Abstract

The Cheonan gold-silver-mineralized area is located approximately 80 km south-southeast of Seoul within the Precambrian Gyeonggi metamorphic belt of the Korean peninsula. The Cheonbo, Ilbo, Seonggeo, Daeheung, and Chungnam mines which comprise the area are each located along gold-bearing hydrothermal quartz veins that crosscut early Proterozoic gneiss and Jurassic and Cretaceous granites. Mineralization can be separated into three distinct stages (I, II, and III) which fill preexisting fault breccia zones. Stages I and II are terminated by the onset of fracturing and brecciation events. Fluid inclusion data suggest that quartz-sulfide-bearing stages I and II each evolved from initial high temperatures (near 400 degrees C) to later lower temperatures (near 200 degrees C). Each of these stages represents a separate mineralizing system which cooled and largely abated prior to the onset of the next stage. Fluid inclusion data from stage III, a postore carbonate stage of mineralization, indicate a much cooler (240 degrees - 115 degrees C), more dilute (3-4 equiv wt % NaCl) hydrothermal system which was the result of increasing influx of meteoric waters.Sulfur isotope and fluid inclusion evidence suggests electrum-sphalerite-galena ore mineralization was deposited during stage I at temperatures between 170 degrees and 255 degrees C from fluids with salinities between 3 and 9 equiv wt percent NaCl. Fluid inclusion evidence of boiling suggests pressures of less than 100 to 300 bars. This range of pressure is consistent with a depth of mineralization near 1 km under pressure conditions that alternated between hydrostatic and lithostatic. Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a delta 34 S (sub Sigma S) value near 5.0 per mil.Measured and calculated delta D and delta 18 O values of hydrothermal fluids in the Cheonan area ore deposits are: stage I, -122 to -127 and 6.2 to 5.9 per mil; stage II, -135 to -143 and 3.6 to 3.2 per mil; stage III, -134 to -137 and 3.4 to -7.4 per mil, respectively. These values are consistent with progressive meteoric water involvement (increasing water/rock ratios) with increasing paragenetic time. Comparison of these values with those of a deeper (2-3 km) Jurassic gold deposit (delta D = -78--102; delta 18 O = 7.7-3.8ppm) suggests a relationship between depth and water/rock ratio in Korean Au-Ag-bearing vein deposits: the Jurassic Au system has ratios of 0.001 to 0.01; the Cretaceous Au-Ag system, 0.02 to 0.5. The relationships among depth, metal contents, and water/rock ratios in these Korean Au-Ag deposits suggest differences in the hydrodynamics and postmagmatic evolutions of granitic hydrothermal systems and may be indicative of the manner in which gold and silver are scavenged from cooling plutons.

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