Origins and applications of size fractions of soils overlying the Beasley Creek gold deposit, Western Australia

Abstract

Primary mineralisation at the Beasley Creek Au deposit, hosted in black shales within mafic and ultramafic rocks, was sulphide- and trace element-rich. The deposit subcrops beneath soil within a small window of deeply weathered Archaean basement in the northeast Yilgarn Craton of Western Australia, surrounded by extensive colluvial–alluvial plains. Regolith overlying the mineralisation and its host sequence is comprised of ferruginous saprolite and some duricrust, overlain by red–brown soil which is strewn with residual ferruginous lag. The colluvial–alluvial wash plains around the weathered basement window are mantled by a thicker red–brown soil, strewn with polymictic lag. Soil samples and surface lag were collected along two traverses across the mineralisation with the aim of determining the optimum geochemical sampling medium for exploration. The 710–4000 μm soil fraction consists largely of black goethite- and hematite-rich nodules, red to yellow ferruginous clay granules, minor quartz, calcrete and, close to subcropping mineralisation, scarce gossan fragments. In contrast, the 75–710 μm fraction, which appears to be largely aeolian in origin, consists mainly of red–brown hematite-coated sand grains, minor feldspar, and small, ferruginous granules. The largely aeolian 4–75 μm fraction is composed mostly of quartz, with minor kaolinite and Fe oxides. A clay-rich <4 μm fraction was removed from the <75 μm material by dispersion in water and sedimentation. The complete soil and the 710–4000, 4–75, and <4 μm fractions were compared as sampling media. Due to its dominant wind-blown quartz component, the 75–710 μm fraction was discarded. The 710–4000 μm fraction, and the fine lag derived from it, are the most effective geochemical sampling media. The distributions of As, Au, Cd, Cu, Sb, Se, W and Zn in this soil fraction are related to mineralisation. Gold gives a 600-m-wide anomaly of >20 ppb, locally reaching 200–300 ppb. The black shales, which host the mineralisation, are indicated by maxima in Ba and Mn. The 4–75 μm fraction is a significantly less effective sample medium than the <4 μm fraction due to dilution by aeolian silt. The <4 μm fraction indicated the host phyllite by an increase in sericite; the phyllites and duricrusts were indicated by an increase in kaolinite. Dispersions of Au, As, Cd and Cu in the <4 μm fraction show that it is also a useful sampling medium.