Geologica Belgica

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Niels HULSBOSCH, Jan HERTOGEN, Stijn DEWAELE, Luc ANDRE & Philippe MUCHEZ

Petrographic and mineralogical characterisation of fractionated pegmatites culminating in the Nb-Ta-Sn pegmatites of the Gatumba area (western Rwanda)

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Mots-clés : Karagwe–Ankole belt, G4-granites, regional pegmatite zonation, LCT-family, mineral geochemistry, fractional crystallisation, alkali metals

Abstract

The Karagwe-Ankole belt in Central Africa hosts numerous rare-metal pegmatites and Sn–W mineralised quartz veins, which are related to the granite generation that formed at 986 ± 10 Ma, i.e. the G4-granites in Rwanda. This early Neoproterozoic granite generation features in the Gatumba area (western Rwanda) a linkage with a zoned cluster of barren and Nb-Ta-Sn mineralised pegmatites. The most distal pegmatite bodies in this area suffered from intense alkali metasomatism, i.e. widespread growth of albite and white mica. A petrographic and geochemical study has been carried out on the pegmatite bodies in order to determine the petrogenetic evolution of the pegmatite zonation. The compositional variation of schorl and elbaite, sampled along the regional zonation sequence, implies that the zonation can be expressed by 4 zones: a biotite, a two-mica, a muscovite and a mineralised pegmatite zone. Alkali element variations and enrichments in muscovite and K-feldspar along the zonation sequence indicate that the origin and the formation of the different pegmatite zones can be explained by a single path of fractional crystallisation. This substantial chemical differentiation accompanies in the Gatumba pegmatite field the evolution from the G4-granite generation to common pegmatites and eventually rare-element pegmatites.

To cite this article

Niels HULSBOSCH, Jan HERTOGEN, Stijn DEWAELE, Luc ANDRE & Philippe MUCHEZ, «Petrographic and mineralogical characterisation of fractionated pegmatites culminating in the Nb-Ta-Sn pegmatites of the Gatumba area (western Rwanda)», Geologica Belgica [En ligne], volume 16 (2013), number 1-2, 105-117 URL : http://popups.ulg.ac.be/1374-8505/index.php?id=4055.

About: Niels HULSBOSCH

Geodynamics and Geofluids Research Group, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, B-3001 Leuven, Belgium

About: Jan HERTOGEN

Geodynamics and Geofluids Research Group, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, B-3001 Leuven, Belgium

About: Stijn DEWAELE

Department of Geology and Mineralogy, Royal Museum for Central Africa (RMCA), Leuvensesteenweg 13, B-3080 Tervuren, Belgium

About: Luc ANDRE

Department of Geology and Mineralogy, Royal Museum for Central Africa (RMCA), Leuvensesteenweg 13, B-3080 Tervuren, Belgium

About: Philippe MUCHEZ

Geodynamics and Geofluids Research Group, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, B-3001 Leuven, Belgium