Airborne Electromagnetic Survey over the Touro Copper VMS World Class Deposit (NW Spain): Geological and Geophysical Correlation

Abstract

Electromagnetic (EM) methods belong to the main geophysical techniques used in the mineral exploration of massive sulphides. For selecting EM anomalies as possible massive sulphide targets, it is important to combine the geophysical results with other geological and/or geochemical techniques. In 2015, Atalaya Mining started a new mineral exploration project in the Touro Cu deposit, combining geological, geochemical (ore over 0.2% Cu), and geophysical techniques. The geophysical survey consisted of helicopter-borne EM using the versatile time-domain electromagnetic (VTEM™) max system operated by Geotech Ltd. with full-waveform processing. In total, 509 line-km of geophysical data were acquired during the survey that was completed in 2018. The results showed the massive sulphide Touro ore to be typically of the order of 0.25 ohm·m (4S/m conductivity) and host rock in the range of 1000–30,000 ohm·m, measured directly on the drill core. This modelling agreed well with the sub-horizontal dips observed for the known Touro ore bodies. The conductance modelled by the plate estimation of the VTEM data were also in good agreement with those provided by Geotech Ltd. and the resistivity/conductivity measurements we made on the massive sulphide samples from several Touro ore bodies. The combination of flat dips, good conductance, shallow depth, and, lastly, lack of conductive overburden or noneconomic conductive stratigraphy, i.e., graphitic shales and sulphide iron formation make the Touro project an ideal target for airborne electromagnetic prospecting. This paper presents the excellent correlation observed between the EM airborne anomalies and the massive sulphide blocks of the Touro copper deposit. Favourable factors contributing to the success of the survey were the high contrast in resistivity/conductivity between the massive sulphide Touro ore and the amphibolite host rock and minimal interference from “nuisance” conductors, such as graphitic shales.