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.