An unique, fluocerite-rich REE deposit in Henan province, Central China: the missing link in magmatic-hydrothermal REE mineralizing systems?

Fluocerite has been shown experimentally and by thermodynamic modeling to be an important phase for the evolution of magmatic-hydrothermal REE mineralizing systems, but one that is rarely observed in nature. The reason for the latter is poorly understood. The newly discovered Taipingzhen REE deposit in Henan province, Central China, is unique for containing abundant fluocerite and thus provides a great opportunity to explore the role of fluocerite in REE mineralization. Fluocerite was the earliest REE mineral to crystallize in this deposit and was replaced extensively by bastnäsite. It has a higher La/Nd ratio than the later-depositing REE minerals, e.g., bastnäsite and monazite. This decrease in the La/Nd ratio with time is interpreted to have resulted from the fluocerite-mediated fractionation of REE (fluocerite prefers the lightest REE during its deposition from a high temperature, sulfate-rich hydrothermal fluid, in which the REE were transported as sulfate complexes). This study provides compelling evidence that fluocerite is, indeed, an important, early crystallizing mineral in magmatic-hydrothermal REE systems, which elsewhere was replaced entirely by bastnäsite during subsequent fluid evolution. Moreover, the study shows that the bastnäsite, which replaced fluocerite, inherited its high La/Nd ratio, suggesting that this ratio could be used as a tool with which to identify the former presence of fluocerite. The preservation of fluocerite in the Taipingzhen deposit is interpreted to have been due to the lack of significant cooling until a late stage of mineralization, at which time most of the fluid was lost from the system.