Testing EQ3/6 and GEMBOCHS Using Fluid-Mineral Equilibria in the Wairakei Geothermal System

The ability of the EQ3 and EQ6 geochemical modeling codes and the GEMBOCHS thermodynamic data bases (Wolery, 1992; Wolery and Daveler, 1992) to simulate geochemical changes in the post-emplacement environment at the potential Yucca Mountain, Nevada repository is being tested using observed mineral-fluid relations in the Taupo Volcanic Zone of New Zealand. In this report, comparisons between observed equilibria and simulations of field relations in the Wairakei geothermal system are used to test the codes and data bases in high temperature (>200°C) systems. Analysis of trends in water and gas chemistries and well discharge characteristics with time were used to identify a set of representative water and gas analyses from zones producing at about 25O’C. The most common vein minerals at this temperature are: wairakite, adularia, epidote, quartz, albite, chlorite, calcite, prehnite and pyrite (Reyes, Giggenbach and Christenson, 1993). Calculations were carried out using version 7.2a R134 of EQ3 and version 7.2a R130 of EQ6 and the SUPCRT and COM subsets of the R24 version of GEMBOCHS. Thermodynamic data bases using different data for Al aqueous species were used to identify the data set which produced the best matches between observed and calculated equilibria. Geochemical calculations with EQ3/6 using measured Al concentrations and thermodynamic data for Al aqueous species from Pokrovskii and Helgeson (in press) suggest that vein minerals are presently in equilibrium with subsurface fluids. Matrix replacement minerals, although of much greater variety and number than vein minerals owing to initial rock mineralogy (Reyes, Giggenbach and Christenson, 1993), are consistent with calculated mineral stabilities at downhole temperatures. The quartz geothermometer of Fournier (1983) describes aqueous SiOz concentrations at temperatures >200°C better than calculations using quartz data from Walther and Helgeson (1977). Calculations suggest that the free energy of wairakite, a high temperature Ca-zeolite, should be decreased by about l-l 5 kcal/mol at 25O’C. Calorimetric data for lower temperature zeolites such as clinoptilolite and stilbite from Johnson and co-workers (Johnson et al., 1991; Howell et al., 1990) appear to over-estimate the stability of these zeolites at temperatures > 2OOOC. The simulations described in this paper suggest that EQ6 can be used to identify facies of minerals that will be stable in various environments, but can not be used to predict the exact phase assemblage that is in equilibrium with a given water.