Integrated Multi-Elemental Soil Geochemical Survey as a Tool in Geological Mapping in the Tropics: A Case Study of Songbe Area of Osun State, West Africa

Abstract

Soil geochemical survey of Songbe area, a typical tropical environment of Nigeria was carried out to determine the concentrations and distribution patterns of trace elements in order to decipher the underlying geology in the area, as a complimentary tool to geological mapping, especially in areas with little or no rock exposure. The soil samples were collected by driving a 1-meter auger down to a depth of 50 - 80 cm into the ground after removing the top soil, to ensure adequate penetration into the “B-horizon”. The samples were air-dried at room temperature for four weeks. Each sample was disaggregated gently with porcelain mortar and pestle. The pestle and mortar were washed and dried after each sample is processed to avoid inter-sample contamination. The samples were subjected to standard preparation and treatment prior to geochemical analyses determined by X-Ray Fluorescence (XRF) method on a PANalytical Axios Wavelength Dispersive spectrometer and LA-ICP-MS (Laser Ablaser Inductively coupled Plasma Mass Spectrometry) method  at the Central Analytical Facility (CAF) of Stellenbosch University, South Africa. The point location maps were plotted using ArcView-Gis.The location point symbol map of each of the elements analysed revealed diverse patterns of distribution and showed considerable variations from one area to the other. Some of these distribution patterns did not perfectly correspond with the inferred geological boundaries, especially in areas with thick overburden. The point symbol maps of magnesium, iron, nickel and chromium (Mg, Fe, Ni and Cr) revealed anomalous and relatively high values in some areas underlain by quartzite, whereas anomalous and relatively high values of these ferromagnesian minerals are expected dominantly in areas underlain by mafic or ultramafic rocks. The symbol maps of tantalum and thorium showed dominantly low values for areas deduced to be underlain by charnockite with relatively high concentrations for pegmatitic bodies.The integration of the interpreted soil data with the point symbol maps of element concentrations superimposed on the produced geological map showed that most of the inferred rock boundaries were overestimated or underestimated and some mineral occurrences were yet to be accounted for. Hence, it was concluded that multielemental chemical analyses using soil as sampling media is a vital tool to delineate underlying geology in the tropics and is also an aid to geological mapping

DOI: 10.7176/JEES/13-5-03

Publication date:July 31^st 2023