Solubility of Ore Minerals and Complexation of Ore Metals in Hydrothermal Solutions
Scott A. Wood, Iain M. Samson, 1998. "Solubility of Ore Minerals and Complexation of Ore Metals in Hydrothermal Solutions", Techniques in Hydrothermal Ore Deposits Geology, Jeremy P. Richards, Peter B. Larson
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Knowledge of the solubility of ore minerals and the speciation of ore metals in hydrothermal solutions is required for a complete understanding of the genesis of hydrothermal ores. In this chapter, we explore the factors that control solubility and speciation, demonstrate how to carry out quantitative calculations, and review the current state of knowledge for a number of economically important metals. The term solubility refers to the sum of the concentrations of all dissolved forms of a given metal in a hydrothermal solution in equilibrium with a mineral (or minerals) containing that metal. We use the term speciation to denote the relative concentrations of the various forms of a metal in solution.
The solubility of a mineral provides an upper limit to the amount of dissolved metal that a hydrothermal fluid can transport, assuming thermodynamic equilibrium. Although a given solution may temporarily carry more metal than permitted by the equilibrium solubility of relevant minerals owing to sluggish reaction kinetics, the equilibrium solubility is nevertheless an important benchmark. Given enough time, equilibrium solubility cannot be exceeded, and systems will proceed in a direction toward the equilibrium state. Also, knowledge of equilibrium solubilities is required for modeling rate processes. Metal concentrations may be maintained below the equilibrium solubility either by sorption processes, which remove metals from solution before saturation is reached with respect to a given mineral, or if there is insufficient metal available in the system to saturate the solution. pointed out in Chapter 1, the extent to which a solution
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Techniques in Hydrothermal Ore Deposits Geology
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.