Petrography and Geochemistry of Orin-Ekiti Basement Rocks, Southwestern Nigeria: Implications on Bauxitization

Abstract

The petrography and geochemistry of Orin-Ekiti Basement rocks were evaluated to determine their mineralogical and chemical composition, assessed their petrogenesis and susceptibility to the formation of bauxitic clay. Ten representative rock samples were selected for thin section petrography and bulk rock geochemical analysis using Petrographic microscope and XRF, Rh Tube, 3k Watt respectively. Field observations revealed charnockites as the dominant rock in the study area with other lithotypes including granite, granite gneiss and banded gneiss. Optical examinations indicated the dominance of quartz in all the rocks suggesting the rocks are product of acidic magma. The order of oxides concentration (wt%) is SiO₂>Al₂O₃>Fe₂O₃>CaO>K₂O>MgO>Na₂O>TiO₂>P₂O₅. Based on silica content only, the rocks in this study are classified as felsic (granite, granite gneiss and banded gneiss) with SiO2>65% and ultramafic (charnockites) with SiO₂<45%. Charnockites with low SiO₂, high Al₂O₃ and TiO₂ compared to other rocks in the study area is considered more favourable to form bauxite. The rocks in the study area are classified into calcic-alkali and calcic groups with igneous protolith.