Effects of normal faulting on fluid flow in an ore-producing hydrothermal system, Comstock Lode, Nevada

Abstract

Normal faults in the Comstock Lode mining district, NV, USA, are delineated by low 18O belts and strong isotopic gradients perpendicular to strike. These faults perturb and also offset the regional pattern of δ18O contours produced by pervasive hydrothermal alteration of the Alta formation, and have preserved the down-dropped hanging wall rocks while exposing the footwall rocks to greater erosion. Fluid–rock interaction was greatly enhanced within >200-m-wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane. The effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the Comstock and Occidental faults and across steeper secondary normal faults such as the Coryell fault that may have formed after the principal faults were rotated to more shallow dips.Oxygen isotope profiles record both post-hydrothermal displacement and syn-deformational enhancement of fluid flow around the faults. Fluid focusing along normal faults perturbs the δ18O values by increasing the water–rock ratio. This isotopic effect can be mathematically described for any distance x from the fault using the formula:Δf=A·e−bx2which is analogous to the fundamental solution for diffusion away from a plane source. The maximum magnitude of 18O depletion, A, depends on the effects of permeability, temperature, mineralogy, fluid isotopic composition and integrated fluid flux. The constant b is related to the scale of exchange enhancement around the fault. The characteristic width (w=1/√b) of the fluid focusing effects is remarkably similar at 100 m for all three faults examined. Coupling this isotope exchange effect with a step-function, which accounts for fault displacement, provides a good fit to the observed 18O fault profiles. The 18O distribution around faults can be used to infer relationships and timing among the processes of structural deformation, hydrothermal convection, and ore deposition in the Comstock district.