Rare earth element enrichment through multi-stage magmatic differentiation and tectono-metamorphic processes: an example from the Hongcheon carbonatite complex, central Korean Peninsula

This study investigates igneous and metamorphic pathways of Rare Earth Elements (REE) enrichment in the Hongcheon carbonatite complex in the central part of the Korean Peninsula and its tectonic implications. This Fe- and P-rich carbonatite complex hosts an estimated 382,600 tons monazite ore reserve and is compositionally analogous to the world’s largest Mesoproterozoic Bayan Obo carbonatite complex in China. The Hongcheon carbonatite complex comprises four main units: a dolomite carbonatite (DC), a magnetite segment (MS), a phosphate segment (PS), and an aureole of Na-rich fenite. The DC unit shows an orthocumulate texture, composed of medium-grained dolomite with minor calcite, suggesting early stages of carbonatite melt differentiation. Whereas subsequent stages formed magnetite and phosphate-rich melt segments, the Na-rich fenite aureole indicates the expulsion of alkali-rich, especially Na-rich, melt/fluid from the carbonatite melt during crystallization. Our geochemical analyses indicate that interaction between the alkali-rich melt/fluid and the country rock does not result in significant REE concentration. Within the Hongcheon carbonatite complex, the PS hosts REE-rich minerals, such as apatite and monazite, whereas apatite and monazite are largely absent from the DC and MS units except where PS occurs. Textural relationships among minerals in the PS suggest that apatite might have preferentially crystallized in Ca- and P-rich melt. Extensive monazite mineralization occurred through apatite dissolution by fluid-assisted recrystallization under amphibolite facies metasomatic conditions, with subsequent calcite and quartz precipitation in the matrix and fracture-filling veins. These results and published (Permo-)Triassic monazite age data suggest that extensive monazite formation was likely influenced by tectono-metamorphic processes, reflecting later metamorphic overprinting shortly after peak (Permo-)Triassic collisional orogeny in the central Korean Peninsula. Overall, this study highlights that Ca-phosphate melts may have an important role in extracting REEs from carbonatite melt, whereas reactive fluids associated with later tectono-metamorphic events likely influence the redistribution and final mineralogical composition of REE ores (e.g., phosphate, fluoride, etc.) in large carbonatite complexes.