Ingebrisen and Appold reported in early 2012 that hydrothermal ore deposits represent a convergence of fluid flow, thermal energy, and solute flux that is hydrogeologically unusual, [but not uncommon]. From the hydrogeologic perspective, hydrothermal ore deposition represents a complex coupled-flow problem—sufficiently complex that physically rigorous description of the coupled thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes (THMC modeling) continues to challenge our computational ability. Though research into these coupled behaviors has found only a limited subset to be quantitatively tractable, it has yielded valuable insights into the workings of hydrothermal systems in a wide range of geologic environments including sedimentary, metamorphic, and magmatic.
Examples of these insights include the quantification of likely driving mechanisms, rates and paths of fluid flow, ore-mineral precipitation mechanisms, longevity of hydrothermal systems, mechanisms by which hydrothermal fluids acquire their temperature and composition, and the controlling influence of permeability and other rock properties on hydrothermal fluid behavior.
The authors reviewed some of the fundamental theory needed to characterize the physical hydrogeology of hydrothermal systems and discuss how this theory has been applied in studies of Mississippi Valley-type, tabular uranium, porphyry, epithermal, and mid-ocean ridge ore-forming systems. A key limitation in the computational state-of-the-art is the inability to describe fluid flow and transport fully in the many ore systems that show evidence of repeated shear or tensional failure with associated dynamic variations in permeability. However, the authors discussed global-scale compilations that suggest some numerical constraints on both mean and dynamically enhanced crustal permeability. Principles of physical hydrogeology can be powerful tools for investigating hydrothermal ore formation and are becoming increasingly accessible with ongoing advances in modeling software.
The authors concluded that hydrothermal ore deposits are the products of complex couplings between fluid flow, heat transport, solute transport, chemical reaction, and mechanical deformation. Quantitativemodels have provided valuable insights into the relevant hydrothermal processes, one of the most important of which is the fundamental role of permeability. High permeability promotes high fluid-flow rates that lead to the large heat and solute fluxes conducive to formation of ore deposits. Where permeability is low, heat and solute transport occur mainly by the relatively slow processes of conduction and diffusion. Low permeabilities, however, help make possible elevated fluid pressures due to processes such as sediment compaction, tectonic compression, fluid generation, or heating.
In low-permeability rocks, fluid pressures can eventually become high enough to create new fractures, induce slip on existing faults, or trigger porosity waves to produce temporary high-permeability conduits that episodically transmit fluid, heat, and solutes. Both distribution and magnitude of permeability are important. Ore zones typically require (episodically) high permeability surrounded or capped by low-permeability zones that maintain and focus the system. Low-permeability surroundings can limit the overall permeability, allowing high temperatures to occur relatively near the surface, and stabilize the ore-forming environment, so that ore-forming processes such as boiling or mixing do not migrate widely — thus, the depth of boiling, for instance, will not change drastically as the hydrothermal system waxes and wanes. Lowpermeability barriers can also allow hydrothermal systems to overpressure, fail, and depressurize, leading to boiling, and then reseal.
Transport and deformation theory have made important contributions to the understanding of ore deposits in sedimentary settings. Among the most studied are MVT deposits in the United States, where much of the focus has been on evaluating driving mechanisms for the flow of mineralizing fluids. Compaction due to sedimentation or tectonic compression has been widely discounted as an important driving mechanism for MVT ore-forming fluids, as the resulting rates of heat advection are too low to raise temperatures above conductive levels, insufficient for MVT ore formation at the expected formation depths of 1 to 1.5 km. Topographic gradients created during the orogenesis with which MVT deposits are commonly associated produce a much stronger driving force
for fluid flow and, provided that permeabilities in the recharge area and ore zones were sufficiently high, topography-driven flow could have elevated ore zone temperatures to the required levels.
More complex models of MVT formation have yet to be tested by quantitative coupled-process modeling. Although topography-driven fluid flow occurs in any uplifted, subaerial terrane, it may be temporarily overwhelmed, significantly supplemented, or overprinted by other flow-driving mechanisms. Examples include the episodic flow caused by seismicity or hydrocarbon generation and the convective flow caused by a magmatic heat source. The flow regimes around magma bodies create many important classes of ore deposits, including epithermal, porphyry, and volcanogenic massive sulfide deposits.
Applications of transport and deformation theory have helped to clarify the geometries of these flow regimes, the roles of magmatic and meteoric water, the locations within the flow regime where, based on maximal fluid fluxes and temperature gradients, mineralization should be concentrated, and the pressure evolution of the flow regime, which has implications for hydraulic fracturing and the formation of veins that can host mineralization. Studies of mid-ocean ridge magmatic-hydrothermal systems have shown that the transport properties of water itself may cause hydrothermal systems to self-organize into pipelike, buoyancy-driven upflow zones that are surrounded by narrow zones of focused, hot recharge.
The permeability of many ore zones is fracture dominated. Data from various modern hydrogeologic studies (e.g., Barton et al., 1995; Townend and Zoback, 2000) demonstrate that most hydraulically conductive fractures are critically stressed under the existing state of stress, whereas most fractures that are not critically stressed are nonconductive. Thus, fracture orientation relative to the (paleo) stress field may ultimately be among the most important determinants of fluid flow, transport, and ore-hosting potential.
Although much of the governing hydrogeologic theory is long established, rigorous quantification of complex processcouplings in models that incorporate site-specific geologic detail continues to be computationally challenging and represents a major research frontier. The issue of inelastic deformation and associated dynamic variations in permeability, for which fully satisfactory theory has yet to be developed, is a particular challenge. Fortunately, this problem is now under attack from several directions, as it is a core issue for emerging technologies such as Enhanced Geothermal Systems (EGS) and geologic carbon sequestration, as well as economic geology.
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