MINERALOGY AND GEOCHEMISTRY OF TROPICAL RAIN-FOREST SOILS - ASHANTI, GHANA

A detailed study of the pedology, mineralogy and geochemistry of soils and underlying saprolite has been made in the Ashanti area of southwest Ghana. The soils in the area are kaolinite-muscovite ferralsols and vary in thickness from 1 to 10 m, depending on saprolite lithology, topography and drainage. Porosity and Eh decrease with depth in the soil profile while pH and grain density increased. The dominant minerals are kaolinite-muscovite-goethite and this is reflected in the major-element chemistry which is essentially SiO2-Al2O3-Fe2O3-H2O with minor amounts of K2O and TiO2. Similar geochemical dispersion patterns are present for most elements in the lower or B soil horizons as in the saprolite with a similar correlation between the elements. However, these patterns are greatly modified in the surface or A soil horizon and the correlation is lost. Elements such as Se, Mo, As, Cd, Sb and Pb appear to be naturally concentrated by soil processes in the A horizon while other elements, such as Th, Y and Zr, appear concentrated in the B horizon. Consideration of the trace-element distribution patterns indicates that the present weathering profile appears to represent a truncated mature lateritic profile capped by a thin recent organic-rich surface soil.