Stillwater igneous complex



The Stillwater igneous complex is a large platinum group elements.[1]

Geology

The Stillwater complex is a large layered sedimentary rock sequence.[3] The intrusion forms a linear body stretching some 30 miles (48 km) and striking roughly N 60 °W and dipping from 50° to near 90° to the northeast. The exposed thickness is around 18,000 feet (5500 m) with an additional extimated 5000 to 15,000 feet having been removed from the top by pre-Cambrian erosion. [4]

The cumulate stratigraphy comprises three distinct zones:[4]

  • The basal zone consists of a chilled fine grained pyroxenites. Thickness up to 700 feet (210 m).
  • The ultramafic zone is composed of a lower pyroxenite. The upper third is massive bronzite pyroxenite. The ultramafic zone averages around 3500 feet (1100 m) in thichness.
  • The banded zone is composed of alternating norite, gabbro and anorthosite. The banded zone has a maximum thickness of 14,000 feet (4300 m).

Orebodies

The chromium orebodies are restricted to the peridotite member of the ultramafic zone. The platinum group orebodies are located in the lower part of the banded zone within a horizon referred to as the J-M reef.[1]

The J-M Reef is similar to the cooperite ((Pt,Pd,Ni)S), braggite ((Pt,Pd,Ni)S), kotulskite (Pd(Te,Bi)1-2) and platinum-iron alloys. Overall the reef contains an average of 20-25 ppm platinum plus palladium in an average two meter thicknesss. It has a Pd/Pt ratio of about 3.6.[1]

References

  1. ^ a b c The Stillwater Complex: A review of the geology by I. S. McCallum http://serc.carleton.edu/files/NAGTWorkshops/petrology03/McCallum.Stillwater.doc
  2. ^ American Mineralogist, Volume 75, pages 37-45, 1990 The Stillwater Complex and its anorthosites: An accident of magmatic underplating? GERALD K. CZAMANSKE, STEVEN R. BOHLEN http://www.minsocam.org/ammin/AM75/AM75_37.pdf
  3. ^ Jackson, Everett D., The Chromite Deposits of the Stillwater Complex, Montana in Ore Deposites of the Unitrd States, 1933-1967 (The Graton-Sales Volume) Vol. 2, pp. 1495-1509, 1968
  4. ^ a b Stanton, R. L., 1972, Ore Petrology, Ch 11 Ores of Mafic and Ultramafic Association, pp. 305-351, McGraw Hill
 
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