CHEMICAL EVOLUTION AND MINERAL DEPOSITION IN BOILING HYDROTHERMAL SYSTEMS.

A thermodynamic model of boiling hydrothermal solutions is developed and applied over a wide range of physical and chemical conditions. Metals that are complexed by chloride are deposited largely as a result of the decreasing proton concentration associated with CO//2 exsolution during boiling. Metal bisulfide complexes are destabilized most when the decrease in proton concentration is sluggish relative to the loss of H//2S. The relationship between the proton, CO//2, and SIGMA SO//4 concentrations prior to boiling to the proton concentration after boiling is defined explicitly by a few simple equations. These equations along with the solubilities of calcite and anhydrite constitute the chemical boundary conditions for significant mineral deposition by boiling. Physical variables such as the heat budget and the restrictions on the partitioning of mass between liquid and vapor, although significant, are subordinate to the compositional variables in determining the chemical evolution of a boiling hydrothermal solution. Refs.