HYDROTHERMAL As Bi MINERALIZATION IN THE NAKDONG DEPOSITS, SOUTH KOREA: INSIGHT FROM FLUID INCLUSIONS AND STABLE ISOTOPES

Abstract

At the Nakdong As–Bi deposits, South Korea, Cambro-Ordovician sedimentary sequences are cut by numerous dykes of quartz monzodiorite and porphyritic granite. In the deposit, stage I involves arsenic mineralization, chiefly associated with arsenopyrite, pyrite, sphalerite and chalcopyrite, whereas bismuth mineralization characterizes stage II, with the coprecipitation of pyrrhotite, chalcopyrite, galena, bismuth, bismuthinite, cosalite, matildite, schirmerite, Au–Ag alloy, and argentite. The mineralization was initiated with the introduction of heterogeneous fluids of high salinity, presumably owing to the prominence of Ca, Mg, Na and K. At stage II, fluid immiscibility, which led to bismuth mineralization, produced (halite ± sylvite)-bearing highsalinity fluids of 27.6 to 49.3 wt.% NaCl equivalents, as well as low-salinity vapor-rich fluids. The homogenization temperatures of mineralizing fluids decreased only slightly from stage I, 283–416°C, to stage II, 222–395°C. A decrease in 18 O as well as 13 C in going from calcite in fresh limestone ( 18 O in the range +17.5 to +22.4‰, 13 C in the range +2.3 to +4.4‰) to silicified limestone ( 18 O in the range +13.3 to +18.3‰, 13 C in the range –2.5 to +1.3‰) were promoted not only by Rayleigh volatilization, but also by fluid–rock interaction, with the influx of magmatic fluids into carbonate rocks during the mineralization process. The fluid–rock interaction contributed to the CO2 and CH4 components in type-III fluids and also to the Ca-enrichment in type-I fluid inclusions. The prevailing species of sulfur in the mineralizing fluids is estimated to have been H2S. The 34 SH2S values obtained from the sulfide minerals increased from stage I, +3.2 to +4.4‰, to stage II, +4.1 to +4.8‰, values typical of magmatic sulfur, and the temperatures of homogenization of fluid inclusions increased as well. The decrease of sulfur fugacities, from 10 –9.1 –10 –6.4 atm at stage I to 10 –15.7 –10 –9.2 atm at stage II, through sulfide precipitation and H2S loss, induced the destabilization of bisulfide complexes and characterized the change of mineral stabilities from arsenopyrite – pyrite – sphalerite to bismuthinite – native bismuth assemblages.