Gold-rich mesothermal vein deposits of the Republic of Korea; geochemical studies of the Jungwon gold area

Abstract

Gold vein deposits of the Jungwon area are among the first documented examples of mesothermal gold deposition in the Republic of Korea. The deposits are composed of massive quartz veins up to 1.0 m wide which occupy fault planes in Precambrian gneiss. K-Ar dating of alteration sericite (146-156 m.y.) indicates a Late Jurassic age for ore mineralization, similar to that of a nearby granodiorite. The veins are gold rich and sulfide poor with ore shoots up to 200 m in horizontal and vertical extent. Ore grades average 10.2 g/ton gold with an average gold/silver production ratio of 6:1.Fluid inclusion data indicate that gold was deposited between 375 degrees and 290 degrees C from relatively dilute fluids ([asymp]4 equiv wt % NaCl) containing [asymp] 15 to 25 mole percent CO 2 . Evidence of fluid unmixing (CO 2 effervescence) indicates pressures of 1,200 to 1,500 bars. Gold deposition was likely a result of decrease of sulfur activity caused by sulfide deposition and/or H 2 S loss accompanying fluid unmixing.Measured and calculated compositions of ore fluids in the Jungwon area (delta 18 O = 5.0 to 7.7ppm; delta D = -78 to -113ppm) indicate that gold was deposited from a highly evolved meteoric water that underwent extreme 18 O enrichment and moderate D enrichment. Mineralogy and ore fluid chemistry of the Jungwon area deposits are similar to those of mesothermal gold vein deposits of the Canadian Cordillera, Canadian Archcan greenstone belts, and the Mother Lode district of California. The diversity of host rocks in these deposits indicates that host-rock petrology is not a controlling factor in the formation of mesothermal gold vein deposits. These observations, together with our geochemical studies of Korean deposits, may validate a more general application of meteoric water models for the formation of mesothermal gold vein deposits.