Role of sulfate in the transport and enrichment of REE in hydrothermal systems

•REE/sulfate-bearing fluids separate into REE-rich and REE-poor liquids at ≥140°C.•Raman analyses reveal new REE3+-SO42−/HSO4− complexes in hydrothermal fluids.•Sulfate can efficiently transport and enrich REE, favouring REE mineralization. Sulfate is a common and important component in the mineralizing fluids of some world-class rare earth element (REE) deposits. However, the roles of sulfate during the transport and mineralization of REEs remain unclear and need further investigation. In this study, liquid–liquid immiscibility is proposed to be a potential mechanism for the enrichment of REE in hydrothermal fluids based on a series of experiments. REEs were considerably concentrated into the separated dense liquid phase, whereas the remaining light liquid phase was depleted in REEs. In situ Raman spectroscopic analyses characterized more complicated REE3+-SO42− complexes and some bisulfate species in addition to the documented REESO4+ and REE(SO4)2−; the newly proposed sulfate species can transport one third of the total REEs at >200°C. Therefore, the transport capacity of sulfate was underestimated in previous studies. Overall, the presence of sulfate can significantly fertilize the REE-mineralizing fluids.