Quartz vein fabrics coupled to elevated fluid pressures in the Stawell gold deposit, south-eastern Australia

Abstract

Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia, contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution (pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution) developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures.