Stillwater igneous complex

The Stillwater igneous complex is a large ultramafic to mafic layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles (48 km) of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of chromium production. More recent mining activity has produced palladium and other platinum group elements.^[1]

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Geology

The Stillwater complex is a large layered intrusion with many similarities to the Bushveld igneous complex of South Africa. The complex was intruded into existing gneisses during the Archean at about 2700 Mya.^[2] The region was subsequently intruded by a quartz monzonite stocks and underwent extensive metamorphism, faulting and folding during the Archean at about 2500 Mya. The area was intruded by north trending mafic dikes before being unconformably covered by a middle-Cambrian 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 gabbro overlain by gabbro, norite and feldspar pyroxenites. Thickness up to 700 feet (210 m).
• The ultramafic zone is composed of a lower peridotite member consisting of alternating dunite, chromitite, harzburgite and bronzite 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 Merensky Reef of the Bushveld complex of South Africa. It is a continuous layer near the base of the banded zone. It consists of one to three meter thick pegmatitic peridotite and troctolite with disseminated sulfide minerals. Common sulfides include pyrrhotite, pentlandite (containing up to 5% Pd), and chalcopyrite along with lessor amounts of moncheite ((Pt,Pd)(Te,Bi)₂), 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