THE WORLD’S LARGEST FOUTA DJALLON–MANDINGO BAUXITE PROVINCE (WEST AFRICA): PART 4. THE MECHANISM OF ZONING FORMATION IN LATERITE BOXITE-BEARING WEATHERING CRUSTS

Abstract

Obtained as a result of detailed exploration and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the lateritic weathering crust of the largest bauxite-bearing province FDM, thanks to a reliable factual base, allowed us to draw a number of important conclusions on the conditions and features of the formation of zoning in the lateritic weathering profile for the first time. The weathering profile consists of two metasomatic columns spatially combined in plan, but vertically separated: the upper, lateritic cover, composed mainly of the products of the final hydrolysis of lateritic weathering, that is, hydroxides and oxides of iron and aluminum; the lower one, composed of clayey eluvium, products of hydration and decomposition of aluminosilicates - polymineral clays and kaolinite. In both columns, the processes of changing begin with the same rainwater, but differ in hydrodynamic parameters in each of the columns. Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism. The modern hydrogeological and gas regime in the bauxite-bearing lateritic covers controls the mineralogical and geochemical zoning. Peculiarities of biological and soil processes, hydrodynamics and hydrochemistry in the weathering profile lead to the threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical (oxidative, reducing, gley, adsorption) barriers - the main mechanism of zonal distribution of matter in lateritic crusts.