The Chungju Fe-REE deposit is located in the Kyemyeongsan Formation of the Ogcheon Group. The Kyemyeongsan Formation includes metavolcanic-hosted and pegmatite-hosted REE deposits which contains different kinds of REE minerals. REE minerals of the metavolcanic-hosted REE deposits are allanite, zircon, apatite, and sphene, whereas those of the pegmatite-hosted REE deposits are mainly composed of fergusonite, and karnasurtite, zircon and thorite.
The major REE mineral of the metavolcanic-hosted REE deposits are allanite, which characteristically concentrated in form of aggregate. The chemical compositions of allanite are 23.89-29.19 wt% TR2O3 (Total Rare Earth Oxide), 4.71-9.92 wt% La2O3, 11.30-14.33 wt% Ce2O3, 0.11-0.29 wt% Y2O3, 0-0.94 wt% ThO2, as a formula of (Ca, Y, REE, Th)2.10(Mg, Al, Ti, Mn, Fe3+)2.77(SiO4)2.98(OH). They belong to ferrialanite.
The major REE mineral of pegmatite-hosted REE deposit is fergusonite. Fergusonite occur as dominant REE mineral is fergusonite as prismatic or subhedral grains associated with zircon, fluorite and karnasurtite. Chemical compositions of the fergusonite(YNbO4) indicate substitutions of Y-REE and Y-Th in A-site, and Nb-Ta-Ti in B-site. The proportion of Y2O3 and Nb2O5 is unnormally 1:1.5 comparing to the ideal ratio 1:1 and Nb is higher than Y, also A-site Y actively substitutes with REE. Karnasurtite variously ranges 9.16-22.88 wt% Ce2O3, 2.15-9.16 wt% and La2O3, 0.44-10.8 wt% ThO2 0.44-10.8 wt%, as a calculated formula (Y, REE, Th, K, Na, Ca)1.48(Ti, Nb)1.30(Mg, Al, Mn, Fe3+)0.99(Si, P)1.43O7(OH)4?3H2O.
SHRIMP U-Pb zircon age occurring in metavolcanic rocks and pegmatite are reported as 870-860 Ma and 190 Ma respectively (Koh et al., 2012). The igneous activity related to the REE mineralization might be caused in Neoproterozoic and Jurassic period. Kyemyoungsan Formation with the abundant metavolcanic rocks(mainly metatrachyte) was formed by A-1 volcanism in Neoproterozoic Era. The allanite mineralization might be formed syngenetically by alkali magmatism. Another possible geologic event which formed allanite is regional metamorphism, which formed allanite with biotite, and amphibole in late Paleozoic Era(300-280 Ma, Cho et al., 2002). REE mineralization caused in pegmatite was caused in Jurassic period(190 Ma, Koh et al., 2012), associated with granite intrusion in chungju area. Two types of REE mineralization was caused in Chungju area, the ore is alkali volcanism related REE deposits in Neoproterozoic Era and the other in pegmatite related REE deposit in Jurassic period. They contain different kinds of REE minerals by distinctive genesis.