CRC LEME
Open File Report 91
ABSTRACT
Investigation of the hydrogeochemical dispersion of gold and
other elements at Lawlers, Western Australia
Gray, D.J.
The hydrogeochemistry of the Lawlers district was investigated,
with a view to understanding the interaction of groundwaters with
mineralized rocks, and the potential for exploration, particularly
in areas of extensive overburden. Groundwaters at Lawlers are neutral,
with a similar Eh range to other neutral groundwaters. However,
they differ from groundwaters investigated at other Yilgarn sites
in having low salinities and marked divergence from sea water ratios,
suggesting that the major ion concentrations are strongly controlled
by local lithological or hydrological factors. Elements that appear
to be controlled by mineral equilibration in some or all groundwaters
are Ba (barite equilibration), Ca, Mg and HCO3 (carbonate), Mn (rhodochrosite),
Sb [Sb(OH)3], Pb and V (Pb2V2O7).
The most marked feature of the Lawlers groundwaters is the extremely
high Au concentrations, with the highest observed concentration
of 40 µg/L being about ten times greater than any other previously
reported values. These high concentrations are correlated with highly
anomalous Co concentrations at Hidden Secret and in the Gt. Eastern/Caroline
pits area but not at Genesis. Under the groundwater conditions observed
at Lawlers, the most likely mechanism for dissolution is expected
to be as the thiosulphate complex Au(S2O3)23-. The generation of
thiosulphate requires oxidation of sulphides to occur under neutral
conditions, with the acidity produced being neutralized by the dissolution
of minerals such as primary carbonates. This is consistent with
limited results at Hidden Secret suggesting that higher Au concentrations
are correlated with increased Mg, Ca, HCO3 (from carbonate dissolution)
and SO4 (from sulphide oxidation).
Most of the specific sites investigated showed specific groundwater
element anomalies:
- Hidden Secret - Au, TDS, SO4, P, Mn, Fe, Co, Ni, Y, REE, W,
As, Sb, I, U, and possibly Ba, Ga, Sc and Ge;
- Genesis - Au, Sc, As, and possibly Ni and Sb;
- Mineralization at Gt. Eastern-/-Caroline - Au, Co, and possibly
SO4, Sc and As;
- Mineralization at Four Corners - SO4, Al, Ga, Mn, Fe, Co, Ni
and Mo (though generally based on only one sample at the centre
of the mineralized area);
- Granitic rocks at Gt. Eastern-/-Caroline - Al, Ga, Ba, Mn, Mo
and U;
- Ultramafic rocks - Cr, Cs and Sr;
- Southern part of Four Corners - TDS, B, Sr, HCO3, I and U (possibly
indicating a different groundwater system in this area).
The differences in the minor element 'signatures' for the groundwaters
in areas of major Au mineralization (Genesis, Gt. Eastern/Caroline,
Hidden Secret) may reflect differences in the lithologies of the
three sites and/or the geochemistry of the Au mineralization. The
Gt. Eastern/Caroline and Hidden Secret groundwaters have groundwater
signatures similar to other mineralized sites elsewhere in the Yilgarn,
with anomalies in a range of chalcophile and other pathfinder elements.
In contrast, groundwaters at Genesis are anomalous in only Au, Sc
and As, possibly due to the Genesis ore deposit being hosted in
a quartzite unit with free-milling Au and disseminated arsenopyrite.
At the Four Corners area, which is covered with extensive transported
overburden, there are anomalous concentrations of varying elements
(but not Au), with size of the groundwater anomaly appearing to
be less than 250 m diameter. The exception to this is SO4, which
has a more disperse anomaly.
Groundwater at Lawlers appears to have exploration potential, particularly
in the residual areas. In areas of extensive transported overburden,
there is no observed groundwater Au response, though some elements
(SO4, Ga, transition metals, Mo) may have potential as pathfinders
for Au.
Last updated: Friday, July 21, 2000 04:24 PM
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