Integrated Magnetic-Radiometric and Geochemical Assessment of an Orogenic Gold Province in the Okpella Area, Igarra Schist Belt, Southwestern Nigeria
Abstract
The Okpella area within the Igarra Schist Belt of southwestern Nigeria represents a poorly understood yet potentially significant orogenic gold province within the Pan-African mobile belt. Although there have been some intermittent artisanal mining activities in several decades past, there has been a lack of systematic geophysical and geochemical characterization of mineralization controls. This research combines high-resolution aeromagnetic and radiometric data with sequential extraction geochemistry to define the structural and lithological controls on gold mineralization and evaluate metal mobility patterns. First Vertical Derivative (1VD) and Tilt Derivative (TDR) filters applied to the Total Magnetic Intensity (TMI) grid delineated the principal NE-SW shear system and subsidiary NW-SE cross cutting structures. Radiometric data processing identified discrete high-potassium zones overlapping magnetic lows. These elevated K/Th anomalies are interpreted as phyllic (sericitic) alteration assemblages, a hallmark of orogenic gold systems worldwide. Sequential extraction of 47 soil samples indicated that gold is mainly located in the fraction of Fe-Mn oxide (43-58%), residual silicate phase (31-42%), but a small amount in the exchangeable or carbonate bound fraction, which shows that it does not have significant secondary mobility in tropical weathering. Coherent pathfinder element anomalies — elevated As (12–87 ppm), Sb (0.8–4.2 ppm), and Bi (0.3–2.1 ppm) — are spatially aligned with NE-trending shear corridors. Combination of both geophysical and geochemical data sets outlined four high-priority targets which were defined by coincidence in structural complexity, footprints of hydrothermal alteration and concurrent multi-element geochemical signatures. These results form a strong exploration model that can be used in other Proterozoic terrains that host gold such as schist in West Africa.