PETROGRAPHY, GEOCHEMISTRY AND EVOLUTION OF ROCK UNITS IN SHEET 241, OYO S.E., SOUTHWESTERN NIGERIA

Abstract

The Basement Complex rocks of Southwestern Nigeria have been mapped regionally on a scale of 1:250,000. On this scale, many rock units are grouped together with very limited information on component members. However, such details are necessary for the understanding of the mineral potentials. Thus, the production of maps on larger scales, with more geologic information are very important. Sheet 241 Oyo SE falls within the Iseyin-Oyan Schist belt whose detailed geology and mineral potentials are yet to be fully understood. Therefore, this study was aimed at detailed mapping and assessment of mineralisation potentials of rock units of Sheet 241 Oyo SE, Southwestern Nigeria. A geological field mapping exercise was undertaken by compass-traversing using a topographical map Sheet 241 (Oyo SE) (1:50 000) as base map. This involved detailed observation of outcrops for structural features, field relationship, measurements of trends and orientation of the rock units. The obtained rock samples were prepared into thin sections. Fifty-four rock samples were purposively analysed using inductively coupled plasma-mass spectrometry and X-ray fluorescence. Selected polished samples were analysed for specific minerals using SEM-EDX, while others were analysed for mineralogical content using X-ray Diffractometer. The rock units were characterised petrologically, while the geochemical data were interpreted using various geochemical binary and discriminatory diagrams. Quartzite; quartz-schist and muscovite-garnetiferous schist; biotite hornblende gneiss; granodiorite gneiss; and biotite granite gneisses as well as leucogranite, variably intruded by pegmatite, quartz veins and dolerite dykes of varying thickness and lengths were identified. Texturally, the rocks exhibited granoblastic, porphyroblastic and augen textures. Folds, joint, shear zones, and foliation trending in NW-SE directions were observed. The compositional range of plagioclase feldspars in the granodiorite gneiss were oligoclase (An10.43-27.82) to low andesine (An31.42-31.81), while low oligoclase (An11.15-11.75andAn11.46-27.82) were observed in the biotite hornblende and biotite granite gneisses. Biotite in granodiorite and biotite granite gneisses was phlogopitic, and siderophyllitic in biotite hornblende gneiss. Amphibole in the granodiorite gneiss was ferro-hornblende in character. Garnet, chlorite, apatite, zircon, rutile and magnetite occurred as accessory minerals in the gneisses. The schists were of sedimentary protolith emplaced in the passive margin field and had undergone low to moderate degree of chemical weathering. The gneisses were dominantly derived from igneous origin of calc-alkaline magma type, which were peraluminous and emplaced in pre-plate to syn-collision granite field. Leucogranite rock was of I-type, emplaced in orogenic unfractionated granite field. All the rocks displayed enrichment in large ion lithophile elements and light rare earth elements, while they were depleted in the high strength field elements and heavy rare earth elements, with distinctive negative Eu anomalies, strongly indicating continental sources for all the rock units. Anomalous concentrations were observed for Au (<0.5-30.8ppb), Cu (0.7-64.2ppm), Pb (2.9-41.8ppm), Zn (18.6-136ppm) and Ni (1.8-13.4ppm) in the quartz schists, gneisses and pegmatite. Ten rock units were delineated on Sheet 241, Oyo SE on a scale of 1:50,000 with the schist, gneisses and pegmatite found to be mineralised with anomalous gold concentrations.