Genesis of the Ulaan silver-lead–zinc deposit in Northeast Mongolia: Constraints from S and Pb isotopes, together with U-Pb and Rb-Sr geochronology

The Ulaan silver-lead–zinc deposit (hereinafter referred to as the Ulaan deposit) is identified as the largest silver-lead–zinc polymetallic deposit in Mongolia, with proved reserves including 2,240 tons of silver (Ag; average grade: 49 g/t), 440,000 tons of lead (Pb; average grade: 1.13 %), and 810,000 tons of zinc (Zn; average grade: 2.07 %). However, the genesis of this deposit remains unclear. The Ag-Pb-Zn ore bodies in the deposit, occurring as cylinders in shape within the Middle-Late Jurassic rhyolites, are governed by a concealed breccia pipe. The ore minerals include galena, sphalerite, and pyrite, followed by chalcopyrite, hematite, stibnite, and siderite. The primary alterations of the surrounding rocks include silicification, chloritization, kaolinization, argillization, carbonatization, and skarnization. The Rb-Sr dating of sulfide minerals and associated vein minerals in the ores yielded isochron ages varying in a range of 146 ± 3 Ma (n = 6, MSWD=1.3), suggesting mineralization during the Late Jurassic. The δ³⁴S values of sulfide minerals in the ores range from 1.6 ‰ to 4.3 ‰, suggesting that the sulfur originated primarily from magmas or deep sources. The isotopic compositions of coexisting sphalerite-galena minerals in the deposit revealed mineralization temperature estimates ranging between 331 °C and 449 °C, indicating a medium- to high-temperature ore-forming conditions. The sulfide minerals exhibit ²⁰⁸Pb/²⁰⁴Pb ratios ranging from 38.138 to 38.301, ²⁰⁷Pb/²⁰⁴Pb ratios from 15.543 to 15.594, and ²⁰⁶Pb/²⁰⁴Pb ratios from 18.318 to 18.354, suggesting that ore-forming metals, represented by Pb, also originated primarily from mantle source. The zircon U-Pb dating of rhyolites in the ore-hosting strata and ore-controlling breccia pipes yielded ages of 160.6 ± 1.7 Ma (n = 24, MSWD=0.68) and 161.6 ± 1.6 Ma (n = 30, MSWD=0.89), respectively, indicating volcanic eruptions during the early Late Jurassic. These ore-hosting rhyolites exhibit characteristics of A-type rhyolites, suggesting that they were formed in an intracontinental extensional environment. These rhyolites share similar rare earth element (REE) distribution patterns with fluorite formed in the main mineralization stage, suggesting a genetic link between the mineralization and magmatic processes. This study proposes that the Ulaan deposit was a hydrothermal deposit formed in an extensional environment following the closure of the Mongol-Okhotsk Ocean, with ore-forming metals and hydrothermal fluids associated with volcanic rocks or magmatic-hydrothermal processes.