1 MINERALOGY AND GEOCHEMISTRY OF BERYL-BEARING PEGMATITES FROM GBAYO, SOUTHWESTERN NIGERIA * Razak O. Jimoh 1 , Akinade S. Olatunji, Jimoh Ajadi 3 , and Adegoke Afolabi 4 1 Department of Chemical and Geological Sciences, Al-Hikmah University, P. M. B. 1601, Ilorin, Nigeria 2 Department of Geology, University of Ibadan, Ibadan, Nigeria 3 Department of Geology and Mineral Science, Kwara State University, Ilorin, Nigeria 4 Department of Earth Sciences, Ladoke Akintola University, Ogbomoso, Nigeria * Corresponding author: Email: rojgems@yahoo.com, Telephone: +2348035702475 ABSTRACT The Gbayo granitic beryl-bearing pegmatites occur as discontinuous dykes and veins, intruding into the host rock of mica schist in the study area. Systematic geological mapping revealed the NE-SW trending beryl-bearing pegmatites as members of the felsic dykes, belonging to the suites of undeformed acid and basic dykes, while the thin section studies showed the pegmatites to contain mainly quartz, feldspar and muscovite, with beryl as accessory mineral. Results from the geochemical analysis showed high enrichments in SiO2 and Al2O3, fair enrichments in the alkali metal oxides, Na2O and K2O, but depletion in the remaining major oxides. While SiO2 ranged between 64.88% and 81.94%, with a mean value of 75.05%, Al2O3 ranged from 11.54% to 19.11%, with an average value of 14.97%. The alkali metal oxides, Na2O and K2O have their respective mean values of 4.10% and 3.58%. The relatively high aluminum and alkaline compositions might have promoted beryl crystallization from the pneumatolytic melt that produced the pegmatite. Incompatible elements are only fairly enriched in most of the analyzed samples, except for Rb which has the highest concentration and greatest variability, with values ranging from 30.10ppm to 1,528.40ppm and a mean value of 533.52ppm, which could be indicative of high degree of melts fractionation and evolution, resulting from long travel distance of pegmatite forming melts from its parental source. The K/Rb ratios for most of the analyzed beryl-bearing pegmatites are less than 100, with a mean value of 75.11, which is indicative of mineralization. Elevated Be values were observed for samples; R02 (174ppm), R09 (142ppm) and R10 (170ppm), indicating some level of beryl mineralization. The dominance of Fe-bearing beryl chromophore but depletions of V, Cr and Mn chromophores in the pegmatites account for the manifestations of aquamarine, goshenite and rarely heliodor beryl mineral types as opposed to the absence of others. Be, Nb, Ta and Sn show consistent enrichments in some particular pegmatite samples, indicating possible associated beryl, tantalite-columbite and tin mineralization in the pegmatites. The fluids precipitating the beryl-bearing pegmatites are probably a mixture of expelled magmatic and hydrothermal melts from some plutons and mobilized metamorphic fluids from the surrounding metasedimentary rocks of the host mica schist. Keywords: Pegmatites, Beryl, Incompatible Element, Mineralization, Hydrothermal Melt