Origin and evolution of W-Mo-producing fluids in a granitic hydrothermal system; geochemical studies of quartz vein deposits around the Susan Granite, Hwanggangri District, Republic of Korea

The Hwanggangri mineralized district is located approximately 130 km southeast of Seoul within the Paleozoic Ogcheon belt of South Korea. The Jungbo, Suri, and Deogman mines in the district occur around the Late Cretaceous (whole-rock K-Ar age = 89.0 + or - 2.0 Ma) Susan Granite and are located on W-Mo-bearing quartz veins. These veins fill fractures in the granite and its host rocks which comprise early Paleozoic limestone-rich sedimentary rocks. The K-Ar date of vein muscovite (88.58 + or - 1.95 Ma) is very similar to that of the Susan Granite (89.04 + or - 1.95 Ma), indicating that W-Mo mineralization was associated with intrusion of the granite. Molybdenite- and wolframite-rich deposits occur within or near (<500 m) the granite, whereas scheelite and base metal sulfide-rich deposits, and fluorite- and carbonate-rich deposits occur mainly at distances of 500 to 1,300 m from the granite. Studies of the deposits provide an understanding of the evolution of ore-forming processes in a granite stock-related hydrothermal system.The ore mineral paragenesis can be divided into three stages, based on the textural relationships of different mineral assemblages: I (W-Mo stage), II (base metal sulfides stage), and III (barren carbonate-fluorite stage). Early, higher temperature veins occur near the Susan Granite, with younger, lower temperature veins located progressively farther from the granite. Fluid inclusion data indicate that deposition of molybdenite and wolframite occurred at temperatures of 370 degrees to >285 degrees C from H (sub 2) O-CO (sub 2) fluids with salinities of 0.4 to 9.7 wt percent NaCl equiv, probably in response to pH increase due to boiling (including CO (sub 2) unmixing) of the hydrothermal fluid at pressures between 150 and 220 bars. Following W-Mo deposition, the high-temperature fluids gave way to progressively cooler, more dilute fluids associated with scheelite-base metal sulfide deposition (200 degrees -320 degrees C, 0.7-7.2 wt % NaCl equiv) and carbonate-fluorite deposition (105 degrees -225 degrees C, 0.0-3.2 wt % NaCl equiv). These trends are interpreted as the result of progressive mixing of magmatic fluids with cooler, more dilute meteoric waters.There is a systematic decrease in the measured and calculated isotopic compositions of hydrothermal waters with increasing paragenetic time and decreasing temperature: stage I, delta (super 18) O (sub water) = 8.0 to 1.5 per mil and delta D (sub water) = -56 to -74 per mil; stage II, delta (super 18) O (sub water) = 1.9 to -5.6 per mil and delta D (sub water) = -88 to -95 per mil; stage III, delta (super 18) O (sub water) = 0.9 to -3.0 per mil and delta D (sub water) = -93 to -97 per mil. The trend reflects the progressive increase of meteoric water involvement in the mineralizing system. During the cooling history of the Susan Granite stock, molybdenum and tungsten were partitioned preferentially into a coexisting magmatic hydrothermal fluid (delta (super 18) O = 8ppm, delta D = -56 to -72ppm). Sulfur isotope analyses of hydrothermal sulfide minerals suggest an igneous source of sulfur with a delta (super 34) S (sub Sigma S) value near 4 per mil. Following deposition of molybdenite and wolframite from magmatic fluids, wolframite was dissolved or replaced by scheelite in the outer and/or shallower portions of the hydrothermal system. This remobilization of tungsten and later redeposition as scheelite (at temperatures of 200 degrees -280 degrees C from fluids with delta (super 18) O = 1-2ppm) accompanied mixing with low-temperature meteoric fluids. Increasing involvement of progressively larger volumes of meteoric water resulted in successive deposition of base metal sulfides, fluorite, and carbonates.