THE IMPACT OF PRECIPITATION-STRENGTHENING ON FAULT ZONE EVOLUTION IN MINERALIZING EPITHERMAL ENVIRONMENTS

Faults range in architectural style from discrete planes to regions of distributed deformation across wider and more complex core and damage zones. Various mechanisms are invoked to explain increasing fault zone width and complexity, from growth and linkage of antecedent structures to the amount of accumulated slip. Water-rock interaction is typically attributed with weakening and shear localization. However, we hypothesize that precipitation, healing, and strength recovery can contribute to the development of thick fault cores in mineralizing systems. We present results from field mapping and petrographic, mineralogical, and mechanical characterization to assess the impact of alteration and cementation on brittle deformation at the Dixie Comstock epithermal gold deposit, Dixie Valley, Nevada, USA. We observe 1) strong, thick silicified fault cores and wide, weak damage zones, 2) evidence for widening of the silicified core through embrittlement and dilation as well as entrainment of damage zone material, 3) repeated fracturing and sealing recorded by multiple breccia and cement types and textures, and 4) breakup of the fault core as deformation outpaced cementation. Based on these observations, we present a conceptual model of fault zone evolution that includes paths through alteration-weakening and precipitation-strengthening regimes, with notable differences in fault zone architecture and hydromechanical properties: alteration-weakening favors localization of the fault zone into thinner, clay-rich, low permeability fault cores, whereas precipitation-strengthening promotes thick, strong yet brittle, low permeability fault cores and enhances transient permeability following co-seismic failure and dilation.