K-Ar ages of some W-Mo deposits and their bearing on metallogeny of South Korea.
K-Ar ages were determined on eight specimens from five W-Mo deposits in South Korea. Muscovites purified from tungsten ore and pegmatitic quartz vein of the Ssangjeon W deposits give ages of Proterozoic. Although biotite in pegmatitic quartz vein of the Ogbang W mine yields much younger age, the occurrences of the deposits in both mines are so similar that the Ogbang deposits seem to have been formed at almost the same time as the Ssangjeon deposits. Muscovites from the Garisan W-Mo and Jangsu Mo deposits give ages of about 170 Ma, and confirm wide distribution of W-Mo mineralization at early Jurassic time. The results for sericites from the Cheongyang W deposits are around 80 Ma, and are almost same as those of the Daehwa W-Mo and Sangdong WMo-Bi deposits. In South Korea, W-Mo mineralization is prior to Au-Ag mineralization in Jurassic time, whereas vice versa in Cretaceous time. At the northeastern end of the Ryeongnam massif,tin (•`1800 Ma), tungsten (•`1500 Ma) and gold (•`1100 Ma) mineralizations are overlapped in Proterozoic time. Introduction The Korean peninsula is famous for its large production of tungsten as well as gold. In South Korea, for example, the Sangdong mine is one of the most productive tungsten mines in the world. In his excellent pioneer work on the metallogeny of South Korea, KIM (1971) thought that most W-Mo deposits were formed in Jurassic time with genetical relation to Daebo granitic rocks. Recent age determinations, however, have revealed that some major W-Mo mineralizations are related genetically to Cretaceous Bulgugsa granitic activity. They include such as the Sannae W-Mo (65 Ma; FLETCHER and RUNDLE, 1977), Ilgwang Cu-W (66-81 Ma; FLETCHER and RUNDLE, 1977), Sangdong W-Mo-Bi (81-84 Ma; FARRAR et al., 1978), Shinyemi Zn-Pb-Mo (75 Ma; SATO et al., 1981), Daehwa W-Mo (89 Ma; SHIBATA et al., 1983) (88 Ma; So et al., 1983a) (85 Ma; KANEOKA et al., 1983), Geumseong Mo (157-179 Ma; SHIBATA et al., 1983), and Garisan (171 Ma; KIM and SEO, 1986) deposits. The first two deposits are situated in the Gyeongsang basin, which is covered by thick accumulations of Cretaceous sedimentary rocks with intermediate volcanic rocks. One more occurrence of tungsten is known in the basin, that is, the Ulsan Fe-W deposits. Although no direct age determination has yet been carried out, the deposits are thought to have been formed with genetical relation to Gadaeri granodiorite (PARK and PARK, 1980), which is dated as 58 Ma (LEE and UEDA, 1977). Those given above are all the data so far appeared in the literature, on the formation age of W-Mo deposits in South Korea. Formation age data on Au-Ag deposits in South Korea have been also accumulated in recent publications, and three epochs, Precambrian, Jurassic and Cretaceous, are clearly recognized (SHIMAZAKI et al., 1986). In the present paper, five W-Mo deposits situated out -Received on July 4, 1987, accepted on October 8, 1987* Geological Institute, Fac lty of Science, Univ rsity of Tokyo, Hongo, T ky 113, Japan.** Geological Survey of Japan, Higashi 1-1-3, Tsukuba,lbaraki 305 Japan.*** Department of Earth Sciences, Coll ge of Educa-tion, Seoul National U iversity, S oul 151, Republic ofKorea.** Depa tment of Min ral Deve opment E gineeri g, Faculty f E gineeri g, U iver it of Tokyo, Hongo, Tokyo 113, Japan.Keyw rds: K-Ar ag , W-M deposits, K rea Precam-br an , Jurassic, Cretaceous, Metallogeny.