Petrography and geochemistry of pegmatite and leucogranite of Ntega-Marangara area, Burundi, in relation to rare metal mineralisation

Abstract

Abstract The Ntega-Marangara area, part of the Kanyaru supergroup, Western Domain of Karagwe Ankole Belt, hosts numerous pegmatite veins related to the leucogranite. This investigation aims to characterise the granitoids, their alteration products, and rare metal mineralisation potentials. Quartz, plagioclase, microcline, albite, and muscovite are the essential minerals in both the pegmatite and leucogranite. The ΣREE of the pegmatite and leucogranite are 147 and 102 ppm, respectively. The greisen with Rb, Cs, Ta, Li, Nb, and Sn of 5,940, 1,015, >2,500, 130, 1,595, and 671 ppm, respectively, is higher than the corresponding values of 636, 62, 32, 74, 58, and 110 ppm of the kaolinised pegmatites. This enrichment classifies them as lithium–caesium–tantalum pegmatite. The mean K/Rb, K/Cs, and Nb/Ta in the leucogranites are 106.86, 2819.24, and 4.30, pegmatite is 35.33, 469.47, and 3.1, and greisen is 14.05, 82.2, and 0.64, respectively, which suggest that greisenisation due to metasomatic reactions of late hydrothermal activity could have increased the potential for rare metal mineralisation of the altered pegmatite. Pegmatite of Ntega-Marangara area is enriched in LREE relative to HREE with K/Rb ratio <100, indicating a high level of fractionation and mineralisation in Ta–Nb, Sn, and Li in contrast to the leucogranite that is less fractionated and barren.