UBC Theses and Dissertations

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UBC Theses and Dissertations

A genetic model for alpine-type chromite deposits in the Josephine peridotite, Northwest California and… Grond, Helen C. 1982

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- p A G E N E T I C M O D E L F O R A L P I N E - T Y P E C H R O M I T E D E P O S I T S I N T H E J O S E P H I N E P E R I D O T I T E , N O R T H W E S T C A L I F O R N I A A N D S O U T H W E S T O R E G O N by H e l e n C . G r o n d B S c . U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1 9 8 0 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E i n t h e D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A M a y 1 9 8 2 c © Helen C. Grond 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) ABSTRACT The J o s e p h i n e P e r i d o t i t e o f n o r t h w e s t C a l i f o r n i a and s o u t h w e s t O r e g o n i s h o s t t o numerous a l p i n e - t y p e c h r o m i t e d e p o s i t s . The d e p o s i t s were c l a s s i f i e d i n t o two g r o u p s a c c o r d i n g t o t e x t u r a l v a r i e t i e s , h o s t r o c k a s s o c i a t i o n s , d e g r e e o f d e f o r m a t i o n and r a t i o o f d u n i t e t o c h r o m i t e . T h e s e i n c l u d e : c l a s s I ( c u m u l a t e c h r o m i t e ) and c l a s s I I ( d e f o r m e d c h r o m i t e ) . C l a s s I d e p o s i t s a r e c h a r a c t e r i z e d by r e l a t i v e l y u n d e f o r m e d c u m u l a t e t e x t u r e s , d u n i t e h o s t r o c k s and h i g h d u n i t e / c h r o m i t e r a t i o s ( 9 5 : 5 ) . C l a s s I I d e p o s i t s g e n e r a l l y o c c u r a s m a s s i v e t o d i s s e m i n a t e d , h i g h l y d e f o r m e d p o d s and l e n s e s . They o c c u r w i t h i n s h e a r z o n e s i n h a r z b u r g i t e s and have a v e r y low d u n i t e / c h r o m i t e r a t i o ( 1 0 : 9 0 ) . F i e l d e v i d e n c e from s h e a r z o n e s w i t h i n c l a s s I t y p e d e p o s i t s i n d i c a t e s t h a t t h e d e f o r m a t i o n o f c u m u l a t e c h r o m i t e may r e s u l t i n t h e f o r m a t i o n o f c l a s s I I d e p o s i t s . T h i s e v i d e n c e i n c l u d e s t h e o c c u r r e n c e o f s m a l l m a s s i v e p o d s and b l o c k s o f d e f o r m e d c u m u l a t e c h r o m i t e w h i c h a p p e a r t o be d e r i v e d f r o m t h e s u r r o u n d i n g c u m u l a t e c h r o m i t e b u t h a v e many s i m i l a r i t i e s w i t h c l a s s I I d e p o s i t s . A g e n e t i c model i s p r o p o s e d i n w h i c h c h r o m i t e o r i g i n a l l y f o r m e d b y c u m u l a t e p r o c e s s e s ( c l a s s I d e p o s i t s ) h a s u n d e r g o n e i n t e n s i v e s o l i d s t a t e d e f o r m a t i o n i n t h e u p p e r m a n t l e . T h i s r e s u l t e d i n m e t a m o r p h i c d i f f e r e n t i a t i o n b e t w e e n c h r o m i t e a n d o l i v i n e , p r o d u c i n g t h e m a s s i v e c o n c e n t r a t i o n s o f c h r o m i t e ( c l a s s I I d e p o s i t s ) . i v T A B L E O F C O N T E N T S 1 . I N T R O D U C T I O N 1 1 .1 G e n e r a l S t a t e m e n t 1 1 . 2 P u r p o s e O f S t u d y A n d F i e l d W o r k 2 1 . 3 P r e v i o u s W o r k A n d C h r o m i t e M i n i n g H i s t o r y 4 1 . 4 L o c a t i o n O f S t u d y A r e a s 5 2 . G E O L O G I C S E T T I N G . . 8 1 .1 K l a m a t h M o u n t a i n s 8 2 . 2 T h e J o s e p h i n e P e r i d o t i t e A n d O u t l y i n g E x t e n s i o n s 11 3 . G E O L O G Y O F T H E C H R O M E R I D G E A R E A 17 3 . 1 I n t r o d u c t i o n 17 3 . 2 P e t r o l o g y 19 3 . 2 . 1 . D u n i t e 21 3 . 2 . 2 . H a r z b u r g i t e 22 3 . 2 . 3 . C h r o m i t i f e r o u s D u n i t e 23 3 . 3 S t r u c t u r e O f T h e C h r o m e R i d g e A r e a 24 3 . 3 . 1 . F a u l t i n g A n d F o l d i n g 27 4 . C H R O M I T E D E P O S I T S 2 9 4 . 1 I n t r o d u c t i o n 2 9 4 . 2 D e s c r i p t i o n O f C h r o m i t e T e x t u r e s . 30 4 . 2 . 1 . C u m u l a t e T e x t u r e s 30 4 . 2 . 2 . N o n - c u m u l a t e C h r o m i t e 37 4 . 3 C l a s s i f i c a t i o n O f C h r o m i t e C o n c e n t r a t i o n s 40 4 . 4 C h r o m i t e D e p o s i t s O f T h e C h r o m e R i d g e A r e a 4 5 V 4.4.1. I n t r o d u c t i o n 45 4.4.2. B u s t e r M i n e 46 4.4.3. V i o l e t M i n e 47 4.4.4. Shady Cove Mine 47 5. OTHER CHROMITE OCCURRENCES IN THE JOSEPHINE PERIDOTITE 49 5.1 The Bar R i c k M i n e 49 5.2 The T y s o n Mine 50 5.3 Low D i v i d e 51 6. ORIGIN OF ALPINE ULTRAMAFIC ROCKS 54 7. ORIGIN OF CHROMITE 59 7.1 C o n c e n t r a t i o n Of C h r o m i t e By D e f o r m a t i o n 64 7.2 F i e l d E v i d e n c e " 69 8. CONCLUSIONS 73 v i LIST OF FIGURES Figure 1. D i s t r i b u t i o n of Josephine P e r i d o t i t e and l o c a t i o n 3 of study areas. Figure 2. Regional geology showing d i s t r i b u t i o n of the four 10 l i t h o t e c t o n i c b e l t s . Figure 3. Section through the Josephine O p h i o l i t e 13 Figure 4. C l a s s i f i c a t i o n of p e r i d o t i t e 14 Figure 5. Geologic map of the Chrome Ridge Area 19 Figure 6. Geologic map of Chrome Ridge JbrPT~ Figure 7. Ink t r a c i n g showing composite nature of the layers 26 and gradational edges. Figure 8a. Antinodular ore..... 26 Figure 8b. Nodular chromite showing pu l l - a p a r t textures 26 Figure 9. Plan-view sketch of Low-Divide area 52 Figure 10. SEM energy dispersive data showing chemical 62 homogeneity between core and rim of deformed chromite grain. Figure 11. Idealized sketch of t y p i c a l dunite hosted shear zone...71 LIST OF TABLES Table 1. C l a s s i f i c a t i o n of chromite deposits i n the Josephine...43 P e r i d o t i t e . LISTS OF PLATES Plate 1. Serpentinized dunite i s strongly f o l i a t e d i n upper 22 half of photo but has an i s o l a t e d block of u n f o l i a t e d dunite i n lower h a l f . P l a t e 2. Weathered harzburgite showing nubbly surface due to....22 r e s i s t a n t pyroxene grains. v i i P l ate 3. Euhedral and subhedral chromite grains i n serpentine 31 matrix. Plate 4. Layers of cumulate chromite i n serpentinized dunite 31 Plate 5. Disseminated chromite from c l a s s I I deposit showing 32 strong f o l i a t i o n d i r e c t i o n marked by clumps of elongate chromite grains. Plate 6. Microphotograph from same sample as p l a t e 5. showing 32 i r r e g u l a r boundaries of elongate chromite grain. Plate 7. Nodular chromite from Shady Cove Mine 34 Plate 8. I s o c l i n a l l y folded chromite from'Seiad ultramafic body...34 Plate 9. Cumulate net textures occur i n 10 cm t h i c k layers at 36 Low Divide. Plate 10. Chromite breccia from a shear zone at Low Divide has 36 well-developed v e r t i c a l f a b r i c formed by elongation of dunite c l o t s and surrounding chromite. Plate 11. A small massive chromite pod from a shear zone at 37 Chrome Ridge. Plate 12. A s e r i e s of small pods connected by chromite stringers...37 from a highly serpentinized shear zone at Chrome Ridge. Plate 13. C a t a c l a s t i c textures i n a -massive chromite pod from 39 the Tyson. Plate 14. Massive chromite from Tyson deposit i s greater than 39 95% chromite. Homogeneous colour indicates chemical homogeneity. Plate 15. Crosscutting seams of chromite from a small (50 meter)...40 dunite body near Low Divide. Plate 16. Irregular chromite body from the same l o c a l i t y as 40 p l a t e 15. Plate 17. Layered t e c t o n i t e showing dunite l a y e r with c e n t r a l 66 chromite seam. v i i i A C K N O W L E D M E N T S I s i n c e r e l y t h a n k D r . J . H . M o n t g o m e r y f o r i n s t i g a t i n g t h e p r o j e c t a n d p r o v i d i n g a d v i c e a n d e n c o u r a g m e n t t h r o u g h o u t t h e c o u r s e o f t h e s t u d y . I am g r a t e f u l t o D r . K . C . M c T a g g a r t f o r t h e c o n s i d e r a b l e a m o u n t o f t i m e a n d p a t i e n c e h e d o n a t e d w h i l e o v e r s e e i n g t h e p r o j e c t . T h a n k s t o D r . C . I . G o d w i n f o r h i s c o n s t a n t e n t h u s i a s m a n d e n l i g h t e n i n g i d e a s . A p p r e c i a t i o n i s e x t e n d e d t o D r . R . L . A r m s t r o n g , D r . R a n d y P a r r i s h a n d G e o r g e G o r z y n s k i f o r c r i t i c a l l y r e a d i n g t h e t h e s i s . S p e c i a l t h a n k s t o N i c k B a r r a n d D i t a R u n k l e f o r a s s i s t a n c e i n t h e f i e l d a n d c o n t i n o u s e n c o u r a g m e n t t h r o u g h a l l p h a s e s o f t h e s t u d y . M u c h g r a t i t u d e i s e x t e n d e d t o D e l N o r t e C h r o m e C o r p . f o r p r o v i d i n g t h e o p p o r t u n i t y t o c o m p l e t e t h e p r o j e c t a n d a l s o f o r f i n a n c i a l a s s i s t a n c e i n t h e f o r m o f a g r a n t . 1 1 . I N T R O D U C T I O N  1 .1 G e n e r a l S t a t e m e n t T h e p r o b l e m s i n v o l v i n g o r i g i n a n d e m p l a c e m e n t o f a l p i n e p e r i d o t i t e s h a v e b e e n g r e a t l y d i m i n i s h e d w i t h t h e u n d e r s t a n d i n g g a i n e d t h r o u g h t h e a d v a n c e m e n t o f p l a t e t e c t o n i c t h e o r y . W i d e s p r e a d o p i n i o n f a v o u r s a m o d e l i n w h i c h o b d u c t i o n o f o c e a n i c c r u s t a n d u p p e r m a n t l e r e s u l t s i n t h e e m p l a c e m e n t o f o p h i o l i t e a s s e m b l a g e s a l o n g o r o g e n i c b e l t s . T h e a l p i n e p e r i d o t i t e s , w h i c h r e p r e s e n t t h e d e e p e s t s e c t i o n o f t h e o p h i o l i t e s e q u e n c e ( F i g u r e 3 ) , a r e w i d e l y b e l i e v e d t o b e t h e r e f r a c t o r y r e s i d u u m r e s u l t i n g f r o m p a r t i a l m e l t i n g o f p r i m i t i v e m a n t l e m a t e r i a l a t a s p r e a d i n g r i d g e . M a n y o f t h e e a r l y p r o b l e m s a s s o c i a t e d w i t h t h e o r i g i n o f a l p i n e - t y p e c h r o m i t e d e p o s i t s w i t h i n t h e p e r i d o t i t e s t i l l p e r s i s t . A t r e m e n d o u s v a r i a b i l i t y i n t h e d e s c r i p t i v e c h a r a c t e r i s t i c s o f t h e d i f f e r e n t d e p o s i t s h a s r e s u l t e d i n t h e p r o p o s a l o f s e v e r a l d i f f e r e n t g e n e t i c m o d e l s . I n c l u d e d w i t h i n t h e s e a r e h y d r o t h e r m a l ( S a m p s o n 1 9 2 9 ) , p r i m a r y m a g m a t i c ( T h a y e r 1 9 6 9 ) l a t e s t a g e m a g m a t i c ( D i c k 1 9 7 5 ) a n d m e t a s o m a t i c ( D u n g a n a n d A v e ' L a l l e m a n t 1 9 7 7 ) . T h e k e y t o e x p l a i n i n g c h r o m i t e d e p o s i t s , i s a b e t t e r u n d e r s t a n d i n g o f m a n t l e p r o c e s s e s i n u l t r a m a f i c r o c k s . I n t h i s s t u d y , " o r e " i s u s e d i n t h e g e o l o g i c a l r a t h e r 2 t h a n t h e e c o n o m i c s e n s e , t o r e f e r t o z o n e s e n r i c h e d i n c h r o m i t e . 1 . 2 P u r p o s e O f S t u d y A n d F i e l d W o r k T h e p u r p o s e o f t h e s t u d y w a s t o g a i n a b e t t e r u n d e r s t a n d i n g o f t h e c h a r a c t e r i s t i c s a n d v a r i e t y o f c h r o m i t e d e p o s i t s a n d p r o v i d e a g e n e t i c m o d e l w h i c h c o u l d a d e q u a t e l y e x p l a i n f i e l d o b s e r v a t i o n s . I t w a s f e l t t h a t f i e l d e x a m i n a t i o n s o f t e x t u r a l v a r i e t i e s a n d s t r u c t u r a l r e l a t i o n s b e t w e e n o r e a n d h o s t r o c k s w o u l d p r o v i d e t h e n e c e s s a r y d a t a f o r a c o m p r e h e n s i v e c l a s s i f i c a t i o n o f c h r o m i t e d e p o s i t s i n t h e J o s e p h i n e P e r i d o t i t e . T h e e x i s t e n c e o f s u c h a c l a s s i f i c a t i o n s c h e m e w o u l d a i d i n t h e d e v e l o p m e n t o f o n e o r s e v e r a l m o d e l s o f o r i g i n w h i c h w o u l d s e r v e t o e x p l a i n t h e d i f f e r e n t t y p e s o f c h r o m i t e d e p o s i t s o b s e r v e d . F i e l d w o r k w a s c a r r i e d o u t d u r i n g t h e 1 9 8 0 a n d 1981 f i e l d s e a s o n s i n t h e J o s e p h i n e P e r i d o t i t e i n t h e K l a m a t h M o u n t a i n s o f n o r t h w e s t C a l i f o r n i a a n d s o u t h w e s t O r e g o n ( F i g u r e 1 ) . T e r r a i n w i t h i n t h e K l a m a t h M o u n t a i n s i s g e n e r a l l y r u g g e d . A l a c k o f v e g e t a t i o n o v e r u l t r a m a f i c r o c k s p r o v i d e d e x c e l l e n t e x p o s u r e . A l l a r e a s v i s i t e d w e r e a c c e s s i b l e b y f o u r - w h e e l d r i v e v e h i c l e s . 4 1 . 3 P r e v i o u s W o r k A n d C h r o m i t e M i n i n g H i s t o r y T h e r e g i o n a l g e o l o g y o f t h e a r e a w a s f i r s t s t u d i e d b y H e r s h e y ( 1 9 0 1 ) , w h o o u t l i n e d t h e m a i n r o c k u n i t s . R e c o n n a i s s a n c e s t u d i e s b y I r w i n ( 1 9 6 0 , 1 9 6 6 ) , w e r e c a r r i e d o u t i n t h e K l a m a t h M o u n t a i n s i n C a l i f o r n i a , a n d l a t e r i n O r e g o n , w h e r e t h e d i v i s i o n i n t o m a j o r g e o l o g i c b e l t s w a s f i r s t e s t a b l i s h e d . D a v i s e t a l . ( 1 9 6 5 ) a n d K a y s ( 1 9 6 8 ) s u m m a r i z e d t h e m e t a m o r p h i c a n d p l u t o n i c h i s t o r y o f t h e K l a m a t h M o u n t a i n s a n d d i s c u s s e d t h e t e c t o n i c c o r r e l a t i o n s . D e t a i l e d s t u d i e s o n t h e p e t r o g e n e s i s o f t h e p e r i d o t i t e s w i t h i n a n d a d j a c e n t t o t h e J o s e p h i n e c o m p l e x w e r e c o m p l e t e d b y M e d a r i s ( 1 9 7 2 , 1 9 7 0 ) , H i m m e l b e r g a n d L o n e y ( 1 9 7 3 ) , L o n e y , H i m m e l b e r g a n d C o l e m a n ( 1 9 7 1 ) , a n d L o n e y a n d H i m m e l b e r g ( 1 9 7 6 ) . C o l e m a n ( 1 9 7 2 , 1 9 7 6 ) p r o v i d e d i n s i g h t o n t h e t e c t o n i c e v o l u t i o n o f t h e a r e a . D i c k ( 1 9 7 5 ) a n d E l l i s ( 1 9 7 7 ) d e a l w i t h t h e p e t r o l o g i c a s p e c t s o f t h e J o s e p h i n e C o m p l e x a n d c o m p l e t e d d e t a i l e d w o r k o n t h e c h r o m i t e d e p o s i t s . C h r o m i t e s t u d i e s i n t h e J o s e p h i n e P e r i d o t i t e a r e c l o s e l y a s s o c i a t e d w i t h g o v e r n m e n t s t o c k p i l i n g p r o g r a m s d u r i n g b o t h t h e f i r s t a n d s e c o n d W o r l d W a r s . D i l l e r ( 1 9 2 1 ) s u m m a r i z e d t h e g e o l o g y o f t h e c h r o m i t e d e p o s i t s e x p l o i t e d d u r i n g W o r l d W a r I . M a x o n ( 1 9 3 3 ) e x a m i n e d t h e m i n e r a l r e s o u r c e s i n p o r t i o n s o f D e l N o r t e a n d S i s k i y o u c o u n t i e s w h i l e J o h n s t o n ( 1 9 3 6 ) m a d e d e t a i l e d d e s c r i p t i o n s o f 5 c h r o m i t e t e x t u r e s . E x t e n s i v e r e c o n n a i s s a n c e s t u d i e s d u r i n g t h e 1940's were u n d e r t a k e n by v a r i o u s government a g e n c i e s t o d e t e r m i n e r e s e r v e s and p o t e n t i a l . T h e s e i n c l u d e : W e l l s e t a l . ( 1 9 4 0 ) , A l l e n ( 1 9 4 1 ) , W e l l s e t a l . ( 1 9 4 6 ) , W e l l s e t a l . ( 1 9 4 9 ) , W e l l s and C a t e r ( 1 9 5 0 ) , W e l l s and W a l k e r ( 1 9 5 3 ) , and C a t e r and W e l l s ( 1 9 5 4 ) . A summary of c h r o m i t e o c c u r r e n c e s i n s o u t h w e s t e r n O r e g o n by Ramp (1961) p r o v i d e s i n f o r m a t i o n on t h e g e o l o g y and m i n i n g h i s t o r y o f t h e d e p o s i t s . A l a r g e number o f s m a l l c h r o m i t e d e p o s i t s , p a r t i c u l a r i l y m a s s i v e d e f o r m e d c h r o m i t e d e p o s i t s , had been mined i n t h e a r e a d u r i n g s t o c k p i l i n g p r o g r a m s . Most o p e r a t i o n s were s m a l l a n d g e n e r a l l y i n v o l v e d one t o two p e o p l e . E x a c t f i g u r e s on c h r o m i t e p r o d u c t i o n from t h i s a r e a a r e n o t a v a i l a b l e . A summary o f U.S. c h r o m i t e p r o d u c t i o n by T h a y e r (1973) i n d i c a t e s t h a t t o t a l p r o d u c t i o n f r o m a l l p o d i f o r m c h r o m i t e d e p o s i t s i n t h e U n i t e d S t a t e s was 900,000 t o n s . T h i s i n c l u d e s c h r o m i t e o c c u r r e n c e s i n C a l i f o r n i a , O r e g o n , P e n n s y l v a n i a , M a r y l a n d and A l a s k a . 1.4 L o c a t i o n Of S t u d y A r e a s The main s t u d y s i t e , known as t h e Chrome R i d g e a r e a , o c c u r s i n a n o r t h e r n o u t l y i n g e x t e n s i o n o f t h e J o s e p h i n e P e r i d o t i t e . The a r e a mapped o c c u p i e s a b o u t e i g h t s q u a r e 6 kilometers in Township 36S, Range 9W, Josephine county, Oregon. Access i s via 32 km of paved forest service road leading southwest from the town of Galice. During winter months the road i s made inaccessible by snow. Topography of the Chrome Ridge area i s rugged with an average elevation of 1200 m. Chrome Ridge i t s e l f trends north through the center of the area and forms a drainage divide between Briggs Creek to the east and Todd Creek to the west. Figure 5 shows the boundaries of the mapped area. Other areas studied in the Josephine P e r i d o t i t e include the Tyson Mine, the Bar-Rick Mine and the Low-Divide area. Their locations are shown in Figure 1. The Tyson Mine i s located on the north side of Craig's Creek in Del Norte county, C a l i f o r n i a . The deposit is at an elevation of 460 m and is accessible by road from Gasquet, C a l i f o r n i a . The French H i l l road (county road #411) leaves Highway 199 at a point about 3 km west of Gasquet "and runs southwest approximately 5 km to the turnoff for the Tyson Mine. The Low Divide area i s located west of Stone Corral in Del Norte County, C a l i f o r n i a at an elevation of 450 m. The area i s accessible by the Wimer Road which leaves Highway 101 at Smith River, C a l i f o r n i a and then continues to the area, a distance of 15 km. The Bar Rick mine i s situated approximately 25 km, southeast of Crescent City, in Del Norte County, C a l i f o r n i a . The area, which i s at an average elevation of 7 700 m i s crossed by L i t t l e Rattlesnake Creek. Access to the Bar Rick Mine i s by 25 km of paved highway followed by 12 km of gravel road. i 8 2 . G E O L O G I C S E T T I N G  1 .1 K l a m a t h M o u n t a i n s T h e K l a m a t h M o u n t a i n s h a v e b e e n s u b d i v i d e d i n t o a s e r i e s o f f o u r c o n v e x - w e s t w a r d , e a s t w a r d ^ d i p p i n g l i t h o -t e c t o n i c b e l t s ( I r w i n 1 9 6 0 , 1 9 6 4 ) ( F i g u r e 2 ) . T h e b o u n d a r i e s b e t w e e n t h e b e l t s c o n s i s t o f e a s t w a r d d i p p i n g t h r u s t f a u l t s . F r o m e a s t t o w e s t , t h e b e l t s i n c l u d e t h e E a s t e r n K l a m a t h , t h e C e n t r a l M e t a m o r p h i c , t h e W e s t e r n P a l e o z o i c a n d T r i a s s i c , a n d t h e W e s t e r n J u r a s s i c . T h r e e o f t h e s e f o u r b e l t s a r e i n t e r p r e t e d a s h a v i n g s t r a t i g r a p h i c a n d s t r u c t u r a l c o u n t e r p a r t s i n t h e n o r t h w e s t e r n S i e r r a N e v a d a ( D a v i s , 1 9 6 9 ) . T h e E a s t e r n K l a m a t h B e l t , c o n s i s t i n g o f e u g e o c l i n a l a n d s h a l l o w - w a t e r s e d i m e n t s , i s c o n s i d e r e d t o r e p r e s e n t a D e v o n i a n - J u r a s s i c i s l a n d a r c w i t h a s s o c i a t e d s e d i m e n t s t h a t d e v e l o p e d o n a n u l t r a m a f i c b a s e m e n t ( B u r c h f i e l a n d D a v i s 1 9 7 2 ) . T h e C e n t r a l M e t a m o r p h i c B e l t i s r e g a r d e d a s a t h r u s t s l i c e o f D e v o n i a n ( ? ) m e t a m o r p h i c r o c k s r e l a t e d t o s u b d u c t i o n , w h i c h w a s c o n t e m p o r a n e o u s w i t h v o l c a n i s m i n t h e E a s t e r n K l a m a t h B e l t ( B u r c h f i e l a n d D a v i s 1 9 7 2 ) . T h e W e s t e r n P a l e o z o i c a n d T r i a s s i c B e l t i s a s t r u c t u r a l l y c o m p l e x e u g e o s y n c l i n a l t e r r a n e c o n s i s t i n g o f u l t r a m a f i c r o c k s , m a f i c v o l c a n i c r o c k s , t h i n - b e d d e d c h e r t s a n d p h y l l i t i c d e t r i t a l r o c k s ( I r w i n 1 9 6 0 , 1 9 6 6 ) . I r w i n ( 1 9 7 2 ) , EXPLANATION S U P E R J A C E N T R O C K S Rocks of Cenozoic age Rocks of Late Jurassic (Tithonian) to Late Cretaceous age S U B J A C E N T R O C K S Western Jurassic belt Western Paleozoic and Triassic belt Central metamorphic belt Eastern Paleozoic belt MM Granitic rocks Ultramafic rocks, in both subjacent and superjacent terranes Jncl-udet tome gabbrou rocks Figure 2. Regional Geology showing distribution of the four lithotectonic belts (Irwin, 1966) 10 11 i n t e r p r e t s t h e s e u n i t s a s a s a l t e r n a t i n g s e q u e n c e o f o p h i o l i t i c r o c k s . T h e W e s t e r n J u r a s s i c B e l t w h i c h r a n g e s f r o m e i g h t t o 2 5 km w i d e a n d i s o v e r 2 7 5 km l o n g , c o n t a i n s t h e g r e a t e s t v o l u m e o f u l t r a m a f i c r o c k s . T h e l a r g e s t s i n g l e u l t r a m a f i c b o d y , t h e J o s e p h i n e P e r i d o t i t e , i s p a r t o f t h e J o s e p h i n e O p h i o l i t e a s d e s c r i b e d b y V a i l a n d D a s c h ( 1 9 7 7 ) a n d H a r p e r ( 1 9 8 0 ) . T h e o p h i o l i t e h a s b e e n d a t e d b o t h i s o t o p i c a l l y a n d p a l e o n t o l o g i c a l l y . Z i r c o n f r o m a p l a g i o g r a n i t e h a s y i e l d e d a U - P b a g e o f 157 Ma ( H a r p e r a n d S a l e e b y 1 9 8 0 ) , w h i l e f o s s i l s ( B u c h i a c o n c e n t r i c a ) c o l l e c t e d f r o m s e v e r a l h u n d r e d m e t e r s a b o v e t h e m a f i c v o l c a n i c r o c k s c o n s i d e r e d t o b e r e l a t e d t o t h e J o s e p h i n e P e r i d o t i t e h a v e a k n o w n r a n g e o f l a t e O x f o r d i a n t o e a r l y K i m m e r i d g i a n ( H a r p e r , 1 9 8 0 ) , a b o u t 150 M a . T h e W e s t e r n J u r a s s i c B e l t a l s o c o n t a i n s s h a l e s , g r e y w a c k e s a n d m e t a v o l c a n i c s o f t h e G a l i c e a n d R o g u e F o r m a t i o n s . T h e s e r o c k s a r e c o n s i d e r e d t o b e t h e r e m n a n t s o f a n a n c i e n t i s l a n d - a r c c o m p l e x ( G a r c i a 1 9 7 9 ) . 2 . 2 T h e J o s e p h i n e P e r i d o t i t e A n d O u t l y i n g E x t e n s i o n s T h e J o s e p h i n e P e r i d o t i t e i s p a r t o f t h e J o s e p h i n e O p h i o l i t e , e x p o s e d i n t h e W e s t e r n J u r a s s i c B e l t o f t h e K l a m a t h M o u n t a i n s . T h e J o s e p h i n e O p h i o l i t e h a s a l l t h e m e m b e r s o f a c o m p l e t e o p h i o l i t e s u i t e , i n c l u d i n g : 12 p e r i d o t i t e , g a b b r o , d i k e c o m p l e x a n d p i l l o w l a v a . A b o v e t h i s , a f l y s c h s e q u e n c e c o r r e l a t i v e w i t h t h e G a l i c e F o r m a t i o n c o n f o r m a b l y o v e r l i e s a t h i n b a s a l p e l a g i c s e q u e n c e . F i g u r e 3 i s a s k e t c h o f a r e a s s e m b l e d s e c t i o n o f t h e J o s e p h i n e O p h i o l i t e ( a f t e r H a r p e r 1 9 8 1 ) . T h e J o s e p h i n e P e r i d o t i t e r e p r e s e n t s t h e u l t r a m a f i c r o c k s o f t h e m a n t l e a n d i s s e p a r a t e d f r o m t h e r e s t o f t h e s e c t i o n b y a f a u l t ( F i g u r e 3 ) . T h e l a r g e p e r i d o t i t e b o d y a n d i t s o u t l y i n g t e c t o n i c s l i c e s c o v e r a p p r o x i m a t e l y 2 0 0 0 s q u a r e km a n d a r e c o n c e n t r a t e d a l o n g t h e w e s t e r n b o u n d a r y o f t h e b e l t w h e r e t h e y f o r m a l a r g e e a s t d i p p i n g t h r u s t s h e e t . T h e p e r i d o t i t e i s l a r g e l y h a r z b u r g i t e (>95%) w i t h s c a t t e r e d d u n i t e b o d i e s (<5%) a n d i r r e g u l a r s e a m s o f o r t h o p y r o x e n i t e a n d o l i v i n e w e b s t e r i t e (<1%), ( D i c k 1 9 7 7 ) . F i g u r e 4 s h o w s t h e I U G S c l a s s i f i c a t i o n o f p e r i d o t i t e s . T h e m o d a l m i n e r a l o g y o f t h e J o s e p h i n e h a r z b u r g i t e i s o l i v i n e ( 7 8 %) , o r t h o p y r o x e n e ( 2 0 %) , c l i n o p y r o x e n e (1 % ) , c h r o m i a n s p i n e l ( l %) ( H i m m e l b e r g a n d L o n e y 1 9 7 3 , D i c k 1 9 7 6 , E l l i s 1 9 7 7 ) . S e r p e n t i n i z a t i o n i s u b i q u i t o u s , r a n g i n g f r o m 50 t o 100 p e r c e n t o f t h e r o c k . H i g h l y s h e a r e d s e r p e n t i n i t e f o r m s a p e r i p h e r a l s h e l l b e t w e e n t h e p e r i d o t i t e a n d t h e c o u n t r y r o c k s . P e r v a s i v e s e r p e n t i n i z a t i o n i s a l s o c h a r a c t e r i s t i c o f t h e n u m e r o u s n o r t h - n o r t h e a s t t r e n d i n g s h e a r z o n e s w h i c h c u t t h e b o d y . A l t e r a t i o n t o s e r p e n t i n e i n i t i a l l y t a k e s t h e f o r m o f c h r y s o t i l e v e i n l e t s c u t t i n g o l i v i n e a n d e n s t a t i t e g r a i n s . P s e u d o m o r p h i c r e p l a c e m e n t b y l i z a r d i t e f o l l o w s a n d 13 Chert, argillite Pillow lava, pillow breccia Pillow lava screens Strong epidotization Diabase dikes Red mudstone Sulfide zone Gobbro screens Rare plagiogran ite Massive gabbro Bended gabbro Wehrlite, dunite pyroxenite gabbro Fault Gabbro dikes Harzburgite tectonite, dunite Podiform chromite Epidote veins Figure 3. Section through the Josephine Ophiolite (after Harper 1981) 14 CLINOPYROXENITE DUNITE ORTHOPYROXENE Figure 4. Classification of peridotite 15 r e s u l t s in the formation of bastite textures in enstatite ( E l l i s 1977). A well developed compositional layering, represented by alternating o l i v i n e - r i c h and olivine-poor p e r i d o t i t e layers occurs throughout much of the harzburgite. This layering i s generally on a centimeter scale and i s consistent over large areas. In some places, l a t e forming o l i v i n e - r i c h and orthopyroxenite sheets crosscut compositional layering at high angles. Dunite in the form of ir r e g u l a r patches, tabular dikes and large podiform bodies i s generally discordant to the compositional layering. Areas in the harzburgite where compositional layering i s poorly developed or nonexistent are usually near large dunite bodies. In these areas a f a i r to poor mineral f o l i a t i o n i s present. This mineral f o l i a t i o n overprints both the compositional layering and the discordant dunite bodies and i s therefore younger. Multistage deformation has been observed in other areas of the Josephine P e r i d o t i t e including the Vulcan Peak area at the Western edge of the Josephine. Loney and Himmelberg (1976), describe three stages of p l a s t i c deformation within the Vulcan Peak Pe r i d o t i t e body, a l l producing c h a r a c t e r i s t i c f o l i a t i o n s . Abundant i s o c l i n a l f o l d i n g with a x i a l planes p a r a l l e l to compositional layering has also been recognized in the Vulcan Peak area (Loney and Himmelberg 1976). Layers of concentrated chromite are found only in 16 l a r g e d u n i t e b o d i e s . T h e s e l a y e r s r a n g e i n t h i c k n e s s f r o m a f e w m i l l i m e t e r s t o s e v e r a l c e n t i m e t e r s . C h r o m i t e l a y e r s i n l a r g e d u n i t e b o d i e s g e n e r a l l y a r e n o t c o n c o r d a n t w i t h f o l i a t i o n a n d h a r z b u r g i t e l a y e r i n g . I n f a c t , s u b p a r a l l e l c h r o m i t e l a y e r s c o m m o n l y o c c u r a t r i g h t a n g l e s t o f o l i a t i o n / l a y e r i n g i n t h e a d j a c e n t h a r z b u r g i t e . 17 3 . G E O L O G Y O F T H E C H R O M E R I D G E A R E A 3 . 1 I n t r o d u c t i o n T h e C h r o m e R i d g e a r e a l i e s n o r t h o f t h e m a i n b o d y o f t h e J o s e p h i n e P e r i d o t i t e ( F i g u r e 1 ) . T h e n a m e " C h r o m e R i d g e " a p p l i e s t o a n o r t h - n o r t h e a s t t r e n d i n g , e l o n g a t e b o d y o f s e r p e n t i n i z e d d u n i t e w h i c h f o r m s a r i d g e , a p p r o x i m a t e l y 8 km l o n g a n d 1 . 5 km w i d e . T h e d u n i t e b o d y , w h i c h i s h o s t t o n u m e r o u s c h r o m i t e d e p o s i t s , i s a l s o k n o w n a s t h e B r i g g s C r e e k A r e a ( W e l l s , P a g e a n d J a m e s 1 9 4 0 ) . T h e d u n i t e b o d y i s s u r r o u n d e d o n a l l s i d e s b y h a r z b u r g i t e w h i c h i s i n f a u l t c o n t a c t w i t h s u r r o u n d i n g c o u n t r y r o c k s . F i g u r e 5 i s a g e o l o g i c m a p o f t h e C h r o m e R i d g e a r e a ( r e v i s e d a f t e r R a m p 1 9 6 1 ) . S h o w n o n t h i s m a p i s t h e a r e a w i t h i n t h e d u n i t e b o d y w h i c h w a s m a p p e d i n d e t a i l d u r i n g t h e c o u r s e o f t h i s s t u d y ( F i g u r e 6 ) ( s e e b a c k p o c k e t ) . T h e a r e a , w h i c h w a s m a p p e d a t a s c a l e o f 1 : 1 0 0 0 , i s a p p r o x i m a t e l y 4 km l o n g a n d 1 km w i d e . 3 . 2 P e t r o l o g y T h e I U G S c l a s s i f i c a t i o n s c h e m e i s s h o w n o n F i g u r e 4 . U l t r a m a f i c r o c k t y p e s f o u n d w i t h i n t h e m a p p e d a r e a a t C h r o m e R i d g e i n c l u d e d u n i t e a n d h a r z b u r g i t e . C h r o m i t e o c c u r r e n c e s w h i c h a r e r e s t r i c t e d t o d u n i t e b o d i e s a r e Figure 5. Geologic Map of the Chrome Ridge Area (revised after Ramp 1961) LEGEND Illinois River Gabbro Iffi&ft? Briggs Creek Amphibolite WS$ Harzburgite I Dunite *:":*:*:" Contact Fault Contact 1 Gradational Contact Mineral Foliation Attitude (inclined, vertical) Outline of mapped area (Figure 6) 19 r e f e r r e d t o a s c h r o m i t i f e r o u s d u n i t e . 21 3.2.1. D u n i t e D u n i t e i s t h e most common r o c k t y p e i n t h e a r e a . Most w e a t h e r e d s u r f a c e s a r e b u f f - c o l o u r e d b u t f r e s h s u r f a c e s r a n g e f r o m p a l e g r e e n t o g r e e n i s h - b l a c k , d e p e n d i n g on t h e d e g r e e o f t h e a l t e r a t i o n . S e r p e n t i n i z a t i o n o f t h e d u n i t e i s i n t e n s i v e , a v e r a g i n g 95 t o 100 p e r c e n t . M i c r o s c o p i c s t r u c t u r e s w i t h i n some o f t h e d u n i t e c o n s i s t s o f a s t r o n g p r e f e r r e d m i n e r a l o r i e n t a t i o n f o r m e d d u r i n g t h e f l a t t e n i n g o f o l i v i n e g r a i n s . P l a t e 1 shows t h e m i n e r a l f o l i a t i o n i n a s e r p e n t i n i z e d d u n i t e . E n s t a t i t e i s m i n o r , c o m p r i s i n g 2 t o 3 p e r c e n t o f t h e r o c k . The e u h e d r a l e n s t a t i t e g r a i n s a r e much l a r g e r (3 mm) t h a n t h e o l i v i n e g r a i n s and t e n d t o be l e s s s e r p e n t i n i z e d t h a n t h e s u r r o u n d i n g o l i v i n e . A c c e s s o r y c h r o m i a n s p i n e l w h i c h i s p r e s e n t i n c o n c e n t r a t i o n s a v e r a g i n g 0.5 %, o c c u r s a s r o u n d e d t o s u b h e d r a l g r a i n s . A v e r a g e g r a i n s i z e i s a b o u t 0.5 mm. S e r p e n t i n i z a t i o n has d e s t r o y e d e s s e n t i a l l y a l l p r i m a r y t e x t u r e s i n v o l v i n g o l i v i n e . 3.2.2. H a r z b u r g i t e The d u n i t e body i s s u r r o u n d e d on a l l s i d e s by h a r z b u r g i t e . The h a r z b u r g i t e i s t y p i c a l l y f o l i a t e d b u t u n l a y e r e d and i s h i g h l y (60 %) s e r p e n t i n i z e d . The r o c k Plate 1. Serpentinized dunite i s strongly f o l i a t e d i n upper half of photo but has an i s o l a t e d block of un f o l i a t e d dunite i n lower h a l f . \Jfir' # 3 CM 4 Plate 2. Weathered harzburgite showing nubbly surface due to r e s i s t a n t pyroxene grains. 23 has a x e n o b l a s t i c , i n e q u i g r a n u l a r t e x t u r e and had an o r i g i n a l c o m p o s i t i o n a v e r a g i n g 83% o l i v i n e , 16% pyroxene and 1 % s p i n e l . T y p i c a l l y , o l i v i n e and e n s t a t i t e a r e c o a r s e g r a i n e d (5 mm), w i t h n o r m a l l y c u r v i l i n e a r g r a i n b o u n d a r i e s . H a r z b u r g i t e has a m o t t l e d i r r e g u l a r appearance i n hand specimen, caused by clumps of e n s t a t i t e g r a i n s w i t h i n the o l i v i n e m a t r i x . P l a t e 2 shows the knobby s u r f a c e of weathered h a r z b u r g i t e . A c c o r d i n g t o . the t e x t u r a l c l a s s i f i c a t i o n i n t r o d u c e d by M e r c i e r and N i c o l a s ( 1 9 7 5 ) , the h a r z b u r g i t e has a t y p i c a l p o r p h y r o c l a s t i c t e x t u r e . Most a c c e s s o r y c h r o m i t e g r a i n s i n the h a r z b u r g i t e a r e v e r y i r r e g u l a r i n shape and s i z e . Equant, e u h e d r a l g r a i n s a r e r a r e . Chromite g r a i n s i n t h i n s e c t i o n range i n c o l o u r from r e d d i s h - b r o w n t o y e l l o w - b r o w n and a r e p r e s e n t i n amounts from 0.5-1.0%. The s i z e of the g r a i n s ranges from 0.1 mm t o 4 mm. I n d i v i d u a l g r a i n s a r e c h a r a c t e r i z e d by i r r e g u l a r l o b e s and hooks p r o j e c t i n g from the c h r o m i t e i n t o the o l i v i n e . The h i g h l y i r r e g u l a r g r a i n b o u n d a r i e s of the s p i n e l s s u g g e s t s t h a t they formed l a t e by c h e m i c a l p r e c i p i t a t i o n . D i c k (1975) d e s c r i b e s the p r o c e s s as one i n w hich r e s o r p t i o n of pyroxene l e a v e s i s o l a t e d s p i n e l g r a i n s i n the o l i v i n e m a t r i x . 3.2.3. C h r o m i t i f e r o u s D u n i t e C h r o m i t i f e r o u s d u n i t e i s d u n i t e w i t h g r e a t e r than 3% 24 c h r o m i t e . The c h r o m i t e c o n t e n t can range up to 60 %. The rock c o n s i s t s of e q u i d i m e n s i o n a l e u h e d r a l to s u b h e d r a l c h r o m i t e g r a i n s w i t h i n a s e r p e n t i n e m a t r i x . The d i s t r i b u t i o n of c h r o m i t e g r a i n s ranges from randomly d i s semimated to l a y e r e d t e x t u r e s to r o u g h l y pod-shaped mass ive c h r o m i t e b o d i e s c o n t a i n i n g up to 60% c h r o m i t e . Average g r a i n s i z e of the c h r o m i t e i s about 1.5 mm. The equant s u b h e d r a l shape of the c h r o m i t e g r a i n s sugges t s t h a t the s p i n e l grew r e l a t i v e l y f r e e l y i n a m e l t . M i c r o c u m u l a t e s t r u c t u r e s such as s i z e graded l a y e r s and mesh t e x t u r e s a l s o suggest a cumulate o r i g i n f o r much of the c h r o m i t i f e r o u s d u n i t e . 3.3 S t r u c t u r e Of The Chrome Ridge Area C o n t a c t s between the d u n i t e and the e n c l o s i n g h a r z b u r g i t e are g r a d a t i o n a l over a zone up to 20 m wide . S i m i l a r i l y , g r a d a t i o n a l c o n t a c t s e x i s t between the d u n i t e and c h r o m i t i f e r o u s d u n i t e b o d i e s . In the l a t t e r c a s e , the c o n t a c t zones are o n l y 1 or 2 m wide . The o u t l i n e of the main d u n i t e body i s i r r e g u l a r ( F i g u r e 6 ) . In f a u l t c o n t a c t w i t h the h a r z b u r g i t e on the e a s t e r n and most of the western s i d e , i s the B r i g g s Creek A m p h i b o l i t e ( F i g u r e 5 ) . T h i s u n i t c o n s i s t i n g of a m p h i b o l i t e s and a m p h i b o l i t e g n e i s s e s has y i e l d e d a K - A r date on hornb lende of 128 Ma ( G a r c i a 1979). I n f a u l t e d b l o c k s of the a m p h i b o l i t e g n e i s s , r a n g i n g from 1 to 20 m 25 i n d i a m e t e r , a r e p r e s e n t i n b o t h h a r z b u r g i t e a n d d u n i t e . F o l i a t i o n m e a s u r e m e n t s o f t h e f a u l t e d b l o c k s s h o w t h e y h a v e u n d e r g o n e r a n d o m r o t a t i o n w i t h r e s p e c t t o t h e n o r t h -s o u t h f o l i a t i o n t r e n d w i t h i n t h e m a i n b o d y o f a m p h i b o l i t e . W e l l d e v e l o p e d c h r o m i t e - r i c h l a y e r s o c c u r w i t h i n s e v e r a l a r e a s o f d u n i t e a t C h r o m e R i d g e . T h e c h r o m i t e l a y e r s a p p e a r t o b e c u m u l a t e i n o r i g i n , w i t h t h i c k n e s s e s r a n g i n g f r o m a f e w m i l l i m e t e r s t o a p p r o x i m a t e l y 6 c m . L o c a l l y , i n d i v i d u a l l a y e r s o c c u r p a r a l l e l t o o n e a n o t h e r b u t t h e y v a r y w i d e l y i n o r i e n t a t i o n b e t w e e n o c c u r r e n c e s . T h e i n d i v i d u a l c h r o m i t e l a y e r s h a v e a v e r y l i m i t e d l a t e r a l e x t e n t , n o r m a l l y 20 t o 3 0 c m . A n o c c u r r e n c e o f w e l l l a y e r e d c h r o m i t e , d e s c r i b e d b y R a m p ( 1 9 6 1 ) , w a s c o n t i n u o u s f o r a b o u t 2 m b e f o r e b e i n g t r u n c a t e d b y a f a u l t . F i g u r e 7 i s a d i a g r a m b y R a m p ( 1 9 6 1 ) s h o w i n g t h e c o m p o s i t e n a t u r e o f t h e l a y e r s a n d g r a d a t i o n a l e d g e s o n t h e l o w e r e n d . T h i s d e p o s i t , w h i c h w a s l o c a t e d i n t h e s o u t h w e s t c o r n e r o f t h e C h r o m e R i d g e a r e a ( V i o l e t M i n e ) ( F i g u r e 6 ) , h a s b e e n e i t h e r b u r i e d o r m i n e d o u t . C o m p o s i t i o n a l l a y e r i n g w i t h i n t h e h a r z b u r g i t e i s n o t p r e s e n t i n t h e v i c i n i t y o f t h e d u n i t e b o d y . H o w e v e r , w e l l - d e v e l o p e d c o m p o s i t i o n l l a y e r i n g i s p r e s e n t a t o n e l o c a l i t y s e v e r a l km n o r t h o f t h e m a i n d u n i t e b o d y . T h e a t t i t u d e o f t h e l a y e r i n g i s r o u g h l y p a r a l l e l t o c o n t a c t s o f t h e u l t r a f m a f i c b o d y w i t h t h e s u r r o u n d i n g c o u n t r y r o c k . A w e l l - d e v e l o p e d m i n e r a l f o l i a t i o n w a s o b s e r v e d i n s o m e h a r z b u r g i t e s n e a r t h e 26 5 cm Figure 7. Ink tracing by Ramp (1961) showing composite nature of the bands and gradational edges. Figure 8a. Antinodular ere Figure 8b. Nodular chromite showing (Cassard et al. 1981) pull-apart textures 27 d u n i t e b o d y , a s w e l l a s i n s o m e p a r t s o f t h e d u n i t e b o d y . T h e f o l i a t i o n i s p r o d u c e d b y t h e a l i g n m e n t o f f l a t t e n e d p y r o x e n e g r a i n s . A t t i t u d e s a r e s h o w n i n F i g u r e 6 . 3 . 3 . 1 . F a u l t i n g A n d F o l d i n g T h e u l t r a m a f i c r o c k s o f t h e C h r o m e R i d g e a r e a a r e b o u n d e d b y L a t e J u r a s s i c n o r t h - s o u t h t r e n d i n g h i g h a n g l e f a u l t s t h a t h a v e j u x t a p o s e d t h e r o c k s a g a i n s t t h e B r i g g s C r e e k A m p h i b o l i t e . A s e c o n d s e t o f f a u l t s i s c o n f i n e d t o t h e u l t r a m a f i c r o c k s a n d g e n e r a l l y a r e n o r t h e a s t t r e n d i n g w i t h s t e e p s o u t h e a s t d i p s . T h e s e f a u l t s a r e c h a r a c t e r i z e d b y b r o a d , h i g h l y s e r p e n t i n i z e d s h e a r z o n e s . E v i d e n c e o f p l a s t i c d e f o r m a t i o n w i t h i n t h e s e s h e a r z o n e s s u g g e s t s t h a t t h e s e f a u l t s a r e m a n t l e r e l a t e d , a l t h o u g h t h e s e r p e n t i n i z a t i o n w a s m u c h l a t e r . S i m i l a r n o r t h e a s t t r e n d i n g p l a s t i c s h e a r z o n e s h a v e b e e n d e s c r i b e d i n t h e V u l c a n P e a k a r e a t o t h e e a s t o f C h r o m e R i d g e ( L o n e y a n d H i m m e l b e r g 1 9 7 6 ) . E x p e r i m e n t a l e v i d e n c e i n d i c a t e s t h a t o l i v i n e f a b r i c s i n t h e V u l c a n P e a k s h e a r z o n e s p r o b a b l y f o r m e d i n t h e 8 0 0 t o 1 0 0 0 ° C t e m p e r a t u r e r a n g e . L a r g e s c a l e f o l d i n g h a s b e e n d e s c r i b e d b y W e l l s e_t a l . ( 1 9 4 0 ) , a n d R a m p ( 1 9 5 6 , 1 9 6 0 ) f o r t h e C h r o m e R i d g e a r e a . T h e y r e f e r t o a l a r g e s y n c l i n e w h o s e a x i s t r e n d s n o r t h w e s t w a r d t h r o u g h t h e m a i n d u n i t e b o d y . H a r p e r ( 1 9 8 0 ) d e s c r i b e s i s o c l i n a l f o l d i n g w h i c h o c c u r r e d p r i o r t o t h e m a j o r n o r t h - s o u t h f a u l t i n g e p i s o d e . T h e a u t h o r h a s 28 o b s e r v e d m i n o r m e s o s c o p i c k i n k a n d c h e v r o n f o l d i n g i n t h e v i c i n i t y o f l a r g e s h e a r z o n e s i n t h e m a p p e d a r e a . A g e n e r a l l a c k o f w e l l - d e v e l o p e d l a y e r i n g t h r o u g h o u t t h e d u n i t e a n d s u r r o u n d i n g h a r z b u r g i t e h a s m a d e i t d i f f i c u l t t o e s t a b l i s h t h e p r e s e n c e o f m a c r o s c o p i c f o l d s . 29 4. CHROMITE DEPOSITS 4.1 I n t r o d u c t i o n T h a y e r (1960) d i s t i n g u i s h e d and d i s c u s s e d i n d e t a i l two t y p e s o f c h r o m i t e d e p o s i t s r e l a t e d t o i g n e o u s r o c k s . The f i r s t t y p e i s s t r a t i f o r m c h r o m i t e d e p o s i t s i n b a s i c l a y e r e d i n t r u s i v e b o d i e s i n n o n - o r o g e n i c c o n t i n e n t a l e n v i r o n m e n t s s u c h a s t h e B u s h v e l d and S t i l l w a t e r c o m p l e x e s . In t h e s e , c h r o m i t e forms r e l a t i v e l y t h i n (30 cm) r e g i o n a l l y e x t e n s i v e l a y e r s t r a c e a b l e f o r t e n s o f k i l o m e t e r s . The l a y e r s a r e p a r a l l e l t o one a n o t h e r and t o t h e o v e r a l l i g n e o u s l a y e r i n g of t h e c o m p l e x . The c h r o m i t e l a y e r s a r e made up o f e u h e d r a l c r y s t a l s e x h i b i t i n g t y p i c a l c u m u l a t e t e x t u r e s . The s e c o n d t y p e of c h r o m i t e d e p o s i t o c c u r s w i t h i n t h e u l t r a m a f i c p a r t of o p h i o l i t e c o m p l e x e s and i s commonly r e f e r r e d t o as a l p i n e - t y p e . C h r o m i t e d e p o s i t s o f t h i s t y p e a r e f o u n d i n o p h i o l i t e s i n Cuba, T u r k e y , t h e P h i l i p p i n e s , U.S.S.R. a n d t h e west c o a s t o f t h e U n i t e d S t a t e s . A l p i n e - t y p e c h r o m i t e o r e b o d i e s a r e i r r e g u l a r w i t h a v a r i e t y o f t e x t u r e s r a n g i n g f r o m d i s s e m i n a t e d t o m a s s i v e . N o d u l a r a n d o r b i c u l a r t e x t u r e s , u n i q u e t o a l p i n e - t y p e d e p o s i t s have been d e s c r i b e d w o r l d w i d e . One o f t h e most c h a r a c t e r i s t i c s t r u c t u r a l f e a t u r e s o f a l p i n e - t y p e d e p o s i t s 30 i s t h e c o m m o n d i s c o r d a n c e b e t w e e n o r e b o d i e s a n d t h e s u r r o u n d i n g h o s t r o c k s . I n a p p r o x i m a t e l y 50% o f o c c u r r e n c e s , o r e b o d y s t r u c t u r e s a r e f o u n d t o b e d i s c o r d a n t w i t h h o s t r o c k s t r u c t u r e s ( C a s s a r d e t a l . 1 9 8 1 ) . 4 . 2 D e s c r i p t i o n O f C h r o m i t e T e x t u r e s M o s t c h r o m i t e t e x t u r e s c a n b e c o n v e n i e n t l y d e s c r i b e d a s b e i n g e i t h e r c u m u l a t e o r o r n o n - c u m u l a t e . T h e c u m u l a t e t e x t u r e d c h r o m i t e i s t y p i c a l o f c l a s s I t y p e d e p o s i t s , w h i l e n o n - c u m u l a t e c h r o m i t e t e x t u r e s a r e c h a r a c t e r i s t i c o f c l a s s I I t y p e d e p o s i t s ( s e e c l a s s i f i c a t i o n i n n e x t s e c t i o n ) . 4 . 2 . 1 . C u m u l a t e T e x t u r e s T h a y e r ( 1 9 6 9 ) d e s c r i b e s c u m u l a t e c h r o m i t e t e x t u r e s i n p o d i f o r m d e p o s i t s a s ( 1 ) t h o s e r e s e m b l i n g o r r e l a t e d t o s e t t l e d t e x t u r e s i n s t r a t i f o r m c o m p l e x e s a n d ( 2 ) n o d u l a r , o r b i c u l a r a n d v e r y c o a r s e t e x t u r e s t h a t a r e u n i q u e t o a l p i n e - t y p e d e p o s i t s . T e x t u r e s a r e s i m i l a r t o t h o s e i n s t r a t i f o r m d e p o s i t s i n c l u d e d i s s e m i n a t e d , l a y e r e d a n d n e t t e x t u r e d c h r o m i t e . C u m u l a t e f e a t u r e s i n c l u d e s u b h e d r a l t o e u h e d r a l g r a i n s ( p l a t e 3 ) a n d i n t h e c a s e o f l a y e r e d c h r o m i t e , s i z e g r a d i n g o f g r a i n s ( F i g u r e 7 ) . D i s s e m i n a t e d D i s s e m i n a t e d g r a i n s i n c o n c e n t r a t i o n s u p t o 31 Plate 4. Layers of cumulate chromite i n serpentinized dunite (from c l a s s I de p o s i t ) . Plate 5. Disseminated chromite from cl a s s II deposit showing strong f o l i a t i o n d i r e c t i o n marked by clumps of elongate chromite grains. Plate 6 . Microphotograph from same sample as plate 5. showing i r r e g u l a r boundaries of elongate chromite grain. 33 10% a r e g e n e r a l l y e u h e d r a l t o s u b h e d r a l . T h e g r a i n s a r e e v e n l y s c a t t e r e d t h r o u g h o u t t h e r o c k . P l a t e 3 i s a m i c r o g r a p h s h o w i n g t h e e u h e d r a l t o s u b h e d r a l n a t u r e o f t h e d i s s e m i n a t e d g r a i n s . L a y e r e d C h r o m i t e c o n s i s t s o f d i s s e m i n a t e d o r m a s s i v e c o n c e n t r a t i o n s o f c h r o m i t e g r a i n s . M o s t o f t h e g r a i n s a r e s u b h e d r a l t o e u h e d r a l . S o m e l a y e r s s h o w s i z e g r a d i n g ( F i g u r e 7 ) . P l a t e 4 s h o w s l a y e r s o f c h r o m i t e i n s e r p e n t i n i z e d d u n i t e . N e t N e t t e x t u r e c o n s i s t s o f s m a l l c h r o m i t e g r a i n s w h i c h f i l l t h e i n t e r s t i c e s b e t w e e n l a r g e o l i v i n e g r a i n s ( p l a t e N o d u l a r N o d u l a r c h r o m i t e ( p l a t e 7 ) c o n s i s t s o f r o u n d o r e l l i p t i c a l m a s s e s o f c h r o m i t e t h a t r a n g e f r o m a b o u t 5 t o 2 5 mm i n d i a m e t e r . T h e n o d u l e s a r e a g g r e g a t e s o f g r a i n s w h i c h a v e r a g e 1 t o 3 mm i n d i a m e t e r . T h e f a c t t h a t t h e y o c c u r i n l a y e r s a n d a r e e v e r y w h e r e c l o s e l y a s s o c i a t e d w i t h c u m u l a t e t e x t u r e d c h r o m i t e s u p p o r t s a c u m u l a t e o r i g i n f o r t h e n o d u l e s . I n g e n e r a l , c u m u l a t e t e x t u r e s a r e r e l a t i v e l y u n -d e f o r m e d . H o w e v e r , i n l o c a l i z e d s h e a r z o n e s w h i c h ' c u t l a y e r e d c h r o m i t e , d i s r u p t i o n o f c h r o m i t e t e x t u r e s r e s u l t s i n d e f o r m e d c u m u l a t e t e x t u r e s . T h e s e i n c l u d e , c h r o m i t e b r e c c i a s , a n t i n o d u l a r ( s e e b e l o w ) a n d m a s s i v e t e x t u r e s . C h r o m i t e b r e c c i a C h r o m i t e b r e c c i a c o n s i s t s o f m i l d l y d e f o r m e d c h r o m i t e n e t t e x t u r e . O l i v i n e g r a i n s h a v e b e e n 34 Plate 8. I s o c l i n a l l y folded body chromite from Seiad ultramafic 3 5 d e f o r m e d a n d e l o n g a t e d . I n t e r s t i t i a l c h r o m i t e h a s b e e n s o m e w h a t c o n c e n t r a t e d b u t g r a i n s a r e s t i l l s u b h e d r a l ( p l a t e 10 ). A n t i n o d u l a r A n t i n o d u l a r c h r o m i t e ( F i g u r e 8 a ) c o m p r i s e s h i g h l y d e f o r m e d c h r o m i t e n e t t e x t u r e s . O l i v i n e g r a i n s a r e i n t e n s e l y d e f o r m e d a n d e l o n g a t e d . C h r o m i t e i s h i g h l y c o m p a c t e d a n d d e f o r m e d ( C a s s a r d e t a l . 1 9 8 1 ) . M a s s i v e M a s s i v e p o d s o f c h r o m i t e (1 t o 2 m i n d i a m e t e r ) o c c u r i n s h e a r z o n e s i n d u n i t e . T h e c o n c e n t r a t i o n o f c h r o m i t e i s 3 0 % t o 4 0 % . T h e p o d s a r e f r i a b l e a n d t h e c h r o m i t e i s " s o m e w h a t d e f o r m e d . P l a t e 11 i s a s m a l l c h r o m i t e p o d f r o m a s h e a r z o n e a t C h r o m e R i d g e . P l a t e 12 s h o w s a s e r i e s o f s m a l l p o d s c o n n e c t e d t o g e t h e r b y c h r o m i t e s t r i n g e r s f r o m t h e s a m e l o c a l i t y a t C h r o m e R i d g e . P l a t e s 15 a n d 16 s h o w s o m e e x a m p l e s o f v e i n s a n d s e a m s o f c h r o m i t e . T h e s e o c c u r i n d u n i t e b o d i e s a n d c o n s i s t o f s u b h e d r a l t o e u h e d r a l g r a i n s . 4 . 2 . 2 . N o n - c u m u l a t e C h r o m i t e M o s t o f t h e n o n - c u m u l a t e t e x t u r e s e n c o u n t e r e d o c c u r i n l a r g e p o d s i n s h e a r z o n e s w i t h i n h a r z b u r g i t e . T h e t e x t u r e s a r e t y p i c a l l y h i g h l y d e f o r m e d a n d h a v e n o c u m u l a t e f e a t u r e s . T e x t u r e s i n c l u d e d i s s e m i n a t e d a n d m a s s i v e . Plate 10. Chromite breccia from a shear zone at Low Divide has w e l l -developed v e r t i c a l f a b r i c formed by elongation of dunite c l o t s and surrounding chromite. Plate 11. A small massive chromite pod from a shear zone at Chrome Ridge. Plate 12. A serie s of small pods connected by chromite s t r i n g e r s from a highly serpentinized shear zone at Chrome Ridge. 38 D i s s e m i n a t e d T h e s e t e x t u r e s o c c u r w i t h i n s h e a r e d s e r p e n t i n i z e d d u n i t e a n d a r e t y p i c a l l y h i g h l y d e f o r m e d . T h e g r a i n s a r e e l o n g a t e a n d h a v e i r r e g u l a r h o o k s a n d l o b e s p r o t r u d i n g f r o m t h e m . T h e r o c k h a s a w e l l - d e v e l o p e d f o l i a t i o n p r o d u c e d b y t h e a l i g n m e n t o f e l o n g a t e c h r o m i t e g r a i n s . P l a t e 6 i s a m i c r o g r a p h o f a t y p i c a l d e f o r m e d g r a i n w h i l e p l a t e 5 s h o w s t h e f o l i a t i o n i n h a n d s p e c i m e n . T h e e l o n g a t e g r a i n s c o m m o n l y o c c u r i n c l o t s . M a s s i v e T h i s t e x t u r e i s t y p i c a l o f t h e p o d s o f c h r o m i t e f o u n d i n s h e a r z o n e s i n h a r z b u r g i t e . C h r o m i t e c o n c e n t r a t i o n s a v e r a g i n g 90 % a r e c o m m o n a n d t h e c h r o m i t i t e h a s a w e l l - d e v e l o p e d i n t e r n a l f a b r i c f o r m e d b y o r i e n t a t i o n o f g r a i n f r a g m e n t s . T h e p o d s r a n g e i n w e i g h t f r o m a f e w p o u n d s t o t h o u s a n d s o f t o n s . P l a t e s 13 a n d 14 a n d a r e m i c r o g r a p h s s h o w i n g t h e h i g h l y c o n c e n t r a t e d n a t u r e a n d d e f o r m e d t e x t u r e s o f t h e c h r o m i t e . F i g u r e 8 b s h o w s p u l l - a p a r t t e x t u r e t y p i c a l o f d e f o r m e d c h r o m i t e . 4 . 3 C l a s s i f i c a t i o n O f C h r o m i t e C o n c e n t r a t i o n s A c l a s s i f i c a t i o n s c h e m e f o r a l p i n e - t y p e c h r o m i t e d e p o s i t s h a s b e e n d e v e l o p e d b y C a s s a r d e t a l . ( 1 9 8 1 ) b a s e d o n t h e p o s i t i o n o f t h e d e p o s i t s a n d t h e r e l a t i o n s h i p o f d u n i t e s t r u c t u r e s t o c o r r e s p o n d i n g s t r u c t u r e s i n t h e s u r r o u n d i n g p e r i d o t i t e . R e l e v a n t s t r u c t u r a l f e a t u r e s i n c l u d e : Plate 14. Massive chromite from Tyson Mine deposit i s greater than 95% chromite. 40 Plate 16. Irregular chromite body from the same l o c a l i t y as plate 15. 41 f o l i a t i o n s , l i n e a t i o n s , c o m p o s i t i o n a l l a y e r i n g a n d c h r o m i t e l a y e r i n g . T h e y d i s t i n g u i s h t h r e e b a s i c t y p e s : d i s c o r d a n t , s u b - c o n c o r d a n t a n d c o n c o r d a n t . A s s o c i a t e d w i t h t h e t h r e e t y p e s o f d e p o s i t s a r e d i f f e r e n t l e v e l s o f d e f o r m a t i o n w i t h t h e m o s t s t r a i n b e i n g a s s o c i a t e d w i t h c o n c o r d a n t d e p o s i t s a n d t h e l e a s t w i t h d i s c o r d a n t d e p o s i t s . T e x t u r a l o r e t y p e s c a n a l s o b e r e l a t e d t o d e p o s i t t y p e . C h r o m i t e t e x t u r e s w i t h i n d i s c o r d a n t - t y p e d e p o s i t s a r e t y p i c a l l y c u m u l a t e i n n a t u r e a n d i n c l u d e : b a n d e d , n o d u l a r , d i s s e m i n a t e d a n d / o r b r e c c i a t e d ( b r e c c i a t e d f r a g m e n t s c o n s i s t s o f d e f o r m e d o l i v i n e c l o t s a n d t e n d t o b e f a i r l y r o u n d e d ) . T h e s e t e x t u r e s a r e w e l l p r e s e r v e d w i t h l i t t l e e v i d e n c e o f d i s r u p t i o n b y d e f o r m a t i o n . I n c o n c o r d a n t a n d s u b c o n c o r d a n t d e p o s i t s , t h e m o s t c o m m o n t e x t u r a l t y p e s a r e m a s s i v e a n d r i c h l y d i s s e m i n a t e d . T h e s e o r e s a r e c o m m o n l y h i g h l y d e f o r m e d i n t h a t n o ' e u h e d r a l g r a i n s a r e p r e s e n t a n d s t r o n g f o l i a t i o n o f c h r o m i t e o c c u r s w i t h i n t h e d e p o s i t . T h e l a c k o f g e o c h e m i c a l d i s t i n c t i o n s b e t w e e n d e p o s i t t y p e s w i t h i n a g i v e n a r e a , s u g g e s t s t h a t t h e y h a v e a s i m i l a r g e n e s i s . , C a s s a r d e t a l . ( 1 9 8 1 ) s u g g e s t t h a t i n c r e a s i n g d e f o r m a t i o n i s r e s p o n s i b l e f o r t h e d i f f e r e n t t y p e s o f o r e d e p o s i t s . P r e v i o u s t o C a s s a r d e t a l . ( 1 9 8 1 ) , n o a t t e m p t w a s m a d e t o s u b d i v i d e a l p i n e - t y p e c h r o m i t e d e p o s i t s . T h e s t r u c t u r a l c l a s s i f i c a t i o n s c h e m e p r e s e n t e d b y C a s s a r d e t a l . ( 1 9 8 1 ) f o r a l p i n e c h r o m i t e d e p o s i t s i n s o u t h e r n New C a l e d o n i a , h o w e v e r , w a s f o u n d n o t t o b e a p p l i c a b l e t o t h e 42 chromite d e p o s i t s of the Josephine P e r i d o t i t e . The main reason f o r t h i s i s that the s t r u c t u r a l r e s t r i c t i o n s imposed by Cassard on the v a r i o u s t e x t u r a l types do not correspond to s i m i l a r ones i n the Josephine P e r i d o t i t e . For example, Cassard d e s c r i b e s d i s c o r d a n t d e p o s i t s as having p r i m a r i l y cumulate t e x t u r e s which are r e l a t i v e l y undeformed. S i m i l a r cumulate t e x t u r e s are found at both Chrome Ridge and Low D i v i d e , however, and both d u n i t e bodies are concordant to l a y e r i n g i n the surrounding h a r z b u r g i t e . Thus i t i s f e l t t h a t the s t r u c t u r a l r e l a t i o n s between the d u n i t e bodies and host p e r i d o t i t e s are poor c h a r a c t e r i s t i c s to use i n c l a s s i f y i n g the chromite d u n i t e bodies. The h i g h l y d i s r u p t e d nature of many d e p o s i t s r e s u l t s i n much of the o r i g i n a l s t r u c t u r a l i n f o r m a t i o n being obscured. I t would a l s o appear that even i n cases where o r i g i n a l s t r u c t u r a l f e a t u r e s are r e l a t i v e l y i n t a c t , the trends d i s c u s s e d by Cassard are not always observed. A more u s e f u l s t r u c t u r a l f e a t u r e i n c l a s s i f y i n g the d e p o s i t s i s the r e l a t i o n s h i p between pres e n t s t r u c t u r e s w i t h i n the chromite c o n c e n t r a t i o n s themselves and the surrounding p e r i d o t i t e . A t t i t u d e s of chromite l a y e r i n g and f a b r i c s w i t h i n deformed chromite pods g e n e r a l l y p r o v i d e much more c o n s i s t e n t c l a s s i f i c a t i o n d a ta. A new c l a s s i f i c a t i o n scheme (Table 1 ) , which i n c o r p o r a t e s some f e a t u r e s of the Cassard c l a s s i f i c a t i o n , was developed f o r the Josephine P e r i d o t i t e . I t i s based p r i m a r i l y on the degree of deformation of the chromite, TABLE 1. CLASSIFICATION OF CHROMITE DEPOSITS IN THE JOSEPHINE PERIDOTITE CLASS I Ratio of dunite to chromite Textures Host (Cumulate Chromite) 95-99% 5-1% dunite chromite Cumulate: layered, disseminated, nodular, chromite breccia, massive, (non-deformed) Large dunite bodies i n harzburgite. (50 to 8,000 m) CLASS II (Deformed Chromit e) 50-1% 50-99% dunite chromite Non-cumulate: r i c h l y disseminated to massive. (Highly deformed) Shear zones i n harzburgite, (minor dunite s h e l l surrounding chromite pod) . CO 44 although the volume of dunite associated with the deposits has also proven to be an extremely useful parameter in c l a s s i f y i n g them. There are e s s e n t i a l l y two main deposit types which have been distinguished in the Josephine P e r i d o t i t e . The f i r s t type, class I, i s everywhere associated with large dunite bodies. The volume r a t i o of dunite to chromite ranges from 95:5 to 99:1. dunite bodies which are later than the surrounding harzburgite have variable concordancy to the harzburgite layering. Chromite concentrations within the dunite are generally cumulate in o r i g i n and usually occur as layers or disseminations. Nodular chromite and net textures are also common. In general, class I chromite textures are r e l a t i v e l y undeformed. The chromite-rich layers tend to be l a t e r a l l y discontinuous but most are p a r a l l e l to one another over the length of the dunite body. In two locations in the Josephine Peri d o t i t e where measurements could be made (Chrome Ridge and Low Divide), the chromite layering occured at high angles to that in the harzburgite (Figures 5 and 9). Class I deposits, in some places contain shear zones which are host to chromite deposits of higher grade than the surrounding dunite. The chromite occurrences within the shear zones consist of chromite-rich (30 to 40 %) lenses and blocks of deformed layered chromite. The second type of chromite deposit (class I I ) , t y p i c a l l y has l i t t l e dunite associated with i t . In general 45 a 50:50 r a t i o b e t w e e n c h r o m i t e a n d d u n i t e i s t h e m a x i m u m a m o u n t o b s e r v e d , w h i l e c o n c e n t r a t i o n s o f 5 % d u n i t e a n d 95 % c h r o m i t e a r e c o m m o n . T h e c h r o m i t e c o n c e n t r a t i o n s o c c u r a s m a s s i v e (85 t o 95 %) c h r o m i t e b o d i e s i n t h e f o r m o f l e n s e s o r s h e e t s . T h e i n t e r n a l f a b r i c o f t h e c h r o m i t e b o d i e s i s g e n e r a l l y c o n c o r d a n t t o t h e s u r r o u n d i n g h a r z b u r g i t e l a y e r i n g . R a r e l y , t h e c h r o m i t e o c c u r s a s d i s s e m i n a t i o n s . T h e i n d i v i d u a l , e l o n g a t e g r a i n s h a v e i r r e g u l a r b o u n d a r i e s . T h e p a r a l l e l d i r e c t i o n o f t h e e l o n g a t e g r a i n s p r o d u c e s a s t r o n g f o l i a t i o n i n t h e r o c k ( p l a t e 5 ) . I n b o t h t h e m a s s i v e a n d d i s s e m i n a t e d d e p o s i t s , i n t e n s i v e d e f o r m a t i o n i s e v i d e n t . T h e m a s s i v e p o d s a r e c h a r a c t e r i z e d b y p e r v a s i v e c a t a c l a s t i c t e x t u r e s i n w h i c h o n l y f r a g m e n t s o f c h r o m i t e g r a i n s a r e v i s i b l e ( p l a t e s 13 a n d 14) . E v i d e n c e o f d e f o r m a t i o n i n d i s s e m i n a t e d d e p o s i t s i s i n t h e f o r m o f a s t r o n g m i n e r a l f o l i a t i o n . 4 . 4 C h r o m i t e D e p o s i t s O f T h e C h r o m e R i d g e A r e a 4 . 4 . 1 . I n t r o d u c t i o n T h e r e a r e t h r e e m a i n c h r o m i t e d e p o s i t s i n t h e C h r o m e R i d g e a r e a , i n c l u d i n g t h e V i o l e t M i n e , t h e B u s t e r M i n e , a n d t h e S h a d y C o v e M i n e . T h e d e p o s i t s o c c u r i n e i t h e r l e n t i c u l a r b o d i e s , l a r g e s h e a r z o n e s , o r i n a c o m b i n a t i o n o f b o t h . E a c h o f t h e t h r e e m a i n d e p o s i t s i s t e x t u r a l l y 46 u n i q u e , a l t h o u g h c e r t a i n s i m i l a r i t i e s b e t w e e n t h e m d o o c c u r . W e l l - d e v e l o p e d c h r o m i t e - r i c h l a y e r i n g i s t h e o n e t e x t u r e w h i c h o c c u r s i n a l l t h r e e d e p o s i t s , a l t h o u g h t h i s l a y e r i n g i s h i g h l y d i s t u r b e d i n t h e B u s t e r d e p o s i t . A l l t h r e e d e p o s i t s f a l l i n t o t h e c l a s s I c a t e g o r y . 4.4 . 2 . B u s t e r M i n e T h e c h r o m i t e c o n c e n t r a t i o n s a t t h e B u s t e r M i n e a r e c l a s s I t y p e d e p o s i t s a n d a r e r e s t r i c t e d t o a l a r g e N W - S E t r e n d i n g s h e a r z o n e , a p p r o x i m a t e l y 30 m w i d e . T h e d u n i t e i s h i g h l y s h e a r e d a n d c o m p l e t e l y s e r p e n t i n i z e d w i t h a p e r v a s i v e f a b r i c t h a t g i v e s t h e d u n i t e a f l a k y t e x t u r e . T h e s h e a r e d d u n i t e i s a l m o s t c o m p l e t e l y d e v o i d o f a c c e s s o r y c h r o m i t e ; h o w e v e r , w i t h i n t h e d u n i t e a r e l e n s e s o f m o r e c o m p e t e n t c h r o m i t i f e r o u s d u n i t e a v e r a g i n g 15 % c h r o m i t e . T h e t e x t u r e s o f c h r o m i t e w i t h i n t h e l e n s e s r a n g e f r o m f i n e l y ( 2 - 3 %) d i s s e m i n a t e d t o r i c h l y ( 1 0 - 1 5 %) d i s s e m i n a t e d w i t h s o m e i s o l a t e d b l o c k s o f l a y e r e d c h r o m i t e . T h e a t t i t u d e o f t h e c h r o m i t e l a y e r s w i t h i n t h e b l o c k s r a n g e s f r o m h o r i z o n t a l t o v e r t i c a l a n d i n d i c a t e s t h a t t h e b l o c k s h a v e u n d e r g o n e m o v e m e n t a n d r o t a t i o n . T h e d i s s e m i n a t e d c h r o m i t e g r a i n s a r e f a i r l y i r r e g u l a r a n d e x h i b i t d e f o r m a t i o n t e x t u r e s , s u c h a s s l i g h t l y e l o n g a t e d g r a i n s w i t h i r r e g u l a r b o u n d a r i e s . 47 4.4.3. V i o l e t Mine C h r o m i t e i n t h e V i o l e t M i n e o c c u r s i n b o t h l e n t i c u l a r b o d i e s i n competent s e r p e n t i n i z e d d u n i t e , and i n a l a r g e N-S t r e n d i n g s h e a r z o n e . C h r o m i t e t e x t u r e s w i t h i n t h e c o m p e t e n t d u n i t e a r e n o d u l a r , l a y e r e d , m a s s i v e a n d / o r d i s s e m i n a t e d w i t h c o n c e n t r a t i o n s r a n g i n g f r o m 3 t o 15 % . The c h r o m i t e t e x t u r e s a p p e a r p r i m a r y and r e l a t i v e l y u n d e f o r m e d , t h e r e b y q u a l i f y i n g as c l a s s I t y p e d e p o s i t . The c h r o m i t e - r i c h d u n i t e z o n e s a r e v e r y i r r e g u l a r and d i s c o n t i n u o u s but show l o c a l t e x t u r a l s i m i l a r i t i e s . L o c a l i z e d c h r o m i t e c o n c e n t r a t i o n s w i t h i n t h e s h e a r zone o c c u r as i r r e g u l a r m a s s i v e seams and s m a l l p o d - l i k e b o d i e s o f 30 t o 40 % c h r o m i t e and a r e 10 t o 20 cm i n d i a m e t e r . B e s i d e s t h e l o c a l c h r o m i t e c o n c e n t r a t i o n s , t h e s h e a r zone i t s e l f c o n s i s t s p r e d o m i n a n t l y o f f r i a b l e s e r p e n t i n i t e w i t h v e r y l i t t l e v i s i b l e a c c e s s o r y c h r o m i t e (<0.3 % ) . 4.4.4. Shady Cove Mine C h r o m i t e o r e a t t h e shady Cove Mine o c c u r s w i t h i n b l o c k y c o m p e t e n t d u n i t e and i s a c l a s s I t y p e d e p o s i t . S e v e r a l z o n e s c h a r a c t e r i z e d by u n i q u e t e x t u r a l v a r i e t i e s o c c u r t h r o u g h o u t t h e d e p o s i t . T h e r e a r e a v a r i e t y o f magmatic t e x t u r e s , t h e most s p e c t a c u l a r i s t h e n o d u l a r c h r o m i t e as shown i n p l a t e 7. The n o d u l a r o r e o c c u r s i n i r r e g u l a r p a r a l l e l l a y e r s w i t h t h e n o d u l e s b e i n g s l i g h t l y 48 f l a t t e n e d i n t h e p l a n e o f l a y e r i n g . A s s e e n i n p l a t e 7 , n o d u l a r c h r o m i t e i s m a d e u p o f i n d i v i d u a l n o d u l e u n i t s s u r r o u n d e d b y n e t t e x t u r e d c h r o m i t e . E a c h u n i t c o n s i s t s o f a c e n t r a l r o u n d e d , c h r o m i t e a g g r e g a t e n o d u l e e n c a s e d - i n a d u n i t e s h e l l . S o m e o f t h e u n i t s a r e m i s s i n g t h e c e n t r a l c h r o m i t e n o d u l e . T h e o r i g i n o f t h i s r o c k i s c o m p l i c a t e d i n t h a t t w o s t a g e s o f m a g m a t i c c h r o m i t e s e e m n e c e s s a r y t o e x p l a i n t h e t e x t u r e s . I n i t i a l l y , t h e c h r o m i t e n o d u l e s f o r m e d b y s o m e u n k n o w n p r o c e s s . T h e n o d u l e s w e r e t h e n i n t r o d u c e d i n t o a magma w h e r e p o s s i b l e p a r t i a l m e l t i n g c a u s e d t h e n o d u l e s t o b e s m o o t h l y r o u n d e d . T h e n o l i v i n e c r y s t a l l i z e d a r o u n d t h e n o d u l e s f o r m i n g t h e u n i t . F i n a l l y , i n t e r s t i t i a l o l i v i n e a n d c h r o m i t e f o r m e d , p r o d u c i n g t h e n e t t e x t u r e s . O t h e r t e x t u r e s o c c u r r i n g a t t h e S h a d y C o v e i n c l u d e , c u m u l a t e l a y e r e d c h r o m i t e a n d d i s s e m i n a t e d c h r o m i t e . T h e d e p o s i t a t S h a d y C o v e i s d i v i d e d i n t o a n u p p e r a n d l o w e r o c c u r r e n c e w i t h a f a u l t s e p a r a t i n g t h e t w o ( F i g u r e 6 ) . T h e c h r o m i t i f e r o u s d u n i t e b o d i e s i n e a c h r a n g e i n s i z e f r o m 10 t o 30 m i n l e n g t h a n d a r e r o u g h l y l e n t i c u l a r i n s h a p e . T h e c h r o m i t i f e r o u s d u n i t e b o d i e s h a v e g r a d a t i o n a l c o n t a c t s w i t h t h e s u r r o u n d i n g d u n i t e . 4 9 5. OTHER CHROMITE OCCURRENCES IN THE JOSEPHINE PERIDOTITE 5.1 The Bar R i c k M i n e The B a r R i c k c h r o m i t e d e p o s i t o c c u r s -in a t a b u l a r d u n i t e body, on t h e s o u t h e r n e x t e n s i o n o f t h e J o s e p h i n e P e r i d o t i t e Complex ( F i g u r e 1 ) . The d u n i t e body a t t h e Bar R i c k i s e x p o s e d a l o n g s t r i k e f o r 600 m w i t h t h e w i d t h v a r y i n g f r o m 200 t o 300 m. The c o n t a c t s between d u n i t e and s u r r o u n d i n g h a r z b u r g i t e a r e i n most p l a c e s c o n c e a l e d by t a l u s a n d v e g e t a t i o n , b u t a p p e a r t o be g r a d a t i o n a l t o t h e n o r t h and west and f a u l t e d t o t h e s o u t h and e a s t . D e f o r m e d l e n s e s o f c h r o m i t e o c c u r w i t h i n s h e a r e d and f r i a b l e s e r p e n t i n i z e d d u n i t e , r o u g h l y m a r k i n g t h e p e r i p h e r y o f t h e o r e body. S h e a r i n g i s g e n e r a l l y p a r a l l e l t o t h e d u n i t e / h a r z b u r g i t e c o n t a c t . C h r o m i t e o c c u r s a s m a s s i v e pods and l e n s e s , t r a c e a b l e f o r up t o 5 m and a s i r r e g u l a r s t r e a k s and lumps. The o r e - b e a r i n g s h e a r zone i s p r e s e n t a l o n g much o f t h e u p p e r c o n t a c t o f t h e d u n i t e body, b u t e x c e p t f o r l o c a l o c c u r r e n c e s i s e s s e n t i a l l y d e v o i d of c h r o m i t e . The c h r o m i t e a t t h e Bar R i c k o c c u r s i n c l a s s I t y p e d e p o s i t s . I t i s c o n t a i n e d w i t h i n h a r d and b l o c k y d u n i t e w h i c h i s much l e s s s e r p e n t i n i z e d (30-50 %) t h a n t h e s h e a r z o n e s . C h r o m i t e o c c u r s i n f a i r l y c o n t i n u o u s z o n e s a s 50 l a y e r s , s t r e a k s and p a r a l l e l s e t s o f s t r i n g e r s made up o f p r e d o m i n a t e l y n e t t e x t u r e d c h r o m i t e . L a y e r e d o r e i s commonly o b s e r v e d t o g r a d e s i n t o i n t o h e a v i l y d i s s e m i n a t e d z o n e s and may be t r a c e a b l e f o r up t o 45 m. I n d i v i d u a l l a y e r s s t r i k e N o r t h 6 0 ° - 8 0 ° west and d i p 65° n o r t h . 5.2 The T y s o n Mine The T y s o n c h r o m i t e d e p o s i t o c c u r s i n a n a r r o w e l o n g a t e e x t e n s i o n on t h e s o u t h e r n end o f t h e J o s e p h i n e P e r i d o t i t e . The narrow N-S t r e n d i n g body o f p e r i d o t i t e a t t h e T y s o n i s i n t e r p r e t a t e d as t h e c r e s t o f an a n t i c l i n a l f o l d w i t h b o t h f l a n k s o v e r l a i n by h i g h l y w e a t h e r e d v o l c a n i c r o c k s ( W e l l s e t a l . 1946). The c o n t a c t zone i s h i g h l y s h e a r e d , c o n s i s t i n g o f s e r p e n t i n i z e d p e r i d o t i t e , w i t h l e s s e r d u n i t e . Ore d e p o s i t s of t h e c l a s s II t y p e o c c u r a s t a b u l a r e l o n g a t e b o d i e s a l i g n e d a p p r o x i m a t e l y p a r a l l e l a n d a d j a c e n t t o t h e c o n t a c t o f t h e s h e a r z o n e . A r i n d o f h i g h l y s h e a r e d d u n i t e and r o d i n g i t e o f t e n e n v e l o p e t h e p o d i f o r m o r e d e p o s i t s . W i t h i n t h e main w o r k i n g s , o l d r e p o r t s i n d i c a t e d t h a t pods r a n g i n g from one t o f o u r t h o u s a n d t o n s o c c u r r e d f a i r l y c l o s e l y s p a c e d , o c c a s i o n a l l y j o i n e d by narrow one t o two meter c h r o m i t e s t r i n g e r s ( W e l l s e t a l . 1946). Ore b o d i e s were g e n e r a l l y q u i t e m a s s i v e w i t h gangue c o n t e n t r a n g i n g from 2 t o 15 % by 51 volume. 5.3 Low D i v i d e The d u n i t e body i n t h e Low D i v i d e a r e a i s a N-S t r e n d i n g e l o n g a t e l e n s . C o n t a c t s between t h e d u n i t e and h a r z b u r g i t e a r e g r a d a t i o n a l • o v e r a d i s t a n c e o f 5-10 m. No c o m p o s i t i o n a l l a y e r i n g was o b s e r v e d i n t h e e n c l o s i n g h a r z b u r g i t e . However, a w e l l d e v e l o p e d m i n e r a l f o l i a t i o n o c c u r s t h r o u g h o u t b o t h t h e h a r z b u r g i t e a n d d u n i t e . The t r e n d o f t h e f o l i a t i o n i s s u b - p a r a l l e l t o t h e d i r e c t i o n o f e l o n g a t i o n o f t h e d u n i t e body. F i g u r e 9 i s a d i a g r a m a t i c s k e t c h o f t h e d u n i t e body s h o w i n g t h e r e l a t i o n s h i p s between t h e v a r i o u s s t r u c t u r e s . W e l l - d e v e l o p e d c u m u l a t e l a y e r i n g o c c u r s i n t h e c h r o m i t e t h r o u g h o u t t h e body, q u a l i f y i n g i t a s a c l a s s I t y p e d e p o s i t . P a r a l l e l l a y e r s , composed m a i n l y o f n e t t e x t u r e d c h r o m i t e , have an e a s t - w e s t t r e n d t h r o u g h o u t t h e body and a r e a t r i g h t a n g l e s t o t h e f o l i a t i o n . T h e s e l a y e r s show no e v i d e n c e o f f o l d i n g . S e v e r a l e a s t - w e s t t r e n d i n g s h e a r z o n e s o c c u r i n t h e d u n i t e body and a r e h o s t t o d e f o r m e d c h r o m i t e o c c u r r e n c e s . In two a r e a s , t h e s e s h e a r z o n e s c r o s s c u t c h r o m i t e - r i c h l a y e r i n g . I n b o t h p l a c e s , t h e s h e a r z o n e s c o n t a i n a h i g h e r c o n c e n t r a t i o n o f c h r o m i t e t h a n t h e s u r r o u n d i n g l a y e r e d r o c k (15-25 % c h r o m i t e i n t h e s h e a r zone compared t o 3-5 % i n l a y e r e d c h r o m i t e ) . The t e x t u r e s o f c h r o m i t e w i t h i n t h e 52 Figure 9. Plan-view Sketch of Low-Divide Area meters 53 s h e a r z o n e s r a n g e f r o m c h r o m i t e b r e c c i a , w h i c h i s w e l l f o l i a t e d i n a d i r e c t i o n p a r a l l e l t o t h e s h e a r f a b r i c , t o m a s s i v e f r i a b l e c h r o m i t e w h i c h o c c u r s i n s m a l l p o c k e t s (10 t o 20 c m i n d i a m e t e r ) . T h e s h e a r z o n e s a r e c h a r a c t e r i z e d b y a s t r o n g p l a t y f a b r i c p a r a l l e l t o s h e a r i n g a n d i n t e n s e s e r p e n t i n z a t i o n . T h e c h r o m i t e o c c u r r e n c e s w i t h i n t h e s h e a r z o n e s a r e i n s o m e w a y s s i m i l a r t o c l a s s I I c h r o m i t e d e p o s i t s a n d i n d i c a t e t h a t t h e r e i s a p o s s i b l e g e n e t i c l i n k b e t w e e n t h e t w o t y p e s . 54 6 . O R I G I N O F A L P I N E U L T R A M A F I C R O C K S A l p i n e p e r i d o t i t e c o m p l e x e s a r e g e n e r a l l y a c c e p t e d a s b e i n g t h e u l t r a m a f i c p a r t o f a n o p h i o l i t e s e q u e n c e w h i c h w a s s o l i d , a n d i n s o m e e x a m p l e s h o t , w h e n e m p l a c e d o n t o o r i n t o t h e c r u s t b y o b d u c t i o n ( C o l e m a n 1 9 7 7 ) . O p i n i o n s d i f f e r a s t o how t h e u l t r a m a f i c r o c k s o r i g i n a l l y f o r m e d . T h a y e r ( 1 9 6 4 , 1 9 6 7 , 1 9 6 9 ) a d v o c a t e d a n o r i g i n b y m a g m a t i c d i f f e r e n t i a t i o n i n w h i c h a magma o f m a n t l e c o m p o s i t i o n f r a c t i o n a t e d t o p r o d u c e a t y p i c a l o p h i o l i t e s e q u e n c e . H e c o n s i d e r s t h e u l t r a m a f i c p o r t i o n t o r e p r e s e n t h i g h t e m p e r a t u r e m i n e r a l s w h i c h h a v e c r y s t a l l i z e d e a r l i e r a n d s e t t l e d o u t b y g r a v i t y p r o c e s s e s . O n t h e o t h e r h a n d a w i d e l y a c c e p t e d h y p o t h e s i s d e s c r i b e s t h e p e r i d o t i t e a s b e i n g t h e d e p l e t e d r e s i d u e o f p r i m i t i v e m a n t l e m a t e r i a l f r o m w h i c h b a s a l t i c a n d o t h e r m a t e r i a l s h a v e b e e n r e m o v e d b y p a r t i a l f u s i o n ( H e s s 1 9 6 4 , G r e e n a n d R i n g w o o d , 1 9 6 7 , D i c k e y 1 9 7 0 , D e w e y a n d B i r d 1 9 7 0 , D i c k 1 9 7 5 ) . T h e r e s i d u e i s d e p l e t e d i n a l k a l i s , a l u m i n a , s i l i c a a n d F e a n d e n r i c h e d i n r e f r a c t o r i e s s u c h a s Mg a n d C r . T h e u l t r a m a f i c r o c k s , t y p i c a l l y h a r z b u r g i t e a n d d u n i t e , a r e c h a r a c t e r i z e d b y a f o l i a t i o n a n d a r e o f t e n r e f e r r e d t o a s t e c t o n i t e s . T h e d u n i t e t e c t o n i t e s o c c u r i n t w o c h a r a c t e r i s t i c f o r m s : 1) d u n i t e l e n s e s c o n f o r m a b l e t o c o m p o s i t i o n a l l a y e r i n g i n h a r z b u r g i t e , a n d 2 ) d u n i t e b o d i e s d i s c o r d a n t t o l a y e r i n g . T h e l a t t e r t y p e r a n g e f r o m l a r g e ( 2 x 1 0 km) l e n s e s , t o s m a l l ( 1 5 x 2 0 c m ) c o m p a c t b o d i e s 55 i n s c a l e . S t u d i e s b y H o p s o n e_t a l . ( 1 9 8 1 ) a n d B o u d i e r a n d C o l e m a n ( 1 9 8 1 ) o n t h e O m a n O p h i o l i t e s h o w t h a t t h e c o n c o r d a n t d u n i t e i s t h e r e , g e n e r a l l y m o r e c o m m o n i n t h e l o w e r p a r t o f t h e t e c t o n i t e a n d h a s u n d e r g o n e t h e s a m e h i g h - t e m p e r a t u r e , l o w - s t r e s s d e f o r m a t i o n a s t h e t e c t o n i t e . D i s c o r d a n t d u n i t e b o d i e s o c c u r t h r o u g h o u t t h e a l p i n e p e r i d o t i t e s . A l t h o u g h t h e d i s c o r d a n t d u n i t e s a r e r e l a t i v e l y u n d e f o r m e d c o m p a r e d t o t h e c o n c o r d a n t o n e s , t h e y h a v e a w e a k m e t a m o r p h i c f o l i a t i o n w h i c h i s g e n e r a l l y c o n c o r d a n t t o f o l i a t i o n i n t h e h a r z b u r g i t e . T e x t u r e s o f c h r o m i t e g r a i n s f o u n d i n t h e d i s c o r d a n t • d u n i t e b o d i e s a r e s i m i l a r t o t h e p r i m a r y c u m u l a t e t e x t u r e s f o u n d i n s t r a t i f o r m d e p o s i t s ( T h a y e r 1 9 6 9 ) . T h e s e t e x t u r e s , w h i c h a r e l e s s d e f o r m e d t h a n t h o s e i n c o n c o r d a n t d u n i t e s , c o m m o n l y p a r a l l e l n e i t h e r t h e e l o n g a t i o n d i r e c t i o n o f t h e b o d y n o r t e c t o n i c l a y e r i n g a n d f o l i a t i o n ( C a s s a r d e t a l . 1 9 8 1 ) . I t i s w i d e l y a c c e p t e d t h a t t h e r e l a t i v e l y u n d e f o r m e d p r i m a r y t e x t u r e s w i t h i n p o d i f o r m c h r o m i t e d e p o s i t s a r e s i m i l a r i n n a t u r e t o t h o s e f o u n d i n s t r a t i f o r m c h r o m i t e d e p o s i t s ( T h a y e r 1 9 6 9 , J a c k s o n a n d o t h e r s 1 9 7 5 , D i c k 1 9 7 5 , C o l e m a n 1 9 7 7 , K a c i r a 1 9 8 1 ) . O p i n i o n s d i f f e r g r e a t l y a s t o t h e o r i g i n o f t h e c u m u l a t e s . T r a d i t i o n a l l y , w h e n a f r a c t i o n a t i o n m o d e l f o r t h e p e r i d o t i t e s w a s f a v o u r e d a n d t h e e n t i r e u l t r a m a f i c m a s s r e p r e s e n t e d a c u m u l a t e , i t w a s e a s y t o v i e w c u m u l a t e s i n t h e t e c t o n i t e s a s r e l i c s o f t h i s o r i g i n . H o w e v e r , w i t h o v e r w h e l m i n g c u r r e n t s u p p o r t i n 56 f a v o u r o f a p a r t i a l r e s i d u e m o d e l f o r h a r z b u r g i t e , c u m u l a t e s a r e m o r e d i f f i c u l t t o a c c o u n t f o r . A n u m b e r o f h y p o t h e s i s h a v e b e e n o f f e r e d . D i c k e y (1975) a r g u e s t h a t t h e c u m u l a t e s o c c u r r i n g w i t h i n a l p i n e r i d g e magma c h a m b e r s h a v e s u n k t h r o u g h t h e M o h o i n t o t h e u l t r a m a f i c t e c t o n i t e s a s s o l i d m a s s e s . D i c k (1975) p r o p o s e s a m o d e l , i n v o l v i n g i_n s i t u f u s i o n o f t h e h a r z b u r g i t e s . F r o m s u c h m e l t s , o l i v i n e , p y r o x e n e a n d c h r o m i t e a c c u m u l a t e a s l a y e r s o f d u n i t e , p y r o x e n i t e a n d c h r o m i t e i n s m a l l m a g m a c h a m b e r s . T h i s m o d e l i n v o l v e s r e m e l t i n g o f h a r z b u r g i t e a b o v e a y o u n g , s u b d u c t i o n z o n e , p r o d u c i n g a n a l u m i n o u s m e l t w h i c h i s f i l t e r - p r e s s e d t h r o u g h t h e p e r i d o t i t e . D i c k d e s c r i b e s t h e p o d i f o r m c h r o m i t e b o d i e s a n d e n c l o s i n g d u n i t e s a s l a g d e p o s i t s . M e t a s o m a t i c t r a n s f o r m a t i o n o f h a r z b u r g i t e t o d u n i t e a n d a c c e s s o r y c h r o m i t e h a s b e e n h y p o t h e s i z e d b y s e v e r a l w o r k e r s , i n c l u d i n g : M c T a g g a r t ( 1 9 7 2 ) , D u n g a n a n d A v e ' L a l l e m a n t ( 1 9 7 7 ) a n d W r i g h t e t a l . ( 1 9 8 2 ) . I n s i t u t r a n s f o r m a t i o n o f h a r z b u r g i t e t o d u n i t e i s a c c o m p l i s h e d b y a q u e o u s v a p o r s w h i c h a t t a c k p y r o x e n e a n d r e m o v e S i a n d C a , l e a v i n g o l i v i n e a n d s p i n e l b e h i n d . E v i d e n c e f o r t h i s p r o c e s s i n c l u d e s t h e p r e s e n c e o f m i n e r a l f o l i a t i o n a c r o s s t h e b o d i e s w h i c h i s c o n s i d e r e d t o b e a r e l i c t t e x t u r e . T h i s p r o c e s s s e e m s p l a u s i b l e f o r t h e s m a l l p a t c h e s o f i r r e g u l a r s h a p e d d u n i t e b o d i e s ; h o w e v e r , i t i s d i f f i c u l t t o i m a g i n e a m e t a s o m a t i c p r o c e s s w h i c h c o u l d r e s u l t i n t h e f o r m a t i o n o f l a r g e d u n i t e b o d i e s w i t h t h e i r e n c l o s e d 57 p r i m a r y magmatic c h r o m i t e t e x t u r e s . C a s s a r d e_t a_l . (1981) d e s c r i b e t h e d i s c o r d a n t d u n i t e b o d i e s a s h a v i n g f o r m e d w i t h i n s t e e p c o n d u i t s w h i c h f e d c h r o m e - r i c h u l t r a m a f i c magmas a c r o s s h a r z b u r g i t e s t o a magma chamber u n d e r n e a t h an o c e a n i c s p r e a d i n g r i d g e . The c o n c e p t o f d i s c o r d a n t d u n i t e s m a r k i n g f o s s i l c o n d u i t s i s a l s o s u p p o r t e d by Hopson e t a l . (1981) a n d B o u d i e r and Coleman ( 1 9 8 1 ) . They p r o p o s e two h y p o t h e s e s : (1) t h e d u n i t e and a s s o c i a t e d c h r o m i t e w i t h i n t h e c o n d u i t r e p r e s e n t d e p l e t e d r e s i d u u m r e s u l t i n g f r o m s e c o n d s t a g e p a r t i a l m e l t i n g o f t h e r e f r a c t o r y h a r z b u r g i t e , o r (2) d u n i t e c u m u l a t e s a r e t h e r e s u l t o f f r a c t i o n a l c r y s t a l l i z a t i o n o f o l i v i n e f r o m a s c e n d i n g p a r e n t a l b a s a l t i c l i q u i d s w i t h i n t h e h a r z b u r g i t e . I n t h e J o s e p h i n e P e r i d o t i t e , f i e l d o b s e r v a t i o n s of two l a r g e d i s c o r d a n t d u n i t e b o d i e s s u p p o r t an o r i g i n i n c o n c o r d a n c e w i t h t h e f i r s t h y p o t h e s i s . In b o t h t h e Low D i v i d e and Chrome R i d g e a r e a s , t h e d u n i t e s a r e l e n s - s h a p e d and e l o n g a t e d i n a N-S d i r e c t i o n . F o l i a t i o n d i r e c t i o n s w i t h i n t h e d u n i t e and e n c l o s i n g h a r z b u r g i t e a r e r o u g h l y c o n c o r d a n t t o t h e c o n t a c t s . P r i m a r y l a y e r i n g w i t h i n t h e c h r o m i t e i s d i s c o r d a n t t o t h e f o l i a t i o n a nd e l o n g a t i o n d i r e c t i o n o f t h e d u n i t e body ( F i g u r e s 6 and 9 ) . The c o n t a c t s between t h e d u n i t e body and e n c l o s i n g h a r z b u r g i t e a r e g r a d a t i o n a l o v e r a p p r o x i m a t e l y 20 m. T h e r e i s a g e n e r a l l a c k o f c o m p o s i t i o n a l l a y e r i n g i n t h e h a r z b u r g i t e e n c l o s i n g t h e d u n i t e body, a l t h o u g h m i n o r l a y e r i n g was 58 o b s e r v e d a n d m e a s u r e d . T h e u n l a y e r e d h a r z b u r g i t e z o n e i s 1 t o 2 km t h i c k f o r t h e C h r o m e R i d g e a r e a i n w h i c h t h e d u n i t e i t s e l f i s 1 . 5 km w i d e . N o d a t a a r e a v a i l a b l e f o r t h e t h i c k n e s s o f t h e c o r r e s p o n d i n g z o n e a t L o w D i v i d e . A l a c k o f t e c t o n i c l a y e r i n g i n t h e h a r z b u r g i t e s u r r o u n d i n g t h e d u n i t e b o d y m a y i n d i c a t e t h a t h o t b a s a l t i c m a g m a m o v i n g t h r o u g h t h e h a r z b u r g i t e h a s e l i m i n a t e d t e c t o n i c l a y e r i n g . A n o r i g i n f o r t h e s e d u n i t e b o d i e s a s f i l l i n g s o f f o s s i l c o n d u i t s i s c o m p a t i b l e w i t h f i e l d o b s e r v a t i o n s m a d e o n t h e s e d e p o s i t s . A m e l t p r o d u c e d b y s e c o n d s t a g e f u s i o n o f h a r z b u r g i t e , d e e p e r i n t h e m a n t l e c o u l d g i v e r i s e t o t h e f r a c t i o n a t i o n p r o d u c t s f o u n d i n t h e d u n i t e b o d i e s . A l e n t i c u l a r p y r o x e n i t e b o d y w h i c h o c c u r s s e v e r a l k i l o m e t e r s f r o m t h e d u n i t e b o d y a t C h r o m e R i d g e c o u l d b e t h e r e s u l t o f c r y s t a l l i z a t i o n o f t h e l o w e r t e m p e r a t u r e m i n e r a l s f r o m t h e s a m e m e l t . T h e f o r m a t i o n o f p a r a l l e l l a y e r i n g w i t h i n t h e c h r o m i t e a t r i g h t a n g l e s t o t h e f o l i a t i o n , e l o n g a t i o n d i r e c t i o n o f t h e d u n i t e b o d y a n d i n t h e c a s e o f C h r o m e R i d g e , t h e c o m p o s i t i o n a l l a y e r i n g , i n d i c a t e t h e f o r m a t i o n o f t h e d u n i t e b o d i e s t o o k p l a c e w h e n t h e m a i n s t r e s s d i r e c t i o n s w e r e v e r t i c a l . T h e m o s t l i k e l y t i m e f o r t h i s e v e n t w a s d u r i n g u p w a r d r i s e o f t h e t e c t o n i t e a t a s p r e a d i n g r i d g e o r i n a m a n t l e d i a p i r . 59 7 . O R I G I N O F C H R O M I T E A s d i s c u s s e d i n p r e v i o u s s e c t i o n s , c e r t a i n a l p i n e -t y p e c h r o m i t e c o n c e n t r a t i o n s f o r m b y c u m u l a t e p r o c e s s e s ( c l a s s I t y p e d e p o s i t s ) . T h e s e d e p o s i t s a r e r e s t r i c t e d t o l a r g e d u n i t e b o d i e s . O t h e r c h r o m i t e o c c u r r e n c e s ( c l a s s I I t y p e d e p o s i t s ) s h o w n o e v i d e n c e o f h a v i n g f o r m e d a s c u m u l a t e s . T h e y o c c u r i n s h e a r z o n e s i n h a r z b u r g i t e . V a r i o u s t h e o r i e s h a v e b e e n p r o p o s e d t o e x p l a i n t h e n o n -c u m u l a t e c h r o m i t e o c c u r r e n c e s . E a r l y t h e o r i e s w h i c h e x p l a i n e d t h e o r i g i n o f c h r o m i t e c o n c e n t r a t i o n s b y s e v e r a l s t a g e s o f c h r o m i t e c r y s t a l l i z a t i o n , a p p e a r e d t o s a t i s f y f i e l d o b s e r v a t i o n s b u t w e r e n o t c o m p a t i b l e w i t h e x p e r i m e n t a l d a t a . F i s h e r ( 1 9 2 9 ) a n d S a m p s o n ( 1 9 3 1 ) d e s c r i b e a s e q u e n c e o f c h r o m i t e c r y s t a l l i z a t i o n f r o m a p e r i d o t i t e magma a s f o l l o w s : 1) C r y s t a l l i z a t i o n o f e a r l y m a g m a t i c c h r o m i t e i s w e l l a d v a n c e d b e f o r e t h e i n i t i a l c r y s t a l l i z a t i o n o f t h e o l i v i n e a n d p y r o x e n e m i n e r a l s . 2 ) C r y s t a l l i z a t i o n o f l a t e m a g m a t i c c h r o m i t e w h i c h r e p l a c e s , e i t h e r p a r t i a l l y o r c o m p l e t e l y , t h e g r o u n d m a s s m i n e r a l s a n d s u r r o u n d s , e m b a y s o r c u t s t h e s e m i n e r a l s . 3 ) C h r o m i t e o f t h e h y d r o t h e r m a l v a r i e t y i s d i s t i n c t l y l a t e r t h a n t h e m a g m a t i c p e r i o d . C h r o m i t e o f t h i s p e r i o d w a s p r o b a b l y d e r i v e d b y t h e h y d r o t h e r m a l s o l u t i o n s f r o m t h e p a r e n t r o c k o r f r o m o t h e r r o c k s w h i c h t h e y t r a v e r s e d . 60 \ T h e c o n c e p t o f l a t e s t a g e c r y s t a l l i z a t i o n o f c h r o m i t e a n d h y d r o t h e r m a l r e m o b i l i z a t i o n w a s d i s p u t e d o n t h e g r o u n d s o f t h e e x t r e m e l y r e f r a c t o r y n a t u r e o f c h r o m i t e a n d a l a c k o f e v i d e n c e t h a t c h r o m i t e i s e a s i l y r e m o b i l i z e d c h e m i c a l l y . I n o r d e r t o p r o d u c e t h e m a s s i v e c h r o m i t e p o d s t h a t c o n t a i n o n l y m i n o r s i l i c a t e m i n e r a l s ( 2 - 8 % ) , D i c k p r o p o s e s e x t e n s i v e o v e r g r o w t h o f t h e o r i g i n a l c u m u l a t e g r a i n s b y c h r o m i t e r i c h m e l t s w h i c h p a s s t h r o u g h t h e r o c k s . E x t e n s i v e m e l t i n g o f e n s t a t i t e i n s u r r o u n d i n g h a r z b u r g i t e s i s i d e n t i f i e d a s t h e s o u r c e o f t h e c h r o m i t e - r i c h m e l t s . E v i d e n c e i n c l u d e s , t h e i r r e g u l a r p a t c h e s o f d e n s e , c o a r s e c h r o m i t e i n d i s s e m i n a t e d o r e , t h e c o a r s e n i n g o f g r a i n s i z e o f c h r o m i t e w i t h i n c r e a s i n g i r r e g u l a r i t y i n g r a i n s h a p e , t h e a p p a r e n t c o a l e s c e n c e o f i n d i v i d u a l g r a i n s , t h e p r e s e n c e o f t r a p p e d p y r o x e n e w i t h i n c h r o m i t e g r a i n s , a n d f i n a l l y t h e m a s s i v e c h r o m i t i t e s t h e m s e l v e s . T h e r e d o n o t a p p e a r t o b e a n y c h e m i c a l d a t a w h i c h s u p p o r t s D i c k ' s o v e r g r o w t h t h e o r y . T h e m e c h a n i s m s f o r p r o d u c i n g t h e n e c e s s a r y c h r o m i t e - r i c h m e l t s a n d t h e p h y s i c a l e v i d e n c e i t s e l f a r e b o t h r a t h e r t e n u o u s . T h e m a i n p r o b l e m s w i t h h i s m o d e l a r e : '1 ) t h e p h y s i c a l c h a r a c t e r i s t i c s o f c h r o m i t e m a k e i t d i f f i c u l t t o r e m o b i l i z e a n d t r a n s p o r t i n s o l u t i o n , a n d 2 ) t h e c h r o m i t e b o d i e s a r e e x t r e m e l y h o m o g e n o u s t h r o u g h o u t w i t h n o c h e m i c a l e v i d e n c e o f o v e r g r o w t h s . T h e w i d e p o t e n t i a l v a r i a b i l i t y i n C r : F e : A l r a t i o s w i t h i n c h r o m i t e l e a d s o n e 61 t o e x p e c t t h a t s o m e n o t i c a b l e c h e m i c a l v a r i a b i l i t y w i t h i n m u l t i s t a g e c h r o m i t e d e p o s i t s s h o u l d b e p r e s e n t . I n s p e c i m e n s e x a m i n e d u n d e r b o t h p o l a r i z e d a n d r e f l e c t e d l i g h t , i n w h i c h c h a n g e s i n C r : F e : A l r a t i o s s h o w u p a s d i s t i n c t c o l o u r c h a n g e s , n o d e t e c t a b l e v a r i a t i o n w a s o b s e r v e d . I n v e s t i g a t i o n s w i t h t h e e n e r g y d i s p e r s i v e s y s t e m o f t h e s c a n n i n g e l e c t r o n m i c r o s c o p e c o n f i r m m i c r o s c o p i c e v i d e n c e t h a t t h e c h r o m i t e g r a i n s a r e c h e m i c a l l y v e r y h o m o g e n e o u s . F i g u r e 10 s h o w s t h e r e l a t i v e a m o u n t s o f C r a n d F e f o r a c h r o m i t e g r a i n f r o m t h e T y s o n M i n e . T w o s e p a r a t e r e a d i n g s o f c o u n t s p e r 100,000 w e r e t a k e n f r o m t h e c o r e a n d t h e r i m o f t h e g r a i n y i e l d i n g v e r y s i m i l a r r e s u l t s . A n i d e a w h i c h h a s r e c e n t l y b e e n i n t r o d u c e d ( C a s s a r d e t a l . 1 9 8 1 ) i n v o l v e s i n t e n s i v e d e f o r m a t i o n a s a p r o c e s s b y w h i c h v a r i o u s t e x t u r e s w i t h i n c h r o m i t e d e p o s i t s m a y b e p r o d u c e d . A v e r y s t r o n g c o r r e l a t i o n w a s m a d e b e t w e e n t h e d e g r e e o f c o n c o r d a n c y o f o r e b o d i e s t o t e c t o n i c t r e n d s i n t h e h o s t r o c k s a n d d e g r e e o f c o n c e n t r a t i o n o f , a n d d e f o r m a t i o n w i t h i n t h e c h r o m i t e i t s e l f . T e x t u r e s s i m i l a r t o c u m u l a t e t e x t u r e s i n s t r a t i f o r m d e p o s i t s w e r e g e n e r a l l y u n d e f o r m e d a n d d i s c o r d a n t t o h o s t r o c k c o n t a c t s a n d s t r u c t u r a l t r e n d s . C o n s i d e r i n g t h e d a t a o f t h i s t h e s i s , t h i s m o d e l a p p e a r s m o s t p l a u s i b l e , a s i t c a n e x p l a i n m o s t o f t h e f e a t u r e s s e e n i n t h e C h r o m e R i d g e a r e a . T h e o r i g i n o f t h e s e m a s s i v e p o d s o f c h r o m i t e h a s a l w a y s b e e n e n i g m a t i c , 62 PR= 168KI 39SEC 180860 INT; U*S132 H=48KEU 2 - 2 H A8=48KEU 2Hi 8951 CR i ; i 1916 FE i>9i24keu.L, (RIM) ;PR= 1M0KI 46 SEC • ,.2=91 H 100008 INT U=8192 H=40KEU 2=2H AG=40KEU 2H (CORE) 18 .24KEU> , Figure 10. SEM energy dispersive data showing chemical homogeneity between core and rim of deformed chromite grain. 63 mainly due to the lack of magmatic features and the massive nature of the chromite i t s e l f . Studies by Wager and Brown (1967) have shown that in simple packing experiments, a cumulate w i l l contain a minimum of 40% i n t e r s t i t i a l melt. In order to produce chromitites with only 5-10% i n t e r s t i t i a l material, a secondary enrichment process must be adopted. Monomineralic phases do occur within magmatic sediments, and are generally considered to be the result of adcumulate growth (Wager and Brown 1967, Jackson 1975). If t h i s were the mechanism responsible for the formation of massive chromite pods, one would have to assume the following: a) The massive pods represent broken segments of cumulate layers which in some cases would be tens of meters thick. b) In e s s e n t i a l l y a l l cases, massive chromite layers were separated from the enormous dunite sections one would expect to f i n d associated with the chromite. c) Magma chambers capable of producing layers t h i s thick would be extremely large (compare t h i s to the 30 cm thick layers of chromite in the 100 km wide Bushveld complex). d) Somewhere between the o r i g i n a l formation and f i n a l emplacement, a l l magmatic textures are destroyed. e) Nowhere have the thick cumulate layers (> 10-20 cm) been preserved in an o r i g i n a l non-deformed condition. Problems with t h i s model are seemingly 64 insurmountable. A more reasonable approach to the problem derives from the ubiquitous association of highly deformed chromite textures with massive chromite pods. This association has been obsered in a number of l o c a l i t i t e s in the Josephine Peridotite and also in New Caledonia (Cassard et a l . 1981). This strong c o r r e l a t i o n points to the p o s s i b i l i t y that the process of extreme deformation of o r i g i n a l l y cumulate chromite has resulted in the formation of massive deformed chromite pods. 7.1 Concentration Of Chromite By Deformation Metamorphic d i f f e r e n t i a t i o n or the segregation of a homogenous rock into mineralogically d i f f e r e n t sub-units, through the application of heat and pressure, has been well established (Turner and Verhoogen 1960, Ramburg 1954, e t c . ) . Ramburg (1954), describes metamorphic segregations in the form of v e i n l e t s , lenses, i r r e g u l a r bodies or plane p a r a l l e l repeating layers. These are a l l d e s c r i p t i v e of the various forms which one finds chromite occurrences in the Josephine P e r i d o t i t e . Chemical metasomatism has generally been considered by most workers as being the most important process responsible for metamorphic segregation. The e f f e c t s of mechanical deformation processes on metamorphic d i f f e r e n t i a t i o n , however, have not been ignored. Schmidt 65 ( 1 9 3 2 ) , d e v e l o p e d a h y p o t h e s i s o f m e t a m o r p h i c d i f f e r e n t i a t i o n b y p u r e l y m e c h a n i c a l m e a n s . H e s u g g e s t e d t h a t m a j o r s l i p s u r f a c e s w h i c h f o r m i n t h e e a r l y s t a g e s o f d e f o r m a t i o n o f a h e t e r o g e n o u s r o c k a r e i r r e g u l a r a n d d i s c o n t i n u o u s . T h e s l i p s u r f a c e s d e v e l o p i n m i n e r a l s w h i c h a r e m o s t a b l e t o a d j u s t t h e i r l a t t i c e s t o t h e i m p o s e d s t r e s s e s a n d a r e i n t e r r u p t e d b y c r y s t a l s a n d c r y s t a l l i n e a g g r e g a t e s w h i c h a r e l e s s p l i a b l e . A s d e f o r m a t i o n p r o c e e d s , t h e o b s t r u c t i n g m i n e r a l s a r e m e c h a n i c a l l y e l i m i n a t e d a n d s e g r e g a t e d , w h i l e t h e p l a s t i c a l l y d e f o r m e d m i n e r a l s f l o w i n t o m o r e c o n t i n u o u s l a y e r s . B h a t t a c h a r j i ( 1 9 6 7 ) p r e s e n t e d a n e x a m p l e o f t e c t o n i c a l l y i n d u c e d m e c h a n i c a l s e g r e g a t i o n ( f l o w d i f f e r e n t i a t i o n ) w h e r e i n i t i a l l y d i s p e r s e d p a r t i c u l a t e m a t t e r w a s c o n c e n t r a t e d a t t h e c e n t e r d u r i n g f l o w o f a f l u i d t h r o u g h a p i p e . H i s e x p e r i m e n t s h e l p e d t o e x p l a i n t h e c o n c e n t r a t i o n o f p h e n o c r y s t s o b s e r v e d i n t h e c e n t r e o f v e r t i c a l d i k e s i n N o r t h S k y e . A l t h o u g h B h a t t a c h a r j i * s e x p e r i m e n t s p e r t a i n t o f l o w d i f f e r e n t i a t i o n i n m a g m a , a r e a s o n a b l e a n a l o g y c a n b e m a d e t o t h e t e c t o n i t e s i n a l p i n e u l t r a m a f i c s . V i s c o s i t i e s o b t a i n e d b y p l a s t i c a l l y f l o w i n g o l i v i n e i n t h e h i g h s t r e s s e n v i r o n m e n t m a y b e l a r g e e n o u g h t o r e s u l t i n a s i m i l a r t y p e o f d i f f e r e n t i a t i o n b e t w e e n o l i v i n e a n d c h r o m i t e . A m e c h a n i s m s u c h a s t h i s c o u l d e x p l a i n t h e s y m m e t r i c a l l a y e r i n g o b s e r v e d i n t e c t o n i t e s . D u n i t e l a y e r s w i t h c e n t r a l c h r o m i t i t e s e a m s a s s e e n i n p l a t e 17 w o u l d b e t h e e x p e c t e d r e s u l t o f p l a s t i c f l o w d i f f e r e n t i a t i o n . 66 Plate 17. Layered t e c t o n i t e showing dunite l a y e r with c e n t r a l chromite seam. 67 B u r c h (1968) c a l l e d upon s o l i d s t a t e d e f o r m a t i o n t o a c c o u n t f o r t h e d i f f e r e n t i a t i o n o f o l i v i n e a n d p y r o x e n e w i t h i n t h e B u r r o M o u n t a i n u l t r a m a f i c body. He s u g g e s t e d t h a t t h e d i f f e r i n g b e h a v i o r o f o l i v i n e and p y r o x e n e d u r i n g d e f o r m a t i o n , i n w h i c h o l i v i n e y i e l d s r e a d i l y t o p l a s t i c f l o w a g e and p y r o x e n e d e f o r m s i n a more b r i t t l e manner, r e s u l t s i n t h e i r u l t i m a t e s e g r e g a t i o n i n t o l a y e r s . B u r c h d e s c r i b e s t h e a c t u a l mechanism as one i n w h i c h l o c a l t e m p e r a t u r e and p r e s s u r e i n c r e a s e s as a r e s u l t o f f r i c t i o n c o m b i n e t o e s t a b l i s h an a c t i v i t y g r a d i e n t a c r o s s t h e band o f d e f o r m a t i o n . As a r e s u l t , u n s t a b l e p y r o x e n e i s removed f r o m t h e d e f o r m a t i o n z o n e , w h i l e p l a s t i c a l l y y i e l d i n g o l i v i n e i s s t a b l e w i t h i n z o n e s o f weakness. The m e c h a n i c a l p r o c e s s by w h i c h c h r o m i t e a p p e a r s s e g r e g a t e d f r o m o l i v i n e and c o n c e n t r a t e d , i s i n p r i n c i p l e t o be t h e same as t h o s e p r o p o s e d by S c h m i d t (1932) and B u r c h (1968) . The p r o c e s s r e l i e s on t h e d i f f e r i n g b e h a v i o r o f o l i v i n e and c h r o m i t e under s t r e s s . T h e r e i s a b u n d a n t e x p e r i m e n t a l e v i d e n c e t h a t u nder h i g h s t r e s s , o l i v i n e d e f o r m s p l a s t i c a l l y ( C a r t e r and A v e * L a l l e m a n t 1970, A v e ' L a l l e m a n t a n d C a r t e r 1970, N i c o l a s e t a l . 1971, N i c o l a s and Le P i c h o n 1980 and Ross e t a l . 1980). Under t h e same c o n d i t i o n s , a c c e s s o r y c h r o m i t e g r a i n s , b e i n g o c t a h e d r a l and s i m i l a r t o g a r n e t a r e much more r e s i s t a n t t o s t r e s s and would r e t a i n t h e i r o r i g i n a l s h a p e ( R o s s , p e r s o n a l comm.). As a n t i c i p a t e d , t h e r e s i s t a n t c h r o m i t e 68 g r a i n s w o u l d b e s e g r e g a t e d f r o m t h e p l a s t i c a l l y f l o w i n g o l i v i n e a n d c o n c e n t r a t e d . T h e r e s u l t i n g c o n c e n t r a t i o n s o c c u r i n t h e o b s e r v e d f o r m s o f v e i n l e t s , l e n s e s , i r r e g u l a r b o d i e s , e t c . A s e c o n d h y p o t h e s i s f o r t h e s o l i d s t a t e c o n c e n t r a t i o n o f c h r o m i t e i n v o l v e s t h e p h y s i c a l s q u e e z i n g o u t o f s i l i c a t e s f r o m b e t w e e n c h r o m i t e g r a i n s . T h i s r e s u l t s i n t h e c o m p a c t i o n o f r e s i s t a n t c h r o m i t e g r a i n s . T h e p h y s i c a l m e c h a n i s m i n v o l v e d i s s i m i l a r i n c o n c e p t t o t h a t o f " f i l t e r p r e s s i n g " i n a m a g m a t i c e n v i r o n m e n t , e x c e p t t h a t i t o c c u r s i n a s o l i d s t a t e . A f i n a l m e c h a n i s m i s o b s e r v e d i n i s o c l i n a l l y f o l d e d c h r o m i t e r i c h l a y e r s . O n c e a g a i n t h e p l a s t i c a l l y d e f o r m e d o l i v i n e w i l l b e h a v e d i f f e r e n t l y t h a n w i l l t h e r e s i s t a n t c h r o m i t e . I n t h i s c a s e t h e c o n c e n t r a t i o n o f c h r o m i t e i n t h e n o s e o f t h e f o l d a s w e l l a s t h e s q u e e z i n g o u t o f i n t e r l a y e r e d o l i v i n e p r o d u c e s c o n c e n t r a t e d m a s s i v e b o d i e s o f c h r o m i t e ( p l a t e 8 ) . 69 7 . 2 F i e l d E v i d e n c e A n u m b e r o f f i e l d r e l a t i o n s h i p s i n t h e J o s e p h i n e P e r i d o t i t e l e n d s u p p o r t t o t h e m e t a m o r p h i c d i f f e r e n t i a t i o n m o d e l . T h e m o s t s t r i k i n g f e a t u r e s o c c u r i n t h e l a r g e d u n i t e b o d i e s w i t h e n c l o s e d a c c e s s o r y c h r o m i t e . I n e a c h o f t h e C h r o m e R i d g e , L o w D i v i d e a n d B a r R i c k d u n i t e b o d i e s , a l l o f w h i c h c o n t a i n p r i m a r y c u m u l a t e c h r o m i t e l a y e r i n g , t h e r e a r e l a r g e s h e a r z o n e s c o n t a i n i n g m a s s i v e c h r o m i t e b o d i e s . T h e s h e a r z o n e s a p p e a r t o b e r e s t r i c t e d t o t h e d u n i t e b o d i e s . T h e m a s s i v e p o d - l i k e c h r o m i t e . b o d i e s a r e w h o l l y c o n t a i n e d w i t h i n t h e s h e a r z o n e s a n d b e a r a s t r o n g r e s e m b l a n c e t o t h e m a s s i v e p o d s n o t c o n t a i n e d w i t h i n l a r g e d u n i t e b o d i e s . T h e m a i n d i f f e r e n c e s b e t w e e n p o d - l i k e b o d i e s i n d u n i t e h o s t e d s h e a r z o n e s a n d i s o l a t e d p o d s i n h i g h l y d e f o r m e d r o c k s i n c l u d e : a ) T h e p o d - l i k e b o d i e s i n d u n i t e a r e g e n e r a l l y q u i t e s m a l l (< 1 m e t e r i n d i a m e t e r ) , a n d a r e m u c h l e s s m a s s i v e ( 4 0 - 5 0 % c h r o m i t e ) , b ) T h e s e p o d s t h e m s e l v e s a r e f a r m o r e f r i a b l e a n d g e n e r a l l y h a v e p o o r l y d e f i n e d b o u n d a r i e s . I t w o u l d a p p e a r t h a t t h e p o d -l i k e b o d i e s w i t h i n t h e s h e a r z o n e s a r e t h e p r e c u r s e r s t o t h e l a r g e m a s s i v e c h r o m i t e b o d i e s f o u n d e l s e w h e r e . T h e l i n k b e t w e e n t h e p o d - l i k e b o d i e s a n d t h e p r i m a r y c u m u l a t e l a y e r i n g i n t h e s u r r o u n d i n g d u n i t e o c c u r s w i t h i n t h e s h e a r z o n e s . I n a l l t h r e e d u n i t e b o d i e s , r o t a t e d b l o c k s o f l a y e r e d c h r o m i t e , s i m i l a r i n t e x t u r e t o c h r o m i t e 70 i n t h e d u n i t e b o d i e s , o c c u r a d j a c e n t t o t h e p o d s i n t h e s h e a r z o n e s . I n m o s t c a s e s t h e t e x t u r e s w i t h i n t h e b l o c k s a r e d e f o r m e d y e t a r e u n m i s t a k a b l y r e l a t e d t o t h e l a y e r e d c h r o m i t e s u r r o u n d i n g t h e s h e a r z o n e . F i g u r e 11 s h o w s t h e r e l a t i o n s h i p b e t w e e n c u m u l a t e c h r o m i t e i n d u n i t e a n d d e f o r m e d c h r o m i t e i n s h e a r z o n e s . W h a t t h e s e f i e l d o b s e r v a t i o n s s u g g e s t i s t h a t w h e n p r i m a r y c u m u l a t e c h r o m i t e w i t h i n d u n i t e i s i n t e n s e l y s h e a r e d , i t b e c o m e s c o n c e n t r a t e d i n t o p o d - l i k e b o d i e s o f m a s s i v e c h r o m i t e . A t t h e B u s t e r M i n e o n C h r o m e R i d g e , t h e c h r o m i t e b o d y l i e s w i t h i n a l a r g e s h e a r z o n e . M o s t o f t h e o r e c o n s i s t s o f s m a l l p o d - l i k e b o d i e s o f m a s s i v e c h r o m i t e . R i c h l y d i s s e m i n a t e d c h r o m i t e ( 1 0 - 1 5 %) a l s o o c c u r s . R o t a t e d b l o c k s o f . l a y e r e d c h r o m i t e t h a t o c c u r w i t h i n t h e s h e a r z o n e a r e t e x t u r a l l y s i m i l a r t o t h e l a y e r e d c h r o m i t e f o u n d t h r o u g h o u t t h e C h r o m e R i d g e a r e a . T h e s e b l o c k s a p p e a r t o b e r e l i c t s w h i c h h a v e b e e n i n c o r p o r a t e d i n t o t h e s h e a r z o n e f r o m t h e s u r r o u n d i n g d u n i t e b o d y , b u t h a v e m a n a g e d t o e s c a p e t h e c o n c e n t r a t i n g m e c h a n i s m s . A t t h e B a r R i c k M i n e , a s i m i l a r s i t u a t i o n o c c u r s . T w o l a r g e s h e a r z o n e s c r o s s c u t t i n g t h e l a y e r e d c h r o m i t e c o n t a i n n u m e r o u s m e t e r -l o n g p o d s o f m a s s i v e c h r o m i t e . R e m n a n t l a y e r e d t e x t u r e s i n b l o c k s w h i c h h a v e e s c a p e d t h e c o n c e n t r a t i n g m e c h a n i s m a r e i d e n t i f i a b l e w i t h i n t h e s h e a r z o n e . L o w D i v i d e l o c a l i z e d s h e a r z o n e s w i t h i n t h e l a y e r e d d u n i t e s c o n t a i n l a y e r e d c h r o m i t e , m a s s i v e c h r o m i t e a n d t e x t u r e s w h i c h m a r k t h e t r a n s i t i o n b e t w e e n t h e t w o . P l a t e 71 Idealized sketch of typical dunite hosted shear zone i . ; 1 20 meters > Figure 11. Shear zone crosscutting chromite layers large dunite body contains massive pod-like chromite bodies, richly disseminated chromite and blocks of layered chromite. 72 9 shows the t y p i c a l net t e x t u r e s which a r e c o n t a i n e d w i t h i n p a r a l l e l l a y e r s t h r o u g h o u t the d u n i t e body. P l a t e 10 shows c h r o m i t e b r e c c i a found w i t h i n a shear zone. I t i s more deformed and c o n t a i n s a h i g h e r p e r c e n t a g e of c h r o m i t e t h a n the net t e x t u r e s but s t i l l r e t a i n s a s t r o n g s i m i l a r i t y t o them. F i n a l l y , i n a l l t h r e e l o c a l i t i e s , the s h e a r zones do not appear t o c o n t a i n v o l u m e t r i c a l l y more c h r o m i t e than the s u r r o u n d i n g d u n i t e . A l t h o u g h some a r e a s a r e much r i c h e r i n c h r o m i t e , o t h e r l a r g e a r e a s of h i g h l y s h e a r e d and s e r p e n t i n i z e d d u n i t e a r e e s s e n t i a l l y b a r r e n of a l l c h r o m i t e , even i n a c c e s s o r y amounts. These l i n k s between cumulate c h r o m i t e and massive deformed c h r o m i t e i n a l l t h r e e a r e a s p r o v i d e s sound e v i d e n c e f o r d e f o r m a t i o n as a c o n c e n t r a t i n g p r o c e s s . F i e l d e v i d e n c e from u l t r a m a f i c r o c k s i n New C a l e d o n i a , c i t e d by C a s s a r d e t a l . ( 1 9 8 1 ) , l e n d s a d d i t i o n a l s u p p o r t t o a m e c h a n i c a l c o n c e n t r a t i n g p r o c e s s . They d e s c r i b e a s i t u a t i o n i n which c h r o m i t e b r e c c i a was a p p a r e n t l y t r a n s f o r m e d i n t o a n t i - n o d u l a r o r e . The a n t i - n o d u l a r ore shown i n F i g u r e 8a i s a deformed c h r o m i t e t e x t u r e which i s b e g i n n i n g t o a p p r o a c h b o t h the degree of d e f o r m a t i o n and m a s s i v e n a t u r e found i n l a r g e h i g h l y deformed massive c h r o m i t e pods. They c o n c l u d e t h a t t h e a n t i - n o d u l a r ore i s a more h i g h l y deformed and c o n c e n t r a t e d form of c h r o m i t e b r e c c i a . 73 8. C O N C L U S I O N S T h e e x t r e m e v a r i a b i l i t y b e t w e e n d i f f e r e n t o c c u r r e n c e s o f c h r o m i t e w i t h i n t h e J o s e p h i n e p e r i d o t i t e , p r e c l u d e s a n y s i n g l e t h e o r y o f o r i g i n . I n g e n e r a l , t h e r e a r e t w o m a i n t y p e s o f c h r o m i t e d e p o s i t s : p r i m a r y c u m u l a t e c h r o m i t e r e s t r i c t e d t o d u n i t e b o d i e s , a n d m a s s i v e p o d i f o r m c h r o m i t e b o d i e s r e s t r i c t e d t o s h e a r z o n e s . P r i m a r y c u m u l a t e c h r o m i t e i n d u n i t e b o d i e s a r e m a g m a t i c i n o r i g i n . T h e y a r e p r o b a b l y t h e c u m u l a t e r e s i d u u m o f s e c o n d s t a g e m e l t i n g a n d d e p l e t i o n o f p a r t i a l l y d e p l e t e d h a r z b u r g i t e . T h e o r i g i n o f t h e m a s s i v e p o d i f o r m c h r o m i t e b o d i e s i s s e c o n d a r y . A l a c k o f m a g m a t i c t e x t u r e s a n d a h i g h c h r o m i t e c o n c e n t r a t i o n p r e c l u d e s i t b e i n g a m a g m a t i c c u m u l a t e . T h e i n t e n s e l y d e f o r m e d t e x t u r e s o f t h e c h r o m i t e g r a i n s w i t h i n t h e p o d s r e f l e c t t h e e x t r e m e d e f o r m a t i o n w h i c h t h e u l t r a m a f i c r o c k s h a v e u n d e r g o n e . T h e h i g h l y d e f o r m e d c a t a c l a s t i c t e x t u r e s a s s o c i a t e d w i t h a l l m a s s i v e c h r o m i t e p o d s s u g g e s t a n o r i g i n i n t i m a t e l y r e l a t e d t o d e f o r m a t i o n p r o c e s s e s . F i e l d e v i d e n c e f r o m t h r e e c h r o m i t e - b e a r i n g d u n i t e b o d i e s i n d i c a t e t h a t i n t e n s i v e s h e a r i n g i s a b l e t o c o n c e n t r a t e c u m u l a t e c h r o m i t e i n t o m a s s i v e p o d - l i k e b o d i e s . T h e c o n c e n t r a t i n g m e c h a n i s m a p p e a r s t o b e d y n a m i c m e t a m o r p h i c d i f f e r e n t i a t i o n . 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