GEOLOGICAL SETTING, COMPOSITIONAL FEATURES AND K-Ar DATING OF PEGMATITE BODIES AROUND OSOGBO-OKINNI AREA, SOUTHWESTERN NIGERIA

Abstract

Pegmatites hosta variety of economic rare metals and this has led to increased search and study of pegmatite belts globally. The Nigerian pegmatites belts continue to attract considerable attention due to their economic potentials. However, detailed geologic information on many of the pegmatites are still lacking and one of such are the pegmatite bodies of the Osogbo-Okinni area. Therefore, this study was aimed at geological evaluation of the pegmatite bodies around Osogbo-Okinni area southwestern Nigeria to elucidate their compositional features, mineralisation potentials and age of their emplacement.

Geological mapping on a scale of 1:25,000was undertaken. Slides from 39 rock chips were made for petrographic study. Twenty-two whole pegmatite, 22 feldspar and 17 mica extracts of the pegmatite were crushed, pulverised, and homogenised. These were analysed for major, trace and rare earth elements using Inductively Coupled Plasma Atomic Emission Spectroscopic and Inductively Coupled Plasma MassSpectroscopy. Age determination was achieved using the Cassignol technique for unspikedK-Ar method on mica extracts. Data interpretation was by descriptive statistics and geochemical variation plots. The pegmatite intruded granite gneiss, quartzite, amphibolite, and granite as low-lying, thin, and massive dykes. Quartz (25.0%), microcline (34.0%) and plagioclase (24.0%) were the main mineral constituents, while muscovite (5.0%), biotite (5.0%), and tourmaline (7.0%) were minor constituents. The mean concentration of SiO2, Al2O3, Na2O, K2O and TiO2were 70.92, 15.95, 3.5, 4.23, and 0.02wt%, respectively. Average whole pegmatite trace elements contents(ppm) were283.95, 101.94, and 70.35 for Ba, W, and Ta. The light rare earth elements sum (18.92ppm) was greater than that of heavy rare earth elements (2.22ppm) indicating high fractionation for the pegmatites. The mica extracts average trace elements contents (ppm) was 510.86, 104.86, 376.88, 1101.97, and 272.41 for Sn, Sc, Li, Rb, Nb,respectively. Low K/Rb (50.37-340.77) and Mg/Li (1.23-36.54) ratios indicated fractionated pegmatites. The plots of Na2O+K2O versus SiO2, Al2O3/Na2O+K2O versus Al2O3/CaO+Na2O+K2O and K/Rb versus Cs indicated igneous origin, peraluminous nature and placedthe pegmatites in the muscovite class of lithium-caesium-tantalum pegmatite family. The pegmatiteswere products of syn-collisional and within-plate tectonic environments and are classified as complex and simple types based on enrichment in Ta, Nb, Sn and Li content. Furthermore, there were clear differences in fractionation indicesof Mg/Li, K/Rb, and K/Cs (1.23-10.64, 50.37-152.88 and 1,375.22-5,570.54) for the complex and the simple (13.75-36.54, 51.60-309, and 2,065.04-17,645.77) pegmatites. The complex pegmatite accounted for 36% of the total pegmatite samples analysed. The age of thepegmatites was put at 774-692 Ma, corresponding to evolution during the early Pan-African orogeny. The Osogbo-Okinni pegmatites are of simple and complex types and are of early Pan African age. The complex ones have noticeable Ta-Sn-Nb and Limineralisation.