Mineral Chemistry of Talc Schist in Wonu-Apomu Area, Southwestern Nigeria

Abstract

Talc schist occur within the Precambrian basement complex rocks of Wonu-Apomu area, southwestern Nigeria and have been investigated with divergent views on their nomenclature and petrogenesis. Earlier research works on the origin of talc schist in Nigeria have been inconclusive due to insufficient compositional data, the earlier studies relied mainly on geochemical data, however, this research is unique because mineral chemistry was utilized for the first time to elucidate the transition phases and evolution of talc schist in the study area. Field mapping on a scale of 1:25,000 was carried at Wonu-Apomu area. The petrographic analysis, Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS) and Electron Probe Micro-Analysis (EPMA), were carried out to determine the mineral morphology and chemistry. The petrographic analysis, the SEM-EDS and the EPMA revealed that the talcose rock samples comprises mainly of talc, anthophyllite, Cr-bearing clinochlore, olivine, tremolite, amesite, saponite, serpentine (antigorite), ferrichromite and magnetite. Two major groups of talc schist were distinguished, from the mineral chemistry plots. The first group is made up of talc-chlorite with FeO concentration of 2.86-4.69 wt.% and Mg/(Mg+Fe_total) of 0.92-0.95, while the second group is the talc-magnesian saponite with FeO concentration of 11.51-14.01 wt.% and Mg/(Mg+Fe_total) of 0.77-0.79. The mineral chemistry analysis revealed that talc and saponite of the talc schist had low TiO₂ content of (0.00-0.02 wt%) which are characteristic of dunite-type magma. The Electron Probe Micro-Analysis revealed the alteration phases in the talc-schist, with talc composition ranging from pure talc to talc-chlorite, talc-chlorite-saponite and talc-chlorite-saponite-antigorite-chrysotile. The mineral chemistry further revealed that the talc schist is a dunite-type magma which is boninitic in composition.