Hydrothermal alteration mineralogy and zonation in the orogenic Frog's Leg gold deposit, Yilgarn Craton, Western Australia

The Frog’s Leg gold deposit (~1.7 Moz Au produced and reserves) and surrounding deposits of the Kundana mining district (locally called the Kundana Trend) are located at or near the Zuleika Shear Zone, NW of Kalgoorlie, in the Eastern Goldfields Superterrane, Western Australia. The Frog’s Leg deposit is hosted in mafic volcanic rocks, shales and volcaniclastic rocks that are metamorphosed at greenschist to lower amphibolite facies. Mineralised structures are indicative of three stages of development :(1) Reverse thrusting and development of NNW striking laminated shear veins with gold (2) Steep tension and flat veins with gold that developed during oblique strike slip transitioning into transitioned to a second stage of thrusting. (3) Tabular zones of quartz-breccia containing gold and stockwork veins that both overprint initial shear veins and crosscut them in NW striking splays. The host rocks are hydrothermally with distinct equilibrium assemblage zonation with evidence of overprinting alteration: (1) distal alteration zones: actinolite-chlorite-calcite (2) intermediate alteration with early biotite-quartz –calcite and late actinolite-chlorite-ankerite-pyrrhotite (3) early stage proximal alteration with biotite-K-feldspar-plagioclase and late stage biotite-ankerite-plagioclase-K-feldspar-tourmaline with 2 corresponding stages of retrograde sericite alteration. Alteration indices are indicative of K alteration in intermediate and proximal zones of the main host rocks with additional strong carbonate alteration in the hanging wall (Catrock Basalt). Mass balance shows enrichment of CO2, K2O, S, As, Au, Ag, Pb, Cu and W in proximal alteration. W and As are found to be the best pathfinder elements for gold. Four stages of Arsenopyrite can be recognised at Frog’s Leg with 3 of which correlate with gold mineralisation. Arsenopyrite geothermometry indicates that early arsenopyrite with gold formed at 325-403 ºC with a second stage of arsenopyrite with electrum at 445-514 ºC and late stage arsenopyrite with gold at 321-472 °C. Utilising biotite-apatite geothermometry, the temperature of early stage biotite is predicted to have formed at 417-492 °C with later stage biotite at 533-582 °C. Thermometry results are compatible with multiple stages of gold mineralisation and associated hydrothermal alteration.