Abstract
The Bajiazi Zn–Pb–Ag skarn deposit is located in northwest Liaoning Province, China. The orebodies occur in a contact zone between Yanshanian (170–177.4 Ma) quartz monzodiorite and Middle Proterozoic dolomitic marble of the Gaoyuzhuang Formation as well as along the NW- and NS-trending fault systems in carbonate wall rocks. Metasomatic mineralization zoning in the deposit is very pronounced. From the intrusive contact zone to dolomitic marble along the NW-trending fault system, three ore-bearing skarn zones may be distinguished: magnesian skarn zone (Fe–Mo)→manganoan–magnesian skarn zone (Cu–S–Fe–Pb–Zn–Ag)→manganoan skarn zone (Zn–Pb–Ag). Magnesian skarn is composed of diopside, forsterite, tremolite, spinel, phlogopite, chondrodite, and serpentine, accompanied by magnetite, pyrite, and molybdenite. Manganoan–magnesian skarn consists mainly of manganoan diopside, manganoan tremolite, mangano-anthophyllite, and manganocummingtonite with minor spessartine and pyroxmangite, The associated ore minerals are pyrite, pyrrhotite, and chalcopyrite, with subordinate magnetite, sphalerite, galena, and Ag-bearing minerals. Manganoan skarn is composed chiefly of spessartine, pyroxmangite, magnesian rhodonite, manganocummingtonite, and manganopyrosmalite, with the associated ore minerals sphalerite, galena, alabandite, and Ag-bearing minerals. Studies of fluid inclusions and S, Pb, O, and H isotopic compositions suggest that ore-bearing fluids flowed from the deep contact zone of the intrusion in the southeast to the shallow depth in the northwest with decreasing temperatures, oxidation state, and increasing pH. When the ore-bearing fluids migrated, their frontal part constantly reacted with the dolomitic carbonate wall rocks to form different types of skarn and mineralization. Sulfur and lead (zinc) were mainly derived from the deep lower crust, while the Middle Proterozoic wall rocks may have provided most Al, Si, Mg, Ca, and Mn for the formation of skarns. The zonal model reflecting the variations in metallization assemblage, skarn mineralogy, composition, δ 34S isotope values, and fluid inclusion characteristics can be used as a guide to exploration for Zn–Pb–Ag skarn deposits.
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