Gold deposit type and implication for exploration in the Abiete-Toko Gold District, South Cameroon: constraint from morphology and microchemistry of alluvial gold grains

The morphology and quantitative chemical analyses of fifty alluvial gold grains from fourteen studied sites were used to constraint gold deposit type and its implications to exploration in the Abiete-Toko Gold District in South Cameroon. The main results revealed that the gold grains show a core-rim zonation marked by Ag depleted rims as a result of leaching during transportation. The fineness of grains ranges from 826 to 1000 and their composition is almost binary gold-silver. Gold fineness refers to the relative amounts of Ag and Au present, given as a number out of 1000 and defined by Au/(Ag + Au)∗1000. The chemical composition of the gold grains are range as thus: 83.40–100 wt.% Au, 0.07–17.45 wt.% Ag, 0–0.96 wt.% Cu, 0–0.01 wt.% As, 0–0.02 wt.% Ni, 0–0.02 wt.% Co, 0–0.01 wt.% Se, 0–0.08 wt.% Hg and 0.003–0.03 wt.% S. The high sulphur concentrations of the gold grains probably imply primary deposition of gold by sulphidation; The ranges of Ni and Co concentrations suggest an interaction with greenstones. Such findings indicate that the Abiete-Toko area hosts a mesothermal-orogenic gold deposit. Exploration operations must target fractures and contact zones between ironstones, felsic gneiss and greenstones. Soil sampling is not recommended in this orogenic gold district due to the very thick and transported nature of the weathering mantle. Detailed mapping coupled to geophysical surveys are the recommended key exploration methods prior to drilling program.