THE RELATIONS BETWEEN MEAN ROCK STRESS AND FLUID-FLOW IN THE CRUST - WITH REFERENCE TO VEIN-STYLE AND LODE-STYLE GOLD DEPOSITS

Fluid movement in the crust at the typical depths of formation of vein- and lode-style gold deposits will be dominated by upward flow. Focussing of upward flow into discrete channelways, as required to form gold deposits, is due to lateral gradients in fluid pressure. Fluid channelways should have lower fluid pressures than surrounding rock, if lateral gradients are induced by variations in permeability. This is, however, at variance with inferences from quartz-vein structures of high relative fluid pressures during the formation of gold deposits. Focussing into zones of low mean rock stress will; in contrast, be associated with high relative fluid pressures in the zone of focussed fluid flow. Variations in mean rock stress are a direct consequence of a regional deviatoric stress acting on an inhomogeneous rock sequence. Analysis of stress fields shows a wide variety of potential sites of low mean stress, dependent on the geometry of rock units, and on patterns of faults, fractures or shear zones. A model of fluid focussing in the crust due to variations in mean stress is thus consistent with the large variety of structural setting of vein- and lode-gold deposits observed in nature. Use of the model, through the technology of 'stress-mapping', has the potential to generate viable exploration targets.