Rare Earth MRI discovery in Maine
USGS geophysical survey identifies critical minerals target Metal Tech News - November 14, 2022
Last updated 4/16/2023 at 6:59am
A recent rare earth elements discovery in the far northern reaches of Maine demonstrates the value of the U.S. Geological Survey's Earth Mapping Resources Initiative, or Earth MRI, an aptly named project under which the USGS is partnering with state geologists to scan America's geology and critical mineral resource potential.
Much like the Earth MRI name implies, the Maine project began with airborne magnetic and radiometric geophysical surveys that provided a picture of the rock formations hidden beneath the surface along a 100-mile-long area of northern Maine.
The magnetic survey allows geoscientists to "see through" nonmagnetic rocks and cover such as vegetation, soil, and water to identify magnetic anomalies that reveal geological features deep within the Earth.
The radiometric data shows natural low-level radiation in rocks and soil that provide geoscientists with clues to the type of mineralization that lies below.
The data from these geophysical scans offer an MRI-like view of otherwise unseen geology and information on where buried mineral deposits might be located.
It did not take USGS research geophysicist Anji Shah long to recognize a nearly half-mile-wide bright red anomaly on a map from the northern Maine Earth MRI scan that represented a radiometric hot spot in the field of greens and blues that looked a lot like the signature of other areas where deposits of rare earths and other critical minerals had been discovered.
"This is really exciting scientifically," said Shah. "As we were examining the data, all of a sudden, this feature caught my eye. I knew immediately we had something special here."
The USGS informed Maine geologists of the exciting geophysical anomaly with the fingerprint of a significant rare earth discovery.
Exploring the Maine anomaly
Chunzeng Wang, professor of earth and environmental sciences at the University of Maine at Presque Isle, wasted no time heading 40 miles northwest to Pennington Mountain where the anomaly was identified.
"It's a perfect example of the science working just as intended and the importance of scientific collaboration," said Amber Whittaker, a scientist 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."
After hiking up to the area of the radiometric anomaly, Wang began gathering samples of some very interesting rocks in the area.
Lab analysis showed the rocks collected by the University of Maine professor contained rare earths needed for electric vehicle motors and high-tech devices, niobium used in high-strength steels and superalloys, and zirconium for 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," Wang explained. "Northern Maine is full of amazing geologic wonders. You never know what is next to discover."
Wang and his colleagues discovered that the rare earths and other critical minerals at Pennington Mountain were found 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.
Mapping northern Maine
With the Earth MRI geophysical scan identifying an intriguing anomaly and Wang's sampling expedition providing the geochemical proof that the big red target on Pennington Mountain did indeed represent a potentially significant critical minerals discovery, Maine geologists were eager to carry out further investigations.
These endeavors are being funded by STATEMAP, a project within the USGS National Cooperative Geologic Mapping Program aimed at bolstering the geological mapping of areas determined to be vital to the economic, social, or scientific welfare of states such as Maine.
"Northern Maine was geologically ignored for many decades until recently, when MGS (Maine Geological Survey) received funding to conduct a multi-year bedrock mapping project with the support of the USGS STATEMAP program," said Wang. "The MGS geologic maps helped narrow down the locations where more information could make a difference."
Future investigations of Pennington Mountain will aim to determine the depth of this critical mineral deposit, how it formed, and which specific minerals contain the rare earth elements identified to date.
Years of geological and environmental baseline studies will need to be carried out by a mining or mineral exploration company before it could be determined whether the geophysical anomaly identified by Earth MRI could support a mine that is both economically and environmentally sound.
Given what they learned so far, Wang and Slack believe similar rare earths-enriched deposits may exist in other areas of northern Maine, and scientists hope to apply the lessons learned at Pennington Mountain elsewhere in the U.S. and in the world.
"It shows how much there is still to discover about Maine's geology," said Whittaker.
Which is further evidence that Earth MRI is achieving the USGS's primary objective for the program – "identify areas with potential for undiscovered critical mineral deposits that could reduce U.S. mineral import dependence, thereby strengthening national security, creating jobs within the private sector, and generating ancillary economic and social benefits."
Further details of the Pennington Mountain rare earths discovery can be read at A Recently Discovered Trachyte-Hosted Rare Earth Element-Niobium-Zirconium Occurrence in Northern Maine, USA published in the scientific journal Economic Geology on Nov. 4.