Changes in fluid migration in ductile shear zones during tectonic switching may explain the formation of hydrothermal ore deposits

A common feature of many hydrothermal ore deposits is that they formed during tectonic switches between extension and shortening on plate boundaries. Several theories to explain this relationship have been proposed but evidence for a mechanism remains elusive. Many of these ore deposits occur within or adjacent to ductile shear zones that changed movement direction during the tectonic switch. Prior to tectonic switches, shear zone structures evolve to orientations optimised to accommodate deformation, which maximises strain rate and creates permeable pathways for fluid migration. But when a tectonic switch occurs the structures are misoriented and must reconfigure to accommodate the new shearing direction. Using numerical models of shear zone evolution, we determined that during tectonic switches the microstructural reconfiguration reduces the strain rate and mean stress, causing fluid influx into the shear zone. To further explore the effect of this microstructural reconfiguration on fluid migration we examined rocks of the Bergen Arc shear zone in Norway in a transition zone where sinistral shearing is progressively overprinted by dextral shearing. We find that during the tectonic switch, accretionary veins of quartz, ankerite and calcite formed in dilatational spaces that opened as the sinistral structures were reconfigured to accommodate dextral shearing. With increasing strain, fluid migration into the shear zone became more pervasive, evidenced by larger vein networks and hydrothermal breccias. Coincident with vein formation there is a statistically significant increase in the water content in quartz as determined by synchrotron FTIR. These data indicate that the microstructural reconfiguration in shear zones during tectonic switching causes fluid influx into shear zones. This process may be responsible for the introduction of ore fluids into the shear zone and the formation of hydrothermal ore deposits during tectonic switching.