Mineralogical, fluid inclusion and oxygen isotope studies for the evolution of pegmatites in Igangan area, southwestern Nigeria

Abstract

The fluid characterisation and oxygen isotope studies of pegmatites in the Igangan area in southwest Nigeria was carried out to delineate the source and evolution of mineralising fluid, and to determine the crystallisation temperature of the pegmatites that are associated with I-type metaluminous granites. The geochemical analysis was carried out using ICPMS, the mineralogical analysis was carried out with the aid of a Scanning Electron Microscope. In contrast, microthermometry measurements were carried out on pegmatitic quartz using a Linkam THMSG600 Conventional and laser fluorination was used to determine the oxygen isotope composition of selected minerals from the pegmatites.The geochemical analysis revealed a low concentration of Ta, Nb, W, Cs and low mineralisation potential for the pegmatites; while the mineralogical analysis of accessory spinels revealed a high concentration of Zn. Two types of hydrocarbon/CO2 free aqueous inclusions were observed; which were Primary/pseudo primary (Type 1) inclusion with 0–11 wt % NaCl eq. salinity and secondary (Type 2) inclusions with a salinity range of 15–22 wt % NaCl eq. The density of the inclusions ranged from 0.57 to 1g/cc. δ18O values in quartz varied from 9.88 to 14.63‰ with an average of 13.13‰; feldspar, 8.9–13.85‰ with an average of 11.9‰ and tourmaline, 10.63–11.96‰ with an average of 11.40‰. Homogenisation temperature ranged from 175 to 370 °C with all inclusions homogenising into the liquid phase.Bivariate plot of salinity against Thtotal reveals the possibility of magmatic-meteoric mixing with Na/Ca rich fluids in the pegmatite evolution and equilibrium temperature calculations yield values of 496–685 °C and 392–713 °C for the quartz–tourmaline pair and the quartz-feldspar pair, respectively. The pegmatites are muscovite class LCT pegmatites with low rare metal mineralisation potential and crystallised from a H2O–NaCl (±CaCl2) fluid which was preserved in the primary/pseudo-primary inclusions.