Abstract The rare earth elements (REE) have potential as tracers in geothermal systems, but the scarcity of data on REE systematics in thermal waters, and on REE mineral solubility and complex stability at elevated temperatures and pressures, has impeded progress. In this paper, information relevant to the use of REE in geothermal systems is reviewed. The REE form their strongest complexes with ligands such as hydroxide, fluoride, carbonate, sulfate, and phosphate. Chloride complexes are very weak at standard conditions, but much experimental data at higher temperatures suggest that their stability increases considerably with increasing temperature. Although chloride complexation by itself is unlikely to result in inter-REE fractionation, experiments show that chloride brines exsolved from silicate melts are enriched in the light REE compared to the melt. Chloride complexes are likely to be the dominant REE species in sea-floor hydrothermal vent fluids and many chloride-rich continental thermal waters. Experimental data at elevated temperatures for chloride complexes, combined with those for acetate and very limited data for hydroxide complexes, suggest that published theoretical estimates of REE complex stability constants are in need of revision. No reliable data exist for fluoride, carbonate, sulfate, or phosphate complexes at elevated temperature. Only very limited experimental data exist for the solubility of REE minerals (e.g., monazite, allanite, xenotime, and bastnäsite), the partitioning of REE between aqueous fluids and minerals (e.g., apatite, fluorite, scheelite, zircon), the sorption of REE onto mineral surfaces, and the behavior of REE during water-rock interaction. The scarcity of these types of data is currently the biggest impediment to quantitative modeling of the behavior of the REE in geothermal systems. In the last couple of decades, there has been a large increase in the amount of information on REE systematics of geothermal waters, owing in large part to analytical advances. Considerable data on thermal waters from mid-ocean ridge sea-floor hydrothermal vents demonstrate that such fluids generally contain 10 to 10,000 times as much REE as seawater, and have light REE-enriched, chondrite-normalized patterns with strong positive Eu anomalies. However, such fluids are not net sources of REE to seawater owing to scavenging of REE by Fe oxyhydroxides formed on mixing of hydrothermal fluids with oxidized seawater. On the other hand, off-axis hydrothermal vent fluids may suffer removal of REE before emerging onto the sea floor. The REE geochemistry of continental geothermal waters shows a greater variety of patterns. However, pH is a major control on both absolute REE contents and chondrite-normalized patterns. Low-pH, acid-sulfate waters typically have the highest REE concentrations and most of the REE appear to be present in true solution. Many, but not all, such waters have a distinctive “gull's wing” chondrite-normalized pattern, in which La, Ce, and Pr are depleted with respect to host rocks. Near-neutral chloride- and bicarbonate-type waters have much lower overall REE concentrations, and filtered aliquots typically contain lower REE concentrations than unfiltered aliquots of the same sample, suggesting that the REE are present dominantly in or on suspended particles. Unfiltered and, to a lesser extent, filtered aliquots of these near-neutral waters commonly have chondrite-normalized patterns that closely parallel those of their reservoir rocks, indicative of minimal REE fractionation during water-rock interaction. However, some fluids exhibit strong positive Eu anomalies not necessarily seen in their reservoir rocks, possibly a result of water-rock interaction at higher temperatures (≥250°C) or under unusually reducing conditions. Finally, there is some evidence to suggest that REE are lost to solid phases upon vapor-liquid separation.

Abstract 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

Abstract 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.

Abstract Supergene enrichment and dispersion involve similar chemical and physical processes operating near the surface of the Earth. In both cases, elements are mobilized from their source, transported and fixed at some new site. In supergene enrichment, elements are concentrated by these processes. Supergene enrichment is of importance because it can upgrade otherwise uneconomic primary deposits to ore grade. During dispersion, elements are spread over a greater volume of space and diluted. In this chapter, use of the term dispersion is restricted to what Rose et al. (1979, p. 17) referred to as “secondary dispersion,” i.e., the redistribution of elements by processes occurring after the main ore-forming event, usually in the surficial environment. Dispersion often results in anomalous elemental concentrations in rocks, soils, lake and stream sediments, plants, or natural waters in the vicinity of ore deposits, and thus impacts on geochemical exploration by increasing the probability that a geochemical survey will uncover the anomaly. Giordano (2000) summarized the various roles of organic matter as transport agents in ore-forming and related systems. The inorganic geochemical processes which govern supergene enrichment and dispersion have been given considerable attention. Somewhat less attention has been paid to the role of organic matter. However, the potential roles for organic matter in supergene enrichment and dispersion are numerous and include (see also Schnitzer and Khan, 1972, 1978; Reuter and Perdue, 1977; and Wood, 1996): (1) increasing the solubility of minerals or decreasing the amount of sorption of ions onto mineral surfaces as a result of the formation of aqueous metal-organic complexes and/or increased acidity; (2) increasing metal mobility via coating and protection of colloids from coagulation; (3) metal fixation, either by reduction or through sorption onto solid organic material; (4) modification of the sorption-ion exchange properties of mineral surfaces; and (5) alteration of the rates of sorption, dissolution, and precipitation.