경북ㆍ의성 동남부 오토산 주변의 동광화작용

Copper mineralization around the Ohto mountain in the southeastern part of Euiseong, Gyeongsangbug-do, Republic of Korea

The Ohto and Tohyun copper mine which are located 4 km southeast of Euiseong, Gyeongsangbukdo, Republic of Korea show various common geologic and mineralogic features. Both copper deposits are of hydrothermal-vein types, and associated with fracture system developed during formation of the Geumseong-san caldera in late Cretaceous age. According to structures and mineral assemblages, the mineralization processes have progressed in four stages: three hypogene mineralization stages and one supergene stage. Three hypogene stages are 1) stage Ⅰ forming N5~20°E veins in the Ohto mine, 2) stage Ⅱ building N5°W~N5°E veins in the Tohyun mine, and 3) stage Ⅲ bringing N80°E veins which crosscut veins of the stage Ⅱ. The vein ores consist mainly of pyrite, arsenopyrite, galena and chalcopyrite, minor or trace amounts of magnetite, hematite, pyrrhotite, stannite, bournonite, boulangerite, stibnite, galenobismutite, native bismuth, marcasite, geothite and malachite. The main gangue minerals are quartz and calcite. Wallrock is altered by sericitization, chloritization, pyritization, carbonitization and argillization. Arsenic and copper contents in arsenopyrite increase from stage Ⅰ to stage Ⅲ (from 31.28 to 33.43 atom.% As) and (from 0.04 to 0.40 atom.% Co). Going from stage Ⅰ to stage Ⅲ Fe and Mn contents in sphalerite decreases from 12.56 to 0.44 wt.% and from 0.24 to 0.01 wt.%, respectively. The compositional data of arsenopyrite in the early stage Ⅰ indicate a temperature of 420~365℃ and sulfur fugacity of 10-6.5~10-8.3 atm. Chalcopyrite and pyrrhotite assemblage suggest that Middle stage Ⅰ was deposited at below 334℃. The compositional data of arsenopyrite in early stage Ⅱ suggest a temperature range of 425~390℃ and sulfur fugacity codition of 10-6.4~10-7.3 atm. Based on fluid inclusion the Middle stage Ⅱ was regarded as to be deposited at 185~337℃ (Chi et al., 1989). Referring composition of sphalerite and stannite middle-late stage Ⅱ seem to be deposited around 246℃ and 10-16.5 atm. sulfur fugacity. The δ³⁴S values of sulfide minerals in the Stage Ⅰ, Ⅱ, Ⅲ range from 4.9 to 7.6‰ and indicate igneous ore fluid origin. Based on differences in mineral assemblages, chemical composition and chemical environments of Ohto and Tohyun mine its mineralization are considered to be formed at differnt mineralization ages and by different ore fluids.