Geophysical and geostatistical reserve estimates of migmatite-gneiss deposits from parts of southwestern nigeria

Abstract

The plan for durable rock base road construction and other civil engineering works necessitated this study to establish the thickness and quantify migmatite-gneiss deposits. This intends to facilitate its exploitation and proffer specific details for diverse applications. Comprehensive geological field mapping, laboratory density measurements and geoelectrical resistivity were employed for the resource quantification. The extent of migmatite-gneiss deposits and its contacts with other rock types were identified. Seventeen (17) fresh migmatite-gneiss rock samples collected from different rock outcrops were taken to the laboratory for measurement of their density. Schlumberger vertical electrical sounding (VES) technique with a total of forty-two sounding stations were employed across the area with electrode spread (AB/2) ranging 1 - 200 m. The average density of the deposits is 2.70 ± 0.10 g/cm³. Three to five geologic layers characterized the area subsurface sequence. Thick fractured rock layer across the area would facilitate the exploitation of the deposits as construction aggregates. Basement topographical highs at northeastern, northwestern and southeastern parts serve as the best zones viable for mining. The basement resistivity (> 3000 Ωm), resistivity contrast, reflection coefficient values and high transverse layer resistivity (ρ_T) (> 500 Ωm) corroborate the freshness of the deposits. The average thickness of the deposit is 29.3 m, though, thicker at northeastern and northwestern parts of the area where the deposit is fresh and less weathered. The study area occupies a total area of 71,300,000 m² with volume of the deposits calculated as 2,089,090,000 m³. Distantly-spaced data points of the variogram reveal high degree of variability with respect to locations. The estimated migmatite-gneiss resource tonnage of 5,640,543,000 tonnes shows prospects for sustainable large scale mining as construction aggregates and diverse applications of economic purposes using open pit coupled with underground mining for deeper sections of the migmatite-gneiss deposit.