Utility of Magnetic and Gravity Data in Evaluating Regional Controls on Mineralization: Examples from the Western United States
T. G. Hildenbrand, Byron Berger, R. C. Jachens, Steve Ludington, 2001. "Utility of Magnetic and Gravity Data in Evaluating Regional Controls on Mineralization: Examples from the Western United States", Structural Controls on Ore Genesis, Jeremy P. Richards, Richard M. Tosdal
Download citation file:
Interacting fractures enhance and localize permeability in the Earth's crust and are, therefore, important phenomena in localizing magmatic and hydrothermal systems. The ability to identify where such interactions are present is useful in evaluating likely areas of mineralized rock, particularly in covered terrains. Regardless of map scale, the interpretation of gravity and magnetic data can define deep-seated crustal fractures and faults that may have guided emplacement of igneous rocks and large ore deposits. Here we emphasize recurring regional-scale structural relationships mainly from the western United States based on the interpretation of potential-field data, which can elucidate areas of past and present fluid flow in the crust.
In particular, we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large Mesozoic and Cenozoic ore deposits (primarily epithermal and pluton-related precious and base metal deposits, and sediment-hosted gold deposits) in the western United States cordillera. On the broadest scale, most ore deposits lie within areas characterized by low magnetization. The Mesozoic Mother Lode gold belt displays characteristic geophysical signatures (regional gravity high, regional low-to-moderate background magnetic field anomaly, long curvilinear magnetic highs) that might serve as an exploration guide. Geophysical lineaments characterize the Idaho-Montana porphyry belt and the La Caridad-Mineral Park belt (from northern Mexico to western Arizona) and, thus, indicate deep-seated control for these mineral belts. At a more local scale, in Nevada, geophysical data define deep-rooted faults and magmatic zones that correspond to patterns of epithermal precious-metal deposits, and that may relate to the Carlin gold trend and the Battle Mountain-Eureka mineral belt. One recurring structural model evolving from this study is that mineralization in the western United States may be localized along strike-slip fault zones where pull-apart basins or releasing bends provided the increased fracture permeability for the migrating ore-forming fluids (e.g., the Butte, Tombstone, Bagdad, and Battle Mountain districts).
Many deposits discussed in the paper appear, at least in part, to be associated with reactivated older faults as well as with faulting contemporaneous with ore deposition. We conclude that at a local scale, structural elements work together to localize mineral deposits within regional zones or belts. Perhaps the greatest utility of regional geophysical data is the identification of structural relationships that help narrow the study area, where more intensive multidisciplinary team studies can be carried out in a concerted effort to evaluate the mineral potential.
Figures & Tables
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.