Alluvial gold in the Bétaré Oya drainage system, east Cameroon

The morphology and elemental composition of alluvial gold grains from the Bétaré Oya gold district were investigated as part of a district exploration strategy. The morphology and general chemistry of the grains, determined using scanning electron microscopy (SEM)—energy dispersive spectroscopy and electron probe microanalyzer (EMPA), respectively, revealed three categories of grains: (1) gold grains with irregular to regular, bent-up and folded outlines with irregular and pitted surfaces and a flatness index ranging from 2.1 to 4.6; (2) gold grains with regular and polished outlines, smooth surfaces with few or no cavities and a characteristic flatness index range between 3.0 and 8.6; (3) and elongate grains. Rounding of the grains, physical abrasion and bent/folded features suggest that the alluvial gold grains have been transported in a high-energy environment, but not necessarily over long distances from their source rock(s). The gold grains are alloyed with Ag and Cu with concentrations of Ag ranging from 0 to 14.19 wt% whereas Cu concentrations are between 0.03 and 0.15 wt%. SEM images of sites where active weathering of gold is indicated by the presence of colloidal gold, i.e., crevices possessing sedimentary materials on the surface of the grains, revealed the presence of bacteria. All of the gold grains analyzed possess high purity (~ 100% Au) at the water–sediment–gold interface demonstrating that Ag and Cu are highly dispersed in placer systems relative to gold, and that gold can be dissolved and subsequently re-precipitated possibly with the aid of bacteria in alluvial systems.