Sn-Ta-Nb mineralization potential of pegmatitic bodies in Bunu area, part of Kabba-Lokoja-Igarra Schist Belt, SW Nigeria

Abstract

The Bunu area in Southwestern Nigeria lies within the Basement Complex that forms part of the Pan-African platform of the crystalline rocks. The complex lies within the vast Trans-Saharan Pan African mobile of Neoproterozoic. The belt is characterized by voluminous post-collision granitoid plutons that are well exposed around the study area. The Bunu area consist of two major lithologic suites namely: (i) the gneiss-schist suite which comprised of Migmatized-gniess/ Migmatized Schist and quartz/mica schist with interbedded amphibolites (ii) intrusive suite that include fine-medium grained granite, medium-coarsed grained granodiorite and porphyritic granite and the minor intrusions like pegmatite, aplite, dolerite and quartz veins. Pegmatite outcrops in the Bunu area of the Kabba-Lokoja-Igarra Schist belt belong to the rare-element type and are mined for columbite-tantalite and gem tourmaline. Pegmatitic textures are well developed, these include large K-feldspar (comb textures), layering and graphic intergrowth of K-feldspar + albite + quartz + schorl and linearly arranged garnet crystals that may be developed at deeper levels of the dyke. Ductile and brittle structural features mapped in the area from remotely sensed and field data are generally consistent with the N-S and NE-SW regional orientation associated with Pan-African event and with minor axes of NNW-SSE and E-W planar structures. Trace elements geochemistry of the pegmatite (bulk rock and mineral extracts of K-feldspars and Mica) revealed that Rb, Sr, Ba, Li and Cs are enhanced in the Otafun and Okutose pegmatites indicating higher fractionation levels. Also, the rare-metals Nb, Ta and Sn were also enriched in the most evolved Otafun and Okutose pegmatites. Trace-element variations in mica from the pegmatites indicate that they are per-aluminous LCT (lithium, cesium, and tantalum) family and bery-columbite subtype. The origin of the gradual composition variation trend observed from the host granite bodies to the pegmatites reflects the increasingly fractionated melt composition and systematic enrichment of the incompatible trace elements Li, Rb, Cs, Be, Na, Ta and Sn. The Zr-SiO2 and Sr-Rb plots of the pegmatite bulk rock samples suggest that they are genetically magmatic and poorly differentiated with little or no post-magmatic metasomatism. Ta vs Cs and Ta vs Ga plots for the chemistry of K-feldspar and muscovite mineral extracts from the pegmatites in Bunu area show potential for tantalum and niobium (>75 ppm) mineralization, thus implying that the mineralization in these pegmatites is probable, therefore investors should use caution.