Fluid inclusion and H-O-C isotope geochemistry of the Yaochong porphyry Mo deposit in Dabie Shan, China: A case study of porphyry systems in continental collision orogens

The Yaochong porphyry Mo deposit in Xinxian County, Henan Province, China, is located in the Hong'an terrane, that is, the western part of the Dabie orogen. The Dabie orogen is part of a >1,500 km long, Triassic continental collision belt between the North China Block and the South China Block. Four types of vein are present. Paragenetically, from early to late, they are as follows: stage 1 quartz + K-feldspar ± pyrite ± magnetite vein; stage 2 quartz + K-feldspar + molybdenite ± pyrite vein; stage 3 quartz + polymetallic sulfides ± K-feldspar vein; and stage 4 quartz ± carbonate ± fluorite vein. Four compositional types of fluid inclusion, pure CO₂, CO₂ bearing, aqueous, and solid bearing, are present in quartz from the first three stages; only low-salinity aqueous fluid inclusions occur in quartz from the last stage. All the estimated salinities are ≤13.1 wt% NaCl eq., and no halite crystals were identified. Homogenization temperatures for the fluid inclusions from stages 1 to 4 are in the ranges of 262-501, 202-380, 168-345, and 128-286 °C, respectively, and estimated depths decrease from 6.9 to 8.9 km, through 6.2-7.2, to ~4.7 km. Quartz separates from the veins yielded a δ¹⁸O value of 7.7-11.2 ‰, corresponding to δ¹⁸O_H2O values of -1.3 to 6.9 ‰ using temperature estimates from fluid inclusion data; δD_H2O values of fluid inclusion vary from -80 to -55 ‰, and δ¹³C_CO2 from -2.3 to 2.7 ‰, suggesting that the ore-fluids evolved from magmatic to meteoric sources. We conclude that the ore-forming fluid system at Yaochong was initially high temperature, high salinity, and CO₂-rich and then progressively evolved to CO₂-poor, lower salinity, and lower temperature, by mixing with meteoric water, which results in ore precipitation.