Metamorphic and structural setting of Archaean amphibolitehosted gold deposits near Southern Cross, Southern Cross province, Yilgarn Block, Western Australia

Abstract

The Southern Cross Archaean greenstone belt comprises dominantly low- to mid-amphibolite-facies, high-strain domains which show evidence of a complex deformational and metamorphic history. Aeromagnetic, field and petrographic data suggest that this complex structural-metamorphic history was related to a single progressive deformation event, which was broadly synchronous with granitoid emplacement between 2.68 and 2.62 Ga. Lode gold deposits at Corinthia, Hopes Hill, Fraser's and Polaris South, occur, in sequence from north to south, along the Fraser's-Corinthia shear zone in the Southern Cross greenstone belt. Corinthia and Hopes Hill are situated along a sheared granitoid-greenstone contact, whereas Fraser's and Polaris South occur within the greenstones about 200 m west of the contact. Gold mineralisation along the Fraser's-Corinthia shear zone is associated predominantly with variably deformed quartz-pyrrhotite-pyrite±plagioclase and/or pyroxene-calcite±quartz±plagioclase±apatite veins with zoned alteration haloes of quartz±plagioclase, pyroxene-calcite±apatite and amphibole±biotite±chlorite. There are accessory amounts of scheelite, chalcopyrite, arsenopyrite and galena with the Fe-sulphides, bismuth and gold. A variety of evidence, including structural timing, P- T conditions deduced from critical metamorphic (±550°C, 2.5–3.7 kbars) and metasomatic alteration assemblages (±520°C, 3–4 kbars), the high thermodynamic variance of the metasomatic mineral assemblages, and the zoning of individual alteration minerals, indicates that gold-related wallrock alteration occurred during or immediately after peak metamorphism. There is some evidence for a sympathetic decline in metamorphic grade and the temperature of gold related alteration from south (Polaris South) to north (Corinthia) along the mineralised shear zone. The shear zone acted as a major conduit for ore fluids at a depth of about 12–14 km ( ∼3–3.5 kbars lithostatic pressure) in the crust. Gold mineralisation along the shear zone is related to large- and small-scale extensional crenulation cleavages. The gold deposits might thus represent a relatively deep section of the crustal continuum of lode gold deposits proposed for the Yilgarn Block.