The Effect of Temperature Gradient on the Interaction between Geothermal Water and Rock—An Approach by Numerical Simulation

Reaction between rhyolitic welded tuff and geothermal solution cooled or heated from the temperature in equilibrium with alteration minerals is numerically examined by the geochemical simulation code. Six varieties of the initial solutions are calculated assuming that total chloride concentrations are 0.01, 0.05 and 0.1 mole/kg and that they are in equilibrium with quartz, muscovite, calcite, albite, K-feldspar, clinochlore and laumontite at 225•Ž or wairakite instead of laumontite at 275•Ž in the H+-Na+-K+-Ca+2-Mg+2-Al+3-H4SiO4-H2O-Cl--CO3-2 system. The initial solutons are cooled or heated by 5•Ž to remove saturated minerals, and subsequently reacted with rlyolitic welded tuff. Quartz and muscovite precipitated in the processes of 5•Ž cooling and the subsequent rock-solution reaction at low rock/water ratio, which results in enrichment of K2O and SiO2 in the reaction product. Calcite always precipitates in the process of 5•Ž heating, and laumontite (or wairakite), albite and clinochlore precipitated in the process of the subsequent rock-solution reaction at low rock/water ratio, which results in enrichment of Na2O, CaO and MgO in the reaction product. These cooling and heating experiments represent respectively thermal condition of discharge area and recharge area in the geothermal system, and this study shows possibility of the hydrological interpretation of geothermal system from the rock alteration.