Loki’s Castle on the Arctic Mid-Ocean Ridge (AMOR) is an area of possible seafloor massive sulphide (SMS)-style mineralisation under Norwegian jurisdiction, which, due to mounting social pressure, may be a strategic future source of base and precious metals. The purpose of this study is to characterise mineralised material from a hydrothermal vent system on the AMOR in detail for the first time, and to discuss the suitability of methods used; reflected light microscopy, X-ray diffraction (XRD), whole rock geochemistry, electron probe micro-analysis (EPMA), and QEMSCAN. The primary sulphide phases, identifiable by microscopy, are pyrite and marcasite with minor pyrrhotite and galena, but multiple samples from the Loki’s Castle contain economically interesting quantities of copper (hosted in isocubanite and chalcopyrite) and zinc (hosted in sphalerite), as well as silver and gold. This reinforces the notion that slow spreading ridges may host significant base metal deposits. Micro-textures (chalcopyrite inclusions and exsolutions in sphalerite and isocubanite respectively) are typically undefinable by QEMSCAN, and require quantitative measurement by EPMA. QEMSCAN can be used to efficiently generate average grain size and mineral association data, as well as composition data, and is likely to be a powerful tool in assessing the effectiveness of SMS mineral processing.

This paper reports the leaching of seafloor massive sulphides (SMS) from the Loki’s Castle area at the Arctic Mid-Ocean Ridge in sulphuric acid with manganese dioxide and sodium chloride. The results presented are of various leaching experiments conducted under different conditions in order to optimise the dissolution of copper and silver. It was shown that the main copper bearing minerals in the SMS were chalcopyrite and isocubanite, while silver could occur as an admixture in the crystallographic lattice of sulphides or as disseminated micro inclusions. Based on the results, the leaching mechanism was discussed and elucidated. It was shown that the dissolution of the SMS was mainly due to galvanic interactions between the primary marine minerals of SMS and manganese dioxide. Addition of sodium chloride promoted the extraction mechanism.