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Open File Report 143

Mineralogical and geochemical aspects of the regolith at the Brahman Au Prospect, Charters Towers area, N.E. Queensland

K.M. Scott and S.J. Fraser

The regolith at the Brahman Au prospect, is characterised by the presence of ferruginous pisoliths above mottled transported sandstone (Campaspe Formation) which unconformably overlies residual clay and saprolitic granite. Within the profile, Au is laterally dispersed for up to several hundred metres at the top and bottom of the saprolite as well as in the surficial pisoliths. Arsenic, Cu and Pb contents are also elevated in the ferruginous pisoliths. Zones of lateral dispersion of Au with a zone of depletion beneath the surficial anomaly are similar to those commonly observed in theYilgarn Craton of Western Australia. However, at Brahman, the Au-enriched pisolitic unit and Au-depleted mottled zone are developed in transported material, i.e. Au has been hydromorphically dispersed into the overlying sediments. Manganese oxides and associated elements (Ce, Co, La and Zn) are well developed in the clay zone of the profile, but are also present in transported material suggesting that they are also dispersed late in the history of regolith development.

Despite their probable development in transported material, the ferruginous pisoliths represent a good sampling medium. Orientation results suggest that low level Au, As, Cu end Pb halos at least 700 x 500 m are present even in five metre composite samples that include a large component of non--ferruginous material. More intense anomalies, with better signal-to-noise ratios, would be expected with purer pisolitic samples which have not been diluted with depleted material. The development of such extensive hydromorphic dispersion over minor mineralisation is encouraging for exploration within the Charters Towers region. Anomalous Th (+U) with the pisoliths suggest that airborne radiometrics may be a useful method to assist in the planning a regional pisolith sampling programme.


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