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Geodynamic Setting and Controls on Iron Oxide Cu-Au (±U) Ore in the Gawler Craton, South Australia
Nicholas Hayward, Teck Australia Pty Ltd., Perth, Western Australia  and  Roger Skirrow, Minerals Division, Geoscience Australia, Canberra, ACT., Australia.

in   Porter, T.M. (Ed), 2010 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective,   Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide.   v. 3,  pp. 119-146.


   The Olympic IOCG (iron oxide copper-gold) Province occurs along the eastern margin of the Gawler craton nucleus where oxidised (magnetite-series), A-type granitoid plutons of the 1595 to 1575 Ma Hiltaba Suite were emplaced into an accreted Palaeoproterozoic terrane, and where mafic volcanic rocks of the lower Gawler Range Volcanics are most abundant. This magmatism comprises the central part of a diachronous corridor of bimodal I-, A- and subordinate S-type intrusions that extends across the Gawler and Curnamona cratons, and was emplaced in a distal continental retro-arc setting during amalgamation of the North and South Australia cratons. IOCG mineralisation mostly formed during a short lived episode of northnorthwest-southsoutheast extension that approximately coincided with eruption of the Gawler Range Volcanics (ca. 1595 to 1590 Ma), but was preceded and followed by more protracted northwest-southeast to northnorthwest-southsoutheast contraction. The deposits were emplaced along eastnortheast- to northeast-trending extensional faults near their intersections with major northnorthwest- to northwest-trending faults in the hangingwall of first-order terrane boundary faults, such as the Elizabeth Creek and Pine Point Fault Zones.
   The 3D crustal-scale architecture of the supergiant Olympic Dam ore system comprises a discrete lower- to mid-crustal zone of seismic transparency that may relate to voluminous Hiltaba-age migmatites and altered felsic batholiths, localised above a crust-penetrating fault zone at the edge of an inferred mafic underplate. Inversions of geophysical data suggest that magnetite-rich alteration extends several kilometres beneath the Olympic Dam deposit to near the top of these interpreted batholiths. A zone of low resistivity in the mid-crust beneath the deposit, imaged in magnetotelluric data, may be associated with conductive mineral seams, probably graphite, related to this alteration event. Large scale IOCG-related alteration systems in the province are generally zoned from distal, high-temperature, deep level albite-actinolite±magnetite assemblages, through biotite-magnetite and then magnetite-K feldspar±carbonate assemblages, to proximal lower temperature, shallow hematite-sericite-chlorite-carbonate alteration. IOCG mineralisation typically occurs at the Fe
2+ to Fe3+ redox boundary which frequently has a distinct geophysical expression. Magnetite-rich alteration assemblages were deposited from high-temperature, hypersaline, CO2-bearing brines of magmatic and deeply circulated bittern origin that isotopically equilibrated with metasedimentary and meta-igneous units. Although these high-temperature brines commonly transported significant copper (>300 ppm), their low sulphur (plus high iron and chlorine) contents severely limited their capacity for sulphide saturation and copper-gold mineralisation. Ore deposition only occurred where mixing or overprinting occurred with lower temperature, more oxidised, SO4-rich brines, derived from either playa lake (bittern) sources or evolved from cooled and extensively equilibrated magmatic brines. In either case, external leaching of copper, gold and sulphur from buried basaltic units, and/or uranium from exposed felsic volcanics and radiogenic granites, is deemed essential for supplying sufficient metals to form economic IOCG(±U) deposits in the Olympic IOCG Province, and explains the spatial restriction of IOCG deposits to only a small portion of the broader magmatic province. Area selection guidelines for further discoveries beneath the extensive cover are considered at subprovince, district and deposit scales.

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