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.
Figures & Tables
This text is designed to introduce you to the practical concepts and calculations involved in interpreting the chemistry of high-temperature fluids in geothermal systems and hydrothermal ore-forming environments. It is intended that the energetic reader will learn to understand chemical principles, handle routine calculations and follow specialized chemical studies involved in geothermal exploration and exploitation and in ore genesis.
Although the emphasis of the text is on the interpretation of the chemistry of active geothermal systems, the principles involved are equally relevant to the interpretation of fossil hydrothermal ore-forming environments. Many gold-silver ore deposits, for example, have been shown to have formed in the near-surface region of hydrothermal systems similar in fluid chemistry and setting to those active today (White, 1981; Henley and Ellis, 1983). Combination of a knowledge of the principle processes within the active geothermal systems, the thermodynamics of complex ion formation, mineral-fluid equilibria and stable isotope systematics provide a framework which may assist in reconstruction of the hydrological regime within a fossil hydrothermal system where ore deposition occurred. This in turn may become useful in ore search. A chapter dealing with the hydrothermal chemistry of magmatic systems is included later in order to encompass a wider range of ore depositing environments and perhaps the root zones of the active geothermal systems.
After a short introduction to the types of geothermal fluids and chemical calculations, successive chapters will address the interpretation of water and gas analyses from geothermal wells. When we understand the reservoir compositions of some geothermal fluids and their relations to rock chemistry and temperature, we will consider the chemical and isotopic changes that occur in the natural transport of this fluid to the surface, derive and use chemical geothermometers and mixing relations, and map the surface chemistry of a hot spring system. After these studies of natural fluids at depth and at the surface, we will study chemical changes that occur during the exploitation of geothermal fluids and how to anticipate and avoid some of the problems of scaling and corrosion.