Mode of silver occurrence in pyrite from the Edmond hydrothermal field, Central Indian Ridge: mineralogical evidence
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摘要:
随着对海底热液多金属硫化物矿床的研究越来越深入，贵金属金（Au）和银（Ag）的赋存形式和沉淀机制被科学家广泛关注。相比于Au，前人对大洋中脊热液区中Ag的产出状态和富集机制研究相对较少。中印度洋Edmond热液区Ag平均含量为47×10−6，明显高于洋中脊环境产出的多金属硫化物中的平均Ag含量（2.78×10−6）。通过光学显微镜和扫描电镜对Edmond热液区硫化物样品进行了详细的观察，确定了该热液区矿物组合、分期以及自然银的赋存形式，并初步探讨了自然银的沉淀机制。Edmond热液区硫化物主要为闪锌矿，其次是黄铁矿、黄铜矿和白铁矿，此外还观察到针钠铁矾、重晶石、硬石膏以及自然银等矿物。根据矿物结构和共生组合，Edmond热液区硫化物成矿过程大致可以分为3个阶段：阶段I的主要矿物组合为一期黄铁矿（Py1）、重晶石、硬石膏等；阶段II主要矿物为白铁矿；阶段III则有二期黄铁矿（Py2）、黄铜矿、粗粒闪锌矿、等轴古巴矿等矿物结晶。自然银主要以细小颗粒的形式存在于Py1的边缘或者内部包体之中。Ag在Edmond热液区的主要迁移形式为AgCl2−，高温热液与海水混合作用导致的温度和Cl−浓度降低以及pH值的升高是导致自然银沉淀的主要影响因素。
Abstract:With the increase in study on submarine polymetallic sulfides, the mechanisms of occurrence and precipitation of gold and silver have become a hotspot of research. Compared with gold, the precipitation mechanism of silver from the hydrothermal field at mid-ocean ridge is poorly studied. The sulfide samples from Edmond hydrothermal field were studied in optical microscopy and scanning electron microscopy. The mineral assemblage, stages of mineralization and the occurrence of native silver were determined, and precipitation mechanism of native silver were also discussed. Results show that the average silver content in the samples was 47×10−6, which is significantly higher than that (2.78×10−6) in sulfide ores from hydrothermal fields of the mid-ocean ridge. Sphalerite was the most abundant sulfide, followed by pyrite, marcasite and chalcopyrite; other minerals including ferrinatrite, barite, anhydrite, and native silver were also observed. In mineral texture and assemblages, the sulfide mineralization process could be divided into three stages. The mineral assemblages in first stage contained pyrite (Py1), barite, and anhydrite; the second stage contained marcasite, and the third stage included pyrite (Py2), chalcopyrite, coarse sphalerite, and isocubanite. Native silver existed mainly in the form of fine particles at the edge or inner inclusions of Py1. The main existing form of silver in the Edmond hydrothermal field was AgCl2-. The decrease in Cl- concentration, the increase in pH value, and the decrease in temperature caused by the mixing of high temperature hydrothermal and seawater were the main factors on the native silver precipitation.
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