GEOLOGICAL CHARACTERIZATION OF THE BASEMENT ROCKS OF OGOTUN, IPETU AND IKEJI-ILE AREA, SOUTHWESTERN NIGERIA

Abstract

The study area (Ogotun, Ikeji-Ile and Ipetu environs) belongs to the Precambrian shield of southwestern Nigeria. Also, the study area belongs to the South-eastern part of Ilesha schist belt which is also known for its gold mineralization and associated metallic mineral deposits. However, there is no available information on the geology, structure and mineralization potentials of the study area which necessitates this research. Sentinel-2A Imagery of the study area was acquired and digitally processed using the standard procedures to obtain the remote sensing data followed by automatic extraction of lineament using Envi 5.0, Geomatical and Arc Gis 10.2 software .These were also used to generate the required lineaments, lineament density map as well as the mineralogical map of the study area. Geological mapping at a scale of 1:25,000 was carried out to determine the underlying rocks followed by sampling at a sampling density of one rock per 3.25km2. Thirty-two rock samples collected were carefully selected for different studies (Petrographic, Ore microscopic and Geochemical). Remote sensing result showed paucity of lineaments in the study area due to the nature of the rocks and thick vegetation cover. The major lineaments trend in the NE-SW direction while the subsidiary ones in the E-W direction has been obliterated by several deformation episodes. The lineament density (fractures) are more pronounced on the migmatite and porphyritic granites. The areas with higher lineaments density are not hosting any mineralization which confirmed lithological control of mineralization. Geological field mapping revealed five major rock types such as migmatites, quartzites, quartzschists, meta-conglomerates and porphyritic granite with well delineated boundaries and the cross sectional map confirmed antiformal fold. Petrographic studies revealed the dominance of minerals such as quartz, biotite, feldspar (microcline and plagioclase) and opaque minerals. However, five rocks samples whose thin sections revealed opaque minerals under transmitted light were prepared for ore microscopic study and the result confirmed a brassy yellow mineral called chalcopyrite. Geochemical analysis revealed that SiO2 (67.37%-62.46%) and Al2O3 (17.35%-11.62%) have iv higher concentrations, indicating siliceous and aluminosilicate nature of the rocks while Na2O, K2O, Fe2O3 occurred in minor/ trace amounts. The result of trace elemental analysis revealed very high concentration of Ba (2533-10)ppm and Fe(55000-4100)ppm, and moderate to low and very low concentration of Cu (38.40-12.50)ppm, Pb (.90-39.40)ppm, Zn(156.00-7.00)ppm, Co(65.20- 1.40)ppm, Mn(148.00-981.00)ppm,, As (1.00-3.00-1.00)ppm ,Th (.00-50.10-00)ppm ,Rb (311.60- 50)ppm ,Cr(1310.-5.00)ppm. Also, the higher concentrations of Cu, Pb, Zn is highly suggestive of sulphide mineralization in the study area. Factor analysis showed Cu-Pb-Zn-Mn-Fe mineral associations with 35.24% covariance thereby suggesting the type of mineralization in the study area. The enrichment factor revealed that Cr (10%) has very high enrichment (10%) followed by Pb (20%) Zn (20% and Co (30) with moderately enrichment. Therefore, the result of factor analysis and enrichment factor all confirmed sulphide mineralization in the study area. In addition, Th, Rb and U showed a positive anomaly suggesting that rocks in the study area may be radio active in nature.