Correlations between microstructures, K-feldspar triclinicity and trace element geochemistry in stanniferous and barren granites, northern Nigeria

Abstract

The biotite granites in the Nigerian Younger Granites are a major source of the economic metals Sn, Nb, Zn and W. The textural variations in these rocks are considered together with the composition and structural state of the K-feldspars. The curious disparity in the degree of mineralization of these rocks of similar mineral assemblage is considered to be related to the textural and structural variations. The stanniferous granites are characterized by medium- to fine-grained texture, miarolitic cavities, subsolidus crystallization of biotite and albite; turbidity, coarsening and incoherent perthitic texture of the K-feldspar. The barren granites are usually coarse-grained with few druses, silvery irridescent feldspar grains and coherent perthitic texture. K-feldspars in the stanniferous granites are mainly of maximum microcline structure while the barren ones are characterized by intermediate microcline structure. Compositionally, all the K-feldspars have less than 3% An; they have crystallized in the binary system at temperatures close to 700°C but have now re-equilibrated to various lower temperatures as a result of variable rock-fluid interaction. Positive correlations are observed between Nb, Sn, Rb, Li and F in granites and triclinicity of their K-feldspars. Enhancement of Rb-values is observed in both the stanniferous rocks and their K-feldspar constituents. The variable textures, degree of Al, Si order in the feldspars and enrichment in ore and trace elements are attributed to the presence and abundance of a fluid phase. It is suggested that in the course of magmatic evolution of these rocks an alkali-rich volatile phase developed, accumulated and was trapped in the roof zone of the stanniferous granites.