광양-승주지역 금은광상의 광화작용

Gold-silver mineralization in the Kwangyang-Seungju area

Gold-silver deposits in the Kwangyang-Seungju area are emplaced along N4°~10°W to N40°~60°W trending fissures and fault in Pre-cambrian Jirisan gneiss complex or Cretaceous diorite. Mineral constituents of the ore from above deposits are composed mainly of pyrite, arsenopyrite, pyrrhotite, magnetite, sphalerite, chalcopyrite, galena and minor amount of electrum, tetrahedrite, miargyrite, stannite, covellite and goethite. The gangue minerals are predominantly quartz and calcite. Gold minerals consist mostly of electrum with a 56.19~79.24 wt% Au and closely associated with pyrite, chalcopyrite, miargyrite and galena. K-Ar analysis of the altered sericite from the Beonjeong mine yielded a date of 94.2±2.4 Ma (Lee, 1992). This indicates a likely genetic tie between ore mineralization and intrusion of the middle Cretaceous diorite (108±4 Ma). The δ³⁴S values ranged from +1.0 to 8.3‰ with an average of +4.4‰ suggest that the sulfur in the sulfides may be magmatic origin. The temperatures of mineralization by the sulfur isotopic composition with coexisting pyrite-galena and pyrite-chalcopyrite from Beonjeong and Jeungheung mines were 343℃ and 375℃ respectively. This temperature is in reasonable agreement with the homogenization temperature of primary fluid inclusion quartz (330℃ to 390℃; Park.1989). Four samples of quartz from ore veins have δ¹⁸O values of +6.9~+10.6‰ (mean=8.9‰) and three whole rock samples have δ¹⁸O values of +7.4~+10.2‰ with an average of 7.4‰. These values are similar with those of the Cretaceous Bulgugsa granite in South Korea (mean=8.3‰; Kim et al. 1991). The calculated δ¹⁸Owater, in the ore-forming fluid using fractionation factors of Bulgugsa et al. (1973) range from -1.3 to -2.3‰. These values suggest that the fluid was dominated by progressive meteoric water inundation through mineralization.