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‰, ␦ 13C 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 CO 2 and CH 4 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 H 2 S. The ␦ 34S H2S 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 H 2 S loss, induced the destabilization of bisulfide complexes and characterized the change of mineral stabilities from arsenopyrite -pyrite -sphalerite to bismuthinite -native bismuth assemblages.

Chalcophile element systematics in volcanic glasses from the northwestern Lau Basin

“…Geochemical studies of pristine and hydrothermally altered oceanic crust, and black smoker‐type hydrothermal fluids have shown Tl is particularly susceptible to leaching during hydrothermal alteration by high temperature (>250–300°C) fluids [ Nielsen et al , 2006]. Similarly, trace element analyses of sulfides associated with black smokers and other types of hydrothermal deposits have noted enrichments in As, Sb, Tl, Bi and Pb [ Hannington et al , 1991; Shin et al , 2004; Kristall et al , 2011]. Hence, enrichment of the mantle wedge via subduction of hydrothermally altered oceanic crust and associated low‐temperature As‐Sb‐Tl‐Bi‐Pb sulfides, as previously suggested to explain the enrichments in Pb in arc magmas and the continental crust compared to MORB [ Chauvel et al , 1995], could explain the enrichment of the source region of the Manus Basin magmas in these chalcophile elements relative to others such as Cu, Ag.…”

Regional variations of sulfur isotope compositions for metallic deposits in South Korea

“…Metallic deposits in relation to Proterozoic-Cenozoic granitic magmatism are widely distributed over the southern Korean peninsula (Figure 1), and the sulfur isotope values of metallic minerals in each deposit have been used to interpret the source of sulfur related to mineralization (e.g., Nakai, 1980, 1982;So et al, 1993So et al, , 2002So and Yun, 1997;Ishihara et al, 2002b;Shin et al, 2004;Lee and Shin, 2009;Yoo et al, 2010a;Im et al, 2018). The sulfur isotope distributions of the metallic deposits are known to be influenced by intrusives and wall rocks on a broad scale, which provide a spatiotemporal and physicochemical information (e.g., pH, redox, and temperature) associated with magma and crustal evolution (Ohmoto and Goldhaber, 1997;So et al, 2002;Ishihara et al, 2002a;Yoon et al, 2003;Bradshaw et al, 2008;Jo and Shin, 2015;Choi et al, 2018).…”