Acoustic data were acquired during the Cambon (2014) (https://doi.org/10.17600/14000100) cruise. Regional acoustic surveys were acquired between 8 and 10 knots using the hull-mounted Reson SeaBat 7150 MBES. High-resolution (HR) seafloor mapping was performed using the ROV Victor-mounted Reson SeaBat 7125 MBES (400 kHz). The surveys were made at 0.3 knots at a near-constant altitude of 50 m above seafloor. Horizontal resolutions of 1 m and 0.5 m were obtained for bathymetric maps and acoustic backscatter maps, respectively. Post-processing of raw data files was carried out with SonarScope and GLOBE software (©Ifremer).

Thirteen dives with sampling and temperature measurements (HOV T-probe) using the HOV Nautile and ROV Victor were made during the Cambon (2014), Fouquet et al. (2017) (https://doi.org/10.17600/17000200), and Pelleter and Cathalot (2022) (https://doi.org/10.17600/18001851) cruises.

To avoid strong sampling bias (e.g., focusing on chimney samples), we favored systematic sampling across SMS deposits (Supplementary Figure S1). When achievable, we focused on massive blocks outcropping from the scarp fault. This strategy is thought to provide a representative view of surface and subsurface mineralization (first 5 m) and, if strongly oxidized samples are excluded, relatively good information of the SMS deposit grades in the upper zones.

Sixty-six sulfide-rich rocks and 55 Fe–Mn oxides and silica-rich mineralization were analyzed (Supplementary Tables S1, S2). Mineral identification was done using X-ray diffraction (XRD) analyses conducted with a Bruker AXS D8 Advance diffractometer. Sample powder was top loaded into circular cavity holders, and all analyses were run between 5° and 70°2θ, with 0.01°2θ step at 1s/step (Cu–Kα radiation, 40 kV, 30 mA). Mineral identification was done using DIFFRAC.SUITE EVA software, and semi-quantitative estimation was performed using TOPAS software.

Major and trace element analyses were conducted with a wavelength-dispersive X-ray fluorescence spectrometer (WD-XRF; Bruker AXS S8 TIGER) on fusion beads or compressed pellets, respectively. After data acquisition, measured net peak intensities corrected from inter-element effects were converted into concentrations using relevant calibration curves generated from the analysis of certified reference materials. For gold analyses, samples were digested in aqua regia. Au was adsorbed on a membrane filter, and WD-XRF analysis was performed directly on the filter (Etoubleau et al., 1996).

The geological limits of the SMS deposits are based on integration of dive observations, sample analyses, and bathymetry and backscatter data interpretation. Some outlines (and related volume calculation) must be taken with caution, particularly when no or low coverage of HR acoustic data was available and/or only limited length of the dive track passed through the SMS deposits.

Volume estimations were performed in ArcGIS software. Most of the volume calculations were done using a 2-m resolution digital terrain model (DTM). A lower-resolution DTM (i.e., 20 m) was used for some deposits that were not covered by HR acoustic data surveys. The first step is to use the outlines of SMS deposits for clipping the bathymetric data. In the second step, we generate a reference surface using interpolation methods. The reference surface will represent the expected topography before the formation of SMS deposits (Graber et al., 2020; Sanchez-Mora et al., 2022). Two interpolation methods were tested using the ArcGIS spatial analyst toolbox: (i) inverse distance weighted interpolation and (ii) “Topo to Raster” interpolation tool (see Supplementary Material for details). After interpolation is done, we calculate the volume between the reference surface and the initial DTM. Tonnage is calculated with an average density value of 3.5 t/m³ (see Supplementary Material for details).

According to the spatial distribution of hydrothermal deposits, type of mineralization, and morphology of the mounds, we propose to distinguish four hydrothermal zones (Figure 2): (1) MIR zone, (2) Nautile zone, (3) Alvin zone, and (4) shimmering zone. Note that the Alvin zone defined here corresponds to the Alvin relict zone described by Tivey et al. (1995) and to the southern part of the Alvin relict zone (also called North relict hydrothermal zone) defined by Rona et al. (1986); Rona et al. (1993a). Similarly, the Nautile hydrothermal zone hosts the Abyss mound (discovered by Graber et al., 2020) and most likely includes the northern part of the historical MIR zone defined by Rona et al. (1993a).

There are also two scattered deposits to the southwest (mound #11 and chimney cluster; Figure 2). Mound #11 is a deposit discovered 750 m northwest of the active mound by Graber et al. (2020) and is the most western deposit of the TAG district. Surface mineralization is mainly composed of the Mn±Fe±Si hydrothermal crust covered by pelagic sediment. The summit exhibits a relatively flat, hill-shaped morphology with no visible chimney structure. The small cluster (10–20 m length) of chimneys is located 1.2 km northeast of the active mound. Chimneys lie on pelagic sediments and are distributed along a NNE–SSW fault (axis-parallel faults) and close to an axis-oblique fault.

Volume and tonnage estimations are summarized in Table 1. The cumulative tonnage of the TAG hydrothermal field is 21.1 Mt for sulfides deposited onto the seafloor. The Alvin zone represents more than half of this tonnage with three mounds (i.e., Shinkai, Southern, and Double mounds) containing 10.2 Mt of sulfides altogether. The largest SMS deposits are the Southern mound (4.4 Mt), Shinkai mound (3.3 Mt), MIR mound (3.4 Mt), and Double mound (2.8 Mt). At the MIR zone, all but the MIR mound are small SMS deposits with a tonnage less than 0.12 Mt. The Shimmering zone contains four mounds with a tonnage over 0.38 Mt and two hydrothermal deposits with a tonnage approximately 1 Mt. The cumulative tonnage of the Shimmering zone is equivalent to the tonnage of the MIR zone. The Nautile zone has a cumulative tonnage for the five mounds of 1.8 Mt, with most of the tonnage contained in the Abyss deposit (ca. 1.3 Mt). The calculated tonnage for Menez Du and Menez Du #2 is approximately 0.2 Mt. However, calculated tonnages at the Nautile zone must be taken with caution since calculations were based on the lower-resolution digital terrain model (DTM), i.e., 20 m. Indeed, the volumes calculated on the basis of 2 m-resolution DTM are greater by a factor of 1.5–1.8 than the volumes calculated on the basis of 20 m-resolution DTM (Supplementary Table S3).

Extensive exploration of the TAG hydrothermal field led to the discovery of thirteen new hydrothermal deposits. Six newly discovered deposits have dimensions over 5,000 m² and can be seen as “significant SMS deposits” (Hannington et al., 2010). The total tonnage of approximately 21.1 Mt of the hydrothermal material deposited on the seafloor is similar, though slightly lower than recent estimation (22.1 Mt; Graber et al., 2020). The lower surface area we defined from extensive dive explorations explains most of the discrepancy between our tonnage estimations and those of Graber et al. (2020). However, we believe that the calculation method used to estimate the mound mass may also lead to an overestimation of tonnages. This is particularly true for MIR deposits, where the IDW interpolation generates an unlikely topography for the reference surface (Supplementary Figure S2), leading to an increase of more than 50% of the tonnage. Thus, caution must be paid when interpolating a reference surface, especially when deposits lie on a complex topography. At TAG, according to Murton et al. (2019) and Graber et al. (2020), total sulfide mass including subseafloor mineralization might be as high as 26 or 29 Mt. A similar tonnage of 27 Mt can be estimated from our results considering that subseafloor mineralization represents 30% of the surface tonnage (Hannington et al., 1998; Graber et al., 2020). These recent estimations indicate that the TAG hydrothermal field hosts one of the largest accumulations of sulfide materials known in the ocean and remains remarkable even in comparison with that in other large districts, such as Middle Valley (10–15 Mt; Hannington et al., 2011) or Semenov hydrothermal fields (9 Mt; Monecke et al., 2016; 13.95 Mt; Cherkashev et al., 2010; or 40 Mt; Cherkashev et al., 2013).

In the Shimmering zone and Nautile zone, surface mineralization mainly comprises LT hydrothermal precipitates (e.g., Mn–Fe–Si ± Cu crusts), metalliferous sediments, Si–Fe mineralization (e.g., jasper and silicified sulfide breccia), and rare fully weathered sulfide breccias. The average concentrations of economic metals (e.g., Cu, Zn, and Au) are low in many, but not all, samples, indicating that the shallow part of the deposits can be seen as nearly barren. No sulfide-rich rocks were sampled in the Nautile zone, and only two pyrite-rich massive sulfides were recovered at the Shimmering mound, making any interpretation of the economic potential of these deposits not possible. Metalliferous sediments with variable thickness may cover the massive sulfides in the Alvin zone, and a silica cap was identified at the summit of Southern and Rona mounds from drilling operations (Lehrmann et al., 2018; Murton et al., 2019). However, sulfide-rich rocks (i.e., massive to semi-massive sulfides and sulfide chimneys) dominate surface and subsurface mineralization sampled in the Alvin zone. Massive sulfides are mainly composed of pyrite and marcasite with only less amounts of sphalerite and chalcopyrite (Figure 6), and the average copper and zinc concentration is remarkably low (Figure 7B). It is particularly true for Southern and Shinkai mounds where the average copper and zinc contents (Cu + Zn < 1.4 wt%) are lower than that calculated for the TAG active mound (Cu + Zn: 3.2 wt%; Hannington et al., 1998). Such low Zn coupled with rather low Cu is unusual for surface and subsurface massive sulfides as the refining process is thought to trigger Zn remobilization (and to a lesser extent Cu) and its subsequent enrichment in the upper part of an SMS deposit (Hannington et al., 1998; Lehrmann et al., 2018). The presence of Co ± Cu ± Se enrichment (i.e., HT elements; Fouquet et al., 2010) in some sulfides might indicate a preferential sampling of the deepest Cu-rich and Zn-poor parts of the mound (>5 m). However, many samples record relatively high concentrations in Zn-associated LT elements (i.e., Ag: mean approximately 150 ppm; Pb: up to 543 ppm) supporting sampling of the upper 5 m of the sulfide mounds (Hannington et al., 1998). Moreover, drilling results obtained by Murton et al. (2019) for the Southern mound show that Zn concentration is systematically lower than 0.3 wt%. Although the reasons for such zinc depletion (e.g., “over-refining” processes and supergene remobilization of Zn; Lehrmann et al., 2018) are beyond the scope of this study, our results indicate that the Zn (±Cu)-poor upper orebody is distinctive of large SMS deposits hosted in the Alvin zone. On the contrary, smaller mounds such as the Rona mound and Mont de reliques (immature SMS deposits) seem to have higher Zn ± Cu grades (Murton et al., 2019). This questions the economic interest of large mafic-hosted SMS deposits, especially when compared to volcanogenic massive sulfides characterized by mean Cu + Zn greater than 2.66 wt% (Franklin et al., 2005). The gold content is generally low (<1 ppm), except for the strongly silicified sulfides that may enclose secondary gold related to reworking processes (Firstova et al., 2019).

The surface and subsurface mineralization in the MIR zone is composite in nature, divided into 1) Fe ± Mn ± Si mineralization (e.g., east of MIR, MIR #2, #3, #6, and #7) and 2) sulfide chimneys and massive sulfides (e.g., West of MIR, MIR #4, and #5). Several LT Mn-rich crusts exhibit high copper and zinc concentrations that are atypical for this type of hydrothermal mineralization (see Section 5.4). Sulfide mineralization processes in the MIR zone have distinct mineralogical and chemical signatures compared to those collected in other TAG hydrothermal zones. Significant amounts of chalcopyrite and to a lesser extent sphalerite were identified (Figure 6), and high copper (±zinc) concentrations are recorded (Figure 7). Our observations confirm that the MIR deposit is actually a large area of coalesced mounds (Rona et al., 1993a; 1993b) with limited thickness (<30 m). These mounds are most likely immature compared to large SMS deposits of the Alvin zone (i.e., Southern mound, Double mound, and Shinkai mound), where HT hydrothermal fluid flow appears more focused through time. This may explain why copper in the upper orebody at MIR and MIR #3 is highly (6–9-fold) enriched compared to Southern and Shinkai upper orebodies. Though our sampling at the MIR is principally restricted to the west area (subzones A and B) and mostly corresponds to chimneys, drilling conducted on the western border of subzone C confirms the presence of a Cu-rich zone at 4 mbsf (Murton et al., 2019). Gold concentrations over 1 ppm are recorded for more than half of MIR zone samples (Zn-rich chimneys, MIR; silicified sulfides, MIR #3). With all the uncertainties associated with these results, it seems that the MIR hydrothermal zone has the highest SMS resource potential in the TAG field.

More than two-thirds (20/28) of hydrothermal deposits belong to the contiguous MIR, Nautile, and Alvin hydrothermal zones and span over a narrow (ca. 500 m) curved surface extending 2.5 km from south to north (Figure 2). In the south, i. e., MIR and Nautile zones, the distribution of hydrothermal mounds shows a general N–S trend, whereas SMS deposits in the Alvin zone are roughly aligned along a NW–SE direction. The general distribution of the deposits appears controlled by two major structures: 1) the N–S west-facing fault in the south and 2) the NW–SE trough interpreted by Graber et al. (2020) as an old transfer fault connecting the southern and the northern domains. Considering the distribution and density of hydrothermal deposits, these two major structures have probably maintained a high permeability in a restricted zone through time. At the MIR mound, local modification of permeability and subsequent migration of vent activity to the west is expected according to the composite nature of mineralization from the east (LT crusts) to the west (standing sulfide chimneys) (see also Section 5.5).

The Shimmering zone is formed by a dense cluster of five mounds apparently spatially disconnected from the other contiguous hydrothermal zones located to the south (Figure 2). Deposits are within the “smooth northern block” (Graber et al., 2020) in an area where oblique and axis-perpendicular faults intersect axis-parallel faults. Moreover, a major N–S-trending, west-facing fault scarp bounds the western part of the hydrothermal zone and is interpreted as a detachment surface related to the initiation of a new detachment fault (Szitkar et al., 2019). A consequence would be that the Shimmering zone is structurally distinct from the southern hydrothermal zones (Szitkar et al., 2019; Graber et al., 2020). The “chaotic zone” between the Alvin and Shimmering zones remains poorly explored, so we cannot rule out the presence of small (<< 5,000 m²) inactive or weakly active deposits. However, with exception to the Cyana mound, no large mounds are inferred from Graber et al. (2020) HR bathymetric maps in this “chaotic zone.” This apparent absence of large SMS and thus long-duration HT hydrothermal activity between the Alvin zone and Shimmering zone should be investigated in future work to ascertain if the shimmering zone and the southern hydrothermal zones are connected or not, i.e., if hydrothermal fluids are linked to a single convection cell or different ones (Humphris et al., 2015).

The sparse distribution of the three last deposits (TAG active mound, mound #11, and chimney cluster; Figure 2) compared to that of the other hydrothermal zones, as well as the diverse size, activity, morphology, and mineralization type, is intriguing. Additional exploration of the hydrothermal mounds hosted in the “extensional area” defined by Graber et al. (2020) is necessary to understand the tectonic controls and/or hydrothermal history in this area.

The extensive dive observations and measurements performed so far have provided the most comprehensive present-day distribution of hydrothermal activity in the TAG field. A significant result is the presence of low-temperature diffuse flow (<30°C) in nearly all of the investigated hydrothermal zones. Beyond questioning the inactive nature of some old hydrothermal deposits of the TAG hydrothermal field (section 5.6), this brings insights on the current upflow zones. The maximum temperatures measured vary in range, from slightly over the seawater temperature (3°C; Rona mound) to up to 29°C (east of the MIR). The contiguous MIR, Nautile, and Alvin hydrothermal zones show a gradual decrease in maximum temperature from south to north (Figure 2). Furthermore, morphologies of currently forming LT precipitates vary from 7-m-tall hills at the MIR mound to scarce centimeter-scale orange patches at the Rona mound supporting the progressive weakening of the LT activity toward the Alvin zone. Further north, the shimmering zone hosts three areas with significant LT hydrothermal activities (with the maximum temperature ranging from 14°C to 26°C). Presently forming LT precipitates are distributed along cracks in Mn–Fe–Si crusts and span over a surface of tens of meters. At Shimmering zones #2 and #4, the distribution of diffuse fluids and precipitates as well as the intensity of venting are similar to those of Menez Du #2. The relatively intense LT activity (up to 26°C) recorded in the Shimmering zone contrasts with the inactive or very weakly active nature of the Alvin hydrothermal zone yet closest to the HT active mound and MIR and Nautile diffusive zones. If the spatial distribution of venting is representative of the current upflow zones in the subsurface, a lack of visible diffuse venting at the Alvin zone (contrary to all other hydrothermal zones) may provide constraints on subsurface circulation patterns in the hanging wall. Considering a single convection model (i.e., fluids from the same upwelling flow supply all hydrothermal zones), inactivity or very weak activity of the Alvin zone deposits could be seen as reflecting a strong decrease of permeability in the hanging wall below this area. According to the mass of sulfides deposited in the Alvin zone (i.e., half of the total tonnage of the TAG field) and occurrence of standing chimneys, evidence of long-lived upflow zones in the area, the decrease of permeability would have to be recent (<5,000 years). LT hydrothermal activity recorded at the MIR, Nautile, and Shimmering zones (2 km WNW and 4 km NNE of Active mound) might be linked to the presence of major N–S-trending faults that would act as preferential pathways for fluids and thus sustain diffuse venting. However, the apparent spatial disconnection of the Shimmering zone and the absence of visible diffuse venting at the Alvin zone might also be the expression of different circulation cells (Humphris et al., 2015). Additional exploration in the vicinity of the Shimmering zone as well as the east of the MIR (faulted upper terrain described by Graber et al., 2020), i.e., toward the LTZ, together with investigation of fluid geochemistry, is crucial to decipher between a single or multiple convection cell model.

Several but not all SMS deposits lacking upright chimneys are sealed by widespread Mn–Fe–Si hydrothermal crusts attesting to the longevity of diffuse flow at specific locations even long after ceasing of the last HT hydrothermal activity. The LT hydrothermal crust covers all deposits in the Shimmering zone and Nautile zone. Only Cyana and mound #11 deposits in the Alvin and southwest hydrothermal zones, respectively, display widespread LT hydrothermal crusts at the surface. In the MIR zone, the LT hydrothermal crust entirely covers MIR #2, #3, and the eastern part of the MIR. Hydrothermal mounds covered by LT hydrothermal crusts are characterized by a relatively flat top morphology with rare visible fault scarps indicating that low-temperature activity still occurs after the dissection and collapse of the sulfide mounds. A significant renewal of HT activity after sustained diffuse flow was only seen at the MIR mound (between subzones A/B and subzone C) where small sulfide chimneys developed on Mn–Fe–Si crusts. In the Alvin zone, a decrease in the hydrothermal activity is evident from major silicification of sulfide-rich rocks at the Shinkai mound and the presence of jasper formation at the summit of the Southern mound and Rona mound (Murton et al., 2019). However, large SMS deposits in the Alvin zone are mostly devoid of widespread LT crusts, suggesting no sustained LT diffuse venting in the area since the last HT hydrothermal activity. The spatial distribution of LT hydrothermal crusts is intriguing and points to longevity of the LT discharge zones in the Shimmering, Nautile, and MIR zones (confirmed by current LT activity). On the contrary, SMS deposits of the Alvin zone located in the heavily tectonized area seem affected by radical changes of upflow zones after HT activity ceased.

LT hydrothermal crusts exhibit a wide variety of Mn, Fe, and Si concentrations and a large range of Fe/Mn that support a hydrothermal origin and different precipitation temperatures. Very high copper and zinc concentrations (up to 10 wt%) recorded make these crusts atypical for low-temperature precipitates and could give misleading information on the origin of this mineralization (Figure 8A), apart from using recent discrimination diagrams based on immobile elements (e.g., Zr, Y, and Ce; Figure 8B; Josso et al., 2017). In the TAG hydrothermal field, five deposits (MIR #3, Menez Du #2 and #3, Cyana, and Shimmering #2) host a hydrothermal crust with copper and zinc concentrations higher than 1 wt%. Mn oxyhydroxides are thought to be the main Cu–Zn-bearing phase since the highest copper and zinc contents (up to 10 wt% Cu and 1 wt% Zn) are recorded for Mn-rich crusts. Metal enrichments (e.g., Ni, Co, Cu, and Zn) in hydrothermal Mn ± Fe ± Si mineralization have already been reported in previous studies (e.g., Hein et al., 2008; Hein et al., 1996; Hein et al., 1992; Hein et al., 1990; Conly et al., 2011; González et al., 2016; Pelleter et al., 2017). Several hypotheses may account for the metal enrichment of hydrothermal Mn mineralization, such as the leaching of magmatic or ultramafic rocks, sulfide mineralization, or biogenic sediments (Hein et al., 2008). Sulfides were sampled under the Mn–Fe–Si crusts at MIR #3 and Cyana mounds and thus are most likely the source of the Cu and Zn recorded in Mn-rich hydrothermal precipitates. Sustained LT hydrothermal activity attested by the presence of widespread Mn–Fe–Si crusts could thus fundamentally alter the pristine surface and subsurface sulfide mineralization and release metals. Though Mn oxyhydroxides appear as potential sinks for copper and to a lesser extent zinc, the ultimate fate of metals during LT hydrothermal activity needs investigations of diffuse fluid and associated LT precipitate chemistry.

Several geological parameters can help define the relative age/chronology of the hydrothermal processes and age(s) of deposits (e.g., size, morphology, surface, and subsurface geology, including faults).

A first-order criterion is the presence of standing or toppled sulfide chimneys (Figure 4A, C, K) for identifying deposits characterized by a recent cessation of HT activity. Standing and/or toppled chimneys are present in the Alvin zone (Shinkai, New Mound #2 and #3, and Mont de reliques), MIR zone (MIR subzones A and B), and the southwest hydrothermal zone (chimney cluster). No relict chimney was observed at the top of large mounds in the Shimmering zone nor Nautile zone, attesting to long-ceased HT activity. At the MIR, the dating of several standing and toppled chimneys provided ages spanning between 3,000 and 500 years (Lalou et al., 1995). Based on these results, we assume that last HT activity at Shinkai, New mounds, Mont de reliques, and chimney cluster most likely belongs to this period of time (3,000 to approximately 500 years). The TAG active mound is thought to be inactive during this period of time until the recent reactivation of HT activity 60 years ago (Lalou et al., 1995). Therefore, we assume that last HT activity (3,000–500 years) was focused in the western part of the Alvin zone and in a restricted area west of the MIR. Chimney clusters discovered between the Alvin zone and active mound indicate a very short-lived hydrothermal activity (transitional?) in the extensional area.

Other criteria that can be used for deciphering a relative chronology of hydrothermal processes include indirect geological factors such as morphology, pelagic sediment cover, weathering of sulfides, or tectonic dissection (e.g., Murton et al., 2019). The size of the deposit may help localize areas of long-duration HT activity. In the TAG hydrothermal field, thirteen deposits consist of very large deposits (>10,000 m²), with sulfide accumulation of over 1 Mt and up to 4.4 Mt. The Shimmering zone and Alvin zone have four very large SMS deposits. The Nautile zone hosts three large deposits, and the last two are the MIR mound and Active mound. Dating studies have shown that the active mound has accumulated 2 Mt of sulfides over 50 kyrs, including several episodes of activity (2%–20% of this time) and inactivity (Lalou et al., 1995; Humphris and Cann, 2000). Shinkai, Southern, and Double mounds exhibit circular, conical, to sub-conical shapes suggesting a sustained (yet probably episodic), focused upflow of HT fluids in the Alvin zone. Relatively steep slopes and surface mineralization dominated by sulfides (only oxidized in outer rims) imply that these three SMS deposits are not significantly older than the Active mound. It is in agreement with ages of approximately 50 kyrs, obtained on sulfide samples attributed to the Double mound (Lalou et al., 1995). Based on the morphology, sediment cover, and absence of tectonic dissection, several authors proposed that the age of the Shinkai mound might be less than 40 kyrs (Murton et al., 2019; Graber et al., 2020). Actually, these geological features mostly suggest that the Shinkai mound has experienced a sustained HT hydrothermal episode more recently than Southern and Double mounds, which is different than being intrinsically younger, and a way to illustrate this is to consider the Active mound as an analog. The Active mound is not dissected by axis-oblique or axis-parallel faults, and it has relatively steep mean slopes (28.7°; Table 1) and a thin pelagic sediment cover. However, the maximum age recorded, i.e., the potential first HT hydrothermal activity, is around 50 kyrs (Lalou et al., 1993). Considering the large sulfide volume and assuming episodic HT hydrothermal episodes, the Shinkai mound may be as old as the Active, Double, and Southern mounds. New geochronological studies are needed to determine precisely the maximum age for the Shinkai mound and the age of the last HT event at Southern and Double mounds (i.e., before the spreading-parallel extensional faulting). The morphology and surface geology of the Cyana mound are unique in the Alvin hydrothermal zone with a gentle slope, flat surface, and a widespread layer of the hydrothermal manganese crust. Contrary to the observation by Graber et al. (2020), Cyana is affected by axis-parallel faults and should not be considered certainly younger than the Double mound. Actually, the widespread Mn crust and lack of relict chimneys indicate that last HT activity at the Cyana mound is probably as old as, or older, than that at the Double mound. Interestingly, Lalou et al. (1995) reported one age of 74 kyrs for the Mn crust in the Alvin zone. Though the precise location of the dated sample is unknown, several lines of evidence support an old HT event (>74 kyrs) in the Alvin zone, which might be attributed to the formation of the Cyana mound.

Gentle slopes and relatively flat morphologies sometimes barely visible on the HR bathymetric map characterize hydrothermal mounds in the Nautile and Shimmering zones. Together with the locally thick sediment cover and presence of oxide-dominated boulders, this supports a very long-ceased HT activity and relatively increased ages for sulfide mineralization (i.e., > 50 kyrs). No absolute ages are presently ascribed for these deposits. However, re-examination of data from the historical MIR zone described by Rona et al. (1993a); Rona et al. (1993b) in regards with new observations suggests that samples collected for dating during Alvin 2188 and 2195 dives do not belong to the MIR mound. This is in agreement with the observation of recent studies highlighting that the MIR mound appears smaller than previously thought (Murton et al., 2019; Graber et al., 2020). Alvin 2188 and 2195 dives most likely occurred several hundreds of meters north to the actual MIR mound, i.e., a zone between MIR #3 and Menez Du #2. High conductive heat flows measured during Alvin 2188 and 2195 dives at 3463, 3483, and 3484 mbsl (Rona et al., 1993b) are also in good accordance with diffuse venting discovered in the southern part of the Nautile zone (up to 19°C measured at Menez Du #2). Two samples collected in this area (i.e., Alvin 2188 3-1 and Alvin 2195 1-1) provided radiometric ages of 102 and 50 kyrs, respectively (Lalou et al., 1995). If our assumptions on localization of historical sample stations and interpretations of geological features are correct, hydrothermal mounds located in the Nautile zone and/or the northern edge of the MIR zone are the oldest deposits in the TAG hydrothermal field. Shimmering zone deposits share common geological features with Nautile zone deposits and the Cyana mound (morphologies, surface mineralization, and sediments cover). Though radiometric dates are needed, deposits hosted in the Shimmering zone appear older than those in Alvin zone mounds (with exception of Cyana) and were probably first active before 74 kyrs (i.e., higher age reported for LT activity in the Alvin zone).

The MIR mound is a complex mineralized area with relatively thin and immature sites with standing and toppled chimneys to the west (subzones A and B) and larger site(s) with the LT Mn–Fe–Si crust to the east (subzone C). Samples collected in the western part (subzones A and B) during dive MIR 3.76 are no older than 20 kyrs (Lalou et al., 1995), attesting to the relatively recent hydrothermal activity to the west. Though no age is available for the eastern part of the mound, we assume higher ages (>20 kyrs) based on geological characteristics (size, morphology, and surface mineralization). Additional sampling of the eastern zone and geochronological studies would help deciphering the polyphase history of the MIR mound.

Within the limitation of indirect geological characteristics and previous dating studies, the HT hydrothermal history of the TAG hydrothermal field may be summarized as follows:

1) The HT hydrothermal activity that began at least 105–75 kyrs ago was mainly focused in the Nautile zone and most likely in the Shimmering zone. Some SMS mounds might be synchronous in the north of the MIR zone (e.g., MIR #3) and in the Alvin zone (i.e., Cyana).

Possible older HT hydrothermal events (>125 kyrs) may have occurred in the TAG hydrothermal field. Our observations indicate that SMS deposits can be concealed by LT hydrothermal crusts and pelagic sediments. Hence, this may lead to misinterpretation on the nature of the deposit (i.e., SMS deposit versus LT deposit only) without extensive exploration and sampling operations. The LTZ localized on the upper eastern flank of the TAG massif (Figure 1) hosts several Mn–Fe deposits (Rona et al., 1975; Rona et al., 1984; Thompson et al., 1985). Even though Mn–Fe–Si crusts in the LTZ (see Plate I of Thomson et al., 1985) are very similar to those covering sulfide mineralization in the HTZ (Figures 4F, M, N) including copper enrichments (Mills et al., 2001), no sulfide mineralization is presently known. Yet, over the last decades, studies have shown that black smoker-type deposits can develop several kilometers off-axis in association with detachment faults (Petersen et al., 2009; Fouquet et al., 2010). The analogy between the Mn–Fe–Si crusts studied here and those described by Thompson et al. (1985) in the LTZ encourages further exploration to the east to find potential old SMS deposits and thus constrain the spatial and temporal extent of the TAG HT hydrothermal field.

The first step of hydrothermal activity led to the formation of chimney clusters and, if HT activity persists, to small SMS mounds (Figure 9A). For the Active mound, formation of the deposit is complex and is linked to cycles of anhydrite precipitation and dissolution, which is responsible for flat tops and platform-like morphology (Humphris and Kleinrock, 1996; Petersen et al., 2000). Anhydrite dissolution and subsequent collapse of the structure may explain the formation of the circular depression observed at Mont de reliques and New mounds #2 (Figure 9A). Nevertheless, the morphology of the Active mound characterized by stacked flat platforms is rather uncommon inside the TAG hydrothermal field. Only Shimmering #2 and Abyss mounds exhibit a similar morphology (i.e., platform(s) shape with a flat top or depression) that may be related to massive anhydrite dissolution and related collapse of the mound. Conversely, large SMS deposits hosted in the Alvin zone display rather conical to sub-conical shapes (e.g., Shinkai, Double, and Southern mounds; Supplementary Figure S3) without significant depression or large flat-top excluding massive anhydrite dissolution. Even though anhydrite dissolution cannot be ruled out from the presence of brecciated sulfides (Murton et al., 2019), the collapse of these mounds is most likely controlled by tectonic dissection and/or mass wasting. Additionally, high silica recorded in massive sulfides at Shinkai may stabilize sulfide structures (Delaney et al., 1992) and, together with filling of some collapse structures by the last ceased HT activity, may explain the striking conical shape of this mound (Figure 9B). A silica cap at depth has also been reported near the summit of the Southern mound and Rona mound and is attributed to a waning phase occurring at the closing stage of the hydrothermal activity (Murton et al., 2019) (Figure 9B).

A second major step of hydrothermal activity in the TAG hydrothermal field is the development of a long-lived, possibly episodic, LT hydrothermal activity. Widespread Mn ± Fe ± Si crusts cover mounds with gentle slopes and relatively flat morphologies with no more standing sulfide chimneys (Figure 9C; Supplementary Figure S3). Together with the presence of active LT venting in the Shimmering zone, Nautile zone, and MIR zone, these observations indicate that LT activity is a major event in the TAG hydrothermal field. This is in good agreement with the large time span (i.e., 140–4 kyrs) recorded for Mn crusts (Lalou et al., 1986; Lalou et al., 1995). LT hydrothermal events are most likely concomitant with oxidation of sulfide minerals in seawater (e.g., Abyss mound), eventually leading to oxide-dominant surface and subsurface mineralization. Oxidation of pristine hydrothermal sulfides by LT hydrothermal fluids and/or seawater may release metals (e.g., Cu, Zn; Dekov et al., 2011) that would either be trapped into secondary minerals (e.g., Fe oxyhydroxides, Mn oxyhydroxydes, and atacamite) or discharged into seawater. Sulfide oxidation and related leaching of metals should be more efficient during the LT hydrothermal activity than at seawater temperature (e.g., Chandra and Gerson, 2010; Fallon et al., 2017). Then, if the long-lived LT activity identified in the TAG hydrothermal field reveals to be a common process in other hydrothermal fields hosting old SMS deposits, this process should be considered in future studies on biogeochemical cycles of metals in the deep ocean. In addition, such sustained LT activity (10°C–30°C) in a sulfide-rich environment (i.e., SMS) may support diverse mesophilic microbial populations for long durations (Zeng et al., 2021).

With increasing interest in polymetallic sulfides, part of the scientific community working on hydrothermal systems calls for the protection of active hydrothermal sites (Van Dover et al., 2018). As a consequence, potential mining activity would be focused on so-called inactive/extinct sites, though only few studies have been carried out on these deposits. Some authors recently advocated for research efforts to propose a classification of inactive and extinct deposits/vent fields that could help for mining regulations (Jamieson and Gartman, 2020; Van Dover et al., 2020). To our knowledge, the International Seabed Authority (ISA) has only proposed a definition for inactivity and extinction at vent-scale with distinction based on potential hydrothermal rejuvenation from mining activity (ISBA/25/LTC/6 Rev.2, 2022). Jamieson and Gartman (2020) argued that the term “extinct” should be only used for deposits hosted in an inactive vent field for which the probability of hydrothermal rejuvenation is low (but not certain). Considering that TAG hydrothermal mounds are all linked to the same heat source and a common detachment fault system that controls the hydrothermal circulation, Jamieson and Gartman (2020) 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. However, defining the extent of a single hydrothermal vent field (i.e., cluster(s) of deposits related to a common heat source and/or subseafloor permeability) may be challenging. That is true for TAG, where the location of the heat source and by extension the pattern of hydrothermal circulation is now disputed (e.g., McCaig et al., 2010; Guo et al., 2023). Moreover, the question of geological continuity may arise if inactive sulfide deposits are discovered toward the LTZ at depths shallower than the detachment fault termination (approximately 3,300 m, Graber et al., 2020). As a consequence, considering knowledge gaps and based on a precautionary approach, all known and to-be-discovered deposits belonging to the TAG area (including HTZ and LTZ, i.e., approximately 45 km²) should be grouped within a single field or district classified as hydrothermally active.

For an inactive field, Jamieson and Gartman (2020) proposed a set of indicators such as the absence of an upright chimney, relatively flat mound, lack of fluid venting, and absence of a vent biological community that may be important to distinguish extinct sites from inactive sites. These criteria are most likely diagnostic for prolonged HT hydrothermal inactivity. The discovery of a long-lived low-temperature hydrothermal activity in the TAG area indicates that deposits with flat morphologies, no more standing chimneys, and a thick Fe–Mn hydrothermal crust may still host hydrothermal activity. Since the episodicity of low-temperature activity is not known, documentation and dating of LT precipitates should be considered to improve the definition of inactivity and extinction. New data arising from active and inactive hydrothermal field exploration programs are essential to fill gaps in scientific knowledge regarding the geology and ecology of inactive/extinct SMS deposits.