CRC LEME
Open File Report 106
ABSTRACT
Geochemical and spatial characteristics of regolith and groundwater
around the Golden Delicious Prospect, Western Australia
Bristow, A.P.J., Gray, D.J. and Butt, C.R.M.
The dispersion and solubility of Au and other elements in the regolith,
and the nature and distribution of regolith materials has been studied
at and around the Golden Delicious deposit near Laverton, Western
Australia. The deposit has a resource of 6.1 Mt ~ 1.3 g/t Au hosted
by a suite of Archaean granitoids that intrude mafic volcanic and
volcaniclastic greenschist host rocks in the southern part of the
Laverton tectonic zone. The mineralised and country rocks are deeply
weathered, and subsequently partly eroded to the clay zone and upper
saprolite. They are now covered by up to 20 m of colluvium - alluvium
derived from eroding mafic regolith several km to the ESE. Late-stage
hematite mega-mottling has overprinted the upper residual horizons.
The entire regolith is saturated with saline, weakly alkaline groundwater
below approximately 10 m. Much of the regolith, including the transported
overburden, has been extensive modified during arid conditions.
Samples of regolith and groundwater from a drill traverse across
the deposit have been examined in detail to determine element distributions,
their relationship to regolith evolution and their significance
in exploration. Elements associated with Au mineralisation were
W, Sb, K and REE, though none displayed a very direct correlation
with Au. Only Au showed evidence of significant remobilization,
the other elements approximating their distribution in the Archaean
prior to weathering.
The distribution, solubility and accessibility of Au in the regolith,
its concentration in groundwater, and the chemistry and depth of
groundwater, suggest that Au has been dissolved from mineralisation,
and re-precipitated in the base of the transported overburden to
give a Au anomaly (12-100 ppb), 500 m across strike, offset slightly
down-slope of underlying mineralisation. No evidence, other than
the Au contents of the groundwater and the slightly soluble nature
of Au in the weathered Archaean, was found to suggest significant
Au depletion or mobilization and it is suggested, based on the distribution
and solubility of other elements associated with Au mineralisation,
that the present Au distribution in the residual regolith is similar
to that prior to weathering. Despite the lack of evidence for significant
Au mobility, there is some enrichment (12-80 ppb) in the top few
metres of the residuum for 300 m across strike, directly overlying
mineralisation, that may be associated with late stage mottling
or residual concentration. Gold in groundwater is anomalous, (0.046-0.18
1µg/L), though slightly offset to the west, for 200 m across
strike of mineralisation. Tungsten is anomalous (8->50 ppm) below
the top few metres of the residuum for 400 m across strike, and
indicates a broader primary halo than Au in the primary mineralisation.
The distribution of Sb concentrations when normalised to Fe is similar
to that of W. Neither Sb nor W are anomalous in the transported
overburden overlying mineralisation and are relatively insoluble
in the weathered Archaean.
Last updated: Sunday, August 05, 2001 13:59:33
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