The Role of Multiple Fluids in Diverse Mineralization of Carbonatite-Related Rare Earth Element Deposits: New Insights from In Situ Elemental and Sr-Nd-O Isotope Analyses of Carbonate Minerals

Abstract A number of Cenozoic carbonatite-related deposits of rare earth elements (REEs), distributed along the western margin of the Yangtze block, southwest China, define the world-class Mianning-Dechang metallogenic belt. Numerous studies of this belt are available, but the potential role of fluids in the development of diverse REE mineralization styles in specific deposits is still poorly understood, particularly for the ~28 Ma Muluozhai deposit, which is distinctly rich in anhydrite. We investigated the evolution of fluids and genesis of diverse REE ores in the Muluozhai deposit, with emphasis on its mineralogical variations and trace element and Sr-Nd-O isotope compositions of calcite and dolomite formed at different evolutionary stages. The REE orebodies in this deposit are lenticular, generally characterized by REE-rich veins or veinlets ranging from <1 cm to several meters in width. On the basis of micro- and macrotextural relationships, we established a paragenetic sequence that includes three main evolutionary stages. Stage I involved fenitization that pervasively overprinted wall-rock syenites. It is characterized by the formation of Na-rich pyroxene (aegirine-augite) and amphibole (magnesio-arfvedsonite) with variable amounts of albite, phlogopite, calcite, celestine, fluorite, apatite, and monazite. The early mineralization stage (stage II) comprises a mineral assemblage of bastnäsite, celestine, calcite, and fluorite, broadly similar to that at other REE deposits in the Mianning-Dechang belt. The late mineralization stage (stage III) is distinguishable from stage II by the occurrence of abundant anhydrite and dolomite in addition to bastnäsite, parisite, monazite, celestine, and fluorite. The stage III mineralization is unique to Muluozhai relative to the other deposits in the Mianning-Dechang belt. Our geochronological data for bastnäsite samples from stages II and III show that both stages have a similar U-Th-Pb age of ca. 28 Ma, indicating that they are products of the same mineralization event. Calcite and dolomite from stages I to III have indistinguishable Sr-Nd isotope compositions that are broadly similar to those of the coeval carbonatites in the Mianning-Dechang belt, strongly indicating that the two stages of mineralization in the Muluozhai deposit were genetically related to a common carbonatitic source. Stage I fluids have calculated δ18OVienna-standard mean ocean water (V-SMOW) values (4.7–8.5‰) comparable to those of typical carbonatites, indicating a dominantly magmatic fluid exsolved from carbonatitic magma. Subsequent stage II fluids have significantly lower δ18OV-SMOW values (–1.9 to 4.1‰), indicating a significant contribution of meteoric water during fluid evolution of the ore system. In contrast, stage III fluids have much higher δ18OV-SMOW values (10.0–13.5 ‰), which are interpreted to result from the incursion of formation water with high δ18OV-SMOW values. This study demonstrates that mixing of REE-rich magmatic fluids with different external fluids (i.e., the meteoric or formation water) played a key role in REE deposition at mineralization stages II and III and was also responsible for their mineralogical differences. This study highlights the potential effects of different external fluids on the diversity of REE ores in a specific carbonatite-related deposit or among different deposits in the same metallogenic belt.