Stable isotope systematics of Ulsan Fe-W skarn deposit, Korea

From the measurements of oxygen and carbon isotope ratios (δ¹³C= 1.2-4.6 ‰ relative to V-PDB, and δ¹⁸O = 13.5-22.1 ‰ relative to V-SMOW) of fresh or partly altered limestones at the Ulsan skarn deposit, it is suggested that the origin of carbonate rocks is marine carbonate rather than carbonatite. Moreover, the calculated isotopic compositions (δ¹³C = -7.67 ‰ and δ¹⁸O = 6.75 ‰) of the hydrothermal fluid in the main Fe stage of skarn formation imply that this fluid was magmatic water derived from differentiated granitic melts. At this prograde stage, carbon and oxygen isotope data of skarn calcites show that X_CO2 was low (<0.1), and temperature was relatively constant (350-450°C). Later, during the retrograde skarn and vein stages, the hydrous silicates, scheelite and polymetallic sulfides were formed from the less saline, lower-temperature ore-forming fluids. This low-temperature environment (from 340°C down to 135°C) related to Zn-Pb-Ag mineralization was inferred not only from the oxygen isotope ratios of siderite, but also from the fluid inclusion data of later quartz-siderite veins. This temperature drop reflects an important change in the hydrology of the hydrothermal system at this vein stage (i.e., mixing with meteoric water).