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#           Mafic alkaline magmatism and rare earth element mineralization in the Mojave Desert, California: The Bobcat Hills connection to Mountain Pass      

      January 9, 2024    

Occurrences of alkaline and carbonatite rocks with high concentrations of rare earth elements (REE) are a defining feature of Precambrian geology in the Mojave Desert of southeastern California. The most economically important occurrence is the carbonatite stock at Mountain Pass, which constitutes the largest REE deposit in the United States. A central scientific goal is to understand the genesis of the carbonatite ore body in the context of widespread REE-rich igneous activity. A swarm of mafic alkaline (shonkinite) dikes has been mapped and sampled at Bobcat Hills, 65 km southeast of the Mountain Pass mine. Whole-rock geochemistry and zircon geochronology demonstrate a clear affinity to the ca. 1.4 Ga Mountain Pass intrusive system. Bobcat Hills dikes have comparably high REE concentrations (La ∼1,000× chondritic) and an error-weighted mean 207Pb/206Pb zircon crystallization age of 1,426 ± 2 Ma (2σ). Unlike the alkaline intrusions at Mountain Pass, which have abundant inherited zircon from Paleoproterozoic basement rocks and crustally influenced oxygen isotope compositions (δ18Ozircon = 6.5–7.5‰), the Bobcat Hills dikes lack any evidence of crustal assimilation and have oxygen isotope values that overlap a mantle range (Bobcat Hills average δ18Ozircon = 5.6 ± 0.3‰). The dikes were a high-temperature, early center of mafic alkaline magmatism in the Mojave Desert that serve as a snapshot of melt generation from a spatially extensive, metasomatized mantle source. We propose that modification of the crust over many tens of Myr at Mountain Pass created an environment that favored crustal assimilation and enabled ascent of late-stage, REE-rich carbonatite magmas.

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January 26, 2023

####                 Multidisciplinary Investigations of REE Mineralization at Mountain Pass and in the Southeast Mojave Desert, California              

Energy and Minerals Mission Area, Mineral Resources Program, Geology, Minerals, Energy, and Geophysics Science Center

link

January 26, 2023

####   Multidisciplinary Investigations of REE Mineralization at Mountain Pass and in the Southeast Mojave Desert, California

Learn More

January 10, 2024

####                 Geological mapping coordinates, and zircon geochemistry, geochronology and isotope geochemistry data, Bobcat Hills, California              

This dataset contains geologic mapping coordinates, and geochemical, geochronologic, and isotopic data collected for shonkinite dikes and zircons from Bobcat Hills, California, USA. See accompanying G-Cubed publication described in the "Larger Work" section of this metadata file for a detailed description of this dataset.

Energy and Minerals Mission Area, Geology, Minerals, Energy, and Geophysics Science Center

#### Kathryn Watts

##### Research Geologist

Geology, Minerals, Energy, and Geophysics Science Center

kwatts@usgs.gov

509-368-3108

#### David M. Miller, Ph.D

##### Scientist Emeritus

Geology, Minerals, Energy, and Geophysics Science Center

dmiller@usgs.gov

650-439-2823

#### David  Ponce

##### Research Geophysicist

Geology, Minerals, Energy, and Geophysics Science Center

ponce@usgs.gov

650-439-2314

## Related Content

link

January 26, 2023

####                 Multidisciplinary Investigations of REE Mineralization at Mountain Pass and in the Southeast Mojave Desert, California              

Energy and Minerals Mission Area, Mineral Resources Program, Geology, Minerals, Energy, and Geophysics Science Center

link

January 26, 2023

####   Multidisciplinary Investigations of REE Mineralization at Mountain Pass and in the Southeast Mojave Desert, California

Learn More

January 10, 2024

####                 Geological mapping coordinates, and zircon geochemistry, geochronology and isotope geochemistry data, Bobcat Hills, California              

This dataset contains geologic mapping coordinates, and geochemical, geochronologic, and isotopic data collected for shonkinite dikes and zircons from Bobcat Hills, California, USA. See accompanying G-Cubed publication described in the "Larger Work" section of this metadata file for a detailed description of this dataset.

Energy and Minerals Mission Area, Geology, Minerals, Energy, and Geophysics Science Center

#### Kathryn Watts

##### Research Geologist

Geology, Minerals, Energy, and Geophysics Science Center

kwatts@usgs.gov

509-368-3108

#### David M. Miller, Ph.D

##### Scientist Emeritus

Geology, Minerals, Energy, and Geophysics Science Center

dmiller@usgs.gov

650-439-2823

#### David  Ponce

##### Research Geophysicist

Geology, Minerals, Energy, and Geophysics Science Center

ponce@usgs.gov

650-439-2314

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