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From rare earths in Northern Maine to lithium in Southern California and graphite in Alaska, the U.S. Geological Survey is on a mission to discover minerals critical to the nation's economy and clean energy goals on American soil. Or, more accurately, under American soil.

This nationwide endeavor is officially called the Earth Mapping Resources Initiative, but is better known as Earth MRI, a clever moniker that reflects the earth penetrating scans that are providing geoscientists with a glimpse of the rocks hidden below the surface.

Carried out in partnership with the Association of American State Geologists, Earth MRI is providing a much more detailed understanding of America's geology and mineral resource potential through new mapping, geophysics, and geochemical sampling.

What makes the name so clever is the subsurface imaging reminiscent of a medical MRI. Much like doctors use magnetic resonance imaging to help diagnose and pinpoint exactly what is going on inside a body, geoscientists can use the Earth MRI data to see what is below the surface without the need for probes or exploratory surgery.

These subsurface images are produced with geophysics, which utilizes a cadre of sensing techniques and equipment to identify various rock characteristics that provide a picture of the geology below the surface.

Two geophysics techniques, airborne magnetic and radiometric geophysical surveys, are particularly well-suited for the initial critical minerals assessment USGS has set out to achieve with its Earth MRI scans.

The magnetic surveys allow geoscientists to "see through" nonmagnetic rocks and cover such as vegetation, soil, and water to identify magnetic anomalies that reveal geological features and potential accumulations of metals within the Earth.

The radiometric surveys identify natural low-level radiation potassium, uranium and thorium in rocks and soil, which provides clues to the type of mineralization that lies below.

Geoscientists are using the data from these surveys to help identify the best locations to search for critical minerals.

"The airborne geophysical surveys help pinpoint the areas where follow-up geologic mapping efforts will provide the most value," said Eric Anderson, a USGS research scientist, explained during a $1.9 million Earth MRI survey of Montana announced in July.

While Earth MRI was launched as part of a 2019 strategy to ensure a reliable domestic supply of critical minerals, this program expanded into a truly nationwide program with the passage of the Bipartisan Infrastructure Law and $320 million to fund the program for five years.

"These historic investments will modernize our mapping of the United States," said Sarah Ryker, USGS associate director for energy and mineral resources.

Federal resources, local knowledge

The USGS partnership with state geologists plays a vital role in determining the best places in the nation to look for the 50 minerals and metals critical to the U.S.

"Merging federal resources with local knowledge of the state surveys creates an efficient and thorough venue to quickly further national understanding of the distribution of our resources," said Erin Campbell, president of the Association of American State Geologists.

Funneling some of the Earth MRI money down to the state level also equates to more rock hammer-toting geologists mapping and sampling the most prospective regions of each state, while the USGS has helicopters and planes equipped with earth-sensing geophysical equipment flying overhead.

Programs to be carried out under the funding include:

National Cooperative Geologic Mapping Program: New maps created by state geological surveys under this program are helping to refine the understanding of the geologic framework of mineral areas of interest. In addition to identifying mineral potential, these maps support decisions about the use of land, water, energy, and minerals, as well as the potential impacts of geologic hazards such as earthquakes, landslides, and volcanoes on communities.

National Geological and Geophysical Data Preservation Program: Earth MRI is partnering with state agencies to support the preservation of physical geological samples that would be costly or difficult to replace. Preserving and cataloging these samples of drill core, rock, and soil is creating rock libraries that can be referenced when geologists have new ideas or are looking for minerals that are critical to future technologies that had little or no use in the past.

USGS 3D Elevation Program (3DEP): This program utilizes private sector contractors to fill data gaps in the nation's topographic mapping in areas with potential for critical mineral resources. Lidar, a laser-based scanning of terrain to create high-resolution digital elevation models, is being used to assist in the development of more accurate maps of surface geology, which geoscientists can use to extrapolate the extent of the rock formations at depth. Lidar data can also help define the location and volume of mine waste materials exposed on the land surface.

Mineral Resources Program: Federal and state geologists are carrying out geochemical reconnaissance surveys over underexplored geologic settings, which provides initial data for planning and prioritizing future programs. This sampling of the soil and rocks exposed at surface is helping the USGS determine where to have private contractors fly airborne geophysical surveys based on the critical mineral potential seen on the surface.

The U.S. Department of Interior, which oversees USGS, says mapping of both unmined geology and the tailings left from previous mining, will help ensure minerals critical to the energy transition can be mined domestically and with strong environmental, social, and governance (ESG) standards.

So far, USGS has completed or is planning to carry out Earth MRI programs in over 39 states and Puerto Rico.

The Maine eureka moment

While the majority of the Earth MRI scans are being carried out over Western U.S. states and Alaska, which are particularly mineral-rich areas of the country, one of the initiative's most exciting early critical minerals discoveries was made in Maine.

An initial look at the geophysical maps generated from an Earth MRI scan carried out in northern Maine was a eureka moment for USGS geophysicist Anji Shah.

"As we were examining the data, all of a sudden, this feature caught my eye. I knew immediately we had something special here," recalled Shah.

This eye-catching feature was a nearly half-mile-wide bright red radiometric hot spot on a map of cooler greens and blues – a geophysical signature similar to other areas where deposits of rare earths and associated critical minerals had been discovered.

"This is really exciting scientifically," she said.

The USGS informed Maine geologists of the exciting geophysical fingerprint of a significant rare earth discovery.

Chunzeng Wang, professor of earth and environmental sciences at the University of Maine at Presque Isle, was as excited as Shah about the discovery and wasted no time trekking 40 miles northwest to Pennington Mountain, where the Earth MRI anomaly was identified.

"It's a perfect example of the science working just as intended and the importance of scientific collaboration," said Amber Whittaker, senior geologist at the Maine Geological Survey. "Having all of us involved meant as soon as we identified the feature, Professor Wang was onsite within a day to do the recon work."

Lab work showed the rocks collected from the Earth MRI anomaly by Wang and his colleagues did indeed contain rare earths, which are in high demand due to their use in electric vehicle motors and a plethora of high-tech devices. The samples also contained niobium and zirconium, critical metals used in high-strength steels, ceramics, and superalloys.

"I was astonished when seeing the analytical results that the rocks were so significantly enriched in rare earth elements and several trace metals," the professor said.

Wang and his colleagues discovered that the critical minerals at Pennington Mountain are hosted in trachyte, a type of volcanic rock known to host similar rare earth-niobium-zirconium mineralization in Australia and China.

"This discovery shows the importance of new evaluations for potential critical mineral resources based on integrated studies involving geophysics, geology, and geochemistry," said John Slack, USGS scientist emeritus from Maine who coauthored a scientific paper on the discovery with Wang.

Given what they learned so far, USGS and Maine geologists believe other rare earth-enriched deposits may exist in northern Maine.

"It shows how much there is still to discover about Maine's geology," said Whittaker.

Last Frontier for critical minerals

While the Maine discovery stands out due to the strength of the rare earth indicating geophysical anomaly in an area not well known for its critical mineral potential, the bulk of the Earth MRI scans are being carried out across the American West and Alaska.

Home to deposits and prospects enriched with 49 out of the 50 minerals deemed critical to the U.S., Alaska is considered by many as the single best place to explore for this suite of high-tech and clean energy metals.

"The Last Frontier remains a frontier for critical mineral resource development," U.S. Geological Survey Director David Applegate said during a 2022 critical minerals summit in Alaska.

Over the past two years, the USGS has allotted more than $12.5 million of Earth MRI funds to better understand the critical minerals potential across America's Last Frontier.

This includes a $6.75 million program in 2022 to explore the Yukon-Tanana Upland, a roughly 100- by 300-mile Alaska's Interior region where the city of Fairbanks is located, and a $5.8 million program this year to scan the more remote Kuskokwim Mineral Belt, which extends the 100-mile-wide critical minerals belt another 600 miles to the southwest.

While both of these regions that form a 900-mile belt across the breadth of the Last Frontier State are highly acclaimed for their gold potential, they also happen to be enriched with at least 30 of the minerals critical to the U.S.

"From the Yukon-Tanana region in the east to the Kuskokwim River in the west, we are working with the entire state of Alaska to assess domestic resource potential and secure a reliable and sustainable supply of critical minerals," said Department of the Interior Assistant Secretary for Water and Science Tanya Trujillo.

Alaska Division of Geological & Geophysical Surveys Director David LePain said, "The State of Alaska and industry exploration geologists are already greatly benefiting from the enhanced understanding of Alaska's geology and mineral-resource potential provided by the new Earth MRI geoscience data."

Scanning the American West

Much like America's Last Frontier, the "Lower 48" states west of the Rockies are abundantly enriched with critical minerals, and the USGS is investing heavily into Earth MRI scans to better quantify this potential.

Earth MRI scans over America's West include:

• More than 90% of Nevada, which is considered the most lithium-rich state in the nation and highly prospective for most of the other critical minerals.

• The majority of Idaho, including an area along the border with Montana, that hosts high-grade deposits of the rare earth element niobium.

• The southern half of Arizona, an area highly prospective for at least 15 critical minerals, including aluminum, cobalt, lithium, and manganese.

• Swaths of Wyoming prospective for more than 40 critical minerals, including indium, gallium, germanium, and rare earths.

• Areas of southwestern Montana with the potential for at least 25 critical minerals, including antimony, cobalt, and tin.

U.S. Geological Survey

A closeup of the geophysical anomaly that led to the rare earths-niobium-zirconium discovery at Pennington Mountain. Click on image for a view of the entire 2021 Earth MRI radiometric survey carried out by USGS in northern Maine.

• A stretch of western Utah enriched with nearly 20 critical minerals that include bismuth, indium, and platinum group metals.

• Parts of northern, central, and southwestern Colorado with critical minerals such as tellurium, vanadium, and tantalum.

• The southwest corner of New Mexico prospective for aluminum, antimony, rhenium, scandium, and 24 other critical minerals.

• A small southeastern Oregon region that hosts the northern half of the McDermitt Caldera, which hosts some of the richest lithium deposits in the U.S.

• A swath of northeastern Washington prospective for rare earths, tin, and critical minerals associated with copper mineralization.

"Through these investments, the U.S. Geological Survey is making critical scientific advancements in data mapping and preservation that will equip future generations," said Trujillo.

Author Bio

Shane Lasley, Publisher

Over his more than 16 years of covering mining and mineral exploration, Shane has become renowned for his ability to report on the sector in a way that is technically sound enough to inform industry insiders while being easy to understand by a wider audience.

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