Gravity and vertical magnetic gradient investigations of a localised area of the Benue Trough, Nigeria

Abstract

Gravity and vertical magnetic gradient studies were carried out in Lokpanta-Lekwesi area in the Lower Benue Trough to investigate the local structures which might be associated with the Lead-Zinc mineralisation as evident in the adjacent area (Ishiagu Pb-Zn mineralisation). For this reason 134 gravity and vertical magnetic gradient readings were taken at intervals of 250 m along major roads and track routes that connect widely separated parts of the 25 km 2 area. The instruments used are the Canadian Scintrex CG-2 gravimeter (No. 318), the American Paulin altimeter and the Proton Precession Magnetometer. The geology of the areacomprises the Ezeaku Formation, and the Awgu Formation, their ages ranging from Middle to Upper Cretaceous. The study was used to delineate two igneous intrusions in the Ezeaku Formation. The intrusive bodies show high positive Bouguer anomalies which ranged from 0.5 mgal to 3.3 mgal in contrast to the negative Bouguer anomaly of the country rock that ranged from 0 to − 3.0 mgal. The vertical magnetic gradient method identified the two intrusive bodies as negative anomalies having vertical magnetic gradient values ranging from −5.0 to −25.0 gammas per meter in contrast to 5.0 to 10.0 gammas per meter values of the country rock. The Profiles were interpreted using computer assisted evaluation procedure of non-linear optimization and interactive techniques. Based on two dimensional model, the minimum depth computations to the upper surfaces of the anomalies A and B by gravity method are 60mm and 160 m respectively while the vertical magnetic gradient method gave a minimum depth estimates of the anomaly A as 75 m and that of B as 180 m.