Hydrolysis Reactions in Hydrothermal Fluids
In previous sections you have considered the components of a geothermal fluid analysis in terms of:
(a.) comparison of conservative element concentrations (like Cl) between wells and effects of boiling and dilution (mixing diagrams).
(b.) relations between component ratios, concentrations and deep temperatures (Na/K, NaKCa, gas and silica geothermometers).
The next step in fully utilizing the chemistry of geothermal discharges is to examine carefully the relations between observed fluid chemistry and alteration minerals occurring in the drillcore. These relations form the basis for chemical geothermometry as well as highlighting some of the important interwoven relationships between the fluid components. To formulate these relations we rely heavily on thermodynamic data which, because of experimental difficulties at high temperatures, Bay sometimes be suspect — we can often recognize these cases by using natural fluid-mineral equilibria as a guide.
In studies of mineral deposits, alteration assemblages are frequently used in conjunction with salinity estimates from fluid inclusion data to indicate the pH of ore-forming fluids. The compatibility of fluid chemistry and mineralogy established through geothermal studies (Browne and Ellis, 1970; Arnorsson et al., 1978; Truesdell and Henley, 1982) provides the confidence to apply this approach in ore forming systems.