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Detailed bedrock geology of the brenda copper- molybdenum mine, peachland, b.c. Oriel, William Michael 1972

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DETAILED BEDROCK GEOLOGY OF THE BRENDA COPPER-MOLYBDENUM MINE Peachland, B r i t i s h Columbia by WILLIAM MICHAEL ORIEL B,Sc.„ 1968 Colorado S t a t e u n i v e r s i t y A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Geology We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1972 In present ing th i s thes is in pa r t i a l f u l f i lmen t o f the requirements fo r an advanced degree at the Un ive rs i t y of B r i t i s h Columbia, 1 agree that the L ibrary sha l l make it f r ee l y ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes i s for s cho la r l y purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or. pub l i c a t i on of th is thes is fo r f i nanc ia l gain sha l l not be allowed without my wr i t ten permiss ion. Department of Geology ^ The Un ivers i ty of B r i t i s h Columbia Vancouver 8, Canada D a t e Mav 15. 1Q72 ABSTRACT D e t a i l e d mapping of the bedrock geology of the Brenda copper-molybdenum deposit has revealed a c o n s i s t e n t sequence of g e o l o g i c a l events. C o n s o l i d a t i o n of the quartz d i o r i t e host rock that forms Brenda stock was followed by i n t r u s i o n of small pegmatite and a p l i t e d ikes that probably r e l a t e to a r e s i d u a l f r a c t i o n of the parent magma. Age da t i n g of variou s quartz d i o r i t e s (White and H a r a k a l , unpublished) i n d i c a t e s that magma s o l i d i f i c a t i o n occurred about 176 m.y. A con c e n t r a t i o n of b i o t i t e model ages at about 146 m.y. seems best i n t e r p r e t e d as the time of ore m i n e r a l i z a t i o n . M i n e r a l i z a t i o n occurred at the beginning of a continuous sequence of r e l a t e d s t r u c t u r a l events. A l l m i n e r a l i z e d ore f r a c t u r e s and younger shears, f a u l t s and i n t r a m i n e r a l dikes e x h i b i t s i m i l a r , s t r o n g l y p r e f e r r e d o r i e n t a t i o n s that are u n l i k e o r i e n t a t i o n s of f e l s i c dikes or primary f o l i a t i o n s i n the quartz d i o r i t e host r o c k . The ore occurs e n t i r e l y i n f r a c t u r e s as products of three s e q u e n t i a l overlapping phases of m i n e r a l i z a t i o n and c o n s i s t s almost e n t i r e l y of the ore minerals c h a l c o p y r i t e and molybdenite w i t h quartz gangue predominanting, Hydrothermal a l t e r a t i o n products e x i s t mostly as t h i n envelopes of potassium f e l d s p a r and/or b i o t i t e and p r o p y l i t i c a l t e r a t i o n . Large a r g i l l i c a l t e r a t i o n zones a l s o e x i s t , however, and are a s s o c i a t -ed commonly w i t h e x t e n s i v e l y sheared zones. An e f f o r t was made at f i n d i n g a zoning p a t t e r n of ore m i n e r a l s , minor elements and a l t e r -a t i o n m i n e r a l s . Ore mineral zoning i n the c l a s s i c a l sense appears to be absent at Brenda. Metal grades show a crude zonal d i s t r i b u t i o n i i t h a t c o r r e l a t e s d i r e c t l y w i t h d e n s i t y of o r e f r a c t u r e s , Hydrothermal a l t e r a t i o n m i n e r a l s and minor elements i n c h a l c o p y r i t e a l s o l a c k any o b v i o u s s y s t e m a t i c z o n a t i o n p a t t e r n on the s c a l e of the mine p i t . I n t r a m i n e r a l d i k e s of v a r i o u s t e x t u r e s and c o m p o s i t i o n s have been d a t e d a t about 130 m.y. They formed a f t e r o r e m i n e r a l i z a t i o n y e t b e f o r e l a t e minor v e i n i n g not a s s o c i a t e d w i t h o r e - p r o d u c i n g v e i n s . i i i TABLE OF CONTENTS CHAPTER PAGE I INTRODUCTION 1 PURPOSE 1 PHYSIOGRAPHY 1 HISTORY 3 METHODS OF STUDY 5 I I GENERAL GEOLOGY 8 INTRODUCTION 8 NICOLA GROUP 8 TERTIARY (?) CONGLOMERATE 10 INTRUSIVE ROCKS 10 I I I GEOLOGY OF BRENDA MINE - STRUCTURE 15 FELSIC DIKES 15 INTRAMINERAL DIKES 22 ZONES OF SHEARING 23 MOLY-SLIPS 25 BLACK CLAYS 25 FAULTS 27 JOINTS 29 AGE DATING 29 SUMMARY 34 IV GEOLOGY OF BRENDA MINE - VEIN DESCRIPTIONS 38 INTRODUCTION 38 STAGE I VEINS 40 MINERALOGY 40 STRUCTURE 42 STAGE I I VEINS 51 MINERALOGY 51 STRUCTURE 54 STAGE I I I VEINS 55 MINERALOGY 55 STRUCTURE 57 STAGE IV VEINS , 58 STRUCTURE 61 SECONDARY MINERALS 61 HYDROTHERMAL ALTERATION" 67 SUMMARY 71 i v CHAPTER PAGE V ZONING 74 MINERAL ZONING 74 COPPER AND MOLYBDENUM 74 LEAD 77 MINOR ELEMENT ZONING 81 VI SUMMARY AND CONCLUSIONS S3 GENERAL GEOLOGICAL EVOLUTION OF THE 83 BRENDA DEPOSIT UNUSUAL ASPECTS OF THE BRENDA DEPOSIT 85 PROBLEMS FOR FUTURE INVESTIGATION 87 BIBLIOGRAPHY 89 APPENDIXES 91 INDUCED POLARIZATION 91 SOIL GEOCHEMISTRY 91 LIST OF FIGURES Page Figure 1 — L o c a t i o n Map 2 Figure 2 - General Geology ( A f t e r Carr) 7 F i g u r e 3 - Contoured Stereonets of S t r u c t u r e O r i e n t a t i o n s 16 F i g u r e 4 - Photograph of Stained A p l i t e Dikes 19 Figure 5 - Photograph of A p l i t e Dike I n t r u d i n g Pegmatite Dike 19 F i g u r e 6 - Photograph of I n t r a m i n e r a l Dike I n t e r s e c t i n g Stage I I V e i n 20 Figure 7 - Photograph of J o i n t i n g Near Surface at Brenda 20 F i g u r e .8 Shear Zone 21 Figure 9 - Contoured Stereonet of J o i n t O r i e n t a t i o n s 28 Figure 10 - Histogram of Potassium-Argon Dating 33 Figure 11 - Summary of S t r u c t u r a l Sequence 36 Figure 12 - Contoured Stereonets of V e i n Stage O r i e n t a t i o n s 41 Figure 13 - D i s t r i b u t i o n of Stage I Vein O r i e n t a t i o n s i n P i t 44 F i g u r e 14 - Photograph of O f f s e t of Stage I Vein by Stage I I V e i n 45 F i g u r e 15 - Photograph of T y p i c a l Stage I I V e i n Surface 45 F i g u r e 16 - Photograph of Banded Stage I I I Vein 46 F i g u r e 17 - Photograph of Stage IV Vein ( C l i n o z o i s i t e ) I n t e r s e c t i n g Stage I I V e i n 46 Figure 18 - D i s t r i b u t i o n of Stage I I Vein O r i e n t a t i o n s i n P i t 47 Figure 19 - Microphotograph of Euhedral P y r i t e i n C h a l c o p y r i t e 48 Figure 20 - Microphotograph of B o r n i t e i n C h a l c o p y r i t e 48 v i Page Figure 21 Microphotograph of Euhedral Magnetite i n C h a l c o p y r i t e 49 Figure 22 - Microphotograph of S p h a l e r i t e i n C h a l c o p y r i t e 49 Figure 23 - Microphotograph of Fractured P y r i t e w i t h C h a l c o p y r i t e 50 F i g u r e 24 - Microphotograph of Molybdenite Containing Wedges of C h a l c o p y r i t e 50 F i g u r e 25 - L i n e Diagram of Mineralogy 63 F i g u r e 26 - Potassium Feldspar A l t e r a t i o n Along Stage I I Vein (Stained) 64 Figure 27 - Potassium Feldspar A l t e r a t i o n Along Stage I I V e i n 64 F i g u r e 28 - Potassium Feldspar A l t e r a t i o n Along Stage I I V e i n (Stained) 65 Figure 29 - B i o t i t e A l t e r a t i o n Along Stage I I I Vein 65 F i g u r e 30 - B i o t i t e A l t e r a t i o n Along Stage I I I Vein 66 F i g u r e 31 - Envelopes of Potassium Feldspar and B i o t i t e Along Stage I I Vein 66 F i g u r e 32 - X-ray D i f f r a c t i o n Traces of A r g i l l i c A l t e r a t i o n Assemblage 70 F i g u r e 33 - D i s t r i b u t i o n of % Copper on 5210 Bench i n P i t 75 F i g u r e 34 - D i s t r i b u t i o n of % Molybdenum on 5210 Bench i n P i t 76 Figure 35 - Cumulative P r o b a b i l i t y Plot-Lead Analyses 79 Figure 36 - D i s t r i b u t i o n s of Lead i n P i t 80 Figure 37 - Induced P o l a r i z a t i o n Anomaly 92 F i g u r e 38 - S o i l Geochemistry Anomaly of Copper 93 Figure 39 - S o i l Geochemistry Anomaly of Molybdenum 94 LIST OF TABLES Page Table I - Table of Model Age-Dates From Brenda V i c i n i t y 32 v i i i ACKNOWLEDGEMENTS The w r i t e r wishes to acknowledge the a s s i s t a n c e o f the l a t e Dr. J . A, Gower under whose s u p e r v i s i o n t h i s t h e s i s was i n i t i a t e d . S p e c i a l thanks a l s o to Dr. A. E. S o r e g a r o l i (Noranda E x p l o r a t i o n , L t d . ) who gave c o n t i n u a l h e l p and encouragement d u r i n g a l l phases of t h i s work; and to Dr. A. J . S i n c l a i r f o r h i s e n t h u s i a s t i c and t i r e l e s s e f f o r t s f o r c r i t i q u e , r e v i e w , and s u p e r v i s i o n o f t h e c o m p i l a t i o n and w r i t i n g phases o f t h i s t h e s i s . Brenda Mines, L t d . and Noranda E x p l o r a t i o n , L t d . p r o v i d e d means and f i n a n c e s w i t h o u t which the t h e s i s c o u l d not have been u n d e r t a k e n . Doug W h i t f o r d , C h i e f G e o l o g i s t ^ and J i m Cahoon were e s p e c i a l l y h e l p f u l a t the mine s i t e . Thanks a l s o go t o Dr. W. White and J . E. H a r a k a l f o r a l l o w -i n g t h e w r i t e r t o use t h e i r u n p u b l i s h e d potassium-argon d a t a f o r Brenda d e p o s i t ; and t o Dr. D. Brabec f o r h i s work on minor element - z o n i n g i n c h a l c o p y r i t e . F i n a l l y , t h e w r i t e r wishes t o thank h i s w i f e f o r her u n d e r s t a n d i n g , s u p p o r t and t i m e l e s s e f f o r t s i n t y p i n g s e v e r a l d r a f t s of t h i s t h e s i s . 1 CHAPTER I INTRODUCTION PURPOSE The purpose of t h i s t h e s i s i s to study and d e s c r i b e i n d e t a i l the bedrock geology of the Brenda open p i t mine w i t h a view towards c l a r i f i c a t i o n of p r e s e n t l y l i t t l e s t u d i e d f e a t u r e s and forwarding an i n s i g h t to the o r i g i n of the min e r a l d e p o s i t . The w r i t e r intends to examine such f e a t u r e s as the d i k i n g , zones of shearing and f a u l t i n g , the mineralogy and s t r u c t u r e of the s u l f i d e bearing v e i n s , the w a l l r o c k a l t e r a t i o n a s s o c i a t e d w i t h v e i n i n g , and the zoning of some s u l f i d e minerals and minor elements w i t h i n the boundaries of the open p i t . The r e g i o n a l geology surrounding the mine and the petrology of the v a r i o u s rock types have been p r e v i o u s l y examined by J . M, Carr i n 1967 and hence are not a subject of t h i s t h e s i s . PHYSIOGRAPHY The Brenda copper-molybdenum deposit i s 18 mile s west of Peachland, B r i t i s h Columbia, and one m i l e east of Brenda Lake at l a t i t u d e 49°52'N and longitude approximately 120°W. I t i s access-able by an improved g r a v e l road extending to the mine from the mouth of Peachland Creek at Highway 97. Roads are e a s i l y c l e a r e d i n winter and provide l i t t l e access d i f f i c u l t y . In a d d i t i o n , there are s e v e r a l s i d e l o g g i n g roads l e a d i n g to f i s h i n g l o c a l i t i e s such as Pennask Lake and Brenda Lake. Other o l d e x p l o r a t i o n roads e x i s t Figure 1. L o c a t i o n Map - The Brenda Mine 3 that normally r e q u i r e c l e a r i n g f o r v e h i c l e use and do not extend f a r i n t o the c o u n t r y s i d e . E l e v a t i o n s range from 3,500 to 6,000 f e e t above mean sea l e v e l i n the general v i c i n i t y of the mine. The uppermost p o i n t of the mine was 5,560 f e e t above mean sea l e v e l before being l e v e l e d to 5,510 f e e t e l e v a t i o n . Slopes are moderate to gentle and t h i c k l y tree-covered. Out-crops are l i m i t e d mostly to the tops of slopes that have been <• scoured g l a c i a l l y (and l o c a l l y p o l i s h e d ) . Most of the ground i s o v e r l a i n by a t h i n cover of g l a c i a l d e b r i s and/or poorly developed s o i l . Topography has been c o n t r o l l e d to a l a r g e extent by g l a c i e r s t h a t , i n the v i c i n i t y of the mine, p o s s i b l y flowed N52°W as suggested by g l a c i a l s t r i a t i o n s . Lineaments that probably r e f l e c t r e g i o n a l s t r u c t u r e s c o n t r o l minor topographic changes. The mine area l i e s between N i c o l a and Similkameen R i v e r s to the west, and Okanagan Lake t r i b u t a r i e s on the e a s t . A s m a l l stream f l o w i n g through the center of the present p i t eroded deeply enough to provide the f i r s t d i s c o very showings. P r e s e n t l y , i t i s d i v e r t e d around the open p i t . HISTORY The f i r s t development work known to have been done on the property was by the Sandberg f a m i l y of Kelowna beginning about 1938. They drove a 30 foot h o r i z o n t a l open-cut, a winze, and an a d i t i n t o the rock along the creek near the center of the present p i t , f o l l o w i n g a l a r g e quartz v e i n c o n t a i n i n g minor masses of c h a l c o p y r i t e and p y r i t e . They apparently were prospecting f o r a 4 g o l d - s i l v e r deposit and abandoned work on the claims (known as the Copper King Groups) a f t e r H. M, A, R i c e v i s i t e d the property i n 1947. R i c e described the showing as being a disseminated copper-molybdenum pro s p e c t . Despite Rice's c a l l f o r f u r t h e r i n v e s t i g a t i o n i n t o the p o t e n t i a l f o r a l a r g e tonnage, low grade m i n e r a l i z e d zone, the property l a y dormant u n t i l 1954 when Bob B e c h t e l , a pr o s p e c t o r , rediscovered the showing. Mr. Bechtel presented h i s showing to Noranda E x p l o r a t i o n who made an arrangement wit h Northwestern E x p l o r a t i o n s to study the a r e a . This f i r s t g e o l o g i c a l study was undertaken i n 1957 by C. S, Ney f o r Northwestern E x p l o r a t i o n s and included geology maps, hornblende-b i o t i t e r a t i o maps, f r a c t u r e d e n s i t y maps, an induced p o l a r i z a t i o n program and a d r i l l i n g program. D r i l l i n g , however, was only c a r r i e d out to a depth of 20 f e e t , not out of the range of l e a c h i n g of copper and molybdenum. Noranda and Kennecott subsequently withdrew from the pr o j e c t and returned the property to Bob Bechtel (Menzies, 1969). Advancing markets and the development of the Endako and Boss Mountain mines gave new impetus and encouragement f o r the prospecting and e v a l u a t i o n of a low grade molybdenum d e p o s i t , which meant Brenda might be considered i n a new l i g h t . Once ag a i n i n 1964 Noranda considered the deposit because of the prompting of B. 0, Brynelsen and M. Menzies but dropped f u r t h e r work because of the apparent low grades i n v o l v e d . Brynelsen and Menzies were pe r m i t t e d , however, to examine and develop the property s e p a r a t e l y . A l s o i n 1964 Chapman of Chapman, Wood and Gr i s w o l d , L t d . examined the property and suggested a thorough examination. F i n a n c i a l support followed through Nippon Mining and M. E. Davis of P e n t i c t o n , B. C. In 1965 the program suggested by 5 Ghapmanwas completed and provided encouraging r e s u l t s . A $700,000 program was recommended. In January 1966, a 15 month $3.5 m i l l i o n f e a s i b i l i t y program was i n i t i a t e d under the d i r e c t i o n of Chapman, Wood, and G r i s w o l d . The f i n a l r e s u l t s were f a v o r a b l e , and i n e a r l y 1970 f u l l p r oduction of the mine was achieved at 24,000 tons per day (Menzies, 1969). The expected l i f e of the mine i s 20 y e a r s . METHODS OF STUDY The w r i t e r spent four months during the summer of 1971 at the mine s i t e . A l l except two weeks of t h i s time was spent studying the geology w i t h i n the l i m i t s of the p i t . Mapping of the p i t took place at a s c a l e of one inc h to 50 f e e t . C o n t r o l s were t r i a n g u l a t e d by the mine's survey crew at approximately 50 foot i n t e r v a l s along bench w a l l s as mapping proceeded. L o c a t i o n e r r o r s are l e s s than two f e e t . At the time of the study only f i v e benches were p a r t l y developed and s u i t a b l e f o r geologic mapping. A l l o r i e n t a t i o n measure-ments on g e o l o g i c a l f e a t u r e s were taken w i t h an i n d u c t i o n damped Brunton compass. L i m i t e d sampling and r e g i o n a l and d e t a i l e d mapping w i t h i n a ra d i u s of three m i l e s from the mine was undertaken during the l a s t two weeks of the summer. This was done to f a m i l i a r i z e the w r i t e r w i t h the geology that Carr (1967) had mapped p r e v i o u s l y . Stereonet p l o t s f o r s t r u c t u r a l data were done on a 10 cm r a d i u s Schmidt n e t . Contours represent percent of poles to planes per one percent a r e a . High d e n s i t y centers were v i s u a l l y estimated to determine predominant a t t i t u d e s of planar f e a t u r e s . Sample p r e p a r a t i o n f o r x-ray d i f f r a c t i o n work was done as 6 follows. Clays were ground in mortor and pestle, suspended in water and siphoned off to be placed on slides that were air dried at room temperature overnight. Standard glycollation and heating procedures were also used for the clays. Identification for minerals other than clays involved the same grinding procedure, but the powder was applied as an acetone slurry. J 10 6 . j x \ J H - 4 - 6 8 'i I3\ 1 I ' x J H - 8 - 6 8 ' ; |*VH-9-68 ' i | i „JH-5-68> 1 H0\ II 7 i 8 Mac Donald ' Laki B r e n d a L a k e / i i I I I I I Figure 2 REGIONAL GEOLOGY of the BRENDA MINE AREA. ( A f t e r C a r r , l 9 6 7 , fig.21.) L E G E N D 116 I C o n g l o m e r a t e PLUTONIC R O CKS |I51 Quartz m o n z o n i t e s 114 I M i x e d q u a r t z d i o r i t e 113 I S a t e l l i t e g r a n o d i o r i t e 112 I S o u t h e r n g r a n o d i o r i t e III! F i n e q u a r t z d i o r i t e 1101 P o r p h y r i t i c q u a r t z d i o r i t e I 91 U n i f o r m q u a r t z d i o r i t e I 81 S p e c k l e d q u a r t z d i o r i t e I 71 M e d i u m q u a r t z d i o r i t e N I C O L A G R O U P R O C K S I 61 Greywacke d i v i s i o n CD L U I 31 P o r p h y r i t i c t u f f d i v i s i o n I 21 S c h i s t o s e h o r n f e l s d i v i s i o n I I 1 U n d i f f e r e n t i a t e d > B r e n d a S t o c k G r a p h i t i c a r g i l l i t e ( 5 ) l i m e s t o n e (4) d i v i s i o n G e o l o g i c c o n t a c t , d e f i n e d or a s s u m e d P i t o u t l i n e , a p p r o x i m a t e F a u l t x A g e d a t i n g s a m p l e l o c a t i o n 10 4 9 ° 5 2 30 -8 CHAPTER I I GENERAL GEOLOGY INTRODUCTION Small s c a l e geologic mapping surrounding the Brenda Mine was done by J . M, Carr i n 1967 f o r the B. C. Department of Mines (Figure 2 ) . I t i s not the purpose of t h i s t h e s i s to expound f u r t h e r on the r e g i o n a l geology, hence Carr's study serves as the main source of r e g i o n a l geologic i n f o r m a t i o n . There are two main rock types: the N i c o l a Group t u f f s and a r g i l l i t e s and the i n t r u s i v e quartz d i o r i t e s that c o n t a i n the Brenda ore body. NICOLA GROUP Carr (1967) defines four w i t h i n the N i c o l a Group u n i t s that crop out west of Brenda s t o c k . These are p o r p h y r i t i c t u f f , s c h i s t o s e h o r n f e l s , greywacke, and a r g i l l i t e - l i m e s t o n e sequence. The s c h i s t o s e h o r n f e l s l i e s i n contact w i t h i n t r u s i v e rocks f o r one mile south and north of the orebody and has an outcrop zone approximately 1/4 of a m i l e wide. Me g a s c o p i c a l l y , i t i s a f i n e - g r a i n e d , s i l i c a -r i c h rock possessing f o l i a t i o n cleavage and l i n e a t i o n s as a r e s u l t of a concentrated development of b i o t i t e p l a t e s up to four m i l l i m e t e r s wide, P e t r o g r a p h i c a l l y , i t i s a " f i n e - g r a i n e d , quartz-f e l d s p a t h i c mosaic c o n t a i n i n g disseminated b i o t i t e and c h l o r i t e , l e n t i c u l a r knots of b i o t i t e , s i n g l e unzoned p l a g i o c l a s e c r y s t a l s and l o c a l l y , g rains of quartz" ( C a r r , 1967, p, 186). S c h i s t o s i t y s t r i k e s c o n s i s t e n t l y N12°W and di p s 40°SW. Probable o r i g i n a l 9 bedding i n the t u f f s around Brenda Lake s t r i k e N80°W and d i p 65°S. The h o r n f e l s probably i s a metamorphic equ i v a l e n t of the p o r p h y r i t i c t u f f and t u f f b r e c c i a u n i t s recognized by C a r r . P o r p h y r i t i c t u f f s crop out i n a wide n o r t h e r l y p i n c h i n g zone c l o s e l y p a r a l l e l l i n g the s c h i s t o s e h o r n f e l s . Exposed width of the outcrop zone at the southern end of the u n i t exceeds one m i l e . Contact r e l a t i o n s w i t h a r g i l l i t e s and graywackes are mostly hidden by deep g l a c i a l d r i f t . They c o n t a i n abundant fragments of v o l c a n i c m a t e r i a l such as p o r p h y r i t i c andesites and d a c i t e s . V o l c a n i c flows were not mapped i n the r e g i o n , however. Carr d e s c r i b e s t u f f s i n the eastern part of the u n i t as a t u f f b r e c c i a c o n t a i n i n g rounded to subangular v o l c a n i c rock fragments up to two inches l o n g . In the western h a l f the t u f f s are c h a r a c t e r i z e d by massive, f i n e - g r a i n e d or g l a s s y rock i n t e r l a y e r e d or intermixed w i t h c l a s t i c sedimentary m a t e r i a l ( C a r r , 1967), G r a p h i t i c a r g i l l i t e and limestone u n i t s (numbers 5 and 4 r e s p e c t i v e l y on Carr's map) crop out about two m i l e s west of the mine over an area extending f o r two m i l e s north and a m i l e south of the p i t . They c o n s i s t of two black limestone beds 200-300 f e e t t h i c k surrounded and separated by tuffaceous greywackes, s i l t s t o n e s and g r a p h i t i c a r g i l l i t e s , "The limestones are g r a p h i t i c , r e c r y s t a l l i z e d , s c h i s t o s e , f i n e - g r a i n e d rocks c o n t a i n i n g r a r e quartz grains and p a r t i c l e s of c h e r t . T h e i r s c h i s t o s i t y shows minor drag-f o l d s plunging southward at 40 degrees," The black a r g i l l i t e s and greywackes "are thin-bedded, incompetent r o c k s , and the s o f t g r a p h i t i c a r g i l l i t e has i n places squeezed d i s c o r d a n t l y i n t o f a u l t s , " ( C a r r , p.185). 10 A nother type o f N i c o l a r o c k i n the a r e a i s the greywacke u n i t . These a r e bedded v o l c a n i c greywackes and s i l t s t o n e s t h a t a p p a r e n t l y grade i n t o i n c r e a s i n g l y metamorphosed h o r n f e l s c l o s e to t h e i n t r u s i v e c o n t a c t . Scour f e a t u r e s i n the s i l t s t o n e s suggest to C a r r t h a t the beds a r e u p r i g h t . The h o r n f e l s a r e brown, q u a r t z i t i c r o c k s w i t h l a r g e amounts o f f i n e l y d i s p e r s e d b i o t i t e . S e r i c i t e was noted on c l e a v a g e s u r f a c e s . F i n e - g r a i n e d q u a r t z , p l a g i o c l a s e , b i o t i t e and hornblende a r e the dominant c o n s t i t u e n t s . I r r e g u l a r p a t c h e s of p a l e and/or d a r k m i n e r a l s a r e common g i v i n g the r o c k a somewhat m o t t l e d appearance, TERTIARY (?) CONGLOMERATE A c o n g l o m e r a t e o f p o s s i b l e T e r t i a r y age ( C a r r , 1967) i s l o c a t e d one m i l e n o r t h o f Brenda Lake on t h e o l d Noranda r o a d . I t c o n s i s t s of rounded p e b b l e s and c o b b l e s i n a f i n e - g r a i n e d m a t r i x ; a l l p o s s e s s -i n g an i r r e g u l a r , b l a c k manganiferous c o a t i n g . The r o c k i s p o o r l y s o r t e d w i t h fragments of t u f f , c h e r t , q u a r t z d i o r i t e and p o r p h y r y i n an a r g i l l a c e o u s m a t r i x . I t was a l s o r e p o r t e d t o have been g l a c i a l l y s t r i a t e d ; however, when t h e w r i t e r examined the expos u r e s , a l l t r a c e s of g l a c i a l p o l i s h i n g had been removed through b u l l d o z e r r o a d c l e a r i n g . INTRUSIVE ROCKS The Brenda o r e body i s w i t h i n a body o f q u a r t z d i o r i t e s termed by C a r r t h e Brenda S t o c k . I t appears more l i n e a r than equant, e x t e n d i n g i n a n o r t h e r l y d i r e c t i o n . To the northwest, n o r t h and e a s t , however, i t s l i m i t s have not been d e f i n e d . The s t o c k has a 11 minimum width of 4 m i l e s east to west and a l e n g t h greater than 7 m i l e s . To the immediate south i s the Similkameen b a t h o l i t h and to the northwest i s the Pennask b a t h o l i t h . The Brenda v i c i n i t y has been included as part of both the Similkameen and the Pennask b a t h o l i t h s by Peto (1970) and Schau (1968) r e s p e c t i v e l y . Further work i s c l e a r l y needed. The western contact of the i n t r u s i v e i s w i t h N i c o l a t u f f s and a r g i l l i t e s as described p r e v i o u s l y . Nearly 3 m i l e s north of the mine the contact d i v e r t s from a n o r t h e r l y to northwesterly t r e n d . Four successive s u b p a r a l l e l quartz d i o r i t e u n i t s and a l a t e r i n t r u s i v e phase were defined by Carr as comprising the Brenda Stock. These are the Medium, Speckled, Uniform, P o r p h y r i t i c , and Fine quartz d i o r i t e s . Separation of u n i t s was based on macroscopic t e x t u r a l d i f f e r e n c e s a r i s i n g p r i m a r i l y from small changes i n the d i s t r i b u t i o n , s i z e , and r a t i o s of b i o t i t e and hornblende. B a s i c a l l y , a l l u n i t s are s i m i l a r ; they are a l l quartz d i o r i t e s possessing s l i g h t l y v a r y i n g amounts of q u a r t z , p l a g i o c l a s e , potash, f e l d s p a r , hornblende, b i o t i t e , and magnetite, sphene and a p a t i t e ; the l a t t e r three occur as accessory m i n e r a l s . Quartz forms about 25 percent, p l a g i o c l a s e 50 percent, o r t h o c l a s e and m i c r o c l i n e up to 20 percent; and hornblende and b i o t i t e ' a c c o u n t f o r 10 to 30 percent of the rock volume. Many outcrops possess a f a i n t f o l i a t i o n due to the alignment of mafic m i n e r a l s , e s p e c i a l l y b i o t i t e . F o l i a t i o n a t t i t u d e s are g e n e r a l l y p a r a l l e l to the contact of the adjacent u n i t of the N i c o l a c o n t a c t . Most contacts d i p from v e r t i c a l to s t e e p l y westward. 12 "Medium Quartz D l o r l t e ( 7 ) . . . . A rough-weathering r o c k , the Medium quartz d i o r i t e i s r e l a t i v e l y dark and inhomogeneous, although I'ess so w i t h d i s t a n c e from the outer c o n t a c t , and i t has a medium-grained appearance due to a h i g h content of c r y s t a l s l a r g e r than 2 m i l l i m e t r e s . F o l i a t i o n i s mostly w e l l developed, and the rock i s commonly streaky due to l o c a l v a r i a t i o n of the dark mineral c o n t e n t . Hornblende mostly exceeds b i o t i t e i n amount, and i t s c r y s t a l s may reach a length of 2 c e n t i m e t r e s . Quartz i s p a r t l y wedge-shaped, and the l a r g e r g r a i n s have a blue colour i n some ou t c r o p s . Potash f e l d s p a r i s l e s s abundant i n t h i s rock than o t h e r s . The estimated range of modal compositions of the Medium quartz d i o r i t e i s as f o l l o w s : Quartz, 10 to 30 percent; potash f e l d s p a r , 0 to 5 percent; p l a g i o c l a s e , 45 to 60 percent; hornblende and b i o t i t e , 10 to 30 percent; magnetite, e t c . , 1 percent." "Speckled Quartz D i o r i t e ( 8 ) . . . . C h a r a c t e r i s t i c f e a t u r e s a i d i n g r e c o g n i t i o n of the rock i n c l u d e : A speckled appearance due to the i n t e r s p e r s i o n of small b i o t i t e and hornblende c r y s t a l s w i t h l a r g e r ones; the presence of b i o t i t e i n amounts e q u a l l i n g or exceeding those of hornblende, and i t s occurrence c h i e f l y i n s i e v e - l i k e shapeless p l a t e s of s i z e to 4 m i l l i m e t r e s ; a p r i s m a t i c or needle-l i k e shape of most hornblende c r y s t a l s ; and a f i n e l y granular appearance of most q u a r t z . Quartz tends to be b l u e - c o l o u r e d , as i n c e r t a i n other u n i t s . The estimated range of modal compositions of the Speckled quartz d i o r i t e i s as f o l l o w s : Quartz, 20 to 25 percent; potash f e l d s p a r , 10 to 15 percent; p l a g i o c l a s e , 45 to 55 percent; hornblende and b i o t i t e , 15 percent; magnetite, etc;., 1 to 2 percent," "Uniform Quartz D i o r i t e ( 9 ) . . . . L i k e the Speckled r o c k , the Uniform quartz d i o r i t e i s predominantly medium grey i n c o l o u r . I t i s d i s t i n g u i s h e d c h i e f l y by the even d i s t r i b u t i o n of i t s dark m i n e r a l s , of which hornblende i s the best shaped and most abundant. B i o t i t e occurs mostly i n small p l a t e s , which are p a r t l y euhedral, and r a r e l y i n books as l a r g e as 3 m i l l i m e t r e s . Quartz i s a l s o mostly f i n e grained but l o c a l l y makes aggregates as l a r g e as one-h a l f centimetre and c o n t a i n i n g small f e l d s p a r s . The estimated range & of modal composition of the rock i s as f o l l o w s : Quartz, 20 to 30 percent; p l a g i o c l a s e , 40 to 55 percent; potash f e l d s p a r , 5 to 15 percent; hornblende and b i o t i t e , 12 to 15 percent; r e s t , 1 percent." " P o r p h y r i t i c Quartz D i o r i t e ( l O ) . . . . T h e rock i s l i g h t e r coloured than o t h e r s , and i t g e n e r a l l y has few i n c l u s i o n . Dark minerals are r e l a t i v e l y sparse and s c a t t e r e d and, together w i t h quartz which forms 1/2-centimetre aggregates, they give the rock a p o r p h y r i t i c appearance. B i o t i t e i s t y p i c a l l y i n well-shaped books of s i z e to one-h a l f centimetre and i t exceeds hornblende i n amount. Hornblende makes r e l a t i v e l y few c r y s t a l s as l a r g e as 2 m i l l i m e t r e s ; quartz f a i l s to occur i n wedge shapes. The estimated range i n mode of the rock i s as f o l l o w s : Quartz, 20 to 30 percent; p l a g i o c l a s e , 45 to 60 percent; potash f e l d s p a r , 8 to 15 percent; b i o t i t e and hornblende, 10 to 15 percent; r e s t , 1 to 2 percent." 13 "Fine Quartz D l o r i t e ( l l ) . . . . A l t h o u g h appearing r a t h e r l i k e the Speckled quartz d i o r i t e , the F i n e quartz d i o r i t e i s somewhat f i n e r grained and l i g h t e r c oloured; i t possesses quartz p a r t l y as small wedges, hornblende p a r t l y i n excess of b i o t i t e , and b i o t i t e i n p l a t e s and t h i c k books mostly smaller than those i n the Speckled quartz d i o r i t e . A d d i t i o n a l u s e f u l f e a t u r e s f o r i d e n t i -f i c a t i o n are blue quartz g r a i n s , a well-developed f o l i a t i o n , unusually abundant s i e v e - c r y s t a l s of potash f e l d s p a r , and w e l l -shaped hornblendes and b i o t i t e s d i s t r i b u t e d p a t c h i l y i n the rock." ( C a r r , 1967, pp. 190-191) I t should be noted here that the w r i t e r c o l l e c t e d more than 70 samples of the above mentioned quartz d i o r i t e s from random out-crops outside the p i t area f o r a r a d i u s of two m i l e s . Without predetermination, the w r i t e r grouped these samples i n t o macroscopic t e x t u r a l d i v i s i o n s which conformed to Carr's as c l o s e l y as p o s s i b l e . I t was found that there i s l i t t l e c o r r e l a t i o n between a mapped u n i t and the t e x t u r a l d e s c r i p t i o n s of the rocks that the w r i t e r c o l l e c t e d w i t h i n i t s boundaries. Each u n i t as shown on Carr's map c o n t a i n s more rock types than the name a p p l i e d to the u n i t . In s e v e r a l l o c a t i o n s , e s p e c i a l l y i n the p o r p h y r i t i c u n i t , there were as many as three d i f f e r e n t rock types from the same outcrop. The w r i t e r noted that i f f o l i a t i o n i s well-developed the t e x t u r e of any p a r t i c u l a r sample can vary c o n s i d e r a b l y depending on whether i t i s broken p a r a l l e l or perpendicular to the f o l i a t i o n . V a r i a b l e character of weathered surfaces can produce s u p e r f i c i a l d i f f e r e n c e s i n rock type that can lead to ambiguity i n a p p l y i n g Carr's c l a s s -i f i c a t i o n . Although Carr's r e g i o n a l mapping has provided much needed i n f o r m a t i o n and i n s i g h t to the problems, the w r i t e r f e e l s that f u r t h e r d e t a i l e d work i s r e q u i r e d . Other separate and p o s s i b l y d i s t i n c t quartz d i o r i t e bodies crop out i n the a r e a . A "Southern Quartz D i o r i t e " occurs i n the southwest corner of Carr's map about 2 m i l e s below the p i t . I t i s 14 similar i n appearance to the aforementioned un i t s , but Carr suggests that the hornblende-biotite r a t i o i s s l i g h t l y different and that hornblende does not have the needle-like appearance commonly found i n the units of the Brenda Stock, Ground cover obscures contact r e l a t i o n s . The S a t e l l i t e quartz d i o r i t e i s a fine-grained d i s t i n c t i v e l y grey, small plug isolated i n Nicola greywackes 2 miles north of the mine. Scattered small (1/2 cm) phenocrysts of plagioclase, hornblende and b i o t i t e appear i n t h i s u n i t . It also has a w e l l -defined f o l i a t i o n . Quartz monzonites appear i n the southeastern part of Carr's map area and are similar i n appearance to the quartz d i o r i t e units except that they contain more potash feldspar than the quartz d i o r i t e s . These rock types are described more thoroughly i n Carr's report to which the reader i s referred. Dikes i n the region vary from a p l i t e s to basalts with several intermediate v a r i e t i e s . Most s t r i k e northwesterly and dip steeply. Carr dates some as being pre-ore and other as post-ore. He describes several s t r u c t u r a l belts as being defined by the presence or absence of aplite-pegmatite dikes of pre-ore age. Detailed data collected by the writer i n and around the p i t do not support these conclusions. A more detailed discussion of the dikes i s presented i n the following chapter. -A suggested age of the quartz d i o r i t e from unpublished potassium-argon age dating data (Harakal and White, personal communication^ 1972) i s about 176 m i l l i o n years. A detailed presentation and analysis of the data i s located i n the following chapter. 15 CHAPTER I I I GEOLOGY OF BRENDA MINE - STRUCTURE Prim a r y s t r u c t u r e f e a t u r e s of t h e S p e c k l e d q u a r t z d i o r i t e t h a t c o n t a i n s the Brenda o r e d e p o s i t have been r e f e r r e d to i n an e a r l i e r s e c t i o n . Secondary s t r u c t u r a l f e a t u r e s i n t h e i r approximate o r d e r of r e l a t i v e age a r e from o l d e s t t o youngest: o r e v e i n s , i n t r a m i n e r a l d i k e s , shear zones, m o l y - s l i p s , and j o i n t s . Ore v e i n s a r e d i s c u s s e d i n d e t a i l i n a subsequent c h a p t e r . -A—geologic—bench-map-can —be f-ound—i-n—the—enve-lope-afc—fche-end-of—the-fches-i-s-. FELSIC DIKES F e l s i c d i k e s appear throughout t h e p i t . They a r e d i v i d e d f o r co n v e n i e n c e i n t o t h r e e t e x t u r a l d i v i s i o n s : a p l i t e , p e g m a t i t e and granophyre d i k e s . As a group, they r e p r e s e n t t h e e a r l i e s t r e c o g n i z e d g e o l o g i c a l event subsequent to development o f t h e S p e c k l e d q u a r t z d i o r i t e . D i k e s range i n w i d t h from 1/4 i n c h to a maximum of 12 i n c h e s . Wider d i k e s a r e more p e r s i s t e n t l a t e r a l l y than a r e s m a l l d i k e s , but o b s e r v e d s t r i k e l e n g t h s do not exceed one hundred f e e t . The d i k e s commonly occur i n s m a l l swarms r e s e m b l i n g f i l l e d , en e c h e l o n , t e n s i o n f r a c t u r e s . Most d i k e s a r e a p a l e to medium p i n k c o l o r ; some, however, a r e g r e y . S t a i n i n g r e v e a l e d t h a t grey d i k e s c o n t a i n l e s s p o t a s h f e l d s p a r than do p i n k d i k e s . ( F i g u r e 4) A p l i t e s have a "sugary t e x t u r e and c o n s i s t of q u a r t z , m i c r o -c l i n e , p l a g i o c l a s e , and minor amounts of gree n b i o t i t e w i t h a v e rage g r a i n d i a m e t e r s r a n g i n g from 0.05 to 0.3 m i l l i m e t e r s . . . . " . Figure 3„ Contoured stereonets and major planes of orientation of secondary structures. "Pegmatite c o n s i s t s , almost e x c l u s i v e l y of quartz and potash f e l d s p a r (mainly m i c r o c l i n e ) . . . . " ( C a r r , 1967). Granophyre r e f e r s to d i k e s possessing i r r e g u l a r subgraphic intergrowths of quartz i n f e l d s p a r . The subgraphic texture i s formed between quartz and potash f e l d s p a r . Black tourmaline, brown b i o t i t e , and a l l a n i t e have been i d e n t i f i e d i i n these d i k e s . Tourmaline occurs i n pegmatites as r a r e anhedral masses up to 1/2 i n c h wide. Most f e l s i c dikes i n the p i t s t r i k e n o r t h w e s t e r l y . Two major a t t i t u d e s are N66°W dip 61°NE and N33°W di p 71°NE (Figure 3a). A t h i r d minor co n c e n t r a t i o n i s n e a r l y f l a t . I n t e r e s t i n g l y , two of the f l a t d i k e s observed were p a r a l l e l l e d by nearby m i n e r a l segrega-t i o n bands that l o c a l l y formed f a i n t g n e i s s i c t e x t u r e s . These were a l l t h i n 1/2 to 1 i n c h dikes w i t h c r y s t a l s v a r y i n g from 1/8 to 1/4 i n c h i n a s l i g h t l y pegmatitic framework. Although most dik e s are unzoned m i n e r a l o g i c a l l y , a combination of textures g e n e r a l l y occurs. The commonest zonal arrangement observed by the w r i t e r i s a granophyric or pegmatitic core grading outwards i n t o an a p l i t i c border zone of v a r y i n g w i d t h . These border zones probably represent c h i l l e d margins. Simple pegmatites w i t h t h i n cores of coarse-grained quartz have a l s o been found but are much r a r e r than other types. Another example of zoning which i s uncommon but not r a r e i s an apparent reverse zoning: a p l i t e forms the c e n t r a l p o r t i o n of the dike w i t h pegmatite and l e s s commonly granophyre i n the border zones. In two such cases the a p l i t e core zone was traced e v e n t u a l l y to a separate l a t e r d i k e that had taken a course along the center of the p r e v i o u s l y emplaced pegmatite, 18 and In other samples streaked remnants of the pegmatite i n d i c a t e f o r c e a b l e i n t r u s i o n of a l a t e r a p l i t e (Figure 5 ) . The pegmatite might have been r e f r a c t u r e d as f u r t h e r s e t t l i n g and c o o l i n g of the quartz d i o r i t e took p l a c e , or i t provided a path of l e a s t r e s i s t a n c e due to coarser c r y s t a l l i z a t i o n and l e s s i n t e r l o c k i n g of g r a i n s than the cooled i n t r u s i v e possessed. E i t h e r e x p l a n a t i o n appears s a t i s f a c t o r y . I t i s p o s s i b l e that the wide range observed i n the types and r e l a t i o n s h i p s of these f e l s i c dikes r e s u l t from an overlapping of more than one pulse or per i o d of i n t r u s i o n . Pegmatites and granophyres are both zoned and unzoned. The unzoned dik e s might be considered the e a r l i e s t of the d i k e s . Because of t h e i r coarse s i z e and u n c h i l l e d margin, they might have intruded an incompletely cooled magma. These are followed by l a t e r a p l i t e d i k e s , probably g e n e t i c a l l y a part of the same system but s l i g h t l y younger i n age, A p l i t e d i k e s cut granophyres and pegmatites. They a l s o have intruded along the centers of p r e v i o u s l y emplaced d i k e s . O f f s e t s of a p l i t e s by other a p l i t e s have a l s o taken p l a c e . P o s s i b l y the l a s t d i k e s to have formed are small grey a p l i t i c d i k e s r i c h e r i n quartz than f e l d s p a r . Some have been observed to i n t e r s e c t other a p l i t e s and pegmatites. Perhaps the a p l i t e s represent i n t r u s i o n during a per i o d when the quartz d i o r i t e had cooled s u f f i c i e n t l y to c h i l l the s i l i c e o u s f l u i d s forming the d i k e s . At s e v e r a l l o c a t i o n s an en echelon arrangement of s e v e r a l d i k e s has been noted both by the w r i t e r and p r e v i o u s l y by Carr (1967), These lend support to the idea that such di k e s f i l l e d t e n s i o n f r a c t u r e s produced by c o n t r a c t i o n of the c o o l i n g mass of Speckled quartz d i o r i t e . Figure 4. Two varieties of aplite dikes. Lighter colored dike contains less feldspar and is later than the other. Specimen has been stained. Figure 5. Small aplite intruding along center of pegmatite dike. Note streaked inclusion in center of dike. 20 Figure 6. I n t r a m i n e r a l d i k e (trachyte) c u t t i n g Stage I I v e i n ( o u t l i n e w i t h red dots) . 21 F i g u r e 8. Shear zone i n a r g i l l i c a l t e r a t i o n . B l a c k r i b b o n i n c e n t e r i s a Stage I I I q u a r t z -m o l y b d e n i t e v e i n t h a t i s s h e a r e d and p a r t i a l l y decomposed. 22 At no time was the w r i t e r able to f i n d evidence supporting . Carr's suggestion that f e l s i c dikes were contemporaneous w i t h ore m i n e r a l i z a t i o n and f r a c t u r i n g . In every observable i n s t a n c e , f e l s i c dikes were e a r l i e r than any other s t r u c t u r a l or m i n e r a l o g i c a l f e a t u r e . O r i e n t a t i o n s of the dikes suggest a l s o that t h e i r causes of formation were d i s s i m i l a r . INTRAMINERAL DIKES I n t r a m i n e r a l dikes are those emplaced between Stage I I I and Stage IV v e i n s . (Kirkham, 1971; Wallace, 1967). They have v a r i a b l e compositions. A l l d i k e s i n the p i t area are i n t h i s category w i t h the exception of two andesite dikes that predate Stage I v e i n s . Many Stage IV v e i n s i n t e r s e c t i n t r a m i n e r a l d i k e s . Despite t h e i r wide range of compositions, i n t r a m i n e r a l d i k e s as a whole have a strong p r e f e r r e d o r i e n t a t i o n averaging about N59°W/80°SW (Figure 3b). S l i g h t d i f f e r e n c e s i n o r i e n t a t i o n are not r e l a t e d to t e x t u r a l or compositional d i f f e r e n c e s . Average width of these dikes i s a p p r o x i -mately one f o o t . An e x c e p t i o n a l t r a c h y t e dike i s 20 f e e t wide. Dike lengths vary considerably i n the p i t , but none have been traced f o r over 1000 f e e t . Normally only lengths l e s s than 100 f e e t have been measured. Rarely a dike b i f u r c a t e s and j o i n s again surrounding an i s l a n d (horse) of quartz d i o r i t e i n the c e n t e r . Rare changes i n s t r i k e and dip take place on some dikes where they i n t e r s e c t e a r l i e r l i n e a r f e a t u r e s such as v e i n s . A l l i n t r a m i n e r a l d i k e s have c h i l l e d contacts and show l i t t l e i f any zoning. No metamorphic a f f e c t s i n adjacent w a l l rock were noted. One d i k e was observed t h a t has a f o u r i n c h m a r g i n a l zone c o n s i s t i n g of v e s j ^ i c l e s p a r t i a l l y or c o m p l e t e l y f i l l e d w i t h c a l c i t e , q u a r t z , and r a r e c h a l c o p y r i t e and p y r i t e . T h i s d i k e s t r i k e s northwest and d i p s s t e e p l y to the southwest. The v e s ^ i c l e s were on the.hanging w a l l of the d i k e . ZONES OF SHEARING Shear zones a r e post o r e zones of c o n c e n t r a t e d s t r i k e - s l i p movement i n pre-weakened a r g i l l i c a l t e r a t i o n zones a s s o c i a t e d w i t h Stage I I I v e i n s . Both the amount of s h e a r i n g and t h e w i d t h of the a c t u a l zone of movement a r e o f t e n o b s c u r r e d by the i n t e n s e a l t e r -a t i o n which so weakens the r o c k t h a t i t crumbles upon exposure to a i r and r a i n . I n two c a s e s shear zones proved t o be o n l y a r g i l l i c a l t e r a t i o n zones a l o n g which no movement a t a l l had o c c u r r e d . In each case f e l s i c d i k e s ( e a r l i e s t f e a t u r e ) passed through t h e zones w i t h no o f f s e t or movement. The maximum w i d t h of t h e s e combined shear and a r g i l l i c a l t e r -a t i o n zones w i t h i n the p i t i s 35 f e e t . Average w i d t h i s about 10 f e e t . A few have been t r a c e d f o r l e n g t h s up to one thousand f e e t . As y e t , an u n e x p l a i n e d f e a t u r e i s t h a t many of the shear zones appear t o p i n c h out upwards and l a t e r a l l y . A zone t e n f e e t wide a t the base of a bench might t h i n to f i v e o r s i x f e e t a t the top of the bench or l a t e r a l l y a l o n g s t r i k e . A l t h o u g h t h i s f e a t u r e has been noted f r e q u e n t l y , i t i s not found i n a l l shear zones. Indeed the o p p o s i t e e f f e c t has been r a r e l y seen when the w i d t h d e c r e a s e s downward i n s t e a d of upward. 24 A few i n t r a m i n e r a l dikes have been observed both to i n t e r s e c t and be i n t e r s e c t e d by shear zones, i n d i c a t i n g that the times of formation must overlap a t l e a s t i n p a r t . Some b r e c c i a t i o n has been observed i n a t l e a s t two of the shear zones. In one sample angular to subrounded quartz fragments i n a matrix of muscovite and minor c l a y c o n t a i n i n g up to 20 per cent euhedral magnetite and minor hematite have been r e f r a c t u r e d . Quartz has p o s s i b l y been introduced i n l a r g e r amounts than normally contained i n the quartz d i o r i t e . Large ( p o s s i b l y annealed) grains up to one inch a l s o o c c u r . Some a p ^ a t i t e i s formed. The other c o n s i s t s of angular quartz and a l t e r e d p l a g i o c l a s e fragments a l t e r e d to c l a y and muscovite. A few Stage I I I veins observed by the w r i t e r are unaffected by shearing i n the a r g i l l i c zones. Most, however, are sheared though not s i g n i f i c a n t l y o f f s e t . F i g u r e 3c d i s p l a y s the p l o t s of the poles to the shear zones. Their o r i e n t a t i o n s possess two d i s t i n c t concentrations a t N68°E/ 78°SE and N61°W/85°SW. The p a t t e r n of o r i e n t a t i o n i s s i m i l a r to those of Stage IV v e i n s , as i s expected s i n c e the a r g i l l i c zones are i n t i m a t e l y r e l a t e d to Stage I I I v e i n i n g , A r g i l l i c a l t e r a t i o n and Stage I I I v e i n s t r u c t u r e are discussed s e p a r a t e l y i n the f o l l o w -i n g chapter. MOLY-SLIPS Bla c k , shiny s l i p surfaces of s l i c k e n s i d e d molybdenite are found commonly i n the p i t . Generally they cover surfaces many tens of square f e e t wide or l a r g e r . A suggestion put forward i n the e a r l y stages of development was that these surfaces " p a i n t e d " w i t h molybdenite c o n s t i t u t e d most of the molybdenum ore (Chapman, Wood, and G r i s w o l d , 1967). The w r i t e r d i s a g r e e s . Most of these moly-s l i p s observed by the w r i t e r are the r e s u l t of post d e p o s i t i o n movement along Stage I I or I I I v e i n s , having a high content of molybdenite along v e i n w a l l s . Even minor movement of an i n c h or so i n many cases would probably be s u f f i c i e n t to smear the moly-bdenite c r e a t i n g the o b t r u s i v e b l a c k , shiny s u r f a c e . Only a few of these v e i n s have had such movementfand by f a r the l a r g e s t amount of molybdenum i s present i n Stage I I and I I I v e i n s that have not undergone subsequent movement. BLACK CLAYS A minor though widespread occurrence w i t h i n and adjacent to h i g h l y a r g i l l i z e d shear zones are dark, black zones c o n t a i n i n g a f i n e granular mixture of c l a y s , c a l c i t e , molybdenite, c h a l c o p y r i t e , p y r i t e , hematite and s i l i c a ( Figure 8 ) . They appear to be a r e s u l t of d e p o s i t i o n from ground water; and a r e , as such, d i s t i n c t l y separate from m o l y - s l i p s discussed e a r l i e r . Most were found to be adjacent to Stage I I I quartz-molybdenite veins of v a r i o u s s i z e s . Apparently the a l t e r a t i o n along the quartz-molybdenite veins provided a zone of p e r m e a b i l i t y f o r p e r c o l a t i o n of ground water which might 26 have broken down and/or d i s s o l v e d the s i l i c a and p h y s i c a l l y t r a n s -ported f i n e l y granular molybdenite downwards along the s i d e s of the vei n s u n t i l t h i c k accumulations of molybdenite, f i n e c l a y s and s i l i c a were deposited i n v a r y i n g amounts. Thicker accumulations were normally found lower i n the p i t . On f r e s h l y b l a s t e d benches, the w r i t e r was able to observe black muddy water t r a n s p o r t i n g and de p o s i t i n g molybdenite, e t c . down these zones of high p e r m e a b i l i t y adjacent to the quartz-molybdenite v e i n s . Late movement on black molybdenite zones along which water was no longer f l o w i n g would produce s l i c k e n s i d e s superimposed on the hardened molybdenite c l a y m i x t u r e . The l a t e s l i c k e n s i d e s probably represent l a t e sporadic T e r t i a r y movement along o l d r u p t u r e s , e n t i r e l y unrelated to development of the ore body. I f most of the quartz i n the quar,tz-molybdenite v e i n s were removed i n any one l o c a l i t y , these black zones might be mistaken f o r f a u l t zones. I t i s the w r i t e r ' s o p i n i o n that t h i s i s what Carr mistakenly i d e n t i f i e d as " l o c a l l y g r a p h i t i c " f a u l t s ( C a r r , p. 201). Although s e v e r a l samples were analyzed by x-ra y , none showed the presence of graphite nor has gra p h i t e been recognized m a c r o s c o p i c a l l y i n the p i t . A n a l y s i s of one of the c l a y s revealed a high z i n c content as w e l l as high copper and molybdenum. De p o s i t i o n of the unaltered molybdenite and minor amounts of c h a l c o p y r i t e apparently took place r a p i d l y and below the zone of o x i d a t i o n . F e r r i m o l y b d i t e or l i m o n i t e were not observed unless w i t h i n 20 f e e t ( r a r e l y up to 90 f e e t ) of the s u r f a c e . Reddish o x i d i z e d hematite might a l s o be presen t . The amount#of molybdenite transported downward i n t h i s f a s h i o n i s not c a l c u l a b l e but i n a l l p r o b a b i l i t y i s only a minor though n o t i c e a b l e amount. 27 FAULTS F a u l t s are not e a s i l y d i s c e r n a b l e . I n many instances t h i s i s due to a l a c k of observable o f f s e t . Although few i n t e r s e c t i o n s w i t h such l i n e a r f e a t u r e s as dikes are a v a i l a b l e f o r every f a u l t r e c o g n i -t i o n , narrow envelopes of a r g i l l i c a l t e r a t i o n are normally i n d i c a t i v e of f a u l t movement. At the present time data are i n s u f f i c i e n t f o r complete under-standing of f a u l t s and t h e i r nature of occurrence. Based on the few r e l i a b l e o b s e r v a t i o n s , however, the w r i t e r t e n t a t i v e l y suggests that two types of f a u l t s developed s u c c e s s i v e l y or perhaps at d i s t i n c t l y separate times. A set of s t r i k e - s l i p f a u l t s are perhaps represented by those that s t r i k e about N66°E and N66°W and d i p s o u t h e r l y (Figure 3d). Due to t h e i r i n d i c a t e d r e l a t i v e movement and a l t e r a t i o n a f f i n i t i e s , they are probably d i r e c t l y r e l a t e d to shear zones. Indeed many of these were at f i r s t c l a s s i f i e d as very small shear zones but subsequently redefined f o r l a c k of evidence of an a c t u a l zone of sh e a r i n g . A p o s s i b l y separate and/or l a t e r set of f a u l t s might be represented by normal f a u l t s s t r i k i n g N50° to 30°W and d i p p i n g more nearly v e r t i c a l . These appear to have l i t t l e a s s o c i a t e d a l t e r a t i o n . Most appear to have an almost i n s i g n i f i c a n t amount of throw. A maximum of 15 f e e t normal d i p - s l i p was recorded on one f a u l t s t r i k i n g N50°W and dip p i n g s t e e p l y southwest. 2b JOINTS N Figure 9. Poles of j o i n t planes p l o t t e d on Schmidt n e t . Line i n d i c a t e s p r i n c i p a l s t r i k e d i r e c t i o n ; N 80°East, d i p 25° K. JOINTS J o i n t s are a l a t e f e a t u r e b e l i e v e d to be an expression of pressure r e l e a s e r e l a t e d to surfaces of e r o s i o n (Figure 7 ) . They are t o t a l l y u n m i n e r a l i z e d . The j o i n t d e n s i t y ( j o i n t s per yard) i s as high as s i x but normally i s l e s s than t h r e e , Conjugate p a i r s appear to form i n some areas but were not observed at a l l l o c a t i o n s . The major importance of these j o i n t s i s t h e i r c o n t r i b u t i o n to breakage of the rock f o r m i l l i n g purposes and t h e i r p o s s i b l e e f f e c t on p i t slope s t a b i l i t y . The major o r i e n t a t i o n i s N80°W/25°NE. Minor o r i e n t a t i o n s occur at N8°E/40°E; N4°W/30°W; and N24°W/23°NE (Figure 9 ) . AGE DATING Potassium-argon model ages were determined f o r eleven samples of quartz d i o r i t e from the Brenda mine and v i c i n i t y by W. H, White and J . E, Harakal (personal communication, 1972), P r e l i m i n a r y r e s u l t s f o r two samples have been published (White e t . a l . , 1968), and those plus more recent data are reproduced w i t h t h e i r p e r m i s s i o n . Sample l o c a t i o n s are shown i n Figure 2 , A n a l y t i c a l data and model ages are l i s t e d i n Table I , i n c l u d i n g d u p l i c a t e and t r i p l i c a t e a n a l y s e s . A histogram of model ages i s shown i n Figure 10 Examinations of the model ages lead to the f o l l o w i n g g e n e r a l i z a t i o n s : (1) Hornblende ages group c l o s e l y around 176 m.y, (2) B i o t i t e ages w i t h i n the p i t area group near 146 m.y. (3) B i o t i t e ages removed from the p i t area give model ages i n the range 131 to 174 m.y. 30 (4) Two i n t r a m i n e r a l d i k e s have whole rock K-Ar ages of about 130 m.y. I t seems reasonable to assume that the consistency of hornblende ages at 176 + 8 m.y. represents the age of s o l i d i f i c a t i o n of the Speckled quartz d i o r i t e and i s , t h e r e f o r e , a reasonable estimate of the age of the e n t i r e Brenda s t o c k . This i s supported by concordant ages f o r b i o t i t e and hornblende of about 176 m.y. (JH-5-68) from an area two miles north of Brenda p i t . Some doubt e x i s t s , however, concerning the i n t e r p r e t a t i o n of the model ages. Consider the f o l l o w i n g p o s s i b l e i n t e r p r e t a t i o n s : (1) M i n e r a l i z a t i o n took place at about 176 m.y. and e i t h e r & a) a l a t e r thermal event unrelated to m i n e r a l i z a t i o n r e s e t the b i o t i t e age at about 146 m.y., or b) the i n t r u s i o n cooled s l o w l y during an i n t e r v a l of about 30 m.y. g r a d u a l l y d e p l e t i n g the argon i n b i o t i t e . (2) M i n e r a l i z a t i o n occurred about 146 m.y. fo l l o w e d by s e q u e n t i a l s t r u c t u r a l events and l a t e minor m i n e r a l i z a t i o n at about 130 m.y. (3) M i n e r a l i z a t i o n occurred at approximately 130 m.y. Considering the t h i r d a l t e r n a t i v e , a p o s s i b i l i t y e x i s t s that some b i o t i t e ages were obtained on mixtures of rock b i o t i t e w i t h an age of 177 m.y. and hydrothermal b i o t i t e (Stage IV) w i t h an age of 130 m.y. The 146 m.y. ages might be a r e s u l t of such a mixture of b i o t i t e s . Hence, the age of a c t u a l m i n e r a l i z a t i o n might be about 130 m.y. Four b i o t i t e ages group about 146 m.y., three of which occur i n or c l o s e to the p i t . One b i o t i t e age occurs at 174 m.y., and two b i o t i t e ages from the s i r a i l a r i l y m i n e r a l i z e d North Brenda 31 v i c i n i t y have lower ages at 131 m.y. and 136 m.y. Such a wide range i n ages between the two extremes would be expected from samples of va r y i n g proportions of d i f f e r e n t age b i o t i t e s . The second p o s s i b i l i t y i s that ore m i n e r a l i z a t i o n occurred at 146 m.y., and ore r e l a t e d events continued i n t e r m i t t e n t l y perhaps u n t i l about 130 m.y. (age of the I n t r a m i n e r a l d i k e s ) . The c o n s i s t e n t grouping of the b i o t i t e s near the p i t at about 146 m.y. seem almost too c o i n c i d e n t a l to r e s u l t from chance mixtures of d i f f e r e n t b i o t i t e ages. On the other hand, 16 m i l l i o n years i s a long though not impossible time f o r m i n e r a l i z a t i o n to endure assuming that model ages f o r the i n t r a m i n e r a l d i k e s are c o r r e c t . The f i r s t a l t e r n a t i v e that m i n e r a l i z a t i o n q u i c k l y f o l l o w e d s o l i d i f i c a t i o n of the i n t r u s i v e n e c e s s i t a t e s a l a t e r thermal event to e x p l a i n the anomalous b i o t i t e ages. Evidence supporting a l a t e r thermal event other than p o s s i b l e m i n e r a l i z a t i o n i s l a c k i n g . S t r u c t u r a l d a t a , f o r i n s t a n c e , does not i n d i c a t e the presence of a l o c a l u n d e r l y i n g i n t r u s i v e as a heat source. And the w r i t e r cannot f i n d g e o l o g i c a l evidence n e c e s s a r i l y r e l a t i n g m i n e r a l i z a t i o n and f r a c t u r i n g to the c o o l i n g h i s t o r y of the quartz d i o r i t e or immediately t h e r e a f t e r . That the i n t r u s i o n cooled s l o w l y over 30 m i l l i o n years and slowly r e s e t the b i o t i t e seem q u i t e u n l i k e l y . Unless the age determinations of the i n t r a m i n e r a l d i k e s are i n c o r r e c t , they would i n d i c a t e that a sequence of events d i r e c t l y and continuously r e l a t e d to ore m i n e r a l i z a t i o n proceeded u n t i l 130 m.y. - an i n t e r v a l of ne a r l y 50 m i l l i o n years! Highly u n l i k e l y . U n t i l f u r t h e r s e l e c t i v e potassium argon d a t i n g has been completed, the three i n t e r p r e t a t i o n s remain. I t seems l i k e l y , TABLE 1. (Harakal - White, unpublished). Sample Number M a t e r i a l Analyzed Rock Unit K(%) *40 Ar *40 ; Ar -5 (10 CC STP/g) *40 Ar Age (my) 40 , T o t a l Ar 40 K W-67-3 Hornblende Medium 0.4% 0.57 6.486 X i o - i 10.766 X 10" 3 176 + 7 W-67-3 Hornblende II 0.4% 0.85 6.298 x io-i 10.456 X 10-3 171 + 7 W-67-3 Hornblende II 0.4% 0.41 6.334 X i o - i 10.515 X l ( p 3 172 + 7 W-67-3 B i o t i t e i II 1.0% 0.85 3.637 9.004 X 1 0 1 148 + 6 W-67-4 Hornblende Speckled 0.7% 0.47 5.363 X i o - i 11.239 X 10~ 3 183 + 8 W-67-4 Hornblende II 0.7% 0.60 4.853 X i o - i 10.170 X 1 0 - 3 166 + 7 W-67-4 Hornblende II - • 0.7% 0.73 5.146 X i o - i 10.784 X l O " 3 176 + 8 W-67-4 B i o t i t e II 0.4% 0.68 4.510 9.033 X l O - 3 148 + 5 JH-1-68 Hornblende Speckled 0.2% 0.60 4.741 X i o - i 10.961 X 179 + 7 JH-1-68 B i o t i t e II 1.1% 0.69 1.154 8.785 X i o " J 145 + 6 JH-2-68 B i o t i t e P o r p h y r i t i c 0.2% 0.91 3.438 8.788 X l O " 3 145 + 5 JH-2-68 Hornblende it 0.6% 0.43 5.245 X i o - i 10.930 X 10"J 178 + 8 JH-3-68 Whole Rock I n t r a m i n e r a l 0.4% 0.86 1.374 7.960 X 131 +_ 5 JH-3A-68 Whole Rock II 0.3% 0.83 1.825 7.659 ;x 127 + 5 JH-4-68 B i o t i t e S a t e l l i t e 0.4% 0.90 4.696 9.464 X 10~ J 155 + 6 JH-4-68 B i o t i t e II 0.4% 0.69 4.615 9.303 X 10" 3 153 + 6 JH-5-68 B i o t i t e ;Fine 0.3% 0.72 2.746 10.677 X 10" 3 174 + 7 JH-5-68 Hornblende II 0.4% 0.46 4.891 X 1 0 _ 1 10.876 X l O " 3 177 + 7 JH-8-68 B i o t i t e T i n e : 0.5% 0.83 3.595 8.247 X l O " 3 136 + 6 JH-9-68 B i o t i t e P o r p h y r i t i c 0.6% 0.91 3.785 8.537 X l O " 3 141 + 6 JH-11-68 B i o t i t e Medium 0.4% 0.92 2.759 7.978 X l O - 3 132 + 6 • n - u r 1 I I 1 I I I I I I I 145 155 i i i i i I i i i i I 175 185 i i | 125 135 Age (my) Figure 10a, Histogram of potassium-argon dates (including error) for Brenda Mine and v i c i n i t y . Average ages were used for multiple analyses for the same concentrate• O C O • • B B Dt | A R | A A M | 125 135 145 155 165 175 185 Age (my) Figure 10b. Diagram of model ages (without assigned errors). Whole rock analyses O t Biotite ages • „ Hornblende ages A • (From unpublished data by White and Harakal) 34 however, that m i n e r a l i z a t i o n was the agent that r e s e t some b i o t i t e and p r e c i p i t a t e d hydrothermal b i o t i t e r e l a t e d to m i n e r a l i z a t i o n . If t h i s were so, then the maximum age of m i n e r a l i z a t i o n i s about 146 m.y. and could be as low as 130 m.y. i f separate b i o t i t e s were mixed f o r a n a l y s e s . The w r i t e r p r e f e r s the second hypothesis because the f i r s t hypothesis appears to be r u l e d out by a l l the data so f a r c o l l e c t e d . And the t h i r d hypothesis r e q u i r e s extreme coincidence that s e v e r a l random samples would a l l c o n t a i n the same r e l a t i v e p r o p o r t i o n s of separate b i o t i t e s . SUMMARY The time of emplacement of the Speckled quartz d i o r i t e appears to be about 176 m i l l i o n years ago or lower J u r a s s i c time as i n d i c a t e d by potassium-argon ages on hornblende by Dr. W, H. White of the U n i v e r s i t y of B r i t i s h Columbia (Harakal and White, personal communication, 1972). This i n t r u s i o n was followed by the i n t r o -d u c t i o n of a p l i t e , pegmatite and granophyre dikes p o s s i b l y r e l a t e d to l a t e magmatic f l u i d s r e s u l t i n g from c o o l i n g c r y s t a l l i z a t i o n and s e t t l i n g of the magma. They s t r i k e predominently northwest and dip moderately to the southeast. Their o r i e n t a t i o n i s s i g n i f i c a n t l y d i f f e r e n t than a l l other secondary s t r u c t u r e s , which leads the w r i t e r to b e l i e v e that they are not contemporaneous w i t h or even caused by the same c o n d i t i o n s that formed l a t e r events as suggested by Carr (1967). F o l l o w i n g these a f t e r some indeterminant time are the ore v e i n s , i n t r a m i n e r a l d i k e s , shears and f a u l t s forming i n overlapping sequence (Figure 11 ) , Age datin g i n d i c a t e s that m i n e r a l i z a t i o n probably commenced about 146 m.y, I n t r a m i n e r a l d i k e s are termed such f o r t h e i r occurrence i n time w i t h Stage IV v e i n s and shears. They are overlapped by both but occur w e l l a f t e r the formation of the ore v e i n s . O r i e n t a t i o n s of i n t r a m i n e r a l d i k e s are remarkably c o n s i s t e n t s t r i k i n g N59°W, dipp i n g 89°SW; however, t h e i r compositions range between a n d e s i t i c and b a s a l t i c . A few t r a c h y t e dikes are present that have been described p r e v i o u s l y by Carr (1967). Two dikes of a n d e s i t i c composition are apparently of premineral o r i g i n . T heir r e l a t i o n to the other dikes are unknown. The a c t u a l time of the sequence of a l t e r a t i o n , m i n e r a l i z a t i o n and the formation of d i k e s and shears cannot as yet be determined a b s o l u t e l y . Two age determinations of mafic d i k e s have been c a r r i e d out (Harakal and White, personal communication, 1972) and i n d i c a t e an age of 130 m i l l i o n y e a r s . I t appears p o s s i b l e that these d i k e s , though l a t e i n the sequence represent the f i n a l events w i t h n e g l i g i b l e but s i g n i f i c a n t m i n e r a l -i z a t i o n f o l l o w i n g emplacement of i n t r a m i n e r a l d i k e s . Large shear zones averaging about 10 f e e t wide are predominant i n the p i t . Shears overlap the i n t r u s i o n of the i n t r a m i n e r a l d i k e s and the end of the period of ore m i n e r a l i z a t i o n . Stage IV veins and a few Stage I I I v e i n s i n part i n t e r s e c t the shear zones. A r g i l l i c a l t e r a t i o n has g r e a t l y extended the l i m i t s of shear zones and e f f e c t i v e l y weakens the rock f o r p e r c o l a t i o n of ground water and f u r t h e r decomposition of the r o c k . Many f a u l t s are r e l a t e d to shearing whereas others appear u n r e l a t e d . Movement d i r e c t i o n and ages are d i f f i c u l t to determine and l i t t l e can be s a i d of them. F a u l t s probably occur contemporaneously 36 S t r u c t u r a l H i s t o r y a t Brenda 176 m.y. ? 146 m.y.(?) Younger Cooline F e l s i r . d i k e s Stage I v e i n s Stage I I v e i n s Stage I I I v e i n s Shearing F a u l t i n g • • ? I n t r a m i n e r a l d i k e s Stage IV v e i n s J o i n t s Weathering & secondary m i n e r a l i z a t i o n F i g u r e 11, Sequence of events showing progressive overlap and r e l a t i v e ages, S t i p l e d area i n d i c a t e s time gap. with and/or s l i g h t l y l a t e r than shear zones. Some fa u l t i n g might be of Tertiary age. Orientation of both shears and f a u l t s are sim i l a r , s t r i k i n g about N68°E and dipping 75° to 85° southeast. One of the most important conclusions i s the apparent significance of the continuity of orientations of structural features of various ages. This suggests that the events were a l l i n response to the same stress environment that existed with i n t e r -mittent release over a protracted time period. Joints are unmineralized l a t e features possibly related to u p l i f t and erosion. Their predominant orientation i s N80°W/25°N. comparing the orientation of these features (including mineralized veins discussed i n the following chapter), a pattern can be observed with two predominant attitudes N60-68°E/75-85°SE and N60° to 70°W/80-85°SW. These orientations appear to be consistent throughout the p i t barring gaps and s l i g h t differences due to inadequate exposure and few recorded observations. 38 CHAPTER IV GEOLOGY OF BRENDA MINE - VEIN DESCRIPTIONS INTRODUCTION C h a l c o p y r i t e and molybdenite are the abundant ore minerals i n the Brenda ore d e p o s i t . They occur i n three p a r a g e n e t i c a l l y and m i n e r a l o g i c a l l y d i s t i n c t v e i n systems. A f o u r t h stage of veins are l a t e and c o n t a i n only infrequent s u l f i d e occurrences. Dominant Gangue B i o t i t e Quartz K - f e l s p a r Quartz C a l c i t e Various P r e v i o u s l y S o r e g a r o l i (1971), mapping w i t h i n the p i t i n 1968, d i v i d e d the veins i n t o four types: 1) quartz-potash, f e l d s p a r -s u l f i d e , 2) b i o t i t e - c h a l c o p y r i t e , 3) quartz-molybdenite, and 4) epidote-magnetite-molybdenite v e i n s , o c c u r r i n g i n that order f o r m a t i o n . The w r i t e r .departs somewhat from t h i s o r d e r . B i o t i t e v e i n s c o n t a i n i n g c h a l c o p y r i t e are found to be c o n s i s t e n t l y separated i n time from barren b i o t i t e v e i n s that crosscut a l l other v e i n s except l a t e c a l c i t e and epidote v e i n s . S p e c i f i c v e i n o r i e n t a t i o n s are s l i g h t l y more emphasized.or de-emphasized by the w r i t e r than by S o r e g a r o l i . These are discussed i n t h e i r a p p r o p r i a t e o r d e r . Much of t h i s work serves to confirm the e x c e l l e n t geology and mineralogy by S o r e g a r o l i w i t h much fewer exposures than were a v a i l a b l e to the w r i t e r . Ore v e i n s Time Primary Opaques f Stage I Veins C h a l c o p y r i t e - Py Stage I I Veins Cpy-Moly-Py Stage I I I Veins Moly-Cpy-Py-Mag-Hem Stage IV Veins (Py, Spec, Hem, Cpy) Average grade f o r the e n t i r e p i t i s 0.18 percent copper and 0.049 percent molybdenum, these metals being present almost e n t i r e l y as c h a l c o p y r i t e and molybdenite r e s p e c t i v e l y . Computer c a l c u l a t e d reserves are 177 m i l l i o n t o n s . Gold and s i l v e r assays i n the copper concentrate average 0.056 oz,/ton and 3.72 oz./ton r e s p e c t i v e l y (D. F. W h i t f o r d , Brenda g e o l o g i s t , personal communication). Rhenium content i n molybdenum concentrate ranges from 120 to 180 ppm (A. E. S o r e g a r o l i , personal communication). Ore minerals occur almost e n t i r e l y i n f r a c t u r e s , and f r a c t u r e d e n s i t y i s the dominant c o n t r o l of ore grade. Disseminations occur only r a r e l y i n areas where a l t e r a t i o n i s i n t e n s e . Other hypogene ore minerals are p y r i t e , magnetite, hematite, b o r n i t e , s p h a l e r i t e , and galena. Galena has thus f a r not been observed i n place or i n p o l i s h e d s e c t i o n s by the w r i t e r . S p h a l e r i t e and b o r n i t e are r a r e l y v i s i b l e m a c r oscopically but are i n places apparent i n p o l i s h e d s e c t i o n s examined by r e f l e c t e d l i g h t microscopy. P y r i t e averages about one percent of v e i n s u l f i d e s . Amounts of the other ore minerals mentioned vary d r a s t i c a l l y from place to place but normally are found only i n tr a c e amounts. Gangue minerals i n c l u d e q u a r t z , potassium f e l d s p a r , c a l c i t e and c l i n o z o i s i t e , B a r i t e was found i n one l o c a t i o n i n the p i t . Occurrences of the above minerals are discussed i n d e t a i l i n f o l l o w i n g paragraphs. STAGE I VEINS The e a r l i e s t s u l f i d e minerals i n the i n t r u s i o n were deposited i n b i o t i t e - l i n e d f r a c t u r e s , 1/32 to 1/16 i n c h wide. S o r e g a r o l i (1971) has considered the quartz-potash-feldspar veins to be o l d e r than b i o t i t e v e i n s . The w r i t e r , however, has c o n s i s t e n t l y observed quartz-potash-feldspar v e i n s t r a n s e c t i n g s u l f i d e bearing b i o t i t e v e i n s . E q u a l l y c o n s i s t e n t l y the w r i t e r has found a second c l a s s of b i o t i t e v e i ns i d e n t i c a l to the above mentioned b i o t i t e veins (excepting f o r a l a c k of v i s i b l e s u l f i d e content) that cut quartz-potash-feldspar v e i n s as w e l l as quartz-molybdenite v e i n s and some i n t r a m i n e r a l d i k e s . The l a t e r b i o t i t e v e i ns are grouped i n the f o u r t h stage of m i n e r a l i z a t i o n and are discussed s e p a r a t e l y i n a l a t e r s e c t i o n of t h i s chapter. The b i o t i t e i s dark brown or black w i t h a f e l t e d t e x t u r e and normally covering the f r a c t u r e w a l l s completely. C h l o r i t e i s g e n e r a l l y present, the amount depending on the extent of a l t e r a t i o n . C h a l c o p y r i t e and minor amounts of p y r i t e are the dominant and almost e x c l u s i v e ore m i n e r a l s ; moly-bdenite and b o r n i t e , occur much l e s s abundantly. A l l b i o t i t e v e i ns combined probably account f o r l e s s than 5 percent of the ore due mostly to t h e i r small s i z e and low s u l f i d e content. Mineralogy C h a l c o p y r i t e appears as patchy, s c a t t e r e d blebs on the b i o t i t e w a l l s , normally not covering more than 10 percent f o r the w a l l a r e a . Aggregates of grains are g e n e r a l l y l e s s than 7 mm wide. I n d i v i d u a l grains are as s m a l l as 0.1 mm. P y r i t e , the most abundant a s s o c i a t e d s u l f i d e , appears to have Figure 12» Contoured stereonets and major planes pf orientation of veins« been deposited e i t h e r contemporaneously w i t h or s l i g h t l y l a t e r than c h a l c o p y r i t e . Although g r a i n s or c r y s t a l s are normally subhedral to a nhedral, euhedral c r y s t a l s are not uncommon. Most cubes and anhedral g r a i n s are 0.5 to 1 mm i n diameter. P y r i t e might form as much as 20 percent of the t o t a l s u l f i d e s , but normally i s about 10 percent or l e s s of t o t a l s u l f i d e s i n Stage I v e i n s . Although the percentage of p y r i t e versus c h a l c o p y r i t e appears high i n t h i s system, i t does not appear to s e r i o u s l y a f f e c t m i l l recovery because the v e i n s are not important c o n t r i b u t o r s to o r e . In the f i e l d b o r n i t e was i d e n t i f i e d i n a s i n g l e instance i n a b i o t i t e v e i n by the w r i t e r but has not been observed i n p o l i s h e d s e c t i o n s . Macroscopic molybdenite i s present i n only a few b i o t i t e v e i n s . Where i t was observed, i t i s s c a t t e r e d as i n d i v i d u a l s mall p l a t e s or small groups of p l a t e s . : Amounts are s i m i l a r to the p y r i t e and i n places equal to c h a l c o p y r i t e . Although c h a l c o p y r i t e -p y r i t e v e i ns are e a r l i e r than second stage v e i n s , most of the f r a c t u r e s bearing molybdenite were c r o s s c u t t i n g and apparently l a t e r . I t i s suggested that these few v e i n s , l e s s than one percent of a l l b i o t i t e v e i n s , are t r a n s i t i o n a l t o , or overlap i n time the i n t r o d u c t i o n of the second stage of m i n e r a l s . Neither molybdenite nor b o r n i t e appear on a s p e c i f i c set of veins or i n any p a r t i c u l a r o r i e n t a t i o n . S t r u c t u r e Although they represent 24 percent of a l l the ore veins measured (Stage I , I I , and I I I ) , Stage I veins probably account f o r no more than f i v e percent of the t o t a l s u l f i d e ore due to the 43 minor concentrations of c h a l c o p y r i t e and almost t o t a l l a c k of molybdenite i n the v e i n s . No o f f s e t s were observed where b i o t i t e v e i ns i n t e r s e c t e d a p l i t e or pegmatite v e i n s . L i n e a t i o n s , i . e . s t r e a k i n g of the f e l t e d b i o t i t e , are minor but present on some of the l a r g e r v e i n -l e t s . Most l i n e a t i o n s , however, are f e l t to be very l a t e , a s s o c i a t e d w i t h u p l i f t , L i n e a t i o n s are commonly v e r t i c a l l y o r i e n t e d showing normal d i p - s l i p movement when obs e r v a b l e . Some a l s o d i s p l a y e d h o r i z o n t a l s t r i k e - s l i p motions w i t h both r i g h t -l a t e r a l and l e f t - l a t e r a l movement. L i n e a t i o n s are a l s o more abundant near shear zones though not tremendously s o. There are two concentrations of b i o t i t e v e i n a t t i t u d e s . By f a r the major con c e n t r a t i o n s t r i k e s about N62°E and di p s 78°SE + 2° (Figure 12a). I t i s o u t l i n e d by two 15 percent concentrations out of 100 veins measured over the e n t i r e p i t . The second high has an a t t i t u d e of N76°W/dip 86°SW and was defined by v i s u a l l y c e n t e r i n g the ten percent contour l e v e l . O r i e n t a t i o n of ve i n s are not found to change s i g n i f i c a n t l y from l o c a t i o n to l o c a t i o n (Figure 1 3 ) , Pr e f e r r e d o r i e n t a t i o n f o r m i n e r a l o g i c a l l y d i s t i n c t v e i n s , i . e . molybdenite or b o r n i t e bearing v e i n s , does not e x i s t as f a r as present data a l l o w s . S o r e g a r o l i (1971) has described two o r i e n t a -t i o n s common to b i o t i t e - c h a l c o p y r i t e v e i n s : "a) 5-20°W/45°E to v e r t i c a l " and "b) N40-60°W/15-30°N" ( S o r e g a r o l i , 1971). Because the w r i t e r has subsequently d i v i d e d b i o t i t e veins between two stages (I and I V ) , the patterns obtained are expectedly d i f f e r e n t , A minor occurrence of veins s t r i k i n g N25°E does e x i s t , however, i n the northeast s e c t i o n of the p i t (Figure 1 3 ) . F r a c t u r e d e n s i t i e s , 44 Figure 13 D i s t r i b u t i o n of Stage I v e i n o r i e n t a t i o n s w i t h i n the Brenda p i t . S t r i k e and dip of major concen-t r a t i o n s are heavy l i n e s , and minor (questionable) concentrations are l i g h t l i n e s . (Schmidt net used f o r c o n s t r u c t i o n ) Primary o r i e n t a t i o n i s N62°E, dip 78°SE. N i n d i c a t e s the number of observations i n each s e c t o r . Dotted l i n e i n d i c a t e s approximate p i t o u t l i n e as of August, 1971. 45 F i g u r e 15. 'Surface of t y p i c a l Stage I I o r e v e i n c o n t a i n -i n g q u a r t z , p o t a s s i u m - f e l d s p a r , c h a l c o p y r i t e , m o l y b d e n i t e and p y r i t e . Figure 16, Large banded Stage I I I v e i n (quartz-moly-bdenite) c h a l c o p y r i t e and p y r i t e s t r i n g e r s crosscut t h i n molybdenite bands. C a l c i t e v e i n runs p a r a l l e l to banding. Figure 17. I r r e g u l a r Stage IV v e i n ( c l i n o z o i s i t e ) i n t e r s e c t i n g Stage I I v e i n . 47 F i g u r e 18 D i s t r i b u t i o n of Stage I I v e i n o r i e n t a t i o n s w i t h i n the Brenda p i t . S t r i k e and d i p of major concen- -t r a t i o n s are heavy l i n e s , and minor (questionable) concentrations are l i g h t l i n e s . (Schmidt net used f o r c o n s t r u c t i o n ) Primary o r i e n t a t i o n i s N66°E/78°SW. N i n d i c a t e s the number of observations i n each s e c t o r . Dotted l i n e i n d i c a t e s approximate p i t o u t l i n e as of August, 1971. 500' i i F i g u r e 20. B o r n i t e w i t h c h a l c o p y r i t e . Stage I I v e i n (X40). Figure 22. S p h a l e r i t e i n c h a l c o p y r i t e w i t h p o s s i b l e e x s o l u t i o n of c h a l c o p y r i t e w i t h i n the s p h a l e r i t e (X40). Figure 24. Molybdenite c o n t a i n i n g wedges of chalco p y r i t e (x40). 51 measured i n terms of number of f r a c t u r e s per y a r d , are commonly h i g h . Due to t h e i r extremely small s i z e , many de n s i t y counts were only best guesses based perhaps on a few inches or a f o o t of exposed r o c k . Even w i t h good exposure, many were not v i s i b l e unless a c c i d e n t a l l y broken along t h e i r p l a n e . B l a s t i n g dust a l s o obscurred v e i n s making accurate counts and measurements d i f f i c u l t . Due to t h i s d i f f i c u l t y , the w r i t e r cannot be c e r t a i n i f Stage I v e i n s are more abundant i n one l o c a l i t y than another i n the mine. STAGE I I VEINS Quartz-potash-feldspar veins are d i s t i n c t l y l a t e r than b i o t i t e veins and c o n s t i t u t e a second separate phase of s u l f i d e m i n e r a l -i z a t i o n . They are the most important source of o r e , probably accounting f o r c l o s e to 80 percent of a l l s u l f i d e s . The reasons are: l a r g e r s i z e v e i n s , greater amounts of c h a l c o p y r i t e and moly-bdenite i n i n d i v i d u a l v e i n s , and greater abundance of the v e i n s , e s p e c i a l l y near the core of the d e p o s i t . Vein thickenesses range from 1/8 i n c h to r a r e l y 1&1/2 inches wide. The average s i z e i s about 3/16 of an i n c h . Generally s u l f i d e s comprise 10 to 30 percent of the v e i n space, but l o c a l l y reach 100 p e r c e n t . Mineralogy Abundant hypogene s u l f i d e s are c h a l c o p y r i t e and molybdenite. Magnetite, hematite, and b o r n i t e appear l e s s commonly as hypogene minerals i n Stage I I v e i n s . Most v e i n s are c l o s e d . Open vuggy ve i n s occur more commonly near the higher grade m i n e r a l i z e d zones but possess e x a c t l y the same mineralogy and v e i n c h a r a c t e r i s t i c s . The bulk of the veins are f i l l e d w i t h quartz unless s u l f i d e s are abnormally abundant. E a r l y quartz i s i n v a r i a b l e anhedral forming g r a i n s as l a r g e as 3/8 i n c h . Rarely quartz has formed doubly terminated c r y s t a l s exceeding an inch i n l e n g t h and embedded i n c h a l c o p y r i t e . Potash-feldspar forms euhedral to subhedral c r y s t a l s up to 1 cm wide p r o j e c t i n g i n t o the quartz along the v e i n w a l l s . I t a l s o forms as a l t e r a t i o n of w a l l rock that w i l l be considered i n more d e t a i l i n a subsequent ch a p t e r . Disseminated c h a l c o p y r i t e and p y r i t e occur r a r e l y where the potassium a l t e r a t i o n i s p a r t i c u l a r l y i n t e n s e . Small euhedral c r y s t a l aggregates of c l i n o z o i s i t e ( i d e n t i f i e d by x-ray) are found s p o r a d i c a l l y upon quartz or c h a l c o p y r i t e . They r a r e l y have lengths of one i n c h , although most are much l e s s than a centimeter l o n g , commonly w i t h a very f i n e - g r a i n e d , anhedral l a y e r i n g . They are f o r the most par t very l a t e i n paragenetic sequence. Molybdenite i s g e n e r a l l y the f i r s t m i neral d e p o s i t e d , e i t h e r along the w a l l s or as s c a t t e r e d , small blades i n the q u a r t z . L o c a l l y small s t r i n g e r s of molybdenite cut c h a l c o p y r i t e . Grain s i z e depends to a l a r g e extent on the t o t a l amount of molybdenite i n the v e i n . Most blades are l e s s than 1 mm to 1 cm wide. Moly-bdenite might be found i n contact w i t h any of the other m i n e r a l s ; and c o n t r a r y to previous thought, much of i t contains i n t i m a t e l y , admixed c h a l c o p y r i t e , and l e s s commonly p y r i t e and b o r n i t e . Disseminated molybdenite i s uncommon but i s found r a r e l y i n intense hydrothermal b i o t i t e halos adjacent to m i n e r a l i z e d v e i n s , ( F i g u r e 29). C h a l c o p y r i t e i s equal to or i n greater p r o p o r t i o n than moly-b d e n i t e . Normally i t i s p a r a g e n e t i c a l l y l a t e r than molybdenite, forming i n the center of v e i n s . Blebs of c h a l c o p y r i t e occur s c a t t e r e d over the v e i n w a l l s , not i n t e r c o n n e c t i n g unless unusually high grades of m i n e r a l i z a t i o n are present (Figure 1 5 ) . Where c h a l c o p y r i t e and molybdenite are i n c o n t a c t , the moly-bdenite i s replaced i n part along (cleavages) planes or between i n d i v i d u a l p l a t e s . These small wedges or l a t h s are i n t i m a t e l y mixed w i t h molybdenite and commonly are as small as one micron (Figure 24), P y r i t e i s i n places corroded by c h a l c o p y r i t e . F r a c t u r e s w i t h i n p y r i t e g r a i n s are g e n e r a l l y f i l l e d w i t h c h a l c o -p y r i t e . Small l a t h s and wedges of p y r i t e i n molybdenite a l s o occur but w i t h l e s s frequency. Other v e i n s show opposite r e l a t i o n s . P y r i t e i n some veins forms euhedral cubes and pyritohedrons i n c h a l c o p y r i t e , which are o f t e n found i n a s s o c i a t i o n s w i t h euhedral epidote and doubly terminated quartz c r y s t a l s , a l l embedded i n masses of c h a l c o p y r i t e (Figure 1 9 ) . No s p e c i f i c o r i e n t a t i o n s f o r t h i s l a t t e r a s s o c i a t i o n of euhedral c r y s t a l s was found. However, a l l were found c l o s e to high grade m i n e r a l i z a t i o n ,5 . B o r n i t e and s p h a l e r i t e are l e s s abundant c o n s t i t u e n t s . B o r n i t e i s seen only r a r e l y i n hand specimens, but mostly occurs as very minute replacements along f r a c t u r e s i n and along g r a i n boundaries of c h a l c o p y r i t e . I t i s g e n e r a l l y l e s s than 1/10 to 1/50 mm. S p h a l e r i t e has been seen only i n p o l i s h e d s e c t i o n s i n a s s o c i a t i o n w i t h c h a l c o p y r i t e . I t occurs as rounded blebs or s t r i n g e r s up to one mm i n diameter on the margins or i n the 54 i n t e r i o r of c h a l c o p y r i t e . Extremely minute d i s s i m i n a t e d g r a i n s of c h a l c o p y r i t e w i t h i n s p h a l e r i t e give the appearance of e x s o l u t i o n t e x t u r e s (Figure 22), P y r r h o t i t e has been reported p r e v i o u s l y to occur i n a s s o c i a t i o n w i t h c h a l c o p y r i t e and p y r i t e ; presumably i n Stage I I veins (Vogan, 1966). The w r i t e r , however, has studied many p o l i s h e d s e c t i o n s and has f a i l e d to observe any p y r r h o t i t e . As f a r as the w r i t e r knows, p y r r h o t i t e has not been reported s i n c e 1966; and then i t p o s s i b l y was m i s i d e n t i f i e d . S t r u c t u r e The predominant s t r i k e and d i p of Stage I I veins (taken from stereogram p l o t s of poles to planes f o r 251 a t t i t u d e s ) i s N66°E/78°SW, and remains r e l a t i v e l y p e r s i s t e n t throughout the mine (Figures 12b and 18). An exception i s the young vuggy veins which s t r i k e mostly NO0 to 10°W and dip v e r t i c a l l y . These are few i n number and occur mostly towards the center of the orebody. They have a l s o been observed by S o r e g a r o l i (1971) who noted a second o r i e n t a t i o n about N30o-45oE15o-25°NW. The w r i t e r has not found a s i g n i f i c a n t c o n c e n t r a t i o n i n t h i s range f o r the vuggy Stage I I v e i n s , however. D e n s i t i e s of v e i n i n g range from 5 to 15 f r a c t u r e s per yard along the periphyry to about 70 per yard and o c c a s i o n a l l y w e l l over a hundred per yard near the center of the orebody. A d e f i n i t e zoning of the f r a c t u r e d e n s i t y of quartz-potassium-feldspar v e i n s i s the main c o n t r o l of ore grade. This aspect i s discussed f u r t h e r i n the f o l l o w i n g chapter. Minor o f f s e t s occur here and t h e r e , but most were observed i n b l a s t d e b r i s ; hence, true o f f s e t i s mostly i n d e t e r m i n a b l e . Because of the steep dips of a l l f r a c t u r e s , i t i s most probable that movements were s t r i k e - s l i p since a large amount of movement i s required for the same throw i f d i p - s l i p movement occurred rather than s t r i k e - s l i p . Maximum o f f s e t i s normally less than one inch. STAGE III VEINS The youngest ore veins at the mine are large quartz veins. Although not nearly abundant as e a r l i e r veins, they enhance the higher grade ore due to t h e i r larger widths and greater s u l f i d e content. Size, properties of the quartz, and lack of potassium feldspar t y p i c a l l y characterize and d i s t i n g u i s h Stage III veins. Vein width averages one to two inches,although they can be as small as 1/2 an inch or r a r e l y as large as 14 inches. Mineralogy Hypogene ore minerals include molybdenite, chalcopyrite, p y r i t e , magnetite and s p h a l e r i t e . Bornite has not been observed i n these veins to date. Gangue minerals are normally quartz and minor amounts of c a l c i t e . Hematite occurs i n small amounts associated with c a l c i t e . Any of the s u l f i d e s can occur s i n g l y or i n combination with others and i n widely v a r i a b l e proportions. Banding of the quartz and s u l f i d e ( s ) might or might not be present. Banded quartz-molybdenite veins consist of 1/4 to 3/4 inch r e p e t i t i o u s layers of quartz separated by t h i n lacy molybdenite covers (Figure 16), The quartz i s generally greyish due to disseminated molybdenite, though t h i s i s l e s s common. Books of 56 s u b p a r a l l e l molybdenite are as l a r g e as 1/8 i n c h , but normally do not exceed 1/16 of an inch i n greatest dimension. C h a l c o p y r i t e and p y r i t e commonly are minor c o n s t i t u e n t s forming i r r e g u l a r blebs of v a r i o u s s i z e s and t i n y , l o c a l l y - c r o s s c u t t i n g v e i n l e t s , i n d i c a t i n g they are probably p a r a g e n e t i c a l l y l a t e r than molybdenite, P y r i t e forms both as s t r i a t e d euhedral cubes and anhedral g r a i n s . Cube edges commonly are l e s s than 1/8 i n c h but can be as l a r g e as 1/4 i n c h . Less commonly, the banded v e i n c o n s i s t s of c h a l c o p y r i t e w i t h molybdenite confined to the very e a r l i e s t outer margin of the v e i n . Although there i s a change i n the predominant s u l f i d e , the shape and s t r u c t u r e i s s i m i l a r to the banded quartz-molybdenite v e i n s . A second type of Stage I I I v e i n s i s massive q u a r t z - c h a l c o p y r i t e or q u a r t z - p y r i t e v e i n . The w r i t e r does not intend that a genetic d i f f e r e n c e be i m p l i e d i n t h i s d i s t i n c t i o n - only a t e x t u r a l and m i n e r a l o g i c a l d i f f e r e n c e . A l l other f e a t u r e s d i c t a t e the same system of m i n e r a l i z a t i o n , C h a l c o p y r i t e and p y r i t e occur as massive lodes i n the v e i n or as smaller blebs s c a t t e r e d among the q u a r t z . Molybdenite i s normally missing or i n subordinate amounts confined to the w a l l s of the v e i n . E i t h e r c h a l c o p y r i t e or p y r i t e can be the most abundant s u l f i d e , P y r i t e forms many euhedral, s t r i a t e d cubes w i t h edges up to one i n c h l o n g . Where present w i t h c h a l c o -p y r i t e , most p y r i t e i s s i t u a t e d along the margins of the v e i n . L o c a l l y , magnetite forms l a r g e v e i n s w i t h minor amounts of p y r i t e up to two inches wide, Euhedral magnetite g r a i n s massively packed together have maximum widths of 1/8 i n c h . Some of these v e i n s have a l a r g e r amount of s p h a l e r i t e ( i n the c h a l c o p y r i t e ) than previous s t a g e s . These veins are more r a r e and are grouped w i t h Stage I I I v e i n s due to t h e i r s i z e and s i m i l a r i t y of occurrence, i n the host r o c k . Most magnetite, where present, i s l e s s than one or two percent of the m e t a l l i c content of the veins (Figure 21). Several samples of w h i t i s h " p y r i t e " were x-rayed to t e s t the p o s s i b i l i t y they might be raarcasite; a l l proved to be p y r i t e . S t r u c t u r e The predominant v e i n a t t i t u d e i s N67°E/77°SE - defined by a 20 percent c o n c e n t r a t i o n of poles to p l a n e s . A secondary concen-t r a t i o n occurs a t N76°W/86°SW ^Figure 1 2 c ) . Stage I I I v e i n s are never densely spaced unless d i r e c t l y adjacent to a shear zone. In the core of the p i t , d e n s i t i e s reach one to two per yard though not c o n s i s t e n t l y . Shears have been observed to t r a n s e c t these quartz v e i n s but a l s o may be unaffected by some. O f f s e t s r e s u l t i n g from the development of the shears have not been observed by the w r i t e r . S i m i l a r l y , o f f s e t s have not been observed due to the formation of the quartz v e i n s . However, i n a few places l e n s i n g of the quartz veins forms a s i g m o i d a l shape. The s i m i l a r i t y i n o r i e n t a t i o n between v e i n stages suggests a s i m i l a r s t r u c t u r a l o r i g i n . STAGE IV VEINS The f o u r t h stage of m i n e r a l i z a t i o n c o n s i s t s of numerous veins and f r a c t u r e s that f a l l i n t o four convenient types; c l i n o z o i s i t e v e i n s , b i o t i t e v e i n s , quartz v e i n s , and c a l c i t e v e i n s . A l l except a few of the e a r l y c l i n o z o i s i t e v e i n s are post ore and i n t e r s e c t a l l p r e v i o u s l y described v e i n s and i n t r a m i n e r a l d i k e s . I t i s p o s s i b l e that a h i a t u s i s present sometime a f t e r the ore veins and before the Stage IV v e i n s except e p i d o t e . B i o t i t e v e i n s are the most numerous and were at f i r s t mistaken f o r Stage I b i o t i t e v e i n s as discussed e a r l i e r . They are q u i t e t h i n , o f t e n d i s c o n -t i n u o u s , not p e r s i s t e n t , and d i f f i c u l t to observe unless the rock was f o r t u i t o u s l y broken along one of the f r a c t u r e s . Minor amounts of s u l f i d e s are present e s p e c i a l l y where the v e i n s i n t e r s e c t mafic d i k e s . C h a l c o p y r i t e , p y r i t e and hematite have been observed i n Stage IV v e i n s i n g e n e r a l l y subordinate amounts. Minor q u a n t i t i e s of molybdenite have been observed i n a few veins - e s p e c i a l l y c l i n o z o i s i t e v e i n s . ' Epidote veins (confirmed by x-ray to be c l i n o z o i s i t e ) are f o r the most part i n s i g n i f i c a n t i n both number and s u l f i d e content. Vein widths range from as small as 1/32 of an inch to 4 i n c h e s . Normally they are present as i r r e g u l a r 1/8 inch v e i n s of f i n e l y granular anhedral c l i n o z o i s i t e (Figure 1 7 ). In the l a r g e r v e i n s c h a l c o p y r i t e , molybdenite, magnetite, (+ hemat i t e ) , and p y r i t e , might be present i n g r e a t l y v a r y i n g amounts and g r a i n s i z e s , The most common assemblages of the l a r g e r veins are c h a l c o p y r i t e -p y r i t e , c h a l c o p y r i t e - m o l y b d e n i t e - p y r i t e , m a g n e t i t e - p y r i t e , and magnetite a l o n e . A l l except c h a l c o p y r i t e occur e i t h e r as anhedral 59 or euhedral c r y s t a l s and c r y s t a l aggregates. Small anhedral grains of quartz can be i n t e r s p e r s e d among the epidote but never i n amounts greater than 10 percent of the v e i n . Paragenesis of the s u l f i d e s i s indeterminable by the w r i t e r , but i t i s l i k e l y that o v e r a l l m i n e r a l -i z a t i o n f o l l o w s the p a t t e r n e s t a b l i s h e d f o r the preceeding two s t a g e s . Only 23 epidote v e i n s were measured i n the p i t , i n s u f f i c i e n t data to g i v e any c o n c e n t r a t i o n when p l o t t e d on a stereogram. O f f s e t along these v e i n s has not been observed. S o r e g a r o l i (1971) has,;:;,/ observed "Epidote-magnetite-molybdenite veins which s t r i k e N45-55°W w i t h v e r t i c a l d i p s " . C l i n o z o i s i t e v e i n s are grouped i n the f o u r t h stage of m i n e r a l -i z a t i o n but are i n part contemporaneous w i t h e a r l i e r v e i n s . The l a r g e r v e i n s are thought to be concurrent w i t h the l a s t phases of Stage I I I m i n e r a l i z a t i o n because the s u l f i d e assemblages are s i m i l a r , and the r a r e l a r g e lenses of s u l f i d e s are a s s o c i a t e d i n space w i t h some l a t e Stage I I I quartz v e i n s . The l a r g e r v e i n s a l s o show strong hydrothermal b i o t i t e envelopes very s i m i l a r to Stage I I I a l t e r a t i o n . F i e l d observation has shown that the smaller v e i n s and v e i n l e t s are younger than mafic dikes and hence are post o r e . A p o s s i b l e m i n e r a l i z a t i o n h i a t u s might be present between the t h i r d and f o u r t h stages of m i n e r a l i z a t i o n , although other s t r u c t u r a l f e a t u r e s t i e them togeth e r . B i o t i t e v e i n s • s i m i l a r to Stage I b i o t i t e v e i n s are a p p r o x i -mately 1/32 inch to 1/16 i n c h but do not c o n t a i n s u l f i d e s . For the most p a r t , they are merely coatings of a dark brown f e l t e d mass of hydrothermal b i o t i t e on f r a c t u r e s u r f a c e s . They are by f a r the most numerous v e i n s of t h i s stage. Rarely p y r i t e and/or c h a l c o p y r i t e 60 occurs as i s o l a t e d i r r e g u l a r blebs l e s s than 2 mm l o n g . Small quartz v e i n s , 1/4 inch or l e s s i n w i d t h , crosscut a l l p r e v i o u s l y mentioned m i n e r a l i z a t i o n . These are few i n number and c o n t a i n no r e l a t e d p o t a s h - f e l d s p a r , b i o t i t e or other a l t e r a t i o n m i n e r a l s . In places they c o n t a i n euhedral p y r i t e or minor amounts of anhedral c h a l c o p y r i t e . S u l f i d e s are r a r e and were observed only where c r o s s i n g post-ore d i k e s . They do not appear i n any way r e l a t e d to ore v e i n s . In one case a vuggy, though unmineralized quartz v e i n , was observed c r o s s c u t t i n g the trachyte d i k e . C a l c i t e v e i n s are numerous though small i n s i z e . They f i l l i n most reopened f r a c t u r e s of a l l ages and form t h e i r own v e i n s , commonly 1/8 i n c h wide and r a r e l y two to three i n c h e s . Hematite i s o r d i n a r i l y the only opaque mineral i n these v e i n s . I t i s thought to be disseminated among the c a l c i t e , producing a black-brown to orange t i n g e . P o s i t i v e i d e n t i f i c a t i o n pf the c a l c i t e c o l o r a t i o n was not p o s s i b l e due to the extremely f i n e - g r a i n e d and dispersed n a t u r e . Rarely c r y s t a l l i n e f e r r o a n dolomite has been found to f i l l open f r a c t u r e s at a very l a t e s t a g e . Specular hematite has, at r a r e l o c a l i t i e s , f i l l e d s m a l l 1/16 inch f r a c t u r e s . Their exact r e l a t i o n to other v e i n s i s unknown, A s i n g l e b a r i t e v e i n , one to one and one-half inches wide, occurs i n the northwest margin of the p i t . The margins of the veins were b r e c c i a t e d w i t h quartz d i o r i t e fragments from 1/2 i n c h to one inch i n s i z e . This i s the only observed occurrance of b a r i t e i n the p i t . I t i s i n an area of hydrothermal b i o t i t e a l t e r a t i o n , but the v e i n i t s e l f was found loose i n the muck; and the w r i t e r was unable to l o c a t e i t s outcrop. 61 S t r u c t u r e F o u r t h stage v e i n s (dominantly b i o t i t e v e i n s ) are s i m i l a r i n a t t i t u d e to the p r e v i o u s v e i n s . High c o n c e n t r a t i o n s on the Schmidt net are N66°E/80°SE and N64°W/89°SW ( F i g u r e 12d) . Greater s c a t t e r of a t t i t u d e s e x i s t s among these v e i n s than i n other v e i n systems; p o s s i b l y because they are l a t e and formed i n a more compl ica ted environment than ore f r a c t u r e s . But a c o n s i s t e n t change i n a t t i t u d e s r e l a t i v e to l o c a t i o n i n the p i t cannot be de termined . The f r a c t u r e d e n s i t y of the b i o t i t e v e i n s i n p l a c e s appears h i g h , but a p r e c i s e count was not always p o s s i b l e . C a l c i t e and q u a r t z v e i n s are most numerous around d i k e s and p e r i p h e r a l to shear zones . SECONDARY MINERALS Secondary m i n e r a l s observed i n c l u d e f e r r i m o l y b d i t e , l i m o n i t e , m a l a c h i t e , a z u r i t e , c h a l c o c i t e , n a t i v e copper , p o w e l l i t e , c u p r i t e , c u p r i f e r o u s manganese o x i d e s , and i l s e m a n i t e , T e n o r i t e and c o v e l l i t e have a l s o been r e p o r t e d by e a r l i e r workers ( S o r e g a r o l i , 1971) . L i m o n i t e i s the predominant secondary m i n e r a l , forming i n weathered r o c k tha t extend r a r e l y to depths of 90 f e e t below the s u r f a c e . The average weathered zone where l i m o n i t e forms i s l e s s than 20 f e e t t h i c k . Greater depths of weather ing and hence of l i m o n i t e f o r m a t i o n are found l o c a l l y i n shear zones . L i m o n i t e forms as t h i n , brown coats and i n some cases as boxwork replacement of p y r i t e and c h a l c o p y r i t e . X - r a y ana lyses d i d not r e v e a l any g o e t h i t e . F e r r i m o l y b d i t e , an a l t e r a t i o n product of m o l y b d e n i t e , i s uncommon at Brenda and has been i d e n t i f i e d by the w r i t e r i n o n l y two samples; Samples were , however, more e a s i l y observed when e a r l y 62 s t r i p p i n g of the cap rock was taki n g place ( S o r e g a r o l i , personal communication). One sample was found i n a one inc h quartz-c h a l c o p y r i t e - p y r i t e v e i n l o c a t e d a m i l e northeast of the mine. The other was found at the t a i l i n g s dam and was presumed to have been hauled from surface muck obtained at the p i t . Favorable c o n d i t i o n s f o r the formation of f e r r i m o l y b d i t e r e q u i r e a low pH on the order of 1.5 to 4. The absence of s i g n i f i c a n t amounts of p y r i t e and a c o l d c l i m a t e both could be r e s p o n s i b l e f o r the o v e r a l l s c a r c i t y of t h i s a l t e r a t i o n product. One small specimen of n a t i v e copper was observed i n a l a t e vuggy c a l c i t e v e i n . M a l a c h i t e and a z u r i t e have been found as t h i n e n c r u s t a t i o n s along j o i n t s or f r a c t u r e s i n the uppermost exposed s e c t i o n near the s u r f a c e . In places the enc r u s t a t i o n s are 1/8 i n c h t h i c k , but normally they are much t h i n n e r . M a l a c h i t e i s more abundant than a z u r i t e . N eglegible supergene enrichment i s recorded i n Brenda d e p o s i t . A d m i t t e d l y , however, i t i s only a s i n g l e occurrence. In a s i n g l e location,sample #4-3 of bench 5060 l o c a t e d near the surface of the o l d stream which transversed through the center of the p i t , sooty c h a l c o c i t e was confirmed as a t h i n c o a t i n g on p y r i t e and c h a l c o -p y r i t e grains along a Stage I I v e i n , A l a y e r of c h a l c o c i t e one to three mm t h i c k i n places almost completely replaces 1/2 cm c h a l c o p y r i t e and p y r i t e g r a i n s . The extent of t h i s supergene enrichment i s l i m i t e d to only a few yards near the former stream-bed where t h i s p a r t i c u l a r v e i n i s exposed. P y r i t e and magnetite, i t should be noted, form as much as 20 percent of t h i s v e i n ; and VEIN STAGES I I I I I IV ' ^ C h a l c o p y r i t e M o l y b d e n i t e P y r i t e M a g n e t i t e Hematite S p h a l e r i t e B o r n i t e Quartz C a l c i t e E p i d o t e A l t e r a t i o n : P r o p y l i t i c B i o t i t e ' K - f e l d s p a r A r g i l l i c Figure 25. L i n e diagram of m i n e r a l o g y of v e i n systems. The t h i c k n e s s o f the l i n e s i n d g e n e r a l r e l a t i v e abundance of t h e m i n e r a l s . i c a t e s ON Figure 27. Same v e i n as Figure 26 although not s t a i n e d . Note greenish p r o p y l i t i c a l e r a t i o n beyond potassium-feldspar envelope. Figure 29. B i o t i t e a l t e r a t i o n along Stage I I I v e i n . Note the disseminated molybdenite i n the b i o t i t e . 66 Figure 30. B i o t i t e a l t e r a t i o n along Stage I I I quartz-p y r i t e v e i n . B i o t i t e envelope overlaps and destroys a l t e r a t i o n of p r e v i o u s l y emplaced Stage I I v e i n . Figure 31. Potassium-feldspar and hydrothermal b i o t i t e a l t e r a t i o n envelopes about a Stage I I v e i n . 67 p y r i t e i s i n great abundance i n an 8 inch q u a r t z - p y r i t e v e i n immediately adjacent. Hence, many of the c o n d i t i o n s appear to have been combined f o r t u i t i o u s l y f o r t h i s r a r e supergene occurrence. HYDROTHERMAL ALTERATION Hydrothermal a l t e r a t i o n i n the Brenda ore body has been described i n more or l e s s general terms by s e v e r a l previous workers (Vogan, 1966; C a r r , 1967; and S o r e g a r o l i , 1968 and 1971). While d i f f e r i n g s i g n i f i c a n t l y from the f i r s t two aut h o r s , the w r i t e r ' s a l t e r a t i o n study i s i n agreement with the types of a l t e r a t i o n presented by S o r e g a r o l i (1968 and 1971). These types are 1) potassium a l t e r a t i o n , 2) p r o p y l i t i c a l t e r a t i o n , and 3) a r g i l l i c a l t e r a t i o n . The f i r s t two types form as narrow envelopes about Stage I and I I v e i n s , w h i l e a r g i l l i c a l t e r a t i o n i s as s o c i a t e d w i t h Stage I I I veins i n zones up to 35 f e e t wide and s e v e r a l hundreds of fe e t l o n g . Although searched f o r d i l l i g e n t l y , a s p a t i a l p a t t e r n of a l t e r a t i o n , other than the small envelopes about ore v e i n s , could not be found. Concentric zoning of a l t e r a t i o n t y p e s , f o r example, does not e x i s t as f a r as the w r i t e r has been able to determine. S o r e g a r o l i (1971) i n d i c a t e s the presence of hydrothermal b i o t i t e i n a zone extending to the northeast beyond the ore body. Potassium a l t e r a t i o n c o n s i s t s of two types: potassium f e l d s p a r ( c h i e f l y o r t h o c l a s e ) and hydrothermal b i o t i t e . Potassium f e l d s p a r mostly forms t h i n envelopes around Stage I I ve i n s (Figures 26, 27, 28, and 31), Generally the f e l d s p a r i s confined to growth along v e i n w a l l s but can form envelopes i n the w a l l rock up to 8 inches wide r e p l a c i n g p l a g i o c l a s e i n the host r o c k . Occasional mica and k a o l i n i t e have been seen r e p l a c i n g mafics and some f e l d s p a r . 68 Where l a r g e amounts of potassium f e l d s p a r are in t r o d u c e d , the hydro-thermal b i o t i t e content a l s o increases and forms p l a t e s up to 1/4 i n c h diameter admixed w i t h the f e l d s p a r . A secondary envelope of hydrothermal b i o t i t e might a l s o develop around the f e l d s p a r envelope before grading i n t o p r o p y l i t i c a l t e r a t i o n . Although hydrothermal potassium f e l d s p a r i s a s s o c i a t e d w i t h the most important ore veins (Stage I I ) , the amount of potassium f e l d s p a r does not appear to be r e l a t e d to the amount of s u l f i d e s p r e s e n t . Hydrothermal b i o t i t e , o f t e n a green b i o t i t e under the microscope, occurs i n a l l stages of v e i n f o r m a t i o n . Only i n the second stage i s i t l e s s developed. In the f i r s t stage of m i n e r a l i z a t i o n , i t forms f e l t e d b l ack coatings as described e a r l i e r . Rarely i s i t v i s i b l y developed i n t o the w a l l rock adjacent to the v e i n . In second stage v e i n s i t i s l e s s developed but can form a t h i n envelope; g e n e r a l l y l e s s than 1/8 inc h and d i f f i c u l t to observe, M a c r o s c o p i c a l l y , i t can be recognized by i t s greater abundance and i t s f i n e - g r a i n e d nature compared to normal host rock b i o t i t e . I t a l s o may be admixed w i t h o r t h o c l a s e as mentioned e a r l i e r or form a secondary envelope about the potassium f e l d s p a r a l t e r a t i o n (Figure 3 1 ) . Th i r d stage veins a l s o c o n t a i n abundant b i o t i t e and g e n e r a l l y l i t t l e or no potassium f e l d s p a r . Commonly, a l t e r a t i o n i s so complete that hydrothermal b i o t i t e forms more than 50 percent of the r o c k . T h i s envelope can be s e v e r a l f e e t wide i n some p l a c e s , w h i l e e l s e -where i t i s l e s s than one i n c h wide. Rarely d i s s i m i n a t e d s u l f i d e s are present i n the b i o t i t e envelope (Figure 29). Stage IV a l t e r a t i o n c o n s i s t s of b i o t i t e f i l l e d f r a c t u r e s s i m i l a r to Stage I v e i n s and envelopes of b i o t i t e a few inches wide around 69 l a r g e epidote v e i n s which are g e n e r a l l y few i n number. In a l l cases b i o t i t e and potassium f e l d s p a r grade outward i n t o p r o p y l i t i c a l t e r -a t i o n , P r o p y l i t i c a l t e r a t i o n i s a term used to d e s c r i b e a weak a l t e r -a t i o n e f f e c t forming t h i n envelopes adjacent to potassium a l t e r a t i o n along v e i n s . Where v e i n d e n s i t i e s are extremely h i g h , pervasive p r o p y l i t i c a l t e r a t i o n might be present, though not n e c e s s a r i l y . Epidote and minor amounts of c h l o r i t e are c h a r a c t e r i s t i c a l t e r a t i o n products of the p r o p y l i t i c zone, Epidote commonly forms i n the cores of p l a g i o c l a s e c r y s t a l s , and o c c a s s i o n a l l y muscovite appears i n the assemblage. M e g a s c o p i c a l l y , the plagioclase:>appears s l i g h t l y greenish and waxy t e x t u r e d . Thin white a l b i t i z e d v e i n l e t s occur r a r e l y , but t h e i r exact r e l a t i o n s h i p to other a l t e r a t i o n types has not yet been determined. Indeed, they are p o s s i b l y l a t e weathering a l t e r a t i o n u nrelated to ore v e i n a l t e r a t i o n . A r g i l l i c a l t e r a t i o n occurs i n l a r g e widely spaced planar zones, g e n e r a l l y a l i g h t tan or brown i n c o l o r (Figure 8 ) . Stage I I I v e i n s are always a s s o c i a t e d , and shearing i s g e n e r a l l y p r e s e n t . X-ray analyses of c l a y f r a c t i o n of nine a l t e r a t i o n zones of v a r i o u s widths and l o c a t i o n s show i d e n t i c a l assemblages of k a o l i n i t e , muscovite and m o n t m o r i l l o n i t e . For an assemblage such as t h i s , Meyer and Hemley (1967) suggest using the term "intermediate" a r g i l l i c a l t e r a t i o n to d i s t i n g u i s h i t from a more advanced a r g i l l i c a l t e r a t i o n which they a l s o d e s c r i b e . Although both b i o t i t e and a r g i l l i c a l t e r a t i o n are a s s o c i a t e d w i t h Stage I I I v e i n s , the w r i t e r could not f i n d a c o n s i s t e n t s p a t i a l 70 a) untreated b) g l y c o l l a t e d c) heated 540°C 31 30 29 28 27 26 25 21 23 22 21 20 19 18 17 16 15 14 13 12 II 10 9 8 7 6 5 1 F i g u r e 32 6 X-ray d i f f r a c t i o n t r a c e s c l a y f r a c t i o n of a r g i l l i c a l t e r a t i o n zon©0 K a o l i n i t e 9 muscovite and montsnorillonite are dominant c o n s t i t u e n t s . Numbers i n d i c a t e degrees 2©0 KV - 40* MA - 20» 71 r e l a t i o n s h i p between b i o t i t e and a r g i l l i c a l t e r a t i o n . I t appears that only c e r t a i n Stage I I I veins ( g e n e r a l l y two or three per shear zone) are s u i t a b l e f o r producing t h i s a l t e r a t i o n . Most of the ve i n s i n these zones were high i n quartz and molybdenum; however, some quartz-molybdenum v e i n s were observed which were not a s s o c i a t e d w i t h a r g i l l i c a l t e r a t i o n . . . Muscovite, much of i t f i n e - g r a i n e d , occurs throughout the shear zone. In places i t might be termed " s e r i c i t e " a l t e r a t i o n ; however, because of i t s extremely r a r e and i n c o n s i s t e n t appearances, the w r i t e r b e l i e v e s i t does not warrant a separate c l a s s i f i c a t i o n . SUMMARY Ore minerals occur almost e x c l u s i v e l y i n veins at the Brenda ore d e p o s i t . These veins have a s p e c i f i c paragenetic o r d e r . Ore forming v e i n s are b i o t i t e , q u a r t z - p o t a s s i u m - f e l d s p a r - s u l f i d e , and quartz-s u l f i d e v e i n s , B i o t i t e veins are the e a r l i e s t . They are t h i n v e i ns c o n t a i n i n g minor yet c o n s i s t e n t amounts of c h a l c o p y r i t e . Predominant o r i e n t a t i o n of these v e i n s i s N62°E/78°S and N76°W/86°S. Quartz-potassium-feldspar v e i n s formed l a t e r , apparently without a s i g n i f i c a n t time gap separating them from the e a r l i e r b i o t i t e v e i n s . Concentrated o r i e n t a t i o n s occur a t an a t t i t u d e of N66°E/78°SE. Vein d e n s i t i e s increase toward the center of the p i t and are the p r i n c i p a l -" r a i s o n d'etre" f o r high grade ore zones. Average v e i n width i s 3/16 of an i n c h . S t r i k e - s l i p o f f s e t s are abundant. The ore m i n e r a l paragenesis i s molybdenite, c h a l c o p y r i t e , p y r i t e , magnetite and hematite, although some r e v e r s a l s do occur. Potassium f e l d s p a r i s the main a l t e r a t i o n type u s u a l l y grading i n t o p r o p y l i t i c a l t e r a t i o n , A t h i n envelope of hydrothermal b i o t i t e might a l s o be p r e s e n t . Stage I I I v e i n s are l a r g e quartz v e i n s c o n t a i n i n g v a r i o u s s u l f i d e s i n s e v e r a l combinations. Banded molybdenite veins and massive c h a l c o -p y r i t e v e ins are the most common. Potassium f e l d s p a r i s normally absent; however, b i o t i t e and a r g i l l i c a l t e r a t i o n are commonly a s s o c i a t -ed. Primary o r i e n t a t i o n s are N69°E/77°SE and N67°W/86°SW. They have i n places the form of sigmoidal l e n s e s . Stage IV v e i n s are post ore and c o n t a i n no s i g n i f i c a n t concen-t r a t i o n of s u l f i d e s . They c o n s i s t of c l i n z o i s i t e v e i n s , b i o t i t e -l i n e d f r a c t u r e s , c a l c i t e and quartz v e i n s . Large epidote ( c l i n o z o i s i t e ) v e i n s overlap i n time of formation w i t h l a t e Stage I I I v e i n s . A l l other Stage IV v e i n s and f r a c t u r e s are s i g n i f i c a n t l y l a t e r and probably d i s t i n c t l y separate from the t h i r d stage of m i n e r a l i z a t i o n . A t t i t u d e s are more widely s c a t t e r e d than i n other stages of m i n e r a l i z a t i o n , A s m a l l 10 percent c o n c e n t r a t i o n occurs w i t h an a t t i t u d e of N66°E/80°SE and N64°W/89°SW. O f f s e t s have not been observed by the w r i t e r . A l l v e i n systems possess remarkably s i m i l a r o r i e n t a t i o n s that do not change s i g n i f i c a n t l y over the p i t a r e a , A s l i g h t progressive change i n a t t i t u d e i s p o s s i b l y i n d i c a t e d f o r Stage I to I I I v e i n s i n c l u s i v e ; Stage I s t r i k e N62°E; Stage I I - N66°E; Stage I I I - N69°E. These s m a l l d i f f e r e n c e s , however, might not be r e a l and could r e s u l t from the r e l a t i v e l y s m a l l amount of data a v a i l a b l e . Stage IV v e i n s are more s c a t t e r e d and have smaller c o n c e n t r a t i o n s , p o s s i b l y due to l a t e r s t r u c t u r a l i n f l u e n c e i n the system long a f t e r the ore forming environment had passed. Hydrothermal a l t e r a t i o n i s l i m i t e d to e n v e l o p e s about v e i n s , except f o r a few l a r g e zones of a r g i l l i c a l t e r a t i o n a s s o c i a t e d w i t h l a r g e Stage I I I v e i n s . Hydrothermal b i o t i t e and p o t a s s i u m f e l d s p a r ( c h i e f l y o r t h o c l a s e ) form about Stage I I v e i n s , w h i l e b i o t i t e a l t e r -a t i o n forms i n v a r y i n g degrees about v e i n s i n Stages I, I I and IV. P r o p y l i t i c a l t e r a t i o n might accompany a l l s t a g e s o f v e i n i n g i n v a r y i n g amounts,although g e n e r a l l y i t too i s l i m i t e d t o e n v e l o p e s about v e i n s . 74 CHAPTER V ZONING MINERAL ZONING At Brenda mine i t i s p o s s i b l e to f i n d a general systematic d i s t r i b u t i o n f o r abundances of c h a l c o p y r i t e , molybdenite, and galena, although no such patterns have been recognized f o r other minerals such as b o r n i t e , s p h a l e r i t e and p y r i t e . Nor have patterns been found f o r the abrupt appearance or disappearance of any of the ore m i n e r a l s . C h a l c o p y r i t e and molybdenite zoning r e s u l t s from a change i n the d e n s i t y of v e i n s across the ore body. Copper and Molybdenum The presence of a c e n t r a l zone of high copper and molybdenum percentages has long been recognized. Because the only s i g n i f i c a n t primary copper and molybdenum minerals are c h a l c o p y r i t e and moly-b d e n i t e , the grade d i s t r i b u t i o n i s d i r e c t l y r e l a t e d to m i n e r a l d i s t r i b u t i o n . Figures 33 and 34 d i s p l a y the zoning p a t t e r n f o r copper and molybdenum grades r e s p e c t i v e l y on the 5210 bench. Cut t i n g s from c l o s e l y spaced 58 f o o t deep r o t a r y d r i l l holes were uniformly sampled and analyzed by atomic a b s o r p t i o n at the mine. Lines of equal abundance were hand contoured. A d i s t i n c t , though i r r e g u l a r c e n t r a l zone of higher grades, e x i s t s i n the center of the ore body. To the west the amount of copper drops q u i c k l y i n an almost l i n e a r zone that c o i n c i d e s roughly w i t h the appearance of l a r g e x e n o l i t h s of N i c o l a Group h o r n f e l s mapped by the w r i t e r . Reductions i n copper and molybdenum take place more g r a d u a l l y to 77 the e a s t . The l i m i t s of the present p i t end i n low grade ore on the eastern margin. The increase of copper and molybdenum assays i n the center of the ore body i s caused mostly by a l a r g e increase i n the d e n s i t y of Stage I I v e i n s accompanied by a s l i g h t i n c r e a s e i n the average width of the v e i n s . Stage I I v e i n s i n the periphyry of the p i t average from 5 to 15 per yard; w h i l e near the center of the p i t , v e i n d e n s i t i e s may be as high as 100 per yard or more i n some l o c a l i t i e s . In a d d i t i o n , a notable increased v a r i a t i o n of a t t i t u d e s of Stage I I v e i n s a l s o occurs i n the c e n t r a l part of the d e p o s i t . Veins w i t h a t t i t u d e s between N35°E to N40°W occur more o f t e n i n areas of higher t o t a l grade. The c o n t r i b u t i o n of Stage I and I I I v e i n s appears to be r e l a t i v e l y l e s s important to the zoning i n the p i t . Indeed Stage I v e i n s seem to decrease i n number i n the center of the deposit r e l a t i v e to the p e r i p h y r y . Lead To date only a small part of the 5110 bench d r i l l holes has been assayed f o r l e a d . Most l e a d i n the mine i s l e s s than 0.005 percent, but small i s o l a t e d patches up to 0.2 percent appear here and t h e r e . Analyses f o r lead on the 5060 and 5110 bench were performed on d r i l l c u t t i n g s i n a s i m i l a r f a s h i o n to copper and molybdenum a n a l y s e s , A s t a t i s t i c a l treatment of 323 values was done i n order to separate p o s s i b l y d i s t i n c t populations according to the methods of Tennant and White (1959), L e p e l t i e r (1969), and S i n c l a i r (1972). The cummulative percent frequencies were p l o t t e d on cummulative l o g 78 p r o b a b i l i t y . Examination of the r e s u l t i n g curve suggests that two lognormal populations are represented i n the d a t a . P o p u l a t i o n R forms 90 percent of the t o t a l , and P o p u l a t i o n S forms 10 percent (Figure 35). R e c a l c u l a t i o n of the values to 100 percent ( f o r each population where not overlapping) d e f i n e s the d i s t i n c t end member po p u l a t i o n (Symbol • i n Figure 35). Because these are two i n t e r p r e t e d lognormal p o p u l a t i o n s , i t i s a u s e f u l check to mix the two i n appropriate proportions f o r comparison w i t h the r e a l d a t a . When mixed i n the r a t i o 10 to 90 ( f o r S and R r e s p e c t i v e l y ) , the r e s u l t i n g curve (Symbol o i n Figure 35) i s seen to match c l o s e l y the r e a l data (Symbol X i n Figure 35). The R p o p u l a t i o n , w i t h a geometric mean of 32 ppm, can be i n t e r p r e t e d as rock background values contained i n K atomic s i t e s i n such minerals as p o t a s h - f e l d s p a r , b i o t i t e , and hornblende. The value compares w i t h a world average of 15 ppm i n intermediate i n t r u s i v e rock (Krauskopf, 1967). The S population probably represents sporadic l o c a l i t i e s of s u l f i d e l e a d , f o r minor amounts of galena have been seen i n the mine. The d i s t r i b u t i o n of these values i s shown i n Figure 36 . The S or assumed s u l f i d e p o p u l a t i o n occurs as i s o l a t e d sporadic h i g h s . Since only a l i m i t e d area of the p i t has yet been sampled, i n t e r -p r e t a t i o n of a p a t t e r n i s h i g h l y u n c e r t a i n . P o s s i b l y f r a c t u r e d e n s i t y might a l s o r e l a t e d i r e c t l y to s u l f i d e l e a d ; and i f so, an expected high i n the center of the p i t should e x i s t , A second p o s s i b i l i t y i s that galena increases away from the center of the deposit?.; however, present data i s too l i m i t e d to confirm or deny 79 Cumulative % (Probability Scale) Figure 35. Cumulative p r o b a b i l i t y p l o t (M) of 323 b l a s t -hole analyses of lead (ppm) from Brenda Mine. Lines R & S are the populations separated from M. Open c i r c l e s represent i d e a l mixing p o i n t s of R.& S i n the proportions 90% R and 10% S. 81 t h i s p o s s i b i l i t y . A d d i t i o n a l analyses are underway at the mine to f u r t h e r examine the d i s t r i b u t i o n of l e ad i n the mine a r e a . Other minerals common i n and about the ore zone are b o r n i t e , magnetite, hematite and p y r i t e . Of these only p y r i t e appears to show even a vague systematic v a r i a t i o n i n abundance, apparently i n c r e a s i n g i n amount to the north and west. MINOR ELEMENT ZONING A study of minor element d i s t r i b u t i o n i n 50 c h a l c o p y r i t e samples from Brenda mine has been undertaken by Dr. D. Brabec. Samples, provided by the w r i t e r , represented the v e i n s of Stages I I and I I I , Samples were analyzed f o r Se, N i , Cd, Mn, Zn, and Ag. The l a t t e r f i v e were analyzed by atomic a b s o r p t i o n spectroscopy, whereas selenium was analyzed by x-ray f l u o r e s c e n c e . Geometric means f o r these elements (lognormal d i s t r i b u t i o n ) were Cd - 31ppm; Mn - 87ppm; N i - 16ppm; Ag - 79 ppm; Zn - 734ppm; and Se - 358ppm. P l o t t e d r e s u l t s do not i n d i c a t e the presence of l a t e r a l systematic v a r i a t i o n s ( i . e . z o n i n g ) . However, sample d i s t r i b u t i o n was e r r a t i c ; and the number of samples was r e l a t i v e l y s mall compared to the s i z e of the p i t . Hence, a p o s s i b i l i t y of other perhaps complicated patterns cannot be r u l e d o u t . S i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s at a p r o b a b i l i t y l e v e l of 0.05 were found w i t h Cd and v e i n width; Cd and Cu/Fe r a t i o and N i and Ag. Negative c o r r e l a t i o n s were found f o r N i and Cu/Fe r a t i o ; Zn and Se; and Se and v e i n w i d t h . Although not a l l have been e x p l a i n e d , cadmium apparently increases i n t h i c k e r v e i n s of Stage I I I type, whereas selenium decreases i n these l a t e r v e i n s compared to Stage I I v e i n s . Except f o r Cd and Ag, the elements show a biomodal d i s t r i b u t i o n on lognormal p r o b a b i l i t y p l o t s (D. Brabec, personal communication, 1972). 83 CHAPTER VI SUMMARY AND CONCLUSIONS GENERAL GEOLOGICAL EVOLUTION OF THE BRENDA DEPOSIT S e v e r a l t e x t u r a l v a r i e t i e s of q u a r t z d i o r i t e s form t h e Brenda Stock t h a t was emplaced i n N i c o l a Group v o l c a n i c r o c k s about 1 7 6 m.y. ago. One phase, the S p e c k l e d q u a r t z d i o r i t e , c o n t a i n s most of the Brenda copper-molybdenum o r e body. S m a l l a p l i t e and p e g m a t i t e d i k e s formed s h o r t l y t h e r e a f t e r , p r o b a b l y d u r i n g a l a t e s t a g e o f t h e c o o l i n g h i s t o r y of the i n t r u s i o n . C o n t r a r y to C a r r ( 1 9 6 7 ) , the w r i t e r found t h a t t h e s e f e l s i c d i k e s , a l t h o u g h more abundant near t h e c e n t r a l p a r t of the p i t , formed i n a l l a r e a s of the o r e body and a r e not c o n f i n e d t o a s t r u c t u r a l b e l t as proposed by C a r r ( 1 9 6 7 ) . F u r t h e r m o r e , t h e s e o r i e n t a t i o n s do not c o r r e s p o n d t o o r e v e i n s or o t h e r younger p l a n a r f e a t u r e s . C o n s e q u e n t l y , the w r i t e r i s i n c l i n e d t o r e l a t e t h e s e f e l s i c d i k e s g e n e t i c a l l y to the Brenda Stock as a l a t e s t a g e r e s i d i u m . T h i s d i s s i m i l a r i t y between f e l s i c ) d i k e s and a l l younger f e a t u r e s a l s o s u p p o r t s the h y p o t h e s i s ( c o n t r a r y to C a r r , 1 9 6 7 ) t h a t a p e r i o d of time , p r o b a b l y l a r g e , e l a p s e d a f t e r t h e i n t r u s i o n o f a p l i t e - p e g m a t i t e d i k e s and b e f o r e f r a c t u r i n g and m i n e r a l i z a t i o n o f the o r e body began. Potassium-argon age d a t i n g d a t a p r e f e r e n t i a l l y s u p p o r t ( i n the w r i t e r ' s view) the h y p o t h e s i s t h a t m i n e r a l i z a t i o n o c c u r r e d a t about 1 4 6 m.y, - w e l l a f t e r the s o l i d i f i c a t i o n of the i n t r u s i v e q u a r t z d i o r i t e a t about 1 7 6 m.y. On t h i s assumption i t appears h i g h l y u n l i k e l y t h a t t h i s d e p o s i t i s a paramagmatic ty p e d e p o s i t , 84 i . e . , "an i n t e g r a l f e a t u r e of a magmatic event." (White e t . a l . , p. 9, 1968). The Brenda ore body was formed by three separate but p a r t i a l l y overlapping p u l s a t i n g phases of m i n e r a l i z a t i o n . An i n c r e a s e i n f r a c t u r e d e n s i t y i n the c e n t r a l part of the p i t causes an i r r e g u l a r zoning of copper and molybdenum grades a t Brenda, There i s , however, an absence of symmetrical l a t e r a l zoning of minerals and elements i n the c l a s s i c a l sense and a s c a r c i t y of hydrothermal a l t e r a t i o n , other than as t h i n envelopes about v e i n s . Because v e i n o r i e n t a t i o n s remain r a t h e r uniform throughout the deposit and because of t h e i r s t r o n g l y p r e f e r r e d o r i e n t a t i o n and s i m i l a r i t y to a i r photo lineament d i r e c t i o n s (Chapman, 1968), the w r i t e r b e l i e v e s that a r e g i o n a l r a t h e r than a l o c a l c o n t r o l caused f r a c t u r i n g that provided channel-ways f o r m i g r a t i n g ore f l u i d s . Although some movement along f r a c t u r e s occurred during m i n e r a l -i z a t i o n as evidenced by minor o f f s e t s of v e i n s , most movement began near the end of the t h i r d stage of m i n e r a l i z a t i o n . Shearing took place i n weakened zones of a r g i l l i c a l t e r a t i o n and f a u l t i n g , both d i p - s l i p and s t r i k e - s l i p ' f o l l o w e d . The f i r s t minor epidote v e i n i n g a l s o took place at the end of the t h i r d stage of m i n e r a l i z a t i o n , but most Stage IV v e i n s developed a f t e r the emplacement of the i n t r a -m i n e r a l d i k e s . Whole rock analyses of two i n t r a m i n e r a l d i k e s i n d i c a t e an age of about 130 m.y. Late T e r t i a r y events are l a r g e l y s p e c u l a t i v e . Movement along shear zones and perhaps some f a u l t s probably continued during the T e r t i a r y but d i d l i t t l e to a f f e c t the ore body, A s m a l l patch of T e r t i a r y (?) conglomerate p o s s i b l y once extended over a l a r g e area 85 but has been l a r g e l y eroded. G l a c i e r s might have eroded ap p r e c i a b l e p o r t i o n s of the ore body. While at the surface and subjected to weathering and weathering processes, features such as the b l a c k c l a y s developed from the combined breakdown of the a r g i l l i z e d zones and the p h y s i c a l t r a n s p o r t of f i n e c l a y s , molybdenite and other opaques downward along accessable f r a c t u r e s . F e r r i m o l y b d i t e , l i m o n i t e , m a l a c h i t e , p o w e l l i t e , and other secondary minerals developed i n a zone of weathering that averaged 20 feet deep, but l o c a l l y a t t a i n e d depths of 90 f e e t . UNUSUAL ASPECTS OF THE BRENDA DEPOSIT The presence or absence of a number of features sets Brenda apart from other l a r g e tonnage, low grade copper and/or molybdenum d e p o s i t s . 1) A w e l l - d e f i n e d s p a t i a l a l t e r a t i o n and ore m i n e r a l sequence i s v i r t u a l l y absent. P o t a s s i c and p r o p y l i t i c a l t e r a t i o n i s l i m i t e d to t h i n envelopes around v e i n w a l l s . Even the highest grade ore can have p r a c t i c a l l y no v i s i b l e a l t e r a t i o n a f f e c t s , whereas some almost barren veins have prominent envelopes up to s i x or e i g h t inches wide. The greatest a l t e r a t i o n e f f e c t , a r g i l l i c a l t e r a t i o n , occurs i n widely spaced planar zones no wider than 35 f e e t and g e n e r a l l y a s s o c i a t e d w i t h a few t h i r d stage veins and s h e a r i n g . What a c o n t r a s t t h i s i s to the a l t e r a t i o n at Climax or Bingham or most l a r g e tonnage deposits one can name. That the deposit a l s o appears to l a c k a c l a s s i c a l z o nation of ore minerals (other than that r e l a t e d to d e n s i t y of f r a c t u r e s ) compounds the d i s p a r i t y . 2) U n l i k e most s i m i l a r d e p o s i t s , disseminated ore m i n e r a l occurrences are extremely l i m i t e d at Brenda. A l l occurrences of disseminated s u l f i d e s observed by the w r i t e r occur i n zones of unusually intense p o t a s s i c a l t e r a t i o n , e i t h e r hydrothermal b i o t i t e or potassium f e l d s p a r . I t i s probable that the l a c k of disseminated m i n e r a l i z a t i o n i s r e l a t e d to the s c a r c i t y of a l t e r a t i o n p a r t i c u l a r l y p o t a s s i c a l t e r a t i o n , 3) Another unusual aspect of the deposit i s the remarkable u n i f o r m i t y of o r i e n t a t i o n of s t r u c t u r e s w i t h time. From the f i r s t phase of m i n e r a l i z a t i o n through s h e a r i n g , f a u l t i n g , and d i k i n g to the l a s t phase of m i n e r a l i z a t i o n only two o r i e n t a t i o n s p e r s i s t . The dominant o r i e n t a t i o n s t r i k e s N60-70°E,and the second o r i e n t a t i o n s t r i k e s N55-65°W. Both have steep sou t h e r l y d i p s . 4) A l s o d i f f e r i n g from most "Porphyry Copper" or "Stockwork-moly" deposits i s the approximately equal economic grades of copper and molybdenum a t Brenda, Most d e p o s i t s of t h i s type predominate i n e i t h e r copper or molybdenum but r a r e l y b o t h . Rhenium values apparently support t h i s "middle of the road" s t a t u s of Brenda. The rhenium content of molybdenite concentrates are about 120 to 180 ppm a t Brenda, whereas most porphyry copper d e p o s i t s have high values ( E l y - 1650 ppm, Chino - 800 ppm, Bingham Canyon - 360 ppm); and stockwork molybdenum dep o s i t s are g e n e r a l l y q u i t e low i n rhenium content (Questa - 12 ppm, Climax - 3 ppm), (Sutulov, 1970). 87 PROBLEMS FOR FUTURE INVESTIGATION A p a r t i c u l a r l y important problem revealed by t h i s t h e s i s i s the need f o r a d d i t i o n a l potassium-argon analyses f o r f i n a l determination of the ages of the quartz d i o r i t e host and ore m i n e r a l i z a t i o n . Present data i n d i c a t e s that m i n e r a l i z a t i o n occurred at a maximum of 146 m.y. and p o s s i b l y as l a t e as 130 m.y. I t i s c r i t i c a l to analyze b i o t i t e s t r i c t l y r e l a t e d to ore m i n e r a l i z a t i o n (Stages I , I I , or I I I ) being c a r e f u l not to i n c l u d e Stage IV b i o t i t e a l t e r a t i o n that a l r e a d y i s i n d i c a t e d to be 130 m.y. In a d d i t i o n , f u r t h e r samples from e n t i r e l y unmineralized areas and from s e l e c t e d i n t r a m i n e r a l d i k e s are needed f o r c o n f i r m a t i o n . C a r e f u l r e g i o n a l mapping i s needed to t i e the s t r u c t u r a l and petrographic h i s t o r y i n t o a r e g i o n a l p i c t u r e . Too l i t t l e i s known about the Brenda Stock. This v i c i n i t y has been termed by some to be part of the Pennask b a t h o l i t h (Schau, 1970) and by others to be part of the Similkameen b a t h o l i t h (Peto, 1970). I n t e r n a l d e t a i l s such as phase r e l a t i o n s and contacts need c a r e f u l mapping, f o r the w r i t e r has found that the r e l a t i o n s between v a r i e t i e s of quartz d i o r i t e might be much more complex than p r e v i o u s l y mapped. C a r e f u l a t t e n t i o n should a l s o be paid to secondary s t r u c t u r a l f e a t u r e s such as v e i n s , d i k e s f a u l t s , and shears i n order to t r y to r e l a t e them to f e a t u r e s observed w i t h i n the Brenda ore body. In a d d i t i o n , the N i c o l a Group rocks are l i t t l e understood i n the v i c i n i t y of Brenda and deserve c o n s i d e r a t i o n . A p r o j e c t f o r f u t u r e c o n s i d e r a t i o n i s an element zoning study from b l a s t - h o l e d a t a . Brenda i s p r e s e n t l y beginning to s t o r e copper and molybdenum concentrate from each b l a s t hole (D. F. W h i t f o r d , Brenda Mines, personal communication). Considering the t i g h t 88 p a t t e r n s of b l a s t h o l e s , i t might be p o s s i b l e a t a l a t e r d a t e t o study b o t h l a t e r a l and v e r t i c a l z o n i n g w i t h thousands o f samples a v a i l a b l e f o r a n a l y s e s by x - r a y f l u o r e s c e n c e . I t has been suggested by Dr. S o r e g a r o l i ( p e r s o n a l communication) t h a t because o f the l i m i t e d a l t e r a t i o n p r e s e n t a t Brenda mine, i t might be much more an/eniable f o r d e t a i l e d study than d e p o s i t s where a l t e r a t i o n i s p e r v a s i v e and o v e r l a p p i n g . The w r i t e r b e l i e v e s t h a t a d e t a i l e d s t u d y of t h e n a t u r e , c h e m i s t r y , p e t r o l o g y and i n t e r d e p e n d e n c e of the w a l l r o c k a l t e r a t i o n assemblages would be a v a l u a b l e c o n t r i b u t i o n . 89 BIBLIOGRAPHY C a r r , J.M. 1967. Geology of the Brenda Lake Area. B.C. Bureau Mines. Mines and P e t . Res. Report, pp. 183-210. Chapman, Wood and G r i s w o l d , L t d * 1966-1967. Brenda F e a s i b i l i t y Study. Brenda Mines L t d . Chapman, E.P. 1968. Geology of the Brenda Molybdenum and Copper De p o s i t , Canadian I n s t . Min. M e t a l , B u l l , A p r i l , 1968. Vancouver, B.C. Drummond, A.W. and K i n u r a , E.T, 1969. Hydrothermal A l t e r a t i o n at Endako - A Comparison to Experimental S t u d i e s . Canadian I n s t . M in. M e t a l l . B u l l . V o l . 62, p. 707-714. Fountain, D.K. 1968. The A p p l i c a t i o n of the Induced P o l a r i z a t i o n Method at Brenda Mine, B.C. I n s t . Min, M e t a l . B u l l . V o l . 61, No. 670, pp. 153-157. Hawley, J . E , and N i c h o l , I . 1959. Selenium i n Some Canadian S u l f i d e s . Econ. Geology. V o l . 54, No. 4, pp. 608-628. Kirkham, R.V. 1971, I n t r a m i n e r a l i n t r u s i o n and t h e i r Bearing on the O r i g i n of Porphyry Copper and Molybdenum D e p o s i t s . V o l . 66. No. 8, pp. 244-249. Krauskopf, K.B. 1967, I n t r o d u c t i o n to Geochemistry, McGraw-Hill, New York. p. 721. L e p e l t i e r , C. 1969, A S i m p l i f i e d S t a t i s t i c a l Treatment of Geochemical Data by Gr a p h i c a l R epresentation. Econ. Geology. V o l . 64, pp. 538-550. L i t t l e , H.S. 1961. V e t t l e River (West H a l f ) . G e o l . Survey Canada. Map 15. Menzies, M.M, 1969. The Brenda P r o j e c t . Canadian Mining J o u r n a l , pp. 120-124. Neyer, C. and Hemley, J . J , 1967, Wall Rock A l t e r a t i o n i n Barnes, H.L. (ed,), Geochemistry of Hydrothermal Ore D e p o s i t s . New York. H o l t , R i n e h a r t , and Winston, pp. 166-235. Nasmith, H, 1962, Late G l a c i a l H i s t o r y and S u r f i c i a l Deposits of the Okanagan V a l l e y , B.C. B. C. Dept. of Mines, B u l l . 46. Ney, C S . 1956. Unpublished company r e p o r t . Northwestern E x p l o r a t i o n , L t d , 90 Ney, C.S. 1957, G e o l o g i c a l and Geophysical Report on the Brenda Prospect. Osoyoos Mining D i v i s i o n , B.C. Assessment Report No. 189. Peto, P. 1970. Unpublished M.Sc. T h e s i s , Univ. of A l b e r t a . R i c e , H.M.A. 1947. P r i n c e t o n Map-area. G e o l . Surv. Canada. Mem 243. Schau, M. 1970. S t r a t i g r a p h y and S t r u c t u r e of the type area of the upper T r i a s i c N i c o l a Group jln South-Central B r i t i s h Columbia. G e o l . Assoc. Canada. S p e c i a l Paper No. 6. S t r u c t u r e of the • Southern Canadian C o r d i l l e r a , p. 123. S i n c l a i r , A . J . 1972. Some S t a t i s t i c a l A p p l i c a t i o n s to Problems i n M i n e r a l E x p l o r a t i o n . B. C. P r o f e s s i o n a l Engineering. March, 1972. S o r e g a r o l i , A.E. 1968. Geology of the Brenda Copper Molybdenum Depo s i t , B r i t i s h Columbia. Paper presented a t Northwest Mining Assoc., Spokane, Washington. S o r e g a r o l i , A.E. 1971. Geology of the Brenda Copper Molybdenum D e p o s i t , B r i t i s h Columbia. Paper presented at Quebec C i t y , IMM 73rd Annual General Meeting. Sutulov, A, 1970. Molybdenum and Rhenium recovery from porphyry coppers. Univ. Concepcion, C h i l e , p. 259. Tennant, C.B. and White, M.L. 1959. Study of the D i s t r i b u t i o n of some Geochemical d a t a . Econ. Geology. V o l , 54, pp. 1281-1290. Vogan, R.S, 1966, G e o l o g i c a l Report — - F e a s i b i l i t y P e r i o d . Brenda Mines, L t d . . Wallace, S.R., Muncaster, N.K., Jonson, D.C., MacKencie, W.B., Brookstrom, A,A. and Surfac e , V.G. 1968, M u l t i p l e I n t r u s i o n and M i n e r a l i z a t i o n at Climax, Colorado i n Ridge, J.H. ( e d . ) . Ore Deposits i n the United S t a t e s , Graton-Sales, New York. Amer. I n s t . M i n, Eng. V o l , 1, pp. 605-640. White, W.H., H a r a k a l , J.E., and C a r t e r , N.C, 1968, Potassium-Argon ages of some ore deposits i n B r i t i s h Columbia. Canadian I n s t . M i n . M e t a l l . B u l l . V o l . 61, pp. 1326-1334. White, W.H,, and H a r a k a l , J.E, 1972. Personal Communication on unpublished Potassium Argon Age Data from the Brenda Mine Dep o s i t . 91 APPENDIX INDUCED POLARIZATION The f i r s t s u c c e s s f u l Induced P o l a r i z a t i o n surveys i n B r i t i s h Columbia were completed at the Brenda Mine i n 1966, The r e s u l t s were published by Fountain i n 1968, Although the estimated t o t a l s u l f i d e s were only 0.5 to 1 percent, s i g n i f i c a n t anomalies out-l i n i n g the ore body were found. The low c o n s i s t e n t background of the q u a r t z - d i o r i t e , probably due p r i m a r i l y to the confinement of a l t e r a t i o n to narrow envelopes along v e i n s , enabled the low but anomalous values to be recognized (Figure 37). Low ^ o v e r a l l p y r i t e content i n the veins a l s o meant that the anomalies might be c o r r e l a t e d more s p e c i f i c a l l y to ore-forming c h a l c o p y r i t e and thus g i v i n g a more p r e c i s e o u t l i n e of the concentrated m i n e r a l i z a t i o n . Fountain (1968) presents a d e t a i l e d d e s c r i p t i o n of procedure and a n a l y s i s i n h i s paper. SOIL GEOCHEMISTRY In c o n j u n c t i o n w i t h the Induced P o l a r i z a t i o n program, a s o i l geochemistry program was undertaken i n 1966 to serve as a guide to d r i l l e x p l o r a t i o n of the property (Chapman, Wood and G r i s w o l d , L t d . , 1967), ( S o r e g a r o l i , 1968 and 1971). R e s u l t s show anomalous copper and molybdenum values (> 200ppm copper and > 20 ppm molybdenum) w i t h s i m i l a r d i s t r i b u t i o n s that o u t l i n e i n a general way the ore body (Figures 38 and 39) ( S o r e g a r o l i , 1968 and 1971). Figure 38. Copper Geochemistry Anomaly ( A f t e r S o r e g a r o l i , 1968) N 4 1000' > 200 ppm Concentrated m i n e r a l i z a t i o n ON Co Figure 3 9 , Molybdenum Geochemistry Anomaly ( A f t e r S o r e g a r o l i , 1968) N 1000' • ^ 20 ppm Concentrated m i n e r a l i z a t i o n 

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