Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria

Abstract

A geochemical investigation was initated on 24 samples from four different weathering profiles—inselbergs—of the Jos Plateau, Nigeria. Results are presented on field-occurrence, macro- and micro-textures, normative mineralogical composition, kaolinite crystallinity, porosity and density measurements and major and trace element compositions. The polygenetic overprinted profiles, up to 80 m thick, show vertical zonation in soil horizons with very strong Fe- or Al-accumulation in the B ox-horizon that classify them as fossil oxisols whose derivation is related to coastal plains during Lower Tertiary times. The fabriques of the in situ weathering profiles show relic textures according to basaltic or magmatic parent rocks in the lower parts. The upper parts of the sections, where relic textures also partly occur, are mainly characterized by neoformed textures like vesicular, concretionary and spongy textures. In the B ox-horizons, which are in all sections of basaltic origin, lateral facies change is developed. In profiles 9 and 10, this horizon is developed as an iron crust (B ox, fe) and in profiles 12 and 13 as bauxite (B ox, al). Partly or completely maintained relictic minerals in the lower parts of the sections are quartz, zircon, monazite, rutile and opaque heavy minerals. Later eolian transport led to infiltration by illuviation in the upper parts of the profiles with some of these minerals. The mineral composition of the profiles is mainly determined by newly formed minerals like kaolinite, gibbsite, hematite, geothite and anatase. Kaolinte crystallinity values are mostly high except for the B/C-horizons of profiles 9 and 10 as well as for the upper part of the B r-horizon in profile 9. Porosity and density in the B/C- and B r-horizons are relatively monotonous. In the B ox-horizons porosity is reduced and density is mainly in relation to Al- or Fe-minerals. Chemical analysis shows enrichment of elements like Al, Fe, Cr, V and Hf in the B ox-horizons, whereas soluble elements like alkalis and earth-alkalis have been leached out. Beside the preserved relic textures of the parent rocks, the origin of the weathering products is determined geochemically by the discrimination diagram ‘Zr/TiO 2: Nb/Y’ after Floyd and Winchester (1978, Chem. Geol. 21, 291–306).