Mineralogy and geochemistry of lateritic weathering profiles on ultramafic rock bodies around Mokuro in Ile-Ife area, southwestern Nigeria

摘要

Two laterite profiles on ultramafic rocks in Mokuro-Ife area of southwestern Nigeria, were investigated for the formation of secondary minerals and trace element behavior during humid tropical weathering. Microscopic and X-ray data indicate that the rocks alter to talc, tremolite and chromiferous chlorite. Chlorite becomes thre major stable component and is concentrated in the weathering profile. Geochemical data further indicated that with increasing depth, Fe_2O_3 and Al_2O_3 contents decrease, whereas SiO_2 and MgO increase. Trace elements such as Ni, Co, Cr, Mn and Cu are all concentrated in the lattice of goethite. Comparison with data on soils overlying associated metapelitic schist shows that geochemical investigation on residual soil may be used for mapping purposes in area with thick weathering overprint.