Mineralogical characterization of cassiterite concentrates from quartz vein and pegmatite mineralization of the Karagwe-Ankole and Kibara Belts, Central Africa

Abstract

The Mesoproterozoic Kibara belt (TUB) and the Karagwe-Ankole Belt (KAB) in Central Africa are characterized by the presence of numerous rare metal mineralized Sn-(Nb-Ta) pegmatites and Sn-W mineralised quartz veins that are related to a S-type granite generation formed at 986 +/- 10 Ma. Cassiterite concentrates have been studied by different petrographic and mineralogical techniques. The concentrates have been collected from the rock and mineral collection of the Royal Museum for Central Africa (RMCA) and originate from historical exploitations of eluvial and alluvial cassiterite deposits. No quantitative study of the concentrates has been envisaged since no information is available about the history of the samples prior to sampling. Microscopic investigation revealed the presence of cassiterite crystals with metallic and non-metallic luster, of which the latter show growth zoning. The color from the cassiterite crystals can vary from transparent colorless to black non-transparent. The variation in color in a single grain can be as varied as the color variation between grains for an entire concentrate. The mineralogical composition of the cassiterite concentrates contains minerals that are typical gangue and accessory minerals in the primary mineralization and that were liberated during weathering. In addition, minerals can be found that result from the weathering from the metasedimentary and doleritic host-rocks of the primary mineralization. Except the occasional presence of a certain mineral or a special color for a certain cassiterite, no systematic variation can be observed between the concentrates from the different locations. Often, the mineral and color variation in one concentrate can be as large as for concentrates found from different locations