He – Ar – S Isotopic Compositions of Polymetallic Sulphides from Hydrothermal Vent Fields along the Ultraslow-Spreading Southwest Indian Ridge and Their Geological Implications

Noble gases have become a powerful tool to constrain the origin and evolution of ore-forming fluids in seafloor hydrothermal systems. The aim of this study was to apply these tracers to understand the genesis of newly discovered polymetallic sulphide deposits along the ultraslow-spreading Southwest Indian Ridge (SWIR). The helium, argon, and sulphur isotope compositions of metal sulphide minerals were measured for a number of active/inactive vent fields in the Indian Ocean. The helium concentrations and isotopic ratios in these ore samples are variable (4He: 0.09–2.42 × 10−8 cm3STP·g−1; 3He: 0.06–3.28 × 10−13 cm3STP·g−1; 3He/4He: 1.12–9.67 Ra) and generally greater than the modern atmosphere, but significantly lower than those in massive sulphides from the fast-spreading East Pacific Rise (EPR), especially for three Cu–Fe-rich samples from the ultramafic-hosted Tianzuo and Kairei vent fields. On the contrary, most of the SWIR sulphide deposits have somewhat higher 40Ar/36Ar ratios of trapped fluids (ranging from 290.6 to 303.4) when compared to the EPR ore samples. Moreover, the majority of sulphide minerals from the Indian Ocean have much higher δ34S values (3.0h–9.8h, ~5.9 on average, n = 49) than other basaltic-hosted active hydrothermal systems on the EPR. Overall, these He–Ar–S results are well within the range of seafloor massive sulphide deposits at global sediment-starved mid-ocean ridges (MORs), lying between those of air-saturated water (ASW) and mid-ocean ridge basalt (MORB) end members. Therefore, our study suggests that the helium was derived mainly from the MORB mantle by degassing during the high-temperature stage of hydrothermal activity, as well as from a mixture of vent fluids with variable amounts of ambient seawater during either earlier or late-stage low-temperature hydrothermal episodes, whereas the argon in ore-forming fluids trapped within sulphide minerals was predominantly derived from deep-sea water. Additionally, relatively high δ34S values exhibit a great estimated proportion (up to nearly 40%) of seawater-derived components. In summary, sub-seafloor extensive fluid circulation, pervasive low-temperature alteration, shallow seawater entrainment, and mixing processes, may make a larger contribution to the SWIR hydrothermal ore-forming systems, compared to fast-spreading centres.


Introduction
Over the past 40 years, submarine hydrothermal venting and associated polymetallic massive sulphides have been found in a variety of tectonic settings on the modern seafloor, including mid-ocean ridges (MORs) with different spreading rates [1][2][3][4][5][6], back-arc basins, and rift environments [7][8][9][10][11].These sulphide deposits represent an immense potential reservoir of various important metal resources, such as Fe, Cu, Zn, Pb, Au, and Ag, thus attracting worldwide attention and research interest [12][13][14].However, relatively little is known about the metallogenic characteristics of massive sulphide deposits from the Indian Ocean [15][16][17][18].In particular, because of their remote locations, ancient and active hydrothermal systems on the ultraslow-spreading Southwest Indian Ridge (SWIR) have been much less extensively explored until recently [19][20][21][22][23][24][25], when compared to those well-investigated vent sites located along the fast-spreading East Pacific Rise (EPR), and the slow-spreading Mid-Atlantic Ridge (MAR) [1][2][3][4].Although preliminary geochemical analyses of sulphide samples and the sporadic occurrence of gold have been reported in several newly discovered hydrothermal fields near the SWIR 49 • -51 • E region [26][27][28][29][30], there is only little data on the physicochemical properties of local vent fluids which may be distinct from other categories of ocean ridges [31].Furthermore, what was the source of ore-forming fluids in such a unique geologic setting, and how did the fluids evolve through multiple stages of hydrothermal circulation, remain poorly understood.
The application of noble gas (including helium and argon) isotope geochemistry is a key to defining the origin of fluids, which can help us to answer the above questions.Helium is one of the most sensitive and indicative geochemical tracers for such studies as it is chemically inert (e.g., 3 He behaves extremely conservatively during mixing of vent fluids with entrained seawater) and has distinct isotopic signatures in different types of geological reservoirs [32][33][34][35][36].It has been well established that, in MOR end-member fluids, 3 He is significantly enriched over air-saturated seawater, with 3 He/ 4 He ratios between about 7 and 9.These ratios represent typical mantle values and correspond to 3 He originating from the MOR magma chambers that has degassed into the hydrothermal system [37][38][39].The He-Ar isotopic compositions of hydrothermal fluids are faithfully preserved when trapped as fluid inclusions by mineral phases such as pyrite, sphalerite, or anhydrite, and thus can be used to reveal the degree of fluid-rock interaction, mantle degassing, magmatic, or tectonic activity [36,[40][41][42][43][44][45][46].Moreover, sulphur isotope features of polymetallic massive sulphides can serve as an important tool to interpret various S sources, multi-stage mineralisation processes, and precipitation mechanisms of seafloor hydrothermal products [47][48][49][50][51][52].Therefore, an integrated study of the He-Ar-S isotope system allows us to further develop our understanding of submarine hydrothermal systems at ultraslow-spreading ridges.

Geologic Setting
The Southwest Indian Ridge (SWIR), a divergent tectonic boundary separating the African and Antarctic plates, is among the world's slowest spreading ridges with an average rate of only 14-15 mm/yr [53,54].It extends ~7700 km from the Bouvet to the Rodriguez Triple Junction (RTJ), and is characterised by asymmetric (oblique/orthogonal) spreading, numerous large non-transform offsets, intermittent volcanism, thin crust, and extensive outcrops of serpentinised peridotite or gabbro exposed by extensional detachment faulting [55][56][57][58].Distinct variations have been observed in axial topography, segmentation patterns, crustal thickness, mantle composition, and melt supply along the strike, from west to east [24,[59][60][61][62][63][64][65][66].It also displays a surprisingly diverse range of seafloor morphology with highly segmented geometry, from long stretches of amagmatic rift valleys to areas with strongly focused magmatism [67][68][69][70].Notably, along-axis geophysical anomalies indicate a complex interaction of the eastern central SWIR with the nearby Marion-Del Cano-Crozet hotspot group, which might have a significant effect on accretionary processes and tectonics of this ultraslow-spreading ridge system [71][72][73][74].Such hotspot-ridge interaction and localised volcanism in some segments make it possible for hydrothermal vent systems to have heat sources and fluid flow channels, which can provide favourable conditions for hydrothermal activity, as well as the formation of massive sulphide deposits [75].In summary, geological background of the SWIR exhibits high complexity and variability.
As shown in Figure 1, our study area is mainly located between the Indomed and Gallieni fracture zones (FZs) at the central eastern portion of the SWIR (~900 km north of the Crozet hotspot), where sediment-starved ridge segments 27-29 lie at a high angle to the regional spreading direction [24].Shallow depth and thicker crust than the adjacent areas reveal that this region has experienced a dramatic increase in magma supply since 10-8 Ma [67,68,[71][72][73][74]. From 2007 to 2010, a number of active/inactive hydrothermal fields have been discovered consecutively in this ridge section (49  [23,24,76].The Longqi (49.6 • E) vent field is located at the high mound on the southeast wall of the ridge valley at a depth of 1755 m, which lies on the junction point of a small non-transform offset and ridge valley [23].Oceanic core complexes form when long-lived detachment faults expose intrusive and ultramafic rocks at the seafloor, which are very ubiquitous on the slow-or ultraslow-spreading ridges, and also important for the structural control of hydrothermal activity (i.e., by providing a pathway and heat source for fluid circulation) [24,77,78].The Duanqiao (50.5 • E) field lies on an axial highland with a shallow depth of ~1700 m.Seafloor morphology of the surrounding terrain is relatively flat [79].Hydrothermal precipitates, including silica-rich metalliferous sediments (mostly amorphous opal), relict chimneys, and massive sulphide talus, are pervasive in this inactive field [23,80].The Yuhuang (49.2 • E) field is situated on the south rift wall of segment 29, approximately 7.5 km from the ridge axis with a water depth ranging from 1400 to 1600 m.Its surrounding terrain exhibits NEE-striking highland features, with an elevation of ~1500 m to the bottom of the rift valley [76].Currently, although water temperature and turbidity anomalies were not detected at Yuhuang, two sites of metal sulphide accumulations have been confirmed in this study area.Recent oceanographic surveys also suggest that there are probably outcrops of ultramafic rocks, further implying the local development of detachment faults [29].Besides, another new hydrothermal field (Tianzuo) was found in the ridge section 63 • -64 • E between the Melville FZ and RTJ.This ultramafic-hosted inactive site is located at 63 • 32 E/27 • 57 S, southwest to the relict Mt.Jourdanne field [24].

Figure 1.
Location map of our study area on the Southwest Indian Ridge (between the Indomed and Gallieni fracture zones (FZs)) as well as the Central Indian Ridge (CIR) north of the Rodriguez Triple Junction (RTJ).Inset shows regional setting of the ultraslow-spreading SWIR.For more detailed bathymetry and topography data on each of these six hydrothermal fields (marked with red circles), please see the previously published literature [23,24,29,[78][79][80][81][82][83].

Sampling Information
With the aim of exploring the SWIR hydrothermal systems, the Chinese research cruises DY115-19, DY115-20, and DY115-21 have been successfully carried out by China Ocean Mineral Resources Research & Development Association (COMRA) during 2007-2010 [23,24,76].Seafloor hydrothermal precipitates were collected by TV-guided grab sampler from one active vent site (Longqi) and three inactive hydrothermal fields (Duanqiao, Yuhuang, and Tianzuo) during these cruises on board the R/V Dayang Yihao.Basic information about the locations of selected sampling stations and detailed mineralogical characterisation of typical sulphide ores are described in Table 1.A suite of hand specimens represents various types of hydrothermal products recovered from the Indian Ocean ridge system, including several fragments of active/extinct "black smoker" chimneys, large blocks of massive sulphides, relict talus and oxidised debris, as well as SiO 2 -rich hydrothermal sediments.The morphology and occurrences of ore mineral assemblages were observed under SEM or using optical microscopy (as shown in Figure 2).Moreover, additional samples from the active mound surface of another four vent fields (Edmond, Kairei, Niaochao & EPR 13 • N), at the CIR and EPR, are also included in this study for comparison.Detailed geographic data and their metallogenic features have already been well-documented in a series of published articles [17,18,[82][83][84][85], which are not presented here due to space limitations.Hydrothermal precipitates collected by TV-Grab from four sampling stations in the Longqi vent field are mainly composed of chimney wall fragments, massive sulphide ores, and silica-rich metalliferous sediments.Based on optical microscopic observations (Figure 2A-C), representative samples can be roughly classified into two major types, including Fe-rich (19II-S7-TVG4A and 20V-S35-TVG17) and Zn-rich (19II-S7-TVG4B) sulphide ores.Two mineralisation stages were also recognised by previous researchers [26,27].Similarly, ore samples recovered from the adjacent Duanqiao and Yuhuang hydrothermal fields also include Fe-rich relict sulphide talus or clasts (20V-S32-TVG14 and 21VII-S35-TVG22) consisting mostly of pyrite/marcasite with minor chalcopyrite and sphalerite (Figure 2D,E), which might be derived from the recent collapse of inactive chimney edifices [24,29,80].By contrast, the mineralisation style is somewhat unique at the Tianzuo field, which reflects complex ore-forming processes related to ultramafic-hosted hydrothermal environments of the ultraslow-spreading SWIR [86].As shown in Figure 2F,G, these porous Fe-Cu-rich sulphide breccias (20VII-S25-TVG21) are principally composed of marcasite (at least two generations) and isocubanite, with a small amount of covellite, whereas pyrite and sphalerite were rarely identified.Detailed mineral assemblages and textures are also characterised by abundant Fe-oxyhydroxides filling in microcavities or fractures, which appear to have undergone extensive hydrothermal alteration and supergene weathering.
Four vent sites from the CIR and EPR are typical of hydrothermal activity at intermediate-to fast-spreading, sediment-starved oceanic ridges.Likewise, we identified two types of polymetallic sulphide samples (Figure 2H,I) from the Edmond vent field: Zn-rich layers of extinct chimney structures dominated by Fe-rich massive to euhedral sphalerite (17A-IR-TVG12), and large blocks of Fe-(Cu)-rich sulphide ores with low porosity due to replacement and infilling of open space by chalcopyrite or marcasite (19III-S18-TVG9).Chalcopyrite-rich massive sulphides (19III-S12-TVG6) and talc-bearing siliceous breccias (19III-S13-TVG7) from the ultramafic-hosted Kairei vent field on the CIR were studied in order to compare their isotopic composition with that of the Tianzuo field.In contrast to the strongly altered Kairei samples with abundant secondary Cu-minerals (Figure 2J), Fe-rich primary sulphide ores (EPR-TVG1, EPR-TVG2, and 20III-S4-TVG1) from the EPR active vent sites were probably formed during the main stage of ~350 • C hydrothermal activity, which represent inner-wall sulphide phases (pyrite-pyrrhotite-chalcopyrite) of black smoker chimneys (Figure 2K,L).In general, a series of common mineralogical associations, characterised by coarse-grained, subhedral to euhedral pyrite/pyrrhotite, chalcopyrite-isocubanite, and Fe-rich sphalerite/wurtzite, points to a high-temperature formation, while those consisting of colloform marcasite, Fe-poor sphalerite and secondary Cu-sulphides (such as covellite-digenite), together with amorphous silica (opal-A) as well as poorly crystalline Fe-Mn oxyhydroxides, indicate a formation under lower temperatures during either earlier or late-stage hydrothermal episodes.

Analytical Methods
Selected fragments of representative hand specimens (n = 21) were coarsely crushed into millimetre-sized granules using agate mortar and pestle.In particular, some chimney walls (such as 19II-S7-TVG4, 20V-S35-TVG17, and 21VII-S35-TVG22) were divided into the interior conduit portions and outer layers, and then cut into pieces of subsamples according to the observed spatial zonation of mineral assemblages.To obtain homogeneous microcrystalline groundmass separates, various types of metal sulphide phases with small diameters (20-60 mesh size fraction) were carefully hand-picked under a binocular microscope.However, it should be noted that a clean separation of pyrite/marcasite, chalcopyrite, and sphalerite was almost impossible to achieve, due to intimate intergrowths of fine-grained sulphide aggregates.Helium and argon isotopic compositions of trapped hydrothermal fluids from the Indian Ocean massive sulphide deposits were determined with an all-metal extraction line and a GV-5400 noble gas mass spectrometer (GV Instruments, UK) at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences (Guiyang).For higher precision, a number of subsamples (denoted by * as shown in Table 2) were further analysed using a Thermo Fisher Helix Split Flight Tube (SFT) multi-collector mass spectrometer at the Analytical Laboratory of Beijing Research Institute of Uranium Geology.
Detailed sample preparation procedures and experimental methods used here were similar to those described in references [87][88][89].Approximately 0.1-0.5 g of coarse-grained (generally 0.2-1.0mm) hand-picked mineral separates were first cleaned ultrasonically several times in alcohol, then dried in vacuo and loaded in on-line crusher buckets.In order to remove adhered atmospheric gases, these samples were baked at about 150 • C in an ultra-high vacuum system for >24 h prior to measurement.Fluid inclusion-hosted volatiles were released from mineral grains into the all-metal extraction system by sequential crushing in modified Nupro-type valves.The released gases were exposed to a titanium sponge furnace at 800 • C for 20 min to remove the bulk of active gases (e.g., CO 2 and H 2 O), and then exposed to two SAES Zr-Al getters (one at room temperature, the other at 450 • C) for 10 min to further purify.He was separated from Ar using an activated charcoal cold finger at liquid N 2 temperature (~196 • C) for 40-60 min to trap Ar, then He and Ar isotopes were analysed using the GV-5400 or Helix SFT with relative errors of <10%.Procedural blanks (below 2 × 10 −10 cm 3 STP 4 He and 2-4 × 10 −10 cm 3 STP 40 Ar) were insignificant.Noble gas abundances were measured by peak-height comparison with known amounts of standard air from an air bottle.The atmospheric 3 He/ 4 He and 40 Ar/ 36 Ar ratio is 1.399 × 10 −6 and 295.5, respectively [90].
S-isotope measurements of fifty-five subsamples from eight different hydrothermal vent fields were performed on a Thermo-Finnigan MAT-253 isotope-ratio mass spectrometer (IRMS) at the Analytical Laboratory of Beijing Research Institute of Uranium Geology (China).For more details on experimental methods and instrumental operating parameters, please see [91].Sulphur isotopic compositions are reported in per mil ( ) relative to the Vienna-Canyon Diablo Troilite (V-CDT) standard.In this study, our data were calibrated against the GBW-04414 and GBW-04415 silver sulphides which were used as certified reference materials (Chinese national standards, according to DZ/T 0184.14-1997) with δ 34 S values of −0.07 ± 0.13 and 22.15 ± 0.14 , respectively.The overall analytical precision is better than ±0.2 .

Helium-Argon Concentrations and Isotopic Ratios
As presented in Table 2, the measured abundances of fluid inclusion-hosted helium and argon in these hydrothermal sulphide samples (n = 13) vary over three orders of magnitude (e.g., 4 He: 0.09-9.70× 10 −8 cm 3 STP•g −1 ; 3 He: 0.06-12.90× 10 −13 cm 3 STP•g −1 ; 40 Ar: 0.28-5.26× 10 −6 cm 3 STP•g −1 ; 36 Ar: 0.95-17.53× 10 −9 cm 3 STP•g −1 ).By comparison, Fe-rich massive sulphide ores from the EPR 13 • N contain the highest 3 He and 4 He concentrations among the entire suite of tested samples, which are significantly higher than the Indian Ocean polymetallic sulphides (especially for those Cu-Fe-rich samples from ultramafic-hosted hydrothermal systems along the SWIR and CIR).On the contrary, most of the 36 Ar and 40 Ar contents in sulphide mineral aggregates from the ultraslow-spreading SWIR are generally higher than those from superfast-to intermediate-spreading ridges (such as Niaochao and Kairei samples).Moreover, we should also note that such variations in He contents may be attributed to differences in the actual size and population densities of fluid inclusions among various sulphide minerals, or even due to the variation in crushing/degassing efficiency (to a certain extent) [36,41].Note: All errors are quoted at the 1σ confidence level.For seven samples denoted by *, experimental uncertainties of their He-Ar concentration data are not given in the laboratory's test report.The atmospheric 3 He/ 4 He ratio (R a ) is ~1.4 × 10 -6 [90].Abbreviations: Ccp = chalcopyrite, Cv = covelite, Icb = isocubanite, Mrc = marcasite, Po = pyrrhotite, Py = pyrite, Sp = sphalerite, Wur = wurtzite.
The 3 He/ 4 He ratios in all of these hydrothermal precipitates vary from 1.12 to ~10.0 R a , while the 40 Ar/ 36 Ar ratios range from 287.4 to 303.4 (Table 2), with an average of 294.9 close to air-saturated water (ASW: 295.5) [90].It is worthy of note that a majority of the SWIR sulphide deposits have somewhat higher 40 Ar/ 36 Ar ratios of trapped fluids (~298.0 on average, n = 7) when compared to the EPR and CIR ore samples (~291.4 on average, n = 6).Furthermore, most of the calculated 3 He/ 4 He ratios (except for the Tianzuo inactive field) are significantly greater than the modern atmosphere, lying within the range of mid-ocean ridge basalts (MORBs: about 7-9 R a ) [23] or even slightly higher.Only two marcasite-dominated samples (20VII-S25-TVG21) from the SWIR near 63.5 • E both have distinctly low 3 He/ 4 He ratios (1.12-1.88R a ) that are very close to the atmospheric end-member value [90].Overall, there appears to be no clear trends/relationships between the 3 He/ 4 He ratios and the MOR spreading rate or water depth.

Helium and Argon Sources
Published analyses of noble gas isotopes show that helium and argon trapped in fluid inclusions have three potential sources.(1) Air-saturated water (ASW), including meteoric fluids and seawater, is characterised by atmospheric He and Ar isotopic compositions ( 3 He/ 4 He = 1R a , 40 Ar/ 36 Ar = 295.5).
(2) Mantle volatiles, including most mantle-derived rocks, are produced from the upper oceanic mantle, which is characterised by a well-defined 3 He/ 4 He ratio of 1-1.3 × 10 −5 (7-9 R a ).At the same time, mantle-derived Ar is dominated by radiogenic 40 Ar, with 40 Ar/ 36 Ar > 40,000 [93][94][95].(3) He and Ar are also produced within the crust.High concentrations of large-ion lithophile elements (such as U and Th) in the continental crust can produce abundant radiogenic/nucleogenic Ar and He, with 40 Ar/ 36 Ar ratios ≥ 45,000 and 3 He/ 4 He ratios ≤0.1 R a , respectively [96], whereas the oceanic lithosphere accreted at mid-ocean ridges is composed mainly of mafic-ultramafic rocks which contain extremely low levels of radiogenic elements.As a result, 3 He/ 4 He ratios of oceanic crust approach to values of the upper mantle (6-9 R a ).Furthermore, it has been suggested that deep-sea water is characterised by lower 3 He concentration but higher 40 Ar abundance when compared with the upper-and lower-mantle end members [42].Therefore, variations in the mixture of different isotopic reservoirs can impart distinct He-Ar isotope compositions to the resulting hydrothermal products during ore-forming processes.
In this study, the 3 He/ 4 He ratios of most massive sulphide samples (except 20VII-S25-TVG21) ranged from 3.37 R a to 10.0 R a .Notably, our results of the SWIR samples (with a mean value as high as 8.4 R a , n = 5) are relatively greater than those obtained from the CIR and MAR hydrothermal precipitates (e.g., Edmond and Kairei: avg.~7.0 R a , n = 3; TAG: avg.7.2 R a [42]).All these results are more or less within the range of mantle-derived He (Figure 3), suggestive of mantle components as the main source of He.In particular, black smoker chimneys and Fe-(Zn)-rich massive sulphides from the Longqi (19II-S7-TVG4B and 20V-S35-TVG17) and Yuhuang fields yield a high 3 He/ 4 He ratio of 9.10, 9.67, and 9.11 R a , respectively.These values are not only higher than the ratios of 3 He/ 4 He in the Indian Ocean Ridge basalts (avg.8.49 R a [35]), but also slightly higher than those of the upper-mantle fluids [97].Thus, we suggest that a small amount of lower-mantle-derived He may contribute to the SWIR hydrothermal systems.At the same time, the input of marine-derived He should not be neglected.For example, some of the Edmond and Kairei sulphide ores have slightly lower 3 He/ 4 He ratios in contrast to those of the mafic-ultramafic host rocks, indicating a mixture of MORB with variable amounts of seawater-derived helium.It is interesting to note that marcasite-dominated sulphide samples from the ultramafic-hosted Tianzuo inactive field (associated with serpentinisation) exhibit uniformly low He-isotope values, which are very close to the ASW end member and significantly different from other vent fields along the ultraslow-spreading SWIR (Figure 3).Such a unique isotopic signature (1.12-1.88R a ) can be easily explained by the dominance of a seawater-derived component due to pervasive low-temperature alteration in this area.Additionally, sub-seafloor extensive fluid circulation and mixing processes facilitated by local detachment faults may also play an important role in enhancing the entrainment of shallow seawater.
In most cases, the Ar isotopic ratios (ranging from 287.4 to 303.4) are generally in agreement with the modern atmospheric value (295.5)and, sometimes, just slightly lower or higher (Figure 4).Their values are considerably lower than those of MORB or OIB mantle end members, thus confirming that Ar is mainly derived from ambient deep-sea water, rather than from upper/lower mantle.This can be interpreted as evidence for a large proportion of seawater entrainment into submarine hydrothermal fluids [36].However, the contribution from mantle-derived Ar cannot be completely ignored.Even minimal changes may occur in the Ar isotopic composition upon mixing with mantle-derived fluids because of the Ar solubility.Ar is soluble in aqueous fluid; thus, Ar concentration in the seawater ( 36 Ar: 1.27 × 10 −6 cm 3 STP•g −1 ; 40 Ar: 3.75 × 10 −4 cm 3 STP•g −1 [90]) is several orders of magnitude higher than that in the mantle ( 36 Ar: 2.7-6.5 × 10 −10 cm 3 STP•g −1 ; 40 Ar: 3.3-7.6× 10 −6 cm 3 STP•g −1 [98][99][100]) owing to its high solubility in seawater.Therefore, marine-derived Ar significantly influences the Ar isotopic compositions in hydrothermal sulphides.In contrast to Ar, the He concentration in mantle is much higher than that in seawater [90].The contribution of mantle-derived fluid to He isotopic composition is more significant in polymetallic sulphides.As presented in Figure 4, hydrothermal precipitates show distinctive isotopic characteristics, implying that He is derived from the mantle.By contrast, Ar isotopic composition is similar to that in air.Analogous to the other hydrothermal fields, most of the He-Ar isotopic compositions in this study fall in the range of typical seafloor massive sulphide deposits with a slightly higher 3 He/ 4 He.However, the majority of Tianzuo samples show an obvious departure from this range, owing to their unusually low 3 He/ 4 He ratios.[36,[40][41][42][43][44][45][46]100,102] and this study).

Sulphur Sources
Sulphide sulphur in global MOR hydrothermal vents can be derived from three main sources: (1) leaching from host rocks; (2) thermochemical reduction of seawater sulphate; and (3) leaching of sulphide minerals in sediments [50,51].S-isotope compositions of hydrothermal precipitates reflect the combination of the processes of simple adiabatic mixing, thermochemical reduction, and/or dissolution of biogenic sulphide minerals.Previous investigations of basaltic-hosted massive sulphide deposits at mid-ocean ridges have shown a wide range of δ 34 S ratios from 1 to 9 with a mean value of 4.5 [49][50][51].In this study, bulk sulphur isotopic compositions of our MOR sulphide minerals fall into the abovementioned range.According to the S-isotope distribution in different vent fields, sulphur sources in polymetallic sulphides can be divided into three types [103].The first type of sulphur source is mainly originating from MORB, and partially from reduced seawater sulphate.Several sediment-starved MOR hydrothermal fields (e.g., EPR 21 • N, southern Juan de Fuca Ridge, and Snakepit from the MAR) are characterised by such type.The second type consists of sulphur that mainly originates from sediments with a mixture of seawater-derived and/or organic sulphur.Sediment-hosted hydrothermal systems, such as Guaymas Basin and Middle Valley, belong to this type.Magmatic and sediment-derived S predominate the third type, also mixing with seawater-derived sulphur.This type consistently exists in hydrothermal fields from back-arc basin, such as the Jade vent field in Okinawa Trough [103,104] (see Figure 5).Therefore, compared with hydrothermal systems at sediment-hosted MORs and back-arc basins, S sources are relatively simple in sediment-starved fields, mainly including seawater-derived sulphate and magmatic sulphur.
Overall, δ 34 S in MORB-hosted sulphide deposits from the sediment-starved SWIR fall within a narrow range, varying from 3.0 to 7.5 (except 20VII-S25-TVG21).Such sulphur isotope features are somewhat similar to the TAG (Trans-Atlantic Geotraverse) hydrothermal field on the slow-spreading MAR [49].We also note that the δ 34 S values of the Longqi samples generally show a slight but significant increase, progressively from high-temperature to relatively low-temperature mineralisation stage.In some cases, the observed mineral zonation in a number of ore samples (e.g., 19II-S7-TVG4 and 20V-S35-TVG17) appears to be related to subtle changes in δ 34 S with a heavier signature from the inner layers to exterior porous walls, implying a moderate admixture of magmatic S and increasing proportions of seawater infiltrating into the chimney.Moreover, these results also indicate that most massive sulphides from the SWIR and CIR failed to reach isotopic equilibrium, the reason of which probably lies in their rapid co-precipitation upon mixing and cooling.This phenomenon (S-isotope disequilibrium) may be ubiquitous in hydrothermal fields along the Indian Ocean Ridges.Since pyrite/pyrrhotite should have higher δ 34 S values than coexisting sphalerite and chalcopyrite under equilibrium conditions [105], the δ 34 S data of some ore samples suggest strong isotopic disequilibrium between pyrite/pyrrhotite, sphalerite and chalcopyrite.Despite that, measured isotopic fractionation between them is negligibly small, particularly in chimney wall fragments or relict sulphide talus.This implies that fractionation did not occur because these minerals were deposited episodically at lower temperatures (<350 • C).Although dissolution and replacement of pre-existing sulphate minerals may be common in some black smoker chimneys, heavy sulphur in pyrite-marcasite/covellite-bearing assemblages and Si-Fe-Mn oxyhydroxides is likely to have been predominantly derived from admixed seawater.
In general, sulphur isotopic data lying between two extreme values (MORB: δ 34 S ≈ ±0 ; seawater: δ 34 S ≈ +21 ) can be explained by non-equilibrium mixing between sulphur of basaltic origin and sulphur from reduced seawater [106,107].Based on a two-component mixing model [92], we simply quantify the proportional contributions of seawater-derived and igneous sulphur to the SWIR hydrothermal precipitates.The mean δ 34 S value of 5.1 for the Longqi massive ores (n = 16) indicates a seawater versus basalt S ratio of about 23:77 on average.By contrast, heavier S isotopes in colloform sulphide aggregates and silica chimney debris from the inactive Duanqiao, Yuhuang, and Tianzuo fields (δ 34 S up to 10.5 ) exhibit a larger contribution of sulphur derived from reduced seawater sulphate, ranging from approximately 22% to 50% (as listed in Table 3).In summary, the δ 34 S values of the SWIR massive sulphides are relatively high in contrast to previously published data reported from the fast-spreading East Pacific Rise at 11 • N, 13 • N and 21 • N [47,108] (see Figure 5).It has been suggested that the geological settings of ultraslow-spreading SWIR (especially for ultramafic-hosted vent systems) include a greater component of shallow seawater entrainment, pervasive low-temperature alteration, and sub-seafloor extensive hydrothermal circulation, when compared to fast-spreading centres [55,109].Obviously, these processes are more conducive to thermochemical reduction of seawater sulphate, which imparts higher δ 34 S values to the resulting sulphides.Our δ 34 S results are generally consistent with precious integrated studies of He-Ar isotopes and, thus, can provide geochemical evidence to support this conclusion.

He-Ar-S Isotopic Constraints on the Proportional Contribution of Mantle Sources
According to the S isotopic data presented above, magmatic sulphur predominates as the major source of sulphur in our SWIR samples.On the other hand, seawater-derived sulphur accounts for 13%-35% in typical ore types from the Longqi, Duanqiao and Yuhuang fields.In relatively acidic environments with low oxygen fugacity, when these sulphide minerals directly precipitate from hydrothermal fluids, their S isotopic composition can represent that in the hydrothermal solution.As a result, up to 40% of hydrothermal fluid is derived from seawater with residues from the mantle.Given the minimal fractionation of S isotope between the upper and lower mantle, δ 34 S cannot reflect the proportions of upper and lower mantle in residual ore-forming fluids.However, an input of lower-mantle-derived He might be introduced to the SWIR hydrothermal products with higher 3 He/ 4 He ratios.Therefore, on the basis of boundary constraints deduced from the δ 34 S ratios of seawater-derived ore-forming fluids, we can estimate the approximate proportions of the upper and lower mantle in hydrothermal fluids by using He-Ar isotopic tracers.
In S7-TVG4B, S35-TVG17, and S35-TVG22 samples, both S and He-Ar isotopic compositions have been analysed.Thus, specific results of certain minerals in corresponding samples were selected for calculation and further discussion.According to δ 34 S values, seawater-derived component accounts for approximately 14%-27% of the SWIR ore-forming fluids.Thus, the residual is derived from the mixture of upper-and lower-mantle fluids.Assuming that the residual fluid is a dual mixed system by upper-and lower-mantle end members, we can simply quantify the proportional contribution of lower-mantle fluids (He LM ) using a computing method modified after [110]: In this formula, the ratio of seawater-derived fluid is expressed as A, which can be calculated by δ 34 S values; ( 3 He/ 4 He) SW refers to the He isotope ratio in seawater, which equals to 1 R a [97]; ( 3 He/ 4 He) UM represents the He isotope ratio in the upper mantle, close to the He isotopic ratio of Indian Ocean MORBs (~8.5 R a ) [35]; and ( 3 He/ 4 He) LM corresponds to the He isotope ratio in the lower mantle with a value of 32 R a [111].The estimation results indicate that lower-mantle-derived He may account for 7%-13% of the total mantle He sources, while 63%-79% of He is probably derived from the upper mantle.As a result, helium in the SWIR hydrothermal sulphides mainly originates from the upper mantle and to a lesser extent, mixed by seawater-derived and lower-mantle-derived fluid as well.
It has been reported that the enrichment of 3 He in hydrothermal sulphide samples from the Okinawa Trough may be attributed to lower-mantle-derived He rising to the upper mantle through mantle plume [112]; as magma is melted and lifted, the lower-mantle-derived He will be introduced into the hydrothermal systems.Likewise, the ultraslow-spreading SWIR is characterised by active hotspot-ridge interaction.Some authors have observed that the He isotopic ratios of some SWIR basalt glasses near the Bouvet mantle plume can reach as high as 14.2 R a [113].The results of Bouguer anomalies and stratigraphic data also confirm the existence of anomalous mantle between Indomed and Gallieni FZs, and incompatible-element-enriched lavas in the vicinity of the Indomed FZ differed from the other segments [67][68][69]71].These characteristics can be related to the initiation of the Marion, Kerguelen and Crozet hotspots [114].Mantle-derived materials (such as peridotites) are extensively exposed on the seafloor, revealing that the SWIR crust is generally thin [55,57].Published studies show that magmatic activity had occurred in this area since 11-8 Ma, and provided massive magma output and heat sources, which were assumed to be related to the Crozet hotspot activity [72].This conclusion implies that a minor proportion of lower-mantle-derived He may be added to the SWIR hydrothermal systems through the Marion and Crozet hotspot activities and, at the same time, mixed with seawater-derived and upper-mantle-derived He.Thus, the He isotopic compositions in three ore samples from the Longqi and Yuhuang fields are products of different mixtures by seawater, upper mantle, and lower mantle.Among them, the upper mantle might play a dominant role.
In summary, we propose a descriptive model with a schematic diagram illustrating the distribution features of He-Ar-S isotopic compositions and multiple sources of ore-forming fluids in hydrothermal systems at SWIR 49 • -51 • E (see Figure 6).

Conclusions
(1) The S isotopic ratios of typical massive sulphide samples in SWIR fall in a narrow range, varying from 3.0 to 7.5 .Seawater-derived and magmatic sulphur are dual sources of S during ore-forming processes, in which the contribution of magmatic sulphur plays a dominate role, approximately accounting for 65%-87% of total S sources, whereas the proportion of seawater-derived sulphur is about 13%-35%.Additional contribution of seawater to No. S25-TVG21 sulphide samples (45%-48%) indicates that the Tianzuo hydrothermal products had undergone extensive hydrothermal reworking, as well as post-depositional alteration by seawater.Sulphur sources in hydrothermal sulphides from the CIR are also characterised by the mixture of two end members dominated by magmatic sulphur.(2) The S isotopic ratios in the Longqi black smoker chimneys show that δ 34 S in the exterior wall is higher than that in the inner portion, suggesting that the contribution of seawater increases from inner to outer layer.The results of δ 34 S in most of the SWIR hydrothermal sulphides suggest that S isotope failed to reach equilibrium due to their rapid precipitation when hydrothermal fluid mixed with seawater after venting.(3) The ratios of 3 He/ 4 He in SWIR hydrothermal sulphides ranged from 9.67 R a to 1.12 R a , along with 40 Ar/ 36 Ar ratios varying from 303.4 R a to 290.6 R a .He isotopic compositions in three representative samples are the products of mixtures with variable amounts of seawater, and upper-mantle and lower-mantle components, among which, the upper mantle plays a dominant role.Ar isotopic analyses indicate that the majority of Ar in these SWIR samples is derived from seawater.(4) According to He-S isotopic compositions, the sources of ore-forming fluids trapped in different types of hydrothermal precipitates can be roughly estimated.Our results indicate that upper-mantle-derived fluid accounts for 63%-79% and 14%-27% from seawater, whereas 7%-13% of He is derived from the lower mantle related to the Marion and Crozet hotspot activities.These isotopic signatures can provide useful insights into the impact of geological control on hydrothermal mineralisation at the ultraslow-spreading SWIR.

Figure 3 .
Figure 3. Scatter plots of R/R a versus 4 He concentrations in polymetallic massive sulphides from active or inactive hydrothermal systems along the SWIR and CIR.Datapoints from other seafloor vent fields in different tectonic settings are also shown for comparison, given by [36,40-46,100-102].

Figure 4 .
Figure 4. R/R a versus 40 Ar/ 36 Ar diagram of fluid inclusions hosted within massive sulphide samples from global seafloor hydrothermal vent fields (data sources from the previously published literature [36,40-46,100,102] and this study).

Figure 5 .
Figure 5.A comparison of variations in sulphur isotopic compositions of different hydrothermal sulphide deposits within a variety of tectonic settings on the modern seafloor (data sources from the published literature [29,47-52,103,104,106-108] and this study).

Figure 6 .
Figure 6.A diagram showing the distribution of He-Ar-S isotopic compositions and their contraints on the proportional contribution of multiple sources to an active hydrothermal vent system located on the ultraslow-spreading Southwest Indian Ridge between 49 • and 51 • .

Table 1 .
Description of the Mid-Ocean Ridge polymetallic massive sulphides and hydrothermal sediment samples analysed for helium-argon and/or sulphur isotopic compositions in this study.

Table 2 .
He-Ar concentrations (cm 3 STP/g) and isotopic compositions of fluid inclusions hosted in seafloor polymetallic massive sulphides from the Mid-Ocean Ridge hydrothermal vent systems.

Table 3 .
Bulk S-isotope compositions of polymetallic massive sulphides from the Mid-Ocean Ridge hydrothermal vent systems and the estimated proportional contributions of dual sulphur sources.*
Author Contributions: Y.W., Z.W. and X.S. conceived and designed the experiments; Y.W. and Z.W. analyzed the data and wrote this paper; X.S., Z.W. and Y.G. have substantially revised the original manuscript; X.D. provided some SWIR sulphide samples for our study; L.X. contributed materials/analysis tools; Y.W., Y.H. and K.C. also made significant contributions to the sample preparation.This research project was jointly funded by National Natural Science Foundation of China (Grant No. 41503036, 41702066, 41273054 and 41602092), National Key R & D Program of China (2018YFC0309902), the doctoral program of Higher Education Research Fund (20120171130005), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2011), the Fundamental Research Funds for Central Universities (No. 12lgjc05) and the China Ocean Mineral Resources Research and Development Association (DYXM-115-02-1-11). Funding: