Beschreibung: Seafloor massive sulfide (SMS) deposits are increasingly seen as important marine metal resources for the future. A growing number of industrialized nations are involved in the surveying and sampling of such deposits by drilling. Drill ships are expensive and their availability can be limited; seabed drill rigs are a cost-effective alternative and more suitable for obtaining cores for resource evaluation. In order to achieve the objectives of resource evaluations, details are required of the geological, mineralogical, and physical properties of the polymetallic deposits and their host rocks. Electrical properties of the deposits and their ore minerals are distinct from their unmineralized host rocks. Therefore, the use of electrical methods to detect SMS while drilling and recovering drill cores could decrease the costs and accelerate offshore operations by limiting the amount of drilling in unmineralized material. This paper presents new data regarding the electrical properties of SMS cores that can be used in that assessment. Frequency-dependent complex electrical resistivity in the frequency range between 0.002 and 100 Hz was examined in order to potentially discriminate between different types of fresh rocks, alteration and mineralization. Forty mini-cores of SMS and unmineralized host rocks were tested in the laboratory, originating from different tectonic settings such as the intermediate-spreading ridges of the Galapagos and Axial Seamount, and the Pacmanus back-arc basin. The results indicate that there is a clear potential to distinguish between mineralized and non-mineralized samples, with some evidence that even different types of mineralization can be discriminated. This could be achieved using resistivity magnitude alone with appropriate rig-mounted electrical sensors. Exploiting the frequency-dependent behavior of resistivity might amplify the differences and further improve the rock characterization.

Beschreibung: Sefloor massive sulfides (SMS) are believed to constitute an important future mineral resource. Nevertheless, little is known about the electrical properties of SMS, in particular under in-situ conditions. We measured electrical impedance spectra of 40 samples, 30 of which are sulfidebearing, and 10 are unmineralized host-rock. The samples were saturated with sodium chloride solution with 5 S/m conductivity. The resistivity magnitude shows a clear difference between mineralized and unmineralized samples, and also a weak grouping between the different types of mineralization. The imaginary conductivity at 1 Hz indicates a more pronounced discrimination between mineralized and unmineralized samples, suggesting that complex measurements might be useful for exploration purposes. We also measured spectra under dry conditions. Surprisingly, the sulfide-bearing samples exhibit significant phase shifts even for dry samples, indicating that the conducting minerals themselves cause a phase shift, and an interaction with an electrolyte might not be necessary.