MINERALIZATION CONTROLS AND PETROGENESIS OF THE RARE METAL PEGMATITES OF NASARAWA AREA, CENTRAL NIGERIA

The pegmatites of Nasarawa area occur in the central part of Nigeria. They are mainly hosted by phyllonites in a NNE-SSW trending shear zone lying east of some foliated Pan-African and West of Jurassic Afu Complex Younger Granites. A geological mapping of the area was followed by petrographic and mineralogical studies of selected rock and mineral samples. A total of 72 samples consisting of 25 rocks, 22 feldspars and 25 white micas were analyzed for various elements.

The pegmatites are peraluminous and are genetically linked to the late Pan-African leucogranite with the shear zone. The Pan-African granites have very low REE abundances and non-chondritic ratios of Zr/Hf and Y/Ho and low Nb/Ta ratios indicating crystallization from a liquid-rich melt. Barren pegmatites are closely associated with the primitive hornblende biotite Pan-African synorogenic granites while Sn- Nb - Ta mineralized granites are correspondingly enriched in pegmatites spatially associated with Pan-African synorogenic granites with enhanced values of rare lithophile elements such as Rb, Cs, Mn, Sn and Nb-Ta. The primary control of rare metal mineralization in the pegmatites is the composition of the source rock since the Ta-Nb-Sn-Li-Be-W mineralized pegmatites crystallized from fluid (H2O-B-P-F) rich melts. It is hereby proposed that the late Pan-African tectonic granite which is parental to the highly mineralized pegmatites in this area originated from anatexis of undepleted mica-rich metasediments at depth, followed by a magmatic fractionation of the fluid rich melt as it ascended through reactivated ancient fractures. The heat for the partial melting might have been supplied mainly by the reactivation of ancient fractures, which controlled the emplacement of the fertile granites and the related pegmatites