58 GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Alx en Provence, France. GEOCHEMISTRY AND MINERALOGY OF SOILS COVERING LATERITES AND THEIR USE FOR GOLD EXPLORATION FREYSSINET Ph. *, ROQUIN C. **, MULLER J.C. **, PAQUET H. ** and TARDY Y. *** * B.R.G.M., BP 6009, Orlrans Cedex, France. ** Centre de Grochimie de la Surface, 1 rue Blessig, 67084 Strasbourg Cedex. *** Institut Franqais pour la Recherche et le Drveloppement en Cooprration (ORSTOM), BP 2528, Bamako, Mali. In West Africa, lateritic profiles and mainly ferricretes are very often capped by a silty-clay soil cover. In topographic depressions of the morpho- logy, such soils can reach several meters thick and constitute flat deposits on several kilometer square. Thus, the geochemical signal of these soils is of considerable importance for mining exploration (FARREL, 1984; CARVER et al, 1987; SCHELLMANN, 1989). Geochemical and mine- ralogical aspects of these soils where examined in detail, as well as the lateral and vertical distribution of gold. The study area is localised in the Kangaba area (South Mali). MORPHOLOGY The morphology of the Kangaba area is slightly undulated. Ferricrete plateaus are capped by gravelly soils mostly made of lateritic nodules derived from the dismantling layer of the ferricrete. In depressions of lateritic plateaus and mainly on thalweg flats, grey silty soils are well developed. These soils consist of an accumulation of fine quartzic and kaolinitic material (ROQUIN et al., 1990). SOIL PROFILES In cross section, soils of flat are composed of several horizons. 1- At the base, the fresh ferricrete made of hematitic nodules included in an indurated ferruginous matrix. 2- An horizon of 30 to 100 cm of weathered ferricrete. The degradation of ferricrete is progressive and results from a transformation of the hematitic matrix into goethite. Tubules in the ferricrete are filled with a silty material coming from the upper horizons. 3- The weathered ferricrete is covered by a nodular horizon of 50 to 120 cm thick. The ferruginous matrix is totally dissolved and hematitic nodules, more resistant, appear then isolated within the silty material. At the top, hematite of nodules is also dissolved or transformed into a pulverulent goethite forming yellow iron stains. 4- The nodular horizon is finally capped by a grey-yellow layer, where iron stains are diffuse. That horizon can reach 100 to 300 cm thick. The soils surface submitted to rainfall and lateral runoff is washed and depleted in clay fraction producing a relative enrichment of quartz in the top 30 cm. The petrographic studies show a weathering process of the ferricrete by a progressive dissolution of the ferruginous material at the contact with the silty material. The ferricrete degradation process occurs in situ. The presence of thick soils creates hydromorphic conditions at their base, where ferricretes are strongly weathered. Iron oxi- hydroxides are dissolved and large voids are invaded by the silty material coming from the upper horizon. ORIGIN OF THE SILTY MATERIAL The average composition of the silty material is 62% quartz, 33 % kaolinite and 2% iron oxyhydroxides. Muscovite and heavy minerals are also frequently observed (ROQUIN et al., 1990). The quartz morphology showed that quartz grains are primary and tittle or not transported. No eolian impacts were observed on grain surfaces. Most of quartz grains don't show