Electrum-sulfide mineralization of the Daeil and Yeongbogari mines in the Youngdong area consists of two stages of quartz and calcite veins that fill fault zones in Precambrian metamorphic rocks. Radiometric dating indicates that mineralization of the Daeil mine is Jurassic age (145 Ma), whereas that of the Yeongbogari mine is Cretaceous (132 Ma). Mineralogy of the two mines differs significantly in that only the Yeongbogari mine contains silver-bearing minerals (argentite, argentian tetrahedrite, pyrargyrite) and its electrums are more silver-rich (49.1 to 71.4 wt.%Ag) than those of the Daeil mine (28.0 to 37.8 wt.%Ag).
Fluid inclusion data show that ore mineralization occurred at temperatures between 375° and 246°C from fluids with salinities between 9.1 and 2.6 wt.% equiv. NaCl. The estimated temperature and sulfur fugacity for gold deposition in the Jurassic Daeil mine are much higher (≈350°C, 10 ^-9 atm.) than those for the Cretaceous Yeongbogari mine (≈275°C, 10 ^-12 atm.). Fluid inclusion evidence of boiling indicates pressures of<200 bars, corresponding to depths of 800 to 2, 600 m assuming lithostatic and hydrostatic pressure regimes, respectively.
Sulfur isotope compositions of sulfide minerals are consistent with sulfur from igneous sources in the Daeil and Yeongbogari mines. Estimated δ³⁴SΣS values for the Daeil and Yeongbogari mines are 8 and 3 per mil, respectively, which may reflect differences in the separation of sulfur-bearing magmatic fluids from their associated granitic magmas.
The difference in ages, mineralogy and ore deposition conditions between the Jurassic Daeil and Cretaceous Yeongbogari deposits further validates the idea of multiple gold-silver metallogenic epochs in Korea. Deposits associated with Jurassic granites are formed at typically deeper and higher temperature conditions and are more gold-rich than those associated with Cretaceous granites.