Geological and Geochemical Characterisation of Pegmatites around Olode, Southwestern Nigeria

Abstract

Olode is prolific in terms of pegmatites, occurring in different ways as large intrusive bodies, as veins and dykes, discordantly intruding the host rocks of mica schist, gneiss and granite. This study is aimed at the geological and geochemical characterisation of pegmatites around Olode. Field investigation was undertaken to determine the occurrences and relationships of the pegmatites with the host rocks. Fresh pegmatite samples were studied petrographically and geochemically. Two main lithologically and chronologically different groups of pegmatites were distinguishable in the study area. These were the tourmaline pegmatites associated with the Older Granite rocks and the NE-SW trending beryl-bearing pegmatites typically occurring as dykes, hosted by the mica schist. Results of the geochemical analysis showed variations in the chemical compositions of the two varieties of pegmatites, reflecting their mineralogical differences, but with common peraluminous provenance. The discrimination plots of Cs and Rb versus K/Rb have classified most of the beryl-bearing pegmatites as mineralised, with mean K/Rb ratio of 85, and the tourmaline pegmatites as barren. The Olode pegmatites have emanated from a common source, a higly mineralised magma, and their differences in chemical composition and occurrences have resulted from fractional crystallisation of the parent magma and varied evolutionary trends.

Keywords: Olode; Pegmatites; Beryl-bearing; Tourmaline; Mineralised;