The Stillwater igneous complex is a large layered mafic intrusion (LMI) located in southern

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

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 Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

ore and has a history of being mined for chromium. More recent mining activity has produced

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

and other platinum group elements.The Stillwater Complex: A review of the geology by I. S. McCallum http://serc.carleton.edu/files/NAGTWorkshops/petrology03/McCallum.Stillwater.doc

Geology

The Stillwater complex is a large layered

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

with many similarities to the

Bushveld igneous complex The Bushveld Igneous Complex (BIC) is the largest Layered intrusion, layered igneous intrusion within the Earth's Crust (geology), crust. It has been tilted and Erosion, eroded forming the outcrops around what appears to be the edge of a great Ba ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

. The complex was intruded into existing gneisses during the

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

at about 2700 Ma. Host rocks on the northern side are Archean sedimentary rocks while the southern boundary includes Precambrian rocks of granite, granite gneiss and schist. The Precambrian rock on its southern side is mostly provided by the Beartooth Mountains. The region had a quartz monzonite intrusion and underwent extensive

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

, 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 The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

sequence.Jackson, Everett D., ''The Chromite Deposits of the Stillwater Complex, Montana'' in ''Ore Deposites of the United States, 1933-1967 (The Graton-Sales Volume)'' Vol. 2, pp. 1495-1509, 1968 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 estimated 5,000 to 15,000 feet (1524 to 4572 m) having been removed from the top by pre-Cambrian erosion.Stanton, R. L., 1972, ''Ore Petrology'', Ch 11 ''Ores of Mafic and Ultramafic Association'', pp. 305-351, McGraw Hill The cumulate stratigraphy comprises three distinct zones: *The Basal series consists of a chilled fine grained

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

overlain by gabbro, norite and feldspar pyroxenites. Thickness up to 700 feet (210 m). *The Ultramafic series is composed of a lower

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

member (Peridotite zone) consisting of alternating dunite, chromitite, harzburgite and bronzite pyroxenite. The upper third is massive, monotonous bronzitite (Bronzitite zone). The Ultramafic series averages around 3500 feet (1100 m) in thickness. *The Banded series is composed of alternating norite, gabbro and

anorthosite Anorthosite () is a phaneritic, intrusive rock, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic ...

. The Banded series has a maximum thickness of 14,000 feet (4300 m).

IUGS geological heritage site

In respect of it being 'one of the world's oldest layered intrusions, containing the world's highest-grade platinum-group element deposit', the

International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world. About Fo ...

(IUGS) included 'The Stillwater Complex' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'

Orebodies

The chromium orebodies are restricted to the Peridotite zone of the Ultramafic series. The platinum group orebody is located in the lower part of the Banded series within a horizon referred to as the J-M Reef. The J-M Reef is similar to the Merensky Reef of the

Bushveld complex The Bushveld Igneous Complex (BIC) is the largest Layered intrusion, layered igneous intrusion within the Earth's Crust (geology), crust. It has been tilted and Erosion, eroded forming the outcrops around what appears to be the edge of a great Ba ...

. 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 Pyrrhotite (''Pyrrhus of Epirus, pyrrhos'' in Greek language, Greek meaning "flame-coloured"'')'' is an iron sulfide mineral with the formula Fe(1−x)S (x = 0 to 0.125). It is a nonstoichiometric compound, nonstoichiometric variant of FeS, th ...

pentlandite Pentlandite is an iron–nickel sulfide with the chemical formula . Pentlandite has a narrow variation range in nickel to iron ratios (Ni:Fe), but it is usually described as 1:1. In some cases, this ratio is skewed by the presence of pyrrhotite ...

(containing up to 5% Pd), and

chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a Mohs scale, hardness of 3.5 to 4 ...

along with lesser amounts of moncheite (), cooperite (), braggite (), kotulskite () and platinum-iron alloys. Overall the reef contains an average of 20-25 ppm platinum plus palladium in an average two meter thickness. It has a Pd/Pt ratio of about 3.6.

Astronaut training

In July 1972, the area was used by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

to geologically train the Apollo Astronauts in recognizing a coarse grained igneous intrusion. Astronauts who would use this training on the Moon included

Apollo 16 Apollo 16 (April 1627, 1972) was the tenth human spaceflight, crewed mission in the United States Apollo program, Apollo space program, administered by NASA, and the fifth and penultimate to Moon landing, land on the Moon. It was the second o ...

's John Young and

Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...

's Gene Cernan and Jack Schmitt. Notable geologist instructors included William R. Muehlberger.

References

External links