Mikhail V. Ivanov
Institute of Precambrian Geology and Geochronology Russian Academy of Sciences, nab. Makarova 2, St. Petersburg 199034, Russia
Numerical thermodynamic models are proposed for the quaternary fluid system H2O-CO2-NaCl-CaCl2 and its ternary subsystems H2O-NaCl-CaCl2, H2O-CO2-NaCl, and H2O-CO2-CaCl2. The models are valid for temperatures from 150 °C to 350 °C, pressures from 0.2 to 1.4 kbar, and for arbitrary concentrations of salts. The latter feature is inherited from the earlier developed models of binary systems H2O-NaCl and H2O-CaCl2. All the models are formulated in terms of the Gibbs free energy. The entropy term in the equation for the Gibbs free energy of mixing is introduced in a general form, based on the number of different ways of arranging particles in the system that lead to the same total energy. The parameters of the energy terms corresponding to the interactions of particles in binary and ternary subsystems are obtained by fitting published experimental data. The concentrations of salts in the gas phase are simulated based on the salt evaporation free energy. Our model, also available as a computer code, makes it possible to predict the physicochemical properties of fluids involved in hydrothermal processes in the upper crust: the phase state of the system (homogeneous or two-phase fluid), activities of the components, densities, and compositions of the (coexisting) fluid phases. The model offers a numerical tool for analyzing fluid inclusion data and better understanding of metamorphic and metasomatic processes in the upper crust. Fluids at studied P-T conditions play a decisive role in the formation of hydrothermal ore deposits, including most of the world's gold deposits.
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