Geochemical patterns in the laterite profile at the Boddington gold deposit,Western Australia

Mineralogical and multi-element chemical analyses of drill-cuttings and surface samples have been used to determine element patterns in residual bauxitic laterite, in southwest Western Australia, as an aid in prospecting for base-metal and gold mineralization. In the profiles studied, components can be divided into those which are generally leached (Ba, Co, La, Mn, Ni, Rb, Sr, Ta, Y, Zn, CaO, K₂O, MgO and Na₂O); those which are mobilized and reprecipitated within the profile (Au, Ce, Cr, Cu, Mo and Fe₂O₃) and those which are essentially residual (Nb, Sn, Th, V, W, Zr, Al₂O₃ and TiO₂). Their mobility depends on rock type, minerals present, Eh/pH fluctuations and degree of mechanical movement in the upper parts of the laterite profile. “Residual” components have been mobilized in the profile but have not been transported any great distance. Anomalous concentrations of As, Au, Cr, Cu, Mo, Nb, Sn, Th, V and W, retained at the surface within recognizable lateral distance (100–150 m) of their bedrock sources, appear suitable as either direct or indirect indicators of bedrock mineralization.Significant concentration of gold occurs within the lower part of the lateritic hardcap and upper part of the underlying nodular “B-zone”. Local concentrations of gold lower in the profile are attributed to still-stands of the water table during periods of generally increasing aridity over the last 30 million years.