Waste rock from long-closed mines in the eastern Adirondacks of New York state may prove valuable due to its rare earth element content, Kallanish learns from new research from the US Geological Survey and its partners.

The REEs are mostly contained in millimetre-sized crystals known as fluorapatite that are found in deposits of iron ore and the mine tailings. When iron ore was mined, the fluorapatite was usually left behind as waste because it was an unwanted impurity. The fluorapatite has elevated levels of heavy REEs such as gadolinium, used in medical imaging, terbium used in cell phones and yttrium used in lasers, the federal agency says.

Now those waste products are of interest as the US tries to ramp up development of REEs that are critically needed for advanced technologies. It must import its REEs from China and other countries.

The eastern Adirondacks in northeast New York were heavily mined for iron ore in the 1800s and 1900s. That mining activity has resulted in piles of waste rock and mill tailings or the residuals of ore processing in various areas throughout the region.

USGS scientists were able to detect both mineral deposits and larger mill tailings from December 2015 aerial surveys of old iron mines. They then analysed samples that showed rare earth content from the deposits, waste and mill tailings.

“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,” says USGS scientist Ryan Taylor who led the analyses of rock samples in the Adirondacks. “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.”

USGS Director Jim Reilly in a statement says the Adirondack discovery has created an “emerging area of study” that is expected to expand to other states.

The amount of REEs in the Adirondacks varies from deposit to deposit but all the deposits were enriched with heavy REEs which are far less common than light REEs. Total REEs range from zero to 2.2% for the waste and tailings piles and zero to 4.8% for ore, the USGS says. The concentrations found are significant and comparable to clay deposits in China and are higher than coal fly ash, also a potential source of REEs, it says.

It notes that the fluorapatite also contains thorium with low levels of radioactivity and that is likely to complicate handling, but the thorium makes it easier to find the mill tailings in aerial surveys. Magnetic surveys have also been used to detect iron ore deposits in the region.