Structural Controls on Ore Genesis

Fluid pathways between metal sources and sites of ore deposition in hydrothermal systems are governed by fluid pressure gradients, buoyancy effects, and the permeability distribution. Structural controls on ore formation in many epigenetic systems derive largely from the role that deformation processes and fluid pressures play in generating and maintaining permeability within active faults, shear zones, associated fracture networks, and various other structures at all crustal levels.
In hydrothermal systems with low intergranular porosity, pore connectivity is low, and fluid flow is typically controlled by fracture permeability. Deformation-induced fractures develop on scales from microns to greater than hundreds of meters. Because mineral sealing of fractures can be rapid relative to the lifetimes of hydrothermal systems, sustained fluid flow occurs only in active structures where permeability is repeatedly renewed.
In the brittle upper crust, deformation-induced permeability is associated with macroscopic fracture arrays and damage products produced in episodically slipping (seismogenic) and aseismically creeping faults, growing folds, and related structures. In the more ductile mid- to lower crust, permeability enhancement is associated with grain-scale dilatancy (especially in active shear zones), as well as with macroscopic hydraulic fracture arrays. Below the seismic–aseismic transition, steady state creep leads to steady state permeability and continuous fluid flow in actively deforming structures. In contrast, in the seismogenic regime, large cyclic changes in permeability lead to episodic fluid flow in faults and associated fractures.
The geometry and distribution of fracture permeability is controlled fundamentally by stress and fluid pressure states, but may also be influenced by preexisting mechanical anisotropies in the rock mass. Fracture growth is favored in high pore fluid factor regimes, which develop especially where fluids discharge from faults or shear zones beneath low-permeability flow barriers. Flow localization within faults and shear zones occurs in areas of highest fracture aperture and fracture density, such as damage zones associated with fault jogs, bends, and splays. Positive feedback between deformation, fluid flow, and fluid pressure promotes fluid-driven growth of hydraulically linked networks of faults, fractures, and shear zones.
Evolution of fluid pathways on scales linking fluid reservoirs and ore deposits is influenced by the relative proportions of backbone, dangling, and isolated structures in the network. Modeling of the growth of networks indicates that fracture systems reach the percolation threshold at low bulk strains. Just above the percolation threshold, flow is concentrated along a small proportion of the total fracture population, and favors localized ore deposition. At higher strains, flow is distributed more widely throughout the fracture population and, accordingly, the potential for localized, high-grade ore deposition may be reduced.
Seismogenic Framework for Hydrothermal Transport and Ore Deposition
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Published:January 01, 2001
Abstract
Fault motion in the upper continental crust is accommodated principally by earthquake rupturing within a seismogenic zone whose base, depending on composition, generally lies in the 300° to 450°C temperature range. Rupture initiation, propagation, and termination within this zone are affected by structural and rheological irregularities. Sawtooth accumulation and release of shear stress on the seis-mogenic structures leads to cycling of both shear and mean stress (affecting fluid content) throughout the surrounding rock mass, with significant fluid redistribution throughout the aftershock phase following large earthquakes. Structural permeability in such regions is intrinsically dynamic: episodic creation of permeability accompanying seismic slip and fracturing is counteracted by the development of low-permeability fault gouge and hydrothermal cementation, so that flow systems are modulated by intercoupled stress and permeability cycling. Because criteria for all modes of brittle failure and fault reshear depend on fluid pressure as well as tectonic stress, a variety of mechanisms may link fluid redistribution to episodic faulting and fracturing. Stress changes accompanying large-scale rupturing on established faults redistribute fluids through subsidiary fracture networks during aftershock periods, but packages of over-pressured fluid migrating through stressed crust may also create new structural permeability by distributed brittle failure, generating earthquake swarms.
The fluid pressure state at different crustal levels is critical to the formation and preservation of void space. Fluid overpressuring above hydrostatic values is generally easier to sustain in compressional tectonic regimes, but maximum sustainable overpressure in any particular setting depends not only on the intrinsic permeability of the rock mass but also on the tectonic stress state and existing fault architecture. Large-scale hydrothermal flow through low-permeability rocks is often channeled within dilatant mesh structures of interlinked shear and extension fractures. These fault-fracture meshes can form and reactivate only under low effective stress (o3’ <0, or Pf > o3) in the absence of throughgoing low-cohesion faults that are well oriented for frictional reactivation. High-flux flow of this kind can, therefore, occur only under special structural circumstances. in extensional-transtensional tectonic regimes, dilatant meshes can be maintained under hydrostatic fluid pressures in the shallow crust to depths dependent on rock tensile strength, defining the epizonal environment for mineralization. However, at all depths within compressional-transpressional regimes, development of fault-fracture meshes involves hydrothermal fluids overpressured to near-lithostatic values. in particular, mesozonal lode mineralization requires the accumulation and intermittent high-flux discharge of strongly overpressured fluids in the midcrust, most commonly around the base of the continental seismogenic zone.
important precipitation mechanisms linked to intermittent seismic slip include the suction-pump mechanism arising from rapid slip transfer across dilational fault jogs and bends, and various forms of fault-valve action where ruptures transect boundaries to overpressured portions of the crust. These mechanisms induce abrupt localized reduction in fluid pressure at specific structural sites, triggering phase separation and hydrothermal precipitation throughout the postseismic period of readjustment (aftershock phase). However, renewal of fault-fracture permeability may also lead to episodic mixing of fluids derived from different sources. For example, each fault-valve discharge may promote precipitation through the mixing of originally deep, hot, overpressured fluids of metamorphic and/or magmatic origin with colder fluids circulating in the near-surface hydrostatic regime.
Regional episodes of fluid redistribution are likely to accompany major tectonic transitions (e.g., tectonic inversion) because of changes in the stress state and sustainable levels of fluid overpressure, and an inherited architecture of faults poorly oriented for slip in the new stress field. Evidence of structural channeling in such settings reveals interesting comparisons between the flow paths of hydrothermal and hydrocarbon fluids.