Triclinicity of K-feldspars and trace-element content of Mesozoic granites of central Nigeria

Abstract

Biotite- and riebeckite-granites together with their albitized varieties are considered to be among the main contributors to the alluvial and elluvial Sn deposits associated with the Mesozoic Younger Granites of Nigeria. Enhanced values of Rb, Nb, Sn, Li and F in some phases of these groups of rocks indicate their ore potential. K-feldspar separates from these rock samples show bimodal distribution in respect of their degree of Al, Si order. Positive correlations are obtained between triclinicity of K-feldspar and enhanced Rb, Nb, Sn, Li and F values in the rocks. The high degree of ordered structure of the K-feldspar and enhanced trace-element values in the host rocks are attributed to the development of an alkali-rich volatile fluid toward the closure of fractionation of the magmas that gave rise to these rocks. Hence the granites with high ore potential are those that represent the late granitic phases that had long duration of rock—fluid interaction in the course of their evolution, while granites with low ore potentials are the early formed rocks which suffered little or no rock—fluid interaction. It is suggested that triclinicity of K-feldspar may be a useful tool in delineating barren and mineralized granites in provinces associated with metallic mineralization through acid magmatism.