Issuu

Geological Society of Australia

ABSTRACTS Number 18

GEOLOGICAL SETTING OF

PRECIOUS METALS IN

NEW SOUTH WALES

One day symposium Sydney 1986

G E O L O G I C A L S O C I E T Y OF A U S T R A L I A ABSTRACT

SERIES

m J M B E R 18

GEOLOGICAL SETTING OF PRECIOUS METALS IN NEW SOUTH WALES

ONE D A Y S Y M P O S I U M U N I V E R S I T Y OP K E W SOUTH W A L E S SYDNEY O C T O B E R 1986

Organized by: - N e w South W a l e s D i v i s i o n of the G e o l o g i c a l Society of A u s t r a l i a - G e o l o g i c a l Survey of N e w South W a l e s - D e p a r t m e n t of Applied G e o l o g y , S c h o o l of M i n e s , U n i v e r s i t y of N e w South W a l e s

Conveners:

ISSN 0729-01 IX

Ian C l a r k e D.W. Suppel Peter L e w i s Ifti Q u r e s h i

PREaOUS

METAL

EXPLORATION

AND MINING IN N E W

Goto Mnnij and frosoecvrg Areas

SOUTH

WALES

COBTENTS FORWARD JOHN CHAPMAN Disseminated gold deposits along the Gilmore Suture. GEOFF CORDERY Epithermal alteration zonation at Peak Hill. L.B. GILLIGAN Precious metal mineralization in the New England region. LAWRENCE M. GLASER Geochemical zonation and source rocks for epithermal goldsilver mineralization, Eden-Yalwal Rift. BRIAN B. GUY The Mineral Hill gold occurrence. DIANA HALL Exploration geochemistry of the London-Victoria gold deposit at Parkes. CHRIS HORSFALL & ANDREW GABELL Remote sensing of alteration associated with precious metal deposits in New South Wales. K.G. MCQUEEN The Cowarra gold deposit - a granitic gold source? R. OVERTON The Sheahan-Grants gold deposit at Junction Reefs. D.W. SUPPEL & L.M. BARRON New data on the occurrence of platinum at Fifield. S.S. WEBSTER The geophysical signature of epithermal gold and other precious metal deposits in the Lachlan Fold Belt.

FOREWORD

The one day s y m p o s i u m on "The g e o l o g i c a l s e t t i n g of p r e c i o u s m e t a l s in N e w South Wales" w a s o r g a n i z e d to be held in c o n j u n c t i o n w i t h the 1986 J . J . P r a n k e l M e m o r i a l L e c t u r e , and the i n a u g u r a l dinner to m a r k the recent f o r m a t i o n of the U n i v e r s i t y of N e w S o u t h W a l e s S c h o o l of M i n e s . The J . J . P r a n k e l M e m o r i a l L e c t u r e w i l l be delivered by Dr R . W . H e n l e y of the B u r e a u of M i n e r a l R e s o u r c e s . Dr H e n l e y w i l l talk on "Modern g e o s c i e n c e as a guide to gold e x p l o r a t i o n " , and his lecture w i l l also serve as a very a p p r o p r i a t e k e y n o t e a d d r e s s for the s y m p o s i u m . P a p e r s on a v a r i e t y of topics p e r t a i n i n g to the g e o l o g y of p r e c i o u s m e t a l s in New South W a l e s w i l l be d e l i v e r e d at the s y m p o s i u m by invited a u t h o r s . To these a u t h o r s the c o n v e n e r s e x p r e s s their a p p r e c i a t i o n . The

conveners

also

thank Dr N . L . M a r k h a m , D i r e c t o r

the

G e o l o g i c a l S u r v e y of New S o u t h W a l e s for p r o v i d i n g s u p p o r t to the s y m p o s i u m . J a c q u e l i n e Cook and J o d i e T e r r y are thanked for their hard w o r k in h a n d l i n g r e g i s t r a t i o n s and p r o d u c i n g the a b s t r a c t s , and Tony N o r m a n for t a k i n g care of the s y m p o s i u m f i n a n c e s . S y m p o s i u m a b s t r a c t s are p r e s e n t e d here in a l p h a b e t i c a l o r d e r , in the form in w h i c h they were received from the a u t h o r s .

Ian C l a r k e 10th O c t o b e r , 1986

DISSEMINATED GOID DEPOSITS ALONG THE GILHORE

SUTURE

John Chapman G e o l o g i c a l S u r v e y of N e w S o u t h W a l e s The m o r e s i g n i f i c a n t of the n u m e r o u s gold d e p o s i t s in the W e s t W y a l o n g - T e m o r a - A d e l o n g d i s t r i c t o c c u r ^ l o s e to a L J o r t e c t o L c the G i l m o r e S u t u r e ( f i g u r e 1 ) . T h i s c o n s t ^ J t e r o n ^ o f sutu^r?rJ d e p o s i t s in N e w S o u t h W a l e s . Th^ boundpri hf? '^ by r e g i o n a l a e r o m a g n e t i o d a t a and f o r m s the boundary between geological provinces (terranes) that have different geological histories. R e c e n t e x p l o r a t i o n has l o c a t e d p r e v i o u s l y u n r e c o g n i s e d d i s s e m i n a t e d gold d e p o s i t s . M a n y of t h e L h a v e f p i t h e r m a l ^^ ^ epithe?mal g o ^ p r o v i n c e . The e p i t h e r m a l d e p o s i t s are h o s t e d by a n d e s i t i c O r d o v i c i a n to S i l u r i a n age w h i c h c r o j out as h i g h s u l f u r " t y p e and f o r m e d f r o m l a r g e h y d r o t h e r m a l s v s t e m s g e n e r a t e d by c o o l i n g m a g m a t i c i n t r u s i o n s . ® T h e s e d e p o s e s a l s ^ -PPer^Jy^i:^?'^^^^'^^^^'^^

^^^ ^ ^ ^ ^

^^^^^

of^orphyrj^^

T h e r e ^ ^ f . ^ h r e e m a i n c e n t r e s of 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 t the Y i d d a h p r o s p e c t a n o m a l o u s gold and w i d e s p r e a d low g r a d e c o p p e r m i n e r a l i z a t i o n are a s s o c i a t e d w i t h i n t e n s e phyllic alteration. - At the G i d g i n b u n g d e p o s i t an in s i t u r e s o u r c e of U t o n n e s g o l d and 59 t o n n e s of s i l v e r o c c u r s in e x t e n s i v e zone of a d v a n c e d argiiiic alteration. - At D o b r o y d e a l i n e a r zone of gold and b a r i t e m i n e r a l i z a t i o n is a s s o c i a t e d w i t h a r g i i i i c a l t e r a t i o n . O t h e r d i s s e m i n a t e d p r o s p e c t s a l o n g the s u t u r e g e n e r a l l y h a v e l e s s p r o m i n e n t a d v a n c f d a r g i i i i c Ind assemblages.

sulphate alteration mineral

L i t t l e is k n o w n a b o u t the n a t u r e of the u n d e r l y i n g i n t r u s i o n s r e s p o n s i b l e for the m i n e r a l i z e d h y d r o t h e r m a l s J s ? L s ? ^hercould a s s o c i a t e d w i t h the a n d e s i t i c ^ v o l c a n i c s t h a t h o s t the d e p o s i t s . A l t e r n a t i v e l y , t h e y could be I - t y p e g r a n i t e s w h o s e e m p l a c e m e n t w e r e c o n t r o l l e d by d e e p p e n e t r a t i n g s t r u c t u r e s a s s o c i a t e d w i t h the s u t u r e . A l i a r ^ ' w h e T ^ o t . ' " ? an I - t y p e p a n i t e o r i g i n o c c u r s at M o u n t A d r a h , w h e r e gold is d i s s e m i n a t e d w i t h i n a s m a l l a l t e r e d

porphyritic biotite granite stock.

Development ^

ofmagnetite

ext^L'^e

thermal

Fig.l-

Gold deposits of ihe West Wyalong-Temora-Adelong Disfrict

EPITHERMAL ALTERATION ZONATION AT PEAK HILL, N.S.W.

Geoff Cordery Alkane Exploration N.L.

INTRODUCTION A well developed epithermal alteration system is associated with disseminated gold mineralisation at Peak Hill in central New South Wales. The system is developed immediately east of Peak Hill township, located on the Newell Highway, 43km north of Parkes and 300km north west of Sydney. The district has been an important gold producer, with a total recorded output of 3238kg from intermittent mining between 1888 and 1947 (de Ferranti 1968 and Bowman et al 1982). Published resource estimates are 1.9 million t at 2.1g Au/t, open pitable, with considerable potential for additional substantial underground resources. Regionallly, Peak Hill lies near the eastern boundary of the Bogan Gate Synclinorial Zone of Scheibner (1972) and within a broad arcuate belt of deformed Palaeozoic sediments and volcanics developed east of the Goonumbla volcanic massif. Host rocks include conglomeratic andesitic wackes (the Peak Hill Volcanics of Krynen et al (1986) interpreted to be mass-flow deposits accumulated at the base of the fore-arc slope of the Goonumbla volcanic complex, and the enveloping flyschoid tuffaceous/lithic siltstones and sandstones of the Cotton Formation. The age of these rocks is Late Ordovician to Early Silurian (Krynen et al, 1986). Deformation within the belt peripheral to the Goonumbla volcanic massif is irregular, with zones of tight folding and meridional shearing being separated by large areas of relatively undeformed rocks. At least three periods of deformation can be identified in the Peak Hill area. The main alteration zone is d e v e l o p e d within a major meridional shear zone coincident with an antiformal hinge zone and an andesitic conglomeratic wacke - siltstone contact. Alteration appears to have occured after initial deformation, though was accompanied by shearing, the sense of which is unknown. Zones of more intense alteration, breccia pipe formation and gold mineralisation, are localised at the intersections of regionally important northwesterly trending fractures (defined by landsat, drainage and aeromagnetic lineaments) and the main shear. A Late Silurian age is suggested for the main alteration event, on the basis of limited structural evidence and by analogy with the Goonumbla porphyry copper mineralisation (Jones 1985) and Gidginbung gold mineralisation (Thompson et at, 1986), both having some similarities with Peak Hill. The system has been subjected to only weak post alteration deformation and is still in its original orientation. Strongly metasomatised rocks crop out in a roughly elliptical zone 120Qm long and up to 600m wide. The main alteration zone is coincident with a broad aeromagnetic low surrounded by a higher rim, reflecting the destruction of magnetite within the altered area, and its redistribution in peripheral less altered zones (see below).

appear to be gradational and partly fault controlled. This paper is based on the results of a detailed inineralogical

for each alteration zone as defined by mineralogy. 400 samples were used in the study.

A total of over

ALTERATION ZONING A total of twelve lateral and verlcal zones are developed ceriphe?at to a majo? fracture controlled conduit. The conduit is

of alteration Table I g i v e s details of the mineralogies of each «nd FiSire 2 displays geochemical trends (note that " r e ; n : S h i r t n n Figure Z^s'equivalent to original lithology described below). ORIGINAL LITHOLOGY Pre-alteration lithologies in the study area are dominated by andesUic conglomeratic wackes. These rocks are typically poorly sorted con?aining rounded to sub-angular pebbles and cobbles of Dlaaioclase-pyrCKene-phyric volcanics and pyroclastics set in a

r ilt:

SjCiB-iJ^^^

siltstone and lithic sandstone are locally important. , plagioclLe-pyroxene phyric andesites crop out immediately v^^^L of the main JlteJation z o n L A regional geochemical study of volcanics ^n the P a r k e r a r e r b y Clarke (1985) showed that most Ordovlcian s series. J l S ^ L r r S c k s belongeS to a potassium-rich, shoshonitic magma Reqional greenschist f a d e s metamorphism has resulted in a chlo?ite-sodic plagioclase-sericite-epidote-actinolite-carbonatemagnetite assemblage. Small quart..-carbonate present and the rock is generally uncleaved. ihis 'f .. . pre-dates the main hydrothermal alteration event and the greenschist asLmblage is consid^ed to represent the pre-alteration mineralogy.

There is also a weak s t r u c t u r a l change in the sericite with d e p t h , which is the subject of f u r t h e r w o r k . This d e e p phyllic assemblage is termed sub-zone c . The p h y l l i c - b sub-zone passes into the a d v a n c e d argillic zone near the s u r f a c e . Advanced A r q l l l l c Zone An a c i d - l e a c h e d m i n e r a l assemblage is t y p i c a l of this z o n e . Pyrophyllite is d i a g n o s t i c and varying proportions of q u a r t z , b a r i t e , a l u n i t e , jarosite and sericite w i t h m i n o r kaolinite make up this zone. The rocks are o f t e n b r e c c i a t e d and have a leached, porous a p p e a r a n c e . T o t a l S i , A l and K are highly c o n c e n t r a t e d together w i t h anomalous Ba and F e . A l l o t h e r elements are strongly d e p l e t e d . The a d v a n c e d argillic assemblage does not exist at depth on the section studied in d e t a i l , b u t is r e s t i c t e d to the s u r f a c e , e s p e c i a l l y adjacent to m i n e r a l i s e d breccia pipes and s t r u c t u r e s , pyrophyllite has been i d e n t i f i e d at depth ( - 2 5 0 m ) in deep d r i l l hole O P H - 1 (Faulkner, 1977) where it occurs witli p y r l t e , quartz and barite. Silicic Zones The silicic zones have the simplest m i n e r a l o g i e s and r e p r e s e n t the sites of m o s t intense h y d r o t h e r m a l a c t i v i t y . They are d o m i n a t e d by q u a r t z , pyrite and barite and coimnonly c o n t a i n economic concentrations of g o l d . On the section studied in d e t a i l , two sub-zones are i d e n t i f i e d . The silicic-a sub-zone is e s s e n t i a l l y an open space filling vein composed of quartz-^barite and p y r i t e , and accompanied by intense silicification of the imiT\ediate w a l l r o c k s . Quartz from this zone is a cherty m i c r o c r y s t a l l i n e v a r i e t y a n d is coiiunonly v u g h y . Barite constitutes 5% to 30% of the zone and forms irregular m a s s e s , b l a d e d crystalline intergrowths w i t h m i c r o c r y s t a l l i n e quartz and c r y s t a l l i n e vugh l i n i n g s . L e s s e r alunite is commonly associated with the barite and forms late v u g h f i l l i n g s . Pyrite is typically very fine g r a i n e d (less than 0.1mm) and d i s s e m i n a t e d . Where m o r e coarsely c r y s t a l l i n e it has a p y r i t o h e d r a l f o r m . Enargite is coimnon in this z o n e , forming sooty coatings on^ an inclusions in p r y i t e . Native g o l d , a g o l d teluride, t e n n a n t i t e , d i g e n i t e and b l a u b l e i b e n d e r covellite also o c c u r in this z o n e . This assemblage gives the silicic-a sub-zone a destinctive g e o c h e m i s t r y , high in B a , S , F e , A u , A g , T e , C u , Mo and A s . The a l k a l i m e t a l s and A l are strongly d e p l e t e d as clays are almost n o n - e x i s t e n t h e r e . T h i s 'vein' passes v e r t i c a l l y upwards into conical breccia pipes. The s i l i c i c - b sub-zone c o n t a i n s up to 10% sericite and is e s s e n t i a l l y a m i n e r a l i s e d and highly s i l i c i f i e d variety of the p h y l l i c - b z o n e . O p e n space textures are r a r e , and f r a c t u r i n g , brecciation and ghosts of s c h i s t o s i t y are c o m m o n . Barite and alunite are u n c o m m o n and d e c r e a s e in abundance w i t h d e p t h , though q u a r t z - b a r i t e - a l u n i t e veins m a y be locally i m p o r t a n t . Possible adularia has been found in this z o n e . G e o c h e m i c a i l y this sub-zone is d i s t i n c t from both the s i l i c i c - a and p h y l l i c - b z o n e , containing e l e v a t e d K , A l , F e , A u (4ppm), S b , A s , T l and Ag values and r e d u c e d Cu.

propyl I hie Zone This outermost .one characterised b plagioclase by sericite and ^he appearance of Y P J ^ ^ ,magnetite L V a n h y d r i t e reiningIjon oxide, in^ ^^ this zone though and primary haematite Deformation is minimal with when it occurs it has ^ °^^ porphyritic textures being primary bedding, ^^^f'^flf^ ^one has high concentrations of Mg preserved. The P f " P ' ^ ^ f L m p L i t i o n ) and dramatically elements enriched In this i^one. t r r i n U n Zones

The a r g l l l l o zone I s c h a r a c t e r i s e d b , a Paragcnite assemblage and the high Na o o L e n t r a t l o n s 1 , with is the dominant ciay ( " " ^ c t e d by high ^

stronger, with nearly

Iiti'-Aiir.t ^/t^s The .one is divided into t - s u b ™ content. The JJ"tic-rsXlone

bSf-iost.

-.ran^^^^Iofit^bi^only Approximately 5% £lne

and more mafic lithic blasts. background levels in the hPGome important. Zinc and hv a factor of two (together a r g l u i c - a zone, though ^re concentrated bV - ^ ^ ^ ^^^^ ^ ^ g ^ i u c - b

with Sb and AS) in the

jlJi^^-^.^^'i^rgeochemistry are noted as a

PhylTic Zones chlorite has totally f f ^ P P ^ ^ ^ J ^ ; t h ^ g e n e r a i composition and includes a group of similar hyLomuscovite. The rocks structure of f ^ ' ^ ' V h a v e lost all primary textures. ?^rrti%ro.rthrs rolirryricJuy'Trs a p y n t o h e d r a l habit. The phyilic .one has ^^en sub-divided into^ t^^^ on the b L i s of quartz and^sulphate ^^^ It has BUb-.one approximately 35 6 guar ^^ anomalous in Au , c

a high concentration 4,, ca, Na and Zn (Figure 3). (O.Sppm), Cu and Te and is depleted in Ca ^^^^^ quart, to ihe inner, phyllic-b ^^^-zone is marked ^y ^ ^^^^ ^ ^ ^ the around 60%, a consequential decrease in sulphates widespread of barite a^^^ and with the occur as veins, disseminations ana p f x Brecciation and paragenetic sequence ^^ff^^z-baiite aiunite^^^ ^^ ^^ similar fracturing are i" ^''^Luqh co ^^ining higher concentrations of o

So^'^:

reduced K, Ai and Tl.

l i t h i n the phyllic-b TesfioLonî^S rrrîeôfSiL^por^lound

. higher up disappears.

oxidised gpixe The oxidised-2one at Peak Mill averages 40in thick, but locally ' ^ ^^ highly variable due to pei-meablllty dlliferences between ^^ silicifled and b r e c c l a t U rocks. Its .nlneralogy and g-^j^'j^'j'i^try are largely dependant on the primary alteration t p e . Hydrous iron o x i L s ate coimion replacing pyrlte and Jarosite ff . . secondary barite are conuiion at an Intermediate deptli in Llie profile but rare at tlie surface. Supergene enrichment of the more mobile elements i®, • ^ near the base of- oxidation. Gold, Ag, Mo, Cu, Sb As Hg and Ee are all enriched at the water table, though concentration factors are not kiiown due to lack of data. XABbE 1

RssEinriAfj

MinRRAiiOoy COllMOll

ACCESROlU

Iron oxldeg G ph»tie opablte caibonabe quar 17i car b'^na be cittfiydt I be ieUGoxeiie

original ilhhoiogy

sprJlc-plfigloalaee chioribe

Ber.lclbe epidobp Bcbiiiolibe

Ptopylltlc

chlcr1 he serIcite

pofrayniil be kaollnibr? Iton oxides

Atgillla-a

pn taaotilte kaoliiilte

chit??:! be ent ic^ibe quarbR

Argillio-b

iifjr97oulte kaollnlte

cliior J te aeticlbe quar br. pytibe

Phylllc-tt

BRtlclfce quarts pvtite gntlclts

pa be kaoiiiiibe

1ron-oHide tublle

barribe aluiiibe

iiiibe diaapote

bar 1 VP a lulli bo

Phylilc-b

Iron oKjde caribou? be nnbydrIbe pytitG ltoi» oKjde Bnliydt Ibe

pvrite 'fierlclbe' llhe mica rUKU tR pvtita qiiath?! pyirlbe barite q\iatfcr. pytibe

alunlbe setIcibe

enargibe qoid

net Id be batibe

pVrnp!»y ilibe 0 lunlb? aduiatia?

Advanced Atgilllo

pytophyillbe quat bz

nlunlbe bairib« Jntosibe

haolJnibe pyirlb? n ? r 1 r? 1 h e diaspote Uate)

CiKldieed

qyarb^

Phylllo-a eilicia-a Bllloia-b

pyrnphylllbe tubi hnrll? qoynr.lbe ? waveliibe Itoti nKlde J cvtoolbcj dependaiib on ptliuary iniiiBtalogy

REb'ERIiltlCliJS

Bownmii, H.N., Richardson, S.J., and Dolanski, J., 1982

Clarke, I., 1985

Naromine 1:250,000 Metallogenia Map SI 55-3, Mine Data Sheets and Metallogenlc Study P e t r o l o g y of I g n e o u s rocks a s s o c i a t e d v/itli gold and o o p p e r m i n e r a l i z a t i o n in the P a r k e s area. G e o l o g i c a l S u r v e y of HSW

Report, GS 1905/121 (unpubl.l de Ferranti, R.Z., 1968

Dowling, J., Cordery, G., Larson, R., Raffan, N., and Sheppard, N., 1983

Faulkner, J.W., 1977

Jones, G.J., 1985

B'inai report on MEL22, Peak Hill, HSW, for Anaconda Inc. NSW Dept. Mill. Resources Report GS 1968/271 (unpubl.) Report on Li'.xpioration of Ii:Ll'164, Peak Hill for six months to October, 198 3, MSW Dept. Min. Resources Report GS 1903/371 (unpubl.) Report on EKploration activities at Peak Hill, Hev/ Soutli Wales for the first twelve months ended 2nd September, 1977. J/V between Occidental Minerals Corporation of Australia and Frio Mining & Exploration Pty. Limited (EL'S 907, 908). MSW Dept. Min. Resources Report GS 1978/090 (unpubl.) The Goonumbia porpliyry copper deposits, Nev; South Wales. Economic Geology,

80, pp.

591-

613. Krynen, J.P., Clarke, 1., and Sherv/in, L. , 1986

Schelbner, E., 1972

Thompson J.F.H., Lessman, J and Thompson, A.J.B., 1986

Geological setting of gold and oopper mineralization in the Parkes area. Geol. Surv. of HSW Report GS 1906/059 (unpubl.) Tectonic concepts and tectonic mapping• Rec. Geol. Surv. HSW, 14(1), 37-03, 5 figs. The Temora Gold-Silver deposit! A newly recognized style of high sulfur mineralization in the Lower Paleozoic ol: Australia. Economic Geology 01, pp. 7 3 27 30

rj(juiu3 I

" Andfl Iflfc " pyf ocloiritrl, C|»5?lo« liGI—A -•"•not llll tor»glofntrol«

brtltletl tul (cK f OMI inJlincMll

SKETCH

MAP

PEAK HILL L6G H MD

N.S.W.

Aieo ol ilfong o»pMllc oIlefQllon WorVjiiQi Deep (jiortiond tlrilliiolri SifiVe & dip ol loliallon 100

200 m

SlilW A fJlp ol bfJJing Bo / s Jc^c^n, A

Dorium yeocltrmicol onomaly Appfon. lock Irpt boonJoff Mo/of tliuclu'al IftnJl topogJoplWc conlouf reol» Mill Itig. ilolion

/ ... h\ '

? ? !1 i}

t • ! ^ ?

I 1 I 1 i

.. V

Vi.li::t

V N

—•

iIIII Q

shallow

! Ii O

tilleniKTcllale

1 I ( ( I. I ^

deep

PRECIOUS METAL MINERALIZATION

IN THE NEW ENGLAND REGION

L.B. Gilligan Geological Survey Of New South Wales Department of Mineral Resources P.O. Box 5288, Sydney, N.S.W.

Well in excess of 50 tonnes of gold have been produced from the New England Fold Belt in New South Wales. Table 1 lists the major gold deposit associations recognised together with their age and recorded production from both hard-rock and associated placer sources.

TABLE 1 indigenous

a. Devono-Carboniferous b. Late Carboniferous

2.3t

Pre-accretionary

Magmatic-related

a. Permo-Carboniferous (S-type) b• Permo-Triassic (Pos t-orogenic)

-^91

Epithermal

Middle Permian

2.5t

Serpentinite-related

Permian

1.5t

Tamworth Belt

Middle Permian

Metahydrothermal

Permo-Triassic

271

Figure 1 shows the areas of significant gold mineralization in the New England Fold Belt and table 2 lists the major gold deposit types together with their interpreted association and relevant geological parameters.

PRE-ACCRETIONARY

Three accretionary prism complexes have been recognised in the New England Fold Belt, viz. the pre-Early Devonian Woolomin Formation (Cawood 1982), the Devono-Carboniferous '^Sandon Association" (Korsch 1977) (Cockburn Formation and Sandon beds) and the Carboniferous accretionary prism rocks oE the Coffs Harbour Block (Fergusson 1982). Gold mineralization occurs in the latter two complexes. The "Sandon Association", comprising turbidites, chert and minor mafic volcanics, hosts stratabound cupriferous-pyrite deposits, stratabound manganese deposits, and auriferous chert and quartz-vein deposits. There is a common areal association between the manganese occurrences and the gold deposits (eg. Limbri, Niangala and ?Cells Creek). The Carboniferous rocks of the southern Coffs Harbour Block host: Au-quartz vein deposits; stratiform auriferous, chert ?exhalites; stratabound cupriferous pyrite and; quartz-magnetite horizons. (Dalmorton and CorambaOrara). The Au-quartz vein deposits in both the "Sandon Association" and Coffs Harbour Block are interpreted to have formed through remobi1ization from stratabound mineralization.

SIGNIFICANT GOLD NEW ENGLAND P , P , , „ P

OCCURRENCES FOLD BELT .

BEENI.EIGII

B L O C K

f f a

G n E A T A U S T R A L I A N B A S I N

HEFEnEMCE MtTAHYDnOlMEnMAl B t H P I M T I N I t E - RELAtEO pnC'ACcnETioMAnv IHDiaCHOUS MAOMAffCOElATeO KPITMEnMAt lAMWOPlM DEPOSITS

SCALE

BELT

PtnMo-cAnnoMiPEnous OnAMITOID IS TYPE) PEnMO-iniASSIC iPOBf.OnoOEMICI

Sii

TibttJ

ORAMIIOID

fof n i t n i i

[T7~

occwnenc«s

H B r b o u r

TABLE 2 MAJOR GOLD DEPOSIT TTPES, HEW EHCLAHD FOLD BELT

Age

Assoca-aLion*

Deposxc cype(aj

Geoiogicai Secrxng

Hose rocks

examples

Valla C16*)

Triassic

Vein Au-Ab

Post-orogenic? I—type coastal granitoids

Valla Adamellite

Triassic

Vein and disseminaced Au in granite

Post-orogenic magmatism (Leucoadamellite Suite)

Poverty Point (2) Stanthorpe Adamellite Oban River Leucoadamellite Oban (6) Glen Elgin (4) Kingsgate Granite

P eraiac-Trias s ic

?Me cahydro thermal vein and replacement Au ± Sb W)

Major fracture and faultcontrolled

Range of host rocks

Hillgrove (17) Nundle (23) Kookabookra ( 7 ) Enmore-Melrose (IB)

Middle Permian

Vein Ag-Au-Aa-Sb Large derived Recent placer and Tertiary deep lead deposits

Post-orogenic magmatism, Uralla Plutonic Suite (IS-type granitoids)

Oralla Granodiorite, Tilbuster Granodiorite

Uraila (19) Tilbuster (15)

iliddle Permian

Vein Au (± Sb)

Surrounding post-orogenic granitoid (Barrington Granodiorite)

Tamworth and Parry Groups

Dpper-Hunter (24) Copeland-Barrington (24)

Hiddle Permian

Epithermal vein, replacement and stockwork Au-Ag

Shallow marine - terrestrial volcanism

Brake Volcanics

Brake ( 1)

Middle Permian

Vein Au-3b

Post-orogenic magmatism (?M-type) Clarence River Plutonic Suite

Dumbudgery Creek Granodiorite

Lionsville-Solferino ( 3)

Early-Middle P ermian

Vein Au

Peel Fault System

Serpentinite and adjacent rocks

Upper Bingara (12) Bingara (11) Woodsreef (13) Crow Mountain (14)

P cmo—Carboniferous

Vein Ag-Au-As-Sb

Hillgrove Plutonic Suite S-cype granitoids

Rockvale Adamellite, Dundurrabin Granodiorite

Rockvale (10) Dundurrabin (19)

Lace Carboniferous

Indigenous mineralization auriferous chert, stratabound, copper, remobilized Au-quartz veins

Accretionary prism rocks

Coffs Harbour Block

Coramba-Orara ( 8 ) Balmorton ( 5)

Devono—Carboniferous

Indigenous mineralization : auriferous, chert (?exhalite) proximaLly related to stratiform nanganese, remobilized Au-quartz ^ veins

Accretionary prism rocks

Sandon beds, Cockbum Formation

Limbri (20) Niangala (21) ?Cells Creek (22)

MAGHATIC-RELATED

Gold .LneraU..atioa appears to be genetically related to both PermoCarbonfferous S-type granitoids and P " n . o - T r l a s s i c post-ocogenrc gra^ The following plutonic suite terminology is after Shaw and Flood (1981). There is a clustering of Au (±Sb) deposits about several HiUgrove P l u t o n i r Suite granitoids (Penno-Carboniferous) but only xn respect of the R k le Ada.nellite and Dundurrabin Granodorite is there a ^ ° a genetic association. The most important occurrence associated "^th the Rockvale Adamellite is the Comet gold mine, a Au-pyrite-pyrrhot.te quart, vexn on the margin of the granite. The

Middle

Permian

Uralla

Plutonic

Suite

(post-orogenic)

the

most

important granitoid suite in New England in terms J-^^J^h;to n hhP llralla area, the Uralla Granodiorite and an afLiliated stock ^Khatoun ^ o n a l U e ) h a ^ r ^ l a t e d Au(±Sb) vein deposits. More importantly, ^^^ J r o f gold have been produced from deep lead and younger placer deposits at u L l l a and this gold is presumably derived from hard rock sources re ated to thfgranitoids. In the Tilbus ter-Puddledock area, near Armidale, similar vein deposits are distributed about the Tilbuster Granodiorite. Gold is also associated with Leucoadamellites in central and the K

^sg^'t " c r -

t n r r u b s r a n t i a l disseminated Au occurs in the Stanthorpe Adamellite at Poverty Point. A small northerly-trending group of granitoids developed along the coast between Kempsey and Coffs Harbour, although characterised by "o-Ag-base iTtlTs, Z a small A u M s deposit at Valla genetically associated with the Valla Adamellite.

EPITHERMAL

The only well established locality of epithermal gold-silver mineralization is at Drake. Here Middle Permian andesites the Drake Volcanics host fissure-type replacement mineralization. The two best examples of this type of mineralization are the w i i t r R o c k Ag-dominant stockwork and the Lady Hampden Au-dominant stratabound infiltration deposit.

SERPENTINITE-RELATEU

A number of small gold fields are distributed along "^t'^ern part of the Peel Fault System. The quartz vein deposits are hosted by serpentinite and adjacent rocks. The origin of such deposits is not known.

TAMWORTU BELT

Two substantial gold fields occur in the southern extension of the Tamworth Belt (forearc basin) viz. Upper Hunter and Gopeland-Barrington gold fields. These gold fields lie to the west and east of the Barrington lops Granodiorite respectively. The occurrences are simple quartz vein or quartzbreccia vein deposits which are joint, fault or bedding-controlled.

In the Upper Hunter gold field major concentrations of Au~quartz veins correspond with vitrinite reflectance anomalies in the host sediments (ie. Stewarts Brook and Omadale Brook areas) (Baker 1931). Minor Sb+Au mineralization lies between these above two concentrations in an area of lower metamorphic grade. In the Copeland-Barrington gold field the gold deposits are restricted to fault-bounded blocks of Devonian sediments. In the eastern block near Copeland the distribution of vein deposits reflects stratigraphy and the largest deposits are fault-controlled occurrences in the hinge area of a regional south-plunging anticline. The origin of the gold mineralization is not known although there may be a genetic relationship with the Barrington Tops Granodiorite. This granitoid may have generated a thermal flux to facilitate leaching of gold out of the host sediments. Sampling of the sediments has revealed anomalous gold concentrations.

METAHYDROTHERMAL

The bulk of New England gold production, both historical and contemporary, is from a poorly-understood association of Au+Sb(±W) vein deposits at Hillgrove and Nundle. The gold deposits at Kookabookra and Enmore-Melrose may be of similar genesis. The Hillgrove, Kookabookra, and Enmore-Melrose occurrences form clusters in or adjacent to Hillgrove Plutonic Suite granitoids. These granitoids have been subjected to regional ductile shearing prior to mineralization and zones of more intense shearing (mylonite zones) have focussed mineralization at Kookabookra, Enmore-Melrose and at some of the Hillgrove deposits. Despite the proximal relationship to S-type granitoids, however, no genetic relationship is argued. The origin of the deposits is probably intimately related to the origin of much of the antimony-only mineralization in central New England. The mineralization may be related to major tectonic processes (e.g. thrusting) with major fracture and shear zones focussing metahydrothermal solutions into the upper crust. The Nundle mineralization is restricted to the eastern boundary of the Tamworth Belt adjacent to the Peel Fault System. No satisfactory explanation for this mineralization has yet been proposed although the proximity of the mineralization to the Peel Fault System may suggest that this fault zone focussed ?metahydrothermal solutions from deeper in the crust. ACKNOWLEDGEMENT

This paper is published with the permission Wales Department of Mineral Resources.

the Secretary, New South

rocks.

University of Newcastle

REFERENCES

Baker, C.K., 1983. Phytoclasts in metamorphic - Ph. D. Thesis (unpubl.).

Fergusson, C.L., 1982. An ancient accretionary terrain in eastern New England - evidence from the Coffs Harbour Block jji NEW ENGLAND GEOLOGY, P.G. Flood and Bruce Runnegar (eds), 63-70, Department of Geology, University of New England and AHV Club, Armidale, N.S.W.

Korsch, R.J,, 1977. A framework for the Palaeozoic geology of the southern part of the New England Geosyncline. Geological Society of Australia Journal 25(6), 339-355. Cawood, P.A., 1982. Structural relations in the subduction complex of the Palaeozoic New England Fold Belt, Eastern Australia. Journal of Geology 90, 381-392. Shaw, S.E., and Flood, R.H., 1981. The Australia: geochemical variations in Geophysical Research 86, 10530-10544.

New England Batholith, Eastern time and space. Journal of

GEOCHEIilCAL ZONATION OF, AND SOURCE ROCKS FOR, EPITHERMAL GOLD-SILVER MINERALIZATION, EDEN-COMERONG-YALWAL RIFT, N.S.W., A U S T R A L I A Lawrence M Glaser Dept. of Geology, University of Melbourne, Parkville, Victoria, 3 0 5 2 Australia

INTRODUCTION AND GEOLOGY

The

During investigations on the genesis of epithermal gold-silver mineral deposits within the

Eden-Comerong-Yalwal

Rift,

became

apparent that despite the physical similarities between the deposits, each displayed a diagnostic geochemical signature which correlated with a depth-temperature profile ctetermlned through fluid inclusion analyses.

Detailed geochemical

studies were commenced to better quantify the chemical zonation pattern displayed within this suite of ^netically related epithermal deposits. In conjunction with these studies, a geochemical

investigation

was commenced

examine possible source rock gold-silver mineralization.

Figure 1. Locality Diagram

Australia

lithologies for

epithermal

gold-silver

deposits

occur within the mid to late Devonian ( 3 7 0 my) Eden-Comerong-Yalwal Rift displayed in Figure 1. Ore genesis is related to extensive intra-rift volcanism which produced a 1 to 2 km. thick bimodal sequence of peraluminous rhyolites and continental to transitional tholeiitic basalts. Intra-rift mineralization consists of the Grassy Gully and Yalwal goldfields in the north, and

the

Pambula,

gjldfields

Wolumla,

in the south.

and

Four

deposits are also known, all

Sugarloaf

pyrophyllite

of which

are

proximally located to the Pambula and Sugarloaf goldfields.

All

mineralization occurs within

extrusive rhyolites proximal to vent features. The style of mineralization is deep epithermal in character,

consisting

high

angle

fault

controlled rhyolite breccias which grade laterally into

pyritiferous

ore

bearing

chalcedonic

stockwork systems. Alteration associated with mineralized systems is typified by: 1) silica flooding with the development

replacement

silicification,

chalcedonic

pyrophyllization

and

veining

and

pyritization,

sericitization,

hematite alteration of pre-existing

pyritized

areas within the boiling zone.

GEOCHEMICAL ENRICHMENT-DEPLETION A suite of high grade ore samples of the 200

S. P a c i f i c

sulphide zones

from

each of the

Pambula,

F i c j u r e

E L E M E N T E N R I C H M E N T - D E P L E T I O N D I A G R A M FOR P A M B U L A - Y A L W A L - G R A S S Y GULLY - W O L U M L A G Q L D F I E L D S W h o l e R o c k O r s G e o c h e m i a t r y N o r m a M z e d to U n m i n e r a l i z e d R h y o l l t e H o s t

Rock

R a n g e in e l e m e n t c o n c e n t r a t i o n s w i t h i n s a m p l e s e t a s e n r i c h m e n t - d e p l e t i o n f a c t o r s -

M e a n v a l u e s of u n m i n e r a i l z e d r h y o l i t e h o s t r o c k s a m p l e s e t ^

u s e d for c a l c u l a t i n g o r e s a m p l e s e t e n r i c h m e n t - d e o l e t i o n f a c t o r s r M a x i m u m v a l u e in s a m p l e set j M e a n value

f - T t ^ I

DATA F O R E A C H G O L O F I E L D A R E

^Minimum value

P R E S E N T E D IM T H E F O L L O W I N G 'AU'

SEQUENCE

• " A - T

l r

I l*hro4ll*

T•^•*l

Wotimlt

T h i s o r d e r of p r e a e n l a l i o n m o v e s f r o m l o w e s t

temperature

of m i n e r a l i z a t i o n on ( h e l e f t ( P a m b u l a ) , to h i g h e s t

temperature

( V / o l u m l a ) o n the right as d e f i n e d by fluid inclusion a n a l y s e s

ilx

A m e a n v a l u e w h i c h p l o t s o n t h e • l O ' x fine i n d i c a t e s t h a t t h e a v e r a g e s a m p l e f r o m t h a t g o l d l l e l d h a s a t e n t i m e s h i g h e r c o n c e n t r a t i o n of that e l e m e n t vs.

the a v e r a g e unminerailzed rhyolite host rock

Mfl

i-iI 1

•

i Im.'.

nil

ttii 1

Wolumla, Orossy 6ully, and Yalwal mines were

Figure 3. Silica Calcite SolubilUy

analyses for 4 2 elements via R.NAA., X.R.F., I.C.P.Q., and A A S . obtained

for

Identical analyses were then

barren

rhyolites

distal

mineralized areas within the volcanic pile to document

normative

values.

This

data

displayed In Figure 2. The sequence of presentation deposit

geochemistry

reflects

the

of ore depth-

temperature profile from fluid inclusion analyses of these same samples.

Pambula Is the lowest

temperature and the shallowest deposit with temperatures between 300® and 320® and a maximum depth of 1200 metres.

Ca data, (Ellis. 1959. Am.J.Sci. 257, p 3 5 ^ - 6 5 . ) Si data, (Holland el.al, 1979. Geo. Hydro. Ore Dep.)

Because the The enrichment trend for Cu also reflects

fluids have boiled, the depth has been caluculated based upon the critical temperature of water with

the

temperature-depth

ameasuredsallnlty of 12 to 15wt.5l5 NaCl equlv.

goldfields, with the greatest enrichment In the

profile

the

four

Moving to the Wolumla goldfield on the right, the

deepest and highest temperature deposit.

measured temperatures average 3 8 0 ° with a

Gold and Ag are the most strongly enriched

maximum depth of 1600 metres. Grassy Gully

elements in all four deposits although the levels of

and Yalwal

are. Intermediate

and

enrichment represent the high grade nature of the

temperature.

Variations in the geochemistries

ore samples used. Despite possible ore mineral

depth

between the deposits are Interpreted as a function

inhomogeneities between samples, Ag enrichment

of variations In element solubilities under these

does

different P , T , conditions.

Pambula deposit

Major

element

enrichment-depletion

increase

zones

replacements.

the

lowest

temperature

to the

higher

temperature

Wolumla deposit. Sulphur and Se are enriched in all four

profiles are variable. Silica is enriched in the ore

from

and

deposits. Although no clear enrichment trends are

Aluminum, Ca, Mn, and Na are

evident, they do display remarkable S : Se ratio

all

deposits

veining

depleted in all deposits, with P, K, and Fe depleted

trends currently under stud/. The deposits ere

in three of the four goldfields.

strongly enriched in Se relative to most other

The depletion trend observed for Ca best exemplifies

the

variations

temperature

between each of the four goldfields. These trends

eplthermal

deposits.

This

enrichment

attributable to a major input from the associated volcanic pile.

can be interpreted in terms of the relative

Arsenic is enriched in all four goldfields

solubilities of silica end carbonate as shown in

(Fig.

Figure 3.

temperature-depth profile is evident.

2),

altiiough

clear

trend

with

It is evident from Figure 3 that the

All of the goldfields are slightly depleted

solubility trends for SI and calcite as functions of

in Zn, Cd, and Mo, although the mean depletion

temperature are Inversely related. In comparing

values are within the max.-mln. range for barren

the Ca solubility trend with the Ca depletion trend

rhyolites. Chromium is significantly enriched in

observed for the goldfields (Fig. 2), a clear

all

correlation

attributable to a geochemical input from the mafic

is evident

between

levels of Ca

depletion, calcite solubility, and temperature.

four ore systems.

This

enrichment

members of the volcanic pile. The

trend

characterized

increasing depletion with increasing temperature.

Thorium, Nd, La, Oa, Nb, and Be display Ihls deplelion Irend.

concentrations In the rhyollle which are unrelated

CORRELATION COEFFICIENTS

enrichment-depletion trends, with fluid inclusion

to mineralization, and thus, no correlation exists. Integration of the correlalion coefficient data and analyses, yields a classical model for base and

Correlation obtained for

geochemical

have data

been

for

precious metals zonation.

each

With Ihe integration of fluid inclusion

The salient Information has been

analyses, we can quantify the exact temperature

extracted from these matrices and is presented in

and depth of each deposit to better define the

goldfield.

all

coefficients

Table 1.

precious-base metal de-coupling scenario.

can define the development of a significant

Table 1. Correlation Coefficients of ores from Pambula, Yalwal, and Grassy Oully EllllSIIRRRIlBflBffl A u / Aq

H.O

Elemtnls

1 Pombulsl Yulwol I Wolumla

Au / Cu

-.39

A u / Pb ;

-.70

-.25

•.43

Ag / Pb

-.70

-.24

•.52

M 5

^56

•.72

Ag/5

-.56

-.51

•.93

Au/Se

^86

Ag/Cu

-.41

•.12

•.98

^15

^56

•.72

Aq/Se

•.17 1 • . 5 9

•.88

5/Se

-.72

*.04

•.95

5e/Cu

-.32

-.07

•.96

5/CI

^78

-.5J

-.60

S e / Pb

-.43

-.07

•.60

Au / A s 1 •.16

suggest that genetically

identical

epithermal

deposits can have radical differences in trace

•.89

Au / 5

M 2

geochemical zonation in mineralization over a vertical depth of 300 to 4 0 0 metres. This data

element geochemistries over

a very

narrow

temperature-depth range.

ELEMENT RATIOS The most apparent feature of all four

1 +.46 1 ^ 2 0

ratios displayed in Figure 4 is the general Gold and Ag are significantly correlated in

conformity in ratio trends between each of the

all three deposits. The correlations between the

four goldflelds. These ratios reflect the trends In

precious and base metals display a remarkably

temperature-depth profiles as indicated by fluid

clear trend which demonstrates the variation in

inclusion data. The large range in values within

the behaviour of these two groups of elements

individual deposits are attributable to sample

with variations in the depth-temperature profile

heterogeneity.

for mineralization. Note that for the deepest and

eg. electrum, have relatively narrow ranges in

highest temperature deposit, Wolumla, there are

composition.

consistently between

the

significant precious

positive and

correlations base

melals,

particularly Cu. At Pambula, these correlations are almost reversed to negative correlations. Yalwal represents an intermediate zone between the two. This data indicates that in the higher temperature deeper zones, precious metals and base metals are all transported and precipitated under similar conditions, l-lowever, for the lower

F i g u r e d . Geochemical Ratios Whole nock Or« SampI* OOLD : S I L V E R YAl WAl 0 nuiir WOLOMUA ' 05 Ah Af

a distinct separation, or de-coupling between the Because of the de-

coupling of precious metals from base melals, with precipitation of the bulk of the base melals in the deeper zones, the only base metals in the nearer surface zones represent near background

1 20

|!35

ISO

165

IBO

noiloi

1^5

112 5

QOLO t S I L V E n natloa of E l i c t r u m Qraint PAMnuiA Y AlWAl a QUI I. V 19 AuAj

temperature shallower Pambula deposit, there is precious and base melals.

Note that single mineral phases,

|I;

I's

i-J

/VuA,

Wholft nock O r t S a m p i t S U L P f l U n : 8 E L E H I U M natloa PAMnUlA YAl WAt a. out IT WOlUUlA I 10 OKO SfS

Mooo rsoo

rioo

iso

no s<s

The Au : Ag ratios In both whole rock and

strongly enriched In gold and are presumed to

eleclrum samples reflect this temperature-depth

have fixed gold out of auriferous solutions during

profile. T h e S : Se ratios are another remarkable

alteration, and may be interpreted as additional

future of these goldfields. Current work at the

evidence of gold leaching from the mafic extrusive

University of Melbourne is progressing towards

rich volcanic pile.

the development of a new model for S and Se

Figure 5. Gold concenlralions in rift related and other rocks.

distribution in boiling epithermal systems.

BOLD SOURCE ROCKS A

number

hypotheses

have

been

forwarded to explain the source of gold within epithermal systems.

These range from

the

occurrence of auriferous porphyry systems at

\ \

depth, to those who propose that gold solubility is so

great

preclude

the

necessity

\ \ \ 0 \ \ tn

- r

"favourable source rocks" as a pre-requisite to mineralization. Geochemical data obtained during

the significance of the extrusive mafic members of the intra-rift volcanic pile as gold source lithologles for gold mineralization. The intra-rlft volcanic pile Is comprised of a bi-modal sequence of rhyolites and basalts with intra-flow sediments.

Both intrusive and

extrusive silicic and mafic rocks have been recognised and sampled, along with co-magmatic A-Type

granites

and

mid

Devonian

I-Type

granites. These rocks have been analysed for a suite of 4 2 elements, the data has been subject to various statistical evaluation. Geochemical

data

suggests

that

the

presence of gold enriched source rocks were one constraint upon intra-rift gold mineralization. Variations in the correlation coefficients for gold and other elements between mafic dikes and mafic flows indicates significant re-distribution of gold

flow base breccias and are ubiquitously altered preferentially to crystalline mafic dykes. Gold concentrations

within

significantly higher

mafic

dikes

than rocks with

are

similar

petrochemical affinities elsewhere in the world, Fig. 5. Carbonaceous Inter-flow sediments within mafic dominant domains of the pile are also

•

3 s 3 —

Other

o e e «

j I S

I M

a 0

1 1

This Study

Studios

1. Z.nlllll, t l al 1B8S 2. Crock.! & TtcuU 1977 3, Tlllln®,

There also

exists

al 1973

a strong

positive

correlation between those elements depleted in mafic

extrusive

units,

and

those

elements

enriched In the Intra-rlft epithermal systems, most notably, gold, silver, sulphur, and selenium. The preferred scenario for

intra-rlft

orogensis is the hydrothermal alteration of the gold anomalous mafic extrusive members of the intra-rift volcanic pile, driven by sub-volcanic granitic intrusion of probable A-Type affinity. Geochemical data supports the influence of the volcanic pile, while petrogenetlc

associations

between gold mineralization and silicic

vent

features support the incorporation of a subvolcanic granitic heal source.

SUMMARY- GEOCHEMICAL EXPLORATION

within extrusive volcanic units. The thin basalt flows are commonly vesicular with flow top and

investigations on the genesis of those deposits within the Eden-Comerong-Yalwal rift support

e C =u

Through the integration of fluid inclusion analyses with geochemical

analyses

including

studies on element enrichment-depletions, ratios, and correlation coefficients, an example of baseprecious metal de-coupling and development of a classical geochemical zonalion model within en epithermal vertical

province has been defined.

component

this

de-coupling

The is

In the

target In contrasting those results to known

Eden-Comerong-Yalwal Rift it occurred between

deposits of that metallo^nic episode. These ratios

3 2 0 ° and 3 8 0 ° and between 1200 and

mey be used as a geothermometer for the system

dependant upon the geolhermal gradient.

metres.

1600

It has been demonstrated that

the

geochemical response in any sampling program will reflect the original P, T, conditions of mineralization. element

The extreme variation in trace

geochemistry

between

genetically

relative to other genetically and spatially related systems. Although

clearly

not

all

epithermal

systems are related to volcanic piles, studies within the Eden-Comerong-Yalwal rift indicate

identical deposits over a depth of 4 0 0 metres

"favourable" source rocks are one constraint upon

must be acknowledged in the development of an

the development of economic concentrations of gold

indicator or

within epithermal systems.

pathfinder suite of elements

exploration for epithermal deposits.

An effort

must be mode to define Ihe exact suite of elements

ACKNOWLEDGEMENTS

which are enriched in aU deposits within the region of interest.

Clearly, the use of Cu

geochemistry

only

delineating

would the

within the rift.

higher

successful

temperature

deposits

Less reliance should be placed

The author is indepted to those minerals exploration

firms

without

whose

financial

support this stud/ would not have been possible. These are Anaconda, Aberfoyle, Billiton (Shell),

upon the "text book" indicator elements, and more

Chevron, C . R A , Esso, and Oold Fields.

emphasis

supported satellite imagery studies, while Gold

should

placed

upon

regionally

confined geochemical orientation studies. Whole rock geochemical ratios may aid in delineating the favourability of an exploration

C.R.A.

Fields made available their own data on the Pambula and Wolumla deposits.

THE M I N E E ^ HILL QOLJD OCCURRENCE Brian B. GUY TRIAKO RESOURCES LID. ABSTRACT Mineral Hill is located 55 kilometres north of Condobolin in central western New South Wales. The area has been the site of mining and exploration activities since early this century. Exploitation has been principally of rich secondary silver /lead mineralization, with some 10,000 tonnes of over 15% lead, 900 g/t silver and 6 g/t gold extracted from the old Iodide workings. The area has been subjected to several phases of exploration since the mid 1960s, however, it was not until the late 1970s that the gold potential was appreciated. Up until late 1984 over 250 percussion and/or diamond holes had been drilled within an area of approximately 1 square kilometre. A low grade copper resource and two small gold resources had been outlined, however, data were inadequate for the classification of such resources in an 'ore reserve' category, and for tlie case of gold, to define the controls of the mineralization. The current phase of exploration has been undertaken by TRIAKO RESOURCES LID. in joint venture with CYPRUS MINES CORPORATION. Since late 1984, 92 holes totalling nearly 4,000 iDetres of reverse circulation drilling have been conpleted. Most of the drilling has been undertaken on a 20 metre by 20 metre grid basis on selected targets. Following such drilling an EXTRACTABLE PROBABLE ORE RESERVE of nearly 300,000 tonnes at 6 g/t gold has been defined, in three near surface bodies, two of which are amenable to open-cut exploitation. Hie ore reserves were estimated using polygonal and geostatistical techniques, with the extractable reserves based in part on cut-off grades reflecting the metallurgical characteristics of the mineralization and the open-pit designs. Preliminary feasibility studies have been completed on the reserves noted and indicate that exploitation of the deposits may prove viable at a gold price of $A 450/oz. The mineralization in the area occurs in the (?) late Silurian Mineral Hill Volcanics which locally have been subdivided into a lower pyroclastic unit and an upper clastic/non-clastic unit. The pyroclastics are the hosts for the major portion of the gold mineralization. The gold occurs in high grade veins and in disseminated zones with the principal localizing features being north-west trending, steep south-westerly dipping breccia zones. Some high grade sections of the mineralization appear to be constrained to steeply pitching 'shoots' within the breccia zones. In the primary zone gold mineralization is associated with chalcopyrite and pyrite. Where free gold has been observed, the gold occurs in chalcopyrite interstitial to fractured subhedral/anhedral pyrite. Alteration of host rocks adjacent to the mineralization is spatially limited with silicification, chloritization and sericitization evident. The gold potential of the area has not, as yet, been adequately evaluated. The gold resources defined are 'open' at depth, and in places, along strike within the associated breccia zones. Many gold targets outlined by early drilling together with silver/gold targets and base metal targets at Parkers Hill and Iodide remain to be tested.

E X P L O R A T I O N GEOCHEMISTRY AT T H E LONDON-VICTORIA GOLD D E P O S I T AT P A R K E S by Diana Hall ABSTRACT

T h e L o n d o n - V i c t o r i a deposit is in an old m i n i n g area. As of May 1984 a reserve of 415 T H tonnes of p r o b a b l e ore g r a d i n g 4.7 g / t plus 33.5 T H tonnes of possible ore g r a d i n g 3.8 g / t (2 g c u t - o f f ) has been proved. T h e p r o p e r t y is owned by A l k a n e E x p l o r a t i o n and Golden Plateau. T h e deposit is in Palaeozoic a n d e s i t i c rocks of the Forbes A n t i c l i n e (part of the L a c h l a n Geosyncline). T h e N / S L o n d o n - V i c t o r i a f a u l t is i m m e d i a t e l y a d j a c e n t to the m i n e r a l i s a t i o n ; a n d e s i t i c t u f f s are sericitised in the vicinity of the f a u l t in a zone up to 100 m wide and c a r r y gold m i n e r a l i s a t i o n . Gold occurs as both small inclusions in p y r i t e a n d as f r e e gold associated with q u a r t z a n d c a r b o n a t e . In a d d i t i o n sphalerite, galena and c h a l c o p y r i t e have been f o u n d as inclusions in pyrite. E x p l o r a t i o n a r o u n d the old m i n e w o r k i n g s has successfully used soil geochemistry to d e f i n e drill targets. Several p a t h f i n d e r elements have been used, i n c l u d i n g Hg, Pb, Zn, Cu. Lead a n d m e r c u r y soil geochemistry h a v e been p a r t i c u l a r l y useful. R e s e a r c h on L i t h o g e o c h e m i s t r y by Govett et al. a n a l y s i n g c h i p samples of core revealed an inconsistent association between Pb and Au m i n e r a l i s a t i o n . T h e study presented has c o n c e n t r a t e d on soil samples. At each site A, B and C horizon samples were collected. T h e A and B horizon samples were sieved into several size f r a c t i o n s . Each size f r a c t i o n and C horizon samples were analysed f o r Au, Fe, Mn, S a n d o r g a n i c carbon. These p a r t i c u l a r elements were selected to d e t e r m i n e if they i n f l u e n c e the Au d i s t r i b u t i o n in soil. T h e salient f e a t u r e s are:

Au is c o n c e n t r a t e d in the f i n e r size f r a c t i o n s in both the A and B horizons the w i d t h of the Au a n o m a l y in the A, B and C horizons is similar, i.e., there has been no noticeable lateral dispersion of Au a strong A u / M n associated in the C horizon is considered to be a lithological association u n r e l a t e d to soil processes. D a t a on the precision of the gold analyses will be presented and f r o m this a m e t h o d of c a l c u l a t i n g the size of gold particles present in the soil.

TITLE!

Remote sensing oE alteration associated with precious metai deposits in NSW

AUTHORS!

airis Horsfaii and Andrew Gabeii

ABSTRACT Modern passive remote sensing for geology uses reflected and emitted radiation in three major windows of atmosplieric transmission:

0.4 to 1.1 |im (Visible,

near infrared; VNIR)

1.1 to 2.5 ^m (Short wave infrared; SWIR) 8 to

14 |im (mid infrared; MIR).

Minerals absorb radi ation at wavelengths determined by the chemical environment of electrons and ions in their crystal lattices.

These

absorptions take place at wavelengths characteristic for eacli mineral. In the VNIR, ferrous and ferric compounds sucli as goethite, hematite and lepidocrocite, afid chromium compounds such as fuchsite can be distinguislied by tlie positions and relative depths of absorption features (i.e. the spectral shape).

Very weak absorption due to

phyilosillcates occurs at around 0.95 |im. In the SWIR phyilosillcates, carbonates, amphiboles and sulphates have characteristic reflectance spectra due to combination and overtone vibrations associated with the relevant anions.

Wltliin these groups it

is possible to distinguish between minerals with A1, Fe, Ca and Mg cations.

In the MIR, the feisic, intermediate, basic or ultrabaslc nature of silicate rocks can be quantified from the positions of the absorption features.

Carbonates can also be distinguished.

The most useful region of the spectrum for remote sensing of alteration associated with precious metal (in particular, gold) deposits is the SWIR, as phyliosllicates are typical alteration products. 1 shows SWIR spectra for some common hydrothermal minerals.

Figure

Table I shows the .ninerals detected in the SWIR spectra of surface samples collected from the altered a..d host rocks of NSW gold deposits.

These results show that alteratlou ^oues can be .napped in the

field using portable apectroradlometers.

Identification of high-grade

areas during selective mining operations may also be possible if a relationship can be established between ore grades and alteration. Airborne spectroradiometer surveys at several sites around Australia

have shown that minerals on the ground can be identified from

aircraft. Uigh-spatial-resolutlon Airborne Thematic Mapper surveys over gold deposits in Queensland have shown that alteration systems can be detected even with broad-band data in the SWiR. provided that processing techniques are used to remove absorption due to vegetation, Recent developments in remote sensing scanner technology will soon make aircraft and satellite mapping of individual mineral species a reality.

Laboratory research indicates that we may be able to

distinguish different chlorites and amphiboles and the degree of order in crystal lattices, as in kaolinite and dickite. from their SWiR spectra.

This technology is expected to bring a revolution in mineral

exploration and monitoring of our changing environment.

ICO FIGURE

V60

ISO

200

2-20

2tO

S H O R T W A V E L E N G T H I N F R A R E D S P E C T R A OF S O M E TYPICAL HYDROTHERMAL ALTERATION MINERALS

TABLE 1.

SUMMARY OF MINERALS DEIECIEU IN SWIU SPECIRA OF SAtlfLES FROM NSW (iOLU PROSPECTS

OJ •H

H H •H W bO C ^pj •H 60 -d 'H

FJ tJ a 3 H

nl H^ C nl M oj

rH H •H a 0 0 M rt W) a cd

'a H

S nl tJ "H

rH rO 0 nj PM

0) C 'H S CL. 0 M >> fl^ nj ^ (Tj 0 JM

<U « H OJ IS 0) u * ox: a cj p RJ >> pq PM

A A

H H •H •H 0 M ^ 0 0

^ p 0J 4 w

<u > u CJ a) rH CJ

nj n 0 BJ O i-i

x: M -a 4-J X

MAJOR ALTERATION MINERALS

Kaolinite Dickite Sericite Pyrophylite Alunite Jarosite Propylltlc Weak, unspecified

A A

* * A

* A

A A

* A A A

MINOR ALTERATION MINERALS A7

Diaspore Other

Pyro? Mon? Kaol? Jar?

Alu?

A7 Aiu?

Pyr? Al? Jar?

Al?

Jar?

MAJOR BACKGROUND MINERALS

Muscovite Kaolinite Mont./Illite Weak unspec. clay Chlorite

SILICA

A A

A * A

A A A

MAJOR MINERALIZED HOST ROCKS

Rhyolite Granodiorite Dacite Dlorite/Andeslte Sediments

AU MINERALIZ. KNOWN

A A A

A A

THE CUWARRA GOLD OEPUSIT - A GRANlTiC GOLD SOURCE? K.G. McQueen Geology, Applied Science, Canberra CAE, Belconnen, A.C.I. The Cowarra gold deposit is the largest oF a number oF epigenetic, gold lode systems whicli make up tlie Cowra Creek goldField, 30 km northeast oF Cooma, N.S.W. (Fig. 1). Ihe deposit is part oF a north-soutli trending belt oF mineralisation that is up to 1.5 km wide and 15 km long. Lodes are developed along this belt at the Bredbo River, Cowra Creek, Macanally and Fiery Creek. Host rocks to all the deposits are tightly Folded Ordovician slates and sandstones within the southern part oF the Cullarin Horst. These rocks are bounded on tlie west by Silurian volcanics and sediments in the Cowra-Yass Synclinorial Zone and on the east by the Bega Batholith. To the north they are intruded by Late Silurian to Early Devonian granites oF the Michelago Igneous Complex (Richardson, 1979). The Ordovician rocks have undergone regional metamorphism oF predominantly low greenschist F a d e s grade. A narrow belt oF higher grade regional metamorphic rocks (biotite- and andalusite-bearing schists) occurs close to the edge oF the Bega Batholith, just east oF Cowarra. Within the Cowra Creek goldField there are at least 5 distinct lode systems (Fig. 2). Total recorded production From these lodes is 923 kg oF gold, and approximately halF oF this was produced From the Cowarra Mine by B.H.P. between 19^0 and 19^2. Proven reserves at Cowarra are approximately 165,000 tonnes grading 8-13 g/t Au, 1.8 g/t Ag and 0.013?^ Cu (N.S.W. Dept. Min. Res., 1984). There is potential For Further reserves, particularly oF lower grade material. The deposit is currently being mined by Horizon PaciFic Ltd.. Most oF the other lodes have only been worked to shallow depth. The lodes at Cowra Creek consist oF steeply east dipping, cleavage-parallel Fractures inFilled with quartz, chlorite sulphide and carbonate vein material. These Fractures have been developed by shearing, particularly in more sandstone-rich parts oF the sequence. The regional deFormation history included an early stage oF Folding with development oF a segregation cleavage (S.), a major period oF tight to isoclinal Folding accompanied by the development oF a penetrative axial plane cleavage (S^), and late stage deFormation resulting in minor kink Folding, crenulation oF the S^ cleavage and Faulting. Peak metamorphic conditions coincided with the early part oF the major deFormation (Haydon, 1980). Shearing and gold lode Formation post-date major Folding and cleavage development. Ihe lodes are not Folded and in places cut across mesoscopic F^ Folds. MicroFabrics also suggest that the main sulphide mineralisation was introduced aFter cleavage development. The shear zones contain steeply south plunging kink Folds and show more intense development oF the crenulation cleavage. On a broad scale the mineralisation at Cowarra plunges north, sub-parallel to the major Fold axes. This is probably due to the eFFect oF the Folding on lithological boundaries. The main local control on mineralisation appears to be the development oF dilational zones, particularly at contacts between slate units and more competent sandstone beds, and at intersecting Fractures. Individual veins show limited wall rock alteration including chloritisation, sericitisation and siliciFication oF the adjacent slates. Ore consists mainly oF massive and irregular sulphide veins in a deFormed quartz-rich gangue, massive sulphide veinlets in slate, and banded carbonate-sulphide veins. The sulphides are dominantly pyrite with

minor arsenopyrite and pyrrhotite and trace chalcopyrite, sphalerite and galena. A number of texturally distinct sulphide types can be recognised and there appears to have been several generations of sulphide deposition. Gold occurs mainly as blebs (sub~microscopic - 0.5min) in inclusion-rich, subhedral pyrite, particularly where this pyrite contains chalcopyrite. Inclusions of gold have also been noted in pyrrhotite, chalcopyrite and arsenopyri±e. Some gold occurs isolated in quartz and in late-stage fractures in pyrite, suggesting some later introduction or redistribution of gold. Electron microprobe analysis of the gold (10 grains in 3 samples) indicates 93.7-95.5 wt?i Au, 2.8-3.7 wt?i Ag and <0.09 wt?o Cu (Gordon, 1985). The main gangue minerals are quartz, chlorite, magnetite, calcite, dolomite and ferroan ankerite with minor sericite, epidote and albite. Fluid inclusions studies indicate temperatures between 300-<^80°C for quartz deposition in the lodes. Geotliermomentry based on partitioning of Fe and Mg between coexisting calcite and ferroan dolofnile and on chlorite compositions indicate temperatures of 290 ± 3a«C and 255-340°C respectively (Gordon, 1985). Phase relations in the sulphides together with compositions of FeS- buffered arsenopyrite inclusions indicate temperatures for major sulphide deposition in the range 320-40G°C. Textural and mineralogical evidence suggests that deposition in the lodes occurred over an extended period with decreasing temperature, and that the main sulphide-gold deposition occurred between 300° and 400°C. The mineralising fluids had low salinity (3-8 eq. wt% NaCl, from fluid inclusions) and were probably neutral to weakly alkaline, as suggested by the stability of carbonate in the veins. They were ricti in S and As and became more oxidising with time and lower temperature. Gold transport was most likely via bisulphide or arsenic-bearing complexes. There are three conceivable sources for tlie mineralising

fluids.

Fluids derived from metamorphic dewatering of the Ordovician sedimentary pile.

Hydrothermal

Convecting brines derived from overlying Silurian seawater.

fluids derived from underlying intrusive rocks.

Low salinities would generally preclude derivation of fluids from seawater or early-stage sediment dewatering. The timing of the mineralisation also indicates that the fluids were introduced into fracture systems late in the metamorphic and structural history. The presence of some wall rock alteration suggests that the fluids were not in equilibrium with the surrounding rocks and if the estimates of fluid temperatures are correct then these are significantly higher than tlie late metamorphic conditions alone would allow. An interesting feature of the area is the presence of magnetic, 1-type granodiorites intruding the Ordovician block (these include the granites in the Michalego Igneous Complex to the north). The regional aeromagnetic data also suggest that there is anotlier similar intrusiofi at shallow depth beneath the mineralised belt and centred at Fiery Creek. In southeastern N.S.W. there are a number of sulphide-gold deposits within and around magnetic I-type granites (e.g. tl)e Braidwood Granodiorite, Sutton Adamellite) and this appears to be an important gold association

3. in this part of the Lachlan Fold Belt. These deposits show mineralogical and alteration features which set them apart from the typical slate-hosted quartz-gold deposits in southeastern Australia. The source granites contain abundant magnetite indicating a high oxygen fugacity in the original magma. High fU^ during granite crystallisation would favour the separation of 5 as immiscible blebs, and also increase the dissolved S in the silicate liquid fraction of the magma (Wyborn, 1985). On cooling this S would be partitioned into the fiuid phase enhancing its potential to carry gold and metals as 5-bearing complexes. Sulphide-gold deposition could occur in the upper levels of the granites and in suitable structures in overlying and adjacent country rocks in response to changes in temperature, pi I or fO^* While the mineralogy, temperature data and timing of the sulphide-gold mineralisation at Cowarra favour a late- or post- metamorphic, graniterelated mineralising process the evidence for a granitic gold source is still only circumstantial. Isotopic studies are needed to help resolve the primary fluid source. Portlier work aimed at gainifig a better understanding of the association of sulphide-gold deposits with particular 1-type granites would have important implications for exploration in the southeastern Lachlan Fold Belt. References Gordon, I.F., 1985. The Cowarra gold mine: A.N.U. ( u n p u b l . ) .

A research report,

Haydon, P., 198U. Structure and Metamorphism of the Urdoviciafi rocks of the Cullarin Horst near Jerangle, N.S.W. Ph.D. thesis A.N.U. (unpubl.). N.S.W. Dept. Min. Res., 1984. Richardson, S.J., 1979. Geol. Surv. N.S.W.

Minfo, 3, 1 - 3 .

Geology of the Michelago 1:10U,UUU Sheet.

Wyborn, D., 1985. Gold in granitic magmas. 14th BMR Symposium, Research Trends in Gold Exploration, B.M.R. Rec. 1985/34, 21-22.

GEOLOGY :::COWRA CREEK :: AREA Silurian Ordovician Granitoid Porphyry dyke Gold lode Major fold Fault

Regional metomorphic zone 2

5 Km KMcQ

Figure 1:

Location of the Cowarra gold deposit and general geology oT the surrounding area

MAIN LODE COWRA

SYSTEMS CREEK

Toilings (60) {A5 1 ( ^ 0am y ^ y

bonder bill

Polar Star

'.Main

Democra Lode System Surface Workings Recorded Production (kg Au) Track Figure 2:

bedding

210

Bredbo

fractures cleavage

Plan showing location and recorded gold production (prior to 1986) for the main lode systems, Cowra rv^'nk Goldfield

300m ^ KMcQ

THE SHEAHAN-^RANTS GOLD DEPOSIT AT JUNCTION REEFS by R. Overton* Introduction Since 1984, Climax Mining and Cyprus Minerals have been joint venture partners in two MLA's covering the Sheahan-Grants (Junction Reefs) deposit and in the surrounding Exploration Licence. After Climax Mining was listed on the Sydney Stock Exchange a year ago, it began an accelerated programme of development drilling and recently completed a comprehensive feasibility study indicating the viability of an open pit gold mine, which could become the second largest in NSW. History Gold was discovered in the Belubula River early in the 1870's and was traced to the Junction Reefs area where hard rock mining soon commenced. A number of companies mined the stratiform ore from 1886 on, by room-and-pillar stoping in the Grants, Sheahans, Cornishmans (Sulphide) and Frenchmans areas. Towards the end of mining, which ceased in 1938, a large pit was excavated in the Grants area and all the material removed as ore. Total recorded production for the above four areas is about 1158 kg of gold from in excess of 130,000 tonnes of ore (average grade 8.9 g/t). Regional Geology The ore deposit lies near the centre of the axis of the Molong Rise. The EL is underlain by Ordovician sediments and intermediate volcanics intruded by dioritic rocks of similar chemistry. The basal unit, the Walli Andesite, is a porphyritic andesite. This is overlain by the Malongulli Formation, towards the top of which the orebody occurs; it comprises siltstone, shale, chert and sandstone with minor intercalated flows, tuffs, conglomerates and calcareous beds. Above this is the Augullong Tuff, a suite of andesitic lavas, tuffs and volcaniclastic sediments. A large diorite stock outcrops north of the Frenchmans and Cornishmans deposits. In general, the volcanics have shoshonitic affinities. The layered sequence dips shallowly to the north and is terminated to the west by the major north-south Marangulla Fault. Metamorphic grade increases towards the diorite intrusive. Local Geology The Sheahan-Grants deposit occurs close to the top of the Malongulli Formation. The mine sequence consists of about 40 metres of intercalated finely laminated siliceous sediments siltstones shales and cherts - and sulphide-rich calc-silicate horizons ranging from 0.5 to 6.0 metres thick and known as "orebeds". As a result of stratigraphic correlation, a detailed nomenclature has been adopted. The present M, N, 0, Q, R, and S subdivisions correspond in part with the old units: "basal 4 foot * Manager, Exploration and Development, Climax Mining Limited, Sydney

bed", "11 foot bed", "20 foot bed", "6 foot bed" "three little beds" and "surface 4 foot bed". The units maintain regular thickness across the deposit but the fine (<lmni) laminations in the siliceous interbeds, indicating a low energy environment, contrast with the faintly layered to massive orebeds; the grain size of these sulphidic calc-silicates ranges up to 10 mm and indicates variable response to metamorphism. The mine sequence is intruded by a number of narrow diorite dykes, several of which are associated with faults with throws of up to 20 m. The dykes are probably related to andesitic lavas within the Angullong Tuff. The rocks dip uniformly at 16-20^ to the northwest and are extensively fractured and jointed. Orebody Discovery; The Sheahan-Grants deposit is the relatively lowgrade gold halo around and down dip of the extensively worked old high-grade Grants open cut and Sheahan's scraping. In 1980 Cyprus Minerals (then Amoco Minerals) drilled a series of vertical percussion and diamond drill holes adjacent to the old workings and defined an envelope of near-surface gold mineralisation with highly variable values. They had previously conducted geophysical (ground magnetic & SP) surveys and a soil geochemical programme, the latter giving clear anomalies in gold and arsenic. Detailed Description; The orebody is about 400 m x 150 m in area and has a geological reserve of 1.4 million tonnes @ 3.4 g/t gold. The ore occurs in two ways; as remobilised sulphide-rich joint fillings, known as "verticals", and as fine grains associated with sulphides, chiefly pyrrhotite, in the orebeds. The verticals constitute ore only when viewed as internal waste to simplify the proposed mining operation. The orebeds are composed of carbonate, quartz, sulphide and stilpnomelane, together with variable clinopyroxene (hedenbergite), amphibole and wollastonite. The sulphides, which make up 10-20% of the orebeds in the ore zone, are predominantly pyrrhotite with minor (0.5 - 1.5%) arsenopyrite, pyrite and marcasite, accessory chalcopyrite, cubanite and covellite, and patchy bismuthinite. The gold occurs essentially as free fine grains (not within the lattice) averaging 10-20 fx in diameter; coarse gold is rare. Silver averages less than 5 g/t. Oxidation of the top 20-40 m of the deposit has destroyed most of the calc-silicates but in places leaves cores of sulphide surrounded by goethite. Gold grades are the same in the oxide and sulphide ores. The gold grades within the deposit are lognormally distributed. Values generally vary smoothly, with a falling off in gold from two higher grade cores to a low grade fringe. Dense rectilinear fracturing and jointing are a feature of the deposit. However, no alteration assemblage has been recognised.

Ore Genesis The sulphides within the deposit show rude banding and may be syngenetlc. However, the gold appears to be eplgenetlc and preferentially deposited at and near sulphide grains within the calc-slllcates, which were probably Impure calcareous sediments. The support for this theory Is largely geometrical, based on the wide and uniform extent of the sulphides, the distinct halo style of the gold distribution and the Intense fracturing and jointing centred on the high grade areas, which may represent a "plumbing system". Sulphides and gold have been remoblllsed to some extent. Into the verticals. Proposed Mining Operations The ore consists about half and half of soft oxide ore, giving high gold recoveries, and harder sulphide ore with a lower (about 88%) recovery. It is proposed to mine the ore by open cut over five and a half years, and treat it by CIP in a plant capable of treating 200,000 tpy of primary and 400,000 tpy of oxide ore. The strip ratio will be about 2.1:1.

NEW DATA ON THE OCCURRENCE OF PLATINUM AT FIFIELD

D.W. Suppel and L.M. Barron Geological Survey of New South Wales

The Fifield district in central New South Wales has been Australia's largest producer of platinum. In all, 639.5 kg of platinum were produced from placer deposits which occur in an area covering approximately 10 km by 5 km. Sampling of drill core from the Owendale Intrusive Complex at Fifield has yielded encouraging results, drawing attention to the potential for the occurrence of economically interesting primary platinum deposits in basic to ultrabasic intrusions which occur in a belt extending north from Fifield to Doradilla, near Bourke (figure 1). Early Devonian intrusions at Fifield, Hylea and Honeybugle are heterogeneous bodies which have been classified as Alaskan type Suppel and Barron 1986). Most of the intrusions are situated in Ordovician or pre-Ordovician metasedimentary rocks of the Girilambone Group within a well defined north-south trending belt of gravity highs, referred to as the Parkes terrace.

Three intrusive complexes occur near Fifield: the roughly circular Owendale Intrusive Complex, the elongate Tout Intrusive Complex and the smaller, irregular Murga Intrusive Complex. Rock types in these complexes include diorite, hornblende melasyenite and meladiorite, quartz-hornblende monzonite, augite monzonite, hornblendite, hornblende-bioti te c1inopyroxenite, and serpentinite after dunite.

Four diamond drill holes were drilled near the southern and southeastern margins of the Owendale Intrusive Complex in 1966 and 1967. Holes FKDl and FKD2 intersected gabbro, diorite, hornblendebiotite clinopyroxenite and c1inopyroxenite. Holes FKD3 and FKD4 intersected serpentinite. Assay results for FKDl and FKD2 are shown on figure 2. Highest results were obtained from a unit of medium- to coarse-grained clinopyroxenite intersected over a rod length of 2.08 m in FKDl. A 1.57 m interval of this unit averaged 13.19 ppm platinum and 0.90 ppm palladium. The highest rhodium value was 0.5 ppm.

The serpentinite rocks in FKD3 and FKD4 are after cumulate, 220 mm dunite (Fo87) with 4-8% Cr-magnetite, 10-100 mm scale layering and relicts of interstitial diopsidic augite.

The mafic suite (gabbro/diorite/hornblendite) , best seen in the top half of FKD2, is typically well layered both in mineralogy and in grainsize (0.1 -0.3 nmi) while there are minor ophitic 5~30 mm eyes of hornblende/biotite and locally high concentrations of An70 or hypersthene or hornblende.

The well banded pyroxenite suite is devoid of plagioclase but has clots, layers and patchy veins of accessory biotite or hornblende or magnetite and is marked by high (h) or low (1) contents of these accessories and coarse (c) or fine (f) grain size as follows: FKDl(mostly ch); FKD2 149-247m(ch), 247~269m(fh), 269m-end of hole (cl). Cumulate textures, slumping, grading, and scouring features are present, while minor remobilized rods and veins of magnetite cut the layering and are associated with margins of dominant biotite or hornblende or diopsidic augite. Anomalous Pt values occur in the (cl) unit and in zones (p) of very coarse monomineralic bladed diopsidic augite which range from isolated 20 mm megacrysts to 30 mm dykes to irregular 1-2 m bodies. Contacts of the (p) units vary from very sharp cross cutting, to conformable with intertexic margins, while the enclosing rocks frequently show 10-50 mm margins which are highly enriched in one of the accessories.

Holes FKDl and FKD2 were collared on or close to a magnetic high. The magnetic susceptibility of the rocks in these holes is high (especially in FKDl), apart from the platinum bearing (cl and p) units, where it is markedly lower (figure 3).

At present, it is not known in what form the platinum occurs. Analytical work has not been completed but results to date indicate that the (p) cl inopyroxeni te unit in FKDl containing very high platinum levels is low in sulphur (< 0.1% S03), selenium, tellurium, antimony, bismuth, arsenic, and tin. These results, together with the very high platinum to palladium ratio and the decreased magnetite content in the unit, suggest that the platinum may occur as a platinum-iron alloy or in native form.

The available whole rock geochemistry shows that on many major element plots there is substantial overlap of the ultrabasics from Fifield with those of the Duke Island (Alaska) and Tulameen (British Columbia) Alaskan type complexes, although alkalies are higher and the slope of the Si02 vs FeO^/FeO^ + MgO wt% trend is substantially steeper for Fifield. The restricted ranges of the Hylea, Dirnaseer (Temora), Mt Derriwong and Honeybugle complexes appear to lie on the Fifield trend, but Broken Hill ultrabasic and basic intrusives do not. When Fifield Pt values are plotted against major and trace elements, they show a distribution curve that links high Pt values to a very restricted compositional range; viz., monomineralic diopsidic augite having Mg/Ca = 0.95 atoms, and FeO^ = 8.8 wt%.

In the Owendale complex, tlie presence of hornfelsed altered microgabbro against the pyroxenite and wedges of pyroxenite "immersed" in serpentinite suggest that the monzogabbro/gabbro units were emplaced first, followed by multiple intrusion of hornblende-

biotite pyroxenite, followed by tectonic emplacement of the serpentinite. A l t h o u g h the d i s t r i b u t i o n of lithologies suggests a broad n o r t h e a s t e r l y s t r i k e , igneous layering in p y r o x e n i t e and gabbro in FKDl and 2 has an e a s t e r l y strike and a v e r t i c a l d i p , parallel to the n e a r e s t ( s o u t h e r n ) m a r g i n of the b o d y . A p r e l i m i n a r y m o d e l for the p l a t i n u m o c c u r r e n c e is that there are low Pt grades and low Pt/Pd ratios in the relatively v o l u m i n o u s (cl) p y r o x e n i t e units w h i l e local r e m o b i l i z a t i o n of this material (caused by the n e a r b y d u n i t e ~ p r o d u c i n g m a g m a ) lead to injection of small s c a l e , irregular m o n o m i n e r a l i c c1inopyroxenite zones (p) h i g h l y e n r i c h e d in Pt but w i t h high Pt/Pd r a t i o s .

Acknowledgements Analytical work was undertaken at the Mineral Resources Development Laboratory by analysts I. H o d g e s , D . Rice and J. Karaolis. R . Spencer of the G e o l o g i c a l Survey of New South Wales is a s s i s t i n g in g e o p h y s i c a l i n v e s t i g a t i o n s . J i m Stroud supplied the g e o c h e m i s t r y of the Broken Hill r o c k s .

Reference S u p p e l , D . W . , and Barron, L.M., 1986. Platinum in basic to ultrabasic intrusive complexes at Fifield: a preliminary report. New South W a l e s G e o l o g i c a l Survey — Q u a r t e r l y Notes 6 5 , 1-8.

(Published w i t h p e r m i s s i o n Mineral Resources).

the S e c r e t a r y , N . S . W . D e p a r t m e n t

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Fifield-noiuke area

(modified from Suppel and Barron

ASSAY

RESULTS

DDH FKD2

FIFIELD

pyroxenlte/ Hornbfendlte

Hornblende horntels

(Intervol) |0-33m)

DDH FKD i Biotile pyroxenlle

Pd g/l

oA gA

"""(Section 187"!)

(interval)

<0 02 |0 77m)' <003 (0-86mh^ 006

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003 |076m)

.<a05 <a02 (025m) ^0-055 0 045|a79m) ^0005 0007 ia51m) 0075 0150 (a53m) 0 21 a23 |023m) ^005 0060 (a46m) <02 a03 |a61m) _0054 0043(a9lm) 020 OdA |0-9lm) _a035 a 16 (a63rT^ SCALE

m. ... Biotlte pyroxenlte

(interval) (0 43f^ (025;;T (0 58 m) (071 (051 r^) (018 m (071 " (053 (061 m)

150

100

Figure 2. ' Assay results, EKDl and FKD2, Fifield

MAGNETIC

SUSCEPTIBILITY FIFIELD

FKD 2

FKD 1

SCALE :50

1CX)

metres

Figure 3.

150

Magnetic .susceptibility, FDKl and FKD2, Fifield

The Geological Signature of Epithermal Gold and Other Precious Metal Deposits in New South Wales by S.S. Webster

Epithermal gold and other precious metal deposits in the Lachlan Fold Belt of New South Wales have characteristic geophysical signatures which may be used in exploration. The regional signature identifies the mineralizing system which has a pattern many times larger than the economic deposit. This pattern is discernible by cheap reconnaissance techniques, for area selection and follow-up to locate the exploitable reserve. At prospect scale other physical properties are utilized by surface and borehole techniques to assist in defining the deposit. The regional signature of an epithermal gold deposit has similar parameters, but not scale, to the signature of porphyry copper systems. The presence of intense alteration and silicification above the source of mineralizing solutions results in a reduction of magnetite content. This magnetite depletion gives a characteristic magnetic low that is often circular (Peak Hill), but may be linear (Gidginbung) if fault or formation controlled. A gravity low may coincide if the system is of significant size (Goonumbla) and radiometric anomalies are observable if potash alteration outcrops. Structural control of mineralizing intrusions and solutions may be reflected in the pattern of magnetic and gravity data. At prospect scale, the high silicification results in cementation of the altered rock giving high resistivities, in the order of several thousand ohm-metres (e.g., Peak Hill and Gidginbung). The precious metal horizon is often accompanied by pyrite which can give significant induced polarization anomalies beneath the resistivity high. These parameters are necessary but not sufficient criteria for the presence of gold mineralization. The search for platinum group metals has concentrated on basic to ultrabasic intrusions along the projected northern extent of the Gilmore Suture, between Fifield and Nyngan. Structural dislocations in the suture are the locus for the mafic intrusions which have a strong magnetic response. The suture is marked by a strong gravity gradient reflecting the younger and denser sediments of the Girilambone Terrane in contrast with Ordovician Wagga-Omeo metamorphics. Detailed magnetic surveys show a fairly uniform magnetite distribution with some zones of higher susceptibility. This has been confirmed by measurements of drill core, however, insufficient data are available to uniquely relate mineralization to the magnetite zoning.