dong&zhou(1996)- zoning in carboniferous-permian cracow epithermal vein system (qld)

7/23/2019 Dong&Zhou(1996)- Zoning in Carboniferous-Permian Cracow Epithermal Vein System (QLD)

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Mineral. Depo sita 31,21 0 224 (1996)

MINER L IUM

DEPOSIT

9 Springer-V erlag 1996

Zoning in the Carboni ferous-Lower Permian Cracow

epitherm al ve in system centra l Q ueen sland A ustra l ia

G . Y . D on g T . Z h ou

National K ey Centre in Econo mic Geology , Geology Dep artment, James Coo k U niversity of North Quee nsland, Q4811, Australia

Received: 16 June 1994/A ccepted: 24 Ap ril 1995

Abstract. Fo u r e p i t h e r m a l v e i n d e p o s i ts ( i .e . D a w n , C e n -

t r a l E x te n d e d , R o s e 's P r i d e a n d K l o n d y k e ) i n t h e C r a c o w

g o l d f i el d, c e n t r a l Q u e e n s l a n d w e r e i n v e s t i g a t e d i n te r ms

o f p a r a g e n e s i s, m i n e r a l o g y , v e i n t e x t u r e s , f l u i d i n c l u s i o n s

a n d s t a b l e i s o t o p e s . T h e Cr a c o w e p i t h e r ma l f i e l d i s c o n -

f i n e d t o a n a r e a a p p r o x i m a t e l y 6 b y 5 k i l o me t e r s . A l l t h e

d e p o s it s a r e h o s t e d b y t h e m a s s iv e C a m b o o n A n d e s i te o f

U p p e r C a r b o n i f e r o u s t o L o w e r P e r m i a n a g e , o c c u r a s

open-space ve in f i l l i ngs , and have s imi l a r paragenes i s .

H o w e v e r , s i g n i fi c a n t v a r i a t i o n s i n m i n e r a l o g y , t e x t u r e s o f

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

f o r a m a i n m i n e r a l i s a t i o n s t a g e ( S t a g e I I ) o f e a c h i n d i -

v i d u a l d e p o s i t s . A t Ro s e ' s P r i d e a n d K l o n d y k e , b a s e -

me t a l s u l p h i d e s a r e v i r t u a l l y a b s e n t , b u t s i g n i f i c a n t

a m o u n t s o f c a lc i te a n d q u a r t z w i t h m i n o r a d u l a r i a a r e

w i d e l y d i s t r ib u t e d . R e p l a c e m e n t t e x t u r e s a r e d i s t in c t , a n d

mi n e r a l i s a t i o n t e mp e r a t u r e i s l es s t h a n 2 2 0 ~ a n d s a l i n i t y

l es s t h a n 0 .2 w t % . T h e c ~1 80 v a l u e s o f q u a r t z a n d c a l c it e

r a n g e f r o m - 2 . 6 5 t o - 2 . 0 6 % ~ a n d f r o m - 6 . 6 6 t o

-6 .3 4% o respec t ive ly , an d c a lcu la t ed 61SOH2o va lue i s

about - 17Toow h i c h r e p r e s e n t s a n e a r l y u n s h i f t e d p a l a e o -

me t e o r i c w a t e r . G o l d mi n e r a l i s a t i o n i s b e s t d e v e l o p e d a t

C e n t r a l E x t e n d e d a m o n g t h e s t u d i e d d e p o s i t s , w h e r e

p a t c h e s r i c h i n e l e c t r u m a r e o f t e n o b s e r v e d i n p o l i s h e d

t h i n s e c t i o n s a n d w h e r e g o l d g r a d e s e x c e e d i n g 1 0 g / t a r e

f r e q u e n t l y i n d i c a t e d b y a s s a y s . Ba s e - me t a l s u l p h i d e s a r e

o n l y p r e s e n t l o c a l l y a n d r a r e l y e x c e ed 5 v o l u me p e r c e n t o f

t h e v e in s a mp le s . Q u a r t z i s t h e d o m i n a n t g a n g u e m i n e r a l ,

b u t s i gn i fi c a nt a m o u n t s o f r h o m b i c a d u l a r i a a n d c h l o r it e

a r e w i d e l y d i s t r ib u t e d . V a r i o u s p r i m a r y a n d r e c r y s t a ll i s a -

t ion t ex tu res poss ib ly inher i t ed f rom s i l i ca ge l a re wel l

d e v e l o p e d a n d w i d e s p r e a d . A t i n d i v i d u a l s it es w h e r e c r u s -

t i f o r m b a n d s d e v e l o p e d f r o m b o t h w a l l s o f a f i ss u r e, te m-

p e r a t u r e s c o u l d d r o p s h a r p l y f r o m 2 75 ~ t o le s s t h a n

2 2 0 ~ T h e o r e - f o r mi n g f l u id a t Ce n t r a l E x t e n d e d , c o m-

p a r e d w i t h t h a t a t Ro s e ' s P r i d e a n d K l o n d y k e , w a s i so -

t o p i c a l l y s h i f t e d f r o m me t e o r i c w a t e r w i t h ~s O H ~o v a l u e

o f - 1 3 . 5 % o c a l c u l a t e d i n e q u i l i b r i u m w i t h q u a r t z ( c 5 1 s O

values o f - 3 .09 to - 1 .44%o). The o re bod ie s a t Da w n are

r i c h i n b a s e - me t a l s u l p h i d e s w h i c h a r e c o mmo n l y c o a r s e -

g r a i n e d a n d f o r m u p t o 2 0 v o l u me p e r c e n t o f t h e v e in

m a t e r ia l s. Q u a r t z is th e p r e d o m i n a n t g a n g u e m i n e r a l, a n d

c o m m o n l y s h ow s a c o a r s e c o m b t e x tu r e . T h e o r e - f o r m i n g

fluid was 275 _+ 10 ~ an d lo w sa l ini ty (0 .4 to 0 .7 wt% ).

T h e 6 1s o v a l u e s o f q u a r t z r a n g e f r o m - 3 . 9 7 t o - 3 . 2 2 % o,

and c a lcu la t e d 61SOn2o va lue is ab ou t -12Too, ind ic a t ing

l a r g e i s o t o p i c s h if t s f r o m p a l a e o - m e t e o r i c w a t e r . A d e p t h

z o n i n g i n t y p i c a l b o i l i n g e p i t h e r ma l s y s t ems , c o r r e s p o n d -

i n g t o d i f f e r e n t f l u id c o mp o s i t i o n s , w a l l r o c k p e r me a b i l i t y

a n d b o i l i n g b e h a v i o r s , w a s i n v o k e d t o e x p l a i n d i f f e r e n t

charac te r i s t i cs o f these se l ec t ed ep i thermal ve ins .

T h e C r a c o w g o l d f ie ld i s a p p r o x i m a t e l y 3 50 k m n o r t h w e s t

o f B r is b a n e . Pa y a b l e g o l d w a s d i s c o v e r e d i n 1 9 3 1 . O v e r

a p e r i o d o f 42 y e a r s o f c o n t i n u o u s u n d e r g r o u n d m i n i n g ,

f r o m 1 9 3 3 t o 1 9 7 6 , t h e m i n e p r o d u c e d 1 8 ,5 2 8 k g o f g o l d

an d 20 ,902 kg o f s i lver f rom 1 ,553,000 to nes o f o re g iv ing

an av erage g r ade 11 .9 g /T A u and 13 .5 g /T A g . S ince 1987,

t h e C r a c o w m i n e h a s b e e n u n d e r t a k i n g a n o p e n c u t o p -

e r a t io n . T h i s l e d a d d i t i o n a l p r o d u c t i o n o f a p p r o x i m a t e l y

5860 kg o f go ld .

T h e m a j o r i t y o f w o r k e d g o l d m i n e s a n d p r o s p e ct s a r e

c o n f i n e d t o a n a r e a a p p r o x i ma t e l y 6 b y 5 k i l o me t r e s

(Fig . 1 ). In ge nera l , o rebo d ies o ccu r as ope n-spa ce v e in

f i ll i ngs , wh ic h d ip v er t i ca l ly to sub-v er t i ca l ly and a re ex -

c l u s i v e l y s t r u c t u r a l l y c o n t r o l l e d . U n l i k e t h e ma i n G o l d e n

P l a t e a u l o d e w h i c h w a s d e s c r i b e d a s ma s s i v e q u a r t z b r e c -

c i as ( Ra n s o m a n d K n i g h t 1 9 7 5; W o r s l e y 1 9 9 2) , t h e o u t s i d e

l o d e s , r e p r e s e n t e d b y D a w n , Ce n t r a l E x t e n d e d , Ro s e ' s

P r i d e a n d K l o n d y k e lo d e s , s h o w a w e l l p r e s e r v e d c r u s ti f i-

c a t i o n v e i n t y p e , a n d h a v e a c o mp a r a b l e p a r a g e n e s i s b u t

w i t h s i g n i fi c a n t v a r i a t i o n s i n t e x t u r e s o f q u a r t z a n d a d u l a -

r i a , m i n e r a l o g y a n d f l u i d g e o c h e mi s t r y .

T h i s p a p e r a t t e mp t s t o d e mo n s t r a t e t h e c h a r a c t e r i s t i c s

o f th e s e s e l e c t e d e p i t h e r ma l v e i n d e p o s i t s ( i .e . D a w n , Ce n -

t r a l E x te n d e d , R o s e ' s P r id e a n d K l o n d y k e ) i n th e C r a c o w

g o l d f i e ld , t o r e p o r t f l u id i n c l u s i o n a n d s t a b l e i s o t o p e d a t a ,

a n d t o e x p l o r e t h e p o s s i b l e g e n e t i c e x p l a n a t i o n f o r t h e

d i f f e r e n c e i n mi n e r a l o g y , t e x t u r e s a n d g e o c h e mi s t r y o f

these ve in depos i t s .



211

1 5 0 o 1 7 ' E

t

==

C a r b o n i fe r o u s) ~ \ S t r i k e n d D ' p r ]

Fig. 1. Simplified geological ma p o f the C raco w gold field. (M odi-

f ied f rom Brook s 1974 and Wo rsley 1992)

nalytical techniques

M i c r o p r o b e a n a l y s i s

E l e c t r o n m i c r o p r o b e a n a l ys e s w e r e p e r f o r m e d o n a J e ol -

8 4 0 A in s t r u m e n t a t J a m e s C o o k U n i v e rs i ty . A n a c c e le r a t-

i n g v o l t a g e o f 25 K V f o r s u lp h i d e s , c o u n t i n g t i m e o f 6 0

s e c o n d s a n d a b e a m c u r r e n t o f 1 5 n A w a s u s ed . A Z F

c o r r e c t io n s w e r e c a r r ie d o u t b y T r a c o r N o r t h e r n M i c r o Q

s o f t w a r e .

F l u i d i n c l u s io n m e a s u r e me n t

M i c r o t h e r m o m e t r i c a n a l y s e s o f f l u id i n c l u s io n s w e r e c a r -

r i e d o u t o n a g a s f l o w h e a t i n g / f r e e z i n g s t a g e o f U . S . G . S .

d e s i gn , c o n s t r u c t e d b y t h e U n i v e r s i t y o f A r i z o n a . T h e

s t a g e w a s c a l i b r a t e d u s i n g s y n t h e t i c f l u i d i n c l u s i o n s t a n -

d a r d s f r o m F l u i d I n c . ( U S A ). B y th is m e t h o d , o p t i m u m

p r e c i s i o n a n d a c c u r a c y w e r e e s ti m a t e d t o b e _+ 0 .1 ~ i n

t h e r a n g e 0 . 0 t o - 2 2 . 8 ~ a n d _+ 1 ~ i n t h e r a n g e 0 t o

3 9 0 ~

c a r b o n . R o u t i n e a n a l y t i c a l p r e c i s i o n is • o x y g e n

a n d c a r b o n .

General geology

The C racow m in ing f ie ld i s located wi th in the Up per P alaeozoic

Cam b o o n V o lcan ic Arc o n th e so u t h -eas t e rn m arg in o f t h e Bo wen

Basin . The go ld deposi t s are hosted in the Camboon Andesi te . To

the east i t borders the Torsdale Beds and to the west i s the Back

Creek G rou p. Outliers of the fluviatile Precipice Sa ndsto ne man tle

the higher hil ls in the area (see Fig. 1).

The s t rat ig raphic un i t s in the area are co mpi led by W hi taker et a l .

(1974) on the M undu bbera 1 :250 ,000 sheet . The Torsdale Beds

consist m ainly of felsic crystal-l i thic welded tufts and a Car bonife r-

ous (proba bly Early Carboniferous) age is suggested. The Camboon

Andesite consists d om inan tly of andesit ic to dac it ic welded tuff, with

min or andesit ic and basal t ic f lows, vo lcan ic b reccia and agglomer-

ate . I t un conform ably over lies the Torsdale Beds and d ips tow ards

the southwest at 15~ to 20 ~ T he massive nature o f the bu lk of the

form ation obscures the regional structure, but a total thickness of

abo ut 3000 m is inferred (W hitaker et al. 1974). Fr om fauna l evid-

ence in the M onto sheet area, the C am boo n Andesite i s regarded as

Low er Pe rm ian in age (Dear et a l . 1971). However , par t o f the

Ca m boo n Andesite m ay be o lder because i t has y ielded i so top ic ages

(Ar-Ar) o f 281 Ma and 294 Ma in the Cra cow area (Runneg ar 1979) .

Th e Back Creek Group i s exposed to the west o f Crac ow and forms

the sou theastern pa r t o f the Bow en Basin. I t consis ts m ain ly of

fossil iferous l imestone, m arine m udston e, l i thic san dston e and argil-

l ite . The Back Creek G rou p d isconform ably overl ies the Cam boo n

Andesite. It dips generally west at 15 ~ o 30 ~ and i s o f Ear ly Perm ian

to ear ly Upper Permian age f rom faunal ev idence. The f luv iat i le

Precipice Sandstone of Early Jurassic age, caps the highe r hil ls in the

d ist ric t and uncon form ably overl ies the Cam boo n Andesite and

Back Creek Gro u p .

Numerous rhyol i te dykes in t rude the Camboon andesi te in the

Cra cow district . These dykes have a porph yrit ic texture with flow

banding at the con tact . The spat ial and temporal relat ionsh ips

between the rhyolites and the mineralisation have been studied in

detail by Wo rsley (1992). This has indic ated th at rhyolites w ere

actively being empla ced during the t ime of epithermal activity an d

gold mineralisation, and som e of the dykes uti lized the same stru c-

tural weaknesses (faults) as the lodes.

T h e U -P b z i r co n d a t i n g f ro m th e rh y o l it e d y k e a t Craco w g av e an

age o f 291.1 _+ 5.3 M a (Perkins, unpubl, report., 1992). Stable iso-

tope data (see below) indicate that there i s appare n t ly no in teract ion

between ore-forming f lu id in R ose s Pr ide and Klon dyke an d the

adjace nt B ack Cre ek l imestone, w hich is unlikely if mineralisation

occ urred after the form ation o f Bac k Creek l imestone. It is, there-

fore, suggested that the mineralisation at Crac ow occu rred during

the early Permian , associated with the emp lacem ent of a large felsic

in t rusion underneath Crac ow which was responsib le fo r the fo rma-

t ion of nume rous rhyol i te dykes in the area.

S tab le i so tope ana lys i s

M i n e r a l s a m p l e s o f q u a r t z a n d c a l c i t e w e r e se l e c te d f o r

s t a b l e is o t o p e a n a l y s i s b y h a n d p i c k i n g u n d e r a b i n o c u l a r

m i c r o s c o p e . W h e r e m i n e r al s w e r e i n t e r g ro w n , s e p a r a t i o n

b y h e a v y l iq u i d ( N a P o l y t u n g s t a t e ) w a s u s e d . T h e p u r i t y

o f t h e s e s a m p l e s w a s c h e c k e d b y X - r a y d i ff r a c ti o n . T h e

i s o t o p e a n a l y s e s w e r e p e r f o r m e d b y t h e s t a ff i n t h e s ta b l e

i s o t o p e l a b o r a t o r y o f t h e D i v i s io n o f E x p l o r a t i o n G e o s c i -

e n c e o f C S I R O , A u s t r al ia . M i n e r a l s w e r e a n a l y s e d b y

u s i ng c o n v e n t i o n a l p r e p a r a t i o n t e c h n i qu e s ( M c C r a e ,

1 95 0; C l a y t o n a n d M a y e d a , 1 96 3) o n a M i c r o m a s s 6 2 0 D

m a s s s p e c t r o m e t e r . I s o t o p e d a t a a r e r e p o r t e d i n t h e f a m i l -

i a r 6 n o t a t i o n r e l a ti v e t o S M O W f o r o x y g e n a n d P D B f o r

Mineralogy and paragenesis

M i n e r a l i s a t i o n i n t h e C r a c o w g o l d f ie ld is c o m p l e x a n d

m u l t i s ta g e . A t m o s t l o c a li ti e s, r e p e a t e d f r a c t u r i n g a n d

b r e c c i a ti o n d u r i n g m i n e r a l i s a ti o n p r o d u c e d c o m p l e x v e i n

t e x tu r e s . O n t h e b a s i s o f a c o m b i n a t i o n o f c r o s s c u t t i n g

r e l a t io n s h i p s , c o m p o s i t i o n o f c l a st s a n d c e m e n t i n g m a t e r -

i al s, m i n e r a l o g y a n d v e i n te x t u re s , t h e p a r a g e n e s i s o f e a c h

l o d e h a s b e e n e s t a b l i s h e d . I n g e n e r a l , i n d i v i d u a l l o d e s

h a v e t h e i r o w n c h a r a c t e r i s t i c s i n t e r m s o f v e i n t e x t u r a l

a s s e m b l a g e a n d t h e p r o p o r t i o n o f m i n e r a l c o m p o n e n t s .

T h e h i s t o r y o f m i n e r a l i s a t i o n o n a d i s tr i c t -w i d e s c a le c a n ,

h o w e v e r b e g e n e r a l is e d i n t o f iv e g r o s s p a r a g e n e t i c s t ag e s ,



212

Table 1. Charac teristics of the deposits at Cracow

Stage I I1 IlI V V

D C R (K) D C R K (D) C R (K) D C (R) (K) D C (R)( K)

Mineralogy

Ore

Pyrite r r m m m m r r r r

Sphaleri te C m

Gal ena C m

Chalcopyrite C m

Hessite(Ag2Te) m m 9 9 t t

Altaite (PbTe ) r r

AuA gTellurides t t

Electrum (AuAg) t r t t r r

Gangue

Q ua rtz M M M M M M M M C M M

Adularia r r m C m m m C

Calcite C C C

Sericite r m m

Chlori te r m

Zeolite r

Vein texture Stockwork Crustiform Massive Veinlet

M M

Veinlet

D, Daw n; C, Central Extended; R, Rose s Pride; K, Klondyke; () , Stage is absent o r is not observed; M, M ajor; C, common; m, minor; r , rare;

t, trace

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

a n d a l s o b a s e d o n c r o s s - c h e c k i n g w i t h t h e p a r a g e n e s i s a t

G o l d e n P l a t e a u w h i c h ha s b e e n w e ll d o c u m e n t e d b y W o r -

s l e y ( 19 92 ). T h e m a i n c h a r a c t e r i s t i c f e a t u r e s o f e a c h s t a g e

a t in d i v i d u a l d e p o s i t s a r e s u m m a r i s e d i n T a b l e 1, a n d

s o m e e x a m p l e s o f c r o s s c u t t i n g r e l a t i o n s h i p s a r e i l lu s t r a t e d

in Fig . 2 .

Stage I

r e p r e s e n t s e a r l y p e r i o d s o f s i l ic i f i c a ti o n a n d

q u a r t z v e i n in g . T h i s p h a s e i s c o m p o s e d p r e d o m i n a n t l y o f

q u a r t z w i t h m i n o r a d u l a r i a , p y r i t e a n d c h l o r i t e , b u t w i t h

t h e a b s e n c e o f o t h e r m i n e r a l s . T h e v e i n le t s r a r e ly e x c e e d

a fe w c e n t im e t r e s in w i d t h a n d f o r m a s t o c k w o r k w i t h n o

o b v i o u s p r e f e r r e d o r i e n t a t i o n . T h i s s t a g e i s w i d e s p r e a d i n

t h e d i s tr i c t, b u t i s n o t a s s o c i a t e d w i t h g o l d m i n e r a l i s a t i o n .

Stage

r e p r e se n t s a m a j o r p e r i o d o f b re c c i a t i o n a n d

f r a c t u r in g . T h e m o s t d i a g n o s t i c f e a t u r e o f t h i s s t a g e i s w e l l

d e v e l o p e d c r u s t if o r m ( c o c k ad e ) b a n d i n g , a c c o m p a n i e d b y

n u m e r o u s v a r i e t i e s o f q u a r t z a n d a d u l a r i a t e x t u r e s. M i n -

e r a l o g i c a ll y , t h e o r e b o d i e s o t h e r t h a n m a i n G o l d e n P l a -

t e a u c a n b e d i v i d e d i n t o t w o g r o u p s , T h e f ir s t o n e , i n c lu d -

i n g D a w n a n d C e n t r a l E x t e n d e d , c o n s i s t s o f v a r i o u s p r o -

p o r t i o n s o f q u a r t z , a d u l a r i a , s e r ic i t e , c h l o r i t e , p y r i t e , b a s e -

m e t a l s u l p h i d e s , t e l l u r i d e s ( c h i e f l y h e s s i t e ) a n d e l e c t r u m .

T h e s e c o n d g r o u p , i nc l u d in g K l o n d y k e a n d R o s e s P r id e ,

c o n s i s t s o f s i g n if i c a n t a m o u n t s o f c a l c it e a n d q u a r t z , a s

w e l l a s m i n o r a d u l a r i a a n d t r a c e s o f e l e c t r u m . P y r i t e is th e

o n l y s u l p h i d e m i n e r a l d e t e c t e d a n d b a s e - m e t a l s u l p h i d e s

a r e v i r t u a l l y a b s e n t i n t h e s e c o n d g r o u p .

T e x t u r a l r e l a ti o n s, s h o w i n g m u t u a l b o u n d a r i e s b e t w e e n

p y r i t e , c h a l c o p y r i t e , s p h a l e r i t e a n d g a l e n a , i n d i c a t e t h a t

t h e s e s u l p h i d e m i n e r a l s w e r e d e p o s i t e d e s s e n t i a l l y c o n -

t e m p o r a n e o u s l y . T e l l u r i d e s , i n c l u d i n g h e s s i t e ( A g 2 T e ) ,

a n d t o a m u c h l e s se r e x t e n t , a l t a i te ( P b T e ) , k r e n n e r i t e a n d

s y l v a n i t e ( A u A g t e ll u r id e s ) , a r e o f te n i n t e r g r o w n w i t h t h e

s u l p h i d e s . E l e c t r u m c o m m o n l y o c c u r s e i t h e r a s i n c l u s i o n s

i n h e s s i t e , o r a s f r e e p a r t i c l e s i s o l a t e d i n q u a r t z , a d u l a r i a

a n d c a l ci te . M i c r o p r o b e a n a l y s e s i n d ic a t e t h a t g o l d f i n e -

n e s s r a n g e s f r o m 6 9 6 t o 8 4 9 w i t h h i g h e r v a l u e s w h e n

e l e c t r u m o c c u r s w i t h i n h e s s i t e .

S t a g e I I i s b e s t d e v e l o p e d a t C e n t r a l E x t e n d e d , D a w n

a n d K l o n d y k e , a n d l es s e x t e n s i v e ly a t R o s e s P r i d e w h e r e

S t a g e I I u s u a l l y o c c u r s a s f r a g m e n t s w i t h i n S t a g e I i i .

S i g n i f i c a n t g o l d v a l u e s a r e c o n t r i b u t e d f r o m S t a g e I I .

S t age I I I r e p r e s e n t s a s e c o n d m a j o r p e r i o d o f b r e c c i a t i o n

a n d f r a c t u r in g . I t i s b e s t d e v e l o p e d a t R o s e s P r i d e a n d

C e n t r a l E x t e n d e d , a n d l e s s e x t e n s i v e ly a t D a w n a n d K l o n -

d y k e . A m a s s i v e t e x tu r e i s p r e d o m i n a n t a n d t h e m i n -

e r a l o g y i s r e l a t i v e l y s i m p l e i n t h is s t a g e . R o s e s P r i d e l o d e

i s c o m p o s e d o f q u a r t z , c a lc i te , a d u l a r i a a n d m i n o r z e o l i t e

( l a u m o n t i t e ) . C e n t r a l E x t e n d e d l o d e c o n s i s t s c h i e f l y o f

q u a r t z w i t h m i n o r a d u l a r i a , i ll it e a n d m i x e d - l a y e r i ll it e -

s m e c t i t e . B a s e - m e t a l s u l p h i d e s a r e v i r t u a l l y a b s e n t a n d

o n l y t r a c e s o f p y r i t e a r e p r e s e n t l o c a l l y in t h i s s t a g e .

N e a r l y a l l e l e c t r u m o c c u r s a s f re e p a r ti c l e s i n q u a r t z ,

c a l c i te a n d a d u l a r i a , w i t h s i m i l a r v a l u e s o f A u f i n e n e s s t o

S t a g e I I ( 7 31 t o 7 47 ). S i g n i f i c a n t g o l d m i n e r a l i s a t i o n i s

a s s o c i a t e d w i t h t h i s s t a g e .

S t a g e I V c o n s i s ts o f c o m b q u a r t z , g e n e r a l l y in t h e a b -

s e n ce o f o t h e r m i n e r a ls . C r u s t i f o r m b a n d i n g m a y o r m a y

n o t b e p r e s e n t . T h i s s t a g e i s e s s e n t i a l l y b a r r e n .

Stage V

c o n s i s t s o n l y o f m a s s i v e c a l c it e . I t o c c u r s e i t h e r

a s v e i n l e t s c r o s s c u t t i n g t h e v e i n s o f t h e e a r l i e r s t a g e s o r a s

i n f i l l o c c u p y i n g t h e c a v i t i e s i n e a r l y v e i n m a t e r i a l . T h i s

s t a g e is e x c l u s iv e l y b a r r e n a n d v o l u m e t r i c a l l y m i n o r .

A l l d e p o s i t s i n t h e C r a c o w f i e l d a r e h o s t e d b y t h e

C a m b o o n A n d e s i te a n d h a v e v e r y si m i la r p a r ag e n e s i s. I t

i s l ik e l y t h a t m a i n m i n e r a l i s a t i o n s t a g e s c ro s s t h e C r a c o w

f ie ld w e r e f o r m e d b y s i m i l a r p r o c e s s e s a n d m a y e v e n h a v e



213

Fig. 2a- f . Character i s t ic fea tures and crosscut t ing re la t ionships of

various vein stages, a Staye I ( s il i c if icat ion and s tockwo rk quar tz

veining) encloses Staqe II mineral iza t ion which i s character ized by

crus t i form bands of coarse comb quar tz and coarse-grained sul -

phides , Dawn, waste dump. b High grade ore (Stage II) at Cent ra l

Extended, showing del ica te crus t i form bands wi th s igni f icant

amo unts of adular ia and chlor ite . An e lect rum- and hess i te- r ich

band, indicated by an ar row on the photo, i s located a t top- r ight s ide

of the handspecimen. Also shown in the photo, Stage 1 ( s tockwork

quar tz veining) is t runcated by

Stage II.

W H D 28/ 58 .05 me t r e s ,

Cen tral Extended. Go ld assays - 30.4 g/T over 2 meters, c Delica te

crus t i form bands (Stage II) are t runcated by mass ive quar tz wi th

clasts of previous phases (Sta e III). Narrow quar tz veinle t (Stage

IV) crosscuts Stage III assemblage, which in turn, is crosscutted by

calci te veinlet (Staye V). WHD30/64.5 met res , Cent ra l Extended.

d Stage II at Rose s Pr ide , showing crus t i form bands of quar tz ,

ca lc i te and adular ia . Smal l amounts of pyr ite and i t s oxidised prod-

ucts are disseminated in the vein phases, e Fragm ents of wall rock

and previous s tage phases are cemented by mass ive calc i te , quar tz

and adular ia aggregates (Stage III), Rose s Pr ide . f Com mo n fea-

tures of samples f rom Klondyke, showing crus t i form bands of

quartz, calci te and pyri te (Staqe II). The m etric scales = 1 cm



214

b e e n s y n c h r o n o u s . C o m p a r i n g w i th t h e m a i n G o l d e n P l a -

t e a u l o d e , t h e o u t s i d e l o d e s c o n t a i n m u c h b e t t e r d e -

v e l o p e d S t a g e I I g o l d m i n e r a l i s a t i o n , c h a r a c t e r i s e d b y

c r u s t if o r m a n d c o c k a d e b a n d i n g . T h i s m a y b e a t t r ib u t e d

t o m o r e e x t e n si v e b r e c c ia t i o n s d u r in g S t a g e I I I a t G o l d e n

P l a t e a u , w h i c h d e s t ro y e d c o n t i n u o u s b a n d s o f S t a g e I I

a n d g a v e a m o r e m a s s i v e c h a r a c t e r t o t h e G o l d e n P l a t e a u

l o d e o v e r a l l .

Mineralogical and textural zon ing in Stage

T h e d i s t r i b u t i o n o f m i n e r a l o g i c a l a n d t e x t u r a l a s s e m -

b l a g e s in S t a g e I I i s w o r t h y o f f u r t h e r a t t e n t i o n b e c a u s e

t h is s t a g e i s w e ll d e v e l o p e d a t a l l s t u d i e d o u t s i d e d e p o s i t s

a n d i s a m a j o r g o l d m i n e r a l i s a t i o n s t a g e . D e t a i l e d s t u d i e s

o f v e r ti c a l z o n a t i o n a t D a w n a n d K l o n d y k e a r e d i ff ic u lt

b e c a u s e o f th e s p a r s i t y o f d r il l h o l e s a n d t h e s h a l l o w

d e p t h s ( l e s s t h a n 5 0 m et~ re s b e l o w t h e p r e s e n t s u r f a c e )

t o w h i c h t h e y w e r e d r i l l e d o r m i n e d . T h e r e f o r e , l a t e r a l

z o n i n g o f S t a g e I I o n a d i s t r i c t w i d e s c a l e i s t h e m a i n

s u b j e c t o f t h i s i n v e s t ig a t i o n . I n a d d i t i o n , v e r t i c a l z o n a t i o n

o f S t a g e I I w a s a ls o i n v e s t i g a t e d a t C e n t r a l E x t e n d e d

w h e r e o p e n c u t o p e r a t i o n s a n d a n e x t e n s i v e d r i ll in g

p r o g r a m h a v e p r o v i d e d g o o d c o n t r o l o v e r a v e rt ic a l

d i s t a n c e o f 1 0 0 m e t r e s , a n d a t R o s e ' s P r i d e w h e r e t h e r e

a r e a t l e a s t f o u r a v a i l a b l e d r il l h o le s d o w n t o 1 20 m e t r e s

b e l o w t h e s u r fa c e .

T h e m i n e r a l o g y o f S t a g e I I i s r e f e r r e d to T a b l e 1. T h e

t e x tu r e s o f q u a r t z a n d a d u l a r i a i n S t a g e I I a r e s u m m a r i s e d

i n T a b l e 2 , a n d s o m e o f t h e e x a m p l e s a r e i l lu s t r a t e d i n

F i g s . 3 a n d 4 ( a l s o s e e F i g . 2 ) . T h e r e a r e s o m e c o n s i s t e n t

c h a n g e s i n t e r m s o f m i n e r a l o g y a n d t e x tu r e f r o m e a s t t o

w e s t a c r o s s t h e C r a c o w f ie l d ( i.e . f r o m D a w n , C e n t r a l

E x t e n d e d t o R o s e ' s P r i d e a n d K l o n d y k e ) :

Gangue mineralogy

I n g e n e r a l, S t a g e I I o f e a c h l o d e h a s i ts o w n p r e d o m i n a n t

g a n g u e m i n e r a l a s s e m b l a g e ( se e T a b l e 1). T h e m o s t d i s-

t i n c t d if f e re n c e i s t h a t t h e o r e b o d i e s i n t h e w e s t p a r t o f th e

C r a c o w d i s t r i c t ( K l o n d y k e a n d R o s e ' s P r i d e ) c o n t a i n s i g -

n i f ic a n t a m o u n t s o f c a l c it e , w h e r e a s c a l c i t e is a b s e n t i n

S t a g e I I o f o t h e r o r e b o d i e s . V e i n s e r ic i te , a d u l a r i a a n d

c h l o r it e a r e p r e s e n t n e a r l y e v e r y w h e r e b u t a d u l a r i a a n d

c h l o r i t e a r e t h e m o s t a b u n d a n t a t C e n t r a l E x t e n d e d .

The amount and grain size of base metal sulphides

A t D a w n , b a s e - m e t a l s u l ph i d e s a r e c o m m o n l y c o a r s e -

g r a i n e d a n d f o r m u p t o 2 0 v o l u m e p e r c e n t o f t h e a s s e m -

b l a g e . A t C e n t r a l E x t e n d e d , b a s e - m e t a l s u l p h i d e s a r e u s u -

a l ly f i n e - g r a i n e d a n d r a r e l y e x c e e d 5 b y v o l u m e . B a s e -

m e t a l s u l p h i d e s a r e v i r t u a l l y a b s e n t a t K l o n d y k e a n d

R o s e ' s P r i d e .

Mole percent FeS in sphalerite

O p t i c a l e x a m i n a t i o n o f a b o u t 15 t h in s e c ti o n s sh o w e d n o

o b v i o u s i n t e r n a l z o n i n g w i t h i n t h e s p h a l e r i t e c r y s t a l s . A s

i l l u s t r a t e d i n T a b l e 3 , t h e a v e r a g e m o l e p e r c e n t F e S i n

s p h a l e r i t e d e c r e a s e s f r o m 1 . 6 2 a t D a w n t o 0 . 2 6 a t C e n t r a l

E x t e n d e d . S p h a l e r i t e l o c a l ly c o n t a i n s v e r y h i g h p e r c e n t -

a g e o f c a d m i u m ( u p to 8 .5 w t ) a t C e n t r a l E x t e n d e d .

Table 2. T extures of Stage II of the studied deposits at Cracow

Locations DA CE RP KL

Quartz textures

Crystalline **** *

Microc rystalline ** **** **** ****

Chalce donic * ** **

Ma ssive ** **

Crustiform **** **** *** ****

Colloform *** *** ***

Mo ss *** ** **

Com b **** ** *

Zonal ** *

Mosaic *** * *

Feathery * *

Flamb oyant ** * *

Gho st-sphere *** ** **

Sacch aroidal *** ***

Pseudo -acicular ** ** ***

Pseudo-bladed *

Varieties of adularia

Sub-rhombic **** *

Rhom bic ** **** *** **

Tabular **

Pseudo -aicular * * **

DA, Dawn; CE, Central Extended; RP, Rose's Pride; KL Klondyke

****Predominant; ***comm on; **present; *rare

GoM mineralisation

G o l d m i n e r a l i s a t i o n i s b e s t d e v e l o p e d a t C e n t r a l E x -

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

e l e c t r u m , u s u a l l y a s s o c i a t e d w i t h r h o m b i c a d u l a r i a a n d

f i n e - g r a i n e d q u a r t z w i t h r e c r y s t a l l i s a t i o n t e x t u r e s , a r e o f -

t e n o b s e r v e d i n p o l i s h e d t h i n s e c t i o n s , a n d w h e r e h i g h

g o l d g r a d e s e x c e e d i n g 1 0 g / t a r e f r e q u e n t l y i n d i c a t e d b y

a s s a y s f r o m d r i l l c o r e s a n d r o c k c h i p s . A t o t h e r d e p o s i t s ,

e l e c t r u m , w h i c h i s c o m m o n l y s p a r s e l y d i s t r i b u t e d , i s v e r y

d i f f i c u l t t o b e d e t e c t e d i n p o l i s h e d t h i n s e c t i o n s , a n d t h e

g o l d t e n o r s a r e u s u a l l y l e ss t h a n 1 0 g / t .

Quartz textures

A c l a s s i fi c a t io n o f q u a r t z t e x t u r e s i n e p i t h e r m a l v e i n s f r o m

Q u e e n s l a n d h a s b e e n d e v e l o p e d ( D o n g e t a l . 1 9 9 5 ) . T h i s

c l a s s i f i c a t i o n c a n b e w e l l a p p l i e d i n t h e C r a c o w g o l d f i e l d .

T h e c r u s t i f o r m t e x t u r e i s d i a g n o s t i c o f S t a g e I I t h r o u g h -

o u t t h e d i s t r i c t ( se e F i g . 2 ). A d d i t i o n a l l y , c o a r s e c o m b

t e x tu r e p r e d o m i n a t e s o n l y a t D a w n ( Fi g. 3 a). T h e p r i m a r y

a n d r e c r y s t a l l i s a t i o n t e x t u r e s p o s s i b l y i n h e r i t e d f r o m s i l -

i c a g el , i n c l u d i n g c o l l o f o r m , m o s s , g h o s t - s p h e r e , f l a m b o y -

a n t a n d m o s a i c t e x t u r e s ( F i g . 3 b , c a n d d ) a r e b e s t d e -

v e l o p e d a n d w i d e s p r e a d a t C e n t r a l E x t e n d e d . I n a d d i t io n ,



215

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m i croc rys t a l l i ne com b , f ea t he ry , p s eudo -ac i cu l a r and

cha l cedon i c t ex t u res a re p res en t l oca l l y . The t ex t u ra l a s -

s em b l age a t K l ond yke and R os e s P r i de is cha rac t e r i s ed

by r ep l acem en t t ex t u res i nc l ud i ng s accharo i da l (F i g . 3e )

and pseudo-acicular (F ig . 3f) .

dularia textures

Adul a r i a t ex t u res and t he i r o r i g i n s i n ep i t he rm al ve i n s

f r o m Q u e e n s l a n d h a v e b e e n r e v ie w e d ( D o n g a n d M o r -

r i s on 1995 ). In C raco w, rh om bi c adu l a r i a (Fi g. 4a ) i s t he

m ai n va r i e t y a t a l l t he o re bod i es excep t Dawn where t he

s ub - rho m b i c t ype (F ig . 4b ) i s p redom i nan t . P s eudo -ac i cu -

l a r adu l a r i a (Fi g. 4c ) i s com m o n l y fou nd a t R os e s P r i de

a n d K l o n d y k e b u t i s v o l u m e t ri c a ll y m i n o r . T a b u l a r a d u -

lar ia (F ig . 4d) i s only seen a t Cent ra l Extended.

S t age I I a t C en t ra l E x t ended i s m i ne ra l is ed by s equen -

t ia l precip i ta t ion in open f ractures . In general , there i s

a cons i s t en t qua r t z t ex t u re a s s em b l age wi t h i n 100 m e t res

ve r t ica l i n te rva l, cha rac t e r i s ed b y c ru s t ifo rm , and va r i ous

p r i m ary and r ec ry s t a l li s a t ion t ex t u res p os s i b l y i nhe r i ted

from s il ica gel . The only d i f ference i s that the pr op or t ion

o f s om e t ex t u res va r i e s s li gh tl y from t o p t o bo t t om . F o r

exam pl e , m os s , ghos t - s phere an d cha l cedon i c t ex t u res a re

b e t t e r d e v e l o p e d a t t h e t o p , a s a r e m o s a i c a n d c o m b

t ex t u res a t t he bo t t om . R hom bi c adu l a r i a i s w i des p read

but r i ches t a t the in termediate l evel in corre la t ion wi th

high grade ore .

The genera l i m pres s i on f rom l i m i t ed s am p l e co l l ec t i on

at R ose s P r ide i s that calc i te is presen t in the vein a t a l l

depths (120 met res below the present surface) , as are the

rep l acem en t t ex t u res . C are fu l i n s pec t i on o f t h in s ec t i ons

reveals that some speci f ic t ex tures l ike ghos t -sphere and

f l am boya n t occu r on l y a t s ha l l ow l eve ls , and g i ve way t o

e u h e d r a l q u a r t z t e x t u r e a p p r o x i m a t e l y 7 0 m e t r e s b e l o w

the present surface .

In s um m ary , t he o rebod i es a t Dawn a re r e l a t i ve l y r i ch

i n bas e -m et a l s u l ph i des . Quar t z i s a p redom i nan t gangue

m i nera l , and nea r l y a l ways s hows coa r s e com b t ex t u re i n

as s oc i a t i on wi t h m i no r s ub - rhom bi c adu l a r i a . The o rebo -

d i es a t C en t ra l Ex t ended con t a i n t he bes t go l d g rades

am ong t he s t ud i ed depos i t s i n t he d i s t r i c t . B as e -m et a l

s u l ph i des a re on l y p res en t l oca l l y and t o a m uch l e s s e r

ex t en t com pared wi t h Dawn . Quar t z i s s t i l l t he dom i nan t

gangue m i ne ra l w i t h we l l deve l oped va r i ous p r i m ary and

recrys ta l l i sa t ion tex tu res po ss ib ly inher i ted f r om s i li ca gel,

and s i gn i fi can t am oun t s o f rhom bi c adu l a r i a a re wi de l y

d i s t r ibu t ed . A t R os e s P r i de an d Kl on dyke , bas e -m et a l

s u l ph i des a re v i r tua l l y abs en t , bu t s i gn i fi can t am ou n t s o f

ca l c it e a re wi de l y d i s t r ibu t ed . Qu ar t z i s cha rac t e r i s ed by

rep l acem en t t ex t u res i nc l ud i ng s accharo i da l and p s eudo -

acicular types .

lu id inc lus ions s tudie s on Stage II mine r a l i sa t ion

M o n o p h a s e l i q u id i n c lu s io n s a r e p r e d o m i n a n t a n d a b u n -

dan t i n qua r t z c ry s ta l s . The l ack o f vap ou r bubb l es in t h is

t ype o f i nc l u s ion p r oba b l y r e s u l t s f rom m e t as t ab l e nuc l ea -

t ion du e to thei r smal l s izes of < 2 ~ tm (Ro edd er 1984) .

L i qu i d - r i ch i nc l u s i ons a re com m on l y s een i n ca l c i t e and



216

Fig. 3a-f . Q uar tz textures in the Stage 1I veins, a Com b q uart z

tex ture , showing groups of para l le l o r subpara l le l cys ta ls perpen-

d icu la r to ve in bands , is p redominant a t Dawn. Crossed pola r s .

b F lamboyant tex ture , showing a r ad ia l o r f lamboyant ex t inc t ion of

indiv idua l quar tz c rys ta ls with mo re or less rounded c rys ta l ou t l ine .

is well developed in the r ims of quartz crystals with euhedral cores.

WILD30/65 .05 mete r s , Centra l Extended . Crossed pola r s , c and

d Moss (groups of spheres ) and co l loform (cont inuou s ban ding with

rounded sur face) tex tures a re h ighl igh ted by the d is t r ibu t ion of

impurities . In this case, s ilica gel has transformed to microcrystalline

quar tz , and or ig ina l ge l s t ruc tures a re prese rved in quar tz c rys ta ls .

Centra l ex tended open p i t , p lane pola r ised l igh t for c and c rossed

pola r s for d . e Saccharo ida l tex ture , showing rand om dis t r ibu t ion of

s lender subhedra l q uar tz c rys ta ls , is p redom inan t a t Rose s Pr ide .

Crossed pola r s , f Pseudo-ac icu la r tex ture , ind ica ted by l inear a r -

rangem ent of l ine e longa ted qu ar tz gra ins, is a r ep lacem ent tex ture

af te r ca lc i te . In th is photomicrograph , r emnants of ca lc i te can he

observed. Klon dyke, crossed polars . All scale burs - 0.5 mm



217

Fig. 4a-d. Characteristics of adularia from diff ere nt deposits.

a Rhom ic adularia, typically very fined-graind (< 0.2 mm) and hav-

ing perfect rhombic forms, is predominant at Central Extended.

Plane polarised light, b Sub-rhombic adularia crystals, typically

coarse-grained ( > 2 mm) and subhedral with a rhombic termination,

is mainly found at Dawn in association with comb quartz. Crossed

polars, e Pseudo-acicular adularia aggregates, displaying a radial-

acicular appearance, is commonly found at Rose s Pride and Klon~

dyke (this sample). Plane polarised light. When crossing polars, each

needle actularly consists of several elongated or ragged adularia

grains, d Tabu lar adualria, showing lath-shaped and perpendicular

to vein bands, is only found at Central Extended. Crossed polars. All

scale bars = 0.5 mm

s p h a l e r i t e , b u t f a r l e s s c o mmo n l y i n a d u l a r i a a n d q u a r t z .

T h e i r s i ze s r a n g e f r o m < 2 t o 3 0 g m i n d i a me t e r c o m -

mo n l y w i t h 5 2 0 v o l u me p e r c e n t o f v a p o u r . Se v e n t e e n

doubly po l i shed th in sec t ions were carefu l ly checked fo r

t h e p r e s e n c e o f v a p o u r - r i c h i n c lu s i on s , l i qu i d CO 2 o r

c la th ra te in inc lus ions , bu t these were no t observed .

D u e t o t h e f in e g r a i n e d h o s t m i n e r a l s o f mo s t o f t he

sam ples a nd smal l s i ze (< 2 Jam) o f the ma jor i ty o f flu id

inc lus ions, on ly e igh t ou t o f seven teen do ub ly p o l i shed

th in sec t ions were p roved to con ta in workab le f lu id inc lu -

s i o n s . T h e t o t a l me a s u r e me n t s o f 1 2 0 h o mo g e n i s a t i o n

t e mp e r a t u r e s a n d 6 4 i c e me l t i n g t e mp e r a t u r e s f r o m p r i -

ma r y o r p s e u d o - s e c o n d a r y i n c l u s i o n s w e r e o b t a i n e d a n d

t h e r e s u l t s a r e s u mma r i s e d i n T a b l e 4 .

I n g e n e r a l , t h e re i s a w i d e r a n g e o f h o mo g e n i s a t i o n

tem per a tu res o f f lu id inc lus ions d i s t ri c t -wide , wi th Th

values be twe en 153 ~ and 287 ~ and be ing s ign i f i can t ly

h i g h e r a t D a w n a n d Ce n t r a l E x t e n d e d ( 2 4 7 t o 2 8 7 ~

t h a n a t K l o n d y k e a n d Ro s e s P r i d e ( 15 3 t o 2 2 6 ~

A n u m b e r o f i n c lu s i o ns w i t h v a r i a b l e p h a s e r a t i o s w e r e

a p p a r e n t l y n o t h o m o g e n e o u s w h e n t r a p p e d ( b u b b l e v o l -

um e on ly s l igh tly decreased du r ing h ea t ing up to 400 ~

T h e i c e me l t i n g t e mp e r a t u r e s me a s u r e d i n a l l s a mp l e s a r e

g e n e r a ll y c l o se t o z e r o , w i t h a n a v e r a g e o f - 0 . 1 ~ f r o m

R o s e s P r i d e a n d K lo n d y k e , - 0 . 3 ~ f r o m D a w n a n d

- 0 . 6 ~ f r o m C e n t r a l E x t en d e d.

T h e f i n e - g r a i n e d n a t u r e o f v e in ma t e r i a l a n d p r e v a l e n t

r e c r y s t a l l i s a t i o n i n e p i t h e r ma l e n v i r o n me n t s ma k e f l u i d

inc lus ion s tudy ex t remely d i f f i cu l t . The da ta ob ta ined

therefo re a re usua l ly insuf f i c i en t to represen t en t i re range

o f f lu id c o n d i t io n s . T h e m e a s u r e me n t s f o r S t a g e I I a t

Ce n t r a l E x t e n d e d w e r e ma d e o n l y f r o m f l u i d i n c l u s i o n s

w i t h i n c o mb q u a r t z a n d t a b u l a r a d u l a r i a w h i c h r e p r e s e n t

a v e r y s ma l l p r o p o r t i o n o f t h e v e in ma t e r i a l . T h e ma j o r i t y

o f ve i n ma t e r i a l , c o mp o s e d o f fi n e - g r a in e d q u a r t z w i t h

wel l -developed recrys ta l l i sa t ion t ex tu res , d id no t con ta in

s u i t a b l e f l u i d i n c l u s i o n s f o r me a s u r e me n t s . H o w e v e r , t h e

presence o f the s i l ica ge l p recursor , ev idenced by var iou s



218

Table 4. Fluid inclusion data from Stage II veins

Sample Ho st Location RL Th (~ Tmice (~ Equiv. b

no. mineral (m et re s) Range Average Range Average wt% NaCI

KL2 1 Cal K L waste du m p 280-320 181 to 211 202.4 (42) 0.0 to - 0.1 - 0.1 (18) 0.18

KL 52 Cal K L ou tcro p 320 153 to 189 171.5 (11)

RP6 6 Cal RP RPD 9/154.2 159 187 to 226 211.7 (20) - 0.0 to 0.2 - 0.1 (20) 0.18

RP4 9 Cal RP ou tcro p 290 162 to 183 171.6 (12) - 0.0 to - 0.2 - 0.1 (3) 0.18

CE1 6 Adu CE W HD 30/65.5 335 264 to 278 271.4 (8) - 0.3 to - 0.6 - 0.5 (5) 0.87

CE 04 Q tz CE ope n pit 375 247 to 269 252.1 (7) - 0.4 to - 0.6 - 0.6 (4) 1.05

DA 64 Qtz Daw n NS 64/54.7m 275 254 to 285 270.3 (8) - 0.2 to - 0.4 - 0.3 (7) 0.53

DA08 Sph Daw n waste du m p 285-305 264 to 287 271.0 (12) - 0.2 to - 0.3 - 0.2 (7) 0.35

Cal, Calcite; Adu, Adularia; Qtz, Quartz; K L, Klondyke; R P, Rose's Pride; CE, Central Extended

Elevations of the sam ples, i t has been assumed that the sam ples from w aste dumps c ame from between the lowest level of the mining and the

present surface

bCalculated fro m P otter et al . (1978)

p r i m a r y a n d r e c r y s t a l l i s a ti o n t e x t u r e s (e .g ., c o l l o f o r m ,

m o s s , f l a m b o y a n t , a n d g h o s t - s p h e r e ) , m a y i n d i c a t e t h a t

t h e m a j o r i t y o f v e i n m a t e r i a l , p r e c i p i t a t i n g i n it ia l ly f r o m

s i l i c a g e l , i s v e r y l i k e l y t o b e f o r m e d a t r e l a t i v e l y l o w

t e m p e r a t u r e , p r o b a b l y b e l o w 2 2 0 ~ ( D o n g e t a l. 1 99 5).

T h e t e m p e r a t u r e s o b t a i n e d f r o m f lu id i n c lu s io n m e a s u r e -

m e n t s i n t a b u l a r a d u l a r i a a n d c o m b q u a r t z ( 24 5 ~ t o

2 7 5 ~ m a y r e p r e s e n t t h e u p p e r te m p e r a t u r e l im i t o f

S t a g e I I a t C e n t r a l E x t e n d e d .

I t is p r o p o s e d t h a t t h e d a t a c o l l e c te d f r o m i n c l u s i o n s i n

s p h a l e r i t e a n d q u a r t z a t D a w n , c a n r e p r e s e n t t h e f l u i d

c o n d i t i o n f a i r l y w e l l , s i n c e t h e q u a r t z t e x t u r e i s q u i t e

s i m p le , b e in g d o m i n a t e d b y c o a r s e c o m b t e x t ur e . I n a d d i -

t i o n , q u a r t z a n d s p h a l e r i t e a r e th e m a i n p h a s e s i n th e v e i n

m a t e r i a l . A t R o s e ' s P r i d e a n d K l o n d y k e , o n l y c a l c it e c o n -

t a i n s m e a s u r a b l e f l u i d i n c l u s i o n s . T h e c o n s i s t e n t l y l o w

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 m e a s u r e d f r o m f l u i d i n c l u -

s i o n s i n c a l c i t e a r e c o n s i d e r e d t o b e r e a s o n a b l e , s i n c e

a p p r o p r i a t e p r e c a u t i o n s ( e.g ., re p r o d u c i b i l i ty o f t h e r m o -

m e t r i c r e su lt s) h a v e b e e n t a k e n t o a v o i d l e a k a g e p r o b l e m

w h i c h i s f a i r l y c o m m o n i n m i n e r a l s w i t h g o o d c l e a v a g e

s u c h a s c a lc it e. T h e c o m m o n p r e s e n c e o f v e in l a u m o n t i t e

a t R o s e 's P r id e a n d K l o n d y k e , i n d ic a t in g a n a p p r o x i m a t e

t e m p e r a t u r e r a n g e o f 15 0 ~ t o 2 0 0 ~ ( B r o w n e 1 98 7) , f u r -

t h e r c o n f ir m s t h e l o w t e m p e r a t u r e n a t u r e o f t h e v e i n

f o r m a t i o n . U n l i k e o t h e r d e p o s i t s w h e r e c a l c i t e o c c u r s

a l o n e i n v e i n l e t s o r i n c a v i t i e s w i t h i n q u a r t z v e i n s ( i . e .

S t a g e V ) , c a l c i t e a t R o s e ' s P r i d e a n d K l o n d y k e m a n i f e s t s

i ts e l f i n c r u s t i f o r m b a n d s r e p e a t e d l y a l t e r n a t i n g w i t h

q u a r t z a n d a d u l a r i a i n S t a g e I I . I t i s t h e r e f o r e r e a s o n a b l e

t o a s s u m e t h e d a t a m e a s u r e d f r o m c a l c it e re p r e s e n t a p -

p r o x i m a t e l y t h e fl u id c o n d i t i o n s a t w h i c h q u a r t z a n d

a d u l a r i a c o - p r e c i p i t a t e d w i t h c a l c i t e .

I t a p p e a r s t h a t b o t h c h l o r id e a n d C O 2 c o n c e n t r a t i o n s

i n t h e i n c l u s i o n f l u i d s a r e g e n e r a l l y l o w s i n c e a n o v e r -

w h e l m i n g m a j o r i t y o f i ce m e l t i n g t e m p e r a t u r e s a r e h i g h e r

t h a n - 0 . 6 ~ w h i c h is e q u i v a l e n t to l e ss t h a n 1 .0 5 w t %

N a C 1 o r l e s s t h a n 0 . 3 2 m o l a l d i s s o l v e d C O 2 , c o n s i d e r i n g

t h e s o l e c o n t r i b u t i o n o f e l e c t r o l y t e ( e. g., N a + , C 1 - ) a n d

n o n e l e c t r o l y t e ( e . g . , C O 2 ) t o i c e m e l t i n g t e m p e r a t u r e r e -

s p e c t i v e ly ( H e d e n q u i s t a n d H e n l e y 1 98 5).

Stable isotope studies on Stage mineralisat ion

T h e 6 1 8 0 v a l u e s o f q u a r t z a n d c a l c it e , 5 1 3C v a l u e s o f

c a l c i te a n d t h e c a l c u l a t e d i s o t o p i c c o m p o s i t i o n s o f t h e

f l u i d a r e l i s t e d i n T a b l e 5 . T h e i s o t o p i c c o m p o s i t i o n s o f

t w o s a m p l e s f r o m B a c k C r e e k l i m e s t o n e a d j a c e n t t o

R o s e ' s P r i d e a n d K l o n d y k e a r e a l s o i n c l u d e d i n T a b l e 5 .

T h e e q u a t i o n s o f C l a y t o n e t al . ( 19 7 2) a n d O ' N e i l e t a l.

( 19 6 9) a r e u s e d i n t h is s t u d y f o r o x y g e n i s o t o p e f r a c t i o n a -

t i o n b e t w e e n q u a r t z a n d w a t e r , a n d c a l c i t e a n d w a t e r ,

r e s p e c t i v e l y . I n e p i t h e r m a l e n v i r o n m e n t s w h e r e t h e t e m -

p e r a t u r e i s g e n e r a l l y b e l o w 3 0 0 ~ a n d t h e fl u id is b u f f e r e d

b y t h e s il ic a t e a n d c a r b o n a t e a s s e m b l a g e s o f w a l l r o c k s ,

t h e a q u e o u s c a r b o n s p e c i e s i n t h e fl u id a r e d o m i n a t e d b y

HzCO a n d H C O 3 ( R e e d a n d S p y c h e r 19 85 ). S i n ce t h e

r a t i o o f HzCO a n d H C O ~ - i s d i f f i c u l t t o d e t e r m i n e , t w o

e x t r e m e c a s e s , i . e . f l u i d w i t h a q u e o u s s p e c i e s o f

HzCO

o n l y a n d t h a t o f H C O ~ - o n l y , a r e c o n s i d e r e d i n th e c a l c u -

l a t i o n o f t h e c a r b o n i s o t o p i c c o m p o s i t i o n i n t h e f lu id . I n

d o i n g s o , e q u i l i b r i u m i s o t o p i c f r a c t i o n a t i o n f a c t o r s o f

c a r b o n c o m p o u n d s w i th r e sp e c t t o C O 2 f r o m O h m o t o

a n d R y e ( 1 9 7 9 ) a r e u s e d i n t h i s s t u d y .

S i n c e f r a c t i o n a t i o n f a c t o r s r e l a t i n g t h e i s o t o p i c c o m -

p o s i t i o n o f t h e m i n e r a l s a n d t h a t o f th e f l u id a r e s ig n if i-

c a n t l y t e m p e r a t u r e d e p e n d e n t , u n c e r t a i n ti e s i n e s t i m a t i o n

o f t h e t e m p e r a t u r e o f c r y s t a l l i s a ti o n l e a d t o u n c e r t a i n t ie s

i n t h e c a l c u l a t e d i s o t o p i c c o m p o s i t i o n o f t h e fl u id . T h e

a v e r a g e v a l u e o f 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 is u se d i n

t h e c a l c u l a t i o n w h e r e a v a i l a b l e . F o r t h o s e s a m p l e s w h i c h

c o n t a i n n o w o r k a b l e f l u i d i n c l u s i o n s , o t h e r t e m p e r a t u r e

c o n s t r a i n t s o n t h e b a s i s o f t e x t u r a l , m i n e r a l o g i c a l a n d

g e o l o g i c a l e v i d e n c e a r e a p p l i e d i n t h i s s t u d y ( s e e T a b l e 5 ) .

I n g e n e r a l, t h e t e m p e r a t u r e u s e d f o r i n d i v i d u a l s a m p l e s

h a v e a r a n g e o f + 2 0 ~ t h e r e f o r e t h e c a l c u l a t e d f lu i d

i s o t o p e v a l u e s r e p r e s e n t a v e r a g e c o m p o s i t i o n s o f t h e f lu id .

A fe w i m p o r t a n t o b s e r v a t i o n s c a n b e m a d e r e g a r d i n g

t h e i s o t o p i c c o m p o s i t i o n s o f f l ui d s a n d m i n e r a ls :

1. T h e v a l u e s o f ~ 1 8O n 2 o c a l c u l a t e d f r o m q u a r t z r a n g e

f r o m - 1 7 . 4 t o - 1 1 .8 % o , a n d s h o w a d e c r e a s e t r e n d f r o m

D a w n w i t h a n a v e r a g e o f - 1 2.2 % o, t o C e n t r a l E x t e n d e d



T a b l e

5. Oxygen and carbon isotope data from Stage II veins

219

Sample Loc ation Mineral

T ~

n o .

Minerals

Fluid ~

1~ S O s M o w 6 1 3 C p D B ~ l S O H 2 0 ~ 13 C1 ~13C2

0 0 0 0

DA08 DA, waste dum p Qtz 270 - 3.22

DA10 As above Qtz

270 b

3.97

CE22 CE, W HD28/58.1m Qtz 230 ~ - 3.09

CE16 CE, WH D30/64.5m Qtz 230~ - 2.99

CE04 CE, open pi t Qtz 210~ - 1.44

RP49-1 RP, outc rop Qtz 170e - 2.06

RP49-2 As abo ve Cal 170 - 6.66

KL52-1 KL, outcrop Qtz 170a - 2.65

KL52-2 As abo ve Cal 170 - 6.34

KLB 1 Back Creek l imestone 19.9

RPB1 As abo ve 18.0

- 6.23

- 5.79

4.7

2.6

- 1 1 . 8

- 1 2 . 5

- 1 3 . 5

- 1 3 . 4

- 1 3 . 0

- 1 6 . 4

- 1 7 . 4

- 1 7 . 0

- 1 7 . 1

- 7 . 0 - 8 . 2

- 6 . 5 - 7 . 7

Some of the samples contain no workable fluid inclusions, therefore following assumptions have to be m ade:

a 6lS O fluid da ta are calculated from m ineral data; 613C1 (613 Cmco3 and 613C2 (613 Cnco~ are calculated asssuming fluid with aqueous

species of H2CO3 on ly and that of HC O 3 only, respectively. Uncertainit ies in the calculated isotopic com position of the fluid are mainly due

to uncertainties in est imation of the tem pera ture of crystall isat ion w hich have a range of _+ 15~ This would result in an uncertainty in

calculated 6lSOn2o value of ab ou t +0 .6 at 280~ ab ou t +_ 1.4 at 150~ an d uncertainties in calculated 61aCH2co3 an d 613 CHCO3values

o f

less than 0.5 0 and 0.3 0, respectively at tem peratu re between 150~ to 175~

b Using the Th of DA08, considering the similari t ies in location and mineralogy between the two sam ples

c Assuming 40~ below the fluid inclusion measurements from adjacent, early com b quartz o r tabu lar adularia banding, see text

f o r

explanation

d Using Th from calcite w hich coexists with qua rtz

w i t h a n a v e r a g e o f 1 3.3 o , t o R o s e ' s P r i d e a n d K l o n d y k e

w i t h a n a v e r a g e o f - 1 6 . 7 o .

2 . C a l c u l a t e d v a l u e s o f 61 S O n2 o f o r t h e c a l c i t e f l u i d s a t

R o s e ' s P r id e a n d K l o n d y k e s h o w s i m i la r v a l u e s a s t h o se

c a l c u l a t e d f r o m q u a r t z , s u g g e s t i n g t h a t i s o t o p i c e q u i l i b -

r i u m w a s a c h i e v e d b e t w e e n t h e t w o m i n e r a l s .

3 . I n t h e t e m p e r a t u r e r a n g e o f c a lc i t e p r e c i p i t a t i o n a t

C r a c o w ( 1 50 t o 2 0 0 ~ c a l c u l a t e d 6 1 3 C z c v a l u e s i n t h e

f l u i d s , e i t h e r a s s u m i n g H 2 C O 3 a n d H C O 3 a s t h e p r e -

d o m i n a n t a q u e o u s c a r b o n s p e c i e s ,

a r e n o t

s i g n i f i c a n t l y

d i f fe r e n t t o t h e v a l u e s o f m e a s u r e d 6 X 3 C o f c a lc i te .

4 . T h e i s o t o p i c c o m p o s i t i o n s o f c a l c i te in t h e v e i n s a r e

s i g n i fi c a n t l y d i f fe r e n t f r o m t h o s e o f th e n e a r b y B a c k C r e e k

l i m e s t o n e .

r igin o f Stage l l f luids

O n e o f t h e p r o m i n e n t c h a r a c t e ri s ti c s o f t he d a t a is t h a t t h e

6 1 80 H 2 0 v a l u e s in t h e R o s e ' s P r i d e a n d K l o n d y k e v e i n s

a r e l o w , r a n g i n g f r o m - 1 7.4 t o - 1 6. 4 o . T h e e p i t h e r m a l

v e i ns a t C r a c o w w e r e f o r m e d d u r i n g t he E a r l y P e r m i a n

w h e n a p a l a e o l a t i t u d e o f 6 5 ~ f o r t h e B o w e n B a s i n is

s u g g e s t e d ( V e e v e r s 1 9 8 4 ) . A t t h i s p o l a r o r s u b - p o l a r t e m -

p e r a t u r e o f c o n d e n s a t i o n , t h e m e t e o r i c w a t e r is e x p e c te d

t o b e e x t r e m e l y d e p l e t e d i s o t o p i c a l ly . B i r d a n d C h i v a s

( 19 8 8) i n d i c a t e d t h a t p a l a e o - m e t e o r i c w a t e r in e q u i l i b r i u m

w i t h e a r l y P e r m i a n k a o l i n i te f r o m t h e G u n n e d a h B a s i n i n

N e w S o u t h W a l e s h a d

t ~18

o f ~ < - 1 7 o a n d ~ D

o f

~< - 1 2 5 o . T h i s i s o to p i c c o m p o s i t i o n o f p a l a e o - m e -

t e o r i c w a t e r i s c o n s i d e r e d t o b e s i m i l a r t o t h a t w h i c h

o c c u r e d i n t h e C r a c o w f i e ld d u r i n g t h e m i n e r a l i s a t i o n .

T h e 6 1 S O , ~ o v a lu e s c a l c u la t e d f r o m b o t h q u a r t z a n d

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

R o s e ' s P r i d e a n d K l o n d y k e r e p r e s e n t v i rt u a l ly u n s h i ft e d

p a l a e o - m e t e o r i c w a t e r .

T h e 6 1 a O H 2 o v a l u e s c a l c u l a t e d f r o m S t a g e I I q u a r t z a t

D a w n a n d C e n t r a l E x t e n d e d v e i n s, h o w e v e r , a r e m u c h

e n r ic h e d , r a n g i n g f r o m - 1 3 . 5 t o - 1 1 . 7 . T h i s 6 1S OH 2o

e n r i c h m e n t i s m a i n l y a t t r ib u t e d t o t h e e x c h a n g e o f l ig h t

p a l a e o - m e t e o r i c w a t e r a t C r a c o w ( 6 1 8 0 = - 1 7 o ) w it h

r e l a t iv e l y h e a v y o x y g e n ( 6 1 8 0 = + 6 t o + 8 0 ) i n t h e

v o l c a n i c r o c k s d u r i n g t h e f l u i d c i r c u l a t e s , a s c o m m o n l y

f o u n d e d i n g e o t h e r m a l s y s t e m i n N e w Z e a l a n d ( H e d e n -

q u i s t 1 98 6), a l t h o u g h t h e i n p u t o f a s m a l l c o m p o n e n t o f

m a g m a t i c w a t e r i n to t h e o r e - f o r m i n g f lu i ds a t D a w n a n d

C e n t r a l E x t e n d e d c a n n o t b e c o m p l e t e l y d i s m i ss e d i f t h e

m a g m a t i c w a t e r w a s d i l u t e d .

P r e c i p i t a t i o n o f o r e m i n e r a l s f r o m S t a g e I I f l u i d

I n e p i t h e r m a l e n v i r o n m e n t s , p r e c i p i ta t i o n o f m e t a l s i s

g e n e r a l ly t h o u g h t t o b e i n d u c e d b y p r o c e s s e s s u ch a s

c o n d u c t i v e c o o l i n g , b o i l i n g o r m i x i n g w i t h o t h e r f l u i d s

( e. g., B u c h a n a n 1 98 1; R e e d a n d S p y c h e r 1 9 85 ; B e r g e r a n d

H e n l e y 1 9 8 9 ; S e w a r d 1 9 8 9 ) .

C o o l i n g o f h y d r o t h e r m a l f l u id s b y s i m p l e c o n d u c t i v e

h e a t l o s s c o u l d c a u s e p r e c i p i t a t i o n o f o r e m i n e r a l s , p a r t i -

c u l a r l y b a s e - m e t a l s u l p h i d e s . H o w e v e r , a s i n d i c a t e d b y

S e w a r d ( 19 8 9 ) a n d S a n d e r a n d E i n a u d i ( 19 90 ), s u c h a

p r o c e s s w o u l d l e a d t o m e t a l s b e i n g s p a t i a l l y d i s t r i b u t e d

t h r o u g h o u t t h e p e r m e a b i l i t y p a t h t r a v e r s e d b y th e

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

g e o c h e m i c a l a n o m a l y w i t h r e s p e c t t o m e t a l s, b u t n o t t o

a n o r e d e p o s i t .



220

Mi x i n g o f n e a r - n e u t r a l , me t a l s - t r a n s p o r t i n g f l u i d w i t h

coo ler , descend ing ac id su lpha te f lu ids o r g round water

would a l so cause meta l depos i t ion in response to decreas -

i n g te mp e r a t u r e , c o n c e n t r a t i o n s o f c h lo r i d e a n d r e d u c e d

su lphur . H en ley (1991) po in ted ou t tha t th i s p rocess is

u s u a l ly a s s o c i at e d w i t h t h e w a n i n g s ta g e o f h y d r o t h e r ma l

ac t iv i ty , s ince a dec l ine o f hea t supp ly f rom the source

reg ime changes the ba lance o f p ressu re be tween h o t upf low

(and meta l supp ly ) and su r round ing (descend ing) coo ler

f lu ids , and l eads to p rogress ive inundat ion o f the sys tem.

The m ix ing model i s un l ike ly fo r S tage H m inera l i sa t ion a t

Cracow because Stage I1 i s paragenet i ca l ly o ld .

Ba s e d o n e x p l o r a t i o n o f a c t i v e g e o t h e r ma l s y s t e ms ,

n u m e r o u s s t u d i e s ( e.g ., B r o w n e a n d E l li s 1 9 7 0 ; B r o w n e

1 97 8; H e n l e y 1 9 8 5 ; a n d H e n d e n q u i s t 1 9 9 0 ) s h o w e d t h a t

v e i n a d u l a r i a , f o r me d d u e t o t h e f l u i d b e c o mi n g mo r e

a lka l ine , and ve in ca lc i t e , depos i t ed due to CO2 loss , a re

mi n e r a l o g i c a l i n d i c a t o r s o f b o i l i n g . T h e s e o b s e r v a t i o n s

w e r e s t r o n g ly s u p p o r te d b y a n u m b e r o f t h e r m o d y n a m i c

s tu d ie s (e .g ., D r u m m o n d a n d O h m o t o 1 9 8 5 ; R e e d a n d

Spyc her 1985 ; Spy cher an d R eed 1989 ; Ca th les 1991) . The

i n v e s t i g a t io n o f t h e s t r u c t u r a l s t a t e o f a d u l a r i a i n e p i t h e r-

ma l v e i n s ( D o n g a n d M o r r i s o n 1 9 9 5) , s h o w i n g r e l a t i v e ly

d i s o r d e r e d s t r u c t u re s , f u r t h e r i n d i c a t e s t h a t v e i n a d u l a r i a

w a s f o r me d u n d e r r a p i d c r y s t a l l i s a t i o n c o n d i t i o n s w h i c h

a r e mo s t l i k e l y i n d u c e d b y b o i l i n g o f h y d r o t h e r m a l f l u id s .

I n t h e Cr a c o w d i s t r i c t , v e i n a d u l a r i a i s w i d e s p r e a d i n

Stage I I o f a l l s tud ied ve in depos i t s . Ca lc i t e occurs main ly

a t K l o n d y k e a n d R o s e s P r i d e . H o w e v e r , p s e u d o - a c i c u la r

q u a r t z t e x t u r e , a r e p l a c e m e n t t e x t u r e a f t e r c a l c it e ( D o n g e t

a l. 1995) , i s o f t en foun d in S tage I I a t Ce n t ra l Ex ten ded ,

i n d i c a t i n g t h e p r e s e n t o f c a l c i t e p r e c u r s o r s . T h e r e f o r e ,

mi n e r a l o g i c a l a n d v e i n t e x t u r a l f e a t u r e s s u g g e s t t h a t

b o i l in g t o o k p l a c e d u r i n g t h e S t a g e I1 mi n e r a l i s a t i o n i n a l l

s tud ied ve in depos i t s .

M a n y o f th e e f f e ct s o f b o il i n g o n o r e d e p o s i t i o n h a v e

b e e n c o m p u t a t i o n a l l y m o d e l le d a n d d i sc u s se d b y a n u m -

b e r o f r e se a r ch e r s, in c l u d i n g D r u m m o n d a n d O h m o t o

(1985) , Reed and Spycher (1985) , Seward (1989) and

Ca t h l e s ( 1 99 1) . I n a n e x a mp l e , t h e e v o l u t i o n o f f lu i d c o m-

p o s i ti o n h a s b e e n m o d e l le d u s i n g t h e c o m p u t e r p r o g r a m

C H I L L E R ( S p y c h e r a n d R e e d 1 9 90 ) . T h i s p r o g r a m is

d e s i g n e d t o mo d e l b o i l i n g , c o o l i n g , a n d v a r i o u s o t h e r

c h e mi c a l i n t e r a c t i o n s i n a q u e o u s s y s t e ms , i n c l u d i n g s o l id

and gas phases . The in i t i a l f lu id compos i t ion used in th i s

c a l c u l a t i o n is s im i l a r t o t h a t a t t h e D a w n d e p o s i t , w h i c h i s

in equ i l ib r ium wi th q uar t z , a du lar i a , a lb i t e , il li te , ch lo r i t e ,

pyr i t e , ep id o te an d ca lc i t e a t 285 ~ (Do ng 1993) . F lu id

b u f f e r e d c o n d i t i o n s a r e a s s u m e d d u r i n g v e i n f o r m a t i o n

because f lu ids a re e f fec t ive ly i so la t ed f rom wal l rocks a s

soon as ear ly ve in minera l s (par t i cu la r ly quar t z ) p rec ip i -

t a t e on the wal l s o f f lu id condu i t s .

I n mo s t n a t u r a l s y s t e ms , w h e n a f l u i d f i rs t b o il s a t d e e p

leve ls where perm eab i l i ty is low, very l i tt l e gas can escap e

f r o m t h e s y s t e m a n d t h e f l u i d e x c h a n g e s s o me h e a t w i t h

wal l rocks (mos t l ike ly hea t i s los t th rough the wal l rock ,

i . e . , sub i soen tha lp ic bo i l ing , c f . Spycher and Reed 1989)

because i t f lows re l a t ive ly s lowly . However , as the f lu id

m o v e s u p a n d e n c o u n t e r s a n e n v i r o n m e n t o f i n c re a s e d

p e r me a b i l i t y w h i c h i n d u c e s e f f e c ti v e g a s r e m o v a l a n d

a h i g h f l o w r a t e , b o i li n g w i ll p r o c e e d m o r e v i g o r o u s l y a n d

h e a t c o n d u c t i o n t h r o u g h w a l l r o c k s c a n b e i g n o r e d ( i . e . ,

i soen tha lp ic bo i l ing) . Accord ing ly , the ca lcu la t ions as -

sum e sub i soen tha lp ic bo i l ing (hea t loss o f 0 .8 kca l per

degree C. ) in a c losed sys te m f rom 285 ~ to 265 ~ and

f o l l o w e d b y is o e n t h a l p i c b o i l i n g in a n o p e n s y s t e m b e l o w

2 65 ~C . T h e t e m p e r a t u r e 2 6 5 ~C a n d t h e h e a t l o ss o f

0 .8 k c a l p e r d e g r e e C . a r e s o me w h a t a r b i t r a r i l y c h o s e n ,

b u t t h i s d o e s n o t m a t t e r f o r t h e p u r p o s e o f t h e e x e rc i se

w h i c h i s ma i n l y f o r mo d e l l i n g a q u a l i t a t i v e t r e n d i n t h e

chan ges o f the so lub i l i t i es o f var ious spec ies dur ing bo i l ing

o f th e i n i t ia l f lu i d a t C r a c o w . D u e t o t h e l a c k o f c o n c e n t r a -

t i o n d a t a f o r me t a l s p e c i e s a n d o t h e r a q u e o u s s p e c i e s i n

t h e i n i ti a l f l ui d , n o a t t e m p t is ma d e r e g a r d i n g q u a n t i t i e s o f

mi n e r a l s d e p o s i t e d d u r i n g b o i l i n g .

" 8

> .

" 6

E

2 ....

o2

- 3

- 4

- 2

' i , i i i

T 4 ~ - - - - - - " " ' ' " ~ _ _

H S O 4 "

- 6

0 "

r I I i . l 9 I

- ,I k2 -, 2~ : . . . . . ~ = = : = : 9 9 _ - : - -

- : : : : . . . .

- 6 -

-6 ~ . , , ,

- 8

- 9

- 1 0

A g C b . -

1 1

N

-5 ' | ' i , i . i 1

- 6

q 0 "

-5 . , I

" 6 " ! ~ , . ~ . . ~

7

-1 2 1 . , 9 ~ . ~ . t 9

2 8 5 2 6 5 2 4 5 2 2 5 2 0 5

T e m p e r a t u r e ( ~

7 . 5

p H

6 . 5

5 . 5

Fig. 5. Ch ang es in the solubilities of m etal com plexes and several

important chem ical param eters during bo iling of a fluid close to

com positions of the initial fluid at D awn



221

The resu l t s are i l lus t ra ted in F ig . 5 . W hen sub- i soe n-

thalp ic b oi l ing f i rs t s t ar t s in a c losed sys tem, the so lubi l i ty

o f go l d is v i r t ua ll y unchang ed un t il abo u t 270 ~ ch i ef ly

because the ef fect of an increase in pH (s tab i l i s ing

Au(H S )2 ) ove r r i des t ha t o f HzS l o s s (des t ab i li s ing

Au(H S)s W i th fur ther boi l ing , the ef fect of H2S degass -

i ng becom es m ore com pe t i t i ve . In add i t i on , when t he pH

becom es s o h i gh t ha t t he dom i nan t s u l ph i de aqueou s

s p e ci e s b e c o m e s H S - , f u r th e r i n c r ea s e i n p H w o u l d h a v e

an op pos ing effect on gold so lubi l i ty ( i. e . des tab i l i s ing

Au(H S)2) . In th i s case, gold so lubi l i ty decreases , especia l ly

when t he s y s t em i s t o t a l l y open . The s o l ub i l i t i e s o f bas e

m et a l s i nva r i ab l y d ro p as t he bo i l ing p roceeds , m a i n l y i n

res pons e t o dec reas i ng t em pera t u re and i nc reas i ng pH.

The beh av i o u r o f si lve r depen ds on t he r a t i o o f Ag ch l o r -

ide-complex to Ag b i su lphide-complex in the f lu id . In the

cas e o f t he f l ui d a t Daw n , w h i ch s hows A gC 12grea t e r t han

Ag(H S)2, the so lubi l i ty of to ta l s i lver dro ps as boi l ing

proce eds , s imi lar to that of base metal s . In th i s way,

a m e t a l dep t h zon i ng m a y be deve l ope d i f t he i n it ia l f l u id

i s sa turated wi th respect to Au, Ag and base metal s , i . e .

bas e m e t a l s u l ph i des p rec i p i t a t e i m m ed i a t e l y a f t e r t he

f lu id boi l s a t deep level s , whereas gold preferent ia l ly de-

pos i t s a t shal lower l evel s .

Z o n i n g m o d e l f o r S t a g e II m i n e r a l i s a t io n a t r a c o w

The mineralogical , t ex tural , f lu id inclus ion and s tab le

i s o t ope s t ud i es p l ace cons t r a i n t s on any m ode l t ha t m ay

be u s ed t o exp l a i n t he zon a t i on o f S tage I I m i ne ra l i s a ti on

i n t he C racow d i s t r i c t . S uch a m ode l m us t be cons i s t en t

wi th a l l the fo l lowing observat ions :

1 . T he o re bod i es a t Daw n a re r e la t i ve ly r ich in bas e -m et a l

s u l ph ides . Quar t z i s t he p redom i nan t gangue m i ne ra l , and

c o m m o n l y s h o w s a c o a r s e c o m b t e x t u r e . A d u l a r i a i s

m a i n l y p res en t i n a s ub - rhom bi c va r i e t y . The hea t i ng -

f r eez i ng r e s u l t s i nd i ca t e a m i ne ra l i s a t i on t em pera t u re o f

275 4- 10~ and a sa l in i ty range of 0 .35 to 0 .67 w t% . The

i s o t op i c s t udy i nd i ca t e s an pa l aeo -m et eo r i c wa t e r s ou rce

wi t h heav i l y en r i ched 61s On2o va l ue o f -12% o .

2 . The o rebod i es a t C en t r a l Ex t ended con t a i n t he bes t

go l d g rades am ong t he s t ud i ed depos i t s i n t he d i s t r i c t .

B as e -m et a l s u l ph i des a re on l y p res en t l oca l l y and t o

a m u ch l e s s e r ex t en t com pare d w i t h Dawn . Q uar t z i s s ti ll

t he dom i nan t gangue m i ne ra l , bu t s i gn i f i can t am oun t s o f

rhom bi c adu l a r i a an d ch l o r i t e a r e w i de l y d i s t r ibu t ed .

Var i ous p r i m ary and r ec ry s t a l l i s a t i on t ex t u res pos s i b l y

inher i t ed f rom s i l i ca gel are wel l developed and wide-

s p read . A t i nd i v i dua l s i t e s where c ru s t i fo rm bands de -

ve l oped f rom bo t h wa l l s o f a f is su re , t em pera t u res cou l d

d ro p s ha rp l y f rom 275 ~ (m eas u red f rom f lu i d i nc l u s ions

i n e a r ly f o r m e d b a n d s o f c o m b q u a r t z a n d t a b u l a r a d u l a -

r ia) to l ess than 220 ~ ( indicated by the presence o f s il ica

gel t ex tures ) . The hydrothermal f lu id a t Cent ra l Extended,

com pared wi t h t ha t a t Dawn , was m ore d i l u t e and l e s s

i s o t op i ca l l y en r i ched (618Ou2o va l ue o f -13 .5%o) m e-

teor ic water .

3 . A t R os e ' s P r i de and Kl ondyke , bas e -m et a l s u l ph i des

a re v i r t ua l ly abs en t , b u t s ign i fi can t am ou n t s o f ca l c it e and

quar t z w i t h m i no r a du l a r i a a r e w i de l y d i s t r ibu t ed . R e-

placement t ex tures are d i s t inct , and tex tures inher i t ed

from s i li ca gel in the upp er sect ions of the de pos i t g ive way

t o euhed ra l t ex t u res w i t h dep t h . The m i ne ra l i s a t i on t em -

pera ture i s determ ined to be l ess than 220 ~ and sal in i ty

t o be l e ss t han 0 .18 wt % . I s o t op i c da t a i nd i ca t e nea r l y

uns h i f ted m e t eo r i c wa t e r , w i t h 618On2o va lue o f -1 7% o.

The d i f f e ren t cha rac t e r i s t i c s docum en t ed above m ay be

re l a t ed t o s eve ra l pos s i b l e f ac t o r s i n ep i t he rm al depos i t s

(He ald e t a l. 198 7) . The se include:

1. M et a l con t en t and m i ne ra l ogy o f t he s ou rce rock w i th

respect to the a vai lab i l i ty of metal s for ex t ract ion;

2 . C hem i s t ry o f hyd ro t he rm al f l ui d s w i t h r e s pec t t o t he ir

re la t ive ab i l i ty to ex t ract , t ranspor t , and precip i ta te meta-

ls ; and

3 . Evo l u t i on o f t he o re - fo rm i ng hyd ro t he rm al s y s t em (a)

in t ime (e .g . , d i f ferent s tages o f mineral i sa t ion) and (b) in

space (e .g . , zonat ion wi th depth) .

As d i scussed in previous sect ions , a l l s tudied depos i t s in

t he C rac ow d i s t r ic t a r e con f i ned to an a rea o f app rox i m -

a t e l y 30 km 2, and a re hos t ed i n t he C am bo on Andes i t e

wh i ch has a m as s i ve na t u re and abou t 3000 m e t res i n

th ickness . S tudies of the parag enes i s indica te that a ll de-

pos i t s exper i enced es s en t i a l l y t he s am e hyd ro t he rm al ac -

t iv i t i es across the camp, and S tage I I i s a major gold

mineral i sa t ion s tage. Therefore , d i f ferences in the chem-

i s try o f the hy d ro t he rm a l f l ui d s (f ac t o r 2) and evo l u t i on o f

t he hyd ro t he rm al s y s t em s i n s pace ( f ac t o r 3b ) a re co n -

s i de red t o be t he m a j o r f ac t o r s accoun t i ng fo r d i f f e ren t

character i s t i cs of S tage I I in indiv idual depos i t s a t

C r a c o w .

The ac t i ve geo t he rm al s y s t em s a re cons i de red t o be t he

a rche t ypes o f t hos e s y s t em s r e s pons i b l e fo r ep i t he rm al

p rec i ous - and bas e -m et a l de pos i t s (Whi t e 1 955 , 1981 ;

Hen l ey and E ll is 1983 ; He n l ey 1985 ) . There a re t h ree

m a j o r e n d - m e m b e r f l ui d ty p e s in N e w Z e a l a n d g e o t h e r -

mal sy s tems (He den quis t 1986, 1990). The se are: the

chlor

ide f lu id

or i g i na ti ng f rom deep l y c i rcu l a t ing m e t eo r i c

wa t e r , and cha rac t e r i s ed by i ts h igh t em pera t u re , nea r

neu t r a l pH, ch l o r i de a s t he dom i nan t an i on , a va r i ab l e

C O2 con t en t , and en r i ched 6180 wi t h r e s pec t t o l oca l

m e t eo r i c wa t e r due t o ex t ens i ve wa t e r - rock i n t e rac t i on ;

the

ac id su lphate s t ream h eated f lu id

f o r m e d b y c o n d e n s a -

t i on o f s t eam i n t o g rou ndw at e r ne a r t he s u r face where

H2S i s oxid i sed to su lphate ; and the

CO2 r ich s team

he a te d f l u id

f o r m e d b y c o n d e n s a t i o n o f s te a m i n to n o n -

o x y g e n a t e d g r o u n d w a t e r a n d c h a r a c te r is e d b y v e r y l o w

sal in i ty (c lose to zero) and low temperature (120 to

180 ~ B o t h s t eam -he a t ed fl u id s have i s o t op i c com pos i -

t i ons s i m i l a r t o g round wa t e r , and genera l l y fo rm an

um b re l l a ove r t he deep ch l o r ide up fl ow.

Many concep t ua l and des c r i p t i ve zon i ng m ode l s fo r

ep i t he rm al go l d - s il ve r depos i t s have been p rop os ed ( e. g.,

B u c h a n a n 1 9 8 1 ; G i l es a n d N e l s o n 1 9 8 2 ; B e r g e r a n d

E i m o n 1982 ; He n l ey and E ll is 1983 ; S i ll it oe and B on ham

1984; S i lberm an and Berg er 1 985; H eald e t a l. 1987).

B u c h a n a n ( 1 9 8 1 ) s u m m a r i s e d a n d t a b u l a t e d d a t a f r o m

over 60 ep i t he rm al depos i t s hos t ed by vo l can i c i n t he

wes t e rn US , and p res en t ed an em p i r i ca l zon i ng m ode l

that ef fect ively i l lus tra tes the geom et r ic ar rang em ent of



222

Depos i t s O r e Z o n e s F l u i d T ~ 1 : 0 H 2 0 O r e G a n g u e Q u a r t z t e x t u r e s d u l a r i a t e x t u r e s

t y p e s (~ (%0)

Present surface .=~

R o s e 's . ~ . ~ 1 50 - 1 7 . 4 p y ri te q u a r t z C r u s t i f o r m , a c c h a r o i d a l , R h o m b ic

Pride ~'~ -o / ~ +hessite calcite Pseudo-acicular, Pseudo-acicular

& Precious ~ ~ , ~ 220 -16 .4 +ele ctrum ad ula r ia +col loform, +moss , _+comb

Klondyke ~ laumon tile

pyr i te qu ar tz Crust iform, CoUoform, Mo ss , Rhombic,

metal .~ <210 -13.5 chalcopyri te adu lar ia Flamboyant , Gh ost-sphe re, Pseudo-acicular

Central

Extended

Dawn

interval

r

Base metal

interval

(not to scale)

o

'S 270 -13.0

.t-,

o

250 -12.5

280 11.8

sphaleri te chlor ite Mosaic, Pseu do-ac icular , Ta bu la r

he ss i te +ser ici te +com b, +feathery, +zonal +sub -rhom bic

galena

electrum

sphalerite quartz

gal ena +_adular i a

chalcopyrite +chlorite

pyrite

hessite

+elecmam

Crust iform, Comb, Zonal , Sub -rhom bic

+feathery, +mosaic :t:rhombic

Intrusive +

+ + +

Heat Source

Fig. 6. A schem atic zoning model for Stage II mineralisation in the Cracow hydrothermal system

o r e , g a n g u e a n d a l t e r a t i o n z o n e s i n e p i t h e r m a l v e i n s .

M o r e r e c e n t ly , b a s e d o n a n e x t e n s i ve r e c o n n a i s s a n c e o f

a n u m b e r o f e p i t h e r m a l v e in s in Q u e e n s l a n d , c o u p l e d w i t h

d e t a i l e d w o r k o n a f e w s e l e c t e d o c c u r r e n c e s , M o r r i s o n

e t a l. ( 19 9 0) d e v e l o p e d a n e m p i r i c a l t e x t u r a l z o n i n g m o d e l

i n e p i t h e r m a l v e in s , i n w h i c h t h r e e s u p e r z o n e s , r e p r e s e n t -

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

a r e d e f i n e d .

T h e f o l l o w i n g C r a c o w m o d e l i s e s t a b l is h e d o n t h e b a s is

o f t h e c u r r e n t u n d e r s t a n d i n g o f f l u id t y p e s in e p i t h e r m a l

s y s t em s a n d c o m p a r i s o n o f t h e c h a r a c t e r i s ti c s o f th e

C r a c o w d e p o s i t s w i t h th e a v a i l a b l e m o d e l s o f e p i t h e r m a l

depos i t s (F ig . 6 ) .

T h e d e e p l y c i r c u l a t i n g d i lu t e c h l o r i d e f l u i d is c o n s i d e r e d

t o b e d o m i n a n t f l u i d t y p e d u r i n g m i n e r a l i s a t i o n a t D a w n

a n d C e n t r a l E x t e n d e d . G i v e n a s o u r c e o f h e a t , p r e s u m a b l y

a c o o l i n g m a g m a a t d e p th , a n y a v a i l a b le g r o u n d w a t e r

w o u l d b e c a u g h t u p i n a h y d r o t h e r m a l c o n v e c t i o n s ys te m .

As t h e g ro u n d wa t e r c i r c u l a t e s , i t i n t e r a c t s w i t h wa l l r o c k ,

b e c o m i n g e n r i c h e d i n l ig a n d s a n d m e t a l s , a s we l l a s ~ 8 0 .

T h i s d e e p l y c i r c u l a t i n g d i l u t e c h l o r i d e f l u i d e v e n t u a l l y

s t a r t s b o i l i n g a t s h a l l o w l e v e ls , l e a d i n g t o t h e fo rm a t i o n o f

o re b o d i e s . T h e d i f f e r e n c e i n c h a ra c t e r i s t i c s b e t w e e n t h e

o r e b o d i e s a t D a w n a n d C e n t r a l E x t e n d e d is t h o u g h t t o b e

r e l a t e d t o t h e d e p t h o f e r o s i o n a n d b o i l i n g in d i f f e r e nt

c o n d i t i o n s .

T h e o r e b o d i e s a t D a w n a r e m o s t l i k e ly t o b e s it u a t e d i n

t h e b o t t o m z o n e o f a b o i li n g s y s t e m w h e r e s u b - i s o e n t h a l -

p i c b o i l i n g i n i t ia t e s w i t h v e r y li t tl e g a s r e m o v a l . I n t h i s

e n v i r o n m e n t , b a s e m e t a l s u l p h i d e s p r e c i p i t a t e w h e r e a s

m o s t o f th e g o l d s ti ll r e m a i n s i n t h e f l u i d . R e l a t i v e l y sl o wl y

c h a n g i n g c o n d i t i o n s a l s o a c c o u n t f o r t h e c o m m o n p r e s -

e n c e o f c o a r se c o m b q u a r t z t e x t u r e a n d s u b - r h o m b i c

a d u l a r i a v a r i e t y . T h e d e e p p o s i t i o n i n t h e s y s t e m a l l o w s

e x t e n s iv e w a t e r - r o c k i s o t o p i c e x c h a n g e t o t a k e p l a c e a n d

p r e v e n t s t h e p a r t i c i p a t i o n o f f r e sh m e t e o r i c w a t e r , t h u s t h e

6 1 8 0 v a l u e s o f t h e f l u id w e r e s h i ft e d f r o m p a l a e o - m e t e o r i c

w a t e r t o a s h ig h a s - 1 2 T oo a t D a w n .

T h e f o r m a t i o n o f o r e b o d ie s a t C e n t r a l E x t e n d e d i s

p e r h a p s a g o o d e x a m p l e o f e x t e n s i v e b o i l in g a t s h a l l o w

l e v e l s . H e r e i s o e n t h a l p i c b o i l i n g t o o k p l a c e i n a n o p e n

s y s te m . T h e r a p i d l o ss o f v o l a t il e c o m p o n e n t s a n d c o o l i n g

l e d t o a s h a rp d e c re a s e i n g o l d s o l u b i l i t y , w i t h e s s e n t i a l l y

a l l o f th e g o l d b e i n g d e p o s i t e d o v e r a n a r r o w d e p t h

i n t e rv a l . T h e r a p i d l y c h a n g i n g c o n d i t i o n s a l so a c c o u n t f o r

t h e c o m m o n p r e s e n c e o f m i c r o - r h o m b i c a d u l a ri a a n d

w i d e s p r e a d q u a r t z t e x t u r e s p o s s i b l y i n h e r i t e d f r o m s i l i c a

g e l. C o n v e n t i o n a l l y , th e r e s h o u l d b e a d e e p e r z o n e r i c h in

b a s e m e t a l s u l p h i d e s . H o w e v e r , if t h e i n i t i a l fl u i d wa s

d i l u t e a n d t h e r e f o r e c o u l d o n l y c a r r y l i m i t e d a m o u n t s o f

b a s e m e t a l s , o r t h e i n i t i a l f l u i d wa s s u b s t a n t i a l l y u n d e r -

s a t u r a t e d w i t h r e s p e c t t o b a s e m e t a l s , a b a s e - m e t a l s u l -

p h i d e - r i c h z o n e m a y n o t b e d e v e l o p e d a t d e p t h .

C O 2 - r i c h - s t e a m - h e a t e d f l u i d is c o n s i d e r e d t o h a v e b e e n

p r e d o m i n a t e d i n t h e u p p e r p a r t s o f t h e s y s te m s a s r e p r e -

s e n t e d a t R o s e s P r i d e a n d K l o n d y k e . H e a t i n g o f t h is

C O 2 - r i c h s t e a m - h e a t e d f l u i d w i l l l e a d t o p r e c i p i t a t i o n o f

c a l c i t e d u e t o i t s in v e r s e s o l u b i l i t y . Ho we v e r , t h i s p ro c e s s

c a n n o t a c c o u n t f o r t h e c o e x i s t e n c e o f c a l ci te , a d u l a r i a a n d

q u a r t z - t h e m i n e r a l a s s e m b l a g e c o m m o n l y o b s e r v e d a t

R o s e s P r i d e a n d K l o n d y k e , b e c a u s e b o t h t h e s o l ub i l it y o f

q u a r t z a n d t h e A I ( O H ) 2 s t a b il i ty i n c r e a s e w i t h i n c r e a s in g



223

t e m p e r a t u r e s w h i c h r e s u l t s i n a f l u i d u n d e r s a t u r a t e d w i t h

r e s p e c t t o q u a r t z a n d a d u l a r i a . O n t h e o t h e r h a n d , b o i l i n g

o f t h e C O 2 - r i c h s t e a m - h e a t e d f l u id , r e s u lt i n g i n lo s s o f

C O 2 , c o o l i n g , e n r i c h m e n t o f s o l u t e s a n d i n c r e a s e i n p H , i s

a b e t t e r m e c h a n i s m o f m i n e r a l d e p o s i t i o n a t R o s e s P r id e

a n d K l o n d y k e . T h e l o w t e m p e r a t u r e a n d l o w sa l in i ty o f

t h e f lu id p r e c l u d e t h e t r a n s p o r t o f b a s e m e t a l c o m p l e x e s ,

h e n c e b a s e m e t a l s u l p h i d e s a r e v i r t u a l ly a b s e n t a t R o s e s

P r i d e a n d K l o n d y k e . A s i n d i c a t e d b y H e d e n q u i s t ( 1 9 8 6

a n d 1 99 0), t h e m i x e d - l a y e r i l li t e -s m e c t i t e a l o n g w i t h m i n o r

k a o l i n i t e i s c o n s i s t e n t w i t h t h e p r e s e n c e o f C O z - r i c h

s t e a m - h e a t e d w a t e r s a t s h a l l o w l e v e l s i n N e w Z e a l a n d

g e o t h e r m a l s ys t em s . H o w e v e r , f r o m t h e e x a m i n a t i o n o f

a v a i l a b l e s a m p l e s a n d d r i l l c o r e l o g s , t h e s e m i n e r a l s

a r e o n l y p r e s e n t in m i n o r a m o u n t s a t R o s e s P r i d e a n d

K l o n d y k e t h u s i n d i c a t i n g t h a t t h e a r g i l l i c a l t e r a t i o n

w a s n o t w e l l d e v e l o p e d i n a s s o c i a t i o n w i t h S t a g e I I m i n -

e r a l i s a t i o n a t t h e s e d e p o s i t s . A p o s s i b l e e x p l a n a t i o n i s t h a t

t h e C O 2 - r i c h s t e a m - h e a t e d f l ui d u n d e r w e n t b o i li n g s o o n

a f t e r c o n d e n s a t i o n o f t h e s t e a m s i n t o g r o u n d w a t e r , s h i ft i-

n g m i n e r a l - f l u i d e q u i l i b r i a b a c k t o K - f e l d s p a r s t a b i l i t y

field.

C o n c l u s i o n s

T h e d i f fe r e n t c h a r a c t e r i s t i c s o f s e l ec t e d e p i t h e r m a l v e i n s i n

t h e C r a c o w g o l d f i e l d c a n b e e x p l a i n e d b y t h e d e p t h

z o n i n g i n t y p i c a l b o i li n g e p i t h e r m a l s y s t e m s . F r o m t o p t o

b o t t o m , Z o n e I ( c o r r e s p o n d i n g t o R o s e s P r id e a n d K l o n -

d y k e ) is c h a r a c t e r is e d b y w i d e s p r e a d r e p l a c e m e n t t e x t u re s

a f t e r c a r b o n a t e , c o r r e s p o n d i n g t o t h e C O : - r i c h s t e a m -

h e a t e d f l ui d w h ic h i s p r e d o m i n a t e d i n th e u p p e r p o r t i o n o f

a n e p i t h e r m a l / g e o t h e r m a l s y s te m . Z o n e I I (c o r r e s p o n d i n g

t o C e n t r a l E x t e n d e d ) is m a r k e d b y t h e d e v e l o p m e n t o f

d e l ic a t e c r u s t i f o r m - c o l l o f o r m b a n d i n g , v a r i o u s t e x t u r e s

p o s s i b l y i n h e r i t e d f r o m o r i g i n a l s i l i c a g e l ( e. g., m o s s , c o l l o -

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

a c i c u l a r ) a n d t h e p r e s e n c e o f s i g n i fi c a n t a m o u n t s o f m i c r o -

r h o m b i c a d u l a r i a . T h e d e v e l o p m e n t o f Z o n e I I is t h o u g h t

t o b e r e l a t e d t o e x t e n s i v e b o i l i n g a t r e l a t i v e l y s h a l l o w

l ev e ls w h i c h i n d u c e s a r a p i d c h a n g e i n m a n y i m p o r t a n t

p a r a m e t e r s i n c lu d i n g t e m p e r a t u r e , p H , r e d o x a n d a c t iv i ty

o f H 2 S . G o l d m i n e r a l i s a t i o n i s m o s t l i k e l y t o o c c u r w i t h i n

t h is z o n e . Z o n e I I I ( c o r r e s p o n d i n g t o D a w n ) is d o m i n a t e d

b y c o a r s e c o m b q u a r t z t e x t u r e i n a s s o c i a t i o n w i t h s u b -

r h o m b i c a d u l a r i a . T h i s z o n e c o u l d b e r e l a t iv e l y r i c h in

b a s e m e t a l s u l p h i d e s a n d i s c o n s i d e r e d t o b e d e v e l o p e d

w h e n s u b - i s o e n t h a l p i c b o i l i n g i n i t i a t e s a t d e e p l e v e l s

w h e r e p e r m e a b i l i t y i s l o w a n d f lu i d c o n d i t i o n s d o n o t

c h a n g e d r a m a t i c a l l y .

Acknowledgements.

This pap er represents pa rt of a Ph .D thesis by

the senior author. Sincere thanks are due to M yles Worsley, Greg g

M orrison, S ubhash Jaireth, Neil Phillips, Chris Cuff, Dick E ngland,

Mike Rubenach, a nd Pe ter Pollard for their valuable comm ents and

supports. Funding for the project was provided by the Austral ian

Mineral Industries Association (AMIRA), who and sponsor com-

panies are gratefully acknowledged. Additionally, the Cracow Min-

ing Venture is thanked for providing logist ical support during the

field work. This pa per has benefi ted greatly from the com ments and

editorial suggestions by T.M. Seward and one anonymous

Min

eralium Deposita reviewer.

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dong&zhou(1996)- zoning in carboniferous-permian cracow epithermal vein system (qld)

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