Fault-related dilation, permeability enhancement, fluid flow and mineral precipitation patterns: numerical models

Abstract

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.

A new 3D geological model and interpretation of structural evolution of the world-class Rio Tinto VMS deposit, Iberian Pyrite Belt (Spain)

“…In addition, as has been shown (i.e. Connolly and Cosgrove, 1999;Drew 2006, Zhang et al, 2008, in a transtensional pull-apart the most favorable areas for increased fluid flow and highest concentration of mineralization are located near the boundaries, such as the areas delimited by red ellipses in Fig. 16.…”

The Paleoproterozoic metaconglomerate-hosted Castelo de Sonhos gold deposit, Tapajós Gold Province, Amazonian Craton: a modified paleoplacer origin

“…Parallel to the diagenesis stages, deformation plays important role favoring fluid movement and inducing local and regional gradients in the hydraulic potential. In the process, the following effects are common: (1) increase in the fluid pressure related to compression, which usually occurs in depth, in association with ductile deformation and metamorphism, leading to mineral dehydration reactions; and (2) decrease in the fluid pressure in shallower depths, induced by space opening by fracturing (Sibson 1994;Oliver 1996;Cox 2005;Zhang et al 2008). The resulting gradient in the hydraulic potential provides the necessary energy for fluid ascent in the crust, and this fluid may be channelized to dilation zones related to shallow brittle deformation, according to the pump-suction model of Sibson (1994).…”

An overview of hydrodynamic studies of mineralization

“…Although much has been focused on the effect of fracturing on enhancing permeability and fluid flow (e.g., Zhang and Cox, 2000;Cox, 2005;Zhang et al, 2008), deformation is also a major driving force of fluid flow as it induces gradients of hydraulic potential both locally and regionally. Two types of effect of deformation on hydraulic potential may be envisaged: one is development of fluid overpressure due to compression, which is often accompanied with mineral dewatering in metamorphic conditions, and the other is development of fluid underpressure (pressures < hydrostatic values) due to creation of open space during fracturing.…”