Sulfur isotope characteristics of mesothermal-type gold deposits in the Boseong-Jangheung area, Korea

Abstract

Within the Boseong-Jangheung area of Korea, several hydrothermal gold(-silver) quartz vein deposits exist. They have characteristic features as follows: a relatively gold-rich nature of electrums; the absence of Ag-Sb(-As) sulfosalt mineral; and a massive and simple mineralogy of veins. It is suggested that gold mineralization in this area is correlated with Jurassic to Early Cretaceous mesothermal-type gold deposits in Korea. Fluid inclusion data show that primary +pseudosecondary fluid inclusions in stage I quartz of the mine area homogenize over a wide temperature range of 200degrees to 460degreesC with 0.2 to 13.8 equiv. wt. NaCl. Evidence of fluid boiling including CO2 effervescence indicates that pressures during entrapment of auriferous fluids in this area range up to 770 bars. Calculated sulfur isotope composition of auriferous fluids in this mine area (delta(34)S(SigmaS) = 0.2-3.3parts per thousand) indicates an igneous source of sulfur in hydrothermal fluids. Within the Sobaegsan Massif of Korea, the delta(34)S values of sulfide minerals from the Youngdong area range from - 6.6 to 2.3 parts per thousand (average = -1.4 parts per thousand, N = 66), and those from Boseong-Jangheung area range from -0.7 to 3.6parts per thousand (average = 1.6parts per thousand, N = 39). These delta(34)S values of both areas are comparatively lower than those of most Korean metallic ore deposits (3 to 7parts per thousand). Within the Sobaegsan Massif, the delta(34)S values of Youngdong area are lower than those of the Boseong-Jangheung area. It is inferred that the difference of delta(34)S values within the Sobaegsan Massif can be caused by different degrees of the oxidation of an H2S-rich, magmatically derived sulfur source (delta(34)S = 22parts per thousand) during the ascent to the mineralization sites. According to the observed differences in ore mineralogy (especially, iron-bearing ore minerals) and fluid inclusions of quartz from the mesothermal-type deposits in both areas, we suggest that pyrrhotite-rich, mesothermal-type deposits in the Youngdong area formed from higher temperatures and more reducing fluids than did the pyrite(- arsenopyrite)-rich mesothermal-type deposits in the Boseong-Jangheung area. In order to elucidate the origins of ore sulfur more systematically, we need to determine the delta(34)S values of the Precambrian metamorphic and Paleozoic sedimentary rocks forming the basement of the Korean Peninsula, including the Sobaegsan Massif.