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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
worlds 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 delta 34S sulfide values (-10 to -4 per mil), consistent with sulfide
precipitation from an oxidized (sulfate-predominant) magmatic fluid at 450 to 400 degrees C. Pyrite grains that occur in the propylitic alteration halos typically have delta 34S sulfide values near 0 per mil. There is a gradual increase in delta 34S sulfide 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 delta 34S sulfide 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
Date Deposited: 10 Aug 2007
Last Modified: 18 Nov 2014 03:20
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