GOLD MINERALIZATION AT THE LADY BOUNTIFUL MINE, WESTERN-AUSTRALIA - AN EXAMPLE OF A GRANITOID-HOSTED ARCHEAN LODE GOLD DEPOSIT

The Lady Bountiful granitoid-hosted lode gold deposit, located in the mid-greenschist facies metamorphosed Ora Banda greenstone sequence, is hosted predominantly by the late-tectonic Liberty Granodiorite. Gold mineralisation is localised along quartz-veined, sinistral, brittle fault-zone(s) that transect the boundary between the Liberty Granodiorite and Mt Pleasant sill. Quartz vein textures indicate two stages of a single gold-related vein-development event, with high-grade gold mineralisation restricted to the second stage. Ore minerals include pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, Au-Ag-Bi-Pb-tellurides, and native gold. Fluid infiltration has resulted in narrow (< 1 m) bleached wallrock alteration envelopes to the fault zones comprising albite-K-mica +/- chlorite +/- calcite +/- rutile assemblages. Temperature-pressure conditions varied from Stage I (300-degrees +/- 50-degrees-C, almost-equal-to 2 kbar) to Stage II (250-degrees +/- 50-degrees-C, almost-equal-to 0.5 to 2 kbar), with the hydrothermal fluid in both stages characterised by X(CO2) less-than-or-equal-to 0.15 and moderate salinity (almost-equal-to 1.28 m NaCl). Intermittent phase separation of Stage II mineralising fluids, initiated by pressure fluctuations in dilational sites, and/or fluid-dominated fluid:wallrock interaction, are invoked as the dominant depositional mechanisms. The granitoid-hosted Lady Bountiful lode gold deposit shares many features with other granitoid-hosted lode gold deposits in the Yilgarn Craton and the Superior Province. Granitoid-hosted lode gold deposits, such as the Lady Bountiful deposit, provide additional evidence that the dominant control on the localisation of gold mineralisation within a granitoid host is structure, with competency contrasts playing a significant role. Furthermore, the hydrothermal wallrock alteration and orefluid chemistry characteristics of the granitoid-hosted lode gold deposits are comparable to those established for greenstone-hosted lode gold mineralisation.