Chemical Geology, 106 ( 1993 ) 345-358 Elsevier Science Publishers B.V., Amsterdam [MBI Mineralogy and geochemistry of tropical rain forest soils: Ashanti, Ghana 345 R.J. Bowell" The Department of Geology, The University, Southampton, UK ( Received February 24, i 992; revised and accepted January 6, 1993) ABSTRACT A detailed study of the pedology, mineralogy and geochemistry of soils and underlying saprolite has been made in ,he Ashanti area of southwest Ghana. The soils in the area are kaolinite-muscovite ferralsols and vary in thickness from I to 10 m, depending on saprolite lithology, topography and drainage. Porosity and Eh decrease wilh depth in the soil profile while pH and grain densiW increased. The dominant minerals are kaolinite-muscovite-goethite and this is reflected in the major-element chemistry which is essentially SiO2-AI20~-Fe:OrH,O with minor amounts of K20 and TiOz. 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 repre- sent a truncated mature lateritic profile capped by a thin recent organic-rich surface soil. 1. Introduction The Ashanti area is 160 km northwest of Ac- era, in southwest Ghana (Fig. I ). Active gold mining has been continuous in the area since 1895 by Europeans, though native workings have been recorded as early as 1471. Mining, cocoa, palm and timber production are the main land uses of the Ashanti region with ex- tensive areas of primary rain forest (of Triplo- chiton-Celtis forest association) still pre- served (Survey of Ghana, 1969 ). The Ashanti area is characterized by heavily forested uplands rising up to 400 m above sea- level. The soils in the area can be defined as ferralsols (FAO/UNESCO, 1965) and have a ~'Present address: Department of Mineralogy, The Natu- ral History Museum, Cromwell Road, London SW7 5BD, UK. saprolite zone ~ 70 m thick with an the over- lying soil thickness of between I and > 10 m, depending on saprolite lithology, topography and drainage. Most profiles, however, average I-3 m in thickness. Average rainfall is ~ 1250 mm yr-~, and daily temperature rises to 30°C, falling to ~ 20°C at night. The soil profiles at Ashanti can be broadly divided into A, A/B, B and B/C horizons. The B/C horizon is transitional to the saprolite and the more porous soils. The B horizon (from 0.6 to 10 m thick) is composed of a massive clay unit. The surface or A horizon represer~ts an admix of reworked soil and saprolite interdis- persed with residual heavy minerals and forest debris (organic matter). Despite intensive agriculture and mining throughout southwest Ghana, little is known about the mineralogical and chemical charac- teristics of the weathering profiles. In order to 0009-2541/93/$06.00 © ! 993 Elsevier Science Publishers B.V. All rights reserved.