Chemistry and origin of monazites from carbonatite dikes in the Hongcheon-Jaeun district, Korea

The Hongcheon-Jaeun district is an important Precambrian iron mineralization zone in Korea. Fe-rare-earth elements (REE) ore bodies in the study area occur within carbonatite dikes composed mostly of Fe-dolomite. These ore bodies shows orientations of N-S to N10°E/30°NW, are traced for 1-1.5 km, and range from 15 to 50 m in thickness. Besides dolomite, the most important minerals are magnetite, monazite, strontianite and barite. Monazite coexists with magnetite in early-stage carbonatite and is associated with strontianite in late-stage carbonatite in this area. Monazites in the study area have small positive or zero Eu anomaly patterns and are similar to rocks of granitic or hydrothermal origin. The high contents of rare earth and trace elements (Ba, Sr, Mo, Ni) suggest that these compositions originated from magmatic carbonatite. δC13 values range from −5.3 to −7.2‰, and δO18 values are from +8.4 to +10.1‰. These isotopic compositions from the Fe-REE ore deposit in the Hongcheon-Jaeun district belong to the typical carbonatite field. The textures, monazite and stable isotopic compositions indicate that REE minerals were derived from carbonatite of igneous origin.