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

Mesothermal gold mineralization of the Seolhwa mine occured in a single stage of massive quartz veins which filled the mainly NE-trending fault shear zones exclusively in the granitoid within the Gyeonggi Massif. The vein quartz contains three main types of fluid inclusions at 25oC: low-salinity (<5 wt. % NaCl), liquid CO2-bearing, type IV inclusion; gas-rich (>70 vol. %), vapor-homogenizing, aqueous type II inclusions; and aqueous type I inclusions (0~15 wt. % NaCl) containing small amounts of CO2. Fluid inclusion chronologies may suggest a progressive decrease in pressure during the auriferous mineralization. The H2O-CO2-NaCl inclusions represent immiscible fluids trapped earlier along the solvus curve at temperatures of 430o to 250oC and pressure of 1 kbars. The aqueous inclusion fluids represent either later fluids evolved through extensive fluid unmixing from a homogeneous H2O-CO2-NaCl fluid due to decreases in temperature and pressure, or the influence of deep circulated meteoric waters possibly related to uplift and unloading of the mineralizing suites. Measured and calculated isotopic compositions of the mesothermal fluids [d18O = 5.3 to 6.5‰; dD = -69 to -84‰ ] provide evidence of CH4-H2O equilibria and may further indicate that the auriferous mesothermal fluids were derived magmatically. The available d13C data (-4‰) are consistent with their deep igneous source. The Seolhwa mesothermal gold deposits were probably formed by extensive fracturing and veining due to thermal expansion of waters derived from the granitoid melt (probably formed by dehydration of mica-rich Precambrian paragneiss). The chemical characteristics of the fluid and the pressure-depths of the Seolhwa auriferous mesothermal deposits are similar to those estimated in the Archean gold deposits, possibly indicating not only the general application of the magmatic model to the mesothermal deposits worldwide but also extending to non-Archean mineralization epochs related to postorogenic granitoid intrusion.