Self‐Potential Tomography of a Deep‐Sea Polymetallic Sulfide Deposit on Southwest Indian Ridge

Abstract

Deep-sea polymetallic sulfides associated with hydrothermal systems are considered a potential viable resource for base and precious metals. The Southwest Indian Ridge (SWIR, Indian Ocean) hosts active and inactive hydrothermal systems. Inactive hydrothermal fields are more abundant than active fields but are difficult to remotely characterize. We report a deep-sea self-potential investigation to locate inactive ore deposits at the Yuhuang hydrothermal field on the ultraslow-spreading SWIR. A horizontal array of six nonpolarizing AgCl electrodes are attached to a deep-towed transient electromagnetic system to record the electrical field at a height of 40 m above the seafloor. The observed electrical field strength near a previously recognized sulfide deposits (verified by drilling) reaches an amplitude of 0.5 mV/m. Negative self-potential anomalies (~−27 mV) were observed by integrating the measured electrical field. The high electrical conductivities (up to 12 S/m) of sulfide samples measured in the laboratory and the oxidized sulfides recovered from the surface of the deposit suggest that the self-potential anomalies are due to sulfide mineralization and corrosion of the polymetallic sulfides. Inversion of the self-potential data reveals a localized body with a thickness of~65 m, which is interpreted as the Yuhuang ore deposit. Our field data demonstrate that the self-potential method is a useful exploration method to identify and image seafloor massive sulfide deposits hosted in inactive hydrothermal fields at mid-ocean ridges.

Rock Magnetic Signatures of Hydrothermal Mineralization in the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field

“…Migrating electrons produce current in SMS deposits, creating spontaneous "geobattery" (Sato & Mooney, 1960). Seafloor self-potential surveys have shown that SMS deposits in inactive hydrothermal have strong electric anomalies (Szitkar et al, 2021;Zhu et al, 2020) and therefore confirm the existence of galvanic reactions in SMS deposits. Laboratory simulations have verified that galvanic reactions can accelerate the oxidation rates of sulfides (Knight et al, 2018).…”

Deep‐Sea Electric and Magnetic Surveys Over Active and Inactive Basalt‐Hosted Hydrothermal Sites of the TAG Segment (26°, MAR): An Optimal Combination for Seafloor Massive Sulfide Exploration

“…It should be noted that the electric data we present in this study were acquired as horizontal electrical field components. A self‐potential (SP) could either be derived by integration of electrical fields along the flight path (e.g., Zhu et al., 2020) or by deriving the self‐potential field from the measured electrical fields by means of a regularized optimization process (Constable et al., 2018). However, as self‐potentials are only a derived quantity from the measured electrical field components, we will mostly refrain from using them in the scope of this article, similar to the aforementioned authors, who also mostly rely on electrical fields in the display and interpretation of their marine datasets.…”

“…To improve the reliability of the measurements, we recommend distortion-reducing options, partially inspired by successful Japanese and Chinese experiments (Kawada & Kasaya, 2017Zhu et al, 2020):…”

“…To improve the reliability of the measurements, we recommend distortion‐reducing options, partially inspired by successful Japanese and Chinese experiments (Kawada & Kasaya, 2017, 2018; Zhu et al., 2020): When the bathymetry is not too steep, favoring AUV dives at a constant water depth rather than a constant altitude above the seafloor to limit attitude variations during the dive. We strongly encourage, not to compensate this need by following the bathymetry isocontours, as such AUV dive geometries would result in complex and undulating routes that are not optimal for an efficient processing of the geophysical data. In the case of torpedo‐shaped AUVs, attaching a rigid array of electrodes with a neutral buoyancy would act as a damping system suppressing limited attitude variations.…”

Joint Interpretation of Marine Self‐Potential and Transient Electromagnetic Survey for Seafloor Massive Sulfide (SMS) Deposits: Application at TAG Hydrothermal Mound, Mid‐Atlantic Ridge

“…In the last decades, galvanometric and electromagnetic methods have been used for the exploration and assessment of SMS resources. These methods include the controlled source electromagnetic (CSEM) method, the direct current (DC) resistivity imaging method, the transient electromagnetic (TEM) method, and the passive self‐potential (SP) method (see case studies in Cairns et al., 1996; Constable et al., 2018; Galley et al., 2021; Gehrmann et al., 2019; Haroon et al., 2018; Ishizu et al., 2019; Kawada & Kasaya, 2018; Müller et al., 2018; Su, Tao, Shen et al., 2022; Szitkar et al., 2021; Zhu et al., 2020). However, ore bodies and metallic deposits are not always characterized by conductivity contrasts (Mao & Revil, 2016; Mao et al., 2016).…”