Introduction: Although, there is an increasing focus on inactive or extinct seafloor massive sulfide (SMS) deposits driven by the possibility of marine mining, only few studies have been devoted to them so far. The Trans-Atlantic Geotraverse (TAG) hydrothermal field is probably one of the best-studied hydrothermal systems even if the relict SMS deposits known since the mid-1980s have not been thoroughly explored.Objectives: The main objective of this study was to describe the characteristics of these so-called inactive sites.Methods: During four different expeditions, we acquired high-resolution acoustic data and performed numerous human occupied vehicle (HOV) dive operations including extensive rock sampling and in-situ temperature measurements.Results and Discussion: We discovered thirteen new hydrothermal mounds including six large (i.e. > 5,000 m²) deposits making the TAG hydrothermal field one of the largest accumulation of hydrothermal materials (21.1 Mt) known on the seafloor. However, copper and zinc grades of the largest SMS deposits remain low (i.e. < 1.4 wt%) even compared to on-land volcanogenic massive sulfide deposits. Additionally, eight areas of diffuse hydrothermal fluid flow were identified challenging the presumed inactivity of these SMS deposits and, for the first time, emphasizing the importance of low temperature (LT) hydrothermal activity in whole the TAG field. Inactive and weakly active SMS deposits exhibit a large diversity of surface mineralization (e.g. sulfides, Fe-Mn mineralization, jasper) illustrating complexity of hydrothermal activities but also different ageing history. Several mounds no longer have visible sulfide chimneys and are covered by a widespread layer of manganese and iron oxyhydroxides attesting the longevity of diffuse fluid flow at specific locations even long after last high-temperature (HT) hydrothermal activity has ceased. This contrasts with SMS deposits that are devoid of extensive LT precipitates but characterized by standing or topped sulfide chimney indicating a relatively abrupt cessation of HT hydrothermal activity.Conclusion: Together these results allow us to propose evolution models to explain the diversity of active, weakly active and inactive SMS deposits in the TAG hydrothermal field.

Located in the South-West Pacific, at the northern extremity of the mostly submerged Zealandia continent, the New Caledonian Exclusive Economic Zone (EEZ) covers 1,470,000 km² and includes basins, ridges and seamounts where abundant ferromanganese crusts have been observed. Several investigations have been conducted since the 1970s on the nature and composition of ferromanganese crusts from New Caledonia’s seamounts and ridges, but none have covered the entire EEZ. We present data from 104 ferromanganese crusts collected in New Caledonia’s EEZ during twelve oceanographic cruises between 1974 and 2019. Samples were analysed for mineralogy, geochemical compositions, growth rates, and through a statistical approach using correlation coefficients and factor analysis. Crust thicknesses range from 1 mm to 115 mm, with growth rates between 0.45 mm/Ma and 102 mm/Ma. Based on textures, structures, discrimination plots, and growth rates, we distinguish a group of hydrogenetic crusts containing the highest mean contents of Co (0.42 wt%), Ni (0.31 wt%), and high contents of Mo, V, W, Pb, Zn, Nb, from a group of hydrothermal and/or diagenetic deposits showing high mean contents of Mn (38.17 wt%), Ba (0.56 wt%) and low contents of other trace metals. Several samples from this later group have exceptionally high content of Ni (0.7 wt%). The data shows that crusts from the southern part of the EEZ, notably seamounts of the Loyalty Ridge and the Lord Howe Rise, present high mineral potential for prospectivity owing to high contents of valuable metals, and constitute a great target for further investigation.

Since 1977 and the discovery of the first high temperature (HT) hydrothermal vent, more than 300 sites are known (about 600 including inferred ones). Among these hydrothermal sites, the talc-rich deposit is the most recent class of hydrothermal system discovered on the seafloor [1]. Only three talc-rich deposits have been described so far: (i) the active Von Damm Vent Field (VDVF), (ii) the inactive St Paul’s and (iii) Conrad fracture zones deposits [2]. These hydrothermal sites are associated with lower crustal rocks and/or serpentinized peridotites and might be widespread at slow or ultraslow spreading ridge. However, no clear spatial or temporal relationship of this new class of hydrothermal system and the “black smoker”-like system has been highlighted. During the HERMINE (March-April 2017) and HERMINE2 (July-August 2022) cruises [3], [4], two hydrothermal areas with talc-rich deposits have been discovered during Nautile HOV dives. The first one (23°N) is an inactive hydrothermal area located 28km northwest of the Snake Pit vent field (25km west of the axial rift). At least two deposits have been observed: (i) a talc-silica deposit and (ii) a fully oxidized SMS-type deposit characterized by copper concentrations up to 3.3wt.%. The second hydrothermal area (26°N) is composed of one large and weakly-active deposit composed of silica-sulfides rocks and at least two small talc-silica deposits. To our knowledge, this is the first time that such a spatial relationship has been described between these two classes of deposits. The preliminary results on these newly discovered hydrothermal field will be presented here. [1] Hodgkinson et al. (2015) Nat.. Commun 6:10150doi: 10.1038/ncomms10150 .[2] D’Orazio et al. (2004) Eur. J. Mineral. 16, 73-83[3] Fouquet and Pelleter (2017), https://doi.org/10.17600/17000200[4] Pelleter and Cathalot (2022),https://doi.org/10.17600/1800185

The Pompeii hydrothermal field was discovered in July 2022 during the HERMINE2 cruise [1]. It is located on an inside corner high (21°20 N) at the northern end of a “doomed” ridge segment [2] located just south of the TAMMAR propagating rift. The inside corner high is a domal bathymetric high with gentle slope ridgewards and spreading-parallel lineations (corrugations) characteristic of an oceanic core complex (OCC). The OCC is dissected by several faults including a ridge-perpedicular fault and a series of smaller ridge-parallel faults.The main Pompeii hydrothermal site is located on the corrugated surface atop a spreading-perpendicular rubble ridge. The mound is about 150 m in diameter and 30-40 m high and mainly composed of sulfide-bearing rocks partly covered by Fe-Mn hydrothermal crusts. Sulfide-bearing mineralization mainly consist of quartz and pyrite and are characterized by low copper and zinc concentrations (i.e.

With the world’s growing demand for metals, Seafloor Massive Sulfides (SMS) deposits are now seen as a possible mineral resource that could contributes to secure metal supply for human needs. Although inactive or extinct SMS deposits are likely to be the main target for potential exploitation, only few studies have been devoted to them so far. Relict SMS deposits in Trans-Atlantic Geotraverse (TAG) hydrothermal field are known since the mid-1980s but these so-called inactive sites have only been recently further investigated [1,2,3]. High-resolution acoustic surveys and extensive human occupied vehicle (HOV) dive operations performed during four different expeditions led to the discovery of thirteen new SMS deposits including six large (i.e > 5000 m2) deposits making the TAG hydrothermal field the largest accumulation of SMS deposits (i.e. 21.1 Mt) known on the seafloor. However, copper and zinc grades of the largest SMS deposits remain low (i.e. < 1.4 wt.%) even compared to on-land volcanogenic massive sulfides. Additionally, eight areas of diffuse hydrothermal fluid flow were identified challenging the presumed inactivity of these SMS deposits and, for the first time, emphasizing the importance of low temperature (LT) hydrothermal activity in the whole TAG field.Some authors [4] classified the TAG field as hydrothermally active. This recommendation is relevant, particularly now knowing that all hydrothermal zones in the TAG field exhibit temperature anomalies. Other inactive sulfide deposits might be discovered further east. However, considering knowledge gaps (e.g. patterns of hydrothermal circulation) and based on a precautionary approach, all to-be-discovered deposits belonging to TAG area should be grouped within a single area classified as active and thus protected from exploitation.References:[1]. Murton, B.J., et al., 2019. Geological fate of seafloor massive sulphides at the TAG hydrothermal field (Mid-Atlantic Ridge). Ore Geology Reviews 107, 903–925. https://doi.org/10.1016/j.oregeorev.2019.03.005[2]. Graber, S., et al., 2020. Structural Control, Evolution, and Accumulation Rates of Massive Sulfides in the TAG Hydrothermal Field. Geochemistry, Geophysics, Geosystems 21, e2020GC009185. https://doi.org/10.1029/2020GC009185[3]. Pelleter E., et al., 2024. Diversity, spatial distribution and evolution of inactive and weakly active hydrothermal deposits in the TAG hydrothermal field . Frontiers In Earth Science , 12, 1304993 (25p.) . Publisher's official version : https://doi.org/10.3389/feart.2024.1304993[4]. Jamieson, J.W., Gartman, A., 2020. Defining active, inactive, and extinct seafloor massive sulfide deposits. Marine Policy 117, 103926. https://doi.org/10.1016/j.marpol.2020.10392