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UBC Theses and Dissertations

A new look at the Afton copper mine in the light of mineral distributions, host rock geochemistry and… Kwong, Yan-Tat John 1981

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A NEW LOOK AT THE AFTON COPPER MINE IN THE LIGHT OF MINERAL DISTRIBUTIONS, HOST ROCK GEOCHEMISTRY AND IRREVERSIBLE MINERAL-SOLUTION INTERACTIONS by YAN-TAT JOHN KWONG M . S c , McMaster U n i v e r s i t y , 1975 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES We accept t h i s t h e s i s as conforming to the r e q u i r e d s tandard THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1981 © Yan-Tat John Kwong, 1981 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 agree 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 Q n 1 n g i c a 1 Sc iences The 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 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 D a t e Feb . 12. 1982 DE-6 (2/79) i i ABSTRACT Embodied i n th ree main p a r t s , t h i s study i n c o r p o r a t e s f i e l d o b s e r v a t i o n s , t h e o r e t i c a l m o d e l l i n g and geochemical data to i n t e r p r e t the v a r i o u s aspects of the copper m i n e r a l i z a t i o n at A f t o n M i n e s . P a r t I d e s c r i b e s and i n t e r p r e t s m i n e r a l d i s t r i b u t i o n s w i t h i n and about the A f t o n orebody. The copper m i n e r a l i z a t i o n occur red i n a roof pendant environment c h a r a c t e r i z e d by a d i v e r s i f i e d a r ray of rock types w i t h i n t r i c a t e mutual r e l a t i o n s h i p s . E x c l u d i n g the l a r g e s c a l e zon ing of p y r i t e and magnet i te e s t a b l i s h e d by Car r and Reed (1976) and the apparent a s s o c i a t i o n of hydrothermal b i o t i t e w i t h o r e , d i s t r i b u t i o n of most pr imary and secondary m i n e r a l s i s r a t h e r s p o r a d i c . High temperature a l t e r a t i o n s at A f t o n p robab ly proceeded w i t h a low r a t i o of f l u i d to rock such t h a t p r imary minera logy was ab le to c o n t r o l the appearance of secondary m i n e r a l s . The absence of i n t e n s e p h y l l i c and a r g i l l i c a l t e r a t i o n s i s i n t e r p r e t e d to be caused by the l a c k of abundant meteor i c water p a r t i c i p a t i n g i n the hypogene m i n e r a l i z a t i o n event . In tense carbonate a l t e r a t i o n , however, i s l i k e l y to be i n d i c a t i v e of concent ra ted j u v e n i l e hydrothermal a c t i v i t y . The mode of occur rence of ore m i n e r a l s , on the o ther hand, suggests that supergene a l t e r a t i o n a t A f t o n was dominated by i r r e v e r s i b l e m i n e r a l - s o l u t i o n i n t e r a c t i o n s . The extent of the a l t e r a t i o n at depth appears to be l a r g e l y f r a c t u r e - c o n t r o l l e d . P a r t I I p resents a thermodynamic r e c o n s t r u c t i o n of the supergene a l t e r a t i o n at A f t o n . I t i s demonstrated tha t the dominance o f n a t i v e copper and the l a c k of copper enr ichment i n the supergene zone are r e l a t e d to the r e l a t i v e l y m a f i c compos i t ion of the w a l l rocks and the absence of abundant s u l f i d e s i n the hypogene zone . Supergene a l t e r a t i o n of porphyry copper i i i d e p o s i t s i n genera l can be i n t e r p r e t e d i n the framework of a l o g f - l o g f p l o t w i t h respec t to two a r b i t r a r y " l i m i t i n g " curves c h a r a c t e r i z e d by extreme v a l u e s o f a c i d i c and b a s i c pH r e s p e c t i v e l y . The c l o s e r a r e a c t i n g f l u i d e v o l v e s p a r a l l e l to the upper or a c i d i c " l i m i t i n g " c u r v e , the more l i k e l y i s the f o r m a t i o n of a w e l l - d e v e l o p e d leached cap w i t h an enr i ched supergene ore b l a n k e t underneath. In c o n t r a s t , a r e a c t i n g f l u i d e v o l v i n g p a r a l l e l and c l o s e to the lower or b a s i c " l i m i t i n g " curve w i l l l e a d to a spectrum of copper ox ides and n a t i v e copper w i t h l i t t l e enr ichment . P a r t I I I summarizes and r e l a t e s major element geochemical d a t a , p e t r o g r a p h i c observa t ions and f i e l d ev idence to the e v o l u t i o n and copper m i n e r a l i z a t i o n of the v a r i o u s i n t r u s i v e phases of the I ron Mask b a t h o l i t h . The four major phases of the b a t h o l i t h , namely, the I ron Mask H y b r i d , Pothook, Sugar loaf and Cherry Creek u n i t s , appear to be r e l a t e d by the p rocess of f r a c t i o n a l c r y s t a l l i z a t i o n . The A f t o n p r o t o r e i s l i k e l y a b y - p r o d u c t of magmatic d i f f e r e n t i a t i o n generated at the e a r l y Cherry Creek s tage when p o r t i o n s of the f r a c t i o n a t i n g magma i n t r u d e d i n t o a s u b v o l c a n i c env i ronment . M i n e r a l o g i c a l c h a r a c t e r i s t i c s of the o r e b o d y , . i n c l u d i n g the s c a r c i t y of m o l y b d e n i t e , r e f l e c t s i g n i f i c a n t i n f l u e n c e of host rock geochemist ry on l a t e s tage copper m i n e r a l i z a t i o n and the a s s o c i a t e d hypogene a l t e r a t i o n . Taking i n t o account a l l the i n f o r m a t i o n a c q u i r e d , the e v o l u t i o n o f the A f t o n orebody appears to have i n v o l v e d f o u r major e v e n t s : i ) f r a c t i o n a l c r y s t a l l i z a t i o n of a parent I ron Mask magma at depth w i t h c o n t i n u a l magnet i te s e p a r a t i o n ; i i ) t r a n s f e r of p o r t i o n s of the e a r l y Cherry Creek magma to a s u b v o l c a n i c environment where r a p i d c o o l i n g enhanced s u p e r s a t u r a t i o n of copper s u l f i d e s and where some l o s s of s u l f u r occur red through v o l c a n i c emanat ions ; i i i ) l o c a l t r a p p i n g of l a t e magmatic f l u i d to r e m o b i l i z e and c o n c e n t r a t e the o r i g i n a l l y d isseminated s u l f i d e s to form the hypogene o r e ; and £v) ..subsequent i v supergene a l t e r a t i o n c o n s t r a i n e d to take p l a c e i n a b a s i c pH domain to s t a b i l i z e abundant n a t i v e copper w i thout much enr i chment . V TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS v LIST OF FIGURES v i i LIST OF TABLES i x LIST OF PLATES ' x ACKNOWLEDGEMENT x i INTRODUCTION 1 PART I - DISTRIBUTION OF MINERALS AT AFTON MINES AND ITS IMPLICATIONS 3 I n t r o d u c t i o n 4 Genera l Geology 6 L o c a l Geology and Petrography of the A f t o n O p e n - p i t 9 a) Genera l aspec ts 9 b) D e t a i l e d p i t petrography 11 M i n e r a l D i s t r i b u t i o n s and A l t e r a t i o n P a t t e r n s 17 a) I n t r o d u c t i o n 17 b) L a t e r a l m i n e r a l d i s t r i b u t i o n s 17 c) V e r t i c a l d i s t r i b u t i o n of m i n e r a l s 27 d) D e t a i l s o f the copper m i n e r a l i z a t i o n 33 D i s c u s s i o n and I n t e r p r e t a t i o n 38 a) Absence of abundant s e r i c i t e and c l a y m i n e r a l s 38 b) P r e f e r e n t i a l a s s o c i a t i o n of m i n e r a l s 39 c) T iming of the carbonate a l t e r a t i o n 41 d) Sporad ic d i s t r i b u t i o n of hydrothermal a l t e r a t i o n m i n e r a l s 42 Summary and C o n c l u s i o n 43 v i Page PART I I - A THERMODYNAMIC SIMULATION OF THE SUPERGENE ALTERATION 45 AT AFTON MINES I n t r o d u c t i o n and Method of Approach 46 S p e c i f i c a t i o n of Computer Runs 48 R e s u l t s and I n t e r p r e t a t i o n 53 a) Genera l aspects 53 b) Enrichment versus non-enr ichment and dominance of a p a r t i c u l a r 58 phase c) D i s c r e p a n c i e s between t h e o r e t i c a l and observed m i n e r a l 65 assemblages d) Other c o n s i d e r a t i o n s 70 Summary and C o n c l u s i o n 72 PART I I I - MAJOR ELEMENT GEOCHEMISTRY AND ITS IMPLICATIONS ON THE 73 EVOLUTION AND COPPER MINERALIZATION OF THE VARIOUS INTRUSIVE PHASES OF THE IRON MASK BATHOLITH I n t r o d u c t i o n 74 Genera l Geology and Petrography of the I r o n Mask P l u t o n 74 R e s u l t of Major Element A n a l y s e s 80 D i s c u s s i o n and I n t e r p r e t a t i o n 97 a) E v o l u t i o n of the I r o n Mask b a t h o l i t h 97 b) E v o l u t i o n of the copper m i n e r a l i z a t i o n w i t h i n the I r o n Mask 100 b a t h o l i t h Summary and C o n c l u s i o n 106 SUMMARY AND OVERVIEW 108 BIBLIOGRAPHY 112 APPENDIX I 116 APPENDIX I I 119 v i i LIST OF FIGURES FIGURE Page 1 L o c a t i o n of porphyry copper d e p o s i t s of the a l k a l i n e s u i t e 5 i n the Canadian C o r d i l l e r a 2 L o c a t i o n and g e n e r a l g e o l o g i c a l s e t t i n g of A f t o n Mines 7 3 A g e o l o g i c a l map (a) and a g e o l o g i c a l s e c t i o n (b) of the A f t o n 10 p r o p e r t y 4 A g e o l o g i c a l s e c t i o n a c r o s s the e a s t e r n h a l f of the A f t o n open - 12 p i t , i l l u s t r a t i n g the complex i ty of host rocks i n v o l v e d 5 A h i g h l y g e n e r a l i z e d g e o l o g i c a l map of the A f t o n o p e n - p i t a t 13 610-640 meter l e v e l s as of June , 1979 6 M i n e r a l d i s t r i b u t i o n s w i t h i n the A f t o n o p e n - p i t at the 613 meter 18-19 l e v e l as r e v e a l e d by x - r a y d i f f r a c t i o n ana l yses of b l a s t h o l e composite samples 7 a - c D i s t r i b u t i o n of s e l e c t e d m i n e r a l s as determined from p e t r o g r a p h i c 22 -24 study of 173 specimens from the 613-640 meter l e v e l s of the A f t o n o p e n - p i t 7d D i s t r i b u t i o n of ore and accessory m i n e r a l s i n the 613-640 meter 25 l e v e l s of the A f t o n o p e n - p i t 8 Hydrothermal a l t e r a t i o n observed i n a g e o l o g i c a l s e c t i o n a c r o s s 28 the e a s t e r n p o r t i o n of the orebody 9 D i s t r i b u t i o n of h i g h temperature a l t e r a t i o n m i n e r a l s i n a 2 9 - 3 0 g e o l o g i c a l s e c t i o n a c r o s s the western p o r t i o n of the orebody 10 D i s t r i b u t i o n o f copper and g o l d a t v a r i o u s benches i n the A f t o n 37 p i t 11 Change i n s o l u t i o n chemis t r y and the p r o d u c t i o n and d e s t r u c t i o n 54 of m i n e r a l s as a r e s u l t of r a i n w a t e r - e a s t p i t rock i n t e r a c t i o n s at 25 degree Cent ig rade and a p ressure of 1 bar 12 Change i n s o l u t i o n chemis t r y and the p r o d u c t i o n and d e s t r u c t i o n 55 of m i n e r a l s as a r e s u l t of r a i n water -west p i t rock i n t e r a c t i o n s at 25 degree Cent ig rade and a p ressure of 1 bar 13 Change i n s o l u t i o n chemis t r y and the p r o d u c t i o n and d e s t r u c t i o n 56 of m i n e r a l s as a r e s u l t of r a i n w a t e r - h y p o t h e t i c a l monzonite i n t e r a c t i o n s at 25 degree Cent ig rade and a p ressure of 1 b a r 14 The e v o l u t i o n paths of the r e a c t i n g s o l u t i o n s f o r the t h r e e 5 9 - 6 0 s i m u l a t i o n runs over the s t a b i l i t y f i e l d s of m i n e r a l s i n the C u - F e - S - 0 system at 25 degree Cent ig rade and a p r e s s u r e of 1 b a r v i i i FIGURE Page 15 D i s t r i b u t i o n of s u l f u r s p e c i e s a t 25°C and 100°C as a 62 a f u n c t i o n of f ( P ) and s o l u t i o n pH 16 Paragenes is of product m i n e r a l s r e s u l t i n g from the w a t e r - 66 r o c k i n t e r a c t i o n c a l c u l a t i o n s 17 Genera l geology of the I r o n Mask p l u t o n 76 18 Harker p l o t s of ana lyses of the major i n t r u s i v e phases of 83 -86 the I ron Mask p l u t o n 19 A t o t a l a l k a l i - t o t a l i r o n - m a g n e s i a t r i a n g u l a r p l o t of the 89 the ana l yses of the four major i n t r u s i v e phases of the I r o n Mask p l u t o n 20 A comparison of ana l yses of v a r i o u s rock groups r e l a t e d to 9 0 - 9 3 the I r o n Mask b a t h o l i t h w i t h those of the four major i n t r u s i v e phases of the I ron Mask p l u t o n 21 A t o t a l a l k a l i ve rsus s i l i c a p l o t f o r the ana lyses of v a r i o u s 96 rock groups r e l a t e d to the I ron Mask p l u t o n i x LIST OF TABLES TABLE Page IA Compos i t ion of s t a r t i n g m a t e r i a l ( i n modal pe rcent ) 49 IB Compos i t ion of h y p o t h e t i c a l r a i n water used i n the s i m u l a t i o n 50 runs I I Compos i t ion o f northwest p i t w a l l seepage 69 I I I P e t r o g r a p h i c c h a r a c t e r i s t i c s o f v a r i o u s rock u n i t s of the 78 I ron Mask b a t h o l i t h IV Major element geochemistry of v a r i o u s rock u n i t s making up or 81 a s s o c i a t e d w i t h the I ron Mask p l u t o n \ X LIST OF PLATES PLATE Page I P r i m a r y , p o i k i l i t i c b i o t i t e i n c o m p l e t e l y e n c l o s i n g p l a g i o c l a s e 32 i n medium gra ined d i o r i t e I I C l u s t e r s of f i n e g ra ined hydrothermal b i o t i t e commonly a s s o c i a t e d 32 w i t h ore i n m i c r o d i o r i t e b r e c c i a . I I I N a t i v e g o l d c o a t i n g c h a l c o c i t e which i n c o m p l e t e l y r e p l a c e s b o r n i t e 35 IV An example of d i s c o n t i n u o u s s e g r e g a t i o n bands of magnet i te and 98 p l a g i o c l a s e observed i n gabbro specimens of the I r o n Mask H y b r i d u n i t x i ACKNOWLEDGEMENT The main theme of t h i s t h e s i s was developed a f t e r my two y e a r s ' f u l l - t i m e attendance at the U n i v e r s i t y of B r i t i s h Co lumbia . I am indebted to many i n d i v i d u a l s and i n s t i t u t i o n s f o r i t s c o m p l e t i o n . At the Department o f G e o l o g i c a l Sc iences of U . B . C . , D r . H . J . Greenwood, my s u p e r v i s o r , has p rov ided c o n t i n u a l guidence and encouragement; Dr . T . H . Brown p a t i e n t l y a d v i s e d on mass t r a n s f e r c a l c u l a t i o n s ; Dr . C . I . Godwin o f f e r e d v a l u a b l e comments throughout the course o f t h i s s t u d y ; D r s . K . C . McTaggart , H . J . Greenwood and T . H . Brown c r i t i c a l l y read the e a r l y manuscr ip ts r e s u l t i n g i n much improvement; John Kn ight he lped a l o t w i t h microprobe a n a l y s e s . B .C . M i n i s t r y of Energy, Mines & Pet ro leum Resources approved and supported the p a r t - t i m e d o c t o r a l program. Mr . G .E . White ( D i s t r i c t G e o l o g i s t at Kamloops) f i r s t aroused my i n t e r e s t i n A f t o n and p rov ided a s u i t e of 173 samples from the o p e n - p i t . D i s c u s s i o n s w i t h D r s . W . J . M c M i l l a n , K . E . Nor thcote ( then a l s o w i t h the M i n i s t r y ) and V . A . P r e t o c l a r i f i e d many g e o l o g i c a l aspects of the I r o n Mask p l u t o n and the A f t o n d e p o s i t . Dr . K . E . Nor thcote a l s o p r o v i d e d r e p r e s e n t a t i v e samples from the p l u t o n w h i l e Dr . W . J . M c M i l l a n c r i t i c a l l y rev iewed p a r t s of the e a r l y m a n u s c r i p t . Co l leagues i n the A n a l y t i c a l L a b o r a t o r y , Mac Chaudhry i n p a r t i c u l a r , o f f e r e d much encouragement and a d v i c e on wet chemica l a n a l y t i c a l t e c h n i q u e s . A f t o n Mines L t d . g ranted p e r m i s s i o n to e n t e r the A f t o n p r o p e r t y f o r f i e l d s t u d i e s and s u p p l i e d company g e o l o g i c a l and geochemical data as w e l l as b l a s t h o l e samples . I am e s p e c i a l l y g r a t e f u l to Mr . A . J . Reed, Ch ie f Mine E n g i n e e r , f o r h i s c o - o p e r a t i o n and h o s p i t a l i t y d u r i n g my study a t A f t o n . F i n a l l y , I would l i k e to thank C e c i l i a , my s i s t e r , and Nancy Lamb f o r k i n d l y s h a r i n g the t y p i n g and d r a f t i n g of the e a r l y and f i n a l m a n u s c r i p t s . 1 INTRODUCTION A f t o n M i n e s , l o c a t e d i n s o u t h c e n t r a l B r i t i s h Co lumbia , i s one of the most r e c e n t l y opened porphyry copper d e p o s i t s i n the p r o v i n c e . I t s h i s t o r y of development has been o u t l i n e d by P r e t o (1972) and Car r and Reed (1976) . Bes ides b e i n g hosted by d i o r i t i c igneous rocks of a l k a l i n e a f f i n i t y , the s a l i e n t c h a r a c t e r i s t i c s of the d e p o s i t i n c l u d e two unique f e a t u r e s . These are the absence of a w e l l - d e f i n e d hypogene a l t e r a t i o n p a t t e r n about the orebody and the abundance of n a t i v e copper w i thout apparent supergene enrichment (Car r and Reed, 1976) . S ince the b e g i n n i n g of the o p e n - p i t o p e r a t i o n i n December, 1977, b e t t e r exposure of the orebody and accumulated new d a t a have s t i m u l a t e d a new and c l o s e r l ook at the ore d e p o s i t as p resented i n t h i s s tudy . Th is study i s d i v i d e d i n t o th ree main p a r t s , P a r t I p r e s e n t s and i n t e r p r e t s m i n e r a l d i s t r i b u t i o n s w i t h i n and about the A f t o n orebody. A f t e r a b r i e f rev iew of the g e n e r a l geology of the d e p o s i t , the new data are p resented and d i s c u s s e d . These data d e r i v e from three main s o u r c e s . F i r s t , new d e t a i l e d mapping of the o p e n - p i t , c o r e - l o g g i n g and o ther f i e l d s t u d i e s c a r r i e d out i n the summer of 1979. Second, p e t r o g r a p h i c s t u d i e s of 173 samples accumulated through the j o i n t e f f o r t of A. Reed, Ch ie f Mine E n g i n e e r , A f t o n Mines L t d . , and G. E. W h i t e , D i s t r i c t G e o l o g i s t at Kamloops, B . C . M i n i s t r y of Energy , Mines and Pet ro leum Resources , s i n c e the b e g i n n i n g of the o p e n - p i t o p e r a t i o n to the f a l l of 1979. T h i r d , x - r a y d i f f r a c t i o n a n a l y s i s of b l a s t h o l e samples at s e l e c t e d benches. The samples were s u p p l i e d to the author through the cour tesy of A f t o n Mines L t d . P a r t I I d e a l s p r i m a r i l y w i t h supergene a l t e r a t i o n . D e t a i l e d m i n e r a l compos i t ions determined by e l e c t r o n microprobe ana lyses have been used to 2 t e s t the p o s s i b i l i t y of r e p r o d u c i n g the supergene assemblage by the i n t e r a c t i o n of downward p e r c o l a t i n g r a i n water w i t h the pr imary m i n e r a l assemblages, u s i n g the thermodynamic mass t r a n s f e r approach of Helgeson (1968, 1971 and 1979) and Helgeson et a l . (1970) . S p e c i a l emphasis i s p l a c e d on i d e n t i f y i n g those parameters which a f f e c t the development o f supergene enr ichment . Par t I I I c o n s i d e r s the major element geochemistry of r o c k s w i t h i n the c l o s e v i c i n i t y of the I r o n Mask p l u t o n . A t o t a l of 97 samples were a n a l y s e d . These i n c l u d e a s u i t e of 39 specimens k i n d l y s u p p l i e d by Dr . K. E. Nor thco te as r e p r e s e n t a t i v e of the v a r i o u s rock types present i n the a r e a . The r e s t were c o l l e c t e d by A . Reed, G. E. White and the author f rom the A f t o n p r o p e r t y . The ana lyses i n d i c a t e t h a t the major phases o f the I r o n Mask b a t h o l i t h are r e l a t e d by f r a c t i o n a l c r y s t a l l i z a t i o n . P o s s i b l e e f f e c t s of the f r a c t i o n a t i o n process on the copper m i n e r a l i z a t i o n of the d i f f e r e n t i n t r u s i v e phases of the b a t h o l i t h a re then e l a b o r a t e d . P a r t i c u l a r emphasis i s p l a c e d on the comparison of the major element geochemistry of the A f t o n r o c k s w i t h the genera l t r e n d d e r i v e d f rom the ana lyses of the major phases of the b a t h o l i t h . S a l i e n t m i n e r a l o g i c a l c h a r a c t e r i s t i c s o f the A f t o n orebody a re a l s o cons idered to support the c o n t e n t i o n that the pr imary copper m i n e r a l i z a t i o n has o r i g i n a t e d from the magmatic d i f f e r e n t i a t i o n o f the b a t h o l i t h . 3 PART I DISTRIBUTION OF MINERALS AT AFTON MINES AND ITS IMPLICATIONS 4 I n t r o d u c t i o n A f t o n M i n e s , w i t h l a t i t u d e 50°39'N and l o n g i t u d e 120°31'W i s l o c a t e d b e s i d e the Trans-Canada Highway No. 1 , 13 k i l o m e t e r s west of Kamloops and 420 k i l o m e t e r s n o r t h e a s t of Vancouver. The orebody c o n t a i n s 30.8 m i l l i o n tonnes of o p e n - p i t ore g rad ing 1 .0 per cent c o p p e r , 0 .58 ppm go ld and 4.19 ppm s i l v e r at a 0 .25 per cent copper c u t o f f . I t i s one of the a l k a l i n e s u i t e porphyry copper d e p o s i t s r e s t r i c t e d i n the Canadian C o r d i l l e r a to the Intermontane B e l t ( F igure 1 , a f t e r B a r r et a l . , 1976) . More recent works d e a l i n g w i t h v a r i o u s aspects of the d e p o s i t i n c l u d e the f o l l o w i n g : P r e t o (1972) , based on f i e l d , mapping, c o r e - l o g g i n g and mine r e c o r d s , cons idered the geology and s t r u c t u r e of the A f t o n d e p o s i t i n some d e t a i l ; as a c o n t r i b u t i o n to the unders tand ing of porphyry d e p o s i t s of the Canadian C o r d i l l e r a , Carr and Reed (1976) d e s c r i b e d the o v e r a l l s a l i e n t f e a t u r e s of the d e p o s i t ; H o i l e s (1978) , upon s t u d y i n g the n a t u r e and genes is of the A f t o n orebody, f u r n i s h e d s u l f u r i s o t o p e d a t a on the p r e v a l e n t s u l f i d e s ; Cann (1979) s t u d i e d the geochemistry of magnet i tes i n the I r o n Mask b a t h o l i t h and made s p e c i a l r e f e r e n c e to t h e i r s i g n i f i c a n c e i n the genes is of the A f t o n orebody; the d i s c u s s i o n s of Reed (1980a and 1980b) on the s t r u c t u r a l geology and s l o p e s t a b i l i t y of the A f t o n o p e n - p i t and on the r e l e v a n t g e o s t a t i s t i c a l parameters make up the l a t e s t p i e c e s of i n f o r m a t i o n on the d e p o s i t . P rev ious i n v e s t i g a t i o n s , w i t h the e x c e p t i o n of those of Reed (1980a and 1980b) , a re based on m a t e r i a l s gathered p r i o r to the s t a r t i n g of the o p e n - p i t o p e r a t i o n i n December, 1977. S ince t h e n , new d a t a t h a t , have been accumulated warrant a c l o s e r l o o k at the ore d e p o s i t . T h i s study focusses on the petrography of the A f t o n orebody and on the s i g n i f i c a n c e of m i n e r a l d i s t r i b u t i o n s not p r e v i o u s l y d e l i n e a t e d . 5 F i g u r e 1 . L o c a t i o n of porphyry copper d e p o s i t s of the a l k a l i n e s u i t e i n the Canadian C o r d i l l e r a ( a f t e r Bar r e t a l . , 1976) . 6 Genera l Geology A f t o n Mines l i e s i n the nor thwestern e x t r e m i t y of the I r o n Mask p l u t o n , w h i c h , together w i t h the Cherry Creek p l u t o n i n the immediate n o r t h , make up the I r o n Mask b a t h o l i t h (F igure 2 ) . These p l u t o n s , once b e l i e v e d to form a s i n g l e connected body ( C o c k f i e l d , 1948) , f o l l o w a n o r t h w e s t e r l y t r e n d , c u t t i n g a c r o s s the n o r t h e r l y t r e n d i n g b e l t of the Upper T r i a s s i c N i c o l a Group r o c k s . The N i c o l a b e l t , c o n t i n u i n g n e a r l y 200 k i l o m e t e r s to i t s t e r m i n a t i o n at the border between Canada and the Un i ted S t a t e s , forms the southern par t of the c o p p e r - r i c h Quesnel Trough r e c e n t l y d i s c u s s e d by Campbell and T ipper (1970) . P roduc ing porphyry copper mines hosted by the t rough i n c l u d e , from n o r t h to s o u t h , B e l l , G r a n i s l e , M o r r i s o n , G i b r a l t a r , A f t o n , the H igh land V a l l e y d e p o s i t s , Brenda, I n g e r b e l l e and Copper Mounta in . The geology of the I ron Mask p l u t o n and rocks i n i t s v i c i n i t y has r e c e n t l y been s t u d i e d by P r e t o (1967) and Nor thcote (1974, 1976 and 1977) . B r i e f l y , the p l u t o n i s a m u l t i p l e i n t r u s i o n comprised of the I r o n Mask H y b r i d , Pothook, Sugar loa f and Cherry Creek u n i t s . The f i r s t two of these u n i t s c o n s i s t main ly of d i o r i t e and gabbro. Rocks of the l a s t two u n i t s , however, a re t y p i c a l l y f i n e g ra ined and p o r p h y r i t i c w i t h c o m p o s i t i o n r a n g i n g from gabbro to s y e n i t e though d i o r i t e predominates . In the nor thweste rn p o r t i o n of the p l u t o n , an a d d i t i o n a l u n i t of P i c r i t e i s l o c a l l y i m p o r t a n t . The d e t a i l e d g e n e t i c r e l a t i o n s h i p among these i n t r u s i v e u n i t s w i l l be p resented i n a l a t e r s e c t i o n . In the v i c i n i t y of the I ron Mask p l u t o n , the N i c o l a Group c o n s i s t s l a r g e l y of p y r o c l a s t i c rocks w i t h some interbedded f l o w s , of both a n d e s i t i c and b a s a l t i c i n c o m p o s i t i o n . Nor th of Hughes Lake near the nor thweste rn end of the p l u t o n , minor amounts of sedimentary b l o c k s i n v o l c a n i c b r e c c i a are a l s o p r e s e n t . C lose to the p l u t o n , these N i c o l a Group r o c k s are c h l o r i t i z e d , e p i d o t i z e d and commonly m i n e r a l i z e d w i t h copper and i r o n . The i n t e n s i t y of i g u r e 2 . L o c a t i o n and genera l g e o l o g i c a l s e t t i n g of A f t o n Mines (modi f ied from Cann, 1979) 8 metaraorphi&ra and m i n e r a l i z a t i o n i s v a r i a b l e but g e n e r a l l y decreases a b r u p t l y a shor t d i s t a n c e from the p l u t o n . On the other hand, the presence of rock fragments o f the Cherry Creek u n i t i n the N i c o l a v o l c a n i c rocks cut by i d e n t i c a l m a t e r i a l poses c o n f l i c t i n g age r e l a t i o n s h i p s between the i n t r u s i v e and e x t r u s i v e r o c k s . To e x p l a i n these and o ther i n t r i g u i n g f i e l d o b s e r v a t i o n s ( e . g . the resemblance of some f i n e g ra ined Sugar loaf rocks to some v o l c a n i c rocks of the N i c o l a Group) , Nor thcote (1976 and 1977)contended t h a t N i c o l a v o l c a n i c r o c k s and u n i t s of the I ron Mask b a t h o l i t h were comagmatic and emphasized the p a r t l y c o e v a l nature of N i c o l a v o l c a n i s m , emplacement of the younger phases of the m u l t i p l e i n t r u s i o n , 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 . Unconformably o v e r l y i n g the N i c o l a rocks and the I ron Mask p l u t o n i n the v i c i n i t y of A f t o n Mines a re T e r t i a r y sedimentary and v o l c a n i c rocks of the Kamloops Group. These i n c l u d e t u f f a c e o u s sandstone , s i l t s t o n e and s h a l e w i t h minor cong lomerate , as w e l l as b a s a l t i c to a n d e s i t i c f l ows and agglomerates w i t h minor d a c i t e , l a t i t e and t r a c h y t e b e i n g l o c a l l y abundant. A more d e t a i l e d d e s c r i p t i o n of them has been presented by Ewing (1979) . S t r u c t u r a l l y , major systems of n o r t h w e s t e r l y , n o r t h e r l y and n o r t h -e a s t e r l y t r e n d i n g f r a c t u r e s or f a u l t s c o n t r o l l e d the emplacement of the v a r i o u s u n i t s of the I ron Mask p l u t o n (Nor thcote , 1974, 1976 and 1977) . Renewed f a u l t movement i n the Paleocene or E a r l y Eocene around the margin of the p l u t o n l e d to the emergence of a h i g h - r e l i e f t e r r a i n of s t r u c t u r a l l y bounded p l a t e a u s and a d j o i n i n g graben s t r u c t u r e s where t h i c k sequences of Kamloops Group r o c k s accumulated (Carr and Reed, 1976 and P r e t o , 1972) . Deep weather ing a t the p l a t e a u margins promoted the supergene a l t e r a t i o n at A f t o n u n t i l the e l i m i n a t i o n of extreme r e l i e f by the g radua l f i l l i n g of the b a s i n s by M idd le Ecocene (Carr and Reed, 1976) . The Eocene s t r a t a a l s o p r o t e c t e d the orebody f rom the a b r a s i o n by P l e i s t o c e n e g l a c i a t i o n . 9 L o c a l Geology arid Petrography of the Af tbr i Open -p i t a) Genera l aspec ts F i g u r e s 3a and 3b ( a f t e r Car r and Reed, 1976) d e p i c t the l o c a l geology and a v e r t i c a l g e o l o g i c a l s e c t i o n of the A f t o n p r o p e r t y . As shown, the orebody l i e s complete ly w i t h i n the Cherry Creek u n i t of the I ron Mask p l u t o n and i s separated from the T e r t i a r y rocks to the n o r t h main l y by f a u l t s and l o c a l l y by a r a t h e r f l a t - l y i n g unconformi ty which pos tdates the supergene event (Carr and Reed, 1976, p . 3 8 0 ) . The Cherry Creek rocks i n t r u d e the N i c o l a Group rocks w i t h s t e e p l y sou thward -d ipp ing and p a r t l y sheared c o n t a c t . W i t h i n t h i s s i m p l i f i e d framework, l o c a l c o m p l i c a t i o n s a r i s e because o f the h i g h l y f r a c t u r e d and a l t e r e d nature of the hos t r o c k s , s i m i l a r i t y i n both t e x t u r e and compos i t ion of the i n t r u s i v e and e x t r u s i v e r o c k s , f requent g r a d a t i o n a l c o n t a c t s between v a r i o u s phases o f the i n t r u s i v e rocks and the i n c l u s i o n o f l a r g e s l a b s of one rock type i n a n o t h e r . Consequent ly , g e o l o g i c a l c o n t a c t s i n many cases are drawn r a t h e r s u b j e c t i v e l y . The amount of K - f e l d s p a r p resent has been u t i l i z e d by Nor thcote (1974, 1976 and 1 9 7 7 ) , C a r r and Reed (1976) and Pre to (1972) to d i f f e r e n t i a t e v a r i o u s phases o f the I r o n Mask p l u t o n and between the e x t r u s i v e and i n t r u s i v e rocks where b r e c c i a t i o n p r e c l u d e s easy i d e n t i f i c a t i o n of pr imary f e a t u r e s (G. W h i t e , p e r s o n a l communicat ion) . In v iew of the f i n e g r a i n s i z e of most rocks found i n the A f t o n p r o p e r t y , two a d d i t i o n a l c r i t e r i a are used i n the c l a s s i f i c a t i o n o f rock t y p e s . I n t r u s i v e rocks are d i s t i n g u i s h e d from the e x t r u s i v e ones by t h e i r h o l o c r y s t a l l i n e t e x t u r e and l a c k of fragments of d i f f e r e n t c o m p o s i t i o n or c o n t r a s t i n g t e x t u r e s . A p p l y i n g these r u l e s i n f i e l d mapping and c o r e -l o g g i n g , i t became ev ident that w i t h i n the c l o s e v i c i n i t y of the A f t o n orebody, c o n t i n u i t y of any s i n g l e rock u n i t over a reasonable d i s t a n c e to se rve as a marker i s s imply n o n - e x i s t e n t , a t l e a s t as f a r as the f i r s t 10 * * • • • sHughes Lake_ • • • KIDDLE EOCENE & (?) EARLIER I | V o l c a n i c & sedimentary rocks UPPER TRIASSIC Iron Mask P l u t o n : Cherry Creek d i o r i t e , monzonite sy e n i t e and b r e c c i a s [SSSN Sufcarloaf p o r p h y r i t i c d i o r i t e • • • P i c r i t e b a s a l t V///A Iron Mask, Pothook d i o r i t e & gabbro N i c o l a Group: V o l c a n i c (• minor sedimentary rocks METRES lOpO A f t o n orebody (>0.25%Cu)| P y r i t e zone TRANS CANADA HIGHWAY 500 'METRES l^ f r^ l SUPERGENE It 1 tl HYPOGENE I P P i PYRITE o l 500 J I i » METRES 1000 F i g u r e 3 . A g e o l o g i c a l map (a) and a g e o l o g i c a l s e c t i o n (b) of the A f t o n p roper t y ( a f t e r Carr and Reed, 1976) . 11 600 meters of the exp lo red orebody i s concerned. F i g u r e 4 , d e p i c t i n g a g e o l o g i c a l s e c t i o n ac ross the eas te rn p o r t i o n of the orebody, s u f f i c e s to i l l u s t r a t e the c o m p l e x i t i e s i n v o l v e d . Because s t r u c t u r a l d « f o r m a t i o n w i t h i n the T r i a s s i c r o c k s r a r e l y i n v o l v e d l a r g e , r e a d i l y r e c o g n i z a b l e d isp lacements (Carr and Reed, 1976) , the d i s c o n t i n u i t y of rock types must r e f l e c t an i n t r u s i v e r o o f - p e n d a n t envi ronment , where s u c c e s s i v e i n t r u s i v e events would cause b r e c c i a t i o n of the e a r l y s o l i d i f i e d u n i t s and l e a d to i n t r i c a t e i n c l u s i o n r e l a t i o n s h i p s . Under the c i r c u m s t a n c e s , a r i g o r o u s d e f i n i t i o n and assignment of rock types to v a r i o u s phases of the I r o n Mask p l u t o n would not be as f r u i t f u l as the mapping of a l t e r a t i o n m i n e r a l o g y , which i s more c l o s e l y r e l a t e d to the copper m i n e r a l i z a t i o n . T h e r e f o r e , the g e n e r a l i z e d g e o l o g i c a l map of the A f t o n o p e n - p i t as of June, 1979 (F igure 5) has been c o n s t r u c t e d and subsequent ly d i s c u s s e d w i t h respect to m i n e r a l d i s t r i b u t i o n s . S i m i l a r l y , the g e o l o g i c a l s e c t i o n s shown i n F igu res 3b and 4 w i l l be r e c o n s i d e r e d i n o rder to e l u c i d a t e the m i n e r a l d i s t r i b u t i o n p a t t e r n s i n r e l a t i o n to the copper m i n e r a l i z a t i o n , b) D e t a i l e d p i t petrography F i g u r e 5 i s a h i g h l y g e n e r a l i z e d g e o l o g i c a l map of the A f t o n o p e n - p i t at the 610-640 meter l e v e l s . I t was c o n s t r u c t e d based on f i e l d mapping c a r r i e d out by the author i n the summer of 1979, p e t r o g r a p h i c and x - r a y d i f f r a c t i o n s t u d i e s of samples c o l l e c t e d and s u p p l i e d by A . Reed (Chief Mine E n g i n e e r , A f t o n Mines L t d . ) and G. White ( D i s t r i c t G e o l o g i s t at Kamloops, B .C . M i n i s t r y of Energy, Mines and Pet ro leum Resources) d u r i n g v a r i o u s s tages of the s t r i p p i n g of the orebody, p r e l i m i n a r y company maps of the r e l e v a n t l e v e l s and d r i l l h o l e d a t a . R e a d i l y d i s c e r n a b l e rock u n i t s are d e s c r i b e d as f o l l o w s . Southwest of the o p e n - p i t , e x c a v a t i o n i n 1979 exposed a narrow s t r i p of p i c r i t e b a s a l t . I t i s made up of brownish g reen , s l i g h t l y e longated 12 F i g u r e 4 . A g e o l o g i c a l s e c t i o n a c r o s s the e a s t e r n h a l f of the A f t o n open -p i t , i l l u s t r a t i n g the complex i t y of the host rocks i n v o l v e d . The orebody a t a c u t o f f grade of 0.5% Cu i s d e p i c t e d i n broken l i n e s . ( S y m b o l s : D, d i o r i t e ; M, monzon i te ; S , s y e n i t e ; L, l a t i t e dyke; N, N i c o l a Group v o l c a n i c r o c k s ; T, T e r t i a r y v o l c a n i c and sedimentary r o c k s ; C, a n k e r i t e ; s o l i d b l a c k , m a g n e t i t e ) . Note the apparent l a c k of c o n t i n u i t y of any s i n g l e rack type over a g reat d i s t a n c e , e i t h e r v e r t i c a l l y or l a t e r a l l y . 13 Tertiary volcanic rocks Tertiary sedimentary rocks Intense carbonate alteration Latite dyke Cherry Creek syenite Cherry Creek diorite & monzonite Pi c r i t e basalt Nicola Group volcanic rocks -— Geological contact •v~ Fault Sample location Figure 5. A highly generalized geological map of the Afton open-pit at the 610-640 meter levels as of June, 1979. 14 nodules of d iameter up to h a l f a meter a l i g n e d i n a y e l l o w i s h , f r i a b l e m a t r i x which a l s o occurs i n cont inuous bands of v a r y i n g t h i c k n e s s s e p a r a t i n g the n o d u l e - r i c h l a y e r s . The nodules c o n s i s t s main ly of s e r p e n t i n i z e d o l i v i n e and r a t h e r f r e s h c l inopy roxene w i t h s m a l l but v a r y i n g amounts of p h l o g o p i t e . The y e l l o w m a t r i x i s s i m i l a r i n c o m p o s i t i o n but f i n e g r a i n e d and shows a l t e r a t i o n t o s e p i o l i t e . P o s s i b l y , the rock u n i t was formed by the i n t r u s i o n of an u l t r a m a f i c c r y s t a l mush to near s u r f a c e c o n d i t i o n s r e s u l t i n g i n the rough a l ignment of the nodules and f a s t quenching of the m a t r i x . Subsequent d i f f e r e n t i a l weather ing of the nodules and the m a t r i x gave r i s e to the l a y e r e d appearance of the whole assemblage. As a r e s u l t of the presence of an i n t e n s i v e carbonate a l t e r a t i o n and poor exposures , the con tac t between the p i c r i t e b a s a l t and the N i c o l a v o l c a n i c b r e c c i a to the immediate n o r t h i s obscure . The v o l c a n i c b r e c c i a i s made up almost e n t i r e l y of f i n e g ra ined h o r n b l e n d i t e f ragments . Ep idote v e i n l e t s , some of which c a r r y p y r i t e and c h a l c o p y r i t e , are q u i t e common. P r o g r e s s i n g towards the e a s t , the b r e c c i a becomes so d e f i c i e n t i n m a t r i x t h a t i t c o u l d e a s i l y be mis taken f o r h o r n b l e n d i t e except where l o c a l i z e d i n t e n s i v e carbonate replacement b r i n g s out the f ragmenta l t e x t u r e of the r o c k . With the g radua l i n c r e a s e i n g r a i n s i z e and d isappearance of the f ragmenta l s t r u c t u r e f u r t h e r e a s t , contac t w i t h the medium gra ined d i o r i t e i s a g a i n obscure . W i t h i n the o p e n - p i t i t s e l f , s i x rock u n i t s are r e a d i l y d i s t i n g u i s h e d . They are des ignated as Cherry Creek d i o r i t e and monzoni te , Cherry Creek s y e n i t e , N i c o l a v o l c a n i c r o c k s , l a t i t e dykes , T e r t i a r y sediments and T e r t i a r y v o l c a n i c r o c k s . The Cherry Creek i n t r u s i v e r o c k s , the most abundant rock type i n the o p e n - p i t , are t y p i c a l l y f i n e g r a i n e d , s l i g h t l y p o r p h y r i t i c i n t e x t u r e and range from s y e n i t e to d i o r i t e . i n c o m p o s i t i o n , w i t h d i o r i t e p r e d o m i n a t i n g . G r a i n s i z e i n c r e a s e s and the ex tent o f development o f p o r p h y r i t i c t e x t u r e 15 decreases southward and eas tward . Rocks w i t h f i f t y pe rcent or more by volume of g r a i n s s m a l l e r than 1 mm i n l a r g e s t dimension are d e s c r i b e d as the m i c r o -v a r i e t i e s . Even a f t e r b r e c c i a t i o n , these f i n e g ra ined i n t r u s i v e r o c k s may be d i s t i n g u i s h e d from the N i c o l a v o l c a n i c rocks on the b a s i s o f o v e r a l l u n i f o r m i t y of c o m p o s i t i o n , h o l o c r y s t a l l i n i t y and l a c k of ev idence of p r e - e x i s t i n g g l a s s . The Cherry Creek rocks c o n t a i n 50-70% f e l d s p a r s . P l a g i o c l a s e g r a i n s i n a l l v a r i e t i e s o f rock are comple te l y a l b i t i z e d . P o t a s s i c f e l d s p a r , t y p i c a l l y m i c r o c l i n e , occurs main ly as f i n e g r a i n e d m a t r i x , f r e q u e n t l y rimming p l a g i o c l a s e and r a r e l y occurs as p h e n o c r y s t s . In the d i o r i t i c and monzon i t i c v a r i e t i e s , K - f e l d s p a r f r e q u e n t l y occurs i n a patchy f a s h i o n i n c l u d i n g and g r a d i n g to aggregates of e p i d o t e or b i o t i t e . D i s c r e t e K - f e l d s p a r v e i n s commonly show a c h l o r i t i c e n v e l o p e . S e r i c i t e , carbonate and c l a y replacements are p resent i n f e l d s p a r s but are r a r e l y very i n t e n s e . C l i n o p y r o x e n e , up to 10% and most ly p a r t i a l l y a l t e r e d , i s found o n l y i n the e a s t e r n p o r t i o n o f the p i t w h i l e amphibole and i t s a l t e r a t i o n products are the p r i n c i p a l maf ic m i n e r a l s e lsewhere . E q u i g r a n u l a r , medium gra ined and more " n o r m a l " l o o k i n g i n t r u s i v e rocks are observed o n l y i n d r i l l cores recovered from moderate depth . N i c o l a v o l c a n i c rocks w i t h i n the A f t o n p i t and i t s v i c i n i t y are of s e v e r a l v a r i e t i e s . In the southern p o r t i o n of the o p e n - p i t , h a r d , massive green v o l c a n i c b r e c c i a w i t h predominant ly h o r n b l e n d i t e f ragments are p a r t i c u l a r l y abundant. A l t e r a t i o n to carbonate ( i n some p l a c e s ve ry severe) + c h l o r i t e replacement and e p i d o t e + carbonate + s u l f i d e s v e i n i n g are common. In the n o r t h e a s t e r n p o r t i o n , the N i c o l a v o l c a n i c rocks c o n s i s t o f h i g h l y s a u s s u r i t i z e d a n d e s i t e , which are h a r d l y d i s t i n g u i s h a b l e i n hand-spec imen from the n e i g h b o r i n g f i n e g r a i n e d Cherry Creek i n t r u s i v e r o c k s . Elsewhere and i n d r i l l c o r e s , the N i c o l a Group rocks range from f i n e g r a i n e d a n d e s i t e , 16 g e n e r a l l y darker i n c o l o r than most Cherry Creek r o c k s , to pebb ly sandstone made up e n t i r e l y of igneous c l a s t s cemented commonly by i r o n o x i d e s . L a t i t e dykes observed i n the A f t o n p i t are i n v a r i a b l y m a s s i v e , f i n e g ra ined and very s l i g h t l y p o r p h y r i t i c r o c k s . The t o t a l f e l d s p a r v a r i e s from 50 to 65% but i s g e n e r a l l y g r e a t e r than 60%. L i k e many Cher ry Creek r o c k s , K - f e l d s p a r i s more concent ra ted i n the m a t r i x . Amphibo le , o c c u r r i n g u s u a l l y i n excess of 20% and main ly as p h e n o c r y s t s , i s the predominant m a f i c m i n e r a l . Quartz amounts up to 10% and i s e s s e n t i a l l y i n t e r s t i t i a l . Hornblende can be f r e s h , r e p l a c e d by c h l o r i t e , carbonate and r a r e l y hydromica . P l a g i o c l a s e i s t y p i c a l l y a l b i t i z e d . Some r o c k s show s i g n s of o x i d a t i o n i n the form"of an o v e r a l l r e d d i s h tone and s e v e r a l samples c o n t a i n n a t i v e copper . They are most r e a d i l y d i f f e r e n t i a t e d from the Cherry Creek i n t r u s i v e r o c k s by the presence of quar t z and K - f e l d s p a r " e y e s " . Based on minera logy and a l t e r a t i o n , i t ! i s concluded tha t these l a t i t e s are p robab ly of p r e - T e r t i a r y age. Arranged i n order of d e c r e a s i n g abundance, sands tone , a r k o s e , conglomerate , l i t h i c wacke, carbonaceous a r g i l l i t e and s t r e a k s o f c o a l are among the T e r t i a r y sedimentary m a t e r i a l s observed i n the A f t o n p i t . In the c e n t r a l p a r t of the p i t ad jacent to the i n f e r r e d f a u l t c o n t a c t between the T e r t i a r y and Cherry Creek r o c k s , l i t h i c wackes w i t h heavy i r o n - s t a i n i n g and i r o n ox ide cement c o n t a i n c l a s t s c a r r y i n g n a t i v e copper . I t has not been p o s s i b l e to determine the exact age o f these l i t h i c wackes. T e r t i a r y v o l c a n i c rocks o c c u r r i n g i n the A f t o n p r o p e r t y i n c l u d e d a c i t e , t r a c h y t i c f l ow b r e c c i a , amygdalo ida l a n d e s i t e and massive a n d e s i t e . Whereas most p l a g i o c l a s e g r a i n s i n these rocks remain i n t a c t , m a f i c m i n e r a l s are t y p i c a l l y r e p l a c e d by a n k e r i t e . 17 M i n e r a l D i s t r i b u t i o n s arid A l t e r a t i o n P a t t e r n s a) I n t r o d u c t i o n P rev ious s t u d i e s on the g e r e r a l f e a t u r e s and the ore genes is of the A f t o n d e p o s i t d i d not p l a c e great emphasis on m i n e r a l d i s t r i b u t i o n s . Other than the w e l l - e s t a b l i s h e d p y r i t e and magnet i te zon ing about the orebody ( F i g u r e s 3a and 3b, a f t e r Car r and Reed, 1976) , v a r i o u s c l a s s i c a l hydrothermal a l t e r a t i o n assemblages were r e p o r t e d to be p resent but not d e l i n e a t e d . In the above p e t r o g r a p h i c d e s c r i p t i o n , i t has a l r e a d y been mentioned t h a t w i t h i n the Cherry Creek rocks i n the A f t o n o p e n - p i t , pyroxene i s l i m i t e d to the e a s t e r n p o r t i o n w h i l e amphibole i s more abundant to the s o u t h . P r e l i m i n a r y s t u d i e s (Kwong and Greenwood, 1980) a l s o i n d i c a t e d tha t as f a r as the carbonate a l t e r a t i o n s are concerned, c a l c i t e predominates, over a n k e r i t e i n the e a s t e r n p o r t i o n of the p i t and the reverse i s t r u e e l sewhere . Thus, i t appears t h a t bo th pr imary and a l t e r a t i o n m i n e r a l zonings are p resent even w i t h i n the orebody. In the present s t u d y , l a t e r a l m i n e r a l d i s t r i b u t i o n s were i n v e s t i g a t e d through x - r a y d i f f r a c t i o n ana l yses o f b l a s t h o l e samples a t the 613 meter l e v e l augmented by p e t r o g r a p h i c s t u d i e s of samples c o l l e c t e d from the 640 to 613 meter l e v e l s . P e t r o g r a g h i c s t u d i e s o f s e l e c t e d samples from two g e o l o g i c a l c r o s s - s e c t i o n s suppor ted by x - r a y d i f f r a c t i o n work f u r n i s h e d v e r t i c a l d i s t r i b u t i o n d a t a . b) L a t e r a l m i n e r a l d i s t r i b u t i o n s F i g u r e 6 i l l u s t r a t e s the l a t e r a l m i n e r a l d i s t r i b u t i o n s w i t h i n the A f t o n p i t a t the 613 meter l e v e l as r e v e a l e d by x - r a y d i f f r a c t o m e t r y work on b l a s t ho le composite samples s u p p l i e d by A f t o n Mines L t d . The p l a n of sampl ing i s i n c l u d e d i n the legend of the d iagram. W i t h i n each sampl ing b l o c k , b a r r e n T e r t i a r y rocks were d e l i b e r a t e l y exc luded to a l a r g e ex tent from the compos i te . L i m i t e d by i n t e r f e r e n c e p rob lems, q u a n t i t a t i v e es t imate 18 F i g u r e 6 . M i n e r a l d i s t r i b u t i o n s w i t h i n the A f t o n o p e n - p i t a t the 613 meter l e v e l as r e v e a l e d by x - r a y d i f f r a c t i o n a n a l y s e s of b l a s t h o l e composite samples. A legend f o r the boundary of the m i n e r a l domains d e p i c t e d i n the diagram and the sampl ing p l a n are as f o l l o w s : 1 KAOLINTTE ANKERTTE EPIDOTE HEMATITE L MONTMORILLONITE NATIVE COPPER 4 — AMPHI BOLE 1 PYRITE ..t K-FELDSPAR CLTNOPYROXENE + MAGNETITE Note tha t arrows p o i n t towards the occur rence of the m i n e r a l d e p i c t e d ; Sampling p l a n 13000N 50 ion The samples ana lyzed here a re prov ided through the c o u r t e s y of A f t o n Mines L t d . 20 of m i n e r a l abundance was not a t tempted . Boundaries of m i n e r a l domains are based on p o s i t i v e i d e n t i f i c a t i o n of the m i n e r a l from the d i f f r a c t o g r a m s o f n o n - o r i e n t e d mounts of the composite samples. For most m i n e r a l s , t h i s means an abundance of about 5% by weight except f o r c a l c i t e , q u a r t z , mica and k a o l i n i t e , the d e t e c t i o n l i m i t s of which are approx imate ly 2%. However, i t i s wor thwhi le to p o i n t out t h a t among the minera l s s p e c i f i e d i n F i g u r e 6 , o n l y a l b i t e , q u a r t z , a n k e r i t e and r a r e l y K - f e l d s p a r and amphibole can be cons idered as major components, i . e . capable of e x i s t i n g i n excess of 20%. C h l o r i t e , e p i d o t e and c a l c i t e commonly assume a subord ina te r o l e , o c c u r r i n g i n 10-20% whereas m i c a , n a t i v e copper , the o x i d e s , s u l f i d e s and p a r t i c u l a r l y the c l a y m i n e r a l s are f r e q u e n t l y o c c u r r i n g j u s t i n s u f f i c i e n t amounts to be d i s c e r n a b l e from the d i f f r a c t o g r a m s . Amorphous m a t e r i a l s , i f p r e s e n t , defy easy q u a n t i f i c a t i o n by x - r a y d i f f r a c t o m e t r y . One prominent f e a t u r e of the m i n e r a l d i s t r i b u t i o n s i l l u s t r a t e d i n F i g u r e 6 i s the overwhelming abundance of n o r t h w e s t e r l y t r e n d s a n d , to a l e s s e x t e n t , n o r t h e a s t e r l y t rends of the boundar ies of the m i n e r a l domains. These t rends are p a r a l l e l to those of the major f a u l t s throughout the I ron Mask p l u t o n , which c o n t r o l l e d the emplacement of v a r i o u s i n t r u s i v e phases of the p l u t o n . The n o r t h w e s t e r l y t r e n d , i n p a r t i c u l a r , i s p a r a l l e l to the i n t r u s i v e c o n t a c t of the I ron Mask p l u t o n w i t h the N i c o l a Group r o c k s . A comparison w i t h F i g u r e 5 r e v e a l s the co inc idence of the m o n t m o r i l l o n i t e boundary w i t h t h a t of the T e r t i a r y sediments and t h a t k a o l i n i t e i s abundant i n an area dominated by T e r t i a r y v o l c a n i c r o c k s . Consequent l y , the presence of these c l a y s may be a r e s u l t of contaminat ion from the T e r t i a r y r o c k s . The narrow n o r t h w e s t e r l y t r e n d i n g s t r i p s of k a o l i n i t e observed i n the southwestern quadrant of the p i t may be caused by near s u r f a c e a l t e r a t i o n s i n r e c u r r i n g f a u l t s d u r i n g the T e r t i a r y p e r i o d . 21 I g n o r i n g the c l a y minera l s and n a t i v e copper and a l l o w i n g f o r minor e x c e p t i o n s , i t i s a l s o ev ident i n F i g u r e 6 tha t the o p e n - p i t can be d i v i d e d d i a g o n a l l y i n t o two p a r t s w i t h d i f f e r e n t m i n e r a l assemblages. K - f e l d s p a r , e p i d o t e , h e m a t i t e , magneti te and, to a l e s s e r e x t e n t , c l i nopy roxene dominate the n o r t h e a s t e r n h a l f whereas the southwestern h a l f i s c h a r a c t e r i z e d by a n k e r i t e and/or amphibole and p y r i t e . O b v i o u s l y , a great p r o p o r t i o n of the hemat i te i s d e r i v e d from a l t e r a t i o n of m a g n e t i t e , and e p i d o t e and at l e a s t some of the K - f e l d s p a r and magnet i te are e i t h e r d e u t e r i c or hydrothermal i n o r i g i n . However, i t i s not c l e a r whether the occurrence of c l i nopy roxene and amphibole i n d i f f e r e n t domains r e f l e c t s an obscure g e o l o g i c a l c o n t a c t o r i s s i m p l y a r e s u l t of i n - s i t u d i f f e r e n t i a t i o n . Among the u b i q u i t o u s m i n e r a l s , q u a r t z i s more abundant i n the southwest w h i l e c a l c i t e i s more c o n c e n t r a t e d i n the n o r t h e a s t . A l b i t e , c h l o r i t e and mica are r a t h e r even ly d i s t r i b u t e d except where carbonate a l t e r a t i o n i s ex t remely i n t e n s e . O c c u r r i n g i n r a t h e r s m a l l amounts, the micas cannot be d i s t i n g u i s h e d w i t h r e s p e c t to k i n d and type based on ev idence ob ta ined from b u l k sample d i f f r a c t o g r a m s . In a d d i t i o n , as a r e s u l t of i n t e r f e r e n c e of r e f l e c t i o n peaks and the g e n e r a l l y d i f f u s e nature of the d i f f r a c t i o n p a t t e r n s of the copper s u l f i d e s , these m i n e r a l s are not d e s c r i b e d i n F i g u r e 6 . These l i m i t a t i o n s , however, can r e a d i l y be e l i m i n a t e d by d e t a i l s of t h i n - s e c t i o n s t u d i e s of specimens c o l l e c t e d from the same g e n e r a l a r e a . F i g u r e s 7a to 7d d e p i c t the m i n e r a l d i s t r i b u t i o n s as determined f rom p e t r o g r a p h i c examinat ions of 173 samples secured from the 640-613 meter l e v e l s . D e s p i t e minor sporad ic e x c e p t i o n s , these r e a f f i r m the g e n e r a l abundance o f c l i n o p y r o x e n e , ep ido te and K - f e l d s p a r i n the n o r t h e a s t e r n h a l f and hornblende and a n k e r i t e i n the southwestern quadrant of the p i t . Hornblende pseudomorphs made up of c h l o r i t e , a m i c a - l i k e m i n e r a l O B i o t i t e A K - f e l d s p a r patches & v e i n l e t s x S e r i c i t e / m u s c o v i t e F i g u r e 7 a . D i s t r i b u t i o n of s e l e c t e d m i n e r a l s as determined from p e t r o g r a p h i c examinat ion of 173 specimens from the 613-640 meter l e v e l s of the A f t o n o p e n - p i t . w o o o cr. 13000N LEGEND O P e r v a s i v e A n k e r i t e A l t e r a t i o n • P e r v a s i v e C a l c i t e A l t e r a t i o n © A n k e r i t e + C a l c i t e A Hornblende X C l inopy roxene F i g u r e 7b. D i s t r i b u t i o n of s e l e c t e d m i n e r a l s i n the A f t o n p i t a t 613-640 meter l e v e l s . M i n e r a l have been i d e n t i f i e d through p e t r o g r a h i c examinat ion of 173 specimens. 8000E 8500E X w o o c cy> X X A O O O X A X X XX X X X 13000N A X X X X O A A X O A A I A O X X 9 x > X X o O A O 12500N X O o X ® 0 LEGEND H y d r o b i o t i t e A K a o l i n i t e X E p i d o t e F i g u r e 7c . D i s t r i b u t i o n of s e l e c t e d m i n e r a l s as determined by p e t r o g r a p h i c examinat ion of 173 samples from the 613-640 meter l e v e l s of the A f t o n o p e n - p i t . 25 ng.to.pr ap,sp,pu ap.sp.pu A ap • dg A "S.P* >ap. 'ap.mg img qt.dg • mg.ku, •qt,ku,ap,sp ap.ng py py.™g py.ap i p y •dg • mg • mg.ap ap.sp ap py . s i [py " p y to.py I p y . r a i . s f l • M p y . p r p y . t o . s i ap ap ap rj A A ap.mg ! * n g ' a p 13000N A Aug ap.mg ap pr py i py.to py.mo.ap py.ajp >ne,ap,pu 12500N mg LECEND • C h a l c o p y r i t e + b o r n i t e + o t h e r s as s p e c i f i e d A N a t i v e copper + c h a l c o c i t e + o t h e r s as s p e c i f i e d • Other m i n e r a l s as s p e c i f i e d A b b r e v i a t i o n s : ap=apatite, d g = d i g e n i t e , ku=kutnohorite, mg=magnetite, m i = m i l l e r l t e mo-=raolybdenite, p r = p r e h n i t e , p u = p u n p e l l y i t e , p y = p y r i t e , qt=quartz, s c = s c a p o l i t e s i = s i d e r i t e , sp=sphene, to=tourmaline Figure 7d. Distribution of ore and accessory minerals in the 613-640 meter levels of the Afton open-pit. Petrographic examination aided by x-ray diffractometry has been used i n the identification of the v specified minerals. 26 ( h y d r o b i o t i t e ? ) and carbonates are a l s o abundant i n the nor thweste rn quadrant . In F i g u r e 7 c , i t i s ev ident t h a t b i o t i t e i s a l s o concent ra ted i n the n o r t h e a s t e r n h a l f whereas s e r i c i t e / m u s c o v i t e replacement i s more abundant i n the southwestern h a l f of the p i t . In a s i m i l a r f a s h i o n , F i g u r e 7d shows t h a t supergene copper m i n e r a l s ( n a t i v e copper ± c h a l c o c i t e ) are more c h a r a c t e r i s t i c of the n o r t h e a s t e r n h a l f whereas hypogene s u l f i d e s ( b o r n i t e + c h a l c o p y r i t e ± p y r i t e ± minor molybdeni te ) are more abundant i n the southwestern h a l f . The s p a r s e l y sampled r e g i o n between them i s c h a r a c t e r i z e d by h i g h grade supergene o r e . C h l o r i t e a s s o c i a t e d w i t h the supergene assemblage i s o f the F e - r i c h v a r i e t y , o c c u r r i n g as y e l l o w i s h , f i n e g r a i n e d aggregates and e x h i b i t i n g normal i n t e r f e r e n c e c o l o r s . C h l o r i t e a s s o c i a t e d w i t h the hypogene assemblage, i n c o n t r a s t , i s t y p i c a l l y coarse g r a i n e d Mg-Fe c h l o r i t e commonly showing anomalous brown i n t e r f e r e n c e c o l o r . Regard ing the accesso ry m i n e r a l s , a p a t i t e i s more or l e s s u b i q u i t o u s ; tou rmal ine i s found ad jacent to i n t e n s e carbonate a l t e r a t i o n s o n l y ; sphene, aga in i s more commonly observed i n rocks of the n o r t h e a s t e r n p o r t i o n of the o p e n - p i t . The d i s t r i b u t i o n of m i n e r a l s g e n e r a l l y c h a r a c t e r i s t i c of low grade metamorphism, i . e . p u m p e l l y i t e and p r e h n i t e , i s r a t h e r s p o r a d i c but i s c l e a r l y a n t i p a t h e t i c to i n t e n s e carbonate a l t e r a t i o n s . Not i n c l u d e d i n F i g u r e s 6 and 7a to 7d are minor to moderate amounts o f dominant ly amorphous m a t e r i a l and t r a c e amounts of gypsum commonly a s s o c i a t e d w i t h supergene copper o r e s . The amorphous m a t e r i a l i n c l u d e s r e d d i s h , ear thy h e m a t i t i c c l a y , g r e e n i s h and p o o r l y c r y s t a l l i n e mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e c l a y s and r a r e l y t r a c e amounts of suspected amorphous copper s u l f a t e s . The gypsum, copper s u l f a t e s (?) and some l a t e f r a c t u r e -f i l l i n g c a l c i t e are p robab ly p roducts of recent w e a t h e r i n g . They occur i n too s m a l l an amount to be of any p e t r o l o g i c s i g n i f i c a n c e . 27 c) V e r t i c a l d i s t r i b u t i o n o f m i n e r a l s F i g u r e s 8 and 9 i l l u s t r a t e the major p a t t e r n s 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 i n the v e r t i c a l d imens ion . F igu re 8 , r e s u l t i n g m a i n l y from d e t a i l e d c o r e - l o g g i n g a i d e d by x - r a y d i f f r a c t o m e t r y , d e p i c t s the presumably h i g h temperature hydrothermal a l t e r a t i o n s i n r e l a t i o n to the orebody (at a c u t - o f f grade of 0.5% Cu) and host rock l i t h o l o g i e s i n the e a s t e r n h a l f of the p i t . Other than the u b i q u i t o u s occurrence of c h l o r i t e and the apparent development of a p y r i t e zone about the orebody both near the s u r f a c e and a t d e p t h , occurrences of abundant K - f e l d s p a r , b i o t i t e , e p i d o t e and carbonates appear to be s p o r a d i c . In d e t a i l , however, s e v e r a l f e a t u r e s are wor th n o t i n g . F i r s t l y , e p i d o t e , f r e q u e n t l y ex tend ing w e l l beyond the orebody, shows a p o s i t i v e c o r r e l a t i o n w i t h the occurrence of m a g n e t i t e . Wherever d i s s e m i n a t e d magnet i te i s abundant, o r i n the v i c i n i t y of l a r g e magnet i te v e i n s ( i n excess o f 30 cent imeters i n w i d t h ) , e p i d o t i z a t i o n i s p e r v a s i v e . To the eas t of the r e f e r e n c e l i n e " H " i n the d iagram, e p i d o t e i s a c t u a l l y c l o s e l y a s s o c i a t e d w i t h magnet i te v e i n l e t s too s m a l l to be shown. Second ly , the boundary of b i o t i t e appearance c l o s e l y approximates tha t of the hypogene ore zones . As l a r g e s t r u c t u r a l d isp lacements are not r e c o g n i z e d , the s e p a r a t i o n of these hypogene zones probably r e s u l t s from f r a c t u r e - c o n t r o l l e d supergene a l t e r a t i o n . T h i r d l y , whereas most r o c k s t h a t have undergone i n t e n s e carbonate a l t e r a t i o n are b a r r e n , a few c a r r y a s m a l l amount of t e n n a n t i t e , c h a l c o p y r i t e and p y r i t e . In f a c t , t e n n a n t i t e appears to be l i m i t e d to rocks r e p l a c e d by carbonate and t h e i r v i c i n i t i e s . F o u r t h l y , though d e s c r i b e d as u b i q u i t o u s , c h l o r i t e i s n e a r l y absent i n most carbonate a f f e c t e d rocks and i t occurs i n a much l e s s conspicuous amount i n l a t i t e s . L a s t l y , not d e p i c t e d i n the d iagram, are ve ry s m a l l amounts of c l i n o p t i l o l i t e and f e r r i e r i t e o c c u r r i n g as f r a c t u r e f i l l i n g s p a r t i c u l a r l y obv ious i n d r i l l h o l e #73-23 a s s o c i a t e d w i t h the more 28 S e c t i o n 11 30 I metres LEGEND f a u l t s - - boundary of orebody a t 0.5% Cu c u t o f f hypogene ore l i m i t of p y r i t e occur rence : b i o t i t e • e p i d o t e x** a n k e r i t e + c a l c i t e o o o o patchy and v e i n K--f e l d s p a r Note tha t c h l o r i t e i s u b i q u i t o u s . F i g u r e 8 . Hydrothermal a l t e r a t i o n observed i n a g e o l o g i c a l s e c t i o n (#11) a c r o s s the e a s t e r n p o r t i o n of the orebody. ( L i t h o l o g i e s : D, d i o r i t e ; M, monzon i te , S , s y e n i t e ; L, l a t i t e ; N, N i c o l a Group v o l c a n i c r o c k s ; T, T e r t i a r y v o l c a n i c and sedimentary r o c k s ; C, a n k e r i t e ; s o l i d b l a c k , m a g n e t i t e . ) 29 Figure 9. An i l l u s t r a t i o n of ve r t i c a l distribution of minerals at Afton Mines: Distribution of high temperature alteration minerals i n a geological section (#18) across the western portion of the orebody. Note that only minerals occurring in excess of about 5% by volume are depicted in the diagram. LEGEND Symbols: ovb Lithology: • a 0 o a * » «| 11 0 0 0 «*0 6 | Mineralogy: C C E sample location overburden d r i l l hole # x Tertiary volcanic and sedimentary rocks equigranular diorite diorite porphyry, microdiorite, micromonzonite and microsyenite Nicola Group volcanic rocks inferred contact between Nicola volcanic rocks and the Iron Mask intrusive rocks ankerite biotite epidote limit of orebody at a cutoff grade of 0.5% Cu limit of hypogene ore Note that chlorite i s ubiquitous and pyrite i s present above the upper boundary of the orebody. These minerals are not plotted i n the diagram for cl a r i t y . 30 31 K-feldspar r i c h sections; some celadonite has been observed i n d r i l l hole #73-39, again associated with the K-feldspar r i c h sections; l a t i t e dykes i n d r i l l hole #73-43, on the other hand, contain marcasite l i n i n g micro-fr a c t u r e s . These, together with the hematite and amorphous clay a l t e r a t i o n p a r t i c u l a r l y abundant east of the d r i l l hole #73-9, are int e r p r e t e d to be the r e s u l t of low temperature hydrothermal events or supergene a l t e r a t i o n . The respective a s s o c i a t i o n of a l t e r a t i o n minerals and s p e c i f i c wall rocks appears to i n d i c a t e a b u f f e r i n g of the re a c t i n g f l u i d s by the host rocks. Figure 9 i s a section across the western h a l f of the p i t . It i s modified from Figure 3b ( a f t e r Carr and Reed, 1976). D i s t r i b u t i o n of a l t e r a t i o n mineralogy i s derived mainly from petrographic studies of t h i n -sections and rock slabs supplied by Afton Mines L t d . Sporadic d i s t r i b u t i o n of a l t e r a t i o n mineral domains i s again obvious. In comparison with that noted i n the eastern>tsection, however, hydrothermal b i o t i t e , while confined within the orebody, extends w e l l into the supergene zone. The development of carbonate a l t e r a t i o n i s more extensive, both near the surface and at depth, thus p a r t i a l l y surrounds the orebody. The f a c t that b i o t i t e survived supergene a l t e r a t i o n under the apparent "blanketing" e f f e c t of the carbonate suggests that the presence of abundant carbonate might have affected the s o l u t i o n chemistry during the supergene event, enhancing the perseverence of b i o t i t e . The equigranular d i o r i t e beneath the orebody and west of the reference l i n e "G" also contains abundant b i o t i t e . However, i n t h i s case, the mica, considered to be primary, displays a well-developed p o i k i l i t i c texture (Plate I) i n contrast to the c l u s t e r s of f i n e grained hydrothermal b i o t i t e , which are obviously f r a c t u r e - c o n t r o l l e d (Plate I I ) . Epidote i s again not l i m i t e d within the orebody and c h l o r i t e , while s t i l l being more or l e s s ubiquitous, occurs i n greatly diminished amounts. F i n a l l y , p y r i t e , 32 PLATE I. Primary, p o i k i l i t i c biotite incompletely enclosing plagioclase (whitish) in medium grained diorite (sample 963LL). Length of photograph (taken under open nicols) i s 3.5 mm. PLATE II. Clusters of fine grained hydrothermal biotite (brown and colorless) commonly associated with ore i n microdiorite breccia (sample GWA 127). Length of photograph (taken with open nicols) i s 3.5 mm. 33 wh-i le more concent ra ted i n rocks above the orebody,. a lso , commonly occurs i n s m a l l amounts i n h i g h l y c a r b o n a t i z e d r o c k s , d) D e t a i l s o f the copper m i n e r a l i z a t i o n As a p r e r e q u i s i t e f o r i n t e r p r e t i n g the o r e - w a l l rock r e l a t i o n s h i p s , the m i n e r a l d i s t r i b u t i o n s d i s c u s s e d above must be c o - o r d i n a t e d w i t h the mode of occurrence o f the ore m i n e r a l s . At A f t o n , the supergene m i n e r a l i z a t i o n dominates to a depth of approx imate ly 400 meters and 250 meters i n the western and e a s t e r n p o r t i o n s of the orebody r e s p e c t i v e l y . I t i s c h a r a c t e r i z e d by a n a t i v e copper to c h a l c o c i t e r a t i o i n excess o f two to one. The c h a l c o c i t e i n t h i s zone i s most ly of the sooty v a r i e t y and occurs main ly i n v e i n s l e s s than 3 mm wide . N a t i v e copper occurs i n stockworks i n the form of s c a l e s , f i l m s and d e n d r i t e s as w e l l as g ranu les a s s o c i a t e d w i t h s p e c u l a r i t e . In the eas t end of the p i t , i s o l a t e d specks o f n a t i v e copper may be observed i n magnet i te , which occurs as v e i n s , patches and r a r e l y as d i s s e m i n a t i o n s . Whereas n a t i v e copper and c h a l c o c i t e commonly occur i n the same h o r i z o n , they r a r e l y occupy the same v e i n . Thus replacement f e a t u r e s between the two are f a r l e s s common than c r o s s - c u t t i n g r e l a t i o n s h i p s w i t h v e i n l e t s of n a t i v e copper always b e i n g the p a r a g e n e t i c a l l y l a t e r s p e c i e s . Th in c o a t i n g s of c u p r i t e are found on many n a t i v e copper c r y s t a l s and m a l a c h i t e , a z u r i t e , c o n i c h a l c i t e and some p o o r l y c r y s t a l l i n e copper s u l f a t e hydrates (?) a l s o show l i m i t e d o c c u r r e n c e s . O c c u r r i n g g e n e r a l l y i n t r a c e amounts, these m i n e r a l s , l i k e minor amounts o f gypsum and l a t e c a l c i t e v e i n l e t s , are p r o b a b l y p roducts of very recent a l t e r a t i o n p r o c e s s . In the hypogene zone, b o r n i t e and c h a l c o p y r i t e are e q u a l l y important w h i l e grey c h a l c o c i t e assumes a more subord inate r o l e . B o r n i t e e n c l o s i n g c h a l c o p y r i t e and more se ldomly v i c e v e r s a , are commonly observed i n c a r b o n a t e -34 f r e e v e i n s c u t t i n g h i g h l y c h l o r i t i z e d rocks or as s u l f i d e patches l o c a t e d w i t h i n c h l o r i t i z e d maf i c phenocrysts themselves . In e i t h e r c a s e , b o r n i t e commonly e x h i b i t s e x s o l u t i o n r ims o f c h a l c o c i t e and c o v e l l i t e . B o r n i t e and grey c h a l c o c i t e , e i t h e r together or s e p a r a t e l y , a l s o occur as d i s s e m i n a t i o n s i n f e l d s p a r - r i c h monzon i t i c rocks w i t h accessory b i o t i t e . D e t a i l e d s t u d i e s of p o l i s h e d s e c t i o n s i n d i c a t e , however, tha t these s u l f i d e s do not occur as i n d i v i d u a l g l o b u l e s but as impregnat ions connected by m i c r o f r a c t u r e s . In a d d i t i o n to i t s a s s o c i a t i o n w i t h b o r n i t e , c h a l c o p y r i t e a l s o occurs w i t h p y r i t e i n carbonate v e i n s p a r t i c u l a r l y abundant i n e p i d o t e - r i c h r o c k s . In the southern r i m of the o p e n - p i t at an e l e v a t i o n of 610 mete rs , i s o l a t e d occur rences of c h a l c o p y r i t e and molybdenite and c h a l c o p y r i t e and hemat i te v e i n s about one cent imete r wide have been n o t e d . These v e i n s commonly show a K - f e l d s p a r enve lope up to two c e n t i m e t e r s wide on "each s i d e . Very r a r e l y , monomineral ic c h a l c o p y r i t e v e i n s of up to two c e n t i m e t e r s wide are a l s o observed e x h i b i t i n g a prominent a l b i t e enve lope . In r o c k s i n t e n s i v e l y r e p l a c e d by c a r b o n a t e , c h a l c o p y r i t e i s o c c a s i o n a l l y seen accompanying t e n n a n t i t e and/or p y r i t e . Such an assemblage a l s o p e r s i s t s l o c a l l y i n the supergene zone. Whereas g o l d and s i l v e r are s i g n i f i c a n t byproducts of the copper o r e , t h e i r mode o f occurrence i s s t i l l open t o s p e c u l a t i o n . In the e a s t e r n p o r t i o n of the orebody, o n l y one example of n a t i v e go ld c o a t i n g d i s s e m i n a t e d b o r n i t e , which i s i n c o m p l e t e l y a l t e r e d to c h a l c o c i t e , has been observed . The t e x t u r a l r e l a t i o n s h i p i n v o l v e d i s i l l u s t r a t e d i n P l a t e I I I . P r o b a b l y , n a t i v e go ld occurs as m i c r o - d i s p e r s i o n s i n the copper s u l f i d e phases . The d i s p e r s e d go ld p a r t i c l e s aggregate and become v i s i b l e o n l y a f t e r p a r t i a l d e s t r u c t i o n of the host m i n e r a l s . Th is may e x p l a i n the c o n c e n t r a t i o n of go ld w i t h i n the PLATE I I I . N a t i v e go ld (white) c o a t i n g c h a l c o c i t e ( l i g h t g rey ) which i n c o m p l e t e l y r e p l a c e s b o r n i t e (dark grey) (sample 7 3 - 2 3 - 1 0 1 3 . 8 ) . Length of p o l i s h e d s e c t i o n i n photograph i s 1.0 mm. 36 copper orebody as shown i n F i g u r e 10 but the h i g h e s t ore grades f o r the two elements may not n e c e s s a r i l y c o i n c i d e (Reed, 1980a) . T e n n a n t i t e , commonly observed i n rocks a f f e c t e d by carbonate and t h e i r c l o s e v i c i n i t i e s , i s a l s o a l i k e l y host f o r s i l v e r and some g o l d . In f a c t as f i r s t suggested by H o i l e s (1978) , the q u a n t i t a t i v e l y i n s i g n i f i c a n t s u l f o s a l t s a s s o c i a t e d w i t h the copper s u l f i d e s may a l s o p l a y a s i g n i f i c a n t r o l e i n h o s t i n g go ld and s i l v e r con ta ined i n the o r e . F i g u r e 10. D i s t r i b u t i o n of copper and gold at v a r i o u s benches i n the A f t o n p i t . Note the c o n c e n t r a t i o n of go ld roughly w i t h i n the l i m i t of the copper orebody at the co r respond ing benches. 38 D i s c u s s i o n and I n t e r p r e t a t i o n In the l i t h o l o g i c d e s c r i p t i o n o f the host rocks p resented above, i t has a l r e a d y been argued that the p r e v a l e n t environment i s t h a t of a roo f pendant c h a r a c t e r i z e d by i n t r i c a t e and s p o r a d i c d i s t r i b u t i o n s o f rock types whereas d e t a i l e d d i s t i n c t i o n o f rock types p r o v i d e s l i t t l e h e l p i n f i n d i n g o r e , d e t a i l s of d i s t r i b u t i o n of a l t e r a t i o n m i n e r a l s should r e v e a l impor tant a s p e c t s of the copper m i n e r a l i z a t i o n . In the f o l l o w i n g , the s i g n i f i c a n c e of a few prominent d i s t r i b u t i o n a l f e a t u r e s i s suggested , a) Absence of abundant s e r i c i t e and c l a y m i n e r a l s . In c o n t r a s t to the a l t e r a t i o n minera logy i n the L o w e l l and G i l b e r t (1970) model f o r c a l c - a l k a l i n e porphyry copper d e p o s i t s , s t r o n g p h y l l i c and a r g i l l i c a l t e r a t i o n s are absent at A f t o n , which i s hosted by a l k a l i n e p l u t o n i c r o c k s . In the d i o r i t e model of H o l l i s t e r (1978) f o r a l k a l i n e porphyry coppers , the d i f f e r e n c e s i n a l t e r a t i o n p a t t e r n s are e x p l a i n e d i n terms of the d i s p a r i t y i n the chemica l compos i t ion of the host r o c k s . From e x t e n s i v e s t u d i e s of the oxygen and hydrogen i s o t o p i c compos i t ion of m i n e r a l s f rom porphyry d e p o s i t s throughout North A m e r i c a , Sheppard e t a l . (1969, 1 9 7 0 ) , however, conc luded tha t secondary b i o t i t e from the cores o f s e v e r a l d e p o s i t s formed i n e q u i l i b r i u m w i t h dominant ly magmatic w a t e r , whereas s e r i c i t e and - hypogene c l a y m i n e r a l s g e n e r a l l y found p e r i p h e r a l to the core of p o t a s s i c a l t e r a t i o n formed i n e q u i l i b r i u m w i t h a l a r g e component of meteor i c w a t e r . At A f t o n , the g e n e r a l l y q u a r t z - f r e e (when u n a l t e r e d ) nature and the more m a f i c c o m p o s i t i o n of the host p l u t o n i n comparison to most o ther porphyry copper d e p o s i t s ought to r e s u l t i n a h i g h e r temperature o f s o l i d i f i c a t i o n and a lower v i s c o s i t y of the i n t r u s i v e magma. A low v i s c o s i t y of magma coupled w i t h i n t r u s i o n a long steep f r a c t u r e zones would g i ve r i s e to a 39 h i g h e r l i k e l i h o o d of fo rmat ion o f s h e e t - l i k e b o d i e s , the d e t a i l e d shape o f which would of course be dependent u l t i m a t e l y on the morphology of the c o n d u i t f r a c t u r e s . In any c a s e , more r a p i d r a t e of c o o l i n g l e a d i n g to l e s s t ime and o p p o r t u n i t y f o r the i n t e r v e n t i o n o f m e t e o r i c water i s to be e x p e c t e d . In s h o r t , though u l t i m a t e l y r e l a t e d to the chemica l compos i t ion of the hos t p l u t o n , the immediate cause of the l a c k o f p h y l l i c and a r g i l l i c a l t e r a t i o n s a t A f t o n a t l e a s t i s p robably due to the f a i l u r e of abundant meteor ic water to p a r t i c i p a t e i n the hypogene hydrothermal a l t e r a t i o n event , b) P r e f e r e n t i a l a s s o c i a t i o n of m i n e r a l s In the hypogene zone, the predominance of c h l o r i t e where magnet i te i s not abundant and the p r e f e r e n t i a l c o r r e l a t i o n of ep ido te w i t h magnet i te suggest t h a t d u r i n g the hydrothermal a l t e r a t i o n , hos t rock mineralogy was ab le to c o n t r o l the fo rmat ion of secondary m i n e r a l s . M a g n e t i t e , i n the p resent c a s e , p robab ly l i m i t e d the oxygen f u g a c i t y so t h a t the presence of abundant magnet i te s t a b i l i z e d e p i d o t e i n p r e f e r e n c e to c h l o r i t e ± p l a g i o c l a s e . In o ther words , a low r a t i o of f l u i d to rock i s i n f e r r e d to have accompanied the hypogene a l t e r a t i o n . The f requent patchy occur rence of K - f e l d s p a r , most l y r imming p l a g i o c l a s e r a t h e r than as d i s c r e t e v e i n s c o u l d a l s o be taken as an i n d i c a t i o n of v a n i s h i n g magmatic a c t i v i t y w i t h a min imal amount of s i l i c a t e -f r e e f l u i d b e i n g a v a i l a b l e . The e n v e l o p i n g of c h a l c o p y r i t e v e i n s w i t h a l b i t e i s a l s o not at odds w i t h such an i n t e r p r e t a t i o n . However i n s i g n i f i c a n t i t might be i n comparison w i t h the t o t a l amount of rock mass, the hydrothermal f l u i d was presumably ab le to l e a c h a l l p r imary s u l f i d e s (and p o s s i b l y some p r imary magnet i te ) and r e d e p o s i t e d them i n the form of v e i n s and f r a c t u r e -c o n t r o l l e d impregnat ions at a temperature of about 300°C as i n d i c a t e d by the s u l f u r i s o t o p e data of H o i l e s (1978) . By analogy w i t h the study of B r i m h a l l (1980) on the deep hypogene o x i d a t i o n o f the porphyry copper po tass ium s i l i c a t e 40 p r o t o r e at B u t t e , Montana, i t can be i n f e r r e d t h a t such a low f l u i d t o rock r a t i o accompanying the hypogene m i n e r a l i z a t i o n at A f t o n would r e s u l t i n a m i n i m a l d isp lacement of the ore elements from the p r o t o r e zone. Under such a c i r c u m s t a n c e , an unders tand ing of the magmatic p rocesses i n v o l v e d i n the f o r m a t i o n of the v a r i o u s phases of the I ron Mask b a t h o l i t h , i n o r d e r to r e c o g n i z e the exact stage of pr imary s u l f i d e p r e c i p i t a t i o n , would h e l p i n f u r t h e r e v a l u a t i o n of the e x p l o r a t i o n p o t e n t i a l of the b a t h o l i t h . Such an e x p o s i t i o n , a ided by chemica l d a t a , i s the main theme of the t h i r d p a r t o f t h i s s t u d y . ^ The supergene a l t e r a t i o n , i n a s i m i l a r f a s h i o n , appears t o be c o n t r o l l e d by the compos i t ion of the w a l l r o c k s . The c o n c e n t r a t i o n of n a t i v e copper ± c h a l c o c i t e i n the e a s t e r n h a l f of the o p e n - p i t a t the 6 1 3 -640 meter l e v e l s appears to r e s u l t from the presence of abundant magnet i te which p r o v i d e s f o r o x i d a t i o n to hemat i te w i t h the coupled r e d u c t i o n of hypogene b o r n i t e and c h a l c o p y r i t e to n a t i v e copper and c h a l c o c i t e . The f requent p e r s i s t e n c e of hypogene s u l f i d e assemblages i n the weste rn h a l f of the p i t seems to be r e l a t e d to the b u f f e r i n g e f f e c t of the carbonates p r e s e n t . The predominant occurrence of supergene m i n e r a l s i n v e i n s and s tockworks w i t h r e l a t i v e l y few replacement f e a t u r e s suggests a c t u a l d i s s o l u t i o n o f the hypogene s u l f i d e s and r e d e p o s i t i o n o f the ore elements as supergene m i n e r a l s . The supergene event shou ld t h e r e f o r e be r e a d i l y s t u d i e d and s i m u l a t e d i n terms of m i n e r a l - s o l u t i o n i n t e r a c t i o n s making use o f a v a i l a b l e thermochemical d a t a . Such a thermodynamic r e c o n s t r u c t i o n of the supergene event i s p resented i n the second p a r t of t h i s s t u d y . 41 e) T iming of the carbonate a l t e r a t i o n I m p l i c i t i n the sugges t ion t h a t the carbonates i n f l u e n c e the s u r v i v a l of hypogene s u l f i d e s i n the supergene zone i s the i m p l i c a t i o n t h a t tha carbonates are h i g h temperature a l t e r a t i o n p r o d u c t s . Evidence abounds to support the p r o p o s i t i o n . F i r s t l y , i t i s the pe rvas i veness of the a l t e r a t i o n which i s p a r t i c u l a r l y abundant i n the western h a l f of the p i t . Supergene and post - supergene c a l c i t e , r a t h e r than a n k e r i t e , on the o ther hand i s main l y v e i n - f o r m i n g . Second ly , m i n e r a l s a s s o c i a t e d w i t h a n k e r i t e suggest e l e v a t e d temperature of f o r m a t i o n . Where p e r v a s i v e c a r b o n a t i z a t i o n p r e v a i l s , o n l y t o u r m a l i n e , mica (mainly muscovi te and m a r i p o s i t e ) , q u a r t z , f e l d s p a r and r a r e l y c h l o r i t e and s c a p o l i t e are n o t e d . Such an assemblage i n d i c a t e s the p a r t i c i p a t i o n of magmatic v o l a t i l e s i n t h e i r c r y s t a l l i z a t i o n . L a s t l y , i n the southwestern quadrant of the o p e n - p i t , i n tense carbonate replacement i s cut by a l a t i t e dyke w h i c h , as d e s c r i b e d above, i s p r e - T e r t i a r y . . . Thus, major carbonate a l t e r a t i o n preceded the supergene e v e n t . The f a c t t h a t p a r t o f the l a t i t e i s a l s o c a r b o n a t i z e d and t h a t m a f i c m i n e r a l s i n the T e r t i a r y v o l c a n i c rocks are a l s o r e p l a c e d by a n k e r i t e can be i n t e r p r e t e d i n terms of contemporaneity of the carbonate a l t e r a t i o n and the l a t i t e i n t r u s i o n and t h a t s i m i l a r p rocesses are i n v o l v e d i n the e v o l u t i o n of CO^ d u r i n g Mesozo ic magmatism and T e r t i a r y v o l c a n i s m . In o ther words , the most p robab le source of carbon d i o x i d e c a u s i n g the carbonate a l t e r a t i o n i s t h a t f l u i d e v o l v i n g d u r i n g secondary b o i l i n g a s s o c i a t e d w i t h the s o l i d i f i c a t i o n o f d i f f e r e n t magmas. As rev iewed by M c M i l l a n and Pante leyev (1980) f o r models o f porphyry copper d e p o s i t s , m u l t i p l e b o i l i n g events are common i n an orthomagmatic hydrothermal system. 42 d) Sporad ic d i s t r i b u t i o n of hydrothermal m i n e r a l s Given a roof pendant environment c h a r a c t e r i z e d by a d i v e r s i f i e d a r r a y o f hos t rocks and a low f l u i d to rock r a t i o , s p o r a d i c d i s t r i b u t i o n of secondary m i n e r a l s i s an expected r e s u l t . In the case of A f t o n M i n e s , at l e a s t one more f a c t o r i s ev ident as a p o s s i b l e cause of s p o r a d i c m i n e r a l d i s t r i b u t i o n s . Th is i s the f a c t tha t the Cherry Creek phase o f the I ron Mask p l u t o n , which h o s t s the A f t o n orebody, c o n s i s t s of a v a r i e t y o f rocks r e s u l t i n g from m u l t i p l e i n t r u s i v e events r a t h e r than a s i n g l e l a r g e i n t r u s i o n . P a r a g e n e t i c a l l y l a t e r i n t r u s i o n s w i t h t h e i r a s s o c i a t e d hydrothermal a c t i v i t i e s are l i k e l y to a l t e r e a r l i e r developed p a t t e r n s . Consequent l y , one must examine the development of a l t e r a t i o n p a t t e r n s at A f t o n i n the p e r s p e c t i v e o f an eve r - chang ing w a l l rock env i ronment . However, i f the o v e r a l l a l t e r a t i o n does not i n v o l v e a great d isp lacement of the ore e lements from the p r o t o r e , then the fo rmat ion of ore would c o i n c i d e w i t h the t r a p p i n g of magmatic f l u i d s . At A f t o n , the p e r v a s i v e carbonate replacement (dominant ly a n k e r i t e w i t h minor amounts of c a l c i t e ) , b e i n g so e v i d e n t l y a h i g h temperature a l t e r a t i o n p r o d u c t , seems a good i n d i c a t o r o f l o c a l c o n c e n t r a t i o n of hydro thermal a c t i v i t y . As i n d i c a t e d by the i n f r e q u e n t t r a p p i n g of specks of c h a l c o p y r i t e or i t s a l t e r a t i o n p r o d u c t , n a t i v e copper , i n m a g n e t i t e , p r imary copper s u l f i d e s p robab ly p r e c i p i t a t e d from the I ron Mask i n t r u s i v e magma s h o r t l y a f t e r the s e p a r a t i o n of m a g n e t i t e . Then, areas bounded by abundant magnet i te on the one hand and in tense a n k e r i t e replacement on the o ther are a t t r a c t i v e e x p l o r a t i o n t a r g e t s f o r s i m i l a r copper o res i n the I r o n Mask b a t h o l i t h . A more d e t a i l e d model o f the copper m i n e r a l i z a t i o n a t A f t o n w i l l be p r e s e n t e d i n the t h i r d p a r t of t h i s s t u d y . 43 Summary and C o n c l u s i o n In summary, the petrography of the w a l l rocks of the A f t o n orebody has been o u t l i n e d i n t h i s study w i t h emphasis p l a c e d on d i s t r i b u t i o n of m i n e r a l s . I t i s conc luded tha t the m i n e r a l i z a t i o n p rocess o c c u r r e d i n a roof pendant environment c h a r a c t e r i z e d by a d i v e r s i f i e d a r r a y of rock types w i t h i n t r i c a t e mutual r e l a t i o n s h i p s . W i t h i n the boundary o f a l a r g e s c a l e z o n i n g of p y r i t e and magneti te e s t a b l i s h e d by C a r r and Reed (1976) , ore appears to be most c o n s i s t e n t l y a s s o c i a t e d w i t h hydrothermal b i o t i t e . Other than t h a t , pr imary and hydrothermal m i n e r a l d i s t r i b u t i o n s w i t h i n the orebody and i t s v i c i n i t y are found to be r a t h e r s p o r a d i c i n d e t a i l , p a r t i c u l a r l y i n the v e r t i c a l d imension . N e v e r t h e l e s s , s e v e r a l i n t e r e s t i n g c o n c l u s i o n s can s t i l l be d r a w n : -1 . ) The absence of i n tense p h y l l i c and a r g i l l i c a l t e r a t i o n s at A f t o n i s p r i m a r i l y the r e s u l t of the l a c k of a major meteor i c water component p a r t i c i p a t i n g i n the hypogene m i n e r a l i z a t i o n event . 2 . ) High temperature hydrothermal a l t e r a t i o n s at A f t o n proceeded w i t h a low r a t i o of f l u i d to rock such tha t pr imary minera logy was ab le to c o n t r o l the appearance of secondary m i n e r a l s . 3 . ) The na tu re and ex tent of carbonate ( a n k e r i t e w i t h a minor amount o f c a l c i t e ) a l t e r a t i o n s at A f t o n , p o o r l y known p r e v i o u s l y , a re documented i n t h i s s tudy . Be ing an i n d i c a t o r of concent rated hydrothermal a c t i v i t y and r e a d i l y r e c o g n i z a b l e , p e r v a s i v e carbonate a l t e r a t i o n c o u l d be o f great v a l u e i n e x p l o r i n g f o r s i m i l a r copper d e p o s i t s i n a l k a l i n e i n t r u s i o n s . 4 . ) Supergene a l t e r a t i o n at A f t o n appears to be dominated by i r r e v e r s i b l e m i n e r a l - s o l u t i o n i n t e r a c t i o n s . The ex tent of the a l t e r a t i o n a t depth i s l a r g e l y f r a c t u r e - c o n t r o l l e d . M i n e r a l 44 d i s t r i b u t i o n s p resented above serve to p rov ide background f i e l d data f o r t e s t i n g the f e a s i b i l i t y o f r e c o n s t r u c t i n g the supergene a l t e r a t i o n event through a thermodynamic approach , which i n v o l v e s m a s s - t r a n s f e r c a l c u l a t i o n s making use of a v a i l a b l e thermochemical data on r e l e v a n t m i n e r a l and s o l u t i o n s p e c i e s . 45 PART I I A THERMODYNAMIC SIMULATION OF THE SUPERGENE ALTERATION AT AFTON MINES 46 I n t r o d u c t i o n arid Method of Approach A f t o n Mines i s unusual among porphyry copper d e p o s i t s i n the Canadian C o r d i l l e r a i n d i s p l a y i n g a s i g n i f i c a n t supergene m i n e r a l i z a t i o n c h a r a c t e r i z e d by a dominance of n a t i v e copper . To an average depth of about 500 meters , the orebody, ave rag ing 1% Cu, possesses n a t i v e copper and c h a l c o c i t e i n a r a t i o i n excess of 2 : 1 as the main copper m i n e r a l s . Whereas t h i n smears of c u p r i t e may be found c o a t i n g n a t i v e copper , m a l a c h i t e , a z u r i t e , amorphous hydrated copper s u l f a t e s (?) and c o n i c h a l c i t e occur very r a r e l y . D e s p i t e the f a c t tha t supergene a l t e r a t i o n extends to a c o n s i d e r a b l e depth , the zone i s not e n r i c h e d i n copper content i n comparison to the hypogene zone . Car r and Reed (1976) suggested t h a t the c o e x i s t e n c e of magneti te and h i g h - s u l f u r copper m i n e r a l s cou ld be e s s e n t i a l to the fo rmat ion of the unusual supergene assemblage by f a c i l i t a t i n g the necessary o x i d a t i o n -r e d u c t i o n p r o c e s s . P r e t o (pe rsona l communciat ion) , on the o ther hand, emphasized the absence o f abundant p y r i t e as the pr ime f a c t o r r e s p o n s i b l e f o r the l a c k of supergene enr ichment . Ney e t a l . (1976) , upon d i s c u s s i n g the genera l a s p e c t s of supergene copper m i n e r a l i z a t i o n , summarized the c o n d i t i o n s r e q u i r e d f o r enrichment to i n c l u d e abundant hypogene i r o n s u l f i d e s , o x i d a t i o n under low pH c o n d i t i o n s , a water t a b l e at c o n s i d e r a b l e depth , h i g h p e r m e a b i l i t y and p o r o s i t y , absence o f r e a c t i v e w a l l rocks and abundant r a i n f a l l i n a warm c l i m a t e . At a g i ven l o c a l i t y and t i m e , s o l u t i o n chemis t r y ( i n c l u d i n g pH and Eh , i n p a r t i c u l a r ) and the a s s o c i a t e d m i n e r a l assemblage are not independent of each o t h e r . I f the o v e r a l l chemica l system i s not i n e q u i l i b r i u m or i s o n l y i n p a r t i a l e q u i l i b r i u m , mass t r a n s f e r w i l l occur among the phases i n the sys tem, t e n d i n g to b r i n g about t o t a l e q u i l i b r i u m , i . e . a s t a t e of min imal 47 Gibbs f r e e energy a t a g i ven temperature and p r e s s u r e . In as much as supergene m i n e r a l i z a t i o n i s a consequence of i n t e r a c t i o n s between downward p e r c o l a t i n g m e t e o r i c water w i t h host rocks c o n t a i n i n g s u i t a b l e hypogene m i n e r a l assemblages , any thorough attempt to understand and e x p l a i n the process has to c o n s i d e r s imultaneous r e a c t i o n s i n v o l v i n g the b u l k rock minera logy and the r e a c t i n g f l u i d r a t h e r than c o n c e n t r a t i n g on a l i m i t e d subset o f m i n e r a l s , e . g . the s u l f i d e s . For t h i s purpose , the mass t r a n s f e r approach expounded i n d e t a i l i n Helgeson (1968, 1970 and 1979) , Helgeson e t a l . (1969) and Helgeson et a l (1970) i s a p p r o p r i a t e . In e s s e n c e , the approach makes use o f thermodynamic c o n s t r a i n t s imposed by l o c a l and " p a r t i a l e q u i l i b r i u m to c a l c u l a t e the ex tent to which components are r e d i s t r i b u t e d among m i n e r a l s and aqueous s o l u t i o n as the s o l u t i o n r e a c t s i r r e v e r s i b l y w i t h i t s m i n e r a l o g i c env i ronment . Separa t ion of phases caused i n d i r e c t l y by a p p a r e n t l y u n r e l a t e d processes i n d i f f e r e n t p a r t s of the system are r e a d i l y r e v e a l e d i n the mass t r a n s f e r c a l c u l a t i o n s . Consequent ly , cause and e f f e c t i n the g e o l o g i c r e c o r d are more e a s i l y r e l a t e d and s u b t l e i n d i c a t o r s of f a v o r a b l e environment f o r ore d e p o s i t i o n can subsequent ly be r e c o g n i z e d . U t i l i z i n g the d e t a i l e d m i n e r a l d i s t r i b u t i o n data d e s c r i b e d i n P a r t I and the mass t r a n s f e r computer program d e s c r i b e d i n P e r k i n s (1980) , t h i s study aims to s i m u l a t e the development of supergene a l t e r a t i o n at A f t o n M i n e s . For compar ison , i n t e r a c t i o n between a h y p o t h e t i c a l m i n e r a l i z e d monzonite and r a i n water under s i m i l a r c o n d i t i o n s i s a l s o p r e s e n t e d . S i m i l a r i t i e s and d i s c r e p a n c i e s between the s i m u l a t i o n r e s u l t s and the a c t u a l f i e l d o b s e r v a t i o n s are e l a b o r a t e d . Parameters c o n t r o l l i n g the supergene enr ichment a t A f t o n Mines i n p a r t i c u l a r , and o t h e r porphyry copper d e p o s i t s i n g e n e r a l are then e v a l u a t e d and c l a r i f i e d . 48 S p e c i f i c a t i o n o f Computer Runs Tables IA and IB summarize the compos i t ions of the i n i t i a l r e a c t a n t s o l i d s and f l u i d s f o r three c r i t i c a l s i m u l a t i o n runs w h i c h , as w i l l be demonstrated l a t e r , o u t l i n e the v a r i a t i o n s i n s t y l e of supergene a l t e r a t i o n s . The m i n e r a l o g i c a l compos i t ions f o r the two p o r t i o n s of the A f t o n o p e n - p i t have been e s t i m a t e d from f i e l d mapping, and from p e t r o g r a p h i c and x - r a y d i f f r a c t i o n s t u d i e s of samples c o l l e c t e d at v a r i o u s s tages o f s t r i p p i n g of the orebody. L i m i t e d areas of i n t e n s i v e carbonate replacement and magnet i te v e i n i n g have been d e l i b e r a t e l y exc luded from c o n t r i b u t i n g to the average . These e s t i m a t e s are a d m i t t e d l y rough but are c o n s i d e r e d to be much more r e a l i s t i c than any a r t i f i c i a l a p p r o x i m a t i o n s . For m i n e r a l s commonly e x h i b i t i n g s o l i d s o l u t i o n , c u r s o r y microprobe ana l yses t a b u l a t e d i n Appendix I i n d i c a t e that c o m p o s i t i o n v a r i a t i o n s are f r e q u e n t l y e v i d e n t i n g r a i n s of the same t h i n s e c t i o n . N e v e r t h e l e s s , the f o l l o w i n g approx imat ions i n terms of end members can be made: pyroxene i s 0 . 8 d i o p s i d e and 0 . 2 h e d e n b e r g i t e ; e p i d o t e i s 0 . 8 e p i d o t e and 0 . 2 c l i n o z o i s i t e ; b i o t i t e i s low i n f l u o r i n e and i s rough ly 0 . 8 p h l o g o p i t e and 0 . 2 a n n i t e , i g n o r i n g i t s r e l a t i v e l y h i g h t i t a n i u m c o n t e n t ; the compos i t ions of f e l d s p a r s are e x p l i c i t l y s t a t e d i n Table IA; carbonates i n the east p i t are main ly c a l c i t i c and are taken as 0 .95 c a l c i t e , 0 . 0 3 magnesite and 0 .02 s i d e r i t e ; i n the western p o r t i o n of the p i t , a n k e r i t e i s more prominent and the carbonates are expressed as 0 . 6 d o l o m i t e , 0 . 1 s i d e r i t e and 0 . 3 c a l c i t e ; hypogene c h l o r i t e i s main ly p y c n o c h l o r i t e w i t h an average Mg/Fe r a t i o of 2 / 1 , the magnesium p o r t i o n i s rep resented by 7A c l i n o c h l o r e and the Fe i s expressed by a cor respond ing amount of a n n i t e b i o t i t e due to l a c k of thermodynamic data on end-member i r o n c h l o r i t e s (the a d d i t i o n a l po tass ium i n t r o d u c e d i n t o the system through t h i s s u b s t i t u t i o n i s c o n s i d e r e d to be TABLE I A. COMPOSITION OF STARTING MATERIAL ( in modal percent ) 1 2 3 EAST PIT ROCK WEST PIT ROCK HYPOTHETICAL MONZONITE FELDSPAR 60 60 65 ( A b.95° r.03 A n.02 ) < A b.95 0 r.03 A a.02 ) ( A b.A9° r.3A A n.17 ) EPIDOTE 12 -CHLORITE 5 20 BIOTITE 5 2 7 MAGNETITE 5 -CHALCOPYRITE 1 1 3 BORNITE 1 1 2 CHALCOCITE 1 1 CARBONATES 4 5 -PYROXENE 5 - -HORNBLENDE 5 10 PYRITE 3 10 QUARTZ - 1 3 APATITE 1 1 TABLE I B. COMPOSITION OF HYPOTHETICAL RAIN-WATER USED IN SIMULATION RUNS RUN # 1 RUN # 2 RUN // 3 log f -0.69 i.e. in equilibrium with oxygen in the atmosphere 2 log frn -3.5 i.e. in equilibrium with carbon dioxide in the atmosphere PH Cu* Mg"1 SO,— 4 H.SiO. 4 4 Fe 1 x 10"8 m 1 x 10" 1 1 m 1 x 10"9 m 1.48xl0"5 m* -v -5 * Ca K+ 6.6 x 10"6 m* N a+ . 4.78 x 10"5 m* 1 x 10' 1 2 m 1 x 10" 1 1 m 1 x 10" 1 1 m 1 x 10"8 m 1 x 10" 1 1 m 1 x 10" 1 2 o constrained by the presence of hematite * from Wedepohl, 1966, 1.25x10 J mK o 2.42 x 10~5 m* 51 i n s i g n i f i c a n t , a l l o w i n g f o r the v a r i a b l e amount of K - f e l d s p a r a c t u a l l y p resent i n the w a l l r o c k s ) ; hornblende i s 0 . 7 p a r g a s i t e and 0 . 3 f e r r o h a s t i n g s i t e but a v a i l a b l e thermodynamic data on f i l e f o r c e one to t r e a t i t as pure p a r g a s i t e . A p a t i t e has s u r v i v e d the supergene a l t e r a t i o n and i s t h e r e f o r e not i n c l u d e d as a r e a c t a n t m i n e r a l . A f t e r these ad justments , the t o t a l number o f s o l i d r e a c t a n t s c o n s i d e r e d i n each of the three runs are 1 8 , 16 and 10 r e s p e c t i v e l y . The data used f o r the compos i t ion of r a i n water have been adapted from Wedepohl (1966) . The m i s s i n g but r e q u i r e d independent components have been ass igned a r b i t r a r y s m a l l v a l u e s . Some of t h e s e , as shown i n Table IB , are a l l o w e d to vary s l i g h t l y to b r i n g out the p o i n t tha t as long as the i n i t i a l c o n c e n t r a t i o n s are low enough, i t i s through r e a c t i o n s w i t h host rocks t h a t the r e a c t i n g r a i n water d e r i v e s i t s c o n s t i t u e n t ions which subsequent ly determine the sequence of p r e c i p i t a t i o n of v a r i o u s s o l i d p r o d u c t s . For the case of i r o n , c a l c u l a t i o n s on i n i t i a l d i s t r i b u t i o n of s p e c i e s have shown that under atmospher ic c o n d i t i o n s and an assumed pH o f 7 , water i s s a t u r a t e d w i t h - 2 3 r e s p e c t to hemat i te at a t o t a l i r o n content as low as 1 x 10 m o l a l . S ince h e m a t i t e , among a l l the F e - m i n e r a l s c o n s i d e r e d i n the system, i s bound to be the ve ry f i r s t r e a c t i o n product p r e c i p i t a t e d soon a f t e r the i n i t i a l • • contact of r a i n water w i t h the r e l a t i v e l y i r o n - r i c h r e a c t i n g r o c k s , i t i s assumed to be a l r e a d y i n e q u i l i b r i u m w i t h the r a i n water as a matter of convenience. F i n a l l y , i t must be p o i n t e d out t h a t a l l the mass t r a n s f e r c a l c u l a t i o n s p resented here have been performed assuming i s o b a r i c and i s o t h e r m a l c o n d i t i o n s i n a c l o s e d sys tem. R e a c t i o n products are not separated from the bu lk system a f t e r t h e i r i n i t i a l p r e c i p i t a t i o n but a re a l l o w e d t o p a r t i c i p a t e i n subsequent r e a c t i o n s accompanying changes i n c o m p o s i t i o n of the r e a c t i n g f l u i d . The p r e s s u r e and temperature s p e c i f i e d f o r a l l the th ree runs are 1 bar and 25°C 52 r e s p e c t i v e l y . Thermodynamic data f o r v a r i o u s aqueous and s o l i d phases i n v o l v e d i n the c a l c u l a t i o n s are d e r i v e d main ly f rom Helgeson (1969) and Robie e t a l . (1978) . 53 R e s u l t s and I n t e r p r e t a t i o n a) Genera l aspects The r e s u l t s of i n t e r a c t i o n s o f r a i n water w i t h the t h r e e r e a c t i n g rock compos i t ions ob ta ined from mass t r a n s f e r c a l c u l a t i o n s are p o r t r a y e d i n F i g u r e s 11 to 1 3 . The p r o g r e s s v a r i a b l e , £ (the l o g a r i t h m of wh ich i s chosen as the h o r i z o n t a l a x i s i n the d i a g r a m s ) , i s a measure of the e x t e n t o f a g i ven r e a c t i o n . I t i s n u m e r i c a l l y e q u a l to the number of m o l e s , n , of a g i ven r e a c t a n t tha t has been conver ted i n t o products by the r e a c t i o n , d i v i d e d by the s t o i c h i o m e t r i c c o e f f i c i e n t of the r e a c t a n t . Since the s t o i c h i o m e t r i c c o e f f i c i e n t i s a c o n s t a n t , the f o l l o w i n g d i f f e r e n t i a l e q u a t i o n can be w r i t t e n : dn — j — = k where k i s a c o n s t a n t . d£ Consequent ly , i t can be shown that i n the f i g u r e s , a l l the r e a c t a n t s e x h i b i t a p o s i t i v e s l o p e of 1 u n t i l they become s a t u r a t e d w i t h r e s p e c t to the r e a c t i n g f l u i d . For the sake of s i m p l i c i t y , o n l y two r e a c t a n t s are d e p i c t e d i n each d iagram, i n broken l i n e s . A l l p roduct m i n e r a l s except h e m a t i t e , which i s assumed to be a l r e a d y i n e q u i l i b r i u m w i t h the i n i t i a l r e a c t i n g s o l u t i o n , assume an i n i t i a l i n f i n i t e s lope i n d i c a t i n g i t s f i r s t appearance as a new s o l i d phase i n the system c o n s i d e r e d . The net amount of any product m i n e r a l p r e s e n t i n the system at any i n s t a n t of the o v e r a l l r e a c t i o n p rogress can r e a d i l y be read o f f from the co r respond ing e v o l u t i o n a r y curve of the m i n e r a l ( d e p i c t e d i n s o l i d l i n e s and l a b e l l e d to the r i g h t of tha t p o r t i o n of the curve i n d i c a t i n g i t s i n i t i a l p r e c i p i t a t i o n ) . The p o r t i o n of the curve w i t h a p o s i t i v e s l o p e i n d i c a t e s p r e c i p i t a t i o n w h i l e t h a t showing a negat i ve s lope i n d i c a t e s subsequent d i s s o l u t i o n due to changes of s o l u t i o n c h e m i s t r y . P l o t t e d i n d o t t e d l i n e s are the v a r i a t i o n i n a c t i v i t y of s e l e c t e d s p e c i e s , i n the aqueous phase , w i t h r e a c t i o n p r o g r e s s . Note the s e q u e n t i a l change i n copper Figure 11. Change in solution chemistry and of Qminerals as a result of rain water -25 C and a pressure of 1 bar (see text the production and destruction east pit rock interactions at for detailed discussion). 55 F i g u r e 12. Change i n s o l u t i o n chemis t r y and the p r o d u c t i o n and d e s t r u c t i o n of m i n e r a l s as a r e s u l t of r a i n w a t e r - w e s t p i t r o c k i n t e r a c t i o n s at o 25 C and a p r e s s u r e of 1 bar (see t e x t f o r d i s c u s s i o n ) . 56 0 _ C M 3 Q W o o Pi Q w o w w l-t S o e> o • J Pi o in U3 M O w to o > HI H U < s - 2 - 5 -7 ."2. C0„ -220 -240 -8 -7 -5 -4 - 3 LOG F i g u r e 13 . Change i n s o l u t i o n chemis t r y and the p r o d u c t i o n and d e s t r u c t i o n of m i n e r a l s as a r e s u l t of r a i n w a t e r - h y p o t h e t i c a l monzonite i n t e r a c t i o n s a t 25 C and a p r e s s u r e of 1 bar (see t e x t f o r d i s c u s s i o n ) . 57 minera logy ( t e n o r i t e — c u p r i t e — n a t i v e c o p p e r — c h a l c o c i t e ) accompanying the monotonic decrease i n oxygen f u g a c i t y and concomitant i n c r e a s e i n s u l f u r f u g a c i t y . T h i s change takes p l a c e i n nature when the downward p e r c o l a t i n g r a i n water has been dep le ted of most of i t s d i s s o l v e d oxygen due to r e a c t i o n w i t h i n c o m p a t i b l e m i n e r a l s encountered i n the w a l l r o c k s . L i m i t e d by the i s c a l e o f the h o r i z o n t a l a x i s , the e v o l u t i o n a r y paths of the c o p p e r - c o n t a i n i n g m i n e r a l s cannot be p l o t t e d f u l l y i n F i g u r e s 11 to 13. D e t a i l s of the s e q u e n t i a l change w i l l be d e s c r i b e d and d i s c u s s e d w i t h r e s p e c t to a l a t e r l o g f_ ve rsus l o g f_ d iagram. U 2 b 2 Before l e a v i n g F i g u r e s 11 to 1 3 , i t i s wor thwhi le d i r e c t i n g the r e a d e r ' s a t t e n t i o n t o the d i f f e r e n c e i n e v o l u t i o n p a t t e r n o f the hydrogen i o n a c t i v i t y i n the cases i l l u s t r a t e d . Because pH i s d e f i n e d as the n e g a t i v e l o g a r i t h m of the a c t i v i t y o f the hydrogen i o n , r e a d i n g the v e r t i c a l a x i s w i t h p o s i t i v e numbers, the H + v a r i a t i o n curves are a c t u a l l y l o c i o f pH accompanying the a l t e r a t i o n s . Note tha t i n Run #1, pH tends to i n c r e a s e throughout the p r o c e s s ; Run #2, w h i l e e n j o y i n g an a l k a l i n e aqueous s o l u t i o n a l l the t i m e , does show a maximum pH va lue o f about 9 ; pH i n Run #3 keeps on d e c r e a s i n g , passes through the n e u t r a l p o i n t before f i n a l l y r e v e r s e s i t s course a f t e r the s a t u r a t i o n of c h a l c o c i t e . As w i l l be d i s c u s s e d be low, these d i f f e r e n c e s have a ve ry s t r o n g e f f e c t on the ex tent of development of supergene enr i chment . L a s t l y , note a l s o t h a t none o f the runs has been c a r r i e d through to reach o v e r a l l e q u i l i b r i u m . The i m p r o b a b i l i t y of rep roduc ing nature i n every s m a l l d e t a i l suggests t h a t once f i r m t rends are e s t a b l i s h e d by computat ion , f u r t h e r e f f o r t s towards r e t r i e v i n g t r i v i a l supplememtary i n f o r m a t i o n are not w a r r a n t e d . 58 b) Enrichment versus non-enr ichment and dominance o f a p a r t i c u l a r phase F i g u r e 14 shows, f o r the th ree s i m u l a t i o n s d e s c r i b e d , the paths of e v o l u t i o n of the r e a c t i n g r a i n water over the s t a b i l i t y f i e l d s i n the F e - C u - S - 0 system at 25°C and a p r e s s u r e o f 1 b a r . B o r n i t e i s s t a b l e i n a t h i n s h e l l around the s t a b i l i t y f i e l d of c h a l c o p y r i t e but i s omi t ted from the diagram f o r c l a r i t y . The two p a r a l l e l t h i c k l i n e s , des ignated by constant p roducts of the a c t i v i t y o f s u l f a t e i o n and the square of the a c t i v i t y of hydrogen i o n are approx imat ions to the c o m p o s i t i o n a l " l i m i t s " of aqueous s o l u t i o n s commonly found i n a supergene env i ronment . They are d e r i v e d as f o l l o w s : Fo r the r e a c t i o n , 2 H + + SO " = 3/2 0 o + 1/2 S„ + H„0 4 2 2 2 l o g K = 3/2 l o g f + 1/2 l o g f + l o g a „ °2 S 2 H 2 ° - 2 l o g a - l o g a (1) H SO. 4 where K = e q u i l i b r i u m constant of the r e a c t i o n , and f , = f u g a c i t y and a c t i v i t y of spec ies x r e s p e c t i v e l y . A l s o , l o g K i s r e l a t e d to the e q u i l i b r i u m constant of f o r m a t i o n , K , of r , x i n d i v i d u a l s p e c i e s x i n v o l v e d i n the r e a c t i o n by l o g K = 3/2 l o g K f ] Q + 1/2 l o g K f ) S + l o g K f R Q - 2 l o g K - l o g K _ (2) 'H 'SO ~ 4 Upon s u b s t i t u t i o n of r e l e v a n t data a t 25°C and a p ressure of 1 bar from Robie e t . a l . (1978) , e q u a t i o n (2) g i v e s l o g K = - 9 5 . 8 7 (3) S u b s t i t u t i n g (3) i n (1) and assuming tha t a n = 1 , one gets 2° 3/2 l o g f f t + 1/2 l o g f = - 9 5 . 8 7 + 2 l o g a , + l o g a _ (4) °2 S 2 H SO." 4 59 F i g u r e IA. The e v o l u t i o n paths of the r e a c t i n g s o l u t i o n s f o r the t h r e e s i m u l a t i o n runs over the s t a b i l i t y f i e l d s of m i n e r a l s i n the C u - F e - S - 0 system a t 25 C and a p ressure of 1 bar (see t e x t f o r d e t a i l e d d i s c u s s i o n ) . — phase boundary i n the F e - S - 0 subsystem phase boundary i n the C u - S - 0 subsystem Note that the s t a b i l i t y f i e l d of c h a l c o p y r i t e and s u l f u r s a t u r a t i o n a r e d e p i c t e d i n t h i n s o l i d l i n e s . A l i s t of m i n e r a l s i n v o l v e d i n the diagram i s as f o l l o w s : Fe F e 3 ° A F e 2 0 3 FeS F e S 2 FeSO^ CuFeS. • 7H 2 0 m e t a l l i c i r o n magnet i te hemat i te p y r r h o t i t e p y r i t e m e l a n t e r i t e c h a l c o p y r i t e Cu Cu 2 0 CuO Cu 2 S CuS CuSO. .5H.0 4 2 Cu.SO.(OH). n a t i v e copper c u p r i t e t e n o r i t e c h a l c o c i t e c o v e l l i t e c h a l c a n t h i t e b r o c h a n t i t e 60 61 A r b i t r a r i l y s e t t i n g a = 1 , the upper houndary curve (A) corresponds SO. 4 to the case of an ext remely a c i d i c environment c h a r a c t e r i z e d by pH = 0 , i . e . a , , a _ = (10 ) . l = 1 H SO." 4 w h i c h , upon s u b s t i t u t i o n i n t o e q u a t i o n ( 4 ) , g i v e s 3/2 l o g f . + 1/2 l o g f . = - 9 5 . 8 7 °2 b 2 and the lower boundary curve (B) d e p i c t s an ext remely b a s i c environment c h a r a c t e r i z e d by pH = 14 , i . e . a , . a = (10 ) . 1 = 10 H S 0 ~ ~ 4 w h i c h , upon s u b s t i t u t i o n i n t o e q u a t i o n (4) , g i v e s 3/2 l o g f + 1/2 l o g f_ = - 9 5 . 8 7 - 28 = - 1 2 3 . 8 7 °2 b 2 S t r i c t l y s p e a k i n g , these " l i m i t i n g " curves apply o n l y to s i t u a t i o n s where s u l f a t e i o n i s the dominant s u l f u r - c o n t a i n i n g s p e c i e s i n s o l u t i o n . Under such c o n d i t i o n s , the p o s s i b l e change i n o rder of magnitude of the a c t i v i t y of s u l f a t e i o n i s s m a l l compared to tha t of hydrogen i o n . Consequent ly , the a c t i v i t y of s u l f a t e i o n i s l e s s important than pH i n a f f e c t i n g the p o s i t i o n and shape o f the " l i m i t i n g " c u r v e s . F i g u r e 15 shows t h a t s u l f a t e i o n i n f a c t dominates over o ther s u l f u r - c o n t a i n i n g s p e c i e s over a wide range of pH and f^ c o n d i t i o n s a t 25°C and 1 bar and thus accounts f o r the f a c t tha t most n a t u r a l s o l u t i o n s do p l o t w i t h i n the " l i m i t i n g " curves of F i g u r e 14. Under ex t remely r e d u c i n g c o n d i t i o n s , i . e . f^ l e s s - 7 0 2 than about 10 and c o n s i d e r a b l y h i g h e r at s l i g h t l y e l e v a t e d temperature , d i m i n i s h i n g abundance o f s u l f a t e i o n i n s o l u t i o n would l e a d to a d e f l e c t i o n of the " l i m i t i n g " curves towards lower f v a l u e s i n F i g u r e 14. b 2 L i n e s l a b e l l e d 1 to 3 i n F i g u r e 14 cor respond to the e v o l u t i o n a l paths of r a i n water i n the th ree s i m u l a t i o n r u n s . Broken p o r t i o n s i n d i c a t e -30 -AO U 63 tha t though w i t h i n the l o g f - l o g f_ framework, the c o m p o s i t i o n of 2 S 2 the s o l u t i o n p l o t s i n the s t a b i l i t y f i e l d o f the r e l e v a n t copper s o l i d phase , a c t u a l p r e c i p i t a t i o n i s not t a k i n g p l a c e because the s o l u t i o n i s not s a t u r a t e d w i t h r e s p e c t to the m i n e r a l concerned; copper s t i l l remains as a p p r o p r i a t e ions i n s o l u t i o n . In c o n t r a s t to Run #3, i n i t i a l w a t e r - r o c k i n t e r a c t i o n s i n both Runs #1 and #2 l e a d to a decrease i n s u l f u r f u g a c i t y i n the l i q u i d phase. Subsequent r e a c t i o n paths i n a l l th ree c a s e s , however, are s u b p a r a l l e l to the a r b i t r a r y boundary curves A and B. T h i s p a r a l l e l i s m r e f l e c t s the preponderant e f f e c t of pH on supergene a l t e r a t i o n . The pH domain w i t h i n which the p rocess i s to take p l a c e i s determined by r e a c t i o n s between the hos t rocks and the p e r c o l a t i n g w a t e r . Because the h y d r o l y s i s of p r imary s u l f i d e s f u r n i s h e s a ready source of hydrogen i o n , i t i s v a l i d to assume t h a t the presence of p r imary s u l f i d e s c o n t r o l s the e x t e n t of supergene enr ichment . T h i s i s r e a d i l y brought out by comparing the r e s u l t s of the th ree r u n s . Reactants i n the th ree runs d i f f e r p r i m a r i l y i n the p r o p o r t i o n of s u l f i d e s and maf i c m i n e r a l s . In Run #1, the d e f i c i e n c y i n p y r i t e and the r e l a t i v e abundance of m a f i c s i l i c a t e s r e s u l t i n a h i g h and e v e r - i n c r e a s i n g pH throughout the c a l c u l a t e d p a t h . T e n o r i t e i s p r e c i p i t a t e d soon a f t e r hemat i te and i n t u r n i s succeeded by c u p r i t e , n a t i v e copper and c h a l c o c i t e . Because a copper m i n e r a l i s s t a b l e at each stage of the a l t e r a t i o n , r e l a t i v e l y l i t t l e copper i s h e l d i n s o l u t i o n to be t r a n s f e r r e d away. Supergene enr ichment i s consequent ly suppressed . Run #2 shows a very s i m i l a r b e h a v i o r except t h a t the presence of more p y r i t e b r i n g s about s l i g h t l y l o w e r , though s t i l l on the b a s i c s i d e , v a l u e s of pH. Supergene enrichment i s s t i l l n e g l i g i b l e . In both of these r u n s , e a r l y c o - p r e c i p i t a t i o n of t e n o r i t e w i t h hemat i te would p r e c l u d e an e x t e n s i v e 64 development of c o p p e r - d e f i c i e n t leached capp ing . In Run //3, the h y d r o l y s i s e f f e c t of abundant s u l f i d e s outweighs that of maf i c s i l i c a t e s . pH i s h e l d a c i d i c and decreases c o n t i n u a l l y u n t i l c h a l c o c i t e i s s t a b l e w i t h r e s p e c t t o the r e a c t i n g w a t e r . A very minor amount of t e n o r i t e i n i t i a l l y c o - p r e c i p i t a t e d w i t h h e m a t i t e i s q u i c k l y r e d i s s o l v e d and copper i s h e l d i n s o l u t i o n u n t i l the c o m p o s i t i o n of the r e a c t i n g s o l u t i o n p l o t s w e l l i n t o the s t a b i l i t y f i e l d of c u p r i t e . From then on , the u s u a l sequence of n a t i v e copper and c h a l c o c i t e f o l l o w s . In t h i s i n s t a n c e , supergene enr ichment i s f i r s t ev ident a t the c u p r i t e zone which should be o v e r l a i d by a w e l l -developed leached cap c h a r a c t e r i z e d by abundant hemat i te and/or l i m o n i t e . The r e a c t i o n path o f Run //l a l s o d i f f e r s from t h a t of Run #3 i n another a s p e c t : i t c r o s s e s the s t a b i l i t y f i e l d of n a t i v e copper over a much g rea te r range o f f_ and f than the l a t t e r . T h i s i m p l i e s t h a t U 2 b 2 Run #1 would be c h a r a c t e r i z e d by a g rea te r abundance of n a t i v e copper , w i t h the consequent ly reduced chance that i t would be c o m p l e t e l y r e p l a c e d by l a t e r c h a l c o c i t e . Here l i e s the prime reason f o r the preponderance of n a t i v e copper and the l a c k of supergene enrichment a t A f t o n M i n e s . The s i t u a t i o n w i t h Run #2 i s i n t e r m e d i a t e between Run //l and Run #3 but a b i t c l o s e r to the fo rmer . S t a y i n g i n the n a t i v e copper s t a b i l i t y f i e l d f o r a much s h o r t e r i n t e r v a l , the r e a c t i n g s o l u t i o n i n Run #2, i n comparison to t h a t of Run #1, i s more r e a d i l y brought i n t o e q u i l i b r i u m w i t h the copper s u l f i d e s . T h i s e x p l a i n s why n a t i v e copper i s more widespread i n the e a s t e r n p o r t i o n of the o p e n - p i t than i n the western one, where l o c a l l y some pr imary s u l f i d e s are i n t a c t l y p r e s e r v e d . The dominance o f n a t i v e copper a t A f t o n Mines c o u l d a l s o have been enhanced by the occur rence of the supergene process a l o n g deep f r a c t u r e s where the ambient temperature was i n excess of 25°C. As b r i e f l y mentioned 65 above, the e f f e c t of the d i m i n i s h i n g s t a b i l i t y f i e l d of SO^ i n F i g u r e 15 w i t h i n c r e a s i n g temperature , when t r a n s l a t e d i n t o F i g u r e 14 , i s the d e f l e c t i o n of the " l i m i t i n g " curves towards lower f a t the same f n 2 °2 E v o l u t i o n of the r e a c t i o n paths of s o l u t i o n s i n Run #1 and Run #2 p a r a l l e l to the lower " l i m i t i n g " curve would then r e s u l t i n a f u r t h e r pro longed s t a y i n the s t a b i l i t y f i e l d of n a t i v e copper (because the r e a c t i o n paths are d e f l e c t e d i n t o i t ) . Consequent ly , more n a t i v e copper would be produced i n the p r o c e s s . In the case of the E l Salvador porphyry copper d e p o s i t i n C h i l e , d e s c r i b e d i n d e t a i l by Gustafson and Hunt (1975) , the r e a c t i o n p a t h of the downward p e r c o l a t i n g water p l o t s p robab ly c l o s e to the upper " l i m i t i n g " curve , so t h a t the s t a b i l i t y f i e l d of n a t i v e copper i s bypassed a l t o g e t h e r . The p r e c i p i t a t i o n of abundant l i m o n i t e - and j a r o s i t e i n the l e a c h e d cap p robab ly h i n d e r s the s e p a r a t i o n of c h a l c a n t h i t e and m e l a n t e r i t e . Copper i s kept i n s o l u t i o n u n t i l the s t a b i l i t y f i e l d of c h a l c o c i t e i s encountered , thus l e a d i n g t o the f o r m a t i o n o f an enr i ched b l a n k e t of supergene o r e . Moreover , s i n c e the p r o j e c t e d r e a c t i o n path misses both the b o r n i t e and c h a l c o p y r i t e s t a b i l i t y f i e l d s but not that of p y r i t e , i t i s easy to understand why a t a p p r o p r i a t e l e v e l s of the d e p o s i t , supergene s u l f i d e s r e p l a c e b o r n i t e and c h a l c o p y r i t e but leaves p y r i t e i n t a c t . Such a t e x t u r a l r e l a t i o n s h i p i s not a t a l l prominent at A f t o n M i n e s . c ) D i s c r e p a n c i e s between t h e o r e t i c a l and observed m i n e r a l assemblages F i g u r e 16 summarizes the paragenes is of p roduct m i n e r a l s r e s u l t i n g from the w a t e r - r o c k i n t e r a c t i o n c a l c u l a t i o n s d e p i c t e d i n F i g u r e s 11 to 13. The t h e o r e t i c a l m i n e r a l assemblage c l o s e l y a s s o c i a t e d w i t h supergene n a t i v e copper and c h a l c o c i t e i s expected to be composed of t a l c , m u s c o v i t e , k a o l i n i t e , h e m a t i t e , q u a r t z ± l a u m o n t i t e . However, as e l a b o r a t e d i n P a r t I, 66 Run #1. Alteration of East Pit Rock HEMATITE -GIBBS1TE _ TENORITE '; — TALC MUSCOVITE KAOLIN ITE LAUMONTITE CUPRITE NATIVE COPPER CHALCOCITE Run # 2 . Alteration of West Pit Rock HEMATITE ! GIBBSITE TENORITE TALC MUSCOVITE KAOLINITE-QUARTZ CUPRITE Time Time V Run #3. Alteration of a Hypothetical Monzonite HEMATITE GIBBSITE-TENORITE KAOLINITE QUARTZ CUPRITE NATIVE COPPER CHALCOCITE Time Figure 16. Paragenesis of product minerals resulting from the water-rock interaction calculations 67 the a c t u a l m i n e r a l s ohserved are s p e c u l a r i t e , ea r thy h e m a t i t e , l i m o n i t e , F e - c h l o r i t e , q u a r t z , carbonates and a p o o r l y c r y s t a l l i n e mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e s w e l l i n g c l a y . A minor amount of k a o l i n i t e i s observed i n the western p o r t i o n of the o p e n - p i t but i s main l y a s s o c i a t e d w i t h c h a l c o c i t e o n l y . T a l c i s suspected merely i n a couple of specimens. Smal l amounts of z e o l i t e s l i k e f e r r i e r i t e and c l i n o p t i l o l i t e have been i d e n t i f i e d i n l a t e f r a c t u r e s i n some potash r i c h w a l l rocks and i n the T e r t i a r y rocks o v e r l y i n g the orebody. They are c o n s i d e r e d to be p o s t - o r e a l t e r a t i o n p r o d u c t s . The d i s c r e p a n c i e s between the t h e o r e t i c a l and observed supergene m i n e r a l assemblages a r e , however, not too d i f f i c u l t to r a t i o n a l i z e . In the f i r s t p l a c e , thermodynamic c a l c u l a t i o n s p r e d i c t an e q u i l i b r i u m m i n e r a l assemblage under s p e c i f i e d p h y s i c a l c o n d i t i o n s by c o n s i d e r i n g a l l r e l e v a n t m i n e r a l s p e c i e s p resent i n the data f i l e . They are i n c a p a b l e of p r e d i c t i n g any phases which are not i n c l u d e d . Due to the l a c k of thermodyanmic d a t a , many s p e c i e s , e . g . F e - c h l o r i t e , amorphous c l a y s and hydrated F e - o x i d e s are not cons idered i n the c a l c u l a t i o n s . The p r e d i c t e d assemblage of t a l c , k a o l i n i t e , l a u m o n t i t e and some of the h e m a t i t e , t h e r e f o r e , has to be i n t e r p r e t e d to have expressed themselves i n na tu re as F e - c h l o r i t e and m o n t m o r i l l o n i t e . In the second p l a c e , k i n e t i c e f f e c t s are not accounted f o r by t h i s thermodynamic model . Commonly, c l a y m i n e r a l s p r e c i p i t a t i n g from a s o l u t i o n under low temperature and p r e s s u r e c o n d i t i o n s are amorphous. I t i s o n l y n a t u r a l to p e r c e i v e the p r e d i c t e d muscov i te as the i l l i t e component of the m i x e d - l a y e r c l a y and hemat i te as m a n i f e s t e d i n a v a r i e t y o f forms of hydrous and anhydrous f e r r i c o x i d e s . The s p e c t a c u l a r a s s o c i a t i o n o f w e l l c r y s t a l l i n e s p e c u l a r i t e w i t h e u h e d r a l n a t i v e copper c r y s t a l s observed l o c a l l y at A f t o n M i n e s , on the o ther hand , may i n d i c a t e that a t some stage o f the supergene 68 e v e n t , the p r e v a i l i n g temperature cou ld have been a b i t h i g h e r than 25 C. In the t h i r d p l a c e , a l l c a l c u l a t i o n s presented here have been performed assuming r e a c t i o n s w i t h i n a c l o s e d system. At A f t o n M i n e s , the d e t a i l e d host rock l i t h o l o g y changes r a p i d l y over a shor t d i s t a n c e . The h i g h l y f r a c t u r e d n a t u r e of these rocks p robab ly f o r c e s the r e a c t i n g water to encounter a new m i n e r a l o g i c environment every now and t h e n . C o m p l i c a t i o n s brought about by the r e l a x a t i o n of c l o s e d - s y s t e m c o n d i t i o n s can be i l l u s t r a t e d w i t h the r e s u l t s of shor t e x p l o r a t o r y c a l c u l a t i o n s of i n t e r a c t i o n s between the seepage water at the northwest p i t w a l l and the average m i n e r a l assemblage of the e a s t e r n p o r t i o n of the p i t . Table I I o u t l i n e s the compos i t ion of the seepage water as f u r n i s h e d by A f t o n Mines i n the s p r i n g of 1979. A f t e r c o n v e r s i o n to a p p r o p r i a t e u n i t s , v a r i a b l e s marked w i t h a s t e r i s k s were a p p l i e d to the c a l c u l a t i o n s . The m i s s i n g Na and K data were es t imated by assuming a Na/K r a t i o of 2 / 1 , rough ly a c c o u n t i n g f o r the abundance of s y e n i t i c rocks i n the v i c i n i t y , and m a i n t a i n i n g a t the same t ime e l e c t r i c n e u t r a l i t y among the major d i s s o l v e d I | | components. The r e q u i r e d A l and I^SiO^ components were aga in a s s i g n e d s m a l l a r b i t r a r y v a l u e s as i n Run #1. , C a l c u l a t i o n s of i n i t i a l d i s t r i b u t i o n of s p e c i e s i n d i c a t e d tha t the water i s s a t u r a t e d w i t h r e s p e c t to d o l o m i t e , h e m a t i t e and t e n o r i t e . T h i s f i n d i n g i s not at a l l s u r p r i s i n g because the seepage was c o l l e c t e d i n an a rea showing p e r v a s i v e a n k e r i t e a l t e r a t i o n . In o rder to enable the mass t r a n s f e r c a l c u l a t i o n to p r o c e e d , the independent ++ + v a r i a b l e s CC^, Fe and Cu were a l lowed to be c o n s t r a i n e d by d o l o m i t e , h e m a t i t e and t e n o r i t e r e s p e c t i v e l y . S a l i e n t r e s u l t s of the spontaneous r e a c t i o n s c a l c u l a t e d i n c l u d e the c o n t i n u a l s e p a r a t i o n of d o l o m i t e , hemat i te and t e n o r i t e , the s e q u e n t i a l p r e c i p i t a t i o n of g i b b s i t e and m u s c o v i t e , f o l l o w e d by an e a r l y s t a b i l i z a t i o n 69 TABLE II COMPOSITION OF NORTHWEST PIT WALL SEEPAGE (Courtesy of Afton Mines Ltd.) March 14,1979 Physical Parameters o * Field: Temperature ( C) 11.0 pH 8.4* Diss. Oxygen (ppm) 5.6 Conductivity (/umhos/cm) 5375 Lab: pH 8.39 Turbidity (JTU) 0.3 Total Suspended Solids (ppm) 1 Major Dissolved Components (ppm) Hardness CaC03 Calcium Ca Magnesium Mg Sodium Na Potassium K Sulphate SO^— Fluoride F-221* (treated as HCO 27.5* 37.0* 2380 * 0.64 Metals (ppm) Dissolved Total Arsenic As < 0.005 < 0.005 Copper Cu 0.002 * 0.007 Lead Pb 0.002 0.005 Mercury Hg - 4 0.0005 Zinc Zn 0.002 0.022 Iron Fe Molybdenum Mo 0.007 * 0.034 Values used in the calculation of interactions with east pit rock. 70 of K - f e l d s p a r and p h l o g o p l t e p r i o r to the monotonic d e p l e t i o n o f d i s s o l v e d oxygen i n the r e a c t i n g s o l u t i o n . I f , i n accordance w i t h the f i e l d o b s e r v a t i o n t h a t c a l c i t e i s the p r e f e r r e d supergene a l t e r a t i o n p r o d u c t , d o l o m i t e i s suppressed i n the c a l c u l a t i o n s , then c a l c i t e takes the p l a c e of do lomi te and the s t a b i l i z a t i o n of p h l o g o p i t e precedes that of m u s c o v i t e . Thus an e x p l a n a t i o n i s obta ined f o r the i n c l u s i o n of carbonates i n the a c t u a l supergene m i n e r a l assemblage and f o r the p e r s i s t e n c e of some b i o t i t e i n l o c a l i z e d areas hav ing abundant n a t i v e copper . S ince n o t h i n g i s d r a s t i c a l l y d i f f e r e n t from the r e s u l t s of p rev ious runs and the pH throughout the r e a c t i o n s i n both e x p l o r a t o r y runs has been b u f f e r e d to a v a l u e o f s l i g h t l y g r e a t e r than 8 , no supergene enrichment i s to be expected . The p r o j e c t e d e v o l u t i o n a l paths of the r e a c t i n g water of these r u n s , i f p l o t t e d i n F i g u r e 1 4 , would l i e p a r a l l e l and c l o s e to the r i g h t of Run #2. I n c i d e n t a l l y , at A f t o n Mines , the re e x i s t s no d i s c r e t e , i n t e n s e zone of t e n o r i t e or c u p r i t e m i n e r a l i z a t i o n . E r o s i o n p r o i r to the d e p o s i t i o n o f the p r o t e c t i v e Eocene s t r a t a i s the most l i k e l y e x p l a n a t i o n f o r t h e i r absence, d) Other c o n s i d e r a t i o n s So f a r , d i s c u s s i o n s have been focused on the c h e m i c a l a s p e c t s of supergene a l t e r a t i o n w i t h l i t t l e a t t e n t i o n p a i d to p h y s i c a l parameters l i k e the p o r o s i t y and p e r m e a b i l i t y of the host r o c k s , the depth of the p r e v a l e n t water t a b l e , c l i m a t i c c o n d i t i o n s accompanying the a l t e r a t i o n s et c e t e r a . B r i e f l y , these parameters a f f e c t the rock to f l u i d r a t i o which i n t u r n c o n t r o l s the ex tent of r e a c t i o n d e s c r i b e d above. The case o f an ex t remely low rock to f l u i d r a t i o such that w a l l rock minera logy f a i l s to a f f e c t the c h e m i s t r y of the r e a c t i n g s o l u t i o n to any s i g n i f i c a n t ex tent i s i r r e l e v a n t h e r e . Other than t h a t , a low r a t i o of r o c k to f l u i d c o u l d , i f g i v e n enough t ime f o r r e a c t i o n s to f u l l y p roceed , l e a d to a complete d e s t r u c t i o n of the 71 p r imary m i n e r a l assemblage. In c o n t r a s t , a h i g h r a t i o of rock to f l u i d would r e s u l t i n a ready e q u i l i b r a t i o n o f the r e a c t i n g s o l u t i o n w i t h the w a l l r o c k , thus enhancing the p r e s e r v a t i o n of p r imary s u l f i d e s i n the supergene zone. In column a d s o r p t i o n chromatography, g i ven i d e n t i c a l c o n d i t i o n s , a l onger column g e n e r a l l y f a c i l i t a t e s b e t t e r s e p a r a t i o n of components. By ana logy , the depth of the p r e v a l e n t water t a b l e , coupled w i t h other p h y s i c a l pa ramete rs , a f f e c t s the ex tent of development of a leached c a p , determines whether t h e r e i s a w e l l - d e f i n e d s u c c e s s i o n of secondary copper m i n e r a l s and whether t e l e s c o p i n g of a l t e r a t i o n minera logy o c c u r s . As presented i n P a r t I, supergene a l t e r a t i o n at A f t o n probab ly took p l a c e a long deep f r a c t u r e s c h a r a c t e r i z e d by a moderately h igh r a t i o of rock to s o l u t i o n , w i t h the consequence t h a t hypogene copper s u l f i d e s are l o c a l l y important i n the supergene zone. 72 Summary and Conc lus ion In t h i s s e c t i o n , a thermodynamic approach to the unders tand ing of the supergene a l t e r a t i o n at A f t o n Mines has been p resented . Whereas i t i s not p o s s i b l e t o reproduce the d e p o s i t i o n sequence of secondary s i l i c a t e m i n e r a l s due t o the l a c k of r e l e v a n t thermodynamic d a t a , c o n s t r a i n t s i n r e a c t i o n k i n e t i c s and l i m i t a t i o n s o f c l o s e d - s y s t e m c a l c u l a t i o n s , v a l i d c o n c l u s i o n s r e g a r d i n g the s e q u e n t i a l changes i n minera logy i n the C u - F e - S - 0 system can s t i l l be drawn. I t i s demonstrated tha t the presence of an overwhelming amount of n a t i v e copper and the l a c k of supergene enrichment at A f t o n Mines are caused by the combined e f f e c t of the r e l a t i v e l y m a f i c c o m p o s i t i o n o f the host rocks and the low s u l f i d e content of the hypogene m i n e r a l assemblage. As a r e s u l t , w a t e r - r o c k i n t e r a c t i o n s d u r i n g the supergene event are c o n s t r a i n e d to occur w i t h i n an a l k a l i n e pH domain. Supergene a l t e r a t i o n of porphyry copper d e p o s i t s i n genera l can c o n v e n i e n t l y be i n t e r p r e t e d i n the framework of a l o g f - l o g f p l o t w i t h r e s p e c t to two °2 b 2 a r b i t r a r y " l i m i t i n g " c u r v e s , which a r e c h a r a c t e r i z e d by extreme v a l u e s of a c i d i c and b a s i c pH r e s p e c t i v e l y . The c l o s e r the r e a c t i n g f l u i d approaches the upper o r a c i d i c l i m i t i n g c u r v e , the more l i k e l y i s the f o r m a t i o n of a w e l l - d e v e l o p e d leached cap w i t h an e n r i c h e d supergene ore b l a n k e t underneath . In c o n t r a s t , a r e a c t i n g f l u i d e v o l v i n g c l o s e to the lower or b a s i c l i m i t i n g curve would r e s u l t i n a spectrum of supergene copper ox ides and n a t i v e copper w i t h l i t t l e supergene enr ichment . 73 PART I I I MAJOR ELEMENT GEOCHEMISTRY AND ITS IMPLICATIONS ON THE EVOLUTION AND COPPER MINERALIZATION OF THE VARIOUS INTRUSIVE PHASES OF THE IRON MASK BATHOLITH y 74 I n t r o d u c t i o n The I r o n Mask b a t h o l i t h has l o n g been known f o r i t s minor magnet i te and copper m i n e r a l i z a t i o n ( C o c k f i e l d , 1948 and C a r r , 1956) . The d i s c o v e r y of the A f t o n orebody l o c a t e d at the nor thwestern e x t r e m i t y of the I ron Mask p l u t o n has l e d to a renewal of e x p l o r a t i o n a c t i v i t y i n the d i s t r i c t . The recent s t u d i e s of P re to (1967) and Nor thcote (1974, 1976 and 1977) f u r n i s h e d v a l u a b l e i n f o r m a t i o n on the s t r u c t u r e and g e n e r a l geology of the p l u t o n and shed l i g h t on the m i n e r a l p o t e n t i a l of the v a r i o u s phases o f the p l u t o n . Upon s t u d y i n g the t r a c e element contents o f magnet i te from the p l u t o n , Cann (1979) suggested a consanguineous r e l a t i o n s h i p between the i r o n and copper m i n e r a l i z a t i o n and the d i o r i t e - and s y e n i t e - d o m i n a t e d p h a s e s - o f the p l u t o n r e s p e c t i v e l y . Few major element geochemical d a t a , however, were p resented i n these s t u d i e s to support the v a r i o u s i n t e r p r e t a t i o n s made. In the present s t u d y , major element compos i t ions of the prominent phases of the p l u t o n are presented and d i s c u s s e d i n c o n j u n c t i o n w i t h the host rock geochemistry of the A f t o n orebody. Re levant p e t r o g r a p h i c f e a t u r e s are a l s o cons idered to a i d i n t e r p r e t a t i o n s of f i e l d and geochemica l r e l a t i o n s h i p s . The main purpose of the e x e r c i s e i s to e l u c i d a t e the d e t a i l s of the processes i n v o l v e d i n the e v o l u t i o n of the I ron Mask b a t h o l i t h and t h e i r s i g n i f i c a n c e to copper m i n e r a l i z a t i o n . Genera l Geology and Petrography of the I ron Mask P l u t o n The g e o l o g i c a l s e t t i n g o f the I r o n Mask b a t h o l i t h has been p r e s e n t e d i n P a r t I. To p r o v i d e the necessary background f o r the i n t e r p r e t a t i o n o f the major element geochemical d a t a , i t s u f f i c e s here to summarize the d e s c r i p t i o n o f Nor thcote (1974, 1976 and 1977) of the geology of the I r o n 75 Mask p l u t o n i t s e l f . The I ron Mask p l u t o n i s c o n s i d e r e d to be a s u b v o l c a n i c , m u l t i p l e i n t r u s i o n which i s comagmatic and c o e v a l w i t h Upper T r i a s s i c v o l c a n i c r o c k s of the N i c o l a Group. F i v e d i f f e r e n t phases of the i n t r u s i o n have been recogn i zed (F igure 17) . The I r o n Mask H y b r i d u n i t , the o l d e s t o f a l l based on c r o s s - c u t t i n g r e l a t i o n s h i p s , i s most ly a g m a t i t i c , c o n s i s t i n g of rounded and angu lar fragments of v a r i o u s s i z e s , t e x t u r e and c o m p o s i t i o n i n a d i o r i t e m a t r i x . The f ragments , which make up more than 80 per cent by volume of a t y p i c a l outcrop of t h i s u n i t , i n c l u d e main ly coarse and f i n e g ra ined d i o r i t e and coarse g r a i n e d gabbro w i t h l e s s e r amounts of medium and coarse g ra ined h o r n b l e n d i t e and s c a t t e r e d x e n o l i t h s of N i c o l a Group v o l c a n i c r o c k s . They are i n t e r p r e t e d to represent s l i g h t l y o l d e r i n t r u s i v e e q u i v a l e n t s of lower N i c o l a v o l c a n i c rocks c a r r i e d up to a h i g h e r l e v e l by resurgence of magmatic a c t i v i t y . The Pothook u n i t i s u n i f o r m i l y of d i o r i t i c compos i t ion and i s medium to coarse g r a i n e d . Rocks o f t h i s u n i t f r e q u e n t l y d i s p l a y g r a d a t i o n a l c o n t a c t s w i t h those of the I r o n Mask Hybr id u n i t and are consequent ly b e l i e v e d to be most c l o s e l y r e l a t e d to the l a t t e r . The p i c r i t e u n i t i s composed of rocks o f b a s a l t i c compos i t ion w i t h abundant s e r p e n t i n i z e d o l i v i n e and c l i n o p y r o x e n e . The g e n e t i c r e l a t i o n s h i p of t h i s u n i t w i t h the o ther phases of the p l u t o n i s not c l e a r . Rocks o f the S u g a r l o a f u n i t are main ly p o r p h y r i t i c w i t h ho rnb lende , minor pyroxene and p l a g i o c l a s e phenocrysts and are of f a i r l y un i fo rm d i o r i t e - a n d e s i t e c o m p o s i t i o n . F ine g ra ined v a r i e t i e s of t h i s u n i t are not r e a d i l y d i s t i n g u i s h e d from the N i c o l a v o l c a n i c f l o w s . The Cherry Creek u n i t , the youngest among the f i v e , c o n s i s t s of rocks w i t h compos i t ion ranges through d i o r i t e , monzoni te , s y e n i t e and t h e i r p o r p h y r i t i c and f i n e g r a i n e d e q u i v a l e n t s . They are b e l i e v e d to d e r i v e from s m a l l , l o c a l l i z e d 76 [W-n-KA-7 l-20l±6 -72-KA-3 l-20l±6 -72-KA-4 1-2091 6 VP-72-KA B-I94J6 £ 5 n. !i 3 a: I-L E G E N D KAMLOOPS GROUP -EROSIONAL UNCONFORMITY NICOLA GROUP POTASSIUM FELDSPATHIZATION \ AND MINERALIZATION 5 CHERRY CREEK IRON 4 SUGAR LOAF MASK 3 PICRITE BATHOLITH 2 POTHOOK 1 IRON MASK HYBRID SYMBOLS ZONE OF INTENSE BRECCIATION LINEARS FROM AIR PHOTOGRAPHS MAJOR HIGHWAY SECONDARY ROAD _ SAMPLE LOCATION CROSS-HATCHING INDICATES SUPERIMPOSED UNITS S C A L E : K I L O M E T R E S 2 3 t Geology after Northcote, 1977. Figure 17. General geology of the Iron Mask pluton (modified from Preto et a l . , 1979). 77 d i f f e r e n t i a t i n g o f f s h o o t s i n t r u d i n g i n t o a w i d e l y y a r i e d p h y s i c a l and chemica l environment. C o m p o s i t i o n a l d i f f e r e n c e s are m a i n l y i n the amount of K - f e l d s p a r . Whereas minor copper m i n e r a l i z a t i o n has been d e t e c t e d i n a l l phases of the b a t h o l i t h , major c o n c e n t r a t i o n s of copper l i k e t h a t of the A f t o n orebody are b e l i e v e d to be c o e v a l w i t h the K - f e l d s p a r metasomatism. As w i l l be d i s c u s s e d l a t e r , the author d i sag rees w i t h t h i s i n t e r p r e t a t i o n . A f t o n Mines l i e s a t the western t e r m i n a t i o n of a nar row, e a s t e r l y t r e n d i n g zone of i n t e n s e i n t r u s i v e b r e c c i a t i o n l o c a t e d a t the n o r t h e r n edge of the p l u t o n . Th is b r e c c i a t i o n i n v o l v e s main ly rocks of the Cherry Creek u n i t and i s cons idered to have r e s u l t e d from h i g h l e v e l v e n t i n g e v e n t s . As d i s c u s s e d i n P a r t I, a few l a t i t e dykes are a l s o p resent i n the A f t o n o p e n - p i t . Whi le s t i l l cons ide red to be p r e - T e r t i a r y , they a re d e f i n i t e l y l a t e r than the pr imary copper m i n e r a l i z a t i o n . With r e s p e c t to d e t a i l e d pet rography , Cann (1979) has f u r n i s h e d p o i n t c o u n t i n g data on 25 rock specimens c o v e r i n g t y p i c a l v a r i e t i e s of rocks of the Cherry Creek, Pothook and P i c r i t e u n i t s . P r e t o (1967) i n c l u d e d a d e t a i l e d p e t r o g r a p h i c d e s c r i p t i o n of the Sugar loaf and Cherry Creek i n t r u s i o n s i n h i s d i s c u s s i o n on the geology of the e a s t e r n p a r t of the I r o n Mask b a t h o l i t h . S a l i e n t m i n e r a l o g i c a l and t e x t u r a l c h a r a c t e r i s t i c s of rocks w i t h i n the A f t o n o p e n - p i t have been presented i n P a r t I. These, toge ther w i t h t h i n - s e c t i o n s t u d i e s of an a d d i t i o n a l e i g h t specimens of the S u g a r l o a f and I r o n Mask Hybr id rocks and h a n d - l e n s examinat ion of the s u i t e of 39 specimens ana lysed i n t h i s s t u d y , a l l o w the p e t r o g r a p h i c c h a r a c t e r i z a t i o n of the v a r i o u s rock u n i t s . P e r t i n e n t data are summarized i n Table I I I . In s p i t e of many i n - g r o u p v a r i a t i o n s , p a r t i c u l a r l y i n the I r o n Mask H y b r i d and Cherry Creek u n i t s , i t i s ev ident t h a t there e x i s t s between-group d i f f e r e n c e s which may be r e l a t e d to the process of d e r i v a t i o n o f the v a r i o u s u n i t s . 78 TABLE I I I . PETROGRAPHIC CHARACTERISTICS OF VARIOUS ROCK UNITS OF THE IRON MASK BATHOLITH Rock u n i t * P i c r i t e I r o n Mask H y b r i d M inerao lgy o l i v i n e phenocrysts >20% c l inopy roxene phenocrysts f r e q u e n t l y >15% f e l d s p a r p r a c t i c a l l y absent accessory p h l o g o p i t e and magnet i te fragments of v a r i a b l e compos i t ion but gabbro i s most common; t y p i c a l modes: c l inopy roxene 55% p l a g i o c l a s e >25% magnet i te >10% accessory hornblende ± b i o t i t e no o l i v i n e S a l i e n t t e x t u r e p o r p h y r i t i c w i t h a p h a n i t i c to very f i n e g ra ined m a t r i x i n excess of 35%, c . f . b a s a l t o c c a s i o n a l l a y e r e d appearance most l y a g m a t i t i c w i t h medium to coarse g r a i n e d d i o r i t e as m a t r i x gabbro fragments f r e q u e n t l y show apparent s e g r e g a t i o n banding ( e . g . P l a t e IV) Pothook S u g a r l o a f Cherry Creek c l inopy roxene 25% p l a g i o c l a s e 50% magnet i te 6-15% accessory b i o t i t e , hornblende ± K - f e l d s p a r hornblende >45% p l a g i o c l a s e <45% accessory magnet i te and K - f e l d s p a r t o t a l f e l d s p a r =70% w i t h K - f e l d s p a r g e n e r a l l y >20% c l inopy roxene 2-30% but g e n e r a l l y <15% magnet i te 2-10% v a r i a b l e amounts of accessory b i o t i t e and hornblende medium to coarse g r a i n e d , p a r t l y cumulate (Cann,1980) p o r p h y r i t i c ; t y p i c a l l y hypabyssa l w i t h a l i g n e d p l a g i o c l a s e and hornblende t y p i c a l l y f i n e g r a i n e d , p o r p h y r i t i c ; m i c r o -v a r i e t i e s common; o c c a s i o n a l b r e c c i a L a t i t e dykes at A f t o n t o t a l f e l d s p a r >60% K - f e l d s p a r g e n e r a l l y >20% hornblende g e n e r a l l y >20% quar tz 5-10% no magnet i te i n v a r i a b l y m a s s i v e , f i n e g r a i n e d and s l i g h t l y p o r p h y r i t i c i n t e r s t i t i a l and eyes of quar t z and K - f e l d s p a r common Note tha t the rock u n i t s , w i t h the e x c e p t i o n of P i c r i t e , a re d e s c r i b e d ' i n o rder of younger age to b r i n g out the resemblance of the s e q u e n t i a l m i n e r a l o g i c a l change to Bowen's r e a c t i o n s e r i e s . See t e x t f o r d i s c u s s i o n . 79 For example, down the column under m i n e r a l o g y , the r e p o r t e d m i n e r a l s rough ly cor respond to Bowen's (1928) r e a c t i o n s e r i e s f o r the c r y s t a l l i z a t i o n of an i d e a l m a f i c magma. The prominent p e t r o g r a p h i c f e a t u r e s w i l l be d i s c u s s e d l a t e r i n c o n j u n c t i o n w i t h major element geochemist ry of the r o c k s . 80 R e s u l t of Major Element Ana lyses 39 samples from the I ron Mask p l u t o n and i t s v i c i n i t y and 58 samples from the A f t o n p r o p e r t y i t s e l f were ana lysed f o r n i n e major elements u s i n g the atomic a b s o r p t i o n s p e c t r o s c o p i c method d e s c r i b e d i n Ralph and Chaudhry ( i n p r e p a r a t i o n ) . The r e s u l t s are t a b u l a t e d i n Appendix I I toge ther w i t h a b r i e f d e s c r i p t i o n of each sample a n a l y s e d . The c h e m i c a l data are a l s o summarized i n Table IV, i n which each rock group i s d e s c r i b e d by the mean and s tandard d e v i a t i o n of each of i t s n ine c o n s t i t u e n t major o x i d e s . Assuming t h a t a l t e r a t i o n of the rocks r e s u l t s ma in l y i n the a d d i t i o n of v o l a t i l e components, the d e v i a t i o n of the t o t a l sum of major ox ides from a hundred can be i n t e r p r e t e d to be i n d i c a t i v e of the ex tent of a l t e r a t i o n . In t h i s r e s p e c t , the i n t r u s i v e r o c k s , w i t h the e x c e p t i o n of those c o l l e c t e d from the A f t o n p r o p e r t y , are l e s s a l t e r e d than the e x t r u s i v e ones . The group of a l t e r e d Cherry Creek rocks at A f t o n i s comprised most l y of m i n e r a l i z e d samples secured from the o p e n - p i t and the n i n e major ox ides of which sum to l e s s than 93 per cent by w e i g h t . The group of f r e s h Cherry Creek rocks a t A f t o n , on the o ther hand, are l e a s t a l t e r e d samples c o l l e c t e d ad jacent t o the orebody and which show l i t t l e s i g n of copper m i n e r a l i z a t i o n . These two groups of rocks w i l l be r e f e r r e d to below as a l t e r e d and f r e s h A f t o n host r o c k s r e s p e c t i v e l y . The group of I r o n Mask H y b r i d r o c k s i n c l u d e , i n p r o p o r t i o n to the a c t u a l f i e l d o c c u r r e n c e , the most abundant v a r i e t i e s of coarse g r a i n e d fragments i n the a g m a t i t i c exposures , f i n e to medium g ra ined and m a g n e t i t e - r i c h i n t r u s i v e b r e c c i a and coarse g r a i n e d d i o r i t e b e l i e v e d to be t r a n s i t i o n a l to r o c k s of the Pothook u n i t . The h e t e r o g e n e i t y of rock types ana lysed i s r e f l e c t e d i n the h igher v a r i a b i l i t y i n most of the n i n e major elements i n comparison to t h a t of the o ther r o c k groups , whereas data 81 TABLE IV. MAJOR ELEMENT GEOCHEMISTRY OF VARIOUS ROCK UNITS MAKING UP OR ASSOCIATED WITH THE IRON MASK PLUTON S10 2 AljOj F c 2 0 3 MgO CaO K20 NajO TiOj MnO Total Tertiary Volcanics x 3 53.48 16.77 8.44 3.98 5.85 2.66 3.79 1.64 0.13 96.72 s 5.47 1.08 3.60 1.29 2.07 1.27 0.17 1.12 0.05 2.45 Tertiary Volcanics at Afton x"2 44.02 14.10 7.28 5.57 7.87 2.19 3.38 1.06 0.13 85.58 s 4.21 0.59 0.73 0.67 1.19 0.01 0.40 0.11 0.02 2.69 Latites 75 62.38 15.28 4.66 3.34 1.16 2.67 4.41 0.72 0.04 94.65 a 4.03 0.69 1.17 1.29 1.09 0.60 0.68 0.31 0.03 1.17 Fresh Cherry Creek Rocks at Afton (fresh Afton host rocks) "19 52.99 17.90 5.97 3.84 4.87 2.91 5.20 0.75 0.19 94.50 s 3.10 1.13 2.51 1.01 2.25 2.16 1.92 0.12 0.11 1.11 Altered Cherry Creek Rocks at Afton (altered Afton host rocks) 7 49.88 17.13 6.34 4.22 4.58 2.34 5.05 0.77 0.22 90.51 s 3.96 0.99 2.80 1.18 2.20 1.50 1.33 0.10 0.19 2.37 Cherry Creek Rocks x 1 Q 57.23 18.06 5.18 2.20 4.46 3.51 5.54 0.59 0.10 96.87 a 4.45 1.37 3.02 1.41 1.81 2.93 2.10 0.21 0.04 1.90 Sugarloaf Rocks x, 49.37 17.13 10.47 5.34 7.71 2.44 3.57 0.64 0.09 96.77 2.92 1.51 4.19 0.99 1.76 0.90 0.84 0.12 0.03 1.04 s Picrite *2 s 41.89 5.13 9.50 30.30 4.71 1.08 0.37 0.21 0.16 93.32 1.54 1.51 0.05 6.08 1.80 0.42 0.25 0.08 0.02 0.45 Pothook Diorite x. 46.26 17.54 11.42 5.66 11.10 1.73 2.73 0.82 0.21 97.46 4 s 4.00 0.79 2.27 1.10 1.44 0.88 1.29 0.16 0.04 0.78 Iron Mask Hybrid rocks x"9 43.44 16.20 12.64 7.57 14.19 1.09 1.43 0.93 0.16 97.64 s 3.60 4.70 4.50 2.75 2.92 0.77 1.09 0.16 0.04 0.97 ** Nicola Volcanics xfi 47.78 16.56 8.53 4.73 8.45 2.67 3.39 0.69 0.17 92.97 s 3.18 1.53 2.61 2.32 1.76 1.69 0.70 0.18 0.06 2.12 Symbols: x n - mean of n samples s - standard deviation * Only five least altered samples are included. ** Nicola volcanic rocks within the Afton open-pit are not included. 82 p resented i n Table i y shed l i g h t on the v a r i a t i o n w i t h i n a r o c k group and d i f f e r e n c e s among the groups w i t h r e s p e c t to chemica l c o m p o s i t i o n , c o m p o s i t i o n a l t rends and s u b t l e d i f f e r e n c e s among r e l a t e d rock groups a re more r e a d i l y r e v e a l e d i n v a r i a t i o n d iagrams. To a f f o r d comparison t o be made on an equa l b a s i s , data p resented i n Appendix I I have been r e c a s t to a dry b a s i s b e f o r e b e i n g p l o t t e d onto the v a r i a t i o n diagrams d i s c u s s e d be low. F i g u r e 18 p resents Harker p l o t s of the ana lyses of the f o u r major i n t r u s i v e phases of the I ron Mask b a t h o l i t h , namely the I ron Mask H y b r i d , Pothook, Sugar loaf and Cherry Creek u n i t s , which are cons idered to bear c l o s e g e n e t i c r e l a t i o n s h i p s ( N o r t h c o t e , 1974, 1976 and 1977 and Cann, 1979) . In g e n e r a l , i n s t e a d of c l u s t e r i n g to form d i s c r e t e domains, a n a l y s e s of s u c c e s s i v e rock phase tend to have s i g n i f i c a n t o v e r l a p . However, i t i s e v i d e n t t h a t p r o g r e s s i v e l y younger u n i t s are c h a r a c t e r i z e d by an i n c r e a s i n g s i l i c a c o n t e n t . Fur thermore , c o n s i d e r i n g the ana l yses as a whole and a l l o w i n g f o r minor e x c e p t i o n s , s e v e r a l smooth t rends are r e a d i l y d i s c e r n a b l e from the p l o t s . Thus, CaO, MgO and t o t a l i r o n as ~Fe^O^ p r o m i n e n t l y decrease w i t h i n c r e a s i n g s i l i c a c o n t e n t . S i m i l a r v a r i a t i o n s of MnO and TIO^ w i t h s i l i c a , w h i l e s t i l l appear ing to be t r u e , are l e s s c o n v i n c i n g because of the r e l a t i v e l y g rea te r s c a t t e r and s m a l l e r range of v a r i a b i l i t y o f the MnO and T i 0 2 d a t a . Save the e x c e p t i o n of a few s i l i c a - p o o r samples o f the I r o n Mask Hybr id u n i t , which c o n t a i n ex t remely to moderate ly low a l u m i n a , the a lumina content of the ana lysed r o c k s do not vary much w i t h s i l i c a . K^O and Na^O g e n e r a l l y i n c r e a s e w i t h s i l i c a but much s c a t t e r i s e v i d e n t i n the h i g h - s i l i c a end of both p l o t s . For the P 2 U 5 versus S i 0 2 p l o t , r o c k s o f the I r o n Mask Hybr id u n i t are o b v i o u s l y low i n P2°5> Otherwise , s c a t t e r of d a t a p r e c l u d e s any s i g n i f i c a n t c o r r e l a t i o n to be made. The ev ident c o m p o s i t i o n a l t r e n d s i n the Harker p l o t s a re d e p i c t e d 83 LEGEND a • V O Analyses of Cherry Creek rocks Analyses of the Sugarloaf unit. Analyses of the Pothook unit Analyses of the Iron Mask Hybrid unit Average of the respective rock groups W W Q M X o 0.4 0.2 0.0 MnO, 20 U 10 CaO _ l _ _L_ _J_ 40 50 60 70 WT. Z SiO„ Figure 18. Harker plots of analyses of the major intrusive phases of the Iron Mask pluton. Compositional trends (solid lines) have been constructed by fitting a smooth curve (estimated by eye) through the average analysis of each rock group. Alternative interpretations in the respective K_0, Na 0 and TiO. versus SiO plots are shown in broken lines. 84 Figure 18 (continued). 85 w o M X o 0.5 P 2 ° 5 o.oL • •• F i g u r e 18 ( c o n t i n u e d ) . 86 F i g u r e 18 ( c o n t i n u e d ) . 87 i n s o l i d l i n e s . These have been c o n s t r u c t e d by f i t t i n g a smooth curve ( e s t i m a t e d by eye) through the average a n a l y s i s ( a r i t h m e t i c mean of a l l a n a l y s e s ) of each rock group. D i s t r i b u t i o n of data p o i n t s i n the Harker p l o t s f o r K^O, N a 2 0 and TiC^ suggests t h a t a l t e r n a t i v e t rends are p o s s i b l e f o r a n a l y s e s w i t h g rea te r than about 55% s i l i c a . At t h i s apparent t h r e s h o l d s i l i c a content and f o r the cases of K^O and N a 2 0 , the i n i t i a l sympathet ic v a r i a t i o n curve appears to branch out i n t o two, one d e p i c t i n g a sympathet ic and the o t h e r an a n t i p a t h e t i c r e l a t i o n s h i p w i t h s i l i c a . For the v a r i a t i o n of TiC>2 w i t h s i l i c a , a l o c a l maximum i s apparent at a s i l i c a v a l u e of about 57%. These a l t e r n a t i v e i n t e r p r e t a t i o n s of t r e n d s a re d e p i c t e d i n each Harker p l o t i n broken l i n e s . Because d i f f e r e n t i a t i o n of a m a f i c magma l e a d s g e n e r a l l y to a p r o g r e s s i v e i n c r e a s e i n s i l i c a content i n the more evo lved l i q u i d (except f o r the l a t e d i f f e r e n t i a t e s of an undersa tu ra ted a l k a l i c magma), c o m p o s i t i o n a l t rends e s t a b l i s h e d i n F i g u r e 18 are i n f a c t approx imat ions to the d i f f e r e n t i a t i o n t r e n d s of the I ron Mask p l u t o n i f the i n v e s t i g a t e d u n i t s were de r i ved f rom. the same parent magma. For th ree reasons a more e l a b o r a t e method of d e r i v a t i o n of the c o m p o s i t i o n a l t rends has not been u s e d . F i r s t , i n s t e a d of b e i n g r e p r e s e n t a t i v e specimens secured from known s t r a t i g r a p h i c h o r i z o n s of a w e l l - l a y e r e d i n t r u s i v e body, the ana lysed s u i t e of samples represent on ly t y p i c a l v a r i e t i e s of r o c k s i n each major u n i t of the I r o n Mask p l u t o n , which i s d i s t i n g u i s h e d m a i n l y on t e x t u r a l e v i d e n c e . The l i m i t e d number of samples ana lysed and the i n t r a - g r o u p v a r i a t i o n s as ev ident i n outcrop exposures argue a g a i n s t the u s e f u l n e s s of d e t a i l e d r e g r e s s i o n a n a l y s i s of any i n d i v i d u a l data group. Second, the c o m p o s i t i o n a l t rends are c o n s t r u c t e d m a i n l y to serve as r e f e r e n c e f o r compar ison of data w i t h other r e l a t e d r o c k groups. The s imple approach p resented here s a t i s f a c t o r i l y serves the purpose . T h i r d , a l though h i g h l y 88 a l t e r e d samples have been e x c l u d e d , most ana lysed rocks s t i l l show s i g n s o f minor a l t e r a t i o n which cou ld be of d e u t e r i c , metasomat ic , hydro thermal or metamorphic i n o r i g i n . Whereas the net change i n major element geochemist ry r e s u l t i n g from these minor a l t e r a t i o n s c o u l d be s m a l l , a b s o l u t e t r e n d s a r e u n l i k e l y to be a c q u i r e d . F i g u r e 19 d e p i c t s the v a r i a t i o n of the ana lyses of the f o u r major i n t r u s i v e phases of the I r o n Mask p l u t o n i n a t r i a n g u l a r AFM p l o t . With a few e x c e p t i o n s , a n a l y s e s of s u c c e s s i v e u n i t s f o l l o w a path s u b p a r a l l e l t o the t r e n d of d i f f e r e n t i a t i o n of a c a l c - a l k a l i n e magma. I f the f o u r u n i t s share a common source magma, t h e n , as i s ev ident from the p l o t , the d i f f e r e n t i a t i o n of the I r o n Mask p l u t o n must have i n v o l v e d l i t t l e i r o n enr ichment throughout the p r o c e s s . F i g u r e 20 compares ana lyses of the two groups of A f t o n hos t r o c k s , the l a t i t e dykes of the A f t o n p i t , N i c o l a Group v o l c a n i c rocks and the P i c r i t e samples w i t h those of the four major i n t r u s i v e phases of the I r o n Mask p l u t o n . The c o m p o s i t i o n a l f i e l d s of the l a t t e r are approximated by c o n s t r u c t i n g the s m a l l e s t convex polygon c o n t a i n i n g a l l a n a l y s e s of the same u n i t . To a l l o w f o r a l t e r n a t i v e t rends i n the v a r i a t i o n of I^O and Na^O w i t h s i l i c a , a r e - e n t r a n t polygon i s drawn to c o n t a i n the Cherry Creek a n a l y s e s i n the Harker p l o t s f o r these two o x i d e s . In g e n e r a l , the range i n s i l i c a of the ana l ysed rocks of the N i c o l a Group approximates t h a t of the S u g a r l o a f i n t r u s i v e phase . The A f t o n host rocks show a l a r g e range i n s i l i c a but most a n a l y s e s l i e c l o s e to a r e g i o n of o v e r l a p p i n g Sugar loaf and Cherry Creek a n a l y s e s , the co r respond ing rock specimens of which were c o l l e c t e d away f rom the A f t o n p r o p e r t y . The a n a l y s e s of bo th the N i c o l a v o l c a n i c r o c k s and the A f t o n hos t r o c k s f a l l i n the c l o s e v i c i n i t y of most of the c o m p o s i t i o n a l t r e n d s e s t a b l i s h e d i n the Harker p l o t s f o r the f o u r major i n t r u s i v e phases of Figure 19. A total alkali - total iron - magnesia triangular plot of the analyses of the four major intrusive phases of the Iron Mask pluton. The Skaergaard and idealized calc-alkaline trends are adapted from Ernst (1976, figure 4.50). 90 LEGEND Z "1 Compositional f i e l d of Cherry Creek analyses i . . C o m p o s i t i o n a l f i e l d of Sugarloaf analyses I I Compositional f i e l d of Pothook analyses I I Compositional f i e l d of Iron Mask Hybrid analyses X Latite dykes within Afton * Altered Afton host rocks • Relatively fresh Afton host rocks_ O P i c r i t e analyses E> Nicola Croup volcanic rocks F i g u r e 20 . A compar ison of a n a l y s e s of v a r i o u s rock groups r e l a t e d to the I r o n Mask b a t h o l i t h w i t h those of the four major i n t r u s i v e phases of the I r o n Mask p l u t o n . The c o m p o s i t i o n a l t rend of the l a t t e r i n each Harker p l o t ( s o l i d curve) has been reproduced from F i g u r e 18 . F i g u r e 20 ( c o n t i n u e d ) , 92 F i g u r e 20 ( c o n t i n u e d ) . F i g u r e 20 ( c o n t i n u e d ) . 94 the I ron Mask p l u t o n . The on ly e x c e p t i o n i s observed i n the Harker p l o t f o r MnO, i n which some of the A f t o n host rocks and , to a l e s s e r e x t e n t , some N i c o l a v o l c a n i c rocks are a p p a r e n t l y e n r i c h e d i n MnO. Because the sum of n ine major o x i d e s of these rocks i s g e n e r a l l y low compared to t h a t of the f r e s h i n t r u s i v e r o c k s , the apparent MnO enrichment may p a r t i a l l y be caused by the r e n o r m a l i z a t i o n of the raw d a t a . Otherwise , hydro thermal a l t e r a t i o n and/or weather ing of these rocks p robab ly concent ra te manganese i n these r o c k s i n p r e f e r r e n c e to o ther e lements . L y i n g r e s p e c t i v e l y i n the low and h i g h s i l i c a end o f the Harker p l o t s , the a n a l y s e s of the P i c r i t e u n i t and the l a t i t e dykes w i t h i n A f t o n show s i g n i f i c a n t d e v i a t i o n from the o u t l i n e d c o m p o s i t i o n a l t r e n d s except i n the MnO and p o s s i b l y the K^O p l o t s . These r o c k s , i n comparison w i t h the N i c o l a v o l c a n i c rocks and the A f t o n host r o c k s , p robab ly bear a l e s s d i r e c t g e n e t i c r e l a t i o n s h i p w i t h the major i n t r u s i v e phases of the I r o n Mask p l u t o n . In d e t a i l , the m a j o r i t y of a n a l y s e s of the two groups of A f t o n host r o c k s l i e above the c o m p o s i t i o n a l t r e n d s i n the Na^O, MgO, T i 0 2 and A l j O ^ Harker p l o t s and below the c o m p o s i t i o n a l t rends i n the CaO and ^ 2 ^ 3 p l o t s . T h e r e f o r e , the A f t o n host r o c k s may be c h a r a c t e r i z e d by a s l i g h t enrichment i n the former group of o x i d e s and a s l i g h t d e p l e t i o n i n the l a t t e r group of o x i d e s when cons ide red i n the p r o g r e s s i v e change i n c o m p o s i t i o n of the major i n t r u s i v e u n i t s of the p l u t o n . One c o m p o s i t i o n a l d i f f e r e n c e between the f r e s h and a l t e r e d groups of A f t o n hos t rocks i s ev ident i n the v a r i a t i o n p l o t s of r e s p e c t i v e l y K^O and N a 2 0 ve rsus s i l i c a . A g r e a t e r p r o p o r t i o n of the ana lyses of the a l t e r e d group, i n comparison w i t h the a n a l y s e s of the f r e s h group, l i e above the e s t a b l i s h e d c o m p o s i t i o n a l t r e n d i n the N a 2 0 p l o t and below the e s t a b l i s h e d t r e n d i n the K 2 0 p l o t . Thus, the a l t e r e d r o c k s appear to be r e l a t i v e l y e n r i c h e d i n N a 9 0 . 95 F i n a l l y , F i g u r e 21 f u r n i s h e s a t o t a l a l k a l i ve rsus s i l i c a p l o t f o r the ana l yses of p e r t i n e n t groups of r e l a t i v e l y f r e s h rocks r e l a t e d to the I r o n Mask p l u t o n . Whi le the m a j o r i t y of the data p o i n t s of the P i c r i t e u n i t and l a t i t e dykes w i t h i n A f t o n l i e w i t h i n the s u b a l k a l i n e f i e l d , almost a l l da ta p o i n t s f o r the r e s t of the ana l ysed rocks l i e i n the a l k a l i n e f i e l d . With a few e x c e p t i o n s , ana l yses w i t h low s i l i c a content l i e c l o s e r to the d i v i d i n g l i n e of MacDonald (1968) f o r d i s c r i m i n a t i n g a l k a l i n e r o c k s from s u b a l k a l i n e ones than those c h a r a c t e r i z e d by h i g h s i l i c a . I f the p l o t t e d r o c k s are g e n e t i c a l l y r e l a t e d at a l l , i t appears tha t t h e i r geochemical a f f i n i t y i s b e t t e r d e f i n e d w i t h more advanced degree of d i f f e r e n t i a t i o n . 12 Figure 21. A total-alkali versus silica plot for the analyses of various rock groups related to the Iron Mask pluton. 97 D i s c u s s i o n arid I n t e r p r e t a t i o n a) E v o l u t i o n of the I ron Mask b a t h o l i t h Three phases of the I ron Mask b a t h o l i t h , namely the I ron Mask H y b r i d , Pothook and Cherry Creek u n i t s , have been dated by the K -Ar method (Pre to e t . a l . , 1979) and shown to be i n d i s t i n g u i s h a b l e i n age by t h a t techn ique (see F igure 17 f o r the repor ted data and sample l o c a t i o n s ) . F i e l d ev idence ( P r e t o , 1967 and N o r t h c o t e , 1974, 1976 and 1977) i n d i c a t e s t h a t the r e l a t i v e age sequence i s I ron Mask Hybr id f o l l o w e d by Pothook, then Sugar loaf and l a s t l y Cherry Creek. Moreover , g r a d a t i o n a l c o n t a c t s a re common between the I r o n Mask Hybr id and Pothook u n i t s and , to a l e s s e r e x t e n t , between the S u g a r l o a f and Cherry Creek u n i t s . These i n t i m a t e age and g e o l o g i c a l c o n t a c t r e l a t i o n s h i p s combined w i t h the smooth c o m p o s i t i o n a l t rends and the o v e r l a p p i n g c o m p o s i t i o n a l boundar ies observed among these four phases i n the Harker p l o t s suggest a common parent magma. Many coarse g ra ined specimens of the I ron Mask Hybr id u n i t examined by the author show a l i g n e d c l i n o p y r o x e n e and/or p l a g i o c l a s e w i t h or w i thout s e g r e g a t i o n bands of magnet i te ( e . g . P l a t e IV) s u g g e s t i v e of c r y s t a l s e t t l i n g from a c r y s t a l l i z i n g magma. Cann (1979) a l s o r e p o r t e d cumulus t e x t u r e i n some rocks of the Pothook u n i t . In a d d i t i o n , upon d e s c r i b i n g the coarser g ra ined b a t h o l i t h i c r o c k s , which i n the p resent contex t are e q u i v a l e n t to those of the I ron Mask H y b r i d and Pothook u n i t s , Ca r r (1956, p.49) n o t e d , "An i n c o n s t a n t banding i s developed i n p l a c e , " . These o b s e r v a t i o n s suggest tha t the b a t h o l i t h evo lved m a i n l y by the p rocess of f r a c t i o n a l c r y s t a l l i z a t i o n . D i s r u p t e d development of cumulus t e x t u r e and the o v e r a l l decrease i n g r a i n s i z e from rocks of the I r o n Mask H y b r i d u n i t to those of the Pothook u n i t cou ld have r e s u l t e d from s low upward movements of the c o o l i n g magma i n i t i a t e d s h o r t l y a f t e r the commencement of 98 PLATE IV . An example of d i s c o n t i n u o u s s e g r e g a t i o n bands of magnet i te (b lack ) and p l a g i o c l a s e (white) observed i n gabbro specimens of the I r o n Mask Hybr id u n i t . The dark green m i n e r a l i s s u b a l i g n e d c l i n o p y r o x e n e . Magnet i te g r a i n s i n t h i s sample (74KN-279) a r e e u h e d r a l . In many o t h e r s , m a g n e t i t e i s i n t e r s t i t i a l to cumulate p l a g i o c l a s e and c l i n o p y r o x e n e . The b l a c k arrow marked on the specimen i s 1.5 cm. l o n g . 99 c r y s t a l l i z a t i o n . In c o n t r a s t , p o r p h y r i t i c and hypabyssa l t e x t u r e s c h a r a c t e r i s t i c of most r o c k s of the Sugar loaf u n i t suggest r a p i d t r a n s f e r of a c o o l i n g magma to s h a l l o w depths . The ana lyses of N i c o l a Group v o l c a n i c r o c k s , w h i l e conforming to the c o m p o s i t i o n a l t rend of the major i n t r u s i v e phases of the b a t h o l i t h , resemble the Sugar loaf ana l yses most. T h e r e f o r e , v e n t i n g of some magma d u r i n g the Sugar loaf stage seems h i g h l y l i k e l y . One of the consequences of the v e n t i n g event may have been the l o c a l i z a t i o n of some magma i n a s u b v o l c a n i c environment where the Cherry Creek phase s t a r t e d to c r y s t a l l i z e . Cont inuous b u i l d up of v o l a t i l e p ressure a p p a r e n t l y caused p e r i o d i c r u p t u r e of the roof of the magma chamber r e s u l t i n g i n the f o r m a t i o n of b r e c c i a s and r o c k s of d i f f e r e n t t e x t u r e and c h e m i c a l c o m p o s i t i o n . The g e n e t i c r e l a t i o n s h i p of the P i c r i t e u n i t and l a t i t e dykes w i t h the f o u r major i n t r u s i v e u n i t s of the b a t h o l i t h i s more d i f f i c u l t to a s s e s s . Wi th data p o i n t s b e i n g m a i n l y i n c o m p a t i b l e w i t h the e s t a b l i s h e d t r e n d s i n the Harker p l o t s and f a l l i n g i n t o the s u b a l k a l i n e f i e l d i n the p l o t of t o t a l a l k a l i ve rsus s i l i c a , complete independence from the major i n t r u s i v e phases seems a l o g i c a l c o n c l u s i o n . However, t h e i r minera logy and t e x t u r e as d e s c r i b e d i n Table I I I a re a t t r a c t i v e a d d i t i o n s to both ends of the p e t r o g r a p h i c sequence ( f o r the main phases of the b a t h o l i t h ) i n o rder to complete Bowen's (1928) r e a c t i o n s e r i e s . P re to (1967) b r a c k e t e d the r e l a t i v e age o f the P i c r i t e u n i t as b e i n g between the Pothook and S u g a r l o a f u n i t s . For the P i c r i t e u n i t to be i n c l u d e d i n the g e n e t i c scheme of the I r o n Mask b a t h o l i t h , a two -s tage model has to be i n v o l v e d : - i ) F o l l o w i n g the p r e c i p i t a t i o n of o l i v i n e and some c l i n o p y r o x e n e , the b u l k of the parent I r o n Mask magma was t r a n s f e r r e d to a secondary magma chamber where the d i f f e r e n t i a t i o n s e r i e s of I ron Mask H y b r i d - P o t h o o k - S u g a r l o a f - C h e r r y Creek proceeded as d i s c u s s e d b e f o r e ; i i ) the c r y s t a l mush l e f t beh ind i n the 100 pr imary magma chamber l a t e r i n t r u d e d the s o l i d i f i e d I ron Mask Hybr id and Pothook phases p r i o r to the v e n t i n g event o c c u r r i n g i n the Sugar loaf s tage of d i f f e r e n t i a t i o n . For the case of l a t i t e dykes , i t i s p o s s i b l e that d u r i n g the ascent of the b u l k magma, a s m a l l p o r t i o n was t r a p p e d , l o c a l l y d i f f e r e n t i a t e d at depth w i t h a s l i g h t l y d i f f e r e n t t rend and then i n t r u d e d i n t o the Cherry Creek r o c k s . With the compos i t ion of a parent magma l y i n g c l o s e to the d i v i d i n g l i n e of F i g u r e 2 1 , s t r a d d l i n g of the d e r i v a t i v e l i q u i d s i n e i t h e r c o m p o s i t i o n a l f i e l d i s not d i f f i c u l t to r a t i o n a l i z e , e s p e c i a l l y i f d i f f e r e n t i a t i o n proceeded under h i g h pressure or i n wet c o n d i t i o n s . However, the s i m p l e s t h y p o t h e s i s remains tha t the p i c r i t e s and p o s s i b l y the l a t i t e s , are u n r e l a t e d to the other i n t r u s i v e rocks of the b a t h o l i t h . In s h o r t , w i t h the p o s s i b l e e x c e p t i o n of the P i c r i t e rocks and the l a t i t e dykes , the major rock u n i t s of the I ron Mask b a t h o l i t h appear to have d e r i v e d from a common parent magma and to be r e l a t e d by a s i n g l e c y c l e of f r a c t i o n a l c r y s t a l l i z a t i o n . T e x t u r a l ev idence sugges ts , however, tha t the f o r m a t i o n of the Sugar loaf and Cherry Creek phases took p l a c e a t a much s h a l l o w e r depth than t h a t r e q u i r e d f o r the c r y s t a l l i z a t i o n of the I ron Mask Hybr id and Pothook r o c k s . P e r i o d i c v e n t i n g of the Sugar loaf phase at the s u r f a c e c o u l d e x p l a i n the e x t r u s i o n o f c o m p o s i t i o n a l l y s i m i l a r rocks i n c l u d e d i n N i c o l a Group and the i n t r o d u c t i o n o f the embryonic Cherry Creek phase to a s u b v o l c a n i c env i ronment . b) E v o l u t i o n of copper m i n e r a l i z a t i o n w i t h i n the I ron Mask b a t h o l i t h At t h i s t i m e , A f t o n Mines i s the on ly major copper producer l o c a t e d w i t h i n the I r o n Mask b a t h o l i t h . An e l u c i d a t i o n of the r e l a t i o n s h i p of the e v o l u t i o n of the b a t h o l i t h to i t s a s s o c i a t e d copper m i n e r a l i z a t i o n i s most a p p r o p r i a t e l y d i s c u s s e d w i t h r e f e r e n c e to the A f t o n orebody. 101 Data p resented i n F i g u r e 20 i n d i c a t e that Cherry Creek r o c k s w i t h i n A f t o n are compat ib le w i t h the c o m p o s i t i o n a l t rends d e f i n e d by the major phases of the b a t h o l i t h . T h i s i m p l i e s that the A f t o n r o c k s a r e a l s o p roducts of f r a c t i o n a l c r y s t a l l i z a t i o n of the same source magma g i v i n g r i s e to the major i n t r u s i v e phases . As i n d i c a t e d by the p o s i t i o n of the d a t a p o i n t s i n F i g u r e 2 0 , the f o r m a t i o n of A f t o n rocks probably took p l a c e i n the t r a n s i t i o n a l s tage between the Sugar loaf and Cherry Creek phases o f i n t r u s i o n . The copper m i n e r a l i z a t i o n found i n A f t o n Mines , t h e n , c o u l d w e l l be a m o d i f i e d by -p roduc t of the magmatic d i f f e r e n t i a t i o n . Th is v iew w a r r a n t s a d i s c u s s i o n of c e r t a i n d e t a i l s of the d i f f e r e n t i a t i o n p r o c e s s . -Cons ider f i r s t the obv ious c o n c l u s i o n drawn from the AFM p l o t ( F igure 19) tha t there i s no prominent i r o n enrichment i n the r e s i d u a l l i q u i d s i n v o l v e d i n the f o r m a t i o n o f the f o u r major s u c c e s s i v e phases o f the I r o n Mask p l u t o n . A r e a s o n a b l e e x p l a n a t i o n i s cons idered to be c o n t i n u a l magnet i te p r e c i p i t a t i o n , s t a r t i n g w i t h the e a r l i e s t events r e c o r d e d . T h i s r e s u l t s i n concomitant s i l i c a enr ichment o f the m e l t . Such an e x p l a n a t i o n i s i n agreement w i t h f i e l d o b s e r v a t i o n s t h a t magnet i te i s prominent i n a l l fou r major u n i t s of the b a t h o l i t h but i s p a r t i c u l a r l y abundant i n r o c k s of the I r o n Mask Hybr id u n i t (Table I I I ) . The c o n t i n u a l p r e c i p i t a t i o n o f magnet i te as the magma c r y s t a l l i z e d has th ree important e f f e c t s on the f o r m a t i o n of copper m i n e r a l i z a t i o n . These are o u t l i n e d as f o l l o w s : ( i ) I n f l u e n c e on s o l u b i l i t y of s u l f i d e s : - The f o r m a t i o n o f i m m i s c i b l e s u l f i d e l i q u i d s i n m a f i c magmas has been d i scussed by Haughton e t a l . (1974) , i n which i t was shown t h a t the s a t u r a t i o n and subsequent s e p a r a t i o n of an i m m i s c i b l e s u l f i d e l i q u i d shares an a n t i p a t h e t i c r e l a t i o n s h i p w i t h the FeO content of the c o o l i n g m e l t . Consequent ly , d u r i n g the c r y s t a l l i z a t i o n of the parent I r o n Mask magma, a c o n t i n u a l decrease i n s u l f i d e s o l u b i l i t y i s 102 expected to be concomitant w i t h the magnet i te p r e c i p i t a t i o n . Whereas the s e p a r a t i o n of the b u l k of copper s u l f i d e s might be de layed to the e a r l y Cherry Creek or A f t o n s t a g e , minor amounts of s u l f i d e s are expected to be found a l s o i n o t h e r phases of the b a t h o l i t h except p o s s i b l y the v e r y f i r s t cumulus phase. Moreover , i f one f u r t h e r accepts the c o n t e n t i o n tha t the d i f f e r e n t i a t i n g magma was sub jec ted to cont inuous upward movement, the s e p a r a t i n g s u l f i d e g l o b u l e s would remain suspended and e v e n t u a l l y be p r e c i p i t a t e d as d i sseminated s u l f i d e s u n l e s s scavenged by l a t e magmatic f l u i d s to form v e i n s . The mode of occurrence of copper s u l f i d e s d e s c r i b e d i n P a r t I s t r o n g l y suggests that r e m o b i l i z a t i o n of some p r imary s u l f i d e s has taken p l a c e . Moreover , a c c o r d i n g to Nor thcote (1974, 1976 and 1977) , minor copper m i n e r a l i z a t i o n i n f a c t c h a r a c t e r i z e s a l l phases of the I r o n Mask b a t h o l i t h . ( i i ) I n f l u e n c e on the compos i t ion of copper m i n e r a l i z a t i o n : - With abundant i r o n b e i n g f i x e d i n the ox ide phase , the s e p a r a t i n g s u l f i d e should have a r e l a t i v e l y low i r o n content and the f o r m a t i o n of abundant hydrothermal i r o n - r i c h s u l f i d e s d u r i n g the waning stage of magmatic a c t i v i t y shou ld be g r e a t l y h indered u n l e s s a new source of i r o n i s made a v a i l a b l e . I t i s not s u r p r i s i n g thus to f i n d b o r n i t e , w i t h a Cu/Fe r a t i o of 4 / 1 , as an e s s e n t i a l component i n the d i s s e m i n a t e d , hypogene ore zone of the A f t o n orebody and t h a t , i n comparison w i t h most c a l c - a l k a l i n e porphyry copper d e p o s i t s , A f t o n i s c h a r a c t e r i z e d by the presence of o n l y minor hydro thermal p y r i t e . The l a t t e r f e a t u r e , however, may a l s o have been caused by the l o s s o f s u l f u r under the then p r e v a l e n t s u b v o l c a n i c c o n d i t i o n s . ( i i i ) I m p l i c a t i o n on the immediate cause o f p r imary s u l f i d e s e p a r a t i o n : -R e c e n t l y , Cann (1979) i n t e r p r e t e d the occur rence of m a g n e t i t e - a p a t i t e lodes i n the b a t h o l i t h as due to i m m i s c i b i l i t y between m a g n e t i t e - a p a t i t e and an 103 a l k a l i c magma. In t h i s , c o n t e x t , the e a r l y c o n t i n u a l p r e c i p i t a t i o n of magnet i te c o u l d have g r a d u a l l y changed the Fe/P r a t i o i n the me l t such tha t at some p o i n t d u r i n g the c r y s t a l l i z a t i o n h i s t o r y , a e u t e c t i c compos i t ion of approx imate ly t w o - t h i r d by volume of magnet i te and o n e - t h i r d a p a t i t e ( P h i l p o t t s , 1967) was reached and l i q u i d i m m i s c i b i l i t y was i n i t i a t e d . I f the f o r m a t i o n of an i m m i s c i b l e m a g n e t i t e - a p a t i t e melt took p l a c e i n the e a r l y Cherry Creek s t a g e , i t would r e a d i l y e x p l a i n the s l i g h t i r o n impoverishment of the A f t o n r o c k s i n comparison w i t h o t h e r f r e s h Cherry Creek rocks and c o u l d cause sudden s u l f i d e p r e c i p i t a t i o n i n the conjugate s i l i c a t e melt by d r a s t i c a l l y r e d u c i n g the s u l f i d e s o l u b i l i t y . However, i n the v a r i a t i o n diagram of ^2^5 v e r s u s SiO^ (F igure 1 8 ) , o ther than the apparent d e p l e t i o n o f the I r o n Mask Hybr id rocks i n ?2°5» t h 6 1 ^ : L S n o obv ious v a r i a t i o n i n phosphorus content among the other phases to s u b s t a n t i a t e the r e q u i r e d a p a t i t e p r e c i p i t a t i o n . T h e r e f o r e , the s e p a r a t i o n of an i m m i s c i b l e m a g n e t i t e - a p a t i t e m e l t , though f e a s i b l e i n p r i n c i p l e , does not seem to have developed to a s u f f i c i e n t q u a n t i t y to a f f e c t the o v e r a l l d i f f e r e n t i a t i o n t r e n d of the b a t h o l i t h . Rapid i n t r u s i o n of some e a r l y Cherry Creek magma i n t o a s u b v o l c a n i c environment w i t h r e s u l t i n g f a s t e r c o o l i n g r a t e that caused a d r a s t i c decrease i n s u l f i d e s o l u b i l i t y would be a more p l a u s i b l e e x p l a n a t i o n f o r the f o r m a t i o n of the A f t o n p r o t o r e . The l a c k of i r o n enrichment i n the rock s e r i e s of the I r o n Mask b a t h o l i t h has another i m p l i c a t i o n — the p reva lence of h i g h o x i d a t i o n s t a t e throughout the course of magmatic d i f f e r e n t i a t i o n (Muan and Osborn, 1956 and E r n s t , 1976) . Coupled w i t h the r e l a t i v e l y maf i c c h a r a c t e r of the A f t o n host r o c k s , t h i s may e x p l a i n the s c a r c i t y of molybdenum i n the A f t o n orebody. The phase e q u i l i b r i a s t u d i e s of Korenbaum (1970) was quoted by Landergren and Manhein (1978) to e x p l a i n the hydrothermal t r a n s p o r t of molybdenum. In 104 essence , at r e l a t i v e l y low tempera tu res , near 200°C, and n e u t r a l pH v a l u e s , molybdenum remains s o l u b l e i n the form of th iomolybdates even at r e l a t i v e l y h i g h t o t a l s u l f u r (=1 m o l a l ) a v a i l a b l e i n the system. Wi th h i g h e r temperatures , near 400°C, the th iomolybdate f i e l d r e t r e a t s towards b a s i c pH v a l u e s but to an a p p r e c i a b l e ex tent o n l y under reduc ing c o n d i t i o n s . Mo lybden i te p r e c i p i t a t i o n thus takes p l a c e o n l y under more a c i d i c c o n d i t i o n s w i t h r e l a t i v e l y low oxygen f u g a c i t y . For the f o r m a t i o n of the A f t o n orebody, n e i t h e r of these c o n d i t i o n s was met, e i t h e r at the i n i t i a l stage of pr imary s u l f i d e s e p a r a t i o n or d u r i n g the l a t e s tage of hydrothermal r e d i s t r i b u t i o n of s u l f i d e s . At l e a s t f o r the porphyry copper d e p o s i t s of s o u t h c e n t r a l B r i t i s h Co lumbia , such a " h o s t - r o c k c o n t r o l l e d " m i n e r a l i z a t i o n concept i s a p l a u s i b l e a l t e r n a t i v e to the p r o p o s a l t h a t the h i g h go ld content and low c o n c e n t r a t i o n of molybdenum i n d i o r i t e p o r p h y r i e s are the consequence of l i t t l e s i a l i c c r u s t a l contaminat ion ( H o l l i s t e r , 1978) . For example, though e x h i b i t i n g d i f f e r e n t chemica l a f f i n i t i e s , bo th the Guichon Creek b a t h o l i t h ( c a l c - a l k a l i n e ) and the I ron Mask b a t h o l i t h ( a l k a l i n e ) are c l o s e l y r e l a t e d i n space and t ime (same age: 200 ± 6 m.y. ) and share the same average i n i t i a l 87 86 Sr /Sr r a t i o of 0.7036 (Preto e t . a l . , 1979) . I t i s d i f f i c u l t to env isage why c o p p e r - r i c h p l u t o n s i n one s u f f e r e d s i a l i c c r u s t a l contaminat ion w h i l e 87 86 those i n the o ther d i d n o t . In a d d i t i o n , the common, low i n i t i a l Sr /Sr r a t i o argues a g a i n s t c r u s t a l c o n t a m i n a t i o n . More l i k e l y , i t i s the b u f f e r i n g e f f e c t s of the host rock geochemistry on important parameters of the m i n e r a l i z i n g f l u i d ( l i k e pH, f , e t c . ) t h a t u l t i m a t e l y determine the k i n d 2 of ore that w i l l be formed. The more a c i d i c c h a r a c t e r of the Guichon Creek rocks p r o v i d e s a more s u i t a b l e geochemical environment f o r the p r e c i p i t a t i o n of molybdenum w h i l e the A f t o n host r o c k s are too m a f i c to do s o . F i n a l l y , minor anomal ies observed i n the v a r i a t i o n diagrams a l s o 105 c a r r y i n f o r m a t i o n about the e v o l u t i o n a l h i s t o r y of the copper m i n e r a l i z a t i o n . The r e l a t i v e enrichment of the A f t o n r o c k s ( p a r t i c u l a r l y the a l t e r e d ones) i n N a 2 0 r a t h e r than I^O i n d i c a t e s t h a t the copper m i n e r a l i z a t i o n was a s s o c i a t e d w i t h sodium metasomatism r a t h e r than K - f e l d s p a r m i n e r a l i z a t i o n as p r e v i o u s l y advocated by Nor thcote (1974, 1976 and 1977) . A l b i t e cou ld be e a s i l y mis taken f o r K - f e l d s p a r i n the I r o n Mask b a t h o l i t h because of the p i n k i s h tone imparted from the o x i d a t i o n of f e r r o u s m i n e r a l s i n the v i c i n i t y . In v iew of the h igh T i 0 2 content of hydrothermal b i o t i t e a s s o c i a t e d w i t h the orebody (Appendix I ) , i t i s s p e c u l a t e d here t h a t the apparent enrichment of the A f t o n a n a l y s e s i n TiO^ (F igu re 20) c o r r e l a t e d w i t h the p r e c i p i t a t i o n of b i o t i t e . To f u l l y a p p r e c i a t e the s i g n i f i c a n c e of the apparent enr i chment , however, the compos i t ion of p r imary b i o t i t e must a l s o be s t u d i e d . 106 Summary arid C o n c l u s i o n Major element geochemical data are p resented i n t h i s s e c t i o n f o r the v a r i o u s phases of the I ron Mask b a t h o l i t h and the rocks i n i t s v i c i n i t y . Augmented by f i e l d o b s e r v a t i o n s , these data l e a d to the deduct ion tha t the f o u r major u n i t s of the b a t h o l i t h , namely the I ron Mask H y b r i d , Pothook, S u g a r l o a f and Cherry Creek u n i t s , are c l o s e l y r e l a t e d through the p rocess of f r a c t i o n a l c r y s t a l l i z a t i o n . The d i f f e r e n t i a t i o n of the b u l k o r i g i n a l magma i s c h a r a c t e r i z e d by an absence o f i r o n enr ichment as a r e s u l t of c o n t i n u a l magnet i te s e p a r a t i o n . A comparison of the major element ana l yses of the host r o c k s of the A f t o n orebody w i t h those o f the o ther i n t r u s i v e r o c k s of the b a t h o l i t h i n d i c a t e s that they too f o l l o w the same d i f f e r e n t i a t i o n t r e n d . T h e i r c r y s t a l l i z a t i o n commenced i n an e a r l y Cherry Creek stage marked by a dominant ly d i o r i t i c c o m p o s i t i o n , i . e . i n t e r m e d i a t e between the S u g a r l o a f h o r n b l e n d i t e s and l a t e Cherry Creek s y e n i t e s . The e v o l u t i o n of the A f t o n orebody p r o b a b l y i n v o l v e d th ree important s t e p s : 1) f r a c t i o n a l c r y s t a l l i z a t i o n of an o r i g i n a l magma under h i g h o x i d a t i o n s t a t e s d u r i n g i t s slow ascent w i t h c o n t i n u a l s e p a r a t i o n of magnet i te so as to b r i n g about i n c i p i e n t s a t u r a t i o n of copper s u l f i d e phases ; 2) r a p i d t r a n s f e r of p o r t i o n s of the d i f f e r e n t i a t i n g magma at the e a r l y Cherry Creek stage to a s u b v o l c a n i c environment i n which a more r a p i d c o o l i n g r a t e gave r i s e to a s u p e r s a t u r a t i o n w i t h r e s p e c t to copper s u l f i d e s ; and 3) t r a p p i n g of l a t e magmatic f l u i d s to r e m o b i l i z e and concent ra te the d i s s e m i n a t e d orthomagmatic copper s u l f i d e s to form the stockwork copper o r e . Such a g e n e t i c model e x p l a i n s many m i n e r a l o g i c a l c h a r a c t e r i s t i c s of the A f t o n orebody i n c l u d i n g the l i m i t e d occur rence o f m o l y b d e n i t e , abundance of b o r n i t e , low t o t a l p y r i t e , e t c . The model i s a l s o compat ib le w i t h the 107 s u l f u r i s o t o p e d a t a of H o i l e s (1978) t h a t the s u l f i d e s at A f t o n are 34 c h a r a c t e r i z e d by a near zero per m i l 6 S w i t h a s m a l l s tandard d e v i a t i o n and the c o n t e n t i o n advocated i n P a r t I t h a t o n l y a l i m i t e d amount of m e t e o r i c water p a r t i c i p a t e d i n the hypogene m i n e r a l i z a t i o n p r o c e s s . Future e x p l o r a t i o n s f o r s i m i l a r copper m i n e r a l i z a t i o n s i n the b a t h o l i t h should focus on areas where the above c o n d i t i o n s are s a t i s f i e d . A combinat ion of i n t e n s e carbonate a l t e r a t i o n and sodium metasomatism found ad jacent to at l e a s t moderate magnet i te m i n e r a l i z a t i o n s may a c t as gu ides f o r l o c a t i n g f a v o r a b l e t a r g e t s . 108 SUMMARY AND OVERVIEW To complete the a n a l y s i s of the A f t o n orehody i n the p e r s p e c t i v e of geochemis t r y , t h i s f i n a l chapter i s intended to r e c a p i t u l a t e the h i g h l i g h t s of each of the t h r e e main p a r t s of the study and to suggest f u r t h e r r e s e a r c h tha t may h e l p c o n f i r m i n g the more s p e c u l a t i v e i n t e r p r e t a t i o n s made or enhancing u n d e r s t a n d i n g of the processes i n v o l v e d . In p a r t I, the petrography of the A f t o n w a l l rocks has been presented and the r e l e v a n t pr imary and secondary m i n e r a l d i s t r i b u t i o n s d e l i n e a t e d and d i s c u s s e d . I t i s concluded tha t the pr imary m i n e r a l i z a t i o n took p l a c e i n a roo f pendant environment w i t h a d i v e r s i f i e d a r r a y of rock types tha t l e d to a r a t h e r sporad ic d i s t r i b u t i o n of a l t e r a t i o n m i n e r a l s . The nature and ex tent of an in tense carbonate a l t e r a t i o n , p r e v i o u s l y o v e r l o o k e d , a r e documented. I t i s i n t e r p r e t e d to be i n d i c a t i v e of l a t e stage magmatic a c t i v i t y i n c l u d i n g hydrothermal a l t e r a t i o n . In a d d i t i o n , the absence of prominent p h y l l i c and a r g i l l i c a l t e r a t i o n s at A f t o n i s i n t e r p r e t e d to have r e s u l t e d from the minor r o l e p layed by meteor i c water i n the hypogene m i n e r a l i z i n g event . Oxygen and hydrogen i s o t o p i c s t u d i e s might e f f e c t i v e l y t e s t t h i s p r o p o s i t i o n . F i n a l l y , a great p r o p o r t i o n of the new data p resented i n t h i s study i s de r i ved from m a t e r i a l s secured from the f i r s t f o r t y meters of the A f t o n o p e n - p i t . S i m i l a r s t u d i e s concomitant w i t h f u r t h e r s t r i p p i n g of the orebody would s u r e l y c o n t r i b u t e to the development of a more d e t a i l e d th ree d imens iona l m i n e r a l i z a t i o n model . Such a model would f a c i l i t a t e e x p l o r a t i o n f o r s i m i l a r d e p o s i t s i n o ther a l k a l i n e p l u t o n s . In P a r t I I , m i n e r a l d i s t r i b u t i o n data d e s c r i b e d i n P a r t I and thermochemical d a t a have been u t i l i z e d i n a thermodynamic r e c o n s t r u c t i o n of the supergene a l t e r a t i o n at A f t o n . I t i s demonstrated t h a t the dominance 109 of n a t i v e copper and the l a c k of copper enrichment i n the supergene zone are the consequences o f the r e l a t i v e l y maf i c compos i t ion of the w a l l r o c k s and the l a c k of abundant s u l f i d e s i n the hypogene zone. I t i s f u r t h e r suggested tha t supergene a l t e r a t i o n i n genera l can be i n t e r p r e t e d w i t h i n the framework of a l o g f u g a c i t y of oxygen versus l o g f u g a c i t y of s u l f u r p l o t w i t h respec t to two a r b i t r a r y " l i m i t i n g " curves c h a r a c t e r i z e d by extreme v a l u e s of pH, 0 and 14 r e s p e c t i v e l y . In any supergene p r o c e s s , the c l o s e r the e v o l u t i o n a l path of the r e a c t i n g f l u i d approaches the upper or a c i d i c c u r v e , the g r e a t e r the extent of supergene enrichment i n secondary copper s u l f i d e s . In c o n t r a s t a r e a c t i n g f l u i d e v o l v i n g p a r a l l e l and c l o s e to the lower o r b a s i c curve w i l l l e a d to a w e l l - d e v e l o p e d spectrum of copper ox ides and n a t i v e copper w i t h l i t t l e enr ichment . In many porphyry copper d e p o s i t s , however, a z u r i t e and m a l a c h i t e are a l s o important supergene a l t e r a t i o n p r o d u c t s . To ach ieve g e n e r a l a p p l i c a b i l i t y , the present approach must be expanded to i n c l u d e CO^ as an a d d i t i o n a l component so that the r o l e of the c a r b o n a t e - c o n t a i n i n g phases can a l s o be a s s e s s e d . In P a r t I I I , major element geochemical data and f i e l d ev idence have been cons idered to a r r i v e a t two important c o n c l u s i o n s . F i r s t , the f o u r major i n t r u s i v e phases o f the I ron Mask b a t h o l i t h , namely the I r o n Mask H y b r i d , Pothook, Sugar loa f and Cherry Creek u n i t s , a re r e l a t e d by a gross c y c l e of f r a c t i o n a l c r y s t a l l i z a t i o n . Second, the A f t o n p r o t o r e i s a b y -product of the d i f f e r e n t i a t i o n p r o c e s s . I t was generated a t the e a r l y Cherry Creek stage when p o r t i o n s of the f r a c t i o n a t i n g magma i n t r u d e d i n t o a s u b v o l c a n i c env i ronment . The f i r s t c o n c l u s i o n can be t e s t e d . u s i n g the computat iona l method d e s c r i b e d by Nathan and Van K i r k (1978) i f more microprobe a n a l y s e s of f r e s h m i n e r a l s cou ld be o b t a i n e d . The second c o n c l u s i o n , on the o ther hand , i s supported by many m i n e r a l o g i c a l 110 c h a r a c t e r i s t i c s o f the orebody and the s u l f u r i s o t o p e data of H o i l e s (1978) . The concept advocated , of c o n t r o l of m i n e r a l i z a t i o n by the host r o c k s , should a l s o f i n d g e n e r a l a p p l i c a t i o n i n e x p l a i n i n g the molybden i te content of o ther porphyry copper d e p o s i t s . One important f e a t u r e r e g a r d i n g the geochemistry of the I r o n Mask b a t h o l i t h has been l e f t untouched i n the p resent s t u d y . I t i s the i n t r i g u i n g occur rence of the a l k a l i n e p l u t o n amidst o t h e r l a r g e r and contemporaneous c a l c - a l k a l i n e b a t h o l i t h s i n c l u d i n g the c o p p e r - r i c h Guichon Creek b a t h o l i t h . V o l c a n i c rocks i n t r u d e d by the p l u t o n s a l s o e x h i b i t both chemica l a f f i n i t i e s ( P r e t o , 1979 and P r e t o et a l . , 1 9 7 9 ) . Presumably , d i f f e r e n t degrees of p a r t i a l m e l t i n g of some p a r e n t a l m a t e r i a l at depth or the v a r y i n g depth at which p a r t i a l m e l t i n g occurs are r e s p o n s i b l e f o r the c o m p o s i t i o n a l d i f f e r e n c e s . However, much more work, e s p e c i a l l y t r a c e element r e s e a r c h , i s r e q u i r e d to e l u c i d a t e the s i t u a t i o n i n d e t a i l . In c o n c l u s i o n , the present study i n c o r p o r a t e s f i e l d o b s e r v a t i o n s , t h e o r e t i c a l m o d e l l i n g and geochemical data to i n t e r p r e t the v a r i o u s aspects of the copper m i n e r a l i z a t i o n at A f t o n . The e v o l u t i o n o f the orebody appears to have i n v o l v e d f o u r major e v e n t s : 1) f r a c t i o n a l c r y s t a l l i z a t i o n of the b u l k o r i g i n a l I r o n Mask magma at depth w i t h c o n t i n u a l magnet i te s e p a r a t i o n , thus b r i n g i n g about i n c i p i e n t s u l f i d e s a t u r a t i o n , p a r t i c u l a r l y at the e a r l y Cherry Creek s t a g e ; 2) t r a n s f e r of p o r t i o n s of the e a r l y Cherry Creek magma to a s u b v o l c a n i c environment where r a p i d c o o l i n g enhanced s u p e r s a t u r a t i o n of copper s u l f i d e s and where some l o s s of s u l f u r occur red through i n t e r m i t t e n t v o l c a n i c emanat ions ; 3) l o c a l t r a p p i n g of l a t e magmatic f l u i d to r e m o b i l i z e and concent ra te the o r i g i n a l d i sseminated s u l f i d e s to form the hypogene o r e ; and 4) subsequent supergene a l t e r a t i o n c o n s t r a i n e d to take p l a c e i n a I l l b a s i c pH domain to s t a b i l i z e abundant n a t i v e copper w i thout much enr ichment . 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S o c , V o l . 3 9 , p. 1 2 1 - 1 4 0 . 114 Nathan , H.D. and C.K, Van K i r k , 1978: A model of magmatic c r y s t a l l i z a t i o n ; J o u r . P e t r o l . , V o l . 19 , p. 6 6 - 9 4 . Ney, C . S . , R . J . C a t h r o , A . Pante leyev and D.C. Rotherham, 1976: Supergene copper m i n e r a l i z a t i o n ; i n A . Suther land Brown ( e d . ) , Porphyry D e p o s i t s of the Canadian C o r d i l l e r a ; Can. I n s t , of M i n . and Met . Spec. V o l . 1 5 , p. 7 2 - 7 8 . N o r t h c o t e , K . E . , 1974: Geology of the northwest h a l f of the I ron Mask b a t h o l i t h ; i n G e o l o g i c a l F i e l d w o r k , 1974, B.C. Dept . of Mines and P e t r o l . R e s . , p. 2 2 - 2 6 . N o r t h c o t e , K . E . , 1976: Geology of the southeast h a l f of the I r o n Mask b a t h o l i t h ; i n G e o l o g i c a l F i e l d w o r k , 1976, B.C. Dept . of Mines and P e t r o l . R e s . , p. 4 1 - 4 6 . N o r t h c o t e , K . E . , 1977: P r e l i m i n a r y map No. 26 ( I ron Mask b a t h o l i t h ) and accompanying n o t e s ; B .C. Dept . of Mines and P e t r o l . R e s . , 8 pp. P e r k i n s , E . H . , 1980: A r e i n v e s t i g a t i o n of the t h e o r e t i c a l b a s i s f o r the c a l c u l a t i o n of i s o t h e r m a l - i s o b a r i c mass t r a n s f e r i n geochemical systems i n v o l v i n g an aqueous phase; unpub. M .Sc . t h e s i s , U .B .C . P h i l p o t t s , A . R . , 1967: O r i g i n of c e r t a i n i r o n - t i t a n i u m o x i d e and a p a t i t e r o c k s ; Econ . G e o l . , V o l . 6 2 , p. 303 -315 . P r e t o , V . A . , 1967: Geology of the e a s t e r n p a r t of the I r o n Mask b a t h o l i t h ; i i n Annual Report of the M i n i s t e r of Mines 1967, B.C. Dept . of Mines and P e t r o l . R e s . , p. 137 -147 . P r e t o , V . A . , 1972: Report on A f t o n , Pothook; B.C. Dept . of Mines and P e t r o l . R e s . , G . E . M . , p. 209 -220 . P r e t o , V . A . , 1979: Geology of the N i c o l a Group between M e r r i t t and P r i n c e t o n ; B u l l e t i n 6 9 , B.C. M i n i s t r y of Energy, Mines and P e t r o l . R e s . , 90 pp. P r e t o , V . A . , M . J . Osatenko, W . J . M c M i l l a n and R . L . Armstrong , 1979: I s o t o p i c d a t e s and s t r o n t i u m i s o t o p i c r a t i o s f o r p l u t o n i c and v o l c a n i c r o c k s i n the Quesnel Trough and N i c o l a B e l t , s o u t h c e n t r a l B . C . : Can. J . E a r t h S c i . , V o l . 16, p. 1658-1672. R a l p h , P . F . and M.A. Chaudhry ( i n p r e p . ) : Accurate r o u t i n e a n a l y s i s of g e o l o g i c a l samples u s i n g atomic a b s o r p t i o n f lame spect rophotometry . Reed, A . J . , 1980a: S t r u c t u r a l geology of the A f t o n c o p p e r - g o l d m i n e , Kamloops, B .C . and i t s i n f l u e n c e on p i t w a l l s l o p e s t a b i l i t y ; Paper No. 90 presented a t the 82nd. AGM of CIM, Toronto , 28th A p r i l , 1980. Reed, A . J . , 1980b: G e o s t a t i s t i c a l parameters of the A f t o n ore d e p o s i t ; Paper No. 38 presented a t the 5th Annual Meet ing of CIM D i s t r i c t 6 , K i m b e r l e y , B . C . , October 2 5 , 1980. 115 R o b i e , R . A . , B . S . Hemingway and J . R . F i s h e r , 1978: Thermodynamic p r o p e r t i e s of m i n e r a l s and r e l a t e d substances a t 298.15 K and 1 bar p r e s s u r e and at h i g h temperatures ; U . S . G . S . B u l l e t i n 1452, 456pp. Sheppard, S . M . F . , R . L . N i e l s e n and H .P . T a y l o r , J r . , 1969: Oxygen and hydrogen i s o t o p e r a t i o s of c l a y m i n e r a l s from porphyry copper d e p o s i t s ; Econ . G e o l . V o l . 6 4 , p. 7 5 5 - 7 7 7 . Sheppard , S . M . F . , R . L . N i e l s e n and H.P . T a y l o r , J r . , 1971: Hydrogen, and oxygen i s o t o p e r a t i o s i n m i n e r a l s from porphyry copper d e p o s i t s ; Econ . G e o l . V o l . 66 , p. 5 1 5 - 5 4 2 . Wedephol, K . H . , 1966: D ie Geocheraie der Gewasser; N a t u r w i s s e n s c h a f t e n 5 3 , 14, 3 5 2 - 3 5 4 . 116 APPENDIX I. RESULTS OF MICROPROBE Mineral analysed Sample number GWA 35 Number of analyses 5 Na20 0.30±0.02 MgO 19.6711.25 A1 20 3 12.9310.46 s i o 2 38.9110.54 K20 9.28±0.58 CaO 0.0310.03 Ti0 2 6.8311.48 Cr 20 3 0.03±0.01 MnO 0.18±0.05 FeO 7.41±0.35 F - (<0.5 Total 95.57±0.63 Formula and Kx. 6 aNa. 08 remarks Mgi,.aaMn.02 F e . e 9 T i . 7 I > S i s • 6 o A l j • i » Oao(OH)., ANALYSES OF SELECTED MINERALS* GWA 38 GWA 112 3 6 0.27±0.04 0.2410.03 18.49±0.52 18.8110.73 14.87±0.08 13.2910.23 36.74±0.37 38.6310.33 9.6110.13 9.5910.22 - 0.0210.05 6.2810.66 5.3710.80 0.0110.01 0.0710.06 0.2010.05 0.2410.03 8.8610.25 8.6010.40 95.3410.04 94.8410.51 Kj . 7 gNa.08 Mg <,.oaMn.0a F e i . o s T i . 6 9 Sis•s«Ala.36 • 7 9N3. • 0 7 M g 4 . - 0 7 M n . 0 9 Fex•osTi.s 9 Cr . 0 1 Sis•6 u Oao(OH) 4 Al a.a90ao(0H) 1, *Feldspar and carbonate were also analysed in most samples listed here. The former varies from plagioclase with Ab. 8 3_. 9 9 to K-feldspar vith Or.9 « _ . 9 9 . The latter varies from calcite with an average calcite component of 93 mole per cent to ankerite with an average composition of Ca. i,8Mg. 3 9Fe. 1 3C0 3. Distribution of feldspar and carbonate in the Afton open-pit are not uniform. Data used for these two minerals in the text are approximations corresponding mainly to the modal abundance in microdioritic rocks which contain well preserved hypogene ore. Analyses presented here were obtained using an ARL-SEMQ instrument with 1 the following operating conditions: 15 kv accelerating voltage. 10 um beam diameter, 40 uA beam current and 10 second counting time. Data were computer reduced using Bence-Albee correction factors (Bence and Albee, 1968). APPENDIX I (continued) Mineral analysed Epidote Sample number GWA 2 GWA 112 GWA 22 Number of analyses 4 4 3 Na20 0.0110.01 - -MgO 0.03±0.03 0.0410.01 0.0310.02 A1203 23.84±0.21 23.3810.18 23.5410.28 Si0 2 37.54±0.05 37.2510.02 37.2510.22 K20 - 0.0110.01 0.0110.01 CaO 23.4310.24 22.8310.01 23.1210.15 Ti0 2 0.0210.02 0.0110.01 0.0110.01 C r2°3 0.0210.01 0.0210.02 -MnO 0.0310.02 0.0310.02 0.0210.02 FeO 11.8210.32 12.8310.40 12.7210.29 Total 96.7410.23 96.4010.20 96.7010.35 Formula and remarks C a 1 . 9 9 F e . 7 s Cai.9t,Fe. B5 C a 1 . 9 7 F e . a s Al2>2sSi2>98 AI2 . 1 9 S I 2 . 9 6 A l 2 . 2 1 S i j . 9 7 0i,(0H)i. o s 0ia(0H)i.i3 0n(0H) FeO has been converted to Fe 2°3 prior to calculation of chemical formula. _+ is taken as the difference of the sum of the major oxides from a hundred. H20 Clinopyro: GWA 38 4 0.5510.20 14.7511.19 2.1410.54 52.1010.55 0.0110.01 23.2810.10 0.3510.10 0.0410.04 0.4010.12 6.3911.04 99.9810.35 Na.ofcCa.93 Fe.2oMg.ea Mn >oiAl>o9 T i . o i S i i . 9 0« GWA 100 2 0.5110.06 15.6010.30 1.8210.15 52.5610.32 0.0110.01 23.2010.08 0.3510.05 0.4710.03 5.3910.21 99.8910.35 Na.0*Ca.93Fe.1 Mg. 8«Mn.oiTi. A l . o e S i i APPENDIX I (continued) Mineral analysed Chlorite Sample number GWA 95 GWA 32 Number of analyses 6 5 Na20 0.01+0.01 O.OliO.01 MgO 19.7110.63 21.19±1.23 A1 20 3 18.86±0.A8 17.70+1.23 Si0 2 27.91+0.32 29.29+1.11 K20 0.03+0.02 0.11±0.13 CaO 0.0510.04 0.03+0.03 Ti0 2 0.20±0.19 0.23+0.18 Cr 20 3 0.02+0.02 O.OllO.Ol MnO 0.3010.06 0.4410.05 FeO 19.96+0.77 18.07+1.61 Total 87.0410.23 87.0810.40 Formula and K.0iMg«.09 K.o3Mg6««.s remarks Ca.0iMn.0s Ca.0iMn.08 Fes . i,i,Ti. os Fes.osTi'o* All, . seSls . 7 s All, . 2sSis . 97 0 a o(0H) 1 6 0 2 O(0H) 1 6 GWA 20 Amphibole GWA 145 GWA 73 6 0.0110.01 22.1711.10 18.4710.27 29.1510.43 0.0210.01 0.1510.09 0.0410.01 0.0110.01 0.0610.03 17.0110.71 87.0910.33 Mg6•6aCa.03 Mn.oiFej • e? Ti. oiAl{, • *o Sis.89020 (0H) 1 6 5 2.4510.07 13.5910.40 12.7510.06 41.6610.50 0.9710.01 11.6610.14 2.6510.44 0.0210.01 0.1910.02 11.4810.42 97.4110.33 Na. 7 0K. l 8 Ca i.8sMn.oi Mg3 • ooEe t. i,a T i . 3 0 A l a • as Si.•17O2 2 (OH) 2 5 2.4510.04 14.7210.24 11.8810.12 41.89+0.25 0.8910.04 11.5310.12 3.1010.14 0.0210.03 0.1310.01 10.1410.28 96.7510.41 Na. 7 0 K . 1 7 Cax . B 9Mn. Mg3 •asEex Ti. 3 3 A I 2 . Si« .20O22 119 APPENDIX II. (A) MAJOR ELEMENT GEOCHEMISTRY AND BRIEF SAMPLE DESCRIPTION OF THE MAJOR INTRUSIVE PHASES OF THE IRON MASK PLUTON AND ROCKS IN ITS VICINITY FIELD * 77KN-2 77KN-9 77KN-59 sio 2 47.13 58. OA 55.23 A1 20 3 17.86 15.71 16.74 F e 2 ° 3 12.51 5.67 7.14 MgO 5.47 3.20 3.28 CaO 8.18 5.11 4.25 K 20 1.32 2.81 3.85 Na 20 3.95 3.61 3.80 T i 0 2 2.93 0.87 1.12 77KN-35 761MU-III 74KN-332 76LM10-I 761-82-11 74KN-342 MnO 0.19 0.10 0.10 P2°5 0.41 0.20 0.48 TOTAL 99.95 95.32 95.99 : 44.07 50.03 51.59 47.56 44.24 45.85 15.56 18.45 17.54 13.74 17.08 17.13 74KN-99 II 74KN-28 I 74KN-104 I 74KN-15 III 74KN-36 74KN-53 I 74KN-130 74KN-27 74KN-49 II 74KN-84 II 11.25 9.11 6.22 9.95 10.37 9.00 5.29 3.39 2.27 9.42 5.05 3.45 10.49 9.01 8.68 9.51 8.25 8.38 3.16 0.84 1.20 2.09 2.44 2.96 2.21 4.06 3.38 2.70 3.95 3.48 0.77 0.82 0.46 0.76 0.89 0.67 0.17 0.17 0.14 0.17 0.25 0.24 0.23 0.61 0.50 0.23 0.38 0.76 60.22 56.72 56.29 62.10 55.27 57.19 54.82 51.39 65.98 52.30 18.20 15.71 19.41 19.15 18.60 16.81 20.03 17.99 16.43 18.26 93.20 96.49 91.98 96.13 92.90 91.92 4.58 3.39 5.04 3.34 6.94 5.02 2.53 10.75 0.97 9.24 1.91 0.70 1.74 0.61 2.92 1.19 4.77 2.89 1.24 4.04 4.67 5.87 4.76 2.69 1.50 5.15 5.97 7.21 2.18 4.62 6.87 8.44 5.37 0.04 5.75 3.44 1.54 1.47 0.26 1.95 3.76 2.56 4.45 9.40 4.39 5.70 5.76 5.28 8.70 5.41 0.48 0.38 0.77 0.39 0.67 0.42 0.91 0.81 0.33 0.72 0.16 0.14 0.09 0.09 0.06 .08 .11 .14 .05 .07 0.32 0.07 0.22 0.07 0.62 0.11 0.08 0.32 0.18 0.18 76LM255 VI 76LM133 IV 76LM131 I 74KN-167 74KN-84 III 48.96 45.80 52.20 52.45 47.43 17.22 14.73 17.35 18.94 17.41 9.23 17.80 8.75 20 39 5.46 6.41 5.15 3.79 5.90 10.56 7.03 7.16 5.86 7.95 101.17 93.98 98.14 97.88 96.72 95.11 96.52 98.25 96.32 96.79 2.30 2.19 1.23 3.68 2.80 2.99 2.64 4.81 3.88 3.55 0.64 0.79 0.71 0.46 0.59 0.14 0.06 0.07 0.07 0.11 74KN-62 74KN-14 74KN-42 74KN-140A 0.61 0.39 0.52 0.34 0.16 47.93 41.50 50.79 44.82 16.83 17.84 18.51 16.96 11.54 13.70 8.29 12.16 6.13 6.40 4.02 6.08 98.11 97.84 97.95 96.67 95.29 9.68 11.50 10.28 12.93 1.91 2.89 0.89 1.22 3.11 1.32 4.32 2.18 0.71 1.00 0.67 0.90 0.25 0.23 0.19 0.16 0.73 0.76 0.39 0.22 74KN-51 74KN-189 74KN-182 761M7-II 74KN-126 74KN-203 II 74KN-180 74KN-214 74KN-304 II 43.85 38.53 39.04 47.86 45.60 40.14 43.16 48.33 44.47 18.10 17.26 5.97 12.68 13.35 20.13 19.37 19.29 19.63 9.49 16.27 23.12 10.10 11.74 12.69 10.92 8.76 10.64 6.93 7.26 12.16 11.53 8.68 6.20 6.16 4.08 5.11 16.43 15.12 17.73 10.14 13.02 16.39 14.53 9.08 15.24 0.69 1.32 0.07 1.40 2.39 0.30 1.55 1.70 0.35 1.13 0.55 0.29 2.11 1.22 0.88 0.97 3.90 1.86 0.83 0.88 1.14 0.65 1.12 0.92 0.84 1.10 0.87 98.82 97.14 98.35 97.63 0.16 0.13 0.18 0.17 0.26 0.14 0.15 0.10 0.15 0.08 0.08 0.08 0.22 0.09 0.07 0.07 0.11 0.07 74KN-125 741CN-135 97.69 97.40 99.78 96.86 97.47 97.86 97.72 96.45 98.39 UNIT FIELD NAME & REMARKS Tertiary mesocrystalline basalt; c.f. mesocratic gabbro; strongly magnetic, volcanic dark grey andesite. rocks dark grey andesite with obvious trachytic texture; s l i g h t l y magnetic. purplish basaltic andesite; epidote alteration prominent. Nicola yellowish green tuffaceous andesite. Group red tuff breccia, volcanic porphyritic dark green flow breccia, rocks massive, dark green basaltic(?) tuff. reddish tuff breccia. medium grained syenite with minor d i o r i t e fragments (?). porphyritic mlcrosyenlte breccia. micromonzonite, s l i g h t l y magnetic. Cherry micromonzonite; a l b i t i z e d ? micromonzonite breccia. f i n e grained monzonite. Creek mlcrodlorlte. microdlorlte; moderately magnetic. medium grained d i o r i t e with epidote patches; a l b i t i z e d ? fi n e grained d i o r i t e , s l i g h t l y magnetic. fine grained gabbro with monzonite fragments; weakly magnetic. porphyritic hornblende-plagioclase andesite; moderately magnetic. Sugarloaf porphyritic hornblende intrusive breccia. hornblende porphyry. porphyritic hornblende gabbro; moderately magnetic. Pothook dark grey, coarse grained diorite, weakly magnetic, dark grey, coarse grained di o r i t e , magnetic, dark grey, medium grained d i o r i t e . coarse grained greenish di o r i t e . Iron Mask Hybrid 40.80 42.98 4.06 6.19 9.46 9.53 34.60 26.00 3.43 5.98 0.78 1.38 0.19 0.54 0.15 0.26 0.17 0.14 0.08 0.08 93.72 93.08 Pi c r i t e coarse grained, dark grey gabbro with minor epidote-albite veins, very coarse grained leucogabbro dominated by plagioclase & pyroxene, massive, magnetite- and pyroxene-rich gabbro. fine grained intrusive breccia with prominent c h l o r i t e a l t e r a t i o n , gabbro, magnetic. intrusive breccia; magnetic; epidote + prehnite a l t e r a t i o n common, magnetite r i c h gabbro. coarse grained d i o r i t e with discontinuous magnetic bands. medium grained magnetite rich gabbro with patchy epidote a l t e r a t i o n . black, porphyritic basalt; sl i g h t l y magnetic. black, porphyritic basalt; s l i g h t l y magnetic. 120 APPENDIX II. (B) MAJOR ELEMENT GEOCHEMISTRY AND BRIEF SAMPLE SAMPLE t SiO 6-3-3 6-2-4 GWA-40 6-3-1 GWA-171 GWA-27 GWA-87 6-4-8 GWA-6 GWA-32 GWA-95 GWA-22 GWA-31 GWA-7 6-2-5 6-1-13 6-4-6 GWA-128 GWA-102 GWA-161 GWA-37 GWA-14 v-35 c\; v-43 CWA-77 GWA-98 GWA-20 GWA-29 6-3-2 30-1 GWA-86 6-4-7 6-1-2 GWA-126 GWA-42 6-2-21 GWA-76 53.49 51.11 40.02 46.73 45.34 47.37 49.94 49.38 50.55 57.35 55.75 54.05 47.75 50.40 54.94 49.61 47.30 52.41 56.75 54.80 52.74 49.94 51.07 53.03 54.01 49.46 47.22 56.56 57.11 50.38 56.90 49.31 54.96 52.67 52.15 54.07 48.09 A1 20 3 17.85 17.96 15.72 17.89 15.74 17.73 18.63 17.96 17.88 18.21 15.92 18.88 15.60 17.01 19.32 16.07 17.23 17.92 15.46 17.90 18.13 18.78 17.20 18.50 18.58 17.68 16.80 17.73 19.00 17.81 14.98 18.07 18.22 18.81 17.46 17.55 16.17 Fe 20 3 5.31 5.95 8.53 8.55 7.43 10.21 6.22 8.93 5.00 4.82 5.05 4.08 3.27 7.50 3.61 14.05 11.00 4.84 3.32 3.06 6.21 6.19 4.41 5.52 2.71 6.71 5.90 4.50 2.80 9.45 6.08 6.86 6.71 4.10 2.35 5.62 10.59 MgO 4.88 4.28 4.56 2.40 5.87 4.72 4.00 4.30 4.52 3.63 3.09 2.95 4.40 3.37 3.59 5.80 4.48 4.34 2.64 3.12 5.39 4.53 5.85 4.05 4.22 5.24 3.71 1.68 2.50 3.41 4.83 2.80 2.90 2.21 3.57 5.59 5.53 CaO 2.15 4.67 9.32 6.28 4.75 4.62 3.71 6.08 1.79 3.54 6.56 3.19 7.60 5.14 4.75 0.90 6.85 2.30 4.27 5.39 1.95 6.45 4.45 4.31 4.35 5.07 7.67 2.55 2.26 2.96 4.16 7.42 0.95 6.24 5.28 4.67 5.72 Na20 5.69 4.88 3.95 3.32 6.05 4.39 5.62 3.23 3.13 7.77 6.86 4.55 2.76 6.13 7.01 4.86 4.89 4.62 6.45 7.75 7.13 4.95 4.31 6.20 6.11 5.43 5.04 8.14 6.66 1.85 4.93 2.69 5.60 2.36 6.06 5.98 3.56 K 20 2.21 2.23 1.59 4.55 1.12 2.18 1.74 4.53 6.03 0.54 1.50 5.10 3.42 1.38 0.99 0.67 1.55 4.57 1.20 0.93 0.79 1.71 3.60 1.21 1.76 1.03 2.16 0.83 2.98 7.91 3.29 4.79 4.42 6.88 1.54 1.27 2.57 T i 0 2 0.85 0.85 0.74 0.72 0.79 0.66 0.74 0.74 0.81 0.79 0.66 0.68 0.79 0.96 0.76 0.70 0.98 0.87 0.72 0.51 0.79 0.73 0.89 0.64 0.81 0.84 0.56 0.75 0.70 0.69 1.01 0.84 0.67 0.56 0.60 0.81 0.88 MnO 0.04 0.05 0.63 0.30 0.14 0.15 0.11 0.21 0.11 0.09 0.15 0.21 0.21 0.10 0.05 0.11 0.26 0.19 0.67 0.06 0.08 0.08 0.37 12 47 33 07 05 10 0.27 0.09 0.36 0.26 0.27 0.25 0.22 0.36 TOTAL 92.47 91.98 85.06 90.74 87.23 92.03 90.71 95.36 89.82 96.74 95.54 93.69 85.80 91.99 95.02 92.77 94.54 92.06 91.48 93.52 93.21 93.36 92.15 93.58 93.02 91.79 89.13 92.79 94.11 94.73 96.27 93.14 94.69 94.10 89.26 95.78 93.47 DESCRIPTION OF ROCKS COLLECTED FROM WITHIN THE AFTON PROPERTY FIELD NAME & REMARKS D.g. d i o r i t e ; magnetite v e l n l e t with c a l c i t e envelope common, brownish grey, ra.g. d i o r i t e ; p y r i t e velnlets common. f.g. diorite with diss, c h a l c o c i t e ; mafic minerals replaced by chlorite & c a l c i t e . pale greenish'brown, ra.g. monzonlte with pervasive carbonate alteration. porphyritic diorite breccia with abundant carbonate patches and chalcopyrite v e l n l e t s . porphyritic monozodiorite with m.g. matrix; epidote-pyrite velnlets common. pale green, f.g. d i o r i t e with d i s s , and veined pyrite. m.g. monzonlte with no obvious mineralization. d i o r i t e breccia with d i s s , bornite and chalcopyrite; K-feldspar and chlor i t e a l t e r a t i o n s . porphyritic diorite with p y r i t e and chalcopyrite velnlets; chlorite alteration common. m.g. d i o r i t e ; streaks of K-feldspar and epidote common. micromonzonite breccia; c h l o r i t e alteration prominent. carbonatized Intrusive breccia; c . f . Nicola Group volcanic rocks. porphyritic diorite with f.g. matrix; patchy K-feldspar and chalcocite common. lig h t grey, e.g. d i o r i t e with minor fracture-lining pyrite. chloritized, m.g. d i o r i t e breccia with included Nicola volcanic fragments. microdiorlte with c h l o r i t e and epidote alterations; trace diss, chalcocite also present. reddish brown micromonzonite with disseminations and velnlets of chalcocite. reddish brown microdiorlte breccia with prominent native copper velnlets. rather fresh, light grey m.g. d i o r i t e . f.g. & tuffaceous-looking monzonlte; chalcopyrite velnlet with alblte envelope observed, massive porphyritic monzonlte with f.g. matrix; trace pyrite along fractures, microdiorlte breccia with patchy K-feldspar, diss. f.g. b i o t i t e & native copper v e l n l e t s . porphyritic hornblende microdiorlte; minor pyrite along fractures, f.g. d i o r i t e breccia with very minor native copper velnlets. microdiorlte breccia with pervasive epidote alteration; native copper velnlets common, f.g. d i o r i t e breccia with chalcopyrite + pyrite + carbonate velnlets & trace molybdenite, carbonatized microdiorlte breccia with chalcopyrite. pale greenish brown micromonzonite with fracture linings of calcite & chalcocite. pinkish, f.g. syenite; c h l o r i t e + epidote alterations prominent; trace diss, magnetite, l i g h t brown porphyritic monzonlte with f.g. matrix; non-mineralized, m.g. syenite with prominent epidote alteration, f.g. syenite with prominent c h l o r i t e alteration. porphyritic syenite with f.g. matrix; mafic minerals replaced by epidote patches, highly altered microdiorlte breccia (?); abundant native copper + chalcocite + amorphous, m.g. monzonlte with pervasive epidote alteration & very minor native copper v e l n l e t s . microdiorlte breccia with intense chlorite + epidote alterations & specks of copper. 121 APPENDIX II. (B) continued. SAMPLE l S i 0 2 A 1 2 ° 3 F e2°3 M s ° Tertiary Volcanic Rocks CWA-101 46.99 14.52 6.76 5.09 CWA-9 41.04 13.68 7.79 6.04 Latltes GWA-5 60.24 14.06 6.44 5.08 GHA-8 63.95 15.54 4.18 2.98 GWA-90 66.88 15.79 3.40 1.60 GWA-139 64.26 15.55 4.14 3.05 GWA-150 56.55 15.45 5.15 3.97 GWA-154 50.39 14.73 6.88 7.21 GWA-4 41.85 13.03 6.15 5.45 GWA-24 42.90 12.88 6.35 6.56 Carbonat es GWA-14 7 32.15 5.39 8.01 9.03 6-2-2 49.68 18.54 4.61 2.73 GWA-155 52.55 18.74 1.80 2.64 6-1-1 46.07 15.65 8.02 3.58 30-10 56.44 15.40 2.61 2.95 Nicola Volcanics 6-11-3 44.58 10.51 6.25 6.16 6-11-4 40.46 9.71 10.79 10.89 6-3-11 46.50 11.12 12.12 12.81 6-11-6 47.45 14.42 12.01 8.57 Pic r i t e Basalts PC-1A 40.84 6.43 8.72 24.91 PC-IB 41.61 6.68 8.76 21.75 CaO Na 20 K 2° T i 0 2 MnO TOTAL 7.03 3.66 2 .19 1.13 0.11 87.48 8.71 3.10 2.18 0.98 0.15 83.67 1.05 3.74 2 .64 1.10 0.06 94.41 0.59 3.93 2.49 0.49 0.02 94.17 0.48 5.05 2 .29 0.50 0.02 96.01 0.61 5.23 2 22 0.49 0.03 95.58 3.08 4.10 3 69 1.02 0.08 93.09 4.83 3.44 2 89 0.92 0.09 91.38 9.23 1.46 3 72 1.01 0.14 82.04 8.55 2.03 2 64 1.07 0.14 83.12 15.63 0.06 1. 45 0.28 0.23 72.23 4.79 3.86 3 50 0.69 0.07 88.47 4.94 6.45 2. 37 0.64 0.20 90.33 . 5.23 3.95 2. 68 0.63 0.50 86.31 5.14 0.15 4. 27 0.54 0.07 87.57 9.55 0.09 3. 03 0.63 0.17 80.97 8.07 0.18 1. 89 0.56 0.21 82.76 7.73 2.29 0.84 0.80 0.27 94.48 7.43 3.91 0. 27 0.81 0.23 95.10 4.78 0.18 3. 17 0.33 0.15 89.51 6.83 0.43 3.01 0.33 0.15 89.55 FIELD NAME 6, REMARKS greenish grey tuffaceous andesite with minor carbonate veins, massive, medium grey andesite; mafic minerals replaced by carbonate. reddish porphyritic hornblende l a t i t e ; c h l o r i t e alteration common. fresh, massive porphyritic hornblende l a t i t e . reddish l a t i t e , s l i g h t l y porphyritic with hornblende; some quartz & K-feldspar eyes. reddish l a t i t e , s l i g h t l y tuffaceous looking; miarolitic cavity f i l l e d with c h l o r i t e & pink l a t i t e with eyes of quartz and carbonate. dark green porphyritic hornblende l a t i t e , c.f. lamprophyre. massive l a t i t e with pervasive carbonate alteration. li g h t grey porphyritic l a t i t e ; carbonate veins and alteration common. essentially a quartz-ankerite rock; r e l i c texture c.f. p i c r i t e ; minor sulfides observed, pinkish porphyritic microdiorlte (?) with pervasive carbonate alteration, pervasively altered, flesh-colored d i o r i t e breccia (?). brownish, f .g. d i o r i t e breccia (?) with intense carbonate alteration, carbonatized, greyish white microdiorlte (?) with green spots (mica ?). porphyritic hornblende andesite with pervasive carbonate alteration, porphyritic hornblende andesite with intense carbonate alteration, hornblende-rich l a p l l l i s t o n e . hornblende-rich tuffaceous andesite; epidote alteration prominent. porphyritic green nodule with abundant clinopyroxene and serpentinized o l i v i n e . fine grained, yellowish green, f r i a b l e crust surrounding the fresher-looking nodules. 

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