Aeromagnetic Anomalies, Limited Ground Magnetic and Gravity Survey of NE Kogi State / NW Benue, Nigeria

SUMMARY The study area has had a good coverage of aeromagnetic survey sponsored by the Geological Survey of Nigeria. A combination of ground surface gravity and magnetic survey was conducted to tie to the aeromagnetic profiles. The terrain is made up Basement Complex rocks bounded at the west by the River Niger, and sedimentary rock type in the east. The Basement Complex comprises schists and meta-igneous rocks. Evidence of igneous activities within the Basement includes igneous intrusions of granites, pegmatites and gabbro. Density measurements of various rock types were taken. Magnetic susceptibility is highest in the igneous intrusive, with the magnitude of 0.00095 emu. The metamorphic rock of high susceptibility (0.00015 emu) is the Cordierite-Tourmaline Schist. Aeromagnetic interpretation of the study area yielded a range of total magnetic field intensity 8000 gammas to 8030 gammas, with mean maximum horizontal gradient of 30gammas/km in the sedimentary terrain, and about 35 gammas/km over the Basement rocks. The ground magnetic survey indicated an average vertical field intensity of about 3140 gammas for the area. An integrated interpretation of geophysical results revealed the existence of a fracture at Gboloko NE-SW axis, a synclinal fold axis at about 5.5km west of Gboloko (between the Staurolite Schist and Cordierite-Tourmaline schist). The Basement-Sedimentary boundary is characterized by a drop in residual Bouguer anomaly from positive to negative at about 6km east of Gboloko, and a decline in magnetic horizontal gradient 20.40 gammas/km. The thickness of the sediments is about 0.90km at the northern part of the Basement-Sedimentary boundary, and about 2.0km in the south, thus suggesting a progressive increase in sedimentary thickness at the western edge of the Benue trough. The magnetic highs correspond to a combination effect of topographic irregularities and some bands of weathered, lateritized sediments. The olivine gabbro covers a diameter of about 1.5km in the subsurface, extending to Emi-Adama area indicative of significant iron-content for the magnetic anomaly and density contrasts in the Bouguer anomalies.