STRUCTURAL AND GEOCHEMICAL FRAMEWORK FOR THE MOUNTAIN PASS REE MINE, SOUTHEASTERN CALIFORNIA

Mesoproterozoic (1.4 Ga) carbonatite and related ultrapotassic intrusions of the Mountain Pass (MP) district have exceptionally high concentrations of light rare earth elements (LREE). The MP carbonatite, averaging ~9 wt% rare-earth oxides, is a vital mineral resource. Our studies of the regional geologic setting of MP reveal that: (1) A Paleoproterozoic granulite-facies gneiss complex, widespread in the Ivanpah and New York Mountains, and comprising diverse orthogneisses ranging from 1.76 to 1.66 Ga, hosts the MP intrusions. (2) Ultrapotassic dikes and associated veins like those at MP are common across this same region, as much as 30 km from MP. (3) In contrast, fenitization associated with 1.4-Ga carbonatite and ultrapotassic rocks is restricted to an area 3–10 km wide, limited largely by the North and South faults, and by the Wheaton fault to the east. Ultrapotassic dikes regionally strike east-west, but in the heavily fenitized MP district swing to north-northwest, parallel to gneissic foliation. Older Paleoproterozoic orthogneisses approximate average upper continental crust in composition, but the younger orthogneisses are enriched 2–3 times in Rb, Ba, LREE, Zr, and Th. The youngest orthogneisses in the New York Mountains host 1.65-Ga Th-LREE mineralization, as monazite veins. This suite of elements is then further enriched, ~10 times, in the 1.4-Ga ultrapotassic intrusions at MP to as much as Ba 8400 ppm, LREE 2200 ppm, Th 140 ppm. Progressive enrichment of incompatible elements over the 100 m.y. intrusion history of the Paleoproterozoic gneisses followed by geochemically unusual, highly enriched melts at 1.4 Ga indicates that melting of metasomatized mantle in this region was unusual both before and during MP intrusion. Whether a single or multiple metasomatic events in the mantle transferred melts to the crust, and what the enrichment process(es) entailed, remain to be elucidated but the MP source area appears to be nearly unique.

Our mapping shows that the MP district has no Cenozoic faults that have significantly displaced the ore body. However, previously unrecognized thrust faults structurally below the Cretaceous frontal thrust west of the carbonatite body cut the Paleoproterozoic gneiss complex, and potentially could have affected the ore body at depth.