0016-7622/2017-90-2-147/$ 1.00 © GEOL. SOC. INDIA | DOI: 10.1007/s12594-017-0692-x JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.90, August 2017, pp.147-153 Gold - Sulphide Mineralization in Ultramafic-Mafic-Granite Complex of Jashpur, Bastar Craton, Central India: Evidences from Geophysical Studies Dewashish Kumar*, D.V. Subba Rao, Setbandhu Mondal, K. Sridhar, K. Rajesh and M. Satyanarayanan CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad – 500 007 *E-mail: dewashishkumar@ngri.res.in ABSTRACT Time Domain Induced Polarization (TDIP) investigation in conjunction with high resolution electrical resistivity tomography surveys carried out at seven sites along a 5.2 km line in Jashpur and Raigarh districts of Chhattisgarh State yielded promising results leading to the demarcation of mineralized zones. These sites are confined to ultramafic-mafic-felsic intrusive complexes representing gabbro-pyroxenite-granite sequences characterized by gold - sulphide mineralization. Two dimensional (2D) geophysical datasets yielded interesting results and a good correlation is found between 2D inverted resistivity tomography and time domain induced polarization (IP) models. Both high resistivity-high chargeability as well as low resistivity-high chargeability conditions have been inferred from these datasets. This observation is consistent with conductive metallic mineralization. Interpretation of the Pharsabahar-Pandripani site, reveals a chargeability magnitude of 9-14 mV/V, which represents a strong metallic conductor associated with gold-sulphide ore body. The results at Samarkachar site are equally significant, showing a good correlation with the presence of a low resistivity and high chargeability zone. Hence, the present study suggests a new scope for application of resistivity and IP tomography surveys for mineral prospecting in favourable geological settings. INTRODUCTION Mineral prospecting and exploration is important to identify and locate new mineral resources as it improves the socio-economic scenario of a nation. The present study is an attempt to delineate and evaluate concealed mineral deposits in central India. Electrical and electromagnetic methods are generally employed for prospecting metallic mineral deposits associated with sulphide and oxide minerals (Sumner, 1976; Moon et al., 2006). In particular, Induced Polarization (IP) method is normally applied due to the presence of high polarizability of ore minerals, which are disseminated in various host lithologies. Auriferous zones are often associated with metallic sulphide mineralization, which creates an IP effect i.e., chargeability in the mineralized zone. Gold itself will not contribute significantly to the IP effect, but in the presence of sulphide minerals, the scope of finding gold is greatly enhanced (Hallof and Yamashita, 1990). The present study documents the combined use of geophysical methods correlated with the observed geological features of the mineralized zones, considered to be a technically feasible procedure for the exploration of mineral deposits. Utilizing the above integrated approach, the present work was taken up in Raigarh, Jashpur and the surrounding areas within Chhattisgarh State of central India to delineate the anomalous sulphide bearing mineralized zones. REGIONAL GEOLOGY The central part of the Indian Precambrian shield consists of two distinct crustal provinces viz., the northern (Bundelkhand) crustal province and the southern (Bastar) crustal province (Roy and Chakraborty, 2008; Sarkar and Gupta, 2012). The Bastar crustal province is an Archaean cratonic nucleus represented by the widely dispersed older Sukma Group of metamorphosed supracrustals, with a older crust (> 3.0 Ga) and the younger (Neoarchaean to Mesoproterozoic) supracrustals occurring as well defined N-S trending volcano-sedimentary belts (Sharma, 2010; Sarkar and Gupta, 2012) (Fig.1a). Younger granites intrude both the older and younger supracrustals. The study area is located in the north-eastern part of Bastar craton, and forms the western continuation of the Chhotanagpur gneissic complex (CGC). The CGC comprises of mainly metasediments and meta-basic/ultramafic rocks, occurring as linear bands or lensoidal patches/enclaves in younger granitoids (Fig.1b). GEOLOGY OF THE STUDY AREA Amphibolite-biotite-chlorite-gneiss (tonalite) also called as banded felsic-biotite gneiss is the oldest rock type in the area (Fig.1b) and is possibly the basement for the younger rocks (Roy and Chakraborty, 2008). Mafic-ultramafic, granodioritic and granitic rocks have intrusive relationship with the gneissic basement. These ultramafic-mafic bodies of varying dimensions occur within the gneisses and granitoids in Pathalgaon area (Fig.1b) (Roy and Chakraborty, 2008). Two prominent bodies: (1) Kotba-Lailanga body (22 o 22'N; 83 o 42'E) (~7.5×2 km), trending WNW-ESE and (2) Samarkachar body (22 o 35'N; 83 o 47'E), which is ellipsoidal in shape (~6×5 km) (Fig. 1b) and are intruded by thin quartz-diorite, aplite and quartz veins. Recently Geo Mysore Services (India) Pvt. Ltd., (GMSI) explored the Pathalgaon area for gold, nickel, copper and platinum in their reconnaissance permit (RP) lease areas at Lailanga-Samarkachar and Pharsabahar-Pandripani section. The gabbroic rocks of Lailanga and Samarkachar complexes are characterized by visible sulphide mineralization and alteration (Fig.2a-c). The vast area around Pandripani and Pharsabahar (Fig.1c) is occupied by coarse to medium grained granitoids, chert and tourmalinite (Fig. 1c and 2d). Both concordant and discordant quartz veins rich in sulphide minerals are abundant (Fig.2d and 2e). Sulphidic alteration zones are also noticed in the chert (Fig. 2f). SULPHIDE MINERALIZATION Disseminated sulphide mineralization is common in different host rocks namely, chert-tourmalinite of Pandripani-Pharsabahar areas as well as in metagabbroic rocks in the Samarkachar ultramafic-mafic intrusive complex (Figs. 2a-f and 3a-h).