Granitoids and their magnetic susceptibility in South Korea

Magnetic susceptibility (MS) measurements were carried out for 1,120 sample s in the Middle Proterozoic to Early Tertiary granitoids so far recognized in South Korea, and the lateral and spatial variation of their magnetic sus ceptibility, i.e., content of magnetite, is studied. The Middle Proterozoic two mica granitoids related to cassiterite (Sn) depo sits in northeastern part of the Sobaegsan Massif show very low MS (less th an 03x10(-3) SI unit), and the Permo-Triassic tonalitic to granodioritic an d monzonitic rocks which are barren in mineralization, distributed in the m iddle part of South Korea also show low MS (less than 1 x 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. Parti cularly in the Andong, Igsan, Gimcheon and Geochang areas, the granitoids w hich are barren in mineralization, are characterized by high MS (more than 10x10(-3) SI unit). The Chuncheon, Jecheon, Namyang and Geumsan plutons rel ated to molybdenite (Mo) and/or wolframite or scheelite (W), and fluorite ( F) mineralizations show a little high MS (more than 3x10(-3) Sl unit). Howe ver, more than 60% of the Daebo granitoids show low MS (less than 3x10(-3) SI unit) and the rest show a little high MS (more than 3x10(-3) SI unit). H eterogeneous distribution of magnetite content in the Daebo granitoids is c onsidered to reflect heterogeneity of redox state of the source materials f or these granitoids. The Cretaceous to Early Tertiary granitoids (=Bulgugsa granitoids) in the G yeongsang Basin had been generally evolved in the order of tonalite, diorit e, granodiorite, granite and alkali-feldspar granites, which are closely re lated to base metal ore deposits, and mostly show higher MS (more than 3x10 (-3) Sl unit) than other granitoids mentioned above, although some exceptio ns are recognized in highly evolved alkali-feldspar granites (SiO2 > 76%). In contrast, as most of the highly oxidized or evolved Cretaceous granitoid s distributed in areas other than the Gyeongsang Basin show lower MS than t hose of the Gyeongsang Basin, and appear to be magnetite free, ilmenite-ser ies granites, but they might be hematite bearing magnetite-series granitoid s. Highly oxidized nature of the Bulgugsa granitoids may be due to high Fe2 O3/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 mo re than 60% of the Late Triassic to Jurassic (Daebo granitoids) belong to i lmenite-series, however less than 40% of the Daebo granitoids and most Cret aceous ones are magnetite-series. Thus, the granitic magma intruded in Kore an Peninsula became oxidized while the intrusive ages become younger.