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Geology and metamorphism of the Mitchell Mountains ultramafite, Fort St. James map area, British Columbia Elliot, Andrew James Mather 1975

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GEOLOGY AND f^ETAMORPHT3K OK THE MITCHELL FOUNTAINS ULTRA^AFITE, FORT ST. JAMES WAP AREA, T-PTTTSH COLUMBIA by ANDREW JA1VES MATHER ELLIOT B.Sc.(Eng.), Queen's U n i v e r s i t y , 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR THE DEGREE Of WASTER OF APPLIED SCIENCE i n the Department o f G e o l o g i c a l Sciences We a c c e p t t h i s t h e s i s as c o n f o r m i n g to t h r e q u i r e d s t a n d a r d THE UNIVERSITY OF' BRITISH COLOMBIA J a n u a r y 1975 In presenting th i s thesis in pa r t i a l f u j f i lment-of the requirements f< an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree tha the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thes i s for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t i on of th is thesis fo r f inanc ia l gain shal l not be allowed without my writ ten permission. Department of e p [ pcjyj The Univers i ty of B r i t i s h Columbia Vancouver 8. Canada Date I *T«,K , « 9 1 S ."•PS TRACT The a r c s mapped i n c l u d e s two u l t r a m a f i t e s o f the a l p i n e t y p e , emplaoed i n , and wi t h i n c l u s i o n s o f , r o c k s of the Cache Creek. Group. Northwest l i e s the M i t c h e l l E a t h o l i t h , a L a t e J u r a s s i c ( ? ) i n t r u s i o n o f d i o r i t e to g r a n i t e . The a r g i 1 1 i t e , c h e r t , g r e e n s t o n e , and 1imestone o f the Cache Creek Group are s t r o n g l y t o i . s o o l i r a l l y f o l d e d about n o r t h - n o r t h e a s t e r l y t r e n d i n g axes. They have m i n e r a l assemblages c h a r a c t e r i s t i c o f the middle to upper g r e e n -s c h i s t f a c i e s . The u l t r a m a f i . c r o c k s are f r a c t u r e d and sheared i n a n o r t h - n o r t h e a s t e r l y d i r e c t i o n , and may be f o l d e d . The o v e r a l l shape o f the Vn t c h e l l f o u n t a i n s Ul t r a m a f i t e i s not known. The M i t c h e l l f o u n t a i n s II1 tramaf i t e has t h r e e rnetamorphic zones, p a r a l l e l to the edge o f the R i t e he 11 E a t h o l i t h . They c o n s i s t of s e r p e n t i n i t e , t a l c - o l i v i n e r o c k , and o l i v i n e - e n s t a t i te-anthophy 11 i te r o c k . The Goat Mountain Ultrama.fi te c o n s i s t s m o s t l y o f s e r p e n t i n a t e , The observed m i n e r a l assemblages may be e x p l a i n e d i n terms of the system CaO-iVgO-SiOp-HpO-COg. At an assumed p r e s s u r e of 2000 b a r s , the temperature d u r i n g metamorphism range d from V-0 C a t the e a s t e r n edge o f the i v i t c n e l 1 Mountains Ul. t ramaf i t e , to almo s t 700° a t the western edge. T h i s c o n t a c t metamorphism was caused by the M i t c h e l l B a t h o l i t b . i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i i LIST OF PLATES x LIST OF MAPS X i ACKNOWLEDGEMENTS x i i INTRODUCTION 1 REGIONAL GEOLOGY 4 INTRODUCTION 4 CACHE CREEK GROUP 4 TREMBLEUR ULTRAMAFITES 5 TAKLA GROUP 5 TOPLEY INTRUSIONS 6 OMINECA INTRUSIONS 6 REGIONAL STRUCTURE 7 GEOLOGY OF THE SOUTHERN MITCHELL MOUNTAINS 8 INTRODUCTION 8 CACHE CREEK GROUP 8 L i t h o l o g y 8 TREMBLEUR ULTRAMAFITES 10 L i t h o l o g y of the M i t c h e l l Mountains U l t r a m a f i t e 10 S e r p e n t i n i t e 10 T a l c - O l i v i n e Rock 11 i v Page O l i v i n e - E n s t a t i t e - A n t h o p h y l l i t e Rock 13 Basic and Intermediate I n c l u s i o n s 14 Contact Zones 15 Carbonate Veins 15 L i t h o l o g y of the Goat Mountain U l t r a m a f i t e 17 TOPLEY INTRUSIONS (MITCHELL BATHOLITH) 18 FELSITE DIKES 19 STRUCTURE 2 0 I n t r o d u c t i o n 20 Mesoscopic Structure 20 Cache Creek Group 20 U l t r a m a f i c Rocks 21 I n c l u s i o n s 24 Contacts 24 Macroscopic Structure 25 METAMORPHISM 38 I n t r o d u c t i o n 38 M i n e r a l Chemistry 39 Mineral' Assemblages 4 l Theory 47 D i s c u s s i o n 4-9 Serpentine Zone 49 T a l c - O l i v i n e Zone 51 O l i v i n e - E n s t a t i t e - A n t h o p h y l l i t e Zone 52 Changes i n F l u i d Composition 56 Carbonate Veins Goat Mountain U l t r a m a f i t e Non-Ultramafic Rocks P-T Conditions During Metamorphism GEOLOGIC HISTORY OF THE MAP AREA REFERENCES APPENDIX I i MINERALOGY ELECTRON MICROPROBE ANALYSIS UNIVERSAL STAGE DETERMINATIONS APPENDIX I I : MINERAL COMPOSITIONS OF ROCK X-RAY DIFFRACTION THIN SECTION MICROSCOPY Page 59 60 61 62 71 73 76 76 83 SPECIMENS 84 84 95 v i LIST OF TABLES Page 1. MINERAL ASSEMBLAGES 42 2 . REACTIONS IN THE SYSTEM CaO-MgO-SiOg.-HgO-COg 44 3. MICROPROBE ANALYSES OF OLIVINE ?8 4. MICROPROBE ANALYSES OF TALC 79 5 . MICROPROBE ANALYSES OF ENSTATITE 80 6. MICROPROBE ANALYSES OF TREMOLITE AND ANTHOPHYLLITE 81 ?. MICROPROBE ANALYSES OF CHLORITE 82 8 . ENSTATITE COMPOSITIONS 83 9 . WHOLE-ROCK X-RAY DIFFRACTION ANALYSIS: ULTRAMAFIC ROCKS.(MITCHELL MOUNTAINS) 85 10. WHOLE-ROCK X-RAY DIFFRACTION ANALYSIS: ULTRAMAFIC ROCKS (GOAT MOUNTAIN) 92 11. WHOLE-ROCK X-RAY DIFFRACTION ANALYSIS: ALTERATION ZONES 93 12. WHOLE-ROCK X-RAY DIFFRACTION ANALYSIS: RODINGITES AND CARBONATE ROCKS 9^ 13. VISUALLY ESTIMATED MODAL ANALYSES: ULTRAMAFIC ROCKS (MITCHELL MOUNTAINS) 95 14. VISUALLY ESTIMATED MODAL ANALYSES: ULTRAMAFIC ROCKS (GOAT MOUNTAIN) 99 15. VISUALLY ESTIMATED MODAL ANALYSES: CHERT AND ARGILLITE 100 v i i Page 16. VISUALLY ESTIMATED MODAL ANALYSES: CARBONATE ROCKS 101 17. VISUALLY ESTIMATED MODAL ANALYSES: BASIC ROCKS 102 18. VISUALLY ESTIMATED MODAL ANALYSES: FELSIC ROCKS 103 v i i i LIST OF FIGURES Page 1. Loc a t i o n and r e g i o n a l geology 3 2 . D i s t r i b u t i o n o f veins 16 3 . Some s t r u c t u r e s i n u l t r a m a f i c rocks 2 3 4. S t r u c t u r a l sub-areas f o r stereonets 27 5 . Stereographic p r o j e c t i o n s of s t r u c t u r e s i n the Cache Creek Group 28 6. Stereographic p r o j e c t i o n s of s t r u c t u r e s i n the u l t r a m a f i c rocks 30 7. A i r photo lineaments 32 8. S t r i k e frequency diagram of topographic lineaments 33 9 . Geologic cross s e c t i o n s 3^ 10. H y p o t h e t i c a l cross s e c t i o n ( f o l d e d sheet) 36 11. H y p o t h e t i c a l cross s e c t i o n ( f a u l t e d !pod) 37 12. P a r t i t i o n i n g of Mg between o l i v i n e and other phases 40 13. T-X diagram f o r r e a c t i o n s i n the f l u i d - s a t u r a t e d system Mg0-Si0 2-H 20-C0 2 64 14. Schematic T-X diagram of r e a c t i o n s a f f e c t i n g s e r p e n t i n i t e , i n the system Ca0-Mg0-Si0 2-H 20-C0 2 65 15. S e r p e n t i n i t e f i e l d of T-X'.diagram f o r r e a c t i o n s In the f l u i d - s a t u r a t e d system Ca0-Mg0-Si0 2-H 20-C0 2 66 16. P a r t of the T-X diagram f o r r e a c t i o n s i n the f l u i d - s a t u r a t e d system CaO-MgO-SiOp-HpO-COp 67 i x Page 17. Schematic P-T diagram f o r part of the system Mg0-Si0 2-H 20 68 18. P o s s i b l e mineral assemblages and r e a c t i o n s i n the system Mg0-Si0 2-H 20 69 19. P o s s i b l e temperature gradients across the M i t c h e l l Mountains U l t r a m a f i t e 70 X LIST OF PLATES Page 1. Specimen 357, t h i n s e c t i o n , plane l i g h t . 104 2 . Specimen 164, t h i n s e c t i o n , p o l a r i z e d l i g h t 104 3 . Specimen 164, t h i n s e c t i o n , plane l i g h t 105 4 . Specimen 138, t h i n s e c t i o n , plane l i g h t 105 5 . Specimen 74 , t h i n s e c t i o n , plane l i g h t 106 6. Specimen 34, t h i n s e c t i o n , p o l a r i z e d l i g h t 106 7. Specimen 79» t h i n s e c t i o n , plane l i g h t 107 8 . Specimen 247, t h i n s e c t i o n , plane l i g h t 107 9 . Specimen 2 1 9 , t h i n s e c t i o n , plane l i g h t 108 10. Specimen 22 6, t h i n s e c t i o n , plane l i g h t 108 11. Specimen 736, t h i n s e c t i o n , p o l a r i z e d l i g h t 109 12. Specimen 488, t h i n s e c t i o n , p o l a r i z e d l i g h t 109 13. Specimen 7 0 1 , t h i n s e c t i o n , p o l a r i z e d l i g h t 110 14. T a l c - o l i v i n e rock: coarse compositional banding 110 15. S e r p e n t i n i z e d harzburgite b r e c c i a 111 16. Rodingite pod i n s e r p e n t i n i t e 111 17. I n c l u s i o n s of c h l o r i t e rock i n t a l c - o l i v i n e rock 112 18. A large i n c l u s i o n of a r g i l l i t e i n t a l c - o l i v i n e rock 112 19. A large i n c l u s i o n of a r g i l l i t e , w i t h f o l d e d lenses of limestone 113 x i LIST OF MAPS 1. GEOLOGY i n pocket 2. STRUCTURE i n pocket 3. SPECIMEN LOCATIONS i n pocket x i i ACKNOWLEDGEMENTS The author i s indebted to Dr. P.B. Read, formerly of the U n i v e r s i t y of B r i t i s h Columbia, who suggested the p r o j e c t and provided s u p e r v i s i o n . Drs. H.J. Greenwood and K.C. MacTaggart of the U n i v e r s i t y of B r i t i s h Columbia made many h e l p f u l suggestions and c r i t i c i s m s . Ms. L. L e i t z a s s i s t e d i n the use of the e l e c t r o n microprobe at the U n i v e r s i t y of Washington. Mr. Roderic Mamu gave valuable a s s i s t a n c e during the f i e l d work. This work was supported w i t h funds made a v a i l a b l e by the G e o l o g i c a l Survey of Canada. INTRODUCTION Armstrong (19^9) mapped a number of u l t r a m a f i t e s i n the F o r t St. James area. D e t a i l e d mapping of two of these bodies was undertaken as p a r t of the U n i v e r s i t y of B r i t i s h Columbia programme of i n v e s t i g a t i n g the u l t r a m a f i c rocks of B r i t i s h Columbia. These two bodies are a d d i t i o n a l l y i n t e r -e s t i n g i n that they have been metamorphosed. The t h e s i s area i s i n the M i t c h e l l Mountains, one of the southern ranges of the Omineca Mountains, i n c e n t r a l B r i t i s h Columbia. Nearest towns are F o r t St. James, seventy miles to the southeast, and Germansen Landing, f i f t y miles to the northeast. Charter f l o a t p l a n e s and h e l i c o p t e r s are g e n e r a l l y a v a i l a b l e at both towns. A rough road leads to Tsayta Lake (Figure 1), f i f t e e n miles north o f the t h e s i s area. Tchentlo Lake, ten miles east of the t h e s i s area, i s e a s i l y a c c e s s i b l e by boat from Chuchi Lake, which i s i n turn a c c e s s i b l e by road. Takla Lake, e i g h t miles southwest of the t h e s i s area, i s e a s i l y a c c e s s i b l e by boat from F o r t St. James, and barges run r e g u l a r l y from the town to small settlements along the lake. The new B r i t i s h Columbia Railway l i n e , under c o n s t r u c t i o n along the north shore of Takla Lake, w i l l pass w i t h i n f i v e miles of the area. The topography i s a l p i n e , w i t h e l e v a t i o n s ranging from 2 4500 to 6790 f e e t above sea l e v e l . U l t r a m a f i c rocks, r e s i s t a n t to er o s i o n , stand higher than the surrounding sedimentary rocks. Much of the u l t r a m a f i t e crops out, but the surrounding sedimentary rocks are mostly d r i f t - c o v e r e d . Sedimentary rocks are well-exposed only on one of the western r i d g e s . The area was a f f e c t e d by c o n t i n e n t a l g l a c i a t i o n , w i t h i c e movement roughly from west to east. G l a c i a l d r i f t , t h i c k and extensive on the west f l a n k of the u l t r a m a f i t e , contains l a r g e boulders of quartz monzonite from the mountains to the west. A f t e r c o n t i n e n t a l g l a c i a t i o n , a l p i n e g l a c i e r s carved s e v e r a l U-shaped v a l l e y s , w i t h c i r q u e s and tarns at t h e i r heads. L a t e r a l moraines are conspicuous i n two v a l l e y s . The f i e l d area i s on the c o n t i n e n t a l d i v i d e between the P a c i f i c and A r c t i c Ocean drainage basins. The eastern p a r t of the area d r a i n s i n t o Tchentlo Lake, then east i n t o Parsnip and MacKenzie R i v e r s , and the A r c t i c Ocean. The western p a r t of the area d r a i n s i n t o Takla Lake, then south i n t o Nechako and Fraser R i v e r s , and the P a c i f i c Ocean. In the summer of 1972 the area was mapped at a scale of one inch to h a l f a mile. A e r i a l photographs were used f o r l o c a t i o n of s t a t i o n s , and base maps were p r e l i m i n a r y topographic maps showing only drainage and ridge tops. 3 4 REGIONAL GEOLOGY INTRODUCTION The M i t c h e l l Mountains U l t r a m a f i t e l i e s i n the P i n c h i A n t i c l i n e , near the eastern margin of the Intermontane B e l t of the Canadian C o r d i l l e r a (see Figure 1). The P i n c h i F a u l t Zone, which s t r i k e s north-northwest, separates the Intermontane B e l t from the Omineca C r y s t a l l i n e B e l t to the east. CACHE CREEK GROUP The o l d e s t rocks i n the area are sedimentary and v o l c a n i c rocks of the Upper Paleozoic Cache Creek Group, which form three main d i v i s i o n s i n t h i s p a r t of the I n t e r -montane B e l t (Armstrong, 19^9)! 1. Greenstone D i v i s i o n : A n d e s i t i c flows, t u f f , and b r e c c i a , w i t h minor basic i n t r u s i o n s ; basic s c h i s t s ; and minor a r g i l l i t e , r i b bon c h e r t , and limestone. 2. Ribbon Chert D i v i s i o n : Ribbon c h e r t , a r g i l l a c e o u s q u a r t z i t e , a r g i l l i t e , s l a t e , greenstone; minor greywacke, conglomerate, limestone. In p a r t metamorphosed. 3. Limestone D i v i s i o n : Massive limestone; minor a r g i l l i t e , s l a t e , r i bbon c h e r t , greenstone. 5 These d i v i s i o n s are not s t r a t i g r a p h i c u n i t s , but may appear s e v e r a l times i n the s e c t i o n . The s e c t i o n has no exposed base, and may be as much as 20,000 f e e t t h i c k , although probably repeated due to f o l d i n g and f a u l t i n g . The l i m e -stone d i v i s i o n , which probably o v e r l i e s the others (Paterson, personal communication), has been dated on the b a s i s of f u s i l i n i d s as ranging from Middle Pennsylvanian to Late Permian (Monger and Ross, 1 9 7 1 ) . The probable t o t a l range of Cache Creek d e p o s i t i o n i s from Middle M i s s i s s i p p i a n to E a r l y Middle T r i a s s i c (Souther and Armstrong, 1 9 6 6 ) . TREMBLEUR ULTRAMAFITES The Trembleur U l t r a m a f i t e s are mostly dunite and p e r i d o t i t e of the alp i n e type, although one has some c h a r a c t e r i s t i c s of the zoned Alaskan type. A l l the u l t r a -mafites are h i g h l y s e r p e n t i n i z e d or a l t e r e d to talc-carbonate rock. They occur mainly w i t h i n rocks of the Cache Creek Group, near the P i n c h i F a u l t , and range i n s i z e from 80 square miles to small s i l l - l i k e bodies. TAKLA GROUP The Takla Group i s a succession of interbedded sedimentary and v o l c a n i c rocks, ranging i n age from Late T r i a s s i c to E a r l y J u r a s s i c . The b a s a l u n i t i s a conglomerate c o n t a i n i n g pebbles of u l t r a m a f i t e s and rocks of the Cache Creek Group. The upper u n i t s are b a s a l t i c and a n d e s i t i c 6 flows, t u f f s , and b r e c c i a s , w i t h interbedded c l a s t i c sedimentary rocks. TOPLEY INTRUSIONS The Topley I n t r u s i o n s are a group of mesozonal p l u t o n s , ranging i n composition from d i o r i t e to g r a n i t e . Potassium-argon ages of rocks which have been c a l l e d Topley range from 180 M.Y. ( m i l l i o n years) to 138 M.Y. (White e t a l . , 1970). Probably the o l d e r dates are more r e p r e s e n t a t i v e . Wheeler et e l . (1972) and Souther and Armstrong (1966) assigned an E a r l y J u r a s s i c age to these i n t r u s i o n s . The M i t c h e l l B a t h o l i t h , j u s t northwest of the t h e s i s area, i s probably one of the Topley I n t r u s i o n s (Douglas et a l . 1970, Armstrong and Souther, 1966) although Armstrong (1948) grouped i t w i t h the Omineca I n t r u s i o n s . 0MINE0A INTRUSIONS The Omineca I n t r u s i o n s are a group of mesozonal plutons, ranging i n composition from d i o r i t e to quartz monzonite. The Hogem B a t h o l i t h , one of these i n t r u s i o n s , has been assigned an E a r l y Cretaceous age, although other Omineca bodies have been dated as Middle J u r a s s i c (165 M.Y.). The Omineca I n t r u s i o n s l i e to the east of the P i n c h i F a u l t Zone, and the Topley I n t r u s i o n s to the west (Douglas e_t a l . , 1970, Armstrong and Souther, 1 9 6 6 ) . 7 REGIONAL STRUCTURE Rocks of the Cache Creek Group are c l o s e l y f o l d e d about nor t h w e s t e r l y axes. The o v e r l y i n g rocks of the Takla Group are l e s s s e v e r e l y f o l d e d . The P i n c h i F a u l t Zone, about 150 miles long, s t r i k e s north-northwest and at the surface dips s t e e p l y westward. Several l e s s prominent f a u l t zones have the same s t r i k e . 8 GEOLOGY OF THE SOUTHERN MITCHELL MOUNTAINS  INTRODUCTION The map area includes two u l t r a m a f i t e s of the a l p i n e type: the M i t c h e l l Mountains U l t r a m a f i t e , which u n d e r l i e s about 2 0 square miles, and the Goat Mountain U l t r a m a f i t e , about one square mile. They have been emplaced i n s t r o n g l y f o l d e d and f a u l t e d rocks of the Cache Creek Group, and show some of the same s t r u c t u r e s . The M i t c h e l l B a t h o l i t h , a l a r g e g r a n i t i c i n t r u s i o n which l i e s j u s t west of the map area, has caused contact metamorphism of u l t r a m a f i c and other rocks. CACHE CREEK GROUP In the map area, a r g i l l i t e , ribbon c h e r t , limestone, and greenstone comprise the Cache Creek Group. A r g i l l i t e and ribbon c h e r t are c l o s e l y a s s o c i a t e d , and may be i n t e r -bedded on a sca l e of inches. Limestone and greenstone g e n e r a l l y form t h i c k e r u n i t s . A l l four rock types occur both as i n c l u s i o n s i n the M i t c h e l l Mountains U l t r a m a f i t e and as country rocks. L i t h o l o g y Ribbon c h e r t c o n s i s t s of l a y e r s of metachert, 1 - 5 cm. t h i c k , separated by t h i n s l a t e y l a y e r s . The metachert c o n s i s t s of f i n e - g r a i n e d quartz w i t h a l i t t l e s c a t t e r e d 9 f i n e - g r a i n e d b i o t i t e , g e n e r a l l y a l i g n e d p a r a l l e l to bedding. The s l a t e y l a y e r s c o n s i s t of about equal amounts of quartz and b i o t i t e , w i t h minor magnetite (P l a t e l ) . Some s p e c i -mens have v e i n l e t s of K-feldspar, and one specimen at a quartz monzonite contact near Goat Mountain has small garnets. Common accessory and a l t e r a t i o n minerals are a p a t i t e , opaques, z o i s i t e , muscovite, and c h l o r i t e . The a r g i l l i t e i s a massive, f i n e - g r a i n e d , black rock, commonly w i t h quartz v e i n l e t s , and i n places with l a y e r s of metachert. I t c o n s i s t s of quartz and b i o t i t e i n about equal amounts, d i s t r i b u t e d i n patches r a t h e r than i n l a y e r s . Some specimens have t i n y needles of a c t i n o l i t e . V e i n l e t s of quartz, q u a r t z - b i o t i t e , q u a r t z - K - f e l d s p a r , or quartz-c a l c i t e are common. A p a t i t e and opaques are common accessory minerals. Greenstone i s a massive, dark green rock, i n places w i t h hornblende porphyroblasts up to 1 cm. long. Small quartz v e i n l e t s are common. The greenstone c o n s i s t s of a f i n e - g r a i n e d groundmass of p l a g i o c l a s e ( a l b i t e to o l i g o c l a s e (?) ), z o i s i t e , a c t i n o l i t e , and c a l c i t e . Commonly i t contains s a u s s u r i t i z e d p l a g i o c l a s e phenocrysts ( 1 - 3 mm.) and porphyroblasts of hornblende ( 5 - 1 0 mm.). Some specimens c o n t a i n b i o t i t e or c h l o r i t e . The limestone i s a medium-grained grey rock, composed of c a l c i t e w i t h a l i t t l e i n t e r s t i t i a l g r a p h i t e . 10 TREMBLEUR ULTRAMAFITES Two u l t r a m a f i t e s of the a l p i n e type l i e i n the map area (see Map 1 ) . The M i t c h e l l Mountains U l t r a m a f i t e i s about seven miles long by three miles wide, and i s elongated i n a n o r t h - n o r t h e a s t e r l y d i r e c t i o n . I t was a h a r z b u r g i t e , but l i t t l e of the o r i g i n a l mineralogy now remains. Contact metamorphism by the adjacent M i t c h e l l B a t h o l i t h has produced three p a r a l l e l metamorphic zones, c o n s i s t i n g of s e r p e n t i n i t e , t a l c - o l i v i n e rock, and o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rock. There are two types of i n c l u s i o n s : small pods of m e t a d i o r i t e , r o d i n g i t e , and c h l o r i t e rock, and l a r g e r bodies of rocks resembling the Cache Creek Group. Carbonate a l t e r a t i o n i s found along some f r a c t u r e s . The Goat Mountain U l t r a m a f i t e , two miles to the southwest, i s about one mile i n diameter. I t i s an o l i v i n e -bearing s e r p e n t i n i t e , except f o r a t h i n contact metamorphic zone of t a l c - o l i v i n e rock at the quartz monzonite contact. L i t h o l o g y of the M i t c h e l l Mountains U l t r a m a f i t e S e r p e n t i n i t e S e r p e n t i n i t e of the M i t c h e l l Mountains U l t r a m a f i t e i s dark green and f i n e - g r a i n e d . In most places i t i s massive i n hand specimen, but i n some i t has a contorted f i n e l a y e r i n g . L o c a l concentrations of coarse (1 cm.) chromite g r a i n s form patches. B a s t i t e s and r e l i c t e n s t a t i t e s are r a r e . 11 The rock c o n s i s t s mainly of bladed serpentine and magnetite. X-ray d i f f r a c t i o n (Aumento, 1969) r e v e a l s t h a t the predominant serpentine polymorph i s a n t i g o r i t e , w i t h subordinate l i z a r d i t e . C h l o r i t e and chromite are common accessory minerals. Magnesite and dolomite are common. Tremolite i s r a r e , and diopside i s present i n only two specimens. R e l i c t e n s t a t i t e i s r a r e . Metamorphic o l i v i n e i s common; r e l i c t o l i v i n e i s thought to be absent. In t h i n - s e c t i o n , b a s t i t e s are patches of f i n e - g r a i n e d serpentine, o r i e n t e d p a r a l l e l to former g r a i n boundaries of the e n s t a t i t e ( P l a t e 2 ) . They are almost free of f i n e -grained magnetite, which i s i n t i m a t e l y mixed w i t h most serpentine ( P l a t e J). Pseudomorphs a f t e r o l i v i n e are l e s s obvious. Magnetite i s r a r e l y arranged i n a network of small g r a i n s ( P l a t e 4 ) which probably represents f r a c t u r e s along which s e r p e n t i n i z a t i o n began. O l i v i n e i s u s u a l l y i n small anhedral g r a i n s ( P l a t e 5 ) . S e r p e n t i n i t e ranges from massive to f o l i a t e d . F o l i a t i o n i s shown by a predominant o r i e n t a t i o n of serpentine l a t h s , i n places emphasized by p a r a l l e l t r a i n s of small o l i v i n e or magnetite gr a i n s ( P l a t e 6). R e l i c t textures are found i n massive rocks, but not i n f o l i a t e d rocks. Most specimens, however, show n e i t h e r f o l i a t i o n nor r e l i c t t e x t u r e s . T a l c - O l i v i n e Rock T a l c - o l i v i n e rock i s dark grey, and i s e a s i l y 12 d i s t i n g u i s h e d i n the f i e l d from the dark green s e r p e n t i n i t e . I t weathers to a smooth, pink, mottled surface. I t c o n s i s t s o f o l i v i n e and t a l c , commonly wi t h c h l o r i t e and t r e m o l i t e . Magnesite or dolomite i s present i n some specimens. Accessory minerals are magnetite, chromite, and r a r e l y a sulphide. The rock has elongate i d i o b l a s t i c o l i v i n e g r a i n s (up to 1 cm.) i n a matrix of scaley, f i n e - g r a i n e d t a l c ( P l a t e 7). In d e t a i l , however, the o l i v i n e g r a i n boundaries are i r r e g u l a r . In some specimens the t a l c i s medium-grained, reaching the same s i z e as o l i v i n e . Most specimens have approximately equal amounts of o l i v i n e and t a l c , but where o l i v i n e predominates, the o l i v i n e g r a i n s impinge on each other and a g r a n o b l a s t i c texture r e s u l t s . Deformation r e s u l t s i n g r a n u l a t i o n of o l i v i n e l a t h s ( P l a t e 8), and produces a rock composed of rounded o l i v i n e fragments i n a matrix of t a l c ( P l a t e 9). T a l c - o l i v i n e rocks are not f o l i a t e d on a microscopic s c a l e . Many specimens c o n t a i n a l i t t l e t r e m o l i t e ( l e s s than 2%) as small r a d i a t i n g needles, u s u a l l y i n the t a l c matrix. Some,' however, have up to $0% t r e m o l i t e , i n c l u s t e r s of needles c u t t i n g a l l other phases, w i t h l i t t l e or no t a l c ( P l a t e 10). These rocks may o r i g i n a l l y have been l h e r z o l i t e . Magnetite occurs as medium-sized anhedral g r a i n s , commonly associated w i t h c h l o r i t e , i n the t a l c matrix, suggesting that near o l i v i n e , i r o n has been incorporated 13 i n t o a f a y a l i t e component. O l i v i n e i n almost a l l specimens i s s l i g h t l y a l t e r e d along f r a c t u r e s to serpentine and magnetite dust. No d i l a t i o n can he seen at the f r a c t u r e s , so replacement appears to have been at constant volume. Patches of magnesite or dolomite may i n some cases be a l t e r a t i o n products o f t a l c or t r e m o l i t e . O l i v i n e - E n s t a t i t e - A n t h o p h y l l i t e Rock O l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rock i s dark grey, w i t h coarse (1 cm.) e n s t a t i t e g r a i n s and small a n t h o p h y l l i t e or t r e m o l i t e needles, i n a f i n e - g r a i n e d o l i v i n e matrix. Some specimens have coarse, r a d i a t i n g , honey-coloured a n t h o p h y l l i t e blades. In places secondary t a l c , a f t e r e n s t a t i t e , gives a c h a r a c t e r i s t i c g l i t t e r to the rock. The rock c o n s i s t s of o l i v i n e (about 50%>) w i t h e n s t a t i t e and a n t h o p h y l l i t e (about 50% each). C h l o r i t e i s common, t r e m o l i t e i s abundant i n some specimens, and chromite and magnetite are ubiq u i t o u s . Talc i s a common a l t e r a t i o n product of e n s t a t i t e . Carbonate was not detected. O l i v i n e i s commonly p a r t l y s e r p e n t i n i z e d (up to 50%). O l i v i n e v a r i e s from medium-grained g r a n o b l a s t i c to granulated. E n s t a t i t e i s coarser-grained, anhedral, and i n places encloses o l i v i n e ( P l a t e 1 1 ) . A n t h o p h y l l i t e forms medium-sized prisms or c l u s t e r s of prisms which p i e r c e a l l other phases ( P l a t e 1 2 ) . Tremolite, which commonly occurs 14 . i n small amounts, has a s i m i l a r h a b i t . Some specimens apparently l a c k a n t h o p h y l l i t e , but are r i c h i n t r e m o l i t e . E l e c t r o n microprobe a n a l y s i s r e v e a l s that some amphibole l a t h s are made up of lamellae of t r e m o l i t e and a n t h o p h y l l i t e . E n s t a t i t e i s e x t e n s i v e l y a l t e r e d to t a l c . A l l stages can be seen from f r e s h to completely a l t e r e d ( P l a t e 1 3 ) . A n t h o p h y l l i t e i s l e s s a l t e r e d . In these rocks, a l l of the t a l c i s probably an a l t e r a t i o n product, even where the connection w i t h e n s t a t i t e i s not obvious. M e t a d i o r i t e , Rodingite, and C h l o r i t e Rock I n c l u s i o n s Pods of me t a d i o r i t e , r o d i n g i t e , and c h l o r i t e rock occur as i n c l u s i o n s i n the u l t r a m a f i t e . M e t a d i o r i t e forms la r g e pods (10 to 400 f e e t l o n g ) , w i t h an outer s k i n of c h l o r i t e rock (2 to 12 inches t h i c k ) . I t c o n s i s t s of medium-s i z e d hornblende g r a i n s i n a matrix of f i n e - g r a i n e d p l a g i o -c l a s e . P l a g i o c l a s e i s granulated and s a u s s u r i t i z e d , and some i s metamorphosed to medium-grained a l b i t e and z o i s i t e . Hornblende i s a l t e r e d to f i b r o u s a c t i n o l i t e and to c h l o r i t e . Some of the l a r g e r g r a i n s of hornblende have cores of c l i n o -pyroxene. Grain s i z e ranges from f i n e - g r a i n e d at the edge of an i n c l u s i o n to medium-fine-grained at the centre, and one l a r g e body i s p a r t l y coarse-grained and peg m a t i t i c . Rodingite occurs as pods, 2 to 10 f e e t long, w i t h a s k i n o f c h l o r i t e rock 6 to 18 inches t h i c k ( P l a t e 1 6 ) . The r o d i n g i t e i s pink and green, and c o n s i s t s of c a l c i c and 15 aluminous minerals: c h l o r i t e and s e v e r a l of g r o s s u l a r , hydro-g r o s s u l a r , epidote, idocrase, p l a g i o c l a s e , d i o p s i d e , t r e m o l i t e , and dolomite. C h l o r i t e rock occurs as swarms of small pods (1 to 10 f e e t long) ( P l a t e 17). These i n c l u s i o n s of three d i f f e r i n g rock types occur together, and probably represent o r i g i n a l d i o r i t e or diabase i n t r u s i o n s which have been a l t e r e d to d i f f e r i n g degrees depending on t h e i r s i z e . The metadiorite i s e s s e n t i a l l y unchanged c o m p o s i t i o n a l l y . The r o d i n g i t e preserves the c a l c i c and aluminous chemistry of the m e t a d i o r i t e , but has l o s t a l k a l i e s . The s m a l l e s t bodies, now c h l o r i t e rock, are d i s t i n g u i s h e d c h e m i c a l l y from the s e r p e n t i n i t e only by t h e i r aluminous character. Contact Zones Chemical gradients between the u l t r a m a f i c rocks and rocks of the Cache Creek Group have caused r e a c t i o n zones at the contacts. The zones c o n s i s t of t a l c and c h l o r i t e , w i t h t r e m o l i t e at contacts w i t h limestone. They range from s i x inches to three f e e t t h i c k around i n c l u s i o n s , and widen up to twenty f e e t t h i c k at e x t e r n a l c o n t a c t s . Carbonate Veins A l t e r a t i o n i s developed along f r a c t u r e s i n the Serpentine Zone and at the eastern edge of the T a l c - O l i v i n e 16 VEINS 0 Talc-Carbonate A Quarti-Carbonate Serp / Oi-Ta ' Ol - Ta / Ot * En * AntTl Figure 2: Quartz-Magnes ite and Talc-Magnesite Veins 17 Zone (see Figure 2 ) . There are two types of a l t e r a t i o n : talc-magnesi-te and quartz-magnesite. Quartz-magnesite a l t e r a t i o n i s r e s t r i c t e d to the eastern edge of the Serpentine Zone. A d d i t i o n a l minerals may he dolomite, c h l o r i t e , or magnetite. One specimen contains both t a l c and quartz. The a l t e r a t i o n i s g e n e r a l l y s i x inches to three f e e t t h i c k , and widens up to ten f e e t where i t crosses a sheared zone. L i t h o l o g y of the Goat Mountain U l t r a m a f i t e The Goat Mountain U l t r a m a f i t e c o n s i s t s e n t i r e l y of s e r p e n t i n i t e , except f o r a t h i n (5 to 10 f e e t ) zone of t a l c - o l i v i n e rock where quartz monzonite intrudes the u l t r a m a f i t e . The s e r p e n t i n i t e d i f f e r s from that of the M i t c h e l l Mountains U l t r a m a f i t e i n having a more magnesian bulk composition, and so c o n t a i n i n g more o l i v i n e (up to ^Ofo). O l i v i n e g r a i n s are medium-coarse (up to 5 mm.) and are equant, anhedral, and undeformed. They tend to be choked w i t h f i n e - g r a i n e d magnetite dust. A l i t t l e b r u c i t e i s found i n these rocks, and both magnesite and dolomite are common. Otherwise the s e r p e n t i n i t e i s s i m i l a r to that of the M i t c h e l l Mountains U l t r a m a f i t e . 18 TOPLEY INTRUSIONS (MITCHELL BATHOLITH) The M i t c h e l l B a t h o l i t h , ranging i n composition from d i o r i t e to g r a n i t e , u n d e r l i e s about 160 square miles, j u s t west of the M i t c h e l l Mountains U l t r a m a f i t e . I t was not examined by the w r i t e r , Armstrong (19^9) showed the contact about j/k mile west of the u l t r a m a f i t e (Figure 1 ) . The v a l l e y between the g r a n i t i c and u l t r a m a f i c rocks i s d r i f t -covered, so the p o s i t i o n of the contact i s not w e l l known. I t i s t h i s b a t h o l i t h which i s res p o n s i b l e f o r the metamorphism of the M i t c h e l l Mountains U l r a m a f i t e . Near the Goat Mountain U l t r a m a f i t e , quartz monzonite intrudes rocks of the Cache Creek Group and u l t r a m a f i c rocks. I t i s l i g h t pink to grey, medium- to coarse-grained, wi t h coarse b i o t i t e f l a k e s , quartz, m i c r o c l i n e , and s l i g h t l y k a o l i n i z e d p l a g i o c l a s e ( A n 2 Q to An^). B i o t i t e u s u a l l y makes up 10$ of the rock, but reaches 30% w i t h i n 50 f e e t of the u l t r a m a f i t e . A few percent of muscovite i s commonly present, and accessory minerals are z i r c o n , garnet, and magnetite. At the southern contact between quartz monzonite and u l t r a m a f i t e the quartz monzonite has a c h i l l e d margin 18 inches t h i c k , and the u l t r a m a f i t e has a zone of t a l c - o l i v i n e rock 5 "to 10 f e e t t h i c k . At the western contact between quartz monzonite and a r g i l l i t e , the a r g i l l i t e i s r e c r y s t a l l i z e d and garnet i s developed. A dike, 500 f e e t t h i c k , of medium-to f i n e - g r a i n e d quartz monzonite w i t h quartz phenocrysts, intrudes the s e r p e n t i n i t e . 19 Minor contact metamorphism of the u l t r a m a f i t e a s s o c i a t e d w i t h these bodies of quartz monzonite suggests t h a t they are s m a l l , and not p a r t of the M i t c h e l l B a t h o l i t h proper, as shown by Armstrong (19^9) (Figure 1). FELSITE DIKES Ha l f a dozen small dikes occur i n the area, ranging from dark grey (andesite) to l i g h t pink w i t h quartz pheno-c r y s t s ,(.£hyolite). Most are 6 to 18 inches t h i c k , although one i s 5 f e e t t h i c k . Grain s i z e v a r i e s from medium- to f i n e - g r a i n e d , and c h i l l e d borders are common. Several of the dikes are intruded along contacts between other rock types. 20 STRUCTURE  I n t r o d u c t i o n The predominant s t r u c t u r a l trend i n the southern M i t c h e l l mountains i s north to north-northeast. Rocks of the Cache Creek Group (both i n c l u s i o n s i n the u l t r a m a f i t e and country rocks) are moderately to i s o c l i n a l l y f o l d e d , and the u l t r a m a f i c rocks are s t r o n g l y sheared and b r e c c i a t e d . I t i s not c l e a r whether the M i t c h e l l Mountains U l t r a m a f i t e i s a f o l d e d sheet or a v e r t i c a l l y extended s l a b . Mesoscopic Structure Cache Creek Group The ribbon c h e r t i s well-bedded, and a r g i l l i t e i s normally p o o r l y bedded. Fracture cleavage i s commonly developed i n the a r g i l l i t e , and where strong, i t obscures bedding. Fracture cleavage i s g e n e r a l l y weak i n the ribbon c h e r t . Greenstone i s massive, and g e n e r a l l y shows no s t r u c t u r e . Limestone shows a weak f r a c t u r e cleavage which i s emphasised by d i f f e r e n t i a l weathering. These s t r u c t u r e s are common to both i n c l u s i o n s and country rocks. F o l d i n g i s evident i n ribbon chert and a r g i l l i t e . In the northeastern p a r t of the area, the contact between c h e r t and a r g i l l i t e i s moderately fol d e d about s t e e p l y plunging, n o r t h e a s t e r l y trending axes, w i t h s t e e p l y d i p p i n g a x i a l planes. On the west r i d g e , h a l f a mile from the u l t r a m a f i t e , the ribbon c h e r t and a r g i l l i t e are g e n t l y 21 f o l d e d about a moderately plunging, n o r t h e a s t e r l y t r e n d i n g a x i s , w i t h a s t e e p l y northwesterly dipping a x i a l plane. C l o s e r to the u l t r a m a f i t e , the f o l d i n g becomes t i g h t , then i s o c l i n a l w i t h r o o t l e s s f o l d hinges. The c e n t r a l large i n c l u s i o n i s moderately folded, apparently about s t e e p l y d i p p i n g , n o r t h e r l y trending axes, w i t h v e r t i c a l a x i a l planes. East of the Goat Mountain U l t r a m a f i t e , the c h e r t and a r g i l l i t e are i s o c l i n a l l y f o l d e d , about s u b - h o r i z o n t a l , n o r t h e r l y - t r e n d i n g axes wi t h v e r t i c a l a x i a l planes. West of the Goat Mountain U l t r a m a f i t e , a r g i l l i t e i s c l o s e l y f o l d e d about s u b - h o r i z o n t a l , n o r t h e a s t e r l y trending axes, w i t h v e r t i c a l a x i a l planes. Strong rodding i s found here at f o l d axes. There are f a u l t s of two ages. One set i s p a r a l l e l to and g e n e r a l l y r e l a t e d to a x i a l planes of f o l d s . A second set , of s t e e p l y d i p p i n g normal f a u l t s , cuts the f i r s t s et. Ult r a m a f i c Rocks The s e r p e n t i n i t e i s traversed by shear zones h a l f an i n c h to s e v e r a l inches wide. The serpentine i n these zones i s p o l i s h e d and p l a t e y . ' Shearing i s e s p e c i a l l y intense along the eastern edge of the u l t r a m a f i t e . A c l o s e l y -spaced f r a c t u r e cleavage i s common, and grades i n t o shearing. Microscopic l a y e r i n g i s obscured by the l a t e r shearing, but can be observed i n unsheared blocks. T a l c - o l i v i n e and o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rocks 22 are sheared i n a d i f f e r e n t manner. The shearing causes a c l o s e l y - s p a c e d f r a c t u r e cleavage, producing small p l a t e s p a r a l l e l to the cleavage (Figure 3 ) . This grades i n t o a l e s s intense, more widely-spaced f r a c t u r e cleavage. Some of the t a l c - o l i v i n e rocks are segregated i n t o crude discontinuous bands of t a l c - r i c h and o l i v i n e - r i c h rock. An u n u s u s l l y c l e a r example i s shown i n P l a t e 14. F o l i a t i o n , shearing, and banding g e n e r a l l y s t r i k e approximately north-northeast, and dip away from the a x i s of the u l t r a m a f i t e . There are two types of serpentine b r e c c i a . The f i r s t i s formed of equidimensional blocks of massive s e r p e n t i n i t e i n a matrix of the same rock type ( P l a t e 15)• This b r e c c i a may have been an o r i g i n a l harzburgite b r e c c i a , subsequently s e r p e n t i n i z e d . I t l a c k s p o l i s h e d , p l a t e y serpentine. The second type of s e r p e n t i n i t e b r e c c i a c o n s i s t s of elongate, streamlined blocks of massive s e r p e n t i n i t e , separated by l a y e r s of f o l i a t e d , p o l i s h e d serpentine (Figure 3 ) . This type of b r e c c i a forms zones up to s e v e r a l hundred f e e t wide, e s p e c i a l l y towards the eastern margin of the u l t r a m a f i t e , T a l c - o l i v i n e and o l i v i n e - e n s t a t i t e -a n t h o p h y l l i t e rocks form a s i m i l a r b r e c c i a , composed of elongate, rounded blocks i n a matrix of f o l i a t e d u l t r a m a f i t e (Figure 3 ) - Shearing and shear b r e c c i a t i o n are post-metamorphic. 1 FOOT b -1 FOOT F i g u r e ' 3 : S o m e s t r u c t u r e s in u l t r a m a f i c r o c k s a : S h e a r i n g in t a l c - o l i v i n e a n d o i i v i n e -. e n s t a t i t e - a n t h o p h y l l i t e r o c k , b: S h e a r b r e c c i a in s e r p e n t i n i t e . c : S h e a r b r e c c i a in t a l c - o l i v i n e a n d o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e r o c k . 24 I n c l u s i o n s M e t a d i o r i t e , r o d i n g i t e , and c h l o r i t e rock i n c l u s i o n s occur as swarms of pods (boudins), elongated i n a n o r t h e r l y to n o r t h - n o r t h e a s t e r l y d i r e c t i o n . They are most common i n sheared zones, and in.the western part of the u l t r a m a f i t e . Their deformation i n d i c a t e s that some north-south extension has taken pl a c e . A second type of i n c l u s i o n c o n s i s t s of c h e r t , a r g i l l i t e , limestone, and greenstone, resembling rocks of the Cache Creek Group. Small i n c l u s i o n s , up to 500 f e e t , are g e n e r a l l y round pods ( P l a t e 18). Larger ones are more i r r e g u l a r , and form v e r t i c a l s l a b s , trending approximately north-northeast. The l a r g e s t i n c l u s i o n i s fo l d e d , and one a r g i l l i t e i n c l u s i o n contains f o l d e d lenses of limestone (P l a t e 1 9 ) . Contacts The M i t c h e l l Mountains U l t r a m a f i t e i s b e l i e v e d to be i n f a u l t c ontact w i t h rocks of the Cache Creek Group. On the West Ridge cherty a r g i l l i t e i s m y l o n i t i z e d near the contact, but an andesite dike intrudes the contact i t s e l f . On the eastern r i d g e s the contact i s not seen, but nearby a r g i l l a c e o u s c h e r t i s m y l o n i t i c . Contacts between the u l t r a m a f i t e and i n c l u s i o n s of sedimentary or v o l c a n i c rocks may also be f a u l t e d , but i f so, subsequent metamorphism has o b l i t e r a t e d any evidence of i t . I n f e r r e d v e r t i c a l f a u l t s separate the Goat Mountain 25 U l t r a m a f i t e from rocks of the Cache Creek Group. At i t s southern edge, the u l t r a m a f i t e i s intruded by quartz monzonite. Macroscopic Structure Structures i n the u l t r a m a f i c and Cache Creek Group rocks are roughly p a r a l l e l (Figures 5» 6 ) , showing t h a t the l a s t phase of deformation was shared by both rock types. The Cache Creek Group rocks are f o l d e d , most i n t e n s e l y near the u l t r a m a f i t e . The kind of deformation i n the u l t r a m a f i c rocks i s l e s s c l e a r . Some f o l d i n g has taken p l a c e , as shown by f o l d i n g w i t h i n i n c l u s i o n s , and e s p e c i a l l y by f o l d i n g of the contact between the u l t r a m a f i t e and the l a r g e c e n t r a l i n c l u s i o n . The main deformation i s by shearing, w i t h north-south extension. A rose diagram (Figure 8) of a i r photo lineaments (thought to represent sheared zones) r e v e a l s that most l i e w i t h i n 40° of north, w i t h maxima at 1 7 0 ° and 3 0 ° . The shape of the u l t r a m a f i t e i s unknown. The w r i t e r concludes that i t i s e i t h e r a f o l d e d sheet (Figure 1 0 ) , o r a v e r t i c a l l y extended pod (Figure 1 1 ) , but cross s e c t i o n s (Figure 9) do not d i s c r i m i n a t e between these two p o s s i b i l i t i e s . A sheet of u l t r a m a f i c rocks could have been t e c t o n i c a l l y t r ansported over or i n t o rocks of the Cache Creek Group. This would have i n v o l v e d east-west compression, f o l d i n g about north-south axes, and perhaps north-south extension. 26 The i n c l u s i o n s would represent cores of f o l d s no longer recognizeable. A large pod of u l t r a m a f i t e could have been emplaced v e r t i c a l l y along a north-south f a u l t zone. The i n c l u s i o n s would represent pieces of w a l l rock, engulfed by the u l t r a -mafite which was moving as s e v e r a l f a u l t s l i c e s , r a t h e r than as one homogeneous block. This f a u l t would be r e a c t i v a t e d to produce the post-metamorpism shearing and b r e c c i a t i o n . The w r i t e r p r e f e r s the v e r t i c a l l y extended shape. F i g u r e 4 : S t r u c t u r a l s u b - a r e a s f o r s t e r e o n e t s F i g u r e 5 : S t e r e o g r a p h i c p r o j e c t i o n s ( l o w e r h e m i s p h e r e ) o f s t r u c t u r e s in t h e C a c h e C r e e k G r o u p , a : N o r t h - e a s t c o r n e r of m a p a r e a . (1) b : W e s t r i d g e . ( 2 ) c : E a s t o f G o a t M o u n t a i n u l t r a m a f i t e . ( 3 ) d : W e s t of G o a t M o u n t a i n u l t r a m a f i t e . ( 4 ) F i g u r e 5: L e g e n d : P o l e s t o b e d d i n g M i n o r f o l d a x e s , r o d d i n g N . N S F i g u r e 6 : S t e r e o g r a p h i c p r o j e c t i o n s ( l o w e r h e m i s p h e r e ) of s t r u c t u r e s in t h e u l t r a m a f i c r o c k s , a : M i t c h e l l M o u n t a i n s u l t r a m a f i t e : n o r t h . ( 5 ) b : c e n t r e . ( 6 ) c : s o u t h . ( 7 ) d : G o a t M o u n t a i n u l t r a m a f i t e . ( 8 ) 33 N F i g u r e 8 : S t r i k e f r e q u e n c y d i a g r a m of t o p o g r a p h i c l i n e a m e n t s , a s s e e n o n a i r p h o t o g r a p h s . 4 0 0 0 -A 7000-. 6000-5000-4000-B F i g u r e 9 : G e o l o g i c c r o s s s e c t i o n s a l o n g lines s h o w n o n M a p s 1 L e g e n d / Foliation / Shear y Contact—def ined —approx imate Limit of exposure Rock types as on Map 1. Scale: 1 inch = V2 mile 7000-6000-5000-4000-1 35 7000-| 6000-5000-4000-D 7000-1 6000-5000-4000H E F i g u r e 9 F i g u r e 10: H y p o t h e t i c a l c r o s s s e c t i o n of t h e M i t c h e l l M o u n t a i n s u l t r a m a f i t e . ( F o l d e d s h e e t ) F i g u r e 11: H y p o t h e t i c a l c r o s s s e c t i o n o f t h e M i t c h e l l M o u n t a i n s U l t r a m a f i t e . ( F a u l t e d p o d ) 38 METAMORPHISM  In t r o d u c t i o n Metamorphic assemblages i n the M i t c h e l l Mountains and Goat mountain U l t r a m a f i t e s (Table 1) can be represented i n the system CaO-MgO-SiOg-HgO-COg, modified by some s u b s t i t u t i o n of FeO f o r MgO. A d d i t i o n a l components (CrgO^i AlgO-^t FegO^) are represented by a d d i t i o n a l phases (chromite, c h l o r i t e , magnetite, and s u l p h i d e s ) . Small amounts of AlgO^ may be incorporated i n t o some phases, e s p e c i a l l y serpentine, t a l c , and a n t h o p h y l l i t e , but t h i s i s ignored i n the f o l l o w i n g d i s c u s s i o n . The mole f r a c t i o n Xp g i s g e n e r a l l y l e s s than 0.10, but i s nevertheless s i g n i f i c a n t . Before contact metamorphism the u l t r a m a f i t e s were composed mainly of s e r p e n t i n i t e . The Trembleur u l t r a m a f i t e s as a whole are at l e a s t 60% s e r p e n t i n i z e d (Armstrong, 19^9). Primary t e x t u r e s , except b a s t i t e s , are absent i n the Goat Mountain U l t r a m a f i t e and the Serpentine Zone of the M i t c h e l l Mountains U l t r a m a f i t e , and r e l i c t e n s t a t i t e i s r a r e . None of the o l i v i n e i s thought to be r e l i c t . I n c l u s i o n s of Cache Creek Group rocks, some wi t h limestone, c o n t r i b u t e d COg to the f l u i d phase during contact metamorphism. The contact metamorphism may be regarded as the dehydration of serpentine, i n the presence of an unusually COg-rich f l u i d , complicated by the presence of CaO-bearing s i l i c a t e s and carbonates. 39 M i n e r a l Chemistry Microprobe analyses of o l i v i n e , e n s t a t i t e , t a l c , a n t h o p h y l l i t e , t r e m o l i t e , and c h l o r i t e are given i n Tables 3 to 7, Appendix I. Figure 12 shows X M of each mineral, p l o t t e d against X^ of c o e x i s t i n g o l i v i n e . Data of Trommsdorff and Evans (1972) are compatible w i t h data of the present work, but Reamsbottom's (1974) minerals are more i r o n - r i c h f o r a given value of X^ i n o l i v i n e . This may be due to a change i n the d i s t r i b u t i o n c o e f f i c i e n t s because of higher pressures and temperatures during meta-morphism i n Reamsbottom's area. Microprobe a n a l y s i s r e v e a l s that the c h l o r i t e i s a c l i n o c h l o r e , with the approximate formula: M g 5 ( A l > 8 6 F e t l i f ) A l S i 3 0 1 0 ( O H ) 8 . Only one specimen of c h l o r i t e was analysed. 1.00 4 0 T .98 .96 .94 .92 UJ > _ J O .9 0 en X .88 O o 0 • TALC TREMOLITE ENSTATITE ANTHOPHYLLITE .86 .84 / A' I ! i I i I i i / /° o / «/ / i i i / . i / o A I A/ / / / / 'A /A /' !88 .90 .92 .94 .96 .98 X Mg 1.00 Figure 12: Partitioning of Mg between olivine and other phases. Solid symbols and dot-dashed lines a r e a f te r Trommsdorff and Evans, 1972. 41 Mineral Assemblages Mineral assemblages are presented i n Table 1. The d i f f e r e n t zones are defined as f o l l o w s : Serpentine Zone: Characterized by the presence of serpentine, o f t e n w i t h e i t h e r o l i v i n e or t a l c , but not both. T a l c - O l i v i n e Zone: Cha r a c t e r i z e d by the presence of the p a i r o l i v i n e + t a l c , o f t e n w i t h serp-entine (considered r e t r o g r a d e ) . O l i v i n e - E n s t a t i t e - A n t h o p h y l l i t e Zone: Cha r a c t e r i z e d by the presence of a n t h o p h y l l i t e or e n s t a t i t e or both, w i t h o l i v i n e . Serpentine or t a l c may be present, but are considered retrograde. A d d i t i o n a l minerals which may be found w i t h the above zone-defining assemblages are t r e m o l i t e , diopside ( i n the serpentine zone), c h l o r i t e , magnesite, dolomite, b r u c i t e , magnetite, chromite, and a sulphide mineral. 4 2 Table Is Mineral Assemblages. Serpentine Zone s s + M s + 0 s + 0 + Dol s + 0 + G s + 0 + ( r e l i c t E) s + 0 + Tr s + Di s + T + M s + T + Dol s + T + M + (B) T a l c - O l i v i n e Zone 0 + T + (S) 0 + T + M + (S) 0 + T + Dol + (S) 0 + T + Tr + (S) 0 + T + Tr + Dol + (S) 0 + (S) 0 + Tr + (S) Tr + (S) M + (S) Tr + M Olivine-"Ens t a t i t e - A n t h o p h y l l i t e Zone 0 + E + A + (T) + (S) 0 + E + A + Tr + (T) + (S) 0 + E + (T) + (S) 0 + E + Tr + (T) 0 + E + Tr + G + (T) + (S) 0 + A + (T) + (S) 0 + Tr + (T) + (S) 0 + Dol + (T) + (S) T + Tr + (S) Tr + (S) A l l assemblages may have c h l o r i t e , magnetite, chromite, and a sulphide mineral. Table 1 (cont.) Abb r e v i a t i o n s : S : Serpentine F : F o r s t e r i t e T s Talc E : E n s t a t i t e A : A n t h o p h y l l i t e Di : Diopside Tr s Tremolite Dols Dolomite Gc : C a l c i t e M : Magnesite B : Bruc i t e P : P e r i c l a s e C : Carbonate ( u n i d e n t i f i e d ) Q : Quartz 44 Table 2: Reactions i n the system Ca0-Mg0-Si02;vH20-CO 1. T + 3M + 3H 20 = 2S + 3 C0 2 2. T + 5M = 4F + 5G02 + H2Q 3 . 5S = 6F + T + 9H 20 4. S + M = 2F + 2H 20 + C0 2 5 . 3M + 4Q + H 20 = T + 3C02 6. S + 2Q = T + H 20 7. 2Q.+ 3M + 2H 20 = S + 3 C 0 2 8. 3Dol + 4Q + H 20 = T + 3Gc + 3 C 0 2 9. 5T + 6Gc + 4Q =",3Tr + 6C0 2 + 2H 20 10. 2T + 3Cc = Tr + Dol + G0 2 + HgO 11. 8Q + 5Dol + H 20 = Tr + 3Cc + 7C0 2 12. T + 2Dol + 4Q = Tr + 4C0 2 13. S + 2Dol + 6Q = Tr + 4C0 2 + H 20 14. Tr + 3Cc + 2Q = 5Di + 3C02 + H 20 15. 2Dol + 3T + 2H 20 = Tr + 2S + 4C0 2 16. 13T + lODol = 5Tr + 12F + 20C0 2 + 8H 20 17. 2Dol + 1 3 S = 18F + Tr + 4C0 2 + 2 5 H 2 0 18. Tr + 11D61 = 8F + 13Cc + 9C0 2 + HgO 19. U S + 2Cc = 14F + Tr + 2G0 2 + 21H 20 20. S + Dol = 2F + Gc + C0 2 + 2H 20 21. 9Gc + 4S + 5G02 = 7Dol + Tr + 7H 20 22. 3Tr + 5Cc = H D i + 2F + 5G0 2 + 3H 20 2 3 . 2Di + 5S = Tr + 6F + 9H 20 45 Table 2 (cont.) 24. 3S • + Cc = Di + 4F + C 0 2 + 6H 20 2 5 . 2Tr + 3Cc = 7Di + S + 3 C 0 2 2 6 . M + H2° = b ' + C 0 2 2 7 . Dol + H 2 0 = i Cc + B + C 0 2 28. F + Cc + H 2 0 = Di + B + C 0 2 2 9 . S + Cc = Di • + 2B + C 0 2 3 0 . S + B = 2F + H 2 0 3 1 . M = p + c o 2 3 2 . B = P + HgO 3 3 . Dol = Cc + P + c o 2 3^. 2F • +• Cc = Di + 3P + c o 2 3 5 . 4F • * 9T = 5A + 4H 2 0 3 6 . A + F = 9E + H 2 0 37 . 7T = = 3A + 4Q + 4H 2 0 3 8 . T + E = A 39. T + F = 5 E + H 2 0 40. A = 7E + Q + H 2 0 4 1 . T = 5 E + Q + H 2 0 4 6 Table 2 (cont.) Min e r a l Formulae Used f o r Reactions Serpentine M g 3 S i 2 0 5 ( 0 H ) ^ F o r s t e r i t e Mg 2SiO^ Talc M g 3 S i 4 0 1 0 ( 0 H ) 2 E n s t a t i t e MgSiO^ A n t h o p h y l l i t e M g ? S i 8 0 2 2 ( 0 H ) 2 Diopside CaMgSi 20 6 Tremolite C a 2 M g 5 S i 8 0 2 2 ( O H ) 2 Dolomite GaMg(C0 3) 2 Calc i t e CaCO^ Magnesite MgCO^ Bruc i t e Mg(0H) 2 P e r i c l a s e MgO Quartz S i 0 2 4 7 . Theory Greenwood ( 1967) and Johannes ( 1969) have determined ex p e r i m e n t a l l y many of the r e a c t i o n s i n the system MgO-SiOg-H 20-C0 2 (Figure 1 3 ) . Skippen ( 1 9 7 1 , 1 9 7 4 ) has determined and c a l c u l a t e d r e a c t i o n s i n the system Ca0-Mg0-Si0 2-H 20-C0 2 which are r e l e v a n t to the metamorphism of s i l i c e o u s dolomites. Figures 14, 15, and 16 are derived from the work of the above three authors, and show r e a c t i o n s r e l e v a n t to the metamorphism of GaO-bearing s e r p e n t i n i t e s . Reaction 2 was determined by both Greenwood (1967) and Johannes ( 1 9 6 9 ) , and t h e i r r e s u l t s are i n c l o s e agreement. Reaction 1 was c l o s e l y determined by Johannes. The p o s i t i o n i n P-T space of these two r e a c t i o n s f i x e s the p o s i t i o n s of the other two r e a c t i o n s which are generated at t h e i r i n t e r -s e c t i o n , r e a c t i o n s 3 and 4 . Reaction 4 was not r e v e r s i b l y determined by Johannes. Figure 15 shows r e a c t i o n 4 as c a l c u l a t e d from the two well-determined r e a c t i o n s , and c o n s i d e r a b l y more HgO-rich than determined e x p e r i m e n t a l l y by Johannes, The data of Skippen (1971.197^) and Johannes (19&9) are not e n t i r e l y c o n s i s t e n t . In d e r i v i n g Figures 14, 15, and 16, more r e l i a n c e was placed on Johannes than on Skippen, since Johannes' r e a c t i o n s are d i r e c t l y determined, whereas Skippen's are, f o r the most p a r t , c a l c u l a t e d from other determinations. Greenwood (1971) has suggested three p o s s i b l e topologies 4 8 f o r r e a c t i o n s i n v o l v i n g a n t h o p h y l l i t e (Figure 1 7 ) . The p o s i t i o n of the vapour-absent r e a c t i o n T + 4E = A ( r e a c t i o n 38) cannot be c a l c u l a t e d w i t h confidence, due to imprecise thermodynamic data, and t h i s r e a c t i o n has been shown to be very s e n s i t i v e to s u b s t i t u t i o n of Mg by Fe (Trommsdorff and Evans, 1971. Reamsbottom, 1974). With mineral compositions of specimen 2 60 (Xp g of a n t h o p h y l l i t e = 0.894; see Appendix I ) , the r e a c t i o n takes place 155° lower than i n the pure magnesian system. Reactions i n v o l v i n g vapour have been shown to be r e l a t i v e l y i n s e n s i t i v e to s o l i d s o l u t i o n of FeO (Trommsdorff and Evans, 1971» Reamsbottom, 1974). I t i s l i k e l y (Zen, 1971. Evans and Trommsdorff, 1974, Reamsbottom, 1974) that c o n f i g u r a t i o n B or C of Figure 17 i s s t a b l e f o r the pure magnesian system, but t h a t the a d d i t i o n of moderate amounts of Fe lowers the temperature of r e a c t i o n 38 and s t a b i l i z e s c o n f i g u r a t i o n A, which i s compatible w i t h Figures 13 to 16. 4 9 D i s c u s s i o n Serpentine Zone Common assemblages i n the serpentine zone are S + 0 , and S. In the c e n t r a l p a r t of the zone, w i t h i n one mile of i t s western edge, S + 0 + Tr i s common, and at i t s eastern edge S + Di i s found. These two assemblages l i e on opposite s i d e s of r e a c t i o n 2J, Unfortunately t h i s r e a c t i o n cannot be l o c a t e d with c e r t a i n t y , because there i s a gap of 6500* f e e t between the c l o s e s t d i o p s i d e - and t r e m o l i t e - b e a r i n g assemblages. Figure 15 shows a very small temperature d i f f e r e n c e ( 2 . 5 ° ) between r e a c t i o n s 23 and 3 compared wi t h a I 8 3 0 d i f f e r e n c e between r e a c t i o n s 3 and 35 (see Figure 1 6 ) . However, the most w e s t e r l y p o s s i b l e p o s i t i o n of the western boundary of the diopside-serpentine sub-zone gives a width of about 1300 f e e t f o r the t r e m o l i t e - s e r p e n t i n e sub-zone, compared w i t h 5000 f e e t f o r the t a l c - o l i v i n e zone. This i n d i c a t e s that the temperature d i f f e r e n c e i s i n f a c t l a r g e r : i n the order of 4 0 ° . Trommsdorff and Evans ( 1972) also i n d i c a t e that t h i s temperature d i f f e r e n c e i s l a r g e . I f , as seems l i k e l y , r e a c t i o n s 3 and 4 are a c c u r a t e l y determined, t h i s i n d i c a t e s that r e a c t i o n 22 occurs at a higher temperature than c a l c u l a t e d by Skippen ( 1 9 7 1 ) . In the northern p a r t of the area, the assemblage S + Dol i s found at the eastern edge of the u l t r a m a f i t e . This assemblage i s r e s t r i c t e d to the low-temperature, 59 high-COg side of r e a c t i o n 20. The assemblage S + 0 + Dol or S + 0 + C i s common near the large c e n t r a l i n c l u s i o n . This assemblage i s r e s t r i c t e d to a narrow f i e l d between r e a c t i o n s 4 and 20 (or i s u n i v a r i a n t along r e a c t i o n 4 i f the u n i d e n t i f i e d carbonate i s magnesite). With v a r y i n g X^Q t h i s assemblage i s s t a b l e from 380° to 490°. Two specimens, one from the contact of an i n c l u s i o n and one from the eastern edge, c o n t a i n the assemblage S +' T + M. This u n i v a r i a n t assemblage i s s t a b l e from 330° to 490°, along r e a c t i o n 1, and represents the maximum p o s s i b l e X p n i n a s e r p e n t i n i t e . One specimen, from near o u2 a limestone i n c l u s i o n , contains the assemblage S + T + M + Dol. This assemblage i s also u n i v a r i a n t along r e a c t i o n 1. I t i s probable that l o c a l l y the composition of the f l u i d was buffered by r e a c t i o n s 4 and 17, and p o s s i b l y by r e a c t i o n 2 0. This would increase the X^Q of the f l u i d u n t i l i t was 0.06 at the i n v a r i a n t p o i n t FSTM. U n i v a r i a n t assemblages on r e a c t i o n 1 would reach t h i s i n v a r i a n t p o i n t w i t h very l i t t l e change i n X^0 of the f l u i d . However, the common occurrence of the assemblage S + 0 shows that the f l u i d was normally able to cross r e a c t i o n 4, and remain more C0 ?-rich than the i n v a r i a n t p o i n t FSTM. 51 T a l c - O l i v i n e Zone The t a l c - o l i v i n e zone i s d i s t i n g u i s h e d from the serpentine zone by the presence of the p a i r T + 0 , i n d i c a t i n g t h a t the temperature of the i n v a r i a n t p o i n t FSTM has been exceeded. At 2000 bars, t h i s i n v a r i a n t p o i n t i s at 4 9 0 ° . Common assemblages are T + 0 and T + 0 + Tr. Less common are 0 , T, and 0 + Tr. . These f i v e assemblages are st a b l e throughout the P-T c o n d i t i o n s of the t a l c - o l i v i n e zone. The assemblages T + 0 + M and T + 0 + Dol are common at the eastern edge of the zone, near the larg e c e n t r a l i n c l u s i o n . T + 0 + M i s u n i v a r i a n t along r e a c t i o n 2 , r e p r e s e n t i n g the low-temperature l i m i t of the t a l c - o l i v i n e zone at X C Q g r e a t e r than 0 . 0 6 . T + 0 + Dol i s d i v a r i a n t , but i s r e s t r i c t e d to a small temperature range between r e a c t i o n s 2 and 16. One specimen has the assemblage T + 0 + Tr + Dol. This assemblage i s u n i v a r i a n t along r e a c t i o n 16, and represents the high-temperature l i m i t of the p a i r T + Dol. Because of extensive secondary serpentine, serpentine formed during contact metamorphism i s obscured. However, i t i s probable that metamorphic serpentine i s present i n at l e a s t some of the above assemblages, g i v i n g S + T + 0 + M and S + T + 0 + Dol. The assemblage S.+ T + 0 + M i s i n v a r i a n t , at 490°, and 0 . 0 6 X c o^ (P = 2000 b a r s ) . The assemblage S + T + 0 + Dol i s u n i v a r i a n t , on r e a c t i o n 3 between r e a c t i o n s 2 and 16. The occurrence of the assemblage T + 0 + M shows that 5? at l e a s t l o c a l l y , the f l u i d composition was buffered by r e a c t i o n 2, and i t s Xnr. would have increased w i t h i n c r e a s i n g temperature. Several carbonate-bearing assemblages (0 + T + Tr + G, 0 + T + C , 0 + C ) are found i n the c e n t r a l or western p a r t of the zone; these are thought to be retrograde. O l i v i n e - E n s t a t i t e - A n t h o p h y l l i t e Zone Common assemblages i n the o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e zone are 0 + E + A and 0 + E. A + 0 i s l e s s common. A l l assemblages commonly occur w i t h t r e m o l i t e , and wit h secondary t a l c and serpentine. Retrograde (?) magnesite i s found i n one specimen. The assemblages 0 + T + Tr, 0 + T + Dol, T + Tr, and Tr are e i t h e r not di a g n o s t i c of grade, or are retrograde. The t y p i c a l assemblage of the zone i s 0 + E + A, wi t h a d d i t i o n a l phases f o r a d d i t i o n a l components. E n s t a t i t e commonly encloses o r i e n t e d c r y s t a l remnants of o l i v i n e , and a n t h o p h y l l i t e p i e r c e s a l l other phases. This may be an e q u i l i b r i u m t e x t u r e , but p o s s i b l y i n d i c a t e s the order of c r y s t a l l i z a t i o n . The occurrence of these three phases (where two phases are d i v a r i a n t i n the water-saturated three-component system) must be explained. I t i s p o s s i b l e that the zone may c o n s i s t of three sub-zones: o l i v i n e - a n t h o p h y l l i t e , o l i v i n e - e n s t a t i t e -a n t h o p h y l l i t e , and o l i v i n e - e n s t a t i t e , from east to west 53 (see Map 3 ) - This would be c o n s i s t e n t with the sequence of r e a c t i o n s i n Figure 13 . However, the evidence i s tenuous, and may be due to imperfect sampling. Of these three sub-zones, the o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e sub-zone would be much the widest. The three-phase assemblage F + E + A i s d i v a r i a n t i f the system i s not f l u i d - s a t u r a t e d (see Figure 18). A l l r e a c t i o n s considered i n t h i s system (except r e a c t i o n 38) involve p a r t i c i p a t i o n of components of a f l u i d phase, and most produce a net increase i n gaseous components w i t h i n c r e a s i n g temperature. Thus, up to t h i s p o i n t the system has been considered to be f l u i d - s a t u r a t e d . However, the system may have become undersaturated i n f l u i d , by over-shooting the s t a t e represented i n Figure l8g, g i v i n g o f f too much f l u i d , then r e v e r t i n g to t h a t s t a t e . The s e r i e s of r e a c t i o n s to accomplish t h i s can be seen i n Figure 18. Figure l8e represents the t a l c - o l i v i n e zone. Reaction 39 occurrs, forming the assemblage E + F + HgO. Some of the water escapes. Reaction 38 occurs,., not a f f e c t i n g the assemblage, but c r e a t i n g the p o t e n t i a l f o r a n t h o p h y l l i t e to e x i s t as a s t a b l e phase. Now r e a c t i o n 36 occurs, forming A + F at the expense of E + H 20. Since some of the water has escaped, t h i s r e a c t i o n does not consume a l l the e n s t a t i t e , and the f i n a l s t a t e i s represented by a p o i n t i n the three-phase t r i a n g l e AEF of Figure l 8 g . This sequence of events could be caused by a r e d u c t i o n 5^  i n pressure during metamorphism (Evans and Trommsorff, 197^ » Reamsbottom, 1974), causing a P-T curve such as l i n e g on Figure 17a. I t could also be caused by the d i f f i c u l t y of n u c l e a t i n g a n t h o p h y l l i t e (Greenwood, 1963» F r o s t , 1973)' Reaction 35 might not occur due to t h i s d i f f i c u l t y . E n s t a t i t e would form by the metastable r e a c t i o n 39t and subsequent n u c l e a t i o n of a n t h o p h y l l i t e would allow a n t h o p h y l l i t e to form by r e a c t i o n 36. Reamsbottom (1974) and Evans and Trommsdorff (1974) had independent evidence of u p l i f t , causing a r e d u c t i o n i n pressure during metamorphism. In the Fo r t St. James area, however, the r e g i o n a l metamorphism i s g r e e n s c h i s t . f a c i e s ; b l u e s c h i s t metamorphic rocks and e c l o g i t e s are r e s t r i c t e d to s m a l l , fault-bounded blocks at the P i n c h i F a u l t (Paterson, 1973)« Therefore, although u p l i f t was p o s s i b l e during contact metamorphism, there i s no independent evidence f o r i t . The sequence of events caused by the m e t a s t a b i l i t y of a n t h o p h y l l i t e i s a complicated and c i r c u i t o u s process to achieve the dehydrated s t a t e . There i s some doubt of the p o s s i b i l i t y of dehydrating a rock while nearby rocks are undergoing r e a c t i o n s which produce large volumes of water. However, t h i s remains a p o s s i b l e process to cause the three phases to occur together. The f l u i d - s a t u r a t e d assemblage 0 + E + A may be s t a b i l i z e d i n b. d i v a r i a n t f i e l d by the presence of i r o n i n the system. Trommsdorff and Evans (1972) have shown that 55 Xp,g of 0.10 i n o l i v i n e produces a temperature s h i f t of -8° i n r e a c t i o n 35. The same Xp e produces a s h i f t of + 2° i n the temperature of r e a c t i o n 36. P a r t i t i o n i n g of Fe between the. d i f f e r e n t phases would r e s u l t i n the c r e a t i o n of a three-phase f i e l d at each r e a c t i o n , over a small p a r t of the c a l c u l a t e d temperature change. The assemblage 0 + E + A may be u n i v a r i a n t (or d i v a r i a n t over a small temperature range due to Fe p a r t i t i o n i n g ) , but be s t a b i l i z e d over a lar g e temperature range by a f l u i d phase of v a r i a b l e composition. A p l a u s i b l e case (see f o l l o w i n g s e c t i o n ) w i t h Xnr. of the f l u i d at 0.37 on ^u2 encountering r e a c t i o n 35i causes r e a c t i o n 35 "to proceed over a 22° temperature range, but r e a c t i o n 36 to proceed over a temperature range of only 8°. Thus, both p a r t i t i o n i n g of Fe between phases, and changing f l u i d composition, cause the assemblage 0 + T + A to be stable over a l a r g e r temperature range than the assemblage 0 + E + A. In the f i e l d , the assemblage 0 + E + A i s widespread. A zone of the assemblage 0 + T + A i s not recognized, and only one specimen has t h i s assemblage. I t must be concluded that these two mechanisms cannot be c a l l e d upon to e x p l a i n the occurrence of the three phases 0 + E + A. In the absence of a more s a t i s f a c t o r y e x p l a n a t i o n f o r the occurrence of the three-phase assemblage 0 + E + A, the w r i t e r p r e f e r s the dehydration of the rock due to the d i f f i c u l t y of n u c l e a t i n g a n t h o p h y l l i t e . 56 Changes i n F l u i d Composition I t i s i n s t r u c t i v e to f o l l o w the f l u i d composition as the rock i s heated (see Figure 13)• The s t a r t i n g composition chosen f o r these c a l c u l a t i o n s i s a s e r p e n t i n i t e , w i t h 90 grams o f serpentine, 5g magnesite, and 5 g magnetite. The i n t e r s t i t i a l f l u i d i s assumed to be of n e g l i g i b l e volume, w i t h a composition between r e a c t i o n s 1 and 4. The i n i t i a l temperature i s about 350°C. The f l u i d composition remains constant as the tempera-ture i s r a i s e d , u n t i l r e a c t i o n 4 i s reached. I t then f o l l o w s r e a c t i o n 4 to the i n v a r i a n t p o i n t FSTM, at 490°and X c o = 0.06. The amount of r e a c t i n g m a t e r i a l i s very s m a l l , since the . i n i t i a l volume of i n t e r s t i t i a l f l u i d was chosen to be very sm a l l . At the i n v a r i a n t p o i n t , r e a c t i o n s 1 and 4 occur simultaneously, i n such proportions as to keep X^Q = 0.06. The composite r e a c t i o n i s 1.5^?S + 0.18 M = 2F + 0.273T + 2.82H 20 + 0.l8CO 2. The complete e l i m i n a t i o n of serpentine r e s u l t s i n the formation of a rock c o n s i s t i n g of: 59.10g F 21.77g T 5.00g Mt 1. 8 l g M 87.68g wit h the e v o l u t i o n of 12.32 grams of f l u i d of composition X Q0 2 = °»06. I f none of the f l u i d escapes, r e a c t i o n 2 w i l l 57 consume the magnesite, changing the f l u i d composition to Xnn - 0 . 0 8 7 . I f most of the f l u i d escapes, l e a v i n g perhaps 0.5% hy weight, r e a c t i o n 2 w i l l be more i n f l u e n t i a l i n a f f e c t i n g the f l u i d composition. Under t h i s assumption, a l l the magnesite would be consumed at X^Q = 0 . 3 7 . and 5 5 6 ° . The rock would now be composed of: 6 0 . 6 8 g F 2 0 . 7 1 g T 5 . 0 0 g Mt 8 6 . 3 9 g Again l e t us take the l i m i t i n g assumption that a l l the f l u i d escapes except 0.5%. I f the metastable r e a c t i o n 39 i s the next one encountered, the f l u i d reaches i t at 650° , and proceeds along i t to X^Q = 0 . 0 8 , at 6 7 8 0 ( i f the r e a c t i o n goes to completion). However, i f a n t h o p h y l l i t e nucleates at some p o i n t i n t h i s process, HgO i s consumed from the f l u i d and the composition becomes more COg-rich. I f the s t a b l e r e a c t i o n 35 i s encountered i n s t e a d of r e a c t i o n 39» the f l u i d reaches i t at 6 4 0 ° , and proceeds along i t to X^Q = 0,15* at 6 6 2 ° , where a rock of the f o l l o w i n g composition i s produced: 5 7 . 2 6 g F 2 3 . 6 8 g A 5 . 0 0 g Mt 8 5 . 9 4 g The remaining 0,5% of the f l u i d remains at X^Q = 0 .15 u n t i l r e a c t i o n 36 i s encountered at 6 8 7 ° , I t then f o l l o w s 58 r e a c t i o n 36 to X^Q = 0.06, at 695°i producing a rock of: 5 2 . 9 9 g F 2 7 . 4 l g E 5.00g Mt 85.40g This i s the same composition as would "be produced by-r e a c t i o n 3 9 . I t can be seen that 0.5% magnesite i n a s e r p e n t i n i t e can cause a s u b s t a n t i a l increase i n X^Q of the f l u i d , provided that most of the f l u i d escapes. One-half percent has been taken as a lower l i m i t of r e t e n t i o n . However, only about h a l f the s e r p e n t i n i t e specimens c o n t a i n carbonate, g e n e r a l l y 2% to 5%t so t h i s must be considered a COg-rich extreme. S e r p e n t i n i t e s c o n t a i n i n g o l i v i n e are as common as those c o n t a i n i n g carbonate. An o l i v i n e s e r p e n t i n i t e has already -passed (or i s on) r e a c t i o n 4. As the temperature r i s e s , o n l y HgO w i l l be c o n t r i b u t e d by i t to the f l u i d phase, which must remain at X^Q l e s s than 0.06. Such a large volume of f l u i d i s given o f f during the metamorphism of a s e r p e n t i n i t e (about 15% by weight, 35% by volume) that the f l u i d composition was probably averaged over l a r g e areas, and Xn~ exceeded about 0 .15 only l o c a l l y . 59 Carbonate Veins Carbonate ve i n s are of two types. Talc-magnesite veins have the assemblage T + M + Dol + C h l o r i t e + Magnetite. Quartz-magnesite veins have the assemblage Q + M + Dol + C h l o r i t e + Magnetite. One specimen has both t a l c and quartz. Talc-magnesite veins are found i n the western three quarters of the serpentine zone and at the eastern edge of the t a l c - o l i v i n e zone. This i s c o n s i s t e n t w i t h phase r e l a t i o n s h i p s as shown on Figure 13. A s t a b i l i t y f i e l d f o r the p a i r T + M e x i s t s at the temperatures of the upper serpentine f i e l d , but at higher Xnn . Thus the talc-magnesite v e i n s were caused by COg-rich s o l u t i o n s r i s i n g along f r a c t u r e s , and l o c a l l y i n c r e a s i n g the Xnn of the f l u i d phase enough to cause r e a c t i o n 1 or 2 to occur. Quartz-magnesite veins are found only at the eastern edge of the serpentine zone. I t can be seen from Figure 13 that at low temperatures, i n c r e a s i n g X^Q w i l l cause r e a c t i o n 7 to occur instead of r e a c t i o n 1, producing Q + M instead of T + M. The quartz-magnesite veins are analogous to the talc-magnesite v e i n s , but were formed.at lower temperatures. No analogous a l t e r a t i o n zones were found west of the talc-magnesite veins. Figure 13 shows that at temperatures higher than the T + M f i e l d , a moderate increase i n Xnr. produces no change i n the mineral assemblage F + T. 60 Goat Mountain U l t r a m a f i t e The Goat Mountain u l t r a m a f i t e i s composed of s e r p e n t i n i t e , except f o r a t h i n c ontact metamorphic zone near the quartz monzonite. The common mineral assemblages are S and S + Dol, Two specimens contained S + 0 + M, which i s u n i v a r i a n t along r e a c t i o n 4 , at temperatures from 380° to 490° and X c o l e s s than 0 . 0 6 (at 2000 b a r s ) . One of these specimens has la r g e equant o l i v i n e s , choked w i t h magnetite dust, which have probably formed by r e a c t i o n 4 . One specimen has the assemblage S + M + Dol, which i s s t a b l below and to the COg-rich side of r e a c t i o n 4 . The assembla B + S + M i s found i n one specimen. This i s u n i v a r i a n t along r e a c t i o n 27 at temperatures l e s s than 380° (P = 2000 b a r s ) , but may be due to retrograde a l t e r a t i o n . A l l the observed assemblages could have been formed near the i n v a r i a n t p o i n t BFSM, but there could also have been a large v a r i a t i o n i n temperature and Xrn . u u 2 The contact zone contains the assemblage T + 0 , w i t h retrograde serpentine, and the assemblage 0 + T + M, which u n i v a r i a n t along r e a c t i o n 2 at Xnn g r e a t e r than 0 . 0 6 and temperatures g r e a t e r than 4-90°. The small width of the contact metamorphic zone (5 to 10 f e e t ) i n d i c a t e s that the i n t r u s i v e body which caused t h i s zone was much smaller than that which metamorphosed the M i t c h e l l Mountains U l t r a m a f i t e . 61 Non-Ultramafic rocks Rocks of the Cache Creek Group (country rocks and i n c l u s i o n s ) and m e t a d i o r i t e s are r e l a t i v e l y i n s e n s i t i v e to contact metamorphism. The Cache Creek Group rocks are r e g i o n a l l y metamorphosed to lower to middle g r e e n s c h i s t f a c i e s (Armstrong, 19^9»' Paterson, 1 9 7 3 ) . Outside the area s t u d i e d here, contact metamorphism has produced rocks as high-grade as augen gniess (Armstrong, 1 9 4 9 ) . Here, however, these rocks have assemblages c h a r a c t e r i s t i c of the middle g r e e n s c h i s t f a c i e s , except f o r some greenstones, which have hornblende porphyroblasts, i n d i c a t i n g the upper g r e e n s c h i s t f a c i e s . One specimen of a r g i l l a c e o u s c h e r t from the quartz monzonite contact, near the Goat Mountain u l t r a m a f i t e , has small garnets, a l s o i n d i c a t i n g upper gr e e n s c h i s t f a c i e s . Apart from t h i s ' i n s t a n c e , no c o r r e l a t i o n of the mineralogy of the p e l i t i c and basic rocks w i t h the metamorphic zones of the u l t r a m a f i t e i s p o s s i b l e . 6 2 P-T Conditions During Contact Metamorphism There i s no i n d i c a t i o n of pressure at the time of contact metamorphism, except that f r a c t u r e s could form to l e t COg-rich f l u i d s pass through. An assumed value of 2 0 0 0 bars has been used i n Figure 1 9 . Figure 1 9 shows a p o s s i b l e temperature gradient during contact metamorphism. The absence of b r u c i t e f i x e s the lowest p o s s i b l e temperature at 3 8 0 ° , Assuming th a t X^Q was 0 . 3 0 i n the l a t e COg-rich f l u i d s , the boundary between quartz-magnesite and talc-magnesite veins i s at 400°. The boundary between the serpentine and t a l c - o l i v i n e zones i s at 4 9 0 ° . The boundary between the t a l c - o l i v i n e and o l i v i n e -e n s t a t i t e - a n t h o p h y l l i t e zones i s probably r e a c t i o n 3 9 , at about 6 8 5 ° , but may l o c a l l y be r e a c t i o n 35» at 6 7 5 ° . The temperature of these two r e a c t i o n s may be lowered by the presence of C 0 2 i n the f l u i d phase. At X Q^ = 0 . 1 5 , both would be lowered by about 1 3 ° . A high-temperature l i m i t of the o l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e zone i s imposed by the breakdown of A + F by r e a c t i o n 3 6 , at 7 0 0 ° . . The temperature g r a d i e n t appears to become gentle at the high-temperature end. Part of t h i s may be only apparent, due to lowering of the temperature of r e a c t i o n 3 5 or 3 9 hy s i g n i f i c a n t X^Q . Par t of i t i s probably r e a l , and suggests that the M i t c h e l l B a t h o l i t h i s very c l o s e to the edge of the exposed u l t r a m a f i t e , and may i n f a c t u n d e r l i i t . The metamorphic aureole i s wide (exposed f o r three mile 63 but the M i t c h e l l B a t h o l i t h i s also l a r g e (160 square m i l e s ) . The b a t h o l i t h ranges from true g r a n i t e to d i o r i t e , and could have had an i n t r u s i o n temperature of 800°. This i s j u s t s u f f i c i e n t to cause the observed metamorphism, provided that the b a t h o l i t h cooled by means of convection w i t h i n the magma. 900 800 U o Z Ul tr z> t-< a: UJ o. UJ 6 4 ^ s t c m M g O - S i 0 2 - H 2 0 - C 0 2 PFLUID = PTOTAL= 2 0 0 0 Ba r s 200 .00 "H 2 0 .20 .40 .60 MOLE FRACTION C 0 2 .80 1.00 c o 2 Figure 13: T-X diagram for reations in the fluid-saturated s y s t em M g O - S i 0 2 - H 2 0 - C 0 2 . (After Greenwood, 1967, and Johannes, 1969.) Heavy line shows evolution of the fluid associated With a serpentinite containing 5 % magnesite, assuming 0-5 w t . % retent ion of fluid. 6 5 H 2 0 Increasing Xcc>2 — ' — ^ * Figure 14: Schematic T-X diagram of reactions affecting serpentinite, in the system CaO-MgO-S i < 0 2 - H 2 O -C02- A probable configuration, derived from Greenwood (1967), Johannes (1969), and Skippen (1971). 66 H2O MOLE FRACTION C O z F i g u r e -15: S e r p e n t i n i t e f ield of T-X d i a g r a m f o r r e a c t i o n s in t h e fluid -s a t u r a t e d s y s t e m C a O - M g O - S i C > 2 - H 2 O - C O 2 . ( A f t e r G r e e n w o o d , 1967 , J o h a n n e s , 1 9 6 9 , a n d S k i p p e n , 1971) L e g e n d : • — — De f i n i t e e x p e r i m e n t a l d e t e r m i n a t i o n . — — C a l c u l a t e d , c o n s i s t e n t w i t h e x p e r i m e n t s . — — T o p o l o g i c a l l y c o r r e c t , .no t p r o v e n . 500-1 A • Q F @" .5JZ S y s t e m C a O - M g O - S i 0 2 - H 2 0 - C 0 2 p F L U i D = PTOTAL = 2 0 0 0 B a r s .06 .08 MOLE FRACTION CO2 F i g u r e 16: P a r t of t h e T - X d i a g r a m f o r r e a c t i o n s in t h e f l u i d - s a t u r a t e d s y s t e m C a O - M g O - S i 0 2 - H 2 O - C O 2 . ( A f t e r G r e e n w o o d , 1967 , J o h a n n e s , 1 9 6 9 , and S k i p p e n , 1971) Figure 17:Schematic P-T diagram fo r par t of the s y s t em MgO - S i 0 2 -H 2 0 J showing th ree possible configurations of the reactions, with different positions of the react ion T+E = A, re lat ive to the other reactions. (After G reenwood , 1971) Figure 18: Possible mineral assemblages and reactions in the system MgO-S i 0 2 -H 2 0 . A r r o w s point t o w a r d s higher temperatures. Paths A, B, and C correspond 'to lines 'a'; 'b' and 'd'; 'c', e/ and T, respectively. 800 700 Ul 600 cc z> t-< cc Ul Q. :E Ul 500 400 T e m p e r a t u r e - D i s t a n c e G r a d i e n t O-E-A k TALC-OLIVINE ^ SERPENTINE X C O 2=0.3 T-M VEINSb-M VEINS _L _L o S-B DISTANCE IN MILES Figure 19: Possible temperature gradients across the Mitchell Mountains Ultramaf ite. 71 GEOLOGIC HISTORY OF THE MAP AREA The a s s o c i a t i o n of c h e r t , a r g i l l i t e , and greenstone of the Cache Creek Group, and the Trembleur U l t r a m a f i t e s suggests ancient ocean f l o o r (Mongerand Ross, 1971» Monger, 1 9 7 2 ) . The Limestone D i v i s i o n of the Cache Creek Group i s thought to represent shallow-water s h e l f or seamount sediment. The u l t i m a t e o r i g i n of the u l t r a m a f i c rocks i s s p e c u l a t i v e , but a p o s s i b l e o r i g i n i s by d i f f e r e n t i a t i o n of a b a s a l t i c magma at a.mid-oceanic ridge (Aumento, 1 9 7 1 ) . The i n t r u s i o n of the dikes which l a t e r became me t a d i o r i t e , r o d i n g i t e , and c h l o r i t e rock probably occurred soon afterwards. E a r l y deformation caused b r e c c i a t i o n , and perhaps f o l i a t i o n . R o d i n g i t i z a t i o n and perhaps s e r p e n t i n i z a t i o n may have occurred quite e a r l y . The P i n c h i F a u l t Zone may represent a Late Paleozoic n o r t h e a s t e r l y d i p p i n g subduetion-obduction zone. Rocks of the Cache Creek Group may have been t h r u s t against and under the c o n t i n e n t a l Omineca C r y s t a l l i n e B e l t , and gained t h e i r north-northwesterly t r e n d i n g s t r u c t u r e . B l u e s c h i s t meta-morphism occurred at t h i s time i n p a r t of the Cache Creek Group near the P i n c h i F a u l t Zone (Paterson, 1 9 7 4 ) . U l t r a m a f i c rocks of an. o p h i o l i t e sequence may have been t h r u s t over the Cache Creek Group and deformed w i t h i t . I f the M i t c h e l l Mountains U l t r a m a f i t e a r r i v e d i n t h i s way i t may have sunk, perhaps along a f a u l t zone, to i t s present s t r a t i g r a p h i c l e v e l . While moving, i t apparently engulfed i n c l u s i o n s or 72 i n f o l d s of Cache Creek Group rocks. By Late T r i a s s i c time some of the Trembleur U l t r a m a f i t e s were exposed at the surface, since the bas a l conglomerate of the Takla Group contains u l t r a m a f i c pebbles. The M i t c h e l l B a t h o l i t h was probably intruded i n E a r l y J u r a s s i c time, although i t may be l a t e r . I t caused metamorphism of the M i t c h e l l Mountains U l t r a m a f i t e , and probably d i s t o r t e d the s t r u c t u r e away from the p r e v a i l i n g n orth-northwesterly trend. Later the u l t r a m a f i t e was sheared and b r e c c i a t e d by north-south movement along the f a u l t zone. 73 REFERENCES Armstrong, J.E., 194-9, F o r t St. James map-area, C a s s i a r and Coast D i s t r i c t s , B r i t i s h Columbia: Geol. Surv. Canada, Mem. 2 5 2 . Aumento, F., 1969, Serpentine mineralogy of u l t r a b a s i c i n t r u s i o n s i n Canada and on the M i d - A t l a n t i c Ridge: Geol. Surv. Canada, Paper 6 9 - 7 3 . Aumento, F., and Loubat, H., 1971* The M i d - A t l a n t i c Ridge near 4-5°N, IV: S e r p e n t i n i z e d u l t r a m a f i c i n t r u s i o n s : Can. Jour. Earth S c i . , V o l . 8 , p. 6 3 1 . Bowen, N.L., and T u t t l e , F., 19^9. The system MgO-SiO?-H ?0: Geol. Soc. Am. B u l l . , V o l . 6 0 , p. 4 3 9 . Evans, B.W., and Trommsdorff, V., 1970, Regional metamorphism of u l t r a m a f i c rocks i n the c e n t r a l Alps: parageneses i n the system Ca0-Mg0-Si0 ?-H ? 0: Schweiz. Min. Pet. M i t t . , V o l . 5 0 / 3 . p. 4 8 1 . , 197^i S t a b i l i t y of e n s t a t i t e + t a l c , and C0 2-meta-somatism of m e t a p e r i d o t i t e , V a l d'Efra, Lepontme Alps: Am. Jour. S c i . , v o l . 2 7 4 , p. 2 7 4 . F r o s t , B.R., 1973» Contact metamorphism of the I n g a l l s p e r i d o t i t e at Paddy-Go-Easy Pass, C e n t r a l Cascades, Washington: Geol. Soc. Am. Abstr., v o l . 5» no. 1, p. 4 5 . G a b r i e l s e , H., and Wheeler, J.O., i 9 6 0 , Tectonic framework of the southern Yukon and northwestern B r i t i s h Columbia: Geol. Surv. Canada, Paper 6 0 - 2 4 . Greenwood, H.J., 1963» The sy n t h e s i s and s t a b i l i t y of a n t h o p h y l l i t e , Jour. Pet., v o l . 4 , p. 317. , 1967» Mineral e q u i l i b r i a i n the system MgO-SiOg-HgO-COg: i n Researches i n geochemistry, v o l . 2 , Abelson, P.H., ed., John Wiley and Sons, New York, London, Sydney, p. 5^2. , 1971i A n t h o p h y l l i t e . C o r r e c t i o n s and comments on i t s s t a b i l i t y : Am. Jour. S c i . , v o l . 2 7 0 , p. 151. , 1975» B u f f e r i n g of pore f l u i d s by metamorphic r e a c t i o n s : Am. Jour. S c i . , ( i n p r e s s ) . Johannes, W., 1969, An experimental i n v e s t i g a t i o n o f the system Mg0-Si0?-H P 0-C0?: Am. Jour. S c i . , v o l . 2 6 7 , p. I O 8 3 . 74 McTaggart, K.G., 1971» On the o r i g i n of u l t r a m a f i c rocks: Geol. Soc. Am. B u l l . , v o l . 82, p. 2 3 . Monger, J.W.H., 1972, Oceanic rocks of the Canadian C o r d i l -l e r a s Canada Dept. Energy, Mines, and Resources, Publ. E a r t h Physics Branch, v o l . 42, p. 5 9 . Monger, J.W.H., and Ross, C.A., 1971, D i s t r i b u t i o n of f u s i l i n a c e a n s i n the western C o r d i l l e r a s Can. Jour. Earth S c i . , v o l . 8 , p. 2 5 9 , Monger, J.W.H., Souther, J.G., and G a b r i e l s e , H., 1972, E v o l u t i o n of the Canadian C o r d i l l e r a : a p l a t e t e c t o n i c model: Am. Jour. S c i . , v o l . 2 7 2 , p. 5 7 7 . Paterson, I.A., 1973» The geology of the P i n c h i Lake area, c e n t r a l B r i t i s h Columbia: unpublished Ph.D. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B r i t i s h Columbia. Reamsbottom, S.B., 1974, Geology and metamorphism of the Mount Breakenridge area, H a r r i s o n Lake, B r i t i s h Columbia: unpublished Ph.D. t h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, B r i t i s h Columbia. Robie, R.A., and Waldbaum, D.R., 1968, Thermodynamic p r o p e r t i e s of minerals and r e l a t e d substances at 298.15°K (25 .0°C) and one atmosphere ( 1 . 0 1 3 bars) pressure and at higher temperatures: U.S. Geol. Surv. B u l l . 1259. Souther, J.G., and Armstrong, J.E., 1966, North c e n t r a l b e l t of the C o r d i l l e r a of B r i t i s h Columbia: i n Tectonic h i s t o r y and mineral deposits of the western C o r d i l l e r a , Can. I n s t . Min. Metal., Spec. V o l . 8 , p. 171. Skippen, G.B., 1971, Experimental data f o r r e a c t i o n s i n s i l i c e o u s marbles: Jour. Geol., v o l . 79 , p. 4 5 7 . , 1974, An experimental model f o r low pressure metamorphism of s i l i c e o u s d o l o m i t i c marbles: Am. Jour. S c i . , v o l . 2 7 4 , p. 4 8 7 . Trommsdorff, V., and Evans, B.W., 1969, The s t a b l e a s s o c i a t i o n e n s t a t i t e - f o r s t e r i t e - c h l o r i t e i n amphibolite f a c i e s u l t r a m a f i c s of the Lepontine Alps: Schweiz. Min. Pet. M i t t . , v o l . 4 9 , p. 325 . _ , 1972, Progressive metamorphism of a n t i g o r i t e s c h i s t i n the B e r g e l l T o n a l i t e aureole ( I t a l y ) : Am. Jour. S c i . , v o l . 2 7 2 , p. 4 2 3 . 75 Wheeler, J.O., and G a b r i e l s e , H., 1972, ( c o - o r d i n a t o r s ) , The C o r d i l l e r a n s t r u c t u r a l p r o v i n c e i i n V a r i a t i o n s i n t e c t o n i c s t y l e s i n Canada, ed. P r i c e , R.A., and Douglas, R.J.W.s Geol. Assoc. Canada, Spec. Paper 11, p. 1. White, W.H., S i n c l a i r , A.J., Harakal, J.E., and Dawson, K.M., 1970, Potassium-argon ages of Topley i n t r u s i o n s near Endako, B r i t i s h Columbia: Can, Jour. E a r t h S c i . , v o l . 7, p. 1172. Zen, E., 1971, Comments on the thermodynamic constants and hydrothermal s t a b i l i t y of a n t h o p h y l l i t e : Am. Jour. S c i . , v o l . 2 7 0 , p. 136. APPENDIX I: MINERALOGY 76 E l e c t r o n Microprobe A n a l y s i s Four specimens were s.tudied w i t h an ARL EMX-SM e l e c t r o n microprobe at the U n i v e r s i t y of Washington. The f o l l o w i n g standards were used: O l i v i n e ENAL 2 0 A l Diopside Glass Ca F19.7 S i , Fe, Mg YS-24 S i , Fe, Mg 174.1 S i , Fe, Mg O l i v i n e T S i , Fe, Mg Quebec S i , Fe, Mg MG7 S i , Fe, Mg E n s t a t i t e ENAL 2 0 A l Diopside Glass Ca R62 S i , Fe, Mg R104 S i , Fe, Mg F19.7 S i , Fe, Mg Es k o l a 22 S i , Fe, Mg E n s t a t i t e Glass S i , Mg Amphiboles ENAL 2 0 A l 116 A c t i n o l i t e S i , Fe, Mg, Ca Campolungo Tremolite S i , Fe, Mg, Ca 77 Talc C h l o r i t e ENAL 2 0 A l E n s t a t i t e Glass S i , Mg Campolungo Tremolite S i , Fe, Mg, Ca O l i v i n e T S i , Fe, Mg ENAL 2 0 A l Campolungo Tremolite Ca Mg7 C h l o r i t e S i , Fe, Mg, A l LA 10 K C h l o r i t e S i , Fe, Mg, A l LA 25 C C h l o r i t e S i , Fe, Mg, A l The r e s u l t s were processed through computer program UWPROBE, which made c o r r e c t i o n s f o r deadtime, "background, and d r i f t , and f i t t e d the data to a curve. The standards and the program UWPROBE are the property of the U n i v e r s i t y of Washington, S e a t t l e . 78 Table 3 s Microprobe Analyses of O l i v i n e Specimen No. 258 . 7?6 2 60 580 S i O ? 4 0 . 5 0 40 . 5 8 40.41 ^ 0 . 5 5 FeO * 9.29 5 . 7 0 9 . 0 6 7. 62 MgO 4 9 . 9 4 5 2 . 7 2 50.04 5 1 . 2 9 CaO 0. 02 0 . 0 1 0. 01 0. 01 T o t a l 9 9 . 7 5 9 9 . 0 1 9 9 . 5 2 9 9 . 4 7 No. of Ions on basis : of 4 ( 0 ) . S i 0 . 9 2 6 0 . 9 8 7 0 .992 0 . 9 9 0 Fe 0 . 1 9 0 0 .116 0 . 1 8 6 0 . 1 5 6 Mg 1.824 1.9H 1 .830 1 .865 Ca 0 . 001 — — Fe 0. 094 0 . 0 5 7 0 . 0 9 2 0 . 0 7 7 XMg 0 . 9 0 6 0 . 9 4 3 0 . 9 0 8 0 . 9 2 3 * T o t a l Fe expressed as FeO. 79 Table 4 : Microprobe Analyses of Talc Specimen No. 580 2 60 2^8 S i 0 2 AlpO~ FeD J * MgO H 2 0 * * 6 3 . 0 9 0 . 2 0 0 . 9 2 3 0 . 9 4 4 . 7 6 2 . 4 3 0 . 3 4 1 .44 3 0 . 69 4 . 7 6 1 . 8 3 0 . 5 9 1 .20 3 0 . 7 4 4 . 7 T o t a l 9 9 . 8 5 9 9 . 60 9 9 . 0 6 No of Ions on b a s i s of 24(0,OH). S i 8 . 0 0 8 7 . 9 6 7 7 . 9 2 9 A l 0 . 0 3 1 0 . 0 5 4 0 . 0 9 2 Fe 0 . 0 9 8 0 . 1 5 3 0.129 Mg 5 . 8 5 0 5 . 8 3 5 5 . 8 7 2 OH 3 . 9 8 1 4 . 0 0 2 4 . 0 2 2 X p e 0 . 0 1 6 0 . 0 2 6 0 . 0 2 1 X M g 0 . 9 8 4 0 . 9 7 4 0 . 9 7 9 * T o t a l Fe expressed as FeO ** 4 . 7 weight percent H 2 0 added to each a n a l y s i s . 8 0 Table 5* Microprobe Analyses of E n s t a t i t e Specimen No. 580 2 60 736 SiOp Alp D o FeO J * MgO CaO 5 7 . 2 4 o. 63 5 . 3 8 3 6 . 3 1 0 . 0 5 5 6 . 8 0 1 . 4 6 5 . 5 3 3 5 . 3 3 0 . 0 3 5 7 . 5 7 1.07 4 . 4 5 3 6 . 4 3 0. 0 4 T o t a l 9 9 . 61 9 9 . 1 5 9 9 . 5 6 No. of Ions on b a s i s of 6 ( 0 ) . S i 1 .971 1 .964 1 .973 A l 0 . 0 2 6 0 . 0 6 0 0 . 0 4 3 Fe 0 . 1 5 5 0 . 1 6 0 0.127 Mg 1 .863 1 . 8 2 1 1 .861 Ca 0 . 0 0 2 0 . 0 0 1 0 . 0 0 1 X.„ 0 . 0 7 7 0 . 0 8 1 0 . 0 6 4 Fe x M g 0 . 9 2 3 0 . 9 0 9 0 . 9 3 6 T o t a l Fe expressed as FeO. 81 Table 6s Microprobe Analyses of Tremolite and A n t h o p h y l l i t e Tremolite A n t h o p h y l l i t e Specimen No. 2 60 2 60 736 SiCU 56 .40 5 6 . 1 0 5 6 . 9 6 A l ? t U 0 . 7 6 O.23 0. 34 FeO J * 1 .90 7 .51 4 . 1 3 MgO 28 .07 35.48 36. 61 GaO 12 .55 0.12 -H 2 0 * * 2 . 2 2 . 2 2 . 2 T o t a l 101 .88 101 .43 1 0 1 . 9 3 No. of Ions on b a s i s of 24(0,OH). S i 7 . 5 9 0 7.512 7 . 4 7 6 A l 0 . 120 0 . 0 3 6 0 . 0 5 3 Fe 0.210 0.841 0 . 4 5 3 Mg 5 .592 7. 082 7 . 4 7 6 Ca 1.810 0.017 -OH 1.975 1 .965 1 .926 Fe o. 036 0 . 1 0 6 0 . 0 5 7 XMg 0 . 9 6 4 0 . 8 9 4 0 . 9 4 3 T o t a l Fe expressed as FeO. 2 . 2 weight percent H ? 0 added. 82 Table 7: Microprobe Analyses of C h l o r i t e Specimen No. 2 60 S i 0 2 3 1 . 3 3 Alp 6- 16. 30 FeD J * 1.94 MgO 3 4 . 6 3 CaO 0.10 H 20 ** 1 2 . 5 T o t a l 96.80 S i A l A l Fe Mg Ca OH No. of Ions on b a s i s of 18(0,OH). 0 . 8 7 8 0.157 4 . 9 8 1 0 . 0 1 0 J 8.046 6. 026 X Fe X Mg 0 . 0 3 0 0 . 9 7 0 T o t a l Fe expressed as FeO. 1 2 . 5 weight percent H ?0 added. 83 U n i v e r s a l Stage Determinations Table 8 : E n s t a t i t e Compositions Sample No. X M g 2 2 9 a 90 8 6 4 8 8 92 513 97 5 7 4 a 9^ 93 577 95 8 4 APPENDIX H i MINERAL COMPOSITION OF ROCK SPECIMENS X-Ray D i f f r a c t i o n Specimens were studied by whole-rock X-ray d i f f r a c t i o n , as a quick method of d e t e c t i n g the presence of e s s e n t i a l minerals. Minerals present i n q u a n t i t i e s l e s s than about 5% would not be detected by t h i s method. No attempt was made to analyse q u a n t i t a t i v e l y or to f i n d compositions of minerals by X-ray d i f f r a c t i o n . Some success was achieved using Aumento's (1969) method f o r determining the three forms of serpentine; however t h i s was found to be quite s u b j e c t i v e . The u n i t used was a P h i l l i p s 1011 d i f f r a c t o m e t e r at the U n i v e r s i t y of B r i t i s h Columbia. Table. 9» Whole-Rock X-Ray D i f f r a c t i o n A n a l y s i s ; Ul-tramaf i c Rocks ( M i t c h e l l Mountains) Specimen No. ^ l b ^ ^ ^ 2 6 ^ ^ 5 J ^ 5 i 5 ^ 5 ^ ^ 6 2 6 2 j X X X X Bruc i t e A n t i g o r i t e x x x x x x L i z a r d i t e x x x Serpentine x x x O l i v i n e x x x x x x x x Talc x x x x x x x x E n s t a t i t e A n t h o p h y l l i t e Diopside Tremolite C h l o r i t e x x x x oo Dolomite x x x ^ Magnesite x x x x x x x Magnetite x x x x x Table 9 (continued); Specimen No. §2 67b 68 Bruc i t e A n t i g o r i t e x L i z a r d i t e x Serpentine x O l i v i n e x x x x Talc x x x x E n s t a t i t e A n t h o p h y l l i t e Diopside Tremolite x x C h l o r i t e x x . x x Dolomite x Magnesite Magnetite x 21 ZZb 77c 21 80a 81 82 8_3_ 84 84a x x X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X 00 ON Table 9 (continued): Specimen No. 86 8£ 9.2 100 1QT Bruc i t e A n t i g o r i t e x x x L i z a r d i t e x x Serpentine x O l i v i n e x x x x Talc x x x x x E n s t a t i t e A n t h o p h y l l i t e Diopside Tremolite C h l o r i t e x Dolomite x x Magnesite x x Magnetite 103a 107 109a 113 114-115 116 168 189 196 x X X X X X X X X X X X X X X X X X X CO X X X X Table 9 (continued) Specimen No. 1?7 218 219 2 2 0 a 229b 244a B r u c i t e A n t i g o r i t e L i z a r d i t e Serpentine X X X X X X O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e X X X X X X X X X X X Diopside Tremolite X C h l o r i t e X X X X X Dolomite Magnesite Magnetite x x x x x x x x x X X X X X 00 CO X X X X X X Table 9 (continued) Specimen No. 45_8 4 6 ^ 4?> $18 5_2J. 530b 533 537 5^7 559 569 579 580 636a 637 Bruc i t e A n t i g o r i t e x x x x L i z a r d i t e x x x x Serpentine x x x x x x x x O l i v i n e x x x x x x x x x x x x Talc x x x x x x x > x x E n s t a t i t e x A n t h o p h y l l i t e x Diopside Tremolite x x x x CO C h l o r i t e x x x x x x x x >^ Dolomite x Magnesite Magnetite x x Table 9 (continued) Specimen No. 639 6^9 65? 655 663 Bruc i t e A n t i g o r i t e L i z a r d i t e Serpentine X X X X X O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e X X X X X X Diopside Tremolite X X C h l o r i t e X X Dolomite Magnesite X Magnetite 680 685 688 689 698 ?04 712 221 755 x x X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X o Table 9 (continued) Specimen No. Bruc i t e A n t i g o r i t e L i z a r d i t e Serpentine O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e Diopside Tremolite C h l o r i t e Dolomite Magnesite Magnetite 757 763 770 773 776 781 785 794 801 814 827 x x x X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Table 10: Whole-Rock X-Ray D i f f r a c t i o n A n a l y s i s : Ultramafic Rocks (Goat Mountain) Specimen No. 8^9 8 9 9 a 901 901a 901c 2^W.2112lA2112U^12^ B r u c i t e x x A n t i g o r i t e x x x x x x x x L i z a r d i t e x x x x Serpentine x-O l i v i n e x x x x x x x Talc x x x E n s t a t i t e A n t h o p h y l l i t e Diopside Tremolite Chlori.te x x x ^ Dolomite x x x Magnesite x x x x Magnetite x x Table 12; Whole-Rock X-Ray D i f f r a c t i o n A n a l y s i s ; Rodingites and Carbonate Rocks Specimen No. Grossular Epidote Idocrase C h l o r i t e Dolomite Rodingites  22b 22e 80 4 5 2 a 590 x x X X X X X X X X Specimen No. Quartz C a l e i t e W o l l a s t o n i t e Diopside Tremolite Carbonate Rocks  178 487 614 969 x x X X X X Table 11; Whole-Rock X-Ray D i f f r a c t i o n A n a l y s i s ; A l t e r a t i o n Zones Specimen No. Quartz Serpentine Talc Diopside Tremolite Cale i t e Dolomite Magnesite C h l o r i t e Veins 77d 154a 175 x x X X X X X X X Contact Zones 5_a 5_0a 5_0b 5_6 179a 696 916 x x X X X X X X X X X X X X X X I n c l u s i o n 4o_ NO 95 Thin Section Microscopy CO TH NO o o o CO col CM CO SI co o ON ON VO o CO toco -3" o 3^-UN, -3-o-co C O ON o 00 V UN. CM U N O CM ' -f UN. UN, CM CM O C O -d- UN. UN. C O UN. UN. o CM 5^-U N o CM i-H M o CO o CM CM CM V N U N CM U N U N •H CD > • -P o CD li c CD H CD •rH -P >> CD -P CD TH CD CD -P CD •H ,c -H -P s -P C c •p ft •rH H •H CD •rH •rH CD •H cd o CQ O o O ft > ft e O CD 3 U •H rH CQ -P O CD H cd ft £H CD H cd c c •H ^ 45 EH Ul pq w O EH fx} < Q EH O 0 •p •H O H cd o CD •p cd C o W> SH cd cd S o TH V U N V U N U N CO CM H H V TH UN. CD -P •H -P CD CD CD -P -d • rH .H Cd 45 45 ft H 3 Table 13 (dontinued) Specimen No. 160 161 164 168 180 B r u c i t e Serpentine 80 75 95 50 50 O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e <1 30 1 1 Diopside Tremolite 1 1 C h l o r i t e 18 20 44 14 C a l e i t e Dolomite Magnesite Carbonate 2 Magnetite Chromite Sulphide 2 2 1 4 4 1 5 T o t a l 100 100 100 100 100 189 219 2 2 0 a 223 225 22 6: 227 229a 229b 45 10 10 10 40 30 40 40 40 34 39 50 27 x x X X 23 50 8 10 4 39 20 x 5 5 10 2 15 5 5 1 1 1 1 2 <1 1 < 1 x 100 100 100 100 100 100 100 100 100 Table 13 (continued) Specimen No. 24? 249 258 2 60 488 Bruc i t e Serpentine 3 20 O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e 45 49 40 50 49 ^5 50 3^ 15 40 10 10 15 Diopside Tremolite 5 1 C a l e i t e Dolomite Magnesite Carbonate 5 Magnetite Chromite Sulphide < 1 1 l 1 < 1 < l <1 T o t a l 100 100 100 100 100 J-94 495 507 510 513 H i 518 574a 577 30 15 10 8 30 10 25 10 5 48 50 50 65 30 57 35 35 35 20 35 5 23 < 1 33 15 15 8 10 15 20 10 5 2 15 5 20 10 2 30 2 1 <1 2 1 5 1 1 1 <1 1 <1 1 < l 100 100 100 100 100 100 100 100 100 Table 13 (continued) Specimen No, 580 602 608 703 704 721 B r u c i t e Serpentine 25 85 O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e 40 5 10 5 5 3 35 50 45 30 10 40 33 5 5 40 29 Diopside Tremolite 10 10 C h l o r i t e 5 10 1 5 C a l e i t e Dolomite Magne s i t e Carbonate 5 Magnetite Chromite Sulphide 7 3 l l 5 5 1 1 1 T o t a l 100 100 100 100 100 100 15 60 13 40 20 25 48 33 x x 50 10 10 x 2 2 1 5 x x 100 100 100 100 Table 14: V i s u a l l y Estimated Modal Analyses; Ultramafic Rocks (Goat Mountain) Specimen No. 899a 901a 917 925 B r u c i t e Serpentine 35 5 84 50 O l i v i n e Talc E n s t a t i t e A n t h o p h y l l i t e 40 10 40 45 Diopside Tremolite C h l o r i t e Calc i t e Dolomite Magnesite Carbonate 10 10 40 15 2 Magnetite Chromite Sulphide 5 2 3 1 3 T o t a l 100 100 100 100 Table 15: V i s u a l l y Estimated Modal Analyses: Chert and A r g i l l i t e Specimen No. 1 8 2£c 41 88 l46a 198 2 07 255 357 400 432 873 9^0 Quartz 80 60 60 83 93 90 75 ^5 40 85 80 10 48 90 K-Feldspar 5 10 Muscovite 1 10 Z o i s i t e 1 10 B i o t i t e 10 20 15 1 5 20 49 35 5 18 40 Hornblende 2 0 10 5 C h l o r i t e 3 5 1 30 Garnet 1 Carbonate 2 0 30 A p a t i t e 5 1 Magnetite 5 20 20 2 2 3 5 5 1 2 1 5 o Hematite 1 5 1 Sulphide 1 o T o t a l 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Table 16: V i s u a l l y Estimated Modal Analyses; Carbonate Rocks Specimen No. 5_ Quartz Carbonate 100 W o l l a s t o n i t e Diopside Tremolite Talc C h l o r i t e Magnetite Graphite <1 T o t a l 100 Limestone  151 178 273 969 10 90 99 55 100 40 5 1 100 100 100 100 Vein i n Limestone 487 614 10 5 5 80 45 <1 48 5 2 100 100 Contact Zone 84i 5 10 53 30 100 Table 17'• V i s u a l l y Estimated Modal Analyses; Basic Rocks Greenstone Metadiorite Rodingite Specimen No. 6 . 22 399 667 1Z 501 4 5 2 a P l a g i o c l a s e Z o i s i t e 20 10 15 9 5 45 20 25 25 40 10 Diopside Hornblende A c t i n o l i t e C h l o r i t e 60 70 5 45 5 5 75 15 20 15 1 20 39 C a l e i t e 5 Grossular 90 Magnetite Leucoxene 1 4 l 1 T o t a l 100 100 100 100 100 100 100 TaMe 18; V i s u a l l y Estimated Modal Analyses: F e l s i c Rocks Quartz __Monzonite F e l s i t e Specimen No. 916c 9^1 9 4 l a 2 5d Quartz 25 30 45 K-Feldspar 25 25 75 P l a g i o c l a s e 40 30 Muscovite 5 5 10 20 S c a p o l i t e 5 B i o t i t e 5 i o 10 5 Hornblende 25 Garnet l Z i r c o n l C a l c i t e 3 Magnetite l 2 T o t a l .100 100 100 100 1 0 4 P l a t e 1: Specimen 357» t h i n s e c t i o n , plane l i g h t . A r g i l l a c e o u s metachert, with b i o t i t e - r i c h "bands and small b i o t i t e f l a k e s . P l a t e 2s Specimen 164, t h i n s e c t i o n , p o l a r i z e d l i g h t . S e r p e n t i n i t e , w i t h b a s t i t e pseudomorphs. 105 P l a t e 3 ' Specimen 164, t h i n s e c t i o n , plane l i g h t . S e r p e n t i n i t e , showing d i s t r i b u t i o n of magnetite i n serpentine mesh and i n b a s t i t e . P l a t e 4 : Specimen 138, t h i n s e c t i o n , plane l i g h t . S e r p e n t i n i t e , with magnetite network. 1 0 6 P l a t e 5i Specimen ?4, t h i n s e c t i o n , plane l i g h t . S e r p e n t i n i t e , w i t h o l i v i n e g r a i n . P l a t e 6s Specimen 34, t h i n s e c t i o n , p o l a r i z e d l i g h t . S e r p e n t i n i t e , f o l i a t e d . P l a t e 8: Specimen 2^7, t h i n s e c t i o n , plane l i g h t . T a l c - o l i v i n e rock, p a r t l y granulated. 108 P l a t e 10: Specimen 226, t h i n s e c t i o n , plane l i g h t . T r e m o l i t e - o l i v i n e rock. 109 P l a t e H i Specimen 736, t h i n s e c t i o n , p o l a r i z e d l i g h t . O l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rock: E n s t a t i t e encloses o l i v i n e , a n t h o p h y l l i t e p i e r c e s other phases. P l a t e 12: Specimen 488, t h i n s e c t i o n , p o l a r i z e d l i g h t . O l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rock: A n t h o p h y l l i t e i n c l u s t e r s , c u t t i n g a l l other phases. 110 P l a t e 13: Specimen 701 , t h i n s e c t i o n , p o l a r i z e d l i g h t , O l i v i n e - e n s t a t i t e - a n t h o p h y l l i t e rock: e n s t a t i t e p a r t l y a l t e r e d to t a l c . P l a t e 14: T a l c - o l i v i n e rock: coarse compositional banding. I l l P l a t e 16: Rodingite pod i n s e r p e n t i n i t e , w i t h s k i n of c h l o r i t e rock. P l a t e 18: A large i n c l u s i o n t a l c - o l i v i n e rock. (400 f e e t ) of a r g i l l i t e i n 113 P l a t e 19: A l a r g e i n c l u s i o n (|- mile) of a r g i l l i t e , w i t h folded lenses of limestone. Foliation, F r a c t u r e Cleavage, in Ultramafic Rocks Shear Bedding, F r a c t u r e Cleavage, in Cache Creek Group Rodding, Minor Fold Axes /Hf~" Syncline Anticline rJs^J Fault: defined approximate assumed -DLine of Cross Section M I T C H E L L M O U N T A I N S U L T R A M A F I C B O D Y F o r t St. J a m e s M a p A r e a B r i t i s h C o l u m b i a STRUCTURE M i l e s Rock Type s o T E A Tr Di Dol M C B Serpentinite Talc-Olivine Rock Olivine-Ens t a t ite-Anthophyllite Rock Mineral Assemblage Boundaries Defined Position Approximate Minerals Present S e r p e n t i n e Olivine T a l c E n s t a t i t e A n t h o p h y l l i t e Tremolite Diopside Dolomite Magnesite C a r b o n a t e Brucite M I T C H E L L M O U N T A I N S U L T R A M A F I C B O D Y F o r t St. J a m e s M a p A r e a B r i t i s h C o l u m b i a SAMPLE LOCATIONS M i l e s L E G E N D QUATERNARY 8 i G l a c t a l D r i f t JURASSIC 7 Topley or Omineca Intrusions Granite PERMIAN TO TRIASSIC Trembleur Intrusions 6 I U l t r a m a f i c M e t a d i o r i t e PENNSYLVANIAN TO TRIASSIC Cache Creek Group § A rg i l l i t e , m ino r C h e r t , G r e e n s t o n e , L i m e s t o n e I 3 | C h e r t , m i n o r A rg i l l i t e G r e e n s t o n e H H H L i m e s t o n e Geological Contact P o s i t i o n A p p r o x i m a t e Edge of Dr i f t -covered A r e a Limit of Geological Mapping / Approximate Edge of / Granite Batholith Isograds S e r p / O U T a Ol • T a / Ol • En * A n t h Fault D e f i n e d A p p r o x i m a t e A s s u m e d F o r t St J a m e s M a p A r e a B r i t i s h C o l u m b i a A J M E 1973 M i l e s • 

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