Laboratory investigations and theoretical considerations on the electrical properties of seafloor massive sulfides

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In order to satisfy future global demands for metals, there is an increasing interest in deep-sea mining. Seafloor massive sulfides (SMS) are considered an important marine metal resource. They are generated in regions of submarine volcanic activity, in particular mid-ocean ridges. Since underwater mining is difficult and expensive, a careful exploration and characterisation of seafloor deposits is essential. Electrical methods, and in particular induced polarization, are considered to play a key role, because they determine the electrical conductivity of the rocks, which depends on mineral content. Unlike for continental ores, where a wealth of data exists, the electrical properties of SMS are poorly known. Seafloor samples are rare and difficult to obtain, and only a small number of studies exists. Results obtained for continental sulfides cannot easily be transferred to seafloor conditions, because the marine fluids are considerably more saline, and the rock chemistry and texture may differ. In this project, we measure the frequency-dependent electrical properties of a large number of SMS samples from several worldwide locations, including deposits in the Atlantic, Pacific and Mediterranean. The results will be analysed in the context of mineralogical data, in order to identify potential relationships between electrical parameters and mineral content, that will constitute a basis for future exploration programs. The measurements will be preceded by methodological studies to define the optimum procedure to measure electrical properties of samples of irregular shapes. The SMS samples are valuable and it is generally not desired to destroy them, such that standard methods using cylindrical plugs cannot be applied to all samples. The data interpretation will be supported by extensive theoretical considerations, in order to obtain a fundamental understanding of the physical processes that control the electrical impedance of mineralized rock. Existing theories exclusively assume that the minerals are disseminated, whereas experimental evidence suggests that connected veins have a huge influence on electrical conductivity and cannot be neglected. Our theoretical advancements will combine existing models of the electrochemistry with new methods recently developed for unmineralized sediments.