Identification of ~1.3 Ga hydrothermal zircon from the giant Bayan Obo REE deposit (China): Implication for dating geologically-complicated REE ore system

Abstract

The Bayan Obo deposit in China hosts the largest REE resource in the world. However, the genesis of this deposit is still highly debated, largely due to a lack of consensus on the mineralization age. The Banyan Obo deposit has experienced intensive post-ore metamorphic and metasomatic overprints, so it is crucial to choose robust chronometers to determine the primary mineralization age. In this study, zircon is identified in the REE ore of the Bayan Obo deposit. The zircon grains are in contact with or contain inclusions of hydrothermal minerals (e.g., magnetite, biotite, monazite, allanite, apatite and ilmenorutile). They have variable Th contents (14 to 1139 ppm) and are poor in U (<0.8 ppm), consistent with the Th-rich and U-poor feature of the Bayan Obo REE ore. Moreover, they have substantially low contents of light REE (La to Nd; LREE), which may be due to zircon coprecipitation with LREE-rich monazite and allanite. The combined textural and compositional features imply a hydrothermal origin of the zircon grains. The hydrothermal zircon grains have a weighted mean 208Pb-232Th age of 1297 ± 13 Ma, which places a direct constraint on the timing of REE mineralization at Bayan Obo. Notably, substantial LREE depletion and Th/U fractionation have not been documented before for hydrothermal zircon from other environments, and thus they could be taken as discriminative features for hydrothermal zircon from carbonatite-related REE deposits.Thorium-lead dating was also conducted on monazite from the zircon-bearing sample, yielding 208Pb-232Th ages (993 to 319 Ma) variably younger than that of zircon. The younger monazite Th-Pb ages could have resulted from disturbance of the Th-Pb isotopic system during post-ore modification process. Similarly, monazite has younger Th-U-Pb dates than zircon in many other geologically-complicated carbonatites or REE deposits. Such a phenomenon implies that the zircon Th-U-Pb chronometer is robust even if the host-rocks have experienced intensive modification, whereas monazite Th-U-Pb chronometer is more susceptible to modification. Therefore, younger Th-U-Pb dates of monazite should be interpreted with caution as they may record the timing of post-ore modification event or represent partial resetting, and hence geologically meaningless results.