The Moolart Well gold deposit lies in the Duketon Greenstone Belt in the Western Australian Goldfields in an area that has seen nearly 150 years of Au exploration with limited success due to the transported cover masking deposits. Here, the site displays no anomaly indicative of underlying mineralization within surface soils. Termites have the ability to burrow to the subsoil and contribute to the development of soil profiles through bioturbation. This study discusses the use of mounds formed by the termite Tumulitermes tumuli from a site where shallow ferruginised palaeochannel sediments with secondary Au enrichment overlays deeper primary mineralization. A series of samples from termite nests and surrounding soil were sampled along a transect from background areas to over mineralization. Various fractions of these samples were analysed with ICP-MS/AES. Tumulitermes tumuli is able to bring sub-surface mineralized material to the surface from 1 to 4 m depth. Termite mounds over mineralization and soil immediately adjacent to the mounds display an Au anomaly in both <250 and >250 µm fractions. Very high concentrations (>5000 ppb) were found in >2000 µm fractions in nests over mineralization as a result of vertical transport of anomalous pisolitic gravels by termites. These results suggest termite-driven local soil heterogeneity and termite mounds being a consistent geochemical and mineralogical sample medium for the discovery of ore deposits beneath weathered cover and shallow sediments.
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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