Abstract

Subsurface analysis of the South Prairie fault, a Laramide basement backthrust located in the Stillwater Complex of the Beartooth Mountains, Montana, has allowed the heretofore unparalleled physical-chemical transect study of a deep subsurface Laramide fault. Fracturing exhibits a bimodal network interpreted as Riedel R and R′ fractures and, along with alteration, increases toward the heavily altered and cataclasized core zone. The width of the core zone is dependent on the host rock. Mineralogy varies from unaltered norite and gabbronorite in the host rock to characteristic alteration products of clinozoisite and serpentine within the damage zone, with minor tremolite. Veins are composed of Ca-stilbite and minor late-stage carbonate with talc. The core zone contains abundant serpentine/chlorite. Plagioclase is observed to withstand heavy stable fracturing and minor alteration to clinozoisite at grain boundaries before further alteration to clinozoisite. Orthopyroxene readily undergoes serpentinization at the same conditions. Evidence of pre-, syn-, and postkinematic fluids is abundant and consistent with the estimated South Prairie fault permeability. These observed characteristics suggest synkinematic conditions of temperature ≤300 °C and pressure <400 MPa, consistent with brittle to brittle-plastic deformation, which may have been ideal for chemical ore remobilization into the South Prairie fault via the fracture network and hydrothermal fluids.

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