The mine once supplied most of the world's rare-earth elements. Owned by MP Materials, it is the only rare earth mining and processing facility in the United States.
The Mountain Pass deposit is in a 1.4 billion-year-old Precambrian carbonatite intruded into gneiss. It 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, and europium.
In 2008, remaining reserves using a 5% cutoff grade were estimated to exceed 20 million tons of ore that averages 8.9% rare-earth oxides.
According to wikipedia