Structural and mineralogic characterization of the South Prairie Fault in the Stillwater Complex, Beartooth Mountains, Montana

Abstract

The South Prairie fault (SPF) is a Laramide basement-involved fault located in the Archean ultramafic-mafic layered Stillwater Complex of the Beartooth Mountains, southern Montana. Laramide movement on the fault is suggested by the sub-greenschist alteration within the fault zone, as well as its similar strike geometry to the Laramide Horseman and Beartooth thrusts. The SPF is rarely visible in outcrop, but is exposed in the subsurface at several levels via active Pt/Pd mining by the Stillwater Mining Company to depths of 2400 feet below the Stillwater River. Thus, the SPF offers a unique and rare opportunity to study an in-situ basement fault never exposed to surface alteration processes, thereby giving a "pristine" look at the micro- and mesoscopic characteristics of a Laramide fault zone from which interpretations of Laramide fault behavior may be made. This study also offers the chance to characterize brittle deformation characteristics of mafic/ultramafic rocks. Both of these aspects have received little attention in previous Laramide fault studies, which have focused more on geometric and kinematic history or faulting in quartzo-feldspathic basement rocks. To characterize the SPF, X-ray diffraction, thin-section analyses and subsurface observations have been conducted. Fracturing and alteration increase towards the heavily altered and cataclasized core zone. Width of the core zone is dependent on host rock. Mineralogy varies from essentially unaltered noritic and gabbronoritic compositions in the host rock to mainly tremolite, clinozoisite and serpentine/chlorite within the damage zone. Veins are composed of stilbite and minor carbonate with talc. The core zone is abundant in serpentine-chlorite, as evidenced by XRD. Plagioclase is observed to withstand heavy stable fracturing and minor alteration to clinozoisite at grain boundaries before complete alteration. Orthopyroxene succumbs to serpentinization much sooner at the same conditions. Evidence of pre-, syn- and post kinematic fluids are abundant, and are consistent with the estimated architecture of the SPF. These characteristics observed suggest syn-kinematic sub-greenschist conditions of T<300 °C and P<400 MPa, consistent with brittle to brittle-plastic deformation.