STRUCTURAL AND MINERALOGIC CHARACTERIZATION OF THE SOUTH PRAIRIE FAULT IN THE STILLWATER COMPLEX, BEARTOOTH MOUNTAINS, MT

The South Prairie fault (SPF) is a Laramide basement fault located in the Archean ultramafic-mafic layered Stillwater Complex of the Beartooth Mountains. Laramide movement for the fault is suggested by the sub-greenschist alteration mineralogy 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 on 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. X-ray diffraction work on samples across the SPF has shown an abundance of prehnite, pumpellyite, clinochlore-chlorite and serpentine. Minor amounts of what is likely laumontite are observed in the damage zone. Gouge mineralogy within the 2.5 m thick core of the SPF is dominated by serpentine and chlorite and an absence of laumontite. Illite and smectite, common in previous fault studies, are absent and may have implications for fault zone processes. A thin section transect across the SPF suggests that pyroxenes succumb to alteration before heavily fractured plagioclase at these conditions, and may be the means by which deformation progresses. Alteration within the fault zone progresses from the unaltered footwall to the pervasively altered core zone, becoming less altered towards the hanging wall. These minerals observed in XRD and thin section suggest syn-kinematic conditions of T < 300 ºC and P < 400 MPa and are consistent with grades ranging from zeolite facies in the outer damage zone to uppermost prehnite-pumpellyite facies within the core zone.