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Fault-related host rock deformation and dilation control fluid flow and mineralization in many world-class mineral deposits. This numerical modelling study explores the interactions between deformation, faulting, dilation, fluid flow and chemical processes, which are suggested to result in this control, with special attention to fault dilatant jog structures. Our two-dimensional numerical models focus on faulting-related deformation, dilation and permeability enhancement, fluid flow patterns and fluid focusing/mixing locations, while three-dimensional models examine several different cases of fault underlap and overlap. The results show that fault-dilation and faulting-induced permeability enhancement, which are closely associated with tensile failure, represent important ways to generate fluid flow conduits for more effective fluid flow and mixing. Dilation during strike–slip faulting is localized near fault tips (wing crack locations) and jog sites, where fluids are strongly focused and mixed. These locations are the tensile domains of the strike–slip regime. In overlapping-fault (dilatant jog) cases, the magnitude of dilation and the extent of the dilatant region are closely related to the extent of fault overlap. These results provide insight into the transport of fluids through low-permeability rocks with isolated, but more permeable, faults. Gold and quartz precipitation patterns as a result of the coupling of chemical reactions to deformation induced fluid flow velocities are also computed. The rates of precipitation depend on structural and fluid flow conditions and on the geometrical relation between local fluid velocity and chemical concentration gradients generated by mixing. Maximum precipitation rates for gold occur in the dilation zones and in faults where high fluid flow rates, sufficient fluid mixing and high concentration gradients of critical chemical species are all present, while the quartz precipitation rate is predominantly controlled, in this isothermal situation, by the rate of fluid flow across concentration gradients in the aqueous silica concentration.

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The McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody. Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid. We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.

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This paper presents the results of a set of numerical models focussing on structural controls on hydrothermal mineralization. We first give an overview of natural phenomena of structurally-controlled ore formation and the background theory and mechanisms for such controls. We then provide the results of a group of simple 2D numerical models validated through comparison with Cu-vein structure observed near the Shilu Copper deposit (Yangchun, Guangdong Province, China) and finally a case study of 3D numerical modelling applied to the Hodgkinson Province in North Queensland (Australia). Two modelling approaches, discrete deformation modelling and continuum coupled deformation and fluid flow modelling, are involved. The 2D model-derived patterns are remarkably consistent with the Cu-vein structure from the Shilu Copper deposit, and show that both modelling approaches can realistically simulate the mechanical behaviours of shear and dilatant fractures. The continuum coupled deformation and fluid flow model indicates that pattern of the Cuveins near the Shilu deposit is the result of shear strain localization, development of dilation and fluid focussing into the dilatant fracture segments. The 3D case-study models (with deformation and fluid flow coupling) on the Hodgkinson Province generated a number of potential gold mineralization targets. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

3D numerical models of coupled deformation and fluid flow provide a useful tool for exploration in orogenic-gold systems. Numerical modelling of ore-forming processes can lead to a reduction in targeting and detection risk, thus improving the value proposition of mineral exploration. Hydrothermal mineralisation arises from a complex interplay of deformation, fluid flow, conductive and advective heat transport, solute transport and chemical reactions. Coupled simulation of all of these processes represents a significant computational challenge that cannot be solved within the time-scale of a mineral exploration program. However, the problem can be simplified by identifying a subset of processes representing the first-order controls on mineralisation at the scale of interest. For most orogenic-gold systems, it is argued that the first-order controls on mineralisation at the camp to deposit scale are deformation-induced dilation, fluid flow and fluid focusing. Hence, numerical models of coupled deformation and fluid flow can provide a quantitative insight into the localisation of oreforming fluids in this type of system. In two case studies, known deposits were modelled in order to determine the critical deformation and fluid-flow-related factors controlling the localisation of mineralisation in these systems. The quantitative results from the forward models were then used as a basis for constructing predictive models that were applied to regional targeting, prospect ranking and selecting the choice of detection methods. Both case studies show that numerical modelling is capable of reproducing the distribution of known anomalism, and that it can predict anomalies that were not expected or accounted for by purely empirical analysis.