The role of fluids in syntectonic mass transport, and the localization of metamorphic vein-type ore deposists

Abstract

In low- to medium-grade metamorphic environments, the mobilization of species is closely associated with the operation of solution-precipitation creep processes during foliation development. Such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields. Mass transfer occurs via solute diffusion in an essentially static fluid network, or by transport of dissolved species in a migrating fluid. Precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces, grain-scale microcracks, and veins. Sink sites may be close to, or distantly removed from source sites. The transport of material during deformation involving solution-precipitation creep is largely controlled by the mobility of metamorphic fluids. In regions of high fluid mobility, significant mass transfer may occur over large distances. Fluid flow, mass-transfer paths and the development of sink sites, in such cases, are constrained by the structurally controlled development of high fluid-pressure domains and the consequent mechanical enhancement of permeability by hydraulic-fracture processes and the development of grain-scale dilatancy. These processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions. A proportion of syntectonically mobilized material is precipitated to form vein-type deposits in hydraulic-fracture arrays which develop in high fluid-pressure environments. The microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization. The formation of syntectonic gold-quartz vein deposits in central Victoria (Australia) illustrates the importance of structural controls on both the large-scale migration of metamorphic fluids, and the development of sites suitable for the precipitation of metamorphically mobilized species.