Contribution of 3D automated mineralogy in unraveling the formation history of Nb-Ta-Sn mineralized LCT pegmatites in the Karagwe-Ankole belt (Central Africa)

The Mesoproterozoic Karagwe-Ankole belt (KAB) in Central Africa hosts important mineralized lithium-cesium-tantalum pegmatites enriched in Sn, Nb, Ta and W. Microscopic techniques are required to further refine the paragenetic history of mineralized pegmatites in the KAB and to understand the pegmatite-forming processes leading to an enrichment of economically important minerals. Using a combination of scanning electron microscopy (SEM)based automated mineralogy systems with X-ray computed tomography (µCT), we investigated the mineral relationships in a three-dimensional (3D) space. This paper presents new insights into the crystallization sequence of mineralized pegmatites in the KAB. A stage of albitization followed the primary crystallization, where muscovite probably plays a role in the fluid migration and the subsequent precipitation of ore minerals. Primary saccharoidal albitization was followed by tourmaline crystallization, but before the secondary cleavelandite-type albitization. The ‘Cornish Type’ cassiterite originating from greisen pockets confirms the suggested hydrothermal formation conditions. A first-time description of oriented zircon and the association with flat tabular columbite-tantalite points to the importance of local magmatic-hydrothermal conditions. This study emphasizes the possibilities of using 3D automated mineralogy for the identification of minerals and their position in the paragenetic history of mineralized pegmatites and by extension of microscopically complex and diverse ore deposits.