A continuous-flow, high temperature-pressure apparatus has been used to simulate the reaction of greywacke with distilled water and re-injection brine. During the initial room temperature period of approximately one week, the effluents contained elevated Ca and Mg and had increased pH values due to calcite dissolution. After a temperature increase to 210 degrees C at 35 bars, the distilled water simulation reached quartz saturation (similar to 320 mg/kg SiO2) after two days. In the re-injection brine experiment, Al dropped below detection limit and SiO2 concentration reached a steady state value of similar to 460 mg/kg after the temperature shift to 203 degrees C. Except for amorphous silica, all SiO2 polymorphs remained supersaturated. Newly-formed secondary minerals observed in the distilled water experiment included a Ca-Na-K-Mg-Fe aluminosilicate (a clay mineral), a Ca-Na aluminosilicate (possibly clinoptilolite) and calcite. From the re-injection brine simulation, secondary minerals included a Ca-Na-K-Mg-Fe aluminosilicate (clay), a Ca-Na aluminosilicate (possibly clinoptilolite) and chlorite. These minerals are consistent with those expected during hydrothermal alteration under the P-T conditions of the experiment. Slow precipitation kinetics explains the lack of equilibration with respect to quartz in the re-injection experiment using Wairakei brine; however, other cation activities appear consistent with the thermodynamically predicted mineral assemblages. The experiments underline the utility of fluid-rock laboratory simulations in studying the effects of brine re-injection into aquifers with which the fluid is no longer in chemical equilibrium. (c) 2012 Elsevier Ltd. All rights reserved.