100th Anniversary Special Paper: Numerical Models of Extensional Deformation, Heat Transfer, and Fluid Flow across Basement-Cover Interfaces during Basin-Related Mineralization

Abstract

Fluid circulation within low-permeability basement rocks has been proposed to occur beneath many sediment-hosted mineral deposits, in some cases contributing substantial metals or sulfur to the deposits in overlying cover sequences. However, mechanisms proposed for fluid transport and mass transfer within and through basement rocks are diverse, some models appealing to thermal circulation but others appealing more to deformation- or topography-driven flow. We address some of these issues here by a series of numerical models designed to compare and then couple thermally and mechanically driven fluid flow (and incorporate temperature-dependent fluid properties), starting with generic problems and then using a simulation of coupled deformation, heat transfer, and fluid flow that may be applicable to the formation of Mount Isa-style Pb-Zn ores and other extension-related basinal deposits. Results from deformation-only models show that downward penetration of near-surface fluids into relatively low permeability basement rocks may occur along fault zones at high strain rates during extension, because local deformation rates may exceed the capacity for fluid to move through the basement rocks due to their low permeability, leading to periods of underpressure. For our thermal fluid-flow models, in the absence of deformation and with elevated basal heat flows, large differences in basement and cover permeability tend to restrict thermal convection to the permeable units. Downflow into low-permeability basement may occur by a reduction of the permeability of cover sequences, because larger convection cells are possible as permeability approaches common, optimal values throughout the rock mass. The normal reduction in porosity and permeability of cover sequences with burial may thus lead to progressively deepening convection cells and an enhanced potential for extraction of components from basement rocks. Long-lived, stable convection is generated with ≤2 order of magnitude permeability difference between basement and cover. Such convection has the potential to lead to near-surface mineralization (e.g., sediment-hosted syngenetic or diagenetic deposits), particularly if an initial overpressure stimulates convection cells toward upflow along basin-bounding faults. These models also serve to indicate the inadequacy of models that do not incorporate thermal dependencies of fluid viscosity and density, because the upward fluid velocity generated by buoyancy is of the same order of magnitude as the downward fluid velocity generated by extension-related underpressure in models that do not incorporate these properties. In numerical models of coupled deformation, heat transfer and fluid flow in which high basal heat flow is coupled with extensional deformation, the effects of the deformation dominate flow regimes, rather than the thermal structure. A model with initial heating and fluid flow established large convection cells with basement fluid circulation, prior to deformation being incorporated. The convection cells are effectively destroyed by extension at geologically reasonable strain rates around 10–14s–1, with surface fluids driven downward and meeting remnants of the decaying convection deep in the system. This simulation provides a possible solution for mixing of near-surface and deep fluids in unconformity-related U deposits and Olympic Dam-style iron oxide Cu-Au deposits. Geological models for shale-hosted base metal deposits (e.g., Mount Isa Zn-Pb) appeal to transitions from active rifting to blanketing by mineralized sag-phase shales, requiring reduction or cessation of extension with time. We simulate this here by stopping the deformation component of the coupled model and allowing the heating and fluid-flow parts to continue. Initial or periodic fluid overpressures (140% of hydrostatic) applied at the base of our coupled numerical models during extension (rift phase) cause initial upflow along faults and sufficient heat advection to generate steep near surface thermal gradients. When deformation ceases, convection progressively deepens with time, but upflow continues along faults, producing perfect conditions for exhalation of fluids that have circulated through basement. From all of the coupled models, we infer that active extension or extensional reactivation of basin-bounding faults is generally destructive with respect to potential fluid upflow and generation of near-surface deposits. Exhalative or other near-surface ores are likely to form when extension ceases and the thermal structure becomes the driver of fluid flow.