Numerical modelling of fluid infiltration constrained by fault and bedding relationships in the Fosterville goldfield, Victoria, Australia

The Fosterville gold deposit hosted by a folded Ordovician turbidite sequence, in Victoria, Australia, contains major gold mineralisation. This is related to the reactivation of low-displacement faults and a network of hydrofractures adjacent to the faults. The location of the hydrofractures was controlled by the angular relationship between the fault and bedding within both the hangingwall and footwall of the fault. Numerical models are used to investigate how the orientations of faults and bedding influence dilation (positive volumetric strain) and fluid infiltration. The initial velocity boundary conditions applied to the models, as a compression direction and strike-slip to compression ratio, are constrained by the orientation of quartz-carbonate slickenline lineations associated with the reactivated mineralised faults.The models show that variations in the magnitudes of positive volumetric strain and fluid flow are associated with the orientations of bedding and faults. High magnitudes of positive volumetric strain and fluid flow occur at low (35-55°) and intermediate (90-110°) bedding-fault angles. The models also illustrate that fluid infiltration within meta-sandstones adjacent to a fault is affected by the dip angle of the fault with a 70° dipping fault promoting greater fluid infiltration than a 60° dipping fault. The magnitudes of positive volumetric strain and fluid flow increase as the magnitude of the applied strike-slip to compression ratio is increased. However, when a velocity field with a low strike-slip component is applied to the models higher fluid flows occur within steep-plunging (30°) folds, whereas a velocity field with a high strike-slip component results in higher fluid flows within gentle-plunging (15°) folds. These results suggest that the orientation of bedding and the plunge angle of folds are exerting a control on mineralisation. Such coupling of numerical simulations of deformation and fluid flow provides a useful framework that can be applied to better understand structural control on fluid infiltration in hydrothermal mineral systems.