Abstract It is hardly possible to read a single paper in the literature on the origin of hydrothermal ore deposits without encountering activity-activity, log -ph, or related diagrams. Such diagrams are immensely useful in graphically depicting phase relationships, solution speciation, mineral solubilities, and fluid evolution. Nevertheless, despite their wide usage, there are few published accounts fully illustrating construction of these diagrams. The subject is treated at various levels of detail by (Holland 1959, 1965),Barnes and Kullerud (1961),Garrels and Christ (1965),Barton and Skinner (1979),Henley et al. (1984),Nesbitt (1984),Faure (1991),Anderson and Crerar (1993),Nordstrom and Munoz (1994),Krauskopf and Bird (1995),Stumm and Morgan (1996), andDrever (1997), among others. In this chapter, a step-by-step description of the methods of construction of activity-activity and log -pH diagrams from tabulated thermodynamic data (Gibbs free energies of formation, equilibrium constants), as well as some of the possible pitfalls, is provided. It is assumed throughout the chapter that reliable, internally consistent thermodynamic data are available for all phases and species in the systems of interest. This will not be the case for every system of relevance to the economic geologist.Henley et al. (1984)discuss some alternatives for constraining the construction of activity-activity and log -pH diagrams in the event that some of the necessary thermodynamic data are not available or reliable. consist of straight lines. Such activity-activity diagrams are particularly helpful in visualizing wall-rock alteration processes. A useful compendium of various activity-activity diagrams over a range of pressures and temperatures has been published byBowers et al. (1984). We first discuss some of the general principles involved in the construction of activity-activity diagrams using a single reaction boundary as an example. This is followed by two worked examples of the construction of entire activity-activity diagrams.

Abstract Examination of igneous rocks and/or alteration products associated with geothermal systems, ore deposits, and volcamsm suggests that magmatic volatiles may be active agents of acid alteration, ore-metal transport, magma ascent, and volcanic eruption. Thus, an understanding of the magmatic volatile phase (MVP) is critical to pure and applied geology. However, because of its fugitive nature, the magmatic volatile phase is difficult to sample or study. Therefore, experimental and theoretical modeling plays an important role in our attempt to understand magmatic-hydrothermal processes such as those thought to be active in the generation of granite-related ore deposits. Geologic studies of past magmatic-hydrothermal activity include a combination of experimental and field-based methods of analysis. However, the relationship between static, microscale, equilibrium experiments (e.g., studies of element partitioning, phase equilibria, etc.), and the complex, time-integrated natural world is a tenuous one. Without models, the deductive consequences of experiments cannot be tested against field observations. Candela and Piccoli (1995) refined a model (now called MVPart) that can be used to predict the concentration of ore metals in successive aliquots of a (Rayleigh) fractionating aqueous phase during second boiling. Here the term “second boiling” indicates volatile exsolution from a melt due to crystallization of the melt at a constant pressure. This model is available in the form of a DOS/PC executable file (see Appendix 1). The model requires several different types of input, such as the estimation of intensive parameters (e.g., temperature, pressure, the initial ratio of chlorine to water in the melt)

Abstract Geochemical processes in hydrothermal systems are complexly interconnected. Quantitative, whole-system modeling is one of the most effective ways to untangle the interdependent and counterposing chemical effects to determine what the actual natural processes may be. Examples of basic processes include precipitation of ore and gangue minerals in open space to form veins, metasomatic replacement of wall rock by alteration minerals, boiling of an ascending fluid, mixing of ascending and descending fluids, and condensation of boiled gases into cold ground water or into aerated fractures. The course of attendant chemical reactions depends on the combined effects of several of the processes, and the linkages among processes may be subtle. The quantitative details of species concentrations, as determined from chemical reactions, make the difference between whether a mineral precipitates or not.

Abstract Anyone studying an ore deposit winds up with a lot of data: field observations in the form of maps, sections and drill logs, chemical analyses, isotope analyses, fluid inclusion data, paragenetic relations, and so on. In addition, there is a vast amount experimental data in the literature on systems relevant to the deposit being studied, in the form of data on the chemical and physical properties of solids and fluids. The investigator then tries to come up with a model of ore formation that is consistent with all these data. Naturally, the model must also be consistent with accepted principles of chemistry and physics, and one of the subjects most useful, in fact essential, in assembling all these data into consistent models is thermodynamics. The purpose of this chapter is to introduce the concepts and terms of chemical thermodynamics that are useful in constructing models of hydrothermal systems. These will be used extensively in the chapters to follow. The concepts covered in this chapter normally occupy a complete book; the coverage is therefore necessarily brief. We can save considerable space, for example, by assuming that we are all familiar with the concepts of energy, work, heat, and temperature. These are in fact quite difficult subjects, but an intuitive understanding is usually sufficient for us. Some Basic Definitions In this chapter we will not describe any natural system, and only one model of a simple natural system (H2-N2). Most of the discussion will be about a model of energy relationships, called thermodynamics. Although natural systems and thermodynamic models of natural systems are described using many of the same terms, there are some subtle differences. To begin with, a natural system is any part of the universe we choose to consider, such as the contents of a beaker, a crystal of quartz, the solar system, or a bacterium. Thermodynamic systems, on the other hand, are not real but conceptual and mathematical, and are of three types. The three types are used to distinguish between the ways that changes in composition and energy content can be effected, and therefore they are defined basically by the nature of their boundaries. Isolated systems can exchange neither matter nor energy with their surroundings. They are therefore described as having walls that are rigid (preventing any change in volume and hence any energy change due to work), and impermeable to matter and energy.

Abstract The basic problem of thermodynamics is the determination of the equilibrium state that eventually results after the removal of internal constraints in a closed composite system. – Callen (1960), p. 24. Anyone studying an ore deposit winds up with a lot of data: field observations in the form of maps, sections and drill logs, chemical analyses, isotope analyses, fluid inclusion data, paragenetic relations, and so on. In addition, there is a vast amount experimental data in the literature on systems relevant to the deposit being studied, in the form of data on the chemical and physical properties of solids and fluids. The investigator then tries to come up with a model of ore formation that is consistent with all these data. Naturally, the model must also be consistent with accepted principles of chemistry and physics, and one of the subjects most useful, in fact essential, in assembling all these data into consistent models is thermodynamics. The purpose of this chapter is to introduce the concepts and terms of chemical thermodynamics that are useful in constructing models of hydrothermal systems. These will be used extensively in the chapters to follow. The concepts covered in this chapter normally occupy a complete book; the coverage is therefore necessarily brief. We can save considerable space, for example, by assuming that we are all familiar with the concepts of energy, work, heat, and temperature. These are in fact quite difficult subjects, but an intuitive understanding is usually sufficient for us.

Abstract Knowledge of the solubility of ore minerals and the speciation of ore metals in hydrothermal solutions is required for a complete understanding of the genesis of hydrothermal ores. In this chapter, we explore the factors that control solubility and speciation, demonstrate how to carry out quantitative calculations, and review the current state of knowledge for a number of economically important metals. The term solubility refers to the sum of the concentrations of all dissolved forms of a given metal in a hydrothermal solution in equilibrium with a mineral (or minerals) containing that metal. We use the term speciation to denote the relative concentrations of the various forms of a metal in solution. The solubility of a mineral provides an upper limit to the amount of dissolved metal that a hydrothermal fluid can transport, assuming thermodynamic equilibrium. Although a given solution may temporarily carry more metal than permitted by the equilibrium solubility of relevant minerals owing to sluggish reaction kinetics, the equilibrium solubility is nevertheless an important benchmark. Given enough time, equilibrium solubility cannot be exceeded, and systems will proceed in a direction toward the equilibrium state. Also, knowledge of equilibrium solubilities is required for modeling rate processes. Metal concentrations may be maintained below the equilibrium solubility either by sorption processes, which remove metals from solution before saturation is reached with respect to a given mineral, or if there is insufficient metal available in the system to saturate the solution. pointed out in Chapter 1, the extent to which a solution

Abstract Few areas of geochemistry have challenged the ingenuity and patience of researchers as much as the analysis of fluid inclusions (see reviews by Roedder, 1972, 1984, 1990; Hollister, 1981; Shepherd et al., 1985; Boiron and Dubessy, 1994). From simple optical techniques to the use of particle accelerators, no stone has been left unturned in the search for analytical perfection. Progress has been painfully slow and, by comparison with methods for the analysis of rocks and minerals, we are still in the rudimentary stages of development. The last five years, however, have seen a quantum leap in progress, largely as a result of rapid advances in microbeam technology that have established new standards in sensitivity, precision, and accuracy. Though rooted historically in the study of ore deposits, many of the recent breakthroughs have been pioneered by analysts in the petroleum and materials science industries. in the wider field of chemistry, techniques tend to fall into two categories: those for organic and those for inorganic constituents. The situation is very similar with regard to elemental and isotope analysis. This has tended to polarize studies of the composition of fluid inclusions much to the detriment of all concerned, in particular those geologists concerned with the genesis of low-temperature, sediment-hosted hydrothermal deposits where organic material plays an important role in the distribution of ore minerals. Fortunately, technology transfer is now resulting in hybrid instruments that offer multiple capabilities and should lead to a substantial broadening of opportunities and applications.

Abstract Fluid inclusion analysis has the potential to provide some of the clearest data regarding the chemical and physical processes that result in mineral growth, deformation, and recrystallization. The purpose of this chapter is, first, to briefly introduce microthermometry, the most common analytical technique used to gain information from fluid inclusions and second, to discuss how to model and interpret the analytical data. The well-informed user must understand both how the data are gathered and how calculations are made. A detailed summary and critique of various analytical techniques and the thermodynamic data for the various chemical systems is beyond the scope of this chapter. The interested reader will need to follow up on the references throughout the text. However, what follows provides a solid basis to evaluate and interpret publications that use fluid inclusion data to constrain geochemical, geological and geophysical processes. In the previous chapter, Shepherd and Rankin reviewed a variety of analytical techniques to determine the chemical and isotopic composition of either individual fluid inclusions or whole populations of inclusions in a sample. In this chapter, I will review microthermometry, the most widely used technique, and discuss how to interpret the data obtained with this method. The following Glossary defines several phase equilibria terms and abbreviations used in the sections that follow.

Abstract Stable isotope and ore deposit studies have a long common history because many of the early developments in the application of stable isotopes to geological problems were from investigations of ore forming processes. Stable isotopes have now become an integral part of studying ore deposits. They provide information in four critical areas: (1) temperature of mineral deposition, (2) sources of the hydrothermal fluids, (3) sources of sulfur and carbon (and by extrapolation, metals), and (4) water-wall rock interactions. One of the most important roles that hydrogen and oxygen isotope studies have played is in the modern recognition that shallow, surface derived fluids are important components in many ore deposits. As stable isotope labs have become automated and the cost per analysis dropped, stable isotopes are also being used more commonly in mineral exploration. For example, isotopes can be used to define alteration halos and to aid in discriminating between mineralized and unmineralized quartz veins. The purpose of this chapter is to provide a basis for understanding and utilizing light stable isotope data in the study of ore deposits. No previous knowledge of stable isotope geochemistry is assumed. However, one must recognize that stable isotopes can seldom provide unequivocal answers by themselves, and thus must be used in conjunction with other geological, mineralogical, petrological, and geochemical data. In other words, the knowledge in this chapter needs to be integrated with the types of studies described in the other chapters in this book in order to make sound interpretations of stable isotope data.

Abstract The potential use of radiogenic isotopes in the study of geological problems was recognized at an early stage in the investigation of nuclear science. At the turn of the century, F. Soddy and E. Rutherford first proposed the law of radioactive decay, and in 1905, Rutherford obtained the first age estimates of uraniferous minerals by measuring their helium content. The first U-Pb chemical dates for uraninites were published two years later byB.B. Boltwood (1907). F.W. Aston's development of the mass spectrometer shortly after the end of World War I led to the confirmation that many elements consist of isotopes having different atomic mass (Aston, But it was A.O. Nier's refinements of mass spectrometer design during and after World War II that provided the technological breakthrough required for routine geochronological measurements (Nier, 1940). Subsequent instrumental developments have principally involved improvements in precision and sensitivity, with the current generation of thermal ionization multi-collector mass spectrometers (TIMS) offering rapid simultaneous measurement of several isotopes from nanogram-sized samples.