Denton, and three co-authors, report that Mountain Pass, California (USA), located in the eastern Mojave Desert, hosts one of the world’s richest rare earth element (REE) deposits. The REE-rich terrane occurs in a 2.5-km-wide, northwest-trending belt of Mesoproterozoic (1.4 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) metamorphic block that extends ~10 km southward from Clark Mountain to the eastern Mescal Range.
To characterize the REE terrane, gravity, magnetic, magnetotelluric, and whole-rock physical property data were analyzed. Geophysical data reveal that the Mountain Pass carbonatite body is associated with an ~5 mGal local gravity high that is superimposed on a gravity terrace (~4 km wide) caused by granitic Paleoproterozoic host rocks. Physical rock property data indicate that the Mountain Pass REE suite is essentially nonmagnetic at the surface with a magnetic susceptibility of 2.0 × 10−3 SI (n = 57), and lower-than-expected magnetizations may be the result of alteration. However, aeromagnetic data indicate that the intrusive suite occurs along the eastern edge of a distinct northwest-trending aeromagnetic high along the eastern Mescal Range.
The source of this magnetic anomaly is ~1.5 to 2 kms below the surface and coincides with an electrical conductivity zone that is several orders of magnitude more conductive than the surrounding rock. The source of the magnetic anomaly is likely a moderately magnetic pluton. Combined geophysical data and models suggest that the carbonatite and its associated REE-enriched ultrapotassic suite were preferentially emplaced along a northwest-trending zone of weakness, which has potential implications for regional mineral exploration.
The authors conclude that geophysical results indicate that the entire REE intrusive suite is located within a gravity terrace (anomaly G3) and east of a prominent magnetic feature (anomaly M7) and a conductive anomaly (anomaly C2). The source of anomalies M7 and C2 has a sharply defined geometry, with a top that probably occurs at a depth of ~1.5 to 2 km and extending >4 km based on matched filtering of magnetic data and geophysical modeling. The gravity terrace (anomaly G3) is defined by steep, northwest-trending gravity gradients and is bounded by the Clark Mountain and South-Kokoweef faults to the west and near the Wheaton fault to the east. The gravity terrace reflects relatively lower-density granitic gneiss (e.g., unit Xg1) and other granitic intrusive rocks (e.g., units Ji, Kt) present in models AA′–CC′ . The source of the magnetic anomaly (M7) shown in models BB′ and CC′ is a relatively low-density and moderately magnetic intrusive granitic pluton (body ma) that is not exposed at
the surface and is spatially associated with the 1.4 Ga Mountain Pass terrane.
Combined geophysical and geologic investigations of the eastern Mojave Desert carbonate terrane provide new insights into the structural framework of the Mountain Pass REE deposit. This geophysical study of the eastern Mojave Desert carbonatite terrane demonstrates the effectiveness of a multi-technique approach to studying the supporting structures that host REE deposits in the Mountain Pass area. Finally, future studies of Mountain Pass would benefit from high-resolution airborne geophysical surveys capable of continuous data gathering over much broader regions to include coverage of rugged and otherwise inaccessible mountain ranges. For example, high-resolution aeromagnetic, gravity gradiometry, lidar (light detection and ranging), and radiometric data would be ideal for evaluating the region in greater detail. In
addition, better understanding of faulting, structural analysis, and kinematics associated with 1.4 Ga structures would dramatically improve the overall constraints and insights related to the REE mineralization in the Mojave Desert.
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