东太平洋海隆9°～10°N热液烟囱体矿物成分、结构和形成条件

为研究东太平洋海隆9°～10°N热液活动特征,采用成因矿物学方法,通过矿相显微镜、扫描电镜、X射线衍射分析以及电子探针等手段,对烟囱体矿物成分、结构和地球化学特征进行了研究.该区烟囱体硫化物矿物有3种矿物组合:(1)硬石膏 白铁矿 黄铁矿;(2)黄铁矿 闪锌矿 黄铜矿;(3)黄铜矿 斑铜矿 蓝辉铜矿 铜蓝.成矿热液流体温度经历了低-高-低的变化,最高温度可达到400℃以上.该热液烟囱为典型的"黑烟囱"类型,早期硬石膏沉淀形成烟囱体的框架,后期的金属硫化物在烟囱体内表面沉淀,由烟囱壁向内形成了硬石膏-黄铁矿、多金属硫化物和黄铜矿及次生铜矿物的矿物分带.     

现代海底烟囱中流体包裹体的研究

大洋中脊的海底块状硫化物矿床是当前地球科学的一个热点。因为他不仅具有经济效益,而且可以直接观测到这些矿床的形成过程。这些矿床均有许多黑烟囱和白烟囱。在这些烟囱中成矿流体不断地流过,并同时沉淀出Cu,Pb,Zn硫化物和脉石矿物。本研究的样品是取自北纬21°N的太平洋洋脊中现代大洋海底的Zn(Cu)型硫化物烟囱。对这些烟囱的矿物共生组合进行研究发现其矿物组合相当简单,分硫化物和脉石矿物两种。硫化物中以其含量多少排列为闪锌矿,黄铁矿,黄铜矿和磁黄铁矿。脉石矿物以硬石膏为主,见少量重晶石。在烟囱中这些硫化物和脉石矿物的分布是有分带性的。从外到内为硬石膏带,黄铁矿带,闪锌矿带。闪锌矿带向里可见到少量黄铜矿和磁黄铜矿,及硬石膏,但它们尚未构成一个带。烟囱的中心常常是空的,因为是原来的热液通道。 发现两类流体包裹体:一类为水溶液相包裹体。另一类为CH_4的包裹体。其均一温度是从180～265℃。最外面的硬石膏带,均一温度从180～245℃(平均212℃)。黄铁矿带均一温度从185～260℃(平均218℃)。闪锌矿带均一温度从190～265℃(平均229℃)。从平均的均一温度看似乎从边上到中心从212—218—229℃向上升,说明一个趋势,中间的温度较高,边上的较低。成矿溶液的盐度与海水相差不大,但略大於海水     

中印度洋海岭Edmond热液区块状硫化物中自然金的发现及其意义

位于中印度洋中速扩张洋脊的Edmond热液区块状硫化物矿石样品主要分为以黄铁矿-黄铜矿为主的富Fe块状硫化物、热水沉积成因的富含硅质块状矿石和以硬石膏为主的硫酸盐矿石等3种不同类型.通过扫描电镜观察和X射线光电子能谱分析,在硫酸盐矿石和富Fe块状硫化物中首次发现了自然金,最大粒径可达20 μm左右,主要呈不规则粒状或板状与硬石膏、闪锌矿等硫化物颗粒紧密共生,少量以次显微金形式沉淀在自形黄铁矿晶体表面.电子探针分析结果显示,晚期形成于中低温条件下的贫Fe闪锌矿中Au富集程度普遍较高(平均含量约为6700×10-6);Ag主要以类质同象形式赋存于与闪锌矿、黄铜矿伴生的硫盐矿物中(5.0％～6.7％ Ag),这表明贵金属元素的富集成矿作用与海底热液活动晚期的中低温成矿阶段有关.推断Au在该研究区以高温、酸性和氯度较高为特征的热液流体中主要呈AuCl2-或AuHS0形式迁移.而海水与热液流体混合、喷口流体发生相分离以及传导冷却作用,被认为是导致Au有效沉淀的重要控制因素.     

金在现代海底热液系统中的分布及演化

现代海底热液系统内冷海水的下渗和热液流体的上涌及由此引发的水岩反应驱动着金的循环演化,并可在海底形成极具经济前景的富金矿床。海底下覆基岩深部层位中的金会因为水岩反应而大量溶解迁移到热液流体中,金含量可比海水高千倍的热液流体在向海底表面运移过程中发生的相分离作用会进一步富集金,当遭遇冷海水发生混合作用后引发H_2S浓度和温度的降低,由此导致金因溶解度降低而从热液流体中迁移出来并赋存在一同形成的硫化物矿物中。海底硫化物中金含量的分布受到围岩性质和构造环境的控制,形成在岛弧和弧后环境中的硫化物一般比洋中脊环境中的硫化物的金含量高,而在相似构造环境下形成在长英质围岩系统和超镁铁质围岩系统中的硫化物其金含量比镁铁质围岩系统中的高。相对于高温阶段形成的黄铜矿,低温成因的黄铁矿和闪锌矿普遍更富集金。在分布最广的黄铁矿的晶格内,固溶体态金的溶解度受到砷含量的控制,当超过矿物的溶解度时则会出现纳米到微米级金颗粒聚合体。虽然热液硫化物中的金含量比热液沉积物高的多,但由于受其捕获效率的制约,随热液流体运移到海底表面的金还是有相当一部分最终随着羽状流扩散到了远端沉积物中或被海洋水体所接纳。若要更清晰甚至定量化地厘清金在现代海底热液系统中的迁移演化过程及控制因素,则微区、原位和高精度的实验方法、分析技术和模拟研究将是下一步工作的重点。     

水热高压釜内流体温度的直接测量

温度是影响地质作用过程中元素活化、迁移和沉淀的一个重要因素,因此在高温高压水热实验模拟研究中,体系温度的准确测量和精确控制至关重要.此外,在海底热液与海水的混合过程中,温度的变化控制着体系的生物系统和硫化物矿物的沉淀,因此,温度的直接测量也成为海底热液探测的首要任务.     

现代海底热液过程及成矿

原始的海水成分、基岩的组分及结构、热源性质等因素决定着现代海底热液喷口系统的流体成分, 同时, 各种地质构造背景下的岩浆脱气作用也在不同程度上影响热液流体的组成.热液流体一旦喷出海底, 就能形成不同类型的热液沉积体, 包括高温流体形成的金属硫化物或硫酸盐烟囱体、热液丘以及由低温弥散流及非浮力羽流形成的含金属沉积物堆积体.高温烟囱体的形成受控于海水和热液的混合比例, 常常表现为典型的两阶段模式, 即先形成环状硬石膏表层, 然后在其内部发生富Cu硫化物的沉淀.这一模式在更大尺度上也可以观察到, 如TAG热液丘.含金属沉积物遍布海底, 除热液羽流外, 金属硫化物烟囱体在氧化环境中氧化蚀变的产物也是其重要来源.生物的活动贯穿于现代热液过程的始终, 并在烟囱体的形成、分解以及羽流的扩散沉淀过程中起到了重要作用.当前, 热液生物矿化机理、Lost City型热液场以及超慢速扩张洋脊的有关研究是海底这一系统研究的热点, 前两者研究能使人们更好地理解地球早期的演化和生命的起源, 而后者的考察和研究能进一步丰富海底热液成矿理论, 并有助于寻找更大规模的热液矿体.      

大西洋洋中脊TAG热液区中块状硫化物的Os同位素研究

新测得TAG热液区中5件海底块状硫化物样品的锇含量及其同位素组成,187Os/186Os比值在2.305～7.879之间,均值为5.986,介于现代海水和上部洋壳岩石的锇同位素组成之间,表明该区海底块状硫化物中锇是海水和上部洋壳来源锇混合的产物.在海底热液循环过程中,海水的混入对该区热液流体的Os浓度及其同位素组成产生了明显的影响。     

庐枞盆地泥河铁矿床中硬石膏微量元素特征、沉淀机制及其对铁成矿作用的制约

泥河铁矿床是长江中下游成矿带庐枞火山岩盆地中典型的磁铁矿-磷灰石型铁矿床。硬石膏是矿床中的主要脉石矿物,在矿床的各个阶段均有发育。本文在详细的野外地质工作和岩相学观察基础上,系统的对矿床各成矿阶段硬石膏开展LA-ICP-MS分析测试工作对硬石膏形成机制进行了分析,初步探讨了硬石膏在铁成矿过程中的作用及意义。矿床中的硬石膏分为三类:与透辉石、磁铁矿、黄铁矿、磷灰石等矿物共生的紫色板状硬石膏(Type Ⅰ);与黄铁矿共生的白色板状硬石膏(Type Ⅱ)以及呈独立脉状产出的白色糖粒状硬石膏(Type Ⅲ)。Type Ⅱ和Type Ⅲ硬石膏REE含量远低于Type Ⅰ硬石膏,Type Ⅱ和Type Ⅲ硬石膏的稀土配分模式相似,二者与Type Ⅰ硬石膏具有很大差别。泥河铁矿床硬石膏中REE含量受结晶学因素的影响较小,主要影响因素为温度、流体的演化过程以及络合物阴离子的类型和数量。早期高温热液中REE主要以Cl络合物形式迁移,矿床中较早沉淀的富稀土矿物捕获了大量REE,使体系中REE含量大幅度减少,致使Type Ⅱ和Type Ⅲ硬石膏稀土含量偏低。晚期流体盐度的降低导致了Cl-/SO42-比值变低,热液中稀土元素的减少以及络合物配位体的改变导致了Type Ⅱ和Type Ⅲ硬石膏稀土配分模式趋于平滑。此外Type Ⅲ硬石膏个别点显示出轻稀土富集的特征,说明热液晚期可能存在硬石膏的溶解再沉淀过程,该过程会改变硬石膏中的稀土配分模式。硬石膏的加入可以提高体系氧逸度,提高热液中铁元素的形成磁铁矿的比率从而利于形成大规模铁矿体。     

层控铅锌矿中两类矿化的成因及相互关系——以陕南八方山为例

陕西八方山铅锌矿床存在两类密切共生的矿化:富含铅锌的整合层状矿和富含黄铜矿的网脉矿.两种矿化均由海底热液活动一次形成.前者代表矿质沉淀于海底,后者代表矿液补给带.成矿热液来自海水,于同沉积断裂中经循环加热而成.     

安徽铜陵冬瓜山矿床矿石硫化物环带及地质意义

安徽冬瓜山矿床是铜陵矿集区内一个重要的大型铜(金)矿床,颇受关注,成因认识分歧较大。矿床内块状硫化物矿石中普遍发育以黄铁矿为核部、黄铜矿为中间带、磁黄铁矿为边部带的硫化物环带。这些环带核部黄铁矿多呈自形-半自形晶,黄铜矿呈他形晶围绕黄铁矿沉淀,磁黄铁矿呈他形分布在黄铜矿外围,内带常被外带硫化物溶蚀交代。硫同位素分析结果显示,环带中硫化物矿物的硫同位素(δ34S=1.6‰~5.1‰)具有岩浆硫源特征,同时从核部黄铁矿到中间带黄铜矿,再到边部磁黄铁矿δ34S值逐渐降低。以上特征表明环带从内到外硫化物之间并非平衡共生关系,而是黄铁矿、黄铜矿和磁黄铁矿先后依次晶出。硫化物环带核部粗粒黄铁矿(粒径大于1.5cm)的Co、Ni含量分别为292×10-6~1504×10-6和32.7×10-6~39.9×10-6,Co/Ni=7.32~46.0(平均26.7),与海底火山喷流沉积型黄铁矿的Co、Ni特征基本一致。核部黄铁矿由颗粒中心向边缘,Fe/S原子比值、Mo和Co含量先逐渐升高,再逐渐降低,而Cu、Zn等成矿元素主要富集于颗粒边缘,并向边缘有逐渐升高趋势。与此同时,细粒黄铁矿(粒径小于0.5cm)中的Cu、Zn等元素的含量明显高于粗粒黄铁矿。环带中三种硫化物矿物的REE配分曲线和微量元素蛛网图极为相似,相对富集LREE、Rb、Th等元素,而亏损Nb、Ta、Zr、Hf、Sr、Ba和HREE等元素,由环带核部到边部δEu逐渐减小,与矿区石英二长闪长(玢)岩表现出较高的相似性。以上特征综合分析表明,冬瓜山铜(金)矿床中硫化物环带经历了以下形成过程:石炭纪海底喷流沉积作用在矿区形成沉积黄铁矿,到燕山期,在区内强烈的构造-岩浆活动作用下,致使早期沉积的黄铁矿首先发生变质重结晶作用,形成粒状黄铁矿,随后岩浆热液对其进行叠加改造,并在岩浆热液作用下相继围绕粒状黄铁矿增生,依次沉淀出热液型黄铁矿、黄铜矿和磁黄铁矿,最终形成硫化物环带。这一认识,结合硫化物环带中元素及硫同位素特征进一步表明冬瓜山铜(金)矿床的形成先后经历了古生代海底喷流沉积成矿作用和燕山期岩浆热液成矿作用,矿床中的成矿物质(特别是Cu、Zn等成矿元素)主要来源于燕山期岩浆热液,但石炭系海底喷流沉积作用也提供了部分物质(例如,Fe、S、Mo、Co和Ni等)。此外,环带中微量元素的变化特征表明,随着硫化物环带的形成,成矿热液系统的温度、硫逸度和氧逸度逐渐降低和(或)p H值升高。     

Formation of massive sulfide deposits on oceanic ridge crests: Incremental reaction models for mixing between hydrothermal solutions and seawater

Equilibrium path calculations have been used to model mixing between hot (350°C) hydrothermal solutions and ambient seawater, in an attempt to simulate mineral precipitation at seafloor vents. These calculations predict temperatures of precipitation, paragenetic sequence of minerals, and chemical composition of chimney deposits associated with vents on the seafloor at 21°N, EPR. Assuming sulfate-sulfide disequilibrium during mixing, the paragenetic sequence revealed is: chalcopyrite, anhydrite, pyrrhotite, pyrite, sphalerite, graphite, and barite. When sulfate-sulfide equilibria is permitted during mixing, however, reduction of small amounts of sulfate results in early precipitation of pyrite and a sequence of Cu-rich sulfide minerals (chalcopyrite-bornite-chalcocite-covellite). This sequence is analogous to that observed in thin chimney walls. The calculations indicate that sulfide mineral precipitation occurs in response to both cooling and change in composition of the hydrothermal solutions as a result of mixing. Varying the amount of mixing with respect to temperature, simulating conductive heating of seawater prior to mixing, results in only minor variations in the sequence and abundance of precipitated phases.Anhydrite precipitation during mixing occurs early, which is consistent with formation of an anhydrite leading edge of chimney structures. Similarly, extrapolation of warm spring data from Galapagos to zero SO₄ concentration suggests anhydrite formation due to mixing with seawater beneath the seafloor, most likely below the level of reactive calcareous sediments. Subsequent interaction of the mixed hydrothermal solution with those sediments results in elevated and variable Ca concentrations estimated for end-member solutions from the Galapagos.Precipitation of Mg hydroxide sulfate hydrate in the walls of the vent chimneys at 21°N, EPR, occurs as a result of conductive heating of ambient seawater with only very minor amounts of mixing. In contrast, precipitation of amorphous silica in the vents must be due to conductive cooling of the hydrothermal solutions.Thus, incremental reaction calculations demonstrate that reactions occurring in and associated with venting ridge crest hydrothermal solutions can be effectively modeled using the thermodynamic data and reaction modeling codes available today. Departures from equilibrium required to accurately model the mixing process are easily accommodated and consistent with data from the vents and vent forming materials.     

Mineralogy and trace element geochemistry of sulfide minerals from the Wocan Hydrothermal Field on the slow-spreading Carlsberg Ridge,Indian Ocean

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000 m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990 m, and Wocan-2 site, located at a water depth of 3100 m, ∼1.7 km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.     

Interpretation of Mineral Zoning in Submarine Hydrothermal Ore Deposits in Terms of Coupled Fluid Flow‐Precipitation Kinetics Model

Major minerals (sulfates, sulfides, quartz) are distributed in different parts of submarine hydrothermal ore deposits. For instance, the abundance of barite increases stratigraphically upwards in the massive orebodies of the Kuroko deposits (black and yellow ores), while quartz is abundant in the lower parts (siliceous ore). The different distribution of barite and quartz in the Kuroko deposits can not be accounted for by thermochemical equilibrium calculations based on the precipitation due to mixing of ascending hydrothermal solutions with ambient cold seawater. In the present study, a coupled fluid flow‐precipitation kinetics model was used to calculate the amounts of quartz, barite, and anhydrite precipitated from a hydrothermal solution mixed with seawater, assuming reasonable values for temperature, precipitation rate, fluid flow velocity, mineral surface area/fluid mass ratio (A/M), and initial concentrations of hydrothermal solution and seawater before mixing occurred. The results indicate that barite precipitates more efficiently than quartz from discharging fluids with relatively higher flow velocity, lower temperatures and under the condition of lower A/M ratios on the seafloor (black ore), whereas quartz precipitates more effectively from solutions with lower flow velocity, higher temperatures and higher A/M ratios beneath the seafloor (siliceous ore) and in the orebody (barite ore, ferruginous chert ore). Anhydrite precipitates in shallow sub‐seafloor environments with lower precipitation rates and higher A/M ratios than barite and higher precipitation rates and lower A/M ratios than quartz. These results explain the observed occurrences of barite, anhydrite, and quartz in the Kuroko deposits. Namely, barite is abundant in black ore and barite ore which formed above the seafloor, anhydrite formed in high‐permeability tuff breccias, and quartz formed in low permeability dacite intrusive bodies in the sub‐seafloor environment.     

吉林小西南岔金铜矿床主要金属矿物的成因矿物学特征

本文通过对矿床中主要金属矿物黄铁矿、磁黄铁矿、黄铜矿的成因矿物学研究,指出黄铁矿中Co/Ni、S/Se、Se/Te、Au/Ag比值均显示出岩浆热液成因特征;黄铁矿、磁黄铁矿、黄铜矿的硫同位素组成具深源岩浆源流特征,并随成矿深度发生有规律的变化;黄铁矿、磁黄铁矿、黄铜矿中某些微量元素质量分数北山矿段与南山矿段之间有一定差...     

Seafloor bioalteration of sulfide minerals: results from in situ incubation studies

We present results of incubation studies conducted at low temperatures (∼4°C) in the vicinity of a seafloor hydrothermal vent system. We reacted Fe-, S-, Cu-, and Zn-bearing minerals including pyrite, marcasite, chalcopyrite, sphalerite, elemental sulfur, and a portion of a natural chimney sulfide structure for 2 months at the Main Endeavour Segment of the Juan de Fuca Ridge in the Pacific Ocean. Our study utilizes Fluorescent In Situ Hybridizations (FISH), Scanning and Transmission Electron Microscopy (SEM, TEM), and light microscopic analysis. The surfaces of these minerals are solely colonized by Bacteria and not by Archaea. Colonization densities vary over an order of magnitude with the following sequence: elemental sulfur > chimney sulfide > marcasite > pyrite > sphalerite > chalcopyrite, and correspond well with the abiotic oxidation kinetics of these materials, excepting elemental sulfur, which is both the least reactive to oxidizing species and the most heavily colonized. Colonization densities also correspond with apparent degree of reaction (dissolution pitting + accumulation of secondary alteration products). Heavy accumulations of secondary Fe oxides on Fe-bearing minerals, most notably on the chimney sulfide, form in situ as the result of mineral dissolution and the activity of neutrophilic Fe-oxidizing bacteria. Results suggest that mineral-oxidizing bacteria play a prominent role in weathering of seafloor sulfide deposits, and that microbial utilization of mineral substrates contributes to biomass production in seafloor hydrothermal environments.     

Diverse Range of Mineralization Induced by Phase Separation of Hydrothermal Fluid: Case Study of the Yonaguni Knoll IV Hydrothermal Field in the Okinawa Trough Back-Arc Basin

The Yonaguni Knoll IV hydrothermal vent field (24°51′N, 122°42′E) is located at water depths of 1370–1385 m near the western edge of the southern Okinawa Trough. During the YK03–05 and YK04–05 expeditions using the submersible Shinkai 6500, both hydrothermal precipitates (sulfide/sulfate/carbonate) and high temperature fluids (Tmax = 328°C) presently venting from chimney‐mound structures were extensively sampled. The collected venting fluids had a wide range of chemistry (Cl concentration 376–635 mmol kg^?1), which is considered as evidence for sub‐seafloor phase separation. While the Cl‐enriched smoky black fluids were venting from two adjacent chimney‐mound structures in the hydrothermal center, the clear transparent fluids sometimes containing CO₂ droplet were found in the peripheral area of the field. This distribution pattern could be explained by migration of the vapor‐rich hydrothermal fluid within a porous sediment layer after the sub‐seafloor phase separation. The collected hydrothermal precipitates demonstrated a diverse range of mineralization, which can be classified into five groups: (i) anhydrite‐rich chimneys, immature precipitates including sulfide disseminations in anhydrite; (ii) massive Zn‐Pb‐Cu sulfides, consisting of sphalerite, wurtzite, galena, chalcopyrite, pyrite, and marcasite; (iii) Ba‐As chimneys, composed of barite with sulfide disseminations, sometimes associated with realgar and orpiment overgrowth; (iv) Mn‐rich chimneys, consisting of carbonates (calcite and magnesite) and sulfides (sphalerite, galena, chalcopyrite, alabandite, and minor amount of tennantite and enargite); and (v) pavement, silicified sediment including abundant native sulfur or barite. Sulfide/sulfate mineralization (groups i–iii) was found in the chimney–mound structure associated with vapor‐loss (Cl‐enriched) fluid venting. In contrast, the sulfide/carbonate mineralization (group iv) was specifically found in the chimneys where vapor‐rich (Cl‐depleted) fluid venting is expected, and the pavement (group v) was associated with diffusive venting from the seafloor sediment. This correspondence strongly suggests that the subseafloor phase separation plays an important role in the diverse range of mineralization in the Yonaguni IV field. The observed sulfide mineral assemblage was consistent with the sulfur fugacity calculated from the FeS content in sphalerite/wurtzite and the fluid temperature for each site, which suggests that the shift of the sulfur fugacity due to participation of volatile species during phase separation is an important factor to induce diverse mineralization. In contrast, carbonate mineralization is attributed to the significant mixing of vapor‐rich hydrothermal fluid and seawater. A submarine hydrothermal system within a back‐arc basin in the continental margin may be considered as developed in a geologic setting favorable to a diverse range of mineralization, where relatively shallow water depth induces sub‐seafloor phase separation of hydrothermal fluid, and sediment accumulation could enhance migration of the vapor‐rich hydrothermal fluid.     

Tellurium-Bearing Mineralization in Clastic Ores at the Yubileynoe Copper Massive Sulfide Deposit (Southern Urals)

At the well-preserved Yubileynoe VMS deposit (Southern Urals), sulfide breccias and turbidites host abundant tellurides represented by hessite, coloradoite, altaite, volynskite, stützite, petzite, and calaverite, as well as phases of the intermediate tellurobismuthite → rucklidgeite solid solution. Three telluride generations were highlighted: (1) primary hydrothermal tellurides in fragments of chalcopyrite and sphalerite of chalcopyrite-rich black smoker chimneys; (2) authigenic tellurides in pseudomorphic chalcopyrite and chalcopyrite veins after fragments of colloform and granular pyrite; and (3) authigenic tellurides in pyrite nodules. Authigenic tellurides are widespread in pyrite-chalcopyrite turbidites. Primary hydrothermal and authigenic tellurides are less common in sulfide turbidites and gritstones with fragments of sphalerite-pyrite, pyrite-sphalerite paleosmoker chimneys and clasts of colloform and fine-grained seafloor hydrothermal crusts. Siliceous siltstones intercalated with sulfide turbidites contain pyrite nodules, whose peripheral parts contain inclusions of epigenetic tellurides. It is assumed that Te for authigenic tellurides originated from fragments of colloform pyrite and hydrothermal chalcopyrite of pyrite-chalcopyrite chimneys, which dissolved during the postsedimentation processes. The main Te concentrators in clastic ores include pseudomorphic chalcopyrite, which inherits high Te, Bi, Au, Ag, Co, Ni, and As contents from the substituted colloform pyrite, and varieties of granular pyrite containing microinclusions of tellurobismuthite (Bi, Te), petzite (Au, Ag, Te), altaite (Pb, Te), coloradoite, and hessite (Ag, Te).

     

Geology,mineralogy, and sulfur isotope geochemistry of the Sargaz Cu–Zn volcanogenic massive sulfide deposit,Sanandaj–Sirjan Zone,Iran

The Sargaz Cu–Zn massive sulfide deposit is situated in the southeastern part of Kerman Province, in the southern Sanandaj–Sirjan Zone of Iran. The stratigraphic footwall of the Sargaz deposit is Upper Triassic to Lower Jurassic (?) pillowed basalt, whereas the stratigraphic hanging wall is andesite. Mafic volcanic rocks are overlain by andesitic volcaniclastics and volcanic breccias and locally by heterogeneous debris flows. Rhyodacitic flows and volcaniclastics overlie the sequence of basaltic and andesitic rocks. Based on the bimodal nature of volcanism, the regional geologic setting and petrochemistry of the volcanic rocks, we suggest massive sulfide mineralization in the Sargaz formed in a nascent ensialic back-arc basin. The current reserves (after ancient mining) of the Sargaz deposit are 3 Mt at 1.34% Cu, 0.38% Zn, 0.08%Pb, 0.24 g/t Au, and 7 g/t Ag. The structurally dismembered massive sulfide lens is zoned from a pyrite-rich base, to a pyrite?±?chalcopyrite-rich central part, and a sphalerite–chalcopyrite-rich upper part, with a sphalerite-rich zone lateral to the upper part. The main sulfide mineral is pyrite, with lesser chalcopyrite and sphalerite. The feeder zone, comprised of a vein stockwork consists of quartz–sulfide–sericite pesudobreccia and, in the deepest part, chlorite–quartz–pyrite pesudobreccia. Footwall hydrothermal alteration extends at least 70–80 m below the massive sulfide lens and more than a hundred meters along strike from the massive sulfide lens. Jasper and Fe–Mn bearing chert horizons lateral to the sulfide deposit represent low-temperature hydrothermal precipitates of the evolving hydrothermal system. Based on mineral textures and paragenetic relationships, the growth history of the Sargaz deposit is complex and includes: (1) early precipitation of sulfides (protore) on the seafloor as precipitation of fine-grained anhedral pyrite, sphalerite, quartz, and barite; (2) anhydrite precipitation in open spaces and mineral interstices within the sulfide mound followed by its subsequent dissolution, formation of breccia textures, and mound clasts and precipitation of coarse-grained pyrite, sphalerite, tetrahedrite–tennantite, galena and barite; (3) replacement of pre-existing sulfides by chalcopyrite precipitated at higher temperatures (zone refining); (4) continued “refining” led to the dissolution of stage 3 chalcopyrite and formation of a base-metal-depleted pyrite body in the lowermost part of the massive sulfide lens; (5) carbonate veins were emplaced into the sulfide lens, replacing stage 2 barite. The δ³⁴S composition of the sulfides ranges from +2.8‰ to +8.5‰ (average, +5.6‰) with a general increase of δ³⁴S ratios with depth within the massive sulfide lens and underlying stockwork zone. The heavier values indicate that some of the sulfur was derived from seawater sulfate that was ultimately thermochemically reduced in deep hydrothermal reaction zones.     

Isocubanite-chalcopyrite intergrowths in the Mid-Atlantic Ridge 26°S hydrothermal vent sulfides

Petrographic, SEM, and EPMA analyses are used to study the micro-textures and mineralogical composition of samples collected by a TV-grab from the 26°S SMAR (southern Mid-Atlantic Ridge) hydrothermal field. The investigated samples include the outermost chimney walls and sulfide debris. Isocubanite-chalcopyrite intergrowths are the major Cu-Fe sulfide phase in the chimney wall samples. These intergrowths include normal chalcopyrite, anomalous chalcopyrite (Cu-poor, Zn- and Fe-rich), normal isocubanite with Cu/Fe < 0.50, and Cu-rich isocubanite with Cu/Fe > 0.50. Anomalous chalcopyrite and Cu-rich isocubanite represent the intermediate phases between stoichiometric chalcopyrite and isocubanite in the Cu-Fe-S system. Anomalous chalcopyrite occurs as cores or thin rims bordering isocubanite, which associated with sphalerite. While Cu-rich isocubanite commonly associates pyrite. Based on textural relationships and microanalytical data of both phases, we interpret the abundant anomalous chalcopyrite and Cu-rich isocubanite as metastable or as high-temperature (~300 °C) rapidly precipitated hydrothermal sulfides. This interpretation advocates the SMAR 26°S hydrothermal field as an immature and short-living system.     

Mineralogy,microchemistry and fluid inclusion studies of the Besshi-type Nudeh Cu-Zn VMS deposit,Iran

The southwestern Sabzevar basin is the north of Central Iranian Microcontinent hosts abundant mineral deposits, including exhalative Mn mineralization and Cu-Zn volcanogenic massive sulfide (VMS) deposits. Amongst them, the Nudeh Besshi-type Cu–Zn volcanogenic massive sulfide (VMS) deposit is hosted within the lower part of a Late Cretaceous volcano-sedimentary sequence composed of alkali olivine basalt flows and tuffaceous silty sandstone. Based on investigations into the ore geometry, mineralogy, and texture, we recognized three different ore facies: (1) a stockwork of sulfide-bearing quartz veins cutting across the footwall volcano-sedimentary rocks and representing the stringer zone; (2) a massive ore type, displaying replacement texture with pyrite, chalcopyrite, sphalerite, friedrichite, and minor magnetite; and (3) a bedded ore type, with laminated to disseminated pyrite and chalcopyrite. EPMA studies indicate a distinctive minor element distribution between the different ore types of the Nudeh deposit. The Fe content in the sphalerite ranges from 0.65–1.80?wt.%, indicating the Fe-poor nature of the sphalerite. However, the Cd content in sphalerite ranged between 0.164–0.278?wt.%. According to the mineral compositions, Zn, Se, and Ag are found in bornite as minor elements. In the bedded ore facies, the pyrite contains higher levels of Se (up to 0.35?wt.%). The Zn content in the friedrichite in all of the ore samples is low. The Co/Ni ratios in pyrite from the Nudeh ore are lower than those of most magmatic deposits, but are similar to those from volcanogenic deposits, and hence support the proposed hydrothermal origin of the deposit. Two generations of quartz, Q1 and Q2 in the stockwork veins, contain primary fluid inclusions and these contain two phases (liquid and vapor). The lack of vapor-rich inclusions or variable liquid/vapor ratios indicate that the fluids did not boil at the site of trapping. Salinity for both Q1 and Q2 fluid inclusions ranges between 2.2–6.8?wt.% eq. NaCl. Homogenization temperatures for inclusions in the Q1 and Q2 veins average at about 296?°C and are similar to the temperatures of hydrothermal fluids discharged through vents in many modern seafloor VMS deposit. The Nudeh Besshi-type VMS deposit appears to have formed on the seafloor and based on the salinity and temperature constraints from the underlying stockwork, a buoyancy plume model is proposed as a mechanism for precipitation.