GEOCHEMISTRY OF HORIZONS OF BLACK FERRO-M ANGANIFEROUS OXIDES RICH INGOLD IN MISSENI LATERITE IN MALI

The laterite overlying the Misseni gold occurrence in Mall is characte rized by the presence of remarkable black layers of ferro-manganiferou s oxides. In an attempt to study the origin of the concentrations in t hese layers, 56 rock and soil samples were collected from three wells and from a borehole near the occurrence. The bedrock of the area is co mposed of Lower Proterozoic volcano-sedimentary formations, within whi ch primary gold is associated with copper and iron sulphides dissemina ted in crushed and silicified zones in andesites and andesitic volcani clastites. The general profile of the laterite at Misseni, 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 o xides is dominated by geothite, hematite, psilomelane, and quartz enve loped in a kaolinite matrix. There is a significant increase of the fo llowing elements in these layers: (i) trace elements: Au, Cu, Co, Ba, W, Pb, V, Be, As, Sb, Cs, Sc, U, acid Th; (ii) rare-earth elements: La , Ce, Sm, Yb, Eu, Lu, and Tb. The average gold content measured in the se 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 r ange from 0.25 to 7.7 ppm. The black layers of Fe-Mn oxides of Misseni are probably related to the remnants of an old phreatic surface, abov e which the oxides of Fe-Mn were deposited as a result of the prevaili ng oxidation conditions in this environment. The gold would have preci pitated synchronously, by reduction from gold thiosulphate and its ads orption on the newly formed oxides.