Geochemistry of the granitoids hosting the seolhwa au mine, asan district, chungcheongnamdo province, Korea: Genetic implication on the mesothermal gold mineralization

Gold-silver vein deposits in South Korea are associated intimately with major periods of granitic plutonism. Among these, mesothermal-type gold-rich deposits appear to be associated genetically with Jurassic Daebo Series (210 to 150 Ma) granitoids. The Seolhwa mesothermal gold mineralization were deposited in quartz veins which filled the fault shear zones in the granitoid within the Gyeonggi Massif. These granitoids(161±4 Ma) have the following geochemical characteristics: 1), the relatively high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7224 of S-type nature; 2), the trace-element characteristics of continent- continent collision tectonic environment.; 3), the high A/CNK value of 1.4 to 1.6 of peraluminous nature; 4) the nondectectable gold of granitoid. The mineralogy is simple and consists mainly of rare sulfides and gold. Gold deposition occurred at temperatures and salinities of 250°C to 430°C and 0.4 to 4.9 wt% NaCl. and most fluid inclusions contain X_CO2 of 0.16 to 0.62. Recently, it has been conjectured that the Chinese collision belt crosses Korea as either the Imjingang belt or a suture complex, including not only the Imjingang belt but also the Gyeonggi massif and the Ogcheon belt. And Mesozoic (ca. 160–150 Ma) granitoids in East Shandong within this Chinese collision belt contain some 30 Moz gold, making East Shandong the largest granitoid-hosted gold province in the world. Geochemically, the Mesozoic granitoids in East Shandong have the following characteristics: 1), the relatively high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7095 to 7273 of S-type nature; 2), the trace-element characteristics of continent-continent collision tectonic environment.; 3), the relatively high A/CNK value of 0.92 to 1.06 of peraluminous nature. Most of the gold occurs as quartz-veins in the granitoids. The major gold mineralization generally display the characteristics of gold-only provinces, namely CO₂-rich, low salinity and reduced fluids. Gold mainly occurs as gold and electrum in the quartz veins. The main minerals closely associated with gold mineralization are pyrite and pyrrhotite, and locally chalcopyrite, galena and sphalerite. Fluid inclusion studies indicate that the fluid temperature range from 170°C to 350°C, salinity varies from 5 to 13 wt% NaCl equivalent (mostly 10%), and that most fluid inclusions contain 10 to 25% CO₂. The gold mineralization may be more tectonically and structurally controlled rather than have a direct genetic relationship with the granitoid itself in terms of nondetectable gold. The major gold in East Shandong may have been formed during the subsequent exhumation stage after the collision of the North and South China cratons. Considering to the above-mentioned similarities between the Seolhwa mine district and the East Shandong mine district, it may be conjectured that the major gold in the Seolhwa mine district may have been formed during the subsequent exhumation stage after the collision of the Sino-Korean and Yangtze cratons.