Granitoids and Their Magnetic Susceptibility in South Korea

Abstract: Magnetic susceptibility (MS) measurements were carried out for 1,120 samples in the Middle Proterozoic to Early Tertiary granitoids so far recognized in South Korea, and the lateral and spatial variation of their magnetic susceptibility, i.e., content of magnetite, is studied. The Middle Proterozoic two mica granitoids related to cassiterite (Sn) deposits in northeastern part of the Sobaegsan Massif show very low MS (less than 0.3 A‐ 10^‐3 SI unit), and the Permo‐Triassic tonalitic to granodioritic and monzonitic rocks which are barren in mineralization, distributed in the middle part of South Korea also show low MS (less than 1 A‐ 10^‐3 SI unit). On the contrary the Late Triassic to Jurassic granitoids (= Daebo granitoids) which were evolved from tonalite through granodiorite to granite, and are most widely distributed in South Korea, show a wide variation on MS. Particularly in the Andong, Igsan, Gimcheon and Geochang areas, the granitoids which are barren in mineralization, are characterized by high MS (more than 10 A‐ 10^‐3 SI unit). The Chuncheon, Jecheon, Namyang and Geumsan plutons related to molybdenite (Mo) and/or wolframite or scheelite (W), and fluorite (F) mineralizations show a little high MS (more than 3 A‐ 10^‐3 SI unit). However, more than 60% of the Daebo granitoids show low MS (less than 3 A‐ 10^‐3 SI unit) and the rest show a little high MS (more than 3 A‐ 10^‐3 SI unit). Heterogeneous distribution of magnetite content in the Daebo granitoids is considered to reflect heterogeneity of redox state of the source materials for these granitoids. The Cretaceous to Early Tertiary granitoids (= Bulgugsa granitoids) in the Gyeongsang Basin had been generally evolved in the order of tonalite, diorite, granodiorite, granite and alkali‐feldspar granites, which are closely related to base metal ore deposits, and mostly show higher MS (more than 3 A‐ 10^‐3 SI unit) than other granitoids mentioned above, although some exceptions are recognized in highly evolved alkali‐feldspar granites (SiO₂ > 76%). In contrast, as most of the highly oxidized or evolved Cretaceous granitoids distributed in areas other than the Gyeongsang Basin show lower MS than those of the Gyeongsang Basin, and appear to be magnetite free, ilmenite‐series granites, but they might be hematite bearing magnetite‐series granitoids. Highly oxidized nature of the Bulgugsa granitoids may be due to high Fe₂O₃/FeO ratio of the source materials and also high level intrusion style of the granitic magma activities. Most of the granitic rocks of the Middle Proterozoic, Permo‐Triassic and more than 60% of the Late Triassic to Jurassic (Daebo granitoids) belong to ilmenite–series, however less than 40% of the Daebo granitoids and most Cretaceous ones are magnetite–series. Thus, the granitic magma intruded in Korean Peninsula became oxidized while the intrusive ages become younger.