A model for the solubility of anhydrite in H2O-NaCl fluids from 25 to 800 °C, 0.1 to 1400 MPa, and 0 to 60 wt% NaCl: Applications to hydrothermal ore-forming systems

Abstract

Anhydrite (CaSO4) is a common gangue mineral in hydrothermal ore-forming systems, but currently there is no comprehensive numerical model for the solubility of anhydrite in H2O-NaCl hydrothermal fluids over wide ranges of pressure, temperature and fluid salinity (P, T and X). Here, we present a new thermodynamic model for the solubility of anhydrite in saline aqueous fluids throughout a wide range of P-T-X conditions relevant to crustal hydrothermal settings. A database of experimentally determined solubility of anhydrite was used to calibrate the model, and the experimental data covers P-T-X ranges of 0.1–1400 MPa, 25–800 °C, and 0–60 wt% NaCl. The model is successful at reproducing solubility values and overall trends from the experimental dataset within error, and also reproduces results for experimental solubility data that were not included in the calibration. We demonstrate that the model can perform well over a wide-range of P-T-X conditions, including conditions of liquid + vapor coexistence. We also provide examples of applications of the solubility model to predict the precipitation and/or dissolution of anhydrite along fluid flow paths representative of hydrothermal systems.