Sulfur Isotope Study of Precambrian Basement and Mesozoic Intrusive Rocks in the Southwestern Part of Ryeongnam Massif, Korea

Abstract

Isotope composition of whole rock sulfur has been measured on 14 schists, 10 gneisses, 7 gabbroids, 7 granitoids and 2 sedimentary rocks, with of 9 sulfide (pyrite) sulfurs in gabbros and granitoids, from the southwestern part of the Ryeongnam Massif, Korea. The δ 34 S values of schists range from -4.6 to +6.1 ‰ (average +0.9 ‰), those of gneisses from -4.0 to +0.8 ‰ (-1.9 ‰), those of gabbroids from -2.3 to +3.7 ‰ (+1.0 ‰), and those of granitoids from -5.9 to +3.2 ‰ (-1.9 ‰). The δ 34 S values of pyrite separated from gabbros and granitoids show rather heavier values ranging from +3.1 to +9.4 ‰ with an average of +5.8 ‰.Though the δ 34 S values of whole rock sulfur give wide range of -5.9 to +6.1 ‰, the average of about -0.5 ‰ is close to the mantle value. The granitoids sampled at the central parts of intrusive bodies or at the contacts with other plutonic rocks tend to show positive values, while those sampled near the boundary with basement rocks such as granitic gneiss and porphyroblastic gneiss show negative values. Though the reason of this tendency is not clear at present, the δ 34 S values of some granitoids in this area seem to represent possible influence by the assimilation of country rocks, particularly of gneisses.Average isotopic compositions of ore sulfur from individual metal deposits in the studied area are summarized to have a range of +1.0 to +7.8 ‰ with an average value of +3.2 ‰. The values are consistent with the previous finding that the ore sulfur isotopic values of the Ryeongnam Massif are the lowest among the four tectonic belts in Korea; Gyeonggi Massif, Ogcheon Belt, Ryeongnam Massif, and Gyeongsang Basin. This feature may reflect the isotopic compositions of plutonic rocks and basements in this area, which are characterized by relatively low values around zero permil.

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

“…+1.6‰, n = 14) for Jurassic, and −3.2 to +9.4‰ (av. +4.5‰, n = 9) for Cretaceous (Park et al, 1991;Ishihara et al, 2000a;Yoon et al, 2003). The continual increase in δ 34 S values over time for the granitoid rocks is comparable to that of the metallic deposits.…”

“…For example, the δ 34 S values of the Sambo Pb-Zn deposits (−7.1‰) (Kim and Nakai, 1980) and the Youngbogari Au-Ag deposits (−3.6‰; So et al, 2002) have fairly low δ 34 S values (Table 1). They probably reflect the characteristics of sulfur sources from the Precambrian wall rocks of metasedimentary rocks such as schist and gneiss (−15.0 to +6.1‰) (Yoon et al, 2003;Kim, 2020) and/or from related igneous rocks (−5.9 to +9.4‰) (Yoon et al, 2003), which are comparatively enriched in 32 S (Figure 9). Among the five provinces, the Taebaeksan basin exhibits the highest δ 34 S value of metallic deposits, with av.…”

“…See Table 1 for the data source Precambrian gneiss (2.1-1.9 Ga), with minor schist, amphibolite, quartzite, and sedimentary rocks (Chough et al, 2000;Jin et al, 2001;Lee and Cho, 2012). The boundary of the southern area is partly overlain by volcanoclastic rocks (Yoon et al, 2003) (Figure 5). The metamorphic process was significantly complex because the basement rocks mixed with metasedimentary rocks (mica-schists, quartzites, and calc-silicate rocks) and orthogneiss had simultaneously undergone metamorphism (Park et al, 2010).…”

“…However, those of the ore-bearing fluids of the Cretaceous deposits show a F I G U R E 8 Sulfur isotope compositions of metallic deposits by mineralization age (a), deposit type (b), and principal ore species (c). See Table 1 for the data source F I G U R E 9 Compositional range of sulfur isotope values for wall rocks and igneous rocks in South Korea (data from Shelton et al, 1988;Park et al, 1991;So and Yun, 1997;Ishihara et al, 2000a;Ishihara et al, 2002aIshihara et al, , 2002bYoon et al, 2003;Shin et al, 2016;Kim, 2020) mixing trend between magmatic water and meteoric water (Kim et al, 1992). It was, therefore, interpreted that magmatic contribution was important in the genesis of the Jurassic Au-Ag deposits, and its influence decreased during the Cretaceous (Choi et al, 2005).…”

“…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).…”