ABSTRACT

Mesothermal gold-quartz veins of the Youngbogari deposit in the Youngdong area, South Korea, are hosted in faulted shear zones in Precambrian gneiss of the Sobaegsan massif. The Youngbogari deposit shares many features with other metamorphic rock-hosted gold deposits in the area, including a massive single-stage nature of veins and a simple vein mineralogy. However, its ore mineralogy is unique in that (1) arsenopyrite and pyrite occur dominantly with only minor pyrrhotite and (2) electrum grains are very poor in gold content (<40 atom. % Au). A variety of geochemical data indicate that deposition of the mesothermal gold–silver ores of the Youngbogari deposit resulted mainly from cooling of ore fluids accompanying decreasing sulfur fugacity. Fluid inclusion evidence indicates that gold deposition was not related to an early history of CO₂ phase separation (at temperatures of 300° to 420°C), but rather was tied to a later history of cooling and dilution of H₂O–NaCl ore fluids at temperatures of <300°C. These temperature estimates for gold deposition agree with those based on chlorite thermometry (270° to 320°C) and sulfide mineral assemblages (<290°C). Measured and/or calculated, stable isotope compositions of hydrothermal fluids (δ¹⁸O_Water = 5.8 to 8.0‰, δD_Water = −75 to −54‰) indicate an important role of rock-dominated (possibly magmatic) fluids in the mesothermal gold–silver system. The unusual arsenopyrite pyrite-rich nature of the Youngbogari ores and their anomalously negative δ³⁴S values (−2.9 to −5.1‰), compared to other mesothermal gold deposits of the Youngdong area, are interpreted to reflect a more oxidizing nature of the Youngbogari ore fluids, likely due to less reaction with graphite in wallrock gneiss.