Redox Reactions in Hydrothermal Fluids
Many elements participate in oxidation-reduction reactions in the geothermal/epithermal environment. These include C, S, H, O, N, Fe, Mn, U, W, As, Sb, Bi, Cu, Ag, Au, Te, and Sn. The first six or seven elements listed are much more abundant than the rest and they interact to buffer the redox state; the remaining (and to a large extent the most interesting economically) elements are usually much less abundant, and they only respond to the chemical environment imposed by the dominant redox systems. In this chapter we shall investigate methods of determining the oxidation state of a system, either directly by calculations based on the chemistry of geothermal gases and liquids, or indirectly by interpreting the phases and phases assemblages observed in fossil hydrothermal systems.
Redox reactions are important in such diverse areas as the corrosion and scaling of geothermal production pipes, the interaction of organic matter with fluids, the oxidation of H2s, the precipitation of native metals and pyrite and other sulfides, the destruction of sulfides by oxidation, and the disproportionation of SO2 into H2S and SO4 on cooling from high temperature.
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.