Origin of the Erdaohe Ag–Pb–Zn deposit, central Great Xing’an Range, northeast China: Constraints from fluid inclusions, zircon U–Pb geochronology, and stable isotopes

Abstract

The large-scale Erdaohe Ag–Pb–Zn deposit in the central Great Xing’an Range, China, contains > 9.04 Mt of ore reserves. The deposit consists of skarn-type Ag–Pb–Zn orebodies located within a contact zone that is mainly between an Early Cretaceous porphyritic granite and Jurassic calcareous tuff. The deposit records four stages of formation, which are skarn, early quartz–sulfide (disseminated ores), late quartz–sulfide (veined ores), and carbonate stages. Fluid inclusion microthermometry indicates that skarn stage fluids were higher temperature (330 °C to > 500 °C) and more saline (up to 59.8 wt% NaCl equivalent) than the early quartz–sulfide (250 °C–300 °C, <36.1 wt% NaCl equivalent) and late quartz–sulfide stage (160 °C–220 °C, <32.1 wt% NaCl equivalent) fluids. Fluid inclusions of the carbonate stage record even lower temperatures (150 °C–170 °C) and have generally low salinity (2.1–5.1 wt% NaCl equivalent). Raman microprobe data indicate that these fluids are dominated by H2O–NaCl ± CO2 system phases. Sulfides formed during the early and late quartz–sulfide stages of mineralization have a relatively narrow range of δ34S values (4.4‰–8.4‰) that are indicative of sulfur derived from a magmatic source. Quartz samples from the early and late quartz–sulfide stages have δ18Ofluid values of 0.1‰–2.1‰ and from − 5.0‰ to − 3.2‰, respectively, suggesting that the mineralizing fluids were originally magmatic with later stage mineralization involving the addition of meteoric water. LA–ICP–MS U–Pb dating of zircons from the mineralization-related porphyritic granite yielded a weighted mean age of 131.4 ± 0.3 Ma (MSWD = 1.5), indicating that the deposit formed during the Early Cretaceous. Deposit formation involved an aqueous fluid that was exsolved during the final crystallization of the porphyritic granite and ascended along fracture zones until it reacted with Ca-rich wall rocks, cooled to < 300 °C, and boiled under hydrostatic conditions. This caused a decrease in temperature and sulfur fugacity in the ore-forming fluid system, which triggered Ag–Pb–Zn mineralization.