Petrographic and geochemical investigation of Sn - W - Nb - Ta pegmatites and mineralized quartz veins in southeastern Rwanda

Abstract

The Musha-Ntunga, Bugarura-Kuluti-Bibare and Rwinkwavu mineral districts of Eastern Rwanda are historically known to host cassiterite, wolframite and columbite-tantalite mineralization. The geology of that area is dominated by meta-sedimentary rocks of Mesoproterozoic age deformed during the Kibaran Orogeny (1.4 to 1 Ga) and intruded by two granite generations: G1-3 granites of ca. 1380+/- 10 Ma and G4- granites of ca. 986+/-8 Ma. The Sn, W and coltan deposits exploited in the E-Rwanda are associated with late magmatic phases of the youngest granite generation (G4) which probably functioned as the heat source for the mineralizing fluids. The trace and major element analyses of igneous rock samples from the study area resulted in peraluminous and S-type granites deplete in Sn, W, Nb and Ta corresponding possibly to the G1 – 3 granites. Moreover, Sn, W, Nb-Tarich pegmatities derived from granites equivalent to the ‘tin’ granites (or G4-granites), were also identified. Meta-sediments hosting pegmatities/hydrothermal veins contain very low amounts of Sn, W, Nb and Ta and therefore, here like or no direct important as source for the Sn, W andNb-Ta mineralization. In cassiterite samples from Bugarura-Kuluti, the dominant substitution was Sn⁴⁺ replaced by (Ta, Nb)⁴⁺ whereas in cassiterite from Musha-Ntunga and Rwinkwavu prospects, the 3 Sn⁴⁺ replaced by 2(Ta, Nb)⁵⁺ + (Fe, Mn)²⁺ and/or that of Sn⁴⁺ + O² by Fe³⁺ + OH types were predominant. The cassiterite samples also showed intergrowths with increasingly distal veins showed evolution trends from ferrocolumbite to mangano-tantalite compositions indicating the increase of Ta and Mn with advanced differentiation of pegmatites. Fluid inclusion studies showed a wide variation in salinities of fluids (0.5-17.5 wt. % NaCl equivalent) and formation temperatures (Tᶠ) ranging from 150°C to 560°C confirming that the coltan precipitation closer to the granites occurred from intermediate to high temperature and relatively more saline fluids. This may indicate penumatolytic conditions. The cassiterite and ferberite mineralization precipitated from less saline, relatively low temperature fluids possibly representing a mixture between primary magmatic fluids and meteoric to connate waters. Based on this study and various works on granite-related ore deposits in the Kibaran Belt and worldwide, a conceptual six – phase metallogenetic model involving multi-stage circulation of hydrothermal fluids caused by progressive granitic magmaticsm is suggested.