Gold mineralisation at the Lady Bountiful Mine,Western Australia: An example of a granitoid-hosted Archaean lode gold deposit |
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| Authors: | K F Cassidy J M Bennett |
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| Institution: | (1) Key Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology, The University of Western Australia, 6009 Nedlands, Western Australia;(2) Present address: Department of Geological Sciences, University of Saskatchewan, S7N OWO Saskatoon, Saskatchewan, Canada |
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| Abstract: | 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 infitration 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°±50°C, ≈2 kbar)
to Stage II (250°±50°C, ≈0.5 to 2 kbar), with the hydrothermal fluid in both stages characterised by X(CO2)≤0.15 and moderate salinity (≈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 alteraction
and orefluid chemistry characteristics of the granitoid-hosted lode gold deposits are comparable to those established for
greenstone-hosted lode gold mineralisation. |
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