Stable isotope and fluid inclusion studies of gold-silver-bearing hydrothermal-vein deposits, Cheonan-Cheongyang-Nonsan mining district, Republic of Korea : cheongyang area

청양지역 광화대 금-은 광상들은 화강편마암내의 단층대를 충진한 수개조의 함 금-은 열수맥상 광체로 구성된다. 광화작용의 시기는 127.1±2.8 Ma이고, 열수광화작용은 구조운동에 시기적으로 2회에 걸쳐 진행되었다. 공생광물의 유체포유물 및 안정동위원소 연구에 의하면, 초기 약 340°의 고온에서 180°에 이르는 제Ⅰ광화시기에는 일렉트럼(42.2-66.8mole % Au), 농홍은석, 함은사면동석, 휘은석, 유비철서, 방연석, 황동석 및 섬아연석 XFeS≒0.13)등이, δ³⁴S∑S=2-5%, fs₂＜10-9.8atm., 1-8wt. % NaCl 상당염농도를 갖는 유체로부터 비등현상과 함께 침전되었다. 광화작용시의 압력은 ＜200-700기압이고, 광화작용의 심도는 약 1.5km였다. 열수 광화유체내 물의 수소(-90~-130‰), 산소(-5.9~+0.1‰) 동위원소값은 광화유체의 기원이 천수임을 뜻하고, 광화유체의 비등현상과 냉각작용은 340℃이하에서 유체내 금-은 복합체(Au (HS)₂, AgCl₂)의 파괴를 초래하여 금-은 광물의 침전을 유도하였다.

Electrum-sulfide mineralization of the Samgwang and Sobo mines of the Cheongyang Au-Ag area was deposited in two stages of quartz and calcite veins that fill fault zones in granite gneiss. Radiometric dating indicates that mineralization is Early Cretaceous age (127 Ma). Fluid inclusion and sulfur isotope data show that ore mineralization was deposited at temperatures between 340° and 180℃ from fluids with salinities of 1 to 8 wt. % equiv. NaCl and a δ³⁴S∑S value of 2 to 5 per mil. Evidence of fluid boiling (and CO₂ effervescence) indicates a range of pressures from ＜ 200 to ○ 700 bars, corresponding to depths of ○ 1.5±0.3 km in a hydrothermal system which alternated from lithostatic toward hydrostatic conditions. Au-Ag deposition was likely a result of boiling coupled with cooling. Meaured and calculated hydrogen and oxygen isotope values of ore-forming fluids indicate a significant meteoric water component, approaching unexchanged paleometeoric water values. Comparison of these values with those of other Korean Au-Ag deposits reveals a relationship among depth, Au/Ag ratio and degree of water-rock interaction. All investigated Korean jurassic and Cretaceous gold-silver-bearing deposits have fluids which are dominantly evolved meteoric waters, but only deeper systems (≥ 1.5 km) are exclusively gold-rich.