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, though currently there is no robust numerical model for the solubility of anhydrite in H2O-NaCl hydrothermal fluids. Here, we present a thermodynamic model for the solubility of anhydrite in saline aqueous fluids for a broad range of pressures, temperatures, and fluid salinities. The generalized solubility model is adapted from Brooks and Steele-MacInnis (2019), which is a combined approach of the previous solubility models of Akinfiev and Diamond (2009) and of Dolejš and Manning (2010). A dataset of experimentally determined solubilities for 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 is also capable of reproducing results for experimental solubility data that was purposefully not included in model calibration. We demonstrate that this solubility model can perform well over a wide-range of P-T-X conditions, including conditions of liquid + vapor coexistence. We also show that the solubility model is a robust tool that can be used to predict the potential precipitation and/or dissolution of anhydrite along representative fluid flow paths within various hydrothermal systems.