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Open File Report 124

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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