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Sulfur isotopic zonation in the Cadia district, southeastern Australia: exploration significance and implications for the genesis of alkalic porphyry gold–copper deposits

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Wilson, AJ and Cooke, DR and Harper, BJ and Deyell, CL (2007) Sulfur isotopic zonation in the Cadia district, southeastern Australia: exploration significance and implications for the genesis of alkalic porphyry gold–copper deposits. Mineralium Deposita, 42 (5). pp. 465-487. ISSN 0026-4598

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Abstract

The alkalic porphyry gold–copper deposits of the
Cadia district occur in the eastern Lachlan Fold Belt of New
South Wales, Australia. The district comprises four
porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill,
and Cadia East) and two iron–copper–gold skarn deposits
(Big Cadia and Little Cadia). Almost 1,000 tonnes of
contained gold and more than four million tonnes of copper
have been discovered in these systems, making Cadia the
world’s largest known alkalic porphyry district, in terms of
contained gold. Porphyry gold–copper ore at Cadia is
associated with quartz monzonite intrusive complexes, and
is hosted by central stockwork and sheeted quartz–sulfide–
(carbonate) vein systems. The Cadia porphyry deposits are
characterized by cores of potassic and/or calc–potassic
alteration assemblages, and peripheral halos of propylitic
alteration, with late-stage phyllic alteration mostly restricted
to fault zones. Hematite dusting is an important component
of the propylitic alteration assemblage, and has produced a
distinctive reddening of feldspar minerals in the volcanic
wall rocks around the mineralized centers. Sulfide mineralization
is strongly zoned at Ridgeway and Cadia East, with
bornite-rich cores surrounded by chalcopyrite-rich halos and
peripheral zones of pyrite mineralization. The Cadia Hill
and Cadia Quarry deposits have chalcopyrite-rich cores and
pyrite-rich halos, and Cadia Hill contains a high-level
bornite-rich zone. Distinctive sulfur isotopic zonation
patterns have been identified at Ridgeway, Cadia Hill, and
Cadia East. The deposit cores are characterized by low
δ34Ssulfide values (−10 to −4‰), consistent with sulfide
precipitation from an oxidized (sulfate-predominant) magmatic
fluid at 450 to 400°C. Pyrite grains that occur in the
propylitic alteration halos typically have δ34Ssulfide values
near 0‰. There is a gradual increase in δ34Ssulfide values
outwards from the deposit cores through the propylitic
halos. Water–rock interaction during propylitic alteration
caused magmatic sulfate reduction and concomitant oxidation
of ferrous iron-bearing minerals, resulting in enrichment
of 34S in pyrite and also producing the distinctive reddened,
hematite-rich alteration halos to the Cadia deposits. These
results show that sulfur isotope analyses have potential
applications in the exploration of alkalic porphyry-style
deposits, with zones of depleted δ34Ssulfide values most
prospective for high-grade mineralization.

Item Type: Article
Keywords: Sulfur isotopes . Gold–copper porphyry . Alkalic porphyry systems . Cadia . Lachlan orogen . Australia
Journal or Publication Title: Mineralium Deposita
Page Range: pp. 465-487
ISSN: 0026-4598
Identification Number - DOI: 10.1007/s00126-006-0071-9
Additional Information:

The original publication is available at www.springerlink.com

Date Deposited: 25 Jun 2008 03:50
Last Modified: 18 Nov 2014 03:43
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