Use of regolith geochemistry to delineate gold mineralisation under cover: a case study in the Lawra belt, NW Ghana

摘要

The Birimian rocks of southern Ghana host world-class gold deposits yet no equivalent-sized deposit has been found in the Birimian rocks in northwest Ghana. The reported gold occurrences in the area suggest a favourable environment for concealed mineralisation if the regolith and landscape evolution can be unravelled. A geochemical exploration model based on the evolution of the regolith-landform can help guide the choice of exploration methodology appropriate for the region.\udThe developed regolith map classifies the landscape into ferruginous (F), relict (R), erosional (E) and depositional (D) domains. Depositional areas cover 72% whereas the remaining 28% represents residual environments. Discrimination, characterisation and identification of regolith materials were carried out by pit and outcrop mapping. Regolith geochemical data provided regolith profile information which helped to distinguish residual regolith from transported types. Binary plots of major and trace element geochemical data were used to determine the compositional variability and different regolith types. Superimposing geochemical data on the regolith map identified residual and transported anomalies, and thus prioritized the weak, subtle and discontinuous anomalies. Gold, Ag and As relations in the regolith were also useful in determining the anomaly type and presence of gold mineralisation. High Au-low Ag with smooth dispersal patterns represents residual anomalies whereas spiky dispersion patterns with high Au-high Ag characterize transported anomalies. The released As into the regolith appears counteracted by the precipitation of Fe-oxyhydroxides, which efficiently scavenge As3+ and As5+ at neutral pH in the regolith, resulting in the weak As concentrations in the analysed samples. Hence it may not be an appropriate pathfinder element for Au in the study area.\udThe regolith mapping techniques devised for the study can be used to map complex regolith terrains. The landscape evolution model developed for the area will provide useful insight into the irregular distribution of the regolith and help in designing exploration techniques suitable for the Lawra belt and similar complex regolith terrains of the savannah regions.