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Geology, mineral equilibria, sulfur, rubidium-strontium and lead isotopes and intrusion chemistry of… Cooke, Bradford James 1983

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c \ GEOLOGY, MINERAL EQUILIBRIA, SULFUR, RUBIPIUM-STRONTIUM AND LEAD  ISOTOPES AND INTRUSION CHEMISTRY OF THE McDAME TUNGSTEN SKARN  PROSPECT, NORTH CENTRAL BRITISH COLUMBIA by BRADFORD JAMES COOKE B . S c , Queen's U n i v e r s i t y , 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l Sciences We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA y October, 1983 M B r a d f o r d James Cooke, 1983 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a gree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f GjGtJ\jpG((M S C t g W C f c * ^ , The U n i v e r s i t y o f B r i t i s h C olumbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) i i ABSTRACT The McDame tungsten skarn prospect occurs i n Hadrynian to Or d o v i c i a n c l a s t i c and carbonate metasediments near Cretaceous f e l s i c s t o c k s . Three prograde skarn and ore f a c i e s and one retr o g r a d e f a c i e s are 1 i t h o l o g i c a l l y and s t r u c t u r a l l y c o n t r o l l e d . Prograde massive c a l c s i l i c a t e W-MO-Fe f a c i e s , c h a r a c t e r i z e d by garnet skarn with disseminated s c h e e l i t e and molybdenite, has r e p l a c e d g r a p h i t i c marble i n cont a c t with b i o t i t e h o r n f e l s and i s t h i c k e r near a northeast s t r i k i n g f a u l t . Banded c a l c s i l i c a t e Fe f a c i e s , t y p i f i e d by quartz skarn, has re p l a c e d b i o t i t e h O r n f e l s as envelopes around f r a c t u r e s . Banded oxide W-MO-Fe f a c i e s , d i s t i n g u i s h e d by magnetite skarn with l a m i n a t i o n s of mOlybdoscheelite, has r e p l a c e d g r a p h i t i c dolomite as v e i n s w i t h i n f r a c t u r e s . Retrograde massive s u l f i d e Fe-Zn-Cu-W f a c i e s , i d e n t i f i e d by p y r r h O t i t e skarn with disseminated s p h a l e r i t e , c h a l c O p y r i t e and s c h e e l i t e , has re p l a c e d the other skarn f a c i e s , and l o c a l l y g r a p h i t i c marble, i n vei n s and pods. L i t h o s t a t i c pressure d u r i n g formation Of McDame skarns was in the order of 1500 bars. Temperature and mole f r a c t i o n c o n d i t i o n s Of the metasOmatic f l u i d are estimated f o r the d i f f e r e n t skarn f a c i e s by assuming an i d e a l i z e d i r o n - f r e e system. Massive garnet skarn was s t a b l e to Tmax = 555°C and Xmax = 0.14; ga r n e t - b e a r i n g quartz skarn formed below Tmax = 475°C and Xmax = 0.08; f e l d s p a r - b e a r i n g magnetite skarn formed above Tmin = 430°C and Xmin = 0.06; and massive p y r r h O t i t e skarn a l t e r e d p r e v i o u s l y formed skarns at lower T and XC0 2. C a l c s i l i c a t e m i n e r a l zoning r e s u l t e d from d i s s o l u t i o n , i n f i l t r a t i o n / d i f f u s i o n and d e p o s i t i o n Of S i 0 2 , CaO, A l 2 0 3 , MgO, i i i H 20 and C0 2 i n the skarn p r o t o l i t h s . M e t a l l i c m ineral zoning, on the other hand, formed from W-, Mo-, 0 2- and S 2 - b e a r i n g magmatic f l u i d s that reacted with country rOcks to produce prograde f a c i e s , and mixed with Fe-, Zn-, and Cu-bearing f o r m a t i o n a l waters to form r e t r o g r a d e skarn. S u l f u r i sotope data from McDame porphyry, skarn and h o r n f e l s form s i x d i s c r e t e groups (skarn average 6 3 f tS = +7.7) between o r d i n a r y magmatic s u l f u r ( 6 3 4 S = 0) and Cambrian sedimentary s u l f a t e (6 3"S = +30). T h e i r d i s t r i b u t i o n can be ex p l a i n e d by f r a c t i o n a t i o n Of magmatic s u l f u r through r e a c t i o n of metasomatic f l u i d s with w a l l rocks to form porphyry p y r i t e , skarn p y r i t e and skarn p y r r h o t i t e , and mixing with f o r m a t i o n a l s u l f u r i n connate waters t o produce skarn p y r r h o t i t e -s p h a l e r i t e - c h a l c o p y r i t e and porphyry p y r r h O t i t e . Rubidium-st r o n t i u m i s o t o p e s from Kuhn stock q u a r t z f e l d s p a r porphyry d e f i n e a 69 i 2 Ma isOchron with a high i n i t i a l r a t i o Of 0.712, i n d i c a t i n g that the Kuhn stock had a s i a l i c c r u s t a l component. IsOtOpic d i s e q u i l i b r i u m between m i n e r a l phases suggests p r o g r e s s i v e contamination Of the g r a n i t i c melt by c o n t i n e n t a l c r u s t d u r i n g magma ascent and c r y s t a l l i z a t i o n . Lead isotope data from v e i n and skarn d e p o s i t s near C a s s i a r c l u s t e r around the upper c r u s t a l " s h a l e " curve f o r the Canadian C o r d i l l e r a , i n d i c a t i n g an upper c r u s t a l source f o r the l e a d . Thus, a n a t e x i s and a s s i m i l a t i o n Of c o n t i n e n t a l c r u s t produced g r a n i t i c melts r i c h i n l i t h o p h i l e elements. The Kuhn stock i s an o x i d i z e d g r a n i t o i d Of sedimentary o r i g i n , geOchemically s p e c i a l i z e d i n K 20, K/Rb, U and U/Th and anomalous i n W and MO. D i f f e r e n t i a t i o n processes have c o n c e n t r a t e d these l i t h o p h i l e elements i n magmatic f l u i d s that produced the McDame tungsten skarn depo V TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS ix INTRODUCTION 1 REGIONAL GEOLOGY 1 LOCAL GEOLOGY 4 Ingenika Group 4 Atan Group 8 Kechika Group 9 Mafic I n t r u s i o n s 9 F e l s i c I n t r u s i o n s 10 SKARN AND ORE FACIES 10 Massive C a l c s i l i c a t e F a c i e s 10 Banded C a l c s i l i c a t e F a c i e s 16 Banded Oxide F a c i e s 17 Massive S u l f i d e F a c i e s 17 MINERAL EQUILIBRIA 18 Massive C a l c s i l i c a t e F a c i e s 18 Banded C a l c s i l i c a t e F a c i e s 28 Banded Oxide F a c i e s 31 Massive S u l f i d e F a c i e s 32 SULFUR ISOTOPES 33 RUBIDIUM-STRONTIUM ISOTOPES 42 LEAD ISOTOPES 48 INTRUSION CHEMISTRY CONCLUSIONS REFERENCES v i i LIST OF TABLES TABLE 1 TABLE 2 TABLE 3 TABLE 4 TABLE 5 TABLE 6 TABLE 7 TABLE 8 C o r r e l a t i o n of g e o l o g i c a l u n i t s near 5 C a s s i a r M i n e r a l assemblages and abundances i n 11 dolomite, marble, h o r n f e l s and skarn F l u i d i n c l u s i o n data from f l u o r i t e i n 24 garnet skarn S u l f u r i s o t o p e data from s u l f i d e s i n 34 porphyry, skarn and h o r n f e l s Potassium-argon i s o t o p e data from 43 quartz f e l d s p a r porphyry of the Kuhn stock Rubidium-strontium i s o t o p e data from 44 quartz f e l d s p a r porphyry Of the Kuhn stock Galena-lead i s o t o p e data from gold or 49 s i l v e r v e i n s and tungsten skarn Chemistry and mineralogy of the 51 C a s s i a r i n t r u s i o n s v i i i LIST OF FIGURES FIGURE 1 Geology of the C a s s i a r area FIGURE 2 Geology Of the A zone FIGURE 3 G e o l o g i c a l plan of the Kuhn showing FIGURE 4 G e o l o g i c a l s e c t i o n Of the Kuhn showing FIGURE 5 P a r t i a l P-T diagram f o r the system SiO z-Al 20 3-MgO-CaO-C0 2-H 20 FIGURE 6 P a r t i a l T-XC0 2 diagram a t 1500 bars f o r the system Si0 2-Al 20 3-MgO-CaO-C0 2-H 20 FIGURE 7 Modes Of occurrence and mineral zoning of the four skarn f a c i e s FIGURE 8 Ternary composition diagrams f o r the four Skarn f a c i e s FIGURE 9 S u l f u r i sotope p l o t f o r McDame data FIGURE 10 P a r t i a l T-a0 2 diagram f o r the system S i 0 2 - F e 2 0 3 - F e O - F e S - F e S 2 - 0 2 - S 2 - H 2 at 1000 bars FIGURE 11 Rubidium-Strontium i s o t o p e p l o t f o r m i n e r a ls from the Kuhn stock 3 7 1 3 15 20 22 26 30 36 40 46 ACKNOWLEDGEMENTS F i r s t and foremost, I would l i k e to thank Dr. C o l i n Godwin for h i s i n s p i r a t i o n and guidance i n s u p e r v i s i n g my t h e s i s . From f i e l d and l a b work to t e x t w r i t i n g and e d i t i n g , C o l i n ' s input has been i n v a l u a b l e . The manuscript has a l s o b e n e f i t t e d from c r i t i c a l reviews by Drs. A.J. S i n c l a i r , H.J. Greenwood, T.H. Brown and R.L. Armstrong. My thanks go to S h e l l Canada Resources L t d . and, i n p a r t i c u l a r , Dr. A.B. Baldwin and G. Moffat f o r funding the f i e l d work and p r o v i d i n g chemical analyses and t h i n s e c t i o n s of the rocks. The B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources i s g r a t e f u l l y acknowledged f o r a f i n a n c i a l grant towards t h e s i s r e s e a r c h and I thank Dr. A. Panteleyev f o r h i s f r u i t f u l d i s c u s s i o n s on geology near C a s s i a r . Many people at U.B.C. c o n t r i b u t e d to my t h e s i s but I wish to acknowledge K r i s t a S c o t t f o r potassium, rubidium and stront i u m i s o t o p e a n a l y s e s ; Bruce Ryan f o r l e a d isotope a n a l y s e s ; Joe Harakal f o r argon i s o t o p e a n a l y s e s ; E r n i e Perkins fo r computer programs and e x p e r t i s e ; John Newlands f o r d r a f t i n g and photography; and Ed Montgomery f o r g e n e r a l l y being h e l p f u l . Last but not l e a s t , I thank my wonderful w i f e , Susan Banks Cooke, f o r her moral and f i n a n c i a l support throughout the past three y e a r s . Her pa t i e n c e and understanding made my work more rewarding and the t h e s i s a success. 1 INTRODUCTION C a s s i a r , i n n o r t h - c e n t r a l B r i t i s h Columbia, has been an important mining d i s t r i c t s i n c e 1873 when p l a c e r gold was d i s c o v e r e d i n T r o u t l i n e Creek ( F i g . 1). C a s s i a r Asbestos Mine was not developed u n t i l the e a r l y 1950's, but more r e c e n t l y the C a s s i a r area has undergone a rennaissance i n mineral e x p l o r a t i o n f o r W, Sn, Mo, U, and Au. The McDame tungsten skarn prospect, s i x k i l o m e t r e s north of C a s s i a r , was d i s c o v e r e d i n 1978 by the l a t e B i l l Kuhn who panned s c h e e l i t e from stream sediments. There are two zones Of economic i n t e r e s t : 1) the A zone, which i n c l u d e s the Kuhn tungsten-molybdenum skarn showing, and t o the west, 2) the B zone, which hosts the Dead GOat tungsten skarn and Contact s i l v e r - l e a d v e i n occurrences ( F i g . 1). D e t a i l e d g e o l o g i c , m i n e r a l e q u i l i b r i a , f l u i d inclusion', and s u l f u r , l e a d and rubidium-strOntium i s o t o p e s t u d i e s were undertaken on A zone skarns i n an attempt to understand the genesis of skarn and ore m i n e r a l s . REGIONAL GEOLOGY C a s s i a r l i e s w i t h i n the Omenica C r y s t a l l i n e B e l t ( F i g . 1), One Of two major r e g i o n a l metamOrphic-plutOnic b e l t s i n the Canadian C o r d i l l e r a (Monger et a l . , 1982). Geology i n the C a s s i a r area has been mapped on a r e g i o n a l Scale by G a b r i e l s e (1963) and Panteleyev (1979; 1980). Three major l i t h o t e c t o n i c elements have been i d e n t i f i e d ( F i g . 1): 1) the C a s s i a r p l a t f o r m , a m i o g e O c l i n a l c o n t i n e n t a l t e r r a c e wedge developed along the western margin of the North American c r a t o n i n l a t e P r o t e r O z o i c t o e a r l y P a l e o z o i c times (Monger and P r i c e , 1979), 2) the 2 FIG. 1. Geology of the C a s s i a r area (from Panteleyev, 1979; 1980), showing major l i t h o t e c t o n i c u n i t s and Pb isotope sample s i t e s ( s o l i d t r i a n g l e s ) . McDame prospect l o c a t i o n and r e g i o n a l g e o l o g i c a l b e l t s (ISB = I n s u l a r B e l t , CPC = Coast P l u t o n i c complex, 1MB = Intermontane B e l t , OCB = Omenica C r y s t a l l i n e B e l t , RMB = Rocky Mountain B e l t ) are i n s e t lower l e f t . McDame mineral occurrences and g e o l o g i c a l u n i t s (IG = Ingenika Group, AG = Atan Group, KG = Kechika Group, CS = Contact stock, KS = Kuhn stock) are i n s e t upper r i g h t . 3 59°30'N ' -' ? 7 ^ z r z s z s r ^ ^ - s - - r - - r - J ! \ v v v v v v v v v v ' - ; \ ' < ^ ^ - ^ _ - ^ _ — _ - ^ - T - _ — _ — _ ^ X - " r \ v V V V V V V V V V W ' •* 7 T. \ <.' . /^ fc — —~ — — I _ j r ^ _ T " ! — V V V V V V V V V PLATFORI I'-iTi V/-+ + + + • 11 / j i^i^McDAME PROPERTY vv v v v V V V v v v v v V V — _ J - I _ - ^ V v v v i r J f v v v v "-£Vv v v v v ij-Zy v v v v v • _ - J / v v v v v j _ - X v v v v v r-_—_rV v v v v v w v v v i v v v v v ' W W ' W W I v v v C A S S I A R ' ' [ K G ] S T O C K • , Q J K u h n I I W j M o \DeaJ IGoatl »KSJ I v \ M C D A M E U P R O P E R T Y 3 0 1 2 K i l o m e t r e s CASSIAR? BATHOLITH + + + + + ++++ C A S S I A I r 4 + + + + + + + + + + + + C A S S I A R A S B E S T O S ^ V W W W W | i u l e V V V V V V V V V V V W W V W IVIIIMC v v v v v v v v v v v w v w v v v v v v w w v w v 3 7 y v v v v v v v v v v v v w w v w v v v v v v v v v v v v v v v k l^f^ft 'w\ . . 0 v v W V V V i V V V V V V>T V W *\»\ v v V V V V V v vy ,w v v v v v v v v v v v v v v v r ^ - ^ \ v v/?vv w w w w w w w v sifeKv SYLVESTER ?: + + + + + + r-_3k + + + + + + ALLOCHTHON -A\ \ O C B O \ O C B \ i Kilometres M c O A M E > A \ P R O S P E C T I M B \ \ \ i v \ ^ . 500 ^ 7 v ' ~ V VJ[V V vv v v v v V V vv v v V V V V V V V V V V V V V V V V V v v v v v v v v v v v v v v v v v i v w v w v v v v v v v v v v w w w w v v v v v v v v v v w v v v v v w v v v w v v v v w v v v v v v v v v v v v w v v w v v v - ^ v v - 5 10 ^ V V V \ Kilometres w v \ v v v v v v v v v w w 59°00' N 4 S y l v e s t e r a l l o c h t h o n , a l a t e P a l e o z o i c oceanic basin assemblage obducted onto the c o n t i n e n t a l margin i n middle Mesozoic times (Monger et a l . , 1972), and 3) the C a s s i a r complex, a composite p l u t o n i c b e l t probably r e l a t e d to l a t e Mesozoic a n a t e x i s of c o n t i n e n t a l c r u s t (Tempelman-Kluit, 1979). V o l c a n i c rocks of the S y l v e s t e r a l l o c h t h o n now occupy the core of the northwest t r e n d i n g McDame s y n c l i n o r i u m , f l a n k e d to the west by C a s s i a r p l a t f o r m a l sediments i n t o which are in t r u d e d the C a s s i a r b a t h o l i t h and r e l a t e d stocks ( F i g . 1). LOCAL GEOLOGY McDame pr o p e r t y rocks have been s u b d i v i d e d i n t o s i x main g e o l o g i c a l u n i t s (Table 1). Ca s s i a r . p l a t f o r m s t r a t a i n c l u d e Lower Hadrynian to Lower O r d o v i c i a n metasediments of the Ingenika, Atan and Kechika Groups ( u n i t s 1 to 3 r e s p e c t i v e l y ) , c r o s s c u t by Mesozoic mafic dikes ( u n i t 4) and Upper Cretaceous f e l s i c stocks ( u n i t 5). Skarn ( u n i t 6) r e p l a c e s marble, dolomite and h o r n f e l s up to s e v e r a l hundred meters away from the nearest f e l s i c i n t r u s i o n . U n i t s 1 to 3 form the s t e e p l y e a s t - d i p p i n g western limb of the McDame s y n c l i n o r i u m . Only minor f o l d i n g i s apparent on the pr o p e r t y and the a s s o c i a t e d f o l i a t i o n , l i n e a t i o n , and j o i n t d i r e c t i o n s correspond r e s p e c t i v e l y with the a x i a l plane, f o l d a x i s , and a-c plane of the McDame s y n c l i n d r i u m (Cooke and Godwin, 1982). One f a u l t , with an apparent l e f t - h a n d s t r i k e s l i p of about 120 m, was mapped i n the A zone ( F i g . 2). Ingenika Group Ingenika Group rocks u n d e r l i e the B zone and c o n t a i n the 5 TABLE I . C o r r e l a t i o n o f i d e o l o g i c a l U n i t s N»«r C a s s i a r T e c t o n i c E l v w i t G a b n e l a * P a n t e l e y e v f l a n s y F r i t z r h i s P a p e r Age * n d G r o u p 1963 1790 1979 l ^ B O YQL&.gL e H . r n C a s s i a r C o m p l e x L a t e C r e t a c e o u s C a s s i a r b a t h o i i t h C a s s i a r s t o c k C o n t a c t s t o c k • - -> uhn stOC* w i n d y StncV q u a r t z m o n t o m t e g r a n o d i o r i t e g r a m t e p o r p h y r y a p l i t e p e g m a t i t e r i e t a s o m a t i c C o n t a c t p o r p h y r l t i c b i O t i t e q u a r t z m o n z o n i t t t U Q l t - f i l c c a r n e g r a i n e d U Q i t - f i l m a n t l e d f e l d -s p a r U O l t - Q l q u a r t : f e l d -s p a r D o r p h y r * W"Lt_fiL medium g r a i n e d a) b i o t i t e q u a r t : "ton i on 11 e or q u a r t z f e l d s p a r p o r p h y r y c a p l i t e - o e g m a t i t e s i l l s I n t r u s i v e C o n t a c t f i d d l e r i e s o z o i c I n t r u s i v e C o n t a c t yni.£_4i_ b i o t i t e l a m o r o -p h y r e d i I e « S v l v e s t e r A l l o c h t h o n D e v o n i a n t o P e r m i a n J V t v a s t e r G r o u p g r e e n s t o n e s s e d i m e n t s lJGLfi_Si d i o r i t e g r e e n s t o n e s F a u l t C o n t a c t C a s s i a r P l a t f o r m UBBtCl b l a c t . s h a l e s h a l e C a m b r i a n t o O r d o v i c i a n b l a c t s l a t e s l a t e r - e c h l l . a G r o u p m i n o r L i men t o n e ^ S B S t i l i m e s t o n e p h y l l i t e m i n o r c o n g l o m -e r a t e ( U p p e r a n d L o w e r ZZv t o 8-0 m i ' s i 1 1 s t o n e s h a l e a> g r a p h i t i c m a r b l e b> w h i t e m a r b l e g> q r a p M t i c m a r b l e h> b i o t i t e h o r n * e l s ( 3 2 0 m) C o n f o r m a d I e C o n t a c t C a s s i a r P l a t f o r m L o w e r C a m b r i a n A t an G r o u p y a a K i l i m e s t o n e d o t o m i t e m i n o r s h a l e USS^T-i. q u a r t i i t e a r q i 1 I i t e s l a t e s h a l e s i 11 a t o n e c o n g l o m e r a t e <?ao m U p p e r a n d ' L o w e r ) WQQ9C.1 d ° l omi t e roarble 1 i m e s t o n e m i n o r s i a t e ' 1*9*8?.I h o r n f e l s a r g i 1 t i t * q u a r t z i t e q u a r t r i t e s i 1 t s t o n e s h a l e I t m e s t o n e 1 I m e s t o n e m i n o r d o l o m i t e m i n o r s h a l e b a s a l s a n d s t o n e <69T m) egy . i-pgc'MUgp. q u a r t r i t e s i l t s t o n e s h a l e a> g r a p h i t i c m a r b l e b i g r e y m a r b l e c ) w h i t e m a r b l e d> g r a p h i t i c d o l o m i t e e> g r e y d o l o m i t e f> w h i t e d o l o m i t e -( l - * . . ' m) q» g r a p h i t i c h o r n * e l s -h) b i o t i t e h o r n f e l s i ) c o r d i e r i t e h o r n f e l a j> m u s c o v i t e h o r n f e l s t- t q u a r t z h o r n t t l * ( * 8 0 m> C o n f o r m a b l e C o n t a c t C a s s i a r P l a t f o r m H i d r v n i a n - C a m o r i a n I n g e n i t:a G r o u p Qggd_HQBfl-f iCGuB 1 i m e s t o n e d o l a m i t e q u a r t z i t e s a n d s t o n e s i 1 t s t o n e a r g i 1 1 i t e s h a l e s l a t e 1 l m t l t o n s ( 1310 m) UfiDCCL m a r b1 e I i m e s t o n e (.Qwerj, h o r n f e l s q u a r t z i t e p h y l 1 \ t e s c h i s t s V a r n s h a l e s i 1 t s t o n e l a n d s t o n e <21Z mi 1 i n e t t. on e d a l o s t a n e s i I t s t o n e m i n o r q u a r t z I t e m i n o r s h a l e < i : r e d b e d l i m e s t o n e d o l o s t o n e s l a t e (86 m) s i a t e mi n o r 11m e s t o n e <32*» m) UQLt-U 1> c o r d i e r \ t (-.400 m> m a r bI e d o l o m i t e '100 mi h a r n f * t s m a r b l • s^ a r n i 200 m> e h o r n f e l s 6 FIG. 2. Geology of the A zone, showing d i s t r i b u t i o n of the main rock and s k a r n t y p e s ( 1 i = I n g e n i k a Group c o r d i e r i t e h o r n f e l s ; 2a,d,h,i,k = Atan Group g r a p h i t i c m arble, g r a p h i t i c d o l o m i t e , b i o t i t e h o r n f e l s , c o r d i e r i t e h o r n f e l s , q u a r t z h o r n f e l s ; 3g,h = K e c h i k a Group g r a p h i t i c h o r n f e l s , b i o t i t e h o r n f e l s ; 4a = b i o t i t e lamprophyre d i k e s ; 5a = C o n t a c t s t o c k b i o t i t e q u a r t z monzonite, 5b = Kuhn s t o c k q u a r t z f e l d s p a r p o r p h y r y ; 6a = massive c a l c s i l i c a t e s k a r n , 6g = banded c a l c s i l i c a t e s k a r n , 6h = banded o x i d e skarn) and the l o c a t i o n of S i s o t o p e ( s o l i d t r i a n g l e s ) and Rb-Sr i s o t o p e ( s o l i d square) samples. 8 Dead Goat and Contact showings ( F i g . 1). Ingenika Group rocks represent the westernmost exposures of s t r a t i f i e d rocks on the McDame prop e r t y and form three metasedimentary bands ( F i g s . 1 and 2). From west to eas t , these a r e : 1) interbanded b i o t i t e h o r n f e l s , white marble and bimetasomatic garnet-pyrOxene skarn, i n contact with the C a s s i a r stock to the west, 2) banded g r a p h i t i c and massive grey marbles, with minor g r a p h i t i c dolomite l e n s e s , c o n t a i n i n g r a r e zebra t e x t u r e d patches ( s t r o m a t o l i t e s ? ) up to s e v e r a l meters t h i c k and c o n c e n t r i c a l l y banded pods (strOmatopOrOids?) up to 20 cm ac r o s s , and 3) s p o t t e d c o r d i e r i t e h o r n f e l s ( F i g . 2: u n i t 1 i ) , with minor b i o t i t e and muscOvite h o r n f e l s and rare white marble bands. These three rock bands c o r r e l a t e r e s p e c t i v e l y with the redbed (interbedded limestone, dolOstOne and s l a t e ) , carbonate and c l a s t i c l a y e r s i n the upper p a r t of the St e l k u z Formation mapped by Mansy (Mansy and G a b r i e l s e , 1978; Mansy, 1979) east Of the McDame s y n c l i n o r i u m (Table 1). Atan Group Atan Group rocks u n d e r l i e the western p a r t of the A zone and i n c l u d e the Kuhn showing. The s t r a t a are conformable with the Ingenika Group and c o n s i s t Of two metasedimentary bands ( F i g . 2 ) . From west to e a s t , these a r e : 1) banded b i o t i t e h o r n f e l s ( u n i t 2h), with l a y e r s Of spot t e d c o r d i e r i t e and massive quartz h o r n f e l s ( u n i t s 2 i , 2k) and p a r t i n g s Of f o l i a t e d muscOvite h o r n f e l s ( u n i t 2 j ) , 9 2) banded g r a p h i t i c marble ( u n i t 2a), with l a y e r s of massive grey and white marbles ( u n i t s 2b, 2c) south of the Kuhn showing that i n t e r f i n g e r towards the north i n t o zebra t e x t u r e d g r a p h i t i c dolomite ( u n i t 2d), i n t e r l a y e r e d with massive grey and white dolomite ( u n i t s 2e, 2 f ) . Rare dolomite b r e c c i a s c o n t a i n c o n c e n t r i c a l l y banded lenses and nodular pods with o v o i d horns up to 5 cm long ( a r c h e O c y a t h i d s ? ) . These two metasedimentary bands are l i t h o l o g i c a l l y s i m i l a r to c l a s t i c and carbonate sedimentary rocks of the BOya and ROsella Formations r e s p e c t i v e l y as mapped by F r i t z (1978; 1980) elsewhere i n north-c e n t r a l B r i t i s h Columbia (Table 1). Kechika Group Kechika Group rocks u n d e r l i e the eastern p a r t Of the A zone ( F i g . 2 ) . They are conformable with the Atan Group and c o n s i s t Of t h i n l y interbanded h o r n f e l s , i n c l u d i n g banded g r a p h i t i c and b i o t i t e h o r n f e l s ( u n i t s 3g, 3h), with minor l a y e r s of banded g r a p h i t i c and massive white marbles ( u n i t s 3a, 3 c ) . These rocks are comparable to interbedded " c l a s t i c and carbonate rocks Of the Kechika GrOup as mapped by G a b r i e l s e (1963) and Panteleyev (1979; 1980) near C a s s i a r (Table 1). Mafic I n t r u s i o n s B i o t i t e lamprOphyre d i k e s i n t r u d e Older s t r a t i f i e d rocks, but are themselves c r o s s c u t by younger skarn v e i n s . The d i k e s appear to have been emplaced along pre-skarn f a u l t s ( F i g . 2) and are probably MesOzOic i n age (Table 1). F e l s i c I n t r u s i O n s F e l s i c i n t r u s i o n s comprise the C a s s i a r b a t h o l i t h and four d i s c r e t e stocks on the McDame property ( F i g . 2 ) . B i o t i t e q uartz monzOnite ( u n i t 5a) i s t y p i c a l of the C a s s i a r and Contact stocks whereas quartz f e l d s p a r porphyry ( u n i t 5b) c h a r a c t e r i z e s the Kuhn and Windy s t o c k s . Minor a p l i t e and pegmatite s i l l s ( u n i t 5c) i n t r u d e both sedimentary and p l u t o n i c rocks (Table 1). SKARN AND ORE FACIES Skarns on the McDame prop e r t y can be c l a s s i f i e d i n t o four metasOmatic f a c i e s (Table 2). The massive c a l c s i l i c a t e (MCF) W-Mo-Fe, banded c a l c s i l i c a t e (BCF) Fe and banded Oxide (BOF) W-Mo-Fe f a c i e s are time e q u i v a l e n t prograde skarns that formed i n d i f f e r e n t rOck types. Massive s u l f i d e (MSF) W-Cu and Zn-W zones are r e t r o g r a d e skarns that r e p l a c e d the other skarn f a c i e s . C a l c s i l i c a t e m i n e r a l zoning i s t y p i c a l of prograde skarn and m e t a l l i c m i n e r a l zoning c h a r a c t e r i z e s r e t r o g r a d e skarn. Massive C a l c s i l i c a t e F a c i e s Massive c a l c s i l i c a t e skarn (MCF) Occurs as semi-cOntinuOus bands up to 10m t h i c k along the western Or lower c o n t a c t s Of the marble bands i n the Ingenika and Atan Groups, as smaller lenses at t h e i r e a s t e r n Or upper c o n t a c t s , and as small pods w i t h i n the marbles themselves ( F i g s . 3, 4). At the Kuhn showing, massive garnet skarn ( u n i t 6a) passes e a s t e r l y through massive pyroxene skarn ( u n i t 6b) i n t o dolomite, and w e s t e r l y through massive epidote skarn ( u n i t 6c) i n t o h o r n f e l s . In the A zone, massive amphibole Skarn ( u n i t 6d) comprises small pods i n IQBkE_2i_Mineral_Assemblages_and_Abund MINERAL ASSEMBLAGES b c P d m h 1 c o t o o a s P a a 5 o 01 r CI 5 s 1 1 P V ' • P m 9 C r c u t u 9 d f C c y c h s r a h y e P i a d h b 5 h r a r o c 1 h h b o a t r 9 e 9 r P h o P i 1 i C o 5 m P z a 1 a q u a e d p 1 i h P n • a o i i c D e o o O c P a a i P o 1 u o e e e y e b o y e • r X d b 1 1 r r t t V 1 h 1 t r h m c a r 1 1 n r r n t r t t n e o D a i i i a i i a i? i i c i i i r l l i l l 1 i 1 i i i e n t 1 5 t t t 1 t t s n n t o t t t t t t t t t t t t t t t t e e' e e e e e c e i? e' e e e n e e e z e e e e e e e e e e • G P E A P S C C T B M a S T A I G D C Q F S M M C S S P P H H A X P M L P n L A I U R P • P I R 0 A 1 L H s • P L B R Y B E SKARN Massive C a l c s i l i c a t e Facies garnet skarn pyroxene skarn epidote skarn amphibole skarn b i o t i t e skarn s c a p o l i t e skarn . Banded C a l c s i l i c a t e Facies garnet zone epidota zone pyroxene zone . amphibole zone Eianded Oxide Facies feldspar zone c a l c i t e zone t a l c zone Massive S u l f i d e Facies p y r r h O t i t e skarn s p h a l e r i t e skarn KECHIKA GROUP B i o t i t e Hornfels ATAN GROUP MCF GS PS ES AS BS SS BCF GZ EZ PZ AZ BUF FZ CZ TZ MSF YS HS BH G r a p h i t i c Marble G r a p h i t i c Dolomite? RM (3D Assemblages and abundances estimated from t h i n and polished sections. Q u a n t i t a t i v e code for abundance i s : 5 = 507. to 1007., 4 = 107. to 502, 3 ~ 57. to 10%, 2 Q u a l i t a t i v e code for abundance i s : C - common, L «= I O N or occasional, R = rare. IX t o 57., 1 = 07. t o 17.. 1 2 FIG. 3. G e o l o g i c a l plan of the Kuhn showing, l o c a t e d i n F i g u r e 2, i n d i c a t i n g the l e n s o i d a l nature of massive c a l c s i l i c a t e f a c i e s garnet skarn (6a) that has r e p l a c e d R o s e l l a Formation g r a p h i t i c marble (2a, b ) . At t r e n c h A-2, massive c a l c s i l i c a t e skarn i s zoned from banded b i o t i t e h o r n f e l s (2h), through 2 m of massive epidote skarn (6c), 8 m of massive garnet skarn (6a), and 20 cm of massive pyroxene skarn (6b), to zebra t e x t u r e d g r a p h i t i c dolomite (2d). Other g e o l o g i c a l symbols are d e f i n e d i n Table 1. 13 1 4 FIG. 4. G e o l o g i c a l s e c t i o n of the Kuhn showing, l o c a t e d i n F i g u r e 2, i n d i c a t i n g the development of banded c a l c s i l i c a t e f a c i e s quartz skarn that r e p l a c e d Boya Formation bleached b i o t i t e h o r n f e l s (2h), and banded oxide f a c i e s magnetite skarn that r e p l a c e d R o s e l l a Formation bleached g r a p h i t i c dolomite ( 2 f ) . The banded c a l c s i l i c a t e and banded oxide f a c i e s are both f r a c t u r e c o n t r o l l e d . Other g e o l o g i c a l symbols are d e f i n e d i n Table 1. 15 TRENCH A-1 EAST TRENCH A-1 WEST 1580 metres • 1560 1540 • 1520 1500 -1480 1460 0+00 0+50E D R I L L L I N E 0 + 0 0 N 1 6 dolomite adjacent to quartz f e l d s p a r porphyry d i k e s . Spotted b i o t i t e skarn ( u n i t 6e) occurs w i t h i n h o r n f e l s where marble and skarn are absent at the o v e r l y i n g dolomite c o n t a c t , north of the Kuhn showing. Rare s c a p o l i t e skarn ( u n i t 6f) i s found as t h i n l a y e r s and coarse d i s s e m i n a t i o n s in Kechika Group h o r n f e l s and metasomatized mafic dikes near the Kuhn showing. S c h e e l i t e ( o c c a s i o n a l l y rimmed by m o l y b d o s c h e e l i t e ) , molybdenite, p y r i t e , p y r r h o t i t e and r a r e magnetite form coarse d i s s e m i n a t i o n s i n t e r s t i t i a l to c a l c s i l i c a t e s at the Kuhn showing. Tungsten and molybdenum grades are higher near the core of the massive c a l c s i l i c a t e f a c i e s whereas t r a c e contents of copper and z i n c ( a s s o c i a t e d with p y r r h o t i t e and p y r i t e ) i n c r e a s e at the skarn c o n t a c t s , p a r t i c u l a r l y i n pyroxene skarn at i t s upper co n t a c t with carbonate. Banded C a l c s i l i c a t e F a c i e s Banded c a l c s i l i c a t e skarn (BCF) c o n s t i t u t e s a bleached, q u a r t z - r i c h zone up to 200 m t h i c k w i t h i n h o r n f e l s of the Kechika Group, surrounding and o v e r l y i n g quartz f e l d s p a r porphyry of the Kuhn stock ( F i g s . 1 and 2). Banded quartz skarn ( u n i t 6g) occurs as narrow, c o n c e n t r i c a l l y zoned, q u a r t z - r i c h envelopes c o n t a i n i n g garnet, e p i d o t e , pyroxene, and amphibole around bedding plane and a-c j o i n t f r a c t u r e s . Where f r a c t u r e d e n s i t y i s h i g h , the o r i g i n a l rock can be bleached throughout; near the Kuhn stock, quartz skarn forms a d i s t i n c t a l t e r a t i o n halo i n b r e c c i a t e d Kechika Group roc k s . Two s m a l l e r quartz skarn zones occur i n Atan Group h o r n f e l s next to quartz f e l d s p a r porphyry d i k e s . Minor f i n e l y disseminated p y r r h o t i t e and p y r i t e 1 7 occur i n quartz skarns. At the Kuhn showing, quartz-molybdenite v e i n s c r o s s c u t Atan h o r n f e l s in d r i l l core where quartz skarn i s absent. Banded Oxide F a c i e s Banded oxide skarn (BOF) i s composed of c r i s s c r o s s i n g v e i n s i n zones up to tens of meters wide w i t h i n Atan Group dolomites ( F i g s . 3, 4), l o c a l l y b o r d e r i n g massive c a l c s i l i c a t e skarn north of the Kuhn showing. Banded magnetite skarn ( u n i t 6h) forms r h y t h m i c a l l y banded magnetite v e i n s up to 1 m wide c o n t a i n i n g pyroxene- and p l a g i o c l a s e - b e a r i n g zones. T a l c skarn ( u n i t 6 i ) has r e p l a c e d magnetite skarn near massive s u l f i d e v e i n s and c o n s t i t u t e s narrow f r a c t u r e f i l l i n g s up to 1 cm wide south of the Kuhn showing. M o l y b d o s c h e e l i t e , magnetite, p y r i t e and r a r e hematite form f i n e d i s s e m i n a t i o n s and l a m i n a t i o n s i n banded oxide skarn. High tungsten and molybdenum grades are a s s o c i a t e d with high f l u o r i t e content i n the v e i n s . Minor amounts of copper and z i n c ( a s s o c i a t e d with p y r i t e and p y r r h o t i t e ) occur at the skarn c o n t a c t s . At the Kuhn showing, s t i b n i t e and s p h a l e r i t e v e i n s c r o s s c u t Atan dolomite where magnetite skarn i s absent. Massive S u l f i d e F a c i e s Massive s u l f i d e skarn (MSF) c o n s i s t s of pods and v e i n s up to 1 m wide r e p l a c i n g other skarn f a c i e s , e s p e c i a l l y massive c a l c s i l i c a t e skarn. Massive p y r r h o t i t e skarn ( u n i t 6j) has r e p l a c e d garnet and magnetite skarns at the Kuhn Showing but grades southwards i n t o more s p h a l e r i t e - r i c h skarn ( u n i t 6k) that has r e p l a c e d pyroxene skarn. S c h e e l i t e , c h a l c o p y r i t e and r a r e 18 p y r i t e form f i n e d i s s e m i n a t i o n s i n both p y r r h o t i t e and s p h a l e r i t e skarns. Tungsten and copper grades are higher i n the former and z i n c grades higher i n the l a t t e r . MINERAL EQUILIBRIA Pressure, temperature and mole f r a c t i o n of the skarn-forming f l u i d s can be estimated from m i n e r a l e q u i l i b r i a on P-T ( F i g . 5) and T-XC0 2 ( F i g . 6) diagrams c a l c u l a t e d by computer from thermodynamic data (Helgeson et a l . , 1978) f o r an i r o n -f r e e , s i x component system ( S i 0 2 , A l 2 0 3 , MgO, CaO, C0 2 and H 2 0 ) . Q u a l i t a t i v e s t u d i e s of skarn i n t h i n s e c t i o n u s i n g p e t r o g r a p h i c and SEM-EDS microscopes i n d i c a t e that garnet, e p i d o t e , pyroxene, amphibole and b i o t i t e at McDame c o n t a i n i r o n . Because the s t a b i l i t y f i e l d s of i r o n - b e a r i n g phases (e.g. garnet, pyroxene) in these diagrams expand at the expense of i r o n - f r e e phases (e.g. w o l l a s t o n i t e , a n o r t h i t e ) and minerals with lower p a r t i t i o n c o e f f i c i e n t s f o r i r o n (e.g. e p i d o t e , amphibole, b i o t i t e ) , the r e s u l t i n g e stimates of Pmax, Tmax and Xmax from the i r o n - f r e e system are lower than the a c t u a l maximum c o n d i t i o n s of formation f o r i r o n - b e a r i n g rocks at McDame. Massive C a l c s i l i c a t e F a c i e s The massive c a l c s i l i c a t e f a c i e s (MCF) i s c h a r a c t e r i z e d by garnet skarn with the m i n e r a l assemblage garnet + pyroxene + c a l c i t e + quartz ± amphibole ± c h l o r i t e (Table 2). G r o s s u l a r i t e -r i c h garnet (GS) and quartz (QZ), i n the absence of w o l l a s t o n i t e (WO) and a n o r t h i t e (AN), l i m i t the s t a b i l i t y of garnet skarn i n P-T space ( F i g . 5) by the r e a c t i o n : 19 FIG. 5. P a r t i a l P-T diagram f o r the system Si0 2-Al 20 3-MgO-CaO-C0 2-H 20, showing the i n t e r s e c t i o n s of two f l u i d - a b s e n t r e a c t i o n s : (1) GS + QZ = 2WO + AN, and (2) CZ = ZO, with a Tmax iso c h o r e from f l u i d i n c l u s i o n data (Table 3) on f l u o r i t e i n massive garnet skarn; mineral symbols are d e f i n e d i n Table 2 and t e x t . Garnet skarn i s s t a b l e to Pmax = 2600 bars and Tmax = 605°C, and where epidote i s present, to Pmax = 1700 bars and Tmax = 505°C. The c o e x i s t e n c e of garnet and epidote in the massive and banded c a l c s i l i c a t e f a c i e s l i m i t s l i t h o s t a t i c p r e s sure d u r i n g formation of McDame skarns to about 1500 bars i n an i r o n - f r e e system. Higher p r e s s u r e s occur i n an i r o n - b e a r i n g system. CO - Q 111 OC w CO LU CC 2 8 0 0 h 2 4 0 0 r-2 0 0 0 h 1600 h 1200 h 8 0 0 4 0 0 h 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 600 650 7 0 0 TEMPERATURE °C to o 21 FIG. 6. P a r t i a l T-XC0 2 diagram at 1500 bars f o r the system S i 0 2 -Al 20 3-MgO-CaO-C0 2-H 20, showing s e l e c t e d skarn m i n e r a l e q u i l i b r i a (symbols are d e f i n e d i n Table 2): (1) GS + QZ = 2WO + AN, (2) CZ = ZO, (3) QZ + CA = WO + C0 2, (4) AN + QZ + 2CA = GS + 2C0 2, (5) 2CZ + 3QZ + 5CA = 3GS + H 20 + 5C0 2, (6) 2CZ + C0 2 = 3AN + CA + H 20, (7) 4CZ + QZ = GS + 5AN + 2H 20, (8) 6CZ + TR + 2QZ = 5DI + 9AN + 4H 20, (9) TR + 2QZ + 3CA = 5DI + H 20 + 3C0 2, (10) 5PH + 6CA + 24QZ = 50R + 3TR + 2H 20 + 6C0 2, (11) 3DO + 4QZ + H 20 = TA + 3CA + 3C0 2, (12) AN + WO + CA = GS + C0 2: a) In the massive c a l c s i l i c a t e f a c i e s (MCF), garnet skarn (crosshatched) i s s t a b l e to Tmax = 555°C and Xmax = 0.14, and zoned epidote ( c r o s s e s ) , garnet, and pyroxene (coarse s t i p p l e ) skarns c o e x i s t below Tmax = 475°C and Xmax = 0.09; b) In the banded c a l c s i l i c a t e f a c i e s (BCF), quartz skarn ( f i n e s t i p p l e ) c o n t a i n i n g a c e n t r a l garnet zone has maximum T and X c o n d i t i o n s s i m i l a r to zoned massive c a l c s i l i c a t e skarn but i s s t a b l e to lower T and X where pyroxene occupies the c e n t r a l zone by r e a c t i o n (9) and amphibole forms from d e s t r u c t i o n of b i o t i t e ( f i n e s t i p p l e ) by r e a c t i o n (10); and c) In the banded oxide f a c i e s (BOF), magnetite skarn ( h o r i z o n t a l l i n e s ) i s s t a b l e to Tmin = 430°C at Xmin = 0.06, but t a l c skarn ( i n t e r m e d i a t e s t i p p l e ) forms at lower temperatures and mole f r a c t i o n C0 2 by react ion (11). 0 . 0 1 0 . 0 5 0 . 1 0 0 . 1 5 0 . 2 0 0 . 2 5 0 . 3 0 MOLE FRACTION C 0 2 /^/'f. iff- Choice u*uL CXeaU^,'^ 23 (1) GS + QZ = 2W0 + AN Rare epidote i s c l i n o z o i s i t e - r i c h (CZ) rather than z o i s i t e - r i c h (ZO) , l i m i t i n g i t s s t a b i l i t y ( F i g . 5) by the r e a c t i o n : (2) CZ = ZO Because r e a c t i o n s (1) and (2) are f l u i d - a b s e n t , t h e i r phase boundaries are not a f f e c t e d by the mole f r a c t i o n s of C0 2 and H 20. T h e i r i n t e r s e c t i o n d e f i n e s a maximum temperature f o r ep i d o t e - b e a r i n g garnet skarn at 545 °C ( F i g . 5). F l u i d i n c l u s i o n s t u d i e s on r a r e f l u o r i t e i n garnet skarn (Table 3) i n d i c a t e that the skarn-forming f l u i d c o n t a i n e d approximately 7% C a C l 2 at a maximum homOgenization temperature of 350°C. Since the f r e e z i n g p o i n t depression of a C a C l 2 f l u i d i s s i m i l a r to that Of a NaCl f l u i d (Crawford, 1981), a constant volume and maximum temperature isOchOre can be p l o t t e d i n P-T space (Roedder and Bodnar, 1980). The i n t e r s e c t i o n of r e a c t i o n (1) with the Tmax isOchOre ( F i g . 5) l o c a t e s the uppermost s t a b i l i t y l i m i t Of garnet skarn at Pmax = 2600 bars and Tmax = 605 °C, assuming that P f l u i d = P t O t a l . Where epidote i s present, the i n t e r s e c t i o n of r e a c t i o n (2) with the Tmax iso c h o r e ( F i g . 5) puts the upper s t a b i l i t y l i m i t Of garnet skarn at Pmax = 1700 bars and Tmax = 505 °C ( F i g . 5). Th e r e f o r e , we assume that l i t h o s t a t i c p r e ssure d u r i n g formation Of McDame skarns i s i n the Order Of 1500 bars. Although garnet skarn i s common i n the A zone ( F i g . 7a), l a t e r a l l y zoned epidote (ES), garnet (GS) and pyroxene (PS) skarns r e p l a c e g r a p h i t i c marble (GM) between b i o t i t e h o r n f e l s (BH) and g r a p h i t i c dolomite (GD) at the Kuhn showing ( F i g . 3). Garnet skarn i s r e s t r i c t e d i n T-XC0 2 space to Tmax = 555 °C and TABLE 3. Fluid Inclusion Data from Fluorite in Garnet Skarn Total No Type Inclusions Phases V Total (cc) V Vapor (7.) V Sol id (7.) T F i r s t Melt < C) Solid T Last Density T Homogeniz-Composition Melt ( C> (Wt7. Salt) at ion ( C) Primary L+V+2S 10 34.: (2) CaCl MgCl (2) 6.S 272.0 (2) Primary L+V+S :i>:10 -49.0 (2) CaCl -3.9 (3) 7.5 334. 4 (2) Primary L+V : Ix 10 234.7 <1> Secondary L+V+2S 3 K 10 CO. 1 -21.0 NaCl -7.8 12.5 15 Secondary L+V+S : ix 10 •21.0 (15) NaCl -6. 3 (5) 10.O 127.0 (3) Secondary L+V C3x 10 •21 .0 (27) NaCl -5.8 (16) 9.3 1. Calculated temperatures are based on the most r e l i a b l e data (number o-f inclusions are in parentheses) as many inclusions leaked during runs due to Fractures -from weathering. 25 FIG. 7. Modes of occurrence and mineral zoning of the four skarn f a c i e s , showing l i t h o l o g i c and f r a c t u r e c o n t r o l s on skarn formation: a) massive c a l c s i l i c a t e f a c i e s (MCF): garnet skarn (GS) has r e p l a c e d g r a p h i t i c marble (GM) between Boya Formation b i o t i t e h o r n f e l s (BH) and R o s e l l a Formation g r a p h i t i c dolomite (GD); b) banded c a l c s i l i c a t e f a c i e s (BCF): quartz skarn (QS) i n c l u d e s garnet (GZ), epidote (EZ), pyroxene (PZ) and amphibole (AZ) zones enveloping f r a c t u r e s i n Kechika and Atan Group b i o t i t e h o r n f e l s (BH); c) banded Oxide f a c i e s (BOF): magnetite skarn (MG) comprises f e l d s p a r (FZ) and c a l c i t e (CZ) zones forming v e i n s i n R O s e l l a Formation bleached g r a p h i t i c dolomite (GD); and d) massive s u l p h i d e f a c i e s (MSF): p y r r h O t i t e skarn (YS) has r e p l a c e d garnet skarn and g r a p h i t i c marble i n pods and v e i n s . 27 Xmax = 0.14 at P = 1500 bars by r e a c t i o n s i n v o l v i n g c a l c i t e (CA) and quartz ( F i g . 6): (3) QZ + CA = WO + C0 2 (4) AN + QZ + 2CA = GS + 2C0 2 (5) 2CZ + 3QZ + 5CA = 3GS + H 20 + 5C0 2 Epidote skarn forms below 475 °C and, where amphibole i s present, below 435 °C by the r e a c t i o n s ( F i g . 6): (6) 2CZ + C0 2 = 3AN + CA + H 20 (7) 4CZ + QZ = GS + 5AN + 2H 20 (8) 6CZ.+ 2QZ + TR = 9AN + 5DI + 4H 20 D i O p s i d e - r i c h (DI) pyroxene and t r e m O l i t e - r i c h (TR) amphibole skarns are s t a b l e under wider T-X c o n d i t i o n s than garnet skarn by the r e a c t i o n ( F i g . 6): (9) TR + 2QZ + 3CA = 5DI + H 20 + 3C0 2 Amphibole, c a l c i t e and quartz cOmmOnly re p l a c e pyroxene, suggesting that u n i v a r i a n t e q u i l i b r i u m governed t h i s r e t r o g r a d e a l t e r a t i o n . M i n e r a l zoning which r e s u l t s dOminantly from composition, pressure and temperature g r a d i e n t s i n a flowing aqueous s o l u t i o n i s t y p i c a l of i n f i l t r a t i o n a l metasomatism ( K O r z h i n s k i i , 1970). D i f f u s i o n metasomatism, On the other hand, forms mineral zOneS in response to chemical a c t i v i t y g r a d i e n t s in a s t a t i o n a r y pore f l u i d . In the A zone, S i - and Ca-bearing skarn f l u i d s i n f i l t r a t e d marble i n co n t a c t with h o r n f e l s to form garnet skarn. At the Kuhn showing, however, c h a n n e l l i n g Of the f l u i d s between h o r n f e l s and dolomite steepened the a c t i v i t y g r a d i e n t s Of S i 0 2 and CaO causing d i f f u s i o n a c r o s s the c o n t a c t s to produce zoned garnet, epidote and pyroxene skarns. On an ACS t e r n a r y 28 phase diagram ( F i g . 8a), a path from h o r n f e l s through skarn to dolomite i s one of major i n c r e a s e i n CaO and major decrease i n S i 0 2 . A minor drop i n A l 2 0 3 a l s o occurs at low but r e l a t i v e l y constant MgO ( b u f f e r e d by dolomite and h o r n f e l s ) and i n c r e a s i n g XC0 2 ( F i g . 6). Banded C a l c s i 1 i c a t e F a c i e s The banded c a l c s i l i c a t e f a c i e s (BCF) i s t y p i f i e d by quartz skarn with the mineral assemblage quartz ± amphibole ± epidote ± pyroxene ± garnet ± p l a g i O c l a s e ± c h l o r i t e ± sphene (Table 2). Within 10 m Of the Kuhn stock, quartz skarn i s zoned c o n c e n t r i c a l l y inwards from u n a l t e r e d b i o t i t e h o r n f e l s (BH) through amphibole (AZ), pyroxene (PZ) and epidote (EZ) zones to a c e n t r a l garnet (GZ) zone ( F i g . 7b). The absence of w o l l a s t o n i t e and a n o r t h i t e , and the c l i n O z o i s i t e - r i c h composition of e p i d o t e , r e s t r i c t the s t a b i l i t y of t h i s assemblage to P-T and T-X c o n d i t i o n s s i m i l a r to those d e f i n e d f o r the massive c a l c s i l i c a t e f a c i e s . More d i s t a n t from the Kuhn stock, the garnet and ep i d o t e zones are absent, pyroxene forms the c e n t r a l zone, and i t Occurs i n the amphibole zone i n s t e a d Of e p i d o t e . T h i s pyroxene-bearing assemblage i s s t a b l e to lower T and higher XC0 2 c o n d i t i o n s than the gar n e t - b e a r i n g zone ( F i g . 6). Assuming that b i o t i t e i s p h l O g O p i t e - r i c h (PH) and the Orthoclase (OR) component s t a y s i n s o l u t i o n , the d e s t r u c t i o n of b i o t i t e i n h o r n f e l s occurs at even lower temperatures ( F i g . 6) by the r e a c t i o n : (10) 5PH + 6CA + 24QZ = 50R + 3TR + 2H 20 + 6C0 2 On an ACM diagram ( F i g . 8b), a path from u n a l t e r e d b i o t i t e 29 FIG. 8. Ternary composition diagrams f o r the four skarn f a c i e s , showing the v a r i a t i o n i n c o n c e n t r a t i o n of chemical components fo r d i f f e r e n t mineral assemblages d e f i n e d i n Table 2. a) massive c a l c s i l i c a t e f a c i e s (MCF): b i o t i t e h o r n f e l s (BH) through zoned ep i d o t e (ES), garnet (GS) and pyroxene (PS) skarns to g r a p h i t i c dolomite (GD) i s a path Of major i n c r e a s e i n CaO and major decrease i n S i 0 2 , accompanied by a minor drop i n A 1 2 0 3 at r e l a t i v e l y constant MgO and i n c r e a s i n g XC0 2 ( F i g . 6); b) banded c a l c s i l i c a t e f a c i e s (BCF): u n a l t e r e d b i o t i t e h o r n f e l s (BH) through amphibole (AZ), pyroxene (PZ) and epidote (EZ) zones to a c e n t r a l garnet (GZ) zone i s a path Of major i n c r e a s e i n CaO c o i n c i d i n g with minor decreases i n A l 2 0 3 and MgO at r e l a t i v e l y constant S i 0 2 and low XC0 2 ( F i g . 6); c) banded oxide f a c i e s (BOF): bleached g r a p h i t i c dolomite (GD) through a c a l c i t e (CZ) zone to a c e n t r a l f e l d s p a r (FZ) zone i s a path Of major i n c r e a s e i n S i 0 2 with a minor drop i n MgO and r i s e i n A l 2 0 3 at r e l a t i v e l y constant CaO but v a r i a b l e XC0 2 ( F i g . 6); t a l c skarn (TS) r e p l a c e s magnetite skarn (MS) at lower S i 0 2 , CaO, A l 2 0 3 and XC0 2 but higher MgO; and d) massive s u l f i d e f a c i e s (MSF): prograde skarns vary i n 0 2 suggesting p r o t o l i t h c o n t r o l , but t h e i r replacement by r e t r o g r a d e massive s u l f i d e f a c i e s (MS) i s One Of major decrease i n S 2. (A = A 1 2 0 3 , C = CaO, M = MgO, S = S i 0 2 , f = Fe, 0 = 0 2, s = S 2.) (c ) BOF M (d) MSF 31 h o r n f e l s through zoned skarn to the c e n t r a l garnet zone i s one of major i n c r e a s e i n CaO ( c a l c i t e occurs r a r e l y i n the garnet zone). Minor decreases i n A l 2 0 3 and MgO occur at r e l a t i v e l y constant S i 0 2 (quartz i s present in a l l zones) and low XC0 2 ( F i g . 6). Temperatures decrease away from the Kuhn stock causing lower grade pyroxene metasomatism. However, the primary c o n t r o l on formation Of the banded c a l c s i l i c a t e f a c i e s was the i n f i l t r a t i o n of Ca- and S i - b e a r i n g f l u i d s along f r a c t u r e s i n h o r n f e l s . Banded Oxide F a c i e s The banded Oxide f a c i e s (BOF) normally c o n s i s t s Of magnetite skarn with the mineral assemblage magnetite + c a l c i t e ± pyroxene ± p l a g i O c l a s e ± t a l c ± c h l o r i t e (Table 2). North of the Kuhn showing, magnetite skarn i s zoned c o n c e n t r i c a l l y from bleached g r a p h i t i c dolomite (GD) through a pyroxene (XZ) zone to a c e n t r a l f e l d s p a r (FZ) zone ( F i g . 7 c ) . The absence of w o l l a s t o n i t e , garnet, epidote and amphibole l i m i t i t s s t a b i l i t y to Tmin = 430 °C and Xmin = 0.06 ( F i g . 6). T a l c and c h l o r i t e , common a l t e r a t i o n m i n e r a l s Of magnetite skarn, a l s o form minor t a l c skarn v e i n s i n dolomite south Of the Kuhn showing. T h i s m i n e r a l assemblage i s s t a b l e to lower temperatures than the pyroxene-bearing zone ( F i g . 6) by the d e s t r u c t i o n of dolomite: (11) 3DO + 4QZ + H 20 = TA + 3CA + 3C0 2 On an SCM diagram ( F i g . 8c), a path from g r a p h i t i c dolomite through zoned skarn to the c e n t r a l f e l d s p a r zone i s One Of major i n c r e a s e i n S i 0 2 (quartz Occurs r a r e l y i n the f e l d s p a r zone). A minor decrease i n MgO and r i s e i n A l 2 0 3 Occur at r e l a t i v e l y 32 constant CaO ( c a l c i t e i s present in a l l zones) but v a r i a b l e XC0 2 ( F i g . 6 ) . LOwer temperatures south of the Kuhn showing r e s u l t in lower grade t a l c metasomatism. However, the main c o n t r o l on formation of the banded oxide f a c i e s was the i n f i l t r a t i o n of S i -and Ca-bearing f l u i d s along f r a c t u r e s i n dolomite. Massive S u l f ide Fac i e s The massive s u l f i d e f a c i e s (MSF) g e n e r a l l y c o n s i s t s of p y r r h O t i t e skarn with the mineral assemblage p y r r h o t i t e ± s p h a l e r i t e ± quartz ± f l u o r i t e ± c h a l c o p y r i t e ± p y r i t e ± garnet ± pyroxene (Table 2). South of the Kuhn showing, s p h a l e r i t e - r i c h skarn becomes dominant as a replacement of marble as w e l l as early-formed skarn ( F i g . 7d). L a t e r a l zoning Of i r o n m i n e r als i n the d i f f e r e n t skarn f a c i e s i s a f u n c t i o n Of the r e l a t i v e c o n c e n t r a t i o n s of 0 2 and S 2. The banded oxide (BO) f a c i e s r e f l e c t s higher 0 2 than the massive (MC) and banded (BC) c a l c s i l i c a t e f a c i e s , and a l l three f a c i e s are r e p l a c e d by lower S 2 assemblages of the massive s u l f i d e (MS) f a c i e s ( F i g . 8d). Through r e a c t i o n with the Ore f l u i d , dolomite was a source of C0 2 (NOkleberg, 1973) and t h i s i n c r e a s e d 0 2 i n magnetite skarn. The c o n c e n t r a t i o n of S 2 was probably c o n t r o l l e d by mixing of a r e l a t i v e l y S 2 - r i c h magmatic s o l u t i o n with a r e l a t i v e l y S 2-pOor formation f l u i d (see s u l f u r isotope d i s c u s s i o n ) . However, the major c o n t r o l On metasomatic replacement of skarn was the i n f i l t r a t i o n of Fe- and S-bearing f l u i d s along permeable zones i n the Other skarn f a c i e s . 33 SULFUR ISOTOPES S u l f u r i s o t o p e s were analyzed i n 24 s u l f i d e samples from McDame skarn, porphyry and h o r n f e l s (Table 4). Data are reported as d e l t a values (8), the per m i l d e v i a t i o n s of 3 " S / 3 2 S i n the samples r e l a t i v e to that of the Canon D i a b l o m e t e o r i t e standard. High l e v e l s of sample p u r i t y (99% mineral) and a n a l y t i c a l p r e c i s i o n (±0.2 S 3 4 S ) were a t t a i n e d during sample p r e p a r a t i o n and a n a l y s i s . A p l o t of McDame s u l f u r i s otope d e l t a v a l u e s ( F i g . 9) shows t h a t : 1) i n t r u s i v e rock p y r i t e 6 3*S (group 1) averages +4.7 (n=2), s l i g h t l y h e a v i e r than magmatic 6 3"S (0±3 from Ohmoto and Rye, 1979) but l i g h t e r than i n t r u s i v e rock p y r r h O t i t e 6 3"S (group 5), which averages +9.3 (n=2); 2) country rock p y r r h o t i t e 63'1S (group 6) averages +18.8 (n = 2), much l i g h t e r than Cambrian sedimentary 6 3 i tS (+30±3 from Faure, 1977); 3) skarn 6 3 U S f o r p y r i t e , p y r r h o t i t e , s p h a l e r i t e and c h a l c O p y r i t e averages +7.7 (n=l9) and forms three groups (2, 3, and 4) that l i e between the i n t r u s i v e rock and country rock data, and between the i n t r u s i v e rock p y r i t e and p y r r h o t i t e data; 4) skarn p y r i t e 5 3 f lS (group 2) f a l l s between + 6.6 and +7.4 and averages +7.0 (n=6), with one exception ( 6 3 a S = +8.2); 5) skarn p y r r h o t i t e 5 3 a S (group 3) f a l l s between +7.3 and +7.6 and averages +7.5 (n=6), with One e x c e p t i o n (6 3 f tS = +5.0); 6) skarn p y r r h o t i t e , s p h a l e r i t e and c h a l c O p y r i t e 5 3"S from the massive c a l c s i l i c a t e and massive s u l f i d e f a c i e s (group 4) f a l l s between +8.1 and +8.7 and averages +8.4 (n=7); and 34 TABLE 4. S u l f u r Isotope Data -from S u l f i d e s i n Porphyry, Skarn and H o r n f e l s Sample Number Rock Type Mineral 1029A 1075 1077 1077 B1A5-144.35 80A3-2555 B1A5-157.00 8 IAS-157 ..00 1063A 1063A 80A3-39.85 80A3-39.85 1024E 1024E 1066C 81A6-19.30 1064D 1064D 1064D 1110 1086 1071A 1074A 1080E 1047B Kuhn stock: quartz -feldspar porphyry Kuhn stock: q u a r t 2 f e l d s p a r porphyry Kuhn stocks quartz f e l d s p a r porphyry Kuhn stock: quartz f e l d s p a r porphyry Banded oxide f a c i e s skarn Banded oxide f a c i e s skarn Banded oxide f a c i e s skarn Banded oxide f a c i e s skarn Banded c a l c s i l i c a t e f a c i e s skarn Banded c a l c s i l i c a t e f a c i e s skarn Massive c a l c s i l i c a t e f a c i e s Massive c a l c s i l i c a t e f a c i e s Massive c a l c s i l i c a t e f a c i e s Massive c a l c s i l i c a t e f a c i e s Massive c a l c s i l i c a t e f a c i e s Massive c a l c s i l i c a t e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Massive s u l f i d e f a c i e s Kechika Group: g r a p h i t i c h o r n f e l s Atan Group: c o r d i e r i t e h o r n f e l s P y r r h o t i te P y r i t e P y r r h o t i t e P y r i t e P y r i t e P y r i t e P y r i t e P y r r h o t i t e P y r i t e P y r r h o t i t e P y r i t e P y r r h o t i t e P y r i t e P y r r h o t i t e P y r r h o t i t e P y r r h o t i te P y r r h o t i t e Sphaleri te C h a l c o p y r i t e P y r r h o t i te P y r r h o t i t e P y r r h o t i t e P y r r h o t i t e P y r r h o t i t e P y r r h o t i t e +9.33 +3.77 +9.35 +3.72 +6.59 +7. 10 +6.96 +7. S3 +7. 11 +7.42 +7.37 +7.47 +8. 17 +4.96 +8.40 +8.46 +7.31 +8. 31 +8. 11 +7.58 +8.21 +8.38 +8.68 +16.69 +20.87 i . S u l f u r i s o t o p e s were analysed by C. Rees i n the s u l f u r i sotope l a b o r a t o r i e s at McMaster U n i v e r s i t y ; per mil d e v i a t i o n s are r e l a t i v e to the Canon D i a b l o meteorite standard. 35 FIG. 9. S u l f u r isotope p l o t f o r McDame data, showing the d i s t r i b u t i o n of porphyry, skarn and h o r n f e l s 6 3"S i n t o s i x groups between o r d i n a r y magmatic and Cambrian sedimentary v a l u e s : 1) porphyry p y r i t e 6 3"S (n=2) averages +4.69 (x) ± 0.97 (S//n); 2) skarn p y r i t e 5 3 l tS (n=5) averages +7.03±0.13; 3) skarn p y r r h o t i t e 5 3 4 S (n=5) averages +7.46±0.05; 4) skarn p y r r h o t i t e , s p h a l e r i t e and c h a l c o p y r i t e 8 3 aS (n=7) averages +8.36±0.07; 5) porphyry p y r r h o t i t e 5 3"S (n=2) averages +9.34±0.01; 6) h o r n f e l s p y r r h o t i t e 6 3 U S (n=2) averages +18.78±2.96 . F i v e of s i x p y r i t e -p y r r h o t i t e p a i r s are i n reverse order to that expected from i s o t O p i c e q u i l i b r i u m . The e x c e p t i o n a l p a i r (marked with s t a r ) g i v e s an u n r e a l i s t i c a l l y low temperature of +34°C (T = [550 / V 6 3 < tSPY - 6 3 f lSPR] - 273, from OhmOto and Rye, 1979) which a l s o suggests i s O t o p i c d i s e q u i l i b r i u m . VEIN SULFIDE F A C I E S 5 B A N D E D C A L C S I L I C A T E L F A C I E S 5 M A S S I V E C A L C S I L I C A T E ^ F A C I E S 5 I I I M A S S I V E SULFIDE L F A C I E S 5 MAGMATIC | SULF.UR GROUP 1 . P O R P H Y R Y ! P Y R I T E I 6 GROUP 2 S K A R N P Y R I T E 6 6 6 GROUP 5 P O R P H Y R Y P Y R R H O T I T E 10 8 8 GROUP 3 S K A R N P Y R R H O T I T E 8 GROUP 4 S K A R N P Y R R H O T I T E S P H A L E R I T E C H A L C O P Y R I T E 8 _ i 9 GROUP 6 H O R N F E L S P Y R R H O T I T E 15 -e- - e -2 0 SYMBOLS O P Y R I T E © S P H A L E R I T E Q C H A L C O P Y R I T E # P Y R R H O T I T E A N A L Y T I C A L E R R O R 8 3 4 S = ±0.2 CAMBRIAN SEDIMENTARY SULFUR 2 5 3 0 « 3 4 S % 0 37 7) i n f i v e of s i x p y r i t e - p y r r h o t i t e p a i r s , p y r i t e 6 3 US i s lower than p y r r h o t i t e 5 3 4 S , reverse to the order expected f o r i s o t o p i c e q u i l i b r i u m . Temperatures of m i n e r a l i z a t i o n c a l c u l a t e d f o r the normal p y r i t e - p y r r h o t i t e p a i r (34 °C) and a s p h a l e r i t e -c h a l c o p y r i t e p a i r (593 °C), using equations from Ohmoto and Rye (1979), are u n r e a l i s t i c a l l y low and high, r e s p e c t i v e l y , again suggesting i s o t o p i c d i s e q u i l i b r i u m . Primary disseminated p y r i t e i n the Kuhn stock (group 1) probably r e f l e c t s a g r a n i t i c magma s u l f u r source whereas l a t e p y r r h o t i t e m i n e r a l i z a t i o n (group 5) and quartz a l t e r a t i o n may represent mixing of s u l f u r from two sources. I n t r u s i v e rock p y r i t e (5 3 f lS = +4.7) i s heavier than normal magmatic s u l f u r (5 3"S = 0) but t h i s c o u l d be due to a r e l a t i v e l y o x i d i z i n g and t h e r e f o r e SO„- and 3 " S - r i c h magmatic f l u i d , b u f f e r e d by 3 2 S - r i c h SH remaining i n a r e l a t i v e l y reducing magmatic melt (OhmOtO and Rye, 1979). Evidence f o r a Cambrian sedimentary s u l f u r source comes from the ext e n s i v e b l e a c h i n g , by d e s t r u c t i o n Of g r a p h i t e and p y r r h o t i t e (group 6), of h o r n f e l s and dolomite a s s o c i a t e d with banded c a l c s i l i c a t e and banded Oxide skarns. Country rock p y r r h o t i t e ( 6 3 4 S = +18.8) i s l i g h t e r than Cambrian sedimentary s u l f a t e ( S 3 , S = +30) because the k i n e t i c e f f e c t s of b a c t e r i a l or in o r g a n i c seawater s u l f a t e r e d u c t i o n e n r i c h sediments i n 3 2 S (Faure, 1977). Two s u l f u r sources are t h e r e f o r e i n d i c a t e d and both mixing and f r a c t i o n a t i o n are necessary to e x p l a i n the p o s i t i v e values Of McDame s u l f u r i s o t o p e s and t h e i r present d i s t r i b u t i o n between Ordinary magmatic and Cambrian sedimentary v a l u e s . Mixing of s u l f u r sources can a l s o Occur w i t h i n g r a n i t i c magmas by 38 a s s i m i l a t i o n of c r u s t a l rocks i n t o mantle melts (Sasaki and I s h i h a r a 1979; 1980) Or by m e l t i n g of sedimentary or igneous lower c r u s t (Coleman, 1979). I n t r u s i v e rock p y r i t e may a c t u a l l y represent an i s o t O p i c a l l y heavy s u l f u r source f o r the Kuhn stock magma. However, a combined m i x i n g - f r a c t i o n a t i o n model p r e d i c t s t h a t : 1) upon i n t r u s i o n and c r y s t a l l i z a t i o n of the Kuhn stock, hot, a c i d i c , o x i d i z i n g and s u l f i d i z i n g magmatic waters moved upwards and Outwards, p r e c i p i t a t i n g p y r i t e ( F i g . 9 , group 1: 5 3 f lS = +4.7) i n s o l i d i f i e d i n t r u s i v e rock ( F i g . 10, p o i n t 1); 2) r e l a t i v e l y c o o l e r , more a l k a l i n e , reducing and l e s s s u l f i d i z i n g f o r m a t i o n a l waters c i r c u l a t e d i n c o n v e c t i o n c e l l s , d i s s o l v i n g p y r r h o t i t e ( F i g . 9, group 6: 5 3 a S = +18.8) from the country rocks; 3) p r o g r e s s i v e f r a c t i o n a t i o n of s u l f u r i s o t o p e s and mixing of magmatic waters with up to 33% f o r m a t i o n a l water along permeable c o n t a c t s and f a u l t s produced the observed s u l f u r i s o tope r a t i o ( F i g . 9, groups 2, 3, 4: 83ltS = +7.7) and sequence of ore f a c i e s i n skarn ( F i g . 10). O x i d a t i o n Of g r a p h i t e , d i s s o l u t i o n of c a l c i t e , p r e c i p i t a t i o n Of s u l f i d e s and a r e s e r v o i r e f f e c t due to country rocks becoming d e p l e t e d i n 3"S a l l tend to decrease T, a 0 2 , aS 2 and aH 2 i n the ore f l u i d r e s u l t i n g i n i t s p r o g r e s s i v e enrichment in 3"S and H 2S ( F i g . 10) . 4) skarn p y r i t e ( F i g . 9, group 2: 6 3 < tS = +7.0), magnetite and molybdOscheelite p r e c i p i t a t e d i n the banded oxide, banded c a l c s i l i c a t e and massive c a l c s i l i c a t e f a c i e s from a r e l a t i v e l y small but 3 4 S - r i c h H 2S f r a c t i o n i n the s t i l l dOminantly hot and 39 FIG. 10. P a r t i a l T-a0 2 diagram f o r the system S i 0 2 - F e 2 0 3 - F e O -FeS-FeS 2- 0 2-S 2-H 2 at P = 1000 bars (Ohmoto and Rye, 1979), showing, by dashed l i n e s , s e l e c t e d ore mineral e q u i l i b r i a (13) S0 2 + H 20 = H 2S + 20 2, (14) 6HE = 4MG + 0 2, (15) MG + 3PY = 6PR + 20 2, (16) 2MG + 3QZ = 3FY + 0 2, and, by s o l i d l i n e s , 6 3 f lS values f o r H 2S i n the ore f l u i d . M i n e r a l symbols are d e f i n e d i n Table 2. Porphyry p y r i t e (group 1) p r e c i p i t a t e d from a hot (T = 650°C), o x i d i z i n g ( l o g a0 2 = -15) magmatic ore f l u i d , which f r a c t i o n a t e d with d e c r e a s i n g T (500°C) and a 0 2 (-21) to produce skarn p y r i t e (group 2) and p y r r h o t i t e (group 3). Mixing of magmatic water with c o o l e r , more reducing f o r m a t i o n a l water at lower T (350°C) and l o g a 0 2 (-32). r e s u l t e d i n replacement of e a r l i e r skarns by p y r r h o t i t e , s p h a l e r i t e and c h a l c o p y r i t e (group 4) and m i n e r a l i z a t i o n of porphyry by p y r r h o t i t e (group 5). 41 o x i d i z i n g magmatic Ore f l u i d ( F i g . 10, p o i n t 2); 5) skarn p y r r h o t i t e a s s o c i a t e d with p y r i t e ( F i g . 9, group 3: 8 3"S = +7.5), s c h e e l i t e and molybdenite were d e p o s i t e d i n the massive c a l c s i l i c a t e f a c i e s from a l e s s hot and o x i d i z i n g magmatic ore f l u i d ( F i g . 10, p o i n t 3); 6) skarn p y r r h o t i t e , s p h a l e r i t e , c h a l c o p y r i t e ( F i g . 9, group 4: S 3 4 S = +8.4) and s c h e e l i t e i n massive s u l f i d e f a c i e s formed from a more cOOl and reducing mixed magmatic and fo r m a t i o n a l ore f l u i d ( F i g . 10, p o i n t 4); and 7) c o l l a p s e Of the hydrothermal system r e s u l t e d i n f i n a l m i n e r a l i z a t i o n Of p y r r h o t i t e ( F i g . 9, group 5: 5 3 < tS = +9.3) and quartz a l t e r a t i o n i n the i n t r u s i v e rOcks themselves ( F i g . 10, po i n t 5). Comparison Of McDame S u l f u r i sotope data with those of Other tungsten skarns shows that 5 3 4 S values f a l l i n a very narrow range f o r each d e p o s i t . At MOina, Tasmania, 5 3 U S Of p y r i t e , p y r r h o t i t e and s p h a l e r i t e range from +8.4 to +9.3 (Kwak and Aski n s , 1981). Shimazaki and Yamamoto (1979) re p o r t 5 3*S values f o r p y r i t e , p y r r h o t i t e , s p h a l e r i t e , galena and c h a l c O p y r i t e i n tungsten d e p o s i t s Of Japan, i n c l u d i n g Kagata mine (-1.3 t o -1.1), Tsumo mine (-1.7 to +0.7), Kawayama mine (-6.3 to -4.6, Kuga mine (-9.2 to -8.3), F u j i g a t a n i mine (-12.2 t o -6.9), and Kiwada mine (-8.5 to -7.1). P y r i t e 8 3"S values from the Osgood Mountains, Nevada (T a y l o r and O' N e i l , 1977) range from +5.4 t o +6.7 (Marcus mine), +2.0 t o +4.6 (Kir b y mine), +1.5 to +3.2 (A l p i n e mine) and around +2.4 ( R i l e y e x t e n s i o n mine). Since high temperature i s a unique f e a t u r e Of skarn d e p o s i t s r e l a t i v e t o Other hydrothermal d e p o s i t s , i t must be a primary 42 c o n t r o l on the r e l a t i v e l y minor s u l f u r i s otope f r a c t i o n a t i o n i n skarns, as shown by t h e i r narrow v a r i a t i o n in 6 3*S. RUBIDIUM-STRONTIUM ISOTOPES Rubidium-strontium and potassium-argon ages were determined from a 25 kg sample of Kuhn stock quartz f e l d s p a r porphyry. The stock t y p i c a l l y i s p o r p h y r i t i c with 10% phenocrysts of corroded q u a r t z , 20% megacrysts of p e r t h i t e mantled by a l b i t e , and 15% glomerocrysts of p l a g i d c l a s e showing o s c i l l a t o r y , normal zoning (An, 5 to A n 2 5 ) . Quartz, f e l d s p a r and b i o t i t e form a f i n e g r a i n e d matrix c o n t a i n i n g t r a c e s of muscovite, c h l o r i t e , sphene, a p a t i t e , z i r c o n , c a l c i t e , p y r i t e and magnetite (Table 2). Potassium-argon d a t i n g of b i o t i t e (Table 5) g i v e s a Late Cretaceous age of 72.4 ± 2.5 Ma, concordant with a rubidium-s t r o n t i u m whole rock(WR) - b i o t i t e ( B I ) isochron of 69 ± 2 Ma which has an i n i t i a l s t r o n t i u m i s o t o p e r a t i o of 0.712 (Table 6 and F i g . 11). S i m i l a r Late Cretaceous ages have been obtained from the nearby C a s s i a r stock by C h r i s t o p h e r et a l . (1972) and Panteleyev (1980). An o l d e r i s o c h r o n of 139 ± 16 Ma with an i n i t i a l 8 7 S r / B 6 S r r a t i o of 0.709 i s given by the whole rock -l i g h t matrix f e l d s p a r ( L F ) - heavy matrix f e l d s p a r ( H F ) -megacrystic p e r t h i t e ( K F ) system ( F i g . 11) i n d i c a t i n g that i t i s not i n e q u i l i b r i u m with the whole r o c k - b i o t i t e system. Such d i s e q u i l i b r i u m has been i n t e r p r e t e d as evidence f o r p r o g r e s s i v e contamination of a mantle melt by c r u s t a l m a t e r i a l d u r i n g magma ascent and c r y s t a l l i z a t i o n (Schuler and S t e i g e r , 1978). Hence, s u b t r a c t i o n of these l e s s r a d i o g e n i c components from the whole rock a n a l y s i s should produce a s l i g h t l y younger is o c h r o n and T A B L E 5 . P o t a s s i u m - A r g o n I s o t o p e D a t a f r o m Q u a r t : F e l d s p a r P o r p h y r y o f t h e K u h n S t o c k S a m p l e L o c a t i o n R o c k U n i t , a n d M i n e r a l XK . Q r j __fic* A p p a r e n t A g e T i m e N u m b e r L a t ( N ) L o n g ( W ) R o c k N a m e D a t e d A r t o t a l 1 0 c m S T P / g ( M a ) 1 0 7 5 5 9 . 3 4 1 2 9 . 8 5 K u h n s t o c k , q u a r t z B i o t i t e 6 . 5 0 0 . 9 2 1 1 . 8 6 7 7 2 . 4 + 2 . 5 L a t e C r e t a c e o u s f e l d s p a r p o r p h y r y 1 . A r g o n a n a l y s e s w e r e d o n e b y J . H a r a k a l a n d K . S c o t t d i d t h e p o t a s s i u m a n a l y s e s . A l l a n a l y s e s w e r e d o n e a t t h e G e o c h r o n o l o g y L a b o r a t o r y , T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . 2 . A r t i n d i c a t e s r a d i o g e n i c a r g o n . 3 . C o n s t a n t s u s e d a r e f r o m S t e i g e r a n d J a g e r ( 1 9 7 7 ) : = 0 . 5 8 1 >: 1 0 y r ; = 4 . 9 6 2 x 1 0 y r ; K / K = 1 . 1 6 2 x 1 0 4 . T i m e d e s i g n a t i o n i s f r o m A r m s t r o n g ( 1 9 7 8 ) . TABLE 6. Rubidium-Strontium Isotope Data from Quartz•Feldspar Porphyry of the Kuhn Stock Sample Number Location Rock Unit and Lat(N) Long(W) Rock Name Mater i al Analysed Sr(ppm) Rbippm) 1075WR 59.34 129.BS 1075BI 59.34 129.85 Kuhn stock:, quarts Whole rock feldspar porphyry Kuhn stock', quartz B i o t i t e feldspar porphyry 207 265 35.0 859 1075k F 59.34 129.85 Kuhn stock, quartz K-feldspar 504 108 feldspar porphyry megarryst 1075HF 59.34 129.85 Kuhn stock, quartz Hc?avy matri feldspar porphyry feldspar 122 1075LF 59.34 129.85 Kuhn stock, quartz Light matri:: 292 250 feldspar porphyry feldspar Sr / br 0.7141 O.7820 0.7104 0.7117 O. 7143 Rb/ 71.6 0. 62" 1.51 2.48 1. A l l analyses by K. Scott, Geochronology Laboratory, Department of Geological Sciences, The U n i v e r s i t y of B r i t i s h Columbia. 45 FIG. 11. Rubidium-strontium isotope p l o t f o r m i n e r a l s from the Kuhn stock ( F i g . 2), showing a b i o t i t e (Bl)-whole rock (WR) iso c h r o n that g i v e s an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.712 and a date of 69 ± Ma. The whole r o c k - l i g h t matrix f e l d s p a r (LF)-heavy matrix f e l d s p a r (HF)-megacryst p e r t h i t e (KF) system produces an o l d e r isochron (139 ± 16 Ma) with a lower i n i t i a l B 7 S r / 8 6 S r r a t i o (0.709), i n d i c a t i n g that i t i s out of e q u i l i b r i u m with the whole r o c k - b i o t i t e system. Such d i s e q u i l i b r i u m suggests p r o g r e s s i v e contamination of a g r a n i t i c melt by s i a l i c c r u s t d u r i n g magma c r y s t a l l i z a t i o n . The high i n i t i a l s t r o n t i u m isotope r a t i o i n d i c a t e s that the Kuhn stock magma had a s i a l i c c r u s t a l component. 0 .79 n 8 7 R b / 8 6 S r higher i n i t i a l 8 7 S r / 8 6 S r r a t i o (Godwin et a l . , 1980). High i n i t i a l 8 7 S r / 8 6 S r r a t i o s (>0.706) c h a r a c t e r i z e g r a n i t i c rocks d e r i v e d by me l t i n g and/or a s s i m i l a t i o n Of o l d s i a l i c c r u s t d u r i n g magma genesis and low i n i t i a l r a t i o s (<0.704) i n d i c a t e mantle and/or young s i m a t i c c r u s t a l source (Faure, 1977). Thus, the Kuhn stock magma (0.712) probably had a s i a l i c c r u s t a l component. S i m i l a r high i n i t i a l 8 7 S r / 8 6 S r r a t i o s have been obtained from Other Cretaceous g r a n i t i c plutOns i n the Omenica C r y s t a l l i n e and Rocky Mountain B e l t s of F i g u r e 1 (<0.710 from F a i r b a i r n et a l . , 1964; 0.725 from Wanless et a l . , 1968; 0.723 from Godwin et a l . , 1980; 0.712 from Kuran et a l . , 1982; 0.712 from MatO et a l . , 1983; and 0.725 from Armstrong, 1983). Radiogenic Sr i n these i n t r u s i o n s i m p l i e s that Precambrian c o n t i n e n t a l c r u s t underlay the ea s t e r n Canadian C o r d i l l e r a d u r i n g CretaceOus time. In the more western Intermontane, COast P l u t o n i c and I n s u l a r B e l t s ( F i g . 1), u n d e r l a i n by younger oceanic c r u s t , f e l s i c i n t r u s i o n s have i n i t i a l s t r o n t i u m i s o t o p e r a t i o s lower than 0.706 (Le Couteur and Tempelman-Kluit, 1976; Morrison et a l . , 1979; Anderson et a l . , 1983). Metamorphosed m i o g e o c l i n a l sediments Of the Omenica C r y s t a l l i n e B e l t have been d i s p l a c e d from s i m i l a r unmetamOrphosed rOcks i n the Rocky Mountain B e l t by more than 1000 km Of r i g h t l a t e r a l s t r i k e s l i p movement d u r i n g l a t e Cretaceous to e a r l y T e r t i a r y time ( G a b r i e l s e , 1983). I s o t o p i c evidence suggests that stocks i n t r u d i n g Rocky Mountain B e l t sediments have higher i n i t i a l 8 7 S r / 8 6 S r r a t i o s than b a t h o l i t h s i n t r u d i n g Omenica C r y s t a l l i n e B e l t metasediments. Higher i n i t i a l s t r o n t i u m isotope r a t i o s , s m a l l e r s i z e of i n t r u s i v e bodies and 48 lower grade r e g i o n a l metamorphism Of country rocks c o i n c i d e with g r e a t e r c r u s t a l t h i c k e n i n g i n the Rocky Mountain B e l t than i n the Omenica C r y s t a l l i n e B e l t (Armstrong, 1983). Rocky Mountain B e l t p l u t o n s t h e r e f o r e have a g r e a t e r l i t h o p h i l e component than Omenica C r y s t a l l i n e B e i t i n t r u s i o n s and h o l d more p o t e n t i a l to produce tungsten skarn d e p o s i t s . LEAD ISOTOPES Lead isotope r a t i o s were measured f o r galena from seven s i l v e r or g o l d v e i n s and one tungsten skarn (Table 7). These d e p o s i t s are r e l a t e d c l o s e l y to the f e l s i c i n t r u s i o n s near C a s s i a r ( F i g . 1). Lead r a t i o s c l u s t e r on the upper c r u s t a l " s h a l e " curve that i s g e n e r a l l y a p p l i c a b l e to the Omineca C r y s t a l l i n e B e l t i n the Canadian C o r d i l l e r a (Godwin and S i n c l a i r , 1982; Andrew et a l . , 1983; 1984); a 2 0 6 P b / 2 0 4 P b model age, based On the shale curve, from the average and standard d e v i a t i o n i n Table 7, y i e l d s an E a r l y Cretaceous age Of 137 ± 24 Ma. Although the p r e c i s i o n Of t h i s age i s pOOr, compared to other methods d e s c r i b e d above, the l e a d c l e a r l y has an upper c r u s t a l source rather than a mantle or lower c r u s t a l source. If the source Of the l e a d i s from the i n t r u s i o n s t h i s would be i n agreement with the rubidium-strontium i n t e r p r e t a t i o n that a s s i m i l a t e d c r u s t a l rocks formed an important component Of i n t r u s i o n s i n the C a s s i a r a r e a . INTRUSION CHEMISTRY On the b a s i s of accessory muscovite, normative corundum, K 20 > Na 20, A 1 2 0 3 > K 20+Na 20+l/2CaO and 8 7 S r / 8 6 S r > 0.712 (Table TABLE 7: Galena-Lead Isotope Analyses From Gold or Silver Veins and Tungsten Skarn Samp1e Anal - Map Lat. Long. Lead Isotope Ratios (Relative IS Error as '/.) Number yst Deposit Name Name North West 206/204 207/204 20B/204 Remar k s 303B3-001 Coast Silver 383 59.26 129.83 19.243 (. 07) 15.6B2 (. 15> 39.416 (.14) Ven. Atan Bp MARB 3O384-001 1 Ray 2 384 59.27 129.85 19.199 (. 03) 15.667 (. 09) 39.309 (.13) Veil. Atan Gp MARB 303B5-001 1 Lower Granite Cl:: D-Zone 385 59.28 129.82 19.326 (. 0B) 15.770 (. 15) 39.571.(.15) Ven. Atan Gp DCJLM 3O386-O01 1 Wei seman 3U6 59. 13 129.77 19.224 (. 02) .15.735 (. 09) 39.349 (.04) Ven. Ingenika MARB 30387-001# 1 Contact - Telemac 387 59.32 129.87 18.990 (. IB) 15.666 (. 25) 39.222 (.33) Ven. Ingenika MARB 30387-0O2 • 1 Contact - Telemac 3B7 59.32 129.87 19.317 (. 20) 15.BOB (. 45) 39.582 (.30) Ven. Ingenika MARB 30399- 001 1 Skarn Showing 399 59. 33 129.88 19.272 (. 07) 15.7O0 (. 17) 39.360 (.17) Sl;r. Ingenika LIMS 30400- 001 1 Cusac 400 59. 19 129. 70 19.271 (. 07) 15.709 (. 12) 39.256 (.13) Ven. Sylvest. VQLC JMMARY: CASSIAR MINERAL OCCURRENCES Number of deposits (n) = C7] Number of analyses (a) - 183 Std. Arithmetic average (::) = Standard deviation (S) = error of mean (S n ) = C19.2651 [0.047] CO.OIBJ C15.727] CO.051] CO.019] 139.406] [0.126] 10.04BJ All analyses, by B. Ryan, were done in the Geology-Geop) ysics L atipratory, Jhe University of British Columbia. Analysis marked with an "*" was deleted from calculations because of suspected poor quality. Abbreviations used in remarks are: Gp = Group, Ingenika - Ingenika Group, Sylvest. = Sylvester Group, Skr. = skarn, Ven. = vein, D0LM = dolomite, LIMS = limestone, MARB = marble, and V0LC - volcanics. 50 7), the Kuhn and C a s s i a r stocks are c l a s s i f i e d as S-type g r a n i t e s (Chappell and White, 1974) formed by a n a t e x i s of sedimentary rocks. Such an o r i g i n i s c o n s i s t e n t with the me l t i n g of c o n t i n e n t a l c r u s t to produce a g r a n i t i c magma (White and Ch a p p e l l , 1977; White et a l . , 1977), but d i f f e r e n t i a t i o n of a magma from an igneous source rock can a l s o produce a g r a n i t e with s i m i l a r chemical trends ( C h a p p e l l , 1979). Accessory magnetite and p y r i t e > 0.1%, magnetic s u s c e p t i b i l i t y > 0.0004 emu/g, and 5 3"S > 0 (Table 8) i n d i c a t e that the Kuhn and C a s s i a r stocks a l s o conform to magnetite-s e r i e s g r a n i t e s ( I s h i h a r a , 1977) that formed from r e l a t i v e l y O x i d i z i n g magmas. I s h i h a r a (1981) notes that i n A u s t r a l i a , m a g n e t i t e - s e r i e s g r a n i t e s c o r r e l a t e only with I-type i n t r u s i o n s whereas i l m e n i t e - s e r i e s p l u t o n s can form from magmas Of e i t h e r S~ Or I-type o r i g i n . However, an O x i d i z i n g magma does not preclude d e r i v a t i o n of the Kuhn and C a s s i a r Stocks from a source rock Of mixed or sedimentary provenance. Both I s h i h a r a (1977) and C h a p p e l l and White (1974) suggest that tungsten skarn and porphyry molybdenum Ore d e p o s i t s are more t y p i c a l of I-type and ma g n e t i t e - s e r i e s g r a n i t i c p l u t o n s than S-type and i l m e n i t e -s e r i e s g r a n i t o i d s . In c o n t r a s t , the McDame W skarn and Casmo MO porphyry d e p o s i t s (Panteleyev, 1979) are r e l a t e d to S-type and m a g n e t i t e - s e r i e s g r a n i t e s , i n d i c a t i n g that both sedimentary source rocks and d i f f e r e n t i a t i o n Of an o x i d i z i n g magma were important at C a s s i a r . Anomalously high K 20, K/Rb, U and U/Th (Table 7) i n d i c a t e that the Kuhn stock i s geochemically s p e c i a l i z e d (TischendOrf, 1977) i n these r a d i o a c t i v e elements. Although not s p e c i a l i z e d i n ielLl-§i--G&affliStrv_and._MinsraIggv._gi_^ CSSSISB !JUHN_1 S _ I Y P i CLIYPE. SPgGIALIJgQ M A J O R E L E M E N T S 7. SiO 68. 0 66.0 68.5 68.07 75. 40 73. 38 TiO 0. 54 • 0.71 0.62 0.63 0.13 0. 16 Al 0 14. 3 14.8 14. 1 14.49 • 13.29 13. 97 Sum FeO 2. 92 3.06 2.45 4. 40 J.41 1. 90 MnO 0 . 08 0.05 i.j. ij 3 0.07 . 0. 04 0. 05 MgO 1. 01 1.28 0.88 2. 13 0. 43 0. 47 CaO 1. 98 2. 11 1.45 n 55 1. 30 0. 75 Na • 3. 54 3.55 3. 16 2.07 3.51 3. 20 0 4. 09 4. 58 5.20 3. 30 3. 93 4. 69 P 0 0. 21 •' 0. 23 0 . 23 0 . 15 0.05 LOI 77 1.16 1 .00 1.93 0.47 TOTALS 97. 44 97. 53 97.62 9 9 . 7? 99. 9b 98. 57 TRACE ELEMENTS ppm Rb 240 - 265 169 550 Sr •" 214 - 287 129 - 100 U S - 11 3 -Th 30 35 18 -UJ - 4 - - 7 Mo r> 6 - - 3 5 Sn 2 - 1 - - .30 F .532 - 583 - - 3180 Li 55 29 - - 400 Be ' 4 — 4 13 ELEMENT RATIOS « 0 /(K O+Na 0+1/2 CaO) 1. 7 1.6 1.6 2.2 1 . 6 1 7 K O/Na 0 1. 2 1.3 1.5 1.6 1.1 1 5 K/Rb . 141 - 163 162 - 71 Rb/Sr 1. 1 - 0 . 9 1 . 3 - *t 5 U/Th 0 3 - 0. 3 'I1. 2 -Mg/Li • 110 182 7 1 NORMATIVE MINERALS Quart: 26. 43 21.82 26.62 3.3. 38 35.97 33 79 Orthoclase 25. 01 28. 10 31.82 19.91 23. 34 2e 1 1 Albite 30 18 30.39 27.01 17.85 29. 83 27 46 Anorthi te S. 75 9.31 5.89 11.92 6. 15 3 77 Hypersthene 6. 26 6.84 4.98 1 1. 07 3.01 3 84 Corundum 1. 15 0. 90 . 1.36 3.31 1. 03 2 30 Apatite 0. 50 0. 55 0. 55 0. 35 0.12 0 00 11 mem te 1. 06 1.40 1. 22 1. 22 0. 25 0 31 Magnetite. 0. 66 0.69 0.55 0 . 9B 0. 3 1 42 MODAL MINERALS 7. Quartz 40 40 44 K-Feldspar 15 20 5 - 33 Plagioclase 35 30 33 - 25 Biotite a 7 5 IB - 3 Muscovite 0 1 . 3 0 - 3 Chlorite l 1 1 0 - 0 Sphene < I < 1 < 1 0 - 0 Apati te < i •. 1 •: 1 0. 1 - 0 Zircon <i < 1 \ 1 0. 1 - 0 Magneti te 0 < 1 < 1 0.2 - 1 Pyrite ,;' 1 - - -MAGNETIC SUSCEPTIBILITY emu/g : 10 17 4.2 3. 6 .1 Chemical analyses by Chemex Labs Ltd., Vancouver, B.C. -Mineral modes are from visual estimates of thin sections. 'CIF'W norms are calculated using standardized FeO-Fe 0 ratios ! B r o o l 5 , 1774). Magnetic susceptibility was measured on pulverized samples. Data, sample BB34, are from White et al. (1977). Data, average of nine samples, are from Tanaka and Nozawa il977>. Magnetite content is greater than 0 .1 volume percent in M-Type granitoids (Ishihara, 1981). Data, average of 962 samples, are from Tischendorf (1977). 52 W or Mo r e l a t i v e to a world average, the Kuhn stock does c o n t a i n more W and Mo than the C a s s i a r stock (Table 7) and t h e r e f o r e holds g r e a t e r p o t e n t i a l to produce W-Mo skarns. Strong (1981) notes that b i o t i t e q uartz monzonites are g e n e r a l l y barren of m i n e r a l i z a t i o n whereas more d i f f e r e n t i a t e d f e l s i c plutons form g r a n o p h i l e Sn-W-U-Mo ore d e p o s i t s and l e s s d i f f e r e n t i a t e d mafic p l u t o n s form porphyry Cu-Mo ore d e p o s i t s . However, tungsten skarns i n the Canadian C o r d i l l e r a are uniquely a s s o c i a t e d with b i o t i t e q uartz monzonites (Dick and Hodgson, 1982) and the Kuhn stock i s a b i o t i t e q uartz monzonite ( S t r e c h e i s e n , 1974) of c a l c a l k a l i n e a f f i n i t y ( I r v i n e and Baragar, 1971). Thus, something other than magmatic d i f f e r e n t i a t i o n must i n f l u e n c e the m i n e r a l i z i n g p o t e n t i a l of a g r a n i t i c p l u t o n . Accounting f o r a magma's source rock as w e l l as i t s d i f f e r e n t i a t i o n h i s t o r y may hel p to eva l u a t e the p o t e n t i a l f o r min e r a l d e p o s i t s a s s o c i a t e d with a c i d i c i n t r u s i o n s . A simple model f o r the o r i g i n of g r a n i t i c p l u t o n s and ore minerals at McDame thus i n c l u d e s : 1) a s i a l i c c r u s t a l source to the C a s s i a r and Kuhn stocks and t h e i r l i t h o p h i l e elements through m e l t i n g of lower c r u s t and/or a s s i m i l a t i o n of upper c r u s t ; 2) magmatic d i f f e r e n t i a t i o n and geochemical s p e c i a l i z a t i o n of an o x i d i z i n g magma to produce A l 2 0 3 - K 2 0 - U r i c h g r a n i t i c p l utons and W-Mo skarn d e p o s i t s . CONCLUSIONS 1) The McDame tungsten skarn prospect i s c o n t a i n e d i n Lower Hadrynian to Lower O r d o v i c i a n c l a s t i c and carbonate metasediments of the Ingenika, Atan and Kechika Groups where 53 they are i n t r u d e d by f e l s i c s t o c k s of the C a s s i a r I n t r u s i o n s . The C a s s i a r area has been o f f s e t along the T i n t i n a and r e l a t e d f a u l t s from a world c l a s s tungsten skarn d i s t r i c t i n s i m i l a r unmetamorphosed rocks of the Rocky Mountain B e l t . T h e r e f o r e , Omenica C r y s t a l l i n e B e l t rocks near C a s s i a r h o l d p o t e n t i a l to host economic tungsten skarns. 2) Four metasdmatic f a c i e s at McDame are l i t h o l o g i c a l l y and s t r u c t u r a l l y c o n t r o l l e d . Prograde massive c a l c s i l i c a t e W-Mo-Fe, banded c a l c s i l i c a t e Fe, banded oxide W-Mo-Fe and retro g r a d e massive s u l f i d e Fe-Zn-Cu-W skarns r e p l a c e marble, h o r n f e l s , dolomite and skarn, r e s p e c t i v e l y , along c o n t a c t s , f r a c t u r e s and f a u l t s . Only the massive c a l c s i l i c a t e f a c i e s a t t a i n s ore th i c k n e s s and grade but the banded c a l c s i l i c a t e and oxide f a c i e s are u s e f u l i n min e r a l e x p l o r a t i o n because they s i g n a l the presence of b u r i e d f e l s i c i n t r u s i o n s . 3) An estimated l i t h o s t a t i c pressure of 1500 bars suggests formation of McDame skarns at about s i x k i l o m e t e r s depth. Temperatures f o r prograde skarn was about 500°C at C0 2 mole f r a c t i o n s l e s s than 0.15. Retrograde skarn formed at lower T and XC0 2. 4) C a l c s i l i c a t e m i neral zoning r e s u l t e d from d i s s o l u t i o n , i n f i l t r a t i o n / d i f f u s i o n and d e p o s i t i o n of S i 0 2 , CaO, A l 2 0 3 , MgO, H 20 and C0 2 i n marble, dolomite and h o r n f e l s by magmatic f l u i d s . Higher grade m i n e r a l s such as garnet i n quartz skarn and p l a g i o c l a s e i n magnetite skarn s i g n a l p r o x i m i t y to a f e l s i c i n t r u s i o n ( w i t h i n tens of meters). M e t a l l i c mineral zoning was formed by i n f i l t r a t i o n of r e l a t i v e l y W-, Mo- 0 2- and S 2 - r i c h magmatic f l u i d s and mixing with r e l a t i v e l y Fe-, Zn- and C u - r i c h , 54 0 2- and S 2-poor f o r m a t i o n a l waters along permeable zones i n skarn. T h i s zoning i s u s e f u l i n g u i d i n g mineral e x p l o r a t i o n from d i s t a l s p h a l e r i t e - and c h a l c o p y r i t e - r i c h skarns to more proximal s c h e e l i t e - and m o l y b d e n i t e - r i c h skarns. 5) S u l f u r i s otope data suggest that f r a c t i o n a t i o n of magmatic s u l f u r ( S 3 U S = 0) produced porphyry p y r i t e ( 5 3 4 S = +4.7), skarn p y r i t e (5 3"S = +7.0) and skarn p y r r h o t i t e (6 3 f lS = +7.5) and mixing with f o r m a t i o n a l s u l f u r formed skarn p y r r h o t i t e - s p h a l e r i t e - c h a l c o p y r i t e (8 3 < tS = +8.4) and porphyry p y r r h o t i t e ( 6 3 " S = +9.3). High temperature of skarn formation r e s u l t s i n the r e l a t i v e l y minor f r a c t i o n a t i o n of s u l f u r i s o t o p e s and the narrow v a r i a t i o n s i n 8 3 < ,S. 6) Rubidium-strontium and l e a d i s o t o p e s i n d i c a t e that the Kuhn stock had a strong s i a l i c component from m e l t i n g and/or a s s i m i l a t i o n of c o n t i n e n t a l c r u s t . Thus, d i f f e r e n t i a t i o n Of an o x i d i z i n g , c r u s t a l l y d e r i v e d magma e n r i c h e d i n l i t h o p h i l e elements produced the McDame tungsten skarn d e p o s i t . Higher i n i t i a l s t r o n t i u m isotope r a t i o s from Cretaceous f e l s i c p l utons in the Rocky Mountain B e l t suggest that they h o l d more p o t e n t i a l to produce tungsten skarns than s i m i l a r , younger i n t r u s i o n s i n the Omenica C r y s t a l l i n e B e l t . 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