEs with very high loading scores (>0.95), explained a disproportionate fraction of the dataset variance and likely diluted the contrast among the remaining dataset variables.

As suggested by the chondrite-normalized plots discussed above, the REEs behave as a coherent group of elements, except for Eu, which shows variable enrichment and depletion patterns. In general, correlations among the heavy REEs are somewhat stronger that those among the light REEs, reflecting the observed variability in the slope of the normalized REE plots. The factor group indicated co-association between the REEs and As, Au, Sb, Tl, and total S, suggesting that REE enrichment correlates with mineralization trends. However, close inspection of factor scores showed that this relationship was strongly dependent on two samples having anomalously high REE concentrations. These two samples were collected proximal (<5 m) to ore. The next 20 mudstone samples with the highest REE concentrations were largely collected

- 56 -

FIG. 2-17: Scatter plots illustrating relationships between trace element concentrations in carbonaceous mudstone from Eskay Creek. from the contact mudstones. Although no unequivocal correlation with the distance from known ore bodies could be established for these samples, it is likely that anomalous REE enrichment is related to the hydrothermal alteration of these contact mudstone samples. For the remaining mudstone samples, an observed high correlation between REEs and Hf, Nb, Ta, and Zr (following removal of the two samples with the highest REE concentrations) suggests that these elements are mostly contained in heavy minerals such as zircon, which form part of the volcaniclastic component of the rocks.

To facilitate the analysis of the full data set, the REEs were removed from the analysis and new factor groups and loadings were generated. The results of this PCA analysis provided additional important insights into mineralogical and geochemical correlations (Table 2-2).

Factor group 1 summarizes variables related to the precipitation of ore minerals within carbonaceous mudstone samples affected by hydrothermal alteration and matches the observations from the PCA performed on the mineral and trace element subsets. There is strong correlation between base metals, base metal sulfides, Ga and the elements enriched in hot spring deposits, including Ag, As, Sb, and Te. This correlation confirms that base metal enrichment and elevated Ag concentrations are genetically related. However, factor group 4 confirms that Au enrichment is decoupled and genetically related to Tl as well as pyrite and Fe2O3. The Fe2O3

- 57 - content in the mudstone samples is primarily controlled by pyrite abundance and to a lesser extent by chlorite. Factor group 8 indicates a genetic relationship between Ag and chalcopyrite.

Factor group 2 emphasizes the contrast between detrital and hydrothermal minerals as well as immobile and mobile chemical components. The immobile trace elements Zr, Th, Ta, and Hf correlate strongly. Al2O3 and TiO2 were identified as conserved elements by the Pearce element evaluation and are, therefore, interpreted to be related to the amount of volcaniclastic material originally contained in the mudstone. TiO2 resides principally in anatase (and sometimes in rutile) while Al2O3 in mostly contained in illite, chlorite, and the feldspars.

Factor group 3 reflects the bulk protolith and alteration mineralogy of the mudstone. Relationships within this factor group are complex and could only be deconstructed from the bulk dataset by careful evaluation of similar relationships within subset PCA results. The factor group primarily expresses an inverse relationship between quartz, and related whole-rock SiO2, and the conserved components Al2O3 and TiO2. This relationship reflects the inverse correlation between quartz-rich mudstones and mudstones rich in Al2O3 phases such as illite, chlorite, and microcline. Figure 2-18 indicates that high quartz concentrations, and related high whole-rock

SiO2 contents, are characteristic of contact mudstone samples collected close to the contact with the footwall rhyolite. However, the bulk of the variance of this factor group is reflected by the fact that the contents of the aluminosilicates increase higher up stratigraphy where mudstone samples typically show lower quartz contents. While both the contact and hanging-wall mudstone samples contain abundant aluminosilicate minerals, the relative abundance of these phases is clearly higher in samples that have not been silicified. Pearce element analysis confirms that illite enrichment occurs proximal to both ore and the contact with the footwall rhyolite. The majority of samples containing high illite concentrations (>15 wt %) was collected from the contact mudstone. However, the existence or abundance of illite does not strongly correlate with the proximity to known ore lenses or the contact with the footwall rhyolite in either the contact or hanging wall-mudstone as least-altered mudstone can also contain abundant illite (Fig. 2-16) The results of the PCA further show that microcline abundance does not correlate with proximity to known ore lenses or the contact with the footwall rhyolite. Pearce

- 58 -

FIG. 2-18: Scatter plots illustrating relationships between whole-rock geochemical data and mineral abundance in the carbonaceous mudstone from Eskay Creek (contact mudstones- purple diamonds, hanging wall mudstones- green triangles).

- 59 - element analysis confirms that there is no clear relationship between K-feldspar alteration and the distance to ore or the contact with the footwall rhyolite. On average, the concentrations of chlorite are also more than double (and the concentrations of Fe2O3 one and a half times higher) in the hanging-wall mudstone samples when compared to the contact mudstone. As discussed above, the REEs patterns suggest that this could be related to the presence of a high amount of volcaniclastic material derived from mafic to intermediate sources in the hanging-wall mudstone. The Pearce element ratios indicate that increased chlorite concentrations correlate with proximity to mineralization in the contact mudstone (Fig. 2-16).

Factor group 3, in combination with factor groups 2 and 5, further explain the mineral enrichment trends observed in altered carbonaceous mudstone at Eskay Creek, including carbonate alteration, chlorite enrichment, and silicification. The relationship expressed in these three factor groups can be most readily visualized in terms of a combination of whole rock major element components and mineral abundances (Fig. 2-18). As discussed above, mudstones containing more than 70 wt. % quartz are most likely affected by silicification. This alteration style occurs predominantly in samples from the contact mudstone. The high quartz content clearly correlates with high whole-rock SiO2 contents. Pearce element analysis further confirms that quartz and silica enrichment occurs proximal to rhyolite and known ore zones. Carbonate enrichment and increases in the relative abundance of chlorite correlated with proximity to known ore lenses and are, therefore, also interpreted to be related to the hydrothermal alteration of the carbonaceous mudstone. The samples with the lowest quartz content contain more than 30 wt. % calcite, 30 wt. % modal chlorite, or 25 wt. % minerals of the dolomite-ankerite solid solution. Although the occurrence of these minerals as such is not indicative of proximity to ore, high concentrations of these phases have only been noted in samples collected close to known ore lenses. Pearce element analysis corroborates that carbonate and chlorite enrichment occurs proximal to mineralization (Fig. 2-16). The analysis also indicates that the occurrence of calcite and chlorite does not correlate with decreased distance to the contact with the footwall rhyolite. Such a correlation only exists for members of the dolomite-ankerite and magnesite-siderite solid solutions. Calcite enrichment, while often occurring proximal to ore, occurs distal to the contact with the footwall rhyolite, which means that that calcite is primarily enriched in the hanging-wall mudstone samples.

- 60 -

Factor group 3 indicates that hydrothermally altered mudstone samples with elevated concentrations of carbonate minerals and chlorite are also typified by enrichments of MgO and MnO. Figure 2-18 shows that MnO increases correlate with high concentrations of dolomite- ankerite or calcite. Figure 2-18 also illustrates that MgO enrichment correlates with the presence of elevated concentrations of minerals of the dolomite-ankerite solid solution. Furthermore, MgO enrichment may also occur in calcite contained in samples collected close to hydrothermal upflow zones (Chapter 3). Pearce element analysis confirms that MgO and MnO enrichment occurs proximal to ore. Samples affected by carbonate alteration, and related MgO and MnO enrichment, typically do not show correspondingly elevated precious and base metal contents. There is also no clear relationship between samples having high chlorite contents and samples with either high carbonate contents or high precious and base metal concentrations. However, due to the spatial correlations established by Pearce element analysis, it has to be assumed that mineralization and carbonate alteration as well as increases in whole-rock chlorite content are spatially related.

Factor group 5 emphasizes the correlation between elements contained in carbonate species

(CO2, CaO, Sr, and MnO) and the abundant carbonate minerals identified such as dolomite- ankerite, and calcite. Fe2O3 does not correlate with these chemical components, highlighting the fact that minerals of the magnesite-siderite solid solution are not a major host to Fe2O3.

Factor 6 indicates Cs and Rb substitution for K2O in illite and concentrations of this oxide in the mudstones are primarily controlled by illite and, to a lesser extent, microcline (Fig. 2-18). The results of the PCA further indicate that mica enrichment near the contact with the rhyolite footwall correlates with Na2O and albite depletion. The latter is also established by the

Pearce element evaluation which indicates that Na2O loss occurs proximal to mineralization, explaining the absence of albite in zones of hydrothermal upflow (Fig. 2-18). Factor 6 also suggests that increasing albite and Na2O contents present in samples collected distal to ore correlate with increasing iron concentrations in chlorite.

- 61 - Factor group 7 confirms the previously established relationships among the organophile elements and the organic carbon content of the samples. It is well known that Ni and V can substitute for Mg in chlorophyll porphyrin molecules derived from decaying phytoplankton in the hemi-pelagic water column (Treibs, 1936). Mudstone samples with high organic carbon contents are, therefore, enriched in Ni and V. There is also a notable correlation with the base metals (Fig. 2-17) and platinum group elements. In samples collected close to ore, base metal concentrations are controlled by base metal sulfides (chalcopyrite, galena, and sphalerite). However, away from zones of intense alteration, the base metal content of the mudstones is controlled by the organic carbon content, presumably via a simple sorption mechanism. Co- association of the organophile suite in Factor 7 with distance to ore, albite and Na2O concentrations, and an inverse association with Mg enrichment in chlorite suggests that organophile sorption of base and platinum group metals to the organic fraction decreases with proximity to mineralization and represents another potential vector to ore.

The other factor groups identified explain a small fraction of the overall variance. Most notable is an observation indicated by factor group 10. This factor group indicates that F enrichment occurs both proximal to ore and to the contact with the footwall rhyolite. Hanging- wall mudstones have an average of 450 ppm F while contact mudstone has an average of 1,000 ppm F.

2.7 Discussion

Mudstone compositional variation and styles of hydrothermal alteration are discussed in this section. Additionally, an alteration halo model is posited, and implications for gold enrichment in submarine hydrothermal systems as well as implications for exploration are outlined.

2.7.1 Mudstone Compositional Variations

The laminated, thinly bedded or massive mudstone at Eskay Creek represents a suspension sediment that contains fine-grained volcaniclastic material deposited through settling from dilute

- 62 - currents accompanying subaqueous mass flows or settling of volcanic ash derived from subaerial or submarine volcanic explosions, intermixed with the remains of marine organisms and potentially other hemipelagic material. Volcaniclastic material in the mudstone samples includes abundant particles of quartz, plagioclase, and microcline crystal fragments as well as relict glass shards (Chapter 3). Remains of marine organisms include the high proportion of carbonaceous material and abundant microfossils present in the mudstone (Chapter 3).

The whole-rock geochemical analyses of the present study suggest that the mudstone contains variable proportions of volcaniclastic material derived from mafic and more felsic volcanic sources. The normalized REE patterns of least-altered mudstone show variably negative Eu anomalies. Samples that have fairly flat REE patterns with only a small Eu anomaly probably contain a high relative proportion of volcaniclastic material derived from basaltic to andesitic sources. In contrast, mudstone sample with pronounced negative Eu anomalies probably contain a higher proportion of volcaniclastic material sourced from rhyolitic sources. This interpretation is based on earlier analyses of coherent volcanic rocks from the Upper Hazelton Group that established that mafic rocks typically lack significant Eu anomalies while felsic rocks are characterized by pronounced negative Eu anomalies (Barrett and Sherlock, 1996).

As individual mudstone samples can contain variable concentrations of mafic and felsic volcaniclastic material as well as variable amounts of microfossil remains, the mineralogical and geochemical composition of least-altered mudstone is quite variable when compared to other rock types present in the host rock successions of massive sulfide deposits such as coherent volcanic rocks. The high primary compositional variability of mudstone clearly poses significant issues when interpreting alteration-induced whole-rock mineralogical and geochemical changes as this rock type does not have a unique precursor composition that could be used as a baseline for the identification of compositional trends or mass balance calculations.

Although the fine-grained and carbonaceous nature of the rock largely precludes the identification of alteration style and intensity in hand specimen, a range of alteration-induced mineralogical and geochemical changes could be recognized. This includes silicification of the

- 63 - mudstone, alteration of the volcaniclastic component of the mudstone, carbonate alteration and related kaolinite formation, K-feldspar alteration, and sulfide impregnation.

In addition to primary heterogeneity and alteration-induced mineralogical and geochemical variations, it has to be taken into account that the composition of the carbonaceous mudstone was probably further modified during diagenesis and low-grade greenschist facies metamorphism (Britton et al., 1990; Roth et al., 1999). Diagenesis and low-grade metamorphism undoubtedly caused the devitrification of volcanic glass contained in the least-altered mudstone (cf. Gharrabi et al., 1998; Merriman and Peacor, 1999; Monecke et al., 2007). At diagenetic to low-grade greenschist facies conditions, any mafic glass originally contained in the carbonaceous mudstone would have been transformed to chlorite and quartz, while felsic volcanic glass would have recrystallized to illite and quartz (Hay and Iijima, 1968; Furnes and El-Anbaawy, 1980; Ghiara et al., 1993; Tomita et al., 1993; Alt et al., 1998; De La Fuente et al., 2002). Depending on the precursor composition, least-altered mudstone samples from Eskay Creek now contain variable amounts of quartz, chlorite, and illite. In some of the most mafic carbonaceous mudstone samples, prehnite porphyroblasts were formed during regional metamorphism. In addition to devitrification, diagenesis and low-grade metamorphism will have resulted in a conversion of plagioclase to secondary albite and concomitant formation of metamorphic calcite (cf. Merino, 1975; Boles, 1982; Ramseyer et al., 1992).

2.7.2 Styles of Hydrothermal Alteration

One of the principal processes of hydrothermal alteration recognized in the carbonaceous mudstone samples at Eskay Creek is related to the interaction of the hydrothermal fluids with the volcaniclastic component contained in the carbonaceous mudstone samples. The mineralogical and geochemical data suggests that hydrothermal alteration of the mudstone caused a conversion of volcanic glass and plagioclase to clay minerals. Strongly altered samples show a depletion of

Na2O and variable enrichment of MgO.

The destruction of volcanic glass and plagioclase in areas affected by intense fluid-rock interaction, coupled with the formation of secondary fine-grained white mica (sericite) and

- 64 - chlorite, is a characteristic alteration feature in VHMS deposits (Ishikawa et al., 1976; Large et al., 2001a). Hydrothermal alteration typically results in well-developed alteration halos that show a chlorite core mantled by a sericite halo or a quartz-sericite core mantled by a chlorite halo (Morton and Franklin, 1987; Gemmell and Large, 1992; Large et al., 1992). Geochemical modeling has suggested that the temperature and acidity of alteration are key factors controlling alteration mineralogy (Schardt et al., 2001). Chlorite alteration mantled by a sericite halo is thought to develop in VHMS deposits that formed from hydrothermal fluids having high temperatures (250-350°C) and a fairly low acidity while a quartz-sericite core surrounded by a chlorite halo forms when the hydrothermal fluids are of distinctly lower temperature (200- 250°C) and slightly higher acidity (Schardt et al., 2001).

As hydrothermal alteration of the carbonaceous mudstone at Eskay Creek probably largely occurred at temperatures below 200°C, which is the maximum temperature of the hydrothermal fluids as inferred from fluid inclusion studies (Sherlock et al., 1999), it is not likely that fluid- rock interaction at Eskay Creek resulted in sericite and chlorite alteration. Constraints from active geothermal systems on land (cf. Monecke et al., 2007) imply that the alteration of volcanic glass and plagioclase at low temperatures (<200°C) results in clay alteration, typified by the formation of dioctahedral and trioctahedral smectite or mixed-layer illite/smectite and chlorite/smectite. The fact that the evaluation of Pearce element ratios did not identify a significant K2O enrichment is consistent with the fact that clay minerals, not white mica, were formed during hydrothermal alteration. Whole-rock Na2O depletion and MgO enrichment associated with low-temperature hydrothermal alteration, resembling the depletion and enrichment pattern at Eskay Creek, has previously been observed at the active Pacmanus vent field in the Manus Basin (Monecke et al., 2007).

During diagenesis and regional metamorphism at Eskay Creek, any clay minerals originally formed through alteration of volcanic glass and plagioclase at low temperatures (<200°C) would have been converted to more stable sheet silicates (cf. Gharrabi et al., 1998; Merriman and Peacor, 1999). Dioctahedral smectite or mixed-layer illite/smectite originally present would have been converted to illite while trioctahedral smectite or mixed-layer chlorite/smectite were converted to chlorite (Monecke et al., 2007). As devitrification of volcanic

- 65 - glass yields the same minerals, least-altered as well as hydrothermally altered carbonaceous mudstone samples now contain illite and chlorite as the only sheet silicate minerals.

The present study also shows that carbonate alteration is a widespread style of hydrothermal alteration affecting the carbonaceous mudstone at Eskay Creek. The carbonate mineralogy changes broadly with increasing distance away from known ore zones and up stratigraphy. More intensely altered mudstone samples collected close to ore frequently contain carbonate minerals belonging to the dolomite-ankerite solid solution, with members of the magnesite-siderite solid solution occurring in some samples. Calcite is more abundant away from known ore zones and in least-altered mudstone, probably reflecting the fact that this mineral can form during distal hydrothermal alteration as well as low-grade metamorphism. The observed spatial relationships suggest that carbonate alteration in the contact and hanging-wall mudstone occurs in association with zones of hydrothermal upflow in the footwall rhyolite, marked by the presence of discordant zones of hydrothermal alteration and disseminated sulfide ores.

Carbonate alteration of carbonaceous mudstone has also been observed in some samples collected along strike at a distance of over four kilometers away from the deposit. The occurrence of gossanous bluffs and other signs indicative of hydrothermal alteration of the volcanic rocks outcropping in this area on the western limb of the Eskay Creek anticline already attracted the attention of early prospectors (Britton et al., 1990). Although no economic ore zones have been identified, the Eskay Creek hydrothermal system must have extended along strike into this area.

Members of the dolomite-ankerite and magnesite-siderite solid solutions are relatively common products of hydrothermal alteration in VHMS deposits (Franklin et al., 1975; Urabe and Scott, 1983; Gemmell and Large, 1992; Offler and Whitford, 1992; Gemmell and Fulton, 2001; Huston and Kamprad, 2001; Paulick et al., 2001; Large et al., 2001b). The occurrence of kaolinite in zones of intense carbonate alteration has, however, only been reported in comparably few cases (Cagatay, 1993; Shikazono et al., 1998).

- 66 - Thermal stability constraints and observations in natural geothermal systems suggest that kaolinite represents a stable alteration product only at temperatures below approximately 300°C (Velde and Kornprobst, 1969; Thompson, 1970; Hemley et al., 1980; Reyes, 1990). As kaolinite was only observed in samples also containing hydrothermal carbonate minerals, it is likely that kaolinite formation and carbonate precipitation in the carbonaceous mudstone were coupled. Experimental constraints by Bischoff and Rosenbauer (1996) and Gysi and Stefansson (2012) showed that CO2 contained in hydrothermal fluids becomes chemically reactive at temperatures below 270°C. Kinetic and thermodynamic constraints suggested that hydrogen metasomatism of volcanic rocks and coupled bicarbonate production caused by the reaction of CO2 with water is most efficient at lower temperatures. Kaolinite present in the contact and hanging-wall mudstone at Eskay Creek can, therefore, be interpreted to represent a low temperature alteration product, which is consistent with fluid inclusion data suggesting that the ore forming fluid only reached temperatures of approximately 200°C (Sherlock et al., 1999).

The high quartz and SiO2 content of some carbonaceous mudstone samples suggests that parts of the lower portion of the contact mudstone has been affected by silicification. Silicification is one of the most common styles of hydrothermal alteration in VHMS deposits (Morton and Franklin, 1987; Gemmell and Large, 1992; Large et al., 1992). This style of alteration develops in response to rapid cooling of the hydrothermal fluids as quartz solubility is strongly temperature dependent (Fournier, 1983). Silicification at Eskay Creek occurred in proximity to upflow zones of hydrothermal fluids as marked by zones of discordant alteration and mineralization in the footwall rhyolite.

Some of the carbonaceous mudstone samples at Eskay Creek have been affected by hydrothermal K-feldspar alteration. Widespread K-feldspar alteration zones is not common in VHMS deposits although this mineral has been observed as a distal alteration product in some alteration halos associated with massive sulfide deposits (Offler and Whitford, 1992; McGoldrick and Large, 1992). Geochemical modeling suggests that K-spar alteration associated with VHMS deposits develops at lower temperatures (200°C) from fluids that have been largely equilibrated with the volcanic wall-rocks (Schardt et al., 2001), which is consistent with the observations at Eskay Creek.

- 67 - Carbonaceous mudstone at Eskay Creek has also been affected by sulfide impregnation. Hydrothermal pyrite appears to be enriched in arsenic, explaining enrichment patterns of this element in samples collected close to the deposit. Base metal enrichment in mudstone collected close to ore is related to the presence of hydrothermal sulfides such as chalcopyrite, sphalerite, and galena. Formation of sulfide minerals during hydrothermal alteration implies that the fluids - were reduced, with H2S or HS representing the principal sulfur species.

The carbonaceous nature of the mudstone apparently controls the enrichment pattern of some metals. Trace elements such as Sc, Ni, V, Se, U, and the platinum group elements correlate with the organic carbon content of the mudstone. In addition to these organophile elements, the base metal content of mudstones located distal to known ore bodies appears to correlate with the organic carbon content.

2.7.3 Alteration Halo Model

Hydrothermal activity at Eskay Creek and background sedimentation of the carbonaceous mudstone were essentially contemporaneous. The clastic ores at Eskay Creek occur within the carbonaceous contact mudstone at or near the contact with the footwall rhyolite, implying that background sedimentation commenced prior to mass-flow emplacement of the ore. Hydrothermal activity was ongoing during mudstone deposition as massive sulfides such as those of the HW zone are located within the contact mudstone and likely formed at the ancient seafloor during ongoing background sedimentation. Alteration of mudstone above the HW lens and of basaltic sills and dikes emplaced within the lower portion of the hanging wall suggest that hydrothermal activity at Eskay Creek was long-lived, accompanying suspension sedimentation of tens of meters of carbonaceous mudstone. Waning of the hydrothermal activity occurred at some stage during the deposition of the hanging-wall mudstone (Roth et al., 1999).

Due to the longevity of hydrothermal activity, temporal overlap between hydrothermal activity and background sedimentation, and lateral transport of clastic ore away from its source, hydrothermal alteration of the carbonaceous host rock of the Eskay Creek does not define a well- developed zoned alteration halo around the ore lenses that would be comparable to footwall

- 68 - alteration pipes and hanging-wall alteration plumes associated with mound-style VHMS deposits formed in flow-dominated volcanic successions (Riverin and Hodgson, 1980; Gemmell and Large, 1992; Gemmell and Fulton, 2001). However, the results of the present study demonstrate that broad alteration patterns can be recognized in the carbonaceous mudstone that map out the location of discordant ore zones within the footwall rhyolite, which presumably formed in areas of structurally-controlled hydrothermal upflow (Roth et al., 1999).

Intense hydrothermal alteration of the carbonaceous mudstone at Eskay Creek was presumably limited due to the nature of this unusual host rock. At the time of mineralization, the character of the mudstone must have ranged from fairly consolidated at the base of the contact mudstone to unconsolidated at the seafloor. Some of the clastic ores hosted by the contact mudstone contain abundant imbricated, laminated mudstone rip-up clasts (Roth et al., 1999), confirming that the mudstone was already at least partially consolidated through compaction prior to mineralization. This fairly consolidated mudstone was presumably only of low permeability, limiting fluid flow away from zones of major fluid upflow. Reaction with the volcaniclastic component contained in the mudstone and cooling through mixing with pore water must have caused a rapid equilibration of hydrothermal fluids percolating through the mudstone.

The temporal relationships between hydrothermal activity and background sedimentation and the variably consolidated nature of the mudstone at the time of alteration explain the observation that intense hydrothermal alteration of the carbonaceous mudstone only occurred locally, close to the contact with the footwall rhyolite and above hydrothermal upflow zones as indicated by the occurrence of discordant zones of alteration and mineralization in the footwall. The alteration of the carbonaceous mudstone occurred at comparably low temperatures from a fluid that rapidly equilibrated with the host rock and cooled due to mixing with pore water during fluid flow up stratigraphy and away from zones of intense fluid upflow.

2.7.4 Implications to Gold Enrichment in Submarine Hydrothermal Systems

The observations of the present study have some implications for models explaining enrichment of precious metals in volcanic-hosted massive sulfide deposits. Total gold

- 69 - endowment in all currently known volcanic-hosted massive sulfide deposits is estimated to be approximately 120 million ounces (data from Franklin et al., 2005). Approximately 40 percent of the total gold endowment is hosted by eleven gold-rich deposits, which includes the Jurassic Eskay Creek deposit in the Iskut River area of British Columbia (Mercier-Langevin et al., 2011).

Previous research has shown that a number of these gold-rich deposits are associated with acid-style hydrothermal alteration. In the Archean LaRonde and Bousquet 2-Dumagami deposits of the Abitibi greenstone belt in Quebec (Dubé et al., 2007, 2014) as well as the Proterozoic Boliden deposit in the Skellefte district in Sweden (Bergman Weihed et al., 1996) the footwall alteration associated with the massive sulfides is characterized by metamorphic mineral assemblages that contain aluminosilicates such as andalusite or kyanite. The aluminous alteration halos associated with these deposits have been interpreted to be the result of intense acid leaching of the volcanic host rocks (Sillitoe et al., 1996; Dubé et al., 2007, 2014), requiring the presence of highly acidic hydrothermal fluids. The presence of strong acid species, such as sulfuric acid formed through disproportionation of SO2 (Herzig et al., 1998; Gena et al., 2001), has been explained by magmatic contributions to the hydrothermal system derived from an actively degassing magma chamber at depth (cf. Giggenbach, 1992). However, the findings of the present study show that gold enrichment in the volcanic environment is not limited to massive sulfide deposits associated with aluminous alteration halos. This in turn implies that active magmatic degassing is not a prerequisite for gold enrichment in the massive sulfide environment.

Hydrothermal alteration of the carbonaceous host rocks of the Eskay Creek deposit also contrasts alteration patterns observed in other gold-rich massive sulfide deposits, such as the Archean Horne deposit in the Abitibi greenstone belt in Quebec (Kerr and Mason, 1990; Kerr and Gibson, 1993; Gibson et al., 2000; Monecke et al., 2008) and the nearby Quemont deposit (Ryznar et al., 1967; Weeks, 1967), where fluid upflow zones are characterized by a chloritic core surrounded by extensive sericite alteration, or the Triassic gold-rich Greens Creek deposit in Alaska that is typified by a quartz-sericite core and a chlorite mantle (Sack, 2009). Hydrothermal alteration of the carbonaceous host rocks of the Eskay Creek deposit occurred at temperatures that were distinctly lower (<200°C) than those causing alteration in these other VHMS deposits.

- 70 - The results of the present study, thus, imply that fluid temperature as constrained by alteration mineralogy is also not the principal control on synvolcanic gold enrichment. VHMS deposits formed at high temperatures (250-350°C; Schardt et al., 2001) having a chlorite core surrounded by a sericite halo as well as deposits formed at intermediate temperatures showing a quartz- sericite core and sericite halo (200-250°C; Schardt et al., 2001) and deposits characterized by low temperature alteration (<200°C; this study) can be gold-rich.

The observed alteration characteristics suggest that Eskay Creek formed from low temperature hydrothermal fluids that were characterized by elevated CO2 contents. The fluids generally were of low acidity, with the exception of acidity produced by the reaction of CO2 with water at low temperatures. The formation of sulfides implies that the hydrothermal fluids were reduced in character with H2S being the dominant sulfur species. The sulfide mineralogy in altered carbonaceous mudstone samples as well as the ore itself (cf. Roth et al., 1999) implies that the fluids were of a low sulfidation state. These observed characteristics are consistent with current models suggesting a continuum of fluid characteristics in the submarine environment ranging from higher temperature hydrothermal systems forming VHMS deposits to lower temperature hot spring systems that have characteristics not unlike geothermal systems and associated low-sulfidation epithermal systems occurring in extensional settings on land (Hannington et al., 1999; Hannington and Herzig, 2000; Monecke et al., 2014).

2.7.5 Implications to Exploration

The mineralogical and geochemical analyses of carbonaceous mudstone collected at different distances to ore at Eskay Creek permitted the identification of a number of potentially useful exploration vectors despite the fact that the ore lenses hosted within this fine-grained sedimentary rock are not enveloped by a well-developed zoned alteration halo.

Mineralogical vectors identified that allow targeting within less than 100 to 200 meters from ore included high illite concentrations and low plagioclase contents, occurrence of anomalously high chlorite content, quartz enrichment through silicification of the carbonaceous mudstone, and locally high K-feldspar contents. The presence of intense carbonate alteration

- 71 - provides the most important mineralogical alteration vectors at Eskay Creek. Members of the dolomite-ankerite and magnesite-siderite solid solutions were only recognized in hydrothermally altered mudstone proximal to ore or fluid upflow zones. Minerals of the dolomite-ankerite solid solution are associated with small amounts of kaolinite. Calcite also represents an important alteration mineral in the periphery of zones of hydrothermal alteration. Within the deposit area, carbonate alteration is most pronounced within 100 m from known ore lenses.

The mineralogical gradients identified correlate with notable variations in major element geochemistry within approximately 100 to 200 meters from ore. Hydrothermal alteration of volcanic glass and plagioclase resulted in a depletion of Na2O. Enrichment of MgO and MnO in altered mudstone is associated with the formation of carbonates of the dolomite-ankerite solid solution, hydrothermal calcite, and chlorite. Due to the low temperature of alteration, K2O enrichment is not pronounced as alteration of the volcanic glass and plagioclase resulted in clay formation. Silicified mudstone samples are typified by SiO2 contents exceeding 70 wt %.

Notable K2O enrichment is mostly associated with the formation of secondary K-feldspar.

Carbonaceous mudstone collected within approximately 100 m of known ore zones are also characterized by anomalous trace element concentrations, in particular enrichment of Au and Ag. In addition, elevated concentrations of As, Hg, Sb, and Tl are indicative of proximity to ore. The base metals Cu, Zn, and Pb are generally enriched within hydrothermally altered mudstone, with proximal base metals incorporated in base metal sulfides, and distal base metals are sorbed to the organic fraction, representing a potential vector to ore. Carbonaceous mudstone samples collected within 100 meters from ore can also show pronounced Ba and sometimes REE enrichments.

The present study highlights that alteration vectoring is possible within carbonaceous mudstone even if cryptic alteration largely precludes identification of alteration styles and intensities in hand-specimen. Alteration vectoring within carbonaceous mudstone intervals is not unlike vectoring in other rock types. However, at Eskay Creek the alteration halo surrounding the ore is not quite as well defined as in other VHMS systems due to the longevity of hydrothermal

- 72 - activity, temporal overlap between hydrothermal activity and background sedimentation, and lateral transport of clastic ore away from its source.

2.8 Conclusions

The present study demonstrates that carbonaceous mudstone hosting the Eskay Creek deposit has been affected by intense hydrothermal alteration. As macroscopic identification of alteration styles and intensities is largely hampered by the fine-grained nature of these rocks and the high abundance of dispersed organic carbon, alteration trends need to be determined using laboratory based techniques. It is shown here that XRD analysis followed by full-pattern fitting of the diffraction patterns using the Rietveld method provides a means of reliably identifying and quantifying the composition of a large number of samples. Combined with whole-rock major and trace element geochemical analysis, this technique yields a large data set of compositional variables that can be screened for vectors to ore. The present study shows that principal component analysis and mass transfer analysis can be most efficiently employed for the exploration of such a large data set of compositional variables and the reduction of the data. Based on these screening techniques, a number of compositional vectors to ore at Eskay Creek were identified.

The observed mineralogical and geochemical alteration patterns suggest that hydrothermal alteration of the immediate host rocks to the Eskay Creek deposit involved the devitrification of originally glassy particles contained in the mudstone and the destruction of feldspars, resulting in the formation of clay minerals such as smectite. Carbonate precipitation represented the most widespread style of hydrothermal alteration. Silicification of the mudstone occurred where the hydrothermal fluids underwent rapid cooling. Hydrothermal K-feldspar formed locally in the carbonaceous mudstone host, mostly peripheral to the main hydrothermal upflow zones. These alteration characteristics, combined with the known ore mineralogy, suggest that Eskay Creek formed from a reduced hydrothermal fluid of variable acidity that contained elevated CO2 concentrations and had a low sulfidation state. Hydrothermal alteration of the carbonaceous mudstone is consistent with Eskay Creek being a low-temperature subaqueous hot spring deposit.

- 73 - 2.9 Acknowledgements

We thank F. Robert for facilitating field work at Eskay Creek. Special thanks are to D. Gale, S. Hasek, and D. MacNeil for their help provided in the field. This paper has benefited from discussions with A. Gysi, E. Holley, N. Kelly, and R. Tosdal. Field work was kindly supported by Barrick Gold. Initial research on the mudstones by T. Monecke was supported by the Michael-Jürgen-Leisler-Kiep Foundation and the Emmy Noether Program of the German Research Foundation and a discovery grant by the Natural Science and Engineering Research Council of Canada awarded to M.D. Hannington. Subsequent research was conducted under the Canadian Mining Industry Research Organization Project 08E04. Geoscience BC provided additional financial support. T. Meuzelaar also benefited from financial support by the Stewart R. Wallace Endowment in Economic Geology at Colorado School of Mines, RockWare, Inc., and Golder Associates, Inc. A research grant by the Society for Economic Geologists Canada Foundation is gratefully acknowledged.

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- 77 - Hay, R.L., and Iijima, A., 1968, Nature and origin of palagonite tuffs of the Honolulu Group on Oahu, Hawaii: Memoir of the Geological Society of America, v. 116, p. 331–376. Hemley, J. J., Montoya, J. W., Marinenko, J. W., and Luce, R. W., 1980, Equilibria in the system Al2O3-SiO2-H2O and some general implications for alteration/mineralization processes:

ECONOMIC GEOLOGY, v. 75, p. 210–228. Herzig, P.M., Hannington, M.D., and Arribas, A., Jr., 1998, Sulfur isotopic composition of hydrothermal precipitates from the Lau back-arc: Implications for magmatic contributions to seafloor hydrothermal systems: Mineralium Deposita, v. 33, p. 226–237. Huston, D. L., and Kamprad, J., 2001, Zonation of alteration facies at Western Tharsis: Implications for the genesis of Cu-Au deposits, Mount Lyell Field, western Tasmania:

ECONOMIC GEOLOGY, v. 96, p. 1123–1132. Idziszek, C., Blackwell, J., Fenlon, R., MacArthur, G., and Mallo, D., 1990, The Eskay Creek discovery: Mining Magazine, March 1990, p. 172–173. Ishikawa, Y., Sawaguchi, T., Iwaya, S., and Horiuchi, M., 1976, Delineation of prospecting targets for Kuroko deposits based on modes of volcanism of underlying dacite and alteration haloes: Mining Geology, v. 26, p. 105–117. Kerr, D.J., and Gibson, H.L., 1993, A comparison of the Horne volcanogenic massive sulfide

deposit and intracauldron deposits of the Mine Sequence, Noranda, Québec: ECONOMIC

GEOLOGY, v. 88, p. 1419–1442. Kerr, D.L., and Mason, R., 1990, A re-appraisal of the geology and ore deposits of the Horne mine complex at Rouyn-Noranda, Québec, in Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J.M., Riverin, G., and Simard, A., eds., The northwestern Québec polymetallic belt: A summary of 60 years of mining exploration: The Canadian Institute of Mining and Metallurgy, Special Volume 43, p. 153–165. Kleeberg, R., and Bergmann, J., 2002, Quantitative phase analysis using the Rietveld method and a fundamental parameter approach, in Gupta, S.P.S., and Chatterjee, P., eds., Powder diffraction. Proceedings of the II international school on powder diffraction: New Delhi, Allied Publishers, p. 63–76. Large, R.R., 1992, Australian volcanic-hosted massive sulfide deposits: features, styles and

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- 78 - Large, R.R., Gemmell, J.B., Paulick, H., and Huston, D.L., 2001a, The alteration box plot: A simple approach to understanding the relationship between alteration mineralogy and

lithogeochemistry associated with volcanic-hosted massive sulfide deposits: ECONOMIC

GEOLOGY, v. 96, p. 957–971. Large, R.R., Allen, R.L., Blake, M.D., and Herrmann, W., 2001b, Hydrothermal alteration and volatile element halos for the Rosebery K lens volcanic-hosted massive sulfide deposit,

western Tasmania: ECONOMIC GEOLOGY, v. 96, p. 1055–1072. Macdonald, A.J., Lewis, P.D., Thompson, J.F.H., Nadaraju, G., Bartsch, R.D., Bridge, D.J., Rhys, D.A., Roth, T., Kaip, A., Godwin, C.I., and Sinclair, A.J., 1996, Metallogeny of an

Early to Middle Jurassic Arc, Iskut River Area, northwestern British Columbia: ECONOMIC

GEOLOGY, v. 91, p. 1098–1114. McGoldrick, P.J., and Large, R.R., 1992, Geologic and geochemical controls on gold-rich

stringer mineralization in the Que River deposit, Tasmania: ECONOMIC GEOLOGY, v. 87, p. 667–685. Mercier-Langevin, P., Hannington, M.D., Dube, B., and Becu, V., 2011, The gold content of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 509–539. Merino, E., 1975, Diagenesis in Tertiary sandstones from Kettleman North Dome, California – Part I, Diagenetic mineralogy: Journal of Sedimentary Petrology, v. 45, p. 320–336. Merriman, R.J., and Peacor, D.R., 1999, Very low-grade metapelites: Mineralogy, microfabrics and measuring reaction progress, in Frey, M., and Robinson, D., eds., Low-grade metamorphism: Oxford, Blackwell Science, p. 10–60. Monecke, T., Köhler, S., Kleeberg, R., Herzig, P.M., and Gemmell, J.B., 2001, Quantitative phase-analysis by the Rietveld method using X-ray powder-diffraction data: Application to the study of alteration halos associated with volcanic-rock-hosted massive sulfide deposits: Canadian Mineralogist, v. 39, p. 1617–1633. Monecke, T., Gale, D., Roth, T., and Hannington, M.D., 2005, The submarine volcanic succession hosting the massive sulfide and sulfosalt Eskay Creek deposit, Canada, in Mao, Y. and Bierlein, F.P., eds., Mineral deposit research: Meeting the global challenge, Proceedings of the 8th biennial SGA meeting, Beijing, China, 2003: Berlin, Springer, p. 655–658.

- 79 - Monecke, T., Giorgetti, G., Scholtysek, O., Kleeberg, R., Götze, J., Hannington, M.D., and Petersen, S., 2007, Textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine PACMANUS hydrothermal system, eastern Manus Basin: Journal of Volcanology and Geothermal Research, v. 160, p. 23–41. Monecke, T., Gibson, H., Dubé, B., Laurin, J., Hannington, M.D., and Martin, L., 2008, Geology and volcanic setting of the Horne deposit, Rouyn-Noranda, Quebec: Initial results of a new research project: Geological Survey of Canada Current Research 2008-9, 16 p. Monecke, T., Petersen, S., and Hannington, M.D., 2014, Constraints on water depth of massive sulfide formation: Evidence from modern seafloor hydrothermal systems in arc-related

settings: ECONOMIC GEOLOGY, v. 109, p. 2079–2101. Morton, R.L., and Franklin, J.M., 1987, Two-fold classification of Archean volcanic-associated

massive sulfide deposits: ECONOMIC GEOLOGY, v. 82, p. 1057–1063. Offler, R., and Whitford, D.J., 1992, Wall-rock alteration and metamorphism of a volcanic- hosted massive sulfide deposit at Que River, Tasmania: Petrology and mineralogy:

ECONOMIC GEOLOGY, v. 87, p. 686–705. Paulick, H., Herrmann, W., and Gemmell, J.B., 2001, Alteration of felsic volcanics hosting the Thalanga massive sulfide deposit (northern Queensland, Australia) and geochemical

proximity indicators to ore: ECONOMIC GEOLOGY, v. 96, p. 1175–1200. Pearce, T.H., 1968, A contribution to the Theory of Variation Diagrams: Contributions to Mineralogy and Petrology, v. 19, p. 142–157. Ramseyer, K., Boles, J.R., and Lichtner, P.C., 1992, Mechanism of plagioclase albitization: Journal of Sedimentary Petrology, v. 62, p. 349–356. Reyes, A.G., 1990, Petrology of Philippine geothermal systems and the application of alteration mineralogy to their assessment: Journal of Volcanology and Geothermal Research, v. 43, p. 279–309. Riverin, G., and Hodgson, C.J., 1980, Wall-rock alteration at the Millenbach Cu-Zn Mine,

Noranda, Québec: ECONOMIC GEOLOGY, v. 75, p. 424–444. Robinson, H.M., 1991, Mineralisation and alteration patterns of the Central Lens (21B Zone), Eskay Creek, British Columbia, Canada: Unpublished M.Sc. thesis, London, U.K., Imperial College of Science, Technology and Medicine, University of London, 129 p.

- 80 - Roth, T., 2002, Physical and chemical constraints on mineralization in the Eskay Creek deposit, northwestern British Columbia: Evidence from petrography, mineral chemistry, and sulfur isotopes: Unpublished Ph.D. thesis, Vancouver, Canada, University of British Columbia, 401 p. Roth, T., Thompson, J.F.H., and Barrett, T.J., 1999, The precious metal-rich Eskay Creek

deposit, northwestern British Columbia: REVIEWS IN ECONOMIC GEOLOGY, v. 8, p. 357– 373. Ryznar, G., Campbell, F.A., and Krouse, H.R., 1967, Sulfur isotopes and the origin of the

Quemont ore body: ECONOMIC GEOLOGY, v. 62, p. 664–678. Sack, P., 2009, Characterization of footwall lithologies to the Greens Creek volcanic-hosted massive sulfide (VHMS) deposit, Alaska, USA: Unpublished PhD thesis, Hobart, Australia, University of Tasmania, 832 p. Schardt, C., Cooke, D.R., Gemmell, J.B., and Large, R.R., 2001, Geochemical modeling of the zoned footwall alteration pipe, Hellyer volcanic-hosted massive sulfide deposit, western

Tasmania, Australia: ECONOMIC GEOLOGY, v. 96, p. 1037–1054. Sherlock, R.L., Roth, T., Spooner, E.T.C., and Bray, C.J., 1999, Origin of the Eskay Creek precious metal-rich volcanogenic massive sulfide deposit: Fluid inclusion and stable

isotope evidence: ECONOMIC GEOLOGY, v. 94, p. 803–824. Shikazono, N., Hoshino, M., Utada, M., Nakata, M., and Ueda, A., 1998, Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan: Mineralium Deposita, v. 33, p. 346–358. Sillitoe, R.H., Hannington, M.D., and Thompson, J.F.H., 1996, High sulfidation deposits in the

volcanogenic massive sulfide environment: ECONOMIC GEOLOGY, v. 91, p. 204–212. Thompson, A.B., 1970, A note on the kaolinite-pyrophyllite equilibrium: American Journal of Science, v. 268, p. 454–458. Tomita, K., Yamane, H., and Kawano, M., 1993, Synthesis of smectite from volcanic glass at low temperature: Clays and Clay Minerals, v. 41, p. 655–661. Treibs, A., 1936, Chlorophyll- und Häminderivate in organischen Mineralstoffen: Angewandte Chemie, v. 49, p. 682–686.

- 81 - Urabe, T., and Scott, S.D., 1983, Geology and footwall alteration of the South Bay massive sulphide deposit, northwestern Ontario, Canada: Canadian Journal of Earth Sciences, v. 20, p. 1862–1879. Velde, B., and Kornprobst, J., 1969, Stabilité des silicates d'alumine hydratés: Contributions to Mineralogy and Petrology, v. 21, p. 63–74. Weeks, R., 1967, Quemont Mining Corporation, Limited, in Abel, M.K., ed., CIMM Centennial Field Excursion Northwestern Québec - Northern Ontario: Montreal, Canadian Institute of Mining and Metallurgy, p. 46–51.

- 82 - CHAPTER 3 HYDROTHERMAL ALTERATION OF CARBONACEOUS MUDSTONE HOSTING THE ESKAY CREEK SULFIDE AND SULFOSALT DEPOSIT, BRITISH COLUMBIA: IDENTIFICATION OF ALTERATION PROCESSES THROUGH CORRELATIVE MICROSCOPY

Chapter 3 identifies mineralogical and petrographic characteristics of the variably altered carbonaceous mudstones hosting the stratiform ore lenses at Eskay Creek using results from detailed correlative microscopy, which included optical microscopy, optical cathodoluminescence microscopy, scanning electron microscopy, and electron microprobe analysis.

3.1 Abstract

The Jurassic Eskay Creek deposit in the Iskut River area of British Columbia represents an unusual, precious metal-rich, polymetallic volcanic-hosted sulfide and sulfosalt deposit. Economic concentrations of precious metals at Eskay Creek are mainly confined to stratiform lenses of clastic ores hosted by a thick interval of carbonaceous mudstone at the contact between a footwall rhyolite and overlying interval of basalt. Due to the fine-grained nature and highly carbonaceous nature of the sedimentary host rock, identification of alteration styles and intensities is not readily possible in hand specimen, which limits the use of alteration vectoring during exploration.

Correlative microscopy on variably altered mudstone has demonstrated that this fine- grained and carbonaceous rock has been affected by a range of post-depositional processes that have modified the original mudstone mineralogy. The carbonaceous mudstone represents a hemipelagic or pelagic suspension sediment that is composed of a mixture of volcaniclastic detritus and organic material. Hydrothermal alteration of the mudstone resulted in the devitrification of the originally glassy volcaniclastic detritus and the formation of abundant clay minerals. In addition to this style of alteration, carbonate infiltration and replacement is widespread, resulting in the abundant occurrence of secondary ferroan dolomite, ankerite, and

- 83 - calcite. Locally, the carbonaceous mudstone has been affected by intense silicification and K- feldspar alteration. Hydrothermal alteration of the mudstone is cryptic and difficult to recognize unless the secondary minerals occur in high concentrations or form distinctive textures such as pervasive carbonate infiltration or K-feldspar flooding. The petrographic evidence suggests that diagenesis of originally glassy volcaniclastic material contained in mudstone that has not been affected by hydrothermal alteration also resulted in the formation of abundant secondary clay minerals and diagenetic pyrite. During low-grade regional metamorphism, clay minerals contained in hydrothermally altered and unaltered mudstone were transformed into illite and chlorite. Transmission electron microscopy demonstrated that two distinct illite polytypes occur suggesting that metamorphism at Eskay Creek did not exceed conditions of the lower anchizone. Plagioclase has been pervasively replaced by secondary albite and calcite.

The results of the present study demonstrate that low-temperature (<200ºC) hydrothermal alteration of the carbonaceous mudstone at Eskay Creek, diagenesis, and 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. In samples not containing these indicator minerals, hydrothermal alteration can only be identified based on textural observations or mineral abundance data. Samples affected by weak hydrothermal alteration are petrographically essentially indistinguishable from carbonaceous mudstone that has only undergone alteration through incipient diagenesis.

3.2 Introduction

The Eskay Creek ore deposit in northwestern British Columbia, Canada, represents a world-class, precious metal-rich synvolcanic sulfide and sulfosalt deposit. Mined between 1995 and 2008, the deposit yielded 2.25 million metric tons of ore at average grades of 48.9 g/t Au and 2,334 g/t Ag. In addition to the anomalous concentrations of precious metals, Eskay Creek was characterized by a number of unique deposit characteristics, including the commonly graded and bedded nature of the ore and an unusual sulfide and sulfosalt mineral assemblage that formed at

- 84 - temperatures of less than 200˚C (Britton et al., 1990; Idziszek et al., 1990; Macdonald et al., 1996; Roth et al., 1999; Sherlock et al., 1999).

Economic enrichment of precious metals at Eskay Creek is mostly restricted to several stratiform lenses of clastic sulfide and sulfosalt ores located within a thick interval of carbonaceous mudstone at the contact between felsic volcanic rocks and overlying basalt (Britton et al., 1990; Ettlinger, 1992; Barrett and Sherlock, 1996; Macdonald et al., 1996). Exploration for the clastic sulfide and sulfosalt ore was challenging because the high-grade stratiform ore lenses are laterally discontinuous and interlayered with low-grade or barren mudstone intervals. Alteration vectoring could not be effectively conducted during exploration because the fine- grained and carbonaceous nature of the host rock precluded the identification of macroscopic alteration styles and intensities in drill core.

The present study reports the findings of a comprehensive petrographic study conducted on the carbonaceous mudstone hosting the Eskay Creek deposit to better characterize the nature of the cryptic alteration. Variably altered mudstone samples collected at various distances to ore were analyzed through a combination of optical microscopy, optical cathodoluminescence microscopy, scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Based on correlative microscopy, it was possible to constrain a range of processes that modified the mudstone mineralogy subsequent to deposition. This included the hydrothermal alteration of the carbonaceous mudstone in areas of hydrothermal upflow, diagenetic compaction of the fine-grained sedimentary material and devitrification of the originally glassy particles, and recrystallization of the mudstone during low-grade metamorphism. Although hydrothermal alteration is cryptic and not easily recognizable in hand- specimen, it is demonstrated that hydrothermal alteration of the fine-grained carbonaceous mudstone can be recognized up to tens to hundreds of meters from the orebodies using the appropriate microscopic techniques.

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3.3 Geological Setting

The Eskay Creek deposit is located at the western margin of the allochthonous Stikinia terrane of the northern Canadian Cordillera (Fig. 3-1). The Middle Jurassic (Childe, 1996) submarine volcanic succession hosting the deposit is assigned to the Hazelton Group that was deposited within an extensional environment during a period of intense continental margin arc volcanism (Barrett and Sherlock, 1996; Macdonald, et al., 1996).

The volcanic host rocks of the deposit were folded into a shallowly north plunging, north- northeast trending, upright, open anticline (Fig. 3-2). The deposit is located on the western limb of this fold, near the fold hinge. The stratigraphy within the deposit gently dips (30 to 45˚) to the west. Normal faults that trend north-northwest to north-northeast crosscut the anticline and disrupt the ore lenses.

The stratigraphic footwall to the Eskay Creek deposit is composed of rhyolite (Fig. 3-2; Britton et al., 1990). The footwall rhyolite has maximum apparent thickness of about 100 m (Britton et al., 1990) and is composed of multiple coherent emplacement units that grade into extensive zones of rhyolite breccia that have formed through quenching, autobrecciation, and hydraulic brecciation (Allen, 1993; Monecke et al., 2005). The central part of the deposit is underlain by a texturally distinct stratified rhyolite sandstone and breccia facies (Allen, 1993; Monecke et al., 2005). Hydrothermal alteration is widespread throughout the footwall rhyolite (Robinson, 1991; Barrett and Sherlock, 1996). Peripheral to the stratiform ores of the Eskay Creek deposit and in the deeper parts of the footwall, hydrothermally altered rhyolite is typified by the occurrence of secondary potassium feldspar and is moderately silicified. Intense and texturally destructive propylitic alteration is largely restricted to the upper contact of the footwall rhyolite, immediately underlying the stratiform mineralization. The tabular shape of this zone, which contains abundant chlorite and white mica, coincides spatially with the inferred distribution of the stratified rhyolite breccia and sandstone facies (Ettlinger, 1992; Roth et al., 1999).

- 86 -

FIG. 3-1: Geological map of the Iskut River area and location of the Eskay Creek deposit. The inset shows the distribution of the Stikinia terrane in British Columbia (modified from MacDonald et al., 1996).

The footwall rhyolite is overlain by a thick package of carbonaceous mudstone, which also represents the immediate host to the stratiform sulfide and sulfosalt ore lenses. The carbonaceous mudstone interval ranges from 1 m to greater than 60 m in thickness (Britton et al., 1990; K. Rye, unpub. report for International Corona Corp., 1992, 125 p.). Basalt sills and dikes occur throughout the carbonaceous mudstone unit (Fig. 3-2). The occurrence of mudstone-matrix basalt breccia along the bottom and top margins of coherent basalt intervals indicates that the basalt intruded the still wet and unconsolidated mudstone (Roth et al., 1999; Monecke et al., 2005). Based on the first occurrence of basalt sills, the carbonaceous mudstones can be subdivided into two units, namely the contact mudstone that occurs between the footwall rhyolite

- 87 -

FIG. 3-2: Geological map of the Eskay Creek anticline. The map also shows the surface projection of the ore bodies (modified from Roth, 2002). and the stratigraphically lowest basalt unit in the mudstone interval and the hanging-wall mudstone, which occurs higher up in the stratigraphy (Britton et al., 1990).

Basalt prevails further up in the mine succession. The hanging-wall basalt locally exceeds 150 meters in thickness, by generally thins southward away from the deposit (Britton et al., 1990; K. Rye, unpub. report for International Corona Corp., 1992, 125 p.). The basalt units are intercalated with variably thick intervals of hanging-wall mudstone. Well-preserved pillow basalts and talus breccia occur towards the top of the hanging-wall basalt (Britton et al., 1990; Barrett and Sherlock, 1996; Roth et al., 1999; Monecke et al., 2005).

- 88 -

At Eskay Creek, economic concentration of precious metals is confined to a number of distinct stratiform ore lenses (Fig. 3-3) that range from 5 to 45 m in true stratigraphic thickness. The ore lenses have a maximum strike length of about 1,500 meters and down-dip extent of 350 meters (Roth et al., 1999).

The 21B zone located within the contact mudstones at or near the contact with the underlying footwall rhyolite represents the main ore lens at Eskay Creek. The ore zone is comprised of clastic ores comprising pebble to cobble-sized sulfide and sulfosalt clasts (Britton et al., 1990; Idziszek et al., 1990; Roth et al., 1999). This tabular ore zone is approximately 650 m long, 60 to 150 m wide, and locally exceeds 20 m in thickness. The stratiform ores of the 21A zone occur at the southern end of the deposit. This ore lens is comprised if massive to semi- massive ores that are characterized by high concentration of stibnite, realgar, cinnabar, and arsenopyrite (Britton et al., 1990; Idziszek et al., 1990; Roth et al., 1999). Ore zone reaches an average thickness of approximately 10 m and covers an area that is 240 meters long and 180 meters wide (Roth et al., 1999). Sulfide ore in the 21C zone in the western part of the deposit occur at the same stratigraphic position at or close to the contact with the footwall rhyolite. The ores of this lens are clastic and consist of beds of massive to bladed barite and calcite containing abundant, very fine-grained sulfides and sulfosalts (Roth et al., 1999).

Several smaller ore lenses hosted by carbonaceous mudstone have been identified in the northern part of the deposit (Fig. 3-3). Near the north end of the 21B zone, the precious metal- rich mineralization extends over the top of the anticline into the complexly folded and faulted East Block (Roth et al., 1999). The NEX zone lies north and consists of mainly fine-grained massive sulfide (Roth et al., 1999). The HW zone, containing fine-grained and finely banded semi-massive to massive sulfide is present in the contact mudstone stratigraphically above the NEX zone (Britton et al., 1990; Roth et al., 1999).

- 89 -

FIG. 3-3: Plan view of the spatial distribution of ore zones at Eskay Creek (modified from Roth, 2002).

In addition to the stratiform ore lenses, high-grade discordant zones of sulfide mineral- bearing veins and disseminations, referred to as the Pumphouse and Pathfinder zones, have been recognized within the footwall rhyolite below and adjacent to the 21B zones. A smaller discordant sulfide zone occurs in the 109 zone that underlies the north end of the 21B zone and the HW zone (Roth et al., 1999).

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3.4 Materials and Methods

The present study is based on 50 mudstone samples collected from exploration drill core as well as surface and underground exposures. The samples were taken at various distances from ore, ranging from the immediate host of ore zone to a maximum distance of approximately 4.4 km from ore.

Initially, polished thin sections were prepared and inspected by transmitted and reflected light microscopy using an Olympus BX51 microscope. Subsequent to optical microscopy, the thick sections were carbon coated and studied by optical CL microscopy using a HC5-LM hot- cathode microscope by Lumic Special Microscopes, Germany. The microscope was operated at 14 kV and with a current density of ca. 10 µA mm-2 (Neuser, 1995). CL images were captured using a high sensitivity, double-stage Peltier cooled Kappa DX40C CCD camera.

Textural relationships not resolvable optically were visualized by back-scattered electron (BSE) imaging using a FEI Quanta FEG 450 field emission scanning electron microscope, housed at the Denver Microbeam Facility, USGS Lakewood, CO. High-resolution imaging was performed using an accelerating voltage of ~20 keV, a beam current of ~1.5 nA, and a working distance of ~10 mm. Electron microprobe (EMP) analyses of carbonate minerals and sheet silicates were performed using the JEOL 8900 Superprobe, also housed at the Denver Microbeam Facility, which is equipped with five wavelength dispersive X-ray spectrometers. Spot analysis of carbonate minerals was conducted using an accelerating voltage of 10 kV, a beam current of 30 nA, measured on the Faraday cup. A 5 µm defocused beam was used. Counting times were 30 seconds on the peak and 15 seconds on the background on each side of the peak. Ca, Sr, Mg, Fe, and Mn were measured. Calcite (Ca), dolomite (Mg), siderite (Fe), rhodonite (Mn), and celestine (Sr) were used as primary standards. Illite and chlorite were analyzed at15 kV accelerating voltage, 20 nA beam current, and a focused beam. Si, Al, Ti, Fe, Mn, Mg, Ca, Na, K, and Cl count times were 20 seconds on the peak and 20 seconds on the background, Cr and Ba were measured 30 seconds on peak and background, and F was measured at 120 seconds on peak and background. Fluorite (F), synthetic fayalite (Fe), orthoclase (K, Si),

- 91 - albite (Na), spessartine (Mn), anorthite (Ca, Al), forsterite (Mg), synthetic TiO2 (Ti), sodalite (Cl), Cr-spinel (Cr), and barite (Ba) were used as primary standards.

Three representative samples (CA90-291-96.6; 1021-27.2; CA90-345-57.0) were selected for combined transmission electron microscopy-analytical electron microscopy (TEM-AEM) analysis to study the textural relationships between the illite polytypes identified by X-ray diffraction (XRD) analysis. Sample preparation for TEM-AEM analysis initially involved the preparation of sticky-wax-polished thin sections. Copper disks with a central hole of 400-600 µm were then glued on the areas targeted for analysis. A total of six mounted samples, two from each sample, were subsequently removed from the thin sections and ion-milled using a Gatan Dual Ion Mill 600. The TEM observations and AEM analyses were conducted on a JEOL 2010 transmission electron microscope, housed at the University of Siena, Italy, operated at 200 kV. The semi-quantitative AEM analyses were carried out employing an X-ray energy dispersive Link Isis EDX system and a beam diameter of ~20 nm. The AEM spectra were processed using K-factors determined on natural ion-milled standards (albite, clinochlore, fayalite, muscovite, paragonite, rhodonite, and titanite) as described by Li et al. (1994).

3.5 Results

Results of petrography, optical cathodoluminescence microscopy, scanning electron microscopy, electron microprobe and transmission electron microscopy are detailed in this section.

3.5.1 Mudstone Petrography

The carbonaceous mudstone samples investigated are black to dark gray in hand sample and laminated, thinly bedded, or massive. Locally abundant intercalated, tan-colored fine-grained volcaniclastic material occurs in layers that are graded to massive and typically less than five millimeters in thickness. Individual beds are commonly planar and laterally continuous at the sample scale, although sedimentary structures including load casts and flames were locally

- 92 - observed. The mudstone also contains rare out-of-size pumice clasts and laterally attenuated, smeared and deformed, soft mudstone clasts.

The mudstone is commonly too fine-grained to allow identification by transmitted light microscope of the detrital component. The fine-grained mudstone matrix contains abundant illite and chlorite and generally only a few, dispersed, silt-sized quartz and feldspar crystal fragments and broken ash shards. The quartz fragments appear to be of dominantly volcanic origin. They are internally clear, inclusion-free, and monocrystalline, as evidenced by even extinction across the entire grain surface upon rotation. The monocrystalline fragments show rounded corners and occasional resorption embayments, which is consistent with these quartz grains representing phenocrysts fragments. Plagioclase fragments are subhedral or angular (Fig. 3-4A). Twins and compositional zones are commonly diffuse, suggesting that the plagioclase is albitized. Some plagioclase grains contain inclusions of chlorite, which are interpreted to represent altered glassy inclusions commonly occurring in volcanic plagioclase. In addition to albitized plagioclase crystal fragments, some samples were found to contain fragments of K-feldspar. The K-feldspar is commonly clouded and partially replaced by illite.

The tan-colored beds exhibit a vitriclastic texture, suggesting the beds represent ash layers dominated by formerly glassy shards (Fig. 3-4B, C). The basal contacts of the ash beds are either planar, with sharply defined contacts, or show evidence of soft-sediment deformation such as load casts and flames. The upper contacts of the beds are usually capped by very fine-grained mudstone. The formerly glassy shards are variably shaped, and as large as 200 µm; rod- and spike-shaped shards are particularly common. Some shards are distinctly cuspate, presumably fragmented at adjacent vesicle junctions (Fig. 3-4C). Flat or slightly curved platy shards probably represent fragmented walls separating adjacent vesicles. Outlines of the delicate shards are typically well preserved although pervasive quartz or carbonate replacement is common. In addition to the relict shards, the ash layers contain subordinate feldspar and quartz crystal fragments. The size range of the crystal fragments is similar to that of the formerly glassy shards.

- 93 -

Fig. 3-4: Microphotographs of carbonaceous mudstone from Eskay Creek. A. Dispersed angular plagioclase (Plg) grains. One of the crystal fragments is partially replaced by illite (arrow). B. Ash layer exhibiting a vitriclastic texture. Formerly glassy shards (S) show rod and spike shapes. The shards as well as rare crystal fragments are set in a very fine carbonaceous matrix that is rich in illite. C. Cuspate shards (arrow) in ash beds are interpreted to represent fragments derived from junctions of adjacent vesicles. Individual shards are replaced by quartz or carbonate minerals. D. Pyrite-rich layers composed of small pyrite (Py) euhedra dispersed in a fine-grained matrix. E. Framboidal pyrite. F. Ankerite (Ank) patches in a hydrothermally altered contact mudstone. The fine-grained matrix is composed of small quartz and feldspar grains and contains a high illite content. G. Prehnite porphyroblasts that range up to 1 mm in size and have radial shapes.

- 94 -

Most mudstone samples contain abundant microfossil remains. Most of the microfossils visible in optical microscopy are small siliceous skeletal fragments that range in shape from oval to circular and are interpreted to be radiolaria or dinoflagellates.

Some mudstone samples contain discrete layers of pyrite that are parallel to the bedding planes of the mudstone. Pyrite layers are composed of thinly dispersed euhedral cuboids as well as aggregates of smaller crystals (< 100 µm) dispersed in a very fine-grained mudstone matrix (Fig. 3-4D, E). Euhedral pyrite grains frequently display overgrowths. The pyrite is typically non-porous and microscopically free of inclusions. The discrete layers of pyrite grains present in the mudstone are interpreted to be the distal equivalents to the mass-flow-emplaced ore beds. In addition to the euhedral pyrite, pyrite framboids are common in the fine-grained matrix of most mudstone samples (Fig. 3-4E). In rare cases, small veinlets of pyrite crosscut bedding in the mudstone.

Many of the mudstone samples from Eskay Creek have been affected by carbonate alteration. Due to the fine-grained and carbonaceous nature of the rocks investigated, weakly altered mudstone samples are generally difficult to distinguish from their protoliths by standard optical microscopy. In more intensely altered mudstone, secondary carbonate minerals are present in concentrations exceeding 10 modal %. The carbonate minerals occur in stringers composed of variably-sized patches and spots within the fine-grained mudstone matrix (Fig. 3- 4F). In addition to the carbonate stringers, carbonate minerals also form small veinlets that crosscut bedding. In some cases, these veinlets terminate abruptly and are then continued as stringers of bedding parallel patches and spots suggesting that carbonate alteration occurred in response to hydrothermal infiltration of the carbonaceous mudstone. The carbonate trails are crosscut by coarsely crystalline calcite veinlets that are interpreted to be unrelated to the hydrothermal alteration of the mudstone.

Porphyroblasts of prehnite are common in some hanging-wall mudstone samples, occurring up to several millimeters in size and constituting up to 40 modal % of the mudstone. The prehnite occurs as isolated porphyroblasts, as stringers, or in groups of impinging and

- 95 - coalesced aggregates (Fig. 3-4G). Most porphyroblasts consist of arrays of closely packed prehnite arranged in radial habit around a common center. In cross-polarized light, these radial porphyroblasts exhibit a cross-shaped extinction pattern, with the extinction centered at the porphyroblast origin. The limbs of the extinction cross are oriented parallel to the transmission directions of the microscope polarizer and analyzer. In addition to the radial prehnite porphyroblasts, fan and sheaf-like aggregates occur. The latter consist of two conical bundles of prehnite crystals jointed at their apices. The prehnite crystals forming the radial, fan, or sheaf- like porphyroblasts are elongate lath-like or wedge-shaped and typically less than 300 µm in maximum dimension. The prehnite grains appear murky in thin section, are commonly studded with minute inclusions of carbonaceous material, and indicate formation temperatures of 200ºC and higher (Schiffman and Fridleifsson, 1991; Neuhoff et al., 1999). In some samples, the prehnite is crosscut by late carbonate veinlets.

3.5.2 Optical Cathodoluminescence Microscopy

In least-altered mudstone samples, optical CL microscopy allowed the identification of a range of mineral phases contained in the volcaniclastic component of the fine-grained carbonaceous mudstone (Fig. 3-5A). These minerals typically occur as small angular to rounded particles, probably largely representing phenocrysts fragments. Most abundant are small feldspar grains. Albite grains show a mottled brown to dark brown CL color. The albite is frequently intergrown with calcite, exhibiting a bright orange CL. The mottled appearance of the plagioclase and the intergrowth relationship may suggest that the albite formed through replacement of plagioclase grains. In addition to the brown albite, abundant bluish feldspar grains can be observed in some samples, which are interpreted to represent K-feldspar phenocrysts fragments. Quartz grains have a dark blue to purple stable CL color. Apatite, characterized by a green to yellow green CL emission, is also present as small angular to rounded grains. Other minerals occurring within least-altered mudstone such as illite and chlorite do not show a CL emission.

Optical CL microscopy proved to be most efficient for the identification of carbonate alteration as carbonate minerals such as dolomite-ankerite and calcite exhibit bright CL

- 96 -

Fig. 3-5: Optical CL images of carbonaceous mudstone from Eskay Creek. A. Crystal fragments of quartz (qtz), K-feldspar (Ksp), albitized plagioclase (ab), and apatite (ap) contained in mudstone matrix affected by weak carbonate alteration. B. Pervasive carbonate flooding of mudstone matrix. C. Irregular carbonate veinlets in intensely altered mudstone. D. Pervasive K- feldspar flooding. The blue secondary K-feldspar is texturally associated with calcite. E. Blocky prehnite (prh) crosscut by a late calcite (cal) veinlet. F. Radial prehnite (prh) porphyroblasts overgrowing a calcite core.

- 97 - emission. The mudstone samples investigated exhibit various intensities of carbonate alteration. In many samples, carbonate minerals only occur as small grains and patches in the matrix of the fine-grained volcaniclastic rocks (Fig. 3-5A). However, pervasive carbonate alteration also resulted in extensive flooding of the mudstone matrix in some sample (Fig. 3-5B). Variations in CL intensity allowed the distinction between dark red-brown dolomite-ankerite and the bright orange calcite. In intensely altered mudstone, feldspar is almost entirely replaced by the secondary carbonate minerals. In addition to pervasive replacement, small, irregular shaped carbonate veins of intergrown dolomite-ankerite and calcite also occur (Fig. 3-5C). The margins of these veins are typically not sharp due to carbonate infiltration of the surrounding matrix.

Some samples are characterized by intense K-feldspar flooding (Fig. 3-5D), which was not readily identified by optical microscopy. The feldspar in these samples forms wavy replacement textures and is associated with small calcite patches.

The blocky to radial prehnite porphyroblasts present in some hanging wall mudstone samples show CL colors varying from greenish to yellowish and brown (Fig. 3-5E,F). Calcite commonly occurs as small inclusions within the prehnite (Fig. 3-5E, F). In some cases, the centers of the prehnite porphyroblasts are composed of calcite, possibly suggesting that the prehnite preferentially formed in mudstone having a high CaO content (Fig. 3-5F). The prehnite porphyroblasts are locally crosscut by late calcite veinlets (Fig. 3-5E).

3.5.3 Scanning Electron Microscopy

SEM imaging was conducted on selected samples containing multiple carbonate minerals as suggested by variations in orange to red brown color of the CL signal. EDX analysis showed that bright orange luminescent carbonate is calcite whereas the darker orange to red-brown carbonate compositionally belongs to the dolomite-ankerite solid solution. In many of the samples, the carbonate phases are finely intergrown (Fig. 3-6). In some cases, calcite forms the core to patches and spots of carbonate minerals and is mantled by minerals of the dolomite- ankerite solid solution. The secondary carbonate minerals are frequently associated with quartz and kaolinite, suggesting that these minerals formed as a result of carbonate alteration of the

- 98 -

Fig. 3-6: Back-scatter electron microscope images of mudstone from Eskay Creek. A. Contact mudstone containing carbonate patches that formed by hydrothermal infiltration and replacement processes. The carbonate patches consist of intergrown calcite (Cal) and ankerite (Ank). Calcite is typified by a high contrast, whereas ankerite is more difficult to distinguish from the fine- grained matrix of the mudstone. Abundant disseminated pyrite (Py) occurs throughout the mudstone matrix. B. Carbonates forming irregularly shaped patches and small veinlets in hydrothermally altered contact mudstone are frequently associated with secondary quartz (Qtz) and kaolinite (Kln). carbonaceous mudstone. The secondary kaolinite typically forms small aggregates with maximum sizes of approximately 20 µm (Fig. 3-6).

- 99 -

3.5.4 Electron Microprobe Analysis of Carbonate Minerals

Electron microprobe analysis on carbonate minerals was conducted on several representative mudstone samples to establish compositional variations (Fig. 3-7; representative EMP data are given in Table 3-1). The microanalytical research demonstrated that calcite in the carbonaceous mudstone samples contain variable FeO and MgO concentrations. On average, calcite contained in the contact mudstone contains 0.21 wt. % FeO while calcite in the hanging- wall mudstone averages 0.34 wt. % FeO. The MgO content calcite contained in the contact mudstone is slightly elevated (average of 0.58 wt. % MgO) when compared to the hanging wall samples (average 0.16 wt. % MgO). The SrO content of contact and hanging-wall calcite is similarly low, with an average of 0.04 wt. %.

The microanalytical data indicate that the dolomite-ankerite grains analyzed predominantly plot into the ferroan dolomite field (Fig. 3-7). Although FeO contents of the ferroan dolomite is somewhat variable, the FeO metal content typically does not exceed 9.0 wt. %. MnO concentrations range from 0.12 to 0.79 wt. %. The SrO content of the ferroan dolomite typically does not exceed 0.20 wt. % SrO. Siderite and magnesite are comparably rare in the samples investigated (Fig. 3-7; Table 3-1). Both minerals contain low concentrations of MnO and SrO.

3.5.5 Electron Microprobe Analysis of Illite and Chlorite

Electron microprobe analysis of sheet silicate minerals was conducted on representative contact and hanging-wall mudstone samples. Spot analysis demonstrated that illite in the carbonaceous mudstone at Eskay Creek has a variable Si contents ranging from 6.31 to 7.63 apfu (Table 3-2). The Fe and Mg contents are commonly elevated (Fe up to 0.35 apfu and Mg up to 1.16 apfu). The interlayer position of the illite is predominately occupied by K. However, the K+Na values of illite only vary from 0.80 to 1.60 apfu, indicating that the illite is interlayer deficient. There is a negative correlation between the Si content of the illite and the interlayer occupancy, which confirms that the illite is interlayer deficient.

- 100 -

Fig. 3-7: Electron microprobe analyses of carbonate minerals contained in carbonaceous mudstone from Eskay Creek.

Chlorite contained in the carbonaceous mudstone hosting Eskay Creek has Si concentrations ranging from 5.58 to 6.80 apfu. The Fetot (Fe2++Fe3+) content of the chlorite varies from 2.49 to 6.68 apfu. The EMP analyses (Table 3-3) showed that chlorite contained in the contact mudstone is relatively enriched in Mg and Mn when compared to chlorite in the hanging- wall mudstone, which shows higher Fe concentrations. Furthermore, there is a clear relationship between chlorite composition and proximity to ore (Fig. 3-8). Chlorite contained in samples collected away from known mineralization exhibits a wider range of Fe and Mg concentrations and contains a higher Fe and lower Mg content than samples collected closer to ore. Within the ore itself, chlorite shows high Fe and low Mg concentrations.

3.5.6 Transmission Electron Microscopy of Illite

To constrain the textural, crystallographic, and mineral chemical characteristics of illite contained in the carbonaceous mudstone from Eskay Creek, TEM investigations were conducted on six ion-milled mounts obtained from three representative mudstone samples.

- 101 -

TABLE 3-1: Representative electron microprobe analyses of carbonate minerals contained in mudstone from Eskay Creek. All data in wt. %. CO2 determined by stoichometric calculation (nd = not detected).

FeO MnO MgO CaO SrO CO2 Total

Calcite C96-738-123.5 0.28 1.28 0.41 54.53 0.016 44.23 100.74 C97-865-78.3 0.19 0.05 0.01 56.5 0.09 44.54 101.39 C98-893-87.8 nd 0.08 nd 56.54 0.03 44.43 101.07 C99-953-127.1 0.47 0.17 0.24 54.75 0.11 43.67 99.41 C99-953-127.1 0.51 0.01 0.19 55.99 0.06 44.49 101.26 Ferroan dolomite C96-783-97.6 7.43 0.49 13.22 33.85 0.15 45.91 101.05 C96-783-99.5 1.60 1.26 20.74 29.49 0.03 47.57 100.70 C96-783-99.5 6.34 0.28 20.13 26.69 0.17 47.06 100.66 C97-866_158.7 8.26 0.64 15.43 29.52 0.05 45.49 99.39 C97-866_158.7 9.43 0.40 15.88 27.93 0.07 45.31 99.02 Siderite C97-865-78.3 53.54 0.15 0.10 6.64 nd 38.21 98.64 C97-865-78.3 53.66 0.09 0.13 6.23 nd 37.97 98.10 C97-865-78.3 52.88 0.08 0.18 6.91 nd 38.06 98.13 C97-865-78.3 52.80 0.37 0.16 7.57 nd 38.69 99.60 Magnesite C96-783-99.5 13.78 0.14 36.87 1.13 nd 49.68 101.60 C96-783-99.5 10.98 0.26 36.68 2.11 0.05 48.61 98.69

In all three samples, illite occurs as two distinct polytypes. Flakes of the 2M polytype form large (100-300 nm thick) crystals that are largely defect-free and have a high crystallinity (Fig. 3- 9). Selected area electron diffraction patterns (inset in Fig. 3-9) show sharp spots, with no diffusiveness. The 00l reflections have a 10 Å periodicity and the 02l reflections show a 20 Å spacing, indicative of a 2-layer polytype. Sometimes, extra spots appear along c* due to dynamic effects. Illite of the 1M polytype forms small (up to 50 nm-thick) and elongated crystals. These crystals are commonly grouped in bundles. Individual crystals are bent, with high angle grain boundaries and are highly defective (Fig. 3-10). The illite 1M flakes are commonly intergrown with titanium oxide grains (anatase or rutile). Selected area electron diffraction shows that the 1M polytype occur in crystals with the c axes being randomly oriented (inset in Fig. 3-10). The illite 2M and 1M crystals commonly occur in separate areas, with no textural relation between them. However, sometimes large 2M crystals appear to be truncated by the smaller 1M illite flakes (Fig. 3-11).

- 102 -

TABLE 3-2: Representative EMP analyses of illite contained in the carbonaceous mudstone host of the Eskay Creek deposit. Mica formulae were calculated on the basis of 22 oxygen equivalents (nd = not detected).

3XL32 C96-737-150.0 C97-866-158.7

SiO 2 50.41 51.11 51.20 49.63 50.31 50.72 52.44 52.78 54.89 TiO2 0.10 0.20 0.14 0.05 0.02 0.16 0.13 0.38 0.14 Al2O3 31.32 30.38 30.99 37.40 37.15 34.45 30.69 30.02 31.98 Cr2O3 0.01 0.03 Nd 0.09 0.08 0.09 nd 0.04 0.05 FeO 0.33 2.07 0.36 0.73 0.19 0.51 1.53 2.32 1.22 MnO 0.04 0.04 0.05 nd 0.04 0.02 nd nd 0.03 MgO 5.48 3.73 5.21 1.34 1.26 1.88 1.84 1.56 2.20 CaO Nd 0.60 0.06 0.14 0.57 1.24 0.08 0.09 0.05 BaO 0.72 0.78 0.69 0.12 0.26 0.35 0.19 0.35 0.12 Na2O Nd 0.06 0.03 0.22 0.63 0.29 0.08 0.23 nd K2O 6.42 6.09 6.21 6.84 6.39 6.69 8.24 9.35 6.11 F 0.08 0.14 0.05 0.05 0.02 0.02 0.05 0.04 0.09 Cl Nd Nd Nd nd nd 0.01 nd nd 0.01 H2O 4.57 4.52 4.61 4.69 4.73 4.68 4.58 4.61 4.72 O=F 0.03 0.06 0.02 0.02 0.01 0.01 0.02 0.02 0.04 O=Cl Nd Nd Nd nd nd nd nd nd nd Sum 99.45 99.68 99.56 101.29 101.64 101.10 99.83 101.75 101.58

Si 6.55 6.67 6.63 6.31 6.37 6.49 6.83 6.83 6.90 AlIV 1.45 1.33 1.37 1.69 1.63 1.51 1.17 1.17 1.10 AlVI 3.35 3.35 3.37 3.92 3.90 3.68 3.54 3.41 3.65 Ti 0.01 0.02 0.01 nd nd 0.02 0.01 0.04 0.01 Cr Nd Nd Nd 0.01 0.01 0.01 nd nd nd Fe 0.04 0.23 0.04 0.08 0.02 0.05 0.17 0.25 0.13 Mn Nd Nd 0.01 nd nd nd nd nd nd Mg 1.06 0.73 1.01 0.25 0.24 0.36 0.36 0.30 0.41 Ca Nd 0.08 0.01 0.02 0.08 0.17 0.01 0.01 0.01 Na Nd 0.02 0.01 0.05 0.15 0.07 0.02 0.06 nd K 1.06 1.01 1.03 1.11 1.03 1.09 1.37 1.54 0.98 Ba 0.04 0.04 0.03 0.01 0.01 0.02 0.01 0.02 0.01 F 0.03 0.06 0.02 0.02 0.01 0.01 0.02 0.02 0.04 Cl Nd Nd Nd nd nd nd nd nd nd OH 3.97 3.94 3.98 3.98 3.99 3.99 3.98 3.98 3.96

- 103 -

TABLE 3-3: Representative EMP analyses of chlorite contained in the carbonaceous mudstone host of the Eskay Creek deposit. Chlorite formulae were calculated on the basis of 28 oxygen equivalents and with Fe2+/Fe3+ and OH calculated assuming full site occupancy (nd = not detected).

AD9771-540.1 C97786-59.9 C97865-78.3

SiO 2 26.20 26.34 27.91 27.29 27.39 27.47 26.79 27.04 27.19 TiO2 4.07 2.08 Nd 0.06 nd 0.02 0.04 nd 0.01 Al2O3 17.83 17.57 18.32 19.45 19.83 19.54 19.84 19.77 19.25 Cr2O3 Nd Nd Nd nd nd nd nd nd nd Fe2O3 3.59 2.12 0.83 0.67 0.58 0.72 0.74 0.90 0.86 FeO 22.21 28.09 23.24 22.98 22.26 22.71 24.64 24.01 24.74 MnO 0.24 0.20 0.20 0.17 0.19 0.21 0.32 0.26 0.26 MgO 10.29 10.38 16.41 16.61 17.36 16.50 15.22 15.44 15.13 CaO 2.48 0.50 0.03 nd nd nd 0.01 0.02 0.03 Na2O 0.04 0.03 0.04 0.05 0.05 0.03 nd nd 0.01 K2O 0.03 0.01 0.07 0.06 0.03 0.23 nd 0.01 0.01 BaO 0.16 0.07 Nd 0.02 nd 0.04 0.02 nd nd F 0.02 nd Nd nd nd nd nd nd nd Cl 0.01 0.01 Nd nd nd 0.01 nd 0.01 nd H2O* 11.08 10.98 11.39 11.46 11.56 11.48 11.38 11.39 11.36 O=F,Cl 0.01 nd Nd nd nd nd nd nd nd Total 98.23 98.38 98.43 98.82 99.25 98.96 99.00 98.85 98.85

Si 5.60 5.71 5.86 5.70 5.67 5.72 5.63 5.67 5.73 Al iv 2.40 2.29 2.14 2.30 2.33 2.28 2.37 2.33 2.27 Al vi 2.13 2.24 2.40 2.50 2.52 2.53 2.56 2.58 2.52 Ti 0.65 0.34 Nd 0.01 nd nd 0.01 nd nd Cr Nd nd Nd nd nd nd nd nd nd Fe3+ 0.58 0.35 0.13 0.11 0.09 0.11 0.12 0.14 0.14 Fe2+ 3.97 5.10 4.08 4.01 3.85 3.95 4.33 4.21 4.36 Mn 0.04 0.04 0.04 0.03 0.03 0.04 0.06 0.05 0.05 Mg 3.27 3.36 5.13 5.17 5.36 5.12 4.77 4.83 4.75 Ca 0.57 0.12 0.01 nd nd nd nd nd 0.01 Na 0.03 0.03 0.03 0.04 0.04 0.02 nd nd 0.01 K 0.01 nd 0.04 0.03 0.01 0.12 nd nd nd Ba 0.03 0.01 Nd nd nd 0.01 nd nd nd F 0.03 nd Nd nd nd nd nd nd nd Cl Nd 0.01 Nd nd nd 0.01 nd 0.01 nd OH* 15.97 15.99 16.00 15.99 16.00 15.99 16.00 15.99 16.00

- 104 -

TABLE 3-3 (continued) C96786-39.6 C97 868 145.7 C98893-87.8

SiO 2 27.18 27.28 27.37 28.36 28.56 28.75 26.29 26.65 27.02 TiO2 nd 0.05 Nd 0.02 nd nd 0.07 nd 0.03 Al2O3 19.47 19.21 19.01 18.77 19.91 19.53 18.71 19.66 20.85 Cr2O3 nd nd Nd nd nd nd nd nd nd Fe2O3 0.65 0.72 1.04 0.02 0.38 0.52 0.74 1.16 0.72 FeO 23.50 23.84 23.64 15.32 15.87 15.33 33.60 32.00 24.15 MnO 0.18 0.16 0.18 0.33 0.30 0.32 0.30 0.17 0.24 MgO 16.06 15.93 15.29 23.14 22.57 22.38 8.79 9.62 15.57 CaO 0.12 0.10 0.30 0.54 0.20 0.49 0.27 0.29 0.05 Na2O 0.08 0.04 Nd 0.01 0.04 0.02 0.13 0.06 0.03 K2O 0.04 0.04 0.03 0.02 nd 0.03 0.07 0.02 0.16 BaO nd nd 0.02 0.04 nd 0.01 nd nd nd F nd nd Nd 0.06 0.04 nd nd nd nd Cl 0.01 nd Nd nd nd 0.01 nd 0.01 nd H2O* 11.41 11.41 11.33 11.77 11.96 11.94 10.98 11.18 11.59 O=F,Cl nd nd Nd 0.03 0.02 nd nd nd nd Total 98.70 98.78 98.22 98.37 99.82 99.32 99.94 100.80 100.42

Si 5.70 5.72 5.78 5.74 5.70 5.76 5.72 5.69 5.58 Al iv 2.30 2.28 2.22 2.26 2.30 2.24 2.28 2.31 2.42 Al vi 2.52 2.49 2.53 2.22 2.39 2.39 2.54 2.66 2.66 Ti nd 0.01 Nd nd nd nd 0.01 nd nd Cr nd nd Nd nd nd nd nd nd nd Fe3+ 0.10 0.11 0.17 nd 0.06 0.08 0.12 0.19 0.11 Fe2+ 4.12 4.18 4.17 2.59 2.65 2.57 6.11 5.72 4.17 Mn 0.03 0.03 0.03 0.06 0.05 0.05 0.06 0.03 0.04 Mg 5.02 4.98 4.81 6.98 6.71 6.69 2.85 3.06 4.79 Ca 0.03 0.02 0.07 0.12 0.04 0.11 0.06 0.07 0.01 Na 0.06 0.03 Nd 0.01 0.03 0.01 0.11 0.05 0.03 K 0.02 0.02 0.02 0.01 nd 0.01 0.04 0.01 0.09 Ba nd nd Nd 0.01 nd nd nd nd nd F nd nd Nd 0.08 0.05 nd nd nd nd Cl nd nd Nd nd nd nd nd nd nd OH* 16.00 16.00 16.00 15.92 15.95 16.00 16.00 16.00 16.00

- 105 -

FIG. 3-8: Compositional variations of chlorite as function of distance to ore.

Tables 3-4 and 3-5 list the chemical compositions of representative 2M and 1M illite grains analyzed by analytical electron microscopy, respectively. No systematic differences in chemical composition can be observed between the three analyzed samples. The 2M and 1M polytypes overlap compositionally (Fig. 3-12). In general, the illite show a high Si content (>6.6 apfu). There is no correlation between the Si content of the illite and the Mg+Fe content. Tables 3-4 and 3-5 show that the totals of the alkali elements are low. Although such low totals could be related to alkali volatilization during analysis, a negative correlation between the Si content of the illite and the interlayer cation content can be observed (Fig. 3-12). This suggests that the excess Si content of the illite of both polytypes and the low interlayer cation content are coupled, confirming that the illite at Eskay Creek is interlayer-deficient.

- 106 -

FIG. 3-9: Low magnification TEM image showing large, defect-free, illite 2M crystals. The inset shows the corresponding SAED pattern. Along the c*-b* direction sharp diffraction effects occur. Along the 00l direction, a 10 Å spacing is observed while a 20 Å spacing can be seen along 0kl directions. Extra spots along the 00l direction are due to dynamic effects.

- 107 -

FIG. 3-10: Low magnification TEM image showing thin and elongated illite 1M crystals surrounding Ti oxide crystals. The inset shows the corresponding SAED pattern demonstrating that the c axes of the crystals are randomly oriented.

- 108 -

FIG. 3-11: Low-magnification TEM image showing illite 1M crystals crosscutting a much larger illite 2M crystal.

- 109 -

FIG. 3-12: Plots illustrating compositional variations in illite contained in hydrothermally altered carbonaceous mudstone from Eskay Creek. The plots show that there is no significant phengitic substitution in the illite and that the illite is inter-layer cation deficient.

- 110 -

TABLE 3-4: Representative AEM analyses of illite 2M in carbonaceous mudstone from the Eskay Creek deposit. All data in apfu.

CA90-291-96.6 1021-27.2 CA90-345-57.0

Si 7.03 6.79 6.77 6.66 6.64 6.94 6.72 6.84 6.74 7.24 6.56 Al IV 0.97 1.21 1.23 1.34 1.36 1.06 1.28 1.16 1.26 0.76 1.44 Al VI 3.57 3.31 3.44 3.30 3.32 3.54 3.37 3.58 3.46 3.28 3.51 Ti 0.04 0.08 0.05 0.07 nd 0.05 0.05 0.05 nd nd 0.07 Mg 0.47 0.58 0.55 0.49 0.64 0.48 0.50 0.52 0.57 0.74 0.53 Fe 0.07 0.07 0.04 0.05 0.08 0.03 0.06 0.06 0.16 nd 0.07 Mn nd nd nd nd nd nd nd nd nd nd nd Ca nd nd nd nd nd 0.02 0.17 nd nd nd nd Na 0.27 0.06 0.07 0.17 0.30 0.06 nd 0.04 nd 0.23 0.05 K 0.76 1.64 1.5 1.85 1.47 1.13 1.42 1.01 1.27 1.02 1.44 VI 4.15 4.04 4.08 3.91 4.04 4.10 3.98 4.21 4.19 4.02 4.18 XII 1.03 1.70 1.57 2.02 1.77 1.21 1.59 1.05 1.27 1.25 1.49

TABLE 3-5: Representative AEM analyses of illite 1M in carbonaceous mudstone from the Eskay Creek deposit. All data in apfu.

CA90-291-96.6 1021-27.2 CA90-345-57.0

Si 7.06 6.86 6.89 6.99 6.91 6.70 6.82 6.74 6.84 7.03 6.97 Al IV 0.94 1.14 1.11 1.01 1.09 1.30 1.18 1.26 1.16 0.97 1.03 Al VI 3.52 3.51 3.39 3.32 3.30 3.31 3.25 3.42 3.39 3.43 3.29 Ti 0.04 0.04 0.03 nd 0.03 0.06 0.08 0.04 0.04 0.03 0.04 Mg 0.52 0.54 0.52 0.57 0.68 0.60 0.61 0.65 0.59 0.52 0.60 Fe 0.07 0.04 0.09 0.12 0.10 0.05 0.11 0.10 0.09 0.09 0.13 Mn nd nd nd nd nd nd nd nd nd nd nd Ca nd 0.03 0.03 Nd nd 0.04 0.06 0.05 0.03 0.16 nd Na 0.27 0.09 0.21 0.25 0.25 0.25 0.14 0.08 0.07 0.17 0.14 K 0.76 1.17 1.35 1.29 1.29 1.58 1.49 1.17 1.38 0.85 1.4 VI 4.15 4.13 4.03 4.01 4.11 4.02 4.05 4.21 4.11 4.07 4.06 XII 1.03 1.29 1.59 1.54 1.54 1.87 1.69 1.30 1.48 1.18 1.54

- 111 -

3.6 Discussion

This discussion focuses on the overall composition of the mudstones and a variety of alteration processes, including the nature of volcanic glass devitrification as well as carbonate and feldspar alteration and silicification. Sulfide mineral formation processes are also discussed. Finally, the effects of the diagenetic and low-grade metamorphic overprint are discussed.

3.6.1 Primary Mudstone Composition

The carbonaceous mudstone at Eskay Creek represents a suspension sediment that consists of fine-grained volcaniclastic material that is intermixed with the remains of marine microorganisms and potentially other hemipelagic material. The volcaniclastic material was deposited through settling from dilute currents trailing subaqueous mass flows or settling of volcanic ash derived from subaerial or submarine volcanic explosions. The mudstone contains abundant glass shards and occasional out-of-size pumice fragments, which are interpreted to have been derived from an explosive volcanic source located outside the immediate Eskay Creek area. In addition to originally glassy fragments, all investigated mudstone samples contain small crystal fragments of quartz, plagioclase, and microcline. Remains of marine organisms in the mudstone include the high proportion of carbonaceous material and abundant microfossils such as radiolarians and dinoflagellates (Roth et al., 1999). Locally occurring belemnite and coral fragments represent the main macrofossils recognized during core logging (Roth et al., 1999; Monecke et al., 2005).

3.6.2 Devitrification of Volcanic Glass

The petrographic investigations of the present study suggest that the carbonaceous mudstone at Eskay Creek originally contained a significant amount of volcanic glass, which has since been devitrified.

In mudstone that has not been affected by hydrothermal alteration, devitrification of the originally glassy particles likely commenced soon after suspension sedimentation and was

- 112 - probably largely completed during early diagenesis. Previous studies have shown that devitrification of the volcanic glass commonly results in the formation of smectite as the principal alteration product at low temperatures. However, other phases including zeolites and carbonate minerals may form, depending on factors such as temperature, reaction progress, fluid/rock ratio, and the glass chemical composition (Hay and Iijima, 1968; Furnes and El- Anbaawy, 1980; Ghiara et al., 1993; Tomita et al., 1993; Alt et al., 1998; De La Fuente et al., 2002).

Hydrothermal alteration of the carbonaceous mudstone likely also resulted in the widespread formation of clay minerals. As hydrothermal fluids at Eskay Creek did not exceed about 200ºC in temperature (Sherlock et al., 1999), hydrothermal alteration of volcanic glass contained in the mudstone must have resulted in the formation of smectite and mixed-layer clays. The study of modern geothermal systems suggests that dioctahedral smectite is a stable alteration product at temperatures below approximately 150ºC while trioctahedral smectite is stable to temperatures of about 200ºC. Mixed-layer illite/smectite and chlorite/smectite overlap with smectite stability and occur towards higher temperatures (cf. Monecke et al., 2007). Illite or muscovite and chlorite are stabilized at temperatures exceeding about 250ºC. Therefore, at Eskay Creek, zones of intense illite or muscovite or chlorite alteration could not have formed within the carbonaceous mudstone host during the deposit formation as the alteration temperatures were not high enough (Sherlock et al., 1999).

Occurrences of smectite and mixed-layer clays as products of hydrothermal alteration have also been documented to occur in ancient volcanic-hosted massive sulfide deposits. Well- documented examples deposits in the Upper Cretaceous Pontides in Turkey (Cagatay, 1993) or the Miocene Hokuroko district in Japan (Date et al., 1983; Pisutha-Arnond and Ohmoto, 1983; Marumo, 1989), which have only experienced very low-grade metamorphism. In these deposits, smectite and mixed-layer clays typically occur in the periphery of hydrothermal upflow zones where temperatures of alteration ranged from approximately 150 to 250ºC. Within the hydrothermal upflow zones where alteration occurred at temperatures exceeding 250ºC, muscovite and chlorite are the dominant alteration products (Pisutha-Arnond and Ohmoto, 1983; Monecke et al., 2007).

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In contrast to deposits in the Pontides of Turkey or the Hokuroko district in Japan, alteration vectoring using clay mineralogy is not possible at Eskay Creek. Regional metamorphism at Eskay Creek occurred at conditions of the low anchizone (see below), which resulted in the recrystallization of hydrothermally formed smectite and mixed-layer clays into illite and chlorite. The same minerals were formed at the expense of the devitrification products contained in mudstone not affected by hydrothermal alteration.

Distinguishing between illite and chlorite formed through these different processes may only be possible using mineral chemistry. Hydrothermal alteration of the mudstone appears to be associated with whole-rock MgO enrichment (Chapter 2). The results of the present study suggest that chlorite formed from recrystallization of hydrothermal clay minerals is typically Mg-rich whereas chlorite, formed after clays in least-altered mudstone, is typically more Fe-rich. Studies focusing on low-temperature hydrothermal alteration of felsic glassy volcanic rocks in modern seafloor hydrothermal vents have demonstrated that the composition of hydrothermally formed smectite can indeed vary as function of alteration intensity. In weakly to moderately altered felsic glass, dioctahedral smectite is the most common alteration product while triocathedral smectite occurs adjacent to fractures in more intensely altered glassy rocks. At the Pacmanus vent field in the Manus basin, formation of Mg-rich trioctahedral smectite in areas of intense alteration of felsic volcanic glass resulted in notable whole-rock MgO enrichment (Giorgetti et al., 2006; Monecke et al., 2007). Other researchers have noted that Mg and Fe in chlorite vary with temperature and equilibration time, with magnesium rich end members reflecting a longer equilibration time and/or higher temperature (Gysi and Stefansson, 2012).

3.6.3 Feldspar Alteration

The petrographic observations of the present study further suggest that plagioclase contained in the carbonaceous mudstone hosting the Eskay Creek deposit was not stable during hydrothermal alteration. Alteration of plagioclase grains resulted in the formation of secondary clay minerals and quartz as suggested by the observed textural relationships. Feldspar alteration

- 114 - may explain the observation that whole-rock Na2O concentrations of the mudstone decrease towards hydrothermal upflow zones and known ore lenses (Chapter 2).

Some of the samples investigated have been affected by pervasive K-feldspar alteration. This alteration style resulted in the K-feldspar flooding of the mudstone matrix, which was most readily identified by optical CL microscopy. The secondary K-feldspar is texturally associated with calcite. Widespread K-feldspar alteration is not common in volcanic-hosted massive sulfide deposits although this mineral has been observed as a distal alteration product in some alteration halos associated with massive sulfide deposits (Offler and Whitford, 1992; McGoldrick and Large, 1992). Geochemical modeling suggests that K-spar alteration may occur at lower temperatures (<200°C) from fluids that have been largely equilibrated with the volcanic wall- rocks (Schardt et al., 2001).

Plagioclase contained in least-altered mudstone has been pervasively albitized. Albitization of plagioclase and associated calcite precipitation are common processes occurring during diagenesis and very low-grade metamorphism (cf. Merino, 1975; Boles, 1982; Ramseyer et al., 1992). Albitization, historically referred to as spilitization in mafic volcanic rocks and keratophyre formation in felsic volcanic rocks (Hughes, 1973), is in fact one of the most common regional alteration processes affecting submarine volcanic successions (Eastoe et al., 1987).

Some of the carbonaceous mudstone samples investigated have been affected by hydrothermal K-feldspar alteration. This alteration style resulted in the K-feldspar flooding of the mudstone matrix, which was most readily identified by optical cathodoluminescence microscopy. The secondary K-feldspar is texturally associated with calcite. Widespread K- feldspar alteration is not common in volcanic-hosted massive sulfide deposits although this mineral has been observed as a distal alteration product in some alteration halos associated with massive sulfide deposits (Offler and Whitford, 1992; McGoldrick and Large, 1992). Geochemical modeling suggests that K-spar alteration may occur at lower temperatures (<200°C) from fluids that have been largely equilibrated with the volcanic wall-rocks (Schardt et al., 2001).

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3.6.4 Carbonate Alteration

The textural relationships documented in this study suggest that minerals of the dolomite- ankerite and magnesite-siderite solid solutions in the carbonaceous mudstone host rocks represent hydrothermal precipitates that formed during or after the deposition of the clastic sulfide and sulfosalt mineralization at Eskay Creek. Pervasive carbonate alteration and veining are abundant in the contact mudstone, but also occur in the hanging-wall tens of meters above the stratigraphic interval hosting the bulk of the clastic mineralization. Dolomite-ankerite and magnesite-siderite are commonly associated with calcite. Compositional zoning is observed in some larger secondary carbonate aggregates, implying that different carbonate minerals have formed through time due to changing physicochemical composition of alteration. In some samples, calcite flooding is intense. This suggests that calcite can also represent a hydrothermal precipitate at Eskay Creek.

Samples containing abundant dolomite-ankerite and magnesite-siderite have been found to also contain trace amounts of kaolinite. Kaolinite is typically directly intergrown with the carbonate minerals or replaces silicate minerals in the groundmass of the mudstone. The occurrence of kaolinite in carbonate-altered rocks is consistent with the low temperatures of hydrothermal alteration inferred from fluid inclusion studies (Sherlock et al., 1999). Kaolinite represents a stable alteration product only at temperatures below approximately 300°C (Velde and Kornprobst, 1969; Thompson, 1970; Hemley et al., 1980; Reyes, 1990). Experimental constraints by Bischoff and Rosenbauer (1996) showed that CO2 contained in hydrothermal fluids becomes chemically reactive at temperatures below 270°C. Kinetic and thermodynamic constraints suggested that hydrogen metasomatism of volcanic rocks and coupled bicarbonate production caused by the reaction of CO2 with water is most efficient at even lower temperatures, with kaolinite forming as a byproduct of hydrothermal alteration.

Although hydrothermal carbonate alteration at Eskay Creek represents the most widespread style of hydrothermal alteration, clearly not all calcite contained in mudstone samples from Eskay Creek is of hydrothermal origin. Disseminated small calcite aggregates within the matrix of the mudstone may be of hydrothermal, diagenetic, or very low-grade

- 116 - metamorphic origin. Calcite formation in association with albitized plagioclase and calcite precipitation in veinlets crosscutting the slaty cleavage is likely unrelated to the hydrothermal activity at Eskay Creek. Calcite veinlets crosscutting prehnite porphyroblasts are clearly texturally late and presumably formed during metamorphism.

3.6.5 Silicification

In addition to carbonate alteration, some of the samples investigated have been affected by silicification. Silicification resulted in pervasive quartz flooding of the matrix of the fine- grained and carbonaceous rock. Silicification is a common style of hydrothermal alteration in volcanic-hosted massive sulfide deposits (Morton and Franklin, 1987; Gemmell and Large, 1992; Large et al., 1992). It develops in response to rapid cooling of the hydrothermal fluids as quartz solubility is strongly temperature dependent (Fournier, 1983). Silicification of carbonaceous mudstone at Eskay Creek was only observed in samples collected from the contact mudstone that were presumably located close to zones of hydrothermal upflow. Rapid cooling of the hydrothermal fluids resulting in silicification of the contact mudstone was probably related to quenching of the hydrothermal fluids close to the seafloor and mixing with cold pore waters.

3.6.6 Processes of Sulfide Mineral Formation

The carbonaceous mudstone samples at Eskay Creek contain abundant sulfide minerals. Pyrite represents the most abundant sulfide mineral, with sphalerite, galena, and chalcopyrite occurring in trace amounts. Macroscopically and microscopically, several distinct textural settings of sulfide minerals can be distinguished.

Samples collected close to ore, may contain laminations rich in pyrite. The presence of flame structures at the base of these sulfide laminations indicates that the pyrite is at least in part clastic in origin. The occurrence of graded beds and fine disseminations of hydrothermal pyrite in the mudstone is linked to the processes that involved in the deposition of the clastic sulfide and sulfosalts mineralization at Eskay Creek.

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The mechanism that led to the deposition of the clastic sulfide and sulfosalt mineralization at Eskay Creek have been investigated in detail by Allen (1993) and Roth et al. (1999). Based on a study of ore textures and their spatial distribution within the mine area, Roth et al. (1999) established that the clastic ore deposited into a northward trending basin or sub-basin that formed in the immediate hanging wall of the extrusive portion of the footwall rhyolite. The observed lateral facies variations suggest that the coarse clastic sulfides were deposited by numerous small-volume mass flows into the center of the proposed basin or sub-basin, whereas the finer- grained material was deposited along its margins. The location of the source for the clastic sulfide-sulfosalt material has been a matter of debate. However, the spatial association with intense alteration and mineralization in the footwall rhyolite suggests that the source area was located within or immediately adjacent to the proposed trough.

Roth et al. (1999) proposed that the sulfide and sulfosalt minerals formed initially in mounds or chimney structures similar to those in modern black smoker environments and that a redistribution of broken and fragmented sulfide minerals into the basin or sub-basin led to the formation of the stratiform mineralization. The occurrence of mudstone between successive beds of clastic sulfide minerals was interpreted to indicate that the sulfide-sulfosalt debris was emplaced into the basin by episodic events, separated by relatively short intervals of background sedimentation. Recurrent deposition of fragmented sulfide minerals is probably best explained by the occurrence of hydrothermal eruptions in the vent area. Hydrothermal eruptions in the vent area may have generated the small-volume mass flows of sulfide and wall rock debris that formed the coarse clastic mineralization of the 21B zone, whereas the more widespread fine- grained ore beds were probably deposited from subaqueous fall-out (Allen, 1993; Roth et al., 1999). Roth et al. (1999) further showed that the size of the sulfide and sulfosalt clasts decreases stratigraphically upward and that the thickness of the sulfide beds progressively thins to fine laminations. These vertical facies changes suggest that the hydrothermal activity gradually declined during the ongoing sedimentation (Roth et al., 1999).

In addition to the clastic pyrite laminations, thin veins and veinlets of pyrite crosscutting bedding are widespread throughout the mine area (Monecke et al., 2005). The occurrence of abundant sulfide veinlets in the mudstone records hydrothermal fluid flow during and after the

- 118 - deposition of the clastic sulfides that occur largely at or near the contact to the underlying footwall rhyolite. However, field observations suggest that the relative abundances of hydrothermal sulfide minerals decrease with increasing distance laterally away from, and stratigraphically above, the orebodies (T. Monecke, pers. commun. 2014). This further confirms that hydrothermal activity at Eskay Creek was waning during the ongoing sedimentation of the hanging-wall mudstone.

Most of the mudstone samples investigated contain abundant disseminated pyrite. The disseminated pyrite exhibits wide variations in crystal morphology as described by Ritts (2012). The fine-grained disseminated pyrite contained in the mudstone may be of both hydrothermal and diagenetic origin. However, the occurrence of abundant framboidal pyrite may suggest that the pyrite is at least in part of biogenetic origin. Sulfur isotopic studies by Roth and Taylor (2000) and Roth (2002) indeed suggest that bacterial seawater sulfate reduction indeed played a role in the pyrite formation at Eskay Creek. These authors established that the sulfur isotopic composition of bulk pyrite concentrates extracted from mudstone hosting the Eskay Creek deposit varies systematically with proximity to ore. Pyrite from the contact mudstone has 34S values ranging from -48.4 to +6.3 ‰ (normalized to V-CDT) with a median value of -8.9 ‰ (n=41). The sulfur was found to become progressively lighter with increasing distance from the orebodies. Pyrite in the hanging wall mudstone was found to be typified by values ranging from - 39.1 to +10.3 ‰, with a median value of -20.4 (n=28). The low median value in the typically less intensely altered hanging-wall mudstone further points to the importance of biogenic processes.

3.6.7 Diagenetic and Low-Grade Metamorphic Overprint

The main effects of diagenetic and low-grade metamorphism are related to the recrystallization of smectite interpreted to have initially formed through hydrothermal alteration of the volcaniclastic component of the mudstone or devitrification of volcanic glass contained in mudstone not affected by hydrothermal alteration. With increasing temperature during progressive diagenesis and low-grade metamorphism, dioctahedral smectite is transformed through mixed-layer illite/smectite into illite at lower anchizone conditions. Trioctahedral smectite transforms through mixed-layer chlorite/smectite to chlorite. The conversion of smectite

- 119 - to illite or chlorite commences at temperatures of approximately 90ºC and is essentially completed at conditions equivalent to the boundary between upper diagenetic and lower anchizone conditions (200 to 250ºC; Hower et al., 1976; Hillier, 1993; Merriman and Peacor, 1999; Monecke et al., 2007).

During further prograde recrystallization, illite formed, which is typically composed of the

1Md polytype, transforms further to the 2M1 polytype. The increased dominance of the 2M1 illite causes crystal thickening and hence illite crystallinity, which contributes to the development of a bedding-parallel microfabric. This is typically accompanied by a change in composition, resulting in the transformation from illite to muscovite (Dong et al., 1997; Merriman and Peacor, 1999). This metamorphic re-equilibration of illite occurs at the transition from low to high anchizone conditions (temperatures of approximately 250 to 300ºC; Gharrabi et al., 1998). The results of the present study indicate that illite contained in the carbonaceous mudstone host occurs as both a 1M and 2M polytype. In addition, the illite flakes analyzed are interlayer deficient. This implies that metamorphic conditions of the high anchizone were not reached at Eskay Creek.

The low-grade metamorphic recrystallization of the carbonaceous mudstone at Eskay Creek is an important process obscuring the effects of hydrothermal alteration. As metamorphic grade exceeded maximum temperatures of hydrothermal alteration, hydrothermal alteration products such as smectite and mixed-layer clays have been recrystallized and are no longer present in the mudstone. The sheet silicate mineral assemblage of least-altered and hydrothermally altered mudstone are identical, although chemical differences were noted for chlorite.

The most macroscopically recognizable effect of low-grade metamorphism at Eskay Creek is the development of incipient slaty cleavage in the carbonaceous mudstone. Previous studies have demonstrated that the transition from the upper diagenetic to lower anchizone conditions is associated with the development of this cleavage (Merriman and Peacor, 1999). The cleavage is produced through a combination of mechanical rotation of mineral grains and pressure solution (Merriman and Peacor, 1999). The development of incipient slaty cleavage in mudstone from

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Eskay Creek is consistent with the above inference based on illite polytypism that regional metamorphism occurred at lower anchizone conditions.

Core logging has demonstrated that the occurrence of prehnite porphyroblasts is fairly common in mudstone from Eskay Creek. The prehnite porphyroblasts, which commonly are sheaf-like in shape, occur mostly in the hanging-wall mudstone (Monecke et al., 2005). Although radiating porphyroblasts of prehnite, variably altered to white mica, calcite and barite also occur within individual bets within the contact mudstone and mudstone adjacent to basalt intrusions. Previous workers interpreted these prehnite porphyroblasts to have formed as a result of contact metamorphism due to the emplacement of basaltic sills or dikes (Roth et al., 1999). However, at least locally, the prehnite porphyroblasts overgrow the incipient slaty cleavage, suggesting that prehnite is not always synvolcanic in origin.

3.7 Conclusions

The carbonaceous mudstone hosting the Eskay Creek deposit represents a complex rock type that was initially composed of variable proportions of volcaniclastic detritus and organic material. Subsequent to deposition and compaction, the composition of the mudstone has been extensively modified during hydrothermal alteration as well through diagenesis and low-grade metamorphism. Due to the fine-grained and carbonaceous nature of the rocks, the effects of these different post-depositional processes modifying the mudstone composition are not easy to separate or quantify petrographically.

The present study demonstrates that correlative microscopy employing a combination of optical microscopy and various electron microscopy techniques represents a powerful tool in the study of this fine-grained material. Hydrothermal alteration of the mudstone can be easily recognized in intensely altered mudstone based on microtextural criteria. Mudstone alteration at low temperatures (<200ºC) resulted in the widespread formation of clay and carbonate minerals. Pervasive quartz flooding and K-feldspar alteration occur only locally, near the contact between the contact mudstone and footwall rhyolite. With increasing distance to hydrothermal upflow zones, hydrothermal alteration took place at decreasing temperatures from hydrothermal fluids

- 121 - that were increasingly buffered by the mudstone and diluted by pore waters. Based on petrographic criteria, it is essentially impossible to distinguish the effects of hydrothermal alteration from those caused by incipient diagenesis in mudstone collected away from hydrothermal upflow zones. Minerals of the dolomite-ankerite and siderite-magnesite solid solutions are the only indicator minerals not encountered in least-altered mudstone.

Low-grade metamorphic recrystallization of the mudstone played a significant role in modifying the mudstone mineralogy. As hydrothermal alteration at Eskay Creek occurred at low temperatures, alteration products such as clay minerals were not stable during regional metamorphism and recrystallized into illite and chlorite. The same minerals were formed as a result of regional metamorphism of mudstone not affected by hydrothermal alteration as smectite also represents the most abundant product of diagenetic devitrification of volcanic glass originally contained in the mudstone.

The results of this research indicate that the limited mineralogical variance between least- altered mudstone and hydrothermally altered mudstone may complicate the use of qualitative alteration vectors in mineral exploration. Unless detailed petrographic investigations are performed, which may not be feasible in routine exploration, it may be necessary to combine whole-rock quantitative mineralogical data with major and trace element analyses to identify spatial alteration trends that can be used for vectoring.

3.8 Acknowledgements

The authors thank F. Robert for facilitating field work at Eskay Creek. D. Gale, S. Hasek, and D. MacNeil provided logistical support in the field. This paper has benefited substantially from discussions with A. Gysi, E. Holley, and N. Kelly. Field work was supported by Barrick Gold and the Michael-Jürgen-Leisler-Kiep Foundation. Initial research on the samples by T. Monecke was supported by the Emmy Noether Program of the German Research Foundation. Subsequent research was conducted under the Canadian Mining Industry Research Organization Project 08E04, with additional financial support from Geoscience BC. T. Meuzelaar benefited from financial support by the Stewart R. Wallace Endowment in Economic Geology at Colorado

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School of Mines, RockWare, Inc., and Golder Associates, Inc. T. Meuzelaar also gratefully acknowledges a research grant by the Society for Economic Geologists Canada Foundation.

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deposit, northwestern British Columbia: REVIEWS IN ECONOMIC GEOLOGY, v. 8, p. 357– 373. Schardt, C., Cooke, D.R., Gemmell, J.B., and Large, R.R., 2001, Geochemical modeling of the zoned footwall alteration pipe, Hellyer volcanic-hosted massive sulfide deposit, western

Tasmania, Australia: ECONOMIC GEOLOGY, v. 96, p. 1037–1054. Schiffman, P. and Fridleifsson, G.O., 1991, The smectite-chlorite transition in drillhole NJ-15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations, v. 9 (6), p. 679-696. Sherlock, R.L., Roth, T., Spooner, E.T.C., and Bray, C.J., 1999, Origin of the Eskay Creek precious metal-rich volcanogenic massive sulfide deposit: Fluid inclusion and stable

isotope evidence: ECONOMIC GEOLOGY, v. 94, p. 803–824. Thompson, A.B., 1970, A note on the kaolinite-pyrophyllite equilibrium: American Journal of Science, v. 268, p. 454–458. Tomita, K., Yamane, H., and Kawano, M., 1993, Synthesis of smectite from volcanic glass at low temperature: Clays and Clay Minerals, v. 41, p. 655–661. Velde, B., and Kornprobst, J., 1969, Stabilité des silicates d'alumine hydratés: Contributions to Mineralogy and Petrology, v. 21, p. 63–74.

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CHAPTER 4 GEOCHEMICAL MODELING OF FLUID-ROCK INTERACTION WITHIN THE MUDSTONE HOST AT THE ESKAY CREEK SULFIDE AND SULFOSALT DEPOSIT, BRITISH COLUMBIA

4.1 Abstract

The Jurassic Eskay Creek deposit in the Iskut River area of northern British Columbia represents an unusual gold-rich volcanic-hosted sulfide and sulfosalt accumulation that consists primarily of stratiform clastic ores hosted within a thick unit of carbonaceous mudstone. Proximal to hydrothermal upflow zones, the carbonaceous mudstone shows clay alteration and is affected by widespread carbonate formation. Minerals of the dolomite-ankerite and magnesite- siderite solid solutions are most abundant in the mudstone close to ore while calcite is a common product of hydrothermal alteration away from hydrothermal upflow zones.

Thermodynamic equilibrium reaction pathway modeling was performed to better understand the physicochemical conditions of alteration of the carbonaceous mudstone host at

Eskay Creek. To correctly model the reaction of the hydrothermal fluids with the mudstone, CO2 and H2S solubility models were developed and incorporated into the numeric simulation code used. The modeling confirms that interaction of hydrothermal fluids with carbonaceous mudstone at temperatures ranging from 50 to 250ºC could indeed have resulted in the observed clay and carbonate alteration mineral associations. The modeling demonstrates that the alteration mineralogy of the mudstone host is strongly influenced by fluid pH, which is directly related to the concentration of dissolved CO2. Proximal to hydrothermal upflow zones, alteration of the mudstone occurs by a fairly acidic and CO2-rich fluid. Feldspar dissolution provides cations required for the formation of montmorillonite and carbonate minerals. Kaolinite is stabilized at

- 129 - temperatures below 150˚C while secondary mica forms at temperatures above 150˚C. At increased CO2 contents and correspondingly lower pH conditions (pH 4 to 5), minerals of the dolomite-ankerite solid solution and calcite are destabilized and saponite, magnesite, and siderite are formed. Mudstone distal to hydrothermal upflow zones interacts with neutral to alkaline hydrothermal fluid as the mudstone represents an effective buffer of the fluid pH. Calcite represents the most important carbonate species formed during alteration at neutral to alkaline conditions. The modeling further demonstrates that the composition of the mudstone represents an important factor influencing fluid pH during fluid-rock interaction. The acid buffering capacity of mudstone primarily composed of rhyolitic volcanic glass is limited. In contrast, more compositionally complex mudstone, derived from a bimodal volcaniclastic source, represents a much stronger wall-rock buffer. Polythermal mixing models suggest that the entrainment of seawater in the hydrothermal upflow zones of the fluids or the mixing of the hydrothermal fluids with pore water during fluid-rock interaction provides an important mechanism controlling the alteration mineralogy. Seawater represents an important local source for cations such as calcium, iron, and magnesium. Abundant magnesium- and iron-bearing clay minerals and carbonate minerals could not have formed without seawater entrainment.

The results of the thermodynamic reaction path modeling suggest that the observed alteration patterns within the carbonaceous mudstone host at Eskay Creek are primarily related to the temperature-dependent equilibration of the hydrothermal fluids with the mudstone and polythermal mixing of the hydrothermal fluids with seawater or pore water. The predicted fluid chemistry of the hydrothermal fluids is consistent with models assuming that Eskay Creek represented a submarine hot spring deposit.

4.2 Introduction

Eskay Creek is an unusual, precious metal-rich, polymetallic volcanic-hosted sulfide and sulfosalt deposit located in northwestern British Columbia, Canada. Between 1995 and 2008, the deposit produced a total of 2.25 million metric tons of ore grading 48.9 g/t Au and 2,334 g/t Ag (unpubl. data T. Monecke), making it the volcanic-hosted massive sulfide deposit with the highest precious metal grades discovered so far (Mercier-Langevin et al., 2011).

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In addition to the unusual precious metal grades, Eskay Creek is typified by a number of other deposit characteristics not commonly encountered in massive sulfide deposits. In particular, the ore lenses were primarily composed of bedded and graded clastic ores. In addition to precious and base metals, the ores showed a pronounced enrichment of elements typically only enriched in subaqueous hot spring deposits, including As, Bi, Hg, and Sb (Macdonald et al., 1996; Roth et al., 1999). Fluid inclusion data showed that the complex sulfide and sulfosalt ores at Eskay Creek formed from low-temperature (<200ºC) hydrothermal fluids (Sherlock et al., 1999).

The stratiform sulfide and sulfosalt ore lenses at Eskay Creek are hosted by a thick carbonaceous mudstone interval at the contact between felsic volcanic rocks in the footwall and basalt in the hanging-wall (Britton et al., 1990; Ettlinger, 1992; Barrett and Sherlock, 1996; Macdonald et al., 1996; Roth et al., 1999; Monecke et al., 2005). The footwall rhyolite locally hosts discordant zones of sulfide veinlets and disseminated sulfide minerals that probably represent structurally controlled upflow zones of hydrothermal fluids. Hydrothermal alteration of the footwall rhyolite is most pronounced in these discordant zones as well as in the immediate footwall of the clastic ores (Robinson, 1991; Barrett and Sherlock, 1996). Detailed mineralogical and geochemical studies demonstrated that the carbonaceous mudstone host of the stratiform ore lenses at Eskay Creek has also been affected by intense alteration. However, due to its fine- grained and carbonaceous nature, alteration of this rock type is generally cryptic and essentially impossible to identify in hand specimen (Chapters 2 and 3).

In this present paper, equilibrium thermodynamic reaction path modeling is performed to gain insights into the chemical processes that took place during the interaction of the hydrothermal fluids with the carbonaceous mudstone host at Eskay Creek. The relatively low (<200ºC) temperature and low salinity (<10 wt. % equiv. NaCl; Sherlock et al., 1999) of the ore forming fluids facilitate numeric simulation of hydrothermal fluid-rock interaction because most thermodynamic databases employed in aqueous geochemical modeling (e.g., Delaney and Lundeen, 1990) are parameterized up to temperatures of about 300ºC and salinities of up to 3 molal (15 wt. % NaCl), assuming that NaCl is the predominant solute (Bethke, 2008). Furthermore, given the fact that hydrothermal fluids altering the carbonaceous mudstone host at

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Eskay Creek were probably highly evolved, reflecting partial or full equilibrium with the mudstone due to long transport distances and low fluid flow rates, it is relatively straight forward to constrain hydrothermal fluid compositions using saturation limits of the rock forming minerals. The present study demonstrates that the equilibrium thermodynamic reaction path modeling correctly reproduces key alteration-induced mineralogical and geochemical trends identified at Eskay Creek.

4.3 Geological Background

The Eskay Creek deposit is located in the Iskut River area at the western margin of the allochthonous Stikinia terrane in British Columbia, Canada (Fig. 4-1). The deposit is hosted by volcanic rocks of the Jurassic Hazelton Group, which record a stage of intense, extensional, continental margin arc volcanism (Barrett and Sherlock, 1996; Macdonald et al., 1996). In the deposit area, the rocks of the Upper Hazelton Group are folded into a shallowly north plunging, north-northeast trending, upright, open anticline (Fig. 4-1). The deposit occurs on the western limb of the anticline, near the fold closure. The deposit area is intersected by a number of north- northwest to north-northeast trending normal faults (Fig. 4-1). Primary relationships are well- preserved at Eskay Creek as the metamorphic grade in the mine area is only of the lower greenschist facies (Britton et al., 1990; Roth et al., 1999).

The Eskay Creek deposit comprises a number of polymetallic ore lenses that occur over a strike length of approximately 1,500 m and a maximum down dip extent of 350 m (Roth et al., 1999). The main stratiform ores at Eskay Creek form the 21B zone that is comprised of clastic sulfide and sulfosalt ores. The zone is a tabular body that is about 650 m long, 60 to 150 m wide, and locally in excess of 20 m thick. In the central portion of the 21B zone, the beds are formed by pebble- to cobble-sized sulfide and sulfosalt clasts (Britton et al., 1990; Idziszek et al., 1990; Roth et al., 1999). The beds grade laterally into thinner, finer grained, clastic beds, and laminations. The clast size and bed thicknesses also decrease stratigraphically upward, progressively thinning to fine laminations and disseminated sulfides and sulfosalts (Roth et al., 1999).

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FIG. 4-1: Geological map of the Iskut River area and location of the Eskay Creek deposit. The inset shows the distribution of the Stikinia terrane in British Columbia (modified from MacDonald et al., 1996).

The stratiform ore lens of the 21B zone is hosted within carbonaceous mudstone that overlies a thick package of rhyolite (Fig. 4-2). In the mine area, the footwall rhyolite has a maximum apparent thickness of approximately 100 m (Britton et al., 1990). The footwall rhyolite is composed of multiple rhyolite generations, including both intrusive and extrusive units (Allen, 1993; Monecke et al., 2005). The footwall rhyolite is extensively altered and crosscut by discordant zones of sulfide veins and disseminations, the largest of which are the Pumphouse and Pathfinder zones, which are located below and adjacent to the 21B zone as well as the 109 zone that underlies the northern part of the 21B zone (Roth et al., 1999).

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Hydrothermal alteration of the footwall rhyolite is often texturally destructive. Chlorite, sericite, and quartz are the dominant minerals present in the intense and texturally destructive alteration immediately below the stratiform ore lenses and within the discordant zones of sulfide veins and disseminations (Robinson, 1991; Ettlinger, 1992; Barrett and Sherlock, 1996; Roth et al., 1999) that are interpreted to represent hydrothermal upflow zones developed along synvolcanic faults (Monecke, pers. commun. 2014). Secondary K-feldspar alteration and moderately strong silicification are common in the periphery of the hydrothermal upflow zones and in the deeper parts of the footwall (Barrett and Sherlock, 1996).

The black, carbonaceous mudstone unit hosting the stratiform sulfide and sulfosalt ores at Eskay Creek ranges from less than 1 m to more than 60 m in thickness (Britton et al., 1990; K. Rye, unpub. report for International Corona Corp., 1992, 125 p.). The mudstone is laminated, thinly bedded, or massive and contains abundant intercalated, tan-colored beds of fine-grained volcaniclastic material (Britton et al., 1990; Monecke et al., 2005).

Careful petrographic investigations (Chapter 3) have shown that the carbonaceous mudstone at Eskay Creek has been affected by widespread hydrothermal alteration. Hydrothermal alteration is most intense close to the contact with the footwall rhyolite within areas of intense fluid flow as recorded by the occurrence of discordant zones of sulfide veins and disseminations. Mudstone in these areas has commonly been silicified. Alteration of the volcaniclastic component of the mudstone and the destruction of plagioclase resulted in the widespread formation of clays that transformed to illite and chlorite during subsequent diagenesis and low-grade metamorphism (Chapters 3). In proximity to ore, carbonate alteration has been recognized. Minerals of the dolomite-ankerite solid solution, and to a lesser extent the magnesite-siderite solid solution, formed proximal to ore while calcite is the dominant carbonate mineral further up stratigraphy and laterally away from the major upflow zones of hydrothermal fluids. Carbonate alteration was accompanied by the formation of trace amounts of kaolinite (Chapters 2 and 3). In the periphery of the hydrothermal upflow zones, the carbonaceous mudstone has been locally affected by K-feldspar alteration (Chapters 2 and 3). Mudstone alteration also resulted in the widespread formation of hydrothermal pyrite and trace amounts of base metal sulfides including sphalerite and galena.

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FIG. 4-2: East-west cross-section of the western limb of the Eskay Creek anticline, showing the location of stratiform and discordant zones hosted by the carbonaceous mudstone and the footwall rhyolite, respectively (modified from Roth, 2002).

Further up stratigraphy, the carbonaceous mudstone unit hosts abundant basalt sills and dikes (Roth et al., 1999; Monecke et al., 2005). Basalt units in the lower part of the carbonaceous mudstone unit are typically affected by weak hydrothermal alteration as evidenced by extensive bleaching of the rocks (Britton et al., 1990) while basalt in the upper part of the carbonaceous mudstone unit is least-altered. The relative proportion of basalt increases in the upper part of the mine succession. An approximately 150 m thick package of basalt forms the upper portion of the hanging-wall (Britton et al., 1990).

4.4 Methods

The modeling of the present study aims to test the hypothesis that the observed alteration mineralogy of the carbonaceous mudstone at Eskay Creek and the mineral zoning of the alteration halo is related to temperature-dependent equilibration of the hydrothermal fluids with the mudstone host. To evaluate the role of CO2 as an acid species, modeling is performed at variable partial pressures. The modeling further aims to constrain to what extent the mudstone mineralogy controls the acid buffering capacity of this fine-grained rock and what role mixing of

- 135 - hydrothermal fluids with seawater entrained in the mudstones played a control on alteration mineralogy. The modeling was conducted using the numeric simulation code The Geochemist’s Workbench Standard (GWB; Bethke, 2008). The GWB is a multi-component reaction modeling software code that by default employs the Lawrence Livermore National Laboratory (LLNL) thermodynamic database (Delaney and Lundeen, 1990).

4.4.1 H2S Solubility

Numerical simulation of the alteration processes at Eskay Creek required an adequate description of H2S solubility in hydrothermal fluids at elevated confining pressures. Meuzelaar and Monecke (2012) showed that correct description of the H2S solubility in the modeling software GWB could be achieved through implementation of the solubility model of Duan et al. (2007). The present study follows the approach described in Meuzelaar and Monecke (2012; this article is included in the Appendix).

Duan et al. (2007) summarized the results of previous experimental investigations, which have thus far been conducted only under a comparably narrow range of salinities, temperatures, and pressures. Duan et al. (2007) discussed the limitations of the various experimental studies and derived a new thermodynamic model calculating the H2S solubility in pure water and aqueous NaCl solutions. The model calculates the chemical potential of H2S in the vapor phase using the equation of state presented in Duan et al. (1996), whereas the chemical potential of H2S in the liquid phase is modeled using the approach described by Pitzer (1973). The model proposed by Duan et al. (2007) is valid for solutions of varied electrolyte compositions, with ionic strengths of 0 to 6 molal NaCl, temperatures ranging from 0 to 227°C, and H2S fugacities ranging from 0 to 200 bar.

Previous modeling by Meuzelaar and Monecke (2012) using the modified H2S solubility data was performed to understand the controls of modern day submarine hydrothermal vent fluid compositions on polymetallic seafloor sulfide precipitation. Results indicated that sulfide mineral precipitation was controlled by fluid pH and H2S fugacity, with higher vent H2S gas pressures and lower fluid pH resulting in sulfide mineral precipitation at lower temperatures. Based on the

- 136 - observation that homogenization temperatures of fluid inclusions in sphalerite at Eskay Creek range as high as 200˚C (Sherlock et al., 1999), the results suggested that a moderately acidic vent fluid with pH of 4.5 to 5.0 could have precipitated sphalerite at Eskay Creek in this temperature range. Such a minimum fluid pH is necessary given that the cross-over pH between CO2 dissolved in chloride waters and the generation of bicarbonate waters is approximately 4.5 at 200˚C (Bischoff and Rosenbauer, 1996), above which the alkalinity is sufficient to produce key alteration minerals observed proximal to hydrothermal upflow zones, including dolomite- ankerite and siderite-magnesite solid solutions, kaolinite, and base metal sulfides.

As equilibrium thermodynamic reaction path modeling of the present study primarily involved silicate minerals, the use of the improved H2S solubility data by Duan et al. (2007) overall only had a minor impact on the modeling results. The carbonaceous mudstones at Eskay Creek contain abundant pyrite. As the pyrite in the mudstone is not only of hydrothermal origin, pyrite abundances in the host rock are of limited use in target generation. For simplicity, the thermodynamic modeling assumed that redox conditions prevailing during fluid-rock interaction were controlled by sufficient H2S fugacity to maintain pyrite stability throughout the simulation. This is justified by the fact that there is no textural evidence for diagenetic pyrite contained in the mudstone being dissolved during alteration (Ritts, 2012). Furthermore, the existence of a significant organic fraction in the mudstone probably contributed to the attainment of reduced conditions during alteration. Macroscopically, the organic material does not appear to have been degraded during alteration of the carbonaceous mudstone.

Previous modeling by Meuzelaar and Monecke (2012) showed that high dissolved H2S concentrations (>1000 mmol), and corresponding fugacities, are capable of producing very acidic hydrothermal fluids (pH < 3). This process was not modeled in the present study as mineralogical investigations on the mudstone (Chapters 2 and 3) indicated that carbonaceous mudstone at Eskay Creek only contains low concentrations of base metal sulfide minerals and that the overall alteration mineralogy is not indicative of highly acidic conditions. As carbonate alteration is interpreted to be the principal alteration process (Chapters 2 and 3), it was assumed that CO2 played a more significant role in controlling fluid pH than H2S.

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4.4.2 CO2 Solubility

As a second step, the thermodynamic database used for GWB modeling of the present study had to be modified to adequately describe CO2 solubility in hydrothermal fluids. This was achieved through implementation of the CO2 solubility model of Duan et al. (2006).

Duan et al. (2006) presented an improved model for calculating CO2 solubility in a CO2-

H2O-Na-Cl-K-Ca-Mg-SO4 system for temperatures ranging from 0 to 260ºC, pressures between 0 and 1975 atm, and ionic strengths ranging from 0 to 4.5 molal. The model represented an improvement over a previous thermodynamic model which predicted CO2 solubility for the CO2-

H2O-Na-Cl system over the same P-T-XNaCl range (Duan and Sun, 2003). The original model calculated the chemical potential of CO2 in the vapor phase using an equation of state (Duan et al., 1992) and the chemical potential of CO2 in the liquid phase according to the ion-solvent interaction model of Pitzer (1973).

Duan et al. (2006) compared their model with previous CO2 solubility experiments, and calibrated the model taking the wide range of available experimental data into account. The improved model included a non-iterative equation that replaces the original equation of state for calculating CO2 fugacity coefficients and provided a greater predictive accuracy at low temperatures.

As part of the present study, the CO2 solubility data from Duan et al. (2006) were used to revise CO2 equilibrium constants at a range of temperatures given in the LLNL thermochemical database (Delaney and Lundeen, 1990). This database is configured for modeling at 1 atm, temperatures of 0 to 300ºC, and activities calculated according to the B-dot equation of Helgeson et al. (1969) and Helgeson and Kirkham (1974), which represents an extension of the Debye- Hückel equation (e.g., Robinson and Stokes, 1968). The database was also extended to include various confining pressures, ranging from 1 to 200 atm, using the revised CO2 solubility data.

Modeling in the present study had to take such elevated confining pressures into account as Eskay Creek undoubtedly formed in a submarine setting at high hydrostatic pressures. Volcanological evidence suggests that Eskay Creek formed below the storm-wave-base

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(Monecke pers. commun. 2014), which is generally thought to be at a hundred to several hundreds of meters of water depth (Brenchley, 1989; Cheel and Leckie, 1993; Johnson and Baldwin, 1996). Fluid inclusion data from Eskay Creek further suggested that ore formation occurred at a water depth of less than 1,500 meters (Sherlock et al., 1999). The limited geological information implies confining pressures ranging from 10 to 150 atm. The range of 1 to 200 atm was chosen for modeling to conservatively bracket conditions expected at Eskay

Creek. It was further assumed that the gas phase exists as pure CO2, such that the CO2 fugacity (non-ideal partial pressure) is equal to the total confining pressure. While experimental data show that the mole fraction of CO2 in water vapor varies significantly within the pressure and temperature ranges selected for modeling (i.e. Dubacq et al., 2013), the range of CO2 fugacities was chosen to fully encompass all pressure conditions possible at Eskay Creek, and to encompass the range of P-T-XNaCl conditions described by the model of Duan et al. (2006). Furthermore, the reaction path models were constructed to reflect the range of physicochemical conditions anticipated for this hydrothermal system and do not necessarily trace the actual P-T- fCO2 path of an evolving hydrothermal fluid.

CO2 solubility data was obtained by first calculating dissolved CO2 concentrations at variable P-T-XNaCl conditions using an online solubility calculator (see http://calc.kl- edi.ac.cn/Pages/Solubility.aspx) that follows the model of Duan et al. (2006). Dissolved CO2 and

CO2 fugacity values were used to calculate the equilibrium constant according to the chemical reaction and corresponding mass action equation

CO2(aq) = CO2(g); k1 = f[CO2(g)] / a[CO2(aq)], (1)

where f is the fugacity of the gas and a the activity of CO2 in the solution. The fugacity of CO2 as a gas and the activity of CO2 in solution in Eqn. (1) were made dimensionless based on the choice of an appropriate standard state (Bethke, 2008). Solubility data for dissolved CO2 and vapor phase CO2 in the LLNL databases are given by the following two reactions and corresponding mass action equations:

+ - + - CO2(aq) + H2O = H + HCO3 ; k2 = a[H ]·a[HCO3 ] / a[CO2(aq)] (2)

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+ - + - CO2(g) + H2O = H + HCO3 ; k3 = a[H ]·a[HCO3 ] / f[CO2(aq)]. (3)

- + HCO3 and H partitioning data was not determined within the P-T-XNaCl range of the solubility model by Duan and Sun (2003) and Duan et al. (2006). Thus it was not possible to quantify the equilibrium constants for these reactions. As Eqn. (1) is simply Eqn. (3) subtracted from Eqn. (2), the LLNL database was configured to calculate the equilibrium constant k1 for

Eqn. (1) from Duan et al. (2006) followed by the subtraction of log k1 from log k2, the equilibrium constant for Eqn. (2). This yields log k3, the equilibrium constant for Eqn. (3), which was used to replace the existing values in the LLNL database.

Figure 4-3 shows a comparison of the CO2 solubility (assuming a 0 molal NaCl solution at

1 atm CO2 and temperatures ranging from 0 to 100ºC) predicted by the LLNL and LLNL extended thermochemical databases (calculated using GWB) and the model by Duan et al. (2006). The data from the LLNL extended thermochemical database are added for comparison even though the simulations on this study use only the original LLNL database with modifications according to Duan et al. (2006). The extended version of the LLNL database is a later release which contains extensive additional organic species.

The plot indicates reasonable agreement among the three models, although there is divergence by as much as 0.01 molal (~450 mg/kg dissolved CO2) especially at low and high temperature. However, as CO2 fugacity increases, modeling using the LLNL databases (which are configured to the steam saturation curve at 1 atm) results in a significant under-prediction of the CO2 solubility. The solubility data from Duan et al. (2006) indicate that at higher confining pressures (assuming 100% CO2 vapor pressure) use of the standard databases can result in under- prediction of dissolved CO2 levels by as much as 2 molal (88,000 mg/kg dissolved CO2). The data also indicates that CO2 solubility decreases with increasing temperature, except at very high

CO2 fugacities (>100 atm), where a solubility minimum occurs at a temperature of 50 to 150ºC, while maxima occur in the lower (0 to 50ºC) and upper (150-250ºC) ranges. Figure 4-4 illustrates the effects of changing salinity on the CO2 solubility constant at a range of temperatures and pressures which, unlike the effects of changing confining pressures, shows consistent, proportional change over the salinity range. In this study, CO2 solubility constants at 0 molal

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FIG. 4-3: Comparison of CO2 solubility data available in the literature. A. Solubility data predicted by the LLNL and LLNL extended thermochemical databases compared to the solubility model proposed by Duan et al. (2006). B. CO2 solubility predicted by the model proposed by Duan et al. (2006) for a range of pressures and temperatures.

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FIG. 4-4: Effects of changing fluid salinity on CO2 solubility at a range of temperatures and pressures as predicted by the solubility model proposed by Duan et al. (2006).

NaCl were employed in the model since this resulted in higher (and maximum) CO2 dissolved in hydrothermal fluids, and since fluid inclusion data from Eskay Creek suggest that the ore- forming fluids were generally of low salinity (Sherlock et al., 1999).

4.4.3 The Role of Boiling

Previous studies indicated that ore formation at Eskay Creek occurred from low- temperature (<200ºC) hydrothermal fluids (Sherlock et al., 1999). Although phase separation may have occurred within the upflow zone of the hydrothermal fluids at Eskay Creek, conclusive evidence for the occurrence of phase separation during ore formation at the seafloor could not be identified (Sherlock et al., 1999).

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For this reason, processes of phase separation were not modeled in the present study. The assumption that fluid-rock interaction at Eskay Creek did not occur from a hydrothermal fluid undergoing phase separation also facilitated the use of GWB as this program does not explicitly describes phase separation. Only a limited number of codes, including CHILLER (Reed, 1982) and WATCH (Arnórsson et al., 1982; Bjarnason, 1994), adequately perform calculations for fluids undergoing phase separation. However, these codes are insufficient to model the hydrothermal alteration processes of silicate minerals as required for the present study and are difficult to merge with GWB.

4.4.4 Equilibrium Assumption

The Eskay Creek alteration system formed part of a larger dynamic hydrothermal system. Fluid-rock interaction within the fluid upflow zones at Eskay Creek took place in a flow- dominated environment under open system conditions. Equilibration between the hydrothermal fluids and the surrounding volcanic rocks could not have occurred under these conditions due to the high fluid flux. However, within the periphery of the fluid upflow zones, the fluid flux likely diminished and alteration must have taken place in a rock-dominated environment. Much of the alteration of the carbonaceous mudstone probably took place under such conditions. In this rock- dominated environment, equilibration between the hydrothermal fluids and the rocks was more likely to occur, assuming that the amount of fluid entering into the area of low fluid flow was sufficiently high (Giggenbach, 1984).

The reaction path modeling performed in the present study is based on the assumption that equilibrium is achieved during the interaction of the hydrothermal fluids and the volcanic rocks. Although such conditions are idealized, equilibrium thermodynamic modeling of the alteration mineral assemblages provides an important end-member reference point, allowing the discussion of actual mineral assemblages at Eskay Creek in terms of deviations from these idealized conditions.

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4.4.5 Thermodynamic Models for Carbonate Minerals

As hydrothermal alteration of the volcanic rocks at Eskay Creek resulted in widespread carbonate alteration, the thermodynamic database used in the GWB calculations of the present study was updated using recently published carbonate solubility data. In particular, the data for ankerite (Holland et al., 1998), dolomite (Robie et al., 1995; Rock et al., 2001), magnesite (Robie et al., 1995; Rock et al., 2001), and siderite (Preis et al., 2002) were added to the database.

4.5 Results

Equilibrium thermodynamic reaction path modeling was conducted by equilibrating hydrothermal fluids with carbonaceous mudstone at temperatures ranging from 50 to 300ºC. Two different starting compositions for the mudstone were assumed. One model was based on the assumption that the carbonaceous mudstone is composed of volcaniclastic material derived from a felsic volcanic source, which essentially implies that the mudstone is compositionally identical to a rhyolite. In a second model, a more complex mudstone composition was assumed, implying a chemically less evolved or bimodal provenance. The importance of CO2 contained in the hydrothermal fluids was investigated by conducting the calculations using variable CO2 fugacities ranging from 1 to 200 atm via a series of sliding fugacity path models. As a third model, the role of mixing with seawater was investigated in a series of reaction path mixing models.

4.5.1 Reaction with Mudstone of Rhyolitic Provenance

Interaction of the hydrothermal fluids with the carbonaceous mudstone was modeled in the + + 3+ - + seven component system represented by the LLNL basis species K -Na -Al -SiO2-Cl -H - - HCO3 . The mudstone was assumed to be of rhyolitic provenance, implying that the mudstone is composed of quartz, K-feldspar, and plagioclase. To simplify modeling, the plagioclase was assumed to be an albite. Although the volcaniclastic detritus contained in the mudstone was probably initially glassy (Chapter 3), dissolution of volcanic glass was not modeled given the lack of detailed compositional data and the fact that glass dissolution is a complex, multi-step

- 144 - process (Oelkers and Gislason, 2001; Wolff-Boenisch et al., 2004), which cannot be modeled by way of the simple equilibrium assumption adopted in this study.

In the model, the mudstone was titrated into a dilute, one liter, neutral pH fluid at 50 ºC temperature steps past the point of equilibrium saturation or the point at which mineral saturation values stabilized. Each resulting fluid was then equilibrated with discrete CO2 fugacities (1, 50,

100, 150, and 200 atm), assuming 100% CO2 vapor pressure. The basic model parameters are summarized in Table 4-1. The models did not converge at temperatures above 250ºC.

The equilibrium thermodynamic reaction path modeling showed that the pH value of the fluid equilibrated with the rhyolitic mudstone varies as function of temperature and pressures

(Fig. 4-5). The modeling indicates that, in the absence of CO2, the hydrothermal fluids equilibrate with the mudstone at alkaline pH (~8 to 10). The pH decreases as fluid temperature increases. This is related to increases in dissolved silica SiO2(aq) causing a decrease in dissolved

H3SiO4, releasing protons into solution.

Small increases in CO2 fugacity resulted in significant effects on the pH value of the hydrothermal fluids. At a CO2 fugacity of 1 atm, the pH value of the hydrothermal fluids equilibrated with the mudstone is near neutral (~6 to 8). The pH value increases with increasing fluid temperature.

At high CO2 fugacities, the fluids equilibrated with the mudstone are mildly acidic (pH ~4 to 5). The pH value of the hydrothermal fluids increases with increasing fluid temperature. The increased acidity results in a high solute loading. The total dissolved solids levels are as high as 60,000 mg/kg in the acidic hydrothermal fluids, but 99% of the loading is due to the dissolved

CO2. The actual cation solute loads are below 500 mg/kg across the entire investigated range in

CO2 fugacity and temperature.

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TABLE 4-1: Model parameters for the equilibration of hydrothermal fluids with mudstone of rhyolitic provenance.

Parameter Model values

Temperature (˚C) 50, 100, 150, 200, 250, 300 CO2 fugacity (atm) 0, 1, 50, 100, 150, 200 Un-equilibrated fluid composition Na+ = 1.0 mg/kg; K+ = 1.0 mg/kg; Al3+ = 1 - µg/kg; SiO2(aq) = 1.0 mg/kg; Cl = 3.0 mg/kg; pH = 7.0 Mineral titrants 1 g quartz, 1 g K-feldspar, 1 g albite

Examples of reaction path model output diagrams for selected CO2 fugacity values are given in Figure 4-6. The diagrams, showing mineral stabilities as a function of pH, demonstrate that the primary minerals in the mudstone of rhyolitic provenance are stable at neutral and alkaline pH. However, the silicate mineral are destabilized as fluid pH drops in response to increasing CO2 fugacity levels and decreasing temperatures. Equilibration of the igneous minerals with a dilute hydrothermal fluid results in quartz and K-feldspar undersaturation and dissolution. Above 200˚C, corresponding muscovite precipitation leads to a quartz-K-feldspar- mica assemblage at a pH range of 8 to 10.

As the pH drops, albite becomes unstable and is completely dissolved in all simulations at pH values below 7.9. Potassium feldspar (represented by the LLNL phase ‘maximum microcline’, which is thermodynamically more stable than the LLNL phase ‘K-feldspar’) becomes unstable at further pH decreases, generally dissolving completely between pH 7.3 and 7.5. Above 200˚C, muscovite replaces the K-feldspar and is, in turn, replaced by kaolinite between pH 5.3 and 5.5.

At lower temperatures, muscovite stability, while predicted, is clearly unrealistic (see Monecke et al., 2007 and references therein). Its exclusion from the model results in K-feldspar stability down to pH 6.5, and subsequent kaolinite replacement, with no mica being formed. Quartz is more or less stable throughout all simulations, slightly increasing or decreasing in

- 146 -

FIG. 4-5: Results of equilibrium thermodynamic pathway modeling predicting the pH of a hydrothermal fluid in equilibrium with mudstone of rhyolitic provenance at variable pressures and temperatures. Note that models with pCO2 > 0 did not converge at temperatures exceeding 250ºC. abundance as this mineral participates in the dissolution and precipitation reactions involving the aluminosilicate phases.

4.5.2 Reaction with Mudstone of Bimodal Provenance

In a second model, the mudstone composition was modified to model the interaction of hydrothermal fluids with a chemically less evolved rock. It was still assumed that the principal minerals contained in the mudstone are quartz, K-feldspar, and plagioclase. The plagioclase was modeled at a mixture of albite and anorthite. In addition, the mudstone was assumed to contain diagenetic pyrite prior to hydrothermal alteration. As the presence of volcanic glass could not be modeled, it was assumed that the mudstone contained equal amounts of montmorillonite and nontronite to account for the Mg and Fe content of the rock prior to alteration. Both smectites are common products of diagenetic devitrification of volcanic glass (see Monecke et al., 2007). As

- 147 -

FIG. 4-6: Results of equilibrium thermodynamic pathway modeling predicting mineral stabilities for the equilibration between a hydrothermal fluid and mudstone of rhyolitic provenance at different temperatures and CO2 partial pressures. A. Mineral stabilities for a reaction occurring at 200 ºC and 20 atm CO2. B. Mineral stabilities for a reaction occurring at 100 ºC and 50 atm CO2.

- 148 -

the LLNL database does not contain montmorillonite, this phase was added for the present study using data from the extended LLNL database.

The equilibration of the hydrothermal fluids with the mudstone was simulated using a 12- + + 3+ - + - component system as represented by the LLNL basis species K -Na -Al -SiO2-Cl -H -HCO3 - 2+ 2- 2+ 2+ Fe -SO4 -O2-Ca -Mg . Equilibration with the mudstone was simulated via titration reaction path models using the same discrete temperatures and CO2 fugacities as in the rhyolitic mudstone equilibration simulations. The basic model parameters are summarized in Table 4-2.

Throughout all simulations, pyrite remained stable and provided the primary redox control, keeping fluids under reducing conditions, although iron oxides provided an additional redox constraint at higher temperatures (200-250˚C), with magnetite being stable above pH 6.5, and hematite being stable at lower pH values. To allow carbonate precipitation, which requires a significant calcium source, it was assumed that much of the calcium required for carbonate formation was supplied by the hydrothermal fluids. To model calcium transport by the hydrothermal fluids, the fluids were equilibrated with anorthite, presumably present at depth within the upflow zone or representing the anorthitic component of the plagioclase present in the mudstone (Chapters 2 and 3).

The modeling suggests that the pH-fCO2 relationships for the mudstone reaction is similar to those observed in the more simple simulations discussed above. Without considering dissolved CO2, solute loading levels are also the same. A noticeable increase in dissolved silica (up to ~400 mg/kg) occurs at higher model temperatures (250ºC).

A summary of the reaction path modeling, showing mineral stability as a function of pH, is given in Figure 4-7. Not unlike the simpler reaction models described above, equilibration of the mudstone without CO2 leads to an alkaline fluid (pH 8 to 10), with pH decreasing as temperature increases. Quartz, K-feldspar, and albite remain stable, and are supplemented by secondary calcium aluminum silicates in the form of zeolites or epidote. The latter occurs only at temperatures above 150˚C. Kaolinite is stable below 100˚C, while muscovite is stable at

- 149 -

TABLE 4-2: Model parameters for the equilibration of hydrothermal fluids with mudstone of bimodal provenance.

Parameter Model values

Temperature (˚C) 50, 100, 150, 200, 250 CO2 fugacity (atm) 50, 100, 150, 200 Un-equilibrated fluid composition Na+ = 1.0 mg/kg; K+ = 1.0 mg/kg; Al3+ = 1 µg/kg; Mg2+ = 1.0 mg/kg; Ca2+ = 1.0 mg/kg; 2+ - Fe = 1 ug/kg; SiO2(aq) = 1.0 mg/kg; Cl = 3.0 2- mg/kg; SO4 = 1.0 mg/kg; O2(aq) = 0.1 mg/kg; - HCO3 = 1.0 mg/kg; pH = 7.0 Mineral titrants 1 g quartz, 1 g K-feldspar, 1 g albite, 1 g pyrite, 1 g anorthite, 1 g montmorillonite, 1 g nontronite

temperatures of 200˚C and higher. Calcium montmorillonite and nontronite are stable at all temperatures except at 250˚C, where only montmorillonite remains stable.

As pCO2 levels increase and pH decreases, mineral stability trends broadly favored dissolution the primary mudstone phases and the formation of secondary clays and carbonate minerals. Anorthite is unstable and dissolves under all model conditions. Albite is stable only above pH 8, and appears most sensitive to decreasing fluid pH caused by increased levels of dissolved CO2. K-feldspar is generally stable above pH 6.5 to 7, below which it is replaced by clay minerals (smectite or kaolinite) and calcite. Above 200˚C, K-feldspar is replaced by muscovite. At lower temperatures (<100˚C), montmorillonite and nontronite are replaced by more magnesian smectite and carbonate minerals below pH 6 to 7. Above 100˚C, the calcium smectite becomes increasingly stable at acidic pH.

Predicted secondary phases include muscovite, clays (kaolinite and various smectites) and carbonate minerals including ankerite, calcite, dolomite, and siderite. Chlorite was not observed in any of the simulations. As in the earlier simulations, muscovite forms after K-feldspar above 200˚C and is generally stable between pH 5 to 7. At 150˚C, K-feldspar is replaced by a secondary aluminum-rich smectite (beidellite). At temperatures below 150˚C and fluid pH below

- 150 -

FIG. 4-7: Results of equilibrium thermodynamic pathway modeling predicting mineral stabilities for the equilibration between a hydrothermal fluid and carbonaceous mudstone of bimodal provenance at different temperatures and CO2 partial pressures. A. Mineral stabilities for a reaction occurring at 100ºC and 50 atm CO2. B. Mineral stabilities for a reaction occurring at 50 ºC and 50 atm CO2.

- 151 -

6.5, secondary kaolinite forms after K-feldspar. Secondary clays form in response to increases in fluid CO2 and the corresponding pH decrease. Calcium and magnesium-bearing montmorillonite show stability across a broad pH range, while iron-rich smectite (nontronite) is generally stable from pH 5 to 7, and aluminous smectite (beidellite) forms below pH 6. In general, clays are stable across a wider pH range as temperature increases, and become more magnesian as fluid pH decreases.

The dissolution of plagioclase and clay provides the calcium, iron, and magnesium cations as fluid pH decreases, while increases in dissolved CO2 result in corresponding increases in alkalinity at lower pH. The increase in cations and alkalinity result in supersaturation for several carbonate phases. In general, carbonate minerals are less stable at higher temperatures. The different carbonate phases precipitating each indicate stability under unique pH conditions. Calcite demonstrates the widest stability, precipitating from mildly acidic (pH 5) to alkaline fluids. In the mudstone simulations, calcite formation occurs at circum-neutral pH according to the following reaction:

2+ - + K-feldspar + 0.5 H2O + Ca + HCO3 = K + 0.5 kaolinite + 2 quartz + calcite. (4)

Calcium is provided by the dissolution of anorthite, while alkalinity is a direct function of the dissolved CO2 content in the fluid. As temperature increases, the reaction occurs at lower pH, as muscovite delays the release of cations required to make secondary clay (beidellite rather than kaolinite). Further decreases in pH and concomitant smectite dissolution result in calcite instability and precipitation of dolomite, ankerite, and siderite, respectively.

Dolomite precipitation was only observed in the 50˚C simulations, with the dolomite forming as a result of calcium and magnesium cations released by montmorillonite dissolution (and partial calcite dissolution) below neutral pH. As montmorillonite is stable at lower pH as temperature increases, dolomite formation was not further observed. Ankerite replaces calcite at low temperatures (<100˚C), below pH 6.5 as iron rich smectite (nontronite) becomes instable and its partial dissolution releases iron cations. As is the case with dolomite, ankerite formation is restricted to low temperatures given that the increased stability of iron smectites prevents cation

- 152 - release at higher temperatures. This is consistent with experimental results of Gysi and Stefansson (2012). Siderite forms at the lowest pH conditions (below pH 6) and below 250˚C, with iron provided by complete dissolution of iron-rich smectite (nontronite) and bicarbonate by the dissolution of calcite (or ankerite at lower temperature). Siderite forms at lower pH as temperature increases, a function of iron smectite stability across an increased pH range at higher temperatures. Precipitation of siderite at lower pH is frequently accompanied by formation of a more magnesian nontronite smectite. Siderite formation is not supported at 250˚C. Magnesite was not observed in any of the mudstone equilibria models.

Quartz is generally stable in all simulations. At temperatures below 200˚C, secondary quartz is added to the system according to Eqn. (4) which occurs below neutral pH. Above neutral pH, quartz dissolves as corresponding silica release buffers fluid pH.

4.5.3 Mixing with Seawater

A final set of simulations was completed to evaluate the effects of mixing of seawater with hydrothermal fluid equilibrated with mudstone of bimodal provenance at a 1:1 ratio. Seawater entrainment into the rising hydrothermal fluids is probably pronounced within the upflow zones of hydrothermal fluids (Hannington et al., 1995; Franklin et al., 2005). In addition, it has to be taken into account that the mudstone probably contained a high proportion of pore waters during hydrothermal alteration, resulting in mixing and dilution of the hydrothermal fluids during wall-rock interaction. Heating of seawater over the temperature range of 50 to 250ºC was initially simulated via a polythermal reaction path model, using average modern seawater concentrations (Hannington et al., 2005), with dissolved aluminum concentrations estimated from Hydes (1977). Heated seawater was subsequently mixed with hydrothermal fluids equilibrated with the mudstone of bimodal provenance at the same range of temperatures and CO2 fugacities used in the previous simulations. General model parameters are as follows are given in Table 4-3.

- 153 -

TABLE 4-3: Model parameters for the reaction of hydrothermal fluids and seawater equilibrated with mudstone of bimodal provenance.

Parameter Model values

Temperature (˚C) 50, 100, 150, 200, 250 CO2 fugacity (atm) 50, 100, 150, 200 Seawater composition Na+ = 10225 mg/kg; Cl- = 14385 mg/kg, K+ = 367 mg/kg; Al3+ = 9.6 µg/kg; Mg2+ = 1230 mg/kg; Ca2+ = 392 mg/kg; Fe2+ = 0.05 ug/kg; 2- SiO2(aq) = 11.5 mg/kg; SO4 = 2570 mg/kg; - HCO3 = 136 mg/kg; pH = 7.8

Results of the polythermal model are given in Figure 4-8 and indicate that seawater pH drops from circum-neutral down to approximately pH 5 upon heating. Consequently, simulation of seawater admixing during fluid-rock interaction at variable temperatures and CO2 fugacities results in further pH reduction. Mixing also results in a significant increase in solute loading. For instance, at 50˚C, predicted TDS loads in the seawater mixing models range from 17,000 mg/kg

(0 atm dissolved CO2) to 70,400 mg/kg (200 atm dissolved CO2), whereas the range for the hydrothermal fluids equilibrated with the mudstone without seawater admixing was 120 to 56,000 mg/kg. The significant increase is especially pronounced at low temperatures and low dissolved CO2 concentrations, where it reflects the increased solute load of seawater (TDS ~30,000 mg/kg) relative to the low temperature solute loading predicted by the mudstone equilibria modeling (~500 mg/kg, not considering dissolved CO2). The increase primarily reflects addition of the major cations and anions present in seawater. At higher temperatures, this increase in salinity is offset by considerable additional precipitation of secondary phases. Figure 4-8 demonstrates that anhydrite and magnesium hydroxide sulfate hydrate control calcium, magnesium, and sulfate concentrations at high temperatures. Serpentine and magnesium chlorite also control magnesium concentrations, but to a lesser extent.

Significant mineral precipitation and dissolution trends along the mixing path are summarized as follows, to the extent that they differ from trends already established by rhyolite and mudstone equilibria modeling. Mixing of seawater with equilibrated mudstones in the

- 154 -

FIG. 4-8: Results of equilibrium thermodynamic pathway modeling predicting mineral stabilities for a polythermal seawater model (A), and a model assuming that hydrothermal fluid equilibrated with a carbonaceous mudstone undergoes mixing with seawater (B). A. Mineral stabilities as function of temperature. B. Mineral stabilities as function of fluid pH at 250ºC and 200 atm CO2.

- 155 - absence of CO2 results in pH 5 (at 250˚C) to pH 6.5 fluid in equilibrium with quartz, pyrite, and secondary magnesium end member smectites (montmorillonite, nontronite, and saponite) which form after primary feldspars, dissolved by the seawater influenced hydrothermal fluid. Since the fluid has little CO2, carbonate minerals do not form.

The pH of mudstone-equilibrated, seawater influenced hydrothermal fluids at higher dissolved CO2 concentrations ranges from 4 to 5. The primary difference between these fluids and those not influenced by seawater is that the increased calcium and magnesium concentrations result in calcium and magnesium-bearing montmorillonite and nontronite stability over a greater pH range. The increased stability of smectite results in overall decreased kaolinite stability and occurrence. Furthermore, formation of magnesium-rich saponite occurs in the models with seawater. This phase did not occur in any of the mudstone equilibration models without seawater influence. The presence of seawater also leads to the conversion of calcium bearing montmorillonite, nontronite, and saponite to pure magnesian end members between pH 5 and 6. The formation of additional clays results in significant quartz instability, with frequent dissolution/precipitation as hydrothermal fluid pH shifts.

The additional cations resulting from seawater mixing in CO2-enriched fluids also have a significant effect on the formation and distribution of carbonate minerals. The primary difference is that calcite is stable at higher temperatures (250˚C), but over a narrower pH range. Calcite is replaced by dolomite as pH drops, with the cations required for dolomite formation also supplied by the conversion of calcium-bearing nontronite and saponite to more magnesian end members. Dolomite was rarely observed in the mudstone models without seawater mixing, but is ubiquitous in the seawater models. Additionally, magnesite, which does not occur in the earlier models, forms above 150˚C and below pH 5 as dolomite becomes unstable and dissolves. This trend towards magnesian enrichment reflects that observed within the smectite clays. Anhydrite is also observed at temperatures above 150˚C, forming between pH 5 and 6, replacing calcite and calcium bearing montmorillonite.

- 156 -

Results of the mixing simulations indicate that seawater places important controls on the distribution of secondary clay and carbonate minerals during hydrothermal alteration of the carbonaceous mudstone at Eskay Creek.

4.6 Discussion

In this discussion, model results are compared to modern day vent analogues and observed alteration mineralogy. The results are subsequently framed into a new alteration model.

4.6.1 Modern Day Vent Analogues

Comparison of the modeling parameters to the chemistry of chloride waters venting on the modern seafloor (see data compilation by Hannington et al., 2005) provides some important additional information on the hydrothermal system that formed the Eskay Creek deposit. To be consistent with the temperature range chosen for modeling, this comparison is only made for end-member vent data with temperatures of less than or equal to 300ºC.

Figure 4-9 shows a comparison between pH and solute load for selected modern seafloor hydrothermal fluids sampled in different tectonic environments. The majority of vent fluids have a calculated TDS ranging from below seawater (~30,000 mg/kg) up to about 55,000 mg/kg. Solute loads in the vent fluids with pH values between 4 to 6 are typically higher than the solute loads predicted for the model at Eskay Creek that assumes mixing between hydrothermal fluids and entrained seawater at 250ºC (7,000 to 15,000 mg/kg). This may indicate that seawater mixing with hydrothermal fluids occurs at a greater ratio than the 1:1 ratio assumed in the modeling. In contrast to slightly acidic to alkaline hydrothermal fluids of the model, high TDS values can also be achieved in strongly acidic chloride waters, presumably under the influence of high dissolved H2S concentrations. Figure 4-9 shows two very acidic fluids (pH 2.8 and 3.2) that have TDS values of 75,000 to 90,000 mg/kg.

- 157 -

FIG. 4-9: Comparison of end-member fluid compositions of modern seafloor hydrothermal vents. The diagram shows the pH of the end-member fluids plotted as function of ionic strength (see data compilation by Hannington et al., 2005). AS = Axial Seamount; BH = Bent Hill; DD = Dead Dog; EP = 21ºN EPR; GY = Guaymas; MG = Menez Gwen; NC = North Cleft; PC = Pacmanus; SC = South Cleft; SE = 17ºN SEPR; VW =Vienna Woods.

It is important to note that the calculated seafloor vent TDS values do not account for contributions from dissolved CO2, measured at eight of the twelve vent sites, which range from 200 to 10,000 mg/kg (~0.005 to 0.25 molal). Given that the measured end-member compositions represent disequilibrium fluids that are rapidly cooling and exsolving gases, the dissolved CO2 concentrations are not likely indicative of equilibrium CO2 fugacities, and should be considered a minimum estimate. Comparison with the revised CO2 solubility data (Fig. 4-3) suggests that minimum equilibrium CO2 fugacities for the vent floor fluids range from 0 to 50 atm.

The pH range of modern seafloor hydrothermal vents varies significantly from site to site and between tectonic settings (Fig. 4-9). Currently, few data are available for hydrothermal systems forming in arcs and related rift settings. Some of the most comprehensive data are available from the Pacmanus vent field in the Manus basin, which represents one of the best studied modern seafloor analogues of ancient volcanic-hosted massive sulfide deposits. The vent

- 158 - site is located on Pual ridge and largely developed on the flanks of dacitic to rhyolitic knolls (Binns and Scott, 1996; Thal et al., 2014). Although the hydrothermal fluids at the Pacmanus vent field are also enriched in CO2 (Reeves et al., 2011) the fluid chemistry at this site is distinct from the predicted composition of the mineralizing fluids at Eskay Creek. The acidic nature of the hydrothermal fluids at Pacmanus likely prevents the formation of significant carbonate alteration within the footwall volcanic rocks. Limited ODP drilling at this vent field indeed showed that carbonate alteration is not widespread at Pacmanus (Binns et al., 2002).

4.6.2 Comparison of Model Results to Observed Mudstone Alteration Mineralogy

The geochemical modeling of mudstone alteration by hydrothermal fluids undergoing mixing with seawater generally predicts the observed alteration mineralogy at Eskay Creek (Chapters 2 and 3).

The main difference between the modeled and observed mudstone mineralogy is that the modeling predicts the widespread occurrence of smectite in the mudstone, which are stable hydrothermal alteration products over the modeled ranges of fluid temperatures and CO2 concentrations. Furthermore, the modeling did not predict the occurrence of chlorite in hydrothermally altered mudstone. The apparent discrepancy can be explained by the fact that the mudstone has been affected by metamorphism at lower greenschist facies conditions (Britton et al., 1990; Roth et al., 1999; Chapter 3 of this thesis). During prograde metamorphism, dioctahedral smectite is typically transformed to illite while trioctahedral smectite is transformed to chlorite (cf. Gharrabi et al., 1998; Merriman and Peacor, 1999). As metamorphic grade exceeds maximum temperatures of hydrothermal alteration, the products of hydrothermal alteration initially formed in the carbonaceous mudstone are recrystallized to their metamorphic equivalents.

The geochemical modeling further predicted enrichment trends of magnesium in smectite at lower fluid pH, which is more extensive in the seawater mixing models. This prediction corresponds well with the observed magnesium enrichment in chlorite and carbonates proximal to mineralization (Chapter 2 and 3) and indicates that seawater entrainment and mixing with pore

- 159 - water were likely important factors in constraining the alteration mineralogy of the carbonaceous mudstone host at Eskay Creek. The influence of seawater is further supported by the fact that kaolinite, although present, only occurs at low concentrations within intensely altered mudstone, which mirrors model predictions that seawater influence increases the stability of smectite relative to that of kaolinite. Evidence for seawater entrainment and mixing with pore water is also supported by the widespread occurrence of members of the ankerite-dolomite solid solution series, and by the local presence of siderite and magnesite. The latter is only predicted to form by the seawater mixing models as high magnesium concentrations are required in the hydrothermal fluids to form this mineral.

However, it is important to note that the predicted alteration mineralogy of the carbonaceous mudstone strongly depends on the composition of the mudstone prior to alteration. The entrainment of seawater and mixing with pore water may only be required to explain the alteration mineralogy in the case that the volcaniclastic component of the mudstone is of rhyolitic provenance. In such a case, the mudstone would not be able to supply sufficient amounts of calcium, iron, and magnesium to explain the widespread occurrence of hydrothermal magnesium and iron bearing clay and carbonate minerals. In mudstone containing abundant mafic minerals, the secondary magnesium and iron bearing clay and carbonate minerals may develop at mixing ratios between hydrothermal fluid and seawater that are significantly lower than those modeled in the present study.

Whole-rock mineralogical investigations on the carbonaceous mudstone from Eskay Creek indicate that minerals of the dolomite-ankerite solid solution occur in many samples, while members of the magnesite-siderite solid solution are comparably rare. According to the reaction path modeling, ankerite formation occurs at very low temperatures, typically below 50˚C. This may indicate that the widespread alteration of the carbonaceous mudstones only occurred at such low temperatures at pH between 5.5 and 6.5. Samples containing abundant dolomite-ankerite (>5 wt %) occur on average 110 meters away from ore and about 30 meters above the contact with the footwall rhyolite. This may suggest that high-temperature alteration associated with fluid upflow through the mudstone was spatially restricted, or that ankerite alteration occurred during

- 160 - waning stage of the hydrothermal activity when the hydrothermal fluids were significantly cooler.

The local occurrence of magnesite on the other hand is suggestive of seawater entrainment and higher temperatures of hydrothermal alteration. Magnesite precipitation is only predicted in the polythermal mixing models at temperatures of 200 to 250˚C. The average distance to ore for mudstone samples containing magnesite is only 90 m, while the average distance to the contact with the footwall rhyolite contact is 4 m. This spatial configuration supports the modeling, indicating that magnesite formation is restricted to zones of high temperature alteration. In contrast, modeling suggests that siderite is stable over a wide range of temperatures. However, this carbonate mineral only occurs rarely in the carbonaceous mudstone at Eskay Creek.

The calcite predicted in the mudstone equilibrium models occurs over a relatively broad pH range, and is a function of dissolution of feldspars in the presence of dissolved CO2. This broadly agrees with the observation that anorthitic plagioclase buffers CO2 and is replaced by calcite and clay minerals in low temperature (<300ºC) hydrothermal environments (Giggenbach, 1980, 1981; Simmons and Christenson, 1994). However, significant calcite is also present distal to hydrothermal upflow zones within apparently least-altered mudstone. Textural evidence (Chapter 3) suggests that this calcite was largely formed during low-grade metamorphism. The calcite forms small veinlets that crosscut foliation in the carbonaceous mudstone.

4.6.3 Eskay Creek Alteration Model

The predictive geochemical modeling supports a number of vectors to ore and mineral alteration trends identified by whole-rock mineralogical and geochemical investigations (Chapter 2) and detailed petrographic studies (Chapter 3). In general, the models indicate an increase of magnesium- and iron-bearing carbonate minerals and clays proximal to hydrothermal upflow zones. The modeling also correctly reproduces the observation that plagioclase is completely replaced by clay minerals in altered mudstone.

- 161 -

The alteration mineralogy of the carbonaceous mudstone is primarily controlled by fluid pH, which is related to the concentrations of dissolved CO2. Seawater entrainment and pore water contained in the mudstone provided an additional important source of cations for the formation of secondary clay and carbonate minerals within the mudstone. The equilibrium modeling suggests that alteration proximal to areas of hydrothermal upflow occurred from moderately acidic (pH 4 to 5) and CO2-rich fluids. Under these conditions and temperatures of up to 200ºC as predicted from fluid inclusion research (Sherlock et al., 1999), all feldspar originally contained in the carbonaceous mudstone was destroyed. Magnesium-rich smectite formed as the principal alteration mineral. At this pH, magnesite and siderite were the main carbonate minerals stabilized. Further out from the hydrothermal upflow zones, alteration of the mudstone occurred under decreasingly less acidic conditions. Feldspar dissolution provided the required cations for the formation of secondary mica (above 150˚C) or kaolinite (below 150˚C), smectite, and carbonate minerals. The clay and carbonate minerals formed were enriched in calcium relative to clay and carbonate minerals formed in mudstone close to the areas of high hydrothermal up-flow. Distal to the hydrothermal upflow zones and outside of the alteration halo, the mudstone was in equilibrium with neutral to alkaline pore water.

The relatively low vent fluid pH range (pH 4 to 6) indicated by the proximal alteration mineralogy is consistent with earlier geochemical modeling that showed that sphalerite precipitation at seafloor hydrothermal vents can indeed occur at fluid pH values ranging from 4.5 to 5.0 (Meuzelaar and Monecke, 2012). Using modern seafloor vent chemistry data in a series of simulations, sphalerite precipitation at these pH values was predicted at temperatures ranging from 180 to 220ºC. The fluid properties indicated by the alteration mineralogy are, therefore, consistent with the mineralogy of the ore at Eskay Creek.

Polythermal seawater titration models indicate that seawater has a potentially important influence on the alteration mineralogy. Entrainment of seawater into the hydrothermal upflow zones or mixing of the hydrothermal fluids with pore water during mudstone alteration results in an increase in solute load and further decreases in fluid pH. The heated seawater supplies the cations to support conversion of mudstone smectites and carbonates to more magnesian varieties.

- 162 -

The modeling of the present study also showed that the mineralogical composition of the mudstone represents a key control on the acid buffering capacity of this rock. Mudstone composed of rhyolitic volcaniclastic detritus only has a very limited capacity to buffer the acidity of the hydrothermal fluids. In contrast, mudstone containing an increased amount of volcaniclastic material of bimodal provenance has a higher acid buffering capacity due to the presence of calcium-rich plagioclase and ferroan and magnesian minerals. Previous investigations demonstrate that the mudstone chemistry broadly changes up stratigraphy. Mudstone of a bimodal provenance appears to be more abundant in the hanging-wall (Chapter 2). Intense hydrothermal alteration only affected the contact mudstone and the stratigraphically lower portion of the hanging-wall mudstone. Further up stratigraphy, the alteration intensity decreases rapidly as the hydrothermal activity was waning at some stage during the deposition of the hanging-wall mudstone. The hanging-wall mudstone equilibrated with pore water at low temperatures and minerals such as calcite were formed under neutral to alkaline conditions.

Based on the thermodynamic modeling performed in the present study, a new conceptual model for synvolcanic mudstone alteration can be derived. It is proposed here that there is a continuum of alteration conditions from proximal intense alteration where mudstone equilibrates with fairly high-temperature and acidic hydrothermal fluids to distal alteration where mudstone equilibrates with low-temperature pore water at neutral to alkaline conditions. With increasing transport distance and decreasing temperatures, the properties of the hydrothermal fluids change as the fluids equilibrate with the host rock. As a consequence, hydrothermal mudstone alteration in the periphery of the alteration halo essentially occurs at conditions identical to diagenetic conditions. Diagenesis also represents a low-temperature alteration process of the mudstone by fluids that are equilibrated with the carbonaceous rocks.

The proposed continuum of alteration conditions explains the fact that there are no clear petrographic characteristics in mudstone collected distal to alteration allowing a definitive distinction between hydrothermally altered mudstone and diagenetic processes. The presence of finely dispersed calcite cement in the mudstone matrix or the abundant occurrence of framboidal pyrite recorded in thin section (Chapter 3) are expressions of low-temperature interaction of the mudstone with fluids equilibrated with the mudstone. A distinction between hydrothermal and

- 163 - diagenetic processes is essentially impossible in the absence of constraints on timing of the fluid- rock interaction. Due to the continuum of alteration conditions occurring in the synvolcanic environment, there are no clear mineralogical or geochemical threshold values that could be used in exploration to distinguish hydrothermal alteration from diagenetic alteration. Univocal identification of hydrothermal alteration is only possible proximal to ore where alteration of the mudstone clearly occurred at elevated temperatures from hydrothermal fluids characterized by elevated CO2 contents. Vectoring to ore can only be conducted by examining spatial trends that are related to the progressive changes in alteration temperature and degree of mixing with pore water.

4.7 Conclusions

The present study demonstrates that thermodynamic reaction path modeling represents a powerful tool to understand synvolcanic alteration processes as reliable thermodynamic data exist for the relevant temperature range (<300ºC) and fluid salinity (<15 wt. % NaCl). As equilibrium reaction pathway modeling correctly predicted the observed alteration mineralogy, predicted fluid parameters can be used to derive an alteration model for the carbonaceous mudstone host of the unusual Eskay Creek sulfide and sulfosalt deposit.

The modeling shows that the observed alteration patterns can be primarily related to the temperature-dependent equilibration between the CO2-rich hydrothermal fluids and the volcaniclastic component of the carbonaceous mudstone. The CO2 content of the hydrothermal fluids controls the acidity of the hydrothermal fluids over the temperature range considered. In general, the acidity of CO2-bearing fluids increases with decreasing temperature, explaining the occurrence of minerals in the alteration halo that are only stable at pH ranges of 4 to 6. However, the decrease in fluid pH upon cooling is counteracted by consumption of CO2. Alteration distal to hydrothermal upflow zones occurs at neutral to alkaline conditions due to the lower CO2 content of the fluids interacting with the mudstone in this part of the alteration halo. The modeling further demonstrates that polythermal mixing of the hydrothermal fluids with entrained seawater or pore water was an important process controlling the alteration mineralogy. The

- 164 - modeling also shows that the mudstone composition plays an important role in controlling fluid chemistry as the acid buffering capacity of the rock is related to the mudstone mineralogy.

The predicted fluid parameter are consistent with a genetic model for Eskay Creek that assumes that mineralization and alteration occurred at fairly low temperatures (<200ºC) from a

CO2-rich hydrothermal fluid. The predicted fluid parameters for this hot spring deposit, especially the pH range of hydrothermal alteration, are not unlike those of geothermal systems and related low-sulfidation state precious metal deposits occurring on land.

4.8 Acknowledgements

This paper has benefited from discussions with E. Holley and N. Kelly. The authors gratefully acknowledge financial support from the Canadian Mining Industry Research Organization Project 08E04. Additional financial support was provided by Geoscience BC, the Stewart R. Wallace Endowment in Economic Geology at the Colorado School of Mines, Rockware, and Golder Associates. A research grant by the Society for Economic Geologists Canada Foundation is gratefully acknowledged.

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4.8 References

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Brenchley, P.J., 1989, Storm sedimentation: Geology Today, v. 5, p. 133–137. Cheel, R.J., and Leckie, D.A., 1993, Hummocky cross-stratification: Sedimentology Reviews, v. 1, p. 103–122. Delaney, J.M., and Lundeen, S.R., 1990, The LLNL thermochemical database: Lawrence Livermore National Laboratory Report UCRL-21658, 150 p.

Duan, Z., and Sun, R., 2003, An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar: Chemical Geology, v. 193, p. 257–271.

Duan, Z., Møller, N., and Weare, J.H., 1992, An equation of state for the CH4-CO2-H2O system: I. Pure systems for 0 to 1000ºC and 0 to 8000 bar: Geochimica et Cosmochimica Acta, v. 56, p. 2605–2617. Duan, Z., Møller, N., and Weare, J.H., 1996, Prediction of the solubility of H2S in NaCl aqueous solution: An equation of state approach: Chemical Geology, v. 130, p. 15–20.

Duan, Z., Sun, R., Zhu, C., and Chou, I.M., 2006, An improved model for the calculation of CO2 + + 2+ 2+ - 2- solubility in aqueous solutions containing Na , K , Ca , Mg , Cl , and SO4 : Marine Chemistry, v. 98, p. 131–139. Duan, Z., Sun, R., Liu, R., and Zhu, C., 2007, Accurate thermodynamic model for the calculation

of H2S solubility in pure water and brines: Energy & Fuels, v. 21, p. 205–2065. Dubacq, B., Bickle, M.J., and Evans, K.A., 2013, An activity model for phase equilibria in the

H2O-CO2-NaCl system: Geochimica et Cosmochimica Acta, v. 110, p. 229–252. Ettlinger, A.D., 1992, Hydrothermal alteration and brecciation underlying the Eskay Creek polymetallic massive sulphide deposit (104B/9W): British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological field work 1991, Paper 1992-1, p. 535–541. Franklin, J.M., Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic massive

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Giggenbach, W.F., 1984, Mass transfer in hydrothermal alteration systems - a conceptual approach: Geochimica et Cosmochimica Acta, v. 48, p. 2693–2711.

Gysi, A.P., and Stefánsson A., 2012, Mineralogical aspects of CO2 sequestration during

hydrothermal basalt alteration – An experimental study at 75 to 250°C and elevated pCO2: Chemical Geology, v. 306-307, p. 146–159. Hannington, M.D., Jonasson, I.R., Herzig, P.M., and Petersen, S., 1995, Physical and chemical processes of seafloor mineralization at mid-ocean ridges: Geophysical Monograph 91, p. 115–157. Hannington, M.D., de Ronde, C.E.J., and Petersen, S., 2005, Sea-floor tectonics and submarine

hydrothermal systems: ECONOMIC GEOLOGY 100TH ANNIVERSARY VOLUME, p. 111–142. Helgeson, H.C., Garrels, R.M., and Mackenzie, F.T., 1969, Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions - II. Applications: Geochimica et Cosmochimica Acta, v. 33, p. 455–481. Helgeson, H.C., and Kirkham, D.H., 1974, Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressures and temperatures: II. Debye-Hückel parameters for activity coefficients and relative partial molal properties: American Journal of Science, v. 274, p. 1199–1261. Holland, T.J.B., and Powell, R., 1998, An internally consistent thermodynamic data set for phases of petrological interest: Journal of Metamorphic Geology, v. 16, p. 309–343. Hydes, D.J., 1977, Dissolved aluminum concentration in sea water, Nature, v. 268, p. 136–137. Idziszek, C., Blackwell, J., Fenlon, R., MacArthur, G., and Mallo, D., 1990, The Eskay Creek discovery: Mining Magazine, March 1990, p. 172–173. Johnson, H.D., and Baldwin, C.T., 1996, Shallow clastic seas, in Reading, H.G., ed., Sedimentary environments: Processes, facies, and stratigraphy: Oxford, Blackwell, p. 232– 280. Macdonald, A.J., Lewis, P.D., Thompson, J.F.H., Nadaraju, G., Bartsch, R.D., Bridge, D.J., Rhys, D.A., Roth, T., Kaip, A., Godwin, C.I., and Sinclair, A.J., 1996, Metallogeny of an

Early to Middle Jurassic Arc, Iskut River Area, northwestern British Columbia: ECONOMIC

GEOLOGY, v. 91, p. 1098–1114. Mercier-Langevin, P., Hannington, M.D., Dube, B., and Becu, V., 2011, The gold content of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 509–539.

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Merriman, R.J., and Peacor, D.R., 1999, Very low-grade metapelites: Mineralogy, microfabrics and measuring reaction progress, in Frey, M., and Robinson, D., eds., Low-grade metamorphism: Oxford, Blackwell Science, p. 10–60. Meuzelaar, T., and Monecke, T., 2012, Fluid controls on ore genesis in the Eskay Creek deposit, northwestern British Columbia (NTS 104B/09,/10): Geoscience BC Summary of Activities 2011, Geoscience BC, Report 2012-1, p. 41–52. Monecke, T., Gale, D., Roth, T., and Hannington, M.D., 2005, The submarine volcanic succession hosting the massive sulfide and sulfosalt Eskay Creek deposit, Canada, in Mao, Y. and Bierlein, F.P., eds., Mineral deposit research: Meeting the global challenge, Proceedings of the 8th biennial SGA meeting, Beijing, China, 2003: Berlin, Springer, p. 655–658. Monecke, T., Giorgetti, G., Scholtysek, O., Kleeberg, R., Götze, J., Hannington, M.D., and Petersen, S., 2007, Textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine PACMANUS hydrothermal system, eastern Manus Basin: Journal of Volcanology and Geothermal Research, v. 160, p. 23–41. Oelkers, E.H., and Gislason, S.R., 2001, The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25˚C and pH = 3 and 11: Geochimica et Cosmochimica Act, v. 65, p. 3671–3681. Pitzer, K.S., 1973, Thermodynamics of electrolytes. I. Theoretical basis and general equations. Journal of Physical Chemistry, v. 77, p. 268–277. Preis, W., and Gamsjäger, H., 2002, Critical evaluation of solubility data: Enthalpy of formation of siderite: Physical Chemistry Chemical Physics, v. 4, p. 4014–4019. Reed, M.H., 1982, Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals, gases, and an aqueous phase: Geochimica et Cosmochimica Acta, v. 46, p. 513–528. Reeves, E.P., Seewald, J.S., Saccocia, P., Bach, W., Craddock, P.R., Shanks, W.C., Sylva, S.P., Walsh, E., Pichler, T., and Rosner, M., 2011, Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea: Geochimica et Cosmochimica Acta, v. 75, p. 1088–1123.

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Ritts, A., 2012, Texture and composition of pyrite contained in the mudstone host of the Eskay Creek sulfide and sulfosalt deposit, British Columbia: Unpublished M.Sc. thesis, Golden, Colorado, Colorado School of Mines, 185 p. Rock, P.A., Mandell, G.K., Casey, W.H., and Walling, E.M., 2001, Gibbs energy of formation of dolomite from electrochemical cell measurements and theoretical calculations: American Journal of Science, v. 301, p. 103–111. Robie, R.A., Hemingway, B.S., and Fisher, J.R., 1995, Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 pascals) pressure and at higher temperatures: U.S. Geological Survey Bulletin 2131, 461 p. Robinson, R.A., and Stokes, R.H., 1968, Electrolyte solutions: London, Butterworths, 571 p. Robinson, H.M., 1991, Mineralisation and alteration patterns of the Central Lens (21B Zone), Eskay Creek, British Columbia, Canada: Unpublished M.Sc. thesis, London, U.K., Imperial College of Science, Technology and Medicine, University of London, 129 p. Roth, T., 2002, Physical and chemical constraints on mineralization in the Eskay Creek deposit, northwestern British Columbia: Evidence from petrography, mineral chemistry, and sulfur isotopes: Unpublished Ph.D. thesis, Vancouver, Canada, University of British Columbia, 401 p. Roth, T., Thompson, J.F.H., and Barrett, T.J., 1999, The precious metal-rich Eskay Creek

deposit, northwestern British Columbia: REVIEWS IN ECONOMIC GEOLOGY, v. 8, p. 357– 373. Sherlock, R.L., Roth, T., Spooner, E.T.C., and Bray, C.J., 1999, Origin of the Eskay Creek precious metal-rich volcanogenic massive sulfide deposit: Fluid inclusion and stable

isotope evidence: ECONOMIC GEOLOGY, v. 94, p. 803–824. Simmons, S.F., and Christenson, B.W., 1994, Origins of calcite in a boiling geothermal system: American Journal of Science, v. 294, p. 361–4400. Thal, J., Tivey, M., Yoerger, D., Jöns, N., and Bach, W., 2014, Geologic setting of PACManus hydrothermal area – High resolution mapping and in situ observations: Marine Geology, v. 355, p. 98–114. Wolff-Boenisch, D., Gislason, S.R., Oelkers, E.H., and Putnis, C.V., 2004, The dissolution rates of natural glasses as a function of their composition at pH 4 and 10.6, and temperatures from 25 to 74˚C: Geochimicia et Cosmochimica Acta, v. 68, p. 4843–4858.

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CHAPTER 5 CONCLUSIONS

The present thesis represents the first comprehensive study of the mineralogical and geochemical characteristics of the carbonaceous mudstone host of the unusual Eskay Creek sulfide and sulfosalt deposit, British Columbia. The detailed investigations have shown that these fine- grained rocks are highly susceptible to hydrothermal alteration despite the fact that alteration is cryptic and cannot be easily recognized in hand specimen. Alteration-induced mineralogical and geochemical gradients within the carbonaceous mudstone extend from tens to hundreds of meters away from the areas of hydrothermal upflow. The observed alteration patterns broadly reflect changes in fluid temperature and acidity that can be primarily related to progressive wall-rock buffering of the fluids. The mineralogical and geochemical investigations along with thermodynamic reaction path modeling were used to constrain the properties of the hydrothermal fluids interacting with the carbonaceous mudstone:

1) In comparison to other seafloor hydrothermal systems, the hydrothermal fluids forming Eskay Creek were of comparably low temperature (<200˚C). The fluids contained elevated

dissolved CO2. The dissolved CO2 resulted in low fluid pH and corresponding wall rock

alteration and high solute loading. The effects of CO2-induced alteration became more pronounced with decreasing temperatures (i.e. 100˚C, rather than 200˚C). The pH of the fluids was buffered by the carbonaceous mudstone during fluid-rock interaction.

2) Alteration of carbonaceous mudstone in proximity to fluid upflow occurred at relatively acidic conditions (pH 4 to 6). The mudstone only possessed a limited capacity to buffer the

effects of CO2-induced alteration. Alteration likely occurred at higher fluid-rock ratio. Fluid flow through the mudstone was more sustained. This resulted in more intense wall rock alteration.

3) Alteration of the mudstone distal to fluid upflow occurred at more alkaline conditions (pH 7 to 9). This reflects the fact that equilibration between the hydrothermal fluids and the

- 171 - mudstone could be more readily achieved at lower fluid-rock ratio and regions of decreased fluid flow.

4) The alteration mineralogy and ore petrography suggests that the hydrothermal fluids forming Eskay Creek were reduced in character and stabilized sulfide minerals of low to intermediate sulfidation states.

The decrease in fluid temperature and progressive neutralization of the hydrothermal fluids with increasing distance from areas of upflow were the key changes in physiochemical factors controlling the zoning pattern of the alteration halo. Subsequent to hydrothermal alteration, further mineralogical and geochemical changes occurred during diagenesis and low-grade metamorphism. The principal alteration styles recognized are:

1) Clay alteration of the volcaniclastic component of the mudstone. Fluid-rock interaction resulted in the destruction of volcanic glass and feldspars and most likely in the formation of dioctahedral smectite that subsequently transformed to illite during diagenesis and low- grade metamorphism. The destruction of volcanic glass and feldspars resulted in decreases

in the whole-rock Na2O content. Dissolution of volcanic glass and addition of MgO (from the hydrothermal fluids that underwent mixing with seawater) allowed the formation of abundant trioctahedral smectite in the altered carbonaceous mudstone. This smectite was converted into chlorite during diagenesis and low-grade metamorphism. Chlorite in hydrothermally altered mudstone is typically Mg-dominant chlorite whereas chlorite present in less intensely altered mudstone and least-altered mudstone is typically Fe- dominant chlorite.

2) Carbonate alteration is the most pronounced style of alteration. Systematic changes in carbonate mineralogy with increasing distance to hydrothermal upflow zones reflect changes in fluid pH and the buffering capacity of the host rocks. Members of the dolomite- ankerite and magnesite-siderite solid solution series formed proximal to ore, resulting in whole-rock increases in FeO, MnO, and MgO. Thermodynamic reaction path modeling suggests that these minerals were stable under slightly more acidic conditions (pH 4 to 6).

- 172 - Carbonate alteration at these acidic conditions was accompanied by kaolinite formation. Calcite is the predominant hydrothermal carbonate mineral distal to ore. This reflects the fact that fluids were increasingly neutralized away from the hydrothermal upflow zones. Significant late calcite was formed due to the conversion of plagioclase to albite during diagenesis and low-grade metamorphism.

3) Silicification of carbonaceous mudstone occurred in areas of hydrothermal upflow and can be related to rapid cooling of the hydrothermal fluids and the temperature-dependent decrease in silica solubility, as well as to pH dependent destruction of feldspars which released excess silica into the hydrothermal fluids.

4) Secondary K-feldspar alteration occurred locally away from the major hydrothermal upflow zones. The secondary K-feldspar alteration is interpreted to have occurred at near

neutral conditions and resulted in significant enrichment of whole-rock K2O concentrations.

In addition to the alteration-induced changes in mudstone mineralogy and major element geochemistry, the following trends were observed at Eskay Creek:

1) The base metal content of hydrothermally altered carbonaceous mudstone can be primarily related to the presence of base metals sulfides that can precipitate from hydrothermal fluids at comparably low pH (pH 4 to 5). In less intensely altered mudstone and least-altered rocks, base metals appear to be associated with the organic fraction contained in the carbonaceous rocks.

2) The Ag content of the carbonaceous mudstone, and well as the concentrations of elements typically enriched in hot spring deposits such as As, Hg, and Sb, broadly correlate with the base metal enrichment of the carbonaceous mudstone.

- 173 - 3) In contrast, Au enrichment of the carbonaceous mudstone cannot be directly correlated to enrichment of these metals, implying decoupling of Au from the other metals in the hydrothermal system.

4) Several additional trace elements show enrichment close to ore. In particular, systematic increases in the Tl and Ba concentrations have been recorded in mudstone collected close to hydrothermal upflow zones. Sporadic enrichment of the REEs occurs in intensely altered carbonaceous mudstone.

The findings of the present study have important implications to the understanding of processes resulting in gold-enrichment in synvolcanic massive sulfide deposits:

1) Synvolcanic gold enrichment is not limited to shallow marine hydrothermal systems. The carbonaceous mudstone hosting the stratiform ores at Eskay Creek is a suspension

sediment that formed below the storm-wave base. The elevated dissolved CO2 content of the hydrothermal fluids required to explain the observed alteration patterns is also inconsistent with a shallow marine origin of Eskay Creek.

2) Synvolcanic gold enrichment can occur from hydrothermal fluids having a range of temperatures. Mineralization at Eskay Creek is related to a low temperature (<200˚C) hydrothermal system while other gold-rich massive sulfide deposits likely formed at higher (>300˚C) temperatures.

3) Hydrothermal alteration at Eskay Creek occurred from fluids that were relatively acidic

(pH 4 to 5), with CO2 being the main acid-generating species. However, in other gold-rich massive sulfide deposits, alteration must have occurred as much more acidic conditions as recorded by the occurrence of aluminosilicates in the alteration halos. Acid alteration in

these deposits may be best explained through the disproportionation of magmatic SO2. The research at Eskay Creek suggests that gold enrichment in massive sulfides is not restricted to strongly acidic hydrothermal systems.

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4) The observed characteristics are consistent with genetic models suggesting a continuum of fluid characteristics in the submarine environment ranging from higher temperature and lower pH hydrothermal systems forming classical volcanic-hosted massive sulfide deposits to lower temperature, higher pH hot spring systems that have characteristics not unlike geothermal systems and associated low sulfidation epithermal systems occurring in extensional settings on land.

Based on the results of the present study, the following recommendations are made for future work at Eskay Creek or other gold-rich massive sulfide deposits:

1) The present study did not focus on the paragenesis of K-feldspar. It is recommended to further study the K-feldspar using microanalytical techniques. It is possible that this would allow the identification of additional trace element vectors to ore. Further thermodynamic reaction path modeling may provide additional critical information on the physicochemical conditions of hydrothermal K-feldspar alteration.

2) It is proposed to further evaluate the thermal and compositional maturity of the mudstone organic fraction. It is possible that alteration of carbonaceous mudstone proximal to hydrothermal upflow zones may have affected the organic fraction of the mudstone. Despite of the low-grade metamorphic overprint, it appears possible that altered and unaltered carbonaceous mudstone samples show differences in their organic fraction. Further work would also be needed to better understand the observed correlation between the organic carbon content and the precious and base metal content of samples collected away from known ore zones.

3) Further microanalytical research is needed to study the deportment of Au in the carbonaceous mudstone. It is not clear at present whether the Au is contained in As-bearing pyrite or occurs in other minerals such as tellurides. Similarly, the mineralogical reasons for Tl enrichment in proximity to ore are currently poorly constrained.

- 175 - 4) Future geochemical modeling should test which effects phase separation of the hydrothermal fluids would have on the alteration of carbonaceous mudstone at Eskay Creek. At the same time, it should be evaluated whether phase separation could play a role as a precipitation mechanism for Au-bearing minerals.

5) At present very little is known on the alteration of basalt units that are hosted by the hanging-wall mudstone. Macroscopically observed bleaching of the basalt units has been reported, implying that these mafic rocks have at least locally been affected by hydrothermal alteration. From an exploration point of view, it would be important to better understand host rock controls on hydrothermal alteration. This could be achieved by comparing existing information on the alteration of the footwall rhyolite, with the results of the present study constraining alteration patterns in the carbonaceous mudstone and an additional study focusing on alteration-induced mineralogical ad geochemical changes in the basalt.

- 176 - APPENDIX SUPPLEMENTAL ELECTRONIC FILES

A list of supplemental data that support the thesis work at Eskay Creek is summarized in the table below. The files include laboratory data, geochemical modeling files, and principal component analysis data files and results. There is also a directory of abstracts, posters and presentations from SEG and AMEBC Roundup conferences, and two publications for Geoscience BC. Conferences and Publications – AMEBC Mineral Roundup This subdirectory contains two abstracts and two posters (subdirectory) from various AMEBC Mineral Roundup conferences. Conferences and Publications – Geoscience This subdirectory contains 2010 and 2011 Summary of BC (subdirectory) Activities publications. Conferences and Publications – International Geological Congress This subdirectory contains one abstract for an oral presentation. Conferences and Publications – Society for Economic Geology This subdirectory contains three abstracts, one poster and one (subdirectory) presentation from various SEG conferences. Geochemical Modeling – This subdirectory contains a single GWB file containing Analog Vent Data vent chemistry data (T < 300ºC) from Hannington et al. (subdirectory) (2005). Geochemical Modeling – This subdirectory contains two Microsoft Excel files, one Gas Solubility for CO2 and one for H2S, with revised solubility data and (subdirectory) equilibrium constant calculations. Geochemical Modeling – Mudstone Equilibria This subdirectory contains GWB model files for (subdirectory) equilibration with mudstones. Geochemical Modeling – Rhyolite Equilibria This subdirectory contains GWB model files for (subdirectory) equilibration with rhyolite. Geochemical Modeling – Seawater Mixing This subdirectory contains GWB model files for seawater (subdirectory) heating and mixing with mudstone-equilibrated fluids. Geochemical Modeling – This subdirectory contains modified LLNL thermodynamic Thermo Databases databases with additional carbonate phases, (subdirectory) montmorillonite data, and revised CO2 and H2S data.

- 177 - This subdirectory contains one Microsoft Excel file with Laboratory Data – Bulk drillhole coordinates, XRD, XRF, ICP, bulk stable isotope, Chemistry and Mineralogy carbonate/pyrite isotope, and pyrite geochemistry data for (subdirectory) all 180 mudstone samples.

Laboratory Data – Cathodoluminescence (subdirectory) This subdirectory contains 130 cathodoluminescence images.

Laboratory Data – Electron This subdirectory contains two Microsoft Excel files with Microprobe (subdirectory) EMPA data for carbonates and sheet silicates.

Laboratory Data – Transmission Electron Microscopy (subdirectory) This subdirectory contains TEM data for two samples. This directory contains seven subdirectories, each containing three files: 1) Microsoft Excel data for input to NCSS, 2) NCSS data file and 3) Microsoft Excel data containing NCSS output and sorted factor loading groups. The seven subdirectories represent PCA of various mudstone compositional data and sample subgroups, as follows:

1. All 180 samples, all mineralogy and chemistry data. 2. The entire dataset minus two outliers with anomalously high REE concentrations. 3. All 180 samples, mineralogy and major elements only. 4. All 180 samples, trace and major elements only. 5. All 180 samples, trace elements only. 6. All 180 samples, major elements only. Principal Component Analysis 7. All 180 samples, minerals only.

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