Magmatic Contributions to Hydrothermal Ore Deposits: An Algorithm (MVPart) for Calculating the Composition of the Magmatic Volatile Phase
P.A. Candela, P.M. Piccoli, 1998. "Magmatic Contributions to Hydrothermal Ore Deposits: An Algorithm (MVPart) for Calculating the Composition of the Magmatic Volatile Phase", Techniques in Hydrothermal Ore Deposits Geology, Jeremy P. Richards, Peter B. Larson
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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)
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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.