Precipitation-Strengthening and Fault Zone Evolution, Dixie Comstock Epithermal Deposit, Dixie Valley, Nevada, USA

Fault-fracture networks vary from single planes to wide core and damage zones. Various mechanisms are invoked to explain increased fault zone width, 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 hydrothermal alteration resulting in mineral precipitation and strengthening can contribute to the development of thick fault cores. We conducted field mapping and petrographic, mineralogical, and mechanical characterization to assess the impact of alteration and cementation on brittle deformation in chloritized footwall gabbro and silicified fault breccia at the Dixie Comstock epithermal gold deposit, Dixie Valley, Nevada, USA. We observed 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 conceptional model of fault zone evolution in crystalline rock that includes alteration-weakening and precipitation-strengthening paths with observable differences in fault zone architecture and hydromechanical properties. Alteration-weakening favors localization of the fault zone into thinner, clay-rich, low permeability fault cores. Precipitation-strengthening promotes thick, strong yet brittle, low permeability fault cores, and enhances transient permeability following co-seismic failure and dilation.