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MacLean 1993

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  • 8/12/2019 MacLean 1993

    1/25

    J o u r n a l o f G e o c h e m i c a l E x p l o r a ti o n , 4 8 ( 1 9 9 3 ) 1 0 9 - 1 3 3 1 09E l s e v i e r S c i e n c e P u b l is h e r s B . V ., A m s t e r d a m

    L ithogeo chem ical techniques us ing im m ob i lee lementsW . H . MacLean and T.J. Barrett

    De partm ent of Earth and Planetary Sciences, McG ill University, 3450 University Street, M ontreal,Quebec H3A 2A 7, Canada(Rece i v ed 1 3 J an u a ry 1 9 9 2 ; accep t ed a f t e r r ev i s i o n 6 A p r i l 1 9 9 3 )

    A B S T R A C TE l e m e n t s o f d e m o n s t r a t e d i m m o b i l it y in h y d r o t h e r m a l a l t e r a t io n a n d m e t a m o r p h i s m p r o v i d e u se -f u l l i th o g e o c h e m i c a l p a r a m e t e r s f o r m i n e r a l e x p l o r a t i o n i n g r e e n s t o n e t e r r an e s . T h e y y i e ld p r e c is ei d e n t i f ic a t i o n s o f p re c u r s o r v o l c a n i c ro c k t y p e a n d m a g m a t i c a f f i n it y , a n d q u a n t i t a t i v e e s t i m a t e s o fm a s s , v o l u m e a n d m i n e r a l o g i c a l c h a n g e s. I m m o b i l e i n c o m p a t i b l e e le m e n t s ( Z r , Y , N b , R E E ) e s ta b -

    l is h a f fi n i ti e s , a n d m o n i t o r f r a c t i o n a t i o n o f c o m p a t i b l e e l e m e n t s i n t h o l e ii t ic a n d t r a n s i ti o n a l v o l c a n i cs u i t e s . M a s s c h a n g e s i n m o b i l e c o m p o n e n t s c a n b e d e t e r m i n e d f o r a h o m o g e n e o u s r o c k u n i t o r ac o n t i n u o u s v o l c a n i c s e r ie s . M a s s c h a n g e r e s u l t s a ls o b e a r d i r e c t l y o n t h e i n t e r p r e t a t i o n o f R E E p r o -f il e s i n a l t e r e d r o c k s. N o r m a t i v e c a l c u l a t io n s a l l o w c h e m i c a l a n a ly s e s t o b e p o r t r a y e d a s a l t e r a t io nm i n e r a l s , w h i c h a r e a l s o u s e f u l i n i n t e r p r e t i n g o x y g e n i s o t o p e d a t a . I m m o b i l e e l e m e n t t e c h n i q u e sp r o d u c e a la r g e n u m b e r o f p a r a m e t e r s t h a t c a n b e i l lu s t r a te d o n m a p s a n d d i a g r a m s , a n d o t h e r w i s eu s e d i n e x p l o r a t io n . U s a g e o f t h e s e te c h n i q u e s b r i n g s d e e p e r i n s ig h t t o t h e e v o l u t i o n o f v o l c a n i c s t ra -t i g r a p h y a n d t h e h y d r o t h e r m a l p r o c e s s e s th a t f o r m e d t h e a l t e r a t i o n z o n e s a n d o r e d e p o s i ts .

    I N T R O D U C T I O N

    U n r a v e l l in g v o l c a n i c s t r a t ig r a p h y i s a m a j o r t a sk i n m i n e r a l e x p l o r a t i o n i ng r e e n s t o n e b e lt s. T h e m i n e r a l o g i c a l , t ex t u r a l a n d c h e m i c a l i d e n t i ti e s o f r o c k sa r e o f te n o b s c u r e d b y h y d r o t h e r m a l a l te r a t io n a n d m e t a m o r p h i s m , l e a d i n g toi n c o r r e c t id e n t i f i c a t io n o f v o l c a n i c r o c k t y p e s ( b a s a lt , a n d e s i t e , r h y o l i te ) a n dr o c k a f f i n i t i e s ( e . g . t h o l e i i t ic , c a l c - a l k a l i n e ) . C o r r e c t i d e n t i f i c a t i o n s a r e p a r -t ic u la r l y n e e d e d i n d e e p d r i l li n g e x p l o r a t i o n p r o g r a m s w h e r e d r i ll h o l e s a r en e c e ss a r i ly w i d e l y s p a c e d o w i n g t o d r i ll in g c o st s. A c c u r a t e i n t er p r e t a ti o n o fd r i l l c o r e d a t a is n e c e s s a r y n o t o n l y f o r o u t l i n i n g m i n e r a l i z a t i o n , b u t a ls o f o rd e f i n i t i o n o f a l t e r a t i o n p a t t e r n s , b e t w e e n - h o l e l i t h o l o g i c c o r r e l a t i o n , a n d e s -t a b l i s h m e n t o f v o l c a n i c - t e c t o n i c s e tt in g .

    A n u m b e r o f r e c e n t s t u d i e s in e x p l o r a t i o n l i t h o g e o c h e m i s t r y h a v e s tr e s se dt h e id e n t i f i c a ti o n a n d u s e o f e l e m e n t s t h a t h a v e r e m a i n e d i m m o b i l e d u r i ngh y d r o t h e r m a l a lt er a t io n a n d m e t a m o r p h i s m a r o u n d o r e d e p o s it s ( e . g. ,M a c L e a n a n d K r a n i d i o t is , 1 9 8 7 ; M a c L e a n , 1 9 8 8 , 1 9 9 0 ; R i c h a r d s e t a l. , 1 9 8 9 ;

    0 3 7 5 - 6 7 4 2 / 9 3 / $ 0 6 . 0 0 1 9 9 3 E l s e v i e r S c i e n c e P u b l i s h e r s B .V . A l l r i g h ts r e s e r v e d .

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    1 1 0 W . H . M a c L E A N A N D l '. J. B A R R E T q

    E l l i o t t- M e a d o w s a n d A p p l e y a r d , 1 9 91 ; B a r r e tt a n d M a c L e a n , 1 9 9 1 ) . S o m ei m m o b i l e e l e m e n t s a r e a ls o i n c o m p a t i b l e i n i g n e o u s p r o c es s es , a n d c a n b eu s e d t o m o n i t o r f r a c t i o n a t i o n p r o c e s s e s . C o m b i n a t i o n s o f i m m o b i l e - c o m -p a ti b le a n d i m m o b i l e - i n c o m p a t i b l e e l e m e n t s p r o d u c e p e r m a n e n t a n d d ia g-n o s t i c i m p r i n t s o f f r a c t i o n a t i o n l i n e s a n d p l o t s th a t d e f i n e m a g m a t i c a ff in i -t ie s. U s a g e o f t h e s e i m p r i n t s l e a d s to i m p r o v e d l i th o s t r a t ig r a p h i c c o r r e l a ti o n ,i d e n t i f i c a t i o n o f a l t e r e d r o c k p r e c u r s o r s , r e c o g n i t i o n o f a l t e r a t i o n h a l o e s , c a l-c u l a ti o n o f m a s s c h a n ge s , a n d a n i m p r o v e d u n d e r s t a n d i n g o f o x y g e n is o to p e ,R E E a n d t ra c e e l e m e n t d i st r ib u t i o n s.TECHNIQUES

    M a p p i n g a n d c o r e lo g g in g f o l lo w e d b y p e t r o g r a p h y a n d l i th o g e o c h e m i s tr yi s t h e l o g i ca l o r d e r o f i n v e s t ig a t i o n , w i t h n e w i n f o r m a t i o n o n r o c k t y p e a n da f f in i t y g a i n e d a t e a c h s t e p . G o o d f i e ld a n d c o r e s a m p l e i d e n t i f i c a t i o n is c ri t-i ca l f o r p e t r o g r a p h i c - l it h o g e o c h e m i c a l s a m p l i n g w h i c h p r o v i d e s t h e f ra m e -w o r k f o r s u b s e q u e n t c h e m i c a l c a l cu l a ti o n s .L a r g e f il es o f l it h o g e o c h e m i c a l d a t a a r e a m a s s e d i n m a n y e x p l o r a t i o n p r o -g r a m s i n g r e e n s t o n e b e lt s. D a t a a r e c o m m o n l y t r e a t e d u s i n g w e l l -e s t a b li s h e dg e o c h e m i c a l d i s c r i m i n a n t s , s u ch a s S iO 2 c o n t e n t , M g - n u m b e r , d i f f e r e n t i a t i o ni n d e x , a l k a li - S iO 2 a n d A F M p l o ts , w h i c h w e r e c r e a te d p r i m a r i l y t o c l as si fyp r i s t in e i g n e o u s ro c k s. M o s t o f t h e e l e m e n t s in t h e s e d i s c r i m i n a n t s a r e i g-n e o u s - c o m p a t i b l e , a n d a re a ls o m o b i l e d u r i n g w a t e r - r o c k i n t e ra c t io n . T h e yc a n b e c o m e h ig h l y d is p e r s ed d u r i n g h y d r o t h e r m a l a lt e ra t io n a t m o d e r a t e t oh i g h w a t e r - r o c k r a ti o s . A l t h o u g h th e i r m o b i l i t y ac t u al ly m a k e s t h e m u s e fu li n o u t l i n i n g h y d r o t h e r m a l a l te r a t i o n z o n e s , i t al so m a k e s t h e m u n f i t f o r i d e n -t i f ic a t i o n o f p r e c u r s o r r o c k t y p e a n d m a g m a t i c a f f in i ty . B y c o n t r a s t, t r ea t -m e n t o f t h e c h e m i c a l d a ta u s i n g i m m o b i l e e l e m e n t t e c h n i q u e s i d e n ti f ie s ro c kt y p e a n d a f fi n it y , a n d p r o v i d e s a n u m b e r o f e x t ra , f i r m l y b a s e d q u a n t i t a t i v ep a r a m e t e r s .

    M e t h o d s f or d e t e r m i n i n g w h e t h e r c e r ta in e l e m e n t s h a v e b e e n i m m o b i l e inm e t a s o m a t i s m a n d h y d r o t h e r m a l a l te r a ti o n h a v e b e e n d i s cu s s ed , a m o n g o t h -e r s, b y G r e s e n s ( 1 9 6 7 ) , B a b c o c k ( 1 9 7 3 ) , F i n l o w - B a t e s a n d S t u m p f l ( 1 98 1 ),G r a n t ( 1 9 8 6 ) , K r a n i d i o t i s a n d M a c L e a n ( 1 9 8 7 ) , M a c L e a n ( 1 98 8, 1 9 9 0 )a n d E l l i o t t -M e a d o w s a n d A p p l e y a r d ( 1 9 9 1 ) . I n t h i s a r ti c le w e r e v i e w t h em e t h o d s e m p l o y e d re c e n tl y b y K r a n i d i o t i s a n d M a c L e a n ( 1 98 7 ) , M a c L e a n( 1 9 8 8 , 1 9 9 0 ) , a n d B a r r e t t a n d M a c L e a n ( 1 9 9 1 ) . W e a ls o o u t l i n e t h e se -q u e n c e o f s t ep s r e q u i r e d i n a l i t h o l o g i c a l i n v e s t i g a t i o n t h a t u s e s i m m o b i l ee l e m e n t s t o e s t a b l is h c h e m o s t r a t i g r a p h i c u n i t s i n a l t e r e d a n d u n a l t e r e d v o l -c a n i c s e q u e n c e s , a n d t o c a l cu l a te t h e m a s s c h a n g e s t h a t o c c u r w i t h i n t h e a l-t e r a t i o n z o n e s . T h i s m a s s - b a s e d a p p r o a c h f i n d s d i r e c t a p p l i c a t i o n i n e x p l o -r a t i o n a n d d r i l li n g p r o g r a m s .

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    LITHOGEOCHEM1CAL ECHNIQUESUSING MMOBILEELEMENTSImmobile elements

    111

    E l e m e n t s s h o u l d b e t e s t e d f o r i m m o b i l i t y i n e a c h a r e a o f s t u d y , p r e f e ra b l yo n a n i d e n t i f i a b le s i n gl e v o l c a n i c f l o w t h a t c a n b e t r a c e d l a te r al ly t h r o u g h a na l te r a t i o n z o n e . U s i n g th e p r o c e d u r e s o u t l i n e d b y M a c L e a n a n d K r a n i d i o t i s( 1 9 87 ) f o r a si n gl e p r e c u r s o r s y s t e m , a n i m m o b i l e e l e m e n t p a i r i n t h i s s a m p l ed a t a s e t w i ll f o r m a h ig h l y c o r r e l a t e d ( r - - 0 . 9 0 - 0 . 9 9 ) l i n e a r t r e n d o w i n g t om a s s g a i n s a n d l o ss e s o f m o b i l e c o m p o n e n t s i n t h e a l t e r e d p a r t o f t h e f l ow ; a ti n f i n i t e m a s s g a i n , t h e r e g r e s se d l i n e id e a l l y p a s s e s t h r o u g h t h e o r i g i n ( F ig .1 ) . R e s u l t s o f t h i s t e s t s h o w t h a t A 1, T i a n d t h e h i g h - f i e l d s t r e n g t h t r a c e e le -m e n t s ( H F S E ) Z r , N b , Y a n d t h e R E E a r e i m m o b i l e i n a l te r a ti o n z on e sa r o u n d a l ar ge n u m b e r o f v o l c a n o g e n ic m a s s i v e s u l p h id e ( V M S ) d e p o s it s i ng r e e n s t o n e b e l ts ( M a c L e a n a n d K r a n i d i o t i s , 1 98 7, M a c L e a n , 1 98 8 ) . W i t h i nt h i s g r o u p o f e l e m e n t s , Y a n d t h e l i g h t- R E E a r e s li g ht ly m o b i l e u n d e r s o m ec o n d i t i o n s o f a l t e r a ti o n , a n d r -v a l u es a r e lo w e r . C a r e m u s t b e t a k e n t o e x c l u d ef r o m t h i s t e s t p a ir s o f e l e m e n t w i t h h i g h c h e m i c a l a f fi n i ty w i t h o n e a n o t h e r ,s u ch a s K - R b a n d Z r - H f .

    30 Phelps Dodge depos it : . ~ j ~ e25 altered rhyoli te J _ . , . ~

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    s Ss5 ,,,, / Alteratton hne

    , y * P r e c u r s o r y = 0 . 0 3 4 x + 0 . 57 9 ;s S, ,, r = 0 . 99 3

    0 I I I I l I I0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

    r p p r nFig. 1. E ffects of m obile m ass gain and loss on imm obile AI and Zr during alteration of aninitially hom ogeneous hyolite unit (single precursor system) at the P helps Do dge VMS deposit(data from M acL ean and Kranidiotis, 1987 .

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    1 12 W H . M a c L E A N A N D T .J . B A R R E T T

    Incompatible elementsA s e c o n d r e q u i r e m e n t f o r o u r p r o c e d u r e s is th e p r e s en c e o f h ig h l y i n c o m -

    p a t i b l e t r a c e e l e m e n t s ( b u l k d i s t r i b u t i o n c o e f f i c i e n t s D s o ] i d / m e l t < 0 . 1 ) tom o n i t o r d e g r e e o f f r a c ti o n a t i o n a n d m a g m a t i c a f fi ni ty . A p a i r o f i n c o m p a t i -b l e el e m e n t s i n a v o l c a n i c s a m p l e s e t p r o d u c e s a l in e a r m a g m a t i c e n r i c h m e n tt r e n d t h a t i n v a r i a b l y p a s s e s t h r o u g h t h e o r i g i n ( F i g . 2 ) . I n c o m p a t i b l e e l e -m e n t e n r i c h m e n t i s m o d e l l e d b y t h e R a y l e i g h f r a c ti o n a t i o n e q u a t i o n :Ci=Co FD I ( 1 )w h e r e C o a n d C i a r e c o n c e n t r a t i o n s o f a n i n c o m p a t i b l e e l e m e n t in i n i t ia l a n df r a c t i o n a t e d m a g m a s , r e s p e ct iv e l y ; F i s t h e f r a c t io n o f r e m a i n i n g m a g m a ; a n dD is t h e b u l k ( s o l i d / m e l t ) d i s t r i b u t i o n c o e f f ic i e n t. W h e n D is s m a l l ( e st i-m a t e d a t ~ 0 .0 5 f o r Z r ) , t h e e l e m e n t c a n b e u s e d t o m o n i t o r t h e f r a ct i o n a ti o npr ocess .T h e H F S E a r e h i g h ly i n c o m p a t i b l e d u r i n g f r a c ti o n a t i o n i n t h e r an g e o f basalt and basaltic andesite i n m o s t t h o l e i i t i c a n d c a l c - a l k a l i n e s u i te s , b u t , i no u r e x p e r i e n c e , t h e y r e m a i n i n c o m p a t i b l e i n a n d e s i t e, d a c i t e a n d r h y o l it e o n l yi n s u i t e s o f t h o l e i i t ic t o t r a n s i t i o n a l ( m i l d l y c a l c - a l k a li n e ) a f f in i ty . T h e t h o l -e i i t ic - t r a n s it i o n a l s u i te s a r e l o w in K ( g e n e r a l ly < 0 . 5 w t % K 2 0 ) a n d f ra c -t i o n a t e t o s o d i c r h y o l i t e , w i t h a l b i te a s t h e s o l e f e l d s p a r p h e n o c r y s t p h a s e . I n

    1 0 0

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    Host vo l can ics a t t he Ans i l m ine :f r a c t i o n a t i o n t r e n d g i v e nby l eas t a l t e r ed sam p l es

    Zr /Y = 5 0

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    O 0 y - 5 5 4 0 . 2 0 1 x r - 0 9 5i

    1 0 0 2 O O 3 0 0

    r p p mF i g . 2 . I n c o m p a t i b l e Y a n d Z r e n r i c h m e n t i n v o l c a n i c s tr a t i g ra p h y a t t h e A n s i l m i n e , N o r a n d a( d a t a f r o m B a r r e t t e t a l ., 1 9 9 1 a ) .

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    LITHOGEOCHEMICALTECHNIQUESUSING IMMOBILEELEMENTS 1 13120

    100

    80,

    60

    40

    20

    0 100 200 300 400 500r ppm

    F ig . 3. S c h e m a t i c Y - Z r p l o t u s e d f o r d e t e r m i n a t i o n o f m a g m a t i c a f f in i ty .t h e m a j o r i t y o f f r a c t i o n a t e d c a l c -a l k a l in e s u it es , w h e r e K - f e l d s p a r is a l so ap h e n o c r y s t p h a s e i n d a c i t e s a n d r h y o l i t e s , t h e H F S E a r e m o r e c o m p a t i b l e ,a n d D v a l u es a r e v a ri a b le a n d m a y b e >t I . T h e H F S E s h o u l d b e t e st e d f o ri n c o m p a t i b i l i t y i n a r o c k s u i t e , u s i n g p l o t s s u c h a s Y - Z r o r L a - Y b , b e f o r ec o n t i n u i n g w i t h t h e s e p r o c e d u r e s . H o w e v e r , c a l c - a lk a l i n e s u i te s c a n s ti ll b et r e a t e d u s i n g i m m o b i l e c o m p a t i b l e e l e m e n t s , as o u t l in e d b e l ow .S o m e r a t io s b e t w e e n t h e H F S E c h a n g e w i t h r o c k a f fi n it y , a n d t h e r e b y p r o -v i d e d i s c r i m i n a n t s u n a f f e c t e d b y a l t e r a t io n . T h e c h a n g e i s p a r t i c u l a r ly l a rg ea n d d i s t i n c t i v e fo r Z r / Y r a ti o s : th e s e r a n g e f r o m ~ 3 t o 5 f o r t h o l e ii t es , t o ~ 7t o ~ 3 0 f o r c a l c - a l k a l i n e r o c k s ( F i g . 3 ) ( c f. L e s h e r e t a l ., 1 9 8 6 ) . W h e r e a sp l o t s o f Z r v e r s u s Y a r e l i n e a r f o r t h o l e ii t es , a n d e x h i b i t s t e a d y i n c o m p a t i b l ee n r i c h m e n t f r o m b a s a l t t o r h y o l i t e , t h o s e f o r a s i m i l a r r a n g e o f c a lc - a lk a l i n er o c k s p r o d u c e p o o r l y c o r r e l a t e d t r e n d s , w i t h d e p l e t i o n s c a u s e d b y v a r i a b l ec o m p a t i b i l i t y o f b o t h Z r a n d Y . R e l a t i v e d e p l e t i o n is , h o w e v e r , g re a t e r f o r Y ,s o t h a t t h e Z r / Y v a l u e i s h i g h e r f o r c al c - a lk a l i n e r o c k s . W e d e s i g n a t e s a m p l e sw i t h i n t e r m e d i a t e Z r / Y v a l u e s ( ~ 5 - 7 ) " t ra n s i t i o n a l " ; th e y h a v e li n e a r H F S Ee n r i c h m e n t t re n d s , b u t s li g h tl y st e e p e r R E E p r o f il e s a n d s li g h tl y h i g h e r A 1-c o n t e n t s t h a n t h o l e ii t es .M a g m a t i c a f fi n it y c a n b e c o n f i r m e d u s in g ch a n g es in L a / Y b a n d Z r / Y br a ti o s , a s th e h e a v y R E E a r e g e o c h e m i c a l l y s i m i l a r to Y . S l o p e s o f Z r v e r s u sN b p l o t s c o i n c i d e i n s o m e t h o l e i i t ic to m i l d l y c a lc - a lk a l i n e s u it es , b u t d i v e r g ei n o t h e r s ( B a r r e t t e t a l ., 1 9 91 a ,b , 1 99 2 ) . T h e Z r / N b r a t i o i s d i s t i n c t l y h i g h e r

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    1 1 4 W . H . M a c L E A N A N D T .J . B A R R E T T

    i n a n o r o g e n i c ( r i f t - r e l a t e d ) t h a n i n o r o g e n i c ( s u b d u c t i o n - r e l a t e d ) a l k a l i n er o c k s, a n d p r o v i d e s a r e li a b le d i s c r i m i n a n t f o r m a g m a t i c s u i te s a n d t e c t o n i cse t t ing.O t h e r , l e ss a b u n d a n t H F S E ( e. g ., H f , T a , W , T h ) c a n a l so b e u s e d to d e t e r -m i n e r o c k a f fi n i ty , b u t , b e c a u s e o f t h e i r l o w a b u n d a n c e s ( ~ 0 .1 - 1 0 p p m ) a n dh i g h a n a l y t i c a l c o s ts , t h e y a r e n o t a s s u i t a b l e a s th o s e d e s c r i b e d a b o v e . I n o u re x p e r i e n c e , Z r i s t h e m o s t u s e f u l e l e m e n t t o m o n i t o r f r a c t i o n a t i o n ; i t is i m -m o b i l e , h i g h ly i n c o m p a t i b l e i n t h e r o c k s e r ie s n o t e d a b o v e , a n d s u f fi c ie n t lya b u n d a n t ( ~ 5 0 - 1 0 0 0 p p m ) f o r p r e ci se c h e m i c a l a n al y se s . T h e Z r / Y r a ti o ist h e m o s t c o n v e n i e n t a n d l e a s t e x p e n s i v e d i s c r i m i n a n t f o r t h o l e i i ti c a n d c a lc -a l k a l i n e r o c k s , a n d c a n b e a p p l i e d o n a s i n g l e s a m p l e b a s is ; h o w e v e r , a f f i n i ti e ss h o u l d b e c o n f i r m e d b y o t h e r te s ts o n a p o r t i o n o f t h e s a m p l e s e t.

    CHEMICAL ANALYSES

    C h e m i c a l a n a ly s e s fo r t h e H F S E m u s t b e p r e ci s e a n d a c c u r a te i n o r d e r t od i s t in g u i s h t h e s m a l l d i f f e re n c e s b e t w e e n f r a c t i o n a t i o n t r e n d s a n d e l e m e n tr a ti o s o f i n d i v i d u a l s a m p l e s . I n o u r e x p e r i e n c e , o n l y t h e d a t a o b t a i n e d f o r Z r ,Y a n d N b b y X - r a y f lu o r e s c e n c e u s i n g p r e s s e d r o c k p o w d e r p e l le t s a r e p re s -e n t l y s u it a b le f o r e s t a b l i s h m e n t o f m a g m a t i c a f f in i ty a n d f r a c ti o n a t i o n t r e n d s .W i t h th i s te c h n i q u e t h e r e is n e i t h e r t h e s a m p l e d i l u t i o n p r o b l e m i n h e r e n t i nb o r a t e b e a d p r e p a r a t io n , n o r t h e p r o b l e m o f in c o m p l e t e so l u t io n i n a c i dd i g e s ti o n m e t h o d s . G o o d q u a l i ty a n a ly s e s f o r R E E a n d le ss a b u n d a n t H F S Ea r e o b t a i n e d b y i n s t r u m e n t a l n e u t r o n a c t i v a ti o n a n a ly s e s ( I N A A ) , o r a c o m -b i n a t io n o f i n d u c e d c o u p l e d p la s m a a n d m a s s s p e c t ro m e t r y ( I C P - M S )t e c h n i q u e s .

    SINGLE PRECURS OR

    A l t e r a ti o n o f a c h e m i c a l l y h o m o g e n e o u s r o c k p r o d u c e s a s in g le p r e c u r s o rs y s te m . A p l o t o f a n i m m o b i l e e l e m e n t p a i r i n s u c h a u n i t f o r m s a n " a l t e ra -t i o n l i n e " t h a t i s l i n e a r a n d p a s s es t h r o u g h t h e o r i g in ( F i g . 1 ) . F o r i m m o b i l e -i n c o m p a t i b l e e l e m e n t p a i rs , a l t e r a t io n a n d f r a c t io n a t i o n l in e s c o i n c i d e f o r r o c kt y p e s o f t h e s a m e m a g m a t i c a ff in i ty ( F ig . 4 a ) . W h e n t h e p a i rs a r e co m p a t i -b l e - i n c o m p a t i b l e , o r b o t h a r e c o m p a t i b l e , a s e p a r a t e a l t e r a ti o n l in e is f o r m e df o r e a c h c h e m i c a l l y d i s t i n c t h o m o g e n e o u s r o c k u n i t ( F ig . 4 b ). S i n g le p r e c u r -s o r a lt e r a t i o n l i n e s a r e t h e r e f o r e v e r y u s e f u l i n d i s t i n g u i s h i n g h o m o g e n e o u sv o l c a n i c u n i t s a n d c o r r e l a t in g t h e m b e t w e e n d r i l l h o l e s a n d t h r o u g h a l te r a-t i o n z o n e s .

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    LITHOGEOCHEMICALTECHNIQUESUSINGIMMOBILEELEMENTS100

    8 0

    E oQ .> 4 0

    2 0

    A n s i l v o l c a n i c r o c k s :o l e a s t a l t e r e d . / . a l t e r e d .

    . . ~ , f po , f ~ . , . o e l 0 o

    O o 0 ~ 1 ~ ' u~ , , : ,

    i E lI I I0 100 200 300 400

    Z r p p m

    115

    3 0S c h e m a t i c a l t e r a t i o n l i n e s

    T w o r h v o l i t e u n i t s A a n d B2 s / ~ / ~

    P r e c u r s o r B , r2 0 I . i r e ' ' `

    O ~ D , t L /o / - j I/ A ~ . . , P r e c u r s o r A

    5 . : 2 : , , 2 : : /~ b

    I I I I I I I0 1 0 0 2 0 0 3 0 0 4 0 0Z r p p m

    F ig . 4 . ( a ) Y - Z r p l o t o f le a s t a l t e re d a n d a l t e re d s a mp les f r o m t h e A n s i ] m in e ( se e F ig . 2 ) .Immobile-incompatibleelement pairs in least altered and altered samples plot along the sameregression line. (b) Alteration lines for immobile compatible -incompati ble element pair canidentify two distinct rhyolite units, A and B, within a mine stratigraphy.

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    116 W.H. MacLEAN AN[) T., I BARRETTMULTIPLE PRECURSORSFractionation trends

    Once an HFSE is shown to be highly incompa tible and immobile, it can beused instead of the mobile mon itors (e.g SiO2, Mg-number, differentia tionindex) which supply only semi-quantitative estimates of degree of igneousfractionation. The HFS E are qua ntitat ive mon itors because of their highincompatibility.

    It is important to establish the fractionation lines as accurately as possible,as they form the basis of the precursor identification and the mass changecalculations. Plut onic rocks, such as tonalites and gabbros, can be included inthe sample set if field relations indicate tha t th ey are synvolcanic. Stronglyfeldspar-porphyritic rocks should be excluded from Al-bearing plots, owingto plagioclase accu mul ati on and resulting high A1 contents. There is always acertain a mou nt o f scatter in the least altered sample data used to fit the frac-tio nat ion lines. With few data at the start o f explorat ion, the line fits are ap-

    2.0 ,,e ~ Sch em a t i c t ho le i i ti cee f rac t iona t ion t rend/ ' . . . . . . A l t e r a ti o n l in e s

    0 1.o. , 4X0.5 / o . . . . - ' x ~ IR hyoliteJ

    , ' 1 I _ . . , , . . o O - . - ~ ' ~J ~ - ' ~ 0 ._ . 0 - - ' 0 0, . . - _ . . . . . . . . . . a

    0 . 0 ' ' - - . , . , , 0 1 0 0 2 0 0 3 0 0 4 0 0

    Z r p p mFig. 5. Idealized fractionation curves for immobile elements in a tholeiitic volcanic suite, withsuperposed chemical data for altered rocks. The two well defined alteration lines represent ho-mogeneous units within the stratigraphy. Their intersection with the fractionation curves iden-tifies the units as an andesite and a rhyolite. Other altered samples on these plots have each adistinct alteration line emanating from the origin. (a) TiO2-Zr: The fraetionation curve ex-hibits strong Ti-enrichment in the basalts (paralleling typical iron-enrichment in tholeiites),followed by strong Ti depletion from andesite to rhyolite.

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    1 1 8 W . H . M acL E A N A N D T .J . B A R R E T T

    p r o x i m a t e , b u t e v e n t h e s e ar e s u p e r i o r t o h a v i n g n o g u i d i n g f r a c t io n a t i o nt r e n d s a t a ll. A s e x p l o r a t i o n p r o c e e d s , a n d m o r e l e a st a l t e r e d s a m p l e d a t a b e -c o m e a v a i la b l e, t h e l in e f it s s h o u l d i m p r o v e .I n t h o le i i t ic v o l c a n ic s ui te s , t h e s h a p e s o f i d e a l F e O - Z r a n d T i O 2 - Z r ( F ig .5 a ) f r a c t i o n a t i o n c u r v e s e x h i b i t i n i t i a l s t r o n g e n r i c h m e n t s ( D 1 ) f r o m a n d e s it e t o rh y o li te . M g O - Z r a n d C a O - Z r f r a c ti o n a t io nc u r v e s a r e c o n c a v e ( D > 1 ) o v e r t h e f u ll r a n g e o f b a s a l t t o r h y o l it e . B y c o n -t r a st , A I 2 0 3 - Z r i s n e a r l i n e a r w i t h a s l ig h t ly n e g a t i v e s lo p e ( D ~ 1 ) ( F i g. 5 b ) .S i O 2 - Z r i s n e a r l i n e a r f r o m b a s a lt t o d a c i t e w i t h a p o s i t i v e s l o p e ( D < 1 ) , b u tf l a t te n s o f f ( D ~ 1 ) i n t h e r h y o l i t e s d u e t o q u a r t z p r e c i p i t a t i o n .

    Cu r ve f i t t i n gU s i n g l e a st a l te r e d s a m p l e s , c h e m i c a l d a t a f o r c o m p a t i b l e e l e m e n t s ex h i b -i t in g c o n c a v e d e p l e t i o n - t y p e f r a c t i o n a t i o n c a n b e f it u s i n g e x p o n e n t i a l o r s ec -o n d - o r d e r p o l y n o m i a l c u r v e s. B o t h c u r v e s c o m m o n l y g i v e s i m i l a r d a t a f it s,a s e x p r e s s e d b y r v a l u e s f o r t h e c u r v e f it . A p o l y n o m i a l s h o u l d b e f it to t h ei m m o b i l e e l e m e n t p ro f il e ( A l 2 0 3 - Z r o r T i O 2 - Z r ) u s e d t o e s t ab l is h th e p r e-c u r s o r Z r m o n i t o r v a l u e s f o r a l t e r e d s a m p l e s ( s ee b e l o w ) . W i t h t h i s f o r m o fe q u a t i o n , i t is s i m p l e r t o c a l c u l a te t h e i n t e r s e c t i o n o f f r a c t i o n a t i o n a n d a lt er -

    a t i o n l i n e s .T i O 2 - Z r a n d F e O - Z r d a t a f o r l e as t a l t e r e d b a s a lt s c a n b e fi t w i t h a s e p a r a tes e t o f c u r v e s . F r a c t i o n a t i o n t r e n d s f o r t h o l e i i t i c r o c k s r i se s t e e p l y in t h e b a s a l tt o b a s a l t i c - a n d e s i t e r a n g e , b u t f o ll o w a d e p l e t i o n t r e n d f r o m a n d e s i t e t o r h y -o l i t e . I n t h e b a s a l t t o b a s a l t i c - a n d e s i t e r a n g e , t h e t r e n d s a r e s u b - p a r a l l e l t oa l t e r a t io n l i n e s e m a n a t i n g f r o m t h e o r i g i n a n d , t h e r e f o r e , n o t u s e fu l in m a s sc h a n g e c a l c u l a t io n s . I n t h i s c o m p o s i t i o n a l r a n g e , i t i s s i m p l e r t o u s e t h e A 1 2 0 3 -Z r f r a c t i o n a t i o n c u r v e t o c a l c u l a t e m a s s c h a n g e s , as a l t e r a t io n a n d f r a c ti o n a -t i o n l i n e s i n t e r s e c t a t h i g h a n g l es .Mu l t ip le f rac t iona t ion curves

    I n s o m e v o l c a n i c s e q u e n c e s , t w o o r m o r e f r a c t i o n a t i o n t r e n d s c a n b e r e co g -n i s e d o n a T i O 2 - Z r p l o t , e v e n t h o u g h s e p a r a t e t r e n d s a r e n o t a p p a r e n t i np l o t s o f o t h e r m a j o r o x i d e s v e r s u s Z r . U s u a l ly , t h e s e p a r a t e t r e n d s a r e a l soe v i d e n t i n Y - Z r a n d o t h e r H F S E p l ot s, a n d i n d i c a te t h e p re s e n ce o f m o r et h a n o n e m a g m a t y p e a s th e s o u r c e o f t h e v o l c a n i c r o c k s in t h e s t r at i g ra p h y .I n a ll s u c h c as e s, t h e s a m p l e s e t s h o u l d b e d i v i d e d f i r st i n t o s u b s e t s b a s e d o nt h e s e tr e n d s . I t is f o r t h e H F S E p l o ts , i n p a r ti c u l a r , t h a t h i g h q u a l i t y c h e m i c a la n a l y s e s fo r Z r , Y a n d N b a r e r e q u i r e d , e s p e c i a l l y i n t h e b a s a l t r a n g e w h e r et h e i r c o n t e n t s a r e l ow a n d t h e f r a c t i o n a t i o n l i n e s c o n v e r g e .

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    LIT H OG E OCH E MICAL T E CH NIQ UE S USING IMMOBILE E LE ME NT S 1 19W h e n a l te r e d s a m p l e s a r e s u p e ri m p o s e d o n t h e T i O 2 - Z r a n d Al203-Zrplots ,t h e i r l o c at io n s m u s t b e c o n s i d e r e d i n r e l a ti o n t o t h e a p p r o p r i a t e f r a c ti o n a t i o nc u r v e , o r m a s s c h a n g e c a l c u l a t io n s w i l l b e i n e r r o r . T h i s e m p h a s i z e s t h e i m -

    p o r t a n c e o f s e p a r a t in g t h e a l t e r e d s a m p l e s i n t o t h e i r p r o p e r a f f i n i t y g r o u p s .A no m a l ous Z r and o t her H F SE behavior

    I n s o m e t h o l e i i t i c - t r a n s i t i o n a l s u i t e s , Z r b e c o m e s m o d e r a t e l y d e p l e t e d i nt h e r h y o li te s , w h i l e t h e o t h e r H F S E m o s t l y r e m a i n i n c o m p a t i b l e ( K r a n i d i -o t is , 1 98 5 ) . T h i s i s u s u a ll y d e t e c t e d o n t h e Y - Z r p l o t , a lt h o u g h t h e s c a t t e r o fd a t a c o u l d b e i n t e r p r e t e d a s e it h e r Z r lo s s o r Y e n r i c h m e n t . T h e p r o b l e m isr e s o lv e d u si n g p l o ts o f N b - Z r a n d Y - N b , w h i c h i so l at e t h e d e p l e t e d c o m p o -n e n t . Z r - d e p l e t e d r h y o l i te s a r e lo c a l ly p r e s e n t a t t h e P h e l p s D o d g e ( K r a n i d i -o t i s , 1 9 85 ) a n d A l d e r m a c ( B a r r e t t e t a l. , 1 99 l b ) V M S d e p o s i t s .A L T E R A T I O N L I N E S

    I n p l o ts o f i m m o b i l e c o m p a t i b l e - i n c o m p a t i b l e e l e m e n t pa ir s, s a m p l e s th a ti n it ia l ly la y a l o n g a f r a c t io n a t i o n c u r v e m a y b e d i l u t e d o r c o n c e n t r a t e d b ym o b i l e m a s s c h a n g e . A l l s u c h s a m p l e p o i n t s w i l l b e d i s p l a c e d f r o m t h e f ra c -t i o n a t i o n c u r v e a n d e a c h w i l l c r e a t e a li n e a r " a l t e r a t i o n l i n e " t h a t p a s se st h r o u g h t h e o ri g in ( M a c L e a n , 1 9 9 0 ) . T h e p r e c u r s o r c o m p o s i t i o n f o r e a c hs a m p l e l ie s a t t h e i n t e r s e c t i o n o f t h e a l t e r a t i o n a n d f r a c t i o n a t i o n l i n e s i n e a c hp l o t u s i n g a n i m m o b i l e c o m p a t i b l e - i n c o m p a t i b l e e l e m e n t p a ir .I n p lo t s w i th m o b i l e e le m e n t s v e r su s Z r ( e.g . S i O 2 - Z r , F e O - Z r , C a O - Z r ,e t c . ) , t h e m o b i l e e l e m e n t s d o n o t b e h a v e i n a n o r d e r l y f a s h i o n . T h e i r d i s -p l a c e m e n t f r o m t h e f r a c t io n a t i o n t r e n d is r a n d o m , a n d t h e i r p r e c u r s o r c o m -p o s i ti o n s c a n n o t b e d e t e r m i n e d d i re c t ly f r o m t h e i r l o c a t io n r e l a ti v e to t h ef r a c t i o n a t i o n c u r v e . I n s t e a d , th e e s t a b l i s h e d p r e c u r s o r Z r - v a l u e ( f r o m A 1 2 0 3 -Z r o r T i O 2 - Z r ) is s u b s t i tu t e d i n t o t h e m o b i l e e l e m e n t - Z r f r a c t io n a t i o n c u r v ee q u a t i o n s t o s o l v e fo r th e p r e c u r s o r c o n t e n t o f ea c h m o b i l e e l e m e n t . As p r e a d s h e e t c o m p u t e r p r o g r a m is c o n v e n i e n t f o r t h e se c a l c u la t io n s .T h e h i g h a n g l e o f in t e r s e c t i o n o f t h e a l t e r a t i o n a n d f r a c t i o n a t i o n l i n e s fo ri m m o b i l e c o m p a t i b l e - i n c o m p a t i b l e e l e m e n t p a ir s p r o v i d e s a v e ry u s ef u l t o o lf o r i d e n t i f y i n g d i s t in c t i v e r o c k u n i t s i n a l t e r a t i o n z o n e s ( F i g . 5 b ) . A s t h en u m b e r o f s a m p l e s i n t h e d a t a s et is b u il t u p w i t h c o n t i n u e d e x p l o r a t io n , a l-t e r a t io n l in e s f o r th e s e u n i t s b e c o m e m o r e e v i d e n t . I n d i v i d u a l a l t er a t io n l in e si d e n t i f y d i s t i n c t i v e r o c k u n i t s a n d p r o v i d e a p o w e r f u l t o o l f o r l i t h o s t r a t i -g r a p h i c c o r r e l a ti o n .A I- T i plots fo r calc-alkal ine rocks

    I n c a lc - a lk a l in e r o c k s u it es , w h e r e t h e H F S E c o m m o n l y a re c o m p a t i b l e a n dd o n o t m o n i t o r f r a c t i o n a t io n w e ll , a n a l t e r n a t i v e p r o c e d u r e i s a d o p t e d t h a t

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    120 W.H. MacLEAN AND T.L BARRETTu t il i ze s i m m o b i l e , c o m p a t i b l e e l e m e n t s . E t h e r i d g e e t al. ( 19 83 ) a n d R i c h a r d se t al. ( 1 9 89 ) a s s u m e d t h a t A l , T i a n d Z r w e r e i m m o b i l e d u r i n g a l te r a t i o n a n dm e t a m o r p h i s m , a n d u s e d th e s e e l e m e n t s t o e s t im a t e m a s s c h an g e s in r o c ks y s t e m s t h e y a s s u m e d t o b e h o m o g e n e o u s ( i. e. s in g le p r e c u r s o r ) . B e r n i e r a n dM a c L e a n ( 1 9 8 9 ) a n d L i a gh a t a n d M a c L e a n ( 1 9 9 2 ) e m p l o y e d A l 2 03 -T i O 2p l o t s t o e s t a b l is h t h e i m m o b i l i t y o f A1 a n d T i i n s i ng l e h o m o g e n e o u s p re c u r -s o r u n i t s , a n d t h e n u s e d t h e s e e l e m e n t s t o c o n s t r u c t f r a c t i o n a t i o n t r e n d s f o rm u l t i p l e p r e c u r s o r s y s t e m s in a s s o c i a t e d t h o l e i i t i c a n d c a l c - a lk a l i n e su i te s .O n a p l o t o f A 1 2 0 3 -T i O 2 ( F ig . 5 c ) , v o l c a n i c r o c k s s h o w T i e n r i c h m e n t i nt h e b a s a l ti c r a n g e o f f r a c t i o n a t i o n , w h i c h i s f o l lo w e d b y T i d e p l e t i o n f r o ma n d e s i t e t o r h y o l i te . T h i s p r o d u c e s a s h a r p r e v e r s a l in t h e f r a c t i o n a t i o n l i n e .A s a r e s u l t , s t r o n g l y a l t e r e d b a s a l t s a r e n o t e a s i l y d i s t i n g u i s h e d f r o m o t h e rl i t h o l o g i e s i n t h i s p l o t . T h i s i s g e n e r a l l y n o t a s e r i o u s p r o b l e m f o r c a lc - a l k a li n ev o l c a n i c r o c k s w h i c h a r e m o s t l y i n t h e a n d e s i t e - r h y o l i t e r a n g e. I t w o u l d b et r o u b l e s o m e i n t h e b i m o d a l t h o l e i i ti c v o l c a n i c s u it e s, b u t t h i s p l o t g e n er a ll yi s n o t u s e d f o r s u c h r o c k s . C a l c - a l k a l i n e r o c k s m a y h a v e m o r e s c a t t e r i n A la l o n g t h e A 1 2 0 3 - T i O 2 f r a c t i o n a t i o n l in e t h a n t h o l e i i t i c - t r a n s i t i o n a l r o c k s ,w h i c h p r o d u c e s l a r g e r e rr o r i n m a s s c h a n g e c a l c u l a ti o n s .MASS CHANGES

    M a s s c h a n g e s c a n b e c a l c u l a t e d f o r e a c h m o b i l e e l e m e n t b a s e d o n t h e d i l u -t io n o r c o n c e n t ra t i on o f a n i m m o b i l e c o m p o n e n t ( M a c L e a n a n d K r a n i d io t is ,1 9 87 ) . T h e c a l c u l a t i o n s h a v e s o m e s i m i l a r i t i e s t o t h o s e o f G r e s e n s ( 1 9 6 7 )a n d t h e m o d i f i c a t i o n s b y B a b c o c k ( 1 9 73 ) a n d G r a n t ( 1 9 86 ) , w h o u s e d v a r -i o u s e l e m e n t s t o e s t i m a t e m a s s a n d v o l u m e c h a n g e s i n si ng l e p r e c u r s o r s y s-t e m s . R e g a r d l e s s o f t h e m e t h o d e m p l o y e d to e s t a b li s h i m m o b i l i t y , c e r ta i n e le -m e n t s c a n b e u s e d a s t h e b a s i s f o r m a s s c h a n g e c a l c u l a t io n s i n b o t h s in g le a n dm u l t i p l e p r e c u r s o r a l t e ra t i o n s y s t em s . I n th e i m m o b i l e e l e m e n t p r o c e d u r e sw e u s e ( o u t l i n e d b e l o w ) , v o l u m e c h a n g e s d o n o t e n t e r i n t o m a s s c h a n g e ca l-c u l a t io n s . V o l u m e c h a n g e s c a n , h o w e v e r , b e c a l c u l a t e d f ro m m a s s c h a n g ere s u l t s .T h e m a s s o f a s a m p l e a f te r a l t e ra t i o n ( r e c o n s t r u c t e d c o m p o s i t i o n , R . C . ) isc a l c u l a t e d f ro m t h e i n it i a l c h e m i c a l a n a l y si s o n t h e b a s i s o f 1 00 m a s s u n i t s( m a s s u n i t s a n d w t % v a l u e s a re i n t e r c h a n g e a b l e ) o f p r e c u r s o r r o c k s, u s i n g Z ra s t h e i m m o b i l e m o n i t o r :R .C . = Zrprecu . . . /Zraltered X O/ocomponentaltered (2 )T o t a l m a s s c h a n g e a n d m a s s c h a n g e s f or e a c h e l e m e n t a re c a l c u l a t e d as:M a s s C h a n g e = R . C . - P r e c u r s o r C o m p o s i t i o n ( 3 )I m m o b i l e c o m p o n e n t s i n t h e c h e m i c a l an a l y si s o f t h e a l t e r e d s a m p l e i n F i g .

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    L I T H O G E O C H E M I C A L T E C H N I Q U E S U S I N G I M M O B IL E E L E M E N T S 1216 h a v e a p p a r e n t l y b e e n d e c r e a s e d b y o n e - t h i r d r e l a t i v e t o t h e p r e c u r s o r a n a l -y s is . T h e r e c o n st r u c te d c o m p o s i t i o n a f te r a l te r a ti o n, w i t h t h e i m m o b i l e c o m -p o n e n t s n o w r e s t o r e d t o t h e i r i n i t i a l a b u n d a n c e s , i s 1 5 0 u n i t s . T h i s y i e l d s am o b i l e m a s s g a i n o f 5 0 u n i t s ( i . e ., 5 0 w t ) .I n a m u l t i p l e p r e c u r s o r s y s t e m , e a c h a l t e r ed s a m p l e i s r el a te d t o i ts p a r t ic -u l a r p r e c u r s o r a s p a r t o f t h e m a s s c h a n g e c a l c u l a t i o n . I n s m a l l d a t a s e ts , e r ro ri s i n t r o d u c e d i n t o t h e c a l c u l a t i o n i f t h e i g n e o u s f r a c t i o n a t i o n c u r v e i s n o t w e l ld e f i n e d , o r i f a si n g le p r e c u r s o r i s t a k e n a s r e p r e s e n t a t i v e o f a g r o u p o f s a m -p l e s th a t i n it i a ll y h a d s li g h t c o m p o s i t i o n a l v a r i a t i o n ( i .e . , r a n g e d a l o n g a n a r -r o w s e c t i o n o f t h e f r a c t io n a t i o n c u r v e ) . N e w d a t a su p p l i e d b y o n g o i n g e x p l o -r a t io n r e f i ne t h e c u r v e e q u a t i o n s , s h a r p e n t h e c o m p o s i t i o n a l r a n g es o f v o l c a n i cf o r m a t i o n s , a n d d e f i n e t h e a l t e r a t io n l i n e s i n g r ea t er d e t a il .

    I n c r e a s e s i n m a s s m o s t l y r e s u l t f r o m s i l i c i f i c a t i o n o r c a r b o n i t i z a t i o n a s v o i d

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    p r e c u r s o r ) : i : : i i : : i i : : : i i i i : i i1 0 0 . . . . . . . . . . . . . . . . . - i i ; i ~ , . : i ~u n i t s i i ii i i i i ii l il i i i i ~ i i i . . . . . . . . . . . i : : : ii i ii i i i : :i i ~ i i i i i i i i i i i i i i i l i i : i ~ i i i. . . . . . : i i i i i i :

    i i i i ii i i i ii i i i i i i i ii i i i ii i : : ~ : : . . . . ii i i i i i i i i i i i i i i l i l i i i i i i : . . . . . . . . . . . .i i i i i i i ~ i i i i i i i i ii i i : : : ~ : : :i i i ii i i ii i i i ii i i i i i i ii i i ~ i : ~ : : : : : : :i i i i i i i i i i i i i i i i i i i i i i i i M o b i l e ::~:: ~:: :~:~i~ i~:~ c o m p o n e n t s iiiiii i~i i i ii . . . .: i i i i i i i : i i i i i i i i i i : i i i i i i i i . . . . . . : :i i i i i i i i i i i i i i i l i i i i i i i ; i i i i i i i i i i i i i i i i : : : : :iiiiiiiii iiiiiiiiii~ iiiii ~ : : : . . . . . : : : ~~ i i i , ~ i i i i i i i i i ~ , ~ i i i i i ~ i i i i i

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    Fig . 6 . I l lu s t r a ti o n o f m a s s c h a n g e c a l c u l a t i o n s f o r a n a l te r e d r o c k s a m p l e , b a s e d o n 1 0 0 u n i ts o fp r e cu r so r . T h e a m o u n t s o f i m m o b i l e e l e m e n t s i n t h e c h e m i c a l a n a l y s i s o f t h e a lt er e d r o c k a p -p e a r to h a v e d e c r e a se d , b u t s i m p l y h a v e b e e n d i l u t e d b y a d d e d m o b i l e m a s s. S i n c e t h e i m m o b i l ee l e m e n t s h a v e n o t l o s t m a s s , t h e y m u s t b e r e s to r e d t o t h e ir i n i t ia l c o n t en t , w i t h t h e o th e r c o m -p o n e n t s r e s to r e d b y t h e s a m e f ac to r . T h i s p r o d u c e s t h e r e c o n st r u c te d c o m p o s i t i o n ( R . C . ) . M a s sc h a n g e f or e a ch c o m p o n e n t i s t h e d i ff e r e n c e b e tw e e n R . C . a n d t h e p r ec u r so r c o m p o s i t i o n .

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    LITHOGEOCHEMICAL ECHNIQUESUSING MMOBILEELEMENTS 123f il li n gs ( a m y g d a l e s , v e in l e t s, a n d m a t r i x t o b r e c c ia s , t u f ts a n d s e d i m e n t s ) . Int h e s e c a s e, t h e o v e r a l l v o l u m e o f t h e v o l c a n i c u n i t s g e n e r a l l y is r e t a i n e d . M a s sl os s is c o m m o n l y d u e t o l e a c h i n g o f S i, N a a n d C a d u r i n g c h l o r i t i z a t io n a n ds e r i c i t i z a t i o n , b u t c a n i n v o l v e F e , M n , M g o r K l o s s a s w e l l. T h e l o s s i s r e c o g -n i s e d b y i n c r e a s e d p r o p o r t i o n s o f i n e r t m i n e r a l s s u c h a s c h l o r i te a n d s e ri ci te ,s h r i n k a g e c r a c k s , a n d s li g ht a r c u a t e d e p r e s s i o n s o f t h e o r e - f o o t w a l l c o n t a c to v e r a l t e r a t i o n p i p e s . M a s s l o s s m a y b e d i f f i c u l t t o v i s u a l ly r e c o g n i s e o n am e t e r - s c a l e ( p i l lo w s , f l o w s ) . F o r e x a m p l e , a 1 0 % v o l u m e lo s s p r o d u c e s o n l ya 3 % d e c r e a s e i n t h e r a d i u s o f a s p h e r i c a l b a s a l t pi ll o w .S o m e m o b i l e c o m p o n e n t s ( Si , K , C a ) r e m o v e d f r o m z o n es o f m a s s lo ss a r et r a n s f e r r e d t o t h e o u t e r p a r t s o f a l t e ra t i o n z o n e s , s o m e c o n t r i b u t e t o t h e o r e -b o d y ( F e , S , b a s e m e t a l s ) a n d m i n e r a l i z e d c h e r t b e d s ( S i, F e , S ) , a n d o t h e r s( C a , N a ) a r e w i d e l y d i s p e r s e d i n s e a w a t e r . M u c h o f t h e a l t e r a t i o n i n p i p e s,p a r t i c u l a r l y i n r h y o l i te , r e p r e s e n t s a l t e r a t i o n o f v o l c a n i c g la ss , w h i c h is v e r yr e a c t i v e ( t h e s o l u b i l it y o f s i l ic a in t h e a m o r p h o u s s t a te is m u c h h i g h e r t h a nf o r q u a r t z ) . D e p l e t i o n o f s i l ic a in t h e c o r e s o f a l t e r a t i o n p i p e s r e s u l ts f r o mt h e h i g h s o l u b i l it y o f a m o r p h o u s s il ic a , as w e l l a s f r o m i n c r e a s e s in t e m p e r a -t u r e a n d w a t e r / r o c k r a t io s .M a s s c h a n g e s c a n b e r e a d i l y i l lu s t r a t e d o n b i n a r y a n d t e r n a r y p l o ts . A u s e -f ul d i a g r a m f o r m o n i t o r i n g a l t e r a t i o n i s A ( F e O + M g O ) v s. A ( C a O + N a 2 0 ) ,w h i c h s h o w s i n c r e a s i n g c h l o r it i z a t i o n a n d F e + M g a d d i t i o n a s th e f e l d sp a rc o m p o n e n t o f t h e r o c k is d e s t r o y e d ( F ig . 7 a ) . A p l o t o f A S iO 2 vs . A K 2 0 ( F ig .7 b ) d i s t i n g u i s h e s s e r i c i t i z e d r o c k s w i t h s i l ic a a d d i t i o n f r o m t h o s e w i t h s il ic al os s. I n b o t h t y p e s o f p lo t s, s i li c a lo s s is i m p o r t a n t a s a g u i d e to h y d r o t h e r m a ll e a ch i n g, w h i c h o c c u r s i n t h e m o s t i n t e n s e p a r t s o f t h e h y d r o t h e r m a l s y s te m .C a l c u l a t e d m a s s c h a n g e s c a n b e e x a m i n e d a l o ng i n d i v i d u a l d r il l h o le s , o r c o n -t o u r e d i n v e r t ic a l a n d p l a n s e c t i o n s i f s u f f i c i e n t d a t a a r e a v a i la b l e . T h e m a s sc h a n g e s al so r e v e a l d i f f e r e n c e s w i t h i n b r o a d a l t e r a t i o n z o n e s t h a t a r e m i n e r -a l o gi c aU y s i m i l a r ( e .g . s e r i c it i z e d z o n e s ) . T r e n d s i n m a s s c h a n g e s a c r o s s l it h -o l o g ic a l u n i t s o r o n a p r o p e r t y s c a l e a r e t h u s u s e f u l i n f o c u s i n g e x p l o r a t i o np r o g r a m s .ALTER ATION MINER ALOG Y

    A b u n d a n c e s o f a l t e r a t i o n m i n e r a l s i n d ri ll c o r e p r o v i d e a s i m p l e a n d d i r e c tm e a n s t o p o r t r a y t h e e x t e n t a n d i n t e n s it y o f h y d r o t h e r m a l a l te r a t i o n . I t is ,Fig. 7. Plots of mass changes for selected elements. (b ) Correlation of mass changes n K20 andSiO2 in the alteration at the H om e mine (data from M acL ean and H oy, 1991 ). The dom inantalteration was a sericitization accom panied by silicification. Small zone s of chlorite-sericitealteration, accom panied by leaching of silica, occur adjacent to th e C u-bearing VM S ores. ( a)L arge losses of CaO + N a20 mass during chlorit ization o f andesite in the footwall of the Corbetmine are accompanied by major additions o f FeO + M gO mass.

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    LITHOGEOCHEMICALTECHN IQUES USING IMMOBILE ELEMENTS 12 5h o w e v e r , d if f i c u lt a n d t i m e c o n s u m i n g t o m e a s u r e t h e m o d a l p r o p o r t i o n s o fa l t e r a t i o n m i n e r a l s i n t h i n - s e c t i o n , o r b y e x p e n s i v e m e t h o d s s u c h a s X - r a yd i f f r a c t io n o r X - r a y i m a g i n g . A n e q u a l l y g o o d e s t i m a t e c a n b e m a d e b y c al -c u l a ti n g n o r m a t i v e a l t er a t io n m i n e r a l a b u n d a n c e s f r o m b u l k r o c k c h e m i c a ld a t a .I n p r i n c i p le , a m i n e r a l a s s e m b l a g e p o r t r a y s t h e c h e m i c a l c o m p o s i t i o n o f ar o c k f o r m e d u n d e r s p e c if i c p h y s i o - c h e m i c a l c o n d i t i o n s . A l t e r a t i o n a r o u n dV M S d e p o s i t s f o r m s u n d e r l o w p r e s s u r e ( s e a f lo o r ) g r e e n s c h i st fa c ie s c o n d i -t i o n s ( 2 0 0 - 4 5 0 C ) , p r o d u c i n g al te r a t i o n m i n e r a l a s se m b l a ge s c o n s is t in gm a i n l y o f q u a r t z , c h l o r i t e , se r i c it e , al b i te , e p i d o t e , c a lc i t e a n d s u l p h i d e . W eu s e t h e N i g g l i- B a r t h c a t i o n n o r m a t i v e c a l c u l a ti o n p r o c e d u r e ( B a r t h , 1 9 6 2 ) ,w h i c h is r e a d i ly a d a p t a b l e t o a c o m p u t e r s p re a d s h e e t , a n d r e p r e s e n t s m o d a lp e r c e n t a g e m o r e c l os e ly t h a n a w e i g h t p e r c e n t n o r m a t i v e c a l c u la t i o n. C a r -b o n a t e a n d s u l p h i d e a re c o m p u t e d w h e n C O 2 a n d S a n a ly s e s a r e a v a il a b le , o ra r e a s s i g n e d a s r e si d u a l s . A l u m i n o - s i l i c a t e c l ay s , t a lc , a n d o t h e r m i n e r a l s a r ec o m p u t e d w h e n a p p r o p r ia t e . C o m p a r i s o n o f n o r m a t i v e r es u lt s w i t h m o d a le s t i m a t e s i n d i c a t e s t h e m e t h o d i s r e li a bl e . T h e n o r m a t i v e d a t a a r e t h e n u s e dt o c o n s t r u c t t e r n a r y a l t e r a t io n m i n e r a l p l o t s ( F ig . 8 a , b ) , o r to p r o d u c e c o n -t o u r e d a l t e r a ti o n m i n e r a l i n te n s i ty m a p s . N o r m a t i v e m i n e r a l a b u n d a n c e s a r ea ls o u se f u l i n th e i n t e r p r e ta t i o n o f ox y g e n i s o to p e d a t a ( M a c L e a n a n d H o y ,1 9 9 1, B a r r e t t a n d M a c L e a n , 1 99 ) .H y d r o t h e r m a l a l t e r a t io n m i n e r a l s d o n o t r e a d i l y in t e r a c t o r r ec r ys ta l li z ed u r i n g s u b s e q u e n t r e g i o n a l g r e e n s c h i s t f a c i e s m e t a m o r p h i s m , w h i c h o c c u r su n d e r s i m i l a r t e m p e r a t u r e c o n d i t i o n s . B u l k c h e m i s t r y , o t h e r t h a n l o ss o f v o l-a t il e s ( w a t e r , C O 2 , S ) , d o e s n o t a p p e a r t o b e n o t ic e a b l y c h a n g e d b y a m p h i -b o l i te f ac i es m e t a m o r p h i s m , w h i c h c o n v e r t s a l t e ra t i o n m i n e r a l o g y t o c o r d i e r-i t e - a n t h o p h y l l i t e - g a r n e t - b i o t i t e a s se m b l a ge s ( R i v e r i n a n d H o d g s o n , 1 98 0;B e r n i e r e t a l. , 1 9 87 ; B e r n i e r a n d M a c L e a n , 1 9 89 ) . N o r m a t i v e a l t e r a t io n m i n -e r al s p r o v i d e a c o n v e n i e n t m e a n s t o c o m p a r e a l t e r a ti o n z o n e s in g r e e n s c h is tf ac i es a n d h i g h e r g r a d e m e t a m o r p h i c t e r ra n e s .NORMA TIVE MINERA L DIAGRAMS

    H y d r o t h e r m a l a l t e ra t io n o f s o d ic rh y o l it e a t t h e H o m e M i n e i s i l l u s t r at e di n F ig . 8 a . A s a l t e r a t i o n p r o g r e s s e d , N a a n d C a w e r e l e a c h e d f r o m a l b i t ic f el d -s p a r a n d r h y o l i te g la ss , a n d t h e A 1 -S i r e si d u e c o m b i n e d w i t h F e + M g a n d Kf r o m t h e r o c k a n d h y d r o t h e r m a l f l u id t o f o r m c h l o r it e a n d s e ri c it e , r e sp e c -Fig. 8. Triangular plots of cation norma tive alteration minerals. (a) Quartz-albite-sericite: Thediagram illustrates the sericitization and silicification of rhyolite in the alteration zon e at theH ome mine (from M acL ean and H oy, 1991 ). (b) Quartz-chiorite-sericite: Chlorite and seri-cite, formed by alteration of plagioclase com ponents in M ontbray rhyolite, were subsequentlyconcentrated by wholescale eachingof silica (from Barrett and M acL ean, 1991 ).

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    LITHOGEOCHEMICALTECHNIQUES USING IMMOBILEELEMENTS 127t iv e l y . B e c a u s e A 1 w a s i m m o b i l e , t h e a b u n d a n c e o f c h l o r i t e a n d s e r ic i te ( b o t hA l - b e a r i n g ) w a s l i m i t e d t o t h e i n i t i a l A l - c o n t e n t o f t h e r h y o l i t e. T h e l e as t a l-t e r e d r h y o l i t e s a m p l e s p l o t c l o se t o t h e a l b i t e - c h l o r i t e ti e l in e , b u t a l t e r a t io nh a s s h i f t e d o t h e r s a m p l e s to w a r d s t h e s e r i c i t e - c h l o r i t e b o u n d a r y , w i t h a m a -j o r i t y o f t h e m n e a r t h e s e r ic i te ap e x . A p l o t o f q u a r t z - s e r i c i t e - c h l o r i t e r e v e a lst h a t t h e d o m i n a n t a l t e ra t i o n a s se m b l a g e i s q u a r t z - s e r i c it e ( M a c L e a n a n d H o y ,1 9 9 1 ) .I n t h e e a r ly s t a g e o f a l te r a t i o n o f a l b i te t o c h l o r i t e a n d s e r ic i te a t l o w w a t e r /r o c k r at io s , r h y o l i t e r e t a i n s m o s t o f it s in i t i a l q u a r t z c o n t e n t ( ~ 5 0 % ) . W i t hi n c r e a s e d a l t e ra t i o n , t h i s a s s e m b l a g e c a n b e r e d u c e d t o s e r ic i te a n d c h l o r it e ,a s o c c u r s i n a l t e ra t i o n z o n e s i n th e M o n t b r a y r h y o l i t e n e a r N o r a n d a ( B a r r e t ta n d M a c L e a n , 1 99 1 ) . R e s i d u a l c o n c e n t r a t i o n o f t h e s e t w o m i n e r a l s is c a u s e db y w h o l e sc a le h y d r o t h e r m a l d i s so l u t i o n o f q u a rt z a t h i g h w a t e r / r o c k r a ti o s( F ig . 8 b ) . E x t r e m e l y a l te r e d r h y o l i t e s p l o t a l o n g t h e c h l o r i t e - s e r i c i t e t i e li n e .C h l o r i t e a n d s e r ic i te a re s t a b le u n d e r t h e a l t e ra t i o n c o n d i t i o n s o f V M S d e -p o s i t s in g r e e n s t o n e b e lt s ( R i v e r i n a n d H o d g s o n , 1 98 1 ) , w h e r e a s A I - S i c la ym i n e r a l s a r e n o t . O t h e r s t a b l e p h a s e s i n c l u d e r u ti le , z i r c o n a n d R E E - b e a r i n gm i n e r a l s .B IN A R Y C A T I O N D I A G R A M S

    A l t e r a t i o n o f v o l c a n i c r o c k s i s a l s o w e l l p o r t r a y e d o n b i n a r y c a t i o n d i a -g r a m s . Q u a r t z , a l b i t e , c h l o r i t e , s e r i c i t e a n d o t h e r m i n e r a l s c a n b e p l o t t e d a sm i n e r a l f o r m u l a e a l o n g w i t h c a t i o n p e r c e n t v a l u es o f e le m e n t s i n t h e a l t e re dr oc ks . S a m p l e s o f si li c if ie d r h y o l i te a t t h e H o m e m i n e p l o t b e t w e e n t h e p r e -c u r s o r a n d q u a r t z o n a n A 1 - S i d i a g r a m ( F i g . 9 a ) . R h y o l i t e s a f f e ct e d b y s t r o n gs i li c a -l e a ch i n g m i g r a t e t o w a r d s t h e s e r i c i t e - c h l o r i t e ti e l in e .T h e S i - ( F e + M g ) d i a g r a m ( F ig . 9 b ) i s u s e fu l i n r e v e a li n g q u a r t z lo ss a n dt h e i n e r t n e s s o f c h l o r i t e a n d s e ri c it e . In i t i a l a lt e r a t i o n o f t h e M o n t b r a y r h y o -l i te s p r e a d s a m p l e s a l o n g a n a p p r o x i m a t e l y c o n s t a n t S i t r e n d , l i m i t e d b y t h eq u a r t z - s e r i c i t e a n d q u a r t z - c h l o r i t e t ie l i n e s. L o s s o f S i t h e n s h i f t e d t h e s a m -p l e s t o t h e s e r i c i t e - c h l o r i t e ti e l i n e w h e r e t h e y f o r m e d s t ab l e r e si d u e s .V O L U M E C H A N G E S

    G e n e r a l l y w e e x a m i n e a l te r a t io n i n t e r m s o f a b so l u t e ch a n g e s i n t h e a m o u n to f m a s s t r a n s fe r r ed i n t o o r o u t o f a r o ck b y h y d r o t h e r m a l fl ui ds . T h i s is m o r eF i g . 9. P l o t s o f p e r c e n t a g e s o f c a t i o n s i n b u l k r o c k c h e m i c a l a n a l y se s . M i n e r a l f o r m u l a e a r e a l s os h o w n i n t e r m s o f c a t i o n c o m p o n e n t s . ( a ) A 1 v s . S i p l o t i l lu s t r a t in g a l t e r a t i o n o f rh y o l i t e a t th eH o m e m i n e . S i l i c i fi e d s a m p l e s p l o t b e t w e e n p r e c u r s o r a n d q u a r t z . T h e s e r i c i t e - c h l o r i t e t i e li n ef o r m s a b a r r i e r t o f u r th e r a l t e r a t io n ( d a t a f r o m M a c L e a n a n d H o y , 1 99 1 ) ( b ) S i v s . ( F e + M g )p l o t i l l u s t r a ti n g a l t e r a t i o n o f t h e M o n t b r a y r h y o l i t e . S e r ic i t e a n d c h l o r i t e f o r m e d b y a l t e r a t io no f p l a g i o c l a s e a r e c o n c e n t r a t e d a l o n g t h e s e r i c i t e - c h l o r i t e t i e l in e b y w h o l e s a l e l e a c h i n g o f q u a r tz .

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    128 W.H. MacLEANANI)1 .J. BARRETT

    i m p o r t a n t t h a n v o l u m e ch a n g e s a s a p a r a m e t e r t o e x a m i n e a n d i n t e rp r e t h y-d r o t h e r m a l p r o c e ss e s. O f c o u rs e , v o l u m e w i ll u su a l l y c h a n g e a s m a s s o f t h er o c k c h a n g e s , b u t a n e x t r a s t e p i s r e q u i r e d t o c a l c u l a t e t h i s v a r i a b le .C h a n g e s i n r o c k v o l u m e o w i n g t o a l te r a t i o n c a n b e e s t i m a t e d f r o m m e a -s u r e d p r e c u r s o r a n d a l t e r e d r o c k d e n s i t ie s a n d t h e m e a s u r e d r a t i o s o f a n i m -

    m o b i l e e l e m e n t , e .g ., Z r p r e c u r s o r Zraltered ( S h r i v e r a n d M a c L e a n , 1 99 3 ). R o c kd e n s i t ie s a n d v o l u m e c h a n g e s c a n a l so b e e s t i m a t e d c lo s e ly fr o m n o r m a t i v em i n e r a l c o n t e n t s o f p r e c u r s o r s a n d r e c o n s t i t u t e d s a m p l e s , a n d m i n e r a l d e n -s it ie s . A b s o l u t e v o l u m e o f a r o c k u n i t m a y n o t c h a n g e a s a re s u l t o f v o i d f i l li n go r v o i d f o r m a t i o n , a l t h o u g h th e v o l u m e s o f i n d i v i d u a l m i n e r a l s c h an g e . M a n ya l t e r a t i o n m i n e r a l s h a v e f i x e d o r n a r r o w r a n g e s i n d e n s i t y ( q u a r t z , a l b i t e ,s e ri c it e , e p i d o t e ) , b u t c h l o r it e h a s a w i d e r r a n g e b e c a u s e o f F e - M g s u b s ti t u -t i o n ( c h l o r i te d e n s i t i e s c a n b e e s t i m a t e d f r o m m i c r o p r o b e m i n e r a l a n a l y se s ).V o l u m e lo s s i n s t r on g l y c h l o r i t i z e d rh y o l i te a t t h e P h e l p s D o d g e V M S d e -p o s i t w a s a s m u c h a s 6 0 % ( M a c L e a n a n d K r a n i d i o t i s , 1 9 8 7 ) . B y c o n t r a s t, a10% v o l u m e in c r e a se w a s e s t i m a t e d f o r t h e b u l k o f t h e H o r n e m i n e a l t e r a t i o nz o n e , w h e r e si l ic i f ic a t io n w a s w i d e s p r e a d ( M a c L e a n a n d H o y , 1991 ).EF FEC T ON

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    LITHOGEOCHEMICALTECHNIQUES USING IMMOBILE ELEMENTS 129o f a ll t h e R E E i n a s u i t e o f a lt e r e d r o c k s b e f o r e u s i n g th e s lo p e s o f t h e R E Ep a t t e r n s t o o b t a i n p e t r o g e n e t i c i n f o r m a t i o n . T h i s i s d o n e b y p l o t t i n g e a c hR E E a g a i n s t a n i m m o b i l e e l e m e n t su c h a s Z r .

    I f t h e R E E h a v e b e e n i m m o b i l e , t h e a d d i t i o n o f m a s s t o a r o c k c a u se s R E Ep r o f i l e s i n a s i n g l e p r e c u r s o r a l t e r a t i o n s y s t e m t o s h i f t v e r t i c a l l y d o w n o nc h o n d r i t e - n o r m a l i z e d d ia g r a m s , w h e r e a s t h e r e m o v a l o f m a s s c a u se s R E Ep r o f il e s to s h i ft v e r t i c a ll y u p w a r d s . I f t h e R E E a ls o h a v e b e e n m o b i l e , i t isd i f f i c u l t t o d i s t i n g u i s h t h e s e v a r i o u s e f f e c t s . T o e v a l u a t e t h e s e c h a n g e s , t h ep r o fi le s ar e re s t o r e d ( n o r m a l i z e d ) t o th e o r i g in a l L u ( o r Y b ) v a l u e o f t h ep r e c u r s o r . I n s o m e a l t e r a t i o n z o n e s , a s a t P h e l p s D o d g e ( M a c L e a n , 1 9 8 8 )a n d A n s i l ( B a r r e t t e t al ., 1 9 9 1 a ) , t h i s p r o d u c e s a f a n - s h a p e d a r r a y o f p r o f il e sw i t h L u a s t h e h i n g e . L a i s t h e m o s t m o b i l e o f t h e t r i v a l e n t R E E , w i t h t h eo t h e r s d e c r e a s in g u n i f o r m l y i n m o b i l it y t o w a r d s i m m o b i l e L u ( M a c L e a n ,1 9 88 ) . A f a n o f p r o fi le s r e s u lt s f r o m R E E m o b i l i ty , w h i c h r a n g e s f r o m u n i -f o r m a d d i t i o n o f t h e l i g h t e r R E E r e l a t iv e to t h e p r e c u r s o r , to u n i f o r m d e p l e -t i o n o f t h e l i g h t er R E E .M o b i l e R E E d e p l e t i o n i s e n c o u n t e r e d i n t h e m o s t i n t e n s e l y c h l o r i ti z e d p a r tso f a l t e r a t i o n z o n e s b e l o w V M S d e p o s i t s ( C a m p b e l l et a l., 1 9 84 ; M a c L e a n ,1 98 8, B a r r e t t et a l. , 1 9 9 1 ) . H y d r o t h e r m a l f l u id t e m p e r a t u r e s a r e h i g h e s t ( 3 0 0 -4 0 0 C ) i n t h e s e z o n e s, w h i c h w e r e s u b j e c t e d t o s i li c a d e p l e t i o n a n d c o m -m o n l y c o n t a i n s t r i n g e r - c o p p e r m i n e r a l i z a t i o n . R E E a d d i t i o n i s m o s t l y i nn e a r b y z o n e s o f s e ri c it e a l t e r a t io n a n d s i li c if ic a t io n , w h e r e f l u id s w e r e c o o l e ra n d p r e c i p i t a t e d R E E m i n e ra l s . L i g h t- R E E d e p l e t i o n an o m a l i e s e n c o u n t e r e di n a l t e re d r o c k d u r i n g m i n e r a l e x p l o r a t i o n a r e i n d i c a t iv e o f c en t e rs o f h i g ht e m p e r a t u r e h y d r o t h e r m a l f lu i d f lo w . R E E a n o m a l i e s o f e i t h e r t y p e a r e i m -p o r t a n t t o d o c u m e n t f r o m a n e x p l o r a ti o n v i e w p o i n t .DISCUSSION

    A h y p o t h e t i c a l e x a m p l e o f a s e q u e n c e o f g r e e n s t o n e v o l c a n ic r o c k s r a n g in gf r o m b a s a l t t o r h y o l i t e b e i n g e x p l o r e d f o r V M S d e p o s i t s i s i ll u s t r a te d i n F i g.1 0 t o h i g h l i g h t ro u t i n e p r o c e d u r e s u s e d f o r l i th o l o g i c a l i d e n t i f i c a t i o n s a n d t h ei m p r i n t s o f a l te r a t io n p r o v i d e d b y i m m o b i l e e l e m e n t s . O n e d r i ll h o l e e n c o u n -t e rs r e l a ti v e l y u n a l t e r e d ( l ea s t a l t e r e d ) s t ra t i g ra p h y , a n d f r o m s a m p l i n g d a t a ,t h e v o l c a n i c u n i t s a r e d i v i d e d i n t o t w o v o l c a n i c a f fi n i ty g r o u p s , o n e t h o l e i i ti ca n d t h e o t h e r m i l d l y c a l c -a l k a l in e , u s i n g a Y - Z r p l o t . T h e m a i n l it h o l o g ie s( b a s a l t - a n d e s i t e - d a c i t e - r h y o l i t e ) a r e id e n t i f ie d f o r ea c h a f fi n it y ty p e u s i n gl e as t a l te r e d s a m p l e s a n d T i O E - Z r a n d A 1 2 O a -Z r p l o ts ; f r a c t i o n a t i o n l in e s a r ef i t t e d t o e a c h p l o t .G e o c h e m i c a l d a t a f o r t h e l e a st a l t e r e d ro c k s ( F i g . l 0 ) r e v e a l t h a t a t h o l ei -

    i t ic b a s a l t t o r h y o l i t e s e q u e n c e i s o v e r l a i n b y m i l d l y c a l c -a l k a l in e r h y o l i te t ob a s a l t u n i ts . R h y o l i t e - r h y o l i t e c o n t a c t s o f t w o t y p e s a r e p r e se n t : b e t w e e nt h o l e i i ti c r h y o l i te s A a n d B ( B is m o r e f r a c t i o n a t e d t h a n A ) , a n d t h o l e i i t ic

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    13 0 W.H. MacLEAN AND T.J. BARRETT

    C h e m o s t r a t i g r a p h y b a s e d o nl e a s t a l t e r e d s a m p l e s f r o msu r face and d r i l l i ng t ransec ts

    C o n t a c t o f In t e r e f

    M i l d l yTIO2%

    1 5 1 B a s ~ A n dac

    iZ r 500

    c a l c - a l k a l i n e v o l c a n i c s e q u e n c eAI203% Y ppm

    1 5- ~ X o o ~ . ~ y= 7

    Zr 500 Zr 500 ppmT h o l e i i t i c v o l c a n i c s e q u e n c e

    TIO2% AI203% Y ppm

    1 .5 - 15 -

    , J ,Zr 500 Zr 500 Zr 500 ppm

    B a s a l t t o a n d e s i t e I I R h y o l i t e S ynvo l can i c fe l s i cD a c i t e ~ i n t r u s i o n ( t h o l e i i t i c )~,, F ra c t i ona t i on t ren ds

    . . . . . . . . . . . . . A l te r a ti on l in e sS t r o n g l y a l t e r e d r o c k snea r mass i ve su l f i de l ens

    ] TiO2

    : - _ - ~O~RhyAF - ~ ' IZr 500

    Zr 500

    A l t e r ed m i l d ly c a l c - a l k a l i n e v o l c a n i c s e q u e n c eTIO2% AI203% Y ppm

    1 5 1 5/ ~ s .And / ' , ? ~l ' , ~ l l ~= .R hy C l ' , ~ '~ 'a'k\1

    Zr 500 Zr 500 Zr 500 ppm

    A l t e r e d th o l e i l t i c v o l c a n i c s e q u e n c eTIO2% AI203/o Y ppm ~o O

    Zr 5(~0 Zr 500 Zr 500 ppm

    ( ~ ) - ( ~ ) R h y o l i t e u n i t s

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    LITHOGEOCHEMICAL TECHNIQUES USING IMMOBILE ELEMENTS 131

    r h y o l i t e B a n d m i l d l y c a lc - a lk a l in e r h y o l i te C . B o t h t y p e s o f c o n t a c t a r e s it eso f fe l s ic - h o s t e d V M S d e p o s i t s i n t h e N o r a n d a d i s tr i ct , Q u e b e c : t h e f o r m e r f o rt h e A l d e r m a c o re s ( B a r r e t t e t a l., 1 99 l b ) , a n d t h e l a t te r f o r t h e M o b r u n a n dD e l b r i d g e o r e s ( B a r r e t t e t a l., 1 9 91 , 1 9 9 3 ) . A n u m b e r o f V M S d e p o s i t s a tN o r a n d a a l s o o c c u r a t a n d e s i t e - r h y o l i t e c o n t a c t s ( e. g. , A n s i l , N o r b e c , M i l le n -b a c h : G i b s o n a n d W a t k i n s o n , 1 99 0; C h a r t r a n d a n d C a t t a l an i , 1 9 9 0 ) . U s i n gt h e l i t h o g e o c h e m i c a l f r a c t i o n a t i o n - a l t e r a t i o n r e l a ti o n s o u t l i n e d e a r li e r, l i t h -o l o g i c a l c o n t a c t s c a n b e t r a c e d f r o m l e a st a l t e r e d s t r a t i g r a p h y t h r o u g h a l te r a -t i o n z o n e s , a n d c o r r e l a t e d a c r o s s f a u l ts .T h e p o s s i b i l i t y o f a g e n e t i c r e l a t i o n s h i p b e t w e e n t h e v o l c a n i c r o c k s a n di n t r u s i v e b o d i e s ( F i g . 1 0 ) a l s o c a n b e e s t a b l i s h e d w i t h m a j o r ( G o l d i e , 1 97 8 )a n d t r a ce ( P a r a d i s e t al ., 1 9 8 8 ) e l e m e n t s , u s i n g t h e f r a c t i o n a t i o n r e l a ti o n sa n d e l e m e n t a l r a ti o s d e s c r i b e d a b ov e . A c o - m a g m a t i c i n t r u s io n i n t h e u p p e rl e v e ls o f a v o l c a n i c p i le w i l l p r o v i d e a n i m p o r t a n t s o u r c e o f h e a t t o d r i v eh y d r o t h e r m a l c i rc u l at io n .I t i s p o s s i b l e t o i d e n t i f y t h e p r e c u r s o r s l i t h o l o g i e s o f v o l c a n i c ro c k s t h a th a v e b e e n i n t en s e l y h y d r o t h e r m a l l y a l te r e d a n d m i n e r a l i z e d t o t h e p o i n t o fb e i n g u n r e co g n i z a b le . T h e i m m o b i l e e l e m e n t i m p r i n t n o t o n l y y ie l ds th e p r e-c i se ro c k t y p e , b u t a ls o t h e m a g m a t i c a ff in i ty . I n t h e N o r a n d a V M S d e p o s i ts ,t h e p a t h s o f a lt e r a t io n c a n b e f o l l o w e d b y m a s s c h a n g e a n d n o r m a t i v e a l t e ra -t i o n m i n e r a l p l o ts . F o r e x a m p l e , si l ic i f ic a t io n a n d s e r i ci t iz a t i o n a r e d o m i n a n tp r o c e s se s i n r h y o l i t e a t th e H o r n e m i n e ( F ig s . 7 a, 8 a, 9 a; M a c L e a n a n d H o y ,1 99 1; B a r r e t t e t a l., 1 9 9 1 a ) ; a t t h e C o r b e t m i n e p l a g i o c l a s e b r e a k d o w n i na n d e s i t e i s c o m b i n e d w i t h c h l o r i t i z a t i o n a n d s il ic a l e a c h i n g ( F ig . 7 b ; B a r re t te t al ., 1 99 3 ) ; a n d i n t h e M o n t b r a y r h y o l i t e e x t r e m e m a s s l os s, c h l o r i t i z a t i o na n d r e s i d u a l b u i d u p o f s e r i c i t e o c c u r s i n m i n e r a l i z e d z o n e s ( F ig . 8 b , 9 b ; B a r-r e t t a n d M a c L e a n , 1 99 1 ) .SUMMARY

    I m m o b i l e e l e m e n t s p r o v i d e n e w l i th o g e o c h e m i c a l t o o ls f o r m i n e r a l e x p lo -r a ti o n . I m m o b i l e H F S E a r e e m p l o y e d t o d i s t in g u i s h b e tw e e n t h o l ei i ti c a n dFig. 10. Schematic example of lithogechemical procedures applied to an exploration drillingprogram. In the upper figure, a drill hole intersects relatively unaltered volcanic stratigraphy.Samples are first divided using Zr /Y ratios into an uppe r mildly calc-alkalinegroup and a lowertholeiitic group (magmatic affinities should be confirmed with further trace el ement and REEdata). Fracti onation curves in TiO 2-Zr and A1203-Zr space are then established for each mag-matic group. Core is generally sampled every 10-20 m. In the lower figure, a drill hole pene-trates a strongly altered (light gray) part o f the same stratigraphy in the vicini ty of a VMSdeposit (black lens). Mass changes due to alteration displace samples of each volcanic unit fromthe fract ionation curves along distinct alteration lines. Three rhyolite types (A -C) are present,as in the unaltered sequence, but the three rhyolites are more readily recognized in the alteredrocks. The felsic int rus ion in this example is of tholeiitic affinity and would be related to thelower volcanic sequence.

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    132 W.H. MacLEAN AND T J BARRETTc a l c - a l k a l i n e a f f i n i t i e s i n f r e s h a n d a l t e r e d r o c k s u i t e s , a n d t o m o n i t o r f r a c -t i o n a t io n o f th e c o m p a t i b l e e l e m e n t s . T h e H F S E , A I a nd T i f o r m t h e b a si s fo rq u a n t i t a ti v e e s t i m a t i o n o f m o b i l e e l e m e n t m a s s c h a n g es . S e v e r e ly a lt er e d r o ck st h a t h a v e l o s t p r i m a r y t e x t u r a l , m i n e r a l o g i c a l a n d c h e m i c a l i d e n t i t y c a n b ec o n f i d e n t l y c l a s s if i e d a s t o i n it i a l r o c k t y p e a n d m a g m a t i c a f f i n it y .

    M a j o r e l e m e n t c h e m i c a l d a t a r e q u ir e d f o r t h e c a l c u l a t io n s a n d p l o t ti n g p r o -c e d u r e s a r e a v a i l a b l e f r o m r o u t i n e b u l k r o c k c h e m i c a l a n a l y s e s , g e n e r a l l ya v a i l a b l e i n m i n e r a l e x p l o r a t i o n p r o g r a m s . H o w e v e r , p r e c i s e a n d a c c u r a t et r ac e e l e m e n t a n a l y s e s a r e n e c es s a r y f o r t h e e s t a b l i s h m e n t o f r o c k a f f i n i t ie s ,a n d f r a c t i o n a t i o n a n d a lt e r a t io n l in e s . T h e l i t h o g e o c h e m i c a l p a r a m e t e r s p r o -d u c e d b y t h e s e t e c h n i q u e s c a n b e p l o t t e d o n e x p l o r a t i o n m a p s , c r o s s - s e c t i o n sa n d d i s c r i m i n a t i o n d i a gr a m s . T h e e x a c t i d e n t i f i c a ti o n o f r oc k ty p e s a n d r o cka f f i n i t i e s a l l o w s c o r r e l a t i o n o f v o l c a n i c u n i t s b e t w e e n d r i l l h o l e s a n d a c r o s sz o n e s a f f e ct e d b y h y d r o t h e r m a l a l te r a t i o n a n d f a u l ti n g . T h i s i n f o r m a t i o n g e n -e r a t e s a c o m p r e h e n s i v e i n s i g h t t o v o l c a n i c l i t h o s t r a t i g r a p h y , h y d r o t h e r m a la l t e r a ti o n a n d o r e - f o r m i n g p r o c e s s e s i n g r e e n s t o n e b e l ts .

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