Rare earth elements deposits in China: Spatio-temporal distribution and ore-forming processes

Melt–Fluid and Fluid–Fluid Immiscibility in a Na2SO4–SiO2–H2O System and Implications for the Formation of Rare Earth Deposits

“…A common phenomenon observed in the carbonatiterelated REE deposits is the coexistence of syn-ore fluid/ melt inclusions with varying contents of dissolved sulfates, ranging from sulfate melt inclusions, to sulfaterich fluid inclusions that contain voluminous daughter minerals, to fluid inclusions with various contents of dissolved sulfate (Xie et al, 2009(Xie et al, , 2015(Xie et al, , 2016(Xie et al, , 2019(Xie et al, , 2020. This phenomenon could be interpreted in the context of the liquid-liquid immiscibility observed in this study.…”

“…Sulfate minerals are commonly associated with the formation of carbonatite-related REE deposits, such as Maoniuping (Xie et al, 2009(Xie et al, , 2015(Xie et al, , 2016(Xie et al, , 2019(Xie et al, , 2020Li et al, 2019), Bayan Obo (Fan et al, 2016;Xie et al, 2019Xie et al, , 2020, Miaoya (Xu et al, 2015), and Huanglongpu (Cangelosi et al, 2020) REE deposits. The sulfate-rich hydrothermal fluids are believed to be responsible for the enrichment and mineralization of REE in these deposits (Xie et al, 2009(Xie et al, , 2015(Xie et al, , 2016(Xie et al, , 2019(Xie et al, , 2020. Moreover, the Na-sulfate is often observed in natural geological environment (Boiron et al, 1999) and it can even be the predominant sulfate daughter mineral in the Maoniuping deposit (Xie et al, 2015).…”

“…The sulfate ion has been proved to be an ideal ligand for the transport of REE (III) (Migdisov et al, 2016;Thomas and Davidson, 2017;Xie et al, 2019Xie et al, , 2020, and our previous experiment has shown that a silica-saturated concentrated sulfate solution has a great ability in migrating REE (Cui et al, 2020). The experiments of this study illustrated that the fluid-fluid immiscibility can simultaneously generate a dilute aqueous solution and a concentrated sulfate-rich aqueous solution, indicating that fluids with high concentrations of sulfates can be generated by fluid immiscibility from a moderately concentrated parental fluid.…”

“…Till now, two types of liquid-liquid immiscibility have been identified for sulfate-water systems: (i) fluid-melt immiscibility between an aqueous solution and a sulfate melt, which has been observed in natural fluid inclusions (Xie et al, 2009(Xie et al, , 2015(Xie et al, , 2016(Xie et al, , 2019(Xie et al, , 2020 and hydrothermal experiments (Kotel'nikova and Kotel'nikov, 2010;Cui et al, 2020); and (ii) fluid-fluid immiscibility between two aqueous fluids with different sulfate concentrations. Previous experiments showed that a homogeneous sulfate-bearing fluid can separate into two immiscible fluids with different sulfate concentrations, when heated above ~260°C and under vapor-saturated pressure (Schmidt, 2009;Wang et al, 2013Wang et al, , 2016Wang et al, , 2021Wan et al, 2015).…”

“…It is noted here that, in this literature, the term 'fluid' denotes only an aqueous solution, rather than a melt. These two processes are believed to play significant roles in the mineralization of carbonatiterelated REE deposits (Xie et al, 2009(Xie et al, , 2015(Xie et al, , 2016(Xie et al, , 2019(Xie et al, , 2020 and submarine hydrothermal vents (Wang et al, 2013(Wang et al, , 2016(Wang et al, , 2021.…”