Dispersion of gold and base metals in the Mborguéné lateritic profile, east Cameroun

Abstract In lateritic profiles, the behavior of trace elements is difficult to predict and may vary significantly from one element to another. As this behavior directly controls the formation of the secondary dispersion halo around mineralization, it has a dramatic influence on the effectiveness of geochemical techniques in such environments. In the Betare Oya region, in east Cameroun, gold mineralization was indicated by contents of As, Bi, Ag, Mo, W, Pb, and Au. The dispersion of Au and associated trace elements was studied in detail from the fresh ore in deep drill hole samples to soils in surface materials. The local geology consists of Proterozoic (Birrimian) volcanosediments dipping northwest at 35 degrees. Bedrock is covered by a deep (25 m) weathered mantle with ferruginous induration at the top. The mineralization is related to silicified structures and is completely oxidized and weathered at the surface. Trace elements have been dispersed in the lateritic profile. Four main chemical dispersion modes have been defined here for base metals: Bi is residual and is little dispersed; W, Mo, Pb, and Au are progressively remobilized and well dispersed; As is strongly associated with Fe and widely distributed, while Ag is completely leached. An attempt is made to evaluate and compare the size of the geochemical dispersion haloes in the different horizons of a laterite profile and to define an order for halo size at the top of the profile: Ag Little dependent on the hydromorphic processes, a new model for the formation of secondary haloes in laterites is proposed. Haloes could also be produced during lateritization by downward progress of the weathering front. Mineralized veins are dissolved in the top horizons, and trace elements are dispersed and concentrated in the surrounding ferruginous horizons.