Mineralogy and geochemical dispersion in lateritic soil profiles of northern Nigeria

Abstract

Six lateritic soil profiles, overlying metasedimentary and crystalline basement rocks of northern Nigeria have been investigated in order to study the mineralogical and geochemical relationships between bedrock and weathering products. A total of 96 samples have been collected including 26 from exposed quarry and road-cut faces and 70 from auger-holes to a depth of 8 m. Two selective leaching methods (cold extraction with 0.1 M HCI and hot extraction with aqua regia) have been applied to the < 75- μm fraction and up to sixteen elements determined by AAS. Conclusions are based largely on the auger-hole samples where element distributions have not been modified by subaerial exposure. Quartz is the dominant mineral in the upper part of the soil profile while kaolinite increases strongly with depth. At the base of some profiles mica is also abundant. Elements are distributed through the soil profile according to their association with clays, iron oxides and to a lesser extent, carbonates. While intercalations of lateritic crusts obscure general trends with depth, dispersion and accumulation processes appear to be similar within each profile. In spite of the intensive weathering and secondary accumulation processes characteristic of the savanna belt, the lateritic soils within surface sampling depth appear to retain a geochemical imprint of the underlying bedrock. Criteria and potential problems in the application of soil geochemistry to bedrock mapping are discussed.