Geophysical Characterization of a Rare Earth Element Enriched Carbonatite Terrane at Mountain Pass, California Eastern Mojave Desert

Abstract

Mountain Pass, California, located in the eastern Mojave Desert, hosts one of the world's richest rare earth element (REE) deposits. The REE-rich rocks occur in a 2.5 kmwide, north-northwest trending zone of Mesoproterozoic (1.4-1.42 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) schist-gneiss terrane that extends ~10 km southward from Clark Mountain to the Mescal Range. Several REE-enriched bodies make up the Mountain Pass intrusive suite including shonkinite, syenite, and granite comprising an ultrapotassic intrusive suite and the Sulphide Queen carbonatite body. Two-dimensional modeling of gravity, magnetic, and electrical resistivity data reveals that the Mountain Pass intrusive suite is associated with a local gravity high that is superimposed on a ~4-km wide gravity terrace. Rock property data indicate that the Mountain Pass intrusive suite is unusually nonmagnetic at the surface (2.0 x 10-3 SI, n=67). However, aeromagnetic data indicate that these rocks occur along the eastern edge of a prominent north-northwest trending aeromagnetic high of unknown origin. The source of this unknown magnetic anomaly is ~2-3 km below the surface and coincides with a body of rock having high electrical conductivity. Electrical resistivity models indicate that this unknown magnetic anomaly is several orders of magnitude more conductive (10 3 Ω•m) than the surrounding rock. Combined geophysical data suggest that the carbonatite and its associated ultrapotassic intrusive suite were preferentially emplaced along a northwest zone of weakness and/or a fault. vi