Contrasting styles of Sn-Nb-Ta-Zn mineralization in Nigeria

Abstract

Two distinct and economically important types of primary Sn-Nb-Ta-Zn mineralization occur in Nigeria.The early stage is related to Palaeozoic pegmatites which occur sporadically in a 400 km long north-east trending belt. Mineralogically the pegmatites consist of quartz, potash feldspar, albitized mica and are characterized by extensive development of tourmaline. Economically they are important for minerals of the columbo-tantalite series.The later stage of mineralization is found in granites of the Mesozoic ring complexes. Tantalum-bearing columbite, accompanied by cassiterite, and rare earth minerals have been disseminated in the cupola zones of biotite granites during an albitization process. A similar albitization process in the arfvedsonite granites has led to the dissemination of pyrochlore, thorite, monazite and xenotime. Mineralized veins, which are limited in occurrence to the biotite granites, form four major types which contain either an oxide series of minerals dominated by cassiterite and wolframite or a series of sulphides dominated by sphalerite.Fluid inclusion studies related to the Younger Granite mineralization show a gradual evolution of fluids from highly saline, at around 500°C, to dilute fluids at 320°C or below. It is from these later dilute fluids that the sulphble ore assemblage is deposited.The contrasting styles of Palaeozoic pegmatite and Younger Granite mineralization are related to granites of differing chemistry whose tectonic setting may also be different.