In this time of increased focus on renewable energy technologies, rare earth elements (REEs) are of critical importance. For example, neodymium (Nd) is a REE used in the generator and motor magnets of wind turbines and electric vehicles. Reliance on REE imports puts the U.S. at high risk for supply disruption. The project will integrate geology, geophysics, petrology, geochronology, and economic geology to study domestic sources of REEs, focusing on the world-class Mountain Pass REE mine in southeastern California.
Science Issue and Relevance:
Mountain Pass is the only operating REE mine in the U.S. (Fig. 1). It is located it the Mojave Desert of southeastern California. As the sole domestic source of REEs, Mountain Pass is crucial to ensuring economic and national security. This project plans to study the formation of the Mountain Pass REE ore deposit and its geologic context in the Mojave Desert. Preliminary data support a genetic connection between the Mountain Pass system and alkaline intrusions in the Mojave Desert, which may contain additional undiscovered REE deposits.
Methodology to Address Issue:
Geology
The main rocks that host the ore at Mountain Pass are a rare type of igneous rock called carbonatite. These unusual rocks form from magmas that crystallize over 50% carbonate minerals. This contrasts with most igneous rocks that chiefly consist of silicate minerals. As carbonatite magmas intruded the crust at Mountain Pass around 1.4 billion years ago (Ga), they were accompanied by a suite of alkaline silicate intrusions (shonkinite, syenite, alkali granite). Understanding the relationships between these rock types is crucial to understanding the genesis of the Mountain Pass deposit. The Mesoproterozoic (ca. 1.4 Ga) carbonatite and alkaline intrusive bodies trend NW-SE and are oriented along the foliation of Paleoproterozoic (ca. 1.6-1.8 Ga) gneiss host rocks (Fig. 2).
Geophysics
The most recent set of tools available for 3D geologic mapping allow scientists to develop complex 3D models of the subsurface that integrate multiple data types, including surficial geologic data, geophysical data, borehole data, and other field measurements. We plan to construct a 3D model of the Mountain Pass REE deposit using multiple high-resolution geophysical datasets, including gravity, gravity gradiometry, magnetics, and magnetotellurics (Fig. 3). To ensure that our model reflects the characteristics of the deposit well, we plan to check its accuracy against direct field observations at the Mountain Pass deposit. Once that model is constructed and tested, we plan to compare it to a regional model of the southeast Mojave Desert to find other possible REE deposits.
Figure 4. Zircon U-Pb geochronology results for alkaline silicate rocks from “Site 1,” located 500 m northeast of the Mountain Pass mine pit. Uppermost panels show all age data, concordant and discordant. Middle panels show concordant age data with calculated concordia ages. Bottom panels shown error-weighted mean ages and probability density functions for concordant age data. From Watts et al. (2022).
Petrology
Petrologic investigations at Mountain Pass are planned to assess genetic relationships between carbonatite and alkaline intrusions, their mantle, crustal, and fluid sources, and distributions in space and time. Though carbonatite is the only rock type we expect to have economic REE mineralization, elevated REEs also occur in the alkaline intrusive suite rocks at Mountain Pass. Because alkaline intrusions constitute the largest volume of ca. 1.4 Ga Mesoproterozoic rocks in the Mojave Desert, developing predictive criteria for cogenetic carbonatite will be essential in delineating favorable areas for REE deposits.
Geochronology
Geochronologic work is planned to support the petrologic investigations. U-Pb dating of zircon and Th-Pb dating of monazite can be used to establish crystallization ages. These geochronologic techniques can be combined with complementary trace element (including REE) and isotopic (O, Nd, Hf) analyses on the same crystals. Zircon U-Pb geochronology of the alkaline silicate rocks has refined the intrusive history at Mountain Pass to ca. 1410-1415 Ma (Fig. 4).
Economic Geology
Bastnäsite, a light REE (LREE) fluorocarbonate mineral, is the dominant REE ore mineral at Mountain Pass (Fig. 5). It has been traditionally ascribed an igneous origin. However, preliminary petrography and mineral chemistry studies indicate that while some bastnäsite and other associated LREE carbonate minerals (e.g., parisite, synchysite, sahamalite) appear to be igneous (Figs. 5-6), many appear to be secondary (hydrothermal), occurring as interstitial crystals in the groundmass, as pseudomorphic rims on primary minerals, and in veins that cross-cut primary minerals. Understanding igneous and hydrothermal conditions of REE mineralization in the Mountain Pass system is fundamental to understanding this world-class deposit.
- Overview
In this time of increased focus on renewable energy technologies, rare earth elements (REEs) are of critical importance. For example, neodymium (Nd) is a REE used in the generator and motor magnets of wind turbines and electric vehicles. Reliance on REE imports puts the U.S. at high risk for supply disruption. The project will integrate geology, geophysics, petrology, geochronology, and economic geology to study domestic sources of REEs, focusing on the world-class Mountain Pass REE mine in southeastern California.
Sources/Usage: Public Domain.Figure 1. The Mountain Pass REE mine in the Mojave Desert, California. Photo by T. Morton. Science Issue and Relevance:
Mountain Pass is the only operating REE mine in the U.S. (Fig. 1). It is located it the Mojave Desert of southeastern California. As the sole domestic source of REEs, Mountain Pass is crucial to ensuring economic and national security. This project plans to study the formation of the Mountain Pass REE ore deposit and its geologic context in the Mojave Desert. Preliminary data support a genetic connection between the Mountain Pass system and alkaline intrusions in the Mojave Desert, which may contain additional undiscovered REE deposits.
Methodology to Address Issue:
Sources/Usage: Public Domain.Figure 2. Geologic map of the Mountain Pass intrusive system draped over a shaded relief map. Major intrusive stocks are labeled. Modified after Watts et al. (2022). Geology
The main rocks that host the ore at Mountain Pass are a rare type of igneous rock called carbonatite. These unusual rocks form from magmas that crystallize over 50% carbonate minerals. This contrasts with most igneous rocks that chiefly consist of silicate minerals. As carbonatite magmas intruded the crust at Mountain Pass around 1.4 billion years ago (Ga), they were accompanied by a suite of alkaline silicate intrusions (shonkinite, syenite, alkali granite). Understanding the relationships between these rock types is crucial to understanding the genesis of the Mountain Pass deposit. The Mesoproterozoic (ca. 1.4 Ga) carbonatite and alkaline intrusive bodies trend NW-SE and are oriented along the foliation of Paleoproterozoic (ca. 1.6-1.8 Ga) gneiss host rocks (Fig. 2).
Geophysics
The most recent set of tools available for 3D geologic mapping allow scientists to develop complex 3D models of the subsurface that integrate multiple data types, including surficial geologic data, geophysical data, borehole data, and other field measurements. We plan to construct a 3D model of the Mountain Pass REE deposit using multiple high-resolution geophysical datasets, including gravity, gravity gradiometry, magnetics, and magnetotellurics (Fig. 3). To ensure that our model reflects the characteristics of the deposit well, we plan to check its accuracy against direct field observations at the Mountain Pass deposit. Once that model is constructed and tested, we plan to compare it to a regional model of the southeast Mojave Desert to find other possible REE deposits.
Sources/Usage: Public Domain.Figure 3. Geophysical maps of the Mountain Pass intrusive system. The star shows the location of the Mountain Pass mine. A. Light detection and ranging (LiDAR) map, with depth of electrical resistivity model contoured in purple lines (lighter is deeper). B. Magnetic intensity map. C. Vertical gravity map. D. Cross section of electrical resistivity model along M-M'. E. Potential field model of the geology along M-M'. From Peacock et al. (2021).
Sources/Usage: Public Domain.
Figure 4. Zircon U-Pb geochronology results for alkaline silicate rocks from “Site 1,” located 500 m northeast of the Mountain Pass mine pit. Uppermost panels show all age data, concordant and discordant. Middle panels show concordant age data with calculated concordia ages. Bottom panels shown error-weighted mean ages and probability density functions for concordant age data. From Watts et al. (2022).Petrology
Petrologic investigations at Mountain Pass are planned to assess genetic relationships between carbonatite and alkaline intrusions, their mantle, crustal, and fluid sources, and distributions in space and time. Though carbonatite is the only rock type we expect to have economic REE mineralization, elevated REEs also occur in the alkaline intrusive suite rocks at Mountain Pass. Because alkaline intrusions constitute the largest volume of ca. 1.4 Ga Mesoproterozoic rocks in the Mojave Desert, developing predictive criteria for cogenetic carbonatite will be essential in delineating favorable areas for REE deposits.
Geochronology
Geochronologic work is planned to support the petrologic investigations. U-Pb dating of zircon and Th-Pb dating of monazite can be used to establish crystallization ages. These geochronologic techniques can be combined with complementary trace element (including REE) and isotopic (O, Nd, Hf) analyses on the same crystals. Zircon U-Pb geochronology of the alkaline silicate rocks has refined the intrusive history at Mountain Pass to ca. 1410-1415 Ma (Fig. 4).
Economic Geology
Bastnäsite, a light REE (LREE) fluorocarbonate mineral, is the dominant REE ore mineral at Mountain Pass (Fig. 5). It has been traditionally ascribed an igneous origin. However, preliminary petrography and mineral chemistry studies indicate that while some bastnäsite and other associated LREE carbonate minerals (e.g., parisite, synchysite, sahamalite) appear to be igneous (Figs. 5-6), many appear to be secondary (hydrothermal), occurring as interstitial crystals in the groundmass, as pseudomorphic rims on primary minerals, and in veins that cross-cut primary minerals. Understanding igneous and hydrothermal conditions of REE mineralization in the Mountain Pass system is fundamental to understanding this world-class deposit.
Sources/Usage: Public Domain.Figure 5. Thin section of a Mountain Pass carbonatite ore sample showing mineralogy and texture. The slider alternates between maps of back-scattered electrons (BSE; grayscale) and layered elements (Nd, Ca, Pb; colored). Note the large, euhedral, tabular crystals of bastnäsite. Images were collected with a scanning electron microscope (SEM).
Sources/Usage: Public Domain.Figure 6. High-resolution detail of Fig. 5 (shown by box inset) of a bastnäsite crystal with parisite rim. The energy dispersive spectra (EDS) to the right shows the chemical variation between these minerals. Note the higher calcium (Ca) contents in parisite. The image and EDS were collected with a scanning electron microscope (SEM).