Mesothermal gold vein mineralization of the Samdong mine, Youngdong mining district, Republic of Korea - A geochemical and fluid inclusion study

Mesothermal gold mineralization at the Samdong mine (5.5-13.5 g/ton Au), Youngdong mining district, is situated in massive quartz veins up to 1.2 m wide which fill fault fractures within upper amphibolite to epidote-amphibolite facies, Precambrian-banded biotite gneiss. The veins are mineralogically simple, consisting of iron- and base-metal sulfides and electrum, and are associated with weak hydrothermal alteration zones (<0.5 m wide) characterized by silicification and sericitization. Fluid inclusion data and equilibrium thermodynamic interpretation of mineral assemblages indicate that the quartz veins were formed at temperatures between 425 and 190°C from relatively dilute aqueous fluids (4.5-13.8 wt. % equiv NaCl) containing variable amounts of CO₂ and CH₄. Evidence of fluid unmixing (CO₂ effervescence) during the early vein formation indicates approximate pressures of 1.3-1.9 kbars, corresponding to minimum depths of ≈ 5-7 km under a purely lithostatic pressure regime. Gold deposition occurred mainly at temperatures between 345 and 240 °C, likely due to decreases in sulfur activity accompanying fluid unmixing. The δ³⁴S values of sulfide minerals (-3.0 to 5.3 ‰), and the measured and calculated O-H isotope compositions of ore fluids (δ¹⁸O = 5.7 to 7.6‰; δ = -74 to -80‰) indicate that mesothermal gold mineralization at the Samdong mine may have formed from dominantly magmatic hydrothermal fluids, possibly related to intrusion of the nearby ilmenite-series, 'Kimcheon Granite' of Late Jurassic age.