ZIRCON-RICH REE MINERALIZATION AND ITS RELATION TO ALKALINE MAGMATISM IN CHUNGJU AREA, SOUTH KOREA

Whole rock chemistry and Nd isotopic compositions were investigated in order to constrain the origin of the zircon-rich schistose rocks of the Kyemyeongsan Formation in Chungju area, South Korea. The schistose rocks are enriched in HFS (high field strength) elements and relatively depleted in LIL (large ion lithophile) elements. The schistose rocks are peralkaline, and their chemistry is similar to that of A-type granite. The element ratios of the schistose rocks mostly belong to the A1 group of A-type granite, suggesting that these rocks have put in an extensional environment. According to the data for rare earth elements (REE) and other trace elements, the schistose rocks were produced by fluorine rich, alkaline magma. The Sm-Nd age and åNd(t) values of the schistose rocks are 343±30 Ma and -1.78 to -2.00, suggesting that the source magma was derived from the mantle and had a little interaction with sialic continental crust. In conclusion, the zircon-rich metavolcanics rocks of the Kyemyeongsan Formation probably formed from a peralkaline magma that was emplaced in a continental rift environment, during the early Carboniferous. Zr-rich REE ore deposits are developed in the metavolcanics and peralkaline granite of the Kyemeyongsan Formation. The ore deposits occur in two rock types with very different paragenetic sequences; (1) early rare metal bearing alkali granite, (2) late metasomatic ore. The late metasomatic zone contains abundant microcline, albite, and quartz with minor biotite, magnetite, hornblende, allanite, and zircon containing REE bearing-minerals. Locally the ores contain as much as 50% magnetite. The ore contains mainly Ce-La, Ta-Nb, Y, Y-Nb, Ti-Nb-(U), Nd-Th group minerals, which are divided into four types (zircon type, allanite veins, feldspar type, and fluorite type) based on occurrence and mineral association.