High sulfate solubility induced by silicate minerals: Implications for the formation of carbonatite-related rare earth element deposits

• Silicate minerals significantly enhance sulfate solubility under hydrothermal conditions. • Adding silicon and increasing pressure show different mechanisms on sulfate solubility. • Findings explain common silicate-sulfate assemblages in carbonatite-related REE deposits. Sulfate plays a crucial role in the formation of carbonatite-related rare earth element (REE) deposits, and it has been proved that sulfate solubility can be significantly enhanced with the presence of quartz by incorporating Si into sulfate melt. However, whether other silicate minerals have the same effect on sulfate solubility is still unclear. This study investigated how silicate minerals, including K-feldspar, nepheline and olivine, modulate sulfate solubility using in situ hydrothermal diamond anvil cell (HDAC) experiments coupled with Raman spectrometer. Our results reveal that silicate minerals, particularly K-feldspar and nepheline, can significantly enhance sulfate solubility by lowering the melting point of Na 2 SO 4 (to below 300 °C) and inducing a shift of sulfate solubility from retrograde to prograde. In addition to silica, pressure has also been proved to show significant effects on sulfate solubility, but adding silica and increasing pressure enhance sulfate solubility via different mechanisms. Moreover, energy dispersive spectrometry (EDS) results of quenched samples confirm the silica incorporation (∼10 wt%) into sulfate melts via forming sulfate-silicic complexes (e.g., Si(OH) 4 SO 4 2– ). In contrast, Fe 2+ in olivine suppresses sulfate dissolution, causing a slightly lower sulfate solubility in olivine-bearing system than these in K-feldspar- and nepheline-bearing systems. These findings are consistent with field observations from carbonatite-related REE deposits and REE-bearing carbonatite-syenite complexes around the world, in which hydrothermal veins containing sulfate and K-feldspar widely exist. Considering that SO 4 2− can form strong complexes with REE 3+ during wide P – T conditions, the silica-induced sulfate-rich fluids are therefore efficient agents to contribute the formation of carbonatite-related REE deposit. This can be also proved by the close association among silicate, sulfate and REE minerals in carbonatite-related REE deposits.