Mode of silver occurrence in pyrite from the Edmond hydrothermal field, Central Indian Ridge: mineralogical evidence

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.