Zircon from the East Orebody of the Bayan Obo Fe-Nb-REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events

Extremely U-depleted (<1 ppm) zircons from H8 banded ores in the East Orebody of the Bayan Obo REE-Nb-Fe deposit are presented, with mineral compositions, textures, ²³²Th-²⁰⁸Pb SHRIMP ages and petrological context. Cores of East Orebody zircon contain up to 7 wt% HfO₂ and are zoned, depicting bipyramidal crystal forms. A distinct generation of patchy, epitaxial rim zircon, similarly depleted in U, is intergrown with rare earth ore minerals (bastnäsite, parisite, monazite). Overprinting aegirine textures indicate paragenetically late, reactive Na-rich fluids. Chondrite-normalized REE patterns without Eu anomalies match closely with those from the Mud Tank and Kovdor carbonatitic zircons. Increased HREE in rims ((Lu/Gd)_N 43-112) relative to cores ((Lu/Gd)_N 6-7.5) and the localized presence of xenotime are attributable to reactive, mineralizing fluid compositions enriched in Y, REE and P. Cathodoluminescence further reveals HREE fractionation in rims, evidenced by a narrow-band Er³⁺ emission at 405 nm. The extreme depletion of U in core and rim zircon is characteristic for this mineral deposit and is indicative of a persistent common source. U depletion is also a characteristic for zircons from carbonatitic or kimberlitic systems. ²³²Th-²⁰⁸Pb (SHRIMP II) geochronological data reveal the age of zircon cores as 1,325 ± 60 Ma and a rim-alteration event as 455.6 ± 28.27 Ma. The combined findings are consistent with a protolithic igneous origin for zircon cores, from a period of intrusive, alkaline-carbonatitic magmatism. Fluid processes responsible for the REE-Nb mineralizations affected zircon rim growth and degradation during the widely reported Caledonian events, providing a new example in a localized context of HREE enrichment processes.