Geology of the cassiterite mineralisation in the Rutongo area, Rwanda (Central Africa): current state of knowledge

The Mesoproterozoic Kibara orogen in Central Africa hosts different granite-related rare element deposits that contain cassiterite, columbite-tantalite ("coltan"), wolframite, beryl, spodumene, etc. as typical minerals. The primary deposits of these minerals are formed by pegmatites and quartz veins that have historically been related to the youngest, most evolved G4-granite generation in the northern part of the Kibara orogen. This study focuses on quartz vein-type cassiterite mineralisation in the Rutongo area in Rwanda. The Rutongo area consists of a large anticline that is characterised by the presence of cassiterite-mineralised quartz vein sets that dominantly occur in quartzites. The emplacement of the quartz veins has been related to a later phase in the deformation history of the Kibara orogeny. The mineralised quartz veins are associated with intense alteration, comprising silicification, tourmalinisation, sericitisation and muscovitisation. Cassiterite itself is associated with muscovite in fractures in and along the margins of the quartz veins. Cassiterite crystallisation is followed by the precipitation of different sulphides, such as arsenopyrite, pyrite, chalcopyrite and galena. Cassiterite mineralisation resulted from the circulation of high-temperature and moderate-salinity fluids with a H2O-CO2-(CH4-N2)-NaCl composition. The stable isotopic composition of the cassiterite mineralising fluids indicates precipitation during metamorphic hydrothermal conditions, during which the metamorphic fluids where in isotopic equilibrium with granitic rocks. The circulation of these fluids probably resulted in the remobilisation of the Sn from these magmatic rocks, as indicated by the relative low Sn concentration of the specialised G4-granites. 40Ar-39Ar age dating of muscovite associated with the mineralisation gives an integrated age of 869  7 Ma, which is clearly younger than the age of the G4-granites (~986 Ma) and the pegmatites with associated columbite-tantalite mineralisation (~965 Ma) in the area. Based on this large time gap, the 40Ar-39Ar age is interpreted to reflect a hydrothermal event post-dating the emplacement of the Kigali granite, only indicating a possible minimum age for the formation of the cassiterite mineralisation. Based on the structural setting, petrographical observations, the geochemistry of the G4-granites, stable isotope geochemistry, we therefore propose a model in which Sn was mobilised from primary magmatic rocks by a m