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Geological setting of the volcanic-hosted Silbak Premier Mine, northwestern British Columbia, (104 A/4,… Brown, Derek Anthony 1987

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GEOLOGICAL SETTING OF THE VOLCANIC-HOSTED SILBAK PREMIER MINE, NORTHWESTERN BRITISH COLUMBIA (104 A/4 , B/I) by DEREK ANTHONY BROWN B. Sc., Carleton University, Ottawa, Ontario, 1981 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 this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA OCTOBER 1987 © Derek Anthony Brown, 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 of The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6(3/81) Frontispiece View southeast to Silbak Premier mine area and Bear River Ridge. iii ABSTRACT Detailed mapping of a 7.5 km2 area at 1 -.2,500 and a 1:10,000 compilation map over 60 km2 have established Hazelton Group stratigraphy and structure. Hazelton Group stratigraphy begins with at least +24 1,000 metres of Late Triassic-Early Jurassic (210 _ 1 4 Ma; U-Pb zircon) green andesite flows, breccias and tuffs. Less than 1750 metres of green and maroon andesitic to dacitic volcaniclastic rocks overlie the andesite unit. North of Silbak Premier, at Slate Mountain, the volcaniclastic unit is overlain by up to 200 metres of a black tuff unit containing characteristic fresh biotite and white plagioclase fragments. The top of the Hazelton is a regional marker horizon, the Monitor rhyolite breccia and tuff (197 * 14 Ma; zircon U-Pb). Hazelton volcanics are overlain by three different units. At Slate Mountain the Bowser Lake Group Bathonian/Callovian argil lite and siltstone (at least 1500 m thick) lie above Hazelton rocks. Farther north on Mount Dilworth, Monitor rhyolite is succeded by black tuff or a Toarcian buff carbonate. East of Monitor Lake, less than 75 metres of Bajocian Spatsizi Group silicic shale and tuff overlies Hazelton volcanic rocks. Three intrusive episodes are discerned through isotopic dating: Early Jurassic (190 ± 2 Ma; U-Pb zircon) Texas Creek plutonic suite dacitic porphyries; Eocene Hyder suite leucocratic dykes; and 01 igocene-Miocene (25.2 ± 2.3 Ma; K-Ar biotite and 18 ± 6 Ma; Rb-Sr) biotite lamprophyre dykes. The Jurassic suite includes K-feldspar megacrystic "Premier porphyry" sills and dykes that are in part spatially and possibly genetically associated with mineralization. Structural features include disharmonic tight folds, ductile shear zones, and brittle faults. At least 4 phases of pre-Eocene deformation are defined by: (1) moderate west-plunging recumbent folds, (2) north-plunging inclined folds, (3) north-plunging upright folds, and (4) moderate west-plunging pencil lineations. iv The map area is divisible into three structural domains: the North, East and Silbak domains. The North  domain is characterized by a marked structural discordance between warped Hazelton volcanic rocks and disharmonically folded Bowser Lake Group argillite and siltstone. Three phases of folding are: first phase tight to isoclinal disharmonic, recumbent folds; second phase open folds with shallow northwest-dipping axial planar cleavage; and a third phase upright, shallow north-plunging synclinorium. Structural continuity is difficult to establish due to lack of marker horizons 8nd inferred detachments. The East  domain is characterized by phase 3 gently north-northwest-plunging folds and locally east-verging asymmetric chevron folds in the Spatsizi Group. In contrast to North domain, Monitor rhyolite and/or Spatsizi Group are structurally conformable with Bowser Lake Group rxks. The Silbak domain is characterized by phase 4 pencil lineations and quartz veins. Stope geometry illustrates that mineralization occurs along two trends (1) northeast zone and (2) northwest zone of unknown phase. Steeply dipping, east-striking ductile fabrics occur in the Texas Creek batholith at the Riverside mine, Alaska and in maroon volcaniclastics along Bear River Ridge. Mylonitic fabrics at Riverside mine suggest a dextral sense of shear. A biotite lineation in the mylonitic foliation yields a totally reset Eocene K-Ar date. The width of Eocene Hyder dyke swarms indicates that there has been at least one kilometre of northeast brittle crustal extension. About 1400 metres of dextral transcurrent movement along the Long Lake-Fish Creek fault is post-Eocene dyke emplacement. 01 igocene-Miocene lamprophyre dykes fill fractures produced during east-west extension. Regional syntectonic greenschist grade metamorphism produced a carbonate-chlorite-sericite-pyrite mineral assemblage, probably in Middle Cretaceous time, bracketed by isotopic dating results. Hazelton Group volcanic rocks and coeval Texas Creek porphyritic rocks are subalkaline high-K to very high-K andesites and dacites. Tectonic discrimination diagrams indicate a calcalkaline, volcanic arc setting, with similar geochemical patterns to those for Andean volcanic rocks. V Mineralization is hosted in Hazelton Group andesites and coeval Texas Creek porphyritic dacite sills and dykes. Mineralization and porphyry emplacement appear to have been controlled by northeast- and northwest-striking structures. Ore is predominantly discordant but locally concordant with moderately northwest-dipping andesite flows and breccias. No mineralization occurs in or above overlying maroon volcaniclastic rocks. Sericite alteration gives a Paleocene K-Ar date (63 ± 5 Ma); this is interpreted to be partially reset. The spatial link with Texas Creek K-feldspar porphyry and discordant nature of the ore suggests mineralization is Early Jurassic age and supports an epigenetic model. vi TABLE OF CONTENTS FRONTISPIECE ii ABSTRACT iii TABLE OF CONTENTS vi LIST OF TABLES ix LIST OF FIGURES x LIST OF PLATES (in pocket) xiv CHAPTER I INTRODUCTION 1 1.1 INTRODUCTION AND OBJECTIVES 1 1.2 LOCATION, ACCESS AND TOPOGRAPHY 2 1.3 CLIMATE AND YEGETATION 3 1.4 PREVIOUS GEOLOGICAL WORK 3 1.4.1 British Columbia 3 1.4.2 Alaska 8 1.5 SILBAK PREMIER HISTORY AND PRODUCTION 8 1.6 FIELDWORK AND METHODS 14 1.7 ACKNOWLEDGEMENTS 14 CHAPTER 2 6E0L06Y AND TECTONIC SETT I NO OF NORTH-CENTRAL BRITISH COLUMBIA 16 2.1 INTRODUCTION 16 2.2 STIKINIA STRATIGRAPHY 16 2.2.1 Introduction 16 2.2.2 Paleozoic rocks 20 2.2.3 Stuhini (or Takla) Group 20 2.2.4 Early Jurassic paleogeography 20 2.2.5 StikineArch 21 2.2.6 Hazelton Group 21 2.2.7 Toodoggone (Cold Fish) volcanics 22 2.2.8 Spatsizi Group 22 2.2.9 SkeenaArch 22 2.2.10 Bowser B8sin 23 2.2.11 Bowser Lake Group 23 2.2.12 Skeena Group 23 2.2.13 Kasalka Group 24 2.2.14 Sustut Basin and Group 24 2.3 INTRUSIVE ROCKS 29 2.3.1 Late Triassic (215-230 Ma) 29 2.3.2 Late Triassic to Early Jurassic (210-187 Ma) :.. 29 2.3.3: Middle Jurassic (180-170 Ma) 30 2.3.4 Late Early to Middle Cretaceous (110-90 Ma) 30 2.3.5 Late Cretaceous (85-64 Ma) 31 2.3.6 Early Tertiary (55-45 Ma) 31 2.3.7 Oligocene to Recent (31 Ma-Recent) 32 2.4 STRUCTURAL HISTORY AND STYLE 32 2.4.1 Pre-Late Triassic 34 2.4.2 Late Triassic 34 2.4.3 Middle Jurassic 34 vii 2.4.4 Pre-Early Cretaceous (pre-Albian) 34 2.4.5 Cretaceous to Early Cenozoic 35 2.4.6 Eocene-Oligocene and Miocene to Recent 36 CHAPTER 3 SALMON RIVER VALLEY AND BEAR RIVER RID6E 6E0L06Y -STEWART AREA 38 3.1 INTRODUCTION 38 3.2 STRATIGRAPHY 38 3.2.1 HAZELTON GROUP 38 3.2.1 .A Argillite, siltstone and minor greywacke (unit Hs) 38 3.2.1 .B Andesite, tuff and breccia (unit Hv) 39 3.2.1 .C Green and maroon andesitic volcaniclastic rocks (units H, Hg and Hm)39 3.2. l.D Black tuff and greywacke (unit Hw) 45 3.2.1 .E Monitor/Dilworth rhyolite tuff and breccia (unit Hr) 45 3.2.1 .F Hazelton Group paleoenvironment 46 3.2.2 SPATSIZI GROUP (unit S) 47 3.2.3 BOWSER LAKE GROUP 48 3.2.3.A Argillite, shale and siltstone (unit B) 48 3.3 INTRUSIVE ROCKS 51 3.3.1 Texas Creek plutonic suite (unit Jp) 51 3.3.2 Hyder plutonic suite (unit Th) 52 3.4 METAMORPHISM 54 3.5 STRUCTURAL STYLE 58 3.5.1 Introduction 58 3.5.2 Phases of deformation 59 3.5.3 Structural domain summaries 59 3.5.4 North domain 62 3.5.5 East domain 67 3.5.6 Silbak domain 74 3.5.7 Phase ? Steeply dipping easterly-striking mylonite and flattening fabric 89 3.5.8 Pre-Eocene brittle faults 90 3.5.9 Eocene to Present dyke emplacement and brittle faulting 94 3.5.10 Conclusion, regional synthesis, and deformational history 99 CHAPTER 4. SILBAK PREMIER GEOLOGY 100 4.1 INTRODUCTION 100 4.2 STRATIGRAPHY 100 4.3 INTRUSIVE ROCKS 113 4.3.1 Texas Creek plutonic suite (units 4, 4A and 4B) 113 4.3.2 Granitic intrusion (unknown age or affiliation 118 4.3.3 Hyder plutonic suite 118 4.3.4 Biotite lamprophyre dykes 121 4.4 MINERALIZATION 121 4.4.1 Introduction 121 4.4.2 Styles of mineralization 121 4.4.3 Mineralization zones 128 4.4.4 Northern Light orebody surface expression - Tank trenches 134 4.4.5 Mount Welker - Mineral Basin area, Alaska 134 4.4.6 Additional mines in the Salmon River Valley 134 4.4.7 Alteration 135 4.4.8 Generalizations and genetic implications 138 4.4.9 Genetic model and comparisons with other deposits 139 viii CHAPTER 5 8E0CHEMISTRY 143 5.1 INTRODUCTION 143 5.2 ROCK SAMPLES 143 5.3 ALTERATION AND LIMITATIONS 143 5.4 MAJOR ELEMENT CHEMISTRY 144 5.5 TRACE ELEMENT CHEMISTRY 155 5.7 TECTONIC DISCRIMINATION DIAGRAMS 156 5.8 CHONDRITE-NORMALIZATION DIAGRAMS 157 5.9 CONCLUSIONS 157 Chapter 6 6E0CHR0N0METRY 161 6.1. INTRODUCTION 161 6.2 U-Pb DATING 161 6.2.1 Introduction 161 6.2.2 U-Pb sample collection 161 6.2.3 Zircon morphology 162 6.3.4 U-Pb results and discussion 162 6.3 K-Ar DATING 175 6.3.1. Introduction 175 6.3.2 K-Ar sample collection 175 6.3.3 K-Ar results and discussion 175 6.4 Rb-Sr DATING 179 6.4.1 Introduction 179 6.4.2 Rb-Sr discussion 179 6.4.3 Rb-Sr results 179 6.4.4 Initial 8 7Sr/ 8 6Sr ratios 180 6.5 GEOCHRONOMETRY CONCLUSION AND SYNTHESIS 183 CHAPTER 7 SUMMARY AND CONCLUSIONS 184 REFERENCES CITED 190 APPENDIX 1.1 Silbak Premier production (1918-1968) 204 APPENDIX 1.2 Unpublished Silbak Premier reports (not cited in text) 205 APPENDIX 5.1 Geochemical techniques 207 APPENDIX 5.2 Rock geochemistry sample location and lithology 209 APPENDIX 5.3 Chemical composition of andesite samples 210 APPENDIX 5.4 Chemical composition of porphyritic dacite samples 211 APPENDIX 5.5 Chemical composition of tuff samples 212 APPENDIX 5.6 Chemical composition of miscellaneous samples 213 APPENDIX 6.1 U-Pb, K-Ar and Rb-Sr analytical techniques 214 i i LIST OF TABLES Table 1.1 Previous reports and maps pertaining to Silbak Premier mine 7 T8ble 3.1 Comparison of regional tables of formations for northwestern British Columbia 41 Table 3.2 Jurassic correlation chart for Salmon River Valley-Bear River Ridge area 43 Table 3.3 Structural elements for Be8r River Ridge and Salmon River Yalley 61 Table 4.1 Styles of mineralization at Silbak Premier mine 123 Table 5.1 Average andesite compositions from published data and this study 152 Table 5.2 Summary of chemical plots 158 Table 6.1 Summary of U-Pb zircon and K-Ar isotopic determinations 168 Table 6.2 Zircon characteristics 169 Table 6.3 Previous U-Pb isotopic age determinations 170 Table 6.4 U-Pb geochronometry results 171 Table 6.5 K-Ar isotopic age determinations 178 Table 6.6 Rb-Sr analytical data for Silbak Premier area 181 Table 7.1 Geologic history of Silb8k Premier area, northwestern British Columbia 189 X LIST OF FIGURES Figure 1.1 Location of thesis area in northwestern British Columbia 4 Figure 1.2 Aerial view of Stewart and Silbak Premier 5 Figure 1.3 Previous regional studies in north-central British Columbia and southeastern Alaska 9 Figure 1.4 Historical photographs of Silbak Premier 12 Figure 1.5 Silbak Premier ore, gold and silver production (1918 to 1968) 13 Figure 2.1 Study area location relative to (a) five geological belts of the Canadian Cordillera and (b) Insular and Intermontane superterranes 17 Figure 2.2 Geological events in north-central British Columbia 18 Figure 2.3 Terrane map, tectonic elements and rock distribution for north-central British Columbia 26 Figure 2.4 Paleocurrents during deposition of Bowser Lake Group sedimentary rocks 27 Figure 2.5 Paleocurrents in Sustut B8sin during deposition of (a) Tango Creek Formation and (b) Brothers Peak Formation 28 Figure 2.6 Plutonic rock suites in north-central British Columbia 33 Figure 2.7 Structural styles and major faults of north-central British Columbia 37 Figure 3.1 Idealized stratigraphic columns of (a) Mount Dilworth, (b) Slate Mountain, (c) Monitor Lake and (d) Silbak Premier 40 Figure 3.2 Chloritized fiamme in Monitor (Dilworth) rhyolite near Fetter Lake 49 Figure 3.3 Well bedded "pajama beds" northeast of Monitor Lake 49 Figure 3.4. a Spatsizi Group, "Pajama beds" with soft sediment deformation 50 Figure 3.4. b Radiolariaghosts in "Pajama beds", Spatsizi Group 50 Figure 3.5 Typical Hyder biotite-hornblende quartz monzodiorite with blocky fractures 53 Figure 3.6 Pressure-temperature diagram illustrating grades of metamorphism 56 Figure 3.7 Albite-actinolite-chlorite zone of greenschist grade matamorphism shown on ACF diagram 56 Figure 3.8 Rotated pyrite crystal with quartz-chlorite-sericite pressure shadow 57 Figure 3.9 Structural domains, structural styles and deformational phases of Long Lake, Bear River Ridge and Silbak Premier areas 60 Figure 3.10 North domain equal area stereonet plots of (a) bedding, (b) foliations, and (c) lineations 64 xi Figure 3.11 Folded argillite on Slate Mountain 65 Figure 3.12 F2 gentle, inclined fold in north domain, Slate Mountain 66 Figure 3.13 East domain equal area stereonet plots of (a) bedding, (b) foliations and (c) lineations 69 Figure 3.14 East-verging phase 3 asymmetric angular folds of "Pajama beds", Spatsizi Group 70 Figure 3.15 Close phase 3 fold of Bowser Lake Group argil lite and siltstone 71 Figure 3.16 Tight phase 3 folds of Bowser Lake Group argil lite and carbonate 72 Figure 3.17 Folded and faulted Hazelton-Bowser Lake Group contact 73 Figure 3.18 Silbak Premier domain equal area stereonet plot of foliations 76 Figure 3.19 Silbak Premier domain equal area stereonet plot of (a) bedding, (b) joints and (c) extensional quartz veins 77 Figure 3.20 Silbak Premier domain equal area stereonet plot of (a) fault planes, (b) lineations and (c) dykes 78 Figure 3.21 Prominent Phase 4 pencil lineation in Hazelton Group andesite 79 Figure 3.22. a Ph8se 4 pencil lineations in Monitor rhyolite 80 Figure 3.22. b Ph8se 4 elongate Monitor rhyolite fragments 80 Figure 3.23 Ph8se 4 pencil lineations in Bowser Lake Group siltstone 81 Figure 3.24 Technically strained volcaniclastic rock 84 Figure 3.25 Flattened felsic lapilli in strained tuff. 84 Figure 3.26 Brittly deformed plagioclase crystal 85 Figure 3.27 C-S fabric developed in volcaniclastic rock 86 Figure 3.28 Joints and slaty cleavage in Hazelton andesite 88 Figure 3.29. a Foliated to mylonitized Tex8S Creek granodiorite near Riverside mine, Alaska 91 Figure 3.29. b Photomicrograph of mylonitic Texas Creek granodiorite, Riverside Mine, Alaska 91 Figure 3.30. a Strained heterolithic volcaniclastic breccia 92 Figure 3.30. b Intensely flattened maroon volcaniclastic rock 92 Figure 3.31 Mylonitized dacitic hornblende-plagioclase porphyry 93 Figure 3.32 Airphoto linears in Salmon River Yalley-Bear River Ridge area 96 Figure 3.33 North-striking, steep east-dipping "110 fault" in Glory Hole 97 xii Figure 3.34 "Cl fault" in Glory Hole 98 Figure 4.1 Interbedded shale and greywacke turbidites 102 Figure 4.2 Heterolithic breccia 106 Figure 4.3 Quartz vein in fragment of andesite breccia 106 F igure 4.4 Wei 1 bedded maroon epiclastic rocks 109 Figure 4.5 Maroon breccia and green lithic lapilli tuff contact 110 Figure 4.6. a Irregular green-maroon contact in tuff 111 Figure 4.6. b Green-maroon colour change in Monitor rhyolite I l l Figure 4.7 Concentric hematitic fractures 112 Figure 4.8 Two-level K-feldspar porphyry 115 Figure 4.9 Embayed, unstrained quartz phenreryst in K-feldspar porphyry 115 Figure 4.10 Partial carbonate replacement of K-feldspar megacryst 116 Figure 4.11 Maroon porphyry with altered K-feldspar megacryst 119 Figure 4.12 Ladder vein in 2160 trench low sulphide stockwork mineralization 124 Figure 4.13 Two-level trench high sulphide breccia ore type 124 Figure 4.14 Northern Light orebody high sulphide breccia ore type 125 Figure 4.15. a High sulphide breccia ore 126 Figure 4.15. b Vugpy high sulphide breccia ore 126 Figure 4.16 Massive to layered pyrite high sulphide ore 127 Figure 4.17 Glory Hole col lapse 130 Figure 4.18 Idealized x-section through northeast 2one mineralization 131 Figure 4.19 Idealized representation of alteration zones 137 Figure 4.20 Potassic alteration in drill core samples 137 Figure 4.21 Schematic ore deposition model for Silbak Premier mineralization 142 Figure 5.1 Geochemical sample locations 145 Figure 5.2 Diagrams to differentiate altered and unaltered volcanic rocks 146 Figure 5.3 Irvine and Baragar's alkaline/subalkaline classification diagram 147 xiii Figure 5.4 AFM diagram 148 Figure 5.5 Si02 versus FeO*/MgO calcalkaline/tholeiitic classification diagram 148 Figure 5.6 S1O2 versus K2O classification diagram 150 Figure 5.7 Total alkalies-silica diagram 150 Figure 5.8 Variation diagrams of AI2O3, T1O2, Fe203 and MnO with respect to S1O2 153 Figure 5.9 Variation diagrams of MgO, CaO, N82O, K2O and P2O5 with respect to Si02 154 Figure 5.10 T1O2 - MnO - P2O5 diagram to separate tectonic environments 159 Figure 5.11 Chondrite-normeli2ed, trace element diagrams 160 Figure 6.1 Geochronometry sample localities near Silbak Premier 164 Figure 6.2 Stratigraphic location of geochronometry samples near Silbak Premier 165 Figure 6.3 SEM-EDS images of typical zircons 166 Figure 6.4 Eutaxitic andesite sample 167 Figure 6.5 Monitor rhyolite at Monitor Lake 167 Figure 6.6 Concordia diagram for Hazelton eutaxitic andesite and green dust tuff. 173 Figure 6.7 Concordia diagram for ctecitic hornblende-plagioclase porphyry, Texas Creek plutonic suite 174 Figure 6.8 Concordia diagram for Monitor rhyolite tuff. 174 Figure 6.9 Photomicrograph of biotite lamprophyre 177 Figure 6.10 Rb-Sr isochron diagram for Hazelton volcanic rocks, Texas Creek plutonic suite and Tertiary dykes 182 xiv LIST OF PLATES (IN POCKET) P-l Big Missouri- Silbak Premier regional geology compilation (1:10,000). P-2 Silbak Premier-Big Missouri vertical x-sections and longitudinal section (1:10,000). P-3 Silbak Premier surface geology 1985/86 (1:2,500). P-4 Silbak Premier geology (south half) (1:500). P-5 Map unit correlation chart. 1 CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION AND OBJECTIVES The Silbak Premier mine in northwestern British Columbia has produced 1.7 million ounces gold and 38 million ounces silver (worth almost a billion dollars at 1987 metal prices) and is under study by Westmin Resources Ltd. for redevelopment as 8n open pit mine. They have submitted feasibility studies for a 2,000 tonnes per day mill operation to the provincial government and joint venture partners. A $4 million pre-development program is proceeding that includes site preparation for a mill, road building, and initial work on hydroelectric power development Field relationships, geochronometry, geochemistry and mineralization in the mine area and its surroundings were investigated. This study provides the first complete surface map of the mine, a 1:2,500 scale geologic map of thel 0 km2 mine area and a 1:10,000 map of 60 km2 that links regional scale geology provided by British Columbia Ministry of Energy, Mines and Petroleum Resources (BCMEMPR) and Geological Survey of Canada (GSC) geologists with property-scale geology. Mapping and compilation of a 60 km2 area (Fig. 1.1) at 1:10,000 scale established the nature of the Hazelton-Bowser Lake Group contact east of Monitor Lake and determined the structural history on Slate Mountain. Geochronometry (U-Pb zircon, K-Ar, and Rb-Sr methods) of volcanic and intrusive rocks was used to date magmatic and metamorphic/alteration events. Investigation of Silbak Premier geology was hampered by uncertainty about the local stratigraphic sequence and structural setting; difficulty in distinguishing stratigraphic, Intrusive, fault, and alteration contacts; vague definition of porphyritic intrusive suites (especially the "Premier porphyry"); irregularly distributed hematitic alteration zones; and uncertainty as to depositional evironment of the volcaniclastic rocks. 2 1.2 LOCATION. ACCESS AND TOPOGRAPHY The thesis area includes the Silbak Premier mine, 22 km north of Stewart, British Columbia in the southeastern corner of the Iskut River 1:250,000 scale map sheet {56*03' latitude (436800 E) and 130*02' longitude (6212500 N); Fig. 1.1}. Regional mapping concentrated on the Slate Mountain and Monitor Lake area 5.5 km north of the mine. The Oranduc road, an all weather gravel road, provides vehicle access from Stewart Seven kilometres of mine roads permit vehicle access to 675 m elevation. Old horse trails, abandoned prospectors trails, creek beds and cut grids provide additional foot access. Vancouver Island Helicopters service provided access to alpine areas for seven days of the regional work. Silbak Premier is situated along the eastern edge of the Coast Mountains, typified by high relief with precipitous glaciated slopes (Fig. 1.2). Bear River and Mount Dilworth-Big Missouri Ridges (Plate 1) are two prominent north-trending ridges. Below these ridges are three major, north-trending, U-shaped valleys: Bear River, which flows south into the Portland Canal at Stewart; Long Lake; and Salmon River, containing the retreating Salmon Glacier and the self-dumping, glacier-dammed Summit Lake (Clarke, 1986). At Silbak Premier local relief is 1700 m (Salmon River, elevation 125 m to Bear River ridge, elevation 1,825 m). Steep-walled creek canyons dissect the property. The west-facing mountainside on which the property is situated has a moderate slope( <30*) below 1,000 m but above this it steepens (>45*) to culminate in Bear River Ridge (Fig. 1.2). Mount Dilworth is named after two pioneer prospectors, the Dilworth brothers. Subsequent spelling of their name varies with author: McConnell (1913) spelt it "Dillsworth", Schofield and Hanson (1922) "Dilsworth", Grove (1971) "Dillworth", and Surveys and Mapping Branch of the Energy, Mines and Resources (EMR) topographic maps "Dilworth". Dilworth is adopted here to conform with more widely published EMR topographic maps (1:250,000 Iskut River 104 B and 104 C; and 1:50,000 Leduc Glacier 104B/1 and 104B/2). 3 1.3 CLIMATE AND VEGETATION Climate is mild and extremely wet (maritime). Palpitation is abundant throughout the year. Mean annual precipitation at Stewart and Silbak Premier is 175.8 cm and 220.0 cm. Mean annual snowfall varies greatly according to elevation: Stewart and Hyder at sea level receive 520 cm and 430 cm, Silbak Premier at 300 m elevation 1,100 cm, and Tide Lake Flats at 915 m elevation, along the northeast end of the Salmon Glacier, 2,250 cm. Extreme twenty-four hour rainfall and snowfall at Silbak Premier are 9.3 and 110 cm. January is the coldest month and July and August are the warmest months. Vegetation varies with elevation. Below 800 m, coastal rain forests comprise mature western hemlock with some mountain hemlock and blue spruce floored amid thick fern and moss ground cover that obscures most outcrops. Tree line ranges from 1,050 to 1,300 m elevation. Subalpine spruce thickets, heather and alpine meadows occur from 800 to 1,300 m elevation, above which are lichen- and moss-covered talus slopes and outcrops. Avalanche paths and old clearings are overgrown by a impassable cover of slide alder, mountain ash, huckleberry, stinging nettle, and devil's club. 1.4 PREVIOUS GEOLOGICAL WORK Previous regional geology studies in north-central British Columbia and Southeastern Alaska are listed in the caption of Figure 1.3. 1.4.1 BRITISH COLUMBIA Initial geological mapping in the area concentrated on mineral prospects end mining operations as described in British Columbia Department of Mines Annual Reports (1904,1906,1909 ,1911, 1912,1915,1917,19l8and 1919). In 1910 and 1911, McConnell conducted the first geological mapping of the Silbak Premier (then "Premier") and Indian mines during their initial development. Descriptions of the main geological features and the first geological map of the Portland Canal region resulted. I30°00' Figure t. 1 Location of thesis area in northwestern British Columbia The vertical and inclined rectangles correspond to 1:10,000 and 1:2,500 scale maps in pocket (Plate 1 and 3). Figure 1.2 (top) Yiew to north along Bear River ridge, Stewart and Hyder in foreground, at the head of the Portland Canal. The U.S.-Canada border is cut north from Hyder townsite, Salmon and Bear River valleys on west and east, and Salmon glacier in northwest background. (Photo by John Hembling). (bottom) View to east of Silbak Premier: 6-level dump and mill beside Oranduc road, large clearing is 4-level millsite, 8nd the highest surface expression is the Glory Hole collapse. Bear River ridge in background with Lesley Creek to the north. (Photo by D.Alldrick). 6 Schofield and Hanson (1922) continued reconnaissance mapping of the Salmon River area begun by O'Neill (1919). They included volcanic rocks with Bear River Formation, conglomerates with Salmon River Formation, and sediments on Slate Mountain with Nass Formation and provided a regional correlation of earlier work (Table 3.1) including a summary of Silbak Premier mine geology. They considered the quartz porphyry ("Premier sills") to be sills intruded parallel to bedding and to be related to Coast Mountain plutonism. Their structural interpretation was that Slate Mountain consisted of a shallow northwest-plunging syncline and that the Silbak Premier area was a westerly-dipping monocline. Hanson's (1929 andl 935) geological summaries of Bear River and Stewart map-areas and Portland Canal Area suggested that the Bitter Creek argillites were overlain by Bear River volcanic rocks (Table 3.1). Grove's (1971) detailed geological descriptions and 1:32,190 scale maps interpreted the Premier porphyry as a metasomatic alteration within predominantly cataclastic rocks around the mine and fringing the Texas Creek granodiorite. His fieldwork from 1964 to 1970 with contributions from N.H. Haimila, R.V. Kirkham, J.T. Fyles, and N.C. Carter resulted in three 1:100,000 scale geology maps (Unuk River, Salmon River, and Anyox) published by the BCMEMPR in 1982. A list of other Silbak Premier reports and papers is tabulated in Table 1.1. Galley (1981) described volcanic stratigraphy and mineralization on the Big Missouri claim group, seven kilometres north of Silbak Premier. Tipper and Richards (1976) defined the Hazelton Group in north central British Columbia in terms of the Telkwa, Nilkitkwa, and Smithers Formations and constituent facies and members. Recent revision of Mesozoic stratigraphy and structural style in the northern Bowser and Sustut basins has been undertaken by Evenchick (1986 and 1987), Moffat and Bustin (1984), Moffat (1985), Smith et al. (1984), Thomson (1985), and Thomson et al. (1986). 7 Table 1.1. Burton White Langille Kidd* Plumb * Starck* Previous reports and maps pertaining to Silbak Premier mine, in chronological order. Those not referenced in text are listed in Appendix 1.2 and indicated by an asterisk. (1926) (1939) (1945) (1954) (1955) (1955) Ewanchuk (1961) Hill and Starck *( 1961) Best* (1962) H111 and Starch *(1963) Seraphim (1979) Kretschmar* (1981) Wojdak* (1982) Wojdak etal.* (1983) Brown* (1984) Wojdak and Brown (1985) Alldrick (1985) Alldricketal. (1986) Wodjak&Randall* (1986) Wodjak& Randall* (1986) Alldrick (1987) Alldricketal. (1987) Alldricketal. (1987a) Ore deposition at Premier Mine, B.C. Geology and ore-deposition of Silbak Premier Mine. Some controls of ore deposits at the Premier Mine. The Premier mine 1954. Geological report 1955 exploration program. A preliminary study of the factors involved in reopening the Silbak Premier and Premier Border mines. Geology and ore-deposition of the Silbak Premier mine Glory Hole. The Silbak Premier mine. The Cascade Creek area, Salmon River district, Portland Canal. Provisional geological report on the Silbak Premier mine. Report of Silbak Premier properly near Stewart, B.C. Summary report and maps, 1980 surface exploration program. Silbak Premier geology and mineralization, 1982 report. Exploration at Silbak Premier, Stewart, B.C., 1983 report. Silbak Premier geology. Silbak Premier, Stewart, B.C., 1984 summary exploration report. Stratigraphy and petrography of the Stewart mining camp (104 B/1). Uranium/lead age determinations in the Stewart area (104 B/1). The Silbak Premier gold-silver deposit: Mineralization and alteration; C1M talk, Victoria, B.C. Silbak Premier, Stewart, B.C., 1985 exploration report. 1:50,000 Open File map 1987-22. Lead isotope data from the Stewart Mining Camp (104 B/1). Geochronology of the Stewart Mining Camp (104 B/1). 8 British Columbia Ministry of Energy, Mines and Petroleum Resources renewed its interest in the Stewart area in 1982. Alldrick (1983,1984,1985, 1987) and Alldrick et al. (1986 and 1987) studied the geological setting of mineralization and geochronometry in the Stewart area On a larger scale R.G. Anderson, Geological Survey of Canada, is currently remapping the Iskut map sheet at 1 -.250,000. 1.4.2 ALASKA Westgate (1921) and Buddington (1925 and 1929) produced the first United States Geological Survey (USGS) maps for the Hyder area Buddington's descriptions of the Texas Creek batholith, Hyder batholith, and Boundary stock have remained unchallenged and his view that Silbak Premier mineralization was related to and probably genetically associated with the Texas Creek batholith remains current. Petrography and geochemistry of Tertiary lamprophyre dykes in southeastern Alaska were documented by Smith (1973). Smith (1977) extended Buddington's coverage in Ketchikan D-1 and Bradfield Canal A-1 Quadrangles, in southeast Alaska; and described the characteristic fabrics, and K-Ar geochronometry for five plutons. He discussed two periods of right-lateral movement on the Fish Creek fault zone (Fig. 3.32), and mylonitic deformation in the Texas Creek granodiorite. Intrusive and plutonic belts of southeastern Alaska were summarized by Brew and Morrell (1983). At least five periods of plutonism and three episodes of metamorphism have been identified by Smith et al. (1979) and Brew and Morrell (1983). Recent U-Pb geochronology and Sr and Nd initial isotopic ratios combined with petrography and chemistry allowed Arth et al. (1986) to recognize six plutonic suites near Ketichikan. 1.5 SILBAK PREMIER HISTORY AND PRODUCTION In 1898 placer gold prospectors arrived in Stewart but panning local streams, met with limited success. Lode gold prospecting followed and prospectors discovered gold and silver showings in the Silbak Premier area between 1910 and 1916. The original Premier claim group (Cascade Fall No. 4 and 8, Pictou, Simpson, Essington, Rupert, and Hazelton claims) was staked by the Bunting brothers and W. Dillworth in 1910 , and tunnelling on 1 - and 2-levels was conducted by the Salmon Bear River Mining Company Ltd. in 1912 and 1913 (Grove, 1971). I34 -W I26'w 59' N 54* N Tulsequan 104K 1 Dease Lake I04J 2 Cry Lake 104 1 3-7 Sumaum 104 F(US) Telegrapn Creek 104 S 8,9 Spatsizi I04H 4 ,10-18 Tooaoggone River 94 E 19-26 Petersourg I04C(U5) Iskut River I04B 27 -32 THC913 AREA Bowser Lake 104 A 12.32 HcConneil Creek 94 0 33 -36 Ketcnikan 103 0 (US) A.S.C.D. E.F.8 Nass River 103 P & O Hazelton 93 M 37.38.39 Prince Rupert 103 J Terrace 103 1 40 Smithers 93 L 37 13 4' w 53'N Whitesail Lake 93 E 41,42,43 9 128'W 126'W Figure 1.3 Previous regional studies in north-central British Columbia and southeastern Alaska References are listed numerically from northwest to southeast 1:250,000 map sheets: BRITISH COLUMBIA 1. Souther, J.6. (1971) 25. 2. Gabrielse, H.( 1980) 26. 3. Tipper, H.W. (1978) 27. 4. Monger, J.W.H. and Thorstad.L.E. (1978) 28. 5. Anderson, R.G. (1978-80,1983,1984) 29. 6. Gabrielse, H.( 1979) 30. 7. Thorstad, L.E. and Gabrielse, H. (1986) 31. 8. Kerr, FA (1948) 32. 9. Souther, J.G. (1972) 33. 10. McKenzie,K.J.(1985) 34. 11. Bustin, R.M. and Moffat, I.W.( 1983) 35. 12. Moffat, I.W. and Bustin, R.M.( 1984) 36. 13. Evenchick, CA (1986 and 1987) 37. 14. Gabrielse, H. and Tipper, H.W. (1977) 38. 15. Smith,R.L. et al. (1984) 39. 16. Thomson, R.C.( 1985) 40. 17. Thomson, R.C.etal. (1986) 41. 18. E1sbaDher,G.H(1974a,b,c, 1977,1981) 42. 19. Carter, N.C. (1972) 43. 20. Forster, D.B. (1984) 21. Gabrielse, H.etal.( 1977) 22. Diakow, L.J. (1983-85) 23. Diakow et al. (1985) 24. Schroeter, T.G. (1981-85) Bold rectangle --Schroeter et al. (1986) Panteleyev, A. (1982-84) Alldrick, D.J. (1983-85) Alldrick, D.J. et al. (1986) Alldrick, D.J. (1987) Alldrick, D.J. et al. (1987a) Brown, DA (1987) Grove (1971,1972 and 1986) Lord, C.S.( 1948) Richards, TA( 1976) Monger, J.W.H. (1977b) Monger, J.W.H. and Church, B.N. (1977) Armstrong, J.E. (1944 a,b) Kindle, E.D.O940) Richards, TA (1974 and 1980) Woodsworth G.J. et al. (1985) MaclntryeD.G.0985) Woodsworth G.J. (1980) van der Heyden, P. (1982) Operation Stikine, Geological Survey of Cananda (1959) Double rectangle - -Tipper and Richards (1976) ALASKA (A) Brew, DA and Morrell, R.P. (1983) (B) Smith, J.G. (1973, 1977) (C) Buddington(1925, 1929) (D) Westgate(1921) (E) Beyers, F.M. and Sainsbury, Cl . (1956) (F) Smith, J.G. etal. (1979) (G) Arth.J.G. etal. (1986) 10 Bonanza high grade ore was discovered between 1916 and 1919 at which time ASARCO (American Smelting and Refining Company) acquired 52% controlling interest in the Premier Gold Mining Company Ltd. The initial discovery was a gossan above 2-level portal. At the same time B. C. Silver Mines Ltd was formed by 0. B. Bush to develop the easterly extension of the Premier ore zone. Construction of a 200 tons per day (tpd) mill was completed by 1921; prior to this, all ore was shipped directly to the Tacoma, Washington smelter. In 1926 mill capacity was increased to 400 tpd. Sebakwe and District Mines Ltd., which gained control of the adjacent Bush property in 1926, tunnelled 300 m from Cooper Creek to intersect mineralization in 1926 (Grove, 1971). From 1924 to 1931 45% of the mine production was direct shipping ore, indicative of the ore's spectacular quality. Mine cutoff grade was estimated to be about 10 grams gold per ton (g Au/T; 0.30 oz Au/ton) equivalent until 1931 (Wojdak, pers. comm., 1985). The mill expanded to 500 tpd in 1933 after which all ore was milled. Post-1935 cutoff grade was about 5.1 g Au/T (0.15 oz Au/ton) equivalent; which included B. C. Silver, the northeastern extension of the Premier deposit and Northern Light deposit, which came on stream after 1935. On January 1,1936 Silbak Premier Mines Ltd. was formed by the amalgamation of Premier Gold Mining Company with B. C. Silver Company (owners of B. C. Silver Ltd. and Sebakwe and District Mines Ltd.; Appendix 1.1), prior to this, the operations were independent. Life was added to the mine in 1940 when the Northern Light orebody was discovered. Labour shortages and disputes plagued the mine from 1943 to 1946. ASARCO withdrew from the management position at Silbak Premier in 1947 . In the same year the aerial tramway (at that time the world's longest) which carried ore from Silbak Premier to the Portland Canal dock was abandoned due to the cost of repairs and was replaced by truck haulage. Indian Mines Ltd. reached a milling agreement with Silbak Premier in 1950 and ore production began in 1951. Low base metal prices in 1953 forced closure of the mine. In 1955, an exploration program aimed 11 at increasing reserves was conducted by W. N. Plumb for Henry Hill and Associates. The mine re-opened in 1956 but only one month later fire destroyed the mill. In 1959 a one year lease was granted for ground above 2-level, including the Glory Hole well which Silbak Premier Mines Ltd. believed contained no significant ore. A Bonanza high grade ore lens was discovered by the leasees, containing electrum and ruby silver and was mined from 1959 to 1960. A new 75 tpd mill and cyanide plant was constructed at 6-level portal in 1964 but was never brought to full capacity and ceased operation in 1967. In 1968 the mine closed and has remained inoperetive. By 1987 all the old buildings have collapsed or been demolished. From 1918 to 1968 over 4.29 million tonnes (4.73 million tons) of ore were mined from Silbak Premier (Fig. 1.4 and 1.5, and Appendix 1.1). Approximately 52,400 kilograms (1.7 million ounces) of gold and 1.2 million kilograms (38 million ounces) of silver were produced. Silbak Premier is the third largest gold producer in British Columbia after Bralorne-Pioneer arid Rossland mines, and the second largest silver producer in the province after Sullivan (Grove, 1971). Gold and silver recovery at Silbak Premier is reported to be 95* and 88-90% (Wojdak, per. comm., 1986). Westmin Resources Ltd. is currently conducting pre-development work at Silbak Premier and plans to proceed with construction of a 2,000 tonnes per day mill to extract the low-grade, bulk tonnage ore. Present geological reserves of 5.8 million tonnes (6.4 million tons) grading 2.36 g Au/T (0.069 oz Au/ton) and 92.2 gAg/T (2.69 czAg/ton), or a gold equivalent of 3.60 g/T( 0.105 oz/ton),ata4.4 to 1 stripping ratio (waste to ore) indicate Silbak Premier will re-open (The Northern Miner, March 9,1987). 12 Figure 1.4 Historical photos of Silbak Premier (1930's). Top photo: View to east of 4- and 2-level buildings before the Glory Hole collapse formed in the area west of 1 -level. Bottom photos: 4-level main haulage route and bunk house in background, all buildings have since been demolished or collapsed. cr O GOLD PRODUCTION SILVER PRODUCTION Total - 1,185,438 kg (38.113,318 oz) Average grade • 297 g/T (8.67 oz/t) i—•—i—•—i—•—i—">—i—"—i 1 9 1 0 1 9 2 0 1930 1940 1 9 5 0 1980 1 9 7 0 YEAR Figure l .5 Silbak Premier ore, gold, and silver production (1918 to 1968). 14 1.6 F1ELDW0RK AND METHODS One hundred and five days of fieldwork from August to September, 1984, June to August, 1985, and February, 1987 were spent on the project, including ten days of regional mapping in the Long Lake, Slate Mountain and Bear River Ridge area (Plate 1). Nine days were spent underground mapping in 6-, 4-, 2-, end 110-levels. Airphotos, hip chain and compass or soil grids provided ground control necessary for plotting field data on 1:500 and 1:2,500 scale contour maps. Regional observations were compiled on 1:5,000 scale maps from 1:10,000 scale aerial photographs. Approximately 1,350 structural measurements were recorded. About three hundred and fifty rock samples were collected; from these, 100 thin sections were cut and studied. Whole rock chemical composition was determined for 27 "least altered" volcanic and plutonic rocks by the Geological Survey of Canada. Four U-Pb and three K-Ar isotopic age determinations of plutonic, volcanic and altered rocks and fourteen Sr isotopic analyses of whole rock and mineral separates were made. 1.7 ACKNOWLEDGEMENTS The author gratefully acknowledges the contribution of Westmin Resources Ltd. in providing summer employment, travel expenses, field support, drafting, and thin section preparation. R. Ivany of Westmin Resources patiently drafted the geological meps. Special thanks are extended to D.J. Alldrick, R. G. Anderson, C. I. Godwin and P.J. Wojdak for their time, advice, discussions, and permission to cite unpublished information, reports, and maps. Helicopter, computer, analytical support (27 whole rock samples), and some drafting for regional studies was provided by the Geological Survey of Canada (Project 840046, R.G. Anderson). R. L. Armstrong (U.B.C. - thesis supervisor) and R. G. Anderson (Geological Survey of Canada - external supervisor) provided encouragement and guidance throughout this study. P. van der Heyden gave thorough guidance in zircon extraction techniques, did zircon sample dissolution, U-Pb column chemistry, and 15 supervised operation of the mass spectrometer. J. Harakal, and K. Scott conducted Ar and K analyses, respectively. J. Mortensen produced U-Pb plots with calculated error envelopes. Natural Sciences and Engineering Research Council of Canada grant number 5-88841 awarded to R. L. Armstrong covered costs of lab studies. 0. Hodge and E. Montgomery provided drafting and photography advice and services, their cheerful help is much appreciated. Fossil identifications were conducted by E.S. Carter, T. Poulton, and H.W. Tipper. The author benefitted from discussions with S. Dykes, P. McGuigan, J. Mortenson, end P. van der Heyden. H. Gabrielse loaned DNAG manuscripts, co-authored by R.G. Anderson, J.W.H. Monger, G.J. Woodsworth, L.C. Struik, and D.J. Tempelman Kluit, on plutonic rocks of the Cordillera and the Coast and Intermontane belt structures. Such contemporary information made a valuable contribution to the preparation of this thesis. The Science Council of British Columbia generously supported the author for two years through the G.R.EAT. (Graduate Research Engineering and Technology Awards) program. 16 CHAPTER 2 6E0L06Y AND TECTONIC SETT I NO OF NORTH-CENTRAL BRITISH COLUMBIA 2.1 INTRODUCTION The Silbak Premier area is situated at the western margin of the Intermontane Belt, very close to the Coast Belt (Fig. 2AA). Regionally, the Intermontane Belt comprises Middle and Upper Paleozoic limestone reefs, island arc volcanic rocks, and oceanic cherts and volcanic rocks and Lower Mesozoic island arc volcanics and inter-arc sediments overlain by mid-Mesozoic volcanics and volcanogenic sediments and upper Mesozoic and Cenozoic volcanics and sediments of successor basins (Thorstad and Oabrielse, 1986). The Intermontane Belt (approximately coincident with the eastern Intermontane superterrane I (Fig. 2.1.B) of Monger, 1984) is divided into three allochthonous tectonostratigraphic terrenes: Stikinia (the largest and westernmost), Cache Creek, 8nd Quesnellia. Stikinia, which contains the study area, comprises middle Paleozoic to Lower Mesozoic eugeoclinal rocks. The Middle to Upper Jurassic Bowser successor basin sediments, an overlap assemblage wholly on Stikinia, unequivocally demonstrate the linkage between Stikinia and the Cache Creek Terrane at least by Middle to Late Jurassic time. The Coast Belt contains the greatest abundance of plutonic rocks (predominantly tonalite) in the Cordillera, and lesser metamorphic rocks, up to granulite-grade. Roof pendants range in age from Devonian or older to Middle Cretaceous, Albian (Roddick, 1983). 2.2 STIKINIA STRATIGRAPHY 2.2.1 Introduction Stikinian stratigraphy comprises Paleozoic rocks, Triassic Stuhini-Takla, and Jurassic Hazelton-Torjdoggone-Spatsizi groups, overlain by Middle and Upper Jurassic Bowser Lake Group, and Cretaceous to Tertiary Skeena, Kasalka, and Sustut Groups (Fig. 2.2). 17 Figure 2.1 The study erea in the Stewart area, northwestern British Columbia relative to: (a) the five geological/morphological belts of the Canadian Cordillera, and (b) the Insular and Intermontane superterranes (from J. W. H. Monger, written comm., 1987). \ 18 Figure 2.2 Geological events in north-central British Columbia. 1. DNAG time scale (not to scale, after Palmer, 1983); Quat. = Quaternary; Ma = million years; Pleisto. = Pleistocene. 2. accret = accretion; T-ll = superterrane II; References: (A) Alldrick et al., 1987. (B) Armstrong, pers. comm., 1987. (C) Evenchick, 1986. (D) Gabrielse et al., In prep. (E) Monger, 1984. (F) Monger and Price, 1979; Monger etal., 1982; Monger, pers. comm., 1987. (G) Souther, 1972. (H) Thorstad, 1983. (I) Thorstad and Gabrielse, 1986. (J) venderHeyden, 1982. 3. References: Eisbacher( 1974 a, b,c, and 1981) and Tipper and Richards (1976). 4. CB = Coast Belt; Mu - detrital muscovite; — • = sediment transport direction;! = uplift; E = east; W - west; N = north; S = south. 5. Adapted after Tipper and Richards (1976) and Eisbacher (1981). 6. Sub. = submerged. 7. Emer. = emerged; from Tipper and Richards (1976), Thomson (1985) and Thomson et al. (1986). 8. Adapted after Smith et al. (1979) and Woodsworth et al. (in prep.); Cret. = Cretaceous, Jur. = Jurassic, Mid. = Middle, Tri. = Triassic. -1 CD tsj Period A g r / o Epoch Ma (1)  •Pleisto 4\ Recent 16 •Pliocene 5.3 •Miocene 2 J 7 • Oligocene •• • 36 O • Eocene „ 37.8 • Paleocene . 66 4 Maastrichtian Campanian Santo man Coniacian Turonian Cenomanian 97 5 Albian Aptian Barremien Hauterivien to Berriasien 144 Tithonian 152 Kimmeridgian 156 Oxfordian 163 Callovian - - - 169 Bat ho ni an 176 Bajocian - 1 8 3 Aalenian 187 Toarcian 19S Pliensbachian - - - 1 9 8 Sinemurian - 204 Hettenojen 208 Norian 225 Karnian Orogenic events Structural style (2) Metamorphism S 8 3 zr i (A 9 c 3 cr «-af> QUAT in ZD o < in in < a: ZD U in in < Western Intermontane Belt: -NNE-directed thrusting (E) -dextral transcurrent faulting, transpression A\ extension (0) Block faulting (E) 1 \ 1 I N 4\ NE-trendmg ext*nstonal I & contractional faults (D) NE contraction (C and 0) _ NE-verging folds eV thrusts • Coast Belt u ' " ' ' f ' mylonttizetion (K) L Alexander /Wrangellia Terrenes' convergence with Stikinia (C) 1 l T-ll accretion (north)(G) T-ll accretion (southXG) T-ll accretion Major NE contraction(D), east-verging structures (G) + pre-Albian block faulting (B) Stikinia-Cache Creek Terrane collision (0) Superterrane I accretion SW directed structures, contractional faulting 4V folding • transcurrent faulting(') (E) - unknown style 6V vergence (D) + King Salmon Fault (J) TAHLTANIAN OROGENY (Permian to Mid. Tr i ) * (H) I J metamorphism L + reset K-Ar | dates j (A.C,I and K) I I Greenschist & blueschist metamorphism (J) Whitehorse • Hazelton • Quesnel marine basins ISLAND ARC Takla sea • arc 20 2.2.2 Paleozoic rocks Paleozoic rocks include the Cache Creek Group in the Atlin map area and the Stikine Assemblage (now referred to as the Asitka Assemblage; H. Gabrielse, pers. comm., 1987) in the Tulsequah, Telegraph and Iskut map areas. The Cache Creek Group contains alpine-ultramafics, basic volcanics, carbonates, chert, cherty argillite of Late Mississippian to Permian age. The Stikine Assemblage comprises Devonian, locally Mississippian and Permian basalt to rhyolite flows and pyroclastics, interbedded carbonate, shale and minor chert (Monger, 1977 a). Argillite and cherty argillite are the oldest rocks. They grade upwards into basic to intermediate tuff and breccia, which are overlain by massive to bedded Mississippian calcarenitic limestone. Conformably overlying crystal and lithic tuff are themselves overlain by basic volcanics that grade into argillite. Permian carbonate caps the sequence (Monger, 1977 a). These units are all regionally extensive, in contrast to the lenticular and disrupted character of Cache Creek rocks. 2.2.3 Stuhini (or Takla) Group The Karnian and Norian Stuhini and correlative Takla volcano-sedimentary sequences flank the Bowser Basin to the north, west and east (Fig. 2.3). The Takla consists of distal turbiditic argillite and volcanic sandstone, pillowed augite porphyry basalt flows and coarse volcaniclastics, and basic to intermediate volcaniclastic rocks (Monger end Church, 1977). Stuhini Group is characterized by augite porphyry basalt and coarsely bladed feldspar porphyry volcaniclastics and volcanogenic sediments (Souther, 1971). 2.2.4 Earty Jurassic paleogeography Rock distribution and depositional environments define Jurassic D8leogeography (Fig. 2.3 and 2.4). The volcano-sedimentary facies changes mapped by (Tipper and Richards, 1976) and the typical calcalkaline volcanic trends indicate the Hazelton Group accumulated in an island arc environment. The arc was probably analogous to the modern western Pacific with volcanic arcs separated from the continent by marginal sedimentary basins (Monger, 1984). 21 2.2.5 StikineArch The Stikine Arch, defined by a belt of Upper Triassic plutonic (Stikine suite; discussed later in this chapter) and volcanic Stuhini rocks, was the source area during Late Triassic to Earliest Jurassic time for the Whitehorse trough to the north and east. The volcanogenic Hazelton trough developed to the south (Tipper and Richards, 1976). 2.2.6 Hazelton Group The Lower to Middle Jurassic Hazelton Group consists of calcalkaline basalt to rhyolite volcanic and volcaniclastic rocks with sedimentary rocks and minor limestone (Tipper and Richards, 1976). In the Terrace map area Hazelton Group (Teklw8 Formation) is as old as Late Triassic (Karnian; Woodsworth et al.,1985). In the Smithers area Tipper and Richards (1976) divided the group into three formations (Fig. 2.2 and Table 3.1): Sinemurian Telkwa volcanics, Lower Pliensbachian to Middle Toarcian Nilkitkwa sediments and tuffs, and Middle Toarcian to Middle Bathonian (or Callovian) Smithers Formation sediments. Hazelton volcanics in the Stewart area may correlate with the Telkwa and in part Nilkitkwa Formations. The Hazelton trough presently is bordered by the Pinchi belt and Hogem batholith on the east, the younger Coast Belt on the west, Stikine Arch on the north, and the Yalakom fault on the south (Tipper and Richards, 1976). Deposition of Sinemurian age, Telkwa Formation volcanic and sedimentary rocks in the Smithers area, transgressed westwards until an Early-Middle Pliensbachian, Stikinia-wide marine regression (Tipper and Richards, 1976 and Thomson et al., 1986; Fig. 2.2). Nilkitkwa Formation clastic-tuff deposition followed until Middle Toarcian when a pyroclastic event marked the end of accumulation of thick volcanic piles in the Hazelton Group (Tipper and Richards, 1976). Thomson (1985; Fig. 2.2) identified an Early Toarcian and Aalenian transgression in the Spatsizi Group. A sub-Toarcian unconformity is widespread in northern Stikinia (Anderson, pers. comm., 1987). In the south, Middle Toarcian to Middle Bajocian Smithers Formation sedimentation occurred but in the north, Thomson (1985) noted a Late 22 Toarcian regression. Deposition continued or renewed in Bathonian and possibly into earliest Callovian time (Tipper and Richards, 1976). 2.2.7 Toodoggone(Cold Fish) volcanics Toodoggone volcanics occur northeast of the Sustut Basin. Thomson et al. (1986) used the name Cold Fish volcanics (previously informally called the Toodoggone volcanics) for com positional ly similar, coeval volcanics, which form a prong in the northern Bowser Basin (Fig. 2.3). The unit comprises predominantly subaerial calcalkaline rhyolitic, andesitic, and basaltic breccias, tuffs, and flows that are in part coeval with Lower Jurassic parts of the Hazelton Group (Pliensbachian). Biotite and hornblende potassium-argon isotopic age determinations range from 202 to 182 Ma (Pleinsbachian to Bajocian; Diakow, 1985; Gabrielseetal., 1980; and Carter, 1981). 2.2.8 Spatsizi Group The Spatsizi Group along the northern margin of the Bowser basin is in part coeval with Cold Fish volcanics, and Hazelton Group (Fig. 2.2 and Table 3.1). The sediments are Early Pliensbachian to Early Bajocian age (Smith et al., 1984). The group comprises marine shale, siltstone, tuffaceous siltstone, sandstone, and rare limy lenses and conglomerate. Spatsizi Group is unconformably overlain by the Bowser Lake Group. 2.2.9 SkeenaArch As volcanism waned in Middle Bajocian to Callovian time, the northeasterly trending Skeena Arch was uplifted and divided the Hazelton trough into the northern Bowser Basin and southern Nechako Basin (Fig. 2.3; Tipper and Richards, 1976). 23 2.2.10 Bowser Basin The Bowser Bssin, a Middle to Late Jurassic successor basin accumulated molasse derived from elevated terrains to the north (Stikine Arch), east (Hogem batholith), and south (SkeenaArch; Fig. 2.3). Paleocurrents (Fig. 2.4) trend inwards radially, this suggests subsidence kept pace with sedimentation. 2.2.11 Bowser Lake Group The Bowser Basin is underlain by the Bowser Lake Group which comprises Middle and Upper Jurassic (Late Bajocian to ?Ear ry Kimmeridgian) shale, siltstone, and conglomerate, and to the south an interbedded volcanic assemblage of green and grey, feldspathic, andesitic breccia, tuff and flows (Tipper and Richards, 1976; Fig. 2.2). Chert pebble conglomerates ere diagnostic of Bowser Lake sediments and commonly occur at the basal contact. Bowser lake Group is divided into et least two formations: Ashman formation, Late Bajocian to Early Oxfordian, and younger unnamed formations (eg. Trout Creek Formation in Spatsizi map area). The widespread Ashman Formation is dark grey to black shale and siltstone with chert-pebble conglomerate lenses in the northern Bowser Basin, and green sandstone end tuffaceous sandstone to the south. Younger, coarsening-upwards facies are alluvial-deltaic-delta slope sediments that represent a Callovian marine regression (Tipper and Richards, 1976). Conglomerates in the southern part of the basin contain volcanic, granitic, and sedimentary detritus derived from the Skeena Arch to the south (Fig. 2.3; Tipper and Richards, 1976). The northern and northeastern portion of the basin comprises volcanic-, argillite-, and chert-bearing channel gravel deposits, but lacks granitic detritus. The abundance of chert indicates Bathonian erosion of the Cache Creek Terrene, farther to the north (Thomson et al., 1986, and Currie, 1984). The northeastern Bowser Basin is a thick sequence of turbidites with west- and south-trending paleocurrents (Fig. 2.4; Eisbacher, 1981). 2.2.12 Skeena Group A major hiatus marked by regional uplift, faulting and erosion in the Kimmeridgian to Early Cretaceous ended with deposition of Skeena and Sustut groups on deformed basement (Tipper and Richards, 1976). The largely Early Cretaceous Skeena Group consists of marine and nonmarine sedimentary rocks (Red Rose 24 Formation) that is conformably overlain by volcanic rocks (Brian Boru Formation, which is at least 1800 m thick; Souther, in prep.). Oreywacke, sandstone, shale, and conglomerate similar to Hazelton and Bowser Lake sediments are distinguished by an abundance of quartz and fine detrital muscovite, indicating uplift and erosion of the Omineca belt as a source area (Tipper and Richards, 1976, and Eisbacher, 1981). Minor and major coal seams (e.g. Telkwa coal) are characteristic of Skeena sedimentation. Volcanics are varicoloured basaltic to rhyolitic breccias, tuffs, and flows (Tipper and Richards, 1976). Skeena Group, deposited in the Sustut Basin (Fig. 2.3), is Early Cretaceous to ? Earliest Late Cretaceous in age (Hauterivian to Albian or Cenomanian; Tipper and Richards, 1976). Deposition was both marine and nonmarine with shoreline, beach, deltas, and lagoonal coal swamps grading eastward into alluvial plains (Eisbacher, 1981). Sparse paleocurrent determinations suggested to Eisbacher (1981) that fluvial transport was to the south-southwest. This deposition correlates with deformation following accretion of the western (T-il) superterrane (Fig. 2.1 .b and 2.2). 2.2.13 Kasalka Group The Kasalka Group, exposed in the Tahtsa Lake area, comprises a volcanic sequence over 1000 m thick that rests unconformably or disconformably on Albian marine sediments of the Skeena Group (Souther, in prep.) and Hazelton and Bowser Lake Groups. The lower felsic fragmentel unit is overlain by porphyritic andesite, lahar, columnar jointed basalt and rhyodacite flows (Maclntyre, 1985). A distinctive reddish brown basal conglomerate contains clasts of Hazelton and Skeena Group (Maclntyre, 1985). Comagmatic dykes, sills, plugs, laccoliths, and plutons provide K-Ar isotopic ages that bracket the group between 108 and 90 Ma (Middle to Late Cretaceous; Maclntyre, 1985). 2.2.14 Sustut Basin and Group The Sustut Group, Middle Albian to Maastrichtian (A.R. Sweet, in Evenchick, 1986), comprises continental sediments that were deposited in the structurally controlled Sustut Basin (Fig. 2.5), and records two distinct source areas: the Omineca Belt to the east and the Coast Belt to the west. The Group was divided into two formations by Eisbacher (1974 a): the lower nonmarine, sandstone-mudstone Tango 25 Creek, and the upper nonmarine mudstone, tuff and conglomerate Brothers Peak Formation. Large detrital muscovite flakes in coarse sandstones are distinctive, and distinguish the Tango Creek Formation from Bowser Lake Group (Evenchick, 1986). The muscovite, represents detritus derived from the uplifting Omineca Belt, deposited by southwest flowing rivers, across the Sustut Basin (Fig. 2.2 and 2.3; Eisbacher, 197-4). The Tango Creek Formation comprises detritus shed from the east in the Albian to Santonian (Fig. 2.5.a; Omineca Belt), whereas, the Brothers Peak Formation (Maastrichtian) sediments were derived from the west (Fig. 2.5.b; Coast Belt), as alluvial fans prograded eastwards over swampy floodplains across the Sustut Basin. Paleocurrents in the upper member of Tango Creek Formation and overlying units indicate a change to longitudinal flow, southeasterly down the axis of Sustut Basin (Eisbacher, 1974 and 1981). As the Coast Belt was uplifted further, Bowser Lake Group sediments were uplifted, eroded and chert clasts shed into Sustut Basin, in Brothers Peak time. The Sustut Group records the timing of the Omineca and Coast Belts' uplift and deformation (Fig. 2.2; Eisbacher, 1981). 26 134° 124° • a f\ T - r 60° DISPLACED TERRANES Cassiar is. f / V V V V V V " v J J C L E G R A P H : BHgr ^ C A S S I A R ACCRETED TERRANES i. ) Cache Creek Quesnellia ISIIj Slide Mountain Stikinia . C E A S E •COLDFISH-.VOLCANICS! POST-TERRANE ACCRETION OVERLAP ASSEMBLAGES Skeena and Sustut Groups Bowser Lake Group (on Stikinia) ^ 1^ l n k l i n Forma"0" (on Cache Creek Terrane) Intrusive Rocks Coast Belt 1 j Undivided •:BOWSER BASIN; metamorphic rocks I I Young cover T E W A R f • H A Z E L T O N ' " J S V . S T A T E S O F ^ E R . C ^ ^ " ^ ' •0 :0; C A N A D A 50 KILOMETRES 134° P R I N C E ' R U P E R T ^^a^ mm mm / ^ v v v v v INTERMONTANE BELT • Figure 2 . 3 Terrane map, tectonic elements and rock distribution for north-central British Columbia (modified after DNAG map, Brookfield, in prep.). 27 Figure 2.4 Paleocurrents during deposition of Bowser Lake Group, Late Bajocian to Early Kimmeridgian sedimentary rocks (modified from Eisbacher, 1981). 28 Figure 2.5 Paleocurrents in the Sustut Basin during deposition of: (a) Tango Creek Formation, Albian to Santonian sedimentary rocks, and (b) Brothers Peak Formation, Maastrichtian volcanic rocks (modified from Eisbacher, 1981). 29 2.3 INTRUSIVE ROCKS There are seven Intrusive periods in northern British Columbia (Fig. 2.2): Late Triassic, Early Jurassic, Middle Jurassic, Late Early to Early Late Cretaceous, Late Cretaceous, Early Tertiary (Eocene), and Late Miocene to Recent. The plutonic suite distribution is illustrated in Figure 2.6 and their characteristics are described below. 2.3.1 Late Triassic (215-230 Ma) There are two suites of Late Triassic plutonic rocks: an Alaskan-type ultramafic suite, and a tholeiitic to calcalkaline granitoid suite. The latter, Stikine suite, are found along the Stikine Arch (f- ig. 2.6 and 2.6). These hornblende-rich granitoids are spatially and genetically associated with Middle to Upper Triassic volcanics (Woodsworth et al., in prep.). The Stikine suite includes the Hotailuh (Anderson, 1983), Hickman (Souther, 1972) and Stikine (Anderson, 198-4) batholiths (Fig. 2.6). Part of the composite Hotailuh batholith is typical of the suite and yielded K-Ar and U-Pb zircon isotopic ages of 215-230 Ma (Anderson, pers. comm., 1987). These rocks are cut by Middle and Late Jurassic plutonic rocks (Anderson, 1983). These batholiths are typical I-type granitoids, intruded at shallow levels, and are comagmatic and coeval with Stuhini Group volcanics (Woodsworth et al., in prep.). Magmas trend towards more alkaline and less sodic from west to east (Armstrong, in press). The batholiths were uplifted and eroded during Early Jurassic time. 2.3.2 Late Triassic to Early Jurassic (210-187 Ma) This plutonic episode includes several suites of economic significance: Guichon and Copper Mountain in southern British Columbia, Texas Creek suite near Stewart, and Island suite on Vancouver Island (Woodsworth et al., in prep.). The plutons represent the subvolcanic roots of several Late Triassic to Early Jurassic island arcs. 30 The Topley suite comprises small, Early Jurassic (210-178 Ma, K-Ar dates) calcalkaline plutons and stocks distributed in a northeast-trending belt along the Skeena Arch (Fig. 2.3 and 2.6). They are massive to weakly foliated biotite- and hornblende-bearing granite to quartz diorite epizonal stocks and batholiths (Woodsworth et al., in prep.). The Tachek batholith, along the Skeena Arch, and Texas Creek grenodiorite, near Stewart, contain K-feldspar megacrysts. Both are interpreted to be the root or source of Telkwa volcanics. The Texas Creek granodiorite is an irregular-sheped intrusion cut by at least two zones of west-northwest trending, steeply dipping mylonitic fabric. It intrudes Hazelton andesite and siltstone. Coeval end comagmatic, potassium-feldspar-hornblende-plagioclase phyric "Premier porphyry" dykes intrude massive Texas Creek granodiorite end andesite. 2.3.3 Middle Jurassic (180-170 Ma) The Three Sisters suite of the Stikine Arch, part of the younger phases of the Hotailuh Batholith, are heterogeneous bodies (Fig. 2.6). Compositions range from diorite through quartz monzodiorite or granodiorite to quartz monzonite, all plutons contain biotite and/or hornblende (Woodsworth et al, in prep.). These Middle Jurassic plutons may be related to Middle Jurassic Whitesail Formation (top of Lower Jurassic; Woodsworth, 1980) of the Hazelton Group in the Whitesail Lake map area (Woodsworth et al., in prep.). In northern British Columbia parts of the Spatsizi Group may be the volcanic equivalent. 2.3.4 Late Early to Middle Cretaceous (110-90 Me) Plutons with K-Ar dates of 120 to 98 Ma are abundant in the western portion of the Coast Belt (Fig. 2.6) and Foreland Belt, and scattered across central British Columbia They are pre-, syn-, and post tectonic (Woodsworth et al., in prep.). During this period garnet-bearing granodiorite end tonalite intrusions were emplaned in Southeastern Alaska (Smith et al., 1979). The Albian (?) to early Upper Cretaceous Kasalka Group could be the extrusive equivalent to plutons of this period. 31 2.3.5 Late Cretaceous (85-6-4 Ma) Late Cretaceous (85-64 Ma; average 70 Ma), small, high-level, calcalkaline intrusions are common in the Intermontane Belt end abundant along the Skeene Arch (the Bulkley intrusions; Carter, 1981; Armstrong, in press; Fig. 2.6). The intrusive rocks ere massive to weakly foliated granodiorite and quartz diorite. Woodsworth et al. (in prep.) suggested Bulkley intrusions were comagmatic with widespread nonmarine Upper Cretaceous volcanism, e.g. Rocher Deboule stock (Sutherland Brown, 1960) and the Brian Boru volcanics. The Bulkley plutonic suite and associated volcanics were contemporaneous with deep-seated plutonism and metamorphism in the Coast Belt to the west (Woodsworth et al., in prep.). 2.3.6 Early Tertiary (55-45 Ma) Small high-level Eocene plutons, stocks and dykes mark a period of extensive intrusive activity. Woodsworth et al. (in prep.) define three Intermontane Belt suites: the granitic Nanika, the granodioritic Babine suite, and the gabbroic Goosly Lake suite. The Nanika suite occurs south of the Bowser Basin (Fig. 2.3 and 2.6). It includes the Alice Arm intrusions, a belt parallel to the eastern margin of the Coast Plutonic Complex west of the Bowser Basin (Carter, 1981 and Maclntyre, 1985); and the Kastberg intrusions east of the basin. The Berg (Panteleyev, 1981) and Kitsault Cu-Mo deposits (Steininger, 1985) are related to the Nanika suite (Woodsworth et al., in prep.). The Babine suite, east of Smithers, hosts the Oranisle and Bell porphyry Cu-Mo deposits (Carter, 1981). As a group, these bodies are the intrusive equivalents of acidic to intermediate explosive eruptions of the Sloko (Tulsequah map area), and Ootsa Lake (Whitesail Lake map area) Groups (Souther, 1970). Coast Belt intrusions of this period include the Hyder suite, around Hyder Alaska and Ponder pluton, south of Stewart (Fig. 2.6). Hyder quartz monzonite, Boundary granodiorite, and Davis River pluton and related dyke swarms (Portland Canal end Premier or Mount Welker) are all included in the Hyder plutonic suite. The Ponder pluton (Fig. 2.6) is a hornblende-biotite granite to granodiorite with an andalusite-bearing contact aureole on the east side and with a complex contact of interfolding the Central Gneiss Complex on 32 the west (Woodsworth et al, in prep.). In the Ketchikan area, southeastern Alaska, Arth et al. (1986) divide the Coast Belt into three suites: (I) the western tonalite-suite (55-57 Ma); (2) the central orthogneiss-suite (ca 127 Ma); and (3) the eastern granite-suite (52-53 Ma). 2.3.7 Oligocene to Recent (31 Ma-Recent) An alkali olivine basalt suite of vertical northeast-trending lamprophyre dykes (Smith, 1973) are common in the Stewart area They are post Hyder and pre-Pleistocene. Smith (1973) noted that their chemistry and petrography was akin to flat lying Miocene and Pliocene basalts in Central British Columbia Hill (1984) postulated that these dykes reflect minor crustal extension during Coast Belt uplift. Widely scattered, 31 to 19 Ma old, high-level granite and granite porphyry plutons are reported by Smith et al. (1979) in southeastern Alaska 2.4 STRUCTURAL HISTORY AND STYLE Structural style in the Intermontane Belt varies with each Terrane and rock type (Fig. 2.7). Hazelton volcanics are dominated by brittle structures and broad warps. Spatsizi Group sediments and Bowser and Sustut Basin sediments are deformed in a complex thrust end fold belt. Recent work in the Spatsizi map area by Evenchick (1986 and 1987) in the northeast edge of the Bowser Basin is unravelling the structural history, but little research has concentrated on the western margin of the basin. Stikinia and Cache Creek Terrenes deformational events for north-central British Columbia are summarized below largely from Gabrielse et al. (in prep.). 33 Figure 2.6 Plutonic rock suites in north-central British Columbia (modified after Woodsworth at al., in prep.). 34 2.4.1 Pre-Late Triassic Stikine Assemblage (Asitka Assemblage) rocks were deformed in the Tahltanian orogeny prior to deposition of Upper Triassic rocks (Souther, 1971; Fig. 2.2). These rocks are characteristically more highly cleaved and metamorphosed than younger strata (Gabrielse et al., in prep.). In the Tulsequah map area folds are tight and north-trending (Fig. 2.7). In the Stikine River's Grand Canyon two fold trends are evident, a north-northeast trend and a younger west-northwest trend (Gabrielse et al., in prep.). 2.4.2 Late Triassic Latest Triassic deformation is evident in the Cache Creek Group as strong cleavage and tight folds (Monger et al, 1978). Melange and blueschist minerals formed during subduction of an accretionary wedge (Gabrielse et al., In prep.). 2.4.3 Middle Jurassic King Salmon Assemblage (Inklin, Sinwa, and Kutcho formations) were isoclinally folded and thrust southwest along the northward-dipping King Salmon Fault in Toarcian to Bajocian (Thorstad and Gabrielse, 1986; Fig. 2.7), when Stikinia was overridden by the Cache Creek Terrene (Gabrielse et al., in prep.). Some dextral transcurrent motion may have occurred along the northern portion of the King Salmon Fault (Gabrielse et al., in prep.). A minimum age for this southwesterly-directed shortening is pre-160 Ma, the concordant hornblende and biotite date in the Snowdrift Creek pluton which intrudes structures related to the King Salmon Fault (Gabrielse, pers. comm., 1987). In the Middle to Late Jurassic there was brittle, block faulting without significant folding or plutonism along northwest-trending faults around the Skeena Arch (Gabrielse et al., in prep.; Fig. 2.3 and 2.7). 2.4.4 Pre-E8rly Cretaceous (pre-Albian) In the northern Bowser Basin tight, northeast-verging folds ere common and are a result of major southwest-northeast contraction during the pre-middle Albian? (Gabrielse et al., in prep.). Eisbacher (1974 a), in the Mosque Mountain area of the Sustut Basin, attributed these structures to the final 35 convergence of Stikinia end North America. In the northwest corner of the Bowser Basin folds are open to tight, northwesterly overturned and east-northeast-trending (Oabrielse et el., in prep.). The structural discontinuity between tightly folded sediments and warped volcanics indicates Bowser sediments must be underlain by a detachment surface that roots in Lower Jurassic volcanics. In the northeast corner of the Bowser Basin the "Spatsizi" thrust fault, juxtaposes Lower Jurassic Cold Fish volcanics on Middle Cretaceous Tango Creek Formation (Evenchick, 1987). Sustut basin deformation is a function of basement lithology. Broad open folds occur in the sediments where they lie on Triassic and Jurassic volcanics (Eisbacher, 1974 a). Farther west, where lower Sustut sediments lie on Bowser Lake Group sediments the structure is similar to that of Bowser sediments; rocks ere tightly folded, thrust faulted, and east-verging (Oabrielse et al., in prep.). Northeast-verging folds and related gently dipping thrusts characterize deformation in Lower Jurassic volcanics between Bowser and Sustut basins (Oabrielse et al., in prep.). Locally, northeast-trending folds and northwest directed thrust faults occur. 2.4.5 Cretaceous to Early Cenozoic Post-Middle Albian, pre-Campanian northeast contraction and a younger period of northeast contraction in the Bowser and Sustut Basins were documented by Evenchick (1986). The post-Campanian (?) event affected Brothers Peak Formation, Sustut Group, (Evenchick, 1986). Transpression in Cretaceous and Earliest Tertiary resulted in right-lateral transcurrent faulting displaced terrenes northward (Monger, 1984 and Oabrielse, 1985). Movement along the west side of the Omineca Belt during the Late Cretaceous would have moved the Bowser Basin hundreds of kilometres northward during deposition of the Upper Cretaceous clastic wedge (Sustut Group; Eisbacher, 1981). Block faulting in the Skeena Arch area was prominent, related to regional uplift and in part dextral transcurrent faulting (Fig. 2.7; Oabrielse et al., in prep.). 36 In the western Intermontane Belt most of the deformation occurred in this period (Gabrielse et al., in prep.). Structural style is variable; sediments are open to isoclinally folded, verging to the northeast but more massive Hazelton volcanic rocks are openly warped. The Hazelton-Bowser Lake Group contact is commonly a detachment zone. Gabrielse et al. (in prep.) suggest that most folds in the western Intermontane Belt are related to north-northeasterly directed thrusting of Late Cretaceous or earliest Tertiary age. Ductile fabrics are gentle to moderately dipping foliations and stretching lineations as exposed in the Telkwa Formation, northeast of Terrace (Gabrielse et al., in prep.). Chert- pebble conglomerate of the Skeena Group is also deformed, therefore indicating the deformation is Late Cretaceous or younger. Van der Heyden (1982) studied the Coast- Intermontane Belt structural transition in the Tsaytis River area and documented Middle to Late Cretaceous metamorphism, contraction and mylonitization, overprinted by a younger, Late Cretaceous-Early Tertiary, phase of northeast-directed thrusting and strike-slip faulting. 2.4.6 Eocene-Oligocene and Miocene to Recent Eocene-Oligocene faulting followed the Late Cretaceous northeast-compression, but ended before extrusion of Miocene plateau basalts that are not faulted (Woodsworth, 1979 and 1980, and Gabrielse et al., in prep.). Younger, Oligocene to Recent northeast trending faults and joints controlled Miocene to Recent alkalic volcanic rock distribution such as the l8mprophyre dyke swarms that Smith (1973) documented. In the Redcap Mountain area, Hill (1984) postulated that the east-west extension was due to cooling and uplift of the Coast Belt. 37 Figure 2.7 Structural trends and major faults of north-centr8l British Columbia (modified after DNAG map, Brookfield, In prep.; Oabrielse, 1985; Oabrielse et al., in prep.). CHAPTER 3 SALMON RIVER VALLEY AND BEAR RIVER RID6E 6E0L08Y - STEWART AREA 3.1 INTRODUCTION The Salmon River Valley and Bear River Ridge areas are underlain by Hazelton Group volcanic rocks and Bowser Lake Group sedimentary rocks. The oldest rocks are part of the Upper Triassic to Lower Jurassic Hazelton Group, and host Silbak Premier ore. Lower to Middle Jurassic Spatsizi Group and Middle to Upper Jurassic Bowser Lake Group sedimentary rocks outcrop two kilometres north of Silbak Premier (Plate 1; Table 3.1 and 3.2). The Early Jurassic Texas Creek plutonic suite intrudes Hazelton Group near the Silbak Premier mine (Smith, 1977; Alldrick, 1986, and this study). Eocene Coast Belt plutonic suite batholiths and related dykes are exposed to the west and south. Stratigraphy (oldest to youngest units), intrusive rocks, metamorphism, and structural style mappable at a scale of 1:10,000 (Plates 1 and 2; in pocket) provide the regional framework. More detailed lithologic descriptions of 1:2,500 scale map units (units Hv, Hg, Hm, and Jp), which host Silbak Premier ore, ere given in Chapter 4 (Plates 3, 4 and 5). 3.2 STRATIGRAPHY In the Silbak Premier area, the Hazelton Group (Table 3.2 and Fig. 3.1) consists of a thick accumulation (> 1000 m) of greenschist grade andesite, tuff and breccia overlain by alternating green and maroon volcaniclastic rocks. The andesite and green and maroon volcaniclastic rocks are overlain by black tuff and felsic to rhyolitic breccia end welded tuff. The felsic rocks are overlain by isolated buff carbonate and/or thinly bedded siliceous shale and tuff of the Spatsizi Group. Finely laminated argillite, siltstone and shale of the Bowser Lake Group cap the sequence. 3.2.1 HAZELTON GROUP 3.2.1A Argillite, siltstone and minor greywacke (unit Hs) A sedimentary member of intensely folded and fractured black (NI; Goddard, 1980, Rxk Colour Chart) to brownish grey (5 YR 4/1) argillite, siltstone, cherty argillite end minor greywacke (unit Hs) occurs within the andesite. It is exposed 1,250 m south of the Westmin camp 8nd on the Granduc road northwest of Indian Mine (P. McGuigan, pers. comm., Sept., 1985; Plate I and 3). It was mapped as Nass Formation by Schofield and Hanson (1922) and as the Bowser Lake Group by Grove (1971); however, recent 39 mapping by tha author includes it with Hazelton stratigraphy (Table 3.1 and 3.2), as advocated by Galley (1981) and Alldrick (1985). 3.2.1 B Andesite, tuff and breccia (unit Hv) Greyish green to dark (dusky) green (5 G 5/2 to 5 G 3/2) weathering, fine grained, massive andesite and monotonous greenstone are characteristic. Andesite, tuff and breccia outcrop along the Granduc road at Silbak Premier (Plate 1), the succession is at least 1,000 m thick but no base is recognized (Table 3.2). The unit hosts Big Missouri mineralization in siliceous and carbonate-rich layers (Galley, 1981) and a portion of Silbak Premier ore. Fragmental lithologies include tuff, lapilli tuff and monolithic breccia Angular fragments in the letter are up to 50 cm long. Eutaxitic textures are observed locally (Fig. 6.4). 3.2.1 .C Green end maroon andesitic volcaniclastic rocks (units H, Hg end Hm) Interfingering pale green (5 G 7/3) and maroon (dusky red; 5 R 3/5) volcaniclastics are characterized by subangular heterolithic volcanic and crystal fragments supported in volcanic detritus. Andesitic to dacitic volcaniclastic rocks (units H, Hg and Hm) outcrop from Mount Dilworth south through Silver Lekes, on west side of Slate Mountain, above Silbak Premier and along Bear River Ridge (Plate 1). The volcaniclastic rocks overlie andesite (unit Hv), however the contact is not exposed. Boulder conglomerate at station B-102, near B.C. Silver (Plate 3), contains cl8sts derived from underlying dacitic porphyries and andesite and suggest an unconformable relationship. Total thickness of the volcaniclastic rocks along the west flank of Bear River ridge is less than 1,750 m. Cascade River fault zone Mt. Dilworth North South Long Lake-Fish Creek fault zone ate Mtn. North South Monitor Lake West East 40 Silbak Premier and* Bear River Ridge East LEGEND Silbak Premier mineralization Bowser Lake Grouo (Bathonian). fin Siltstone, greywacke & argillite. Spatsizi Group (Bajocian ?). L§J Bedded siliceous sediments, "Pajama beds" Buff carbonate (Toarcian ?). Hazelton Group (Upper Triassic-Lower Jurassic). [Hr] Monitor -Dilworth rhyolite breccia & tuff. [HW] Black carbonaceous crystal tuff. IH 1 Undifferentiated volcaniclastic rocks. |Hm| Maroon volcaniclastic rocks. fhlvl Andesitic greenstone. [Hi] Argillite & siltstone. INTRUSIVE ROCKS Texas Creek Plutonic Suite (Early Jurassic) Dacitic porphyry Jp |Jmp| Maroon dacitic porphyry. Figure 3.1 Idealized stratigraphic columns, Stewart, northwestern British Columbia: (a) Mount Dilworth and (b) Slate Mountain sections are drawn north-south, (c) Monitor Lake and (d) Silbak Premier/Bear River Ridge sections are east-west. Faults have oblique slip displacement. Sections are not to scale. 41 Table 3.1 Comparison of regional tables of formations for northwestern British Columbia. Brian Boru volcanics are about 72 Ma (Maastrictian age), based on K-Ar dates reported by Wanless et al. (1979). The volcanic rocks are no longer considered part of the Skeena Group. Quat. = Quaternary; = unconformity. 1. McConnell, 1913 — Stewart area. 2. Schofield and Hanson, 1922 — Stewart area. 3. Hanson, 1929 — Stewart area. •4. Buddington, 1929 ~ Hyder area. 5. Grove, 1971, 1972,1986 — Stewart area. 6. Tipper and Richards, 1976 and Eisbacher, 1981 ~ Sustut Basin. 7. Thomson et al., 1986 and Thomson, 1985 — Spatsizi area, northeast corner Bowser Basin. 8. Souther (in prep.) refers to Kasalka Group. 9. DNAG time scale (after Palmer, 1983) not to scale. TRIASSIC JURASSIC CRETACEOUS TERTIARY Paleogene Neogene CO <c Upper Lower Middle Upper Lower Upper Series « CD 3* 70 00 O < CO 3+ r 5 " ? O * g-• 3 3 ex z o a» VI *o A u *0 cr ?a o * » o ui TJ > © 8-3! " JC 5- X CL 2.'' * O CD — O ° 3 * Z O a <* <t -, - l =3 o 3 i—i ^ < CD O Q> <D => O vi * .£ ;t vi <» -» 5 °- -i •» 3 3 ;V 3 » 2. VI S o 2. S 3 o o o 3 £ « & X JT VI • «< * O ng VI 3 - q i CL TJ f-<C 3" Si JC«£ g IA i% S 9» TJ 70 T «V VI •» o 3 s Coast Range Intrusions X o -* a * > 9 o * a c r-g TJ > i • 3 xr 70 O CD CM O 3 * «• — 70 01 CD 3' a 3 o • 3 • 3 ^ o 3 O TI 2 O 3 OJ 3 VI 3 « HAZELTON GROUP CO a TJ cn _ _ « < CD 3 •» 2 -Q s I s 1 cr w |5! 2. ( - 1 Z 1 x-i M 3 ~U \ O 7T 3 ! 3 o * o o 3 o 1 CO o o to 3 4 3 a. HAZELTON GROUP BOWSER LAKE GROUP » SKEENA J SUSTUT GROUP STUHINI GP. H 0 s ^ o O o cn z - I n VI ,c I"! 2° 2 3 SPATSIZI GROUP 3 £ BOVSER LAKE GROUP w M cn is): o CC O <1 3 J- 0 \0 CD SO CM CO ON CD a a 3 o a o a 1. 3 ffi o X o 3 CD » a T3 0-1 n o ) f i 3 — o a a -3 3 3 3 3 0» *» em- 3 — CH cn cr cn \0 n -si 3 01 as —• o 3 3 2CT.5 cn cjjs jo' 5«. CO Cn cn CH 43 Table 3.2 Jurassic correlation chart for Salmon River Velley-Bear River Ridge area, northwestern British Columbia. The rock correlation is consistent but poor age control prevents time correlation. The tightest chronological control (fossil end isotopic) is utilized in the section from this report. Abbreviations used in Table 3.2 are: arg. = argillite, cgl. = conglomerate, Cr. = Creek, FM. = formation, frags. = fragments, gd. = granodiorite, 1st. = limestone, por. = porphyry, rhy. = rhyolite, slst = siltstone, vole. = volcanic rocks, ~ = unconformity. * * Grove's (1986) age control modified to correlate lithologies based on new chronologic information. * Galley's (1981) pre-Bowser Lake Group stratigraphy is inverted on the correlation chart to conform with sections by Grove, Read, Alldrick end this study. Alldrick's (1985) unit 3 = "Mount Dilworth formation" (Alldrick, 1987). Rock unit nemes in "THIS REPORT" column refer to 1:10,000 and 1:2,500 maps in pocket (Plates 1 and 3). Key horizons are (shown by numbers in boxes): (1) the Bowser Lake Group argillite, shale, and siltstone, ' (2) the Monitor/ Dilworth rhyolite tuff and breccia, and (3) the andesite/volcaniclastic contact. 44 CL Grove (1971) Grove (1986) ** Read (1 979) Galley (1981)* Alldrick (1985) Alldrick (pers. comm. ,1986) THIS REPORT + Anderson (pers. comm., 1986) u CO co at CL o. 3 younger rocks not exposed CO <: siltstone, greywacke, argillite o 2: I co — rrT-~ siltstone, M greywacke, chert pebble cgl., slst., calcareous sst., rhy flows v"L. <u Bowser Lake Group h ml (unit 1) dark grey breccia (unit 5) Bowser Lake Group El (unit 4) Sedi mentary sequence (>300m) younger rocks not exposed [Hargillite. shalelj 6\ siltstone (unit B) on Troy 1 | i Ridge S § ? 1st. C E red & green volcanic cgl. CO co «x Q£ 3 1 ~3 U J CO en. ZD Z 3 green & some red volcanic cgl,slst., & breccia Q i CD U j CO C L « C L C C 3 r-(unit 1 a to 1 d) tuff limestone SALMON RIVER FM. (unit 4a) Transitional unit + 1st. I Dilworth rhy (unit 2) green tuff + volcanic breccia (unit DR)rhy + 1st, blocks fjl (unit 3) Felsic vole. + black tuff i (unit 3) maroon tuff + breccia (unit 2 a) tuff w plutonic & volcanic frags (unit 1 e) maroon tuff -1 (unit 4) andesite (unit 2 b) green andesite tuff & flow (unit 3) basaltic andesite (unit 4) slst. base not exposed BETTY CREEK FM (unit 2) andesite to dacite tuffs and flows + epiclastics hiatus r Slate ^ Mountain section . hiatus T a s t ^ Monitor Lake section . volcaniclastics (units H, Hg & Hm) U f & g) por. flows (unit 1 e) andesite (1 d) siltstone (unit 1 c) andesite (1 b) argVslst (unit 1 a) andesite not exposed \%%% AAA 45 The andesitic volcaniclastic rocks weather pale greyish green (5 0 5/2), maroon (dusky red; 5 R 3/5) end purple (greyish red purple; 5 RP 4/2). Lithologies include varieties of ash tuff intercalated with coarse volcanic breccia and conglomerate. The finer tuffs have a well developed phyllosilicate foliation. Fragments are predominantly feldspar porphyry similar to unit Jp, with minor andesite end fine grained volcanic material. Crystal fragments include plagioclase, and rounded quartz. In a correlative unit, Galley (1981) noted sanidine, apatite, zircon, and biotite replaced by sericite and magnetite. 3.2. l.D Black tuff and greywacke (unit Hw) Prominent white plagioclase laths in a black (N1) to dark grey (N3) weathered matrix, characterize the biotite-plagixlase crystal-lithic tuff and lapilli tuff (unit Hw; Plate 1). The distribution is limited; it extends south from Mount Dilworth to the southern edge of Slate Mountain. The unit is up to 200 m thick and is assumed to be conformable with green end maroon volcaniclastic rocks (unit H, Hg, and Hm), however, the contact was not observed. Accessory lithic fragments include aphanitic, dark grey lapilli, and aphanitic trachyte. Read (1979) included porphyritic flows of the black tuff unit in his "unit 1". Characteristic biotite crystals are fresh with slightly corroded rims. Plagioclase is twinned and fresh with minor carbonate-sericite altered cores. Apatite is accessory. Crystal fragments are supported in an aphanitic matrix of quartz, albite, prehnite, end opaque amorphous material. 3.2.1 .E Monitor/Dilworth rhyolite tuff and breccia (unit Hr) A regionally extensive rhyolite tuff and breccia marker horizon (unit Hr) is exposed at Monitor Lake and Mount Dilworth. It conformably overlies the black tuff at Monitor Lake, and underlies the black tuff on the southwest flank of Mount Dilworth (Meade et al., 1982; Plates 1 and 2, and Fig. 3.1). Its thickness, greatest at Monitor Lake (200 m), is variable. Mount Dilworth exposures ere rusty weathering due to disseminated pyrite (< 55B). Monitor Lake outcrops weather greenish grey (5 GY 6/1) to buff (light olive grey; 5 Y 6/1); but farther south, the rhyolite is pale green with irregular maroon (dusky red) to moderate red (hematitic; 5 R 3/5 to 5 R 4/6) patches (Fig. 4.6). The unit consists of rhyolitic tuff, welded tuff and breccia with silicic to felsic angular fragments up to 25 cm long. Fiamme are well 46 preserved (Fig. 3.2) 125 m south of Fetter Lake on the Big Missouri claims. The Monitor rhyolite unit at station B-384 (Plate 1) contains rare subrounded limestone fragments. On Mount Dilworth rhyolite breccia contains fragments of recrystallized, sparry calcite and is overlain by fossiliferrous (belemnite- and Welva-bearing) buff sandy limestone which is regionally correlated with other middle Toarcian localities. Alldrick (1987) has informally named this unit the "Mt. Dilworth formation". There is a thin (< 20 m thick) transitional unit between Monitor rhyolite (unit Hr) and black tuff (unit Hw) that outcrops east of Monitor Lake (station B-520; Plate 1), end at the black tuff-Bowser Lake Group contact on Slate Mountain (station B-490; Plate 1). The unit is dark grey (N3), fine grained to aphanitic, with angular quart2 and lithic fragments. It is fissile, characterized by anatomosing pressure solution cleavage, represented by microscopic layers of amorphous opaque material, that crosscut lithic fragments of similar composition and texture to the matrix. 3.2.1 .F Hazelton Group peleoenvironment Depositional environment of Hazelton Group andesite and volcaniclastic rocks in the Stewart region is controversial. The basal Hazelton Group (Unuk River Formation; Table 3.2) on Bear River Ridge was inferred to be subaeriel (Grove, 1971 and 1972), due to a lack of invertebrate fossils. Alldrick (1983), based on absence of pillow structures and other submarine features, reached a similar conclusion. At the Big Missouri mine (Plete I) Galley (1981) proposed that the "lower strete" (volcaniclastic rocks and rhyolite) were subaeriel but "succeeding strata" (andesite) were deposited in a submarine environment after caldera subsidence (Galley's stratigraphy is inverted relative to other workers; Table 3.2). Smith (1977) proposed a submarine environment for andesite and argillite near Silbak Premier. The interbedded argillite, siltstone and minor greywacke (unit Hs) are unequivocally subaqueous but the overlying andesites (unit Hv) are less confidently interpreted to be subaqueous and marine. Higher in the stratigraphic column, mixed green and maroon volcanicl8stics (unit H, Hg, and Hm) are fluvial deposits 47 (Fig. 4.4). The amount of maroon relative to green volcaniclastic rocks increases up-section end to the east of Silbak Premier. This is interpreted to indicate the emergence of a subaerial stratavolcano. Although precise time constraints are lacking, this emergence marked by the influx of coarse maroon volcanic breccia end conglomerate, could correlate with a regional Early Jurassic (Sinemurian) regression (Fig. 2.2). Grove (1971) proposed that the mode of deposition for the majority of the volceniclastics was reworking (epiclastics). Schofield end Hanson (1922), Buddington (1929), Hanson (1935), and Galley (1981) preferred dominantly explosive, pyroclastic with minor epiclastic environments. Andesite and fragmental andesite are probably flows end flow breccia with little pyroclastic (Plinian-type) component. Rounded fragments, cross-bedding end channel fills in the mixed green and maroon volcaniclastic rocks indicate reworking (epiclastic) and fluvial deposition. The black tuff and greywacke unit represents ash flow tuffs that accumulated in a small, restricted reduced basin. Monitor rhyolite fiamme, glass shards, and breccia fragments are an indication of Hazelton Group explosive volcanism. 3.2.2 SPATSIZI GROUP (unit S) The Spatsizi Group is represented by distinctly well bedded, siliceous rocks (unit S; Plate I; Fig. 3.3 and 3.4) that outcrop east of the Monitor Lake rhyolite, as shown in the "Monitor Lake" stratigraphic section. They form normally graded, alternating light grey tuffaceous (N6) and dark grey siliceous shale (N3) bedss. Beds ere planar to undulatory and less than 5 cm thick (Fig. 3.3 and 3.4.a) and indicate an upright, east-facing sequence. Spherical (less than 0.1 mm in diameter) and elongate outlines of radiolaria (Fig. 3.4.b) are well preserved in the silicic shale (Fig. 3.4.a). Spherical forms with radial symmetry are spumellarians; elongate (conical) forms with axial symmetry end prominent apical spines are nassellarians (Haz and Boersme, 1981), probably Parvicinqule. which ere diverse and stratigraphically useful in the Middle Jurassic (Elizabeth Carter, written comm., Feb., 1987). The tuffaceous beds contain glass shard outlines, bubble well fragments, plagioclase microlites, and quartz fragments. Calcite patches ere concentrated along particular layers suggesting a syndepositional concentration in some beds. 48 The beds are correlated with regionally extensive Bajocian chert end shale (informally called the "pajama beds": Tipper, pers. comm., 1986). They correlate with part of the Quock Formation, Spatsizi Group (Thomson et el., 1986). The radiolaria-volcanic glass association links pyroclastic volcanism with contemporaneous marine deposition of the radiolaria-bearing strata in a low energy marine basin. Although unmineralized at Monitor Lake, there is potential for syngenetic, exhalative ore deposition within the unit 3.2.3 BOWSER LAKE GROUP The Middle to Late Jurassic Bowser Lake Group is regionally comprised of marine and locally, in the Klappan coal area, non-marine argillite, siltstone, sandstone and chert pebble conglomerate. Commonly the base of the Bowser Lake Group comprises chert pebble conglomerate. Isolated exposures of chert pebble conglomerate have been mapped beside the Granduc road near Tide Lake Flats (Dykes et al., 1984), possibly indicating basal Bowser sedimentation. 3.2.3.A Argillite, shale and siltstone (unit B) Argillite, fissile shale and siltstone (unit B; Plate 1) are exposed along the shores of Long Lake, and on Slate Mountain and Mount Dilworth. At Mount Dilworth, black tuff and argillite (units Hw and B) are separated by a minor steeply east-dipping bedding plane fault. Southeast of Monitor Lake, Bowser Lake Group bedding is subparallel to rhyolite tuff (unit Hr), but the contact is a bedding plane parallel fault. On Slate Mountain, Monitor rhyolite is absent and there is a marked structural discordance between black tuff (unit Hw) and overlying tightly folded sedimentary rocks (unit B). 49 Figure 3.3 View northeast to steeply northwest-dipping, well bedded "pajama beds" (station B-398, Plate 1; So = 240V75" NW; Sy = 235V80* NW). Outcrop is 800 m northeast of Monitor Lake, Stewart, northwestern British Columbia 50 Figure 3.4.a "Pajama beds", Spatsizi Group, with soft-sediment deformation. The radiolaria are concentrated in dark grey silicic shale beds and glass shards in pale grey tuffaceous beds from station B-385, Stewart, northwestern British Columbia; Plate 1. Figure 3.4.b Radiolaria ghosts in a thin section from "Pajama beds", Spatsizi Group (station B-399, Stewart, northwestern British Columbia; Plate 1). The conical form, 0.15 mm long, is a nassellarian. probably Parvicinqula (E. Carter, written comm., Feb., 1987), and spherical forms are spumellarians. 51 Thesedimentary rocks weather light (N6) to dark grey (N3). Siltstone is well bedded 8nd characterized by fine laminations, whereas shale and argillite are massive or intensely cleaved (fissile to weakly phyllitic). Rare limy beds, with centimetre to decimetre scale buff carbonate concretions in sh8le, and dismembered, brown iron-carbonate pods and chert pebbles in siltstone are characteristic. Millimetre scale laminated siltstone contains fresh detrital muscovite flakes (< 5%; < 1 mm long), with plagioclase and quartz fragments, along the west shore of Long Lake at station B-438 (Plate 1). Detrital muscovite is typical of the Skeena Group between Smithers and Terrace. Poorly preserved belemnoids 1.35 km southeast of Mount Dilworth suggest a possible Callovian and/or( ?) Bathonian age and correlation with the Bowser Lake Group, Ashman Formation for the unit (H. W. Tipper and R.G. Anderson, pers. comm., 1987). A muscovite K-Ar isotopic date could help to determine a potential muscovite provenance. A probable middle Bajocian fossil collection (Grove, 1971 ;GSC locality 86260, field no. BRR, Mount Dilworth; lat. 56*09' N, long. 130*02*30" W) was re-examined by T. P. Poulton (Paleontology Subdivision, GSC, Calgary). He noted a rich and diverse shallow marine bivalve assemblage. The collection could represent any part of the Middle Jurassic through Lower Oxfordian (Poulton, written comm., Feb., 1987). McConnell (1913) named these rocks the Nass Formation, and Grove (1971), Grove et al. (1982), and Alldrick (1987) continue to use the name. However, Ashman Formation, basal Bowser Lake Group, is a preferred name to establish regional correlation with the Spatsizi, Hazelton and Smithers map areas where more detailed stratigraphy and age control Is established. 3.3 INTRUSIVE ROCKS 3.3.1 TEXAS CREEK PLUTONIC SUITE The Texas Creek granodiorite (Buddington, 1929), related dykes, and "Premier porphyry" (Grove, 1971) comprise the Texas Creek plutonic suite. The Texas Creek granodiorite batholith is 10 km in diameter and has a medium grained, biotite-hornblende-plagioclase phyric granodiorite core with a narrow border phase of equigranular quartz diorite (Buddington, 1929;Grove, 1971 ;Smith,1977). The granodiorite contains accessory titanite and zircon. Chlorite and epidote alteration of mafic components and veinlets are characteristic. 52 Oreen dacitic feldspar porphyry (unit Jo): (1) hornblende-plagioclase (± K-feldspar) and (2) hornblende-plagioclase phyric porphyry are characteristic at Silbak Premier (Plate 1). The porphyry intrudes andesite (unit Hv) but not the structurally overlying green end maroon volcaniclastic rocks (units H, Hg, and Hm). The porphyry weathers shades of green (100 4/2) except adjacent to mineralization where it is commonly bleached pale yellowish green (10 OY 7/2). At higher structural levels it is locally maroon (moderate red: 5 R 3/6). The porphyritic rocks very from tabular bodies to irregular lenses, that intrude andesite below 2-level at Silbak Premier. At structurally higher levels the porphyry may be extrusive (Fig. 3.1 and Plate 2) due to its intimate association with maroon volcaniclastics and its parallelism with stratigraphy. Auoite porphyry is exposed east of Long Lake (unit Jap, Plate 1). It is massive with augite phenocrysts, up to 8 mm long, in a greyish green (505/2) aphanitic groundmass. Its affinity is uncertain (Plate 1 end 5), Dupes (1985) interpreted these rocks to be Middle Jurassic extrusive flows. Alldrick (pers. comm., 1987) was unsuccessful in extracting zircons for U-Pb isotopic dating. 3.3.2 HYDER PLUTONIC SUITE (unit Th) Hyder quartz monzonite (Smith, 1977) covers more than a seventy-five squsre kilometre area south of Silbak Premier around the town of Hyder, Alaska. It is fresh grey (light bluish grey: 5 B 6/1), massive medium grained, biotite-hornblende quartz monzonite with blocky fractures (unit Th; Plate 1 and Fig. 3.5). Several monzodiorite to quartz phyric granite dyke swarms (i.e. "Portland Canal", and "Premier"; Grove, 1971), are associated with this intrusive event. The Portland Canal quartz phyric granite dyke swarm has a more easterly-striking orientation than the southeast-striking, steep southwest-dipping swarm at Silbak Premier. It is more differentieted then the Silbak Premier biotite-hornblende quartz monzodiorite dyke swerm and is possibly slightly younger. The dykes cut the southern end of the Silbak 53 Premier property (Plates 1 and 3). The dyke swarms are outlined on Figure 3.32, the Portland Canal dykes crosscut the Big Missouri property and Salmon glacier valley, but they are not beside the Portland Canal. Figure 3.5 Typical massive, medium grained Hyder biotite-hornblende quartz monzodiorite with blocky fractures. Photo of U-Pb sample locality of R.6. Anderson on Silbak Premier road just above 6-level mill, Stewart, northwestern British Columbia (station AT-84-34-3; Plates 1 and 3). 54 3.4 METAMORPHISM Rock units of different composition end intensity of deformation display variable signs of metamorphism. Greenschist grade metamorphism of andesite and porphyritic dacite occur at the base of the stratigraphic column (Fig. 3.1); up-section and eastwards, the metamorphic grade decreases to subgreenschist in the Spatsizi and Bowser Lake Group sedimentary rxks (units S and B). Metamorphic mineral assemblages were identified with a petrographic microscope. Seven groundmass samples were analysed using XRD techniques to identify the fine groundmass minerals. Greenschist grade metamorphism is documented in Hazelton andesite (unit Hv), volcaniclastics (units H, Hm and Hg) and the Texas Creek plutonic suite (unit Jp). Secondary chlorite, carbonate, pyrite, sericite, epidote and leucoxene( ?) are common and indicate subamphibolite grade. The term "sericite" refers to any fine grained white mica. Minor epidote and absence of subgreenschist grade minerals indicate greenschist grade (Fig. 3.6 and 3.7). Fine grained tuff members of the green and maroon volcaniclastic rocks (units H, Hg and Hm) exhibit prominent phyllosilicate growth that produces a slaty cleavage to phyllitic cleavage. Lapilli tuff and breccia do not have a prominent phyllosilicate cleavage east of the Long Lake fault (Plate I). Galley (1981) noted fan-like aggregates of calcite, quartz and albite that were interpreted to be pseudomorphs after a zeolite, that he felt was probably mordenite. Prehnite identified in the black tuff (unit Hw), is a diagnostic subgreenschist grade mineral (Winkler, 1979). Structurally overlying units are probably subgreenschist grade but are not the suitable composition to form diagnostic minerals. Pajama beds (unit S) of the Spatsizi Group are dominated by a subgreenschist or greenschist grade assemblage consisting of quartz-albite-clay minerals or carbonate-sericite-chlorite ± clay minerals ± albite. Carbonate occurs as blebs or patches. 55 Pelitic Bowser Lake Group sedimentary rocks show little evidence of regional metamorphism. Fresh detrital muscovite flakes (< 2 mm long) show no signs of retrograde alteration. Phyllosilicate growth is minor, restricted to areas of weak pressure solution cleavage and patches of chlorite. Grove (1971) identified possible andalusiteon Slate Mountain; however, pseudo-porphyroblasts found there in argillite were detrital quartz and lithic fragments. Therefore, Slate Mountain is subgreenschist, not amphibolite grade. Tertiary intrusions are unmetamorphosed. Local alteration occurs in fracture zones as chlorite, sericite, and clay retrograde alteration of plagioclase and mafic constituents. PT conditions and timing of metamorphism are only qualitatively estimated. The absence of lawsonite suggests pressures were less than 3 kbars (Fig. 3.6). Temperatures could have varied from 200-500°C, the range of subgreenschist to greenschist grade (Fig. 3.6; Winkler, 1979). Textural evidence indicates syndeformational mineral growth (Fig. 3.8): sericite, quartz, and chlorite have grown in asymmetrical pressure shadows around euhedral pyrite cubes and in pyrite pull-aparts. Commonly sericite and quartz crystals are curved. Timing of metamorphism and deformation is post-Bathonian and pre-Eocene. The Bowser Lake Group (Ashman Formation) is tectonized but Tertiary (Eocene) dykes and stocks are undeformed and unmetamorphosed. A Middle Cretaceous (Albian) thermal event is suggested from K-Ar and Rb-Sr isotopic studies (Chapter 6 and Alldrick et al, 1987a). It is not known whether deformation observed in the region is Cretaceous or Late Jurassic. Further research could include microprobe and XRD identification of the phyllosilicates to improve metamorphic pressure and temperature estimates. Isotopic dating by the 4 0 Ar/ 3 9 Ar technique is planned by R.G. Anderson that may elucidate more details of the thermal history. 56 Figure 3.6 Pressure-temperature diagram illustrating metamorphic grades (Ps = P H20) modified from Winkler, 1979, p. 65 and 70), fine stippled (shaded) area is expected field for metamorphic event. (1) zeolite facies, (2) laumontite-prehnite facies, (3) pumpellyite-prehnite facies, and (4) lawsonite-elbite facies. Figure 3.7 The albite-actinolite-chlorite zone of greenschist grade metamorphism shown on ACF diagram (modified from Winkler, 1979). Most rocks from units Hv, H, Hm and Hg would plot in the stippled and shaded fields. 57 1.0 mm Figure 3.8 Rotated euhedral pyrite crystal with quarts, chlorite, and sericite pressure shadow indicating apparent sinistral sense of shear. Large pressure shadow relative to the pyrite crystal indicates significant strain. Planar contact of chlorite (toned area) with quartz suggests chlorite broke from pyrite then the void was filled by quartz. Sample is from DDH 81-26 at 96.7 m depth, Stewart, northwestern British Columbia (Fig. P-3). 58 3.5 STRUCTURAL STYLE 3.5.1 INTRODUCTION The Salmon River Valley is underlain by rocks that exhibit diverse fabrics and structures produced by at least four phases of pre-Eocene deformation. The aim of this chapter is to document these structures and attempt to distinguish phases of deformation. Regional strike is northerly, and dip and facing direction are generally east. This is based on measured bedding attitudes with normal graded bedding and cut and fill marks, but around Silbak Premier deposit stratigraphy dips moderately to the northwest. Structural style and fabrics are lithology dependent. Styles include disharmonic folds, ductile shear zones, cleavage, and brittle faults. Bowser Lake Group sedimentary rocks and shale-argil lite-siltstone within Hazelton Group volcanic rocks are isoclinally to tightly, disharmonically folded with prominent spaced cleavage and continuous cleavages (phase 1 and 2). Competent Monitor rhyolite breccia, is deformed into map-scale phase 3 open folds with moderate north-northwest-plunging fold axes. Andesite exhibits a moderate northwest-dipping chloritic foliation. Shear zones, cleavages, phase 4 pencil lineation end en-echelon extensional quart2 veins have consistent orientations. Discrete narrow east-west striking, steeply-dipping to vertical highly strained zones crosscut the area. Tertiary transtensional features include brittle, in part transcurrent faults, extensional fractures filled with Eocene felsic to intermediate dyke swarms, and extensional fractures filled by 01 igocene-Miocene mafic dykes. Minor structures, mineral fabrics and microfabric analysis establish the multiphase structural style. The study area is divided into three structural domains, each of which have consistent planar and linear structures, and folds (Table 3.3). This section defines the phases of deformation, structural domains and describes in detail structural features for each domain of the Salmon River Valley and Bear River Ridge area Non-genetic morphologically-based rock cleavage names used here are "continuous" or "spaced cleavage" 59 (Powell, 1979). "Continuous cleavage" results from continuous parallelism of platy minerals throughout the rock (Dennis, 1972) or from cleavage with domains spaced closer than 0.01 mm (Powell, 1979). Spaced cleavage refers to a cleavage spaced at finite intervals; a lower spacing limit of 0.01 mm and an upper limit of less than 10 cm is adopted as proposed by Powell (1979). 3.5.2 PHASES OF DEFORMATION The four phases of deformation, from oldest to youngest, ere defined by: (1) moderate west-plunging recumbent folds, (2) north-plunging inclined folds, (3) north-plunging upright folds, and (4) moderate west-plunging pencil lineations. Only the last two phases are traceable from the north to east domain. North-striking, steeply west-dipping and east-striking, steeply dipping to vertical mylonite end zones of flattening display a dextral sense of motion. 3.5.3 STRUCTURAL DOMAIN SUMMARIES Three domains (Fig. 3.9) were selected where detailed structural data defined characteristic folds, cleavage, and lineations. Coplanar and collinear fabrics link domains and suggest they ere elements of at least four phase structural history. The domains defined in Figure 3.9 and described in detail below are: north, east and Silbak domains. Not every phase is evident in a particular domain (Table 3.3). The north end east domains ere separated by the major north-striking, steeply west-dipping dextral transcurrent Long Lake-Fish Creek fault zone (Fig. 3.9 and 3.32). The area between the north and Silbak domains is poorly exposed and was not mapped in detail. INSERT A Phase 1: A X P - 1 2 2 / 2 8 ° S W Phase 2T AXP=215 /30°NW ,6> Phase 3: SE-verging folds EAST V DOMAIN SE-overturned folds :2-LEVEL# GLORY HOLE SILBAK DOMAIN 35. m 25 B-449 2 km south PHASE 1 Recumbent folds PHASE 2 fnclined folds } PHASE 3 NNW plunging, ^ l ' upright folds PHASE 4 2g-*— Pencil lineation UNKNOWN PHASE = Ductile shear zones 0 1 2 Kilometres Figure 3.9 Structural domains (north, east, and Silbak), structural styles and deformational phases of Long Lake, Bear River Ridge, and Silbak Premier areas, Stewart, northwestern British Columbia. 61 Table 3.3 Structural elements for Bear River Ridge end Salmon River Yalley area, Stewart, northwestern British Columbia Abbrevietions: FAX = fold axis, N.R. = not recognized. Symbols: V , = axial, \ = dyke, and \ = joints. DOMAIN PHASE 1 PHASE 2 PHASE 3 PHASE 4 UNCERTAIN (DUCTILE) LATE (BRITTLE) brittle faults North recumbent folds TFVJY 3<«\ north-plunging inclined f \ow north-plunging upright east-west warping East not recognized (N.R.) N.R. 28 k north-northwest plunging upright pencil lineations brittle faults Silbak N.R. northwest-dipping foliation southeast-overturned folds N.R. pencils, joints, quartz veins east-west folds v _ east-west shears Hyder dykes/ joints W 75 \)35 N Outside domains E-WtoSE-NW (Riverside mine) N-S (Long Lake fault) 62 3.5.4 NORTH DOMAIN The north domain encompasses a four square kilometre area, west of the Long Lake fault, extending north from Slate Mountain to Fetter Lake (Fig. 3.9). Several folding styles end cleavage orientations developed in Bowser Lake sedimentary rocks indicate at least three phases of deformation. The first phase produced tight to isoclinal disharmonic recumbent folds. The second phase formed open folds with shallow northwest-dipping axial planar cleavage. The third phase formed an upright, shallow north-plunging synclinorium. Although it is apparent from the map pattern (Fig. 3.9 and Plate 1), no minor folds or axial planar cleavage are associated with the synclinorium. It was recognized as a regional doubly-plunging structure by Grove (1971), and Alldrick (1985). The study area included the north-plunging, southern end of this synclinorium. Bedding orientations vary (Fig. 3.10 a) but commonly are moderately north-dipping and crudely indicate the broad shallow north-plunging syncline. Cleavage orientations are variable but cluster around an average orientation of 185°/30°W (Fig. 3.10 b). Cleavage may have been openly folded about an east-trending axis during phase 4 (?) deformation. Phase 4 pencil lineations range from 235V39* to 280V20*, crenulation fold axes (Ls2) everage 262V10* but are veriable (Fig. 3.10 c). Slate Mountain area folds, in the north domain, observed by the author, R.G. Anderson (pers. comm., 1987) and Grove (1971) are complex and disharmonic. On Slate Mountain small-scale open to isoclinal folds display an axial planar cleavage. This cleavage is sub-parallel to the foliation in underlying Hazelton rocks (Read, 1979; Grove, 1971). At least two other cleavage sets are evident. Recumbent folds in siltstone beds (unit B, Plate I) along the penstock from Long Lake dam to Big Missouri powerhouse have about 2 m fold amplitudes and 0.5 m wavelengths. Recumbent isoclinal folds in the Bowser Lake Group on Troy Ridge indicate southeast vergence (Grove, 1971, Plate XI). Phase 1 tight to isoclinal moderate west-plunging recumbent folds on Slate Mountain are refolded, indicating a second phase of deformation (R.G. Anderson, pers. comm., 1987; Fig. 3.11 and 3.12). The younger, more open north-plunging inclined folds have a moderate northwest-dipping disjunctive axial planar cleavage(Fig. 3.12). Structural continuity on Slate Mountain and elsewhere in the Bowser Lake Group is difficult to establish due to the 63 lack of marker beds, and inferred detachments. Underlying Hazelton volcanic rocks are relatively massive, structurally disconformable with Bowser Lake sedimentary rocks. The youngest ductile structure, an easterly-trending warping is locally evident on the Big Missouri property. A northwest-trending foliation becomes eesterly-striking (Read, 1979). Cleavage locally is openly warped about an easterly-striking axis on Slate Mountain. This late warping may correlate with the development to phase 4 pencil lineations. Regional structural doming of Hazelton volcanics over Texas Creek batholith to produce a broad arch with a gently west-plunging axis (Smith, 1977) is not evident in structures in the Silbak Premier area. BEDDING (So) n - 46 64 N FOLIATIONS L " n - 78 n - 8 n - 9 L s 2 n = 13 Figure 3.10 North domain equal area stereonet plots of: (a) poles to bedding (So), (b) foliations (Si), and (c) lineations (L) for first and second phase of deformation. Abbreviations: Lp = pencil lineation, L si = bedding and S-surface intersection lineation, and Ls2 = crenulation fold axes. 65 Figure 3.11 View northwest to folded argillite on Slate Mountein illustrating two phases of orthogonal deformation: F1 = tight inclined folds with amplitudes greater than 20 cm and wavelenghts of about 10 cm (FAX = 255V25* but variable, AXP = 122 V28* SW). F2 = open inclined folds, FAX = 349V16* and AXP = 215V30* NW (Fig. 3.12). Photo and data from R.6. Anderson at station AT-84-66,3 m below station B-486, north domain, Stewart, northwestern British Columbia (Plate 1). 66 West East V / Figure 3.12 Close-up view north to F2 gentle, inclined fold with irregular, axial planar, spaced disjunctive cleavage (SI = 215°/30° NW; unit B; station B-486, north domain, Stewart northwestern, British Columbia, Plate 1). Flexural slip mechanism is indicated by slickensides on bedding planes, normal to fold axis, minimum fold amplitude is about 0.5 m and wavelengh of 2-3 m. Hammer handle points down plunge of fold axis (B FAX = 349°/23°). 67 3.5.5 EAST DOMAIN The east domain is characterized by well exposed phase 3 gently north-northwest-plunging anticline/syncline peirs, as defined by bedding measurements and mesoscopic folds. Phase I and 2 struotwnw are not ovidont. The domain oonsiets of a ten square kilometre area, east of Long Lake-Fish Creek fault zone, from Long Lake south five kilometres elong the west slope of Bear River ridge (Fig. 3.9). Folds in Monitor rhyolite and Spatsizi-Bowser Lake Group sediments are structurally conformable (i.e. characterized by parallel bedding end coaxial fold axes), in contrast to Slate Mountain, in the north domain. Bedding plane intersections define a moderate north-northwest plunging p-fold axis orientation (000V40*; Fig 3.13 a) as earlier described. The synclinorium is collinear with shallow to moderately north-northwesterly plunging mesoscopic fold3 (Fig. 3.13 c; average FAX (Lsi) = 005V25*). There is no evident axial planar cleavage associated with the synclinorium. The synclinorium is correlated with Slate Mountain phase 3 syncline based on similar open fold style and orientation. Cleavage orientations are variable (Fig. 3.13 b). Phase 3 east-verging asymmetric folds ere well displayed in pajama bed siliceous shsle and tuff (unit S; Plate 1 and 3.14). Macroscopic asymmetric, east-verging folds are exposed on the west limb of a the broad phase 3 synclinorium in the east domain (Plates 1 and 2 and Fig. 3.9). The tight folds, with amplitudes less than 10 metres, have no axial planar cleavage. Recumbent isoclinal structures in the Bowser Lake Group on Troy Ridge indicate southeast vergence (Grove, 1971; Plate XI). Phase 3 moderate north-northwest-plunging folds in Bowser Lake sedimentary rocks and Monitor  rhyolite are prominent in the north and east domains. Open folds in rhyolite (unit Hr, Plate 2), 1.5 kilometres southeast of Monitor Lake, plunge gently north-northwest with steep axial planes. The minor folds are on the limb of a related major synclinorium involving Hazelton and Bowser Lake Group rocks (Fig. 3.9). Open to tight folds deform interbedded argillite, siltstone, and shale (Fig. 3.15 and 3.16 and 3.17), and Monitor rhyolite is interpreted to be more openly folded (Plate 2). The orientation of these folds suggests they could be related to the north-striking dextral transcurrent Long Lake-Fish Creek fault system. 68 The Hazelton-Bowser Lake Group contact changes attitude but is a link between north and east domains. Along the west flank of Mount Dilworth, it is a steep easterly-dipping bedding plane parallel fault with limited displacement South from Mount Dilworth to Fetter Lake the contact remains steep (Plates 1 and 2) but near Fetter Lake, the contact follows topography and defines a broad bend which suggests it either flattens, or is steep and warped eastward. Between Fetter Lake and Slate Mountain it is steeply east-dipping; but at the southern end of Slate Mountain, the contact is shallow north-dipping and cut by a steep northeast-striking fault (Plates 1 and 2). According to Grove (1971) on Slate Mountain, cleavage and Hazelton-Bowser Lake (locally Spatsizi) Group contact are coplanar and almost horizontal, cutting steep axial planes of minor folds. This is apparent only at Fetter Lake and the south end of Slate Mountain. Competency contrast between Bowser Lake Group shale and siltstone and massive Hazelton Group volcanic rocks in the north domain is manifested in a complex, multiphase structural style in Bowser Lake Group, inferred open folds in Hazelton Group, and a decollement at the contact between Bowser Lake Group and Hazelton Group black tuff (unit Hw; Plates 1 and 2). East of Monitor Lake, in the east domain, the contact is openly folded into a series of phase 3 gently north-northwest-plunging folds but layering and structural style above and below the contact are concordant (Fig. 3.17). These folds form the west limb of an inferred broad synclinorium, that lies to the northeast, and that is cored by Bowser Lake sedimentary rocks and outlined by Monitor Rhyolite (Fig. 3.9 and 3.17). (a) N BEDDING (So) 69 T Figure 3.13 East domain equal area stereonet plots of poles to: (a) bedding(So), (b) foliations (SO, and (c)lineations (L) for first and second phase of deformation. Abbreviations: Lp = pencil lineation, L si = bedding and S-surface intersection lineation, and Ls2 = crenulation fold axes. (Stewart, northwestern British Columbia). 70 Figure 3.14 Yiew northeast to "pajama beds", Spatsizi Group (unit S; Plate 1), phase 3 east-verging folds. Tight folds with angular closures have about 3:1 long to short limb ratios (station B-399, Plate 1, east domain, Stewart, northwestern British Columbia). Beta fold axes vary from 352°/22° to 358°/25° and axial planes from 210°/55° NW to 2300/80° NW. 71 Figure 3.15 View south to phase 3 tight, subhorizontal, steeply inclined Bowser Lake Croup argillite and siltstone fold (unit B; station B-386, east domain, Stewart, northwestern British Columbia, Plate 1). Slightly asymmetrical, flexural slip fold (steep west and moderate east limbs indicating anticline to the east and syncline to the west). Horizontal fold axis trends northerly and axial plane dips steeply (70*) westerly. 72 Figure 3.16. a (left) View northeast down-plunge of phase 3 tight flexurel slip and flexural flow folds with angular closures developed in Bowser Lake Group argillite and carbonate-rich siltstone (unit B; station B-385, east domain, Stewart, northwestern British Columbia, Plate 1; AXP = 240V80'NW, FAX = 040V45"). 3.16. b (right) Yiew north to phase 3 tight to open flexural flow folds with attenuated limbs and thickened carbonate-rich hinge zones (unit B; station B-385, east domain, Stewart, northwestern British Columbia, Plate 1; AXP = 200V75* NW, FAX = 015V40*). 73 Figure 3.17 View southeast, from station B-394 toward B-385 (Plate 1), to phase 3 folded and faulted Hazelton-Bowser Lake Group contact exposed along the west flank of Bear River ridge (east domain, Stewart, northwestern British Columbia; Fig. 3.9). Bowser Lake sedimentary rxks (unit B; Plate 1) are concordant with underlying Monitor rhyolite (unit Hr) of the Hazelton Group. 74 3.5.6 SILBAK DOMAIN The Silbak domain is characterized by phase 4 pencil lineation, quartz veins and joints (Fig. 3.19,3.20 end 3.21), and noteworthy phase 4 easterly-striking, steeply-dipping planar fabrics. Silbak domain includes the seven and one-half square kilometre area including the mine area north to Lesley Creek and east to the Long Lake fault shown in the 1:2,500 scale map (Plate 3 end Fig. 3.9). The domain lacks stratigraphic markers; shows phyllitic to spaced cleavage, that could have been a product of phase 2 deformation, and fractures. There are four foliation populations (Fig. 3.18): (1) 197*752* NW,(2) 133V58" SE, (3) 010V82* E, and (4) 090V75* S. Set (1) has a similar trend but is steeper-dipping then sets (2) end (3) in the east domain. Volcaniclastic rocks (unit H) become progressively less deformed eastward of the Long Lake fault zone (east of Silbak Premier domain), except for narrow steep easterly-striking high strain zones (set 4 in Fig. 3.18). These zones contain flattened clasts (Fig. 3.30). Bedding attitudes at Silbak Premier are difficult to establish, controversial and critical to interpretation of mineralization models. Greywacke and shale beds (unit 1 A, Plate 3) in Logan Creek are overturned to the southeast but the beds are faulted against the andesite stratigraphy. The Silbek Premier hillside was considered to be essentially a dip slope with northeast striking, moderate northwesterly-dipping stretigrephy (Burton, 1926). A'purple tuff" horizon (pert of unit 7 on Plates 3 and 5) that unconformably overlies andesite-andesite (unit 2 end 3 on Plate 3) was used by Langille (1945) to estimete a northerly-strike and a precise westerly-dip ("17" W"). The gross distribution of the volcaniclastics (unit 7, Plate 3) indicates a steeper, northwesterly-dip, as indicated by drill core data Porphyry sills (unit Jp on Plate 1 , and unit 4 and 4A on Plate 3) or andesitic volcaniclastic rocks (unit Hv on Plate 1, and unit 2 and 3 on Plate 3) northwest of the Glory Hole indicate a moderate northwesterly dip (Wojdak, pers. comm., 1986; Fig. 3.19. a and Plate 2). Some beds exhibit this northwest-dipping attitude (Plate 3). However, other isolated measured bedding attitudes (Fig. 3.19. a and Plate 3) are variable and steep. Southeast overturned bedding appears common (Figure 3.19. e), however, all ten overturned attitudes were collected at one outcrop. 75 All lineations (pencil, crenulation, minerel, elongate fragments, and fold axes; Fig. 3.20. b) plunge moderately to the west, the average pencil attitude is 260V30*. Fault attitudes ere distributed along a girdle but they form five crude clusters (Fig. 3.20. a): (1) 176V70' W, (2) 243V764 NW, (3)132°/68° SW, (4) 349V80* NE, and (5) 080°/80° S. Hyder dykes display a consistent steep southwest-dipping orientation (138V76* SW; Fig. 3.20. c). Extensional quart? veins dip moderately to the northeast (353V65' NE; Fig. 3.19. c). Fractures end joints display three prominent attitudes (Fig. 3.19. b): (1) the main 346V56* NE group, (2) 144V68* SW, end (3) a less defined 232V64* NW group. Phase 1 (?) south-southeast to southeast-overturned folds ere exposed in two locations on the Grenduc road: 1250 m south of the Westmin camp near Logan Creek and the international border (Plate 3), and north of the Silbak domain, below Consolidated Silver Butte property (Plate 1). Phyllite and slaty argillte (unit Hs) are deformed into tight to isoclinal folds with angular closures, amplitudes up to 10 m, gentle east- or west-plunging fold axes, and moderate to steep southerly-dipping axial planes. Tops, determined by flame structures, cut and fill, and load casts, indicate bedding and cleavage (axial planar) is overturned to the southeast. Smith (1977) recognized similar structures in the seme area. Syncline/anticline pairs, inferred from bedding-cleavage intersections ere disrupted by north-striking, steep west-dipping faults. The moderately northwest-dipping foliation is a variably developed penetrative phyllosilicate foliation in the Silbak domain developed in andesitic volcanic rocks (unit Hv). The phase I southeast-overturned folds and northwest-dipping foliation are interpreted, by similarities in orientation of planar structures, to be related to the same period of southeasterly-directed compression. The foliation is syn-metamorphic. Linking phase 1 structures between the north and Silbak domains is tenuous because of the Slate Mountein decollement. 76 FOLIATIONS Foliation sets: 1 197°/52°NW 2 = 133°/58°SE 3 = 010°/82°E 4 - 090°/75°S Chlorite n = 27 Mineral n = 13 Flattening n = 2 Undifferentiated n * 252 Spaced cleavage n 3 16 Continuous cleavage n • 7 Figure 3.18 Silbak domain equal 8rea stereonet plot of poles to foliations, Stewart, northwestern British Columbia. 77 • n = 16 • Overturned n - 10 Figure 3.19 Silbak domain equal arestereonet plots of: (a) poles to bedding, (b) joints, and(c) extensional quartz veins, Stewart, northwestern British Columbia. 78 FAULT PLANES n - 191 LINEATIONS • L p n - 30 • ^(Calculated) n - 7 A L F R A G M E N T S n - 6 • L SO FAX " " 3 ^MINERAL " " 2 • UNDIFFERENTIATED n - 10 * ANDESITE n - 8 • MONZONITE n • 6 a RHYOLITE n - 3 O BASALT n - 1 Average attitude 138°/76°SW Figure 3.20 Silbak domain equal area stereonet plots of: (a) poles to fault planes, (b) lineations and (c) poles to dykes, Stewart, northwestern British Columbia. 79 Phase 4 west-plunging pencil lineation is the most prominent 8nd consistent structural feature of all three domains and the region. The intersection of two S-surfaces define individual pencils of various shapes and si2es, most are 2-10 cm in width and height with variable interface angles, and planar to irregular faces. Both S-surfaces consist of cleavage and/or phyllosilicate foliation. The lineation, averaging 260*/35* (Fig. 3.20.b) is developed consistently in Hazelton Group, especially inaphanitic andesite (Silbak domain, Fig. 3.21), Monitor rhyolite (east domain, Fig. 3.22.a) and Bowser Lake Group sedimentary rocks (North domain, Fig. 3.23) despite variations in the orientations of S-surfaces. Elongation axes in strained Monitor rhyolite siliceous fragments (unit Hr; Fig. 3.22.b), volcaniclastic clBSts (unit H), the chlorite-biotite mineral lineation (Fig. 3.30), and some phyllosilicate crenulation fold axes are coaxial with the pencil lineation. The consistent orientation of the pencil lineation indicates that it is not refolded. Thus it must be a product of later deformation, possibly associated with an inferred phase 4 easterly-directed contraction. Figure 3.21 View northwest to prominent phase 4 pencil lineation in Hazelton Group andesite (unit Hv, Plate 1; station B-332, Silbak domain, Stewart, northwestern British Columbia, unit 2, Plate 3) formed by two intersecting spaced cleavages. Figure 3.22. a Yiew south to phase 4 pencil lineations developed in Monitor rhyolite tuff East domain, Stewart, northwestern British Columbia (Lp = 270*/25", poorly developed Si = 195V40* NW; station B-397, Plate 1). Figure 3.22. b Yiew east to phase 4 elongate Monitor rhyolite fragments on a foliation and bedding plane (Lp = 260V45" W; So = Si = 195V55* W; unit Hr; station B-393, East domain, Stewart, northwestern British Columbia, Plate 1). 81 Figure 3.23 View west to intersecting continuous (Si = 185V25* W) and spaced disjunctive cleavage (Sy - 215V30* NW), producing phase 4 pencil lineation (Lp = 265V25") in Bowser Lake Group siltstone (unit B, station B-472, North domain, Stewart, northwestern British Columbia, Fig. P-1). 82 Phase 4( ?) moderate west to north-northwest-dippino strained volcaniclastic rxks and moderate  westerlv-dippino foliations occur in Silbak domain. Narrow west to west-northwest dipping phyllitic chlorite-hematite zones (Si = 170/45" W) with flattened and elongated L-S tectonite, volcaniclastic rocks (unit H, Hm and Hg; Fig. 3.24 to 27) are exposed along the Big Missouri road at Lesley Creek (station B-96 and B-380; Plate 3). Elongated heterolithic lapilli to block size fragments have length to width ratios of 6:1 to 10:1. The elongation direction is coaxial with the pencil lineation (Lp = 245V20' to260V45#). The fabric could be the result of primary flattening during welding of hot pyroclastic flows or of tectonic shearing or both. Evidence for a tectonic cause is threefold: fragment elongation is collineer with the pencil lineation and is areally consistent, and phyllosilicate foliation in nearby andesite (unit Hv) is sub-parallel to the plane of flattening in strained volcaniclastics; it is only locally parallel to bedding (Plete 3). The heterolithic epiclastic rock units would probably be cold during deposition. Fragment asymmetry indicates elements of eastward tectonic transport (west hanging wall over east footwall). Therefore the textures indicate easterly-directed thrusting, probably on a regional scale, as suggested by R.G. Anderson (pers. comm., 1985) and Gunning (1986). In contrast to a tectonic origin, Galley (1961) advocated primary bedding with a parallel foliation marked by partly-flattened pumice 8nd elongated lithic fragments. He noted two tectonic foliations 30-50" to each other, with "S2 subparallel to primary compositional layering but with a steeper dip"; a third foliation (S3 = 070V70" SE) warps So and Si. These observations we conflicting and indicate that detailed structural mapping is required. 83 Narrow, less than 10Q m wide, zones of chlorite-sericite phyllite and schist (Plate 3) mark shear zones or thrust faults. Porphyritic dacite (unit Jp, Fig. or unit 4A, Plate 3); for example, can be traced into a 100 m-wide zone of chlorite-sericite schist along Lesley Creek (Plate 3). The north-striking, shallow westerly-dipping foliation near Big Missouri was interpreted to be a zone of cataclasis by Grove (1971). Late phase 4 extensional quartz veins and joints in the Silbak domain are en echelon crystalline, milky quartz veins up to 1 m thick that crosscut andesite (unit Hv). The veins and parallel planar joints (Fig. 3.28) have a consistent moderate to steep northeast-dipping attitude (330-345V50-75* NE; Fig. 3.19.b and 3.19.c). Rare sigmoidal quartz veins indicate local contemporaneous shearing. The veins are syntaxial growth type (new fibre growth elong central axis of a vein; Durney and Ramsay, 1973), indicated by quartz crystals oriented perpendicular to vein walls, and the absence of country rock fragments within the veins. They are interpreted to have formed during pressure solution deformation. The veins are approximately perpendicular to the pencil lineation and elongation direction; which implies they formed in the same deformational event. 84 Figure 3.24 Yiew southeast to tectonically strained heterolithic volcaniclastic fragments along Big Missouri road, Silbak domain, Stewart, northwestern British Columbia (unit Hw on Plate 1 or unit 7 on Plate 3; station B-380; foliation = 180°/30° NYV). Fragment asymmetry and pulled-apart fragment are compatible with easterly-directed thrusting. Figure 3.25 Flattened felsic lapilli in strained tuff at station B-97, Silbak oomain, Stewart, northwestern British Columbia, unit 7, Plate 3. Large (5 mm wide) euhedral pyrite cube in top left corner with asymmetric pressure shadow filled with white mineral and quartz, indicates apparent dextral sense of shear. Figures 3.26 and 3.27 are photomicrographs from this hand specimen. 85 1 mm Figure 3.26 Plagioclase phenocryst brittly deformed in a ductile groundmass (sample B-97A; Plate 3, Silbak domain, Stewart, northwestern British Columbia). Prominent chlorite-sericite foliation and antithetic displacement of plagixlase suggests an apparent dextral sense of sheer. Interpretation of kinematic indicators in camera lucida sketch modified from Simpson and Schmid (1983). Quartz, in left corner, exhibits extension compatible with dextral sense of shear. Samples from Figures 3.26 and 3.27 were not oriented in the field, therefore their true sense of shear is indeterminate. Note that an alternate interpretation could be made if the sense of shear is opposite to that shown in plagixlase. Abbreviations: ch = chlorite and ser = sericite. 86 1 mm Figure 3.27 C-S fabric developed in volcaniclastic rock (unit 5, station B-97, Plate 3, Silbak domain, Stewart, northwestern British Columbia) illustrating an apparent dextral sense of shear by the C (shear surface; "cisaillement") and S (schistosity; "schistosite") intersection (after Simpson and Schmid, 1983). 87 Silbak Premier mineralization orientation and style of deformation Stopes at Silbak Premier display consistent planar attitudes, possibly offset by faults. At the mine scale, ore zones trend northeast and northwest (Chapter 4), and intersect et right angles. Individuel ore shoots plunge moderately to steeply westerly to northwesterly. Some are col linear with the pencil lineation and elongated volcanic fragments. Vuggy open-spaces end symmetrically sulphide-rimmed fragments within siliceous breccia indicate most parts of the ore zones are undeformed. Siliceous breccia is competent with no internal fabric other then pre-existing foliations displayed in host rock fragments. It deformed brittly, in contrast to the sulphide-dominated zones that flowed and developed layering. Locally, a prominent easterly-striking, steeply-dipping layering is developed in sulphide-rich zones (Chapter 4). The layering is believed to be e tectonic fabric correlated with other easterly-striking structures (e.g. mineral foliations and strained volcaniclastics). A secondary group of narrow quartz-sphalerite-galena veins strike northeast and dip steeply southeasterly. Superimposed on these older structures are north-striking brittle faults with horizontal slickensides that offset "west zone" mineralization to the north, indicating a dextrel sense of displacement. These faults are sub-parallel to and presumably related to the Long Lake-Fish Creek fault system (Fig. 3.32). 88 Figure 3.28 View southeast to joints (000V40' E) and penetrative slaty cleavage (Si = 220V55* NW) in Hazelton Group andesite 150 m north of 4-level mill along the Silbak Premier road (outcrop is 2.5 m high; Plate 3, Silbak domain, Stewart, northwestern British Columbia). 89 3.5.7 PHASE ? STEEPLY DIPPING EASTERLY-STRIKING MYLONITE AND FLATTENING FABRIC Several steeply dipping easterly-striking zones of intense deformation occur in the region. They exhibit synmetarnorphic ductile fabrics end younger brittle features. Commonly they are narrow zones, less than 2 m wide, but in the Riverside mine area mylonites are exceptionally wide, over 500 metres in width. Riverside mvlonite zones in Texas Creek granodiorite ere characterized by chlorite and biotite lineations (260V45*) lineations. Lineations 8re within steep southerly-dipping, easterly-striking mylonitic and foliated zones in the Texas Creek hornblende granodiorite. Outcrops displaying these features occur at the Riverside Mine and one to two kilometres to the south, along the Granduc road (Fig. 3.29). C-S fabrics, asymmetric augen, and broken quartz and plagioclase crystals indicate a dextral sense of shear on the mylonite zone. Quartz exhibits ductile end brittle features, therefore it was at the brittle-ductile transition, in a rather water-rich rock, if hydrous minerals like chlorite and biotite can be used as H2O indicators. Smith (1977) described two mylonite zones: a northern zone, 6.5 km long end less than 1.6 km wide, and a southern zone, about 9.7 km long by 4.8 km wide. He bracketed the age of deformation to be post-Texas Creek batholith but pre-Bowser Lake Group because he felt Bowser Lake rocks were not deformed. The mineral lineation is coaxial with the west-plunging pencil lineation and some crenulation-cleavage fold axes end is considered part of phsse 4 deformation since it is not folded. Bear River ridge easterly-striking, steeply-dipping flattening fabric zones in the maroon volcaniclastic rocks are parallel to the Riverside zones. The fabric is normal to bedding (L-S tectonite; unit H; Fig. 3.30). It is exposed along Bear River ridge, 0.5 km north of Mount Welker. The heterolithic clasts are flattened with length to width ratios up to 12:1 (Fig. 3.30). Plagioclase phenocrysts within clasts are aligned in the plane of flattening (Fig. 3.30.b), in contrast to flow aligned phenocrysts described in unit 7C (Chapter 4). The zones are included in the fourth phase because they ere not folded. 90 Other easterly-striking structures include vertical, tabular to lensoidal, layered sulphide-rich veins et 6-level, 2-level end the Olory Hole (Fig. 3.9). Some plagixlase porphyry-andesite contacts, base metal quartz veins, siliceous breccia zones, bleached alteration zones, strike-slip faults, hornblende and/or plagioclase foliation and cleavages at Silbak Premier trend easterly as well. The Long Lake-Fish Creek schist, mylonite and fault zone is north-striking, orthogonal to the east-striking mylonites. The zone had Late Jurassic and Tertiary movement along it according to Smith (1977). The Middle Jurassic movement formed catacl8site, schist and mylonite along the dextral strike-slip fault. Four kilometres of dextral offset is estimated from the displacement of the Texas Creek batholith-Hazelton Group contact which is exposed four kilometres south of Silbak Premier in Alaska (Smith, 1977). Mylonitic to foliated dacitic porphyry is exposed in an isolated outcrop elong Lesley Creek (about 780 m elevation; Fig. 3.31 and Plate 1). Its fabric strikes north and dips steeply west. Massive medium grained hornblende-plagixlase porphyry grades Into well foliated porphyry, this is interpreted to be mylonitized porphyry. 3.5.8 PRE-EOCENE BRITTLE FAULTS Some faults are older than the Eocene Hyder dykes. These include faults that: (1) bound siliceous breccia mineralization and layered sulphide, and (2) are contacts between altered and fresh rxk. Figure 3.29. a View east to transition from massive to well foliated, mylonitized Texas Creek granodiorite near Riverside Mine, Hyder, southeastern Alaska. Mylonitic foliation strikes easterly and dips steeply southerly. (Photo by R.G. Anderson; station AT-84-3-16; UTM Zone 9, 6204625 N 433185 E). Figure 3.29. b Photomicrogreph of mylonitic Texas Creek granodiorite, two kilometres south of Riverside mine, near Hyder, southeastern Alaska (station B-383, UTM Zone 9, 6204625 N 433185 E) with strained, polygonized quark, recrystallized groundmass, and foliated secondary biotite that was separated for K-Ar dating, see Chapter 6 (field of view is 4.25 mm wide). Figure 3.30. a Strained heterolithic volcaniclastic breccia where plagioclase phenocrysts are aligned in the plane of flattening Si = 270V70* S which is normal to bedding (not visible in photograph but measured at same outcrop); unit H; station B-449, Plate 1, on Bear River ridge, Stewart, northwestern British Columbia Length to width strain ratio is up to 10:1. Figure 3.30. b Yiew down 5\ pl8ne to intensely flattened maroon volcaniclastic rock (Si = 265V65* N). Bedding, not visible on the photograph, is at a high angle to Si (So • 135V65* SW; unit H, station B-449; on Beer River ridge, Stewart, northwestern British Columbia, Plate 1) and a chlorite lineation plunges-westerly ( chlorite lineation, not visible = 280V4S*). 93 Figure 3.31 View north to mylonitized dacitic hornblende-plagioclase porphyry, Texas Creek plutonic suite, along Lesley Creek between the east and Silbak domains, Stewart, northwestern British Columbia (unit Jp on Plate 1; station B-56; S\ = 175V70* W). 94 3.5.9 EOCENE TO PRESENT DYKE EMPLACEMENT AND BRITTLE FAULTING Eocene northeast-southwest crustal extension is reflected by the Hyder plutonic suite granodiorite and quartz phyric rhyolite dykes. They have a consistent attitude at Silbak Premier (138V76* SW; Plate 3 and Fig. 3.20). At least 30* Eocene northeast-southwest crustal extension is indicated over the one kilometre width of the Portland Canal leucogranite and microdiorite dyke swarm along the Salmon glacier (Plate 1 and Fig. 3.32). The change in width is related to depth of emplacement, host lithology end attitude relative to bedding. The Silbak Premier dyke swarm Indicates similar extension. The dyke swarm orientations suggest significant northeast-southwest crustal extension that is not related to north-striking dextral strike-slip faulting along the Long Lake-Fish Creek fault zone. Oliqocene to Miocene west-northwest-east-southeast crustal extension is represented by intrusion of north to northeast-striking mafic dyke swarms (Smith, 1973). In the Redcap Mountain 8rea, Hill (1984) postulated that east-west extension was due to uplift of the Coast Belt as mentioned In Chapter 2. Brittle faults are common, the regionally dominant groups are: a north-striking, steeply west dipping set and a moderate to steep northwest dipping set. The faults are characterized by fault breccia zones less than one metre wide (Fig. 3.33), clay gouge zones less than four centimetre thick, polished planar surfaces (Fig. 3.34), slickensides, and limonitic staining. Although stereonet plots show a scatter of points, detailed mapping suggests thst there are five predominant fault sets, most pronounced in Silbak domain Fig. 3.20.a): (1) the youngest, north-striking, steep west-dipping locally transcurrent set (176V70* W); (2) a southwest-striking, steeply dipping (243V68* NW) set; (3) a set oriented 132V68* SW which include some listric faults that crosscut and parallel Hyder dykes, (4) 349V80* NE oriented dip-slip slickensides along reactivated tension cracks, and (5) 080Vsteep southeast-dipping locally some are sinistral strike-slip faults that offset quartz veins and horizontal slickensides at the metre scale. The scatter of measurements (Fig. 3.20.e) is attributed to the curviplanar and listric nature of the faults and refraction in lithologies of contrasting competencies. A larger data set used by Piteau end Associates Engineering Ltd. yielded two additional fault sets: 200V65* NW, and 225V75* NW. 95 Physiography and drainage (e.g. Long Lake-Fish Creek, Cascade Creek and possibly Salmon River) are controlled by steeply west-dipping, north-striking curvi-planar faults whose average attitude is 175V75* W (Fig. 3.32). Kinematic indicators such as slickensides suggest horizontal and dip-slip motion. One kilometre of dextral post-Eocene movement is indicated by offset of the vertical Texas Creek-Hyder granodiorite contact (Smith, 1977). Grove (1971) estimated 450 m dextral displacement of the Hazelton-Bowser Lake Group contact At Silbek Premier, the Tertiary Hyder plutonic suite dykes ere not significantly offset within fault blreks; but across the northern extension of the Fish Creek fault at Long Lake and near Mount Welker there is about 1,400 m post-Eocene dextral offset of the dykes (Fig. 3.32). Fractures and minor faults within the main fault-bounded blocks cut the dykes with dextral movement limited to a few metres. Mineralization in the Glory Hole collapse area of Silbak Premier is right-laterally displaced more than 10 m. Faults oriented at 150-165V35-75' SW ere cut by the steeper north-south set. Secondary drainage and gullies follow these moderate dipping structures. Coplanar fractures and slaty cleavage suggest that some of these faults are reactivated cleavage planes (Fig. 3.18). Figure 3.32 Airphoto linears in the Salmon River Yalley-Bear River Ridge 8rea, Stewart, northwestern British Columbia. Rectangle outlines location of Plate 1 (11.5 km x 6.0 km). Dashed lines outline the approximate extent of Eocene dyke swarms and illustrates their estimated 1,400 m post-Eocene dextral offset across Long Lake-Fish Creek fault zone. 97 Figure 3.33 View south to major north-striking, steep east-dipping fault ("110 fault", set (1) of Fig. 3.19; Plate 4) cutting cleavage in andesite in the Glory Hole collapse, Silbak domain, Silbak Premier mine, Stewart, northwestern British Columbia 98 Figure 3.34 Yiew southwest to major steeply-dipping north-striking fault ("Cl fault", set (1) of Fig. 3.19; Plate 4) exposed in the Glory Hole collapse, Silbak domain, Silbak Premier mine, Stewart, northwestern British Columbia Extensional quartz veins are on the hanging wall of a chloritic "Glory Hole" fault zone (Plate 4; fault = 180V50* W), interpreted to be a thrust. Quartz vein attitude suggests easterly-directed motion on the "Glory Hole" fault, if veins and fault are related. 99 3.5.10 INCLUSION, REGIONAL SYNTHESIS, AND DEFORMATIONAL HISTORY Bowser Lake Group sediments are disharmontcally and isoclinally folded, whereas the structurally underlying Hazelton volcanics are broadly folded and sheered. The Hazelton-Bowser Lake Group contact is a tectonic break (decollement) which also follows a regional unconformity. A younger deformational event produced shallow north-northwest-plunging open folds snd east-west fabrics. Tertiary faults and dykes cut the region and Indicate significant, locally up to 30%, crustal extension. The complex brittle fault pattern evolved from Jurassic to present as regional stress fields changed and strain increased, the youngest faults are the most prominent Timing constraints for deformation are sparse. Phase 2 predates undeformed Eocene Hyder plutonic suite and postdates deformed Middle Jurassic Bowser Lake Group sedimentary rocks. Syn-metamorphic mineral growth in asymmetrical pressure shadows link greenschist grade metamorphism with phase 2 foliations. Based on reset K-Ar and Rb-Sr isotopic dates, a thermal event is estimeted to be Middle Cretaceous (Albian; Alldrick et al., 1987). Phase 3 easterly warping probably occurred in the Late Cretaceous. Eocene and Miocene extensional episodes are dated by their respective dyke swarms. 100 CHAPTER 4. SILBAK PREMIER GEOLOGY 4.1 INTRODUCTION The regional geological setting described in Chapter 3 establishes a framework for 1:2,500 and 1:500 scale surface mapping, drill core examination and underground mapping. Lithological and interpretational changes to Grove's (1971) Silbak Premier property map indicate the importance of 8 mappable maroon porphyry unit and suggests that deformation wss not as intense as advocated by Grove (1971). Results support original ideas by Buddington (1929) linking ore genesis with the Texas Creek plutonic suite. Rock units described in this chapter refer to the 1:2,500 map (Plate 3). The rock unit correlation chart (Plate 5) allows comparisons among regional scale (1:10,000) and property scale (1:2,500 and 1:500) rock units. 4.2 STRATIGRAPHY An important araillite-siltstone marker (unit 1) at Silbak Premier can be traced at least fifteen kilometres north along strike (D.J. Alldrick, pers. comm., 1985) and five kilometres south to the west slope of Bear River ridge (P.J. McGuigan, pers. comm., 1986). The grey argillite and siltstone unit is intercalated with dominant green andesite and minor porphyritic dacite. Argillite and siltstone outcrop on the Granduc road, 1,250 m south of the Westmin camp and 4 km northwest of Silbak Premier, below Consolidated Silver Butte. The sediments are structurally complex but at least 200 m thick, although the top 8nd base of the unit was not mapped. No argillite or siltstone is present in Silbak Premier mine workings. Contacts of these sediments with overlying andesite are sheared and fractured. Outcrops weather medium (N5) to dark grey (N3) 8nd rusty light brown (6 YR 5/6). Argillite is massive to fissile with laminated siltstone interbeds and locally pale grey cherty horizons. Normally graded bedding, exposed 100 m south of Logan Creek on the Granduc road, indicates stratigraphy is overturned to the southeast (Plate 3; also noted by Smith, 1977). Siliceous and carbonate-rich beds are apparently barren of radiolaria and conodonts. 101 Alldrick (1985) reported two argillite horizons. Unit 1 correlates with the upper argillite zone; the second argillite horizon is not recognized in the map area (Table 3.2). Shale, argillite and siltstone apparently collected in a small low energy basin during a period of volcenic quiescence. Finely disseminated pyrite (< 158) and the black shale indicate deposition in an anaerobic environment. The unit is more extensive near Consolidated Silver Butte than at Silbak Premier, implying the unit thins toward Silbak Premier. The Silbak Premier area may have been a Jurassic paleo-high, coinciding with the eastern-most exposures of the Texas Creek batholith. A well bedded shale and orevwacke sequence (unit 1 A) is markedly coarser than unit 1. Unit 1A is limited in areal extent, only about 75 m thick, and outcrops in Logan Creek at the Granduc road (Plate 3). South of Logan creek, bedding cleavage data suggest sedimentary rocks are folded and that andesite is structurally andstratigraphically above them (Plate 3). Medium (N5) grey to light brown (5 YR 5/6) outcrops comprise alternating black (N1) shale and fine-to medium-grained greywacke. The greywacke and shale are massive to faintly bedded. Thin, planar shale beds constitute approximately 15% of the exposure and are distinguished from thicker and irregular greywacke beds. Large bedded shale rip-up fragments (1.5 cm thick) are contained in thicker greywacke beds (Fig. 4.1). Normally graded bedding, 0.5 to 10 cm in thickness, consists of coarse to fine greywacke, to siltstone terminating in black shale. Shale-siltstone contacts are sharp and planar. Centimetre scale syndepositional growth faults are common in shale beds. Normal grading, flame and load structures indicate that beds are overturned to the southeast. Fine to medium grained plagioclase-rich greywacke groundmass supports angular shale rip-ups, and small (less than 1 cm long) subrounded quartz clasts. Clasts are extensively altered to carbonate and clay minerals, plagioclase is variably retrograde altered to albite with recrystellized quartz around some grains. Figure 4.1 Yiew south to interbedded black shale and greywacke turbidites in Logan Creek, Silbak Premier, Stewart, northwestern British Columbia (station B-86, Plate 3). Note shale rip-up clasts in greywacke (So = 215/70* NW). 103 The alterneting greywacke-shale beds are interpreted to be AE bourne turbidites (Walker, 1979): possibly analogous to Walker's (1979) mid-fan facies. The provenance is unknown. Qreenstone/andesite (unit 2) consists of greyish green (10 G 4/2) to greyish blue green (5 BG 5/2) weathering, massive and fine grained, aphanitic andesite (Fig. 3.21). Small (< 0.5 mm long) plagioclase and subordinate hornblende phenocrysts are rare. Disseminated euhedral pyrite (<3£; < 1 mm long) is ubiguitous and is an inferred propylitic alteretion product. Phyllitic chlorite-rich andesite predominates. Narrow zones, less than 25 m wide, of chlorite schist ere common. Andesite is common in the map area west of B.C. Silver portal and underlies most of the property (Plate 3). It is intruded by Texas Creek plutonic suite porphyries and is unconformably overlain by volcaniclastic and epiclastic rocks (units 5 to 7). Total stratigraphic thickness at Logan Creek is estimated to be over 750 m. Previous workers felt stratigraphy was west-dipping (McConnell ,1913; Schofield and Hanson, 1922). Isolated bedding attitudes measured in the volcaniclastic rocks (units 5 and 7; Plate 3) and gross map pattern indicate a moderate west- to northwest-dipping stratigraphy. These west and northwest attitudes contrast the predominantly east-dipping regional stratigraphy (Plates 1 and 2). Rare irregular chlorite- and calcite-filled amygdules, less than 5 mm in diameter, are evident in drill core. The least altered samples contain plagioclase and minor hornblende phenocrysts in an aphanitic groundmass. Pervasive carbonate, chlorite and clay propylitic alteration of primary phenocryst and groundmass minerals is characteristic and the most striking feature of the andesite around Silbak Premier. A foliation is defined by aligned chlorite and subparallel carbonate veinlets. Fragmental textures are common and characterize unit 3, andesitic breccia. Andesitic breccia (unit 3) is greyish green (5 6 5/2) fragmental andesite (a subdivision of the unit 2 andesite). It is recognized on the weathered surface by preferential weathering of the matrix compared to 104 more resistant fragments. Fragmental andesite consists of monolithic, angular to rounded matrix supported andesitic tuff, lapilli tuff and breccia fragments. Dark green (5 6 3/2) angular to subrounded chloritic andesite fragments (typically 1 to 3 cm and up to 25 cm long) occur in a matrix of similar composition. Individual volcaniclastic units are discontinuous and there is no systematic variation of fragment size. The eutaxitic texture as exposed at station B-294 on the Granduc road (Plate 1; Fig. 6.4) and monolithic fragments suggest rocks are flow breccia and ash flows, rounded clasts indicate local reworking. Units 2 and 3 are interpreted to be andesitic flows and related flow breccia. Andesitic to dacitic tuff (unit 5) is pale green (5 6 7/3) and consists of soft, foliated, ephanitic to fine grained tuff. This unit 5 correlates with Alldrick's (1985) "unit 2A dust tuff". Dacitic tuff has a limited distribution but is best exposed at the second bridge on the Big Missouri road (station DB-40; Plate 3). The nature of contacts with underlying andesite or overlying units are uncertain because they are poorly exposed. Scattered, angular fragments of felsic, and chlorite-altered lithic lapilli, plagioclase and hornblende( ?) crystals and scattered, large (5-15 mm diameter) pyrite euhedra occur. Along the Big Missouri road tuff exhibits faint, moderate northwest-dipping bedding with subparallel foliated sericite that produces a phyllitic fabric. Maroon and green phvllitic andesitic tuff (unit 6) form a distinct unit (Plate 3). Maroon (dusky red, 5 R 3/5) and greyish green (56 5/2) phyllitic tuff is characterized by a prominent, pervasive chlorite and hematite foliation and the maroon colour. This unit represents deformed units 5 and 7. Maroon andesitic volcaniclastic rocks (unit 7) are heterolithic breccia and fine grained volcaniclastic rocks. They are exposed above B.C. Silver and up the east side of Bear River Ridge (Plate 3, Fig. 4.2 and 105 4.3). The volcaniclastic rocks are inferred to unconformably overlie andesite (unit 2 and 3) southeast of Silbak Premier and porphyries (units 4,4A, and 4B) because green hornblende-plagioclase porphyry fragments (derived from unit 4) occur in purple tuff (station B-99)-- no porphyry intrudes units 5 to 7 and they structurally cap the maroon porphyry (unit 4B) where it may be a flow. Along the Indian Mine tramline, volcaniclastics are in fault contact with hornblende-plagioclase porphyry (unit 4A). The volcaniclastics are typically maroon (dusky red, 5 R 3/4), purple (greyish red purple, 5 RP 4/2), or brick red (moderate red, 5 R 3/6); locally they are moderate green (5 G 5/4). Talus boulders in Lesley Creek derived from unit 7 on Bear River Ridge comprise all combinations of mixed green end maroon fragments and matrix. Hornblende- and plagioclase-bearing matrix supports angular to rounded heterolithic volcanic fragments (up to a few metres long, but on average < 5 cm long) and rare angular shale rip-up fragments. Figure 4.2 Heterolithic breccia (unit H in Plate 1) with matrix-supported hornblende-plagioclase porphyry (unit 4A), and green andesite (unit 2) fragments, one kilometre north of Mount Welker, Bear River Ridge, Stewart, northwestern British Columbia (station B-449, Plate 1). Figure 4.3 Quartz vein in plagioclase porphyry fragment of andesite breccia, Bear River Ridge, Stewart, northwestern British Columbia (unit H in Plate 1 station B-449). 107 Brick red to maroon siltstone and mudstone, a subunit of unit 7, occur 100 m above B.C. Silver's 2080-level portal (unit 7B, Plate 3). The rocks are likely reworked tuff because they lack any ductile fabric rather than tectonites as proposed by Grove (1971). Coarse channel-fill conglomerate and bedded maroon volcaniclastics indicate reworking (Fig. 4.4). Unit 7 tuff, lapilli tuff and volcanic breccia are pyroclastic and epiclastic in origin. Fragment size and heterogeneity increases eastwards, toward Bear River ridge; therefore, it is presumed that a Jurassic volcanic vent lay east of Silbak Premier. Green and purple volcanic breccia (unit 7C) occurs in an isolated outcrop 150 m north-northeast of the B.C. Silver-Glory Hole road junction (station B-102; Plate 3). It comprises greyish green (5 G 5/2) and purple (greyish red purple, 5 RP 4/2) breccia The outcrop forms a 10 m high knob that is differentially weathered according to fragment type. The southwest face is a north-striking, steeply west-dipping planar fault surface. The basal unit is green, monolithic framework-supported breccia consisting of subrounded plagioclase porphyry pebbles and cobbles up to 7 cm long Plagioclase and hornblende phenocryst mineral foliation in porphyry fragments are randomly oriented and indicates a pre-depositional, primary flow alignment texture, in contrast with other penetrative tectonic foliations, prior to erosion and incorporation in the volcaniclastic. Purple heterolithic breccia lies above the green unit with matrix-supported, subrounded pebble and cobble fragments (some > 20 cm long), comprised of rare brick red tuffaceous mudstone (unit 7B), altered K-feldspar porphyry clasts (unit 4) and maroon porphyry (unit 4B), in a volcanic matrix of hornblende and plagioclase crystal fragments. At the same outcrop, a green weathering heterolithic breccie contains ellipsoidal fragments of hornblende-plagioclase porphyry fragments derived from unit 4A, and scattered andesite (unit 2). 108 Because green and maroon K-feldspar, and hornblende-plagioclase porphyry (units 4 , 4B, and 4A) fragments are components of the breccia, the porphyries must have been uplifted, eroded and redeposited. The maroon porphyry clasts imply that some maroon colouration occurred before incorporation in the volcanic breccia and probably was coeval with intrusion (and/or extrusion) of the porphyry (unit 4 to 4B). Green-maroon contacts, regionally and at Silbak Premier, have variable significance -- from lithogic unit contacts to reduction-oxidation fronts in a uniform lithology. Hematitic rocks are both stratabound and discordant, locally certain units are consistently maroon or brick red and can therefore be used as marker beds. Regionally maroon colouration exists in various rock types end only reflects en oxidizing environment during or after deposition. Maroon units are common along Beer River ridge. All mineralization et Silbak Premier occurs structurally below maroon lithologies. The maroon to green colour transition is abrupt and does not necessarily have a compositional or bedding control in the Silbak Premier area However, it is locally stratabound along Bear River ridge. The colour contact at station B-236, on the Silbek Premier property (Plate 3 and Fig. 4.5), is a lithologic contact of green lapilli tuff and purple to maroon phyllitic tuff. A faint sericite foliation crosses the contact at a high angle, but is overgrown by post-tectonic chlorite concentrated in a zone less than 1 cm wide parallel to the lithologic contact (Fig. 4.5). Chlorite is zoned with Mg-rich cores (anomalous brown birefringence) with Fe-rich rims (anomalous blue birefringence). Near Silver Lakes (Plate I) a gradational colour change occurs within a texturally and compositionally uniform unit. The change probably marks a diagenetic reduction-oxidation front. Irregular green or maroon patches occur in southern exposures of Monitor rhyolite and could reflect devitrification of primary glassy material (Plate 1 and Fig. 4.6). Maroon, hematitic rings are evident in some 109 volcaniclastic rock fragments (Fig. 4.7). Elsewhere clasts have dark green chloritic rims and bleached pale green cores supported in a maroon mudstone matrix. Figure 4.4 Yiew west to well bedded maroon epiclastic rocks with channel-fill conglomerate (station B-449, one kilometre north of Mount Welker, Bear River Ridge, Stewart, northwestern British Columbia, unit H in Plate 1; So = 280/60* N). Figure 4.5 TOP: Yiew east to coarse maroon breccia and green lithic lapilli tuff contact and bleached zone (station B-236, Silbak Premier, Stewart, northwestern British Columbia; Plate 3). An example of a green-maroon colour contact that corresponds to a lithologic contact. BOTTOM: Close-up sketch illustrates fine sericite foliation, Si = 160V30* SW and elongation axis of lithic fragments crosscut bedding (bottom sketch), So = 050V60* SE at a high angle. A B fold axis = 224V27*. Chlorite is concentrated along the contact overgrowing the older sericite foliation. Extensional quartz veins just outside the photograph dip moderately to the northeast (vein = 350V40* NE). Abbreviations: ch = chlorite and ser = sericite. I l l Figure 4.6.a Irregular green-maroon change in Monitor rhyolite tuff southeast of Monitor Lake, Stewart, northwestern British Columbia Colour changes are in a uniform lithology. (Photo by D. Alldrick) Figure 4.6.b Green-maroon colour change in uniform lithology of Monitor rhyolite tuff. Horizontal lines parallel to hammer head are glacial striae (station B-384, Plate 1; Stewart, northwestern British Columbia). 112 Figure 4.7 Concentric hematitic precipitation in spheroidal fractures within a large plagioclase-porphyry fragment (7 cm long) in a volcaniclastic breccia (unit H; station B-449, Plate 1; Bear River Ridge,.Stewart, northwestern British Columbia). 113 Syndepositional and post-depositional hematite formation are evident from observations of maroon porphyry and maroon volcaniclastics. Hematite in veins may have been deposited by ascending, oxidizing hydrotherma] fluids or by descending oxidized connate or meteroic water. 4.3 INTRUSIVE ROCKS 4.3.1 TEXAS CREEK PLUTONIC SUITE (units 4, 4A and 4B) Three varieties of porphyritic dacite (units 4,4A and 4B; Plate 3) are hypabyssal members of the Texas Creek plutonic suite. Phenocryst content and red or green groundmass colour distinguish each variety. Porphyries intrude andesite (at 2-level trench), are faulted against andesite or are altered and/or mineralized along contacts. Tuff and volcaniclastics (units 5 to 7) locally unconformably overlie porphyritic dacites. Porphyry characteristically is blocky weathering and less foliated than andesite or tuff. It weathers pale green (10 0 6/2), has visible phenocrysts, rare subangular andesite xenoliths and is massive to weakly foliated Hornblende and plagioclase exhibit primary and tectonic foliations. Primary flow alignment of phenocrysts parallels the walls of the tabular intrusions that strike northeast and dip moderately northwesterly. Local tectonic foliations vary from moderate northwest-dipping to vertical easterly-striking (Piste 3). Dacitic potassium feldspar porphyry ("Premier porphyry", unit 4) is discussed below. The term "Premier porphyry" was used extensively but inconsistently for verieties of potassium feldspar-quartz-hornblende-plagioclase porphyry spetially associated with ore at Silbak Premier. Fresh potassium-feldspar porphyry adjacent to ore zones (Fig. 4.8), is the typical "Premier porphyry". This porphyry is not unique to the mine area, because it occurs regionally. Compositionally identical potassium-feldspar porphyry is recognized in areas devoid of alteration as well as altered and mineralized zones. Variation in potassium feldspar phenocryst abundance and intense carbonate alteration of plagioclsse and K-feldspar megacrysts obscure unequivocal assignment of rocks to the "Premier porphyry" unit. More confusion 114 t arises when the name "Premier dykes" (Grove, 1971) is used. These Eocene Hyder plutonic suite monzodioritic dykes crosscut the Texas Creek plutonic suite "Premier porphyry" (sensu stricto). Schofield and Hanson (1922) used "Premier Sills" synonymously for "Premier porphyry" because of their interpretation of the intrusions as sills. The term potassium-feldspar porphyry is proposed as a descriptive alternative to "Premier porphyry". Potassium-feldspar porphyry contains K-feldspar (0.1 -1058 and average about 1 cm but up to 5 cm long), quartz (<558; < 2 mm; Fig. 4.9), hornblende (2-858; average 3 mm long) and plagioclase (25-3558; average 2 mm long) phenocrysts in a fine grained altered groundmass. K-feldspar megacrysts, remain only as euhedrel pits on the weathered surface due to carbonate and/or sericite alteration (Fig. 4.10). Simple twinning, and hornblende end plagioclase inclusions ere common; inclusions are generally finer grained than groundmass equivalents. X-ray detraction (XRD) analysis of one sample by E.P. Meagher (University of British Columbia) identified a greater degree of Al/Si disorder than that found in orthoclase and suggested a composition greater then 9558 potassium-feldspar. He classified the megacrysts as "low sanidine". XRD analysis of one megacryst from 2-level trench, by the euthor, yielded 8858 potassium-feldspar composition; the degree of disorder was not determined. It seems unlikely that hornblende, potassium feldspar and plagioclase phenocrysts are metasomatic or detritial as proposed by Grove (1971) because: (1) they are symmetrically zoned towards the core of the 2-level porphyry dyke, and (2) they are present in unaltered dykes within the Texas Creek batholith at the toe of the Salmon glacier. Vernon (1986) presented evidence to support a primary phenocryst origin for potassium feldspar megacrysts. Figure 4.9 Embayed, unstrained quartz phenocryst from K-feldspar porphyry from unit 4 (field of view is 4.25 mm wide; station B- 181, Plate 3). 116 Figure 4.10 Partial carbonate replacement of Carlsbad twinned K-feldspar megacryst from K-feldspar porphyry, unit 4 (3.25 mm long; station B- 366; 2-level at Silbak Premier Stewart, northwestern British Columbia; Plate 3). Unstrained quartz phenocryst is black, optically extinct, in upper right corner of photo. 117 Diffuse phenocryst outlines, a result of intense propylitic alteration, prohibit precise estimates of modal abundance. Quartz is equant (1 to 3 mm in diameter), commonly embayed (probably incompletely grown) and unstrained (Fig. 4.9). Prismatic hornblende is 3 mm long on average (up to 6 mm long) and is pervasively altered to chlorite ± pyrite ± leucoxene. Relict biotite, predominantly replaced by chlorite, albite and opaques is rare. Plagioclase phenocrysts 2 to 3 mm long ere altered to carbonate and sericite or locally, in maroon units, to epidote-carbonate-sericite. Apatite, titanite (sphene) and zircon are accessory minerals. The presence of hornblende and minor biotite phenocrysts but absence of pyroxene indicate a hydrous magma. Dacitic hornblende-olaQioclase porohvrv (unit 4A) is texturally similar to K-feldspar porphyry (unit 4) but contains few or no quartz or potassium feldspar phenocrysts. Variable hornblende and plagioclase phenocryst content produce two varieties that are not separable at 1:2,500 scale mapping: quartz- and hornblende-rich plagioclase porphyry and hornblende-poor porphyry. In areas of poor exposure unit 4A cannot be confidently distinguished from unit 4. Plagioclase porphyry dykes crosscut greenstone (unit 2) in DDH 81 -22 to 26, in the M-Creek area (north of Lesley Creek), and 8t 6-level (Plate 3). Rare subrounded andesite xenoliths occur within the dykes but the lack of other fragmental textures (e.g., flow top breccia) suggests unit 4A may be shallow level sills or dykes rather than flows. Dacitic maroon porphyry (unit 4B: Fig. 4.11) is distinguished from similar units 4 and 4A by its maroon (dusky red, 5 R 3/5) to purple (greyish red purple, 5 P 4/2) colour. Hematitic alteration is abundant compared with green porphyritic units. It is higher structurally. Maroon porphyry is exposed along cliffs above B.C. Silver 2080-level portal and extends 2 km north to Cooper Creek and the Big Missouri road (Plate 3). A discontinuous veneer of maroon volcaniclastic rocks (crystal-lithic tuff with andesite and porphyry fragments; unit 7, Plate 3) lie stratigraphically and structurally above this porphyry. All known mineralization lies stratigraphically and structurally below maroon porphyry. Previous workers 118 called this unit "purple tuff" and "purple porphyry". The unit is equivalent to Grove's (1971) purple mylonite, brittle deformation but not mylonitization of the maroon porphyry was observed. The iron oxidation reflects a higher structural level, either subaerial or in the realm of oxidizing groundwater. The maroon porphyry is inferred to be a flow, at least in part, because of its lateral continuity and intimate association with maroon volcaniclastic rocks of the overlying unit. 4.3.2 GRANITIC INTRUSION (unknown age or affiliation; unit 8) Unit 8, a sericite-pyrite altered intrusion, outcrops along the banks of the Cascade River one kilometre southwest of the Westmin camp (Plate 3). It is pyritic, fine grained to medium grained, massive greenish grey (5 G 6/2) granite. It was not studied. 4.3.3 HYDER PLUTONIC SUITE A variety of quartz monzonite, rhyolite, microdiorite, and monzodiorite dykes intrude the property and strike southeasterly to Mount Welker. They all have about the same northwestern strike and steep southwestern dip. Intrusive chronology among the phases varies. Details of these units are descibed below. Quartz monzodioritic dykes (unit 9A) are generally less than 50 m wide (but up to about 100 m wide where named "Main Dyke* on mine sections). They consistently cross the property at 135V70* SW. Wilson and South Fork Fletcher Creek drainage patterns reflect the dyke-andesite (unit 2 and 3) contact. A small quartz monzodiorite stock, Mineral Hill stock (Alldrick, pers. comm., 1986), outcrops along the international border, along the Granduc road (Plate 3). Figure 4.11 Maroon porphyry from unit 4 with altered K-feldspar megacryst (station B - 2 5 5 , Plate 3 ) . Relatively fresh plagioclase laths and faintly visible prismatic hornblende show a weak preferred orientation. Length of the sample is 7 . 5 cm. 120 Unit 9A consists of very light grey (N8) to light bluish grey (5 B 7/1) weathering, massive to porphyritic monzodiorite and granodiorite. Outcrops exhibit blocky fracturing with one joint set parallel to intrusive contacts (Fig. 3.5). Contacts in detail are extremely irregular, following angular brittle fractures. Medium grained biotite end hornblende phenocrysts are fresh to locally altered to chlorite in fractures. Andesite-microdiorite dykes (unit 10) ere pale green to greenish grey (5 0 7/2 to 5 6 6/1), fine groined, aphanitic massive and amydaloidal. They crosscut monzodiorite dykes (unit 9A). Dykes are narrow (<2 m wide) with blocky fracturing and chilled margins. Mine production era literature referred to these dykes as lamprophyre, but most ere microdiorite. Unit 9 and 10 dykes cut mineralization at Silbak Premier with e consistent steeply southwest-dipping attitude. These dykes are correlated with the Portland Canal dyke swarm of Orove (1971). K-Ar isotopic results of Alldrick et al. (1987), and K-Ar and Rb-Sr results from this study (Chapter 6) suggest some of the andesite dykes are of Early Jurassic age. Rhyolite dvkes (unit 11) are northerly-striking as exposed beside the Oranduc road 500 m southeast of Indian Lake (Plate 3). They crosscut a Hyder quartz monzonite dyke (unit 9A) 150 m south of the road. A similar steeply west-dipping dyke is exposed neer the intersection of Logan and Cascade Creeks. Rhyolite is creamy grey (very pale orange, 10 YR 8/2), flinty, and fine grained with rare quartz phenocrysts. Millimetre scale flow banding along dyke margins is locally present. Weathered surfaces commonly have irregular, dentritic manganese oxide staining. Plagioclase phenocrysts are less than one mm long and clay altered. Finely disseminated pyrite is present in minor amounts (<0.5X). The rhyolite (unit 11 )-Hyder quartz monzonite (unit 9A) dyke contact is irregular. The rhyolite exhibits flow banding and an aphanitic chilled margin (station B-51, Plate 3). The unit had previously 121 been considered part of the Hazelton Group. But it was observed by the author to crosscut Hyder dykes, and Rb/Sr isotopes (Chapter 6), indicate an Eocene age (late or post-Hyder plutonic suite). 4.3.4 BIOTITE LAMPROPHYRE DYKES Biotite lamprophyre dykes, minettes, exposed along the Granduc road at Indian Lake (station AT84-27-5; Plate 3), are the youngest dykes in the region. Characteristically the dyke hand specimens are strongly magnetic. Prominent biotite phenocrysts, up to 3 mm in diameter, are hosted in a massive greyish black (N2) aphanitic groundmass. Dykes are diabasic, and contain biotite, augite, and feldspar phenxrysts enclosed in a finer grained groundmass of pyroxene, plagixlase and magnetite (Fig. 6.9). Biotite phenocrysts end lack of olivine distinguish these dykes from lamprophyre dykes described by Smith (1973). A biotite K-Ar and Rb-Sr biotite-whole rxk isxhron yield Oligocene to Miocene ages for the dykes (Chapter 6). 4.4 MINERALIZATION 4.4.1 INTRODUCTION Surface geological mapping at Silbak Premier was incomplete and ore controls and genesis poorly understood previous to this study. Syngeneic and epigenetic models proposed have been based on inadequate field evidence. Detailed examination of surface showings established a better geological framework to guide exploration and develop ore deposition models for mineralization in the northeast and northwest zones. 4.4.2 STYLES OF MINERALIZATION There are at least four styles of mineralization: low sulphide stxkwork and breccia, and high sulphide stxkwork and breccia (Table 4.1). Low and high sulphide are relative terms used to describe mineralization, less than 1 S%, and greater than 1 S% (commonly > 505?) sulphide are considered low and high, respectively. Exh ore type is an end member with a spxtrum of variations among exh. Low sulphide, stxkwork type mineralization comprises quartz stockwork ± narrow, less than 3 cm wide base 122 metal veins (Fig. 4.12), with local crustiform banding, predominantly in porphyry and along porphyry-andesite contacts. The stockwork grades into siliceous breccia Bonanza ore is classified as low sulphide siliceous breccia type ore. It consists of silver sulphosalt-rich breccia with abundant electrum and native silver in late fractures. Lower grade low sulphide siliceus breccia is more common and occurs in porphyry and andesite. Siliceous breccia is characteristically composed of bleached porphyry and/or andesite fragments within a variable stockwork of quartz-K-feldspar-sericite veins and stringers. Sulphide content is generally less than 5SR, predominately pyrite with lesser sphalerite and galena Examples of high sulphide breccia type ore occurs at 2-level trench and in the Northern Light orebody, they comprise pyrite- end gold-rich stringers in endesite breccie (Fig. 4.13), and andesite breccie fragments with sulphide rims (Fig. 4.14). Locally siliceous breccia contains a polysulphide matrix (sphalerite, galena, pyrite end tetrahedrite; Fig. 4.15.8 and 4.15.b), hence it is grouped as high sulphide breccia type. A variety of high sulphide type ore is semi-massive to massive, polymetellic ore that is locally layered (Fig. 4.16). Table 4.1 Silbak Premier styles of mineralization, Stewart, northwestern British Columbia (this study and Wo]dak, pers. comm., 1987). Ore type Location Mineralogy Textures Host lithology Comments Figure u Q stock work 2160 upper trench quartz veins porphyry variable alteration Fig. 4.12 >W SULPH breccia Glory Hole Ag-sulphosalts, native Ag siliceous breccia, late fractures filled with native Ag altered porphyry silicifled, K-feldspar altered; = Bonanza ore u _j Glory Hole disseminated siliceous breccia porphyry & andesite altered porphyry &. andesite fragments stcckwork 2-level tr. veinlets porphyry grades into siliceous breccia Ul Q 2-level tr. pyrite py veinlets/stcckwork andesite high grade Au, low Ag Fig. 4.13 HIGH SULPH breccia Northern Light orebcdy py,gal, sph, ± chalco breccia andesite gal rimmed andesite fragments, dissem. py, interstitial sph Fig. 4.14 HIGH SULPH Glory Hole sph, gal, py ± tetra breccia, vuggy altered porphyry silicified angular, some quartz rimmed fragments Fig. 4.15 2-level pyrite podiform to layered andesite/porphyry contact deformational layering Fig. 4.16 Abbreviations: Ag = silver, Au = gold, chalco = chalcopyrite, dissem. = disseminated, gal = galena, py = pyrite, sph = sphalerite, tetra = tetrahedrite, tr = trench. Figure 4.12 Yiew northeast to a classic "ladder vein" in 2160 upper trench (Plate 3; Silbak Premier Stewart, northwestern British Columbia). An example of low sulphide stockwork type mineralization, base metal quartz veins, in K-feldspar porphyry. Quartz-pyrite veins at 215V80* NYY and extensional quartz veins at 340*/60* NE. Figure 4.13 High sulphide breccia ore in 2-level trench. This andesite breccia with a pyrite matrix is gold-rich (Silbak Premier, Stewart, northwestern British Columbia). 125 Figure 4.14 High sulphide breccia type ore from Northern Light orebody, Silbak Premier, Stewart, northwestern British Columbia. Andesite breccia contains galena (silver) rimmed fragments, interfragment space is filled with euhedral pyrite (bronze) and sphalerite (black). Photograph by R. Player, BCMEMPR. 126 Figure 4.15.a (left) High sulphide breccia ore type with silicified-K-feldspar altered porphyry fragments from the Glory Hole, station DB-143 (Plate 4; Silbak Premier, Stewart, northwestern British Columbia). Some fragments are rimmed by aphanitic quartz. Figure 4.15.b (right) Close-up of high sulphide breccia with vugs and quartz rimmed fragments. 127 Figure 4.16 View east to massive to layered (westerly-striking, steeply north dipping) pyrite high sulphide ore at 2-level trench, Silbak Premier, Stewart, northwestern British Columbia. Pyrite pod with adjacent extensional quartz veins in green andesite terminate before the sulphide pod. 128 Open space-filling textures such as quartz and carbonate-filled inter-fragment space, cockade structure (quartz and sulphide rimmed breccia fragments) and vugs lined with quartz are common. Ore minerals include pyrite, sphalerite, galena with minor tetrahedrite, chalcopyrite, local pyrrhotite (replacing pyrite), native gold, native silver, electrum, acanthite, pyrargyrite and polybasite. Much work remains to characterize mineralization. It is not known whether native silver, argentite and polybasite are supergene (Burton, 1926) or hypogene (White, 1939). Dean MacDonald, a Ph. D. candidate at the University of Western Ontario, has undertaken a study of the mineralization. 4.4.3 MINERALIZATION ZONES Vein mineralization and associated porphyry bodies occur along two trends. The northeast zone extends 1.5 km northeast from the Glory Hole and includes the "main ore zone", and B.C. Silver. The northwest zone is about 0.5 km long, extends west to 6 level millsite, and includes the "west ore zone". Approximately 8035 of the ore occurs within 500 metres of the intersection of these two trends. The area corresponds to the converging porphyry geometries (dykes and sills) and is now the location of the Glory Hole collapse (Fig. 4.17). It is a 70 m wide by 200 m long by 50 m deep hole, formed in the 1940's where extensive mining near surface finally resulted in the entire area collapse. The intersection area contained the widest ore shoots (up to 17 m), which also generally carried the higher ore grades of gold and silver. The northeast zone mineralization dips steeply northwest at surface but the dip moderates to approximately 65* immediately below 2-level, and to about 35" at 6-level (Fig. 4.18). The northeast zone occurs at a footwall K-feldspar porphyry contact at depth and a hanging wall contact above 3-level (i.e. ore zone dip is steeper than porphyry; P. Wojdak, pers. comm., 1986; Fig. 4.18 and Plate 2). Ore shoots in the southwest end of the zone consist of long tabular sub-parallel "tandem" (Langille, 1945) shoots (ie. two parallel shoots). In the northeast end of the zone, en echelon shoots are as little as 30 m long (Langille, 1945). The apparent left-lateral offset of stopes could be due to faulting or original mineralization geometry. Individual ore shoots plunge consistently 55 to 70" westerly. 129 Typical northeast zone mineralized shoots, exposed along the south side of the Glory Hole (2190 N trench), consists of quartz, K-feldsper and/or carbonate veins with disseminated sulphides within porphyry and andesite. Northeast-striking porphyry is conformable with andesite stratigraphy 8S determined by tuff and flow contacts (P. Wqjdak, pers. comm., 1986). Quartz-sericite-K-feldspar alteration and brecciation within porphyry are characteristic. Sericite alteration is more intense and mineralization broader (up to 50 m wide) than in the northwest zone (Wojdak et al., 1985). Host rocks become progressively more altered towards ore end are crosscut by several generations of quartz veins end stringers which grade into quartz vein stockwork and siliceous breccia. Porphyry and andesite fragments occur in breccia but intense alteration obscures their distinction et many localities. Siliceous breccia (SIBX) consists of andesite and/or porphyry that is pervasively altered (sericite, silica, and K-feldspar), brecciated and veined, and containing disseminated sulphides. Average sulphide content is low, fine to medium grained sulphide minerals comprise 1 to 32 of the rock and include irregularly disseminated pyrite, sphalerite, galena, with minor inclusions end stringers of sulphosalts (including tetrahedrite), rare pyrargyrite and native silver (Wojdak et al., 1985). Figure 4.17 Yiew east to north end of the Glory Hole collapse, Silbak Premier, Stewart, northwestern British Columbia Two people in circled area for scale. Siliceous breccia and altered K-feldspar porphyry collapse blocks constitute ore. 131 W SECTION 2395N — 1 Level — 2 Level .— 3 Level — 4 Level Metres Andesite (undifferentiated) Porphyritic dacite Stope Siliceous breccia Figure 4.18 Idealized x-section through northeast zone mineralization (modified after Wojdak, unpub. x-section, 1987). The discordant nature of Silbak Premier deposit is evident where stopes and mineralization crosscut moderate northwest-dipping stratigraphy. 132 Vertical zoning of silver at the mine scale was noted by Hanson (1929); more silver (polybasite, pyrargyrite, acanthi te, native silver, electrum, and tetrahedrite) occurs between surface and 3-level. Below 3-level ore minerals are predominantly pyrite, galena, sphalerite and minor chalcopyrite. Gold, on the other hand, is more uniformly distributed. Lateral metal zoning has not been studied. The northwest zone at surface is a 10 m wide vertical ore and breccia zone. Northwest zone mineralization and alteration is common along a K-feldspar porphyry-andesite contact west of the Glory Hole (Plate 3). Base metal sulphides are medium to coarse grained, and commonly are associated with open space filling in contrast to northeast zone ore (Wojdak. et el., 1985). Veins and breccia zones also display open space textures and contain vugs lined with quartz, pyrite end rare sphalerite. Breccia zones have irregular geometry and distribution. Two-level and ladder trenches exhibit characteristic northwest zone features as descibed below. Two-level trench comprises an unaltered K-feldspar porphyry dyke in andesite. The trench exhibits diverse mineralization, comprised from north to south of: (1) veins andveintets (at least four generations: Wojdak, 1984) within K-feldspar porphyry, (2) a bleached zone of K-feldspar (K-feldspar), sericite, and quartz, (3) a breccia zone of angular, green chloritic sulphide-rimmed andesite fragments in a sulphide-matrix and quartz- and pyrite-filled vugs, (4) a zone of layered sulphides in sharp contact with dark green chloritic andesite on the southeast end of the trench (Fig. 4.16), and (5) dark green chloritic andesite cut by pyrite stringers (Fig. 4.13). Vertical sulphide layering strikes easterly. It is coplanar with several hornblende-plagioclase mineral foliations, other layered sulphide zones, and high strain zones in volcaniclastics. Sulphide layering is not parallel to any bedded strata, and is interpreted as tectonic (not primary bedding). The stockwork of pyrite veins in andesite generally contains high gold (10 to 60 g Au/T) but low silver (Wojdak et al., 1985). Free gold in the bonanza lens (on the south wall of the Glory Hole) occurs as minute inclusions in pyrite (Ewanchuk, 1961), and native silver fills late fractures. 133 The ladder trench, between the Glory Hole and 2-level trench, consists of layered base metal sulphides (pyrite, sphalerite, and galena) in andesite. A bleached siliceous zone forms the northeastern boundary near but not at a porphyry contact Sulphide layering striking easterly with a southerly dip of about 75*, is crosscut by post-ore coarse white vuggy quartz veins. Additional showings within the northwest zone include: Hope showing, Fork zone, and Woodbine. They occur at surface, from 6-level west across Cascade Creek, and are described below. The Hope showing comprises a siliceous zone and layered base metal sulphides along the southwestern contact of a vertical (at surface) northwest-striking K-feldspar porphyry that is intruded into massive and fragmental andesite (Plate 3). The K-feldspar porphyry is a continuous body about 50-75 m wide that extends east to 2-level and the Glory Hole. A limonitic weathering, silicified-pyritized altered zone occurs along the length of the porphyry-andesite contact (Plate 3). The siliceous zone consists predominantly of pyrite, with lesser sphalerite, galena and chalcopyrite veins in altered porphyry and subordinate^ in andesitic volcanic breccia. Base metal sulphide layers dip 80* northerly and warp around altered siliceous and mineralized fragments. Massive andesite with disseminated pyrite is crosscut by pyrite veinlets. Fork zone which is up to about 15 m wide and extends 400 m from the Hope showing to Woodbine Portal No. 2. It does not appear to be significantly offset across Cascade or Lesley Creek faults (Plate 3). The Fork zone is similar to Hope showing and Woodbine mine (Plate 3). It is characterized by silicified, pyritized and brecciated rock along the same K-feldspar porphyry-andesite contact 8S the Hope showing and contains pyrite- and sphalerite-rich altered rock, massive sphalerite pods and disseminated chalcopyrite. Chloritic andesite contains disseminated pyrite (1 to 205?) and the K-feldspar porphyry is bleached and siliceous. 134 Woodbine workings (Portals No. 1 and 2) are on the west side of Cascade Creek, 500 m northwest of Silbak Premier (6-level). A large area of K-feldspar porphyry is brecciated and qu8rtz-pyrite-sericite-limonite altered. Breccia fragments are supported in a siliceous sulphide-rich (sphalerite, galena and lesser pyrite) matrix. The Woodbine area is intruded by Hyder plutonic suite dykes (Plate 3). 4.4.4 NORTHERN LIGHT OREBODY SURFACE EXPRESSION - TANK TRENCHES Upper and lower tank trenches adjacent to the B. C. Silver road junction are believed to be the surface expression of the underlying, blind, Northern Light orebody (Plate 3). The two trenches contain extensively bleached sericitic hornblende-plagioclase porphyry. Porphyry is pale green to grey with aphanitic grey pyrite stringers and patches in addition to finely disseminated pyrite. Quartz- pyrite-sphalerite veins are cut by white quartz-carbonate veins. 4.4.5 MOUNT WELKER - MINERAL BASIN AREA, ALASKA Precious- and base metal-bearing veins extend 7 km south from Silbak Premier into Mount Welker-Mineral Basin area, Alaska Fault and fracture controlled quartz veins contain galena, pyrite, sphalerite, tetrahedrite and chalcopyrite with minor amounts of electrum, boulangerite, pyrargyrite, pyrrhotite, scheelite, arsenopyrite, and native silver (Holbek, 1984). Five veins (Onlione, Silver Point, Ruby Silver, Olympie and Roanan veins) are hosted in Texas Creek granodiorite and in Hazelton volcanic rocks. The Roanan vein with best economic potential within Texas Creek granodiorite, consists of quartz with layered sulphides (Holbek, 1984). 4.4.6 ADDITIONAL MINES IN THE SALMON RIVER VALLEY Big Missouri, Indian and Riverside mines are three past producers in the Salmon River valley. Together with Silbak Premier they demonstrate the economic significance of the valley. The Big Missouri mine. 8 km north of Silbak Premier (Plate 1), is hosted in Hazelton andesite similar to Silbak Premier. Three mineralized horizons (Dykes et al., 1984) are moderately westerly-dipping zones 135 comprised of free gold, pyrite, galena, sphalerite, and chelcopyrite (Grove, 1971). Syngenetic stratabound and epigenetic vein models both have been adovocated, but ore genesis remains controversial. The geology and mineralization is described by Alldrick (1984), Galley (1981), Holbek (1983), and Soregaroli 8nd Meade (1983). Indian mine, five kilometres north of Silbak Premier (Plate 1), lies along a north-striking fault that juxtaposes K-feldspar porphyry on the west with andesite on the east (Plate 1). Ore consisted of lenticular, irregular sulphide-bearing (galena, pyrite, and sphalerite) quartz veins and stockwork (Grove, 1971). Alldrick et al. (1987) report Tertiary galena Pb-Pb and sericite K-Ar ages for Indian ore and alteration, and argue on these isotopic grounds that Indian mineralization is distinct and younger than that at Silbak Premier. The Riverside gold-tungsten mine. Alaska is a quartz vein deposit within Texas Creek granodiorite 7.5 km south of Silbek Premier. Three mineralized veins are known 8s the Ickis vein, Cross vein (both in granodiorite), and the Lindeborgvein or "main lead" (in mylonite; Buddington, 1929). Scheelite and base metal-bearing (pyrite, galena with minor chalcopyrite, pyrrhotite and sparse sphalerite and tetrahedrite: Allen, 1964) quartz veins occur within mylonitic portions of the Texas Creek granodiorite. Veins are discordant to the mylonitic fabric. Ductile deformation and cataclasis are evident in outcrop and microscopically (Fig. 3.29.8 and 3.29.b). The Lindeborg shear (Allen, 1964, and Smith, 1977)strikes westerly to northwesterly and dips steeply to moderately northeasterly. Post-tectonic Hyder plutonic suite dykes cut across the veins and mylonitic fabrics at high angles. 4.4.7 ALTERATION Hydrothermal alteration zones (Fig. 4.19) related to the mineralizing system are represented by a proximal silicification/quartz stockwork and K-feldspar and/or sericite facies potassic alteration (Wojdak and Brown, 1985). Peripheral to mineralization is a propylitic alteration assemblage of carbonate, chlorite and pyrite. The variable intensity and type of alteration is partially controlled by 136 fracture intensity and host lithology, and presumably, elevation in the hydrothermal system (e.g. there is abundant chlorite but little sericite et depth; 6-level; P. Wojdak, pers. comm., 1987). Secondary quartz and potassic alteration (including secondary K-feldsper flooding, veins, and pervasive sericite) are recognized in extremely altered bleached rocks using sodium cobeltinitrite staining techniques (Fig. 4.20). Primary K-feldsper phenocrysts are not altered in potassic zone but outside it they can be replaced by carbonate and sericite (Fig 4.10). Pervasive sericite is diagnostic of the potassic zone, but greenschist grade metamorphic sericite also occurs regionally within the andesite. Silicification and potassic alteration are specially associated with mineralization and provide important guides to ore. Silica replacement of host porphyry or andesite ranges from negligible to complete and is paralleled by a change from quartz veining to quertz stockwork. The intensity of bleaching is an indication of the extent of silicification and potassic alteration. Propylitic alteration and secondary pyrite, characteristic of the Silbak Premier area, are peripheral to minereliz8tion, therefore they are not effective exploretion guides to ore. Propylitic alteration, ubiquitous and most intense in andesite, consists of secondary chlorite, carbonate, sericite and pyrite. Abundance of chlorite and paucity of epidote suggest H20-rich conditions. Carbonate replacement is widespread within and outside mineralization and indicates a C02-rich environment. Secondary pyrite occurs in veins and fine grained disseminations. The most intense alteration is proximal to the Texas Creek batholith and therefore appears to be related to an associated hydrothermal system. 137 Mineralization, silica and K-feldspar Sericite ± silica ± K-feldspar Carbonate Chlorite and pyrite o- \ o • O o 0 * o 9 Figure 4.19 Idealized representation of alteration zones adjacent to mineralization at Silbak Premier, Stewart, northwestern British Columbia. Potassic alteration is typically several metres wide, total alteration halo in 10's of metres scale. Figure 4.20 Potassic alteration, stained yellow, from drill core DDH 84-25 and 40 (Photo by M. Marback). Two samples on right are K-feldspar flooded siliceous breccia and left sample is dacitic plagioclase porphyry. Scale is in 10 mm gradations. 138 Epidote-rich saussauritization and quartz veins in maroon porphyry (unit 4B) could be a distal manifestation of the mineralizing system. Plagioclase and K-feldspar phenocrysts are pale green, fresh or saussauritized (altered to epidote ± sericite) contrasted with carbonate ± sericite feldspar alteration in units 4 and 4A. Epidote alteration (bleaching) occurs in asymmetric vein envelopes up to 4 cm wide around epidote-quartz veins. Maroon porphyry (unit 4B) is crosscut by several generations of veins (oldest to youngest): hematite, epidote, and quartz. In addition, there are isolated coarsely crystalline chlorite-epioote-quartz veins, with chlorite coating euhedral quartz crystals. 4.4.8 GENERALIZATIONS AND GENETIC IMPLICATIONS Mineralization is spatially and probably genetically associated with intrusion of "Premier porphyry" (K-feldspar porphyry), as advocated by Buddington (1929). Ore is discordant, structurally controlled and distributed as veins in K-feldsp8r porphyry, along K-feldspar porphyry-andesite contacts, in andesite, and crosscutting andesite and K-feldspar porphyry. Mineralization at K-feldspar porphyry-andesite contacts typically occurs in a zone of brecciation. Several episodes of brecciation and introduction of ore-bearing solutions are apparent from mineralized fragments within veins, and veins of varying compositions crosscutting mineralized breccia. Ore zones are narrower in andesite than in porphyry which Hanson (1935) related to competency contrast, where foliated andesite is more ductile and porphyry is more brittle. There are four alteration zones: (1) silica-K-feldspar flooded ore, (2) ser1cite-K-feldsp8r, (3) carbonate, and (4) chlorlte-pyrite. Ore type and textures are diverse and gradational with one another; they are divided into two types: (1) low and (2) high sulphide with stockwork and/or breccia textures. Large amounts of carbonate, sulphides, and propylitic alteration associated with mineralization indicate ore fluids were H 2 O - , C O 2 - andS-rich. The diverse Au and Ag mineralogy and textures indicate episodic mineralization under changing physicochemical conditions. Ore deposition occurred under low pressures as inferred from vuggy open space textures, and crustiform banding in dilatant veins. 139 Ore deposition may nave been caused by a temperature drop associated with mixing cool, oxidized meteoric water circulating through maroon hematitic volcaniclastics with hotter, reduced mineralized waters circulating through chlorite- end pyrite-bearing andesite and K-feldspar porphyry. Siliceous breccie zones could indicate local pressure-controlled isothermal boiling (Barnes, 1979) and hydrothermal brecciation. Further insight might result from fluid inclusion and stable 0, S, and H isotope studies. The time span between host porphyry intrusion and mineralization is inferred to be less than five million years. Stratigraphic constraints yield a maximum 5 million year period for mineralization: between intrusion of K-feldspar porphyry (about 190 Ma; Toarcian) and middle Toarcian fossil-bearing impure carbonate unit on Mount Dilworth. Potassium feldspar megacrystic porphyritic intrusions are common in the subvolcanic environment (Vernon, 1986) which adds credence to placing K-feldspar porphyry in a shallow, subvolcanic setting. 4.4.9 GENETIC MODEL AND COMPARISONS WITH OTHER DEPOSITS Traditionally, Silbak Premier had been regarded as a quartz-vein replacement deposit (Grove, 1971). Barr (1980) classified Silbak Premier as a telescoped epithermal deposit. In contrast, a volcanogenic model with stratiform and vent ore facies was suggested by Seraphim (1979). Ore genesis model possiblities include syngeneic, partially submarine exhalative and stratigraphically controlled or epigenetic, entirely subvolcanic and structurally controlled. The spatial association of mineralization to "Premier porphyry" and general geological setting are indicative of a subvolcanic environment, no exhalative horizons or conclusive marine features are evident. The strong structural control and discordant ore suggest mineralization is vein-type, epithermal or mesothermal. Open space filling textures are common to both environments. Lack of As, Sb, TI, Se or Te-enrichment, significant base metal content, and high Ag:Au ratios suggest Silbak Premeir is near the epithermal-mesothermal transition, as schematically illustrated by Panteleyev (1986). 140 The model proposed here follows that of a porphyry copper hydrothermal system (Lowell and Guilbert, 1970). The Texas Creek batholith, as a heat source (Fig. 4.21), intruded into submarine andesite and drove a subvolcanic (shallow level) hydrothermal system. Northeast and northwest structures controlled emplacement of the last potassium-rich phases of the Texas Creek granodiorite (K-feldspar porphyry bodies). Hydrothermal circulation was localized along northeast and northwest structures after crystallization and brittle fracturing. Close spatial association between porphyry and mineralization suggest a temporal relationship. Ore deposition occurred in open spaces produced by fracturing of andesite and porphyry, preferentially along lithologic contacts. Mineralization was capped by maroon volcaniclastics, which may have been an additional structural and chemical (oxidation) control to ore deposition as suggested by Langille (1945). Silbak Premier has strong similarities with adularia-sericite-type epithermal deposits of Heald et al. (1987) and Hayba et al. (1985). These features include complex volcanic structural setting, andesitic host rocks, ore in several lithologies, vein K-feldspar-sericite-chlorite alteration, high Ag:Au ratios with abundant electrum and silver sulphosalts, ore deposition occurred at least 1 million years after formation of host rocks, large vertical extent to ore (over 400 m), and inferred paleodepth. Silbak Premier is silver-rich and Heald et al. (1987) suggest these deposits usually have high base metal production, just as is borne out by Silbak Premier's production data (Appendix 1.1). They noted barite in districts with relatively high base metal production, Silbak Premier has geochemically anomalous Ba and local barite crystals. The Silbek Premier model, spatial association with Texas Creek plutonic suite and related porphyritic ascites and its general porphyry copper-type setting are characteristic of acid-sulphate systems rather than adularia-sericite type of Heald et al. (1987). Adularia-sericite systems are typically not directly associated with an intrusion (Heald et al., 1987) but Silbak Premier could be a hybrid variant of these models. 141 The Goonumbla Cu-deposits, in New South Weles, Australia, as described by Jones (1985), display features akin to Silbak Premier. They are associated with high-K, calcalkaline volcanic pile intruded by small, irregular late stage quartz-rich, K-feldspar megacrystic monzonite porphyry intrusions. Alteration geometry end mineralogy is strikely similar to that outlined at Silbak Premier: pervasive K-feldsper flooding end quartz vein stockworks are developed at or near porphyry-andesite contacts; quartz-sericite-pyrite alteration zones surround the potassic centre; carbonate totally replaces plagioclase and orthoclase in porphyry and it occurs as irrregular blebs in volcanic rocks; and propylitic alteration is widespread. A significant difference is the low sulphur content, expressed by almost total absence of pyrite in the Goonumbia deposit, in contrast to the Silbak Premier pyrite-rich mineralization.. 142 West East - 0 km - 2 km - 4 km - 6 km 0 km HAZELTON GROUP Undifferentiated volcaniclastics (H) I I Massive and fragmental andesite (Hv) 2 km LEGEND 4 km 6 km INTRUSIVE ROCKS TEXAS CREEK PLUTONIC SUITE Maroon dacitic porphyry (in part extrusive ?) (Jmp) E3 Dacitic porphyry (Jp) i H Argillite-siltstone-greywacke (Hs) Figure 4.21 Schematic ore deposition model for Silbak Premier mineralization, Stewart, northwestern Bristish Columbia. Large rectangle represents the estimated location of Silbak Premier mineralization. Units correspond to those in Plate I. 143 CHAPTER 5 GEOCHEMISTRY 5.1 INTRODUCTION Chemical analyses were done to classify volcanic and plutonic rock units, establish their tectonic affinities, and determine their degree of alteration. Twenty-seven "least altered" representative specimens were collected from surface outcrops and analysed by the Geological Survey of Canada. These samples consisted of five andesite, eight porphyries, eight tuffs, end six miscellaneous samples (including two maroon porphyry). Data and analytical techniques used are described in Appendices 5.1 to 5.6 with sample locations shown on Figure 5.1, and Plates 1 and 3. 5.2 ROCK SAMPLES The five greenschist-grade andesites are fine groined, messive to fragmental, monolithic rocks. Rare hornblende and plagioclase phenocrysts, less than 1 mm long, and less than 22 disseminated pyrite are in a carbonate-sericite altered groundmass. Hornblende, plagioclase, groundmass and fragments are variably altered to calcite, sericite, and chlorite. The eight medium grained porphyritic ascites contain variable amounts of quartz, K-feldspar, hornblende, and plagirelase phenocrysts. Phenocryst end groundmass alteration is dominated by calcite, sericite, chlorite, and less than 235 disseminated pyrite. Accessory minerals are titanite, zircon, apatite, and relict ilmenite. The eight tuff samples are more intensely altered than andesite or porphyry. 5.3 ALTERATION AND LIMITATIONS Element mobility in greenschist grade metamorphic and hydrothermal ly altered rocks must be considered before interpretation and rock classification utilizing chemical data. The most mobile elements are Na, K, Mg, Ca, and Si. However, "least altered" samples can be assessed by utilizing the screening procedure devised by de Rosen-Spence and Sinclair (1987). Three consecutive diagrams eliminate "altered" samples from further classification schemes. All but one andesite and two porphyry samples plot in the "unaltered" field of the CaO versus MgO diagram (Fig. 5.2). In contrast, tuffs are clearly altered and fall in the "spilite" field and therefore should not be classified using major elements (Fig. 5.2). In their 144 second diagram that indicates N a 2 0 loss, four of six "unaltered" porphyry samples fall in the "unaltered" field ( S i 0 2 versus N 8 2 O diagram; Fig. 5 . 2 ) . The third plot does not indicate a MgO enrichment pattern, suggesting minimel effects due to alteration. Rock classification will therefore be based on "unaltered" samples, following criteria developed by de Rosen-Spence and Sinclair (1987). 5.4 MAJOR ELEMENT CHEMISTRY Chemical rock classification corroborates assigned field names. All samples plot in subalkaline field of Irvine and Baragar's S 1 O 2 versus ( N a 2 0 + K 2 O ) diagram (Fig. 5 . 3 ) . Andesite and porphyry samples plot in calcalkaline field on the AFM diagram and have characteristic calcalkaline trends (Fig. 5.4). The hornblende-plagioclase phyric nature of the andesites is petrographic support of the calcalkaline affinity (Gill, 1981). Similar calcalkaline trends were noted by Galley (1981) in his study of Big Missouri volcanic rocks. On a regional scale, Silbak Premier volcanic rocks are akin to the western belt trend of the Hazelton Group, Telkwa Formation (cf. Tipper and Richards, 1976). Tuff samples plot in the tholeiitic field with a crude but distinct iron enrichment trend (Fig. 5.4). It is not known whether this trend reflects primary chemical composition or if it is a product of secondary alteration. Similar results to the AFM diagram are evident on Miyashiro's (1974) S i 0 2 versus FeO*/MgO diagram. Andesite and porphyry display flat calcalkaline trends and tuffs poorly define a tholeiitic Fe-enrichment trend (Fig. 5 . 5 ) . In rock unit classification two diagrams were utilized: S1O2 versus K 2 O (Gill, 1981) and total alkalies silica diagram (TAS; LeBas et al., 1986). Samples informally named "andesites" and "porphyry" are chemically classified as andesite and dacite, respectively (Fig. 5.6 and 5.7). Two porphyries plot outside the dacite field, in the trachydacite field. The majority of samples are high- to very high-K andesites and dacites (Fig. 5.6 and Table 5.1). 145 i c e « v. <lf CD 00 i C 0 *B-383 (9 kni south) 0 1 2 A vffi'i'j'i'i':':^ Kilometres 3 Figure 5.1 Oeochemical sample locations Silbak Premier, Stewart, northwestern British Columbia. Rock unit legend is given in Figure 6.1 and Plate I. 5i 4 6 CaO (wt. X) o ANDESITE x PORPHYRY + TUFF • nisc 146 i o CM » ANDESITE x PORPHYRY • TUFF • MISC 55 60 Si02 (Wt. X) o 50 » ANDESITE • x PORPHYRY + TUFF • MISC - i r 55 60 65 Si02 (wt. Z) l 70 Figure 5.2 Diagrams to differentiate altered and unaltered volcanic rocks: (a) CaO versus MgO, (b) Si02 versus Ne^ O, and (C) Si02 versus MgO (after de Rosen-Spence and Sinclair, 1987). Figure 5.3 Irvine and Baragar's (1971) alkaline/subalkaline classification diagram. 148 Figure 5.4 AFM diagram for andesite, tuff, porphyry and miscellaneous rocks (after Irvine and Baragar ,1971). Andesite and porphyry have calcalkaline trends and tuff have a crude tholeiitic trend. Si02 (wt. 5f) Figure 5.5 Si02 versus FeO*/MgO calcalkaline/tholeiitic classification diagram (after Miyashiro, 1974). Andesite and porphyry h8ve flat calcalkaline trends. Tuffs display a poorly defined Fe-enrichment trend in tholeiitic field. 150 rhyolite very high-K 0 ANDESITE x PORPHYRY + TUFF T— 1— i — 1 — r 50 52 54 56 58 60 62 64 66 68 70 Si02 (Wt. %) Figure 5.6 Si02 versus K2O classification diagram for volcanic rocks (after Peccerillo and Taylor, 1976; and modified by Gill, 1981). Figure 5.7 Total alkalies-silica (TAS) diagram after LeBas et al. (1986). 151 The chemical characteristics of Silbak Premier andesites are similar to published dats (Table 5.1). They contain slightly higher A I 2 O 3 and P 2 O 5 , and lower MgO and CaO than average andesite compositions (Table 5.1). Silbak Premier and Big Missouri andesites contain very high-<20 and corresponding low Na20. Sixteen samples from the Big Missouri mine are8, from a suite collected by Galley (1981), are similar to Silbak Premier except they have lower CaO and Na20 and slightly higher K 2 O , perhaps reflecting more intense alteration. Alumina content compares with typical calcalkaline orogenic andesite with values between 16 and 18 wt. % A I 2 O 3 (cf. Gill, 1981). Major element variations are similar to those noted for andesites by Gill (1981): A I 2 O 3 , T 1 O 2 . Fe203, and MnO show a negative correlation relative to S 1 O 2 , Na20 has a positive correlation with Si02, and MgO and CaO are almost constant. In contrast to Gill (1981), K 2 O and FeO show a negative Si02 correlation that could be related to crystal fractionation of K-bearing minerals (hornblende, biotite or K-feldspar) or secondary propylitic alteration. Samples with more than 70 wt. % Si02 represent strongly altered samples and consequently were not plotted. Texas Creek plutonic suite porphyries exhibit chemical variations of A I 2 O 3 , Ti02, and MnO, with respect to silica, similar to andesites. MgO decreases as Si02 increases. N82O is roughly constant with increasing silica content. The altered tuff chemistry is shown on silica variation diagrams (Fig. 5.8 and 5.9). Patterns are distinctly different from andesite or porphyry samples. 152 Table 5.1 Average andesite compositions from published data and this study. 1 2 3 4 5 6 7 8 Si02 58.2 57.9 57.6 54.2 60.9 57.20 55.93 59.0 Ti02 0.82 0.87 0.77 1.31 0.9 0.95 0.99 0.81 A I 2 O 3 17.2 17.0 17.3 17.17 18.18 16.28 17.37 17.8 Fe203 3.1 3.3 3.1 3.48 7.80* 2.78 4.24 2.46 FeO 4.0 4.0 4.3 5.49 — 5.34 4.71 5.89 MnO 0.15 0.14 0.15 0.15 0.26 0.13 0.16 0.28 MgO 3.2 3.3 3.6 4.36 1.99 4.56 3.65 2.53 CaO 6.8 6.8 7.2 7.92 3.50 7.78 7.33 6.10 N820 3.3 3.5 3.2 3.67 0.84 2.46 3.33 1.16 K20 1.7 1.6 1.5 1.11 5.23 1.05 1.25 3.73 P 2 O 5 0.23 0.21 0.21 0.28 0.33 0.15 0.19 0.33 H20 1.3 1.2 1.0 0.86 —- 1.19 0.97 TOTAL 100.00 99.82 99.93 100.00 99.93 99.87 100.12 100.09 1. Chayes(1975) 2. LeMaitre(1976) 3. Gill (1981) 4. Nocko1ds(1954) 5. Galley (1981) * = total iron; normalized assuming 1 % H20. 6. Miyashiro (1974) Calcalkaline andesite, outer volcanic zone, northeast Japan. 7. Miyashiro (1974) Calcalkaline andesite, inner volcanic zone, northeast Japan. 8. This study (unit Hv or units 2 and 3; samples B-294, -332, -340, -381, -553; Appendix 5.3; Plates 1,3, and 5). Normalized assuming 1 % H2O. > o • ANDESITE x PORPHYRY + TUFF • MISC 153 o ANDESITE x PORPHYPY + TUFF • MISC o ANDESITE x PORPHYRY + TUFF o MISC o ANDESITE x PORPHYRY + TUFF a MISC * ANDESITE x PORPHYRY + TUFF a MISC S i 0 2 (Wt. I ) 6 5 Figure 5 . 8 Variation diagrams of A I 2 O 3 , TiC?, Fe203, FeO, and MnO with respect to Si02 for rocks around Silbak Premier, Stewart, northwestern British Columbia. 154 > o <M 10 i 8 6 4-2' 0-50 > K o O ID-S ' 6H 4 2 0 > o r 50 5 t 4-3-2 -1 -0 55 60 65 55 65 50 —r— 55 • ANDESITE x PORPHYRY + TUFF n MISC 70 • ANDESITE x PORPHYRY + TUFF • MISC * ANDESITE x PORPHYRY + TUFF o MISC — i 70 ANDESITE PORPHYRY TUFF MISC 60 65 — i 70 5 5 Si02 (wt. S) 6 5 ANDESITE PORPHYRY TUFF MISC Figure 5 . 9 Variation diagrams of MgO, CaO, N a 2 0 , K 2 0 , and P 2 O 5 with respect to S i 0 2 for rxks around Silbak Premier, Stewart, northwestern British Columbia. 155 5.5 TRACE ELEMENT CHEMISTRY Trace elements can be used to classify rxks and determine tectonic affinity. Silbak Premier trace element data is listed in Appendices 5.3 to 5.6. The andesites contain 82 to 110 ppm Zr and have Zr/Nb ratios slightly less than 10, values considered normal for high-K-andesites(cf. Gill, 1981). The relatively high Be and Rb (andesite average = 100 ppm) correlate with the high-K nature of the rocks. The andesite average has lower K/Rb ratios (about 320) as expected for high-K volcanic rxks, in xntrast to low-K volcanic rxks with K/Rb ratios of around 1000 (Gill, 1981). Transition metals (compatible group: Ni, Co, Cr, end V) show a negative correlation with silica in andesites, possibly reflxting crystallization of ferromagnesian minerals (Gill, 1981). Low Ni values (< 71 ppm), Co about 16 ppm and an average Ni/Co ratio of 0.36 are normal for andesites. In xntrast, the Cr/Ni ratio is 0.29, rather low considering average andesites range from 1 to 3 (Gill, 1981). Compared with average andesites 40 ppm Cu is relatively low, 8nd 100 ppm Zn and 4.6 ppm Mo are high (Gill, 1981). Y and heavy rare earth elements (HREE: Yb and Lu) values are approximately constant between samples with different S i 0 2 xntents, the limited data suggests a decrease in Y and Yb as silica increases. This could reflect frxtionation, with Y and Yb accomodated in hornblende, biotite and zircon (cf. Pearce, 1982). Incorporating trace element data into classification diagrams such as the Zr / T i 0 2 versus S1O2 plot (Winchester and Floyd, 1977) provides a classification compatible with major element plots (Fig. 5.3, 5.4 end Table 5.2): andesite samples are andesite, porphyry samples are rhyodxite/dxite and tuffs are andesite. Winchester and Floyd's (1977) Nb/Y versus S i 0 2 and Nb/Y versus Zr / T i 0 2 plots give inconsistent and conflicting results, with points more scattered (Table 5.2), porphyry and tuff plot in trxhyte or trachyandesite fields. 156 5.7 TECTONIC DISCRIMINATION DIAGRAMS Tectonic environments have been characterized in literature using immobile element diagrams. Most plots were developed for basalts (< 54 wt. % Si02) because basalts closely represent primitive, least evolved melt compositions. The majority of Hazelton Group-Texas Creek plutonic suite rocks have more than 58 wt. % silica, but despite this limitation, several diagrams do indicate e calcalkaline volcanic arc setting which is compatible with field data All andesite T 1 O 2 results are less than 1.2 wt.X (Fig. 5.8 and Appendix 5.3), which based on criteria from Gill (1981) indicate a convergent margin, island arc environment. As Si02 increases T 1 O 2 decreases, a trend common to calcalkaline rocks. The decrease is possibly due to crystallization, where titanium is incorporated in magnetite (Gill, 1981), hornblende, and any excess in titenite, ilmenite or other Fe-Ti oxides. Titenium-bearing phases in the porphyry are magnetite, hornblende, titanite, ilmenite or Fe-Ti oxides, but in andesite primary phases are altered to leucoxene. The Mullen (1983) ternary discriminant diagram (MnO x I0)-Ti02-(P205 x 10)) was developed for mafic rocks with silica between 45 and 54 wt SB. The Hazelton-Texas Creek rocks have higher silica contents (52-70 wt. % Si02). All porphyry and andesites plot in "island arc calcalkaline" field (CAB), tuffs fall in "island arc tholeiite" and "oceanic arc tholeiite" fields (IAT and OIA), results coincident with other tectonic discrimination diagrams listed in Table 5.2 (Fig. 5.10). Convergent margin basalts have higher abundances of large ion lithophile elements (LIL Ba, Rb, K and Sr) but lower Nb.andZr than normal- andenriched-mid-ocean ridge basalt (N-MORB andE-MORB; Erdman, 1985). Rb/Nb and K/Nb ratios are much higher than from either MORB or within-plate basalts (WPB; Erdman, 1985) and provide tectonic indicators. The Rb/Nb and K/Nb ratios are high (andesite = 11 and 3400, porphyry = 11 and 3600, and tuff = 9.3 and 2830), suggestive of an arc setting. All but one andesite sample have La/Nb ratios less than 2, typical of N-MORB, not the expected 2 to 5 range characteristic of convergent plate margins. The results are suspicious, La and Nb analyses should be 157 checked to verify if they are actually as low and high as reported. Ba/La ratios are greater than 80 indicating E-MORB affinity; however, Erdman (1985) noted that Gill's orogenic andesite field was too constrained end there was probably overlap between orogenic andesite and E-MORB fields. This data illustrates that overlap. 5.8 CHONDRITE-NORMALIZATION DIAGRAMS Multi-element diagrams can be used to establish tectonic setting. Chemical data is chondrite-normalized to compare data from different suites. The order of elements plotted on Figure 5.11 is based on their mobility in an aqueous fluid end their incompatibility at partial melting (Sun, 1980; Pearce, 1983; Thompson et al., 1983). Andesite, porphyry and tuff /chondrite plots display similar patterns (Fig. 5.11): enriched LIL elements, Nb-dip, Sr-dip, and level Ti, Y, and Yb. These patterns are characteristic of convergent plate margins (Erdman, 1985). A similar pattern is evident with Andean arc andesite data from Gill (1981) and Ewart (1979 and 1982; Fig. 5.11). Ba and K appear enriched in the study suite. An Andean dacite (Ewart, 1979) shows the same pattern of Ba enrichment as porphyritic dacite at Silbak Premier (Fig. 5.11). Although the paleogeography of the Hazelton arc remains obscure it has chemical similarities with volcanic rxks associated with Andean convergent margin. 5.9 CONCLUSIONS Hazelton Group volcanic rxks and comagmatic Texas Creek plutonic suite are subalkaline, calcalkaline high-K to very high-K andesites and dxites. The high-K chemical classification is borne out by the phenocryst assemblage that includes K-feldspar and biotite (Ewart, 1979). Hornblende andesites are derived from high-K, silica-, soda-, water-enriched magma between 900-1000 * C (Gill, 1981). Gill's (1981) conclusion that "hornblende is restricted to topographically Bnd stratigraphically high levels of most andesite stratovolcanoes" fits the interpreted Silbak Premier geological setting and ore deposition model, exh of which were developed independently. Txtonic dixrimination diagrams indicate a calcalkaline, volcanic arc setting. Chondrite-normalized trace element patterns are similar to those for Andean volcanic rxks. Low Ti02, the Nb-dip, and high-K suggest Hazelton volcanic rxks and comagmatic Texas Creek plutonic suite rocks were generated on the back-arc side of an inner island arc. 158 Table 5.2 Summary of chemical plots. Figure 5.3 5.4 5.5 5.6 5.7 5.10 diagrams not included Plot Si02 v$ Na20+K20 AFM Si02 vs FeO*/MgO Si02 vs K20 TAS MnO-Ti02-P205 Zr/Ti02 vs Si02 MgO-FeO*-A1203 Reference 1 1 2 3 4 5 6 7 UNIT ANDESITE Si02 = 53.4-67.0 Av. » 5 9 * Subalkaline CA CA High- and very high-K andesite Subalkaline; basaltic andesite, andesite CAB andesite E-MORB (spreading center island) PORPHYRY Si02 = 60.7-67.6 Av. = 6 9 * Subalkaline CA CA High- and very high-K andesite, dacite Subalkaline; dacite, andesite CAB rhyodacite, dacite, andesite E-MORB TUFF Si02 = 57.7-62.7 Av =60* Subalkaline TH TH High- & very high-K andesite Subalkaline; andesite, trachy-andesite OIT IAT CAB andesite E-MORB References. (1) Irvine and Btnqar (1971), (2) Miyashiro (1974), (3) Gill(l9S1), (4) LeBas et al. (1986), (5) Mullen (1983), (6) Winchester and Floyd (1977), and (7) Pearce et al. (1977). Abbreviations: CA = calcalkaline TH = tholeiitic CAB = island arc calcalkaline IAT = island arc tholeiitic OIT * oceanic arc tholeiitic E-MORB = oceanic island which is adjacent to or straddling a mid-ocean ridge 159 Figure 5.10 Ti02-Mn0-P205 diagram used to separate tectonic environments (after Mullen, 1983). Abreviations are: CAB = island arc calcalkaline, IAT = island arc tholeiite, OIA = ocean island alkaline, OIT = ocean island tholeiite, and MORB = mid-ocean ridge basalt. 160 PORPHYRIES • DB-70 x DB-105 B-15 o B-366 • B-382 o B-455 s B-524 a B-546 + South American dacite (Ewart. 1979) — i 1 i Rb Ba K — i 1 i 1 i i i i— Nb La Sr P Zr Ti Y Yb ! ! I i a g TUFFS 1 * • • * • o a • • I I I DB-40 B-231 B-96 B-317 B-357 B-543 B-359 B-458 oo w o> g » w ' * - ci b o- 1 1 r Rb Bi K - i 1 1 r Nb La Sr P • S 8 5 9 ci o T Yb Zr Ti Figure 5.11 Chondrite-normalized, trace element abundance diagrams (spidergrams) for Hazelton andesites 8nd tuff, and Texas Creek plutonic suite (porphyries). The normalization factors are listed along the x-axis (from Thompson et al., 1983). 161 Chapter 6 6E0CHR0N0METRY 6.1. INTRODUCTION Hazelton strata are rarely fosslliferous in the Stewart area; therefore geochronometry is important in establishing their age. For this thesis U-Pb, K-Ar, and Rb-Sr techniques were applied to determine: (1) the age of emplacement or deposition of key Jurassic units, (2) the age of easterly-trending biotite lineation developed in the Texas Creek batholith, (3)the age of potassic alteration associated with Silbak Premier mineralization, (4) the age of l8mprophyre dyke emplacement, amongst the youngest of igneous events in the erea, and (5) Rb-Sr isochron dates and initial 8 7Sr/ 8 6Sr ratios for Jurassic and Tertiary rocks. 6.2 U-PB DATING 6.2.1 Introduction U-Pb geochronometry W8S conducted to provide reliable ages for critical units where K-Ar results would reflect isotopic resetting during regional greenschist grade metamorphism or other thermal events. Zircon closure temperatures are variable. Discordant zircons can yield good ages if several fractions are analysed and a best-fit line extrapolated to determine the upper intercept along concordia. 6.2.2 U-Pb sample collection Five samples were collected from four Hazelton volcanic units for zircon U-Pb age determinations (Fig. 6.1). The oldest rocks are unfossiliferous andesite with lithological similarity to the Hazelton volcanics (Fig. 6.2). Fragmental andesite (unit Hv on Plate 1 or unit 2 and 3 on Plate 3) at the Qlory Hole did not yield enough zircon for enelysis. A second, eutaxitic andesite sample collected beside the Oranduc road (B-294; Fig. 6.1), 12 km north along strike of Silbek Premier, provided four zircon fractions. Dacitic ash tuff along the Big Missouri road represents the volcaniclastic part of the stratigraphic section (DB-40; . Fig. 6.2). The tuff sample collected contained only enough zircon for a single analysis. Unaltered hornblende-plagioclase porphyry represents either a high-level sill or a flow along the andesite/volcaniclastic lithologic change (B-382; Fig. 6.2). The unit contains fresh hornblende, and zircon in suffient quantity to give two fractions for analysis. Four zircon fractions were analysed from the Monitor rhyolite tuff, which caps the Hazelton volcanic sequence (B-underlies Spatsizi and Bowser Lake Group sedimentary rocks. 162 394; Fig. 6.2) and unconformably 6.2.3 Zircon morphology The eutaxitic andesite (B-294) contains uniform pink short prismatic zircons (Fig. 6.3 a and b). These are similar in appearance to zircons from the porphyry (B-382). The dacitic tuff (DB-40) contains subrounded zircon crystal fragments, thet may have been reworked. Zircons from hornblende plagioclase porphyry (B-382; Fig. 6.3 c end d) are pink, euhedral, end short prismetic crystals that contain rare apatite inclusions as determined with the scanning electron microscope with an energy dispersion system (SEM-EDS). Monitor Lake rhyolite breccia (B-394) sampled at Monitor Lake (Fig. 6.3 and Fig. 6.1 e to h) has a bimodal zircon population consisting of colourless, long prismatic zircons and rare equant, pink crystals which were not analysed. Attached plagioclase is common as identified in the SEM-EDS (Fig. 6.3 e to h); therefore, two fractions were abraded and hand-picked to remove unsuitable crystals. Table 6.2 summarizes zircon characteristics. 6.3.4 U-Pb results and discussion Uranium-lead data are presented in Teble 6.4; analytical techniques are provided in Appendix 6.1.1. Zircons from the eutaxitic andesite (B-294; Fig. 6.3 and Fig. 6.4) are discordant; the four fractions are col linear, lying along a Pb-loss trajectory with upper and lower concordia intercepts, of 210*^ and 12 Ma. It should be noted that the commonly observed correlation between U-content, grein size, and magnetic character does not apply to this population of zircons. However the degree of discordance is proportional to the measured U-content, supporting a Pb-loss hypothesis, the lower intercept probably h8S no geological significance (Parrish and Roddick, 1985) and in any case h8s errors too large for meaningful interpretation. The Late Triassic to Early Jurasic, upper intercept age supports field relations which shows K-feldspar porphyry (Texas Creek batholith age equivalent) intruding the andesite. This result suggests that Hazelton Group volcanism may have commenced in the Latest Triassic, a 163 conclusion also reached by Woodsworth et al. (1983) and van der Heyden (pers. comm., 1987) in the Terrace and Whitesail Lake map areas. A single isotopic analysis from dacitic ash tuff (DB-40) is discordant; therefore, no age determination is possible. It is discordant due to inferred Pb-loss and its isotopic composition overlaps the eutaxitic andesite Pb-loss curve and it could be of approximately the same age. Hornblende-plagioclese porphyry (B-382) is concordant at 190 * 2 Ma (Table 6.4). This result is similar to previously published results for porphyries associated with the Texas Creek batholith (Alldrick et al., 1986). Zircons from the Texas Creek plutonic suite ere distinctly pink as noted in this study, in Alldricketal. (1986), and by van der Heyden (pers. comm., 1987). Zircon U-Pb ratios for Monitor rhyolite tuff (B-394) are discordant and are not collinear on concordia diagram (Fig. 6.8). The zircons have high non-radiogenic 204pD values: probably due to minor attached feldspar (Fig. 6.1). Two age interpretations are possible: (1) Pb-loss from 197 ± 14 Ma old zircons (Fig. 6.8) or (2) possible inheritance with subsequent irregular Pb-loss, which could generate the observed array of points. A York linear regression through a fixed lower intercept of 0 Ma, gives a geologically reasonable age, 1971 14 Ma. This is consistent with Middle Toarcian fossils that were mapped above the rhyolite on Mount Dilworth. If there is an inherited component, then the 175 Ma lower 206pD/238u intercept age represents only a minimum age. The youngest Pb/Pb age could therefore approximate the maximum age (about 183 Ma; Fig. 6.8). Older inherited Pb is unlikely in this tectonic setting because older crust is unknown. Further sample collection and zircon analyses are required to refine the result. The presence of rare pink zircons like those in the Texas Creek plutonic suite suggests both units may have a common m8gma chamber or Monitor rhyolite assimilated part of the Texas Creek batholith. 164 • B-294 (2.7 km north) t A B-1 (6 km south) • B-383 (7 km south) Figure 6.1 Geochronometry sample localities near Silbak Premier, Stewart, northwestern British Columbia. Samples for dating are: U-Pb—DB-40, B-294, B-382 and B-394. K-Ar DB84-25,AT84-27-5,B-383. Rb-Sr—DB-40, B-1, B-22, B-79, B-294, B-295, B-332, B-340,B-366,B-382,B-394,AT84-27-5, and AT84-34-3. 165 Mount Dilworth BOWSER LAKE GROUP &Hr&i H v v " Black tuff and greywacke HAZELTON GROUP SILBAK PREMIER MINERALIZATION DB84-25* ? ? ? Monitor Lake-Slate Mountain Siltstone, gregwacke and argillite (Bathonian/C8llovian) Buff carbonate (Toarcian) Siliceous "Paiama beds" (Bajocian) - -SPATSIZI GROUP ? Monitor/Dilworth rhyolite breccia { 1 9 7 ± 1 4 Ma) B-394* * Green and maroon andesitic to dacitic volcaniclastic rocks 0 6 - 4 0 * * { i n d e t e r m i n a t e } Local porphgrg boulder conglomerate ) Dacitic porphyritic sills or flows (unit Jp) { 1 9 0 ± 2 Me} ("Premier porphyry") B-366* B - 3 8 2 * * •+- Andesitic flows, breccias and tuffs / 2 i r j • 2 4 Ma) B -294* * , B-332*, B-340*. - 14 ' Argillite, siltstone and greywacke Texas Creek granodiorite B-1 * LEGEND * U-Pb sample {date} * Rb-Sr sample n> K-Ar sample Figure 6.2 Schematic stratigraphic location of geochronometry samples near Silbak Premier, Stewart, northwestern British Columbia. 166 Figure 6.3 SEM-EDS images of typical zircons: A and B are from Hazelton eutaxitic andesite (sample B-294), C and D are extracted from hornblende-plagioclase porphyry (sample B-382), E to H are from Monitor rhyolite (sample B-394) illustrating attached feldspar (back scatter images on the right, F and H). B and C have euhedral P-rich (determined by EDS) casts believed to be apatite ghosts. 167 Figure 6.4 Eutaxitic andesite sample collected along the Granduc road at station B-294, Plate 1, near Silbak Premier, Stewart, northwestern British Columbia (Photograph by R.L. Armstrong). Figure 6.5 View northeast towards outcrops of Monitor rhyolite at Monitor Lake, station B-394, near Silbak Premier, Stewart, northwestern British Columbia, parallel east-dipping fractures do not represent bedding (Photograph by R.G. Anderson). Table 6.1. Summary of U-Pb zircon and K-Ar Isotopic ages determinations from this stud/-Sample Location (UTM zone 9) Easting Northing Rock type' Regional map unit 2 Isotopic method^ Isotopic age«(Ma) B-294 434950 6224650 Eutaxitic andesite Unit Hv U-Pb 5 + 24 210-14 DB-40 436850 6213700 Dacitic ash tuff Unit Hg U-Pb ** B-382 436750 6216200 Hb-plag porphyry Unit Jp U-Pb 190 ± 2 B-394 438800 6217000 Monitor rhyolite tuff Unit Hr U-Pb 197 ± 14 DB-84-25 438150 6212375 Siliceous breccia Alt. UnitJp W.R. K-Ar 63 ± 5 B-383 433185 6204625 Biotite lineation in mylonitic monzodiorite Texas Creek monzodiorite Bi. K-Ar 45.2 ± 3.2 AT84-27-5 435300 6214550 Biotite lamprophyre dyke Bi. K-Ar 25.2 ± 2.0 1. Hb = hornblende; plag = plagioclase. 2. Alt. = altered. 3. W.R. = whole rock. 4. 2 sigma errors; ** = indeterminate with present data. 5. Isotopic age determined using York (1969) regression of four data points. O N 00 Table 6.2. Zircon characteristics, Sample 1 Shape, external features 2 Colour Internal features Morphology B-294 9051 euhedral, remainder broken L/W = 2:1 to 4:1 pink rare fractures and colourless inclusions S. prismatic DB-40 euhedral to rounded (most sub rounded) L/W = 3:1 colourless to pink rare colourless inclusions ovoid B-382 euhdral, short prismatic bimodal size distribution L/W = 4:1, rarely = 6:1 light to medium pink numerous colourless inclusions, rarely cracked prismatic B-394 50% euhedral; 505? fragments <200m 205? with attached silicates L/W = 3:1, rarely = 4:1 colourless 105? colour less inclusions, rare opaque inclusions L. prismatic bypyramidal B-394 655€ euhedral; 3535 fragments >200m L/W = 3:1, rarely = 5:1 colourless rare colourless inclusions L. prismatic bypyramidal B-394 most euhedral, rare attached colourless <200m silicates, rare fragments tofaintpink NM1.0/5D L/W = 2:1 to 3:1, rarely = 5:1 minor colourless inclusions 5. prismatic 1. < 200m = zircons coarser than 200 mesh seive screen; > 200m = zircons finer than 200 mesh screen; NM = nonmagnetic; 1.0/5° = amperage on magnet and tilt of Franz magnetic separator ramp. 2. L/W = length/width ratio (visually estimated). 3. L. = long; S. = short. Table 6.3. Previous U-Pb isotopic age determinations for the Stewart area, northwestern British Columbia. Sample Location (UTM Zone 9) Easting Northing Rock type Rock name Isotopic age (Ma) 1 Ref. A84-I 432250 6232320 Hor n b lends gr anodior ite Summit Lake granodiorite 192.8 ± 2.0 2 A84-2 434400 6216475 K-feldspar porphyritic granodiorite dyke Texas Creek plutonic suite 189.2 i 2.2 2 A84-3 434400 6216475 K-feldspar porphyritic granodiorite stock Texas Creek plutonic suite (border phase) 195.0 i 2.0 2 A84-5 436760 6208240 K -feldspar / p lagioclase porphyry (dioriticd/ke) Texas Creek plutonic suite (Premier porphyry) 194.8 i 2.0 2 AT-84-34-3 436300 6212130 Biotite-hornblende monzonite dyke Hyder plutonic suite 54.8 ± 1.3 3 1. 2 sigmaerrors. 2. Alldrick etal., 1986. 3. R. G. Anderson, unpub. Tab le 6.4. U-Pb geoch ronometry resu Its. 1 Fractions U/Pb chemistry6 Sample Map Abr. Grain Magnetic Weight U Pb Measured unit 2 Tech.3 Size 4 Properties^ (mg) (ppm) (ppm) 206pD/204pD B-294 Hv AIR 100-200m M 1.0/3" 3.7 339.1 10.03 3809 B-294 Hv N.A. <100m NM 1.8/1° 7.8 499.7 12.85 6035 B-294 Hv N.A. >200m M 2;NM 1.5/2.5° 2.3 425.0 12.38 2726 B-294 Hv Apyr 100-200m NM 2.1/1° 9.4 330.2 10.10 10143 DB-40 Hg N.A. BULK NM 0.5/5° 0.5 400.5 11.90 705 B-382 Jp N.A. <200m NM 1.8/1° 4.4. 490.8 14.47 8664 B-382 Jp N.A. >200m M 1.0/3° 1.9 503.2 14.87 5407 B-394 Hr N.A. <200m NM 1.0/5° 2.3 299.6 10.14 341 B-394 Hr N.A. >200m NM 1.0/3° 3.0 304.2 10.46 275 B-394 Hr Apyr <200m M 1.0/5; NM 0.5/5" 5.1 277.2 9.73 307 B-394 Hr N.A. >200m M 1.0/3 2.2 327.7 10.65 432 1. See Appendix 6.1.1 for analytical methods and uncertainitles. 2. Hg = Hazelton green volcaniclastic; Hr = Monitor Lake rhyolite breccia; Hv = Hazelton andesite; Jp = Dacitic hornblende-plagioclase porphyry (Texas Creek plutonic suite). 3. AIR= air abraded, Apyr = pyrrhotlte abraded; N.A. = not abraded. 4. Grain size >200m indicates finer than 200 mesh size; < 200m indicates coarser than 200 mesh size. 5. NM = non-magnetic, M = magnetic; 1.0/3° = amperage on magnet and tilt of Franz magnetic separator ramp. 6. Com. = common; Blank composition: 206p D /204p D = 1 7 7 5 . 207p 0 / 204p b = 15.50; 2 0 8 P b / 2 0 4 P b = 37.30 Common Pb isotop ic com position deter mined from Stacey and Kramer (1975) growth curve at 220 Ma for first two B-294, 225 Ma for last two B-294, and DB-40; 190 Ma for B-382, and 185 Ma for B-394. Decay constants: ^238 = 0.155125 x 10 " 9 / y r ; A . 2 3 5 = 0.98485 x 10 "9/yr ; 238u/235u = 137.88. ^ 7. Errors are 1 sigma and refer to last significant figure of atomic ratio. ^ Table 6.4. U-Pb geochronometry results (cont'd). Atomic ratios 7 ± 1 a errors Isotopic ages (Ma) ± 2 o error Concordia Intercept age (Ma) 206 P b /238u 207pD/235ij 207p D /206p b 206p 0 /238ij 207 P b /235u 207p D/206 P b ± 2 a errors 0.02939* 17 0.02573* 15 0.2043* 16 0.1783* 15 0.05042* 25 0.05026*32 186.7 ± 2.0 163.8 t 1.8 188.7* 2.6 166.6 ±2.6 214 ± 23 207 ± 30 + 24 210±_J\J 0.02829* 18 0.1954* 19 0.05009* 37 179.8 ± 2.2 181 ± 3 199 ± 35 0.03068* 19 0.2127* 15 0.05029* 16 194.8 i 2.4 195.8 ±2.4 209 ± 15 0.02811 * 24 0.1965* 42 0.05070* 95 179 ±3 182 ± 7 ™ - I I indeterminate** 0.02996* 17 0.02980* 17 0.2066* 15 0.2052* 14 0.04998* 20 0.05001 * 11 190.4* 2.2 189.3 ±2.2 190.7 ± 2.4 189.5 ± 2.4 194 ± 19 193 ± 19 190 ± 2 0.02781 * 17 0.1928* 22 0.05027* 47 176.8* 2.2 179± 4 2 0 8 - 4 4 0.02703* 17 0.1867* 25 0.05010*59 171.9 ± 2.2 174± 4 + 44 2 0 0 -55 197 ± 14 0.02807* 18 0.02755* 17 0.1945* 15 0.1889 * 15 0.05025* 29 0.04974* 28 178.5 ± 2.2 1 75.2 ± 2.2 180.4 ± 2.6 175.7 ± 2.6 206 ± 27 183 ± 26 ** DB-40 age is indeterminate but compatible with concordia line for B-294. 173 0.17 0.18 0.19 0.20 0.21 0.22 0.23 2 0 7 p b / 2 3 5 u Figure 6.6 Enlargement of Concordia diagram between 165 and 220 Ma for: (a, top) the Hazelton eutaxitic andesite (B-294), and (b, bottom) green dust tuff (DB-40). Andesite points give York (1967) linear regression upper intercept of 210 + 24-14 Ma. Abbreviations are given in Table 6.4. 174 0.033 0.031 -00 CO CM - s . jp 0.029 Q. CO O CM 0.027-0.025 NM < 200 m (491 U) M > 200 m (503 U) 1 1 1 1 1 1 0.17 0.18 0.19 0.20 0.21 0.22 0.23 2 0 7 P b / 2 3 5 U Figure 6.7 Enlargement of Concordia diagram between 160 and 210 Ma showing dacitic hornblende-plagioclase porphyry, Texas Creek plutonic suite (B-382) zircon points that are concordant at 190 ± 2 Ma. Abbreviations are given In Table 6.4. oo CO CM n o_ CO o CM 0.033 - i 0.031 -0.029 -0.027 -0.025 NM < 200 m Apyr (227 U) NM < 200 m (300 U) NM > 200 m (304 U) 1 1 1 1 0.17 0.18 0.19 0.20 0.21 —T 1 0.22 0.23 2 0 7 p b / 2 3 5 u Figure 6.8 Enlargement of Concordia diagram between 160 and 210 Ma illustrating positions of Monitor rhyolite tuff zircon fractions (B-394). The York (1967) upper intercept is 197 ± 14 Ma. Abbreviations are given in Table 6.4. 175 6.3 K-AR DATING 6.3.1. Introduction The Stewart area has undergone Jurassic hydrothermal ore deposition, Jurassic and/or Cretaceous greenschist grade metamorphism, possibly a Cretaceous deformation, and Tertiary plutonism which have variably reset the K-Ar isotopic system, therefore, results must be carefully interpreted. Argon loss, producing younger apparent ages, is a function of solid state diffusion which varies with temperature and fluid availability. Mineral species, grain size, groundmass, and rock fabrics will influence how the K-Ar isotopic system reacts to subsequent deformational and thermal events. Finer grained minerals with greater surface area will tend to lose more argon. Closure temperatures depend on mineral species largely (but also grain size and cooling rate) and are reported by Parrish and Roddick (1985) to be: hornblende (530 ± 40 *C), muscovite (about 350 *C), biotite (280 ± 40 *C), and K-feldspar (130 ± 15 *C). Previous conventional K-Ar dates are reported by Smith (1977), Alldrick (1985), and Alldrick, Brown et al. (1987). The Texas Creek batholith results are discordant and indicate that a Cretaceous deformational/thermal (?) event affected biotite but not hornblende. Alternatively, these results could be attributed to partial resetting by intrusion of the Hyder plutonic suite. In either case the thermal event is bracketed between 530 "C and 280 *C, the respective hornblende and biotite closure temperatures. 6.3.2 K-Ar sample collection Three samples for K-Ar analysis were collected: (I) whole rock potassic alteration in siliceous breccia (DB-84-25), (2) a biotite lineation in the Texas Creek batholith (B-383), and (3) a biotite lamprophyre dyke at Indian Lake along the Granduc road (AT84-27-5; Plate 2). Isotopic results are listed in Table 6.6, and analytical procedures outlined in Appendix 6.1.2. 6.3.3 K-Ar results and discussion The 62.9 ± 4.6 Ma (Paleocene) whole rock K-Ar result for potassic altered siliceous breccia (sample DB-84-25; Table 6.5) lies between the Eocene plutonic episode and the 89-101 Ma whole rock K-Ar dates of 176 Alldrick et al. (1987a). Petrographically the sample is dominated by fine K-feldspar (< 4058) as veinlets and groundmass replacement. There are sporatic primary K-feldspar megacrysts (sanidine; < 558), and sericite is a minor consistent (< 558). Although a whole rock sample was analyzed, the material was predominantly fine K-feldspar with a low closure temperature of about 130 ± 15 *C (Parrish and Roddick, 1985). Therefore, the 62.9 Ms age is interpreted to be partial Eocene resetting of Jurassic or Cretaceous minerals. Alldrick's et al. (1987a) results could represent Cretaceous thermal resetting of Jurassic minerals or a Middle Cretaceous deformation event The metamorphic biotite and chlorite lineation formed in a mylonitic foliation in the Texas Creek quartz monzodiorite, 10 km south of Silbak Premier, and 2 km south of the Riverside Mine, Alaska. The 45.2 ± 3.2 Ma date is reset because the lineation-forming deformational event is known to be post-Texas Creek batholith and pre-Teriary intrusions, based on field relationships. Massive Hyder plutonic suite dykes cut across the lineation/foliation at high angles, indicating the deformation and mineral growth must be pre-Eocene. Smith (1977) also found no evidence of Eocene intrusions displaying mylonitic fabric or lineation. The K-Ar result, from a clean biotite mineral separate, indicates total Eocene resetting of an older mineral phase. Sample AT-84-27-5 from a biotite lamprophyre dyke (Fig. 6.9) gives a Miocene-01 igocene age (25.2 ± 2.0 Ma; Table 6.5). The isotopic age is supportive of field relations which suggest that lamprophyre dyke emplacement post-dates the Eocene Portland Canal dyke swarm and is a late intrusive event. Smith (1973) considered the lamprophyre dykes to be feeders to flows, since removed by erosion, and he noted the petrographic and chemical similarity with Miocene basalts of the Telegraph Creek map area (Souther, 1972). 177 Figure 6.9 Plane light photomicrograph of biotite lamprophyre (minette, sample AT84-27-5; Plate 1), illustrating large fresh biotite plates and clear, colourless euhedral pyroxene, and cloudy feldspar in an aphanitic groundmass. The field of view is 4.25 mm wide. Table 6.5. K-Ar isotopic age determinations.1 Sample/lithology2 Material % K = mean of duplicate analysis A r 4 0 * * [ I A r 4 0 ] Isotopic date analysed ± = range of duplicates from mean radiogenic ± 1 a error (Ma) DB-84-25 pyrite-sericite-adularia-siliceous breccia whole reck (-40 to + 70 mesh) 5.92 ±0.05 14.721 x 10" 6 cc/gm 6.569 x 10" 1 0 mol/gm 82.2 62.9 ± 2.3 B-383 Biotite lineation in Texas Creek granodiorite Biotite 4.17±0.02 7.416 x 10" 6 cc/gm 3.309 x 10-'°mol/gm 84.4 45.2 ±1.6 AT84-27-5 Biotite lamprophyre2 Biotite 7.22 * 0.01 7.133 x \0~6 cc/gm 3.183 x 10" 1 0 mol/gm 68.0 25.2 i 1.0 1. See Appendix 6.1.2 for analytical methods and uncertainties. K analyses by K. Scott and Ar analyses by J. Harakal (U.B.C. geochron. lab.). 40Kdecaycontants: X c = 0.581 x 10-^0 y e a r-1. jt^ = 4.96 x 10" 1° year" 1. Abundance ratio: 4 0K/K = 1.167 x 10~ 4 atom percent. 2. Collected by R. G. Anderson (Geological Survey of Canada). CO 179 6.4 RB-SR DATING 6.4.1 Introduction Thirteen samples were analyzed for Rb-Sr content and Sr isotopic ratios: four Texas Creek plutonic suite, four Hazelton volcanics, and five Tertiary intrusive rocks. Analytical procedure and errors are given in Appendix 6.1.3. Emphasis wss on determining the initial ratios for each group. Usable isochron dates were not expected and the isochrons obtained did not compare well with zircon U-Pb or K-Ar ages. Assumptions of Rb-Sr whole rock dating of magmatic suites outlined by Faure (1977) are: (I) during crystal fractionation, which produces the required spread in Rb-Sr ratios, it is assumed that the initial Sr isotopic ratio in all differentiates will be the same, (2) time of crystallization was relatively short, (3) all rocks regressed together are of one age. 6.4.2 Rb-Sr discussion The interpretation of Rb-Sr analytical results (Table 6.6 and Fig. 6.10), that do not coincide with U-Pb zircon, K-Ar, or stratigraphic age constraints, must consider the effects of a possible middle Cretaceous or later metamorphic disturbance. Fine grained and fragmental volcanic rocks are susceptible to Rb-Sr resetting. 6.4.3 Rb-Sr results A four point whole rock Hazelton volcanics isochron gives a model age of 112118 Ma and an initial 8 7Sr/ 8 6Sr intercept of 0.7050 ± 0.0006 (2 sigma errors used in text; Fig. 6.10). It is heavily weighted to the dacitic ash tuff point (DB-40). The result is obviously not the age of deposition but could represent an Albian or younger metamorphic event that reset or partially reset the Rb-Sr system. The date falls within the 110 ± 5 Ma thermal peak of Alldrick et al. (1987) that was based on K-Ar isotopic results. The Texas Creek plutonic suite isochron date lies between the age of emplacement and the period of metamorphism probably because intrusive rxks are less susceptible to Rb-Sr resetting than volcanic rxks. The four points yield a 148 ± 12 Ma date and 0.7043 ± 0.0002 initial ratio. The Tertiary rxks, 180 heavily weighted towards the rhyolite dyke (DB-22) produce an expected 44 ± 4 Ma whole rock isochron date with a 0.7056 ± 0.0001 initial ratio. The initial ratio compares with the 0.7046 to 0.7061 range reported by Arth et al. (1986) for their "52 and 53 Ma eastern-granite suite" in southern Alaska. The Eocene age results when the endesite dyke (B-295) is excluded in the linear regression for the slope of the isochron. Two andesite samples were mapped, one by the author and one by D. Alldrick (pers. comm., 1986) as Tertiary dykes based on field characteristics and fresh appearance. However, the andesite dyke of this study (B-295) plots along Texas Creek plutonic suite isochron, suggesting on isotopic evidence that it may be associated with the Jurassic suite. A similar andesite dyke from the Oranduc mill area gives a Jurassic K-Ar date (Alldrick et al., 1987). Additional mapping and analyses could resolve differences between Jurassic and Eocene andesite dykes. The whole rock and biotite separate for a lamprophyre dyke produce an 18 ± 6 Ma date with a 0.7047 ± 0.0001 initial ratio, essentially concordant and generally confirming the biotite K-Ar age of 25.2 ± 2.3 Ma. 6.4.4 Initial 8 7Sr/ 8 6Sr ratios Armstrong (in press) summarized the westward decrease of initial 8 7Sr/ 8 6Sr ratios for Mesozoic plutonic and Cenozoic volcanic rocks across the Cordillera. Hazelton Group volcanic, Texas Creek plutonic and Tertiary intrusive rocks around Silbak Premier were analyzed to better constrain initial ratio trends in the region. Hazelton volcanics, Texas Creek porphyritic rocks, and Hyder plutonic suite monzonites have initial ratios of 0.7050,0.7043, and 0.7047. The Jurassic rxks are probably partially reset; therefore, values appear high. These transitional values are characteristic of rxks in northern Intermontane Belt, and are part of a southward extending "prong" of transitional values in the Coast Belt (Armstrong, in press). Table 6.6 Rb-Sr analytical data1 for Silbak Premier area, northwestern British Columbia. Location (UTM zone 9) Sample number Description Easting Northing ppmSr ppm Rb Rb/Sr 87Rb/86Sr 8 7 S r / 8 6 S r 2 DB-40 dacitic ash tuff 436850 6213700 46.3 189 4.08 11.83 0.72346 ± 4 B-294 eutaxitic andesite 434975 6224625 198 90.9 0.459 1.327 0.70701 * 6 B-332 andesite 437175 6211940 193 140 0.727 2.10 0.70873 * 6 B-340 andesite 437150 6212460 147 114 0.777 2.25 0.70842 ± 6 B-394 Monitor rhyolite 438800 6217000 108 75.0 0.697 2.018 0.70828 * 7 tuff/breccia B-366 K-feldspar porphyry 436825 6212250 195 199 1.019 2.95 0.71070 * 4 B-382 nor n b lende- p lagioclase 436750 6216200 571 58.4 0.102 0.296 0.70498 * 9 porphyry B-1 hornblende diorite 433690 6207025 620 62.0 0.100 0.2896 0.70488 * 6 AT84-34-3 Hyder quartz 436225 6212400 698 69.4 0.099 0.288 0.70582 * 5 monzonite dyke B-79 Hyder quartz 435725 6211150 793 57.5 0.073 0.209 0.70563 * 6 monzonite stock B-22 rhyolite dyke 435680 6213525 48.2 199 4.13 11.96 0.71312 ± 3 B-295 andesite dyke 434950 6222125 778 41.0 0.053 0.152 0.70436 ± 3 AT84-27-5WR biotite lamprophyre 435300 6214530 1332 76.7 0.058 0.167 0.70476 ± 3 AT84-27-5 Bi biotite separate 435300 6214530 261 219 0.837 2.42 0.70532 ± 8 1. See Appendix 6.1.3 for analytical procedure and error estimates. 2. Errors are 1 sigmaand refer to last significant figure of 8 ? S r / 8 6 S r ratio. 182 0.725-1 0 2 4 6 8 10 12 8 7 Rb 8 6 S r Figure 6.10 Whole rock and one biotite Rb-Sr isochron diagram for the Hazelton volcanic rxks, Texas Creek plutonic suite, and Tertiary dykes. Error in age and initial 87Sr/8&Sr ratio (1 sigma) refer to last significant figure, but note that errors in text are 2 sigma. 183 6.5 GEOCHRONOMETRY CONCLUSION AND SYNTHESIS This study and previous U-Pb age determinations (Alldrick et al., 1986; Tables 6.3 and 6.-4) establish that Hazelton Group andesitic volcanism began at the end of Triassic time. The comagmatic Texas Creek plutonic suite, comprised of porphyritic sills, dykes and extrusive equivalents intruded the andesitic volcanic pile in Early Jurassic. Potassium-argon and Rb-Sr isotopic results suggest there was a Middle Cretaceous thermal (Tables 6.5, 6.6 and Fig. 6.10) and this may have been the time of major deformation of the rocks. An Eocene, thermal/plutonic event is recorded by U-Pb zircon and K-Ar hornblende and biotite dates for the Hyder plutonic suite. The biotite lineation in Early Jurassic Texas Creek granodiorite (sample B-383; Table 6.5) yielded a Tertiary isotopic age, but is interpreted to be totally reset. Whole rock dates from 60 to 80 Ma are interpreted as partial resetting of older rocks because no deformational or thermal events for this period are known or suspected. The Eocene, Hyder plutonism and related dykes is the dominant and most intense thermal event that has influenced K-Ar results, as shown from interpretation of U-Pb, K-Ar, and Rb-Sr dates (Tables 6.3 to 6.6). 01 igocene-Miocene biotite lamprophyre dykes , which cross-cut Hyder felsic dykes along the Granduc road, are the youngest intrusive rxks in the Salmon River valley. They correlate with olivine alkaline basalt dykes of Smith (1973). Rubidium-strontium model dates indicate the isotopic systematics were affected by an Albian thermal event. The initial ratios are compatible with regional patterns discussed by Armstrong (in press) where Mesozoic and Cenozoic rxks in the region have "transitional" values between 0.704 and 0.706, with Jurassic initial ratios lower than Eocene. The lowest values obtained, from a lamprophyre dyke, could reflect a deep primitive, possibly more directly from mantle, source with the least contamination from an evolved, but by then cool, Mesozoic crust. 184 CHAPTER 7 SUMMARY AND CONCLUSIONS 1. Silbak Premier is the second largest silver and third largest gold producer in British Columbia, with production between 1918 and 1968. Westmin Resources is currently undertaking a pre-development program for open pit mining at Silbak Premier and Big Missouri. 2. Silbak Premier is situated along the western edge of the Intermontane Belt adjacent to the Coast Belt, within the Stikine Terrene. Mineralization is hosted by Hazelton Group volcanic rocks and the coeval and probably comagmatic Texas Creek plutonic suite rocks. 3. Mapping and compilation of a 60 km2 area established the geologic setting of Silbak Premier, local stratigraphy, and structural discordance between Upper Triassic to Lower Jurassic Hazelton Group volcanics and overlying Middle Jurassic Bowser Lake Group sedimentary rocks. 4. The stratigraphic succession consists of Hazelton Group greenschist grade andesite flows, breccia and tuff (which together host the mineralization), overlain by heterolithic green and maroon andesitic to dacitic volcaniclastics (with rounded dacite porphyry fragments), and black tuff and greywacke with characteristic fresh biotite and white plagioclase crystal fragments. Above and below that are Monitor rhyolite breccia and tuff with fiamme, an important regional marker horizon. Silicic shale and tuff "pajama beds", possibly correlative with Bajocian Spatsizi Group, locally overlie Hazelton volcanics. Elsewhere Toarcian buff carbonate pods or Bowser Lake Group argillite and siltstone may overlie Hazelton volcanics. Bowser Lake Group, Ashman Formation, argillite and siltstone cap the sequence. It locally contains detrital muscovite. Poorly preserved belemnoids give a Callovian and/or (?) Bathonian age. 5. Hazelton andesitic volcanic rocks in the Stewart area are lithologically similar to Telkwa Formation around Smithers and Terrace. Monitor rhyolite has lithologic and possibly age similarities with Nilkitkwa Formation in Smithers area and Spatsizi Group to the northeast. 185 6. Hazelton paleogeography is unclear but a turbiditic argillite member with carbonate-rich beds in andesite suggests aqueous deposition. Maroon epiclastic rxks with fluvial sedimentary structures suggests that part of the green and marxn volcaniclastic rxks was subaerial. Black tuff was deposited into a reduced basin. Essentially contemporaneous Monitor rhyolite xntains fiamme, probably related to subaerial explosive volcanism. Toarcian carbonate suggests marine conditions. Spatsizi Group "pajama beds" contain both radiolaria and relict glass shards, this links pyrxlastic volcanism with contemporaneous marine deposition in a low energy basin. Bowser Lake Group argillite and siltstone were deposited in the marine Bowser Basin. 7. There are three intrusive rxk episodes. The oldest, Late Triassic to Early Jurassic Texas Creek plutonic suite, is characterized by K-feldsper porphyry ("Premier porphyry" that is spatially and probably genetically assxiated with mineralization at Silbak Premier) and some andesite dykes. "Premier porphyry" is green weathering dacitic sills and dykes. At higher structural levels it is locally marxn and could be extrusive. The green porphyries intrude andesite but not structurally overlying green and marxn volcaniclastic rxks. The second, Tertiary age Hyder plutonic suite is bimobal with monzodiorite to andesitic and quartz phyric dykes and stxks. The youngest event is represented by 01 igocene-Miocene lamprophyre to basalt dykes. 8. Whole rxk gexhemistry indicates Hazelton Group volcanic rxks and comagmatic Texas Creek porphyritic rxks are subalkaline, calcalkaline high-K to very high-K andesites and dxites. Txtonic discrimination diagrams indicate a calcalkaline, volcanic arc setting, with similar geochemical patterns to those for Andean volcanic rxks. 9. U-Pb zircon gexhronometry yields similar results to those reported by Alldrick et al. (1986), Texas Creek plutonic suite is Early Jurassic age. Hazelton andesite gave a Late Triassic to early Jurassic age. Potassium-argon and Rb-Sr isotopic results suggest there was a Middle Cretaceous thermal event which could be the time of major deformation of the rxks. A Tertiary thermal/plutonic event is evident from U-186 Pb zircon, K-Ar and Rb-Sr isotopic results from hyder plutonic suite. A biotite lamprophyre dyke is the youngest intrusive rxk of the area, dated by K-Ar and Rb-Sr methods to be Oligocene-Miocene age. U-Pb results: Hazelton andesite eutaxitic flow 210 + 24- 14 Ma (all errors are 2 o) Monitor rhyolite tuff 197 *. 14 Ma Texas Creek porphyry flow (?) 190 ± 2 Ma K-Ar results: Pot8ssicalteration 62.9 ± 4.6 Ma (partially reset) Biotite lineation in Texas Creek diorite 45.2 ± 3.2 Ma (reset) Biotite lamprophyre dyke 25.2 ± 2.0 Ma Rb-Sr results: Hazelton andesite whole rxk date 112 ± 18 Ma (reset) Texas Creek plutonic suite 148 +. 12 Ma (partially reset) Hyder plutonic suite 44 ± 4 Ma Biotite lamprophyre 18 ± 5 Me 10. Metamorphic grade dxreases from greenschist-grade in the basal andesites to sub-greenschist in Bowser Lake Group sedimentary rxks. Pressure shadows and asymmetric mineral fibers indicate syn-txtonic metamorphism. 11. The structural style varies with lithology and structural level and includes disharmonic tight folds, ductile shear zones, and brittle faults. At least 4 phases of pre-Eocene deformation are evident. They are defined by: (1) moderate west-plunging recumbent folds, (2) north-plunging inclined folds, (3) north-plunging upright folds, and (4) moderate west-plunging pencil lineations. 12. The map area was divided into three structural domains (North, East and Silbak) on the basis of structural style and structural orientation. The North domain is charxterized by the marked structural discordance between openly warped to massive Hazelton volcanic rxks and tightly disharmonically folded Bowser Lake Group argillite and siltstone. Three phases of folding are evident: first phase tight to isxlinal disharmonic, recumbent folds; second phase open folds with shallow northwest-dipping axial planar cleavage; and third phase upright, shallow north-plunging synclinorium. Structural continuity is difficult to establish due to lxk of marker horizons and inferred detxhments. 187 The East domain is characterized by phase 3 gently north-northwest-plunging anticline/syncline pairs and locally well exposed east-verging angular asymmetric folded Spatsizi Group rocks. The orientation of these folds suggests they could be related to movement along the north-striking dextral transcurrent Long Lake-Fish Creek fault zone. In contrast to North domain, Monitor rhyolite and/or Spatsizi Group rocks are structurally conformable with Bowser Lake Group rxks. The Silbak domain structure is obscured by pxr exposure and paucity of bedding. The domain is characterized by phase 4 pencil lineations, quartz veins, joints and brittle faults. Stope geometry illustrates the two mineralized structures (1) northeast zone and (2) northwest zone. The zones intersect at right angles in plan view. Research of ore geometry with respect to regional structure could provide insight into ore controls and distribution and age of ore with respect to structural features. Ductile fabrics of uncertain phase are steeply dipping east-west-striking zones in Texas Creek batholith at Riverside mine, Alaska and in marxn volcaniclastics along Bear River Ridge. Mylonitic fabrics are well exposed at Riverside mine and suggest a dextral sense of shear. This could be assxiated, but orthogonal, to the north-striking Long Lake-Fish Creek fault zone. 13. The structural history is pxrly constrained but northeast-verging folds could correlate with Late Cretaceous deformation as dxumented by Evenchick (1986) and Oabrielse et al. (in prep.)(Table 7.1). Significant Eocene northeast-southwest crustal extension is evident from Hyder dykes, this extension is not related to north-striking dextral transcurrent faults, because its extension direction is opposite to that produced with dextral offset. The dextral transcurrent movement on the Long Lake-Fish Creek fault zone continued after Hyder dyke emplacement. 01 igocene-Miocene east-west to southeast-northwest extension is represented by lamprophyre dykes. 188 14. Mineralization at Silbak Premier occurs elong two trends (1) a steeply dipping at surface but moderate dipping at depth (6-level) "northeast zone" and (2) a steep to vertical "northwest zone". Most production came from within about 500 m of the intersection of these zones. These trends are believed to represent structural controls to mineralization and emplacement of dacite porphyry intrusions. 15. There are at least four styles of mineralzation. Sulphide content varies, generally less than 52 but can be as high as 752. Textures range from stockwork and siliceous breccia to local layered to massive sulphide-rich mineralization. Such ore diversity is an indication of the complex and episodic nature to ore deposition at Silbak Premier. 16. Hydrothermal alteration associated with mineralization consists of a proximal potassic facies, within ten's of metres of ore, and peripheral propylitic facies. The potassic alteration is characterized by silicification/quartz stockwork and adularia and/or sericite. It is an important exploration guide to mineralization. Propylitic alteration, prominent in andesite, comprises carbonate-chlorite-pyrite. 17. A hydrid ore genesis model combining epigenetic vein and porphyry copper characteristics compare well with features observed at Silbak Premier. 189 Table 7.1 Geological events in the Silbak Premier area, northwestern British Columbia. EPOCH AGE (Ma) ROCK UNIT EVENTS Miocene Oligocene Eocene Paleocene Late Cretaceous Early Cretaceous Late Jurassic Middle Jurassic Early Jurassic Late Triassic 23.7 Lamprophyre dykes mafic dyke emplacement WNV-ESE block faulting crustal extension 36 6 Hyder dykes and stocks (Th) 57.6 ' 66.4 •transcurrent faulting • felsic dyke emplacement N-S and NE-SV crustal extension 97.5 P h a s e 3 and 4 -easterly-directed structures -moderate west-plunging pencil lineations + greenschist grade metamorphism (?) '144 163 Phase 1 and) 2 tight folding Bowser Lake Group, Ashman Fm. (B)--argillite, siltstone deposition in Bowser Basin Spatsizi Group (S)-- silicic shale/tuff deposition 167 Buff carbonate. Monitor rhyolite (Hr), Black Tuff (dacitic to rhyolitic) Texas Creek Hazelton calcalkaline volcanism ( . ^ p ^ ™ J p ) (dominantly andesitic) '208 (Hv) ORE DEPOSITION 190 REFERENCES CITED Alldrick, D.J. 1983 Salmon River Project, Stewart, British Columbia (104 B/1); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1982, Paper 1983-1, p. 182-195. 1984 Geological Setting of the Precious Metal Deposits in the Stewart Area (104 B/1); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1983, Paper 1984-1, p. 149-164. 1985 Stratigraphy and Petrology of the Stewart Mining Camp (104 B/1); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1984, Paper 1985-1, p. 316-341. 1987 Geology and mineral deposits of the Salmon River Valley, Stewart area (NTS 104 A,B); 1:50,000 Open File 1987-22, British Columbia Ministry of Energy, Mines and Petroleum Resources. Alldrick, D. 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A. 1974 Geology, Hazelton m8p area, British Columbia; Geological Survey of Canada, Open File map 215. 1976 Geology, McConnell Creek map area, British Columbia; Geological Survey of Canada, Open File map 342. 1980 Geology, Hazelton m8p ares, British Columbia; Geological Survey of Canada, Open File map 720. Roddick, J.A. 1983 Geophysical review and composition of the Coast Plutonic Complex, south of latitude 55^ N; in Circum Pacific Plutonic Terranes; edited by J.A. Roddick, Geological Society of America, Memoir 159, p. 195-212. Schof ield, S. J. and Hanson, G. 1922 Geology and Ore Deposits of the Salmon River District, B. C.; Geological Survey of Canada, Memoir 132,81 p. Schroeter, T. G. 1981 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological F ieldwork ,1980, Paper 1981-1, p. 124-131. 1982 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1981, Paper 1982-1, p. 122-133. 1983 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources,Geological Fieldwork, 1982, Paper 1983-1, p. 125-132. 1984 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1983, Paper 1984-1, p. 134-135. 1985 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1984, Paper 1985-1, p. 291-297. Schroeter, T. G., Diakow, L. J. and Panteleyev.A. 1986 Toodoggone River (94 E); British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1985, Paper 1986-1, p. 167-174. Seraphim, R. H. 1979 Report on Silbak Premier Property Near Stewart, B. C.; Unpublished report, 46 p. Simpson, C. andSchmid, S. M. 1983 An evaluation of criteria to deduce the sense of movement in sheared rocks; Geological Society of America Bulletin, v. 94, p. 1281 -1288. Smith, J. G. 1973 A Tertiary dike province in southeastern Alaska; Canadian Journal of Earth Science, v. 10, no. 3, p. 408-420. 201 Smith, J.6. 1977 Geology of the Ketchikan D-1 and Bradfield Canal A-1 Quadrangles, Southeastern Alaska; United States Geological Survey, Bulletin 1425,49 p. Sm ith, J. G., Stern, T. W., and Arth, J. G. 1979 Isotopic ages indicate multiple episodes of plutonism and metamorphism in the Coast Mountains near Ketchikan, Alaska; Geological Society of America, Abstracts with Programs, v. 11, p. 519. Smith, R. L, Thomson, R. C, and Tipper, H. W. 1984 Lower and Middle Jurassic sediments and volcanics of the Spatsizi map area, British Columbia; Current Research, Part A, Geological Survey of Canada, Paper 84-1 A, p. 117-120. Soregaroli, A. and Meade, H. 1983 Promise of the Stewart Area, British Columbia; Western Miner, v. 56, no. 5, p. 27-29. Souther, J.G. 1971 Geology and Mineral Deposits of Tulsequah map-area, British Columbia; Geological Survey of Canada, Memoir 362,84 p. 1972 Telegraph Creek map-8rea, British Columbia; Geological Survey of Canada, Paper 71-44,38 p. in prep. Post accretionary volcanism; in Decade of North American Geology special volume (part of the volcanism chapter). Souther, J. G., Brew, D. A.,andOkulitch, A. Y. (compilers) 1979 Map 1418A, Iskut River, British Columbia-Alaska, Sheet 104, 114, Geological Survey of Canada, 1:1,000,000 scale. Stacey, J. S. and Kramers, J. D. 1975 Approximation of terrestrial lead isotope evolution by a two-stage model; Earth and Planetary Science Letters, v. 26, p. 207-221. Steiger, R. H. and Jager, E 1977 Sub-commission on geochronology: convention on the use of decay constants in geo- and cosmxhronology; Earth and Planetary Science Letters, v. 36,p.359-362. Steininger, R.C. 1985 Geology of the Kitsault molybdenum deposit, British Columbia; Economic Geology, v. 80, p. 57-71. Sun, S.-S. 1980 Lead isotopic study from mid-ocean ridges, ocean islands and island arcs: Philosophical Transactions of the Royal Society of London Series A, v. 297, p. 409-445. Sutherland Brown, A. 1960 Geology of the Rocher Deboule Range; British Columbia Ministry of Energy, Mines and Petroleum Resources, Bulletin 43,78 p. 202 Tipper, H. W. 1978 Jurassic stratigraphy, Cry Lake map area, British Columbia; Current Research, Part A, Geological Survey of Canada, Paper 78-1 A, p. 25-27. Tipper, H. W. and Richards, T. A. 1976 Jurassic Stratigraphy and History of North-Central British Columbia; Geological Survey of Canada, Bulletin 270, 73 p. Tipper, H. W., Woodsworth, G. J., and Gabrielse, H. (co-ordinators) 1981 Tectonic assemblage map of the Canadian Cordillera and adjacent parts of the United States of America; Geological Survey of Canada, map 1505 A. Thompson, R.N., Morrison, M.A., Dickin, A.P., and Hendry, Q.L. 1983 Continental flood basalts... Arachnids rule OK?; in Continental basalts and mantle xenoliths, Hawkesworth, C.J. and Norry, M.J. (Editors), Shiva Publishing Limited, p. 158-185. Thomson, R.C. 1985 Lower to Middle Jurassic (Pleinsbachian to Bajocian) stratigraphy and Pliensbachian ammonite fauna of the northern Spatsizi Area, North Central British Columbia; M. Sc. thesis, University of British Columbia, 211 p. Thomson, R. C, Smith, P. L. and Tipper, H. W. 1986 Lower to Middle Jurassic (Pliensbachian to Bajxian) stratigraphy of the northern Spatsizi area, north-central British Columbia; Canadian Journal of Earth Science, v. 23, p. 1963-1973. Thorstad, L.E. 1983 The Upper Triassic "Kutcho Formation", Cassiar Mountains, north-central British Columbia, M. Sc. thesis, University of Britsih Columbia, 271 p. Thorstad, L.E., and Gabrielse, H. 1986 The Upper Triassic Kutcho Formation Cassiar mountains, north-central British Columbia; Geological Survey of Canada Paper 86-16,53 p. van der Heyden, P. 1982 Tectonic and stratigraphic relations between the Coast Plutonic Complex and Intermontane Belt, west-central Whitesail Lake map area, British Columbia; Unpublished M.Sc. thesis, University of British Columbia, 172 p. Vernon, R. H. 1986 K-feldspar megacrysts in granites--phenocrysts, not porphyroblasts; Earth-Science Reviews, v. 23, p. 1-63. Walker, R.G. 1979 Turbidltes and associated coarse clastic deposits; in Facies Models, Geoscience Canada, Reprint Series 1, p. 91 -103. Wanless, R.K., Stevens, R.D., Lachance, G.R. and DeLabio, R.N. 1979 Age determinations and geological studies, K-Ar isotopic ages, report 14; Geological Survey of Canada, Paper 79-2, p.23 203 Westgate, L. 6. 1921 Ore Deposits of the Salmon River District, Portland Canal Region, Alaska; United States Geological Survey, Bulletin 722 C, p. 117-140. White, W. H. 1939 Geology and Ore-deposition of Silbak Premier Mine Limited; Unpublished MASc., University British Columbia, 78 p. Winchester, JA and Floyd, PA 1977 Geochemical discrimination of different magma series and their differentiation products using immobile elements; Chemical Geology, v. 20, p. 325-343. Winkler, H.J.F. 1979 Petrogenesis of Metamorphic Rxks; Fifth edition, Springer-Yerlag, New York, 348 p. Wojdak, P. J. and Brown D. A. 1985 Silbak Premier, Stewart, B.C., 1984 Summary Exploration Report; Westmin Resources Limited, company report, 55 p. Wojdak, P. J., Dykes, S. M. and LeBlanc, E. R. 1984 Exploration at Silbak Premier, Stewart B.C., 1983 Report; Westmin Resources Limited, company report, 72 p. Woodsworth.G.J. 1979 Geology of the Whitesail L8ke map-area, British Columbia; Current Research, Part A, Geological Survey of Canada, Paper 79- IA, p. 25-29. 1980 Geology of Whitesail Lake (93E) map area, B.C.; Geological Survey of Canada, Open File map 708. Woodsworth, G.J., Anderson, R.G, Struik, L.C, and Armstrong, R.L. In prep. Plutonic Regimes; Chapter 15, in Decade of North American Geology Spxial Volume. Woodsworth, G.J., Hill, M.L., and van der Heyden, P. 1985 Preliminary geologic map of Terrace (NTS 103 I east half) map area, British Columbia; Geological Survey of Canada, Open File 1136. 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APPENDIX 1.1 SILBAK PREMIER PRODUCTION (1918-1968). 204 Year Tons Auoz/t Agoz/t PbX Zn£ Cu£ Cd* Ag:Au 1918 26 7.0 88.0 - - - - 12.6 19 268 6.6 221.0 - - - - 33.5 20 1,061 2.86 97.0 - - - - 33.9 21 17,591 1.88 60.5 - - - - 32.2 22 102,334 1.21 41.8 - - - - 34.5 23 145,665 0.80 18.9 2.1 - - - 23.6 24 159,014 0.875 19.0 1.42 - - - 21.7 25 168,557 0.70 14.4 2.34 - - - 20.6 26 230,987 0.527 12.6 0.97 - - - 23.9 27 244,172 o;48 12.8 4.70 - 2.57 - 26.7 28 275,811 0.47 8.6 5.30 - 3.73 - 18.3 29 266,972 0.364 8.7 3.94 - 2.43 - 23.9 1930 256,836 0.338 9.9 2.40 - 0.98 - 29.3 31 242,317 0.328 6.25 2.42 - 0.76 - 19.3 32 221,718 0.343 7.03 2.45 - 0.78 - 20.5 33 185,421 0.268 5.43 - - - - 20.3 34 153,693 0.17 4.38 - - - - 25.8 35 149,673 - - - - - - -36 192,442 0.225 5.2 - - - 23.1 37 201,206 0.237 4.54 - - - - 19.2 38 184,606 - - - - - - -39 169,164 0.24 5.25 - - - - 21.9 1940 171,504 0.216 3.58 - - - - 16.6 41 170,504 0.229 3.18 - - - - 13.9 42 140,567 0.259 3.51 - - - - 13.6 43 93,003 0.238 3.58 - - - - 15.0 44 68,496 0.223 1.68 2.33 - - - 7.5 45 65,801 0.220 1.30 2.05 - - - 5.9 46 34,804 0.234 1.10 1.68 - - - 0.5 47 59,343 0.22 1.49 2.25 - - - 6.8 48 41,360 0.207 1.46 1.97 1.36 - 0.014 7.1 49 10,142 0.164 0.81 1.62 0.97 - - 4.9 1950 79,167 0.205 1.69 2.0 2.66 - - 8.2 51 67,844 0.101 1.95 2.63 3.87 - 0.027 19.3 52 90,762 0.098 1.73 2.23 3.37 - 0.025 17.7 53 40,332 0.123 1.94 2.70 3.48 - 0.068 15.8 1954 to 1958 - — mine closed — - - - -1959 1,282 5.89 158.7 6.41 7.85 0.64 - 26.9 1960 62 10.48 271.4 5.15 8.64 - - 25.9 61 831 7.78 135.8 3.36 3.48 - - 17.5 62 465 7.0 112.4 3.15 4.6 - - 16.1 63 96 6.6 98.0 3.12 4.6 - - 14.8 64 2,712 0.71 14.4 - - - - 20.3 65 2,336 0.28 6.4 0.15 0.28 - - 22.9 66 14,189 0.57 11.6 0.30 0.42 - - 20.4 67 6,694 0.54 12.4 0.35 0.46 - - 23.0 1968 4 12.75 182.0 1.28 2.15 — — 14.3 Weighted averages: 0.390 oz/t 8.67oz/t2.14* 2.84* 1.93* 0.031* 19.1 Totals: 4,292,247 tonnes ore (4,731,834 tons); 53,393 kg gold (1,716,657 oz); 1,185,438 kg silver (38,113,318 oz) 1. B.C. Silver (1924-27); Premier (1918-37); Silbak Premier (1936-68); and Premier Border (1950-53). APPENDIX 1.2 UNPUBLISHED SILBAK PREMIER REPORTS (NOT CITED IN TEXT). 205 Best, R. Y. 1962 The Cascade Creek Area, Salmon River district, Portland Canal; Unpublished report, 15 p. Brown, D. A. 1984 Silbek Premier Geology; Westmin Resources Limited, company report, 13 p. Buchland, C. C. 1963 Geology of the Indian Mine, Cascade Creek, B.C.; Unpublished B.Sc. thesis, University of British Columbia, 25 p. Bunting, C. 1919 The Premier Gold Mine, Portland Canal, B.C.; Mining and Scientific Press, v. 119, p. 670-672. Dolmage, V. 1920 The High Grade Silver Ores of the Stewart District, B.C.; Canadian Mining Journal, v. 41, p. 454- 458. Fyles.J. T. 1948 Ores of the Silbak Premier; Unpublished 524 report, University of British Columbia, 36 p. Hanson, G. 1922 The Premier Mine; Transactions Canadian Institute of Mining and Metallurgy, v. 25, p. 225-232. Hill, H.L. and Starck, LP. 1961 The Silbak Premier Mine; A paper presented at the annual meeting, B.C. Section, C.I.M., Victoria,Sept. 29,4p. Hill, H. and Starck and Associates Limited 1963 Provisional Geological Report on the Silbak Premier Mine, Unpublished report, 15 p. Hughes, N. 1984 Paragenesis of the Woodbine Deposit; Unpublished 418 report, University of British Columbia, 21 p. Kidd.D. F. 1954 The Premier Mine 1954, Unpublished report, 14 p. Kretschmar, D. and Kretschmar, U. 1981 Summary Report, 1980 Surface Exploration Program: Unpublished report for British Silbak Premier Limited, 25 p. McGuigan, P.J. 1985 Summary report of 1985 exploration on the Indian Property; British Columbia Ministry of Energy, Mines and Petroleum Resources, Assessment Report 14111. 206 Plumb, W. N. 1955 White, W. H. 1962 Geological Report 1955 Exploration Program, Silbak Premier Mines Limited, Unpublished report, 53 p. Starck, L.P. 1955 A preliminary study of the factors involved in opening the Silbak Premier and Premier Border mines; Unpublished report, 27 p. Examination of Property of New Indian Mines, Limited; Unpublished report, 12 p. Wojdak, P.J. 1982 Silbak Premier Geology and Mineralization, 1982 Report; Westmin Resources Limited, company report, 17 p. Wojdak, P. J. and Randall, A.W. 1986 Silbak Premier, Stewart, B.C., 1985 Exploration Report; Westmin Resources Limited, company report, 57 p. 207 APPENDIX 5.1 GEOCHEMICAL TECHNIQUES. Twenty-seven "fresh" rocks were submitted to the Geological Survey of Canada, Economic Geology and Mineralogy Division, Analytical Chemistry Section, ICP-Emission Spectrometry Laboratory in Ottawa, Ontario. All major elements were analysed by inductively coupled plasma spectroscopy (ICP), except Pb by atomic adsorption (A.A.) and FeO, H 2 O T , C O 2 , C, Sand loss-on-ignition (LOI) by chemical methods. The trace elements As, Br, Mo, Nb, Rb, Sr, Th, U, Y, and Zr were determined by X-ray fluorescence energy dispersion system (XRF-EDS) using compton scatter to correct for mass absorption effects. Fe203 is calculated using Fe203 = Fe203 T (ICP) - 1.11134 * FeO (volumetric). ICP data for Si02, T i 0 2 , A I 2 O 3 , Fe^T, Fe203, FeO, MnO, MgO, CaO, Na20, K20, H 2 O T , C O 2 T , P 2 O 5 , and S are obtained on 0.5 g of sample fused with lithium metaborate, dissolved in 5S H N O 3 and diluted to 250 ml. ICP data for Ba, Be, Co, Cu, La, Ni, Pb, Y, Yb, and Zn are obtained on 1.0 g of sample (acid and fusion residue) dissolved in 10XHC1 and diluted to 100 ml. 208 Results are listed in Appendices 5 . 3 to 5 . 6 . Estimates of the validity of results from the GSC laboratory are listed below: ELEMENT ± (ABSOLUTE + RELATIVE) Si02 ± (0.4 58 + 2 58 of concentration) Ti02 ± (0.02 58 2 58 of concentration) A I 2 O 3 ± (0.2 58 + 2 58 of concentration) Fe203T (0.01 58 + 2 58 of concentration) MnO ± (0.1 58 + 2 58 of concentration) MgO 1 (0.1 58 + 2 58 of concentration) CaO ± (0.1 58 + 2 58 of concentration) Na20 ± (0.1 58 + 2 58 of concentration) K20 (0.1 58 + 2 58 of concentration) H 2 O T ± (0.1 58 + 5 58 of concentration) C O 2 (0.1 58 + 3 58 of concentration) P 2 O 5 ± (0.02 58 + 1 58 of concentration) S ± (0.04 58 + 5 58 of concentration) Rb (10 ppm + 10 58 of concentration) Sr ± (10 ppm + 10 58 of concentration) Ba ± (20 ppm + 5 58 of concentration) U ± (30 ppm + 10 58 of concentration) Zr t (10 ppm + 10 58 of concentration) Be ± (0 . 5 ppm + 5 58 of concentration) As ± (30 ppm + 10 58 of concentration) Br ± (10 ppm 10 58 of concentration) Mo ± (10 ppm + 10 58 of concentration) Co ± ( 5 ppm + 5 58 of concentration) Cr ± (10 ppm + 5 58 of concentration) Ni ± (10 ppm + 5 58 of concentration) Y ± ( 5 ppm + 5 % of concentration) Cu ± (10 ppm + 5 58 of concentration) Pb ± (20 ppm + 10 58 of concentration) Zn ± ( 5 ppm + 5 58 of concentration) La (10 ppm + 5 58 of concentration) Y ± (10 ppm + 10 58 of concentration) Yb ± (0 . 5 ppm + 5 58 of concentration) Nb 1 (10 ppm + 10 58 of concentration) F ± (20ppm + 20 58 of concentration) 209 APPENDIX 5.2 Rock geochemistry sample location and lithology. Sample Map UTM Location Lithology number Easting - Northing DB i-40 P-2 436875 6213700 Big Missouri rd. (1450') green dust tuff DB -70 P-2 438350 6213750 Bush adit (2100') K-feldspar porphyry OB i-105 P-2 436775 6212425 Silbak road (1620') purple hb-plag porphyry B- 15 P-2 436125 6214200 Oranducroad (1125") foliated K-feldspar B-22 P-•2 435675 6213525 Oranducroad(1300"0 dacite-rhyodacite B-96 P-•2 436650 6213150 Big Missouri road maroon lapilli tuff B-251 P-•2 437400 6212500 250 mNE of B.C. Silver green dust tuff B-255 P-•2 437500 6212350 200 m E of B.C. Silver maroon K-spar porphyry B-294 P-•2 434925 6224650 Oranduc road welded tuff [U-Pb] B-317 P-•2 437650 6212200 Cooper Creek (2600') maroon tuff B-332 P-•2 437200 6211950 Wilson Creek (2075') massive andesite B-340 P-•2 437400 6212250 North end Glory Hole fragmental andesite B-357 P-•2 436700 6212350 Silbak road purple tuff B-359 P-•2 436700 6212400 Silbak road green tuff B-360 P-•2 437150 6212200 2190 trench (2190') silicified K-spar porphyry B-366 P-•2 436800 6212250 2-Level trench K-feldspar porphyry B-379 P-•2 436600 6213100 Big Missouri road fragmental andesite B-381 P-•2 437050 6214050 Big Missouri rd. (1800*) fragmental andesite B-382 P-•1 436900 6216200 Big Missouri road hb-plag porphyry [U-Pb] B-394 P-•1 438800 6217000 Monitor Lake rhyolite breccia [U-Pb] B-454 P--2 437300 6212800 Lesley Creek trail (2125") purplish K-spar porphyry B-455 P--2 436350 6212425 Big Missouri rd. (1020") hb-plag porphyry B-458 P-•2 436400 6213000 Big Missouri rd. (1140") foliated andesite (tuff?) B-•524 P--2 436125 6212225 Granduc road (740') K-feldspar porphyry B-535 P--2 436650 6212150 Silbak rd. 4-Level altered hb-plag porphry B-543 P--2 437150 6212450 End B.C. Silver rd. (2190') maroon lapilli tuff B-546 P--2 437200 6212175 2190 trench K-feldspar porphyry 210 APPENDIX 5.3 Chemical composition of andesite samples. Abbreviations are B.D. = below detection limit and N.A. = not analyzed. MAJOR ELEMENTS SAMPLES B-294 B-332 B-340 B-381 B-533 AVERAGE STANDARD DEVIATION S102 57 .1 17 .9 56.4 64.2 50.1 55.1 6.4 A1203 16 .7 18 .0 15 .4 14.9 18 .0 16 .6 1.4 T102 0.61 0.78 0 .73 0.59 1.10 0.76 0.21 Fe203 0.4 1.7 6.3 0.9 2.1 2.3 2.3 FeO 1.3 6.2 N.A. 3.7 7.8 5.5 1.9 MnO 0.11 0 .29 0.48 0 .13 0.31 0.26 0.15 MgO 1.62 2 .18 2.08 1.84 4.11. 2 .37 1.00 CaO 5.36 7.53 5 .65 4.86 5.09 5.70 1.07 Na20 1.89 0.20 0.95 2.23 0.14 1.08 0 .96 K20 3.23 4.55 3.47 2.17 4.03 3 .49 0 .90 P205 0.25 0 .30 0.30 0.23 0.48 0.31 0 .10 H20t 3.3 4.1 N.A. 3.0 N.A. 2.1 1.9 C02 3.9 5.8 4.3 3.0 3-5 4.1 1.1 S 0.33 0.04 1.45 0 .03 0 .57 0.48 0 .58 TOTAL: 99.10 99.57 97 .52 101.78 97 .33 100.16 TRACE ELEMENTS AVERAGE STANDARD SAMPLES B-294 B-332 B-340 B-381 B-533 DEVIATIC Rb 90 130 87 58 130 99 31 Sr 230 150 150 260 92 176 68 Ba 1300 1700 2400 2500 1300 1840 581 U B.D. B.D. B.D. B.D. B.D. B.D. 0 Zr 86 82 110 100 94 94 11 Be 1.3 1.3 1.5 1.2 1.5 1.4 0.1 As 2 0 0 0 4 1 2 Br B.D. B.D. B.D. B.D. B.D. B.D. Mo 3 4 10 2 4 5 3 Co 11 17 13 11 27 16 7 Cr 12 10 14 15 13 13 2 Ni 41 51 28 71 47 48 16 V 140 160 100 87 260 149 68 Cu 28 28 33 25 85 40 25 Pb 9 8 22 13 16 14 6 Zn 67 110 98 77 130 96 25 La 13 14 18 20 16 16 3 Y 14 21 18 14 21 18 4 Yb 1.5 1.8 1.9 1.5 1.9 1.7 0.2 Nb 10 8 11 11 8 10 2 F 720 810 670 540 930 734 146.7 APPENDIX 5.4 211 Chemical composition of porphyritic dacite samples. Abbreviations are B.D. = below detection limit and N.A. = not analyzed. SAMPLES DB-70 DB-105 B-15 B-366 B-382 B-455 B-524 B-546 AVERAGE STANDARD DEVIATION S102 59.6 63.3 62.5 61.6 62.1 58.6 62.8 60.1 62.2 3.0 A1203 1U.8 14.3 15.0 15.1 17.4 15.1 16.0 14.5 15.2 1.0 T102 0.50 0.48 0.51 0.49 0.64 0.56 0.55 0.52 0.54 0.06 Fe203 0.7 1.8 0.9 0.8 4.0 5.6 0.6 0.6 1.9 1.8 FeO 3.6 2.5 3.4 3.4 2.0 N.A. 3.9 4.0 3.4 0.9 MnO 0.28 0.11 0.15 0.30 0.16 0.17 0.14 0.26 0.19 0.07 MgO 1.55 0.99 1.20 1.49 1.87 1.08 1.82 1.60 1.58 0.50 CaO 4.46 4.86 4.96 3.83 4.29 6.01 3.49 7.20 4.44 1.74 Na20 0.39 2.39 2.47 0.17 5.25 2.37 3.31 0.16 1.91 1.73 K20 4.96 2.64 2.83 7.89 1.46 3-33 3.99 3.89 3.74 1.85 P205 0.19 0.23 0.22 0.22 0.24 0.28 0.26 0.19 0.22 0.03 H20t 3.0 2.4 2.7 2.5 2.0 N.A. N.A. 3.4 2.0 1.3 C02 4.5 3.6 3.6 2.8 0.5 4.2 2.3 5.2 3-0 1.7 S 0.36 0.00 0.10 0.42 0.00 3.38 0.50 0.33 0.63 1.05 TOTAL: 98.89 99.60 100.54 101.01 101.91 100.68 "99766 101.95 TOO.93 SAMPLES AVERAGE STANDA DB-70 DB-105 B-15 B-366 B-382 B-455 B-524 B-546 DEVIATI Rb 150 69 79 170 29 100 99 120 99 43 Sr 260 230 260 210 750 290 230 180 277 187 Ba 4300 1500 1900 7200 13OO 2200 3500 2300 2889 1884 U B.D. B.D. B.D. B.D. B.D. B.D. B.D. B.D. Zr 88 100 88 100 120 89 97 98 99 11 Be 1 1 1 1 1 1 1 1 1 0 A3 33 0 34 2 40 22 20 110 30 33 . 1 Br B.D. 2 B.D. B.D. B.D. B.D. B.D B.D. B.D. Mo 2 4 3 3 0 3 3 3 3 1 Co 11 10 10 10 11 11 12 12 11 1 Cr 14 15 12 17 14 15 15 17 15 2 Ni 110 45 55 86 45 17 89 68 63 28 V 91 70 74 76 99 93 94 88 88 13 Cu 26 13 32 26 23 42 24 25 25 8 Pb 15 14 41 19 21 23 15 17 20 8 Zn 76 89 190 72 88 51 79 80 98 45 La 15 12 11 16 22 15 17 18 15 4 Y 13 13 9 10 19 11 9 11 12 3 Yb 1.4 1.4 1.3 1.4 2.0 1 5 1.5 1.5 1.50 0.21 Nb 9 13 8 9 10 8 10 9 9.4 1.5 F 530 70 80 510 640 730 300 630 436.25 256.12 APPENDIX 5.5 212 Chemical composition of tuff samples. Abbreviations are B.D. = below detection limit and N.A. = not analyzed. MAJOR ELEMENTS SAMPLES DB-UO B-251 B-96 B-317 B-357 B-543 B-359 B-458 AVERAGE STANDARD DEVIATION Si02 57.4 53.7 56.2 60.9 56.7 57.7 57.0 59.4 57.4 2- 1 A1203 22.6 22.0 19.4 17.3 21.1 14.2 17.8 20.5 19.4 2.8 Ti02 0.83 1.09 0.81 1.06 0.96 0.68 0.71 1.03 0.90 0.16 Fe203 1.6 1.6 7.4 7.1 8.1 3-9 2.5 8.0 5.0 2.9 FeO 5.1 8.0 1.4 2.9 1.5 2.6 9.8 N.A. 3.9 3.4 MnO 0.09 0.12 0.14 0.07 0.08 0.26 0.35 0.11 0.15 0.10 MgO 1.02 1.83 0.98 1.14 1.02 1.09 3.60 1.84 1.57 0.90 CaO 0.30 0.09 1.20 1.25 0.20 7.02 0.22 0.57 1.36 2.33 Na20 0.26 1.02 0.61 0.87 0.45 0.71 0.22 0.19 0.54 0.31 K20 5.93 3.57 5.45 3.14 5.85 3.55 2.79 5.29 4.45 1.30 P205 0.09 0.02 0.12 0.23 0.12 0.30 0.16 0.17 0.15 0.09 H20t 4.3 5.0 3.5 3.4 3.6 2.3 5.1 N.A. 3.4 1.7 C02 0.1 0.0 1.0 0.7 0.0 7.2 0.2 0.2 1.2 2.5 S 0.00 0.00 0.00 0.00 0.02 0.01 0.03 1.01 0.13 0.35 TOTAL: 99.62 98.04 98.21 100.06 99.70 101.52 100.48 98.31 99.49 TRACE ELEMENTS AVERAGE STANDARD SAMPLES DB-40 B-251 B-96 B-317 B-357 B-543 B-359 B-458 DEVIATION Kb 180 92 170 79 180 100 88 150 130 44 Sr 44 270 110 170 83 110 74 48 114 . 75 Ba 5300 4400 3000 2500 3700 1700 2600 4400 3450 1206 U B.D. B.D. B.D B.D. B.D. B.D. B.D. B.D. Zr 180 200 140 130 160 120 130 150 151 27 Be 2.2 1.3 1.9 1.8 1.7 ' 1.3 1.0 2.0 1.7 0.4 A3 0 0 1 7 0 0 16 10 D 6 Br B.D. B.D. B.D. B.D. B.D. B.D. B.D. B.D. Mo 1 3 1) 4 l( 1) 4 7 4 2 Co 13 19 14 16 17 14 38 14 18 8 Cr 12 23 11 13 16 12 16 14 15 4 Ni 140 120 83 72 100 52 81 27 84 36 V 110 170 • 160 200 170 83 130 150 147 37 Cu 15 6 19 18 14 11) 36 34 20 10 Pb 8 13 16 11 12 11 5 13 11 3 Zn 88 150 85 87 100 110 310 100 129 76 La 29 10 13 22 27 17 11 26 18 7 Y 28 17 22 25 20 18 16 19 21 4 Yb 2.9 1.7 2.3 2.4 1.8 1.6 1.6 2.0 2.0 0.5 Nb 15 14 13 15 16 10 12 16 14 2 F 400 530 660 850 620 340 580 650 579 160 213 APPENDIX 5.6 Chemical composition of miscellaneous samples. Abbreviations are B.D. = below detection limit and N.A. = not analyzed. SAMPLES B-255 B-454 B-394 B-22 B-360 B-379 S102 55.9 59-4 67.5 76.7 78.1 68.9 A1203 18.11 17-0 13 .6 12.7 10.0 14.2 T102 0.82 0.76 0.65 0.07 0.31 0.64 Fe203 7.7 7.5 1.0 0.4 2.95 1.9 FeO O.i) 0.7 7.0 0.4 N.A. 4.7 MnO 0.18 0.16 0.18 0.05 0.05 0.14 MgO 2.59 1.82 4.30 0.15 0 .32 2.62 CaO 3-22 4.06 0.50 0.51 1.10 0.90 Na20 7.53 4.35 1.02 2.70 0.63 0.67 K20 1.07 2.03 1.40 4.15 2.64 2.63 P205 0.26 0.39 0.05 0.01 0.16 0.18 H20t 1.8 2.0 4.1 1.0 N.A N.A. 002 0.3 1.0 0.1 0.4 0.8 0.6 S 0.03 0.04 0.00 0.03 1.70 0.61 TOTAL: 100.20 101.21 101.40 99.27 98.81 98.69 SAMPLES B-255 B-454 B-394 B-22 B-360 B-379 Rb 9 52 53 210 71 76 Sr 500 590 100 69 150 87 Ba 1100 2300 1600 710 3900 1800 0 B.D. B.D. B.D. B.D. B.D. B.D. Zr 99 100 220 64 64 110 Be 1 2 2 2 1 1 A3 14 37 0 24 72 8 Br B.D. 8 B.D. B.D. B.D. B.D. Mo 2 0 3 0 12 5 Co 16 15 16 1 9 9 Cr 13 12 16 3 11 13 Ni 37 69 53 22 18 54 V 90 140 61 1 50 110 Cu 7 12 20 2 26 16 Pb 23 24 18 31 140 19 Zn 92 86 150 25 260 160 La 17 22 20 7 11 9 Y 22.0 15.0 46.0 9.0 10.0 16.0 Yb 1.8 1.6 2.4 1.0 1.1 1.7 Nb 9 9 11 16 9 9 F 680 690 1000 100 520 680 214 APPENDIX 6.1 U-PB, K-AR AND RB-SR ANALYTICAL TECHNIQUES. 6.1.1 U-PB ANALYTICAL TECHNIQUES Approximately 40 kg of rock sample were reduced to powder by jaw crushing, and grinding in a disc mill. The powder was passed over a wet shaking (Wilfley) table to obtain a bulk heavy mineral concentrate. Zircons were purified by heavy liquid and magnetic separations (Frantz isodynamic separator; Krogh, 1982 b). They were washed in strong aqua regia, and sized using nylon mesh screens. Uniform clean zircon populations were hand-picked from zircons with attached silicates or inclusions. The zircon extraction, dissolution, U and Pb isolation followed procedures of Krogh (1973) and in the U.B.C. Geochronology Labratory Manual (Mortensen and Parkinson, Unpub., 1985). Three separates required air or pyrrhotite abrasion techniques as developed by Krogh (1982 a). Dissolution and isolation of U and Pb were done by P. van der Heyden using Krogh's (1973) method. Solutions were loaded onto single rhenium filaments coated with H3P04~silica gel. Analyses were conducted on an automated Yacuum Generators 1S0MASS 54R solid source mass spectrometer in single collector mode (Faraday cup). Automation and data reduction was done with a dedicated Hewlett Packard HP-85 computer. Precisions for 207pb/206pD a n ( j 208pb/206pb w e r e better than 0.1 *, and for 204pb/206pb they were better than 0.5*. Total Pb blanks were 0.12 ± 0.05 ng and total U blanks were 0.05 ± 0.02 ng, based on repeated procedural blank runs. U-Pb and Pb-Pb errors for individual zircon fractions were obtained by individually propagating all calibration and analytical uncertainities through the entire date calculation program and summing the individual contibutions to the total variance. Upper 8nd lower concordia intercepts from discordant fractions were determined by fitting a straight line using a routine based on York (1969), and extrapolating to concordia using the algorithm of Ludwig (1980). Errors on individual dates and on calculated intercept ages are quoted at the 2 sigma level (95* confidence level). 215 6.1.2 K-AR ANALYTICAL TECHNIQUES Whole rock samples were ground and sieved to -40 to + 70 mesh. Biotite mineral separates were obtained after grinding and sieving using standard heavy liquid separation methods. Frantz magnetic separator W8S used to provide a cleaner biotite separate before analysis. Potassium chemistry and argon mass spectrographic analyses were done by K. Scott and J. Harakal, respectively. Two K-analyses were obtained for each sample. Argon analysis involves sample fusion using an induction furnace, addition of pure 3 8Ar spike, and conventional purification techniques. Prior to fusion all whole rxk samples were baked at 130*C for approximately 15 hours to reduce atmospheric argon contamination. The gas mixture was purified by passing it over titanium "getter" furnaces and the argon isotopic ratios were measured on an an Associated Electrical Industries MS-10 mass spectrometer equipped with a Carey model 31 vibrating reed electrometer, operated in a static mode. When necessary, spike calibrations are.performed against known aliquots of "purified" air and interleboratory mineral standards such as the GL-0 glauconite. The errors reported are for one sigma in Table 6.5 but are 2 sigma in the text. Analyses of standards indicate K and Ar accuracy better than 258. For samples containing less than 5058 atmospheric argon contamination, results are internally consistent to within 1 58 and the range of the age determination (accuracy) lies within 358 of the calculated age (Harakal, pers. comm., 1986). All ages are calculated using IUOS-recommended decay constants (Steiger and Jager, 1977). 216 6.1.3 RB-SR ANALYTICAL TECHNIQUES Rb and Sr concentrations were determined by replicate analysis of pressed powder pellets using X-ray florescence. US6S rxk standards were used for calibration; mass absorption coefficients were obtained from Mo K-alpha Compton scattering measurements. Rb/Sr ratios have a precision of 2* (I sigma) and concentrations a precision of 5* (1 sigma). Sr isotopic composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer, a Vacuum-Generators ISOMASS 54 R, has data aquisition digitized and automated using a Hewlett Packard HP-85 computer. Experimental data have been normalized to a 86sr/88sr ratio of 0.1194 and adjusted so that the NBS standard SrC03 (SRM 987) gives a 8 7 S r / 8 6 S r ratio of 0.71020 ± 0.00002 and the Eimer and Amend Sr a ratio of 0.70800 ± 0.00002. The precision of a single 87Sr/8&Sr ratio is better than 0.00010 (I sigma). Rb-Sr dates are b8sed on a Rb decay constant of 1.42 x 10"'1 year"'. The regressions are calculated according to the technique of York (1967). 


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