John G. McLellan

Numerical models of extensional deformation, heat transfer, and fluid flows across basement-cover interfaces during basin-related mineralization

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.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

A numerical modelling approach to fluid flow in extensionalenvironments: implications for genesis of large microplaty hematite ores

Abstract Numerical modelling of deformation and fluid flow on a regional scale was utilised to test recent models for deep (≥ 5 km)penetration of surface fluids involved in genesis of Whaleback style microplaty hematite ores in the Pilbara, Western Australia. Current models suggest they formed in the waning stages of the ca. 2300 Ma Ophthalmian Orogeny. This study uses a finite difference continuum modelling code, Fast Lagrangian Analysis of Continua (FLAG), to test whether deep penetration of surface fluids is mechanically feasible in relation to ore formation. Several “conceptual models” were tested and model boundary conditions were conducive to an extensional collapse of the mountain range. During extensional deformation, downward fluid flow is evident along a subvertical fault for reasonable strain rates and topographic elevation. Throughout deformation, fluids move progressively deeper into the model. Deeper seated fluid is forced upwards from the base of the model due to perturbations in hydraulic head and pore pressure. Both fluids mix at intermediate levels and this mixing process becomes progressively deeper within the model as extension takes place. Fluid mixing is apparent at the banded iron formation (BIF) and fault boundaries, as is lateral fluid migration along the BIF layers. This study supports the hypothesis that downward flow of meteoric fluid may have played a role in the evolution of the microplaty hematite ores. However, unlike the model of Morris [Morris. R.C., 1985. Genesis of iron ore in banded iron formation by supergene and supergene-metamorphic processes—a conceptual model. In: Wolff, K.H. (Ed.), Handbook of Strata-Bound and Stratiform Ore Deposits, vol. 13, Elsevier, Amsterdam, pp. 73–235] in which downward penetration is essentially superficial, the association of fluid flow with extension may have allowed deep fluid penetration (≥ 5 km) and potential fluid mixing, as proposed by Powell et al. [Geology 27 (1999) 175] and Taylor et al. [Econ. Geol. 96 (2001) 837].

Delineating the structural controls on the genesis of iron oxide–Cu–Au deposits through implicit modelling: a case study from the E1 Group, Cloncurry District, Australia

Abstract Iron oxide–Cu–Au (IOCG) deposits encompass a range of ore body shapes, including strata-bound replacement ores and hydrothermal breccias. We use the implicit method to make a detailed three-dimensional geological model of a strata-bound IOCG in the Cloncurry District, the E1 Group, to elucidate structural controls on mineralization. This model is compared with the nearby, world-class, Ernest Henry breccia-hosted IOCG deposit. Cu–Au mineralization in the E1 Group occurs as structurally controlled, mainly strata-bound, replacement bodies hosted in metasedimentary and metavolcaniclastic rocks intercalated with barren meta-andesite. Replacement bodies in the E1 Group conform to a series of NNW-plunging folds formed in regional D2 during peak metamorphism. Folding was followed by local D3/regional D4 shortening, which formed a dextral, transpressional Riedel brittle to ductile system along the regional Cloncurry Fault Zone. Modelling suggests that much of the Cu–Au mineralization is controlled by synthetic R structures associated with this Riedel system. The deformation sequence at Ernest Henry is comparable, but differences in host rock rheology, permeability and fluid pressure may explain the variation in ore body types and total Cu–Au resource between the two deposits. The results carry implications for other districts containing these styles of IOCG mineralization.

New faults v. fault reactivation: implications for fault cohesion, fluid flow and copper mineralization, Mount Gordon Fault Zone, Mount Isa District, Australia

Abstract Fluid flow leading to mineralization can occur both on newly formed faults and on faults that are reactivated subsequent to their initial formation. Conventional models of fault reactivation propose that, under high pore-fluid pressures, misorientated faults may reactivate due to low fault cohesion. Timing and orientation data for a mineralized Palaeo- to Mesoproterozoic terrain (Mount Gordon Fault Zone (MGFZ)) indicate that multiple successive new orientations of predominantly strike-slip faults developed (between 1590 and c. 1500 Ma), requiring that during the younger deformations some earlier formed faults were too cohesive and/or had insufficient pore-fluid pressures (or other potential fault-weakening effects) to induce reshear. Low pore-fluid pressures were probably not to blame for failed reactivation on all older faults because some young faults did form or reactivate due to high pore-fluid pressures, as evidenced by jigsaw-fit dilatant breccias, hypogene copper mineralization in veins and breccia infill, and subvertical tensile quartz veins aligned subparallel to σ1. The assumption that old faults consistently have little or no cohesion appears to be incorrect in this terrain. Many older faults display prominent quartz veins along their length, which may explain this conclusion. Furthermore, faults with high cohesion may have acted as barriers and compartments, so that intersections between them and newly formed faults host mineralization, not because of reactivation, but because of interaction between new faults and cohesive materials defined either by fault precipitates or rock juxtaposition. Together, these results and observations provide new, simple tools to stimulate copper exploration within the region and in fault-hosted terrains.