Factors contributing to enrichment and crystallization of niobium in pyrochlore in the Kaffo albite arfvedsonite granite, Ririwai Complex, Younger Granites province of Nigeria

The Ririwai Complex of northern Nigeria comprises an outer ring dyke of granite porphyries surrounding down-faulted volcanic and basement rocks with a core of composite granitic rocks made up of biotite, aegirine arfvedsonite and albite arfvedsonite granites. The granites show coherent major element chemistry typical of the Nigerian Younger Granites, and are enriched in Nb, Ta, Rb, Y, Zr, Hf, Zn, U, Th and REE compared to the world average for rocks of similar composition.The albite arfvedsonite granite, which is noted to host disseminated pyrochlore mineralization exhibits a four- to eightfold increase in concentration of Nb relative to (unmineralised) aegirine arfvedsonite granite. Microprobe analysis of the pyrochlore shows that it contains 45–62.5% Nb2O5 and up to 6% Ta2O5. Elpidite, a hydrate sodium zirconium silicate which typically, is a primary mineral in nepheline pegmatite has been recorded as metasomatic replacement of quartz in the albite arfvedsonite granite.Evidence from the present study suggests that Nb in the granites was derived from the magmatic fluids. Its concentration was facilitated by fluorine carrying incompatible Nb in the volatile fraction of the granitic magma; and together with water, fluxing the magma and lowering the temperature of the residual fluid. An increase in alkalinity of the residual fluid into which Nb has been concentrated resulted in its crystallization in pyrochlore during the postmagmatic sodic metasomatism. The concentration of Nb in the albite arfvedsonite granite also appears to be primarily a consequence of the relative abundance and composition of the accessory iron-, fluoride-bearing minerals in the host rock. As the amount of these host minerals increased, they scavenged Fe and increased the opportunity for direct crystallization of pyrochlore rather than columbite.