Carbonatite dyke and related REE mineralization in the Bayan Obo REE ore field, North China: Evidence from geochemistry, CO isotopes and RbSr dating

The Bayan Obo deposit in North China is famous for its enormous rare-earth element (REE) resources in the world. The carbonatite dykes occurring around the orebodies are considered to be closely related to the REE mineralization. Here we present new geochemical and CO isotopic data of the ‘Wu’ carbonatite dyke, together with integrated studies of fluid inclusion preserved in the quartzite spatially associated with the dyke and RbSr dating to reveal the dyke genesis and metallogenic process. The dyke has a geochemical signature and C isotopic composition similar to those of typical magmatic carbonatite, suggesting an igneous origin. It is also characterized by significant enrichment in REEs with extreme differentiation between light and heavy REEs, and negative Sr, Zr, and Ti anomalies, indicating that the magma evolved through fractional crystallization. The quartzite surrounding the dyke underwent strong fenitization. The types, microthermometric data and chemical compositions of the fluid inclusions suggest that the fluids preserved in the quartzite have great similarity with carbonatitic fluids, and RbSr dating of these fluid inclusions from the quartzite yielded an isochron age of 368 ± 43 Ma, indicating that there was an early Paleozoic activity of carbonatitic fluids caused by intrusion of the carbonatite dyke. During the early Paleozoic, the Bayan Obo deposit was under a subduction setting. The REE- and CO2-rich fluids/melts were released from subducted sediments during the subduction, and metasomatized and overprinted the pre-existing mineralization hosted in the dolomite, resulting in large-scale REE enrichment and mineralization.