Abstract

The laterite overlying the Misséni gold occurrence in Mali is characterized by the presence of remarkable black layers of ferro-manganiferous oxides. In an attempt to study the origin of the concentrations in these layers, 56 rock and soil samples were collected from three wells and from a borehole near the occurrence. The bedrock of the area is composed of Lower Proterozoic volcano-sedimentary formations, within which primary gold is associated with copper and iron sulphides disseminated in crushed and silicified zones in andesites and andesitic volcaniclastites. The general profile of the laterite at Misséni, from bottom to top is as follows: (i) saprolite (including the Fe–Mn oxides rich layers), (ii) ferruginous hardpan, (iii) ferruginous cuirasse, and (iv) eluvium. The mineralogical composition of the layers rich in Fe–Mn oxides is dominated by geothite, hematite, psilomelane, and quartz enveloped in a kaolinite matrix. There is a significant increase of the following elements in these layers: (i) trace elements: Au, Cu, Co, Ba, W, Pb, V, Be, As, Sb, Cs, Se, U, and Th; (ii) rare-earth elements: La, Ce, Sm, Yb, Eu, Lu, and Tb. The average gold content measured in these layers of black Fe–Mn oxides is 672 ppb. These gold concentrations are three times higher than the amount of gold found in the saprolite (233 ppb) surrounding these layers and six times superior to the gold content measured in the country rock (109 ppb) hosting the auriferous zone. The gold content of oxide grains derived from the black layers range from 0.25 to 7.7 ppm. The black layers of Fe–Mn oxides of Misséni are probably related to the remnants of an old phreatic surface, above which the oxides of Fe–Mn were deposited as a result of the prevailing oxidation conditions in this environment. The gold would have precipitated synchronously, by reduction from gold thiosulphate and its adsorption on the newly formed oxides. [Journal Translation]

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