CRC LEME
Open File Report 96
ABSTRACT
Selective extraction techniques for the recognition of buried
mineralization, Curara Well, Western Australia
Gray, D.J.
The Curara site was selected for initial investigations of the
utility of partial/selective extraction for mineral exploration
in the Yilgarn. There are up to 20 m of barren transported overburden
over laterite, which is enriched in Au above primary mineralization.
It was considered that MMI (mobile metal ion) extractions were successful
in locating the buried mineralization. This was tested, using one
soil profile in the middle of the mineralized zone, two traverses
across the region of Au-rich laterite and one traverse in a background
area, which were treated using standard selective extraction techniques.
The first extraction, acetate, which measures pH 5 soluble metals,
shows no direct correlations with underlying Au-rich laterite, except
for Mn, which appears to be moderately enriched directly above mineralization.
The second extraction, hydroxylamine, which extracts Mn oxides and
amorphous Fe oxides, gives higher levels of Fe, Mn, Cu, Zn and Pb
overlying mineralization. The best signal to noise contrast was
obtained for Mn, and is clearly caused by significant concentrations
of separate phase Mn oxides above the Au-rich laterite. Analysis
of the data suggests that the higher concentrations of extractable
base metals are primarily due to association of these metals with
Mn oxides. Furthermore, examination of the MMI results suggested
that those MMI extractable metals showing high contents over mineralization
(e.g., Cu and Cd) were also associated with Mn oxides.
The enrichment in Mn over mineralization is greater than 800 ppm,
which cannot be characterized as a subtle extraction anomaly. Despite
some evidence for moderate Mn enrichment in groundwaters contacting
mineralized areas, it is considered that there is unlikely to be
a direct link between the surface Mn anomaly and the Au-rich laterite
ten or more metres below. Instead, it is likely that surface phenomena,
such as an observed drainage system through the anomalous area,
is the cause of this effect. This suggests that the MMI and CSIRO
extraction results showing a correlation of greater extractable
metals with buried Au-rich laterite at Curara is coincidental.
It is considered that partial extractions (such as MMI or enzyme
leach) can only be understood and interpreted correctly if conducted
in conjunction with determinations of the levels of critical soil
phases such as Mn oxides, amorphous Fe oxides and/or any other materials
expected to adsorb or otherwise accumulate dissolved ions at the
site under investigation.
Last updated: Friday, July 21, 2000 04:24 PM
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