COMPOSITIONAL STUDIES OF THE RESIDUAL LATERITIC PROFILES OVER MINERALISED AND BARREN PEGMATITES IN PARTS OF THE PRECAMBRIAN BASEMENT COMPLEX OF SOUTHWESTERN NIGERIA

Abstract

Pegmatites (mineralised and barren) occur as intrusive bodies within the basement rocks of southwestern Nigeria. They are overlain by residual lateritic profiles which hinder access to the rocks thus making the discrimination between barren and mineralised pegmatites difficult. Geological information on lateritic profiles over mineralised and barren pegmatites in Nigeria is rare. The aim of this study was to determine the mineralogical and geochemical indicators in the overlying residual lateritic profiles that could be used to discriminate between barren and mineralised pegmatites. One hundred and twenty-three lateritic profile samples were collected from five selected vertical profiles over mineralised (Ijero, Komu, Ofiki and Iwere) and non-mineralised (Osu) pegmatites. Thirty mineralised and seven barren bedrock pegmatite samples were collected directly from exposed rocks underlying the profiles using standard geological method. Discrete mineral phases in the pegmatite samples were isolated and identified using standard methods. The mineralogy of the whole-rock and lateritic samples was determined using petrographic and X-ray diffraction techniques. The mineral chemistry of discrete phases in the pegmatites was determined by electron microprobe and laser-ablation-inductively coupled plasma-mass spectrometry. Elemental compositions of the pegmatites and their lateritic profile samples were determined, using X-ray fluorescenceand inductively coupled plasma-mass spectrometry. Data were analysed using descriptive statistics. The essential minerals in the barren pegmatite were microcline-perthite, quartz and muscovite, with accessory biotite, garnet and schorl; while the mineralised pegmatites contained accessory beryl, lepidolite, tourmalines, topaz, tantalo-columbite, rutile, ilmenite and magnetite in addition to the essential minerals. The lateritic profile over barren pegmatite was dominated by quartz and kaolinite with minor amounts of microcline, albite and muscovite. The mineralised pegmatite profiles contained quartz, kaolinite, muscovite, microcline and albite, with minor amounts of schorl, magnetite, dilithium tungstate, rutile and ilmenorutile. K-feldspar and muscovite showed low K/Rb(45.00-143.00 and 15.20-29.80), K/Cs(123.00-448.00 and 20.12-77.42) and Nb/Ta(1.30-1.50 and 1.10-1.56) ratios, reflecting moderate degree of fractionation and rare-metal enrichment of the mineralised pegmatites; whereas higher K/Rb(911.00 and 368.00), K/Cs (4456.00 and 2378.00) and Nb/Ta (3.70 and 1.64) ratios indicated very low fractionation and poor rare metal-bearing potential of the barren pegmatite. Immobile trace elements including Nb, Ta, Sn, YandTh were enriched in the mineralised pegmatite profiles, relative to the barren pegmatite. The mineralised pegmatite profiles showed low Nb/Ta (0.9-3.9) and high Th/U (0.5-3.6) ratios, while the barren pegmatite profile had high Nb/Ta (>6.0) and low Th/U (0.3-0.7) ratios. The residual profiles were clearly differentiated into their respective clusters on the Zr-Th, Y-Nb, Nb-Ta-Sn and Hf-Th-Ta discrimination plots. These geochemical patterns reflected the varying abundances of accessory minerals hosting the immobile elements in the respective pegmatite protoliths and the differential susceptibility of the mineral phases to intensive weathering conditions. Mineralogical indicators, including schorl, rutile, ilmenorutile, dilithium tungstate and magnetite, and geochemical indicators (niobium, tantalum, tin, yttrium and thorium) which showed abundance in the mineralised pegmatite profiles clearly distinguished them from the barren pegmatite profile. These mineralogical and geochemical indicators can be used as exploration guides for mineralised pegmatites underneath lateritic profiles.