Abstract High-temperature, volcanic-centre-related hydrothermal systems involve large fluid-flow volumes and are observed to have high discharge rates in the order of 100–400 kg/s. The flows and discharge occur predominantly on networks of critically stressed fractures. The coupling of hydrothermal fluid flow with deformation produces the volumes of veins found in epithermal mineral deposits. Owing to this coupling, veins provide information on the fault–fracture architecture in existence at the time of mineralization. They therefore provide information on the nature of deformation within fault zones, and the relations between different fault sets. The Virginia City and Goldfield mining districts, Nevada, were localized in zones of strike-slip transtension in an Early to Mid-Miocene volcanic belt along the western margin of North America. The Camp Douglas mining area occurs within the same belt, but is localized in a zone of strike-slip transpression. The vein systems in these districts record the spatial evolution of strike-slip extensional and contractional stepovers, as well as geometry of faulting in and adjacent to points along strike-slip faults where displacement has been interrupted and transferred into releasing and restraining stepovers.

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 Quartz and chalcedony are the silica minerals commonly found in hydrothermal ore deposits. However, in many places there is textural evidence that chalcedony formed after amorphous silica, probably with poorly crystalline cristobalite or opal- CT as an intermediate phase. Fournier (1973) and apparently White (1965) used the term β -cristobalite both for poorly crystalline cristobalite and for opal- CT. Poorly crystalline cristobalite shows broad X-ray diffraction peaks centered at about 4.1 and 2.5 A. Opal-CT also shows these same X-ray peaks plus an additional low-tridymite peak at about 4.3 A (Jones and Segnit, 1971). Quartz is the stable form of silica at pressure- temperature conditions found in convecting hydrothermal systems. Faceted quartz crystals generally grow in solutions that are not greatly supersaturated with silica, indicating relatively slowly changing conditions. In contrast, the deposition of amorphous silica requires high degrees of silica supersaturation with respect to quartz, and generally indicates large and rapid changes in the physical or chemical nature of the solution. These large and rapid changes may also affect the capacity of a solution to transport and deposit ore. There are various ways to bring about this supersaturation such as rapid cooling (generally with decompressional boiling), mixing of different waters, pH changes, and reaction of the solution with volcanic glass. Each of these processes will be discussed in the subsequent sections. Much of what follows is taken from Fournier (1985).

Abstract The factors affecting the transport and deposition of carbonate in hydrothermal systems have been discussed in detail by Holland and Malinin (1979). Solubilities of carbonates are strongly influenced by pH, P CO2 temperature, and the presence of other dissolved salts. The alkali carbonates, Na, K, and Li, are relatively soluble at all temperatures and generally precipitate only where there is extreme evaporation. In contrast, the alkaline earth carbonates, Ca, Mg, Sr, and Ba, are moderately to sparingly soluble and commonly precipitate in hydrothermal systems. Calcite is by far the most abundant and important carbonate found in the epithermal environment, and more solubility data at hydrothermal conditions are available for it than for any of the other carbonates. Therefore, after briefly reviewing the system CO 2 -water, the discussion will focus on the transport and deposition of calcite in hydrothermal solutions. The behaviors of other moderately to sparingly soluble carbonates in hydrothermal solutions are similar to that of calcite.

Abstract Fluid-inclusion analyses have provided some of the most useful information for determining the physical and chemical environments of mineral formation. The purpose of this chapter is to describe those fluid-inclusion characteristics which serve to distinguish relatively near-surface, epithermal formation conditions from deeper and, potentially, higher temperature formation conditions, and to discuss several techniques and problems which are specific to fluid inclusions trapped in the epithermal environment. A detailed summary and critique of fluid-inclusion literature related to epithermal systems has not been attempted. For this information the reader is referred to the recent compilations of Buchanan (1981), Heald-Wetlaufer et al. (1983), Roedder (1984), and Hedenquist and Henley (1985). Moreover, we have not attempted to relate any particular fluid-inclusion characteristic to a specific type or stage of mineralization, because an adequate data base to do so does not presently exist. This presentation is limited to two subjects--the petrography and petrology of fluid inclusions from the epithermal environment--and is intended to provide the explorationist with a basic understanding of the criteria for recognizing and interpreting inclusions trapped in this environment. Two important topics will be discussed in detail: (1) the identification and interpretation of fluid inclusions trapped from boiling fluids, and (2) the identification of gases (mainly CO 2 ) in fluid inclusions and the effect of volatiles on calculated pressures and depths of trapping. We will not, however, discuss the important chemical consequences of boiling and dissolved volatiles, as these subjects are covered in detail in other chapters in this volume (see Henley)

Abstract Stable-isotope geochemistry has made important contributions to the widely acknowledged “renaissance” in the earth sciences for more than three decades. This status may be ascribed both to theoretical and practical considerations. First, the isotopic species of an element may be fractionated (partitioned unequally) between two or more coexisting phases because of mass-dependent differences in their chemical and physical behaviors, and the amount of such fractionation normally varies inversely with temperature and independently of pressure. Accordingly, the isotopic abundances of an element may serve to define the mechanisms of formation, thermal environment, and provenance of rocks, minerals, and fluids. Second, the analytical procedures now available render most geologic materials well suited for routine and rapid isotopic measurements. Some important milestones of the 1930's and 1940's leading to our present understanding include the discovery of deuterium and formulation of the theoretical basis for stable-isotope fractionation by Harold C. Urey and colleagues at the University of Chicago and the development of improved mass spectrometers by Alfred O. Nier at the University of Minnesota. The subsequent construction of laboratory facilities elsewhere was commonly directed by graduates and associates of these pioneers and their respective institutions. As of today, the literature relevant to stableisotope geochemistry is voluminous and far beyond the scope of this topical overview. Most investigations, apart from those concerned with theory or laboratory experimentation, have been focused on one or more of the following objectives: (1) the conditions and mechanisms of rock or mineral formation; (2) the sources of magma, sediment

Abstract In Chapter 1, R. W. Henley summarized our understanding of the chemical and hydrodynamic structure and the transport properties of active hydrothermal systems, with particular emphasis on terrestrial magmatic-hydrothermal systems. Such an overview is especially valuable because active geothermal systems are modern “archetypes” of the ancient systems which concentrated metals in their upper portions to form epithermal ore deposits. More than any other factor, the study of active systems has provided the framework on which the observations on epithermal deposits have been arranged in the relatively recent development of comprehensive models of epithermal ore formation. The Principle of Uniformitarianism has served us well in this instance. In this chapter, we focus on observations on epithermal ore deposits in continental silicic to andesitic volcanic terranes. Volcanic-hosted deposits offer the most direct comparison with many of the well-studied modern geothermal systems. We first compare the attributes from a number of epithermal ore deposits and show how they may be used to identify two important, and distinct volcanic-related hydrothermal environments. We then examine the best-studied deposit of each type: Creede and Summitville, both of which are located in the San Juan Mountains in southwest Colorado. In so doing, we are able to examine epithermal deposits for evidences of processes that are now occurring in geothermal systems. Finally, we use the observational base and interpretations derived from each deposit type to develop generalized “geothermal” models of mineralization. The models have been taken, in large part, from the excellent synthesis by Henley and Ellis

Abstract Sediment-hosted precious-metal deposits are typically formed in carbonaceous, silty dolomites and limestones or calcareous siltstones and claystones. Gold mineralization is disseminated in the host sedimentary rocks and is exceedingly fine grained, usually less than one micron in size in unoxidized ore. Primary alteration types include silicification, decalcification, argillization, and carbonization. Supergene alteration is dominated by oxidation resulting in the formation of numerous oxides and sulfates and the release of gold from its association with sulfides. Commonly associated trace elements are arsenic, barium, mercury, antimony, and thallium. Deposits of this type are commonly referred to as either Carlin-type deposits, after the large bulk- minable, disseminated-gold deposit in northern Nevada, or as fine-grained or “invisible-gold” deposits. We refer to deposits of this type as sediment-hosted, disseminated precious-metal deposits. This chapter presents a classification scheme and reviews the geologic characteristics of sediment- hosted, precious-metal deposits. The influences of geology on both mining and the development of genetic and exploration models are discussed. Although deposits of this type occur throughout the western United States, the largest concentration of deposits and also the best understood are in Nevada. We have chosen, therefore, to use selected individual deposits from Nevada as type examples to support the classification scheme and to provide the student with an understanding of the similarities and differences that occur in these deposits. This chapter is thus designed to develop and nurture the knowledge of the comparative geology of sediment-hosted, disseminated precious-metal deposits. This is accomplished by reviewing and comparing regional-, district

Abstract An epithermal ore deposit is defined as a relatively near-surface deposit formed in a hydrothermal system under low to moderate pressure and a temperature range below about 300°C (Barrett, 1985). This concise definition is a restatement of Lindgren's characteristics of hydrothermal systems of “epithermal” character. A modification of Lindgren's characteristics is tabulated in Table 9.1. These characteristics are both physical and chemical, and we will, in this and the following paper (Berger and Silberman, 1985, this volume), attempt to relate them. Epithermal lode deposits in the Circum-Pacific region produce approximately 30 million grams of gold annually (Giles and Nelson, 1982) and a larger, but indeterminate, amount of silver. Many epithermal deposits are closely associated with convergent plate boundaries related to present and relatively recent regimes of plate tectonic interaction (Giles and Nelson, 1982; Sawkins, 1984). These mobile regions of the earth's crust are characterized by recent volcanism, high heat flow and tectonic activity, and by the presence of active and recently active geothermal fields, some of which have deposited precious metals and associated metals (Table 9.1) in similar concentrations (but not volumes) to those found in the epithermal ore deposits (Weissberg et al., 1979; Henley, 1985, this volume). The understanding of processes that occur during the formation of epithermal ore deposits has been advanced in the recent past by the suggestion that these ore deposits are essentially fossil geothermal systems (e.g., White, 1955, 1981 ; White, 1974; Wetlaufer et al., 1979; Henley and Ellis, 1983; Henley, 1985, this volume).

Abstract Those epithermal precious-metal deposits where ore was precipitated within 100-300 m of the earth's surface such that the direct interaction of hydrothermal fluids with the surface is a major cause of ore-mineral precipitation in the upper part of the system make up the subclass known as hot-spring-type deposits (Berger and Eimon, 1983; Berger, 1985). The deposits were emplaced as small veins, stockworks, and explosive breccias in association with non-marine volcanism, generally calc-alkaline in composition. Henley (1985b, this volume) and Hayba et al. (1985, this volume) prefer to not separate hot-spring deposits as a separate class or subtype of epithermal deposits. However, we have chosen to treat hot-spring related deposits separately because of the importance of hydrothermal eruptions and accompanying brecciation to near-surface ore deposition and exploration recognition criteria (Adams, 1985, this volume). Active geothermal systems have long been thought to be modern analogs of epithermal systems (cf. White, 1955; Weissberg et al., 1979), but it wasn't until the recent discovery of the McLaughlin gold deposit in California and the publication of data on Round Mountain, Nevada (Berger and Tingley, 1980; Tingley and Berger, 1985) and Hasbrouck Mountain, Nevada (Silberman et al., 1979; Graney, 1984) that there became a widespread recognition among explorationists of the geological and geochemical characteristics and resource importance of fossil hot- spring systems. Subsequently, study in the Bodie, California mining district by P. Herrera and M. L. Silberman (Silberman and Berger, 1985, this volume) has further linked fossil hot-spring systems to the deeper-emplaced bonanza-type epithermal vein

Abstract Some active geothermal systems are currently depositing gold, silver, and base metals, and most “epithermal” ore deposits formed in once-active geothermal systems (e.g., White, 1981; Henley, 1985, this volume). Boiling of hot (l00°-300°C) ground water in such systems is a process of fundamental significance because it fixes temperature gradients (e.g., White et al., 1971; Muffler et al., 1971; Henley and Ellis, 1983) and causes precipitation of sulfide, carbonate, and silicate minerals (e.g., Buchanan, 1981; Berger and Eimon, 1983). The gas phase, including H 2 O, CO 2 , and H 2 S, when condensed and oxidized near the surface, produces acid waters that generate argillic alteration of rocks and which may trigger deposition of precious metals. The geologic and hydrologic framework of a boiling geothermal system is depicted in Figure 11.1, based in part on White et al. (1971), Henley and Ellis (1983), Berger and Eimon (1983), and Steven and Eaton (1975). Figure 11.2 corresponds to Figure 11.1, showing in flow-diagram form the chemical components and processes in the hydrothermal system. These include boiling (A, Figs. 11.1 and 11.2), condensation of the boiled gas in rock (B), oxidation of the gas by the atmosphere (C), condensation followed by oxidation of the gas in cool, fractured ground (D), mixing of acid ground waters with the boiled liquid (E), and mixing of cold ground water with the boiled liquid (F). All of these processes shape the chemistry of geothermal systems and several of them are responsible for ore formation in epithermal systems. We present here some results

Abstract This paper presents a rationale for (a) the integrated use of geologic information in the development of exploration strategies, (b) an increased awareness of the impact of human factors on the geologists' use of geologic information in exploration, and (c) the effective use of available geologic information through the development of more predictive and reliable models for exploration. This is accomplished, first, by illustrating how geologic information is used in the development of exploration strategies, and how other strategic factors influence the selection and use of geologic information in' exploration. Second, human factors are identified and discussed that most significantly influence how geologists use geologic information. Finally, an approach for the organization and use (i.e., modeling) of geologic information in mineral exploration is presented. This modeling approach is referred to as a data-process-criteria model, and it is illustrated with examples for the hot-spring-type epithermal precious- metal deposit.

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, 1985; 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, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).