西南印度洋中脊龙旂热液区中硫化物的结构和微量元素特征对金赋存形式和沉淀机制的启示

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。     

On the role of abiogenic factors in the bioaccumulation of heavy metals by the hydrothermal fauna of the Mid-Atlantic Ridge

The distributions of Fe, Mn, Zn, Cu, Ni, Co, Cr, Pb, As, Ag, Cd, Se, Sb, and Hg in 128 samples of tissues of the organisms that inhabit hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, and Rainbow) depending on the abiotic environmental parameters were studied. The majority of the elements studied showed direct correlations between their concentrations in the fluids released and in the tissues of hydrothermal organisms. A higher degree of bioaccumulation of metals was revealed in the Bathymodiolus mussels and Rimicaris shrimps from the Rainbow hydrothermal vent field as compared to their analogues from Menez Gwen and Snake Pit. This corresponds to the maximum concentrations of the majority of the metals studied in the Rainbow high-temperature hydrothermal fluids. The highest degree of bioaccumulation of heavy metals was found in gills of the symbiotrophic mussels Bathymodiolus and in maxillipeds of the ectosymbiotic shrimps Rimicaris, i.e., in the organs that function in dependence on chemosynthetic bacteria. Within the Rainbow vent field, the shrimps, which inhabit in biotopes with more high-temperature conditions and therefore are more strongly subjected to the influence of fluids, were found to contain higher metal contents than mollusks. The Fe-Mn hydroxide films that cover mussel shells might serve as important reservoirs of other metals related to Fe and Mn.     

太平洋中脊热液硫化物地球化学特征

对东太平洋海隆9°~10°N(9°50.01'N, 104°17.44'W,水深2 200 m)热液硫化物的主要组成元素测试资料研究表明,硫化物具有高Ca、Cu、Co、Se含量,低Pb、Mn、As、Cd、Ba和Au含量的特点,为富Cu型热液硫化物。热液硫化物中Cu与Co、Se、Ag、Cd等元素呈显著的正相关关系(R2>0.8),而与Ca、Ba、Mn、Pb等元素呈负相关关系(R2>0.8)。热液硫化物烟囱体中不同圈层化学成分的变化较大,稀土元素(REE)的含量从内层到外层逐渐变高,可以反映出硫化物生长过程中的不均一性和矿化条件的不同。热液硫化物中REE含量较低,具有明显的Ce负异常和LREE富集的特征,与海水的REE配分曲线基本一致,表明热液硫化物中的REE主要来源于海水。     

西南印度洋脊龙旂热液场金属硫化物的矿物学组成及指示意义

西南印度洋脊(SWIR)是当前洋底超慢速扩张洋脊的典型代表, 具有独特的热液硫化物矿床形成机理, 对位于该洋脊的龙旂热液场硫化物样品进行了系统的矿物学分析, 结果表明:该区硫化物为高温热液喷溢活动所形成的富Fe型硫化物, 目前已经历了一定程度的氧化蚀变; 硫化物矿物组合以磁黄铁矿、黄铁矿为主, 其次是少量黄铜矿、白铁矿和(铁)闪锌矿; 黄铜矿出溶等轴古巴矿现象普遍, 部分样品中可见自然金颗粒。经综合分析, 该区热液成矿作用可划分为3个成矿阶段和1个后期海底风化阶段: (1)高温的黄铁矿+黄铜矿(等轴古巴矿) +磁黄铁矿阶段, (2)中高温的黄铁矿+闪锌矿阶段, (3)低温的胶状或莓球状黄铁矿+白铁矿+自然金阶段, (4)后期硫化物海底氧化性蚀变阶段主要是形成Fe的氧/羟化物。在整个成矿期间, 流体温度有不同程度的波动, 主要硫化物矿物形成时端元流体的温度应在335℃以上, 瞬间(短时)或局部热液的最高温度推测超过400℃。本区的磁黄铁矿属于富钴型磁黄铁矿亚类, 经历了六方磁黄铁矿+黄铁矿→单斜磁黄铁矿+黄铁矿的变化, 表明该区热液流体发生了快速降温的演化过程。     

Geochemistry of pyrite and chalcopyrite from an active black smoker in 49.6°E Southwest Indian Ridge

Active hydrothermal chimneys, as the product of submarine hydrothermal activity, can be used to determine the fluid evolution and formation process of potential volcanic-hosted massive sulfide deposits. A hard-won specimen from an active hydrothermal chimney was collected in the 49.6°E ultraslow-spreading Southwest Indian Ridge (SWIR) field through a television-guided grab. A geochemical study of prominent sulfide (e.g., pyrite and chalcopyrite) included in this sample was performed using laser ablation inductively coupled plasma mass spectroscopy. The early sulfides produced at low temperature are of disseminated fine-grained anhedral morphology, whereas the late ones with massive, coarse euhedral features precipitated in a high-temperature setting. The systematic variations in the contents of minor and trace elements are apparently related to the crystallization sequence, as well as to texture. Micro-disseminated anhedral sulfides rich in Pb, As, Ni, Ba, Mn, Mo, U, and V were formed during the initial chimney wall growth, whereas those rich in Sn, Se, and Co with massive, coarse euhedral morphology were formed within the late metallogenic stage. The hydrothermal fluid composition has experienced a great change during the chimney growth. Such a conclusion is consistent with that indicated by using principal component analysis, which is a powerful statistical analysis method widely used to project multidimensional datasets (e.g., element contents in different mineral phases) into a few directions. This distribution pattern points to crystallographic controls on minor and trace element uptake during chimney growth, occurring with concomitant variations in the fluid composition evolutionary history. In this pyrite-chalcopyrite-bearing active hydrothermal chimney at the SWIR, the metal concentration and precipitation of sulfides largely occurred at the seafloor as a result of mixing between the upwelling hot hydrothermal fluid and cold seawater. Over the course of mixing, significant variations in metal solubility were caused by changes in temperature, pH, and redox conditions in the parental fluid phase.     

Metal and metalloid concentrations in the giant squid Architeuthis dux from Iberian waters

This study investigated 14 trace elements (Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, V and Zn) in the tissues of the giant squid Architeuthis dux from the Mediterranean and Atlantic Spanish waters. As for other families of cephalopods, the digestive gland and the branchial hearts of Architeuthis showed the highest concentrations of Ag, Cd, Co, Cu, Fe, Ni, Se, V and Zn, highlighting their major role in the bioaccumulation and detoxification processes. With the exception of Hg, the muscles showed relatively low trace element concentrations. Nevertheless, this tissue contained the main proportion of the total As, Cr, Hg, Mn, Ni, and Zn body burden because muscles represent the main proportion of the squid mass. These findings suggest that the metal metabolism is overall the same as other cephalopod families from neritic waters. In females, Zn concentrations increased in the digestive gland with the squid's weight likely reflecting physiological changes during sexual maturation. Comparing the trace element concentrations in the tissues of Architeuthis, higher Ag, Cu, Hg and Zn concentrations in the squid from the Mediterranean reflected different exposure conditions. In comparison to other meso-pelagic squids from the Bay of Biscay, Cd concentrations recorded in the digestive gland suggest that Architeuthis might feed on more contaminated prey or that it displays a longer life span that other cephalopods.     

西南印度洋脊龙旂热液区蚀变岩岩石学特征及对热液流体循环的指示

慢速−超慢速扩张洋脊的海底热液活动区多出露类型多样的蚀变岩石,记录了地壳深部的流体与围岩的相互作用,为研究深部热液流体特征以及循环过程提供了样本。本研究选取了中国大洋第30、34和40航次在超慢速扩张西南印度洋脊龙旂热液区(A区、B区和C区)利用电视抓斗采集的蚀变玄武岩、蚀变辉长岩、蚀变辉石岩和蛇纹岩等蚀变岩样品,利用光学显微镜、电子探针开展了岩相学和矿物化学分析。岩相学结果表明,龙旂热液区蚀变岩石样品约95%发生了地壳浅部的脆性变形作用,靠近龙旂1号热液区(A区)约有5%的蚀变岩石混合发育了脆性变形及脆性−塑性变形特征。研究区岩石蚀变属于中−低温变质作用,变质相近似绿片岩相,变质矿物组合为绿泥石−绿帘石−钠长石−阳起石−榍石。其中,A区的蚀变岩中的绿泥石形成温度(201～341℃)以及蛇纹石、阳起石、绿泥石等蚀变矿物的Fe元素含量(17.5%～27.5%)都高于龙旂3号热液区(B区和C区)的绿泥石形成温度(239～303℃)和Fe元素含量(16.8%～26.5%),这也与在该区观测到高温的热液喷口相符合。本研究认为龙旂热液区所在洋脊段发育的拆离断层为热液流体的向上运移提供了通道,洋壳扩张后期轴部的岩浆熔体在轴侧区域的岩浆侵入或喷发活动可能为热液循环提供了热源。     

Geochemical features of heavy metal bioaccumulation in the Guaymas Basin of the Gulf of California

Atomic absorption spectroscopy (flame and graphite furnace techniques) and instrumental neutron activation analysis were used for determining the Fe, Mn, Zn, Cu, Pb, Cd, Ag, Co, Cr, As, Se, Sb, Ba, Au, and Hg contents in 25 samples of different tissues and whole organisms inhabiting the southern trough in the Guaymas Basin (Gulf of California) and in several samples of its bottom waters. It is shown that the habitat environment of this hydrothermal field with high primary production of both photosynthetic and bacterial chemosynthetic origin influences the Fe and Mn ratios in the waters of the microbiotopes and the distribution patterns of the metals in the external and internal organs of the benthic animals. In the dominant specialized taxa, the maximal bioaccumulation of metals is registered both in the organs related to bacterial chemosynthesis such as the trophosome of Vestimentifera Riftia pachyptila and the gills of the vesicomyid clam Archivestica gigas and in other organs. The other organisms such as the mollusks Nuculana grasslei, actinias Paraphelliactis pabista, Actinaria, and Spongia and the crabs Munidopsis alvisca demonstrate high bioaccumulation properties as well. The metal concentration coefficient is highly variable ranging from 10 to 10⁴. The changes in the molar Fe/Mn ratio values imply the partitioning of these two metals in the following migration succession: microbiotope water-external organs-internal organs.     

Specific features of the distribution of heavy metals in bottom sediments in the Northeast part of the Black Sea

We study the distribution of Fe, Mn, Co, Ni, Cu, Zn, Cd, Hg, Ag, and Au in present-day and ancient bottom sediments in the shelf, slope, and deep-trench areas of the northeast part of the Black Sea. The samples were taken in 1990 in the Crimean, Kerch-Taman, and Caucasian regions. The revealed character of the fields of concentration of metals does not allow one to establish exact regularities in the distribution of Mn, Co, and Cu with Fe. It is shown that the values of the ratios of concentrations of metals increase in the following direction: Au/Cu → Zn/Cu → Co/Fe → Ni/Fe → Mn/Fe → Hg/Cu → Ag/Cu → Cu/Fe. The data on Ag are presented for the first time.     

Mineralogy,geochemistry, and sulfur isotope characteristics of sediment-hosted hydrothermal sulfide minerals from the southern Okinawa Trough

Studies of the mineralogy and sulfur isotope composition of sediment-hosted hydrothermal sulfide minerals in cores are important for understanding the seafloor mineralization environment and material source and reconstructing the hydrothermal history. However, the source of ore-forming materials and the history of hydrothermal activity in the southern Okinawa Trough (SOT) remain unclear. Here, the mineralogy and sulfur isotope characteristics of sulfides from gravity core HOBAB4-S2, collected between the Yonaguni Knoll IV hydrothermal field (HF) and the Tangyin HF, was investigated. Enrichments in Zn (up to 321×10?6), Cu (up to 73.7×10?6), and Pb (up to 160×10?6) and the presence of pyrite, galena, pyrrhotite and minor sphalerite and chalcopyrite provide evidence for the input of hydrothermal materials into the sediments. The pyrite morphologies include disseminated cubic, pentagonal dodecahedron, and framboidal forms. Except for minor framboidal pyrite, euhedral pyrite from core HOBAB4-S2 is mainly of hydrothermal origin with Co/Ni ratios >1 and S/Fe atomic ratios <2 in the Cu-Zn-Pb-rich layers. The occurrences of hexagonal pyrrhotite, high-Co (up to 0.17%) pyrite and high-Fe sphalerite indicate that the hydrothermal precipitates formed at medium-high temperatures and low-sulfur fugacity (fS₂) environments. The δ34S values of sulfides (0.21‰–3.45‰) with low- fS₂ mineral assemblages (e.g., pyrrhotite ± high-Fe sphalerite) in the core are significantly lower than those of magmatic rocks and seawater, indicating possible incorporation of previously formed biogenic sulfur in the sediment. Combined with the age model of the core, it is suggested that hydrothermal activity likely began in the Tangyin HF before AD 1445–1483 and that at least three active episodes may have occurred since then.     

Delivery of chemical elements with atmospheric precipitations to the coastal regions of the Crimea in 2004–2008

We study the space-and-time variations of the delivery of fluxes of 26 elements (Na, K, Rb, Cs, Cu, Ag, Au, Ca, Sr, Ba, Zn, Cd, Hg, Hf, Ta, As, Sb, Cr, Se, Mo, W, Mn, Br, Fe, Co and Ni) with atmospheric precipitations to the sea surface in the regions of Sevastopol and the South Coast of Crimea. It is established A series of general regularities of the delivery of the sum of these elements and their soluble and insoluble forms. We also give the characteristics of their space-time variability.     

Variability of the fields of concentration of microelements and strontium in seawater near the South Coast of the Crimea in 2002–2007

We study the space-and-time variability of the concentrations of 27 microelements (Sc, Cr, Mn, Fe, Ni, Co, Cu, Zn, As, Mo, Ag, Cd, Sn, Se, Sb, Ba, La, Ce, Sm, Eu, Tb, Hf, Hg, Pb, Au, Th, and U) and Sr in the sea, river, and waste waters of the South Coast of Crimea in 2002–2007. Parallel with the data about elements studied earlier, we present, for the first time, the data of investigations of the contents of rareearth elements (Lu, Nd and Yb) whose concentrations in the major part of the analyzed samples of seawater were lower than the limit of their quantitative determination. We present the ranges of the concentrations of elements and perform their comparison with the data of the previous period of monitoring.     

Trace metal concentrations in sediments of the Eastern Mississippi Bight

Sediments collected from 3 onshore-offshore transects (12 stations total) in the Eastern Mississippi Bight at 5 different times during 1987–1988 have been analyzed for total Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Sn, Th and Zn. The data show considerable variation both spatially and temporally, largely as a result of natural variability in grain size and mineralogy. Clay-rich samples from deep water were always more metal-rich than sandy samples from shallow areas. Many samples from near the Mississippi River Delta appeared to be enriched in Ba by about a factor of two, probably as a result of contamination from oil well drilling mud, but there were few other indications of pollution influences. Many samples from throughout the area gave Mn/Fe and Cd/Fe ratios a factor of two or more lower than their Mississippi River source material. This shows that biochemical activity in the sediment is capable of solubilizing reducible and adsorbed metals.     

The geochemistry of deepwater particulate matter over the hydrothermal field at 9°d50′ N (the East Pacific Rise)

The results of geochemical studies of particulate matter in the water mass over the hydrothermal field at 9°50′ N on the East Pacific Rise are presented. The particulate matter was tested in background waters, in the buoyant plume, and in the near-bottom waters. The contents of Si, Al, P, C_org, Fe, Mn, Cu, Zn, Ni, Co, As, Cr, Cd, Pb, Ag, and Hg were determined. No definite correlations were found between the ele-ments in the background waters. Many of the chemical elements correlated with Fe and associated with its oxyhydroxides in the buoyant plume. In the near-bottom waters, microelements are associated with Fe, Zn, and Cu (probably, to their sulfides formed under fluid mixing with seawater). The matter precipitated in a sed-imentation trap was similar to the near-bottom particulate matter in the elemental composition.     

Trace elements in ferromanganese nodules from the central indian ocean basin

Thirty-three bulk ferromanganese nodules from the sediment–water interface of siliceous sediment domain from the Central Indian Ocean Basin were analyzed for 50 elements including 6 new (Be, As, Se, Sn, Sb, and Bi) using inductively coupled plasma–mass spectrometer. The Mn/Fe ratio and triangular plot (Fe-Mn-{Cu+Ni+Co?×?10}) suggest that ferromanganese nodules are of hydrogenetic, early diagenetic, and diagenetic origin. In the ferromanganese nodules, Mo, Sb, Bi, and As are highly enriched ～320, 160, 90, and 50 times compared with upper continental crust, respectively. A majority of the elements such as Be, Sc, Ti, V, Co, As, Se, Sr, Y, Zr, Nb, Sn, rare earth elements (REEs), Pb, Bi, P, Th, U, Hf, and Ta are associated with Fe, whereas, Cu, Ni, Zn, Mo, Li, Ga, Sb, Mg, and Cs are associated with Mn in the ferromanganese nodules. Redox proxies such as U/Th (0.14) and Mo/Mn (0.0019) ratio in the ferromanganese nodules suggest their formation under oxic conditions.     

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O₂ within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer.The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.     

台湾北部四种软珊瑚体内十八种金属累积于共肉组织与中轴骨骼的特性

分别检测采自台湾北部和平岛沿岸海域及淡水河出海口海域的侧扁软柳珊瑚(Subergorgia suberosa)、扁刺柳珊瑚(Echinogorgia complexa)、网刺柳珊瑚(Echinogorgia reticulata)和1未定种柳珊瑚4种软珊瑚样品体内18种金属元素。比较同株珊瑚中金属累积于共内组织与中轴骨骼含量的特性．研究结果显示铜(Cu)及钒(V)于4种珊瑚中累积于中轴骨骼的量高于共内组织，Zn、Fe、Mn、Ca、Cd、Ni、Pb及Se则恰相反，Al、As、Co、Cr及Sn累积部位没有规则，Ag,Au及Ba因有ND值(未检出值)无法归纳其累积倾向，但若忽略ND值，则Ag与Ba累积于共肉组织较高，Au累积于中轴骨骼较高．此研究结果显示环境中金属倾向累积于软珊瑚共肉组织或中轴骨骼可能有一定之脉络可循．     

Geochemistry and stable isotope constraints on high-temperature activity from sediment cores of the Saldanha hydrothermal field

The Saldanha hydrothermal field is hosted atop a mafic–ultramafic seamount, located at a non-transform offset on the Mid-Atlantic Ridge. Previous observations revealed a field where transparent low-temperature fluids discharge through centimeter-sized vents without the formation of chimney structures. We present geochemical and stable isotope (O and C) analyses from sediment samples collected at this field, both at and far from the vent area. Most sediments, including some directly adjacent to orifice vents, are pelagic oozes with only a weak hydrothermal overprinting. Hydrothermal precipitates are characterized by Fe–Mn oxyhydroxides and a minor amount of Cu–Zn sulphide minerals. However, one of the cores (SCD7) collected at the vent area shows a much stronger hydrothermal signature. This core is composed of a matrix of serpentine + talc ± chlorite with high porosity, where calcite + chalcopyrite + sphalerite/wurtzite ± pyrite–pyrrhotite were precipitated. In this core, metal enrichments, REE patterns, and the oxygen and carbon isotope composition of calcites indicate that mineralization must have occurred in the subsurface by high-temperature fluids, with minor mixing with seawater and with a significant magmatic contribution. Thus, while most samples confirm previous findings indicating that Saldanha hydrothermal fluid discharge is mainly diffuse and of low temperature, data from core SCD7 suggest that areas of high-temperature hydrothermal activity also occur, where temperatures of the fluids could reach > 260 °C and maximum temperatures of 330 °C. We suggest that fluids can flow through faults at the top of the mount and discharge in a more focused way through vent orifices, producing intense hydrothermal alteration of the sediments. At these locations complex hydrothermal processes occur, including reactions of the hydrothermal fluids with mafic and ultramafic rocks and magma degassing, as suggested by the carbon isotope composition of hydrothermal calcites. The high temperature of the fluid inferred from the geochemistry of the hydrothermal minerals requires a significant heat input to the system, suggesting an additional magmatic heat source to the already proposed exothermic serpentinization reactions.     

Geological characteristics of the Qiaoyue Seamount and associated ultramafic-hosted seafloor hydrothermal system (～52.1°E,Southwest Indian Ridge)

Hydrothermal vent incidence was once thought to be proportional to the spreading rate of the mid-ocean ridges (MORs). However, more and more studies have shown that the ultraslow-spreading ridges (e.g., Southwest Indian Ridge (SWIR)) have a relatively higher incidence of hydrothermal venting fields. The Qiaoyue Seamount (52.1°E) is located at the southern side of segment #25 of the SWIR, to the west of the Gallieni transform fault. The Chinese Dayang cruises conducted eight preliminary deep-towed surveys of hydrothermal activity in the area during 2009 and 2018. Here, through comprehensive analyses of the video and photos obtained by the deep-towed platforms, rock samples, and water column turbidity anomalies, a high-temperature, ultramafic-hosted hydrothermal system is predicted on the northern flank of the Qiaoyue Seamount. We propose that this hydrothermal system is most likely to be driven by gabboric intrusions. Efficient hydrothermal circulation channels appear against a backdrop of high rock permeability related to the detachment fault.     

Metalliferous Sediment and a Silica-Hematite Deposit within the Blanco Fracture Zone,Northeast Pacific

A Tiburon ROV dive within the East Blanco Depression (EBD) increased the mapped extent of a known hydrothermal field by an order of magnitude. In addition, a unique opal-CT (cristobalite-tridymite)-hematite mound was discovered, and mineralized sediments and rock were collected and analyzed. Silica-hematite mounds have not previously been found on the deep ocean floor. The light-weight rock of the porous mound consists predominantly of opal-CT and hematite filaments, rods, and strands, and averages 77.8% SiO₂ and 11.8% Fe₂O₃. The hematite and opal-CT precipitated from a low-temperature (≥ 115° C), strongly oxidized, silica- and iron-rich, sulfur-poor hydrothermal fluid; a bacterial mat provided the framework for precipitation.

Samples collected from a volcaniclastic rock outcrop consist primarily of quartz with lesser plagioclase, smectite, pyroxene, and sulfides; SiO₂ content averages 72.5%. Formation of these quartz-rich samples is best explained by cooling in an up-flow zone of silica-rich hydrothermal fluids within a low permeability system. Opal-A, opal-CT, and quartz mineralization found in different places within the EBD hydrothermal field likely reflects decreasing silica saturation and increasing temperature of the mineralizing fluid with increasing silica crystallinity.

Six push cores recovered gravel, coarse sand, and mud mineralized variously by Fe or Mn oxides, silica, and sulfides. Total rare-earth element concentrations are low for both the rock and push core samples. Ce and Eu anomalies reflect high and low temperature hydrothermal components and detrital phases.

A remarkable variety of types of mineralization occur within the EBD field, yet a consistent suite of elements is enriched (relative to basalt and unmineralized cores) in all samples analyzed: Ag, Au, S, Mo, Hg, As, Sb, Sr, and U; most samples are also enriched in Cu, Pb, Cd, and Zn. On the basis of these element enrichments, the EBD hydrothermal field might best be described as a base- and precious-metal-bearing, silica-Fe-oxide-barite deposit. Such deposits are commonly spatially and temporally associated with volcanogenic massive sulfide (VMS) ores. A plot of data for pathfinder elements shows a large hot spot at the northwestern margin of the field, which may mark a region where moderate to high temperature sulfide deposits are forming at depth; further exploration of the hydrothermal field to the northwest is warranted.