Geochemical and mineralogical aspects of hot rain water-rock interaction: A view from chemical equilibrium approach

Equilibrium chemical modeling calculations were performed to investigate fluid composition and mineral assemblage of HOT WET ROCK (HWR) systems mainly at 300°C. Fluid chemistry and pH reached steady state after a small amount of rock reacted with fluid. Influence of rock-type (i.e., basalt, andesite, granite, on fluid chemistry) was found to be small at the same temperature. Equilibrium pH of fluid is the same in the three rock-types. Alteration mineral assemblages are slightly different in the three rock-types and reflect the difference of chemical composition of the host rock. Effects of biotite reactivity were found to be significant for redox related aqueous species and Fe-bearing mineral assemblages. Soluble chloride in granitic rock decreases fluid pH slightly. pH changes greater than calculated in this study may indicate the addition of volcanic gas or brine to the system. Disproportionation reaction of native sulfur was also found to decrease fluid pH slightly. This suggests that natural acidic high temperature fluid is produced in an environment where more than one mineral of a pH buffer mineral assemblage is completely dissolved due to pervasive fluid flow.