| A magmatic volatile phase (MVP) may be a supercritical fluid, low salinity vapor, or high salinity brine. The MVP causes hydrothermal alteration and can also transport metals in ore-forming environments. Unfortunately, there is a lack of experimental data regarding mineral alteration by high salinity liquids (brines) in porphyry systems. This study is designed to fill the gaps in the experimental database of mineral alteration by chloride-rich subcritical, vapor-undersaturated high-salinity brines. The phase stability and equilibrium brine compositions in the K-feldspar-andalusite/sillimanite-quartz-brine (70 wt. % KCl equivalent; KCl-HCl-H2O) system were determined from 600 to 750°C and 50 to 80 MPa. The stable mineral phase was ascertained by an examination of crystal morphology and by the K/Al, K/Si and Al/Si of the run products as determined by using a JEOL JSM-5610LV Scanning Electron Microscope with Energy Dispersive Spectrometry (SEM-EDS). The phase stability fields were plotted as a function of temperature, pressure and molar (KCl/HCl) ratios of the fluid. The K-feldspar-andalusite/sillimanite-quartz-brine equilibrium boundaries at 80 MPa and 600, 650, 700, and 750°C are at molar (KCl/HCl) of 37+/-5, 29.94+/-0.02, 22+/-8, and 5+/-02, respectively (uncertainties are half-widths of the reversal brackets). Pressure effects were examined by fixing temperature at 600°C while varying pressure to 50, 65 and 80 MPa. The K-feldspar-andalusite/sillimanite-quartz-brine phase boundaries at 600°C and 50, 65 and 80 MPa were determined to be (KCl/HCl) of 10+/-2, 30+/-10 and 37+/-5, respectively. These data show that with increasing pressure and/or decreasing temperature, the equilibrium boundaries shift to higher molar (KCl/HCl) values in the brine. These phase boundaries are not consistent with the metastable extensions of the reaction boundaries involving low salinity fluids at 100. Thus, the (KCl/HCl) of high-salinity brines cannot be estimated accurately by applying data derived from experiments on low salinity fluids. A thermodynamic model was developed from the determined equilibrium boundaries that allowed for the calculation of equilibrium constants (K) and apparent equilibrium constants (K'), and the estimation of activities aBrineKCl/a BrineHCl and activity coefficients gBrineKCl/g BrineHCl . |
