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Geology and mineralization of the stikine assemblage, Mess Creek area, northwestern British Columbia Holbek, Peter Michael 1988

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GEOLOGY AND MINERALIZATION OF THE STIKINE ASSEMBLAGE, MESS CREEK AREA, NORTHWESTERN, BRITISH COLUMBIA by PETER MICHAEL HOLBEK B . S c , The U n i v e r s i t y o f B r i t i s h Columbia, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l S c i e n c e s We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1988 © P e t e r M i c h a e l Holbek, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date Our- V^V J l^PJPs DE-6G/81) - i i -ABSTRACT The S t i k i n e Assemblage i n the Mess Creek a rea c o n s i s t s o f M i s s i s s i p p i a n aged, v a r i a b l y a l t e r e d , deformed, meta-morphosed and m i n e r a l i z e d s c h i s t s , p h y l l i t e s and g r e e n s t o n e s . O r i g i n a l l i t h o l o g i e s were m a f i c p y r o c l a s t i c s and e p i c l a s t i c s , f e l s i c v o l c a n i c b r e c c i a s t o c r y s t a l - l i t h i c ash t u f f s , g r a p h i t i c sed iments , and g a b b r o i c s i l l s . R e g i o n a l dynamothermal metamorphism t o lower g r e e n s c h i s t f a c i e s o c c u r r e d between La te Permian and M i d d l e T r i a s s i c t i m e . Four phases o f de fo rma t i on a re r e c o g n i s e d i n c l u d i n g two e a r l y phases o f n o r t h w e s t e r l y t r e n d i n g i s o c l i n a l , recumbent f o l d i n g f o l l o w e d by e a s t e r l y t r e n d i n g k i n k bank and chev ron s t y l e f o l d i n g , and n o r t h e r l y t r e n d i n g p a r a l l e l -s t y l e f o l d i n g . The f i r s t phase o f f o l d i n g was synchronous w i t h metamorphism and produced a p e n e t r a t i v e a x i a l p l a n a r f o l i a t i o n . Second phase f o l d s s i g n i f i c a n t l y o u t l a s t e d t h e r m a l e f f e c t s and produced a l o c a l l y pronounced c r e n u l a t i o n c l e a v a g e . T h i r d and f o u r t h phase f o l d i n g a l s o a f f e c t e d o v e r l y i n g L a t e T r i a s s i c sediments and c o u l d be r e l a t e d t o t e r r a n e c o l l i s i o n i n E a r l y J u r a s s i c t i m e . Three s t ages o f p l u t o n i s m o c c u r r e d w i t h i n the Mess Creek a r e a . Potas s ium-argon and r u b i d i u m - s t r o n t i u m dates - i i i -from p l u t o n s o f the Hickman b a t h o l i t h i n d i c a t e E a r l y t o M idd le T r i a s s i c ages f o r quar tz d i o r i t e s and a M idd le J u r a s s i c age f o r q u a r t z monzoni tes . A l k a l i c , g e n e r a l l y s y e n i t i c p l u t o n s , commonly a s s o c i a t e d w i th porphyry Cu-Au d e p o s i t s y i e l d E a r l y J u r a s s i c ages. G o l d and s i l v e r m i n e r a l i z a t i o n i s hos ted by s t r u c t u r a l l y c o n t r o l l e d qua r t z and q u a r t z - c a r b o n a t e v e i n s and a s s o c i a t e d a l t e r a t i o n . Two ages o f a l t e r a t i o n have been i d e n t i f i e d . Widespread and commonly conformable s i l i c i f i -c a t i o n and potas s ium metasomatism, c h a r a c t e r i z e d by a q u a r t z - m u s c o v i t e - ca rbona te assemblage, i s p r e - k i n e m a t i c and a t t r i b u t e d t o v o l c a n o g e n i c hydrotherma l systems. F o l i a t i o n - p a r a l l e l q u a r t z v e i n s a s s o c i a t e d w i t h t h i s a l t e r a t i o n a r e u b i q u i t o u s but g e n e r a l l y ba r ren o f p r e c i o u s m e t a l s . L a t e r a l t e r a t i o n , which c o n s i s t s o f i n t e n s e c a r b o n a t i z a t i o n , s i l i c i f i c a t i o n and s e r i c i t i z a t i o n , i s f r a c t u r e c o n t r o l l e d and commonly ho s t s g o l d and s i l v e r b e a r i n g v e i n s t h a t c u t a l l de fo rmat i on f a b r i c s . Gangue and a l t e r a t i o n m inera l ogy i n c l u d e s a n k e r i t e , s i d e r i t e , q u a r t z , a l b i t e , muscov i t e , f u c h s i t e , c h l o r i t e , mangani ferous d o l o m i t e , b a r i t e and l a n t h a n i d e group phosphates . Ore m ine ra l o gy i n c l u d e s p y r i t e , s p h a l e r i t e , t e t r a h e d r i t e , a r s e n o p y r i t e , c h a l c o p y r i t e , g a l e n a , s i l v e r and b ismuth - i v -t e l l u r i d e s , electrum and g o l d . Sulphide c o n c e n t r a t i o n s w i t h i n v e i n s range from massive to s p a r s e l y disseminated. Hydrogen metasomatism a s s o c i a t e d with m i n e r a l i z a t i o n produced c o m p o s i t i o n a l changes w i t h i n host r o c k s t h a t i n c l u d e the removal of Ca and Na and the a d d i t i o n of K and H. P r e c i p i t a t i o n of q u a r t z , carbonates and s u l p h i d e s was caused by a pH i n c r e a s e due to h y d r o l y s i s r e a c t i o n s with w a l l r o c k s . Potassium-argon and rubidium-strontium d a t i n g of a l t e r a t i o n and m i n e r a l i z a t i o n y i e l d s E a r l y J u r a s s i c ages, which c o i n c i d e w i t h r e g i o n a l a l k a l i c p l u t o n i s m and p o s s i b l e time o f t e c t o n i c a c c r e c t i o n o f allochthonous t e r r a n e s . TABLE OF CONTENTS PAGE NO. ABSTRACT i i TABLE OF CONTENTS v LIST OF TABLES v i i LIST OF FIGURES LIST OF PLATES x i ACKNOWLE DGEMENTS x i v FRONTISPIECE x v i CHAPTER 1: INTRODUCTION 1.1 General Statement 1 1.2 L o c a t i o n and Access 3 1.3 Physiography and V e g e t a t i o n 3 1.4 Previous Work 6 1.5 Present Work 9 CHAPTER 2: REGIONAL GEOLOGY 2.1 T e c t o n i c S e t t i n g 12 2.2 General Geology and S t r a t i g r a p h y Summary 15 2.3 S t r a t i f i e d Rocks 18 2.3.1 S t i k i n e Assemblage 18 2.3.2 Stuhuni Group 23 2.3.3 Upper T r i a s s i c Sediments 27 2.4 I n t r u s i v e Rocks 3 0 2.4.1 Hickman P l u t o n 32 2.4.2 Nighout P l u t o n 3 6 2.4.3 Y e h i n i k o P l u t o n 37 2.4.4 Other P l u t o n i c Rocks 39 2.5 Age and Metallogeny 41 CHAPTER 3 : GEOLOGY OF STIKINE ASSSEMBLAGE 3.1 I n t r o d u c t i o n 4 6 3.2 S t r a t i g r a p h y 47 3.2.1 S t r a t i g r a p h i c Summary 47 3.2.2 A r g i l l a c e o u s S c h i s t 51 3.2.3 Ma f i c P y r o c l a s t i c and E p i c l a s t i c U n i t 55 3.2.4 F e l s i c P y r o c l a s t i c U n i t 57 3.2.5 Greenstone U n i t 59 3.2.6 A l t e r a t i o n U n i t s 61 3.3 S t r u c t u r e 65 3.3.1 I n t r o d u c t i o n 65 3.3.2 S t r u c t u r a l Sequence 66 3.3.3 Stereonet A n a l y s i s 69 - v i -PAGE NO. 3.3.4 F o l d Sets and Deformation F a b r i c s 70 3.3.5 F a u l t s 75 3.3.6 S t r u c t u r a l Comparison of the Study 76 Area w i t h o t h e r Exposures of the S t i k i n e Assemblage 3.3.7 Timing of Deformation 78 3.4 Metamorphism 81 3.4.1 Metamorphic Mineralogy 92 3.4.2 C o n d i t i o n s o f Metamorphism 94 3.4.3 Timing of Metamorphism 97 3.5 Age of the S t i k i n e Assemblage 99 3.5.1 S t r a t i g r a p h i c Evidence 100 3.5.2 I s o t o p i c Ages 101 3.6 Chemistry 105 3.6.1 General Statement 105 3.6.2 Methods and Q u a l i t y C o n t r o l 106 3.6.3 Major Element Chemistry 107 3.6.4 Trace Element Chemistry 111 CHAPTER 4 : ECONOMIC GEOLOGY 4.1 I n t r o d u c t i o n 120 4.2 M i n e r a l i z a t i o n 121 4.2.1 F o l i a t i o n - P a r a l l e l Quartz Veins 123 4.2.2 Q u a r t z - B r e c c i a V e i n s 125 4.2.3 Carbonate-Sulphide V e i n s 127 4.2.4 Carbonate-Arsenopyrite V e i n s 128 4.2.5 Other V e i n Types 130 4.2.6 Sulphide Mineralogy 131 4.3 A l t e r a t i o n 138 4.3.1 D e s c r i p t i o n and Petrography 138 4.3.2 Chemical Changes A s s o c i a t e d 142 with A l t e r a t i o n 4.3.3 C o n d i t i o n s and O r i g i n o f A l t e r a t i o n 145 4.4 Comparison o f M i n e r a l i z a t i o n w i t h Other 158 Deposits CHAPTER 5: CONCLUSIONS 5.1 Summary of G e o l o g i c a l H i s t o r y o f the 164 Study Area 5.2 T e c t o n i c I m p l i c a t i o n s 168 5.3 M i n e r a l E x p l o r a t i o n P o t e n t i a l 171 REFERENCES 176 - v i i -T a b l e 2.1 Tabl e 3.1 Tabl e 3.2 Tabl e 3.3 Ta b l e 3.4 Ta b l e 3.5 Ta b l e 4.1 LIST OF TABLES Page I s o t o p i c Age Determinations f o r P l u t o n i c 35 Rocks of the Hickman B a t h o l i t h Summary of S t r u c t u r a l Elements of the 67 S t i k i n e Assemblage, Mess Creek Area Summary of Metamorphic M i n e r a l s and 93 Textures o f S t i k i n e Assemblage Rocks, Mess Creek Area Rubidium-Strontium Data f o r Rocks of 104 the S t i k i n e Assemblage, Mess Creek Area Major Element Chemistry of the S t i k i n e 108 Assemblage Rocks, Mess Creek Area Trace Element Chemistry o f the S t i k i n e 109 Assemblage Rocks, Mess Creek Area Summary of M i n e r a l i z a t i o n and A l t e r a t i o n 122 C h a r a c t e r i s t i c s , BJ Prospect, Mess Creek Area - v i i i -F i g u r e 1.1 F i g u r e 1.2 F i g u r e 1.3 F i g u r e 2.1 F i g u r e 2.2 F i g u r e 2.3 F i g u r e 2.4 F i g u r e 2.5 F i g u r e 2.6 F i g u r e 2.7 F i g u r e 3.1 F i g u r e 3.2 F i g u r e 3.3 F i g u r e 3.4 L I S T O F F I G U R E S L o c a t i o n Map of Study Area, Mess Creek, Northwestern B r i t i s h Columbia Topography and M i n e r a l Claims, BJ Prospect, Mess Creek Area L o c a t i o n Map f o r Major M i n e r a l D e p o s i t s , Northwestern B r i t i s h Columbia and Adjacent P a r t s of A l a s k a T e c t o n i c Features of the S t i k i n e Arch Regio n a l Geology, South-Central p a r t of the Telegraph Creek Map Sheet Area S t r a t i g r a p h i c R e l a t i o n s of P a l e o z i c and Mesozoic Rocks of the S o u t h - C e n t r a l p a r t of the Telegraph Creek Map Sheet Area D i s t r i b u t i o n of Late T r i a s s i c V o l c a n i c Rocks R e l a t i v e t o the Cache Creek Group i n the Canadian C o r d i l l e r a S t r a t i g r a p i c Column f o r the Upper T r i a s s i c Sediments, Mess Creek Area D i s t r i b u t i o n o f P l u t o n i c Rocks o f the Hickman B a t h o l i t h , S t i k i n e R i v e r Area Summary o f Age R e l a t i o n s h i p s f o r Rocks and S e l e c t e d M i n e r a l D eposits o f the S t i k i n e Arch Area Schematic S t r a t i g r a p h i c Column o f S t i k i n e Assemblage Rocks, Mess Creek Area G e n e r a l i z e d Geology of the Mess Creek Area, Northwestern B r i t i s h Columbia 1:12,500 S c a l e Geology of the Western Headwaters of Mess Creek, T e l e g r a p h Creek Map Sheet Area, Northwestern B r i t i s h Columbia G e o l o g i c a l Cross S e c t i o n s f o r the Mess Creek Area Page 4 11 14 19 21 24 29 33 43 49 50 PojeneetV X - i x -F i g u r e 3.5 F i g u r e 3.6 F i g u r e 3.7 F i g u r e 3.8 F i g u r e 3.9 F i g u r e 3.10 F i g u r e 3.11 F i g u r e 3.12 F i g u r e 3.13 F i g u r e 3.14 F i g u r e 3.15 F i g u r e 3.16 F i g u r e 4.1 F i g u r e 4.2 F i g u r e 4.3 Page Summary of F o l d S t y l e s and O r i e n t a t i o n s , 68 Mess Creek Area Contoured Stereonet P l o t s of S t r u c t u r a l 71 Data from the Mess Creek Area Schematic Diagram I l l u s t r a t i n g the 74 E f f e c t s of Superposed F o l d i n g ACF, AKF, and QCM diagrams of 95 Metamorphic M i n e r a l Assemblages of the Mess Creek Area Some I s o b a r i c E q u i l i b r i u m Curves f o r 98 Metamorphic Reactions i n the System SiO 2-MgO-CaO-H 2O-C0 2 Rubidium-Strontium Isochron Diagram f o r 103 Rocks of the S t i k i n e Assemblage, Mess Creek Area Harker V a r i a t i o n Diagrams of Major 112 Elements A l k a l i e s - S i l i c a P l o t of S t i k i n e 113 Assemblage Rocks from the Mess Creek Area AFM Diagram 114 FeO-MgO-Al 20 3 T r i a n g u l a r Diagram 116 T i 0 2 - Z r Diagram 118 A l 0 /TiO , A l 0 /lOOZr, 100Zr/TiO 2 119 versus MgO Diagrams Composite Vanderveer Type Diagram 137 showing s u l p h i d e m i n e r a l p a r a g e n e s i s f o r m i n e r a l i z a t i o n of the BJ P r o s p e c t Isochon Diagrams D i s p l a y i n g 144 Compositional Changes a s s o c i a t e d w i t h a l t e r a t i o n , BJ Prospect Mess Creek Area Chemical and M i n e r a l o g i c a l Changes 146 A s s o c i a t e d w i t h the T e l l u r i d e V e i n , BJ Prospect - x -Page F i g u r e 4.4 Phase R e l a t i o n s i n the System 152 K 2 0 - N a 2 0 - A l 2 0 3 - S i 0 2 - H 2 0 . F i g u r e 4.5 Phase R e l a t i o n s i n the Systems 153 Na 0-A1 0 -SiO -H 0-HC1 and K 0-A1 0 -SiO -H 0-HC1 as a Fu n c t i o n of Log (a K+/ a H+) a n d  L ° g ( a H a + / a „ + ) - x i -P l a t e 3.1a 3. l b 3. l c 3. l d 3.1e 3. I f 3. l g 3.1h P l a t e 3.2a 3.2b LIST OF PLATES Coarse c l a s t - s u p p o r t e d b r e c c i a of f e l s i c p y r o c l a s t i c u n i t F i n e bedding w i t h i n the mafic p y r o c l a s t i c / e p i c l a s t i c u n i t V e s i c u l a r v o l c a n i c bombs w i t h i n the M a f i c p y r o c l a s t i c / e p i c l a s t i c u n i t Deformed fragmental-fragment w i t h i n t r a n s i t i o n zone between mafic and f e l s i c p y r o c l a s t i c u n i t s M i c r o l i t h o n s t r u c t u r e (coarse c r e n u l a t i o n cleavage) i n f e l s i c p y r o c l a s t i c u n i t Folded a l t e r a t i o n r e a c t i o n f r o n t and u n a l t e r e d remnant w i t h i n m afic p y r o c l a s t i c u n i t Warped c o n t a c t between massive greenstone u n i t and f e l s i c fragmental u n i t F e l s i c ash fragment w i t h i n m a f i c p y r o c l a s t i c u n i t d i s p l a y i n g i n t e r f e r e n c e f i g u r e View l o o k i n g south a c r o s s Wishbone G l a c i e r showing c l i f f f a ce composed o f q u a r t z - s e r i c i t e - c a r b o n a t e s c h i s t o v e r l a i n by a t h i n l a y e r of greenstone View l o o k i n g northwest a c r o s s Wishbone G l a c i e r showing l a r g e zone od s e r i c i t i c a l t e r a t i o n Page 83 3.2c Laminated c h l o r i t e s c h i s t w i t h i s o c l i n a l f o l d s and a l a t e , c r o s s - c u t t i n g o x i d a t i o n zone 3.2d Greenstone u n i t showing laminar t e x t u r e d e f i n e d by c o n c e n t r a t i o n s of o l i v i n e and i l m e n i t e 3.2e T h i n e p i c l a s t i c beds with i n t e r b e d carbonate l a y e r s i n mafic p y r o c l a s t i c e p i c l a s t i c u n i t 3.2f Weathered s u r f a c e of greenstone u n i t showing c o m p o s i t i o n a l l a y e r i n g d e f i n e d by c o n c e n t r a t i o n of p l a g i o c l a s e c r y s t a l s 85 - x i i -P l a t e 3.3a 3.3b 3.3c 3.3d 3.3e 3.3f 3.3g P l a t e 3.4a 3.4b 3.4c 3.4d 3.4e 3.4f 3 .4g 3.4h P l a t e 3.5a Page 87 Late T r i a s s i c a r g i l l i t e and c h e r t beds o v e r l y i n g S t i k i n e Assemblage Rocks Laminated ash t u f f and a r g i l l a c e o u s t u f f w i t h i s o c l i n a l f o l d Minor f o l d nose(s) i n f e l s i c t u f f - s e r i c i t e s c h i s t u n i t s L_ l i n e a t i o n s produced by the i n t e r s e c t i o n of two s e t s o f a x i a l plane cleavage Chevron f o l d s i n f e l s i c c r y s t a l ash t u f f Fe-carbonate impregnated v o l c a n i c bombs i n c h l o r i t e s c h i s t u n i t R efolded f o l i a t i o n , producing coarse c r e n u l a t i o n cleavage i n f e l s i c p y r o c l a s t i c u n i t Photomicrograph of in t e n s e c r e n u l a t i o n 89 cleavage i n f e l s i c p y r o c l a s t i c rock Photomicrograph showing second f o l d phase cleavage and primary l a y e r i n g Photomicrograph o f a l t e r e d imtermediate rock Photomicrograph o f f e l s i c , l i t h i c ash t u f f w i t h carbonate p o r p y r o b l a s t s Photomicrograph of c r y s t a l - l t h i c ash t u f f Photomicrograph of q u a r t z - a l b i t e v e i n l e t c u t t i n g a mass o f a n t i g o r i t e Photomicrograph of a l b i t e porphyry greenstone Photomicrograph of c h l o r i t e , t r e m o l i t e and opaques r e p l a c i n g o l i v i n e i n greenstone Photomicrograph of c r e n u l a t i o n cleavage 91 i n s e r i c i t e s c h i s t - x i i i -Page 3.5b Photomicrograph of c r e n u l a t e d q u a r t z -muscovite s c h i s t 3.5c Photomicrograph showing micro-kink bands i n S 2 c r e n u l a t i o n cleavage 3.5d Photomicrograph o f c r e n u l a t e d q u a r t z -muscovite s c h i s t 3.5e Photomicrograph of i s o c l i n a l y f o l d e d quartz l a y e r i n s e r i c i t e s c h i s t 3.5f Photomicrograph of i s o c l i n a l y f o l d e d quartz l a y e r i n s e r i c i t e s c h i s t showing development of a x i a l p l a n e r c r e n u l a t i o n cleavage 3.5g Photomicrograph o f broad c r e n u l a t i o n cleavage i n c h l o r i t e - s e r i c i t e s c h i s t 3.5h Photomicrograph showing cleavage d i s c o n t i n u i t i e s i n c r y s t a l ash t u f f P l a t e 3.7a 3.7b 3.7c Quartz-carbonate c r a c k l e b r e c c i a i n greenstone u n i t Banded c a r b o n a t e - s u l p h i d e v e i n sample Carbonate a l t e r a t i o n along f o l i a t i o n p l anes i n mafic e p i c l a s t i c rock 3.7d A l t e r e d l a p i l l i t u f f with s u l p h i d e fragments 3.7e Small s t r a t b o u n d s u l p h i d e l e n s i n t r a n s i t i o n zone from mafic t o f e l s i c p y r o c l a s t i c rocks 3.7f Ferruginous dolomite b r e c c i a from M c l a u g h l i n zone 3.7g Quartz-carbonate stockwork i n greenstone boulder w i t h i n l a n d s l i d e d e b r i s 133 P l a t e 3.8a 3.8b Back s c a t t e r e d e l e c t r o n image from scanning e l e c t r o n miscroscope showing p y r i t e f ormation along f o l i a t i o n p l a n e s , away from quartz-carbonate v e i n l e t Back s c a t t e r e d e l e c t r o n image from scanning e l e c t r o n microscope showing muli-phase c a r b o n a t e - s u l p h i d e g r a i n from carbonate a l t e r a t i o n zone w i t h i n greenstone u n i t 135 - x i v -ACKNOWLEDGEMENTS There are many people who c o n t r i b u t e d , both d i r e c t l y and i n d i r e c t l y , t o t h i s t h e s i s ; t h e i r e f f o r t s are very much a p p r e c i a t e d . B. Meyers, P. F o l k and W. S p i l l s b u r y of Teck E x p l o r a t i o n s L t d . supported the o r i g i n a l concept, a t l e a s t i n i t i a l l y , both m o r a l l y and f i n a n c i a l l y . Dr. H. Greenwood and Dr. C. Godwin of U.B.C. s u p e r v i s e d the r e s e a r c h and p r o v i d e d v a l u a b l e guidance and d i r e c t i o n . Dr. R. Armstrong and K. S c o t t performed most of the analyses r e l a t e d t o geochronology and made u s e f u l suggestions f o r the i n t e r p r e t a t i o n o f t h a t data. H. Smit and P. S h i p l e y p r o v i d e d competent and enjo y a b l e a s s i s t a n c e i n the f i e l d . J . Knight p r o v i d e d e x c e l l e n t d i r e c t i o n s on the use of the scanni n g e l e c t r o n microscope, not t o mention many enjo y a b l e e x c u r s i o n s away from r e s e a r c h . S. Horsky was a tremendous h e l p i n o b t a i n i n g XRF r e s u l t s and i n oth e r f a c e t s o f geochemistry. E. Montgomery and B. Cranston d i d a f i r s t r a t e j o b i n the p r e p a r a t i o n o f t h i n and p o l i s h e d t h i n s e c t i o n s and kept the r i g h t machines going a t the r i g h t t i m e s. Drs. J . Monger, J . Souther and R. Anderson of the G e o l o g i c a l Survey of Canada were v e r y h e l p f u l i n p r o v i d i n g u n p u b l i s h e d data and answering a myriad o f q u e s t i o n s . D i s c u s s i o n s with f e l l o w graduate students p r o v i d e d much entertainment and some v a l u a b l e i n s i g h t s . In p a r t i c u l a r , - X V -s t i m u l a t i n g and f r u i t f u l c o n v e r s a t i o n s on the nature of g o l d d e p o s i t s and the geology of northwestern B.C. were had wit h J . Nelson and J . Mortinson. R. P a r r i s h , R. Berman, P. VanderHeyden and G. Nixon p r o v i d e d computer programs and many u s e f u l comments on v o l c a n i c rock geochemistry. P. Matysek, K. Shannon and e s p e c i a l l y D. H e b e r l e i n served as sounding boards and made many u s e f u l s u g g e s t i o n s . G. Hodge, K. Simpson, P. T h i e r s c h and E. Montgomery a s s i s t e d w i t h the p r o d u c t i o n of f i g u r e s and p l a t e s . The manuscript was g r e a t l y improved by c r i t i c a l comments from C. Godwin, H. Greenwood, R . L. Armstrong, D. H e b e r l e i n , J . McDonald, H. Marsden and R . B r i t t e n . To a l l of those mentioned and t o many who weren't, thankyou. F i n a l l y , I would l i k e t o thank G. Woodsworth and L. Werner, whose p a t i e n t encouragement so many y e a r s ago helped me make i t through t o t h i s p o i n t , and my w i f e , C a r o l e , who taught me t h a t t h e r e i s more t o l i f e than r o c k s . - x v i -F R O N T I S P I E C E Oblique, westward looking, a e r i a l view of the central part of the study area. Foreground shows rocks of the Paleozoic Stikine Assemblage. The trace of the a n t i c l i n a l axis related to the fourth phase of folding crosses through the center of the photographed area. Potassium metasomatic (beige) and ferruginous carbonate (orange) a l t e r a t i o n zones can be seen in outcrop areas. The ridges that trend westwards into the ice f i e l d are composed of Late T r i a s s i c sediments. Mt. Hickman can be seen i n the extreme right hand corner. Ambition Mtn. i s i n the center background. - 1 -CHAPTER 1: INTRODUCTION 1.1 General Statement This report describes the geology, alteration and mineralization of the BJ gold-silver prospect, and the intrusive and structural history of the surrounding region. Objectives of this study were to evaluate the potential of mineralization on the BJ prospect, to develop c r i t e r i a to guide mineral explorationists, both on the prospect and regional scales, and to provide data useful to a regional synthesis of tectonic activity. Regional stream sediment sampling and consequent prospecting in 1980 led to the discovery of precious metal mineralization (BJ prospect) along the western head waters of Mess Creek, northwestern British Columbia. Rocks hosting the mineralization belong to the Paleozoic Stikine Assemblage. I n i t i a l attempts to explore and evaluate the mineralization were frustrated by complex structure, unknown stratigraphy and a variety of alteration types. A better understanding of the geology and geological history of the area would significantly contribute to cost effective exploration. - 2 -Rocks o f the P a l e o z o i c S t i k i n e Assemblage occur i n i s o l a t e d exposures a l o n g a 500km north-south t r e n d i n northwestern B r i t i s h Columbia. R e l a t i v e i n a c c e s s i b i l i t y and l a c k o f c o n t i n u i t y o f many o f these exposures has r e s u l t e d i n ah incomplete understanding of the geology of t h e Assemblage. Recent work has shown t h a t base and p r e c i o u s metal d e p o s i t s o r i g i n a l l y thought t o occur w i t h i n T r i a s s i c r o c k s a r e a c t u a l l y hosted by t h e S t i k i n e Assemblage (Nelson and Payne, 1984). Of p a r t i c u l a r importance i n d e t e r m i n i n g both t h e t e c t o n i c h i s t o r y o f t h e area and t h e e x p l o r a t i o n p o t e n t i a l i s the t i m i n g o f metamorphism and deformation w i t h i n the S t i k i n e Assemblage r e l a t i v e t o m i n e r a l i z a t i o n and d e p o s i t i o n o f o v e r l y i n g r o c k s . I t was i n i t i a l l y p l a n e d t h a t two f i e l d seasons would be r e q u i r e d t o meet the o b j e c t i v e s o u t l i n e d above. The second f i e l d season was never r e a l i z e d due t o c u r t a i l e d e x p l o r a t i o n f u n d i n g r e s u l t i n g from plummeting metal p r i c e s and consequent l o s s o f revenue. A t the time i t was f e l t t h a t s u f f i c i e n t f i e l d work had been conducted and enough hand specimens c o l l e c t e d t o a l l o w t h e p r o j e c t t o proceed. Data c o l l e c t e d d u r i n g r e g i o n a l e x p l o r a t i o n by the author from the p r e c e e d i n g f i e l d season was i n c o r p o r a t e d i n t o t h i s study t o h e l p f i l l any gaps w i t h i n t h e database and i n d e f i n i n g r e g i o n a l t e c t o n i c h i s t o r y and metallogeny. - 3 -1.2 L o c a t i o n and Access The study area, centered on the BJ m i n e r a l c l a i m groups (57 08'north, 130 50'west - NTS: 104G/2W), i s on the e a s t e r n edge o f the Coast Mountains, approximately 150km n o r t h of Stewart and 90km south o f Telegraph Creek, B r i t i s h Columbia ( F i g . 1.1). Access t o the r e g i o n i s by f i x e d - w i n g a i r c r a f t t o a g r a v e l a i r s t r i p a t Teck E x p l o r a t i o n L i m i t e d ' s S c h a f t Creek d e p o s i t , 24km n o r t h o f the study area. T h i s a i r s t r i p was s e r v e d d u r i n g the p r o j e c t by Trans P r o v i n c i a l A i r l i n e s from T e r r a c e , B.C. Access t o the study area was by h e l i c o p t e r from the S c h a f t Creek a i r s t r i p . More d i r e c t a c c e s s t o the study area c o u l d be a c h i e v e d by s m a l l f l o a t p l a n e t o Round Lake, 6km t o the southwest, o r p o s s i b l y t o t h e s m a l l l a k e s i n the pass between Mess and More Creeks ( F i g . 1.2). The S t e w a r t - C a s s i a r highway passes 50km e a s t o f the a r e a . 1.3 Phys iog raphy and V e g e t a t i o n Topography o f the study area i s moderately rugged w i t h e l e v a t i o n s r a n g i n g from 1,000 t o 2,000m. Large i c e f i e l d s bound th e area t o the south and west. E x c e l l e n t exposure i s p r o v i d e d by r e c e n t g l a c i a l r e t r e a t . Both g l a c i a t i o n and d r a i n a g e have e x p l o i t e d zones o f s t r u c t u r a l o r l i t h o l o g i c a l weaknesses d i s p l a y i n g a p r e f e r r e d n o r t h - s o u t h or n o r t h -e a s t e r l y o r i e n t a t i o n . - .4 -Figure 1.1 Location Map of the Study Area, Mess Creek, Northwestern B.C. Figure 1.2 1:50,000 scale topographic map showing BJ claim group. - 6 -Retreat of g l a c i e r s i n areas of a l t e r a t i o n has produced oversteepened slopes r e s u l t i n g i n recent landslide a c t i v i t y . Large, gravel outwash plains cover the southern map area. Vegetation i s alpine and sub-alpine with t r e e l i n e between 1200 and 1300m. A young forest of spruce and sub-alpine f i r blankets the lower elevations along the Hess Creek v a l l e y . In some areas, t h i c k growths of stunted sub-alpine f i r make passage d i f f i c u l t . Higher elevations are e i t h e r barren or carpeted with alpine meadows. S o i l s are reasonably well-developed between the 1600 and 1100m elevations, p a r t i c u l a r l y over al t e r e d bedrock. Overburden below the 1100m l e v e l or v a l l e y bottoms consists predominately of g l a c i a l l y derived material. 1.4 Previous Work The f i r s t published geological work i n the region was c a r r i e d out by F.A. Kerr (1948a), f o r the Geological Survey of Canada, who mapped areas along the Stikine and Iskut Rivers during the l a t e 1920's. Kerr (1948a and b) recognized a th i c k sequence of pre-Permian s t r a t a which was characterized by extensively deformed volcano-sedimentary rocks and separated from other units by unconformities. On the basis of s i m i l a r stratigraphy and composition, Kerr (1948b) correlated the pre-Permian rocks of the Stikine River area with exposures i n the Taku River area. He also noted abundant quartz veins associated with these rocks. - 7 -The G e o l o g i c a l Survey o f Canada resumed work i n the area i n 1956 w i t h O p e r a t i o n S t i k i n e ( G a b r i e l s e , 1961). Rocks o f the T e l e g r a p h Creek map sheet area were examined i n t e r m i t t e n t l y f o r the next 14 y ears by J . Souther (1972). P a l e o z o i c r o c k s near the t h e s i s area were i n v e s t i g a t e d by J . Monger (1977). B i o s t r a t i g r a p h y of the Permian carbonate u n i t s near G a l o r e Creek was the s u b j e c t o f an MSc t h e s i s by P i t c h e r (1966). More r e c e n t l y , 1:50,000 s c a l e mapping o f v a r i o u s areas w i t h i n t h e Telegraph Creek map sheet, r e l a t e d t o damsite f e a s i b i l i t y s t u d i e s by B.C. Hydro, has been done by Read (1983). D e f i n i t i o n o f t e c t o n o s t r a t i g r a p h i c assemblages (Monger and P r i c e , 1979; Monger, 1977 and 1975) and o t h e r r e f i n e m e n t s i n models o f C o r d i l l e r a n e v o l u t i o n have p l a c e d renewed emphasis on the geology and g e o l o g i c a l r e l a t i o n s o f t h e o l d e r r o c k s w i t h i n the S t i k i n e r e g i o n . M i n e r a l e x p l o r a t i o n began i n the area i n the e a r l y 1920's w i t h d i s c o v e r y o f p l a c e r g o l d along the S t i k i n e R i v e r . P l a c e r d e p o s i t s o f the S t i k i n e , however, were i n s i g n i f i c a n t and the r i v e r was mainly used as an access r o u t e t o t h e r i c h e r g o l d f i e l d s of the K l o n d i k e . E x p l o r a t i o n was renewed i n the 1950's w i t h the s e a r c h f o r l a r g e tonnage, low grade porphyry copper and molybdenum - 8 -deposits. Two important discoveries i n the mid 1950's helped maintain exploration a c t i v i t y i n the area f o r the next two decades. The Galore Creek deposits (Allen et a l . , 1976), 24km west of the thesis area, were discovered i n 1955 by prospectors working f o r Hudson Bay Exploration and Development Company Limited. Kennco Explorations Limited c a r r i e d out work on the deposits from 1960 to 1967. Hudson Bay Mining and Smelting Company Limited became the operators i n 1972. Reserves are given (Allen et a l . . 1976) as 125 m i l l i o n tonnes grading 1.06% Cu, 0.40 g/t Au and 7.7g/t Ag. The Schaft Creek deposit was discovered i n 1957 by Nicholas Bird of the Bik Syndicate. Work was c a r r i e d out by S i l v e r Standard Mines NPL, Asarco, Hecla Operating Company Limited and eventually Teck Explorations Limited. Current reserves are one b i l l i o n tons grading 0.30% Cu, 0.034% Mo, 1.09 g/t Ag and 0.12g/t Au (Financial Post, Oct. 1981). Exploration f o r precious metals i n the region was minimal u n t i l 1980 when the increases i n the p r i c e of base and precious metals had a consequent a f f e c t on exploration. Teck Explorations Limited, Dupont Exploration Canada Ltd., and others were a c t i v e l y involved i n the region during 1980 and 1981. Exploration slowed with declining copper pr i c e s and became concentrated i n more accessible areas along Highway 37 and the Iskut River. Resurgence of exploration - 9 -accompanied the advent o f flow-through share f i n a n c i n g and r e s u l t e d i n some s i g n i f i c a n t new d i s c o v e r i e s i n 1986 and 1987. The n e a r e s t p a s t o r pr e s e n t p r e c i o u s metal producers a r e l o c a t e d 150km t o the south i n the Stewart area and 250km t o the northwest ( P o l a r i s Taku and A l a s k a Juneau Gold mines). F i g u r e 1.3 shows the major d e p o s i t s and p r o s p e c t s of t h e r e g i o n . 1.5 P r e s e n t Work A r e g i o n a l , m i n e r a l e x p l o r a t i o n program i n t h e Te l e g r a p h Creek map sheet was undertaken by Teck E x p l o r a t i o n s L t d . i n 1980. Stream sediment sampling and consequent p r o s p e c t i n g r e s u l t e d i n the s t a k i n g o f the BJ c l a i m groups along t h e western headwaters o f Mess Creek. I n i t i a l work on the c l a i m s proved encouraging and a subsequent program was undertaken i n 1981. A f i e l d and l a b o r a t o r y study, which forms a p o r t i o n o f t h i s t h e s i s , was designed t o determine: the c o n t r o l s o f l i t h o l o g y and s t r u c t u r e on m i n e r a l i z a t i o n , the r e l a t i o n s h i p o f a l t e r a t i o n t o l i t h o l o g y and m i n e r a l i z a t i o n , the e x p l o r a t i o n p o t e n t i a l o f t he m i n e r a l showings and, the s t r u c t u r a l and metamorphic h i s t o r y o f the h o s t r o c k s , and t h e i r r e l a t i o n s h i p s t o surr o u n d i n g r o c k s . In c o n j u n c t i o n w i t h c o n t i n u i n g r e g i o n a l work, approximately 110 square k i l o m e t e r s were mapped a t a 1:12,500 s c a l e on and around the BJ c l a i m groups. - 10 -P l u t o n i c h i s t o r y o f t h e a r e a was determined based on s t u d i e s i n the Hickman b a t h o l i t h , approximately 15km northwest o f the BJ c l a i m group, as t h i s area o f f e r s the b e s t exposure of i n t r u s i v e - e x t r u s i v e and i n t r u s i v e - i n t r u s i v e r e l a t i o n s h i p s . L a b o r a t o r y work c o n s i s t e d o f major and t r a c e element d e t e r m i n a t i o n s on a v a r i e t y o f rock samples, t h i n and p o l i s h e d s e c t i o n s t u d i e s and i s o t o p i c age d e t e r m i n a t i o n s . Figure 1.3 Major mineral deposits of Northwestern British Columbia and adjacent parts of Alaska. - 1 2 -CHAPTER 2: REGIONAL GEOLOGY 2.1 Tectonic Setting The thesis area i s located within the S t i k i n e Arch on the western edge of the Intermontane Belt (Wheeler and Gabrielse, 1972; Monger et a l . , 1972) near i t s contact with the Coast Plutonic Complex. The main tectonic features of the area, shown i n Figure 2.1, include: the S t i k i n e Arch," the Whitehorse Trough, the A t l i n Terrane (Horst), the Quesnel Trough, and the Bowser and Sustut Basins. The Stikine Arch i s a triang u l a r shaped area that has remained t e c t o n i c a l l y p o s i t i v e throughout most of the Mesozoic (Souther, 1972) and transects the Intermontane Belt, separating the Bowser Basin from the Whitehorse Trough. Material eroded from the St i k i n e Arch was shed both northwards into the Whitehorse Trough and southwards into the Bowser Basin (Souther, 1971; Eisbacher, 1974). T r i a s s i c and J u r a s s i c plutons and coeval volcanics of the Stuhini, Takla and Hazelton Groups are interpreted to be remnants of magmatic arcs (Monger and Price, 1979) that form the core of the S t i k i n e Arch, and that extend around the northeastern edge of the Bowser Basin into the Quesnel Trough. These rocks are underlain by Paleozoic s t r a t a of the Stikine \ - 13 -Assemblage (Monger, 1977). Pre-Middle T r i a s s i c r o c k s of the S t i k i n e A r c h underwent deformation and metamorphism d u r i n g the Permian t o m i d - T r i a s s i c T a h l t a n i a n orogeny (Souther, 1971 and 1972; Read and O k u l i t c h , 1977). On i t s western edge t h e S t i k i n e Arch i s i n t r u d e d by g r a n i t i c r o c k s o f the Coast P l u t o n i c Complex. The A t l i n Terrane, n o r t h e a s t o f the Whitehorse Trough, i s composed of o p h i o l i t e - l i k e assemblages o f u l t r a m a f i c s , c h e r t s l i m e s t o n e s , a r g i l l i t e s and minor v o l c a n i c s . These r o c k s range i n age from M i s s i s s i p p i a n t o L a t e T r i a s s i c but a r e d i s t i n c t , both l i t h o l o g i c a l l y and p a l e o n t o l o g i c a l l y , from s i m i l a r aged rocks o f the S t i k i n e Assemblage (Monger, 1977). Paleomagnetic, p a l e o n t o l o g i c a l and s t r a t i g r a p h i c s t u d i e s have shown t h a t most o f t h e r o c k s w i t h i n the Intermontane B e l t are a l l o c h t h o n o u s w i t h r e s p e c t t o the North American c o n t i n e n t (Monger e t a l . . 1972; Monger, 1977; Monger and P r i c e , 1981; Saleenby, 1985). The S t i k i n e A r c h forms t h e n o r t h e r n p a r t o f the S t i k i n e Terrane o r S t i k i n i a ( S t i k i n e B l o ck o f Monger e t a l . . 1972), which i s a composite t e r r a n e c o n s i s t i n g o f P a l e o z o i c , T r i a s s i c and J u r a s s i c i s l a n d a r c s (Monger and P r i c e , 1981) t h a t u n d e r l i e s much o f th e Intermontane B e l t . S t i k i n i a had amalgamated w i t h the Cache Creek Group ( A t l i n Terrane) by a t l e a s t the Middle t o Late J u r a s s i c , as rocks of t h a t age w i t h i n the Bowser B a s i n Figure 2.1 Tectonic elements of the northwestern Canadian Cordillera and relative locations of the Hickman and Hotialuh batholiths (after Souther, 1972). - 15 -contain fossiliferous Cache Creek detritus (Eisbacher, 1974; Monger et a l . . 1978). If the Cache Creek rocks are inter-preted as a subduction complex for Upper Triassic volcanism, as suggested by Monger and Price (1979), then amalgamation would have had to have taken place by Late Triassic. The c o l l i s i o n of Stikinia with continental North America i s less well-defined, but l i k e l y took place between the Middle Jurassic and Late Cretaceous (Monger and Price, 1981; Templeman-Kluit, 1982). Emplacement of the Coast Plutonic Complex along the suture between the Intermontane and Insular Belts began by at least Lower Cretaceous time (Monger, 1982). Right Lateral strike s l i p displacement along the Queen Charlotte Fault system produced an extensional environment resulting in late faulting (graben development ?) and eruption of Tertiary alkalic volcanic rocks of the Edziza Complex (Souther, 1974 and 1984). 2.2 General Geology and Stratigraphic Summary Distribution of the major rock units within the study region i s shown in Figure 2.2. A table of formations and stratigraphic summary i s given in Figure 2.3. The oldest stratigraphic unit in the study region i s the Stikine - 16 -Assemblage (Monger, 1977) which e s s e n t i a l l y includes a l l pre-Mesozoic rocks i n the area. I t i s separated from younger rock units by a pronounced unconformity. Rocks of the S t i k i n e Assemblage were mapped i n d e t a i l west of the headwaters of Mess Creek and b r i e f l y examined within the Forrest Kerr and Scud River drainages. Large exposures of Paleozoic rocks at the north end of Mess Creek and Zagoddetchino Mountain were not examined. A l l observed contacts between the S t i k i n e Assemblage and T r i a s s i c volcanics are eit h e r f a u l t s or i n s u f f i c i e n t l y exposed to determine t h e i r nature, although elsewhere i n the Telegraph Creek map sheet they are mapped as unconformable contacts (Souther, 1972). T r i a s s i c volcanic rocks i n the region are correlative.... with, and l i t h o l o g i c a l l y s i m i l a r to, the Takla Group. Within the S t i k i n e Arch area these rocks are referred to as the Stuhini Group (Souther, 1972; Souther et a l . . 1979). Age of the Stuhini Group i s reasonably well determined as Late Carnian to Late Norian (Souther, 1971). I t i s not c l e a r from f i e l d mapping whether or not rocks of the Stuhini Group have been intruded by the T r i a s s i c plutonic suite of the Hickman batholith. The T r i a s s i c plutons y i e l d radiometric ages between 230 and 220 Ma (Table 2.1). The s i m i l a r i t y of ages between the Stuhini volcanics and the - 17 -plutonic rocks suggests intrusion into coeval volcanics t y p i c a l of an arc-type se t t i n g . Ages and i n t r u s i v e -extrusive r e l a t i o n s h i p s of the Hickman batholith are s i m i l a r to those of the Hotialuh b a t h o l i t h (Anderson, 1983) located towards the northeastern apex of the Stikine Arch (Fig. 2.1) . The narrow b e l t of sediments (Fig. 2.2) that l i e s immediately west of the study area does not have formational status. F o s s i l s indicate an Upper T r i a s s i c (Norian) age. Coarse boulder conglomerates of t h i s b e l t contain c l a s t s having l i t h o l o g i e s s i m i l a r to the T r i a s s i c plutonic suite, which implies r e l a t i v e l y rapid u p l i f t and erosion during mid- to Late T r i a s s i c time. Contact relationships between these sediments and Stuhini Group volcanics are not cl e a r , as most contacts are faulted or poorly exposed. On the basis of present fieldwork, these sediments appear c o r r e l a t i v e with, or to s l i g h t l y predate, the upper parts of Stuhini stratigraphy. The pronounced north-south alignment of l i t h o l o g i e s within the Telegraph Creek map sheet i s a s i g n i f i c a n t contrast to the east-west orientation along the north edge of the S t i k i n e Arch. The north-south orientation r e s u l t s from horst and graben type features related to the Edziza - 18 -Complex, Mess Creek v a l l e y , and Hickman bat h o l i t h . These features may have been caused by variable u p l i f t along north-south s t r i k e - s l i p f a u l t s . I n i t i a l formation of these f a u l t s could have resulted from d i f f e r e n t i a l motion as the S t i k i n e Arch was forced under the A t l i n Terrane during accretion of S t i k i n i a . Deposition of various sedimentary rock units within narrow s t r u c t u r a l depressions along the margins of the Hickman b a t h o l i t h suggests that both v e r t i c a l and l a t e r a l motion took place along these f a u l t s over considerable lengths of time. 2.3 S t r a t i f i e d Rocks 2.3.1 S t i k i n e Assemblage The S t i k i n e Assemblage i s one of s i x groupings of mid-to Upper Paleozoic rocks i n the western C o r d i l l e r a (Monger, 1977a). These groups are defined by s i m i l a r i t y of l i t h o l o g i e s , gross stratigraphy, faunal assemblages and external r e l a t i o n s h i p s . The S t i k i n e Assemblage, which includes a l l p r e - T r i a s s i c rocks within the study region, i s exposed i n windows of younger material around the northern and western periphery of the Bowser Basin (Fig. 2.1). This assemblage includes the Asitka Group (Lord, 1948; Gabrielse, 1978) on the northeast edge of the Bowser Basin, and rocks that have been described i n the Taku River area (Kerr, 1948; Nelson and Payne, 1983), and i n an area about 100km south -19 -TERTIARY AND QUATERNARY Tev Edziza Volcanics: b a s a l t flows and re l a t e d p y r o c l a s t i c rocks CRETACEOUS Kss JURASSIC m.T 1J TRIASSIC Sustut Group: chert pebble conglomerate, granite boulder conglomerate, sandstone, s i l t s t o n e ; non-marine granite, quartz-monzonite; medium to coarse-grained, pink to orange i n colour; Yehiniko Pluton syenite, orthoclase porphyry monzonite, pyroxenite Galore Creek Intrusions amphibolite, amphibolite gneiss; age uncertain p e r i o d o t i t e , gabbro; age uncertain Ld < _ l CQ Li) Ul Ul < Ld Z Ul Tsv mT PERMIAN greywacke, s i l t s t o n e , chert, granite boulder conglomerate, polymictic v o l c a n i c conglomerate Stuh i n i Group and Unnamed Rocks: angite prophyry andesite, andesite t u f f , b r e c c i a , v o l c a n i c conglomerates, derived e p i c l a s t i c rocks B i o t i t e hornblende quartz d i o r i t e , granodiorite, d i o r i t e , Hickman and Night-Out Plutons limestone, b i o c l a s t i c limestone, c h l o r i t e p h y l l i t e s , f e l s i c t u f f MISSISSIPPIAN M c r i n o i d a l limestone, ferruginous limestone, maroon t u f f and p h y l l i t e MISSISSIPPIAN TO DEVONIAN MD f e l s i c to mafic p y r o c l a s t i c rocks, p h y l l i t e , greenstone, q u a r t z - s e r i c i t e s c h i s t , c h l o r i t e s c h i s t , a r g i l l a c e o u s s c h i s t , limestone L E G E N D F O R F I G U R E 2 .2 - 2 0 * Figure 2.2: Regional geological map of the Mess Creek-Stikine River area. The detailed study area is outlined in the lower right hand corner. (Modified after Souther, 1972.) PERIOD/ EPOCH QUATERNARY TERTIARY C R E T A C E O U S JURASSIC O co co < cc o o N o UJ < Q. L A T E MID PERMIAN < o. p. CO CO CO CO DEVONIAN DIAGRAMATIC SECTION INTRUSIVE ROCKS AGE * A A A V A A A A A A A" A A A A A A A n-ir A A A A A A A A A A A A A A A A A A k . A A A A A A A A A A A A A f l n A M l l V r i i H irn II « fl H « iS II " n » f* A A A / A A A A A A A A A A A A A A A A A A A A A A / A A A A AAAA_A.A-A.A-- * • • • • .••*.* A* y* ^ /A GROUP OR NAME LITHOLOGY YEHINIKO PLUTON HICKMAN PLUTON NIGHTOUT PLUTON 17616 Ma 230i16 Ma EDZIZA VOLCANICS UNCONFORMITY SUSTUT UNCONFORMITY H A Z E L T O N (?) FAULT CONTACT STUHINI JNCONFORMITY/FAUL JNCONFORMITY/FAUL1 UNCONFORMITY ? STIKINE ASSEMBLAGE -Basalt flow* and related pyroclastic rocks: minor rhyolite Conglomerate , quartzose sandstone, arkose Maroon 1o green lithic tuffs, crysta l ash tuffs, volcanic conglomerates, minor grey wacke Auglte andesite flows, pyroclastic rocka, maroon volcanic conglomerate, ash tuffs volcanic sandstones/wackes Siltstone, greywacke, chert.shale, volcanic conglomerate, granite boulder conglomerate Limestone, chloritic ash tuffs, minor chert Limestone, chlorite phyllltes, greenstone, quartz sericite schist, lithic tuffs, argillite See Table 2.1. Figure 2.3 Table of formations for the Mess Creek area, northwestern B.C. - 2 2 -of Terrace (Evenchick, 1980). Correlation i s based p r i m a r i l y on the widespread, f o s s i l i f e r o u s Permian limestone common to a l l areas. Li t h o l o g i e s of the Stikine Assemblage include a r g i l l i t e s , cherts, limestones, b a s a l t i c to r h y o l i t i c flows, p y r o c l a s t i c s and derived e p i c l a s t i c s . Metamorphism and deformation i s variable depending upon age and loca t i o n . I n s u f f i c i e n t work has been done to subdivide the Assemblage into formations; i t i s u n l i k e l y that i t i s composed of a s i n g l e s t r a t i g r a p h i c succession within a l l areas. There i s substantial evidence in d i c a t i n g a separation between Permian and pre-Permian rocks. The Permian limestone i s geographi-c a l l y widespread over a north-south distance i n excess of 500km, t y p i c a l of miogeoclinal deposition (Monger, 1977a), whereas a Mississippian limestone i s much l e s s continuous and often intercalated with volcanic rocks, which suggests deposition on seamount reefs. Near the study area, 1,500m of missing stratigraphy between Permian and Mississippian limestones over a 5km s t r i k e distance suggests an unconformity (Monger 1977a). This relationship, however, i s complicated by f a u l t s . Within the study region, pre-Permian rocks are t y p i c a l l y metamorphosed to mid-greenschist facies and display three or more phases of deformation, whereas presumed or known Permian-aged rocks are commonly less metamorphosed and deformed. - 23 -Faunal ages of the Sti k i n e Assemblage range from Permian to Devonian. Fauna c o l l e c t e d from the Permian limestone near Scud River are s i m i l a r to those of the same age found i n northern C a l i f o r n i a and Nevada (Pitcher, 1961). Devonian conodonts have been recovered from grey limestones along Forrest Kerr River and from s t r a t a 12km south of the study area (P. Read, pers. comm., 1984, and R. Anderson, pers. comm., 1987, re s p e c t i v e l y ) . Rocks studied i n d e t a i l (Chapter 3) are s t r a t i g r a p h i c a l l y below the Mississippian limestone and consist of a r g i l l i t e s , f e l s i c to mafic pyro-c l a s t i c s , and massive greenstones. Relationships between Miss i s s i p p i a n and Devonian aged stratigraphy are not known. 2.3.2 Stuhini Group A more or less continuous b e l t of Upper T r i a s s i c volcanic rocks runs the length of the c o r d i l l e r a (Fig. 2.4). This b e l t consists of the Nicola Group i n southern B r i t i s h Columbia, the Takla Group within the Quesnel Trough, the Stuhini Group within the S t i k i n e Arch and the Lewis River Group i n the Yukon T e r r i t o r y (Souther, 1977). The Nicola and Takla Group rocks are c h a r a c t e r i s t i c a l l y a l k a l i n e (Preto, 1977; Souther, 1977) and although the Stuhini Group rocks are l i t h o l o g i c a l l y s i m i l a r they tend towards more c a l c - a l k a l i n e chemistry (Souther, 1979). Differences i n chemistry may r e f l e c t t e c t o n i c p o s i t i o n as only the Stuhini Group occurs west of the Cache Creek rocks. -24 " FIGURE 2.4 D i s t r i b u t i o n of Late T r i a s s i c volcanic rocks i n the Canadian C o r d i l l e r a , and t h e i r s p a t i a l r e l a t i o n s h i p to the Paleozoic Cache Creek Group; a possible subduction complex f o r T r i a s s i c arc volcanism - 25 -Age of the Stuhini Group i s f a i r l y well confined to Late T r i a s s i c (Souther, 1977) and therefore contact re l a t i o n s h i p s with other units are important f o r determining the tectonic h i s t o r y of the area. Stuhini Group rocks a t t a i n a thickness of over 3,600m i n the Tulsequah map sheet area (Souther, 1971) but only 1,200m i n the Telegraph Creek map sheet area (Souther, 1972). Within the regional study area, Stuhini volcanics occur as a disrupted fringe around the Hickman bath o l i t h . Approximately 600m of pale green to maroon volcanic breccias and conglomerates are exposed on the ridge south of the Schaft Creek deposit. Immediately north of the deposit, augite porphyry flows predominate i n the sequence. Souther (1972) noted that the volcanics east of Mess Creek are l e s s massive and contain more sedimentary beds than rocks to the west. A s i m i l a r d i s t r i b u t i o n was observed i n a southwesterly d i r e c t i o n away from the Hickman ba t h o l i t h . The thick breccia and conglomerate sequences observed along the south-eastern and northwestern margins of the Hickman bath o l i t h could have accumulated i n trough-like depressions formed c o i n c i d e n t a l l y with u p l i f t of the b a t h o l i t h . L i t h o l o g i c a l l y , Stuhini volcanics are characterized by augite phenocrysts and a n d e s i t i c composition. Internal stratigraphy of the Group has not been defined (within the region) and i s l i k e l y subject to strong l o c a l controls. - 26 -L i t h o l o g i e s exposed within the study region generally co r r e l a t e well with those reported elsewhere within the S t i k i n e Arch by Anderson (1983), Schink (1977) and Souther (1972 and 1971). In the area immediately east of the Hickman ba t h o l i t h , flows are volumetrically subordinate to coarse p y r o c l a s t i c s and f i n e r grained, reworked material. Four l i t h o l o g i c a l / d e p o s i t i o n a l groups are recognized: (1) coarse-grained, fragment supported, grey-green to maroon volcanic breccias and h e t e r o l i t h i c conglomerates; (2) pillowed to massive, dark green augite porphyry flows; (3) andesitic, dark to l i g h t green l i t h i c t u f f s , wackes and volcanic sandstones; and (4) maroon p y r o c l a s t i c s , e p i c l a s t i c s and minor flows. The f i r s t three groups are commonly i n t e r -layered. Spilsbury (1982) describes the Stuhini Group stratigraphy which o v e r l i e s the Schaft Creek deposit as a sequence of f i n e grained andesite flows, augite p o r p h y r i t i c andesites, andesitic l a p i l l i t u f f s and intercalated t u f f and volcanic sandstone over l a i n by more andesite flows. The upper part of t h i s sequence i s weathered and unconformably ov e r l a i n by maroon subaerial p y r o c l a s t i c s . The maroon p y r o c l a s t i c s are post-mineralization and therefore, i f they are not emplaced by a thrust f a u l t , they l i k e l y belong to the J u r a s s i c Hazelton Group. - 27 -A t h i c k u n i t of volcanic conglomerate conformably o v e r l i e s massive augite-bearing greenstone i n the area west of Round Lake, about 6km southwest of the study area. This polymictic conglomerate has a maroon matrix and, i n addition to a v a r i e t y of volcanic c l a s t s , contains p y r i t i c quartz d i o r i t e fragments i n d i c a t i n g that at l e a s t some of the Stuhini Group post-dates i n t r u s i v e a c t i v i t y i n the area. Granitoid c l a s t s within demonstrably Stuhini aged volcanics i s reported by Anderson (1983) fo r the area f r i n g i n g the Hotailuh b a t h o l i t h . Contact rel a t i o n s h i p s between Stuhini volcanics and i n t r u s i v e rocks are discussed i n section 2.4. 2.3.3 Upper T r i a s s i c Sediments Sediments of probable Upper T r i a s s i c age are exposed as a t h i n b e l t , one to three kilometers wide, along the western edge of the study area (Fig. 2.2). True s t r a t i g r a p h i c thickness of t h i s b e l t i s i n the order of 1,000m. The sequence consists of arkosic sandstones, int e r c a l a t e d shales and cherts, maroon agglomerates, a r g i l l i t e s , limey s i l t s o n e s and granite boulder conglomerates (Fig. 2.6). Load casts within the interbedded cherts and a r g i l l i t e s suggest that the sequence i s r i g h t s i d e up but no other top indications were observed. Contacts on both sides of the b e l t , l a r g e l y obscured by i c e f i e l d s , appear to be tectonic. A steeply dipping f a u l t , marked by a 50m t h i c k zone - 2 8 -of ankerite cemented breccia, separates the sediments from Stuhini volcanics on the northeastern edge of the b e l t . At the southern end of the study area the breccia zone i s absent and the sediments may conformably o v e r l i e the volcanics. On the western edge of the b e l t , augite-bearing volcanics appear to be thrust over the sediments along a shallow, westerly dipping f a u l t . L i t h o l o g i e s of the Upper T r i a s s i c sequence, p a r t i c u l a r l y the granite boulder conglomerate, correlate well with the base of the J u r a s s i c s t r a t a mapped by Souther (1971 and 1972) i n the Tulsequah and Telegraph Creek map sheets. L i t h o l o g i c a l s i m i l a r i t y , the absence of meta-morphism, and a s i m i l a r degree of deformation (consisting of low amplitude, easterly plunging, open chevron folds within the T r i a s s i c sequence), make t h i s a l o g i c a l c o r r e l a t i o n . However, f o s s i l c o l l e c t i o n s indicate a l a t e Norian age. F o s s i l s were c o l l e c t e d from two horizons and i d e n t i f i e d by Paul Smith (pers. comm., 1981). Rhynchonellids and Terebratulid brachiopods were recovered from t h i n shaley units at the base of the boulder conglomerate. A 30m t h i c k bed of f i n e arkose at the base of the sequence contained i abundant Monotis (cf. s u b c i r c u l a r i s ) i n d i c a t i n g a l a t e Norian (Suessi zone) age (Westerman, 1973). Souther (1972) reports sedimentary beds containing Monotis that are -29 -METERS fc A A A A A A A V ^ A A A A A A A A A A A A A A A A A A A A V V V V V V V V V V V V V V vvvvvvvvvvvvvv Feldspar porphyry dacite flow (?) Sheared mafic rock (basalt or gabbro) THRUST (?) A A A A A . A A A A A A A A A A i A A A A A A A A A A i A A A A A k V " " * Granitic boulder conglomerate with thin faiterbeds ol siltstone Limey shale/slltstone Maroon volcanic' conglomerate Friable shales with interbedded argillites Maroon volcanic conglomerate Grey quartz-feldspar, hornblende sandstone and greywackes, minor Interbedded maroon Dthlc tufts Thin bedded shales and cherts Arkose sandatones. Interbedded siltstones Ankerite/quartz cemented breccia with maroon volcanic fragments Massive maroon tuff-breccias Figure 2.5 Stratigraphic section of unnamed Triassic sediments on western edge of detailed study area. - 30 -o v e r l a i n by Stuhini Group volcanics near B a l l Creek, 20km east of the study area. The t h i n maroon volcanic conglomerates within the sedimentary sequence are indistinquishable from outcrops of si m i l a r l i t h o l o g y within the Stuhini volcanics, suggesting a common source and therefore a close a f f i n i t y between the two rock groups. Hornblende granodiorite c l a s t s i n the boulder conglomerates are s i m i l a r i n appearance to phases of the Hickman ba t h o l i t h . The presence of these conglomerates around the periphery of the Hickman bat h o l i t h implicate i t as the source area, and provide s t r a t i g r a p h i c constraints for minimum ages of some of the plutonic phases. S i m i l a r l y , Anderson (1983) describes c l a s t s from the 218 to 227 m i l l i o n year o l d Cake H i l l pluton, of the Hotialuh b a t h o l i t h (Fig. 2.1), within the base of onlapping Upper T r i a s s i c volcanics. 2.4 Intrusive Rocks Intrusive rocks underlie about 30 percent of the Telegraph Creek map sheet area and play a s i g n i f i c a n t r o l e i n the tectonic and metallogenic history of the region. There i s a strong s p a t i a l association between rocks of the St i k i n e Assemblage and plutonic rocks within the Tulsequah and Telegraph Creek map sheet areas; possibly due to coincident u p l i f t of Stikine Assemblage and plutonic rocks - 31 -through younger s t r a t a . Plutonic rocks occur to the north, east and northwest of the study area. Only two small outcrops of granitoid rocks occur within the area of d e t a i l e d mapping. Plutons of the Hickman batholith provided the best combination of ranges i n age, l i t h o l o g i e s and i n t r u s i v e relationships f o r study. Study of these rocks was only cursory, but data are rendered more useful when interpreted i n l i g h t of exhaustive studies by Anderson (1983) on the nearby, and s i m i l a r , Hotailuh batholith. Souther (1972) used the term Hickman ba t h o l i t h for the roughly c i r c u l a r pluton approximately centered on Mount Hickman, i n the south c e n t r a l area of the Telegraph Creek map sheet. The term Hickman b a t h o l i t h i s herein extended to encompass the adjoining plutons to the north. The Hickman ba t h o l i t h would then be composed of three plutons (Fig. 2.6), informally named, from north to south, the Nightout pluton, the Yehiniko pluton, and the Hickman pluton, and have an exposed area of about 1,100 square kilometers. Samples c o l l e c t e d from the three plutons were dated (Table 2.1) at the geochronolgy lab of The University of B r i t i s h Columbia and indicate a Middle T r i a s s i c age f o r the Hickman and Nightout plutons and a Middle Ju r a s s i c age f o r the Yehiniko pluton. - 32 -2.4.1 Hickman Pluton The Hickman pluton i s a roughly c i r c u l a r intrusion, approximately 16km i n diameter, which forms the most rugged topography i n the region, with peaks i n excess of 3,000m elevation. About one h a l f of the area of the pluton i s covered by g l a c i a l i c e . The pluton i s zoned from a more mafic i n t e r i o r to a le s s mafic outer phase (Souther, 1972). Rocks range from medium grained hornblende b i o t i t e quartz d i o r i t e s i n the i n t e r i o r , to f i n e r grained b i o t i t e hornblende granodiorites towards the exterior. Mafic minerals make up from 10 to 20 percent of the rock volume i n both phases. Accessory minerals include sphene, apatite and magnetite. The Mount Hickman area i n the southwestern part of the pluton i s underlain by ultramafic rocks which form a north-easterly oriented e l l i p t i c a l shaped body 6km i n length. Mount Hickman i t s e l f , i n the southern end of the ultramafic body, consists of hornfelsed volcanic and sedimentary rock of probable Permian age. The ultramafic body appears to be in t r u s i v e into the Hickman pluton and i s zoned from an o l i v i n e bearing pyroxenite core through to hornblende gabbro with rare b i o t i t e at the margin. Foliated hornblende - 3 3 -Figure 2.6. Distribution of the plutonic components of the Hickman batholith, and location of dated samples. - 34 -d i o r i t e s occur around the margin of the ultramafic body. F o l i a t i o n i s oriented p a r a l l e l to the contacts. Small plugs of gabbro, from 20 to 200m i n s i z e , were also observed on the northern edge of the Hickman pluton. Dykes and plugs of magnetite-rich syenite, possibly related to the Galore Creek bodies which occur 10km to the west, and/or quartz monzonites of the Yehiniko pluton, occur i n the northern h a l f of the pluton. Steep dipping f a u l t s bound the pluton on both i t s western and eastern margins. Intrusive contacts between the Hickman pluton and Stuhini volcanics were not observed. A remnant of Stuhini volcanics appears to cap the pluton on i t s northwestern edge, but the contact between mineralized v o l c a n i c s — c o n s i s t i n g of plagioclase porphyry flows and an d e s i t i c fragmentals—and the Hickman pluton i s obscured by dykes and s i l l s of younger quartz monzonite. Age of the Hickman pluton has been determined by potassium-argon dating as Middle T r i a s s i c . Dates of 209 15 Ma and 221±8 Ma (Table 2.1) were obtained from b i o t i t e and hornblende separates, respectively, from a sample c o l l e c t e d near the center of the pluton (Fig. 2.6). A whole rock rubidium-strontium date i s calculated as 233±23 Ma using an 87 86 assumed i n i t i a l Sr/ Sr of 0.7038. TABLE 2.1 Radiometric Age Determinations for Plutons of the Hickman Batholith and Mineralization Within the Stikine Assemblage, Northwestern B.C. SAMPLE Latitude Loneitude • LITHOLOGY UNIT/FORMATION OR GROUP METHOD1 AGE3 (Ma) INITIAL MINERAL and error 87„ /86. Sr Sr TIME SCALE4 81-HAl7a 57"30' Hornblende biotite granodiorite Nightout Pluton K-Ar K-Ar Bi Hb 236±9 228±8 A G E (Ma) j PERIOD EPOCH AGE 131°03' Rb-Sr Bi 232±5 0.7038 1 5 0 - TITHONIAN LATE KIMMERIDGIAN 81-HAlOa Biotite hornblende Hickman Pluton K-Ar Bi 209±15 • 1 6 0 - OXFORDIAN • o CALLOVIAN 57° l3 ' 131°l6' granodiorite Hb 221±8 1 7 0 -ASSI MIDDLE BATHONIAN Rb-Sr Bi 216±4 0.7038 1 8 0 - ASSI BAJOCIAN 233±23 0.7038 • CC AALEN1AN WR 1 9 0 - D TOARCIAN _ PLIENSBACHIAN - EARLY 81-HAl9a Leucocratic Yehiniko K-Ar Bi 172±6 2 0 0 - SINEMURIAN quartz monzonite Pluton HETTANGIAN 57°25* 131°15' Rb-Sr Bi WR 170±16 0.7038 178±11 0.7038 2 1 0 -220 -OISSV LATE NORIAN 230 -OISSV CARNIAN 81-HRC4 Siderite-quartz Stikine K-Ar Ms 192±7 TRL MIDDLE LADINIAN 57"09'08" 130°58'10" vein Assemblage J-0 - TRL ANISIAN TRL EARLY SCYTHIAN 1 A l l analyses were performed by K. Scott, R.L. Armstrong and J. Harakal, Geochronology Lab, The University of British Columbia Bi = Biotite, Hb = Hornblende, WR = Whole Rock, MS «= Muscovite (± fuchsite)' Decay constants after Steiger and Jager (1977) Geological Society of America, DNAG - 1983 Time Scale (Kent et a l . , 1984; Armstrong, 1982) - 36 -2.4.2 Nightout Pluton The Nightout pluton forms the northern part of the Hickman b a t h o l i t h and i s bisected into northern and southern bodies by the Yehiniko pluton. Topography i s moderate i n the northern h a l f , near Nightout Mountain, but more rugged i n the southern portion. Compositionally, the pluton i s r e l a t i v e l y homogeneous with l i t h o l o g i e s ranging from hornblende b i o t i t e quartz d i o r i t e to granodiorite, which i s compositionally equivalent to the outer phase of the Hickman pluton. D i s t i n c t i v e features are abundant coarse sphene grains and commonly c h l o r i t i z e d mafics. Thin sections show that feldspars are weakly to moderately altered to s e r i c i t e and carbonate. Intrusive relationships of the Nightout pluton are not c l e a r . A remnant of Stuhini volcanics l i e s along the northern edge of the southern body but contacts are covered by extensive t a l u s . On the east side of Schaft Creek, below Mt. Lacasse, an apophysis of the pluton appears to have intruded and metamorphosed pyroxene-plagioclase p o r p h y r i t i c flows. However, abundant dyke swarms of quartz monzonite complicate the exposed contacts. The Nightout pluton has r e l a t i v e l y sharp i n t r u s i v e contacts with the Paleozoic sediments of Zagoddetchino Mountain (Fig. 2.2). - 37 -Age of the Nightout pluton, determined from samples c o l l e c t e d from the southern body, i s Middle T r i a s s i c . Potassium-argon dates of 236±9 Ma and 228±8 Ma (Table 2.1) were determined from b i o t i t e and hornblende separates, respectively. Samples c o l l e c t e d from an apophysis on the east side of Schaft Creek were too altered f o r potassium-argon determinations. Ages of the Hickman and the Nightout plutons are concordant. Four whole rock samples from both the Nightout and Hickman plutons yielded a whole rock rubidium-strontium age (190±70 Ma) that, although concordant with the ages above, has excessive calculated error. 2.4.3 Yehiniko Pluton Flesh coloured, le u c o c r a t i c quartz monzonite to granite make the Yehiniko Pluton d i s t i n c t i v e . The pluton has a vaguely c i r c u l a r outline within the center of the Hickman ba t h o l i t h , but dykes, s i l l s and other apophyses extend f o r considerable distances away from the main mass. Souther (1972) noted the strong j o i n t patterns, which r e s u l t i n impressive monolithic peaks, c h a r a c t e r i s t i c of t h i s pluton. - 38 -The i n t r u s i o n i s compositionally homogeneous, being composed of roughly equal amounts of orange potassium feldspar, grey plagioclase, and c l e a r to smokey quartz. Mafic minerals seldom t o t a l more than f i v e percent by volume, with b i o t i t e predominating over hornblende. Magnetite i s a common constituent, l o c a l l y reaching two to three percent of rock volume. A large pluton west of the Stikine River (Fig. 2.2) i s l i t h o l o g i c a l l y i d e n t i c a l to the Yehiniko pluton. The quartz monzonites are unequivocally i n t r u s i v e into a l l surrounding rocks except f o r the Cretaceous and T e r t i a r y Sustut conglomerates which contain c l a s t s of quartz monzonite (Souther, 1972). Xenoliths and screens of granodiorite within the quartz monzonite, between the north and south parts of the Nightout pluton suggest that these bodies were previously joined. Near the headwaters of Schaft Creek and below the Schaft Creek porphyry deposit, extensive dyke swarms of quartz monzonite and granite i n t e r d i g i t a t e with Hickman plutonic and Stuhini volcanic rocks. Quartz monzonite intrudes a sedimentary package j u s t west of Yehiniko Lake. These sediments are mapped as Lower Ju r a s s i c by Souther (1972), but they may be equivalent to the Upper T r i a s s i c sediments described i n section 2.3.3. - 39 -Rubidium-strontium whole rock and b i o t i t e potassium-argon determinations give respective dates of 178+11 Ma and 172±6 Ma (Table 2.1) for the quartz monzonite, i n d i c a t i n g a Middle J u r a s s i c age, which i s i n accordance with observed geology. 2.4.4 Other Plutonic Rocks An elongate pluton of r e l a t i v e l y homogeneous hornblende quartz d i o r i t e occurs approximately f i v e kilometers east of the d e t a i l e d study area. Souther (1972) correlated t h i s pluton with the Nightout pluton. On the basis of l i t h o l o g y and f i e l d r e l a t i o n s i t i s most l i k e l y part of the T r i a s s i c p l u t o n i c s u i t e . The pluton, which i s roughly 6km wide by 30km long, was not examined i n d e t a i l except f o r the contact zone with S t i k i n e Assemblage rocks, 10km south of the d e t a i l e d study area. L i t h o l o g i c a l l y , t h i s pluton i s s i m i l a r to the Nightout and Hickman plutons. Compositional zonation, f o l i a t i o n , and gabbro or syenite dykes and plugs a l l appear to be absent. Contacts with the Stikine Assemblage rocks are gradational over a one km wide zone along the southwestern margin of the pluton. Within the contact zone, f o l i a t e d S t i k i n e Assemblage volcanics are r e c r y s t a l l i z e d , disrupted, granitized (Souther, 1972) and r a r e l y p y r i t i z e d , but no other forms of a l t e r a t i o n were - 40 -observed. Contacts with Mesozoic aged rocks were not investigated. The small e l i p t i c a l pluton, 2km northeast of the study area (Fig. 2.2) has l i k e l y been severed from the main i n t r u s i v e mass by f a u l t s . Apart from sparse molybdenum stream sediment geochemical anomalies, no mineralization was observed to be associated with t h i s pluton. A l i t h o l o g i c a l l y and chronologically d i s t i n c t plutonic s u i t e i s formed by r e l a t i v e l y small a l k a l i c plutons, ranging from p o r p h y r i t i c syenites to magnetite b i o t i t e pyroxenites, that occur throughout the study region (Souther, 1972). Some of the plutons of t h i s suite host p o t e n t i a l l y economic mineral deposits, and deposits hosted i n other rocks appear to be g e n e t i c a l l y related to t h i s plutonic s u i t e . The Galore Creek prospect i s described by A l l e n et a l . (1977) as ten tabular to manto shaped copper-gold deposits, hosted by altered T r i a s s i c volcanic rocks and associated with syenite porphyry dykes and plugs. The main deposit occurs within a breccia pipe associated with f i n e grained orthoclase porphyry syenite. Four potassium-argon dates (recalculated to new decay constants from White et a l . , 1968) on hydrothermal b i o t i t e range from 178 to 203 m i l l i o n years i n d i c a t i n g an Early to Middle Ju r a s s i c age of mineralization. - 41 -The Red Chris deposit, located 35km southwest of the Hotialuh batholith, i s a porphyry copper deposit within a composite stock of altered hornblende monzonite (Schink, 1977). Host monzonites intrude Norian (Late T r i a s s i c ) age volcanic rocks. Two potassium-argon determinations (Schink, 1977) , on hornblende monzonite and a related dyke, give dates of 210±7 Ma and 195±8 Ma respectively, i n d i c a t i n g an Early J u r r a s i c age. 2.5 Summary of Age Relationships and Metallogeny T r i a s s i c plutons of the Hickman bath o l i t h y i e l d roughly concordant potassium-argon and rubidium-strontium dates that range from 209 to 236 Ma (Table 2.1) suggesting an early Carnian to early Norian age. A s i m i l a r range of dates (Fig. 2.7) was obtained from the T r i a s s i c plutonic s u i t e of the Hotialuh b a t h o l i t h (Anderson, 1983). Stuhini volcanics range i n age from early Carnian to Late Norian based on both i s o t o p i c dates (Anderson, 1983) and f o s s i l ages (Souther, 1972). Unequivocal i n t r u s i v e relationships between the Hickman or Nightout plutons and Stuhini volcanics were not observed. Granodiorite boulders, l i t h o l o g i c a l l y s i m i l a r to the Hickman pluton, within f o s s i l i f e r o u s l a t e Norian sediments close to the Hickman pluton, suggest that i n t r u s i o n of the pluton took place p r i o r to Norian time. - 42 -Mapped re l a t i o n s h i p s (Souther, 1972) indicate that at le a s t some of the Stuhini volcanics post date the Norian sediments. Plutonic rocks that have c l e a r l y i n t r u s i v e contacts with Stuhini volcanics y i e l d potassium-argon dates of 222±16 and 218±12 Ma, from the Katsetsa stock on the eastern side of the Sti k i n e Arch (recalculated to new decay constants from McMillan, 1974), and 210±9 Ma, from the Red stock in" the south central Stikine Arch area (Schink, 1977). I t i s probable that the older phases of the Hickman b a t h o l i t h predate some or a l l of Stuhini volcanism, and were the source of g r a n i t o i d c l a s t s within volcanic and sedimentary conglomerates, while the younger T r i a s s i c phases may be coeval and comagmatic with the volcanics. Anderson (1983) argues that although the Hotailuh b a t h o l i t h i n part predates the overlying Stuhini volcanics, i t i s the source area f o r the volcanics i n that region, with the ultramafic i n t r u s i v e s representing feeders for the overlying volcanics. Similar r e l a t i o n s h i p s between volcanics and the Hickman ba t h o l i t h are suggested by the presence of in t r u s i v e pyroxenites, close temporal association of in t r u s i v e and extrusive rocks, and the t h i c k accumulations of coarse volcanic conglomerates and breccias along the margins of the bath o l i t h . o CA 9- s> c » to 3 •O CJ » • CD " s 3 5T c CQ "O CD _ 2 =>" o o CO < DEVONIAN CARBONIFEROUS MlUmiPrHAM PEMNITLVANIAN PERMIAN i T i 11 ? . TRIASSIC JURASSIC s I • I . . S 8 CD I T O Q. ro 35 CD —^  CD s o CD^ Stikine Assemblage Hotialuh batholith Hickman batholith Stuhini Group Triassic sediments Galore Creek Intrusions BJ quartz-carbonate vein Red Chris deposit Schaft Creek deposit © approx. 30 dates 4 dates © = Fossil Age 12 12 FIGURE 2.7 Summary of age r e l a t i o n s h i p s f o r rocks and s e l e c t e d mineral d e p o s i t s of the S t i k i n e Arch area, northwestern B.C. Potassium argon dates r e c a l c u l a t e d to recent decay constants (Steiger and Jager, 1977) by the method of Dalrymple, 1979). References: (1) Anderson, 1983, (2) t h i s study, (3) Nelson and Payne, 1984 ,(4) Pantaleyev 1973,(5) P. Read pers. comm., 1983,(6) Schink, 1977,(7) Souther, 1972 (8) White et a l . , 2 1 9 6 8 Radiometric Dating Methods: (1) K-Ar hornblende (2) K-Ar b i o t i t e (3) K-Ar muscovite (4) Rb-Sr b i o t i t e (5) Rb-Sr whole rock, - 44 -There i s a notable absence of s i g n i f i c a n t mineralization associated with the T r i a s s i c suite of intr u s i v e s within the Sti k i n e Arch area. Granodiorite outcrops near the Schaft Creek porphyry deposit led to the association of the Hickman pluton with mineralization. More det a i l e d investigation r e l a t e d the genesis of the deposit to quartz monzonites of the Yehiniko pluton (Spilsbury, 1982). A potassium-argon date of 186±5 Ma on hydrothermal b i o t i t e (recalculated to new decay constants from Pantaleyev, 1973) from the deposit supports t h i s genetic interpretation. Barr et a l . (1977) demonstrated a strong association between Early J u r a s s i c (Pleinsbachian) age a l k a l i c porphyry deposits and Upper T r i a s s i c volcanic rocks along the length of the C o r d i l l e r a . In many cases chemistry of the volcanics and i n t r u s i v e s suggest a comagmatic l i n k . The common gap i n age between the volcanics and intrusives may be because these intrusives are l a t e stage d i f f e r e n t i a t e s and that dated minerals r e f l e c t cooling ages or l a t e hydrothermal events. The Galore Creek, Red Chris and Schaft Creek deposits a l l belong to t h i s age and s t y l e of mineralization. A l l of these deposits, p a r t i c u l a r l y Galore Creek, are enriched i n precious metals r e l a t i v e to other porphyry deposits of the C o r d i l l e r a . Notably, the two major precious metal camps peripheral to the Bowser B a s i n — t h e Toodoggone - 45 -camp (Schroeter, 1982; Forster, 1984) to the east, and the Stewart camp (Aldrick, 1986; Brown, 1986) to the south—are hosted by intrusive-extrusive complexes of Bajocian to Sinemurian (176 - 205 Ma) age. Muscovite and fuchsite from a gold and s i l v e r bearing quartz-siderite vein (see Chapter 4) within highly a l t e r e d S t i k i n e Assemblage p y r o c l a s t i c rocks yielded a potassium-argon age of 192±7 Ma suggesting that precious metal vein type mineralization i s also related to lower Jurassic magmatic a c t i v i t y . - 46 -CHAPTER 3: GEOLOGY OF THE STIKINE ASSEMBLAGE 3.1 Introduction This chapter describes the stratigraphy, petrology, structure and chemistry of Sti k i n e Assemblage rocks exposed within the study area. Rocks of the Stikine Assemblage are exposed i n u p l i f t e d , generally fault-bounded, blocks throughout the Stikine Arch. Lithologies and the int e r n a l structure and stratigraphy of these blocks are known i n only a general way. Age determination and c o r r e l a t i o n between blocks i s l a r g e l y by f o s s i l i f e r o u s limestones. Rocks described herein underlie, with apparent conformity, Mississippian age limestones (Souther, 1972; Monger, 1970) and are considered time-correlative with these limestones. The base of the section i s not exposed. A l l of the rock units within the study area have been complexly deformed, metamorphosed and varia b l y altered. The in t e n s i t y of deformation and widespread a l t e r a t i o n i s unusual r e l a t i v e to other exposures of Stikine Assemblage rocks within the Telegraph Creek map sheet area. The r e l a t i v e timing of the deformation and a l t e r a t i o n , and the cause of a l t e r a t i o n i s s i g n i f i c a n t , not only as a f i r s t step i n i n t e r p r e t i n g the geological history, but also to the evaluation of the exploration pot e n t i a l of the area. - 47 -Map units were defined i n the f i e l d on the basis of compositional and text u r a l s i m i l a r i t i e s . Many of the l i t h o l o g i e s have gradational contacts and/or are interbedded within the contact area; consequently, placement of map contacts i s commonly approximate, and i n places even a r b i t r a r y . A c l e a r understanding of f o l d geometry i s hampered by lack of consistent s t r a t i g r a p h i c markers and probable rapid facies changes. This, i n turn, severely l i m i t s the qu a l i t y of st r a t i g r a p h i c interpretations. S t r u c t u r a l data c o l l e c t e d i n the f i e l d was interpreted with the help of stereonets and oriented specimens. As i n many geo l o g i c a l l y complex areas, f i e l d data are variable i n qu a l i t y and i n places contradictory; many problems remain. 3.2 Stratigraphy 3.2.1 Stratigraphic Summary The S t i k i n e Assemblage i s composed predominately of v o l c a n i c l a s t i c s with subordinate, interbedded sediments and minor flow and/or i n t r u s i v e rocks (Fig. 3.1). No unambiguous indicators of tops were observed and thus the st r a t i g r a p h i c sequence i s interpreted to be r i g h t side up on the basis of s t r u c t u r a l continuity with a defined Permian-Mississippian contact (Souther, 1972) located 1km to the southwest of the study area. At the regional scale, units appear to be l e n t i c u l a r and stratigraphy cannot be followed - 48 -f a r beyond the l i m i t s of the mapped area. The volcanic sequence i s , for the most part, bimodal consisting of either r h y o l i t i c or b a s a l t i c compositions. Pyr o c l a s t i c units display evidence of mixing from two or more sources. Grain s i z e of these rocks can change from coarse to very f i n e over short distances, both l a t e r a l l y and v e r t i c a l l y . Nomenclature used i n descriptions of p y r o c l a s t i c rocks follows that of Fisher (1961), and Fisher and Schminke (1984). Volcanic and sedimentary rocks have been subdivided into seven units on the basis of textural and mineralogical c h a r a c t e r i s t i c s (Fig. 3.2). Two of these units are non-stratigraphic and characterized by a l t e r a t i o n assemblages with indeterminable p r o t o l i t h s . Chemical analyses support these groupings. The lowest unit within the s t r a t i g r a p h i c section i s a s i l i c e o u s to gr a p h i t i c a r g i l l i t e or argillaceous s c h i s t with intercalated l i t h i c wackes and ash t u f f s . This u n i t i s overla i n by, and gradational with, mafic py r o c l a s t i c s characterized by a wide v a r i e t y of fragment types and interlayered green and maroon beds. C h l o r i t e s c h i s t , a sub-unit of the mafic p y r o c l a s t i c s , i s interbedded with the argillaceous s c h i s t . The next overlying unit i s f e l s i c i n composition and ranges from coarse volcanic breccias through l a p i l l i t u f f s to c r y s t a l - l i t h i c ash t u f f s . A dark green, basic, coarse to f i n e grained, t y p i c a l l y massive unit caps - 4 9 -*» tt 9 Mississippian o -2 •Q S « «l «« <0 - Shales, cherts, volcanic ssts, grits, boulder conglomerates Unconformity / Thrust - Crinoidal limestone - Undifferentiated I volcanics Unit 5 GRST Gabbro sills Greenstone unit - Unit 4 F V C S Felsic tuffs and breccias - Unit 3 PGST Hematitic schists, pyroclastics - Unit 2 CHST Massive chlorite schists - Unit 1 ARGL Meta-argillite, graphitic schist Figure 3.1 Schematic stratigraphic column of Stikine Assemblage rocks, Mess Creek area, northwestern B.C. - 50 -F i g u r e 3 .2 G e n e r a l i z e d g e o l o g y of the M e s s C r e e k a r e a , n o r t h w e s t e r n B . C . - 5 1 -the sequence. This uppermost unit, which i s interpreted to be an in t r u s i v e s i l l , may also contain minor flow and e p i c l a s t i c rocks. A l l l i t h o l o g i e s have been metamorphosed to lower greenschist f a c i e s . Intense shearing related to polyphase f o l d i n g has l o c a l l y o b l i t e r a t e d primary textures (Plate 3.3c). Hydrothermal a l t e r a t i o n , characterized by s e r i c i t e -carbonate-quartz s c h i s t , i s widespread. At lea s t part of t h i s a l t e r a t i o n appears to be pre-deformation and could have served to l o c a l i z e s t r a i n , thereby accounting f o r the higher i n t e n s i t y of deformation i n t h i s area. This type of a l t e r a t i o n a f f e c t s a l l units but i s most common within the f e l s i c rocks. 3.2.2 Argillaceous Schist Argillaceous s c h i s t s and p h y l l i t e s form the base of the st r a t i g r a p h i c sequence, and are exposed i n the core of the large antiform that extends the length of the map area (Fig. 3.3). Although re g i o n a l l y widespread, t h i s unit i s not demonstrably time-specific enough to use for c o r r e l a t i o n of stratigraphy. The maximum observed thickness i s approximately 200m but as the base i s not exposed thickness i s greater. - 52 -A r g i l l i t e i s intercalated with beds and lenses of fine-grained v o l c a n i c l a s t i c rock, predominately mafic l i t h i c wackes s i m i l a r to the overlying uni t . Contacts are both sharp and gradational, with the l a t t e r being more common. Gradational contacts can extend over a v e r t i c a l distance of 30m. Apart from the v o l c a n i c l a s t i c layers, bedding i s not observed. Colour of the argillaceous rocks ranges from black to pale grey depending upon the r e l a t i v e amounts of quartz and graphite present. Outcrops are commonly riddled with f i n e quartz or carbonate v e i n l e t s r e s u l t i n g from pressure solution and r e c r y s t a l l i z a t i o n i n d i l a t a n t zones during deformation. Mineralogy of the argillaceous s c h i s t , as observed i n t h i n section, consists of very f i n e grained quartz, muscovite, carbonate, graphite, orthoclase, plagioclase, c h l o r i t e , epidote and various oxides. D e t r i t a l minerals such as apatite and zircon are also present. Euhedral p y r i t e grains are common. In sections taken from crenulated rocks, a secondary compositional layering i s produced by concentration of graphite and opaques along second-phase cleavage surfaces between carbonate and quartz layers (Plate 3.5c). Rare e l l i p t i c a l fragments up to 5mm i n si z e can also be seen i n sections. Graphite content varies from 2 to 20 percent with the average at about 5 percent. P e l i t i c - t y p e metamorphic minerals were not observed i n outcrop or i n th i n section. - 53 -Macro f o s s i l s were not observed and samples digested for conodonts were s i m i l a r l y barren. This may be attributed to the in t e n s i t y of deformation and/or to the absence of fauna during deposition. 3.2.3 C h l o r i t e Schist Fine-grained, laminated to t h i c k l y bedded, blue green to pale green c h l o r i t e s c h i s t s are interpreted to be metamorphic equivalents of mafic ash t u f f s . These t u f f s form part of the overlying mafic p y r o c l a s t i c / e p i c l a s t i c unit within which they are interbedded. At the south and north ends of the map area, within and adjacent to the argillaceous s c h i s t , the c h l o r i t e s c h i s t s are s u f f i c i e n t l y homogeneous and thick enough to be mapped as d i s t i n c t units. These units are l e n t i c u l a r and pinch out ra p i d l y along s t r i k e ; a feature possibly exaggerated by competency contrast with the adjacent a r g i l l i t e during deformation. Maximum observed thickness i s 70m. Outcrops of t h i s u n i t t y p i c a l l y display laminations within compositional layers presumed to be bedding (Plate 3.3). Quartz and/or carbonate layers p a r a l l e l to f o l i a t i o n and bedding planes are also common. Massive, r e s i s t a n t outcrops occur l o c a l l y and r e f l e c t a mineralogical change from dominantly c h l o r i t e to dominantly epidote. Near the - 54 -upper and lower contacts, muscovite becomes an important constituent and the rock takes on a p h y l l i t i c appearance. Fine l i t h i c fragments, r e a d i l y apparent i n t h i n section, are not obvious i n outcrop. A notable exception i s where 3 to 10cm long e l l i p t i c a l , f i n e l y v e s i c u l a r , orange weathering fragments occur within t h i c k l y bedded chlorite-epidote s c h i s t s . These fragments are well preserved with v e s i c u l a r f i l l i n g s of ferruginous carbonate (Plate 3.3f), and resemble frothy b a s a l t i c bombs of the type commonly associated with spatter cones (McBirney and Williams, 1979). Thin sections of the s c h i s t reveal f i n e layers of c h l o r i t e , epidote, a l b i t e , carbonate, muscovite (plus or minus t a l c ? ) , quartz, tremolite and opaques. Chlorite, which can form up to 35 percent of the s l i d e , occurs as fi n e wisps within the matrix or as coarse e l l i p t i c a l grains or aggregates; the l a t t e r appear to be replaced fragments rather than porphyroblasts. Matrix c h l o r i t e usually has anomalous b e r l i n blue interference colours in d i c a t i n g a high-magnesium (peninite) composition. Chemical analyses indicate that the rocks of t h i s unit have an unusual composition that i s characterized by low s i l i c a , high iron and high magnesium contents (section 3.6). Compositions are s i m i l a r to those of the mafic p y r o c l a s t i c u n i t supporting the inter p r e t a t i o n of a common parentage. - 55 -I f chemistry of these rocks has not been modified markedly by diagenetic and metsomatic processes i t i s reasonable to conclude that they represent fine-grained, but proximal, e p i c l a s t i c and minor p y r o c l a s t i c d e t r i t u s derived from a l k a l i n e o l i v i n e basalts. 3.2.3 Mafic Py r o c l a s t i c and E p i c l a s t i c Unit The mafic p y r o c l a s t i c and e p i c l a s t i c u n i t i s a heterogeneous c o l l e c t i o n of l i t h o l o g i e s that includes coarse agglomerates, debris flows, l a p i l l i t u f f s , l i t h i c wackes and ash t u f f s . The unit i s best exposed i n the southwestern part of the study area where i t forms steep south-facing slopes. Here, thickness of the unit may be up to 500m, although some st r u c t u r a l r e p e t i t i o n i s l i k e l y . Exposure i n the c e n t r a l part of the map area i s minimal as overlying units form dip slopes i n t h i s area. In the northern map area an upper and a lower layer are exposed. These layers are interpreted to be f o l d r e p e t i t i o n s . In general, the northern exposures have a greater proportion of e p i c l a s t i c material while the southern outcrops are predominatly p y r o c l a s t i c . The uni t i s characterized by heterogeneity of t h i n l y interbedded fine-grained p y r o c l a s t i c and e p i c l a s t i c material (Plates 3.1b and 3.2e). Matrix material ranges from c h l o r i t i c to hematitic and argillaceous with corresponding - 56 -colour ranges of dark green to maroon and grey. Maroon and green layers are frequently interbedded. Thickness of i n d i v i d u a l beds varies from 30cm to 10m. Coarse fragmental units are more common i n the central and southern map area while f i n e grained units tend to dominate i n the northern map area. On the very north end of the study area, mafic p y r o c l a s t i c s and wackes become progressively f i n e r grained and more argillaceous s t r u c t u r a l l y up section. Near the top of the section (or bottom i f the sequence i s inverted) th i n limestone lenses appear suggesting marine transgression. The alternating maroon and green colours could correspond to changes from subaerial to submarine depositional environments. Thin sections show a varied mineralogy commensurate with te x t u r a l v a r i a t i o n . C h l o r i t e , epidote, a l b i t e , orthoclase, muscovite, carbonate, quartz, graphite and oxides are a l l present i n variable proportions. Textures are often more noticable on weathered outcrop surfaces than they are i n t h i n section. Bright green micaceous spots are very conspicuous within maroon coloured p h y l l i t e s at a few locations. These spots were confirmed (by scanning electron microscopy and X-ray d i f f r a c t i o n techniques) to be chrome-bearing c h l o r i t e (kammererite) and muscovite (fuchsite) which formed around and replaced chromite grains. - 57 -Samples c o l l e c t e d for chemical analyses from t h i s unit were fine-grained maroon l i t h i c t u f f s which are not necessarily representative of the entire u n i t . Chemistry i s characterized by high i r o n and magnesium, and low s i l i c a , s i m i l a r to the c h l o r i t e s c h i s t discussed i n the preceeding section. These samples are also distinguished by t h e i r high chromium (300ppm) and n i c k e l (200ppm) values. The combination of textures and chemistry indicate that these rocks formed as a r e s u l t of both d i r e c t deposition and reworking of b a s a l t i c p y r o c l a s t i c and possibly flow material. 3.2.4 F e l s i c Pyroclastics F e l s i c p y r o c l a s t i c s are the most abundant rock type i n the area, although t h i s i s a function of exposure rather than deposition. This u n i t forms a dip slope that underlies much of the central map area. Thickness of the u n i t i s v a r i a b l e . In the east-central part of the study area, maximum thickness could be i n excess of 400m. At the north and south ends of the study area, where both the top and bottom of the u n i t i s exposed, i t i s usually l e s s than 150m t h i c k . The u n i t i s primarily composed of l a p i l l i t u f f s with l e s s e r l i t h i c ash t u f f s and breccias. Fragments are usually angular to sub-rounded, poorly to moderately well-sorted and - 58 -of s i m i l a r l i t h o l o g i e s . In contrast to the mafic p y r o c l a s t i c s , reworked material i s minimal. Most of the outcrops with v i s i b l e fragmental textures are c l a s t supported (Plate 3.1a). S t r a t i f i c a t i o n i s not often v i s i b l e but a crude layering defined by fragment character (size, colour or type) occurs on some exposures. Fragmental rocks commonly grade l a t e r a l l y , and more r a r e l y v e r t i c a l l y , into quartz-sericite-carbonate s c h i s t s . This i s attributed p r i m a r i l y to a l t e r a t i o n but could also correspond to changes i n fragment s i z e and/or composition. The f e l s i c p y r o c l a s t i c u n i t i s overlain by the s t r u c t u r a l l y competent and massive greenstone u n i t . This contact i s usually sharp and undulating. In the central map area the greenstone i s separated from the f e l s i c rocks by a t h i n layer (3 to 20m) of q u a r t z - s e r i c i t e mylonite. This mylonite zone was probably produced by l o c a l i z a t i o n of s t r a i n within the l e s s competent f e l s i c rocks during f o l d i n g . Hydrothermal a l t e r a t i o n p r i o r to onset of deformation may have contributed to t h i s process by decreasing competency of the f e l s i c rocks even further. Other areas of the f e l s i c u n i t may also have been affected i n t h i s manner. The lower contact of the f e l s i c p y r o c l a s t i c unit with the mafic p y r o c l a s t i c u n i t i s usually gradational or i n t e r d i g i t a t i n g . This t r a n s i t i o n a l area commonly hosts - 59 -minor sulphide occurrences that range from zones of disseminated p y r i t e and chalcopyrite to small lenses of massive p y r i t e , chalcopyrite and sphalerite. Sulphide-rich fragments occur sporadically throughout the f e l s i c p y r o c l a s t i c s (Plate 3.7d). Mineralogy of the f e l s i c fragmentals consists of quartz, muscovite, carbonate, a l b i t e , minor c h l o r i t e , epidote, orthoclase and opaques. Orthoclase i s often fringed with s e r i c i t e and carbonate. With the exception of some l a t e carbonate microveins and large quartz grains, grain s i z e i s very f i n e . Fragments are commonly r e - c r y s t a l l i z e d into f i n e masses of quartz, s e r i c i t e , carbonate and indistinguishable oxides and s i l i c a t e s . Chemistry of the f e l s i c rocks i s not well understood as almost a l l of the samples analyzed have undergone moderate to intense a l t e r a t i o n . These samples are characterized by high s i l i c a and potassium contents with low iron, sodium and magnesium concentrations (section 3.6). 3.2.5 Greenstone Unit The greenstone unit forms topographically prominent kn o l l s and ridges over most of the study area, lar g e l y owing to i t s greater resistance to erosion. Thickness of the unit ranges from 20 to 140m but seldom are both the upper and - 60 -lower contacts exposed. The greenstone forms a t h i n veneer over the central part of the map area with topographical v a r i a t i o n s exposing the underlying f e l s i c units and causing i r r e g u l a r map patterns. Immediately north of Wishbone Gla c i e r (Figure 3 . 3 ) , the greenstone i s c h a r a c t e r i s t i c a l l y c l i f f - f o r m i n g , caused by near v e r t i c a l j o i n t s and g l a c i a l erosion. For the most part, the greenstone appears to be conformable, but i t could cross-cut stratigraphy at a shallow angle i n the extreme north and south ends of the map area. Textures within the greenstone range from fine-grained, weakly f o l i a t e d along the margins to coarse-grained hypidiomorphic near the center. In t h i n section, i t i s composed of coarse-grained, p o r p h y r i t i c a l b i t e i n a matrix of c h l o r i t e , epidote, a l b i t e , carbonate and iron-titanium oxides. Mafic phenocrysts, replaced by c h l o r i t e , are too d i s t o r t e d to d i s t i n g u i s h psuedomorphed hornblende from pyroxene. B i o t i t e i s r a r e l y observed i n t h i n section. In one section round masses of c h l o r i t e and opaques pseudomorph what appears to have been o l i v i n e . Coarse a l b i t e c r y s t a l s are t y p i c a l l y fractured, non-aligned and i n c i p i e n t l y altered to very f i n e grained masses of s e r i c i t e and carbonate, which indicates that they are not porphyroblastic. The o r i g i n of the greenstone u n i t i s ambiguous. Fine-grained massive outcrops and grain s i z e changes over 4 to 5m - 61 -i n t e r v a l s are suggestive of basalt flows. Elsewhere, the coarsely c r y s t a l l i n e texture suggests an i n t r u s i v e o r i g i n . On g l a c i a l l y polished outcrops, i n the west central part of the map area, subtle but d i s t i n c t layering defined by changes i n the quantity and grain s i z e of a l b i t e and ilmenite suggest a layered s i l l (Plates 3.2d and e). Chemical analyses of t h i s u n i t display an extensive range of compositions attributed to both hydrothermal a l t e r a t i o n and o r i g i n a l inhomogeneity. T i ° 2 values range from 0.62 to 6.9 percent (with consistent A l 2 ° 3 v a l u e s ) ' t h i s i s more pl a u s i b l y explained by c r y s t a l s e t t l i n g than by magmatic changes within c o r r e l a t i v e basalt flows. Chemical s i m i l a r i t y among the greenstone, the mafic p y r o c l a s t i c s , and the c h l o r i t e s c h i s t (section 3.6) suggests a genetic l i n k . I t i s probable that the greenstone represents a gabbroic s i l l that i s comagmatic with the mafic extrusive rocks. 3.2.6 A l t e r a t i o n Units Two a l t e r a t i o n assemblages were mapped as separate units? i n part to emphasize them because of t h e i r possible s i g n i f i c a n c e to mineral exploration, but also because i n many locations t h e i r p r o t o l i t h was u n i d e n t i f i a b l e . The assemblages are quartz-muscovite-carbonate and ferruginous carbonate-quartz-hematite. The former i s widespread and volumetrically s i g n i f i c a n t whereas the l a t e r i s r e s t r i c t e d to narrow, s t r u c t u r a l l y controlled zones. - 62 -Quartz-muscovite-carbonate s c h i s t s are developed most intensely within the f e l s i c t u f f unit, however, they are developed l o c a l l y within a l l other units except the greenstone unit. They occur as conformable lenses from 1 to 60m t h i c k and as large, i r r e g u l a r cross-cutting zones, p r i m a r i l y i n the northern part of the study area (Fig. 3.2). On the south side of Wishbone g l a c i e r , which cuts across the north-central study area, a 200m high c l i f f exposes an equivalent thickness of quartz-muscovite-carbonate s c h i s t that i s overla i n by a t h i n layer of greenstone (Plate 3.2a). In t h i s area, remnants of unaltered rock, which are up to 30m square, can be seen. F o l i a t i o n i s consistent among the remnants, the altered s c h i s t s , and the unaltered rocks that bound the altered zone. Reaction rims around the unaltered remnants, which are defined by carbonate composition and r e l a t i v e abundance, are deformed into s i m i l a r s t y l e folds (Plate 3.1f) which suggests, although not unequivocally, that f o l d i n g post-dates the a l t e r a t i o n . Outcrops and hand specimens of the quartz-muscovite-carbonate s c h i s t are pale grey to creamy yellow with a d i s t i n c t i v e bleached appearance. Primary textures are usually o b l i t e r a t e d . Mineralogicaly, the s c h i s t s consist of quartz, muscovite, carbonate, a l b i t e , and minor p y r i t e and opaques. Relative proportions of quartz, carbonate and muscovite are variable, which i s l i k e l y a r e f l e c t i o n of the p r o t o l i t h composition. F e l s i c rocks form muscovite and - 6 3 -quartz-rich s c h i s t s that are pale grey to silvery-white and extremely f r i a b l e . (On steep slopes of t h i s material, footing can be treacherous.) The more mafic units are r i c h e r i n carbonate, more competant, and range i n colour from creamy yellow to pale orangey-brown. The compostion of the carbonate, as determined q u a l i t a t i v e l y using the energy dispersive system of a scanning electron microscope, i s highly v a r i a b l e , even within the same sample, but i s usually a form of iron-bearing dolomite. The maroon, mafic p y r o c l a s t i c rocks tend to form carbonate-quartz-muscovite s c h i s t s with bright green micaceous spots i n f e r r e d to be fuchsite. T y p i c a l l y , grain s i z e within a l l of the altered rocks i s very f i n e . Coarse grained bands of quartz and/or carbonate and porphyroblastic cubes of p y r i t e , up to 2cm, do occur l o c a l l y . Formation of the muscovite appears to pre-date the l a t e r phases of deformation as most t h i n sections display crenulated muscovite flakes or grains with minimal r e c r y s t a l i z a t i o n . Chemistry of t h i s u n i t i s d i s t i n c t i v e , and characterized by high s i l i c a and potash and low sodium, magnesium and ir o n . Ratios of T i 0 2 and Zr suggest that a l t e r a t i o n samples analyzed are l i t h o l o g i c a l l y d i s t i n c t from the mafic units (section 3.6). - 64 -Origin of t h i s a l t e r a t i o n i s uncertain. Some of the conformable lenses are s p a t i a l l y associated with zones of s i l i c i f i c a t i o n and are interpreted to have formed from hydrothermal a c t i v i t y related to mineralization. The large cross-cutting zones are more problematical. These zones pervade an area roughly 4 by 4km and extend to depths of greater than 200m. The a l t e r a t i o n appears to cut through the e n t i r e s t r a t i g r a p h i c sequence. Contacts between the alt e r e d and unaltered rock are commonly gradational but l o c a l l y , they can be sharp. Ferruginous carbonate a l t e r a t i o n zones form conspicuous rusty weathering areas along f a u l t s and fractures. Unlike the quartz-muscovite-carbonate a l t e r a t i o n the ferruginous carbonate i s best-developed within the greenstone unit f o r both s t r u c t u r a l and chemical reasons. The zones are t y p i f i e d by pervasive carbonate replacement of ferro-magnesium minerals, commonly accompanied by the conversion of ilmenite to r u t i l e (or anatase) and hematite. Contacts are t y p i c a l l y gradational. Carbonate textures are var i a b l e and range from fine-grained massive to open space f i l l i n g s of coarse-grained botryoidal, colloform ferroan c a l c i t e . These zones are discussed i n more d e t a i l i n Chapter 4. - 65 -3.3 Structure 3.3.1 Introduction Four phases of f o l d i n g are recognized within the S t i k i n e Assemblage rocks exposed within the study area. The degree and i n t e n s i t y of deformation within t h i s area i s anomalous r e l a t i v e to other exposures of Stikine Assemblage rocks within the Stikine Arch. Two phases of recumbent fo l d i n g are interpreted from minor structures and deformation f a b r i c s . These e a r l i e r phases were refolded into easterly trending kinks and chevron folds, and a gentle northward plunging a n t i c l i n e . Late T r i a s s i c sediments (Chapter 2), which o v e r l i e the Stikine Assemblage volcanics have undergone only a s i n g l e phase of folding and are v i r t u a l l y free of metamorphic e f f e c t s . The purpose of t h i s section i s to describe the s t r u c t u r a l c h a r a c t e r i s t i c s , as determined from f i e l d mapping, and interpret the relationships between deformation and metamorphism, a l t e r a t i o n and mineralization. The s t r u c t u r a l s t y l e of the study area i s then compared to that of other exposures of the S t i k i n e Assemblage within the S t i k i n e Arch area. Relative ages of the s t r u c t u r a l elements were determined from cleavage intersections and cleavage-bedding - 66 -relationships, or from f o l d i n g of l i n e a r structures. The lack of a well defined stratigraphy and s t r a t i g r a p h i c markers prevents a clear understanding of the e a r l i e r f o l d morphologies, however, t h e i r orientations r e l a t i v e to l a t e r structures were determined from stereonet analysis, and approximate f o l d s t y l e s i n f e r r e d from minor structures. 3.3.2 Structural Sequence A l l Paleozoic rocks within the map area have undergone moderate to intense deformation. A summary of the s t r u c t u r a l elements associated with each phase i s i n Table 3.1. Fold s t y l e s and orientations are summarized i n Figure 3.5. The e a r l i e s t structures (FI) are a northwesterly trending f o l d set, recognized by r e f o l d i n g of a x i a l planer cleavage, lineations and minor f o l d s . Map-scale f o l d hinges of t h i s phase are not observed, but a large recumbent f o l d with westerly closure i s interpretted. Morphology of t h i s f o l d set has been modified by a second f o l d set (F2) which i s superimposed upon i t . The second f o l d phase i s also recumbent with an interpretted closure to the east, below Mess Creek. Both phases are i s o c l i n a l i n s t y l e , with nearly coincident f o l d axes trending northwest and north-northwest, respectively. Penetrative a x i a l planer f o l i a t i o n (S2) associated with the second f o l d phase i s the dominant rock TABLE 3.1 Summary of Structural Elements of Stikine Assemblage Rocks, Mess Creek Area, Northwestern, B.C. PHASE FOLD STYLE PLANAR STRUCTURES LINEAR STRUCTURES MINERAL GROWTH ORIENTATION SQI Compositional l a y e r i n g / b e d d i n g S i m i l a r ; i s o c l i n a l S c a l e unknown Ro o t l e s s minor f o l d s observed S^: A x i a l p l a n a r f o l i a t i o n L^: I n t e r s e c t i o n between S Q and S 1 muscovite, c h l o r i t e , e p i d o t e , a l b i t e and others n o r t h w e s t e r l y S i m i l a r ; t i g h t to i s o c l i n a l Some r o o t l e s s minor f o l d s Sub-round t o angular hinge on minor f o l d s S2: A x i a l p l a n a r f o l i a t i o n L^: I n t e r s e c t i o n between S. and - m i n e r a l l i n e a t i o n s L ^ : I n t e r s e c t i o n between S. and S^; c r e n u l a t i o n cleavage and p e n c i l l i n e a t i o n s minor p o l y g o n a l i z a -t i o n o f f o l d e d sheet s i l i c a t e s carbonate growth n o r t h -n o r t h w e s t e r l y Chevron to Kink Band F o l d s , low amplitude open, p a r a l l e l f o l d s and warps Lg: Kink Band/Chevron f o l d axes east-west to n o r t h e a s t e r l y 4 P a r a l l e l and open AC j o i n t s o r i e n t e d F olds are t i g h t e r east-west to the west n o r t h e r l y - 68 -F-j Tight - Isoclinal (refolded?) Fold axes trended NE Axial planes dipped moderately NW Greenschist facies metamorphism Large scale Fold - Thrust structure Summary of fold styles and orientations within the Stikine Assemblage, Mess Creek area, northwestern B.C. - 6 9 -f a b r i c i n the area. Phase 3 folds range from kink bands and chevron folds to broad open warps. These folds are developed i n S2 cleavage and have variable trends, generally along an east-west o r i e n t a t i o n . They do not s i g n i f i c a n t l y a l t e r the geometry of mapped contacts. The fourth phase of f o l d i n g (F4) i s manifested as a large, open, assymmetrical, north trending antiform. There are no f a b r i c s associated with t h i s phase. Planar f a b r i c s become progressively steeper dipping away from the study area i n a southwesterly d i r e c t i o n . This could be caused by an increase i n amplitude and tightening of fourth phase folds i n a westerly d i r e c t i o n . 3.3.3 Stereonet analysis Structural data were plotted on stereonets by microcomputer. Both p l o t t i n g and contouring programs were written by Pigage (1986) following the algorithm of Woodsworth (1983). Data plotted include poles to f o l i a t i o n s , l i n e a t i o n s , and f o l d axes. D i s t i n c t i o n between f i r s t and second phase f o l d axes and mineral l i n e a t i o n s i n the f i e l d was not usually possible and therefore these data are plotted on a single diagram (Fig. 3.6), which shows a r e l a t i v e l y homogeneous data set with a maximum concentration of points at 155°/00°. - 70 -The weaker concentration at 300°/08° might be due to FI data. F3 f o l d axes have a maximum concentration at an azimuth of 075° with a 20° plunge. The plunge d i r e c t i o n i s a function of the dip of F2 f o l i a t i o n surfaces. A plo t of poles to f o l i a t i o n i s not p a r t i c u l a r l y informative (Fig. 3.6). I t shows the two limbs of the F4 antiform with considerable dispersion caused by F3 folds and warps. The e f f e c t s of the F3 folds on e a r l i e r data i s seen i n Figure 3.6d. This data was taken from a small area about 50m square. The average F3 f o l d axis plots at 075°/20° which i s v i r t u a l l y i d e n t i c a l to the f o l d axis calculated from the poles to f o l i a t i o n s . L2 and F2 f o l d axes l i e along a small c i r c l e , confirming that the F3 f o l d s t y l e i s p a r a l l e l (Ramsey, 1967). 3.3.4 Fold Sets and Deformation Fabrics Outlines of phase 1 and phase 2 folds were not observed, with the exception of minor folds, and most of the information about these phases i s derived from analysis of tectonic f a b r i c s . Because the major hinge areas of phase 1 and 2 folds were not known and phase 3 folds were small and pervasive, separation of the map area into domains of homogeneous st r u c t u r a l data was not possible. Figure 3.6 Contoured Stereonet Plots of Structural Data from the Mess Creek Area, Northwestern B.C. - 72 -Compositional layering can be found i n most units, but i t i s best developed and preserved i n the mafic p y r o c l a s t i c and e p i c l a s t i c unit, where i t i s equivalent to bedding. F o l i a t i o n i s usually p a r a l l e l to compositional layering, except i n the hinge region of minor f o l d s . Most outcrops display only a single f o l i a t i o n (S2), however i f the rock fractures along a primary bedding plane then SO/SI in t e r s e c t i o n l i n e a t i o n s (Ll) can be seen. In t h i n section, S2 can be seen to be a crenulation cleavage formed from SI (Plate 3.8). L l lineations generally i n t e r s e c t L2 l i n e a t i o n s (S0/S2) at angles between 5 and 20 degrees. The L l l i n e a t i o n s form st r a i g h t l i n e s as they cross over the hinge regions of F2 minor folds, i n d i c a t i n g that F2 folds are s i m i l a r and of type 2 (Ramsey, 1967), formed by inhomogeneous simple shear. I t i s d i f f i c u l t to discern phase 1 minor folds from phase 2 fo l d s i n outcrop as both occur as flattened, rootless i s o c l i n a l f o l d s . Where a f i r s t phase f o l d has been refolded by the second phase, the two phases appear to be approximately c o - a x i a l . Intersections of SI and S2 a x i a l planer cleavages occur i n the hinge areas of F2 minor folds and produce a coarse crenulation cleavage (S2) with attendant p e n c i l l i n e a t i o n s that are equivalent to L2 i n orientation. Fragments within the p y r o c l a s t i c units are generally flattened with the axis of elongation co-linear to either L l or L2. Large scale interference figures defined by bedding were not observed, but rare, fine lenses or fragments of f e l s i c ash within a mafic matrix - 73 -appear to be deformed into interference patterns (Plate 3.1h) which approximate a 31 type (Ramsey, 1967). This type of pattern forms where recumbent folds are refolded and the angle between the two f o l d axes i s small; consistent with previously described data. Figure 3.7 i l l u s t r a t e s the r e l a t i o n s h i p between the f i r s t and second phase fo l d s . L i t h o l o g i c a l contact geometry and the relationships between f o l i a t i o n s and bedding indicates that F l and. F2 f o l d s are large scale recumbent structures. The major hinge region of e i t h e r phase i s not recognized. Mapping suggests that the F2 hinge may l i e to the east of Mess Creek (see cross sections: Figure 3.4) The t h i r d phase of deformation i s characterized by kink bands, chevron fo l d s , and broad open warps. The former range i n s i z e from microscopic to a few metres, whereas the l a t e r range from 2 to 40m. On the eastern side of the F4 antiform, chevron and single kink band f o l d axes generally have northeasterly to east-northeasterly trends. Fold plunges are c o n t r o l l e d by the dip of F2 f o l i a t i o n surfaces. Folds plunge i n the opposite d i r e c t i o n on the western side of the antiform. Conjugate kink bands straddle an east-west trend. Kink bands and chevron folds are pervasive throughout the study area and represent shortening (Wiess, 1980) along a north-south d i r e c t i o n . - 74 -FIGURE 3.7 Schematic i l l u s t r a t i o n of superimposed fold i n g of the S t i k i n e Assemblage, Hess Creek area, northwestern, B.C. F^ and F_ are recumbent i s o c l i n a l f o l d phases with sub-parallel a x i a l planes and s l i g h t l y divergent f o l d axes. - 75 -The f i n a l phase of folding (F4) i s expressed as an open antiform plunging gently i n a north-northeast d i r e c t i o n . This f o l d has modified a l l previous structures. Limbs are planer to gently curving. The hinge region i s not exposed having been mostly eroded. Mylonitization of incompetent s e r i c i t e s c h i s t s immediately below the competent greenstone u n i t suggest that t h i s f o l d formed by f l e x u r a l s l i p . 3.3.5 Faults Rocks within the map area are affected by a multitude of f a u l t s and fractures. R e l a t i v e l y minor f a u l t s with l i m i t e d o f f s e t are ubiquitous. The greenstone unit i n the east-central study area i s segmented by orthogonal fractures which are are commonly open, up to 2m wide, and extend to depths of of up to 30m. These open fractures are presumed to be caused by i n s t a b i l i t y of the underlying quartz-carbonate-muscovite s c h i s t s . Potential for f o l i a t i o n p a r a l l e l thrust f a u l t s , p a r t i c u l a r l y i n attenuated f o l d hinges i s high. These f a u l t s are d i f f i c u l t to recognize during f i e l d mapping, e s p e c i a l l y i n areas of metasomatic a l t e r a t i o n ; consequently, substantial amounts of movement could occur along f a u l t s and be undetected. Faults of t h i s type are interpretted to cause the p a r t i a l repeats of folded stratigraphy i n the area north of Wishbone Glacier (Fig. 3.3). - 76 -Easterly to northeasterly trending, near v e r t i c a l f a u l t s and fractures are common throughout the map area. These f a u l t s , which commonly host mineralization, could be l o c a l i z e d by A-C j o i n t s related to fourth phase fo l d s . Displacement on these structures appears to be minimal. Northeasterly directed, steeply dipping normal and/or l e f t l a t e r a l s t r i k e s l i p f a u l t s with up to four kilometers of l a t e r a l displacement occur i n the western and northern parts of study area (Fig 2.2). Related structures may have shuffled s l i c e s of Paleozoic limestone into younger rocks along the Mess Creek v a l l e y (Souther, 1971). These same structures may also have been the locus of u p l i f t , bringing the S t i k i n e Assemblage rocks up through Mesozoic s t r a t a . 3.3.6 Structural Comparisons of the Study Area with other Exposures of the St i k i n e Assemblage Comparison of the s t r u c t u r a l s t y l e between the study area and nearby exposures of s t r a t i g r a p h i c a l l y s i m i l a r rocks reveals some marked differences. The most pronounced difference i s i n the development of deformational f a b r i c s . S t i k i n e Assemblage rocks immediately east of Mess Creek and rocks south of Spahler Creek (Fig. 2.2) display a single, bedding p a r a l l e l f o l i a t i o n . Primary textures within these rocks are well preserved. Demonstrable Permian aged - 7 7 -v o l c a n i c l a s t i c rocks along the north side of Sphaler Creek, 12km southeast of Mess Creek, also show a single penetrative cleavage, usually p a r a l l e l to bedding. A l l of these rocks contain mineral assemblages compatible with greenschist f a c i e s metamorphism. Differences i n deformation s t y l e are exhibited by S t i k i n e Assemblage rocks exposed i n u p l i f t s throughout the S t i k i n e Arch. Mississippian limestone near the Scud River, 15km northwest of the study area, shows l i t t l e evidence of deformation (Pitcher, 1961). However, possible Permian age limestones, along the northwest edge of the Stikine Arch, are deformed into i s o c l i n a l and d i a p i r i c structures with northerly trending axes (Souther, 1971). Nelson and Payne (1984) describe north-northwesterly trending t i g h t to n e a r - i s o c l i n a l folds within four fault-bounded blocks of S t i k i n e Assemblage rocks near the confluence of the Taku and Tulsequah r i v e r s , on the west-central edge of the Stikine Arch. Stratigraphic continuity e x i s t s within the blocks but not between them. The east-central block hosts limestones which contain mid-Pennsylvanian f u s i l i n i d s (Nelson and Payne, 1984). A l l of the blocks are metamorphosed to lower greenschist f a c i e s . Deformation of the northernmost block, which contains a l i t h o l o g i c a l s u i t e that i s s i m i l a r to that of the study a r e a — i n c l u d i n g q u a r t z - s e r i c i t e schists, i s more intense than the other blocks (J. Nelson, pers.comm., - 78 -1985). The Polaris-Taku g o l d - s i l v e r and Tulsquah Chief massive sulphide deposits occur within these rocks. 3.3.7 Timing of Deformation Timing of deformation i s c r i t i c a l to understanding the tectonic h i s t o r y of the study area. The r e l a t i v e timing of deformation to metamorphism and a l t e r a t i o n can be determined from the analysis of rock f a b r i c s . Stratigraphic r e l a t i o n s h i p s and radiometric dates allow temporal constraints to be placed on some of the def ormational events. Micro-fabrics within deformed rocks indicate that metamorphic minerals were formed during the f i r s t and the earl y part of the second deformational events (section 3.4.3). Evidence of s t r a i n within synkinematic c r y s t a l s indicate that the l a t e r phases of deformation outlasted thermal e f f e c t s . The large areas of quartz-muscovite-carbonate a l t e r a t i o n formed p r i o r to the second phase of fo l d i n g as f o l i a t i o n related to t h i s phase i s commonly formed from crenulated muscovite grains. Localized f o l d i n g of altered-unaltered rock contacts suggests that at lea s t some of the a l t e r a t i o n occurred p r i o r to fold i n g . - 79 -A potassium-argon date on chrome-bearing muscovite from the selvages of a carbonate-sulphide vein yielded an age of 192+7 Ma. This vein, and others l i k e i t , are undeformed and emplaced i n fractures (possibly A-C j o i n t s related to fourth phase folds) that cut a l l deformation f a b r i c s and kinkbands i n d i c a t i n g that Early Jurassic mineralization post-dates a l l but the l a s t phase of deformation. Upper T r i a s s i c (Norian) sediments that o v e r l i e intensely deformed Paleozoic rocks display large amplitude, easterly oriented chevron folds and broad, open, north trending f o l d s . Therefore, these rocks were l i k e l y deposited, or f a u l t emplaced, p r i o r to the t h i r d and fourth phase of fol d i n g but a f t e r the early phases of i s o c l i n a l f o l d i n g . Absolute timing of the early phases of deformation i s more d i f f i c u l t . Differences i n the s t r u c t u r a l s t y l e among exposures of Paleozoic rocks within the St i k i n e Arch may be att r i b u t e d to one or more of the following: (a) Location—deformation may be l o c a l i z e d by proximity to a major structure or intr u s i v e complex. (b) L i t h o l o g y — a l t e r e d , or otherwise l e s s competant rocks w i l l become more strained than the competant rocks which surround them. - 80 -(c) Age—pre-Permian deformation resulted i n an additional f o l d phase within the older rocks. Evidence for a pre-Permian orogenic event i s s l i g h t . Monger (1977) has documented the disappearance of at lea s t 1,500m of Mississippian s t r a t a over a 5km long trend j u s t south of the study area, which suggests a profound pre-Lower Permian unconformity—although the re l a t i o n s h i p i s complicated by f a u l t s . I f t h i s was an erosional unconformity, the products of that erosion are not observed within the rock record. Although the volcanic rocks within Permian stratigraphy are s i m i l a r to those within the Mississippian sections, the associated limestones are quite d i f f e r e n t . The Mississippian limestones are t y p i c a l of those deposited on seamounts whereas the Permian limestones are platformal i n character (Monger, 1977). This change i n depositional s i t e s does not imply an orogeny so much as allow f o r one. The various exposures of the St i k i n e Assemblage have d i f f e r i n g s t r u c t u r a l s t y l e s , but appear to have shared the same metamorphic event. The r e l a t i o n s h i p between f o l d textures and metamorphic mineral growth suggests, although not unequivocally, that a single episode of dynamothermal metamorphism took place during or a f t e r Permian time, but p r i o r to Upper T r i a s s i c time. Souther (1971) defined the Tahltanian orogeny as a period of u p l i f t , folding, metamorphism and intrusion that took place between Middle and Upper T r i a s s i c time. This - 81 -orogeny accounts for the depositional hiatus i n early to mid-Triassic time that i s recognized throughout most of west-central B r i t i s h Cloumbia (Souther, 1971; White, 1959; Read and Okulitch, 1977). Deformational h i s t o r y of the study area supports, at the very l e a s t , the upper time boundary fo r t h i s orogenic event. The Tahltanian Orogeny appears to be confined to the Stikine Terrane (Monger, 1977; Read and Okulitch, 1977) predominately i n the western part and could be related to the amalgamation of blocks which form the terrane. 3.4 Metamorphism Paleozoic rocks within the study area have a complex h i s t o r y of mineralogical and textural changes incurred by hydrothermal metasomatism, metamorphism and deformation. Relative timing of these changes i s not well understood. Extensive, possibly syngenetic, potassium metasomatism i s overprinted by pre-Upper T r i a s s i c regional dynamothermal metamorphism of lower greenschist f a c i e s . Deformation outlasted peak metamorphism and metamorphic textures are l a r g e l y overprinted by s t r a i n features. Locally, prograde and retrograde e f f e c t s have been superimposed upon regional metamorphism by Early J u r a s s i c hydrothermal a l t e r a t i o n . - 82 -PLATE 3.1 a) R h y o l i t e b r e c c i a o f f e l s i c p y r o c l a s t i c u n i t . Note e l o n a t i o n o f c l a s t s p a r a l l e l t o f o l i a t i o n . b) T h i n bedded r o c k s o f t h e m a f i c p y r o c l a s t i c / e p i c l a s t i c u n i t . Beds v a r y from f i n e - g r a i n e d c h l o r i t e s c h i s t s t o l a p i l l i t u f f s . c) C l o s e up photograph o f t h e m a f i c p y r o c l a s t i c / e p i c l a s t i c u n i t showing a l a p i l l i t u f f l a y e r . V e n t i c u l a r , h e t e r o l i t h i c fragments s i t i n a c h l o r i t e - e p i d o t e m a t r i x . Note p r e s e r v e d v e s c u l a r fragment i n c e n t e r o f photo , p o s s i b l y r e p r e s e n t i n g a s m a l l b a s a l t i c bomb. -d) E p i v o l c a n i c l a s t i c b r e c c i a t r a n s i t i o n a l from m a f i c p y r o c l a s t i c t o f e l s i c p y r o c l a s t i c u n i t . The l a r g e fragment i s i t s e l f f r a gmenta l i n d i c a t i n g rework ing o f v o l c a n i c m a t e r i a l . Fragment shape which c o u l d r e p r e s e n t an i n t e r f e r e n c e f i g u r e produced by po lyphase f o l d i n g . Lens cap i n bottom l e f t c o r n e r i s 55mm i n d i amete r . e) Coarse c r e n u l a t i o n c l eavage ( m i c r o l i t h o n s t r u c t u r e ) i n f e l s i c p y r o c l a s t i c u n i t . f ) U n a l t e r e d remnant o f m a f i c e p i c l a s t i c m a t e r i a l i n q u a r t z - m u s c o v i t e - c a r b o n a t e s c h i s t . R e a c t i o n h a l o e s around t h e remnant a r e more c a r b o n a t e - r i c h and appear t o be i s o c l i n a l l y f o l d e d . g) G r e e n s t o n e - f e l s i c p y r o c l a s t i c c o n t a c t . Contac t i s sharp and u n d u l a t i n g . F e l s i c u n i t i s u n a l t e r e d but d i s p l a y s s m a l l - s c a l e i s o c l i n a l f o l d s i n e a r l i e r f o l i a t i o n w i t h a x i a l p l a n e s p a r a l l e l t o the g reens tone c o n t a c t . h) Ash fragment i n m a f i c e p i c l a s t i c u n i t which d i s p l a y s a t ype 31 (Ramsey, 1967) i n t e r f e r e n c e p a t t e r n i n d i c a t i n g two phases o f t i g h t f o l d i n g w i t h s m a l l ang le s between f o l d axes . V e r t i c a l s t r u c t u r e s a re caused by l i g h t r e f l e c t i n g from water f l o w i n g over the o u t c r o p . PLATE 3.1 - 8 4 -P L A T E 3 . 2 a) View across Wishbone Glacier showing c l i f f face composed of quartz-muscovite-carbonate s c h i s t overlain by a t h i n layer of greenstone. The appearance of a f l a t - l y i n g large i s o c l i n a l f o l d i s p a r t l y caused by t a l u s along the base of the c l i f f and an actual f o l d structure could not be substantiated. The greenstone-s e r i c i t e s c h i s t contact displays small-scale folds on the r i g h t hand side of the photograph. b) View looking northwest across the Wishbone Gl a c i e r showing large zone of s e r i c i t i c a l t e r a t i o n . Area i n center of photograph i s landslide debris. Note the stratabound nature of intense s e r i c i t i c a l t e r a t i o n between mafic p y r o c l a s t i c unit and carbonate altered greenstone (orange) i n l e f t center of photograph. c) Laminated c h l o r i t e s c h i s t unit with i s o c l i n a l folded quartz and c a l c i t e layers. Orange coloured area r e f l e c t s l a t e oxidation feature. d) Greenstone unit showing laminar texture defined by concentrations of o l i v i n e and ilmenite. e) Mafic p y r o c l a s t i c / e p i c l a s t i c u n i t showing t h i n beds of e p i c l a s t i c material with interbed layers of carbonate (pale brown layer 5cm above the top of the hammer handle). f) Weathered surface of the greenstone u n i t showing subtle composition changes defined by concentration of plagioclase c r y s t a l s . PI A T F 3 ? - 86 -PLATE 3.3 a) Unmetamorphosed, f o s s i l i f e r o u s Late T r i a s s i c (Norian) a r g i l l i t e and chert on western edge of the study area. Granite boulder conglomerates forms the c l i f f outcrop i n the upper r i g h t corner of the photograph. b) Laminated ash t u f f and argillaceous t u f f displaying i s o c l i n a l folds (F 1) and conjugate kink bands ( F 3 ) . c) Shallow, northwesterly plunging minor f o l d nose(s) i n f e l s i c t u f f - s e r i c i t i c a l t e r a t i o n units. F i r s t f o l d phase f o l i a t i o n (S 1) has been re-folded producing a "pencil cleavage". d) L_ l i n e a t i o n s , produced by in t e r s e c t i o n of two sets of a x i a l planar cleavage (S. and S_), i n f e l s i c t u f f u n i t . Kink bands are related to t h i r d phase of deformation and Fe-carbonate f i l l e d fractures may represent A-C j o i n t s r e l a t e d to the fourth phase of deformation. e) Chevron f o l d i n g (F_) i n strongly f o l i a t e d f e l s i c crystal-ash t u f f . Quartz blob with minor p y r i t e i s t y p i c a l of material i n f o l i a t i o n - p a r a l l e l quartz veins. f) Fe-carbonate impregnated volcanic bombs (see Plate 3.1c) i n c h l o r i t e s c h i s t u n i t . Pen i s 14cm long. g) Re-folded f o l i a t i o n , producing coarse crenulation cleavage that has been re-folded into a chevron-type f o l d by t h i r d phase of deformation. PLATE 3.3 - 88 -PLATE 3-4 Photomicrographs of S t i k i n e Assemblage Rocks, Mess Creek Area, Northwestern, B.C. a) Carbonate-sericite-rich fragment, showing pervasive second phase f o l i a t i o n , i n f e l s i c t u f f u n i t . Note s t r a i n shadows around rounded p y r i t e grain i n the upper l e f t central part of the photograph. Scale i s the same as Plate b. b) Pervasive S 2 f o l i a t i o n (lower l e f t to upper right) at a small angle to compositional layering. Quartz-albite-carbonate-rich layer (lower) i n contact with a sericite-chlorite-opaque-rich layer (upper). c) S e r i c i t e , c h l o r i t e , opaques, quartz, a l b i t e , carbonate and epidote i n a f e l s i c ash t u f f . Quartz-carbonate-r i c h layer i s p a r a l l e l to f o l i a t i o n . Scale i s the same as i n Plate b. d) Carbonate porphyroblasts(?) and very f i n e quartz-rich e l i p t i c a l fragments i n f e l s i c ash t u f f . Scale i s the same as i n Plate b. e) Quartz and a l b i t e phenocrysts and l i t h i c fragments i n a c r y s t a l - l i t h i c t u f f . Matrix i s composed of c h l o r i t e , s e r i c i t e , quartz, carbonate and opaques. f) Quartz-albite v e i n l e t i n a mass of an t i g o r i t e from shear zone i n altered greenstone unit. g) A l b i t e c r y s t a l s separated by rims of c h l o r i t e , s e r i c i t e , b i o t i t e and opaques i n greenstone unit. h) Sprigs of tremolite on the edge of r e l i c o l i v i n e grain i n greenstone unit. O l i v i n e i s mostly replaced by c h l o r i t e , quartz and opaques. Matrix i s predominantly c h l o r i t e . PLATE 3.4 - 90 -PLATE 3.5 Photomicrographs of Deformation Textures i n S t i k i n e Assemblage Rocks, Mess Creek Area, Northwestern, B.C. a) Crenulation cleavage developed i n s e r i c i t e (talc?) r i c h layer i n quartz-sericite-carbonate s c h i s t . Cleavage displays minor o f f s e t s . Same sacle as that of Plate b. b) Crenulated muscovite-quartz-carbonate s c h i s t . Crenula-t i o n cleavage i s preserved i n muscovite whereas quartz-r i c h domains have r e c r y s t a l l i z e d . Scale bar should read 0.1mm. c) Micro kink bands (F_) i n S 2 cleavage of quartz-muscovite-carbonate s c h i s t . d) Same rock as i n Plate c but at greater magnification. S. f o l i a t i o n i s sub-vertical, S_ crenulation cleavage i s diagonal (lower r i g h t to upper l e f t ) . S_ cleavage traces defined by residue of insoluble opaque material. Scale bar should read 0.1 mm. e) I s o c l i n a l micro-fold (F 2) of quartz-rich layer i n ser i c i t e - q u a r t z s c h i s t . Scale i s the same as Plate c. f) Same specimen as Plate e, showing well-defined a x i a l planar crenulation cleavage ( S 2 ) . Scale i s the same as Plate c. g) Broad spaced crenulation cleavage i n c h l o r i t e s e r i c i t e s c h i s t . Scale i s the same as that i n Plate c. h) Crenulation with cleavage d i s c o n t i n u i t i e s i n c r y s t a l ash t u f f . O r i g i n a l (S.) f o l i a t i o n trends diagonally across photograph (lower l e f t to upper r i g h t ) . An F. f o l d closure appears to be present i n the center of the photograph. S_ cleavage defined by t h i n black l i n e s (insoluble opaques); some with minor o f f s e t s . Scale i s the same as Plate c. PLATE 3.5 - 92 -3.4.1 Mineralogy-Mineral assemblages were determined primarily from t h i n and polished t h i n section analysis. V e r i f i c a t i o n of mineral phases was done with the energy dispersive system of a scanning electron microscope or X-ray d i f f r a c t i o n where appropriate. Many of the t h i n sections were of specimens so fi n e grained that only the main mineral phases could be i d e n t i f i e d o p t i c a l l y . The predominant mineral assemblages and t h e i r interpreted precursors are i n Table 3.2. The greenstone and mafic p y r o c l a s t i c / e p i c l a s t i c units are mainly composed of c h l o r i t e , epidote, a l b i t e , carbonate and opaques. Tremolite was only observed within specimens that previously contained o l i v i n e , and i n a sing l e specimen of the c h l o r i t e s c h i s t u n i t . B i o t i t e i s rare. Coarse p l a g i c l a s e phenocrysts within the greenstone are psuedomorphed by a l b i t e . The core areas of these phenocrysts are t y p i c a l l y f i n e masses of s e r i c i t e , carbonate and epidote. Talc may be present but i s d i f f i c u l t to d i s t i n g u i s h from muscovite i n t h i n section. The c h l o r i t e s c h i s t u n i t l i k e l y contained primary carbonate but other pre-metamorphic minerals can only be inferred. The metamorphic mineral assemblage includes Mg-rich c h l o r i t e ( i n d e n t i f i e d by b e r l i n blue interference colour), epidote, a l b i t e , quartz, carbonate and rare tremolite. TABLE 3.2 Summary of Mineral Changes due to Metamorphism, Stikine Assemblage Rocks, Mess Creek Area, Northwestern, B.C. METAMORPHIC MINERALS PRIMARY MAFIC FELSIC MINERALS GREENSTONE TUFFS TUFFS ARGILLITE TEXTURES OF SECONDARY MINERALS ( i n f e r r e d ) O l i v i n e H o r n b l e n d e / A u g i t e P l a g i c l a s e O r t h o c l a s e G r a p h i t e Quartz K e o l i n i t e ^ " C a l c i t e T r e m o l i t e C h l o r i t e M a g n e t i t e A n t i g o r i t e C h l o r i t e B i o t i t e Carbonate A l b i t e C h l o r i t e E p i d o t e Carbonate Pseudomorphs Quartz Carbonate C h l o r i t e T r e m o l i t e Carbonate A l b i t e C h l o r i t e E p i d o t e Carbonate Quartz M u s c o v i t e Carbonate C h l o r i t e A l b i t e M u s covite Carbonate M u s c o v i t e Quartz Quartz M u s c o v i t e Carbonate A l b i t e M u s c o v i t e Carbonate M u s c o v i t e Quartz G r a p h i t e Carbonate Quartz Carbonate Pseudomorphs, medium-grained e u h e r a l a g g r e g a t e s , f i n e - g r a i n e d i r r e g u l a r masses Pseudomorphs, f i n e - g r a i n e d i r r e g u l a r masses I r r e g u l a r masses A c c i c u l a r c r y s t a l s I r r e g u l a r masses G e n e r a l l y , r e d u c e d g r a i n s i z e F i n e s h e a t h - l i k e a g g r e g a t e s F i n e t o c o a r s e a n h e d r a l g r a i n s 1. See S e c t i o n 4.3 - 94 -The f e l s i c t u f f u n i t i s characterized by an assemblage of quartz, muscovite, carbonate, a l b i t e and opaques. C h l o r i t e and epidote are l o c a l l y present. The quartz-muscovite-carbonate assemblage l i k e l y r e f l e c t s potassium metasomatism (section 4.3), which may have been a k a o l i n i t e r i c h assemblage before being metamorphosed. Mineral textures, p a r t i c u l a r l y of muscovite and c h l o r i t e , are inconclusive i n e s t a b l i s h i n g the r e l a t i v e timing of a l t e r a t i o n and metamorphism. However, v i r t u a l l y . a l l muscovite and c h l o r i t e i s crenulated by the second phase of f o l d i n g and therefore both a l t e r a t i o n and metamorphism preceeded t h i s event. Argillaceous s c h i s t s consist of quartz, muscovite, carbonate, graphite and minor opaques (pyrite +/- hematite), a l b i t e , c h l o r i t e and epidote. 3.4.2 Conditions of Metamorphism The main metamorphic mineral assemblages are displayed on ACF and AKF diagrams (Fig. 3.8; Miyashiro, 1973), and indicate that rocks of the Stikine Assemblage have been metamorphosed within the quartz-albite-epidote-biotite subfacies of the greenschist fa c i e s (Greenwood, 1976). This i s equivalent to the albite-epidote-amphibolite facies of Turner (1968). The lower temperature boundary for t h i s - 95 -Q calcite dolomite Figure 3.8 Mineral assemblages which define metamorphic grade of the Stikine Assemblage rocks, Mess Creek area, northwestern B.C. projected on ACF, AKF and QCM diagrams. Shaded areas show the common mineral assemblage of the quartz—albite-epidote-biotite subfacies of the greenschist facies (Greenwood, 1976) Stipled areas show the rare assemblage containing tremolite. - 96 -f a c i e s i s 300°C and i s recognized by the absence of z e o l i t e s (Winkler, 1974). The upper boundary, i n the order of 450°C, i s usually indicated by the reaction: c h l o r i t e + muscovite + quartz = b i o t i t e + almandine + H 20. The albite-epidote assemblage i s stable over the range from 1 to 7 kilobars (Winkler, 1974). Composition and mineral assemblages of the c h l o r i t e s c h i s t u n i t are best displayed on the QCM diagram (Fig. 3.8). The occurence of t a l c was suspected from f i e l d mapping but could not be confirmed by X-ray d i f f r a c t i o n techniques, consequently t h i s phase i s omitted from the QCM diagram. However, an equilibrium assemblage of quartz-tremolite-carbonate was observed i n t h i n sections (H. Greenwood, pers. comm., 1982). I f tremolite formed from e i t h e r of the reactions: 5 t a l c + 6 c a l c i t e + 4 quartz = 3 tremolite + 6 C0 2 + 2 H 20. or 1 t a l c + 2 dolomite + 4 quartz = tremolite + 4 C0 2 then a minimum temperature f o r a given P and X(C0 2) can be determined from an isobaive T-X(C0 2) diagram (Fig. 3.9a). An upper temperature constraint i s provided by the reaction: 3 c a l c i t e + 2 quartz + tremolite = 5 diopside + 3 C0 2 + H 20. The absence of both t a l c and diopside, represented by the s t i p l e d area i n Figure 3.9b, indicates a temperature range of 420° to 530° for X(C0 ) between 0.1 and 0.9. - 97 -Buffering of the pore f l u i d by metamorphic reactions within c a l c - s i l i c a t e rocks drive the f l u i d towards increasing X(C0 2) (Fig. 3.9c) and suggests a possible mechanism fo r generating C0>2 r i c h f l u i d s during prograde metamorphism. 3.4.3 Timing of Metamophism Textural evidence suggests that metamophism took place during the early stages of deformation but had greatly diminished during l a t e r stages. Mineral growth was larg e l y synkinematic to formation of early f o l i a t i o n . A pronouced crenulation cleavage crumpled e a r l i e r s i l i c a t e s with only minor polygonalization. Deformation twins within carbonates and undulatory extinc t i o n (progressive subgrains) within quartz are ubiquitous. These textures generally disappear with s l i g h t heating (D. Murphy, pers. comm., 1984) and therefore deformation must have outlasted thermal e f f e c t s . 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 X(C02) at P = 2000 bars REACTION LIST isemblages on the l e f t are s t a b l e on the: high s i d e of the Y-axis v a r i a b l e , or the high s i d e of the X-axis v a r i a b l e f o r v e r t i c a l r e a c t i o n s 1) : 2 C02 + D i = Do + 2 aQz 2) : H20 + 3 C02 + S D i = 3 CCss + 2 aQz + Tr 3) : Wo + C02 » aQz + CCss 4) : 3 C02 + Tc + 3 CCss = 3 Do + 4 aQz + H20 5) : 7 C02 + Tr + 3 CCss - 5 Do + 8 aQz + H20 6) : 4 Di + Do + C02 + H20 = Tr + 3 CCss 7) i 2 H20 + 6 C02 + 3 Tr - 6 CCss + 4 aQz + 5 Tc 8) : 4 C02 + Tr - 2 Do + 4 aQz + Tc 9) : H20 + C02 + Tr + Do = 3 CCss + 2 Tc ID CO O i l 0 . J 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ° - 1 ° . 2 0.3 0 > 4 0 .5 0.6 0.7 0.8 0.9 B X(C02) at P = 2000 bars C XCC02) at P = 2000 bars Figure 3.9A. T - X C Q 2 diagram for the system CaO-SiC^-COg-HgO at P*2 Kbars from GEOCALC (Perkins et. a I., 1986). B. Shaded.area indicates the stability field for the quartz-calcite-tremolite + dolomite assemblage with diopside and talc absent. C. Arrows show reaction path towards increasing X Q Q ^ if reactions buffer the pore fluid during prograde metamorphism. - 99 -3.5 Age of the Stikine Assemblage Precise age of the Stikine Assemblage rocks within the study area i s uncertain. Correlation of Stikine Assemblage rocks within the Stikine Arch i s based on f o s s i l i f e r o u s Permian and Mississippian limestones. However, other ages also e x i s t ; f o r example, i n the Tulsequah map sheet area, Middle Pennsylvanian f u s i l i n i d s have been recovered from limestones within rocks that are l i t h o l o g i c a l l y s i m i l a r to those of the study area (Nelson and Payne, 1984) . Argillaceous limestones adjacent to the Forrest Kerr River, 30km southeast of the map area, contain Lower Devonian conodonts (P. Read, pers. comm., 1983). Stratigraphic evidence indicates that a Mississippian age i s most probable for rocks within the thesis area. Other methods for age determination were inconclusive with regards to absolute age. Attempts to obtain zircons and conodonts, from the appropriate l i t h o l o g i e s , were unsuccessful. Isotopic dating methods indicate Early J u r a s s i c ages f o r a l t e r a t i o n and mineralization. A - 100 -whole-rock rubidium-strontium isochron on altered and unaltered rocks gives a date of 201+16 Ma. A potassium-argon date of 193±7 Ma was obtained from muscovite and fuchsite taken from the selvage of a carbonate-sulphide vein. 3.5.1 Stratigraphic Evidence Stratigraphy of the thesis area has been assigned a Mississippian age on the basis of s t r a t i g r a p h i c continuity with an overlying, f o s s i l i f e r o u s limestone (Souther, 1972; Monger, 1970) located 1km to the west. Calcareous t u f f s with Permian f o s s i l s o v e r l i e the Mississippian limestone immediately to the west (Souther, 1972). I f t h i s contact i s at l e a s t approximately conformable then d i r e c t i o n of younging to the west can be assumed. Permian limestones also occur to the east of the study area, along the east side of Mess Creek, maintaining the appropriate s t r a t i g r a p h i c relationships i n accordance with the north-trending a n t i c l i n a l structure centered within the map area. Limestones, 12km southeast of the head waters of Mess Creek, contain Mississippian (Visean to Namurrian) f o s s i l s (R. Anderson, pers. comm., 1988) supporting t h i s age assignment for the study area. - 101 -3.5.2 Isotopic Dating Methods Thirteen whole rock rubidium-strontium determinations were c a r r i e d out on alte r e d and unaltered rocks c o l l e c t e d from the study area (Table 3.3). A single potassium-argon date was obtained from chromium muscovite associated with vein s t y l e mineralization (Table 2.1). A l l age determinations were performed i n the geochronology labs at U.B.C. according to the methods described i n Thorstad (1983) . Ty p i c a l l y , Rb-Sr determinations on mineral separates can be used to determine metamorphic ages, while whole rock determinations give the o r i g i n a l age of the rock. On highly altered or metasomatized rocks, as i s the case here, whole rock Rb-Sr determinations give the a l t e r a t i o n age, r e f l e c t i n g p a r i t a l to complete r e - e q u i l i b r a t i o n of Sr: Sr r a t i o s . An isochron calculated by the York (1976) technique of l e a s t squares regression gives an age of 201+16 Ma (Fig. 3.10). Potassium-argon determinations on chromium muscovite gives a date of 192±7 Ma., in d i c a t i n g an Early Jurassic age for mineralization. Consistency between the Rb-Sr and the K-Ar ages suggests that the a l t e r a t i o n and mineralization 87 86 are related. The samples with high Rb/ Sr exert the most control on the slope of the isochron and thus the assigned - 1 0 2 -age. These samples were co l l e c t e d i n the v i c i n i t y of the T e l l u r i d e vein (section 4.2.1) which i s apparently related to the Early Jurassic mineralization. Therefore the Rb-Sr isochron does not indicate the metamorphic age of these rocks. The paucity of in t r u s i v e rocks of t h i s age within the area makes i t unl i k e l y that ages have been reset by a regional thermal event. The a l t e r a t i o n and mineralization ages are s i m i l a r to those obtained from int r u s i v e phases at the Galore Creek and Schaft Creek porphyry deposits, c i r c a 198+7 Ma (White et a l . f 1968) and 185+6 Ma (Panteleyev, 1973) respectively (Fig. 2.7), suggesting that a l t e r a t i o n and mineralization may be associated with s i m i l a r plutonic a c t i v i t y . Figure 3.10 Rubidium strontium isochron indicating a 201±16Ma date for metamorphic and metasomatically altered samples from the Stikine Assemblage, Mess Creek area n o r t h -western B.C. Isochron calculated using a1 regression analysis after York (1967). R b - S r data is given in Table 3.3. TABLE 3.3 Rub id ium-S t ront ium Data f o r Rocks f rom the S t i k i n e Assemblage, Mess Creek A r e a , Northwestern B.C. SAMPLE I .D. DESCRIPTION LATITUDE LONGITUDE ppm Sr ppm Rb Rb/Sr Sr 87 8 Sr/ Sr 81-BJ23d Chlor i te schist 5 7 ° 0 9 ' 1 0 " 1 3 0 ° 5 7 ' 1 5 " 226 8.7 0.038 0.111 0. 70406 81-BJ24a Quartz - c h l o r i t e -muscovite schist 5 7 ° 0 8 ' 4 0 " 1 3 0 ° 5 7 ' 1 0 " 162 17.6 0.109 0.315 0. 70644 81-BJ27 Metapelite 57°07'30" 1 3 0 ° 5 9 ' 0 0 " 1883 61.0 0.032 0.094 0. 70594 81-BJRC4 Quartz-muscovite schis t 5 7 ° 0 9 ' 0 8 " 1 3 0 ° 5 8 ' 1 0 " 195 33.9 0.173 0.502 0. 7064 81-BJ136b Greenstone 5 7 ° 0 9 ' 5 0 " 1 3 0 ° 5 7 ' 4 0 " 65.7 68.5 1.043 3.02 0. 7127 81-BJ144a Quartz-muscovite schist 5 7 ° 1 1 ' 3 0 " 1 3 0 ° 5 6 ' 3 0 " 119 21.5 0.181 0.524 0. 7064 81-BJ134a Chlorite-muscovite carbonate schis t 5 7 ° 1 0 ' 2 0 " 130'57'45" 418 31.4 0.075 0.217 0. 7059 81-BJ173 Greenstone 5 7 ° 1 0 ' 5 0 " I S O ^ ' O S " 243 0.8 0.003 0.009 0. 7042 81-BJ181d. Chlori te-quartz schist 5 7 ° 0 6 ' 4 0 " 1 3 0 ° 5 9 ' 0 0 " 165 49.2 0.298 0.864 0. 7078 81-BJ181e Chlorite-muscovite schist 5 7 ° 0 6 ' 4 0 " 1 3 0 ° 5 9 ' 0 0 " 226 84.3 0.373 1.078 0. 7082 81-BJ181f Muscovite-quartz schist 5 7 ° 0 6 ' 4 0 " 1 3 0 ° 5 9 ' 0 0 " 9.1 88.0 9.70 28.399 0. 7860 81-BJ181g Chlor i te schist 5 7 ° 0 6 ' 4 0 " 1 3 0 ° 5 9 ' 0 0 " 533 16.7 0.031 0.091 0. 7047 81-BJ188 Greenstone 5 7 ° 1 0 ' 2 0 " 1 3 0 ° 5 8 ' 5 0 " 173 ' 4.4 0.025 0.074 0. 7061 - 105 -3.6 Chemistry 3.6.1 General Statement Chemical compositions of Stikine Assemblage rocks from the study area were determined to chemically characterize the l i t h o l o g i c a l map units, to document chemical changes associated with a l t e r a t i o n , and to c l a s s i f y the rocks according to type and plate tectonic setting. C l a s s i f i c a t i o n of rocks i s hampered by metasomatic a l t e r a t i o n and chemical inhomogeneity caused by c r y s t a l s e t t l i n g or c l a s t i c deposition. Therefore, r e s u l t s from c l a s s i f i c a t i o n schemes, such as those discussed below, can be misleading and should be used with caution. V i r t u a l l y a l l of the rocks analyzed show evidence of chemical modification. V o l a t i l e content, as indicated by loss on i g n i t i o n (LOI) , i s generally i n the range of 10% with a minimum of 2% and a maximum of 21%. The main v o l a t i l e s are C0 2 (carbonates), C (graphite), and H 20 (both intergranular and bound to hydrous phases). Sulphur, which occurs i n eit h e r sulphide or sulphate minerals, could be a s i g n i f i c a n t v o l a t i l e component i n the more altered samples. 2+ 3+ Mass gains caused by the oxidation of F to F during determination of LOI are i n s i g n i f i c a n t r e l a t i v e to v o l a t i l e contents. Major element analyses were normalized to a v o l a t i l e - f r e e basis to f a c i l i t a t e comparisons and allow the use of conventional chemical p l o t s . - 106 -Sixty-three rock samples, representing t h i r t y one sample locations, were analyzed for major and trace elements by X-ray fluorescence methods. Results are discussed i n the following sections. 3.6.2 Methods and Quality Control Rock samples were c o l l e c t e d as chips from two to ten square metre areas of unweathered outcrop. Approximately every t h i r d sample was c o l l e c t e d i n duplicate to monitor chemical variance due to sampling. Duplicate samples were c o l l e c t e d from adjacent outcrops where possible. Rock samples, usually from one to three kg i n s i z e , were crushed and ground to -240 mesh. The powdered samples were s p l i t and lab duplicates prepared at that stage. A l l elements, except for gold, were analyzed on four gram pressed powder p e l l e t s by X-ray fluorescence following the technique of Brown et a l . (1973) . Analyses and data reduction were performed at U.B.C. Equipment s p e c i f i c a t i o n s and operating procedures for major and trace elements can be found i n Horsky et a l . (1985) and Berman (1979). Gold analyses were performed by Acme A n a l y t i c a l Laboratories Limited of Vancouver, B.C., on lOg subsamples. - 107 -A n a l y t i c a l precision was determined on lab duplicates using the method of Thompson and Howarth (1976 and 1978). Precisi o n at the 90% confidence l e v e l (n=19) i s + 3% for S i 0 2 ; + 5% f o r A1 20 3, CaO and FeO t o t; + 7% for MgO; and 10% for Na 20, K 20, T i 0 2 ' M n 0 ' a n d P2°5* V a r i a t i o n between duplicate samples taken at the sample s i t e i s approximately twice the a n a l y t i c a l v a r i a t i o n . 3.6.3 Major Element Chemistry and C l a s s i f i c a t i o n Chemical c l a s s i f i c a t i o n schemes f o r volcanic rocks provide consistancy i n nomenclature (Irvine and Baragar, 1971; Cox et a l . . 1979) and help i d e n t i f y o r i g i n a l plate tectonic s e t t i n g (Pearce and Cann, 1975 and 1978). The c l a s s i f i c a t i o n system of Irvine and Baragar (1971) i s l a r g e l y based on normative mineral concentrations calculated from major element abundances. Rocks of the study area c l a s s i f i e d according to t h i s system (normative mineralogy determined a f t e r the method of Nixon (1985)) yielded nomenclature such as K-rich basalt f o r quartz carbonate s e r i c i t e s c h i s t , and p i c r i t e basalt for c h l o r i t e s c h i s t . These c l a s i f i c a t i o n s are inappropriate and r e f l e c t the high degree of a l t e r a t i o n (and/or unusual chemistry caused by c l a s t i c deposition) of these rocks. Nevertheless, some useful general chemical c h a r a c t e r i s t i c s can be derived from elemental p l o t s used to c l a s s i f y chemical data. TABLE 3.4 Major Element Chemistry of the Stikine Assemblage Rocks, Mess Creek Area, Northwestern B.C., A n a l y t i c a l Techniques and Precision are Described i n Text 1 . 2 Sample U n i t S i 0 2 A l 2 ° 3 FeO MgO CaO N a 2 0 K 2 0 T i 0 2 MnO P 2 ° 5 T o t a l LOI R81 -27 ARGL 65. 14 17 . 65 6 . 00 2. 18 1. 84 2 . 41 3 . 76 1. 00 0. 17 0. 24 100. 39 21. 05 R81 -78b PGST 45. 29 15. 65 10 .62 11. 39 11. 23 4 . 60 0. 35 1. 08 0. 10 0. 15 100. 46 10 . 92 R81 -189a PGST 44 . 73 14. 95 10 .33 11. 76 11. 83 4 . 10 0. 91 1. 33 0. 17 0. 20 100. 32 11. 63 R81 -189b PGST 46. 90 15. 22 9 .95 15. 11 6. 46 3. 73 0. 98 0. 98 0. 17 0. 11 99. 61 5. 59 R81 -186 CHST 46. 59 14. 15 13 .26 14. 90 6. 10 1. 78 1. 97 0. 89 0. 15 0. 15 99. 94 5. 56 R81 -181g CHST 45. 09 13 . 99 13 . 37 15. 85 7. 31 2 . 21 0. 86 0. 92 0. 12 0. 15 99 . 87 9. 61 R81 -166b FVCS 63 . 64 15. 06 7 . 64 6. 85 1. 79 3 . 54 0 . 58 0 . 66 0 . 17 0. 11 100 . 04 1. 97 R81 -126e GRST 50. 47 15. 68 14 . 83 8. 30 4. 24 4. 11 0. 33 2 . 36 0. 17 0. 38 100. 87 4 . 33 R81 - 1 2 6 f GRST 48. 20 14 . 88 14 .21 8. 69 7. 37 4. 41 0. 44 1. 97 0. 16 0. 37 100. 70 11. 46 R81 -136b GRST 49. 64 17. 45 14 .21 9. 59 1. 85 2 . 83 3 . 93 1. 04 0. 17 0. 23 100. 94 4. 74 R81 -142 GRST 48. 65 15. 42 15 .16 9. 60 5. 12 3 . 06 1. 47 1. 87 0. 14 0. 36 100. 85 10. 40 R81 -143 GRST 48. 98 15. 50 14 . 56 10. 39 4 . 63 3 . 46 0 . 82 1. 94 0. 15 0. 32 100. 75 12. 71 R81 -173 GRST 39. 04 15. 82 20 .96 11. 97 4 . 58 2. 88 0. 10 6. 68 0. 16 0. 61 102 . 80 5. 57 R81 -176 GRST 49. 18 14 . 45 13 . 67 9. 18 7 . 69 3 . 74 0. 49 1. 66 0. 09 0. 40 100. 55 11. 61 R81 -188 GRST 50. 32 14 . 09 16 . 10 7. 41 3 . 76 4 . 17 0. 32 3 . 66 0. 29 0. 53 100. 65 5. 17 R81 -126a QMCS 67. 19 12 . 09 4 . 86 5. 55 5. 92 1. 64 1. 72 0. 42 0. 17 0. 08 99. 64 9. 06 R81 -126b QCMS 68. 45 15. 60 4 .59 3. 47 2. 78 1. 79 2. 49 0. 59 0. 12 0. 27 100 . 15 4 . 90 R81 -144 QMCS 69. 32 15. 24 6 . 61 2. 95 2. 71 . 1. 32 1. 71 0. 56 0. 13 0. 08 100. 63 12 . 47 R81 - 1 8 1 f QCMS 73 . 33 13 . 90 2 .46 4. 03 0. 23 0. 83 4 . 82 0. 28 0. 14 0. 03 100. 05 3 . 67 R81 -190 QMCS 67. 88 13 . 11 6 . 58 3 . 78 6. 55 0. 76 0. 55 0. 39 0. 17 0. 09 99. 86 7 . 72 R81 -180 CALT 57. 84 15. 91 8 .34 4 . 65 8. 89 1. 46 2 . 21 0. 66 0. 17 0. 12 100. 25 14 . 60 R81 -184a CALT 55. 87 18. 58 11 .75 3 . 60 4 . 17 3 . 88 1. 74 0. 98 0. 17 0. 24 100 . 98 7 . 70 R81 -188C CALT 53. 53 16. 16 13 .31 3. 73 7. 46 1. 25 3 . 06 2 . 22 0. 27 0. 33 101. 32 5. 80 R81 -188d CALT 69. 73 12. 36 9 .24 1. 34 2 . 23 1. 21 1. 82 1. 40 0. 17 0. 38 99. 88 8 . 64 R81 - 1 8 8 f CALT 64. 32 13 . 90 9 . 06 1. 93 4 . 98 o. 81 2 . 50 1. 97 0. 17 0. 31 99. 95 10. 70 A v e r a g e o f f i e l d a n d l a b o r a t o r y d u p l i c a t e s ( s e e A p p e n d i x I ) . ARGL - A r g i l l a c e o u s s c h i s t u n i t , PGST - M a f i c p y r o c l a s t i c / e p i c l a s t i c u n i t , CHST - C h l o r i t e s c h i s t u n i t , FVSC - F e l s i c p y r o c l a s t i c u n i t , GRST - G r e e n s t o n e u n i t , QMCS - Q u a r t z - m u s c o v i t e - c a r b o n a t e s c h i s t , QCMS - Q u a r t z - c a r b o n a t e - m u s c o v i t e s c h i s t , CALT - C a r b o n a t e a l t e r e d g r e e n s t o n e o r c h l o r i t e s c h i s t TABLE 3 . 5 Selected trace element geochemistry for rocks of the Stikine Assemblage, Mess Creek area, northern, B.C. 1 £pm (PPb) SAMPLE I.D. ROCK TYPE Nb Zr Y Sr Rb NI Cr V Ce Nd Au R81-27a ARGL R81-78b PGST R81-126a CQMS 15 R81-126b QCMS 20 R81-126d CHST 20 R81-126e CALT/GRST 20 R81-126f GRST 20 R81-128a ? R81-136b CALT 20 R81-142 GRST 18 R81-143a GRST 19 R81-143b GRST R81-144 QMCS 14 R81-145b GRST R81-166B VFCS R81-173a GRST 21 R81-175 CALT/GRST R81-176a GRST R81-180a ? R81-181a QMCS 20 R81-181C QMCS 18 R81-181d CHST 17 R81-181e CHST 14 R81-181f QMCS 24 R81-181g CHST 18 R81-184 CALT/CHST 17 R81-186a CHST R81-188 GRST 23 R81-188c CALT R81-188d CALT R81-188f CALT 15 R81-189 PGST 17 R81-190 QMCS 18 13 208 121 22 173 31 45 146 43 238 40 62 145 32 162 29 32 138 38 102 4 20 154 35 415 8 23 122 32 81 78 3 169 42 445 23 158 40 260 12 23 155 35 153 32 8 22 110 198 27 252 <1 4 33 11 99 26 199 57 3 114 39 207 112 5 99 26 199 57 5 110 18 270 104 11 271 <1 9 96 11 64 20 566 13 58 110 24 234 36 4 63 194 45 196 2 5 4 106 24 1031 30 77 25 206 16 118 130 32 118 • 6 12 99 72 36 1 323 172 43 7 5 110 71 65 9 -68 95 77 12 1 65 381 53 19 1 78 358 36 9 12 78 350 36 10 1 108 135 13 207 35 7 60 329 26 34 59 345 16 12 7 26 383 21 19 2 45 89 53 10 2 18 508 67 5 74 83 27 6 15 412 30 48 34 86 132 240 31 47 370 26 25 1 118 126 62 11 2 29 161 56 10 1 5 235 36 10 2 29 210 36 7 4 58 194 22 <1 6 53 71 174 48 1 21 449 15 5 2 39 154 44 14 58 452 24 18 1 10 440 34 14 1 61 378 919 15 89 134 215 <1 87 182 106 28 212 233 8 5 1 47 15 2 See Table 3.4 f o r d e f i n i t i o n o f r o c k t y p e s . - 110 -For the purposes of p l o t t i n g , data from laboratory and f i e l d duplicates (Appendix II) have been averaged to a sin g l e value and samples with highly anomalous values or excessive error (>+30% for lab duplicates) have been discarded (Table 3.3). Major element analyses, shown gra p h i c a l l y on Harker v a r i a t i o n diagrams (Fig. 3.11), display t y p i c a l negative correlations between s i l i c a and MgO, FeO, CaO, Na 20 and T i 0 2 . The two c l u s t e r s along the s i l i c a axis demonstrate the bimodal character of rock compositions. Ou t l i e r s from these c l u s t e r s are the more alt e r e d specimens. Cox et a l . (1979) have demonstrated the usefulness of an a l k a l i - s i l i c a p l o t f o r a general c l a s s i f i c a t i o n of normal (non-potassic) volcanic rocks. This type of p l o t (Fig. 3.12) with the alkaline-subalkaline d i v i d i n g l i n e of Irvine and Baragar (1971), shows that rocks of the study area c l u s t e r within the basalt and alk a l i n e f i e l d s . Samples within the subalkaline f i e l d are, for the most part, v i s i b l y altered. Rocks that l i e outside of the general c l a s s i f i c a t i o n f i e l d s suggest extensive chemical modification involving mobility of S i 0 2 , Na 20 and K 20. The AFM diagram (Fig. 3.13) i s commonly used to d i f f e r e n t i a t e c a l c - a l k a l i n e rocks from t h o l e i i t e s (Irvine and Baragar, 1971). This p l o t i s not p a r t i c u l a r l y e f f e c t i v e i n c l a s s i f i n g rocks of the study area, but does show an enrichment trend of the a l k a l i e s for the al t e r e d samples. - I l l -Previously reported analyses for Stikine Assemblage volcanic rocks p l o t within the c a l c - a l k a l i n e f i e l d of the AFM diagram (Souther, 1977) and were interpreted to represent an island arc-type s e t t i n g . Pearce (1977) demonstrated the use of an MgO-FeO-A1203 t r i a n g u l a r p l o t to c l a s s i f y volcanic rocks by tectonic s e t t i n g . This i s based on observations l i k e those of T i l l e y (1950) that c a l c - a l k a l i c suites of orogenic b e l t s usually have more A1 20 3 and lower Fe/Mg r a t i o s than t h e i r t h o l e i i t i c counterparts. The average values for the unaltered units of the study area p l o t within the f i e l d s i n d i c a t i n g rocks of oceanic o r i g i n (Fig. 3.14). Altered samples trend towards the alumina apex due to loss of Mg and Fe. The general scatter of data points indicates the o v e r a l l inconsistency of chemical trends within the rocks of the study area. 3.6.4 Trace Element Chemistry The use of minor or trace elements, p a r t i c u l a r l y those of l i m i t e d mobility during metamorphism or a l t e r a t i o n , for c l a s s i f i n g volcanic rocks according to tectonic s e t t i n g i s well-documented (Pearce and Cann, 1975; Garcia, 1978; Floyd and Winchester, 1978 and Holm, 1982). The use of 'immobile' trace elements for c l a s s i f i c a t i o n has some d i s t i n c t advantages i n older, chemically corrupted terranes, - 1 1 2 -6 4 2 0 4 2 0 -L 4 2 0 10 5 H 15 io H 5 A 1 2 ° 5 20-15-10 • 5 • 20-15 H 10 40 IT • i * - 9 - r ft" ' ° o n • * A o A O 50 60 % S i 0 2 70 O A R G L • Q M C S • G R S T • P G S T A F V C S • C A L T 0CHST Figure 3.11 Harker variation d agrams of major element geochemistry, Stikine Assemblage rocks, Mess Creek area, northwestern B.C. See Table 3.4 for explanation of legend. F i g u r e 3.12 A l k a l i e s - S i l i c a p lo t of S t i k i n e A s s e m b l a g e Rocks , Mess Creek A r e a . C l a s s i f i c a t i o n f ie lds a f t e r Cox et al_. (1979) - 114 -FeO to t Figure 3.13 AFM diagram for Stikine Assemblage rocks, Mess Creek area, northwestern B.C. Dashed line which sparates the tholeiitic field (upper) from the calc-alkal ine field (lower) from Irvine and Baragar (1971). - 115 -i n c l u d i n g t h e i r l a r g e v a r i a t i o n i n c o n c e n t r a t i o n among magma types, ease o f measurement and r e l a t i v e i n s e n s i t i v i t y t o secondary processes (Pearce, 1974). The disadvantages t o t h e i r use i s t h a t a p p l i c a t i o n s hould be l i m i t e d t o rocks of b a s i c composition i n order t o a v o i d unknown changes caused by p a r t i t i o n i n g between f l u i d and c r y s t a l phases i n a d i f f e r e n t i a t i n g magma. T h e r e f o r e , rocks o f the study area are not w e l l - s u i t e d f o r the d e t e r m i n a t i o n o f p l a t e t e c t o n i c s e t t i n g . However, the use o f 'immobile' elements can be a p p l i e d t o determine the parentage o f a l t e r e d rocks and p o s s i b l y the degree o f a l t e r a t i o n . Before the s o - c a l l e d immobile elements can be used t h e i r l a c k o f m o b i l i t y should be e s t a b l i s h e d . Winchester and F l o y d (1977), Pearce (1974), and Pearce and Cann (1975) demonstrate the r e l a t i v e i m m o b i l i t y o f the elements Zr, T i and Y among o t h e r s . Although these elements may have l i m i t e d m o b i l i t y d u r i n g metamorphic o r a l t e r a t i o n processes, they a re s t i l l s u s c e p t i b l e t o c o n c e n t r a t i o n changes by the a d d i t i o n o r s u b t r a c t i o n o f o t h e r rock-forming components. Such changes can be deduced from a Z r - T i 0 2 p l o t a f t e r the method of Finlow-Bates and Stumpfl (1981). The amounts of T i 0 2 and Zr w i t h i n a rock w i l l v a r y i n v e r s e l y w i t h the ext e n t o f magmatic d i f f e r e n t i a t i o n . T h e r e f o r e , a s e r i e s o f p o i n t s which r e p r e s e n t the products o f a d i f f e r e n t i a t i n g magma would form a l i n e w i t h a n e g a t i v e s l o p e on the TiO vs - 116 -FeO Total MgO A 1 2 ° 3 Figure 3.14 MgO - FeO (total) - A l 2 0 3 d i a g r a m for rocks of the S t ik ine Assemblage, Mess Creek Area. Sol id l ines divide tec tonic env i ronments (after Pearce et a t , 1977). - 117 -Zr diagram. D i l u t i o n o r c o n c e n t r a t i o n of e i t h e r o r both elements due t o a d d i t i o n o r s u b t r a c t i o n o f ot h e r components would d r i v e the p l o t t e d t r e n d e i t h e r towards or away from the o r i g i n . The T i 0 2 ~ Z r p l o t f o r the study area ( F i g . 3.15) shows d i s t i n c t grouping by l i t h o l o g y . A p o s s i b l e d i f f e r e n t i a t i o n t r e n d i s i n d i c a t e d f o r the mafic p y r o c l a s t i c u n i t t o the quartz-muscovite-carbonate s c h i s t s . The o u t l y i n g c l u s t e r o f greenstone u n i t samples may be a t t r i b u t e d t o c r y s t a l s e t t l i n g o r t o a d i f f e r e n t magma source. A l l of the u n i t s show some spread along t r e n d s r a d i a t i n g out from the o r i g i n . R a t i o s o f two r e l a t i v e l y immobile elements minimize the e f f e c t o f major changes t o rock composition. P l o t s o f A l 2 0 3 / T i 0 2 , A l 2 O 3 / 1 0 0 Z r and 100Zr/TiO 2 a g a i n s t MgO ( F i g . 3.16) d i s p l a y t r e n d s t h a t suggest a comagmatic r e l a t i o n s h i p among the mafic u n i t s , and a d i s t i n c t magmatic source f o r the f e l s i c and quartz-muscovite-carbonate a l t e r a t i o n u n i t s . 3 . 0 -2 . 0 T i 0 2 Wt.% 1 0 0 1 5 0 Zr (ppm) 2 5 0 Figure 3.15. Ti02-Zr diagram for rocks of the Stikine Assemblage, Mess Creek Area. Distribution trends with a negative slope indicate typical magmatic differentiation (Finlow-Bates and Stump, 1981). Trends towards or away from the origin reflect mutual concentration or dilution due to secondary (alteration) processes. O A R G L • Q M C S • G R S T • P G S T A F V C S • C A L T 0CHST oo i - 1 1 9 -O O X o • r-H O O *—I X u O CO CM 3 0 -2 0 -1 0 -O ARGL • QMCS • GRST • PGST A FVCS • CALT 0CHST Figure 3.16 Harker—type variation diagrams showing AlgOg/TiOg, A l 2 0 3 / 1 0 0 Z r and lOOZr/TiOg plotted against MgO for Stikine Assemblage Rocks of the Mess Creek Area. Segregation of the quartz—muscovite—carbonate schist samples suggest that these rocks were derived from the felsic pyroclastic unit. - 120 -CHAPTER 4. ECONOMIC GEOLOGY 4.1 Introduction Gold and s i l v e r mineralization was discovered on the BJ claim group area by routine prospecting consequent to a regional stream sediment geochemical survey. I n i t i a l follow-up work proved encouraging with rock chip sampling returning values of up to 75.4g/t Au over 1.5m wide carbonate-sulphide vein. A large variety of mineralization and a l t e r a t i o n types were noted but r e l a t i o n s h i p s among mineralization, a l t e r a t i o n , l i t h o l o g y and structure appeared complex. To date, i n s u f f i c i e n t work has been done to formulate a s p e c i f i c and p r e d i c t i v e exploration model fo r the mineral-i z a t i o n . This study documents the surface c h a r a c t e r i s t i c s of the mineralization and a l t e r a t i o n i n order to: i d e n t i f y general geological processes that may have resulted i n , or influenced, the development of a l t e r a t i o n and mineral-i z a t i o n ; determine the timing of mineralization and a l t e r a t i o n r e l a t i v e to l o c a l plutonism and dynamothermal metamorphism; and define features that may a s s i s t explora-t i o n i s t s i n the discovery and evaluation of additional mineralization. Comparisons of the mineralization and a l t e r a t i o n c h a r a c t e r i s t i c s with those of known deposits could aid i n the evaluation of exploration p o t e n t i a l and the development of exploration tools and strategy. - 121 -4.2 Mineralization A v a r i e t y of mineralization types occur within and around the property area. A majority of the mineralization, and the primary exploration targets, consist of precious metal-bearing mesothermal s t y l e veins. Volcanogenic massive sulphide (VMS) deposits may be a secondary target but are not the focus of current exploration. Potential f o r VMS deposits i s indicated by: widespread, p a r t i a l l y stratabound a l t e r a t i o n ; presence of sulphide fragments within altered f e l s i c l a p i l l i t u f f s ; zones of disseminated chalcocite within altered quartz c r y s t a l ash t u f f s ; and 2 to 8m long lenses of massive p y r i t e and chalcopyrite hosted by f e l s i c fragmental rocks (Plate 3.7e). The large v a r i e t y of vein types and the numerous showings (of v a r i a b l e and often indeterminable significance) make i t necessary to c l a s s i f y veins into groups. Five groups or vein types are defined on the basis of morphology and gangue and sulphide mineralogy (Table 4.1) and consist of: (1) f o l i a t i o n - p a r a l l e l quartz veins; (2) quartz breccia veins; (3) carbonate-sulphide veins; (4) carbonate-arsenopyrite veins; and (5) others. D i s t i n c t i o n between groups can be vague, and sometimes subjective. The f o l i a t i o n - p a r a l l e l quartz veins are commonly deformed and TABLE 4.1 Mineralization Characteristics, BJ Prospect, Mess Creek Area, Northwestern B.C. GANGUE SULPHIDE VEIN TYPE MINERALOGY MINERALOGY ALTERATION MORPHOLOGY GOLD GRADES Foliation parallel quartz veins Quartz Pyrite (Tetrahedrite, chalcopyrite, tellurides) Electrum S i l i c i f i c a t i o n S e r i c i t i c Fuchsite Minor carbon-atization Foliation parallel, commonly folded discontinuous Low Quartz breccia veins Quartz Pyrite Fe-carbonate Chalcopyrite s i l i c i f i e d fragments Galena Tetrahedrite Sphalerite Arsenopyrite Hematite Gold S i l i c i f c a t i o n S e r i c i t i c CarbonatizatIon Cross-cutting Structural control Stockworks Breccias Variable 1 to 20 g/t Au: Ag - 1:1 to 1:1 Carbonate-sulphide veins Ankerite, siderite Manganiferous Fe-carbonates Quartz Pyrite Sphalerite Chalcopyrite Freibergite Galena Arsenopyrite Gold Fe-carbonates Seri c i t i c S i l i c i f i c a t i o n Fuchsite Chlorite Cross-cutting Fracture controlled Tabular Variable 0.5-50 g/t Au:Ag ~ 1:1 to 1:1 Arsenopyrite veins Siderite Ankerite Quartz Arsenopyrite Pyrite Sphalerite Fe-carbonates Minor sericite Shear zones Lenticular Banded Brecciated Low 0.1 to 2 g/t Quartz Albite veins Albite Quartz Fe-carbonate Pyrite ?? Weak, quartz carbonate Irregular veins, plug? Moderate to 7 g/t Au:Ag 1:3 Barite veins Barite Jasper Pyrite Chalcopyrite Tetrahedrite S i l i c i f i c a t i o n Carbonatization Narrow Sinuous Breccias Moderate 3 - 7 g/t - 1 2 3 -therefore were an early event, possibly related to dynamothermal metamorphism, whereas the other vein types are approximately c o r r e l a t i v e and g e n e t i c a l l y related to an Early J u r a s s i c mineralizing event. 4.2.1 F o l i a t i o n - P a r a l l e l Quartz Veins F o l i a t i o n - p a r a l l e l quartz veins are ubiquitous within the study area. They occur i n a wide v a r i e t y of sizes and within a l l units except for the greenstone unit, where fracture controlled cross-cutting quartz veins occur. The majority of stratabound veins appear to be related, at lea s t temporally, to metamorphic-deformational events. These veins are from 0.5 to 3m wide and 5 to 50m long and contain traces of p y r i t e and rare base metal sulphides. Quartz i s c h a r a c t e r i s t i c a l l y milky and coarse grained. Euhedral, rusty weathering carbonate c r y s t a l s usually account f o r 1 to 2% of vein volume. Weak to moderate i n t e n s i t y s e r i c i t e a l t e r a t i o n haloes are common. Locally, veins and a l t e r a t i o n envelopes are deformed into i s o c l i n a l f o l d s . Minor enrichment of precious metals do occur and although, not economically s i g n i f i c a n t , contribute to s o i l geochemical anomalies. The most s i g n i f i c a n t f o l i a t i o n - p a r a l l e l quartz vein i s the T e l l u r i d e vein which i s located i n the south-central part of the study area (Fig. 3.3). This vein i s f l a t - l y i n g , - 124 -from 1 to 2m thick and has been traced for approximately 85m. Vein contacts are gradational with wall rocks becoming progressively more s i l i c i f i e d towards the vein centre. F o l i a t i o n of xenoliths within the vein i s concordant with that of wall rocks suggesting 'passive' emplacement of the vein. Vein quartz ranges from milky to l i g h t . grey and vitreous. Fine spots of rusty weathering carbonate are disseminated throughout the vein. A l t e r a t i o n peripheral to the vein consists of 3 to 10m wide haloes of s i l i c i f i c a t i o n and s e r i c i t i z a t i o n with patchy development of carbonate. Green muscovite, possibly fuchsite, occurs l o c a l l y as vein selvages. Sulphide mineralogy of the vein i s unique to the property and includes: p y r i t e , tetrahedrite, chalcopyrite, sphalerite and hessite with traces of tellurobismuthite, hematite, pyrrhotite and electrum. T e l l u r i d e minerals were noted i n the f i e l d as bright, s i l v e r y s e c t i l e spots and appear to be r e s t r i c t e d to a single l o c a t i o n within the vein. Chip samples across the vein yielded only trace amounts of precious metals. Although t h i s vein i s morphologically stratabound, the d i s t i n c t i v e sulphide assemblage indicates that i t i s u n l i k e l y to be co-genetic with the other f o l i a t i o n - p a r a l l e l quartz veins. - 125 -4.2.2 Quartz-Breccia Veins Quartz-breccia veins are cross-cutting, generally steeply dipping, and contain a gangue mineralogy dominated by quartz with minor carbonate or a l b i t e . Breccia textures are common but not c r i t i c a l to the c l a s s i f i c a t i o n . Large a l t e r a t i o n envelopes of moderate to intense carbonatization are c h a r a c t e r i s t i c . Veins are t y p i c a l l y fracture controlled and well-defined with sharp contacts, but ir r e g u l a r quartz-carbonate stockworks are also included with t h i s group. Vein trends are easterly i n the northern part of the map area and northeasterly i n the southern part. Sulphide mineralogy i s t y p i f i e d by p y r i t e , tetrahedrite, chalco-p y r i t e , sphalerite and trace amounts of arsenopyrite, galena, hematite and gold. Quartz and quartz-breccia veins are di s t r i b u t e d over much of the prospect area but most of the s i g n i f i c a n t showings occur within the area surrounding the Wishbone Gla c i e r . On the north end of the study area (Windy claim) a group of easterly trending quartz veins are the target of current exploration (Folk, 1987). These veins have pri m a r i l y been exposed by hand-trenching and are hosted by the mafic p y r o c l a s t i c / e p i c l a s t i c unit, quartz-muscovite-carbonate s c h i s t s and greenstones. The main vein has an infe r r e d s t r i k e length of 500m and widths up to 6m. Gold grades of 7g/t or more, over mineable widths, are common - 126 -(Folk, 1988). Veins consist of grey to white quartz with rare p y r i t i c and s i l i c i f i e d wall rock fragments. Carbonate a l t e r a t i o n envelopes are most conspicuous within the greenstones. Narrow tetrahedrite and gold bearing veins within the intensely carbonate altered greenstone b l u f f immediately adjacent to the northeast corner of Wishbone G l a c i e r (Plate 3.2b) are on s t r i k e with the Windy veins and suggest a po t e n t i a l structure, subparallel to the g l a c i e r , with a s t r i k e length i n excess of 3kms. The quartz-carbonate stockwork (Plates 3.7a and g) associated with the McLaughlin Creek carbonate breccia zone (Plate 3.7f) probably belongs to the quartz-breccia vein group. This stockwork forms a crackle breccia within the greenstone unit and c a r r i e s anomalous to sub-ore grades of gold (0.1 to 3.0g/t Au). At lea s t two stages of quartz and carbonate veins are indicated by cross-cutting r e l a t i o n s h i p s . The stockwork i s rela t e d to the same structure that controls the carbonate breccia and probably McLaughlin Creek as well. - 127 -4.2.3 Carbonate-Sulphide Veins Carbonate-sulphide veins occur i n both the north and south ends of the study area. They are t y p i c a l l y sulphide r i c h (greater than 10% by volume) and have carbonate as the dominant gangue mineral. A v a r i e t y of d i f f e r e n t carbonate minerals within a single vein i s common. Dark brown weathering manganiferous s i d e r i t e or ankerite i s generally a good indicator of precious metal values, p a r t i c u l a r l y s i l v e r . Other gangue minerals include quartz, s e r i c i t e , fuchsite and c h l o r i t e . Banding of carbonate-rich, q u a r t z - s e r i c i t e - r i c h and sulphide-rich layers, and breccia textures caused by disruption of bands suggest that these veins are a sub-type of the quartz-breccia veins. Sulphide mineralogy includes, i n order of abundance: p y r i t e , sp h a l e r i t e , chalcopyrite, tetrahedrite ( f r i e b e r g i t e ) , and trace amounts of galena, arsenopyrite and gold. The best exposure of t h i s vein type i s the Rat vein, which i s hosted by quartz-muscovite-carbonate s c h i s t s on the southwestern side of the Wishbone Gl a c i e r (Fig. 3.3). G l a c i a l a c t i v i t y has stripped o f f the hanging wall leaving a roughly e l l i p t i c a l , 90 by 30m section of the steeply north-dipping vein well-exposed. Thickness of the vein v a r i e s from 1 to 2m. The outer margins of the vein are composed of s i d e r i t e and ferroan dolomite while the inner - 128 -part of the vein consists of semi-massive sulphides i n a grey vitreous quartz gangue. Footwall rocks are carbonate altered, cut by a f i n e stockwork of s i d e r i t e and pyrite v e i n l e t s , and contain patches and clumps of quartz with c h l o r i t e , muscovite, and fuchsite. I t was the muscovite-fuchsite that yielded a potassium-argon date of 194+6 Ma (section 3.7). Precious metal content of the vein i s e r r a t i c with c l o s e l y spaced chip samples y i e l d i n g values ranging from 74g/t Au to 2g/t Au. Gold to s i l v e r r a t i o s vary, from about 1:1 to 1:5. Carbonate-sulphide veins are exposed i n the southwest part of the study area but are usually narrow (less than 30cm), sinuous, and poorly mineralized. An abundance of rounded, well-mineralized boulders up to 87cm i n diameter indicates that more veins of t h i s type occur below the g l a c i e r that flows from the west-central part of the study area. 4.2.4 Carbonate-Arsenopyrite Veins Carbonate-arsenopyrite veins display features common to both quartz-breccia and carbonate-sulphide veins, and may be ge n e t i c a l l y related to eith e r . The veins, which are cross-cutting and usually hosted by shear zones, are defined by containing arsenopyrite as the dominant sulphide mineral. - 129 -Gangue mineralogy i s mainly carbonate (siderite) with lesser quartz. Only two l o c a l i t i e s are currently known but f l o a t boulders indicate that other showings remain to be discovered. The showings and f l o a t samples a l l occur i n the northern part of the map area. The two showings are located on steep slopes; one on the south side of the Wishbone Glacier, the other on the south side of the g l a c i e r to the north (Fig. 3.3). Both veins have east-northeast trends with steep northerly dips. Veins range from 10cm to 1.8m i n thickness but other dimensions are not known due to rugged topography. Precious metal enrichment appears to be lim i t e d with the best samples to date containing from 0.5 to 1.5g/t gold and ne g l i g i b l e s i l v e r . Veins are banded with alternating sulphide-poor and sulphide-rich breccias. Within the sulphide-rich bands fine rounded fragments of s i l i c e o u s or carbonate material s i t i n a matrix of very f i n e grained arsenopyrite, p y r i t e and sphalerite. The sulphide-poor bands consist of s i l i c e o u s fragments and lesser sulphide fragments i n a ferroan dolomite matrix. Narrow p y r i t e v e i n l e t s and coarser grained carbonate veins cut e a r l i e r breccia bands. I t i s not clear whether the breccia textures are tectonic or hydrothermal i n o r i g i n . Vein morphology i s l e n t i c u l a r , possibly owing to emplacement within lens-shaped voids, or to shearing a f t e r - 130 -emplacement or to a combination of both. A l t e r a t i o n associated with the veins i s primarily carbonate with patchy s e r i c i t e . Because of the low assay re s u l t s from these veins no d e t a i l e d work was undertaken on t h e i r mineralogy. 4.2.5 Other Vein Types Two other vein types e x i s t on the prospect but t h e i r s i g n i f i c a n c e i s unknown. An albite-quartz body occurs approximately 800m southeast of the Rat vein (Fig. 3.3). This body i s e l l i p t i c a l i n plan and may have a p i p e - l i k e morphology. Mineralogy consists of a l b i t e , r u t i l a t e d quartz, ferroan dolomite and p y r i t e . A weak to moderate carbonate a l t e r a t i o n halo surrounds the zone. A single chip sample returned a 6.3g/t Au assay over the maximum 2.8m width. Thin, discontinuous b a r i t e veins, are rare but widespread over the prospect area. Most occurrences consist of white coarse-grained b a r i t e with minor p y r i t e and appear to be l a t e features. A notable exception i s found i n the extreme west-central map area where a b a r i t e matrix breccia occurs within an extensive zone of carbonate a l t e r a t i o n . The b a r i t e breccia varies from 1 to 2m i n width and consists of angular carbonate and jasperoid fragments within a white to grey b a r i t e matrix. A 15m chip sample across the zone contained 2.9g/t Au. - 131 -4.2.6 Sulphide Mineralogy Sulphide mineralogy consists of p y r i t e , tetrahedrite, chalcopyrite, sphalerite, arsenopyrite, galena, pyrrhotite, hessite, telluorbiomuthite, electrum and gold. Cuprite and hematite occur as oxidation rinds on chalcopyrite and p y r i t e or arsenopyrite, respectively (Bibby, 1981). Pyrite, chalcopyrite, tetrahedrite and sphalerite are common _ to v i r t u a l l y a l l mineralization types within the prospect area. Arsenopyrite, galena and pyrrhotite are widespread but usually minor constituents with rare l o c a l enrichments. S i l v e r and bismuth t e l l u r i d e s are known from a single l o c a l i t y . Sulphide contact relationships are displayed i n a composite Vandeveer'type diagram (Fig. 4.1). Most of the gold and s i l v e r occurs i n sulphide associations, either i n contact with sulphide minerals or adjacent to them i n both quartz and carbonate gangue (Bibby, 1981). The p r i n c i p a l repository f o r s i l v e r i s i n tetrahedrite ( f r i e b e r g i t e ) ; minor amounts also occur as electrum. - 132 -PLATE 3.7 Mineralization and A l t e r a t i o n Features i n S t i k i n e Assemblage Rocks, Mess Creek Area, Northwestern, B.C. a) Quartz-carbonate stockwork (crackle breccia) i n greenstone unit, McLaughlin Creek Zone. b) Carbonate sulphide vein showing pyri t e - c h a l c o p y r i t e -tetrahedrite-manganiferous ankerite and quartz-muscovite-dolomite bands. c) Partly carbonate altered mafic e p i c l a s t i c rock. A l t e r a t i o n has p r e f e r e n t i a l l y t r a v e l l e d p a r a l l e l to f o l i a t i o n . d) Sulphide fragments (black) i n f e l s i c l a p i l l i t u f f . Matrix i s altered to sericite-carbonate and quartz. e) Stratabound sulphide lens near the contact between f e l s i c and mafic p y r o c l a s t i c rocks. Sulphide lens has a p y r i t e - p y r r h o t i t e - r i c h bottom and a copper-rich top. Malchite s t a i n defines a minor f o l d . Note e l l i p t i c a l jasperoid fragment i n center r i g h t of photograph. Photograph i s approximately 2m across. f) Monomineralic Fe-dolomite breccia from the McLaughlin Creek Zone. g) Quartz-carbonate stockwork i n greenstone boulder within landslide rubble i n Cauldron Creek area (Figure 3.3). PLATE 3.7 - 1 3 4 -PLATE 3.8 Back scattered electron images from the scanning electron microscope. Specimens from S t i k i n e Assemblage Rocks, Ness Creek Area, Northwestern, B.C. Fine quartz v e i n l e t (bottom center to upper right) and p y r i t e grains along f o l i a t i o n planes. Pyr i t e formed from d i f f u s i o n of sulphur away from the vein. Bright l e n t i c u l a r spots within the vein are b a r i t e . Fine carbonate-sulphide fragment from carbonate altered greenstone, McLaughlin Creek area (Figure 3.3). Darker areas represent s i x d i s t i n c t carbonate compositions ranging from r e l a t i v e l y pure s i d e r i t e , c a l c i t e and dolomite to Fe-dolomite, F e - c a l c i t e and manganiferous s i d e r i t e . Brighter spots are sulphides, sulphates and phosphates including p y r i t e , galena, sphalerite, arsenopyrite, b a r i t e , xenotime and tetrahedrite. -A £ » 4 0 M PLATE 3.8 - 136 -Pyrite, the most abundant sulphide mineral, i s widespread and occurs within veins as anhedral to euhedral disseminations and as semi-massive c r y s t a l aggregates. Coarse euhedral porphyroblasts up to 2cm occur within the quartz-sericite-carbonate s c h i s t . Pyrite commonly encapsulates quartz and carbonate gangue, and i s rar e l y intergrown with arsenopyrite. Minor amounts of pyrrhotite replace p y r i t e along cracks and cleavage planes. Hematite replacement of p y r i t e also occurs and i s p a r t i c u l a r l y severe i n areas with extreme carbonate a l t e r a t i o n . Plate 3.8a shows' very f i n e grained p y r i t e d i s t r i b u t e d along vein selvages and subordinate branch structures; these textures are interpreted to indicate a reaction front and either r e f l e c t sulphur moving from the vein into the wall rock or iron moving from the wall rock into the vein. Arsenopyrite usually occurs as fine anhedral grains with rare coarse euhedral grains within p y r i t e aggregates. Arsenopyrite appears to be r e s t r i c t e d to the northern part of the prospect area. Chalcopyrite i s common to a l l mineralized structures and i s usually associated and/or intergrown with tetrahedrite and sphalerite. I t i s para-g e n e t i c a l l y l a t e r than p y r i t e as i t i n f i l l s cracks within p y r i t e . Oxidation of chalcopyrite to cuprite occurs l o c a l l y . Sphalerite i s less common than chalcopyrite but s t i l l widespread. Tetrahedrite occurs l a t e i n the paragenetic sequence as i t encloses grains of p y r i t e , sphalerite and arsenopyrite. - 137 -Figure 4.1 Composite Vandeveer diagram showing sulphide mineral paragenesls of vein mineralization on the BJ prospect, Mess Creek area, northwestern B. C . Modified from Bibby (1981). - 138 -4.3 ALTERATION 4.3.1 Description and Petrography Two predominant a l t e r a t i o n types have been defined for the study area: a pre- or syndeformational s e r i c i t i c zone characterized by a quartz-muscovite-carbonate assemblage; and a post deformational, mineralization related carbonate-oxide zone characterized by pervasive carbonate replacement and oxidation. The r e l a t i v e proportions of the a l t e r a t i o n minerals within the s e r i c i t i c zone are controlled, at least p a r t l y , by composition of the host rock. Consequently, i t can be d i f f i c u l t to d i f f e r e n t i a t e between the two a l t e r a t i o n types i n areas where mafic s c h i s t s are altered to a carbonate dominant assemblage. S e r i c i t i c a l t e r a t i o n i s widespread within the map area and occurs as both stratabound and cross-cutting zones (although these re l a t i o n s h i p s are often complicated by f a u l t s ) . This a l t e r a t i o n a f f e c t s most units but i s concentrated within the f e l s i c rocks. Hand specimen and outcrop character of t h i s a l t e r a t i o n i s discussed i n the previous chapter. The two most s i g n i f i c a n t features of the s e r i c i t i c a l t e r a t i o n are the volume of rock that i t has affected (Fig 3.1) and the i n t e n s i t y , or completeness of the a l t e r a t i o n process. - 139 -Mineralogy of the s e r i c i t i c a l t e r a t i o n includes quartz, muscovite, and carbonate with minor sulphides, oxides and r e l i c accessory minerals. A l b i t e commonly occurs with the a l t e r a t i o n assemblage but i t i s not always c l e a r whether t h i s i s primary or secondary i n o r i g i n . Euhedral, r e l a t i v e l y coarse-grained, i n t e r l o c k i n g a l b i t e c r y s t a l s within quartz and/or carbonate v e i n l e t s indicate that a l b i t e was stable during the l a t e r stages of the a l t e r a t i o n process. Epidote and c h l o r i t e may p e r s i s t as r e l i c grains i n areas of less intense a l t e r a t i o n . Replacement textures between s e r i c i t e and c h l o r i t e do not unequivocally indicate the r e l a t i v e timing of metamorphism and s e r i c t i c a l t e r a t i o n . Secondary c h l o r i t e i s associated with some l a t e quartz veins. There does not appear to be any mineralogical zoning within the a l t e r a t i o n , that i s , the a l t e r a t i o n mineralogy remains consistent but does vary i n abundance r e l a t i v e to unaltered mineralogy. Compositions of the opaque and accessory minerals were determined q u a l i t a t i v e l y by scanning electron microscopy of polished t h i n sections. Accessory minerals include apatite, monazite, zircon and ilmenite or leucoxene. Monazite grains within carbonate r i c h s e r i c i t i c a l t e r a t i o n can exhibit heavy-rare earth element enriched overgrowths. Vaslov (1967) attributes rare earth enriched monazite with co-e x i s t i n g non-enriched apatite to hydrothermal a c t i v i t y . - 140 -Very fine grains of galena and ba r i t e are disseminated throughout intensely s e r i c i t i z e d sections. These grains may have formed from lead and barium released from potassium feldspar during i t s conversion to muscovite. Pyr i t e occurs throughout the s e r i c i t i c zone as randomly disseminated fine-grained rounded c r y s t a l s to coarse euhedral porphyroblasts. Rutilated quartz occurs within the coarser quartz-albite veins and indicates l o c a l titanium mobility. Carbonate-oxide a l t e r a t i o n i s associated with f a u l t and fracture controlled mineralization and overprints a l l other deformation and a l t e r a t i o n features. The oxidation reactions may be temporally and/or ge n e t i c a l l y unrelated to the carbonate a l t e r a t i o n but because the oxidation gives the carbonate a l t e r a t i o n a d i s t i n c t i v e orange colour the two features are usually mapped as a single zone. The carbonate-oxide a l t e r a t i o n t y p i c a l l y occurs as a halo or envelope around veins or fractures. In extensively veined or stockworked greenstone the a l t e r a t i o n zone can be up to 100m i n width. More commonly, widths of the a l t e r a t i o n zones vary from 2 to 40m, or f i v e to ten times the thickness of the associated vein. Mineralogy of the carbonate-oxide a l t e r a t i o n (Table 4.2) consists of carbonate, s e r i c i t e , a l b i t e , sulphides, ferrous or t i t a n i f e r o u s oxides and r e l i c minerals of the p r o t o l i t h . Quartz usually occurs as v e i n l e t s or micro-- 141 -stockworks and only r a r e l y occurs within the matrix. Development of s e r i c i t e i s much more evident i n t h i n section than i n outcrop. Carbonate composition can be highly v a r i a b l e even within s i n g l e grains (Plate 3.8b), but within fracture f i l l i n g s i t i s r e l a t i v e l y homogeneous. Grains of banded hematite and r u t i l e or anatase ( t r e l l i s texture) are probably produced from the oxidation of ilmenite (Hayes, 1984). Monazite occurs l o c a l l y as fine-grained disseminations and may be secondary as suggested by substantial increases i n the cerium contents of altered rocks (Table 3.5). Sulphide minerals are sparsely disseminated, generally very fine grained, and consist of the same assemblage as found within the veins, namely: p y r i t e , pyrrhotite, spahalerite, galena, chalcopyrite and arsenopyrite. Textures of the carbonate a l t e r a t i o n are varied and range from p a r t i a l replacement of e x i s t i n g minerals to coarse-grained open space f i l l i n g i n stockwork veins. A large (20 by 800m) and conspicous zone of carbonate a l t e r a t i o n , which follows a northeasterly trending structure i n the north-central study area, displays some unusual features. The north or footwall side i s a carbonate-quartz stockwork (Plate 3.7a) which grades into unaltered greenstone. The south side consists of a monomineralic - 142 -carbonate (Ca:Fe:Mg = 7:2:1) breccia with fragments of coarse-grained botryoidal and colloform carbonate (Plate 3.7f). The stockwork c a r r i e s geochemically anomalous gold and s i l v e r , whereas the carbonate breccia i s barren. The orange weathering carbonate-oxide a l t e r a t i o n does not appear to be a supergene feature. In numerous l o c a l i t i e s , outcrops show sharp t r a n s i t i o n s between orange-weathering and green rocks. Boundaries between the orange and green areas are commonly eithe r quartz or carbonate v e i n l e t s (Plate 3.3g). No discernable difference i n mineralogy between the orange and green areas could be seen i n t h i n sections which straddle the colour change. Local control of t h i s colouration, which may r e f l e c t a change i n the oxidation state of iron, appears to be a function of f l u i d pressure and permeability. I t i s not known whether the f l u i d s were ascending and hydrothermal or descending and meteoric. 4.3.2 Chemical Changes Associated with A l t e r a t i o n Changes i n the chemical composition of al t e r e d rocks can be used, i n conjunction with mineralogical and textural changes, to draw inferrences regarding the a l t e r a t i o n processes. To accurately determine the gains and losses of material that took place during a l t e r a t i o n , r e l i a b l e chemical compositions of both the unaltered and altered - 143 -rocks are required. Additionally, changes to rock density and volume must also be considered. Problems r e l a t i n g to t h i s procedure are summarized i n Meyer and Hemley (1967) and Ri v e r i n (1977). Estimates of the chemical compositon of a rock p r i o r to a l t e r a t i o n are usually made by analysis of the le a s t a l t e r e d l i t h o l o g i c a l equivalent (Gresens, 1967). This procedure i s t r i c k y for rocks within the study area because of probable chemical inhomogeneity within rock units and because a l t e r a t i o n transgresses l i t h o l o g i c a l contacts so that the pr e - a l t e r a t i o n l i t h o l o l g y i s not always known. Determination of changes i n density or volume can be made i f immobile elements can be i d e n t i f i e d and plotted on a isochon diagram (Grant, 1986), which i s a graphical so l u t i o n to Gresens' (1967) volume-concentration equations. Major element data for samples from the s e r i c i t i c and carbonate-oxide a l t e r a t i o n zones are plotted against t h e i r most l i k e l y unaltered l i t h o l o g i c a l equivalents on isochon diagrams of Figure 4.2. The extreme chemical v a r i a b i l i t y of the greenstone unit (section 3.6) and an almost a r b i t r a r y s e l e c t i o n of unaltered p r o t o l i t h for the s e r i c i t i c l y altered rocks prevents a meaningful estimate of volume change. However, the isochon diagrams remain useful f o r displaying r e l a t i v e elemental changes between altered and unaltered rocks. Destruction of primary rock texture and fin e stockwork veins within the carbonate a l t e r a t i o n indicate - 144 -5 10 15 20 26 30 % Unaltered Greenstone R81-188 Figure 4.2 An isochon diagram (Grant, 1986) for greenstone unit, comparing fresh rock with three zones of progressive carbonate alteration. Heavy lines show isochons for constant volume and constant aluminium, dashed lines show changes in components. - 145 -possible volume change. Choosing best f i t or constant aluminium isochon l i n e s (Fig. 4.2) shows increases of S i 0 2 , K 20 and C0 2 and decreases i n Na 20, FeO, MgO and CaO within both the carbonate and s e r i c i t i c a l l y a ltered rocks. Chemical and corresponding mineralogical changes i n s e r i c i t i c a l t e r a t i o n around the T e l l u r i d e vein and a related quartz vein are shown i n Figure 4.3. Host rocks for t h i s a l t e r a t i o n are c h l o r i t e s c h i s t s i n the southern part of .the study area. Increases i n S i 0 2 and K 20 coincide with intense s e r i c i t i z a t i o n and r e l a t i v e depletion i n Na 20, MgO, FeO and CaO. Secondary a l b i t e and an increase i n carbonates peripheral to the t e l l u r i d e vein increases the N a 2 ° concentration and tends to sta b a l i z e the carbonate forming components, CaO and MgO. Trends i n FeO concentration can be l o c a l l y e r r a t i c due to formation of p y r i t e and s i d e r i t e . 4.3.3 Conditions and Origin of A l t e r a t i o n Mineralogical, chemical and textural changes associated with a l t e r a t i o n can be used to i n f e r the nature of the hydrothermal f l u i d ( s ) and conditions under which the a l t e r a t i o n took place. These inferences allow a better understanding of the processes responsible for a l t e r a t i o n and related mineralization. A l t e r a t i o n takes place through f l u i d - r o c k i n t e r a c t i o n where the f l u i d and rock are i n i t a l l y out of chemical equilibrium. Unlike metamorphism, - 146 -MINERALOGY QUARTZ MUSCOVITE CHLORITE ALBITE PYRITE CARBONATE LU O DC III Q. 30 25 -20 -15 10 -5 -VEIN VEIN 181 G 181 F i i 181 E 181 D 181 C 181 A SAMPLE Distance 10 m FIGURE 4 . 3 Chemical and M i n e r a l o g i c a l Changes A s s o c i a t e d w i t h t h e T e l l u r i d e V e i n , BJ P r o s p e c t , N o r t h w e s t e r n B.C. - 147 -a l t e r a t i o n i s usually an open system where components may be added or subtracted (metasomatism). Moreover, hydrothermal f l u i d s may change with time and reactions with host rocks may be p a r t i a l , leaving disequilibrium mineral assemblages (Rose and Burt, 1979; Henley et a l . . 1984). Metamorphism of e a r l i e r a l t e r a t i o n assemblages adds an additional complicating factor. The s p a t i a l extent and i n t e n s i t y of the s e r i c i t i c a l t e r a t i o n suggests high f l u i d to rock r a t i o s . The more or l e s s uniform mineral assemblage of muscovite, quartz, carbonate, a l b i t e and opaques over large areas indicates that chemical equilibrium was attained, at l e a s t l o c a l l y , although t h i s could be caused by l a t e r metamorphism. Changes i n mineralogy (Table 4.2) were primarily caused by two types of reactions: hydrolysis and carbonation. The formation of quartz and muscovite from feldspars i s con t r o l l e d by exchange of cations K +, Na +, and Ca + with H + i n the hydrothermal f l u i d . A reaction showing the hydrolysis of a l b i t e i s as follows: 3NaAlSi 3O g + K + + 2H + = K A l 3 S i 3 0 1 ( ) (OH) 2 + 6Si0 2 + 3Na + a l b i t e muscovite quartz Any of the feldspar compositions could be substituted i n for a l b i t e to produce a s i m i l a r reaction. Continued hydrolysis would produce k a o l i n i t e according to the reaction: K A l 3 S i 3 0 1 Q ( O H ) 2 + H + + 3/2H20 == 3/2Al 2Si 20 5(OH) 4 + K + muscovite k a o l i n i t e - 148 -The r e l a t i v e abundance or a c t i v i t e s of K and H w i l l drive these reactions to the l e f t or rig h t , respectively. The chemical s t a b i l i t e s for these reactions as a function of H + and K + a c t i v i t e s i s displayed graphically i n Figure 4.4. Textural evidence from t h i n sections suggests that i n i t i a l l y a l b i t e was unstable and decomposed to form muscovite (or k a o l i n i t e ) and quartz. This reaction may have buffered the f l u i d , eventually d r i v i n g i t into the a l b i t e - s t a b l e f i e l d (Fig 4.4). At 250°C the pH for muscovite + quartz + a l b i t e stable i s approximately 5.5 (Henley et a l . . 1984) which i s near n e u t r a l i t y for that temperature. S t a b i l i t y of k a o l i n i t e , muscovite and feldspar also varies with temperature. The reaction curves for the K20-A1203-Si02-H20 system (Fig. 4.5) show that the t r a n s i t i o n from muscovite to k a o l i n i t e requires increasing H + a c t i v i t y with increasing temperature. A k a o l i n i t e r i c h assemblage produced by early hydrogen or potassium metasomatism could have been converted to a muscovite r i c h assemblage during a l a t e r metamorphic event. Muscovite and quartz react to form andalusite and/or p y r o p h y l l i t e and/or potassium feldspar at temperatures between 300 and 600°C depending on aK +/aH + (Montoya and Hemley, 1975). Neither andalusite or secondary potassium feldspar are observed in d i c a t i n g probable a l t e r a t i o n or metamorphic temperatures of less than 350°C. - 149 -Presence of dissolved C0 2 within the hydrothermal or metamorphic f l u i d can contribute to hydrogen metasomatism by the following reaction: C0 2 + H 20 == HC03~ + H + This reaction i s controlled by temperature and pressure of the system and w i l l be buffered by reactions which consume C0 2, such as: 3 ( M g F e ) 5 A l 2 S i 3 0 1 0 ( O H ) 8 + 15HC03~ + 13H + +2K+ == c h l o r i t e 2KAL 3Si 30 1 Q(OH) 2 + 15(MgFe)C0 3 + 3Si0 2 + 24H20 muscovite ankerite quartz Carbonation reactions could take place during metamorphism, a l t e r a t i o n or both. Harte and Graham (1975) have shown that greenschist facies metamorphism of mafic igneous rocks w i l l produce d i f f e r i n g mineral assemblages, consisting of c h l o r i t e , c a l c i t e and ankerite, depending on the amount of C0 2 present during metamorphism. The source for C0 2 within metamorphic f l u i d s could be graphite-water reactions or decarbonation reactions at depth (section 3.4.2). However, the extensive area that received carbonate addition and the i n t e n s i t y of that addition within the study area, i s not t y p i c a l for metamorphic terranes. Additionally, CO w i l l - 150 -remain at r e l a t i v e l y low concentrations within a metamorphic f l u i d i f pH i s buffered (Kerrich and Fyfe, 1981). Therefore hydrogen metasomatism with the addition of carbonate i s more reasonably caused by magmatic related a l t e r a t i o n processes i n the Mess Creek area. The o r i g i n of the s e r i c i t i c a l t e r a t i o n within the map area remains unclear. Rock textures displayed i n outcrop and t h i n sections indicate that s e r i c i t i c a l t e r a t i o n occurred p r i o r to the second phase of deformation and therefore, i s temporally d i s t i n c t from the carbonate-oxide a l t e r a t i o n which i s post-deformation. D i s t r i c t - s c a l e a l t e r a t i o n with a muscovite, plagioclase, c h l o r i t e , quartz, carbonate and p y r i t e assemblage has been mapped i n the Stewart area by Grove (1968). Grove a t t r i b u t e s t h i s a l t e r a t i o n , which hosts many of the mineral deposits of the region, to a process involving dynamothermal metamorphism, c a t a c l a s i s , g r a n i t i z a t i o n , and migrating meteoric waters. There are s i m i l a r i t i e s between the a l t e r a t i o n mapped by Grove and that of the study area. However, although a temporal r e l a t i o n s h i p between a l t e r a t i o n and metamorphism cannot be ruled out, a genetic r e l a t i o n s h i p i s rejected (but remains moot) fo r the following reasons: - 151 -Di s t r i b u t i o n of s e r i c i t i c a l t e r a t i o n i s widespread but appears l o c a l i z e d by a combination of structure and l i t h o l o g y ; i t i s d i f f i c u l t to explain t h i s d i s t r i b u t i o n i n a regional metamorphic context. The degree of hydrogen and carbonate metasomatism i s uncharacteristic of greenschist fa c i e s metamorphism. Both xenotime and monazite appear to occur as secondary phases. These minerals are regarded as unstable during greenschist facies metamorphism (Overstreet, 1961) and t h e i r growth with rare-earth element enrichment are considered most c h a r a c t e r i s t i c of hydrothermal systems by Vlasov (1967). - 1 5 2 -D O O — I T" T=200*C P=500 bars ALBITE PARAGONITE - KAOLINITE MUSCOVITE K-SPAR L O G ( a K + / a H + ) F i g u r e 4 .4 P h a s e re la t i ons in the s y s t e m K 2 0 - N a 2 0 - A l 2 0 3 - S i 0 2 - H C I - H 2 0 in the p r e s e n c e of q u a r t z a s a f u n c t i o n of l0g(d^/a H+) a n d log(aN£/a,}) fo r unit a c t i v i t y of H 2 0 at 5 0 0 b a r s ar id 2 0 0 ° C . T a k e n f rom J a c k s o n and H e l g a s o n ( 1 9 8 5 ) . 153 -IOO -i i i 1 1 i 0 1 2 3 4 5 L O 9 (mKCI /mHC|) Figure 4.5 Phase relations in the systems I^O-AlgOjj-SiOg-HgO Na 2 0-A l203-S i0 2 -H 2 0 as a function of temperature and calculated activity ratios. From Henley et al. (1984). - 154 -Potassium metasomatism with corresponding elemental losses and gains and a quartz-muscovite-carbonate-albite a l t e r a t i o n assemblage i s commonly associated with porphyry type deposits (Gilbert and Lowell, 1974; Drummond and Godwin, 1976), certain volcanogenic massive sulphide deposits (Ohomoto and Skinner, 1979; S i l l i t o e , 1978), and quartz vein systems (Casaderval and Ohmoto, 1977; Rose and Burt, 1979). Within the map area, there are large volumes of metasomatized rock with few or no associated quartz veins i n d i c a t i n g that, although veins are associated with a l t e r a t i o n , the source of the hydrothermal f l u i d s l i e s elsewhere. Zonation of a l t e r a t i o n assemblages, t y p i c a l of a porphyry system, would be d i f f i c u l t to recognize following r e - e q u i l i b r i a t i o n during regional metamorphism and d i s t i n c t i o n between porphyry and volcanogenic massive sulphide related a l t e r a t i o n on the basis of zoned assemblages i s not f e a s i b l e . Intrusive rocks are not observed to be s p a t i a l l y associated with the s e r i c i t i c a l t e r a t i o n zone. Therefore, i f an intrusion i s related to the a l t e r a t i o n i t must ex i s t at depth, and the d i s t i n c t i o n between porphyry and volcanogenic systems becomes i n d i s t i n c t (Britten, 1981). Geological features, including coarse-grained f e l s i c fragmental rocks, sulphide p y r o c l a s t i c fragments and small stratiform sulphide lenses, favour a volcanogenic o r i g i n for the s e r i c i t i c a l t e r a t i o n . Massive sulphide deposits i n l i t h o l o g i c a l l y s i m i l a r and c o r r e l a t i v e - 1 5 5 -stratigraphy i n the Tulsequah River area (Nelson and Payne, 1984) support t h i s concept. The o r i g i n of the carbonate-oxide a l t e r a t i o n i s s p a t i a l l y and temporally associated with quartz, carbonate and sulphide veins. These associations and a s i m i l a r sulphide assemblage between the a l t e r a t i o n and the veins implies a genetic r e l a t i o n s h i p . The c o - p r e c i p i t a t i o n of quartz and carbonate minerals necessitates sp e c i a l physiochemical conditions, at l e a s t t h e o r e t i c a l l y , due to the reverse temperature s o l u b i l i t y c o e f f i c i e n t s of these minerals. Chemical changes related to carbonate a l t e r a t i o n are very s i m i l a r to changes related to s e r i c i t i c a l t e r a t i o n , namely the loss of Na 20, MgO, CaO and FeO and gains of S i 0 2 , K 20 and C0 2. These changes could be produced by a combination hydrolysis-carbonation reaction: 3(MgAlFe) 5Si 40 1 ( )(OH) 8+ 3 C a 2 A l 3 S i 3 0 1 2 (OH) + 12C0 3" + 5K + 25H + c h l o r i t e epidote = 5 K A l 3 S i 3 O 1 0 ( O H ) 2 + 6Ca(MgFe)(C0 3) 2 + 6Si0 2 + 3 F e + 2 + 6Mg + 2 muscovite dolomite quartz + 21H20 Depending upon the Na/K r a t i o of the hydrothermal f l u i d , a l b i t e could be formed instead of muscovite. Reactions of t h i s sort are compatible with textures observed i n t h i n section. - 156 -Sudden gas loss due to adiabatic b o i l i n g of hydrothermal f l u i d s i s a mechanism invoked for carbonate deposition i n many epithermal deposits (Meyers and Hemley, 1985). Textures associated with b o i l i n g such as brecciation are not commonly observed within the mineralization of the BJ prospect, except for the carbonate breccias of the McLaughlin Creek zone which could well be a b o i l i n g phenomenon. Carbonate may also be p r e c i p i t a t e d i f C0 2 enriched f l u i d s are cooler than t h e i r host rocks. This type of s i t u a t i o n i s noted where seawater convects through b a s a l t i c rocks of the ocean f l o o r and carbonate i s deposited i n downward-flowing "recharge zones" (Kerrich and Fyfe, 1981). An analogous process may be envisioned for the o r i g i n of the oxidation zones but i t i s not compatible with the geometry of the carbonate a l t e r a t i o n or the geology at the time of formation. Kerrich and Fyfe (1981) demonstrate that metamorphic f l u i d s undergoing C0 2-forming reactions w i l l l a t e r reverse those reactions and p r e c i p i t a t e carbonate within wall rock a l t e r a t i o n zones. I f the Jurassic age of mineralization, indicated by a potassium-argon date on vein selvage muscovite, i s correct then metamorphic f l u i d s are an u n l i k e l y source for a l t e r a t i o n within the BJ prospect as there i s no evidence of regional metamorphism i n t h i s area during Jurassic time. - 157 -Adjustment of pH within hydrothermal f l u i d s by hydrolysis reactions (and possibly, pressure and temperature) can lead to the deposition of carbonate (Holland, 1967). The d i s s o c i a t i o n of C0 2 dissolved i n water i s given by the reaction: C0 2 + H 20 == HC0 3" + H + Analyses of geothermal f l u i d s show that HC03 i s the dominant carbon-bearing species (Henley e t _ a l . , 1984) and therefore other reactions are of less importance. This process i s suggested f o r the a l t e r a t i o n at the Con mine near Yellowknife by Kerrich and Fyfe (1981). Thus, the carbonate a l t e r a t i o n and the s e r i c i t i c a l t e r a t i o n may have been produced by s i m i l a r processes. Differences i n the character of the two a l t e r a t i o n types would be caused by l i t h o l o g y , s t r u c t u r a l and l i t h o l o g i c a l controls on f l u i d flow, and possible differences i n the r e l a t i v e amounts of HC1 and C0 2 within the a l t e r a t i o n f l u i d s . Source of the hydrothermal f l u i d s related to the carbonate a l t e r a t i o n i n the Mess Creek area i s not evident from f i e l d mapping. However, the Early J u r a s s i c age for the mineralization i n v i t e s comparison with the s i m i l a r aged Red Chris and Galore Creek porphyry Cu-Au deposits, p a r t i c u l a r l y as there i s no evidence f o r a s i m i l a r age metamorphic event i n the immediate area. At the Red Chris deposit, a l t e r a t i o n associated with the Red stock, an Early J u r a s s i c hornblende - 158 -monzonite, consists of potassium metasomatism and carbonatization indicated by three a l t e r a t i o n assemblages: o r t h o c l a s e - a l b i t e - b i o t i t e , a l b i t e - c h l o r i t e - c a l c i t e , and ankerite-quartz-sericite (Schink, 1977). Ankerite veins and breccia zones are noted within andesitic rocks peripheral to the Red stock. Temperatures of a l t e r a t i o n and mineralization within the Galore Creek deposits were i n the 450 to 600°C range which produced an a l t e r a t i o n assemblage co n s i s t i n g primarily of orthoclase, b i o t i t e and andradite-grossular garnet (Allen et a l . , 1976). Carbonate veins and a l t e r a t i o n zones are observed peripheral to the deposit area (D. Forster, pers. comm., 1988). 4.4 Comparison of Mineralization with Other Deposits Mineralization of the BJ prospect has many features i n common with other mesothermal vein-type deposits. Comparisons between the study area and other, better understood, deposits could lead to more e f f i c i e n t exploration. Mineral and chemical c h a r a c t e r i s t i c s of veins and a l t e r a t i o n on the BJ prospect have strong s i m i l a r i t i e s to those of the Mother Lode deposits i n north-central C a l i f o r n i a and vein deposits i n Archean greenstone b e l t s of the Canadian Shield. - 159 -The Mother Lode i s a 1 to 5km wide system of en echelon quartz veins which extends over a length of 190km, primarily along the Melones f a u l t zone, a major tectonic break i n C a l i f o r n i a (Dodge and Lloyd, 1984). Gold and minor sulphide minerals i n quartz veins are hosted by carbonate-altered mafic rocks. In the northern part of the d i s t r i c t , most of the gold i s contained within the quartz veins, whereas i n the southern part of the d i s t r i c t the quartz veins are generally barren and the gold occurs within enormous bodies of carbonate rock and associated carbonate-altered and mineralized country rock (Dodge and Lloyd, 1984). The mineralized country rock, usually greenstone, i s composed of ferro-magnesium carbonate and lesser amounts of s e r i c i t e (± fuc h s i t e ) , a l b i t e , quartz and sulphides (Knopf, 1929). Gold deposits within Archean greenstones have s i m i l a r vein and a l t e r a t i o n mineralogy to that described for the Mother Lode and study area mineralization. Additionally, chemical changes associated with a l t e r a t i o n of Archean gold deposits are si m i l a r to those documented for the study area. P i r i e (1982) demonstrates wallrock gains of S i 0 2 and K 20, and losses of MgO, FeO and Na 20 related to vein-associated a l t e r a t i o n i n metavolcanic rocks of the Red Lake d i s t r i c t , Ontario. Andrews and Wallace (1983) note that regional carbonate a l t e r a t i o n , i n the Red Lake d i s t r i c t , overprints greenschist facies metamorphic assemblages but preceeds l o c a l i z e d , intense Fe-carbonatization, s i l i c i f i c a t i o n and s e r i c i t i z a t i o n associated with gold mineralization. - 160 -Both the Mother Lode and Archean greenstone mineralization d i f f e r from the study area i n tectonic s e t t i n g , s t r u c t u r a l controls and scale of mineralization. Therefore, evolution of hydrothermal systems within the d i f f e r e n t areas could also be quite d i f f e r e n t . However, the strong s i m i l a r i t y between the chemistry and mineralogy of the host rocks, mineralization and a l t e r a t i o n of these areas indicates a commonality of hydrothermal f l u i d chemistry and fluid-wallrock interactions. Studies of the carbonate-quartz-gold association, p a r t i c u l a r l y i n the s h i e l d deposits (Dube et a l . . 1987; Kerrich and Fyfe, 1981; Fyon and Crockett, 1982) have shown that deposition of quartz, carbonate and gold i s controled by f l u i d pH and redox reactions much more than by pressure and temperature (see section 3.3 f o r further discussion). A l t e r a t i o n mineralogy i s compatible with formation at temperatures from 200 to 400°C and can be derived from a s i n g l e f l u i d with a high C0 2/H 20 which i s i n i t i a l l y reducing but through water-rock i n t e r a c t i o n gradually becomes ox i d i z i n g (Dube et a l . , 1987). This provides a reasonable explanation f o r why deposits of t h i s type t y p i c a l l y occur i n greenschist metamorphic terranes. Deposits which occur at shallow depths are more l i k e l y to have mineral deposition c o n t r o l l e d by pressure and temperature, whereas chemically c o n t r o l l e d mineral deposition would occur at greater depths. Rocks buried to such depths would l i k e l y have undergone at l e a s t some metamorphic e f f e c t s . - 161 -Deposits i n northwestern B.C. and Alaska that have features i n common with the study area include the Alaska Juneau (AJ) Gold Mine, the Golden Bear deposit and the Polaris-Taku mine (locations given i n Figure 1.3). These deposits share, perhaps, a more common tectonic s e t t i n g and scale of mineralization with the study area than those deposits previously discussed. The Alaska Juneau Mine has produced over 100,000kg of gold (3 m i l l i o n ounces) and i s hosted i n T r i a s s i c to lowermost Jurassic mafic volcanics and p h y l l i t e s . These rocks have been regionally metamorphosed to upper green-s c h i s t f a c i e s ( M i l l e r , 1988). Pyr i t e , pyrrhotite, sphalerite, galena, tetrahedrite and gold are contained i n quartz veins that occur within a 5,000m long zone that ranges i n width from 30 to 120m. Individual quartz veins, usually l e s s than 1.5m i n width and 100m i n length, are most numerous within or near the mafic metavolcanic units ( M i l l e r , 1988). Intense b i o t i t e and Fe-carbonate a l t e r a t i o n associated with veining overprints the metamorphic assemblage. The mineralogical c h a r a c t e r i s t i c s of the Alaska Juneau deposit are s i m i l a r to those of the study area, although the presence of b i o t i t e i n the a l t e r a t i o n assemblage suggests higher mineralization temperatures. The contrast between the si z e of i n d i v i d u a l veins and the o v e r a l l s i z e of the system could have profound implications - 162 -for exploration. Like the BJ prospect, mineralization at the Alaska Juneau mine i s l a t e r than regional dynamothermal metamorphism. Plutonic rocks are not recorded within the mine area. The Golden Bear deposit, a recent discovery by Chevron Minerals Ltd. 100km west of Dease Lake and 170km northwest of the study area, has indicated reserves of 1.6 m i l l i o n tonnes grading l l g / t Au (Franzen, 1987). The deposit i s hosted by sch i s t s and limestone of the S t i k i n e Assemblage. Min e r a l i z a t i o n i s s t r u c t u r a l l y c o n t r o l l e d and post-dates regional metamorphism. A l t e r a t i o n consists of carbon-a t i z a t i o n and s e r i c i t i z a t i o n within the s c h i s t s and s i l i -c i f i c a t i o n within the limestone (K. Shannon, pers. comm., 1986). Detailed mineralization c h a r a c t e r i s t i c s of the Golden Bear deposit are not p a r t i c u l a r l y s i m i l a r to those of the study area, l i k e l y r e f l e c t i n g a difference i n st r u c t u r a l and l i t h o l o g i c a l controls. The Polaris-Taku mine i s situated 250km northwest of the study area. Past production and remaining reserves when the mine closed i n 1949 totaled 700,000 tonnes of approximately l l g / t Au (Smith, 1950). Mineralization consists of quartz and quartz-carbonate veins within al t e r e d , deformed and metamorphosed p y r o c l a s t i c rocks and greenstones of the Stikine Assemblage (Nelson and Payne, 1984). A l t e r a t i o n i s widespread and i s composed of Fe-- 1 6 3 -carbonate, quartz, a l b i t e , s e r i c i t e and l o c a l fuchsite (Bacon, 1942). About ninety percent of production came from areas of intersecting fractures within the s i l l - l i k e greenstone rocks (Smith, 1950). Mine r a l i z a t i o n cuts a l l deformation features (Smith, 1950). Sulphide minerals i d e n t i f i e d by Bacon (1942) include p y r i t e , arsenopyrite and s t i b n i t e . The widespread q u a r t z - a l b i t e - s e r i c i t e a l t e r a t i o n (Smith, 1950) i s e a r l i e r or synchronous with dynamothermal metamorphism (J. Nelson, pers. comm., 1985). The Polaris-Taku mine appears to have had a s i m i l a r t e c t o n i c h i s t o r y to that of the study area. S t r u c t u r a l l y c o n t r o l l e d , post-deformation, greenstone hosted mineralization with associated intense carbonate a l t e r a t i o n suggests a s i m i l a r ore genesis with that of the study area. - 164 -CHAPTER 5: CONCLUSIONS 5.1 Summary of Geological History The geological h i s t o r y of the study area i s determined from observed or inf e r r e d s t r u c t u r a l , s t r a t i g r a p h i c and in t r u s i v e relationships. The key features of the Mess Creek area documented by t h i s study are reviewed i n the following section. The Stikine Assemblage i s a c o l l e c t i o n of Palezoic rocks which form the basement of S t i k i n i a , an allocthonous or 'suspect' terrane, which formed to the west of continental North America (Monger, 1977). Paleomagnetic data from Late T r i a s s i c rocks within S t i k i n i a indicate that when those rocks formed they were 13 to 14° south of t h e i r present position, possibly near the present C a l i f o r n i a -Oregon border (Irving, 1981). This i s supported further by the s i m i l a r i t y between Mississippian fauna from limestones of the Stikine Assemblage and species found i n northern C a l i f o r n i a (Pitcher, 1 9 6 0 ) — i n marked contrast to exotic Permian age Tethyian f u s i l i n i d s found i n Cache Creek limestones. Py r o c l a s t i c and e p i v o l c a n i c l a s t i c l i t h o l o g i e s within the Mississippian stratigraphy exposed i n the study area are compatible with an arc-type s e t t i n g . Local basins, - 165 -presumably between volcanic e d i f i c e s , appear to have received f i n e e p i c l a s t i c material from one source while receiving proximal p y r o c l a s t i c debris from another. A singl e t r a n s i t i o n from mafic to f e l s i c volcanism i s preserved within the study area. Argillaceous sediments mark period(s) of volcanic quiescence. Large areas of potassium metasomatism, defined by a quartz-muscovite-carbonate a l t e r a t i o n assemblage, were caused by syngenetic, submarine (?) geothermal systems. Related volcanogenic massive sulphide deposits are possible. The st r a t i g r a p h i c sequence i s capped by limestones deposited on emergent seamounts (Monger, 1977). Permian stratigraphy was l i k e l y deposited disconform-ably on the Mississippian rocks. More work i s required to determine the precise nature of the Permian-Mississippian contact. Permian volcanic rocks are l i t h o l o g i c a l l y s i m i l a r to the Mississippian units but are generally f i n e r grained and more d i s t a l i n character. The widespread Permian carbonate rocks suggest deposition i n a platformal environment (Monger, 1977). Both the Mississippian and Permian rocks were reg i o n a l l y metamorphosed to lower greenschist f a c i e s . Early deformation was synchronous with metamorphism, but deformation s i g n i f i c a n t l y outlasted thermal e f f e c t s . Two phases of recumbant i s o c l i n a l f o l d i n g with northwesterly - 166 -trending shallowly plunging f o l d axis were followed by easterly trending kink bands and chevron folds, and by open, north-trending p a r a l l e l - t y p e folds. The i n t e n s i t y of deformation exposed i n the study area i s a t t r i b u t e d to decreased s t r u c t u r a l competence caused by early, widespread a l t e r a t i o n . Penetrative a x i a l planar f o l i a t i o n accompanied both phases of i s o c l i n a l f olding. This f o l i a t i o n i s cut, 10km south of the study area, by plutonic rocks which are t e n t a t i v e l y correlated with the Hickman pluton of Middle T r i a s s i c age. Therefore, regional dynamothermal metamorphism occurred before Middle T r i a s s i c time. Upper T r i a s s i c (Norian) f o s s i l i f e r o u s sediments, which are unmeta-morphosed, unconformably o v e r l i e the S t i k i n e Assemblage rocks providing additional constraints to the upper time l i m i t of regional metamorphism. These Norian sediments, which include boulder conglomerates with c l a s t s l i t h o g i c a l l y s i m i l a r to the Hickman pluton, were folded by the t h i r d and fourth phases of deformation. Plutonic rocks of the Hickman bat h o l i t h are l i t h o -l o g i c a l l y and chronologically s i m i l a r to rocks of the Hotialuh b a t h o l i t h i n the eastern part of the S t i k i n e Arch. Both the Hickman and Hotialuh batholiths are s p a t i a l l y associated with, but pre-date, eruption of Stuhini volcanic rocks. Chemical differences between some of the plutonic rocks of the Hotialuh b a t h o l i t h (calc-alkaline) and the - 167 -Stuhini rocks (alkaline) suggests d i f f e r e n t magmatic sources (Anderson, 1983). Stuhini volcanic rocks i n the Stikine Arch area are Middle to Late T r i a s s i c i n age (Souther, 1971) and are unmetamorphosed and only l o c a l l y deformed into open fo l d s . Tectonic a c t i v i t y within the Stikine Arch during J u r a s s i c time was concentrated along the northern edge where rocks within the King Salmon allocthon were metamorphosed, deformed and transposed along southwest-verging thrust f a u l t s (Monger, 1978; Thorstad, 1983). Westerly trending f o l d axes related to t h i s deformation occur within the Cache Creek Group rocks of the A t l i n Terrane (Souther, 1971) and the T r i a s s i c Kutcho formation. The east-west oriented kink bands and chevron folds within the Stikine Assemblage rocks and the overlying Late T r i a s s i c sediments of the Mess Creek area could be related to t h i s deformational event. However, there i s l i t t l e evidence f o r Jurassic or younger metamorphism within or near the study area. The Galore Creek and related intrusions, the Yehiniko pluton, and various other intrusions and re l a t e d porphyry copper-molybdenum-gold deposits were emplaced within the S t i k i n e Arch during Early to Middle J u r a s s i c time. - 168 -Potassium-argon and rubidium-strontium dating of s t r u c t u r a l l y c o n t r o l l e d quartz and quartz-carbonate veins, and associated a l t e r a t i o n within the study area give Early J u r a s s i c ages which coincide with the ages of a l k a l i c plutons and related porphyry deposits.. 5.2 Tectonic Implications Current models of tectonic evolution of the S t i k i n e Arch r e l y heavily on the larger plate tectonic model fo r the C o r d i l l e r a . The C o r d i l l e r a n model involves the accretion of a "collage of allocthonous terranes" to the western margin of the North American craton by convergent and transform plate motions (Price et a l . . 1981; Monger and Price, 1979). Timing of terrane accretion i s determined from geological events or features that e i t h e r l i n k or separate adjacent tectonostratigraphic assemblages or terranes. The S t i k i n e terrane, or S t i k i n i a , had coalesced with three more easterly terranes: Cache Creek, Quesnellia, and the Eastern Assemblage, to form a composite terrane, p r i o r to emplacement i n t h e i r present locations (Price et a l . , 1981). This composite terrane i s thought to have c o l l i d e d with North America between Middle Ju r a s s i c and Middle Cretaceous time (Price, 1981). Evidence f o r t h i s c o l l i s i o n p r i m a r i l y occurs external to the Stikine Arch and includes extensive r i g h t - l a t e r a l displacement on s t r i k e - s l i p f a u l t - 169 -systems on the eastern margin of the composite terrane which occurred during middle Jurassic to Middle Cretaceous time (Price et a l . , 1981). The magnitude of displacement on these s t r i k e - s l i p f a u l t systems (up to 500km) so l a t e i n C o r d i l l e r a n h i s t o r y renders tectonic models of e a r l i e r events speculative (Monger and Price, 1979). Dynaraothermal metamorphism of St i k i n e Assemblage rocks, within the study area took place between Late Permian .and the i n t r u s i o n of the Hickman pluton and r e l a t e d intrusions i n Early-Middle T r i a s s i c time. This period of metamorphism and deformation marked the beginning of the Tahltanian orogeny (Souther, 1971) that culminated i n the u p l i f t of Early to Middle T r i a s s i c plutons and eruption of Stuhini volcanics. Relationships between the Tahltanian orogeny and terrane c o l l i s i o n remain unclear. The Cache Creek Group i s thought to have been linked to S t i k i n i a by Late T r i a s s i c time on the basis that uppermost T r i a s s i c rocks of S t i k i n i a possibly overlap Cache Creek Group rocks (Monger i n Price et a l . , 1981). This i s consistent with a c o l l i s i o n between S t i k i n i a and more easterly terranes during the time of the Tahltanian orogeny. However, i f the Cache Creek Group represents a subduction complex for the Stuhini volcanic rocks, then i t i s u n l i k e l y that S t i k i n i a c o l l i d e d with the more inboard assemblages p r i o r to Stuhini volcanism. - 170 -Evidence for the Tahltanian orogeny i s widespread within the Stikine Arch but s i g n i f i c a n t l y l e s s so i n southern B r i t i s h Columbia. Anderson (1983) documents a northwesterly trending f o l i a t i o n within Early to Middle T r i a s s i c plutons of the Hotialuh b a t h o l i t h . The compression which caused t h i s f o l i a t i o n has a s i m i l a r o r i e n t a t i o n to that which caused i s o c l i n a l f o l d i n g i n the Stikine Assemblage rocks, and could be related to terrane c o l l i s i o n (Anderson, 1983). Corroborating evidence f o r terrane-continent c o l l i s i o n could be preserved south of the i n t e r -national border where the Sonoma orogeny i s approximately time equivalent with the Tahltanian orogeny. More d i r e c t evidence of c o l l i s i o n between S t i k i n i a and North America i s preserved i n Middle to Late J u r a s s i c rocks and events. Southwesterly directed f o l d s i n the A t l i n Terrane (Monger, 1975) and southwesterly verging thrust f a u l t s i n the King Salmon Assemblage (Tipper, 1978) are suggestive of terrane c o l l i s i o n . D i s t i n c t i v e chert and carbonate fragments from Cache Creek Group rocks occur within Middle to Upper Jurassic Bowser Lake Group sediments which indicates amalgamation of S t i k i n i a and the A t l i n Terrane by t h i s time (Monger et a l . , 1978) . A s l i g h t l y d i f f e r e n t l i n e of reasoning for terrane-continent c o l l i s i o n i s presented by Monger and Price (1979). They point out the d i s p a r i t y between the configuration of the T r i a s s i c volcanic arcs (Takla, Stuhini, Nicola) and t h e i r associated - 171 -subduction complex (Cache Creek Group) and the Jurassic Hazelton arc and i t s proposed subduction complex, the Bridge River Formation. This reversal i n arc-subduction complex p o l a r i t y i s best explained by c o l l i s i o n of the Stikine block with the craton. Accretion of the Stikine Block with North America i n J u r a s s i c time does not exclude the p o s s i b i l i t y of terrane c o l l i s i o n within S t i k i n i a , or between S t i k i n i a and a more westerly terrane, before Jurassic time. However, d i r e c t evidence to support such models would be l a r g e l y masked by younger events and Late Mesozoic cover. 4.3 Exploration Potential Although the c h a r a c t e r i s t i c s of mineralization and a l t e r a t i o n of the BJ prospect are known i n only a general way, comparisons with better understood deposits and t h e i r geological settings allows the formulation of a generalized genetic model. This, i n turn, can lead to more e f f i c i e n t exploration and evaluation of the prospect and surrounding areas. Two temporally d i s t i n c t a l t e r a t i o n types are recognized within the property area. The f i r s t , characterized by a quartz-muscovite-carbonate + p y r i t e assemblage ( s e r i c i t i c ) , i s widespread and pre-kinematic. Chemical changes - 172 -associated with t h i s a l t e r a t i o n include additions of quartz and potassium and losses of magnesium, sodium, iron and calcium. This a l t e r a t i o n i s attributed to pre-metamorphic hydrothermal a c t i v i t y rather then to metamorphic processes (section 4.3). The presence of sulphide fragments within the f e l s i c t u f f unit and small stratabound sulphide lenses suggest that t h i s early a l t e r a t i o n was caused by a submarine volcanogenic hydrothermal system. Related volcanogenic massive sulphide deposits are a d i s t i n c t p o s s i b i l t y . F o l i a t i o n - p a r a l l e l quartz veins are s p a t i a l l y , and possibly temporally, associated with zones of s e r i c i t i c a l t e r a t i o n . With a few exceptions, these veins are barren with respect to precious metals. Therefore, the s e r i c i t i c a l t e r a t i o n does not serve as a guide to g o l d - s i l v e r mineralization except i n a regional sense where o r i g i n a l s t r u c t u r a l weaknesses serve to focus l a t e r hydrothermal a c t i v i t y . The second type of a l t e r a t i o n consists of fracture-c o n t r o l l e d zones of carbonatization, s i l i c i f i c a t i o n and s e r i c i t i z a t i o n . This a l t e r a t i o n overprints s e r i c i t i c a l t e r a t i o n but i s most noticeable where i t has affected previously unaltered mafic l i t h o l o g i e s . Controlling structures cut a l l deformation-related f a b r i c s and are commonly coincident with A-C j o i n t s associated with the - 173 -fourth phase of fold i n g . Veins related to t h i s a l t e r a t i o n consist of a vari e t y of mixtures of quartz, Fe-carbonates and sulphides. Vein morphology varies from tabular to l e n t i c u l a r . Textures range from massive to banded and less commonly brecciated. Sulphide mineralogy includes p y r i t e , sphalerite, tetrahedrite, arsenopyrite, chalcopyrite, galena, s i l v e r and bismuth t e l l u r i d e s , electrum and gold. Late hypogene oxidation l o c a l l y overprints sulphide minerals with a pyrrhotite, hematite, and cuprite assemblage. Chemical changes associated with carbonate a l t e r a t i o n include the loss of MgO, Na 20 and FeO, and gains of S i 0 2 , K 20, C0 2 and l o c a l l y Ce. Change i n concentration of CaO i s var i a b l e . Co-precipitation of quartz, carbonate and sulphides was l i k e l y due to an increase i n the pH of the slowly cooling hydrothermal f l u i d caused by hydrolysis reactions with wall-rocks. Oxidation-reduction reactions could also have contributed to mineral deposition, p a r t i c u l a r l y the sulphides. This form of vein formation allows for considerable v e r t i c a l continuity of vein mineralogy. Potassium-argon and rubidium-strontium dates from vein associated a l t e r a t i o n indicates a minimum Early Jurassic age for mineralization. This age i s supported by geological constraints. Mineralization i s l a t e r than or synchronous - 1 7 4 -with the l a t e r phases of deformation, which have affected Late T r i a s s i c sediments. Re-setting of i s o t o p i c ages due to a thermal event i s u n l i k e l y because of the paucity of Early J u r a s s i c intrusions. More than a single o r i g i n of the hydrothermal f l u i d s r e l a t e d to carbonate a l t e r a t i o n and precious metal mineral-i z a t i o n i s possible. Comparisons with other deposits shows that s i m i l a r a l t e r a t i o n and mineralization mineralogy can be produced i n a v a r i e t y of tectonic settings. Kerrich and Fyfe (1981), P h i l l i p s (1986) and Kerrich (1986) demonstrate how dewatering and decarbonation reactions during prograde metamorphism at the greenschist-amphibolite f a c i e s t r a n s i t i o n can produce s u f f i c i e n t f l u i d s of the appropriate chemistry to cause quartz-carbonate-gold deposits. A l t e r n a t i v e l y , the extensive zones of quartz-a n k e r i t e - s e r i c i t e a l t e r a t i o n associated with the Red Chris porphyry deposit (Schink, 1977) demonstrate that a l k a l i c i n t r u s i v e rocks can produce s i m i l a r type f l u i d s (cf. Henely and E l l i s , 1983, and White, 1981). The age of mineralization on the BJ prospect i s concordant with the age of a l k a l i c intrusions at Galore Creek and elswhere i n the S t i k i n e Arch. This, and the lack of evidence f o r an Early J u r a s s i c metamorphic event i n the prospect area, implicates a moderate to deep seated a l k a l i c - 175 -in t r u s i o n as the source of hydrothermal f l u i d s . This r e l a t i o n s h i p i s r e f l e c t e d on a regional scale by the s i m i l a r i t y between the geographical d i s t r i b u t i o n of precious metal deposits along the western edge of the Stikine Arch and the northwesterly trending b e l t of a l k a l i c plutons as noted by Barr et a l . (1976). Regional exploration for mesothermal vein type deposits should focus on pre-Middle Ju r a s s i c rocks of mafic composition that show evidence of at l e a s t moderate b u r i a l (greenschist facies metamorphism) and s p a t i a l proximity to observed or infe r r e d a l k a l i c plutons. At the property scale, mineralization w i l l be primarily c o n t r o l l e d by structure; l i t h o l o g i c a l controls w i l l be of secondary importance. Structures with associated a l t e r a t i o n warrant attention. Carbonate a l t e r a t i o n haloes are t y p i c a l l y from 2 to 20 times the thickness of associated veins, and commonly contain a s i m i l a r sulphide assemblage as the veins. Therefore, geochemical based exploration techniques are well-suited to search f o r mineralized structures i n areas of overburden. - 176 -REFERENCES ALDRICK, D. 1986. Geology of the Stewart area, northwestern B. C., Unpub. PhD thesis, Univ. of B.C. ALLEN, D.G., PANTELEYEV, A., ARMSTRONG, A.T. 1976. Galore Creek i n porphyry deposits of the Canadian C o r d i l l e r a . C. I.M. Spec. Vol. 15, pp. 402. AMLI, R. 1975. Mineralogy and rare earth geochemistry of apatite and xenotime from gloserheia pegmatite, Froland, Norway. Am. Mineralogist, Vol. 60, pp. 607-620. ANDERSON, R.G. 1983. The Hotailuth Batholith, Cry Lake,. S p a t s i z i and Dease Lake map-areas north-central, B.C., Unpub. PhD thesis Carlton Univ. ARMSTRONG, R.L. 1984. 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