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Emanated Carbonatite Fluids for REE Mineralization: From Fenitization to Late-Stage Metasomatism

Carbonatite melts are highly enriched in incompatible elements, which results in carbonatites as important critical metal resources. The carbonatite fluids emanated from the melt or evolved from carbonatites in the late stage transport high quantities of volatiles and also REE. Fenitization and/or metasomatism play important roles in the formation of carbonatite related REE deposits, especially those in cratonic margins and orogenic belts (e.g., Bayan Obo and Miaoya). Fenitization is widely distributed both around the carbonatite dykes and in the ores at the Bayan Obo REE-Nb-Fe deposit. Bulk rock chemical compositions suggest that the initial alkali fluids derived from the Wu calciocarbonatite dyke exhibit enrichments in Ca, Sr, Ba, Mn, REE and Nb. Of interest, from the central carbonatite to the outer low-grade fenite, REE content decreased whereas HREE abundance increased in the high-grade and medium-grade fenites. The HREE enrichments are associated with apatite mineralization which contains hundreds ppm of HREE. This suggests that the carbonatite-emanated (carbo)-hydrothermal fluids can remobilize and fractionate REE. and detailed investigation can be remobilized, and redeposited in late-stage metasomatic processes.