CRC LEME
Open File Report 68
ABSTRACT
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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