Geology of the Stillwater Complex Exposed in the Mountain View Area and on the West Side of Stillwater Canyon
Start pointCarter's Camp at the intersection of Montana Highways 419 and 420.
End pointMount Belmont, MT
Roads (and Trails)Montana 419, West Fork Stillwater River Road (Forest Service Road 846), Mouat Mine Road, trails along the Mountain View cliffs
Total distance78.6 miles
SummaryThe part of the Stillwater Complex exposed on the west side of the Stillwater canyon has been the site of concentrated prospecting and geology activity since Jack Nye and Jimmy and Jonas Hedges found sulfide-rich rocks there in 1883. Readily seen in this area are well-studied and excellent exposures of: (1) most rock types, (2) ore deposits and mineralized occurrences, (3) structural features and (4) stratigraphic and age relations between the metamorphosed sedimentary rocks and the layered ultramafic-mafic stratiform intrusion. In addition, there are good exposures of a quartz monzonitic suite, which intrudes the complex and the metamorphosed sedimentary rocks, and of the Beartooth gneisses.
Because the rocks in this area have been deformed by several episodes of faulting, a synopsis of the structural relations is given to clarify the geologic setting. Detailed overviews of the geology specific to the area are presented, and the geology of the more interesting exposures in the area is described in a road log that begins in the Ultramafic series, proceeds through the Basal series, the Banded series, and the quartz monzonite that cuts the complex, and ends in the Beartooth gneisses.
Key Lithologic Features
- cyclic units and chromitite seams in the Peridotite zone of the Ultramafic Series
- Ultramafic Series/Banded Series contact
- mafic sills and dikes (diabases and norites) in the Stillwater Complex (and associated Ni-sulfide mineralization)
- Late Archean granitoids
- metasedimentary rocks (diamictite)
- layered mafic rocks (troctolite, anorthosites, and gabbros) of the Banded Series, including outcrops showing inch-scale rhythmic layering
- J-M Reef (PGE deposit)
- Hamslice Fault
- syndepositional structures in layered igneous rocks
- Mouat Mine (chromite in the Ultramafic Series)
- Monte Alto tunnel