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Open File Report 127

Soil, bedrock and profile geochemistry at Police Creek, Drummond Basin, Queensland

K. M. Scott

Anomalies within residual soils along the 20300E traverse at Police Creek are defined by both the +2 mm and -75 µm fractions. Although Au, As and Sb are anomalous in both soil fractions, Au is preferentially concentrated in the fine fraction and As and Sb in the coarse fraction. Significant Mo may also be present, especially in the coarse fraction in anomalous areas. Gold >40 ppb in the fine fraction and As >100 ppm in the coarse fraction define an anomaly at least 1 km x 600 m (southern anomaly) in alluvium and residual soil. As in the previous survey at 20050E, a second anomaly was found to the north of the major geochemical anomaly and again identified by anomalous Au, As and Sb contents in both soil fractions. Arsenic >100 ppm defines the 400x150 m northern anomaly in residual soil. Comparison of results from residual soils and adjacent outcrop reveals that the fine fraction of the soils concentrate Au with respect to the rock. These features indicate that the soils at Police Creek are a good sample medium whether they are transported or residual.

The effect of weathering on three distinctive profiles have been observed. Highly mineralized samples with illite, adularia and kaolinite associated with arsenical pyrite (up to 7.3% As, 2.3% Sb and 0.4% Hg) weather to illite, kaolinite, jarosite and Fe oxides. In less mineralized samples, chlorite and calcite occur with adularia and illite, with the latter two minerals retained in outcrop with neoformed kaolinite. Areas of argillic alteration contain dickite and pyrophyllite, with the former being retained in outcrop. Such mineralogical and consequent geochemical variation in the feldspar and phyllosilicate stabilities/abundances could be defined by PIMA and radiometric survey of outcrop (or shallow drill spoil) to provide vectors to the most altered areas during exploration.


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