SUBSURFACE STRUCTURAL AND MINERALOGIC CHARACTERIZATION OF THE ESSENTIALLY UNALTERED LARAMIDE SOUTH PRAIRIE FAULT IN THE STILLWATER COMPLEX, BEARTOOTH MOUNTAINS, MONTANA

The South Prairie fault (SPF) is a Laramide basement fault located in the Archean ultramafic-mafic Stillwater Complex of the Beartooth Mountains, MT. Laramide movement for the fault is suggested by geometric and kinematic analyses, as well as the sub-greenschist alteration mineralogy within the fault zone. Rarely visible in outcrop, the SPF is exposed in the subsurface at several depths via active mining by the Stillwater Mining Company down to 2400 feet below the Stillwater River. Thus, the SPF has offered 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. This study has also offered the chance to characterize brittle deformation characteristics of mafic/ultramafic rocks. Both of these aspects have received less attention in previous Laramide fault studies, which have focused more on the geometric-kinematic history or fault characteristics in quartzo-feldspathic basement rocks.

X-ray diffraction, scanning electron microscopy, thin section analyses, and subsurface observations at three different levels within the mine have been conducted. Fracturing and alteration increase towards the heavily comminuted core zone. Width of the core zone appears dependent on host rock. Mineralogy varies from unaltered noritic and gabbronoritic host rock to mainly tremolite, clinozoisite, serpentine and chlorite alteration within the damage zone. Post-kinematic veins are composed of stilbite with minor carbonate and talc. The core zone is abundantly serpentine-chlorite. Plagioclase is observed to endure heavy stable fracturing and minor alteration to clinozoisite at grain boundaries before complete alteration by Al³⁺ remobilization and albitization. Orthopyroxene succumbs to serpentinization sooner while Clinopyroxene resists alteration and deforms by cleavage plane fracture. These characteristics support syn-kinematic sub-greenschist conditions of T<300ºC and P<400 MPa and are consistent with brittle to brittle-plastic deformation. Evidence of hydrothermal fluids is shown by the mineral assemblage talc-tremolite-chlorite-serpentine and likely has implications for Pt/Pd remobilization from the JM Reef ore body to within the core zone.