HIGH-TEMPERATURE MINERAL ALTERATION IN THE ALBITE-ANDALUSITE-QUARTZ-BRINE SYSTEM

During the ascent of magma to shallow portions of the crust, decompression of the melt and/or the crystallization of anhydrous phases may cause one or more magmatic volatile phases (MVP) to be exsolved. MVPs are commonly chloride-rich fluids and may be a supercritical fluid, low salinity vapor, or high salinity brine. They are hypothesized to be responsible for some areas of hydrothermal alteration and the transfer of metals from a crystallizing melt to areas of ore deposition. The importance of the subcritical, vapor undersaturated high-salinity liquid phase (brine) is widely known, however, a dearth of data exists on the properties of that phase at elevated temperatures and pressures. This study addresses the influence of the brine’s composition on the alteration of minerals commonly found in magmatic-hydrothermal systems. The phase stability and equilibrium brine compositions in the albite-andalusite-quartz-brine (65 wt. % NaCl equivalent; NaCl-HCl-H₂O) system were determined from 600 to 700°C and 50 to 80 MPa. The stable mineral phase was ascertained by an examination of crystal morphology, optical microscopy, and by the compositions of the run products. The phase stability fields were plotted as a function of temperature, pressure and molar (NaCl/HCl) concentration ratios of the brine. The data show that with decreasing temperature, the equilibrium boundaries shift to higher NaCl/HCl values in the brine, consistent with trends seen in low-salinity fluids at lower temperatures. However, the NaCl/HCl values at the phase boundaries are not consistent with the metastable extensions of the reaction boundaries involving low salinity fluids and illustrate that the chloride content of the MVP has an impact on the equilibrium NaCl/HCl during albitic alteration. Furthermore, our preliminary results illustrate that the equilibrium boundaries are not equivalent to the recently reported KCl/HCl boundaries observed for potassic alteration.