Journal of the Korean Society of Mineral and Energy Resources Engineers ISSN:2288-0291(Print) 2288-2790(Online) 한국자원공학회지

The Mugeuk mine was historically the largest gold-silver producer in South Korea, and yielded more than ten metric tons of gold with an average grade of about 8 g/ton Au . Ore deposits consist of more than 10 subparallel quartz-calcite veins that fill faults and fractures in Jurassic granodiorite and Cretaceous quartz porphyry. Veins formed during six successive hydrothermal events of Late Cretaceous age. Ore mineralogy of the veins is complex, consisting mainly of base-metal sulfides with electrum (23-51 atom. % Au), native silver, argentite and silver sulfosalts. Stage I veins do not contain gold and silver minerals. During stages II to V, economic quantities of gold and silver were precipitated. Stages III and IV represent the culmination of gold precipitation in distinct sulfide bands, whereas stage V represents a shift to dominantly silver deposition. Stage VI veins are post-ore barren quartz-calcite-fluorite. A variety of types of geochemical data indicate that deposition of gold and silver resulted mainly from cooling and mixing of ore fluids, accompanying successive incursions of meteoric water into the hydrothermal system. Fluid inclusion data show general decreases of temperature and salinity within each stage. Ore mineral assemblages indicate decreases in the fugacity of sulfur with decreasing temperatures during the deposition of gold and silver. Measured and calculated δ18O values of hydrothermal fluids decrease generally from +3.0 to 7.4‰ from stages II through VI, and δD values range from 66 to 84‰. The Au-Ag deposits at Mugeuk, in particular, mineralogy, Au/Ag ratios and fluid geochemistry, are the result of episodic meteoric water events within the granite-hosted hydrothermal system.