Lithogeochemical dispersion associated with Ririwai zinc-tin lode, northern Nigeria

Abstract

The Ririwai complex forms a classic younger granite ring complex, 180 km 2 in size within the anorogenic younger granite province of Nigeria. An outer ring dyke of granite porphyry defines the caldera ring fracture in which fragmental rhyolite units, largely ignimbritic and intercalated thin basalt flows are preserved. Into this volcanic cover a biotite granite with widely varying texture and degree of mineralization has been centrally emplaced. Related to this biotite granite are two phases of mineralization, an early dispersed phase in which the granite is texturally modified and recrystallized (by intense albitization) and columbite, xenotime and thorite are introduced and a later phase of fracture-controled lode mineralization associated with acidic hydrothermal solutions in which sulphides are common. The Ririwai tin-zinc deposit is one of the major lode mineralizations of considerable economic importance so far located in the Younger granite province of Nigeria. It is localized within a major fracture zone approximately 5 m wide and extending east west for 5 km in the Ririwai biotite granite. Principal ore minerals are sphalerite, cassiterite and minor galena, chalcopyrite and wolframite, concentrated within quartz veins and greisens. Studies in wall-rock alteration show that the central quartz veins are bordered by zones of greisen and greisenized granite that are characterized by zinnwaldite, quartz, fluorite and minor topaz. Inward from the greisen zones are envelopes of K-feldspathized and reddened granite (orthoclase + chlorite + sericite + biotite) which locally pass outwards into kaolinized or unaltered granite. The mineralogy and chemistry of alteration products indicate that they were produced by hydrogen and potassium metasomatism. Geochemical analysis of unaltered bedrock samples indicate that the Ririwai biotite granite is characterized by an enrichment in Sn, Li, U, Th, Rb, F, Nb, W, Zn and Y and a depletion in Ba, and Sr relative to barren rocks of similar composition. It is also characterized by high values for the following whole-rock element ratios, Rb/Zr, Rb/Sr, Y/Sr and low Ba/Rb and K/Rb. These anomalous element concentrations are interpreted as reflecting geochemical specializations and an indication of the ore-bearing potential of the granite. The above mineralogical and chemical changes are probably the result of extreme fractionation with superimposed autometasomatic alteration.