Magnetotelluric evidence for massive sulphide mineralization in intruded sediments of the outer Voring Basin, mid-Norway

A highly conductive body (0.1-0.8 Omega.m) is identified at mid-crustal depth (8-13 km) in the north Gjallar Ridge from magnetotelluric (MT) data and further investigated in light of other remote-sensing geophysical data (seismic reflection, gravity, aeromagnetic). A commercial 3D controlled-source electromagnetic survey was conducted in the Voring Basin in 2014 and, although primarily designed for hydrocarbon exploration, good quality MT data were extracted at periods ranging from 10 to 10(3) s. Dimensionality analysis indicates clear 1D to 2D characteristics in the MT data. 2D inversion was carried out on four profiles (totalling-94 km) oriented perpendicular to the electromagnetic strike and one profile along strike (similar to 45 km), using a 1D subset of the data. All inversions converged quickly to RMS values close to unity and display a very good agreement with borehole resistivity data from well 6705/10-1 located in the survey area. A striking feature on all profiles is a highly conductive (0.1-0.8 Omega.m) body at 8-13 km depth. To explain the prominent conductive anomaly, integration of geophysical data favours the hypothesis of electrical conduction across well-connected mineral network in pre-Cretaceous sediments. Seismic interpretation suggests a link between the conductor and intruded sedimentary successions below a detachment level and associated low-angle faults. In the Voting Basin, low magnetic signal and temperature at the conductor's depth indicate that such thick mineral deposits could display non-magnetic behaviour while occurring well below the magnetite Curie isotherm (similar to 585 degrees C). Natural occurrences and magnetic properties of common iron-sulphide minerals favour a geological interpretation of mid-crustal conductivity as thick pyrrhotite deposits formed in intrusion's contact metamorphic aureoles. (C) 2017 Elsevier B.V. All rights reserved.