On the Effects of NaCl on Convective Fluid-Flow in Magmatic-Hydrothermal Systems

Previous studies of convective fluid-flow and heat transport in magmatic-hydrothermal systems have approximated the fluid as pure H₂O, ignoring the major concentrations of dissolved NaCl. The presence of NaCl in water has profound effects on the thermodynamics and hydrodynamics of hydrothermal systems. NaCl-H₂O fluids can separate into a high-density, high-salinity liquid (brine) phase and low-density, low-salinity vapor phase at pressures and temperatures well above the critical point of pure water. This process is particularly common in magmatic-hydrothermal systems and likely a key driver for the formation of the world's major ore deposits of Cu, Mo, and Au. During convective transport of heat and NaCl, so-called double-diffusive, double-convective systems form because heat diffuses faster than salt, but is advected at a slower rate. The presence of NaCl can hence stabilize the flow, preventing convection cells to form, or amplify flow-instabilities, leading to chaotic convection behavior. Using numerical simulations, we discuss the effects of NaCl on hydrothermal convection at geologically realistic pressure, temperature, and salinity conditions. The results show that five general flow-patterns, ranging from single-phase diffusive to multiphase convective, can be identified. Classical parameters such as the Rayleigh number or buoyancy ratio fail to predict the evolution of convection patterns, because they do not account for the non-linearity of the fluid properties and are intrinsically not defined at two-phase conditions. Geological implications are that the transport of NaCl, and probably that of many other ore-forming metals, is maximized at single-phase conditions. Because of its high density and low volume fractions, the brine is immobile at two-phase conditions and sub-lithostatic pressures. Many geochemical fractionation processes and ore-mineral precipitation, however, occur near the transition from single to two-phase flow.