Stable isotope systematics of Ulsan FeW 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 ℵrelative 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 ℵ and δ18O=6.75 ℵ) 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 XCO2 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 oreforming fluids. This low-temperature environment (from 340 °C down to 135 °C) related to ZnPbAg 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).