This study is one of the two pioneering ’Regional Geochemical mapping programmes of Nigerian Geological Survey Agency which commenced in 2007. Global Geochemical Reference Network (GRN) covering Nigeria territory consist of 44 cells out of which 35 are either completely within the Nigerian territory or have over 50% of its area within Nigeria. Nine other cells have the larger area within neighbouring nations. The cell covered is the GRN cell N06E04 with the sampling density of 1:150km2 chosen for the study.
The aims of the study are to provide- a baseline geochemical database to monitor contamination and a multi-element geochemical database for future mineral exploration. The standard sampling and analytical procedures of International Geological Correlation Programme (IGCP 259 and 360), were adapted for this study.
This area lies within the basement complex of southwestern Nigeria. Three media were sampled, namely residual soil, stream sediment and stream water. In all 540 stream sediment and residual soil samples were collected and analysed by ICP-MS for 56 elements. The subsoil samples were collected from a 25cm thick section within a depth of 20-200cm (C-horizon) while the stream sediments were wet sieved in the field to <0.150mm fraction. The gamma ray spectrometer measurements were obtained at each sampling point for potassium, thorium and uranium.
Enhanced values of Ni, Co, Ta, Sn and Au were observed within the pelitic schist belt of Ilesa area. Strong anomalous values of U and Th revealed a noticeable NNW-SSE anomaly around the granitic bodies of Osi area.