The Influence of Geochemical Techniques on the Development of Genetic Models for Porphyry Copper Deposits
Jeffrey W. Hedenquist, Jeremy P. Richards, 1998. "The Influence of Geochemical Techniques on the Development of Genetic Models for Porphyry Copper Deposits", Techniques in Hydrothermal Ore Deposits Geology, Jeremy P. Richards, Peter B. Larson
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In the previous chapters, we have seen how a variety of theories and geochemical techniques can be applied in practice to real geological situations. In isolation, these techniques may provide important constraints on variables such as temperature, fluid composition, or age of ore deposition. But their real strength is realized when data from a number of different lines of investigation are combined and their interpretations are integrated, each providing independent constraints on a model. This integrated approach represents a huge advance on the isolated knowledge obtained from individual techniques, and provides information of fundamental and practical value.
We aim in this chapter to illustrate the value of such a multidisciplinary approach by using as an example the development of ideas concerning the genesis of porphyry Cu deposits, which have provided over 50 percent of the world's Cu production this century. The fundamental constraints on the genesis of porphyry deposits are based on geological observations. Nevertheless, geochemical studies have helped to refine our understanding of the hydrothermal processes that lead to the formation of these deposits.
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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.