Flow mechanisms in sulphide minerals

Abstract

The sulphide minerals as a group display a diverse range in strength and mechanical behaviour in crustal environments, deforming by a range of different flow mechanisms which can lead to the development of characteristic microfabrics. The dependence of creep rate on imposed parameters, such as stress difference, temperature, grainsize, effective confining pressure, and the activities of various components, varies for each type of flow mechanism. For particular phases, changes in the dominant operative flow mechanism may occur in response to changes in the imposed environmental variables. At high applied stress differences, low effective confining pressures, and low temperatures, brittle failure and cataclastic flow are significant deformation mechanisms in some sulphide minerals. At elevated confining pressures, brittle failure can be suppressed and low-temperature plasticity processes involving dislocation glide and/or deformation twinning may operate. With decreasing applied stress difference, and at elevated temperatures, even though strain may be accommodated largely by dislocation glide, deformation may be rate-controlled by recovery processes, and dynamic recrystallization may be significant in influencing microfabric development. The operation of dislocation glide processes leads to the development of crystallographic preferred orientations. At very low applied stress difference, various grainsize-dependent atomic-transfer deformation mechanisms may be important. In many fluid-present, low- to medium-grade metamorphic environments, solution-precipitation creep processes are active in the deformation of some sulphide phases; in fluid-absent situations, grain-boundary diffusion creep may be significant. At higher temperatures, where intragranular diffusion is relatively rapid, lattice-diffusion creep processes are expected to be a major deformation mechanism. In fine-grained sulphide assemblages at elevated temperatures, grain-boundary sliding may be an important deformation mechanism.