Petrochemical Characteristics and Age of Rare Metal (Ta-Nb)Mineralization in Precambrian Pegmatites,Komu, Nigeria

Abstract

The Precambrian pegmatites of Komu area intrude semi discordantly older rock types, such as biotite gneiss,amphibolites and pelitic schists. The pegmatites occur as near flat lying bodies. This study aims at elucidating thegeological setting, petrography, geochemical features and age of mineralization of these pegmatite bodies, with a viewto classifying them and knowing their economic potential. The petrographic analysis shows that the pegmatite samplescontain mainly quartz (35%), plagioclase (15%), microcline (10%) and muscovite (12%) with accessories like tourmaline,tantalum, niobium and illmeno-rutiles. Geochemical analysis of the muscovites extracted from pegmatites show thatthese rocks are enriched in silica (>60%) and Al2O3 (>12%) and depleted in Fe2O3, MgO and TiO2. Trace element analysisshows that the pegmatites contain rare metals with moderately high Ta, Nb, Sn, Rb, Li and Cs values and depleted in Ti,Ba and Zr. Elemental ratios indicate low ratios of K/Cs, Th/U and K/Rb. Variation plots of Ta/(Ta+Nb) versus Mn/(Mn+Fe) show that the pegmatites plot in the complex (beryl subtype) field. The Na/K versus Sn, Nb, Rb variation plotsshow that the pegmatites of Komu area are mineralized and compare favourably with those of other mineralized pegma-tite areas like Egbe and Ijero in southwestern Nigeria. The variation plots of Ta versus K/Cs, and Ta/W versus Cs, alsoconfirm rare metal mineralization of Komu pegmatites, which plot over the mineralized line of Beus and Gordiyenko. TheK/Rb versus Rb, Cs and Sn plots indicate low K/Rb ratios indicating moderate differentiation. The Rare Earth Elements(REE) show high heavy REE values and lower light REE values with prominent positive Ce anomaly and negative Euanomaly from normalized chondrite plots. K/Ar dating of the age of mineralization of muscovites extracted from thepegmatite yielded late Pan-African ages between 502.8±13Ma and 514.5±13.2Ma. This period represents the coolingages of the muscovite and suggests that the pegmatites may not be cogenetic with the surrounding Pan-Africangranitoids