THE MOUNTAIN PASS RARE-EARTH CARBONATITE AND ASSOCIATED ULTRAPOTASSIC ROCKS, CALIFORNIA

The Sulphide Queen carbonatite, which contains the rare-earth orebody at Mountain Pass in California, is a moderately dipping tabular intrusion associated with 1.40 Ga alkaline plutons of similar size and orientation, as well as abundant carbonatite and alkaline dikes. The Mountain Pass carbonatite and alkaline rocks lie in a narrow belt of occurrences of alkaline rocks 130 km long. Both the carbonatite and the associated alkaline rocks are chemically and mineralogically unusual. The mafic members of the Mountain Pass alkaline rock suite belong to the ultrapotassic group, and are most akin to the lamproite subdivision of that group. These rocks are part of a unique ultrapotassic suite in that it ranges from typical mafic lithologies to silicic granite. The association with significant amounts of carbonatite is also unique. Chemical similarities suggest that the carbonatite, which has yielded slightly younger (1.38 Ga) ages, is related to the ultrapotassic rocks. It is not clear if the carbonatite and associated alkaline rocks came from a common magmatic parent, but they likely came from a common enriched source in the mantle.