The Physical Hydrology of Ore-Forming Magmatic-Hydrothermal Systems
Classifications of magmatic-hydrothermal ore deposits are largely geochemical, based on metal associations and characteristic alteration types, but the process of metal enrichment is primarily controlled by the physical hydrology of fluids flowing through rocks. Physical hydrology plays a decisive role in forming distinct ore deposit types, including volcanogenic massive sulfide deposits at mid-ocean ridges or submarine arc volcanos, porphyry-style ore deposits in continental collisional arcs, and epithermal vein and replacement deposits. Results from simulations of magmatic-hydrothermal systems using a new numerical modeling platform for thermohaline convection are used to determine the implications for ore formation in light of the different structural styles, timing, and igneous characteristics of major magma-related ore deposit types. Thermal convection, volatile expulsion, and salt water dynamics are shown to be the first-order hydrologic components, and different combinations or successions of general hydrologic patterns characterize particular oreforming systems.
Due to the nonlinear properties of fluids and rocks as a function of pressure, temperature, and composition, the physical behavior of hydrothermal systems can be counterintuitive, and understanding their self-organization requires numerically rigorous models. Thus, mid-ocean ridge hydrothermal systems do not involve broadscale lateral infiltration of seawater; instead, focused warm downflow in the immediate vicinity of hot upflow zones provides a more efficient mechanism for metal leaching and ore formation in Cyprus-type massive sulfide deposits. Phase separation in submarine magmatic-hydrothermal systems can lead to a decoupling of vapordominated venting, which is expected to favor sulfur complexing of some metals leading to the formation of Au-rich chimneys, whereas chloride-complexing metals may precipitate during the waning stages, favoring the formation of base metal-rich sulfide deposits from negatively buoyant brines. Porphyry copper mineralization is localized by a self-stabilizing hydrologic front, located at the transition from brittle to ductile rock behavior and controlled by the heat balance between an external convective cooling engine and an overpressured magmatic fluid plume. This hydrologic divide also provides a mechanism for the transition to epithermal-style deposits where magmatic and meteoric fluids mix on ascent to the surface.
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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).