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.
At 1 July 2020, Proven and Probable Reserves, using a 3.83% total rare-earth oxide (REO) cutoff grade, were 18.9 million tonnes of ore containing 1.36 million tonnes of REO at an average grade of 7.06% REO.