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Use of vegetation and soil in mineral exploration in areas of transported overburden, Yilgarn Craton, Western Australia: a contribution towards understanding metal transportation processes

CRC LEME, c/- CSIRO Exploration and Mining, PO Box 1130, WA 6102, Australia (email: ravi.anand@csiro.au)

Five sites were selected in the semi-arid northern Yilgarn Craton (Jaguar VHMS Cu-Zn-Ag, Moolart Well Au, Rumour Au, Gossan Hill VHMS Cu-Zn-Au and McGrath North Au deposits) to test the use of vegetation as an exploration sampling medium in areas of transported overburden. A variety of vegetation samples (litter, roots, bark, branch wood and phyllodes) were collected from Mulga (Acacia aneura) plants. In addition, the soil from 10–20 cm below surface was sampled and analysed following total, partial and selective digests to investigate any potential chemical signature of bedrock mineralization at surface.

At all sites, selective extractions of the <250 µm fraction of soil show either no expression of buried mineralization or, where present, only a weak or ambiguous signature. In contrast, the vegetation survey shows a multi-element signature in different plant organs. Branches show very low anomaly contrasts or none at all. Phyllodes and bark generally show weak to moderate anomaly contrasts. Litter shows the greatest anomaly contrast of all plant material.

There is also an enrichment in metals and an expression of the bedrock mineralization with good anomaly contrast in the uppermost soil horizon (0–4 cm) at Jaguar and this may indicate some mixing of litter and soil by bioturbation, and fixing on organic material, and Fe and/or Mn oxides. This contrasts with greater depths (10–20 cm) where no significant metal enrichment appears to have occurred, suggesting that most contained metals were fixed near-surface and do not percolate deeper into the profile. This is possibly due to slow decomposition of litter in arid terrains, with continuous loss of soil and fine litter particles by wind erosion and bush fires. Our data support the hypothesis that vegetation plays an important role in bringing metals to surface in areas with a semi-arid to arid climate and a low water-table.

KEYWORDS: biogeochemistry, soil geochemistry, vegetation, metal migration mechanisms, Yilgarn Craton