Oxygen Isotope Characteristics of the Footwall Alteration Zones in the Hongtoushan Volcanogenic Massive Sulfide Deposit, Liaoning Province, China and Restoration of Their Formation Temperatures

The Hongtoushan Volcanogenic Massive Sulphide Deposit (VMSD) occurs in the Hunbei granite‐greenstone terrane, Liaoning Province, NE China. Rocks in the mining area have been metamorphosed around 3.0–2.8 Ga to upper amphibolite facies at temperatures between 600°C and 650°C. Cordierite‐anthophyllite gneiss (CAG) in the Hongtoushan mining area, which occurs hundreds of meters below the ore horizon, corresponds to the metamorphosed semi‐conformable alteration zone of the VMSD hydrothermal system, whereas the one immediately below the main ore layer represents the metamorphosed pipe‐like alteration zone. Whole‐rock oxygen isotope signatures were well preserved in both types of CAGs, although the mineral components have been entirely changed during regional metamorphism. Therefore, whole‐rock oxygen isotopes can be used to estimate the formation temperature of both types of alteration zone. Calculations show that the semi‐conformable and pipelike alteration zones for the Hongtoushan submarine hydrothermal system were formed at 290–360°C and 285–320°C, respectively, whereas estimates for the former were slightly higher than that of the latter, indicating that the semi‐conformable alteration zone represents the deep part of the Hongtoushan seafloor hydrothermal system, while the pipe‐like alteration zone represents the discharge conduits for metal‐rich fluids, which is closer to the seafloor.