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Physical and chemical constraints on mineralization in the Eskay Creek deposit, northwestern British… Roth, Tina 2002

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PHYSICAL AND CHEMICAL CONSTRAINTS  O N M I N E R A L I Z A T I O N IN T H E  E S K A Y C R E E K DEPOSIT, NORTHWESTERN EVIDENCE FROM PETROGRAPHY, AND SULFUR  BRITISH  MINERAL  CHEMISTRY,  ISOTOPES  by TINA R O T H  B. Sc. (Hons.), The University of Waterloo, 1989 M.Sc,  The University of British Columbia, 1993 A thesis submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in  T H E FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences)  W e accept thesis standard as conforming tc/tljethis required  T H E UNIVERSITY OF BRITISH COLUMBIA  October 2002 © Tina Roth, 2002  COLUMBIA:  In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  Department of Earth and Ocean Sciences The University of British Columbia Vancouver, C a n a d a  Date: December 18,  2002  ABSTRACT  The Eskay Creek deposit is an unusual, polymetallic, precious metal-rich, volcanogenic massive sulfide and sulfosalt deposit located in northwestern British Columbia. Combined production and current reserve and resource estimates total 2.34 million tonnes, grading 51.3 g/t A u and 2 326 g/t A g , and are contained in a number of stratiform and stockwork vein zones that display a variety of textural and mineralogical characteristics. The bulk of the ore is hosted in the 21B zone, a tabular stratiform lens that consists of well-bedded, clastically reworked sulfides and sulfosalts interbedded with unmineralized, carbonaceous argillite. In addition to extremely high precious metal grades, Eskay Creek is distinguished from conventional V M S deposits by its association with elements of the 'epithermal suite' ( S b - H g ± A s ) , sulfosalt-rich mineralogy, and the dominance of clastic sulfides and sulfosalts. Physical and chemical constraints on the deposition of the Eskay Creek deposit have been investigated by detailed field studies, supplemented by petrographic, geochemical and stable isotopic analysis of the ore minerals. The deposit formed during two periods of hydrothermal activity, reflecting stratiform lenses occurring at two stratigraphic levels within argillite at the contact between rhyolite and basalt. During the first stage of activity, early sphalerite, Ag-rich tetrahedrite, galena, pyrite and electrum deposited on the sea floor near the center of the present 21B zone. T h e resulting seafloor mound and chimneys were periodically fragmented, transported, and redeposited in a basin adjacent to the vent site. Hydrothermal fluids migrated laterally along the permeable clastic bed to deposit progressively more Sb and Hg-rich sulfides and sulfosalts including dominantly boulangerite and bournonite, associated with significant electrum, followed by stibnite. Later minerals are progressively more restricted towards the center of the deposit. The latest stage of hydrothermal overprint in the 21B zone is characterized by minor cinnabar. T h e second stage of hydrothermal activity resulted mainly in deposition of the stratigraphically higher HW massive sulfide lens. The HW zone is typical of mineralized lenses where sulfides were not transported from the vent site. Ore of this type is characterized by replacement textures, presence of chalcopyrite and absence of lead sulfosalts.  Silver is hosted principally in near end-member tetrahedrite and in electrum. Sphalerite, tetrahedrite and electrum locally contain significant Hg due to reaction with late stage, Hg-rich fluids. Low  temperatures of formation are reflected in the low F e S contents in sphalerite, as well as the sulfosalt-rich mineral assemblage, and are optimal for transport of gold as a sulfide complex. Sulfur isotope signatures in the sulfides are consistent with a magmatic source, either derived by leaching the underlying volcanic pile, direct magmatic input, or both. Sulfur isotope values in pyrite from the host argillite grade rapidly to biogenic signatures within tens of metres from the edge of the orebodies, indicating that hydrothermal fluids are focussed and had a limited extent of lateral migration.  Processes responsible for formation of the Eskay Creek deposit are not unique in the V M S environment, but require the coincidence of several favourable conditions to optimize the precious metal grade in the deposit. Continued removal of material deposited at the vent site is essential to prevent sealing and subsequent increase of temperatures of the hydrothermal system, therefore providing sustained low temperature conditions favourable for transport of gold. Redeposition of the clastic sulfides adjacent to the vent site allowed access for hydrothermal fluids to deposit additional gold. A reduced basinal environment with an appropriate depositional geometry is therefore necessary to preserve transported sulfides.  TABLE OF CONTENTS  ii  Abstract Table of Contents  iv  List of Tables  xii  -  xiv  List of Figures  xviii  Foreword  xx  Acknowledgements  Chapter 1: Introduction 1  Introduction  2  Previous Work  3  Objectives Methods  "  4  Presentation References  3  —-  5  Chapter 2: The Precious Metal-Rich Eskay Creek Deposit, Northwestern British Columbia Introduction  10  History  13  Geological Setting  14  Structure and Metamorphism  16  Mine Sequence  16  Lower Footwall Units  18  Rhyolite  18  Contact Mudstone  19  Basalt_  20  Intrusive Rocks  20  Primary Geochemistry of the Volcanic Rocks  21  Alteration  23  ..25 Mineralization 28  21A Zone,  28  21B Zone  32  East Block  33  NEX Zone,  34  21C Zone.  34  Hanging Wall Zone,  35  Pumphouse and Pathfinder Zones.  35  109 Zone Summary of Mineralization  37 Hydrothermal Fluids 39 The Eskay Creek Model  "" 44  Acknowledgements .... 44 References  —-  —-  Chapter 3: Mineralogy, Paragenesis and Chemistry of Sulfides and Sulfosalts in the Eskay Creek Deposit Introduction  49  Regional Geologic Setting....,  52  Local Geology  53  Geometry and Textural Characteristics of the Orebodies,  57  Stratiform Orebodies  58  Discordant Orebodies,  68  Structural Control and Deformation Part I: Mineralogy and Textural Characteristics of the Ore Zones  21B Zone,  69 72  86  Sphalerite  87  Tetrahedrite  88  Galena  88  Lead sulfosalts  _  88 v  Pyrite  88  Electrum  89  Minor sulfides  89  Ganaue minerals  90  Textural relationships Pangenesis East Block  92 94 94  Mineralogy  95  Clastic ore  96  Massive ore  96  Paragenesis  97  NEXZone  97  Mineralogy  98  Ore textures  99  Hanging Wall (HW) Zone Mineralogy Ore textures and mineral paragenesis 109 Zone  99 102 102 103  21C Zone  104  21EZone.  106  Mineralogy  106  Ore textures  107  Pumphouse Zone.  107  21A Zone,  108  Mineralogy and textures in the 21A-contact zone  108  Mineralogy and textures in the 21A-rhyolite zone  109  Genetic Constraints from Ore Textures and Mineralogy  Relative Timing Among the Orebodies. Mineral Paragenesis.  110  110 113  vi  Part II: Mineral Chemistry  118  Sphalerite  118  Compositional variations within grains  119  Compositional variations within samples  121  Compositional variations between subzones  125  Constraints from sphalerite chemistry  128  Tetrahedrite  130  Constraints from tetrahedrite chemistry Electrum,  136 138  Constraints from electrum chemistry Lead Sulfosalts  141 142  Constraints from lead sulfosalt chemistry  145  Silver Sulfosalts.  145  Galena  145  Stibnite  145  Pyrite  147  Lateral, vertical and paragenetic variations in mineral composition  147  Metal Zonation  155  ;  Correlation between elements.  155  Bulk grade distribution  155  Normalized grade distribution  159  Vertical variations in assay grades.  161  Distribution of gold and silver  163  Summary  163  References  166  Chapter 4: Sedimentary Characteristics and Depositional Environment of the Eskay Creek 21B Zone Introduction  176  Depositional Setting  177 vi  Geometry of the Orebody  179  Sedimentary Features in the 21B Zone  181  Sulfide-Sulfosalt Beds.  181  Clasts: characteristics and composition  .186  Matrix  186  Clast-size distribution  187  Bed-thickness  188  Sorting and grading within beds  188  Other sedimentary features  189  Barite.  191  Unmineralized Interbeds  194  Bed Relationships  194  Bed Continuity: Deposition and Deformation Diagenetic Features  196 '.  Distribution of B e d T y p e s  199 199  Area A  199  Area B  200  Area C  200  Area D  201  Area E  201  Area F  201  Area G  201  Discussion  202  Direction of Transport and Evidence for Source Area. Formation Processes for the 21B Zone. Fragmentation Transport and Deposition Paleoen vironment Clastic Sulfides in Other V M S Deposits  203 204 204 207 210 212  viii  Implications for Precious Metal Enrichment  214  Conclusions  215  References  216  Chapter 5: Variations in Sulfur and Carbon Isotopes in the Eskay Creek Deposit and Related Argillite Host Rocks Abstract  2  2  3  Introduction  224  Geologic Setting  225  Mine Sequence  226  Orebodies.  228  Textural Variations in Pyrite  2 3 1  Sample Selection and Analysis  234  Sulfur Isotopes.  2  3  4  Comparison of MILES with Standard S0 Extraction Method  .238  Carbon  239  2  Results  2  3  9  Ore Sulfides  2 3  Pyrite Separates  243  Textural Variations  248  Carbon Isotopes  248  Geothermometry in Ore Sulfides  252  Discussion  257  Variation ofS? S in the Orebodies 4  Distribution offfS in the Argillite Hostrocks.  2  9  57 259  Sulfur Sources  266  Textural Controls on Isotopic Composition  270  Comparison to Other Deposits  272  Carbon Isotopes.  274  Exploration Significance,  276 ix  Summary.  277  References  278  Chapter 6: Physical and Chemical Controls on Mineralization in the Eskay Creek Deposit and Implications for Exploration Introduction  287  Depositional Setting  289  Evolution of the Eskay Creek Hydrothermal System  291  Lower Stratiform Lenses (Stage I)  293  Upper Stratiform Lenses (Stage II).  297  Chemical Constraints on the Ore-Forming Fluids  297  Fluid Temperature and Chemistry.  298  Source of Metals and Sulfur.  301  Metal Transport  303  Precipitation Mechanisms Cooling and dilution by mixing with seawater  305 305  Boiling  306  Reduction of the Fluids  307  Sulfide Deposition and Precious Metal Enrichment  Vein and Disseminated Sulfides and Gold Enrichment. 109 zone 21C-rhvolitezone Clastic Sulfides and Sulfosalts. 21B zone Massive Sulfides and Sulfosalts  307  308 308 308 308 309 311  East Block  311  NEX zone  311  HWzone  312  Physical Constraints on Formation of the Eskay Creek Deposit  313  x  Modern and Ancient Analogues for the Eskay Creek Deposit  314  Implications for Exploration  316  Concluding Statements  318  References  —-  —-  319  Appendix A: Drillhole and Sample Distribution  328  Appendix B: Analytical Methods  332  Electron Microprobe Analysis  332  Appendix C: Results Tables  340  Appendix D: Calculation of sulfur activity from FeS content in sphalerite  345  Appendix E: Distribution of Trace Elements in Pyrite Around the Eskay Creek Deposit  347  Appendix F: Publications Relevant to This Study Origin of the Eskay Creek Precious Metal-Rich Volcanogenic Massive Sulfide Deposit: Fluid Inclusion and Stable Isotope Evidence;  R.L. Sherlock, T. Roth, E.T.C. Spooner, and C.J. Bray.  364  Mineralogical Characterization and Hg Deportment in Field Samples from the Polymetallic Eskay Creek Deposit, British Columbia, Canada;  T.A. Grammatikopoulos and T. Roth  386  LIST O F T A B L E S  Table 2.1  Summary of ore reserves and precious metal resources in the Eskay Creek 21  10  Zone. Table 2.2  Average values for major and trace elements in least altered rhyolite and basalt at  25  Eskay Creek. Table 2.3  Summary of mineralization styles in the Eskay Creek 21 Zone (as of 1997).  26  Table 2.4  Summary of the dominant characteristics of the Eskay Creek deposit, including  39  those typical of V M S deposits, and unusual characteristics.  Table 3.1  Characteristics of the Eskay Creek #21 Subzones.  50  Table 3.2  Mineralogy of subzones in the Eskay Creek #21 zones.  73  Table 3.3  Characteristics of the principal ore textures observed in the Eskay Creek deposit.  75  Table 3.4  Criteria used to distinguish bedding parallel veins from thin clastic beds.  92  Table 3.5  Summary of E P M A data for sphalerite.  119  Table 3.6  Compositional variations of sphalerite in subzones of the Eskay Creek deposit.  126  Table 3.7  Summary of E P M A data for tetrahedrite.  131  Table 3.8  Compositional variation of tetrahedrite in subzones of the Eskay Creek deposit.  132  Table 3.9  Summary and compositional variation of electrum, determined by E P M A .  139  Table 3.10  Summary of E P M A data for Pb - Sb - S ± C u ± Ag ± Fe ± A s minerals in the  143  Eskay Creek deposit. Table 3.11  Summary of E P M A data for polybasite in the N E X zone.  146  Table 3.12  Summary of E P M A data for galena and stibnite in the Eskay Creek deposit.  146  Table 3.13  Summary and compositional variation of pyrite, determined by E P M A .  148  Table 3.14  Pearson correlation coefficients for assay data in the 109 zone  156  Table 4.1  Characteristics of sulfide-sulfosalt bed types in the Eskay Creek 21B zone.  184  Table 4.2  Classification of sediment gravity flows.  208  Table 5.1  Summary of subzones in the Eskay Creek 21 Zone.  229  xi  Table 5.2  Comparison of 8 S values from MILES vs. S 0 2 extraction.  239  Table 5.3  Results of in situ 8 S analysis of pyrite, sphalerite and galena in ore.  240  Table 5.4  Results of 5 S analysis in pyrite separates.  244  Table 5.5  Comparison of 8 S in two electromagnetically separated pyrite fractions.  249  Table 5.6  Results of 5 C analysis of organic carbon in argillite.  250  Table 5.7  Calculated temperatures for mineral pairs in ore.  253  Table A.1  Distribution of polished sections and mounts  329  Table B.1  Acquisition parameters for microprobe analysis of sphalerite  332  Table B.2  Acquisition parameters for microprobe analysis of sulfosalts  332  Table B.3  Acquisition parameters for microprobe analysis of electrum  332  Table B.4  Acquisition parameters for microprobe analysis of pyrite  333  Table B.5  Standards used for E P M A calibration for sphalerite  333  Table B.6  Standards used for E P M A calibration for sulfosalts  333  Table B.7  Standards used for E P M A calibration for electrum  334  Table B.8  Standards used for E P M A calibration for pyrite and Ni sulfides  334  Table B.9  E P M A detection limits  335  Table B.10  E P M A error estimates - from replicate analysis of standards  336  Table B.11  E P M A error estimates - from analytical statistics  336  Table C.1  Average E P M A results for sphalerite  341  Table C.2  Average E P M A results for tetrahedrite  343  Table E.1  Trace elements in pyrite separates  348  34  34  3 4  34  1 3  LIST O F FIGURES  Figure 1.1  Location map of the Eskay Creek deposit.  1  Figure 2.1  Regional geology of the Iskut River Area of northwestern British Columbia.  11  Figure 2.2  Geology of the Eskay Creek area.  12  Figure 2.3  Stratigraphy of the Eskay Creek area.  17  Figure 2.4  Trace element and R E E contents in the immediate host rocks of the Eskay Creek  22  deposit. Figure 2.5  Plan view of ore subzones in the #21 Zone (as of 1997).  27  Figure 2.6  Photos of clastic sulfide-sulfosalt ore in the 21B zone.  30  Figure 2.7  Genetic model for the Eskay Creek deposit.  40  Figure 3.1  Plan view of resource and reserve models in the Eskay Creek #21 zone (as of  51  2001). Figure 3.2  Geology of the Eskay Creek anticline.  54  Figure 3.3  Isopach map of true thickness for the contact argillite.  56  Figure 3.4  Cross sectional views through the subzones of the Eskay Creek deposit. Legend.  59  Section A-A' - 21A and southern 21C zones Section B-B' - 21B and Pumphouse zones  60  Section C - C - 21C zone Section D-D' - 21B zone and East Block  61  Section E - E ' - 21E zone Section F-F' - 1 0 9 zone  62  Section G - G ' - 1 0 9 , N E X and HW zones  63  Figure 3.5  Ore textures in the 21B, East Block and N E X zones.  64  Figure 3.6  Ore textures in the N E X , HW, 21E, Pumphouse-rhyolite and 21A zones.  66  Figure 3.7  Distribution of sulfides in the footwall rhyolite under the 21B zone.  70  Figure 3.8  Photomicrographs of coarse clastic ore in the 21B zone.  Figure 3.9  Photomicrographs of fine clastic ore beds and thin sulfide-sulfosalt veinlets in the 21B zone.  • 76 78  Figure 3 10  Photomicrographs of clastic, deformation and hydrothermal overprint and  80  replacement in the 21B zone. Figure 3 11  Photomicrographs of tetrahedrite in sphalerite, sphalerite fragments, colloform  82  pyrite and pyrite overgrowths in the 21B zone. Figure 3 12  Photomicrographs of electrum, carbonaceous material in argillite, replacement and  84  veining (21B, East Block and NEX). Figure 3 13  Photomicrographs in the N E X , H W , 109, 21C rhyolite, 2 1 E , Pumphouse-rhyolite  100  and 21A zones. Figure 3 14  Genetic history of the Eskay Creek deposit showing relative timing of major  111  depositional events. Figure 3 15  Paragenetic sequence of ore minerals in the Eskay Creek orebody.  114  Figure 3 16  Distribution of sulfides and sulfosalts in the stratiform subzones hosted in the lower  115  contact argillite. Figure 3 17  Distribution of minor sulfides in stratiform subzones hosted in the base of the lower  116  contact argillite. Figure 3 18  Scatterplots of E P M A data for sphalerite in the 21B zone.  120  Figure 3 19  Compositional variation in a zoned sphalerite grain from the 21B zone.  122  Figure 3 20  Box and whisker plots of ranges in trace element abundance within individual  123  sphalerite grains from the 21B zone. Figure 3 21  Compositional variation in sphalerite adjacent to a cinnabar vein in the 21B zone.  124  Figure 3 22  Histograms of F e S contents in sphalerite for each subzone.  127  Figure 3 23  Histograms of X  134  Figure 3 24  XFe-Xzn-XHg in tetrahedrite from the 21B zone.  135  Figure 3 25  Scatterplots of compositional variations in tetrahedrite.  137  Figure 3 26  Ternary scatterplots of E P M A data for electrum  140  Figure 3 27  Compositional variations in lead-bearing sulfosalts: P b S - S b S 3 - A s S 3 ternary plot  144  Figure 3 28  Lateral variations in the iron content of sphalerite and tetrahedrite  149  Figure 3 29  Lateral variations in the mercury content of sphalerite, tetrahedrite and electrum  150  Figure 3 30  Vertical variations in composition of sphalerite, tetrahedrite and electrum in the 21B  152  A g  in tetrahedrite for each subzone.  2  2  zone Figure 3 31  F e S content in two textural varieties of sphalerite in the 21B zone  153  Figure 3 32  Vertical variations in composition of sphalerite in the 109 zone.  154  Figure 3 33  Scatterplot matrix of assays in the 21B zone.  156  Figure 3 34  Grade distribution of Au, A g , total base metals (Zn + Pb + Cu), and total epithermal  157  elements (Sb + Hg + As) in the lower stratiform subzones. Figure 3 35  Grade distribution of A s in the lower stratiform subzones.  158  Figure 3 36  Grade distribution of Sb, Hg, and A g normalized to total base metals, and  160  distribution of Ag:Au ratio in the lower stratiform subzones. Figure 3 37  Assay profiles through the 21B zone.  162  Figure 3 38  Histograms of Ag:Au in each subzone.  164  Figure 4 1  Geometry of the 21B zone: a series of cross sections.  180  Figure 4 2  Isopach map of the thickness of argillite occurring between the top of the rhyolite  182  and the base of the 21B zone ore. Figure 4 3  Isopach map of the 21B zone orebody.  183  Figure 4 4  Photos of bed types 1A, 1B, 1C, 2Aj 2B, 3, and 4.  185  Figure 4 5  Photos of structural features in the 21B beds.  190  Figure 4 6  Drillhole profiles and distribution of bed types in the 21B zone.  192  Figure 4 7  Changes in bedding profiles along the strike length of the 21B zone.  193  Figure 4 8  Wall maps of #1 Stope cross cuts.  195  Figure 4 9  Wall map of #3 Stope cross cut.  197  Figure 4 10  Back map and graphic drillhole profiles in the 867-sill drift (#1 Stope).  198  Figure 4 11  True thickness of rhyolite under the 21B zone.  210  Figure 4 12  Conceptual block diagram of the paleodepositional environment of the 21B zone  211  Figure 5 1  Schematic cross-section of the Eskay Creek mine sequence.  227  Figure 5 2  Spatial distribution map of mineralized subzones in the Eskay Creek deposit.  230  Figure 5 3  Photomicrographs and S E M backscatter images showing textural variations of  233  pyrite in argillite around the Eskay Creek deposit. Figure 5 4  Photomicrographs of ore sulfides analyzed in situ.  236  Figure 5 5  Plot of in situ 8 S values of ore sulfides.  242  34  Figure 5.6  Histograms of 834S in pyrite separates.  246  Figure 5.7  Box and whisker plot of 8 S in pyrite vs. distance from ore.  247  Figure 5.8  Histogram of 8 C in argillite and the 109 zone.  251  Figure 5.9  8-8 distribution plot for contact mineral pairs in ore.  254  Figure 5.10  Fractionation vs. temperature plot for ore sulfides.  260  Figure 5.11  Contour map of 8 S in the contact argillite.  261  Figure 5.12  Contoured cross-sections of 834S distribution in the Eskay Creek mine sequence.  262  Figure 5.13  Evolution of  Figure 5.14  Variation in 834S between magnetically separated pyrite fractions and comparison  34  1 3  34  5 Sj_i s 34  2  o v e r  t i m e  -  2  6  8  271  to isotopic differences between pyrite morphologies worldwide. Figure 5.15  Range in sulfur isotopes for various V M S deposits, associated volcanic host rocks  273  and seafloor sulfide deposits in comparison to data from Eskay Creek.  Figure 6.1  Comparative grade, tonnage and metal ratios in gold-rich V M S deposits  288  Figure 6.2  Plan view of Eskay Creek orebodies, precious metal grades, gold to base metal and  290  gold to epithermal element ratios Figure 6.3  Timing of depositional events in the Eskay Creek deposit  292  Figure 6.4  Genetic model for formation of the 21B and 21C zones  295  Figure 6.5  Solubility and stability fields of gold chloro and thio-complexes in fluids typically  300  associated with V M S deposits  Figure A.1  Location map of E P M A samples  330  Figure A.2  Distribution of D D H logged in detail in the 21B zone  331  Figure B.1  Estimates of precision for trace elements measured by E P M A in sphalerite  337  Figure B.2  Estimates of precision for trace elements measured by E P M A in electrum  337  Figure B.3  Estimates of precision for trace elements measured by E P M A in pyrite  337  Figure B.4  Estimates of precision for trace elements measured by E P M A in sulfosalts  338  xvii  Figure E.1  Distribution of Au in pyrite separates  356  Figure E.2  Distribution of A g in pyrite separates  357  Figure E.3  Distribution of Hg in pyrite separates  358  Figure E.4  Distribution of Sb in pyrite separates  359  Figure E.5  Distribution of A s in pyrite separates  360  Figure E.6  Distribution of Zn in pyrite separates  361  Figure E.7  Distribution of B a in pyrite separates  362  xviii  FOREWORD This thesis represents research with both academic and economic applications produced within a university, in collaboration with both industry and government. This foreword is presented to acknowledge the specific contributions of researchers who collaborated in this study. Research for this thesis began in early 1994, shortly after completion of an M S c research project that was confined exclusively to the 21A zone at Eskay Creek. Most of the older drillcore for the current study was re-logged during the summer of 1994. From January 1995 to the end of 1998, the author worked full-time at the Eskay Creek Mine as a mine geologist, and subsequently as an in-mine exploration geologist. This provided an excellent opportunity to record numerous observations while mapping and sampling underground, and logging fresh drill intersections, to understand the geometry and distribution of the various ore types presented in this thesis. Limited laboratory research was completed during this time, however. Most of the petrography, microprobe and isotopic data were collected, compiled and interpreted since early 1999. In accordance with the guidelines of The University of British Columbia and the doctoral committee, the thesis is presented as a collection of published papers, reviewed manuscripts, and chapters that will provide the basis for future publications. Chapters 2 and 5 are published or accepted papers in refereed publications. Chapter 4 is a paper that will be submitted for future publication, and Chapters 3 and 6 provide the basis for future manuscripts. Additional papers relevant to the thesis, where the thesis author was not the senior author, are presented in Appendix F. The paper that comprises Chapter 2 is an overview of the Eskay Creek deposit that summarizes previous work and presents observations from early fieldwork by the author to provide a geological framework and genetic model. It is published in Reviews in Economic Geology as part of a collection of papers presented at a Geological Association of C a n a d a - Mineralogical Association of C a n a d a ( G A C MAC) meeting short course in 1997 and was co-authored by John Thompson and Timothy Barrett. The paper was formally reviewed by Tucker Barrie, Suzanne Paradis and Mark Hannington (Geological Survey of C a n a d a (GSC)). T h e co-authors played an editorial role and helped to develop and shape the ideas in the paper. The section presenting lithogeochemistry of the host stratigraphy was contributed entirely by Barrett. Chapter 5 is presented as it was submitted to Economic Geology. T h e manuscript has been accepted pending revisions, and therefore will not be published exactly as it is included here. The paper  xix  is co-authored by Bruce Taylor ( G S C ) and reflects his guidance in the laboratory, assistance with interpretation of the data, and editorial comments. This paper was also informally reviewed by John Thompson, Jim Mortensen and Dick Tosdal. The papers that comprise Chapters 2 and 5, and that are included in Appendix F are as follows:  Chapter 2: Roth, T., Thompson, J . F . H . , and Barrett, T . J . , 1999. The Precious Metal-Rich Eskay Creek Deposit, Northwestern British Columbia, in Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings, C T . Barrie and M.D. Hannington (editors),  Reviews in Economic Geology, Volume 8, pp. 357-373  Chapter 5: Roth, T., and Taylor, B . E . ,  accepted.  Variations in Sulfur and Carbon Isotopes in the Eskay Creek  Deposit and Related Argillite Host Rocks. Economic Geology.  Appendix F 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.  Grammatikopoulos, T., and Roth, T., 2002. Mineralogical Characterization and Hg Deportment in Field Samples from the Polymetallic Eskay Creek Deposit, British Columbia, Canada. International Journal of Surface Mining, v.16, no.3, p. 180-195.  xx  ACKNOWLEDGEMENTS  This thesis has benefited from the expertise, assistance and support of numerous people over the past several years. First of all I would like to thank my supervisor, John Thompson, for his support, guidance, knowledge, and willingness to read and re-read my endless edits. Thank you for your encouragement and patience. This thesis would not have been possible without the support of Homestake C a n a d a Inc., who generously provided permission for this research and unlimited access to data, reports, drillcore, and computer equipment. I am grateful to the exploration teams, past and present, for their willingness to share their wealth of knowledge, ideas and time. Thanks to Ron Britten, Dave Kuran, Carl Edmunds, Andrew Kaip, Henry Marsden and Percy Pacor for helping me get started. I would also like to thank Ian Cunningham-Dunlop, Aletha Buschman, Dave Gale, Jeff Lewis, and Theresa Anderson (nee Fraser) for collecting samples, providing data, and for numerous discussions (serious and otherwise), and Chuck and Mike Chadaway for digging to the bottom of so many stacks in 'coreland' to help me find the right sample intervals. I am grateful to Jim Rogers, Chief Geologist at Eskay Creek, for his support of this thesis by giving me the opportunity to become a mine geologist and 'Vulcanaut', and finding research funds when I needed them. I would also like to thank the geologists at the mine for all of their help and camaraderie over the years: Earl Masarsky, Dan Lampman, Barry McDonough, John Dadds, Joe Horawski, and Rob Boyce. The assistance of everyone at the Eskay Creek Mine is greatly appreciated. Although four years of full-time work as a geologist at the mine added substantially to the completion time of this study, the thesis benefited enormously from the opportunity to observe the deposit from inside, and provided me with a valuable experience. Chapter 5 of this thesis is based on a sulfur isotope study completed in Bruce Taylor's laboratory at the G S C in Ottawa. I am grateful to Bruce for his collaboration in this study and his patience in teaching me the art of sulfur fluorination. Many thanks also to Adrian Timbal for his assistance in the lab. I would like to thank Mark Hannington for initiating this part of the project and overseeing preparation of the samples, as well as sharing his vast knowledge of gold-rich V M S deposits.  xxi  Tim Barrett, Ross Sherlock and Fiona Childe, formerly of the Mineral Deposit Research Unit Volcanogenic  Massive Sulfide Deposits of the Cordillera Project ( M D R U - V M S project), are thanked for  their input, collaboration, and many contributions to the understanding of Eskay Creek. Dick Tosdal, Jim Mortensen and Lee Groat served as thesis advisory committee members and are thanked for their comments and input. Thank you also to Colin Godwin for helping me get started on this project. I am grateful to Mati Raudsepp for his patient instruction and assistance in the use of the S E M and microprobe. Thanks also to Arne T o m a for all his help with assorted computer problems and diagrams. Financial support for this research was provided by the M D R U - V M S project, which was funded by the Natural Science and Engineering Research Council of C a n a d a ( N S E R C ) , the Science Council of British Columbia, and eleven member mining companies: Granges Inc., Homestake C a n a d a Inc., Inco Ltd., Inmet Mining Corp. (formerly Metall Inc. and Minova Inc.), Kennecott C a n a d a Inc., Lac Minerals, Placer Dome Corp., Teck Corp., TVI Pacific Inc., Westmin Resources Ltd., and W M C International Ltd. Additional funding was provided by Homestake C a n a d a Inc., and a Hugh E . McKinstry Grant from the Society of Economic Geologists. The author was supported in part by an N S E R C scholarship and a Thomas and Marguerite MacKay Scholarship from the University of British Columbia.  I am grateful to my friends for their encouragement over the past several years, and to David and Joan Moors for their welcoming and generous hospitality during my stay in Ottawa. I would particularly like to thank my parents, Dieter and Heike Roth, for their constant support and for not asking the dreaded 'is it done yet?' question more than once every few months.  Finally, special thanks to my husband, James Moors, who has endured much these past few years. Thank you for your continued support, tolerance, sense of humour and understanding.  xxii  CHAPTER 1  INTRODUCTION  Stratiform gold and silver rich mineralization in the Eskay Creek deposit has attracted much interest since its discovery in 1988. Located in the Iskut River area of northwestern British Columbia (Fig. 1.1), the deposit is one of the highest-grade gold and silver deposits in the world. It is currently owned and operated by Barrick Gold Corporation, following a merger with Homestake C a n a d a Inc. in December 2001. Commercial production commenced in January 1995 and, to the end of 2001, has totaled 1.04 million tonnes containing 2.06 million ounces of gold and 91 million ounces of silver. Proven and probable resources at the beginning of 2002 are estimated to be 1.3 million tonnes containing 1.8 million ounces gold and 84 million ounces of silver (Rogers, 2002). Therefore, precious metal grades for the deposit average 51.3 g/t gold and 2 326 g/t silver.  Based on its environment of deposition, geometry, and associated footwall alteration, Eskay  Figure 1.1: Location of the Eskay Creek deposit in northwestern British Columbia. 1  Creek is classified as a polymetallic volcanogenic massive sulfide (VMS) deposit. Its depositional setting is very similar to that of massive sulfides in the Kuroko district (Ohmoto and Skinner, 1983; Franklin et al., 1981). Stratiform sulfides and sulfosalts in several subzones are hosted in marine mudstone at the contact between underlying rhyolite and overlying basalt that form the uppermost unit of the Lower to Middle Jurassic arc sequence known as the Hazelton Group. The Eskay Creek deposit has several characteristics, however, that differ from ordinary V M S ore models including: high grades of precious metals (Au and A g are the principal commodities), elevated concentrations of elements more typically associated with the epithermal environment (Sb-Hg-As), low temperatures of formation, and dominantly clastic sulfide-sulfosalt stratiform ore. Although some of these features may be observed locally in other V M S deposits, they comprise only a fraction of the ore.  Many V M S deposits contain significant accessory gold, although unlike Eskay Creek, their primary commodities are base metals. Recent investigations of these gold-rich V M S deposits identified distinctive characteristics that are summarized by Hannington et al. (1986), Huston and Large (1987), Large et al. (1989), Huston and Large (1989), Hannington and Scott (1989), Hutchinson (1990), Poulsen and Hannington (1995), Hannington et al. (1999), and Huston (2000). Eskay Creek appears to represent an end-member of this subclass of gold-rich V M S deposits.  Previous Work The deposit and surrounding area have been the subject of several studies. These have included work by exploration and mine geologists, government survey programs, and extensive regional and deposit-scale study by the Mineral Deposit Research Unit at The University of British Columbia. Relevant references include Anderson (1989), Blackwell (1990), Britton et al. (1990), Idziszek et al. (1990), Alldrick (1991), Edmunds and Kuran (1992), Ettlinger (1992), Rye (1992), Roth and Godwin (1992), Bartsch (1993a, b), Nadaradju (1993), Roth (1993a, b), Rye et al. (1993), Edmunds et al. (1994), Sherlock et al. (1994a, b, c), Roth (1995), Barrett and Sherlock (1996), Childe (1996), Macdonald et al. (1996), Sherlock et al. (1999).  2  Objectives This study was undertaken to develop a model of physical and chemical controls on the formation of the unusual Eskay Creek deposit, and to provide a comparative basis for future exploration of similar targets. Research focussed on the characteristics within the orebody to assess:  1.  distribution and physical controls on the subzones,  2.  mineralogical and chemical differences among the subzones,  3.  paragenetic history of the mineralization,  4.  the origin and source of the bedded sulfides,  5.  mechanisms of deposition and the importance of clastic ore, and  6.  the environment of deposition.  Geologic relationships and textural characteristics of the deposit were determined through extensive fieldwork and observations at the deposit, supported by laboratory techniques. Variations in chemistry and isotopic composition of the ore sulfides and sulfosalts were evaluated within this framework.  Methods Most of the observations presented in this study are based on detailed examination and logging of drill core. T h e distribution of ore intercepts investigated in detail are shown in Appendix A. Numerous additional drillcore intercepts were reviewed less formally while the author was employed as a mineexploration geologist at Eskay Creek. Observations of large-scale relationships were collected primarily from underground exposures in the mine. These included routine face-maps, as well as more detailed back and wall maps of cross-cuts through the ore and sill drifts along strike.  Samples collected from drillcore and underground were cut and prepared as polished thin sections and mounts. The mineralogy and textural characteristics of the ore were determined by extensive ore microscopy. Due to the optical similarity of many of the antimony-bearing minerals, a scanning electron microscope (SEM) was used to confirm their identity or to discriminate among them.  Electron microprobe analysis (EPMA) of the sulfides was undertaken to define variations in the chemistry of the sulfides and sulfosalts and to provide constraints on thermochemical conditions in the  3  ore-forming fluids. Details of the analytical conditions, precision and accuracy for the method are presented in Appendix B.  Sulfur isotope compositions were analyzed in the ore minerals, as well as in pyrite collected regionally from argillite that hosts the stratiform ore and that occurs elsewhere in the mine stratigraphy. These data are used to constrain possible sources of sulfur in the orebodies and to determine the extent of the hydrothermal signature in pyrite surrounding the orebodies. In situ analysis of S^S in the ore sulfides provides evidence for physicochemical changes in the hydrothermal fluids. T h e isotopic composition of organic carbon from the host argillite was analyzed to determine whether the extent of hydrothermal activity around the orebodies could be discerned.  Presentation This thesis constitutes a series of manuscripts that each present supporting data for the genetic model summarized in Chapter 6. A s outlined in the Foreword, Chapter 2 has been published in a peer reviewed volume and Chapter 5 has been reviewed and accepted to an international technical journal, pending revisions. T h e remaining chapters (3, 4 and 6) provide the basis for future submissions. Some repetition was unavoidable in order to provide context within each chapter as a separate entity. This style has been adopted for clarity and ease to the reader, and to facilitate publication of portions of the thesis. The subject of each chapter is outlined below. Contributions from co-authors for Chapters 2 and 5 are detailed in the Foreword.  Chapter 2 provides an overview of the geology and characteristics of the Eskay Creek deposit, including a review of previous work. Descriptions of the deposit, host lithology, alteration, depositional setting, and a broad genetic model are presented and provide the context for detailed observations within the orebody that follow in subsequent chapters. This paper was presented as part of a short course on volcanic-associated massive sulfide deposits at the Geological Association of C a n a d a - Mineralogical Association of C a n a d a ( G A C - M A C ) Annual Meeting in 1997, and represents the state of knowledge at that time. T h e paper was subsequently published in a volume of Reviews in Economic Geology.  Chapter 3 presents the bulk of the mineralogical and chemical observations and data collected from the Eskay Creek deposit. T h e chapter begins with updated descriptions of the geometry and  4  characteristics of subzones in the deposit to reflect additional discoveries and changes in the understanding and interpretation of the zones made subsequent to publication of Chapter 2. The rest of the chapter is presented in two parts. Part I documents detailed petrographic observations from each of the subzones and addresses the paragenetic history of the deposit. Part II presents results of electron microprobe analyses in sulfides and sulfosalts, and metal zoning patterns in the deposit. Constraints inferred from the data are summarized within the text. Implications of these constraints and a genetic model for the deposit are discussed in Chapter 6. Chapter 4 documents primary bedforms and variations in the reworked sulfides and sulfosalts of the 21B zone based on extensive, detailed examinations of drillcore and underground workings by the author. Evidence for the local depositional setting, possible source areas, and mechanisms of fragmentation and transport are discussed. These processes provide physical constraints for the genetic history of the Eskay Creek deposit. Chapter 5 presents sulfur and carbon isotope data from argillite in the Eskay Creek host stratigraphy. This paper documents variations in 8 S of pyrite and 5 C of organic carbon with distance, 34  13  both laterally away from and stratigraphically above the orebodies, to evaluate regional influence of hydrothermal activity and trends that might provide directional indicators for mineral exploration. The paper also presents in situ analysis of sulfides within the ore to assess variations in 8 S at the grain scale 34  as well as broader differences between orebodies in the deposit. Chapter 6 discusses the physical and chemical constraints on formation of the Eskay Creek deposit, based on the findings of this study. A genetic model and implications for exploration of this type of deposit are presented.  References Alldrick, D . J . 1991, Geology and ore deposits of the Stewart Mining camp, British Columbia: Unpublished Ph.D. thesis, Vancouver, University of British Columbia, 347 p. Anderson, R . G . , 1989, A stratigraphic, plutonic and structural framework for the Iskut Map area, northwestern British Columbia: Current Research, Part E , Geological Survey of Canada, Paper 89-1E, p. 145-154.  Barrett, T . J . and Sherlock, R . L . , 1996, Geology, lithogeochemistry and volcanic setting of the Eskay Creek A u - A g - C u - Z n deposit, northwestern British Columbia, Exploration and Mining Geology, Vol. 5, No. 4, pp. 339-368.  Bartsch, R . D . , 1993a. A rhyolite flow dome in the upper Hazelton Group, Eskay Creek area (104B/9.10): Geological Fieldwork, 1992, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1993-1, p. 331 -334. Bartsch, R . D . , 1993b. Volcanic stratigraphy and lithogeochemistry of the Lower Jurassic Hazelton Group, host to the Eskay Creek precious and base metal volcanogenic deposit: Unpublished M.Sc. thesis, Vancouver, T h e University of British Columbia, 178 p.  Blackwell, J . , 1990, Geology of the Eskay Creek #21 deposits: T h e Gangue, Mineral Deposits Division, Geological Association of Canada, Number 31, p. 1-4. Britton, J . M . , Blackwell, J . D . and Schroeter, T . G . , 1990, #21 zone deposits, Eskay Creek, northwestern British Columbia: British Columbia Ministry of Energy, Mines and Petroleum Resources, Exploration in British Columbia 1989, p. 197-223.  Childe, F . C . , 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, Vol. 91, pp. 1209-1224.  Edmunds, F . C . and Kuran, D . L . , 1992, The 1992 exploration program: geological and diamond drilling results: Internal report for International Corona Corporation, Vancouver, British Columbia, 27 p.  Edmunds, F . C , Kuran, D.L. and Rye, K.A., 1994, T h e geology of the 21 Zone deposits at Eskay Creek northwestern British Columbia, Canada: Black Hills Fifth Western Regional Conference on Precious Metals, Coal and the Environment, p. 154-175.  Ettlinger, A . D . , 1992, Hydrothermal alteration and brecciation underlying the Eskay Creek polymetallic massive sulphide deposit: Geological Fieldwork, 1991, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1992-1, p. 535-541.  Hannington, M . D . , Peter, J . M . , and Scott, S.D., 1986. Gold in sea-floor polymetallic sulfides. Economic Geology, v. 81, p. 1867-1883.  Hannington, M.D., and Scott, S.D., 1989. Gold mineralization in volcanogenic massive sulfides: Implications of data from active hydrothermal vents on the modern s e a floor. Economic Geology Monograph 6, p. 491-507. Hannington, M . D . , Herzig, P . M . , and Scott, S.D., 1991. Auriferous hydrothermal precipitates on the modern sea floor; in Gold Metallogeny and Exploration, R.P. Foster (ed.), Blackie, Glasgow, p. 249-282. Hannington, M . D . , Poulsen, K . H . , Thompson, J . F . H . , and Sillitoe, R . H . , 1999. Volcanogenic gold in the massive sulfide environment. Reviews in Economic Geology, v. 8, p. 325-356. Huston, D.L., 2000. Gold in volcanic-hosted massive sulfide deposits: Distribution, genesis and exploration. Reviews in Economic Geology, v. 13, p. 401-426. Huston, D.L., and Large, R.R., 1989. A chemical model for the concentration of gold in volcanogenic massive sulfide deposits. Ore Geology Reviews, v. 4, p. 171-200. Hutchinson, R.W., 1990. Precious metals in massive base metal sulfide deposits. Geologische Rundschau, v. 79, no. 2, p. 241-263. Idziszek, C , Blackwell, J . , Fenlon, R., McArthur, G . and Mallo, D., 1990, T h e Eskay Creek discovery: Mining Magazine, March 1990, p. 172-173. Large, R.R., Huston, D.L., McGoldrick, P . J . , Ruxton, P.A., and McArthur, G . , 1989. Gold distribution and genesis in Australian volcanogenic massive sulfide deposits and their significance for gold transport models. Economic Geology Monograph 6., p. 520-535.  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, Vol. 91, pp. 1098-1114.  Nadaraju, G . , 1993, Triassic-Jurassic biochronology of the eastern Iskut River map area, northwestern British Columbia: Unpublished M . S c . thesis, Vancouver, T h e University of British Columbia, 268p.  7  Poulsen, K . H . , and Hannington, M . D . , 1995. Volcanic-associated massive sulfide gold; in Geology of Canadian Mineral deposit types, R . O . Eckstrand, W . D . Sinclair, and R.I. Thorpe (eds.), Geological Society of America, D N A G , v. P-1, Geology of C a n a d a , no. 8, p. 183-196.  Rogers, J . A . , 2002. The Eskay Creek gold and silver mine: Reserve opportunities and exploration success created from metallurgical diversity in a complex mineralized system in northwestern British Columbia. Abstract, Canadian Institute of Mining, Metallurgy and Petroleum, CIM Annual Meeting and Exhibition, Vancouver.  Roth, T., 1993a, Surface geology of the 21A Zone, Eskay Creek, British Columbia (104B/9W): Geological Fieldwork 1992, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1993-1, p. 325-330. Roth, T., 1993b, Geology, alteration and mineralization in the Eskay Creek 21A zone, northwestern British Columbia, Canada: Unpublished M.Sc. thesis, Vancouver, T h e University of British Columbia, 230p. Roth, T., 1995, Mineralization and facies variations in the Eskay Creek 21 Zone polymetallic sulphidesulphosalt deposit, northwestern British Columbia [abs.]: Victoria '95 G A C / M A C Annual Meeting, Final Program with Abstracts, Volume 20, p. A-91. Roth, T., and Godwin, C.I., 1992, Preliminary geology of the 21A zone, Eskay Creek, British Columbia. Geological Fieldwork 1991, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1992-1, p. 529-533.  Rye, K . A . , 1992, Geological and geochemical report on the 1991 Eskay Creek diamond drill relog program: Internal report for International Corona Corporation, 125 p.  Rye, K . A . , Edmunds, F . C . and Kuran, D.L., 1993, Geology of the Eskay Creek 21 Zone deposits [abs.]: The Cordilleran Round-Up, January, 1993, Vancouver, British Columbia.  Sherlock, R . L . , Barrett, T . J . , Roth, T., Childe, F., Thompson, J . F . H . , Kuran, D . L . , Marsden, H . , and Allen, R., 1994a, Geological investigations of the 21B deposit, Eskay Creek, northwestern British Columbia (104B/9W): Geological Fieldwork 1993, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1994-1, p. 357-364.  8  Sherlock, R.L., Barrett, T . J . and Roth, T., 1994b, Preliminary geologic and fluid inclusion investigations of the 21 Zone deposits, Eskay Creek, British Columbia [abs.]: Waterloo '94 G A C / M A C Annual Meeting, Final Program with Abstracts, Volume 19, p. A-107.  Sherlock, R.L., Barrett, T . J . , Bray, C . J . , and Spooner, E . T . C . , 1994c, The Eskay Creek Au-Ag-rich V M S deposit, northwestern British Columbia: Fluid inclusion evidence for a shallow water hydrothermal system [abs.]: Geological Society of America, Programs with Abstracts, v. 26, p. A-380. 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.  9  CHAPTER 2  T H E P R E C I O U S METAL-RICH E S K A Y C R E E K DEPOSIT, N O R T H W E S T E R N BRITISH COLUMBIA  Tina Roth, John F.H. Thompson, and Timothy J. Barrett Introduction The Eskay Creek Mine is an unusual polymetallic, precious-metal rich volcanic-associated massive sulfide and sulfosalt deposit which has generated much interest over the past decade. Located in the metallogenetically rich Iskut River area (Macdonald et al., 1996), 80 kilometers north of Stewart, British Columbia (Fig. 2.1), the deposit is distinct in style and mineralogy from other known deposits in the area and from other volcanic-associated massive sulfide (VMS) deposits. The deposit is hosted by a Middle Jurassic bimodal volcanic sequence on the west flank of an anticlinal structure, at the contact between felsic volcanic rocks and a mudstone-basalt package (Fig. 2.2). Economic concentrations of precious and base metals are contained in the 21 Zone, which is subdivided into a number of distinct sub-zones (Table 2.1). The bulk of the reserves are contained in the stratiform 21B zone, which is characterized by clastic sulfide-sulfosalt beds composed of sphalerite, tetrahedrite, lead sulfosalts, galena, pyrite, electrum-amalgam, and variable amounts of mudstone, chloritized and sericitized rhyolite, and barite fragments. In January 1995, Homestake C a n a d a Inc/Prime Resources Group Inc. commenced production from the 21B zone with a proven and probable mining reserve of 1.08 million metric tonnes (Mt) grading 65.5 grams A u per tonne and 2,930 grams Ag per tonne (g/t) (Prime Resources Group Inc., Annual Report 1994).  T a b l e 2.1: Zone  21A 21B ' NEX HW 1  2 3 4  4  Summary of ore reserves and metal resources in the Eskay Creek 21 zone. Tonnes  1,058,000 1,080,000 99,700 106,000  Au (g/t)  Ag (g/t)  8.9 65.5 40.3 20.6  96 2,930 2,447 1,426  Pb  Zn  (%)  (%)  n/a 2.89 n/a n/a  n/a 5.6 n/a n/a  Cu  (%)  n/a 0.77 n/a n/a  probable resource; Britton et al., 1990 proven and probable mining reserves of the 21B zone and East Block, including 27% dilution; Prime Resouces Group Inc. Annual Report, 1994 Homestake Canada Inc., Press Release, June 1993 probable resource; Homestake Canada Inc., Press Release n/a not available; (Values for the 21C, Pumphouse/Pathfinder and 109 zones are not available.) 1  2  3 4  10  11  LEGEND  Intrusive Rocks  Bowser Lake Group ] Marine sediments  | Felsite  Hazelton Group • ^  #21 Zone projected to surface  ^  Eskay Porphyry  Argillite & turbiditic sediments Hangingwall basalt Contact argillite (bedded argillite & siltstone)  —  Q  Rhyolite  ] Footwall volcanic unit (mixed volcaniclastics & andesitic flows or sills) [7j| Mudstone, sandstone and andesitic tuff [71 Argillaceous to conglomeratic sedimentary rocks  Mackay Adit  SCALE 800  1  400  0  400 800  metres TIE  Figure 2.2: Geological map of the Eskay Creek anticline showing the location of surface projections of the stratiform 21A and 21B zones and historical adits (compiled from mapping by C . Edmunds, D. Kuran, H . Marsden, A. Kaip and R. Bartsch).  12  T h e purpose of this paper is to describe the main features of the Eskay Creek deposit and to present a genetic model for the deposit. Although a syngenetic V M S model is widely accepted for Eskay Creek, the high grades of precious metals, and the unusual textural and geochemical features of the ore indicate that processes atypical of most V M S systems were active at the time of formation. The results and ideas presented in this paper represent the cumulative efforts of many exploration and research geologists (e.g. Blackwell, 1990; Britton et al., 1990; Idziszek et al., 1990; Roth and Godwin, 1992; Ettlinger, 1992; Bartsch, 1993a, b; Roth, 1993a, b; Rye et al., 1993; Edmunds et al., 1994; Sherlock et al., 1994a; Barrett and Sherlock, 1996; Childe, 1996).  History The Eskay Creek area has been explored intermittently since 1932. Much of the original prospecting was carried out by T o m MacKay, his family and associates. The MacKay and E m m a adits (Fig. 2.2) were driven on precious metal-rich stockwork veins during this initial period of exploration. From 1935 to 1938, Premier Gold Mining Company optioned the property and defined 30 mineralized prospects including the 21 zone, which was identified and named during a trenching program. Gold and silver-rich boulders containing orpiment and realgar were discovered in the 21 zone area at this time but were not followed up (Britton et al., 1990). Limited amounts of precious metals were extracted from vein mineralization in the rhyolite at various times, including 8.75 tonnes of hand-cobbed ore from the 22 zone that contained 1,263 g/t A u and 25,490 g/t A g plus lead and zinc (Britton et al., 1990). In the post war period (1945-1979), numerous companies explored the property with the emphasis changing from precious to base metals and volcanic-associated massive sulfide models (e.g., exploration by Texas Gulf in the 1970s). Exploration continued in the 1980s when a junior company, Kerrisdale Resources, drilled four holes near the 21 zone, including one that intersected the stratiform mineralization in the 21A zone (Edmunds et al., 1994). In 1988, a drill program by joint venture partners, Stikine Resources Limited and Calpine Resource Inc., confirmed the existence of Au-Ag-rich massive sulfide mineralization in the 21A zone. Follow-up geophysical and geochemical surveys in 1989 outlined a chargeability and geochemical anomaly that was tested by hole 109. This hole intersected 61 meters averaging 99 g/t A u and 29 g/t A g in an area that became known as the 109 zone (Britton et al., 1990). By mid-1990, 650 holes had been drilled and the 21B zone effectively defined. Since then, surface mapping, underground mapping, and  13  additional drilling have resulted in the definition of several subzones within the 21 zone defined by mineralogy, textures, occurrence and precious metal grade (Edmunds et al., 1994).  It took 19 companies and 57 years of sporadic exploration to discover a major economic ore body at Eskay Creek. The continuing interest in the area over such a long period relates to the presence of extensive visible iron staining and alteration, and localized precious metal-rich mineralization over a strike length of about 5 kilometers. The extent of this alteration and mineralization documents the size of the hydrothermal system in the Eskay Creek area. Although the system is large, the area of economic mineralization in the 21B zone is only 60 x 900 meters in plan. This is a small target for blind drilling and its delineation is further complicated by variable dip angles and structural disruptions. The Eskay Creek exploration history emphasizes the need for careful and sustained exploration in V M S environments.  Geological Setting T h e Eskay Creek property lies in the Stikine terrane near the western margin of the Intermontane tectonic belt (Anderson, 1989; Wheeler et al., 1988). T h e Stikine terrane consists of four assemblages: Upper Paleozoic metavolcanic and metasedimentary rocks, arc-related volcano-sedimentary complexes and related intrusions of the Triassic Stuhini Group and Jurassic Hazelton Group, and basinal sediments of the overlying Middle to Upper Jurassic Bowser Lake Group (Anderson, 1989; Britton et al., 1990). Mesozoic and Cenozoic plutonic suites intrude all of these assemblages. T h e Eskay Creek ore bodies are hosted by Middle Jurassic units in the upper part of the Hazelton Group.  T h e Hazelton Group is conventionally divided into four or five formations (Grove, 1986; Alldrick, 1991; Anderson, 1993). Difficulties in the use of the existing formations have led to problems of correlation and interpretation. For the purposes of this paper, five regional units within the Hazelton Group are described, similar to the approach adopted by Macdonald et al. (1996). The lowest unit in the Hazelton Group consists of fossiliferous conglomerate to sandstone that is separated by a disconformity or angular unconformity from the underlying Stuhini Group. T h e age of the lowest Hazelton Group unit is defined by the presence of Upper Hettangian to Lower Sinemurian ammonites (Nadaraju, 1993). The coarse clastic sediments are overlain in most places by a sequence of andesitic to dacitic flows, sills and volcaniclastic rocks with associated tuffs, greywackes and conglomerates. T h e age of this sequence, which is characterized by extensive variations in thickness and facies, is constrained by the ammonites in 14  the unit below, by Upper Pliensbachian fossils in the overlying unit, and by U-Pb dates from a flow of approximately 193 M a (Macdonald et al., 1996). T h e intermediate volcanic and volcaniclastic strata are overlain locally by a unit characterized by felsic volcanic flows, tuffs and breccias with an age range of about 194 to 185 M a (Macdonald et al., 1996). A mainly sedimentary unit occurs above the intermediate and felsic volcanic rocks. At Eskay Creek, this unit consists of bioclastic sandstone, but elsewhere it varies from limestone to sandstone, and is locally tuffaceous or conglomeratic. Fossil collections from this area span the Upper Pliensbachian to Aalenian (Nadaraju, 1993). T h e uppermost unit in the Hazelton Group is dominantly a bimodal volcanic assemblage with lesser tuffaceous, calcareous and argillaceous sedimentary rocks. T h e stratigraphic and geochemical character of this unit is different from the underlying volcanic units of the Hazelton Group (Macdonald et al., 1996; Barrett and Sherlock, 1996). Although previously divided into lower felsic volcanic (Mount Dilworth Formation) and upper basaltic volcanic-sedimentary packages (Salmon River Formation), as seen at Eskay Creek, these stratigraphic relationships are more complex regionally and are often reversed. Macdonald et al. (1996) have grouped the felsic and basaltic volcanic and sedimentary rocks of the upper part of the Hazelton Group as a single unit and suggest that they document intra-arc rifting. Large variations in the thickness of rock types, particularly sections of pillow basalt, and evidence for syndepositional faulting in the Eskay area suggest that an extensional environment existed at this time. In addition, the geochemistry of the volcanic rocks (Barrett and Sherlock, 1996) and their radiogenic isotopic signatures (Childe, 1994; 1996) are consistent with the involvement of primitive magmas probably related to rifting. T h e age of this upper unit is constrained by fossils and microfossils from intercalated sediments within the sequence, and from the units above and below, to be between Late Aalenian and Early Bajocian (Nadaraju, 1993). U-Pb dating of rhyolites in the Upper Hazelton Group at Eskay Creek and elsewhere in the Iskut River area, indicate a range of 181-172 M a (Childe, 1994, 1996; Macdonald et al., 1996; J . Mortenson, pers comm.).  T h e Hazelton Group is overlain by marine and terrestrial mudstones, sandstones and conglomerates belonging to the Bowser Lake Group. T h e s e basinal sedimentary rocks lack volcanic components and contain clasts of rock types from adjacent terranes, hence indicating a change in the local and regional tectonic setting.  15  Structure and Metamorphism Regional metamorphic grade in the Mesozoic rocks in the Iskut River area is lower greenschist facies (Britton et al., 1990). Metamorphism was probably related to Cretaceous deformation which formed the Skeena fold and thrust belt (Rubin et al., 1990; Evenchick, 1991). In the Iskut River area, deformation is characterized by regional upright anticlinoria and synclinoria, related thrust faults, mesoscopic folds and normal faults, and cleavage development. The McTagg anticlinorium is the dominant feature in the eastern part of the Iskut River area (Fig. 2.1).  T h e stratigraphy at Eskay Creek is folded about a shallowly north plunging, north-northeast trending upright open fold, termed the Eskay Anticline (Fig. 2.2). This is located on the imbricated western limb of the McTagg anticlinorium. Stratiform mineralization dipping at 3 0 ° to 4 5 ° occurs on the western limb of this fold, which is disrupted by a series of north to northeast trending faults. Cleavage is well developed in sedimentary units, and appears to be axial planar to both the Eskay Anticline, and to smallscale, possibly parasitic folds (Rye et al., 1993). Edmunds and Kuran (1992) recognized at least two phases of deformation on the Eskay Creek property. T h e earliest imparts a 0 3 0 ° - trending pervasive cleavage which is axial to the Eskay anticline and discordant to bedding. This early deformational event implies a 1 2 0 ° - trending compressive regime. A later event has locally reoriented cleavage and formed north and north-northeast trending normal faults. The later event implies that compression was directed north-northeast to south-southwest.  Structural analyses and field relations indicate mineralization in the 21 Zone predates cleavage development (Rye et al., 1993). Several porphyroblastic phases, including prehnite, calcite, pyrite and sphalerite, locally overgrow cleavage and provide evidence for a metamorphic overprint of probable Cretaceous age.  Mine Sequence T h e Eskay Creek deposit is located on the western limb of the Eskay anticline, close to the fold closure (Fig. 2.2). T h e stratigraphy and facies relationships at the Eskay Creek deposit and its vicinity have been described by Blackwell (1990), Bartsch (1993a, b), Roth (1993a,b), Edmunds et al. (1994) and Sherlock et al. (1994a). T h e stratigraphic succession of the mine (Fig. 2.3) comprises, from base to top,  16  mudstone-siltstone turbidites; thicklybedded sandstone and lensoidal conglomerate layers; containing Upper Bathonian to Lower Callovian ammonoids tujfaceous mudstone massive and pillowed basalt flows, sills, and volcanic breccia; intercalated mudstone and tuff beds; contain Lower Bajocian bivalves and Aalenian, possibly Lower Bajocian, radiolaria  mudstone containing Aalenian, possibly Lower Bajocian, radiolaria massive, flow banded to brecciated rhyolite overlain by rhyolitic volcaniclastic rocks; U-Pb age: 173.6+5.6/-0.5 Ma (Childe, 1994) mudsiane intermediate pyroclastic and epiclastic rocks intercalated mudstone, siltstone, sandstone, subordinate conglomerate, and minor andesitic tuff; containing Upper Pliensbachian ammonoids andesite breccia and heterolithic volcaniclastic rocks  Figure 2.3: Schematic stratigraphic section in the Eskay Creek area showing the relative distribution of the different mineralized zones within the 21 zone. The N E X zone is not shown; it occurs near the rhyolite-mudstone contact and is spatially related to the 21B zone.  17  andesite, marine sedimentary rocks, intermediate to felsic volcaniclastic rocks, rhyolite flow domes, carbonaceous shale, and basalt. Most of the significant mineralization occurs in carbonaceous shale, termed the 'Contact Mudstone', which occurs immediately above the rhyolite (Fig. 2.3). The strata are disrupted by faults that strike north-northeast, notably the Argillite Creek, Pumphouse, Portal and East Break faults (Edmunds and Kuran, 1992).  Lower Footwall Units Coarse monolithic andesite breccia and heterolithic volcaniclastic rocks are exposed in the core of the Eskay anticline and are the oldest rocks in the mine sequence. T h e andesite is overlain by marine shales and interbedded coarse clastic sedimentary, volcaniclastic and calcareous rocks. A northward change from shallow marine sandstone and conglomerate, to deep marine shale-dominant sedimentary facies was suggested by Bartsch (1993b). Shales in this horizon contain bivalves and ammonites of Late Pliensbachian age (Nadaradju, 1993).  A sequence of volcaniclastic rocks, with compositions that vary from dacite to basalt (Rye, 1992), overlie the marine sedimentary rocks. This unit was previously termed the 'footwall dacite' (Britton et al., 1990; Blackwell, 1990; Edmunds and Kuran, 1992), but due to its variable geochemistry, it is termed more appropriately 'the footwall volcanic unit' (Roth, 1993b; Sherlock et al. 1994a). This unit comprises pumice-rich block and lapilli tuffs, and heterolithic epiclastic rocks which locally contain abundant fossils, including ammonites, brachiopods, molluscs, belemnites and possible wood fragments. A distinctive amygdaloidal, aphanitic flow or sill called the 'datum dacite' forms a marker horizon near the top of the sequence. T h e composition of this marker unit ranges from basaltic andesite to dacite (Roth, 1993b; Barrett and Sherlock, 1996). The footwall volcanic unit is capped by a thin (<3m thick) black mudstone horizon.  Rhyolite Rhyolite forms the immediate footwall to the 21B stratiform deposit and is host to stringer-style discordant mineralization and locally intense alteration. In this area, hydrothermal alteration and related brecciation have destroyed much of the original rock fabric and volcanic minerals. However, the presence of locally preserved flow banding, flow lobes, breccias, hyaloclastite, spherulites and perlitic textures allowed Bartsch (1993a, b) to define several facies within the rhyolite which he interpreted to represent  18  portions of flow-dome complexes. T h e s e facies include basal and peripheral fragmental felsic rocks commonly containing pumiceous clasts, outer zones dominated by chaotic autobrecciated flow banded rhyolite, and central zones of massive to flow-banded rhyolite. T h e total thickness of the sequence ranges up to 200 meters.  The upper contact of the rhyolite with the mudstone is locally marked by a black-matrix breccia, consisting of matrix-supported white rhyolite fragments in a siliceous black matrix (Rye, 1992; Bartsch, 1993a, b; Roth, 1993b). T h e rhyolitic fragments have irregular concave surfaces and locally show jigsawfits with adjacent fragments. T h e s e textures indicate in situ fragmentation, which combined with the siliceous argillite matrix, suggest that the breccias are peperites formed by the intrusion of some rhyolite lobes into unconsolidated wet sediments. This interpretation implies that rhyolitic volcanism was at least partly synchronous with argillaceous sedimentation and that the rhyolitic flow domes were partly intrusive.  T h e rhyolite and black matrix breccia are overlain by black mudstone or graded volcaniclastic sedimentary rocks which are intercalated with, and grade into, mudstone. T h e volcaniclastic beds are dominated by rhyolitic fragments demonstrating that the rhyolite flow domes were also extrusive (Roth, 1993b, 1995). Intervals of coarser rhyolite breccia associated with these clastic intervals are interpreted to be debris flows. T h e thickest accumulations of rhyolitic volcaniclastic detritus occur in the immediate footwall to the 21B zone clastic ore (Rye 1992; Roth, 1995), suggesting that a basin or trough developed in this area prior to mineralization.  Contact Mudstone T h e Contact Mudstone occurs between the rhyolite and the overlying basalt, and is the host to the stratiform mineralization of the 21 A, B, C and Hanging Wall (HW) zones. T h e unit ranges from less than 1 meter to more than 60 meters in thickness (Rye, 1992; Britton et al., 1990) and consists of a laterally extensive, well-laminated, carbonaceous mudstone that is variably calcareous and siliceous. Thin siltstone, sandstone and ash beds and pyritic laminae are common throughout the mudstone. Radiolaria, dinoflagellates, rare belemnites and corals have been identified, indicating deposition in a marine environment during Aalenian to Bajocian time (Nadaraju, 1993). Radiating porphyroblasts of prehnite, variably altered to sericite, calcite and barite (Ettlinger, 1992) are locally abundant. These are generally restricted to individual beds within the contact mudstone and in mudstone intercalated with the  19  overlying basalt. T h e s e porphyroblasts may have formed as a result of contact metamorphism due to emplacement of basaltic sills.  Basalt The hanging wall basalt is the uppermost unit in the Eskay stratigraphy, and locally exceeds 150 meters in thickness, generally thinning southward away from the deposit (Britton et al., 1990). The basalt occurs as both extrusive and intrusive phases, and ranges from aphanitic to medium-grained with local feldspar phenocrysts. Basaltic sills and dikes are common near the base of the sequence. Where these intrude the argillite, their contacts are often brecciated and peperitic; in some cases, the sills merge laterally into a breccia that extends up to 5 meters beyond the massive basaltic body. The well developed mafic peperitic breccias indicate that mafic magmas were intruded into unconsolidated wet mudstone. Higher in the hanging-wall basalt sequence, well-preserved pillow flows and breccias, hyaloclastite, and basaltic debris flows with minor mudstone and rhyolite clasts are intercalated with thin argillite intervals. Chlorite and quartz-filled amygdules are common and tend to be concentrated at the upper contacts of basalt flows (Rye, 1992).  Intrusive Rocks Several intrusions are exposed in the Eskay Creek area. The largest is a monzodiorite sill or stock, called the Eskay Porphyry, which is exposed in the core of the Eskay anticline (Fig. 2.2). The Eskay Porphyry has a U-Pb zircon age of 184 +5/-1 M a (Macdonald et al., 1992; Childe, 1996), and therefore predates the age of the Eskay rhyolite and mineralization in the 21 Zone by 5-10 million years.  O n the west limb of the anticline, intrusive felsic rocks form a series of prominent, gossanous bluffs extending about 7 kilometers to the south of the Eskay Creek deposit (Fig. 2.2). T h e intrusive rocks cut the mine stratigraphic succession and reach their highest stratigraphic level directly below the 21 zone (Edmunds and Kuran, 1992; Bartsch, 1993b). They are aphanitic and strongly altered to an assemblage of quartz, pyrite, potassium feldspar and minor sericite. Geochemically, the felsic intrusive rocks are indistinguishable from the rhyolite (Rye, 1992; Bartsch, 1993b; Roth, 1993b). Bartsch (1993b) suggested these intrusive rocks may represent feeder dikes to a linear rhyolite flow-dome complex. Based on additional drill information, Edmunds et al. (1994) suggested that they represent a sill-like feature.  20  Mafic dikes and sills cut the footwall and hanging wall stratigraphy around Eskay Creek, and are geochemically indistinguishable from the hanging wall basalts (Rye, 1992; Bartsch, 1993b; Roth, 1993b). The mafic dykes within the footwall stratigraphic succession are significantly less altered than the surrounding host rocks.  Primary Geochemistry of the Volcanic Rocks The volcanic rocks in the lower part of the Hazelton Group are mainly intermediate in composition with a calc-alkaline magmatic affinity (Rye, 1992; Marsden and Thorkelson, 1992; Bartsch, 1993b). These rocks, and interbedded sedimentary units, were deposited over an extended time period in the Early Jurassic, from about 200 to 180 M a , in an arc environment which is interpreted to have consisted of emergent volcanic edifices and intervening basins (Macdonald et al., 1996). Both the environment and nature of magmatism changed with the deposition of the upper part of the Hazelton Group in the Middle Jurassic. At Eskay Creek, the footwall rhyolite is severely altered, but immobile trace element ratios (Ti, Zr, Nb and Y) and rare earth element (REE) patterns indicate that it was of near-uniform composition prior to alteration (Fig. 2.4). Zr/Y ratios and R E E data suggest that the Eskay rhyolite is of tholeiitic affinity and is similar in chemistry to other Phanerozoic rhyolites related to V M S mineralization in which the volcanism is interpreted to reflect rifting of a continental margin. A n extensional rifting environment is also postulated for the upper part of the Hazelton Group (Macdonald et al., 1996), although the basement in this case is dominated by the older arcs of the Stikine terrane. The R E E patterns of least altered Eskay rhyolite are also similar to the Fill-type rhyolites of Lesher et al. (1986) which host some Archean V M S deposits in the Superior Province.  Trace element and R E E geochemistry of the basalt in the hanging wall at Eskay Creek demonstrates that they are of tholeiitic affinity and were derived from relatively unfractionated mantle melts of N - M O R B character, possibly with a small E - M O R B component (Barrett and Sherlock, 1996). The chemistry of the basalts is consistent with a rifting rather than a subduction-related tectonic setting.  21  (a)  (b)  140  O  Sericite 120  Tholeiitic (Zr/Y=1.4-3.6)  100  E O  80  a. S  oo 0  o o % O.  >-  ,o  60  \ Mg-chlorites  • o  40  50  Ti0 %  100  150  2  O  o  '  200  250  300 350  Zr ppm  Rhyolite precursor  o Rhyolites Basalt flows, sills  Rhyolite alteration line  (C)  1000 r  Eskay rhyolite 100  32 oo 10  Eskay basalt  ~i  i  La Ce  i  i  i  i  i  i  Nd Sm Eu Gd Tb  r  T  i — i  r  Yb Lu  REE  Figure 2.4: (a) Al 0 versus Ti0 plot. The immediate host rocks in the 21 Zone consist of rhyolite footwall and basalt hanging wall (with some 10 to 20 meters of intervening mudstone and fine sandstone). Least-altered rhyolites are strongly fractionated, with very low Ti0 contents. Although most rhyolites are altered in the vicinity of the 21 Zone deposits, immobile element ratios involving Al, Ti and Zr remain essentially unchanged (with very few exceptions), and indicate that all of the altered rocks are derived from essentially one precursor composition. The mafic rocks are unaltered to weakly altered. Rhyolites with mass gain have been variably affected by silica addition, which in some cases represents more than half of the analysed rock. Mass loss occurs in zones of strong sericitization and/or chloritization, where large losses of Si, Na and Ca more than offset smaller gains in K, Fe and Mg. (b) Y versus Zr plot. The Zr/Y ratios of both the basalts and rhyolites suggest that they are of tholeiitic magmatic affinity (this assessment is corroborated by REE data and other immobile trace element ratios). The rhyolites show wide variations in absolute Y and Zr values due mainly to mass gain-loss effects (as also evident shown in Fig. 4a). In a few extremely chlorite-altered rhyolites, some mobility of Y has occurred, (c) Chondrite-normalized REE plot. Eskay basalts have near-flat patterns indicating that they are relatively unfractionated mantle melts (normal MORBs would show moderate depletion in the light REE). This is also supported by their relatively high MgO, Cr and Ni contents. The rhyolites have high overall REE contents with slight enrichment in the light REE. Rhyolites with these features occur in rifted continentalmargin and some rifted-arc settings. They could be generated by melting of relatively primitive lower crust, although other explanations are possible. Although least altered rhyolites have moderate negative Eu anomalies, the anomalies have been enhanced by alteration in many samples. (From Barrett and Sherlock, 1996). ? 2  3  2  2  2  Although both the rhyolite and basalt at Eskay Creek are of tholeiitic affinity, the relationship between their source magmas is uncertain. Barrett and Sherlock (1996) noted that differences in certain immobile element ratios between rhyolites and basalts are inconsistent with the derivation of the rhyolite solely by fractional crystallization of the basalt. As an alternative, they suggest that the rhyolite may have been derived from the partial melting of crustal rocks of tholeiitic affinity. Regardless of the relationship between the Eskay basalt and rhyolite, their affinity is different from the transitional and calc-alkaline affinities typical of regionally correlative rocks, and also volcanic rocks deeper in the Eskay Creek footwall. These observations support the inferred regional change from an arc setting to an extensional environment during the Middle Jurassic (Macdonald et al., 1996) and suggest a more focused source of primitive magmas was tapped in the Eskay area, possibly related to deep extensional structures.  Alteration Host rocks in the footwall rhyolite and underlying volcanic rocks of the 21 Zone stratiform mineralized zones are strongly to intensely altered with few primary textures remaining (Ettlinger, 1992; Bartsch, 1993b; Barrett and Sherlock, 1996). Alteration in the footwall volcanic unit commonly comprises pervasive quartz-sericite-pyritepotassium feldspar±chlorite. Zones of intense alteration are associated locally with sulfide veins including pyrite, sphalerite, galena and chalcopyrite. In the footwall rhyolite, alteration assemblages vary greatly over short distances. Rhyolite lateral to the area of stratiform ore is typically altered to K-feldspar with moderate silicification. This type of alteration also occurs in the deeper parts of the footwall beneath the 21 zone. Fractures cutting through K-feldspar-silica altered rhyolite typically have sericitic alteration envelopes, often with very fine-grained pyrite. Chlorite appears in and around the fractures as the alteration becomes more intense. The most intense alteration occurs near the upper contact with stratiform mineralization, as a tabular blanket of pervasive chloritization and sericitization. Within this zone, Mg-chlorite has completely replaced the precursor rhyolite to form a dark green, waxy rock consisting of clinochlore. The tabular shape of the intense chlorite-sericite alteration zone below the stratiform mineralization coincides spatially with a thickening of volcaniclastic rhyolitic rocks in the immediate 23  footwall of the 21B zone and extensive brecciation in the upper portion of the rhyolite (deeper portions of the rhyolite are dominated by massive to flow banded facies). The tabular zone of intense alteration therefore may be situated where increased permeability in the fragmental and brecciated rocks allowed greater access to hydrothermal fluids, and provided a greater surface area for fluid-rock interaction. The rhyolite locally contains a black, carbon-rich material which is accompanied by silicification. This style of alteration occurs under the north end of the 21B zone (associated with the 109 zone) and locally under the south end of the 21B zone (not associated with mineralization). However it has not been observed under the bulk of the deposit. Where the carbon-rich alteration is pervasive and it affects flowbanded rhyolite, the banded texture is strongly enhanced. Locally the carbon alteration produces wispy to dendritic textures. In the 109 zone, some of the carbon was remobilized into younger quartz veins during later silicification. Rare earth and trace element studies have shown that virtually all of the rhyolite in the footwall of the 21 zone was derived from a single precursor (Barrett and Sherlock, 1996). The alteration of the rhyolite resulted in strong chemical changes which range from net mass gain in silicified zones to net mass loss in sericitized and chloritized zones. The deposition of quartz in open spaces within porous rhyolite and cross-cutting quartz veins has resulted in significant net mass gains relative to the unaltered precursor. Zones of extreme chlorite-sericite alteration show a net mass loss (addition of MgO and K 0, 2  but major depletion of Si0 ). The chlorite-sericite alteration is interpreted to reflect areas where seawater 2  was drawn laterally into discharging relatively acidic fluids; clinochlore formed where seawater influx dominated for a sustained interval while sericite formed where ascending hydrothermal fluids dominated (Barrett and Sherlock, 1996). The K-feldspar-silica alteration is interpreted as having formed in response to cooler, near-neutral conditions peripheral to, or below, the main feeder zones. In general, the hanging wall basalt is weakly altered chemically. In areas of moderate chloritesericite alteration, the basalts exhibit lower Na and Ca and higher K and MgO than unaltered equivalents (Table 2.2; Macdonald et al., 1993; Barrett and Sherlock, 1996).  24  Table  2.2: Average values for major and trace elements in least altered rhyolite and basalt at Eskay Creek.  Major Elements Rock Si0  2  TiOz  3  MgO  CaO  Na 0  %  %  %  %  %  %  %  %  46.22  1.65  15.57  12.22  8.26  75.98  0.07  13.20  1.51  6.03 0.68  0.35  3.93 4.64  4.93 2.14  Trace Elements Rock Cu ppm  Ni ppm  V ppm  Cr 0 ppm  Zr ppm  Y ppm  Nb ppm  Sr ppm  77 6  342 48  354 83  78 192  37 88  3 44  200 29  Basalt  1  Rhyolite  Basalt Rhyolite  2  48 14  1  i. 1  2  2  Al 0 2  3  Fe 0 2  2  3  K 0  2  2  .  Least altered values in basalt were calculated from 3 samples: C-93-714: 131.2, C-93-714: 141.4 and U-60: 43.9 Least altered values in rhyolite are from Rye, 1992.  Mineralization Several styles of mineralization are present in the Eskay Creek area. Early exploration efforts were focused on precious metal mineralization in sulfide veins within the rhyolite, felsic intrusions and the footwall volcanic unit. Following the recognition of the important stratiform mineralization in the 21 Zone, drilling for more than a kilometer to the north of the discovery holes outlined several styles of stratiform and discordant mineralization. These can be subdivided (Edmunds et al., 1994) into a number of zones which reflect varying locations, mineralogy, textures, and'precious metal grades (Fig. 2.5, Tables 2.1 and 2.3). Due to the complexity of these mineralized zones, the nomenclature applied to some of the mineralized zones has varied somewhat during the past eight years as the number of geologists studying the deposit and the amount of available information increased. In this review, some new terms, such as the East Block, are presented to clarify the nature of the mineralized zones as they are currently understood. As mining and drilling in and around the Eskay Creek deposit proceeds, the nomenclature may continue to evolve further. The spatial distribution of the mineralized zones within the Eskay Creek 21 Zone.as they are currently understood, is shown in Figures 2.3 and 2.5. The 21 A, 21B and 21C zones occur at the same stratigraphic horizon, within carbonaceous mudstone at or near the contact with the underlying rhyolite sequence (Fig. 2.3). The bulk of the ore in the Eskay Creek mine is hosted in the stratiform 21B zone. Fault-complicated mineralization, possibly related to the 21B zone, occurs in the East Block and NEX zones. The Hanging Wall (HW) zone occurs stratigraphically above the north end of the 21B zone, also 25  Table 2.3: Summary of mineralization styles in the Eskay Creek 21 Zone Zone  Associated elements  Characteristics  Stratigraphic Position  21A  As-Sb-Hg-Au-Ag  S t r a t i f o r m l e n s of m a s s i v e t o s e m i - m a s s i v e s u l f i d e s (realgar, stibnite, c i n n a b a r , arsenopyrite) underlain by disseminated stibnite, a r s e n o p y r i t e , t e t r a h e d r i t e a n d v e i n l e t s of pyrite, s p h a l e r i t e , g a l e n a , tetrahedrite, ± chalcopyrite.  s t r a t i f o r m a t b a s e of c o n t a c t mudstone; overlying discordant mineralization within rhyolite  21B  Au-Ag-Zn-Pb-Cu-Sb  Stratiform, b e d d e d clastic sulfides a n d sulfosalts including: sphalerite, tetrahedrite freibergite, Pb-sulfosalts (including boulangerite, boumonite, jamesonite), stibnite, g a l e n a , pyrite, e l e c t r u m , a m a l g a m .  s t r a t i f o r m , a t b a s e of c o n t a c t mudstone  East Block  Ag-Au-Zn-Pb-Cu  F i n e - g r a i n e d m a s s i v e to l o c a l l y c l a s t i c sulfides a n d sulfosalts. M a s s i v e pyritef l o o d i n g in rhyolite g r a d i n g u p w a r d s into m a s s i v e sulfides a n d sulfosalts.  within f a u l t - b o u n d e d block, mainly at contact between rhyolite a n d m u d s t o n e  NEX  Au-Ag-Zn-Pb-Cu  Similar to the E a s t Block a n d locally the 2 1 B z o n e , with f e w e r s u l f o s a l t s a n d l o c a l o v e r p r i n t of c h a l c o p y r i t e s t r i n g e r s .  s t r a t i f o r m , at b a s e of c o n t a c t mudstone  21C  B a (Pb-Zn-Au-Ag)  B e d d e d m a s s i v e t o b l a d e d barite a s s o c i a t e d with v e r y f i n e - g r a i n e d d i s s e m i n a t e d s u l f i d e s i n c l u d i n g pyrite, t e t r a h e d r i t e , s p h a l e r i t e a n d g a l e n a . U n d e r l a i n b y l o c a l i z e d z o n e s of cryptic, d i s s e m i n a t e d , p r e c i o u s - m e t a l b e a r i n g m i n e r a l i z a t i o n in t h e rhyolite.  s t r a t i f o r m , a t b a s e of c o n t a c t m u d s t o n e ; a n d d i s c o r d a n t in t h e rhyolite  Hanging Wall (HW)  Pb-Zn-Cu  M a s s i v e , fine-grained stratabound sulfide l e n s d o m i n a t e d by: pyrite, s p h a l e r i t e , galena, & chalcopyrite (mainly a s stringers) This zone h a s generally lower gold - silver g r a d e s a n d higher b a s e metals relative to t h e 21 z o n e s .  within c o n t a c t m u d s t o n e ; a t a higher stratigraphic level than t h e 21 z o n e s  Pumphouse & Pathfinder  Fe-Zn-Pb-Cu  V e i n s of pyrite, s p h a l e r i t e , g a l e n a , a n d t e t r a h e d r i t e . C o m m o n l y b a n d e d ; l o c a l l y with c o l l o f o r m t e x t u r e s . L o c a l z o n e s of v e r y f i n e g r a i n e d m i n e r a l i z a t i o n in rhyolite.  d i s c o r d a n t , within rhyolite; spatially underlying the 2 1 B zone  109  Au-Zn-Pb-Fe  V e i n s of q u a r t z , s p h a l e r i t e , g a l e n a , pyrite  d i s c o r d a n t , within rhyolite  a n d v i s i b l e g o l d a s s o c i a t e d with s i l i c a flooding a n d fine-grained a m o r p h o u s c a r b o n alteration.  26  Figure 2.5: Plan view of the spatial disribution of the mineralized subzones within the Eskay Creek 21 zone (modified from Edmunds and Kuran, 1992). Refer to the text and Table 2.3 for a description of each zone.  27  within mudstone, and is locally cut by unmineralized basaltic dikes and sills. Discordant, disseminated and vein-style mineralization is present in the rhyolite footwall below and adjacent to the 21B zone in the Pumphouse-Pathfinder and 109 zones (Fig. 2.3). Mineralization in rhyolite also occurs in the immediate footwall of the 21A and 21C stratiform zones. 21A Zone The 21A zone is a small, Au-Ag-rich sulfide lens located approximately 200 meters south of the 21B orebody (Fig. 2.5) on the flank of a small (50 x 70 meters) depression in the rhyolite-mudstone contact. Mineralization in the 21A zone averages 10 meters in thickness and is bounded on the east by a probable growth fault (Roth and Godwin, 1992; Roth, 1993b). The 21A zone contains an estimated resource of 1.06 million tonnes grading 8.9 g/t Au and 96 g/t Ag (Table 2.1; Britton et al., 1990). A discontinuous stockwork of intense Mg chlorite alteration in the rhyolite forms a pipe which underlies the stratiform mineralization. The lens consists of semi-massive to massive stibnite-realgar±cinnabar±arsenopyrite which are hosted in mudstone at the contact with underlying rhyolite. The lens locally contains angular mudstone fragments. Disseminated stibnite, arsenopyrite and tetrahedrite occur in the immediate footwall of the sulfide lens within intensely sericitized rhyolite. Cinnabar occurs mainly in late fractures cutting the sulfide lens, the surrounding mudstone, and locally the underlying rhyolite. Realgar-calcite veinlets locally cut the mudstone in a restricted area adjacent to the sulfide lens. 21B Zone The 21B zone is a stratiform tabular body of Au-Ag-rich mineralization which is about 900 meters long, 60 to 200 meters wide and locally in excess of 20 meters thick. The ore comprises beds of clastic sulfides and sulfosalts with variable amounts of barite, rhyolite and mudstone clasts. Gold and silver occur as electrum and amalgam with silver mainly occurring in sulfosalts. In the thickest, proximal part of the orebody, pebble to cobble-sized clasts occur in thick beds overlying rhyolite. The thick-bedded, coarse-grained sulfide-sulfosalt beds form a northward trending channel. The beds rapidly grade laterally into thinner, finer grained, clastic beds and laminations. In the core of the deposit, stibnite occurs locally as veins and replacement of the clastic ore.  28  The clasts dominantly consist of sulfides and sulfosalts including sphalerite, tetrahedrite, freibergite, various lead-sulfosalts (including boulangerite and bournonite), galena, pyrite, and rare electrum or amalgam. The proportion, texture and grain size of sulfide-sulfosalt minerals is variable between clasts and between beds. Individual beds may be variably dominated by sphalerite or sulfosalts. Locally, banded sulfide fragments are observed, suggesting formation in a sulfide mound or chimney which was subsequently fragmented and reworked. Pervasively sericite-chlorite altered rhyolite and mudstone clasts are common in the sulfide-sulfosalt rich debris. Imbricated, laminated mudstone rip-up clasts have been observed locally at the base of the clastic sulfide-sulfosalt beds, indicating turbiditic emplacement of some beds (Fig. 2.6). Barite clasts are present in some sulfide-sulfosalt beds, often in close association with calcite. In general, barite clasts are more common towards the north end of the deposit. In one area within the 21B zone, several meters of massive, mottled barite were intersected in drillcore. Adjacent drillholes showed no barite accumulation, indicating that the lateral dimension of the massive barite could be no more than 10 meters in diameter. This suggests the formation of barite mounds was active locally within the depositional sub-basin. Locally, beds of massive and clastic, reworked barite occur within the bedded sulfide-sulfosalt sequence. These beds, comprising barite and calcite, are generally found near the base of the ore and range from 10 centimeters to 1 meter in thickness. These baritic horizons are typically low in precious metals. Facies variations laterally and vertically in the 21B zone are defined by changes in clast size, composition and bedding thickness (see examples in Fig. 2.6). Clast size and bed thickness typically decrease stratigraphically upwards, progressively thinning to fine laminations and disseminations of sulfides and sulfosalts. Thick, clastic, ore beds, each up to 1 meter thick, near the base of the sequence commonly appear amalgamated (uninterrupted by intervening mudstone). These accumulations of clastic sulfides and sulfosalts may have formed by deposition from successive debris flows or turbidites, whereby the upper portion of existing beds were truncated and removed by the subsequent flows. Beds of intercalated black mudstone generally become thicker (up to 1 meter) toward the top of the ore sequence as sulfide-sulfosalt beds become progressively thinner (decreasing to fine laminations 1 millimeter thick). In the coarsest beds, near the base of the ore sequence, sulfide-sulfosalt cobbles locally reach 10 centimeters in diameter. More typically, the base of the sequence 29  Figure 2.6 : Examples of clastic sulfide-sulfosalt ore from the 21B zone. Scale bar = 2 cm. a) Imbricated laminated mudstone rip-up clasts at the base of a coarse, clastic heterolithic ore bed. The lighter fragments comprise mainly honey coloured sphalerite. Grey fragments are mudstone and chloritized rhyolite; b) Unsorted, coarse heterolithic ore bed from the core of the orebody, containing angular to subrounded fragments of fine-grained sulfides and sulfosalts, chloritized and sericitized rhyolite and mudstone; c) Bedded clastic sulfides and sulfosalts in mudstone from an area laterally adjacent to the coarse, clastic core of the deposit; d) Thin, graded beds of sulfides and sulfosalts showing load and flame structures at the base of the beds, and small entrained mudstone fragments within the beds.  30  near the core of the deposit contains poorly sorted ore fragments up to 2 centimeters across. The size of the sulfide-sulfosalt clasts rapidly decreases upwards to <1 to 5 millimeters near the top of each graded bed and overall towards the top of the ore sequence. Sedimentary structures are locally well preserved in the sulfide-sulfosalt beds, with load and flame structures at the base of some beds. Successive clastic ore beds may be alternately graded or ungraded, and poorly to well sorted. The precious metal grades generally decrease proportionally with the decrease in total sulfides and sulfosalts. In most places there is a relatively abrupt vertical transition (<1 meter thick) between the last ore lamination and barren mudstone(<1 g/t) containing no visible sulfides other than minor pyrite laminations. However, ore-grade values locally persist into macroscopically barren mudstone above the last visible sulfide-sulfosalt lamination. Precious-metal-enriched mudstone in these areas usually contains microscopically disseminated sulfides and rarely, tiny specks of visible gold. The silver values in these areas are usually low, relative to the rest of the 21B zone, due to the lack of sulfosalts. The clastic ore beds also show lateral facies changes similar to the vertical changes described above. The thick, amalgamated, coarse-grained sulfide-sulfosalt beds grade laterally (generally over less than 10 meters) into finer-grained, thinner beds, generally 1 to 10 centimeters thick, which are usually separated by 1 to 10 centimeters of mudstone (Fig. 2.6). The geometry of the thickly bedded ore suggests that the coarser sulfide-sulfosalt clasts were deposited in a northward-trending channel. The adjacent, thinner, finer-grained ore beds were probably deposited on the margins of this channel. More distally, the sulfide-sulfosalt beds thin to very fine laminations with increasing thicknesses of intervening mudstone. Locally, ore-grade precious metal mineralization on the margins of the orebody is hosted in fine-grained sulfides disseminated in black mudstone. This is similar to the macroscopically barren mudstone described above, which occurs above the clastic ore beds in some areas. The ore horizon is remarkably continuous over several hundred meters of strike length, but individual sulfide-sulfosalt or mudstone beds cannot be traced for more than about 10 meters. This lack of local continuity may be due to both primary depositional processes and the subsequent structural overprint. Both the northward-trending main channel and smaller-scale channels may have restricted the areal distribution of some of the clastic beds. In addition, numerous faults, some of which are shown in Figure 2.2, dissect the 21 zone and locally drag the ore into the fault gouge. Individual ore beds are 31  locally boudinaged or truncated by narrow faults subparallel to cleavage. Offsets along these faults are typically on the order of centimeters to a few meters. Larger graphitic fault zones, subparallel to bedding, commonly truncate the upper limit of the ore. On a smaller scale, the primary sedimentary textures are obscured in some areas by penetrative cleavage associated with folding of the Eskay Anticline, which has also variably flattened the sulfide-sulfosalt clasts parallel to cleavage. Finally, sulfosalts are recrystallized at the microscopic scale, which commonly obscures the clastic texture. The complete textural preservation of the full vertical succession from thick, coarse clastic sulfide-sulfosalt beds at the base of the ore zone to thin laminations at the top has been seen in a only few drill holes. Stibnite also occurs within the 21B zone. It is generally restricted to veinlets and stringers which crosscut bedding, as rims on sulfide-sulfosalt clasts, and as complete replacements of individual beds within the clastic ore. The zone of stibnite replacement is generally confined to the central, thickest part of the 21B orebody, suggesting a locus of late hydrothermal activity spatially related to the original feeder conduit. Relict sphalerite grains, aligned parallel to bedding, are locally observed within massive stibnite "beds". Rarely, the stibnite is accompanied by cinnabar. Precious metals do not appear to have been introduced with the stibnite, and commonly, zones where stibnite has pervasively replaced the mineralization may be subeconomic by Eskay Creek standards; i.e., grades are considerably lower than those typical of thickly bedded sulfides and sulfosalts in the core of the deposit (on the order of 200 g/t Au and 20,000 g/t Ag). East Block  Near the north end of the 21B zone, precious metal-rich mineralization extends over the top of the anticline and is caught up in a series of parallel north-south faults of the Pumphouse and Pathfinder fault zones. Within this fault block lies a complicated zone of mineralization that was originally termed the Pathfinder zone (Edmunds and Kuran, 1992; Edmunds et al., 1994) and is currently referred to, by mine staff, as the #8 Stope-East Limb area. Because this zone of mineralization was initially thought to represent discordant mineralization hosted mainly in the footwall rhyolite, the term "Pathfinder Zone" has been applied by several workers (Roth, 1995; Sherlock et al., 1994a, b; Childe, 1996; Barrett and Sherlock, 1996) to discordant vein-style mineralization in the same area that may be related to discordant mineralization in the Pumphouse Zone, further to the south. In view or recent underground observations, the term East Block mineralization will be used in this review. 32  The East Block is a complexly folded and faulted zone of precious metal-rich mineralization that appears to occur in a steeply dipping, fault bounded slab of mudstone, massive pyrite, and chloritic and sericitic rhyolite located immediately east of the Pumphouse fault (Edmunds et al., 1994). Early drill results indicated that this zone was hosted mainly in rhyolite and represented a discordant zone of mineralization which may have been a feeder to the 21B zone. However, further drilling suggested that the mineralization was stratabound or stratiform at the same horizon as the 21B zone (Edmunds and Kuran, 1992). Some of the mineralization is similar to that in the 21B zone, however the bulk of it is steeply dipping and dominated by fine-grained, massive sulfosalts that grade downward into massive pyrite. Ore-grade concentrations of precious metals are confined mainly to the sulfosalt-rich portion of the mineralization. The Ag/Au ratio in this zone is about 100 times that in the 21B zone. The sulfosalts are commonly overprinted by stringers of chalcopyrite and fine-grained mixtures of chalcopyrite, galena and sphalerite. Fine-grained pyrargyrite occurs locally in hairline fractures cutting mudstone and hosts oregrade mineralization. This style of mineralization is generally difficult to see; it resembles a fine, darkreddish dust on the surface of the fracture. The textures in the East Block mineralization suggest that the pyrite and fine-grained sulfides and sulfosalts were introduced by replacement processes, perhaps controlled along early faults. NEX Zone  Surface drilling in 1995 led to the discovery of the North Extension (NEX) zone. This zone occurs at the nose of the Eskay Anticline; its geometry is complicated by a number of faults associated with the fold closure and is still poorly understood. Textures, mineralogy and precious metal grades in the NEX zone are somewhat variable and locally similar to those in the East Block. In some places, the mineralization resembles distal parts of the 21B zone; however, pyrite and chalcopyrite are more prevalent and Sb-Hg rich minerals are less common. Chalcopyrite occurs mainly in stringers that overprint earlier clastic mineralization and which are probably related to the formation of the HW zone, discussed below. Much of the pyrite may also have been introduced during this later event. The formation of the NEX zone is probably synchronous with the formation of the 21B zone and also related to replacement synchronous with mineralization in the East Block.  33  21C Zone  The 21C zone comprises mainly stratabound to stratiform barite-rich mineralization with associated disseminated base and precious metal-rich mineralization in the rhyolite footwall. The 21C zone has not yet been fully evaluated and is poorly understood. It occurs at the same stratigraphic horizon as the 21B zone and is down-dip and subparallel to it. The two zones are separated by a barren interval of contact mudstone. Mineralization in the 21C zone is generally associated with mottled barite-calcite ±tetrahedrite beds in and near the base of the contact mudstone. Locally, the mudstone is brecciated and infilled with sulfides including sphalerite, pyrite, galena and tetrahedrite. Precious metal grades are variable. Mineralization in the underlying rhyolite forms a cryptic tabular body, subconcordant to stratigraphy. The mineralization is not visible macroscopically and the rhyolite looks like much of the surrounding unmineralized host rock, apart from containing 1 to 2% very fine-grained pyrite and traces of sphalerite, tetrahedrite and galena (Edmunds et al., 1994). However, drill holes intersecting this visually unspectacular zone of mineralization may contain intervals with over 35 g/t Au. Sericitically altered rhyolite in the footwall to the 21 zone contains similar quantities of sulfides, without elevated precious metal values. Hanging Wall Zone  At the northern end of the 21B zone, a second interval of stratiform massive sulfide mineralization, the HW zone, is present within the contact mudstone stratigraphically above the north end of the 21B zone. The geometry of the HW zone is disrupted by fault structures associated with the fold closure. Texturally the HW zone is more typical of classical massive sulfide deposits. Sulfides are typically fine grained, finely banded, and consist of semi-massive to massive pyrite, sphalerite, galena, chalcopyrite and tetrahedrite. Whereas the sphalerite in the 21B zone has a pale honey yellow colour, the sphalerite in the HW zone is generally reddish to brown, suggesting a higher iron content. Overall, the HW zone contains lower precious metal grades than does the clastic 21B ore (Table 2.1). However, where tetrahedrite (± other sulfosalts) is locally dominant, the precious metal grades are significantly higher.  34  Pumphouse and Pathfinder Zones  The Pumphouse and Pathfinder zones are discordant zones of diffuse vein and disseminated sulfide mineralization hosted in rhyolite underlying the 21B zone. Precious metal grades are generally lower than in the other zones. Patchy sulfide mineralization is observed locally throughout the rhyolite immediately underlying the 21B zone mineralization. The veins comprise mainly pyrite, sphalerite and galena with lesser sulfosalts such as tetrahedrite. The occurrence of chalcopyrite increases with depth. In general, sphalerite in footwall veins is darker in colour than sphalerite within the 21B zone, indicating a higher iron content. Colloform banding of pyrite has been observed locally, suggesting open-space filling of the veins. Locally, footwall mineralization is characterized by areas of extremely fine-grained disseminated sulfides enriched in precious metals, similar to the footwall mineralization described below the 21C zone. The Pumphouse and Pathfinder zones are believed to be associated with the hydrothermal feeder system which formed the sulfides of the 21B zone. The geometry of these zones is still somewhat poorly understood as their limits are currently defined by the grade of precious metal and the economic cutoffs. 109 Zone  The 109 zone is named after its discovery hole which intersected 61 meters of 99 g/t Au and 29 g/t Ag (Edmunds et al., 1994). This zone is characterized by a distinctive, siliceous stockwork of crustiform quartz veins with coarse-grained, zoned sphalerite, galena, minor pyrite and chalcopyrite (Blackwell, 1990; Sherlock et al., 1994a). The zone occurs entirely within the rhyolite underlying the north end of the 21B zone and the HW zone. The 109 zone is associated with abundant black carbonaceous material disseminated throughout the rhyolite and locally remobilized into the quartz veins. Gold and silver occur in electrum and sulfosalts. Summary of Mineralization  At Eskay Creek, stratiform mineralization is hosted by the contact mudstone whereas discordant mineralization is hosted mainly by the footwall rhyolite. Because exploration and development have focused on the 21B zone, mineralization in the other zones is less well understood. The 21B zone shows textural evidence for the clastic deposition of sulfides and sulfosalts with associated precious metals. The thickest accumulation of clastic ore in the 21B zone occurs above an 35  accumulation of volcaniclastic rocks near the top of the underlying altered and mineralized rhyolite. The clastic ore appears to have been deposited in a northward trending basin or sub-basin that may have been controlled by synvolcanic faulting. Coarse clastic sulfides were deposited in the center of the basin with finer-grained material on the margins. Mineralized beds grade out rapidly to the east and west, across the trend of the basin, but less rapidly along the trend of the basin. Surface exposures of the contact mudstone, as little as 60 meters up-dip from the 21B zone, are unmineralized. A similar change from coarse to fine clastic ore beds also occurs stratigraphically upwards through the 21B zone. The sedimentary textures in successive sulfide-sulfosalt beds vary considerably, suggesting a variety of mechanisms were active, to emplace the clastic ore beds. The intervening mudstone beds are a mixture of lithified pelagic and volcaniclastic sediment and often contain pyrite-rich laminations. The ocurrence of pelagic mudstone between successive ore beds indicates that sulfide-sulfosalt debris was emplaced into the basin by episodic events, separated by intervals of unknown duration. The accumulation of ore beds was nonetheless sufficiently rapid that the host mudstones remained unconsolidated, at least until the intrusion of basaltic dikes and sills. The other stratiform zones (21 A, 21C, NEX) show some of the sedimentary clastic features of the 21B zone, although stibnite, realgar and cinnabar are more abundant in the 21A zone, and barite is more abundant in the 21C zone. The HW zone differs from the other stratiform zones; it occurs at a higher stratigraphic level within the contact mudstone, is relatively chalcopyrite-rich and is characterized by massive to semi-massive sulfides. Chalcopyrite-rich stringers in the NEX zone and the East Block area suggest that some of the Cu in these areas was introduced relatively late, although the relationship between these stringers and the HW zone is unknown. Discordant mineralization of the Pumphouse, Pathfinder and 109 zones (Figs. 2.3, 2.5) in the footwall rhyolite may represent feeders to stratiform mineralization in the 21 zone. The Pumphouse and Pathfinder zones are of similar mineralogy and may represent a single feeder system which has been offset along the Pumphouse fault (Fig. 2.2; Edmunds and Kuran, 1992). Sulfide mineralization occurs in these zones as veins and disseminations of sphalerite-pyrite -galena-tetrahedrite. The 109 zone consists of crustiform quartz-sphalerite ± pyrite, galena, chalcopyrite, sulfosalts and electrum veins, many of which also contain black carbonaceous material. The 109 zone is located stratigraphically below the HW zone, 36  but the differing mineralogy and relatively low fluid temperatures in the 109 zone (see discussion below) are not consistent with the 109 zone being a feeder to the HW zone. Hydrothermal Fluids Widespread hydrothermal alteration occurs in the footwall rhyolite and parts of the lower footwall units at Eskay Creek. Alteration in the footwall is dominated by pervasive quartz-sericite-pyrite-potassium feldspar ± chlorite. Early alteration in the rhyolite comprised quartz and potassium feldspar. This assemblage is cut by strong sericitic alteration (Bartsch, 1993b). The most intense sericite and chlorite alteration occurs close to the upper contact of the rhyolite, immediately below the thickest accumulation of felsic volcaniclastic rocks under the central part of the 21B zone. A distinct alteration style is prevalent in the 109 zone as fine, black carbonaceous material associated with silicification. The tabular zone of intense chlorite-sericite alteration below the 21B zone may reflect an area of focused hydrothermal discharge and mixing with seawater. The tabular geometry may reflect the increased permeability associated with an accumulation of volcaniclastic rocks near the top of the rhyolite sequence. Addition of MgO and K 0 and severe depletion of Si0 in zones of extreme chlorite-sericite 2  2  alteration may have resulted from seawater being drawn laterally into discharging relatively acidic fluids. Zones of massive waxy clinochlore were probably dominated by the influx of seawater, whereas sericiterich alteration probably represents areas where ascending hydrothermal fluids were dominant. Whole rock 5 0 values from zones of quartz-K feldspar alteration in rhyolite are 11 -13 per mil compared to 8-9 18  per mil for unaltered rhyolite (Barrett and Sherlock, 1996). In contrast, intensely chloritized zones have significantly lower 8 0 values of ~5 per mil Barrett and Sherlock (1996) have interpreted these 18  differences largely in terms modal mineralogy, particularly the addition or removal of quartz. The 8 0 18  data suggest temperatures of alteration in the 150° to 200°C range, assuming that equilibrium between rocks and fluids was attained. Fluid inclusion homogenization temperatures from discordant quartz veins in the footwall rhyolite and from amber sphalerite in the overlying clastic sulfide-sulfosalt beds of the 21B and 21A zones are uniformly low, ranging from 120 to 210°C (Sherlock et al., 1994b, c; 1999). In the 109 and 21B zones, homogenization temperatures average less than 150°C, whereas those in the 21A zone are slightly higher. Ice melting temperatures suggest fluid salinities in the range of 2.6 to 10.5 wt. percent NaCl 37  equivalent. Gas chromatography on fluid inclusion volatiles indicates overall low gas contents (<1 mole %) with variable C0 -CH -N ratios (Sherlock et al., 1994b,c). Ratios of C0 /CH and C0 /N in the fluid 2  4  2  2  4  2  2  inclusions are consistent with model boiling curves, suggesting that the hydrothermal fluid underwent at least sporadic liquid-vapor phase separation. Calculations of fluid pressures using gas ratios and fluid inclusion homogenization temperatures suggest that phase separation may have occurred at water depths as shallow as 160 meters (Sherlock et al., 1994b). However, the calculated depth is affected by the partial pressures of N and CH in the fluid inclusions. Taking this factor into account, the depth of 2  4  water at which boiling may have occurred is probably about 1000 meters (Sherlock et al., 1999). Lower-salinity fluids in the fluid inclusions are consistent with typical modified seawater-dominant hydrothermal vent fluids (Sherlock et al., 1994b,c). However, the higher-salinity fluids have lower Br/CI and Na/CI ratios and higher K/CI ratios than typical seawater-dominated vent fluids. These ionic ratios suggest that the original seawater-dominated hydrothermal fluids were modified by the addition of salts in proportions consistent with magmatic ratios. This may have occurred by direct magmatic input or by leaching of the underlying volcanic rocks (Sherlock et al., 1994b,c; 1999). Childe (1996) has suggested that the lead isotopic data from mineralization at Eskay Creek is consistent with a direct magmatic contribution associated with Middle Jurassic rhyolitic magmatism, although a significant contribution from the leaching of these and other rocks is not ruled out. The sulfur isotope values for sulfides from the 21B, HW and footwall zones range from -3.13 to -0.20 per mil (Sherlock et al., 1999). These values are typical of many VMS deposits and are generally interpreted to reflect the reduction of seawater sulfate with the possibility of some magmatic contribution (e.g., Ohmoto and Rye, 1979). Cinnabar, realgar and stibnite collected from the 21A zone have heavier 8 S values ranging from 1.39 to 11.81 per mil. Barite from all zones ranges between 21.8 and 27.3 per 34  mil. The estimated equilibrium temperatures determined from mineral pairs range between 190° and 497°C (Sherlock et al., 1999). Most of this range is inconsistent with fluid inclusion homogenization temperatures, suggesting that the sulfur isotope data do not reflect equilibrium. The Eskay Creek Model Most of the significant mineralization at Eskay Creek formed on the sea floor in an active volcanic environment, during an interval of mainly pelagic sedimentation. Classification of the orebody as a 38  volcanic-associated or volcanic-hosted massive sulfide deposit is therefore appropriate. Unusual features of the deposit (Table 2.4) include the predominance of clastic sulfide-sulfosalt mineralization, the extremely high precious metal grades, and the elevated levels of other elements (Sb, Hg, As). These features require a specialized model for the genesis of Eskay Creek mineralization, as discussed below and shown schematically in Figure 2.7. The Eskay Creek deposit formed during the waning stages of volcanism in a Lower to Middle Jurassic Hazelton arc. The lower footwall units at Eskay Creek are dominated by fragmental and tuffaceous volcanic rocks of intermediate calc-alkaline composition. Most units show evidence for subaqueous deposition although coarse pumiceous breccias, beds of accretionary lapilli and the presence of large wood fragments all suggest proximity to land. Based on geochronological and fossil data, these units were deposited 5-10 m. y. prior to the formation of the Eskay Creek deposit. In the Middle Jurassic, bimodal volcanism, which became important throughout the Iskut River area, is interpreted to reflect the extension and rifting of the Hazleton arc (Macdonald et al., 1996). At Eskay Creek, this volcanic event is marked by the emplacement of rhyolitic flow domes, the footwall rhyolite, and the subsequent intrusion and extrusion of basaltic magmas, the hanging-wall basalts. The tholeiitic geochemistry of the rhyolites and basalts in the Eskay Creek area is consistent with rifting of the older parts of Hazelton arc and underlying arc sequences of the Stikine terrane (Macdonald et al., 1996; Barrett and Sherlock, 1996). Although the rhyolites and basalts have similar tholeiitic affinities, it is not clear if they are comagmatic. Regardless, their distinctive chemistry at Eskay Creek, relative to regional volcanic rocks, suggests the importance of significant local, deep rift structures that probably also focused hydrothermal activity.  Table 2.4:  Summary of the dominant characteristics of the Eskay Creek deposit, including those typical of volcanogenic massive sulfide (VMS) deposits, and unusual characteristics, some of which are more commonly associated with epithermal mineralization.  VMS characteristics  Unusual characteristics  h o s t r o c k s : rhyolite f o o t w a l l ; b a s a l t h a n g i n g w a l l  a s s o c i a t i o n with S b - H g ± A s s u i t e  submarine setting  h i g h c o n c e n t r a t i o n s of A u - A g  footwall chlorite - sericite alteration  l o w t e m p e r a t u r e s of f o r m a t i o n  s u l f i d e f o r m a t i o n at t h e s e a f l o o r i n t e r f a c e  i m p o r t a n c e of c l a s t i c s u l f i d e - s u l f o s a l t o r e  39  Rhyolite flow d o m e s  Figure 2.7: Block model for the development of the Eskay Creek 21 zone orebodies. a) Rifting, basin development, and intrusion and extrusion of rhyolite flow domes. Coarse volcaniclastic debris from extrusive portions of the rhyolite domes are deposited along the developing 21B zone trough, b) Hydrothermal activity is focused through rift faults forming chimneys and mounds on the seafloor. Collapse or disruption of these mounds forms clastic sulfide-sulfosalt debris which is redeposited in the 21B zone trough. Other smaller basins provide the sites for similar mineralization and barite-rich zones related to white smokers, c) The HW zone of massive sulfide forms higher in the mudstone stratigraphy and basaltic magmatism begins (dikes and flows) during the waning stages of hydrothermal activity. 4 n  The Eskay Creek rhyolite occurs as a series of flow domes, related breccias and volcaniclastic rocks; it may have been emplaced from a linear fissure now marked by the dikes which form prominent gossanous bluffs in the footwall sequence. The occurrence of peperitic breccias flanking portions of the rhyolite domes indicates that domes were partly intrusive into unconsolidated pelagic sediments. Autoclastic breccias and volcaniclastic rocks were generated from extrusive portions of the rhyolite domes with debris flows being concentrated into a north trending trough or basin (Fig. 2.7a). The contact mudstone is carbonaceous and commonly contains pyritic laminae both in the immediate Eskay area and where it occurs more regionally. This suggests that an extensive reduced basin developed during the Middle Jurassic, probably coincident with postulated extensional rifting and bimodal volcanism. Peperitic breccias on some of rhyolite contacts indicate that pelagic sedimentation was initiated prior to rhyolitic volcanism. Microfossils in the mudstone document the submarine setting for the Eskay Creek mineralization, but unfortunately, neither micropaleontological data nor sedimentary textures and the geochemistry of the mudstone provide clear constraints on the depth of water. The mudstone in the Eskay area becomes more calcareous upwards, suggesting a change in basinal conditions. Basaltic volcanism in the Eskay Creek area was initiated towards the end of mudstone sedimentation, although its precise timing has not been established. Peperitic breccias around many of the basaltic dikes and sills indicate that the mudstones were unconsolidated. The presence of these breccias around both rhyolites and basalts suggest that the gap between rhyolitic and basaltic volcanism was relatively small. Regionally, the stratigraphic relationships between felsic and basaltic volcanism in the Upper Hazelton Group suggest that they were erupted more or less synchronously in the Iskut River area. The hydrothermal system at Eskay Creek was probably initiated soon after the emplacement of the rhyolite domes. The hydrothermal fluid was dominated by seawater, some of which was probably drawn down into the upper part of the system causing strong magnesian chlorite alteration of the rhyolite. Some components in the hydrothermal fluid may have been derived directly from rhyolitic magmas. Clastic sulfide-sulfosalt beds of the 21B zone were deposited in the north-trending trough already partially filled with volcaniclastic debris (Fig. 2.7b). The precise location of the source for the clastic 41  sulfide-sulfosalt material is uncertain, but the spatial association with intense alteration and mineralization in the footwall suggests that it was within or immediately adjacent to the trough that hosts the 21B zone. It is likely that the sulfides and sulfosalts formed initially in mounds or chimney structures similar to those in modern black smoker environments. The mineralization formed at low temperatures (120°-210°C) and water depths of less than 1000 meters, and possibly as shallow as 160 meters. At the temperatures indicated for mineralization at Eskay Creek, gold is stable as a bisulfide complex. An effective means of destabilizing bisulfide complexes and precipitating gold is by boiling, which would have partitioned H S 2  into the vapor phase (Seward, 1989; Henley, 1991). Boiling at relatively low temperatures may also at least partly explain the high contents of precious metals and the anomalous levels of Sb, Hg and As in the 21 Zone (cf. Spycher and Reed, 1989). The mound and chimney structures must have collapsed at recurring intervals, redistributing broken and fragmented clasts within the north-trending basin. The fragmentation of mounds may have resulted from a combination of: (1) physical instability due to dissolution and oversteepening, (2) earthquake activity, or(3) hydrothermal eruption through the mounds due to boiling in the hydrothermal system and overpressuring. Poorly sorted, chaotic beds may represent deposition directly from hydrothermal eruptions. Such a mechanism could also explain the more widespread distribution of some of the ore beds as a result of distal fine-grained fallout. Regardless of the importance of hydrothermal eruptions versus collapse and slumping, the resultant debris and turbidite flows were focused along the north-trending trough. Lateral facies changes reflect the geometry of the trough, whereas similar vertical facies changes suggest a gradual decline in hydrothermal activity during pelagic sedimentation. During and following the deposition of sulfide-sulfosalt beds, and burial by pelagic mudstone, the hydrothermal system may have evolved or became reactivated resulting in the local replacement of clastic sphalerite-tetrahedrite-rich mineralization in the 21B zone by stibnitelcinnabar. Precious metals were not introduced during this event. This late replacement of 21B zone mineralization may have resulted from lower temperature fluids, possibly condensates, which carried no gold and silver. The 21A and 21C zones show some similarities to the 21B zone, but with additional stibniterealgar-cinnabar and barite respectively. Although they occur at the same stratigraphic level as the 21B zone, the relative timing of mineralization in each zone is uncertain. These zones may represent 42  mineralization in separate sub-basins. It is possible that stibnite-realgar-cinnabar mineralization in the 21A zone formed at the same time as the areas of stibnite replacement in the 21B zone. The local importance of clastic barite beds in both the 21B and 21C zones suggests that white smokers were active in the area, particularly in, or adjacent to the northern part of the 21B trough. The timing and nature of the formation of the East Block and NEX zones remains uncertain. Within each of these zones are areas of laminated or bedded mineralizaton that are similar in character and stratigraphic position to the 21B zone. However, the more massive, fine grained portions of these zones may represent a somewhat later hydrothermal event in which precious metal-rich sulfides replaced earlier stratiform mineralization along controlling fault structures. The HW zone occurs stratigraphically higher in the contact mudstone (Fig. 2.7c) suggesting that the hydrothermal system operated over a significant time period at Eskay Creek. It is not possible to quantify this period because the contact mudstone is a mixture of pelagic and tuffaceous material and hence the rate of ambient sedimentation is difficult to establish. The higher Cu content of the HW zone suggests that higher fluid temperatures prevailed during this stage of the hydrothermal activity. Basaltic dikes cross-cut the HW zone. These dikes probably fed basaltic flows interbedded with pelagic mudstone in the hanging wall to the HW zone (Fig. 2.7c). Weak alteration occurs in the basalts suggesting that they were emplaced during the waning stages of hydrothermal activity, probably less than 2 to 3 m. y. after rhyolitic magmatism and the initiation of the hydrothermal system. The environment of deposition for Eskay Creek, particularly the importance of water depths which may be less than 1000 meters, may explain some of the features of mineralization at Eskay Creek that differ from more typical VMS deposits. Neither this environment nor the resultant processes, however, are likely to be unique. Potential exists for the discovery of deposits similar to Eskay Creek, particularly in complex arc environments characterized by concurrent tectonic and magmatic activity, basin development, and subaerial to subaqueous conditions. In such systems, volcanism can occur in relatively shallow marine settings. The importance of magmatic-hydrothermal systems in such areas is uncertain, but such systems are likely to be active in many arc environments. Consequently, the type of mineralization found at Eskay Creek may be present in arc terranes that are currently known mainly for porphyry and related types of deposits. 43  )  Acknowledgments Numerous geologists have contributed to the understanding of the geology at Eskay Creek. In particular, the following are acknowledged for their past work and ideas: R. Bartsch, J. Blackwell, R. Britten, F. Childe, C. Edmunds, A. Ettlinger, A. Kaip, D. Kuran, P. Lewis, G. MacArthur, A.J. Macdonald, H. Marsden, G. Nadaraju, R. Sherlock, and K. Rye. Present mine staff are thanked for their help and support to the senior author. We are grateful to Prime Resources Group Inc. and Homestake Canada Inc. for permission to publish this paper. The research described in this paper was largely funded through the MDRU project on Volcanogenic Massive Sulfide Deposits of the Cordillera, which is funded by mining companies (Barrick Gold (incorporating Lac Minerals), Homestake, Granges, Inco, Inmet (previously Metal and Minnova), Kennecott, Placer Dome, Teck, TVI, Westmin, WMC International), the Science Council of British Columbia and the Natural Science and Engineering Research Council of Canada. This paper is MDRU contribution number 80. References Alldrick, D.J. 1991, Geology and ore deposits of the Stewart Mining camp, British Columbia: Unpublished Ph.D. thesis, Vancouver, University of British Columbia, 347 p. 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