Chemical Structure of Geothermal Systems
In this chapter we shall examine the different types of water which may occur in geothermal systems and relate this range of water types to the basic processes which dominate their chemistry. We shall learn how, from the chemistry of water discharged from wells, we can obtain specific information about the deep fluids in a geothermal system and how they relate to natural discharges at the surface. Skills developed in this way may then be used in exploration to obtain deep system information from analyses of natural discharges. In later chapters we shall learn additional techniques based on chemical data to obtain essential information about reservoir behavior before and during exploitation. This chapter is concerned largely with non-volatile components (NaCl, SiO2, etc); gases are briefly mentioned but discussed in much more detail later. The chemical and physical processes discussed here and in later chapters apply equally to hydrothermal ore deposits but in their case chemical data must be estimated from fluid inclusion, stable isotope, mineral paragenesis, and stability data.
It would be impractical to discuss all types of geothermal systems in this text; they occur in a range of tectonic settings (Fig. 2.1) and here we shall focus on the higher temperature systems which occur in areas of active volcanism. The principal features of these systems are outlined below and for more detail the reader is referred to reviews by Ellis and Mahon (1977), Elder (1981), Rybach ana Muffler (1981) and Henley and Ellis (1983). Derivation of the evidence for