Magmatic Arcs and Associated Gold and Copper

7/22/2019 Magmatic Arcs and Associated Gold and Copper

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E L S E V I E R Journal of Geochemical Exploration 50 (1994) 91-14 2

J O U R N L O FG E D C H I l m C NE X P L O R T I O N

Magmat ic arcs and assoc ia ted go ld and copperm inera l iza t ion in Indo nes ia

J . C . C a r li le a, A . H . G . M i t c h e l l bNewcrest Mining Limited, Level 9, 600 St Kilda Road, Melbourne, Victoria 3004, AustraliabThree Trees, Knelle Road, Robertsbridge, E. Sussex TN32 5DZ, UK

( Received 15 January 1993; accepted after revision 8 October 1993 )

A b s t r a c t

G o l d m i n e r a l i z a ti o n i n I n d o n e s i a f o r m e d i n a n d e s i ti c a r c s w h i c h w e r e a c t i v e f o r in t e r v a l s o f b e t w e e n3 a n d 2 0 M y f r o m t h e C r e t a c e o u s t o P l i o c e n e . F i f te e n m a j o r a r c s a r e i d e n ti f ie d w i t h a t o ta l o n l a n de x t e n t o f o v e r 1 5 ,0 0 0 k m s . K n o w n o r e b o d i e s a n d m a j o r p r o s p e c t s a r e c o n f in e d t o s ix a r c s w i t h i n t h em i d - T e r t i a r y to P l i o c e n e a g e r a n g e . I n I n d o n e s i a , t h e s e a r cs t o ta l a p p r o x i m a t e l y 7 , 0 0 0 k m s i n le n g t ha n d c o n t a i n c o m b i n e d h i s to r i c a l p ro d u c t i o n a n d c u r r e n t r e s o u r c e s i n e x c e s s o f 2 ,5 0 0 t o n n e s o f g o l da n d 2 0 m i l li o n t o n n e s o f c o p p e r . I n d i v i d u a l a r c s o r s e g m e n t s o f a r c s a r e c h a r a c t e r i z e d b y s p e c i f ict y p es o f mi n e ra l i za t i o n r e f l ec t i n g bo t h a r c bas emen t r e l a t ed t o ea r l i e r co l l i s i o n s an d r ev e r s a l s i nt ec t o n i c p o l a r i t y , an d e ro s i o n l ev e l .

P o r p h y r y c o p p e r - g o l d , s k a r n , a n d h i g h s u l p h i d a t i o n e n a r g i t e - g o l d d e p o s i t s f o r m e d i n b o t h i s l a n da rc an d co n t i n en t a l s e t t i n g s w h e re i n t ru s i v e s t o ck s a s cen d ed t o s h a l l o w d ep t h s n ea r t h e bas e o f a co -e v a l v o l c a n i c s e q u e n c e . L o w s u l p h i d a ti o n e p i t h e r m a l d e p o s i ts , l a r g e l y c o n f i n e d to t h e w e s t e r n S u n d a -B a n d a a n d C e n t r a l K a l i m a n t a n c o n t i n e n t a l a r cs , f o r m e d p r e f e r e n t i a l l y i n th e l o w e r p a rt o f t he v o l c a n i cp i l e a n d s o m e t i m e s a t t h e b a s e m e n t s u r f a c e . T h e l o w s u l p h i d a ti o n d e p o s i t s m a y b e u n d e r l a i n a t d e p th so f s e v e r a l k i lo m e t r e s b y s i l ls w h i c h p r o v i d e d h e a t f o r t h e h y d r o t h e r m a l s y s t e m s ; t h e d e p o s i ts p r o b a b l yf o r m e d d u r i n g l a t e - s t a g e e x t e n s i o n o n l o w - a n g l e f a u l t s . S e d i m e n t - h o s t e d m i n e r a l i z a t i o n d e v e l o p e dw h e r e l o w s u l p h i d a ti o n f lu i d s e n c o u n t e r e d f a v o u r a b l e c a l c a r e o u s m a r i n e s e d i m e n t a r y r o c k s a t e p it h -e r m a l d e p t h s . T h e d i s t in c t i v e g o l d - s i l v e r - b a r i t e 5- b a s e m e t a l d e p o s i ts o f e a s te r n I n d o n e s i a m a y h a v ef o r m e d b y v e n t i n g o f h i g h s u l p h i d a ti o n f l ui d s i nt o r e s t ri c t e d s h a l l o w m a r i n e b a s in s .

A s m o r e i n f o r m a t i o n b e c o m e s a v a i l a b l e a n d u n d e r s t a n d i n g o f a r c s y s te m s i n c r e a se s , t h e p r o p o s e dr e l a ti o n s h i p s o f d e p o s i t t y p e t o u p p e r b a s e m e n t a n d s t a g e o f a r c e v o l u t io n , a n d d e p o s i t a b u n d a n c e t oe ro s i o n l ev e l , a r e l i k e l y t o beco me i n c reas i n g l y u s e fu l t o r eg i o n a l an d d i s t r i c t s ca l e a s s e s s men t s o fm i n e r a l p o t e n t i a l, b o t h i n I n d o n e s i a a n d i n a r c e n v i r o n m e n t s e l s e w h e r e .

0375-6742/94/$07.00 19 94 Elsevier Science B.V. All rights reservedSSD10375-6742(93) E 0 0 5 1 - W


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92 J. C. Ca r l ile , A.H . G. Mitche ll / Journal o f Geoche mica l Exploration 50 1994) 91-14 2

1 . I n t r o d u c t i o nGold mining in Indonesia dates back to the 4th century when the Chinese, and later, the

immigrant Hindu population worked mainly alluvial deposits in Kalimantan, Sumatra andJava. The first company mining activities were established by the Dutch at the turn of thecentury and continued until the start of the Pacific War in 1941. Recorded production duringthis period was approximately 130 tonnes of gold and 1,300 tonnes of silver, derived mainlyfrom vein deposits in Sumatra ( Van Bemmelen, 1949) and North Sulawesi ( Van der Ploeg,1945). Alter the war, gold mining resumed at Cikotok in West Java in 1957; this was theonly gold producer in Indonesia until 1972 when production started from the copper-goldskarns of the Ertsberg district in Irian Jaya.

The intensive search for gold and copper deposits in Indonesia over the last quarter of acentury has resulted in many new discoveries (Van Leeuwen, 1994). It has also demon-strated a close association of mineralizat ion with magmatic arcs and has produced newinformation on both the regional setting and local geological environment in which min-eralization took place. Deposit types identified include porphyry copper-gold, skarn, highand low sulphidation epithermal systems, sediment-hosted gold, gold-s ilver-barite + basemetal deposits, and at least one deposit, Kelian, that shows geochemical characteristicstransitional between porphyry and epithermal systems (Van Leeuwen et al., 1990).

The combined past production and present reserves/resources of Indonesia are currentlyestimated to exceed 2,500 tonnes of gold and 20 million tonnes of copper with approximately8,000 tonnes of associated silver (Carlile, unpub, data). Seven mines are now producing,two projects are under construction, a number of deposits are undergoing feasibility or areat an advanced exploration stage, and exploration in progress is expected to result in majornew discoveries, particularly in arcs of Neogene age.

Some aspects of these new data have been incorporated into regional reviews (Carlile etal., 1990; Mitchell and Leach, 1991; Kavalieris et al., 1992; Van Leeuwen, 1994), andmajor advances have been made in understanding the tectonic evolution of much of Indo-nesia. However, no overall attempt has been made to relate the recently discovered depositsand major prospects to specific arc systems, or to describe the regional geological featuresof the arcs which may have favoured mineralization.

In this overview, we attempt to identify the main mineralized magmatic arcs in Indonesia,the processes and events that formed them and the nature of basement on which they aresuperimposed. We then highlight the major styles of mineralization within each arc or arcsegment and summarize the main styles of mineralization and aspects of arc geology whichmay have influenced their formation and preservation. Some of our conclusions are neces-sarily speculative, particularly those on the tectonics of individual arcs. In many cases nowidely accepted interpretation exists, highlighting the need for more thorough studies oftectonics, arc magmatism, and mineralization in the region.

2 . R e g i o n a l t e c t o n ic p r o c e s s e s i n a r c s y s t e m sWhile the broad principles of plate interactions in arc systems are more or less universally

accepted, current ideas on many of the tectonic processes are widely scattered in the


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J.c . Carlile, A.H.G. Mitchell/Journal of Geochemical Exploration 50 1994) 91-142 93l it e r at u re . T h e r e f o r e w e s u m m a r i z e t h e m a i n t e c t o n i c p r o c e s s e s w h i c h e x p l a i n f e a t u r e s o fIn d o n es ian g eo lo g y an d th e r eg io n a l g eo lo g ica l se t t i n g s fo r t h e mag mat i c a rcs an d th e i ra s s o c i a t e d g o l d a n d c o p p e r m i n e r a l i z a ti o n .

2 . 1. O p h i o l i t e e m p l a c e m e n t , c o l l i s i o n s , a n d a r c r e v e r s a l

M a n y o f th e I n d o n e s i a n m a g m a t i c a rc s , f o r e x a m p l e t h e S u n d a - B a n d a a r c i n S u m a t r a(C am ero n e t a l. , 1 9 8 0 ) , i n c lu d e o p h io l i t e i n th e basemen t . E ar ly ex p lan a t io n s in v o k edo b d u c t i o n i n t h e s e n s e o f C o l e m a n ( 1 97 1 ), w i th t h r u s t in g o f o c e a n i c c r u st a n d u p p e r m a n t l eo n t o t h e l e a d i n g e d g e o f a n i s l a n d a r c o r c o n t i n e n ta l m a r g i n d u r i n g s u b d u c t i o n b e n e a t h t h ea r c . T h i s e m p l a c e m e n t m e c h a n i s m i m p l i e d th a t a r c m a g m a t i s m t o o k p l a c e o n t h e c o n t in e n t a lmarg in o r i s l an d a rc p r io r t o o p h io l i t e emp lacemen t , an d a l so th a t su bd u c t io n w as acco m-p a n i e d b y c o m p r e s s i o n i n t h e a r c , w i t h s u b d u c t io n d o w n a f i x e d s l o t i n t he m a n t l e . H o w e v e r ,a s p o i n t e d ou t b y H a m i l t o n ( 1 9 8 8 ) , m o s t f e a t u r e s o f s u b d u c t i o n z o n e s a r e b e s t e x p l a in e dby g rav i t a t io n a l s in k in g o f th e su bd u c t in g p l a t e , w i th co n seq u en t ro l l -back o r r e t r ea t o f t h esu bd u c t io n zo n e (F ig . IA ) , an d ex ten s io n a l st r e s s in th e o v er r id in g p la t e p e rm i t t i n g ascen to f m a g m a .

S u b d u c t i o n b y g r a v i ta t io n a l s i n k in g , t o g e t h e r w i t h e v i d e n c e f o r o p h i o l i t e e m p l a c e m e n to n to p ass iv e , r a th e r th an d e s t ru c t iv e co n t in en ta l ma rg in s , h as n o w e f fec t iv e ly r ep lace do b d u c t i o n in t h e s e n s e o f C o l e m a n ( 1 9 7 1 ) a s t h e p r e f e r re d m e c h a n i s m . C o n s e q u e n t l y , i tn o w a p p e a r s t h a t m o s t , i f n o t a l l m a j o r o p h i o li t e b o d ie s a r e e m p l a c e d w h e n t h ic k c r u s t o nth e su bd u c t in g p l a t e en te r s a su bd u c t io n zo n e d ip p in g ben ea th o cean ic l i t h o sp h ere (F ig .1 B ) , w i t h o r w i t h o u t an a c t i v e s p re a d i n g s y s t e m o n t h e o v e r r id i n g p l a te ( F i g . 1 A ) . T h e s es u p r a - s u b d u c ti o n z o n e o p h i o li t e s ( E d e l m a n , 1 9 8 8 ) m a y d i f f e r f r o m o c e a n i c l i th o s p h e r eg en era t ed a t m id -o cean r id g e sp read in g cen t res . P r io r t o in i t i a l emp lacemen t , o p h io l i t e sm a y b e o v e r l a i n b y s u b m a r i n e b a s a l ts , b u t in g e n e r a l a re n o t o v e r l a i n b y p r e - e m p l a c e m e n tan d es i t i c a rc seq u en ces . A bsen ce o f l a t e C en o zo ic o p h io l i t e s su g g es t s t h e p o ss ib i l i t y th a to p h i o l it e g e n e r a t i o n m a y b e c o n f i n e d to p e r i o d s o f r a p id s e a f l o o r sp r e a d in g , a s r e c e n t l yd i scu ssed by L arso n ( 1 99 1 ), a l th o u g h o th e r in t e rp re t a t io n s a re p o ss ib l e ( e .g . , Har r i s , 1 9 9 2 ) .

E m p l a c e m e n t o f a n o p h i o li t e o n t o a c o n t i n e n t ( F i g . 1 B ) o r i sl a n d a rc c a u s e s b a c k -th ru s t in g (F ig . 1 C ) an d in i t ia t io n o f su bd u c t io n ben ea th th e co n t in en ta l marg in o r i s l an darc (F ig . 1 D ) , r e su l t in g in r ev er sa l o f th e su bd u c t io n d i r ec t io n o r p o la r i ty . M o s t a rcs w i tho p h i o l i t e i n t h e b a s e m e n t h a v e p r o b a b l y u n d e r g o n e a t l e a s t o n e p o l a r i t y r e v e r s a l , a n dH a m i l to n ( 1 9 8 8 ) h a s s u g g e s t e d t h a t in i ti a ti o n o f s u b d u c t io n b e n e a t h c o n t i n e n t a l m a r g i n sreq u i res a rc r ev er sa l fo l lo w in g a rc co l l i s io n .

D u r i n g a r c - a r c o r a r c - c o n t i n e n t c o l l i s i o n , o p h i o l i t e a l r e a d y e m p l a c e d o n w h a t s u b s e -q u e n t l y b e c a m e t h e o v e r r i d i n g p la t e m a y b e r e - th r u s t, t o g e t h e r w i t h o v e r l y i n g a r c r o c k s ,o n t o t h e s u b d u c ti n g p l a te m a r g i n ( F i g . 1 E , F , G ) . A r c - a r c o r a r c - c o n t i n e n t c o l li s io n c a na l s o r e s u lt in r e - r e v e r s a l o f t h e s u b d u c t i o n p o l a r it y ( F i g . 1 G ) , f o r e x a m p l e i n t h e B a n d aS e a w h e r e s o u t h w a r d s u b d u c t io n h a s b e g u n b e n e a t h F l o r e s a n d W e t a r f o l lo w i n g c o l l is i o no f t h e B a n d a a r c w i t h T i m o r a n d S u m b a o n t h e n o r t h e r n m a r g i n o f th e A u s t r a l ia n c o n t i n e n t( H a m i l t o n , 1 9 7 9 ) . W h e r e c o l l i s i o n i n v o l v e s an i s l a n d a rc r a t h e r t h a n a c o n t i n e n t o n t h esu bd u c t in g p l a t e , co l l i s io n m ay resu l t i n s t ep p in g -back o f th e su bd u c t io n zo n e ra th e r th anarc r ev er sa l .


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94 J.C. Carlile , A.H.G. Mitch ell~Jo urn al of Geochemical Exploration 50 1994) 91-14 2

A. OPH IOUTE GENERATIONPas.~ve Su pra ~ rus~C o~ner~ I Ophio=te SpreadingM argin Tu rbid i tes Centret Trench /, + + . + + % i . .. . t c o ~_+ + + + - ' - - I~ o s r ; h e ~ c ~ t

Sink bd~n~nRgetSr s /

B . OPH IOUTE EMPLACEMENTR y s c h O p h ~ eM e l a n g e N a p p e

Assimilation~ / i n t o U p p e rMantle

C. ST AR T OF POLAR ITY REVERSALForeland Back ArcTh rus ts , ' .,^.:^,:~^ Thrusts

~ ~ " ~ . . . . . /~ /" O c e a n

E. INTRA-AR C RIFTING

BasalticV o l c an i s m Graben~ " ~ ~ / ' ~ d s e ~ e s O cean

G. ARC-CONTINENT COLLISIONSUBDUCT1ON RE-REVERSAL"" --- - Extension ----- . .- Re-ThrustingErod ed Andes ite of O phioliteBack Arc & Exposed Plutons 1hrust FO c e a n . 7

. . f /

D. REVERSEDSUBDUCTIONExtension

-. ~ Ocean. ~ + I~ v ~ . ~ , t l J [ l l l J l ) i l l [ j j j l l | |

SL~ducting Slab Sinksand RetreatsF. ISLAND ARC DEVELO PMENTExtension =-

AndesiticIsland Arc Ba saltick A rc ,~ ,~ /U pp er B as em ent

[-~+7 Continental Crus tI-; -~T hin ne d C ontinental or Island Arc Crust

BasaRic Cru st/O phioliteTurbidite/Flysch/Melange

~ Plutons

Fig . 1 . M a jo r p l a t e t e c to n i c p ro c e sse s i n a su bd u c t io n se t t i n g .


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J.C. Carlile, A.H.G. Mitchell/Journal of Geochemical Exploration 50 1994) 91-142 952.2. Arc magmatis m

F o r t h e p u r p o s e s o f th i s r e v ie w , a r c m a g m a t i s m i n I n d o n e s i a i s d i v i d e d i n t o m a f i c a n da n d e s i t i c t y p e s . Ma f i c v o l c a n i c r o c k s a r e m o s t l y m a r i n e , p r e d o m i n a n t l y ba s a l t o r ba s a l t i ca n d e s i t e , a n d t h e i r g e n e r a t i o n p r o ba b l y r e q u i r e d s t r o n g e x t e n s i o n w i t h i n t h e a r c ( e . g . ,N a k a m u r a a nd U y e d a , 1 9 8 0 ).

A n d e s i t i c a r c r o c k s i n I n d o n e s i a a r e m o s t l y c a l c - a l k a l i n e a n d e r u p t e d s u b - a e r i a l l y o r i ns h a l lo w m a r i n e e n v i r o n m e n t s , a n d t h e s e q u e n c e s c o m m o n l y i n c l u d e d a c i te s . R h y o l i t e s a ret h o u g h t t o be l a r g e l y c o n f i n e d t o a r c s u n d e r l a i n by c o n t i n e n t a l c r u s t . A n d e s i t i c v o l c a n i s mi m p l i e s a " n o r m a l " w e a k l y e x t e n s i o n a l s t r e s s f i e l d i n t h e a r c , w h i c h m a y b e r e l a t e d t or e t re a t o f t h e h i n g e l in e o r tr e n c h i n th e s u bd u c t i n g o c e a n i c l i t h o s p h e r e ( H a m i l t o n , 1 9 8 8 ) ,p e r m i t t i n g a s c e n t o f m a g m a . I n t r u s i v e e q u i v a l e n t s o f t h e a n d e s i t i c e r u p t i v e s a r e m o s t l ys t o c k s o f d i o r i t e , q u a r t z d i o r i t e , t o n a l i t e , g r a n o d i o r i t e , a n d p o r p h y r i t i c o r s u b - v o l c a n i ce q u i v a l e n t s .

De t a i l e d w o r k o n Ne o g e n e a n d e s i t i c a r c s i n E u r o p e ( e . g . , L e x a , 1 9 9 3 ) s u g g e s t s t h a ta n d e s it ic v o l c a n i s m a n d g e n e r a t io n o f a s t r a to v o l c a n o c o m m o n l y p r e c e d e i n t r u si o n o f s t o ck s .S y n - t o p o s t - m a g m a t i c u p l i f t , c h a r a c t e r i s t i c o f a n d e s i t i c a r c s ; i s p r o ba b l y a r e s u l t o f m a g -m a t i c i n fl at io n a c c o m p a n y i n g e m p l a c e m e n t o f b a t h o li t h s o r s il ls i n th e b a s e m e n t ( H a m i l t o n ,1 9 88 ; L i s t e r a n d B a l d w i n , 1 9 9 3 ) , a n d i s im p o r t a n t be c a u s e o f t h e r e s u l ti n g e r o s i o n a n d i tse f f e c t o n t h e a g e / p r e s e r v a t i o n r e l at io n s h i p s o f o r e b o d i e s .

2.3. Arc baseme ntI n m o s t m i n e r a l i z e d a r c s i n I n d o n e s i a ba s e m e n t i s e x p o s e d a t l e a s t l o c a l l y be n e a t h t h e

v o l c a n i c h o s t r o c k s . T h e ba s e m e n t m o s t l y c o n s i s ts o f n a p p e s o r th r u s t s h e e t s w h i c h i n c l u d em e t a m o r p h i c r o c k s ( g r e e n s t o n e , p h y l l i t e , m i c a s c h i s t , g n e i s s ) a n d o f t e n o p h i o l i t e , s o m e -t i m e s w i t h a t h i n s e d i m e n t a r y c o v e r .

No m i n e r a l i z e d a n d e s i t ic a r c s i n I n d o n e s i a l ie d i r e c t ly o n o c e a n i c c r u s t w h i c h i s sti ll p a r to f , o r r o o t e d i n , th e m a n t l e , a n d t h u s n o n e o f t h e se a n d e s i t i c a r c s c a n be r e g a r d e d a s e n s i m a t i c ,o c e a n i c o r p r i m i t iv e . I s l a n d a r c c r u s t is th i n n e r t h a n t h a t be n e a t h m o s t c o n t i n e n t a l m a r g i na r cs , f o r e x a m p l e t he P e r u v i a n A n d e s o r m a i n la n d N e w G u i n e a . T h e s e c o n t i n e n ta l m a r g i na r c s c o m m o n l y s h o w e v i d e n c e o f a p r e - M e s o z o i c o r o g e n y w i t h a s s o c i a te d a n a te c t ic g r a n it e si n d i c a t i v e o f c r u s t a l m e l t i n g .

2.4. Back-arc spreading

A s a r g u e d by K a r i g ( 19 71 ) a n d P a c k h a m a n d F a l v e y ( 1 9 71 ) , m a n y m a r g i n a l o r ba c k -a r c o c e a n i c b a s in s a r e p r o b a b l y g e n e r a t e d b e h i n d m a g m a t i c a r c s d u r in g s u b d u c t io n f o l lo w -i n g ri f t in g w i t h i n th e a r c ( F i g . I E a n d F ) . Un c e r t a i n t i e s r e m a i n a b o u t t h e p r e c i s e s p r e a d i n gm e c h a n i s m , a n d t h e r e la t io n s h i p o f s p r e a d i n g to a r c m a g m a t i s m . N e v e r t h e l e s s , f o r m a n ya r cs , th e p r o p o s a l o f N a k a m u r a a n d U y e d a ( 1 9 8 0 ) t h at i ni ti al b a c k - a r c s p re a d i n g i s a c c o m -p a n i e d by ba s a l t i c v o l c a n i s m w i t h i n t h e a r c s e e m s t o a p p l y .


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96 J. C. Carlile, A.H. G. Mitch ell/Jou rna l of Geochemical Exploration 50 1994) 91-1422 .5 . M e t a m o r p h i c c o r e c o m p l e x e s

R e c e n t d e s c r i p ti o n s o f m e t a m o r p h i c c o r e c o m p l e x e s i n P a p u a N e w G u i n e a ( H i l l e t al.,1 9 9 2 ) a n d t h e p r e s e n c e o f w h a t w e c o n s i d e r t o be s i m i l a r f e a t u r e s i n th e P h i l i p p i n e s , f o re x a m p l e t h e t r o n d jh e m i t e d o m e a t P a ra c a l e ( M i t c h e l l a n d L e a c h , 1 9 91 ) , s u g g e s t t h a t c o rec o m p l e x e s a n d a s s o c i a t e d l o w - a n g l e e x t e n s i o n a l f a u l t s m a y a l s o o c c u r i n I n d o n e s i a .R e g i o n a l c r u s t a l e x t e n s i o n a s s o c i a t e d w i t h t h e d e t a c h m e n t f a u l t i n g p r o v i d e s a p o s s i b l em e c h a n i s m f o r a rc c o l l a p s e a nd w e a r g u e l a te r t h a t in m a n y a r c s s u b d u c t i o n i s a c c o m p a n i e dby a c h a r a c t e r i s t ic s e q u e n c e o f t e c t o n i c e v e n t s t o w h i c h m i n e r a l i z a t i o n m a y be r e l a t e d .

3 . T e c t o n ic f r a m e w o r k o f I n d o n e si aT h e p r e s e n t c o n f i g u r a t io n o f m a g m a t i c a rc s y s t e m s in I n d o n e s i a ( F i g . 2 ) is t h e re s u l t o f

a c o m p l e x h i s t o ry o f t e c to n i c e v e n t s a n d p r o c e s s e s in c l u di n g s u b d u c t io n a n d a r c m a g m a t i s m ,a r c m i g r a t i o n a n d r o t a t i o n , ba c k - a r c s p r e a d i n g , o p h i o l i t e e m p l a c e m e n t a n d c o l l i si o n r e s u lt -ing in a rc reversa l s , s t r ike - s l ip fau l t ing , and in fe r red c rus ta l ex ten s ion . A s d i scussed in ea r lys y n t h e s e s ( K a t i l i, 1 9 75 , 1 9 8 4; Ha m i l t o n , 1 9 7 9 ) t h e s e e v e n t s a c c o m p a n i e d t h e n o r t h w a r dd r i ft o f t h e A u s t r a l i a - N e w G u i n e a p l a te f r o m G o n d w a n a , a n d t h e l a te C e n o z o i c c o l l i si o no f t h e A u s t r a l i a - Ne w G u i n e a c o n t i n e n t a n d i t s w e s t e r n s a l i e n t i n t o S u l a w e s i w i t h t h eS u n d a l a n d p r o m o n t o r y o f S o u t h E a s t A s ia .

I n t h e l a t e C r e t a c e o u s , t h e n o r t h w e s t e r n o r S u n d a l a n d p a r t o f I n d o n e s i a m a y h a v e be e nor ien ted as sho wn in F ig . 3 . Bo rneo h ad no t ye t ro ta ted an t i c lock wis e ( Fu l le r e t a l. , 1991 )a n d s u b d u c t i o n w a s t a k in g p l a c e in t he p a l a e o - S u n d a tr e n c h w i t h a r c m a g m a t i s m s u p e r i m -p o s e d o n t h e c o n t in e n t a l m a r g i n a n d o n t h e W o y l a G r o u p o c e a n i c n a p p e s ( C a m e r o n e t al.,1 9 80 ; Mi t c h e l l , 1 9 9 2 ) e m p l a c e d d u r in g t h e la t e E a r l y C r e t a c e o u s . No r t h e a s t e r n S u n d a l a n dw a s a p a s s i v e c o n t i n e n t a l m a r g i n a t t h i s t i m e ; w e s u g g e s t t h a t t h e L u p a r o p h i o l i t e w a se m p l a c e d o n t o t h is m a r g i n by t h e e n d o f t h e C r e t a c e o u s , d e f o r m i n g t h e fl y s c h a n d s h e l fs e d i m e n t s d e s c r i b e d b y W i l li a m s e t al. ( 1 9 8 8 ) .

B y t h e O l i g o c e n e ( F i g . 4 ) , B o r n e o h a d be g u n t o r o t a t e a n t i c l o c k w i s e a n d th e n o r t h e a s t e r nm a r g i n o f S u n d a l a n d w a s a n a c ti v e ( s u b d u c t i n g ) m a r g i n . T h e M a k a s s a r S e a m a y h a v e b e e na w i d e b a c k - a r c o c e a n b a s i n . T h e A u s t r a l i a - N e w G u i n e a c o n t i n e n t w a s s o m e 2 , 0 0 0 k m ss o u t h o f i ts p r e s e n t p o s i t i o n in th e O l i g o c e n e ( V e e v e r s e t a l ., 1 99 ) , a n d p r o ba b l y c o n t i n u e dn o r t h w e s t w a r d s a s a c o n t i n e n t a l p r o m o n t o r y i n t o t h e S u l a - B u t o n - B u m b u l a n s a l i e n t a n dp o s s i b l y p a rt o f n or t h er n S u l a w e s i ( F i g . 4 ) . A n o p h i o l i t e n a p p e w a s e m p l a c e d o n t o t h ep a s s i v e n o r th e a s t e rn m a r g i n ( P i g r a m e t al ., 1 9 8 9) o f N e w G u i n e a a n d i ts n o r th w e s t e r ns a l i e n t i n th e O l i g o c e n e , f o r m i n g w h a t w e s u g g e s t w e r e th e f o r m e r l y - c o n t i n u o u s o p h i o l i t e so f N e w G u i n e a ( P i g r a m a n d S y m o n d s , 1 9 9 1 ) , W a i g e o , H a l m a h e r a ( B a l l a n t y n e , 1 99 0 ),S u l a w e s i ( P a r k i n s o n , 1 9 91 ) a n d p e r h a p s e a s t M i n d a n a o i n t he P h i l ip p i n e s .

B y t h e P l i o c e n e ( F i g . 5 ) , w e s u g g e s t t h a t a n o r t h - f a c i n g a r c s y s t e m h a d c o l l i d e d w i t h t h eS u n d a - B a n d a a r c . N e w G u i n e a a n d H a l m a h e r a h a d d r i f t e d n o r t h t o n e a r t h e i r p r e s e n tp o s i ti o n , a n d H a l m a h e r a h a d m o v e d w e s t a n d r o t at e d c l o c k w i s e . T h e M i r i b l o c k o f n o r th -w e s t e r n B o r n e o ( Hu t c h i s o n , 1 9 8 9 ) , p o s s i b l y a c o n t i n e n t a l f r a g m e n t , h a d c o l l i d e d w i t hS u n d a l an d , a n d t h e S u l a - B u t o n - B u m b u l a n s a li e n t o f N e w G u i n e a ( F i g . 4 ) h a d a l s o c o l li d e dw i t h S u n d a l a n d . T h i s c o l l i s i o n w a s f o l l o w e d by m a j o r s t r i k e - s l i p f a u l t i n g in th e B i r d s He a d -


7/52

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102 J . C . C a r l i le , A . H . G . M i t c h e l l / 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 t io n 5 0 1 9 9 4 ) 9 1 - 1 4 2

S u l a r e g io n o f I r ia n J a y a ( M c C a f f r e y a n d A b e r s , 1 9 9 1 ) . T o t h e s o u t h , th e T i m o r s e g m e n to f th e S u n d a - B a n d a a r c h a d b e g u n t o c o l li d e w i th t h e A u s t r a l i a - N e w G u i n e a p l a t e b y th ee n d o f th e M i o c e n e ( e .g . , H a m i l to n , 1 9 8 8 ) .

A r c m a g m a t i s m c o n t i n u e d i n t h e S u n d a - B a n d a a r c, th e n o r th a n d w e s t e r n p a rt o f S u l a w -e s i , a n d H a l m a h e r a t h r o u g h o u t t h e Qu a t e r n a r y , a n d a l s o t o o k p l a c e in t h e A c e h a r c o fn o r t h e r n m o s t S u m a t r a ( A s p d e n e t a l ., 1 9 8 2 a ) , bu t a n y m i n e r a l i z a t i o n a s s o c i a t e d w i t h t h e s ey o u n g v o l c a n o e s i s n o t y e t e x p o s e d .

A s c h e m a t i c c r o s s -s e c t i o n t h ro u g h B o r n e o a n d w e s t S u l a w e s i t o t he N o r t h B a n d a b a s i n( F i g . 6 ) i l l u s t r a t e s o u r i n t e r p r e t a t i o n o f m a n y o f t h e t e c t o n i c p r o c e s s e s w h i c h h a v e t a k e np l a c e i n m u c h o f I n d o n e s i a s i n c e t h e e a r l y C r e t a c e o u s .

4 . I n d o n e s i a n m a g m a t i c a r c sF i f t e e n l a te Me s o z o i c a n d C e n o z o i c m a g m a t i c a r c s a re i d e n ti f ie d i n I n d o n e s i a ( F i g . 2 ) ,

a n d h a v e a to t a l o n l a n d e x t e n t i n e x c e s s o f 1 5 ,0 0 0 k m s ( T a b l e 1 ) . Ho w e v e r , o v e r 9 8 % o ft h e c o u n t r y ' s g o l d a n d c o p p e r ( h i s t o r i c a l p r o d u c t i o n a n d r e s e r v e / r e s o u r c e e s t i m a t e s ) , i sd e r i v e d f r o m o n l y s i x a rc s ( F i g . 7 ) , o f m i d - T e r t i a r y o r y o u n g e r a g e . I n I n d o n e s i a th e t o ta ll e n g t h o f th e s e a r c s i s a p p r o x i m a t e l y 7 ,0 0 0 k m s , bu t t h e y a l s o e x t e n d i n to t h e n e i g h bo u r i n gc o u n t r i e s o f P a p u a N e w G u i n e a , t h e P h i l i p p i n e s a n d M a l a y s i a w h e r e t h e y c o n t a i n m a j o rc e n t r e s o f m i n e r a l i z a t i o n ( F i g . 8 ) . S e v e n o t h e r a r c s a r e e i t h e r d e e p l y e r o d e d o r l i t t l ee x p l o r e d , a n d t h e e x i s t e n c e o f t w o o t h e r s , t h e T a l a u d a n d C o a s t a l I r i a n J a y a a r c s , i s s p e c u -l a t i v e ( F i g . 2 ) .4 .1 . S u m a t r a - M e r a t u s a r c M i d d l e a n d L a te C r e ta c e o u s )Geo log i ca l s e t t ing

T h i s c o n t i n e n t a l a r c ( F i g . 2 a n d T a b l e 1 ) e x t e n d s a l o n g t h e i n f e r re d s o u t h e r n e d g e o fS u n d a l a n d f r o m n o r t h S u m a t r a t h r o u g h t h e t i p o f w e s t J a v a t o ea s t K a l i m a n t a n .

T h e S u n d a l a n d c o n t in e n t a l m a s s i f w i t h P a l a e o g e n e o r o l d e r b a s e m e n t c o n t i nu e s n o r th -w a r d t h r o u g h t h e M a l a y P e n i n s u l a ( F i g . 2 ) i n to T h a i la n d , M y a n m a r a n d I n d o c h in a . S u n -d a l a n d h a d b e c o m e a si n g l e c o n t in e n t a l l a n d m a s s by t h e e n d o f t h e T r i a s s ic o r e a r l y J u r a s s i c ,f o l l o w i n g T r i a s s i c a n d p o s s i b l y e a r l y J u r a s s i c c o l l i s i o n s a l o n g s u t u r e s e x t e n d i n g f r o mo f f s h o r e o f n o r t h e a s t S u m a t r a i n t o K a l i m a n t a n , a n d p r o b a b l y t h r o u g h c e n t r a l S u m a t r a( H a m i l t o n , 1 9 79 ; Hu t c h i s o n , 1 9 89 ; Mi t c h e ll , 1 99 2 ; a n d r e f e r e n c e s th e r e i n ) . T h e s e c o l l i-s i o n s r e s u l te d d i r e c t l y o r in d i r e c t ly i n th e h i g h l y p r o d u c t i v e T r i a s s i c t in be l t o f S o u t h E a s tA s i a , a n d p o s s i b l y t h e m e s o t h e r m a l g o l d b e l t o f m e d i a l M a l a y a a n d T h a i la n d .

F r o m t h e Mi d d l e J u r a s s i c t o l a t e E a r l y C r e t a c e o u s , t h e s o u t h e r n m a r g i n o f S u n d a l a n dw a s p r o b a b l y a p a s s i v e c o n t i n e n t a l m a r g i n , f a c i n g w e s t i n S u m a t r a a n d s o u t h i n K a l i m a n t a n ,p r i o r to p o s t - C r e t a c e o u s a n t i c l o c k w i s e r o t a t io n o f e a s t e rn S u n d a l a n d a n d a l a t e r c l o c k w i s er o t a t i o n o f w e s t e r n S u n d a l a n d ( F u l l e r e t a l ., 1 99 1 ) t o i t s p r e s e n t p o s i t i o n .

O c e a n i c r o c k s o f th e W o y l a G r o u p i n w e s t e r n S u m a t r a h a v e b e e n e x p l a i n e d i n te r m s o fa c c r e t i o n t o a s o u t h - f a c i n g a r c o n t h e c o n t i n e n t a l m a r g i n o f S u n d a l a n d ( C a m e r o n e t a l . ,1 98 0 ; W a j z e r e t a l ., 1 9 9 1 ) . W e c o n s i d e r i t m o r e l i k e l y t h a t i n th e l a t e E a r l y C r e t a c e o u s , an o r t h - f a c i n g m a f i c a r c a n d u n d e r l y i n g o p h i o l i t e c o l l i d e d w i t h , a n d w e r e e m p l a c e d o n t o , th e


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J.C. Carlile , A.H.G. Mitch ell~Jo urn al of Geochemical Exploration 50 1994) 91-14 2 103Table 1Selected features of the magmatic arcs of Inclonesia. Land lengths are approximations and only include the partof an arc contained within Indonesia. Sunda-Banda: (A) Continental crust underlies Sumatra and part of Java;(B) island arc crust underlies east Java and islands to the east. Sulawesi-East Mindanao: (A) Continental crustunderlies the western section of the north arm and the western arm of Sulawesi; (B) island arc crust underliesmost of the north arm of Sulawesi. Halmahera: (A) Continental crust underlies Bacan; (B) island arc crustunderlies western HalmaheraArc Age Crustal type Upper Length Au /km Cu /km

basement (km) (kg) (t)I Middle and Late Continental Ophiolite and 2,000 8 0.121Sumatra Cretaceous pre-TertiaryMeratus schistsI1 Neogene (A) Continental Arc I and itsSunda-Banda basement 3,700 133 730

(B) Island arc basalts1 I I Neogene Continental Pre-Tertiary 320 - 2 , 8 1 3Aceh schistsIV mid Tertiary and Continental Arc VIII and 1,15 0 205Central Kalimantan Neogene its basementV Neogene (A) Continental Schists andSulawesi- ophiolite 1,200 293East Mindanao ( B ) Island arc basaltsVI Neogene (A) Continental SchistsHalmahera 350 44

( B ) Island arc BasaltsVII Neogene Continental Sediments 500 2,688Medial Irian JayaVIII late Early Continental Schists and 1,450Schwanner Cretaceous granitesIX Late Cretaceous Continental Schists 1,600Sunda Shelf

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26,271

X Neogene Flysch over Paleocene 530Northwest Borneo continental flyschXI Neogene Continental Schists and 1,065West Sulawesi ophioliteXII Neogene Continental Schists and 330Moon Utawa ophioliteXIIi Paleogene and Continental Sediments 400Sumba Timor Neogene


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122 J.C. Carlile , A.H.G. Mitch ell~Jo urn al o f Geochemical E xploration 5 0 1994) 91 142s o m e t i m e s h ig h c a r bo n a t e a n d ba s e m e t a l c o n t e n t s , i m p l i e s e r o s i o n l e v e l s p r o b a b l y s u f f i c ie n tt o e x p o s e t h e u p p e r p o r t i o n s o f a n y p o r p h y r y c o p p e r - g o l d s y s t e m s , i f th e y w e r e p r e s en t .

A t T o m b u l i l a t o , s t o c k s i n t r u d e o l d e r ba s a l t ic a n d a n d e s i t i c v o l c a n i c r o c k s , t o l e v e l s j u s tb e l o w o r j u s t a b o v e a n u n c o n f o r m i t y w i t h o v e r l y i n g d a c i ti c v o l c a n i c r o c k s t h a t a r e c o - e v a lw i t h th e m i n e r a l i z e d s t o c k s ( F i g . 1 2 B ) . A c o m p a r a b l e s i t u a ti o n o c c u r s a t B a t u H i j a u , a n da t G r a s b e r g m a a r v o l c a n i c s a r e p r o b a b l y c o - e v a l w i t h t h e e a r l y m i n e r a l i z e d d i a t r e m e( M a c D o n a l d a n d A r n o l d , 1 9 9 4 ) .

T h e s e r e la t io n s h i p s s u g g e s t th a t th e g e n e r a t io n o f p o r p h y r y c o p p e r - g o l d s y s t e m s m a y b ed e p e n d e n t o n a s c e n t o f m a g m a to a h i g h l e v e l , a t o r n e a r th e b a s e o f e r o d i n g a n d e s i t i c -d a c i t ic v o l c a n o e s ( S i l l it o e , 1 9 9 0 ) . W e s u g g e s t t h a t t h e r is e o f m a g m a t o t h e s e s h a l lo wd e p t h s , w h e r e c o o l i n g w o u l d h a v e be e n r e l a t e d t o c i r c u l a t i n g m e t e o r i c w a t e r ( C a t h l e s ,1 98 1 ) , w a s p o s s i b l y f a c i l i t a t e d by t h e a b s e n c e o f s i g n i f i ca n t e x t e n s i o n in t h e c r u s t , i n h i b i t in gp e n e t r a t i o n o f c o o l f lu i d s t o d e e p e r l e v e l s .

A s in m o s t p o r p h y r y s y s t e m s o f t hi s t y p e, t h e m a i n s t o c k w o r k o r d i s s e m i n a t e d m i n e r a l -i z a t io n i s h o s t e d i n t h e p o t a s s i c z o n e w i t h in t h e i n t r u s io n . M i n e r a l i z a t io n i s t h o u g h t t o h a v ea c c o m p a n i e d p h a s e s e p a r a ti o n o f m a g m a t i c f lu id s a n d f r a c tu r i n g o f t h e c r y s ta l l iz e d p o r p h y r ym a r g i n s i n r e s p o n s e t o f l u i d e x s o l u t i o n f r o m a c r y s t a l l i s i n g m e l t ( P h i l l i p s , 1 9 7 3 ; L o w e l la n d G u i l be r t , 1 9 7 0 ; B u r n h a m , 1 9 7 9 ) .

5 . 3 . S k a r n m i n e r a l i z a t i o n

D i s t r i b u t io n a n d c h a r a c t e r i s t i c sT h e o n l y m a j o r s k a r n d e p o s i t s k n o w n i n I n d o n e s i a o c c u r i n t h e Me d i a l I r i a n J a y a a r c .

N u m e r o u s m i n o r e x a m p l e s a re k n o w n in th e S u m a t r a - M e r a t u s a n d C e n t ra l K a l i m a n t a n a r csa n d s o m e s m a l l o c c u r r e n c e s a r e r e c o r d e d i n No r t h S u l a w e s i a n d t h e B i r d s He a d o f I r i a nJ a y a .

T h e t h r e e s k a r n bo d i e s i n I r ia n J a y a , E r ts be r g , E r t s be r g E a s t ( i n c l u d i n g t h e i n t e r m e d i a t ea n d d e e p o r e z o n e ) a n d Do r a a r e h o s t e d by t h e E a r l y E o c e n e d o l o m i t i c ba s a l u n i t s o f t h eN e w G u i n e a L i m e s t o n e . T h e E r t s b e r g o r e b o d y ( n o w m i n e d o u t ) o c c u r r e d a s a p i p e - s h a p e dp l u g o f m a g n e t i t e - c h a l c o p y r i t e o r e s u r ro u n d e d b y c a l c - s i li c a te sk a r n ( m o n t i c e l l i t e - g a r n e t -v e s u v i a n i t e ) w i t h i n t h e 3 . 2 Ma E r t s be r g D i o r i t e ( K a t c h a n , 1 9 8 2 ) .

E r t s be r g E a s t a n d Do m a r e bo t h l o c a t e d a t t h e l i m e s t o n e - d i o r i t e c o n t a c t ( Me r t i g e t a l . ,1 9 9 4 ) . E r t s be r g E a s t , t h e l a r g e s t d e p o s i t , e x t e n d s v e r t i c a l l y f o r o v e r 1 , 5 0 0 m e t r e s . C h a l -c o p y r i t e a n d a c a l c - s i li c a t e a s s e m b l a g e , s i m i l a r to E r t s be r g i n th e u p p e r p o r t i o n s , g r a d e s t oa b o r n i t e - d o m i n a n t c o p p e r m i n e r a l o g y w i t h m a g n e t i t e at d e pt h , a c c o m p a n i e d b y a n i n c re a s ei n f o r s t e r i t e . G o l d g r a d e s a l s o i n c r e a s e w i t h d e p t h f r o m a n a v e r a g e o f 0 . 7 g / t t o 1 . 2 g / t ,p o s s i b l y a s s o c i a t e d w i t h a n i n c r e a s e in t h e a m o u n t o f bo r n i t e ( P o t t e r e t al ., 1 9 9 1 ) . Do r a i sb r o a d l y s i m i l a r t o th e u p p e r l e v e l s o f E r t s be r g E a s t .

O l d e r m i n o r s k a r n m i n e r a li z a ti o n in t h e S u m a t r a - M e r a t u s a r c o c c u r s a t M u a r a S i p o n g ia n d M u a r a S o m a w h e r e p r e - T e r t ia r y g r a n o d i o ri t e s i nt ru d e P e r m i a n m a r b l e s a n d l i m e s t o n e s .A t S e r a n t a k a n d B u d u k i n th e w e s t e r n s e c t i o n o f t h e C e n t ra l K a l i m a n t a n a r c, m i d - T e r t i a r yd i o r i t e s i n t r u d e c a l c a r e o u s s e d i m e n t s , a n d w e a k l y m i n e r a l i z e d s k a r n s a r e a s s o c i a t e d w i t hg o l d - b e a r in g q u a r t z - p y r i t e v e i n s . T h e m i n o r s k a r ns i n W e s t K a l i m a n t a n a p p e a r t o b e o l d e rt h a n a d j a c e n t g o l d be a r i n g q u a r t z - p y r i t e v e i n s .


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,L C . C ar li le , A .H .G . Mi t ch e l l /Jo u rn a l o f Geo ch em ica l Exp lo rat io n 5 0 1 9 9 4 ) 9 1 -1 4 2 123

Geological environment of ormationF o r m a t i o n o f t h e m a j o r s k a m s o f t h e E r t s b e r g d i s tr i ct a p p e a r s t o h a v e t a k e n p l a c e i n a

e n v i r o n m e n t s i m i l a r t o t h a t o f t h e d e e p e r p o r p h y r y - c o p p e r g o l d m i n e r a l i z a t io n a t G r a s b e r g .T h e d e p o s i t s r e fl e c t t h e f o r t u i to u s s u p e r i m p o s i t i o n o f a P l i o c e n e a r c o n a th i c k s e q u e n c e o fc o n t i n en t a l m a r g i n c a r b o n a t e - r i c h s e d i m e n t s w h i c h h a d b e e n t e c t o n is e d a n d f r a c t u r e d d u r in gt h e M e l a n e s i a n o r o g e n y .

T h e v e r t i c a l z o n a t i o n o f c a l c - s il i c a t e m i n e r a l o g y a t E r t s be r g E a s t , w h e r e f o r s t e r i te d o m -i n a t e s a t d e p t h , i s t h o u g h t t o be r e l a t e d t o a n i n c r e a s e i n c o n f i n i n g p r e s s u r e o v e r t h e 1 , 50 0m e t r e v e r ti c a l e x t e n t o f t h e d e p o s i t ( K a t c h a n , 1 9 8 2 ) .5 4 High sulphidation epithermal mineralizationDistribution and characteristics

A n u m be r o f p r o s p e c t s i n I n d o n e s i a d i s p l a y c h a r a c t e r is t i c s o f h i g h - s u l p h i d a t i o n e p i t h e r -m a l m i n e r a l i z a t i o n , a s d e s c r i be d by A s h l e y ( 1 9 8 2 ) , S i l li t o e ( 1 9 83 ) , He a l d e t a l. , 1 9 8 7, a n dH e d e n q u i s t ( 1 9 8 7 ) , b u t w i t h t h e e x c e p ti o n o f M o t o m b o t o n o n e a r e e x t e n s iv e l y e x p l o r e d .

M o t o m b o t o , i n th e T o m b u l i l a t o d i st ri c t o f N o r t h S u l a w e s i , o c c u r s a t t h e c o n t a c t o f a L a t eP l i o c e n e d a c it e p o r p h y r y a n d M i o c e n e - P l i o c e n e a n d e s it e s. T h e d o m i n a n t l y d a c it ic P l i o c e n eh o s t r o c k s ( P e r e l l o , 1 9 9 4 ) u n c o n f o r m a b l y o v e r li e M i o c e n e a n d e s i t es t h a t e l s e w h e r e in t h ed i s tr i ct h o s t th e m a i n p o r p h y r y c o p p e r - g o l d d e p o s i ts . A l t e r at i o n c o m p r i s e s a q u a r t z - a l u n i t e -p y r i t e c o r e , w i t h l o c a l i z e d v u g g y t e x t u r e s , t h a t p a s s e s p r o g r e s s i v e l y o u t w a r d s t h r o u g hq u a r t z - k a o l i n i t e t o c h l o r i ti c a l t e r a ti o n .

M i n e r a l i z a t i o n o c c u r s m a i n l y w i t h i n t h e q u a r t z - a l u n i t e z o n e i n m u l t i p h a s e e n a r g i t e -l u z o n i t e - p y r i t e c e m e n t e d b r e c c i a s a n d v e in s , s o m e o f w h i c h a r e s u r r o u n d e d b y s t ri n g e rz o n e s ( C a r l i l e e t a l. , 1 9 90 ; P e r e l l o , 1 9 9 4 ) . V u g g y s i li c a bo d i e s c o n t a i n o n l y m i n o r m i n e r -a l i z a t i o n w h e r e e n a r g i t e - l u z o n i t e i s i n t e r g r o w n w i t h b l a d e d a l u n i t e a n d i n f i l l s v u g s . An o t a b l e f e a t u r e i s th e o c c u r r e n c e o f o x i d i z e d b r e c c i a z o n e s i n t e r s e c t e d i n d ri ll c o r e a t d e p t h so f o v e r 1 5 0 m e t r e s ( C a r l i l e e t a l. , 1 9 9 0 ) . T h e s e a r e t h o u g h t t o be s l u m p e d b l o c k s o f s u r f a c eo x i d a t i o n t h a t d e v e l o p e d i n t h e P l e i s t o c e n e ( P e r e l l o , 1 9 9 4 ) .

A t t h e r e c e n tl y d i s c o v e r e d M i w a h p r o s p e c t in t h e A c e h c o n t in e n t a l a r c ( A . W i l li a m s o n ,p e r s . c o m m u n . , 1 9 9 2 ) , i n t e r be d d e d P l i o c e n e s e d i m e n t s a n d a n d e s i t i c v o l c a n i c s c o n t a i nd i s s e m i n a t e d p y r i t e , e n a r g i t e , n a t i v e s u l p h u r a n d ba r i t e w i t h i n a n e x t e n s i v e a l t e r a t io n s y s t e m .S p e c t a c u l a r c l i f f s o f s i l i c a p a s s l a t e r a l l y t o s i l i c a - p y r i t e , s i l i c a - c l a y - p y r i t e a n d c h l o r i t e -e p i d o t e a s s e m b l a g e s . G o l d - b e a t i n g q u a r t z v e i n s a n d b re c c i a s a t P e l a n g a n o n L o m b o k c o n -t a i n a l u n i t e , p y r i t e a n d e n a r g i t e , w i t h i n z o n e s o f s i l i c a - k a o l i n i t e - a l u n i t e a l t e r a t i o n ( S .T u r n e r , p er s . c o m m u n . , 1 9 9 2 ) .Advanced argillic zones and silica bodies

I n I n d o n e s i a , m a j o r a d v a n c e d a r g i l l i c z o n e s d o m i n a t e d by s i l i c a , bu t w i t h o u t c o n t a i n e de c o n o m i c m i n e r a l i z a t i o n , o c c u r bo t h i n c o n t i n e n t a l a n d i s l a n d a r c s e t t i n g s a n d a r e a l w a y sh o s t e d i n a n d e s i t i c - d a c i t i c v o l c a n i c r o c k s . A t Ma s u p a R i a i n t h e C e n t r a l K a l i m a n t a n a r c ,s i l ic i f ic a t i o n c o v e r i n g a n a r e a o f s e v e r a l s q u a r e k i l o m e t r e s is u p t o 2 0 0 m e t r e s t h i c k , a n d i sc u t b y t h e lo w s u l p h id a t i o n O n g k a n g v e i n ( T h o m p s o n e t a l. , 1 9 9 4 ) . T h e b l a n k e t - li k e b o d yi s c o m p o s e d d o m i n a n t l y o f s il ic a b u t a l s o c o n t a in s a l u n it e a n d p y r o p h y l l it e . I n t h e B a g u i od i s t r i c t o f the Ph i l ipp ines , a s im i la r b lan ke t o f s i l i c i f ica t ion and a lun i t e a l t e ra t ion , a t t r ibu ted


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124 J. C. C arlile, A.H .G. Mitchell/Journal of Geochemical Exploration 50 1994) 91-142t o h ig h - su lp h id a t io n ac t iv i ty (A o k i e t a l. , 1 9 9 3 ) i s a l so cu t by lo w -su lp h id a t io n v e in s w i thg o l d m i n e r a li z a t io n . C o m p a r a b l e b l a n k e t - li k e f e a tu r e s a t B u a t a a n d B a y a d e i n n o r t h S u l a w -e s i, a n d o n e a s t e rn S u m b a w a a t D o n g g o m a s s a a r e o f u n k n o w n t h i c k n e ss b u t a l so c o v e rex ten s iv e su r face a reas . P a tch y g o ld an d a r sen ic an o m al i es , lo ca l ly sp ec tacu la r b recc ia t io nf e a t u r e s a n d r e m n a n t d i s s e m i n a t e d p y r i t e a r e c o m m o n . W h e t h e r o r n o t t h e s e a d v a n c e darg i l l i c r e s id u a l s i l i ca bo d ies a re u n d er l a in by re l a t ed min era l i zed sy s t ems remain s to betes ted .In n o r th S u law es i , a s i li ca b l an k e t u p to 1 5 0 me t res th i ck o v er l ay s p a r t o f th e S u n g a i M akp o r p h y r y c o p p e r - g o l d d e p o s i t, a n d c o m p r i s e s m a s s i ve , d e n s e a n d p a r t ly b r e c c i a t e d s i li ca .T h e l a rg e s t s u p e r g e n e c o p p e r b l a n k e t i n I n d o n e s i a o c c u r s b e n e a t h t h i s l e a c h e d s i l ic a b o d y .G e o l o g ic a l e n v i r o n m e n t o f o r m a t i o n

L a t e r a l l y e x t e n s i v e b l a n k e t s o f a d v a n c e d a r g i l l i c a l t e r a t i o n m a y b e f o r m e d b y r o c kreac t io n w i th co o l ac id i c f lu id s p ro d u ce d by o x id a t io n o f H2 S to su lp h a te in th e v ad o se zo n e(H en ley an d E l l i s , 1 9 8 3 ) , w h ere th e H2 S ev o lv ed o f f a bo i lin g , c ir cu la t in g , n ea r -n eu t ra l p Hlo w -su lp h id a t io n h y d ro th e rmal sy s t em a t d ep th . Z o n a t io n in a l t e ra t io n min era lo g y in th es i l ica b l an k e t a t M asu p a R ia (T h o m p so n e t a l. , 1 9 9 4 ) in d ica t es a co o l su r f ic i a l ac id su lp h a teso u rce fo r t h e s i l i c i f ica t io n abo v e th e O n g k an g v e in .

A d v an ced a rg i l l i c a l t e ra t io n assemblag es asso c ia t ed w i th h ig h - su lp h id a t io n min era ld ep o s i t s a re a t t r ibu ted to ro ck reac t io n w i th h o t ac id f lu id s w h ich w ere p ro d u ced by th ed i s p r o p o r ti o n a t i o n o f m a g m a t i c S O 2 a n d H C 1 g a s e v o l v i n g d i r e c t l y o f f a d e g a s s i n g s t o ck a td ep th ( B e th k e , 1 9 84 ; G ig g en bach , 1 9 9 2 ) . C o n d e n sa t io n o f t h e g as in to g ro u n d w ate r re su t t sin ex t rem ely lo w p H f lu id s , an d p ro bab ly t ak es p l ace in p e rme ab le s t ru c tu res o r l i t h o lo g iesin th e h o s t ro ck s . R eac t io n o f t h e p ro g ress iv e ly co o l in g an d n eu t ra l i s in g h o t ac id f lu id( L e a c h a n d E r c e g , 1 9 9 0 ) w i t h t h e v o l c a n i c r o c k s f o r m s t he a d v a n c e d a r g il li c a s s em b l a g e sa lo n g w i th m in era l s su ch as p y ro p h y l l i t e an d d iasp o re in d ica t in g t em p era tu res in ex cess o f2 0 0 C (R ey e s , 1 9 9 0 ) ; t h e s i l i ca ro c k is a co mb in a t io n o f r e s id u a l s i l ica , fo rm ed by ex ten s iv eleach in g , a n d l a t e r s i l ica ad d i t io n .

Hig h su lp h id a t io n min era l i za t io n a t M o to m bo to , M iw ah an d P e lan g an i s a s so c ia t ed w i thex ten s iv e ad v an ce d a rg i ll i c a lt e ra t io n . M o to m bo to i s h o s t ed by v o lcan ic ro ck s th a t e l sew h e rein th e d i st r i c t li e s t ra t ig rap h ica l ly abo v e min e ra l i zed p o rp h y r i es . T h e re fo re in co m m o n w i thm an y en a rg i t e -g o ld d ep o s i t s e l sew h ere (A sh ley , 1 9 82 , S i l l it o e , 1 9 8 3 ) , i t can be a rg u ed th a tM o t o m b o t o , a n d p o s s i b ly M i w a h a n d P e l a n g a n , m a y b e u n d e rl a i n b y p o r p h y r y c o p p e r -g o l d s y s te m s ( F i g . 1 5 A ) .5 .5 . G o l d - s i l v e r - b a r i t e +_ b a s e m e t a l m i n e r a l i z a t i o n

D i s t r i b u ti o n a n d c h a r a c t e r i s t i c sG o l d - s i l v e r - b a r i t e + b a s e m e t a l m i n e r a l i z a t io n o c c u r s i n t h e S u n d a - B a n d a a r c e a s t w a r d s

f r o m F l o r e s , a n d a t B i n e b a s e o n S a n g i h e i n t h e S u l a w e s i - E a s t M i n d a n a o i s l a n d a r c . As i m i l a r a s s o c i a t i o n i s k n o w n f r o m S a r a n g a n i I s l a n d , 2 0 0 k m s n o r t h o f S a n g i h e a n d a tA n a g a s e a n d B a g a c a y ( e . g ., B r y n e r , 1 9 6 9 ) o n S a m a r i n t h e P h i l ip p i n e s .S t a c k e d s t r a ti f o r m b a r i t e - ja r o s i t e h o r i z o n s c o n t a i n in g g o l d a n d s i lv e r , s o m e t i m e s u n d e r -l a i n b y c o p p e r - b e a r i n g m a s s i v e p y r i t i c b o d i e s a n d p y r i t e s t o c k w o r k s , f o r m a c l u s t e r o f


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J.C. Carlile, A.H.G. Mitch ell/Jo urn al of Geochem ical E~ploration 50 1994) 91 142 125d e p o s i t s o n W e t a r at L e r o k i s , K a l i K u n i n g ( S e w e l l a n d W h e a t l e y , 1 9 9 4 ) a n d M e r o n ( C . J . V .W h e a t l e y , p e r s . c o m m u n . , 1 9 93 ) . T h e f e r r u g i n o u s b a r it e l a y e rs , t y p i c a l l y 3 - 3 5 m e t r e s t h i c ka n d u p t o 4 0 0 m e t r e s i n d i a m e te r , a r e i n t e r c a l a t e d w i th f i n e g r a in e d c l a s t i c s e d im e n t s a n dc h e r t s a n d p a rt l y c o v e r e d b y li m e s t o n e s a n d m a r i n e s e d i m e n t s . A s e q u e n c e o f a n d e s i t i c -d a c i t ic v o l c a n i c l as t i c s , u n d e r l ai n b y p i l l o w b a s a l t, h o s t s t h e s t o c k w o r k z o n e s a n d i n c l u d e st e l s i c d o m e r o c k s n e a r t h e o r e b o d i e s . A l t e r a t i o n i n t h e v o l c a n i c l a s t i c r o c k s a d j a c e n t t o t h es t o c k w o r k s c o m p r i s e s s i l i c a - p y r i t e - c l a y - a l u n i t e - b a r i t e p a s s i n g d i s t a l l y t o i l l i t e - c h l o r i t ep y r i t e . S i l v e r a n d l o c a l l y h ig h l e a d , a r s e n i c a n d a n t im o n y i n t h e b a r i t i c h o r i z o n s a r e a s s o -c i a t e d w i th s u lp h o s a l t s . E n a r g i t e i s a s i g n i f i c a n t c o n s t i t u e n t o f t h e m a s s iv e p y r i t i c l e n s es .

A s i m i l a r b u t l o c a l i z e d g o l d - s i l v e r - b e a r i n g s a n d y b a r i te u n i t a t B i n e b a s e is a ls o u n d e r l a i nb y a s e q u e n c e o f a n d e s i t i c - d a c i t i c v o l c a n i c l a s t i c r o c k s a n d a n d e s i ti c p o r p h y r i e s . T h e m a i ng o l d - s i l v e ~ c o p p e r m i n e r a l i z a t i o n o c c u r s i n c r y s t a l t u f t s a s m a s s i v e s i l i c a - p y r i t e - b a r -it e_ + c h a l c o p y r i t e l e n s e s a n d b r e c c i a s , a d j a c e n t t o a n a n d e s i t e p o r p h y r y . P e r v a s iv e s i l i c ap y r i t e - c l a y - a lu n i t e - b a r i t e a l t e r a t i o n c u t b y l a t e b a r i t e v e in in g p a s s e s l a t e r a l l y t o i l l i t e -c h l o r i t e - p y r i t e . E n a r g i t e f lo a t h a s a l s o b e e n f o u n d a t t h e p r o s p e c t b y o n e o f t h e a u t h o r s( J . C . C . ) .

G e o l o g i c a l e n v i r o n m e n t o J l f o r m a t i o nM i n e r a l i z a t i o n o n S a n g i h e a n d W e t a r h a s m a n y s i m i l a r i t i e s , a n d t h e d e p o s i t s d i s p l a y

c h a r a c t e r is t i c s o f b o t h h i g h s u l p h i d a t io n m i n e r a l i z a t i o n a n d e x h a l a t i v e m a s s i v e s u l p h i d es y s t e m s . T h e y a p p e a r t o b e c o - e v a l w i t h a c i d s u l p h a te a l t e ra t i o n i n d i c a t e d b y t h e p r e s e n c eo f a lu n i te , a n d c o n t a i n e n a r g i t e w h i c h i s a c o m m o n c o n s t i t u e n t o f h i g h s u l p h i d a t i o n g o l dd e p o s i t s e l s e w h e r e ( A s h l e y , 1 9 8 2 , S i l li t o e , 1 9 8 3 ) . T h e h ig h b a ri t e c o n t e n t im p l i e s b a r i u m

0 m (Sea Level)

250

7 5 0 m

Massive Pyri te wi thBase M eta l SulphidesAu Ag in Massive and SulphosaltsBarite Barite Sand / Gold-Silver-Base Metal

X / Ba ry te Ve i ns and S t oc k w ork sX / / ~ e. . . . - - / - - - . ~ ~ V o l c a no c l as t ic s. . . . . . . V7 ~ - - v (N e o g e n e i - -

v ~ ,==~ __ Re p lacem ent Au =AgBase Meta ls Ba O r e^ A A A I ^ 1 ^ ^ A ^ P i l l o w B a s a l t s A A^ m A ^ ^ ^ A ^ ^ (Pa laeogene) A ^h

Fig. 13. Schem atic cross-section illustrating the possible depositional setting for "high sulph idation - subm arinegold-silver-barite-base m etal mineralization in the eastern S unda-Ba nda and Sulawesi-East M indanao islanda r c s .


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126 J .C . Ca r l i l e , A .H .G . Mi t ch e l l / Jo u r n a l o f G eo ch em ica l Exp lo ra t i rm 5 0 1 9 9 4 ) 9 1 1 4 2

in the min era l iz ing f lu id , wi th SO 4 poss ibly from e i ther sea water or ox idize d meteor ic f lu id .The s trong base meta l a ssoc ia t io n and high s i lver to go ld ra t ios are cons is tent wi th chlor idec o m pl e x i n g o f m e t a l s ( H e m l e y a nd Hu nt , 1 9 9 2 ) . T he s t ra t if o rm be dde d na t u r e o f t he ba r it icho r i z o ns s u g g e s t s s ha l l o w s u bm a r i ne v e nt i ng o f t he f l u i ds i n l o c a l i z e d t r o u g hs a t de pt hss u f f ic i e n t t o pre v e nt r e wo r k i ng by wa v e a c t io n . T he m a s s i v e s u l ph i de bo d i e s a nd s t o c k wo r k sprobably form ed a t and be lo w the bedrock sur face ( F ig . 13 ) .

T he m i ne r a l i z i ng f l u i ds m a y ha v e be e n s i m i l a r i n c he m i c a l c o m po s i t i o n t o t ho s e wh i c hf o r m e d a d v a nc e d a r g il li c s i l i c a bo d i e s w i t h hy dr o t he r m a l br e c c ia s a nd m i no r g o l d m i ne r -a l iza t ion wi thin sub-aer ia l andes i te s in eas tern Sumbawa , and a l so poss ib ly to those a sso -c i a te d wi t h t he B a tu H i j au po rphy r y c o pp e r - g o l d de po s i t i n we s te r n Su m ba w a .

5 6 Lo w sulphidat ion epi thermal mineral i za t ion

Dis t r ibut ion an d character i s t icsMaior low sulphidation epithermal veins and lesser stockwork deposits are largely con-

fined to the western Sunda-Banda and the Central Kalimantan continental arcs. Elsewhere,deposits of this type are small and mainly restricted to the North Sulawesi and the easternSunda-Banda island arcs.

Vein and stockwork deposits are hosted mainly by andesitic volcanic rocks and lesscommonly by intrusive equivalents of mid-Miocene to Pliocene age in the Sunda-Bandaarc (Fig. 14), and by Palaeogene to Neogene intermediate intrusions and andesitic to felsic

1 4 0 0 ~1 P o s t - M i n e r a l G r a b e n1 2 g O - t / V E I N1 1 0 0 v v1 0 0 0 . _ v V v ~v v v v l A n d e s i t i c

- v vv v [ V o l c ,a n i c s800 - v v v v v v v v

600

4 0 0 - ~3 0 0 - ~ M a r i e O r T e c t o n i s e d200 B a s e m e n t1 0 0 -

0 -1 0 0 -2 0 0 -

4 0 0 -5 0 0 -

C o n g l o m e r a t e s/ C I K O N D A N G o r L a h e r si V E I NI R O T A N ~ / ~ o o' ~V E I N r/oo ~ % ~ ~

v v v v~ V / A : ~ i t i c v v v ,. . . . . . iq l n a e s . c I . . . . . v / V o l c a n i c s vv v v v v v v v v v vv v v v v v v v v v lv v v ~L

x x x x X x X Ox__ X x x , I ~ qer l ,ments~ ,L E B O N G D O N O K G a b b r o , M a f ic D y k ex x x x x ; , ~ x x X l

V E I N B a s a l t B a s e m e n t A n d e s i t icD y k e

v v v v vv v v v v

v v v v v vv v v v v

v v v v v vv v v v v

. . / / / / / /F o l d e dM u d s t o n e s

Fig. 14. Schematic cross-section illustrating the geological setting of selected gold-bearing low sulphidationepithermal veins in the western continental segment of the Sunda-Banda arc.


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J.C. Carlile, A.H.G. Mitchell / Journal of Geochemical Exploration 50 1994) 91-142 127

v o l c a n i c r o c k s i n t h e C e n t r a l K a l i m a n t a n a r c ( T a b l e 3 ) . I n t e r c a l a t e d s e d i m e n t a r y r o c k so c c u r n o t f a r a b o v e b a s e m e n t i n S u m a t r a a n d W e s t J a v a ( F i g . 1 4 ). T h e l o w e r p a rt o f t h eC i r o t a n v e i n i n t h e C i k o t o k d i s t r ic t is h o s t e d by g a bb r o i c ba s e m e n t ( F i g . 1 4 ) a n d v e i n s i nNo r t h S u l a w e s i o c c u r in s e d i m e n t a r y r o c k s a n d ba s a l t i c a n d e s i t e s ( K a v a l i e r i s e t a l., 1 9 9 2 ) .T h e o n l y t w o m a j o r e x a m p l e s o f d i s s e m i n a t e d m i n e r a l i z a t i o n , a t K e l i a n i n C e n t r a l K a l i -m a n t a n a n d G u n u n g P a n i i n No r t h S u l a w e s i , a r e bo t h s p a t i a l l y a s s o c i a t e d w i t h r h y o l i t i cp y r o c l a s t i c a n d / o r e p i c l a s t i c r o c k s .

A t K e l i a n g o l d m i n e r a l i z a t i o n i s a s s o c i a t e d w i t h t h e d e p o s i t i o n o f c a r bo n a t e a n d ba s em e t a l s i n a d i s s e m i n a t e d v e i n n e t w o r k , w h i c h p o s t - d a t e d , a n d i n p l a c e s b r e c c i a t e d , p o o r l ym i n e r a l i z e d q u a r t z - a d u l a r i a - s e r i c i t e v e i n i n g . O r e g r a d e m i n e r a li z a ti o n f o r m e d i n p e r m e a b l ez o n e s a s s o c i a t e d w i t h f a u l t m o v e m e n t , a n d a r o u n d t h e c o n t a c t s w i t h a n d e s i t e s t o c k s . C a r -bo n a t e s p e c i e s a t K e l i a n a r e z o n e d w i t h d e p t h ; s i d e r i t e - r h o d o c h r o s i t e w i t h k a o l i n i t e o c c u ra t s h a l lo w l e v e l s, c a l c it e a n d M g - c a r b o n a t e s a t d e p th , a n d m i x e d F e - M n - C a - M g c a r b o n a t e si n t h e m i n e r a l i z e d i n t e r m e d i a t e z o n e s ( V a n L e e u w e n e t al., 1 9 9 0 ) , i n d i c a t i n g f lu i d m i x i n ga s t h e p r o ba b l e m e c h a n i s m f o r m i n e r a l i z a t i o n .

T h e v e i n - t y p e d e p o s it s i n I n d o n e s i a m a y b e g r o u p e d i nt o a n u m b e r o f b r o a d c a t e g o r i es .Q u a r t z - a d u l a r i a - c a l c i t e v e i n s a t G u n u n g P o n g k o r an_d L e b o n g D o n o k h a v e v e r y l o w s u l-p h i d e a n d ba s e m e t a l c o n t e n t s . V e i n s a n d s t o c k w o r k s a t B o l a n g i t a n g , L a n u t , a n d Ma s u p aR i a ( a p a r t f r o m t h e a b s e n c e o f a d u l a r i a ) , s h o w b r o a d l y s i m i l a r c h a r a c te r i s ti c s . T h e Mt .Mu r o v e i n s w h i c h h a v e a h i g h e r ba s e m e t a l a n d c a r bo n a t e c o n t e n t a t d e p t h , m a y be m o r ed e e p l y e r o d e d e q u i v a l e n t s . Qu a r t z r h o d o n i t e - r h o d o c h r o s i t e v e i n s , f o r e x a m p l e a t Ma n g a n i .C i r o t a n , C i k o t o k , L e ba k S e m ba d a , a n d p o s s i b l y S a l i d a a n d Do u p , g e n e r a l l y h a v e a h i g hs u l p h i d e a n d ba s e m e t a l c o n t e n t. T h e s e v e i n s d i s p l a y c o m p l e x m i n e r a l o g y , o f t e n i n c lu d i n gt e l l u r i d e s a n d s e l e n i d e s a n d , i n t h e c a s e o f Ma n g a n i a n d C i r o t a n , m i n o r t i n m i n e r a l s .C a r bo n a t e a n d ba s e m e t a l c o n t e n t s c o m m o n l y i n c r e a s e w i t h d e p t h i n t h e s e v e i n s a n d ba s em e t a l - r i c h s h o o t s w i t h i n t h e v e i n s a r e u s u a l ly a s s o c i a t e d w i t h m u l t i p le p h a s e s o f b r e c c i a ti o na n d r e s e a l in g . Qu a r t z v e i n s w i t h s u lp h i d e a n d ba s e m e t a l - r i c h z o n e s a n d s o m e t i m e s c o m p l e xm i n e r a l o g y , b u t l a c k in g m a j o r a m o u n t s o f r h o d o c h r o s it e a n d r h o d o n it e o c c u r a t C i k o n d a n g ,L e b o n g T a n d a i , a n d t h r o u g h o u t t h e C i k o t o k a n d J a m p a n g d i st ri ct s.

G o l d o c c u r s m o s t c o m m o n l y a s e l e c tr u m , s o m e t i m e s a s f r e e g o ld m a i n l y in o x i d e z o n e s ,a n d m a y a l s o be l o c k e d i n p y r i t e o r a r s e n o p y r i t e , a n d i s o c c a s i o n a l l y a s s o c i a t e d w i t ht e l lu r i d e s , s e l e n i d e s a n d s u l p h o s a l ts . S i l v e r t o g o l d r a t i o s a r e h i g h l y v a r i a b l e a n d m a y bez o n e d v e r t i c a l ly a n d l a t e r a l ly w i t h i n o n e d e p o s i t . H i g h r a t i o s in e x c e s s o f 2 5 : 1 a r e c o m m o n l yf o u n d in q u a r t z - r h o d o n i t e - r h o d o c h r o s i t e v e i n s a n d v a l u e s o f a r o u n d 1 0 :1 t y p i f y d e p o s i tss u c h a s G u n u n g P o n g k o r . R a t i o s o f m o r e t h a n 2 0 : 1 a t Mr. M u r o c o i n c i d e w i t h t h e ba s e o fm i n e r a l i z a t i o n .

W i t h t h e e x c e p ti o n o f K e l ia n , w h e r e m i n e r a l iz a t i o n f o r m e d a t 3 0 0 - 3 3 0 C ( V a n L e e u w e ne t a l. , 1 9 9 0 ) , f l u id i n c l u s io n h o m o g e n i s a t i o n t e m p e r a t u r e s o f be t w e e n 1 9 0 a n d 2 9 0 C a r er e p o r t ed a t M t . M u r o ( S i m m o n s a n d B r o w n e , 1 9 9 0 ) , M a s u p a R i a ( T h o m p s o n e t a l., 1 9 9 4 ) ,G u n u n g P o n g k o r ( B a s u k i e t al., 1 9 9 4 ) , a n d C i r o t a n ( K R T A , 1 9 8 8 a , b ) , b u t m o s t d a tai n d i c a t e t e m p e r a t u r e s o f l e s s th a n 2 5 0 C .

W a l l r o c k a l t e r a t i o n t y p i c a l l y c o m p r i s e s a n i n n e r q u a r tz - i ll i te _+ a d u l a r i a - p y r i t e e n v e l o p e ,s o m e t i m e s o v e r p r i n t e d b y k a o l in i te a n d t r an s i ti o n a l o u t w a r d s o v e r a f e w m e t r e s o r t e ns o fm et res to ch lo r i t e _+ i l li t e _+ ca rb ona te + ep ido te _+ pyr i t e . A t M asu pa Ria (T ho m ps on e t a l . ,1 9 9 4 ) , a d v a n c e d a r g i l l i c a l t e r a t i o n f o r m s s p e c t a c u l a r s i l i c a b l u f f s w h i c h o v e r l i e a n d p r e -


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J . C . Ca r l i l e , A .H . G . Mi t ch e l l / Jo u rn a l o f G e o ch e m ica l Exp lo r a t io n 5 0 1 9 9 4 ) 9 1 1 4 2 1 31

date the Ongkang vein. Pyrophyllite is also recorded in a low sulphidation vein at Kaidundu(Perello, 1994) implying the presence of acid sulphate as well as neutral pH chloride fluids,perhaps due to a late collapse of steam-heated acid sulphate fluids, as observed in activegeothermal systems (Reyes, 1990).Typical strike lengths of individual veins range from a few hundred metres to over 4,500metres at Lebong Tandai, with widths commonly between one and five metres but swellingin places to more than 30 metres. Veins at Lebong Tandai extend vertically for over 650metres. In Central Kalimantan and on the Bayah Dome in West Java, veins occur verticallyfor between 100 and 250 metres, reaching a maximum of 350 metres in the carbonate-richdeposit at Cirotan on the Bayah Dome.

eological environment of ormationVein deposits in the western Sunda-Banda and Central Kalimantan arcs are hosted mainlyin eruptive rocks that pass downwards into sedimentary rocks and pre-volcanic basement,

supporting observations in the Philippines (Mitchell and Leach, 1991 ) and Japan (Izawaet al., 1990) that epithermal veins occug preferentially immediately above and occasionallyin the basement (Figs. 14 and 15B). In the western Sunda-Banda arc, a relationship tobasement is most evident on the Bayah Dome where the surface of the mafic basement andthe elevations of veins varies from around 300 metres above sea level on the flanks of thedome to over 700 metres on the crest. Vertical extents of veins on the crest of the BayahDome are not notably less than those of veins at the flanks.

These relationships imply that the concentration of low sulphidation epithermal veins inthe Bayah Dome may be analogous to the concentration of veins in the Paracale Dome andthe Baguio basement antiform, the two most productive gold districts in the Philippines( Mitchell and Leach, 1991 ). In the western Sunda-Banda arc, the association of epithermalveins with basement domes coincides with a scarcity of exposed intrusive stocks and theabsence of porphyry copper-gold deposits.

We suggest that the epithermal mineralization coincided with initiation of extensionaltectonics accommodated by movement on low-angle detachment faults at depth ( Fig. 15B ).Onset of extensional strain in the thermally weakened arc, a response to gravitational pullof the sinking slab, may have permitted descent of meteoric fluids along pre-mineral frac-tures and faults to depths of 7 to 10 kms, near the brittle-ductile transition zone (Nesbitt,1988). Rising magma, perhaps cooled by the descending fluids, crystallized as sills at depthsof at least several kilometres and consequent magmatic inflation resulted in elevation of thearc (Fig. 15B). Mineralization took place as the ascending meteoric fluid boiled (e.g.,Henley, 1985) and mixed with cool ground water in fractures within permeable volcanicand volcanoclastic rocks near and above the basement surface.

The abundance of epithermal deposits in Sumatra and western Java may be relatedindirectly to pre-mineralization arc reversal and emplacement of the mafic Woyla Grouponto the Sundaland margin in the Cretaceous. Crustal extension and emplacement of sillsaccompanied doming of the tectonically thickened basement in regional antiforms; thedomes focused ascending fluids beneath low permeability rocks on the antiIbrm flanks.Overall, the inferred association of low sulphidation mineralization with crustal extension.following early andesitic magmatism, contrasts with the setting of porphyry copper-gold


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132 J.C. Carlile. A.H.G. Mitchell~Journal qf Geochemical Exploratirm 50 1994) 91-142

High Sulphidation H igh Su lphida t ionA l t e r a t i o n A u - A g - C u

lkm ~ ~ IAndesitic/Dacitic . ~ ~ - ~ :~ < :~ 1 VolcanoI ,..

0 V e i n s v ~ ~ / v ~ /~ Andes tic vv i v i i v ( ~ I 1 ~ Volcanics v~ ~ 1 1 1 | + S.,edimen~s v_, ~ : . ~ B.~,^ " j

I rphyry2 " I ' C u - A uShallowPost-VolcanoStock3

- 4

A

Low Su lph ida tionVe ins and Stockwor ksSedim ent Hosted M ost Ore in Upp er LevelsA u AndesiticV V Vv / \ ~ l . . / v ~ : v ' :v, V/~u~ ---~

~ ~ ~ j ~ . ~ e r v~ , . ~ . ~ ~ ~ - ]]~.- Sedimentsand~ ~ ~/? r~ l% c an~s/ ~ : ~ L _ _ ~ C C ~ f \/ ' ~ . ~ i - ~ F ' - P ~ / . M a,cNappe? T e n s , o n V ? ~ I ~ ~ ~ c o 0 . w o y , a )Fractures~ ) ~ / ~ ~ Ex tension

I - ~ ~ ~ F ' ' p a l e z i - M e s z i c2 \ / ~ h ~ ' - ~ . . ~ / IV ~ ta m orp hic s1 / ~ I I W ater

Metam orphic Metamorphism ~..4 Water

BFig. 15. Schematic cross-sectioncomparing he inferred geological settings of (A) porphyry copper-gold andhigh sulphidationmineralization n an islandarc setting based on the Tombulilatodistrict in North Sulawesi, and( B ) low sulphidationepithermal mineralization n a continentalarc based on the Bayah Dome in West Java.deposits (Fig. 15). It suggests that the epithermal minera lization is younger than, and notgeneticall y related to, any porphyry-type minera lizati onor intr usion in the same arc segment.

5.7. Sediment-hosted mineralization

Distribution and characteristicsThe only major deposit of this type in Indonesia occurs at Mesel (Tur ner et al., 1994) in

the North Sulawesi island arc, although mi nor amounts of gold have recently been recoveredfrom small occurrences in the Cikotok district (Carlile and Sitorus, unpublished results).Sediment -hosted mineraliz ation in the Bau district of Sarawak ( Sillitoe and Bonham, 1990)may be related to the projected westward extension of the Central K alima ntan arc.

Mineral ization at Mesel (Tur ner et al, 1994) is hosted in a faulted sequence of LateMiocene carbonate sediments beneath the contact with an overlying pre-mineralizationandesite sill. In unoxidized parts of the deposit, gold occurs mainly with fine-grained


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J.C. Carlile, A.H.G. Mitchell~Journal of Geochemical Exploration 50 1994) 9t-142 133

a r s e n i c a l p y r i t e in d e c a l c i f i e d a n d d o l o m i t i s e d s e d i m e n t s t h a t a r e p e r v a s i v e l y s i li c if i e d a n dc a r r y v e r y l o w b a s e m e t a l v a l u e s a n d o n l y m i n o r s i lv e r .

A l t h o u g h o n l y a m i n o r o c c u r r e n c e , L a n g k o b i n t h e C i k o t o k d i s t r i c t h a s s o m e b r o a ds i m i l a r i t i e s t o Me s e l . Mi n e r a l i z a t i o n i s h o s t e d i n m a r l s a n d c a r bo n a c e o u s s h a l e s , p r o ba b l yo f L a t e Mi o c e n e a g e , t h a t a r e i n t e r be d d e d w i t h a n d e s i t i c v o l c a n i c r o c k s . G o l d o c c u r s i ns i li c if i e d u n i t s a n d i s a s s o c i a t e d w i t h l o w b a s e m e t a l a n d s i l v e r c o n t e n t s a n d e l e v a t e d a r s e n i ca n d a n t i m o n y c o n c e n t ra t io n s .

L o w s u l p h i d a t i o n e p i t h e r m a l v e i n s o c c u r i n bo t h t h e R a t a t o t o k a n d C i k o t o k d i s t r i c t s i ns e d i m e n t a r y a n d v o l c a n i c r o c k s o f s i m i l a r a g e t o t h o s e t h a t c o n t a i n t h e s e d i m e n t - h o s t e dm i n e r a l i z a t i o n . R e s i d u a l p a l a e o - k a r s t b r e c c i a s i n t h e R a t a t o t o k d i s t ri c t a l s o c o n t a i n s i g n i f -i c a n t g o l d ( T u r n e r e t al. , 1 9 9 4 ) .Environment of ormation

M i n e r a l i z a t i o n f o r m s w h e r e r e c e p t i v e h o s t - r o c k s a r e in t e r s e c t e d by f a u l ts th a t h a v e c h a n -n e l l e d g o l d - be a r i n g h y d r o t h e r m a i fl u id s . T u r n e r e t a l. ( 1 9 9 4 ) s u g g e s t t h a t v o l u m e l o s s d u et o d e c a l c i fi c a t i o n a n d d o l o m i t i s a t i o n h a s e n h a n c e d p o r o s i t y a t Me s e l , f a c i l it a t i n g a c c e s s o fm i n e r a l i z i n g f l u i d s . T h e o v e r l y i n g a n d e s i t e a t Me s e l i s w e a k l y m i n e r a l i z e d a n d m a y h a v ea c t e d a s a n i m p e r m e a b l e ba r r ie r t o h y d r o t h e r m a l f lu i d s, a l t h o u g h a d j a c e n t t o L a n g k o b , t h ei n t e r be d d e d a n d e s i t i c r o c k s h o s t v e i n m i n e r a l i z a t i o n .

T h e s p a t ia l a s s o c i a t i o n w i t h l o w s u l p h i d a t i o n v e i n s s u g g e s t s a p o s s i b l e g e n e t i c r e l a t io n -s h i p w h e r e s i m i l a r s t r u c t u r a l l y f o c u s e d m i n e r a l i z i n g f l u i d s m a y e i t h e r p e r v a d e p o r o u sr e a c t i v e h o s t - r o c k s , o r be c o n t a i n e d w i t h i n s t r u c t u r e s in i m p e r m e a b l e ro c k s t o f o r m v e i n s .5.8. Other mineralization s~ les

O t h e r s t y l e s o f m i n e r a l i z a t i o n a r e e i t h e r m i n o r , l a c k s i g n i fi c a n t g o l d , o r a r e n o t d i r e c t lyre la ted to andes i t i c a rcs . These inc lude :Copper-molybdenum and molybdenum porphyries

T a n g s e , t h e o n l y k n o w n p o r p h y r y c o p p e r - m o l y b d e n u m p r o s p e c t in I n d o n e s ia , o c c u r s i nt h e c o n t i n e n t a l A c e h a r c . Mi n e r a l i z a t i o n i s h o s t e d i n Mi d d l e Mi o c e n e q u a r t z d i o r i t e s t h a ti n t ru d e a g a b b r o - g r a n o d i o r i t e p l u to n ; l o w i n i ti a l s t r o n t i u m r a t io s n e v e r t h e l e s s i m p l y m a g m ag e n e r a t i o n w i t h o u t s i g n i f ic a n t c o n t i n e n t a l i n p u t ( V a n L e e u w e n e t a l ., 1 9 8 7 ) . P o r p h y r ym o l y b d e n u m m i n e r a l i z at i o n a t M a l a l a i n th e n e c k o f S u l a w e s i i s a s s o c i at e d w i t h P l i o c e n ei n tr u s io n s w i t h a h i g h c o m p o n e n t o f c r u s ta l m e l t , an d i s p r o b a b l y u n r e l a te d t o a rc m a g m a t i s m( V a n L e e u w e n e t a l. , 1 9 9 4 ) .Veins associated with porphyr) systems

Qu a r t z v e i n s w i t h g o l d a n d b a s e m e t a l s a r e s p a t ia l l y a n d p r o ba b l y g e n e t i c a l l y a s s o c i a t e dw i t h p o r p h y r y m i n e r a l i z a t i o n a t T a w a r e , B u l a g i d u n , T o m bu l i l a t o , a n d B a t u H i j a u , bu t n os i g n i f i c a n t p r o d

Magmatic Arcs and Associated Gold and Copper

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Transcript of Magmatic Arcs and Associated Gold and Copper