Genesis of the Huoshenmiao Mo deposit in the Luanchuan ore district, China: Constraints from geochronology, fluid inclusion, and H-O-S isotopes

The Huoshenmiao deposit is Mo skarn deposit, located in the western part of the Luanchuan ore district. Mineralization process can be divided into a skarn and a quartz-sulfide episodes with six stages: prograde (I), retrograde (Ⅱ), quartz-K-feldspar (Ⅲ), quartz-molybdenite (IV), quartz-pyrite (V), and quartz-calcite (VI). A combined study of geochronology, fluid inclusion (FI), and stable isotopes was conducted to constrain the mineralization age, source of ore materials, as well as the origin and evolution of the ore-forming fluids. Molybdenite Re-Os dating indicates that the deposit was formed in the Late Jurassic (∼145 Ma). The δ³⁴S values of sulfides range from 3.0‰ to 7.1‰, implying that the ore materials in the deposit are magmatic in origin. Three types and six subtypes of FIs are distinguished, namely, aqueous two-phase (W₁- and W₂-type), daughter mineral-bearing multiphase (S₁- and S₂-type), and CO₂-bearing three-phase (C₁- and C₂-type). In stages I and Ⅱ, the W₁-type FIs display homogenization temperatures (Th) from 496 ℃ to >600 ℃, with salinities of 14.9-18.3 wt.% NaCl eqv. The FIs in stages Ⅲ, IV and early stage V composed of coeval S-, C- and W-types, respectively homogenize at similar Th, suggesting the occurrence of boiling. The W₁-type FIs in late stage V and stage VI, yield Th of 102-406 ℃ and salinities of 0-4.7 wt.% NaCl eqv. The δD_H2O and δ¹⁸O_H2O values of the ore-forming fluids in quartz-sulfide episode vary from -112‰ to -76‰, and 11.0‰ to 1.0‰, respectively. All these above observations reveal that the early ore-forming fluids are magmatic in origin, and characterized by high temperature and moderate to high salinity, and gradually evolve to low temperature, low salinity meteoric water. The Huoshenmiao Mo deposit is associated with the magmatism event induced by the protracted subduction of the Izanagi plate beneath the eastern China continent. The decrease in temperature, salinity and f(O₂), as well as change of pH due to boiling and fluid-rock interaction, are the main factors controlling Mo deposition.