Introduction to Stable Isotope Applications in Hydrothermal Systems
Andrew R. Campbell, Peter B. Larson, 1998. "Introduction to Stable Isotope Applications in Hydrothermal Systems", Techniques in Hydrothermal Ore Deposits Geology, Jeremy P. Richards, Peter B. Larson
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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.
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.