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PIERINA AU-AG DEPOSIT, CORDILLERA NEGRA, NORTH-CENTRAL PERU

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PIERINA AU-AG DEPOSIT, CORDILLERA NEGRA,

NORTH-CENTRAL PERU

David F. Volkert and Craig J .A. McEwan

MINERA ABX EXPLORACIONES S.A.

IIIIINTRODUCTIONNTRODUCTIONNTRODUCTIONNTRODUCTIONNTRODUCTION

T he P i e r i na Au- Ag depos i t ( F i g .1 ) i s i n t he C or d i l l e r a N egr a o fn o r t h - c e n t r a l P e r ú a t l a t i t u d e 9 ° 2 6 . 5 ’ S , lo n g i t u d e 7 7 ° 3 5 ’ W. P i e r in ai s a h i g h - s u l f i d a t i o n ( a c i d - s u l f a t e ) e p i t h e r m a l d e p o s i t h o s t e d b ypy r oc l as t i c vo lcan i cs o f t he T er t i a r y C a li puy Gr ou p . In D ecem ber 19 96 ,p r o v e n a n d p r o b a b l e r e s e r v e s a t P i e r in a w e r e 6 7 . 7 M t g r a d i n g 2 . 9 8g / t A u a n d 2 2 g / t A g , g i v i n g a t o t a l o f 2 0 1 , 9 0 0 k g A u a n d 1 , 4 9 0t o n n e s A g ( 6 . 4 9 a n d 4 7 . 9 m i l l i o n o z , r e s p e c t i v e l y ) .

Enrique Garay M.

MINERA BARRICK MISQUICHILCA S.A.

RRRRREGIONALEGIONALEGIONALEGIONALEGIONAL GGGGGEOLOGICEOLOGICEOLOGICEOLOGICEOLOGIC SSSSSETTINGETTINGETTINGETTINGETTING

T he C or d i l l e r a N egr a cons i s t s o f a t h i ck sequence o f Ju r ass i c t oC r e taceous m ar l s , sha l es , l i m es tones , and con t i nen ta l c l as t i c r ocksu n c o n f o r m a b l y o v e r l a i n b y a n d e s i t e s , d a c i t e s , a n d r h y o d a c i t e s o ft h e l a t e E o c e n e t o l o w e r M i o c e n e C a l i p u y G r o u p ( 5 2 . 5 – 1 4 . 6 M a ;Wi lson e t a l . 1 9 9 5 ) . T h e Co r d i l le r a B l a n c a i s t h e l a t e Te r t i a r y ( 1 6 . 0 –2 .7 M a ) g r an od i o r i t e ba tho l i t h t ha t in t r u ded l a te Cr e taceous sed i m entsand i s f l anked by M i ocene i gn i m br i t es .

T h e m a i n s t r u c t u r a l e p i s o d e s i n t h e r e g i o n h a v e b e e n E a r l y – t oM i d - Te r t i a r y c o m p r e s s i o n , fo l d in g , a n d o v e r t h r u s t i n g o f t h e M e s o z o i cs t r a ta , f o l low ed by an ex tens i ona l pe r i od assoc i a ted w i th t he ex t r us i on

o f t he C a li puy vo l can ics , em p l acem ent o f t he C or d i ll e r a B l anca ba tho l it ,a n d f o r m a t i o n o f t h e C a ll e jo n d e H u a y l a s g r a b e n ( R io S a n t a v a l l e y) .T h e P l i o c e n e - P l e i s t o c e n e o r o g e n i c u p l i f t o f t h e A n d e a n z o n e w a sthe last structural episode. Main structural trends are NW (Rio Santa grabe nand gen eral accretionary structures associated with subd uction processes),and NE and EW second ary crossfaul ts.

T h e C o r d i ll e r a N e g r a h o s t s s t r u c t u r a l l y c o n t r o l l e d , P b , Z n , C u ,A g , a n d A u m i n e r a l iz a t i o n , m a i n l y in t h e Ca l i p u y v o l c a n i c s . Th e p r i n -c i p a l p r o d u c e r s i n t h e C o r d i l le r a N e g r a w e r e t h e A l i a n z a ( P b - Z n - A g )a n d S a n t o T o r i b i o ( P b - Z n - A g ) m i n e s . A 7 0 - k m - l o n g b e l t o f

h y d r o t h e r m a l a lt e r a t i o n i s a s s o c i a t e d w it h t h e d e p o s i t s a n d d i r e c t l yc o r r e l a t e s w i t h t h e N W- t r e n d i n g R i o S a n t a g r a b e n a n d a s s o c i a t e db o u n d i n g f a u l t s . In t e r s e c t i n g N E - a n d E W - t r e n d i n g s t r u c t u r e s , a l o n gw i t h c ir c u l a r f e a t u r e s p o s s i b i l y r e l a t e d t o e r u p t i v e c e n t e r s , l o c a l iz em i n e r a l i z a t i o n i n v e i n a n d r e p l a c e m e n t d e p o s i t s .

A l o n g t h e w e s t e r n f l a n k o f t h e C o r d i l le r a B l a n c a , P b - Z n - A g i sp r edo m i nan t and i s l a r ge l y con t r o l led by the deep sea ted , N W- t r end i ngcon tac t be tw een the g r an od i o r i t e ba tho l i t h and sed i m enta r y r ocks . Thecon tac t i s a t leas t i n pa r t a m oun ta i n - f r o n t f au l t . Cur r en t Au p r o duc t i onf r o m d e Co r d i l le r a B l a n c a i s r e l a t e d t o t h e a f o r e m e n t i o n e d h i g h - a n g l es t r uc tu r a l con ta c t a t t he N u eva Ca l i fo r n i a m i ne . T he P i e r i na Au de pos i t

i s a n o m a l o u s i n t h e a r e a i n t h a t i t h a s a t y p i c a l h ig h - s u l f id a t i o nep i the r m a l char ac te r, a depos i t t ype w i ch w as p r ev i ous l y un r eco gn i zedi n t h e r e g i o n .

FIGUREFIGUREFIGUREFIGUREFIGURE 11111

SSSSSIMPLIFIEDIMPLIFIEDIMPLIFIEDIMPLIFIEDIMPLIFIED RRRRREGIONALEGIONALEGIONALEGIONALEGIONAL GGGGGEOLOGYEOLOGYEOLOGYEOLOGYEOLOGY AAAAAROUNDROUNDROUNDROUNDROUND THE THE THE THE THE PPPPPIERINAIERINAIERINAIERINAIERINA AAAAAUUUUU-A-A-A-A-AGGGGGDDDDDEPOSITEPOSITEPOSITEPOSITEPOSIT (((((AAAAAFTERFTERFTERFTERFTER WWWWWILSONILSONILSONILSONILSON ET ET ET ET ET AL AL AL AL AL . 1995).. 1995).. 1995).. 1995).. 1995).



David F. Volkert - Craig J .A. McEwan - Enrique Garay M.

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GGGGGEOLOGYEOLOGYEOLOGYEOLOGYEOLOGY OFOFOFOFOF THE THE THE THE THE PPPPPIERINAIERINAIERINAIERINAIERINA DDDDDEPOSITEPOSITEPOSITEPOSITEPOSIT

T he P i e r i na depo s i t i s in r hyo dac i t i c pum i ce and l i t h i c t u f fs t ha t a r eu n d e r l a i n b y a n d e s i t ic l a v a s . Sm a l l e r a n d m o r e r e s t r ic t e d b o d i e s o fc r y s t a l t u f f o c c u r l o c a ll y a t t h e b a s e o f t h e p u m i c e t u f f, a n d a q u a r t z -f e l d s p a r p o r p h y r y in t r u s i o n o c c u r s o n t h e s o u t h f l a n k o f t h e d e p o s i t .T h e c h a o t i c n a t u r e o f t h e r o c k s o n t h e s o u t h f l a n k o f t h e o r e d e p o s i tsugge s ts t he p r esence o f a do m e com p l ex o r ven t a r ea . A ll r ocks t ypesw i t h in t h e o r e d e p o s i t , e x ce p t t h e b a s a l a n d e s i t e , a r e p r e s e n t i n t h i ssou ther n do m e/ven t a r ea . T he quar t z - fe ldspar po r phy r y i s t ough t t o bee i the r con tem p or ane us w i th t he l a te r s tages o f m i ner a l iza t i on o r pos t -m i ner a l .

T he pum i ce and l it h i c t u f f un i t s w er e dep os i t ed i n a N N W- or i en ted ,r es t r i c ted g r aben o r s t r uc tu r a l t r oug h tha t w as fo r m ed w i th i n t he o l de r,b a s a l a n d e s i t e . A t t h e t i m e o f d e p o s i t io n o f t h e p y r o c l a s t ic r o c k s , t h e

a n d e s i t e f o r m e d h i g h w a l ls o n t h e n o r t h , w e s t , a n d e a s t s i d e s o f t h et r o u g h . T h e s o u t h e r n p a r t o f t h e t r o u g h c o n t a in e d t h e d o m e o r v e n tf r om w i ch the py r oc l as t i c s w er e e r up ted .

Dominan t s t ru c tures in t he d epo s i t t rend NNW, WNW, and NE. Cross-sections of the ore depo sit (Fig. 2) sug gest that pre -mineral faul ts displacedand t i l ted the basal andes i te. Post -mineral faul ts apparent ly down-dropp edthe o r e d epos i t t o t he eas t i n to t he R io San ta g r aben . T he d r o p occur r edalong NNW and NE t rend ing s t ruc tures, som e of w ich may be react ivatedpre-m ineral faul ts .

AAAAALLLLL TERA TERA TERA TERA TERA TI TI TI TI TIONONONONON ANDANDANDANDAND MMMMMINERALIZAINERALIZAINERALIZAINERALIZAINERALIZA TI TI TI TI TIONONONONON

A lt e r a t i o n a n d m i n e r a l iz a t i o n a t t h e P i e r in a d e p o s i t a r e t y p i c a lo f v o l c a n i c- h o s t e d h i g h - s u l f i d a t i o n s y s t e m s . T h e r e i s a s t r o n g

l i t h o l o g i c c o n t r o l o n t h e d e v e l o p m e n t o f t h e a l t e r a t i o n a n dm i n e r a l iz a t i o n . R e s i d u a l v u g g y s i l ic a c o n t a i n i n g a l u n i t e i s t h e h o s tf o r m o s t o f t h e o r e . S u r r o u n d i n g t h e v u g g y s i li c a a lt e r a t i o n i s az o n e o f a l u n i t e w i t h m i n o r p y r o p h y l l it e a n d d i c k it e . T h i s a l t e r a t io n

a l s o h o s t o r e , b u t g e n e r a l l y o f lo w e r g r a d e t h a n i n t h e v u g g y s i li c a .T h e a l u n i t e z o n e i s s u c c e e d e d o u t w a r d b y c l a y a lt e r a t i o n ( k a o l i n i t ct o i l l i t i c ) .

T he vuggy s i l ica and s t r ong a l un i t e a lt e r a t i on a r e b es t deve l oped i nt h e p u m i ce t u f f u n i t . Pe r v a s i v e q u a r t z - a l u n it e a l t e r a t i o n o c c u r s i n t h eover ly ing l ith ic tuf f uni t , w ich is a lso cut by vu ggy-s i l ica-al tered s t ru c tures.T he basa l and es i t e is char ac te r i zed by p er vas i ve c lay - py r i t e a lt e r a t i on ,w i th Au mineral i zat ion con f ined to vein lets . Prop y l it i c a l terat ion is a lmostabsen t , and i n t r oduced s i li ca in t he fo r m o f pe r vas i ve s i l if ica t i on i s r a r e .S t e a m - h e a t e d a l t e r a t io n o c c u r s i n t h e u p p e r p a r t s o f t h e s y s t e m , b u tn o A u a c c o m p a n i e s t h i s s t a g e .

Go l d m i ner a l i za t i on a t P i e r i na i s d i ssem i na ted i n t he vugg y - s i l ica -a l t e r e d r h y o d a c i t ic p u m i c e t u f f. M o r e t h a n 9 5 % o f t h e m i n e r a l iz a t i o nde f i ned a t P i e r i na i s ox id i zed . T he Au i s p r ese n t as m i c r om e te r g r a i nso f na t i ve go l d assoc i a ted w i th Fe ox i des , quar t z , and py r i t e . In pa r t s o ft h e o r e b o d y, k e r n e l s o f p y r i t e - e n a r g i t e - c o v e l l it e - n a t i v e s u l f u r a r ep r e s e n t . T h e s e s u l f id e - r i c h k e r n e l s a r e r e m n a n t s o f t h e e a r l i e s t s t a g eo f m i n e r a l iz a t i o n i n t h e d e p o s i t . Gr a d e s o f A u a n d A g a t P i e r i n a a r ez o n e d , wit h t h e h ig h e s t g r ad e s ( > 8 g / t A u a n d u p t o 3 0 0 g / t A g )l o ca l iz e d o n t h e n o r t h e n d o f t h e d e p o s i t .

A s u l fi d e f e e d e r z o n e h a s b e e n i n t e r s e c t e d i n c o r e h o l e s i n t h esou th - cen t r a l a r ea o f t he d epos i t . Wi th in t he feeder zon e , m i ner a l iza t i oni s m a i n l y assoc i a ted w i th ve i ns and f r ac tu r es . Go l d occur s w i th py r i t e ,e n a r g i t e , a n d s p h a l e r it e . T h e l it h o l o g y i n t h e d e e p e s t e x p l o r e d p a r t

FIGUREFIGUREFIGUREFIGUREFIGURE 22222CCCCCROSSROSSROSSROSSROSS-S-S-S-S-SECTIONECTIONECTIONECTIONECTION OFOFOFOFOF PPPPPIERINAIERINAIERINAIERINAIERINA GGGGGEOLOGYEOLOGYEOLOGYEOLOGYEOLOGY.....



PIERINA AU-AG DEPOSIT, CORDILLERA NEGRA, NORTH-CENTRAL PERU

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o f t h e s y s t e m i s p u m i c e t u f f, s u g g e s t i n g t h a t t h e f e e d e r z o n e t o

the mine ral i zat ion ma y also have b een a so urce o f pyroc las t ic volcanics ,and that the and es i te basament was down- faul ted pr ior to mineral izat ion.

O u t c r o p p in g o r e o n t h e n o r t h e n d t h e d e p o s i t co v e r s a n a r e a o f2 0 0 x 3 0 0 m . T h e t o t a l , ve r t i ca l ly - p r o j e c t e d a r e a o f c u r r e n t l y d e f in e dm i n e r a l i z a t io n i s 3 0 0 x 9 0 0 m , o r i e n t e d N N W. M i n e r a l iz a t i o n i si n t e r p r e t e d t o h a v e f o r m e d i n t h e f o l lo w i n g s e q u e n c e : ( 1 ) F o r m a t i o no f p e r v a s i v e q u a r t z - a l u n i t e a l t e r a t i o n i n t h e p u m i c e , l it h i c a n d lo c a lc r y s ta l t u f f s, w i th s u b s e q u e n t , p r o b a b l y p e n e c o n t a m p o r a n e o u s , a c i d -l e a c h i n g o f t h e p u m i c e t u f f t o f o r m t h e c h a r a c t e r i s t i c v u g g y s il i car o c k . N o A u m i n e r a l iz a t i o n a c c o m p a n i e d t h i s s t a g e . A n a g e o f 1 4 . 5M a h a s b e e n o b t a i n e d f r o m h y p o g e n e a l u n i t e a t P i er i n a . ( 2 ) P e r v a s i vep y r i t e - e n a r g i t e - c o v e l l i t e - n a t i v e s u l f u r, w it h m i n o r A u m i n e r a l i z a t i o n

FIGURE 3FIGURE 3FIGURE 3FIGURE 3FIGURE 3

CCCCCROSSROSSROSSROSSROSS-S-S-S-S-SECTIONECTIONECTIONECTIONECTION OFOFOFOFOF THE THE THE THE THE PPPPPIERINAIERINAIERINAIERINAIERINA DDDDDEPOSITEPOSITEPOSITEPOSITEPOSIT, S, S, S, S, SHOWINGHOWINGHOWINGHOWINGHOWING

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( 0 . 5 - 1 . 0 g / t ) i n t h e l ea c h e d , p u m ic e t u f f. ( 3 ) Fo r m a t i o n o f q u a r t z -

p y r i t e - A u ve i n l e t s , w i t h Au p o s s i b l y g r a d i n g 1 - 2 g / t . ( 4 ) H y p o g e n eo x i d a t i o n t h a t f o r m e d s p e c u l a r h e m a t i t e a f t e r p y r i t e , a n d c o v e l li t er i m s a r o u n d r e l ic t l o c a li z e d z o n e s o f S t a g e 2 p e r v a s i v e s u l f id em i n e r a l iz a t i o n . T h e b u l k o f t h e A u , w i t h g r a d e s t o 9 0 . 0 g / t A u , w a si n t r o d u c e d i n t h i s s t a g e . ( 5 ) F o r m a t i o n o f b a r i t e - Au v e i n l e t s a n db r e c c ia s , w it h A u g r a d e s o f u p t o 1 . 5 g / t .

RRRRREFERENCEEFERENCEEFERENCEEFERENCEEFERENCE

WILSON, J.REYES, L & GARAYAR, J. (1995) Geología de los Cuadrángulosd e P a l l a s c a , T a y a b a m b a , C o r o n g o , P o m a b a m b a , C a r h u a z y H u a r i .Instituto Geológico Minero y Metalúrgico ( L i m a , P e r ú ) , B u l l . 6 0 ,6 4 p .



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