Erstwhile waste rock from abandoned and closed mines in the eastern Adirondack Mountains, New York, may prove valuable source for rare earth elements, said a newly published research after USGS found it following two of its recent surveys.
Rare earth elements are critical for advanced technologies such as cell phones, supermagnets, computers, medical apparatus, renewable energies and advanced defense systems, which are mostly produced in China.
The results from airborne and ground surveys conducted by the U.S. Geological Survey and collaborators. “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 here.
USGS scientists were able to detect both mineral deposits and larger mill tailings piles from airplane surveys of old iron mines in December 2015 and analyzed samples that showed rare earth minerals from the deposits, waste and mill tailings.
In the 1800s and 1900s, the eastern Adirondack Mountains in upstate New York were heavily mined for iron, which played a key role in industrialization of the Northeast prior to and during World War II. The mining activities left behind piles of waste rock and mill tailings (the residuals of ore processing) 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.”
What are rare earth minerals?
The rare earth elements are mostly contained in millimeter-size crystals known as fluorapatite in deposits of iron ore. When the iron was mined, the fluorapatite was usually left behind as waste since it was considered an unwanted impurity. But over a period, these waste products gained value and 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. 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. It may be seemingly low, but these concentrations 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.
“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. When these airplane surveys were conducted for detecting the iron ore bodies based on 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.
Three-dimensional model of the iron ore deposits in eastern Adirondack Mountains, New York. Pink areas show high magnetic susceptibility, representing ore bodies. Small gray spheres represent known mines.
In addition to mine tailings in the Adirondacks, the USGS is looking into rare earth element occurrences elsewhere in the US, 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 too.