Mineralogy, geochemistry and mineralization of the Ririwai complex, northern Nigeria

Abstract

The Ririwai complex represents the eroded roots of an alkaline volcano developed as part of a sequential chain of anorogenic centres in early Jurassic times. An outer ring-dyke fracture which formed a volcanic feeder is filled with quartz porphyry, and granite porphyry surrounds and partly encloses a caldera-collapsed volcanic pile into which peralkaline granite and biotite granite have been emplaced. The volcanic rocks are dominantly rhyolitic ignimbrites with minor basalts, showing petrological and geochemical features of magmatic crystallization and subsolidus re-equilibrium. the volcanic feeder intrusions are partly quenched and partly degassed representatives of the original granite magma but petrological and geochemical data testify to the limited interaction of an alkaline residula fluid phase. Th effects of the fluid phase are seen as a series of metasomatic reactions generating peralkaline granites, biotite granites and their mineralization. The interactions between crystal and fluids have been monitored by XRD, XRF, INAA, wet chemical analyses and fluid inclusion studies. Ore mineralogy confirms the paragenetic columbite (pyrochlore)-cassiterite-sphalerite evolution. The metasomatic reactions commence with Na + metasomatism followed by K +, then H + and finally Si 4+. The latter reactions are best displayed in the Ririwai lode where particle track studies have delineated the relative mobility of U and Th. Mineralization in biotite granite can be grouped according to the dominant metasomatic process. Columbite, minor cassiterite and sphalerite can be equated with albitite formation. Potash metasomatism generated columbite, wolframite, cassiterite and sphalerite deposition, which continued into greisen formation as H + metasomatism developed. This was accompanied by molybdenite, chalcopyrite and galena deposition during silica metasomatism. According to isotopic data the source of the ore metals and the granite magma was probably the Pan-African continental crust with contributions from the mantle.