The Erdaohezi Pb- Zn- Ag polymetallic deposit, one of the largest and most representative deposits, is located in the central part of the Derbugan metallogenic belt. The hydrothermal mineralization process is subdivided into four mineralization stages: quartz- pyrite stage (I); quartz- sulfides polymetallic stage (II); quartz- sphalerite- galena- silver minerals polymetallic stage (III); and quartz- calcite- pyrite stage (IV). Microscope observation and electron probe microanalysis (EPMA) analysis suggest that the occurrence of silver in this deposit is mainly visible silver (＞1 μm), followed by invisible silver (＜1 μm). Visible silver is mainly microencapsulated silver and independent silver minerals, and invisible silver is mainly super- microencapsulated silver and isomorphic silver. Microencapsulated (super- microencapsulated) silver occurs usually as round or irregular inclusions within sulfides or between sulfides and quartzes. Independent silver minerals are mainly distributed as veinlets or aggregates in microfractures or edges of galena (sphalerite) veins. A very small amount of isomorphic silver exists in the crystal lattices of sulfides. The metal ions in the ore- forming hydrothermal fluids, such as Ag, Cu, Fe, Pb and Zn, are transported as chlorine complex ions in the early mineralization stage (quartz- pyrite stage), and as HS- complex ions after fluid boiling. With decreasing temperature and pressure of the fluids and mixing of meteoric water, the ore- forming fluid has migrated upward and experienced two silver enrichment processes: Ag+ released from HS- complex is coprecipitated with Cu+, Sb3+, Pb2+ and S2- to form microencapsulated (super- microencapsulated) silver in stage II; then the instability of Ag(HS)-2 leads to the complete disintegration of silver from its HS- complex and combination with Cu+, Sb3+and S2-to form independent silver minerals (such as argentite and polybasite) in stage III.