GEOCHEMICAL STUDIES OF THE GYEONGCHANG W-MO MINE, REPUBLIC-OF-KOREA - PROGRESSIVE METEORIC WATER INUNDATION OF A MAGMATIC HYDROTHERMAL SYSTEM

Abstract

The Gyeongchang W-Mo mine is located within the Cretaceous Gyeongsang basin along the southeastern margin of the Korean peninsula. The major ore minerals, wolframite and molybdenite, together with galena, sphalerite, scheelite, and minor Ag and Bi sulfosalts, occur in fissure-filling quartz veins contained within a micrographic biotite granite. Radiometric dates of 83.7 +/- 2.2 Ma for the granite and 82.4 +/- 1.4 Ma for vein alteration muscovite indicate a Late Cretaceous age for granite emplacement and associated W-Mo mineralization. The ore mineral paragenesis can be divided into four distinct stages: W-Mo, Fe oxide, Pb-Zn, and carbonate. The shifts in mineralogy reflect decreases in the temperature and fugacity of sulfur with a concomitant increase in the fugacity of oxygen. Fluid inclusion data indicate progressive decreases in temperature and salinity within each stage with increasing paragenetic time. During the W-Mo stage, high-temperature (> 450-degrees-C), high-salinity fluids (30-33 wt % NaCl equiv) formed by condensation during decompression of a magmatic vapor phase at pressures between 300 and 500 bars. In the waning portion of the W-Mo stage, the high-temperature, high-salinity fluids gave way to progressively cooler, more dilute fluids of the Fe oxide (380-degrees-270-degrees-C, 23-5 wt % NaCl equiv) and Pb-Zn (270-degrees-200-degrees-C, 11-1 wt % NaCl equiv) stages. These trends are interpreted to indicate progressive mixing of high-salinity, magmatic hydrothermal fluids with cooler, less acidic, more oxidizing meteoric waters. There is a systematic decrease in calculated delta-O-18water values with decreasing temperature in the Gyeongchang hydrothermal system, from values of almost-equal-to 7 per mil for W-Mo stage mineralization, to 2.8 per mil for the Fe oxide stage, to -0.6 to -2.2 per mil for the Pb-Zn stage. The delta-D values of inclusion waters also decrease with paragenetic time: W-Mo stage, -50 to -57 per mil; Fe oxide stage, -68 per mil; Pb-Zn stage, -69 to -71 per mil. These trends are interpreted to indicate progressive meteoric water inundation of an early magmatic W-Mo hydrothermal system. We suggest that without significant introduction of meteoric waters into the Gyeongchang hydrothermal system, its deposits would be simple quartz-wolframite-molybdenite veins rather than a complex sequence of W-Mo, Fe oxide, Pb-Zn, and precious metal mineralization.