The Mountain Pass rare earth mine is an open-pit mine of rare earth elements (REEs) on the south flank of the Clark Mountain Range, just north of the unincorporated community of Mountain Pass, California, United States. The mine, owned by Molycorp Inc., once supplied most of the world's rare earth elements.

The Mountain Pass deposit is in a 1.4 billion year old Precambrian carbonatite intruded into gneiss, and contains 8% to 12% rare earth oxides, mostly contained in the mineral bastnäsite. Gangue minerals include calcite, barite, and dolomite. It is regarded as a world-class rare-earth mineral deposit. The metals that can be extracted from it include:

Cerium Lanthanum Neodymium Europium

Known remaining reserves were estimated to exceed 20 million tons of ore as of 2008, using a 5% cutoff grade, and averaging 8.9% rare earth oxides.

The mine, once the world's dominant producer of rare earth elements, was closed in large part due to competition from REEs imported from China, which in 2009 supplied more than 96% of the world's REEs. Since 2007 China has restricted exports of REEs and imposed export tariffs, both to conserve resources and to give preference to Chinese manufacturers. Some outside China are concerned that because rare earths are essential to some high-tech, renewable-energy, and defense-related technologies, the world should not be reliant on a single source.

en.wikipedia.org/wiki/Mountain_Pass_rare_earth_mine