20-35. Tracking the spatiotemporal evolution of a world-class carbonatite REE deposit at Mountain Pass, California

CLOSED

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Background: Extraordinary concentrations of rare earth elements (REE) characterize carbonatite intrusive rocks at the Mountain Pass mine in southeastern California. Recent decades have seen a vast expansion in the technological applications of REE, and they are currently at the forefront of revolutions in electronics and clean energy. The ubiquity and importance of REE to the U.S. economy, coupled with vulnerability to supply disruption from predominantly foreign sources, led to the designation of REE as “critical” commodities (Fortier et al., 2018). As the Nation’s premier REE ore deposit, and the only actively producing REE mine in the country, Mountain Pass is essential in bolstering domestic supply of critical REE. The U.S. Geological Survey (USGS) is seeking a postdoctoral researcher through its Mendenhall Research Fellowship Program to establish key parameters in the space and time development of this important critical element ore deposit.

Description of the Research Opportunity: Carbonatite at Mountain Pass comprises a central stock, the Sulphide Queen REE ore body, and dozens of dikes. The stock is westward dipping, 150 m thick and had surface dimensions of 700 m by 150 m prior to mining (Olson et al., 1954). It is compositionally variable between calcite carbonatite (sövite) and dolomite carbonatite (beforsite) end members (Castor, 2008). Bastnäsite, a fluorcarbonate, is the main ore mineral and occurs in both primary (igneous) and secondary (alteration) textural context. Published Th-Pb dates on monazite are 1371 ± 10 Ma for the Sulphide Queen ore body and 1396 ± 16 Ma for a carbonatite dike about 1 km north of the ore body (Poletti et al., 2016). The densest concentration of carbonatite dikes is found near the Sulphide Queen mine site, but sparse carbonatite dikes also occur at least 6 km south of the mine (Olson et al., 1954). Alkaline igneous rocks, primarily shonkinite and syenite, form stocks and dikes that trend southeast of the mine. These intrusions also generally dip west, have elevated REE, and similar ages to carbonatite intrusions (Watts et al, 2021). Evidence for a large volume of hydrothermally altered rocks includes fenitization envelopes, as much as 200 m thick locally, surrounding carbonatite and alkaline intrusive rocks. Some fenitization zones are enriched in REE to economic grades (Castor, 2008).

Despite its status as the Nation’s most significant critical element REE deposit, little is known about processes that generated the Mountain Pass carbonatite ore body. Below are outstanding questions intended as a springboard for Mendenhall candidates to formulate an original proposal that draws on their individual strengths. It is not expected that a single Mendenhall postdoctoral study will address all these questions, or be restricted to them, but candidates are encouraged to think creatively and with latitude to combine questions and research goals.

Outstanding questions:

• What were the roles of the mantle and the crust in carbonatite genesis, and can specific source domains be identified?
• How did the carbonatite intrusions, large and small, form in space and time at Mountain Pass and in the larger Mountain Pass district? How and when were they structurally modified?
• What explains the compositional diversity of the carbonatite and how do the different subtypes relate in its petrogenesis?
• How did the carbonatite evolve in time-temperature-pressure-compositional space?
• What was the source of REE and how did they become concentrated to economic grade?
• What was the role of hydrothermal alteration in REE mineralization?

Experience studying magmatic-hydrothermal systems, carbonatites, and/or REE mineralization would be beneficial, but is not required. New types of data acquired may include: geologic mapping, geochronology, mineral chemistry, stable and radiogenic isotope geochemistry, thermobarometry, and thermodynamic modeling.

Potential tasks:

Determining sequence of events

• Radiogenic and stable isotopes to constrain mantle and crustal sources of magmas and REE.
• Mapping of the carbonatite ore body and carbonatite dikes at Mountain Pass and the surrounding region.
• High-precision geochronology (CA-ID-TIMS) of U- and Th-bearing accessory minerals (primary and secondary) in the carbonatite.
• High-spatial resolution geochronology (SIMS, LA-ICP-MS) of U- and Th-bearing accessory minerals.

Estimating magmatic parameters/conditions/rates

• Temperature and pressure as constrained by trace elements or stable isotopes in minerals.
• Oxygen and sulfur fugacity as determined by major and trace element analyses of minerals, such as in oxide and sulfide minerals.
• Trace element diffusion modeling of domains within minerals to estimate rates of geologic processes.

Constraining hydrothermal processes

• Integration of melt and fluid inclusion analyses to track igneous to hydrothermal conditions of mineral growth.
• Thermodynamic modeling of igneous and hydrothermal processes.

These tasks outline ideas for future work to address outstanding questions, but we seek the unique perspectives of Mendenhall candidates. The Mendenhall Fellow will be aided by a comprehensive inventory of rock samples, drill core samples, maps, polished thin sections, and whole-rock major and trace element geochemistry for the Mountain Pass carbonatite ore body. Access to the Mountain Pass mine site will be accommodated by MP Materials Co-advisors R. Ruesch (Manager of Mining and Ore Delivery) and L. Xia (Senior Mine Geologist). Primary USGS Research Advisor K. Watts and Co-advisors G. Haxel and D. Miller will provide research guidance and support, with decades of combined experience studying and mapping diverse intrusive and metamorphic rocks at Mountain Pass and the surrounding region (e.g., Watts et al., 2021; Haxel, 2005; Miller and Wooden, 1994). The Fellow will also benefit from the knowledge of Co-advisor S. Castor, formerly Chief Geologist at Mountain Pass who has mapped and described rocks of the carbonatite ore body (Castor, 2008). Interested applicants are strongly encouraged to contact the Research Advisors early in the application process to discuss project ideas.

References:

Castor, S.B., 2008, The Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks, California: The Canadian Mineralogist, v. 46, p. 779–806.

Fortier, S.M., Nassar, N.T., Lederer, G.W., Brainard, J., Gambogi, J., and McCullough, E.A., 2018, Draft critical mineral list—summary of methodology and background information—U.S. Geological Survey technical input document in response to Secretarial Order No. 3359: U.S. Geological Survey Open-File Report 2018–1021, 15 p.

Haxel, G.B., 2005, Ultrapotassic mafic dikes and rare earth element- and barium-rich carbonatite at Mountain Pass, Mojave Desert, southern California: summary and field trip localities: U.S. Geological Survey Open File Report 2005-1219, 56 p.

Miller, D.M, and Wooden, J.L., 1994, Field guide to Proterozoic geology of the New York, Ivanpah, and Providence Mountains, California: U.S. Geological Survey Open-File Report 94-674, 40 p.

Olson, J.C., Shawe, D.R., Pray, L.C., and Sharp, W.N., 1954, Rare-earth mineral deposits of the Mountain Pass district, San Bernardino County, California: U.S. Geological Survey Professional Paper 261, 75 p.

Poletti, J.E., Cottle, J.M., Hagen-Peter, G.A., and Lackey, J.S., 2016, Petrochronological constraints on the origin of the Mountain Pass ultrapotassic and carbonatite intrusive suite, California: Journal of Petrology, v. 57, p. 1555–1598.

Watts, K.E., Haxel, G.B., and Miller, D.M., 2021, Temporal and petrogenetic links between Mesoproterozoic alkaline and carbonatite magmas at Mountain Pass, California: Economic Geology, in press, https://doi.org/10.5382/econgeo.4848.

Proposed Duty Station: Spokane, Washington

Areas of PhD: Geologic mapping, structural geology, igneous petrology, carbonate petrology, economic geology, aqueous geochemistry, geochronology, isotope geochemistry, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet the qualifications for: Research Geologist.

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Human Resources Office Contact: Victor Mendoza, 650-439-2454, vjmendoza@usgs.gov