The Stillwater Complex, Southern Montana; A Layered Mafic Intrusion

Click for route map

Route for condensed road log of Benbow area

Start point

Dean, MT

End point

near Benbow Mine

Roads

Benbow Road (U.S. Forest Service Road 1414), road to Benbow Mine

Access

The road to the Benbow chromite mine is open to the public, but the mine is on private property; roads are accessible to 2WD vehicles

Total mileage

12.3 miles

Route for condensed road log of Mountain View area

Start point

Stillwater Mine, near Nye, MT

End point

near Minneapolis adit

Roads

West Fork Stillwater River Road, other roads in Mountain View area

Access

Permission must be obtained from the Stillwater Mining Company; roads are accessible by 2WD vehicles

Total mileage

6.3 miles

Geology

Summary

Layered complexes provide the most visible evidence of processes of magmatic differentiation; thus, they occupy a central place in the study of igneous petrology. Such complexes are not only scientifically rewarding, they are host to several types of mineral deposits, including copper-nickel, chromium, and platinum-group elements. The Stillwater Complex is one of the world's great layered mafic intrusions, distinguished not so much by its size as by the fact that it is tilted on its side, and erosion has exposed the layering to ready access. This fieldguide presents a summary of the geology of the complex in the Benbow and Mountain View areas. The Benbow area offers easy access to a variety of rocks from the Wltramafic series and chromite deposits but only limited exposures of features from the Banded series. The Mountain View area offers easy access to most of the banded series and the platinum deposits.

Key Lithologic Features

  • lower Mesozoic and Paleozoic sedimentary rocks
  • layered igneous rocks of the Stillwater Complex
    • Banded Series: norite, gabbro, anorthosite (including classic outcrops showing fine-scale, rhythmic layering)
    • Ultramafic Series: bronzitite, harzburgite, chromitite

Structures

  • unconformity between the Stillwater Complex and overlying Cambrian sedimentary rocks
  • layering and other structures resembling sedimentary structures in igneous rocks

Landforms

Other Features

  • Benbow chromite mine
  • J-M Reef platinum deposit

Reference

Atkinson, W.W., Jr., and Derkey, R.E., 1987, The Stillwater Complex, southern Montana; a layered mafic intrusion, in Beus, S.S., ed., Centennial Field Guide Volume 2: Rocky Mountain Section of the Geological Society of America , p. 69-74.

Availability

Out of print. Check Amazon.com. Digital access to this publication is available via the links in the references section above.