Hydrothermal bismuth mineralization of the Yucheon mine, South Korea: Oxygen and hydrogen isotope study

Hydrothermal bismuth ore deposit of the Yucheon mine, Kyongsang Basin, consists of quartz veins that filled fault-related fractures within Cretaceous sedimentary and Tertiary igneous rocks. Geologic setting and K-Ar age of muscovite (45.4± 0.6 Ma) indicate that the ore mineralization was associated with calc-alkaline granitism during the Early Tertiary. The three phases of mineralization are: early phase of pyrite+arsenopyrite+pyrrhotite+sphalerite+chalcopyrite; middle phase of base-metal sulfides +bismutninite+electrum+Bi-, Sb- and Te-bearing minerals; and late phase of fluorite+calcite in vugs. Based on fluid inclusion data and oxygen isotope geothermometry, the early mineralization phase was formed at higher temperatures (from 350° to 480°C) than the middle phase (230°-350°C). Such cooling was a result of progressive meteoric water mixing, likely due to further fracturing of veins. The main deposition of bismuth occurred as a result of cooling at temperatures between 250° and 350°C. The early to middle phases of mineralization occurred from high temperature fluid (up to 480°C) with the oxygen isotope composition and salinity of 5.5 to -1.4‰ and 0.7 to 18.4 wt.% eq. NaCl, respectively. The fluid experienced progressive meteoric water inundation toward the late mineralization (248-167°C; δ¹⁸O_water=-3.9 to -13.9‰; down to 1.9 wt.% eq. NaCl). The oxygen and hydrogen isotope data indicate the extensive evolution of hydrothermal fluids at Yucheon, characterized by the progressive increasing influx of an isotopically less-evolved (approaching the meteoric water composition) meteoric water into a deeply circulating highly-evolved meteoric water or a magmatic fluid.