GERAGHTY, ENNIS P.
Stillwater Mining Company, HC 54, Box 365, Nye, Montana 59061
Abstract: Drillhole Test of Beartooth Uplift Frontal Fault: Offset Estimates and Continuation at Depth of J-M Palladium-Platinum Zone, Stillwater Complex, Montana
Stillwater palladium-platinum mine is located on the west-northwest-trending J-M reef, a relatively continuous, 42 km long, thin (about 2.6 m), sulfide mineralized zone in the lower portion of the ultramafic to mafic Stillwater igneous-rock complex of Archean age. This complex is exposed in the upper plate along the northern front of the Beartooth uplift in south-central Montana. Originally gently north-dipping igneous rock layers and mineralized zones were tilted northward varying amounts during the Laramide orogeny of latest Cretaceous to Paleocene time.
A continuously cored, vertical drillhole collared in the Stillwater River canyon (collar elevation 1,510 m) penetrated 1,555 m of Stillwater complex. This hole was drilled to test the continuity at depth of the moderately to steeply north-dipping J-M palladium-platinum zone that is threatened by cutoff at depth by the south-dipping, major thrust-fault zone bounding the northern margin of the uplift. Lack of Phanerozoic sedimentary strata near the base of the drillhole indicates the bounding thrust zone was not completely penetrated. One major (32 m of gouge and breccia) fault was intersected at 1,265 m depth and is interpreted to be a splay of the major thrust. Assuming a moderate south dip based on angles to core axis, igneous-rock stratigraphic relations indicate 1,990 m of northward overthrust with northward rotation and tilt of about 40 degrees in the upper plate, 20 degrees in the lower plate. Accessing the J-M reef in the lower plate will involve southward drifting and down ramping in amounts to be determined by better definition of fault attitude.
AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana