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Ore forming fluids of the Laverton District, Yilgarn Craton, Westen Australia: source, gold precipitation mechanism(s) and extent of alteration

Amanda Stoltze and Ian Campbell

Greenstone-hosted mesothermal gold deposits of the Archaean Yilgarn Craton, Western Australia, have yielded the majority of AustraliaÕs gold. The Laverton region, within the Yilgarn Craton, is Australia's second largest gold producing district and host to the Sunrise Dam, Granny Smith and Wallaby deposits. The gold deposits are structurally controlled, but are hosted by a variety of lithologies; granodiorite at Granny Smith, conglomerate of dominantly basaltic composition at Wallaby, and andesitic volcaniclastic and banded iron formation at Sunrise Dam. Mineralisation in banded iron formation can be explained due to the interaction of ore fluids with the iron-rich wall rock causing precipitation of gold, but this process can not be responsible for mineralisation in iron-poor lithologies. Another possible gold precipitation mechanism is the mixing of two fluids (ore fluid with a second fluid from a different source), which will be investigated using stable isotopes.

The emphasis of this study will be the recently developed Wallaby deposit. Here a suit of monzonite-syenite-carbonatite dykes is concentrated in the centre of a steeply plunging pipe of magnetite-actinolite alteration that hosts the majority of the cross-cutting mineralisation. The relationship between the intrusive suite, the magnetite-actinolite alteration pipe and the overprinting ore alteration will be investigated using stable isotopes and trace element geochemistry. Ore fluid characteristics and the fluid source (metamorphic, magmatic, connate, seawater or mantle source) will also be determined using fluid inclusions and stable isotopes (H, O, C, N, B). The extent and composition of geochemical changes associated with alteration events will be identified by trace element geochemistry using laser ablation ICP-MS analysis.