Immiscibility in sulfate-bearing fluid systems at high temperatures and pressures

Fluid equilibria in the H 2 O-Na 2 SO 4 -SiO 2 system were experimentally studied at 700 and 800°C and pressures of 1, 2, and 3 kbar using synthetic fluid inclusions in quartz. The obtained results indicated a heterogeneous state of fluid within the whole range of experimental parameters of this study. Sodium sulfate underwent high-temperature hydrolysis, whose products chemically reacted with quartz. As a result, a noncrystalline phase containing substantial amounts of silica was formed at a temperature of 800°C and a pressure of 3 kbar. This phase was observed in the inclusions as glass. The thermometric investigation of inclusions that trapped fluid phases immiscible under experimental conditions showed that they can, in turn, become heterogeneous at temperatures of approximately 200–400°C. Under such conditions, three or four noncrystalline phases can be in equilibrium. It was concluded that the modeling of natural fluids on the basis of the structure of binary water-salt systems does not account for all variants of fluid equilibria occurring (a) at an increase in the number of components and (b) in the presence of salts of the second type. In natural multicomponent fluid-silicate systems, the possibility of fluid unmixing increases considerably and, this process may result in the equilibration of not only two, but also three, four, or more noncrystalline immiscible phases. Under the conditions of the separation of a SiO 2 -rich phase, ore components can be transported together with SiO 2 .