Fluid mixing and mineralization along faults – the role of stable and traveling reactive zones

Fluid mixing is one of the important ore-forming processes for hydrothermal mineralization. A common hypothesis envisages hot metal-bearing fluids entering rocks that contain brines derived from paleo-seawater. Upwelling of pressurized hydrothermal fluids may occur along permeable faults or fracture systems along fluid pressure gradients. When the upwelling- and pore fluids mix, the conditions for ore-precipitation are met for instance by varying pH, temperature, or redox-conditions. This would be reflected best by changes in the saturation states of the respective mineral phases.

However, how does this process work in detail? How are the two fluids moving, where are they are meeting, how are they mixing and what minerals are precipitating? In order to study such a system, we link a transport model in the software ELLE with the IPHREEQC library of the USGS. We investigate the case of two fluids, a pore fluid representing paleo-seawater and a hot metal-rich fluid that enters the system at high fluid overpressure. As an initial condition, the pore fluid is equilibrated with different mineral phases reflecting different lithologies with varying permeabilities. Temperature as well as multi-components in the system are advecting along pressure gradients as a function of the local Darcy velocity that is calculated as the influx of a compressible fluid initially entering the system and subsequently leading to a flux through the system. In addition, the different species are diffusing as a function of the timescales set by the pressure diffusion into the system. IPHREEQC then calculates the fluid properties and mineral saturation states for every node in the system.

We show that a reactive wave develops between the two fluids, pore matrix, and infiltrating fluid. The incoming fluid pushes the pore fluid out of the system and the mixing process is mainly governed by diffusion. Depending on the time scales involved, minerals will preferably precipitate between the fluids in relatively “quiet” domains where the reactive zones prevail for an extended time. An example are fault walls where the reactive zone is stable for a longer time, whereas it is moving relatively fast along the faults. We discuss the implications of our observations with respect to low temperature ore deposits, present a first model of reactive domain development in a two-fluid system, and compare the results obtained by utilizing different thermodynamic databases.