MOUNTAIN PASS, CALIFORNIA FROM GOLD TO RARE EARTHS

The Mountain Pass claims originally produced gold and the site was later rediscovered in 1949 by use of a Geiger counter (Th anomaly) during the first postwar uranium rush. Review of samples collected revealed that the rock was an igneous carbonate rock (a carbonatite) and was enriched in bastnaesite (a rare earth fluorocarbonate). Molycorp bought the claims in 1950. Currently the average ore grade is approximately 8% with a cut off 2.3% rare earth elements (REE). The Mt Pass carbonatite is associated with a suite of ultrapotassic rocks with abundant F, Th, K, Ba, Rb, Cs + REE (bastnasite). The gold mineralization consists of native gold in quartz veins that contain abundant barite and galena with the barites being geochemically like those in the carbonatite. A lead isotope study was completed and documented that the Mt Pass carbonatite and Sulfide Queen deposits have Pb isotopic compositions consistent with a model age of about 1400Ma. The timing of the gold mineralization is thus like that of the emplacement of the carbonatite ore body.

As late as the mid-1980’s, Mountain Pass led the world in rare earth production and innovation. This leadership was wrested from Molycorp by Chinese national interests using their own rare earth carbonatite orebody (Bayan Obo) and by acquiring western technology. Mountain Pass is now owned by MP Materials (NYSE:MP). The Mountain Pass mine in California is one of the only locations extracting rare earth elements (REE) in the U.S. The economics of rare earth production by MP Materials is still affected by Chinese interests and competition. Bastnasite concentrate was and continues to be shipped to a Chinese company (Shenghe) on a non-exclusive basis. Production of separated rare earths is now beginning.