Mineralogy and Geochemistry of Beryl-bearing Pegmatite Dykes from Gbayo, Southwestern Nigeria

Abstract

The Gbayo granitic beryl-bearing pegmatites occur as discontinuous dykes, intruding into the host rock of mica schist. This study was aimed at appraising the geochemistry and mineralization potentials of the pegmatites. Systematic geological mapping to ascertain the relationships between the pegmatites and their host rocks was undertaken. Fresh pegmatite samples were collected for both petrographic and geochemical studies. Analytical results showed high SiO2 (64.88 to 81.94%) and Al2O3 (11.54 to 19.11%,), and fair concentrations of Na2O (av. 4.10%) and K2O (av. 3.58%). The relatively high aluminum and alkaline compositions have promoted beryl crystallization from the pegmatitic melt. Incompatible elements are fairly concentrated in most of the analyzed samples, except for Rb which has the highest and greatest variability, with values ranging from 30.10 to 1,528.40ppm which could be indicative of the high degree of melts fractionation, resulting from long travel distance of pegmatite-forming melts. The K/Rb ratios for most of the analyzed beryl-bearing pegmatite samples are less than 100, with a mean value of 75.11, which is indicative of mineralization. Elevated Be values (>142ppm) were observed for some samples, indicating some levels of beryl mineralization. The dominance of Fe chromophore in the pegmatites accounts for the manifestations of aquamarine.