Geochemical trends in the Jos-Bukuru granites of central Nigeria: magmatic and metallogenic implications

Abstract

The Jos-Bukuru Complex is the largest individual ring complex in the Nigerian province occupying an area of approximately 430 km2. The Complex is composed of fayalite granite and porphyrites, hastingsite granites and biotite granites. The biotite granites play host to both disseminated and vein controlled mineralization. Columbite - Cassiterite deposits occur in the Rayfield (Centre and Southern eastern parts of the Complex) and Cassiterite-wolframite bearing quartz-topaz-greisens are found as veins in the Southwestern and Northwestern (Sabon Gida and Ngell phases) parts of the Complex. Geochemical evidence suggest that the fayalite granites and hastingsite biotite granites are metaluminous, while the biotite granites are generally peraluminous. Their sum characteristics point to an A-type classification with high SiO2, K2O and Na2O, low CaO and MgO. Although the major element characteristics are comparatively similar, the trace elements show strong variations. Generally the high concentrations of Rb, Li, F, Nb, Sn, Th, Y and low contents of Ba and Sr compared with low-Ca granites indicate a high degree of fractionation of magma prior to emplacement. However the element enrichment and depletion trends in the biotite granites supports a model of liquid - state diffusion and or convective fractionation analogous to those observed for chemically zoned high silica magma chamber. It is suggested that such processes have been instrumental to the formationof tin and associated mineral deposits in the complex. Thus extreme enrichment of the large ion lithophile elements and depletion of Ba and Sr are useful indicators of mineralizing processes and ore bearing plutons. © 1990.