The role of fluid phase immiscibility in quartz dissolution and precipitation in sub-seafloor hydrothermal systems

A numerical model describing quartz dissolution and precipitation in sub-seafloor hydrothermal systems has been developed that documents the effects of retrograde quartz solubility and fluid phase immiscibility on the transport and deposition of silica in this environment. Rates of dissolution and precipitation increase with increasing permeability and with increasing maximum temperature at the base of the system. At the most optimal conditions considered in this study (425 °C, permeability of 10− 13 m2), quartz is precipitated at rates up to 10− 6 mol/m3·s (equivalent to 700 cm3 of quartz per cubic meter of rock per year). Immiscibility at the base of the system creates a zone in which large amounts of quartz precipitate as a result of phase separation. The high rate of quartz precipitation at the one-fluid-phase/two-fluid phase boundary is consistent with the location of highly silicified zones found beneath volcanogenic massive sulfide deposits. Rapid quartz deposition at this boundary may affect the heat transfer efficiency at the base of the upflow zone and may contribute to immobilizing the brine layer so that it does not rise towards the surface. The process of rapid quartz precipitation at the base of the upflow zone, and its effects on the dynamics of these systems, is only observed under conditions of liquid–vapor immiscibility.