Compositional characteristics and Ta-Sn-Nb rare metal mineralization potential of Egbe Pegmatite, Southwestern Nigeria

Abstract

Pan African (600 ± 150) pegmatite which intrudes gneisses, amphibolites, quartzite and schists around Egbe area were studied for their compositional features. This is with a view to characterizing and assessing their potential for rare metal Ta-Sn-Nb mineralization. Twenty seven samples comprising eleven (11) whole rock, sixteen (12) muscovite extracts and four (4) feldspar extracts samples were analyzed using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Major oxide geochemical assessment of the whole rock pegmatite indicates that the pegmatite is siliceous (79.82% SiO2) and moderately high in Al2O3 (12.22%). The K2O, Na2O contents are averagely 2.25% and 2.87% respectively while Fe2O3, MnO, MgO are each less than 1.0%. Average values of trace elements in the pegmatite are Ta (52.55ppm), Sn (92.3 ppm), Rb (1083.5ppm), Nb (109.35 ppm), Cs (32.12ppm), Sr (17.84 ppm), Y (9.86 ppm), and Zr (41.27 ppm), Ba (50.47ppm), Hf (12.38ppm), Zr (41.27ppm), Ga (32.12ppm) and W(2.91PPM) and ratios of diagnostic elements are K/Rb (30.27), Rb/Sr (298.93), Zr/Hf (7.23), Rb/Cs (65.80), K/Cs (1103.11), Ta/W (41.81), Nb/Ta (55.397) Na/Rb (41.77) and Na/K (92.25) A/CAN >1and Al2O3>CaO+ Na2O+ K2O with enrichment of SiO2, Al2O3, Na2O, K2O and depletion of Fe2O3, MnO2 and MgO suggests that Egbe pegmatite is of peraluminous bulk composition and plots of A/NK vs Al/CNK and Rb vs (Y+Nb) discriminates Egbe pegmatite in the peraluminous LCT (Li, Rb, Cs, Be, Ga, Nb Ta, Sn, Hf, B, P, F) of syncollisional to within plate granitic family. The Ta/W vs Cs plot for the three sample media shows the relationship between increasing Ta/W ratio and elemental fractionation as indicated by Cs. The K/Rb vs Cs and K/Rb vs Rb plots for the three sample media further establishes the rare – metal nature and mineralization potential of Egbe pegmatite. Using the discriminant plots of TaVs Rb, Ta vs Cs .TaVs K/Cs, and Ta Vs (Cs+Rb) where most of the samples of the whole rock pegmatite and mineral extracts plots above the Beus line and about 30% plotting above the Gordinyenko line respectively. Overall geochemical assessment is that Egbe pegmatite is a rare metal, highly fractionated Beryl type with rare metal enrichment trend of Nb>>Sn>>Ta. Keywords: Pegmatite, Peraluminous, Syncollisional, Within-plate, Discriminant, Fractionation, Sample media