Formation stages of the unique sangaredi bauxite-bearing group, Futa Jallon-Mandingo province, West Africa

The middle Miocene continental fluvial sediments of the Sangaredi Group in the western districts of the Futa Jallon-Mandingo bauxite-bearing province were initially more widespread. Their thickness in the axial zones of the main drainage systems (Paleocogon, Paleotomine, etc.) attained 150 m. The lower, early Miocene steps of topography formed during the first phase of regional neotectonic evolution were buried beneath these sediments by the end of middle Miocene. The sediments of the Sangaredi Group underwent substantial epigenetic alteration in the glei geochemical setting with removal of iron and bleaching. At the second stage of neotectonic uplifting and dissection, this low-Fe and high-Al protolith was again affected by lateritic weathering with formation of unique sedimentary-lateritic bauxites. The underlying bedrock was also affected by bleaching and lateritization with formation of high-quality infiltration-metasomatic bauxites with geliform cryptocrystalline structure and massive porcelainlike or oolitic texture. Both classes of bauxites are high-quality and of great economic importance.