QUARTZ TRANSPORT AND DEPOSITION FROM IMMISCIBLE FLUIDS IN PORPHYRY COPPER SYSTEMS

Fluid immiscibility (boiling) is a common process in the earth's crust. In the 1980's Bowers and Helgeson published an equation of state for the H₂O-NaCl-CO₂ system to estimate phase equilibrium properties of this geologically-important ternary system. They also examined the implications of fluid immiscibility on metamorphic phase equilibria at elevated temperatures and pressures using this EOS.

Building upon the contributions of Bowers and Helgeson, we present results of a modeling study of quartz transport and deposition in porphyry copper deposits. Using PVTX data for H₂O-NaCl, the temporal and spatial distribution of fluid immiscibility within the crystallizing porphyry pluton and surrounding country rock has been modeled and the solubility of quartz in the mixed liquid + vapor fluid has been calculated and compared to its solubility in a metastable one-phase fluid at the same P-T conditions. The solubility of quartz in the brine phase is always higher than the solubility of quartz in a metastable one-phase fluid, whereas the solubility in the vapor phase is lower than the bulk solubility. Overall quartz solubility in the immiscible fluids is calculated as a mass-balanced combination of quartz solubility in the brine and vapor phases. The results show that for a fluid mixture flowing upward through an immiscible, two-phase L+V field, quartz exhibits prograde solubility in the brine phase and retrograde solubility in the vapor phase over much of the flow path. While the solubility of quartz in the vapor phase remains significantly lower than that in the brine, the mass fraction of vapor is usually significantly higher than that of the brine, and the bulk quartz solubility in the system is dominated by the vapor portion of the immiscible fluid. These results have important implications for the trapping of fluid inclusions in evolving magmatic hydrothermal systems associated with porphyry copper deposits.