Genetic environment of W skarn and Pb-Zn vein mineralization associated with the Imog granite in the Taebaeksan Mineralized District, South Korea

Abstract

W skarn and Pb-Zn vein mineralization were developed in the Imog and Seojin deposits in association with the Imog granite that intruded the Cambro-Ordovician carbonate rocks in the Taebaeksan Mineralized District, South Korea. Garnet/Pyroxene ratios in the skarn of both deposits decreased as away from the granite. In the Imog deposit, garnet compositions vary from andradite to grossular series with increasing distance from the igneous contact. However, andradite is dominant in the Seojin deposit. In both the deposits, diopside series remained dominant with a slight increase in johannsenite composition in late stage. The skarn ore minerals mainly comprise scheelite and pyrrhotite, whereas the vein ores comprise galena, sphalerite, and arsenopyrite. In sphalerite, a decrease in the CdS contents and an increase in the MnS contents from W skarn to Pb-Zn vein ore can be attributed to leaching from sedimentary rocks and high mobility in magmatic-hydrothermal fluids, respectively. The δ34SH2S values of sulfide minerals were 3.3–4.9‰ for W skarn and 4.1–6.7‰ for Pb-Zn vein ore in the Imog deposit, mostly belonging to magmatic origin, but a slight increase is observed in the vein stage due to the interaction with carbonate host rock. The isotopic depletion of δ18O and δ13C values from host rock to skarn and Pb-Zn vein can be mostly attributed to the infiltration of external fluids involving magmatic components. The effects of organic carbon in the host rock and input of meteoric water resulted in distinctly low δ13C and δ18O values in the skarn and vein samples, respectively. The sulfur isotope geothermometer and mineral stability in combination with fluid inclusion data constrained the upper limit of skarn formation to 520 °C and 3 kbar and a temperature range of <360–460 °C and >0.5 kbar for W and Pb-Zn mineralization in the Imog deposit, but pressure would be higher in the Seojin deposit. The skarn was formed by the intrusion of oxidized magma, and scheelite mineralization occurred with decreasing oxygen fugacity. The Pb-Zn vein ores were precipitated with decreasing sulfur fugacity and increasing pH under the influence of meteoric water and carbonate host rock.