Abstract

The Samkwang gold-silver deposits consist of gold-silver-bearing hydrothermal massive quartz veins which filled the fractures along fault shear (NE, NW) zones within Precambrian banded or granitic gneiss of Gyeonggi massif. Ore mineralization of this deposits occurred within a single stage of quartz vein which was formed by multiple episodes of fracturing and healing. Based on vein mineralogy and paragenesis, massive quartz veins are divided into two main paragenetic stages which are separated by a major faulting event. Main ore mineralization occurred at stage I. Wall-rock alteration from this deposits occur as mainly sericitization, chloritization, silicification and minor amounts of pyritization, carbonitization, propylitization and argillitization. Ore minerals are composed mainly of arsenopyrite (29.21~32.24 As atomic %), pyrite, sphalerite (6.45~13.82 FeS mole %), chalcopyrite, galena with minor amounts of pyrrhotite, marcasite, electrum (39.98~66.82 Au atomic %) and argentite. Systematic studies of fluid inclusions in early quartz veins and microcracks indicate two contrasting physical-chemical conditions : 1). temperature (215~345^oC) and pressure (1296~2022 bar) event with H₂O-CO₂-CH₄-NaCl fluids (0.8~6.3 wt. %) related to the early sulfide deposition, 2). temperature (203~441^oC) and pressure (320 bar) event with H₂O-NaCl ± CO₂ fluids (5.7±8.8 wt. %) related to the late sulfide and electrum assemblage. The H₂O-NaCl ± CO₂ fluids represent fluids evolved through fluid unmixing of an H₂O-CO₂-CH₄-NaCl fluids due to decreases in fluid pressure and influenced of deepcirculated meteoric waters possibly related to uplift and unloading of the mineralizing suites. Calculated sulfur isotope compositions (δ³⁴S_fluid) of hydrothermal fluids (1.8~4.9‰) indicate that ore sulfur was derived from an igneous source. Measured and calculated oxygen and hydrogen isotope compositions (δ¹⁸O_H2O, δD) of ore fluids (-5.9~10.9‰, -102~-87‰) indicate that mesothermal auriferous fluids at Samkwang gold-silver deposits were likely mixtures of H₂O-rich, isotopically less evolved meteoric water and magmatic fluids.

Keywords Samkwang gold-silver deposits, Ore mineralization, Fluid inclusion, Stable isotope.