Abstract

Significant Au discoveries are becoming less common because the remaining prospective, underexplored areas are obscured by transported cover. At Moolart Well (Western Australia), secondary Au deposits hosted in transported pisolitic ferricrete and saprolite are overlain by younger transported cover. Here, we show how Au has been, and is being, dispersed and concentrated in these deposits and the overlying younger transported cover and biota during the evolution of the landscape. We identified coarse (>400 µm), Ag-rich, primary, angular Au accumulated residually along with some precipitated, Ag-poor (<0.5% Ag) secondary Au in saprolite. Gold enrichment in the pisolitic ferricrete is mostly secondary nanometer- to micron-sized spheres, chains, triangles, and wires in precipitates of organic carbon (C), goethite, kaolinite, and amorphous Si within cortices, cracks, and cavities in pisoliths. Spectacular secondary Au as clumps and larger clusters, not previously reported in ferricrete, occurs in organic C-rich zones of the cortices and cavities of pisoliths, implying a role for organic matter in their formation. To our knowledge, the organic C–Au relationship in pisolitic ferricrete described in this study has not been documented previously. We propose that minor Au was recycled from the pisolitic ferricrete formed during humid conditions (Miocene) into an overlying silicified subsoil that was formed in an arid climate of the mid-Miocene to the present. The presence of Au in vegetation (Acacia aneura) and termite mounds indicates active dispersion.

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