Geophysical characterization of a Proterozoic REE terrane at Mountain Pass, eastern Mojave Desert, California, USA

Abstract

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 ∼1.5–2 km 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.

Three‐Dimensional Electrical Resistivity Characterization of Mountain Pass, California and Surrounding Region

“…Depth of the electrical resistivity anomaly is contoured in purple lines (lighter is deeper). Bottom: cross sections along M–M’ of the resistivity model and the B–B’ potential field model from (Denton et al., 2019). (a) Lidar image of Mountain Pass with MT stations (black triangles).…”

“…Denton et al. (2019) interpreted the gravity high as mafic amphibolite veins and dikes intruding Proterozoic gneissic rocks. This intrusion could have preferentially followed the unmapped fault and the basin fault to the east.…”

“…Drawn in are structures identified from potential field modeling (Denton et al., 2019). (e) Potential field model from (Denton et al., 2019) with C1 drawn in. Pink polygons‐syenite and shonkanite granites (Olson et al., 1954); dark purple hashed polygon‐carbonatite deposit (Olson et al., 1954); solid lines‐fault; dashed lines‐inferred fault; dashed lines with question marks‐questionable faults; I‐15‐interstate 15; MTF‐Mesquite Pass thrust fault; KTF, Keaney‐Mollusk Mine thrust fault; NF‐North Fault; SF‐South Fault; MF‐Middle Fault; TF‐Thrust Fault; dark gray circles‐earthquake hypocenters (SCEDC, 2013); Xg‐Proterozoic gneiss; Tg‐tertiary gravels; ma‐possible granitic intrusion; gr‐granitic rock; PDI‐Permian and Devonian limestone; Cd‐Cambrian dolomite.…”

Carbonatite-Related REE Deposits: An Overview

“…The mineralized rocks include the shoshonite, syenite, granite, and the Sulphide Queen carbonatite. However, only the carbonatite orebodies are economically viable [75]. The orebodies intrude into the Precambrian metamorphic basement in plate or lenticular form, mainly composed of calcite, dolomite, barite, bastnäsite in mineralogical composition, and bastnäsite as the main ore mineral accounts for about 10-15%.…”

“…The Sulphide Queen carbonatite is distinct from other carbonatites in the world. Carbonatites are usually spatially and genetically associated with a set of sodic alkaline rocks in a ring-shaped and concentric circle, and rich in LREE, Nb, Ta, Zr, and P. However, the formation of the Sulphide Queen carbonatite is associated with a suite of ultrapotassic intrusive rocks in plate-like form, being rich in LREE and Ba and depleted in Nb and P. Previous studies suggest that the abnormal nature of Sulphide Queen carbonatite is due to the fact that the carbonatite magma and associated ultrapotassic magma have a common source, or alternatively, the carbonatite magma evolved from the ultrapotassic magma [73,75,77].…”