CRC LEME
Open File Report 124
ABSTRACT
The geochemistry of transported soils and weathered bedrock at
Police Creek, Drummond Basin, Queensland - A progress report
K. M. Scott
Two distinct anomalies occur in the soils at the Police Creek epithermal
gold deposit. The major geochemical anomaly occurs within transported
soils overlying bedrockhosted mineralisation. It is best defined
by the occurrence of >100 ppb Au (and elevated S) in the <75
µm kaolinite-rich fraction of the soils and by As >400
ppm (and elevated Sb and Mo) in the >2 mm Fe-rich fraction of
the soils. The second type of anomaly in thin residual soils directly
overlying bedrock is characterised by anomalous Au, As and Sb within
the >2 mm of the soils. Although use of <75 µm fraction
of the soils gives a larger anomaly than the previously used <80
mesh (<180 µm) fraction, use of only that material would
not identify the second type of anomaly. Thus a strategy of analysing
both the fine and coarse material in soils in the region is recommended
if the origin of the soils is not known.
Study of the bedrock in a number of profiles also reveals that
Au is likely to be separated from pathfinders like As and Sb during
weathering process. Under very acid conditions (reflected by the
presence of alunite in rocks), Au is depleted but the pathfinders
(As, Sb, Mo and W) are retained. However, under more alkaline conditions
where near-surface dolomite is present, Au is present but pathfinder
contents are low. Analysis of bedrock material for Au, As, Sb, Mo
and W is thus recommended with knowledge of the mineralogy also
important to understand the acidity of the environment and, hence,
why different suites of elements occur.
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