The active Deyin hydrothermal field (DHF) is located at 15.2 degrees S on the Mid-Atlantic Ridge and is hosted by typical basaltic substrate. Here, we report distinct Pb-S-Zn isotopic compositions of pyrite, sphalerite, and chalcopyrite from the DHF, which differ from those of other mid-ocean ridge hydrothermal systems. Key features include: (i) Pb isotopes in pyrite and sphalerite plot within the field of Mid-Atlantic Ridge basalts, whereas chalcopyrite is characterized by depleted 206Pb and enriched 207Pb and 208Pb, resembling nearby 26 degrees S seamount basalts with an enriched mantle signature; (ii) delta 34S values increase progressively from pyrite (0.86%o-3.40%o) to sphalerite (2.08%o-2.84%o) and chalcopyrite (3.02%o-3.64%o), a trend not commonly observed in other hydrothermal fields; and (iii) sphalerite exhibits lower delta 66Zn values (-0.17%o to 0.05%o) than both pyrite (0.08%o-0.57%o) and chalcopyrite (0.01%o-0.09%o), contrary to expectations from Rayleigh fractionation alone. Integrating Pb-S-Zn isotopic data, we propose that while leaching of normal mid-ocean ridge basalts supplied metals and sulfur for pyrite and sphalerite formation, a local volatile-rich magmatic source, termed Deyin enriched mid-ocean ridge basalts, directly contributed additional metals and sulfur during chalcopyrite deposition. This magmatic input also explains the unusually high delta 66Zn values of chalcopyrite relative to sphalerite. Our findings demonstrate that direct magmatic contributions can play a significant role in mid-ocean ridge hydrothermal systems, a process likely substantially underestimated, particularly when hydrothermal fields are proximal to an enriched mantle source.

The Longqi-1 hydrothermal field (LQ-1 HF), the first active high-temperature vent field discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR), provides a valuable setting for studying seafloor sulfide mineralisation. However, spatially resolved constraints on physicochemical gradients during chimney growth remain limited. This study presents mineralogical, trace element, and sulfur isotopic analyses of Cu-rich, Zn-Ferich, and Zn-rich chimneys from the LQ-1 HF, revealing systematic mineral zonation and metal enrichment across sulfide chimney walls. The Cu-rich chimney displays well-developed zonation, transitioning from anhydrite, pyrite, or marcasite with minor sphalerite at the exterior to massive chalcopyrite at the interior. The presence of bornite, covellite, and digenite suggests episodic seawater infiltration and oxidation during chimney evolution. Pyrite from outer zones is enriched in Mn, Mo, Tl, U, and V due to low-temperature precipitation from seawater mixing, while increasing Co in pyrite and Se in chalcopyrite toward the interior reflect higher temperatures and conditions favouring the stability of Cu-rich sulfides. In contrast, the Zn-Fe-rich and Zn-rich chimneys exhibit colloform to dendritic pyrite, marcasite, and sphalerite with minor chalcopyrite, consistent with lower-temperature, seawater-influenced precipitation conditions. Sulfur isotope gradients (delta 34S: 4.5-9.9 parts per thousand) reflect a mixed sulfur source from seawater sulfate and host rocks, with early-formed zones more seawater influenced. Elevated Sn and low Cd/Zn ratios are consistent with metal inputs from a heterogeneous lithological source, potentially involving ultramafic components and possibly facilitated by deep detachment faults. These results reveal the influence of host-rock composition, temperature, redox state, and seawater mixing on sulfide mineralisation at the detachment fault-controlled LQ-1 HF on the ultraslow-spreading SWIR.

Copper content in sulfides on mid-ocean ridges (MOR) is controlled by the variation and abundance of Cu-rich minerals, that sulfides in ultramafic-hosted hydrothermal fields commonly contain higher Cu, compared with those in mafic-hosted hydrothermal fields. Recently, a new ultramafic-hosted hydrothermal field at 49.28 degrees E was discovered on the ultraslow-spreading Southwest Indian Ridge. Sulfides collected show remarkably high Cu contents (averaged 26.88 wt%), greater than those of typical ultramafic-hosted hydrothermal fields (averaged 17.94 wt%). In this study, we integrate sulfide mineralogy, bulk geochemistry and sulfur-isotope analysis of typical Cu-rich and Fe-rich sulfide samples. Our findings indicate that the collected sulfides consist predominantly of primary chalcopyrite, bornite, pyrite and secondary copper minerals, with bulk compositions enriched in Cu (similar to 26.88 +/- 12.47 wt%, N = 19, 1SD), Au (similar to 5.1 +/- 5.66 ppm), Co (similar to 730.96 +/- 434.27 ppm), and Se (similar to 454.4 +/- 247.21 ppm), while being depleted in Ni (similar to 7.46 +/- 17.54 ppm). Additionally, these sulfides display relatively positive but uniform delta S-34 values, ranging from + 4.49 to + 6.9 parts per thousand (averaged + 5.49 parts per thousand; N = 42), except for one value of + 2.3 parts per thousand. These characteristics combined indicate deep ultramafic sourced metals, with restricted seawater entrainment (19-30%), and sulfide deposition occurring in a high-temperature (>300 degrees C), reducing, and relatively closed system. We propose that these conditions were resulted from the immature Yuhuang detachment fault, which promotes early serpentinization of ultramafic rocks and low permeability, distinguishing from matured detachment fault controlled hydrothermal systems. Our results constrain hydrothermal systems formed at the terminus of immature detachment faults and provide new insights into ore-forming processes in ultramafic-hosted systems along ultraslow-spreading ridges.

The integrated fusion and inversion of seismic data and marine controlled-source electromagnetic (MCSEM) data can identify the gas hydrate distributions. However, due to differences in observation systems and scales between seismic and MCSEM data, current fusion methods have failed to effectively address the critical issue of physical property variation within gas hydrate reservoirs. This research seeks to consolidate the two datasets into a cohesive observational framework. By transforming the MCSEM data into low-frequency constraints applicable to seismic impedance inversion, it is possible to realize an effective integrative interpretation that combines both seismic and MCSEM data. Using a 3 km-long seismic dataset and MCSEM data from the Shenhu Sea area in the South China Sea as a case study, we apply the Poisson blending algorithm to integrate seismic and MCSEM data, enabling precise identification and characterization of gas hydrate reservoirs and underlying gas-bearing fluids. The gas hydrate saturation results, derived from seismic inversion constrained by MCSEM data, demonstrate strong consistency with well logging and geological interpretation. This concordance validates the efficacy of the integrated fusion and inversion methodology and highlights its advantages in accurately predicting the spatial distribution of gas hydrate enrichment. The developmental positions of deep gas-bearing fluid pathways, coupled with the fault locations within the free gas zone and gas hydrate-bearing layer, play significant roles in the heterogeneous enrichment of gas hydrates. This research provides important technical and theoretical support for the precise and efficient prediction of gas hydrate reservoirs.

In this study, we analyzed trace-element and S-Cd isotopic compositions of sphalerite and related igneous rocks from four representative skarn deposits in southern Hunan Province, southeastern China. These include the Shuikoushan (SKS), Tongshanling (TSL), and Baoshan (BS) Zn-Pb(-Cu) deposits associated with granodiorite porphyry, and the Huangshaping (HSP) Zn-Pb-W deposit related to granite porphyry. Our results indicate that sphalerite from the SKS deposit exhibits consistent delta 114/110Cd and delta 34S values near 0 parts per thousand, with Zn/Cd ratios resembling those of typical high-temperature mineralization systems. In contrast, sphalerite from the BS and TSL deposits shows lower delta 114/110Cd and Zn/Cd ratios but higher delta 34S values, suggesting that although the granodiorite porphyry was the primary source of Zn-Pb mineralization in these three deposits, the carbonate strata contributed additional sulfur and metals to the BS and TSL systems. Meanwhile, sphalerite from the HSP deposit is characterized by elevated delta 114/110Cd, delta 34S, and Zn/Cd ratios, implying that hydrothermal circulation through basement rocks played a dominant role in metal and sulfur extraction, with evaporite layers providing heavy sulfur. Integrating these findings with previous petrological data, we propose that lower crustal melting alone was sufficient to generate Zn-Pb mineralization. Additional contributions from dehydrated and melted sub-ducted slabs, together with upper crustal melting, further led to the superposition of Cu and W-Sn mineralization in southern Hunan Province.

AimGenetic diversity and connectivity are crucial to informing the conservation strategy for deep-sea organisms, especially those threatened by deep-sea mining. The vent mussel Bathymodiolus septemdierum has an extraordinary trans-oceanic distribution range across the Indo-West Pacific including numerous sites eyed for exploitation. Here, we aimed to assess the connectivity of B. septemdierum across its entire range, for the first time.LocationHydrothermal vent fields on the Carlsberg Ridge (CR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), as well as those in the Mariana Trough and the Southwest Pacific.Taxon Bathymodiolus septemdierumMethodsCombining new and published data, we studied the genetic structure, historical demography and gene flow of B. septemdierum using three mitochondrial and two nuclear genes from a total of 444 individuals across 16 vent fields.ResultsBathymodiolus septemdierum populations are structured with four regional metapopulations including (1) the whole Western Pacific, (2) CR in the northern Indian Ocean, (3) vents on the SWIR and southern CIR (CIR + SWIR) and (4) Onnuri field on the northern CIR. IMa3 analysis shows divergence first occurred between WP and the Indian Ocean, and then between CR and CIR + SWIR metapopulations. The gene flow towards CR from both CIR + SWIR and WP is higher than the reverse directions, while Onnuri received exceptionally high migrations from both CIR + SWIR and CR metapopulations.Main ConclusionsOur findings support a large-scale dispersal scenario where larvae are carried westwards from the Pacific Ocean into the western Indian Ocean through Indonesia, and then separated into northern and southern Indian Ocean clades. The CR acts as an isolated gene pool for this species among others, and the Onnuri field serves as a key transition zone in the Indian Ocean, and we urge for the prioritised protection of these vents currently targeted by deep-sea mining.

Off-axis inactive hydrothermal fields may harbor significant sulfide deposits. However, their distribution and abundance remain poorly constrained due to the challenges in their detection, as they lack associated hydrothermal plumes. Here, we present the first assessment on the abundance of off-axis inactive fields along four segments of the ultraslow-spreading Southwest Indian Ridge, based on a systematic sediment geochemistry survey. We found distinct dispersion patterns of hydrothermal Cu and Zn between magmatic- and detachment fault- controlled hydrothermal fields. These patterns are likely attributed to multiple factors, including variable physicochemical conditions of the hydrothermal plume,seafloor topography and bottom current. Up to 15 potential fields and their approximate locations were identified by the dispersion distance of hydrothermal Zn, Cu, and Fe. Among these fields, 8-14 are located off-axis, and 3-7 are likely newly identified inactive fields. The latter may have been active within the last 10 ka, with more than 60% spatially associated with detachment faults. The number of inactive fields is at least 2 times greater than previously reported for active fields within 40 km of the axial zone of the studied segments. As the studied sediments represent only the last 10 ka, significantly more buried inactive hydrothermal fields would be expected to be identified over longer time intervals. These findings significantly revise previous estimates of sulfide resources on ultraslow-spreading ridges.

Many locations with concentrated hydrates at vents have confirmed the presence of abundant thermogenic gas in the middle of the Qiongdongnan Basin (QDNB). However, the impact of deep structures on gas-bearing fluids migration and gas hydrates distribution in tectonically inactive regions is still unclear. In this study, the authors apply high-resolution 3D seismic and logging while drilling (LWD) data from the middle of the QDNB to investigate the influence of deep-large faults on gas chimneys and preferred gas-escape pipes. The findings reveal the following: (1) Two significant deep-large faults, F1 and F2, developed on the edge of the Songnan Low Uplift, control the dominant migration of thermogenic hydrocarbons and determine the initial locations of gas chimneys. (2) The formation of gas chimneys is likely related to fault activation and reactivation. Gas chimney 1 is primarily arises from convergent fluid migration resulting from the intersection of the two faults, while the gas chimney 2 benefits from a steeper fault plane and shorter migration distance of fault F2. (3) Most gas-escape pipes are situated near the apex of the two faults. Their reactivations facilitate free gas flow into the GHSZ and contribute to the formation of fracture - filling hydrates.

The Indian Ocean ridges are shaped like the Greek letter lambda (i.e., lambda), and are up to approximately 18,000 km in length with various full spreading rates (<12-80 mm/yr) including the Carlsberg, Central, Southeast, and Southwest Indian Ridges. To date, in total of 15 active hydrothermal fields have been reported at these ridges with varying intensities of activity, but the distribution and morphology of their sulfide structures, which are formed by the venting of hot fluid in cold seawater, have not been well studied. In this paper, we review the published literature and compile descriptions of sulfide structure morphology in the active hydrothermal fields of Indian Ocean ridges by combining relevant video and images. These hydrothermal fields are hosted by ultramafic rocks, basalts, or both, but the host rock is not the sole factor controlling the morphology of sulfide structures. Some hydrothermal fields contain sulfide structures sharing similar features despite different host rocks. In addition, some hydrothermal fields have large sulfide structures indicating a relatively stable tectonic environment while others only have small and short structures indicating a weak supply of materials. Nevertheless, these characteristics alone are not sufficient to demonstrate significant differences in morphology between the sulfide structures in the Indian Ocean and at the slow-spreading Mid-Atlantic Ridge. The Longqi-1 field on the Southwest Indian Ridge is unique for hosting large flanges on sulfide edifices, which contrasts with most hydrothermal fields particularly in sediment-starved areas. Regardless of the level of activity, a long-term locally stable tectonic setting and a continuous heat supply are necessary for the formation of large sulfide structures. The various morphologies in these hydrothermal fields do not correspond strictly to the tectonic settings they're located, and some local conditions such as the various permeability of the subseafloor crust could be more directly influencing the morphology of the hydrothermal structures. This review can aid in evaluating Indian Ocean ridges for mineral resources and enhance understanding of the formation of hydrothermal fields on the seafloor.

In mid-ocean ridge (MOR) hydrothermal systems, the gold grade of sea-floor massive sulfides (SMSs) is neg-atively correlated with the spreading rate of the ridge. Previous investigations have addressed the distribu-tion of gold in sulfides from hydrothermal fields hosted by ultramafic rocks. In contrast, the gold enrichment mechanisms in sulfides from hydrothermal fields hosted by mafic rocks in ultraslow-spreading ridge environ-ments are less well constrained. The basalt-hosted Longqi hydrothermal field, located on the classic ultra-slow-spreading Southwest Indian Ridge, provides an opportunity to examine gold enrichment mechanisms in such an environment. Two ore-forming stages are identified in chimney fragments: anhydrite + barite + col-loidal/porous pyrite (Py1) + marcasite + fine-grained sphalerite (stage 1); euhedral-subhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite (stage 2). Py1 is usually overgrown by marcasite, which is in turn enclosed by Py2. Py2 coexists with coarse-grained sphalerite and chalcopyrite. Abundant native gold nanoparticles occur in Py1 or at the transition zone between Py1 and Py2. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis suggests that Py1 contains higher Mo, V, Sn, and Pb and lower As, Co/Ni, and Se/Tl values compared to Py2. In situ LA-multicollector (MC)-ICP-MS analyses show that Py1 has a higher mean delta 34S (7.1 parts per thousand) value than Py2 (6.6 parts per thousand). Sulfur primarily derives from MOR basalt (MORB) and seawater sulfate, of which the proportion of sulfur from seawater sulfate is between 20.5 and 47.6%. Textures, mineral assemblages, and trace element contents of sulfides indicate that the degree of mixing between hydrothermal fluids and seawater decreases as the chimney grows and is accompanied by a gradual increase in temperature. Based on data compiled from 41 hydrothermal fields hosted by basalt, the large range of sulfide delta 34S from slow- and ultraslow-spreading MORs may be attributed to the wide range of sulfur sources (e.g., leaching from MORB, thermochemical reduction of seawater sulfate, magma degassing, and bacterial activity), fluid-basalt interaction, and redox state (CH4/CO2 ratios). Prolonged fluid-basalt interaction and the type of chimneys, such as beehive chimneys, may lead to higher gold grades in hydrothermal fields. Moreover, low H2S content may be an important contributor to gold enrichment in basalt-hosted SMS deposits in ultraslow-spreading MOR environments.

Deep-sea mining magnifies the release of heavy metals into seawater through oxidative dissolution of seafloor massive sulfide (SMS). At present, there is little information about how the metals released into seawater might be affected by the mineral assemblages, seawater conditions, and solid percentages. Here, leaching experiments were carried out to examine the behavior of three sulfides from the Southwest Indian Ridge, under conditions that replicated deep and shallow seawater environments at three solid-liquid ratios. The results demonstrated that sphalerite dissolved rapidly, and the metals released in both experimental conditions were comparable, potentially reflecting galvanic interactions between the sulfide minerals. Large quantities of the released metals were removed from the solutions when hydrous ferric oxides formed, especially for shallow seawater conditions. A comparison of metal concentrations in the leachates with the baseline metal concentrations in natural seawater indicated that most of the released metals, when diluted with seawater, would not have widespread impacts on ecosystems. Based on the obtained unique oxidative dissolution properties of each SMS at variable solid-liquid ratios, targeted wastewater discharge treatments are proposed to minimize impacts from the dissolved metals. This study will support the development of robust guidelines for deep-sea mining activities.

Reaction of ultramafic rocks with seawater and subsequent serpentinization has been considered one of the most important factors controlling the formation of ultramafic-hosted seafloor massive sulfide (UM-SMS) deposits. However, the mineralization processes responsible for these deposits remain poorly understood, in particular because they are less abundant as compared with their basaltic counterparts. In this work, serpentinites with different alteration grades collected at the Tianzuo hydrothermal field (THF), Southwest Indian Ridge, were studied. Mineralogical and chemical analyses were performed in the secondary opaque minerals resulting from serpentinization to understand the role of this process during the formation of UM-SMS deposits. Our results show that these opaque minerals mainly consist of magnetite, hematite, pentlandite, and minor pyrite, suggestive of high but varying oxygen and sulfur fugacities. The hematite is characterized by an enrichment in Mg, Si, Ni, and Co as compared with magnetite. Pentlandite associated with hematite has elevated and consistent Ni contents as compared with that associated with magnetite. These results indicate that breakdown and decomposition of primary silicate and sulfide minerals during serpentinization has controlled the sources of ore-forming materials. Concentrations of Te are variable and show a positive correlation with Ni in pentlandite associated with magnetite or hematite, suggesting that gabbroic intrusions provided additional material to the hydrothermal system. Oxidation and sulfidation conditions are ideal for the formation of trisulfur ion S3− in THF, which can significantly improve the capability of hydrothermal fluids for leaching ore-forming metals from the wall rocks, promoting the formation of THF. In addition of reduced systems, hydrothermal fluids with high oxygen and sulfur fugacities triggered by extensive seawater infiltration can most likely also develop in ultramafic-hosted systems. These results suggest that the areas with well-developed fractures are promising candidates for further exploration of UM-SMS deposits along mid-oceanic ridges.

The Yuhuang hydrothermal field（YHF） is located between the Indomed and Gallieni fracture zones near the top of the off-axis slope on the south rift wall of Segment 29 on the ultraslow Southwest Indian Ridge（SWIR）. Previous studies have shown that sulfides in the YHF formed during different mineralization episodes and the YHF has the greatest potential for the formation of large-scale seafloor massive sulfide deposits. However, the sulfide chronology and hydrothermal activity of the YHF remain poorly constrained. In this study, mineralogical analyses and 230Th/U dating were performed. Hydrothermal activity may start about（35.9 ± 2.3） ka from the southwest part of the YHF and may cease about（708 ± 81） a ago from the northeast part of the YHF. The 74 nonzero chronological data from hydrothermal sulfide samples provide the first quantitative characterization of the spatial and temporal history along the SWIR. Hydrothermal activity in the SWIR has been relatively active over the past20 ka. In contrast, between 40 ka and 100 ka, hydrothermal activity was relatively infrequently and short in duration. The maximum activity occurred at 15–11 ka, 9–7 ka, 6–0.2 ka. There was a slight positive correlation between the maximal age and estimated surface area or estimated tonnage. The minimum mass accumulation rate of YHF is about 278 t/a, which is higher than most HFs related to ultramafic systems. The ultraslow spreading SWIR has the greatest potential to form large-scale seafloor massive sulfides（SMS） deposits. The results of this study provide new insights into the metallogenic mechanism of hydrothermal sulfides along ultraslow-spreading ridges.

本文报道来自西南印度洋超慢速洋脊49.6° E洋脊处的新鲜洋中脊玄武岩(MORB)玻璃样品的主微量元素和w(H2O).样品w(H2O)范围为0.31％～0.40％,高于其他洋脊一般的N-MORB样品.样品H2O/Ce的比值范围为543～613,显著高于大西洋、太平洋MORB和前人发表的邻近洋脊样品数据.根据熔融模型计算,地幔源区w(H2O)高于450×10-6,显著高于前人估计的亏损地幔源区(DMM)w(H2O)范围50×10-6～200×10-6.流体活动性元素和Nb之间的线性正相关关系,以及w(H2O)与不相容元素之间的相关关系表明,相对高的w(H2O)和H2O/Ce比值并非遭受混染了的蚀变产物造成的.相反,它们反映了源区地幔的特殊性,很有可能来自于源区中小尺度的古老地幔楔熔融残余.

The enrichment of precious elements including Au and Ag in submarine hydrothermal sulfide deposits attracts more and more attention. Previous studies indicate that Au and Ag combine distinctly different ligands under the physico-chemical conditions of submarine hydrothermal systems, therefore, the factors controlling their precipitation may be different. In general, silver mineralization has been much less studied than gold mineralization in submarine hydrothermal sulfide deposits. The Ag mineralization process and precipitation mechanism of submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges are still poorly constrained. In this study, we studied the occurrence and precipitation mechanism of Ag in sulfide deposits of Edmond hydrothermal field, located on the intermediate-spreading Central Indian Ridge (CIR). Three ore-forming stages were identified in chimney fragments collected in the Edmond hydrothermal field. The corresponding minerals are (stage I) anhydrite + barite + colloidal/porous pyrite (Py1) + fine-grained sphalerite; (stage II) marcasite; (stage III) euhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite. Py1, characterized by colloidal, porous, and micro-sized anhedral morphology, was usually overgrown by marcasite, which is, in turn, surrounded by euhedral-subhedral Py2, usually coexisting with coarsegrained sphalerite, chalcopyrite, and isocubanite. Compared to marcasite and Py2, only Py1 contains abundant native silver nanoparticles. LA-ICP-MS analyses suggest that Py1 contains higher Pb, Cu, Ag, Mn, Tl, Mo, Au, and lower Co as compared to Py2. In-situ LA-ICP-MS and nanoSIMS analyses indicate that Py1 with higher Ag contents (101-586 ppm) has larger variation in sulfur isotopic compositions (0.8 to 6.6%0) than those of Py2 with lower Ag contents (0.22-15.5 ppm; delta 34S values: 2.1 to 4.8%0 for Fe-rich samples and 0 to 2.8%0 for Zn-rich samples). Texture, mineral assemblage, pyrite trace element and sulfur isotopic compositions indicate a progressive decrease on the degree of fluid-seawater mixing as the temperature gradually increases during the growth of the chimneys in Edmond hydrothermal field. Phase diagram analysis indicates that the increase in pH and in particular cooling, due to the mixing of the hot hydrothermal fluid with cold seawater, can significantly decrease the solubility of Ag and be the effective mechanisms of silver precipitation. Our results suggest that mixing of hydrothermal fluid with seawater is the main Ag precipitation mechanism for submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges.

To date, discovered ultramafic‐hosted seafloor massive sulfide (UM–SMS) deposits are rare, remain poorly understood in terms of their genesis, and typically form along slow‐ and ultraslow‐spreading mid‐oceanic ridges. The Tianzuo hydrothermal field (THF) is the only UM–SMS deposit identified so far along the ultraslow‐spreading South‐west Indian Ridge (SWIR). This study presents new detailed mineralogical and S–C isotopic data for serpentinized peridotite, gabbro, serpentinite, and sulfide ore samples from the THF and to provide new insights into the processes that formed the deposit. Chromium‐reduced sulfur (CRS) and sulfate–sulfur (SS) in samples of serpentinized peridotite have positive δ 34 S values (up to 10.22‰ and 20.67‰, respectively). The CRS formed through thermochemical reduction of seawater sulfate during reactions between seawater and ultramafic rocks, whereas the SS most likely occurred as dissolved sulfate phases in seawater occupying the pore space of serpentinized peridotite after exposure at the seafloor. The serpentinite samples contain SS with δ 34 S values of 19.21‰–20.9‰, indicating infiltration by seawater that led to the deposition of seawater sulfate. The sulfide ores have acid‐volatile sulfur (AVS), CRS, and SS δ 34 S values of 10.99‰–12.07‰, 2.34‰–10.98‰, and −4.13‰ to 2.76‰, respectively, indicating that the AVS and CRS within the samples are dominated by sulfur reduced from seawater sulfate (e.g., anhydrite) and leached from the surrounding wall rocks, although the re‐oxidization of microbial‐reduced sulfur most likely generated the sulfate with low and negative δ 34 S values. The total carbon and total organic carbon δ 13 C values of the THF samples are negative and range from −21.99‰ to −0.34‰ and −26.7‰ to −21.02‰, respectively, further suggesting that the samples record microbial activity. Gabbro samples have strongly negative AVS (−10.69‰) and CRS (−7.2‰) δ 34 S values, indicating that microbial‐reduced sulfur derived from shallow levels in the crust was incorporated into the hydrothermal system that circulated through the deeper‐seated gabbroic units. Our results suggest that the ultramafic and gabbroic rocks, seawater, and microbial activity all provided sulfur to the THF, with well‐developed fractures and local magma supply probably being the key factors controlling the formation of this field and similar UM–SMS deposits elsewhere.

相比于快速和中速扩张洋中脊,慢速和超慢速扩张洋中脊热液区通常含有丰富的金属硫化物资源.近年来的研究表明大洋中脊的扩张速率与矿石中金的品位呈明显的负相关,即超慢速扩张洋中脊热液区矿石中金的含量高.前人对龙旂热液区的构造环境以及硫化物组合进行了详细研究,但是对龙旂热液区硫化物中贵金属金的赋存形式和沉淀机制研究较少.本文对西南印度洋龙旂热液区中的硫化物进行了精细的矿物结构和微量元素分析,并探讨了金的赋存形式和沉淀机制.龙旂热液区的硫化物主要以黄铁矿为主,其次是黄铜矿和闪锌矿,黄铜矿普遍出溶等轴古巴矿,此外还观察到了少量的针钠铁矾和自然金等矿物.根据矿物结构和形态,黄铁矿明显被划分为两期,一期黄铁矿(Py1)自形度低,呈细粒状或胶状,内部多孔洞;二期黄铁矿(Py2)自形度高,呈自形-半自形,且粒径较大.Py1往往存在于Py2内部或以包体的形式被Py2所包裹,Py2则与自形-半自形黄铜矿和闪锌矿等矿物共生.自然金主要存在于Py1的内部孔洞之中,少量存在于Py2以及Py2与其他硫化物之间.相比于Py2,Py1含有更高的Ni、Zn、Pb、Ba、Mn、V、Mg、U、Au、Ag、Cd元素含量,更低的Co、Se、As、Sb元素含量.在龙旂热液区的物理化学条件下,热液流体中金的主要存在形式为Au(HS),HS-浓度的降低和pH值升高均会促进金的沉淀.龙旂热液区早期热液流体与海水的混合造成热液流体pH值升高,而混合作用导致的热液流体温度降低会促使黄铁矿(Py1)的结晶,从而促使热液流体中HS-浓度的降低,热液流体pH值升高和黄铁矿结晶(Py1)引起的硫逸度降低是龙旂热液区自然金沉淀的主要机制.

The modern seafloor hydrothermal system plays a significant role in the cycling of energy and mass between the internal and external layers of the oceanic crust and upper mantle. It continues to supply hydrothermal fluids containing three to five orders of magnitude more gold into the ocean than the amount typically present in deep seawater. It has a considerable impact on the distribution and budget of gold with respect to the large geological inventory of the ocean along with other input sources such as river water. The large amount of various types of data compiled for this study reveals that only about 0.3% of the annual hydrothermal flux of gold (2618.3 kg/a) injected into the overlying seawater column as a dissolved phase is eventually trapped in sulfide deposits near vent sites on the seafloor, while about 0.8% is trapped in metalliferous sediments that fall out from the distal nonbuoyant plume. The remaining ~98.9% of gold is delivered into the depths of the global open ocean. The global budget of gold in seawater (about 1.4 × 107 kg), the annual flux of hydrothermal fluids at the seafloor (about 2.6 × 103 kg/a), the amount delivered by river water (about 7.2 × 104 kg/a), and significant estuarine removal (15%) allows us to estimate the residence time of gold in the modern ocean to be about 220 years. This value is 70% shorter than that (~1000 years) reported previously. In the future, the use of appropriate artificial means to achieve more efficient precipitation of gold from the hydrothermal system at the seafloor could increase the level of enrichment of gold to obtain gold-rich hydrothermal deposits, yielding greater economic benefits.

Hydrothermal activity in the mid-ocean ridge facilitates the chemical exchange of seawater with new oceanic crusts. This activity mostly occurs on the detachment fault of the asymmetric accretion segment in the slow-ultraslow spreading ridge, which is characterised by limited magma supply. Deep faults can readily extract heat from deeper heat sources. Moreover, the repeated movement of faults activates the permeable fluid channels of the overlying oceanic crust, thus driving long-life hydrothermal circulation. Recent studies have found that the response time of the hydrothermal activity of the intermediate-fast spreading ridges differs from that of the slow-spreading ridge to the glacial cycle, and a unified model is expected to explain it. Also, the response of hydrothermal activity to the glacial cycle must consider the differences between oceanic ridges with different spreading rates and types of hydrothermal systems.Here, based on two sediment cores collected near the Yuhuang hydrothermal field (HF)on ultraslow-spreading Southwest Indian ridge, we obtained high-resolution sediment history records spanning three glacial periods, understood the 160 ka history of hydrothermal, volcanic and tectonic activities in the region and attempted to reveal the response mechanism of hydrothermal activities controlled by detachment faults to the glacial cycle. We discovered that in the Yuhuang HF controlled by detachment faults, hydrothermal activity increased significantly during the glacial period, and more active detachment fault activity appeared at the same time. At the end of the glacial period, both activities are reduced at the same time. We believe that in the slow-ultraslow spreading ridge, the magmatism regulated by sea level changes may regulate the evolution of detachment faults and the hydrothermal circulation, which are recorded in the sediments near the hydrothermal field.We established a response model of Sea level change–Magmatism–Detachment fault activity–Hydrothermal activity and concluded that the magmatism of slow-ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycle.