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 t o 1.2 m wide which fill fault fractures within upper amphibolite to ep idote-amphibolite facies, Precambrian-banded biotite gneiss. The veins are mineralogically simple, consisting of iron- and base-metal sulfid es 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 tem peratures between 425 and 190 degrees C from relatively dilute aqueous fluids (4.5-13.8 wt. % equiv NaCl) containing variable amounts of CO, and CH,. Evidence of fluid unmixing (CO2 effervescence) during the ea rly vein formation indicates approximate pressures of 1.3-1.9 kbars, c orresponding to minimum depths of approximate to 5-7 km under a purely lithostatic pressure regime. Gold deposition occurred mainly at tempe ratures between 345 and 240 degrees C, likely due to decreases in sulf ur activity accompanying fluid unmixing. The delta(34)S values of sulf ide minerals ( - 3.0 to 5.3 %,), and the measured and calculated O-H i sotope compositions of ore fluids (delta(18)O = 5.7 to 7.6 parts per t housand; delta D = - 74 to - 80 parts per thousand) indicate that meso thermal gold mineralization at the Samdong mine may have formed from d ominantly magmatic hydrothermal fluids, possibly related to intrusion of the nearby ilmenite-series, 'Kimcheon Granite' of Late Jurassic age .