Numerical simulation of fluid pressure build-up below the seismic lid: Implications to ‘fault-valve’ mechanism for lode-type gold deposits

Orogenic gold systems are flow-controlled thermodynamic systems and typically occur in mid- to upper crustal environments where there is a strong coupling of deformation and fluid flow with attendant heat transfer and chemical reactions. Fluids are generated during metamorphic devolatilization reactions under greenschist to amphibolite/granulite facies conditions and accumulated below 15 km depth from the earth's surface due to the presence of an impermeable layer (‘seismic lid’) that prevents the upward flow of fluid. Here the fluid pressure regime ranges from hydrostatic above the seismic lid to suprahydrostatic value below the seismic lid. The formation of orogenic gold deposits is associated with fluid pressure variation and rupture of the fault. The ‘fault valve’ mechanism that operates during periodic seismic pumping is widely believed to be responsible for gold mineralization in such systems. A 2D model is generated with the help of COMSOL Multiphysics software to describe the fluctuations of fluid pressure based on fault-valve vis-à-vis seismic pumping mechanism taking various assumptions, standard physical and lithological parameters, and governing equations related to Darcy’s law, storage coefficient equation. The rectangular cross-section covers a region of 50 km long and 25 km deep. The 200 m width seismic lid is located at 15 km depth and a fault of infinite length along a strike of 300m width cutting through the seismic lid. Specific sense of movement on the fault and the inferences of tectonic movements are not considered for this study. The intact rocks have low porosity and low permeability and a fixed heat flux is assigned for the bottom boundary and other boundaries are thermally insulated. Based on results obtained from the numerical simulation, the followings can be concluded. (1) The fluctuation of the fluid pressure shows a larger variation below the seismic lid and the zone where the fault penetrates the seismic lid (Fig 1). (2) A high-angle fault seems favourable for fluid flow and may not give rise buildup of supralithostatic fluid pressure that is essential for fault-valve process to operate. On the other hand, orogenic gold deposits are hosted in high angle reverse faults/ shear zones. Therefore the operation of Sibson’s cycle for the origin of lode-type gold deposit is need to be more critically evaluated.