Late Neoproterozoic overprinting of the cassiterite and columbite-tantalite bearing pegmatites of the Gatumba area, Rwanda (Central Africa)

Abstract

The Mesoproterozoic Kibara belt in Central Africa has recently been redefined and subdivided into the Karagwe–Ankole belt (KAB) and the Kibara belt (KIB), separated by Palaeoproterozoic (Rusizian) terranes. The KIB and KAB are characterised by the presence of numerous rare metal mineralised (Nb–Ta–Sn) pegmatites and Sn–W mineralised quartz veins that are related to the youngest granite generation, i.e. the G4-granites in Rwanda, which formed at 986 ± 10 Ma. The pegmatites of the Gatumba area (western Rwanda) have historically been mined for their columbite-tantalite and cassiterite mineralisation, but contain also beryl, apatite, spodumene, amblygonite, and rare phosphates. Columbite-tantalite formed during the crystallisation of the pegmatites, followed by intense alkali metasomatism, i.e. widespread growth of albite and white mica. The major part of the cassiterite mineralisation is, however, concentrated in zones associated with intense phyllic alteration. U–Pb ages of columbite-tantalite samples vary between ∼975 Ma and ∼930 Ma. The oldest ages (975 + 8.2/−8.3 Ma and 966 + 8.7/−8.6 Ma) overlap with previous reported Rb–Sr ages of the emplacement of the pegmatites (∼965 Ma) and are interpreted to reflect the crystallisation of the Nb–Ta mineralisation. The youngest ages (951 ± 15 Ma to 936 ± 14 Ma) are apparently related to variable degrees of resetting by (metasomatic) post-crystallisation processes. The resetting could either be due to recrystallisation of early Nb–Ta minerals or due to the disturbance of the U–Pb isotopic signature of the Nb–Ta minerals. The 40Ar– 39Ar spectra of muscovite samples associated with different steps in the paragenesis of the pegmatites show a spread of apparent ages between ∼940 Ma and ∼560 Ma that reflect Late Neoproterozoic tectonothermal events. One plateau age of 592.2 ± 0.8 Ma is interpreted to reflect far-field effects of the East African orogeny on the Karagwe–Ankole Belt.