Insights into Extinct Seafloor Massive Sulfide Mounds at the TAG, Mid-Atlantic Ridge

Over the last decade there has been an increasing interest in deep-sea mineral resources that may contribute to future raw metal supply. However, before seafloor massive sulfides (SMS) can be considered as a resource, alteration and weathering processes that may affect their metal tenor have to be fully understood. This knowledge cannot be obtained by assessing the surface exposures alone. Seafloor drilling is required to gain information about the third dimension. In 2016, three extinct seafloor massive sulfide mounds, located in the Trans-Atlantic Geotraverse (TAG) hydrothermal area of the Mid-Atlantic Ridge were drilled. A mineralogical and textural comparison of drill core and surface-grab samples revealed that in recent ceased mounds high-temperature copper assemblages typical for black smoker chimneys are still present whereas in longer extinct mounds the mineralogy is pre-dominated by an iron mineral assemblage. Zinc becomes remobilized early in the mound evolution and forms either a layer in the upper part of the mound or has been totally leached from its interior. Precipitation temperatures of sphalerite calculated using the Fe/Zn ratio can help to identify these remobilization processes. While the Fe/Zn ratios of primary sphalerites yield temperatures that are in very good agreement with fluid temperatures measured in white smokers, calculated temperatures for sphalerites affected by remobilization are too high for SMS. Overall drilling of SMS provides valuable information on the internal structure and mineralogy of the shallow sub-surface, however, additional drilling of SMS, at a greater depth, is required to fully understand the processes affecting SMS and their economic potential.


Introduction
Since the discovery of the first hydrothermal venting at the Galápagos Rift in 1977 [1], the number of discovered vent sites globally has been steadily increasing with more than 600 sites now known in the oceans [2]. The formation of modern black smoker deposits is related to processes that have occurred on Earth since Archean times [3] and are well understood [4]. However, it remains unknown how seafloor massive sulfides (SMS) are affected by alteration and weathering processes once hydrothermal activity ceases, and what impact this has on their economic potential, that is, whether metal tenors become enriched, depleted, or entirely lost.
Current studies, using bulk geochemical data from 95 sites published in the literature [5,6], suggest a global resource potential for modern SMS deposits along the neovolcanic zones of the seafloor of 600 million tons, with a median grade of 3 wt % Cu, 9 wt % Zn, 2 g/t Au, and 100 g/t Ag [5,6]. However, the geochemical data being used largely originated from easily recoverable, mainly  The active TAG Mound, located in the southwest of the area, comprises two distinct zones. These are the so-called "upper platform" in the northwestern part of the mound and "lower platform", also referred to as Kremlin area in the southeastern part of the TAG mound. Multiple chimneys discharge metal-rich black smoke at a temperature of 365 • C [17] on the upper platform. On the lower platform, there is low temperature diffuse flow, with zinc-rich chimneys and current fluid temperatures of about 50 • C that have decreased from about 270 • C since the 1990s [17]. Mineralization typically encountered here comprises chalcopyrite, pyrite, and anhydrite-rich massive sulfides on the upper platform, and sphalerite-marcasite-pyrite-silica-rich massive sulfides on the lower platform [18]. Sub-surface massive pyrite breccia occurs in the upper part of the mound that gives way at depth to anhydrite-rich pyrite breccia [7]. Deeper in the mound, the anhydrite has been replaced by quartz. The deepest known parts consist of altered wallrock and basalt with the deepest hole recovering altered basalt at 125 m below seafloor (mbsf). At the active TAG Mound, minerals of economic interest only seem to be enriched in the upper five metres [13] whereas at depth a barren massive pyrite body occurs. Similar observations are made for the upper part of volcanic hosted massive sulfide deposits on Cyprus [19]. In addition to the active TAG mound, there are several hydrothermally extinct mounds to the north-northeast: for example, Shinkai Mound, Double Mound, Rona Mound, New Mound #2, New Mound #3 (New M. #2 & #3), Southern Mound, as well as the MIR Zone located two kilometers to east-northeast of the active TAG mound ( Figure 1).
Radiometric data indicate that hydrothermal activity started at the active TAG mound 50 ka ago with episodic periods of activity of tens to hundreds of years [20] characterized by accumulation of sulfides and anhydrite, replacement and refining, and hydrothermally inactive periods of 3000 to 5000 years that are characterized by mass wasting, dissolution of anhydrite, and brecciation. Currently, the active TAG Mound is in a high-temperature hydrothermal phase with activity having started 80 years ago [21]. The MIR Zone is the oldest area, with hydrothermal activity first starting 140 ka ago and the last active episode ceasing 0.6 ka ago [20] whereas sulfides from Double and Shinkai Mound yield ages of 50 ka and 2-23 ka, respectively [20]. No radiometric data are available for Rona Mound, Southern Mound, New M. #2 & #3. However, based on the proximity of New M. #2 & #3 to Shinkai Mound and Rona Mound and Southern Mound to Double Mound, and similar slope angles [22], that indicated the degree of anhydrite dissolution at depth [13], similar development stages are assumed for this study.
In 2016, detailed studies of inactive hydrothermal activity and associated extinct seafloor massive sulfides (eSMS) of the TAG area were conducted. A robotic, lander-type seafloor drilling rig (RD 2) from the British Geological Survey was deployed from the British research vessel RRS James Cook during cruise JC138. In this paper we compare the nature and mineralogy of surface and sub-surface ore types from a modern, active SMS system at the TAG site provided by ODP (Ocean Drilling Program) drilling [7] with extinct SMS investigated during "Blue Mining" cruise JC138. The aim is to gain information on how hydrothermal alteration and seafloor weathering change the composition of extinct SMS over time and what implications this may have on the economic potential. The results of this surface sampling and drilling campaign add significantly to our understanding of the nature of the shallow sub-seafloor of eSMS and provide detailed information on composition and mineralogy of eSMS that contrasts with the composition of active chimneys.

Materials and Methods
During RRS James Cook cruise JC138 visual seafloor investigations and 29 surface samples were obtained at Rona Mound, Southern Mound, Shinkai Mound, New M. #2 & #3 using a multipurpose robotic underwater vehicle (HyBIS) from the National Oceanography Centre. In addition, eight holes were drilled on the plateaus of three extinct mounds (Southern Mound, Rona Mound, and MIR Zone) using RD2 (Figure 2). A total of 9.5 m of core was recovered and a maximum penetration of up to 12.5 m below seafloor was achieved. The true core depth was reconstructed from the drill telemetry data recorded. After recovery, samples were handled following the protocol established for cruise JC138 [22]. Where possible, sample orientation (i.e., top and bottom) were marked on the sample, and respected during any further steps. The exteriors of all samples were then petrographically described before being cut using a rock saw. Samples were further texturally and mineralogically described and subsamples were subsequently taken for further studies onshore. All samples were dried and individually packed together with oxygen and silica-gel moisture absorbers in evacuated lay-flat polythene tubes and stored at 4 °C in a reefer container to prevent oxidation. Based on surface location, sub-surface depth, textural appearance, and macroscopically identifiable mineralogy, a total of 73 polished thin and thick Sections were prepared and studied under transmitted and reflected light using a MEIJI MX microscope with the attached Infinity 1 camera system. The relative mineralogical compositions were estimated, at a micro-scale, for each sample and an average composition derived for the mounds interior and exterior.
Major element analyses of sulfide minerals were performed on a Cameca-SX100 5-spectrometer microprobe (EMPA, University of Edinburgh) and a Zeiss Sigma HD Field Emission Gun Analytical scanning electron microscope (SEM, Cardiff University). Sulfides were analyzed using 20 kV accelerating voltage, 10 nA beam current (S, Zn), and 100 nA (Fe) and a 1 μm beam diameter. For SEM analyses, the Oxford Instruments 150 mm 2 energy dispersive X-ray spectrometer was used with an acceleration potential of 15 kV. Calibration of EMPA and SEM was performed on natural minerals (pyrite: Fe, S; sphalerite: Zn) on a daily basis. The SEM data are normalized to 100%, automatically, and a direct comparison with EMPA data from the same sample demonstrated that quantitative data from this specific SEM are of comparable quality to that obtained by EMPA. By comparison, using an older generation of SEM (FEI XL30 Field Emission Gun Environmental SEM, Zeiss, Jena, Germany, Cardiff University), elemental analyses for sphalerite provided only semi-quantitative results and were not used in the geothermometry calculations.
Sphalerite minimum precipitation temperatures were calculated using the equation Fe/Znsphalerite = 0.0013 (T) − 0.2953 of [23] for each individual sample. This geothermometer is based on the assumption that the uptake of iron into the sphalerite lattice is temperature dependent [24]. According to [23] this geothermometer should only be used for samples from sediment-starved sites. This argument is valid for samples from the TAG area as the MIR Zone as well as the Alvin Zone that hosts the Rona Mound, Southern Mound, Shinkai Mound, and New M. #2 & #3 are classified as a sediment-free tectonic settings [25]. In addition, samples should not have been affected by metamorphic overprint higher than upper greenschist fazies [23], which is also valid for our samples. Furthermore, [23] states that samples should not have been affected by Zn mobilization, which could be identified by the presence After recovery, samples were handled following the protocol established for cruise JC138 [22]. Where possible, sample orientation (i.e., top and bottom) were marked on the sample, and respected during any further steps. The exteriors of all samples were then petrographically described before being cut using a rock saw. Samples were further texturally and mineralogically described and subsamples were subsequently taken for further studies onshore. All samples were dried and individually packed together with oxygen and silica-gel moisture absorbers in evacuated lay-flat polythene tubes and stored at 4 • C in a reefer container to prevent oxidation. Based on surface location, sub-surface depth, textural appearance, and macroscopically identifiable mineralogy, a total of 73 polished thin and thick Sections were prepared and studied under transmitted and reflected light using a MEIJI MX microscope with the attached Infinity 1 camera system. The relative mineralogical compositions were estimated, at a micro-scale, for each sample and an average composition derived for the mounds interior and exterior.
Major element analyses of sulfide minerals were performed on a Cameca-SX100 5-spectrometer microprobe (EMPA, University of Edinburgh) and a Zeiss Sigma HD Field Emission Gun Analytical scanning electron microscope (SEM, Cardiff University). Sulfides were analyzed using 20 kV accelerating voltage, 10 nA beam current (S, Zn), and 100 nA (Fe) and a 1 µm beam diameter. For SEM analyses, the Oxford Instruments 150 mm 2 energy dispersive X-ray spectrometer was used with an acceleration potential of 15 kV. Calibration of EMPA and SEM was performed on natural minerals (pyrite: Fe, S; sphalerite: Zn) on a daily basis. The SEM data are normalized to 100%, automatically, and a direct comparison with EMPA data from the same sample demonstrated that quantitative data from this specific SEM are of comparable quality to that obtained by EMPA. By comparison, using an older generation of SEM (FEI XL30 Field Emission Gun Environmental SEM, Zeiss, Jena, Germany, Cardiff University), elemental analyses for sphalerite provided only semi-quantitative results and were not used in the geothermometry calculations.
Sphalerite minimum precipitation temperatures were calculated using the equation Fe/Zn sphalerite = 0.0013 (T) − 0.2953 of [23] for each individual sample. This geothermometer is based on the assumption that the uptake of iron into the sphalerite lattice is temperature dependent [24]. According to [23] this geothermometer should only be used for samples from sediment-starved sites. This argument is valid for samples from the TAG area as the MIR Zone as well as the Alvin Zone that hosts the Rona Mound, Southern Mound, Shinkai Mound, and New M. #2 & #3 are classified as a sediment-free tectonic settings [25]. In addition, samples should not have been affected by metamorphic overprint higher than upper greenschist fazies [23], which is also valid for our samples. Furthermore, [23] states that samples should not have been affected by Zn mobilization, which could be identified by the presence of chalcopyrite and pyrite inclusions in the sphalerite. As no visible inclusions of chalcopyrite and pyrite are present in the samples of this study, the argument of [23] remains valid in the study.
Average and standard deviation were calculated on the fully quantitative sphalerite data set for the two identified sphalerite types for each of the individual mounds.

Seafloor Observations from the Extinct Mounds
The youngest mounds, Shinkai Mound and New M. #2 & #3 are only covered by a thin layer of sediment comprising a mixture of pelagic and weathered iron-rich hydrothermal material that is generally restricted to the flanks of the mounds and small depressions between the mounds. Southern Mound and Rona Mound, that are assumed to be older based on their proximity to Double Mound, are cut by NE-SW-trending, arcuate faults. They are covered by a layer of sediment up to 3 m thick. This comprises a thin layer of pelagic material transitioning into red-brown material of hydrothermal origin at depth ( Figure 3A). On the summit of Shinkai Mound, as well as New M. #2 & #3, upright, extinct chimneys are observed, whereas on the flanks chimney relicts and boulder-sized talus material that is mixed with sulfide-rich sediments occurs ( Figure 3B). These angular sulfide blocks are generally coated by a thin weathering crust ( Figure 3C), comprising iron-oxyhydroxides, jarosite, and copper chloride (atacamite/paratacamite). of chalcopyrite and pyrite inclusions in the sphalerite. As no visible inclusions of chalcopyrite and pyrite are present in the samples of this study, the argument of [23] remains valid in the study. Average and standard deviation were calculated on the fully quantitative sphalerite data set for the two identified sphalerite types for each of the individual mounds.

Seafloor Observations from the Extinct Mounds
The youngest mounds, Shinkai Mound and New M. #2 & #3 are only covered by a thin layer of sediment comprising a mixture of pelagic and weathered iron-rich hydrothermal material that is generally restricted to the flanks of the mounds and small depressions between the mounds. Southern Mound and Rona Mound, that are assumed to be older based on their proximity to Double Mound, are cut by NE-SW-trending, arcuate faults. They are covered by a layer of sediment up to 3 m thick. This comprises a thin layer of pelagic material transitioning into red-brown material of hydrothermal origin at depth ( Figure 3A). On the summit of Shinkai Mound, as well as New M. #2 & #3, upright, extinct chimneys are observed, whereas on the flanks chimney relicts and boulder-sized talus material that is mixed with sulfide-rich sediments occurs ( Figure 3B). These angular sulfide blocks are generally coated by a thin weathering crust ( Figure 3C), comprising iron-oxyhydroxides, jarosite, and copper chloride (atacamite/paratacamite).    the samples is coated by a thin red-brown iron-oxyhydroxides layer that is associated with greenish copper-chloride and yellowish-brown jarosite. The innermost part of the chimney sample, obtained from New Mound #2 ( Figure 4A), comprises an orifice that is lined by idaite ( Figure 5A). Extending outwards, coarse-grained tetrahedral chalcopyrite is present that is altered to covellite along grain boundaries adjacent to the idaite. Other chimney material obtained from New Mound #3 comprises coarse-grained chalcopyrite that hosts micro-meter sized Fe-rich sphalerite inclusions (average Fe: 10.4 wt %, n = 9). Exsolution lamella of isocubanite ( Figure 5B) can also be observed. Pyrrhotite, a common phase in high-temperature chimneys, is absent in the sample from the Shinkai Mound and New M. #2 & #3, however, tabular to lath shaped pseudomorphs now composed of pyrite are present ( Figure 5C). is coated by a thin red-brown iron-oxyhydroxides layer that is associated with greenish copper-chloride and yellowish-brown jarosite. The innermost part of the chimney sample, obtained from New Mound #2 ( Figure 4A), comprises an orifice that is lined by idaite ( Figure 5A). Extending outwards, coarsegrained tetrahedral chalcopyrite is present that is altered to covellite along grain boundaries adjacent to the idaite. Other chimney material obtained from New Mound #3 comprises coarse-grained chalcopyrite that hosts micro-meter sized Fe-rich sphalerite inclusions (average Fe: 10.4 wt %, n = 9). Exsolution lamella of isocubanite ( Figure 5B) can also be observed. Pyrrhotite, a common phase in high-temperature chimneys, is absent in the sample from the Shinkai Mound and New M. #2 & #3, however, tabular to lath shaped pseudomorphs now composed of pyrite are present ( Figure 5C). Sulfide-rich material, characteristic of the outer wall of a classic black smoker chimney was obtained from New Mound #2 and Shinkai Mound ( Figure 4B,C). These samples comprise Fe-poor sphalerite (average 4.17 wt %, n = 6) that is associated with colloform marcasite ( Figure 5D) and aggregates of cubic pyrite that are overgrown by colloform marcasite ( Figure 5E). In both samples, Sulfide-rich material, characteristic of the outer wall of a classic black smoker chimney was obtained from New Mound #2 and Shinkai Mound ( Figure 4B,C). These samples comprise Fe-poor sphalerite (average 4.17 wt %, n = 6) that is associated with colloform marcasite ( Figure 5D) and aggregates of cubic pyrite that are overgrown by colloform marcasite ( Figure 5E). In both samples,   Figure 4D-F) dominate. In the massive sulfide blocks pyrite forms massive recrystallized aggregates that are either intercalated with chalcopyrite layers ( Figure 5F) or overgrow earlier porous colloform pyrite ( Figure 5G). Within the recrystallized pyrite, inclusions of Fe-rich sphalerite (average Fe: 12.74 wt %, n = 7) and chalcopyrite can be observed. The sulfide breccia comprises angular to sub-angular clasts formed of aggregates of recrystallized pyrite that are hosted in a pyrite matrix. Rare fossil remnants of tube worms that are replaced by pyrite can be identified in the breccia ( Figure 5H). At Rona Mound the breccia comprises angular pyrite clasts, hosted in massive chalcopyrite that has been altered along micro-fractures to chalcocite ( Figure 5I). Rare rosettes of barite occur in vugs.

Petrography of Sub-Surface Samples
Drill core recovery from three eSMS mounds varies (Figure 6), ranging from 1.4% at site 068RD to 72.5% at site 031RD. The uppermost lithological layer consists of pelagic sediment where it was observed during HyBIS dives ( Figure 3A) and accidentally sampled by the legs of RD2 ( Figure 2B) indicating a thickness of approximately 20 cm. The pelagic sediment is underlain by 0.7-3.5 m of orange to red-brown hydrothermal sediment. Beneath the sediment a layer of jasper, up to three meters thick occurs. At Southern Mound and Rona Mound, this is intercalated with a layer of jasper-silica breccia (i.e., angular jasper clasts in a silica/quartz matrix) that is up to 0.5 m thick. However, this appears to be absent in the MIR Zone. In addition, at the MIR Zone a 0.1 cm thick layer comprising iron-oxyhydroxides can be observed between the hydrothermal sediments and jasper. hosted in massive chalcopyrite that has been altered along micro-fractures to chalcocite ( Figure 5I). Rare rosettes of barite occur in vugs.

Petrography of Sub-Surface Samples
Drill core recovery from three eSMS mounds varies (Figure 6), ranging from 1.4% at site 068RD to 72.5% at site 031RD. The uppermost lithological layer consists of pelagic sediment where it was observed during HyBIS dives ( Figure 3A) and accidentally sampled by the legs of RD2 ( Figure 2B) indicating a thickness of approximately 20 cm. The pelagic sediment is underlain by 0.7-3.5 m of orange to red-brown hydrothermal sediment. Beneath the sediment a layer of jasper, up to three meters thick occurs. At Southern Mound and Rona Mound, this is intercalated with a layer of jasper-silica breccia (i.e., angular jasper clasts in a silica/quartz matrix) that is up to 0.5 m thick. However, this appears to be absent in the MIR Zone. In addition, at the MIR Zone a 0.1 cm thick layer comprising iron-oxyhydroxides can be observed between the hydrothermal sediments and jasper. The jasper is underlain by massive sulfides at 3.6 mbsf at MIR Zone, 6 mbsf at Rona Mound and 6.6 mbsf at Southern Mound. At Southern Mound, only a pebble of massive sulfide was recovered ( Figure 7A) that comprises massive pyrite with intercalated layers of chalcopyrite. Sulfides from Rona Mound are more diverse. The upper three meters of intersected sulfides (6-9 mbsf) are vuggy (up to 25% porosity) and comprise of Fe-poor sphalerite (average Fe: 2.88 wt %, n = 33), marcasite, and pyrite. These minerals form rhythmic colloform zones and overgrow framboidal pyrite ( Figure 8B,C). A second generation of pyrite exhibits massive recrystallized textures and surrounds the earlier marcasitesphalerite assemblage. Barite forms rosettes that occur in interstices of the sulfides. Another accessory The jasper is underlain by massive sulfides at 3.6 mbsf at MIR Zone, 6 mbsf at Rona Mound and 6.6 mbsf at Southern Mound. At Southern Mound, only a pebble of massive sulfide was recovered ( Figure 7A) that comprises massive pyrite with intercalated layers of chalcopyrite. Sulfides from Rona Mound are more diverse. The upper three meters of intersected sulfides (6-9 mbsf) are vuggy (up to 25% porosity) and comprise of Fe-poor sphalerite (average Fe: 2.88 wt %, n = 33), marcasite, and pyrite. These minerals form rhythmic colloform zones and overgrow framboidal pyrite ( Figure 8B,C). A second generation of pyrite exhibits massive recrystallized textures and surrounds the earlier marcasite-sphalerite assemblage. Barite forms rosettes that occur in interstices of the sulfides. Another accessory phase in the upper part of the sulfides of Rona Mound is amorphous silica that coats the sulfides with a micro-meter thick layer. Beneath the Zn-rich sulfide assemblage massive pyrite that is associated with chalcopyrite occurs (Figure 7C,D). This mineral assemblage is also present in sulfides in the MIR Zone ( Figure 7E). At Rona Mound, porous colloform pyrite is overprinted by massive pyrite that itself is overgrown by coarse-grained aggregates of pyrite ( Figure 8E,G). Chalcopyrite either overprints the pyrite, or is found as inclusions ( Figure 8D) in association with Fe-poor sphalerite (average Fe: 4.41 wt %, n = 7). phase in the upper part of the sulfides of Rona Mound is amorphous silica that coats the sulfides with a micro-meter thick layer. Beneath the Zn-rich sulfide assemblage massive pyrite that is associated with chalcopyrite occurs (Figure 7C,D). This mineral assemblage is also present in sulfides in the MIR Zone ( Figure 7E). At Rona Mound, porous colloform pyrite is overprinted by massive pyrite that itself is overgrown by coarse-grained aggregates of pyrite ( Figure 8E,G). Chalcopyrite either overprints the pyrite, or is found as inclusions ( Figure 8D) in association with Fe-poor sphalerite (average Fe: 4.41 wt %, n = 7). Even at 12 mbsf, rosettes of barite can be observed at sulfide grain interstices ( Figure 8F) where the barite seems to surround earlier chalcopyrite that has been altered to covellite along its grain boundaries. At the MIR Zone, pyrite also exhibits a porous colloform texture ( Figure 8I) that is overgrown by massive recrystallized pyrite. Chalcopyrite occurs in interstices or surrounds earlier cubes of coarse-grained pyrite ( Figure 8H). Very rare micro-sized inclusions of Fe-rich sphalerite (average 17.09 wt %, n = 2) are present in the pyrite. Even at 12 mbsf, rosettes of barite can be observed at sulfide grain interstices ( Figure 8F) where the barite seems to surround earlier chalcopyrite that has been altered to covellite along its grain boundaries. At the MIR Zone, pyrite also exhibits a porous colloform texture ( Figure 8I) that is overgrown by massive recrystallized pyrite. Chalcopyrite occurs in interstices or surrounds earlier cubes of coarse-grained pyrite ( Figure 8H). Very rare micro-sized inclusions of Fe-rich sphalerite (average 17.09 wt %, n = 2) are present in the pyrite.

Iron-Content of Sphalerite and Formation Temperatures
Sphalerite from the extinct mounds in the TAG hydrothermal area can be classified into two types. Type-1 comprises one of the principal phases and is predominately associated with marcasite in sub-surface samples from Rona Mound and in chimney material from Shinkai Mound and New M. #2 & #3. Analysis of this sphalerite reveals that it has a low iron-content (Table 1), ranging from 0.82-8.96 wt %. Type-2 comprises inclusions of sphalerite hosted in pyrite and chalcopyrite. These sphalerites are either iron-poor (Rona Mound) or iron-rich at MIR Zone, Shinkai Mound, and New M. #2 & #3. Based on the temperature-dependent uptake of iron into the sphalerite lattice [24], it is possible to calculate formation temperatures of the sphalerite [23].

Iron-Content of Sphalerite and Formation Temperatures
Sphalerite from the extinct mounds in the TAG hydrothermal area can be classified into two types. Type-1 comprises one of the principal phases and is predominately associated with marcasite in sub-surface samples from Rona Mound and in chimney material from Shinkai Mound and New M. #2 & #3. Analysis of this sphalerite reveals that it has a low iron-content (Table 1), ranging from 0.82-8.96 wt %. Type-2 comprises inclusions of sphalerite hosted in pyrite and chalcopyrite. These sphalerites are either iron-poor (Rona Mound) or iron-rich at MIR Zone, Shinkai Mound, and New M. #2 & #3. Based on the temperature-dependent uptake of iron into the sphalerite lattice [24], it is possible to calculate formation temperatures of the sphalerite [23]. All data were obtained using the SEM with the exception of Rona drill core where an electron microprobe was used.

Sulfide Mineralogy and Textures
The polymetallic, coarse-grained mineral assemblages (i.e., chalcopyrite, isocubanite, idaite, pyrite, marcasite) observed within surface samples from Shinkai Mound and New M. #2 & #3 are similar to those observed in the black smoker complex at the active TAG Mound (Figure 9). Although pyrrhotite and anhydrite, which are representative of the early stage of black smoker formation [4] are absent, tabular pseudomorphs now comprising pyrite indicate that pyrrhotite may have been present at an earlier stage of development, but has been oxidized and replaced by a more stable phase. The chalcopyrite-isocubanite assemblage, formed through the breakdown of an intermediate solid solution, suggests that the chimney-type samples were formed at high temperatures, ranging between 280 • C to 350 • C [4,26]. This is validated by the mineral assemblage comprising chalcopyrite and idaite observed in another chimney sample and data arising from previous work that suggests this assemblage forms at temperatures of 300-350 • C [27].
Further chimney material, recovered from the Shinkai Mound and New M. #2 & #3, and drill core from the upper sulfide layer at Rona Mound comprises dendritic sphalerite that is associated with colloform marcasite and overgrow framboids of earlier pyrite. The presence of framboidal pyrite suggests that the iron-disulfide was precipitated from supersaturated hydrothermal fluids, at temperatures of <150 • C [29] or 200 • C [30], which are typical of the initial phase of black smoker growth [4]. Marcasite, that is overgrowing the earlier framboidal pyrite, forms at temperatures of <240 • C [31]. It is associated with sphalerite forming a typical white smoker mineral assemblage [4] and forms between 250-175 • C [26]. Further chimney material, recovered from the Shinkai Mound and New M. #2 & #3, and drill core from the upper sulfide layer at Rona Mound comprises dendritic sphalerite that is associated with colloform marcasite and overgrow framboids of earlier pyrite. The presence of framboidal pyrite suggests that the iron-disulfide was precipitated from supersaturated hydrothermal fluids, at temperatures of <150 °C [29] or 200 °C [30], which are typical of the initial phase of black smoker growth [4]. Marcasite, that is overgrowing the earlier framboidal pyrite, forms at temperatures of <240 °C [31]. It is associated with sphalerite forming a typical white smoker mineral assemblage [4] and forms between 250-175 °C [26].
Coarse-grained, recrystallized, occasionally euhedral pyrite encountered in the MIR Zone and the deeper parts of Rona Mound suggest that the primary mineral assemblage reacted with hotter, more reduced fluid that could precipitate chalcopyrite [32]. The presence of an amorphous silica coating indicates that the sulfide assemblages are likely to have formed from supersaturated fluids at temperatures <200 °C [33]. Nevertheless this coating helps to protect the sulfides from extensive oxidation by ingressing ambient seawater. However, the presence of barite indicates that seawater was able to penetrate into the deposits, to depths of at least 12 mbsf. Furthermore, the formation of covellite and chalcocite along micro-fractures and grain boundaries indicates the presence of an oxidizing and more acidic, low temperature fluid (<150 °C) in the system [34,35]. In addition, the absence of anhydrite, that is quite abundant at the active TAG mound, suggests that fluid temperatures dropped below <150 °C, at which anhydrite becomes unstable [36].
The sulfide breccia formed during various stages of the mound growth. Anhydrite associated with chimneys dissolved when temperatures dropped below 150 °C resulting in collapsing of the sulfide edifices on the surface but also within the mounds wherever seawater was able to penetrate into its interior, for example, along faults. This process causes extensive in-situ brecciation [37], mass wasting, and ultimately the formation of sulfide sands [13]. For the samples in this study, mass wasting appears to be a minor process as the presence of angular clasts indicates a transport of short distances. Low temperature diffuse flow caused cementation of the sulfides, as observed in samples from Southern Mound and Rona Mound ( Figure 4E,F). Within the mounds, sulfides zone refining is present that was caused by late-stage, upwelling, Cu-rich, high-temperature fluids [38]. This acidic Coarse-grained, recrystallized, occasionally euhedral pyrite encountered in the MIR Zone and the deeper parts of Rona Mound suggest that the primary mineral assemblage reacted with hotter, more reduced fluid that could precipitate chalcopyrite [32]. The presence of an amorphous silica coating indicates that the sulfide assemblages are likely to have formed from supersaturated fluids at temperatures <200 • C [33]. Nevertheless this coating helps to protect the sulfides from extensive oxidation by ingressing ambient seawater. However, the presence of barite indicates that seawater was able to penetrate into the deposits, to depths of at least 12 mbsf. Furthermore, the formation of covellite and chalcocite along micro-fractures and grain boundaries indicates the presence of an oxidizing and more acidic, low temperature fluid (<150 • C) in the system [34,35]. In addition, the absence of anhydrite, that is quite abundant at the active TAG mound, suggests that fluid temperatures dropped below <150 • C, at which anhydrite becomes unstable [36].
The sulfide breccia formed during various stages of the mound growth. Anhydrite associated with chimneys dissolved when temperatures dropped below 150 • C resulting in collapsing of the sulfide edifices on the surface but also within the mounds wherever seawater was able to penetrate into its interior, for example, along faults. This process causes extensive in-situ brecciation [37], mass wasting, and ultimately the formation of sulfide sands [13]. For the samples in this study, mass wasting appears to be a minor process as the presence of angular clasts indicates a transport of short distances. Low temperature diffuse flow caused cementation of the sulfides, as observed in samples from Southern Mound and Rona Mound ( Figure 4E,F). Within the mounds, sulfides zone refining is present that was caused by late-stage, upwelling, Cu-rich, high-temperature fluids [38]. This acidic fluid was capable of remobilizing zinc along the steep thermal gradient, that is, moving it to cooler, outer parts of the mounds [37,39] and simultaneously precipitating Fe sulfides and chalcopyrite [38].

Sphalerite Precipitation Temperatures
The validity for the geothermometer for our samples was verified prior to application (see material and methods). Type-1 sphalerite, that is only present in Shinkai Mound, New M. #2 & #3, Southern Mound and Rona Mound, yield average temperatures of 279 ± 25 • C and 263 ± 32 • C, respectively. The temperatures are slightly higher than the temperatures of 250-175 • C suggested in the literature [26]. However, measurements in the 1990s of white smoker vent fluids at the active TAG Mound recorded temperatures of 270 • C [17] and are in very good agreement with the calculated temperatures from this study. As type-1 sphalerites exhibit dendritic and colloform textures that indicate a direct precipitation from the hydrothermal fluid [40] it can be proposed that these sphalerites are the primary generation and not formed through later remobilization processes. Type-2 sphalerites, present in inclusions, are more complex. At Shinkai Mound, New M. #2 & #3, Southern Mound, and Rona Mound the average temperatures are 377 ± 100 • C and 416 ± 74 • C, respectively. The large standard deviation indicates that inclusions are heterogeneous with regards to their iron/zinc ratio. As the host mineral of these inclusions are coarse-grained, mainly recrystallized pyrite and chalcopyrite, it is proposed that remobilizing and recrystallizing processes affected the sphalerite chemistry yielding temperatures that are too high. Sphalerite inclusions of the MIR Zone appear also to have been altered by remobilizing and recrystallizing processes with their average temperature of 497 ± 27 • C too high for black smoker systems. Inclusions of sphalerite in drill core from Rona Mound are hosted in pyrite and chalcopyrite from the copper-rich zone and yield an average formation temperature of 283 ± 39 • C. This result is within the range of standard deviation from the type-1 sphalerite from this site and it is proposed that these inclusions have not experienced any loss of zinc.

Evolution of Extinct Seafloor Massive Sulfide Deposits
The active TAG Mound represents the earliest formation stage of an SMS deposit with black and white smoker fluids being discharged its surface chimneys ( Figure 10A). Massive sulfides and breccia recovered from the upper 15 m below the surface during ODP drilling in the 1990's [7] reflect metal remobilization and re-precipitation caused by multiple hydrothermal events of different temperature [14]. These caused zone refining and metal enrichment in the upper 5 m of the mound [13]. In addition, dissolution of anhydrite created void space and resulted in collapse of the mound and brecciation. Currently, the TAG Mound is active and high-temperature (>350 • C), copper-rich metal assemblages still form.
Shinkai Mound and New M. #2 & #3 ( Figure 10B) represent the first stage after high-temperature hydrothermal activity has ceased. Although these mounds were not drilled during the Blue Mining cruise, petrological and mineralogical observations are important as the Shinkai Mound and New M. #2 & #3 represents the intermediate step between an active system and a long deceased system. Copper-rich, high-temperature minerals and assemblages, typical of white smokers, are still present. Talus samples recovered from the mounds only comprise chimney fragments, rather than massive sulfide blocks or breccia, suggesting that anhydrite is still present in the interior of the deposit as otherwise the upper-mound surface would have flattened [13]. Since these mounds were not drilled during this study, it remains unknown whether metal zonation occurs in the sub-surface.
The model for Southern Mound and Rona Mound ( Figure 10C) implies that the upper part of the mounds has undergone zone refining, with a zinc-rich sulfide layer being underlain by copper-rich sulfides, like at the active TAG mound [13]. Since this metal zonation is also observed in other SMS systems that have been drilled, such as Bent Hill and Palinuro [8,10], it seems to be a common process in mound evolution. The mound has started to collapse and the surface flattens because of anhydrite dissolution and in-situ brecciation in the mound interior. The MIR Zone, that represents the oldest identified mound in the TAG area [20], has also undergone interior zone refining ( Figure 10D), with the presence of sulfide breccia and a flattened mound shape suggesting anhydrite dissolution has occurred at depth. No-zinc rich layer was encountered in the shallow sub-surface during the drilling campaign, but surface material is enriched in zinc [12,28]. This suggests that that zinc was removed from the shallow sub-surface part of the deposit by a late-stage copper-rich fluid as observed in the other mounds. The presence of such a fluid would explain the presence of chalcopyrite cementing coarse-grained recrystallized pyrite.

Conclusions
The results of this surface sampling and shallow sub-surface drilling campaign demonstrate that the composition of eSMS contrasts with the composition of active chimneys. Once hydrothermal activity ceases zone refining processes result in zinc being remobilized or removed from the mounds. Sphalerite precipitation temperatures can help to distinguish primary from remobilized sphalerite as the Fe/Zn ratio become affected by remobilization leading to unrealistically temperatures that are too high for black smoker systems. Copper appears to be preferentially retained within the upper parts of the structure. This suggests that any mineralization of economic interest is restricted to shallow part of eSMS. However, drilling of additional eSMS, and to a greater depth, is required to fully understand the processes controlling the alteration of eSMS deposits as a whole.  The MIR Zone, that represents the oldest identified mound in the TAG area [20], has also undergone interior zone refining ( Figure 10D), with the presence of sulfide breccia and a flattened mound shape suggesting anhydrite dissolution has occurred at depth. No-zinc rich layer was encountered in the shallow sub-surface during the drilling campaign, but surface material is enriched in zinc [12,28]. This suggests that that zinc was removed from the shallow sub-surface part of the deposit by a late-stage copper-rich fluid as observed in the other mounds. The presence of such a fluid would explain the presence of chalcopyrite cementing coarse-grained recrystallized pyrite.

Conclusions
The results of this surface sampling and shallow sub-surface drilling campaign demonstrate that the composition of eSMS contrasts with the composition of active chimneys. Once hydrothermal activity ceases zone refining processes result in zinc being remobilized or removed from the mounds. Sphalerite precipitation temperatures can help to distinguish primary from remobilized sphalerite as the Fe/Zn ratio become affected by remobilization leading to unrealistically temperatures that are too high for black smoker systems. Copper appears to be preferentially retained within the upper parts of the structure. This suggests that any mineralization of economic interest is restricted to shallow part of eSMS. However, drilling of additional eSMS, and to a greater depth, is required to fully understand the processes controlling the alteration of eSMS deposits as a whole.