January 8. The Stillwater Complex

By Richard I. Gibson


 Listen to the podcast:



Transcript: At about the same time as cyanobacteria were developing photosynthesis, the earth was still separating its various materials. This process continues today, in volcanoes that move rocks from inside the earth to its surface. 2.7 billion years ago, there was still a good bit of molten, metal-rich stuff floating around. Molten material undoubtedly reached the surface in volcanoes – we call those rocks extrusive, on the surface – and there were also intrusives forming, where molten rock was forced through or into pre-existing rocks well below the surface.

When magmas rich in dense elements like iron, nickel, and chromium were intruded into older rocks, sometimes the densest minerals would settle out under gravity, just as dense gold settles in a stream bed. But in this case, the “stream” was molten rock in what is today south-central Montana. The accumulation of different layers of different density led to something called a Layered Igneous Complex, and the one in the Beartooth Mountains is called the Stillwater Complex. Its rocks solidified about 2.7 billion years ago, and they contain some of the richest reserves of platinum, palladium, and chromium in the Western Hemisphere.

The mines in the Stillwater Complex are the only sources of platinum and palladium in the United States, and they manage to make us only 91% dependent on imports for platinum and 54% dependent on palladium imports. These elements are far more important to everyday life than as jewelry. In fact the biggest use of platinum-group elements is in catalytic converters in vehicles. They also serve as catalysts in petroleum refining, and can be found in flat-panel displays such as TVs and Computers. Apart from the Stillwater Complex, most U.S. platinum and palladium come from Russia, South Africa, and specialized metal refineries in Germany.

Map from USGS Misc. Investigations Map I-797, 2002 (1974), by N.J. Page and W.J. Nokleberg.