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.

本研究采用矿相学（mineragraphy）、电子显微镜（electron microscopy）、电子探针微区分析（microprobe analysis）及X射线分析（X-ray analysis）方法，对与超镁铁质岩（ultramafic rocks）相关的大西洋中脊（Mid-Atlantic Ridge, MAR）硫化物矿点（sulfide occurrences）中的矿石矿物组成展开了系统研究。
研究对象按硫化物丘（sulfide mounds）的成熟度等级可划分为多个层级，该差异源于下述热液系统（hydrothermal systems）的形成年代、持续时长与活动强度各不相同：基本处于停滞状态的Logatchev-1热液场（形成年龄可达66.5 ka）、已停止活动的Logatchev-2热液场（形成年龄3.9 ka），以及整体仍处于活动状态的Rainbow热液场（形成年龄可达23 ka）。
相较于玄武岩基底中的大西洋中脊海底硫化物矿点，上述三处热液场的矿化作用具有Au、Cd、Co、Ni含量显著偏高的特征，且伴生有Co、Ni的副矿物（accessory minerals）。
本次研究涉及的硫化物丘在主要矿物的定量占比与矿石的结构构造特征方面存在明显差异，指示其经历了不同程度的后期改造作用。
Logatchev-1热液场中的矿石以最高的Cu含量为典型特征，且发育多期次、反差显著的等轴古巴矿（isocubanite）与斑铜矿（bornite）出溶结构（exsolution structures）。
Logatchev-2热液场中可见闪锌矿-黄铜矿（sphalerite-chalcopyrite）组合与金-砷出溶结构，但等轴古巴矿的出溶结构类型更为单一、结构反差相对较弱。
Rainbow热液场则以均质等轴古巴矿的广泛发育为标志，仅伴随少量出溶结构的形成。
研究者在铜-铁-硫（Cu-Fe-S）体系中识别出4种全新矿物相：相X与相Y分别近似于黄铜矿（chalcopyrite）与等轴古巴矿，二者以叶片状晶层（lamellae）的形式赋存于Logatchev-1与Logatchev-2热液场的等轴古巴矿出溶产物中；相Y则产出在Rainbow热液场的分带热液烟囱（zonal chimneys）均质带内。
相A与相B形成于Logatchev-1热液场中黄铜矿经低温蚀变作用形成的橙色斑铜矿域（orange bornite domain）内。
铜-硫（Cu-S）体系的矿物组合在Logatchev-1热液场中最为丰富多样，而在Logatchev-2热液场中发育程度最低——该热液场中仅识别出锗硫铜矿-铜蓝（geerite-covellite）系列的贫铜硫化物。
上述矿物组合的专属特征可反映硫化物丘的成熟度等级，可作为硫化物丘成熟度的指示标志。