Chemical composition of hydrothermal sulfide minerals from the Rainbow, Logachev-1, and Logachev-2 hydrothermal fields

Composition of ore minerals in MAR sulflde occurrences related to ultramaflc rocks was studied using methods of mineragraphy, electron microscopy, microprobe analysis, and X-ray analysis. Objects are located at various levels of maturity of sulflde mounds owing to differences in age, duration and degree of activity of the following hydrothermal systems: generally inactive Logatchev-1 field (up to 66.5 ka old), inactive Logatchev-2 field (3.9 ka), and generally active Rainbow field (up to 23 ka). Relative to MAR submarine ore occurrences in the basalt substrate, mineralization in the hydrothermal fields mentioned above is characterized by high contents of Au, Cd, Co, and Ni, along with presence of accessory minerals of Co and Ni. The studied mounds differ in quantitative ratios of major minerals and structural-textural features of ores that suggest their transformation. Ores in the Logatchev-1 field are characterized by the highest Cu content and development of a wide range of multistage contrast exsolution structures of isocubanite and bornite. In the Logatchev-2 field, sphalerite-chalcopyrite and gold-arsenic exsolution structures are present, but isocubanite exsolution structures are less diverse and contrast. The Rainbow field is marked by presence of homogenous isocubanite and the subordinate development of exsolution structures. The authors have identified four new phases in the Cu-Fe-S system. Phases X and Y (close to chalcopyrite and isocubanite, respectively) make up lamellae among isocubanite exsolution products in the Logatchev-1 and Logatchev-2 fields. Phase Y includes homogenous zones in zonal chimneys of the Rainbow field. Phases A and B formed in the orange bornite domain at low-temperature alteration of chalcopyrite in the Logatchev-1 field. Mineral assemblages of the Cu-S system are most abundant and diverse in the Logatchev-1 field, but their development is minimal in the Logatchev-2 field where mainly Cu-poor sulfides of the geerite-covellite series have been identified. Specific features of mineral assemblages mentioned above reflect the maturity grade of sulfide mounds and can serve as indicators of maturity.

本研究针对与超镁铁质岩（ultramafic rock）相关的大西洋中脊（Mid-Atlantic Ridge, MAR）硫化物矿化点开展了矿石矿物组成研究，采用了矿相学（mineragraphy）、电子显微镜（electron microscopy）、电子探针分析（microprobe analysis）及X射线分析（X-ray analysis）等方法。研究对象为不同成熟度阶段的硫化物丘（sulfide mound），其差异源于以下三个热液系统（hydrothermal system）的形成年龄、活动时长与活跃程度各不相同：基本处于休眠状态的洛加切夫-1（Logatchev-1）热液场（形成年龄可达66.5 ka）、休眠状态的洛加切夫-2（Logatchev-2）热液场（形成年龄3.9 ka），以及整体仍处于活跃状态的彩虹（Rainbow）热液场（形成年龄可达23 ka）。相较于玄武岩基底（basalt substrate）内的大西洋中脊海底矿化点，上述热液场中的矿化作用（mineralization）具有Au、Cd、Co、Ni含量较高的特征，且伴生有Co、Ni副矿物（accessory mineral）。本次研究的硫化物丘在主要矿物的定量比例及矿石的结构构造特征（structural-textural feature）方面存在差异，指示其经历了不同程度的改造作用。洛加切夫-1热液场的矿石以最高的Cu含量为特征，且发育多期次、反差显著的等古巴矿（isocubanite）与斑铜矿（bornite）出溶构造（exsolution structure）。洛加切夫-2热液场中可见闪锌矿（sphalerite）-黄铜矿（chalcopyrite）及金-砷出溶构造，但等古巴矿出溶构造的多样性与反差均较弱。彩虹热液场则以均质等古巴矿的发育为标志，且出溶构造的发育程度较低。研究团队在铜-铁-硫体系（Cu-Fe-S system）中识别出4种全新物相：X相与Y相分别近似于黄铜矿与等古巴矿，在洛加切夫-1与洛加切夫-2热液场的等古巴矿出溶产物中以晶层（lamellae）形式产出；Y相还存在于彩虹热液场分带烟囱（zonal chimney）的均质区域中。A相与B相形成于洛加切夫-1热液场中黄铜矿经低温蚀变（low-temperature alteration）形成的橙色斑铜矿域内。铜-硫体系的矿物组合（mineral assemblage）在洛加切夫-1热液场中最为丰富多样，而在洛加切夫-2热液场中发育程度最低，该区域仅识别出以吉尔矿-铜蓝（geerite-covellite）系列为主的贫铜硫化物。上述矿物组合的独特特征反映了硫化物丘的成熟度等级，可作为硫化物丘成熟程度的指示标志。