Controls on Solubility and Deposition of Quartz, Calcite, Fluorite and Anhydrite in Boiling Saline Hydrothermal Systems

Abstract

Fluid boiling is a key mechanism for precipitation of minerals in epithermal deposits, and evidence for boiling includes a variety of vein mineral textures that indicate rapid deposition. Hence, the relationships between vein mineral paragenesis and boiling are critical to interpreting processes that drive mineralization. Here, we evaluate quantitatively the factors that control solubility and deposition of four common epithermal vein minerals – quartz, calcite, fluorite and anhydrite – in boiling hydrothermal fluids ranging from 0 to 15 wt% NaCl. Major controls on the precipitation of these minerals include the initial salinity of fluid; the temperature at which boiling commences; the extent of vaporization; and magnitude of temperature decrease during progressive boiling. At 0 wt% NaCl, solubilities of all four minerals in the liquid phase increase with increasing temperature up to a local maximum (336 °C for quartz; 287 °C for calcite; 300 °C for fluorite; and 100 °C for anhydrite). Solubility of each mineral in the liquid phase decreases with increasing temperature thereafter (a phenomenon referred to as retrograde solubility), and converges with that in the vapor phase at the critical point of H2O. As fluid salinity increases, solubilities of all four minerals increase, but to different extents. Specifically, the solubilities of calcite, fluorite and anhydrite show strong salting-in effects in comparison to that of quartz. For quartz, calcite, and fluorite, salting-in eventually overcomes the retrograde solubility trends, such that the temperature dependence of solubility is strictly prograde at salinities exceeding a certain threshold (approximately 6 wt% NaCl for quartz; 3 wt% NaCl for calcite; 1 wt% NaCl for fluorite). Anhydrite stands apart in that it exhibits a strictly retrograde trend for all salinities up to 10 wt% NaCl, and only sees a limited prograde trend at temperatures close to the critical point for a fluid of 15 wt% NaCl. The various factors affecting boiling-induced deposition of all three minerals allow for a variety of distinct paragenetic sequences depending on fluid salinity, temperature and extent of vaporization and cooling. Our results provide a framework for both qualitative and quantitative interpretations of vein paragenesis and relationships between fluid composition, boiling and mineral deposition.