Deformation, Fluid Flow, and Ore Genesis in Heterogeneous Rocks, with Examples and Numerical Models from the Mount Isa District, Australia
Nicholas H. S. Oliver, Alison Ord, Richard K. Valenta, Phaedra Upton, 2001. "Deformation, Fluid Flow, and Ore Genesis in Heterogeneous Rocks, with Examples and Numerical Models from the Mount Isa District, Australia", Structural Controls on Ore Genesis, Jeremy P. Richards, Richard M. Tosdal
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Consideration of the role of rock property variations is crucial in any analysis of the effects of deformation on fluid flow and mineralization. An empirical analysis of any mineralized terrain should consider this factor, in addition to those used in any other analysis of geometry and kinematics, such as orientation, evolution of the stress and strain fields, and the known distribution of veins, shear zones, breccias, and alteration. The conceptual models that arise from such an analysis can be enhanced by computer models. The models shown here are finite difference models that simulate fluid flow in deforming rock masses, one for fluid flow along predefined rock boundaries (Universal Distinct Element Code, UDEC), and another for fluid flow through deforming porous media (Fast Lagrangian Analysis of Continua, FLAC). UDEC modeling of the perturbed stress field around stronger, lower permeability meta-intrusive rocks in the Mary Kathleen district and the Hilton mine of the Mount Isa district, northwest Queensland, reproduces the observed location of the most intense veining and alteration. FLAC modeling of the Mary Kathleen U-REE orebody reproduces the location and geometry of ore shoots and provides an explanation for focusing of regional fluid towards the ore deposition sites. FLAC models of the giant Mount Isa copper deposit reveal that the effect of the rheological heterogeneity on fluid flow and solute transport is amplified if consideration is made of whether or not the rocks are contractant or dilatant, with increasing strain. Multiple working hypotheses can be evaluated quickly by such modeling; therefore, the models can be used in exploration and orebody extension studies. Furthermore, it is suggested from our work that the size and spacing of epigenetic, structurally controlled ore deposits is related in a fairly systematic way to the scale and degree of rock property variations, at least for a given strain history. For giant ore deposits to form, it requires that the gradients in pore pressure generated at local scales by heterogeneous rock packages must be subordinate to those operating at broader scales.
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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.