Fluorine systematics and alteration across the late Archaean East Repulse deposit, St Ives gold camp, Yilgarn craton, Western Australia: implications for fluid-flow pathways and fluid sources

The East Repulse gold deposit occurs in the St Ives gold camp of the eastern Yilgarn craton. The deposit is vertically zoned and consists of four main alteration domains. Domains include: (1) a lower-domain with assemblages of albite-carbonate-quartz-pyrite epidote and biotite-anhydrite-quartz carbonate; (2) an ore-domain with albite-carbonate-quartz-pyrite and biotite-pyrite-quartz alteration; (3) a transitional-domain comprised of amphibole-feldspar and biotite-amphibole alteration and (4) an upper-domain with quartz and clinozoisite veins and veins of biotite-K-feldspar-calcite-pyrite. Chlorite and pyrrohtite alteration are also common in parts of the upper domain. Gold mineralisation dominantly occurs in the ore-domain along a shallowly dipping shear zone, and gold commonly occurs as inclusions in pyrite associated with biotite alteration. We investigated the halogen content of hydrothermal biotite, which is temporally and spatially associated with gold mineralisation. Calculations show that the log(fH(2)O/fHF) values for fluids in equilibrium with hydrothermal biotite decrease with increasing depth from the ore-domain into the low-domain. This trend is consistent with potassic fluids moving upwards beneath the East Repulse shear zone. Fluids likely moved upwards along steeply dipping porphyry dikes, which exhibit porosity developed during an earlier calc-sodic alteration event where andesine/oligoclase was replaced by albite.