earth science

Stillwater igneous complex

Stillwater igneous complex is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stillwater igneous complex rather than just read about it. In short: The Stillwater igneous complex is a large layered mafic intrusion (LMI) 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.

Key takeaways

Stillwater igneous complex belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Stillwater igneous complex to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Stillwater igneous complex from memory before moving on to harder problems.

Reference excerpt

The Stillwater igneous complex is a large layered mafic intrusion (LMI) 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 being mined for chromium. More recent mining activity has produced palladium and other platinum group elements.

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 2.7 Ga. 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, faulting and folding during the Archean at about 2.5 Ga. The area was intruded by north trending mafic dikes before being unconformably covered by a middle-Cambrian sedimentary rock sequence. 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. The cumulate stratigraphy comprises three distinct zones:

The Basal series consists of a chilled fine grained gabbro overlain by gabbro, norite and feldspar pyroxenites. Thickness up to 700 feet (210 m). The Ultramafic series is composed of a lower peridotite 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. 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 (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 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 lesser 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 thickness. It has a Pd/Pt ratio of about 3.6.

Astronaut training In July 1972, the area was used by NASA 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's John Young and Apollo 17's Gene Cernan and Jack Schmitt. Notable geologist instructors included William R. Muehlberger.

Illustrations

Worked examples

Example 1 — a first encounter with Stillwater igneous complex

Start with the simplest possible case. Write down what Stillwater igneous complex claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stillwater igneous complex before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stillwater igneous complex ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stillwater igneous complex

In research: Stillwater igneous complex appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Stillwater igneous complex in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Stillwater igneous complex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Economic geology, First 100 IUGS Geological Heritage Sites, Igneous petrology of Montana, so understanding it makes those chapters shorter.
In everyday life: Look for Stillwater igneous complex outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Stillwater igneous complex in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stillwater igneous complex means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Stillwater igneous complex out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stillwater igneous complex in simple terms?

Why does Stillwater igneous complex matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stillwater igneous complex?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stillwater igneous complex.