Geologic, sulfur isotope, and fluid inclusion studies of the Sannae W-Mo Mine, Republic of Korea; comparison of sulfur isotope systematics in Korean W deposits

Abstract

The Sannae tungsten-molybdenum mine is located approximately 60 km north of Pusan within the Gyeongsang basin of the southern Korean peninsula. The hydrothermal vein-type deposits which comprise the mine occur within an Upper Cretaceous magnetite series quartz monzonite stock which intrudes Middle Cretaceous sedimentary and volcanic rocks. Mineralization can be separated into three stages (with declining temperature): early molybdenum, main tungsten, and late carbonate stages. Fluid inclusion data suggest that most of the molybdenite was deposited at temperatures between 550 degrees and 350 degrees C; wolframite and scheelite, between 450 degrees and 300 degrees C; and carbonates, between 300 degrees and 180 degrees C. Salinities of mineralizing hydrothermal fluids ranged from 5 to 25 equiv wt percent NaCl, with a marked decrease in salinity between main tungsten and late carbonate stages. Fluid inclusion evidence of boiling suggests a range of pressures from about 100 to 300 bars. This range is consistent with a depth of mineralization near 1,250 m under pressure conditions that alternated between hydrostatic and lithostatic.Sulfur isotope compositions of sulfides from the mine have the following ranges of delta 34 S values: molybdenite, 5.2 to 6.0 per mil; and pyrite, 6.0 to 7.1 per mil. The delta 34 S (sub H 2 S) values calculated for these minerals are consistent with an igneous source of sulfur with a delta 34 S (sub Sigma S) value near 4.0 to 5.0 per mil.The geology, mineral paragenesis, and sulfur isotope and fluid inclusion data of the Sannae W-Mo mine are very similar to data from other Korean tungsten deposits. The total range of delta 34 S (sub H 2 S) values of sulfides for all the Korean tungsten deposits is -2 to +7 per mil; however, there is a narrow range of delta 34 S values for individual deposits. The sulfur isotope compositions and systematics of Korean tungsten deposits are nearly identical to those of Japanese deposits associated with magnetite series granitoids and may be explained by the separation of a sulfur-bearing magmatic fluid from a granitic melt under different oxidizing conditions.