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.
Figures & Tables
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.