Fluid flow in extensional environments; numerical modelling with an application to Hamersley iron ores

Abstract

The mechanical feasibility of focusing both surface- and basinal-derived fluids towards sites of iron ore genesis during Proterozoic deformation in the Hamersley Province is tested here by computer simulation. Finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain. The addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow. Upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra-hydrostatic pore pressures. Downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes. The introduction of sedimentary layering into the models allows lateral fluid flow, such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone. Allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis. The involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the Mt Tom Price deposit (and possibly other large deposits) with time.

Introduction

Surface fluids have been implicated in subsurface ore forming processes in several geologic environments (e.g. Epithermal: Sillitoe, 1993; Mississippi Valley Type (MVT): Garven, 1985). Surface fluids moving downwards during sedimentation may become overpressured or underpressured during the burial–compaction–inversion cycle of sedimentary basins and compacting basins can generate abnormal fluid pressures or overpressurised zones (Bredehoeft and Hanshaw, 1968, Kissen, 1978, Bethke, 1985, Neuzil, 1995). Strongly inverted basins with elevated mountain ranges may also be dominated by topographically driven fluid flow (Fig. 1). Regional groundwater flow studies with applications to hydrologic, petroleum and mineral systems have been examined previously (e.g. Tóth, 1962, Tóth, 1963, Freeze and Witherspoon, 1966, Garven and Freeze, 1984a, Garven and Freeze, 1984b, Oliver, 1986, Upton et al., 1995), and these studies recognise the varying effects of topography and deformation in controlling hydraulic head gradients and hence the migration of fluids within the crust. Compaction driven or overpressurised upward flow is a common feature in the generation of hydrocarbon deposits (e.g. Upton et al., 1998) and within sedimentary basins (Bethke, 1985), and has also been related to fault activity (Sibson, 1987, Sibson et al., 1988). Lateral fluid flow is generally influenced by topography and the permeability of geological units, where topographic influences on the hydraulic head have been shown to drive fluid in a downward and lateral direction. For example, Tóth, 1962, Tóth, 1963, Garven and Freeze, 1984a, Nesbitt and Muehlenbachs, 1989 have interpreted that deep gravity-driven flow has significant implications for large-crustal hydrodynamic processes. For contractional deformation, Oliver (1986) has described the ‘squeegee’ effect of thrusts enabling considerable lateral fluid flow and the potential for mixing of surficial and deep-seated fluids during this process. Downward migration of fluids has been attributed to underpressure, or as a result of extensional deformation, where vertical interconnectivity of fractures within rocks allows deep penetration of surface fluids (Nesbitt and Muehlenbachs, 1989). Deformation induced fluid flow has been discussed by several authors (Etheridge et al., 1983, Etheridge et al., 1984, Oliver, 1996, Oliver et al., 2001; Ord and Oliver, 1997, Cox, 1999) and this process undoubtedly has great influence over directions and rates of flow, particularly at depth.

Sedimentary basins are subjected to several forces that are known to cause large-scale fluid migration (Garven and Raffensperger, 1997), which include gravity, thermal and chemical buoyancy, compaction, loading, unloading, dilation, and overpressurisation. Large-scale flow has been linked to many types of ore deposits e.g. Mississippi Valley Type (MVT) deposits, and fluid modelling has shown that flow can be as great as 1 to 10 m yr⁻¹ when associated with topographic driven flow in a foreland basin (Garven and Raffensperger, 1997). Within these low temperature environments the process of fluid mixing has been proposed for many different types of ore deposits (e.g. Kendrick et al., 2002a, Kendrick et al., 2002b, Sharpe and Gemmell, 2002), and in some circumstances evidence of downward meteoric water incursion has been linked to later oxidising stages of ore formation. However, the nature of fluid flow that may allow localisation of ore deposits is primarily controlled by the permeability and porosity of the rocks. Dissolution and cementation are two processes that can alter the permeability of any given rock, fracture or vein. The process of dissolution allows increased fluid flux and hence increasing fluid pathways and the potential to carry dissolved metals, which are of great concern in forming ore deposits, excellent examples being carbonate-hosted MVT deposits.

The basic theory of fluid flow has been realised since the time of Hubbert (1940), although the patterns of flow within porous media can change dramatically as a result of hydraulic head, pressure, structure, heat and mineralogical variations. Numerical models of fluid flow at depth have been utilised for many geological scenarios (e.g. Upton et al., 1995, Upton et al., 1998, Ord and Oliver, 1997, Koons et al., 1998, Upton, 1998, Oliver et al., 1999, Oliver et al., 2001, Gow et al., 2002, Ord et al., 2002, Schaubs and Zhao, 2002), and have become an important tool for simulating the response of geological materials and fluid flow to deformation. In this paper we examine the effects of topography, structure, extension and permeability creation on fluid flow within a collapsing mountain range. These modelling scenarios are applied to the structural setting and ore genesis of the Hamersley Province, Western Australia, but in a generic form could be applied to many other extensional environments worldwide, particularly in the 2 to 8 km-depth range of deep sedimentary basins and the shallow parts of orogenic belts.