Seafloor massive sulfides (SMS) deposits are considered as potential resources to secure and diversify the metal supply in Europe. Estimates of the ore volume but also the spatial and temporal distribution of massive sulfide deposits are, however, imprecise and still debated. It is therefore important to continue exploration cruises at different scales (regional and local) to answer fundamental questions in metallogeny but also define preservation strategies in case of exploitation. The most voluminous SMS deposits and therefore of potential economic interest are located on slow oceanic ridges. In 2014, France signed an exploration contract for polymetallic sulfides located on the slow mid-Atlantic ridge between 21°N and 26°N.

The first campaign carried on in 2017 (HERMINE cruise) explored the 600km of the license, identifying at least six new hydrothermal areas. The data acquired allow performing local exploration operations with the AUV and HOV Nautile. In addition, the operations carried out in the TAG district have resulted in the discovery of numerous inactive deposits which provide a better understanding of the spatial and temporal distribution of massive sulfides sites as well as their aging processes. Finally, study on hydrothermal fluids and plume of the TAG active mound provide answers on the impact of high temperature hydrothermal systems on metal cycles in the oceans. The HERMINE2 cruise revolves around the AUV/Nautile coupling and has two main objectives: (i) characterize and sample the six new hydrothermal fields identified during the HERMINE cruise (2017) and (ii) continue the characterization of SMS deposits in the district of TAG and study the low temperature zone located 5km east of the 'TAG active mound' site. The first part of the HERMINE 2 cruise will therefore aim to locate the new active sites on the seafloor and to sample the mineralization, fluids and associated fauna. Additionally, Nautile dives dedicated to the search for inactive sites will be carried out based on data acquired by AUV (e.g. acoustics, magnetism) and knowledge of the geological parameters controlling the spatial distribution of SMS deposits. The objective is to assess the nature and importance of sulfide mineralization in order (1) to identify or not the presence of targets of economic interest and (2) to assess the perimeter of a potential mining area; a parameter essential for the sizing of future impact studies. This work should also provide a better understanding of the spatial and temporal distribution of hydrothermal systems, from the scale of a district (20-100km2) to the scale of several ridge segments. (10,000km2). The environmental component of the HERMINE2 cruise will provide important data on the biodiversity of the new hydrothermal sites making it possible to assess the consequences of large geological structures (e.g. the transforming Kane fault) on the connectivity of species at the scale of the exploration license. The second part of the cruise will focus on the TAG district including the acquisition of geophysical data on active and inactive sulfide deposits and the detailed study of low temperature hydrothermal deposits (iron-manganese-silica precipitates and diffuse fluids). The aim is to understand the link between these low temperature systems and high temperature sulfide deposits in a tectonic context controlled by a detachment fault. The study of low-temperature precipitates will improve our knowledge of these poorly known systems, which are thought to play a major role in the metal cycles in the oceans.

The strategy defined for this campaign is complementary and coordinated with the approach led by biologists and microbiologists (BICOSE and BICOSE2 cruises). The HERMINE2 cruise will thus make it possible to meet the requirements of support to the public authorities on knowledge of the SMS resource while carrying out fundamental studies on hydrothermal processes.