ABSTRACT

The Namseong gold and silver deposit is located in the Seolcheon metallogenic province, one of the most important gold production areas in the Republic of Korea. It consists of three gold and silver-bearing quartz veins that fill fractures oriented NW along fault zones in Triassic porphyritic granite. The gold and silver-bearing quartz veins are simple and consist of only one mineralization stage on basis of vein mineralogy and paragenesis. Coarse-grained quartz is found in mainly comb, and partially cockade and cavity-filling textures throughout most of the veins. Wall-rock alteration minerals include sericite, minor pyrite and chlorite. Ore minerals deposited along with electrum include pyrite, sphalerite, chalcopyrite, galena, pyrargyrite, argentite and native silver. Its mineralization age (78.2 ± 1.7 Ma) determined on sericite from wall-rock alteration indicates that the gold and silver mineralization was synchronous with Cretaceous igneous activity. Petrographic examination of textural relationships among sulfides, fluid inclusions, and quartz from one stage shows the genetic relationships between Au-Ag deposition and fluid entrapment. Early wall-rock alteration in one stage was produced and sulfides were deposited from H₂O-NaCl-CO₂ fluids (260-1,700 bar, average 1,000 bar) with Th_total values of 250°C to 380°C and salinities less than about 4 wt.% NaCl. The late sulfides and electrum in one stage were deposited from H₂O-NaCl fluids (100 bar) with Th_total values of 195°C to 331°C and salinities less than about 7 wt.% NaCl. The H₂O-NaCl fluids either evolved through unmixing of H₂O-NaCl-CO₂ fluids or through mixing with circulating meteoric waters as a result of uplift or unloading during mineralization, or both. The deposition of electrum and silver minerals was caused by a decrease in sulfur fugacity/aH₂S/aCl, oxygen fugacity and temperature during wallrock alteration, cooling and dilution produced by mixing of the saline aqueous fluids with meteoric water. The calculated sulfur isotope compositions of hydrothermal fluids from the Namseong deposit (δ³⁴S_H2S = 3.7‰ to 7.2‰) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (δ¹⁸O_H2O = -2.2‰ to 1.6‰, δD = -83‰ to -64‰) indicate that the fluids were derived from meteoric water and evolved by mixing with local meteoric water and by limited water-rock exchange during mineralization in uplift zones.