Gold-silver mineralization of the Jungheung and Okdong mines, Kwangyang area, Korea: Mineralogical and geochemical change in a cooling hydrothermal system
Chil-Sup So, ; Seong-Taek Yun, ; Soon-Hak Kwon,
published: May 26, 1999
ArtNo. ESP154017403001, Price: 29.00 €
Hydrothermal gold-silver mineralization of the Jungheung and Okdong mines, Kwangyang area, Korea, was deposited in three stages (I to III) of quartz ± carbonates veins (usually about 0.3 m thick) which filled fault-related fractures in Precambrian gneisses and Cretaceous granitic rocks. Available age data and geologic setting indicate that ore mineralization occurred in association with intrusion of calc-alkaline granitoids during the late Cretaceous (around 100 Ma). Stage I is Au-Ag-bearing, whereas stages II and III are barren. Stage I is divided into three mineralization phases, based on the relative formation time and mineralogical change: an early phase consisting of pyrrhotite (largely hexagonal) + Fe-rich sphalerite + chalcopyrite; a middle phase consisting mainly of pyrite and base-metal sulfides with rare amounts of Au-Ag-, Bi-, and Te-bearing minerals; and a late phase consisting of quartz and carbonates in vugs. The early pyrrhotite-rich mineralization phase dominates at the Jungheung mine and is unique among the Cretaceous epithermal systems recognized in South Korea, whereas the middle and late mineralization phases characteristically occur at the Okdong mine. Based on fluid inclusion data and chlorite and sulfur isotope geothermometers, the early mineralization phase formed at higher temperatures (from >300° to 380 °C) than the middle mineralization phase (260°-270 °C to 310 °C) in a cooling hydrothermal system. The cooling of hydrothermal fluids was a result of progressive admixture of meteoric water, likely due to further fracturing of veins. The main gold and silver deposition occurred as a result of the cooling at temperatures around 260°-270 °C. Sulfur isotope data of vein sulfides reflect an igneous source of sulfur with a δ34SΣs value of 3-5 ‰. Hydrogen and oxygen isotope compositions of hydrothermal fluids evidence the increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the hydrothermal system. The unique, pyrrhotite-rich mineralization phase formed at high temperatures (up to 380 °C) from either a water with some magmatic water component or a highly exchanged meteoric water (δ18O = 5-6 ‰; salinity = up to 9.3 wt. % eq. NaCl), and later evolved through progressive meteoric water inundation toward the common, middle and late mineralization phases (δ18water = -2.3 to 3.8 ‰; temperature = 300° to 180 °C; salinity = down to 1.4 wt. % eq. NaCl).