Ground Magnetic Survey for Mapping Pegmatite Bodies in Oyo State

This study conducts a ground magnetic survey to map pegmatite bodies and associated mineralisation in the basement complex of Oyo State, South West Nigeria. Pegmatites in southwestern Nigeria host commercially important gemstones including tourmaline, aquamarine, topaz, and rare earth mineral-bearing coltan and tantalite. Systematic geophysical mapping of pegmatite distribution can guide artisanal mining towards productive areas while minimising environmental damage from unguided excavation. This study acquires total magnetic intensity data at 15-metre station intervals along 28 profiles covering an area of approximately 8 square kilometres in Ijesa-Isara and Ife East areas. A proton precession magnetometer is used for field measurements with a fixed base station for diurnal correction. Data processing includes diurnal drift correction, IGRF 2020 removal, RTP transformation, and computation of first vertical derivative. Magnetic bodies are modelled using Potent 3D modelling software constrained by geological field observations. Findings reveal discrete, elongated magnetic lows of 200 to 600 nanoteslas amplitude trending north-northeast, interpreted as late-to-post-tectonic granite and pegmatite bodies that are less magnetic than the surrounding amphibolite and schist lithologies. Seven discrete low-amplitude closures are delineated and spatially correlated with known gemstone occurrences in the area. The modelled pegmatite widths range from 15 to 80 metres and exhibit steep eastward dips. Three bodies show the lateral continuity and depth extent most favourable for artisanal mining, and their surface expression is recommended for licensed gemstone development.

Keywords: ground magnetic survey, pegmatite, gemstone mineralisation, Oyo State, basement complex.