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.
The study of a large amount of factual material about the formation conditions of gibbsite or boehmite and their mutual transformations in lateritic bauxites allowed us to solve the problem of boehmite formation and its spatial and genetic relationship to gibbsite. The boehmite formation occurred only during the formation of sedimentary-lateritic bauxites from alluvial and lacustrine sediments that underwent bleaching and resilification at the stage of sediment flooding, as well as on bleached bedrock under sedimentary-lateritic bauxites. The most intense boehmite formation occurred at a depth of 20–30 m from the surface and was accompanied by an alumina input, which was realized here in the form of boehmite along with the gibbsite formation due to the hydrolytic decomposition of kaolinite and partially due to the input alumina. In the upper profiles, the recrystallization of bauxites occurs with the replacement of dissolving boehmite with gibbsite, with a decrease in the total Al2O3 content and an increase in the amount of crystallization water. In classic lateritic bauxites, the boehmite content is highly insignificant; that is, its formation in them practically does not occur or is very much inhibited.
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