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Open File Report 34

Further aspects of the chemistry of gold in some Western Australian soils

Gray, D.J. and Lintern, M.J.

Understanding of the soil chemistry of Au is important for the development and modification of sampling and analysis of soils for exploration. Gold chemistry in a range of soils and regolith materials from the southern Yilgarn, W.A., was investigated by selective extraction, primarily using an iodide reagent or de-ionised water. On the basis of these results, a number of different classes were delineated:

  1. unweathered rock and saprolite - low Au solubility;
  2. laterite and other Fe oxide-dominated regolith - moderate Au solubility, with significant re-absorption of any dissolved Au;
  3. Mn oxide-dominated regolith - very high Au solubility when pulverized;
  4. carbonate - high Au solubility even without pulverizing;
  5. organic-rich - low Au solubility, possibly due to re-absorption of dissolved Au;
  6. hardpan - high Au solubility when pulverized.

The very high solubility of Au in Mn-rich regolith suggests that Mn can act to mobilize Au, which is consistent with previous investigations indicating an important role for Mn in dissolution of Au in chloride-rich groundwaters.

Results for carbonate-rich soils from various sites across the southern Yilgarn craton indicate highly consistent Au behaviour. In general, about 30-50% of the total Au is dissolved in iodide reagent. Indeed, the inherent Au solubility is even higher (up to 100%), with the reduction in observed solubility being due to re-absorption of some of the dissolved Au by surfaces (such as Fe oxides) exposed by grinding. Commonly, Au solubility is as high, or higher, in coarsely ground material as compared with finely pulverized material. These results indicate that Au in these carbonate soils is highly soluble and commonly found on mineral surfaces or otherwise available to solution. Iodide-soluble Au is consistently found to be correlated with friable carbonate, even where other phases occur, such as Au associated with Fe oxides or contained in carbonate concretions. These results suggest that Au is behaving as a soluble element (like Ca or Mg) in soils and is precipitating in response to similar factors.

Pedogenic carbonate forms in soil as a result of evaporative processes, with the carbonate coating the surfaces of other minerals. Gold may be highly mobile in soil, as a result of biologically generated ligands and may be taken up by plants. As plant material is deposited on the ground surface, Au-organic complexes are formed that then percolate down the soil profile. Once these complexes reach the carbonate horizon, they will be immobilized, not necessarily by chemical means, but because these horizons are an evaporative zone. Therefore, the primary controls on Au distribution in soil with pedogenic carbonate may be biological and physical, rather than purely chemical, as first expected. Gold in such soils is in a highly dynamic state, which may explain why Au mobilization processes may give rise to enrichments, even in geologically recent overburden, that reflect underlying mineralization.

Last updated: Thursday, January 06, 2000 08:09 AM


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