Concealed Rare Earth Element (REE) Terranes in the Southern Basin and Range Geologic Province

Science Center Objects

We are working to develop an understanding of the types, sizes, and spatial distribution of rare-earth element (REE) related deposits in the Southern Basin and Range geologic province to help ensure the availability of REEs to industry in the United States. Geologic, geophysical, and geochemical data acquired will be used to develop permissive tracts for REE related deposits and to define areas of high potential within those tracts. The permissive tracts will include regions where deposits may be concealed by sedimentary rock or basin fill sediments. 

Science Issue and Relevance

The Department of the Interior Secretarial Order 3359 'Critical Mineral Independence and Security' directed the USGS to develop a plan to improve the Nation's understanding of domestic critical mineral resources. Critical minerals include REEs which because of their unique physical and chemical properties have multiple uses in the computer industry, energy, and military technology. During the past decade, China has accounted for more than 90 percent of global production and supply of REE resources and even began limiting supply in 2010 through the imposition of quotas, licenses, and taxes. Securing domestic sources of REEs is of great importance. 

Methodology to Address Issue

Geologists examining a carbonatite dike in the Lemitar Mountains, New Mexico

Geologists from the USGS and the New Mexico Bureau of Geology and Mineral Resources examining a carbonatite dike in the Lemitar Mountains, New Mexico

(Public domain.)

Most of the rare earth element deposits associated with Tertiary alkaline igneous rocks are found in the Trans-Pecos of West Texas and parts of southeastern New Mexico while the older rare earth element deposits are found scattered throughout Arizona and New Mexico. We will summarize existing data and prioritize regions for new geophysical data acquisition through the Earth Mapping Resources Initiative (Earth MRI).

The black outline defines the location of the Trans-Pecos aeromagnetic/aeroradiometric survey, Texas and New Mexico

Location of the Trans-Pecos aeromagnetic/aeroradiometric survey, Texas and New Mexico. Survey was flown in January and February of 2021. The location of the Round Top REE deposit is indicated.

(Public domain.)

A belt of alkaline-igneous rocks extends along the eastern boundary of the Rocky Mountains and the Basin and Range geological province. These rocks, primarily Tertiary in age, contain large quantities of many critical minerals including REEs. We are focusing on these alkaline igneous rocks in New Mexico and Texas and not only considering where they can be found outcropping, but also where they may be concealed in sedimentary rocks and by basin-fill sediments.  

One focus area is the Trans-Pecos region of southern New Mexico and far western Texas. This area contains a series of rare earth element enriched ~ 34-36 Ma intrusive rhyolites and syenites and hosts the Round Top REE deposit in the Sierra Blanca Mountains, Texas. This deposit is one of the few REE deposits in the U.S. which has an economic evaluation and is scheduled to go into production around 2025. In addition, Alkaline igneous rocks in the Cornudas and Hueco Mountains, both within the Trans-Pecos focus area, also show high concentration of REEs and other critical elements. The project has recently (early 2021) flown and aeromagnetic and aeroradiometric survey over this region with the following objectives: 1) Detect regional trends, fractures,  or patterns that may be responsible for the locations of Tertiary alkaline intrusions; 2) Identify magma sources and plumbing systems for Tertiary alkaline intrusions; and 3) Identify potential concealed Tertiary alkaline intrusions. Preliminary work on the new data has produced 3-D models of the intrusive bodies, some of which are not found at the surface. 

Another focus area in the alkaline-igneous belt is the Gallinas Mountains in New Mexico, an area which has produced REEs in the past. Acquisition of gravity data and sampling has occurred in this region and we are hoping to also acquire an aeromagnetic survey here. 

We are also looking at Proterozoic to Cambrian alkaline-igneous rocks throughout New Mexico and Arizona. One region, the Lemitar Mountains, New Mexico, contains carbonatite veins outcropping at the surface and may contain a large carbonatite body at depth. Carbonatites are igneous rocks with a large percentage of carbonate minerals and, in general, have some of the highest concentration of REEs in any igneous rocks. We are conducting geophysical modeling and sampling of the carbonatite veins in this region. 

Scientist studying a carbonatite dike

Virginia McLemore, New Mexico Bureau of Geology and Mineral Resources, discussing a carbonatite dike in the Lemitar Mountains, New Mexico

(Credit: Mark Bultman, USGS. Public domain.)

We are also working in conjunction with the New Mexico Bureau of Geology and Mineral Resources and the Texas Bureau of Economic Geology who are providing geological mapping. Both of those institutions are mapping in the Cornudas Mountains and the New Mexico Bureau of Geology and Mineral Resources is also mapping in the Gallinas Mountains. 

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