Geophysical studies in parts of Kolar, Dharmapuri, Chittoor and Thiruvannamalai districts, Karnataka and Tamilnadu

Regional geophysical mapping employing gravity and magnetic methods under National Geophysical Mapping Programme (NGPM) has been carried out in 2160 sq km area of Obvious Geological Potential (OGP) category. The objective of the work was to delineate the subsurface structures for identification of suitable areas for detailed study in future. The Bouguer anomaly (BA) map has brought out negative gravity anomaly with amplitude of 43 mGal in the range from - 41 mGal in the northern & eastern part to - 84 mGal in the western part. The area surveyed is characterized by high gravity anomaly towards northern and eastern part and low to medium gravity anomaly in the southern and central part. The elongated residual high gravity nosing east of Kolar Gold field trending in nearly N-S direction and flanked by residual gravity lows on either side, is inferred as basement high and may assume importance for mineralization in the area. N-S trending residual gravity high in the central portion is possibly attributable to faulted contact where deformation of crustal layers in the form of faults and folds are quite evident over the PGC (Peninsular Gneissic Complex) Group of rocks. Magnetic anomaly (TF) map shows bipolar magnetic anomalies ranging between - 179 nT to 396 nT. Northern and eastern parts are dominated by high intensity magnetic anomaly and central part is associated with small bipolar nature of anomalies. The radially averaged power spectrum of gravity field exhibits three interfaces at depths of 5.6 km, 3.1 km and 1.6 km and similarly, magnetic field exhibits two interfaces at depths of 3.7 km and 2.0 km. The interfaces brought out by both the gravity as well as magnetic data, may represent the depth range of contact between PGC and Kolar Group. The linear and curvilinear clustering of Euler depth solutions has been brought out and may be inferred as geologic/faulted contacts. Major contacts/faults are observed with a depth range of <1000 m due to sharp contrast in density/susceptibility of the subsurface rocks. Depth extension of existing Kolar schist belt (3.3 km) and estimated thickness of inferred schist belt (1.2 km and 1.1 km) are obtained by 2D gravity modelling.