Abstract The timing and duration of volatile generation from crystallizing magma reservoirs and fluid release across the magmatic‐hydrothermal interface depend on complex coupled interactions controlled by non‐linear, dynamic properties of magmas, rocks and fluids. Understanding these mechanisms is essential to explain the rare formation of economic porphyry copper deposits. For this study, we further developed a coupled numerical model that can simultaneously resolve magma and hydrothermal flow by introducing a description of fluid transport within the magma reservoir and volatile release to the host rock. Our simulations use realistic magma properties derived from published experimental and modeling studies and cover different magma compositions and water contents. We show that magma convection at melt‐dominated states leads to homogenization, which delays fluid release and promotes a rapid evolution toward a mush state. The onset of magmatic volatile release can be near‐explosive with a tube‐flow outburst event lasting <100 years for high initial water contents of >3.5 wt% H 2 O that could result in the formation of hydrothermal breccias and vein stockworks or trigger eruptions. This event can be followed by sustained fluid release at moderate rates by volatile flushing caused by magma convection. Subsequent fluid release from concentric tube rings by radial cooling of non‐convecting magma mush with a volume of ∼100 km 3 at ∼5 km depth is limited to remaining water contents of ∼3.1 wt% H 2 O and lasts 50–100 kyr. Ore formation from hydrous magmas may thus involve distinct phases of volatile release.
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).
Abstract Numerical simulation of subaerial, magma‐driven, saline hydrothermal systems reveals that fluid phase separation near the intrusion is a first‐order control on the dynamics and efficiency of heat and mass transfer. Above shallow intrusions emplaced at <2.5 km depth, phase separation through boiling of saline liquid leads to accumulation of low‐mobility hypersaline brines and halite precipitation, thereby reducing the efficiency of heat and mass transfer. Above deeper intrusions (>4 km), where fluid pressure is >30 MPa, phase separation occurs by condensation of hypersaline brine from a saline intermediate‐density fluid. The fraction of brine remains small, and advective, vapor‐dominated mass and heat fluxes are maximized. We thus hypothesize that, in contrast to pure water systems, for which shallow intrusions make better targets for supercritical resource exploitation, the optimal targets in saline systems are located above deeper intrusions.
Traditionally, global ocean tide models are forced by individual partial tides resulting from a decomposition of the complete lunisolar tidal potential in Fourier components. Implicitly, this approach neglects nonlinear interactions between partial tides. This can be partly compensated by a superposition of a selection of partial tides. In order to ensure the full dynamics, the Tidal Model forced by Ephemerides (TiME) incorporates the tidal potential of second degree calculated online from analytical ephemerides and utilizes the classical shallow‐water equations with a horizontal resolution of 5 min globally. Interactions between partial tides generate shallow‐water tides which are shown to form in extended shelf areas where they develop the highest amplitudes. However, as they propagate into the open ocean, they should be regarded as a global phenomenon. Simulations with TiME confirm that M 4 is particularly pronounced in the Atlantic and suggest further areas of strong energy fluxes in the southern Pacific. MN 4 is strongest in the Atlantic and MS 4 , 2 SM 2 , and MK 3 mainly spread out into the Indian Ocean.
Abstract The clastic‐dominant (CD‐type) deposits that are contained within sedimentary basins are major resources of Zn, Pb and Ag, but their formation by basin‐scale hydrothermal mass and energy transport processes is still poorly understood. Using geological constraints from the Late Devonian Selwyn Basin (Canada), we apply quantitative numerical fluid flow modeling to explore the effect of strata permeability, timing of fault opening and increased heat flow in controlling fluid migration, metal leaching and ore formation during an extensional tectonic event. The results indicate that tapping hot fluids from a confined and permeable aquifer at several km depths by means of permeable normal faults is a key factor for the formation of large Zn‐Pb deposits. The hot (282°C) ore‐forming fluids are transported to the shallow subsurface shortly after the initiation of a rifting event (within 100 kyr), before the development of extensive basin‐scale convection patterns that lead to stronger cooling and a reduction in the capacity of the hydrothermal system to make an economic deposit. Such a hydrothermal event can result in metal endowments comparable to the deposits of the Selwyn Basin.
Abstract. To meet the growing global demand for metal resources, new ore deposit discoveries are required. However, finding new high-grade deposits, particularly those not exposed at the Earth's surface, is very challenging. Therefore, understanding the geodynamic controls on the mineralizing processes can help identify new areas for exploration. Here we focus on clastic-dominated Zn–Pb deposits, the largest global resource of zinc and lead, which formed in sedimentary basins of extensional systems. Using numerical modelling of lithospheric extension coupled with surface erosion and sedimentation, we determine the geodynamic conditions required to generate the rare spatiotemporal window where potential metal source rocks, transport pathways, and host sequences are present. We show that the largest potential metal endowment can be expected in narrow asymmetric rifts, where the mineralization window spans about 1–3 Myr in the upper ∼ 4 km of the sedimentary infill close to shore. The narrow asymmetric rift type is characterized by rift migration, a process that successively generates hyper-extended crust through sequential faulting, resulting in one wide and one narrow conjugate margin. Rift migration also leads to (1) a sufficient life span of the migration-side border fault to accommodate a thick submarine package of sediments, including coarse (permeable) continental sediments that can act as source rock; (2) rising asthenosphere beneath the thinned lithosphere and crust, resulting in elevated temperatures in these overlying sediments that are favourable for leaching metals from the source rock; (3) the deposition of organic-rich sediments that form the host rock at shallower burial depths and lower temperatures; and (4) the generation of smaller faults that cut the major basin created by the border fault and provide additional pathways for focused fluid flow from source to host rock. Wide rifts with rift migration can have similarly favourable configurations, but these occur less frequently and less potential source rock is produced, thereby limiting potential metal endowment. In simulations of narrow symmetric rifts, the conditions to form ore deposits are rarely fulfilled. Based on these insights, exploration programmes should prioritize the narrow margins formed in asymmetric rift systems, in particular regions within several tens of kilometres from the paleo-shoreline, where we predict the highest-value deposits to have formed.
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