Use of reaction path modeling to identify the processes governing the generation of neutral Na–Cl and acidic Na–Cl–SO4 deep geothermal liquids at Miravalles geothermal system, Costa Rica

The EQ3/6 Software Package was used to investigate the irreversible water–rock mass exchanges governing the production of neutral Na–Cl and acidic Na–Cl–SO4 geothermal liquids circulating at depth into the Miravalles geothermal system. First, we reconstructed the in-situ reservoir composition, including pH, we evaluated the boundary conditions (PCO2, PH2S, and/or PO2), which are necessary for the implementation of reaction path modeling, we estimated the thermodynamic affinity of the reservoir liquids with respect to relevant hydrothermal solid phases, and we identified possible compositions of the initially acidic meteoric–magmatic liquids. Then we modeled both the interaction between meteoric–magmatic fluids and calc–alkaline volcanic rocks and the interaction between neutral Na–Cl waters and highly altered volcanic rocks. Reaction path modeling was performed through a purely stoichiometric approach without considering the kinetics of irreversible mass exchanges. Results of reaction path modeling suggest that the neutral Na–Cl liquids hosted in the Miravalles geothermal reservoirs are produced through interaction between initially acidic, magmatic–meteoric aqueous solutions and volcanic rocks and represent the final product of this process. However, the deep acid Na–Cl–SO4 liquids of Miravalles are not intermediate products in this process. They are instead generated through interaction of neutral Na–Cl waters with volcanic rocks affected by advanced argillic alteration, representing old solfataras buried by later volcanics, as suggested by Robert O. Fournier about 10 years ago.