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Open File Report 68

Chemistry of gold in some Western Australian Soils

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

The chemistry of Au in sample soils from the southern Yilgarn, W.A. was investigated by a variety of experiments. Soil solution and incubation studies implied that in carbonate-rich soils dolomite was forming as a result of evaporative processes, with the soil solution in the carbonate horizon being more saline than elsewhere, and containing high concentrations of Na and Mg. The dolomite appears to be coating the surfaces of other minerals, and will therefore dominate the chemistry of this zone.

Shaking AuCl4- solutions with various soil samples showed that the samples least likely to adsorb AuC14- over one day are those enriched in carbonates and low in organic matter. This may reflect poor adsorption of anionic Au complexes at the higher pH of the carbonate dominated materials.

Selective extraction data suggested that various forms of Au exist in the soils tested. In the upper part of the profile, where organic matter is high, Au appears to be associated with this phase. Lower in the profile, Au associated with the Fe oxides or other non-carbonate materials generally has a moderate extractability, which is considerably lower for samples that are merely crushed to <10 mm, relative to those pulverised to <75 µm, suggesting that the Au occurs within solid phases or is otherwise not available to solution. Gold associated with carbonates is very soluble, even in crushed samples, suggesting that a substantial part of the Au associated with carbonate is on surfaces or in environments that are readily accessible to the solution.

Soil incubation experiments were performed on three samples from various depths down a carbonate-rich soil profile. Shaking the samples with deionized water resulted in significant dissolution of about 10 µg/L Au over one week for all three samples. This dissolution was suppressed by CO2 bubbling. The dissolved Au then reprecipitated in the mixtures with the two samples that occurred closer to the soil surface. Reprecipitation was prevented by irradiation of the mixtures, indicated it to be a result of biological activity. The deeper, carbonate-rich sample differed in that Au dissolution increased with time and was not suppressed by CO2 bubbling.

When the soil samples were shaken with a solution containing AuCl4- the Au precipitated from solution. After several weeks exposure, there was major redissolution of the Au (up to 2.3 mg/L) in the mixtures with the two samples closer to the surface. This redissolution was completely repressed by CO2 bubbling and strongly reduced by irradiation. The redissolution carbonate-rich sample showed a sustained redissolution, that was less strongly affected by CO2 bubbling or irradiation.

Thus, the incubation experiments indicated that the soil samples tested had the capacity to dissolve major concentrations of Au. Results suggested that Au dissolved more readily from the carbonate-rich sample than from samples further up in the profile.

Results to date suggest that, contrary to expectations, Au is less strongly adsorbed by, and more soluble from, carbonate-rich horizons than other soil zones. The mechanism for Au concentration in this zone is suggested to be evaporative concentration, rather than through chemical factors. This hypothesis may have important applications for the use of soil as an exploration tool north of the Menzies line.

Last updated: Thursday, January 06, 2000 11:38 AM


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