沃川帶의 地質 및 鑛物資源에 關한 硏究

Geology and mineral resources of the Ogcheon zone:mineralization in the Pyeongchang-Jucheon area, Kangwon-do, Korea

A group of 16 Zn+Pb±Ag deposits distributed in the Pyeongchang-Jucheon area, Kangwon-do, South Korea, were semi-regionally investigated. These deposits are contact metasomatic and/or hydrothermal replacement types hosted in the carbonate-dominated Cambrian Machari Formation and Ordovician Ibtanri Formation, and also in the carbonate interbeds of the Precambrian argillic metasediments. Comparing some key aspects of the individual deposits, it is found that the ore deposits hosted in the Machari and Ibtanri Formations are mostly of steeply-dipping chimneys with or without skarn minerals and are rich in Ag and Pb＞Zn in metal grade whereas those occuring in the carbonate interbeds of the Precambrian argillic metasediments are gently-dipping conformable lenticular orebodies mostly with skarn minerals and are generally poor in Ag and Zn＞Pb. The skarn mineralization in the area appears to have occurred during the lower Cretaceous (118.7Ma) to mid-Cretaceous (107.8Ma) time assumed from the K-Ar dates of the Dowon and Pyeongchang granites which are closely associated with the skarn ore deposits. The Rb/Ba/Sr ratios of these granites indicate that they are of strongly differentiated anomalous granites, and the Nb vs. Y and Rb vs. Y+Nb plots fall on the field of volcanic are setting. The contact aureoles are zoned, giving the sequence in order of increasing distance from igneous contact: garnet-wollastonite, granet-wollastonite-clinopyroxene and garnet-clinopyroxene in such as the Pyeongchang and Yeonwol 114 areas. Electron microprobe analyses reveal that garnets and clinopyroxenes are generally low in Fe and Mn. Garnets are grossular to intermediate grandite except for those from the Ogryong exoskarn which are richer in andradite, pyrope and spessartine fractions. This indicates that the oxidation state of skarn-forming environment at Ogryong was higher than at the other deposits. Clinopyroxenes are mostly salitic except for those from the Ogryong exoskarn which involve considerable amounts of hedenbergite and johansenite fractions. The δ¹⁸O value of Jurassic biotite granite at Ogryong is higher (+10.21‰) than that of Cretaceous one at Chodun. (+8.41‰). The δ¹³ values of carbonate rocks range from -0.89‰ to 0.68‰ and the δ¹⁸O values range from +11.91‰ to +19.34‰ indicating that these carbonate rocks are of marine origin. However, the δ¹³C values of skarn calcite and vein calcite are -4.80‰ and -12.92‰, and the δ¹⁸O values arc +5.56‰ and +10.32‰, respectively, indicating that these calcites are of hydrothermal origin. The δ³⁴S values of sulfide minerals range from +4.4‰ to +8.7‰ suggesting that the sulfurs are of magmatic origin.