TEMPERATURE AND PRESSURE AFFECTS ON POTASSIC AND SERICITIC ALTERATION BY HIGH SALINITY FLUIDS

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 detailing the numerous instances of mineral alteration by high salinity brines in porphyry systems. This study is designed to fill the gaps in the experimental database of mineral alteration by chlorine-rich subcritical, vapor-undersaturated high-salinity fluids (brines). The phase stability and equilibrium brine compositions in the K-feldspar-muscovite-andalusite-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 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 log₁₀ KCl/HCl ratios of the fluid. The K-feldspar/andalusite equilibrium boundaries at 80 MPa and 600, 650, 700, and 750°C are at log₁₀ KCl/HCl of the brine of 1.64±0.16, 1.33±0.15, 1.03±0.12, and 0.47±0.44 (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 muscovite-andalusite (+quartz) phase boundaries were determined at 600°C and 50 and 65 MPa to be log₁₀ KCl/HCl of 0.98±0.07 and 1.16±0.44. Muscovite was not observed at 80 MPa; the K-feldspar/andalusite phase boundary was 1.64±0.16 at 600°C and 80 MPa. These data show that with increasing pressure and/or decreasing temperature, the equilibrium boundaries shift to higher KCl/HCl values in the brine. These phase boundaries are not consistent with the metastable extensions of mineral alterations by low salinity fluids at 100 MPa (e.g. Hemley, 1959, Haselton et al., 1991). Thus, the KCl/HCl of high-salinity brines cannot be estimated accurately by applying data derived from low salinity fluids. Equilibrium constants, apparent equilibrium constants, activities and activity coefficient ratios were calculated from the phase boundaries and will be presented.