Influence of the parent rock nature on the mineralogical and geochemical composition of ferralsols used for sedentary agriculture in the Paleoproterozoic Franceville sub-basin (Gabon)

Abstract

The aim of this study was to assess the influence of parent rock nature on the mineralogical and geochemical properties of ferralsols  used by sedentary subsistence farmers in the Proterozoic Franceville subbasin of Gabon. Thus, soils developed from cherts, black shales  and sandstones were investigated for their mineralogical composition and their heavy metal contamination of topsoil using the X-ray  diffraction (XRD), and Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Results show that the dominant mineralogical assemblage is made of quartz, illite, kaolinite and smectite, and this is reflected in the major-element chemistry which include essentially  SiO₂ (46.4 - 89.2%), Al₂O₃ (4.3 - 19.8%) followed by Fe₂O₃ (0.7 - 15.3%), with highest amounts of SiO₂ and lowest amounts of Al₂O₃ and  Fe₂O₃ found in soils developed from sandstone. The geochemical data revealed some doleritic intrusions through the chert formations  with highest values of Fe2O3 and TiO₂ in the overlying soils. Results indicated serious health concern associated with a geogenic source  of As, Ba, Cd, Cu, La, Pb, Rb, Th and U in soils developed from cherts and black shales. Consequently, only the uncontaminated soils  developed from sandstone could be appropriate for smallholder farming communities.