Magnetotelluric exploration of a deposit scale prospecting over a proterozoic volcanics, Eastern India

Abstract

North Singhbhum Mobile Belt (NSMB) is a metallogenic province with dominant rock types of carbon phyllites, graphitic schists, quartzites, tuffs, rhyolites dominated with Dalma basalts of Proterozoic ages. Earlier 2-D Magnetotelluric (MT) studies across Dalma Volcanics (DV), lying almost at the center part of NSMB, hinted towards 3-D behavior at MT short periods for sites at the boundary of DV and Singhbhum Group of Metapelite (SGM). This tempted to acquire Audio MT (AMT) data sets over DV and SGM. The data was acquired at an interval of ~ 500 m for a profile length of ~7.5 km. The 3D joint inversion of MT + AMT data was performed with impedance, tipper and combination of impedance and tipper. Resistivity cross-sections from the 3D joint MT + AMT inversion of the combination of impedance and tipper are in broad agreement with previous 2-D results but with modified dimension and location. It delineated both near surface and deep conductor and establishes the link between these conductors. The near surface conductors are in agreement with the coincident Electrical Resistivity Tomography section. Near surface conducting heterogeneities mostly represents carbon phyllites, graphitic schists and tuffs - indicators of gold deposits. These mostly lie at the northern fringes of DV are surrounded by shallow resistive subsurface rocks. Relatively deep conducting volcanics are mostly dominant below DV. The presence of alternating conductive and resistive rocks below DV also validates the bimodal character of Dalma Volcanics. The sensitivity analysis suggest that the modeling results are mostly sensitive up to the depths of ~ 15 km. Conductivity anomalies of deposit scale both at shallow and deep levels, possibly enriched with sulfide-mineralization representing the metallogeny are associated with the evolution of Proterozoic Volcanics. The obtained depths of DV and SGM are 4–5 km and ~1.5 km, which is in agreement with the gravity data.