Episodic meteoric water interaction in a granite-hosted hydrothermal gold-silver system: Geochemistry of fluids at the Mugeuk mine, South Korea

Abstract

The Mugeuk mine was historically the largest gold-silver producer in South Korea, and yielded more than ten metric tons of gold from ores with an average grade of about 8 g/ton Au and Au/Ag ratios near 1:5. Ore deposits consist of >10 subparallel quartz-calcite veins (typically 0.6 to 1.0 m wide) 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 pyrite and base-metal sulfides with electrum (23‐51 atom. % Au), native silver, argentite and silver sulfosalts (polybasite, pyrargyrite). Stage I veins contain no gold and silver (e.g., barren). 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 barren post-ore quartz-calcite-fluorite. A variety of types of geochemical data indicate that deposition of gold and silver resulted mainly from cooling 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 δ 18 O 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 their mineralogy, Au/Ag ratios and fluid geochemistry, are the result of episodic meteoric water events within the granite-hosted hydrothermal system. Early stages of the hydrothermal history involved successive introduction of new pulses of meteoric ore fluids, each of which equilibrated with the host granitic rocks at successively higher water-to-rock ratios and lower temperatures during deposition of gold-silver mineralization. Later stages were the result of inundation of the hydrothermal system by isotopically unevolved meteoric waters, resulting in a silver-rich overprint on the earlier gold-silver system.