GEOCHEMISTRY AND GENESIS OF HYDROTHERMAL CU DEPOSITS IN THE GYEONGSANG BASIN, KOREA - MASAN MINERALIZED AREA

Hydrothermal vein-type deposits which comprise the Majin and Woongnam mines are primarily copper deposits, but they have associated gold, zi nc, lead, and iron mineralization. The deposits occur within Upper Cre taceous volcanic rocks of the Gyeongsang Basin of Korea. Mineralizatio n can be separated into two distinct stages (stage I, ore-bearing quar tz veins; stage II, barren calcite veins) which developed along pre-ex isting fracture zones. Stage I ore minerals comprise pyrite, arsenopyr ite, pyrrhotite, chalcopyrite, electrum, magnetite, sphalerite, galena and hematite. Fluid inclusion data show that stage I ore mineralizati on was deposited between initial high temperatures (near 330 degrees C ) and later lower temperatures (near 200 degrees C) from fluids with s alinities between 1.4 to 6.9 wt. percent equiv. NaCl. The relationship of salinity and homogenization temperature suggest that ore mineraliz ation at Majin was deposited mainly due to fluid boiling, whereas ore deposition at Woongnam resulted from cooling and dilution via influx o f cooler, more dilute meteoric waters. Evidence of fluid boiling sugge sts pressures decreasing from about 140 to 60 bars. This corresponds t o a depth of about 550 m in a hydrothermal system that changed from li thostatic toward hydrostatic pressure conditions. Sulfur isotope compo sitions of early sulfides of stage I are consistent with an igneous so urce of sulfur with a delta(34)S(Sigma S) value near 2.0 per mil. Calc ulated delta(34)S(H2S) values show a tendency to decrease with paragen etic time during stage I, from about 2.5 parts per thousand to -3.0 pa rts per thousand. This pattern, coupled with fluid inclusion evidence and the occurrence of hematite in later stage I veins, indicate progre ssively increasing oxidizing conditions. Measured and calculated hydro gen and oxygen isotope values of ore-forming fluids indicate meteoric water dominance, with values approaching unexchanged meteoric water wi th increasing paragenetic time. Equilibrium thermodynamic interpretati on of solubility data for copper chloride complexes, and the evolution of the Masan hydrothermal fluids indicate that increasing pH and fo(2 ) concomitant with cooling ore fluids were the main causes of copper p recipitation.