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A lead isotope study of selected precious metal deposits in British Columbia Andrew, Anne 1982

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A LEAD ISOTOPE STUDY OF .SELECTED PRECIOUS METAL DEPOSITS I N B R I T I S H COLUMBIA by ANNE ANDREW B. S c , The U n i v e r s i t y Of E d i n b u r g h , 1980 A T H E S I S SUBMITTED I N P A R T I A L FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES D e p a r t m e n t Of G e o l o g i c a l S c i e n c e s We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA A u g u s t 1982 © A n n e - A n d r e w , 1982 In 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 of t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e 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 D e p a r t m e n t 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 or p u b l i c a t i o n of 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 . D e p a r t m e n t of G e o l o g i c a l S c i e n c e s 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 V a n c o u v e r , Canada V6T 1W5 D a t e : 24 A u g u s t 1982 i i A b s t r a c t Lead i s o t o p e analyses of galena from m u l t i p l e ore d e p o s i t s r e s t r i c t e d to s p e c i f i c t e c t o n o - s t r a t i g r a p h i c t e r r a n e s can pro v i d e i n f o r m a t i o n on the age and o r i g i n of the l e a d . In t h i s t h e s i s , three separate s t u d i e s of l e a d i s o t o p e s a p p l i e d to the m e t a l l o g e n e s i s of p a r t s of the Canadian C o r d i l l e r a are presented. Lead i s o t o p e data from q u a r t z - g o l d v e i n d e p o s i t s and volca n o g e n i c and r e l a t e d d e p o s i t s i n the I n s u l a r B e l t group p l o t i n four d i s t i n c t c l u s t e r s on Pb-Pb p l o t s . Each c l u s t e r corresponds to a s p e c i f i c d e p o s i t type and host rock c a t e g o r y . Two p a r a l l e l e v o l u t i o n a r y trends i n the l e a d i s o t o p i c composition e x i s t : 1 ) S i c k e r - h o s t e d v o l c a n o g e n i c d e p o s i t s to S i c k e r - h o s t e d v e i n s , and 2 ) Karmutsen and Bonanza-hosted volcanogenic and r e l a t e d d e p o s i t s to Karmutsen and Bonanza-hosted v e i n s . The trends i n d i c a t e a g e n e t i c r e l a t i o n s h i p between host rock and i s o t o p i c c o m p o s i t i o n . These o b s e r v a t i o n s favour a host rock source f o r the l e a d i n vein d e p o s i t s and, by a s s o c i a t i o n , a comparable source f o r the g o l d . P l u t o n i c or a b y s s a l d i r e c t sources of metals are not c o n s i s t e n t with the l e a d i s o t o p i c d a t a . It i s suggested that the g o l d was e x t r a c t e d from the country rock, and c o n c e n t r a t e d as v e i n s by hydrothermal a c t i v i t y r e l a t e d to T e r t i a r y p l u t o n s . V e i n d e p o s i t s are i s o t o p i c a l l y d i s t i n c t from volcanogenic and r e l a t e d d e p o s i t s , p r o v i d i n g an e m p i r i c a l t e s t f o r d i s t i n g u i s h i n g syngenetic from e p i g e n e t i c d e p o s i t s . Karmutsen and Bonanza-hosted d e p o s i t s are more i i i d e p l e t e d i n 2 0 7 P b t h a n s i m i l a r d e p o s i t s i n S i c k e r G r o u p r o c k s , i n d i c a t i n g s i g n i f i c a n t l y d i f f e r e n t s o u r c e s f o r v o l c a n i c c o m p o n e n t s o f t h e s e two i m p o r t a n t r o c k u n i t s . L e a d i s o t o p e d a t a f r o m q u a r t z - g o l d v e i n s i n t h e C a r i b o o a r e a o f t h e Omineca B e l t , and f r o m s i m i l a r v e i n s i n t h e a d j a c e n t I n t e r m o n t a n e B e l t i n d i c a t e t h a t t h e s e two v e i n t y p e s a r e g e n e t i c a l l y u n r e l a t e d . A m i d - M e s o z o i c m o d e l age c a l c u l a t e d f o r t h e C a r i b o o g o l d m i n e r a l i s a t i o n e v e n t i n d i c a t e s t h a t a l l of t h e d e p o s i t s e x a m i n e d a r e c l e a r l y e p i g e n e t i c , d e s p i t e r e p o r t e d s t r a t i f o r m t e x t u r e s a t t h e M o s q u i t o C r e e k m i n e . K-Ar d a t e s f r o m a q u a r t z - b a r i t e v e i n and f r o m r e g i o n a l l y m e t a m o r p h o s e d p h y l l i t e s u p p o r t a s y n m e t a m o r p h i c o r i g i n f o r t h e v e i n s , b u t a d i s t a l p l u t o n i c o r i g i n i s n o t r u l e d o u t . R e c e n t work by G o d w i n a n d S i n c l a i r ( 1 9 8 2 ) h a s shown t h a t s y n g e n e t i c , s h a l e - h o s t e d , s e d i m e n t a r y e x h a l a t i v e d e p o s i t s i n t h e a u t o c h t h o n o u s p a r t of t h e C a n a d i a n C o r d i l l e r a c o n t a i n l e a d w h i c h has e v o l v e d i n a h i g h U/Pb e n v i r o n m e n t . T h i s ' s h a l e ' c u r v e e v o l u t i o n m o d el a p p l i e s t o d e p o s i t s w h i c h have an u p p e r c r u s t a l ( h o s t - r o c k ) l e a d s o u r c e . A i n s w o r t h - B l u e b e l l , C a r m i and S l o c a n camps, a n d l e a d a s s o c i a t e d w i t h t h e M o y i e i n t r u s i o n s , a l l c o n t a i n l e a d w h i c h p l o t s s u b s t a n t i a l l y and v a r i a b l y b e l o w t h e ' s h a l e ' c u r v e . T h e i r d e p a r t u r e s f r o m t h i s c u r v e p r o v i d e e v i d e n c e f o r a s e c o n d , u r a n i u m p o o r , p o s s i b l y l o w e r c r u s t a l l e a d s o u r c e , f o r w h i c h a g r o w t h c u r v e , r e f e r r e d t o a s t h e B l u e b e l l c u r v e , c a n be c o n s t r u c t e d . The l e a d d a t a a r e i n t e r p r e t e d w i t h i n t h e f r a m e w o r k p r o v i d e d by t h e s e two g r o w t h c u r v e s . M i x i n g o f l e a d b e t w e e n t h e s e two l e a d r e s e r v o i r s i s i v proposed to e x p l a i n the l i n e a r a r r a y of data from Slocan and Carmi camps. Mixing l i n e s , j o i n i n g p o i n t s of equal time on the two growth cu r v e s , p r o v i d e a method f o r i n t e r p r e t i n g l e a d data from these d e p o s i t s . I n t r o d u c t i o n of r e l a t i v e l y u n r a d i o g e n i c l e a d i n t o the upper c r u s t v i a magmas which o r i g i n a t e d i n the lower c r u s t i s invoked to e x p l a i n the mixing. The three s t u d i e s c o n s i d e r e d here i l l u s t r a t e the d i f f e r e n c e s i n l e a d i s o t o p i c c h a r a c t e r i s t i c s of d i f f e r e n t t e c t o n o - s t r a t i g r a p h i c t e r r a n e s and show that the development of l o c a l models f o r the i n t e r p r e t a t i o n of common lea d i s o t o p e data has a p p l i c a t i o n to e x p l o r a t i o n . V Table of Contents A b s t r a c t i i L i s t of Tables . v i i L i s t of F i g u r e s v i i i Acknowledgment x CHAPTER 1. GENERAL INTRODUCTION 1 1 . 1 SCOPE OF THESIS 1 1.2 HISTORY OF LEAD ISOTOPE INTERPRETATION 2 1.3 COMMON LEAD ISOTOPE SYSTEMATICS 6 CHAPTER 2. AN EXAMINATION OF GOLD METALLOGENY IN IN THE INSULAR BELT OF THE CANADIAN CORDILLERA USING GALENA LEAD ISOTOPE ANALYSES 10 2.1 INTRODUCTION 10 2.2 ANALYTICAL TECHNIQUES 10 2 . 3 DATA 12 2.3.1 C l u s t e r 1: S i c k e r Group Volcanogenic and Related Deposits 12 2.3.2 C l u s t e r 2: Karmutsen and Bonanza Group Volcanogenic and Related Deposits ...18 2.3.3 C l u s t e r 3: T e r t i a r y Veins in Karmutsen or Bonanza Group Rocks 19 2.3.4 C l u s t e r 4: T e r t i a r y Veins i n S i c k e r Group Rocks 20 2.4 LEAD ISOTOPE MODELS 21 2.5 SOURCE OF GOLD 2 3 2.6 CONCLUSIONS 25 CHAPTER 3. GENESIS OF CARIBOO GOLD MINERALISATION AS DEFINED BY LEAD AND K~AR ISOTOPIC STUDIES 2 7 3.1 INTRODUCTION 2 7 3.2 GEOLOGY .32 3.3 LEAD ISOTOPE DATA 33 3.4 DISCUSSION - 36 3.5 CONCLUSIONS 40 CHAPTER 4. A RE-EXAMINATION OF LEAD DATA FROM MOYIE, AINSWORTH-BLUEBELL, SLOCAN AND CARMI CAMPS, SOUTHEASTERN BRITISH COLUMBIA 42 4.1 INTRODUCTION 42 4.2 GEOLOGY 43 4.2.1 T e c t o n o - s t r a t i g r a p h i c S e t t i n g 43 4.2.2 Slocan Camp 45 4.2.3 Carmi Camp 45 4.2.4 A i n s w o r t h - B l u e b e l l Camp 46 4.2.5 Moyie I n t r u s i o n s 47 4.3 SOURCE OF DATA AND ANALYICAL PROCEDURES 47 4.4 DATA INTERPRETATION 48 4.4.1 Shale Curve Model (Godwin and S i n c l a i r , 1982) .48 4.4.2 Evidence f o r the e x i s t e n c e of a second le a d source 50 4.4.3 E s t i m a t i o n of the ' B l u e b e l l ' l e a d e v o l u t i o n curve , 50 4.4.4 Mixing L i n e Isochrons 58 v i 4.5 DISCUSSION 60 4.6 CONCLUSIONS AND SUGGESTIONS FOR. FURTHER WORK 68 CHAPTER 5. CONCLUSIONS 70 REFERENCES 73 APPENDIX A - ORE LEAD SAMPLE PREPARATION AND ANALYSIS 81 APPENDIX B - B R I E F DESCRIPTIONS OF THE DEPOSITS NAMED I N CHAPTER 2 92 APPENDIX C - D E T A I L S OF THE DATA COLLECTION AND REDUCTION OF LE COUTEUR, 1 973 .18:.-•'• v i i L i s t of Tables Table page 1.1 Equations used i n l e a d isotope model c a l c u l a t i o n s 7 2.1 Galena Lead Isotope Analyses from M i n e r a l D e p o s i t s ; I n s u l a r B e l t , B r i t i s h Columbia 13 3.1 Galena Lead Isotope Analyses from M i n e r a l D e p o s i t s ; Cariboo area (Omineca B e l t and adjacent Intermontane B e l t ) , s o u t h - c e n t r a l B r i t i s h Columbia 29 3.2 Sample data f o r K and Ar analyses of whole rock and Muscovite from Cariboo Gold Quartz 31 4.1 Shale Curve ( a f t e r Godwin and S i n c l a i r , 1982) 49 4.2 Galena Lead Isotope Analyses from M i n e r a l D e p o s i t s ; Slocan and Carmi camps, southern B r i t i s h Columbia 51 4.3 Galena Lead Isotope Analyses from M i n e r a l D e p o s i t s ; A i n s w o r t h - B l u e b e l l camp, southeastern B r i t i s h Columbia 52 4.4 Galena Lead Isotope Analyses from M i n e r a l D e p o s i t s ; Moyie i n t r u s i o n s , southeastern B r i t i s h Columbia 53 4.5. B l u e b e l l curve: „ 2 = 9.23, k = 4.22 56 4.6 I s o t o p i c analyses of l e a d in f e l d s p a r from the Nelson b a t h o l i t h . A f t e r Reynolds and S i n c l a i r , 1 971 62 4.7 Percentage contamination of assumed lower c r u s t a l l e a d with upper c r u s t a l lead f o r f a c t o r s of three and ten times more l e a d in the upper c r u s t a l source, Slocan camp 67 v i i i L i s t of F i g u r e s F i g u r e Page 1.1 Holmes-Houtermans model. Upper: The curved l i n e s are l e a d growth curves f o r U-Pb systems having present-day » values of 8 and 10. The s t r a i g h t l i n e s are isochrons f o r two v a l u e s of t , 2.0 and 0.0 Ga. Lower: I s o t o p i c e v o l u t i o n of common le a d in c o - o r d i n a t e s of 2 0 8 P b / 2 C M , P b and 2 0 6 P b / 2 0 4Pb a c c o r d i n g to the Holmes-Houtermans model. Taken from Faure , 1 977 3 2.1 L o c a t i o n of g a l e n a - l e a d i s o t o p e analyses (Table 2.1) i n I n s u l a r B e l t , Canadian C o r d i l l e r a ; symbols are d e s c r i b e d in F i g u r e 2.2 11 2.2 Galena-lead p l o t of 2 0 7 P b / 2 0 6 P b vs. 2 O 6 P b / 2 O 0 P b using data from Table 2.1. C i r c l e s denote d e p o s i t s r e l a t e d to Karmutsen or Bonanza Group rocks; square symbols are used for d e p o s i t s r e l a t e d to the S i c k e r Group rocks; f i l l e d symbols are used f o r v e i n s . Unreasonable analyses are c r o s s e d and are not used i n the i n t e r p r e t a t i o n . Bars mark standard d e v i a t i o n of the mean and l i n e s extend to standard d e v i a t i o n . Lines 1 and 2 represent the e v o l u t i o n of l e a d from t, to t 2 f o r v a r y i n g values of t>; t 2 f o r C l u s t e r 4 i s not known with c e r t a i n t y (see t e x t ) . The average growth curve shown i s that of Stacey and Kramers ( 1 975) 16 2.3 Galena-lead p l o t of 2 0 8 P b / 2 0 " P b vs. 2 0 6 Pb/ 2 0' tPb data from Table 2.1. Symbols are d e s c r i b e d i n F i g u r e 2.2. L i n e s 3 and 4 represent the e v o l u t i o n of lead from t , to t 2 for v a r y i n g values of w i f k i s 3.3 and 3.9 r e s p e c t i v e l y . The growth curve shown i s that of Stacey and Kramers (1975) 17 3.1 L o c a t i o n s of gold d e p o s i t s from which g a l e n a - l e a d i s o t o p e analyses were obtained ( r e f e r to Table 3.1 f o r d e p o s i t names). K-Ar analyses were obtained from a v e i n and p h y l l i t e near CG. The l i n e d i v i d i n g the Omineca and Intermontane -Belts i s probably a g e n e r a l l y west-dipping t h r u s t (see t e x t ) . C i s Cariboo B e l l porphyry, and M i s M i t c h e l l Bay porphyry 28 3 . 2 2 0 7 Pb/ 2 0 1 ,Pb vs. 2 0 6Pb/ 2 0' ,Pb p l o t of galena analyses from Cariboo and adjacent Intermontane B e l t gold d e p o s i t s (Table 3.1). Symbols are the same a those of F i g u r e 3.1 34 i x 3 . 3 2 0 8 P b / 2 0 l , P b vs. 2 0 6 P b / 2 0 , t P b p l o t of galena analyses from Cariboo and adjacent Intermontane B e l t g o l d d e p o s i t s (Table 3.1). Symbols are the same as those of F i g u r e 3.1 35 3.4 Comparison of Omineca, I n s u l a r and Intermontane B e l t g a l e n a - l e a d isotope r a t i o s . T r i a n g l e s represent Intermontane B e l t d e p o s i t s (Table 3.1). E r r o r bars on the f i v e c l u s t e r s mark the standard d e v i a t i o n of f o r each. The two growth curves shown are the 'shale' curve (Godwin and S i n c l a i r , 1982), and Stacey and Kramers (1975) curve 39 4.1 T e c t o n o - s t r a t i g r a p h i c t e r r a n e s of southeastern B r i t i s h Columbia and the approximate l o c a t i o n s of the mining camps d i s c u s s e d i n the tex t 44 4.2 2 0 7 P b / 2 0 " P b vs. 2 0 6 P b / 2 0 * P b p l o t of g a l e n a - l e a d data from Moyie, A i n s w o r t h - B l u e b e l l , Carmi and Slocan camps (Tables 4.2, 4.3 and 4.4). The 'shale' curve (Godwin and S i n c l a i r , 1982) i s shown f o r re f e r e n c e 54 4.3 2 0 7 P b / 2 0 a P b vs. 2 0 6 P b / 2 0 " P b p l o t showing the c l o s e f i t of the data l i s t e d in Tables 4.2, 4.3 and 4.4 with mixing l i n e s drawn between 'shale' curve (Godwin and S i n c l a i r , 1982) and the B l u e b e l l curve at 0.16, 0.58 and 1.44 Ga respect i v e l y 57 4.4 Summary of the changes in mu with time f o r the 'shale' curve (Godwin and S i n c l a i r , 1982), Stacey and Kramers (1975) average l e a d growth curve, and the B l u e b e l l curve 59 4.5 D e t a i l e d 2 0 7 P b / 2 0 a P b vs. 2 0 6 P b / 2 0 " P b p l o t of g a l e n a - l e a d data from Carmi and Slocan camps (Table 4.2) showing the d i s t r i b u t i o n of values along the 0.16 Ga mixing l i n e between the 'shale' curve (Godwin and S i n c l a i r , 1982) and the B l u e b e l l curve. Four analyses of f e l d s p a r l e a d ( a f t e r Reynolds and S i n c l a i r , 1971; Table 4.6) from the Nelson b a t h o l i t h are a l s o shown 61 4.6 Contoured 2 0 6 P b / 2 0 l t P b r a t i o s f o r Slocan camp data (Table 4.2). Geology a f t e r C a i r n e s (1934) and L i t t l e (1960). Adapted from Le Couteur (1973) 64 4.7 2 0 8 P b / 2 0 f l P b vs. 2 0 6 P b / 2 o a P b p l o t of ga l e n a - l e a d data from Moyie, A i n s w o r t h - B l u e b e l l , Carmi and Slocan camps (Tables 4.2, 4.3 and 4.4). The 'shale' curve (Godwin and S i n c l a i r , 1982) i s shown f o r r e f e r e n c e 65 X Acknowledgement I would l i k e to thank my s u p e r v i s o r s , Dr. C.I. Godwin and Dr. A.J. S i n c l a i r , f o r suggesting t h i s study. T h e i r enthusiasm and h e l p at a l l stages i n the development of t h i s t h e s i s i s s i n c e r e l y a p p r e c i a t e d . I am g r a t e f u l to Dr. R.A. P a r r i s h f o r h i s p a t i e n c e in t e a c h i n g me the mysteries of f i l a m e n t l o a d i n g and mass spectrometry and f o r reading the manuscript.- Dr. R.L. Armstrong a l s o p r o v i d e d a s s i s t a n c e i n the l a b and p r o v i d e d i n f o r m a t i v e d i s c u s s i o n s . V i c H o l l i s t e r arranged my v i s i t to the Cariboo area, and F. Beaumann guided me through the geology of the C a r i b o o Gold Quartz p r o s p e c t . I am g r a t e f u l to the s t a f f at Mosquito Creek mine f o r showing me the underground workings at short n o t i c e . H. Meade, D. K l e p a c k i , P. Wodjak and T. Hoy c o n t r i b u t e d ideas which are d i s c u s s e d in t h i s t h e s i s . John Newlands d r a f t e d s e v e r a l f i g u r e s . My husband, E r i c , p r o v i d e d i n c a l c u l a b l e s u p p o r t i v e h e l p , i n c l u d i n g a s s i s t a n c e with d r a f t i n g and t y p i n g . The f i n a n c i a l support of a Graduate Student F e l l o w s h i p bursary i s g r a t e f u l l y acknowledged. 1 CHAPTER 1  GENERAL INTRODUCTION 1.1 SCOPE OF THESIS Galena-lead isotope data from mineral d e p o s i t s in B r i t i s h Columbia, Yukon T e r r i t o r y and Northwest T e r r i t o r i e s have been compiled at the U n i v e r s i t y of B r i t i s h Columbia as part of a systematic study of l e a d i s o t o p e s a p p l i e d to metallogeny i n the Canadian C o r d i l l e r a . Previous s t u d i e s of t h i s type by LeCouteur (1973), S i n c l a i r (1964), Godwin, S i n c l a i r and Ryan (1978, 1979, 1980 and 1982), Godwin and S i n c l a i r (1982) and S i n c l a i r and Godwin (1981) have shown that g a l e n a - l e a d isotope analyses can provide u s e f u l i n f o r m a t i o n on the o r i g i n and age of ore d e p o s i t s . T h i s t h e s i s i s based on an examination of e x i s t i n g and new data from three major p r e c i o u s metal-producing areas of B r i t i s h Columbia, and i s presented i n the form of three papers. The f i r s t of these examines data from volcanogenic and r e l a t e d d e p o s i t s , and q u a r t z - g o l d v e i n s i n the I n s u l a r B e l t , i n an attempt to understand the metallogeny of the g o l d . The second de a l s with data from gold d e p o s i t s i n both the Omineca and Intermontane t e c t o n i c b e l t s in the Cariboo area of south c e n t r a l B r i t i s h Columbia. The p o s s i b i l i t y of syngenetic gold m i n e r a l i s a t i o n in the area i s i n v e s t i g a t e d and a model age f o r the m i n e r a l i s a t i o n proposed. Two K-Ar dates are reported and t h e i r s i g n i f i c a n c e d i s c u s s e d . The t h i r d study re-examines the data of Le Couteur (1973) from Slocan, and A i n s w o r t h - B l u e b e l l 2 camps, and from l e a d i n Moyie i n t r u s i o n s . Anomalous values f o r the i s o t o p i c composition of l e a d at Ainsworth and B l u e b e l l suggest the p o s s i b i l i t y of an unradiogenic l e a d source i n a region otherwise c h a r a c t e r i s e d by upper c r u s t a l l e a d . A t e n t a t i v e model i s presented f o r the e v o l u t i o n of le a d i n an unradiogenic r e s e r v o i r and a process of mixing of le a d between r e s e r v o i r s i s invoked to e x p l a i n l i n e a r a r r a y s which are observed i n the l e a d data from s i l v e r d e p o s i t s i n the Slocan and Carmi camp. The s i g n i f i c a n c e and l i m i t a t i o n s of t h i s new approach to the i n t e r p r e t a t i o n of le a d isotope data are d i s c u s s e d . A f i n a l chapter summarises the major c o n c l u s i o n s and d e s c r i b e s the e x p l o r a t i o n a p p l i c a t i o n s that emerge. The h i s t o r y of the development of lead isotope i n t e r p r e t a t i o n s , an exp l a n a t i o n of common lead s y s t e m a t i c s , and the c o n t r o l s on the composition of l e a d i n an ore d e p o s i t are o u t l i n e d in the f o l l o w i n g s e c t i o n s of chapter one. 1.2 HISTORY OF LEAD ISOTOPE INTERPRETATION The study of lead i s o t o p e s was i n i t i a t e d by Holmes (1946) and Houtermans (1946), who independently d e r i v e d equations to d e s c r i b e the e v o l u t i o n of lead isotope r a t i o s with time, based on the a d d i t i o n of r a d i o g e n i c l e a d to common l e a d by the r a d i o a c t i v e decay of uranium and thorium i s o t o p e s . Thus the Holmes-Houtermans model, i l l u s t r a t e d i n Fi g u r e 1.1, was the f i r s t to be used i n i n t e r p r e t i n g l e a d data. Analyses which gave ages which c o n f l i c t e d with e x i s t i n g age data were termed 'anomalous' by Holmes, and c o u l d not be i n t e r p r e t e d w i t h i n the framework of the model. HOLMES-HOUTERMANS MODEL -Q o D_ o CM -18 H6 H4 H12 hio t = 2.0 by. r4.55 Ga Primeval Lead 10 i 12 14 _ l 16 _J 18 _ j 20 22 _ i 2 0 6 p b / 2 0 4 p b .Q Q_ *T o CM _Q Q. CO o CM T I 1 i i / ° -38 / l Ga -36 Ga -34 X3 Ga --32 / 4 6a --30 Primeval Lead _ 1 P 1,2 14 1,6 1,8 2 0 6 p b / 2 0 4 p b Figure 1.1 Holmes-Houtermans model. Upper: The curved l i n e s are lead growth curves f or U-Pb systems having present-day values of 8 and 10. The s t r a i g h t l i n e s are isochrons f o r two values of t; 2.0 Ga and 0 Ga. Lower: Isotop jggevo^t.ion of Cj^^mon^^ead i n co-ordinates of Pb/ Pb and Pb/ Pb according to the Holmes-Houtermans model. Taken from Faure, 1977. 4 F u r t h e r a n a l y s e s of g a l e n a samples and improvements i n a n a l y t i c a l t e c h n i q u e s - r e v e a l e d t h a t a n a l y s e s o f g a l e n a - l e a d f r o m most d e p o s i t s p l o t t e d c l o s e t o a s i n g l e ' p r i m a r y g r o w th c u r v e ' d i s c u s s e d i n s e v e r a l p a p e r s by R u s s e l l , F a r q u h a r and Cumming ( R u s s e l l and F a r q u h a r , 1960). At t h i s s t a g e t h e a p p r o x i m a t i o n t o a s i n g l e s t a g e g r o w th c u r v e was a t t r i b u t e d t o h o m o g e n i s a t i o n by c r u s t a l m i x i n g p r o c e s s e s . S t a n t o n and R u s s e l l ( 1 9 5 9 ) , however, n o t e d t h a t l a r g e , s t r a t i f o r m v o l c a n o g e n i c o r e d e p o s i t s c o n t a i n e d l e a d t h a t p l o t t e d on a g r o w t h c u r v e , i n d i s t i n g u i s h a b l e from t h e p r i m a r y g r o w t h c u r v e i . e . ' o r d i n a r y ' l e a d , and c o n c l u d e d t h a t s i n c e t h e l e a d was p r e s u m a b l y v o l c a n i c i n o r i g i n , t h e p r i m a r y g r o w t h c u r v e must r e p r e s e n t l e a d e v o l u t i o n i n t h e upper m a n t l e , w h i c h was i n t e r p r e t e d as b e i n g homogeneous i n i s o t o p i c c o m p o s i t i o n . Numerous a n a l y s e s of mid-ocean r i d g e and ocean i s l a n d b a s a l t s have shown t h a t t h i s i s not t h e c a s e . S e v e r a l r e v i s i o n s of t h e p r i m a r y g r o w th c u r v e were made u s i n g more p r e c i s e d a t a ( K o l l a r , 1960; O s t i c e t a l . , 1963; R u s s e l l and R e y n o l d s , 1965; O s t i c e t a l . , 1967; Cooper e t a l . , 1969; and S t a c e y e t a l . , 1969). R e v i s i o n o f u r a n i u m and t h o r i u m d e c a y c o n s t a n t s i n t h e mid-1970s c a u s e d t h e p a r a m e t e r s u s e d i n t h e c a l c u l a t i o n of t h e p r i m a r y g r o w t h c u r v e t o be m o d i f i e d i n s u c h a way t h a t a s i n g l e -s t a g e g r o w th c u r v e c o u l d not p a s s t h r o u g h a l l t h e d a t a p o i n t s a d e q u a t e l y , and t h e c o m p l e x i t y of l e a d i s o t o p e e v o l u t i o n became a p p a r e n t . S t a c e y and Kramers (1975) e m p i r i c a l l y p r o d u c e d a s i m p l e t w o - s t a g e model i n w h i c h t h e 2 3 8 U / 2 0 < t P b (») e n v i r o n m e n t i n c r e a s e d f r o m 7.19 t o 9.74 a t 3.7 b i l l i o n y e a r s . The s e c o n d s t a g e commences a t a t i m e w h i c h r e c u r s i n i s o t o p e s t u d i e s , and 5 corresponds to the approximate age of the o l d e s t known rocks. I t i s probably the time at which the e a r t h ceased to be w e l l mixed and formation of s t a b l e c r u s t caused i s o t o p i c enrichment and d i f f e r e n t i a t i o n processes -to begin. A s i m i l a r curve was generated by Cumming and Richards (1975) using a c o n t i n u o u s l y i n c r e a s i n g \> and w model. Both of these curves c l o s e l y f i t the lea d data f o r many l a r g e conformable d e p o s i t s . A second approach to the study of le a d i s o t o p e s i s to p r e d i c t l e a d e v o l u t i o n by developing computer models to simulate p l a t e t e c t o n i c mixing of c r u s t and mantle r e s e r v o i r s , r a t h e r than, as before, e m p i r i c a l l y f i t t i n g curves to l e a d a n a l y s e s . Armstrong (1968) and Armstrong and Hein (1973) r e c o n c i l e d l e a d and strontium e v o l u t i o n in a heterogeneous e a r t h , assuming i s o t o p i c exchange between c r u s t and mantle at r a t e s which decrease e x p o n e n t i a l l y with time. A s i m i l a r approach was taken by Doe and Zartman (1979), who subdi v i d e d c r u s t i n t o upper and lower c r u s t , based on the ob s e r v a t i o n of d e p l e t e d lead r a t i o s in g r a n u l i t e s . They c a l c u l a t e d l e a d e v o l u t i o n i n each of four environments; the mantle, 'orogene', lower c r u s t and upper c r u s t , assuming mixing of l e a d between r e s e r v o i r s at evenly-spaced orogenies. A l l the parameters used, however, are estimated, and not based on r e a l s i t u a t i o n s , so the r e s u l t i n g curves do not c l o s e l y f i t e x i s t i n g data. The study does, however, i l l u s t r a t e the point that l e a d e v o l u t i o n i s p a r t l y dependent on p l a t e t e c t o n i c s . Thus, v a r i a t i o n s i n i s o t o p i c c h a r a c t e r i s t i c s are to be expected between s e c t i o n s of the c r u s t which have had d i f f e r e n t p l a t e t e c t o n i c h i s t o r i e s . 6 C o r r e l a t i o n between the i s o t o p i c composition of magmatic and ore leads and t h e i r t e c t o n i c s e t t i n g was demonstrated by Zartman (1974) and i s fundamental to the work of Godwin and S i n c l a i r (1982) who e m p i r i c a l l y developed a growth curve a p p l i c a b l e to a s p e c i f i c l a r g e segment of the Canadian C o r d i l l e r a . T h i s t h e s i s a l s o notes d i f f e r e n c e s i n l e a d i s o t o p i c c h a r a c t e r i s t i c s of separate t e c t o n o - s t r a t i g r a p h i c t e r r a n e s i n the Canadian C o r d i l l e r a , and a r r a y s of i s o t o p i c data due to mixing between d i f f e r e n t r e s e r v o i r s w i t h i n these t e r r a n e s . 1.3 COMMON LEAD ISOTOPE SYSTEMATICS There are four s t a b l e i s o t o p e s of l e a d : 2 0 a P b , 2 0 6 P b , 2 0 7 P b and 2 0 8 P b , the heavier three being end-products of the r a d i o a c t i v e decay of parent uranium and thorium i s o t o p e s . The 2 0 " P b isotope i s not r a d i o g e n i c and t h e r e f o r e has a f i x e d t e r r e s t r i a l abundance. 2 0 6 P b / 2 0 a P b , 2 0 7 Pb/ 2 0 a Pb, and 2 0 B P b / 2 0 < , P b r a t i o s i n c r e a s e c o n t i n u o u s l y with time, due to the a d d i t i o n of r a d i o g e n i c l e a d . I s o l a t i o n of the l e a d from i t s parent i s o t o p e s by formation of l e a d minerals such as galena, f r e e z e s the i s o t o p i c composition. A n a l y s i s of these uranium- and thorium-poor minerals r e v e a l s the i s o t o p i c composition of the mineral at i t s time of formation. Galena i s a common ore mineral so any i n f o r m a t i o n which can be d e r i v e d from the a n a l y s i s of galena-l e a d w i l l be u s e f u l f o r a wide v a r i e t y of ore d e p o s i t s . The equations l i s t e d i n Table 1.1 show that the amount of r a d i o g e n i c l e a d produced in a uranium and thorium-bearing c l o s e d system, depends on the length of time that the uranium and thorium i s o t o p e s are able to decay, and on the c o n c e n t r a t i o n s of 7 Table 1.1 Equations Used i n Lead Isotope Model Cal c u l a t i o n s Equation 1: Equation 2: Equation 3: Equation 4: Equation 5: -J u2 J ^2 L 2 0 V j t m 207-206 m Wher e: 238TT,204, u = >U/^Pb 206 Pb 204 Pb. 207 Pb 204 Pb J "1 208. Pb 204 2 Pb + u ( e X l  Zl - e h t 2 ) + 137T88 H ^ ' l - eV2); + wCeM'l - e x3 t2) 137.88 2 07_. .204_~1 _ Fo7 T l U /204^ Pb/ PbJ t - [_ Pb/ Pb 206_, ,204_.-l „ "206^ .204„, ~1 Pb/ PbJ t 2 - Pb/ 208-206 = k e X 2 f c l - e'X2t2 e * l f c l - e ^ l ^ e ^ l - e>3t2 sH*! - e>3t2 k = 2 3 2 T h / 2 3 8 U w = ku ,-9 •\1 = 0.155125 x 10 (Jaffey et. a l . , 1971) x = 0.98485 x 10" 9 (Jaffey et a l . , 1971) -9 X 3 = 0.04 9475 x 10 (LeRoux and Glendenin, 1963) m207-206 a n d m208-206 a r e s : ]- 0P e s °^ isochrons on the Pb-Pb p l o t s . 8 l e a d , uranium and thorium (\x and k ) . Uranium and thorium are l i t h o p h i l e elements and t h e r e f o r e tend to be c o n c e n t r a t e d i n s i a l i c r o c k s . Thus the upper c r u s t a l environment i s more h i g h l y e n r i c h e d i n r a d i o g e n i c lead than i s the mantle or lower c r u s t (Doe and Zartman, 1979). Consequently, the source of l e a d i n an ore d e p o s i t w i l l i n f l u e n c e the i s o t o p i c composition of the contained l e a d . The age of m i n e r a l i s a t i o n of a d e p o s i t ( t 2 ) governs the amount of time that parent U and Th isotopes have been allowed to decay before the l e a d was separated and the r a t i o s frozen by the formation of, f o r example, galena. In a homogeneous system, ore d e p o s i t s which vary only in age of m i n e r a l i s a t i o n should f a l l along a growth curve ( F i g . 1.1) f o r which the ? and k values can be determined i f the ages of m i n e r a l i s a t i o n are known (Table 1.1: Equations 1 to 3). Processes of m i n e r a l i s a t i o n can modify the i s o t o p i c composition of an ore d e p o s i t . Enrichment of v e i n d e p o s i t s i n r a d i o g e n i c l e a d has been observed i n many s t u d i e s ( R u s s e l l and Farquhar, 1960; Heyl et a l . , 1974; Watson et a l . , 1982). T h i s process probably occurs because l a r g e i n t e r s t i t i a l U and Th atoms decay to smaller r a d i o g e n i c lead i s o t o p e s , which are more e a s i l y a v a i l a b l e to ore f l u i d s ( e s p e c i a l l y because the l a t t i c e surrounding the s i t e might be damaged by r a d i a t i o n ) , than are 2 0 " P b atoms which are t i g h t l y h e l d where they were o r i g i n a l l y accommodated in the l a t t i c e s of minerals such as f e l d s p a r or suphides. The e f f i c i e n c y with which t o t a l l e a d versus r a d i o g e n i c lead i s e x t r a c t e d depends on the m i n e r a l i s a t i o n process, and i s l i k e l y to be g r e a t e r f o r l a r g e r , massive s u l p h i d e - t y p e d e p o s i t s 9 than f o r s m a l l e r v e i n d e p o s i t s . T h i s i s probably mainly because formation of l a r g e d e p o s i t s i n v o l v e s the e f f e c t i v e scavenging of lead from c o m p a r i t i v e l y l a r g e volumes of c r u s t with l a r g e volumes of f l u i d s . The l e a d i s o t o p i c composition of galena i n an ore d e p o s i t t h e r e f o r e depends on: 1 ) the source of l e a d , 2) the age of m i n e r a l i s a t i o n , and 3) the m i n e r a l i s i n g p r o c e s s . Thus galena c o n t a i n s i n f o r m a t i o n which may be obtained with c a r e f u l a n a l y s i s and i n t e r p r e t a t i o n . Any combination of the above f a c t o r s can c o n t r i b u t e to the i s o t o p i c composition of a d e p o s i t , making i n t e r p r e t a t i o n d i f f i c u l t unless the analyses are accompanied by sound g e o l o g i c a l i n f o r m a t i o n . The data i n t h i s study were i n t e r p r e t e d w i t h i n a known framework of t e c t o n i c s e t t i n g , d e p o s i t type, host-rock type and age of m i n e r a l i s a t i o n determined, i f p o s s i b l e , by independent i s o t o p i c or s t r a t i g r a p h i c methods. 10 CHAPTER 2 AN EXAMINATION OF GOLD METALLOGENY IN THE INSULAR BELT  OF THE CANADIAN CORDILLERA USING GALENA LEAD ISOTOPE ANALYSES 2.1 INTRODUCTION Galena-lead isotope data from mineral d e p o s i t s i n B r i t i s h Columbia have been c o l l e c t e d and compiled at the U n i v e r s i t y of B r i t i s h Columbia s i n c e 1978 as part of a systematic study of gal e n a - l e a d isotopes a p p l i e d to metallogeny i n the Canadian C o r d i l l e r a . One aspect c o n s i d e r e d here concerns i s o t o p i c compositions of lead from volcanogenic and r e l a t e d d e p o s i t s and q u a r t z - g o l d veins in the I n s u l a r B e l t , i n an attempt to understand the metallogeny of g o l d - b e a r i n g d e p o s i t s . A l l the data used in t h i s study are l i s t e d i n Table 2.1. Information regarding the g e o l o g i c a l s e t t i n g , mineral a s s o c i a t i o n s and d e p o s i t type, was e x t r a c t e d from MINDEP and MINFILE computer f i l e s of mineral d e p o s i t s , i n f o r m a t i o n from annual r e p o r t s of the B.C. M i n i s t r y of Energy, Mines and Petroleum Resources, and from assessment r e p o r t s submitted to the B.C.Government. APPENDIX B d e s c r i b e s the 18 d e p o s i t s used i n t h i s study and l i s t s the c h i e f sources of info r m a t i o n f o r each. L o c a t i o n s of the d e p o s i t s are shown i n F i g u r e 2.1. 2.2 ANALYTICAL TECHNIQUES Galena samples were d i s s o l v e d using HC1. P u r i f i e d lead was obtained using anion exchange columns and anodic e l e c t r o -d e p o s i t i o n , as d e s c r i b e d in APPENDIX A. Samples were analysed Figure 2.1 L o c a t i o n o f galena-lead isotope analyses (Table 2.1) i n I n s u l a r B e l t , Canadian C o r d i l l e r a ; symbols are d e s c r i b e d i n F i g u r e 2.2. 12 using s i n g l e f i l a m e n t , s i l i c a g e l techniques on a 90 degree, 12 inch, s o l i d source mass spectrometer. In-run p r e c i s i o n , r e p o r t e d in the t a b l e s as percent standard d e v i a t i o n w i t h i n the brackets f o l l o w i n g mean i s o t o p i c r a t i o s , i s u s u a l l y b e t t e r than 0.1 percent. M u l t i p l e analyses of Broken H i l l No.1 standard shows that the r e p r o d u c i b i l i t y of sample analyses i s about 0.1 percent at one standard d e v i a t i o n . A l l data in the t a b l e s have been normalised to the Broken H i l l No.1 standard; n o r m a l i s i n g procedures assumed the f o l l o w i n g composition f o r t h i s standard: 2 O 6 p b / 2 O 4 P b = l 6 < 0 Q 3 f 2 0 7 P b / 2 o « p b = 1 5 i 3 8 9 ( 2 0 8Pb/ 2 0 aPb=35.657. A l l analyses were done by B.Ryan, in the Geology-Geophysics Laboratory, The U n i v e r s i t y of B r i t i s h Columbia. 2.3 DATA Galena-lead isotope data from Table 2.1 f a l l i n t o four c l u s t e r s on the Pb-Pb p l o t s of F i g u r e s 2.2 and 2.3. Each c l u s t e r corresponds to a d e p o s i t type and host-rock group. Vein d e p o s i t s are c o n s i s t e n t l y more e n r i c h e d i n 2 0 6 P b than are volcanogenic and r e l a t e d d e p o s i t s . S i c k e r - h o s t e d d e p o s i t s are g e n e r a l l y more en r i c h e d i n 2 0 7 P b than the Karmutsen and Bonanza-hosted d e p o s i t s . The c l u s t e r s , numbered 1 to 4, ( F i g s . 2.2 and 2.3) are d e s c r i b e d in more d e t a i l below. 2.3.1 C l u s t e r 1: S i c k e r Group Volcanogenic and R e l a t e d Deposits The S i c k e r Group i s made up of dominantly a n d e s i t i c v o l c a n i c rocks and interbedded c l a s t i c and limestone u n i t s of C a r b o n i f e r o u s to Permian age (Northcote and M u l l e r , 1972). S i c k e r v o l c a n i c rocks e x h i b i t a c o n s i d e r a b l e range i n chemical composition and are i n t e r p r e t e d as r e p r e s e n t i n g an arc-type / T O O L E 2 . 1 l G A L E N A - L E A D I S O T O P E A N A L Y S E S F R O M M I N E R A L D E P O S I T S I n s u l a r B e l t , B r i t i s h C o l u m b i a S a m p l e A n a l - M a p L a t . L o n g . L e a d I s o t o p e R a t i o s ( R e l a t i v e I S E r r o r am 7.) N u m b e r y s i s D e p o s i t N a m e N a m e N o r t h W e s t 2 0 6 / 2 0 4 2 0 7 / 2 0 4 2 0 8 / 2 0 4 C L U S T E R l i P E N N S Y L V A N I A N - P E R M I A N G 7 9 C L - 0 0 1 G 7 9 C L - 0 0 2 G 7 9 C L - 0 0 3 G 7 9 C L - A V G * G 7 9 I C - 0 0 1 G / 9 L N - 0 0 1 G 7 9 L N - 0 0 2 G 7 9 L N - A V G * G 7 9 T Y - 0 0 I G 7 9 W M - 0 0 1 G 7 9 W M - 0 0 2 B 7 9 W M - 0 0 3 G 7 9 W M - A V G * C o w i c h a n L a k e C o w l c h a n L a t e C o w i c h a n L a k e C o w i c h a n L a k e ( n = 3 ) I r o n C l a d L e n o r a L e n o r a L e n o r a ( n = 2 ) T y e e W e s t m i n : M y r a W p s t m i n : M y r a W e s t « i i n : M y r a W e s t m i n : M y r a ( n = * 3 ) C I . C L C L C L I C L N L N L N T Y WM WM WM WM 4 B . 7 A 1 2 4 . 3 A 4 S . 7 A 1 2 4 . 3 A 4 8 . 7 A 1 2 4 . 3 A 4 8 . 7 A 1 2 4 . 3 A 4 B . 8 5 1 2 3 . 6 0 4 8 . 8 7 1 2 3 . 7 8 4 8 . 8 7 1 2 3 . 7 8 4 8 . 8 7 1 2 3 . 7 B 4 8 . 8 7 1 2 3 . 7 8 4 9 . 5 7 1 2 5 . 5 9 4 9 . 3 7 1 2 5 . 5 9 4 9 . 5 7 1 2 5 . 5 9 4 9 . 5 7 1 2 5 . 3 9 1 8 . 6 4 6 ( . 0 3 ) 1 8 . 6 6 6 ( . 0 2 ) 1 8 . 7 0 2 ( . 0 7 ) 1 8 . 6 7 1 ( . 0 4 ) 1 8 . 6 8 2 ( . 0 8 ) 1 8 . 5 3 4 ( . 0 4 ) 1 8 . 5 6 2 ( . 0 8 ) 1 B . 5 4 B ( . 0 6 ) 1 8 . 5 3 8 ( . 0 8 ) 1 8 . 3 0 6 ( . 0 6 ) 1 8 . 4 B U ( . 0 6 ) 1 8 . 4 8 4 ( . 0 6 ) 1 8 . 4 9 3 ( . 0 6 ) 1 3 . 5 8 1 ( . 0 8 ) 1 5 . 5 0 9 ( . 0 9 ) 1 5 . 5 4 6 ( . 1 7 ) 1 3 . 3 7 2 ( . 1 1 ) 1 5 . 5 8 1 ( . 1 5 ) 1 3 . 5 3 8 ( . 1 3 ) 1 5 . 5 7 2 ( . 1 6 ) 1 3 . 5 3 5 ( . 1 4 ) 1 5 . 5 7 7 ( . 1 5 ) 1 3 . 5 7 9 ( . 1 0 ) 1 3 . 5 3 4 ( . 1 5 ) 1 5 . 5 5 1 ( . 1 5 ) 1 5 . 5 6 1 ( . 1 3 ) 3 8 . 2 7 6 ( . 1 3 ) 3 8 . 3 9 6 ( . 1 2 ) 3 8 . 0 8 6 ( . 3 7 ) 3 8 . 2 5 3 ( . 2 0 ) 3 8 . 3 0 4 ( . 2 0 ) 3 8 . 2 1 6 ( . 0 8 ) 3 8 . 2 3 0 ( . 1 7 ) 3 8 . 2 2 3 ( . 1 2 ) 3 8 . 1 2 3 ( . 1 9 ) 3 8 . 1 8 6 ( . 1 3 ) 3 0 . 0 8 9 ( . 0 9 ) 3 8 . 1 3 0 ( . 1 0 ) 3 8 . 1 1 3 ( . 0 9 ) N u m b e r of d e p o s i t s ( n ) = 5 N u m b e r at a n a l y s e s ( a ) = 1 0 A r i t h m e t i c a v e r a g e ( ~ ) S t a n d a r d d e v i a t i o n (S) S t d . e r r o r o- f m e a n ( S . n " * ^ ) C 1 8 . 5 9 0 ( . 0 6 ) 3 C O . 0 8 2 3 C O . 0 3 / 3 C I S . 5 6 9 ( . 1 4 ) 3 C O . 0 1 1 3 C O . 0 0 5 3 C 3 8 . 2 0 6 ( . 1 6 ) 3 C O . 0 7 9 3 C O . 0 3 5 3 r u i n i n i 21 UTTER Tiunauiu - JIWMSIHC 3 0 3 3 5 - 0 0 1 * 1 N u t c r a c k e r 3 0 3 6 6 - 0 0 1 * 1 B o n 3 0 3 1 4 - 0 0 1 * 1 S t a r l i g h t N u m b e r o f d e p o s i t s <n)= 2 N u m b e r o f a n a l y s e s < a ) = 3 3 3 5 4 9 . 7 5 1 2 4 . 5 " ? 1 8 . 6 0 9 ( . 0 7 ) 3 6 6 5 0 . 2 6 1 2 6 . 6 7 1 8 . 5 8 7 ( . 0 6 ) 3 1 4 4 9 . 0 2 1 2 4 . 7 1 I B . 5 9 2 ( . 0 7 ) A r i t h m e t i c a v e r a g e <>T) = S t a n d a r d d e v i a t i o n ( S ) = S t d . e r r o r o f m e a n ( S - n " f r ) = C 1 8 . 5 9 B ( . 0 7 ) ] C O . 0 1 6 . 1 C O . 0 1 1 ] 1 5 . 5 2 1 ( . 1 6 ) 1 5 . 5 2 6 ( . 1 0 ) 1 5 . 5 6 2 ( . 1 6 ) C I S . 3 2 4 ( . 1 3 ) ] C O . 0 0 4 ] C O . 0 0 3 ] 3 B . 0 4 B ( . 1 7 ) 3 B . 0 8 6 ( . 1 6 ) 3 8 . 1 9 6 ( . 1 2 ) C 3 B . 0 6 7 ( . 1 7 ) ] C O . 0 2 7 ] C O . 0 1 9 3 C L U S T E R 3 s T E R T I A R Y 3 0 3 1 7 - 0 0 1 1 L o n e S t a r - R e y O r o 3 1 7 5 0 . 0 2 1 2 6 . 7 9 1 8 . 9 4 0 ( . 0 8 ) 1 5 . 5 6 9 ( . O B ) 3 B . 4 4 5 ( . 1 5 ) 3 0 3 1 7 - 0 0 2 1 L o n e S t a r - R e y O r o 3 1 7 5 0 . 0 2 1 2 6 . 7 9 I B . 9 5 0 ( . 1 0 ) 1 5 . 5 7 1 ( . 1 6 ) 3 8 . 3 7 8 ( . 1 3 ) 3 0 3 1 7 - A V G * 1 L o n e S t a r - R e y O r o ( n = 2 ) 3 1 7 5 0 . 0 2 1 2 6 . 7 9 1 8 . 9 4 5 ( . 0 9 ) 1 5 . 5 7 0 ( . 1 4 ) 3 8 . 4 1 1 ( . 1 4 ) 3 0 3 1 8 - 0 0 1 * 1 W h i t e S t a r 3 1 B 5 0 . 0 3 1 2 6 . 8 1 - 1 8 . B 6 7 ( . 1 0 ) 1 5 . 4 9 9 ( . 1 4 ) 3 8 . 2 4 5 ( . 1 5 ) 3 0 3 2 0 - 0 0 1 * 1 P e e r l e s s 3 2 0 5 0 . 0 4 1 2 6 . B 4 I B . 9 8 6 ( . 0 6 ) 1 5 . 5 6 0 ( . 1 6 ) 3 8 . 4 5 3 ( . 1 4 ) 3 0 3 3 4 - 0 0 1 * 1 L u c k y S t r i k e 3 3 4 5 0 . 0 6 1 2 6 . 8 4 1 8 . 8 2 7 ( . 0 5 ) 1 5 . 5 3 2 ( . O B ) 3 B . 4 0 3 ( . 1 4 ) 3 0 3 4 9 - 0 0 1 * 1 P r i v a t e e r 3 4 9 5 0 . 0 3 J 2 6 . 8 1 1 9 . 0 1 1 ( . 0 8 ) 1 5 . 5 8 1 ( . 1 7 ) 3 0 . 5 0 3 ( . 1 4 ) G 7 9 A D - 0 0 1 H 1 A l p h a a n d B e t a A B 4 8 . 7 3 1 2 4 . 0 9 1 8 . 8 8 2 ( . 0 3 ) 1 5 . 5 8 1 ( . 0 8 ) 3 8 . 4 0 6 ( . 1 0 ) : i r . : : ? ^ r 1 ^ : : ; ^ : S t d . e r r o r o f m e a n ( S - n ' J ' a ) = C O . 0 3 5 ] C O . 0 1 5 ] C O . 0 9 7 ] C O . 0 4 4 ] C L U S T E R 4 i T E R T I A R Y 3 0 3 1 3 - 0 0 1 « 1 G a l d u n E u g l * 3 1 3 4 9 . 1 1 1 2 4 . 3 9 1 9 . 0 0 7 < . 0 8 ) 1 3 . 6 2 1 ( . 1 3 ) 3 0 . 3 7 7 ( . 2 0 ) 3 0 3 1 3 - 0 0 J » I V i c t o r i a 3 1 3 4 9 . 1Q 1 2 4 . 6 6 1 E l . 8 2 0 ( . 0 4 ) 1 3 . 6 4 6 ( . 0 8 ) 3 8 . 7 3 3 ( . 0 8 ) 3 0 3 2 3 - 0 0 1 H 1 F a n c i e r a 3 2 3 4 9 . 2 3 1 2 3 . 6 0 1 0 . 0 3 6 < . 0 7 ) 1 3 . 3 2 6 ( . 1 1 ) 3 0 . 2 9 0 ( . 1 1 ) 3 0 3 3 5 - 0 0 1 * 1 C r e a m L a l s e 3 3 3 4 9 . 4 9 1 2 3 . 3 4 1 9 . 1 7 1 ( . 0 4 ) 1 5 . 3 B 3 ( . 0 8 ) 3 B . 7 9 6 ( . 1 1 ) N u m b e r o f N u m b e r of d e p o s i t s ( n > = 3 a n a l y s e s < a ) = 4 A r i t h m e t i c a v e r a g e <Sf> = S t a n d a r d d e v i a t i o n < S ) = S t d . e r r o r of m o a n {S<ri'/i)  a C 1 9 . 0 0 2 ( C O . 1 7 2 3 C O . 0 9 9 3 . 0 5 ) 3 C 1 5 . 6 1 7 C O . 0 3 1 3 C 0 . 0 1 B 3 < . 1 0 ) 3 C 3 B . 7 0 2 C O . 1 1 3 3 C O . 0 6 5 3 ( . 1 3 ) l a t i o n s o * m e a n s , s t a n d a r d d e v i a t i o n s a n d s t a n d a r d e r r o r s . T h e l e t t e r A i n d i c a t e s t h a t t h e l o c a t i o n i s a p p r o x i m a t e . 15.6 JQ a. O CM JQ CL o C4 Figure 2.2 15.5 18.1 CLUSTER 4 t2-30Ma Analytical error 2 0 7 . . 2 0 4 . Pb/ . Pb 18.7 18.9 2 0 6 p b / 2 0 4 p b 2 0 6 2 OA Pb/ Pb using data from Table 2.1, 19.3 Galena-lead plot of  vs.  "  using data from Table . C i r c l e s denote deposits related to Karmutsen or Bonanza Group rocks; square symbols are used f o r deposits rel a t e d to the Sicker Group rocks; f i l l e d symbols are used f o r veins. Unreasonable analyses are crossed and are not used i n the i n t e r p r e t a t i o n . Bars mark standard d e v i a t i o n of the mean and l i n e s extend to standard deviation. Lines 1 and 2 represent the evolution of lead from t, to t 2 for varying values of ; t 2 for Cluster 4 i s not known with c e r t a i n t y (see The average growth curve shown i s that of Stacey and Kramers (1975). text) Figure 2.3 Galena-lead plot of Pb/ Pb vs. Pb/ Pb data from Table 2 . 1 , Symbols are described i n Figure 2.2. Lines 3 and 4 represent the evolution of lead from t to for varying values of w i f k i s 3.3 and 3.9 r e s p e c t i v e l y . The growth curve shown i s that of Stacey and Kramers (1975). 18 e n v i r o n m e n t (Monger e t a l . , 1972). An age of 270 Ma has been a s s i g n e d , i n t h i s s t u d y , t o t h e d e p o s i t s t h a t make up t h i s c l u s t e r , on t h e b a s i s o f p a l e o n t o l o g i c a l e v i d e n c e ( M u l l e r and C a r s o n , 1969). T h i s i s t h e minimum p o s s i b l e age f o r t h e v o l c a n o g e n i c d e p o s i t s s i n c e t h e f o s s i l s come from t h e B u t t l e Lake F o r m a t i o n w h i c h o v e r l i e s t h e Myra F o r m a t i o n i n wh i c h t h e v o l c a n o g e n i c d e p o s i t s o c c u r . U-Pb ages o f a p p r o x i m a t e l y 380 Ma have been p r o p o s e d f o r t h e Myra F o r m a t i o n ( M u l l e r , 1979), but t h e i n t e r p r e t a t i o n of t h e s e d a t e s i s u n c e r t a i n ( A r m s t r o n g , 1982, p e r s . comm.). S e l e c t i o n of t h e o l d e r age f o r t h e v o l c a n o g e n i c d e p o s i t s does not s i g n i f i c a n t l y a f f e c t t h e c o n c l u s i o n s drawn from t h i s s t u d y . 2.3.2. C l u s t e r 2: Karmutsen and Bonanza Group V o l c a n o g e n i c and  R e l a t e d D e p o s i t s Karmutsen Group v o l c a n i c r o c k s form a t h i c k s e quence of T r i a s s i c ( c i r c a 220 Ma) t h o l e i i t i c p i l l o w b a s a l t s , f l o w s and b r e c c i a s , w h i c h a r e s i m i l a r i n c h e m i s t r y t o t h o s e of mid-ocean r i d g e s (Monger e t a l . , 1972). M u l l e r (1971) and S o u t h e r (1977) have s u g g e s t e d t h a t t h e y a r e v o l c a n i c a r c r o c k s w h i c h formed c l o s e t o a t r e n c h , b e c a u s e t h e y o v e r l i e t h e S i c k e r G roup i s l a n d a r c a s s e m b l a g e and l a c k g a b b r o , c h e r t and u l t r a m a f i c r o c k s , w h i c h commonly a r e a s s o c i a t e d w i t h o c e a n b a s a l t s . Bonanza Group v o l c a n i c r o c k s a r e f l o w s and p y r o c l a s t i c l a y e r s r a n g i n g i n c o m p o s i t i o n from b a s a l t t o r h y o l i t e . They o v e r l i e t h e Karmutsen Group and a r e d a t e d as M i d d l e J u r a s s i c ( c i r c a 180 Ma) on t h e b a s i s o f i s o t o p i c d a t e s from c o e v a l g r a n i t i c p l u t o n s ( M u l l e r and C a r s o n , 1969). Two d e p o s i t s d e f i n e C l u s t e r 2 and b o t h a r e e n c l o s e d by 19 Karmutsen v o l c a n i c rocks; consequently, an age of 220 Ma was assign e d to both d e p o s i t s i n i n t e r p r e t i n g l e a d i s o t o p i c data. C l u s t e r 2 p l o t s below the growth curve of Stacey and Kramers (1975) f o r average c r u s t a l l e a d (Fig.2.2) and i s more de p l e t e d in 2 0 7 P b than C l u s t e r 1. I f the age d i f f e r e n c e between C l u s t e r s 1 and 2 i s taken i n t o account, the d i f f e r e n c e i n i s o t o p i c composition would be more pronounced, s i n c e C l u s t e r 2 would have been even l e s s r a d i o g e n i c at 270 Ma. The two d e p o s i t s that d e f i n e the Karmutsen c l u s t e r are not demonstrably volcanogenic, and the term paramagmatic (White et a l . , 1971) may be more s u i t a b l e . Paramagmatic i s the name given to e p i g e n e t i c d e p o s i t s which can be shown to be an i n t e g r a l part of a magmatic event. The l e a d i s o t o p i c composition of such a d e p o s i t would t h e r e f o r e probably r e f l e c t the composition of the le a d i n the magma at i t s time of formation. Thus the r e l a t i v e l y unradiogenic Pb-isotope c h a r a c t e r of C l u s t e r 2 probably r e f l e c t s the i s o t o p i c c h a r a c t e r i s t i c s of the e n c l o s i n g Karmutsen v o l c a n i c rocks. The S t a r l i g h t d e p o s i t (Table 2.1, APPENDIX B) might belong to t h i s c l u s t e r , but no mention i s made in the l i t e r a t u r e of i t s host rock group, although i t p l o t s w i t h i n an area u n d e r l a i n by the Karmutsen Group on r e g i o n a l g e o l o g i c a l maps (M u l l e r , 1963: Map 49-1963, A l b e r n i Area 092F/SE). T h i s d e p o s i t i s excluded from the c a l c u l a t i o n s , but i n c l u s i o n of t h i s datum would make l i t t l e d i f f e r e n c e to the mean p o s i t i o n of C l u s t e r 2. 2.3.3 C l u s t e r 3: T e r t i a r y Veins in Karmutsen or Bonanza Group  V o l c a n i c Rocks Bonanza and Karmutsen Group v o l c a n i c rocks c o n t a i n s e v e r a l q u a r t z - g o l d v e i n s from which g a l e n a - l e a d isotope data were 20 obtained. These v e i n s vary i n width, are predominantly of quartz with minor carbonate and g e n e r a l l y c o n t a i n p y r i t e , s p h a l e r i t e , c h a l c o p y r i t e , galena, a r s e n o p y r i t e and gol d . S p a t i a l l y they are c l o s e l y r e l a t e d to T e r t i a r y quartz d i o r i t e stocks (Northcote and M u l l e r , 1972). Three of the v e i n s occur w i t h i n the Eocene Z e b a l l o s pluton (APPENDIX B). Consequently, an age of 30 Ma was assigned to t h i s c l u s t e r ( B a n c r o f t , 1940; Stevenson, 1950; Wanless, 1967). Galena from the veins which make up C l u s t e r 3 has a uniform l e a d composition, the average of which i s d e p l e t e d i n 2 0 7 P b r e l a t i v e to C l u s t e r 1, but i s e n r i c h e d in 2 0 8 P b , 2 0 7 P b and 2 0 6 P b r e l a t i v e to C l u s t e r 2. The data p l o t beneath Stacey and Kramers (1975) curve f o r average c r u s t a l l e a d . Alpha and Beta d e p o s i t (Table 2.1, APPENDIX B) belongs with C l u s t e r 3 on g e o l o g i c a l evidence, but was excluded from c a l c u l a t i o n s because of a h i g h l y anomalous 2 0 7 P b / 2 0 " P b value ( F i g . 2 . 2 ) . I t p l o t s with C l u s t e r 3 i n Fig u r e 2.3, but with C l u s t e r 4 i n Fig u r e 2.2. I t s high value i n 2 0 7 P b p o s s i b l y r e s u l t s from a n a l y t i c a l e r r o r which i s greater f o r 2 0 7 P b than f o r 2 0 6 P b and 2 0 8 P b . 2.3.4 C l u s t e r 4: T e r t i a y Veins i n S i c k e r Group Rocks Quartz-gold v e i n s in S i c k e r Group rocks that form C l u s t e r 4 in F i g u r e 2.2 have the same general appearance and mineralogy as those of C l u s t e r 3 (Muller and Carson, 1969). Although C l u s t e r 4 forms a d i f f e r e n t group r e l a t i v e to C l u s t e r 3, there are only three data p o i n t s which are q u i t e widely s c a t t e r e d on the Pb-Pb p l o t s . The average composition of the c l u s t e r i s more r a d i o g e n i c than the average f o r any of the 21 other c l u s t e r s . The age of these v e i n s i s not known with c e r t a i n t y , but an age of 30 Ma was assign e d on the b a s i s of the s i m i l a r i t y i n 2 0 6 P b content with C l u s t e r 3, and the assumption that they were r e l a t e d to the same phase of i n t r u s i v e a c t i v i t y . V i c t o r i a showing (Table 2.1, APPENDIX B) may be an o l d e r d e p o s i t r e l a t e d to the J u r a s s i c I s l a n d i n t r u s i o n s . The age ( t 2 ) of C l u s t e r 4, however, i s not c r u c i a l to the hypothesis proposed i n t h i s study. Fandora de p o s i t (Table 2.1,*APPENDIX B) belongs to t h i s group on g e o l o g i c a l grounds, but was not i n c l u d e d i n the d i s c u s s i o n because of i t s anomalously unradiogenic l e a d content. 2.4 LEAD ISOTOPE MODELS D i s t r i b u t i o n of the four c l u s t e r s i n F i g u r e 2.2 give s the appearance of two, p a r a l l e l t r e n d s . One i s from S i c k e r - h o s t e d volcanogenic to ve i n d e p o s i t s (Line 1: C l u s t e r s 1 and 4), the other from Karmutsen and Bonanza-hosted volcanogenic and r e l a t e d d e p o s i t s to v e i n d e p o s i t s (Line 2: C l u s t e r s 2 and 3). Three p o s s i b l e e x p l a n a t i o n s f o r these groupings are: 1) v e i n l e a d i s u n r e l a t e d to the volcanogenic l e a d , 2) v e i n l e a d l i e s on an isochron with the volcanogenic l e a d , and 3) ve i n l e a d l i e s on a growth curve with volcanogenic l e a d . The f i r s t hypothesis appears u n l i k e l y on the general grounds that the r e l a t i v e p l o t p o s i t i o n s of the two " v o l c a n i c " c l u s t e r s are i d e n t i c a l with the r e l a t i v e p l o t p o s i t i o n s of the two, younger v e i n c l u s t e r s . T h i s seems an unreasonable c o i n c i d e n c e and one i s l e d to the c o n c l u s i o n that v o l c a n i c - v e i n p a i r s of c l u s t e r s are somehow g e n e t i c a l l y r e l a t e d . The 22 r e l a t i o n s h i p was t e s t e d by c a l c u l a t i n g the s l o p e s of the l i n e s t h a t pass through C l u s t e r s 1 and 2 ( F i g 2.2; L i n e s 1 and 2 ) , w i t h p a i r s of ages 270 Ma t o 30 Ma and 220 Ma t o 30 Ma r e s p e c t i v e l y , u s i n g E q u a t i o n 4 (Table 1.1). In both c a s e s the l i n e s pass through t h e i r r e s p e c t i v e v e i n c l u s t e r s . T h i s r e s u l t shows t h a t the p a r a l l e l t r e n d i s s i g n i f i c a n t and t h e r e f o r e h y p o t h e s i s one can be e l i m i n a t e d . Use of an o l d e r date f o r C l u s t e r 1 o f , f o r example, 350 Ma does not s i g n i f i c a n t l y a l t e r the s l o p e of the i s o c h r o n , L i n e 1, on F i g u r e 4.2. H y p o t h e s i s two can be d i s m i s s e d on g e o l o g i c a l grounds s i n c e t h e r e i s c l e a r l y a l a r g e age d i f f e r e n c e between the s y n g e n e t i c v o l c a n o g e n i c d e p o s i t s and the e p i g e n e t i c v e i n s . The d i s t i n c t i o n between v e i n and v o l c a n o g e n i c l e a d i n 2 0 6 P b / 2 0 l , P b r a t i o i s marked and p r o v i d e s a method f o r d i s t i n g u i s h i n g T e r t i a r y e p i g e n e t i c v e i n s from s y n g e n e t i c d e p o s i t s i n the I n s u l a r B e l t . A l l the T e r t i a r y e p i g e n e t i c d e p o s i t s have r a t i o s g r e a t e r than 18.8, and the s y n g e n e t i c d e p o s i t s have r a t i o s l e s s than 18.7. I r o n C l a d i s an example of one showing which was thought t o be e p i g e n e t i c , b u t which i s l i k e l y s y n g e n e t i c on the b a s i s of i t s i s o t o p i c c o m p o s i t i o n . H y p o t h e s i s t h r e e can be t e s t e d by c a l c u l a t i n g the apparent 2 3 8 U / 2 0 " P b ( t i ) v a l u e which would be r e q u i r e d t o produce the v e i n l e a d c o m p o s i t i o n s from the v o l c a n o g e n i c l e a d c o m p o s i t i o n s i n the time i n t e r v a l s 270 t o 30 Ma and 220 t o 30 Ma, u s i n g E q u a t i o n s 1 and 2 (Table 1.1). T h i s method i s based on the assumption t h a t the v o l c a n o g e n i c l e a d i s r e p r e s e n t a t i v e of the l e a d i n the v o l c a n i c host r o c k s a t t h e i r time of f o r m a t i o n ( t , ) . Galena l e a d s t a y s unchanged i n i t s c o m p o s i t i o n , because i t has been removed 23 from u r a n i u m and t h o r i u m , but t h e l e a d i n t h e v o l c a n i c r o c k s e v o l v e s as u r a n i u m and t h o r i u m i n v a r i o u s m i n e r a l s d e c a y from t h e t i m e of f o r m a t i o n ( t , ) t o t h e t i m e of m i n e r a l i s a t i o n ( t 2 ) . The amount of change i n t h e r a t i o s o v e r any t i m e i n t e r v a l d epends on t h e v a l u e of ** and w. C a l c u l a t e d a p p a r e n t M v a l u e s a r e 10.8 and 10.9 f o r t h e i n t e r v a l s c l u s t e r s 1 t o 4 and c l u s t e r s 2 t o 3 r e s p e c t i v e l y . T h e s e a r e h i g h e r t h a n t h e * v a l u e f o r t h e a v e r a g e g r o w t h c u r v e o f S t a c e y and Kramers ( 1 9 7 5 ) , w h i c h i s 9.23, but r e a s o n a b l e v a l u e s f o r v o l c a n i c r o c k s (Doe, 1970). Thus, t h e l e a d d a t a a r e c o n s i s t e n t w i t h t h e h y p o t h e s i s t h a t v e i n and v o l c a n o g e n i c l e a d p a i r s a r e r e l a t e d t o t h e l e a d e v o l u t i o n g r o w t h c u r v e s a s s o c i a t e d w i t h t h e i r h o s t r o c k s . F u r t h e r s u p p o r t f o r h y p o t h e s i s t h r e e comes from t h e o b s e r v a t i o n s t h a t l i n e s 3 and 4 ( F i g . 2 . 3 ) , w h i c h p a s s t h r o u g h t h e c e n t r e s of b o t h p a i r s of c l u s t e r s , have s l o p e s w h i c h g i v e g e o l o g i c a l l y r e a s o n a b l e 2 3 2 T h / 2 3 8 U (k) v a l u e s . T h e s e k v a l u e s , c a l c u l a t e d u s i n g E q u a t i o n 4 ( T a b l e 1.1), were f o u n d t o be k=3.9 f o r l i n e 3 and k=3.3 f o r l i n e 4, and a r e c l o s e t o t h o s e l i s t e d f o r v o l c a n i c r o c k s by Doe ( 1 9 7 0 ) . 2.5 SOURCE OF GOLD The s o u r c e of g o l d i n v e i n d e p o s i t s i s c o n t r o v e r s i a l . B o y l e (1979) summarised t h r e e p o s s i b i l i t i e s : 1) an a b y s s a l s o u r c e s u c h as t h e m a n t l e , 2) a p l u t o n i c s o u r c e , and 3) t h e h o s t r o c k s by p r o c e s s e s such as m etamorphic s e c r e t i o n o r h y d r o t h e r m a l e x t r a c t i o n and d e p o s i t i o n . E a c h of t h e s e h y p o t h e s e s i s d i s c u s s e d w i t h r e s p e c t t o t h e l e a d i s o t o p e d a t a , b a s e d on t h e a s s u m p t i o n t h a t t h e o r i g i n of t h e l e a d i s t h e same as t h a t of t h e 24 a s s o c i a t e d gold. Lead from an abyssal source would have an unradiogenic i s o t o p i c composition due to the d e p l e t i o n of uranium and thorium in the mantle (Faure, 1977). Galena i n the Bonanza and Karmutsen-hosted veins ( C l u s t e r 3: F i g . 2.2) i s notably depleted in 2 0 7 P b and must therefore have a source which i s l e s s radiogenic than the Sicker v o l c a n i c rocks ( C l u s t e r 1: F i g s 2.2 and 2.3). I f t h i s source was the mantle, then a l l the quartz-gold veins of the same age and w i t h i n a l i m i t e d geographical area, would have a uniform lead composition i r r e s p e c t i v e of host-rock group. Recognition of two groups of veins with d i f f e r e n t i s o t o p i c c h a r a c t e r i s t i c s , corresponding to d i f f e r e n t host-rock groups, seems to negate t h i s hypothesis. The r e l a t i v e l y unradiogenic nature of the Karmutsen v o l c a n i c leads ( C l u s t e r 2: F i g s . 2.2 and 2.3)- suggests that these v o l c a n i c rocks may be the source of the lead. C l u s t e r 3 ( F i g . 2.2) deposits ( P r i v a t e e r , White Star and Lone Star) are a l l s p a t i a l l y r e l a t e d to the Zeballos p l u t o n , the l a t t e r two o c c u r r i n g w i t h i n the pluton i t s e l f (APPENDIX B). If the source of the metals in each vein was a p l u t o n , then one might expect to see e i t h e r a uniform composition of lead i n the v e i n s , unrelated to host rock, or v a r i a t i o n s in the vein lead compositions corresponding to d i f f e r e n t plutons. The c l o s e c o r r e l a t i o n between vein composition and host rock group i s too persuasive to allow for a l l of the metal to have been introduced from plutons. Close s p a t i a l a s s o c i a t i o n of the quartz-gold veins to T e r t i a r y quartz d i o r i t e i n t r u s i o n s suggests that the plutons 25 play a r o l e i n the m i n e r a l i s a t i o n process. I t i s suggested that the plutons provided a heat source that c i r c u l a t e d hydrothermal f l u i d s through the host rocks; these f l u i d s e x t r a c t e d lead and gold from the v o l c a n i c terrane and p r e c i p i t a t e d these metals in quartz v e i n s . This model i s i n accord with Boyle's t h i r d p r o p o s i t i o n , namely that the host rocks provided the metals which were mobilised and concentrated by igneous-related hydrothermal a c t i v i t y . Consequently, the i s o t o p i c composition of the lead i n the veins i s l a r g e l y a r e f l e c t i o n of the host rock surrounding the pluton. 2,6 CONCLUSIONS Lead isotope data from volcanogenic and quartz-gold v e i n deposits i n the Insular B e l t suggest that gold i n veins was ex t r a c t e d , concentrated and deposited by hydrothermal f l u i d s m o b i l i s e d i n geothermal c e l l s generated adjacent to T e r t i a r y p l u t o n i c centres. Lead and gold were derived l a r g e l y from the adjacent rock u n i t s that enclose the vein d e p o s i t s . Lead i s o t o p i c compositions for quartz-gold veins of the southern I n s u l a r B e l t are depleted i n 2 0 7 P b r e l a t i v e to s i l v e r -bearing veins i n the Yukon T e r r i t o r y (Godwin et a l . , 1982), r e f l e c t i n g the d i f f e r e n t source terrane of the I n s u l a r B e l t . Lead i s o t o p i c compositions for deposits of the allochthonous I n s u l a r B e l t are markedly d i f f e r e n t from those of the g e n e r a l l y autochthonous terranes such as the Omineca B e l t , Eastern Fold B e l t and Selwyn Basin. This emphasises a major d i f f e r e n c e in geochemical h i s t o r y between the two types of terra n e , which i s a l s o apparent i n a d i f f e r e n c e i n i n i t i a l 26 strontium r a t i o s (Armstrong, 1979). Lead isotopes i n the autochthonous terranes are more radiogenic (with the exception of p o s s i b l e lower c r u s t a l compositions discussed i n Chapter 4) and f o l l o w a simple e v o l u t i o n a r y path, as i l l u s t r a t e d by Godwin and S i n c l a i r (1982). In the allochthonous terranes, such as the In s u l a r B e l t , lead isotopes are l e s s enriched i n 2 0 7 P b , but more enriched i n 2 0 6 P b for t h e i r age, and have apparently complex h i s t o r i e s . The u n i f o r m i t y of the ve i n lead compositions of C l u s t e r 3 c o n t r a s t s with the s t r o n g l y l i n e a r patterns of data for vein deposits elsewhere in the C o r d i l l e r a (Godwin et a l . , 1982; LeCouteur 1973). From a p r a c t i c a l point of view, i s o t o p i c analyses of galena appears u s e f u l i n d i s t i n g u i s h i n g young epigenetic vein deposits from older volcanogenic deposits (perhaps with r e l a t e d v e i n s ) . This d i s t i n c t i o n w i l l have an e f f e c t on the approach to d e t a i l e d e x p l o r a t i o n w i t h i n the Ins u l a r B e l t . 27 CHAPTER 3 GENESIS OF CARIBOO GOLD MINERALISATION  AS DEFINED BY LEAD AND K~AR ISOTOPIC STUDIES. 3.1 INTRODUCTION The Cariboo area i n southeastern B r i t i s h Columbia ( F i g . 3.1) i s one of the o l d e s t gold-producing regions in Canada, yet the source and age of m i n e r a l i s a t i o n are d i s p u t e d (Sutherland Brown, 1963; K l e p a c k i , 1981, pe r s . comm.). Gold-bearing quartz v e i n s i n the Cariboo area occur i n Hadrynian to Cambrian metasedimentary s t r a t a of the Omineca B e l t . T r i a s s i c v o l c a n i c s t r a t a in the adjacent Intermontane B e l t a l s o host q u a r t z - g o l d v e i n s . Galena-lead isotope data were obtained from v e i n s i n both t e r r a n e s ( F i g . 3.1) in order to determine any g e n e t i c r e l a t i o n s h i p that might e x i s t between them. Galena was a l s o c o l l e c t e d and analysed from s t r a t i f o r m and v e i n type m i n e r a l i s a t i o n i n the Mosquito Creek g o l d mine near Wells ( F i g . 3.1: no. 427), to i n v e s t i g a t e the p o s s i b i l i t y that at l e a s t some of the m i n e r a l i s a t i o n there might be sy n g e n e t i c . K-Ar dates were obtained f o r muscovite from a q u a r t z - b a r i t e v e i n a s s o c i a t e d with the Cariboo Gold Quartz mine, and f o r r e g i o n a l l y metamorphosed p h y l l i t e . L o c a t i o n s of galena and K-Ar samples are shown i n F i g u r e 3.1, and analyses are l i s t e d i n Tables 3.1 and 3.2. Galena samples were c o l l e c t e d by D.Klepacki, except f o r Mosquito Creek and Cariboo Gold Quartz samples which were c o l l e c t e d by the author, and f o r Pin Money which was c o l l e c t e d by C.I.Godwin. 28 CARIBOO GOLD QUARTZ VEINS F i g u r e 3.1 L o c a t i o n s o f g o l d d e p o s i t s f r o m w h i c h g a l e n a - l e a d i s o t o p e a n a l y s e s w e r e o b t a i n e d ( r e f e r t o T a b l e 3.1 f o r d e p o s i t names) . K - A r a n a l y s e s w e r e o b t a i n e d f r o m a v e i n a n d p h y l l i t e n e a r CG. T h e l i n e d i v i d i n g t h e O m i n e c a a n d I n t e r m o n t a n e B e l t s i s p r o b a b l y a g e n e r a l l y w e s t - d i p p i n g t h r u s t . C i s C a r i b o o B e l l p o r p h y r y , a n d M i s M i t c h e l l B a y p o r p h y r y . TABLE 3.1: GALENA-LEAD ISOTOPE ANALYSES FROM MINERAL DEPOSITS Cariboo Area (Omineca Belt and Adjacent Intermontane B e l t ) , South-Central B r i t i s h Columbia Sample Analr Map Lat. Long. Lead Isotope Ratios (Relative IS Error as •/.) Number y s i s Deposit Name Name North West 206/204 207/204 208/204 INTERMONTANE BELT . G79CD-001* G79MR-001* Cedar Creek Marj ner CD MR 52.55 121.47 18.757 (.09) 15.584 (.17) 38.310 (.21) 52.59 121.27 18.953 (.03) 15.654 (.11) 3B.B45 (.13) Number of deposits (n)= 2 Number of analyses (a)= 2 Arithmetic average (if) Standard deviation (S) Std. error of mean (S-n',/»-> C 18. 855 (.06)3 CO.1391 CO.0983 C15.619 (.14)3 CO.0503 CO.0353 C3B.577 (.17)3 C0.37B3 CO.2673 OMINECA BELT (CARIBOO DISTRICT) G79AU-001» 1 Aurum (Island Mountain) AU 53. 10 121. ,58 19. 237 (.07) G79CC-001* 1 Cunningham Creek (A-Zone> CC 52. 93 121. , 3B 19. 209 (.09) G79CG-001 1 Cariboo Gold Quartz CG 08 121 . 55 19. 202 (.06) 30431-001 2 Cariboo Gold Quartz CG 53. OB 121. . 55 19. 163 (.07) 30132-AVG* Cariboo Gold Quartz (n=2> CG 53. 08 121. . 55 19. 183 (. 04) G79CH-001* 1 Cariboo Hudson CH 52. 88 121, .37 19. , 201 (.04) 30426-001 2 Pin Money 426 5*i>. 03 121. .49 19.184 (.23) 30426-0013 2 Pin Money 426 53. 03 121, .49 19. , 195 (. 15) 30426-002 2 Pin Money 426 53. 03 121, .49 19. , 245 (.22) 30426-AVG* 2 Pin Money 426 53. 03 121, .49 19. , 208 (. 20) 30427 .30427 -001 -002 Mosquito Creek Mosquito Creek 427 427 53.10 121. 53.10 121. 19.046 (.20) 19.169 (.06) 15. 759 ( . 14) 39.280 ( . 13) 15. 759 ( • 1 9 > 39.233 ( • 19> 15. 703 ( . 16) 39. 198 ( .08) 15. 692 ( .08) 39.064 ( .08) 15. 690 ( .09) 39.131 ( .05) 15. 752 ( . 12) 39.258 ( . 12) 15. 720 ( .24) 39. 225 ( .25) 15. 729 ( . 17) 39.213 ( . IB) 15. 7B3 ( . 32) 39.417 ( .28) 15. 744 ( . 24) 39.285 ( .24) 15. 745 ( .22) 39.133 ( .24) 15. 759 ( .07) 39.267 ( .07) 30427-AVG* M o s q u i t o Creek 427 53.10 121.57 19.107 (.13) <. 13) 39.200 (.16) Number of d e p o s i t s (n)= 6 Number of a n a l y s e s (a)= 10 A r i t h m e t i c a v e r a g e (x) S t a n d a r d d e v i a t i o n (S) S t d . e r r o r of mean <S-ri"^i> C19.194 (.10)3 CO.045 3 CO.0183 C15.744 (.16)3 CO.023 3 CO.009 3 C39.231 (.15)3 CO.0583 CO.0243 A n a l y s e s by B.D. Ryan, G e o l o g y - G e o p h y s i c s L a b o r a t o r y , The U n i v e r s i t y of B r i t i s h C o l u m b i a . A n a l y s e s by A. Andrew, Ge o l o g y L a b o r a t o r y , The U n i v e r s i t y of B r i t i s h C o l u m b i a . The * symbol d e n o t e s t h o s e a n a l y s e s used i n the c a l c u l a t i o n of a v e r a g e s , s t a n d a r d d e v i a t i o n s and s t a n d a r d e r r o r s , 'the a) symbol d e n o t e s a d u p l i c a t e a n a l y s i s . Two samples of g a l e n a , one c o a r s e g r a i n e d and one f i n e g r a i n e d , were tak e n f r o m t h e same s a m p l e s . Table 3.2 Sample Data for K and Ar analyses on Whole Rock and Muscovite Sample Number Lat. Long. K% A r ^ (mol/gm) rad. Ar% Age (Ma) Whole Rock A81 SN1 53 5' 121 33' 2.49 . 07 8.114 x 10" 1 0 95.2 179 8 Muscovite A81 VN1 53 5' 121 33' 8.61 . 06 21.986 x 10" 1 0 96.9 141 5 K was determined by K r i s t a Scott by atomic absorption using a Techtron AA4 spectrophotometer. Ar was determined by J.E. Harakal, by isotope d i l u t i o n , using an AE1 MS-10 mass spectrometer 38 and high purity Ar spike (White et a l . , 1967). Errors are for two standard deviations. The constants used are: X K e = 0.581 x 10 a , X K & = 4.962 x 10 a , 40 K = 0.01167 atom per cent. 32 Most l e a d isotope analyses were by B.Ryan who used the 12 inch , s o l i d source mass spectrometer i n the Department of Geophysics and Astronomy. Analyses of Pin Money, Cariboo Gold Quartz and Mosquito Creek Samples were obtained by the author on a V.G. Micromass 54R mass spectrometer i n the Department of G e o l o g i c a l s c i e n c e s . Chemical and l o a d i n g procedures f o r both sets of analyses were the same (Appendix A). K-Ar dates were determined by J.Harakal and K.Scott (see Table 3.2). 3.2 GEOLOGY T r i a s s i c a n d e s i t i c v o l c a n i c rocks of the Intermontane B e l t are juxtaposed a g a i n s t P r o t e r o z o i c to Lower P a l a e o z o i c metasedimentary s t r a t a of the Omineca B e l t ( F i g . 3.1). The boundary i s thought to be a low angle, southwesterly d i p p i n g t h r u s t f a u l t ( K l e p a c k i , 1981, pers.comm.; Rees, 1981). East of t h i s boundary the P r o t e r o z o i c to Cambrian Kaza and Cariboo Groups are made up of conglomerates, a r k o s i c and quartzose sandstones, shales and carbonates (Campbell et a l . , 1972). The Snowshoe Formation (Sutherland Brown, 1963) i s e q u i v a l e n t to the Kaza Group (Campbell et a l . , 1973; S t r u i k , 1981a). Kaza Group and Cariboo Group rocks are o v e r l a i n unconformably by rocks of the Black S t u a r t Formation, which are dark a r g i l l i t e s and c h e r t and dolomite b r e c c i a s with Lower Devonian f o s s i l s at t h e i r base (Campbell et a l . , 1973; S t r u i k , 1980). M i s s i s s i p p i a n v o l c a n i c and sedimentary rocks of the Guyet Formation unconformably o v e r l i e the Black S t u a r t Formation. Several minor a n k e r i t i c , a c i d i c dykes of p r e - M i s s i s s i p p i a n age.intrude the Cariboo Group and are known as the Proserpine dykes (Sutherland Brown, 1957). 33 West of the boundary the T r i a s s i c v o l c a n i c s t r a t a of the Intermontane B e l t are predominantly a n d e s i t i c and have been i n t e r p r e t e d as arc-type v o l c a n i c rocks (Monger et a l . , 1972). S e v e r a l i n t r u s i o n s occur w i t h i n t h i s b e l t , and most have J u r a s s i c K-Ar ages of approximately 180 Ma. Cariboo B e l l and M i t c h e l l Bay porphyry d e p o s i t s a l s o have 180 Ma K-Ar ages and are a u r i f e r o u s (Hodgson et a l . , 1976; Shink, 1974). Quartz-gold veins that occur w i t h i n the Snowshoe Formation c o n t a i n galena, s p h a l e r i t e , f r e e g o l d, s c h e e l i t e and l o c a l l y p y r r h o t i t e or a r s e n o p y r i t e . A l l these veins c o n t a i n important values in s i l v e r . Those that occur i n the T r i a s s i c s t r a t a d i f f e r i n that they lack s c h e e l i t e and s p h a l e r i t e ; v e i n s u l p h i d e minerals are mainly galena and c h a l c o p y r i t e . 3.3 LEAD ISOTOPE DATA Lead-lead p l o t s of the data (Table 3.1) from each of the d e p o s i t s (Fig.3.1) are shown in F i g u r e s 3.2 and 3.3. The Omineca B e l t data forms a s i n g l e group ( C l u s t e r 1), which p l o t s above but c l o s e to the 'shale curve' of Godwin and S i n c l a i r (1982), but markedly above the average c r u s t a l curve of Stacey and Kramers (1975); both of these model curves are shown for r e f e r e n c e on F i g u r e 3.2. Data from the Mosquito Creek mine p l o t with C l u s t e r 1 . Vein leads from T r i a s s i c v o l c a n i c rocks are l e s s r a d i o g e n i c than those of the Omineca B e l t , p l o t t i n g below the 'shale curve' in F i g u r e s 3.2 and 3.3. Thus they are d i s t i c t l y d i f f e r e n t from C l u s t e r 1. 34 15.8. 15.75 JQ CL o CJ o CM 15.7 0.5 15.65 CLUSTER 1 / / 0.4 15.6 CD • 0.3 MR o 427 ;427-avg A i A . 'A 427 I \ 426 \ CC \ jAAAU A C H > A" :426-avg A 4 2 6 A 4 2 6 \ 0.2 ACQ / \ A A C G - a v g \ CG / F r a c t i o n a t i o n s l o p e Pb e r r o r 18.6 18.8 19.0 19.2 2 0 6 p b / 2 0 4 p b F i g u r e 3 . 2 P b / " Pb v s . *" P b / Pb p l o t o f g a l e n a a n a l y s e s f r o m C a r i b o o a n d a d j a c e n t I n t e r m o n t a n e B e l t g o l d d e p o s i t s ( T a b l e 3 . 1 ) S y m b o l s a r e t h e same a s t h o s e o f F i g u r e 3 . 1 35 3 9 . 4 3 9 . 2 3 9 . 0 a. ->* o CM CL oo o CN 3 8 . 8 3 8 . 6 3 8 . 4 1 8 . 6 1 8 . 8 1 9 . 0 1 9 . 2 2 0 6 p b / 2 0 4 p b Figure 3.3 2 0 8 P b / 2 0 A P b vs. 2 0 6 P b / 2 ° 4 P b pl o t of galena analyses from Cariboo and adjacent Intermontane Belt gold deposits (Table 3.1). Symbols are the same as those of Figure 3.1. 36 3.4 DISCUSSION Coincidence of C l u s t e r 1 ( F i g s . 3.2 and 3.3) with the 'shale curve' model suggests that a model age can be given to the gold m i n e r a l i s a t i o n event. If the 'shale curve' model a p p l i e s , i t a l s o i m p l i e s that the source of the l e a d , and by i n f e r e n c e g o l d , i s upper c r u s t a l , s i n c e the 'shale curve' r e p r e s e n t s l e a d e v o l u t i o n i n a s i a l i c , upper c r u s t a l environment (Godwin and S i n c l a i r , 1982). J u s t i f i c a t i o n f o r using the 'shale curve' model in t h i s p art of the Omineca B e l t comes from four l i n e s of evidence: 1) Lead data from S u l l i v a n , which i s i n the southern Omineca B e l t , were used i n the c o n s t r u c t i o n of the curve. 2) S i m i l a r i t i e s i n the geology of the B a r k e r v i l i e - C a r i b o o River area with the Selwyn Basin and C a s s i a r P l a t f o r m of northern B r i t i s h Columbia and the Yukon have been noted ( S t r u i k , 1981b). Therefore although most of the l e a d data used in the c o n s t r u c t i o n of the 'shale curve' were from the Selwyn Basin, Omineca B e l t l e a d should have s i m i l a r c h a r a c t e r i s t i c s . 3) The provenance of the sediments i n the Omineca B e l t i s e s s e n t i a l l y the same as f o r those of the Selwyn Shale Basin, s i n c e they are both autochthonous t e r r a n e s which formed at the western margin of the North American c r a t o n . Therefore s i m i l a r geochemical and i s o t o p i c c h a r a c t e r i s t i c s are to be expected f o r the Selwyn Basin and Omineca B e l t . 4) Stratabound carbonate-hosted d e p o s i t s from the Cariboo area a l s o have l e a d i s o t o p i c compositions which f a l l on the 'shale curve', g i v i n g ages which agree with the Cambrian age 37 of t h e i r host rocks. These d e p o s i t s (Maeford Lake and Comin Thro' Bear), c l e a r l y e p i g e n e t i c as c r o s s - c u t t i n g bodies i n carbonate rocks (Holbek, 1982, per s . comm.), are probably only s l i g h t l y younger than t h e i r ho-st rocks. Some e p i g e n e t i c carbonate-hosted d e p o s i t s in the Yukon can appa r e n t l y be dated with the 'shale curve' model (Godwin et a l . , 1980). Therefore carbonate-hosted d e p o s i t s a l s o support the hypothesis that the 'shale curve' model i s g e n e r a l l y a p p l i c a b l e to the Omineca B e l t . The c a l c u l a t e d model age f o r the go l d m i n e r a l i s a t i o n event i s 185 Ma a c c o r d i n g to the 'shale curve' model, but t h i s i s only a c c u r a t e to approximately 50 Ma (Godwin and S i n c l a i r , 1982). K-Ar d a t i n g of a r e g i o n a l l y metamorphosed p h y l l i t e g i v e s an age of 179±8 Ma (Table 3.2) which i s i n t e r p r e t e d as being the age of the l a t e s t metamorphism. T h i s agrees with previous estimates f o r the age of metamorphism by Pigage (1977) and Wanless et a l . (1965, p.15). S t r u i k (1981b) suggests that metamorphism occurred du r i n g the mid-Mesozoic Columbian orogeny. S i m i l a r i t y i n metamorphic and m i n e r a l i s a t i o n ages suggest that the ve i n s may be synmetamorphic, r a t h e r than magmatic in o r i g i n . At l e a s t three phases of ve i n s are present i n the Cariboo Gold Quartz mine, and not a l l v e i n phases are go l d - b e a r i n g (F. Beaumann, 1981, pers. comm.). Muscovite from one q u a r t z - b a r i t e v e i n y i e l d e d a K-Ar model age of 141±5Ma (Table 3.2), which i s the same as ages obtained f o r p o s t - t e c t o n i c g r a n o d i o r i t e plutons to the southeast of the area (Pigage, 1977). Thus at l e a s t one set of ve i n s i s p o s t - t e c t o n i c and may be r e l a t e d to p l u t o n i c a c t i v i t y although no plutons are seen in the immediate area of 38 the mine. The v e i n s may be d i s t a l , or p l u t o n s may be present at depth. Whether the v e i n s are synmetamorphic or magmatic in o r i g i n does not a l t e r the c o n c l u s i o n that most of the l e a d and g o l d were d e r i v e d from the host rocks, e i t h e r by l a t e r a l s e c r e t i o n d u r i n g r e g i o n a l metamorphism (Boyle, 1979), or by hydrothermal a c t i v i t y r e l a t e d to magmatism. Lead isotope data from the Mosquito Creek gold mine are i n d i s t i n g u i s h a b l e from those of the q u a r t z - g o l d v e i n s of C l u s t e r 1 ( F i g s . 3.2 and 3.3). Since the host Snowshoe Formation i s P a l a e o z o i c or o l d e r , and the m i n e r a l i s a t i o n age i s Mesozoic, the d i f f e r e n c e between syngenetic and e p i g e n e t i c l e a d would be r e a d i l y d i s t i n g u i s h a b l e by l e a d i s o t o p e a n a l y s e s . The l e a d from Mosquito Creek i s c l e a r l y e p i g e n e t i c based on l e a d i s o t o p i c composition. Lead i s o t o p i c c h a r a c t e r i s t i c s of q u a r t z - g o l d v e i n s in T r i a s s i c s t r a t a are d i s t i n c t l y d i f f e r e n t from those of the Omineca B e l t d e p o s i t s ( F i g s . 3.2 and 3.3), i n d i c a t i n g a fundamental d i f f e r e n c e i n the source of l e a d (and perhaps gold) in these t e c t o n i c b e l t s . Markedly d i f f e r e n t mineralogy supports the hypothesis that the two v e i n types have l e a d sources which are d i f f e r e n t and u n r e l a t e d . For example, l i t h o p h i l e tungsten in s c h e e l i t e - b e a r i n g v e i n s in the Snowshoe Formation c o u l d have been d e r i v e d from the s i a l i c host rocks; i t s absence from veins in the more mafic Intermontane B e l t i s to be expected on chemical grounds i f the veins have a host rock source. Thus, the d i f f e r e n c e s i n the l e a d i s o t o p e s can be a t t r i b u t e d to growth of l e a d in d i f f e r e n t uranium and thorium environments. F i g u r e 3.4 shows data from t h i s study p l o t t e d with I n s u l a r B e l t data to 39 15.8 C O M P A R I S O N O F O M I N E C A A N D I N S U L A R B E L T S G A L E N A - L E A D I S O T O P E R A T I O S 180 Ma? CARIBOO GOLD VEINS '(OMINECA BELT) 0 Ma QL ^ -o JQ CL f-. o CN 15.7 250 Ma_ 500 Ma 15.6 INSULAR BELT VOLCANOGENIC DEPOSITS 2 2 0 Ma 30 Ma INSULAR BELT GOLD VEINS 155 J I ' I I l ! I I I I I I I ' I 18.0 18.5 19.0 206pb/204p D 19.5 F i g u r e 3 . 4 C o m p a r i s o n o f O m i n e c a , I n s u l a r a n d I n t e r m o n t a n e B e l t g a l e n a - l e a d i s o t o p e r a t i o s . T r i a n g l e s r e p r e s e n t I n t e r m o n t a n e B e l t d e p o s i t s ( T a b l e 3 . 1 ) . E r r o r b a r s o n t h e f i v e c l u s t e r s s h o w n m a r k s t a n d a r d d e v i a t i o n o f t h e s u i t e f o r e a c h c l u s t e r . T h e t w o g r o w t h c u r v e s s h o w n a r e t h e ' s h a l e * c u r v e ( G o d w i n a n d S i n c l a i r , 1 9 8 2 ) , a n d S t a c e y a n d K r a m e r s ( 1 9 7 5 ) ' c u r v e . 40 d i s p l a y the c o n t r a s t between these d i f f e r e n t l e a d p r o v i n c e s . Intermontane B e l t l e a d i s more l i k e l e a d from the I n s u l a r B e l t than l e a d i n the Omineca B e l t . The age of m i n e r a l i s a t i o n of the v e i n s i n the Intermontane B e l t i s not known, but i s p o s s i b l y J u r a s s i c , c a . 180 Ma, based on the p l u t o n i c a c t i v i t y r e l a t e d to a u r i f e r o u s porphyry d e p o s i t s in t h i s p a r t of the Intermontane B e l t . 3.5 CONCLUSIONS The d i f f e r e n c e i n i s o t o p i c composition of v e i n s in Hadrynian metasedimentary rocks and in T r i a s s i c v o l c a n i c rocks p r e c l u d e s d e r i v a t i o n of le a d from the same source, thus r u l i n g out the p o s s i b i l i t y that the gold i n the Cariboo area was d e r i v e d from the Intermontane B e l t . S i m i l a r i t y ' i n lead i s o t o p i c composition of a l l the galena analysed i n the Mosquito Creek d e p o s i t with l e a d in c l e a r l y e p i g e n e t i c q u a r t z - g o l d v e i n s i n the Omineca B e l t suggests that syngenetic m i n e r a l i s a t i o n i s not presen t . Lead is o t o p e and K-Ar evidence i n d i c a t e that the age of m i n e r a l i s a t i o n of the Omineca B e l t q u a r t z - g o l d v e i n s i s Mesozoic. L o c a t i o n of C l u s t e r 1 ( F i g s . 3.4 and 3.3) near the 'shale curve' model suggests that the lead and gold have an upper c r u s t a l , host-rock source. The process by which metals were m o b i l i s e d from t h e i r host rocks i n t o the q u a r t z - g o l d v e i n s was probably by l a t e r a l s e c r e t i o n d u r i n g r e g i o n a l metamorphism. Support f o r t h i s theory comes from c o i n c i d e n t metamorphic and m i n e r a l i s a t i o n ages, but the p o s s i b i l i t y that the v e i n s are r e l a t e d .to d i s t a n t J u r a s s i c plutons i s not r u l e d .out completely. 41 A host-rock source f o r the T r i a s s i c a r c - v o l c a n i c rocks may of the l e a d i n these v e i n s ( c f . gold i n q u a r t z - g o l d v e i n s i n the e x p l a i n the unradiogenic nature Chapter 2). 42 CHAPTER 4 A RE-EXAMINATION OF LEAD DATA FROM MOYIE, AINSWORTH-BLUEBELL,  SLOCAN AND CARMI CAMPS, SOUTHEASTERN BRITISH COLUMBIA 4.1 INTRODUCTION Lead isotope data from mineral d e p o s i t s i n the Kootenay Arc were the sub j e c t of s t u d i e s by S i n c l a i r (1964), Reynolds and S i n c l a i r (1971), and LeCouteur (1973). A l l of these attempted to e x p l a i n the anomalous nature of the lead u sing c o n v e n t i o n a l isochron c a l c u l a t i o n s and g l o b a l average growth curve models. They were unable to i n t e r p r e t the data uniquely, although some of t h e i r e x p l a n a t i o n s came c l o s e to the model proposed here. For example, Reynolds and S i n c l a i r (1971) suggested that Nelson b a t h o l i t h magma and ore f l u i d s c o n t a i n e d l e a d d e r i v e d from the lower c r u s t that mixed with upper c r u s t a l l e a d to produce the observed l i n e a r data a r r a y from Slocan camp. One of the models put forward by LeCouteur (1973) a l s o i n v o l v e d mixing of two end-member l e a d compositions to e x p l a i n the anomalous le a d l i n e , but the end-members were not s p e c i f i e d . The major d i f f e r e n c e between t h i s and pre v i o u s lead i s o t o p e work, i s the i n t e r p r e t a t i o n of the i s o t o p i c composition of lead in B l u e b e l l and Ainsworth d e p o s i t s . A l l of the previous models i n v o l v e an i n t e r p r e t a t i o n which suggests that Ainsworth leads f a l l on an anomalous lead l i n e drawn through Slocan data, and that they are t h e r e f o r e r e l a t e d g e n e t i c a l l y to the Nelson b a t h o l i t h . Here i t i s suggested ( i n accordance with Muraro, quoted i n Hoy et a l . , 1981) that Ainsworth and B l u e b e l l l e a d i s 43 syngenetic and Lower Cambrian, and that Ainsworth lead i s o t o p i c compositions are not c o l i n e a r with data from Slocan camp. New data from Carmi g o l d v e i n s (Watson et a l . , 1982) which are a l s o a s s o c i a t e d with Nelson p l u t o n i c rocks, do not f a l l on the anomalous l e a d l i n e , i n d i c a t i n g that p revious i n t e r p r e t a t i o n s are u n r e a l i s t i c . 4.2 GEOLOGY 4.2.1 T e c t o n o - s t r a t i g r a p h i c S e t t i n g The d e p o s i t s named in t h i s study a l l occur w i t h i n the Omineca B e l t i n southeastern B r i t i s h Columbia. Slocan, Ainsworth and B l u e b e l l camps occur in the Kootenay Arc ( F i g . 4.1), which i s a s t r u c t u r a l l y complex b e l t bounded by the Shuswap metamorphic complex to the west, and by the P u r c e l l a n t i c l i n o r i u m on the e a s t . Deformed and metamorphosed Late P r o t e r o z o i c to Late Mesozoic sedimentary and v o l c a n i c rocks are i n t r u d e d by the Middle J u r a s s i c Nelson b a t h o l i t h and other g r a n i t i c to g r a n o d i o r i t i c i n t r u s i o n s to the west of the a r ea. The Kootenay Arc i s noted for i t s s i l v e r , lead and z i n c m i n e r a l i s a t i o n . Carmi camp occurs to the west of the Kootenay a r c , w i t h i n the Shuswap metamorphic complex, but a s s o c i a t e d with a mass of Nelson p l u t o n i c rock of Middle J u r a s s i c age. The Moyie i n t r u s i o n s l i e to the east i n the P u r c e l l a n t i c l i n o r i u m ( F i g . 4.1). 44 Figure 4.1 Tectono-stratigraphic terranes of southeastern B r i t i s h Columbia and the approximate locations of the mining camps discussed in the text. 45 4.2.2 Slocan Camp S i l v e r - r i c h f i s s u r e v e i n s with s i l v e r v a l u e s commonly as high as 6300 ppm were mined at Slocan camp. These v e i n s are s p a t i a l l y r e l a t e d to the Nelson b a t h o l i t h which i s a Middle J u r a s s i c g r a n i t i c to g r a n o d i o r i t i c i n t r u s i o n , g i v i n g a Rb-Sr isochron age of 162±6 Ma (Duncan, 1982) and a K-Ar age of approximately 160 Ma (Nguyen et a l . , 1968). Many of the vei n s occur w i t h i n the b a t h o l i t h i t s e l f , but others occur w i t h i n the Mesozoic Slocan Group sedimentary and v o l c a n i c rocks which the plu t o n i n t r u d e s (Cairnes, 1934). I t i s g e n e r a l l y thought that the m i n e r a l i s a t i o n i s r e l a t e d t o the i n t r u s i o n and i s t h e r e f o r e approximately the same age as the b a t h o l i t h (Reynolds and S i n c l a i r , 1971; S i n c l a i r , 1964; C a i r n e s , 1934). 4.2.3 Carmi Camp G o l d - r i c h v e i n s occur at Carmi, seven k i l o m e t e r s north of B e a v e r d e l l , at the contact of the Westkettle b a t h o l i t h and the Permian Wallace Formation (Watson et a l . , 1982). The Westkettle g r a n o d i o r i t e has been c o r r e l a t e d with the Nelson b a t h o l i t h ( L i t t l e 1961), so the age of m i n e r a l i s a t i o n of the Carmi veins i s assumed to be the same as f o r the Slocan d e p o s i t s which i s approximately 160 Ma. Comparison of le a d data from Carmi gold v e i n s with data from the T e r t i a r y s i l v e r v eins at B e a v e r d e l l a l s o supports a 160 Ma age f o r the Carmi v e i n s (Watson et a l . , 1982). 46 4.2.4 A i n s w o r t h - B l u e b e l l Camp Ai n s w o r t h - B l u e b e l l camp, l o c a t e d on the shores of Kootenay Lake ( F i g . 4.1), i n c l u d e s many l e a d - z i n c d e p o s i t s in an area of complex geology. B l u e b e l l mine, a major l e a d - z i n c ore d e p o s i t i n B r i t i s h Columbia, on the east shore i s hosted by B l u e b e l l limestone which has been c o r r e l a t e d with the Lower Cambrian Badshot Limestone ( F y l e s , 1967; Shannon, 1970). It might not s t r i c t l y be part of Ainsworth camp, but i t has le a d isotope r a t i o s which are very s i m i l a r ( c l u s t e r s o v e r l a p at two standard d e v i a t i o n s ) to those from the Ainsworth d e p o s i t s , suggesting that i t i s s i m i l a r in age, and o r i g i n . However, d i f f i c u l t i e s encountered in the c o r r e l a t i o n of s t r a t i g r a p h y across Kootenay Lake preclude d i r e c t c o r r e l a t i o n of Ainsworth and B l u e b e l l s t r a t i g r a p h y ( F y l e s , 1967; Hoy, 1982, per s . comm.). Ore minerals at Ainsworth occur in the Cambrian to O r d o v i c i a n Lardeau S e r i e s ( S i n c l a i r 1964). Many sulp h i d e c o n c e n t r a t i o n s are f r a c t u r e - c o n t r o l l e d with s l i g h t w a l l r o c k "replacement", and were d e c r i b e d as "bedded v e i n s " by Eastwood (1951). M i n e r a l i s e d f r a c t u r e s c r o s s - c u t lamprophyre dykes, which are b e l i e v e d to be T e r t i a r y ( F y l e s , 1967), thus some of these are c l e a r l y e p i g e n e t i c d e p o s i t s . I f , however, they are c l o s e l y r e l a t e d i n o r i g i n to the B l u e b e l l d e p o s i t s as suggested by t h e i r i s o t o p i c compositions, the e p i g e n e t i c v e i n s may represent remobi1isat ion of o l d e r stratabound d e p o s i t s as has been noted by Muraro f o r the B l u e b e l l mine i n a personal communication to Hoy et a l . (1981). P r e v i o u s l y , S i n c l a i r (1964) and LeCouteur (1973) assumed that the A i n s w o r t h - B l u e b e l l m i n e r a l i s a t i o n was the same age as 47 that i n the Slocan camp; m i n e r a l i s a t i o n i n both camps was thought to be g e n e t i c a l l y r e l a t e d to the Nelson i n t r u s i o n s . T h i s , as shown below, does not adequately e x p l a i n the le a d i s o t o p e data. However, the time of the i n t r u s i o n of the Nelson b a t h o l i t h may approximately date the r e m o b i l i s a t i o n of the o l d e r d e p o s i t s i f the hypothesis of Muraro (Hoy et a l . , 1981) i s c o r r e c t . 4.2.5 Moyie I n t r u s i o n s Gabbroic to d i o r i t i c s i l l s (up to 245 m t h i c k ) , dykes and stocks i n the A l d r i d g e Formation are known as the Moyie i n t r u s i o n s . They occur to the east of the Kootenay Arc i n the P u r c e l l a n t i c l i n o r i u m , and give i s o t o p i c ages of 1430±20 Ma (Hoy et a l . , 1981; LeCouteur, 1979). These have been g e n e t i c a l l y l i n k e d to the S u l l i v a n ore dep o s i t (Leech and Wanless, 1962), but the le a d in small d e p o s i t s with a c l o s e s p a t i a l a s s o c i a t i o n with these i n t r u s i o n s , i s l e s s r a d i o g e n i c than i s lead i n the S u l l i v a n d e p o s i t , which i s of a major s i z e on a world s c a l e . Lead r e l a t e d to the Moyie i n t r u s i o n s i s assumed to be penecontemporaneous or syngenetic and t h e r e f o r e 1.44 Ga o l d . 4.3 SOURCE OF DATA AND ANALYTICAL PROCEDURES Galena samples from Moyie i n t r u s i o n s , Ainsworth, B l u e b e l l and Slocan camps were analysed by P.C. LeCouteur, using a gas source mass spectrometer i n the Geophysics and Astronomy Department, U n i v e r s i t y of B r i t i s h Columbia. His data have been c o r r e c t e d f o r r e v i s e d a b s o l u t e values of the Broken H i l l standard, which are; 2 0 6Pb/ 2 0*Pb=16.003, 2 0 7Pb/ 2 0"Pb=15.389, and 2 0 8 Pb/ 2 C M ,Pb=35.657. P r e c i s i o n i s estimated to be b e t t e r than 48 0.16 percent at the two sigma l e v e l . D e t a i l s of the a n a l y t i c a l procedure and c a l c u l a t i o n of p r e c i s i o n are given i n APPENDIX C. Carmi camp samples were c o l l e c t e d by P.H. Watson and analysed by B.Ryan using a s o l i d source mass spectrometer i n the Geophysics and Astronomy Department, U n i v e r s i t y of B r i t i s h Columbia. A l l the data have been normalised to a b s o l u t e values (see above) using the Broken H i l l standard (No.1). D e t a i l s of a n a l y t i c a l procedures and data r e d u c t i o n are i n APPENDIX A. 4.4 DATA INTERPRETATION 4.4.1 Shale Curve Model (Godwin and S i n c l a i r , 1982) Galena-lead isotope analyses from shale-hosted l e a d - z i n c d e p o s i t s i n the Selwyn Shale Basin, Wernecke F o l d B e l t , Mackenzie Mountains and Omineca B e l t depart from the average le a d growth curve model of Stacey and Kramers (1975), p l o t t i n g c o n s i s t e n t l y above i t . S t r a t i g r a p h i c d a t i n g of these d e p o s i t s r e v e a l s a systematic change in i s o t o p i c composition of the l e a d with age, a l l o w i n g a b e s t - f i t growth curve to be c a l c u l a t e d from a p o i n t on Stacey and Kramers (1975) curve through the mean values f o r each dated c l u s t e r (Table 4.1: a f t e r Godwin and S i n c l a i r , 1982). The high 2 3 8 U / 2 0 " P b („) value („=12.16) r e q u i r e d f o r the t h i r d stage of t h i s three-stage growth curve model i m p l i e s that the l e a d i n the d e p o s i t s i s upper c r u s t a l in c h a r a c t e r (Doe and Zartman, 1979). T h i s i s c o n s i s t e n t with the s i a l i c c o n t i n e n t a l margin nature of the host terrane f o r these d e p o s i t s , and t h e i r u l t i m a t e d e r i v a t i o n from Precambrian s h i e l d r o c k s . Thus the 'shale curve' i s thought to represent e v o l u t i o n of l e a d in an Table 4.1 Shale Curve (after Godwin and S i n c l a i r , 1982) Time (Ga) 206„, /204„, Pb/ Pb 207T,V/204T11_ Pb/ Pb 208 p b /204 ] 0.0 19.526 15.723 39.829 0.14 19.259 15.710 39.488 0.25 19.04 5 15.699 39.218 0.37 18.808 15.685 38.922 0.50 18.545 15.667 38.599 0.58 18.381 15.655 38.400 0.75 18.026 15.627 37.973 1.00 17.486 15.575 37 .339 1.25 16.924 15.510 36.697 1.44 16.483 15.447 36.204 1.50 16.340 15.425 36.047 1.75 15.734 • 15.317 35.389 1.887 15.391 15.246 35.025 Pb 50 upper c r u s t a l environment (Godwin and S i n c l a i r , 1982) and i s t h e r e f o r e a p p l i c a b l e to d e p o s i t s w i t h i n the autochthonous part of the Canadian C o r d i l l e r a , which have a c o n t i n e n t - d e r i v e d host rock l e a d source. Since a l l of the d e p o s i t s named i n t h i s study occur w i t h i n the g e n e r a l l y autochthonous part of the Canadian C o r d i l l e r a , t h e i r i s o t o p i c compositions (Tables 4.2, 4.3 and 4.4) are i n t e r p r e t e d w i t h i n the framework provided by the shale curve lead e v o l u t i o n model (Godwin and S i n c l a i r , 1982). 4.4.2 Evidence f o r the E x i s t e n c e of a Second Lead Source. Slocan, Moyie and A i n s w o r t h - B l u e b e l l leads p l o t below the shale curve ( F i g . 4.2), suggesting that they a l s o c o n t a i n a component of l e a d which i s not upper c r u s t a l i n c h a r a c t e r . Since a l l three groups of data are anomalously low i n t h e i r r a d i o g e n i c component with respect to the shale curve model, i t i s proposed that there i s a second, l e s s r a d i o g e n i c l e a d r e s e r v o i r which has c o n t r i b u t e d l e a d to the d e p o s i t s of southeastern B r i t i s h Columbia, and which must be at l e a s t 1.44 Ga o l d (the i s o t o p i c age of the Moyie i n t r u s i o n s ) . Thus i t i s proposed that a le a d r e s e r v o i r , d i s t i n c t from the upper c r u s t e x i s t s , and has remained d i s t i n c t f o r at l e a s t 1.44 Ga. 4.4.3 E s t i m a t i o n of the ' B l u e b e l l ' Lead E v o l u t i o n Curve Assuming that l e a d e v o l u t i o n i n t h i s r e l a t i v e l y uranium-poor source f o l l o w s a c o n v e n t i o n a l e v o l u t i o n a r y path, i t i s p o s s i b l e to estimate a growth curve f o r t h i s source. T h i s growth curve must depart from average growth curve models at some time ( t , ) in the past. In order to be c o n s i s t e n t with the 'shale curve' model, and f o r ease of computation, i t i s assumed that t h i s curve may be approximated by a two-stage model in which TABLE 4 . 2 I GALENA-LEAD ISOTOPE ANALYSES FROM MINERAL DEPOSITS SLOCAN AND CARMI CAMPS, SOUTHEASTERN BRITISH COLUMBIA Sample A n a l - Map L a t . Long. Number y s t D e p o s i t Name Name N o r t h West SLOCAN CAMP OAIB71 3 A r l i n g t o n AI 49 79 117 .36 OCA877 3 C h a p l e a u CA 49 74 117 .39 OCN869 3 C o r p o r a t i o n CN 49 82 117 .43 0EN868 3 E n t e r p r i s e EN 49 82 1 17 . 33 OKA903 3 K a l i s p e l l KA 49 86 117 41 OLT902 3 L i t t l e Tim LT 49 B l 1 17 37 O0T870 3 Ottawa OT 49 79 117 40 OSC295 3 S c r a n t o n SC 49 79 1 17 06 OZL867 "3 S i l v e r l e a f ZL 49 86 1 17 32 ORU878 3 Ruth-Hope RU 49 97 117 23 OZI879 3 S i 1 v e r s m l t h ZI 49 97 117 22 0ZT88O 3 S t a n d a r d ZT 49 96 117 32 0MPB8I 3 Mol1y Hughes MP 50 01 117 38 OVRB82 3 Van Roz VR 49 93 1 17 29 OPYB83 3 Payne PY 50 01 117 23 OVT8B4 3 V u l t u r e VT 49 98 117 16 ON0887 3 N o b l e 5 NO 49. 99 117 20 OWH894 3 Whi t e w a t e r WH 50. 04 117 13 OCK895 3 Cork P r o v i n c e CK 49. 91 1)7. 07 OIN89V 3 I ml ex IN 49. 86 117. 13 ODP904 3 Bosun BP 49. 97 117. 36 OIV903 3 Ivanhoe IV 49. 93 117. 24 OCI906 3 C a l i f o r n i a CI 49. 98 117. 32 OFM907 3 F i s h e r Maiden FM 49. 91 1 17. 21 OLJ912 3 L u c k y J i m L J 30. 03 117. 20 OVC294 3 V i c t o r VC 50. 00 117. 27 OD0900 D u b l i n Queen DQ 50. 00 117. 17 }RMI CAMP W79BH-020 1 B l a c k s m i t h BH 49. 43 119. 06 W79CR-004 1 Carmi Dump CR 49. 49 1 19. 12 W79EU-002 1 Eugene Creek EU 49. 39 119. 11 30419-008 1 B e a v e r d e l 1 Show 419 49. 43 119. 06 Lead I s o t o p e R a t i o s ( R e l a t i v e IS E r r o r a s '/.) 206/204 2 07/204 20B/204 Remarks 18.046 15 .639 39.026 L e C o u t e u r , 1973 19.136 15 .671 3B.908 L e C o u t e u r , 1973 18.964 15 . 649 38. 946 L e C o u t e u r , 1973 IB.717 15 . 627 38.951 L e C o u t e u r , 1973 IB.701 15 .624 38.952 L e C o u t e u r , 1973 18.810 15 . 633 39.135 L e C o u t e u r , 1973 18.B73 15 .634 39.052 L e C o u t e u r , 1973 18.834 15 . 653 38.696 L e C o u t e u r , 1973 18.626 15 . 606 38.913 L e C o u t e u r , 1973 18.699 15 . 623 38.998 L e C o u t e u r , 1973 18.721 15 641 39.051 L e C o u t e u r , 1973 •18.664 15 .627 39.006 L e C o u t e u r , 1973 18.918 15 656 38.728 L e C o u t e u r , 1973 18.708 15 639 39.033 L e C o u t e u r , 1973 18.740 15 625 39.030 L e C o u t e u r , 1973 18.704 15 620 39.012 L e C o u t e u r , 1973 18.893 15 656 39.025 L e C o u t e u r , 1973 IB.795 15 648 3B.B25 L e C o u t e u r , 1973 18.709 15 627 39.021 Lij C o u t u u r v 1 773 IB.737 15 641 38.940 L e C o u t e u r , 1973 18.736 13 623 37.019 L e C o u t e u r , 1973 IB.743 15 636 39.001 L e C o u t e u r , 1973 18.619 15 610 39.035 L e C o u t e u r , 1973 IB.739 15 626 38.993 L e C o u t e u r , 1973 18.773 15 625 39.012 L e C o u t e u r , 1973 18.715 15. 616 38.976 L e C o u t e u r , 1973 18.748 15. 631 39.012 L e C o u t e u r , 1973 18.662 (. 09) 15. 600 (.10) 38.653 (. 09) 18.655 <. 09) 15. 591 (.19) 3B.43B (. 15) 18.657 (. 08) 15. 595 (.14) 39.915 (. 16) 1B.5B3 (. 06) 15. 603 (.10) 38.412 (. 14) Wal1 a c e Fm. 1. A n a l y s e s by B.D. Ryan, G e o l o g y - G e o p h y s i c s L a b o r a t o r y , The U n i v e r s i t y of B r i t i s h C o l u m b i a . 3. A n a l y s e s by P.H. L e C o u t e u r , G e o l o g y - G e o p h y s i c s L a b o r a t o r y , The U n i v e r s i t y of B r i t i s h C o l u m b i a ; a n a l y s i s h a s been a d j u s t e d t o c u r r e n t l y u s e d i s o t o p i c v a l u e s f o r t h e Broken H i l l s t a n d a r d (see t e x t ) . TABLE 4.3: GALENA-LEAD ISOTOPE ANALYSES FROM MINERAL DEPOSITS AINSWORTH CAMP, SOUTHEASTERN BRITISH COLUMBIA Sample Anal-Number yst Deposit Name Map Lat. Long. Lead Isotope Ratios (Relative IS Error as 7.) Name North West 206/204 207/204 20B/204 RemarkB OI3L226 3 Bluebell BL OHN9U 3 Highland HN OHI314 3 . Hlqhlandor HI OKF709 3' Kootenay Florence KF OMU901 3 Montezuma MU ONI316 3 Nicolet NI ONU710 3 No. 1 NU oZG89B 3 S i l v e r Glance ZG OZH90B 3 S i l v e r Hoard ZH OTR315 3 Triumph TR 47.76 49. 75 49.72 47.77 47.74 47.75 49. 74 47.77 49.75 47.76 116.B6 116.73 116.71 116.72 1 17.05 116.72 116.75 116.74 116.75 116.94 17 17 17 17 17 17 17 17 17 17 477 530 552 437 579 566 764 4(35 787 ,533 15.480 13.470 13.503 13.480 15.499 15.474 15.522 15.478 15.514 15.478 37.942 38.234 38.227 37. 38. 38. 750 225 186 38.323 37.773 38.277 3B.205 LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, LeCouteur, 1 9 7 3 1 9 7 3 1 9 7 3 1 9 7 3 1 9 7 3 1 7 7 3 1 9 7 3 1 7 7 3 1 7 7 3 1 9 7 3 Analyses by P.H. LeCouteur, Geology-Geophysics Laboratory, The University of B r i t i s h Columbia; analysis has been adjusted to currently used isotopic values for the Broken H i l l standard (see t e x t ) . TABLE 4.41 GALENA-LEAD ISOTOPE ANALYSES FROM MINERAL MOYIE CAMP DEPOSITS Sample Anal- Map Lat. Long. Lead Isotope Ratios (Relative IS Error as V.) Number yst Deposit Name Name North West 206/204 207/204 208/204 Remarks. OHA765 3 Hope HA 47. 40 1 15. ,85 OLE776 3 Leadvi H e LE 49. 22 1 16. 32 OL0769 3 Lone P ine H i l l LO 49. 63 1 15. 91 OMY774 3 Moy i e Tungsten MY 49. . 37 1 16. ,05 OPA775 3 Park PA 49. 63 115. , 91 oPFBSl 3 Fedro ' PF 50. . 43 117. , 37 OPL852 3 Pol 1 en Basin PL 49. 55 1 16. 30 OVL777 3 Vu 1 c an 5111 VL 49. ,80 116. 16.315 13.39B 35.978 LeCouteur , 1973 16.329 15.403 35.979 LeCouteur, 1973 16.427 15.425 36.086 LeCouteur, 1973 16.379 15.415 36.048 LeCouteur, 1973 16.402 15.420 36.061 LeCouteur, 1973 16.434 15.425 36.064 LeCouteur, 1973 16.538 15.436 36.288 LeCouteur, 1973 16.417 13.405 36.054 LeCouteur, 1973 3. Analyses by P.H. LeCouteur, Geology-Geophysics Laboratory, The University of B r i t i s h Columbia) an a l y s i s has been adjusted to curr e n t l y used i s o t o p i c values for the Broken H i l l standard (see te x t ) . 0.0 Ga 206 pb /204 Pb F i g u r e 4 . 2 P b / Pb v s . P b / Pb p l o t o f g a l e n a - l e a d d a t a f r o m M o y i e , A i n s w o r t h - B l u e b e l l , C a r m i a n d S l o c a n c a m p s ( T a b l e s 4 . 2 , 4 . 3 a n d 4 . 4 ) . T h e ' s h a l e ' c u r v e ( G o d w i n a n d S i n c l a i r , 1 9 8 2 ) i s s h o w n f o r r e f e r e n c e . 55 lead e v o l u t i o n follows the f i r s t - s t a g e growth curve of Stacey and Kramers (1975), u n t i l a time ( t , ) at which a second stage begins. Choice of a d i f f e r e n t primary growth curve or average c r u s t a l growth curve from which to develop the f i n a l low n e v o l u t i o n stage would not s i g n i f i c a n t l y a l t e r the parameters used i n developing the B l u e b e l l curve. The uranium-poor lead source must have a composition which i s at l e a s t as unradiogenic as the l e a s t radiogenic a n a l y s i s for each camp. Ainsworth-Bluebell camp, s p e c i f i c a l l y the B l u e b e l l deposit provided the l e a s t radiogenic analyses and was therefore used i n the estimation of a growth curve. B l u e b e l l deposit i s thought to contain syngenetic, Lower Cambrian lead (Muraro, c i t e d above). I f so, the i s o t o p i c composition of the lead in the deposit probably represents the i s o t o p i c composition of the lead in the uranium-poor source at the time of m i n e r a l i s a t i o n , 0.58 Ga. Thus i n developing the lower curve model i t i s assumed that the curve must pass through the composition of the B l u e b e l l lead at 0.58 Ga. Using the f i r s t stage of Stacey and Kramers (1975) curve as a s t a r t i n g p o i n t , and the B l u e b e l l lead composition and age of m i n e r a l i s a t i o n , i t i s p o s s i b l e to c a l c u l a t e a unique growth curve, which w i l l be r e f e r r e d to as the B l u e b e l l curve. The age, t 1 ; of the s t a r t of the second stage of the two-stage growth curve, and the » value for that stage, are 3.78 Ga and 9.23 r e s p e c t i v e l y - Values of 2 0 6 P b / 2 0 a P b and 2 0 7 P b / 2 0 U P b r a t i o s for the B l u e b e l l curve at various times are l i s t e d i n Table 4.5, and i l l u s t r a t e d i n Figure 4.3. Changes i n » with time for each of the three curves; Stacey and Kramers (1975), the 'shale' curve, Table 4.5 Bl u e b e l l Curve: „n2= 9.23, k = 4 .22 Time (Ga) 206^.204. Pb/ Pb 207,,, ,204^, Pb/ Pb 208 p b /204 ] 0.00 18.347 15.532 39.076 0.05 18.275 15.528 38.979 0.15 18.130 15.521 38.786 0.16 18.115 15.520 38.766 0.25 17.982 15.513 38.591 0.37 17.802 15.502 38.357 0.50 17.602 15.489 38.101 0.58 17.478 15.480 37.943 0.75 17.208 15.459 37.604 1.00 16.798 15.420 37.102 1.25 16.372 15.369 36.593 1.44 16.037 15.322 36.202 1.50 15.929 15.305 36.077 1.75 15.468 15.224 35.556 2.00 14 .989 15.119 35.027 2.50 13.9.74 14.814 33.951 3.00 12.877 14.314 32.848 3.77 11.0.12 12.856 31.095 N.B. Usinp, the method of Stacey and Kramers (1975) to c a l c u l a t e a two-stage model curve, as t^ , the s t a r t of the second stage, increases from 3.3Ga to 4.0Ga u^ va r i e s from 7.60 to 6.73 varies from 9.42 to 9.16 The curve l i s t e d i s the so l u t i o n for which ju^ =7.19, which i s the value of p f o r the f i r s t stage of the growth curve of Stacey and Kramers (19.75). 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 206pb/204pb O.OGa Figure 4.3 Pb/ Pb vs. Pb/ Pb plot showing the close f i t of the data l i s t e d in Tables 4.2, 4.3 and 4.4 with mixing l i n e s drawn between the 'shale' curve (Godwin and S i n c l a i r , 1982), and the B l u e b e l l curve at 0.16, 0.58 and 1.44 Ga r e s p e c t i v e l y . 58 and the B l u e b e l l curve, are summarised in F i g u r e 4.4. 4.4.4 Mixing L i n e Isochrons F i g u r e 4.2 shows the l i n e a r a r r a y of data from Slocan and Carmi camps, p l o t t e d with the 'shale' curve. The slope of the Slocan-Carmi a r r a y i s , however, too steep to s a t i s f y the isochron equation (Equation 4: Table 1.1) i f t 2 i s 150 Ma or l e s s . I t i s t h e r e f o r e suggested that the array may have been produced by mixing of two end-member le a d compositions which f a l l on the two extreme lead e v o l u t i o n curves, that i s , the 'shale' curve (Godwin and S i n c l a i r , 1982) and the B l u e b e l l curve. E x t r a c t i o n and mixing of l e a d from these two r e s e r v o i r s must have taken p l a c e at the time of m i n e r a l i s a t i o n , so the mixing l i n e s w i l l j o i n p o i n t s of equal time and are t h e r e f o r e i s o c h r o n s . The term 'mixing l i n e i s o c h r o n ' i s proposed to d e s c r i b e the l i n e a r a r r a y . I s o t o p i c d a t i n g of the Nelson b a t h o l i t h (Nguyen et a l . , 1968; Duncan, 1982) and the occurrence of s i l v e r v e i n s i n a s s o c i a t i o n with the b a t h o l i t h suggest that the age of m i n e r a l i s a t i o n f o r Slocan and Carmi camps i s 160 Ma. The 160 Ma mixing l i n e isochron should, i f the model i s c o r r e c t , correspond c l o s e l y to the spread of data from these camps. The c l o s e f i t of the mixing l i n e isochron with the l e a d data i s i l l u s t r a t e d i n F i g u r e 4.3. Two a d d i t i o n a l t e s t s of the model i n v o l v e comparing the 1.44 Ga mixing l i n e isochron with the data from the Moyie i n t r u s i o n s , and the 0.58 Ga mixing isochron with the spread of the data from Ainsworth camp. The c l o s e f i t of the data to the t h e o r e t i c a l l y p r e d i c t e d l i n e a r a r r a y i s very good, however l a r g e e r r o r s are a s s o c i a t e d with these l i n e s because of the short 12.16r SHALE CURVE 9.74 9.23 7.19 S AND. K BLUEBELL CURVE J U L 4.57 3.77 3.70 1.89 Time b.p. (Ga) o.oo Figure 4.4 Summary of the changes i n mu w i t h time f o r the 'shale' curve (Godwin and S i n c l a i r , 1982), Stacey and Kramers (1975) average l e a d growth curve, and the B l u e b e l l curve. 60 l e n g t h of the data a r r a y s ( F i g . 4.3). 4.5 DISCUSSION The simplest e x p l a n a t i o n f o r the data from ,Slocan and Carmi camps i s mixing of r e l a t i v e l y unradiogenic l e a d from the Nelson and Westkettle batholiths,, respect i v e l y , with l e a d from the upper c r u s t a l environment which the plu t o n s i n t r u d e . Mixing probably took p l a c e w i t h i n the b a t h o l i t h due to the process of a s s i m i l a t i o n of upper c r u s t a l rocks. S e v e r a l examples of contamination of magma with country rocks have been documented to e x p l a i n the p e t r o g e n e t i c f e a t u r e s , i s o t o p i c compositions and rare e a r t h element d i s t r i b u t i o n s of v a r i o u s igneous rocks (Godwin et a l . , 1980; Moorbath et a l . , 1969; Strong and Dupuy, 1982). A s i m i l a r process i s invoked here to e x p l a i n the i s o t o p i c mixing. Four analyses of f e l d s p a r l e a d , reported by Reynolds and S i n c l a i r (1971), are shown i n F i g u r e 4.5 and l i s t e d i n Table 4.6. These analyses i n d i c a t e that the Nelson b a t h o l i t h i s inhomogeneous and deplet e d i n r a d i o g e n i c l e a d r e l a t i v e to the upper c r u s t . If the f e l d s p a r c o n t a i n s uranium as w e l l as l e a d , the i s o t o p i c composition of the le a d w i l l change c o n t i n u o u s l y u n t i l the sample i s analysed. F i g u r e 4.5 shows that these analyses f a l l on or to the r i g h t of the zero age mixing l i n e i s o c h r o n , supporting the mixing h y p o t h e s i s . T h i s a l s o i m p l i e s that the i s o t o p i c compositions of leads i n the Nelson b a t h o l i t h 160 Ma ago would have been i d e n t i c a l to those of the ore lea d s , i f the U/Pb r a t i o s in the b a t h o l i t h are s u i t a b l e , emphasizing the p l u t o n i c c o n t r i b u t i o n of metals to the ore. Furt h e r 15.75 O.OGa i i i i ' i i i i i i i 1 1 1 18.2 18.4 18.6 18.8 19.0 19.2 19.4 2 0 6 P b / 2 0 4 P b 207 2 0^ 206 2 0A Figure 4.5 Detailed Pb/ Pb vs. Pb/ Pb plot of galena-lead data from Carmi and Slocan camps (Table 4.2) showing the d i s t r i b u t i o n of values along the mixing l i n e between the 'shale' curve (Godwin and S i n c l a i r , 1982) and the Blue b e l l curve. Four analyses of feldspar lead (from Reynolds and S i n c l a i r , 1971; Table 4.6) from the Nelson b a t h o l i t h are also shown. Table 4.6 Isotopic analyses of lead i n feldspar from the Nelson b a t h o l l t h . A f t e r Reynolds and S i n c l a i r , 1971,  Rock Type 2 0 6 P b / 2 ° 4 P b 2 ° 7 P b / 2 ° 4 P b 2 ° 8 P b / 2 ° 4 P b R14 Porphyritic granite 19.330 15.657 38.903 R3 Porphyritic quartz monzonite 18.942 15.602 38.601 RIO Pegmatite assoc. with f o l i a t e d d i o r i t e 19.152 15.660 39.039 R9 Porphyritic granite 18.932 15.557 38.639 O N 63 p e t r o l o g i c a l s t u d i e s are needed to determine any r e l a t i o n s h i p between the i s o t o p i c composition and p e t r o l o g y . of the igneous roc k s . Contours of 2 0 6 P b / 2 0 * P b of the Slocan d e p o s i t s ( F i g . 4.6, a f t e r LeCouteur 1973) have a s t r o n g l y zoned p a t t e r n , with the l e a s t r a d i o g e n i c values c e n t r e d around the northern edge of the b a t h o l i t h , suggesting that the b a t h o l i t h continues north beneath the Slocan Group sediments. The Nelson magma must have been generated i n a uranium-poor region such as the lower c r u s t , i f , as suggested here, the Nelson b a t h o l i t h i s the source of the l e a s t r a d i o g e n i c l e a d i n the mixing process. P l o t s of 2 0 S P b / 2 0 ' t P b versus 2 0 6Pb/ 2 0''Pb ( F i g . 4.7) show a negative slope to the Slocan data, implying that the 'unradiogenic' source was enr i c h e d i n thorium r e l a t i v e to uranium. T h i s i s supported by evidence from Ainsworth camp leads which a l s o p l o t with a high Th/U r a t i o on the l e a d - l e a d diagram of Fig u r e of 4.7. Such high Th/U r a t i o s are c o n s i s t e n t with the f i n d i n g s of Doe and Zartman (1979), who p r e d i c t e d that the lower c r u s t has a low U/Pb r a t i o , but a high Th/U r a t i o . In d e t a i l , the values obtained in t h i s study are q u i t e d i f f e r e n t from those p r e d i c t e d by Doe and Zartman (1979), but the general p r i n c i p l e i s the same. I n i t i a l strontium r a t i o s from the Nelson b a t h o l i t h are in the range 0.7052-0.7696 (Duncan, 1982) which i s lower than f o r normal upper c r u s t a l m a t e r i a l . F a i r b u r n and others (1964), i n e a r l i e r work, a t t r i b u t e low, v a r i a b l e i n i t i a l r a t i o s to d e r i v a t i o n of magma from a deep source, such as the base of the c r u s t or the upper mantle. Thus, both l e a d and stront i u m i s o t o p e s support the hypothesis that the Nelson b a t h o l i t h may have been generated in the lower c r u s t . 64 Figure 4.6 Contoured Pb/ Pb r a t i o s f o r Slocan camp data (Table 4.2). Geology a f t e r Cairnes (1934) and L i t t l e (1960). Adapted from Le Couteur (1973). LO 401 3 9 a . o \ -Q CL 00 o eg 3 8 3 7 3 6 0.0 Ga AINSWORTH - BLUEBELL CAMP SLOCAN CAMP y ' y - CARMI CAMP 1 . 4 4 ^ Fractionation slope •H-+ t ^ - MOYIE CAMP Pb error 16.0 16.5 17.0 17 .5 , 18.0 2 0 6 P b / 2 0 4 Pb 18.5 19.0 19.5 Figure 4.7 208T,u/204_ Pb/ Pb vs. 206_ ,204„, . . . , . _ . Pb/ Pb plot of galena-lead data from Moyie, Ainsworth-Bluebell, Carmi and Slocan camps (Tables 4.2, 4;3 and 4.4). The 'shale' curve (Godwin and Sinclair, 1982) i s shown for reference. 66 The Slocan data (Table 4.2) p l o t s c l o s e r to the 'shale curve' than to the B l u e b e l l curve ( F i g s . 4.3 and 4.5), implying that the Nelson b a t h o l i t h must be s u b s t a n t i a l l y contaminated with upper c r u s t a l m a t e r i a l . Concentrations of lead i n the upper crust are much higher than for lower c r u s t (Doe and Zartman, 1979), so even small amounts of contamination would produce the s i g n i f i c a n t changes seen i n the lead r a t i o s . Percentages of lead c o n t r i b u t i o n from each source were estimated, assuming that the end-member compositions are c o r r e c t , and that there i s ten times more lead i n the upper c r u s t a l source. The r e s u l t s show that a range of between s i x and f o r t y percent of upper c r u s t a l m a t e r i a l must have been a s s i m i l a t e d to produce the observed data (Table 4.7). C a l c u l a t i o n s of t h i s type were made by Moorbath et a l . (1964) to estimate the amount of contamination of mantle-derived T e r t i a r y v o l c a n i c s with Lewisian g n e i s s i c basement rocks i n the I s l e of Skye, Scotland. Their values ranged between s i x and eighty percent. D i f f e r e n c e s i n amount of mixing from one camp to another can a l s o be seen. According to the model some of the lead at Ainsworth-Bluebell camp i s almost e n t i r e l y lower c r u s t a l . The mechanism by which lower c r u s t a l lead became incorporated i n t o these deposits i s not yet understood. However, the r a p i d change in d e p o s i t i o n a l environment seen i n the Poplar Creek s t r a t i g r a p h y (Read, 1973), may i n d i c a t e that a north-trending r i f t s t r u c t u r e was present immediately to the north of the Ainsworth area i n the Lower Cambrian (H. Meade and P. Wodjak, 1982, pers. comm.). Rock types change from a coarse p l a t f o r m a l sequence to black shales, capped by s e v e r a l hundred metres of 67 T a b l e 4 . 7 P e r c e n t a g e c o n t a m i n a t i o n o f a s s u m e d l o w e r c r u s t a l l e a d w i t h u p p e r c r u s t a l l e a d f o r f a c t o r s o f t h r e e a n d t e n t i m e s m o r e l e a d i n t h e u p p e r c r u s t a l s o u r c e , S l o c a n Camp. D e p o s i t F a c t o r o f 3 F a c t o r R u t h - H o p e 2 5 . 7 9 . 6 S i l v e r s m i t h 2 7 . 4 1 0 . 0 S t a n d a r d 2 3 . 4 8 . 6 M o l l y H u g h e s 4 4 . 5 1 2 . 0 V a n Roz 2 3 . 8 9 . 9 P a y n e 2 8 . 5 1 0 . 9 V u l t u r e 2 5 . 9 9 . 7 N o b l e 5 3 9 . 8 1 8 . 1 W h i t e w a t e r 3 3 . 0 1 3 . 1 C o r k P r o v i n c e 3 2 . 3 1 2 . 7 U t i c a 3 2 . 0 1 2 . 6 I n d e x 3 0 . 0 1 1 . 6 B o s u n 2 8 . 2 1 0 . 8 I v a n h o e 2 8 . 9 1 1 . 1 C a l i f o r n i a 2 0 . 5 7 . 3 F i s h e r M a i d e n 2 8 . 4 10.9: L u c k y J i m 3 1 . 0 1 2 . 1 V i c t o r 2 6 . 7 1 0 . 0 D u b l i n Q u e e n 2 9 . 2 1 1 . 2 A r l i n g t o n 3 7 . 3 1 5 . 4 C h a p l e a u 7 5 . 0 4 8 . 1 C o r o n a t i o n 4 9 . 5 2 3 . 1 E n t e r p r i s e 2 6 . 9 1 0 . 1 K a l i s p e l l 2 5 . 8 9 . 6 L i t t l e T i m 3 4 . 1 1 3 . 7 O t t a w a 3 9 . 6 1 6 . 8 S c r a n t o n 4 1 . 0 1 7 . 6 S i l v e r L e a f 2 1 . 0 7 . 5 68 b a s i c v o l c a n i c s , i n d i c a t i n g r a p i d deepening of a trough, p o s s i b l y r e l a t e d to the development of a r i f t . A r i f t c o u l d p r o v i d e a s u i t a b l e pathway f o r ore f l u i d s from the lower c r u s t . 4.6 CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK The framework provided by two growth curves, the shale, curve and the B l u e b e l l curve, adequately e x p l a i n s the data from three d i f f e r e n t types of d e p o s i t s and ages of m i n e r a l i s a t i o n i n the southern Omineca B e l t . Close f i t of p r e d i c t e d and a c t u a l l i n e a r a r r a y s of data from Slocan, Carmi and Moyie lead suggests that the assumptions made in developing the model are reasonable. Thus there i s reinforcement f o r the hypothesis of a Lower Cambrian age of m i n e r a l i s a t i o n at Ainsworth and B l u e b e l l . Many of the Ainsworth d e p o s i t s appear to be e p i g e n e t i c , but these probably represent d e p o s i t s formed by the remobi1isat ion of o l d e r , Cambrian d e p o s i t s , as suggested by Muraro (Hoy et a l . , 1981). It may be p o s s i b l e to re - e v a l u a t e the Ainsworth d e p o s i t s and f i n d evidence f o r a p r e - e x i s t i n g s t r a t i f o r m d e p o s i t . Mixing l i n e isochrons of the type d e s c r i b e d here are a new concept i n l e a d isotope i n t e r p r e t a t i o n . Although they have age s i g n i f i c a n c e , they do not obey c o n v e n t i o n a l isochron equations because of the d i f f e r e n t c h a r a c t e r i s t i c s of the growth curves that they connect. They i l l u s t r a t e the importance of having a l o c a l framework w i t h i n which to i n t e r p r e t l e a d data, and provide a new method f o r d a t i n g both e p i g e n e t i c and syngenetic m i n e r a l i s a t i o n i n the Omineca B e l t . S i m i l a r models may be a p p l i c a b l e to other areas. The 2 0 8 P b / 2 O , P b versus 2 0 6 P b / 2 0 * P b p l o t causes s e v e r a l 69 problems i n that data from Slocan camp have a negative slope whereas those from the Moyie i n t r u s i o n s and Ainsworth camp have p o s i t i v e s l o p e s . Sandon and Slocan data support the mixing model, but Moyie, Carmi and Ainsworth lead compositions are d i f f i c u l t to i n t e r p r e t i n terms of mixing on t h i s diagram. V a r i a t i o n s i n r a t i o s of thorium to l e a d and uranium i n d i f f e r e n t environments, however, are not w e l l understood, and are more v a r i a b l e than, and somewhat independent of uranium to lead r a t i o s . The model proposed here has f a r - r e a c h i n g i m p l i c a t i o n s in that i t leads to d i s c u s s i o n of magmatic contamination, a s s i m i l a t i o n processes, geochemical d i s t i n c t i o n s between upper and lower c r u s t , and the e v o l u t i o n of lead in these r e s e r v o i r s , as w e l l as models for ore genesis and the metallogeny of part of the Canadian C o r d i l l e r a . I t would t h e r e f o r e be u s e f u l to v e r i f y the model by f u r t h e r work in d i v e r s i f i e d f i e l d s . A study of the p e t r o l o g y and rare e a r t h element d i s t r i b u t i o n of the Nelson b a t h o l i t h might r e v e a l a l i n k between rock composition and rock lead i s otope composition, which might be expected a c c o r d i n g to t h i s model. Studies of the r e l a t i v e c o n c e n t r a t i o n s of strontium, neodymium, samarium and other i s o t o p e s , i n c l u d i n g uranium and thorium, would give a b e t t e r i n d i c a t i o n of whether the ' B l u e b e l l curve' r e p r e s e n t s lower c r u s t or upper mantle or an intermediate environment, and would f u r t h e r t e s t the mixing model. 70 CHAPTER 5  CONCLUSIONS The composition of lead i n galena depends on many f a c t o r s which are extremely v a r i a b l e from place to place, so that worldwide "average" lead isotope e v o l u t i o n models are not a p p l i c a b l e , i n d e t a i l , to the deposits of the Canadian C o r d i l l e r a . Studies of lead isotopes in the In s u l a r B e l t , Omineca B e l t , and Intermontane B e l t reveal s u b s t a n t i a l d i f f e r e n c e s i n i s o t o p i c c h a r a c t e r i s t i c s between these tectono-s t r a t i g r a p h i c terranes. Since lead isotope data i d e n t i f y major metallogenic and t e c t o n o - s t r a t i g r a p h i c terranes, they are u s e f u l in studying r e g i o n a l metallogeny. D e t a i l e d studies of lead compositions of the deposits in each of these terranes has l e d to the development of l o c a l models that provide a framework for sev e r a l types of e x p l o r a t i o n d e c i s i o n s . Although there are i n s u f f i c i e n t data to develop a d e t a i l e d growth curve model for the In s u l a r B e l t , the e m p i r i c a l d i s t i n c t i o n between syngenetic and epigenetic deposits can be used to d i s t i n g u i s h between these two deposit types in e x p l o r a t i o n . Thus, i t i s p o s s i b l e to d i s t i n g u i s h veins which form part of a stockwork of a massive sulphide ore body from those which are epigenetic and r e l a t e d to plutons, using analyses of the i s o t o p i c composition of the contained lead. Lead data suggest that the metals in quartz-gold vein deposits are derived from the host rocks, implying that s t r a t i g r a p h y as w e l l as plutonism may be important i n the search for these d e p o s i t s . The study of lead isotope compositions for mineral deposits 71 from the Cariboo area i l l u s t r a t e s the use of le a d i s o t o p e s i n t e s t i n g hypotheses put forward f o r the o r i g i n of ore d e p o s i t s . Use of the 'shale curve' model to c a l c u l a t e the age of m i n e r a l i s a t i o n of the Omineca B e l t v e i n s g i v e s a Mesozoic age fo r the event, a p r e v i o u s l y u n c e r t a i n age of m i n e r a l i s a t i o n . Comparison of Mosquito Creek l e a d data with the 'shale curve' c l e a r l y shows that the m i n e r a l i s a t i o n i s e p i g e n e t i c , g i v i n g one example of the use of the 'shale curve' in d i s t i n g u i s h i n g e p i g e n e t i c from syngenetic d e p o s i t s . T h i s d i s t i n c t i o n i s c r i t i c a l to any e x p l o r a t i o n model and c o n v e n t i o n a l methods may not always be c o n c l u s i v e . R e c o g n i t i o n that the Intermontane B e l t and Omineca B e l t v e i n s c o n t a i n galena with d i f f e r e n t l e a d s i g n a t u r e s i s important i n developing models f o r the genesis of the v e i n s , s i n c e i t r u l e s out the p o s s i b i l i t y of a common gen e s i s . Development of a curve d e s c r i b i n g lead e v o l u t i o n i n an unradiogenic environment, which i s p o s s i b l y the lower c r u s t , has many a p p l i c a t i o n s and i m p l i c a t i o n s to e x p l o r a t i o n i n southeastern B r i t i s h Columbia. C o n s i s t a n t f i t of the model to data from three m i n e r a l i s i n g events at widely spaced times i n d i c a t e s that the assumed ages f o r the m i n e r a l i s a t i o n in each case are approximately c o r r e c t . Thus, a Lower Cambrian age of m i n e r a l i s a t i o n f o r the Ainsworth camp seems a p p r o p r i a t e , i n agreement with recent ideas of Muraro ( i n Hoy et a l . , 1981, and S i n c l a i r , 1982, pers. comm.). The two curve model with mixing l i n e isochrons p r o v i d e s a framework f o r d a t i n g l i n e a r a r r a y s from d e p o s i t s r e l a t e d to plu t o n s which have p r e v i o u s l y been c o n s i d e r e d anomalous. I t a l s o c l a r i f i e s the r o l e of plutons i n 72 the search f o r s i l v e r v e i n s . S i m i l a r mixing models may apply to other areas, and may provide a l t e r n a t e e x p l a n a t i o n s f o r l i n e a r a r r a y s of data i n l e a d - l e a d p l o t s . The three s t u d i e s c o n s i d e r e d here i l l u s t r a t e how galena-lead i sotope analyses can c o n t r i b u t e to an improved understanding of ore genesis and m i n e r a l i s a t i o n ages. 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A study of lead isotopes from mineral deposits i n southeastern B r i t i s h Columbia and i n the A n v i l range, Yukon T e r r i t o r y . Unpublished Ph.D t h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 142p. LeCouteur, P.C., 1979. Age of the S u l l i v a n l e a d - z i n c deposit. C o r d i l l e r a n Section Geol. Ass. Can. C o r d i l l e r a n S e c t i o n , Prog. And Abst., February 1979, p. 19. Leech, G.B. and Wanless, R.K., 1962. Lead-isotope and K-Ar studies i n the East Kootenay d i s t r i c t , B r i t i s h Columbia. -Geol. Soc. America, Buddington volume, p. 241-279. LeRoux, L . J . and Glendenin, L.L., 1963. H a l f - l i f e of thorium-232. Proc. N a t l . Conf. on Nuclear Energy, P r e t o r i a , A p r i l . L i t t l e , H.W., 1961. Geology of K e t t l e R i v e r , West h a l f (82E), B r i t i s h Columbia. Geol. Surv. Can. Map 15-1961. 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Rb/Sr dates f o r g r a n o d i o r i t e i n t r u s i o n s on the northeast margin of the Shuswap Metamorphic Complex, Cariboo Mountains, B r i t i s h Columbia. Can. J . E a r t h S c i . , v. 14, pp. 1690-1695. Read, P.B., 1973. P e t r o l o g y and s t r u c t u r e of Poplar Creek map-area, B r i t i s h Columbia. Geol. Surv. Can. B u l l . 193, 144p. Rees, C.J., 1981. Western margin of the Omineca B e l t at Quesnel Lake, B r i t i s h Columbia. In Current Research, Part A, Geol. Surv. Can., Paper 81 - 1A, pp. 223-226. Reynolds, P.H. and S i n c l a i r , A.J., 1971. Rock and ore lead i s o t o p e s from the Nelson b a t h o l i t h and the Kootenay Arc, B r i t i s h Columbia, Canada. Econ. Geol. v.66, pp. 259-266. R u s s e l l , R.D. and Farquhar, R.M., 1960. Lead is o t o p e s i n geology. I n t e r s c i e n c e P u b l i s h e r s , New York, London, 243p. R u s s e l l , R.D. And Reynolds, P.H., 1965. The age of the E a r t h . In: Problems i n geochemistry, Acad. S c i . U.S.S.R., 78 Vinogradov J u b i l e e Volume, p. 37. Schink, E.A., 1974. Geology of the Shiko Lake stock, near Quesnel Lake, B r i t i s h Columbia. B.Sc t h e s i s U n i v e r s i t y of B r i t i s h Columbia, 64p. Shannon, F.G., 1970. Some unique g e o l o g i c a l f e a t u r e s at the B l u e b e l l Mine, R i o n d e l , B.C. In ' l e a d - z i n c d e p o s i t s i n the Kootenay Arc, northeastern Washington and adjacent B r i t i s h Columbia.' ed. A.E. Weissenborn; Dept. N a t u r a l Resources Div. of Mines and Geology, Washington S t a t e , B u l l . 61, pp.107-120. S i n c l a i r , A.J., 1964. A l e a d isotope study of mineral d e p o s i t s in the Kootenay Arc. Ph.D t h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 257 p. S i n c l a i r , A.J. and Godwin, C.I., 1981. I n t e r p r e t a t i o n of l e a d isotope data, southern coast mountains. B.C. M i n i s t r y of Energy, Mines and Petroleum Resources Paper 1981-1, pp. 178-184. Souther, J.G., 1977. Volcanism and t e c t o n i c environments i n the Canadian C o r d i l l e r a - a second look. In ' v o l c a n i c regimes in Canada.' ed. Baragar, W.R.A., Coleman, L.C. and H a l l , J.M. Geol. Surv. Can., S p e c i a l Paper 16, pp. 3-24. Sutherland Brown, A., 1957. Geology of the A n t l e r Creek area, Cariboo d i s t r i c t , B r i t i s h Columbia. B.C. M i n i s t r y of Energy, Mines and Petroleum Resources B u l l . .38, I05p. Sutherland Brown, A., 1963. Geology of Cariboo R i v e r , B.C. B.C. M i n i s t r y of Energy, Mines and Petroleum Resources B u l l . 47, 60p. Stacey, J.S., Delevaux, M.E., and U l r y c h , T.J., 1969. Some t r i p l e - f i l a m e n t lead isotope r a t i o measurements and an absolute growth curve f o r s i n g l e - s t a g e l e a d s . E a r t h P l a n e t . S c i . L e t t . v. 6, pp. 15-25. Stacey, J.S. and Kramers, J.D., 1975. Approximation of t e r r e s t r i a l l ead isotope e v o l u t i o n by a two-stage model. Ea r t h P l a n e t . S c i . L e t t . v. 26, pp. 207-221. Stanton, R.L. and R u s s e l l , R.D., 1959. Anomalous leads and the emplacement of lead s u l p h i d e ores. Econ. Geol. v. 54, pp. 79 588-607. Stevenson, J.S., 1950. Geology and mineral deposits of the Zeballos Mining camp, B.C. B.C. M i n i s t r y of Energy, Mines and Petroleum Resources B u l l . 27, 145 p. Strong, D.F. and Dupuy, C , 1982. Rare earth elements i n the bimodal Mount Peyton b a t h o l i t h : Evidence of c r u s t a l anatexis by mantle-derived magma. Can. J . Earth S c i . , v. 19, pp. 308-315. S t r u i k , L.C. 1979. S t r a t i g r a p h y and s t r u c t u r e of the B a r k e r v i l l e - Cariboo area, c e n t r a l B r i t i s h Columbia. In Current Research, Part B, Geol. Surv. Can., Paper 79-1B, pp. 33-38. S t r u i k , L . C , 1981a. Snowshoe Formation, C e n t r a l B r i t i s h Columbia. In Current Research, Part A. Geol. Surv. Can. Paper 81-1A, pp. 213-216. S t r u i k , L . C , 1981b. A re-examination of the type area of the Devono-Mississippian Cariboo Orogeny, c e n t r a l B r i t i s h Columbia. Can. J . Earth S c i . v. 18, pp. 1767-1775. T i l t o n , G.R., 1973. I s o t o p i c lead ages of c h o n d r i t i c meteorites. Earth Planet. S c i . L e t t . v. 19, pp. 321-329. Wanless, R.K., Stevens, R.D., Lachance, G.R. and Rimsaite, R.Y.H., 1965. Age determinations and g e o l o g i c a l s t u d i e s . Part 1 - I s o t o p i c ages, Report 5. Geol. Surv. Can. Paper 64-17 (part 1), 126p. Wanless, R.K.,1967. Age determinations and g e o l o g i c a l s t u d i e s , K-Ar i s o t o p i c ages, Report 7, Geol. Surv. Can. Paper 66-17, 1 20p. Watson, P.H., Godwin, C.I. and Christopher, P.A., 1981. General geology and genesis of s i l v e r and gold veins i n the Beaverdell area, s o u t h - c e n t r a l . B r i t i s h Columbia. Can. J . Earth S c i . v. 19, pp. 1264-1274. White, W.H., E r i c k s o n , G.P., Northcote, K.E., Dirom, G.E., and Harakal, J.E., 1967. I s o t o p i c dating of the Guichon b a t h o l i t h , B r i t i s h Columbia. Can. J . Earth S c i . v. 4, pp. 677-690. 80 White, W.H., H a r a k a l , J.E. and C a r t e r , N.C., 1968. K-Ar ages of some ore d e p o s i t s i n B.C. Canadian M i n i n g M e t a l l . B u l l . Nov. 1968, v. 61, No. 679, pp1326-l334. Zartman, R.E., 1974. Lead i s o t o p e p r o v i n c e s i n the C o r d i l l e r a of the Western U n i t e d S t a t e s and t h e i r g e o l o g i c a l s i g n i f i c a n c e . Econ. G e o l . v. 69, pp. 792-805. 81 APPENDIX A ORE LEAD SAMPLE PREPARATION AND ANALYSIS 1. LEAD PURIFICATION D i s s o l v i n g Galena Hand p i c k e d galena samples (approx. 1 gram) are p l a c e d i n 50 ml g l a s s beakers with 40 ml 6M HC1. If carbonate i s present 2.5M HC1 i s used to destroy a l l the carbonate before d i s s o l v i n g the l e a d i n d i s t i l l e d 6M HC1. There i s s u f f i c i e n t l e a d f o r the process i f galena i s v i s i b l e . The samples are l e f t f o r s e v e r a l hours i n a fume hood to d i s s o l v e as much lead as p o s s i b l e . P r e c i p i t a t i o n Of Lead C h l o r i d e The l e a d and c h l o r i d e - b e a r i n g s o l u t i o n i s f i l t e r e d through g l a s s f i b r e f i l t e r paper i n t o a c e n t r i f u g e tube to remove the i n s o l u b l e s i l i c a t e s and s u l p h i d e s . I t i s returned to the we l l r i n s e d 50 ml beaker and heated u n t i l only about 10 ml l i q u i d remain; at which time, white l e a d c h l o r i d e c r y s t a l s should appear. The 10 ml of l i q u i d and c r y s t a l s are then t r a n s f e r r e d back i n t o the c e n t r i f u g e tube and c o o l e d to allow the maximum amount of c r y s t a l s to form. C e n t r i f u g a t i o n of samples f o r a few seconds f o r c e s the c r y s t a l s to s e t t l e , and the supernatent l i q u i d i s then decanted. D i s t i l l e d water i s added to wash the c r y s t a l s and the supernate i s decanted; t h i s process i s repeated u n t i l only c l e a n white c r y s t a l s remain. These are r e d i s s o l v e d in d i s t i l l e d water. Heating with hot tap water flowing over the ^2 c e n t r i f u g e tube speeds t h i s p rocess. Anion Exchange Columns A 2 ml sample of the l i q u i d i s taken and passed through an anion exchange column to remove Ag, Bi and Cd ions, which i n t e r f e r e with the a n a l y s i s . 2 ml of d i s t i l l e d water i s used as an e l u e n t . The 4 ml of l i q u i d i s c o l l e c t e d i n a 5 ml t e f l o n beaker which i s then t r a n s f e r r e d to an e l e c t r o p l a t i n g chamber. The anion exchange r e s i n i s cle a n s e d by passing 10 ml HC1 (6M), fol l o w e d by 10 ml H 20 ( d i s t i l l e d ) through the column. Columns are kept wet between runs i n beakers of d i s t i l l e d water. E l e c t r o p l a t i n g Lead D i s t i l l e d water and a few drops of HN03 (6M) are added to the s o l u t i o n in the e l e c t r o p l a t i n g chamber. The s o l u t i o n must cover the e l e c t r o d e s . A volt a g e of 1.8 i s a p p l i e d to the platinum e l e c t r o d e s ; the c u r r e n t i s maximised by s t i r r i n g with a magnetic s t i r r e r . A brown d e p o s i t on the cathode should develop a f t e r a few hours. E l e c t r o p l a t i n g removes t r a c e s of z i n c and potassium. P r e p a r a t i o n Of Lead N i t r a t e . A s o l u t i o n of 1% HN03 with 1% H 20 2 i s used to s t r i p the brown d e p o s i t from the cathode. Only a few drops are r e q u i r e d . The drops are c o l l e c t e d in the 5 ml t e f l o n beakers used p r e v i o u s l y . Evaporation of the drops p r e c i p i t a t e s small white specks of le a d n i t r a t e , which are used in the subsequent a n a l y s e s . The t e f l o n beakers should be covered with p a r a f i l m to prevent contamination of the samples. T e f l o n beakers are cleaned i n aqua r e g i a , and glassware i n 1:1 HN0 3. 2. PREPARATION OF RHENIUM FILAMENTS Mounting Filaments S t r i p s of Rhenium (Re) wire are cut i n t o 1.5cm lengths i n a cl e a n environment (gloves should be worn). Each l e n g t h i s a r c -welded ( s e t t i n g at "45" and 6 amps) to a f i l a m e n t block, e n s u r i n g that the wire i s taut a c r o s s the p o s t s . Outgassing F i l a m e n t s Outgassing or baking out the Rhenium f i l a m e n t s c l e a n s them of adsorbed gasses p r i o r to use in the mass spectometer. I t i n v o l v e s h e a t i n g the f i l a m e n t s by passin g a c u r r e n t through them in a vacuum. Nine f i l a m e n t blocks are p l a c e d i n the holder and connected in s e r i e s (making sure that none of the Re wire touches the holder) to the e l e c t r i c a l c i r c u i t . The c i r c u i t i s t e s t e d f o r c o n t i n u i t y using a m u l t i t e s t e r , then the holder i s c a r e f u l l y p l a c e d i n t o the vacuum chamber. A torque wrench, set at approximately 150 l b s , i s used to t i g h t e n the screws on the vacuum l i d . A f t e r r e - t e s t i n g the c i r c u i t , t e r m i n a l s are connected and pump down begins. F i r s t the r o t a r y pump i s switched on to reduce the pressure to 100 mbars then l i q u i d n i t r o g e n i s a p p l i e d to the c o l d t r a p . T h i s should reduce the pressure f u r t h e r to approximately 60 mbars. The second pump i s switched on and l i q u i d n i t r o g e n 94 a p p l i e d to the second c o l d t r a p . Pressure now f a l l s to near zero w i t h i n h a l f an hour. A f t e r t h i s point the second pressure gauge can be read and various heating elements i n the vacuum system are turned on to increase pressure t e m p o r a r i l y , and f a c i l i t a t e pumping. -1 When pressure i s i n the range 2.5 - 3.0 x 10 mbars the bakeout switch i s turned on. The "power", "DEG" and "FIL" switches should a l l be o f f . The timer i s set to 15 minutes and bakeout w i l l stop a u t o m a t i c a l l y a f t e r that time. The current for bakeout i s set at 3.2 amperes. T h i r t y minutes a f t e r the s t a r t of the bakeout, a l l vacuum pumps except the ro t a r y pump are switched o f f . This i s l e f t on for a f u r t h e r 30 minutes. Dewars with l i q u i d nitrogen are removed, and the bakeout chamber i s o l a t e d from the vacuum pump. The vacuum pump i s switched off and the bakeout chamber i s vented by opening the appropriate glass v a l v e . The system i s l e f t to c o o l for a f u r t h e r hour, or longer as convenient, then the bakeout flange can be disassembled. Baked out filament blocks are t r a n s f e r r e d to a clean box u n t i l ready for loa d i n g . 3. LOADING LEAD ONTO RHENIUM FILAMENTS Lead n i t r a t e i s loaded onto the outgassed filaments in a s o l u t i o n of phosphoric a c i d . S i l i c a g el i s used to enhance and s t a b i l i z e the evaporation of the lead from the filaments i n the mass spectrometer. A piece of spaghetti tubing, about 3cm long i s taken from a b o t t l e with tweezers and i s mounted i n t o a p i p e t t e . The p i p e t t e t i p i s then r i n s e d three times i n d i s t i l l e d HN0 3, and three times i n t r i p l e d i s t i l l e d water. A f t e r shaking the b o t t l e of prepared s i l i c a gel i t i s drawn 2cm up i n t o the p i p e t t e t i p and dropped onto the f i l a m e n t , which i s attached to e l e c t r o d e s with a current of 1 amp across them. Meanwhile, a pipette-measured 2cm of phosphoric a c i d i s placed i n the t e f l o n beaker to d i s s o l v e the lead n i t r a t e sample. The sample i s then added to the filament on top of the s i l i c a g e l , which should have evaporated to near dryness at t h i s p o i n t . The combined load on the filament i s now d r i e d by t u r n i n g the current up to 1.1 amps. The load should turn black. The current i s now turned up to 1.5 amps u n t i l bubbling on the filament stops. Slowly the current i s increased to 2.3 amps, u n t i l a steady stream of white smoke begins to be given o f f . The sample load on the filament should turn white. A 'low glow' i s then obtained by t u r n i n g the current s t i l l higher, for a minute; t h i s i s followed by i n c r e a s i n g the current a few seconds so that a medium red glow i s obtained. The current i s turned o f f and the filament i s ready for loading i n t o the mass spectrometer. 4. LOADING FILAMENTS INTO THE MASS SPECTROMETER Filament blocks from previous samples are removed from the source by i s o l a t i n g the source chamber from the rest of the system and b r i n g i n g i t up to room pressure. This i s done by c l o s i n g the valve to the ion beam path ( 1 ) 1 , and the valve to the t i t a n i u m sublimation pump (2). Nitrogen i s then supplied to the source chamber by opening two valves (one on the nitrogen 1 Numbers in parentheses r e f e r to Figure A.1 SOURCE Ion Path Titanium sublimation pump and ion pump Electromagnet 1 1 — ^ 3 \> 5 N i « Cold trap 4 Rotary pump ' I Nitrogen Supply Titanium sublimation pump and ion pump Col l e c t o r Figure A - l . Schemmatic i l l u s t r a t i o n of the p r i n c i p l e components of the 54R micromass mass spectrometer. 87 tank, 6, and one on the mass spec.,5). Valve 3 i s opened a f t e r making sure that both vacuum pumps are i s o l a t e d and that the power s u p p l i e s to the source and a c c e l e r a t o r are o f f . A pressure gauge i n d i c a t e s when the door can be removed. The f i l a m e n t holder f o r the filam e n t blocks i s removed c a r e f u l l y by unscrewing the c e n t r a l b o l t (without j o l t i n g the fi l a m e n t s a g a i n s t c o n t a c t s ) . The holder i s t r a n s f e r r e d to a cle a n s u r f a c e , where the o l d fi l a m e n t blocks are removed and re p l a c e d by new ones. Filament posts should be s t r a i g h t and touching the s p r i n g c o n t a c t s (none of the s i x copper wires att a c h e d to each fila m e n t should be t o u c h i n g ) . When s i x fi l a m e n t blocks are i n place the fi l a m e n t support i s re p l a c e d i n the source chamber, and the f i r s t f i l a m e n t block i s ce n t r e d on the c o n t a c t s . The door i s c l o s e d g e n t l y and the pump down process begins.-The r o t a r y pump alone i s used f o r about 20 minutes u n t i l the pressure i s reduced to 0.005 atm. L i q u i d n i t r o g e n i s then a p p l i e d to the c o l d t r a p (7) to reduce the pressure to 0.001 atm wi t h i n about h a l f an hour. At t h i s p o i n t the v a l v e s from source chamber to r o t a r y pump are c l o s e d (3,5) and the pump i s switched o f f . The t i t a n i u m pump i s a p p l i e d to the source chamber by opening the va l v e (2) slowly so that the pressure in the pump does not exceed 1 0 5 m bar. The T i pump i s set to c y c l e at a convenient time i n t e r v a l such as every 15 minutes or once every three hours as r e q u i r e d . When the pressure in the chamber reaches 10 mbars the valve l e a d i n g to the ion beam path (1) can be opened. T h i s w i l l cause a s l i g h t r i s e in the pressure i n the system, but the pressure to s h o u l d remain at I0~mbars f o r good r u n s . To commence a run the ' c e n t r e f i l a m e n t ' knob i s r e s e t and the power s u p p l y s w i t c h e d on. F i v e k i l o v o l t s a r e a p p l i e d t o the s o u r c e , then a c u r r e n t of a p p r o x i m a t e l y 2 amps i s a p p l i e d t o the f i r s t f i l a m e n t . A y e l l o w - o r a n g e glow i s o b t a i n e d when the f i r s t f i l a m e n t i s a p p r o x i m a t e l y a t the r i g h t t e m p e r a t u r e , but t h i s must be measured w i t h an o p t i c a l pyrometer. The c u r r e n t i s a d j u s t e d u n t i l a temperature of 1025C ± 50C i s o b t a i n e d ( t h i s i s c r i t i c a l ) . The v o l t a g e s i n the source a r e f i n e l y a d j u s t e d t o o b t a i n a s i g n a l which i s observ e d on the s t r i p c h a r t r e c o r d e r . E i g h t f o c u s i n g knobs are used t o maximise the s i g n a l , and a s c a l e f a c t o r s e l e c t e d a c c o r d i n g l y . A 'SET 1' command on the computer keyboard i n i t i a t e s d a t a c o l l e c t i o n ( e n t e r i n g and u s i n g the "UBCPB" program a r e d e s c r i b e d i n the f o l l o w i n g s e c t i o n ) . Each b l o c k of da t a c o n t a i n s 5 s e t s of measurements each of t h r e e r a t i o s ; 2 0 "Pb/ 2 0 6 P b , 2 0 7 P b / 2 0 6 P b and 2 0 8P.b/ 2 0 6Pb. Mean v a l u e s a re p r i n t e d out a f t e r each b l o c k , w i t h the s t a n d a r d e r r o r at t he 2 sigma l e v e l . A minimum of s i x b l o c k s of da t a a r e c o l l e c t e d f o r each r u n , and more i f the i n - r u n p r e c i s i o n i s poor. The 'summary' command f i n d s the mean v a l u e f o r each r a t i o and c a l c u l a t e s the a s s o c i a t e d s t a n d a r d e r r o r . R a t i o s a re c o n v e r t e d i n t o the c o n v e n t i o n a l r a t i o s by hand, and e r r o r s c o n v e r t e d i n t o p e r c e n t a g e s . Raw d a t a was c o r r e c t e d f o r f r a c t i o n a t i o n u s i n g the c o r r e c t i o n f a c t o r s : 1.001,1.002, and 1.003. E l e v e n runs of the Broken H i l l S t a n d a r d gave mean v a l u e s f o r the c o r r e c t i o n f a c t o r s of 0.9999(0.28), 1.0026(0.28) and 1.0035(0.23). The c a l c u l a t e d si mean v a l u e f o r the s t a n d a r d d i d not f a l l on the f r a c t i o n a t i o n l i n e w i t h the a b s o l u t e v a l u e f o r the s t a n d a r d which has r a t i o s o f : 2 O 6 P b / 2 0 4Pb = 16. 003 , 2 0 7 P b / 2 < M P b = 15.389, and 2 0 8 P b / 2 0 a P b = 35.657. The p r e c i s i o n f o r these c o r r e c t i o n f a c t o r s i s o n l y 0.3% a t the 2 sigma l e v e l . R e p r o d u c i b i l i t y was b e t t e r f o r most of the sample runs than f o r the s t a n d a r d ( f o r example P i n Money, Chapter 3 ) , so i t was f e l t t h a t i m p u r i t i e s i n the s t a n d a r d were p r o d u c i n g m i s l e a d i n g l y poor p r e c i s i o n . An a n a l y s i s of the NBS s t a n d a r d was i d e n t i c a l , w i t h i n a n a l y t i c a l e r r o r , t o a n a l y s e s of the same by R . P a r r i s h over the p e r i o d d u r i n g which the samples were a n a l y s e d . Thus the c o r r e c t i o n f a c t o r s used by R . P a r r i s h were p r o b a b l y more a p p r o p r i a t e . A p p l i c a t i o n of thes e c o r r e c t i o n f a c t o r s t o raw da t a produced good agreement between o l d d a t a of B.Ryan and new a n a l y s e s of the same sample (e.g. C a r i b o o G o l d 1 Q u a r t z . TO RUN THE "UBCPB" PROGRAM I n s e r t the magnetic t a p e , then keypunch: LOAD "UBCPB" ENDLINE RUN ENDLINE When the program i s ru n n i n g the e l e c t r o m a g n e t i s a u t o m a t i c a l l y s e t and v a r i e d f o r each s e t of d a t a . The data c o l l e c t i o n p r o c e s s i s c o n t r o l l e d by e i g h t keys as shown below: KEY K2 SET4:46 K4 SCAN K6 HALT@88 K8 SUMMARY LABEL K1 RESTART K3 INPUTS K5 SET1:76 K7 SET2:86 RESTART:- Erases previous data, and asks f o r 'number of peak se t s i n blo c k ? ' answer: 5 ENDLINE. The spectrometer a u t o m a t i c a l l y stays on the 2 0 8 P b peak 'waiting f o r command'. SET1:76 :- Begins data c o l l e c t i o n a f t e r RESTART command, or continues data c o l l e c t i o n a f t e r SUMMARY or HALT@88 beginning with the 76 r a t i o ( 2 0 7 P b / 2 0 6 P b ) . Previous data s e t s are r e t a i n e d . SET2:86 :- Begins with 86 r a t i o ( 2 0 8 P b / 2 0 6 P b ) . T h i s i s a u s e f u l command i f 2 0 8 P b peak i s o f f s c a l e and data c o l l e c t i o n s tops. When the 2 0 8 P b peak i s brought back i n t o the c o r r e c t range the SET1:76 can be saved. HALT @88 :- T h i s holds the 2 0 8 P b s i g n a l u n t i l SET 1:46 or other command i s gi v e n . I t i s u s e f u l for a l t e r i n g peak height d u r i n g runs ( f o r example i f the 2 0 8 P b peak goes o f f s c a l e ) . SUMMARY :- Summarizes data c o l l e c t e d and is s u e s the standard d e v i a t i o n of the mean and the standard d e v i a t i o n of the mean of the e r r o r s a s s o c i a t e d with the i n d i v i d u a l r a t i o s . SUMMARY h a l t s the program. SCAN :- systemmatically a l t e r s the electromagnet s e t t i n g so that a l l peaks are found. T h i s i s used to de t e c t any i n t e r f e r e n c e or poor peak shapes. 1^ INPUTS :- can change the number of peaks i n set without d e s t r o y i n g e x i s t i n g data b l o c k s . SET 4:46 :- S i m i l a r to SET 2:86, but commences with the 2 o a p b / 2 0 6 P b r a t i 0 > ^1 APPENDIX B B r i e f D e s c r i p t i o n s o f the D e p o s i t s named i n C h a p t e r 2  Western Mines: LAT 49-57°; LONG 125-59°: NTS 092 7 12E Sample Nos: G7 9Vr,l-001, 002. arid' 003-.-D e s c r i p t i o n : A kunoko-cype massive sulphide deposit. Lenses, veins and mosses of p y r i t e , c h a l c o p y r i t e , s p a h l e r i t e and galena i n Si c k e r Group r h y o l i t i c v o l c a n i c s . Metals recovered: Cu, Zn, Pb, Au., Ag, Cd, Ba References: M i n f i l e 092F 071 Lenora: LAT 48-S7°; LONG 123-78°; NTS 092 3 13W Sample Nos: G79LN-001, 002 D e s c r i p t i o n : Described as a 'replacement' ore, but i s probably a volcanogenic massive sulphide i n Sicker Group folded t u f f s . Two ore types occur; b a r i t e ore i s a fi n e - g r a i n e d mixture of p y r i t e , c h a l c c p y r i t e , s p h a l e r i t e , and galena i n a b a r i t e , c a l c i t e , quartz, gangue; and quartz ore i s mainly quartz a n d c h a l c c p y r i t e . Metals recovered: Zn, Cu, Ag, 3a, ?b References: M i n f i l e 092? 001 Tyea: • LAT 4S-S7°; LONG 123-7S°; NTS 052 3 13V; Sample Nos: G79TY-001 D e s c r i p t i o n : (See Lenora) Coviehan Lake: LAT 43-7°; LONG 124-3°; NTS 092 C 16V,' Sa-ple Nos: G7 9CL-001, 002 and 003 D e s c r i p t i o n : Permocarboniferous B u t t l e Lake formation Limestone (Sicker Group) i s und e r l a i n by Si c k e r Group cherts, t u f f s and br e c c i a s and o v e r l a i n by Karmutsen formation basalt and diabase. References: B.C. Dept. of Mines B u l l . 37, p 16 and B u l l . 40, p 4 6 P r i v a t e e r : LAT 50-03°; LONG 126'Sl°; NTS 092 L 02W Sarr.ple Nos. : 30349-001 D e s c r i p t i o n : Quartz veins with p y r i t e , s p h a l e r i t e , galena, arseno p y r i t e , p y r r h o t i t e , and native gold cut massive Bonanza Group v o i c a n i c s , lime s i l i c a t e s and small bodies of J u r a s s i c i n t r u s i v e quartz d i o r i t e . Metals recovered: Ag, Au, Cu, Pb References: M i n f i l e 092 L 008 Bancroft, (1940) Alpha and Beta: LAT 4S-73°; LONG 124-09°; NTS 092 C 09E Sample Nos.: G79AB-001 D e s c r i p t i o n : F r a n k l i n Creek (Karmutsen Formation) andesite and lenses of Quatsino LLmestone are cut by dykes of g r a n o d i o r i t e , g r a n i t e porphyry and d i o r i t e porphyry. Limestone, andesite and g r a n o d i o r i t e are p a r t l y a l t e r e d to garnet-epidote^pyroxene skarn with c h a l c o p y r i t e , magnetite and p y r i t e l o c a l l y . References: B.C. Dept. of Mines, G.E.M. 1970 p 291, 1971 p 226, M i n f i l e 092 C 039 Lucky S t r i k e : LAT 50-06°; LONG 126-84°; NTS 092 L 02W Sample- Nos.: '30334-001 D e s c r i p t i o n : The Vancouver-G i r 1 shear, which follows a f e l d -spar porphyry dyke i n d i o r i t e and g r a n o d i o r i t e b r e c c i a , contains lenses of quartz 5-7.5 cm wide with p y r i t e and f r e e - g o l d . Metals recovered: Au References: M i n f i l e 092 L 030, Mindep 05S56, Geol. Surv. Canada Mem. 272 p 59 V i c t o r i a : LAT 49-18°; LONG 124-66°; NTS 092 F 02E Sample Nos.: 30315-001 De s c r i p t i o n : Gold and p y r i t e are associated with quartz veins i n sheared sections of a n d e s i t i c flows and t u f f s . The rock groups observed were placed t e n t a t i v e l y i n the Sicker Group, though Govern-ment g e o l o g i s t s l e f t them i n the Island s e r i e s . The deposit i s adjacent to a stock r e l a t e d to the coast range b a t h o l i t h ( i . e . t h i s v e i n i s probably an olde r vein than the o t h e r s ) . Metals recovered: Au, Ag and Cu References: M i n f i l e 092F 079, B.C. Dept. of Mines Ass. Rpt. 4914, Muller and Carson (1969) Pandora: LAT 49-25°; LONG 125-68°: NTS 092 F 05E Sample Nos.: 30323-001 D e s c r i p t i o n : Gold and s i l v e r - b e a r i n g veins of quartz with some carbonate, f i n e l y c r y s t a l l i n e p y r i t e and r a r e l y l e r i t e , cut a l t e r e d v o l c a n i c s near a n d e s i t i c dylas i n one working c l o s e to a small mass cf i n t r u s i v e quartz d i o r i t e . Metals recovered: Au, Ag, Cu, Pb, Zn References: M i n f i l e 092F 040 Muller-Carson (1969) White Star: LAT 50-30°; LONG 126-Sl°; NTS 092 L 02W Sample Nos.: 30318-001 D e s c r i p t i o n : Quartz d i o r i t e of Eocene Zeballos pluton i s cut b y feldspar porphyry dykes and subsequently j o i n t e d i n three d i r e c t i o n s . Quartz veins with p y r i t e , galena, arsenopyrite, s p h a l e r i t e and gold White Star (Cont'd) occur i n gangue and. breccia, zones, along;.,vhich f a u l t moveaent has taken p l a c e . The sulphides;, are. u s u a l l y concentrated i n bands along the v a i l s of the veins'. Metals recovered: Au, Ag, ?b, Zn References: . M i n f i l e 092L 010 ' Bancroft, 1940 p 12 Lone Star-Rey-Oro: LAT 50-02°; LONG 126-79°; NTS 092 L 02W Sample Nos: 30317-001 D e s c r i p t i o n : Quartz d i o r i t e of Zeballos pluton i s cut by a p l i t e , a n d e s i t i c and feldspar porphyry. Many quartz veins with galena, arser.opyrite, gold c h a l c o p y r i t e and s p h a l e r i t e occur i n shears i n j o i n t e d quartz d i o r i t e and the dykes. Metals recovered: Au, Ag, Zn, ?b, Cu References: M i n f i l e 092 L 015 Mincep 05S41 P e e r l e s s : LAT 50-04°; LONG 126-54°; NTS 092 L 02iv Sa-ple Nos.: 30320-001 D e s c r i p t i o n : A quartz v e i n 5 cm wide follows a contact between a f e l d s p a r porphyry dyke and f e l d s p a t h i s e d andesite of Lever J u r a s s i c Bonanza group. Quartz contains abundant c a l c i t e and small amounts of s p h a l e r i t e and c h a l c o p y r i t e , Metals recovered: Au, Zn, Cu References: M i n f i l e 092 L 025 Mindep 05851 Iron Clad: LAT 48-85°; LONG 123•6S°; NTS 092 B 13E Sample Nos.: G79IC-001 De s c r i p t i o n : S p h a l e r i t e , p y r r h o t i t e , c h a l c o p y r i t e m i n e r a l i s a t i o n in a shear zone of q u a r t z - s e r i c i t e s c h i s t of Sicker Group. Minerals recovered: Zn, Cu References: B.C. Dept. of Mines Assessment Report No. 19. S.C. M i n i s t r y of Mines Annual Report, 1904, p 253. Bon:' LAT 50-27°; LONG 126-67°; NTS 092 L 07E Sample Nos . : 30366-001 De s c r i p t i o n : The Bon mineral occurrences are in predominantly volcanic rocks of the Karmutsen Group i n the type area (GEM 197 9). P r i n c i p l e showings are replacements of c e r t a i n v o l c a n i c layers by skarn with e i t h e r magnetic or p y r r h o t i t e . One c o n f l i c t i n g report suggests that Bon i s i n 3onanza Group rocks (B.C. Dept. of Mines Ass. Rpt. 1821) Metals recovered: ?e, Cu References: . B.C. Dept. of Mines Ass. Rpt. 1821 G.E.M. 1970 « Nutcracker: LAT 49-75°; LONG 124-59°; NTS Sample Nos.: 30335-001 De s c r i p t i o n : Brown porphyrite (Karmutsen?) i s traversed by sever f i s s u r e zones 3-4 feet wide, r a r e l y completely f i l l e d with quartz. Usually narrow v e i n l e t s occupy the zones. A contact with the Marble Bay Limestone occurs to the southeast. Galena and ch a l c o p y r i t e occur i n the quartz. Nutcracker (Cont'd) Metals recovered: References: BCDM Ass. Rpt. 6414, M i n f i l e 092 F 359 S t a r l i g h t : LAT 49-06°; LONG 124-71°; NTS 092 F 02E Sample Nos.: 30314-001 De s c r i p t i o n : Fine-grained free gold i s associated with galena that i s finely'disseminated through extensively a l t e r e d diabase. Metals recovered:. Au References: M i n f i l e 092F 216, Map 49-1963 Cream Lake: LAT 49-49°; LONG 125-54°; NTS 092 F 05E Sample Nos.: 30355-001 D e s c r i p t i o n : Veins of quartz with l e s s e r s i d e r i t e and c a l c i t e contain values i n s i l v e r , gold, zinc and copper. They are un-d e r l a i n by v o l c a n i c s and l e s s e r sediments of Permian age and o v e r l a i n by Karmutsen v o l c a n i c s . A d i s t i n c t band of Permian limestone occurs between the two. The Western Mines ore bodies l i e to the north w i t h i n t h i s b e l t . Metals recovered: Au, Ag, Cu, Zn References: B.C. Dept. of Mines Ass. Rpt. No. 18S4 Golden Eagle: LAT 49-11°; LONG 124-59°; NTS 092 F 02E Sample Nos.: 30313-001 D e s c r i p t i o n : Gold occurs i n a v e i n of ribbon-quartz and p y r i t e with other minor sulphides i n a small i n t r u s i o n of f e l d s p a r porphyry, Metals recovered: Gold References: M i n f i l e 092F 080, Muller & Carson, (1969) p 38 APPENDIX C D e t a i l s of the data c o l l e c t i o n and reduction of LeCouteur, 1973 Analyses were made on a gas-source mass spectrometer i n the Department of Geophysics and Astronomy of the UNIVERSITY OF BRITISH COLUMBIA. Isotope r a t i o s have been normalised to the absolute r a t i o s of the Broken H i l l Standard r e p o r t e d by Stacey and others (1969). The p r e c i s i o n of the analyses i s b e t t e r than ±0.16% (2 6 sample) for a l l the isotope r a t i o s r e l a t i v e t o 2 0 a P b . Samples were introduced as te t r a m e t h y l lead. Isotope r a t i o s were c a l c u l a t e d from raw an a l y s e s i n the f o l l o w i n g way: S,Sa A,Aa. B, S z SgS^ . C, D,DX c a l c u l a t e mean of 5 , 5 2 5 3 5 , , , c a l c u l a t e (S-A), (S-B) Absolute values were c a l c u l a t e d for each sample by. a d d i t i o n o f the true d i f f e r e n c e s (S-A)',(S-B) 1 to the absolute value of .Broken H i l l Standard as reported by Stacey, Delevaux and U l r y c h ( 1 969) , that i s : 2 0 6 p B / 2 o * p B = 1 6.007, 2 0 7 P B / 2 0 * P B = 15.397, 2 0 E P B / 2 O ' P B = 35.675 Standard d e v i a t i o n s were c a l c u l a t e d i n the f o l l o w i n g way: SET SAMPLES MEAN CALCULATE DIVISOR (N-1) w w, ,w 2 w (w, -wf+(w;L-wf 1 x • x, , x i , x 3 x (x, -xf+(x A-&T+ (x 3-xf 2 Y YitYz,Yn Y ( Y, - Y f + (Y 2 - Y f + ( Yn -Y f~ N - 1 t o t a l of above = t o t a l of above = sum of squares degrees of freedom CT = /sum of squares AJ degrees of freedom The percentage standard d e v i a t i o n s were c a l c u l a t e d by comparing the standard d e v i a t i o n s with the mean values obtained for the Broken H i l l Standard, the sample with the lowest r a t i o s measured i n t h i s study ( i . e . These are maximum % e r r o r s ) . 6 ± 0.007(0.04%) ±0.008(0.05%) ± 0.022(0.06%) 2 6 ± 0.014(0.09%) ± 0.017(0.11%) ± 0.045(0.13%) 

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