Waste rock from long-closed mines in the eastern Adirondack Mountains, New York, may prove valuable due to its rare earth element content, according to newly published research. The results are from airborne and ground surveys conducted by the U.S. Geological Survey and collaborators.
Mine Waste in the Eastern Adirondacks May be Untapped, Accessible Source of Rare Earth Element Minerals
Rare earth elements are critical for advanced technologies such as cell phones, supermagnets, computers, medical apparatus, renewable energies and advanced defense systems. Currently, the U.S. depends heavily on imports for rare earth mineral resources.
“The possibility of accessing rare earth elements from mine waste and mill tailings is attractive partly because the minerals have already been excavated from the ground,” said USGS scientist Ryan Taylor, who led analyses of rock samples from the region. “This would reduce mining costs by making it easier to access the minerals. It also allows ‘recycling’ of discarded materials, which could help to remediate these mined areas,” he said.
USGS scientists were able to detect both mineral deposits and larger mill tailings piles from airplane surveys of old iron mines that occurred in December 2015. They then analyzed samples that showed rare earth content from the deposits, waste and mill tailings.
The eastern Adirondack Mountains in upstate New York were heavily mined for iron in the 1800s and 1900s and played an important role in industrialization of the Northeast prior to and during World War II. The mining activities resulted in piles of waste rock and mill tailings (the residuals of ore processing) in various areas throughout the region.
“This inspired the USGS, New York state, and local groups to work together to analyze waste rock and tailings,” said USGS Director Dr. Jim Reilly. “We look forward to future collaborations with them and potentially other states in this emerging area of study.”
The rare earth elements are mostly contained in millimeter-size crystals known as fluorapatite that are found in deposits of iron ore. When the iron was mined, the fluorapatite was usually left behind as waste because it was considered an unwanted impurity. As the need for certain mineral resources changed with time, these waste products are now of interest. The fluorapatite has elevated levels of heavy rare earth elements such as gadolinium, which is used in medical imaging; terbium, which is used in cell phones; and yttrium, which is used in lasers.
The amount of total rare earth elements varies from deposit to deposit, but each deposit is enriched in the heavy rare earth elements, which are far less common than the light rare earth elements. Total rare earth elements range from zero to nearly 2.2% for the waste and tailings piles and zero to nearly 4.8% for ore. Although seemingly low, these concentrations actually indicate significant potential, with the higher grades even comparable to other heavy rare earth element deposits, such as the clay deposits in South China, which are one of the primary sources for China’s rare earth elements and the primary source of the world’s heavy rare earth elements. These grades are also higher than those reported for coal fly ash, a residual of coal combustion that has also been considered a potential source of rare earth elements.
“There may be some challenges to processing the tailings for rare earth elements,” said USGS scientist and project chief Anji Shah. “While the fluorapatite contains recoverable rare earth elements, it also contains thorium, a weakly radioactive element which has economic uses but also requires careful handling."
That thorium, however, makes mill tailings easier to locate by airplane surveys since natural, low-level radioactivity can be seen on the images, Shah explained.
The airplane surveys were also helpful for detecting the iron ore bodies themselves because the iron is contained in highly magnetic crystals of magnetite. By measuring subtle variations in Earth’s magnetic field from the sky, the researchers created 3D models showing the size and shape of the deposits beneath Earth’s surface.
In addition to mine tailings in the Adirondacks, the USGS is looking into rare earth element occurrences elsewhere in the country, such as in clays in the Southeast or phosphate rocks throughout the U.S. Efforts to map rare earth-bearing formations are also ongoing at the rare earth mine in Mountain Pass, California.