The gold-in-calcrete anomaly at the ET gold prospect, Gawler Craton, South Australia

Abstract

Abstract Much of Australia has an extensive regolith cover that conceals basement rocks and hinders mineral exploration and this situation is particularly acute in the Western Gawler Craton (South Australia), where, in addition to fluvial, marine and colluvial sediments, the land surface is extensively cloaked by sand dunes. This study documents the Au distribution at the ET Au prospect (Great Victoria Desert) in the Western Gawler Craton (South Australia). Although no economic Au mineralisation has yet been found at ET, the prospect hosts one of the larger Au-in-calcrete anomalies in Australia and is typical of many such prospects identified in the region. In addition to calcrete, the distribution of Au in regolith and biotic sample media was also examined. The study at ET shows that: (i) For in situ regolith, Au is concentrated in surficial calcrete and above an upper saprolite zone depleted in Au. (ii) The Au anomaly extends from a ridge of weathered Archaean basement into adjacent and transported regolith dominated by thin (∼5 m) aeolian sand cover. The anomaly appears to be locally broadened by lateral dispersion in the transported overburden, or, possibly, Au additions to the surface from underlying buried mineralisation. (iii) Calcrete appears to be the most consistent sample medium for Au providing coherent anomalies compared with soil, vegetation, near surface drill cuttings and Bacillus cereus . (iv) Gold appears to be the best target element in upper regolith or calcrete, although As may provide supplementary information on the location of prospective mineralisation. Calcrete sampling has been a successful exploration technique to reveal cohesive Au anomalies within in situ regolith. Where transported regolith dominates and landforms are favourable e.g. sloping, calcrete (containing Au) will disperse (in solution and/or mechanically eroding) and thus provide a spatially larger target area for mineral exploration purposes. However, in these settings, the actual source of mineralisation may be difficult to locate due to various factors related to the dispersion processes and weathering history. At ET, other sampling media such as bacteria ( Bacillus cereus ) and vegetation have a greater uncertainty associated with them and their anomalies are not as cohesive.