Stillwater Complex
Quick Summary
The Stillwater Complex is a significant geological feature located in the Beartooth Mountains of Montana, United States. This layered intrusion, approximately 2.7 billion years old, is notable for its substantial size, measuring 48 km (30 miles) long with a stratigraphic thickness of 6 km (3.7 miles).1
It formed when a subhorizontal body of magma intruded and underwent crystal settling, creating its distinct layered structure. The complex is particularly recognized for a 3-meter- (9-foot-) thick layer rich in platinum minerals, which constitutes a major economic deposit. The Stillwater Complex is one of several large layered basaltic intrusions found in stable continental areas, including others in Canada, South Africa, and Greenland. These intrusions are formed from tholeiitic magma that experienced fractional crystallization, resulting in layers of ultramafic rocks topped by gabbroic and anorthositic layers. Some of these layered intrusions are economically important due to deposits of chromite, a source of chromium and platinum.12
Ask Britannica AI about this topic
Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.
PrecambrianSedimentary basins, basic dikes, and layered complexesThe Stillwater Complex is a famous, 2.7-billion-year-old, layered ultrabasic-basic intrusion in the Beartooth Mountains of Montana in the United States. It is 48 km (30 miles) long and has a stratigraphic thickness of 6 km (3.7 miles). It was intruded as a subhorizontal body of magma…
Read MoreThe Stillwater Complex is a famous, 2.7-billion-year-old, layered ultrabasic-basic intrusion in the Beartooth Mountains of Montana in the United States. It is 48 km (30 miles) long and has a stratigraphic thickness of 6 km (3.7 miles). It was intruded as a subhorizontal body of magma…
Read More- PrecambrianArchean crustal growth
…igneous complexes (such as the Stillwater in Montana) formed; and the formation of the first large sedimentary basins (for example, the Witwatersrand in South Africa) also occurred. All of these structures indicate that the continental crust had reached a mature stage with considerable stability and rigidity for the first time…
Read More…igneous complexes (such as the Stillwater in Montana) formed; and the formation of the first large sedimentary basins (for example, the Witwatersrand in South Africa) also occurred. All of these structures indicate that the continental crust had reached a mature stage with considerable stability and rigidity for the first time…
Read More
mineral depositImmiscible melts…the world’s platinum-group metals; the Stillwater Complex, Montana, host to platinum-group deposits similar to the Merensky Reef; and the Norilsk deposits of Russia, containing large reserves of platinum-group metals.
Read More…the world’s platinum-group metals; the Stillwater Complex, Montana, host to platinum-group deposits similar to the Merensky Reef; and the Norilsk deposits of Russia, containing large reserves of platinum-group metals.
Read More
platinum groupOres…the largest deposit is the Stillwater Complex in Montana, but this is substantially smaller than the deposits cited above. The world’s largest producers of platinum are South Africa, Russia, Zimbabwe, and Canada.
Read More…the largest deposit is the Stillwater Complex in Montana, but this is substantially smaller than the deposits cited above. The world’s largest producers of platinum are South Africa, Russia, Zimbabwe, and Canada.
Read More