Experimental Constraints on the Transport and Deposition of Metals in Ore-Forming Hydrothermal Systems
A. E. Williams-Jones, A. A. Migdisov, 2014. "Experimental Constraints on the Transport and Deposition of Metals in Ore-Forming Hydrothermal Systems", Building Exploration Capability for the 21st Century, Karen D. Kelley, Howard C. Golden
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The capacity of hydrothermal fluids to transport metals in concentrations sufficient to form ore deposits is due in large part to the polar nature of the water molecule and the ability of metals to form strong aqueous complexes with a number of ligands commonly found in nature. In this paper, we review the properties of hydrothermal liquids and vapors, show how the hard/soft acid/base (HSAB) principle can be used to predict why certain metals form strong complexes with particular ligands, and review the experimental data on the aqueous speciation of a selection of base, precious, and critical metals in high- and low-density hydrothermal fluids. Based on these data, we identify the important complexes for each metal and determine the physicochemical conditions under which they may predominate and thereby control hydrothermal metal transport. This information is used to quantitatively determine the solubility of the main ore minerals in hydrothermal liquids and vapors, and evaluate the mechanisms of metallic mineral deposition (cooling, fluid mixing, boiling, and fluid-rock interaction) in selected ore-forming systems.
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Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).