Magmatic-Hydrothermal Origin for Tungsten in the Quartz Vein Deposits of the Karagwe-Ankole Belt (Rwanda)

Most of the rare metal mineralisation in highly differentiated facies of zoned granites or in syngenetic pegmatite dykes results from advanced fractional crystallization. The role of the granites as a direct metal source is in these cases mostly well demonstrated. However, the direct involvement of granites as a metal source for peribatholitic metasediment-hosted vein systems has been more strongly debated due to extensive fluid-rock interaction at the deposit site, and overprinting potential magmatic fluid signatures. In addition, ambiguous temporal or geochemical correlations with granitoid rocks in the field obscure the potential magmatic-hydrothermal origin of these vein deposits. This study focuses on the metal source of metasediment-hosted tungsten vein deposits in Rwanda, which are regionally associated with peraluminous granites and pegmatites. 40Ar-39Ar of vein muscovite, the enrichment of granitophile elements in vein muscovite and the existence of an exsolved, aqueous tungsten-rich fluid during granite-pegmatite differentiation, let us conclude that the tungsten mineralising fluids originate from geochemically specialised granites in Rwanda. Strong water-rock interactions have subsequently overprinted this original signal and gave rise to the complex fluid chemistry of the present vein system.