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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 U n i v e r s i t y , A THESIS SUBMITTED  Kingston, O n t a r i o , 1971  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 t h e s i s as conforming to the r e q u i r e d  standard  THE UNIVERSITY OF BRITISH COLUMBIA April @  1989  C r a i g Henry Bowen L e i t c h  In  presenting  degree freely  at  this  the  available  copying  of  or  publication  of  in  partial  fulfilment  of  the  University  of  British  Columbia,  I  agree  for  this  department  thesis  reference  thesis by  this  for  his thesis  and  scholarly  or for  her  Department  77  DE-6  (2/88)  Columbia  I  further  purposes  gain  that  agree  may  be  It  is  representatives.  financial  permission.  T h e U n i v e r s i t y o f British Vancouver, Canada  study.  requirements  shall  not  that  the  Library  permission  granted  by  understood be  for  allowed  an  advanced  shall for  the that  without  make  it  extensive  head  of  my  copying  or  my  written  -i i ABSTRACT  The other  Bridge  camp  River  gold  in British  camp p r o d u c e d  Columbia  t o n n e s o f 18 g / t o r e ) , m a i n l y mesothermal accreted Permian  Bridge  spatially albitite  related  The  east ore  major  shoots  gold-bearing 0  that  Cretaceous  occurred  slickensides  the last  o f t h e major dated  strike  about  that plunge  movement was r e v e r s e .  roughly  after  K-Ar).  v e i n s at B r a l o r n e  with  long  Bralorne d i o r i t e  e>  Major  20'/. o f t h e  perpendicular  rocks  45  to the  for productive and t h e  Cadwallader  s o d a g r a n i t e may have been t o o weak t o s u s t a i n  fractures,  or t o o low i n Fe f o r p y r i t e  Hydrothermal  alteration  envelopes  t o 10 m w i d e and g r a d e o u t w a r d s  quartz  Late  and s o d a g r a n i t e ) ,  The most common h o s t  v e i n s a r e t h e competent  up  of e a r l y  i n t h e v e i n s o c c u p y somewhat l e s s t h a n  slickensides.  greenstone;  which a r e o f  island-arc  u n r e l a t e d t o t h e emplacement  70 ,  i n the  M i n e r a l i z a t i o n i s t e m p o r a l l y and  and p l u n g e s t e e p l y west,  large  and  O 2 7 0 Ma U-Pb; 284 Ma  and i n d i c a t e  vein  Terranes,  age and o f o c e a n i c  to a suite  and d i p n o r t h  0  The d e p o s i t s a r e h o s t e d  (Bralorne d i o r i t e  Permian  million  from t h e B r a l o r n e - P i o n e e r  d y k e s o f 3 6 - 9 1 Ma age, and t h u s  intrusives  as E a r l y  02 o f Au, from 7  and C a d w a l l a d e r  respectively.  i s genetically  host  110  River  to Triassic  character,  and  vein deposit.  than any  i t s 70 y e a r s o f  over  CI 30 t o n n e s or 4 m i l l i o n  operation  more g o l d  - ankeritic  carbonate  from  - sericite  around  precipitation. the veins are  intensely C+fuchsite)  foliated to less  -iii sheared c a l c i t e - c h l o r i t e - a l b i t e to unsheared e p i d o t e calcite.  Chemical s t u d i e s of  volume b a s i s  (normalized  remained r e l a t i v e l y As  and  vein  Au,  but  the  a l t e r a t i o n on  t o A l ^ O s and  depletion Si0  2  and  CaO  are  locally  Arsenopyrite  to vein  S,  as  the  MgO  depleted  and  i n the  s t r o n g l y ribboned veins. i n the quartz,  black  appear  Gold i s only  where i t l o c a l l y  of  tellurides  septae of  veins  forms e x t r e m e l y  flakes the  r a r e l y found  usually in extensional  to  Traces  as t h i n smeared  sulfidic  the  selvages.  s t i b n i t e h a v e been o b s e r v e d but  n a t i v e metal  chalcopyrite  m e t e r s from  of the v e i n s .  Gold i s found p r i n c i p a l l y  than shear veins,  minor  e s p e c i a l l y galena,  gold-rich portions  t e t r a h e d r i t e and  itself  to several  i s confined  s p h a l e r i t e , and  c o r r e l a t e with  of t h e  CO^,  '  w i t h i n e n v e l o p e s f o r up  have not.  have  ( t o t a l . ) and  2  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  Occasional  which  TiQz>,  o f N a 0 , FeO  reconeentrated.  veins.  constant  i m m o b i l e ) , show a d d i t i o n o f K^Q,  i s approached.  occur  a  -  by rather rich  poc k e t s . B a s e d on primary ore low  salinity  significant  fluid  i n c l u s i o n and  deposition  stable isotope  a p p e a r s t o h a v e been f r o m f l u i d s  ( l e s s t h a n 5 wt. 7. N a C l e q u i v a l e n t ) C 0 ' a n d m i n o r ChU 2  studies,  content,  with  of  a  at t e m p e r a t u r e s  of  SSO^C a n d  pressures  secondary  fluids  contents  and  pressures associated per d  l s  mil, Q  of  0.5  +  observed  (log  0  of  and  the  gold  and  mil.  of  -11  +  depth  of  CH*  by  and  The  veins  the  f a u l t s that  2  range  C  lower  COaa  and sulfides  from  ore  permil.  at  <=,  r a t i o s of  The  Later  much  240  -7  fluid  to  had  +9 a  Homogenization  approximately  fluid  30°C/km,  had  a  a  high  model  adjacent,  predicts  a  with  observed CH.*  originally  north-west  4.5,  may  COs* as  f0  10~  about  a  reaction  with  correlation These  of  the  between predictions and  by  the  inclusions.  formed  by  developed  within  a  Bralorne  Na:K  precipitation  have  trending  a  quartz-sericite-  to  fluid  The  reducing,  a l t e r a t i o n assemblages, in  used  dissolved  with  pyrrhotite.  observed  Bralorne  highly  strong  about  strongly  predicts  the  complexes.  of  +0.5),  response a  of  content  fugacity  in  pH  were  for  deposition  fluid  The  not  gold  chloride  wallrock,  but  and  Conditions  - 7  responsible  and  and  lO ".  minor at  with  data,  Clog  also  pyrite,  CO a  Cthe  8:1,  immediately  It  depth).  below  isotopic  per  the  +2.5).  supported  zone  dilute,  0  with  1978  that  altered  rock.  high  and  the  km  gradient.  least  by  wall  of  1969  at  ankerite  are  C  modelling  fSsj a b o u t  in  1 3  (7  mineralization  about d  kb  temperatures  gold  increase  fugacity  gold  the  suggest  suggested 3  more  a l t e r a t i o n assemblages  Helgeson's results  1.75  Sulfur  1 and  Computer  ratio  kb.  geothermal  to  even at  clustering  temperatures normal  CH*,  with  +13  up  were  no  of  of  fault  mineralization Riedel zone)  shear with  an  —veast-west  maximum  compressive  horizontal  minimum  is  for  implied  "fault-valve" euhedral, build-up the  compressive  this  fault  model  best  fluid  failure  by  a  the  to  transpressive discharge  of  regime. fluids,  d e p o s i t i o n of  crystals  were  Sealing  allowed  fluid  Each  the  of  represents included mineral  deposited  a  of  pressure  quartz  the  to  oriented  fault build  by  open  this  and  the  ribbons  of  sulfide,  sliver  of  highly replaced  in the  vein  deposition  when  the  occurred.  with  next  in  openings  sulfides; held  for  stress  c o i n c i d e n t drop  i n space the  reservoir  previously  provided  and  Cyclic below  overpressuring,  compressive  the  A  yet  veins.  geopressured  of  movement  Cretaceous.  ribboned,  caused  Failure and  Late  quartz  unfavourably  maximum  promoted  pressures.  in a  reactivation  faults,  angle  milky  north-south, Sinistral  in the  e x p l a i n s the  transition  steeply-dipping high  system  pressure  brittle-ductile  and  stress.  coarsely crystalline of  invoking  stress  in  cycle  minor  to  gold,  wall rock  episode  of  at  a  pressure  the  mineral  failure  for  zoned by  formed  quartz high  pore  deposition repeat. probably that  was  fracturing  and  F r o n t i s p i e c e , ( a b o v e ) : V i e w s o u t h e a s t up C a d w a l l a d e r C r e e k , a l o n g t h e 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 t o the r i g h t , w i t h S u n s h i n e Peak i n t h e 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 c o a r s e o r e s t o r a g e b i n a t t h e mouth o f t h e 8 l e v e l (main h a u l a g e ) p o r t a l a r e shown, ( b e l o w ) : View n o r t h e a s t down H u r l e y R i v e r t o B r a l o r n e in 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 f o o t o f Mount F e r g u s s o n (below t h e h o r i z o n t a l l o g g i n g s l a s h ) .  -vi iTABLE  OF C O N T E N T S  Page i i vi vi i xi xiii xvi xviii  ABSTRACT FRONTISPIECE TABLE OF CONTENTS L I S T OF TABLES L I S T OF FIGURES L I S T OF PLATES ACKNOWLEDGEMENTS CHAPTER 1: INTRODUCTION 1. 1 Loc a t i on 1 . 2 Hi s t o r y 1.3 Pr e v i ous Wor k 1.4 P u r p o s e o f s t u d y 1.5 S c o p e o f 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 . 2.2 S t r a t i g r a p h i c S e t t i n g 2.2.1 B r i d g e R i v e r Group 2.2.2 C a d w a l l a d e r Group 2.2.3 T y a u g h t o n T r o u g h 2.3 P e t r o c h e m i s t r y o f t h e B r i d g e R i v e r and Cadwallader groups » CHAPTER 3: GE0CHR0N0L0GY OF THE BRIDGE RIVER CAMP 3.1 I n t r o d u c t i o n 3.2 S a m p l i n g D e t a i l s 3.3 S a m p l i n g P r e p a r a t i o n and A n a l y t i c a l T e c h n i q u e s 3.3.1 P o t a s s i u m - A r g o n 3.3.2 Ur an i urn-Lead 3.3.3 R u b i d i u m - S t r o n t i u m 3. 4 R e s u l t s 3.4.1 P o t a s s s i u m - A r g o n 3.4.2 U r a n i u m - L e a d 3.4.2.1 B r a l o r n e D i o r i t e and Soda G r a n i t e 3.4.2.2 A l b i t i t e D y k e s 3.4.3 R u b i d i u m - S t r o n t i u m 3.5 D i s c u s s i o n 3.5.1 Age o f M i n e r a l i z a t i o n 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 C a d w a l l a d e r g r o u p s 3.6 C o n c l u s i o n s CHAPTER 4: GALENA LEAD ISOTOPES OF THE BRIDGE RIVER CAMP 4.1 I n t r o d u c t i o n 4.2 M i n e r a l D e p o s i t s o f t h e B r i d g e R i v e r Camp 4.2.1 B r a l o r n e Type 4.2.2 C o n g r e s s Type 4.2.3 T y a u g h t o n Type  1 1 5 6 & B 12 16 18 13 20 32 35 37 37 39 42 46 46 49 50 54 56 62 62 67 74 77 77 78 79 80  -vi ii -  4.3 4.4 4.5 4. 6  4.2.4 Blackdome Type 4.2.5 Summary G a l e n a Lead I s o t o p e A n a l y s e s G a l e n a Lead D a t a Discussion Cone 1 u s i o n s  CHAPTER 5: MINE GEOLOGY 5.1 I n t r o d u c t i o n 5.1.1 Mine L a y o u t 5.2 Rock T y p e s : L i t h o l o g y , P e t r o g r a p h y , P e t r o l o g y 5.2.1 B r i d g e R i v e r ( F e r g u s s o n ) Group 5.2. 1~1 V o l c a n i c s ~ 5.1.1.2 S e d i m e n t s 5.2.2 C a d w a l l a d e r Group 5.2.2.1 P i oneer F o r mat i on 5.2.2.2 H u r l e y F o r m a t i o n 5.2.3 B r a l o r n e I n t r u s i o n s 5.2.3.1 P r e s i d e n t U l t r a m a f i c s 5.2.3.2 H o r n b l e n d i t e 5.2.3.3 B r a l o r n e D i o r i t e 5.2.3.4 Soda g r a n i t e 5.2.3.5 M i g m a t i t e 5.2.3.6 A p l i t e Dykes 5.2.4 C r e t a c e o u s - T e r t i a r y Dykes 5.2.4.1 G r e y p l a g i o c l a s e p o r p h y r y 5.2.4.2 A l b i t i t e and Green Hornblende P o r p h y r y Dykes 5.2.4.3 Bendor Dykes 5.2.4.4 L a m p r o p h y r e Dykes 5.3 Met amor ph ism 5.3.1 F a d e s 5.3.2 T i m i n g 5.4 S t r u c t u r e and V e i n i n g 5.4.1 F o l d i n g 5.4.2 F a u l t i n g and D y k i n g 5.4.3 Q u a r t z v e i n s 5.4.3.1 M o r p h o l o g y 5.4.3.2 O r e C o n t r o l s 5.4.4 R i e d e l Shear Model 5.5 S e q u e n c e o f E v e n t s CHAPTER 6: ALTERATION 6.1 M i n e r a l o g i c a l Zoning 6.1.1 C h i o r i t e - e p i d o t e ( O u t e r ) Zone 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 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 ( I n n e r ) Zone 6.1.4 O t h e r A l t e r a t i o n T y p e s 6.1.4.1 S i l i c a F l o o d i n g 6.1.4.2 B i o t i t e A l t e r a t i o n 6.1.4.3 B l a c k C a r b o n a t e A l t e r a t i o n 6.1.4.4 T o u r m a l i n e and G a r n e t A l t e r a t i o n  80 SI 83 85 90 '="4 97 100 103 105 106 107 108 110 116 117 118 123 125 130 134 137 139 139 140 145 147 149 149 151 153 155 155 157 163 16& 173 177 178 181 187 189 189 194 196 196  -:ix6.2  6.3  Mineral Chemistry 6.2.1 Zonation i n Carbonate Compositions 6.2.2 F e l d s p a r s 6.2.3 M i c a s ('.Muscovite and B i o t i t e ) 6.2.4 C h l o r i t e s C h e m i c a l Changes 6.3.1 Sample P r e p a r a t i o n 6.3.2 A n a l y t i c a l P r o c e d u r e s 6.3.3 R e l i a b i l i t y o f C h e m i c a l D a t a 6.3.3.1 P r e c i s i o n 6.3.3.2 A c c u r a c y 6.3.3.3 Homogeneity o f Major U n i t s 6.3.4 G r e s p l o t ('.Constant-Volume C a l c u l a t i o n s ) 6.3.5 L o s s e s and G a i n s  197 199 206 207 207 210 211 211 213 213 215 218 218 230  CHAPTER 7s VEIN AND ENVELOPE MINERALIZATION 7. 1 V e i n Types 7.2 V e i n M i n e r a l o g y 7.2.1 Gangue M i n e r a l s 7.2.2 Opaque M i n e r a l s 7.2.2.1 S u l f i d e s and N a t i v e G o l d 7.2.2.2 O x i d e s 7.3 Model f o r t h e V e i n F o r m a t i o n  241 243 245 250 250 259 260  CHAPTER 8: FLUID INCLUSIONS 8.1 I n t r o d u c t i o n 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 P r o c e d u r e 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 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 o f I n c l u s i o n s .. 8.2.1 P r i m a r y (Type 1) I n c l u s i o n s 8.2.2 P s e u d o s e c o n d a r y (Type 2.) I n c l u s i o n s .... 8.2.3 S e c o n d a r y (Type 3) I n c l u s i o n s 8.3 Mi c r o t h e r mometr i c D a t a 8.3.1 F i n a 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 8.3.2 S a l i n i t i e s ( I c e M e l t i n g T e m p e r a t u r e s ) .. 8.3.3 C o m p o s i t i o n s ( E u t e c t i c T e m p e r a t u r e s ) ... 8.3.4 D e n s i t i e s C C 0 H o m o g e n i z a t i o n ) 8.3.5 P r e s s u r e E s t i m a t e s 8.4 Summary  269 272 273 278 279 286 288 289 289 297 298 299 300 304  CHAPTER 9: STABLE ISOTOPE STUDIES 9.1 S u l f u r I s o t o p e s 9.1.1 P r o c e d u r e 9. 1.2 R e s u l t s 9.2 C a r b o n and Oxygen I s o t o p e s o f C a r b o n a t e s 9.2.1 P r o c e d u r e 9.2.2 R e s u l t s 9.3 Oxygen I s o t o p e s 9.3.1 P r o c e d u r e 9.3.2 R e s u l t s 9.3.2.1 L a t e r a l / V e r t i c a l Z o n i n g i n V e i n s  309 309 310 315 315 316 319 320 325 325  a  9.3.2.2 Geothermometry •3.3.2.3 I m p l i c a t i o n s f o r Water/Rock R a t i o 9.3.2.4 C h a r a c t e r i s t i c s o f t h e Ore F l u i d CHAPTER 10: SOURCE, TRANSPORT, AND DEPOSITION OF GOLD 10.1 P-T--X C o n d i t i o n s 10.2 C h a r a c t e r i s t i c s o f t h e O r e - f o r m i n g F l u i d s .... 10.3 D e p o s i t i o n o f g o l d 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 10.3.2 C o m p a r i s o n o f P r e d i c t e d and O b s e r v e d Assemblages 10.4 T r a n s p o r t o f g o l d 10.5 S o u r c e o f f l u i d s and g o l d CHAPTER 11: CONCLUSIONS 11.1 Summary . .' 11.2 Recommendat i ons 11.2.1 R e g i o n a l E x p l o r a t i o n 11.2.2 E x p l o r a t i o n i n t h e B r a l o r n e REFERENCES APPENDICES APPENDIX APPENDIX APPENDIX APPENDIX  APPENDIX APPENDIX  Mine A r e a  328 331 334  339 342 347 347 350 353 355  362 . 370 370 371 374  1: C h e m i c a l c o m p o s i t i o n s o f a l t e r e d r o c k s a r o u n d v e i n s a t 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 2: M i c r o p r o b e a n a l y s e s 406 3: R e p l i c a t e a n a l y s e s o f i n t e r n a l and international standards 412 4: G r e s e n s bar d i a g r a m s o f l o s s e s and g a i n s in detailed traverses across vein e n v e l o p e s a t E i r a l o r n e from s u r f a c e t o 2 km d e p t h 425 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 d a t a CSugiyama, 1986.) 468 6: Maps C F i g s . 5-1 and 5-2) and c r o s s sections i n pocket  L I S T 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, southwestern B r i t i s h Co 1 umb i a  38,  Table 3-2 U-Pb data on zircons for the Bridge Rivercamp, 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 B r i t i s h Columbia  4'3  Table 3-4  45  Latitudes and longitudes  for a l l samples  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 BralornePioneer area  98  Table 5-2 Chemistry of minor units in the BralornePioneer area  99  Table 6-1 Summary of mineral abundances for primary rocks and a l t e r a t i o n envelopes surrounding the Bralorne veins  176  Table 6-2 Typical input data f i l e for Bralorne altered wall rocks  221  for GRESPLOT program,  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 across the footwall of the 51 vein, 15 level  226  traverse  Table 8-1 Locations and detailed descriptions of inclusion samples for the Bralorne deposit Table 8-2 Summary of fluid for the Bralorne deposit  fluid  270  inclusion c h a r a c t e r i s t i c s 280  - xi 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 for the Bralorne deposit  calcites . 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 Carbonates b ) Feldspars c ) Micas (muscovite and b i o t i t e ) (d ) C h l o r i t e s , 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  L I S T OF FIGURES  Figure 1-1 Simplified terrane map of B r i t i s h Columbia, Canada  southwestern 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 basalt samples from the Cadwallader Group .  for  28  Figure 3-1 Geology of the Bralorne-Pioneer mine area, southwestern B r i t i s h Columbia, showing location of dated samples  36  Figure 3-2  Concordia diagram for Bralorne d i o r i t e  51  Figure 3-3  Concordia diagram for Bralorne soda granite  52  Figure 3-4 dykes  Concordia diagram for Bralorne  albitite 55  Figure 3-5 Rb-Sr diagram for different with the Bridge River d i s t r i c t  petrologic  Figure 4-1 Deposit age versus distance PIut on i c Complex  from the Coast for  Figure 4-3 2oop 2o^p versus a©«pb/ °-»Pb i t deposits in the Bridge River camp ..  for  2 0 4  b/  206  :2C  a  b  57 82  Figure 4-2 F ' b / P b versus Pb/ "*Pb plot deposits in the Bridge River camp 2 0 7  units  p  0  86 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  - xi vFigure 5-5b Cross-section showing major vein systems at Bralorne and extent to depth  159  Figure 5-5c Longitudinal section through the BralorneF'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 d i f f r a c t i o n scans of carbonates (a,b) Major c a l c i t e , trace ankerite . . . . . . . . ('. c,d) Major c a l c i t e and anker i t e  Figure 6-2 wal 1 r oc ks  Compositions of carbonates in altered  Figure 6-3  C l a s s i f i c a t i o n of c h l o r i t e minerals  184 184 200 209  Figure 6-4 Typical composition-volume diagram from GRESPLOT program  224  Figure 6-5 (a) and Cb) GRESPLOT program  Typical volume factor plots from  227  Figure 6—6 (a) and <b) GRESPLOT program  Typical bar diagrams from the  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 l e v e l , 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 for various l e v e l s in the Bralorne mine  temperatures  Figure 8-2 Histograms of ice melting temperatures various l e v e l s in the Bralorne mine  291 for  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  -xvFigure S-5  Progression of fluid inclusion compositions  Figure 9-1 Schematic presentation of sulfur isotopic data for various sulfides in the Efridge River camp  305 312  Figure 9-2 (a) Bralorne d 0 whole-rock and ore fluid values compared to some common reservoirs (ta) Plot of d C versus d 0 for carbonates from the Bralorne deposit, compared to the Mother Lode  326  Figure 9-3 (a) Geographic variation of d 0 values in quartz from the Bralorne-Pioneer vein system (b) V e r t i c a l variation of d 0 values in quartz from the Bralorne-Pioneer vein system  327  x e  1 3  1 B  326  ie>  l s  Figure 10-1 Log oxygen fugacity pH diagram for conditions of mineralization in the Bralorne deposit  327 ...  349  — xvi — L I S T OF PLATES  Frontispiece . Plate 5--1 Pioneer Formation aquagene breccia  vi 112  Plate 5-2 (a~d) Photomicrographs of mineralogy and texturesin 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 d i o r i t e  126  Plate 5-6 <'.a,b> Photomicrographs of unaltered and strongly altered d i o r i t e 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 a p l i t e dyke,sample? C121 Plate 5-11  ..  138  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 o s c i l l a t o r y zoning in plagioclase phenocrysts .. 148 Plate 5-14 CI 033  Photomicrograph of lamprophyre dyke,  sample 148  Plate 6-1 Chlorite ( c a l c i t e ) 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, c h l o r i t e and a l b i t e 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  -xviiPlate 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 b i o t i t e a l t e r a t i o n 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 assemblage and in atypical quartz host  sulfide 258  Plate 8-1 (a,b) Growth zones in euhedral quartz c r y s t a l s 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  - xvi 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 f l u i d i n c l u s i o n , stable isotope and thermodynamic modelling presented in t h i s thesis. Dr. K. Fletcher kindly provided guidance and c r i t i c a l review of analytical d e t a i l s . The personnel of Mascot Gold Mines L t d . 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 B r i t i s h 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 i s thanked for assistance in drafting and photographical d e t a i l s . 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 t h i s study could be completed.  CHAPTER 1 INTRODUCTION 1.1 L o c a t i o n  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 km* in an area encompassing the 5  Cadwallader and Lower Hurley valleys,  the head of Carpenter  Lake, and the lower reaches of Gun Creek to Tyaughton Lake (Fig.  1-2).  ;  Access i s by good gravel road from L i l l o o e t on  the Fraser River, or by a poor gravel road over the Hurley Pass from Pemberton on Highway 99. topography, with elevations  The area has rugged  from 600 to 3,000 m above sea  level and land forms characteristic of alpine g l a c i a t i o n . Extensive d r i f t at lower levels in the valleys conceals most of the bedrock.  The climate i s typical of the i n t e r i o r side  of the Coast Ranges of B r i t i s h Columbia. 1.2 H i s t o r y  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 u n t i l 1897 that the main lode  deposits were found.  The f i r s t discovery was apparently on  the Fourty Thieves claim in the BRX area (deposits 5 and 6 on F i g . 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 o f s o u t h w e s t e r n B r i t i s h C o l u m b i a , Canada, e m p h a s i z i n g 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 t h e B r i d g e R i v e r camp C a f t e r Dawson and P a n t e l e y e v , DNAQ C o r d i l l e r a n Volume, i n p r e p . , 1 9 8 7 ) .  Pioneer vein some 20 m above the floor of the Cadwallader valley was exposed fortuitously by a mud s l i d e after rains (Patterson,  1979:  F i g . 5-1).  heavy  In spite of many  attempts by various companies, i t was not u n t i l 1920-1930 that significant  underground development took place with the  formation of the Pioneer and Lome companies 1979).  (Patterson,  Bralorne Mines was formed by the amalgamation of  Bradian Mines and Lome Mines in 1935. of the mine closed in 1962 soon after  The Pioneer section the amalgamation with  Bralorne to form Br al orne-F'i oneer Mines Ltd. mine f i n a l l y closed in 1971, in the price of gold in  The Bralorne  i r o n i c a l l y just before the r i s e  1972.  Only eight of the many other prospects in the camp have recorded production.  Of these only a few—Congress,  K i t t y , Minto and Wayside (Fig. 1-2)—were of  Gloria  significance.  Most were active in the 1930-1940 period (Harrop and Sinclair,  1986).  The Bridge River camp produced more gold  over i t s 70 year l i f e than any other in B r i t i s h Columbia, from a l i t t l e over 7 m i l l i o n tonnes of ore grading about 18 g/t  Au and 4 g/t  Ag.  Total recorded production was about  130 tonnes or 4 m i l l i o n ounces of Au and 30 tonnes or 1 m i l l i o n ounces of Ag (Barr, 1980). the Bralorne-Pioneer vein system.  Most of t h i s came from Thus the deposit  i s 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 m i l l i o n ounces of Au (Ber t oni,  1983).  The giant Ka1goorlie deposi t  Geologic Contact Fault •  Isotoplc date (Ma)  h =hornblende K = K -Ar r =Rb-Sr b=biotite u=U-Pb z=zlrcon m=muscovlte w= whole rock 10 0 I L_ km LEGEND 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  r~MT~  EARLY PERMIAN-EARLY  CD  CRETACEOUS  Methow Terrane CRETACEOUS  Cadwallader Terrane  PERM A N-EARLY  JURASSIC  BRIDGE RIVER  TERRANE  j***",*^] Bralorne Intrusions BR  Bridge River Assembledge 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 A u s t r a l i a , by way of comparison, produced an order of magnitude more gold at 1200 tonnes  (Phillips,  1986).  1.3 P r e v i o u s 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, recent attempt to dewater  1983)  was the outcome of a  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 •  only  (1977).  Prior to t h i s ,  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 e n t i r e l y on the Pioneer deposit.  Other than t h i s , 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  mineralogical  contains detailed petrological and  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  i s unique within the C o r d i l l e r a in i t s great depth extent of almost 2 km that makes i t more l i k e the c l a s s i c 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 BralornePioneer 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 v e r t i c a l zoning in the deposit,  and to  synthesize the r e s u l t s 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 Bridge River d i s t r i c t .  (Chapter 4) of the surrounding  This work was supplemented by a  limited geological  study aimed at c l a r i f y i n g certain  regional concepts,  such as the ages and stratigraphic  r e l a t i o n s 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  and the 8th level underground (Chapter 5).  Key vein  intersections in old d r i l l  surface  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 a l t e r a t i o n at constant volume was studied using whole-rock analyses (Chapter 6). oxygen,  for major and trace  elements  Fluid inclusions and the stable isotopes of  hydrogen, carbon and sulfur were investigated  examine v e r t i c a l zonation within the deposit The results of the above studies,  to  (Chapters 7 - 9 ) .  coupled with computer  modelling of the ore f l u i d s , 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 T e c t o n i c  Setting  The Bridge River camp in southwestern B r i t i s h Columbia (Fig.  1-1'.), in which the Br a 1 or ne-Pi oneer deposit i s  situated,  occurs adjacent  to the Coast Plutonic complex and  i s 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 ( S i l b e r l i n g 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  slices  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 i s composed mainly of the  Wrangellia and Alexander terranes that became amalgamated by the Jurassic (Coney et a l . . 1980).  The Intermontane super-  terrane i s composed of the S t i k i n e , 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 B r i t i s h 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 i g h t - l a t e r a l s t r i k e - s l i p Fraser - Straight Creek Fault by 120-150 km (Monger, 1985). The T r i a s s i c Cadwallader terrane was viewed as the eastern edge of Wrangellia by Kleinsphen (1985).  However,  the r e s u l t s of Rusmore's work (1985; Rusmore et a l . .  1988)  indicate that the Cadwallader i s a d i s t i n c t terrane separated from Wrangellia by an early Late Cretaceous  fault  zone and by Upper T r i a s s i c rocks of unknown a f f i n i t y . The Permo-Jurassic Bridge River terrane was considered by Potter setting,  (1983) to have been deposited in an ocean-margin 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 e n t i r e l y 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) Bralorne intrusives  (section 5.1.3).  Early Permian in age,  and the  These intrusives are  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 T e r t i a r y .  Trace element petrochemistry  (section 2.3)  shows that  the Bridge River rocks are t r a n s i t i o n a l between mid-ocean ridge basalts (MORE) and somewhat a l k a l i c basalt  (OIB) (Potter,  1983), while the Cadwallader  are mainly island-arc t h o l e i i t e towards c a l c - a l k a l i n e basalt places. 2.2).  ocean-island  (IAT) with a tendency  (CAB) or MORE character in  A l l rocks of both packages are submarine Several  volcanics  (section  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, trondjhemite or "soda granite", association.  hornblende d i o r i t e , and  i s typical of an ophiolite  The gradational contact r e l a t i o n s 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 d i o r i t e 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 i n t r u s i o n .  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 t h i s 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 i s 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 a c t i v i t y 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, by northeast-directed compression (section  5.4).  followed  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 S i n c l a i r (1985).  Parts of the area and adjoining areas have recently  been remapped (Church, 1987; Schiarizza,  1987;  Church et a l . , 1988; Glover and  Glover et a l . ,  Schiarizza et a l . ,  1989).  1988; Garver 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 t h o l o g i c assemblages distinguishable in the Bridge River camp. in more detail  in sections 2.2.1  Each i s described  to 2.2.3.  oceanic Bridge River terrane (Fig. 2-la)  The dominantly  i s composed of the  Bridge River Group (or Bridge River Complex, c f. Glover et al.,  1988) and the Shulaps and President ultramafic  The dominantly island arc Cadwallader terrane (Fig.  bodies. 2-lb)  includes the Cadwallader Group, Tyaughton Group, and a Lower Jurassic shale unit (Wheeler and McFeely, 1987). terrane, the Methow (Fig. 2 - l c ) , assemblage ( i . e .  The t h i r d  i s partly an overlap  i t s upper part postdates the accretion of  the Cadwallader terrane to the Intermontane super-terrane by overlapping them both), although i t also includes rocks c o r r e l a t i v e with the Relay Mountain Group in i t s lower part.  /3  Table 2-1: Generalized stratigraphic section l i s t i n g 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  Regi ona1 Name 2  10  a  T  Plateau lavas  T  Eocene volcanics  T  Rexmount porphyry  Mi ne Name  Name and Description (This study ) 3  Lamprophyre dykes  Kersantite  K-T  Coast plutonics  Bendor dykes  Dacitic porphyry  LK  Felsic dykes  Green hornblende porphyry dykes  Andesitic b a s a l t  LK  Felsic dykes  Albitite dykes  Dacite.  8a  Grey plagioclase porphyry dykes K J-K  Dacitic porphyry  Relay Mountain Group Jurassic shale  Tr  Tyaughton Group  Bralorne intrusion! 6a -J2  porphyry  Taylor Creek Group  J  ?P  Soda.  Shulaps u l t r a mafic Complex  Bralorne soda grani te  Albite tonalite or tr ondh.jemi te  Bralorne diorite  Hornblende quartz di or i te  Mafic diorite  Hornblendi te  Presi dent ultramafics  Dunite, peridotite and pyroxenite  Hurley sediments  1 3 3  ?P-Tr Cadwallader Group  Turbidites, wackes and a r g i l l i t e s Pioneer greenstone Aquagene breccias, basaltic andesite Noel a r g i l l i t e s  ?P-J  Bridge River Group  Bridge River Complex  Ribbon chert and argillite Pillow basalts  Unit i s as defined in the Bralorne mine area (this study. Regional name i s taken from Schiarizza et a l . . 1989. Prefix "meta-" i s understood in a l l rocks older than Tertiary.  METHOW TERRANE PLUTONIC EVENTS PEMBERTON  FIGURE  2-1  ET  STRRTIGRAPHIC CHRRT BRIDGE RIVER CPMP  JACKASS MT TAYLOR CR RELAY" MOUNTAiN  (From K.H. Dauson andfl.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 the O a t on and A c c r e t e d T e r r a n e s of the C a n a d i a n Cordillera", Ch. 19, ONflG Cordilleran Volume, i n p r e p . , 1937.)  JK  •  J T (C)  Figure 2-1. Stratigraphic correlations between terranes in the Bridge River area; (a) l i s t s the units found in the Bridge River Terrane, Cb) l i s t s the units for the Cadwallader Terrane, and (c) l i s t s the units for the Methow Terrane (Tyaughton Basin). Symbols are: o = coarse elastics, - = shales, + = intrusives, v = volcanics. CADWALLADER TERRANE  BRIDGE RIVER TERRANE njUTCMC EVENTS  —[cmcoTiN—-  T  T  PLUTONIC EVENTS ET  ET  LK  LK  K  J  ~&4  powe u_  n T i T T i r i -  T  K  —  BRUME  /  Riven  y  *—*•  P (a)  C  +  8MULAP8  J  "  T  •*• •  (b)  TYAUGHTON QWWMliciR  »*  /5  In the Bridge River camp, the Methow terrane i s 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 Cordey,  (Potter,  1983; Rusmore,  1986'.), to be of T r i a s s i c to Jurassic age.  regional correlation to the Hozameen Group (Monger, Haugerud, 1985;  Potter,  1986; Rusmore et a l . ,  1985; However, 1985;  1988),  suggests the Bridge River Group could be, in part, as old as Permian.  This was the view, based on l i t h o l o g i c a l  correlations to the Permo—Triassic Cache Creek Group, held by a l l previous workers before T r i a s s i c conodonts were found by Cameron and Monger (1971). Currently the Cache Creek Group i s 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 d i o r i t e that appears to  intrude the Bridge River and Cadwallader groups.  The  d i o r i t e 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 i s Upper T r i a s s i c , and the  Relay Mountain, Jackass Mountain and Taylor Creek groups are Upper Jurassic to mid-Cretaceous  2.2.1  Bridge River  (Garver et al . . 1989).  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 - l a ) .  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 t h o l o g i c a l complexities prohibit measurement of meaningful type sections 1988),  (Glover et a l . ,  i s not used because of the more general usage of  "Bridge River Group".  Following the subdivision of Cairnes  (1937), the group i s divided into sedimentary and volcanic packages.  The sedimentary package consists of 1000 m or  more thicknesses of ribbon chert and a r g i l l i t e with very minor discontinuous limestone lenses.  The sedimentary  package i s intercalated with the volcanic package consists of large volumes of pillowed and  that  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 KAr 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, deformation accompanying tectonic ribbon cherts, argillite,  1983) during  transport, while the  separated by thin septae of graphitic  buckled and were intensely tectonized,  flowing around the buttresses  literally  of massive basalt.  There i s  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  styles merely reflect  their different  tectonic  competencies.  Alpine-type ultramafic rocks in lensoid to very elongate bodies are s p a t i a l l y associated  with the  sedimentary and volcanic rocks and are therefore thought form part of the assemblage Hodgson et a l . .  (Schiarizza et a l . .  faults that were later  transcurrent movements (Garver et a l . . 1989).  1989; c f•  1982); they may mark the s i t e s of major  sutures or thrust  al•.  to  foci  for  1989; Schiarizza et  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), Leech (1953) and Nagel  described by  (1979), s t r u c t u r a l l y overlies and i s  s p a t i a l l y associated with the Bridge River Group; i t usually considered to be part of the group.  Potter  is (1983)  postulated that the Shulaps i s 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 i s mainly preserved as competent  fault-bounded  blocks, either s t r u c t u r a l l y above or within the Bridge River Group.  The Cadwallader rocks are in fault contact with, but  inferred to be stratigraphical1y and v o l c a n i c l a s t i c  overlain by, the volcanic  arc sequence of the Upper T r i a s s i c  Tyaughton Group, Lower Jurassic shales,  and sedimentary  rocks of the Jura-Cretaceous Relay Mountain Group (Wheeler and McFeely, 1987;  Garver et a l . ,  1989).  T r a d i t i o n a l l y , 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 d i s t i n c t i o n 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).  Formation consists of a basaltic felsie  volcanics.  volcaniclastic  The Pioneer  andesite p i l e with minor  The Hurley Formation i s composed of a  sequence  tectonical1y-transported upper t u r b i d i t e unit.  '.'.containing o l i s t o l i t h s , or blocks, of reefal  limestone) and an  The contact between the two  formations i s 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 i s a package of rocks separate from that o r i g i n a l l y 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 - l c ) .  The assemblage consists p r i n c i p a l l y of:  the Middle Jurassic to Cretaceous continental margin type c l a s t i c  (1)  easterly-derived,  wedge of shale, s i 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 o v e r l i e  the Lower Jurassic Ladner Group, which i s 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  Volcanics  Recent mapping by Church (1987) and Church et a l . (1988) in the Bridge River area has led them to suggest that no d i s t i n c t i o n can be made between the volcanic rocks of the Cadwallader and Bridge River (Fergusson) Groups.  (The  lithology and stratigraphy of these groups i s 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 a r g i 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 t h i s 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 i e l d from the paler green Pioneer volcanics of the Cadwallader Group.  The latter are commonly  closely associated with t u r b i d i t e s and  volcanic1astic  2/ sediments of similar composition and green appearance  (as  also described by Rusmore, 1987), unlike the Bridge River which i s largely devoid of coarse e l a s t i c s  (Chapter  5).  A l 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 l a s s i f i e d  using  trace element discriminant diagrams that were developed expressly  for altered rocks, considering only  immobile elements such as T i , Y, V, Zr and Cr.  relatively 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 i s t e d 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  higher Na 0, S i 0 , 2  3  and A l 0 a  3  in major elements is in the  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. Cadwallader,  For Na Q, the corresponding figures are 2  3.5 to 6.07. and average 4.97., compared to  22. TABLE 2-2 (a): Chemistry of Pioneer v o l c a n i c s (Cadwallader Group) from the Bralorne block, Bridge River d i s t r i c t , B.C. Location BRALORNE BLOCK C096C Sample No. C095 C096A C096B (N) (4) (1) (1) (5) Ma jor Element s (7.) SiOa 47.34 44.8 45.9 63.96 Al O 13.71 11.0 14.8 13.49 1.01 0. 15 0.72 0. 33 TiOa 8.40 4.46 FeaOa 10.32 11.5 MgO 12.41 14.7 15.5 8. 07 CaO 3. 13 9.03 9.0 5. 9 Na,0 1.50 0.00 w• / 0.59 K0 0.04 0. 00 0.03 0. 05 MnO 0. 26 0. 11 0. 15 0.25 0. 11 0.03 0. 06 0.06 Pz0 LOI 4.5B 11.92 3. 16 4.90 TOTAL 100.31 100.39 (100) (100) 2.74 Density 2.90 2.72 2.83 Minor Elements (ppm) As 14 15 4 0 Ba 50 62 52 53 Co 30 64 54 25 Cr* 275 1185 117 495 Cu 75 58 a 135 Nb 3 3 1 3 Ni 70 465 195 34 Pb 6 15 8 11 Rb 2 0.0 0.5 0. 1 S 745 305 220 530 Sb 3 0 4 2 Sr 270 50 240 196 V* 270 54 57 235 Y 22 8 12 25 Zn 105 61 200 66 Zr 63 41 48 65 Normative Minerals (5£> Quartz 1.1 23.4 Corundum 2.2 Orthoclase 0.2 0.2 0.3 Albite 13.8 5.3 30.4 Anorthite 35.6 34.5 31.2 15.8 Diopside 7.0 12.9 Hypersthene 29.4 49.4 54.4 26.2 01ivine 9.3 1.7 Magnetite 2.2 1.9 2.4 0.9 Ilmeni te 2.2 0.3 1.5 0.7 Apatite 0.3 0.05 0. 1 0. 1 D i f f ' n Index 14 1 5 54 Modes (estimated volume %) Quartz 10 8 10 10 Plagioclase 40 44 30 68 Hornblende 58 22 C I i nopyroxene 55 46 R u t i l e . Sphene 3 2 2 P y r i t e (Py/Po) 2 tr (No. samples) (1) (1) <1) 2  a  a  O  C098A (1)  C098D (1)  48.6 14.8 1.38 10.9 10. 1 6.8 3•& 0.59 0. 18 0.20 2.88 (100) 2. 87  52.9 14.2 0.50 7.8 11.7 6. 0 4. 1 0.24 0. 16 0.05 2.50 (100) 2.84  0 72 46 191 8 75 18 5.0 575 0 163 395 35 91 139  3.6 32. 1 23.5 8.7 4.2 22.5 2.2 2.7 0.5 36 50 45 5  0 83 52 348 4 1 135 12 1. 540 0 110 140 16 83 52  1.5 36.5 20.0 8.8 15.8 14.7 1.6 1.0 0.1 38 5 40 54 1 (1)  22 TABLE 2-2 (b>: Chemistry of Pioneer volcanics from other l o c a l i t i e s i n the Bridge River d i s t r i c t , B.C. Location WAYSIDE Sample No. C2003 CN) CI) Major Elements C7.) SiOa 56. 0 A1 0 16.9 TiOa 0.75 Fe Oa 8.2 MgO 5.0 CaQ 1.6 Na O 8.0 0. 15 K0 MnO 0. 15 0.21 P*0» LOI 3. 14 TOTAL C100) 2.65 Density Minor Elements (ppm) Ba 135 Co 21 Cr* 16 Cu 6 Nb 5 Ni 3 Pb 4 Rb 3 Sr 160 v-» 160 Y 18 Zn 68 Zr 80 Normative Minerals C/O Corundum 1.3 Orthoclase 0.9 Albite 70.4 Anorthi t e 6.6 Nepheline Leucite Diopside Hypersthene 1.0 •1ivine 16. 1 Magnetite 1.7 Ilmenite 1.5 Apatite 0.5 D i f f ' n Index 71 Modes (estimated volume JO Quartz <1 Plagioclase 65 Hornblende 25 CIinopyroxene Ch1 or i t e 5 Ilmenite CRu/Sp) 5 Magnetite CNo. samples) CI) 2  a  a  a  3  ELDORADO BASIN C2001 C2001A C2001B C2001C C2001D C1) (1) (1) (1)  cn  49.6 14.9 1. 16 9. 1 12.0 3.0 4.30 0.70 0. 13 0. 14 5.22 C100) 2 • &3 210 29 130 40 6 71 4 7 200 230 21 61 82 2. 1 4.4 35 • 3 14.8  46.3 45.4 16.5 15.9 1. 10 1.01 9.7 9.3 10.0 10.5 7.8 7.0 4.2 3.9 0. 10 0.51 0. 16 0. 16 0. 14 0. 10 4.71 5.62 (100) (100) 2.74 2.78 120 41 210 36 4 105 ND 1 370 250 17 66 60  490 46 180 23 5 170 1 6 340 240 18 56 80  0. 6 29.3 27. 1 4.7  3.2 27.6 25.9 4.4  38. 1 16.4 1.53 12. 1 8.5 17.6 0.77 0.69 0.21 0. 15 4. 11 (100) 2.94  42.0 15.5 0.51 14.9 8.9 9.9 2.0 3. 1 0.25 0. 12 3.01 (100) 2.92  420 32 160 40  430 47 330 100 10 180 5 83 480 240 24 20O 40  7  75 8 21 230 260 28 81 100  5.2  18.7 17 1 1.9 2.3 0.4 43  10.6  8.2  41.4 3.7 3.4 13.7  23.5 1.9 2.0 0.3 35  26.0 2.0 2.2 0.4 35  31.8 2.5 3. 1 0.4 4  45  40  20 25 10  25 35 <1  CI)  CI)  50 4 25 12 4 4 CI)  25.4 9.5 10.9 20.6 24.1 3.0 1.0 0.3 15  45  60*  30 15 10  30* 5 5  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 b a s a l t s . Loc'n ELDORADO CARPENTER Sample AVGCAD AVBRIV (N) (3;10) (3) Major Elements (>0 SiO 50.64 48.4 A 120a 15.26 13.6 TiOa (1.22) 2.05 Fe203 12. 17 11.1 MgO 5.57 5.20 CaO 6.24 10.7 Na 0 4.90 3.31 (0.71) 0.36 K0 MnO (0.16) 0. 15 (0.17) 0.31 PaOo LOI 2.58 4.60 TOTAL 99.65 99.8 Minor Elements (ppm) Ba 220 Co 39 Cr* 115 130 Cu 95 Nb <10 20 Ni 61 Rb 10 Sr 310 210 V* 340 340 Y 24 33 Zn 91 Zr 49 150 Normative Minerals (7.) Quartz 2.0 Orthoclase 3.2 2. 1 Albite 43. 1 29.5 Anorthite 18.8 25.0 Diopside 9.9 23.5 Hypersthene 5.8 8.0 01ivine 12.4 2.5 Magnetite 2.8 3.5 Ilmenite 3. 1 2.8 Apatite 0.5 0.7 D i f f ' n Index 37 25 Modes (estimated volume 7.) Quartz 2 Plagioclase 60 75 CIinopyroxene 35 20 Rut i1e 3 5 (No. samples) (3) (3) 1  3  a  a  a  2  CALK ( >3) 51.0 18.7 0.90 8.9 4.8 10.7 2.9 0.6 0. 17 0.13 1.0 39.80  AVERAGE BASALTS MORE OIB (>14) (100) 49.3 16. 5 . 1.5 10. 0 7.5 11.0 2.8 0.2 0. 18 0. 10  -  99. 1  83  50. 0 13 2.7 11.5 10 10 2.5 0. 5 0. 18 0.3 0.5 101.2  I AT (10) 51.2 18. 1 0.8 10. 1 6.2 11.0 2.0 0. 3 0.2 0. 1 0.7 100.4  <2 65 13 400 255 15  12 50 305 70 2.5 175 3 150 230 35  190 60 170 80 27 75 13 400 240 27  70 30 80 60 1.5 30 5 200 270 18  65  100  155  45  1.2 23.5 32.4 19.2 10.3 7. 1 3.0 2.9 0.3 25  1.0 3.0 19.0 24.0 20.6 22.0 4.0 2.8 4.9 0.6 27  3.4 1.8 16.9 39.5  185 115  1.3 3.5 24.5 31.2 13. 1 14.8 3.2 1.7 0.3 29  11.8  21.8 3.9 1.5 0.3 22  25" Notes t o accompany Table 2 - 2 : Rusmore (1985J. = Potter (1983). Bracketted f i g u r e s are averages of 10 analyses, otherwise of 3 analyses. "* Analyses are adjusted to allow for known contamination due to g r i n d i n g i n Cr-steel Tema m i l l , determined by comparison to same samples ground i n W-carbide m i l l . =» Hughes (1982); trace elements for MORB, OIB and I AT are from Pearce and Qale (1976). Sum of a l t e r a t i o n products ( c h l o r i t e , epidote and carbonate for mafics; s e r i c i t e and carbonate for piagioc1ase). 1  3  s  Analyses from Bralorne block are by XRF: (100)totals are by pressed powder p e l l e t , normalised to 1007.; others are by fused glass d i s k . Analyses from Rusmore are by ICP; those by Potter are by X R F ( d e t a i l s unknown); those by Hughes are by t r a d i t i o n a l wet chemistry. ( N ) = number of r e p l i c a t e analyses averaged i n these columns. Samples are located on Figures 3- 1 and i n Table 3-4.  Bridge River, 2.7 to 4.27. and average 3.37.; for A 1 0 , 2  figures are Cadwallader, Bridge River, 11.8  3  the  13.3 to 16.57., average 15.37.,  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; individual analyses, 1985. >  refer to Potter,  for  1983 and Rusmore,  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: groups of soda contents  C2001 - C2001D) show two  (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 s i g n i f i c a n t l y higher (in the 7 to 147. range, even after allowing for overestimation 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 t h i s 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 o l i v i n e 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 o l i v i n e normative (as i s average mid-ocean ridge basalt  (MORB), and ocean-island basalt  COIB)), the more sodic nature of the Cadwallader rocks shows up in several samples, p a r t i c u l a r l y from Eldorado Basin (C2001A to D), which contain normative nepheline and leucite or corundum.  However, these rocks have probably been  strongly changed chemically ( s p i 1 i t i z a t i o n on the ocean floor,  followed by greenschist  metamorphism, i s probably  responsible for the apparent undersaturation) and the norms may be misleading. the c l a s s i f i c a t i o n s ,  Furthermore such names are at odds with below, by r e l a t i v e l y 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 basalt types given in Table 2-2,  for major  Cadwallader volcanics from  the Eldorado Basin show a f f i n i t i e s with c a l c - a l k a l i n e basalts (CAB) and island-arc t h o l e i i t e s  CIAT).  The samples  from the Bralorne block tend to be t r a n s i t i o n a l 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 i n d i c a t i v e of IAT basalts.  Such  t r a n s i t i o n a l 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 c i r c l e s are samples from the Eldorado Basin (Rusmore 1985), as are f i l l e d c i r c l e s (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 t h o l e i i t e s (IAT) or low-K t h o l e i i t e s (LKT), and within-plate basalts (WPB), ocean-floor basalts (OFB) and ocean island basalts (OIB) are shown as f i e l d s , with their respective averages plotted as squares (from Table 2-2: Pearce and Gale 1976, and Hughes 1982). Note that in (a) the f i e l d s of CAB and LKT overlap the central MORB f i e l d .  29  Ti/100  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 t r a n s i t i o n a l between MORB and somewhat a l k a l i c 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 PaO  a  (Fig. 2-2 a~d), the samples analysed in t h i s 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 t h i s study from both Eldorado Basin and the Bralorne area have more MORB character than Rusmore's samples. versus Cr (Fig. 2-2g)  F i n a l l y , a diagram of Ti  and a comparison of Y/Nb r a t i o s (not  shown) give similar indications of petrotectonic setting  fo  the samples analysed in t h i s study as do Rusmore's ( i . e . they are t r a n s i t i o n a l 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. two may or may not have been adjacent  The  and contemporaneous;  there i s arc d e t r i t u s in both, but the detritus i s 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 i s much younger,  probably early Late Cretaceous.  Isotopic dating of  the plutons c o n f l i c t s with the accepted T r i a s s i c - 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 d i o r i t e and soda granite Joubin,  1948; Stevenson,  1958).  (cf•: Cairnes, 1937; However the results of the  present study show that t h i s cannot be true because the Early Permian Bralorne intrusives are at least years older than the mineralization.  180 m i l l i o n  Instead,  mineralization i s related to a set of pre-mineral to postmineral 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;  to be of T r i a s s i c to Jurassic age.  Cordey,  1986),  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 t h o l o g i c a l correlations to the PermoTriassic Cache Creek Group, held by a l l previous workers before T r i a s s i c conodonts were found by Cameron and Monger (1971).  The Permian or pre-Permian age i s 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 i s 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 t h i s 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) also Church et a l • , 1988).  by Rusmore (1987)  (see  The present study (Chapter 2)  shows that rocks from both areas have similar petrology and major and trace element chemistry.  Possibly the d e f i n i t i o n  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, i s 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 i s 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 d i o r i t e are common.  A l l previous  workers considered the contact to be i n t r u s i v e , and i t was also widely held that the d i o r i t e was contemporaneous with the volcanics, 1337).  grading into them in places  (e.g.  Cairnes,  Based on my observations at Bralorne, Goldbridge,  and Wayside (Fig. 1-2), intrusive.  I believe the contact to be  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. true, the c o n f l i c t would not e x i s t ,  between paleontological  If this were  and isotopic ages  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 t h i s important  contact.  35"  3.2 Sampling Details Sampling for the geochronological the Bralorne block near the mine.  study was mainly in  The r e s u l t s 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) in this study (Table 3-3).  are also l i s t e d  Latitude and longitudes  samples described are in Table 3-4,  for a l l  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 Pioneer mine to the east (Fig. 1-2),  were digested  the  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 Figure 3-1 core.  (C092B, C093A, C094A and C095A) shown in  were taken from underground NQ diamond d r i l l  These were sampled in duplicate,  16-30 m (50-100 ft) kg.  each representing  of core and weighing approximately 25  Other samples (C082A and C082B) of similar s i z e were  taken from adit walls on the main haulage level Empire Crosscut).  (8 Level,  Post-mineral dykes (C083A and C1033) were  sampled from the dump at the portal on the eighth level obtain 20 kg of the most unaltered material possible.  to 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 B r i t i s h 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 i s also well exposed in a quarry at Goldbridge. gabbro,  These rocks,  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 r e s u l t s  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 r e s u l t s of  additional work confirm the r e s u l t s of the i n i t i a l  3.3 Sample P r e p a r a t i o n and A n a l y t i c a l  3.3.1  Potassium-Argon  (Table  studies.  Techniques  3-1)  Hornblende concentrates were made using standard heavy liquid  (methylene iodide) and magnetic  techniques.  separation  B i o t i t e concentrates were made by e l u t r i a t i n g  carefully sized fractions in a water column in which flow from bottom to top. cleaned  is  The r e s u l t i n g concentrates were further  magnetically.  Potassium concentrations were determined by r e p l i c a t e atomic absorption analysis of solutions of the separated  38 TABLE 3-1. Columbia.  K—Ar  data  for the Bridge River  Rock U n i t (Sample No.)  K  Material analyzed  Camp, s o u t h w e s t e r n  British  P;adi o g e n i c R a d i o g e n i c Date*,Ma Ar ( c m x l 0 ) Ar ("/.) ( + l s i gma) 3  s  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 ) (UBC)  m  a. 08  20.47  94.4  63.6+1.S  b h  7.37 0. 356  22.876 1. 036  80.7 55.2  79.6+2.9 74.8+3.6  w  i.  3.714 5.S45  85.5 74.7  69.4+2.4 45 +1.5  3. 004  86.5  67.1+2.2  7. 70 0.382  17.565 0.6235  80.3 34.5  58.9+2.3 42.3+2.9  7. 14  12.323  69.7  44.7+2.4  3  Coast P l u t o n i c (West of s t u d y (QSC 7 6 - 4 9 ) CGSC 7 6 - 5 0 )  Complex area)  1  1  M i n t o Mine: dyke (UBC) vein C o n g r e s s Mine: (UBC) dyke 2  m  3  w  Bendor p l u t o n s (GSC 7 6 - 5 4 ) (oldest) b (GSC 7 6 - 5 9 ) (youngest) h 1  1  Rexmount p o r p h y r y (GSC 7 6 - 6 3 )  b  J J >~\  1. 13  1  NEW Goldbridqe, gabbro (GBQ) Bralorne, d i o r i t e (C093A) B r a l o r n e , l a t e dyke (C083B) Robson ( E l d o r a d o ) s t o c k (DY 3217)"* Lucky Gem v e i n (C2020) Bralorne, lamprophyre (C1033)  DATA  h  0.087  1.053  74.5  287  +  20  h  0.049  0.5861  69. 1  284  +  20  h  0. 162  0.5524  62. 0  85.7+  3.0  b  6.62  16.684  93.7  63.7+  2.2  m  1.53  3.489  61.7  57.7+  2.0  b  6.64  11.353  91.4  43.5+  1.5  Wanless et a l . (1977). Pearson (1977). H a r r o p and S i n c l a i r ( 1 9 8 6 ) . ** K.M. Dawson, p e r s . comm. 1987. 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 r o c k . Ages were c a l c u l a t e d usingA>= 4. 9 6 x l 0 / y r ; /\e+A e" = 0 . 5 8 1 x l 0 / y r ; "°K/K = 1.167xl0-*mole/mole. 1  3  3  a  s  - l o  T  - l o  minerals using a Techtrem AA4 spectrophotometer.  Argon  analyses were performed by isotope d i l u t i o n using an AEI MS10 mass spectrometer  operated in the s t a t i c  measurement, a high purity  3 e  Ar  mode for mass  spike, and conventional  extraction and p u r i f i c a t i o n procedures.  The precision of  the data, shown as the + value in Table 3-1, estimated analytical  gas  i s the  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, iodide, and magnetic techniques. subdivided into several magnetic  methylene  The zircons were 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 e l e c t r o s t a t i c  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.  A l l fractions consisted of up to 60 percent  cracked grains, or fragments of grains.  It was necessary  use these because of the small amounts (1-2 recoverable z i r c o n . the more magnetic  to  mg) of  In addition to their cracked nature,  fractions contained some cloudy or turbid  grains which were probably of lower i n t e g r i t y and could have  TABLE 3-2. U-Pb d a t a on z i r c o n s f o r the B r i d g e R i v e r Canp, southveeterr* B r i t i s h Columbia. Data a r e p l o t t e d on F l q u r c t 3-2, 3-3 and 3-4# as noted below. •  •SBBB.l'SB^BSaB.IEBm^na.aia.H  Fraction  1  Weight  lag)  U (ppm)  Pb (ppm)2-  Pb i s o t o p i c abundance" 20€> Pb/204pb 20fc P b - 1 0 0 measured* £08pb '20.7pt> 204pb  Sample C082B, B r a l o r n e d i o r i t e , ei. a l l •74  NH  l.a/i.5  H  body a d j a c e n t  l. 2  305  13. 1  12. 7040  5. 1629  0.0O15  5190  l. a  420  17. 1  13. asso  5.1718  0.0035  7898  -74 i.a/i.5  Sample CQ93A, B r a l o r n e d i o r i t e , • 74 NH 2. I/O. S •74 HI. 8 / 2 , NH1. 6/3 -74 »44 HI. 6/3, NH1.6/S  s. 7  134  6.0  9. 4  199  a. 4  1. 4  257  11. 0  16. 1897  S. 8730  0. 0454  1890  16.3250  5. 2106  0. 0098  6692  17. 4372  3.2149  O. 0073  4183  -149 »74 HH 1.8/2 -74 *44 H 1.9/1.3  2. 7  12. a  12. 1147  S. 2933  0. 0084  6075  3. 2  384  is. a  13.0184  5. 2725  0. 0074  7720  •44 NH 1.73/2 -44 NH 1.3/3  2. 6  331  24. 1  18. 0600  3.3119  0.0067  8766  1. 0  SS4  25. 1  19.3150  5. 3929  0. 0099  4611  Sample C094A, B r a l o r n e soda g r a n i t e .  2. 1  4.9S  424  6. oa  7.7776  5.3831  0.0383  1826  9. 2495  5.8157  0.0717  1106  (Fig.  (250. 7*274)  3-2)  0. 2999 *.22 (266. 4*1.6) 0.2800*34 (2S0. 6*2. 6)  0. OS141.*26 (259 12) 0.03121 0 4 (250 i 15)  0. 3032*72 (268. 9*3. 6) 0.2825*122 (252.6*9. 4) 0. 28S5*.24 ( 255. 0*1.8)  0.05208^98 (289 i~43> 0.05067*76 (226 *34 -35) 0.05108*24 (245 11 )  3-2)  0.04223*58  (266. 6*37is>  0.04043-166 (255. 5*10. 2> 0. 040531.28 (2S6. 1*.!. 8)  3- 3)  0.04140*30 (261.5*l78> 0. 04046*70 (253.7*4. 4)  0. 2951*24 (262.6*2.0) 0. 2881*52 (257.1*4.1)  0.05171*22 (272 *.~10> 0. 0S164.*2a <270 *12 -13)  major body ( F i g . 3 -3 )  Sample C092B, a l b l t l t e dyke ( F i g . 333  u  0.04232*20 (267. 2*1.~2> 0.03965-40  s m a l l dyke ( F i g .  303  2. 9  t o CO&2A  major body ( F i g .  Sample C082A, B r a l o r n e soda g r a n i t e ,  + 74 NH1. 3/3 -74 NH 1.3/S  Atomic r a t i o s (datee. Ha)' 2.Q7pb/Z3S 2O7Pb/-*06pb l e r r o r c a r e 2 slgma)  2.06pb/238u  0.04259*64 ( 268. 9*4.~o> 0. 04206£.28 (263. 6*_1. 8)  0. 3062*46 (271. 2*3. 6) 0.3044*52 (269. 8*_4. 0)  0.05214*12 (291 *. 5> 0.05248*68 (306 »33~-34)  3-4 > 0.01429*8 (91. 5*0. 4) 0.01427*10 (91. 3*0. 6)  O. 04819*62 (108 »31> O.04759*86 (79 * 43)  0.0950*14 (92. 1*1.~2) 0.0936*20  (90. 9*l7a>  •  u c i c i a i a o i i i i i i ia .  *  -149*74 " s i z e range i n microns; H, NH * magnetic, non-magnetic on F r a n t z lsodynamlc s e p a r a t o r at i n d i c a t e d a m p e r a g e and s i d e t i l t (e.g. 1.5/3 - 1 . 3 amps a t 3* s i d e t i l t ) . R a d i o g e n i c * common Pb R a d i o g e n i c • common Pb, c o r r e c t e d f o r 0. 15X per atomic masB u n i t (AMU) f r a c t i o n a t i o n and f o r 120*50 pg Pb b l a n k w i t h c o m p o s i t i o n o f 208t207i206i204 • 37. 301IS. 50117.75i1. C o r r e c t e d f o r 0. 13X per AHU f r a c t i o n a t i o n . 235 _ 2 Decay C o n s t a n t a » 2 38U * 0.13S123xl0-9/yr. U. - 0.98483xl0-9/yr,38u'»u 137.88) r a t i o s a r e c o r r e c t e d i o r /2 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 c f c o m p o s i t i o n ^ 0 6 p b ' 2 o ^ p . 17.750, 207Pb'- « '*-Pb • 13.500, .20£pb'204 , _ p ( a l b l t l t e ) and 265 HB (soda g r a n i t e a n d d i o r i t e ) u s i n g t h e Stacey and Kramer (1975) growth curve.  * > • •  aa  .a a a a a a a a a a a a a  aa 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 = -;  =  b  ?  J  P  b  3 a  4  6  6  >  B  n  d  l  o  r  C  O  B  B  O  n  D  o  i  9  0 M  b  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!). a  U and Pb concentrations were determined using  2oap _2asu-238 b  together  U  mixed spike.  LJ and Pb were loaded  on Re filaments using the H PCU-si1ica gel 3  tec hn i que. Mass spectrometry was done on a VG Isomass 54R s o l i d source mass spectrometer collector.  with a single  Precisions for = ' Pb/ 20  7  better than 0.1"/., and for 2o=Pb/  were better  :2:0e  ao7  (Faraday cup)  Pb and  a 0 B  Pb/  3 0 6  Pb  Pb and sso^pb/ao-ypt,  than 0.5"/. in most cases.  were t  h  e  y  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 i n d i v i d u a l l y propagating a l l c a l i b r a t i o n 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 i n g data points to a straight  l i n e 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 i n t e r v a l ) .  analytical  Additional information on  technique i s 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 p e l l e t s using X-ray fluorescence. U.S. Geological Survey rock standards were used for c a l i b r a t i o n , and mass absorption coefficients  were obtained  from Mo K-alpha Compton scattering measurements.  Rb/Sr  r a t i o s 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 i s 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  Sr/  e o  Sr  0.1134 and adjusted so that the NBS standard SrCQ gave an  0 , r  Sr/  r a o  sigma).  a 7 ,  Sr/  e , s  3  (SRM 367)  S r r a t i o of 0.71020 + 2, and the Eimer and  Amend standard gave a r a t i o of 0.70800 + 2. single  r a t i o of  Precision of a  S r r a t i o was between 0.00001 and 0.00010 (1  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 s a m p l e s from the B r i d g e River camp, s o u t h w e s t e r n B r i t i s h In F i g u r e 3-7. L a t i t u d e and l o n g i t u d e o f s a a p l e s a r e In T a b l e 3-4. Sample  Rock type  Sr (ppm)  B r a l o r n e ( l a t e dykes) C083A Hb porphyry 366 C092A Albltlte 113  Rb (ppm) 4.1 38.8  C082A C082B  (major i n t r u s i v e masses) Diorite 111 1. S Soda g r a n i t e 77.6 12. 6 (alnor bodies) Soda g r a n i t e 236 1.6 Diorite 200 1. 4  C09SA C096A C096B C096C C098A CQ98D  (Pioneer Basalt Basalt Basalt Daclte Basalt Andealte  C093A C094A  07Rb/86Sr  S^Sr/S^Sr  Ci. 032 0.991  0.70341 0.7O480  0. 039 0. 470  0. 70365 0. 70433  0. 020 0. 021  0. 70336 0.70353  1  Date < n a )  2  Initial  ratio  B r i d g e R i v e r Canyon a r e a ( H e t a p l u t o n l c BRD313A H e t a p i u t o n l c 433 7.2  O. 7O310 0. 70340 0.70297 0. 70319  166 * 27  0.70343 * 0.00008  0.70327 » 0.00008  O. 70380 » O. OOOOS  0.70355 <• O.00003  ( B r i d g e R i v e r Group s c h i s t s ) 42.1 0.793 O. 7063 26.9 0.392 0. 7059 28.6 0.323 0. 7060 36.0 0.607 0. 7061 37.7 0.536 0. 7060  Garnet-blotite Garnet-blctlte Blotite Garnet-blotite Garnet-blotite  133 132 157 172 203  C3-3A  Qtz-ser-chlor Otz-ser-chlor Qtz-aer-chlor Qtz-ser-chlor  111 246 117 59. 4  40. 49. 67. 46.  7 3 4 2  1. 039 0. sai 1. 673 2. 2S  0. 7085 O. 7074 0. 7073 0.7087  Quartz-blotlte Quartz-blotlte Quartz-blotlte Qtz-blo-chlor  186 221 224 128  32.4 17.8 34.2 10. 2  0.504 0.233 0.441 0. 229  O. 7061 0. 7060 0.7062 0. 7062  a c D  CS-7A B C D  320 • 80  0.70302 » 0.OOOOS  85 • 46  0. 7053  • 0. 0004  154 •• 27  0. 7058  ' O. 0004  41 <• 30  0.7060  * O.0002  block i n melange) 0.046 0. 7047  C5-3C D E F H  Initial ratio r e c a l c u l a t e d t o 270 Ha  0.70336 • 0.00010  ( H u r l e y - N o e l sediments o f the Cadwallader Group) Noel Slate 300 38.6 0.373 0.7048 Hurley A r g l l l l t e 144 16. 9 0. 339 0. 7063 dykes) 009 080 O08 027  Data a r e p l o t t  102 * 10  v o l c a n i c s o f the Cadwallader Group) 262 0.00S 0. 5 0.70382 37. 0. O03 0. 04 0. 70372 178 0. 009 0. S 0.70380 240 0. O01 0. 1 0. 70384 136 0. 092 S. 0 0.70382 lOS 0. 034 1. 9 0.70384  G o l d b r l d q e Quarry (major I n t r u s i v e , f e l s l c GBQ Diorite 349 1.1 0. GBO-1 Soda g r a n i t e 16.9 0.3 O. GBO-2 Soda g r a n i t e 204 0. 3 0. GBQ-3 Soda g r a n i t e 133 1.1 0.  Coluaiblu.  TABLE 3-3 D E  (continued).  • t z-b i o-ch1or Qtx-chlor-blot  Phyl1ite BR-1 Sch i et 2 Argillite 6 7 Sandstone MV77BR5 B r i t MV77BR7 B a s a l t  128 106  10. 2 22.2  0. 229 0. 604  0. 7062 0. 7064  122 283 119 336 72. 9 213  48. 1 22. 0 38.2 15. 3 52. 4 17. 3  1. 138 0. 223 1.419 0. 133 2. 080 0. 235  O. 7071 0. 7049 0.7068 0. 7046 0. 7105 0. 7050  North s i d e o f C a r o e n t e r Lake (Bridge R i v e r Group c h e r t s ) BR-* Chart 21.0 15.6 2. 14 0. 7138 23.9 5 Chert 40. 3 1.849 0. 7140 Hope S l i d e (Hozameen Group v o l c a n i c s ) HozHBl Greenstone 182 3.9 HozHS2 Greenstone 188 2.8  0.062 0.043  0.7033 0.7033  C o o u i h a l l a Road (Hozameen Group sediments) HozCqR P e n c i l s l a t e 77.8 1.2 0.045  O. 7068  142 ± £0  0. 703O  191 228  0. 7080 assumed*' 0. 7080 assumed^-  if if  aaO(anumed)  + O. 0002  0.7031  1 P r e c i s i o n i s l e s s than o r equal t o +0.0001 where 4 decimal f i g u r e s a r e g i v e n 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 c o n s t a n t used = 1. 42x10-'*/yr. } C a l c u l a t e d I n i t i a l r a t i o at 270 Ma (before Mesozoic rehomogenization) u s i n g the appi-ox irnat i o n • Change i n «'Sr/«»Sr -(Rb/Sr)»(0. 0000413)«(270 Ma - age o f isochron) where Rb/Sr i s t h e a r i t h m e t i c average o f present-day measured c o n c e n t r a t i o n s , 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 s t r o n t i u m o f a p p r o p r i a t e age.  45  TABLE 3-4: Latitude and longitude of samples in t h i s study SAMPLE  LATITUDE CN!)  LONGITUDE (W)  C082A/B C0S3A/B CQ92A/B C093A/B C094A/B C095A/B C096A,B,C C098A,D CI 033 C2001 C2001A,B C2001C, D C2003 C2020  50° 50 50 50 50 50 50 50 50 50 50 50 50 50  46.2' 46. 2 46.2 46.2 46.2 45.6 45.6 45.6 45.6 59.4 59.3 59. 0 59. 3  122° 48. 0' 122 48. 0 48.0 122 48. 0 1 22 48. 0 122 47.4 1 47.4 122 47.4 122 47. 4 56.3 1 JCJC 56.5 122 54. 5 1 '"2*2 47.0 i •->*""• 55.7  Noel Slate Hurley Arg  50 50  46.7 46.8  122 122  49.3 51.3  GBQ GBQ-1,2,3  50 50  51.6 51.6  122 122  50.8 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  BR-1 BR-2 BR-4 BR-5 BR-6 BR-7  50 50 50 50 50 50  50. 9 46.9 46.0 46.0 50. 6 53.2  122 122 122 122 122  12.0 14.6 17.2 17.6 28. 9 37.7  MV77BR5 MV77BR7  50 50  50. 5 53. 6  122 122  27.6 47.7  HozHSl HozHS2  49 49  18. 1 18. 1  121 121  14.9 14.9  HozCqR  49  26.2  121  18.0  err>  o  12. 0  3.4  Results  3.4.1  Potassium-Argon  A summary of a l l K-Ar data for the camp i s presented in Table 3-1. granite)  The Bralorne intrusive suite north of Goldbridge (Fig. 1-2)  ( d i o r i t e and soda gives an Early  Permian or Late Pennsy1 vanian age of 287 + 20 Ma by K-Ar on hornblende. equivalent  At the Bralorne mine the d i o r i t e gives an 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, (Fig.  1-2)  at the BRX mine north of Bralorne  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, b i o t i t e from the Robson (Eldorado) stock dates at 63.7 + 2 . 2 Dawson, pers. comm., 1987).  Ma (K.M.  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 b i o t i t e  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 a c t i v i t y igneous a c t i v i t y  (Wanless et al . , 1977!).  Other  in the'Bridge River camp coincides with  t h i s Pal eocene to mid-Eocene magmatic episode:  late  lamprophyre dykes at Bralorne (43.7 + 1.5 Ma by K-Ar on biotite—this  study) and the Rexmount porphyry (44.7 + 2.4  Ma by K-Ar on b i o t i t e ,  F i g . 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 t h i s age; a l l that i s known i s that i t i s post 51 Ma and older than 24 Ma (Faulkner, 1986). A large sample of the Bralorne d i o r i t e CC093A) yielded fine pale green hornblende with only traces of c h l o r i t e . 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 i s t e d 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 d i o r i t e and Rb-Sr date for the gabbro and leucocratic dykes near Goldbridge.  If excess Ar  i s involved, however, i t has almost exactly compensated for any loss of Ar due to resetting by later magmatic or hydrothermal events;  t h i s seems an unlikely interpretation  for these dates. A great gap in time i s present between the d i o r i t e intrusion and a largely post-mineral dyke set which contains large black euhedral hornblende c r y s t a l s .  The hornblende  c r y s t a l s in the dated dyke (C083A) preserve their o r i g i n a l magmatic zoning when seen in thin section, with dark brown rims surrounding pale green i n t e r i o r s (Plate 5-12'.).  This i s  unlike plagioclase in the same rock, which has undergone the regional degradation to a l b i t e , z o i s i t e , c h l o r i t e 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 t h i s 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 a l t e r a t i o n 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 vein.  Thus they are either post-mineral or late  to a major intra-  mineral in character, and mineralisation probably occurred mainly before 36 Ma. Black lamprophyre dykes with a markedly different orientation to a l l the e a r l i e r dykes d i s t i n c t l y cross-cut mineralized veins at Bralorne. coarse brown b i o t i t e ,  These dykes (C1033) contain  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 514).  Plagioclase has been reported in these dykes by  Stevenson  (1958), and thus they would be c l a s s i f i e d as  kersantites  (Hughes, 1982).  Their K-Ar date of 43.5 + 1.5  Ma on a clean b i o t i t e concentrate puts another younger l i m i t on the timing of mineralization at Bralorne.  3.4.2  Uranium-Lead  Two samples each of the Bralorne d i o r i t e 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 a v a i l a b l e ,  areas of the underground workings.  are from widely separated Another pair of smaller  samples, d i o r i t e (C082B) and soda granite (C082A) were sampled in close proximity to each other. are differences  Although there  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 d i o r i t e (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. (soda granite)  In contrast,  the zircons of C094A  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 i k e 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 prisms.  elongate  The soda granite i s more mafic than the average at  t h i s l o c a t i o n , 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 d i o r i t e are the same within analytical  uncertainty,  t h i s 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 y i e l d any zircons.  3.4.2.1 Bralorne D i o r i t e and Soda Granite Zircons from the d i o r i t e and soda granite y i e l d 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 d i o r i t e (two fractions  !Sh J — " ^iSr.j: ?rFf r s=?." W  samp^C^A  ::zi:  th  2  S  l  9  m  a  i  r  e  r  r  rror  g  o  o  o  o n  8E2  °  r  S  '  " >  o / C | d  a  n  d  t h  "  e e  faction  D a t a a r e f r o m T a b i e 3  o  902  o  "  - > 2  o  and  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 F i g . 3-1.  S3  abbreviations in Table 3-2),  a l l zircon fractions plot as  points with 2 sigma error envelopes overlapping concordia. The d i o r i t e has a minimum age of 257 Ma based on the 2 0 S  Pb/  2 3 B  U date for the coarse, non-magnetic  sample C082B.  fraction from  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  j u s t i f i e d on the basis of lower Pb/U r a t i o s for the magnetic zircons from both samples.  fine,  Although these other  fractions plot as concordant points, their lower Pb/U r a t i o s can be reasonably interpreted as due to Pb loss, along trajectories  post-crystallization  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 t h i s discrepancy,  which could  be due to inheritance of old radiogenic Pb, sporadic contamination in the laboratory, or to = *F'b analytical so,  errors.  Inaccuracy in measuring  "*Pb (with a Faraday cup,  SEO  since the UBC Geochron Laboratory did not have a Daly photomultiplier at the time of analysis) would have propagated into inaccurate holds for the non-magnetic  ao  ' Pb/ r  a:38S  U ratios.  fraction of C094A.  The same It i s for  5*  t h i s reason that the  2 0 S  Pb/  2 3 a  U dates are preferred in  assigning minimum aiges to these rocks. Using the same reasoning, the other sample of soda granite Ma.  (C082A) yields the minimum  However, the r e l a t i v e l y better  both fractions sso  ' 'Pb/ 7  S!os  2 0 6  Pb/  a 3 B  U date of 261  analytical  quality  from t h i s sample indicates that  Pb date of 272.4 Ma for the coarse,  fraction i s a better  for  the  non-magnetic  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  :aos  Pb/  date of 272.4 Ma overlap the minimum age estimates Bralorne d i o r i t e .  :20  ' Pb 7  for the  Although the f i e l d relations show the  soda granite to be younger than the d i o r i t e (Chapter 5 ) ,  it  i s clear that no d i s t i n c t i o n 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 t h i s i s 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 o l d .  3.4.2.2 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:> y i e l d unambiguously concordant U-Pb dates (Table 3-2  ?rlTr2lt\  co ia E ° n and ^ dsample diagram for Bralorne locations are in  from Table 3-2,  a l b i t i t e dykes. F i g . 3-1.  Dat a are  and F i g . 3-4).  The two zircon fractions have ^oepb' " ^ 23  dates of 91.3 and 91.5 Ma.  5  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 p a r a l l e l attitudes to mineralized veins, plus their common intense alteration to quartz, sericite,  fuchsite,  carbonate, and p y r i t e , suggests that  they were also closely related temporally to mineralization, probably immediately preceding i t .  3.4.3 Rubidium—Strontium A l l the rubidium-strontium data from t h i s 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, i s mainly from p e l i t i c metasediments Monger).  (samples donated by C . J . Potter and J.W.  Two samples of Cadwallader Group sediments  included with t h i s data also plot within the envelope of the Bridge River Group sediments. suites from specific  Individual isochrons for  l o c a l i t i e s give dates (Table 3-3)  range from 41 to 154 Ma, and average 106 Ma. together,  that  Taken a l l  they plot in a band about 0.002 wide in ' S r / S r , s  7  o s  with an i n i t i a l r a t i o 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. »'Sr/'«*Sr vs. R b / 9 r 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. ^ P a r t i a l l y reset at age given; data give "scatterchrons" which are believed to be p a r t i a l l y reset isotopic systems so that the calculated ages l i e between age of the p r o t o l i t h and the Late Cretaceous mineralizing/metamorphic event. Samples from the Bralorne d i o r i t e and soda granite intrusions are shown as X's. Only late dykes at Bralorne, located i n F i g . 3-1 (C092B, a l b i t i t e = Ab C0B3A green hornblende porphyry = GHP, shown as squares), and GBQ suite (crosses) from Goldbridge quarry, located i n F i g . 1-2, give reasonable isochron dates. C l a s t i c and p e l i t i c sediments. a  M  ay  e<>  f  2  of sedimentary rocks (Armstrong and Misch, 1987), t h i s must be a minimum stratigraphic youngest f o s s i l  age,  and i s younger than the  (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 . , separates  1989).  White mica  from a nearby outcrop y i e l d 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 i s thus dated as Permo-  T r i a s s i c to T r i a s s i c (220 to 250 Ma), and i s similar to ages of metamorphism of other blueschist  complexes in B.C.,  Washington, Oregon and C a l i f o r n i a (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 c o r r e l a t i v e 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 p l o t . Two samples of r e l a t i v e l y unmet amorphosed ribbon chert from the Bridge Fliver Group l i e s i g n i f i c a n t l y  (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 T r i a s s i c - J u r a s s i c  5?  boundary, i f a Sr i n i t i a l r a t i o of 0.7030 for Mesozoic seawater  i s assumed.  The Sr isotopic signature of the E-fridge River suite  is  similar to that of the Darrington P h y l l i t e , part of the Shuksan Metamorphic Suite located in northern Washington (Armstrong and Misch, 1987). ' Rb/® Sr  s 7  6  On a plot of  (Fig. 3-5), the envelope  P h y l l i t e almost exactly River suite  Q  ' Sr/ 7  s<5  S r versus  for the Darrington  overlies the envelope of the Bridge  (and therefore i s not plotted).  In other words,  the suites art? identical in average and range of Rb, Sr , and a 7  Sr/  8 6  Sr  values.  S i m i l a r l y , the envelope for the  radiogenic Shuksan blue- and greenschist  less  samples overlies  the d i s t r i b u t i o n 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 C a l i f o r n i a , which are more  radiogenic than associated c l a s t i c  sedimentary rocks, as  discussed by Armstrong and Misch (1987).  This i s not to  imply that the Bridge River rocks are of precisely Jurassic age,  but they are i s o t o p i c a l l y 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 u n l i k e l y , given the apparent i n i t i a l r a t i o of 0.705 and high average Rb/Sr r a t i o of the sample s u i t e .  60  The attempt to obtain the age of the Cadwallader Group by Rb-Sr dating of the Pioneer volcanics,  after  proved barren of zircons, was unsuccessful.  they had  They gave only  an isochron with almost zero and very uncertain slope, probably due to pervasive Mesozoic rehomogenization of the Sr isotopic composition and r e d i s t r i b u t i o n and gain or loss of the Rb (Faure, 1982) by metamorphic and/or mineralizing fluids in the Bralorne area. (.0.001-0.094)  The range of  E3  i s very limited in t h i s s u i t e ,  ' Rb/ 'Sr ratios 7  ss  even though  some r e l a t i v e l y s i l i c e o u s rocks were sampled (quartz keratophyres with up to 667. SiGV). therefore the Rb contents The i n i t i a l  " 'Sr/ Sr  ra  t o 270 Ma) o f  ?  06  (1-5  The Kx-Q CO. 03 to 0.597.) and ppm) were a l l extremely  r a t i o of these volcanics  low.  (recalculated  0.70372 + 0.00005, i s c ompat i b1e with an  oceanic arc setting  (section 3.5),  petrochemical data of Table 2-1,  as suggested by the  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 c o r r e l a t i v e 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 » ? ' S r / S r r a t i o of 0.70302 + as  0.00005 (Fig. 3-5).  The date, although quite uncertain,  lends support to the K-Ar evidence for a Late Paleozoic rather than T r i a s s i c age for these rocks.  The i n i t i a l  r a t i o s 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 i k e ocean floor basalts and plagiogranites,  which have  i n i t i a l r a t i o s 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 produce an e s s e n t i a l l y two-point age of 166 +27  Ma (Fig. 3-5,  analyses  isochron with an apparent  inset).  This has presumably  also been reset by hydrothermal a c t i v i t y  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  r a t i o 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 t r a n s i t i o n a l 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 a c t i v i t y or by Eocene magmatic/metamorphic a c t i v i t y , isochron (Fig. 3-5).  also gave a two-point  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 r a t i o s  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 Bridge River Group.  from the  This i s 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 r o l e .  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. evidence  Petrographic  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 i s o t o p i c a l l y reset but retain o s c i l l a t o r y 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 t h i s study constrain the age of mineralization, which cannot be older than the 91.4  million  63  year a l b i t i t e dykes that are strongly altered and mineralized.  Indeed the mineralized veins often  follow the a l b i t i t e dykes, relation.  closely  implying a close genetic  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 i s 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 porphyries. orientations:  Also, the latter one sub-parallel  oblique to them.  to the green hornblende  usually display two prominent to the a l b i t i t e s ,  Mineralization must, however,  and one 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 b i o t i t e s 1977).  CWanless et a l . .  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 o s c i l l a t o r y zoning of clear, glassy plagioclase that from oligoclase to andesi ne (An2o~An4o-' •  ranges  Neither zoned  plagioclase nor such c a l c i c plagioclase compositions are seen in any e a r l i e r rocks in the Bralorne block,  possibly  because a l l these e a r l i e r rocks have been subjected extensive  hydrothermal alteration or lower  to  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 their compositions to a l b i t e  (An -An» ). a  0  and homogenized 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 alteration,  by the metamorphism and/or  i t i s l i k e l y that the mineralization/metamorphi  event died out during the interval 85-60 Ma. Congress mine 15 km to the northeast,  At the  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 r e s t r i c t e d 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. probably largely pre-85.7 Ma.  It i s post 91.4 Ma, and  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  t h i s i s regarded as u n l i k e l y .  This Late Cretaceous  mineralizing episode i s unusual amongst gold deposits of the Canadian C o r d i l l e r a , which are generally of T r i a s s i c 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 i s different  from other  vein deposits in the Canadian C o r d i l l e r a , 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 S i n c l a i r 1986) to about 45 Ma (actually, 51.5  between  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) i s not unexpected, and i s in agreement with the mineralization zonation outlined by Woodsworth et a l • (1977).  In t h i s zonation they postulated  that mineralisation became more epithermal, or lower temperature, in character with increasing distance from the CPC.  This i s 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 r a t i o s 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 i s provided by the stable isotope and f l u i d inclusion data of Maheux et a l . inclusion data of Vivian et a l . (Chapter 8).  (1987), the  fluid  (1987), and t h i s study  Naturally, erosion depths decrease away from  the margin of the CPC, as u p l i f t associated with the core of the complex decreases. If the known deposit ages are plotted versus their distance from the CPC, a r e l a t i v e l y 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, i t seems that mineralization along t h i s 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 f l u i d s .  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. weaker,  This was followed by perhaps two other  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 s e t t i n g of the B r i d g e River and  Cadwallader Groups  Isotopic dating of t h i s study on the Bralorne d i o r i t e 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 f o s s i l s as Mid- to Upper T r i a s s i c (Ladinian to Norian: 240 to 220 Ma).  None of the f o s s i l 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 f o s s i l 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 t h i s 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 r e a l . subjected  The sample from Bralorne has been  to ** Ar/ Ar dating, which has confirmed i t s Early <s  Permian age,  33  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 i s in conflict  with Middle Triassic  stratigraphic ages (225 Ma) assigned elsewhere on the basis of paleontologic several (1)  evidence.  The age c o n f l i c t  hinges on  key points: Paleontologic  evidence,  all  from outside the  Bralorne block, dates the Cadwallader and Bridge River Groups as younger than the Bralorne i n t r u s i v e s . (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, however,  some contacts are  the Bralorne intrusives;  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 i s 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 i n t e r c a l a t i o n s of volcanics 1ithaiogical1y  similar  to those of the underlying unit. The paleontologic  age of the Cadwallader Group i s  limited to the Upper T r i a s s i c 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  r a d i o l a r i a , have also been found in the Bridge River Group sediments at several  locations north of Bralorne along  Carpenter Lake (Cameron and Monger, 1971). paleontologic Jurassic  Here the  ages range from Middle T r i a s s i c to Lower  (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,  In the mine area, both d r i l l  1958;  Joubin, 1948).  core and underground workings  show Bralorne i n t r u s i v e s with abundant xenoliths,  apparently  To  of Cadwallader rocks, implying an intrusive r e l a t i o n s h i p .  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 comagmatic 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 an intrusive contact that was later  contact—probably  faulted.  Such  hornfelsing i s also evident on the Wayside property 5 km north of Goldbridge, where the adjacent  i n t r u s i v e , although  not containing the network of quartz/epidote  fractures  characteristic of the Bralorne d i o r i t e , i s seen in thin section to actually be a soda granite, with the d i s t i n c t i v e symplectic quartz - a l b i t e 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 d i o r i t e .  The alternative i s 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 i s 1ithological1y similar to the Cadwallader section established by Rusmore (1985). volcanic1astics, volcanic  Both contain volcanics,  and t u r b i d i t e sequences composed of  detritus.  Cadwallader Group sediments at several  locations  outside the Bralorne block (Wayside property, and Eldorado Basin) include a conglomerate unit that contains c l a s t s which resemble soda granite in both hand specimen and thin section.  One possible interpretation of t h i s r e l a t i o n i s  that the t r a n s i t i o n a l and sedimentary units of the Cadwallader are younger than the Bralorne intrusives, while the underlying volcanic unit i s 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 i s younger  (< 230 Ma), i t  would imply a volcanic event spanning at least 40 m i l l i o n years, since the lower portion of the sedimentary unit contains intercalations of the same volcanics. Alternatively,  i t i s possible that the rocks.mapped in  the Bralorne block as Cadwallader are d i s t i n c t  from those of  Carnian to Norian age mapped by Rusmore in the Eldorado Basin some 30 km along s t r i k e to the northwest.  However,  72the rocks from both areas look so a l i k e ,  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 i s that the volcanic unit of the Cadwallader i s 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 t h i s age or older, since they are intruded and assimilated by the Bralorne d i o r i t e along i t s southwest 5.2.3.1).  margin  (section  This appears to be the cause of a contaminated  border phase of the d i o r i t e that contains  c1inopyroxene  mantled by hornblende (this phase might have led to the term "augite d i o r i t e " so common in the Bralorne l i t e r a t u r e : 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,  diorite  ultramafic  appears  of u l t r a m f i c  Permian  suggests  that  t h e basement Groups, to  complex  age  they  may  p o s t u l a t e d by  Potter  Such a m a r g i n a l probably  formed  location  with  suggested  by  unusually  sodic composition  lead  isotopic  5.57.  Sr  low  floor  (Hughes,  evidence  initial  (section  evidence intricate  volcanics.  The  between d i o r i t i c intruding submarine,  i t s own the  and  p.S7;  slices formed  this  craton, 4).  suite,  that  mass  as  The  Coleman and c1ose  and  even  below  Donato,  t o t h ose  1979).  f or oc ean  interpretation. i s found  of  these  emplaced  Further  i n the  between t h e d i o r i t e  and  Pioneer  f i n e g r a i n e d phases i n t e r m e d i a t e  volcanic  volcanic  (1983).  i t s present  a  a  possibility  t e x t u r e s of  the  Cadwallader  K 0),  K 0) , suggests  3.4.3) s u p p o r t  contact r e l a t i o n s  of  g r a n i t e (a  o n l y 0.57.  i s o t op i c r a t i os,  supporting t h i s  from  (Chapter  of the soda  0.27.  1982  1987).  mantle that  American  i n t r u s i v e s were p a r t o f an o p h i o l i t e  The  Potter  possibly  f a r removed  3  2  floor  age  one  b a s i n •- o f f s h o r e a r c  N a 0 and  (4.57. N a 0 ,  the sea  more t h a n  (1983) f o r t h e S h u l a p s  r e s p e c t t o the North  trondh.jemi t e w i t h the d i o r i t e  the J u r a s s i c  represent oceanic  by  i n the  p e r s . comm.,  t o t h e B r i d g e R i v e r and  the east.  be  o f some o f t h e s e u l t r a m a f i c  as s u g g e s t e d  combination  by  east  t h e r e may  Church,  i s supported  ultramafic  The  (B.N.  bodies in  mass f u r t h e r  implying that  rocks  This suggestion Shulaps  fault-bounded  r o c k s of the Shulaps  B r i d g e R i v e r Group, age  as  suggest  products.  i n t r u s i o n s were  also.  that  the d i o r i t e  If the v o l c a n i c s  was were  7*  3.7 C o n c l u s i o n s  Mineralization at the Bralorne vein gold deposit took place long after  emplacement of the major body of d i o r i t e  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 t r a n s i t i o n a l between c a l c - a l k a l i n e and island-arc t h o l e i i t e basalt s. Intrusive contacts of the d i o r i t e 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 diorite intrusion.  Thus not a l l of the deformation of the  Bridge River marginal basin can be r e s t r i c t e d 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 i s in conflict with stratigraphic ages assigned elsewhere on the basis of paleontology.  The discrepancy of about 40  m i l l i o n years might be due to: rocks being stratigraphical1y fossils;  (. 1) the i s o t o p i c a l l y dated under the beds dated by  (2) the volcanics and sediments, long considered as  Cadwallader in the Bralorne area, being d i s t i n c t  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 i s 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 t h i s 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 r e l a t i o n to  the emplacement of the Bralorne d i o r i t e or soda granite, as has long been considered to be the case (cf. Cairnes, 1937). Instead, mineralization i s closely related to a sub-parallel swarm of p r e - , i n t r a - and possibly post-mineral dykes trending approximately p a r a l l e l to the mineralized veins. There appear to have been several pulses of mineralizing a c t i v i t y in the Intermontane Belt adjacent the Coast Plutonic Complex (CPC).  to  These pulses were  probably closely related to pulses of magmatic a c t i v i t y in the CPC.  There i s 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<£  r e l a t i v e l y high temperature (mesothermal, 300 to 450°C) AuAg~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 i s almost 60 km distant. et a l .  The metal zonation established by Woodsworth  (1977) p a r a l l e l s 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 I n t r o d u c t i o n  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 T e r t i a r y .  The data plot along a mixing  l i n e (c_f_. Andrew et al . , 1984) between the "upper c r u s t a l " 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 i s not within the Bridge River  camp but occurs within arc-related rocks of the Stikine Terrane along the western margin of the Intermontane superterrane.  It thus may also be genetically related to  intrusions that are s p a t i a l l y associated with the Coast P1ut on i c Complex.  4.2 M i n e r a l D e p o s i t s  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 t r a n s i t i o n a l 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 d i o r i t e and soda granite, greenstone.  and the Cadwallader Group  The timing of mineralization i s 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 and a K-Ar date on hornblende of 85.7 + 3  set,  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, tetrahedrite and s c h e e l i t e . quartz, c a l c i t e , mariposite.  galena,  sphalerite,  Gangue minerals are mainly  s e r i c i t e and ankerite, with rare  The veins average about one meter thick and are  mostly milky quartz ribboned with thin dark septae of slickensided s u l f i d e s ,  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, sericite,  ankerite, a l b i t e and pyrite to epidote,  chlorite  79  and c a l c i t e zones.  Ore shoots within the veins are  s t r u c t u r a l l y 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 d e t a i l s 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, metal assemblages characteristic  (Ag-Au-Sb-As+Hg)  and vein  of epithermal deposits.  and with  textures  Stibnite i s the  prominent sulfide mineral, while sphalerite,  galena,  chalcopyrite and tetrahedrite are more common than in the Bralorne deposits;  cinnabar i s present in some cases.  Gold-  s i l v e r r a t i o s at around 0.2 are the inverse of the r a t i o s in the Bralorne-type veins.  Microdiorite porphyry dykes at the  Congress and Minto deposits, group of deposits, 67.1  representative of the second  have been dated as Late Cretaceous at  + 2.2 and 69.4 + 2.4 Ma, respectively,  rock (Pearson, 1977;  by K-Ar on whole  Harrop and S i n c l a i r , 1986).  Mineralization in the Bonanza Basin i s 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.  At the nearby Lucky Gem property in Eldorado Basin,  data).  8o  mineralization i s also early Tertiary at 57.7 Ar on muscovite  (Chapter 3).  4.2.3  Type  Tyaughton  + 3.0 Ma by K-  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 (Woodsworth et a l . , deposition.  1977)  typical  These showings,  of  occurrences  low-temperature  which include prospects along  Tyaughton Creek and the Yalakom Fault, are characterized by jamesonite, Isotopic  4.2.4  stibnite,  tetrahedrite,  age data are not available  cinnabar and scheelite. for these occurrences.  Blackdome Type  Epithermal veins, such as at the Blackdome Au-Ag vein (Fig.  1-1;  Vivian et a l . . 1987), are characterized by vuggy,  c r u s t i f i e d and opaline quartz stockworks,  and zones of  s i 1 i c i f i cat i on containing 27. or less opaque minerals. include native gold and s i l v e r , acanthite, other sulphosalts,  s i l v e r selenides,  pyrrhotite, marcasite, chalcopyrite, sphalerite and galena.  These  tetrahedrite and  plus minor pyrite, bornite,  arsenopyrite,  The veins carry higher s i l v e r  than do the Bralorne-type mesothermal veins. i s Early T e r t i a r y , constrained between 51.5  values  Mineralization + 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  precise  information,  an age o f 45 Ma i s assumed  also for  Blackdome.  4.2.5  Summary In t h e B r i d g e R i v e r  to:  (1) t h e emplacement  which  range  biotite 44.7  of l a t e phases  e t a l • , 1977);  + 2.4 Ma by K-Ar on b i o t i t e  on b i o t i t e  ages  are ascribed  o f t h e Bendor  and L o r n a Lake (Archibald  (2) t h e Rexmount p o r p h y r y a t (Woodsworth,  1977);  and  fuchsite  e t a l . . 1989),  at Minto  t h e Red M o u n t a i n  (4) l a m p r o p h y r e  a t 45 + 1.5 by K-Ar  range  (Archibald  d a t e s f o r t h e Coast  to that  detailed  above  t o 55 Ma by K-Ar on b i o t i t e  Plutonic  e t a l • , 1989). Complex  and h o r n b l e n d e .  (Woodsworth e t a l • .  metals  decreasing vein  fluid  inclusion  quartz of the d e p o s i t s  Vivian  e t a l . , 1987).  plutons are  from west  (mesothermal  t o east  to epithermal),  (Au-As~W t o Ag-Sb-Hg) and age ( e a r l y This  84  1977).  t o be a p r o g r e s s i o n  to middle T e r t i a r y ) .  from  T h i s c a n be  included  type  show a s i m i l a r  for the mineralization,  t o 42 Ma i f t h e p o s t - o r o g e n i c Bendor  ( T a b l e 4-1) i n d e p o s i t  reset),  p o r p h y r y a t 47.4  extended  There appears  dykes  (Chapter 3 ) ,  (possibly  hornb1ende-plagioc1ase  + 0.5 Ma by K-Ar on b i o t i t e Overall,  (3) t h e  p l u t o n s a t 43.7 + 0.6 Ma by K-Ar  B r a l o r n e a t 43.5 + 1.5 Ma by K-Ar on b i o t i t e  vein  pluton  from 58.9 + 2.3 t o 42.3 + 2.9 Ma by K-Ar on  (Wanless  Beece Creek  at  camp, T e r t i a r y  Late Cretaceous  i s s u p p o r t e d by t h e t r e n d o f homogenization  temperatures i n  ( C h a p t e r 8; Maheux e t a l . . 1987;  Thus t h e 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 TYPE  BralornePi oneer  1  AQE (Ma) DISTANCE (km)  CongressMi nto  TyaughtonYalakom  Blac kdome  Mes  Mes-Epi  Epi  Epi  85  68  •-.  50-24 (45 )  8  20  40  350  •-.  60  38  T C°C)  3  EROSION DEPTH (km) d 0qt2= ls  Deep (6-10)  Moder ate (?2-4)  + 18 + 1  +22 + 1  d 0 f l u i d = + 12 + 1 lo  1 32 3  53 s 7  275"*  •->  Shallow (0.5-1)-*  Shallow s  + 10 + 1 ~*  +25 + 4  G  + 1 + 1 -*  + 8 + 2  17  -8 + 1 •*  Mes = mesothermal; Epi = epithermal. Distance from eastern margin of Coast Plutonic Complex. Estimated from f l u i d inclusion trapping temperatures. From Vivian et al . (1987). In per m i l , r e l a t i v e to SM0W. From Maheux et a l . (1987). From Nesbitt et a l . (1987).  100  60 km/50 Ma = 0.12 cm/yr  Age (Ma)  • 4-C—M  50  0  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 lower temperature (epithermal) deposits which l i e away.  youngest, farther  The relation between age and distance from the Coast  Plutonic Complex i s almost l i n e a r , 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 i s due not only to their greater age,  but  also to greater u p l i f t closer to the axis of the Coast PIut on i c Complex.  4.3 Galena Lead Isotope A n a l y s e s  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 d e t a i l s are in Godwin et al.  (1982).  Recent analyses by J . E . Gabites, F.R. Goutier,  and the writer were done as follows (cf. Hand-picked galena c r y s t a l s  Andrew, 1982).  (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) final- Deposit name yst 1  Lat. (N) Long.(W) Lead isotope ratios (decinal degrees) 206Pb/204Pb 207Pb7204Pb 208Pb/204Pb  3  a  Deposit type*  50.77 50.77 50.77 50.76 50.76 50.76 50.75 50.84 50.82 50.84 50.80 51.10  122.80 122.80 122.80 122.79 122.77 122.77 122.75 122.84 122.82 122.91 122.77 123.45  18.722 19.014 IB. 727 18.652 18.769 18.709 18.400 18.998 18.968 18.666 18.418 18.549  15.576 15.656 15.579 15.526 15.632 15.564 15.561 15.634 15.574 15.589 15.191 15.546  38.263 38.784 38.278 38.082 39.009 38.197 37.909 38.681 38.529 38.185 37.020 37.980  VeinsMes VeinsMes VeinsMes VeinsMes Vein:Mes Vein:Mes Vein:Mes Vein:Mes Vein:Mes VeimMes VeinsMes ReplsMes?  1 9.Lucky Gen 1 10.Lucky Strike 1 Robson 1 Bonanza Basin  50.99 50.98 51.02 51.02  122.89 122.86 122.88 122.88  18.545 18.923 18.947 18.944  15.671 15.598 15.607 15.560  38.126 38.499 38.490 38.461  VeimMes? VeinsMes? VeinsMes? VeinsMes?  30379-OOlfl <2) 30380-OOlfl (2) 30959-101 30%1-OOlfl <2) 30961-101 30645-101  2 2 3 2 3 3  50.90 50.90 50.90 50.90 50.92 50.89  122.75 122.75 122.75 122.75 122.77 122.78  19.063 18.997 19.042 18.920 18.932 18.912  15.635 15.610 15.612 15.601 15.608 15.605  38.727 38.634 38.633 38.635 38.570 38.615  VeinsMes/Epi VeinsMes/Epi VeinsMes/Epi VeinsMes/Epi VeinsEpi? Vein/Rep:Epi  30611-OOlfl (2) 30609-1/4A (5) 30610-001 30972-001 30612-001 30612-101/2A 30621-OOlft (3) 30941-OOlfl (2) 30065-001  3 14.Peerless 3 15.SuH.it 3 16.Kelvin 4 16.Reliance (Menika) 3 17.Olynpic 3 17.Olympic 3 18.Matson 2 19.Greyrock 4 20.Piebiter(Chopper)  50.93 50.87 50.89 50.89 50.89 50.89 50.77 50.80 50.72  122.79 122.52 122.75 122.75 122.74 122.74 122.21 122.70 122.60  18.935 18.934 18.985 19.030 18.998 19.055 18.834 18.920 18.927  15.603 15.609 15.613 15.633 15.616 15.606 15.5% 15.624 15.613  38.555 38.554 38.617 38.705 38.611 38.580 38.446 38.599 38.560  VeinsEpi VeinsEpi? VeinsMes/Epi Vein/ReplsEpi VeinsEpi VeinsEpi Repl? VeinsEpi? VeinsMes  30916-OOlfl (2) 30903-001  4 4  51.31 53.25  122.45 125.26  18.759 18.903  15.583 15.601  38.322 38.482  VeinsEpi StkwksEpi?  30000-001 -002ft (3) -003 -004 30001-OOlft (3) -002 30002-001ft (3) 30003-001 30350-001 30004-001 30337-001 30322-001  4 4 4 4 4 4 4 4 1 4 2 1  30391-001 30392-1/4B (5) 30346-1/3ft <3) 30414-001  1.Bralorne 1.Bralorne-Pioneer 1. Bralorne 41 Level 2.Bralorne Surface 3.Pioneer 5 Level 3.Pioneer 14 Level 4.P.E.6old<Pio.Ext)' 5.BRX (ftrizona) 6.BRX (California) 7.Veritas 8.Waterloo Tchaikazan 3  3  7  7  7  7  7  ll.Minto <0aega) ll.Minto (Gully) ll.Minto ll.Minto (Beta) 12.Golden Sidewalk 13. Congress Lou Zone  Blackdone Capoose  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 ». In quartz diorite of the Coast Plutonic Complex. 7  85  chloride c r y s t a l s 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, rhenium filament  (cf•  Cameron et a l . , 1969).  single  Lead isotope  ratios were then measured on a Vacuum Generators Isomass 54R s o l i d source mass spectrometer HP-85 computer.  linked to a Hewlett-Packard  Within run precision, expressed as a  percentage standard deviation, i s better than 0.01 "/., and the variation observed in duplicate analyses i s less than 0.1 "/..  Isotope r a t i o s 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, :zoe  P b / " * P b = 35.651.  a©?F /a©-*-p D  D  = 15.390, and  Analytical precision i s monitored by  20  repeated measurement of BHS-UBC1 and systematic analyses of samples.  Fractionation error and  =0  duplicate **Pb error  trends are shown on data plots so that trends in data can be assessed as being real or related to analytical  4.4 Galena Lead  problems.  Data  A l l 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, rocks, vein type and age. 2C6Pb/2°"Pb  and  respectively),  2o p B  b / a  o4  host  In terms of both their P b  r a t i o s (Figs. 4-2 and 4-3  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 a l . (1984), respectively. Ticks mark age in Ga.  O  88  deposits  (solid c i r c l e s ) 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 Sebert,  1987).  (cf•  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  20  '^Pb/ ' *Pb r a t i o s 20  cannot be shown to be s i g n i f i c a n t l y different limited range of ^ T ' b values.  ,  because of the  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 t h i s lead data. Kramers'  For example, Stacey and  (1975) model predicts a future age for the  mineralization.  A somewhat better f i t  curves that reflect  i s provided by growth  the evolution of lead in the radiogenic  upper crustal environment.  These are: (1)  the "shale curve"  model defined by Godwin and S i n c l a i r (1982), which estimates average lead isotopic evolution for shale-hosted deposits i n the autochthonous Selwyn shale basin and Foreland Belt of the western part of the Canadian C o r d i l l e r a  (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 d u p l i c a t e analyses reported i n Table 4-2. Sample No.  1  Deposit  name  of lead  isotope r a t i o s  Lead i s o t o p e r a t i o s Pb/ **Pb =ospKj/=eo»p Pb/=°-*Pb 207  :20  30000-002 -002R -002D  B r a l o r n e - P i oneer  19.012 19.016 19.013  15.656 15.657 15.656  38.785 38.787 38.781  30001-001 -001R -001D  Pioneer  18.772 18.765 18.769  15.637 15.625 15.634  39.021 38.985 39.022  30002-001 -001R -00 ID  P.E. Gold (Pioneer 18.423 Extension) 18.408 18.368  15.581 15.529  37.961 37.934 37.833  30916-001 -00 IP.  Blac kdome  •15.577 15.590  38.303 38.341  5 Level  18.754 18.764  1J  .  /  -Zi  D  S u f f i x e s a r e : R = repeat mass spectrometry of sample s o l u t i o n ; D = d u p l i c a t e a n a l y s i s of separate p i c k i n g s of galena from the hand specimen. A l l analyses were by the w r i t e r . 1  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  2osp /204p b  b  plot  (Fig. 4-2), but generally  follow a  l i n e joining the 0.1 Qa points of the mantle and upper crustal curves of Doe and Zartman (1979). 2o  8  p  b / 2 0  4  P b  versus  206  Pb/  204  p  Pb  l  o  t  S i m i l a r l y , in the  (Fig. 4-3),  the data  follow the 0.1 Ga l i n e from the upper crustal curve to the mantle curve, almost p a r a l l e l 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. source could be either the pelagic River Group or the minor f e l s i c  The upper crustal sediments in the Bridge  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 i n e isochrons" by Andrew et a l . (1984).  Such trends, within analytical  uncertainty,  reflect  91  contemporaneous mineralization. t h i s was not suspected  In the Bridge River camp,  from the deposits,  in type and widely spaced geographically.  which are diverse Even galenas from  deposits as remote from Bralorne as Blackdome, 60 km to the northeast,  plot within the  Similar conclusions  array.  of a single broad mineralizing episode for  (i.e.  deposits  s p a t i a l l y 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 C o r d i l l e r a in general by Nesbitt  et a l •  (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 r e l a t i v e l y uranium-depleted (lower mu, or lower environment.  a 3  «U/  a o ,  *Pb)  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 al•. the  1984). 2 0 B  Pb/  2 0 4  This i s evident Pb  from the pattern displayed in  versus = o»Pb/ "*Pb plot s  (c f. Andrew et  =so  (Fig. 4-3)  that  apparently defines a mixing l i n e which almost p a r a l l e l s the upper crustal and mantle curves.  Although the curves  themselves are almost p a r a l l e l in t h i s 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 207  Pb/ "»Pfa  versus  2C  206  Pb/=°"*Pb  plot.  by the  The trend i s 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 C a l i f o r n i a , Oregon, Washington, and Nevada (Church et a l . ,  1986).  This  i s 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"7p  b/  ao4  Pb  versus  20<a  Pb/ °-»Pb a  plot  almost p a r a l l e l to the curves on an =sos  Pb/  :5SO  "»Pb  plot  (Fig. 4-3),  (Fig. 4-2), but being ao,B  Pb/  ao,  *Pb  i s similar to that shown in  Doe and Zartman (1979, figures 2.4 to 2.10). 20  ' Pb/ -*Pb versus 7  3!0  20&  Pb/ °-*Pb 2  D  Plots of  (Fig. 4-3) allow age  estimates to be made, whereas plots of ao«aptj/2o-*p  versus  2 0 0  Pb/  2 0 4  Pb  versus  do not, in part because the mixing isochrons i n  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,  confirm the values.  since repeat analyses  Furthermore, duplicate  (Table 4-3)  analyses  (separate solution s t a r t i n g from a new fragment of galena; Table 4-3) shows that the v a r i a t i o n also cannot be explained by zonation within or among galena c r y s t a l s . that the large deposits,  Bralorne and Pioneer,  zonation within the deposits themselves.  It i s possible exhibit  It would, however,  93  require further analyses to establish whether or not t h i s i s the case (galena i s exceedingly rare in these deposits and a suitable sample d i s t r i b u t i o n might never be a v a i l a b l e ) . The Bridge River Terrane contains voluminous pelagic sediments which are ultimately of continental derivation. These, possibly with the minor f e l s i c 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 S i n c l a i r (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 i s 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) fields for pelagic sediments and oceanic volcanics could produce the observed f i e l d s of the Bridge River data.  Such mixing could have occurred between  lead derived from pelagic sediments or f e l s i c arc d e t r i t u s , 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. example, Maheux et a l .  For  (1987) and Nesbitt et a l . (1987)  noted progressive evolution in oxygen isotope values compatible with f a l l i n g 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 r a t i o s 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  Conclusions  Evidence from radiogenic and stable isotopes, combined with available fluid al..  1987; Sebert,  inclusion data (Chapter 8; Nesbitt et  1987; Woodsworth et al..1977) suggests an  extended Upper Cretaceous to early Tertiary mineralizing episode for the western Intermontane Belt adjacent Coast Plutonic Complex.  to the  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 t h i s study are consistent with a mixing model (cf. 2 0 7  Pb/  2 0 4  P b versus  2 0 6  Pb/  2 0 4  Andrew et al . . 1984.) for  P b plot, and may also display  mixing in a ^oopb/ao^pb versus  2 0 6  Pb/  2 0 4  P b plot.  Applying  the p r i n c i p l e s of t h i s model permits explanation of galena lead isotope data by allowing for different in different  mining camps.  sources of lead  In the Bridge River camp, both  upper crustal and mantle components are i d e n t i f i e d which may correlate with pelagic sediments of the Bridge River Group and/or f e l s i c 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 t h i s 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 epithermal deposits.  Pb isotope ratios from the smaller  This has also been noted by Doe (1968)  on a d i s t r i c t scale i n the western U.S.A.  It i s also of  possible exploration significance that only the  largest  deposit sampled (Bralorne-Pioneer.') showed significant differences the deposit.  in lead isotope r a t i o s for galenas from within Thus, i f the lead isotope r a t i o s of several  samples from various parts of a prospect were measured,  v a r i a b i l i t y o f the r e s u l t s 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 l a t e r ,  epithermal character.  more d i s t a l deposits of  This prospecting guide i s 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 interpretation of metallogeny.  contribution to make to  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, t h i s supports the premise (e.g. et a l . .  Nesbitt  1986) that the mineral deposits were formed by  widespread c i r c u l a t i o n of fluids focussed along major zones.  fault  Whether these fluids were of meteoric, metamorphic  or magmatic derivation i s not yet clear; evidence bearing on t h i s problem i s 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,  1988) or metamorphic (e.g. 1987)  origin.  Kerrich,  1987;  Nesbitt,  1983; Pickthorn et a l . .  Certainly there i s a magmatic influence in  the form of a heat source to drive convection, but how much of the fluid i s actually of juvenile derivation i s to ascertain.  difficult  Whole-rock lead isotopic measurements could  help to choose between various sources of upper crustal and mantle lead, and thus elucidate f l u i d paths and sources.  CHAPTER 5 MINE GEOLOGY  5.1  Introduction  The Bralorne-Pioneer vein deposits,  hosted in the  Bralorne d i o r i t e , Bralorne "soda granite" t o n a l i t e or trondhjemite)  (actually  and Pioneer volcanics,  the "Bralorne fault zone", or "Bralorne block"  albite  l i e within  (Fig. 5-1).  This block i s bounded by the s u b - p a r a l l e l ,  northwest-  trending Fergusson and Cadwallader  which are marked :  faults,  along their length by narrow, sinuous,  serpentinized  ultramafic bodies (the President i n t r u s i v e s ) . terminology  ("soda granite",  "albitite",  Mine  e t c . ) w i l l be used  throughout t h i s thesis to correlate with previous descriptions of the geology at Bralorne ( e . g . , Cairnes, 1937;  Stevenson,  1958).  with chemical analyses,  A table of formations (Table 2-1), calculated norms and estimated modes  in Tables 2-2 (Pioneer volcanics),  5-1 (major  intrusives)  and 5-2 (minor u n i t s ) , 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, F i g . 5—1, in pocket). soda granite, contact  The  a major dyke-like body, i s emplaced along the  between the Cadwallader Group—Pioneer  or Hurley sediments—and the d i o r i t e .  greenstones  The Bralorne d i o r i t e  and soda granite bodies thin toward the southeast,  so that  only soda granite i s found near the Pioneer mine.  Both  98  TABLE 5-1: Individual  Whole-rock c o m p o s i t i o n s o f major u n i t s i n t h e B r a l o r n e - P i o n e e r a n a l y s e s f o r a v e r a g e d s a m p l e s a r e i n T a b l e A-3-1, A p p e n d i x 3.  Diorites Pioneer Descr i pt i on S a m p l e Number C082B AVGDI C095 CD93 (4) <2> (8) (4) (N) ' Major- E l e m e n t s <*> 59. 74 5 5 . 52 SiO 47. 34 5 5 . 32 10. 91 13. 60 13. 12 01.O, 13. 71 0 . 50 0 . 40 1. 01 0. 23 TiO. 9. 65 9. 28 Fe.O, ( T o t a l Fe> 10. 32 7. 80 12. A l 5 . 07 7. 00 MgO 8. 45 5 . 89 7. 02 CaO 9. 03 6. 86 4. 71 1. 50 3. 60 Na. 0 3. 79 0 . 10 O. 08 0 . 14 K.O 0 . OA 0 . 18 0. 20 MnO 0 . 26 0 . 15 0 . 02 0 . 11 0. 03 P.O. 0 . 02 A. 58 3. 96 2.71 LOI 1.70 99.20 98.76 lOO.31 99. 80 TOTAL 2. 78 2. 85 2. 90 2. 84 Specific Gravity Minor* E l e m e n t s <ppm> 6 8 As 13 1 48 120 Bo 63 65 53 52 26 74 CI 24 30 30 Co 32 35 86 Cr 275 160 69 60 75 Cu 62 1 Mo 1 0 2 1 Nb 3 1 17 43 Ni 74 69 8 6 8 Pb 6 4 3 Rb 3 745 6000 375 S 430 0 ~2 Sb 3 2 190 125 Sr 270 110 V 200 ISO 270 145 20 17 Y 24 16 Zn 65 64 105 56 34 40 Zr 70 49 Normative M i n e r a l s 7. 1 Quart z 8. 1 lO. 5 Corundum O. 6 •rthoclase O. 8 0. 5 0. a 32.8 39.9 30. 5 12.4 Albite 15.7 19. 4 Anorthite 28. 6 12. 8 10.9 12. 3 Diopside 13. 7 17.8 14.7 Hy p e r s t h e n e 32. 5 19. O 20. O 2. 0 Olivine 3. 7 5.4 4. 5 Magnetite 3.8 0.9 0.8 1.8 0.4 Ilmenite 0. 04 O. 1 Apatite O. 3 0. 04 13 47 39 44 Diff»n Index Modes ( E s t i m a t e d V o l u m e X) 6 Quart z 10 10 .11... Albite 40 40 60 55 Mafici(Hblend) 48 30 33 (Cpx) 55* Ilmenite (Rutile) 3 2 1 1 P y r i t e <Py/Po> 2 tr 3 tr (No. o f S a m p l e s ) <1> (1) (1) (10) a  area.  A l b i t i t e H' b por Soda n r a n i t e s COB3 C092 AVGSG COS2A C094 (5) (2) (5) <5> <8> 74. 00 13. 38 0 . 19 2. 42 0. 44 2. OO 5.52 0. 69 O. OS O. 05 2. 47 100.20 2.66 12 102 28 4 10 11 1 3 6 14 760 2 SO 14 17 36 86  66. l O 14. 27 0 . 20 4. 74 1. 61 4. 00 6. OO 0. 12 0. 09 0. 11 2. 30 99. 54 2. 67  a  120 33 16 26  ia 0 4 7 3 4160 3 233 58 6 54 24  70. 00 13. 80 0. 20 3. 60 1. 03 3. 00 5.78 0. 41 0 . 08 0. 07 2. 40 99.78 2.66 10 110 30 13 17 14 1 1 3 7 9 760 2  63. 06 17. 09 0.21 2. 60  1.11  3. 90 1. 98 2. 45 0. 10 0. 12 4. S3 99.81 2. 72  11  706 14 9 12 6  1  2 9 39 950  2\-  51. 40 14. 76 0. 80 l O . 84 6. 48 8.47 3. 10 O. £ 5 0. 18 0. 19 2.72 99.50 2.91 3 245 57 35 135 36 , 2 " 1 36 4 5 310  1  30 12 43 60  29  350 230  32 69  lOO 61  19.7  25. 0  4.5  3. 8  0. 7 50. 8 11.6 6. 2 5. 2  2. 4 48. 9 11.0 3. 1 4. 5  32.8 5. 1  1.5 O. 4 O. 1 82  2. 3 0.4 O. 3 71  1.9 0. 4 0.2 76  30. O. 4. 47. 10.  4 02 1 4 1  ao  40  20  37  30  70 7  52  8 1 1  (1)  1 2  (1)  11 tr 1 (10)  llO  lO.  13.8  16.8 17. 9  4.0  22  1.3 26.2 25. 8 13. 1 19.6  2. 1 0.4 O. 3 63  4.3 1.3 O. 5 32  33* 3-'-  52 33  1. v  3 O  10  l_-. <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 a v e r a g e d i o r i t e and s o d a 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 o n f u s e d b e a d s and p r e s s e d p o w d e r s a t MESA U.K. lab,-England. S a m p l e 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 • A v e r a g e 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 - S o d a g r a n i t e - C082A = same- AVGSS - A v e r a g e o f two s o d a g r a n i t e s ; C092 = A l b i t i t e d y k e , 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 • G r e e n h o r n b l e n d e p o r p h y r y d y k e ; HBITE = a v e r a g e h o r n b l e n d i t e ; AVGAB => a v e r a g e a l b i t i t e d y k e , AVGGHO » a v e r a g e g r e e n h o r n b l e n d e porphyry; LAMP «• l a m p r o p h y r e d y k e . Norms w e r e c a l c u l a t e d w i t h a B A S I C p r o g r a m (FeO/Fe.O, was estimated f o r each r o c k ) .  99  TABLE 5—£: Description S a m p l e No.  Whole—rock  compositions  Grey dike Albitite C193 C038 C022  (N) ' (2) (4) Major Elements <*> SiO« 70. 9 6 3 . 31 01. 0, 12. 7 16. 76 Ti0„ 0. 24 0. 4 0 Fe. 0 , (Total Fe) 2 . 7 4. 00 MgO 1.47 1. 3 5 CaO 2. £ 4 3. 47 Na.O 5. 72 7. o a K.O 0. 86 0. 32 MnO 0. 07 0. 09 0. 06 0 . 17 P . o3 LQI 3. 24 2 . 71 TOTAL (100) 100. 32 Specific G r a v i t y 2. 68 2. 63 Minor Elements (ppm) Ps ND ND Ba 60 275 Co 34 14 Cr» 11 7 Cu 4 6 2 Nb 4 Ni 4 7 Pb 12 12 Rb a 16 S 2000 3700 Sb ND ND Sr 88 280 V» 27 55 Y 15 24 Zn 36 70 Zr 91 110 Normative M i n e r a l s Quart z 29. 6 7. 3 Orthoclase 20 5. 3 Albite 497 63. 6 finorthite 8. 6 a. 7 Diopside 4. 0 a. 7 Hypersthene 4. 8 4. 4 01 i v i n e Magnet i t e 0. 6 0. 8 Ilmenite 0. 5 0. 7 ftpatite 0. 1 0. 4 D i f f n Index ai 76 Modes ( E s t i m a t e d Volume• X) Quartz 34 15 Albite 56 60 MaficsHornblende 10 19 CIinopyx I l m e n i t e <Rut i l e ) t r 3 Sulfide (Py/Po) tr 3 <M> ' (2) <1> 1  of  <2>  minor  Dikes C4141  S5 5 73. 2 16. 5 13. 2 0. 25 0. 16 2. 85 1. 6 5 1. 3 2 0. 64 2. 70 1. 6 5 6. 50 6. 95 0. 62 0. 3 8 0. 09 0. 06 0 . 13 0 . 10 3. 54 2. 03 (100) (100) 2. 67 2. 62 B  14 14 1400 ND 200 25 13 57 84 17. 3. 56. 13.  9 7 0 3  7. 1  2 140 52 15 1 2 3 15 9 570 4 205 12 9 53 120 29. 2. 58. 4. 2. 2.  0 1 5 6 8 1  51. 14. 0. 10. 6. 8. 3. 0. o. 0. 2. 99. 2.  40 76 80 24 48 47 10 25 ia 19 72 50 91  3 245 35 1 35 36 1 36 4 5 310 1 350 230 22 100 61 4. 1. 26. 25. 13. 19.  10 as 5  8 54 36  (1)  2 (8)  (Glass (Biotite 45 2 (Apatite (1)  (2)  H' b 1 e n d i UM/HBITI  (Calc)  49. 5 48. 4 56. 1 15. 2 14. 7 10. 6 O. 3 O. 9 7 0. 72 13. 6 13. 2 9. 0 7. 04 9. 22 10. 7. 25 a. a i a. 4 4. o a 2. 82 3. 4 0 . 19 0. 05 0. 0 0. 24 0. 2 0. 20 0. 08 0. 05 0. 0 1. 9 7 2. 54 1. a (100) (100) 10O 2. 84 2. 96 ND 75 41 90 10 2  15 2200 34 100 130 17 110 27 47 6800 6 3200 300 19 130 430  1.5 4. 7 3. 3 48  20 70 7  (2)  50. 7 14. 0 2. 40 7 . 10 5. 82 7 . 81 2. 84 3 . 55 0. 09 1. 2 5 4. 57 ( 100) 2. 62  0. 3 21.3 26. 0 15. 5 13. 4 14. 0  4. 3 1.5 0. 5 32  area.  Rest i t e s Lamp, dike C043 C085 RESTI CI 0 3 3  5 5 2 8 1 6  0. 3 0. 3 0. 2 90  tr tr (1)  Bralorne-Pioneer  (2)  0. 6 0. 5 0. 3 78  1  i n the  Green dike C083 <5>  (2)  1 260 12 7 4 2 2  units  55 11 3 320 ND 300 290 25 78 42  ND 60 33 160 3 4 77 10 ND 180 3 300 210 25 120 21  1. 1 35. 5 23. 5 10. 9 11.5 12. 8 2. 7 1.9 O. 2 37  0. 3 25. 3 27. 3 15. 0 15. 0 13. 0 2. 6 1.4 0. 1 26  35 60  3 40 53  33)  5 5) (1)  3 1 (2)  54. 6 4. 84 0. 22 7. 20 15. 9 14. 9 0. 58 0. 04 o. i a 0. 02 1. 55 (100) 3 . 01 26 70 75 3800 .; 55 ND 195 35 ND 360 ND 25 ao 10 52 18  5. 4  15)  (2)  28.8 13. 6 22. 4 23. 1 4. 3 0.6 0. 02 34  4. 0. 5. 10. 51. 26.  3 2 0 5 7 3  1.5 0. 4 0 . OS 10 1 20 62 15 2 1 (6)  (N> i s number 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 modes, M i s number 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 known 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 in Ci—steel ring mill. 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. Otherwise a n a l y s i s i s by f u s e d d i s k . 1  *  /oo  these P.E.  u n i t s occur Gold  o n l y as dykes  property.  B r a l o r n e but  Thus t h e major  greenstone  Mineralization Cretaceous quartz  age  (85.7  Ma),  orientation mineral  Ma)  green  related  "albitite"  to post-mineral.  post-mineral. the  other  v e i n s and  than  lower  later  and  t o a swarm o f  60  similar intrarelated  appear  to  be  lamprophyre dykes c r o s s angle.  affected  fades  by  A l l rocks  early  Late  metamorphism.  M i n e Layout  s u r f a c e at  Noelton, area  dykes of  Ma,  W o r k i n g s a t t h e B r a l o r n e mine e x t e n d at  Upper  Slightly  therefore late  dykes at a high  greenschist  at  Pioneer.  "Bendor" d y k e s ,  black  dykes are  is diorite  the v e i n s .  a t about  (45 Ma)  earlier  t h e two  Cretaceous,  5.1.1  Eocene  the  (altered plagioclase-  Paleocene  pluton,  on  Group at  hornblende porphyry  are u s u a l l y u n a l t e r e d  t h e n e a r b y Bendor  cut  of the Cadwallader  is spatially  (31.4  ore host  p o r p h y r y ) dykes t h a t p a r a l l e l  later  to  further southeast  and  1400  down t o t h e  level,  for a t o t a l  During  the  therefore  vertical  c u r r e n t study,  '44'  each o t h e r  the  mine was  a r e each a p p r o x i m a t e l y except  near  the  work.  45  2 km  m below  level. m  (150  together  mine sea  ( F i g . 5-5b).  dewatered,  20th  level  o l d "King"  a t 600  s u r f a c e where t h e  were e s t a b l i s h e d c l o s e r  exploratory  level  "0'  Cosmopolitan,  i n the  r a n g e o f about  a c c e s s i b l e , only to the  mine l e v e l s  levels  m e l e v a t i o n , where t h e  Maud South v e i n s o u t c r o p  ( F i g . 5-1),  from t h e  and  Successive f e e t ) below first  during  few  early  /ol  At the f i r s t  level down in the old "King" mine area,  the o r i g i n a l l y discovered Lome (Shaft) 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 adit at 3 l e v e l ) , on 8 level  vein outcrops.  (accessed by a short  i s the surface expression of the 55 vein  (Fig. 5-2).  The 55 vein i s the offset portion  across the Empire fault of the main 51 vein, which i s stoped to surface near 2 level just south of the Empire fault 5-1)  and i s exposed there in a glory hole.  (Fig.  Much of the  production of the upper l e v e l s 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 Further to the southeast, 77,  (Bellamy and Arnold,  1985).  the largest vein in the mine, the  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 nearsurface angled diamond d r i l l  holes.  This d r i l l i n g was done  by Mascot Gold Mines L t d . during their recent of the mine.  re-evaluation  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 i b r a r y 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 r e - i n t e r p r e t a t i o n of the geology by the writer. Unfortunately, t h i s 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 t h i s 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 all  labelled with aluminum tags on the core boxes and with  footage markers that had been written in graphite and meticulously turned over.  (pencil)  Key intersections of major  veins and their altered wall rocks had fortunately been saved at the 26, 32, 41 and 44 levels before t h i s occurred,  /o3  permitting the study of v e r t i c a l 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 i r e many years ago CP,. Barclay, comm., 1986).  pers.  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 i s  available  regarding the d i s t r i b u t i o n of units with depth in the Bralorne mine. are d i f f i c u l t  Data from old level plans, while voluminous, 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: L i t h o l o q y . Petrography. P e t r o l o g y  The d i s t r i b u t i o n 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 d i s t r i b u t i o n of units in a section through the main adit,  and the main veins are shown in a diagrammatic  (Fig. 5-5b).  section  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 l e v e l , Bralorne Mine. Units, as in Figures 5-1 and 5-2, are defined in Table  /OS  5.2.1  Bridge River  (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 to i t and separated  from i t by the Fergusson  adjacent  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 t h i s group (Monger, 1977; Nagel, et a l . . 1982).  1979; Wright  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 r a d i o l a r i a of T r i a s s i c 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 i s not exposed,  but i t s  thickness i s at least a thousand meters (Cairnes, Because i t i s highly stratigraphy,  Potter  1937).  deformed and lacks a coherent (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 i s divided here into metabasalts 2).  (unit  1),  and ribbon cherts and a r g i l l i t e s  (unit  However, Church (1987) and Church et al . (1988) contend  that no d i s t i n c t i o n i s possible between greenstones of the Bridge River and Cadwallader groups, and place a l l these volcanics into the Cadwallader Group (section  5.2. 1. 1 Volcani c.s (uni t  2.2).  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. flows,  Elsewhere, amygdaloidal  pillow breccias and light green tuffaceous  well developed  (Potter,  1983).  t h o l e i i t i c metabasalts oceanic-island basalt basalt  flows,  The greenstones  of mid-ocean ridge basalt  pillowed  layers are  are (MORB) to  (OIB), or possibly back-arc basin  CBABB) character  (Potter,  1983: Table 2-2 and section  The greenstones are made up p r i n c i p a l l y of  lath-shaped  a l b i t i s e d plagioclase  to 2 mm with trachytic  to 2 mm c1inopyroxene  r e l i c s that are occasionally mantled  by hornblende grains, and amygdules,  rare 1  chlorite-calcite-epidote  a l l of which are surrounded by a fine-grained  groundmass of c h l o r i t e and opaques. than 1 meter thick, cherts,  texture,  Tuffaceous  layers,  less  form pale green interbands in ribbon  and may grade into coarse sandstone  (Potter,  1983).  /o7  5.2.1.2 Sediments (unit 2) Dark grey to black ribbon chert with thin intimately interbedded septa of graphitic a r g i l 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  r a d i o l a r i a n chert with nodular structures are common (Cairnes,  1937).  C h a r a c t e r i s t i c a l l y , the chert i s highly deformed, fractured and veined by white quartz in the Bralorne area. Both Potter  (1983) and Rusmore (1985) have suggested that  folding i s mainly attributable to slumping of  soft  sediments, since the fold axes are markedly disharmonic.  I  agree with t h i s interpretation, even though Church (1987) maintains, probably i n c o r r e c t l y , 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. t h i s limestone.  No dateable  f o s s i l s have been found in  /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,  o r i g i n a l l y 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 d i v i s i o n 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. found i t d i f f i c u l t  The writer also  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, t h i s does not mean that Cairnes'  o r i g i n a l d i s t i n c t i o n in the Bralorne area was not r e a l ; Church (1987) retains the Noel and Hurley d i v i s i o n s . The Cadwallader rocks in the Bralorne mine area consist of a volcanic package (unit 3: the Pioneer formation greenstones)  that i s 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  t u r b i d i t e s 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 T r i a s s i c (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 o r i g i n a l l y 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 t h i s 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 t h i c k .  There i s such a s t r i k i n g s i m i l a r i t y between the 1ithologies of the Cadwallader rocks in the Bralorne block and those of the Eldorado Basin that a correlation seems likely  (see Table 2-2 for chemistry of the Pioneer  volcanics  in the Bralorne, Eldorado and Goldbridge areas).  Rusmores'  (1985) analysis of the Cadwallader  it  indicates that  represents a marine volcanic arc, overlain by a submarine fan of proximal to d i s t a l turbidites that received sediment from fault scarps and terrigineous sources consisting of carbonate and igneous rocks.  5.2.2.1 Pioneer Formation (unit 3) Although commonly termed "greenstones" the Pioneer Formation in the Bralorne block, spi1itisation,  preserve effusive  andesites,  following, the prefix  They should  but in the discussion  "meta-" w i l l be dropped for the sake  The volcanics include aquagene breccia  and pillows,  amygdaloidal  which in places  and pyroclastic textures.  properly be termed meta-volcanics,  5-1)  before  metamorphism and hydrothermal a l t e r a t i o n ,  consisted mainly of basaltic  of brevity.  in mine usage,  lapili  tuffs,  (Plate  and fine grained massive  flows to medium grained feeder dykes or s i l l s .  Paler s i l i c e o u s flows,  some with abundant quartz eyes, are  infrequently seen, but are p a r t i c u l a r l y 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) dating (section 3.4.3).  and Rb-Sr  Analyses of a mafic and a f e l s i c  unit from the Pioneer volcanics are also reported in Church et a l .  (1387).  The volcanics analysed in t h i s study range  from intermediate to f e l s i c and chemical analyses  in appearance, but thin section  (Table 2-2)  suggest that  before  metamorphism they were basalts and basaltic andesites the exception of C096A, a quartz-eye mine).  (with  flow at the Pioneer  These meta-volcanic rocks are now composed  p r i n c i p a l l y of r e l i c t plagioclase  Calbite,  An -Es, 0  andesine, A n ) and hornblende or c1inopyroxene 30  replacing  phenocrysts  in a felted mat of plagioclase microlites and i n t e r s t i t i a l c h l o r i t e and/or a c t i n o l i t e significant  (Plate 5-2 a-d).  Small but  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 q u a r t z - c a l c i t e - e p i d o t e - c h l o r i t e  (Rusmore,  1985 also reports pumpellyite and prehnite) are common and c h a r a c t e r i s t i c of the Pioneer basalts  (Plate 5-2  a,b).  Magnetite and apatite are rare in the Pioneer rocks, but 1 to 3 7. pyrite i s common. altered to c a l c i t e ,  Plagioclase i s usually strongly  c h l o r i t e 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 t h e P i o n e e r F o r m a t i o n o f t h e C a d w a l l a d e r Group ( a s o r i g i n a l l y d e f i n e d i n t h e B r a l o r n e mine a r e a by C a i r n e s , 1937). From t h e P a c i f i c E a s t e r n G o l d p r o p e r t y ( F i g . 3 - 1 ) , DDH-P-85-02, a t a p p r o 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. M i n e r a l o g y and t e x t u r e s of the P i o n e e r 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 ( B r a l o r n e mine, DDH S B - 8 4 - 1 7 / 3 0 0 ) , r e l i c t p l a g i o c l a s e p h e n o c r y s t s 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 d a r k m a t r i x o f c h l o r i t e and o p a q u e s , and f i e l d of v i e w i s 0.25 cm ( c r o s s e d p o l a r s ) . I n ( b ) , from sample C098D ( P a c i f i c E a s t e r n G o l d p r o p e r t y , DDH P-85-03/500m), s i m i l a r m i n e r a l s and t e x t u r e s a r e i l l u s t r a t e d ( f i e l d o f v i e w 1 cm; c r o s s e d p o l a r s ) . I n ( c ) , from sample C2001A ( g r e e n and p u r p l e p i l l o w b a s a l t from h e a d w a t e r s o f E l d o r a d o C r e e k , 1.5 km e a s t o f 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 m a f i c p h e n o c r y s t s a r e a l t e r e d t o e p i d o t e ( b r i g h t ) , c h l o r i t e ( g r e e n ) or c a r b o n a t e ( w h i t e ) . Highly birefringent 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 o f v i e w 1 cm, c r o s s e d p o l a r s ) . In ( d ) , from sample C2001B (on r i d g e above 2001A) t h e t e x t u r e i s a l m o s t g a b b r o i c , 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 ( g r e e n ) , 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 o f v i e w 1 cm, p l a n e p o l a r i z e d l i g h t ) . 1  were analysed in this study, Potter  (1983).  by Rusmore (1985), and by  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 picrites.  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 t r a n s i t i o n a l between island-arc t h o l e i i t e s mid-ocean ridge basalts  (MORB).  (IAT) and  A similar conclusion was  reached by Rusmore (1985) for the volcanics of the Eldorado Basin, although the Bralorne block samples tend towards a more c a l c - a l k a l i n e basalt  (CAB) character  (Fig. 2-2  a-c).  The one sample at Bralorne that i s not basaltic composition i s C096C (Table 2-2:  in  647. SiO*, 3.67. Na 0) .  This  2  rock, as noted by Cairnes in 1937,  i s a quartz  keratophyre  (dacite which has been altered to a l b i t e 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 (i.e.. al..  basalts  basalts altered to a l b i t e and c h l o r i t e : Carmichael et  1974), characteristic of sea-floor  vocanic rocks.  alteration of  Rusmore (19B5) noted an "andesite" and a  " r h y o l i t e " in her volcanic package,  and Church et a l .  (1988)  analysed a "rhyodacitic breccia" from near Gwynneth Lake with 727. SiO.2, 47. Na 0 and 2.57. K 0 . 2  a  Thus there are minor  indications of f e l s i c  volcanics within the Cadwallader p i l e ,  suggesting accumulation in an arc  setting.  5.2.2.2 Hurley Formation (unit 4) Hurley sediments in the v i c i n i t y of the BralornePioneer mine comprise rhythmically interbedded green volcanic wacke and darker a r g i l l i t e or s i l t s t o n e .  They  resemble A-E Bouma cycles of d i s t a l t u r b i d i t e s  (Mutti and  Ritti-Lucci,  as defined by  1978).  Noel Formation sediments,  Cairnes C 1 9 3 7 ) , consist of black a r g i l l i t e s that are less calcareous than those of the Hurley, but the d i s t i n c t i o n 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) fine grained matrix of the same.  Some small  in a very  recognizable  rock fragments are of volcanics similar to the Pioneer Formation (unit 3 ) .  Consequently,  largely v o l c a n i c l a s t i c  the Hurley sediments are  and were derived from the underlying  volcanics; as Cairnes (1937) notes,  "Every gradation was  observed to exist between fragmental formation and tuffaceous formation."  members of the Pioneer  sediments of the succeeding, Hurley  The sediments show a general  black a r g i l l i t e ,  cherty a r g i l l i t e ,  fining upwards:  and green or brownish  //7 cherty tuff  occur higher in the section.  The total  thickness of the sedimentary portion of the Cadwallader about 2000 m (Church,.1987;  is  Cairnes, 1937).  Other features d i s t i n c t i v e of the Hurley Formation are thin green andesitic  flows that are megascopical1y  identical  to those of the underlying Pioneer Formation; Rusmore (1985) also notes t h i s in the Eldorado Basin.  G r i t s or  conglomerates containing well-rounded m u l t i l i t h i c clasts are also c h a r a c t e r i s t i c  (Garver et a l . ,  not seen in the Bralorne block. include f e l s i c granite"  Near Goldbridge, these  igneous c l a s t s that resemble the "soda  (section 5.2.3).  Limestone fragments,  across, contain Mesozoic gastropods olistoliths  1989), but these were  (exotic  blocks)  up to 50 m  (Cairnes, 1937).  Large  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,  "augite d i o r i t e " (unit 6) and "soda granite" mine usage.  called  (unit 7) in  These two units, with the "Pioneer  greenstone"  (unit 3) and "greenstone d i o r i t e " (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, Joubin, 1948; Stevenson,  1958) although a l l that  1937; i s known  //8  about their age i s 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 s t r i k e to the northwest  from the Pioneer, through the King  mine area, to the northern edge of Figure 5-1. outline, the main body of soda granite  In broad  (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 d i o r i t e and soda granite taper out towards the southeast  near the Pioneer mine; here the d i o r i t e i s cut out  by the soda granite, fault.  which i s cut off by the Cadwallader  To the northwest,  between the Bralorne and King  mines, the soda granite fingers out as dykes into the diorite.  At i t s widest,  the soda granite i s about 400 m  thick, and l i k e the d i o r i t e i t also ocurs over almost 4 km of s t r i k e length.  In general, the d i o r i t e and soda granite  widen s l i g h t l y with depth, indicating steeply outward dipping  contacts.  5.2.3.1 President Ultramafic Rocks (unit 5) The ultramafic rocks in the Bralorne-Pioneer mine are mainly r e s t r i c t e d 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 t o t a l l y 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. 2) are less altered and tectonized,  1-  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  t h i s 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 l h e r z o l i t e to minor pyroxenite.  Although variably  serpentinized, cumulate and intercumulate textures are l o c a l l y recognizable.  Some dull black masses represent  total replacement of o l i v i n e by a n t i g o r i t e , (Kerr,  1959,  serpophite  p. 416) and lesser talc cut by s l i p - f i b r e  veinlets of c h r y s o t i l e .  A typical r e l i c t peridotite i s  /2o  P l a t e 5-3. T h i n s e c t i o n views, (1 cm wide, crossed p o l a r s ) of r e l i c t u l t r a m a f i c rocks from the Cadwallader f a u l t zone. (a) i s from sample C1034, u n l o c a t e d core from o l d Bralorne Mines L t d . d r i l l i n g ) , showing f o l i a t e d nature and o u t l i n e s of b a r e l y d i s c e r n i b l e r e l i c t c r y s t a l s , (b) i s from sample C 1 3 8 B (DDH U B - 8 0 - 7 / 4 7 2 ' ), and shows o p t i c a l l y continuous mass of hornblende (dark) e n c l o s i n g cumulate-textured r e l i c t o l i v i n e g r a i n s (now s e r p e n t i n e , white) and l a r g e r pyroxene (orange).  /2/  characterized by abundant coarse euhedral grains of "bastite"  (enstatite altered to antigorite and tremolite),  l o c a l l y up to 1 cm across.  Similar sized subhedral  c1inopyroxene grains are mantled by hornblende, altered to c h l o r i t e , tremolite and t a l c .  subsequently  Like the  orthopyroxene, they p o i k i 1 i t i c a l 1 y enclose small altered o l i v i n e grains.  Both the c l i n o - and orthopyroxenes are set  in a matrix of finer  C<0.5 mm) euhedral o l i v i n e r e l i c t s , now  largely psuedomorphed by t a l c , magnetite or ilmenite. which often  serpentine and fine-grained  Other opaques are mainly chromite,  fringes p i c o t i t e  (iron-chrome s p i n e l ) , and minor  sphene and p y r i t e . Plagioclase,  absent  from the main mass of ultramafics,  increases as the adjacent  d i o r i t e i s approached.  rocks are ophitic-textured,  These  with feldspars i n t e r s t i t i a l  the amphibole mantled pyroxenes.  The plagioclase  to  is  unusually sodic for such mafic rocks, ranging from a l b i t e to oligoclase  CAno-io).  No chemical analyses were obtained for the ultramafic rocks in t h i s study, al.  but both Cairnes (1937) and Church et  (1988) report analyses  for the President intrusions.  They contain about equal amounts of SiO^ and MgO (407. each), about 107. total  i r o n , small amounts of CaO (less than 57.),  and about 107. v o l a t i l e s . relatively  There i s l i t t l e variation from the  fresh to the strongly serpentinized samples,  implying isochemical  alteration.  /22 Although e a r l i e r workers (Cairnes, Stevenson,  1958)  1937,  1943;  considered the ultramafics to be later than  the Eiridge River, Cadwallader,  and Bralorne intrusive rocks,  evidence from t h i s study suggests the opposite.  Dykes of  d i o r i t e 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 p a r t i c u l a r l y 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 a d i t , F i g . Here the gradation, over 50 to 100 m, i s from (1)  5-1). 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) p e r i d o t i t e with r e l i c t  o l i v i n e , enstatite and abundant c1inopyroxene.  This  indicates that the augite of the "augite d i o r i t e " probably came from t h i s 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 d i o r i t e ) 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 c i r c u l a t i n g waters below the sea floor, greenschist  facies metamorphism, or hydrothermal  alteration associated with mineralization.  Hydrothermal  alteration i s also l i k e l y responsible for the t a l c , tremolite, c h l o r i t e , ankerite and z o i s i t e observed in some of the ultramafics.  5.2.3.2 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 i s a variable unit, including  rocks ranging from dark, mafic-rich d i o r i t e s , to ultramaficlooking 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  5.2.3.1) 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 1937)  (this study;  Cairnes,  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 h o r n b l e n d e s u r r o u n d i n g c o r e s o f r e l i c t u l t r a m a f i c r o c k 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 t h e d i o r i t e ; w h i t e i s I n t e r s o d a g r a n i t e .  Plate 5-4(.b).Thin s e c t i o n view of coarse hornblende g r a i n s p o i k i l l t i c a l l y e n c l o s i n g cores of clinopyroxene i n the h o r n b l e n d l t e u n i t (6a) developed by a s s i m i l a t i o n of u l t r a m a f i c rock i n t o 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 p o l a r s .  /2S* d i s t i n c t i v e of the Bralorne d i o r i t e and soda granite. Bralorne intrusions and the ultramafic  rocks are  probably unrelated, and have been introduced  therefore  separately.  Hornblendite at Bralorne represents altered rocks, as discussed above.  The  ultramafic  The texture of the o r i g i n a l  ultramafic rock i s strongly modified to a net of coarse dark green hornblendes (mantling pyroxene) continuity over large areas, pyroxene,  that show optical  and p o i k i 1 i t i c a l 1 y  o l i v i n e , and plagioclase  hornblende i s not serpentinized,  enclose  (Plate 5-4b).  The  but i s altered at  its  margins to tremolite and at i t s centers to c h l o r i t e .  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 a c t i v i t y ultramafic.  in the  In any case, the amphibole c r y s t a l l i z e d  than metamorphism or hydrothermal  earlier  alteration.  5.2.3.3 Bralorne Diorite (Hornblende Quartz D i o r i t e , unit  6)  The main mass of the Bralorne intrusion between the bounding Fergusson and Cadwallader d i o r i t e (Fig. 5-1). the prefix  i s called Bralorne  Although t h i s i s properly meta-diorite,  "meta-" will be omitted (but understood)  discussion following. distances  faults  in the  The d i o r i t e varies l o c a l l y over short  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,  zoisite,  carbonate and prehnite (Plate  5-5).  /26  P l a t e 5-5. Barren (pre-mineral) pale green q u a r t z - e p i d o t e - c a l c i t e prehnite stockvork t h a t d i s t i n g u i s h e s the Bralorne d i o r i t e . From SB-84-34/500 ; f o r t h i n s e c t i o n of s i m i l a r m a t e r i a l , see Plate 6-2 1  /27  T y p i c a l l y the d i o r i t e 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 a l b i t e  ( A n - i o ) and hornblende (Plate 5~6a). 0  There are also accessory, skeletal  distinctive,  altered grains of  ilmenite (Plate 5-6b), and lesser p y r i t e ,  and sphene, and rare zircon.  apatite  It i s a medium-grained rock,  with a 1 to 3 mm average grain s i z e .  Although t r a d i t i o n a l l y  called "augite d i o r i t e " , c1inopyroxene i s conspicuously absent  from a l l the d i o r i t e s studied at Bralorne except  those proximal to ultramafic bodies (section 5 . 2 . 3 . 2 : ) .  The  abundance of hornblende and lack of pyroxene led Cock f i e l d and Walker ( 1 9 3 2 ) to propose the name hornblende d i o r i t e , and the present study confirms t h i s . plagioclase  (albite,  The abundance of sodic  rarely to oligoclase)  seen ubiquitously  in t h i s study i s also out of place with a rock as mafic as diorite.  Although Cairnes (1937) describes plagioclase up  to andesine or even bytownite, Stevenson's  the present work confirms  (1958) findings of almost pure a l b i t e 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 i s rare in the d i o r i t e according to Cairnes (1937). Chemistry of the d i o r i t e i s s i g n i f i c a n t  (Table  Most noteworthy i s the extremely low potash content detection l i m i t of 0.17.  5-1). (at the  for the average of 8 analyses,  although Boyle (1979) reported 0.37. and Cairnes (1937) reports up to 0.87.),  which i s unusually low for d i o r i t e s  /28  P l a t e 5-6. (a) T h i n s e c t i o n v i e w (1 cm w i d e , t r a n s m i t t e d p l a n e p o l a r i z e d l i g h t ) o f t h e B r a l o r n e d i o r i t e i n sample C093A (DDH UB-8117/525') s h o w i n g l a c k o f h y d r o t h e r m a l a l t e r a t i o n ( t h i s sample u s e d a s 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 c h e m i c a l s t u d i e s of a l t e r a t i o n ) . R o u g h l y e q u a l amounts o f p a l e g r e e n h o r n b l e n d e and c l o u d y 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 quartz.  (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 f r o m sample C116 (DDH 41 Q-144/200*) s h o w i n g s k e l e t a l remnants o f f o r m e r 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 t h e 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 t o rutile. F i e l d o f v i e w 0.25 cm, c r o s s e d polars.  /X9  (Nockolds,  1954)  of sea-floor  and suggests a relation to  affinity  (Hughes,  1982).  "piagiogranites"  No K-feldspar occurs  in any of the Bralorne rocks, so i t 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 d i o r i t e (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 t h i s study,  as  compared to the analyses reported by Cairnes (1937), agree with those of Boyle (1979) and reflect  the more typical  quartz d i o r i t e s that were sampled compared to Cairnes', which came from areas near the Cadwallader  fault.  Abundant small areas of "greenstone d i o r i t e " 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 portions.  lighter  The variation i s 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 d i o r i t e , through even-grained and sugary greenstone,  recrystal1ized  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 " .  A l l of t h i s i s then overprinted by  the intrusion of soda granite with similar complex dyke  /30  relations, and then by a l t e r a t i o n , producing a complexappearing migmatite  (section 5.2.3.5).  Several intrusive phases of d i o r i t e are distinguished on the basis of their r e l a t i v e l y These might represent,  fine or coarse nature.  respectively,  syn-volcanic  dykes and batholithic phases (Cairnes, of 1ight-coloured  1937).  feeder  The stockwork  veinlets that distinguishes the d i o r i t e  could be due to late deuteric alteration at the time of intrusion of the soda granite,  which i s also affected  them, or i t could be due to later metamorphism.  by  This  stockwork i s barren of sulfides and i s cut by mineralised veins.  Ubiquituous a l b i t e in the d i o r i t e i s presumably due  to greenschist  facies metamorphism and hydrothermal  al t er at i on .  5.2.3.4 Soda Granite (Trondh.jemite/Albite Tonalite, unit 7) The main body of soda granite i s found along the northeast side of the Bralorne d i o r i t e , but i t also forms many dykes cutting the d i o r i t e . and soda granite,  The contact between d i o r i t e  l i k e the d i o r i t e - greenstone contact,  is  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 r e t i c u l a t e network in t h i s 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 i s  usually p a r a l l e l to a weakly developed, broadly east-west f o l i a t i o n in the d i o r i t e .  The contact with Cadwallader  Group rocks on the other side of the soda granite,  however,  i s sharp, linear and usually sheared, although the Cadwallader rocks look recrystal1ized  and bleached.  Typically the soda granite i s a leucocratic,  coarse-  grained granitic rock composed of 407. quartz as large anhedral grains to 1 cm s i z e , 507. squat a l b i t e (An -o) 0  of 3 to 5 mm average s i z e ,  crystals  and 5 to 107. thoroughly  c h l o r i t i s e d 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 b i o t i t e 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 p y r i t e ,  pyrrhotite, molybdenite, z i r c o n , sphene and apatite.  As in  a l l the other Bralorne intrusions, no orthoclase can be found and therefore the term "soda granite" i s somewhat misleading.  It more properly should be called a  trondhjemite, a l b i t e t o n a l i t e ,  or piagiogranite  (Coleman and  Donate, 1979). Low-grade a l t e r a t i o n of the soda granite i s widespread, with former amphibole s i t e s now replaced by c h l o r i t e , epidote,  s e r i c i t e , carbonate and p y r i t e .  Another  c h a r a c t e r i s t i c 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 , epidote and/or z o i s i t e and carbonate.  chlorite,  This i s commonly very  c l o s e l y spaced, and possibly indicates the b r i t t l e n e s s of the soda granite.  A part of t h i s stockwork i s similar to  the barren pre-mineral stockwork found in the d i o r i t e , although i t lacks the prehnite identified in these veins in  /3Z  P l a t e 5-7. T h i n s e c t i o n v i e w (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 t h e B r a l o r n e s o d a g r a n i t e i n cample C094A (DDH UB-8425/440') s h o w i n g s l i g h t a l t e r a t i o n t o 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 u s e d as t h e l e a s t a l t e r e d r e f e r e n c e f o r s o d a g r a n i t e ) . L a r g e a l b i t e g r a i n s ( c l o u d y , o c c a s i o n a l l y t w i n n e d ) 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 m i g m a t i t e s ( " a g m a t i t e s " ; M e h n e r t , 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 ( d a r k e s t , f i n e g r a i n e d ) i n d i o r i t e (medium g r e y , c o a r s e g r a i n e d ) f u r t h e r c u t by i r r e g u l a r t o s h e e t e d d y k e l e t s of s o d a g r a n i t e ( l i g h t e s t c o l o u r , c o a r s e g r a i n e d ) . From DDH UB-14/150*.  t h e 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 i s d i s t i n c t in  i t s major elements from a c a l e - a l k a l i n e 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 i s more s i l i c e o u s (70V. S i 0 2 ) than the d i o r i t e (55 - 607.  Si0 ). 2  The soda granite i s younger than the d i o r i t e on the basis of i t s dykes that cross-cut the d i o r i t e , although i t i s 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)  it  appears to be related to the d i o r i t e , containing similar minerals although in different  proportions.  e a r l i e r opinions (James and Weeks, 1961;  Contrary to  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 w i l 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 Hughes (1982, p.67).  "plagiogranite" intrusions described by  /34~  5.2.3.5 Migmatite Contacts between the d i o r i t e and soda granite are complex and often resemble migmatites Migmatite does not, however, mine area.  (Plate 5-8).  form a mappable unit in the  P a r t i a l 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 additionsubtraction 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 H 0 and HC1 (Mehnert, 1971). 2  Derivation of the Bralorne soda granite from the d i o r i t e by p a r t i a l melting at the present level of exposure has been suggested (C.I. Godwin, pers. comm., 1985) explain the presence of " r e s t i t e s " ,  to  or dark-coloured masses,  in the Bralorne intrusions, since the d i o r i t e and soda granite show migmatitic contact r e l a t i o n s h i p s .  To test t h i s  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 d i o r i t e 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 r e s t i t e 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 r e s t i t e composition.  They also have normative  o l i v i n e , while the calculated r e s t i t e does not.  Except  for  higher iron they correspond most closely to analyses of Pioneer basalts,  which also have normative o l i v i n e (C095,  C0S6, C098: Table 5-2).  This supports f i e l d  that suggest the " r e s t i t e s " volcanics,  observations  are merely xenoliths of the  i n i t i a l l y partly assimilated by the d i o r i t e and  then both intruded by the soda granite (contact  relations  described in section 5.2.3.3). I favour the alternative explanation for the o r i g i n of the soda granite, diorite.  by normal d i f f e r e n t i a t i o n  from the  Although i t i s clear that the contact zone between  d i o r i t e and soda granite i s a migmatite (variety  agmatite:  Mehnert, 1971)  explained  I feel that the texture i s better  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 i s partly obscured by dark basaltic  xenoliths  ("restites")  intrusion of the soda granite,  included in d i o r i t e prior to and i s therefore not clear in  the two-dimensional exposure of d r i l l core (Plate 5-8), but i t can c l e a r l y 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 diorite,  i t cannot have been derived by p a r t i a l melting of  the d i o r i t e during a younger thermal event such as  /36  P l a t e 5-9. Contact r e l a t i o n s of Bralorne d i o r i t e (grey-green, on r i g h t ) and soda g r a n i t e ( l i g h t g r e y ) . There appears to be a t h i n white c h i l l e d margin on the soda g r a n i t e , which i s s i m i l a r to t h i n d y k e l e t s c u t t i n g the d i o r i t e . X e n o l l t h s of b a s a l t i c 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 i n soda g r a n i t e but d i f f i c u l t to d i s t i n g u i s h i n 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 R i v e r .  /37  emp1acement of the Coast PIut oni c Complex.  Ear 1y Per mi an  p a r t i a l melting of the d i o r i t e at depths below the present level of exposure to produce soda granite, which was then intruded at the present l e v e l , 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 i s a differentiate  of the same magma that produced the  diorite.  5.2.3.6 Aplite Dykes Aplite dykes are thin (less than 1 m) and i r r e g u l a r , and are found cutting the Bralorne soda granite. difficult  They were  to separate from soda granite in d r i l l core and  could be widespread, although volumetrical1y in the Bralorne intrusive mass.  insignificant,  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 a p l i t e 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 a p l i t e .  They probably are final differentiates  the d i o r i t e - soda granite system, and as such are Early Permian in age.  of  /33  P l a t e 5-10. T h i n s e c t i o n v i e w (1 cm wide, c r o s s e d p o l a r s ) o f a p l i t e dyke from sample C121 (DDH 43 Q-163/661*), s h o w i n g f i n e , even g r a i n s i z e (0.5 mm) o f t h e 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. analyses are in Table 5-2. youngest  Chemistry, norms and modal The dyke sets,  from oldest  to  based on cross-cutting relationships and  a l t e r a t i o n , are: grey plagioclase porphyry, a l b i t i t e ,  green  hornblende porphry, Bendor porphyry, and lamprophyre.  5.2.4.1 Grey Plagioclase Porphyry Dykes (unit 8a) Grey plagioclase porphyry dykes are of r e s t r i c t e d d i s t r i b u t i o n and have only been noted in one area between the King and Bralorne mines near 9000N, 7500E, in angle drill  holes from surface (Fig. 5-1).  They seem to be  vol umetrical1y i n s i g n i f i c a n t , traceable along s t r i k e for only 100 m or so, and only up to a few meters wide. they have been found only in d r i l l  Since  core, their s t r i k e i s  unknown but i s 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 (An >. 57. grey quartz eyes and 57. mafic a  remnants, a l l of 2-3 mm s i z e ,  in a grey phaneritic (0.05 to  0.1 mm) groundmass of the same minerals.  These dykes  contain s t r i k i n g symplectic q u a r t z - a l b i t e  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  The  (Table 5-1).  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 e s s e n t i a l l y the  a precursor to  albitites.  5.2.4.2 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 l o c a l l y up to 50 m, can be traced on s t r i k e for up to a kilometer. centrally-located (Fig. 5-2),  They are most numerous and thickest  in a  zone within the Bralorne intrusive mass  where they are most numerous and thickest,  they also occur outside the Bralorne block.  but  Green  hornblende porphyry dykes are more widespread than the albitites,  occurring farther  from the center of  mineralization and with more variety of s t r i k e d i r e c t i o n , 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), the dip of the quarts veins.  sub-parallel  Dips range from nearly  v e r t i c a l to as low as 50 degrees,  based on information from  correlation of dykes in angled surface d r i l l A l b i t i t e s are f e l s i c quartz-plagioclase  holes.  dykes ranging from creamy or buff  porphyry to pale green hornblende-  pi agioclase porphyry. "albitites"  to  They have been referred to as  because of the universal presence of a l b i t e in  phenocrysts and groundmass. flow-banded,  These are definite dykes with  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,  £0% a l b i t e ,  fresh a l b i t i t e dykes contain 25"/. quartz,  137. hornblende remnants, and 2% skeletal  and leucoxene after  rutile  ilmenite, plus minor pyrite and zircon  (Plate 5-11). A l b i t i t e dykes are commonly strongly altered and subp a r a l l e l to, or even adjacent  to, gold-bearing  veins.  Alteration i s intense and consists of fine grained aggregates of ankerite, s e r i c i t e and quartz with coarse pyrite c r y s t a l s ;  often only the o r i g i n a l quartz  are s t i l l v i s i b l e .  phenocrysts  As the name a l b i t i t e implies, these  rocks are r i c h 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 ( d i f f e r e n t i a t i o n indexes are 75 to SO; Hughes,  1982'J,  and their p r o t o l i t h s may have been c a l c -  alkaline dacites to r h y o l i t e s of sub-alkaline potash-deficient)  character  (sodic,  (Irvine and Baragar, 1971).  The green hornblende porphyry dykes (unit 9) can be of substantial s i z e , 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 r e l a t i v e l y 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 512).  These dykes never contain quartz phenocrysts; they are  p r i n c i p a l l y made up of plagioclase phenocrysts or seriate c r y s t a l s 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 c h l o r i t i s e d 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 , c a l c i t e 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 a l t e r a t i o n ( b i o t i t e ,  ankerite,  and arsenopyrite) so some must be intra-mineral.  fuchsite  It was not  P l a t e 5-11. T h i n s e c t i o n v i e w (1 cm w i d e , c r o s s e d p o l a r s ) o f a l b i t i t e dyke from sample C092A (DDH UB-81-18/225*), s h o w i n g e u h e d r a l q u a r t z p h e n o c r y s t s ( g r e y , b l a c k ) 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 (sericitic p a t c h e s o f f i n e g r a i n s i z e ) and m a f i c r e l i c s ( c o a r s e r m u s c o v i t e ) i n a v e r y f i n e groundmass (0.01 t o 0.03 mm) of q u a r t z and s e r i c i t e .  P l a t e 5-12. T h i n s e c t i o n v i e w (1 cm w i d e , c r o s s e d p o l a r s ) o f g r e e n h o r n b l e n d e p o r p h y r y dyke from sample C083A ( u n l o c a t e d , f r o m main dump a t 8 l e v e l a d i t p o r t a l ) , s h o w i n g p r i m a r y magmatic z o n i n g p r e s e r v e d i n m a j o r i t y o f c o a r s e h o r n b l e n d e p h e n o c r y s t s (one i s a l t e r e d t o b i o t i t e ) , s e t i n a f i n e g r a i n e d groundmass o f p l a g i o c l a s e ( g r e y ) , q u a r t z ( w h i t e ) and h o r n b l e n d e (coloured) g r a i n s .  possible to c l e a r l y define age relations of these  sub-types,  since even where one variant cuts another, the contacts are always sheared and s u b - p a r a l l e l , 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 i s postulated  from contact r e l a t i o n s 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 c l e a r l y post-dates and i s  related to the a l b i t i t e dykes, hornblende porphyry dykes.  but i s often cut by the green  However, a few green hornblende  porphyry dykes do show a l t e r a t i o n 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 t r a n s i t i o n a l a l b i t i t e to green hornblende porphyry.  from  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, and hornblende phenocrysts.  Albite and quartz  plagioclase 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 l a s s i f i c a t i o n of sub-alkaline CK-poor) t h o l e i i t i c basalt. differentiated  (differentiation  It i s much less  index i s 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). and chemical c l a s s i f i c a t i o n s  The modal analyses  of green hornblende porphyry  and d i o r i t e are s t r i k i n g l y s i m i l a r , although they share no magmatic r e l a t i o n 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 a l b i t e porphyry, dykes (and  trondhjemitic host rocks) are also found at many other mesothermal gold-quartz vein deposits in Canada and Africa Colvine et a l . . 1984; Wood et a l . ,  (e.g. 1986).  1986; Clark et a l . .  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 i s 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).  5.2.4.3 "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 i s small and their orientation uncertain. albitite  Although mapped as  i n i t i a l l y , these dykes (best seen at 5000E, 12000N:  Fig. 5-1) have several d i s t i n c t i v e 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, o s c i l l a t o r y zonation Coligoc1ase,  Ani , 7  to andesine, An^ ). 2  This o s c i l l a t o r y  zoning, the more c a l c i c composition, and the magnetite, not observed in any of the e a r l i e r intrusive rocks. zoning and c a l c i c composition of plagioclase  is  The  in the e a r l i e r  rocks has presumably been obliterated by the homogenizing effects of greenschist  metamorphism associated  intrusion of the Coast Plutonic Complex (CPC).  with Available  isotopic dates support t h i s conclusion, since the e a r l i e r 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% c l e a r , o s c i l l a t o r y zoned,  intermediate  plagioclase,  and similar amounts of c h l o r i t i s e d 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 .  5.2.4.4 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 e a r l i e r dykes. surface d r i l l Empire fault  Lamprophyre dykes are well defined by  holes in the area to the southeast (Fig. 5-1).  of the  The dykes p a r a l l e l t h i s  fault,  with a consistent north-south s t r i k e and steep dip. of recessive weathering, no lamprophyres crop out. have only been found in d r i l l  Because They  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 t h i n , rarely over a few meters thick, but  commonly persist over several hundred meters of s t r i k e 1ength. The lamprophyres are dark, mafic rocks that might be classed as kersantites  (Hughes, 1982) since Stevenson  describes them as containing plagioclase. contain prominent black b i o t i t e ,  dark green  In the mine, they c1inopyroxene,  and smaller clear apatite phenocrysts in a finer grained dark groundmass of the same minerals plus brown glass 5-14).  They are d i s t i n c t chemically from a l l other  (Plate  /48  P l a t e 5-13. T h i n s e c t i o n v i e w (1 cm wide, c r o s s e d p o l a r s ) o f " B e n d o r " d y k e , 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), s h o w i n g o s c i l l a t o r y z o n e d p l a g i o c l a s e w i t h s e r i c i t i z e d r i m , and c h l o r i t i z e d h o r n b l e n d e r e l i c below i t .  P l a t e 5-14. T h i n s e c t i o n v i e w (1 cm wide, c r o s s e d p o l a r s ) o f l a m p r o p h y r e d y k e , sample C1033 ( f r o m main dump a t 8 l e v e l a d i t portal), s h o w i n g l a r q e c l i n o p y r o x e n e ( y e l l o w ) , s m a l l e r b l o t i t e ( g r e e n ) and a p a t i t e (grey) phenocrysts m i n e r a l s and g l a s s .  i n matrix  of f i n e r  crystals  o f t h e same  /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 t h i s study by K-Ar on biotite  (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  A l l the rocks in the Bralorne area except the postmineral Bendor and lamprophyre dykes are affected grade, sub-greenschist  to lower greenschist  by low-  facies s t a t i c 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 t h i s may only be an enhancement around shear zones that pre-date or accompany metamorphism.  The characteristic  metamorphic minerals in order of overall abundance are a l b i t e , quartz, s e r i c i t e (muscovite), c h l o r i t e , calcite,  a c t i n o l i t e and prehnite.  epidote,  The r e l a t i v e abundances  are, however, variable depending on the composition of the protolith.  Plagioclase i s replaced by a l b i t e and quartz,  with lesser s e r i c i t e and c a l c i t e ; c h l o r i t e , epidote, c a l c i t e ,  mafics are replaced by  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 e q u i l i b r a t i o n , 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. hornfels aureoles are encountered rocks labelled "gneisses" Regionally,  (e.g.  1-2),  biotite-bearing  by Church et a l . .  1988).  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, a l b i t e and opaques, Rusmore (19B5) describes:  (1)  a pumpel1yite-epidote-ch1  calcite+sericite,  orite-  and (2) an a c t i n o l i t e - h y d r o b i o t i t e -  chl or i t e - c a l 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 albite,  c h l o r i t e , pumpel1eyite,  s e r i c i t e and prehnite.  The  absence of a c t i n o l i t e in the Bridge River Group rocks outside the Bralorne block suggests a s l i g h t l y pressure, sub-greenschist  facies that  lower  formed at less than 4  kb and less than 325 degrees Celsius (see below). In the Bralorne block the coexistence, greenstone samples, of a c t i n o l i t e ,  epidote,  in some c h l o r i t e and  a l b i t e suggests that the metamorphic grade was  locally  s l i g h t l y higher than i n surrounding areas mapped by Rusmore (1985) and Potter (1983). lower  greenschist facies  not seen  These minerals are d i a g n o s t i c of (Winkler, 1971).  Pumpelleyite  i n any samples from the Bralorne block,  p r e h n i t e was  was  and  p o s i t i v e l y i d e n t i f i e d only i n v e i n s c u t t i n g  the  Bralorne d i o r i t e . A c t i n o l i t e - p r o d u c i n g r e a c t i o n s r e s p o n s i b l e for the transition  from s u b - g r e e n s c h i s t  facies  p r e h n i t e - c h l o r i t e assemblages) may  (pumpel1eyite-  be s i m i l a r t o those  e x p e r i m e n t a l l y i n v e s t i g a t e d by N i t s c h (1971) and  Schiffmann  and L i o u (1980) about i n v a r i a n t point X ( F i g . 5-4). data suggests that at water p r e s s u r e s of 2 t o 3 kb, t r a n s i t i o n occurs at temperatures Celsius.  of about 325  evidence which suggests that quartz i n the v e i n s  kb  (Chapter 8).  this  degrees  Such c o n d i t i o n s are supported by f l u i d  deposited at about 350 degrees  Their  inclusion was  C e l s i u s , at p r e s s u r e s of  G e o c h r o n o l o g i c a l evidence  (Chapter  1.75  3)  i n d i c a t e s that metamorphism took p l a c e at about the same time as m i n e r a l i z a t i o n .  5.3.2  Timing The timing of metamorphism i n the B r a l o r n e area i s  c o n s t r a i n e d by evidence suites  ( s e c t i o n s 3.6.1  from the a l t e r a t i o n of s e v e r a l dyke and 5.1.4).  U-Pb  z i r c o n and  K-Ar  whole-rock d a t i n g of these dykes i n d i c a t e s that peak c o n d i t i o n s of metamorphism overlapped the i n t r u s i o n of the  /sz.  4  3  .-••"6  100  200 TEMPERATURE  500  m  (°C)  F i g u r e 5-4. S t a b i l i t y r e l a t i o n s i n subgreenschist t o lowermost g r e e n s c h i s t grade metamorphic rocks (from P o t t e r , 1983). Probable s t a b i l i t y f i e l d for r o c k s i n the B r a l o r n e block i s s t i p p l e d ; f i e l d for Bridge River Group r o c k s i s dark. Reaction 1 was e x p e r i m e n t a l l y determined by L i o u (1971a). Reaction 2 was e x p e r i m e n t a l l y determined by L i o u (1971b). R e a c t i o n s 3,4 were c a l c u l a t e d by P e r k i n s et a l . (19B0). R e a c t i o n s 5,6 a r e from Schiffman and L i o u (1980). Reaction 7 i s from N i t s c h (1971). A b b r e v i a t i o n s : ab = a l b i t e , an = a n a l c i n e , 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 = t r e m o l i t e , laum • laumontite, lw = l a w s o n i t e , q = quartz, mg = m a r g a r i t e , pu = pumpel1eyite, pr = p r e h n i t e , zo = z o i s i t e , gr = g r o s s u l a r , cz c l i n o z o i s i t e and v = vapour.  /5i  Bralorne a l b i t i t e - g r e e n 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 i s 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 B r i t i s h 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 c o n t r o l , because a l l the veins are in faults. full  However, only a synopsis i s presented here;  consideration i s beyond the scope of t h i s thesis. The area hosting the Bralorne-Pioneer veins i s  anomalous on a regional scale,  f a l l i n g 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 t h i s s t r i k e length, notably where the structure bends from northwesterly to more northerly (Fig.  1-2).  It i s 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 90 degrees) southwest.  (70 -  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 i s of unknown extent and sense, but i s constrained to the Late Cretaceous  (100 to 85 Ma) by  the r e s u l t s of t h i s 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 Bralorne fault  The Tchaikazan fault (Umhoefer et a l . ,  i s correlated with  1989).  The Eiralorne-Pioneer vein system i s outstanding in i t s 6 km s t r i k e length and 2 km (and open) depth extent. t h i s respect  In  i t i s similar to the major vein deposits of the  Motherlode d i s t r i c t in C a l i f o r n i a (Landefeld,  1988),  the  Canadian Shield (Colvine et a l • , 1984) and the Archean of Western Australia (Groves et a l . , 1984;  1987).  A l l 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. i n t r u s i v e contacts, dykes,  Bedding,  long axes of the intrusive bodies,  fold axes and f o l i a t i o n are a l l sub-parallel to the  major northwesterly trending bounding faults that define the Bralorne shear zone, implying northeast-southwest compression.  directed  The main veins however are oblique to the  /ss  general trend of the shear zone (110° and 070 °) and appear to f i t 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.  5.4.1  Miller,  1388).  Folding  Primary layering in the Hurley Formation within the Bralorne block i s usually steeply dipping and westerly northwesterly trending. isoclinal foliation,  Local dip reversals indicate that  folding may be present.  A weakly developed  p a r a l l e l or subparallel to the bedding, can only  be discerned in a r g i l l i t e s . difficult  to  The timing of folding is  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 1986).  (G. Nordine, pers. comm.,  Joubin's (1948) cross-sections support t h i s ; 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  F a u l t i n g and Dyking  Most faults,  including those hosting the dykes and  veins within the Bralorne block, are p a r a l l e l or sub-  /56  p a r a l l e l to the bounding shears.  The major, Permian  Bralorne intrusions also l i e along the antiformal p a r a l l e l to the bounding shears.  axis,  They may have been  controlled along faults that paralleled t h i s axis, or they may have been rotated into t h i s orientation by later northeast-directed compression (in places,  a weakly defined  f o l i a t i o n i s 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 at their ends (Fig. 5-1).  ends.  The intrusives also dyke out  The Permian Bralorne intrusions  were intruded much later by two sets of dykes: of Cretaceous  (91.4-85.7 Ma) a l b i t i t e - g r e e n  (1) a swarm  hornblende  porphyry dykes that trend subparallel to the veins at and 070 ° ,  110  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  movement at 45-35 Ma: M i l l e r ,  fault  1988) because t h i s direction  p a r a l l e l s features such as the steeply dipping, 45 Ma lamprophyre dykes, (Fig. 5-2),  and the Empire and No. 1,  which offset  2 and 3 faults  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 d i r e c t i o n , since in one place a lamprophyre dyke that cuts off the 51 vein has itself al..  been faulted along the vein for S m (Cleveland et  1938)•  5.4.3 Quartz Veins  5.4.3.1 Morphology Two main vein types are recognized at Br al orne-F'i oneer, and a t h i r d subsidiary type i s l o c a l l y present. (Joubin, 1948; see F i g . 5-5a):  These are  (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 s t r i k e of roughly 110  degrees  azimuth, and for 1,800 m down a dip of about 70 degrees north (Fig. 5-5b).  Their average width i s 1 to 2 m (Bellamy  and Saleken,  1983), although they pinch to a few  centimeters,  swell to as much as 7 m, or splay  horsetail or breccia structures  into  (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  x  ^  M I N E  „ \ ^  N  \ ^ » ^  \  BRALORNE Ml NE  <rn'CROWN (•.'CROWN  \  j  v—. \.->  CMMM SUA*T V?l  MINE  • *^°,  °^ ti  im  *y  »°' 0  Aft«r: P. JouOtn, 1945  111  BRALORNE PROJECT  E & B EXPLORATIONS INC.  1  a ATI  OIMKTMfNT  VEIN M A P I N O O NO.  SYSTEMS SCAUE  i:24/X»arl">2000'  FIG 5-5_a  ...  Figure 5-5<a). Plan view of the main vein systems in the BralornePioneer deposit. The 51 and 55 veins are believed to be offsetportions of the same vein across the Empire f a u l t . Locations of the cross-section of Figure 5-5(b) and the longitudinal section of Figure 5-5<c) are shown.  FIG.  5-5 b  wuuimc MOJCCT •j •hi  a  Tt. a  tt* J M  miWa to'-"  atrrcH O F V E I N S L00OM N *  •••  Figure 5-5(b). Sketch of the major veins i n 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" *' ° 9 ^p relation of the ore shoots *to Z t ? hu .2 -° H ^ ' * antipathetic * *iP*thetic 6 5  W  M  t  a n d  t th h e  5 h  n  W i n  s t  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 s t r i k e 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 s t r i k e 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 r i c h 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 r  ecrystal1ized  filling  (slickensides and  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 i s the faulted extension of the 53 vein (Fig. 52) across the Empire f a u l t , as the major 77B vein.  and i s also therefore the same  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 n o r t h e r l y - s t r i k i n g , steeply-dipping structures.  (55-80 degree)  They were not accessible at the time of t h i s  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. CHedley, 1935)  They have been described  as "discontinuous, rudely l e n t i c u l a r bodies  of more or less fractured quarts between heavy gouge w a l l s . . . w i t h a re-cemented quarts breccia evident".  frequently  They were not important economically in either  s i z e or grade, with only 200 m s t r i k e 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 p r i n c i p a l 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 i m i l a r l y mineralized, implying coeval  formation.  If  the "tension" veins are considered as lying partly along a conjugate  shear direction rather than a true tension  d i r e c t i o n (with the other conjugate main shear veins: F i g . 5-6),  shear represented by the  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 i s presented in section 5.4.4 below.  The age of the s t r i k e - s l i p regime i s  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).  5.4.3.2 Ore Controls Within the main shear vein set, plunge steeply northwest  (Fig. 5-5c:  individual oreshoots James and Weeks, 1961).  This i s approximately perpendicular to s t r i a t i o n s 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 e a r l i e r movement  appears to have been nearly h o r i z o n t a l , 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 d i r e c t i o n of movement (c f. Badgley,  1959).  There i s also a tendency to richer mineralization i n some of the more steeply-dipping sections of the vein (Campbell, 1975).  These steeply i n c l i n e d 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 ( d i o r i t e and soda granite) primarily provided a large competent block of rock that sustained the fractures that hosted the gold quartz veins. In t h i s respect the d i o r i t e 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 , p a r t i c u l a r l y  where they cross into soda granite.  Campbell (1375)  suggested that the ore shoots were peripheral to the bodies of soda granite (see F i g . 5-5c).  However, t h i s i s 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 e a r l i e r than the mineralization (Chapter 3).  Veins passing beyond the  competent intrusions and greenstone into f i s s i l e a r g i l l i t e tend to die out, as pointed out by Cleveland et a l .  (1938)  and Joubin (1948). Another important l i t h o l o g i c control i s exerted by the serpentine belt along the Cadwallader fault zone. been suggested (Joubin, 1948; Poole, 1955; James and Weeks, 1961) effect  It has  Stevenson,  1958;  that the serpentine had a "damming"  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 d i o r i t e 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 p r e c i p i t i a t i o n of p y r i t e , to which the p r e c i p i t a t i o n of Au i s related: see sections 5.3 and 10.3). In the longitudinal section of Figure 5-5c, shoots overlap very l i t t l e 1961).  (Skerl, 1956,  the ore  iri James and Weeks,  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 i s 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 r a t i o n a l i z e 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 F i g . 5-6a)  are : (1) the angle 2 theta  between the two major shear directions i s much too large (actually obtuse, about 100 degreees,  as shown by Joubin)  compared to the 60 degrees currently known to be c h a r a c t e r i s t i c of most b r i t t l e rocks, and (2) the direction of p r i n c i p a l 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 d i r e c t i o n , neither of which i s compatible with the observed northwest-trending major shear zone bounding the Bralorne block. demonstrated, however,  that at least  where conditions are favourable exceeds l i t h o s t a t i c  It has recently been in some situations  (where fluid pressure  load), fractures may develop at  2 theta angles of up to 110 degrees  greater  (Sibson et a l . ,  A r e - i n t e r p r e t a t i o n i s presented in Figure 5-6b.  1988). In  t h i s 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) i s reproduced; i t would not predict s t r i k e - s l i p 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) i s 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 i n (c). Implied maximum compression i s about 090° azimuth, and movement on the bounding f a u l t s i s s i n i 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 d i r e c t i o n , 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 p a r a l l e l a l b i t i t e dykes in  the area east of the King mine area (Fig. 5-1).  It  immediately apparent from the diagram (Fig. 5-6b)  is  that the 2  theta angle between these R and.R' directions at Bralorne i s less than the 60 degrees predicted from f a i l u r e measurements on most rocks. Figure 5-6c,  This may be explained with reference to  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 d i r e c t i o n 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,  reproduced in Figure 5-7. alternate  one of which i s  Here the veins appear to  from the extension direction to the shear  d i r e c t i o n , resulting in an overall 2 theta angle less than 60 degrees. different  The R and R' directions are better  developed in  parts of the mine, with the former best shown in  the main Bralorne mine area, and the l a t t e r  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 l e v e l , 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 d i r e c t i o n bisecting the angle between them. The 51 vein shows segments along both R and R' directions as well as the extensional d i r e c t i o n .  /7o developed, has also been predicted by the clay modelling experiments of Wilcox et a l .  (1973), and i s 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 i s 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 p a r a l l e l i n g the Ida May (or 55) vein at surface, are in this orientation, although i t i s to say i f they reflect shear (D) d i r e c t i o n .  difficult  the P direction or the major external This i s not unexpected, since many  such fractures p a r a l l e l to the overall shear zone are also revealed by the studies of Morgenstern and Tchalenko (1971). The Riedel model, which was o r i g i n a l l y developed in 1929, has also been applied to Archean lode gold deposits i n 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 boundaries).  (parallel to the shear zone  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 c r o s s - f a u l t s 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 s l i p and 370 feet of dip s l i p (Poole,  strike  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.  Miller,  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 dextral  (right-lateral)  tectonics.  (left-lateral)  rather than the  normally associated with Cordilleran  Two possible explanations are: (1) the sense of  translation at t h i s 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  left-  l a t e r a l s l i p has also been suggested for the Yalakom fault, 30 km northeast of Bralorne ( M i l l e r , Pasayten  fault  1988), and on the  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); t h i s i s 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 al..  1988).  At Bralorne, such northeast compression i s  appropriate to subduction associated with the emplacement of the Coast Plutonic complex, and i s indicated by the generally northwest s t r i k e of bedding and fold axes (which do not f i t  with an east-west maximum compressive s t r e s s ) .  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 t h i s was followed by mineralization of these preexisting faults that were reactivated in a compressive regime at about 90-85 Ma. left-lateral  Miller  (1988) suggested such a  s t r i k e 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 movement.  Other workers in the Bridge River area,  have found evidence  (thrust  northeastward-directed  faulting and folding.)  fault  however, for a  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