Solubility of K2SO4 in silica-saturated solutions and applications to determine the composition of ore-forming fluids

Sulfate-bearing geofluids are widespread in many geological environments and play important roles in ore-forming processes. Recent experiments provide evidences that in silica-rich fluids, Na 2 SO 4 solubility exhibits a complex temperature dependence, first decreasing from 25 to 313 °C and then increasing from 313 to 425 °C. The fluid-borne silica have critical implications for sulfate-bearing hydrothermal systems and formations of rare earth element deposits. However, whether other sulfate salts share the same characteristics remains unknown. In this study, fused silica capillary capsules (FSCCs) containing K 2 SO 4 -saturated solutions and K 2 SO 4 crystals are used to quantitatively investigate the solubility of K 2 SO 4 at different temperatures in the K 2 SO 4 -SiO 2 -H 2 O system. In the K 2 SO 4 -SiO 2 -H 2 O system, K 2 SO 4 shows prograde solubility from 50 to 400 °C, and does not melt through the whole investigated temperature range. The relationship between temperature and the solubility of K 2 SO 4 can be described by a simple linear function with R 2  = 0.991: C sulfate  = 4.1307 × 10 −3  ×  T  + 0.5532, where C sulfate and T represent the solubility of K 2 SO 4 in m (mol/kg H 2 O) and temperature in °C, respectively. These results show that the solubility of K 2 SO 4 can be enhanced by 3 to 5 times at 300–400 °C by the presence of silica, compared with that in silica-absent system, even without the formation of sulfate melt. Considering that quartz is ubiquitous in the Earth's crust , the solubility of sulfate in natural geofluids may reach a much higher level than previously expected. The temperature dependences of the solubilities of K 2 SO 4 and Na 2 SO 4 in silica-saturated fluids were used to constrain the compositions of the mineralizing fluids of the synmetamorphic hematite-quartz veins at Ouro Fino (SE Brazil) and the Maoniuping REE deposit (SW China), based on the previously reported microthermometric results of fluid inclusions that contain sulfate daughter minerals. For the hematite-quartz vein at Ouro Fino, the sulfate concentration was calculated to be 8.9–19.2 wt%, which is in line with previous laser ablation measurements (~15.3 wt%). For the Maoniuping REE deposit, the total sulfate concentration was determined to be 16.3–27.4 wt%, and the REE concentration of the ore-fluid was further estimated to be 0.30–0.44 wt%.