Overburden Thickness and Potential Tonnage Estimate of Olode-Gbayo Pegmatite Deposit, Southwestern Nigeria, Using Electrical Resistivity Method

Prominent locations in the Nigerian basement complex are known to harbor precious rocks and gemstones, in addition to the basement rocks being quarried for engineering construction. However, reserve estimates of these rocks and mineral deposits are largely unknown. Geophysical investigations have been undertaken at Olode-Gbayo village in Ibadan, southwestern, Nigeria. The primary objectives are to estimate overburden thickness and extend results to estimate probable gross basement tonnage and probable reserve of mineralized pegmatite in the basement rock. The Olode-Gbayo pegmatite has been found to be mineralized in aquamarine at depths. A total of seventeen vertical electrical soundings (VES) were carried out. Data acquisition involved the use of Schlumberger electrode configuration. Interpretation of the geoelectrical data involved the use of curve matching technique and computer iteration. Geoelectrical cross sections and isopach maps were constructed based on lateral combination of inverted soundings from the VES surveys. Existing mining pits were logged and rock samples were collected to estimate basement rock distribution and determine specific rock gravity. The VES result showed a three to four geo-electric layers, the top soil, weathered layer, partially weathered layer and fresh or partially fractured basement. Overburden thickness varied from about 2.6 to about 7.7 m. The thickest overburden is observed in the southern part whereas thinner overburden is observed in the northern part of the area. Using an exploitable basement rock thickness of 50 m, gross basement rock tonnage was estimated as 1.95 x 108 tons, with pegmatite being estimated to constitute about 0.5% of the basement rock volume. Generally, the basement rock is considered economically viable for mining and engineering construction, whereas the pegmatite rocks could be economically minable as host rock for gemstone.