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

A highly conductive body (0.1–0.8 Ω·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 Vøring Basin in 2014 and, although primarily designed for hydrocarbon exploration, good quality MT data were extracted at periods ranging from 10 0 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 (~ 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 Ω·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 Vøring 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 (~ 585 °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. [ABSTRACT FROM AUTHOR]