Granite Hydrolysis to Form Deep Brines

Reaction path calculations suggest that water fixation by zeolite and chlorite formation can account for much of the high salinity of deep brines in contact with deep granites, as well as their Ca/Na ratios, which reflect the rock-dominated chemistry of such brines. Resultant brines, undiluted by the influx of shallower fresher waters, are likely to be at equilibrium with laumontite, chlorite, calcite, dolomite, anhydrite/gypsum, K-feldspar, quartz, plagioclase, and possibly halite. The growth of laumontite and chlorite consumes water, causing the concentration of residual salts to increase during the formation of such brines. In these analyses, the major trends suggest that these fundamental processes drive this outcome naturally. Predicted phase assemblages and end-point water compositions are relatively unaffected by the chemistry of the starting/reacting fluid. Additionally, mineralogical and mineral compositional variations both appear to have no major impact on brine formational trends. More precise analysis involves the use of Pitzer coefficients and considers Br/Cl exchange in the alteration phases. Explicit consideration of silicate dissolution points to water availability as a key control over granite alteration. Diffusion-limited water availability appears to lead to stagnant systems dominated by the increasing brine density and Ca/Na ratios with depth. Alteration phases tend to decrease permeability and porosity, further isolating such systems from the flow of shallower dilute fluids.

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