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