Induced Polarisation Tomography for Mapping Supergene Gold Deposits in the Birnin Gwari Schist Belt

This study applies induced polarisation tomography to map the distribution and depth extent of supergene gold mineralisation in the Birnin Gwari schist belt of Kaduna State, North West Nigeria. The Birnin Gwari area hosts one of Nigeria's most historically productive gold mining districts, with current artisanal activity concentrated in supergene enrichment zones developed over hypogene sulphide-rich quartz veins. Characterising the three-dimensional distribution of the supergene ore zone is critical for planning formal small-scale mining development. This study deploys dipole-dipole IP-resistivity tomography along 12 profiles totalling 3.6 kilometres across the principal mineralised zone. Data are acquired using a Syscal Pro system with 48 electrodes at 10-metre spacing and current injection of 200 milliamperes. RES2DINV inversion generates 2D sections of chargeability and resistivity. Sections are correlated and interpolated to produce a 3D chargeability volume. Geological mapping and sampling of surface exposures supplement the geophysical survey. Core samples from three shallow boreholes are used for geophysical-geological validation. Findings reveal that chargeability anomalies of 25 to 45 milliseconds occur at 5 to 35 metres depth along two primary zones trending northeast-southwest, consistent with the orientation of gold-bearing shear zones mapped at surface. The higher chargeability values correlate with disseminated arsenopyrite and pyrite assemblages that carry the majority of the gold at depth, confirmed by fire assay results. The supergene oxide cap shows low chargeability (less than 8 milliseconds) but reduced resistivity and is the currently mined zone. The study recommends directional drilling of the primary chargeability targets for sulphide ore evaluation.

Keywords: induced polarisation tomography, supergene gold, Birnin Gwari, schist belt, sulphide mapping.