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
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.