Mineralogy and chemistry of columbite-tantalite from Bugarura-Kuluti area, Karagwe-Ankole Belt, Rwanda: Indicators of pegmatite and granite evolution

Abstract

Our study characterizes in detail the mineralogical, textural, and compositional features of Ta-Nb oxides from the Bugarura – Kuluti deposit located in the eastern Rwanda. The research presents the chemical evolution of oxides at all scales, from a single crystal, through a single host body, to the regional granite – pegmatite scale. Finally, we propose an evolutionary model of a B-rich magmatic – hydrothermal system hosting Ta-Nb-Sn mineralisation. Ta-Nb oxides are hosted by LCT (lithium-caesium-tantalum) pegmatites, rarely in greisens and two-mica peraluminous granites. They display a wide range of composition including all types of columbite group minerals, wodginite, tapiolite and microlite. The internal texture of columbite-tantalite exhibits a multiple zoning pattern indicating primary composition modified by the extreme fractionated, volatile and water saturated residual melt. Hydrothermal origin of columbite-tantalite is observed in greisens, expressed by a higher W concentration and coeval or even younger crystallization compared to the associated cassiterite. Fractionation trend in a crystal scale includes an extreme increase in Ta/(Ta + Nb) ratio with little or no change in Mn/(Mn + Fe), mainly caused by the high Ta solubility in the silicate melt during fractionation. On a regional scale, the pegmatites follow Fe-Mn fractionation trend towards high Mn/(Mn + Fe) compositions. This is due to fractional crystallization of CGM and crystallization of other Fe competing minerals shifting the melt composition towards Mn enrichment. In the outermost pegmatites, the final crystallization stage is characterized by a rapid increase in Fe. Fe enrichment in the residual melt was due to infiltration of meteoric/metamorphic fluids. Tapiolite and wodginite crystallised from the B and H2O undersaturated, Na-rich residual melt carrying Ta mixed with Sn- and Fe-rich hydrous fluids. The difference in fractionation patterns and styles of mineralization between different deposits within Kibara metallogenic province provides the evidence of the complexity of magmatic-hydrothermal systems which developed by partial melting of heterogenous crustal material.