Quantifying the nature of ore-forming fluids in the Dalucao carbonatite-related REE deposit, Southwest China: implication for the transport and deposition of REEs

Carbonatite-related rare earth element (REE) deposits are major hosts of REE resources. REE mineralization in these deposits is generally associated with hydrothermal systems at late stages of carbonatite evolution, but the nature of ore-forming fluids and their role in concentrating REEs are not well understood. In this study, we quantified the nature of ore-forming fluids by conducting a detailed investigation on the fluid inclusions of the Dalucao carbonatite–related REE deposit, Southwest China. Based on the phases present at room temperature, three types of fluid inclusions, namely, high-density, CO 2 -rich inclusions, solid-bearing brine inclusions, and aqueous liquid inclusions, have been distinguished in the deposit. High-resolution Raman mapping reveals that the daughter minerals in the earliest brine inclusions are dominantly composed of sulfate (average mass proportion of 92.3%) with minor chloride (3.1%), carbonate (4.4%), and silicate (0.2%). In addition, a Raman peak of SO 4 2 – has been detected in the liquid phase of the aqueous inclusions. These new results suggest that the early, high-temperature ore-forming fluids are unique for containing extremely high concentrations of sulfate. Such sulfate-rich fluids are confirmed to be responsible for transporting appreciable amounts of REEs in the forms of REE-sulfate complexes. We further propose that deposition of REE minerals was mainly triggered by decreasing temperatures, which, as a result, reduced the solubility of sulfate in the fluids. Our new findings highlight the fact that sulfate-rich fluids tend to be more common in carbonatite-related REE deposits and play key roles in REE mineralization.