Nigerian rare-metal pegmatites and their lithological framework

Abstract

Most of Africa's rare-metal potential is linked to Proterozoic pegmatite fields except for the Jurassic tin-bearing ring complees of the Jos Plateau Nigeria. In order to establish the position of the Nigerian tin province in terms of economic rare-metal mineralization, occurrences of mineralized pegmatitic phases of the Nigerian basement complex, including the so-called ‘Older Tin-Fields’, have been investigated. Rb/Sr geochronology confirms the end Pan-African ages for the pegmatites in the range 580-530 Ma. Although emplaced within the same time span, the mineralogy, geochemistry, and mineralization of these pegmatites differ according to the lithology of their host-rock. Pegmatite occurrences from southwest Nigeria, which are emplaced into Proterozoic meta-sedimentary-metavolcanic sequences, are enriched in tantalum relative to niobium. These geochemical characteristics are also displayed by the composition of eluvial heavy-mineral concentrates as well as by the trace-element distribution in cassiterites. Although Pan-African granites with an age range of 700-520 Ma are dispersed throughout Nigeria, associated rare-metal mineralized pegmatites are known almost exclusively along a southwest-northeast striking belt about 400 km long, which intersects the Jos Plateau tin-fields. A direct genetic link between the rare-metal bearing pegmatites and proximal granite occurrences was never observed. It is suggested that reactivation of old tectonic lineaments during the Pan-African orogeny provided excess heat and fluid to concentrate rare-metal pegmatites by partial melting with selective leaching from the country rocks, i.e. their lithological framework.