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Geology and genesis of the Mount Skukum tertiary epithermal gold-silver vein deposit, southwestern Yukon… McDonald, Bruce Walter Robert 1987

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GEOLOGY AND GENESIS OF THE MOUNT SKUKUM TERTIARY EPITHERMAL GOLD - SILVER VEIN DEPOSIT, SOUTHWESTERN YUKON TERRITORY (NTS 105D SW) By BRUCE WALTER ROBERT MCDONALD B. Sc. (Hons.), The Universi ty of B r i t i sh Columbia, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Geological Sciences We accept th is thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Apr i l 1987 © Bruce Walter Robert McDonald, 1987 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 The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date |VW (ST* ; flE7 ii Abstract The Tertiary Mt. Skukum gold - silver epithermal vein deposit occurs 65 km southwest of Whitehorse in the Yukon Territory. Veins are hosted bv a sequence of nearly flat- lying Eocene Skukum Group andesitic volcanic rocks of the Mt Skukum Volcanic Complex, part of the Sloko Volcanic Province .which unconformably overlies these intrusive complexes as well as metamorphic rocks of the Yukon Group. Maior known mineralized zones occur within a regional halo of propylitic alteration centered on a fault-bounded graben within Main Cirque in the southwestern corner of the Mt. Skukum Volcanic Complex. Each zone consists of steeply-dipping quartz-carbonate-sericite veins associated with major faults and rhyolite dykes which bound blocks of the graben. Precious metals occur as electrum and native silver as fine grains averaging 15 to 20 microns and locaIiy exceeding l mm across, in veins containing only trace amounts of sulphides. Fluid inclusions indicate that vein minerals were deposited from hydrothermal fluids averaging •313"C with an average salinity of 0.7 weight percent NaCl equivalent. Primary inclusions show that depositional fluids existed under two pressure regimes; one close to hydrostatic, the other approaching lithostatic. Both reflect depths of deposition of about 470 m below paleosurface. Variable fluid pressures reflecting similar depths of deposition combined with variable liquid to vapour ratios in primary inclusions as well as abundant textural evidence of hydrothermal brecciation indicate that boiling was common during mineralization. Oxygen and carbon isotope composition of minerals in the deposit and surrounding wall rocks indicate that depositional fluids were meteoric in origin with no contribution from magmatic sources. Large depletions in 0 1 S content of andesitic rocks in the deposit area indicate a minimum v/ater : rock ratio over the lite of the. deposit of 0.81:1. Precious metals at the Mt. Skukum deposit were emplaced at relatively low temperature in a near surface environment by a circulating, meteoric water dominated, hydrothermal system driven by a heat source associated with the rhyolite dykes. Gold, leached from andesitic volcanic rocks and metamorphic and granitic rocks was precipitated with quartz and carbonate in permeable conduits such as fault zones, and breccia bodies. iv Table of Contents Page, Abstract ii List of Tables viii List of Figures .' x Acknowledgements xix 1.0. Introduction 1.1. Location and Access 1 1.2. Climate and Physiography 1 1.3. Previous Work and Exploration History 3 1.4. Obiectives 3 1.5. Fieldwork 4 2.0 Regional Geology 2.1. Introduction and Tectonic Setting 5 2.2. Geology 8 2.2.1. Geological Setting of the Mt. Skukum Volcanic Complex 8 2.2.2. Geology of the Mt. Skukum Volcanic Complex 10 2.2.3. Geoehronology - 2 0 3.0. Geology of Mine Area 3.1. Introduction 23 3.2. Geology and Petrography of Lithologies in Main Cirque 28 3.2.1. Formation 2 28 3.2.2. Formation 3 38 V •Page. 3.2.3. Formation 4 44 3.2.4. Igneous Intrusive Rocks 47 3.3. Whole Rock Geochemistry of Igneous Lithologies in Main Cirque 55 3.3.1. Introduction 55 3.3.2. Classification 55 3.3.3. Discussion 60 3.4 Structure 62 4.0. Mineralization and Alteration 4.1. Introduction 66 4.2. Mineralization of the Main Cirque Zone 69 4.2.1. Introduction 69 4.2.2. Character of Veins in the Main Cirque Ore Zone 69 4.2.3. Ore Petrology of the Main Cirque Zone 73 4.2.4. Distribution of Gold in the Main Cirque Ore Zone 84 4.2.5. Interpretation 92 4.3. Alteration 95 4.3.1. Introduction 95 4.3.2. SupergeneAlteration 95 4.3.3. HypogeneAlteration 100 4.3.4. Hypogene Alteration Facies 102 4.3.5. Discussion 112 5.0. Hydrothermal Environment of Deposition 5.1. Introduction ' 18 5.2. Fluid Inclusions 119 vi 5.2.1. Introduction and Objectives 119 5.2.2. Data Collection 119 5.2.3. Homogenization Data 121 5.2.4. Freezing Data 121 5.2.5.. Pressure Correction 126 5.2.6. Interpretation 127 5.3. Oxygen Isotope Composition of Hydrothermal Minerals, Fluids, and Surrounding Host Rocks 137 5.3.1. Objectives 137 5.3.2. Data 137 5.3.3. Isotopic Composition of Hydrothermal Fluids 139 5.3.4. Water to Rock Ratio 141 5.3.5. Geothermometry ; 142 5.3.6. Conclusions T143 5.4. Carbon Isotopes 149 5.4.1. Introduction 149 5.4.2. Data 149 5.4.3. Conclusions 150 6.0. Conclusions 6.1. Origin 152 6.2. Deposit Model 157 6.3. Consequences of the Proposed Model 164 Bibliography 165 Appendix A 171 Appendix B Appendix C Paoe. .173 .174 List of Tables Table 2.1. Potassium-argon data from whole rock analysis of andesite from the Mt. Skukum property, Main Cirque Zone area, Yukon Territory 21 Table 3.1. Average visually estimated mineralogical modes of igneous rocks present in Main Cirque, Mt. Skukum, Yukon Territory 29 Table 3.2. Major element chemistry of rhyolite dyke (unit 8.) from Main Cirque (Figure 3.1), Mt. Skukum, Yukon Territory. Major element analyses are reported as oxides in weight percent; zirconium analyses are reported in parts per million 56 Table 3.3. Major element chemistry of porphyritic andesite flow rxks(unit 3) from Main Cirque (Figure 3.1), Mt. Skukum, Yukon Territory. Major element analyses are reported as oxides in weight percent; zirconium analyses are reported in parts per million 57 Table 4.1. Hypogene alteration facies and zones at the Mt. Skukum deposit, Yukon Territory 101 Table 4.2. Alteration minerals in rocks of Main Cirque listed in order of abundance as determined through petrography and x-ray diffraction analysis of representative specimens of alteration facies at the Mt. Skukum deposit, Yukon Territory. Numbers in table indicate the number of specimens in which minerals occured with a particular relative abundance 115 Table 5.1. Homogenization and freezing data for fluid inclusions from quartz veins at Mt. Skukum, Yukon Territory 122 Table 5.2. Summary of homogenization temperature (T h) data from fluid inclusions in vein samples, Mt. Skukum, Yukon Territory 121 Table 5.3. Summary of freezing data from fluid inclusions in vein samples, Mt Skukum, Yukon Territory : 121 Table 5.4. Oxygen isotope data From the Mt. Skukum deposit, south-central Yukon Territory 138 Table 5.5. Oxygen isotope composition of hydrothermal fluids at Mt. Skukum, Yukon Territory. Calculations are from data in Table 5.4 and equations 1,2 and 3 in text 140 Table 5.6. Water to rock mass ratio calculations for Mt. Skukum, Yukon Territory. Calculations are from data in Table 5.5 and equation 4 in text 142 Table 5.7. Calculated isotopic temperature of deposition from mineral pairs obtained from Mt. Skukum, Yukon Territory. Calculations use data from Table 5.4 and equation 5 in text 143 .8. Carbon isotope composition of hydrothermal minerals at Mt. Skukum, Yukon Territory List of Figures Page. Figure 1.1. Location map of the Mt. Skukum area showing access ro3d and proximity to Whitehorse, Yukon Territory 2 Figure 2.1. Regional geology of the Mt. Skukum Volcanic Complex surrounding area include the Bennett Lake Cauldera Complex in the Yukon Territory (after Wheeler, 1961 and Pride, 1985) 6 Figure 2.2. Generalized geology map of the Mt. Skukum Volcanic Complex, Yukon Territory, (after Pride, 1985) 11 Figure 2.3. Stratigraphy in the western portion of the Mt. Skukum Volcanic Complex, south-central Yukon Territory. The Mt. Skukum gold deposits occur in Formation 3. 12 Figure 2.4. Photograph of felsic welded tuff typical of volcanic rocks from the lower portion of Formation 2 in the Mt. Skukum Volcanic Complex, Yukon Territory 14 Figure 2.5. Photograph of maroon andesitic ashfall accretionary lapilli tuff typical of volcanic rocks in the upper portion of Formation 2 in the Mt. Skukum Volcanic Complex, Yukon Territory, Arrows on the rock point to two accretionary lapilli near the pen top. 15 Figure 2.6. Photograph of well bedded epiclastic units typical of the upper portion of Formation 2 in the Mt. Skukum Volcanic Complex, Yukon Territory. As pictured here, epiclastic units in the upper portion of this formation are mainly derived from volcanics of andesitic composition 16 Figure 2.7. Photograph showing a typical section of Formation 3 as seen in a cliff exposure near Main Cirque, Mt. Skukum, Yukon Territory. Repetitive, laterally extensive flows of porphyritic andesite are visible as pale and dark coloured bands running across the cliff. Pale bands represent relatively highly altered zones of interflow breccia, dark bands represent relatively unaltered bands which correspond to massive, unfractured flow centers. The eastern wall of Main Cirque and the reddish-brown gossan of the Alunite Cap Zone is seen in the background 18 Figure 2.8. Photograph of monolithic andesite breccia which forms a thick sequence at the top of Formation 3 in the Mt. Skukum Volcanic Complex, Yukon Territory 18 Figure 3.1. Geologic map of Main Cirque, Mt. Skukum, Yukon Territory. 24 Figure 3.2. Geologic cross section of Main Cirque, Mt. Skukum, Yukon Territory. Section is based on field observation and the geologic map (Figure 3.1) 25 Figure 3.3, Photograph of the Main Cirque looking north from the peak of Mt. Skukum, Yukon Territory. Locations of the three major mineralized zones Lake Zone (LZ), Brandy Zone (BZ), and Main Cirque Zone (MZ) are indicated ,.,,26 xi Pare. Figure 3.4. Photograph of a typical outcrop of rhyolite lapilli tuff of Formation 2 from Mt. Skukum, Yukon Territory 31 Figure 3.5. Photograph of rhyolite ash tuff of Formation 2 showing concentrically layered accretionary lapilli from Mt. Skukum, Yukon Territory 31 Figure 3.6. Photograph of handspecimen/outcrop exposure of a lahaar of Formation 2 showing the polymictic content of angular to sub-rounded fragments from Mt. Skukum, Yukon Territory 32 Figure 3.7. Photograph of typical felsic volcaniclastic sandstone from the lower portion of Formation 2 showing the coarse grained nature of some of these rocks at Mt Skukum, Yukon Territory 34 Figure 3.8. Photograph of a typical outcrop exposure of maroon andesite ash tuff from the upper portion of Formation 2 at Mt. Skukum, Yukon Territory 34 Figure 3.9. Photograph of abundant accetionary lapilli common in andesitic ash tuff in the upper portion of Formation 2 at Mt. Skukum, Yukon Territory. Accretionary lapilli occur only in the fine-grained maroon ash layer in this specimen 36 Figure 3.10. Photograph showing a typical specimen of andesitic lapilli tuff from the upper portion of Formation 2 at Mt. Skukum, Yukon Territory 36 Figure 3.11. Photograph of distinctive, coarse andesitic lapilli tuff which forms the top horizon of Formation 2 with grey-green porphyritic andesite fragments enclosed in a fine grained hematitic andesite matrix at Mt. Skukum, Yukon Territory ' 37 Figure 3.12. Photograph of porphyritic andesite flow rocks of unit 2 (lower Formation 3), Mt. Skukum, Yukon Territory showing the characteristically low phenocryst content and maroon colour of these rocks 40 Figure 3.13. Photograph of a typical outcrop of porphyritic andesite flow rock showing characteristic phenocryst abundance and distribution in this rock type in the central and upper portions of Formation 3 at Mt. Skukum, Yukon Territory -. 40 Figure 3.14, Photomicrograph of typical porphyritic andesite flow rock under crossed polars showing the altered nature of plagioclase and pyroxene phenocrysts typical in Formation 3 at Mt. Skukum, Yukon Territory. Magnification is 45 times. ! 41 Figure 3.15. Photograph of oscillatory zonation of a plagioclase phenocryst in porphyritic andesite flow rock of Formation 3 at Mt. Skukum, Yukon Territory. Magnification is 175 times 41 Figure 3.16. Photograph of handspecimens of typical andesitic lapilli tuff of upper Formation 3 from Mt. Skukum, Yukon Territory 43 I l l Pare. Figure 3.17. Photomicrograph of devitrified essential fragments in andesite tuff from the upper portion of Formation 3 showing replacement of andesitic pumice by chlorite at Mt. Skukum, Yukon Territory. Magnification is 175 times 43 Figure 3.18. Photograph of the eastern wall of Main Cirque, Mt. Skukum, Yukon Territory, with lithological boundaries outlined in black. F3 = rocks of Formation 3, F4R = flow-banded rhyolite of Formation 4, F4M = Felsic megabreccia of Formation 4, F4T = densely-welded felsic tuff of Formation 4, ACZ = Alunite cap zone 45 Figure 3.19. Photograph illustrating spherulites in flow-banded rhyolite of Formation 4 at Mt. Skukum, Yukon Territory 45 Figure 3.20. Photograph of flow-banding and autobrecciation textures in flow-banded rhyolite of Formation 4 from Mt. Skukum, Yukon Territory 46 Figure 3.21. Photograph of a hand specimen of densely-welded felsic tuff of Formation 4 showing elongate essential fragments and associated fiamme at Mt. Skukum, Yukon Territory 48 Figure 3.22. Photograph of the rhyolite stock intruding rocks of Formation 3 on the south western wall of Main Cirque on Red Ridge (Figure 3.1)., Mt. Skukum, Yukon Territory 50 Figure 3.23. Photograph of handspecimens of rhyolite dyke, one stained and one unstained showing characteristic distribution of plagioclase and orthoclase phenocrysts as well as two fine cross-cutting fractures infilled by pyrite. Handspecimen on right is 7 cm square 50 Figure 3.24. Photomicrograph of matrix material in a typical sample of rhyolite dyke rock showing randomly oriented prismatic feldspar grains enclosed in a mozaic of coarser grained quartz at Mt. Skukum, Yukon Territory. Magnification is 175 times 51 Figure 3.25. Photograph of a typical outcrop exposure of pebble dyke material from Mt. Skukum, Yukon Territory, showing the polymictic nature of rounded to sub-angular fragments and the framework supported character of this lithology .54 Figure 3.26. AFM plot after Irvine and Baragar illustrating the calc-alkaline nature of igneous rocks at Mt. Skukum. Data is from Appendix 3.A. A = N^O+K^O, F = Total iron.M = MgO 58 Figure 3.27. Plot of Na20+K20 and Si02 after LeBas (1986) illustrating the distribution of lithologies at Mt. Skukum, Yukon Territory using this geochemical classification scheme. Data is from Appendix 3.A. Abbreviations are as follows: F = foidite, Pc = picrobasalt, Ul = basanite-tephrite, U2 = phonotephrite, U3 = tephriphonolite, Ph = phonolite, B = basalt, S1 = trachybasalt, S2 = basaltic trachyandesite, S3 = trachyandesite, T = trachyte, 01 = basaltic andesite, 02 = andesite, 03 = dacite, R = rhyolite. The letters 0, S, and U indicate the general state of silica saturation: 0 = over saturated, S = saturated,and U = undersaturated 59 Page. Figure 3.28. Geochemical classification plot after Winchester and Floyd, 1977 showing the positions of igneous rocks from Main Cirque, Mt. Skukum, Yukon Territory, Data is from Appendix 3.A 60 Figure 3.29. Photograph of Main Cirque, Mt. Skukum, Yukon Territory, looking due south showing the fault scarps forming the east and west walls of the cirque and the step-like topography produced by down dropped blocks. The traces of several faults are visible on the escarpment at the front of the cirque 63 Figure 4.1. Photograph showing typical occurence of chalcedony veinlets in porphyritic andesite of the Main Cirque, Yukon Territory. Veins display pyritic selvages and bleaching associated with argillic alteration which forms an envelope around veins 70 Figure 4.2. Photograph of a chalcedony veinlet flanked on one side by a hematitic selvage and on the other by an epidote selvage. Epidote selvages are not common; however, hematitic selvages as shown are commonly associated with these veins 70 Figure 4.3. Photograph of quartz-carbonate vein-breccia showing open-space filling textures in a drusy, quartz-lined cavity from Mt. Skukum, Yukon Territory 72 Figure 4.4. Photograph of a quartz-carbonate vein showing the distribution and abundance of wail rock, fragments in the Main Cirque Ore Zone at Mt. Skukum, Yukon Territory 72 Figure 4.5. Photograph of vein breccia material illustrating the range in abundance of wall rock fragments in vein material (see Figure 4.4) at Mt. Skukum, Yukon Territory 74 Figure 4.6. Photograph of brecciated quartz-carbonate vein material recemented by a later phase of quartz-carbonate breccia-infilling which illustrates the similarity of appearance and composition between early brecciated vein fragments and later matrix at Mt. Skukum, Yukon Territory 74 Figure 4.7, Photograph of a specimen of high grade ore showing wall rock fragments in relatively coarse-grained quartz and lamellar carbonate. The greenish-brown areas in the vein material reflect localized patches of abundant sericite. Specimen shown comes from an underground drift where assays of 1612.1 grams Au/tonne were obtained. 75 Figure 4.8. Photomicrograph of quartz-carbonate vein material showing the lamellar morphology of calcite crystals in the Main Cirque Zone, Mt. Skukum, Yukon Territory. Magnification is 45 times 77 Figure 4.9. Photomicrograph of quartz-carbonate vein material at Mt. Skukum, Yukon Territory, showing fine-grained bladed calcite with a radiating habit even though the hand specimen megascopically has a sucrosic texture. Magnification is 175 times 77 xiv Page. Figure 4.10. Photomicrograph of quartz-carbonate vein material at tit. Skukum, Yukon Territory, showing randomly oriented, bladed calcite crystals forming a skeletal lattice infilled by later fine-grained sucrosic quartz grains. Magnification is 45 times 78 Figure 4.11. Photomicrograph showing clusters of tightly packed bladed calcite crystals occurring in randomly oriented bundles distributed throughout the vein material at Mt. Skukum, Yukon Territory. Magnification is 45 times 78 Figure 4.12. Photomicrograph showing cockade overgrowth of quartz surrounding a bladed calcite grain at Mt. Skukum, Yukon Territory. Early paragenetic formation of calcite followed by quartz as an interstitial infilling at a later time is indicated. Magnification is 45 times 79 Figure 4.13. Photomicrograph of quartz-carbonate vein material showing exfoliation textures in bladed calcite clusters through replacement of calcite by quartz along cleavage planes an grain boundaries in elongate crystals at Mt. Skukum, Yukon Territory. Magnification is 175 times 79 Figure 4.14. Photograph showing prominant blades of equant, fine-grained quartz pseudomorphing calcite on weathered surfaces of quartz-carbonate veins at Mt. Skukum, Yukon Territory 80 Figure 4.15. Photomicrograph showing brecciated vein material recemented in a chaotic matrix of quartz-carbonate-sericite vein material at Mt. Skukum, Yukon Territory. Magnification is 175 times 82 Figure 4.16. Photomicrograph showing late-stage development of sericite as coatings on cavities in vein material from the Main Cirque Ore Zone, Mt. Skukum, Yukon Territory. Magnification is 175 times 82 Figure 4.17. Photomicrograph showing a typical occurrence of fine-grained euhedral adularia grains intergrown with quartz in a brecciated specimen of quartz-carbonate material which was re-cemented by matrix material containing abundant adularia at Mt. Skukum, Yukon Territory. Magnification is 175 times 83 Figure 4.18. Photograph of stained slabs illustrating the abundance and distribution of adularia in vein material from Mt. Skukum, Yukon Territory. Areas of pale yellow staining are sericitic, but areas of intense yellow staining contain adularia 83 Figure 4.19. Photomicrograph showing a typical occurence of electrum (larger highly reflective grains) and native silver (smaller grey grains) in quartz-carbonate veins from Mt. Skukum, Yukon Territory. Grains of both electrum and native silver average 20 microns across, and are typically distributed as small concentrations of disseminated, co-existing grains. Magnification is 100 times 85 XV Paoe. Figure 4.20. Photomicrograph showing fine inclusions of native silver (grey) in electrum at Mt. Skukum, Yukon Territory. Native silver in this figure occurs as several minute grains disseminated within a coarser grain of electrum; however, the reverse is also seen where electrum occurs as fine disseminations in coarser grains of native silver. Magnification is 700 times. 85 Figure 4.21a. Electron photomicrograph (backscattered electron scan) showing relatively coarse grained electrum grains (lighter grains) some displaying fine inclusions of native silver interspersed with discrete, extremely fine grains of native silver (grey). Several of the finer grains of native silver also show inclusions of electrum 86 Figure 4.21 b. Electron photomicrograph (backscattered electron scan) showing one of the two phase grains of precious metals enlarged from Figure 4.21 a consisting of electrum (lighter phase) and native silver (darker phase) in vein material from Mt. Skukum, Yukon Territory 86 Figure 4.22. Energy dispersive electron microscope scan of the two phases in Figure 4.21. The darker phase in the photomicrograph consists of silver whereas the lighter \ coloured phase consists of electrum about 600 fine 87 Figure 4.23. Contour diagram showing distribution of gold values in the Main Cirque Zone (Figure 3.1) at Mt. Skukum, Yukon Territory. Gold occurs in elongate, irregular, vertically plunging ore shoots that increase in grade upwards. High-grade zones pinch out consistently downward near the 1676 sublevel. Data is from routine mine sampling during development. 89 Figure 4.24. Isopach diagram illustrating vien thickness in the Main Cirque Ore Zone (Figure 3.1) at Mt. Skukum, Yukon Territory. Zones of maximum thickness correspond with zones of maximum grade (Figure 4.23) 90 Figure 4.25. Contour diagram of grade times thickness outlining zones of maximum vein width and gold grade at Mt. Skukum, Yukon Territory. Gold occurs in three vertically plunging ore shoots that reflects the orientation of major hydrothermal channels indicating that fluid flow in the Main Cirque Ore Zone was directed vertically with little lateral flow ...91 Figure 4.26. Air photograph of Main Cirque and the surrounding area showing the locations of the Main Cirque Zone (labelled Main Zone) and the Lake Zone in relation to Mt. Skukum. Zones of super gene alteration surrounding Main Cirque produce a reddish-brown halo centered on the cirque 96 Figure 4.27. Photograph showing limonitic supergene alteration of porphyritic andesite in a zone of chalcedonic stockwork veins approximately 40 m in diameter, located approximately 100 m southwest of the southern end of the Main Cirque Zone, Mt. Skukum, Yukon Territory 96 Figure 4.28. Photograph of typical limonitic supergene alteration in a hand specimen of brecciated porphyritic andesite showing kaolinitic alteration extending into wall rock fragments from pyritic breccia infilling at Mt. Skukum, Yukon Territory 97 Figure 4.29, Photograph showing limonitic supergene alteration halos surrounding centrally located veins in the Lake Zone, Mt. Skukum, Yukon Territory 99 Figure 4.30. Photograph of brightly coloured reddish-brown limonitic alteration in theAlunite Cap Zone, Mt. Skukum, Yukon Territory 99 Figure 4.31. Schematic illustration showing gradation between veins and zones of vein breccia and the distribution of hypogene alteration around quartz-carbonate veins and vein breccia at Mt. Skukum, Yukon Territory 101 Figure 4.32. Photomicrograph, under crossed polars, of typical silicification of andesitic country rock adjacent to mineralization at Main Cirque, Mt. Skukum, Yukon Territory. Primary textures are obscured by pervasive development of fine grained quartz. Sericite occurs as fine grained aggregates replacing feldspar phenocrysts. Magnification is 45 times 103 Figure 4.33. Photograph showing typical appearance of phyllic alteration in a hand specimen of porphyritic andesite from Mt. Skukum, Yukon Territory 106 Figure 4.34. Photomicrograph showing complete replacement of plagioclase phenocrysts through the development of sericite and calcite in zones of phyllic alteration at Mt. Skukum, Yukon Territory. Magnification is 175 times 108 Figure 4.35. Photograph of typical intense phyllic alteration developed in brecciated, flow-banded rhyolite from the Alunite Cap Zone at Mt. Skukum, Yukon Territory 108 Figure 4.36. Photograph of intense argillic alteration in rock of the Alunite Cap Zone, Mt. Skukum, Yukon Territory, showing a gradual decrease in alteration intensity away from fractures and preservation primary fragmental texture 110 Figure 4.37. Photomicrograph showing typical amygdule infilling of radiating chlorite rosettes in propylitically altered porphyritic andesite at Mt. Skukum, Yukon Territory. Magnification is 45 times 113 Figure 5,1. Fluid inclusion homogenization data from Mt. Skukum, Yukon Territory, showing distribution of primary, secondary, and pseudo-secondary (P/S) inclusions from vein material. A trimodal distribution with peaks near 31O'C, 270°C, and 190°C is indicated. Data are from Table 5.1 124 Figure 5.2. Fluid inclusion homogenization data obtained from individual samples of vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1 124 Figure 5.3. Eutectic melting temperatures by distribution of inclusion types (primary, pseudo-secondary, and secondary) in vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1 125 Figure 5.4. Final melting temperatures of fluid inclusions by distribution of inclusion types (primary, pseudo-secondary, and secondary) in vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1 125 xvii Page. Figure 5.5. Frequency distribution of volume percent gas in fluid inclusions from veins, lit. Skukum, Yukon Territory showing the wide variation in LV ratios and their bimodal distribution in primary inclusions at 2 8nd 20 vol. percent gas. The overall mode is at 5 vol. percent. Data is from Table 5.1 ..." 129 . Figure 5.6. Homogenization temperature versus LY ratio for primary fluid inclusions at Mt. Skukum, Yukon Territory. This histogram illustrates the generally consistent homogenization temperatures of inclusions with a wide range of volume percent gas content indicating that inclusions formed from similar fluids under boiling conditions. Data are from Table 5.1 130 Figure 5.7. Distribution of the last melting temperature of primary fluid inclusions from veins at Mt. Skukum, Yukon Territory, showing that those containing different gas contents have different apparent salinities. Data are from Table 5.1 131 Figure 5.8. Distribution of the homogenization temperature of primary fluid inclusions from veins at Mt. Skukum, Yukon Territory, showing that those containing different gas contents had different minimum temperatures of emplacement. Data are from Table 5.1 . 131 Figure 5.9. Primary fluid inclusion data for temperature of last melt ys,. homogenization temperature from veins at Mt. Skukum, Yukon Territory, shows two fluid types, a low temperature, low salinity fluid and a high temperature, high salinity fluid. Points plotted represent all those inclusions for which both homogenization temperature and temperature of last melt were obtained and thus represents only a part of the total dat8 set. Fluid temperature and salinities for each cluster are from Figures 5.7 and 5.8 which represent complete data sets. Data are from Table 5.1 132 Figure 5.10. Phase equilibria in the lower temperature part of the system H 2 O - C O 2 showing the limitations of C O 2 content in depositional fluids at M l Skukum, Yukon Territory (after Roedder, 1984). QI = C O 2 vapour, H20 liquid, C O 2 clathrate, and ice at« -2'0 and 10.4 bars. Q2 = C O 2 vapour, H 2 O liquid, C O 2 liquid, and C O 2 clathrate at« 10°C and 45 bars. The equivalents of curve QI - Q2 for 5 percent added NaCl is also shown 135 Figure 5.11. S 1 $0 V J L S D values showing fields for magmatic and metamorphic water and the possible range of depositional fluid composition at Mt. Skukum, Yukon Territory. Values for some Tertiary volcanic-hosted epithermal deposits of the Basin and Range region of the U.S. also shown are: Bullfrog (BU), Aurora (A), Jarbidge (J), Gilbert (G), Tonopah (T), Bodie (B), Comstock Lode (CD. SMOW indicates the position of Standard Mean Ocean Water. All values in per mil (°/Q.O)- Modified after Taylor (1979) and Field and Fifarek, (1985) 145 Figures. 12. North America with contours of S D values in meteoric surface waters (from Taylor, 1979) 146 xviii Figure 6.1. Series of four schematic cartoons (a to d) illustrating the possible sequence of events forming the excellent ground preparation at Mt. Skukum, Yukon Territory, leading to formation of epithermal vein mineralization at Main Cirque 154 Figure 6,2. Spatial relationships among known mineralized zones in the Main Cirque area, Mt. Skukum, Yukon Territory 158 XIX Acknowledgements The author is indebted to C.I. Godwin for his guidance, encouragement and support throughout this project. I also wish to thank A.J. Sinclair, S. Horsky, K. Fletcher, and J. Knight, and others among the friends and colleagues at U.B.C. who provided helpful suggestions, assistance and encouragement throughout the course of this research. Many thanks are also due to J.A. Morin of the Exploration and Geological Services Division of the Department of Indian and Northern Affairs, Yukon for initiating this project and funding fieldwork and much of the analytical work. Assistance in the form of field accommodation and many useful discussions were generously provided by geologists R.A. Doherty, and E.B. Stewart of Agip Canada Ltd., as well as R. Basnett and R. Somerville of Total-Erickson Resources Ltd. I would also like to express my appreciation to K. Muehlenbachs of The University of Alberta for undertaking oxygen isotope analyses used in this project. .1 t.O. Introduction t.t. Location and Access The nt Skukum deposit, centered near latitude 60* 12' north and longitude 135c 28' west (NTS: I05D SW), is approximately 65 km southwest of Whitehorse, Yukon Territory (Figure. 1.1), in the Wheaton River valley, on the boundary between the Yukon Crystalline Terrane and Coast Crystalline Complex. The deposit consists of low-sulphide, high-level, gold-silver bearing • quartz-carbonate veins -located in Main Cirque at an elevation of 1,800 m in the south central part of the lit, Skukum Volcanic Complex, The area is easily accessible from Whitehorse using a newly upgraded road. 12. Climate and Phvsiograohv Climate of the Mt. Skukum area is typical of uplands in the southern ifukon with harsh winters and cool but pleasant summers enhanced by long periods of daylight lasting up to 20 hours in June (Wheeler, 1961), Weather, at times, can be unpredictable with sunny mornings quickly replaced by strong, highly destructive winds and snowfalls by afternoon; even blizzards lasting several days occur in the middle of July. The Mt. Skukum area is characterized by Jagged, alpine topography with a relief of 1,400 m between the peak of Mt. Skukum at 2,400 m and the floor of the adjacent Wheaton River valley at approximately 1,000 m. The area has been affected by alpine and continental glaciation which at the time of its greatest extent covered the area to an elevation of about 2,000 m leaving peaks such as Mt. Skukum rising above the ice as nunataks (Wheeler, 1961). Alpine glaciation is still active in areas of highest elevation such as Main Cirque which hosts a small glacier at the north foot of Mt. Skukum. S K U K U M A R E A L O C A T I O N MAP Figure 1.1. Location map of the Mt. Skukum area showing access road and proximity to Whitehorse, Yukon Territory. 3 1.3. Previous Work and Exploration History Exploration interest in the Wheaton River District, including the Mt. Skukum area, began near the turn of the century with attention focussed on precious metal and antimony veins first discovered in 1893. Small scale mining at several locations in the Wheaton valley resulted in only limited production. The first geologic investigation of the area was undertaken in 1906 by D.D. Cairns who mapped the district at a scale of 1 inch to 1 mile and studied deposits in the area (Cairns, 1912,1916), In 1940, H.S. Bostock studied antimony properties in the Wheaton River District as part of a study of special minerals (Bostock, 1941). More recently, Monica Pride has described much of the geology of the Mt. Skukum Volcanic Complex as part of a Ph.D. study (Smith, 1982,1983; Pride, 1985,1987). Despite the prolonged period of geologic interest in the area, no record exists of staking in the Main Cirque area prior to 1981. In 1980, a reconnaissance program of preliminary prospecting, mapping, and extensive stream sediment geochemistry conducted by Agip Canada Ltd. of Calgary, indicated anomalous concentrations of gold and arsenic in sediment from Butte Creek (Figure 1.1). Values of up to 630 ppb Au and 192 ppm As were found in these sediments. In all, nine samples containing values in excess of 85 ppb Au were defined against a background of 5 ppb Au in the headwaters of Butte Creek. In May 1981,48 claims were staked to cover this anomaly and several brightly coloured gossanous zones in Main Cirque (McDonald etal. 1986). Exploration drilling and prospecting was conducted over a four year period before mine production began in 1985 at a rate of 300 tonnes per day. Production has continued at the same rate to the date of writing. 1.4. Objectives This field and laboratory study of the Mt. Skukum deposit was designed to investigate the nature of the mineralized veins and their relationship to surrounding rocks of the Mt. Skukum Volcanic Complex. The objectives were to: (I) map the deposit and surrounding volcanic rocks, (2) determine the mineralogical and chemical character of surrounding volcanic rocks, (3) determine the nature and precious metal mineralogy of veins, (4) determine the geochemistry and distribution of hydrothermal alteration, and (5) define the depth of formation and hydrothermal environment of deposition of gold and silver bearing veins using fluid inclusions and stable isotope chemistry. 1.5. Fieldwork Field work for this study was conducted during the summers of 1984 and 1985 while employed by the Geological Services Division of the Department of Indian and Northern Affairs. During this time the area surrounding the Main Cirque Zone was mapped at a scale of 1:1,000 and later incorporated into a map of the Main Cirque at a scale of 1:5,000. Over 6,000 m of core was also logged and sampled during this time. Although the entire Main Cirque area was mapped, detailed examination of vein mineralization in this study Is restricted to the Main Cirque Zone as it was the only zone developed for mining during the fieldwork period. 5 2.0. Regional Geology 2.1. Introduction and Tectonic Setting TheMt, Skukum gold-silver deposit (Figure 2.1) occurs in an early Eocene (51.6 ± 1.8 Ma.; Table 2.1) volcanic complex on the border of the Coast Crystalline Complex and Yukon Crystalline Terrane. It lies approximately 27 km north-northwest of the Bennett Lake Cauldera Complex (Figure 2.1) described by Lambert (1974). These volcanic complexes represent two of several penecontemporaneous centers of continental volcanic activity which occur in southwestern Yukon Territory, and extend southeastward into northern British Columbia. This belt of volcanic centers forms the Sloko Volcanic Province characterized by intermediate to felsic volcanic rocks lying unconformably on Jurassic to Precambrian metasedimentary and Cretaceous granitic rocks (Figure 2.1). Volcanic rocks of Sloko Province lie along the northeast margin of the Coast Crystalline Complex where they 8re characteristically preserved as erosional remnants within down-faulted blocks in areas of high elevation (Souther ,1967). Within the Yukon Territory, rocks of the Mt. Skukum Volcanic Complex, until recently considered correlative with Mt. Nansen volcanic rocks (Templeman-Kluit, 1981), are now thought to represent a group distinct in age from other volcanic rocks in Yukon Territory which unconformably overlie parts of the Yukon Crystal line Terrane, and Coast Crystalline Complex (Templeman-Kluit, pers. comm., 1987). The Coast Crystalline Complex forms a southwesterly facing magmatic arc within the Intermontane Belt which represents an allochthonous block accreted to the North American craton during the Mesozoic (Godwin, 1972; Templeman-Kluit, 1981; Monger and Price, 1979). Northeastward subduction of oceanic lithosphere initiated on the southwestern side of the accreted block ceased in the Late Cretaceous but is considered to have produced this magmatic arc on which Early Tertiary volcanic rocks of the Sloko Volcanic Province unconformably lie (Templeman-Kluit, 1981). Deposition of these Early Tertiary volcanic rocks was coincident with a period of widespread deformation during which large bodies of quartz monzonite were emplaced in both the Primrose M r i ^ * * ^ 1 * * * * * * * 7^ " ~ " — V * A* + * + . * * >^ *>r + 4 f i.a*e+ + + + * + - r — -f;:;i;;:;:;i::;:;;:!;i:;:;|> - V * — - -£ZL£^ + -4 \ + + + + + + + + + + + + + - CT^ *^*- + * - * *"v + + \ X + + + + + + + + + 't*",-',"","~'"*' +. +. +. +. .. +. -^ £2? + + + + + + -h + -*- + + -*--t- + * - - > * - - t - ^ - - r + - - V ^ * 4 . - 1 . - * .+ + -*- + + + + + - 4 - - t - - ^ * - - * - - - > * - - i . > + 4 . - - - * - r * - - ^ * - -A A A A A A A A A A A A A A / A A Bennett Lake A A A A A A A A A A A A A A / * A A A A A A A A A A / " V.V V \+ + + + + + + -f7!""** Cauldera Complex . * • • » ' ^ * * - - - * - r - - - - t - ^ r ' A A A A A A A A A A A ^ LEGEND Granite porphyry, rhyolite Skukum Group, Felsic volcanic rocks Skukum Group, Intermediate volcanic rocks Coast intrusions, Cretaceous Jurassic to Triassic sedimentary and volcanic rocks Yukon Group; PE metamorphic rocks Metavolcanics, age uncertain 0 5 Km Figure 2.1. Regional geology of the Mt. Skukum Volcanic Complex and surrounding area Including the Bennett Lake Cauldera Complex in the Yukon Territory (after Wheeler, 1961 and Pride, 1985) 7 Yukon Crystalline Terrane and the Coast Plutonic Complex, particularly along the eastern margin of the Coast Mountains (Souther, 1966). Magmatic intrusion was associated with widespread eruption of pyroclastic material throughout the southern Yukon Territory and British Columbia forming several volcanic provinces including the Sloko Volcanic Province (Lambert, 1974). 8 2.2. Geology 2.2.1. Geological'Setting'of'theMt. Skukum VolcanicComplex: The fit. Skukum Volcanic Complex, shown in Figure 2.1 .occurs in one of two sub-circular regions of Skukum Group volcanic rocks that comprise the northernmost extent of the Sloko Volcanic Province. The second region of Skukum Group rocks comprises the Bennett Lake Cauidera Complex, approximately 27 km southeast of tit, Skukum. Both have been preserved as erosional remnants in down-faulted blocks which unconformably overlie Cretaceous and older metamorphic and plutonic rocks of the Yukon Crystalline Terrane and Coast Plutonic Complex. Both volcanic centers are of a similar early Tertiary age (Table 2.1) (Lambert, 1974), but the Bennett Lake Cauldera Complex has a relatively higher proportion of felsic volcanic rocks and is partially surrounded by a well-defined felsic ring dyke. The Mt. Skukum Volcanic Complex contains predominantly andesitic volcanic rocks, surrounded by many small, high-level porphyritic rhyolitic to dacitic stocks (Figure 2.1) which may represent late intrusions associated with ring fractures related to a cauldera collapse event (McDonald and Godwin, 1986). Based on lithogeochemical, structural, and lithological evidence, Smith (1982 and 1983) has interpreted these two Tertiary volcanic centers as two distinct expressions of separate magmatic systems rather than as two erosional remnants of the same volcanic system, although the two are closely genetically related. The oldest rocks in the area surrounding the Mt. Skukum Volcanic Complex are Yukon Group metamorphic rocks which occur as a series of irregular, discontinuous masses distributed in a broad northwesterly trending band parallel to the structural grain of the region (Figure 2.1). These rocks, perhaps Precambrian in age (Wheeler, 1961), unconformably underlie rocks of the Tit. Skukum Volcanic Complex in its southern and eastern parts, and are in fault contact with the complex along all of its western margin where large vertical displacements juxtapose rocks high in the voicanic stratigraphy with these older rocks. Lithologies of the Yukon Group in the Mt. 9 Skukum area include interbedded quartz-muscovite and quartz-biotote schist, graphitic and micaceousquartzite, gneiss, marble, andamphibolite (Wheeler, 1961). Two lithologic units in Figure 2.1 are not in contact with the fit. Skukum Volcanic Complex. Lewes River Group sedimentary 8nd volcanic rocks of Upper Triassic to Jurassic age, occur in the northeastern corner of the Mt. Skukum region (Figure 2.1) in elongate northeast trending belts. In this area, lithologies consist mainly of purplish, grey and green volcanic rocks and breccia, probably of andesitic composition, interbedded with lenses of grey and pinkish massive limestone similar to the Late Norian limestone occuring east of the Whitehorse Copper Belt in the Whitehorse map sheet (NTS: 105DSW) (Wheeler, 1961). Unitsshownas metavolcanics of uncertain age occuring in the northeastern part of Figure 2.1 belong to either the Lewes River Group or the Hutshi Group (Wheeler, 1961). These metavolcamc rocks consist of mafic flows and breccias, locally containing granitic clasts; they are isolated from or faulted against sedimentary rocks of Lewes River Group (Wheeler, 1961). Local, minor stocks of porphyritic rhyolite occur sporadically throughout the Mt. Skukum Volcanic Complex and the region surrounding it (Figure 2.1). In the Bennett Lake Cauldera Complex, this intrusive unit, comprised of several intrusive centers, forms an arcuate series of ring fracture intrusions. Rhyolite stocks in the Mt. Skukum area, occur within the complex and the region surrounding it. No consistently elongate or arcuate stocks are apparent; however, those occuring outside the complex surround it in an elliptical arrangement which vaguely follows the outline of the complex and may reflect an elliptical zone of weakness associated with collapse that facilitated emplacement of this unit. These peripheral rhyolite stocks have been dated at 53 ± 1.1 Ma using rubidium-strontium geochronometry (Pride and Clark, 1985). 2.2.2. Oeologyofthetlt. Skukum Volcanic Complex: The Mt. Skukum Volcanic Complex (Figure 2.2) forms a subrounded remnant of fault-bounded Tertiary andesitic and felsic volcanic rocks measuring 20 km in its longest dimension and 11 km in the short dimension with a maximum verticle thickness of approximately 850 m. The volcanic complex is divided into two parts by two north-south trending faults (Figure 2.2). These faults mark structural divisions between the eastern and western parts of the volcanic complex. The eastern part consists mainly of felsic volcanic rocks which are downdropped as much as 300 m (Pride, 1986) relative to the western part along these faults. Felsic units of the eastern portion of the Mt. Skukum Volcanic Complex consist m8inly of interlayered, brecciated, flow-banded and spherulitic rhyolite lava flows and pyroclastic rocks. These felsic units are particularly thick in the northeastern part of the complex where prominant large-scale circular fracture systems visible from the air and the occurence of slump blocks and other features reflect a center of cauldera subsidence, The western part of the Mt. Skukum Volcanic Complex is characterized by andesitic volcanic rocks which host the Mt. Skukum deposit. Geology of the eastern portion is not discussed further. Stratigraphy in the western portion of the Mt. Skukum Volcanic Complex unconformably overlies Cretaceous granitic rocks and Precambrian metasediment8ry rocks which form a highly irregular surface due to paleotopography and post-depositional block faulting. Skukum Group rocks overlying this basement have been divided into five formations by Pride (1986). Pride's stratigraphic divisions generally have been maintained in this work; however, in this study, Pride's Formation 5 is incorporated with Formation 3, and Pride's Formation 4 in this study is the uppermost stratigraphic horizon. This preserves the perceived order in the western portion of the Mt. Skukum Volcanic Complex as illustrated in Figure 2.3. Formation 1 comprises the oldest lithologies of the Mt. Skukum Volcanic Complex that are distributed in rare, isolated areas around the peripheries of the complex and within the complex Figure 2.2. Generalized geology map of the Mt. Skukum Volcanic Complex, Yukon Territory, (after Pride. 1985). 12 Format ion 4. 11 OQm Format ion 3 4@0m Format ion 2 »00m Format ion 1 0m Basement B r e c c l a t e d , f low-banded, and spheru l i t ic rhyo l i te f lows and welded to non-welded fe ls ic py roc las t i c rocks unconformably Over ly ing Format ion 3 Andesi t ic P y r o c l a s t i c and Epiclast ics!-" Thick sequence of interbedded porphyr i t i c andesite f low rocks and arniesitic lapi l l i tuffs in te rspersed wi th andesit ic debris f low mate r ia l and andesit ic py roc las t i c rocks Epic last ic components are most common in the lower part of this Format ion where indiv idual f l ows show dist inct var ia t ions in phenocryst compos i t ion , s ize and abundance' Towards the center of the Format ion Andesi t ic f low rocks become uni form in appearance forming monotonous ser ies of r e p e t i t i v e f lows P y r o c l a s t i c components become abundant towards the top of the Format ion where a thick sequence of poorly bedded monoli thic andesite b recc ia caps the format ion Format ion 3 is a max imum of 700m thick Interbedded py roc las t i c and ep ic las t ic units grading upwards m composit ion f rom fe ls ic to intermediate Li thologies include coarse to fine grained sandstones , s i l ts tones , and debr is f lows in te rspersed wi th welded to non-welded asn and lapi l l i tuf fs ranging in composi t ion f rom rhyo l i t i c to andesit ic conformably ove r l y i ng Format ion 1 and nonconformably ove r l y i ng basement. Format ion 2 is a max imum of 30Qm thick A l luv ia l sediments consist ing of a lower conglomerate compr ised of boulder , cobble , and pebble s i ; e d c las ts of basement mater ia l over la in by interoedded s i l ts tone and sandstone Deds and debr is f low mate r ia l . The sequence is 5 to 100 m thick and nonconformably ove r l i es basement r ocks . Format ion 1 is a max imum of 100m thick Basement r o c k s consist ing of P recambr ian Yukon Group me ta -sediments intruded by Cretaceous grani t ic rocks ] c v <J 1 Andesi t ic Ash Tuff w <j <j v <j v • + + • + * + F+ + +• + • + + + + • + + + + • + ; • + + + + + • '+ + Cre taceou$ + + Gran i t i c , . + + + + i-*f + + + k+ + + 4- + + 4- + + + 4- + 7 + + + + + + + + + + + + ' + + + • + + + + + + • + + > + + + + + + + + + + + + + + + + +• + + + + •f + • + Figure 2,3, Stratigraphy in the western portion of the Mt. Skukum Volcanic Complex, south-central Yukon Territory. The Mt. Skukum gold deposits occur in Formation 3. 13 in areas where deep erosion penetrates volcanic units to the basement (Figure 2.2). This formation unconformable/ overlies basement rocks, and consists of a 5 to 100 m thickness of, coarse, alluvial material (Pride, 1986) dominated by basement fragments. Lithologies include a basal conglomerate composed of boulder to cobble sized clasts of basement granitic rocks overlain by channelled, inter layered siltstone and sandstone beds and debris flow material with increasing frequency of volcanic derived fragments up-section. This formation represents deposition during incipient stages of volcanism where tectonic disruption during emplacement of magma at depth and initial stages of volcanic activity created sufficient topographical relief to allow formation of coarse clastic deposits, derived initially from basement material, and, with passage of time from an increasingly significant volcanic source (Figure 2.3). Formation 2 occurs throughout the western division of the Mt. Skukum Volcanic Complex where it forms a discontinous unit around both the western peripheral margins of the complex and in central areas of the complex where deep erosion penetrates volcanic rocks exposing underlying basement (Figure 2.2). This formation attains a maximum thickness of 300m and lies unconformably on basement rxks and conformably over Formation 1 (Figure 2.3) from which it is distinguished by the presence of primary volcanic rocks and a greater diversity in clast composition and types (Pride, 1986; Smith, 1984). Lithologies include interbedded pyroclastic volcanic rxks and derived epiclastic units both of which xntain abundant basement material including granitic and metasedimentary clasts. Pyrxlastic units grade upward from lower pale brown to green rhyolitic units (Figure 2.4) xntaining basement fragments, to upper olive-green and marxn andesitic ashfall and lapilli tuffs (Figure 2.5) xntaining abundant fragments of lower felsic volcanic material but proportionately less basement fragments. Volcano-sedimentary epiclastic units interbedded with this pyrxlastic material are invariably of a similar xmposition to that of surrounding volcanic rxks (Figure 2.6) and include well bedded, fine to coarse grained siltstones and sandstones as well as debris flow units which bxome less predominant up-sxtion. Units of Formation 2 represent more proximal and widespread volcanism than those of Formation 14 Figure 2.4. Photograph of felsic welded tuff typical of volcanic rocks from the lower portion of Formation 2 in the fit. Skukum Volcanic Complex, Yukon Territory. Figure 2.5. Photograph of maroon andesitic ashfall accretionary lapilli tuff typical of volcanic rocks in the upper portion of Formation 2 in the Mt. Skukum Volcanic Complex, Yukon Territory. Arrows on the rock point to two accretionary lapilli near the pen top. 16 Figure 2.6. Photograph of well bedded epiclastic units typical of the upper portion of Formation 2 in the Mt. Skukum Volcanic Complex, Yukon Territory. As pictured here, epiclastic units in the upper portion of this formation are mainly derived from volcanics of andesitic composition. 17 1. The gradual decrease in accessory basement fragments and debris flow deposits up-section reflect a broad accumulation of volcanic material on surface which covered basement rocks preventing its erosion. This accumulation infilled and leveled paleotopography to produce a relatively flat surface on which the upper, comparatively well sorted and well bedded epiclastics and andesitic tuffs were deposited. The gradational change from rocks of largely rhyolitic composition in the lower parts of Formation 2 to rocks of andesitic composition towards the top of Formation 2, reflects an evolution of magmatic extrusion typical of compositionally stratified magma chambers common in terrestrial volcanism (Fischer and Schmenke, 1984). This same succession of volcanism grading upwards in composition from felsic to intermediate is described by Lambert (1974) in the Bennett Lake Cauldera Complex. Formation 3 underlies almost the entire western portion of the Mt. Skukum Volcanic Complex and conformably overlies rocks of Formation 2 (Figure 2.3). It consists of a sequence of gently dipping porphyritic andesite flow rocks interbedded with andesitic ash and lapilli tuffs and epiclastic rocks reaching a thickness of up to 700 m. Individual porphyritic andesite flow rocks are laterally continuous and can be traced for distances in excess of 1 km with thicknesses which range from 2 to 10 m (Figure 2.7). Vesicular flow tops and brecciation along flow tops and bottoms are common with flow centers remaining massive and locally columnar jointed. Although porphyritic andesite flows are the dominant lithology in this Formation, epiclastic units occur throughout, and are most common in the lower parts of the section where flow rocks are less frequent and display distinct changes in phenocryst morphology, type, size, and abundance (Pride, pers. comm., 1985). At stratigraphically higher levels, porphyritic andesite flows become more abundant and texturally monotonous. The highest stratigraphic levels of Formation 3 occur in the southwestern portion of the Mt. Skukum Volcanic Complex where the proportion of andesitic pyroclastic and epiclastic units increases up-section (Figure 2.3) with porphyritic andesite flows remaining as the dominant lithology. At the highest stratigraphic levels (Figure 2.3) comprising the peak of Mt. Skukum and areas of surrounding high elevation, interbedded andesitic Figure 2,7. Photograph showing a typical section of Formation 3 as seen in a cliff exposure near Main Cirque, Mt. Skukum, Yukon Territory. Repetitive, laterally extensive flows of porphyritic andesite are visible as pale and dark coloured bands running across the cliff. Pale bands represent relatively highly altered zones of interflow breccia, dark bands represent relatively unaltered bands which correspond to massive, unfractured flow centers. The eastern wall of Main Cirque and the reddish-brown gossan of the Alunite Cap Zone is seen in the background. Figure 2.8. Photograph of monolithic andesite breccia which forms a thick sequence at the top of Formation 3 in the Mt. Skukum Volcanic Complex, Yukon Territory. 19 flows and fragmental rocks are capped by a thick section of monolithic andesite breccias (Figure 2.8) comprised of poorly sorted, sub-rounded to angular fragments of boulder to pebble sized andesite in a matrix of accessory, sand sized andesite grains (Pride, 1986). These breccias, which reach a total thickness of approximately 320 m, are generally poorly bedded and locally intruded by pod-like columnar jointed andesite si l ls and dykes in the westernmost exposures. Formation 3 represents proximal and vent facies volcanic r x k s deposited on the upper part of a collapsed strata-volcano (Pride, 1986) with the thick accumulation of monolithic andesite breccia representing the vent facies and deposits produced down-slope from the vent. Porphyritic andesite flow rocks occuring immediately east of this breccia unit are grouped with the breccia to form Pride's Formation 5 interpreted as distinct from porphyritic andesite flow rocks of Formation 3 from which they are separated by felsic volcanics of Formation 4 (Pride, 1985). Here, Pride's Formation 5 is not considered distinct from Formation 3 as the two grade imperceptibly into eachother and are indistinguishable throughout the entire southwestern part of the lit. Skukum Volcanic Complex. Instead, porphyritic andesite flow r x k s and monolithic andesite breccia are included with Formation 3 representing the uppermost stratigraphic levels of this formation. A slight increase in MgO content between porphyritic andesite flows low in the stratigraphy (Pride's Formation 3) and those higher up (Pride's Formation 5) lead Pride (pers. comm., 1985) to draw a distinction between the two. As these units appear in the field to grade into each other without intervening lithologies or textural distinction, this author considers the geochemical distinction to represent a natural sequence of eruptive products from a compositionally stratified magma chamber at depth which gradually becomes more mafic in composition as eruption continues. This process has been illustrated elsewhere in the Mt. Skukum Volcanic Complex by the trend in composition of volcanic units in Formation 3 from rhyolitic to andesitic and has been documented in the Bennett Lake Cauldera Complex (Lambert, 1974) as well as other localities (Fischer and Schmenke, 1984). Formation 4 comprises the uppermost stratigraphic sequence in the Mt. Skukum Volcanic Complex where it is most prevalent in the eastern portion. In the western portion, this formation occurs as several irregular shaped, isolated bodies in areas of high elevation in the southeast and northwest, corners of Figure 2.2. These consist mainly of brecciated, flow-banded and spherulitic rhyolite flows and pyroclastic rocks which lie unconformably over Formation 3 (Figure 2.3). Formation 4 appears to fill a cauldron subsidence structure in the eastern part of the volcanic complex that developed some time after andesitic volcanism in the west ended. It was subsequently down dropped at least 300 m preserving much of the formation in the east and leaving only small erosional remnants at high elevation in the western side of the complex, 2.2,3. etexfiromAw-Geochronometry of two samples of porphyritic andesite from the Main Cirque area at Mt. Skukum was undertaken to aid interpretation of the timing of geological events. Analyses were conducted using whole rock K-Ar methods with potassium analyses done by K. Scott and argon analyses by J . Harakal, both at The University of British Columbia. Potassium was determined in duplicate by atomic absorption using a Techtron AA4 spectrophotometer and Ar by isotope dilution using an AEI MS - 10 mass spectrometer and high purity 3 8 A r spike. Samples analysed included one macroscopically pristine specimen of porphyritic andesite and one specimen of propylitically altered porphyritic andesite. Dates, in Table 2.1, overlap at 50.7 ± 1.8 Ma for the altered porphyritic andesite, and 53.2 ± 1.8 Ma for the unaltered porphyritic andesite sample. Potassium-argon dates for both samples fall within the accepted range for Early Eocene 3pe rocks. This corresponds with K-Ar whole rock analyses of rhyolite porphyry ring-dyke material and associated ashflow tuff from the Bennett Lake Cauldera Complex which yeilded ages of 52 and 51 Ma (Morrison et al.. 1979). It also corresponds with recent work by Pride and Clarke (1985) that gave Rb-Sr dates of 53.3 ± 1.1 Ma for the high level rhyolite intrusions associated with the Mt. Skukum Volcanic Complex. The difference in apparent ages between altered and Table 2.1. Potassium-argon data from whole rock analysis of andesite from the Mt. Skukum property, Main Cirque Zone area, Yukon Territory. 1 Sample Number Location latCN) longCW) Rock Description (±) 4 0 A r » 2 4 ° A r t 0 t a l 40Ar* 2 10" 5 cm 3 STP/g Apparent Age 3 Time^ ASTN-13 DH83-63: 57.61m 60.20 135.47 Fine grained andesite with pervasive propyllitic alteration. 2.71 ±0.01 0.674 0.5416 50.7±1.8 Tert ASTN-14 DH83-63: 60 20 135.47 Fine grained, fresh andesite. 2.18 ±0.04 0.938 0.4574 53.2±1.8 Tert 1. Argon analyses are by J . Harakal and potassium analyses are by K. Scott; all were done at the Geochronology Laboratory, The University of British Columbia. 2. Ar* indicates radiogenic argon. 3 Constants used are from Steiger and Jager (1977): KX a= 0.581 x 10" , 0yr"'; KAp* 4.962 x 10- , 0yr"'; 4 0 K/K = 1.167 x IO"4. 4. Time designation is from Harland et al. (1962). 22 unaltered porphyritic andesite samples in Table 2.1 might reflect re-equilibration of the altered sample with hydrothermal fluids responsible for mineral deposition at Main Cirque although the ages overlap if worst case errors are considered. The data, without considering errors, indicate that vein mineralization might have occured 2.5 Ma after deposition of volcanic rocks. Long delays between the time of formation of host rocks and the time of mineral emplacement in epithermal systems are common, particularly in adularia-sericite type deposits where ore deposition almost invariably occurs more than 1 Ma subsequent to host rock depostion (Hayba etal.. 1985). 23 3.0. Geology of Mine Area 3.1. Introduction The deposit at Mt, Skukum, Yukon Territory, is in the southwestern part of the Mt. Skukum Volcanic Complex in Main Cirque which forms the headwaters of Butte Creek (Figure 2.2). Economic horizons are contained within sub-parallel, steeply-dipping, vein-fault systems trending between N10°W and N45eE , which form three major mineralized zones within Main Cirque as well as several others that occur outside the cirque perimeter. The three zones within the cirque, Lake Zone, Brandy Zone, and Main Cirque Zone, have been extensively explored and dri 1 led; the Lake Zone and Main Cirque Zone are currently in production. As the extent of these three zones encompass the entire Main Cirque area, geology discussed below is restricted to this region of the Mt. Skukum Volcanic Complex; based on detailed mapping of the area at scales of 1 •. 1,000 and 1 ;5,000 as well as detailed logging of over 6,000 m of drill core. Volcanic stratigraphy of the Main Cirque area Include Skukum Group Volcanic rocks of Formations 2 ,3 , and 4 (section 2.2.2) distributed as shown in Figures 3.1 and 3.2, with the majority of the area underlain by andesite flows of Formation 3. Economic zones within Main Cirque (Figure 3.3) occur entirely within these andesitic volcanic rocks which are cross-cut by steeply dipping felsic to intermediate dykes, and pebble dykes concentrated in and around zones of structural weakness between fault-bounded, down-dropped blocks which form a collapse feature centered around Main Cirque. Volcanic stratigraphy in the Mine area has been subdivided into nine mappable units (Figures 3.1 and 3.2) on the basis of field observation, roughly 350 hand specimens and over 65 thin sections. Each unit encompasses one or more related lithologies including strata of Formation 2 (unit 1), interbedded andesite flow rocks and epiclastic rocks of lower Formation 3 (unit 2), monotonous, repetative andesite flow rocks of upper Formation 3 (unit 3), interbedded andesitic pyroclastic and porphyritic andesite flow rocks of upper Formation 3 (unit 4), flow-banded rhyolite of lower Formation 4 (unit 5), felsic megabrecciaof 24-Q u a r t z - c a r b o n a t e v e i n s a n d s t o c k w o r k ( u n i t 1 0 ) E j l A l u n i t e - p y r o p h y l l l t e c l a y a l t e r e d gBJj z o n e |?Jp R h y o l i t e b r e c c i a ( u n i t 9 ) l:J'ig\ R h y o l i t e d y k e s a n d s t o c k s ( u n i t 8 ) •vTT] D e n s e l y - w e l d e d r e l s i c t u f f ivvvl ( u n i t 7 ) gjVj F e l s i c m e g a b r e c c i a ( u n i t 6) [ j F l o w - b a n d e d r h y o l i t e ( u n i t 5 ) I n t e r b e d d e d a n d e s i t i c p y r o c l a s t i c a n d p o r p h y r i t i c a n d e s i t e f l o w r o c k s ( u n i t 4 ) I P o r p h y r i t i c a n d e s i t e f l o w r o c k s ( u n i t 3 ) l> K l I n t e r b e d d e d a n d e s i t i c e p i c l a s t i c a n d I • p o r p h y r i t i c a n d e s i t e f l o w r o c k s ( u n i t 2 ) F o r m a t i o n 2 ~ A n g u l a r U n c o n f o r m i t y (observed'inferred) " F a u l t (observed''inferred) Figure 3.1. Geologic map of Main Cirque, Mt. Skukum, Yukon Territory. Figure 3.2. Geologic cross section of Main Cirque, Mt. Skukum, Yukon Territory. Section Is based on field observation and the geologic map (Figure 3.1). "1 Figure 3.3. Photograph of the Main Cirque looking north from the peak of Mt. Skukum, Yukon Territory. Locations of the three major mineralized zones Lake Zone (LZ), Brandy Zone (BZ), and Main Cirque Zone (MZ) are indicated. Formation 4 (unit 6), densely-welded felsic tuff of Formation 4 (unit 7), rhyolite dykes and stocks (unit 8), and rhyolite breccia (unit 9). All extrusive volcanic units are gently dipping rarely inclined more than 12*. Contacts of intrusive units are steeply dipping to verticle. Several large faults cross-cut stratigraphy with a north-south or east-northeasterly trend. 28 3.2. Geoloov and Petrography of Lithologies in Main Cirque Emphasis of petrography in this chapter is on primary mineralogy and textures in lithologies characteristic of Main Cirque. Highly altered and mineralized horizons are intentionally avoided as they are treated separately in section 4.3. Four varieties of igneous rocks of three compositions are present in Main Cirque, including two varieties of andesite, and one each of rhyolite and dacite. Pyrxlastic and epiclastic rxks present in Main Cirque are xmposed of cognate, essential, and accessory clasts derived mainly from ignxus rxks of these xmpositions. Table 3.1 shows the average mineralogical mode for exh of the ignxus rxk xmpositions in Main Cirque. These and derived volcanic and epiclastic rxks are described below in order of stratigraphic age from oldest to youngest. 3.2.1. Formation2: Rxks of Formation 2 outcrop over a small area in the extreme north-central portion of Figure 3.1 where they are mapped as a single unit (unit 1). This formation, which conformably overlies units of Formation 1 (Figure 3.2), romprises a bedded sequence of felsic to intermediate pyrxlastic and derived epiclastic rxks which form thin individual beds usually about 0.5 m thick and never more than 6.5 m thick. Bedding xntacts, generally sharp, separate pyrxlastic rxks from interbedded epiclastics of similar xmposition emphasising their well-bedded nature in outcrop (Figure 2.6). Although lithologies are gradational with respxt to grain size and xmposition, six lithologies are recognized as follows (in order from oldest to youngest): rhyolite lapilli tuff, rhyolite ash tuff, debris flow/lahaar rxks, volcaniclastic sedimentary rxks, andesitic lapilli tuff, and andesitic ash tuff. Lithological descriptions below are based on field, hand spximen and thin sxtion examination of a measured sxtion of Formation 2 8S it oxurs in the northern part of Figure 3.1; they are not intended to charxterize this formation as it occurs throughout the Mt. Skukum Volcanic Complex. 29 Table 3.1. Average visually estimated mineralogical modes of igneous rocks present in Main Cirque, Mt. Skukum, Yukon Territory. Porphyritic Andesite Andesite Dykes Dacite Rhyolite Whole Rock Mode Phenocrysts 15 2 2 26 Groundmass 85 98 98 74 Mineralogical Mode Primary Minerals: Plagioclase 66 53 41 32 K-Feldspar - - 14 25 Quartz - - 15 33 Pyroxene 3 Tr - -Apatite Tr Tr - Tr Opaques 2 2 - — Secondary Minerals: Chlorite 16 25 15 3 Epidote 16 - 10 1 Sericite 3 10 - 4 Calcite 2 6 - 2 Leucoxene - - 5 -Pyrite - - Tr Tr Quartz 2 4 - -Hematite Tr - - -Zircon - - - Tr Tr = Trace amount 30 Rhyolite tapiffi tuff is relatively uncommon and occurs mainly in the lower part of Formation 2 as thin beds (Figure 3.4) which, in the Main Cirque area, do not exceed 1 m in thickness. Fragments, although primarily rhyolitic in composition, are heterolithic containing up to 15 percent basement fragments including pelitic schist, quartzite, and marble. Essential and cognate rhyolite fragments comprise up to 45 percent of the rxk and commonly display at least a minor degree of welding through the alignment of collapsed, devitrified pumice fragments which can comprise up to 30 percent of the rock. Matrix material is typically composed of an aphanitic pale-green volcanic ash which gives the rxk a pale green colour. Rftyoh'teash tuff is common throughout the lower portion of Formation 2 where it occurs as pale-green, extremely fine-grained rxks comprising individual beds which do not exceed 81 cm thickness in the Main Cirque area. This lithology is typically composed of minute shards of partially devitrified volcanic glass containing 2 mm sized essential and cognate rhyolite fragments, as well as minor basement fragments and rare quartz "eye" and feldspar crystal ejxta. These fragments can locally comprise up to 85 percent of the rxk but usually constitute less than 10 percent of the rxk by volume. Essential fragments are typically most abundant and commonly occur as elongate, xllapsed pumice clasts drawn out parallel to bedding and wrapped around cognate fragments to produce a mildly welded texture. Axretionary and armored lapilli up to 7 mm across are common in this unit and can comprise up to 30 percent of the rxk (Figure 3.5). Debris flow/iahaar oxurs only in the lower part of Formation 2 interbedded with rhyolitic pyrxlastic and minor finer grained epiclastic units. In outcrop, this unit forms the thickest beds in Formation 2 that are commonly 2.5 m thick but locally up to 6.5 m thick, and invariably massive in appearance. In hand specimen (Figure 3.6), these rxks contain an average of 60 percent subrounded fragments that are commonly 1 cm across but locally exceed 26 cm across and are enclosed in a matrix of finer grained, pxrly sorted material of the same composition. Clasts in beds of this lithology grade up-sxtion from coarse to fine in size and from Figure 3.4. Photograph of a typical outcrop of rhyolite lapilli tuff of Formation 2 from Mt. Skukum, Yukon Territory. Figure 3.5. Photograph of rhyolite ash tuff of Formation 2 showing concentrically layered accretionary lapilli from Mt. Skukum, Yukon Territory. Figure 3 6 Photograph of handspecimen/outcrop exposure of a lahaar of Formation 2 showing the polymictic content of angular to sub-rounded fragments from Mt. Skukum, Yukon Territory. 33 clasts of predominantly basement lithologies in the lower part of the formation to clasts of predominantly rhyolitic composition in the upper part of the formation. Beds of this lithology in the lower part of the formation contain clasts averaging 4 cm across comprised of up to 70 percent basement clasts together with minor rhyolitic and rare andesitic clasts. Beds of this lithology in the upper part of the formation contain clasts that do not exceed 2 cm across and are comprised of 50 percent rhyolitic volcanics, 40 percent basement rocks, and 10 percent andesitic volcanics. Individual beds of this unit commonly display crude reverse grading where the coarsest fragments occur towards the top of the bed and the finer fragments occur towards the bottom. Debr is flow/lahaar rocks are interpreted to be products of slope instability that resulted in mud and rock slides containing locally derived volcanic and basement clasts. Vo/cmic/asticsedimentaryrooks 8re most common in the middle and upper portions of Formation 2. Outcrops display a well-bedded nature (Figure 2.6) defined by layers of differing grain sizes which range from coarse sand to silt-sized grains. Clast composition is variable from rhyolitic to andesitic with minor amounts of basement fragments that become less common in beds higher in the stratigraphic pile. Hand specimens of this unit display a fine laminar bedding characteristically consisting of interbedded sandstone and siltstone layers ranging from 0.5 to 2 cm thick and contain sub-rounded clasts which rarely exceed 4 mm across. Beds range from poorly to well-sorted, and display either reverse or normal grading. Individual clasts commonly display weathered rinds and invariably show compositions and textures similar to the volcanic units immediately surrounding them (Figure 3.7). Andesite ash tuff is relatively common throughout the upper portion of Formation 2 where it occurs as massive olive-green to maroon beds ranging from 0.5 to 2.1 m thick. Maroon beds are predominant (Figure 3.8) and consist primarily of ash-sized fragments ranging in size from minute to 1 mm across. Lapilli-sized fragments of cognate and essential porphyritic andesite, rarely exceeding 5 mm across, can comprise up to 3 percent of the rock surrounded by a Figure 3.8. Photograph of a typical outcrop exposure of maroon andesite ash tuff from the upper portion of Formation 2 at Mt. Skukum, Yukon Territory. 35 matrix of finer cognate and essential grains. Accretionary and armoured lapilli (Figure 3.9) are characteristic of this lithology. They range in size from 1.5 to 3 mm across and typically comprise 1 percent of the rock but locally constitute up to 90 percent. These lapilli are typically ovoid in shape with long axes oriented parallel to bedding and display concentric layers of fine ash particles which commonly show an intense hematitic colouration in the outermost layer. Essential fragments are devitrified and commonly highly vesicular with chlorite replacing the andesitic glass and forming rosettes that infill many of the vesicles. Maroon beds of this lithology occur only in the uppermost part of Formation 2 and form an excellent marker horizon in several exposures of these rocks in the western part of the Mt. Skukum Volcanic Complex. AfKfesJtic tapiIJi tuff is relatively uncommon and is found only in the upper portion of Formation 2 where it forms beds ranging in thickness from 10 cm to 2.5 m. Most prominent is a thick bed which is particularly coarse-grained and forms the cap unit of Formation 2. In outcrop, individual beds (Figure 3.10) are massive, dark grey-green to maroon, and locally display fiamme textures and a weakly welded appearance through the presence of elongate, compressed pumice fragments aligned parallel to bedding. Hand specimens display angular to sub-rounded fragments which comprise up to 75 percent of the rock by volume in a matrix of ash-sized particles. Fragment composition is entirely andesitic and is comprised of up to 70 percent devitrified essential fragments altered mainly to chlorite as well as a minor amount of cognate fragments. Fragment sizes range up to 4 cm across but are typically between 4 mm and 1 cm. The bed of this lithology capping Formation 2 is an exception in that it is relatively coarse-grained, approaching a volcanic breccia (Figure 3.11) and consists primarily of cognate fragments of grey-green andesite with a bimodal size distribution of 1 and 20 cm across in a matrix of ash-sized maroon andesite particles. 36 Figure 3.9. Photograph of abundant accetionary lapilli common in andesitic ash tuff in the upper portion of Formation 2 at Mt. Skukum, Yukon Territory. Accretionary lapilli occur only in the fine-grained maroon ash layer in this specimen. 37 Figure 3.11. Photograph of distinctive, coarse andesitic lapilli tuff which forms the top horizon of Formation 2 with grey-green porphyritic andesite fragments enclosed in a fine grained hematitic andesite matrix at Mt. Skukum, Yukon Territory. 38 3.2.2. Formation 3; Rocks of Formation 3 are widely distributed in Main Cirque where they occur as a sequence of andesitic flow rocks intercalated with andesitic pyroclastic rocks and epiclastic rocks containing fragments of similar composition. Formation 3 conformably overlies rocks of Formation 2 and is unconformably overlain by Formation 4. Lithologies recognized in Formation 3 are, from oldest to youngest: porphyritic andesite flow rocks, andesitic debris flows, volcaniclastic sandstone, and andesite tuff. These rocks have been mapped as three units in Figure 3.1 (units 2 ,3 , and 4) that reflect textural differences in andesite flow rocks and different associations between rock types. Unit 2 comprises the lower portion of Formation 3 where relatively thin, commonly maroon andesite flows (Figure 3.12) of variable phenocryst size and composition are intercalated with andesitic debris flow material containing a relatively high proportion of felsic clasts. Unit 3 comprises the central portion of Formation 3 and is characterized by thick, monotonous porphyritic andesite flows, sparsely interbedded with volcaniclastic sediments and rare pyroclastic rocks. Unit 4 comprises the upper portion of Formation 3 and is typified by abundant andesitic pyroclastic rocks interbedded with porphyritic andesite flows. Porphyritic andesite flow rocks form an integral part of Formation 3 and comprise the major rock type present in Main Cirque (Figure 3.1). Rxks of this unit oxur as narrow, extensive, gently dipping flows (Figure 2.7) which may be separated from one another by debris flows and pyrxlastic rxks, or may oxur as a repetitive series of flows distinguishable from one another only on the basis of brxciated flow tops and bottoms marking the margins of individual flows. In hand specimen, these rxks are typically dark grey-green, but can range from a very dark greenish-black to dark marxn. Darkest xloured rxks tend to be least affxted by alteration and are relatively rare, usually present in flow centers which are free of fractures and in areas distant from mineralization. Paler grey-green rxks are invariably propylitically to phyllically altered, and are xmmonly amygdaloidal with abundant chlorite, sericite and minor silicif ication. Dark maroon rocks of this unit only occur in the lower part of the formation (Figure 3.12). Andesitic flow rocks (Figure 3.13) are invariably porphyritic and characteristically contain 15 to 30 percent plagioclase phenocrysts ranging from 2 to 5 mm across as well as up to 10 percent pyroxene phenocrysts averaging 1 to 2 mm across. Phenocryst composition remains relatively constant throughout the central and upper parts of the formation, but andesite flows in the lower part of the formation are characterized by a large variability in phenocryst size and composition. Thin sections (Figure 3.14) show that plagioclase phenxrysts, which may be randomly oriented or display a trachytic alignment, show oscillatory zonation (Figure 3.15) and range in composition from An3o to An^g. with an average of about An^- In the freshest samples, plagioclase is nearly free of alteration; however, more commonly, grains are variably saussuritized, sericitized, and silicified. Augite phenocrysts, ubiquitous in andesite flow rxks, are also nearly free of alteration in the freshest samples but are wmpletely altered to chlorite in rxks which appear only slightly propylitically altered. Augite and plagixlase oxur mainly as isolated crystals in flows but also oxur together as glomerxrystic aggregates distributed randomly throughout the rxk. Magnetite crystals, not visible in handspximen, constitute between 1 and 3 volume percent of fresh andesite flow rxks as fine grains which give the rxk a moderately magnetic character. In altered spximens magnetite is one of the earliest affxted minerals and dexmposes to leuxxene. Andesitic debris flow rocks are common in the lower part of Formation 3 where they oxur interbedded with andesite flow rxks of highly variable phenxryst xntent and size. They become less xmmon in the central and upper portions of Formation 3 where epiclastic units are dominated by sandstones and siltstones. Individual beds are massive, matrix-supported, and rarely over 3 m thick. They consist of pxrly sorted, angular to subrounded fragments averaging 3 to 4 cm across in a finer grained groundmass. These coarse fragments xmprise up to 70 percent of the rxk and are almost entirely xmposed of porphyritic andesite with minor amounts Figure 3.12. Photograph of porphyritic andesite flow rocks of unit 2 (lower Formation 3), Mt. Skukum, Yukon Territory showing the characteristically low phenocryst content and maroon colour of these rocks. Figure 3.13. Photograph of a typical outcrop of porphyritic andesite flow rxk showing characteristic phenxryst abundance and distribution in this rxk type in the central and upper portions of Formation 3 at Mt. Skukum, Yukon Territory. Figure 3.14. Photomicrograph of typical porphyritic andesite flow rock under crossed polars showing the altered nature of plagioclase and pyroxene phenocrysts typical in Formation 3 at Mt. Skukum, Yukon Territory. Magnification is 45 times. 42 of felsic volcanic rock of Formation 2, and rare basement fragments. This unit, similar in most respects to debris flow rocks of Formation 2, has a noteable lack of felsic and basement fragments. Volcaniclastic sandstone occurs in the central and upper parts of Formation 3 as rare layers seldom more than 1 m thick interbedded with andesitic lapilli tuff and porphyritic andesite flow rocks. In hand specimen, this unit is dark grey-green, similar to that of propylitically altered porphyritic andesite flow rocks, and is characterized by a fine laminar inter bedding of silt and sand-sized grains which are well-sorted and range in size from 0.2 to 2 mm. Clast composition is typically exclusively andesitic; coarse-grained beds display a relatively high degree of hydrothermal permeability as quartz, sericite, calcite, and pyrite occur locally as an interstitial precipitate. This unit is similar to finer grained volcaniclastic sedimentary rxks of Formation 2 with the principal difference being a Ixk of marxn xlouration which dominates andesitic volcanic rxks of that formation. Andesite tuff is ubiquitous to Formation 3 but is most xmmon in the central and upper parts of the formation where it oxurs as beds of ash to lapilli tuff interbedded with porphyritic andesite flow rxks. Individual beds of this lithology range from 10 cm to 7 m thick snd 8re typically massive, dark grey-green and, due to a relatively high hydrothermal permeability, tend to be Intensely altered and silicified. Hand spximens (Figure 3.16) display a fragmental charxter with angular to sub-angul8r lapilli to ash-si2ed cognate fragments of porphyritic andesite averaging 3 cm xross in lapilli tuffs and rarely exceeding 2mm xross in ash tuff beds. Coarser fragments are enclosed in a finer grained, blexhed and relatively more intensely altered matrix of angular lithic fragments of the same xmposition, as well as variable amounts of feldspar crystal fragments. Charxteristically, essential fragments (Figure 3.17) are rare; however, in the upper part of the formation essential fragments locally xmprise up to 15 percent of the rxk. Thin sxtions display the high degree of silicification and sericitization of fragments and matrix in these rocks with primary plagixlase phenxrysts xmpletely replaced by sericite, 43 Figure 3.17. Photomicrograph of devitrified essential fragments in andesite tuff from the upper portion of Formation 3 showing replacement of andesitic pumice by chlorite at Mt. Skukum, Yukon Territory. Magnification is 175 times. calcite and epidote; pyroxene phenocrysts are completely replaced by chlorite. Alteration in the matrix is locally so intense that quartz and pyrite can totally replace original constituents. 3.2.3. Formation 4: Rxks of Formation 4 oxur at the highest elevations (1,900 to 2,100 m) on the eastern wall of Main Cirque (Figure 3.18), in the southeastern xrner of Figure 3.1. This formation represents a period of explosive felsic volcanism which followed a period of erosion after deposition of Formation 3. Formation 4 overlies Formation 3 unxnformably and, in the Main Cirque area, is xmposed of thrx distinct lithologies: a thick lower flow-banded rhyolite (unit 5), felsic megabrexia (unit 6), and a densely-welded felsic tuff (unit 7). Flow banded rhyolite forms the lower unit of Formation 4 in the southwestern portion of Figure 3.1. In outcrop, it oxurs as a massive, marxn, porphyritic unit over 200 m thick that has a gentle east southeasterly dip and displays fine flow-banding. Hand spximens xntain rare plagixlase, orthxlase and quartz phenxrysts, typically less than 3 mm xross that are enclosed in an aphanitic felsic groundmass showing fine white and marxn flow-bands wrapped around large, locally abundant spherulites up to 6 cm xross (Figure 3.19). Autobrxciation textures (Figure 3.20) are locally abundant and display angular, marxn, flow-banded, brexia fragments enclosed in a matrix of aphanitic, marxn rhyoliteflow material. This flow unit, in the southernmost end of Main Cirque, forms a circular dome-shaped intrusion which may represent a feeder to the flow. Felsicmegabreccia oxurs in the southeastern xrner of Figure 3.1 where it lines a deeply cut bowl-shaped depression (Figure 3.18) formed in the underlying flow-banded rhyolite. It xnsists of enormous rotated blxks of flow-banded rhyolite, andesite flow rxks of Formation 3 and interbedded lithologies of Formation 2 enclosed in a greenish-brown, unsorted matrix of finer brexia. This unit attains a maximum thickness of about 200 m with steep to vertical xntxts 45 Figure 3.18. Photograph of the eastern wall of Main Cirque, Mt. Skukum, Yukon Territory, with lithological boundaries outlined in black. F3 = rocks of Formation 3, F4R = flow-banded rhyolite of Formation 4, F4M = Felsic megabreccia of Formation 4, F4T = densely-welded felsic tuff of Formation 4, AC2 = Alunite cap zone. Figure 3.19. Photograph illustrating spherulites in flow-banded rhyolite of Formation 4 at Mt. Skukum, Yukon Territory. Figure 3.20. Photograph of flow-banding and autobrecciation textures in flow-banded rhyolite of Formation 4 from Mt. Skukum, Yukon Territory. 47 against underlying rocks in the north and east. Breccia blocks attain sizes several times that of a large house - - the largest are all comprised of flow-banded rhyolite. This unit is interpreted to partially infill a volcanic crater with the largest blocks of flow-banded rhyolite formed through slumping and collapse of the crater margins. Blocks of more highly displaced lithologies of Formations 2 and 3 may have been deposited at this location during eruption. Densefy-weid&f felsic tuff occurs at the highest elevations in Main Cirque where it overlies the felsic megabreccia and forms a final infilling in the crater on the eastern wall of the cirque (Figure 3.1 and 3.18). In outcrop, this unit is dark brown, up to 150 m thick, and displays a pronounced columnar jointing. In the central part of the unit where the contact between felsic tuff and underlying rxks in the crater bottom is close to horizontal, these xlumns form vertical pillars extending upwards to the top of the unit. On the crater margins, however, particularly to the north where the xntx t betwxn felsic tuff and underlying lithologies is almost vertical, xlumns display bends of almost 90° from near horizontal in areas close to the vertical crater walls, to vertical near the top of the felsic tuff unit. In hand spximen, this lithology is pale brown xntaining abundant cognate and crystal fragments as well as elongate essential fragments with well-developed fiamme (Figure 3.21). It is interpreted to have formed as a single cxling unit infilling the crater from which it was erupted. 3.2.4. igneousintrusiveRocks-Ignxus intrusive rxks xcur throughout the Main Cirque as dykes and stxks of felsic to intermediate xmposition. In the Main Cirque area, intrusive rxks primarily cut strata of Formation 3 and are interpreted as intrusive equivalents and feeders for volcanic rxks of Formations 3 and 4. Rhyolitic, dxitic and andesitic dykes are recognized in Main Cirque; all parallel nearby fault zones and dominant local frxtures. Rhyolite dykes and stxks (unit 8) are the oldest and largest intrusive rxks in the cirque. They are cross-cut by younger dxitic and 48 Figure 3.21. Photograph of a hand specimen of densely-welded felsic tuff of Formation 4 showing elongate essential fragments and associated fiamme at Mt. Skukum, Yukon Territory. 49 andesitic dykes most abundant in the Main Cirque Zone that are too narrow to be represented in Figure 3.1. All types of dykes are cross-cut by the veins. Rnyvtite dikes/stacks were intruded as large, irregular bodies into rxks of Formation 3 in the Main Cirque area. Stxks and dykes are similar in texture and xmposition and are assxiated with mineralized zones in Main Cirque. The only rhyolite stxk in Main Cirque (Figure 3.22) is in the southwestern xrner of Figure 3.1 where it underlies an area of red hematite staining known as Red Ridge. Rhyolite dykes in Main Cirque, irregular in shape and thickness, are oriented parallel to fault zones with trends between N10'W and N35'E. These dykes oxur as tan, massive, porphyritic, stxply dipping bodies up to 30 m thick that cross-cut volcanic strata of Formation 3 and, in turn, are cut by relatively narrow but equally extensive andesite and dxite dykes. Hand spximens of rhyolite dykes and stxks display feldspar phenxrysts (Figure 3.23) from 2 to 5 mm xross that are randomly oriented in an aphanitic to fine grained, locally flow-banded groundmass. The average mineralogical mode of this lithology is in Table 3.1. Typically alteration is intense, with phenxrysts xmpletely replaced by sericite, calcite, and epidote and groundmass affxted by patchy to pervasive propylItization and phyllic alteration. In thin sxtion, areas of relatively fresh groundmass oxur as minute, euhedral, prismatic needles of randomly oriented feldspar enclosed in a mozaic of coarser grained anhedral quartz (Figure 3.24). The most prominant rhyolite dyke in Main Cirque oxurs in the Main Cirque Fault Zone where it is flanked on either side by a thick marginal brexia (unit 9) which is evidence of the violent nature of its intrusion. This brexia, up to 10 m thick, is xmposed of a silicxus matrix enclosing angular, flow-banded rhyolite fragments up to 10 cm xross. Fragment xmposition bexmes increasingly andesitic with distance from the dyke and, in areas adjxent to andesite xuntry rxk , brexia fragments are almost entirely andesitic. Rhyolite stxks and dykes in the Main Cirque area are xrrelative with porphyritic rhyolite intrusions which cross-cut rxks of Formation 4 elsewhere in the Mt. Skukum Volcanic Complex (Figure 3.1) and were probably intruded as a late-stage, resurgent event localized in frxtures and faults formed through cauldera xllapse. Figure 3.22. Photograph of the rhyolite stock intruding rocks of Formation 3 on the south western wall of Main Cirque on Red Ridge (Figure 3.1), Mt. Skukum, Yukon Territory. Figure 3.23. Photograph of handspecimens of rhyolite dyke, one stained and one unstained showing characteristic distribution of plagioclase and orthoclase phenocrysts as well as two fine cross-cutting fractures infilled by pyrite. Handspecimen on right is 7 cm square, 51 Figure 3.24. Photomicrograph of matrix material in a typical sample of rhyolite dyke rock showing randomly oriented prismatic feldspar grains enclosed in a mozaic of coarser grained quartz at Mt. Skukum, Yukon Territory. Magnification is 175 times. 52 AncteftedWes form steeply dipping, north to northeast trending bodies which cross-cut rocks of Formation 3 within Main Cirque and the surrounding area. These dykes occur in two texturally distinct forms with distinct distributions, and different times of emplacement in Main Cirque. The earliest formed, most widely distributed andesite dykes are relatively extensive, recessive weathering, widely spaced bodies up to 5 m thick that commonly display a horizontal columnar jointing. In hand specimen these dykes appear similar to porphyritic andesite flow rocks in colour, phenocryst size and composition. They are typically massive, non-vesicular, display narrow chill margins commonly less than 2 cm across, and are locally glomeroporphyritic. These dykes probably formed feeders to porphyritic andesite flows in the upper part of Formation 3. The second type of andesite dyke is far less widely distributed. It occurs as relatively narrow, steeply dipping bodies rarely exceeding 3 m across that are oriented parallel to the trend of the Main Cirque Fault Zone. These dykes are composltlonally similar to the first type but show a lower content of plagioclase phenocrysts that averages 3 percent of the rock. Abundant amygdules comprising up to 5 percent of the rock commonly display a concentric infilling of quartz, chlorite, sericite and calclte. Thin sections show that these dykes invariably display intense propylitlc or phylllc alteration, probably as a result of their close proximity to mineralization. Plagioclase phenocrysts are typically almost entirely replaced by sericite and carbonate with a patchy but pervasive alteration of matrix material to sericite, calclte, epidote, and quartz. This amygdaloidal, sparsely porphyritic variety of andesite dyke displays cross-cutting relationships with rhyolite dykes in the Main Cirque Zone. Andesite dykes in the Main Cirque area are too narrow to appear in Figures 3.1 and 3.2. Dacfteoykes occur as steeply dipping, north to northeast trending bodies cross-cutting other lithologies of Formation 3 and closely associated with amygdaloidal, sparsely porphyritic andesite dykes most abundant in the vicinity of the Main Cirque Fault Zone. As with the andesite dykes, dacite dykes are too narrow to appear in Figure 3.1 or 3.2. Dacite dykes occur as greenish-grey to tan porphyritic rocks with an aphanitic groundmass surrounding sparse quartz, plagioclase and orthoclase phenocrysts between 2 and 6 mm across that comprise up to 10 percent of the rock but average 5 percent. Thin sections show the matrix has a trachytic llneation produced by alignment of prismatic plagioclase and orthoclase crystals averaging 0.016 mm In length. Matrix feldspar crystals are interspersed with up to 20 percent quartz grains which average 0.26 mm across. Euhedral pyroxene phenocrysts present In the rock are Invariably completely replaced by chlorite or calcite. Amygdules, common in the dacite dykes, may comprise up to 5 percent of the rock and are typically infilled with calcite but locally contain sericite, chlorite and quartz. These dykes are penecontemporaneous with the late andesite dykes described above and form similar cross-cutting relationships with rhyolite dykes. Pebbleoykes, typically between 10 and 30 cm and rarely exceeding 1 m across, occur throughout Main Cirque but are most common adjacent to fault zones associated with mineralization. These bodies, which are steeply dipping and trend parallel to faults and dominant local fractures, are framework supported and composed of a variety of rounded to sub-angular clastic components comprised mainly of porphyritic andesite of variable colour and phenocryst size and composition (Figure 3.25). In addition, clasts of granitic rock, rhyolite, politic schist, and marble are common. Lithic fragments, typically 3 mm to 5 cm across, comprise 80 percent of the rock and display intense phyllic and propylUic alteration. Matrix material consists mainly of fine-grained anhedral quartz with sericite, epidote and chlorite occurring as a late-stage Infilling. Locally, space between fragments Is not completely Infilled and the resulting cavities are lined with druzy quartz and calcite crystals up to 7 mm long. Pebble dykes cross-cut rocks of Formation 4 but pre-date mineralization as quartz-carbonate material similar to that of surrounding veins is found infilling cavities in the pebble dykes. 54 Figure 3.25. Photograph of a typical outcrop exposure of pebble dyke material from Mt. Skukum, Yukon Territory, showing the polymictic nature of rounded to sub-angular fragments and the framework supported character of this lithology-55 3.3. Whole Rock Geochemistry of Igneous Lltholooies in Main Cirque 3.3.1. Introduction; Sixteen samples of relatively fresh rhyolite of unit 8 (Figure 3.1) and andesite of unit 3 (Figure 3.1) from Main Cirque were analysed for major elements and zirconium using X-ray fluorescence by X-Ray Assay Laboratories, Don Mills, Ontario (Appendix A). Samples were obtained from surface exposures and drill core within the study area for the sole purpose of geochemically confirming rock names assigned important igneous units in the map area through hand specimen and thin section examination. Specimens used in this chapter have been selected as representative of igneous rocks in Main Cirque. The data include 10 specimens of relatively fresh porphyritic andesite of unit 3, and 6 specimens of rhyolite dyke. Rocks of these two igneous compositions comprise the fundamental elements in all epiclastic and pyroclastic rocks in the area. Consequently, the chemical composition of almost all rocks in Main Cirque, certainly those of Formation 3, is described by the composition of these andesitic, and rhyolitic igneous rxks. Samples were chosen where possible to avoid effxts of hydrothermal alteration, however, as indicated by oxygen isotopes (section 5.2), all have been affected to some extent, even those perceived in handspecimen to be pristine. Complete data sets of chemical analyses used below are listed in Tables 3.2 and 3.3. Locations of drill core samples are listed in Appendix B. 3.3.2. Classification: Igneous rocks from Main Cirque can be broadly classified using the AFM diagram of Irvine and Baragar (1971). The geochemistry of al) 14 samples are plotted using this classification scheme in Figure 3.26 and show a narrow range of hydrothermal alteration reflating the relatively fresh nature of selected samples. All igneous rock samples from Main Cirque are calc-alkaline in nature, analogous to rocks of the Cascades and Mt. Hood (Irvine and Baragar, 1971). Plotted data displays a discontinuous, curvilinear trend between two elongate clusters of points which probably represents 8 fractionation series for the volcanic rocks at Mt. Skukum. The tendency of points to cluster into two elongate groups rather than to form a continous linear 56 Table 3.2. Major element chemistry of rhyolite dyke (unit 8) from Main Cirque (Figure 3.1), Mt. Skukum, Yukon Territory. Major element analyses are reported as oxides in weight percent; zirconium analyses are reported in parts per million. Element* C298? C246 C245 C296 BM840353 BM84069 S102 68.5 67.8 70.5 70.8 75.2 76.6 AI2O3 13.97 14.55 13.34 13.91 11.9 11.8 CaO 1.01 0.98 0.72 0.63 0.08 0.08 MgO 0.29 0.23 0.28 0.32 0.09 0.00 N&2O 3.77 2.74 3.35 3.64 1.82 3.44 K20 5.09 6.63 4.94 4.46 6.81 4.76 F e ^ 2.31 2.02 2.24 2.25 1.19 1.10 MnO 0.07 0.05 0.07 0.04 0.01 NA TIO2 0.20 0.21 0.18 0.19 0.11 0.10 P2O5 0.02 0.02 0.03 0.02 0.03 0.02 LOI 1.80 1.50 1.60 1.20 1.08 1.16 Irest4 2.17 3.58 1.14 1.28 NA NA SUM 99.17 99.48 98.34 98.68 98.4 99.1 Zr NA NA NA NA 120 120 1. F e ^ = Total iron presented as F e ^ ; NA = No Analysis. 2. Samples with the prefix C were obtained from underground mine workings In the Main Cirque Ore Zone in areas adjacent to vein mineralization. 3. Samples with the prefix BM indicate surface samples taken of unit 8 In Main Cirque. 4. Indicates minor elements not reported which contribute to the total. 57 Table 3.3. Major element chemistry of porphyritic andesite flow rocks( unit 3) from Main Cirque (Figure 3.1), Mt. Skukum, Yukon Territory. Major element analyses are reported as oxides in weight percent; zirconium analyses are reported in parts per million. Element1 C2782 C279 C280 C281 BM1853 BM186 BM187 BM023 BM028 BM059 Si02 56.2 58.5 56.8 57.7 58.9 58.2 57.1 55.8 56.4 58.3 A I 2 O 3 16.3 16.4 16.5 16.3 17.7 17.6 17.5 16.8 16.4 16.4 CaO 6.65 6.40 6.51 6.39 5.65 6.26 7.05 6.06 6.09 5.79 MgO 3.49 3.28 3.09 3.24 1.72 2.39 2.20 3.08 3.06 2.51 Na20 2.57 2.70 2.59 2.67 3.10 2.66 2.85 2.94 2.70 3.03 K20 2.68 2.59 2.64 2.52 2.62 2.75 1.77 2.48 2.66 2.69 Fe203 7.88 7.70 7.63 7.57 6.26 6.85 6.88 7.66 7.43 7.18 MnO 0.14 0.14 0.14 0 14 0.11 0.11 0.14 0.14 0.13 0.13 TIO2 0.99 0.97 1.00 0.95 0.95 0.94 0.95 1.05 0.99 0.99 P 2 O 5 0.27 0.28 0.27 0.27 0.29 0.29 0.29 0.28 0.29 0.29 LOI 1.31 1.31 1.70 2.31 2.08 2.00 2.85 3.39 3.08 2.00 SUM 98.7 100.5 99.1 100.3 99.7 100.3 99.8 99.8 99.3 99.4 Zr 170 180 190 180 NA 180 170 170 180 180 7*7507 = Total iron presented as F ^ ; N.A. = No Analysis. 2. Samples with the prefix C were obtained from underground mine workings in the Main Cirque Ore Zone in areas adjacent to vein mineralization. 3. Samples with the prefix BM indicate surface samples taken of unit 8 in Main Cirque. 58 distribution is probably a result of sampling only two units over the restricted area of Main Cirque which probably represents merely a portion of the complete magmatic series in the complex. F A M Figure 3.26. AFM plot after Irvine and Baragar illustrating the calc-alkaline nature of Igneous rocks at Mt. Skukum. Data is from Appendix 3.A. A = Hafl+KjO, F = Total iron, M = MgO. The geochemical classification scheme of LeBas (1986) allows a more detailed characterization of igneous rocks in Main Cirque Involving the plot N^ O+fc^ O Y£.Si02 (Figure 3.27). Using this scheme, most samples of igneous rocks from Main Cirque are oversaturated in silica and plot Just inside the border between saturated and oversaturated rocks. Again the data produces two clusters representing the felsic and the more mafic rocks. Rocks named andesite on the basis of petrography cluster near the junction between four fields of intermediate rocks. All are evenly distributed between the basaltic andesite and andesite fields. Two specimens identified petrographically as rhyolitic plot as trachytes; four others plot in the rhyolite field. Once again, plotted data in Figure 3.27 probably represents the products of fractionation for the Mt. Skukum volcanic rocks. As in Figure 3.26, the lack of a continuous trend between data clusters is probably 59 the result of sampling only two rock units within the limited area of Main Cirque which may have restricted the availability of the full variety of rock types expected from differentiation. The tendency of some rhyolitic samples to plot in fields indicating a relatively high concentration of alkalis elements and silica saturation rather than oversaturatlon, may be due to an influx of hydrothermal fluids rich in potassium (section 4.3). As this process is shown to occur in highly altered rocks, the overlap of points into the trachyte field may not entirely reflect the original chemistry. Rock names assigned through hand specimen and thin section examination. • Rhyolite (unit 8) • Andesite (unit 3) 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 s io 2 Figure 3.27. Plot of Na^O^O and Si02 after LeBas (1986) illustrating the distribution of lithologies at Mt. Skukum, Yukon Territory using this geochemical classification scheme. Data is from Appendix 3.A. Abbreviations are as follows: F = foldite, Pc = picrobasalt, Ul = basanite-tephrite, U2 = phonotephrite, U3 = tephriphonolite, Ph = phonollte, B = basalt, SI = trachybasalt, S2 = basaltic trachyandesfte, S3 = trachyandesite, T = trachyte, 01 = basaltic andesite, 02 = andesite, 03 = dacite, R « rhyolite. The letters 0, S, and U indicate the general state of silica saturation: 0 = oversaturated, S - saturated,and U = undersaturated. The classification scheme of Winchester and Floyd (1977) involves the plot of S102 the ratio Zr/Ti02 (Figure 3.28) (elements usually considered immobile) which is intended to minimize the effects of hydrothermal alteration superimposed on the rocks. Using this plot all samples are classified as subalkaline with several of the more mafic rocks lying very close to the mildly alkaline boundary. Specimens are grouped In two clusters, the largest of which occurs just inside the andesite field,- the second, smaller cluster In the rhyolite field consists of the only two specimens on which zirconium analyses were done. No basalts, dacltes or trachytes are present according to this classification scheme. Despite the occurence of two distinct clusters of data in Figure 3.28 these rocks probably represent a differentiation series from a single magma body as the trend between the two clusters is similar to that seen in the normal differentiation series at several other volcanic centers such as Mt. Misery, Easter Island, and the Dunedin Volcano (Winchester and Floyd, 1977). The lack of a continuous trend between data clusters is probably again the result of sampling only two units within the limited area of Main Cirque to which this study is restricted. Rock names assigned through hand specimen and thin section examination. o Rhyolite (unit 8) • Andesite (unit 3) IO"3 10"2 IO"1 10° Zl7TI02 Figure 3.28. Geochemical classification plot after Winchester and Floyd, 1977 showing the positions of igneous rocks from Main Cirque, Mt. Skukum, Yukon Territory. Data is from Appendix 3.A. 3.3.3. Discussion: The calc-alkaline character of the Main Cirque igneous rocks (Figure 3.26) is consistent with the location and geology of the Mt Skukum Volcanic Complex. Calc-alkaline rocks are prominant throughout the North American Cordillera and 8re typified by basalt-andes1te-d8Cite-rhyollte associations common in andesitic volcanoes in Circum-Paclfic regions (Irvine and Baragar, 1971). In Figures 3.27 and 3.28, Igneous rocks of Mt Skukum were found to be generally oversaturated in silica, and sub-alkaline In nature, an observation that supports the calc-alkalic classification in Figure 3.26. The rock compositions cluster as two lithological groups, one of andesites and the other of rhyolites 1n Figures 3.27 and 3.28. The possibility of minor hydrothermal enrichment of fresh rocks in potassium causing an artificial classification of two rhyolitic rxks in the more alkalic trachyte field in Figure 3.27 is reasonable considering the close proximity of these dykes to mineralized veins (section 4.3). Consequently, major Igneous lithologies in Main Cirque are seen to trend between sub-elkaltc andesite and sub-alkalic rhyolite. Field classification of Skukum Group volcanic rxks In Main Cirque is confirmed on the basis of major element chemical analysis. Field names adequately represents their initial chemistry at the time of their formation. 62 3.4. Structure Volcanic rocks in Main Cirque are dissected by a series of high-angle normal faults that form a down-dropped graben centered on the Main Cirque Zone. Faults in Main Cirque display a dominant northeast trend with a series of major splay-faults branching northward (Figure 3.1). These north and northeast branching faults produce a series of 8t least three wedge-shaped blocks which have been displaced downward forming a collapse feature. The downward displacement of each block appears to increase progressively to the east resulting in a step-like topography (Figure 3.29) downward from west to east. E8ch block is bounded by these normal faults (Figure 3.2) with the eastern and western walls of the cirque marked by fault scarps which form the edges of the collapse feature. The eastern wall of the cirque is the largest of these fault scarps rising steeply to a height of over 400 m above the bottom of Main Cirque thereby exposing rocks of the upper Formation 2 on its northern face (Figure 3.1), and rocks of Formations 3 and 4 at higher elevations (Figure 3.18). Rocks on the eastern wall of Main Cirque are separated from down-dropped rocks of the western blocks of Main Cirque by an inferred major fault. As no rocks of Formation 4 are found on these western down-dropped blocks (Figure 3.1), a minimum fault displacement of approximately J 00 m is indicated — the vertical distance between the eroded top of Formation 3 on the east wall and the highest elevation of Formation 3 rocks on the block immediately adjacent to the west in Figure 3.1. Further evidence of the magnitude of displacement along this fault is seen in the exposure of upper Formation 2 on the escarpment of the east wall approximately 250 m higher in elevation than they occur 600 m west in the north-central part of Figure 3.1. Although structure in this area is complicated by several faults, noteably a east-west trending fault inferred to have down-dropped rocks to the north, a maximum displacement of 250 m is indicated. Two other major faults occuring in Main Cirque are the Saddle Fault Zone and the Main Cirque Fault Zone each of which form boundaries between large down-dropped blocks (Figures 3.1 and 3.2). The Saddle Fault Zone forms a prominant linear depression trending between north 8nd 63 Figure 3.29. Photograph of Main Cirque, Mt. Skukum, Yukon Territory, looking due south showing the fault scarps forming the east and west walls of the cirque and the step-like topography produced by down dropped blocks. The traces of several faults are visible on the escarpment at the front of the cirque. 64 north-northeast along the foot of the western wall of Main Cirque and consists of highly fractured rocks and an en echelon series of small faults dipping westward at approximately 60*. At least four large gold-silver bearing quart2-carbonate veins up to 2 m across are located within 75 m of this fault in an area known as the Brandy Zone. A large rhyolite dyke is also associated with the zone running parallel to the fault for about 700 m. All of these structures are oriented parallel to the Saddle Fault Zone which dips approximately 60° west. The second major fault is the Main Cirque Fault Zone which also forms a prominant linear depression visible in air photographs that trends northeasterly across the lowest part of Main Cirque. This fault zone is divided into southern and northern segments, each with slightly different orientations. The southern segment trends approximately 030* and dips eastward at 80°; the northern segment trends approximately 050° and also dips 80* to the east. This zone ranges from 20 to 30 m across and consists of two to three major faults and associated heavy fracturing that is poorly expressed on surface but apparent in drill core as gouge ranging from 10 cm to 3 m or more in width. Quartz-carbonate veins and felsic to intermediate dykes are common In this zone, especially In the southern segment were gouge material is best developed. In the northern segment, faults and fractures are less abundant. Although quartz-carbonate vein material is present it does not appear to develope the same thicknesses seen in the south and leeks the profusion of intrusive dykes. Sense and degree of displacement along both the Saddle and Main Cirque Fault Zones is Impossible to determine accurately due to the lack of distinct marker horizons In rocks of Formation 3. However, in each case it Is inferred that displacement is downward on the eastern side of the fault. Attitudes of volcanic strata in and around Main Cirque tends to be flat-lying with dips rarely more than 20*. Attitudes of strata in the central area of the cirque vary from one down-dropped block to another; however, dips on the eastern and western walls of the cirque show a systematic divergence. Stratigraphy on the western wall of the cirque dip gently to the west at 10°. Stratigraphy on the eastern wall of the cirque dips the same amount but to the east rather than to the west. These divergent attitudes may reflect the original topography surrounding an eruptive center, or may indicate a doming of the area during insurgence of high-level rhyolitic stxks and dykes analogous to a resurgent doming event. Divergent dips displayed by major faults and assxiated structurally controlled dykes and veins on the eastern and western sides of the cirque, support a resurgent doming event which was probably centered on Main Cirque. 66 4.0. Mineralization and Alteration 4.1. Introduction The Mt. Skukum gold-silver deposit consists of low sulphide, electrum and native silver-bearing, quartz-carbonate-sericite veins which formed in an epithermal environment within several hundred meters of paleosurface. Proven reserves at Mt. Skukum are about 200,000 tonnes of ore with an average grade of about 14 gms Au/tonne (R. B8snett, 1987, pers. comm.). Mineral and alteration assemblages resemble those found in adularia-sericite epithermal systems as described by Hayba et al. (1985). Veins representing all levels of mineral emplacement in a typical epithermal environment, are exposed in the Mt. Skukum area. The main deposit area is in the south central part of the Mt. Skukum Volcanic Complex in Main Cirque which forms the headwaters of Butte Creek. The depostts consist of several similar subparalle! gold-bearing vein-fault systems with trends of N10*W to N50°E and steep dips from 80" east to 60" west. The best known of these vein systems are, from east to west, the Main Cirque Zone, Brandy Zone and Lake Zone. Separation between these vein systems is approximately 300m. The Main Cirque Zone, the largest of these ore-bearing structures, forms a continuous zone of massive or stockwork veins traceable along strike for 1.5 km and continues down dip as a strong structure at over 100m below surface. The Main Cirque Zone and Lake Zone, currently in production, contain proven reserves of approximately 70,000 tonnes and 130,000 tonnes of ore, respectively, at an average grade of 14 gms Au/tonne. These producing zones consist of one or two continuous veins that range in thickness from 30cm to 13m, and are in or near a fault. Average thickness of producing veins in the Main Cirque Zone and Lake Zone are approximately 5 meters and 1.5 meters respectively. Propylitic alteration is widespread and forms an alteration halo within andesitic volcanic rocks of Formation 3 that has a radius of up to 6 km. More intense alteration involving the development of sericite and epldote as well as intense siliclflcation and minor argllllc alteration occurs only adjacent to the veins (section 4.4). Geochemical signatures associated with the deposit are inconsistent. Gold in soil samples gives the most localized indication of mineralization; silver in soil samples also reflects mineralized veins but tends to form dispersed halos that are broad exploration targets. Other common tracer elements such as antimony, arsenic and mercury that typically are associated with gold are present in the veins at background concentrations and do not produce reliable exploration targets. Individual veins at Mt. Skukum have characteristic features of high-level emplacement such as crustification, chalcedonic quartz, brecci8tion textures with cockscomb growth of well-formed quartz, and calcite crystals up to 15 cm in size. Colloform layeringin veins is common as is the presence of large drusy cavities commonly formed in areas of partially infilled framework supported breccias of wall rock material. Two main types of vein-filling material are present in all mineralized zones; an early blue-grey chalcedonic quartz, and later, coarser grained quartz-carbonate mineralization. Early chalcedonic quartz occurs primarily as veinlets commonly with pyritic selvages and envelopes and associated pervasive wall-rock alteration. This chalcedonic mineralization does not contain significant gold or silver. The later, coarser grained mineralization constitutes the majority of vein-filling material in all mineralized zones, cross-cuts earlier chalcedonic veinlets, and forms a final infilling In fractures already partially filled by chalcedonic material. These veins, characteristic of the Main Cirque area, contain most known gold mineralization but rarely contain sulphides. The Main Cirque Zone, Brandy Zone, and Lake Zone, although similar in most respects, display slightly different styles of both mineralization and alteration. The Main Cirque Zone contains variable proportions of quartz and carbonate mineralization surrounded by pervasive propylitic alteration and silicification. In comparison, veins in the Brandy Zone have a consistently higher proportion of carbonate minerals (generally more than 60% calcite) characterized by very coarse crystal growth (calcite crystals up to 15 cm across have been found); associated propylitic alteration has abundant epidote. The Lake Zone displays quartz-dominant veins with halos of silicification and epidote-bearing propylitic alteration which extend on surface up to 4 m from veins. These quartz veins uniquely contain local zones with up to 5 volume percent sphalerite and galena. During study, the Main Cirque Zone was the only vein system under development, consequently, the discussion which follows applies to this zone unless stated otherwise. 69 4.2. Mineralization of the Main Cirque Zone: 4.2.1. Introduction. The Main Cirque Zone constitutes the largest body of mineralization found in the Mt. Skukum Volcanic Complex to date. It is centrally located in Main Cirque (Figures 3.1 and 3.2) in a wide fault zone within one of the central blocks of a down-faulted graben structure (section 3.4), Vein structures within this zone have been traced on surface over 1.5 km and continue as a strong, near-vertical structure down-dip beyond the lowest ore-haulage level of the mine, approximately 100 m below the surface. Ore shoots containing economic mineralization within this zone extend southward 200 m from a flexure in the Main Cirque Fault Zone; they average 5 m in width and 80 m in vertical extent. Before production, this zone contained 149,000 tonnes of ore with gold assavs which averaged 14 grams Au/tonne and ranged from a high of 1,612 grams Au/tonne to lows of less than 1 gram Au/tonne. 4.2,2 Character of Veins in the Main Cirque Ore Zone: Three temporally, texturally and mineralogically distinct phases of vein mineralization are recognized in the Main Cirque Ore Zone. The two earliest formed phases are of hypogene origin and locally gold-bearing whereas the latest-formed phase is of supergene origin and does not contain economic amounts of material. These three phases of mineralization are detailed below. The first and earliest phase of hypogene mineralization is marked by blue-grey chalcedony velnlets (Figure 4.1) rarely more than 2 mm across which display fine-grained pyritic and/or hematitic selvages and pyritic envelopes characterized by a narrow zone of silicification followed by a zone of argil lie alteration. Intergrowths of pyrite and hematite forming selvages to these veinlets (Figure 4.2) in the absence of supergene effects indicates that pvrite and hematite form a primary mineral assemblage precipitated from solutions which fluctuated in oxidation state. Strong argillic alteration envelopes surrounding these veins are commonly up to 7 cm wide. Lithic fragments within the veins are commonly intensely silicified 70 Figure 4.1. Photograph showing typical occurence of chalcedony veinlets in porphyritic andesite of the Main Cirque, Yukon Territory. Veins display pyritic selvages and bleaching associated with argillic alteration which forms an envelope around veins. Figure 4.2. Photograph of a chalcedony veinlet flanked on one side by a hematitic selvage and on the other by an epidote selvage. Epidote selvages are not common; however, hematitic selvages as shown are commonly associated with these veins. 71 and argillically altered. Blue-grey chalcedony also infills hydrothermal breccias either as an early phase that coats fragments in framework-supported breccias, or infilling myriad small hydrothermal breccias, typically no more than 10 cm across, which anastomose throughout andesitic volcanic rocks of the Main Cirque area. Chalcedony veinlets have not been found to host significant gold or silver mineralization, whereas chalcedony-filled breccias locally contain up to 60 grams Au/tonne. The second, most common phase of hypogene vein emplacement in Main Cirque hosts most of the economic mineralization. These veins characteristically consist of quartz with variable amounts of calcite and sericite, minor adularia (Figure 4.18) and trace albite. Other minerals present locally include, in order of abundance, fluorite (purple, green, and transparent varieties), ankerite, anhydrite, rhodochrosite, palygorskite, pyrite, barite, electrum, and native silver. Open-space filling textures are characteristic of these veins and include cockade and comb textured crystal growth around breccia fragments and lining vein walls (Figure 4.3), drusy cavities lined with variable layers of coarse grained quartz and calcite crystals, colliform layering, and ubiquitous breccia textures (Figure 4.4). These veins cross-cut all lithologies as well as chalcedony veins and commonly form a final infilling in breccia cavities already partially filled bv chalcedony. Quartz-carbonate veins form large, continuous bodies in the Main Cirque Zone, contain most of the gold and silver, and range in width from veinlets no more than 2 mm across to major veins 13 m across which branch toward surface. Evidence of multiple emplacement episodes for these veins is abundant as brecciated fragments of older quartz-carbonate vein material are common in younger veins. In many instances these younger veins are emplaced within older veins and can be distinguished by sudden changes in quartz or calcite content, vein textures, or abundance of wallrxk breccia fragments. Brecciation is characteristic of all veins in the Main Cirque Zone with a typical increase in wallrock breccia fragments towards the margins of the veins. Typical contacts between veins and wallrocks show a gradational increase in the quantity and size of breccia fragments until the vein has the appearance of a j igsaw Figure 4.3. Photograph of quartz-carbonate vein-breccia showing open-space filling textures in a drusy, quartz-lined cavity from Mt. Skukum, Yukon Territory. Figure 4.4. Photograph of a quartz-carbonate vein showing the distribution and abundance of wall rock fragments in the Main Cirque Ore Zone at Mt. Skukum, Yukon Territory. 73 breccia (Figure 4.5) which then grades outward into zones of stockwork veins and eventually undisturbed country rxk. In some areas, sharp contacts of veins with wallrock result from fault displacements juxtaposing veins and undisturbed wallrock. Brecciation and re-cementation of earlier formed quartz-carbonate veins by later quartz-carbonate mineralization is ubiquitous, even in veins which appear massive in handspecimen. Thin section microscopy commonly reveals abundant fragments of coarse-grained, earlier formed vein material surrounded by a matrix of fine-grained quartz-carbonate-sericite material which can be difficult to distinguish due to similar compositional and textural characteristics (Figure 4.6). The third phase of mineralization in the Main Cirque area occurs as veins and in breccia bodies but is far less common than mineralization of the first and second phases. This phase occurs as crustiform and drusy coatings of gypsum, and palygorskite on the walls of partially infilled fractures and cavities in breccia bodies. Typically, most of this phase occurs in fractures cross-cutting earlier formed hypogene minerals and andesitic volcanic rxks. Gypsum, dominant mineralization of this phase, is most xmmon infilling fractures in andesitic xuntry rxk with palygorskite xcuring exclusively in frxtures developed in vein material. Gypsum xcurs as medium to coarse-grained euhedral crystals growing outward from vein walls with a cxkade habit. Palygorskite oxurs as a white, flexible, fibrous matte of crystals coating frxtures in mineralized zones. This third phase of mineralization is probably of supergene origin based on the abundance of sulphate minerals and palygorskite,. 4.2.3. Ore Petrology of the Main Cirque Zona-Ore in the Main Cirque Zone is hosted exlusively in sexnd phase, hypogene, quartz-carbonate veins. Hand spximens of these veins are white to pale green, (Figure 4.7), and xmposed of medium to coarse grained quartz and calcite which together characteristically make up 90 volume percent of the vein material; quartz is the dominant mineral in most cases. Sulphides are rare but oxur as trace amounts of fine-grained disseminated pyrite rarely exceeding 0.3 mm -74 Figure 4.5. Photograph of vein breccia material illustrating the range in abundance of wall rock fragments in vein material (see Figure 4.4) at Mt. Skukum, Yukon Territory. Figure 4.6. Photograph of brecciated quartz-carbonate vein material recemented by a later phase of quartz-carbonate breccia-infilling which illustrates the similarity of appearance and composition between early brecciated vein fragments and later matrix at Mt. Skukum, Yukon Territory. 75 Figure 4.7. Photograph of a specimen of high grade ore showing wall rock fragments in relatively coarse-grained quartz and lamellar carbonate. The greenish-brown areas in the vein material reflect localized patches of abundant sericite. Specimen shown comes from an underground drift where assays of 1612.1 grams Au/tonne were obtained. 76 across and present mainly within fragments of brecciated porphyritic andesite. A single grain of galena was identified in one polished section. No other sulphides have been identified in this zone. Thin section microscopy combined with field observation show that ore veins display two styles of infilling, one paragenetically earlier than the other, that can be distinguished on the basis of their regularity of composition and texture. These two styles are described below. The first, and earliest, and most widespread style shows a wide range of quartz-carbonate composition and appearance. Crystals tend to be coarse to medium-grained and, on the scale of a single thin section, variations in texture and composition are negligible. Calcite in these veins typically displays a strikingly elongate, two dimensional, bladed or scalenohedral crystal shape (Figure 4.8) which may involve coarse crystals with lengths of 4 cm or more but widths of no more than 2 mm. Even veins which appear in hand specimen to consist of equant, sucrosic quartz and carbonate, are found in thin section to contain calcite as minute bladed crystals clustered in radiating rosettes (Figure 4.9) growing outward from central quartz grains. These second phase veins contain variable proportions of quartz and calcite. Where calcite occurs as coarse, elongate grains, individual blades intersect one another without truncation; this results in a criss-crossed skeletal lattice of calcite crystals forming a network of angular cavities infilled by fine-grained sucrosic quartz (Figure 4.10). Where vein material is mainly calcite, these skeletal lattices develop as sheaves of many parallel, elongate, hair-thin blades packed together and oriented as random clusters of parallel crystals (Figure 4.11). Locally, quartz grains form cockade overgrowths on bladed calcite crystals (Figure 4.12) and are commonly seen replacing calcite along cleavage planes and boundaries between elongate crystals thereby exfoliating the calcite sheaves (Figure 4.13). In some cases, bladed calcite crystals are replaced entirely by many fine, anhedral quartz grains that produce blade-like pseudomorphs which become prominant on weathered surfaces (Figure 4.14). 77 Figure 4.8. Photomicrograph of quartz-carbonate vein material showing the lamellar morphology of calcite crystals in the Main Cirque Zone, Mt. Skukum, Yukon Territory. Magnification is 45 times. Figure 4.9. Photomicrograph of quartz-carbonate vein material at Mt. Skukum, Yukon Territory, showing fine-grained bladed calcite with a radiating habit even though the hand specimen megascopically has a sucrosic texture. Magnification is 175 times. 78 Figure 4.10. Photomicrograph of quartz-carbonate vein material at Mt. Skukum, Yukon Territory, showing randomly oriented, bladed calcite crystals forming a skeletal lattice infilled by later fine-grained sucrosic quartz grains. Magnification is 45 times. Figure 4.11. Photomicrograph showing clusters of tightly packed bladed calcite crystals occurring in randomly oriented bundles distributed throughout the vein material at Mt. Skukum, Yukon Territory. Magnification is 45 times. Figure 4.12. Photomicrograph showing cockade overgrowth of quartz surrounding a bladed calcite grain at Mt, Skukum, Yukon Territory. Early paragenetic formation of calcite followed by quartz as an interstitial infilling at a later time is indicated, Magnification is 45 times. Figure 4.13. Photomicrograph of quartz-carbonate vein material showing exfoliation textures in bladed calcite clusters through replacement of calcite by quartz along cleavage planes an grain boundaries in elongate crystals at Mt, Skukum, Yukon Territory. Magnification is 175 times. Figure 4.14. Photograph showing prominant blades of equant, fine-grained quartz pseudomorphing calcite on weathered surfaces of quartz-carbonate veins at Mt Skukum, Yukon Territory. 81 The second style of mineralization in the Main Cirque Zone consists of breccia infilling around earlier formed vein fragments that become recemented by this matrix material (Figure 4.15). It occurs as a chaotic, fine-grained assemblage of equant calcite in greater abundance than quartz and accompanied by abundant sericite and adularia which forms irregular patches that appears to brecciate and flood earlier formed mineralization. Evidence of brecciation caused by multiple vein emplacement events is apparent in most specimens of vein material. Sericite, most common in the second style of vein mineralization, is the third most common mineral present in veins. It typically occupies about 2 volume percent of the early-formed style of vein material but locally constitutes up to 10 volume percent in the second style of mineralization discussed above. It is a late-formed mineral in the paragenetic sequence that typically occurs as minute clusters filling interstitial space between quartz and calcite grains, and locally occurs as rims around bladed calcite crystals. Sericite can be a major constituent of the quartz-carbonate breccia infilling described above (Figure 4.15); it also forms patchy to complete pseudomorphic replacement of adularia and albite in both veins and breccia matrix material, and late-stage infillings in vugs within veins where it forms cockade coatings on cavities (Figure 4.16). Veins containing large amounts of sericite are pale green and similar to phyllically altered 2ones of porphyritic andesite. Adularia, found in both the first and second styles of quartz-carbonate mineralization, is more common in the latter. It typically occupies between 1 and 10 volume percent of vein material where it occurs as clusters of fine euhedral grains averaging 0.2 mm across (Figure 4.17) that are rarely visible in hand specimen unless they are stained (Figure 4.18). Thin sections show that adularia characteristically occurs in quart2-calcite breccia as cockade coatings of crystals rimming brecciated fragments of earlier formed vein material, and also as bands lining late veins. Adularia is commonly replaced by pseudomorphic patches of sericite and, as a result, the original abundance of adularia in this vein system may be underestimated. 82 Figure 4.15. Photomicrograph showing brecciated vein material recemented in a chaotic matrix of quartz-carbonate-sericite vein material at Mt. Skukum, Yukon Territory. Magnification is 175 times. Figure 4.16. Photomicrograph showing late-stage development of sericite as coatings on cavities in vein material from the Main Cirque Ore Zone, Mt. Skukum, Yukon Territory Magnification is 175 times. 83 •Tr Figure 4.17. Photomicrograph showing a typical occurrence of fine-grained euhedral adularia grains intergrown with quartz in a brecciated specimen of quartz-carbonate material which was re-cemented by matrix material containing abundant adularia at Mt. Skukum, Yukon Territory. Magnification is 175 times. Figure 4.18. Photograph of stained slabs illustrating the abundance and distribution of adularia in vein material from Mt. Skukum, Yukon Territory. Areas of pale yellow staining are sericitic, but areas of intense yellow staining contain adularia. 84 Other vein gangue minerals in the Main Cirque Zone include ankerite, fluorite, albite, rhodochrosite, barite, anhydrite, and palygorskite. Ankerite and fluorite are locally abundant and can become a major constituent of veins at the expense of calcite. Albite and rhodochrosite are sparsely distributed as fine grains intergrown with quartz but rhodochrosite is most common in veins outside the Main Cirque Zone. Trace amounts of barite, anhydrite, and palygorskite in the Main Cirque Zone fill late fractures which cross-cut the larger veins. Reflected light microscopy confirms the paucity of sulphides in the Main Cirque Ore Zone. Pyrite, the most abundant opaque mineral, is present only in trace amounts. Galena, the only other sulphide mineral in the Main Cirque Ore Zone, was observed as a single grain in only one polished section. Electrum and native silver (Figure 4.19) are present in trace amounts and are only slightly less abundant than pyrite. No other precious metal-bearing minerals were observed. All opaque minerals are fine grained and rarely visible to the eye usually because they are less than one millimeter in size and commonly range from 10 to 40 microns across. Electrum and native silver occur as irregular discrete grains interstitial to aggregates of either quartz or calcite; they show no systematic variations in abundance or morphology between the different styles of quartz-carbonate veins. Immiscibility between electrum and native silver is indicated by the local presence of minute grains of native silver within larger grains of electrum (Figures 4.20, 4.21a, 4.21b, and 4.22) in textures similar to those produced through exsolutionof chalcopyrite in sphalerite. 4.2.4. Distribution of (told in the Main draw Ore/one-The Main Cirque Ore Zone of the Mt. Skukum deposit is characterized by relatively high gold to silver ratios of 1.2:1 based on production history. Figures 4.23,4.24, and 4.25 show the contoured distribution of gold values and vein thickness in the Main Cirque Ore Zone at Mt. Skukum (Figure 3.1). Contoured values were obtained from routine sampling of underground workings 85 Figure 4.19. Photomicrograph showing a typical occurence of electrum (larger highly reflective grains) and native silver (smaller grey grains) in quartz-carbonate veins from Mt. Skukum, Yukon Territory. Grains of both electrum and native silver average 20 microns across, and are typically distributed as small concentrations of disseminated, co-existing grains. Magnification is 100 times. Figure 4.20. Photomicrograph showing fine inclusions of native silver (grey) in electrum at Mt. Skukum, Yukon Territory. Native silver in this figure occurs as several minute grains disseminated within a coarser grain of electrum; however, the reverse is also seen where electrum occurs as fine disseminations in coarser grains of native silver. Magnification is 700 times. 86 Figure 4.21 a. Electron photomicrograph (backscattered electron scan) showing relatively coarse grained electrum grains (lighter grains) some displaying fine inclusions of native silver interspersed with discrete, extremely fine grains of native silver (grey). Several of the finer grains of native silver also show inclusions of electrum. Figure 4.21 b. Electron photomicrograph (backscattered electron scan) showing one of the two phase grains of precious metals enlarged from Figure 4.21 a consisting of electrum (lighter phase) and native silver (darker phase) in vein material from Mt. Skukum, Yukon Territory. 8 7 PR= 188KI 19SEC 188880 INT U»8192 HM8KEU 1=1H AQM8KEU IH AG 18.88KEU> PR= 108KI 16SEC 188888 INT U 4896 H=49KEU 1=1H AQ=48KEU IH Figure 4.22. An energy dispersive electron microscope scan of the two phases in Figure 4.24. The darker phase in the photomicrograph consists of silver whereas the lighter \ coloured phase consists of electrum 8bout 600 fine. Calcium peaks are from calcite which surrounds the electrum grain examined. 88 during mining. Underground workings following veins were sampled every 2.5 m at the working face after each round. At each 2.5 m interval, a line of samples was moiled across the vein. Individual samples were collected for each contrasting part of the vein mineralization; lithological divisions were sampled separately. Each sample location, therefore, consists of one or more samples which were analysed for silver and gold at the mine by fire assay. Using this data base, the length-weighted average value of gold assays from each sample line was calculated to provide a representative value at each point. Vein thickness data was obtained from routine test hole drilling along drifts at approximately 5 m intervals. Figure 4.23 shows that distribution of gold grade forms four irregular, vertically-plunging zones which fan outward, becoming more laterally extensive with increasing elevation. Areas of highest grade in all zones are limited to a horizontal band representing a range of elevations from surface at about 1730 m to about the 1676 m sublevel of the mine; below this 60 m wide band, high-grade zones pinch out laterally but extend as narrow ore shoots below the level of the ore zone. Vein isopachs in Figure 4.24 show that vein thickness is not uniform in the Main Cirque Ore Zone. The thickest part of the vein occurs in the southern part of the zone as a localized area of broad horizontal extent which appears to pinch out towards surface and at depth. At the southern end of this zone of maximum thickness, a near-vertical shoot is developed but this also pinches out with depth. In the northern end of the ore zone an arcuate area of vein thickness extending from the top to the bottom of the ore zone curves around the bulls-eye pattern of isopachs formed by the zone of maximum vein thickness to the south and extends down-dip below the present level of the ore. Zones of maximum gold grade (Figure 4.23) correspond closely with zones of maximum vein thickness. J Figure 4.23. Contour diagram snowing distribution of gold values in the Main Cirque Zone (Figure 3 I) at Mt. Skukum, Yukon Territory. Gold occurs in elongate, irregular, vertically plunging ore shoots that increase in grade upwards. High grade zones pinch out consistently downward near the 1676 sublevel. Data is from routine mine sampling during development. Figure 4.24. Isopach diagram iIlustrating vein thickness in the Main Cirque Ore Zone (Figure 3.1) at Mt. Skukum, Yukon Territory. Zones of maximum thickness arrespond with zones of maximum grade (Figure 4.26) indicating that throttling could have been an important mechanism of mineral precipitation. o Figure 4.25. Contour diagram of grade times thickness outlining zones of maximum vein width and gold grade at Mt. Skukum, Yukon Territory. Gold occurs in three vertically plunging ore shoots which reflect the orientation of major hydrothermal channels indicating that fluid flow in the Main Cirque Ore Zone was directed vertically upward with little lateral flow 92 Figure 4.25 integrates both vein thickness and grade in a grade times thickness contour plot that illustrates distribution of ore shoots in the Main Cirque Ore Zone. Despite the prominance of a horizontal band limiting areas of highest grade and vein thickness in Figures 4.23 and 4.24, ore shoots all display near-vertical plunges in Figure 4.25. Three major but irregularly shaped oreshoots are delineated that tend to narrow with depth but fan outward laterally with increasing elevation and coalesce to form a single large body- Each ore shoot extends below the present level of the ore zone, thus indicating a potential for further mineralization at depth in lower structurally dilatant zones. No consistent textural or mineralogical guide to gold-rich vein material has been noted in mining; however, favourable indicators include veins containing coarse-grained calcite in excess of 50 percent by volume, and areas of abundant coarse-grained calclte exhibiting a well-defined bladed morphology. Veins containing mainly quartz with massive, sucrosic textures commonly yield low precious metal contents. 4.2.5 Interpretation: Hypogene quartz-calcite veins in the Main Cirque Zone host all significant gold and silver mineralization. Open spaces formed dominantly through fault displacement acted as conduits for hydrothermal.fluids. Abundant breccia textures associated with mineralization throughout the Main Cirque indicate that vein emplacement typically occured under conditions of hydrothermal eruption causing rapid precipitation and loss of volatiles - - this is consistent with shallow formation. Bladed calcite crystals forming a skeletal lattice is described in Oatman, Arizona by Lindgren (1933) as a characteristic form of crystal growth in epithermal deposits known as lamellar ore; this also supports a shallow depth of formation. The local presence of chalcedony veinlets attests to periods of supersaturation of silica in hydrothermal fluids that may have been produced by a decrease of volatiles caused by boiling (Fournier, 1985). Strong argil lie alteration is consistent with boiling, as the release and expansion of volatile components (mainly C O 2 , H 2 S , and H2O) during this process forces them into surrounding country rock. In the presence of oxygenated groundwater, this creates acidic conditions leading to local clay alteration surrounding conduits where boiling occured. As boiling has been proposed as a mechanism of gold precipitation in many epithermal deposits, it is possible that it may have been a trigger for formation of mineralized zones at Mt. Skukum. As the development of clay alteration requires an influx of oxygenated surface waters, mixing of upwelling hydrothermal fluids and cooler surface waters may also have contributed to ore deposition. Mineral deposition was also probably associated with fluctuating oxidation-reduction conditions in the fluid allowing near-penecontemporaneous deposition of pyrite and hematite. Concentrations of electrum and native silver form three major, vertically-oriented ore shoots in the Main Cirque Ore Zone (Figure 4.25). These ore shoots fan out upwards and coalesce to form a body of maximum gold grade (Figure 4.23) which corresponds with areas of greatest vein thickness (Figure 4.24) in a horizontal horizon extending from surface at 1730 m to the 1676 m sublevel, a distance of 60 m. Although economic concentrations of lower grade ore extend below this horizon, the consistently limited depth to the highest grade ore zones suggest that gold deposition was controlled by a pressure decrease in fluids migrating upwards allowing boiling at a depth dictated by the boiling curve for the hydrothermal solution (Haas, 1971). The corelation between areas of maximum vein thickness and maximum gold grade supports pressure decrease as a primary mechanism triggering gold deposition. These zones probably existed as open cavities and brecciated zones created by fault displacement prior to vein mineralization. Upward migrating hydrothermal fluids encountering these cavities would undergo a rapid pressure decrease thereby initiating boiling with consequent gold precipitation in accordance with the throttling model of Barton and Toulmin (1961). Grade times thickness contours (Figure 4.25) map zones of maximum vein width and grade which probably formed major channels within the conduit of the fault system. The 94 orientation of these conduits suggests that fluid flow in the Main Cirque Ore Zone was directed generally upwards toward paleosurface controlled by structural constraints produced through fault dislocation creating vertically-plunging cavities or zones of permeability. Mineralization at Mt. Skukum is consistent with that found in low-sulphur epithermal systems characterized by the presence of adularia and sericite that typically display high gold to silver ratios (ejx. Round Mountain, Nevada and Oatman, Arizona: Haybaetal.. 1985). Several other localities of this deposit type display bladed calcite textures similar to those at Mt. Skukum, such as the Bodie Mining District, California, where they are also used as visual indicators of high-grade gold mineralization (M. Silberman, 1985, pers. comm.). Vein textures and mineralogy at Mt. Skukum are consistent with ore deposition, triggered by boiling, from solutions rich in silica, potassium and carbon dioxide,- groundwater mixing likely played a subordinate role. As pressure decrease causing boiling is indicated as an important factor in gold deposition in the Main Cirque Ore Zone, the existance of a horizontal band of highest grade gold values may indicate the lower level of maximum economic potential below paleosurface. On a regional scale, this could delineate the depth below paleosurface at which an orebody could be located. Thus, if the depth below paleosurface of the Main Cirque Ore Zone can be determined, this information may help to define drill target depth to potential ore zones in vein structures located elsewhere in the Mt. Skukum Volcanic Complex. 95 4.3. Alteration 4.3.1. Introduction: Rocks of the Main Cirque area at Mt. Skukum are affected by both supergene and hypogene alteration; both can be important exploration guides. Supergene alteration h3s been subdivided into two facies, one characterized by jarositic brown and yellow-brown gossans and the other by bright red-brown limonitic gossans, that occur in localized zones throughout the Mt. Skukum Volcanic Complex. Hypogene alteration has been subdivided into six facies which are directly related to vein mineralization and form an extensive halo surrounding the vein deposits in Main Cirque. 4.3.2. Supergene Alteration: Supergene alteration forms brightly coloured gossans on the east, south, and west walls of Main Cirque (Figure 4.26), that consist primarily of pervasive clay alteration associated with limonite and jarosite centered in and around fractures in silicified rocks containing disseminated pyrite. The jarositic facies of supergene alteration occurs as small gossans developed in surface outcrop of porphyritic andesite which contains abundant early chalcedonic veinlets and/or breccia. These veinlets can form local areas of stockwork within which more than 60 veinlets per meter occur. In a surficial environment chalcedonic veinlets and breccia, which are commonly associated with pyritic selvages and envelopes, are exposed to oxidized surface waters causing breakdown of pyrite to limonite (Figure 4.27). Resulting acidic conditions cause development of kaolinite from plagioclase. This supergene alteration produces bright, yellowish-brown, jarositic and limonitic gossans. Alteration is weak distal to fractures and the center of larger fragments, but is intense, and obliterates all primary textures immediately adjacent to fractures or breccia fragments (Figure 4.28). Alteration can be so intense in these areas that movement of present-day groundwater through rocks exposed at surface transports clay out of the rock and 96 Figure 4.26. Air photograph of Main Cirque and the surrounding area showing the locations of the Main Cirque Zone (labelled Main Zone) and the Lake Zone in relation to Mt. Skukum. Zones of supergene alteration surrounding Main Cirque produce a reddish-brown halo centered on the cirque. Figure 4.27. Photograph showing limonitic supergene alteration of porphyritic andesite in 8 zone of chalcedonic stockwork veins approximately 40 m in diameter, located approximately 100 m southwest of the southern end of the Main Cirque Zone, Mt. Skukum, Yukon Territory. 97 Figure 4.28. Photograph of typical limonitic supergene alteration in a hand specimen of brecciated porphyritic andesite showing kaolinitic alteration extending into wall rock fragments from pyritic breccia infilling at Mt. Skukum, Yukon Territory 98 deposits it in stalactitic masses on the undersides of exposed ledges. This variety of jarositic supergene alteration also occurs in andesitic volcanic rocks where pervasive silica flooding is accompanied by disseminated pyrite. As this alteration type is commonly associated with pyritic chalcedony veinlets generally low in precious metal content, it is not specific to areas of gold and silver mineralization; however, in the Lake Zone where hypogene alteration creates diffuse halos of disseminated pyrite, formation of supergene limonite staining surrounding veins for several meters on either side creates a similar gossanous appearance which serves as an exploration guide (Figure 4.29). The red-brown limonitic facies of supergene alteration occurs on the southwestern and southeastern walls of Main Cirque where brightly-coloured, red-brown gossans occur on bedrock containing disseminated pyrite. The red-brown gossan on the southwestern wall of the cirque (Figure 3.22) is underlain by a porphyritic rhyolite stock on Red Ridge (Figure 3.1). This stock contains up to 5 volume percent disseminated pyrite which becomes oxidized to produce acidic conditions that alter feldspathic minerals to supergene clays. In this area, iron is oxidized to produce gossans of bright reddish-brown colour that covers the entire surface area of the stxk. Iron oxides occur only in 8 very thin surficial zone involving the topmost 5 cm of soil horizon overlying pyritic bedrock. Brightly coloured zones such as this are common throughout the Mt. Skukum Volcanic Complex and are located almost exclusively over pyrite-bearing felsic volcanics. Soil samples from these areas are commonly anomalous in gold; they may contain up to 10OO parts per billion Au with no apparent source in underlying or surrounding country rock. Pyrite-associated red-brown supergene alteration also occurs in areas of intense hypogene alteration snd quartz-carbonate mineralization. In these areas, brightly coloured iron oxide results in intensely coloured gossans that occur only in the uppermost soil layers and as fracture coatings formed through oxidation of disseminated pyrite in rxks immediately below the surfax. Two such areas are known in the Mt. Skukum region; one on the southeastern wall of Main Cirque, known as the Alunite Cap Zone (Figures 3.1 and 4.30), and the other at Vesuvius Hill (Figure 2.2), an Figure 4.29. Photograph showing limonitic supergene alteration halos surrounding centrally located veins in the Lake Zone, Mt. Skukum, Yukon Territory 100 extension of the Mt. Skukum Volcanic Complex to the northeast. Bright red-brown colouration over an area of about 250 m by 200 m is intense at both locations; both are also associated with pervasive hypogene alteration of underlying rocks to kaolinite, sericite, al unite, pyrophyllite and pyrite with minor local silicif ication. The Alunite Cap Zone occurs along a possible extension of an inferred major fault (Figure 3.1) along the eastern wall of Main Cirque and is an expression of near-surface acidic alteration caused by condensation of volatiles boiled off during hypogene hydrothermal activity. Similar alteration occurs underlying the red-brown gossan at Vesuvius Hill where drill holes have intersected significant thicknesses of quartz-carbonate mineralization in veins and as a matrix infilling large breccia bodies underlying the surface supergene gossan. Thus, in both localities, important and recessive expressions of hypogene mineralization are clearly marked by supergene alteration. Although a promising target at Mt. Skukum, no surface indication of precious-metal mineralization has been found associated with the Alunite Cap Zone in Main Cirque through soil sampling. 4.3.3. typcgeneA/terdtfm Surface and drill core specimens were selected to obtain a representative suite of samples from each hypogene alteration facies present in the Main Cirque Zone. Thin section petrology and x-ray diffraction were used to determine textural and mineralogic details of alteration types. Results are presented below in order of alteration sequence (Table 4.1) which is the same as the spatial distribution outward from the veins (Figure 4.31). Hypogene alteration at Mt. Skukum is ubiquitous in porphritic 8ndesite volcanic rxks of Formation 3 (sxtion 2.2.2), but is only locally intense adjacent to dykes and veins. Facies of pervasive hypogene alteration at Mt. Skukum are characterized by the terms silicic, potassic, phyllic, argillic, and propylitic (Table 4.1), which represent mineral assemblages stable at progressively lower temperature xnditions in the presence of hydrothermal fluid (Meyer and 101 Hemley, 1967; Rose and Burt, 1979). Each facies may be subdivided into zones based on consistent mineral assemblages named after the dominant mineral components present. Table 4.1. Hypogene alteration facies and zones at the Mt. Skukum deposit, Yukon Territory. Facies Zone Mineralogy1 Silicic Qz,Ms,Py,Ad,Cl,Ka Potassic Ad,Qz,Ms,Py,Ca,Cl,Ka Phyllic Ms,Qz,Ca,Cl,Ep,Py, Le.Ad Argil lie Ka.Pr.Oz, Py,Ms,Al,Sm, 11 Propyl itic Chlorite Cl.Ep.Ms.Qz.Ca.Py.Le Propylitic Epidote Ep,Cl,Ms,Qz,Ca,Py, Le 1. Minerals are listed in order of abundance and are coded as follows: Ad = adularia, Ca = calclte, CI = Chlorite, Qz = quartz, Ep = epidote, Ka = kaolinite, Le = leucoxene, Ms = Muscovite (sericitic), Py = Pyrite. Figure 4.31. Schematic illustration showing gradation between veins and zones of vein breccia and the distribution of hypogene alteration around quartz-carbonate veins and vein breccia at Mt. Skukum, Yukon Territory. 102 4.3.4. fyptxm? Alteration facies: Silicic Alteration: This alteration type is one of the most common found in Main Cirque where it generally forms envelopes of silica flooding in rock adjacent to veins extending up to 15 cm into wallrocks. It is also common in brecciated wall-rock fragments within veins and breccia bodies. Silica also forms the major infilling in myriad hydrothermal breccias within the Main Cirque area. Silicification is closely associated with potasslc alteration. Although both usually occur together adjacent to veins, potassic alteration does not invariably accompany silicification. Silicification is typified by a bleached appearance which commonly obliterates primary textures and is invariably accompanied by disseminated pyrite. As silicification is strongly fracture dependant, it is commonly accompanied by brecciation of the host rock followed by 8n infilling of the matrix with quartz. The importance of permeability to silicic alteration is seen in hydrothermal breccias where large rock fragments commonly display only a surflcial rim of silicification whereas smaller fragments are completely silicified and matrix material is completely replaced by quartz and pyrite. Permeable horizons such as lapilli tuffs, brecciated flow tops and bottoms in andesitic volcanic rocks and coarse-grained volcaniclastic sedimentary rocks all display preferential silicification which can involve complete replacement of matrix material yet leave coarse fragments relatively unaffected. In thin section, a fine-grained replacement of all minerals by a mosaic of equant anhedral quartz grains averaging about 5 to 10 microns xross is charxteristic (Figure 4.32). In fragmental rxks, coarse, cxkade-textured quartz grains xmmonly oriented outward from the surfxe of fragments xmpletely replax surrounding matrix and locally produces promlnant white halos around fragments. Silicification is invariably axompanied by 0.1 to 5 volume perxnt pyrite; there is a dirxt relation betwxn intensity of alteration and the amount of pyrite present. Pyrite invariably oxurs as finely disseminated grains no more than 0.2 mm xross. Primary feldspars and pyroxene minerals may be partially or xmpletely replaced by quartz which x n retain the pseudomorphic crystal shape of the original mineral grain. Minerals such as chlorite, epidote, and sericite, which are not 103 Figure 4.32. Photomicrograph, under crossed polars, of typical silicification of andesitic country rock adjacent to mineralization at Main Cirque, Mt. Skukum, Yukon Territory. Primary textures are obscured by pervasive development of fine grained quartz. Sericite occurs as fine grained aggregates replacing feldspar phenocrysts. Magnification is 45 times. 104 characteristic of this alteration type, occur in silicified rocks because silicification tends to overprint other alteration types as it infiltrates outwards from veins into wall rock, thus preserving remnants of overprinted alteration. Silicification represents a wholesale flooding of wall rxk and vein or brexia fragments with hydrothermal fluids rich in silica that prxipitated throughout the rxk. Textural evidence indicates it is a metasomatic process xntrolled by permeable horizons and frxtures which acted as fluid xnduits. PotassicAlteration: Potassic alteration is more closely assxiated with vein mineralization than with dykes and forms very narrow haloes immediately surrounding veins containing potassic minerals. It is not present as a halo around all veins and may display effxts of retrograde overprint by phyllic alteration which obscures identification. It most xmmonly occurs in zones of brexiation rarely extending more than 15 cm into surrounding xuntry rxk adjacent to veins. Potassic alteration is not xmmon or widespread in the Main Cirque Zone and shows no distinctive features in hand spximen. Most xmmonly, potassic alteration is assxiated with silicification in areas within 1 m of a fracture xnduit or permeable horizon; features in hand spximen appear similar to those of silicification. The primary mineral in areas of potassic alteration is adularia which occurs as fine-grained euhedral crystals averaging 0.2 mm xross that are closely intergrown with quartz in silica-flooded areas (Figure 4.17). Adularia appears to have bxn an early phase of alteration because it xmmonly oxurs as a border of cxkade-textured crystals nucleated on relatively unaltered fragments in andesitic lapilli tuffs or brexia bodies where the matrix material has bxn xmpletely replaced. Another xmmon mode of adularia occurence is partial replacement of plagixlase phenxrysts and matrix crystals in altered porphyritic andesite and felsic dykes. Although this form of alteration is most xmmon as a minor part of the silicification process, it can locally dominate silicification. Much original potassic alteration around veins may, in fact, remain undetected as subsequent alteration episodes could have overprinted potassic 2ones by altering adularia to sericite. Masses of sericite have been observed to occur In regular shapes consistent with those expected in pseudomorphs of adularia. Potassic alteration affects all rock types including intermediate volcanics which do not contain the appropriate chemistry to allow such alteration under closed conditions (Grunsky, 1986). Consequently, potassic alteration represents a metasomatic process caused by exposure of the wall rock to hydrothermal fluids rich in potassium. PhyllicAlteration: Phyllic alteration is the most common form of alteration directly associated with veins and dykes. It occurs as halos extending up to 4 m from veins or dykes but most commonly extends no more than 1 m. Phyllic alteration is also common in brecciated rock fragments in veins and breccia bodies or as a border alteration on the margins of rhyolite dykes. Phyllic alteration forms relatively extensive halos immediately adjacent to veins. Phyllic alteration halos are best developed in the Lake Zone where they extend up to 4 m away from veins. Main Cirque and Brandy zones display abundant phyllic alteration associated with veins and dykes; however, halos in these zones are less extensive and average between 2 and 10 cm from veins, locally, they may be absent but elsewhere they may exceed 1m. Phyllic alteration is ubiquitous in dykes that cross-cut andesitic pyroclastic and flow rocks in Main Cirque. Felsic dykes are most intensely affected, whereas some porphyritic andesite dykes are unaffected. Its presence in handspecimen is characterized by a mottled bleaching and pale apple-green colour in a groundmass of tuffaceous and porphyritic rocks (Figure 4.33), by unusually soft feldspar phenocrysts, and by the presence of trace amounts of finely disseminated pyrite. Thin sections of phyllic alteration in lithologies of Main Cirque display consistently high abundances of calcite and variable amounts of introduced quartz and pyrite that are invariably present. Other minerals identified through x-ray Figure 4.33. Photograph showing typical appearance of phyllic alteration in a hand specimen of porphyritic andesite from Mt. Skukum, Yukon Territory. 107 diffraction in this zone include localized pyrophyllite and illite occurences. Plagioclase phenocrysts in rocks of these zones are characteristically clouded or completely replaced by sericite and calcite locally with minor quartz and epidote (Figure 4.34). In areas of intense alteration, primary quartz phenocrysts may be rimmed and corroded by sericite and the groundmass of porphyritic rxks is xmmonly xmpletely altered or mottled by patches of sericite, carbonate and quartz. Intensely altered rxks also xmmonly display fine cross-cutting frxtures filled by sericite and calcite which can also infill amygdules in porphyritic andesite. Phyllic alteration in felsic dykes is usually pervasive in some places however, marginal zones of intense phyllic alteration are apparent which are probably caused by breakdown of fine-grained or vitric chill margins to sericite. The most intense phyllic alteration oxurs in felsic dykes adjacent to large veins in the Main Cirque Zone. It is also xmmon in rxks underlying the brightly xloured red-orange gossan in the Alunite Cap Zone where it is widespread and appears to reduce the rxk to a soapy-textured green xloured mass in which some coarse primary fragmental and flow-banded textures are preserved by differential silicification or alteration intensity (Figure 4.35). Phyllic alteration is closely assxiated with veins and xntrolled by frxture intensity and rxk permeability. It represents a metasommatic process of potassium enrichment that produced sericite in intermediate volcanic rxks and increased the abundance of sericite in felsic dykes. Argil lie Alteration: Argillic alteration oxurs as infrequent, narrow alteration halos around individual chalcedonic veinlets, but rarely extends more than 1.5 cm from the veinlet. It is also locally present as pervasive alteration in the Main Cirque Zone where alteration envelopes in stxkwork chalcedonic veinlets coalesce. A large area of intense argillic alteration also oxurs at high elevation on the southeastern wall of Main Cirque in the Alunite Cap Zone. Figure 4.34. Photomicrograph showing complete replacement of plagioclase phenxrysts through the development of sericite and calcite in zones of phyllic alteration at Mt. Skukum, Yukon Territory. Magnification is 175 times. Figure 4.35. Photograph of typical intense phyllic alteration developed in brecciated, flow-banded rhyolite from the Alunite Cap Zone at Mt. Skukum, Yukon Territory. 109 This form of alteration occurs only locally in the Main Cirque Zone and is rare in the Brandy and Lake zones. It occurs as narrow halos which extend up to 4 cm away from veins and is mainly associated with early chalcedonic veins. Argillic alteration is commonly associated with silica flooding which usually forms an envelope immediately adjacent to veins and passes outward to pervasive argillic alteration. In areas where abundant stockwork chalcedonic veinlets occur, argillic alteration envelopes coalesce forming pervasive zones of intense alteration which are usually of limited extent. Argillic alteration affects all lithologies, obliterates all primary mineralogy, obscures primary textures in handspecimen, and reduces rxk to a soft powdery white mass containing finely disseminated pyrite; it is typically criss-crossed by chalcedonic veinlets. Thin sections display complete replacement of original mineralogy by fine-grained kaolinite, quartz and pyrite with some preservation of original porphyritic textures through kaolinite pseudomorphs of plagioclase or potassium feldspar phenocrysts. Up to 5 volume percent pyrite is commonly present in this zone as finely disseminated grains averaging 0.3 mm across which are associated with silica flooding and occur as envelopes extending away from silica-flooded zones within argillic alteration. Although argillic alteration in the Main Cirque Zone is characterized by its association with chalcedonic veinlets, this alteration facies is also present in rocks underlying the bright, red-orange gossan In the Alunite Cap Zone with no associated veinlets or silicification. Argillic alteration in this zone is intense and reduces the rock to a soapy textured white mass of soft kaolinite and pyrophyllite obliterating primary textures. In this area, alteration extends outward from fractures in the rock leaving relatively unaltered cores In areas farthest from cross-cutting fractures (Figure 4.36). Argillic alteration in Main Cirque is caused by low pH conditions produced by condensation of volatile components released through boiling of hydrothermal solutions. Vein mineralization shows abundant textural evidence that boiling occured (section 4.2) and the Alunite Cap Zone (Figure 3.1) represents a near-surface alteration cap produced by condensation of volatile components (mainly C0 2 , H 2S, and H20). Localized argillic alteration zones associated with 110 Figure 4.36 Photograph of intense argillic alteration in rock of the Alunite Cap Zone, Mt. Skukum, Yukon Territory, showing a gradual decrease in alteration intensity away from fractures and preservation primary fragmental texture. Ill mineralization representing deeper levels of emplacement ( l e Main Cirque Zone) indicate that boiling extended below these levels. Argillic alteration may have been produced in these zones by a localized deep penetration of oxidized surface waters in highly permeable fault zones allowing development of acidic conditions adjacent to boiling zones. PrgpyfttfcAtteratton: This alteration type forms the most widespread halo and is related to zones of maximum permeability. It is present in most areas underlain by andesitic volcanic flow rocks but is less pronounced in andesitic dykes and many intrusive andesite stocks in the area. Propylitic alteration is present to some degree in all rxks of Main Cirque regardless of assxiation with, or distance from veins. It is most abundant in the Main Cirque and Brandy zones, but it is also xmmon in the Lake Zone. Propylitic alteration is not xmmonly found in felsic dykes where phyllic alteration is predominant; however, propylitized rhyolite dykes are present in drill core from some locations. Andesitic flow and pyrxlastic rxks appear to be most readily affxted by propylitic alteration; rxks of this type from all locations in the Mt. Skukum Volcanic Complex display some degree of propylitization. Permeability once again xntrols this form of alteration with the affxt that permeable andesitic lapilli tuffs, ash tuffs, and flow top and bottom brxcias in andesitic flow rxks appear to be more intensely altered than less permeable rxks of similar xmposition. This is apparent in cliff exposures in areas of multiple andesite flows where brecciated flow tops and bottoms show a slightly bleached, greenish xlour characteristic of propylitization whereas the less fractured and altered flow centers remain dark grey in xlour. This variable alteration results in an alternating light and dark banding in cliff exposures (Figure 2.7). Hand spximens of propylitically altered rxks display this blexhed, green xlour brought about by xmplete replaxment of primary pyroxene by chlorite. Plagixlase also is typically green due to sausser itization which includes breakdown of plagixlase to epidote and calcite. Magnetite, a characteristic xmponent of relatively pristine andesite at Mt. Skukum, is replaced in propylitized rocks by leucoxene. A transitional zone exists between the phyllic alteration facies and the propylitic facies which can be identified by the appearance of abundant chlorite and a decrease with eventual disappearance of sericite and secondary quartz outward from the phyllic alteration zone. In hand specimen, this transition is seen as a subtle colour change from pale apple green, to a dark green marked by increasing abundances of epidote and chlorite, and decreasing abundances of pyrite, quartz, sericite, and calcite. Two zones of propylitic alteration facies occur (Table 4. 1). One, involving chlorite as the dominant alteration mineral, is most common in Main Cirque. However, propylitic alteration in the Brandy and Lake zones locally displays epidote as the dominant alteration mineral. Thin sections of propylitically altered rock invariably display a complete replacement of pyroxene by chlorite in both phenocrysts and groundmass, Plagioclase phenocrysts and microlites are commonly clouded by incipient, fine-grained epidote with minor calcite which, in some cases, replaces most of the original crystal. Minor sericite occurs locally as small patchy areas replacing plagioclase phenocrysts or as fine envelopes along fractures and cleavage planes in these phenocrysts. In areas of propylitic alteration, amygdules in andesite flow rocks are commonly filled either entirely by rosettes of radiating chlorite crystals with minor epidote, or by a combination of chlorite and epidote rimming the amygdule followed by a final central infilling of quartz (Figure 4.37). Propylitic alteration throughout the Mt. Skukum Volcanic Complex Is developed mainly In andesitic volcanic rocks. Its distribution is controlled by primary permeability in fractured and volcaniclastic rocks. Propylitization represents a form of isochemical alteration produced by elevated temperature in rocks adjacent to hydrothermal conduits without the introduction of large amounts of volatile components such as potassium, silica, or carbon dioxide. 4.3.5. Discussion: Four of the five alteration facies described - - silicic, potassic, argillic, and phyllic — result from the metasommatic introduction of potassium, carbon dioxide, and silica in varying 113 Figure 4.37. Photomicrograph showing typical amygdule infilling of radiating chlorite rosettes in propylitically altered porphyritic andesite at Mt. Skukum, Yukon Territory. Magnification is 45 times. 114 amounts to rocks along fractures and permeable zones. This produces silicification and the formation of variable amounts of adularia, sericite, kaolinite, and calcite in altered rxks. The fifth alteration fxies (propylitic alteration) represents an isxhemical breakdown of primary minerals in response to increased temperatures with little addition of chemical xmponents from exterior sourxs. The most xmmon form of alteration is propylitization which is found throughout the Mt. Skukum Volcanic Complex. This alteration type is not spxifically assxiated with mineralization although it does bexme most intense near veins and is assxiated with permeable zones in host rxk. All the metasommatic forms of alteration are directly assxiated with veins and may be used as indicators of proximity to mineralization. Silicic and potassic alteration represent the most proximal fxies and although potassic alteration is undisxrnible in hand spximen, silicification is readily identifiable and is the most xmmon form of alteration assxiated with veins (Table 4.2). Phyllic alteration is also closely related to vein mineralization. Although it oxurs in all mineralized areas, it is not assxiated with all veins and xn oxur in areas where veins are not apparent, thereby presenting an unreliable guide to mineralization. Argillic alteration is a lx dirxtly assxiated with vein mineralization in the Brandy, Lake, and Main Cirque zones where it forms envelopes of limited extent around chalcedonic veinlets. The presenx of abundant argillic alteration xnsisting of a kxlinite > pyrophyllite assemblage in the Alunite Cap Zone (Figure 3.1) may represent an important guide to vein mineralization as a similar zone of alteration at Vesuvius Hill (Figure 2.2) displays wide zones of quartz-xrbonate filled hydrothermal brexia material found in drill x re taken from dirxtly beneath the zone. Table 4.2 shows the oxurrenx of alteration minerals in 26 spximens examined using x-ray diffraction on water-oriented mounts. Minerals are listed in order of abundanx based on thin section examination and diffrxtion chart peak height. Spximens were chosen to represent a cross-sxtion of the various alteration types identified in hand spximen and to show the many variations within exh alteration type. Table 4.2. Alteration minerals in rocks of Main Cirque listed in order of abundance as determined through petrography and x-ray diffraction analysis of representative specimens of alteration facies at the Mt. Skukum deposit, Yukon Territory. Numbers in table indicate the number of specimens in which minerals occurred with a particular relative abundance. Alteration 1 2 3 4 5 6 7 8 9 Mineral (order of abundance, 1 = most abundant; 9 = least abundant) Quartz 14 3 2 1 Sericite 4 1 4 7 1 1 Chlorite 1 8 4 2 1 Calcite 1 2 3 4 7 1 Epidote 1 1 2 2 Adularia 3 3 2 1 Pyrite 1 2 1 1 Smectite 1 Kaolinite 2 Ankerite 3 Alunite 1 It is apparent in Table 4.2 that although samples were chosen as a representative suite of alteration styles, silicification is the most abundant form of alteration in all specimens as quartz is the most abundant alteration mineral in more than half (14) of the 26 specimens. Sericite is the most abundant alteration mineral in 4 out of the 26 specimens, reflecting the field observation that phyllic alteration is the second most common form of alteration in Main Cirque. Chlorite, the second most abundant mineral in 8 out of the 26 representative specimens, is the most widespread alteration mineral present to some degree in all rocks of Main Cirque. The abundance of silicification and phyllic alteration in Main Cirque is consistent with thin section observation which showed significant silicification and sericite development along fractures and mineral cleavage planes even in areas of propylitic alteration. As the Main Cirque area was a focus of hydrothermal discharge, the predominance of intense alteration may be expected at this locality even though the Mt. Skukum Volcanic Complex as a whole is characterized by propyl itization. 116 The most intense hypogene alteration in Main Cirque occurs in rxks underlying the area of brightly xloured supergene alteration in the Alunite Cap Zone. This zone is underlain by areas of intense argillic and phyllic alteration xntered around a vertixlly-dipping vein xnsisting of alunite > pyrophyllite > quartz. The presenx of alunite (xmmon in epithermal systems where high level oxidation oxurs) and the abundanx and intensity of argillic alteration in this zone suggest it may represent alteration at or near the palx-surfxe at the time of mineralization in the Main Cirque Zone. This is supported by the high elevation of this area relative to the Main Cirque Zone (Figure 3.1). Alteration fxies and their distribution at Mt. Skukum are xnsistent with those which charxterize a class of epithermal systems known as low-sulphur adularia-sericite depxits (Haybaeial, 1985,-Bonham, 1986). Phyllic alteration assemblages xnsisting of micacxus minerals Including sericite + illlte + quartz + pyrite typically dominate these systems. IllUe and smxtite are also xmmonly present as mixed layer assemblages where illlte layers dominate. Phyllic alteration in this class of deposit charxteristically borders a silicified zone adjaxnt to veins which includes sericite as well as fine-grained potassium feldspar and/or chlorite disseminated throughout silicified wall rxk. Away from veins, phyllic alteration assemblages grade outwards into a propylitic zone; locally, argillic alteration is located between phyllic and propylitic zones (Hayba et al.. 1985). Several examples of this deposit class are Creede, -Colorado; Pxhuca, Mexlx; and 0atm8n, Arizona; exh of which display a serlcitfc cap alteration over the orebody that is interpreted as the result of xndensation of xidic volatiles (mainly H2S and COo) released at depth during boilino (Barton et al.. 1977; Buchanan, 1981). This description fits the observation of intense phyllic and argillic alteration in the Alunite Cap Zone of Main Cirque remarkably well, espxially sinx release of volatiles at depth through boiling xn also cause intense argillic alteration in epithermal systems (Rose and Burt, 1979). Thin section and x-ray study indicates that rocks at Mt. Skukum were altered by hydrothermal fluids rich in silica, potassium and carbon dioxide. The abundance of calcite as an alteration mineral present to some extent in all alteration facies, particularly phyllic and silicic 2ones, is symptomatic of exposure of host rock proximal to veins and fractures to fluids rich in CO2. In phyllic alteration zones, formation of calcite largely through replacement of plagioclase indicates that Ca 2 + was not added to the system but was recombined with externally derived CO2. In addition, chloritic amygdules found locally in andesitic flow rocks may be due to the interaction of CC^-rich fluids with host-rock destabilizing Ca 2 + to form chlorite (Grunsky, 1986). Sausseritization of plagioclase in propylitic zones is indicative of a lack of CO2, and is consistent with its location away from the fractures and veins which channeled hydrothermal fluids. The presence of adularia and abundant sericite in intermediate volcanics which are relatively low in primary potassium minerals suggests that potassium-enriched fluids passed through the rocks. 118 5.0 Hydrothermal Environment of Deposition 5.1 Introduction: The preceding chapters describe many geological features of the deposit at Mt Skukum such as rock types, structural environment, and mineralogy. This chapter examines some features of the hydrothermal environment of deposition. Factors defining hydrothermal fluids that deposited ore and gangue minerals In the deposit such as the temperature and salinity of the depositional fluid, the isotopic composition 8nd origin of the fluid, and considerations of scale involving possible water to rock ratios and total water volumes, are considered. Although estimation of the conditions of deposition Involves many assumptions as to equilibrium and other factors, theoretical guidlines serve to place constraints on the size and character of the hydrothermal system. This information may be used to assess the likelyhood of additional, similar deposits in areas adjacent to the mine and may provide insight into previously untried exploration techniques. Two methods used to measure hydrothermal conditions of deposition are: a) direct measurements of the depositional fluids, and b) indirect measurements from minerals assumed to have been deposited in equilibrium with the hydrothermal fluids. Although the hydrothermal system which formed the deposit at Mt. Skukum became inactive millions of years ago making the direct measurement of the depositional fluid difficult, microscopic analysis of the fine "ore" fluid trapped as fluid inclusions by Imperfect crystal growth allows direct measurements of some of the characteristics of the original fluid (section 5.2). Indirect measurements obtained here include the oxygen and carbon isotopic compositions of the depositional fluid and resultant mineral assemblages (sections 5.3 and 5.4). 119 5.2 Fluid inclusions 5.2.1. Introduction and Objectives: Fluid inclusions, which represent minute samples of the original depositional fluid in a deposit, provide an opportunity to study several aspects of the depositional environment. The primary goals in this survey were to determine the temperature of deposition and the salinity of fluids responsible for the deposit. In addition, calculation of the depth of mineral emplacement, identification of volatile components, and evidence of boiling in the hydrothermal fluids were sought. 5.2.2. Data Collection: Ten specimens were selected from drill intersections at different lateral positions and elevations in mineralization of the Main Cirque Zone to allow representation of ore fluids over a broad extent of the deposit. These specimens can be accurately located from data in Appendix B. All specimens were of quartz-carbonate vein material, some from high-grade gold areas, and others from relatively barren zones. Of the original ten specimens, only three were devoid of usable inclusions. Separation of vein specimens by their paragenetic episode of emplacement was not possible because textural evidence for age relationships are obscure (section 4. 1). A total of 63 fluid inclusions were analysed in this study, using a Chaixmeca heating-freezing stage, Measurements were calibrated using calibration curves derived from nine standards (Appendix C). These curves (Figure C. 1) demonstrate an accuracy of measurement to within 6.7°C with a precision of 0.6°C (Iff) for the temperature range -100* to +40°C. Temperatures in the range +40" to +420°C( Figure C.2)show an accuracy to within 5.3°C and a precision of 2.2°C (Iff). Inclusions in quartz grains yielded the most reliable fluid inclusion data. These inclusions occur in two ways. The first consists of quartz grains hosting enormous quantities of m inute 120 inclusions of all classifications (see Roedder, 1984) which are invariably too small to work with (<2urn across); because optical interference from surrounding inclusions make measurements difficult, and abundant fracture planes in these grains attest to disturbance after emplacement which might have caused leakage. The second occurence of inclusions are relatively easy to measure because they occur in clear quartz grains. Although sparse, they are isolated and moderately sized (2 to 25 urn). inclusions in calcite were commonly very large, some exceeding 20 urn across. However, although some useful measurements were obtained, problems were encountered with optical interference from cleavage planes, poor clarity in calcite crystals, and leakage along cleavage planes during heating. Fluid inclusions studied were first carefully classified by origin as (Roedder, 1979): primary (P), pseudo-secondary (PS), or secondary (S). Primary inclusions were identified by their solitary location or presence on a crystal growth zone, and the lack of any associated fracture planes; these inclusions commonly took a negative crystal or ovoid shape. Pseudo-secondary inclusions, also commonly of negative crystal habit, occured along fracture planes truncated by crystal boundaries. Secondary inclusions with highly irregular, two dimensional shapes, occurred along fracture planes that crossed crystal boundaries. The above characteristics, insufficient alone to uniquely categorize a fluid inclusion, formed the basis of classification of inclusions described in Table 5.1. Size of fluid Inclusions studied ranged from 28.3um to 5.0um across in their longest dimension. All contain at least two phases, a liquid and a vapour phase. The amount of vapor phase present, determined using visual estimation tables (Roedder, 1984), ranges from 1 to 20 volume percent (Table 5.1) around an overall mode of 5 volume percent gas (Figure 5.5). 121 5.2.3. Homogenization Data: A total of 48 measurements of homogenization temperature (Th) are in Table 5.1 and summarized in Table 5.2. Homogenization invariably took place through disappearance of the vapour phase. To obtain the recorded homogenization temperature, each inclusion was heated three times to obtain an average Th. If leakage of fluid, decrepitation or disparate Th determinations occurred, no measurements were recorded. Homogenization temperature data are plotted in Figures 5.1 and 5.2. Table 5.2. Summary of homogenization temperature (Th) data from fluid inclusions in vein samples, Mt. Skukum, Yukon Territory. Inclusion Type Number of AveTh'C Measurements Primary 29 313.5 ±6.5 Pseudo-secondary 10 269.1 ± 8.0 Secondary 9 196.8 ± 11.5 1. Arithmetic means with standard error. 5.2.4. Freezing Data: Freezing temperatures and phase changes were observed in 23 fluid inclusions. The temperature of eutectic melting T(e) as well as the temperature of final melting T(m) are in Table 5.1. Results, plotted in Figures 5.3 and 5.4, are summarized in Table 5.3. Table 5.3. Summary of freezing data from fluid inclusions in vein samples, Mt Skukum, Yukon Territory. Inclusion Type Number of T(e)'C l T(m)°C' Measurements Primary 20 -29.6 i 0.9 -0.4 ±0.5 Pseudo-secondary 2 -28.4 ± 1.8 -2.4 ± 1.3 Secondary 1 -31.4 + 0.1 1. Arithmetic mean with standard error, where calculable. Table 5.1. Homogenization and freezing data for fluid inclusions from quartz veins at Mt. Skukum, Yukon Territory. Sample and Chip Sample Inclusion Type Inclusion Volume ?» Tn(Ave.) Eutectic Melting Final Melting Number1 Elevation (m)1 (P.PS.S)2 Size (urn) Gas CO Temp. CO Temp. CO C177-1 1653.6 P 14.0 x 22.6 10 327.0 C177-1 1653.6 P 11.3x 8.5 2 301.6 C177-1 1653.6 P 19.8 x 6.5 10 -33.3 -0.9 C177-2 1653.6 P 8.5 x 5.1 5 305.3 -32.3 +7.2 C172-1 1690.7 P 19.8 x 5.1 5 255.9 -30.1 -1.1 C172-1 1690.7 P 19.8x 14.1 10 354.2 C172-2 1690.7 S 14.0 x 6.0 5 178.1 -23.7 +0.1 C172-2 1690.7 S 16.0 x 14.0 5 139.2 C172-3 1690.7 S 11.3x 8.5 2 186.7 C172-3 1690.7 S 11.3 x 5.1 2 192.6 C104-1 1690.8 P 14.0 x 6.5 20 -29.2 +0.1 C104-1 1690.8 P 14.0 x 8.5 10 325.3 C104-1 1690.8 PS 17.0 x 14.0 5 263.6 C104-1 1690.8 P 8.5 x 5.1 20 335.5 C104-2 1690.8 P 14.0x11.0 10 297.0 C104-3 1690.8 P 17.0 x 14.0 20 412.7 -34.2 -2.8 C104-4 1690.8 P 11.0 x 5.0 5 294.6 C104-4 1690.8 PS 17.0 x 8.0 5 280.4 C104-4 1690.8 P 14.0 x 5.1 20 340.6 C104-4 1690.8 PS 14.0 x 5.1 5 241.9 C104-5 1690.8 PS 8.5 x 8.5 5 274.8 C104-5 1690.8 PS 11.3 x 5.1 5 266.7 C104-5 1690.8 PS 11.3 x 11.3 10 236.1 C104-5 1690.8 PS 11.3 x 8.5 5 235.4 C104-5 1690.8 S 14.0 x 8.5 2 237.7 C104-6 1690.8 P 28.3 x 8.5 10 313.9 C104-6 1690.8 P 8.5 x 8.5 2 254.6 C078-1 1644.2 S 10.0 x 5.0 2 -31.4 +0.1 C078-1 1644.2 p 22.6 x 85 20 300.0 -29 0 -0.1 C078-2 1644.2 PS 16.9 x 8.5 5 292.4 C078-3 1644.2 p 8.5 x 4.2 1 323.2 C078-3 1644.2 PS 17.0 x 8.5 10 310.4 C078-3 1644.2 P 14.0 x 5.0 2 259.6 C078-4 1644.2 PS 14.0 x 8.5 5 -30.2 -1.1 C078-4 1644.2 P 14.0 x 5.0 5 -30.5 +0.2 C078-4 1644.2 P 14.0x8.5 20 317.1 C078-4 1644.2 P 8.5 x 3.0 5 318.2 C078-4 1644.2 P 14.0 x 5.0 5 317.9 C078-5 1644.2 PS 14.0 x 14.0 5 261.4 -32.3 +0.9 C078-6 1644.2 PS 8.5 x 14.0 10 289.2 C041-1 1633.3 P 19.8 x 8.5 2 -31.1 -1.6 C041-2 1633.3 S 25.5 x 25.0 2 214.0 C04I-3 1633.3 S 19.8 x 14.0 2 172.1 C041-4 1633.3 S 11.3 x 5.1 2 198.0 C022-1 1706.3 P 11.3x5.1 2 -31.3 + 1.4 C022-2 1706.3 P 14.0 x 8.5 20 321.6 0244-1 1706.3 PS 7.1 x 5.7 1 -26.6 -3.8 0244-1 1706.3 P 5.1 x 2.8 20 -24.2 -2.8 0244-1 1706.3 P 7.1 x 5.7 10 -25.3 -4.6 0244-2 1706.3 P 7.1 x 7.1 20 338.5 -29.4 -3.7 0244-2 1706.3 P 5.7 x 4.7 1 337.4 0244-3 1706.3 P 5.7 x 2.9 2 -31.7 -0.5 0244-4 1706.3 P 7.1 x 5.7 2 -0.3 0244-4 1706.3 P 5.7 x 5.7 20 -30.6 0.0 0244-4 1706.3 P 9.9x7.1 20 272.0 0244-5 1706.3 P 7.1 x 5.6 5 -28.8 0.0 0244-5 1706.3 P 5.1 x 4.8 20 -33.1 0.0 0244-5 1706.3 P 8.5 x 7.1 2 274.2 -33.1 0.0 0244-5 1706.3 P 5.1 x 7.1 2 329.0 0244-5 1706.3 S 8.5 x 5.1 10 252.7 0244-5 1706.3 P 14.1 x 8.5 20 349.5 0244-5 1706.3 P 5.1 x 2.0 1 306.2 0244-5 1706.3 P 8.5x2.8 10 348.7 1 Sample location details are in Appendix B. Elevations are calculated from data in Appendix B. 2 (P ,PS,S) denote Primary, Pseudo-secondary and Secondary inclusions respectively. 124 • Primary E3 Secondary • P/S Homogenization Temperature Figure 5.1. Fluid inclusion homogenization data from Mt. Skukum, Yukon Territory, showing distribution of primary, secondary, and pseudo-secondary (P/S) inclusions from vein material. A trimodal distribution with peaks near 310'C, 270°C, and 190*C is indicated. Data are from Table 5.1. • 0244 • C022 LI C041 • C078 • C104 • C172 m C177 S B E g g g g g g g g ' g g g Homogenization Temperature Figure 5.2. Fluid inclusion homogenization data obtained from individual samples of vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1. 125 pj Primary E3 Secondary 0 Pseudo-secondary Eutectic Melting Temperature Figure 5.3. Eutectic melting temperatures by distribution of inclusion types (primary, pseudo-secondary, and secondary) in vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1. Final Melting Temperature Figure 5.4. Final melting temperatures of fluid inclusions by distribution of inclusion types (primary, pseudo-secondary, and secondary) in vein material from Mt. Skukum, Yukon Territory. Data are from Table 5.1. 126 The temperature at which liquid in fluid inclusions commonly froze during cooling was approximately -46.58C. Although nucleation of a clathrate was never observed during the freezing process, appearance of minute amounts of liquid at approximately -56°C was consistently observed. This liquid appears in such small quantity that it would have gone un-noticed but for a minute "frantic" boiling action displayed at that temperature. 5.2.5. Pressure Qyreetton: Homogenization data are not corrected for the effects of pressure. Many of the features of mineralization in the Mt. Skukum area (section 4.1) - - vuggy veins, chalcedony, and extensive fracturing — indicate a high level of emplacement. The presence of abundant hydrothermal breccias adjacent to and surrounding the deposit attest to the existence of hydrostatic pressure conditions over at least short periods. This is only possible in near-surface conditions where lithostatic loads are comparatively low. In addition, the presence of finely banded silica sinter and zones of alunite-clay alteration in areas of the volcanic complex no more than 400 m higher in elevation than the deposit, represent near-surface deposition. Specifically, the position of the Alunite Cap Zone (Figure 3.1), which consists of alunite-clay alteration on the southeastern wall of Main Cirque, is only 320 m above the Main Cirque Zone and represents near-surface mineralization probably developed at depths of less than 150 m. Its presence constrains the minimum depth of emplacement at 320 m. A large fault structure inferred to pass between the Main Cirque Zone and the Alunite Cap Zone is interpreted to have dropped the Main Cirque Zone block down with respect to the Alunite Cap Zone by a minimum of 250 m (section 3.4), These two lines of evidence indicate that the minimum depth of emplacement was probably about 320 m; however, marker horizons are absent, making such qualitative determinations of displacement difficult. Thus, a hydrostatic pressure of approximately 31 bars may have effected this fluid assuming 320 m as a tentative minimum depth of emplacement. Pressure corrections to measured homogenization temperatures in inclusions subjected to this hydrostatic pressure in a solution of 1 weight percent NaCl equivalent are no more than S'C (Potter, 1977) indicating that the actual 127 temperature of deposition probably would be less than 8"C higher than reported homogenization temperatures. 5.2.6 Interpretation: Inclusions at room temperature contain at least one liquid and one vapour phase. As these inclusions freeze at -46.5 0C, a slight contraction of the vapour bubble occurs which is consistent with inclusions being r^O-rich. After being frozen, two distinct melting events occur during warming. One at approximately -56"C, indicates the presence of C O 2 (Hollister etal., 1981), the other at about -0.4"C (Table 5.3.) indicates the presence of minor amounts of dissolved salts (Hollister et al.. 1981). Although the presence of C O 2 is indicated by the melting event at -56°C, no clathrate was seen to nucleate upon cooling. This suggests that clathrates were obscured in the bubble miniscus, and that only low partial pressures of C O 2 are involved. Homogenization temperatures from inclusions at Mt. Skukum range from a maximum of 412.7°C to a minimum of 139.2"C but show a trimodal distribution (Figure 5.1). The three modes at approximately 310'C, 270*C, and 190'C correspond to clusters of primary, pseudo-secondary, and secondary fluid Inclusions respectively. These modal temperatures are consistent with findings in other epithermal deposits formed in sub-aerial conditions where characteristic temperatures range from 200° to 330°C (L&Sunnystde, Colorado at 300°C; Finlandia, Peru at 270*C;Tonopah, Nevada at 300-250*C(Roedder, 1984; Spooner, 1981; Kamll 11 and Ohmoto, 1977)). The three modal temperatures might reflect three major mineralizing events related to periods of peak fluid flow. The first event probably occurred at the highest emplacement temperature where precipitating minerals preserved the event mainly as primary inclusions. The second event, at a lower temperature of 270eC, was preserved partly in primary inclusions of precipitated minerals and partly in pseudo-secondary inclusions in fractures caused by some associated activity. The final pulse, at a temperature of 185°C, was also associated with activity which fractured earlier mineralization to form secondary inclusions. No marked difference in homogenization temperature was noted between inclusions in calcite and quartz. An overall decay is apparent in filling temperatures in Figure 5.1, from the peak of 31O'C to the low of 130eC. This smooth decay in filling temperatures, frequently observed in other hydrothermal deposits, reflects the gradual decline in temperature and activity of the hydrothermal cell as the heat source cools (Spooner, 1981). As a result, this decay may record the thermal history of the system from its beginning at temperatures of 350° to 41O'C, to its peak at 310°C, followed by a waning of activity and fluid temperatures. The presence of low concentrations of dissolved salts in fluids of primary inclusions from Mt. Skukum was determined by the melting point depression of 0.4°C. Eutectic melting in these inclusions at -29.6*C is very close to the metastable eutectic melt in the H 2 O-N3CI system ( « -28°C) indicating that salinity can be largely attributed to NaCl (Roedder, 1984). Using the experimentally derived curve for the H20-NaCl system of Potter et al. (1978) , the average final melting point depression of 0.4"C corresponds to a dissolved salt content of 0.7 weight percent NaCl equivalent. Eutectic and final melting temperatures for pseudo-secondary and secondary inclusions show minor variations from data for primary inclusions but data are too few to be conclusive (Figures 5.3 and 5.4). Salinities lower than that of sea water, as found at Mt. Skukum, are characteristic of epithermal deposits which typically range from 0.1 to 3.6 weight percent NaCl equivalent (Spooner, 1981) with most deposits showing salinities of between 0.5 and 1.0 weight percent NaCl equivalent (1^Mount Kasi, Fiji at <2 weight percent NaCl (Turner, 1986); Au-quartz-adularia veins in Nevada at <2.1 weight percent NaCl equivalent (Nash, 1972)). Similar temperatures and salinities are also recorded in modern geothermal systems (Ellis, 1979). Consequently, an average salinity of 0.7 weight percent NaCl equivalent at Mt. Skukum is within expected limits for epithermal deposits, and is indicative of large amounts of meteoric water passing through the hydrothermal system containing a low initial salt content and maintaining this dilution in its passage through the rock. The presence of COp. noted above, will increase the apparent salinity of the inclusions (Hedenquist and Henley, 1985) indicating that the 129 actual salinity may be even lower than measured. Hedenquist and Henley (1985) found that a solution containing 1 weight percent NaCl equivalent and 1 weight percent C O 2 at 300CC shows an apparent salinity of approximately 1.7 weight percent NaCl equivalent. Liquid to vapour (L:V) ratios of primary inclusions are highly variable and bimodal (Figure 5.5) with peaks at 2 and 20 volume percent vapour. No evidence of necking, which could Volume % 6*s Figure 5.5. Frequency distribution of volume percent gas in fluid inclusions from veins, Mt. Skukum, Yukon Territory showing the wide variation in LV ratios and their bimodal distribution in primary inclusions at 2 and 20 vol. percent gas. The overall mode is at 5 vol. percent. Data is from Table 5.1. account for this variation, was observed. Figure 5.6 shows that the average homogenization temperature for each of the volume percent gas class intervals in Figure 5.5 are remarkably similar. The only minor deviation occurs in inclusions which contain 2 and to a lesser extent 5 volume percent gas each of which display slightly lower homogenization temperatures. Widely ranging L:Y ratios with similar homogenization temperatures such as these are generally considered symptomatic of a boiling event. However, these variations might 130 400 I m ! I m a S I 300-200-100--<L J a 2 5 10 Volume X Gas 20 Figure 5.6. Homogenization temperature versus L:V ratio for primary fluid inclusions at Mt. Skukum, Yukon Territory. This histogram illustrates the generally consistent homogenization temperatures of inclusions with a wide range of volume percent gas content indicating that inclusions formed from similar fluids under boiling conditions. Data are from Table 5.1. also be due to several stages of vein emplacement from fluids of varying composition. No additional fluid inclusion evidence of boiling, such as coexisting liquid-rich and vapour-rich inclusions, was found. Nevertheless, breccia textures within veins and abundant breccia bodies associated with ore indicate that boiling probably was common during mineralization and may be responsible for variable LV ratios. The bimodal distribution of gas content in primary inclusions corresponds with subtle differences in homogenization temperature and apparent salinity (Figures 5.6,5.7, and 5.8). This indicates that at least two different fluids were responsible for mineralization; one of relatively high gas content, high salinity, and high temperature; the other is relatively lower in all of these. Plotting the homogenization temperature vs final melting temperature for those primary inclusions in which both were obtained illustrates this relationship (Figure 5.9). 131 I Primary inclusions containing 20 vol X gas. E3 Primary inclusions containing 2 vol 8 gas. -2.5 -1.5 -0.5 0.5 1.5 Temperature of Last Melt Figure 5.7. Distribution of the last melting temperature of primary fluid inclusions from veins at Mt. Skukum, Yukon Territory, showing that those containing different gas contents have different apparent salinities. Data are from Table 5.1. 2 8 e * » K) » ^ ^ Homogenization Temperature I Primary inclusions containing 20 vol % gas •Primary inclusions containing 2 vol % gas Figure 5.8. Distribution of the homogenization temperature of primary fluid inclusions from veins at Mt. Skukum, Yukon Territory, showing that those containing different gas contents had different minimum temperatures of emplacement. Data are from Table 5.1. 132 X 0 z m © 9 -1 --3 Low temperature low salinity 0.87 wtJJ NaCl eq. 250*C High temperature high salinity 4.12 wtS NaCl eq. 330*C • 200 300 400 500 Homogenization Temperature Figure 5.9. Primary fluid inclusion data for temperature of last melt vs homogenization temperature from veins at Mt. Skukum, Yukon Territory, shows two fluid types, a low temperature, low salinity fluid and a high temperature, high salinity fluid. Points plotted represent all those inclusions for which both homogenization temperature and temperature of last melt were obtained and thus represents only a part of the total data set. Fluid temperature and salinities for each cluster are from Figures 5.7 and 5.8 which represent complete data sets. Data are from Table 5.1. As the fluids are at or near boiling (sections 4.2 and 4.3), the data for the two clusters in Figure 5.9 allow calculation of depths of emplacement based on the boiling curves of Haas (1971) using the different modal salinities and homogenization temperatures for each cluster. Although the points plotted in Figure 5.9 represent only inclusions for which both homogenization temperatures and final melting temperatures were obtained, they apparently are coincident with the two modal salinities and homogenization temperatures in Figures 5.7 and 5.8 that represent complete data sets. Using this evidence, the maximum depth of emplacement for each fluid can be calculated for both lithostatic and hydrostatic pressure conditions. The high temperature fluid in Figure 5.9 has a modal temperature and salinity of 330°C and 4.12 weight percent NaCl equivalent respectively (Figures 5.7 and 5.8), the low temperature fluid in Figure 5.9 has a modal temperature and salinity of 250'C and 0.87 weight percent NaCl equivalent respectively (Figures 5.7 and 5.8), Salinities and temperatures of these fluids indicate densities of 0.789 ± 0.002 133 gms/cm3 for the low temperature, low salinity fluid and a density of 0.675 ± 0.002 gms/cm3 for the high temperature, high salinity fluid if both are considered to lie on the boiling curve (Haas, 1971) at 250°C and 330'C respectively. Calculated vapour pressures for the low and high temperature fluids are 39.5 bars (± 0.5 percent) and 125.2 bars (± 0.5 percent) respectively; these are equal to the confining pressure on the respective liquids during deposition at the maximum depth of boiling. As confining pressure may be hydostatic or lithostatic, different maximum depths of emplacement can be calculated. For hydrostatic conditions, maximum depths of emplacement are 460 ± 4 m for the low temperature fluid and 1,600 ± 19 m for the high temperature fluid. For lithostatic conditions, assuming a mean rock density of 2.7 gms/cm3, the maximum depth of emplacement for the low temperature solution is 149 ± 4 m and for the high temperature solution 473 ± 19 m. The coincidence of calculated lithostatic and hydrostatic depths at about 465 m is consistent with: 1) stratigraphic evidence (section 5.2.5), 2) the epithermal character of the veins, and 3) Lindgren's (1933) estimate that epithermal veins form at depths of less than 1 km. The depth of 1,600 m obtained for the high temperature, high salinity fluid under hydrostatic conditions exceeds the reasonable range of depths expected in an epithermal deposit that exhibits the abundance of open-space filling textures observed at Mt. Skukum, and also contradicts the depth of emplacement indicated by stratigraphic evidence. From the same stratigraphic evidence, the calculated depth of 149 m for the low temperature fluid under lithostatic pressure is unreasonably shallow. This introduces the possibility that these are perhaps not different solutions but rather high pressure and low pressure equivalents of the same hydrothermal fluid. Formation of a silica cap sealing movement of fluids in the Main Cirque Zone would cause an increase in fluid pressure towards lithostatic conditions. Minerals deposited during these sealed periods reflect higher pressure conditions through lower L.Y ratios and higher temperatures of emplacement. Breaking of this seal, perhaps explosively, would allow sudden releases of pressure, and trigger boiling events which would be reflected in a decrease in fluid temperature accompanied by rapid mineral precipitation and loss of volatiles. The result is seen as lower homogenization temperatures and higher L:V ratios in inclusions from mineral phases 134 formed at this time. These periods of free fluid flow might also be accompanied by widespread mixing of upwelling hydrothermal fluids with cooler groundwater which may contribute to the relatively dilute dissolved salt content found in lower temperature inclusions. Carbon dioxide clathrates formed in fluid inclusions from Mt. Skukum indicate that a moderate partial pressure of CO2 existed in the depositional fluid. Only where CO2 pressures within the inclusion lie between 10.4 and 45 bars (Figure 5.10) will a CO2 clathrate (CO2 • 5.75 H2O: Roedder, 1984) form as the inclusion is cooled below 10'C. This partial pressure corresponds to a molal concentration at Pco2" 1 0 - 4 oars of at least 0.85 molal (Hedenquist and Henley, 1985). Where Pcfo is greater than 45 bars (2.2 molal) a liquid CO2 phase will be visible within inclusions at room temperature (Figure 5.10). As this phase was not observed in inclusions from Mt. Skukum, the maximum molal concentration of CO2 present must be less than 2.2 molal. The minor quantities of clathrate present in inclusions from Mt. Skukum indicates a CO2 concentration close to the lower of these limits, probably about 1.0 molal or 4.4 weight percent ( i e just above the minimum concentration required for clathrate formation). Moderate concentrations of CO2 in fluid inclusions of epithermal deposits are common as phase equilibria in the H2O-CO2 system allows almost complete miscibility over a wide range of temperature and pressure above 300°C and a high degree of immiscibility at lower temperatures. Homogenization temperatures determined above, indicate that fluids at Mt. Skukum were sufficiently hot to carry large amounts of CO2 in solution as seen in several active geothermal systems (Ngawha and Broadlands, New Zealand) where carbon dioxide dominates the dissolved gases in geothermal fluids and in some cases exceeds chloride as the major dissolved component (Hedenquist and Henley, 1985). Fluid inclusion evidence from Mt. Skukum indicates that fluids responsible for mineralization were of low salinity and contained an apparent average of 0.7 wieght percent NaCl equivalent. These fluids, which contained approximately 1.0 molal CO2, deposited minerals at an Q - CO^gas) •Lw - HjO (liquid) Lc - CCfefJould) 8Of H - C O j hydrate I - ice co oc < CO Ul oc 3 CO CO UJ ct a. eo • 4 0 -20-CP. c o 2 Lw+a Possibt* ran«« of P Q O ^ In deposHlorwl fluid at Mt. Skukum. - 3 0 -20 -10 0 10 20 TEMPERATURE (°C> 30 Figure 5.10. Phase equilibria in the lower temperature part of the system H20 - CO2 showing the limitations of CO2 content in depositional fluids at tit. Skukum, Yukon Territory (after Roedder, 1984). Q1 = CO2 vapour, H2O liquid, CO2 clathrate, and ice at «-2'Cand 10.4 bars. 02 = CO2 vapour, H20 liquid, CO2 liquid, and CO2 clathrate at * 10'C and 45 bars. The equivalents of curve Q1 - Q2 for 5 percent added NaCl is also shown. 136 average temperature of about 313"C at depths of about 470 m below paleo-surface. Mineral precipitation was probably controlled by boiling events brought about through tectonic activity that fractured overlying rocks thus allowing sudden releases of pressure. Mineral deposition may have been enhanced by mixing of upwelling hot hydrothermal fluids with cooler ground water near surface. At least three major episodes of mineral deposition are recognized; the first involving fluids at 310'C, the second at 270°C, and the third at 190°C. The presence of significant CC»2 in depositional fluids at Mt. Skukum explains the abundance of carbonate minerals occurring as gangue in the deposit and as an alteration mineral permeating the wall rxks (section 4.0). Its presence has been found to cause a decrease in the measured salinity of fluid inclusions which ma/ decrease the calculated emplacement depth of mineralization by as much as 23 percent (107 m). Carbon dioxide may have been incorporated into the hydrothermal system by deep fluid circulation through underlying basement rocks (section 2.2.1) where CO2 probably was incorporated from dissolution of marble abundant in underlying metasedimentary rxks of the Yukon Group. 137 5.3. Oxvoen Isotope Composition of Hydrothermal Minerals. Fluids, and Surrounding Host Rocks: 5.3.1. Objectives: Knowledge of the origin of water, which is the dominant component of any hydrothermal fluid ,is essential to any theory of ore formation. Systematic differences in the ratios of deuterium to hydrogen (D/H), and ratios between oxygen isotopes ( * 8 0/ , 6 0) allows the derivation of naturally occuring waters to be classified broadly Into categories of meteoric, metamorphic and magmatic. Direct measurements of the isotopic composition of extinct hydrothermal systems may be made by crushing fluid inclusion-bearing hydrothermal minerals and measuring the isotopic composition of minute amounts of liquid contained therein. An alternate method, used in this study, involves analysts of oxygen-bearing hydrothermal mineral phases and the application of experimentally derived equations to calculate the Isotopic composition of the depositional fluid. The objectives of the oxygen isotope study were to: a) determine the source of hydrothermal fluids which formed the deposit; b) calculate the isotopic composition of the mineralizing fluids, c) define constraints on the water to rock ratio operative in this hydrothermal system and thereby estimate Its size, and d) calculate an isotopic temperature of deposition for minerals in the deposit. 5.3.2. Date: Fourteen analyses from ten samples from the Mt. Skukum deposit area were used, including three whole rock samples and eleven mineral separates. Two samples of altered wall rock, one of fresh wall rock, two pairs of quartz and calcite mineral separates from coexisting assemblages in quartz-carbonate veins, two of veins containing quartz only, two of early chalcedonic veinlets, and three independently analysed samples were used (Table 5.4). Mineral separation for quartz was achieved by coarsely crushing the sample, removal of carbonate by 138 Table 5.4. Oxygen isotope data' From the Mt. Skukum deposit, south-central Yukon Territory. Sample Sample Description S^O 1 Number (% 0 ) ASTN-15 Unaltered porphyritic andesite with relatively abundant -6.8 magnetite and only slightly sericitized plagioclase phenocrysts. Whole rock analysis. ASTN-13A Propylitized porphyritic andesite exhibiting bleached, -6.7 chloritized appearance. Whole rock analysis. ASTN-IOA Propylitized porphyritic andesite exhibiting bleached, -7.2 chloritized appearance. Whole rock analysis. C049-Qtz Mineral separate of quartz from a 2m thick massive vein - 3.5 consisting of 60$ calcite, 40$ quartz, and 9 gms Au/tonne. C049-Cal Mineral separate of calcite from a 2m thick massive vein -6.9 consisting of 60$ calcite, 40$ quartz, and 9 gms Au/tonne. C005-Qtz Mineral separate of quartz taken from a vein consisting of -4.3 70$ fine grained quartz Intergrown with 20$ bladed calcite, 10$ altered wall rock fragments, and 2.75 gms Au/tonne. C005-Cal Mineral separate of calcite taken from a vein consisting of -6.8 70$ fine grained quartz intergrown with 20$ bladed calcite, 10$ altered wall rock fragments, and 2.75 gms Au/tonne. C042 Quartz in a massive vein containing altered wallrock fragments - 4.3 in a matrix consisting of 90$ quartz with minor calcite, sericite, and 0.07 gms Au/tonne. C040 Quartz in stockwork veins containing 90$ quartz minor calcite -4.0 and sericite, argillized wallrock fragments, and 0.07 gms Au per tonne. CO72 Quartz in a stockwork of pre-ore chalcedonic veins in a highly -5.2 argillized porphyritic andesite containing 0.07 gms Au/tonne. C095 Quartz in a stockwork of pre-ore chalcedonic veins in brecciated, -6.8 propylitized andesitic lapilli tuff. M l 2 Quartz. -4.71 M2 2 Quartz -4.3 M3 2 Calcite -8.6 1 All analyses by K. Meuhlenb8chs, Universtiy of Alberta. Error in analysis ± 0 . 0 0 0 8 °/QO (Field s l a l , 1985) . 2 Samples obtained independently by Karlis Meuhlenbachs, University of Alberta. 139 heating in concentrated HCL followed by hand picking. Calcite separation followed a similar procedure; however, samples were not treated with acid prior to hand-picking. All samples were analyzed by K. Meulhenbachs at the University of Alberta. Results in Table 5.4 are expressed in terms of S 1 8 0 representing the per mil (°/oo) deviation of 1 8 0 in the sample from that in the universal standard, SMOW (standard Mean Ocean Water). All analyses were done on a gas source mass spectrometer. A precision of ±0.02 percent or better is routinely obtained using this technique (Field eiaj., 1985). 5.3.3. Isotopic Composition of Hyxfrothermal Fluids: The isotopic species of oxygen ( 1 8 0 and 1 6 0) become fractionated between a mineral and its depositional fluid during precipitation through mass-dependant differences in chemical and physical behaviour. The degree of fractionation varies inversely with temperature and independantly of pressure in a predictable way( Field § L a J L , 1985;Taylor, 1979). Consequently, if an independant estimation of temperature for the depositional fluids can be made through fluid inclusions or other means, the isotopic composition of that fluid may be calculated using analytical isotope data from a mineral and the fractionation coefficient between that mineral and water at the temperature of deposition. Primary fluid inclusions at Mt. Skukum yield an average temperature of homogenization of approximately 315°C (section 5.2). Using this independant. estimation of temperature (Temperature of formation estimates using oxygen isotope analysis of mineral pairs in section 5.3.5 appear to be invalid.) and isotopic data from analyses of quartz and calcite in the veins, the original isotopic composition of the depositional fluids can be calculated using the following two equations: 1. Quartz-H20 1000 In o y q u a P t Z J W 8 t e r ) - 3 . 3 4 x(10r5+T2)-3.31 where: T = Temperature (°K) Temperature range=250-500'C (Matsuhisaetal, 1979) 2. Calcite-H20 1000 In o^ c a l c i t e _ w a l e r ) -2.78 x(lG*+T2)-2.89 where: T = Temperature (°K) Temperature range = 0-500°C (Friedman andO'Neil, 1977) TheS180 values of whole rock can also be used to calculate the composition of depositional fluids. According to Taylor (1979), it is reasonable to assume that 6 1 80rO C k in equilibrium with hydrothermal fluids is equal to the 8 1 80 value of plagioclase (An3o). Therefore, the following plagioclase- H20 equation can be used: 3. Plagioclase(An30)-H20 1000 In otyiagjoclase-water) * 2 6 8 x Q&+T2)-Z5 where: T = Temperature (°K) Temperature range = any reasonable geologic temperature. (O'Neil and Taylor, 1967) Table 5.5. Oxygen isotope composition1 of hydrothermal fluids at Mt. Skukum, Yukon Territory Calculations are from data in Table 5.4 and equations 1,2 and 3 in text. Sample Material 5 , 80 value of Calculated 5180' Number material of depositional fluid ASTN-15 Unaltered Andesite -6.8 -11.0 ASTN-13A Altered Andesite -6.7 -10.9 ASTN-10A Altered Andesite -7.2 -11.4 C049-Qt2 Quart2 -3.5 -9.8 C049-Cal Calcite -6.9 -12.0 C005-Qtz Quart2 -4.3 -10.6 C005-Cal Calcite -6.8 -11.9 C042 Quartz -4.3 -10.6 C040 Quartz -4.0 -10.3 CO 72 Quartz -5.2 -11.5 Ml Quartz -4.71 -11.1 M2 Quartz -4.3 -10.6 M3 Calcite -8.6 -13.7 Average(n= 13) is -11.2 1 Error in calculated values ± 0.0008 °/QO 141 In the above three equations, 10OO In o t ^ ) « 8 a - 8b. Consequently, they may be used to calculate the 8 1 8 0 composition of depositional fluids for all data in Table 5.4. Results of these calculations (Table 5.5) display a very narrow spread of isotopic composition in the depositional fluids of between 8180=-9.8 and -13.7 °/0o. The average value for calculated 8 1 8 0 composition is -11.2 0 /oo; analytical error gives an uncertainty of ± 0.0008 °/o0. 5.3.4. Water to Rock Ratio Unaltered andesitic rocks have a well-known 8 1 8 0 value of +6.5 ± 1 °/oo (Taylor, 1979). The extent to which 8 1 8 0 values of the andesitic volcanics surrounding the deposit have been lowered from this norm is dependant on temperature, extent of isotopic equilibrium between wall rock and fluids, the 8' 80 value of circulating fluid, and the water to rock ratio (i.e. the amount of exposure the rock has had to the fluids). Since isotopic equilibrium is assumed and the remaining three variables are known — one from independent observation and the others determined above - - the remaining unknown may be calculated using the following equation (after Ohmoto and Rye, 1974, and Field and Fifarek, 1985): 4. _ 7 ~ V w +(1.8R)(8 t 8 0 w ) 1+0.8R) where: r = rock, w = water, f = final, i = initial, R = water to rock mass ratio, and A » calculated fractionation coefficient for isotopic exchange between rock and water This equation assumes continuous recirculation and re-equilibration of the water in a closed system. Although this cannot be rigourously true, it is probably closer to reality than an open system would be (Taylor, 1979). Results of applying the above equation to the available whole rxk isotope data are in Table 5.6; they indicate an average water to rxk ratio for the Mt. Skukum deposit of 0.81. Factors other than simple fluid-rock exchange that may affect the isotopic Table 5.6. Water to rock mass ratio calculations for Mt. Skukum, Yukon Territory. Calculations are from data in Table 5.5 and equation 4 in text. Sample Calculated 5 , 80 value of water in Calculated water:rock mass Number equilibrium with sample (Table 5.5) ratio ASTN-15 -11.0 0.80 ASTN-13A -10.9 0.78 ASTN-10A -11.4 0.86 Average(n = 3) is 0.81 ±0.0008° / 0 0 composition of the fluids include boiling and mixing with unexchanged meteoric water, neither of these conditions are compensated for in the equation (Field and Fifarek, 1985). 5.3.5. Geothermometry: Two minerals deposited in isotopic equilibrium from the same fluid will contain different quantities of 1 8 0 due to a temperature dependant fractionation operating between the two minerals. Assuming that this equilibruim has been preserved and knowing variations in fractionation factor with temperature from experimental data it Is possible to calculate temperature of deposition for the two minerals. In this study, quartz and calcite were used as the mineral pair to determine the isotopic temperature of deposition. The fractionation factor (AqUartz-caicite) between these two minerals in relation to the temperature of deposition can be expressed as follows (Feild and Fifarek, 1985): 143 5. TOK) = (0.74X 103) + ( A p u a r t 2 < a l c j t e + 0.42)1/2 Temperature range = (250-500"C) where: A q u a r l 2 _ c a l c i t e «t$t 80 q t 2 - S 1 8 0 c a , Two mineral pairs from samples C005 and C049 were chosen as they both contained intimately intergrown quartz and calcite, a feature supporting the assumption that the mineral assemblage was deposited in isotopic equilibrium. Applying the above formulae to data from these samples results in the calculated temperatures of deposition in Table 5.7. of 160*C and 106°C or an average of 133°C. Table 5.7. Calculated isotopic temperature of deposition'.from mineral pairs obtained from Mt. Skukum , Yukon Territory. Calculations use data from Table 5.4 and equation 5 in text. Sample 6 , 8 0 Q U A R L7 S 1 80 c aicite ^quartz-calcite Calculated T('C)1 Number deposition COOS -4.3 -6.8 2.5 160° C049 -3.5 -6.9 3.4 106' Average (n = 2) is 133° 1 Error in calculation 0.001 °/ 0o 5.3.6. Conclusions; A significant feature of all analytical results regardless of the mineral or sample type is a consistent depletion in , 8 0 with respect to SMOW. Whole rock analyses show average 8 , 8 0 values at Mt. Skukum of -6.9 °/oo; analyses of quartz show average6,80 values of -4.6°/oo; and calcite analyses show average 6 , 8 0 values of -7.4 %()• As the lower boundary of 8,80 for normal igneous rocks on the earth and moon is + 5.5 ° /QO (Taylor, 1968; Taylor and Epstein, 1970), these materials are depleted in , 80by 10 to 13 °/oo- An average of-6.9 °/oo for the whole rock data is 13.4 °/ 0o below normal for 8n unaltered andesitic igneous rock (Taylor, 144 1979). This is indicative of isotopic exchange at elevated temperature with large volumes of hydrothermal solutions of low 1 8 0 content. Such isotopic exchange occurs during alteration of country rock by hydrothermal fluids; the degree of isotopic exchange at Mt. Skukum is symptomatic of host rocks being completely saturated in this fluid. Calculation of the original 1 8 0 content of the hydrothermal fluid from 8 1 8 0 values for the resultant quartz and calcite mineralization give an average S 1 8 0 of -11.2 °/QO- A S this fluid must have interacted with country rxks xnsiderably higher in 1 8 0 xntent than itself, its original isotopic xmposition can only have had an even lower 6 1 8 0 value than the post-equilibration value calculated above. In xmparing the calculated 8 1 8 0 value for depositional waters to fields for naturally oxurring waters in Figure 5.11, it is apparent that regardless of the 6D xmposition, a fluid this low in 1 8 0 xntent can only have originated from metxric water with little or no xntribution from magmatic or metamorphic sources. The8 1 80 xmposition of metxric water in the Mt. Skukum area today can be calculated using the relation cited by Craig (1966). 6. 8D(°/oo) = 8 $ 1 8 ° ( 0 / o o ) + 10 where: 6D = deuterium values in metxric water at any given point on earth Figure 5.12 indicates that present-day 6D values for metxric water in the Mt. Skukum area are approximately -155 °/oo- Applying this data to the above equation indicates a 8 1 8 0 value of -20.6 ° / 0 o for naturally oxuring metxric water in the 8rea today. Although the isotopic xmposition of metxric waters in any area does not remain xnstant through time, analyses of Tertiary supergene mineral assemblages indicate only minor changes involving a 1 to 2 ° / Q O shift toward heavier values for 8 1 8 0 sinx Tertiary time (Taylor, 1974; O'Neil and Silberman, 1974; Field and Fifarek, 1985). This assumption is reasonable in that as metxric 1 4 5 -20 -15 -10 -5 0 5 10 15 & 0 /oo Possible Range of • . , , isotopic compositions of fluid associated ^ - J with Mt. Skukum ores. Figure 5.11. 5' $0 ys^SD values showing fields for magmatic and metamorphic water and the possible range of depositional fluid composition at Mt. Skukum, Yukon Territory. Values for some Tertiary volcanic-hosted epithermal deposits of the Basin and Range region of the U.S. also shown are: Bullfrog (BU), Aurora (A), Jarbidge (J), Gilbert (6), Tonopah (T), Bodie (B), Comstock Lode (CL). SMOW indicates the position of Standard Mean Ocean Water. All values in per mil ( °/QQ). Modified after Taylor (1979) and Field and Fifarek, (1985). 14-6 Figure 5.12. North America with contours of S D values in meteoric surface waters (from 1979). isotope composition reflects latitude and elevation, no significant changes have occurred since before Tertiary time. Therefore, depositional fluids at Mt. Skukum, having had an original 8 1 8 0 composition of -20.6 per mil, were enriched by 9.4 ° / 0 0 during interaction with the country rock. This enrichment in 1 8 0 composition of the fluid almost exactly matches the 1 8 0 depletion in the surrounding country rocks. This supports the hypothesis that groundwater was the major and probably only constituent of mineralizing fluids; if this was not so there would be some discrepancy between the amount of depletion of 1 8 0 in country rocks and the amount of enrichment in depositional fluids from the pristine meteoric state. Reciprocal shifts in isotopic composition of this sort in both host rxks and metxric fluids have been noted around the world both in modern geothermal systems such as Wairakei, New Zealand and Steamboat Springs, Nevada (Taylor, 1979; White, 1981) and in their fossil equivalents Bodie, California; Tonopah, and Goldfield, Nevada (Taylor, 1973,1974:0'Neil etal.. 1973; Field and Fifarek, 1985). These areas are all charxterized by highly jointed, permeable volcanic rxks assxiatai with high-level ignxus intrusions, usually Tertiary in age, which x t as heat engines driving the circulation of metxric water over a broad area. It is also charxteristic that volcanic xuntry rxks will show depletions in 1 8 0 of between 10 and 13 ° / Q O around xntres of hydrothermal xtivity (Taylor, 1971), Calculations of the mass ratio of W8ter to rxk which prevailed at Mt. Skukum show that for every gram of rxk in the area of the deposit, 0.81 grams of water have bxn moved through the system during its lifetime. This represents a minimum figure xnslderlng that at least some of the water passing through the system may never have made xntx t with the walls and that water in the latter phases would only xntx t wallrxk which had already equilibrated with the fluid. Although this represents an enormous amount of water xnsidering the aerial extent of hydrothermal alteration, it is xnsistent with values obtained in many epithermal districts in the western U.S. where reported water to rxk ratios range from 0.2 to 2 (Taylor, 1974). Low salinities in fluid inclusions (sxtion 5.2) support a high water to rxk ratio to maintain dilution of the dissolved salts. Modern gxthermal systems have similar water to rxk ratios such as 0.45 to 1.3 at the Salton Sea, and 4.3 at Wairakei (Clayton and Steiner, 1975). Typical porosities for host rocks of these deposits limit the water.rock ratio to generally less than 0.1 (Field and Fifarek, 1985). This implies that mineral deposition at Mt. Skukum must have occurred in an open system of fractures and fault zones to allow passage to such large quantities of fluid. Two attempts to calculate an isotopic temperature of mineral deposition yielded divergent results of 106° and 160'C. Not only are these numbers not in agreement with those obtained using fluid inclusions, but they are both below the lower limit of temperatures required to cause the regional propylitic alteration observed. Calculations of temperature based on isotopic evidence requires not only isotopic equilibrium between the depositional fluid and each mineral considered, but also equilibrium between the minerals themselves. As isotopic temperatures are radically lower than homogenization temperatures of fluid inclusions (Tn « 315°C: Section 5.2), the assumptions of equilibrium, and contemporaneity are invalid. Despite the apparent intimate relationship between the quartz and calcite, they may have been deposited at different times by fluids of slightly differing isotopic composition. Thus, the intimate textures displayed by the quartz-calcite pairs are probably a result of replacement rather than equilibrium. Unfortunately, in an epithermal environment, equilibrium conditions that allow temperature calculations are the exception rather than the rule. This fact does not affect other calculations using isotopes as each of the other applications above relies not on mineral-mineral equilibria but mineral-fluid equilibria which is much more reasonable to assume. 149 5.4 Carbon Isotopes 5.4.1 /ntrcductfon: Carbon isotopes provide an indirect method of determining hydrothermal conditions in a fluid. Although carbon is not as fundamental a part of the hydrothermal fluid as oxygen, carbon-bearing species such as CO2, HCO3", H2CO3, and CH3 8re commonly present in hydrothermal fluids. The isotopic ratio between ' 3 C and 12C In minerals deposited from a hydrothermal fluid can supply information about the rocks it moved through and equilibrated with during its history in the hydrothermal system. Unfortunately, isotopic signatures of many varieties of rocks are similar and commonly cannot be satisfactorily distinguished. Also, the occurrence of carbon as either oxidized or reduced species leads to very different isotopic compositions in rocks that are genetically similar (Ohmoto and Rye, 1979). Despite these difficulties, some useful information was obtained from these studies. Analyses from carbonates in vein material were obtained from three specimens examined in section 5.3. All samples were analysed by K. Muehlenbachs at the University of Alberta. Two of the samples were collected and prepared by the author. The third sample was independently Table 5.8. Carbon isotope composition of hydrothermal minerals at fit. Skukum, Yukon 5.4.2 Data: Territory. Sample* 1 Mineral Analysed OO C005 C049 M3 2 Calcite Calcite Calcite -10.3 -10.3 -10.7 Aver8ge(n = 3) -10.4 1. 2. Descriptions of samples are in Table 5.1. All samples were analysed by K. Muehlenbachs at the University of Alberta. Collected and analysed by K. Muehlenbachs. 150 collected by K. Muehlenbachs. Results are reported in Table 5.8 as8 1 3C per mil (O/rjo) representing the deviation from the marine carbonate carbon standard (PDB). 5.4.3 Conclusions-Calcite analyses in Table 5.8 exhibit an extremely narrow isotopic range with an average 813c value of -10.4 O / Q O Due to the relation 8' ^C^ci te « 8' ^Cf}Uj(j, which applies at temperatures greater than 200"C (Ohmoto and Rye, 1979), the ' 3 C composition of the depositional fluid must also have been approximately -10 0 / Q O . This result falls within the overall range in carbon isotope compositions in epithermal deposits which vary world wide from -I0to + 1 O / Q O (Field and Fifarek, 1985). In addition, 8 ' l v a l ues in C O 2 emanating from geothermal sources today range from -8 to -11 °/oo. values that encompass those in calcite at Mt. Skukum. Determining the source of carbon based on carbon isotopic compositions alone is difficult due to the lack of discrimination among isotopic compositions for carbon from different sources. Nevertheless, calculated isotopic compositions at Mt. Skukum of -10 ° /QO are well below average values of 8' ^C for igneous, metamorphic and sedimentary rocks at about -5.5 °/oo (Ohmoto and Rye, 1979). This Indicates significant enrichment in the lighter isotope relative to these rocks. Abundant carbonate minerals present in veins at Mt. Skukum (section 4.2) and the presence of carbon dioxide in fluid inclusions (section 5.2) Indicates that much of this carbon occurred in the system as carbon dioxide. Marble present in underlying Yukon Group metasedlmentary rocks (section 2.2.1) has been proposed as a probable source of this component through dissolution in deeply circulated hydrothermal fluids. From the relation given aboveS^CcaicJte^S^Cnuid (Ohmoto and Rye, 1979), it is apparent that the fractionation coefficient between carbon isotopes in basement marble and interacting hydrothermal fluids is close to unity. This suggests that if these fluids had equilibrated with basement marble, then calcite deposited from these fluids should have the same isotopic composition as that of the marble which is expected to be about -5.5 ° / 0 0 . As carbon isotopes in vein material from Mt. Skukum display values almost twice as negative as that expected in any of the surrounding metamorphic or igneous host rxks, other fxtors must have affxted isotopic xmpositions. Equilibration with reduced carbon is an effxtive way to drive carbon isotopic xmpositions towards significantly lower S 1 3 C values. Thus, the presence of disseminata! graphite in politic schists in underlying Yukon Group (sxtion 2.2.1) metasedimentary units may be the cause of the unusually light carbon isotopic xmposition of calcite at Mt. Skukum. Abundant evidence that these rxks were exposed to and xmmonly involved in much of the hydrothermal xtivity is indicated by frequent inclusions of both marble and pelitic schists in hydrothermal brxcias in the area. Preliminary study of carbon isotopes in calcite from Mt. Skukum veins indicates that carbon was inxrporatx" into a deeply circulating hydrothermal system through lexhing of volcanic and graphitic metasedimentary xuntry rxk. It existed in solution in an oxidized state as reflxted by the abundance of carbonate minerals in the deposit. M8rble, which xmprises a significant proportion of these metasediments, may have been a primary sourx for carbon along with significant xntributions from reduced carbon (graphite) which resultaJ in unexpxtedly negative 6,3C values in vein material. Carbon isotope data indicates that a magmatic sourx for carbon dioxide at Mt. Skukum cannot be ruled out; however, data from oxygen isotope studies makes a magmatic sourx unlikely. 152 6.0. Conclusions 6.1. Qrjgip The Ml. Skukum epithermal gold deposit occurs in one of two early Eocene volcanic complexes in the southern Yukon Territory on the border of the Coast Crystalline Complex and the Yukon Crystalline Terrane (Figure 2.1). These volcanic centers unconformably overlie Cretaceous and older rocks, are preserved as erosional remnants in downfaulted blocks, and display features of cauldera collapse of the complexes as a whole and as smaller features surrounding volcanic centers within the complexes themselves. These volcanic complexes form the northernmost expression of a large belt of Tertiary volcanic rocks extending into northern British Columbia known as the Sloko Volcanic Province. Rocks of the Mt. Skukum Volcanic Complex are divided into four formations (section 2.2.2) reflecting three cycles of volcanic activity. The first cycle involves rocks of Formations 1 and 2, which are largely felsic in composition. The second cycle involves rxks of Formation 3 that were deposited xnformably over Formation 2, and represent a period of andesitic volcanism. The third cycle of volcanic xtivity involves rxks of Formation 4 that were deposited following a period of erosion after cycle 2; an unxnformity separates Formation 3 from Formation 4. This last cycle represents a return to felsic volcanism (Figure 2.3). Post-depositional block faulting, probably assxiated with depletion of a large inferred magma chamber at depth, caused downward displxement of the entire volcanic xmplex along major faults on the eastern and southern margins of the xmplex (Figure 2.2). Downward displaxment was most pronounced on the eastern side of the volcanic xmplex which was separated from the less displaced western side by a major north-trending fault which preserved much of the upper volcanic cycle in the more highly displaced eastern portion. A final magmatic resurgenx caused emplaxment of high- level, tan xloured, porphyritic rhyolite stxks and dykes throughout the volcanic xmplex, as well as several irregular intrusions surrounding the xmplex. This resurgent event is xnsidered analogous to that which accompanied ring dyke intrusion late in the evolution of the nearby Bennett Lake Cauldera Complex (Lambert, 1974) with the outlying, high-level porphyritic rhyolite bodies surrounding the Mt. Skukum Volcanic Complex representing intrusions into a poorly developed ring fracture system. Evolution of the Mt. Skukum Volcanic Complex (Figure 6.1) began with deposition of lower conglomerate and debris flow rocks of Formation 1 that contains abundant basement fragments which reflect an initial period of rapid erosion and instability of basement rocks associated with magmatic intrusion and incipient volcanism in the area (section 3.2). Volcanic rocks of Formation 2 which followed include early thicknesses of felsic tuffs and debris flows that infilled basement topography and reflects periods of violent volcanic eruption. These rxks grade upward from felsic to andesitic in xmposition, and represent the magmatic frxtionation series produced by the parent magma (sxtion 3.3). They also show a consistent dxrease in xntent of basement accessory clasts as well as a dxrease In grain size of assxiated epiclastic rxks; this reflxts an increasing thickness of volcanic stratigraphy that infilled and leveled off underlying basement palxtopography. Lithologtes of Formation 3, Including porphyritic andesite flows, andesitic pyrxl8stic rxks, and epiclastics are xnsldered to have formed as a proximal fxles to an andesitic strat8volcano which covered the western and southern parts of the volcanic xmplex (Pride, in press) xntered in the vicinity of Main Cirque. The uppermost monolithic andesite brexia unit of Formation 3 (Figure 2.2) oxurring in the southeastern xrner of the xmplex south and west of Main Cirque is Interpreted to represent explosion and talus brexia and slump blxks formed on the inclined southeastern side of this volcano. Lithologies of Formation 4 are interpreted as part of a felsic volcanic event that occurred sometime after andesitic volcanism had ceased. Although this latest volcanic cycle appears to have been xnxntrat&l in the east (Figure 2.2), several isolated areas of Formation 4 are present in the west. Sinx erosion may have removed much of this felsic event in the western portion of the xmplex, It may have xtually bxn 154 Figure 6.1. Series of four schematic cartoons (a to d) illustrating the possible sequence of events forming the excellent ground preparation at Mt. Skukum, Yukon Territory, leading to formation of epithermal vein mineralization at Main Cirque. Each diagram represents a distance of about 15 km from west to east. ^monolithic andesite breccia Figure 6.1 a. Early deposition of volcanic rocks of formations 1,2, and 3, lead to the development of an andesitic stratavolcano in the southwestern portion of the volcanic complex centered in the vicinity of Main Cirque and capped by a mantle of monolithic andesite explosion and talus breccia as well as slump blocks. Inactive andesitic volcano Figure 6.1 b. A period of erosion following the end of andesitic volcanism was succeeded by a period of felsic volcanism centered mainly in the west which lead to deposition of formation 4. w E Figure 6.1 c. This final cycle of felsic volcanism lead to a depletion of the parent magma chamber at depth causing a regional subsidence of the volcanic complex and formation of a cauldera bounded by high-angle normal faults. This was accompanied by the development of a major north trending fault dividing the volcanic complex in two with maximum displacement of rxks on the eastern side. Figure 6.1 d. Late stage intrusion of high-level porphyritic rhyolite stxks and dykes in structurally preparx! sites caused an upward doming of volcanic stratigraphy at Main Cirque and provided a heat source to drive hydrothermal circulation that formed mineralizxl veins. 156 widespread. Main Cirque was central to some of this felsic volcanic activity which was preserved as an infilled volcanic crater on the south end of the eastern wall of Main Cirque (Figure 3.1). Structural and stratigraphic evidence in Main Cirque indicates a downward displacement of large blocks along steeply dipping, north-trending normal faults. This produced a graben structure centered on the cirque (Figure 3.2). As the major fault forming the eastern wall of the cirque truncated felsic volcanics in the southwest corner of the map area (Figure 3.1), the collapse event which caused formation of this structure was probably related to depletion of a magma chamber at depth towards the end of felsic volcanism. These faults provided an ideal locus for subsequent dyke intrusion and vein formation (Figure 6.1). Emplacement of high-level porphyritic rhyolite stocks and dykes throughout the Mt. Skukum Volcanic Complex occured after the last cycle of felsic volcanism. This is seen throughout the complex in cross-cutting relationships mapped by Pride (1985) and in the structural control displayed by correlative rhyolite dykes and the stock in Main Cirque (Figures 3.1). Emplacement of these rhyolite intrusions probably coincided with a late-stage doming of rocks in Main Cirque along a north-south axis represented by divergent dips observed in rocks of the eastern and western walls the cirque (Figure 3.1). This relation is compatible with dyke emplacement during a extensional reopening of normal faults by doming associated with a deeper rhyolitic intrusion under Main Cirque (Smith et al.. 1961). Intrusion of minor felsic to intermediate dykes, probably immediately following emplacement of rhyolite dykes, was the last event prior to formation of mineralized veins (Figure 6.1 d). 157 6.2. Deposit Model The model proposed for the Mt. Skukum epithermal gold-silver deposit is depicted in Figure 6.2. Characteristics of this deposit closely fit those described as low-sulphur, adularia-sericite type eoithermal deposits (Heald et al.. 1987. Havbaet al.. 1985; Bonham, 1986; Buchanan, 1981; Berger and Eimon, 1983). Emplacement of veins forming the Mt. Skukum deposit was probably caused by development of a hydrothermal circulation cell driven by the porphyritic rhyolite intrusion inferred below rocks of Main Cirque. Considering the broad extent of propylitic alteration (section 4.3.2) which affects porphyitic andesite flow rxks over an area of at least 66 square km, this hydrothermal x l l was probably regional in extent covering the entire western portion of the volcanic xmplex. Hydrothermal outflow may have bxn xntered on different areas at different times, one of which W8S Main Cirque where previous block-faulting, rextivated and dilated by resurgent doming, provided exxllent ground preparation. In addition, the presenx of a heat sourx xntered below the cirque, fxussed hydrothermal discharge in the area (Figure 6.1). Mineralization in Main Cirque W8S deposited in veins structurally xntrolled by major fault zones betwxn down-dropped blxks (Figure 3.2). The largest mineralized zone is the Main Cirque Zone which xcurs in a large fault in the keystone blxk of the Main Cirque graben. Veins in the southern segment of this zone xhieve thicknesses of up to 13 m and are paralleled by an equally thick rhyolite dyke which xntinues northward past the point at which the Main Cirque Fault Zone assumes a more easterly trend and cross-cuts this rhyolite dyke (Figure 3.1). The extreme thickness of vein material present in the southern segment of the Main Cirque Zone, as opposed to that in the northern segment, may be caused by two factors. Arching, which rextivated faults in Main Cirque creating dilatant cavities for vein formation, oxurred through a broad folding of strata about a north-trending axis which would tend to dilate north-trending faults by a greater amount than those trending north-east. In addition, if even slight rotational or dextral strike-slip displaxment was brought about on the northeasterly-trending segment of the Main Paleosurface hosts zones of silica sinter consisting of vuggy or laminated, opaline or chalcedonic quartz. Alteration cap zone contains pervasive kaolinite, pyrophyllite, alunite. chalcedony, and minor pyrite reflecting a low pH alteration cap. Anomalous Hg values are found in this zone. Upper portion of vein system consists of relatively thin chalcedony-fluorite veins and hydrothermal breccias containing minor calcite. adularia. barite. and alunite. Precious metal values are not found in these veins. Goat Zone (Figure 2.1) best illustrates this mineralization in Main Cirque area. Lower portion of vein system covers the extent of the boiling zone and consists of large, continuous veins localized within faults adjacent to rhyolite dykes. High precious metal values are common in veins consisting of quartz, calcite. and sericite, with minor rhodochrosite, fluorite. ankerite. and traces of pyrite. Precious metals occur as electrum and native silver. Main Cirque Zone and Brandy Zone best represent this level of the hydrothermal system Base metal - precious metal enriched zone extends from the bottom of the boiling zone to as deep as. 1000 m below paleosurface. Lake Zone ore represents the top of this zone. Figure 6.2. Spatial relationships among known mineralized zones in the Main Cirque area, rlt. Skukum, Yukon Territory. CD 159 Cirque Fault Zone (Figure 3.1) during doming, the southern segment would be opened as an enormous cavity due to the change in strike between northern and southern segments, allowing a greater thickness of vein material to be deposited. Vein textures (section 4.2) take the form of open-space fillings with abundant associated brecciation. This indicates deposition in a near-surface environment where lithostatic pressure conditions were low enough to allow open cavities produced by faulting to be maintained for prolonged periods of time. Hydrothermal breccias are common throughout the Main Cirque indicating that at times pressures were built up and released in explosive events leading to localized fracturing of rocks in the area and precipitation of mineralization. Precious metal distribution within the Main Cirque Zone (section 4.2.4) forms a mineralized horizon of maximum values extending over a vertical depth of 59 m to a base elevation of 1,676 m; this supports a pressure-related precipitation mechanism limited by depth to a boiling curve (Haas, 1971) dictated by the temperature and salinity of the hydrothermal fluid. Grade-thickness plots of the Main Cirque Zone (Figures 4.26,4.27, and 4.28) show that mineralization is deposited in steeply dipping ore shoots filling large conduits suggesting that fluid flow in this zone was directed upwards toward paleosurface. This further reinforces pressure decrease as a viable trigger for mineral precipitation. Fluid inclusion study (section 5.2.6) indicates that vein minerals formed from fluids under two pressure regimes at a common depth of about 470 m. Two populations of salinity and temperature are clearly demonstrated by inclusion measurements (Figure 5.9); one population characterized by homogenization temperatures of approximately 330°C and a salinity of 4.12 weight percent NaCl, and the other by homogenization temperatures of approximately 250"C and a salinity of 0.87 weight percent NaCl. A coincident depth of formation (within error margins) can be calculated for these different fluids assuming that high temperature, high salinity fluids were trapped in minerals formed under lithostatic pressure whereas fluids of lower temperature and salinity were trapped in minerals formed under hydrostatic pressure. Postulation that these populations of temperature and salinity formed at a common depth is consistent with sudden boiling events caused by tectonic release of neai—lithostatic pressure built up against a sealed alteration cap near the surface. A wide variation in liquid to vapour ratios in inclusions and abundant zones of hydrothermal brecciation also supports the occurrence of boiling in the deposit as do localized zones of argillic alteration surrounding veins and the occurence of an alteration cap zone. Fluids trapped below a sealed cap would maintain a relatively high temperature and salinity as represented by the high temperature, high salinity population of fluid inclusions at Mt. Skukum, When the cap was fractured, released pressure allowed boiling to occur causing rapid precipitation and mixing of hot hydrothermal fluids with cooler groundwaters during a rapid return to hydrostatic conditions as solutions circulated freely to the surface. Boiling alone causes a decrease in temperature and may account for the temperature difference between inclusion populations; however, the significant decrease in salinity observed suggests interplay between hydrothermal fluids and in_sJlu groundwater which may have both cooled and diluted hydrothermal solutions. The conclusion that inclusion populations represent common solutions under different pressure regimes is supported by stratigraphic evidence which indicates that the determined depth of 470 m is not unreasonable (section 5.2.5). Consequently, fluid inclusion evidence suggests that boiling was common during vein formation and may have been important in mineral precipitation. It also indicates that mixing of hot hydrothermal fluids with cooler in situ groundwater may have played a part in mineral precipitation as the solubility of gold decreases rapidly below 250°C and with decreasing salinities (Helgeson and Garrels, 1968). High homogenization temperatures averaging about 315°C support the presence of an igneous body at depth which supplied thermal energy to drive hydrothermal circulation. Stable isotope composition of minerals in the Mt. Skukum deposit indicate that they have undergone isotopic exchange at elevated temperature with large volumes of hydrothermal fluid (section 5.3). Although the presence of a rhyolitic magmatic body at depth is indicated as a heat engine driving hydrothermal circulation, oxygen isotope evidence clearly shows that circulating solutions were meteoric in origin with little or no contribution from magmatic sources (Figure 5.3.1). The magnitude of this hydrothermal system is indicated in calculations of the mass ratio of water to rock which shows that for every gram of altered rocks in the area, a minimum of 0.81 grams of water moved through the system in its lifetime (section 5.3.4). Considering the 8rea of altered rock covers approximately 66 square km to a depth exceeding the thickness of the volcanic complex (850 m), an enormous volume of water is represented. As an example, assuming that 50 percent of the rocks in this volume 8re affected, an unreasonably small quantity, the minimum amount of water indicated is 7.22 x 1 0 1 3 kg. A preponderance of evidence suggests that boiling of an upward moving low-salinity, hydrothermal fluid at Mt. Skukum was a common phenomenon during mineral deposition and may have been important in precious metal precipitation. Hydrothermal fluids interacting with oxidized surface and groundwaters caused pervasive clay alteration of rocks at the site of hydrothermal discharge which, combined with precipitation of quartz, effectively sealed-off hydrothermal discharge causing pressure below this sealed cap to approach lithostatic conditions. Periodic re-activation of faults in the area fractured these sealed caps and allowed sudden release of pressure as described by the throttling model of Barton and Toulmin (1961); this process likely lead to repetitive boiling events extending significantly deeper than could be expected under normal hydrostatic conditions (Buchanan, 1981; Henley, 1985). This would be accompanied by mineral deposition over a relatively broad range of elevations brought about by free interaction of in situ groundwater with hydrothermal solutions. During periods where this seal restricted flow to the surface, lithostatic pressures which built up are reflected in high temperature, high salinity inclusions containing CO2. This mechanism of precipitation provides an efficient means of gold deposition and aids in the understanding of complexes involved in transporting gold to this location. The complex Au( HS)2~ has been found to be a dominant complexing agent of gold in hydrothermal fluids and allows a relatively high gold solubility in near-neutral pH, low saline 162 solutions at temperatures lower than 350°C(Seward, 1973; Lewis, 1982; Henley, 1985). The dominant dissolution reaction involving this gold species is Au + 2H2S = Au( HS) 2" + H + + 0.5H2 (Henley, 1985) Consequently, increasing H2S increases the solubility of gold carried as sulphide and bisulphide thio complexes. Boiling events cause C O 2 and H2S to be rapidly partitioned into the vapour phase, and formation of only a few percent of vapour allows the loss of more than 90 percent of dissolved C O 2 with an accompanying increase in pH and loss of H2S (Henley, 1985). This leads to supersaturation of gold and silver sulphide and bisulphide thio complexes causing deposition of precious metals. The premise of sulphur complexes forming a ligand allowing transportation of precious metals is reasonable in that although the system is generally low in sulphur, significant amounts of sulphides are present in alteration envelopes and associated with some veins. In addition, the presence of abundant alunite in the Alunite Cap Zone Is indicative of significant amounts of sulphur in the system that was released through a boiling process then cooled and condensed near the surface to form a low pH cap over the hydrothermal system. Due to the small amounts of precious metal required to make a viable deposit, a large supply of sulphur is not necessary for economic amounts of gold to be deposited in a system leaving only trace amounts of sulphide minerals. This boiling model reflects mineral precipitation under two main pressure regimes and explains the observed mineralogic paragenesis in veins of early-formed skeletal bladed calcite later infilled and partially to completely replaced by fine-grained equant quartz crystals locally forming cockade overgrowths on these blades. During periods where hydrothermal circulation was sealed at the surface, precipititation of minerals from the relatively hot hydrothermal fluid under lithostatic pressure, resulted in widespread deposition of calclte which is relatively insoluble in high temperature solutions (Fournier, 1985a). Upon initiation of boiling following tectonic 163 rupture of the sealed cap, a sudden decrease in temperature of the hydrothermal solution caused supersaturation in silica and an attendant increase in solubility of calcite (Fournier, 1985b) thereby initiating rapid precipitation of fine-grained anhedral quartz in skeletal interstices between calcite blades as well as replacement of calcite by quartz. Evaluation of many epithermal deposits (Henley, 1985) has led to the conclusion that the availability of a recognized source of precious metal components found in the deposit is of little consequence considering the volume of solution which moves through a typical epithermal system such as Mt. Skukum (section 6.1) and the area of rock affected. Any rock type contains sufficient quantities of these metals in trace amounts to supply the total metallic content typically found in these deposits many times over. Consequently, the source of precious metals at Mt. Skukum was probably a combination of the Tertiary volcanic pile and underlying basement rocks. The availability of a suitable structure which focused the discharge of hydrothermal fluids is perhaps the most important factor in concentration of precious metals to form 8n epithermal deposit; this occurs at Mt. Skukum in the form of large block faults centered over the graben structure at Main Cirque. Following models of other precious metal districts (Buchanan, 1981; Berger andElmon, 1983; Hayba et al.. 1985) a zone of base metal enrichment may be expected at and below the base of the boiling zone with an accompanying decrease in precious metal values. Although this zone is not reached in the Main Cirque Ore Zone, elevated basemetal contents encountered in the Lake Zone may reflect the upper portion of this basemetal zone, and the lowest extent of the boiling zone. In addition, a mineral prospect presently being explored by Omni Resources Ltd. 5 km southeast of the Main Cirque Ore Zone (Figure 2.1) contains precious met8ls in epithermal veins containing large quantites of base metal sulphides and may reflect this epithermal zonation on 8 regional scale. These latter points are speculative and require further research. 164 6.3. Consequences of the Proposed Model Application of this model as an exploration tool in the Mt. Skukum area allows assessment of the level within the hydrothermal system that prospects occur. This is related to prediction of grade and vertical extent of gold and silver values in prospects at various levels in the epithermal framework of the region. Thus, the model might assist in determining target depths in drilling of prospects deemed to represent near-surface environments, or prospects where depth to paleosurface can be determined. Veins in the Main Cirque area all appear to have formed above the boiling level with lowest level mineralization represented by the Lake Zone with its relatively high base metal content. Consequently, there appears to be potential for a precious metal rich, base metal zone below the Main Cirque Ore Zone. In addition, major inferred north-trending structures such as the fault at the foot of the eastern wall of Main Cirque are potential zones for yet undetected mineralization. The Alunite Cap Zone, which represents a low pH alteration cap on the southeastern wall of Main Cirque, also may mark potential mineralization at depth. Zones of hydrothermal discharge in adularia-sericite deposits are broad and typically cover areas of between 12 square km (Oatman, Arizona) and 190 square km (Guanajuato, Mexico) (Havba. et. al.. 1985). Precious metals carried by these solutions would be hopelessly disseminated without the existence of a structural focus allowing concentrations of mineralization which can constitute an economic deposit. Identification of these structures is imperative to guiding future exploration efforts in the Mt. Skukum Volcanic Complex and the surrounding areas. Evidence of boiling associated with vein deposition may be important in selection of worthwhile exploration targets because boiling is considered to be related to precious metal precipitation in the Main Cirque Ore Zone. Vein textures indicating replacement of lamellar calcite by quartz is an important exploration guide to precious metal veins. 165 Bibliography Barton, P.B., Jr. , and P. Toulmin, 1961, Some mechanisms for cooling hydrothermal fluids: U.S. Geological Survey, Professional Paper, 424-D, p.348-352. Barton, P.B., Jr. , P.M. Bethke.andE. Roedder, 1977, Environment of ore deposition intheCreede mining district, San Juan Mountains, Colorado: Part 111. Progress toward interpretation of the chemistry of the ore forming fluid for the OH vein: Economic Geology, vol. 72, p. 1-24. Berger, B.R., P.I. Eimon, 1983, Conceptual models of epithermal precious-metals deposits; in Shanks, W.C., III (ed.), Cameron Volume on Unconventional Mineral Deposits Society of Mining Engineer, A.I.M.E., p. 191-205. Bloom, M.S., 1979, Calibration and collection of fluid inclusion data on thechaixmeca heating/freezing stage: Unpubl. paper, U.B.C., 20p. Bloom, M.S., 1983, Geochemistry of fluid inclusions and hydrothermal alteration in vein and fracture controlled mineralization, stxkwork molybdenum deposits: Unpubl. PhD. Thesis, U.B.C. Bodnar, R.J., T.J. Reynolds, andC.A. Kuehn, 1985, Fluid inclusion systematica in epithermal systems; la Berger, B.R, and P.M. Bethke, (eds.), Geology and Geochemisty of Epithermal Systems: Society of Ec. Geol. ,p. 73-97 Bonham, H.F., Jr . , 1986, Models for volcanic - hosted epithermal precious metal deposits; a review; |n Proceedingsof'Symposium 5: Volcanism, Hydrothermal systems and related mineralization: International Volcanological Congress, New Zealand, p. 13-17. Bostock, H.S., 1941, Mining industry of Yukon, 1939 and 1940; Geol. Surv., Canada, Mem. 218 Buchanan, L.J., 1981, Precious metal deposits associated with volcanic environments in the southwest; iaDickinson, W.R. and W.D. Payne, (eds.), Relation of Tectonics to Ore Deposits in the Southern Cordillera: Arizona Geological Society Digest, vol. 16,292 p. Burruss, R.C., 1981, Analysis of phase equilibria in C-O-H-S fluid inclusions; laHollister, L.S., M l . Crawford, (eds.), Short course in Fluid Inclusions: Applications to Petrology: Mineralogical Association of Canada, p. 39-74. Cairns, D.D., 1912, Wheaton District, Yukon Territory; Geological Survey of Canada, Mem. 31. Cairns, D.D., 1916, Wheaton District, Yukon Territory; Geological Survey of Canada, Supplement to Sum. Rept. for 1915, p. 36-49. Clayton, R.N., and A. Steiner, 1975, Oxygen isotope studies of the geothermal system at Wairakei, New Zealand: GeochimicaetCosmochimica Acta, vol. 39, p. 1179-1186. Craig, H., 1966, Isotopic composition and origin of the Red Sea and Salton Sea geothermal brines: Science, vol. 154, p. 1544-1548. Crawford, M l . , 1981, Phase Equilibria in Aqueous Fluid Inclusions; in Hollister, L.S., M l . Crawford, (eds.), Short course in Fluid Inclusions: Applications to Petrology: Mineralogical Association of Canada, p. 75-100. 166 Eimon, P.I., 1983, Exploration for epithermal gold and silver deposits: the epithermal model; in Eimon, P. I., (ed.), A Collection of Unpublished. Papers on Epithermal Silver - Gold Deposits Ellis, A.J., 1979, Explored geotherma systems; in Barnes, H.L., (ed.), Geochemistry of Hydrothermal Ore Deposits, Second Edition: John Wiley and Sons, New York, p. 632-683. Field, C.W., and R.H. Fifarek, 1985, Light stable-isotope systematica in the epithermal environment; ia Berger, B.R. and P.M. Bethke, (eds.), Geology and Geochemistry of Epithermal Systems: Society of Economic Geologists, p. 99-128. Fisher, R.V. andH.-U. Schmincke, 1984, Pyroclastic Rxks, Springer-Verlag, New York, 472p. Fournier, R.O., 1985a, Carbonate transport and deposition in the epithermal environment; in Berger, B.R. and P.M. Bethke, (eds.), Geology and Geochemistry of Epithermal Systems: Sxietyof ExnomicGxlogists.p. 63-72. Fournier, R.O., 1985b, The behaviour of silica in hydrothermal solutions; in Berger, B.R. and P.M. Bethke, (eds.), Geology and Geochemistry of Epithermal Systems: Sxietyof ExnomicGxlogists, p. 45-62 Giles, D.L..C.E. Nelson, 1983, Principle features of epithermal lode gold deposits of thecircum-pxif icrim: Circum-Pxific Energy Minerals Conferenx, Honolulu, Hawaii, August 22-28,1982. Godwin, C.I., 1975, Imbricate subduction zones and their relationship with Upper Cretacxus to Tertiary porphyry deposits in the Canadian Cordillera: Canadian Journal of Earth Sciences, vol. 12, no. 8, p. 1362-1378. Grunsky, E.C, 1986, Recognition of alteration and mineralization in volcanic terrains; in Proceedings of Symposium 5: Volcanism, Hydrothermal systems and related mineralization: International Volcanological Congress, New Zealand, p. 13-17. H33S, J.L., 1971, Theeffxt of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressures: Exnomic Gxlogy, vol 66, p. 940-946. Harland, W.B., A.Y. Cox, P.G. Llewellyn, C.A.G. Pickerton, A.G. Smith, and R. Walters, 1982, A Gxloqic Time Scale; Cambridge Earth Sciences Series, Cambridge University Press, 131p. Hayba, D.O., P.M. Bethke, P. Heald.andN.K. Foley, 1985,Gxlogic, mineralogicandgexhemical charxteristics of volcanic-hosted epithermal prxious-metal deposits; in Berger, B.R. and P.M. Bethke, (eds.), Geology and Geochemistry of Epithermal Systems: Reviews in Exnomic Gxlogy, vol. 2, S.E.G., p. 129-167. Heald, P., N.K. Foley, D.O. Hayba, 1987, Comparative anatomy of volcanic-hosted epithermal deposits: acid-sulphate and adularia-sericite types: Exnomic Gxlogy, vol. 82, no. 1, p. 1-26. 167 Hedenquist, J.W. and R.W. Henley, 1985, The importance of C O 2 on freezing point measurements of fluid inclusions: evidence from active geothermal systems and implications for epithermal ore deposition: Ec. Geol., vol. 80, no. 5, p. 1379-1406. Helgeson, H.C. and R.M. Garrels, 1968, Hydrothermal transport and deposition of gold: Economic Geology, vol. 63, p. 622-635. Henley, R.W., 1985, The geothermal framework for epithermal deposits; in Berger, B.R. and P.M. Bethke, (eds.), G&logi'and Gecrtm>istry of Epithermal Systms-tt Economic Geologists, p. 1-25. Hollister, L.S. etal, 1981, Practical aspects of microthermometry; in Hollister, L.S., M l . Crawford, (eds.), Snort course in Fluid Inclusions- Applications to Petrology Mineralogical Association of Canada, p. 278-295. Irvine, T.N. and W.R.A. Baragar, 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, vol. 8, p. 523-548. Kamilli, R.J., and H. Ohmoto, 1977, Paragenesis, zoning, fluid inclusion, and isotopic studies of the Finlandia Vein, Colqui district, central Peru: Economic Geology, vol. 72, p. 950-982. Lambert, M.B., 1974, The bennett lake cauldron subsidence complex, British Columbia and Yukon Territory: GSC Bull., 227p. LeBas, M J . , 1986, Chemical classification of volcanic rocks: Journal of Petrology, vol. 27, p. 745-750. Lewis, A., 1982, Gold geochemistry: new ideas about the paragenesis of hydrothermal deposits have implications for finding undiscovered gold ore: Earth and Mining Journal, December, 1982, p. 56-60. Lindgren, W., 1933, Mineral Deposits 4th ed., McGraw-Hill Book Co., 929p. McDonald, B.W.R., and C.I. Godwin, 1986, Geology of Main Zone at Mt. Skukum, Wheaton River area, southern Yukon; in Yukon Geology, vol. I: Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p.6-10. McDonald, B.W.R., E.B. Stewart, and C.I. Godwin, 1986, Exploration Geology of the Mt. Skukum epithermal gold deposit, southwestern Yukon; in. Yukon Geology, vol. 1: Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 11 -18. Meyer, C and J.J. Hemley, 1967, Wall rock alteration; in Barnes, H.L., (ed.), Geochemistry of Hydrothermal Ore Deposits, Isted: Holt, Rinehart and Winston, p. 166-235. Monger, J.W.H., and R.A. Price, 1979, Geodynamic evolution of the Canadian Cordillera ~ progress and problems: Canadian Journal of Earth Sciences, vol. 16, p. 770-791. Morrison, G.W., C.I. Godwin, and R l . Armstrong, 1979, Interpretation of isotopic ages and 8 7 S r / 8 6 S r initial ratios for plutonic rocks in the Whitehorse map area, Yukon: Canadian Journal of Earth Sciences, vol. 16, p. 1988-1997. 168 Nash, J.T., 1972, Fluid inclusion studies of some gold deposits in Nevada: U.S. Geol. Surv., Professional Paper, 800-C, p. C15-C19. O'Neil, J.R. and H.P. Taylor, 1967, The oxygen isotope and cation exchange chemistry of feldspars: American Mineralogist, vol. 52, p. 1414-1437. O'Neil, J.R. and M i . Silberman, 1974, Stable isotope relations in epithermal Au-Ag deposits: Ec. Geol., vol. 69, p. 902-909. O'Neil, J.R. et al., 1973, Stable isotope and chemical relations during mineralization in theBodie Mining District, Mono County, California: Ec. Geol., vol. 68, p.765-784. Ohmoto, H. and R.O. Rye, 1979, Isotopes of sulfur and carbon; in H.L. Barnes, (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nded.: Wiley, p. 509-567. Potter ,11, R.W., 1977, Pressure corrections for fluid inclusion homogenization temperatures based on the volumetric properties of the system NaCl-H20: U.S. Geol. Surv. J . Res., vol. 5, p. 603-607. Potter, l i , R.W., M.A. Clynne, andP.L. Brown, 1978, Freezing point depression of aqueous sodium chloride solutions: Economic Geology, vol. 73, p. 284-285. Pride, M.J., 1986, Description of the Mount Skukum Volcanic Complex, Yukon Territory; in Yukon Geology, vol. I: Exploration and Geological Services Division, Yukon, Indian and Northern Affairs, Canada. Pride,M.J., 1985, Preliminary geological map of the Mount Skukum Volcanic Complex, 105D -2 , 3 , 4 , 5 ; Exploration and Geological Services Division, Yukon, Indian and Northern Affairs, Canada, Open File, 1:25,000 scale map. Pride, M.J. and G.S. Clarke, 1985, An Eocene Rb-Sr isochron for rhyolite plugs, Skukum area, Yukon Territory: Can J . Earth Sci., vol. 22, p. 1747-1753. Roedder,E., 1979 .Fluid inclusions as samples of ore fluids; in H.L, Barnes, (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nded.: Wiley, p. 684-737. Roedder, E., 1984, Fluid inclusions; Reviews in Mineralogy, vol. 12: Mineralogical Society of America, 644p. Roedder, E. and R.J. Bodnar, 1980, Geologic pressure determinations from fluid inclusion studies: Ann Rev. Earth and Planet. Sciences, vol. 8, p. 263-301. Rose, A.W. and D.M. Burt, 1979, Hydrothermal alteration; la Barnes, H.L., (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nded.: Wiley, p. 173-235. Seward, T.M., 1973, Thio complexes of gold and the transport of gold in hydrothermal ore solutions: Economic Geology, vol. 37, p. 379-399. Smith, M.J., 1982, Petrology and geology of high level rhyolite intrusives of the Skukum Area, 105DSW, Yukon Territory; in Yukon Exploration and Geology, 1981: Department of Indian and Northern Affairs, Whitehorse, p. 62-73. 169 Smith, M.J., 1983, The Skukum Volcanic Complex, 105DSW: geology and comparison to the Bennett Lake Cauldron Complex; in Yukon Exploration and Geology, 1982: Department of Indian and Northern Affairs, Whltehorse, p. 68-72. Smith, R.L., R.A. Bailey, and C.S. Ross, 1961, Structural evolution of the Valleys Cauldera, New Mexico, and its bearing on the emplacement of ring dykes: U.S.G.S., Prof. Paper 424-D, p. D145-D149. Smith-Pride, M.J,, 1985, Interlayered sedimentary - volcanic sequence of the Mt. Skukum Volcanic Complex; in Yukon Exploration end Geology: Departmental Mian md Northern Affairs, Whltehorse, p. 94-104. Souther, J.G., 1966, North-central belt of the Cordillera of British Columbia; in Tectonic History and Mineral Deposits of the western Cordiiiera: Can. Inst. Mining Met. Spec. Vol., no. 8, p. 171-184. Souther, J.G., 1967, Acid volcanism and its relationship to the Cordillera of British Columbia, Canada: Bull. Volcanol., vol. 30, p. 171-176. Souther, J.G., 1970, Volcanism and its relationship to recent crustal movements in the Canadian Cordillera: Can. J . Earth Sci., vol. 7, p. 553-568. Spooner, E.T.C., 1981, Fluid inclusion studies of hydrothermal ore deposits; in Hollister, L.S., M.L. Crawford, (eds.), Short course in Fluid Inclusions: Applications to Petrology: Mineralogical Association of Canada, p. 209-240. Taylor, H.P., 1973,0 , 8 /o '6 Evidence for meteoric - hydrothermal alteration and ore deposition in the Tonopah, Comstock Lode, and Goldfield Mining Districts, Nevada: Ec. Geol., vol. 68, no. 6, p. 747-764. Taylor, H.P., 1974, The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition: Ec. Geol. vol. 69, p. 843-883. Taylor, H.P., 1979, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits; in Barnes, H.L., (ed.), Geochemistry of Hydrothermal Ore Deposits: John Wiley & Sons. Taylor, H.P., Jr . , and S. Epstein, 1970, Oxygen and silicon isotope ratios of lunar rock 12013: Earth and Planetary Science Letters, vol. 9, p. 208-210. Templeman-Kluit, D., 1981, Geology and mineral deposits of southern Yukon; in Yukon Geology and Exploration, 1979: Indian and Northern Affairs, Canada, p. 7-31. Turner, S., 1986, Fluid inclusion, alteration and ore mineral studies of an epithermal vein system: Mount Kasi, Vanua Levu, Fiji; in Proc. of Symposium 5: Volcanism, Hydrothermal Systems and Related Mineralization: I nter national Volcanological Congress, New Zealand, p. 87-94. Wheeler, J.O., 1961, Whitehorse map-area, Yukon Territory, 105D: Geol. Surv. Can., Mem. 312,156p. White, D.E., 1981, Active geothermal systems and hydrothermal ore deposits; in Skinner, B.J., (ed.), Economic Geology, Seventy-Fifth Anniversary Volume: Ec. Geol. Publishing Co., p. 392-423 170 Winchester, J A and P.A. Floyd, 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, vol. 20, Elsevier Sci. Publ. Co., p. 325-343. 171 Appendix A. Geochemical Data for Igneous Rocks at Mount Skukum A.1. Introduction Major element compositions of igneous rocks found in Main Cirque at Mt. Skukum (Tables 3.2 and 3.3) were obtained from a suite of whole rock analyses on 16 samples. Samples include 10 specimens of porphyritic andesite, and 6 of rhyolite reflecting the relative abundances of these lithologies in Main Cirque. Samples taken from drill core are located in Appendix B. A.2. Djta Data comprising a classical whole rock major element analysis and including zirconium are compiled in Tables 3.2 and 3.3 for the purpose of allowing a broad geochemical classification of the two dominant igneous rock compositions found in Main Cirque. Samples were collected by the author during the 1984 and 1985 field seasons. All samples were analysed at X-ray Assay Laboratories Limited, Don Mills, Ontario using x-ray fluorescence techniques with results for major elements calculated as oxides in weight percent, and zirconium reported in parts per million. Samples supplied to X-ray Assay Laboratories Ltd. by the author were taken from hand specimens averaging approximately 1 kg which were pulverized at The University of British Columbia and split to an average size of about 250 grams prior to dispatch. These were then roasted at 950'C by X-ray Assay Laboratories prior to analysis and L.O.I, was determined. Major elements and zirconium analyses were carried out on a 1.7 gram split of the original sample. This was fused with 5 grams of lithium tetraborate to form a 40 mm diameter disk. Fused disks were analysed on a Philips PW1600 simultaneous, multichannel spectrometer with calibration fixed using "preferred" values determined by Abbey (1979) on a collection of 40 international rock standards. A.3. Conclusions Absoluts accuracy and precision for geochemical data cannot be assessed as independent standards and duplicates were not included in assay batches, however, values for accuracy and precision routinely obtained in these analyses by X-ray Assay Laboratories Limited are quoted as ±2 percent and ±4 percent respectively for all major elements (L. MacFarlane, pers. comm., 1987). These quoted values for accuracy and precision in routine analyses are confirmed as maximum values by an on-going survey of this lab conducted over the past 5 years by the Production Research Division of Exxon Corporation (A.E. Bence, verbal communication, 1987). Appendix B. Location of Drill Core Samples Used In Study of the hit. Skukum deposit, Yukon Territory. Sample Drill Hole Sample Northing Easting Collar Dip Azimuth Number Number Interval (m) (UTM) (UTM) Elevation (Degrees) From To (m) C005 82-•10 80.45 80.47 4771.99 4267.24 1729.63 -45 104 C022 83-•35 28.83 28.87 4743.04 4377.11 1731.24 -60 284 C040 83-•35 107.53 107.57 4743.04 4377.11 1731.24 -60 284 C041 83-•35 113.10 113.14 4743.04 4377.11 1731.24 -60 284 C042 83-•35 116.39 116.43 4743.04 4377:11 1731.24 -60 284 C049 83-•37 39.19 39.23 4751.00 4347.78 1733.08 -45 284 C072 82- 25 23.53 23.57 4934.94 4439.19 1720.38 -45 104 C078 82- 25 107.73 107.77 4934.94 4439.19 1720.38 -45 104 C095 83-•68 83.00 83.06 4914.22 4514.91 1730.50 -45 284 C104 83-•51 57.09 57.13 4570.18 4252.33 1734.55 -50 284 C172 83-•63 52.07 52.11 4582.91 4297.92 1736.21 -61 284 C177 83-•63 94.45 94.49 4582.91 4297.92 1736.21 -61 284 C278 84-•83 33.04 33.25 4909.90 3721.67 1880.20 -70 104 C279 84-•83 74.08 74.45 4909.90 3721.67 1880.20 -70 104 C280 84-•83 96.33 96.56 4909.90 3721.67 1880.20 -70 104 C281 82-•17 51.81 52.01 4898.10 3768.43 1890.43 -55 104 ASTN- 10A 83-•51 79.02 79.24 4570.18 4252.33 1734.55 -50 284 ASTN- 13 83-•63 57.61 57.91 4582.91 4297.92 1736.21 -61 284 ASTN- 13A 83-•63 58.08 58.42 4582.91 4297.92 1736.21 -61 284 ASTN- 14 83-•63 59.19 59.58 4582.91 4297.92 1736.21 -61 284 Appendix C. Calibration Data for the Chaixmeca Fluid Inclusion Stage. C.I. Introduction. Nine standards were used to calibrate the fluid inclusion stage, four over the temperature range -100° to +40°C and five over the temperature range +40° to +420°C. Only minor deviations between measured temperature and actual temperature were noted and estimates of the accuracy and precision of measurements using this equipment (section 5.2.2) are similar to those encountered by Bloom (1983) using the same apparatus. C. 2. Calibration of Freezing Stage: The initial melting temperature (T(m)) of each standard was measured twice to obtain data in Table C. 1 for the calibration curve in Figure C. 1. Table C. 1. Freezing Calibration Curve Data (-100" to +40"C) for the Chaixmeca stage. Calibration Standard T(m) Measurements Ave.T(m) Expected T(m)1 Acetone -91.6 -91.4 -96.35 -92.2 Chloroform -61.0 -60.6 -63.40 -60.3 Carbon Tetrachloride -25.5 -25.6 -22.90 -25.8 Water 0.0 0.0 0.01 0.0 1. From Roedder (1984) 40 -100 -100 -80 -60 -40 -20 0 20 40 Recorded T(m) Figured . Freezing calibration curve based on data in Table C.1; for the Chaixmeca stage. The above calibration curve and data demonstrate an accuracy, defined as twice the standard deviation of the difference between recorded T( m) and actual T( m), of ± 6.7°C for data in this temperature interval. Precision, defined as the measure of random error, was calculated using the following formula for each pair of measurements: Using the above formula a precision (1c) of within 0.6'C is expected for data in this temperature range. This indicates a 95£ confidence that recorded T(m) is within 6.7°C of the actual T(m). C.3. Calibration of Nesting Stage-The initial melting temperature (T(m)) of each standard was measured twice, and in one case three times, to obtain data in Table C.2 for the calibration curve in Figure C.2. Calibration curve and data demonstrate an accuracy, defined as twice the standard deviation of the difference between recorded T(m) and actual T(m), of ± 5.3'C for data in this temperature interval. Precision Precision, defined as the measure of random error, was calculated using the following formula for each pair of measurements: Precision = .flMMs^F V n-1 Using the above formula a precision (Iff) of within 2.2°C is expected for data in this temperature range. This indicates a 95% confidence that recorded T(m) is within 5.3"C of the actual T(m). Table C.2. Heating calibration curve data (+40° to +420°C) for the Chaixmeca stage. Calibration Standard T(m) Measurements Ave.T(m) Expected T(m)* Naphaline 80.1 80.4 80.25 80.55 Merck 9735 135.5 135.7 135.6 135.0 Merck 9800 202.3 203.1 202.7 200.0 Sodium Nitrate 299.9 304.1 302.7 302.2 306.8 Sodium Acetate 323.4 324.3 323.8 324.0 *From Roedder (1984) 

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