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Geology, wallrock alteration, and characteristics of the ore fluid at the Bralorne mesothermal gold vein… Leitch, Craig Henry Bowen 1989

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GEOLOGY, WALLROCK ALTERATION, AND CHARACTERISTICS OF THE ORE FLUID AT THE BRALORNE MESQTHERMAL GOLD VEIN DEPOSIT, SOUTHWESTERN BRITISH COLUMBIA By CRAIG HENRY BOWEN LEITCH B.Sc., Queen's University, Kingston, Ontario, 1971 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n . THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Apr i l 1989 @ Craig Henry Bowen Leitch In p resen t i ng this thesis in partial fu l f i lment of the requ i remen ts fo r an a d v a n c e d d e g r e e at the Univers i ty of Brit ish C o l u m b i a , I agree that t he Library shal l m a k e it f reely avai lable fo r re fe rence and s tudy. I fur ther agree that p e r m i s s i o n fo r ex tens ive c o p y i n g of this thes is fo r scho lar ly p u r p o s e s may b e g ran ted by the h e a d of m y depa r tmen t o r by his o r her representa t ives . It is u n d e r s t o o d that c o p y i n g o r pub l i ca t i on of this thesis fo r f inancial ga in shal l no t b e a l l o w e d w i t h o u t m y wr i t ten p e r m i s s i o n . D e p a r t m e n t T h e Un ivers i ty of Brit ish C o l u m b i a V a n c o u v e r , C a n a d a 77 D E - 6 (2/88) - i i -ABSTRACT The B r i d g e River gold camp produced more gold than any other camp i n B r i t i s h Columbia over i t s 70 years of op e r a t i o n CI 30 tonnes or 4 m i l l i o n 02 of Au, from 7 m i l l i o n tonnes of 18 g/t o r e ) , mainly from the Bralorne-Pioneer mesothermal v e i n d e p o s i t . The d e p o s i t s are hosted i n the ac c r e t e d B r i d g e River and Cadwallader Terranes, which are of Permian t o T r i a s s i c age and of oceanic and i s l a n d - a r c c h a r a c t e r , r e s p e c t i v e l y . M i n e r a l i z a t i o n i s temporally and s p a t i a l l y r e l a t e d to a s u i t e of e a r l y Late Cretaceous a l b i t i t e dykes of 36-91 Ma age, and thus occurred long a f t e r and i s g e n e t i c a l l y u n r e l a t e d t o the emplacement of the major host i n t r u s i v e s ( B ralorne d i o r i t e and soda g r a n i t e ) , dated as E a r l y Permian O 2 7 0 Ma U-Pb; 284 Ma K-Ar). The major go l d - b e a r i n g v e i n s at B r a l o r n e s t r i k e about 1 1 0 0 and dip north 7 0 0 , with s l i c k e n s i d e s that plunge 45 e > east and i n d i c a t e that the l a s t movement was re v e r s e . Major ore shoots i n the v e i n s occupy somewhat l e s s than 20'/. of the ve i n and plunge s t e e p l y west, roughly p e r p e n d i c u l a r t o the s l i c k e n s i d e s . The most common host rocks for p r o d u c t i v e v e i n s are the competent B r a l o r n e d i o r i t e and the Cadwallader greenstone; soda g r a n i t e may have been too weak to s u s t a i n l a r g e f r a c t u r e s , or too low i n Fe for p y r i t e p r e c i p i t a t i o n . Hydrothermal a l t e r a t i o n envelopes around the v e i n s are up t o 10 m wide and grade outwards from i n t e n s e l y f o l i a t e d quartz - a n k e r i t i c carbonate - s e r i c i t e C+fuchsite) t o l e s s - i i i -s h e ared c a l c i t e - c h l o r i t e - a l b i t e t o unsheared e p i d o t e -c a l c i t e . Chemical s t u d i e s of t h e a l t e r a t i o n on a c o n s t a n t volume b a s i s ( n o r m a l i z e d t o Al^Os and TiQz>, which have remained r e l a t i v e l y i m m o b i l e ) , show a d d i t i o n of K^Q, CO^, S, As and Au, but d e p l e t i o n of Na 20, FeO ( t o t a l . ) and MgO as t h e v e i n i s approached. S i 0 2 and CaO are l o c a l l y d e p l e t e d and r e c o n e e n t r a t e d . ' D i s s e m i n a t e d p y r i t e , p y r r h o t i t e , and minor c h a l c o p y r i t e occur w i t h i n e n v e l o p e s f o r up t o s e v e r a l meters from t h e v e i n s . A r s e n o p y r i t e i s c o n f i n e d t o v e i n s e l v a g e s . O c c a s i o n a l s p h a l e r i t e , and e s p e c i a l l y g a l e n a , appear t o c o r r e l a t e w i t h g o l d - r i c h p o r t i o n s of the v e i n s . T r a c e s of t e t r a h e d r i t e and s t i b n i t e have been observed but t e l l u r i d e s have n o t . Gold i s found p r i n c i p a l l y as t h i n smeared f l a k e s of t h e n a t i v e metal i n t h e b l a c k s u l f i d i c s e p t a e of t h e s t r o n g l y r i b b o n e d v e i n s . Gold i s o n l y r a r e l y found by i t s e l f i n t h e q u a r t z , u s u a l l y i n e x t e n s i o n a l v e i n s r a t h e r t han shear v e i n s , where i t l o c a l l y forms e x t r e m e l y r i c h poc k e t s . Based on f l u i d i n c l u s i o n and s t a b l e i s o t o p e s t u d i e s , p r i m a r y o r e d e p o s i t i o n appears t o have been from f l u i d s of low s a l i n i t y ( l e s s than 5 wt. 7. NaCl e q u i v a l e n t ) w i t h a s i g n i f i c a n t C0 2'and minor ChU c o n t e n t , at t e m p e r a t u r e s of SSO^C and p r e s s u r e s o f up t o 1.75 kb (7 km d e p t h ) . L a t e r s e c o n d a r y f l u i d s were even more d i l u t e , w i t h much lower COaa c o n t e n t s and no CH*, at t e m p e r a t u r e s below 240 < = ,C and p r e s s u r e s o f 0.5 kb. S u l f u r i s o t o p i c r a t i o s of s u l f i d e s a s s o c i a t e d w i t h t h e g o l d m i n e r a l i z a t i o n r a n g e from -7 t o +9 per m i l , c l u s t e r i n g about 0 per m i l . The o r e f l u i d had a d l s Q o f +13 + 1 and d 1 3 C o f -11 + 2 p e r m i l . H o m o g e n i z a t i o n t e m p e r a t u r e s i n c r e a s e w i t h d e p t h a t a p p r o x i m a t e l y 30°C/km, a normal g e o t h e r m a l g r a d i e n t . Computer m o d e l l i n g o f t h e f l u i d r e s p o n s i b l e f o r t h e o b s e r v e d a l t e r a t i o n a s s e m b l a g e s and g o l d d e p o s i t i o n u s e d H e l g e s o n ' s 1969 and 1978 d a t a , and c h l o r i d e c o m p l e x e s . The r e s u l t s s u g g e s t t h a t t h e f l u i d had a pH o f about 4.5, a Na:K r a t i o o f a t l e a s t 8:1, and a h i g h c o n t e n t o f d i s s o l v e d COs* ( l o g f u g a c i t y +2.5). C o n d i t i o n s were s t r o n g l y r e d u c i n g , a s s u g g e s t e d by t h e CH* C l o g f u g a c i t y +0.5), w i t h f 0 a about 10~ 3 0 and fSsj about l O - 7 " . The model p r e d i c t s p r e c i p i t a t i o n of g o l d i n t h e i m m e d i a t e l y a d j a c e n t , h i g h l y q u a r t z - s e r i c i t e -a n k e r i t e a l t e r e d w a l l r o c k , i n r e s p o n s e t o r e a c t i o n w i t h t h e w a l l r o c k . I t a l s o p r e d i c t s a s t r o n g c o r r e l a t i o n between g o l d and p y r i t e , but not w i t h p y r r h o t i t e . T hese p r e d i c t i o n s a r e s u p p o r t e d by o b s e r v e d a l t e r a t i o n a s s e m b l a g e s , and by t h e h i g h CO a and minor CH.* o b s e r v e d i n f l u i d i n c l u s i o n s . The v e i n s at B r a l o r n e may have formed by m i n e r a l i z a t i o n o f f a u l t s t h a t o r i g i n a l l y d e v e l o p e d w i t h i n a R i e d e l s h e a r z one Cthe n o r t h - w e s t t r e n d i n g B r a l o r n e f a u l t zone) w i t h an — v -e a s t - w e s t maximum c o m p r e s s i v e s t r e s s and n o r t h - s o u t h , h o r i z o n t a l minimum c o m p r e s s i v e s t r e s s . S i n i s t r a l movement i s i m p l i e d f o r t h i s f a u l t s y s t e m i n t h e L a t e C r e t a c e o u s . A " f a u l t - v a l v e " model b e s t e x p l a i n s t h e r i b b o n e d , y e t e u h e d r a l , c o a r s e l y c r y s t a l l i n e m i l k y q u a r t z v e i n s . C y c l i c b u i l d - u p o f f l u i d p r e s s u r e i n a g e o p r e s s u r e d r e s e r v o i r below t h e b r i t t l e - d u c t i l e t r a n s i t i o n c a u s e d o v e r p r e s s u r i n g , i n v o k i n g f a i l u r e by r e a c t i v a t i o n o f t h e p r e v i o u s l y formed s t e e p l y - d i p p i n g f a u l t s , u n f a v o u r a b l y o r i e n t e d f o r f a i l u r e at a h i g h a n g l e t o t h e maximum c o m p r e s s i v e s t r e s s i n a t r a n s p r e s s i v e r e g i m e . F a i l u r e p r o v i d e d o p e n i n g s f o r d i s c h a r g e o f f l u i d s , and t h e c o i n c i d e n t d r o p i n p r e s s u r e promoted d e p o s i t i o n o f q u a r t z and s u l f i d e s ; zoned q u a r t z c r y s t a l s were d e p o s i t e d i n s p a c e h e l d open by t h e h i g h p o r e p r e s s u r e s . S e a l i n g o f t h e f a u l t by t h i s m i n e r a l d e p o s i t i o n a l l o w e d f l u i d p r e s s u r e t o b u i l d and t h e c y c l e t o r e p e a t . Each o f t h e r i b b o n s o f s u l f i d e , w i t h minor g o l d , p r o b a b l y r e p r e s e n t s a s l i v e r o f h i g h l y r e p l a c e d w a l l r o c k t h a t was i n c l u d e d i n t h e v e i n when t h e next e p i s o d e o f f r a c t u r i n g and m i n e r a l d e p o s i t i o n o c c u r r e d . F r o n t i s p i e c e , (above): View southeast up Cadwallader Creek, along the s t r i k e of the B r a l o r n e - P i o n e e r v e i n system. Noel Creek i s to the r i g h t , with Sunshine Peak i n the d i s t a n c e . The o l d m i l l b u i l d i n g s and coarse ore storage b i n a t the mouth of the 8 l e v e l (main haulage) p o r t a l are shown, (below): View northeast down H u r l e y R i v e r to Bralorne i n the d i s t a n c e , l y i n g i n the northwest-trending Cadwallader v a l l e y at the foot of Mount Fergusson (below the h o r i z o n t a l l o g g i n g s l a s h ) . - v i i -T A B L E OF CONTENTS Page ABSTRACT i i FRONTISPIECE v i TABLE OF CONTENTS v i i LIST OF TABLES x i LIST OF FIGURES x i i i LIST OF PLATES x v i ACKNOWLEDGEMENTS x v i i i CHAPTER 1: INTRODUCTION 1. 1 Loc at i on . 1 1 . 2 Hi s t o r y 1 1.3 Pr e v i ous Wor k 5 1.4 Purpose of s t u d y 6 1.5 Scope of s t u d y • & CHAPTER 2: REGIONAL SETTING OF THE BRIDGE RIVER CAMP 2.1 T e c t o n i c S e t t i n g . B 2.2 S t r a t i g r a p h i c S e t t i n g 12 2.2.1 B r i d g e R i v e r Group 16 2.2.2 Cad w a l l a d e r Group 18 2.2.3 Tyaughton Trough 13 2.3 P e t r o c h e m i s t r y of t h e B r i d g e R i v e r and Cadw a l l a d e r groups » 20 CHAPTER 3: GE0CHR0N0L0GY OF THE BRIDGE RIVER CAMP 3.1 I n t r o d u c t i o n 32 3.2 Sampling D e t a i l s 35 3.3 Sampling P r e p a r a t i o n and A n a l y t i c a l T echniques 37 3.3.1 Potassium-Argon 37 3.3.2 Ur an i urn-Lead 39 3.3.3 R u b i d i u m - S t r o n t i u m 42 3. 4 R e s u l t s 46 3.4.1 Potasssium-Argon 46 3.4.2 Uranium-Lead 49 B r a l o r n e D i o r i t e and Soda G r a n i t e 50 A l b i t i t e Dykes 54 3.4.3 R u b i d i u m - S t r o n t i u m 56 3.5 D i s c u s s i o n 62 3.5.1 Age of M i n e r a l i z a t i o n 62 3.5.2 Age and P e t r o c h e m i c a l S e t t i n g o f B r i d g e R i v e r and Cadwallader groups 67 3.6 C o n c l u s i o n s 74 CHAPTER 4: GALENA LEAD ISOTOPES OF THE BRIDGE RIVER CAMP 4.1 I n t r o d u c t i o n 77 4.2 M i n e r a l D e p o s i t s of t h e B r i d g e R i v e r Camp 77 4.2.1 B r a l o r n e Type 78 4.2.2 Congress Type 79 4.2.3 Tyaughton Type 80 - v i i i -4.2.4 Blackdome Type 80 4.2.5 Summary SI 4.3 Galena Lead Isotope Analyses 83 4.4 Galena Lead Data 85 4.5 D i s c u s s i o n 90 4. 6 Cone 1 usi ons '="4 CHAPTER 5: MINE GEOLOGY 5.1 I n t r o d u c t i o n 97 5.1.1 Mine Layout 100 5.2 Rock Types: L i t h o l o g y , Petrography, P e t r o l o g y 103 5.2.1 Bridge River (Fergusson) Group 105 5.2. 1~1 V o l c a n i c s ~ 106 Sediments 107 5.2.2 Cadwallader Group 108 Pi oneer For mat i on 110 Hurley Formation 116 5.2.3 B r a l o r n e I n t r u s i o n s 117 P r e s i d e n t U l t r a m a f i c s 118 H o r n b l e n d i t e 123 B r a l o r n e D i o r i t e 125 Soda g r a n i t e 130 Migmatite 134 A p l i t e Dykes 137 5.2.4 C r e t a c e o u s - T e r t i a r y Dykes 139 Grey p l a g i o c l a s e porphyry 139 A l b i t i t e and Green Hornblende Porphyry Dykes 140 Bendor Dykes 145 Lamprophyre Dykes 147 5.3 Met amor ph ism - 149 5.3.1 F a d e s 149 5.3.2 Timing 151 5.4 S t r u c t u r e and V e i n i n g 153 5.4.1 F o l d i n g 155 5.4.2 F a u l t i n g and Dyking 155 5.4.3 Quartz v e i n s Morphology 157 Ore C o n t r o l s 163 5.4.4 Rie d e l Shear Model 16& 5.5 Sequence of Events 173 CHAPTER 6: ALTERATION 6.1 M i n e r a l o g i c a l Zoning 177 6.1.1 Chi o r i t e - e p i d o t e (Outer) Zone 178 6.1.2 C a r b o n a t e - a l b i t e - S e r i c i t e ( C e n t r a l ) Zone 181 6.1.3 Q u a r t z - s e r i c i t e - c a r b o n a t e (Inner) Zone 187 6.1.4 Other A l t e r a t i o n Types 189 S i l i c a F l o o d i n g 189 B i o t i t e A l t e r a t i o n 194 Black Carbonate A l t e r a t i o n 196 Tourmaline and Garnet A l t e r a t i o n 196 -:ix-6.2 Mineral Chemistry 197 6.2.1 Zonation i n Carbonate Compositions 199 6.2.2 F e l d s p a r s 206 6.2.3 Micas ('.Muscovite and B i o t i t e ) 207 6.2.4 C h l o r i t e s 207 6.3 Chemical Changes 210 6.3.1 Sample P r e p a r a t i o n 211 6.3.2 A n a l y t i c a l Procedures 211 6.3.3 R e l i a b i l i t y of Chemical Data 213 P r e c i s i o n 213 Accuracy 215 Homogeneity of Major U n i t s 218 6.3.4 Gresplot ('.Constant-Volume C a l c u l a t i o n s ) 218 6.3.5 Losses and Gains 230 CHAPTER 7s VEIN AND ENVELOPE MINERALIZATION 7. 1 Vein Types 241 7.2 Vein Mineralogy 243 7.2.1 Gangue M i n e r a l s 245 7.2.2 Opaque M i n e r a l s 250 S u l f i d e s and Native Gold 250 Oxides 259 7.3 Model for the Vein Formation 260 CHAPTER 8: FLUID INCLUSIONS 8.1 I n t r o d u c t i o n 269 8.1.1 Sample P r e p a r a t i o n / A n a l y t i c a l Procedure 272 8.1.2 C a l i b r a t i o n and E r r o r A n a l y s i s 273 8.2 C l a s s i f i c a t i o n and D e s c r i p t i o n of I n c l u s i o n s .. 278 8.2.1 Primary (Type 1) I n c l u s i o n s 279 8.2.2 Pseudosecondary (Type 2.) I n c l u s i o n s .... 286 8.2.3 Secondary (Type 3) I n c l u s i o n s 288 8.3 Mi cr other mometr i c Data 289 8.3.1 F i n a l Homogenization Temperatures 289 8.3.2 S a l i n i t i e s (Ice M e l t i n g Temperatures) .. 297 8.3.3 Compositions ( E u t e c t i c Temperatures) ... 298 8.3.4 D e n s i t i e s CC0 a Homogenization) 299 8.3.5 Pressure E s t i m a t e s 300 8.4 Summary 304 CHAPTER 9: STABLE ISOTOPE STUDIES 9.1 S u l f u r Isotopes 309 9.1.1 Procedure 309 9. 1.2 R e s u l t s 310 9.2 Carbon and Oxygen Isotopes of Carbonates 315 9.2.1 Procedure 315 9.2.2 R e s u l t s 316 9.3 Oxygen Isotopes 319 9.3.1 Procedure 320 9.3.2 R e s u l t s 325 L a t e r a l / V e r t i c a l Zoning i n Veins 325 Geothermometry 328 • I m p l i c a t i o n s for Water/Rock Ratio 331 C h a r a c t e r i s t i c s of the Ore F l u i d 334 CHAPTER 10: SOURCE, TRANSPORT, AND DEPOSITION OF GOLD 10.1 P-T--X C o n d i t i o n s 339 10.2 C h a r a c t e r i s t i c s of the Ore-forming F l u i d s .... 342 10.3 D e p o s i t i o n of gold 347 10.3.1 T h e o r e t i c a l P r e d i c t i o n s 347 10.3.2 Comparison of P r e d i c t e d and Observed Assemblages 350 10.4 Transport of gold 353 10.5 Source of f l u i d s and gold 355 CHAPTER 11: CONCLUSIONS 11.1 Summary . .' 362 11.2 Recommendat i ons . 370 11.2.1 Regional E x p l o r a t i o n 370 11.2.2 E x p l o r a t i o n i n the B r a l o r n e Mine Area 371 REFERENCES 374 APPENDICES APPENDIX 1: Chemical compositions of a l t e r e d rocks around v e i n s at B r a l o r n e from s u r f a c e t o 44 l e v e l 396 APPENDIX 2: Microprobe analyses 406 APPENDIX 3: R e p l i c a t e a n a l y s e s of i n t e r n a l and i n t e r n a t i o n a l standards 412 APPENDIX 4: Gresens bar diagrams of l o s s e s and gains i n d e t a i l e d t r a v e r s e s a c r o s s v e i n envelopes at Eiralorne from s u r f a c e to 2 km depth 425 APPENDIX 5: F l u i d i n c l u s i o n d e e r e p i t i a t i o n data CSugiyama, 1986.) 468 APPENDIX 6: Maps CFigs. 5-1 and 5-2) and c r o s s -s e c t i o n s i n pocket LIST OF TABLES Table 2-1 Stratigraphic sections for the Bridge River area, and l i s t of formations for the Bralorne mine area 13 Table 2-2 Chemistry of Pioneer volcanics (Cadwallader Group) from Bralorne and E<onanza Basin, Bridge River d i s t r i c t , B.C 22-25 Table 3-1 K-Ar data for the Bridge River camp, south-western B r i t i s h Co 1 umb i a 38, Table 3-2 U-Pb data on zircons for the Bridge River-camp, southwestern B r i t i s h Columbia 40 Table 3-3 Rb-Sr data for whole-rock samples from the E<ridge River camp, southwestern Brit ish Columbia 4'3 Table 3-4 Latitudes and longitudes for a l l samples 45 Table 4-1 Vein progression from west to east in the Bridge River camp, B.C. 82 Table 4-2 Galena lead isotope analyses for deposits in the Bridge River Mining Camp, southwestern B r i t i s h Columbia 84 Table 4-3 Repeat and duplicate lead isotope analyses . . . 89 Table 5-1 Chemistry of major units in the Bralorne-Pioneer area 98 Table 5-2 Chemistry of minor units in the Bralorne-Pioneer area 99 Table 6-1 Summary of mineral abundances for primary rocks and alteration envelopes surrounding the Bralorne veins 176 Table 6-2 Typical input data f i l e for GRESPLOT program, for Bralorne altered wall rocks 221 Table 6-3 Typical tabulation of volume factors from GRESPLOT program, from footwall of 51 vein, 15 level . . . 226 Table 6-4 Tabulation of losses and gains for traverse across the footwall of the 51 vein, 15 level 226 Table 8-1 Locations and detailed descriptions of fluid inclusion samples for the Bralorne deposit 270 Table 8-2 Summary of fluid inclusion characteristics for the Bralorne deposit 280 - x i i -Table 8-3 Summary of fluid inclusion data by level in the Bralorne gold-quartz vein system 290 Table 8-4 Summary of pressure estimates for the Bralorne deposit 305 Table 9-1 Sulfur isotope data for several deposits of the Bridge River camp 311 Table 9-2 Carbon and oxygen isotope data for calcites for the Bralorne deposit . 317 Table 9-3 Oxygen isotope compositions of minerals and rocks in the Bralorne~Pioneer mesothermal gold veins . . . 321 Table 9-4 Duplicate oxygen isotope data for the Bralorne-Pioneer deposit 323 Table 10-1 Reactions between modelled fluid and wall rock at the Bralorne deposit, predicted by PATH program 344 Table 10-2 Characteristics of the ore fluid at the Bralorne deposit, predicted by the PATH program 345 Table A-l-1 Chemical compositions of altered rocks around veins at Bralorne from surface to 44 level 396 Table A-2-1 Operating conditions and standards used for microprobe analyses 406 Table A-: Microprobe analyses of minerals: a b c (d Carbonates ) Feldspars ) Micas (muscovite and biotite) ) Chlorites, hornblendes, and others 407 408 409 410 Table A-3-1 Replicate analyses of duplicate samples and internal standards, analysed at MESA U.K. and U.B.C. . . . 412 Table A-3-2 Comparison of known standards to values obtained for them (by MESA U.K.:) Comparison of known standards to values obtained for them (at Dept. of Oceanography, U.B.C.) 420 LIST OF FIGURES Figure 1-1 Simplified terrane map of southwestern B r i t i s h Columbia, Canada 2 Figure 1-2 Geology and mineral deposits of the Bridge River d i s t r i c t , southwestern B r i t i s h Columbia 4 Figure 2-1 Stratigraphic columns for the terranes making up the Bridge River camp, southwestern B.C 14 Figure 2-2 Ca-g) Trace-element discriminant diagrams for basalt samples from the Cadwallader Group . 28 Figure 3-1 Geology of the Bralorne-Pioneer mine area, southwestern Brit ish Columbia, showing location of dated samples 36 Figure 3-2 Concordia diagram for Bralorne diorite 51 Figure 3-3 Concordia diagram for Bralorne soda granite 52 Figure 3-4 Concordia diagram for Bralorne a l b i t i t e dykes 55 Figure 3-5 Rb-Sr diagram for different petrologic units with the Bridge River d i s t r i c t 57 Figure 4-1 Deposit age versus distance from the Coast PIut on i c Complex 82 Figure 4-2 2 0 7 F ' b / 2 0 4 P b versus 2 0 6 Pb/ : 2 C "*Pb plot for deposits in the Bridge River camp 86 Figure 4-3 2oopb /2o^pb versus a©«pb/a°-»Pb p i 0 t for deposits in the Bridge River camp . . 87 Figure 5-1 Surface geology plan of the Bralorne-Pioneer vein deposit, at a scale of 1:8000 . in pocket Figure 5—2 Eighth level underground geology plan of the Bralorne-Pioneer deposit, at a scale of 1:6800 . . . in pocket Figure 5-3 Cross-section through the Bralorne deposit 104 Figure 5-4 P-T grid for metamorphic mineral assemblages within the Bralorne fault block 152 Figure 5-5a Plan view of the major vein systems at Bralorne, projected to surface 158 - x i v -Figure 5-5b Cross-section showing major vein systems at Bralorne and extent to depth 159 Figure 5-5c Longitudinal section through the Bralorne-F'i oneer mesothermal gold vein deposit 160 Figure 5-6 Structural models for the Bralorne mineralized fault system 167 Figure 5-7 Detail of vein orientations on 15 level of the Bralorne mine 169 Figure 6-1 X-ray diffraction scans of carbonates (a,b) Major calci te , trace ankerite . . . . . . . . 184 ('. c,d) Major calcite and anker i te 184 Figure 6-2 Compositions of carbonates in altered wal 1 r oc ks 200 Figure 6-3 Classif ication of chlorite minerals 209 Figure 6-4 Typical composition-volume diagram from GRESPLOT program 224 Figure 6-5 (a) and Cb) Typical volume factor plots from GRESPLOT program 227 Figure 6—6 (a) and <b) Typical bar diagrams from the GRESPLOT program 230 Figure 6-7 Gresens bar diagrams for the altered traverses in (a) hangingwall of 51 vein, 8 level and (b) footwall of 51 vein on 16 level , Bralorne mine 235 Figure 6-8 Volume factor plots for the loss/gain bar diagrams of Figure 6-7 23.7 Figure 7-1 Sketches of textures in euhedral quartz from the Bralorne veins 247 Figure 8-1 Histograms of homogenization temperatures for various levels in the Bralorne mine 291 Figure 8-2 Histograms of ice melting temperatures for various levels in the Bralorne mine 292 Figure 8-3 Histograms of data for the carbonic component in Types 1 and 2 inclusions 293 Figure 8-4 Plot of variation with depth for homogenization and decrepitation temperatures 295 -xv-Figure S-5 Progression of fluid inclusion compositions 305 Figure 9-1 Schematic presentation of sulfur isotopic data for various sulfides in the Efridge River camp 312 Figure 9-2 (a) Bralorne d x e 0 whole-rock and ore fluid values compared to some common reservoirs 326 (ta) Plot of d 1 3 C versus d 1 B 0 for carbonates from the Bralorne deposit, compared to the Mother Lode 326 Figure 9-3 (a) Geographic variation of d i e > 0 values in quartz from the Bralorne-Pioneer vein system 327 (b) Vertical variation of d l s 0 values in quartz from the Bralorne-Pioneer vein system 327 Figure 10-1 Log oxygen fugacity pH diagram for conditions of mineralization in the Bralorne deposit . . . 349 — xvi — LIST OF PLATES Frontispiece . vi Plate 5--1 Pioneer Formation aquagene breccia 112 Plate 5-2 (a~d) Photomicrographs of mineralogy and textures-in the Pioneer Formation volcanic rocks 112-114 Plate 5-3 Ca,b) Textures of serpentinized ultramafic rocks in the Eiralorne mine area as seen in thin section 120 Plate 5-4 <a,b> Hornblendite textures and mineralogy in hand specimen and thin section 124 Plate 5-5 Barren pre-mineral stockwork cutting diorite 126 Plate 5-6 <'.a,b> Photomicrographs of unaltered and strongly altered diorite showing r e l i c t texture 128 Plate 5-7 Photomicrograph of unaltered soda granite . . . . 132 Plate 5-8 Migmatite textural relations in core 132 Plate 5-9 Migmatite in outcrop near Goldbridge 136 Plate 5-10 Photomicrograph of aplite dyke,sample? C121 . . 138 Plate 5-11 Photomicrograph of a l b i t i t e dyke, sample C092 143 Plate 5-12 Photomicrograph of green hornblende porphyry, sample C083, with zoned magmatic hornblende phenocrysts 143 Plate 5-13 Photomicrograph of "Bendor" dyke, sample C1004, showing osci l latory zoning in plagioclase phenocrysts .. 148 Plate 5-14 Photomicrograph of lamprophyre dyke, sample CI 033 148 Plate 6-1 Chlorite (calcite) alteration in d i o r i t e and soda granite, as seen in d r i l l core 180 Plate 6-2 Massive epidote alteration of d i o r i t e 180 Plate 6-3 Ca,b!> Carbonate alteration of d i o r i t e as seen in d r i l l core 182 Plate 6-4 Ca,b) Photomicrographs of cenral zone alteration of d i o r i t e to carbonate, chlorite and albite 186 Plate 6-5 (a,b) Photomicrographs of intense inner zone alteration of d i o r i t e to s e r i c i t e , carbonate and quartz 188 - x v i i -Plate 6-6 "Crackle breccia" alteration of soda granite as seen in thin section 191 Plate 6-7 (a~e) Photomicrographs of increasing degree of a l b i t i c - s i 1 i c i c alteration of soda granite 191-193 Plate 6-8 (a,b) Photomicrographs of biotite alteration in d i o r i t e and Pioneer greenstone 135 Plate 6-9 (a-c) SEM-EDS backscattered electron images of carbonate alteration in the inner zone, SB84—49/795'.... 202 Plate 6-10 SEM-EDS backscattered electron image of carbonates in the inner zone, 19-51FW1 204 Plate 7-1 Major ribboned "shear" vein, 51Ei on 8 level as seen in underground exposure near the Empire shaft 242 Plate 7-2 Minor, poorly ribboned vein, Alhambra on 8 level as seen in underground exposure near the Empire crosscut 242 Plate 7-3 "Breccia" portion of the 51B vein on 8 level as seen in underground exposure near the Empire shaft 244 Plate 7-4 Spectacular free gold in unribboned quartz from "tension" vein (unlocated) 244 Plate 7-5 (a,b) Photomicrographs to i l l u s t r a t e coarse euhedral quartz crystals in the ribboned "shear" veins 248 Plate 7-6 Ca,b) Photomicrographs (reflected light) of the arsenopyrite-pyrite assemblage typical of the inner zone 252 Plate 7-7 Photomicrograph (reflected light) of pyrrhotite occasionally found in the inner zone 255 Plate 7-8 Photomicrograph (reflected light) of the diverse assemblage (sphalerite, galena) found in richer veins .. 255 Plate 7-9 (a,b) Photomicrographs of gold in typical sulfide assemblage and in atypical quartz host 258 Plate 8-1 (a,b) Growth zones in euhedral quartz crystals outlined by minute primary fluid inclusions 282 Plate 8-2 (a,b) Type 1 primary fluid inclusions showing typical examples of small inclusions 283 Plate 8-3 (a,b) Type 1 primary fluid inclusions showing atypical examples of large 3-phase inclusions 284 Plate 8-4 (a,b) Type 2 pseudosecondary inclusions . . . . . . 287 - x v i i i -ACKNOWLEDGEMENTS I gratefully acknowledge the constructive c r i t i c i s m and supervision of Dr. Colin I. Godwin and the members of the supervisory committee, Dr. A l i s t a i r J . S i n c l a i r , Dr. Richard L . Armstrong and Dr. J . Kelly Russell, under whose direction this thesis was written. I also thank Dr. Tom H. Brown of U.B.C. and Drs. Bruce E. Taylor and Winton C. Cornell of the Geological Survey of Canada in Ottawa for guidance, support and interpretation of the fluid inclusion, stable isotope and thermodynamic modelling presented in this thesis. Dr. K. Fletcher kindly provided guidance and c r i t i c a l review of analytical details. The personnel of Mascot Gold Mines Ltd. and Corona Corp. (John Bellamy, Mark T i n d a l l , and Alan H i l l ) are thanked for granting access to the property, l o g i s t i c a l support in the f i e l d , and valuable discussions of the mine geology. I am grateful to members of the Brit ish Columbia Geological Survey, including Dr. William J . McMillan, Dr. J . Neil Church, Dr. Paul Schiarizza, John I . Garver, and Bob Gaba, for insights into the regional geology of the Bridge River area. Mary Anne Bloodgood and Joanne Nelson of the B.C. Geological Survey, and Drs. Murray Journeay and Don Murphy of the Geolgical Survey of Canada in Vancouver, contributed c r i t i c a l reviews of portions of the manuscript. Technical assistance in analysis was generously provided by Yvonne Douma, John Knight, Bryan Cranston, Dr. Peter van der Heyden, Dr. Steve Juras, Dr. Peter Michael and Maggie Pi rani an. Gord Hodges is thanked for assistance in drafting and photographical detai ls . Financial support was supplied by an Izaak Walton K i l l am Pre-doctoral Fellowship, the B r i t i s h Columbia Ministry of Energy, Mines and Petroleum Resources via the Mineral Development Agreement, and National Science and Engineering Research Council grants to Dr. Godwin. Last by by no means least, I thank my long-suffering wife and family who put up with three years of neglect so that this study could be completed. CHAPTER 1 INTRODUCTION 1.1 Location The Bralorne-Pioneer mesothermal vein deposit in the Bridge River mining camp, southwestern B r i t i s h Columbia, l i e s just east of the Coast Plutonic Complex, 180 km north of Vancouver (Fig. 1-1) and 100 km west of L i l l o o e t . The camp occupies about 300 km5* in an area encompassing the Cadwallader and Lower Hurley valleys, the head of Carpenter Lake, and the lower reaches of Gun Creek to Tyaughton Lake ; (Fig. 1-2). Access is by good gravel road from Lil looet on the Fraser River, or by a poor gravel road over the Hurley Pass from Pemberton on Highway 99. The area has rugged topography, with elevations from 600 to 3,000 m above sea level and land forms characteristic of alpine glaciation. Extensive drif t at lower levels in the valleys conceals most of the bedrock. The climate is typical of the interior side of the Coast Ranges of Brit ish Columbia. 1.2 History Placer gold was found in the Hurley and Cadwallader valleys in 1863 as an outcome of the Fraser gold rush (Barr, 1980). However, i t was not until 1897 that the main lode deposits were found. The f irst discovery was apparently on the Fourty Thieves claim in the BRX area (deposits 5 and 6 on Fig. 1-2: Green, in prep.). That same year, the Lome vein was found and the hanging-wall s p l i t of the main F i g u r e 1-1. S i m p l i f i e d t e r r a n e map of southwestern B r i t i s h Columbia, Canada, emphasizing t h e p r i n c i p a l g o l d d e p o s i t s and t e r r a n e s r e l a t e d the B ridge River camp C a f t e r Dawson and Panteleyev, DNAQ C o r d i l l e r a n Volume, i n prep., 1987). Pioneer vein some 20 m above the floor of the Cadwallader valley was exposed fortuitously by a mud sl ide after heavy rains (Patterson, 1979: Fig. 5-1). In spite of many attempts by various companies, i t was not until 1920-1930 that significant underground development took place with the formation of the Pioneer and Lome companies (Patterson, 1979). Bralorne Mines was formed by the amalgamation of Bradian Mines and Lome Mines in 1935. The Pioneer section of the mine closed in 1962 soon after the amalgamation with Bralorne to form Br al orne-F'i oneer Mines Ltd. The Bralorne mine f inal ly closed in 1971, i r o n i c a l l y just before the r ise in the price of gold in 1972. Only eight of the many other prospects in the camp have recorded production. Of these only a few—Congress, Gloria Kitty, Minto and Wayside (Fig. 1-2)—were of significance. Most were active in the 1930-1940 period (Harrop and S i n c l a i r , 1986). The Bridge River camp produced more gold over i t s 70 year l i f e than any other in Brit ish Columbia, from a l i t t l e over 7 mil l ion tonnes of ore grading about 18 g/t Au and 4 g/t Ag. Total recorded production was about 130 tonnes or 4 million ounces of Au and 30 tonnes or 1 mill ion ounces of Ag (Barr, 1980). Most of this came from the Bralorne-Pioneer vein system. Thus the deposit is the only one in B.C. to approach the output of the famous deposits in the Precambrian Shield, such as the Hoi linger or Maclntyre, each of which produced 300 tonnes or 10 million ounces of Au (Ber t oni, 1983). The giant Ka1goorlie deposi t Geologic Contact Fault • Isotoplc date (Ma) h =hornblende K = K -Ar b=biotite z=zlrcon w= whole rock 0 L _ km L E G E N D r =Rb-Sr u = U - P b m=muscovlte 10 I EOCENE I v v v v v v l Rexmount Porphyry CHE TA CEOUS-PALEOCENE Bendor Plutons I CPC I Coast Plutonic Complex LATE TRIASSIC-UIDDLE CRETACEOUS r~MT~ Methow Terrane EARLY PERMIAN-EARLY CRETACEOUS CD Cadwallader Terrane PERM A N-EARLY JURASSIC BRIDGE RIVER TERRANE j***",*^] Bralorne Intrusions Bridge River Assembledge BR ULTRAMAFIC ROCKS Shulaps Cadwallader Fault Zone Figure 1-2. Geology, mineral deposits and isotopic dates of the Bridge River District, southwestern British Columbia. Geology is after Woodsworth (1977) and Church (1987). Isotopic dates are in Tables 3-1 to 3-3. Mineral deposits corresponding to numbered open triangle symbols are listed in Table 4-2. 4v in Western Australia, by way of comparison, produced an order of magnitude more gold at 1200 tonnes ( P h i l l i p s , 1986). 1.3 Previous Uork Most previous work on the Bralorne-Pioneer vein system was done in the pre-1960 period when the mine was a major producer. The most recently published description of the geology (Bellamy and Saleken, 1983) was the outcome of a recent attempt to dewater and explore the mine by the current owners of the property, Mascot Gold Mines Ltd. Brief overviews of the deposit were given by Barr (1980) and Bacon (1978), and regional studies of the Bridge River camp by Church et a l . (1988) and Church (1987) have aided in understanding the setting of the deposit. Studies by Pearson (1977) led to the description of metallogenic zoning proposed by Woodsworth et a l• (1977). Prior to this, only the rather general description of the Bralorne mine by James and Weeks (1961) breaks the long period devoid of publication that followed Joubin's (1948) primarily structural study that was based almost entirely on the Pioneer deposit. Other than this , one must refer to the detailed work by Cairnes (1937) and a mineralogical study by Dolmage (1934), or McCann's (1922) early description of the deposits of the. Bridge Fsiver camp. Several unpublished works on the area include (1) a master's thesis by Stanley (1960), which was restricted to the Pioneer deposit, and (2) a manuscript by Stevenson (circa 1958), which was a major study of the Bralorne deposit. The latter contains detailed petrological and mineralogical descriptions. 1.4 Purpose of Study No modern detailed study of the important Bralorne deposit had been made before the present work commenced. It was overdue for a careful reappraisal in the light of modern techniques of ore deposit study. In addition, the deposit is unique within the Cordil lera in i t s great depth extent of almost 2 km that makes i t more l ike the classic Archean lode deposits of the Superior Province (Hodgson et a l . , 1982). Many recently published detailed studies of the Canadian Archean deposits (e.g. Colvine et a l . , 1984; Kerrich and Watson, 1984; Kishida and Kerrich, 1987; Robert and Brown, 1986a,b) provide a framework for this study of the Bralorne-Pioneer system. 1.5 Scope of Study The scope of the present study was threefold: to establish a regional framework for the detailed work at Bralorne, to document vertical zoning in the deposit, and to synthesize the results of detailed examinations into a viable genetic model of ore formation that would guide future exploration in the camp and at the deposit. These are the major topics within Chapters 2 to 4, 5 to 9, and 10 to 11, respectively. A regional and tectonic setting for the deposit was established with a geochronology study (Chapter 3) and a galena lead isotope study (Chapter 4) of the surrounding Bridge River d i s t r i c t . This work was supplemented by a limited geological study aimed at clarifying certain regional concepts, such as the ages and stratigraphic relations of the rocks hosting the deposit (Chapter 2). On the Bralorne property, detailed geologic mapping and relogging of available core led to geologic plans at surface and the 8th level underground (Chapter 5). Key vein intersections in old d r i l l core to 1800 m depth in the mine were sampled for detailed investigation. Petrographic examinations helped to reinterpret several important host 1ithologies, and to understand the details of wall rock alteration. The geochemistry of the host 1ithologies and their response to alteration at constant volume was studied using whole-rock analyses for major and trace elements (Chapter 6). Fluid inclusions and the stable isotopes of oxygen, hydrogen, carbon and sulfur were investigated to examine vertical zonation within the deposit (Chapters 7 - 9 ) . The results of the above studies, coupled with computer modelling of the ore f luids, are combined in Chapter 10, which synthesizes the formation of the Bralorne deposit. Conclusions, and recommendations for further exploration, are in Chapter 11. 8 CHAPTER 2 REGIONAL SETTING OF THE BRIDGE RIVER CAMP 2.1 Tectonic Setting The Bridge River camp in southwestern Brit ish Columbia (Fig. 1-1'.), in which the Br a 1 or ne-Pi oneer deposit is situated, occurs adjacent to the Coast Plutonic complex and is contained within three small tectonostratigraphic terranes: Bridge River, Cadwallader and Methow-Tyaughton. Two of these, the Bridge River and Cadwallader, are .; "suspect" terranes that were l i k e l y accreted to North America in Mesozoic time (Silberling and Jones, 1984; Monger and Berg, 1987; Wheeler et a l . . 1987). The Methow-Tyaughton, herein referred to as the Methow after Wheeler and McFeely (1987), i s mostly post-accretionary. The terranes are presently found as small lozenge-like fault-bounded s l ices between the two "super-terranes", the Insular on the west and the Intermontane on the east (Monger, 1984). Both these large composite terranes were assembled ( i . e . came together) prior to their accretion to North America (Monger et a l . . 1982). The Insular•super-terrane is composed mainly of the Wrangellia and Alexander terranes that became amalgamated by the Jurassic (Coney et a l . . 1980). The Intermontane super-terrane is composed of the Stikine, Cache Creek, and Quesnel terranes. The contact between these two super-terranes i s obscured by the intrusion of the Coast Plutonic Complex, and in the part of Brit ish Columbia under discussion here, by the several smaller terranes hosting the ore deposits and the Nooksack terrane. During the Eocene, these terranes were translated northward along the major r ight- lateral s t r i k e - s l i p Fraser - Straight Creek Fault by 120-150 km (Monger, 1985). The Triassic Cadwallader terrane was viewed as the eastern edge of Wrangellia by Kleinsphen (1985). However, the results of Rusmore's work (1985; Rusmore et a l . . 1988) indicate that the Cadwallader is a distinct terrane separated from Wrangellia by an early Late Cretaceous fault zone and by Upper Triassic rocks of unknown aff inity. The Permo-Jurassic Bridge River terrane was considered by Potter (1983) to have been deposited in an ocean-margin setting, close to a volcanic arc. Its a f f i n i t i e s to the adjoining super-terranes are also uncertain. It could represent a collapsed back-arc basin (Potter, 1986) separating the Cadwallader arc from North America (Rusmore, 1987). However, the Bridge River terrane could be the result of the closing of a major ocean basin, an accretionary prism entirely unrelated to the island arc volcanic and c l a s t i c Cadwallader terrane (Wheeler and McFeely, 1987). In either case, lead isotopic evidence for the camp (Chapter 4) suggests that both the Cadwallader arc and the Bridge River basin probably formed far from the influence of North American sources of Pb. Further evidence for the tectonic setting of the camp comes from geochronometry (Chapter 3) and petrochemistry of the Cadwallader and Bridge River rocks (section 2.3) and the Bralorne intrusives (section 5.1.3). These intrusives are Early Permian in age, and the Cadwallader and Bridge River rocks they intrude, are therefore Early Permian or older. The intrusives associated with mineralization in the camp, however, are Late Cretaceous to early Tertiary. Trace element petrochemistry (section 2.3) shows that the Bridge River rocks are transitional between mid-ocean ridge basalts (MORE) and somewhat alkalic ocean-island basalt (OIB) (Potter, 1983), while the Cadwallader volcanics are mainly island-arc t h o l e i i t e (IAT) with a tendency towards calc-alkaline basalt (CAB) or MORE character in places. All rocks of both packages are submarine (section 2.2). Several lines of evidence indicate that the major intrusives hosting the Bralorne deposit were also emplaced below the sea floor, perhaps in a spreading ridge oceanic environment. The petrology of the intrusive suite, which includes serpentinized ultramafite, hornblende d i o r i t e , and trondjhemite or "soda granite", is typical of an ophiolite association. The gradational contact relations between the hornblende d i o r i t e and the intruded Cadwallader volcanics suggest that the d i o r i t e in part intruded i t s own volcanic products. The fact that the diorite intrudes the adjacent elongate ultramafic bodies implies that the ultramafics had themselves been emplaced into a higher structural level than they formed i n , by the time of the d i o r i t e intrusion. Ely the time of major mineralization in the camp in the early Late Cretaceous, however, the tectonic setting had changed to one marked by compression and magmatism of an advancing continental-margin subduction zone. The intrusions associated with the mineralization appear to be part of the Coast Plutonic Complex. The rate of eastward migration of this magmatic belt in the v i c i n i t y of the Bridge River camp can be estimated from the geochronometric evidence as approximately 1.2 mm/year. This is comparable to the transgression rate deduced by Godwin (1975), and Armstrong C198S), for other parts of the Coast Plutonic Complex. Thus the tectonic setting of the rocks hosting the Bridge River camp has gone from divergent (spreading ridge in a back-arc or ocean basin) and convergent (island arc) in the Late Paleozoic, to compressional accretion in the Jurassic, and to transpressional above a continental-margin subduction zone in Cretaceous - early Tertiary time. Gold vein mineralization in the camp was associated with the plutonic activity of the latter compressional regime. The transpression—at least in the early Late Cretaceous— appears to have been east-west, with a s i n i s t r a l sense of shear on the major Bralorne s t r i k e - s l i p fault zone, followed by northeast-directed compression (section 5.4). Z2. 2.2 Stratiqraphic Setting The latest comprehensive geological map of the Bridge River camp (sheet 92J, at a scale of 1:250 000) was by Woodsworth (1977), with contributions also by Roddick and Hutchinson (1973). More recent compilations have been made by Cooke (unpublished, 1984), and Harrop and Sinclair (1985). Parts of the area and adjoining areas have recently been remapped (Church, 1987; Church et a l . , 1988; Glover and Schiarizza, 1987; Glover et a l . , 1988; Garver et a l . . 1989; Schiarizza et a l . , 1989). A synthesis of regional data i s in the stratigraphic chart of Table 2-1, which compares regional units to those l o c a l l y named at Bralorne. The three terranes, Bridge River, Cadwallader, and Methow, correspond to the three main l i thologic assemblages distinguishable in the Bridge River camp. Each i s described in more detail in sections 2.2.1 to 2.2.3. The dominantly oceanic Bridge River terrane (Fig. 2-la) i s composed of the Bridge River Group (or Bridge River Complex, c f. Glover et  a l . , 1988) and the Shulaps and President ultramafic bodies. The dominantly island arc Cadwallader terrane (Fig. 2-lb) includes the Cadwallader Group, Tyaughton Group, and a Lower Jurassic shale unit (Wheeler and McFeely, 1987). The third terrane, the Methow (Fig. 2 - l c ) , i s partly an overlap assemblage ( i . e . i t s upper part postdates the accretion of the Cadwallader terrane to the Intermontane super-terrane by overlapping them both), although it also includes rocks correlative with the Relay Mountain Group in i t s lower part. /3 Table 2-1: Generalized stratigraphic section l isting geological units in the Bridge River area, showing equivalents in usage at the Bralorne mine and updated names from this study (Figs. 5-1 to 5-3). Unit 1 Aqe 10 a 8a Regi ona1 Name2 Mi ne Name Name and Description (This study3) T Plateau lavas T Eocene volcanics Lamprophyre dykes Kersantite T Rexmount porphyry K-T Coast plutonics Bendor dykes LK LK K J-K J Tr Felsic dykes Felsic dykes Green hornblende porphyry dykes Albitite dykes Grey plagioclase porphyry dykes Dacitic porphyry Andesitic basalt Taylor Creek Group Relay Mountain Group Jurassic shale Tyaughton Group Bralorne intrusion! Bralorne soda grani te Bralorne diorite Soda. Dacite. porphyry Dacitic porphyry Albite tonalite or tr ondh.jemi te Hornblende quartz di or i te 6a ?P -J2 Shulaps u l t r a -mafic Complex Mafic diorite Presi dent ultramafics Hornblendi te Dunite, peridotite and pyroxenite ?P-Tr Cadwallader Group ?P-J Bridge River Complex Hurley sediments Turbidites, wackes and argi l l i tes Pioneer greenstone Aquagene breccias, basaltic andesite Noel argi l l i tes Bridge River Group Ribbon chert and a r g i l l i t e Pillow basalts 1 Unit is as defined in the Bralorne mine area (this study. 3 Regional name is taken from Schiarizza et a l . . 1989. 3 Prefix "meta-" is understood in a l l rocks older than Tertiary. METHOW TERRANE F I G U R E 2 - 1 STRRTIGRAPHIC CHRRT BRIDGE RIVER CPMP ( F r o m K.H. D a u s o n a n d fl. P a n t e l e y e v , " R e g i o n a l M e t a l l o g e i i y o f t h e O a t on and A c c r e t e d T e r r a n e s o f t h e C a n a d i a n C o r d i l l e r a " , Ch. 19, ONflG C o r d i l l e r a n V o l u m e , i n p r e p . , 1 9 3 7 . ) PLUTONIC EVENTS PEMBERTON JACKASS MT TAYLOR CR J T RELAY" MOUNTAiN • E T J K (C) Figure 2-1. Stratigraphic correlations between terranes in the Bridge River area; (a) l ists the units found in the Bridge River Terrane, Cb) lists the units for the Cadwallader Terrane, and (c) l ists the units for the Methow Terrane (Tyaughton Basin). Symbols are: o = coarse elastics, - = shales, + = intrusives, v = volcanics. BRIDGE RIVER TERRANE CADWALLADER TERRANE njUTCMC EVENTS (a) T K powe u_ J n T i T T i r i - — " T BRUME / Riven y *—*• •*• P + 8MULAP8 • C T E T L K (b) PLUTONIC EVENTS —[cmcoTiN—-K J T ~&4 T Y A U G H T O N QWWMliciR » * E T LK /5 In the Bridge River camp, the Methow terrane is represented by the Tyaughton Basin consisting of the Relay Mountain Group plus the overlying Taylor Creek Group and Jackass Mount ai n Gr oup. The Bridge River Group and Tyaughton Trough are thought to have been offset since the Cretaceous from their correlatives to the south, the Hozameen Group and Methow Basin, by at least 100 km along the major right-1 ateral s t r i k e - s l i p Fraser - Straight Creek fault systems (Haugerud, 1985; Ray, 1986; Monger, 1985; Monger and Price, 1979). The Methow basin rocks host the Carol in gold deposit that has s i m i l a r i t i e s to the Bralorne deposit. Both the Bridge River and Cadwallader groups are currently considered, on the basis of conodont and radiolarian determinations (Potter, 1983; Rusmore, 1985; Cordey, 1986'.), to be of Triassic to Jurassic age. However, regional correlation to the Hozameen Group (Monger, 1985; Haugerud, 1985; Potter, 1986; Rusmore et a l . , 1988), suggests the Bridge River Group could be, in part, as old as Permian. This was the view, based on l i thological correlations to the Permo—Triassic Cache Creek Group, held by a l l previous workers before Triassic conodonts were found by Cameron and Monger (1971). Currently the Cache Creek Group is considered to range in age from the Mississippian (Monger, 1977) to possibly Jurassic (Cordey, 1986). The Permian or pre-Permian age i s supported by the isotopic dating (Chapter 3) of the Bralorne diorite that appears to intrude the Bridge River and Cadwallader groups. The diorite yielded an Early Permian age, based on a minimum zircon U-Pb date of 270 Ma and a hornblende K-Ar date of 284 + 20 Ma. The Tyaughton Group is Upper Triassic , and the Relay Mountain, Jackass Mountain and Taylor Creek groups are Upper Jurassic to mid-Cretaceous (Garver et al . . 1989). 2.2.1 B r i d g e River Group The Eiridge River terrane, an oceanic assemblage, is represented in the Bridge River camp by the Permian to Lower Jurassic Bridge River Group (Fig. 2-la). The assemblage has been assigned to the Fergusson Series by Cairnes (1937) and Church (1987), and called the Bridge River Complex by Potter (1983). The term "complex", although appropriate because structural and l i thological complexities prohibit measurement of meaningful type sections (Glover et a l . , 1988), i s not used because of the more general usage of "Bridge River Group". Following the subdivision of Cairnes (1937), the group is divided into sedimentary and volcanic packages. The sedimentary package consists of 1000 m or more thicknesses of ribbon chert and argil l i t e with very minor discontinuous limestone lenses. The sedimentary package i s intercalated with the volcanic package that consists of large volumes of pillowed and volcanic1astic basalt. The Bridge River Complex i s heterogeneously metamorphosed, and deformation i s complex and multiple. In addition to the common greenschist facies rocks, blueschist rocks (which are not provably Eiridge River Group; they could be exotic blocks'.) have recently been mapped and dated by K-Ar and Rb~Sr on whole rock and white mica at 195 to 250 Ma (Garver et a l . , 1989). The basalts of the group appear to have acted as large competent blocks (Potter, 1983) during deformation accompanying tectonic transport, while the ribbon cherts, separated by thin septae of graphitic a r g i l l i t e , buckled and were intensely tectonized, l i t e r a l l y flowing around the buttresses of massive basalt. There is therefore no major age difference between the basalts and the sediments of the Eiridge River Group as has been suggested by Church (1987) on the basis of the sediments being more deformed. Instead, their different tectonic styles merely reflect their different competencies. Alpine-type ultramafic rocks in lensoid to very elongate bodies are spatial ly associated with the sedimentary and volcanic rocks and are therefore thought to form part of the assemblage (Schiarizza et a l . . 1989; c f• Hodgson et a l . . 1982); they may mark the sites of major sutures or thrust faults that were later foci for transcurrent movements (Garver et a l . . 1989; Schiarizza et  a l • . 1989). At Bralorne, at least some of the President ultramafics marking the faults are intruded by, and therefore older than, the Early Permian Bralorne intrusives. The large Shulaps ultramafic mass (Fig. 1-2), described by Leech (1953) and Nagel (1979), structurally overlies and is spatial ly associated with the Bridge River Group; it is usually considered to be part of the group. Potter (1983) postulated that the Shulaps is a slab of oceanic crust that was hot enough when obducted to have formed an inverted metamorphic gradient in rocks underlying i t , which implies a Jurassic age for that body. 2.2.2 Cadwallader Group The Cadwallader Group and overlying Jurassic-Cretaceous rocks are included in the Cadwallader terrane in the Etridge River camp (Fig. 2 - l b ) . The Cadwallader island arc assemblage is mainly preserved as competent fault-bounded blocks, either structurally above or within the Bridge River Group. The Cadwallader rocks are in fault contact with, but inferred to be stratigraphical1y overlain by, the volcanic and volcaniclastic arc sequence of the Upper Triassic Tyaughton Group, Lower Jurassic shales, and sedimentary rocks of the Jura-Cretaceous Relay Mountain Group (Wheeler and McFeely, 1987; Garver et a l . , 1989). Traditionally, the Cadwallader Group has been subdivided in the type area at Bralorne into three formations: the Pioneer Formation greenstones and the sedimentary Hurley and Noel Formations (Cairnes, 1937). However, the dist inction between the Hurley and Noel formations i s often d i f f i c u l t to make. Following Rusmore's (1985) work in the Eldorado Basin 30 km north of Bralorne, the Cadwallader Group can be better divided into a lower mafic volcanic unit (the Pioneer Formation) and an overlying sedimentary package (the Hurley Formation). The Pioneer Formation consists of a basaltic andesite p i l e with minor felsie volcanics. The Hurley Formation is composed of a volcaniclastic sequence '.'.containing o l i s t o l i t h s , or tectonical1y-transported blocks, of reefal limestone) and an upper turbidite unit. The contact between the two formations is generally considered to be conformable ('.Church, 1987; Rusmore, 1985), but there could be a gap in time between the Pioneer and the Hurley. It i s also possible that the Cadwallader Group as defined by Rusmore (1985) in the Eldorado Basin is a package of rocks separate from that original ly defined in the Bralorne area. 2.2.3 Tyaughton Basin The Tyaughton Basin marine sedimentary strata of Middle Jurassic to mid-Cretaceous age represents the overlap assemblage of the Methow terrane in the Bridge River area (Fig. 2- lc) . The assemblage consists principal ly of: (1) the Middle Jurassic to Cretaceous easterly-derived, continental margin type c last ic wedge of shale, si Itstone, greywacke and conglomerate comprising part of the Relay Mountain Group, (2) the mid-Cretaceous Taylor Creek Group, and (3) the Early to mid-Cretaceous Jackass Mountain Group (Glover et a l . . 1988; Garver et a l . . 1989). These overlie the Lower Jurassic Ladner Group, which is equivalent to the arc assemblage of the Tyaughton Group in the Cadwallader terrane. The Tyaughton and younger rocks are not considered further because they do not occur in the Bralorne mine area. 2.3 P e t r o c h e m i s t r y of the B r i d g e River and Cadwallader  V o l c a n i c s Recent mapping by Church (1987) and Church et a l . (1988) in the Bridge River area has led them to suggest that no dist inction can be made between the volcanic rocks of the Cadwallader and Bridge River (Fergusson) Groups. (The lithology and stratigraphy of these groups is described in detail in section 5 . 2 ; only the implications of their petrochemistry i s discussed here.) They place a l l the volcanic rocks of the Bridge River and Cadwallader groups in the Pioneer Formation of the Cadwallader Group, of presumably Triassic age, but assign a pre-Permian age to the extensive ribbon chert and argi l l i t e of the Bridge River Group because of their more intense deformation. Earlier workers, including Cairnes (1937), Joubin (1948) and Stevenson (1958), a l l disagree with this interpretation, and most current workers in the camp disagree also CG.E. Woodsworth, J . Garver, P. Schiarizza, pers. comm., 1988). Nor do the observations of the writer, working mainly in the fault-bounded Bralorne block, support Church's subdivision, since the dark Bridge River basalts seem to be generally distinguishable in the f ield from the paler green Pioneer volcanics of the Cadwallader Group. The latter are commonly closely associated with turbidites and volcanic1astic 2/ sediments of similar composition and green appearance (as also described by Rusmore, 1987), unlike the Bridge River which is largely devoid of coarse elastics (Chapter 5). Al l of the volcanic rocks in both packages are commonly altered to greenschist facies assemblages, and the following rock names should therefore a l l be considered as prefixed by the term "meta--", that has been omitted in the interests of brevity. Hence, the volcanic rocks are c lassif ied using trace element discriminant diagrams that were developed expressly for altered rocks, considering only relatively immobile elements such as T i , Y, V, Zr and Cr. The classic major element discrimination diagrams of Irvine and Baragar (1971) and Jensen (1976), or the diagrams of de Rosen-Spence may not be appropriate. A comparison of whole rock chemistry for the two volcanic packages ('.Potter, 1983: Rusmore, 1985), averages of which (AVCAD and AVBRIV) are l isted in Table 2-2, indicates that the Bridge River volcanic rocks are predominantly basalts, but the Cadwallader volcanic rocks are basaltic andesites. The main differences in major elements is in the higher Na 20, S i 0 3 , and A l a 0 3 and lower CaO and P^Qa contents of Rusmore's analyses of Cadwallader volcanics CAVCAD) compared to Potter's of Bridge River volcanics (AVBRIV). For SiOa, the range for Cadwallader i s 49.271 to 51.97. and average 50.67. compared to 44.77. to 50.57. and average 48.47. for Bridge River. For Na2Q, the corresponding figures are Cadwallader, 3.5 to 6.07. and average 4.97., compared to 22. TABLE 2-2 (a): Chemistry of Pioneer volcanics (Cadwallader Group) from the Bralorne block, Bridge River d i s t r i c t , B.C. Location BRALORNE BLOCK Sample No. C095 C096A C096B C096C C098A C098D (N) (4) (1) (1) (5) (1) (1) Ma jor Element s (7.) SiOa 47.34 44.8 45.9 63.96 48.6 52.9 A l 2 O a 13.71 11.0 14.8 13.49 14.8 14.2 TiOa 1.01 0. 15 0.72 0. 33 1.38 0.50 FeaOa 10.32 8.40 11.5 4.46 10.9 7.8 MgO 12.41 14.7 15.5 8. 07 10. 1 11.7 CaO 9.03 9.0 5. 9 3. 13 6.8 6. 0 Na,0 1.50 0.00 0.59 w • / 3 • & 4. 1 Ka0 0.04 0. 00 0.03 0. 05 0.59 0.24 MnO 0. 26 0. 15 0.25 0. 11 0. 18 0. 16 Pz0 O 0. 11 0.03 0.06 0. 06 0.20 0.05 LOI 4.5B 11.92 4.90 3. 16 2.88 2.50 TOTAL 100.31 (100) (100) 100.39 (100) (100) Density 2.90 2.72 2.83 2.74 2. 87 2.84 Minor Elements (ppm) As 14 15 4 0 0 0 Ba 50 52 53 62 72 83 Co 30 64 54 25 46 52 Cr* 275 1185 495 117 191 348 Cu 75 58 135 a 4 Nb 3 3 3 1 8 1 Ni 70 465 195 34 75 135 Pb 6 15 8 11 18 12 Rb 2 0.0 0.5 0. 1 5.0 1. S 745 305 530 220 575 540 Sb 3 0 4 2 0 0 Sr 270 50 196 240 163 110 V* 270 54 235 57 395 140 Y 22 8 25 12 35 16 Zn 105 61 200 66 91 83 Zr 63 41 48 65 139 52 Normative Minerals (5£> Quartz 1.1 23.4 Corundum 2.2 Orthoclase 0.2 0.2 0.3 3.6 1.5 Albite 13.8 5.3 30.4 32. 1 36.5 Anorthite 35.6 34.5 31.2 15.8 23.5 20.0 Diopside 7.0 12.9 8.7 8.8 Hypersthene 29.4 49.4 54.4 26.2 4.2 15.8 01ivine 9.3 1.7 22.5 14.7 Magnetite 2.2 1.9 2.4 0.9 2.2 1.6 Ilmeni te 2.2 0.3 1.5 0.7 2.7 1.0 Apatite 0.3 0.05 0. 1 0. 1 0.5 0.1 D i f f ' n Index 14 1 5 54 36 38 Modes (estimated volume %) Quartz 10 8 10 10 5 Plagioclase 40 44 30 68 50 40 Hornblende 58 22 45 CIi nopyroxene 55 46 54 Rutile. Sphene 3 2 2 5 1 Pyrite (Py/Po) 2 tr (No. samples) (1) (1) <1) (1) 22 TABLE 2-2 (b>: Chemistry of Pioneer volcanics from other l o c a l i t i e s in the Bridge River d i s t r i c t , B.C. Location WAYSIDE ELDORADO BASIN Sample No. C2003 C2001 C2001A C2001B C2001C C2001D CN) CI) C1) cn (1) (1) (1) Major Elements C7.) SiOa 56. 0 49.6 46.3 45.4 38. 1 42.0 A1 20 3 16.9 14.9 16.5 15.9 16.4 15.5 TiOa 0.75 1. 16 1.01 1. 10 1.53 0.51 Fe aOa 8.2 9. 1 9.3 9.7 12. 1 14.9 MgO 5.0 12.0 10.0 10.5 8.5 8.9 CaQ 1.6 3.0 7.8 7.0 17.6 9.9 NaaO 8.0 4.30 4.2 3.9 0.77 2.0 K a0 0. 15 0.70 0. 10 0.51 0.69 3. 1 MnO 0. 15 0. 13 0. 16 0. 16 0.21 0.25 P*0» 0.21 0. 14 0. 10 0. 14 0. 15 0. 12 LOI 3. 14 5.22 4.71 5.62 4. 11 3.01 TOTAL C100) C100) (100) (100) (100) (100) Density 2.65 2 • & 3 2.78 2.74 2.94 2.92 Minor Elements (ppm) Ba 135 210 120 490 420 430 Co 21 29 41 46 32 47 Cr* 16 130 210 180 160 330 Cu 6 40 36 23 40 100 Nb 5 6 4 5 7 10 Ni 3 71 105 170 75 180 Pb 4 4 ND 1 8 5 Rb 3 7 1 6 21 83 Sr 160 200 370 340 230 480 v-» 160 230 250 240 260 240 Y 18 21 17 18 28 24 Zn 68 61 66 56 81 20O Zr 80 82 60 80 100 40 Normative Minerals C/O Corundum 1.3 2. 1 Orthoclase 0.9 4.4 0. 6 3.2 5.2 Albite 70.4 35 • 3 29.3 27.6 Anorthi te 6.6 14.8 27. 1 25.9 41.4 25.4 Nepheline 4.7 4.4 3.7 9.5 Leucite 3.4 10.9 Diopside 10.6 8.2 13.7 20.6 Hypersthene 1.0 18.7 •1ivine 16. 1 17 1 23.5 26.0 31.8 24.1 Magnetite 1.7 1.9 1.9 2.0 2.5 3.0 Ilmenite 1.5 2.3 2.0 2.2 3. 1 1.0 Apatite 0.5 0.4 0.3 0.4 0.4 0.3 Diff'n Index 71 43 35 35 4 15 Modes (estimated volume JO Quartz <1 Plagioclase 65 45 40 50 45 60* Hornblende 25 4 CIinopyroxene 20 25 25 30 30* Ch1 or i t e 5 25 35 12 15 5 Ilmenite CRu/Sp) 5 10 <1 4 10 5 Magnetite 4 CNo. samples) CI) CI) CI) CI) CI) CI) S B 9 S S S S S S S 3 S S 2* TABLE 2—2 (c): Chemistry of Pioneer volcanics from Eldorado Basin (Rusmore, 1985), compared to Bridge River volcanics from Carpenter Lake (Potter, 1985) and to worldwide averages for basalts. Loc'n ELDORADO CARPENTER AVERAGE BASALTS83 Sample AVGCAD1 AVBRIV2 CALK MORE OIB I AT (N) (3;10) 3 (3) ( >3) (100) (>14) (10) Major Elements (>0 SiO a 50.64 48.4 51.0 49.3 50. 0 51.2 A 120a 15.26 13.6 18.7 16. 5 13 18. 1 TiOa (1.22) 2.05 0.90 . 1.5 2.7 0.8 Fe203 12. 17 11.1 8.9 10. 0 11.5 10. 1 MgO 5.57 5.20 4.8 7.5 10 6.2 CaO 6.24 10.7 10.7 11.0 10 11.0 Naa0 4.90 3.31 2.9 2.8 2.5 2.0 K a0 (0.71) 0.36 0.6 0.2 0. 5 0. 3 MnO (0.16) 0. 15 0. 17 0. 18 0. 18 0.2 PaOo (0.17) 0.31 0.13 0. 10 0.3 0. 1 LOI 2.58 4.60 1.0 - 0.5 0.7 TOTAL 99.65 99.8 39.80 99. 1 101.2 100.4 Minor Elements (ppm) Ba 220 185 12 190 70 Co 39 50 60 30 Cr* 115 130 115 305 170 80 Cu 95 70 80 60 Nb <10 20 <2 2.5 27 1.5 Ni 61 65 175 75 30 Rb 10 13 3 13 5 Sr 310 210 400 150 400 200 V* 340 340 255 230 240 270 Y 24 33 15 35 27 18 Zn 91 Zr 49 150 65 100 155 45 Normative Minerals (7.) Quartz 2.0 1.3 1.0 3.4 Orthoclase 3.2 2. 1 3.5 1.2 3.0 1.8 Albite 43. 1 29.5 24.5 23.5 19.0 16.9 Anorthite 18.8 25.0 31.2 32.4 24.0 39.5 Diopside 9.9 23.5 13. 1 19.2 20.6 11.8 Hypersthene 5.8 8.0 14.8 10.3 22.0 21.8 01ivine 12.4 2.5 7. 1 4.0 Magnetite 2.8 3.5 3.2 3.0 2.8 3.9 Ilmenite 3. 1 2.8 1.7 2.9 4.9 1.5 Apatite 0.5 0.7 0.3 0.3 0.6 0.3 Diff'n Index 37 25 29 25 27 22 Modes (estimated volume 7.) Quartz 2 Plagioclase 60 75 CIinopyroxene 35 20 Rut i1e 3 5 (No. samples) (3) (3) 25" Notes to accompany Table 2 - 2 : 1 Rusmore (1985J. = Potter (1983). 3 Bracketted figures are averages of 10 analyses, otherwise of 3 analyses. "* Analyses are adjusted to allow for known contamination due to grinding in Cr-steel Tema m i l l , determined by comparison to same samples ground in W-carbide m i l l . =» Hughes (1982); trace elements for MORB, OIB and I AT are from Pearce and Qale (1976). s Sum of alteration products (chlorite, epidote and carbonate for mafics; s e r i c i t e and carbonate for piagioc1ase). Analyses from Bralorne block are by XRF: (100)totals are by pressed powder pellet, normalised to 1007.; others are by fused glass disk. Analyses from Rusmore are by ICP; those by Potter are by X R F (details unknown); those by Hughes are by traditional wet chemistry. ( N ) = number of replicate analyses averaged in these columns. Samples are located on Figures 3-1 and in Table 3-4. Bridge River, 2.7 to 4.27. and average 3.37.; for A1 2 0 3 , the figures are Cadwallader, 13.3 to 16.57., average 15.37., Bridge River, 11.8 to 15.27., average 13.67.. For CaO, The figures are Cadwallader, 4.6 to 8.47., average 6.2, Bridge River, 7.6 to 14.37., average 10.97.; for P-^ Ots, Cadwallader, 0.06 to 0.267., average 0.177., Bridge River, 0.19 to 0.497., average 0.317.. (Averages for the Cadwallader and Bridge River rocks are quoted in Table 2-2 for brevity; for individual analyses, refer to Potter, 1983 and Rusmore, 1985. > However, analyses of Cadwallader volcanics in the present study (Table 2-2) from both the Bralorne block (Fig. 3-1: C095 - C098D), the Wayside area near Goldbridge (Fig. 1-2: C2003) and the Eldorado (Bonanza) Basin 20 - 30 km northwest of Goldbridge (Fig. 1-2: C2001 - C2001D) show two groups of soda contents (0 to 27. and about 47.), spanning the values l i s t e d in both Potter (1983) and Rusmore (1985). The MgO contents of basalts in the Cadwallader Group analysed in this study CC095 - C098D, C2001 - C2001D) are significantly higher (in the 7 to 147. range, even after allowing for over-estimation of MgO by the pressed-powder XRF technique), than Potter's average MgO for the Bridge River Group (5.27., by unspecified XRF technique). Only C2003 i s close to Rusmore's average of 5.67. MgO (her analyses were done by the induction coupled plasma, or ICP, technique). Some of the Cadwallader volcanics analysed in this study (see C095, 96A,B, C2001 in Table 2-2) could be called p i c r i t e s , or 27 high-Mg basalts (Hughes, 1982). However, there are no r e l i c t olivine phenocrysts v i s i b l e in these rocks. Normative mineralogy (Table 2--2) suggests differences between Cadwallader and Bridge River volcanics. Although many from both groups are ol ivine normative (as is average mid-ocean ridge basalt (MORB), and ocean-island basalt COIB)), the more sodic nature of the Cadwallader rocks shows up in several samples, particularly from Eldorado Basin (C2001A to D), which contain normative nepheline and leucite or corundum. However, these rocks have probably been strongly changed chemically (spi1it ization on the ocean floor, followed by greenschist metamorphism, i s probably responsible for the apparent undersaturation) and the norms may be misleading. Furthermore such names are at odds with the c lassi f icat ions, below, by relatively immobile trace elements. In terms of d i s t i n c t i v e trace elements (Ba, Cr, Nb, Rb, Sr, V, Y and Zr) compared to the average figures for major basalt types given in Table 2-2, Cadwallader volcanics from the Eldorado Basin show a f f i n i t i e s with calc-alkal ine basalts (CAB) and island-arc tholei i tes CIAT). The samples from the Bralorne block tend to be transitional between IAT and MORB. Bridge River basalts, on the other hand, are more similar to MORB or OIB, with only the low Sr contents and high V contents indicative of IAT basalts. Such transitional chemistry i s typical of back-arc basin basalts CBABB), as pointed out by Potter (1983). Z8 Figure 2-2. Trace-element discriminant diagrams for basalt samples from the Cadwallader Group. Diagram 2-2(a) and Y/Nb ratios are from Pearce and Cann (1973); (b) and (d) are from Pearce C1S80); <c) and (f) from Hawkins (1980); (e) from Shervais (1982) and (g) from Garcia (1978). Open circles are samples from the Eldorado Basin (Rusmore 1985), as are f i l l e d circles (this study; Table 2-2). Crosses are from the Bralorne block (Table 2-2). Average for Bridge River basalts i s plotted as a large c i r c l e in a l l diagrams. Calc-alkaline basalts (CAB), back-arc basin basalts (BABB), mid-ocean ridge basalts (MORB), island-arc tholeiites (IAT) or low-K tholeiites (LKT), and within-plate basalts (WPB), ocean-floor basalts (OFB) and ocean island basalts (OIB) are shown as fields, with their respective averages plotted as squares (from Table 2-2: Pearce and Gale 1976, and Hughes 1982). Note that in (a) the fields of CAB and LKT overlap the central MORB field. 29 T i / 1 0 0 The trace element discriminant diagrams in Figure 2-2 (data from Table 2-2) also suggest differences between the two volcanic packages. The Bridge River volcanic rocks appear to be transitional between MORB and somewhat alkalic OIB (Potter 1383, 1986), whereas the Cadwallader volcanic rocks are mainly IAT with a tendency towards some MORB character (Rusmore 1985 and 1987, and this study). In d e t a i l , on the basis of diagrams involving Z r , Y, Ti and PaOa (Fig. 2-2 a~d), the samples analysed in this study fro the Cadwallader Group in Eldorado Basin, Rusmore's type area, show more MORB character, while those from the Bralorne area (this study) are somewhat a r c - l i k e (CAB or IAT) in character. On the basis of diagrams involving V (Fig. 2-2e and f), samples analysed in this study from both Eldorado Basin and the Bralorne area have more MORB character than Rusmore's samples. Final ly , a diagram of Ti versus Cr (Fig. 2-2g) and a comparison of Y/Nb ratios (not shown) give similar indications of petrotectonic setting fo the samples analysed in this study as do Rusmore's ( i . e . they are transitional between IAT and MORB). The postulated petrotectonic setting i s thus back-arc or marginal basin for the Bridge River Group, but island ar close to a marginal basin for the Cadwallader Group. The two may or may not have been adjacent and contemporaneous; there i s arc detritus in both, but the detritus is more felsic in the Cadwallader (Rusmore, 1985; Chapter 5). CHAPTER 3 GEOCHRONOMETRY OF THE BRIDGE RIVER CAMP 3.1 Introduction Potassium-argon, uranium-lead, and rubidium-strontium dating of intrusive rocks at the Bralorne Mine has established that the major plutons hosting the deposits are Early Permian in age and that the mineralization is much younger, probably early Late Cretaceous. Isotopic dating of the plutons conflicts with the accepted Triassic - Jurassic .; pal eontol ogi c ages for the Cadwallader and Eiridge River Groups by implying that at least parts of them are Permian, since in the Bralorne area rocks that are similar in lithology and petrochemistry to these groups appear to be cut by the Bralorne intrusions. The Bralorne deposit has long been considered to be genetically related to the enclosing plutonic rocks, the Bralorne diorite and soda granite (cf•: Cairnes, 1937; Joubin, 1948; Stevenson, 1958). However the results of the present study show that this cannot be true because the Early Permian Bralorne intrusives are at least 180 mill ion years older than the mineralization. Instead, mineralization is related to a set of pre-mineral to post-mineral dykes that appear to constrain the time of mineralization to the early Late Cretaceous, between 90 and 85 Ma. Younger dykes that cross-cut the veins are of Tertiary age (57 and 45 Ma). 33 Both the Bridge River and Cadwallader groups are considered, on the basis of conodont and radiolarian determinations (Potter, 1983; Rusmore, 1985; Cordey, 1986), to be of Triassic to Jurassic age. However, regional considerations, such as correlation to the Hozameen Group, (Monger, 1985; Haugerud, 1985) suggest the Bridge River Group could be in part as old as Permian. This was the view, based on l i thological correlations to the Permo-Triassic Cache Creek Group, held by a l l previous workers before Triassic conodonts were found by Cameron and Monger (1971). The Permian or pre-Permian age is supported by the isotopic dating reported here (minimum zircon U-Pb date of 270 Ma and a hornblende K-Ar date of 284 + 20 Ma) for the Bralorne d i o r i t e , which appears to intrude the Bridge River and Cadwallader groups in the Bralorne block. The Bralorne block is defined herein as that area bounded by the Fergusson and Cadwallader faults (Fig. 3-1). Rocks of the Cadwallader Group, as o r i g i n a l l y defined in this block and in Cadwallader Creek north of Bralorne, are 1ithological1y similar to fossi1-bearing rocks recently described as Cadwallader in the Eldorado (Bonanza) Basin 20 km to the northwest of Goldbridge (Fig. 1-2) by Rusmore (1987) (see also Church et a l • , 1988). The present study (Chapter 2) shows that rocks from both areas have similar petrology and major and trace element chemistry. Possibly the definition of the Cadwallader Group needs to be expanded to include the 34-Triassic rocks mapped by Rusmore (1385) outside the Bralorne block, or the rocks mapped by Rusmore need to be renamed. The contact between the Bralorne intrusives and the Bridge River and Cadwallader rocks, of c r i t i c a l importance to the age of both groups, is not well exposed anywhere at the surface. Where seen in d r i l l core underground at the Bralorne mine, a zone of bleaching extending for 3 to 10 meters out into the volcanics and sediments is usually seen where they are in contact with the soda granite, and a l l stages from unaltered Pioneer volcanics to wel1-digested xenoliths of greenstone in diorite are common. All previous workers considered the contact to be intrusive, and it was also widely held that the diorite was contemporaneous with the volcanics, grading into them in places (e.g. Cairnes, 1337). Based on my observations at Bralorne, Goldbridge, and Wayside (Fig. 1-2), I believe the contact to be intrusive. However, the contact between the large mass of soda granite at Bralorne and the Cadwallader rocks often is strongly sheared or even mylonitic, and B.N. Church (pers. comm., 1387) considers a l l contacts between Bralorne intrusives and s t r a t i f i e d rocks to be faults. If this were true, the conflict between paleontological and isotopic ages would not exist, but the greenstone xenoliths in the d i o r i t e would then be from some completely unknown formation. Further work i s required, and i s planned, to refine understanding of this important contact. 35" 3.2 Sampling Details Sampling for the geochronological study was mainly in the Bralorne block near the mine. The results of previous studies carried out by R.L. Armstrong in the quarry near Goldbridge, and R.L. Armstrong, J.W.H. Monger and C . J . Potter in the Bridge River Canyon (Fig. 1-2) are also l isted in this study (Table 3-3). Latitude and longitudes for a l l samples described are in Table 3-4, and the locations of detailed sampling in the Bralorne mine area are shown in Fi gure 3-1. Three samples of limestone from lenses in Cadwallader Group rocks caught up in one of the major faults bounding the Bralorne block, on the P.E. Gold property adjoining the Pioneer mine to the east (Fig. 1-2), were digested for microfossils by the Geological Survey of Canada in Vancouver. However, they proved to be barren of conodonts. Large samples for isotopic dating of the Bralorne intrusive suite (C092B, C093A, C094A and C095A) shown in Figure 3-1 were taken from underground NQ diamond d r i l l core. These were sampled in duplicate, each representing 16-30 m (50-100 ft) of core and weighing approximately 25 kg. Other samples (C082A and C082B) of similar size were taken from adit walls on the main haulage level (8 Level, Empire Crosscut). Post-mineral dykes (C083A and C1033) were sampled from the dump at the portal on the eighth level to obtain 20 kg of the most unaltered material possible. A few of the samples for Rb-Sr analysis were 30 cm pieces of NQ Figure 3-1. Geology of the Bralorne-Pioneer mine area, southwestern British Columbia, showing location of dated samples referred to in text (projected to surface). 37 core (C036A, B and C from Bralorne, and C098A and D from the P.E. Gold property). The rock types for each of these samples are in Table 3—3. Plutonic rock of the E<ralorne intrusive suite is also well exposed in a quarry at Goldbridge. These rocks, gabbro, intrusive breccia and leucocratic dykes, were sampled in 1979 by R.L. Armstrong and dated in the early 1980's by both K-Ar and Rb-Sr as late Paleozoic, with f a i r l y large errors and uncertainties. At face value these results contradicted the age given in published maps and reports. Reinvestigation of field relationships and further dating of related rocks seemed required before giving great emphasis to possibly anomalous geochronometry (from one very low-K amphibole, and low-Rb whole-rock samples). The results of additional work confirm the results of the i n i t i a l studies. 3.3 Sample Preparation and Analytical Techniques 3.3.1 Potassium-Argon (Table 3-1) Hornblende concentrates were made using standard heavy l iquid (methylene iodide) and magnetic separation techniques. Biotite concentrates were made by elutriat ing carefully sized fractions in a water column in which flow i s from bottom to top. The resulting concentrates were further cleaned magnetically. Potassium concentrations were determined by replicate atomic absorption analysis of solutions of the separated 38 TABLE 3-1. K—Ar data for the Bridge River Camp, southwestern B r i t i s h Columbia. Rock Unit M a t e r i a l (Sample No.) analyzed K P;adi ogeni c Ar ( c m 3 x l 0 s ) Radiogenic Ar ("/.) Date*,Ma ( + l s i gma) PREVIOUSLY PUBLISHED DATA Bralorne i n t r u s i v e s (soda g r a n i t e - s e e t e x t ) m a. 08 20.47 94.4 63.6+1.S (UBC) 3 Coast P l u t o n i c Complex (West of study area) (QSC 76-49) 1 b 7.37 22.876 80.7 79.6+2.9 CGSC 76-50) 1 h 0. 356 1. 036 55.2 74.8+3.6 Minto Mine: dyke w i . J J 3.714 85.5 69.4+2.4 (UBC) 2 v e i n m >~\ 5.S45 74.7 45 +1.5 Congress Mine: (UBC) 3 dyke w 1. 13 3. 004 86.5 67.1+2.2 Bendor p l u t o n s (GSC 76-54) 1 ( o l d e s t ) b 7. 70 17.565 80.3 58.9+2.3 (GSC 76-59) 1 (youngest) h 0.382 0.6235 34.5 42.3+2.9 Rexmount porphyry b 7. 14 12.323 69.7 44.7+2.4 (GSC 76-63) 1 NEW DATA Goldbridqe, gabbro (GBQ) h 0.087 1.053 74.5 287 + 20 Bralorne, d i o r i t e (C093A) h 0.049 0.5861 69. 1 284 + 20 Bralorne, l a t e dyke (C083B) h 0. 162 0.5524 62. 0 85.7+ 3.0 Robson (Eldorado) stock (DY 3217)"* b 6.62 16.684 93.7 63.7+ 2.2 Lucky Gem v e i n (C2020) m 1.53 3.489 61.7 57.7+ 2.0 Bralorne, lamprophyre (C1033) b 6.64 11.353 91.4 43.5+ 1.5 1 Wanless et a l . (1977). 3 Pearson (1977). 3 Harrop and S i n c l a i r (1986). ** K.M. Dawson, p e r s . comm. 1987. a A b b r e v i a t i o n s : h = hornblende, b = b i o t i t e , m = muscovite ( s e r i c i t e or f u c h s i t e ) , w = whole rock. s Ages were c a l c u l a t e d usingA>= 4. 9 6 x l 0 - l o / y r ; /\e+A e" = 0 . 5 8 1 x l 0 - l o / y r ; "°K/K T = 1.167xl0-*mole/mole. minerals using a Techtrem AA4 spectrophotometer. Argon analyses were performed by isotope dilution using an AEI MS-10 mass spectrometer operated in the static mode for mass measurement, a high purity 3 e A r spike, and conventional gas extraction and purification procedures. The precision of the data, shown as the + value in Table 3-1, i s the estimated analytical uncertainty at one standard deviation. 3.3.2 Uranium-Lead (Table 3-2) Zircons were recovered, after crushing and grinding to less than S O mesh, by standard Wilfley table, methylene iodide, and magnetic techniques. The zircons were subdivided into several magnetic fractions, and each of these was again divided, using new nylon sieves, into four size fractions: +149, -149 + 74, -74 + 44, and -44 microns. Where pyrite was present zircons were isolated using a high voltage electrostatic separator. This was followed by hand-picking to as near to 1007. purity as possible. None of the zircons contained r e l i c t cores or inclusions, so abrading to remove outer surfaces was not necessary. Al l fractions consisted of up to 60 percent cracked grains, or fragments of grains. It was necessary to use these because of the small amounts (1-2 mg) of recoverable zircon. In addition to their cracked nature, the more magnetic fractions contained some cloudy or turbid grains which were probably of lower integrity and could have TABLE 3-2. U-Pb data on zircons for the Bridge River Canp, southveeterr* B r i t i s h Columbia. Data are plotted on F l q u r c t 3-2, 3-3 and 3-4# as noted below. • • S B B B . l ' S B ^ B S a B . I E B m ^ n a . a i a . H F r a c t i o n 1 Weight U Pb Pb i s o t o p i c abundance" 20€> Pb/204pb Atomic r a t i o s (datee. Ha)' lag) (ppm) (ppm)2- 20fc Pb -100 measured* 2.06pb/238u 2.Q7pb/Z3Su 2O7Pb/-*06pb £08pb '20.7pt> 204pb l e r r o r c are 2 slgma) Sample C082B, Bralorne d i o r i t e , ei . a l l body adjacent to CO&2A (Fig. 3-2) •74 l . 2 305 13. 1 12. 7040 5. 1629 0.0O15 5190 0.04232*20 0. 2999 *.22 0. OS141.*26 NH l.a/i.5 (267. 2*1.~2> (266. 4*1.6) (259 12) -74 l . a 420 17. 1 13. asso 5.1718 0.0035 7898 0.03965-40 0.2800*34 0.03121 0 4 H i . a / i . 5 (250. 7*274) (2S0. 6*2. 6) (250 i 15) Sample CQ93A, Bralorne d i o r i t e , major body (Fig. 3-2) • 74 s. 7 134 6.0 16. 1897 S. 8730 0. 0454 1890 0.04223*58 0. 3032*72 0.05208^98 NH 2. I/O. S (266. 6*37is> (268. 9*3. 6) (289 i~43> •74 9. 4 199 a. 4 16.3250 5. 2106 0. 0098 6692 0.04043-166 0.2825*122 0.05067*76 HI. 8 /2 , NH1. 6/3 (255. 5*10. 2> (252.6*9. 4) (226 *34 -35) -74 »44 1. 4 257 11. 0 17. 4372 3.2149 O. 0073 4183 0. 040531.28 0. 28S5*.24 0.05108*24 HI. 6/3, NH1.6/S (2S6. 1*.!. 8) ( 255. 0*1.8) (245 11 ) Sample C082A, Bralorne soda granite, small dyke (Fig. 3-3) -149 »74 2. 7 303 12. a 12. 1147 S. 2933 0. 0084 6075 0.04140*30 0. 2951*24 0.05171*22 HH 1.8/2 (261.5*l78> (262.6*2.0) (272 *.~10> -74 *44 3. 2 384 is. a 13.0184 5. 2725 0. 0074 7720 0. 04046*70 0. 2881*52 0. 0S164.*2a H 1.9/1.3 (253.7*4. 4) (257.1*4.1) <270 *12 -13) Sample C094A, Bralorne soda granite. major body (Fig. 3 -3 ) •44 2. 6 331 24. 1 18. 0600 3.3119 0.0067 8766 0.04259*64 0. 3062*46 0.05214*12 NH 1.73/2 ( 268. 9*4.~o> (271. 2*3. 6) (291 *. 5> -44 1. 0 SS4 25. 1 19.3150 5. 3929 0. 0099 4611 0. 04206£.28 0.3044*52 0.05248*68 NH 1.3/3 (263. 6*_1. 8) (269. 8*_4. 0) (306 »33~-34) Sample C092B, a l b l t l t e dyke (Fig . 3-4 > + 74 2. 9 333 4.9S 7.7776 5.3831 0.0383 1826 0.01429*8 0.0950*14 O. 04819*62 NH1. 3/3 (91. 5*0. 4) (92. 1*1.~2) (108 »31> -74 2. 1 424 6. oa 9. 2495 5.8157 0.0717 1106 0.01427*10 0.0936*20 O.04759*86 NH 1.3/S (91. 3*0. 6) (90. 9*l7a> (79 * 43) • u c i c i a i a o i i i i i i i a . a a . a a a a a a a a a a a a a a a a a c a a a a a a a a s s a a x a a a . a a a a a . a a a a = s = a a = a = = =.aa = = = » = a * = = = = =3DC33BBn3 = = S333=C3B = E = 3 3 4 = -; * -149*74 " s i z e range i n microns; H, NH * magnetic, non-magnetic on Frantz lsodynamlc separator at indicated amperage and side t i l t (e.g. 1.5/3 -1.3 amps at 3* side t i l t ) . * Radiogenic * common Pb > Radiogenic • common Pb, corrected for 0. 15X per atomic masB unit (AMU) f r a c t i o n a t i o n and for 120*50 pg Pb b l a n k with composition of 208t207i206i204 • 37. 301IS. 50117.75i1. • Corrected for 0. 13X per AHU f r a c t i o n a t i o n . _ • Decay Constanta » 238U * 0.13S123xl0-9/yr. 235U. - 0.98483xl0-9/yr, 2-38u/-2'»u = 137.88) r a t i o s are corrected i o r U and Pb f r a c t i o n a t i o n (0. 12X/AMU and 0. 15%/AMU r e s p e c t i v e l y ) , blank Pb cf composition ^06pb'2o^p b . 17.750, 207Pb'-?«J'*-Pb • 13.500, .20£pb'204 P b , 3 a_ 4 6 6 > B n d l o r C O B B O n p D o i 9 0 M b ( a l b l t l t e ) and 265 HB (soda g r a n i t e and d i o r i t e ) using the Stacey and Kramer (1975) growth curve. 41 suffered more Pb loss. The ages are therefore to be regarded as mini mums. Zircons were processed using a procedure modified from Krogh (1973!). U and Pb concentrations were determined using a 2oapb_2asu-238U mixed spike. LJ and Pb were loaded together on Re filaments using the H 3PCU-si1ica gel tec hn i que. Mass spectrometry was done on a VG Isomass 54R solid source mass spectrometer with a single (Faraday cup) collector. Precisions for =2 0'7Pb/ : 2 : 0 ePb and a 0 B P b / 3 0 6 P b were better than 0.1"/., and for 2o=Pb/ a o 7Pb and sso^pb/ao-ypt, t h e y were better than 0.5"/. in most cases. Total Pb blanks were less than 0.1 ng; total U blanks were less than 0.03 ng, based on repeated procedural blank runs. U/Pb and Pb/Pb errors for individual zircon fractions were obtained by individually propagating a l l calibration and analytical uncertainties through the entire date calculation program and summing the individual contributions to the total variance. Ages from discordant fractions were determined by f i t t ing data points to a straight l ine using a computer routine based on York (1969), and extraploating to concordia using the algorithm of Ludwig (1980). Errors on individual U-Pb and Pb-Pb dates are quoted at the 2 sigma level (95V. confidence interval) . Additional information on analytical technique is provided in Table 3-2. 4i 3.3.3 Rubidium-Strontium (Table 3-3) Rb and Sr concentrations were determined by replicate analyses of pressed powder pellets using X-ray fluorescence. U.S. Geological Survey rock standards were used for cal ibration, and mass absorption coefficients were obtained from Mo K-alpha Compton scattering measurements. Rb/Sr ratios l i s t e d in Table 3-3 have a precision of 27. (1 sigma) and concentrations a precision of 57. (1 sigma) for samples with higher concentrations of Rb and Sr. For low concentrations a precision of + 1 ppm is applicable. Sr isotopic composition was measured on unspiked samples prepared using standard ion-exchange techniques. The mass spectrometer used was the same as for U-Pb measurements. Experimental data were normalized to an a s S r / e o S r ratio of 0.1134 and adjusted so that the NBS standard SrCQ3 (SRM 367) gave an 0 , r S r / r a o S r ratio of 0.71020 + 2, and the Eimer and Amend standard gave a ratio of 0.70800 + 2. Precision of a single a 7 , S r / e , s S r ratio was between 0.00001 and 0.00010 (1 sigma). The regressions were calculated using the technique of York (1967). The decay constants used for a l l techniques are those adopted by the International Union of Geological Sciences Subcommission on Geochronology (Steiger and Jager, 1977). TABLE 3-3. Rh-Sr data f o r whole-rock samples from the Bridge River camp, s o u t h w e s t e r n B r i t i s h Coluaiblu. Data a r e p l o t t In Figure 3-7. Latitude and longitude of saaples are In Table 3-4. Sample Rock Sr Rb 07Rb/86Sr S^Sr/S^Sr1 Date <na) 2 I n i t i a l r a t i o I n i t i a l r a t i o type (ppm) (ppm) r e c a l c u l a t e d to 270 Ha Bralorne ( l a t e dykes) C083A Hb porphyry 366 4.1 Ci. 032 0.70341 C092A A l b l t l t e 113 38.8 0.991 0.7O480 102 * 10 0.70336 • 0.00010 (major i n t r u s i v e masses) C093A C094A C082A C082B D i o r i t e 111 Soda granite 77.6 (alnor bodies) Soda granite 236 D i o r i t e 200 1. S 12. 6 1.6 1. 4 0. 039 0. 470 0. 020 0. 021 0. 70365 0. 70433 0. 70336 0.70353 166 * 27 0.70343 * 0.00008 0.70327 » 0.00008 (Pioneer volcanics of the Cadwallader Group) C09SA C096A C096B C096C C098A CQ98D Basalt Basalt Basalt Daclte Basalt Andealte 262 37. 178 240 136 lOS 0. 5 0. 04 0. S 0. 1 S. 0 1. 9 0.00S 0. O03 0. 009 0. O01 0. 092 0. 034 0.70382 0. 70372 0.70380 0. 70384 0.70382 0.70384 O. 70380 » O. OOOOS 0.70355 <• O.00003 Noel (Hurley-Noel sediments of the Cadwallader Group) S l a t e 300 38.6 0.373 0.7048 Hurley A r g l l l l t e 144 16. 9 0. 339 0. 7063 Goldbrldqe Quarry (major Intrusive, GBQ D i o r i t e 349 1.1 GBO-1 Soda granite 16.9 0.3 GBO-2 Soda granite 204 0. 3 GBQ-3 Soda granite 133 1.1 f e l s l c d y kes) 0. 009 O. 080 0. O08 0. 027 O. 7O310 0. 70340 0.70297 0. 70319 320 • 80 0.70302 » 0.OOOOS Bridge River Canyon area (Hetaplutonlc block in BRD313A Hetapiutonlc 433 7.2 0.046 melange) 0. 7047 ( B r i d g e R i v e r Group s c h i s t s ) C5-3C G a r n e t - b l o t i t e 133 42.1 0.793 D G a r n e t - b l c t l t e 132 26.9 0.392 E B l o t i t e 157 28.6 0.323 F Garnet-blotite 172 36.0 0.607 H Garnet-blotite 203 37.7 0.536 O. 7063 0. 7059 0. 7060 0. 7061 0. 7060 85 • 46 0. 7053 • 0. 0004 C3-3A a c D Q t z - s e r - c h l o r Otz-ser-chlor Qtz-aer-chlor Qtz-ser-chlor 111 246 117 59. 4 40. 7 49. 3 67. 4 46. 2 1. 039 0. sai 1. 673 2. 2S 0. 7085 O. 7074 0. 7073 0.7087 154 •• 27 0. 7058 ' O. 0004 CS-7A Qua r t z - b l o t l t e 186 32.4 0.504 B Qu a r t z - b l o t l t e 221 17.8 0.233 C Q u a r t z - b l o t l t e 224 34.2 0.441 D Qtz-blo-chlor 128 10. 2 0. 229 O. 7061 0. 7060 0.7062 0. 7062 41 <• 30 0.7060 * O.0002 TABLE 3-3 (continued). D • t z-b i o-ch1or 128 10. 2 0. 229 0. 7062 E Qtx-chlor-blot 106 22.2 0. 604 0. 7064 BR-1 Phyl1i t e 122 48. 1 1. 138 O. 7071 2 Sch i et 283 22. 0 0. 223 0. 7049 6 A r g i l l i t e 119 38.2 1. 419 0.7068 7 Sandstone 336 15. 3 0. 133 0. 7046 142 ± £0 0. 703O + O. 0002 MV77BR5 B r i t 72. 9 52. 4 2. 080 0. 7105 MV77BR7 Basalt 213 17. 3 0. 235 0. 7050 North sid e of Caroenter Lake (Bridge River Group cherts) BR-* Chart 21.0 15.6 2. 14 0. 7138 191 i f 0. 7080 assumed*' 5 Chert 40. 3 23.9 1. 849 0. 7140 228 i f 0. 7080 assumed^-Hope S l i d e (Hozameen Group volcanics) HozHBl Greenstone 182 3.9 0.062 0.7033 HozHS2 Greenstone 188 2.8 0.043 0.7033 aaO(anumed) 0.7031 Coouihalla Road (Hozameen Group sediments) HozCqR Pe n c i l s l a t e 77.8 1.2 0.045 O. 7068 1 P r e c i s i o n i s less than or equal to +0.0001 where 4 decimal f i g u r e s are given and +0.00001 f o r 5 f i g u r e s . 011 e r r o r s are 1 sigma. * Decay constant used = 1. 42x10-'*/yr. } Calculated I n i t i a l r a t i o at 270 Ma (before Mesozoic rehomogenization) using the appi-ox irnat ion • Change i n «'Sr/«»Sr -(Rb/Sr)»(0. 0000413)«(270 Ma - age of isochron) where Rb/Sr i s the arithmetic average of present-day measured concentrations, and 270 Ma i s round-number minimum age from U-Pb. 4 Assumed i n i t i a l r a t i o f o r seawater strontium of appropriate age. 45 TABLE 3-4: Latitude and longitude of samples in this study SAMPLE LATITUDE CN!) LONGITUDE (W) C082A/B 50° 46.2' 122° 48. 0' C0S3A/B 50 46. 2 122 48. 0 CQ92A/B 50 46.2 48.0 C093A/B 50 46.2 122 48. 0 C094A/B 50 46.2 1 22 48. 0 C095A/B 50 45.6 122 47.4 C096A,B,C 50 45.6 1 47.4 C098A,D 50 45.6 122 47.4 CI 033 50 45.6 122 47. 4 C2001 50 59.4 56.3 C2001A,B 50 59.3 1 JCJC 56.5 C2001C, D 50 59. 0 122 54. 5 C2003 50 err> o 1 '"2*2 47.0 C2020 50 59. 3 i •->*""• 55.7 Noel Slate 50 46.7 122 49.3 Hurley Arg 50 46.8 122 51.3 GBQ 50 51.6 122 50.8 GBQ-1,2,3 50 51.6 122 50.8 BRD513A 50 48. 0 122 10.5 C5-3C to H 50 49.2 122 10. 4 C5-5A to D 50 49.7 122 11.8 C5-7A to E 50 50. 1 12. 0 BR-1 50 50. 9 12.0 BR-2 50 46.9 122 14.6 BR-4 50 46.0 122 17.2 BR-5 50 46.0 122 17.6 BR-6 50 50. 6 122 28. 9 BR-7 50 53.2 122 37.7 MV77BR5 50 50. 5 122 27.6 MV77BR7 50 53. 6 122 47.7 HozHSl 49 18. 1 121 14.9 HozHS2 49 18. 1 121 14.9 HozCqR 49 26.2 121 18.0 3.4 Results 3.4.1 Potassium-Argon A summary of a l l K-Ar data for the camp is presented in Table 3-1. The Bralorne intrusive suite (diorite and soda granite) north of Goldbridge (Fig. 1-2) gives an Early Permian or Late Pennsy1 vanian age of 287 + 20 Ma by K-Ar on hornblende. At the Bralorne mine the d i o r i t e gives an equivalent age of 284 + 10 Ma by K-Ar on hornblende. An attempt by Pearson (1977) to date white phengitic mica of the Bralorne soda granite, at the BRX mine north of Bralorne (Fig. 1-2) gave 63.6 + 1.8 Ma. This mica actually came from a quarts porphyritic a l b i t i t e dyke exposed in a road outcrop (my observation, confirmed by G. Woodsworth, pers. comm. 1988) and so i s not representative of the Bralorne soda granite. Also in the Bridge River camp, at the Congress and Minto deposits, dykes possibly associated with mineralization have been previously dated at 67 and 69 Ma (Pearson, 1977; Harrop and S i n c l a i r , 1985). To the north in Eldorado Basin, s e r i c i t e associated with the Lucky Gem mineralization (Fig. 1-2) dates at 57.7 + 2.0 Ma, and in the Bonanza Basin to the north of the Lucky Gem, biotite from the Robson (Eldorado) stock dates at 63.7 +2.2 Ma (K.M. Dawson, pers. comm., 1987). Hornblende plagioclase porphyry dykes in the Noaxe and Warner Pass areas north of the Bridge River area indicate intrusive events at 86, 76 and 65 Ma, 47 and similar dykes that also contain biot i te indicate intrusive events at 57 and 47 Ma (Archibald et, al • . 1989!). Dates from the post-deformation Bendor plutons range from 59 to 42 Ma, and represent the final pulse of Coast Belt plutonic act iv i ty (Wanless et al . , 1977!). Other igneous act iv i ty in the'Bridge River camp coincides with this Pal eocene to mid-Eocene magmatic episode: late lamprophyre dykes at Bralorne (43.7 + 1.5 Ma by K-Ar on b i o t i t e — t h i s study) and the Rexmount porphyry (44.7 + 2.4 Ma by K-Ar on b i o t i t e , Fig. 1-2; Woodsworth, 1977). Confirmation of an important magmatic event during Middle Eocene time i s provided by dates of 44 Ma for the Beece Creek and Lorna Lake plutons in the Warner Pass area north of Bridge River and west of Blackdome (Archibald et a l . . 1989). Mineralization at the Blackdome epithermal Au-Ag vein deposit, which l i e s northeast of the Coast Belt, may also be of this age; a l l that is known is that i t is post 51 Ma and older than 24 Ma (Faulkner, 1986). A large sample of the Bralorne diorite CC093A) yielded fine pale green hornblende with only traces of chlorite . Its age, 284 + 20 Ma, agrees very well with the previous date of 287 + 20 Ma from the gabbro north of Goldbridge that contained black hornblende of very fine quality. Pertinent K-Ar data for both samples are l isted in Table 3-1. However the potassium content of both hornblendes i s so low that the calculated age should be considered a maximum value (below approximately 0.20V. potassium, excess argon i s of concern). The hornblende dates are believable only in view of their mutual consistency, and agreement with the U-Pb date determined for the Bralorne diorite and Rb-Sr date for the gabbro and leucocratic dykes near Goldbridge. If excess Ar is involved, however, i t has almost exactly compensated for any loss of Ar due to resetting by later magmatic or hydrothermal events; this seems an unlikely interpretation for these dates. A great gap in time is present between the diorite intrusion and a largely post-mineral dyke set which contains large black euhedral hornblende crystals. The hornblende crystals in the dated dyke (C083A) preserve their original magmatic zoning when seen in thin section, with dark brown rims surrounding pale green interiors (Plate 5-12'.). This i s unlike plagioclase in the same rock, which has undergone the regional degradation to albite , zois i te , chlorite and s e r i c i t e displayed by a l l the igneous rocks of the Bralorne area except those later than regional metamorphism and intrusion of the Coast Plutonic Complex. More confidence can be placed in the 85.7 + 3 Ma date obtained from this hornblende, although i t s 0. 1&7. potassium content i s s t i l l quite low. Most of these hornblende porphyry dykes are fresh, showing l i t t l e or none of the hydrothermal alteration that has affected the pre-mineral rocks. However, a few samples do contain abundant, strongly altered r e l i c t hornblende phenocrysts where the dyke i s adjacent to a major vein. Thus they are either post-mineral or late i n t r a -mineral in character, and mineralisation probably occurred mainly before 36 Ma. Black lamprophyre dykes with a markedly different orientation to a l l the earl ier dykes d i s t i n c t l y cross-cut mineralized veins at Bralorne. These dykes (C1033) contain coarse brown biot i te , pale green c1inopyroxene, and smaller clear apatite phenocrysts in a finer grained matrix of the same, plus a groundmass of glass and Fe-Ti oxides (Plate 5-14). Plagioclase has been reported in these dykes by Stevenson (1958), and thus they would be c lassi f ied as kersantites (Hughes, 1982). Their K-Ar date of 43.5 + 1.5 Ma on a clean bioti te concentrate puts another younger l imit on the timing of mineralization at Bralorne. 3.4.2 U r a n i u m - L e a d Two samples each of the Bralorne diorite and soda granite, and one of the pre-mineral a l b i t i t e dykes yielded sufficient zircons for U-Pb dating (Table 3-2). Three major samples, which were d i o r i t e (C093A), soda granite (C094A) and a l b i t i t e dyke (C092B), each representing 16 to 30 m of the freshest d r i l l core available, are from widely separated areas of the underground workings. Another pair of smaller samples, diori te (C082B) and soda granite (C082A) were sampled in close proximity to each other. Although there are differences in appearance of the zircons from the d i o r i t e and soda granite, their ages are the same within analytical uncertainty (Table 3-2). Zircons separated from the diorite (C093A and C082EO are generally pink. Coarse, squat, flattened, or mildly deformed ovoid grains are very common in C093A; they are not present in C0S2B, where instead the zircons are d i s t i n c t l y pink, stubby prisms. In contrast, the zircons of C094A (soda granite) are white to s l i g h t l y yellowish and are usually elongate prisms with slanting terminations. In these features they are much more l ike the slender, prismatic zircon crystals of the a l b i t i t e dykes (C092B). Zircons of the other soda granite sample, C082A, are most similar to those of C082B: pink, stubby to s l i g h t l y elongate prisms. The soda granite is more mafic than the average at this location, probably due to border-phase contamination of the soda granite by the intruded d i o r i t e . However, since a l l the dates for both soda granite and diorite are the same within analytical uncertainty, this i s not a problem. Two large samples of the Pioneer basaltic andesites from the Cadwallader Group (C095A and B), and two samples of the late green hornblende dykes (C083A and B) were processed. Both failed to yield any zircons. Bralorne Diorite and Soda Granite Zircons from the diorite and soda granite yield concordant to s l i g h t l y discordant U-Pb and Pb-Pb dates (Table 3-2 and Figs. 3-2 and 3-3). With the exception of a single fraction from the soda granite (C094A, +44 Ml.75/2°; £7 Figure 3-2. Concordia diagram for the Bralorne diorite (two fractions s a m p ^ C ^ A !Sh WJ t h — " 2 S l 9 m a e r r ° r S ' a n d t h " e e faction " ::zi: ^iSr . j : r ?rFf gr is=?." r r o r" >- D a t a a r e f r o m T a b i e 3- 2> a n d o o o o o o o o n 8 E 2 / C | d 9 0 2 52 Figure 3-3. Concordia diagram for the Bralorne soda granite (two fractions for each of C082A and C094A). Data are from Table 3-2, and sample locations are in Fig. 3-1. S3 abbreviations in Table 3-2), a l l zircon fractions plot as points with 2 sigma error envelopes overlapping concordia. The diorite has a minimum age of 257 Ma based on the 2 0 S P b / 2 3 B U date for the coarse, non-magnetic fraction from sample C082B. The other d i o r i t e sample (C093A) also has a minimum age of 267 Ma. The assignment of a minimum age i s just i f ied on the basis of lower Pb/U ratios for the fine, magnetic zircons from both samples. Although these other fractions plot as concordant points, their lower Pb/U ratios can be reasonably interpreted as due to post-crystal l izat ion Pb loss, along trajectories making very low angles with concordia. Such trajectories may have early Mesozoic lower intercepts, but the large overlapping errors for individual fractions preclude f i t t i n g chords to their respective U-Pb points; in any case the resulting intercept errors would be too large for meaningful geological interpretation. Similar considerations suggest a minimum age of 269 Ma for one sample of soda granite CCG94A, +44 Ml.75/2°). The coarse fraction has discordant Pb-U and Pb/Pb dates, but l i t t l e meaning i s attached to this discrepancy, which could be due to inheritance of old radiogenic Pb, sporadic contamination in the laboratory, or to =so,*F'b analytical errors. Inaccuracy in measuring SEO"*Pb (with a Faraday cup, since the UBC Geochron Laboratory did not have a Daly photomultiplier at the time of analysis) would have propagated into inaccurate a o ' r P b / a : 3 8 S U ratios. The same holds for the non-magnetic fraction of C094A. It i s for 5 * this reason that the 2 0 S P b / 2 3 a U dates are preferred in assigning minimum aiges to these rocks. Using the same reasoning, the other sample of soda granite (C082A) yields the minimum 2 0 6 P b / a 3 B U date of 261 Ma. However, the relat ively better analytical quality for both fractions from this sample indicates that the s s o ' 7 'Pb/ S ! o s Pb date of 272.4 Ma for the coarse, non-magnetic fraction i s a better approximation of minimum age. A two point regression ('.York, 1963.) for C082A gives an upper intercept of 278 Ma. The 10 Ma errors on the : a o s P b / : 2 0 ' 7 P b date of 272.4 Ma overlap the minimum age estimates for the Bralorne d i o r i t e . Although the f ield relations show the soda granite to be younger than the diorite (Chapter 5 ) , i t i s clear that no distinction can be made between them on the basis of the U-Pb data. The present data therefore suggest that the d i o r i t e and soda granite form a composite Early Permian intrusion, which i s at least as old as 270 Ma. The U-Pb data do not rule out an older, perhaps Pennsylvanian age, but this is considered unlikely because of the reasonably close clustering of points overlapping concordia, combined with the large amounts of Pb loss that would have to be invoked for the zircons to be that old. A l b i t i t e Dykes Zircons from the pre- or syn-mineral a l b i t i t e dykes (C092B:> yield unambiguously concordant U-Pb dates (Table 3-2 ? r l T r 2 l t \ E ° n c o ^ d i a diagram for Bralorne alb i t i t e dykes. Dat from Table 3-2, and sample locations are in Fig. 3-1. a are and Fig. 3-4). The two zircon fractions have ^oepb' 2 3" 5^ dates of 91.3 and 91.5 Ma. The overlap of both points on the concordia diagram indicates no Pb-loss in these zircons, and gives a c r y s t a l l i z a t i o n age of 91.4 Ma. Thus the a l b i t i t e dykes are early Late Cretaceous in age. Their close association with and parallel attitudes to mineralized veins, plus their common intense alteration to quartz, s e r i c i t e , fuchsite, carbonate, and pyrite, suggests that they were also closely related temporally to mineralization, probably immediately preceding i t . 3.4.3 Rubidium—Strontium All the rubidium-strontium data from this study and previously unpublished work by R.L.Armstrong in the Bridge River d i s t r i c t are l i s t e d in Table 3-3 and plotted in Figure 3-5. A large portion of this previously unpublished data, pertaining to the Bridge River Group, is mainly from p e l i t i c metasediments (samples donated by C . J . Potter and J.W. Monger). Two samples of Cadwallader Group sediments included with this data also plot within the envelope of the Bridge River Group sediments. Individual isochrons for suites from specific l o c a l i t i e s give dates (Table 3-3) that range from 41 to 154 Ma, and average 106 Ma. Taken a l l together, they plot in a band about 0.002 wide in s ' 7 S r / o s S r , with an i n i t i a l rat io of about 0.7050, and with a slope corresponding to a Late Jurassic age of about 152 Ma. Given the uncertainties in interpretation of Rb-Sr analyses 57 Figure 3-5. a»'Sr/'«M*Sr vs. a yRb/ e < >9r plot for different petrologic units from the Bridge River d i s t r i c t . Data are from Table 3-3; note change of scale on both axes from main diagram to inset diagram. ^Partially reset at age given; data give "scatterchrons" which are believed to be partially reset isotopic systems so that the calculated ages l i e between age of the protolith and the Late Cretaceous mineralizing/metamorphic event. Samples from the Bralorne diorite and soda granite intrusions are shown as X's. Only late dykes at Bralorne, located in Fig. 3-1 (C092B, a l b i t i t e = Abf C0B3A green hornblende porphyry = GHP, shown as squares), and GBQ suite (crosses) from Goldbridge quarry, located in Fig. 1-2, give reasonable isochron dates. 2 Clastic and p e l i t i c sediments. of sedimentary rocks (Armstrong and Misch, 1987), this must be a minimum stratigraphic age, and is younger than the youngest fossil (Lower to Middle Jurassic) in cherts associated with these p e l i t i c rocks (Potter, 1983), The Rb-Sr ages of the p e l i t i c metasediments of the Bridge River Group have therefore probably a l l been variably reset by regional metamorphic event(s). The Late Jurassic age i s also possibly indicated to be a minimum by ages of 195 + 6 to 250 + 9 Ma obtained by K-Ar and Rb-Sr whole-rock dating of blueschist samples from Bridge River Group rocks in the Eldorado Mountain area (Garver et a l . , 1989). White mica separates from a nearby outcrop yield a K-Ar age of 244 + 7 Ma and a Rb-Sr age of 217 + 5 Ma (Garver et al . , 1989b). The blueschist metamorphic event is thus dated as Permo-Triassic to Triassic (220 to 250 Ma), and is similar to ages of metamorphism of other blueschist complexes in B.C., Washington, Oregon and Cal i fornia (Garver et al • , 1989b). However, the blueschist rocks may be exotic blocks and not date the Bridge River Group. One sample of Bridge River pillow basalt and two of greenstone from the correlative Hozameen Group plot at the low Rb end of the Bridge River Group envelope on Figure 3-5, close to where the Cadwallader Group basalt samples plot. Two samples of relat ively unmet amorphosed ribbon chert from the Bridge Fliver Group l i e s ignif icantly (several sigma) above the points for the p e l i t i c sediments and suggest an age of approximately 210 Ma, near the Triassic-Jurassic 5? boundary, i f a Sr i n i t i a l ratio of 0.7030 for Mesozoic seawater i s assumed. The Sr isotopic signature of the E-fridge River suite i s similar to that of the Darrington Phyl l i te , part of the Shuksan Metamorphic Suite located in northern Washington (Armstrong and Misch, 1987). On a plot of Q ' 7 S r / s < 5 S r versus  s'7Rb/®6Sr (Fig. 3-5), the envelope for the Darrington Phyl l i te almost exactly overlies the envelope of the Bridge River suite (and therefore is not plotted). In other words, the suites art? identical in average and range of Rb, Sr , and a 7 S r / 8 6 S r values. Similarly, the envelope for the less radiogenic Shuksan blue- and greenschist samples overlies the distribution of the Bridge River and Cadwallader basalts. Also, the two samples of chert above the envelope for the other sediments, occupy a more radiogenic position similar to the Jurassic radiolarian cherts from the Franciscan assemblage in Cal i fornia, which are more radiogenic than associated c last ic sedimentary rocks, as discussed by Armstrong and Misch (1987). This is not to imply that the Bridge River rocks are of precisely Jurassic age, but they are isotopically similar to other Cordilleran Mesozoic subduction-accretion assemblages and may thus be of similar age and provenance, although they have been reset by Jurassic to Eocene metamorphic episodes. An age older than Late Paleozoic i s unlikely, given the apparent i n i t i a l rat io of 0.705 and high average Rb/Sr rat io of the sample suite. 60 The attempt to obtain the age of the Cadwallader Group by Rb-Sr dating of the Pioneer volcanics, after they had proved barren of zircons, was unsuccessful. They gave only an isochron with almost zero and very uncertain slope, probably due to pervasive Mesozoic rehomogenization of the Sr isotopic composition and redistribution and gain or loss of the Rb (Faure, 1982) by metamorphic and/or mineralizing fluids in the Bralorne area. The range of E 3' 7Rb/ s s'Sr ratios (.0.001-0.094) i s very limited in this suite, even though some relat ively si l iceous rocks were sampled (quartz kera-tophyres with up to 667. SiGV). The Kx-Q CO. 03 to 0.597.) and therefore the Rb contents (1-5 ppm) were a l l extremely low. The i n i t i a l ra"?'Sr/06Sr ratio of these volcanics (recalculated t o 270 Ma) o f 0.70372 + 0.00005, i s c ompat i b1e with an oceanic arc setting (section 3.5), as suggested by the petrochemical data of Table 2-1, and of Rusmore (1987). Gabbro and three leucocratic dykes from the quarry 2 km northeast of Goldbridge (unpublished data of F:.L. Armstrong: Table 3-3) are correlative with Bralorne d i o r i t e and soda granite respectively. They are likewise low in Rb and give only a limited spread in whole rock Rb/Sr r a t i o . Nevertheless they provide an isochron date of 320 + 80 Ma and a precisely defined i n i t i a l »?'Sr/ a sSr rat io of 0.70302 + 0.00005 (Fig. 3-5). The date, although quite uncertain, lends support to the K-Ar evidence for a Late Paleozoic rather than Triassic age for these rocks. The i n i t i a l ratios of the Bralorne intrusions at Goldbridge (0.70302 + 61 0.00008) are .lower than those of the Cadwallader volcanics (0.70380 +0.00005) or the dykes associated with mineralization (0.70336 +0.00010). The intrusions are more l ike ocean floor basalts and plagiogranites, which have i n i t i a l ratios of 0.7024 to 0.7030, than oceanic volcanic arc rocks, which have i n i t i a l ratios of 0.7032 to 0.7040 CFaure, 1982). The four Bralorne d i o r i t e - soda granite analyses produce an essentially two-point isochron with an apparent age of 166 +27 Ma (Fig. 3-5, inset). This has presumably also been reset by hydrothermal activity in the Bralorne mine area, since the zircon ages for the same samples are 270 Ma. The unaltered suite from the Goldbridge Quarry has not been reset. The Sr i n i t i a l ratio for the Bralorne plutonic suite, at 0.70327 + 0.00008 (corrected to 270 Ma), l i e s between that of the Goldbridge Quarry suite and that of the Cadwallader volcanics, possibly implying a transitional ocean floor - volcanic arc character. The late dykes ( a l b i t i t e and green hornblende porphyry), which although altered do not appear to have been reset by the hydrothermal act ivi ty or by Eocene magmatic/metamorphic a c t i v i t y , also gave a two-point isochron (Fig. 3-5). The Rb-Sr date of 102 + 10 Ma i s in reasonable agreement with the U-Pb age of 91.4 + 1 Ma for the a l b i t i t e , and the K-Ar date of 85.7 + 3 Ma for the green hornblende porphyry. These dykes have i n i t i a l Sr ratios 62 (0.70336 + 0.00010) between those of the Bralorne intrusions and the Cadwallader volcanics. There appears to have been a major Sr isotopic homogenisation of a l l the samples within the Bralorne block, similar to that noted above for p e l i t i c suites from the Bridge River Group. This is in contrast to the undisturbed state of the Goldbridge quarry suite. The regional homogenization was probably caused by the major hydrothermal event at the time of mineralization, although regional metamorphism occurring at the same time could also have played a role. This has variably reset a l l the Rb-Sr systems except those of the dikes coeval with the mineralization and the rocks at Goldbridge. Petrographic evidence for such homogenization comes from the textures of the plagioclase feldspars, which have lost almost a l l traces of their original zoning in the pre- and syn-mineral rocks, where Sr has been isotopical ly reset but retain osci l latory zonation in post-mineral rocks. Good feldspar zonation i s preserved only in the plagioclases of dykes from the Bendor pluton, dykes at the Congress Mine, and similar dykes in Eldorado Basin, a l l of which are younger than 70 Ma. 3.5 Discussion 3.5.1 Age of Mineralization The results of this study constrain the age of mineralization, which cannot be older than the 91.4 mill ion 63 year a l b i t i t e dykes that are strongly altered and mineralized. Indeed the mineralized veins often closely follow the a l b i t i t e dykes, implying a close genetic relat ion. This feature has been noted before by Cairnes (1937) and Stevenson (1958). A minimum age for the mineralization may be indicated by the 85.7 Ma for the intra-mineral to post-mineral green hornblende porphyry dykes. This is however not as clear-cut for two reasons: CI) some alteration of the same type accompanying mineralization Cfuchsite, pyrite, arsenopyrite) does occasionally cut the green hornblende porphyry, and C2) petrographic study of the dykes suggests a transitional relationship from the a l b i t i t e s to the green hornblende porphyries. Also, the latter usually display two prominent orientations: one sub-parallel to the a l b i t i t e s , and one oblique to them. Mineralization must, however, be older than the 44.7 Ma lamprophyre dykes that are completely unaltered and d i s t i n c t l y cross-cut mineralized veins. Therefore, the mineralizing event can be bracketed between approximately 90 and 45 Ma. The age of mineralization might be further restricted by i t s relationship to apparently post-mineral dykes related to the Bendor pluton, which l i e s 5 km to the east. These dykes have yielded K-Ar dates of 62 to 63 + 3 Ma on hornblende and 57 to 59 + 2 Ma on biotites CWanless et a l . . 1977). One such dyke near the east side of the Bralorne mineralized zone has strong s i m i l a r i t i e s to the Bendor pluton: i t contains augite cores to hornblende grains, and osci l latory zoning of clear, glassy plagioclase that ranges from oligoclase to andesi ne (An2o~An4o-' • Neither zoned plagioclase nor such calcic plagioclase compositions are seen in any earlier rocks in the Bralorne block, possibly because a l l these earlier rocks have been subjected to extensive hydrothermal alteration or lower greenschist facies metamorphism, or both (the two processes probably overlap and cannot be separated here). This destroyed a l l but traces of zoning in the plagioclases and homogenized their compositions to albite (An a-An»0). It seems probable then, that the metamorphic or hydrothermal culmination occurred before emplacement of the Bendor plutons and dykes Since both the a l b i t i t e and green hornblende porphyry dykes have been variably affected by the metamorphism and/or alteration, i t is l i k e l y that the mineralization/metamorphi event died out during the interval 85-60 Ma. At the Congress mine 15 km to the northeast, similar glassy clear zoned plagioclase occurs in a microdiorite porphyry of 67.1 + 2.2 Ma age (B. Cooke, i_n Harrop and S i n c l a i r , 1985). Thi suggests that the decline of the metamorphic episode might be further restricted to between 85 and 70 Ma. This correlates well with many K-Ar dates on b i o t i t e and hornblende from the Coast Plutonic granodiorites in the area, which commonly show the same range from 70 to 85 Ma (Woodsworth, 1977). This time interval coincides with the Late Cretaceous magmatic episode described by Armstrong-(1988) as located along the east side of the Coast Plutonic Complex in southern B.C. Mineralization at Bralorne i s , therefore, most reasonably of Late Cretaceous age. It is post 91.4 Ma, and probably largely pre-85.7 Ma. It could have lasted as late as Paleocene time, i f the green hornblende porphyry dykes are in fact i nt r a-mi ner al and not post--mi ner al , although this is regarded as unlikely. This Late Cretaceous mineralizing episode i s unusual amongst gold deposits of the Canadian Cordil lera, which are generally of Triassic -Jurassic or Early Tertiary (Paleocene -- Eocene) age. The Bralorne deposits are unusual in other respects also: their size and extraordinary depth extent of 2,000 m without notable zonation or change in grade is different from other vein deposits in the Canadian Cordil lera, and instead bears strong resemblance to the large gold vein deposits of the Precambrian Shield (c f. Bertoni, 1983). The other vein deposits of the Bridge River camp and surrounding area (Chapter 4) are mostly of Early Tertiary age, ranging from about 68 + 3 Ma at Congress and Minto (Harrop and Sinclair 1986) to about 45 Ma (actually, between 51.5 and 24 Ma; Faulkner, 1986) at Blackdome 50 km to the northeast of the camp (Fig. 1-1). This arrangement of ages, becoming progressively younger farther from the Coast Plutonic Complex (CPC) is not unexpected, and i s in agreement with the mineralization zonation outlined by Woodsworth et a l• (1977). In this zonation they postulated that mineralisation became more epithermal, or lower temperature, in character with increasing distance from the CPC. This is evidenced by increasing amounts of such elements as As, Sb and Hg further away from the CPC, and the presence of molybdenite and scheelite with high Au/Ag ratios in the mesothermal Bralorne deposits closer to the CPC. A relationship among a l l these mineral deposits i s suggested not only by their progression in ages (90 to 45 Ma) but also by galena lead isotopic evidence (Chapter 4 ) , in which a l l deposits analysed plot in a cluster suggestive of a young (90 to 45 Ma) mixing l i n e between mantle and upper crustal sources. Further confirmation of the zoning from higher temperature deposits close to the CPC to lower temperature deposits further away is provided by the stable isotope and fluid inclusion data of Maheux et a l . (1987), the fluid inclusion data of Vivian et a l . (1987), and this study (Chapter 8). Naturally, erosion depths decrease away from the margin of the CPC, as uplift associated with the core of the complex decreases. If the known deposit ages are plotted versus their distance from the CPC, a relat ively smooth curve i s generated (Fig. 4-1) of decreasing age with increasing distance that gives a slope of about 60 km/50 Ma or roughly 0.12 cm/yr, close to the 0.18 cm/yr rate of eastward regression of the magmatic front noted by Armstrong (1988; cf. 0.25 cm/yr of Godwin, 1975). In summary, it seems that mineralization along this portion of the Coast Belt and the adjacent Intermontane Belt i s strongly tied to the evolution of the granitic plutons of the CPC, which presumably were the source of heat driving the mineralizing fluids. Mineralization appears to have begun at or just before the peak of metamorphism, with early, higher temperature mineralization at Bralorne between 91.4 and 85.7 Ma. This was followed by perhaps two other weaker, lower temperature "pulses" further from the CPC at around 68 Ma (Minto, Congress), and even further away at about 45 Ma (Blackdome). These mineralizing pulses may simply be discernible peaks in a semi-continuous process. 3.5.2 Age and petrochemical se t t i n g of the Bridge River and Cadwallader Groups Isotopic dating of this study on the Bralorne diorite and soda granite gives Early Permian ages O270 Ma U-Pb to 285 Ma K-Ar) apparently older than the rocks they intrude. These, the Cadwallader and Bridge River Groups, have been dated by fossi ls as Mid- to Upper Triassic (Ladinian to Norian: 240 to 220 Ma). None of the fossil l o c a l i t i e s are within the structural block hosting the Bralorne intrusives dated by U-Pb on zircons. However, the fossil l o c a l i t i e s in the Bridge River Group rocks are only 10 km from the Bralorne intrusions in the Goldbridge Quarry that are dated here at 287 + 20 Ma by K-Ar on hornblende. The excellent agreement between this date and the 284 + 20 Ma age obtained 68 for another part of the d i o r i t e , at Bralorne, suggests that the Permian age i s real . The sample from Bralorne has been subjected to ** < s Ar/ 3 3 Ar dating, which has confirmed i t s Early Permian age, with a plateau at 276 + 31 Ma (2 sigma; pers. comm, D. Archibald, 1988). The Permian age i s further supported by the Rb-Sr date of 320 + 80 Ma for the d i o r i t e -soda granite suite from the Goldbridge Quarry. Since both the Bridge River and Cadwallader Groups seem to be intruded by the d i o r i t e and soda granite, the p o s s i b i l i t y i s raised that parts of both groups could be as old as Permian. The Early Permian age implied by the present work for those parts of the Cadwallader and Bridge River Groups in the Bralorne block is in conflict with Middle Triassic stratigraphic ages (225 Ma) assigned elsewhere on the basis of paleontologic evidence. The age conflict hinges on several key points: (1) Paleontologic evidence, a l l from outside the Bralorne block, dates the Cadwallader and Bridge River Groups as younger than the Bralorne intrusives. (2) The rocks mapped as Cadwallader and Bridge River within the Bralorne block are 1ithoiogical1y and chemically similar to those mapped elsewhere. (3) The Cadwallader and Bridge River rocks appear to be intruded by, not laid on top of, the Bralorne intrusives; however, some contacts are faulted. 63 (4) Cadwallader sediments include a conglomerate which contains c l a s t s that resemble soda granite, implying that the soda granite is older than the conglomerate. (5) Although the paleontologic dates in the Cadwallader rocks come from a sedimentary unit overlying the basal volcanic unit, the sedimentary unit in i t s lower parts contains intercalations of volcanics 1ithaiogical1y similar to those of the underlying unit. The paleontologic age of the Cadwallader Group is limited to the Upper Triassic Karnian --• Norian boundary at about 230 to 220 Ma, by conodonts found by Church et al . (1988) 4 km northwest of Bralorne, and by Rusmore (1985) in exposures in the Eldorado Basin 20 km northwest. Rusmore's mapping led her to propose a new subdivision of the Cadwallader Group (section 2.2.2). Microfossils of age similar to those in the Cadwallader, including conodonts and radiolaria , have also been found in the Bridge River Group sediments at several locations north of Bralorne along Carpenter Lake (Cameron and Monger, 1971). Here the paleontologic ages range from Middle Triassic to Lower Jurassic (Potter, 1983; Cordey, 1986). Volcanic and sedimentary rocks in the Bralorne block have long been considered to belong to the Bridge River and Cadwallader Groups, and to be intruded by the Bralorne intrusives (Cairnes, 1937; Stevenson, 1958; Joubin, 1948). In the mine area, both d r i l l core and underground workings show Bralorne intrusives with abundant xenoliths, apparently To of Cadwallader rocks, implying an intrusive relationship. A similar complex inter fingering relationship between the Cadwallader volcanics and the Bralorne intrusives has been described 5 km north of the mine by Cairnes (1937), who further suggested that the volcanics and intrusives were co-magmatic and therefore roughly coeval. The following evidence suggests that Cadwallader Group rocks are intruded by the Bralorne intrusives: (.1.) Up to 5 to 20 m of apparent horn f el sing ('.baking or bleaching) of the Cadwallader rocks occurs in d r i l l holes crossing the linear northeast contact of the Bralorne intrusive mass, where i t i s composed mainly of soda granite (Fig. 3-1). This contact i s , however, marked by a strong zone of shearing, so i t might be a fault contact—probably an intrusive contact that was later faulted. Such hornfelsing is also evident on the Wayside property 5 km north of Goldbridge, where the adjacent intrusive, although not containing the network of quartz/epidote fractures characteristic of the Bralorne d i o r i t e , is seen in thin section to actually be a soda granite, with the dist inct ive symplectic quartz - albite intergrowths of the soda granite at Bralorne (Chapter 5). (2) Complex interfingering of Bralorne d i o r i t e and greenstone occurs on the southwest flank of the intrusive mass. If these greenstones belong to the Cadwallader Group then i t would be d i f f i c u l t to propose that each of these small greenstone bodies were in fault contact with the 77 Bralorne diori te . The alternative is that the greenstone bodies are unrelated to the Cadwallader rocks and are simply fragments of an older greenstone terrane. This does not seem l i k e l y because the section at Bralorne is 1ithological1y similar to the Cadwallader section established by Rusmore (1985). Both contain volcanics, volcanic1astics, and turbidite sequences composed of volcanic detritus. Cadwallader Group sediments at several locations outside the Bralorne block (Wayside property, and Eldorado Basin) include a conglomerate unit that contains clasts which resemble soda granite in both hand specimen and thin section. One possible interpretation of this relation i s that the transitional and sedimentary units of the Cadwallader are younger than the Bralorne intrusives, while the underlying volcanic unit is older. However, both the volcanic and sedimentary portions of the section at Bralorne appear to be hornfelsed by the Bralorne intrusives. If the volcanic unit i s older than the Bralorne intrusives ( i . e . > 270 Ma), and the sedimentary unit is younger (< 230 Ma), i t would imply a volcanic event spanning at least 40 million years, since the lower portion of the sedimentary unit contains intercalations of the same volcanics. Alternatively, i t is possible that the rocks.mapped in the Bralorne block as Cadwallader are distinct from those of Carnian to Norian age mapped by Rusmore in the Eldorado Basin some 30 km along strike to the northwest. However, 72-the rocks from both areas look so al ike, and her descriptions of the stratigraphy there are so similar in many respects to the Cadwallader rocks of the Bralorne block, that a correlation seems l i k e l y . This was also the feeling of previous workers in the area, such as Cairnes (1937, 1943) who mapped in both areas. Another p o s s i b i l i t y is that the volcanic unit of the Cadwallader is older than the overlying transitional and sedimentary units that were the source of Rusmore's conodonts. However, volcanic units intercalated with the dated sediments are similar to those of the underlying volcanic package (Rusmore, 1985). Also, the very similar sedimentary part of the Cadwallader in the Bralorne block appears to be cut and altered (bleached) by the Permian intrusive complex. In summary, i t appears that at least parts of the Bridge River and Cadwallader Groups are as old as Early Permian. Also, at least some of the ultramafic and serpentine bodies of the Bridge River area must be of this age or older, since they are intruded and assimilated by the Bralorne d i o r i t e along i t s southwest margin (section This appears to be the cause of a contaminated border phase of the diorite that contains c1inopyroxene mantled by hornblende (this phase might have led to the term "augite diori te" so common in the Bralorne l i terature: c f. Cairnes, 1937). Other relationships between the Bralorne d i o r i t e and the ultramafics have been postulated. For 73 example, d i o r i t e appears as fault-bounded bodies i n u l t r a m a f i c rocks of the Shulaps mass f u r t h e r east i n the Bridge River Group, implying that there may be more than one age of u l t r a m f i c rocks (B.N. Church, pers. comm., 1987). T h i s suggestion i s supported by the J u r a s s i c age of the Shulaps u l t r a m a f i c complex p o s t u l a t e d by Potter (1983). The Permian age of some of these u l t r a m a f i c s l i c e s suggests that they may represent oceanic mantle that formed the basement to the Bridge River and p o s s i b l y Cadwallader Groups, as suggested by Potter (1983) for the Shulaps mass to the east. Such a marginal basin •- o f f s h o r e arc combination probably formed far removed from i t s present l o c a t i o n with respect to the North American c r a t o n , as suggested by lead i s o t o p i c evidence (Chapter 4). The unusually s o d i c composition of the soda g r a n i t e (a trondh.jemi te with 5.57. Na 30 and only 0.57. K a 0 ) , and even of the d i o r i t e (4.57. Na 20, 0.27. K a0) , suggests that these i n t r u s i v e s were part of an o p h i o l i t e s u i t e , emplaced below the sea f l o o r (Hughes, 1982 p.S7; Coleman and Donato, 1979). The low Sr i n i t i a l i sot op i c r a t i os, c1ose t o t h ose f or oc ean f l o o r ( s e c t i o n 3.4.3) support t h i s i n t e r p r e t a t i o n . Further evidence s u p p o r t i n g t h i s p o s s i b i l i t y i s found i n the i n t r i c a t e contact r e l a t i o n s between the d i o r i t e and Pioneer v o l c a n i c s . The t e x t u r e s of f i n e g rained phases intermediate between d i o r i t i c and v o l c a n i c suggest that the d i o r i t e was i n t r u d i n g i t s own v o l c a n i c products. If the v o l c a n i c s were submarine, the i n t r u s i o n s were a l s o . 7 * 3.7 Conclusions Mineralization at the Bralorne vein gold deposit took place long after emplacement of the major body of diorite and soda granite that host the deposit. This major intrusive may have been emplaced below the sea floor in the Early Permian at about 270 to 235 Ma, roughly coeval with i t s own contemporaneous volcanic products, namely basalts of the Cadwallader Group. These basalts have i n i t i a l Sr ratios and geochemical signatures for major and trace elements transitional between calc-alkal ine and island-arc t h o l e i i t e basalt s. Intrusive contacts of the diorite with elongate ultramafic bodies imply that some of the ultramafics are also of Permian or older age and that they had themselves been emplaced into a higher structural level by the time of d i o r i t e intrusion. Thus not a l l of the deformation of the Bridge River marginal basin can be restricted to the Jurassic, as was suggested by Potter C1983). The Early Permian age implied for at least part of the Cadwallader and Bridge River Groups by the present work is in conflict with stratigraphic ages assigned elsewhere on the basis of paleontology. The discrepancy of about 40 million years might be due to: (. 1) the isotopically dated rocks being stratigraphical1y under the beds dated by fossi ls; (2) the volcanics and sediments, long considered as Cadwallader in the Bralorne area, being distinct from and older than Rusmore's (1987) newly established and dated type section in Eldorado Basin (Fig. 1-2); or (3) the contact between Bralorne intrusives and s t r a t i f i e d rocks being everywhere faulted. Mineralization at Bralorne i s Late Cretaceous and probably is closely defined by pre- and late intra-mineral dykes dated at 91.4 and 85.7 Ma respectively. The i n t r a -mineral character of the 85.7 Ma dyke set makes the latter age most l i k e l y . Mineralization certainly took place before 44.7 Ma, since veins are cut by unaltered lamprophyre dykes of this age. It also probably preceded 60 Ma intrusive rocks nearby, which are not affected by the regional metamorphic/hydrothermal processes associated with the mineralization. Mineralization had no genetic relation to the emplacement of the Bralorne diorite or soda granite, as has long been considered to be the case (cf. Cairnes, 1937). Instead, mineralization is closely related to a sub-parallel swarm of pre-, i n t r a - and possibly post-mineral dykes trending approximately parallel to the mineralized veins. There appear to have been several pulses of mineralizing activity in the Intermontane Belt adjacent to the Coast Plutonic Complex (CPC). These pulses were probably closely related to pulses of magmatic a c t i v i t y in the CPC. There is a trend of decreasing temperature and younger age of mineralization with increasing distance eastward from the eastern margin of the CPC. The eariest mineralization seems to have been about 90 to 85 Ma for 7<£ relat ively high temperature (mesothermal, 300 to 450°C) Au-Ag~As+Mo, W, 3b mineralization at Bralorne (Chapter 7'). This ranges outwards to about 63 Ma for Ag-Au-Bb-As+Hg mineralization at Minto and Congress, and to about 45 Ma for Ag-Au mineralization at Blackdome. Bralorne l i e s only 8 km from the major plutons of the CPC, while the Minto and Congress mines are about 20 km away, and Blackdome is almost 60 km distant. The metal zonation established by Woodsworth et a l . (1977) parallels the progression in age of mineralization (deeper and older — younger and shalower). 77 CHAPTER 4 GALENA LEAD ISOTOPES OF THE BRIDGE RIVER CAMP 4.1 Introduction The E-tridge River camp embraces deposits previously separated into several metallogenic groups, but tentatively considered contemporaneous on the basis of a regional metal zonation (Woodsworth et al . , 1977). Previous lead isotope data from galena specimens, supplemented with further analyses reported here (for a total of 20 deposits), :, indicate that a l l deposits in the camp, regardless of age, location, host rock, or style of mineralization, formed during a protracted mineralizing episode in the early Late Cretaceous to early Tertiary. The data plot along a mixing l ine (c_f_. Andrew et al . , 1984) between the "upper crustal" and "mantle" curves of Doe and Zartman (1979). Data from the Blackdome deposit, 60 km northeast of Bralorne, plot in the same array. This deposit is not within the Bridge River camp but occurs within arc-related rocks of the Stikine Terrane along the western margin of the Intermontane super-terrane. It thus may also be genetically related to intrusions that are spatial ly associated with the Coast P1ut on i c Complex. 4.2 Mineral Deposits Ore deposits of the Bridge River camp can be classed as mesothermal or epithermal, and divided into four groups following Woodsworth et al . (.1377'). These are: Bralorne-type mesothermal ribboned Au quartz veins, Congress-type discontinuous Ag-Au-Sb-As veins transitional to epithermal, lesser-known epithermal Sb-Hg prospects, and Blackdome-type epithermal Au-Ag quartz veins. Details of isotopic dating of deposits in the Bridge River camp are in Chapter 3. 4.2.1 Bralorne Type The Bralorne-type deposits in the Bridge River camp, represented by the large Bralorne-Pioneer Au~As-Ag(W-McO mesothermal quartz vein system, are hosted by the Bralorne diorite and soda granite, and the Cadwallader Group greenstone. The timing of mineralization is constrained at Bralorne to the early Late Cretaceous by a U-Pb date on zircon of 91.4 + 3 Ma on a pre-mineral a l b i t i t e dyke set, and a K-Ar date on hornblende of 85.7 +3 Ma from a late i n t r a - to post-mineral porphyry dyke (Chapter 3D. Opaque minerals in the vein include pyrite, arsenopyrite and pyrrhotite, with traces of chalcopyrite, galena, sphalerite, tetrahedrite and scheelite. Gangue minerals are mainly quartz, calc i te , s e r i c i t e and ankerite, with rare mariposite. The veins average about one meter thick and are mostly milky quartz ribboned with thin dark septae of slickensided sulfides, s e r i c i t e and native gold. Well-developed alteration envelopes around the major veins, as much as several meters wide, grade outwards from quartz, s e r i c i t e , ankerite, albite and pyrite to epidote, chlorite 79 and c a l c i t e zones. Ore shoots within the veins are structurally controlled and make up less than one quarter of the total vein material. Gold-silver ratios average about 2, but range up to 5 (Harrop and S i n c l a i r , 1986). (For further details of the mineralogy, see Chapters 5 to 7.) Many other smaller vein deposits are similar in style of mineralization and host rocks. 4.2.2 Congress Type Congress-type deposits are smaller, and occur as discontinuous veins in shear zones, often with higher sulfide contents than the Bralorne-type deposits, and with metal assemblages (Ag-Au-Sb-As+Hg) and vein textures characteristic of epithermal deposits. Stibnite is the prominent sulfide mineral, while sphalerite, galena, chalcopyrite and tetrahedrite are more common than in the Bralorne deposits; cinnabar is present in some cases. Gold-si lver ratios at around 0.2 are the inverse of the ratios in the Bralorne-type veins. Microdiorite porphyry dykes at the Congress and Minto deposits, representative of the second group of deposits, have been dated as Late Cretaceous at 67.1 + 2.2 and 69.4 + 2.4 Ma, respectively, by K-Ar on whole rock (Pearson, 1977; Harrop and S i n c l a i r , 1986). Mineralization in the Bonanza Basin is associated with the Robson stock, which has a similar but early Tertiary age of 63.7 + 2.2 Ma by K-Ar on b i o t i t e (K.M. Dawson, unpub. d a t a ) . At the nearby Lucky Gem property in Eldorado Basin, 8o mineralization is also early Tertiary at 57.7 + 3.0 Ma by K-Ar on muscovite (Chapter 3). 4.2.3 Tyaughton Type Between the Minto area and the Blackdome mine there are numerous Hg-Sb+W vein and f r a c t u r e - f i l l occurrences (Woodsworth et a l . , 1977) typical of low-temperature deposition. These showings, which include prospects along Tyaughton Creek and the Yalakom Fault, are characterized by jamesonite, st ibnite, tetrahedrite, cinnabar and scheelite. Isotopic age data are not available for these occurrences. 4.2.4 Blackdome Type Epithermal veins, such as at the Blackdome Au-Ag vein (Fig. 1-1; Vivian et a l . . 1987), are characterized by vuggy, crustif ied and opaline quartz stockworks, and zones of si 1 i c i f i cat i on containing 27. or less opaque minerals. These include native gold and s i l v e r , acanthite, tetrahedrite and other sulphosalts, si lver selenides, plus minor pyrite, pyrrhotite, marcasite, chalcopyrite, bornite, arsenopyrite, sphalerite and galena. The veins carry higher si lver values than do the Bralorne-type mesothermal veins. Mineralization i s Early Tertiary, constrained between 51.5 + 1.9 and 24 + 0.8 Ma by K-Ar on whole rock samples (Faulkner, 1986). In light of a clustering of radiometric ages at about 45 Ma in the Bridge River camp (below), and in the absence of more p r e c i s e i n f o r m a t i o n , an age of 45 Ma i s assumed a l s o for Blackdome. 4 . 2 . 5 Summary In the Bridge River camp, T e r t i a r y ages are a s c r i b e d to: (1) the emplacement of l a t e phases of the Bendor pluton which range from 58.9 + 2.3 to 42.3 + 2.9 Ma by K-Ar on b i o t i t e (Wanless et a l • , 1977); (2) the Rexmount porphyry at 44.7 + 2.4 Ma by K-Ar on b i o t i t e (Woodsworth, 1977); (3) the Beece Creek and Lorna Lake p l u t o n s at 43.7 + 0.6 Ma by K-Ar on b i o t i t e ( A r c h i b a l d et a l . . 1989), (4) lamprophyre dykes at B r a l o r n e at 43.5 + 1.5 Ma by K-Ar on b i o t i t e (Chapter 3), vei n f u c h s i t e at Minto at 45 + 1.5 by K-Ar ( p o s s i b l y r e s e t ) , and the Red Mountain hornb1ende-plagioc1ase porphyry at 47.4 + 0.5 Ma by K-Ar on b i o t i t e ( A r c h i b a l d et a l • , 1989). O v e r a l l , dates for the Coast P l u t o n i c Complex show a s i m i l a r range to that d e t a i l e d above for the m i n e r a l i z a t i o n , from 84 to 55 Ma by K-Ar on b i o t i t e and hornblende. T h i s can be extended t o 42 Ma i f the post-orogenic Bendor p l u t o n s are incl u d e d (Woodsworth et a l • . 1977). There appears t o be a p r o g r e s s i o n from west to east (Table 4-1) i n dep o s i t type (mesothermal t o e p i t h e r m a l ) , metals (Au-As~W to Ag-Sb-Hg) and age ( e a r l y Late Cretaceous to middle T e r t i a r y ) . T h i s i s supported by the trend of decreasing f l u i d i n c l u s i o n homogenization temperatures i n ve i n quartz of the d e p o s i t s (Chapter 8; Maheux et a l . . 1987; V i v i a n et a l . , 1987). Thus the o l d e s t , highest temperature 91 TABLE 4-1. Vein progression from west to east across the Bridge River camp, B.C. DEPOSIT Bralorne- Congress- Tyaughton- Blac kdome Pi oneer Mi nto Yalakom TYPE1 Mes Mes-Epi Epi Epi AQE 85 68 •-. 50-24 (Ma) (45 ) DISTANCE 8 20 40 60 (km)38 T C°C) 3 350 •-. •-> 275"* EROSION Deep Moder ate Shallow Shallow DEPTH (km) (6-10) (?2-4) (0.5-1)-* d l s 0qt2= + 18 + 1 +22 + 1 s +25 + 4 G + 1 + 1 -* d l o 0fluid= + 12 + 1 + 10 + 1 ~* + 8 + 2 1 7 -8 + 1 •* 1 Mes = mesothermal; Epi = epithermal. 32 Distance from eastern margin of Coast Plutonic Complex. 3 Estimated from fluid inclusion trapping temperatures. From Vivian et al . (1987). 5 3 In per mil, relative to SM0W. s From Maheux et a l . (1987). 7 From Nesbitt et a l . (1987). Age (Ma) 100 50 0 60 km/50 Ma = 0.12 cm/yr • 4 - C — M 10 20 30 40 50 60 Distance from CPC (km) Figure 4-1: Deposit age versus distance from the Coast Plutonic Complex. B-P = Bralorne-Pioneer, C-M = Congress-Minto, T-Y = Tyaughton-Yalakom, B = Blackdome. 83 (mesothermal) deposits l i e closest to the Coast Plutonic Complex, and have been eroded more deeply than the youngest, lower temperature (epithermal) deposits which l i e farther away. The relation between age and distance from the Coast Plutonic Complex is almost l inear, and the apparent movement of the mineralization and magmatic front eastwards (approximately 60 km, from the edge of the Coast Plutonic Complex to Blackdome, in about 40 Ma) yields a rate of advance of about 1.2 mm/year (Fig. 4-1). It i s possible that the deeper unroofing of deposits closest to the Coast Plutonic Complex is due not only to their greater age, but also to greater uplift closer to the axis of the Coast PIut on i c Complex. 4.3 Galena Lead Isotope Analyses Galena lead isotope data reported in Table 4-2 have been obtained by various workers at The University of B r i t i s h Columbia. Early analyses by B. Ryan were done on a s o l i d source mass spectrometer using standard, single filament, s i l i c a gel techniques; procedural details are in Godwin et  a l . (1982). Recent analyses by J . E . Gabites, F.R. Goutier, and the writer were done as follows (cf. Andrew, 1982). Hand-picked galena crystals (0.5 mg) were converted to pure lead chloride solution by dissolution of the galena in pure 2-normal hydrochloric acid and evaporation to dryness. Lead chloride crystals so formed were cleaned by washing several times in 4-normal hydrochloric acid, and the cleaned lead TABLE 4-2. Galena lead isotope analyses for deposits in the Bridge River Mining Casip, Interraontane Belt, southwestern British Columbia. Location and geology of deposits are in Figure l-£. Repeat and duplicate analyses are reported in Table 4-5. Sample No. (n)1 final- Deposit name3 Lat. (N) Long.(W) Lead isotope ratios Deposit ysta (decinal degrees) 206Pb/204Pb 207Pb7204Pb 208Pb/204Pb type* 30000-001 4 1.Bralorne3 50.77 122.80 18.722 15.576 38.263 VeinsMes -002ft (3) 4 1.Bralorne-Pioneer3 50.77 122.80 19.014 15.656 38.784 VeinsMes -003 4 1. Bralorne 41 Level 50.77 122.80 IB. 727 15.579 38.278 VeinsMes -004 4 2.Bralorne Surface 50.76 122.79 18.652 15.526 38.082 VeinsMes 30001-OOlft (3) 4 3.Pioneer 5 Level 50.76 122.77 18.769 15.632 39.009 Vein:Mes -002 4 3.Pioneer 14 Level 50.76 122.77 18.709 15.564 38.197 Vein:Mes 30002-001ft (3) 4 4.P.E.6old<Pio.Ext)' 50.75 122.75 18.400 15.561 37.909 Vein:Mes 30003-001 4 5.BRX (ftrizona) 50.84 122.84 18.998 15.634 38.681 Vein:Mes 30350-001 1 6.BRX (California) 50.82 122.82 18.968 15.574 38.529 Vein:Mes 30004-001 4 7.Veritas 50.84 122.91 18.666 15.589 38.185 VeimMes 30337-001 2 8.Waterloo 50.80 122.77 18.418 15.191 37.020 VeinsMes 30322-001 1 Tchaikazan7 51.10 123.45 18.549 15.546 37.980 ReplsMes? 30391-001 1 9.Lucky Gen7 50.99 122.89 18.545 15.671 38.126 VeimMes? 30392-1/4B (5) 1 10.Lucky Strike7 50.98 122.86 18.923 15.598 38.499 VeinsMes? 30346-1/3ft <3) 1 Robson7 51.02 122.88 18.947 15.607 38.490 VeinsMes? 30414-001 1 Bonanza Basin7 51.02 122.88 18.944 15.560 38.461 VeinsMes? 30379-OOlfl <2) 2 ll.Minto <0aega) 50.90 122.75 19.063 15.635 38.727 VeinsMes/Epi 30380-OOlfl (2) 2 ll.Minto (Gully) 50.90 122.75 18.997 15.610 38.634 VeinsMes/Epi 30959-101 3 ll.Minto 50.90 122.75 19.042 15.612 38.633 VeinsMes/Epi 30%1-OOlfl <2) 2 ll.Minto (Beta) 50.90 122.75 18.920 15.601 38.635 VeinsMes/Epi 30961-101 3 12.Golden Sidewalk 50.92 122.77 18.932 15.608 38.570 VeinsEpi? 30645-101 3 13. Congress Lou Zone 50.89 122.78 18.912 15.605 38.615 Vein/Rep:Epi 30611-OOlfl (2) 3 14.Peerless 50.93 122.79 18.935 15.603 38.555 VeinsEpi 30609-1/4A (5) 3 15.SuH.it 50.87 122.52 18.934 15.609 38.554 VeinsEpi? 30610-001 3 16.Kelvin 50.89 122.75 18.985 15.613 38.617 VeinsMes/Epi 30972-001 4 16.Reliance (Menika) 50.89 122.75 19.030 15.633 38.705 Vein/ReplsEpi 30612-001 3 17.Olynpic 50.89 122.74 18.998 15.616 38.611 VeinsEpi 30612-101/2A 3 17.Olympic 50.89 122.74 19.055 15.606 38.580 VeinsEpi 30621-OOlft (3) 3 18.Matson 50.77 122.21 18.834 15.5% 38.446 Repl? 30941-OOlfl (2) 2 19.Greyrock 50.80 122.70 18.920 15.624 38.599 VeinsEpi? 30065-001 4 20.Piebiter(Chopper) 50.72 122.60 18.927 15.613 38.560 VeinsMes 30916-OOlfl (2) 4 Blackdone 51.31 122.45 18.759 15.583 38.322 VeinsEpi 30903-001 4 Capoose 53.25 125.26 18.903 15.601 38.482 StkwksEpi? Mn) = nueber of analyses averaged; see Table 4-3. aftnalyses were bys 1 = B.D. Ryan; 2 = F. Goutier; 3 = J.Gabites; 4 = C. Leitch. fill analyses were performed in the Geochronoaetry Laboratory of R.L. flrastrong at The University of British Coluabia; current values for the Broken Hill standard were used (see text). deposit numbers refer to Figure 1-2: deposits not numbered lie beyond Figure 1-2. *Stkwk = stockwork; Rep = replacenent; Mes = nesotherval; Epi = epitheroal. 'Unlocated in the aire (specimen fron 'E' Collection). 'From DDH P-85-03 §450 ». 7In quartz diorite of the Coast Plutonic Complex. 85 chloride crystals were dissolved in ul trap tire water. One microgram of lead in the lead chloride solution was loaded with phosphoric acid and s i l i c a gel onto a cleaned, single rhenium filament (cf• Cameron et a l . , 1969). Lead isotope ratios were then measured on a Vacuum Generators Isomass 54R solid source mass spectrometer linked to a Hewlett-Packard HP-85 computer. Within run precision, expressed as a percentage standard deviation, is better than 0.01 "/., and the variation observed in duplicate analyses is less than 0.1 "/.. Isotope ratios are normalized to the values of Broken H i l l Standard lead (BHS-UBC1) given in Richards et  al . (1981): »©*Pb/»o-*Pb = 16.004, a©?FD/a©-*-pD = 15.390, and : z o e Pb/ 2 0 "*Pb = 35.651. Analytical precision is monitored by repeated measurement of BHS-UBC1 and systematic duplicate analyses of samples. Fractionation error and =0**Pb error trends are shown on data plots so that trends in data can be assessed as being real or related to analytical problems. 4.4 Galena Lead Data All the previous and new galena lead isotope data for the Bridge River camp are compiled in Table 4-2 and plotted in Figures 4-2 and 4-3.' The deposits of the Bridge River d i s t r i c t have been divided, above, into mesothermal and epithermal groups on the basis of their mineralogy, host rocks, vein type and age. In terms of both their 2C6Pb/2°"Pb and 2 o B p b / a o 4 P b ratios (Figs. 4-2 and 4-3 respectively), the early Late Cretaceous mesothermal 96 Figure 4-2. Plot of *<"Pb/*°*Pb versus «°»Pb/*°»Pb for galena from the Bridge River Camp (solid circles = mesothermal deposits, open circles = epithermal deposits). Stippled field = pelagic sediments; open field = oceanic volcanics (both from Doe and Zartman, 1979, as are the mantle and upper crust curves). Shale and Bluebell curves are from Godwin and Sinclair (198S) and Andrew et al. (1984), respectively. Ticks mark age in Ga. O 88 deposits (solid circles) are s t a t i s t i c a l l y less radiogenic than the mainly Tertiary epithermal deposits (open c i r c l e s ) at the 99.9 confidence level based on Student's t test (cf• Sebert, 1987). However, both of these groups appear to l i e within one larger cluster, and therefore presumably belong to the same event. Differences in the 2 0'^Pb/2 0' ,*Pb ratios cannot be shown to be s ignif icantly different because of the limited range of ^ T ' b values. There i s some overlap on the plots; samples from the mesothermal E^ralorne and BRX (Arizona) deposits plot within the radiogenic group. Repeat analyses of these deposits and of the epithermal Blackdome deposit, which plots within the mesothermal group, confirm the overlap (Table 4-3). The data are discussed below with reference to several well known, published growth curves. Some generalized global growth curves are not applicable to the interpretation of this lead data. For example, Stacey and Kramers' (1975) model predicts a future age for the mineralization. A somewhat better fit i s provided by growth curves that reflect the evolution of lead in the radiogenic upper crustal environment. These are: (1) the "shale curve" model defined by Godwin and Sinclair (1982), which estimates average lead isotopic evolution for shale-hosted deposits in the autochthonous Selwyn shale basin and Foreland Belt of the western part of the Canadian Cordil lera (sediment-hosted deposits from eastern Alaska relate to a d i s t i n c t l y different curve: Church et a l . . 1987), and (2) the computer 89 TABLE 4-3. Repeat and duplicate analyses of lead isotope r a t i o s reported in Table 4-2. Sample No.1 Deposit name Lead isotope r a t i o s Pb/=°-*Pb 2 0 7Pb/ : 2 0**Pb =ospKj/=eo»p D 30000-002 Bralorne-Pi oneer 19.012 15.656 38.785 -002R 19.016 15.657 38.787 -002D 19.013 15.656 38.781 30001-001 Pioneer 5 Level 18.772 15.637 39.021 -001R 18.765 15.625 38.985 -001D 18.769 15.634 39.022 30002-001 P.E. Gold (Pioneer 18.423 15.581 37.961 -001R Extension) 18.408 1 J . / -Zi 37.934 -00 ID 18.368 15.529 37.833 30916-001 Blac kdome 18.754 •15.577 38.303 -00 IP. 18.764 15.590 38.341 1 Suffixes are: R = repeat mass spectrometry of sample solution; D = duplicate analysis of separate pickings of galena from the hand specimen. A l l analyses were by the writer. 90 modelled "upper crustal curve" of Doe and Zartman (1979; c f. Zartman and Doe, 1981). The data in the current study plots below both of these upper crustal curves in a ^ T ' b / ^ ^ P b versus 2ospb/204pb plot (Fig. 4-2), but generally follow a line joining the 0.1 Qa points of the mantle and upper crustal curves of Doe and Zartman (1979). Similarly, in the 2 o 8 p b / 2 0 4 P b versus 206Pb/204Pb p l o t (Fig. 4-3), the data follow the 0.1 Ga l ine from the upper crustal curve to the mantle curve, almost parallel to the upper crustal curve. 4.5 Discussion The galena lead isotope data suggest that the samples analysed contain variable proportions of lead with mantle and upper crustal character. The mantle source could be dominantly from the mantle-derived Bridge River Group basalts and associated ultramafics. The upper crustal source could be either the pelagic sediments in the Bridge River Group or the minor felsic arc-related volcanic and volcanic1astic rocks of the Cadwallader Group. Circulating hydrothermal fluids passing through these rocks could have leached and mixed the lead of appropriate, varied character. Alternatively, the mixing could have occurred within the magmas responsible for both the arc volcanic and the intrusive rocks now found within the Bridge River camp. Trends lying between growth curves have been interpreted as "mixing l ine isochrons" by Andrew et a l . (1984). Such trends, within analytical uncertainty, reflect 91 contemporaneous mineralization. In the Bridge River camp, t h i s was not suspected from the deposits, which are diverse in type and widely spaced geographically. Even galenas from deposits as remote from Bralorne as Blackdome, 60 km to the northeast, plot within the array. Similar conclusions ( i . e . of a single broad mineralizing episode for deposits spatial ly related to the Coast Plutonic Complex (see below) have been reached for the Bridge River camp by Maheux et a l . (1987) and for the Cordil lera in general by Nesbitt et al• (1987). Mixing of two lead components to produce the observed isotopic compositions requires that some of the lead evolved along a growth curve well below the shale curve in a relat ively uranium-depleted (lower mu, or lower a 3«U/ a o ,*Pb) environment. Such an environment could be the lower crust, the upper mantle, or both. By comparison with Doe and Zartman's (1979) curves (reproduced in Figs. 4-2 and 4-3), i t i s l i k e l y that such a reservoir represents the uranium and thorium depleted upper mantle, rather than the uranium depleted and thorium enriched lower crust (c f. Andrew et  a l • . 1984). This i s evident from the pattern displayed in the 2 0 B P b / 2 0 4 P b versus =so»Pb/=so"*Pb plot (Fig. 4-3) that apparently defines a mixing l ine which almost p a r a l l e l s the upper crustal and mantle curves. Although the curves themselves are almost paral lel in this plot, they end at 0 Ma at quite different points. Thus the observed array, although i t s e l f sub-parallel to the growth curves, could 92 represent mixing between a more radiogenic source (the upper crust) and a mantle source, as was suggested by the 2 0 7Pb/2 C"»Pfa versus 2 0 6Pb/=°"*Pb plot. The trend is close to the position of the "upper crust contributed to orogene" curve of Doe and Zartman (1979), and plots with their "continental margin" type of deposits from California, Oregon, Washington, and Nevada (Church et a l . , 1986). This is where one would expect the Bridge River d i s t r i c t leads to plot—with other similar epigenetic deposits of the western Cor d i11er a. The pattern of trends crossing between the curves on a 2"7pb/ao4Pb versus 2 0 < aPb/ a°-»Pb plot (Fig. 4-2), but being almost parallel to the curves on an a o , B P b / a o , * P b versus  =sosPb/:5SO"»Pb plot (Fig. 4-3), is similar to that shown in Doe and Zartman (1979, figures 2.4 to 2.10). Plots of 2 0 ' 7 Pb/ 3 ! 0 -*Pb versus 2 0 & Pb/ 2 °-*Pb (Fig. 4-3) allow age estimates to be made, whereas plots of 2 0 0 P b / 2 0 4 P b versus ao«aptj/2o-*pD do not, in part because the mixing isochrons in the former cut the growth curves at a higher angle. The data trends observed are r e a l , and not simply due to analytical uncertainty, since repeat analyses (Table 4-3) confirm the values. Furthermore, duplicate analyses (separate solution starting from a new fragment of galena; Table 4-3) shows that the variation also cannot be explained by zonation within or among galena crystals. It i s possible that the large deposits, Bralorne and Pioneer, exhibit zonation within the deposits themselves. It would, however, 93 require further analyses to establish whether or not this i s the case (galena is exceedingly rare in these deposits and a suitable sample distribution might never be available). The Bridge River Terrane contains voluminous pelagic sediments which are ultimately of continental derivation. These, possibly with the minor felsic volcanic rocks of the Cadwallader Terrane, could constitute the source, rich in both uranium and thorium, for the more radiogenic lead component. Both these terranes, hosting the deposits of the Bridge River camp, are accreted to the North American craton. The shale curve of Godwin and Sinclair (1982) approximates the lead evolution in many sedimentary rocks of western North American provenance. The fact that the Bridge River radiogenic lead component i s more readily interpreted by a curve other than the shale curve helps substantiate a provenance for the Bridge River and Cadwallader groups from sources other than that portion of ancestral North America to which they are currently adjacent. A uranium- and thorium- depleted upper mantle source for the least radiogenic lead component is probably represented best by the Bridge River Group basalts and associated ultramafic rocks. Figures 4-2 and 4-3 show that simple mixing between Doe and Zartman's (1979) f ields for pelagic sediments and oceanic volcanics could produce the observed fields of the Bridge River data. Such mixing could have occurred between lead derived from pelagic sediments or felsic arc detritus, and lead derived from basalts by the action of meteoric 94 and/or magmatic hydrothermal fluids during a period of high heat flow associated with emplacement of the Late Cretaceous - early Tertiary Coast Plutonic Complex. Other workers recently have concluded that deposits lying within the various terranes inboard of the Coast Plutonic Complex may be genetically related to the Coast Plutonic Complex. For example, Maheux et a l . (1987) and Nesbitt et a l . (1987) noted progressive evolution in oxygen isotope values compatible with fa l l ing temperatures of deposition (based on fluid inclusions) as distance increased from the Coast Plutonic Complex. This supports a genetic link among the deposits studied, which extend from the Bridge River camp to the Intermontane Belt. However, Zartman (1974) and Church et a l . (1986) show that the change in lead isotope ratios as one moves into younger belts may also be a function of mixing with younger sources, as predicted by the orogene model of Doe and Zartman (1979). 4.6 C o n c l u s i o n s Evidence from radiogenic and stable isotopes, combined with available fluid inclusion data (Chapter 8; Nesbitt et  a l . . 1987; Sebert, 1987; Woodsworth et al..1977) suggests an extended Upper Cretaceous to early Tertiary mineralizing episode for the western Intermontane Belt adjacent to the Coast Plutonic Complex. This episode appears to have spanned the period from about 90 Ma at Bralorne to between 52 and 24 Ma at Blackdome, with a progression to younger, higher level and lower temperature mineralization as distance from the Coast Plutonic Complex increases. The lead isotope data acquired in this study are consistent with a mixing model (cf. Andrew et al . . 1984.) for 2 0 7 P b / 2 0 4 P b versus 2 0 6 P b / 2 0 4 P b plot, and may also display mixing in a ^oopb/ao^pb versus 2 0 6 P b / 2 0 4 P b plot. Applying the principles of this model permits explanation of galena lead isotope data by allowing for different sources of lead in different mining camps. In the Bridge River camp, both upper crustal and mantle components are identified which may correlate with pelagic sediments of the Bridge River Group and/or felsic volcanic1astics of the Cadwallader Group, and basaltic volcanics and ultramafics of the Bridge River Group respectively. However, the heterogeneity could have been in the Mesozoic-Cenozoic arc volcanics and later intrusions, and not by local mixing of Pb sources. The results of this study may be of use in exploration for further Bralorne-type deposits since the mesothermal deposits, which tend to be larger and more economic, have s t a t i s t i c a l l y different Pb isotope ratios from the smaller epithermal deposits. This has also been noted by Doe (1968) on a d i s t r i c t scale in the western U.S.A. It i s also of possible exploration significance that only the largest deposit sampled (Bralorne-Pioneer.') showed significant differences in lead isotope ratios for galenas from within the deposit. Thus, i f the lead isotope ratios of several samples from various parts of a prospect were measured, v a r i a b i l i t y of the results would be considered encouraging. The v a r i a b i l i t y of the larger mesothermal deposits could be the result of mineralization spanning a range of time and temperature, or to their proximity to the batholiths and stocks, as opposed to the later, more distal deposits of epithermal character. This prospecting guide is counter to the normal pattern, that the most productive deposits are least radiogenic and most homogeneous (e.g. . Gulson, 1986:'. Lead isotopes have a significant contribution to make to interpretation of metallogeny. In the area studied, large-scale mixing of mantle and upper crustal components can explain the lead isotopic data. If mixing was by fluids and not in the magmas, this supports the premise (e.g. Nesbitt et a l . . 1986) that the mineral deposits were formed by widespread circulation of fluids focussed along major fault zones. Whether these fluids were of meteoric, metamorphic or magmatic derivation i s not yet clear; evidence bearing on this problem is presented in Chapter 10. Currently there i s a debate as to whether lode deposits such as Bralorne are of magmatothermal (e.g. Nesbitt et a l • , 1986, 1987; Nesbitt, 1988) or metamorphic (e.g. Kerrich, 1983; Pickthorn et a l . . 1987) o r i g i n . Certainly there is a magmatic influence in the form of a heat source to drive convection, but how much of the fluid is actually of juvenile derivation is d i f f i c u l t to ascertain. Whole-rock lead isotopic measurements could help to choose between various sources of upper crustal and mantle lead, and thus elucidate fluid paths and sources. CHAPTER 5 MINE GEOLOGY 5.1 I n t r o d u c t i o n The Bralorne-Pioneer vein deposits, hosted in the Bralorne d i o r i t e , Bralorne "soda granite" (actually albite tonalite or trondhjemite) and Pioneer volcanics, l i e within the "Bralorne fault zone", or "Bralorne block" (Fig. 5-1). This block is bounded by the sub-parallel , northwest-trending Fergusson and Cadwallader faults, which are marked : along their length by narrow, sinuous, serpentinized ultramafic bodies (the President intrusives). Mine terminology ("soda granite", " a l b i t i t e " , etc.) wil l be used throughout this thesis to correlate with previous descriptions of the geology at Bralorne (e.g., Cairnes, 1937; Stevenson, 1958). A table of formations (Table 2-1), with chemical analyses, calculated norms and estimated modes in Tables 2-2 (Pioneer volcanics), 5-1 (major intrusives) and 5-2 (minor units), define a l l units within the block. Early Permian Bralorne d i o r i t e and soda granite make up the bulk of the elongate stock hosting the gold-bearing veins (see surface geology plan, Fig . 5—1, in pocket). The soda granite, a major dyke-like body, i s emplaced along the contact between the Cadwallader Group—Pioneer greenstones or Hurley sediments—and the d i o r i t e . The Bralorne d i o r i t e and soda granite bodies thin toward the southeast, so that only soda granite is found near the Pioneer mine. Both 98 TABLE 5-1: Whole-rock c o m p o s i t i o n s of major u n i t s i n the B r a l o r n e - P i o n e e r area. I n d i v i d u a l a n a l y s e s f o r averaged samples are i n T a b l e A-3-1, Appendix 3. Descr i pt i on Sample Number P i o n e e r D i o r i t e s Soda n r a n i t e s A l b i t i t e H' b por C095 CD93 C082B AVGDI C094 COS2A AVGSG C092 COB3 (N) ' (4) (4) <2> (8) <8> (2) (5) <5> (5) Major- Elements <*> S i O a 47. 34 59. 74 5 5 . 52 5 5 . 32 74. 00 66. lO 70. 00 63. 06 51. 40 01.O, 13. 71 10. 91 13. 60 13. 12 13. 38 14. 27 13. 80 17. 09 14. 76 TiO. 1. 01 0. 23 0 . 50 0 . 40 0 . 19 0 . 20 0. 20 0.21 0. 80 Fe.O, ( T o t a l Fe> 10. 32 7. 80 9. 65 9. 28 2. 42 4. 74 3. 60 2. 60 lO. 84 MgO 12. A l 8. 45 5 . 07 7. 00 0. 44 1. 61 1. 03 1.11 6. 48 CaO 9. 03 6. 86 5 . 89 7. 02 2. OO 4. 00 3. 00 3. 90 8.47 Na. 0 1. 50 3. 79 4. 71 3. 60 5.52 6. OO 5.78 1. 98 3. 10 K.O 0 . OA 0 . 14 0 . 10 O. 08 0. 69 0. 12 0. 41 2. 45 O. £5 MnO 0 . 26 0 . 15 0 . 18 0. 20 O. OS 0. 09 0 . 08 0. 10 0. 18 P.O. 0 . 11 0 . 02 0 . 02 0. 03 O. 05 0. 11 0. 07 0. 12 0. 19 LOI A. 58 1.70 3. 96 2.71 2. 47 2. 30 2. 40 4. S3 2.72 TOTAL lOO.31 99. 80 99.20 98.76 100.20 99. 54 99.78 99.81 99.50 S p e c i f i c G r a v i t y 2. 90 2. 84 2. 78 2. 85 2.66 2. 67 2.66 2. 72 2.91 Minor* Elements <ppm> As 13 1 6 8 12 a 10 11 3 Bo 48 65 120 63 102 120 110 706 245 CI 26 74 53 52 28 33 30 14 57 Co 30 32 24 30 4 16 13 9 35 Cr 275 160 35 86 10 26 17 12 135 Cu 75 62 69 60 11 ia 14 6 36 Mo 1 0 1 1 1 1 , 2 Nb 3 1 2 1 0 1 2 " 1 Ni 74 69 17 43 3 4 3 36 Pb 6 6 8 8 6 7 7 9 4 Rb 3 4 3 14 3 9 39 5 S 745 430 6000 375 760 4160 760 950 310 Sb 3 2 0 ~2 2 3 2 2\- 1 Sr 270 110 190 125 SO 233 ao l l O 350 V 270 145 200 ISO 14 58 30 29 230 Y 24 16 20 17 17 6 12 lO. 22 Zn 105 56 65 64 36 54 43 32 lOO Zr 70 49 34 40 86 24 60 69 61 Normative M i n e r a l s Quart z lO. 5 7. 1 8. 1 30. 4 19.7 25. 0 32.8 4.5 Corundum O. 02 5. 1 • r t h o c l a s e 0. a O. 8 O. 6 0. 5 4. 1 0. 7 2. 4 13.8 1.3 A l b i t e 12.4 32.8 39.9 30. 5 47. 4 50. 8 48. 9 16.8 26.2 A n o r t h i t e 28. 6 12. 8 15.7 19. 4 10. 1 11.6 11.0 17. 9 25. 8 D i o p s i d e 13. 7 17.8 10.9 12. 3 6. 2 3. 1 13. 1 Hy p e r s t h ene 32. 5 19. O 14.7 20. O 3. 8 5 . 2 4. 5 4.0 19.6 O l i v i n e 2. 0 M a g n e t i t e 3. 7 3.8 5.4 4. 5 1.5 2. 3 1.9 2. 1 4.3 I l m e n i t e 1.8 0.4 0.9 0.8 O. 4 0.4 0. 4 0.4 1.3 A p a t i t e O. 3 0. 04 0. 04 O. 1 O. 1 O. 3 0.2 O. 3 O. 5 Diff»n Index 13 44 47 39 82 71 76 63 32 Modes ( E s t i m a t e d Volume X) Quart z 10 10 6 .11... 40 20 37 10 A l b i t e 40 40 60 55 30 70 52 33* 52 M a f i c i ( H b l e n d ) 55* 48 30 33 8 7 11 3-'- 33 (Cpx) I l m e n i t e ( R u t i l e ) 3 2 1 1 1 1 t r 1. v 3 P y r i t e <Py/Po> 2 t r 3 t r 1 2 1 l _ - . O (No. o f Samples) <1> (1) (1) (10) (1) (1) (10) <I> <!>'.' » Number o f a n a l y s e s <5 s e p a r a t e r o c k s f o r average d i o r i t e and soda g r a n i t e , i n c l u d i n g a n a l y s e s o f t h e f r e s h r o c k s i n t h i s t a b l e and o f u n a l t e r e d r o c k s i n T a b l e fi-1-1). * Sum o f a l t e r a t i o n p r o d u c t s <chl, ep, c a r b f o r m a f l c s ; s e r , c a r b f o r p l a g i o c l a s e ) . A l l a n a l y s e s a r e by XRF on f u s e d beads and p r e s s e d powders at MESA U.K. lab,-England. Sample i d e n t i f i c a t i o n s a r e i C095 = P i o n e e r A n d e s i t e ; C093 = B r a l o r n e q u a r t z d i o r i t e ; C082B •> same* AVGDI • Average o f f i v e d i o r i t e s , 16 L e v e l t o s u r f a c e ' C094 - Soda g r a n i t e - C082A = same- AVGSS - Average o f two soda g r a n i t e s ; C092 = A l b i t i t e dyke, s e r i c i t e - c a r b o n a t e a l t e r e d ; C083 • Green hornblende porphyry dyke; HBITE = average h o r n b l e n d i t e ; AVGAB => average a l b i t i t e dyke, AVGGHO » average green hornblende porphyry; LAMP «• lamprophyre dyke. Norms were c a l c u l a t e d w ith a BASIC program (FeO/Fe.O, was e s t i m a t e d f o r each r o c k ) . 99 TABLE 5—£: Whole—rock compositions of minor u n i t s i n the B r a l o r n e - P i o n e e r area. D e s c r i p t i o n G r e y d i k e A l b i t i t e D i k e s G r e e n d i k e L a m p , d i k e R e s t i t e s H' b 1 e n d i Sample No. C 1 9 3 C 0 3 8 C 0 2 2 C4141 C 0 8 3 CI 0 3 3 C 0 4 3 C 0 8 5 R E S TI UM/HBITI (N) ' (2) (4) <2> (2) <5> (2) (2) (2) ( C a l c ) (2) Major Elements <*> S i O « 7 0 . 9 6 3 . 31 S 5 B 5 7 3 . 2 5 1 . 4 0 5 0 . 7 4 9 . 5 4 8 . 4 5 6 . 1 5 4 . 6 0 1 . 0 , 12 . 7 16 . 76 16 . 5 1 3 . 2 14. 76 1 4 . 0 1 5 . 2 1 4 . 7 10 . 6 4 . 84 T i 0 „ 0 . 24 0. 4 0 0 . 2 5 0. 16 0 . 8 0 2 . 4 0 O. 9 7 0 . 72 O. 3 0 . 22 Fe. 0 , ( T o t a l Fe) 2 . 7 4 . 0 0 2 . 85 1. 6 5 10 . 24 7 . 10 1 3 . 6 1 3 . 2 9 . 0 7 . 2 0 MgO 1. 3 5 1 . 4 7 1. 32 0 . 64 6 . 48 5 . 8 2 7 . 04 9 . 22 10 . 15 . 9 CaO 2 . £ 4 3 . 4 7 2 . 7 0 1. 6 5 8 . 47 7 . 81 7 . 2 5 a . a i a . 4 14. 9 N a . O 5 . 7 2 7 . o a 6 . 5 0 6 . 9 5 3 . 10 2 . 84 4 . o a 2 . 82 3 . 4 0 . 58 K . O 0 . 32 0 . 8 6 0 . 62 0. 38 0 . 2 5 3 . 55 0 . 19 0 . 0 5 0 . 0 0 . 04 MnO 0 . 0 7 0 . 0 9 0 . 0 9 0 . 06 o . i a 0 . 0 9 0 . 2 0 0 . 24 0 . 2 o . i a P . o3 0 . 0 6 0 . 17 0 . 13 0 . 10 0 . 19 1. 2 5 0 . 0 8 0 . 0 5 0 . 0 0 . 02 LQ I 3 . 2 4 2 . 71 3 . 54 2 . 0 3 2 . 72 4 . 5 7 1. 9 7 2 . 54 1. a 1. 55 T O T A L 1 ( 100 ) 1 0 0 . 32 ( 100 ) (100) 9 9 . 5 0 ( 100) ( 100 ) (100 ) 10O (100) S p e c i f i c G r a v i t y 2 . 6 8 2 . 6 3 2 . 6 7 2 . 62 2 . 91 2 . 62 2 . 8 4 2 . 9 6 3 . 01 Minor Elements (ppm) P s ND ND 1 2 3 15 ND ND 26 B a 6 0 2 7 5 2 6 0 140 2 4 5 2 2 0 0 7 5 6 0 7 0 C o 34 14 12 52 35 34 41 33 75 C r » 11 7 7 15 1 35 100 9 0 160 3 8 0 0 C u 4 6 4 1 36 130 10 3 .; 55 Nb 2 4 2 2 1 17 2 4 ND N i 4 7 2 3 36 110 55 7 7 195 P b 12 12 14 15 4 2 7 11 10 3 5 Rb a 16 14 9 5 4 7 3 ND ND S 2 0 0 0 3 7 0 0 1 4 0 0 5 7 0 3 1 0 6 8 0 0 3 2 0 180 3 6 0 S b ND ND ND 4 1 6 ND 3 ND S r 8 8 2 8 0 2 0 0 2 0 5 3 5 0 3 2 0 0 3 0 0 3 0 0 2 5 V» 2 7 55 2 5 12 2 3 0 3 0 0 2 9 0 2 1 0 a o Y 15 24 13 9 22 19 2 5 2 5 10 Z n 36 70 5 7 53 100 130 7 8 120 5 2 Z r 91 110 84 120 61 4 3 0 4 2 21 18 Normative M i n e r a l s Q u a r t z 2 9 . 6 7 . 3 1 7 . 9 2 9 . 0 4 . 5 0 . 3 5. 4 4 . 3 O r t h o c l a s e 2 0 5 . 3 3 . 7 2 . 1 1. 5 2 1 . 3 1. 1 0 . 3 0 . 2 A l b i t e 4 9 7 6 3 . 6 5 6 . 0 5 8 . 5 2 6 . 2 2 6 . 0 3 5 . 5 2 5 . 3 2 8 . 8 5 . 0 finorthite 8 . 6 a . 7 1 3 . 3 4 . 6 2 5 . 8 1 5 . 5 2 3 . 5 2 7 . 3 1 3 . 6 10 . 5 D i o p s i d e 4 . 0 a . 7 2 . 8 13 . 1 1 3 . 4 10 . 9 1 5 . 0 2 2 . 4 5 1 . 7 H y p e r s t h e n e 4 . 8 4 . 4 7 . 1 2 . 1 19 . 6 1 4 . 0 11.5 1 5 . 0 2 3 . 1 2 6 . 3 01 i v i n e 12 . 8 1 3 . 0 M a g n e t i t e 0 . 6 0 . 8 0 . 6 0 . 3 4 . 3 1.5 2 . 7 2 . 6 4 . 3 1 . 5 I l m e n i t e 0 . 5 0 . 7 0 . 5 0 . 3 1.5 4 . 7 1 . 9 1 . 4 0 . 6 0 . 4 ftpatite 0 . 1 0 . 4 0 . 3 0 . 2 0 . 5 3 . 3 O . 2 0 . 1 0 . 0 2 0 . OS D i f f n Index a i 7 6 7 8 9 0 32 4 8 3 7 2 6 34 10 Modes (Estimated Volume • X) Q u a r t z 34 15 2 0 10 8 ( G l a s s 15) 3 1 A l b i t e 5 6 6 0 7 0 a s 54 3 5 4 0 2 0 M a f i c s H o r n b l e n d e 10 19 7 5 36 ( B i o t i t e 33 ) 6 0 5 3 6 2 C I i n o p y x 4 5 15 I l m e n i t e <Rut i l e ) t r 3 1 t r 2 5 3 2 S u l f i d e ( P y / P o ) t r 3 t r 2 ( A p a t i t e 5) 1 1 <M> ' (2) <1> (1) (1) (8) (1) (1) (2) (6) 1 (N> i s n u m b e r o f r e p l i c a t e c h e m i c a l a n a l y s e s ; f o r m o d e s , M i s n u m b e r o f t h i n s e c t i o n s . * C r , V c o n t e n t s a r e a d j u s t e d f o r k n o w n c o n t a m i n a t i o n i n t r o d u c e d d u r i n g g r i n d i n g i n C i — s t e e l r i n g m i l l . 3 T o t a l s i n b r a c k e t s (100) i n d i c a t e p r e s s e d p o w d e r a n a l y s i s , n o r m a l i z e d t o 100X. O t h e r w i s e a n a l y s i s i s b y f u s e d d i s k . /oo these u n i t s occur only as dykes f u r t h e r southeast on the P.E. Gold property. Thus the major ore host i s d i o r i t e at B r a l o r n e but greenstone of the Cadwallader Group at Pioneer. M i n e r a l i z a t i o n i s s p a t i a l l y r e l a t e d t o a swarm of Upper Cretaceous age (31.4 Ma) " a l b i t i t e " ( a l t e r e d p l a g i o c l a s e -quartz porphyry) dykes that p a r a l l e l the v e i n s . S l i g h t l y l a t e r (85.7 Ma), green hornblende porphyry dykes of s i m i l a r o r i e n t a t i o n are u s u a l l y u n a l t e r e d and t h e r e f o r e l a t e i n t r a -mineral to p o s t - m i n e r a l . Paleocene "Bendor" dykes, r e l a t e d to the nearby Bendor p l u t o n , at about 60 Ma, appear to be p o s t - m i n e r a l . Eocene (45 Ma) black lamprophyre dykes c r o s s -cut the v e i n s and e a r l i e r dykes at a high angle. A l l r o c k s other than the two l a t e r dykes are a f f e c t e d by e a r l y Late Cretaceous, lower g r e e n s c h i s t f a d e s metamorphism. 5 . 1 . 1 Mine Layout Workings at the B r a l o r n e mine extend from the "0' l e v e l at s u r f a c e at 1400 m e l e v a t i o n , where the Cosmopolitan, Noelton, and Maud South v e i n s outcrop i n the o l d "King" mine area ( F i g . 5-1), down to the '44' l e v e l at 600 m below sea l e v e l , for a t o t a l v e r t i c a l range of about 2 km ( F i g . 5-5b). During the c u r r e n t study, the mine was dewatered, and t h e r e f o r e a c c e s s i b l e , o n l y to the 20th l e v e l . S u c c e s s i v e mine l e v e l s are each approximately 45 m (150 f e e t ) below each other except near the s u r f a c e where the f i r s t few l e v e l s were e s t a b l i s h e d c l o s e r together during e a r l y e x p l o r a t o r y work. /ol At the f irst level down in the old "King" mine area, the original ly discovered Lome (Shaft) vein outcrops. Further down the h i l l at 2 and 3 level the Wedge, King, Woodchuck and Alhambra veins outcrop. The Ida May (Blackbird) vein, near the Empire shaft (accessed by a short adit at 3 level) , i s the surface expression of the 55 vein on 8 level (Fig. 5-2). The 55 vein is the offset portion across the Empire fault of the main 51 vein, which is stoped to surface near 2 level just south of the Empire fault (Fig. 5-1) and is exposed there in a glory hole. Much of the production of the upper levels of the mine came from the 51 vein; i t s t i l l contains some 150,000 tonnes of 15 g/t material above the 8th level (Bellamy and Arnold, 1985). Further to the southeast, the largest vein in the mine, the 77, i s stoped to surface near the Coronation adit and shaft. The Taylor adit l i e s between the outcrop of the 77 and the Pioneer Main vein, which i s represented at surface by the Pioneer HW s p l i t . Mapping was done at surface (Fig. 5-1) using information from available outcrops and projection of near-surface angled diamond d r i l l holes. This d r i l l i n g was done by Mascot Gold Mines Ltd. during their recent re-evaluation of the mine. Mapping at the 8th level underground (Fig. 5-2), on the main haulage level from the main adit portal at 1,000 m (3440') elevation, was compiled from: workings where accessible, voluminous underground diamond d r i l l core d r i l l e d by Mascot Gold Mines L t d . , old underground and surface diamond d r i l l core and logs d r i l l e d by Bralorne Mines L t d . , and projections from angled surface holes d r i l l e d by Mascot Gold Mines Ltd. and Corona Corp. In areas where workings were caved or dangerous, such as in the King mine area and southeast of the Empire shaft (Fig. 5-2), a less detailed version of the geology was reconstructed from old mine plans and diamond d r i l l logs. This was possible because a core l ibrary of representative samples of the old holes in these areas was s t i l l available for examination at that time. This enabled correlations to be made between the descriptions of individual core loggers in the past and the current re-interpretation of the geology by the writer. Unfortunately, this correlation process could not be extended to the rest of the old diamond d r i l l log information, which extended from the uppermost to the deepest levels of the mine (roughly some 1,500 holes). It had been hoped to construct an up-to-date three-dimensional model of the geology, but before this could be completed the old (Bralorne Mines Ltd.) diamond d r i l l core stored at the dump area at the 8th level portal was bulldozed by Mascot Gold Mines Ltd. This core, although d i f f i c u l t of access in places where the wooden core racks were leaning over, was a l l labelled with aluminum tags on the core boxes and with footage markers that had been written in graphite (pencil) and meticulously turned over. Key intersections of major veins and their altered wall rocks had fortunately been saved at the 26, 32, 41 and 44 levels before this occurred, /o3 permitting the study of vertical zoning of the deposit to go ahead. The workings of the Pioneer Mine (Joubin, .1.948) are sealed, and the old diamond d r i l l logs, plans and sections were destroyed by a f ire many years ago CP,. Barclay, pers. comm., 1986). Apart from sampling the hanging-wall extension to the Main vein at surface (Fig. 5-2), the current study focusses on the Bralorne mine. Very l i t t l e first-hand information is available regarding the distribution of units with depth in the Bralorne mine. Data from old level plans, while voluminous, are d i f f i c u l t to correlate with current interpretations of the geology without the key to the old d r i l l logs that could have been provided by the old core. 5.2 Rock Types: Litholoqy. Petrography. Petrology The distribution of the 1ithologies described below, and the locations of the major veins, are discussed with reference to Figures 5-1 and 5-2 (in pocket) at surface and on the 8th level underground respectively. Figure 5-3 shows the distribution of units in a section through the main adit, and the main veins are shown in a diagrammatic section (Fig. 5-5b). Further cross-sections are on the geology plans (in pocket). Unit numbers are those shown in the legends of Figures 5-1 and 5-2. /o4 Figure 5-3. Cross-section through the main haulage adit, 8 level, Bralorne Mine. Units, as in Figures 5-1 and 5-2, are defined in Table /OS 5.2.1 B r i d g e R i v e r (Fergusson) Group (units 1 and 2) The oldest rocks in the Bralorne mine area belong to the Bridge River Group (Fergusson Series of Cairnes, 1937). They do not occur within the Bralorne block, being adjacent to it and separated from it by the Fergusson fault. However, both the current mapping at surface and old mine plans in presently inaccessible areas of the underground workings indicate that the Bridge River rocks are intruded by s a t e l l i t e bodies of Bralorne d i o r i t e , so i t i s included in the discussion here. The two major components of the group are low grade metabasalts (unit 1), and ribbon chert, a r g i l l i t e and minor limestone (unit 2). Although Potter (1983) indicates that the ultramafic rocks of the Shulaps complex may be as young as Jurassic, they are also usually included with this group (Monger, 1977; Nagel, 1979; Wright et a l . . 1982). In the mine area the ultramafic rocks are described as the President ultramafics (unit 5 below). The age span of the Bridge River Group may be Permian to Jurassic (see Chapter 2). Rocks of the Bridge River Group contain conodonts and radiolaria of Triassic to Jurassic age (Cordey, 1986; Potter, 1983), but appear to be intruded by the Early Permian Bralorne d i o r i t e in the mine area. The base of the group is not exposed, but i t s thickness is at least a thousand meters (Cairnes, 1937). Because i t i s highly deformed and lacks a coherent stratigraphy, Potter (1983) suggested that "Bridge River Complex" would be a more appropriate name for the group. Following Cairnes (1937!), Stevenson (1958) and Pearson (1977), the E<ridge River Group is divided here into metabasalts (unit 1), and ribbon cherts and a r g i l l i t e s (unit 2 ) . However, Church (1987) and Church et al . (1988) contend that no distinction is possible between greenstones of the Bridge River and Cadwallader groups, and place a l l these volcanics into the Cadwallader Group (section 2 . 2 ) . 5.2. 1. 1 Volcani c.s (uni t 1) Volcanics of the Bridge River Group, exposed immediately north of the Fergusson fault, consist of reddish-brown to dark green massive greenstone lacking any primary structures. Elsewhere, amygdaloidal flows, pillowed flows, pillow breccias and light green tuffaceous layers are well developed (Potter, 1983). The greenstones are t h o l e i i t i c metabasalts of mid-ocean ridge basalt (MORB) to oceanic-island basalt (OIB), or possibly back-arc basin basalt CBABB) character (Potter, 1983: Table 2-2 and section The greenstones are made up principal ly of lath-shaped albitised plagioclase to 2 mm with trachytic texture, rare 1 to 2 mm c1inopyroxene r e l i c s that are occasionally mantled by hornblende grains, and chlorite-calcite-epidote amygdules, a l l of which are surrounded by a fine-grained groundmass of chlorite and opaques. Tuffaceous layers, less than 1 meter thick, form pale green interbands in ribbon cherts, and may grade into coarse sandstone (Potter, 1983). /o7 Sediments (unit 2) Dark grey to black ribbon chert with thin intimately interbedded septa of graphitic argil l i t e makes up the bulk of the sedimentary section of the Bridge River Group. Thicknesses of up to 100 m of chert are common. The cherts are composed of fine grained s i l i c a with occasional ankeritic carbonate. Layers a few centimeters thick of gray radiolarian chert with nodular structures are common (Cairnes, 1937). Characteristically, the chert is highly deformed, fractured and veined by white quartz in the Bralorne area. Both Potter (1983) and Rusmore (1985) have suggested that folding is mainly attributable to slumping of soft sediments, since the fold axes are markedly disharmonic. I agree with this interpretation, even though Church (1987) maintains, probably incorrectly, that the ribbon cherts are strongly folded tectonical1y, and that they therefore are older than the greenstones. Limestone as small pods a few meters to a hundred meters long are closely associated with the greenstones (Cairnes, 1937; Stevenson, 1958). Unfortunately, they do not form a horizon that can be defined as a marker for the Bralorne mine area. No dateable fossils have been found in this limestone. /oa 5.2.2 Cadwallader Group ( and 4) Rocks of the Cadwallader Group occur throughout and to the southwest of the Bralorne block. The Cadwallader Group, original ly defined in the Bralorne area, was divided by Cairnes (1937) into three formations, the lower sedimentary Noel Formation, the middle volcanic Pioneer Formation and the upper sedimentary Hurley Formation. Rusmore (1985), mapping in the Eldorado Basin (Fig. 1-2) 20 to 30 km north of Bralorne, could find no evidence to support the division of the Cadwallader sediments into Noel and Hurley. Rusmore (1987) therefore proposed that the Noel Formation terminology be abandoned, although her examinations near Bralorne were of a reconnaissance nature. The writer also found it d i f f i c u l t to separate Noel from Hurley in the Bralorne area, and so prefers to place the large section of black a r g i l l i t e s and t u r b i d i t i c sediments found along Cadwallader Creek in the Hurley Formation as defined by Rusmore (1985). However, this does not mean that Cairnes' original distinction in the Bralorne area was not real; Church (1987) retains the Noel and Hurley divisions. The Cadwallader rocks in the Bralorne mine area consist of a volcanic package (unit 3: the Pioneer formation greenstones) that is present along the northeast and southwest sides, and north end of the Bralorne intrusive mass. The volcanics grade up into and are interbedded with tuffaceous turbidites and wackes, and these in turn grade into the overlying a r g i l l i t e s that form the bulk of the sedimentary Hurley Formation (unit 4) to the northwest of the intrusive mass and in the Cadwallader Creek canyon. Three samples of fossi1iferous limestone from d r i l l core intersecting the Hurley For mat i on on the P.E. Gold property 2 km southeast of the Pioneer mine (Fig. 1-2) were barren of useable microfossiIs CM. Orchard, pers. comm, 1987). Regionally, conodonts found in the Hurley Formation in the Eldorado Basin 25 km northwest of Bralorne (Rusmore, 1985) and at Gwynneth Lake 4 km northwest of Bralorne (Church et al . . 1988) are of Upper Triassic (Kami an to Norian) age, at about 225 Ma. However, as described by Cairnes (1937), Stevenson (1958) and the present work (section 5.2.2), rocks of the Pioneer and Hurley formations are intruded by the Early Permian Bralorne intrusives. Thus either the Cadwallader Group includes rocks as old as the Permian in the Bralorne area, or else there are two separate assemblages now labelled "Cadwallader": the original ly defined one at Bralorne, and the other one outside the Bralorne block as recently defined by Rusmore (1985). Her section i s about 1,500 m thick, but this must be a minimum estimate because she measured only 200 m of volcanics (neither the base nor the top of the section i s exposed: Schiarizza et a l . . 1989). Church (1987), for instance, estimated that the volcanics were at least 300 m thick. He also estimated that the total Cadwallader i s in the order of 2,300 m thick. There is such a striking s imilarity between the 1ithologies of the Cadwallader rocks in the Bralorne block and those of the Eldorado Basin that a correlation seems l i k e l y (see Table 2-2 for chemistry of the Pioneer volcanics in the Bralorne, Eldorado and Goldbridge areas). Rusmores' (1985) analysis of the Cadwallader indicates that i t represents a marine volcanic arc, overlain by a submarine fan of proximal to distal turbidites that received sediment from fault scarps and terrigineous sources consisting of carbonate and igneous rocks. Pioneer Formation (unit 3) Although commonly termed "greenstones" in mine usage, the Pioneer Formation in the Bralorne block, before spi1 i t isat ion, metamorphism and hydrothermal alteration, consisted mainly of basaltic andesites, which in places preserve effusive and pyroclastic textures. They should properly be termed meta-volcanics, but in the discussion following, the prefix "meta-" will be dropped for the sake of brevity. The volcanics include aquagene breccia (Plate 5-1) and pillows, l a p i l i tuffs, and fine grained massive amygdaloidal flows to medium grained feeder dykes or s i l l s . Paler si l iceous flows, some with abundant quartz eyes, are infrequently seen, but are particularly noticeable in dump material near the Pioneer mine. Six samples of Pioneer volcanics from the Bralorne block, one from the Goldbridge area, and four from Eldorado /// Basin were selected to show as great a range of composition as possible for chemical analysis (section 2.3) and Rb-Sr dating (section 3.4.3). Analyses of a mafic and a felsic unit from the Pioneer volcanics are also reported in Church et a l . (1387). The volcanics analysed in this study range from intermediate to felsic in appearance, but thin section and chemical analyses (Table 2-2) suggest that before metamorphism they were basalts and basaltic andesites (with the exception of C096A, a quartz-eye flow at the Pioneer mine). These meta-volcanic rocks are now composed principal ly of r e l i c t plagioclase Calbite, An0-Es, replacing andesine, An 3 0 ) and hornblende or c1inopyroxene phenocrysts in a felted mat of plagioclase microlites and i n t e r s t i t i a l chlorite and/or a c t i n o l i t e (Plate 5-2 a-d). Small but significant amounts of quartz, Fe-Ti oxides and sphene (up to 37.) are present. Amygdules, 0.1 to 1.0 cm in diameter and f i l l e d with quartz-calcite-epidote-chlorite (Rusmore, 1985 also reports pumpellyite and prehnite) are common and characteristic of the Pioneer basalts (Plate 5-2 a,b). Magnetite and apatite are rare in the Pioneer rocks, but 1 to 3 7. pyrite is common. Plagioclase is usually strongly altered to calci te , chlorite and s e r i c i t e . Whole-rock analyses (Table 2-2, section 2.3) show the Pioneer volcanic rocks to be metamorphosed high-Mg basalts or p i c r i t e s (MgO = 8 to 157.5 Hughes, 1982). Potash contents range from not detectable to rarely 0.67., as i s typical of a l l the volcanic rocks in the Bridge River d i s t r i c t that P l a t e 5-1. Aquagene b r e c c i a s i n the Pioneer Formation of the Cadwallader Group (as o r i g i n a l l y d e f i n e d i n the Bralorne mine area by Cai r n e s , 1937). From the P a c i f i c E a s tern Gold p r o p e r t y ( F i g . 3-1), DDH-P-85-02, a t appro x i m a t e l y 500 m. Plate 5-2 («) See 2. page.5 -forward ±e>y- CAptier\ dehx'As • P l a t e 5-2. Mineralogy and t e x t u r e s of the Pioneer v o l c a n i c s i n t h i n s e c t i o n ( t r a n s m i t t e d l i g h t ) . In ( a ) , from sample C095A (Bralorne mine, DDH SB-84-17/300 1), r e l i c t p l a g i o c l a s e phenocrysts and m i c r o l i t e s (white) are a l b i t e ; b r i g h t c o l o u r e d patches are epidote+quartz amygdules i n dark matrix of c h l o r i t e and opaques, and f i e l d of view i s 0.25 cm (crossed p o l a r s ) . In ( b ) , from sample C098D ( P a c i f i c E a s tern Gold property, DDH P-85-03/500m), s i m i l a r minerals and t e x t u r e s are i l l u s t r a t e d ( f i e l d of view 1 cm; crossed p o l a r s ) . In ( c ) , from sample C2001A (green and p u r p l e p i l l o w b a s a l t from headwaters of Eldorado Creek, 1.5 km e a s t of peak 7810) s i m i l a r p l a g i o c l a s e i s a l s o a l b i t e , but r e l i c t mafic phenocrysts are a l t e r e d to e p i d o t e ( b r i g h t ) , c h l o r i t e (green) or carbonate ( w h i t e ) . H i g h l y b i r e f r i n g e n t i n t e r s t i t i a l m a t e r i a l i s sphene ( f i e l d of view 1 cm, crossed p o l a r s ) . In (d), from sample C2001B (on r i d g e above 2001A) the t e x t u r e i s almost gabbroic, with clinopyroxene ( c l e a r , high r e l i e f ) , c h l o r i t i z e d clinopyroxene r e l i c s (green), c l o u d y a l b i t i z e d p l a g i o c l a s e , and i n t e r s t i t i a l opaques and sphene ( f i e l d of view 1 cm, plane p o l a r i z e d l i g h t ) . were analysed in this study, by Rusmore (1985), and by Potter (1983). Soda contents are either low (0 to 1.57.) or high C3.5 to 47.). The high values are characteristic of s p i l i t i z e d basalts produced by low-grade alteration on the sea floor (Carmichael et a l . , 1974). Titania values are low compared to average basalts and very low compared to p i c r i t e s . As noted in Chapter 2, less mobile trace elements (V, T i , Y, Nb, Zr and Cr) in diagrams proposed by Pearce and Norry (.1979), Shervais (1982), and Garcia (1978) indicate that even although they have been exposed to greenschist facies metamoprphism, these Pioneer volcanic rocks probably were transitional between island-arc tholei i tes (IAT) and mid-ocean ridge basalts (MORB). A similar conclusion was reached by Rusmore (1985) for the volcanics of the Eldorado Basin, although the Bralorne block samples tend towards a more calc-alkal ine basalt (CAB) character (Fig. 2-2 a-c). The one sample at Bralorne that is not basaltic in composition i s C096C (Table 2-2: 647. SiO*, 3.67. Na20) . This rock, as noted by Cairnes in 1937, is a quartz keratophyre (dacite which has been altered to albite and c h l o r i t e ) . Such rocks are commonly associated with s p i l i t i z e d basalts ( i . e . . basalts altered to albite and chlorite: Carmichael et  a l . . 1974), characteristic of sea-floor alteration of vocanic rocks. Rusmore (19B5) noted an "andesite" and a "rhyolite" in her volcanic package, and Church et a l . (1988) analysed a "rhyodacitic breccia" from near Gwynneth Lake with 727. SiO.2, 47. Na20 and 2.57. K a 0. Thus there are minor indications of felsic volcanics within the Cadwallader p i l e , suggesting accumulation in an arc setting. Hurley Formation (unit 4) Hurley sediments in the v i c i n i t y of the Bralorne-Pioneer mine comprise rhythmically interbedded green volcanic wacke and darker argil l i t e or s i l tstone. They resemble A-E Bouma cycles of distal turbidites (Mutti and R i t t i - L u c c i , 1978 ) . Noel Formation sediments, as defined by Cairnes C1937) , consist of black a r g i l l i t e s that are less calcareous than those of the Hurley, but the distinction i s uncertain and was not attempted here. Sediments of the Hurley Formation are, however, readily distinguished from those of the Bridge River Group (unit 2) by their lack of ribbon chert. The paler, coarser laminations of the typical turbidites consist of sand-sized, broken quartz and plagioclase grains (no K-feldspar was observed) in a very fine grained matrix of the same. Some small recognizable rock fragments are of volcanics similar to the Pioneer Formation (unit 3 ) . Consequently, the Hurley sediments are largely volcaniclastic and were derived from the underlying volcanics; as Cairnes (1937) notes, "Every gradation was observed to exist between fragmental members of the Pioneer formation and tuffaceous sediments of the succeeding, Hurley formation." The sediments show a general fining upwards: black argil l i t e , cherty a r g i l l i t e , and green or brownish //7 cherty tuff occur higher in the section. The total thickness of the sedimentary portion of the Cadwallader i s about 2000 m (Church,.1987; Cairnes, 1937). Other features dist inct ive of the Hurley Formation are thin green andesitic flows that are megascopical1y identical to those of the underlying Pioneer Formation; Rusmore (1985) also notes this in the Eldorado Basin. Grits or conglomerates containing well-rounded m u l t i l i t h i c clasts are also characteristic (Garver et a l . , 1989), but these were not seen in the Bralorne block. Near Goldbridge, these include felsic igneous clasts that resemble the "soda granite" (section 5.2.3). Limestone fragments, up to 50 m across, contain Mesozoic gastropods (Cairnes, 1937). Large o l i s t o l i t h s (exotic blocks) in the Eldorado Basin yielded Triassic conodonts (Rusmore, 1985). 5.2.3 Bralorne Intrusions (units 5 to 7) The Bralorne intrusions are abundant in the Bralorne mine area (Fig. 5-1). They include two main types, called "augite diorite" (unit 6) and "soda granite" (unit 7) in mine usage. These two units, with the "Pioneer greenstone" (unit 3) and "greenstone diorite" (included with unit 6), constitute the main host rocks to the gold quartz veins. The President ultramafics (unit 5) are t r a d i t i o n a l l y included with the Bralorne intrusions (Cairnes, 1937; Joubin, 1948; Stevenson, 1958) although a l l that is known //8 about their age is that they are older than the d i o r i t e and soda granite by which they are cut. The Bralorne d i o r i t e (unit 6) forms an irregular elongate mass up to 600 m wide, extending over 4 km along strike to the northwest from the Pioneer, through the King mine area, to the northern edge of Figure 5-1. In broad outline, the main body of soda granite (unit 7) forms a large dyke-like mass, apparently intruded along the northeast contact of the d i o r i t e , between the d i o r i t e and the older rocks of the Cadwallader Group (units 3 and 4). Both the diorite and soda granite taper out towards the southeast near the Pioneer mine; here the d i o r i t e is cut out by the soda granite, which is cut off by the Cadwallader fault. To the northwest, between the Bralorne and King mines, the soda granite fingers out as dykes into the d i o r i t e . At i t s widest, the soda granite i s about 400 m thick, and l ike the d i o r i t e i t also ocurs over almost 4 km of strike length. In general, the diorite and soda granite widen s l i g h t l y with depth, indicating steeply outward dipping contacts. President Ultramafic Rocks (unit 5) The ultramafic rocks in the Bralorne-Pioneer mine are mainly restricted to elongate serpentinized bodies, up to 50 m wide by many kilometers long, along the major Cadwallader fault and the north-south faults immediately west of the King mine area (Fig. 5-2). Most of the ultramafic rocks in //9 the Bralorne area are dark green serpentinite (Plate 5-3a), taut in other parts of the Bridge River camp they are often total ly replaced by quartz-anker i te--tal c + fuchsi te. The serpentinized President ultramafic rocks bound the area within which productive veins occur (the Cosmopolitan vein has no production). Gold-bearing quartz veins do not cross the serpentinite masses, but instead feather out into them. The soft serpentinites presumably were unable to maintain a coherent opening and instead responded to stress by flow. Other major bodies 2 km southwest of Bralorne (Fig. 1-2) are less altered and tectonized, forming layered bodies of dunite-harzburgite (Wright et a l • , 1982). Larger bodies, such as the main President mass 2 km south of the mine or the Shulaps ultramafic complex, are partly composed of rythmically layered harzburgite, dunite and peridotite (Leech, 1953; Nagel, 1979). However, Wright (1974) ascribed this layering to secondary processes (tectonic deformation) rather than primary layering. Thin sections of the ultramafic rocks at Bralorne (Plate 5-3b) show that they were o r i g i n a l l y dunite through lherzol i te to minor pyroxenite. Although variably serpentinized, cumulate and intercumulate textures are local ly recognizable. Some dull black masses represent total replacement of ol ivine by antigorite, serpophite (Kerr, 1959, p. 416) and lesser talc cut by s l i p - f i b r e veinlets of chrysotile. A typical r e l i c t peridotite is / 2 o Plate 5-3. Thin section views, (1 cm wide, crossed polars) of r e l i c t ultramafic rocks from the Cadwallader fault zone. (a) is from sample C1034, unlocated core from old Bralorne Mines Ltd. d r i l l i n g ) , showing fo l i a t e d nature and outlines of barely discernible r e l i c t c r y s t a l s , (b) is from sample C138B (DDH UB-80-7/472' ), and shows o p t i c a l l y continuous mass of hornblende (dark) enclosing cumulate-textured r e l i c t o l i v i n e grains (now serpentine, white) and larger pyroxene (orange). / 2 / characterized by abundant coarse euhedral grains of "bastite" (enstatite altered to antigorite and tremolite), locally up to 1 cm across. Similar sized subhedral c1inopyroxene grains are mantled by hornblende, subsequently altered to c h l o r i t e , tremolite and talc . Like the orthopyroxene, they poiki1itical1y enclose small altered olivine grains. Both the c l i n o - and orthopyroxenes are set in a matrix of finer C<0.5 mm) euhedral ol ivine r e l i c t s , now largely psuedomorphed by talc , serpentine and fine-grained magnetite or ilmenite. Other opaques are mainly chromite, which often fringes picotite (iron-chrome spinel), and minor sphene and pyrite. Plagioclase, absent from the main mass of ultramafics, increases as the adjacent diorite is approached. These rocks are ophitic-textured, with feldspars i n t e r s t i t i a l to the amphibole mantled pyroxenes. The plagioclase is unusually sodic for such mafic rocks, ranging from albite to oligoclase CAno-io). No chemical analyses were obtained for the ultramafic rocks in this study, but both Cairnes (1937) and Church et  a l . (1988) report analyses for the President intrusions. They contain about equal amounts of SiO^ and MgO (407. each), about 107. total iron, small amounts of CaO (less than 57.), and about 107. v o l a t i l e s . There i s l i t t l e variation from the relat ively fresh to the strongly serpentinized samples, implying isochemical alteration. /22 Although earlier workers (Cairnes, 1937, 1943; Stevenson, 1958) considered the ultramafics to be later than the Eiridge River, Cadwallader, and Bralorne intrusive rocks, evidence from this study suggests the opposite. Dykes of diorite and soda granite cut the ultramafic rocks in both outcrop and d r i l l core in the mine area. Partial recrystal1ization of the ultramafic rocks, seen at Bralorne and at Wayside north of Goldbridge (Fig. 1-2), leading to a gradational contact with the d i o r i t e , suggests border-phase contamination of the d i o r i t e by pre-existing ultramafic. These features are particularly well developed along the southwest side of the d i o r i t e near the Cadwallader serpentine mass (near the King 4 level adit, Fig . 5-1). Here the gradation, over 50 to 100 m, is from (1) the main mass of hornblende d i o r i t e , to (2) d i o r i t e containing mantled c1inopyroxene cores, to (3) peridotite with r e l i c t ol ivine, enstatite and abundant c1inopyroxene. This indicates that the augite of the "augite d i o r i t e " probably came from this border phase contamination. As shown in Figure 5-2 of the underground geology at Bralorne, large hornblendite masses (unit 6a) in the d i o r i t e are noted only in the southwestern side of the d i o r i t e . It i s also l i k e l y that occasional grains of chromite (commonly surrounded by the chrome mica, fuchsite, in altered diorite) probably were derived from assimilated ultramafic material. The a l b i t i c composition of the plagioclase in the border-phase zone could not be o r i g i n a l , and must reflect A23 sodium metasomatism produced by circulating waters below the sea floor, greenschist facies metamorphism, or hydrothermal alteration associated with mineralization. Hydrothermal alteration is also l i k e l y responsible for the t a l c , tremolite, chlorite, ankerite and zoisite observed in some of the ultramafics. Hornblendite (unit 6a) Hornblendite occurs only along the southwestern flank of the Bralorne d i o r i t e near the ultramafic rocks of the Cadwallader fault zone. It is a variable unit, including rocks ranging from dark, mafic-rich diorites, to ultramafic-looking rocks with a peculiar "network" texture as the contact with the ultramafic i s approached. The "network" is composed of dark, coarse (1 cm) hornblende grains around cores of pyroxene-rich rock, which average 5 - 10 cm across CPlate 5-4a). A chemical analysis of the hornblendite (Table 5-2) shows the low alumina, t i t a n i a , soda, potash, phosphorus and higher magnesia and lime expected of an ultramafic derivative. A comparison of the analysis of the hornblendite and the analyses of the ultramafics (section shows strong modification of the former, which has much lower magnesia and higher s i l i c a and alumina contents. Chemical analyses and thin sections (this study; Cairnes, 1937) show no trace of titanium-bearing minerals in the ultramafic rocks, although they are common in and /2+ P l a t e 5 - 4(a). Network of c o a r s e dark hornblende s u r r o u n d i n g c o r e s of r e l i c t u l t r a m a f i c rock t h a t have b e e n p a r t i a l l y a s s i m i l a t e d by the d i o r i t e ; white i s I n t e r soda g r a n i t e . Plate 5-4(.b).Thin section view of coarse hornblende grains p o i k i l l t i c a l l y enclosing cores of clinopyroxene in the hornblendlte unit (6a) developed by assimilation of ultramafic rock into the d i o r i t e . From sample C117, DDH 32Q-132/82'; f i e l d of view 1 cm wide, crossed polars. /2S* dist inct ive of the Bralorne d i o r i t e and soda granite. The Bralorne intrusions and the ultramafic rocks are therefore probably unrelated, and have been introduced separately. Hornblendite at Bralorne represents altered ultramafic rocks, as discussed above. The texture of the original ultramafic rock is strongly modified to a net of coarse dark green hornblendes (mantling pyroxene) that show optical continuity over large areas, and poiki1 i t ical1y enclose pyroxene, ol ivine, and plagioclase (Plate 5-4b). The hornblende is not serpentinized, but is altered at i t s margins to tremolite and at i t s centers to chlorite. The p o i k i l i t i c hornblende may have been produced by intrusion of the d i o r i t e , and therefore post-dated the serpentinization, which was l i k e l y caused by late magmatic act ivity in the ultramafic. In any case, the amphibole crystal l ized earlier than metamorphism or hydrothermal alteration. Bralorne Diorite (Hornblende Quartz Diorite, unit 6) The main mass of the Bralorne intrusion between the bounding Fergusson and Cadwallader faults is called Bralorne d i o r i t e (Fig. 5-1). Although this is properly meta-diorite, the prefix "meta-" will be omitted (but understood) in the discussion following. The d i o r i t e varies local ly over short distances from fine to coarse grained and light gray to dark green in colour, but i t i s characterized by a stockwork of pale green to buff veinlets composed of a mixture of quartz, epidote, zois i te , carbonate and prehnite (Plate 5-5). /26 Plate 5-5. Barren (pre-mineral) pale green quartz-epidote-calcite-prehnite stockvork that distinguishes the Bralorne d i o r i t e . From SB-84-34/5001; for thin section of similar material, see Plate 6-2 /27 Typically the diorite contains about 5 to 107. i n t e r s t i t i a l quarts, with the bulk being about equal proportions of albite ( A n 0 - i o ) and hornblende (Plate 5~6a). There are also accessory, dist inct ive, altered grains of skeletal ilmenite (Plate 5-6b), and lesser pyrite, apatite and sphene, and rare zircon. It is a medium-grained rock, with a 1 to 3 mm average grain size. Although tradit ionally called "augite d i o r i t e " , c1inopyroxene is conspicuously absent from a l l the diorites studied at Bralorne except those proximal to ultramafic bodies (section 5 . 2 . 3 . 2 : ) . The abundance of hornblende and lack of pyroxene led Cock field and Walker ( 1 9 3 2 ) to propose the name hornblende diorite, and the present study confirms this . The abundance of sodic plagioclase (albite, rarely to oligoclase) seen ubiquitously in this study i s also out of place with a rock as mafic as diori te . Although Cairnes (1937) describes plagioclase up to andesine or even bytownite, the present work confirms Stevenson's (1958) findings of almost pure albite in the d i o r i t e . The small amounts of quartz, almost always present, suggest that the rock should properly be called a hornblende quartz d i o r i t e . Away from the Bralorne mine area, though, quartz is rare in the diorite according to Cairnes (1937). Chemistry of the d i o r i t e i s significant (Table 5-1). Most noteworthy is the extremely low potash content (at the detection limit of 0.17. for the average of 8 analyses, although Boyle (1979) reported 0.37. and Cairnes (1937) reports up to 0.87.), which is unusually low for diorites /28 P l a t e 5-6. (a) Thin s e c t i o n view (1 cm wide, t r a n s m i t t e d plane p o l a r i z e d l i g h t ) of the Bra l o r n e d i o r i t e i n sample C093A (DDH UB-81-17/525') showing l a c k of hydrothermal a l t e r a t i o n ( t h i s sample used as the l e a s t a l t e r e d r e f e r e n c e f o r chemical s t u d i e s of a l t e r a t i o n ) . Roughly equal amounts of pale green hornblende and cloudy a l b i t e , o c c a s i o n a l i n t e r s t i t i a l g r a i n s of white q u a r t z . (b) S t r o n g l y c a r b o n a t e - s e r i c i t e a l t e r e d d i o r i t e from sample C116 (DDH 41 Q-144/200*) showing s k e l e t a l remnants of former i l m e n i t e g r a i n s t h a t c h a r a c t e r i z e the B r a l o r n e i n t r u s i o n s , now a l t e r e d to r u t i l e . F i e l d of view 0.25 cm, cro s s e d p o l a r s . /X9 (Nockolds, 1954) and suggests a relation to "piagiogranites" of sea-floor aff inity (Hughes, 1982). No K-feldspar occurs in any of the Bralorne rocks, so it seems l i k e l y that the low potassium contents reflet original igneous chemistry. The phosphorus and t i t a n i a contents are also very low compared to normal diorite (Hughes, 1982), implying a spreading ridge setting (MORB rocks are notably low in these components compared to QIB rocks: Hughes, 1982). The higher s i l i c a contents of the rocks analysed in this study, as compared to the analyses reported by Cairnes (1937), agree with those of Boyle (1979) and reflect the more typical quartz diorites that were sampled compared to Cairnes', which came from areas near the Cadwallader fault. Abundant small areas of "greenstone diorite" of mine usage are included with the Bralorne d i o r i t e map-unit. These rocks are characterized by variations in colour index and grain size from dark fine portions to coarse lighter portions. The variation is due to variable extents of assimilation of ultramafic and Pioneer volcanic rocks. The contacts with these two older rock units (5 and 3) are complex, being more of the form of agmatitic contact zones up to 100 m or more wide in which occur a l l gradations from diorite, through even-grained and sugary recrystal1ized greenstone, to recognizable volcanic textured basalt. This has caused the formation of a mixed rock, the so-called "greenstone d i o r i t e " . Al l of this is then overprinted by the intrusion of soda granite with similar complex dyke /30 relations, and then by alteration, producing a complex-appearing migmatite (section Several intrusive phases of diorite are distinguished on the basis of their relat ively fine or coarse nature. These might represent, respectively, syn-volcanic feeder dykes and batholithic phases (Cairnes, 1937). The stockwork of 1ight-coloured veinlets that distinguishes the diorite could be due to late deuteric alteration at the time of intrusion of the soda granite, which is also affected by them, or it could be due to later metamorphism. This stockwork is barren of sulfides and is cut by mineralised veins. Ubiquituous albite in the diorite is presumably due to greenschist facies metamorphism and hydrothermal al t er at i on . Soda Granite (Trondh.jemite/Albite Tonalite, unit 7) The main body of soda granite is found along the northeast side of the Bralorne diorite , but i t also forms many dykes cutting the d i o r i t e . The contact between diorite and soda granite, l i k e the diorite - greenstone contact, i s a zone up to 200 m wide rather than a sharp demarcation. Dykes of soda granite from a centimeter to many meters thick form a reticulate network in this border zone. Figures 5-1 and 5-2 show that the majority of the dykes follow a trend oblique to the bounding faults of the block. This trend is usually parallel to a weakly developed, broadly east-west foliation in the d i o r i t e . The contact with Cadwallader Group rocks on the other side of the soda granite, however, is sharp, linear and usually sheared, although the Cadwallader rocks look recrystal1ized and bleached. Typically the soda granite is a leucocratic, coarse-grained granitic rock composed of 407. quartz as large anhedral grains to 1 cm size, 507. squat albite crystals (An 0-o) of 3 to 5 mm average size, and 5 to 107. thoroughly chlorit ised i n t e r s t i t i a l mafic remnants (Plate 5-7). These were probably hornblende o r i g i n a l l y ; no biotite r e l i c s have been observed. Accessory minerals comprise minor skeletal r u t i l e and leucoxene after ilmenite, plus traces of pyrite, pyrrhotite, molybdenite, zircon, sphene and apatite. As in a l l the other Bralorne intrusions, no orthoclase can be found and therefore the term "soda granite" is somewhat misleading. It more properly should be called a trondhjemite, albite tonali te, or piagiogranite (Coleman and Donate, 1979). Low-grade alteration of the soda granite is widespread, with former amphibole s i tes now replaced by chlorite, epidote, s e r i c i t e , carbonate and pyrite. Another characteristic feature i s "crackling" of the soda granite by a fine stockwork of secondary s i l i c a , s e r i c i t e , chlorite , epidote and/or zoisite and carbonate. This is commonly very closely spaced, and possibly indicates the brittleness of the soda granite. A part of this stockwork i s similar to the barren pre-mineral stockwork found in the d i o r i t e , although it lacks the prehnite identified in these veins in /3Z P l a t e 5-7. Thin s e c t i o n view (1 cm wide, t r a n s m i t t e d l i g h t , c r o s s e d p o l a r s ) of the Bralorne soda g r a n i t e i n cample C094A (DDH UB-84-25/440') showing s l i g h t a l t e r a t i o n to c a l c i t e , s e r i c i t e and c h l o r i t e ( t h i s sample i s used as the l e a s t a l t e r e d r e f e r e n c e f o r soda g r a n i t e ) . Large a l b i t e g r a i n s (cloudy, o c c a s i o n a l l y twinned) and g u a r t z g r a i n s ( c l e a r ) , plus o c c a s i o n a l c h l o r i t e a f t e r mafics ( B e r l i n blue, f l a k y ) . P l a t e 5-8. I n j e c t i o n migmatites ("agmatites"; Mehnert, 1971) composed of x e n o l i t h s of b a s a l t i c a n d e s i t e (darkest, f i n e grained) i n d i o r i t e (medium grey, coarse grained) f u r t h e r cut by i r r e g u l a r to sheeted d y k e l e t s of soda g r a n i t e ( l i g h t e s t c o l o u r , coarse g r a i n e d ) . From DDH UB-14/150*. the d i o r i t e , possibly because of the lower Ca in the soda grani te. Chemically (Table 5-1) the soda granite is distinct in i t s major elements from a cale-alkaline granite (Nockolds, 1954), mainly in the total lack of potash and the very high soda content. As may be seen in Table 5-1, the composition of these unusually sodic plutonic rocks is more siliceous (70V. S i02) than the diorite (55 - 607. S i 0 2 ) . The soda granite i s younger than the diorite on the basis of i t s dykes that cross-cut the d i o r i t e , although i t is indistinguishable in i t s Early Permian isotopic age from the d i o r i t e (Chapter 3). On the basis of i t s field relations (occasional gradational contacts and similar met amor phi sin, alteration and pre-mineral stockwork) i t appears to be related to the d i o r i t e , containing similar minerals although in different proportions. Contrary to earlier opinions (James and Weeks, 1961; Cairnes, 1937) the soda granite cannot be genetically related to the gold mineralization, because i t i s much older than the Late Cretaceous mineralization. As wil l be discussed in section 5.4, however, i t s competence could have influenced the locus of gold deposition. The total lack of potash and the very high soda content of the soda granite indicate s i m i l a r i t i e s to the sub sea-floor "plagiogranite" intrusions described by Hughes (1982, p.67). /34~ Migmatite Contacts between the diorite and soda granite are complex and often resemble migmatites (Plate 5-8). Migmatite does not, however, form a mappable unit in the mine area. Partial melting of d i o r i t e , or differentiation of d i o r i t e , are possible origins for the soda granite and explanations for the migmatites. An attempt was made to distinguish between these two alternatives on the addition-subtraction diagrams of Bowen (1956) and on a plot of normative Qz-Or-Ab, but they cannot be distinguished. The temperature required to p a r t i a l l y melt a dry quartz d i o r i t e , which does not l i e on the granite minimum, would be 1050 °C. This could be reduced to 750 °C in the presence of abundant H20 and HC1 (Mehnert, 1971). Derivation of the Bralorne soda granite from the diorite by partial melting at the present level of exposure has been suggested (C.I. Godwin, pers. comm., 1985) to explain the presence of "restites", or dark-coloured masses, in the Bralorne intrusions, since the d i o r i t e and soda granite show migmatitic contact relationships. To test this hypothesis with Bowen's (1956) method, areas of darker coloured material in the migmatite (the r e s t i t e or neosome) presumably left behind from diorite by extraction of felsic material that formed the lighter coloured soda granite (leucosome) were analysed CC043 and C085 in Table 5-2). These analyses do not correspond well with the theoretical calculated rest i te compositions (reproduced here as RESTI, Table 5-2). They are depleted in s i l i c a , and considerably enriched in alumina, t i t a n i a and iron compared to the calculated rest i te composition. They also have normative ol ivine, while the calculated restite does not. Except for higher iron they correspond most closely to analyses of Pioneer basalts, which also have normative ol ivine (C095, C0S6, C098: Table 5-2). This supports f ield observations that suggest the "restites" are merely xenoliths of the volcanics, i n i t i a l l y partly assimilated by the diorite and then both intruded by the soda granite (contact relations described in section I favour the alternative explanation for the origin of the soda granite, by normal differentiation from the diori te . Although i t i s clear that the contact zone between diorite and soda granite is a migmatite (variety agmatite: Mehnert, 1971) I feel that the texture i s better explained by intrusion of soda granite into d i o r i t e . The relationship between d i o r i t e and soda granite is partly obscured by dark basaltic xenoliths ("restites") included in d i o r i t e prior to intrusion of the soda granite, and is therefore not clear in the two-dimensional exposure of d r i l l core (Plate 5-8), but i t can clearly be seen in an excellent outcrop (Plate 5-9) located near the bridge over the Bridge River, 1 km north of Goldbr i dge. Since the soda granite i s so close in age to the d i o r i t e , i t cannot have been derived by partial melting of the diorite during a younger thermal event such as /36 Plate 5-9. Contact relations of Bralorne d i o r i t e (grey-green, on right) and soda granite ( l i g h t grey). There appears to be a thin white c h i l l e d margin on the soda granite, which is similar to thin dykelets cutting the d i o r i t e . Xenollths of basaltic andesite (top l e f t corner and near hammer) are c l e a r l y v i s i b l e in soda granite but d i f f i c u l t to distinguish in the d i o r i t e . In outcrop on road to L i l l o o e t , 1.5 km east of bridge over Bridge River. /37 emp1acement of the Coast PIut oni c Complex. Ear 1y Per mi an partial melting of the diorite at depths below the present level of exposure to produce soda granite, which was then intruded at the present level , cannot be ruled out. However, to judge from their similar mineralogy and closeness in age, and the contact relations described above, the simplest explanation seems to be that the soda granite is a differentiate of the same magma that produced the d i o r i t e . Aplite Dykes Aplite dykes are thin (less than 1 m) and irregular, and are found cutting the Bralorne soda granite. They were d i f f i c u l t to separate from soda granite in d r i l l core and could be widespread, although volumetrical1y insignificant, in the Bralorne intrusive mass. They are even more leucocratic than the soda granite, usually with no mafic minerals v i s i b l e . In thin section, the aplite dykes resemble fine-grained equivalents of the soda granites (Plate 5-10). They consist of roughly equal amounts of fine, even grained (0.5 mm) quartz and plagioclase, and lack the porphyritic character and aphanitic groundmass that characterize the later dykes. No isotopic dates or chemical analyses are available for the aplite. They probably are final differentiates of the diorite - soda granite system, and as such are Early Permian in age. /33 P l a t e 5-10. Thin s e c t i o n view (1 cm wide, crossed p o l a r s ) of a p l i t e dyke from sample C121 (DDH 43 Q-163/661*), showing f i n e , even g r a i n s i z e (0.5 mm) of the a l b i t e ( m i l d l y s e r i c i t i z e d ) and q u a r t z . /J? 5.2.4 Cretaceous-Tertiary Dykes Five Cretaceous-Tertiary dyke types intrude the plutonic rocks at Bralorne. Chemistry, norms and modal analyses are in Table 5-2. The dyke sets, from oldest to youngest based on cross-cutting relationships and alteration, are: grey plagioclase porphyry, a l b i t i t e , green hornblende porphry, Bendor porphyry, and lamprophyre. Grey Plagioclase Porphyry Dykes (unit 8a) Grey plagioclase porphyry dykes are of restricted distribution and have only been noted in one area between the King and Bralorne mines near 9000N, 7500E, in angle d r i l l holes from surface (Fig. 5-1). They seem to be vol umetrical1y insignificant, traceable along strike for only 100 m or so, and only up to a few meters wide. Since they have been found only in d r i l l core, their strike i s unknown but is probably oblique to the main vein trend—in other words along the conjugate R' shear direction (section 5.4.4). They cut and are c h i l l e d against the soda granite, and are cut by a l b i t i t e dykes. These dykes usually consist of 307. white plagioclase phenocrysts (Ana>. 57. grey quartz eyes and 57. mafic remnants, a l l of 2-3 mm size, in a grey phaneritic (0.05 to 0.1 mm) groundmass of the same minerals. These dykes contain striking symplectic quartz-albite intergrowths (Plate S-7e), which are also present in highly altered /4o "quartz core" areas of the soda granite and a l b i t i t e dykes (sect i on 6.1.4). Chemically, grey plagioclase porphyry dykes are very similar to the a l b i t i t e dykes (Table 5-2, sample C133) in a l l respects except for the soda, Eta and Sr contents, which are closer to those of the soda granite (Table 5-1). The grey plagioclase porphyry dykes were not dated isotopical1y. They could be either late-stage dykes related to the soda granite, or they could be more closely related to the time of mineralization, and as such essentially a precursor to the a l b i t i t e s . Albitite/Green Hornblende Porphyry Dykes (units 8/9) A l b i t i t e dykes (unit 3), generally only a few meters thick but local ly up to 50 m, can be traced on strike for up to a kilometer. They are most numerous and thickest in a centrally-located zone within the Bralorne intrusive mass (Fig. 5-2), where they are most numerous and thickest, but they also occur outside the Bralorne block. Green hornblende porphyry dykes are more widespread than the a l b i t i t e s , occurring farther from the center of mineralization and with more variety of strike direction, including oblique cross-cutting of the veins. They range from small faulted segments a few centimeters thick by a few meters long, to major dykes up to 50 m wide by 2 km in strike length. J4I The a l b i t i t e and green hornblende porphyry dykes mainly dip steeply to moderately north (Fig. 5-3), sub-parallel to the dip of the quarts veins. Dips range from nearly vertical to as low as 50 degrees, based on information from correlation of dykes in angled surface d r i l l holes. Albit i tes are felsic dykes ranging from creamy or buff quartz-plagioclase porphyry to pale green hornblende-pi agioclase porphyry. They have been referred to as "albit i tes" because of the universal presence of albite in phenocrysts and groundmass. These are definite dykes with flow-banded, c h i l l e d margins against their host rocks, and can be distinguished from zones of dense cream-coloured ankeritic alteration which also commonly adjoin mineralized veins. Typically, fresh a l b i t i t e dykes contain 25"/. quartz, £0% albite, 137. hornblende remnants, and 2% skeletal r u t i l e and leucoxene after ilmenite, plus minor pyrite and zircon (Plate 5-11). A l b i t i t e dykes are commonly strongly altered and sub-parallel to, or even adjacent to, gold-bearing veins. Alteration is intense and consists of fine grained aggregates of ankerite, s e r i c i t e and quartz with coarse pyrite crystals; often only the original quartz phenocrysts are s t i l l v i s i b l e . As the name a l b i t i t e implies, these rocks are rich in soda (77.). Their chemistry i s not dissimilar from an average dacite (Nockolds, 1954), but they are more leucocratic as evidenced by the low iron and magnesia contents (Table 5-2). Although obviously metamorphosed and altered, these are s t i l l extremely fractionated rocks (differentiation indexes are 75 to SO; Hughes, 1982'J, and their protoliths may have been c a l c -alkaline dacites to rhyolites of sub-alkaline (sodic, potash-deficient) character (Irvine and Baragar, 1971). The green hornblende porphyry dykes (unit 9) can be of substantial size, and display well-defined flow-banded, c h i l l e d margins with a progressive coarsening towards the centers where they are seriate textured and relat ively coarse grained. They are distinguished by about 10"/. large, fresh, black hornblende phenocrysts up to a centimeter across, showing primary zoning near their margins (Plate 5-12). These dykes never contain quartz phenocrysts; they are principal ly made up of plagioclase phenocrysts or seriate crystals up to 3 mm across. These are set in a variable groundmass of fine grained plagioclase with i n t e r s t i t i a l chlorit ised mafics, less than 10% anhedral quartz, and 57. fine Fe-Ti opaque oxides. Sulfides are not present, and no zircons were recovered from two large samples (section 3.4.2). Green hornblende porphyry dykes are mainly fresh to weakly altered to epidote, c h l o r i t e , calcite and s e r i c i t e . Only rarely are they cut by mineralized veins. The bulk of them appear to be post-mineral, but an olive-drab amygdular variant and a fine grained crowded hornblende porphyry are seen with main-stage alteration (biotite, ankerite, fuchsite and arsenopyrite) so some must be intra-mineral. It was not P l a t e 5-11. T h i n s e c t i o n view (1 cm wide, crossed p o l a r s ) of a l b i t i t e dyke from sample C092A (DDH UB-81-18/225*), showing euhedral q u a r t z phenocrysts (grey, black) and a l t e r e d p l a g i o c l a s e r e l i c s ( s e r i c i t i c patches of f i n e g r a i n s i z e ) and mafic r e l i c s (coarser muscovite) i n a v e r y f i n e groundmass (0.01 to 0.03 mm) of quartz and s e r i c i t e . P l a t e 5-12. Thin s e c t i o n view (1 cm wide, crossed p o l a r s ) of green hornblende porphyry dyke from sample C083A (unlocated, from main dump at 8 l e v e l a d i t p o r t a l ) , showing primary magmatic zoning preserved i n m a j o r i t y of coarse hornblende phenocrysts (one i s a l t e r e d to b i o t i t e ) , s e t i n a f i n e g r ained groundmass of p l a g i o c l a s e ( g r e y ) , q u a r t z (white) and hornblende (coloured) g r a i n s . possible to clearly define age relations of these sub-types, since even where one variant cuts another, the contacts are always sheared and sub-parallel , and c h i l l relations are equivocal. A general trend from the early more f e l s i c , pale coloured a l b i t i t e s to later darker green, more mafic dykes is postulated from contact relations in core and supported by the isotopic dating (Chapter 3). Blocks and fragments of a l b i t i t e have been seen in green hornblende porphyry dykes. A l b i t i t e and green hornblende porphyry dykes appear to form a spectrum which spans the pre- to post-mineral range (91.4 to 85.7 Ma). Mineralization clearly post-dates and i s related to the a l b i t i t e dykes, but i s often cut by the green hornblende porphyry dykes. However, a few green hornblende porphyry dykes do show alteration typical of the main stage mineralization, and when the dykes are considered in d e t a i l , there appear to be a l l stages of dykes transitional from a l b i t i t e to green hornblende porphyry. The latter must therefore be viewed as an intra-mineral rather than a s t r i c t l y post-mineral dyke set. The spectrum of dykes i s characterized by varying proportions of quartz, plagioclase and hornblende phenocrysts. Albite and quartz phenocrysts with rare hornblende are typical of the a l b i t i t e s ; the other end-member, green hornblende porphyry, i s typified by lesser albite , major hornblende, and a lack of quartz phenocrysts. The colour index therefore gradually increases from a l b i t i t e to green hornblende porphyry. The green hornblende porphyry end-member of the series, which appears to be a mafic /4S andesite in thin section, has a chemical c lassif ication of sub-alkaline CK-poor) t h o l e i i t i c basalt. It is much less differentiated (differentiation index is 30) and far more mafic than the a l b i t i t e end-member to judge from the chemistry, norms and modes (Table 5-2). The modal analyses and chemical c lassif ications of green hornblende porphyry and diorite are strikingly similar, although they share no magmatic relation because of their different ages. The same may be said about the a l b i t i t e - soda granite pair. From an exploration point of view, the a l b i t i t e dykes are important since they are most closely related to mineralisation. A l b i t i t e , or albite porphyry, dykes (and trondhjemitic host rocks) are also found at many other mesothermal gold-quartz vein deposits in Canada and Africa (e.g. Colvine et a l . . 1984; Wood et a l . , 1986; Clark et a l . . 1986). Both a l b i t i t e and green hornblende porphyry dykes are found outside the Bralorne fault block and have been affected by motion on the bounding Fergusson and Cadwallader faults. The maximum age (85.7 Ma) thus implied for motion on the faults is similar to the 100-85 Ma found by Rusmore (1985) for motion on what she c a l l s the "Bralorne Fault" (the Cadwal1ader-Fergusson fault zone). "Bendor" dykes "Bendor" dykes are so named because of s i m i l a r i t i e s to the nearby Bendor batholith as described by Cairnes (1937). They are not well represented at Bralorne, but are common throughout the Bridge River camp. Bendor dykes were seen only in poor exposures at the north end of the Bralorne block near the Lome vein ('.Fig. 5-1); their size is small and their orientation uncertain. Although mapped as a l b i t i t e i n i t i a l l y , these dykes (best seen at 5000E, 12000N: Fig. 5-1) have several dist inctive petrographic features that set them apart and link them to the nearby Bendor batholith. These dyke rocks are characterized by about 5V. fine-grained, ragged hornblende with r e l i c t clear augite cores, conspicuous sphene and magnetite, and 1 to 2 mm plagioclase phenocrysts with sharp, osci l latory zonation Coligoc1ase, A n i 7 , to andesine, An^ 2 ). This osci l latory zoning, the more calcic composition, and the magnetite, i s not observed in any of the earlier intrusive rocks. The zoning and calcic composition of plagioclase in the earlier rocks has presumably been obliterated by the homogenizing effects of greenschist metamorphism associated with intrusion of the Coast Plutonic Complex (CPC). Available isotopic dates support this conclusion, since the earlier dykes (SO to 85 Ma) pre-date the bulk of CPC intrusion at 85 to 70 Ma (Woodsworth, 1977), and the Bendor plutons are younger than much of the CPC at 63 to 57 Ma (Wanless et a l . . 1977). Dykes at the Congress mine, 15 km north of Bralorne, are also post-CPC at 67 Ma (Harrop and S i n c l a i r , 1986). They contain 30% clear, osci l latory zoned, intermediate plagioclase, and similar amounts of chlorit ised hornblende phenocrysts, as in the "Bendor" dykes (Plate 5-13). These /47 dykes were not chemically analysed, but Cairnes (1937, p. 38) provides an analysis of the Bendor pluton, which approximates normal granodiorite, and has considerably less soda and more potash than the a l b i t i t e s . Lamprophyre dykes (unit 10) Lamprophyre dykes d i s t i n c t l y cross-cut mineralized veins, with orientations roughly perpendicular to the veins and earlier dykes. Lamprophyre dykes are well defined by surface d r i l l holes in the area to the southeast of the Empire fault (Fig. 5-1). The dykes parallel this fault, with a consistent north-south strike and steep dip. Because of recessive weathering, no lamprophyres crop out. They have only been found in d r i l l core and underground, where they have well developed flow-banded, c h i l l e d margins and coarsely porphyritic central portions. Their contacts are often sheared and they appear to taper out into small faults. They are thin, rarely over a few meters thick, but commonly persist over several hundred meters of strike 1ength. The lamprophyres are dark, mafic rocks that might be classed as kersantites (Hughes, 1982) since Stevenson describes them as containing plagioclase. In the mine, they contain prominent black b i o t i t e , dark green c1inopyroxene, and smaller clear apatite phenocrysts in a finer grained dark groundmass of the same minerals plus brown glass (Plate 5-14). They are dist inct chemically from a l l other /48 P l a t e 5-13. Thin s e c t i o n view (1 cm wide, crossed p o l a r s ) of "Bendor" dyke, sample C1004 (4260' e l e v a t i o n , c o - o r d i n a t e s 5150E/12400N), showing o s c i l l a t o r y zoned p l a g i o c l a s e with s e r i c i t i z e d rim, and c h l o r i t i z e d hornblende r e l i c below i t . P l a t e 5-14. Thin s e c t i o n view (1 cm wide, crossed p o l a r s ) of lamprophyre dyke, sample C1033 (from main dump a t 8 l e v e l a d i t p o r t a l ) , showing l a r q e c l i n o p y r o x e n e ( y e l l o w ) , smaller b l o t i t e (green) and a p a t i t e (grey) phenocrysts i n matrix of f i n e r c r y s t a l s of the same minerals and g l a s s . /49 intrusive rocks in the area, containing more abundant Ba, Sr, phosphorus, potash and t i t a n i a (Table 5-2). The lamprophyre dykes, dated in this study by K-Ar on b i o t i t e (44 + 2 Ma), are the same age as the Rexmount porphyry (Fig. 1-2: Woodsworth, 1977), and perhaps the same age as mineralization at Blackdome (Faulkner, 1986). 5.3 Metamorphism 5.3.1 Facies All the rocks in the Bralorne area except the post-mineral Bendor and lamprophyre dykes are affected by low-grade, sub-greenschist to lower greenschist facies static or burial metamorphism, and show l i t t l e or no penetrative fabric. A weak fabric i s developed near some major faults, although this may only be an enhancement around shear zones that pre-date or accompany metamorphism. The characteristic metamorphic minerals in order of overall abundance are albite , quartz, s e r i c i t e (muscovite), chlorite, epidote, c a l c i t e , act inol i te and prehnite. The relative abundances are, however, variable depending on the composition of the p rot ol i th. Plagioclase i s replaced by albite and quartz, with lesser s e r i c i t e and c a l c i t e ; mafics are replaced by c h l o r i t e , epidote, c a l c i t e , a c t i n o l i t e and quartz. Prehnite occurs only in veins within rocks of the Bralorne intrusive suite. Metamorphic minerals are best developed in the greenstones, but even there the primary minerals often have not been completely reconstituted, and textures indicative of equil ibration, such as sharp grain boundaries or t r i p l e junctions, are seldom seen. Outside the Bralorne fault block, as the batholithic rocks of the Coast Plutonic Complex to the west or the Bendor intrusions to the east are approached (Fig. 1-2), hornfels aureoles are encountered (e.g. biotite-bearing rocks labelled "gneisses" by Church et a l . . 1988). Regionally, both Rusmore (1985) and Potter (1983) in Cadwallader Group rocks and Bridge River Group rocks respectively have described similar low-grade metamorphism. The mineral assemblages differ by being s l i g h t l y lower grade than those observed in the Bralorne block. Besides ubiquitous quartz, albite and opaques, Rusmore (19B5) describes: (1) a pumpel1yite-epidote-ch1 o r i t e -calcite+sericite, and (2) an actinolite-hydrobiotite-chl or i te-cal c i t e but pumpel1eyite-free assemblage. In regionally distributed Bridge River Group rocks, Potter (1985) describes similar sub-greenschist, prehnite-pumpellyite facies metamorphism, based on an assemblage of a lbite , c h l o r i t e , pumpel1eyite, s e r i c i t e and prehnite. The absence of act inol i te in the Bridge River Group rocks outside the Bralorne block suggests a s l i g h t l y lower pressure, sub-greenschist facies that formed at less than 4 kb and less than 325 degrees Celsius (see below). In the Bralorne block the coexistence, in some greenstone samples, of a c t i n o l i t e , epidote, chlorite and a l b i t e suggests that the metamorphic grade was l o c a l l y s l i g h t l y higher than in surrounding areas mapped by Rusmore (1985) and Potter (1983). These minerals are diagnostic of lower greenschist facies (Winkler, 1971). Pumpelleyite was not seen in any samples from the Bralorne block, and prehnite was p o s i t i v e l y i d e n t i f i e d only in veins cutting the Bralorne d i o r i t e . Actinolite-producing reactions responsible for the t r a n s i t i o n from sub-greenschist facies (pumpel1eyite-prehnite-chlorite assemblages) may be similar to those experimentally investigated by Nitsch (1971) and Schiffmann and Liou (1980) about invariant point X (Fig. 5-4). Their data suggests that at water pressures of 2 to 3 kb, t h i s t r a n s i t i o n occurs at temperatures of about 325 degrees Celsius. Such conditions are supported by f l u i d inclusion evidence which suggests that quartz in the veins was deposited at about 350 degrees Celsius, at pressures of 1.75 kb (Chapter 8). Geochronological evidence (Chapter 3) indicates that metamorphism took place at about the same time as mineralization. 5.3.2 Timing The timing of metamorphism in the Bralorne area i s constrained by evidence from the a l t e r a t i o n of several dyke suites (sections 3.6.1 and 5.1.4). U-Pb zircon and K-Ar whole-rock dating of these dykes indicates that peak conditions of metamorphism overlapped the intrusion of the /sz. 4 3 .-••"6 100 200 500 m TEMPERATURE ( ° C ) Figure 5-4. S t a b i l i t y r e l a t i o n s in subgreenschist to lowermost greenschist grade metamorphic rocks (from Potter, 1983). Probable s t a b i l i t y f i e l d for rocks i n the Bralorne block i s stippled; f i e l d for Bridge River Group rocks i s dark. Reaction 1 was experimentally determined by Liou (1971a). Reaction 2 was experimentally determined by Liou (1971b). Reactions 3,4 were calculated by Perkins et a l . (19B0). Reactions 5,6 are from Schiffman and Liou (1980). Reaction 7 i s from Nitsch (1971). Abbreviations: ab = a l b i t e , an = analcine, cc = c a l c i t e , ch = c h l o r i t e , , act = a c t i n o l i t e , t r = tremolite, laum • laumontite, lw = lawsonite, q = quartz, mg = margarite, pu = pumpel1eyite, pr = prehnite, zo = z o i s i t e , gr = grossular, cz -c l i n o z o i s i t e and v = vapour. / 5 i Bralorne albitite-green hornblende porphyry dykes (85-90 Ma), but concluded before the emplacement of the Bendor dykes (about 60 Ma) and dykes at the Congress Mine (about 70 Ma). This is close to the time of emplacement of the Coast Plutonic batholiths immediately west of Bralorne, which ranges fr om 85 to 7o Ma (Chapter 3 ) , and coincides with the Late Cretaceous magmatic episode described by Armstrong (1988) on the east side of the Coast Plutonic Complex in southern Brit ish Columbia. 5.4 Structure and Veininq Most of the detailed investigations carried out at Bralorne since i t s discovery have concentrated on structure. It i s the most important ore control, because a l l the veins are in faults. However, only a synopsis is presented here; full consideration i s beyond the scope of this thesis. The area hosting the Bralorne-Pioneer veins i s anomalous on a regional scale, fal l ing within a major northwest trending fault zone that extends discontinuously for over 100 km. The Bralorne block hosting the deposits l i e s near the center of this strike length, notably where the structure bends from northwesterly to more northerly (Fig. 1-2). It is bounded on the north by the Fergusson fault, which dips steeply (70-90 degrees) northeast, and on the south by the Cadwallader fault, which dips steeply (70 -90 degrees) southwest. The lozenge-like block thus formed tends to widen and lengthen at depth (Fig. 5-3). This block contains a l l the major known gold veins. Movement on these bounding faults at Bralorne is of unknown extent and sense, but i s constrained to the Late Cretaceous (100 to 85 Ma) by the results of this study (Chapter 3) and that of Rusmore (1985). This i s similar to the 100 to 85 Ma time interval postulated for thrust faulting in the area north of Bralorne (Garver et a l . , 1989) and 85-87 Ma for granodiorite which truncates the Tchaikazan fault northwest of E<ralorne (McMillan, 1976). The Tchaikazan fault is correlated with the Bralorne fault (Umhoefer et a l . , 1989). The Eiralorne-Pioneer vein system is outstanding in i t s 6 km strike length and 2 km (and open) depth extent. In this respect i t is similar to the major vein deposits of the Motherlode d i s t r i c t in California (Landefeld, 1988), the Canadian Shield (Colvine et a l • , 1984) and the Archean of Western Australia (Groves et a l . , 1984; 1987). All of these authors note the importance of major s t r i k e - s l i p shear zone environments as hosts for mesothermal gold vein mineralization, as also noted by Nesbitt and Muehlenbachs (1988). A major pervasive feature of the Bralorne block i s the general parallelism of structural elements. Bedding, intrusive contacts, long axes of the intrusive bodies, dykes, fold axes and foliation are a l l sub-parallel to the major northwesterly trending bounding faults that define the Bralorne shear zone, implying northeast-southwest directed compression. The main veins however are oblique to the /ss general trend of the shear zone (110° and 070 °) and appear to fit a simple Riedel shear zone model (Aydin and Page, 1984), as discussed below. The veins imply easterly-directed maximum compressive stress in a s t r i k e - s l i p regime that apparently preceeded the northeast-directed compressive regime (cf. M i l l e r , 1388). 5.4.1 Folding Primary layering in the Hurley Formation within the Bralorne block i s usually steeply dipping and westerly to northwesterly trending. Local dip reversals indicate that isocl inal folding may be present. A weakly developed fol iat ion, paral lel or subparallel to the bedding, can only be discerned in a r g i l l i t e s . The timing of folding is d i f f i c u l t to be sure of, but i s l i k e l y to be prior to dyke intrusion (below) since no dykes are observed to be folded. No major fold axes were seen on the Bralorne property, although at the P.E. Gold property, 2 km to the southeast of the Pioneer mine (Fig. 3-1) bedding attitudes suggest an antiformal axis plunging southeast (G. Nordine, pers. comm., 1986). Joubin's (1948) cross-sections support this; he postulated an a n t i c l i n a l axis running through the center of the Bralorne block. 5.4.2 Faulting and Dyking Most faults, including those hosting the dykes and veins within the Bralorne block, are paral lel or sub-/56 parallel to the bounding shears. The major, Permian Bralorne intrusions also l i e along the antiformal axis, parallel to the bounding shears. They may have been controlled along faults that paralleled this axis, or they may have been rotated into this orientation by later northeast-directed compression (in places, a weakly defined foliation is sub-parallel to their length). These major intrusive bodies have steep sided contacts along their length, but the contacts dip gently outwards at both the northwest and southeast ends. The intrusives also dyke out at their ends (Fig. 5-1). The Permian Bralorne intrusions were intruded much later by two sets of dykes: (1) a swarm of Cretaceous (91.4-85.7 Ma) albitite-green hornblende porphyry dykes that trend subparallel to the veins at 110 and 070 ° , and (2) Eocene (45 Ma) lamprophyre dykes oriented at 340 to 360 ° , approximately perpendicular to the veins. Cross-faulting along a more northerly direction could be largely later (possibly at the time of Fraser fault movement at 45-35 Ma: M i l l e r , 1988) because this direction paral lels features such as the steeply dipping, 45 Ma lamprophyre dykes, and the Empire and No. 1, 2 and 3 faults (Fig. 5-2), which offset the veins and a l b i t i t e dykes. Parts of the northerly fracture set must have existed prior to and during the mineralising episode, though, since several minor veins in the King mine—such as the C vein (Fig. 5-2), and portions of vein in the Empire fault—follow this direction. There i s also a suggestion of even later JS7 reactivation along the main "shear" vein direction, since in one place a lamprophyre dyke that cuts off the 51 vein has i t s e l f been faulted along the vein for S m (Cleveland et  a l . . 1938)• 5.4.3 Quartz Veins Morphology Two main vein types are recognized at Br al orne-F'i oneer, and a third subsidiary type is locally present. These are (Joubin, 1948; see Fig. 5-5a): (1) the main "shear" veins from which the bulk of the ore was mined, such as the 51, 55 or 77 veins at Bralorne, or the Main vein at Pioneer; (2) the so-called "cross-over" or "tension" veins, which contained erratic high-grade gold values, such as the Bralorne 75 and 83 veins (parallel to, but deeper than the 59 vein), and the Pioneer 27 vein; and (3) the "cross" veins such as the C vein in the King mine. The main "shear" veins have been traced continuously for up to 1,500 m along a strike of roughly 110 degrees azimuth, and for 1,800 m down a dip of about 70 degrees north (Fig. 5-5b). Their average width is 1 to 2 m (Bellamy and Saleken, 1983), although they pinch to a few centimeters, swell to as much as 7 m, or splay into horsetail or breccia structures (Cleveland et a l . . 1938). Ore shoots within the veins occupy only a small portion of the veins (20% or less, F i g . 5-5c), with the intervening /SB K I N G M I N E x„ N BRALORNE ^ \ \ \ Ml NE ^ ^ » ^ <rn'CROWN v—. (•.'CROW \.-> \ j CMMM SUA*T V?l MINE • *^ °, °ti ^  im *y »°0' Aft«r: P. JouOtn, 1945 BRALORNE PROJECT 111 E & B EXPLORATIONS INC. V E I N S Y S T E M S 1 a A T I O I M K T M f N T M A P I N O O NO. S C A U E i :24 /X»ar l">2000 ' FIG 5-5_a ... Figure 5-5<a). Plan view of the main vein systems in the Bralorne-Pioneer deposit. The 51 and 55 veins are believed to be offset-portions of the same vein across the Empire fault. Locations of the cross-section of Figure 5-5(b) and the longitudinal section of Figure 5-5<c) are shown. F I G . 5-5 b • j a Tt. tt* miWa •hi a J M to'-" • • • wuuimc MOJCCT atrrcH O F V E I N S L00OM N * Figure 5-5(b). Sketch of the major veins in the Bralorne section of the mine, in cross-section (looking to the northwest). The depth extent of the explored system i s almost 2,000 m. /60 " - ^ e s ' a n ^ ^ *5J Bralorne-Pioneer vein system Plunge of the ore shoots to t h i norJhwLt T J " 6 5 * ' 5 h ° W i n 9 s t ^ p relation of the ore *Z ? u 2 ° ^ W M t ' a n d t h e antipathetic shoots to th. - H t h * * n*iP*thetic portions often well below the average ore grade of 17 g/tonne (0.5 oz/ton) of gold. The largest ore shoot, in the 77 vein, extended for 300 m along strike by 1500 m down dip, and was open to depth at average mine grade and mining width. It was d r i l l e d off to 100 m below the lowest developed level by Bralorne Mines Ltd. before the mine closed in 1971 (Stevenson, 1958). The so-called (see section 5.4.4) "tension" veins strike roughly 070 degrees azimuth and dip about 75 degrees north. They are smaller structures (up to 500 m by 500 m) than the shear veins, with smaller ore shoots that are occasionally of very rich but often highly variable tenor (Joubin, 1948). They tend to form "cross-overs", or subsidiary structures between major veins, and also splay near their ends. They do not show the same strong evidence of shear motion during and after f i l l i n g (slickensides and r ecrystal1ized quartz) as the main shear veins do. However, later work by Poole (1955) demonstrated that the "tension" veins may be, in part, extensions of the shear veins (e.g.. the 75 vein is the faulted extension of the 53 vein (Fig. 5-2) across the Empire fault, and i s also therefore the same as the major 77B vein. This interpretation agrees with the Riedel shear model presented below in section 5.4.4. The so-called "cross" veins are oriented approximately perpendicular to the shear and tension veins, and are northerly-striking, steeply-dipping (55-80 degree) structures. They were not accessible at the time of this study, and so were not sampled, but when recently made accessible, they were v i s i t e d ; they looked similar to the other veins of the system. They have been described CHedley, 1935) as "discontinuous, rudely lenticular bodies of more or less fractured quarts between heavy gouge walls. . .with a re-cemented quarts breccia frequently evident". They were not important economically in either size or grade, with only 200 m strike and dip extent. To sum up, within the Bralorne block the productive veins cross the block obliquely (Fig. 5-1, 5-2 and 5-5a). The principal mineralized "shear" veins trend about 110 degrees azimuth and dip 70 degrees north (Fig. 5-5b); the "tension" veins trend about 070 degrees azimuth and dip 75 degrees northwest. The "cross" veins trend roughly north-south and dip 55-80 degrees west. Although they do not display mutual cross-cutting relationships, a l l these veins are s imilarly mineralized, implying coeval formation. If the "tension" veins are considered as lying partly along a conjugate shear direction rather than a true tension direction (with the other conjugate shear represented by the main shear veins: Fig. 5-6), then these relationships suggest an axis of roughly horizontal east-west maximum compression, with s i n i s t r a l motion on the bounding faults. Thus the veins at Bralorne are considered to be mineralized faults that i n i t i a l l y developed during an episode of s i n i s t r a l s t r i k e - s l i p faulting at or just prior to 90 Ma (c f. 100-95 Ma s i n i s t r a l motion on the Yalakom fault zone, /43 30 km northeast of Bralorne: M i l l e r , 1988'.). The evidence for the s i n i s t r a l s t r i k e - s l i p regime is presented in section 5.4.4 below. The age of the s t r i k e - s l i p regime is constrained to be prior to the 90-85 Ma age of mineralization, which appears to have developed (section 7.3) in a northeast-directed compressive regime (cf. the 90 Ma compressive regime evidenced by folding and thrusting in the Eldorado Basin 30 km north of Bralorne: Garver et a l . . 13891). Ore Controls Within the main shear vein set, individual oreshoots plunge steeply northwest (Fig. 5-5c: James and Weeks, 1961). This i s approximately perpendicular to str iat ions on the veins,,which plunge approximately 45 degrees to the east; steps on the fault surface indicate that the last movement was reverse and s i n i s t r a l . Although earlier movement appears to have been nearly horizontal, the sense of shear predicted on the vein faults i s also s i n i s t r a l (Fig. 5-6). It i s possible that movement along the plane of the veins may have been responsible for remobi1ization of gold into dilatant zones that were oriented roughly perpendicular to the direction of movement (c f. Badgley, 1959). There is also a tendency to richer mineralization in some of the more steeply-dipping sections of the vein (Campbell, 1975). These steeply inclined sections might have been oriented most favourably for reactivation during a mineralising episode, postulated to have followed formation of the vein faults when the stress regime had changed to one of compression rather than of s t r i k e - s l i p (Chapter 7) • The Bralorne intrusions (diorite and soda granite) primarily provided a large competent block of rock that sustained the fractures that hosted the gold quartz veins. In this respect the diorite and the Pioneer volcanics were the best host, apparently capable of maintaining fractures over great distances Cup to almost 2 km). The soda granite, however, as pointed out by Campbell (1375) was more b r i t t l e and weak, and responded to stress by forming an intense stockwork of many small fractures subsequently f i l l e d by veinlets similar to that seen in a typical porphyry copper stockwork. This competency contrast between rock types may have influenced the orientation and hence grade and width of ore shoots. Thus, most of the ore was derived from that portion of the Bralorne composite.stock next to the Cadwallader fault; veins die off away from i t , particularly where they cross into soda granite. Campbell (1375) suggested that the ore shoots were peripheral to the bodies of soda granite (see Fig . 5-5c). However, this is possibly because the weaker nature of the soda granite meant i t could not sustain the large fractures that are prevalent in the d i o r i t e and greenstone. It cannot be because the soda granite was the source of mineralization, as he claimed at that time, since the soda granite i s much earlier than the mineralization (Chapter 3). Veins passing beyond the competent intrusions and greenstone into f i s s i l e argil l i t e tend to die out, as pointed out by Cleveland et a l . (1938) and Joubin (1948). Another important l i thologic control is exerted by the serpentine belt along the Cadwallader fault zone. It has been suggested (Joubin, 1948; Poole, 1955; Stevenson, 1958; James and Weeks, 1961) that the serpentine had a "damming" effect on the hydrothermal solutions ( i . e . . the serpentine was less permeable and acted as a barrier to fluid flow), resulting in increases of gold grades towards the contact. Another possible explanation would be the increase in iron in the diorite near the serpentine due to border-phase contamination (the a v a i l a b i l i t y of Fe may control the precipit iat ion of pyrite, to which the precipitation of Au i s related: see sections 5.3 and 10.3). In the longitudinal section of Figure 5-5c, the ore shoots overlap very l i t t l e (Skerl, 1956, iri James and Weeks, 1961). Veins tend to "horsetail" before being repeated en  echelon in the next productive shear, or towards a junction with other veins (Campbell, 1975; James and Weeks, 1961). Occasionally, the widest ore is found at intersections of sub-branches with main veins (Cleveland et a l . . 1938), but vein junctions are not always the loci of richer ore shoots, as can be the case in other vein deposits (McKinstry, 1948). Furthermore, there appears to be a roughly horizontal barren gap at about 1000 m depth (20 level) where values in an upper set of veins (the 51 vein) drop off before increasing again in deeper veins (the 77 vein at Bralorne). This was noted by Joubin (1948) and can be seen in Figure 5-5c (after James and Weeks, 1961). 5.4.4 Riedel Shear Model The early attempt by Joubin (1948) to rationalize a l l the structural data and reconstruct the stress regime responsible for the observed vein pattern has several inconsistencies in the light of modern structural interpretation. Two specific problems with his interpretation (see Fig. 5-6a) are : (1) the angle 2 theta between the two major shear directions is much too large (actually obtuse, about 100 degreees, as shown by Joubin) compared to the 60 degrees currently known to be characteristic of most b r i t t l e rocks, and (2) the direction of principal stress predicted by such a model, oriented northeast-southwest, implies an east-west s i n i s t r a l or north-south dextral shear direction, neither of which i s compatible with the observed northwest-trending major shear zone bounding the Bralorne block. It has recently been demonstrated, however, that at least in some situations where conditions are favourable (where fluid pressure exceeds l i thostat ic load), fractures may develop at greater 2 theta angles of up to 110 degrees (Sibson et a l . , 1988). A re-interpretation i s presented in Figure 5-6b. In this interpretation, the main "shear" veins (Bralorne 51 and 77; Pioneer Main) are s t i l l believed to represent one of the /67 Figure 5-6. Alternative structural interpretations for the Bralorne mineralized fault systems. In (a), the interpretation of Joubin (1948) is reproduced; i t would not predict strike-slip motion on the bounding Cadwallader and Fergusson faults, and predicts a 2 theta of over 90°. The Riedel shear model <cf. Aydin and Page, 1984) is in (b), showing the main shear veins (51, 77) developed along the R direction, and the less well-developed R' direction along the 59 and 73 veins, both with a tendency to splay into the extensional direction, as shown in (c). Implied maximum compression i s about 090° azimuth, and movement on the bounding faults i s sini s t r a l shear. /4>8 shears of a conjugate shear set, or the R direction of a Riedel shear model (modified after Aydin and Page, 1984). The other direction, R', i s not so well developed but may be represented by the so-called "tension" veins such as the Bralorne 59 (Fig. 5-5a) and the parallel a l b i t i t e dykes in the area east of the King mine area (Fig. 5-1). It is immediately apparent from the diagram (Fig. 5-6b) that the 2 theta angle between these R and.R' directions at Bralorne is less than the 60 degrees predicted from failure measurements on most rocks. This may be explained with reference to Figure 5-6c, in which the two shear vein directions are shown to be the average of a series of shorter segments, alternating between the appropriate shear direction and the tension direction that bisects the angle 2 theta between the shears, and l i e s along the axis of maximum compression. This may be seen in detailed views of the veins in old mine plans from the 15, 16 and 20 levels, one of which is reproduced in Figure 5-7. Here the veins appear to alternate from the extension direction to the shear direction, resulting in an overall 2 theta angle less than 60 degrees. The R and R' directions are better developed in different parts of the mine, with the former best shown in the main Bralorne mine area, and the latter in the King mine area, where the major bounding shears swing northerly. Such uneven development was observed by Morgenstern and Tchalenko (1971). The development of en echelon shears, with the set closer to the main bounding shear direction being better /69 Figure 5-7. Detail of quartz vein pattern, 20 level, Bralorne Mine, near Crown shaft, showing the tendency of veins in both the main R direction (77 vein) and the subsidiary R' direction (73 vein) to splay into the extensional direction bisecting the angle between them. The 51 vein shows segments along both R and R' directions as well as the extensional direction. /7o developed, has also been predicted by the clay modelling experiments of Wilcox et a l . (1973), and is also similar to the configurations of " s t r i k e - s l i p duplexes" (Woodcock and Fischer, 1986). The "P' direction of such a model is about 15 degrees to the direction of the major shears bounding the zone. Several features, notably the major green hornblende porphyry dyke near the northeast side of the block, and a major a l b i t i t e dyke paral lel ing the Ida May (or 55) vein at surface, are in this orientation, although it is d i f f i c u l t to say i f they reflect the P direction or the major external shear (D) direction. This is not unexpected, since many such fractures parallel to the overall shear zone are also revealed by the studies of Morgenstern and Tchalenko (1971). The Riedel model, which was original ly developed in 1929, has also been applied to Archean lode gold deposits in the Canadian Shield (Roberts, 1987). He noted that, as at Bralorne, the shear veins in the R direction are generally larger and more economically significant than the extension veins, and also described the development of the P (pressure) set and a D set (parallel to the shear zone boundaries). Other authors have also applied the Riedel model to Archean lode gold deposits (see Roberts, 1987). The Riedel model was recently successfully applied to the San Andreas fault zone by Aydin and Page (1984). Their Figure 5B might also help to explain the north-trending "cross" veins and cross-faults such as the Empire or No. 1, / 7 / 2 and 3 faults at Bralorne (Fig. 5-5a>, as west-dipping thrust faults approximately perpendicular to the axis of maximum compression. Although the last movement on these faults was reverse, composed of 240 feet of dextral strike s l i p and 370 feet of dip s l i p (Poole, 1955), as shown by the offsets of the 51 and 77 veins and the Fergusson and Cadwallader faults by the Empire fault (Fig. 5-1), the faults may have formed i n i t i a l l y as thrusts and later were reactivated. Minor amounts of gold-bearing vein material along these north trending faults (C vein; Empire fault) points to their long-lived history. Their latest movement may represent Tertiary dextral s t r i k e - s l i p movements related to the Fraser-Straight Creek fault (40 Ma: c f. M i l l e r , 1988). The Riedel model for the Bralorne block ( i . e . , for the entire northwest-trending major fault zone) predicts a direction of maximum compression about east-west and the major shear direction directed northwesterly. The east-west compression i s compatible with the supposed stress regime in the Late Cretaceous, as North America moved westwards with respect to the Pacific plates and accretion of al1ochthonous terranes, begun in the Middle Jurassic (Rusmore et a l . . 1988), was completed. However, the sense of shear predicted by such a model i s s i n i s t r a l ( l e f t - l a t e r a l ) rather than the dextral (right- lateral) normally associated with Cordilleran tectonics. Two possible explanations are: (1) the sense of translation at this particular location was different in the Late Cretaceous to what i t has been since, or (2) the bulk overall translation of material was dextral (up the coast of North America), but there were local areas where the sense of shear was the opposite. Similar pre-Cenozoic l e f t -lateral s l i p has also been suggested for the Yalakom fault, 30 km northeast of Bralorne (Miller, 1988), and on the Pasayten fault in northern Washington (Lawrence, 1978; C. Greig, pers. comm., 1989). F i n a l l y , the Riedel model requires a s t r i k e - s l i p regime with the minimum compressive stress directed horizontally (north-south); this is inappropriate for the model for vein formation (the actual f i l l i n g of the veins) developed in Chapter 7, which requires the least stress to be v e r t i c a l , in a northeastward-directed compressive regime (Sibson et  a l . . 1988). At Bralorne, such northeast compression i s appropriate to subduction associated with the emplacement of the Coast Plutonic complex, and is indicated by the generally northwest strike of bedding and fold axes (which do not f i t with an east-west maximum compressive stress). Thus at Bralorne i t appears that the major bounding shear zone, and the fractures that later became mineralized, developed in a s i n i s t r a l s t r i k e - s l i p regime prior to 90 Ma, and this was followed by mineralization of these pre-existing faults that were reactivated in a compressive regime at about 90-85 Ma. Miller (1988) suggested such a l e f t - l a t e r a l strike s l i p regime on the Yalakom fault from 100-95 Ma, and postulated that i t preceeded compression, /73 although he ascribed compression to the time of Fraser fault movement. Other workers in the Bridge River area, however, have found evidence (thrust faulting and folding.) for a northeastward-directed compressive regime at about 90 Ma (Garver et_ al . . 19B9; Schiarizza et a l . , 1989), which was then followed by a s t r i k e - s l i p regime between 85 and 65 Ma. It i s possible that the strike s l i p and compressional regimes were part of one continuous or progressive deformational event, with the compressional event strongly overprinting a relat ively weak, earlier transpressional event ( J . I . Garver and M.A. Bloodgood: pers. comm., 1989). 5.5 Sequence of Events The geology of the Bralorne block appears to record the following sequence of events: (1) Early Permian ocean crust, formed by ocean floor-spreading and represented by the President ultramafic s l i c e s , was emplaced into higher crustal levels, as shown by i t s intrusion by the Bralorne d i o r i t e - soda granite complex. <2) The rocks in (1), variably basement to and possibly intrusive into the Bridge River Group and Cadwallader Group, represent respectively a back-arc basin and an offshore volcanic arc in Permian to Jurassic times. (3) The Bridge River and Cadwallader terranes, containing a l l the l i thologies l isted in (1) and (2), were accreted to North America in the Jurassic. /7<r (4) Development of an easterly-dipping continental <". Andean-type) subduct ion zone produced the magmatism of the Coast Plutonic Complex. One episdode in that long period of magmatism was intrusion of the Bralorne dykes, which was accompanied by greenschist facies metamorphism and major veining, alteration and mineralization, probably at the time of major movement on the Bralorne fault system in the Late Cretaceous at around 90 to 85 Ma. As at the giant Kalgoorlie gold vein deposits in Western Australia ( P h i l l i p s , 1986), where gold mineralization i s found i n , but not temporally related to, the Golden Mile dolerite, at Bralorne the Early Permian diorite-soda granite complex i s closely associated with but not temporally related to gold mineralization (Chapter 3). (5) Magmatism after the orogenic peak, accompanied by lesser mineralization, transgressed eastwards, producing the post-mineral Bendor batholith and dyking, and f inally lamprophyre dykes at around 45 Ma. These also f i l l e d late, north-trending structures. (6) The accreted terranes were disrupted and separated from their correlative terranes along the Fraser fault (possibly evidenced by the north-trending Empire fault at Bralorne) by Early Tertiary dextral s t r i k e - s l i p movement. CHAPTER 6 PETROGRAPHY AND CHEMISTRY OF WALLROCK ALTERATION A striking feature of the Bralorne-Pioneer deposit i s the widespread and intense wallrock alteration that accompanied vein formation and gold deposition. Vein envelopes vary from l e s s than 0.1 m up to 10 m wide, and in places coalesce to form intense .alteration envelopes up to 50 m wide. The dominant envelope alteration i s carDonation, although si 1 ici f ication and s e r i c i t i z a t i o n are common -immediately adjacent to the veins. Biotite alteration, rare in the Bralorne mine, i s more common at the Pioneer and at the P.E. Goid property, 2 km southeast of the Pioneer. Mineralogical and textural features were observed in about 300 thin and polished sections in 10 detailed traverses across the wallrock alteration at various levels from surface to 2 km depth in the Bralorne deposit. Mineralogical zoning around the veins is described in section 6.1. Estimated modal compositions of the altered rocks are summarized in Table 6-1, and the detailed data i s in Appendix 1, Table A - l - 1 . Whole-rock chemical analyses were also obtained for each traverse across altered wall -rocks, and these data are in Table A - l - 1 . Chemical losses and gains in the altered rocks around the veins were investigated with a Pascal program for personal computers, developed for the purpose, that produced bar diagrams of constant-volume loss and gain for each 176 TABLE 6-1s Summary of modal mineralogy 1 for primary rocks and alteration envelopes around the Bralorne veins. Data used to construct this table are in Appendix 1, Table A - l - 1 . Unaltered (modal volume ! Altered (modal volume 7.)^ Di or i te Soda Sranite Outer Central Inner Fresh Fr esh c l - ep cb-ab -ms qz -ms -cl DI SS DI SG DI SQ qz 10 37 5 43 12 40 25 50 ab 55 52 30 42 30 35 5 5 hb 10"* 5 3 ms 10 •—J 13 7 30 25 ca 15 w 13 15 10 ak 5 20 7 cl 20 7 12 bi 2 3 ep 10 ox <1 2 1 1 l tr tr sx tr 1 1 1 Jul 5 o 1 Abbreviations are as follows: qz = quartz, ab = albite, hb = hornblende, ms = s e r i c i t e (muscovite), cb = carbonate (ca = c a l c i t e , ak = ankerite), cl = c h l o r i t e , bi = b i o t i t e , ep = epidote, ox = oxides (ilmenite, r u t i l e , leucoxene, sphene), sx = sulfides (pyrite, arsenopyrite, pyrrhotite, chalcopyrite): DI = d i o r i t e and SG = soda granite. 2 Averaged from thin-section estimates in Table A - l - 1 . 3 Not common. "* A l t e r e d equivalents. . . . / 7 7 traverse across altered envelopes (section 6.3). In order to interpret these chemical changes, chemical compositions of both precursor and alteration minerals were determined with an electron microprobe (section 6.2). 6.1 Mineraloqical Zoning Unaltered wall rocks display lower greenschist or subgreenschist facies metamorphic mineral assemblages, as described in section 5.3. Estimated average modal compositions of the unaltered rocks are in Tables 5-1 and 5-2. The three principal rock types hosting veins are the Bralorne d i o r i t e , soda granite and Pioneer greenstones. Although the latter i s unimportant in the Bralorne mine, i t forms the major ore host in the Pioneer section of the mine. These three rock types vary in their response to alteration, as discussed below. However, in general there i s a consistent zoning of alteration minerals from unaltered wall rock inwards towards the vein over an average distance of 5 m. The sequence i s usually from an outer green c h l o r i t e -epidote zone, through a central buff carbonate-albite+sericite zone, to an innermost cream-coloured quartz-sericite(+ fuchsite)-carbonate zone. The composition of the carbonate changes from calci te to ankerite as the vein i s approached. Associated with the mineralogic zoning, there i s a progressive destruction of texture as the vein is approached. The chlorite-epidote zone faithful ly preserves the original rock textures, as does the outer portion of the carbonate zone. In the inner portion of the carbonate-albite+sericite zone, the original rock texture disappears, and the innermost quartz-sericite-carbonate zone i s commonly a foliated rock, local ly becoming a "paper schist". This sequence of alteration i s l i k e that in other mesothermal gold quartz vein deposits. The zonal arrangement i s similar to that described by Robert and Brown (1986) at the Sigma Mine in Quebec, by Kishida and Kerrich (1987) at the Kerr-Addison Mine in Ontario, and by Albino (in press) for the Motherlode d i s t r i c t in Cal i fornia. 6.1.1 Chlorite-Epidote (Outer) Zone The transition from unaltered wall rock to the outer chlorite-epidote zone i s gradational and often poorly defined for several reasons. F i r s t , a l l the rocks at the mine contain some chlorite after original mafics (hornblende or c1inopyroxene) due to greenschist facies metamorphism. Secondly, much of the epidote i s part of the early widespread "barren" alteration accompanying the dist inct ive quartz-epidote-carbonate-prehnite stockwork in the d i o r i t e and soda granite (Plate 5-5), and thus formed prior to the main alteration-mineralization stage. Also, alteration effects are weaker in the outer part of the chlorite-epidote zone. Consequently, the sharp l imits characteristic of the inner alteration zones are not developed. /79 Chlorite, the characteristic mineral of this outer zone, consists of fine grained, dark green, scaly aggregates usually replacing primary hornblende in the diorite and soda granite. The chlorite is commonly intimately intergrown with lesser amounts of c a l c i t e and s e r i c i t e (Plate 6-1). Chlorite forms up to 30"/. (Table 6-1) of the rock in the altered d i o r i t e , but rarely over 57. in the soda granite. This reflects the more mafic character of the d i o r i t e . There is also a difference in the type of chlorite in the two rocks. In thin section, chlorite in the d i o r i t e i s usually fine (0.05 mm), and lacks pleochroism or anomalous interference colours. In contrast, chlorite in the soda granite i s coarser (flakes up to 0.5 mm long), bright green and pleochroic, and displays strong purple to blue anomalous interference colours. These optical characteristics suggest that the chlorite in the d i o r i t e is a r i p i d o l i t e (prochlorite) with higher Mg and A l , and less Si than the more Fe-rich variety found in the soda granite (Kerr, 1359). This i s supported by the mineral chemistry (section 6.2) and by the more mafic (Mg-rich) character of the d i o r i t e compared to the soda granite." In thin section, epidote usually appears to be an early mineral replaced by the later c h l o r i t e , carbonate, s e r i c i t e and quartz. It most commonly occurs as fine grained, anhedral, semi-translucent grains about 0.01-0.1 mm in diameter. The dark brown colour in plane polarized light appears to be due to minute dark inclusions or alteration of /80 P l a t e 6-1. C h l o r i t e ( + c a l c i t e and epidote) a l t e r a t i o n i n d i o r i t e (lower piece of core) and soda g r a n i t e (upper p i e c e ) . Large pale green patch ion d i o r i t e may be a remnant of pre-mineral epidote stockwork (see P l a t e s 5-4 and 6-2). From DDH-SB-84-36/675'. P l a t e 6-2. Thin s e c t i o n view of massive epidote ( b r i g h t c o l o u r s , r a d i a t i n g ) - c a l c i t e a l t e r a t i o n of d i o r i t e i n sample C117-10, from Bralorne Mines L t d . DDH 32Q-132/90 1, 1 m i n t o the hangingwall of the 79 v e i n on 32 l e v e l . Width of f i e l d of view i s 1 cm; crossed p o l a r s . unknown identity; the epidote i s darkest where i t is most altered. In places where the epidote is particularly well-developed, or massive, i t forms a pale creamy, dense (specific gravity about 3.1), very hard rock. An example would be at 995' or 1276' in DDH-B11-80 at Bralorne, where it looks l i k e a separate (pegmatitic) intrusive phase but i s actually an alteration product. In these zones the epidote-group mineral (Plate 6-2) is relat ively clear, coarse (up to 1 mm across), with l i t t l e or no yellow pleochroism, and may consist of epidote intermixed with Fe-poor c l inozoisi te that replaces the margins of the epidote. Such areas of massive epidote alteration are probably the product of areas of unusually intense, early "barren" quartz-epidote-carbonate-prehnite stockworking that escaped later alteration. 6.1.2 C a r b o n a t e - A l b i t e - S e r i c i t e ( C e n t r a l ) Zone This zone can be divided into an outer sub-zone where the original rock texture i s preserved, and an inner sub-zone where the texture i s destroyed. The outer sub-zone, produced by the onset of carbonate alteration as a vein i s approached, i s distinguished in the d i o r i t e as a buff replacement of hornblende crystals that preserves the original rock texture but makes the rock look l i k e a different rock type (Plate 6-3a). This transition from the outer chlorite-epidote zone to the onset of carbonate alteration roughly corresponds to the "cryptic-v i s i b l e " boundary of Robert and Brown (1986). Their P l a t e 6-3. (a) Buff carbonate a l t e r a t i o n ( c a l c i t e ) a f t e r hornblendes i n the d i o r i t e (top row of core; a l t e r a t i o n i n c r e a s e s from outer zone ( c h l o r i t i c ) a t l e f t to outer sub-zone of the c e n t r a l zone ( c a l c i t e -a l b i t e - s e r i c i t e ) at r i g h t ; rock t e x t u r e i s s t i l l l a r g e l y p r e s e r v e d . Lower rows of core are soda g r a n i t e . From DDH-SB-84-36/370 1. (b) Orange-brown weathering a n k e r i t i c carbonate a l t e r a t i o n near a v e i n i n DDH-SB-84-36/75G" . In lower piece of core, t e x t u r e i s p a r t i a l l y destroyed as a l t e r a t i o n i n c r e a s e s to the r i g h t , c r o s s i n g from the outer to the inner sub-zone of the c e n t r a l zone ( c a r b o n a t e - a l b i t e -s e r i c i t e ) . "cryptic" c h l o r i t e - c a l c i t e - s e r i c i t e zone i s similar to the outer chlorite-epidote zone at Bralorne, in that i t does not markedly change the hand specimen character of the rock but is obvious in thin section. The buff or creamy inner sub - z o n e lacks the original igneous texture because the texture is progressively destroyed as alteration intensity increases towards the vein. This sub-zone commonly weathers a rusty orange-brown due to the increasing amounts of ankeritic alteration carbonate as the vein i s approached (Plate S-3b.'>. The ankeritic carbonate does not react as strongly to cold dilute HC1 as does the calcite that predominates farther from the vein. The carbonate gives only small X-ray diffraction peaks for ankerite (the range of ankerite-calci te ratios i s i l l u s t r a t e d in Fig. 6-1 a to d), but the iron and magnesium contents are confirmed by microprobe analyses (section 6.2.1). Carbonates are usually the most abundant alteration minerals in the central zone in altered d i o r i t e , but are less common in altered soda granite (Table 6-1). Coarse secondary albite i s next in abundance, and forms grains up to 0.5 mm across, with replacement textures, which include "chessboard", "patchwork" and "irregular" (untwinned) albite (Battey, 1951; Leitch, 1981). Albite is more prevalent in alteration of the soda granite; this reflects the differences in original composition between the more mafic diorite and the more s i l i c i c soda granite. The secondary Figure 6-1. X-ray diffraction scans of carbonate minerals from the intensely altered inner zone. Ca) is from sample SB-B4-49/794.5*, <b> is from the same hole at 795'; both show the small peak for ankerite compared to calcite. (c) i s from sample 19-51 FW1, and shows a larger ankerite peak; Cd) in sample C080 shows ankerite i s more abundant than calcite. Note that in (a) and (b) f full-scale deflection i s 4xlO a counts, but in <c) and (d), i t i s 10s" counts. /8S albite i s often d i f f i c u l t to distinguish from the ubiquitous albite of both diorite and soda granite that characterizes their lower greenschist facies metamorphism, except by the textures l isted above, or by i t s more obvious habit in veins and envelopes to veins. Although textures in thin section (Leitch, 1981!) suggest the vein- and fracture-controlled secondary albite is about A n 0-ai i t s composition i s indistinguishable (about A n 2 _ 3 : microprobe analyses in section 6.2.2) from the widespread metamorphic albite, which also appears in thin section to be about An B-to-Carbonate (up to 657.!) and albite (up to 457.) occur together in variable proportions, with lesser amounts of s e r i c i t e , throughout this central alteration zone (Table 6-1) . Chlorite remnants from the outer zone may not be completely replaced and may make up to 157. of the rock. Plate 6-4 (a) and <b) i s typical of this alteration in the central zone. Ankeritic carbonate forms anhedral interlocking grains ranging from 0.1 to 0.5 mm diameter, with higher r e l i e f and s l i g h t l y cloudy character compared to the c a l c i t e , which is commonly associated as clearer, coarser grains (Plate 6-4b). Composite grains are also common (section 6.2.1), in which the clear cores are c a l c i t e , while the rims are ankerite. Other grains are composed of ankerite cores with siderite rims (see Fig. 6-2) . The lack of a strong X-ray peak for ankerite (Fig. 6-1 a,b) may be due to the volumetrically minor ankeritic rim to the composite grains. However, these rim portions may be /86 P l a t e 6-4. (a) Thin s e c t i o n view of outer sub-zone of c e n t r a l zone a l t e r a t i o n ( c a l c i t e , with high b i r e f r i n g e n c e ; a l b i t e , grey-white; s e r i c i t e , white f l a k e s ; c h l o r i t e , green. Sample C116-21, from DDH 41Q-144/212', 3 m i n t o hangingwall of 79 v e i n on 41 l e v e l . Width of f i e l d of view i s 1 cm; c r o s s e d p o l a r s . (b) Thin s e c t i o n view of inner sub-zone of c e n t r a l zone a l t e r a t i o n ( l a r g e p o r p h y r o b l a s t i c g r a i n s of c a l c i t e with b r i g h t e r a n k e r i t e rims, s e t i n a m a t r i x of s e r i c i t e and c a l c i t e , plus minor c h l o r i t e t h a t shows low b i r e f r i n g e n c e ) . Sample C117-1, from DDH 32Q-132/85.5', 0.2 m i n t o f o o t w a l l of 79 v e i n on 32 l e v e l . Width of f i e l d of view i s 1.2 cm; crossed p o l a r s . /87 most exposed to weathering, perhaps explaining the prominent orange-brown weathering colour to this zone. Thus a favourable exploration guide in weathered d r i l l core would be an orange-brown colour of the oxidised rock. In fresh d r i l l core, the carbonate should be tested with dilute HC1, and i f i t reacts strongly, it is probably distant from a productive vein. A further check for ankerite would be a blue stain with potassium ferricyanide (Warne, 1962). 6.1.3 Q u a r t z - S e r i c i t e - C a r b o n a t e CInner) Zone Intense replacement by fine-grained secondary quartz, s e r i c i t e (+ fuchsite) and ankeritic carbonate characterizes the highly altered envelope adjacent to the vein. This zone i s up to 50 cm wide, with schistose or occasionally brecciated textures commonly developed. The foliated texture and intimate intergrowth of the minerals, which average about 0.1 mm across, i s i l l u s t r a t e d in d r i l l core in Plate 6-5a and in thin section in Plate 6-5b. In this inner zone quartz forms up to 60'/. of the rock, although much of t h i s , especially in the soda granite, i s secondary only in terms of i t s texture. It shows sub-grain development: that i s , i t has been isochemically recrystal1ized, rather than having been added to the rock. Chemical analyses, corrected for volume changes (section 6.3) confirm this observation, although sometimes the unavoidable inclusion of minor quartz veins in the sample causes large fluctuations in SiOss content (c f • Sketchley and S i n c l a i r , 1986). Plate 6-5. (a) Section of core around the 51B FW vein in DDH SB-84-42/625' (cross-section 11,200E), showing the progressive destruction of texture in both the soda granite (top) and d i o r i t e (bottom) from a coarse grained, massive rock to a f o l i a t e d , highly q u a r t z - s e r i c i t e -carbonate altered rock in the inner zone. The position of the vein i s at the gap (where the quartz vein was sampled whole) at the black f e l t -t i p mark on the core divider. The increasing brown stain due to ankerite closer to the vein is also apparent. (b) Thin section view of f o l i a t e d , intensely quartz-ser icite-carbonate altered inner zone rock, with cubic pyrite, from immediate footwall of the 51 vein, 8 l e v e l , near the Empire shaft (9600E, 6100N). F i e l d of view i s 1 cm wide; crossed D o l a r s . /89 S e r i c i t e (fine-grained (Muscovite:, analyses in Table A-2-2c) occurs as pale green to yellowish masses composed of fine flakes less than 0.05 mm long. It completely replaces original plagioclase, and forms up to 307. of the rock. Carbonate includes s iderite , ankerite and minor Fe-calcite (section 6.2.1), with the siderite forming rims around ankerite and Fe-calcite cores. The modal estimates of these two minerals in Table 6-1.are tentative because of the d i f f i c u l t y in distinguishing them in thin section; even SEM studies failed to confidently identify them, and microprobe analyses had to be used. Bright green chrome mica (fuchsite) i s locally present, forming up to 5% of the rock. Although Cr was not analysed for, these green micas always rim cores of chromite (identified in reflected l i g h t ) , leaving l i t t l e doubt that i t i s fuchsite. Green mica, variably identified as Cr-bearing, is also known at other meosthermal gold vein deposits such as the Casa Berardi (Pattison et a l . . 1986) and Lac Shortt"(Morasse et a l • . 1986), both in Quebec. 6.1.4 Other A l t e r a t i o n Types S i l i c a Flooding Quartz-albite alteration i s suff iciently different to warrant separate description. One expression of such alteration, " s i l i c a flooding", forms a dist inct ive zone, mainly in the central portions of the large dyke-like mass of soda granite lying northeast of the d i o r i t e (Fig. 5-1), and in grey plagioclase porphyry and a l b i t i t e dykes cutting the soda granite there. This alteration i s fracture-controlled, and grades in intensity from a sparse stockwork of quartz-ser i c i te-carbonate-ch1 or i te-epi dote-albite microveinlets, to a brecia composed of a network of the same minerals enclosing fragments of soda granite (Plate 6-6). Sections of such alteration are well exposed in diamond d r i l l core, in which the typical s i l i c a "crackling", or stockworks of h a i r l i n e quartz-albite-pyrite veinlets in the soda granite, becomes increasingly strong. This leads eventually to a rock composed almost entirely of quartz and albite, with some s e r i c i t e developed after albite, and pyrite after mafics. In these areas, alteration (i l lustrated in Plates 6-7a to 6-7d) proceeds from "chessboard" albite through veinlet-controlled secondary albite to a " s i l i c i f e d " , stockworked soda granite and finally to an intensely " s i l i c i f i e d " white rock. In thin section, this " s i l i c i f i e d " rock i s composed of an unusual symplectic intergrowth of quartz and a l b i t e . These intergrowths are up to 3 mm across, and have a radiating pattern with crudely hexagonal shape, probably controlled by quartz crystallography as the quartz and albite crystal l ized simultaneously. Such textures locally completely destroy the original texture of the soda granite, grey plagioclase porphyry (Plate 6-7e) and a l b i t i t e dykes; since the alteration i s not restricted to the soda granite, and i s /$>/ P l a t e 6-6. "Crackled" soda g r a n i t e , cut by a stockwork of f i n e - g r a i n e d q u a r t z , s e r i c i t e , c a l c i t e , e p i d o t e , p y r i t e and minor c h l o r i t e . Sample C104, from DDH 8E-1174/880•; c r o s s e d p o l a r s , f i e l d of view 1 cm. P l a t e 6-7. (a) Chequerboard a l b i t e r e p l a c i n g a large a l b i t e c r y s t a l i n soda g r a n i t e . Sample C062, from DDH UB-84-25/365 1. F i e l d of view i s 0.26 cm; c r o s s e d p o l a r s . /9Z. (b) V e i n l e t - c o n t r o l l e d a l b i t e (shades of grey, some vaguely twinned, the r e s t " i r r e g u l a r a l b i t e " of L e i t c h , 1981) with f i n e r -g rained q u a r t z ( w h i t e ) . Sample C188, from DDH SB-14-80/492'. F i e l d of view i s 1.2 cm; c r o s s e d p o l a r s . (c) S i l i c i f i e d , b r e c c l a t e d soda g r a n i t e showing stronger q u a r t z - a l b i t e a l t e r a t i o n than i n P l a t e s 6-6 or 6-7(a-b). Large f r a c t u r e d q u a r t z phenocryst v i s i b l e ; sample C190, from DDH S B-13-80/130*. F i e l d of view 1.2 cm; crossed p o l a r s . /93 (d) Primary t e x t u r e of soda g r a n i t e r e p l a c e d by s y m p l e c t i c overgrowths of quartz and a l b i t e i n a r a d i a t i n g , psuedohexagonal p a t t e r n . From sample C187 ( D D H S B - 1 4 - 8 0 / 9 3 ' ) ; f i e l d of view i s 1 cm; crossed p o l a r s . (e) Primary igneous phenocryst of p l a g i o c l a s e i n a grey p l a g i o c l a s e porphyry that has been p a r t i a l l y r e p l a c e d by a s y m p l e c t i c overgrowth of q u a r t z and a l b i t e . From sample C193 (DDH S B - 1 2 - 8 0 / 2 0 8 1 ); f i e l d of view 1 cm wide; crossed p o l a r s . /94 later than the a l b i t i t e dykes, i t appears hydrothermal. However, there are no well-defined quartz veins or gold values associated with such s i l i c a flooded zones. Biotite Alteration Biotite alteration distributed along fractures, and therefore hydrothermal, occurs at Bralorne both near the surface around the '51' vein and at depth around the '77' vein (Fig. 5-1). It i s much more common to the southeast on the Pioneer (Joubin, 1948) and P.E. Gold (Nordine, 1983) properties. The Pioneer greenstones, prevalent on these two properties, alter commonly to b i o t i t e , as do the green hornblende porphyry dykes near the '51' vein on 4 Level at Bralorne. Otherwise, b i o t i t e has only been seen below 40 Level (1,700 m depth) at Bralorne in altered d i o r i t e (Plate 6-8). The occurrence of b i o t i t e may be similar to that at the Sigma mine in Quebec (Robert and Brown, 1986) where biotite only becomes prominent as an alteration mineral in the lower levels of the mine. Consequently, the Pioneer and P.E. Gold properties might rperesent deeper levels of the vein system than at Bralorne. Unfortunately the biotite is always associated with chlorite and s e r i c i t e , and i s not suitable for isotopic dating (J. Harakal, pers. comm., 1987). P l a t e 6-8. (a) B l o t i t e a l t e r a t i o n of d i o r i t e i n sample C128-4 (DDH 44 Q-171/350*) from 1 m i n t o the hangingwall of the 77 v e i n on 44 l e v e l . The b i o t i t e i s brown, and a l b i t e i s white; c r o s s e d p o l a r s , f i e l d of view 1 cm. (b) B i o t i t e a l t e r a t i o n ((brown, f l a k y ) ) of Pioneer greenstone i n sample C1040-1 ( T a y l o r X-cut, F i g . 5-1). Quartz i s white, and l a r g e carbonate g r a i n i s brown. Crossed p o l a r s , f i e l d of view i s 0.26 cm a c r o s s . /94 Black Carbonate Alteration A dist inct ive black calcium carbonate alteration i s occasionally present, most often in the soda granite but occasionally in the d i o r i t e . It begins along hairl ine fractures and eventually replaces the whole rock, turning i t black and nearly opaque in thin section. The black colouration i s caused by myriads of extremely fine (1-2 urn) opaque inclusions that are probably amorphous carbon; tests by X-ray diffraction, SEM-EDS and microprobe failed to identify this material. In places, this black alteration seems to cross-cut the main-stage alteration associated with mineralization, described above; elsewhere, it appears to be coi nc i dent. Tourmaline and Garnet Alteration Two other unusual alteration facies in Pioneer volcanics were found in specimens collected from the Pioneer mine by F.R. Joubin, which are now in the U.B.C. Economic Geology col lect ion. These specimens comprise garnet-quartz-c a l c i t e - p y r i t e and quartz-tourmaline (schorl). Neither alteration facies is common, but the presence of borosil icate i s significant. Although apparently rare at Bralorne, i t i s common in analagous systems in the Canadian Shield, (e.g. Sigma: Robert and Brown, 1986). At Pioneer, the tourmaline is abundant in late fractures cutting the 27 vein, which is a "tensional" vein (Chapter 5). m 6.2 M i n e r a l C h e m i s t r y Detailed chemical compositions of both the precursor and the alteration minerals were studied by microchemical means in order to determine the temperature, pressure and chemical composition of ore fluids responsible for alteration and mineralization. Analytical techniques used included: X-ray diffraction, the scanning electron microscope-energy dispersive system (SEM-EDS), and quantitative analyses with the CAMECA wavelength dispersive electron microprobe (probe). Methods for semi-quantitative estimation of oxide percents in certain minerals with the SEM-EDS were developed by comparison of the SEM-EDS results with the probe analyses. Operating conditions for the SEM-EDS studies were: (1) polished specimens were run with no t i l t on the energy dispersive spectrum, and (2) the tungsten filament was used with 30 kv accelerating voltage and 2.7 amp filament current. The beam curent was 0.5 nanoamps, giving a 0.5 micron (500 angstrom) beam width or resolution for backscattered electrons. A fixed number of counts (42,000 for carbonates; 100,000 for s e r i c i t e s and chlorites, and 50,000 for other s i l i c a t e s such as feldspars, hornblendes, epidote, prehnite, etc.) and fixed vertical scale expansion (V = 4096) were used to standarize the results and make them comparable. For the electron microprobe analyses, operating conditions were 15 kv accelerating voltage, 10 nanoamperes beam current, 0.5 micron beam diameter (approximately 3-4 micron diameter of spot size resolution on the polished surface), and 10 second counting times for peaks, 5 seconds for background. Probe quality polished thin sections prepared in the department by Y. Douma were used in a l l cases. Operating conditions and standards used are in Appendix 2, Table A-2-1. All data was reduced using a PAP correction program that corrected for atomic number, absorption and fluorescence, supplied by the probe maufacturer. Routine analysis of standards were within + 5'/. of the accepted values. The precision of microprobe analysis is d i f f i c u l t to estimate, since there i s no p o s s i b i l i t y of re-analysing exactly the same point (significant "burns" occur in some minerals—especially the micas and the carbonates). However, repeated analyses of the same grain in several locations showed that fluctuations were, usually less than five percent except in the carbonates where fluctuations are usually less than 107. (the carbonates analysed in this study are, however, notably inhomogeneous). The minerals chosen for study were those that are widely present in the altered vein envelopes (section 6.1): carbonate, s e r i c i t e , b i o t i t e , chlorite and feldspar. Feldspars and hornblende in unaltered rocks were also analysed to establish baseline conditions before alteration. In addition to providing details of the physical and chemical nature of the hydrothermal fluids with which they /99 interacted during ore deposition, the changes in chemical compositions of these minerals aided in explaining the bulk chemical changes during alteration (section 6.3'.). It is d i f f i c u l t to be certain that the minerals studied attained equilibrium with each other and the coexisting hydrothermal fluids. One c r i t e r i o n that can be used, although this does not prove equilibrium, i s an absence of chemical v a r i a b i l i t y in the various minerals (Linnen, 1985). The variation about - a median value of major element oxides for a l l the minerals studied was always less than ten per cent in an individual grain or within a sample. Analyses are tabulated in Appendix 2 in Table A-2-2 (a-d) for carbonate, s e r i c i t e , b i o t i t e , chlorite, feldspars, hornblende and other minerals. 6.2.1 Zonation i n Carbonate Compositions Carbonates in the alteration .envelopes around veins at Bralorne are variable (Table A-2-2a and F i g . 6-2). Calcite, which reacts to dilute HC1 acid, i s in general the dominant carbonate in the outer and central zones of altered wallrock (sections 6.1.1, 6.1.2). Ankeritic carbonate ( i . e . . variably ferroan dolomite) i s more common in the inner zone adjacent to the vein and does not react to dilute HC1. The ankeritic carbonate, which belongs to the dolomite group, proved d i f f i c u l t to distinguish by X-ray diffraction; only rarely was a distinct ankerite or dolomite peak identif ied. SEM-EDS studies, however, revealed that the inner zone Lu U < U O U< o U < o £ u oo v 9J . .CO •— LO ' ,1 O") «— £ ^ ° u o) r ro O U D U cn S F ^ S S s H£L>r~!-s rays-ssi' cis,^--"- s i ^ r * carbonates consist of composite grains that contain mainly grains of Fe-calcite rimmed by ankerite, or less commonly, cores of ankerite with siderite rims. Minor calcite is also present. This may explain the lack of strong ankerite XRD peaks, since the bulk of the carbonate is s t i l l calci te and Fe-calci te . The volume of s iderite i s so insignificant that XRD peaks would not be expected, and none were seen ('.siderite i s a member of the c a l c i t e group, but has d i s t i n c t l y different X-ray peaks than either calci te or ankerite). As may be seen from Figure 6-2, there is considerable contamination of some of the ankerite analyses by c a l c i t e , and siderite analyses by ankerite, causing the apparent metastabi1ity of the plotted points (cf• Anovitz and Essene, 1387). In unaltered rocks, such as the freshest soda granite that could be found (C094A), the carbonate i s almost pure end-member CaC03 (0.37 c a l c i t e mole fraction, with 0.02 s i d e r i t e and negligible magnesite). Similar calcite (0.96 c a l c i t e and 0.03 siderite mole fractions) is found in the central alteration zone (sample SB-84-49 @812': Table A-2-2a), over 5 m away from the major 51B vein. Adjacent to the vein (sample SB-84-49 ©795'), much of the carbonate i s '; ankerite, with the cores of 0.03 mm diameter grains (spots 3 and 7, Plate 6-9a,b; analyses in Table A-2-2a) being about 0.63 c a l c i t e mole fraction, 0.30 magnesite, and 0.07 s i d e r i t e . The rims of these grains are only 5 microns thick (Plate 6-9a,b), but are clearly distinguishable in plane— 2o2-P l a t e 6-9. (a) and (b) SEM-EDS b a c k s c a t t e r e d p i c t u r e of composite carbonate g r a i n s i n sample SB-84-49/795*, found i n the inner zone a l t e r a t i o n of d i o r i t e i n the hanging w a l l of the 51B FW v e i n near 5 l e v e l ( c r o s s - s e c t i o n 11,600E). The g r a i n s show a n k e r i t e cores (grey; spots 3 and 7) and s i d e r i t e rims ( b r i g h t e r due to higher atomic number of Fe compared to c a l c i u m and magnesium; spots 2 and 8). Dark spots are holes i n s e c t i o n ; carbonate a n a l y s e s are i n Table A-2-2a, and mica analyses are In Table A-2-2c. Z03 polarized light in thin section by having higher r e l i e f than the cores. "These rims are more Mg- and Fe-rich (0.47 magnesite mole fraction, with 0.15 to 0.45 mole fraction siderite and corresponding calcite mole fractions—see analyses for spots 8 and 2, Table A-2-2a). The rims also have higher Fe:(Fe+Mg) ratios of around 0.4 compared to the 0.1 to 0.2 typical of the ankeritic cores and the calcites in other grains (Table A~2-2a). They plot as siderites in Figure 6-2, after allowing for dif iculty of analysing such thin rims (the analyses are contaminated by the adjacent ankerite); However, even this close to the vein a few grains of nearly end-member calcite are present (Table A-2-2a, analysis 9), and similar calcite is also found adjacent to the 79 vein on the 41 level (sample C-l16-1). Analyses from the inner alteration zone around the 51 vein on 19 level (sample 19-51 FU1), are similar to those adjacent to the vein in DDH SB-84-49 @795'. A composite grain about 0.1 mm across (Plate 6-10) showed a zonation from rim or margin areas, this time composing the bulk of the grains, that average about 0.35 magnesite mole fraction, 0.14 s iderite , and 0.51 c a l c i t e (Table A-2-2a: spots 1, 3, 5 and 6), to a core area of calci te (spot 4). However, small grains of almost end-member calci te (spot 5B) were mixed with the rim ankerites, and many of the apparently metastable ankerite compositions in Figure 6-2 reflect this contamination. A separate grain showed similar ankerite (analyses 6, 6B) mixed with calci te (analysis 6A). P l a t e 6-9. (c) SEM-EDS b a c k s c a t t e r e d overview of the area i n ( a ) , showing l o c a t i o n of analyses 2 and 3 i n carbonate g r a i n a t immediate margin of qu a r t z v e i n . The white spots i n the v e i n are t e t r a h e d r i t e , and the b r i g h t g r a i n s o u t s i d e the v e i n are p y r i t e and a p a t i t e . X -* 1 -.1. P l a t e 6-10. SEM-EDS b a c k s c a t t e r e d p i c t u r e of composite carbonate g r a i n i n sample 19-51FW1, found i n the inner zone a l t e r a t i o n of d i o r i t e 0.1 m i n t o the f o o t w a l l of the 51 v e i n on 19 l e v e l (8650E, 7050N). The g r a i n i s mostly a n k e r i t e ( l i g h t grey; spots 1, 3) with cores t h a t are c a l c i t e ( b r i g h t e r ; spot 4) and t h i n rims t h a t c o n t a i n a n k e r i t e (spots 5, 5A) and c a l c i t e (spot 5B). 26S Calcite formed i n i t i a l l y close to the veins as alteration fluids spread outwards. The calcite close to the veins may then have been progressively replaced by i r o n -magnesium carbonate as the advancing alteration front moved outwards from the veins, at the same time producing c a l c i t e further from the vein. This would explain the zoning seen in the carbonate grains from more Fe-Mg-rich rims to more Ca-rich cores. The calcium, iron and magnesium to form the alteration carbonates was probably derived from alteration of hornblende in the original rock. However, the process cannot have been a simple molecule by molecule replacement since total Ca, Fe, and in particular Mg, show depletion near the veins (section £.3.5). Thus only a portion of the Ca, Fe and Mg was available to react with the ore-forming f l u i d , which was rich in dissolved CQ2 (section 10.2). The rest of the Mg and Fe went into formation of chlorite (section £.2.4), muscovite, and rarely biot i te (section 6.2.3), or was leached out of the parent rock into the ore f l u i d , to be available for deposition elsewhere. Carbonate in late reopenings at the centers of major veins, in cross-cutting veinlets, and in the early "barren" quartz-carbonate-epidote-prehnite stockwork i s a l l calci te that reacts freely to cold dilute HC1. It i s of exploration signficance that only the main-stage (ore-related) carbonate i s ankeritic; i t may be tested for in the f ield with dilute HC1 or the KCN test of Warnes (1962). A similar zoning, of peripheral calci te to ankerite close to the vein has also 206 been described at many other mesothermal gold vein deposits, such as the Yellowknife d i s t r i c t (Boyle, 1961), Lac Shortt, Quebec (Morasse et a l . . 1986), Casa Berardi, Quebec (Pattison et a l . . 1986), the McDermott deposit at Kirkland Lake, Ontario (Workman, 1986), or the Cameron Lake deposit, Ontario (Mel ling et a l . , 1986). In the Red Lake d i s t r i c t , Ontario, the anomalous zone of dolomite-ferrodolomite is a kilometer wide, rather than being confined to the envelopes of single veins (MacGeehan et a l . . 1982). 6.2.2 Feldspars The feldspars are described petrographically in section 6.1; probe analyses are in Table A-2-2b. Ubiquitous plagioclase of almost pure a l b i t i c composition is confirmed by the X-ray, SEM and microprobe studies; there is no K-feldspar present in any of the rocks investigated. The average albite in relat ively fresh, unaltered rock CC093A, C094A) i s about fi\b-9e.—a-yf Ana—* with negligible orthoclase molecule. This may be greenschist facies metamorphic a l b i t e . There i s a slight increase in albite molecule, to about AbsQ-ioo) A n 0 - i , in the texturally distinct albite alteration feldspars in veins and around veins (19-51 FW1 and C182-2, respectively, in Table A-2-2b). ~ The very low anorthite component of both the metamorphic and the hydrothermal plagioclase implies a formation temperature of less than 400 degrees Celsius (Winkler, 1967). This agrees with the estimates of ore 207 fluid temperatures of about 350 degrees from fluid inclusion and sulfur isotope studies (Chapters 9 and 9, respectively). The narrow range of compositional v a r i a b i l i t y observed suggests thorough homogenization of the feldspars. 6.2 .3 Micas (Muscovite and B i o t i t e ) Alteration micas in a l l the vein envelopes (inner to outer) at Bralorne are primarily s e r i c i t e (muscovite). Biotite is rare except at depth in the deposit, and to the southeast in the Pioneer mine and on the P.E. Gold property. Only one analysis of biot i te is reported in Table A-2-2c, from an unusual zone of b i o t i t i c alteration in a green hornblende porphyry dyke at about 4 level near the 51B vein in DDH SB-84-49 @795'. Although not analysed, there is also abundant secondary biot i te near the 77 vein on 44 level . The s e r i c i t e s analysed were from four different samples from various rock types and various parts of the deposit (SB-84-49@795' in green hornblende porphyry, C182-2 in "crackled" soda granite in SB-84-37 @ 516', 19-51 FW1 in d i o r i t e on 19 level near the 51 vein, and C116-1 in diorite at 32 level near the 79 vein). They a l l have similar compositions, with low Fe and Mg (1-27.) and Ti (0.37.) contents. 6.2 .4 C h l o r i t e s Two types of chlorite are distinguishable optically (section 6.1). Chlorite with the optical properties of 208 pennine, such as anomalous interference colours (Berlin blue and purple) and distinct green pleochroism, is present mainly in relat ively fresh soda granite, such as sample C094A (chosen to be as representative as possible of unaltered rock, well removed from any veins). Microprobe analyses (Table A-2-2d) show that this i s a high~Fe chlorite with Fe:CFe+Mg) about 0.7 and Si (cations) about 5.5, that approximates aphrosiderite, a variety of r i p i d o l i t e (Fig. 6-3, after Hey, 1954). The other c h l o r i t e , that has normal interference colours, is very pale green with no detectable pleochroism, and is much richer in Mg with Fe:CFe+Mg) about 0.35. It i s much more widely present, both in unaltered d i o r i t e CC093A: Si cations about 5.6 to 6.0) and strongly altered diorite CC116-1) and soda granite (C182-2) that both have Si cations about 5.0 to 5.4. This second type approximates a composition between r i p i d o l i t e and pycnochlorite (Fig. 6-3). A separate group f a l l s within the pseudothuringite group, with Si cations about 4.7 and Fe:(Fe+Mg) about 0.6, and one analysis plots as corundophi11ite. The r i p i d o l i t e -pseudothuringite compositions are similar to those described by Robert and Brown (1986) for chlorite at the Sigma Au quartz vein deposit in Quebec. The o r i g i n a l , possibly metamorphic, chlorite in the diorite is thus much more magnesian than that in the soda granite. This magnesian chlorite i s also stable in the presence of hydrothermal fluids, since i t i s present in both Z&9 a i J e r V j a i i ? ^ compositions observed in Hey (1954). Bralorne deposxt, according to the scheme of Z/o the altered diorite and soda granite. It implies a high Mg:Fe ratio in the hydrothermal f luids, which is supported by the results of computer modelling (Chapter 10). Analyses of hornblende (Table A-2-2e), the precursor mineral to the chlorite in the diorite , also show Mg:Fe ratios greater than unity. The similarity of Mg:Fe ratios in the precursor hornblende and alteration chlorite suggest that the chlorite replaced hornblende on a one-for-one basis. In other portions of a given hornblende grain, some Fe, Mg and Ca cations were trapped as carbonate, while others were released to the f luid. 6.3 Chemical Changes Chemical changes (losses and gains of elements) occurring during alteration were studied in 10 detailed traverses across alteration envelopes in the footwall and hangingwall of major veins. The detailed chemical analyses are in Appendix 1 (Table A - l - 1 ) . The traverses were made in both soda granite and d i o r i t e host rocks, to evaluate possible different alteration responses of the two rock types. Traverses were also made at various levels from surface to as deep as 44 l e v e l , an interval of 2000 m. At higher elevations (above 20 level) envelopes about the 51 Vein (the principal vein in upper levels) were studied. At deeper elevations (below 20 level) envelopes to the 77 or 79 vein system (the principal vein system at deeper levels) were studied. 2/1 6.3.1 Sample P r e p a r a t i o n Rock samples, collected in plastic bags, varied from 1 to 20 kg in size. Each piece was scrubbed in running water to remove extraneous dirt before being reduced with a hydraulic s p l i t t e r to less than 10 cm. After crushing in a .jaw crusher to less than 0.5 cm size by three or more passes, a 0.5 to 1 kg spl i t was obtained by Jones r i f f l e r for chemical analysis. This s p l i t was subsampled by coning and quartering because this was found to be more rel iable than the Jones r i f f l e r CS. Juras, pers. comm., 1987). This provided approximately 150 g, which was ground in a chrome-steel or tungsten-carbide ring mill (see discussion below on contamination, section 6.3.3) until 95 7. passed a 200 mesh (63 micron) screen. A 4 g subsample of this -200 mesh powder, scooped at random in small portions out of the container, was pelletized in a hydraulic press at approximately 10 tonnes with a PVA binding and boric acid backing. The pellets were then labeled on their edges and placed face down o