GENETIC ENVIRONMENT OF GERMANIUM-BEARING GOLD-SILVER VEIN ORES FROM THE WOLYU MINE, REPUBLIC-OF-KOREA

The Wolyu mine is one of the largest vein-type gold-silver-bearing epi thermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag8GeS6). Mineralized veins (78.9 +/- 1.2 Ma) cross-cut ting Late Cretaceous hostrock tuff and quartz porphyry (81.5 +/- 1.8 M a) consist of three stages of quartz and carbonates, the first of whic h contains pyrite, base-metal sulfides and Au-Ag-minerals. Stage I Au- Ag-Ge-mineralized veins show a systematic variation of mineral assembl age with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfu r activity are: assemblage (1), 360-280-degrees-C and 10(-7)-10(-10); (2), 280-210-degrees-C and 10(-10)-10(-14); (3), 210-180-degrees-C and 10(-14)-10(-16); (4), 180-155-degrees-C and 10(-17)-10(-18). These da ta, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen an d oxygen isotope compositions of ore fluids display a systematic varia tion with increasing time. Within the main Ag-Au-Ge mineralization, de ltaD and deltaO-18 values decrease with the transition from quartz to carbonate deposition (from -78 and -2.8 parts per thousand to -90 and -8.7 parts per thousand, respectively), indicating increasing involvem ent (mixing) of less evolved meteoric water which resulted in progress ive cooling and dilution of ore fluids in the shallow (almost-equal-to 370-600 m) Wolyu epithermal system.