Honoka Umemiya

This study documents the first occurrence of pyrochlore in Mongolian carbonatites and provides a case study of the magmatic–hydrothermal evolution of the Lugiin Gol alkaline–carbonatite complex and its Nb–REE mineralization, offering new insights into Early Triassic magmatism and tectonic processes in southern Mongolia. Pyrochlore is the principal Nb‑hosting mineral in the Lugiin Gol carbonatite, whereas REE‑fluorocarbonates (parisite, synchysite, bastnäsite) represent the dominant REE-bearing phases. Petrographic and mineral-chemical data indicate a coupled magmatic–hydrothermal evolution of Nb–REE mineralization: (i) pyrochlore crystallized as inclusions in calcite and apatite from the primary carbonatite melt, establishing the primary Nb budget with limited redistribution during subsequent hydrothermal overprints; (ii) magmatic processes may have partially incorporated REEs into magmatic minerals, particularly calcite, which were subsequently mobilized during hydrothermal activity, redistributed along mineral cleavages to form subordinate REE‑fluorocarbonates, and concentrated within pre-existing structural discontinuities, producing the main REE‑fluorocarbonates and significant REE enrichment. New in situ zircon U–Pb geochronology of coexisting alkaline–silicate rocks yields a weighted mean age of 246 ± 4 Ma, constraining the timing of magma emplacement, with carbonatite emplacement inferred to have been broadly contemporaneous during the Early Triassic. The complex likely formed in a localized extensional setting, with magmatism potentially associated with the closure of the Paleo-Asian Ocean, which is generally considered to mark the final stage of amalgamation of the southern Central Asian Orogenic Belt. The study underscores both the tectonic framework and the mineralization processes that govern the distribution and concentration of critical metals in orogenic carbonatite systems.