A new National Science Foundation grant is helping researchers at Washington State University investigate how certain geologic systems form critical ore deposits, including at Mountain Pass mine in California’s Mojave Desert, the largest source of rare earth elements in the United States.
Erin Benson, an assistant professor in the School of the Environment and faculty fellow with the Institute for Northwest Energy Futures (INEF), received the funding to study the geologic processes that concentrated rare earth elements at Mountain Pass. The research could ultimately help geologists identify similar deposits elsewhere.
“When we think about critical minerals, we’re thinking specifically about geological commodities that fulfill two criteria,” said Benson. “The first is that they have a supply chain at risk, for instance, if the material is sourced from a country with which we have a fraught relationship. The second criterion is the measure of economic impact if that supply is disrupted. Rare earth elements rank very highly for both measures of criticality, even though they’re only a small number of the minerals on the US critical minerals list.”
Rare-earth elements (REEs) are a set of 17 soft heavy metals that are commonly used in technologies such as advanced electronics, renewable energy, and medical imaging. In most rocks, REEs are dispersed as trace impurities, making extraction much more difficult.
Benson and her collaborators are specifically investigating the geologic formation of carbonatites, a rare type of igneous rock produced in a volatile-rich geologic system. Carbonatites deposits host higher concentrations of REEs than other deposits, making these geologic features a key target for mineral extraction.
“Carbonatites as a whole are not particularly well understood, since they are a really rare rock type,” said Benson. “But carbonatites host over 50% of rare earth element deposits and over 86% of what is mined, so if you’re looking for a rare earth element deposit, you’re probably looking for a carbonatite.”
Benson explains that carbonatites are full of volatile elements, such as carbon dioxide and water, as well as molecules that bond easily with rare earth elements, such as sulfate and fluorine. This geologically unique mix of elements helps to move REEs around within the cooling rock, forming high concentrations of REEs in the carbonatite deposit. The exact balance of elements that forms a large REE ore deposit is still debated.
“We are determining how different fluid compositions result in high concentrations of REEs in the rock, and what fluid composition is needed to create economic deposits,” said Benson. “Eventually we hope to test our model at other carbonatite localities and see how our applied model translates to other occurrences.”
Once this research project is complete, Benson hopes the data model she is developing will help identify additional ore deposits with high concentrations of REEs, creating a domestic and secure source chain for these crucial materials.
“Mineral exploration can be very expensive. If you can look at altered rock and determine that the alteration wasn’t caused by fluids of the correct composition, then you can eliminate the area and move on to other locations, ultimately saving time and resources,” said Benson.