Genesis and age constraints on gold deposits of the Daerae mine, Sangju area, central-northern Sobaegsan Massif, Korea

Gold mineralization of the Daerae mine represents the first recognized example of the Jurassic gold mineralization in the Sangju area, Korea. It occurs as a single stage of quartz veins that fill fault fractures in Precambrian gneiss of the central-northern Sobaegsan Massif. The mineralogical characteristics of quartz veins, such as the simple mineralogy and relatively gold-rich (65-72 atomic % Au) nature of electrum, as well as the CO₂-rich and low salinity nature of fluid inclusions, are consistent with the 'mesothermal-type' gold deposits previously recognized in the Youngdong area (about 50 km southwest of the Sangju area). Ore fluids were evolved mainly through CO₂ immiscibility at temperatures between about 250 and 325°C. Vein sulfides characteristically have negative sulfur isotopic values (-1.9 to +0.2 ‰), which have been very rarely reported in South Korea, and possibly indicate the derivation of sulfur from an ilmenite-series granite melt. The calculated O and H isotopic compositions of hydrothermal fluids at Daerae (δ¹⁸O_water = +5.2 to +5.9 ‰; δD_water = -59 to -67 ‰) are very similar to those from the Youngdong area, and indicate the important role of magmatic water in gold mineralization. The ⁴⁰Ar-³⁹Ar age dating of a pure alteration sericite sample yields a high-temperature plateau age of 188.3 ± 0.1 Ma, indicating an early Jurassic age for the gold mineralization at Daerae. The lower temperature Ar-Ar plateau defines an age of 158.4 ± 2.0 Ma (middle Jurassic), interpreted as reset by a subsequent thermal effect after quartz vein formation. The younger plateau age is the same as the previously reported K-Ar ages (145-171 Ma) for the other 'mesothermal-type' gold deposits in the Youngdong and Jungwon areas, Korea, which are too young in view of the new Jurassic Ar-Ar plateau age (around 188 Ma).