Stillwater igneous complex
The Stillwater igneous complex is a large layered intrusion located in southern Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

 in Stillwater
Stillwater County, Montana
-National protected areas:*Custer National Forest *Hailstone National Wildlife Refuge*Halfbreed Lake National Wildlife Refuge-Demographics:...

, Sweet Grass
Sweet Grass County, Montana
-National protected areas:*Custer National Forest *Gallatin National Forest *Lewis and Clark National Forest -Demographics:As of the census of 2000, there were 3,609 people, 1,476 households, and 987 families residing in the county. The population density was 2 people per square mile...

 and Park Counties
Park County, Montana
Park Counties in Montana and Wyoming are among the very few pairs of counties in the United States with the same name to border each other across state lines.-National protected areas:* Custer National Forest * Gallatin National Forest...

. The complex is exposed across 30 miles (48 km) of the north flank of the Beartooth Mountain Range
Beartooth Mountains
The Beartooth Mountains are located in south central Montana and northwest Wyoming, U.S. and are part of the 900,000 acre Absaroka-Beartooth Wilderness, within Custer, Gallatin and Shoshone National Forests. The Beartooths are the location of Granite Peak, which at 12,807 feet is the highest...

. The complex has extensive reserves of chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

 and has a history of chromium production. More recent mining activity has produced palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

 and other platinum group elements.


The Stillwater complex is a large layered intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

 with many similarities to the Bushveld igneous complex
Bushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...

 of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

. The complex was intruded into existing gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

es during the Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

 at about 2700 Mya. The region was subsequently intruded by a quartz monzonite
Quartz monzonite
Quartz monzonite is an intrusive igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende...

 stocks and underwent extensive metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

, faulting
Geologic fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces...

 and folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 during the Archean at about 2500 Mya. The area was intruded by north trending mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

 before being unconformably
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...

 covered by a middle-Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 sequence. The intrusion forms a linear body stretching some 30 miles (48 km) and striking
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...

 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 5000 to 15,000 feet having been removed from the top by pre-Cambrian erosion.

The cumulate stratigraphy comprises three distinct zones:
  • The basal zone consists of a chilled fine grained gabbro
    Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

     overlain by gabbro, norite
    Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...

     and feldspar pyroxenite
    Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...

    s. Thickness up to 700 feet (210 m).
  • The ultramafic zone is composed of a lower peridotite
    A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

     member consisting of alternating dunite
    Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...

    , chromitite
    Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa and the Stillwater igneous complex in Montana.-References:...

    , harzburgite
    The ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium pyroxene ; it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral...

     and bronzite
    Bronzite is a member of the orthopyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group...

    Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...

    . 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
    Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...

    . The banded zone has a maximum thickness of 14,000 feet (4300 m).


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.

The J-M Reef is similar to the Merensky Reef
Merensky Reef
The Merensky Reef, is a layer of igneous rock in the Bushveld Igneous Complex in the Transvaal which together with an underlying layer, the Upper Group 2 Reef , contains most of the world's known reserves of platinum group metals or platinum group elements - platinum, palladium, rhodium,...

 of the Bushveld complex of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

. It is a continuous layer near the base of the banded zone. It consists of one to three meter thick pegmatitic peridotite and troctolite
Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a...

 with disseminated sulfide minerals. Common sulfides include pyrrhotite
Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...

, pentlandite
Pentlandite is an iron-nickel sulfide, 9S8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt.Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness of 3.5 - 4 and specific gravity of 4.6 - 5.0 and...

 (containing up to 5% Pd), and chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...

 along with lesser amounts of moncheite ((Pt,Pd)(Te,Bi)2), cooperite
Cooperite is a grey mineral consisting of platinum sulfide , generally in combinations with sulfides of other elements such as palladium and nickel . Its general formula is S. It is a dimorph of braggite....

 ((Pt,Pd,Ni)S), braggite
Braggite is a sulfide mineral of platinum, palladium and nickel with chemical formula: S. It is a dense , steel grey, opaque mineral which crystallizes in the tetragonal crystal system...

 ((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.
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