The Gatumba area is characterized by the presence of numerous mineralized pegmatites, which are related to the so-called "tin granites" that formed at -986 Ma. The pegmatites are dated at -968 Ma, of which some are associated with columbite and cassiterite mineralization. The columbite precipitated during crystallization of the pegmatites. Also beryl, spodumene, amblygonite and rare phosphates formed during this crystallization. The pegmatites underwent intense alteration by hydrothermal processes: albitization, sericitization (i.e., greisenization in the old literature) and muscovitization. The cassiterite mineralization in the pegmatitic bodies is associated with the two latter alteration stages. Sericitization and cassiterite precipitation occurred from a H20-C02"(X)-NaCl-KCl fluid (Thror between 240 and 370oC; Tm1ce between -19.2 and -8.2°C}. Based on the structural setting, petrographical observations, geochemistry of the so-called tin granites, stable and radiogenic isotope geochemistry, we propose a model in which Sn was remobilized from primary magmatic rocks by a hydrothermal system that was installed after the crystallization of the granites and pegmatites, and precipitated in structurally controlled locations, together with sericitization/ muscovitization of parts of the pegmatites.