Occurrence and precipitation of gold in pyrite from the Yuhuang hydrothermal field, Southwest Indian Ridge: Evidence from mineralogy and trace elements

With the continuous increase in global demand for metal resources and the gradual depletion of land-based mineral resources, polymetallic sulfide deposits on seafloor have attracted extensive attention from geologists. Previous studies have found that polymetallic sulfide deposits in ultra-slow spreading mid-ocean ridges have higher metal resource contents than those in fast, intermediate, and slow spreading mid-ocean ridges. The Yuhuang hydrothermal field on the Southwest Indian Ridge, an ultra-slow spreading mid-ocean ridge, is one of the largest submarine polymetallic sulfide deposits of the world. However, the occurrence and precipitation of gold in pyrite in the hydrothermal field remain unclear. Taking this hydrothermal field as an example, we applied optical microscopy, scanning electron microscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to study the mineral texture and analyze the trace element on sulfide ores. Results show that in the hydrothermal field, gold is mainly in the form of native gold within pyrite in iron-rich sulfide ores. Pyrites in iron-rich sulfide ores were formed under very intensive mixing of hydrothermal fluids and seawater; this intense mixing caused rapid decrease in temperature and increase in pH, which in turn promoted the precipitation of pyrite and consumes a large amount of H₂S, which consequently resulted in the precipitation of gold in the iron-rich sulfide ores.