Abstract

The Yeoju gold-silver mineralized district is located approximately 60 km southeast of Seoul within the Precambrian Gyeonggi metamorphic belt of the Korean peninsula. Mines of the district are located along gold-bearing hydrothermal quartz veins that crosscut early Proterozoic augen gneiss and Mesozoic granites. Mineralization occurs in three stages (I, II, and carbonate) which fill preexisting fault breccia zones. Fluid inclusion data suggest that quartz sulfide-bearing stages I and II evolved from initial high temperatures (near 350 degrees C) to later lower temperatures (near 180 degrees C). Fluid inclusion data from the postore carbonate stage reflect much cooler (220 degrees -190 degrees C), more dilute (4-5 equiv wt % NaCl) hydrothermal fluids.Fluid inclusion and stable isotope evidence show that electrum, jalpaite, argentite, galena, and sphalerite were deposited at temperatures between 285 degrees and 185 degrees C from fluids with salinities between 14.0 and 2.6 equiv wt percent NaCl. Fluid inclusion evidence of boiling suggests pressures of less than 100 bars during portions of stage I and II mineralization, corresponding to depths between 500 and 1,250 m, assuming lithostatic and hydrostatic loads.Sulfur isotope compositions of stage I sulfide minerals decrease systematically with paragenetic time from calculated delta 34 S (sub H 2 S) values of 7.7 to 0.7 per mil. A gradual increase in the sulfate/sulfide ratio of the fluid, likely due to H 2 S loss during boiling coupled with declining temperatures, not only resulted in the systematic decrease of delta 34 S (sub H 2 S) values with time but may also have caused gold deposition by the breakdown of Au(HS) (super -) 2 .The H and O isotope values of hydrothermal fluids in the Yeoju district are consistent with meteoric water dominance approaching unexchanged meteoric water values. Comparison of these values with those of other Korean Au-Ag deposits reveals a relationship between depth and water to rock ratios: Jurassic Au systems are mesothermal and have ratios of 0.001 to 0.01 whereas Cretaceous Au-Ag systems are epithermal with ratios of 0.02 to 0.5.

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