LITHIUM CONCENTRATION AND REGIONAL ZONATION OF RARE METALS IN GRANITIC PEGMATITES OF THE MUSHA-NTUNGA AREA, RWANDA

Granitic pegmatites are complex coarse-grained igneous rocks that typically form from the last bits of melt isolated from their plutonic source. The concentration of rare metals such as lithium (Li), tantalum (Ta), niobium (Nb) and tin (Sn) is related to fractional crystallization and melt-fluid immiscibility processes within pegmatitic melts. Pegmatites account for approximately half of the world’s lithium production. Understanding their genesis, evolution and mineralization style is therefore a crucial step in their exploration and potential future exploitation. In Central and West-Africa, LCT (lithium, cesium, tantalum) pegmatites are associated with widespread granitic magmatism during the Proterozoic (e.g. the Kamativi, Arcadia and Bikita deposits in Zimbabwe and the Manono and Kitotolo deposits in the Democratic Republic of Congo).

Here we study LCT pegmatites in the Musha-Ntunga area in Eastern Rwanda, where shallow mining activities for Ta-Nb-Sn rich kaolinized pegmatites and Sn-rich quartz veins are upscaled and prospected for lithium potential at depth. Recent drill core campaigns have exposed fresh, unweathered spodumene pegmatites from depths up to 400 m, with significant intervals (>100 m) of pegmatite with Li₂O contents between 1 and 3 wt.%. The pegmatites appear to be unzoned and contain abundant beige to white spodumene up to 10 cm long and minor amblygonite. The spodumene is locally replaced by albite and some core sections show evidence for extensive shearing, with the formation of mica fish. Symplectite and fine needle-shaped spodumene-quartz intergrowths were found at the margins of some spodumene crystals. Detailed characterization of the lithium mineral assemblages, textures, and potential stages of magmatic-hydrothermal replacement are required in order to optimize lithium extraction and processing routes.

We will perform detailed petrographic studies, mineral geochemistry and fluid-melt inclusion analyses to (I) study the unique regional zonation of granitic pegmatites, (II) determine the role of immiscible melts and fluids, as well as the input of (III) metamorphic fluids in concentrating the ore minerals and (IV) constrain the origin and evolution of Li-bearing minerals in the Musha-Ntunga pegmatites.