Petrochemical Characteristics and Ageof 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 the
geological setting. petrograply, geochemical features and age of mineralization of these pegmatite bodies, with a view
to classifying them and knowing their economic potential. The petrographiic analysis shows that the pegmatite samples
contain mainly quartz (3 59%), 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 that
these rocks are enriched in silica (>60%) and A₂O₃  (>12%) and depleted in Fe₂O_3, MgO and TIO₂, Trace element analysis
shows 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 plots
show that the pegmatites of Komu area are mineralized and compare favourably with those of other mineralized pegmatitc areas like Egbe and ljero in southwestern Nigeria. The variation plots of Ta versus K/Cs, and Ta/ W versus Cs, also  confirm rare metal mineralization of Komu pegmatites, which plot over the mineralized line of Beus and Gordiyenko, The
K/Rb versus Rb,Cs and Sn plots indicate low K/Rb ratios indicating moderate differentiation. The Rare Earh Elements
(REE) show high heavy REE values and lower light REE values with prominent positive Ce anomaly and negative Eu
anomaly from nonalized chondrite plos. K/Ar dating of the age of mineralization of muscovites extracted from the
pegmatite yilded late Pan-African ages between 502.8 ± 13Ma and 514.54 ±13.2Ma. This period represents the cooling
ages of the muscovite and suggests that the pegmatites may not be cogenetic with the surrounding Pan-African
granitoids.