Abstract

The stratiform Pb–Ag–Zn and stratabound Cu–Au mineralization of the Thunderdome prospect, located in the Mundi Mundi Plain 50 km NW of Broken Hill, NSW, is covered by c. 200 m of sediments. A downhole mineralogical and geochemical dispersion study was carried out on 245 regolith samples from six exploration drillholes to establish if geochemical dispersion from the mineralization could be identified in the overlying cover. This was achieved using a combination of qualitative and quantitative XRD, Portable Infrared Mineral Analyzer (PIMA) and XRF techniques. The stratigraphy is defined by a variable thickness of diagenetically modified and weathered Cenozoic sediments (and soil) overlying the weathered Proterozoic basement, which hosts the mineralization. The composition of the sediments and saprolite appears to have been influenced by chemical dispersion from underlying mineralization, as shown by anomalous concentrations of Cu (up to c. 13 times above background), Ni (c. 9 times), As, Zn, Pb and Co (all c. 3 times) among others. Some of these anomalies are widespread and may be either clastic or related to adsorption on clay minerals, Fe-oxides and neoformed, poorly crystalline mineral phases. The precise transport mechanisms from the underlying mineralization need to be tested in order to gain further insights into geochemical dispersion processes under cover.

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