Hydromechanical development of fault-controlled orogenic gold deposits: A numerical approach to constrain the spatial distribution of mineral reserve.

Abstract

The genesis of hydrothermal ore deposits is enabled by fluid flow through the crust, which is a complexmulti-physics phenomenon comprised of mechanical, thermal and chemical rock-fluid interactions. Developmentof fracture networks at different scales, along with decompaction and evolution of connected interstitial voids,enhances the rock permeability through extensional deformation. This in turn, allows the hydrodynamicalproperties of syn-kinematic flow to evolve not only transiently, but also heterogeneously in space. Orogenicgold deposits, regardless of the gold origin, requires an efficient fluid flow and storage mechanism, such asfault systems, to provide necessary thermodynamic conditions for gold precipitation. If a fault system geometryis known up hand, it is in the general interest to determine quantitatively its hydromechanical control on thedistribution of precipitation zones within a deposit. Studying this relation would provide further guidelines for theexploration of gold deposits while constraining their economic feasibility before and during exploitation.In this work, we propose a novel finite element modelling framework to study the formation of orogenicgold deposits from an hydromechanical point of view, suitable to an arbitrary fault system geometry andtectonic boundary condition. We determine the spatial correlation between the rock deformation/stress and fluidvelocity/pressure fields with the distribution of gold grade. To model the hydromechanical coupling between fluidflow and host rock deformation, we represent the rock as a poro-elasto-plastic material, where flow is enhancedby both decompaction and permeability evolution through dilational deformation. Pre-existent faults are modeledas frictional contact surfaces, whose hydrodynamical behavior is incorporated with an auxiliary sub-model ofDarcy-Brinkman fluid flow. We address the uncertainty of tectonic boundary conditions by constraining the modelto field fault-slip analysis. Finally, we confront benchmark simplified scenarios (i.e. step-over, duplex and faultintersection) to two real ore deposits mapped in the northern and central Chilean Andes: a kilometric strike/slipfault-vein deposit with overlapped brecciated hydrothermal textures, where mineralization occurs mainly (andheterogeneously distributed) throughout the fault volume; and a duplex fault system, where mineralization existsin both main fault-veins and secondary vein arrays.Results from benchmark scenarios are consistent with previously reported studies. Furthermore, particulari-ties arise for each case-study: (1) For a single vein-fault deposit, geometrical irregularities allow the localizationof fluid overpressure zones. Consecutive faulting implies high fluid pressure drops (flash vaporization) within atrajectory sub-parallel toσ_2 projected onto the fault surface, which may be consistent with high ore grade bodiesand overlapped breccia textures. This is also observed in lithological contacts (intrusive/limestone) cross-cutted bythe fault (2) In a duplex fault system, similar conditions are found in rock volumes adjacent to, rather than within,fault intersections. These results highlight the opportunity of performing numerical modelling for an increasedconstrain of any mineral reserve. Further development of this approach would include (i) best fit techniquesfor in-situ data to improve the model accuracy and (ii) increase the model complexity by incorporating thethermo-chemical evolution of fluids throughout their pathway, while identifying other modes of gold precipitationsuch as boiling and cooling