Mineralogical and Petrographic Features of the Lower Horizon of Ferruginous Laterites and Ferriplantites of the Fouta Djallon–Mandingo Province (Guinea, West Africa)

A ferruginous laterite horizon was established for the first time as a result of the detailed study of the profile of the bauxite-bearing lateritic weathering crust in the Fouta Djallon–Mandingo province. This horizon occurs ubiquitously between the clayey horizon and the bauxite zone and is spatially associated with the hydrogeological zone of groundwater-table fluctuation. It formed in the course of the mass accumulation of iron minerals, due to the effect of intensely oxidative geochemical barrier, as a result of the active ferruginous metasomatism. Plates and lenses of highly ferruginous rocks, ferriplantites, formed in this horizon. The chemical composition of ferriplantites in absolute quantities of rock-forming oxides shows that Fe 2 O 3 accumulation during their formation was 9–11 times more active compared to kaolinite clays and approximately twice as active as compared to the ferruginous horizon as a whole. This was accompanied by the mass removal of silica from the lower part of the profile. The study of the mineralogical and petrographic features of ferriplantites made it possible to establish their composition and the sequence of mineral formation. Several generations of the main rock-forming ferruginous minerals in the sealing wax-red (brownish-red) and tobacco-ocherish (yellowish-brown) ferriplantites—hematite and goethite, respectively—have been identified. The character of interaction and the form of occurrence of the minerals were established. In addition, the participation of biota in the redistribution of substances and bio- and phytomorph formation was established for the first time.