Abstract In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore- bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious- metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985a; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg)Now recogpized as characteristic of epithermal gold deposits (Weissberg, 1969). The term “hydrothermal” encom passes al l types of hot-water phenomena in the earth’s crust although most commonly the term is used in reference to those associated with impressive geyser activity, aesthetically attractive hot pools, etc. These features are most common in volcanic areas such as Yellowstone National Park, U.S.A., Iceland, or in the Taupo Volcanic Zone of New Zealand, but other terranes also host hydrothermal activity even though subsurface temperatures may be relatively low and surface features less impressive. Warm springs in the Rocky Mountains, the European or New Zealand Alps, or in the sedimentary massifs of central Europe are examples, and it is clearly important for mineral exploration to discriminate these types of systems from those in more favorable geological environments.

Abstract In trying to understand the depositional processes which led to ore deposition in fossil hydrothermal systems, we attempt to reconstruct the chemistry of the fluid phase from observation of its relics (e.g., alteration minerals, fluid inclusions). We may also attempt to thermodynamically model the chemical changes experienced by this fluid as it passes upward through a vein, vents to the seafloor, boils or mixes with other waters, etc. A number of important assumptions are made; one is the assumption of equilibrium and another is that the thermodynamic data base is sound. Analyses of fluids discharged from geothermal wells, together with drill-core data, allow the opportunity to independently check the validity of the thermodynamic data base and to observe directly, chemical processes leading to the deposition of gold, base-metal sulfides and common gangue minerals like quartz and calcite. The calculations involved are not trivial, but are essential to the understanding of epithermal or any other type of hydrothermal ore deposit. To illustrate these procedures, we shall examine the discharge of one production well in the Broadlands geothermal field in New Zealand. Through the use of thermodynamics, the amount of information we shall retrieve about the reservoir and depositional processes is quite astonishing. We shall then turn to som e review questions to consider implications for the formation of some hydrothermal ore deposits. In this chapter we have tried to follow a pragmatic course, avoiding the temptation to overindulge in the (essential) nuances of thermochemistry at the expense of the proscribed

Abstract 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 reservoi r compositions of some

Abstract 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, SiO 2 , 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

Abstract In Chapter 2 chemical data from a set of geothermal wells were used to derive an empirical geothermometer based on silica content. In this section we shall derive this geothermometer by an independent method based on experimental data and examine some other empirical geothermometers which are related to mineral equilibria in the system. During exploration the chemical geothermometers provide rare and valuable windows into the deep system through which we see "as through a glass darkly” (St Paul), the choice and interpretation of geothermometer data being the art of the exploration geochemist.

Abstract Gas concentrations in fluids encountered during drilling of geothermal fields range from 0.05 wt% (Wairakei, Ahuachapan) up to about 1 wt% (Ngawha, Broadlands). We discovered in Chapter 2 that carbon dioxide is the dominant gas in geothermal systems and, as we shall see later, plays an important role in controlling the pH of the aquifer fluid. The ratios of the principal gases (e.g., CO 2 , H 2 , CH 4 ) are controlled by reactions such as and may therefore be used as geothermometers in the same way as we have used alkali ion ratios. The development of gas geothermometers is discussed in a later chapter; at this stage we will examine the behaviour of gases when phase separation occurs from an initially singlephase geothermal fluid. This is important when we consider the recalculation of analyses of steam samples separated at the surface to determine aquifer dissolved gas compositions. Gas pressures are also important in reservoir modelling studies as well as in a number of engineering problems associated with geothermal field development; in studies of fossil hydrothermal systems — ore deposits — the constraints imposed by gas contents are just as important and deserve much more attention.

Abstract In this chapter we will consider equilibria among gases in hydrothermal fluids from hot water geothermal reservoirs which produce from a liquid phase at depth. We leave until Chapter 11 consideration of gas equilibria in vapor-dominated reservoirs and hot-water reservoirs with aquifer steam, because discharges from these reservoirs contain gases of mixed origin from vapor and liquid and require more complicated computational methods. The calculations in this chapter will introduce us to the use of thermochemical (free energy) data to derive equilibrium constants and to the conversion of measured gas concentrations in the liquid phase into reservoir gas pressures.

Abstract 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.

Abstract In the last chapter the relationship between alteration mineralogy and fluid chemistry was discussed using a greatly oversimplified scheme for estimating the pH t of deep system fluids. We use this as a starting point to consider some further implications of the fluid- mineral equilibria we have considered and then develop some calculation procedures to improve our calculation of pH t for high temperature fluids from analytical data. Drill core from hydrothermal systems contains a number of secondary minerals such as Kmica, K-feldspar, and epidote that result from the alteration of primary minerals. In many hydrothermal studies it is assumed that fluid composition and pH were controlled--or buffered—by simple alteration phase equilibria. We can examine this proposition as follows. If the pH controlling equilibria are

Abstract 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 H 2 s, the precipitation of native metals and pyrite and other sulfides, the destruction of sulfides by oxidation, and the disproportionation of SO 2 into H 2 S and SO 4 on cooling from high temperature.

Abstract The recognition that some present day geothermal systems may be active analogs of metal- depositing hydrothermal systems of the past has promoted a great deal of interest in metal transport and deposition in present day systems (White, 1981; Weissberg; et al., 1979; Henley and Ellis, 1983). Relatively few thermodynamic data are available from which to calculate the high temperature solubilities for metals and metal sulphides. The calculations discussed below focus on recent experimental data up to about 350°C and on metal transport in geothermal and analogous epithermal environments. In Chapter 14 similar calculation procedures are used to examine metal deposition in some other ore-forming environments. Studies of high temperature metal complexing may ultimately become significant in geothermal corrosion and scale control, mineral recovery from brines and development of chemical processes for the control of toxic metals.

Abstract Isotopes are forms of an element with the same number of electrons and protons but a different number of neutrns and therefore different masses. Although chemical behavior differs very little between isotopes of the same element, the mass differences between isotopes do produce small chemical differences and equilibrium constants (fractionation factors) for isotopic exchanges are typically close to one. The relative abundances of isotopes commonly analyzed in geothermal fluids were given by Panichi and Gonfiantini (1976). Results of the application of isotopes to geothermal systems have been reviewed by Craig (1963), Panichi and Gonfiantini (1976), Truesdell and Hu1ston (1980), and Giggenbach et al. (1983).

Abstract Analytical data presented in earlier chapters were used to calculate the single phase composition of aquifer fluid through a knowledge of the discharge enthalpy of the well. The assumptions made in this calculation are a) that a single phase is indeed present in the feed zone to the well and b) that no phase separation occurs underground to modify the original fluid composition. While valid for many wells, these assumptions are not universally true. In this chapter we shall examine the effects of underground boiling on the composition of well discharges in relation to original aquifer fluid. These effects are frequently seen in exploration well discharges but become common in production wells as a consequence of exploitation.

Abstract The physical and chemical setting of geothermal systems is dominated by waters of surfi- cial origin; nevertheless, the heat sources are believed to be magma-s, and there is also a high probability that magmatic fluids contribute heat and some dissolved components H 2 s, SO 2 , CO 2 , … ) to the modern hydrothermal systems that are tapped for energy. By the same token, epithermal and other fossil hydrothermal deposits may well have received contributions of metals from magmas. Because subsurface zones of magma influence are never directly observed during magmatic activity, the magmatic story is an after-the-fact interpretation of mineral assemblages; and it is an emerging story with many chapters still unwritten. This chapter will develop the basis necessary to deal with the magmatic equilibria responsible for some of the magmatic gases. For the most part we shall deal with silicic magmas because they are much more commonly associated with both geothermal activity and hydrothermal ore deposits than are mafic ones, but the general calculations described here are relatively insensitive to rock type. It is the minor minerals — especially the titanium-iron oxides and pyrrhotite — that are most critical in this discussion. At magmatic temperatures many species are associated as neutral complexes rather than ionic compounds. So, for example, HCl, NaCl, and KCl are not ionic, but neutral molecules at temperatures above about 500°C or so (Franck, 1956; Quist and Marshall, 1968). Similarly, the sulfurous gases are present as H 2 S and SO 2 rather than ionized species. As a consequence, reactions at magmatic