Fractionation-Controlled Nb–Ta Mineralization in Rare-Element LCT Pegmatites of the Lokoja Basement Complex, Nigeria

Abstract

This paper presents a geochemical and petrological assessment of niobium-tantalum-bearing pegmatites around Bakomba village, Lokoja, Kogi State, central Nigeria. Field mapping, structural measurements, thin-section petrography, and major- and trace-element geochemistry were integrated to determine emplacement style, source affinity, degree of fractionation and rare-metal mineralisation potential. The pegmatites occur as concordant flat-lying bodies and discordant dykes/veins hosted by migmatite-gneiss, Obajana gneiss and Older Granite units of the Nigerian Basement Complex. Structural measurements define dominant N-S, NW-SE and NE-SW trends, with NE-SW orientations particularly prominent, suggesting late Pan-African brittle-structure-controlled melt migration and dyke emplacement. Petrographically, the host granitoids are quartz-feldspar-biotite-rich and contain microcline and monocrystalline to polycrystalline quartz. Cross-hatched twinning, sutural contacts, undulose extinction and microfractures record crystallisation, deformation and recrystallisation. Geochemically, the biotite granite shows features compatible with an evolved, weakly to moderately peraluminous S-type affinity and a likely post-orogenic setting, with enrichment in Rb, Sn, Li and Nb relative to the gneissic host. Feldspars are low in Ta but enriched in Rb and Cs, whereas muscovite contains the strongest rare-metal signature, including Nb, Ta, Sn, Rb and Cs. The central Bakomba mica sample BM2 shows higher Ta and lower K/Rb than the northern Bakomba mica sample BM1, indicating stronger fractionation and higher Nb-Ta mineralisation potential. The data support classification of the Bakomba bodies as rare-element LCT pegmatites genetically related to evolved peraluminous granitic melts. Central Bakomba is therefore the preferred target for follow-up Nb-Ta exploration.