Soluble ionic gold in soils

Soluble ionic gold in soils Meivyn J. Lintern Chris G. Ryan Robert M. Hough  

Abstract Mobile Au in soil has been postulated for many years. It has been used by the mineral exploration industry in areas of transported overburden as a vector towards buried deposits. Until now, the nature of this mobile Au has not been known or investigated. Soil samples from a colluvial area above the Bounty Deposit (Yilgarn Craton, Western Australia) investigated by analytical techniques including laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and synchrotron x-ray fluorescence (SXRF) combined with X-ray absorption spectrometry (XAS) have allowed us to map the invisible Au in these soils and suggests that at least some of it occurs in an ionic form. 

Keywords gold, synchrotron XRF, mineral exploration, ionic gold, Yilgarn Craton 

Recently, it has been shown that Au in soil can be dissolved using deionised water, the most benign of lixiviants (Gray et al. 1999). The reason for this is not clear either (i) Au occurs in a soluble form within the soil, and/or (ii) deionised water is dissolving an adsorbed ligand in the soil such as cyanide that subsequently dissolves metallic Au. Notwithstanding these uncertainties, use has been made of this previously recognised soluble Au characteristic in soil by mineral exploration and analytical companies, to provide a possible vector towards mineral deposits. 

The study of natural Au in soil has been hampered by low concentrations, typically in the ppb range. Investigating the presence of metallic ions has been made more difficult with very few traditional techniques that may be used (e.g. polarography, high voltage electrophoresis and ion chromatography). At the low concentrations of Au in soil, these 

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