Fig. 2shows the equilibrium quotients and constant for the potassium feldspar–mica exchange reaction, which controls the pH in the Cerro Prieto reservoir (Truesdell and Henley, 1982). The observed points also fall below the equilibrium boundary, except for fluids higher than 270 °C, which plot above the line. These fluids have shown evidence of boiling processes at the hole bottom that seem to produce change in the ion exchange rate. Therefore, it is possible that this reaction is not the more adequate to trace the pH for the fluids of the southeast part, since the degasification event causes an additional modification in the pH values and possibly precipitation of minerals. As discussed byTruesdell et al. (1987), the excess enthalpy in the discharge makes the reconstruction of the original aquifer composition a
difficult task. Calcite equilibrium was evaluated using the following equation.