ИСТИНА

Facial analysis of continental deposits has been made in surroundings of large high-grade bauxite deposits of the northwestern Fouta Djalon-Mandingo Bauxite-bearing Province to judge in principle if high-grade bauxite deposits can form in mechanical sedimentation. The studied continental deposits represent their marginal facies, namely (1) eluvium on plateaus, as deposits representing the area of matter mobilization, and (2) alluvium as deposits transported to the largest distances and graded most essentially in the continental environment. The eluvium is polygenetic. Coarse to medium debris is autochthonous, formed after bauxites and laterites. Fine sandy, silty, and clayey matter is mainly eolian. Transported and redeposited material is initially quality-graded, depending on granulometry: coarse fractions represent bauxites; gravel to coarse sand fractions are dominated by ferruginous laterites; sand-clay matrix shows elevated SiO2. In order to estimate the redeposition we have studied channel and flood-plain facies of recent alluvium in Kogon and Tingilinta valleys. Dominating in the channel alluvium are gravels and pebbles, whereas fine gravel and sand are essentially subordinate there. Nevertheless, even these deposits, because of their elevated silica, in most cases may not be classified as bauxite and especially as high-grade bauxite. The only exception is a well-washed thin interbed of pebbles in Tingilinta alluvium, representing high-grade bauxite yet with elevated SiO2. Flood-plain sand-clay deposits of both valleys show Al2O3 strongly decreased, Al2O3/SiO2 decreased to lower 1, and organics strongly increased. Thus, only accumulation of coarse material well-washed from medium-fine-grained clastic matrix can produce a small bauxite deposit yet with elevated silica. Continental deposits, whatever their facies, do not form any high-grade bauxite deposit even in the uniquely favorable environment. The mechanical sedimentation does not provide with any environment separating aluminum from iron and silica, and this makes such a formation of high-grade bauxite deposits impossible.