CRC LEME
Open File Report 34
ABSTRACT
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:
- unweathered rock and saprolite - low Au solubility;
- laterite and other Fe oxide-dominated regolith - moderate Au
solubility, with significant re-absorption of any dissolved Au;
- Mn oxide-dominated regolith - very high Au solubility when pulverized;
- carbonate - high Au solubility even without pulverizing;
- organic-rich - low Au solubility, possibly due to re-absorption
of dissolved Au;
- 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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