Mountain Pass Mine

Rare-earth elements, a series of chemically very similar elements, have been much in the news in recent years because they turn out to be critical in many high-tech applications. This deposit at Mountain Pass, California was discovered in the late 1940s and for decades supplied most of global demand. However, the uses of the rare earths then were not nearly so extensive as they are now, and as applications grew so did competition, mostly from China. Meanwhile, production costs at Mountain Pass also soared, so that by 2002 the mine had shut down.

The deposit at Mountain Pass is in an “igneous limestone,” a so-called carbonatite (pronounced “car-BON-uh-tite”). Carbonate minerals are nearly always found in sedimentary rocks, limestone being the typical example, and the carbonate minerals they contain either have been directly precipitated from water, or by living things from water as shells and such, with that biological debris then incorporated into the limestone.

A carbonatite, by contrast, was originally magma and cooled from the molten state. Other carbonatite deposits are known, but they are definitely out of the ordinary. They also commonly concentrate rare earths, and that’s where the overseas competition comes from.

Because of national defense concerns about dependence on a possibly unreliable foreign supplier, beginning in the 2010s efforts to reopen the Mountain Pass Mine began, and after several false starts it seems to be operating routinely. The efforts are an echo of the “strategic minerals programs” that lasted into the 1960s, where the US Government subsidized production of materials that were deemed vital to national security.

One side note is that the Mountain Pass carbonatite is slightly radioactive, from its content of thorium, a natural radioactive element. In fact, this originally led to the discovery of the deposit, when prospecting for uranium with a Geiger counter had become the mid-20th century equivalent of panning for “color.” The slight radioactivity also complicates the safe extraction of the rare earth elements, because waste products tend to be radioactive and therefore require special handling.