Petrogenesis of rare-metal pegmatites in northeastern part of Nasarawa, northcentral basement complex of Nigeria

Abstract

The pegmatites in Nasarawa region of northcentral Nigeria are within the Pan-African remobilized belt of Basement rocks of Nigeria, along Eggon-Wamba axis. Pan African orogenic events gave rise to reworked, crustal thickening and decompression which resulted to syn-to post-collision rocks (granitic rocks and pegmatites). The rare-metal pegmatites are classified as albite-muscovite pegmatites and biotite-microcline pegmatites. The area is predominantly underlain by migmatitic banded gneiss and localized biotite-muscovite gneiss which host most of the rare-metal pegmatites. The pegmatites mostly occur in NNW-SSE direction with few NE-SW orientations. The rare metal pegmatites contain albite, muscovite, quartz and microcline, while garnet, black tourmaline (schorl), apatite, beryl, ilmenites and minor Sn-Nb-Ta oxides occur as accessory. The biotite-microcline pegmatites possess less accessory minerals. The host rocks are made up of quartz, albite-andesine, microcline, biotite, muscovite and hornblende. Minor constituents include orthopyroxene, cordierite, sillimanite, garnet and chlorite. The rocks in the area (pegmatites and migmatitic rocks) show extreme peraluminous and exhibiting similar geochemical signatures suggesting genetic relationship. The albite-muscovite pegmatites are enriched in Rb, B, Li, Cs, Sn, Nb > Ta, W, Be and Ga over the biotite-microcline pegmatites and exhibit low values of K/Rb, Mg/Li and Zr/Hf across the zones. They have higher potential for cassiterite and columbite over tantalite. Modal equilibrium batch melting model supports anatexis of the migmatitic-gneiss complex at depth rather than the Pan-African granites origin as earlier believed.