The DNR said a January Minnesota Court of Appeals decision could affect future mining proposals, the state's currently operating iron mines, and a wide range of other state permit decisions, calling the ruling, "a significant departure from the DNR's long-standing application of state mining law".

Glencore-backed PolyMet filed a similar petition with the Supreme Court seeking to overturn the Court of Appeals decision that remanded the permits back to the DNR for a contested case hearing.

"We are respectfully asking the Supreme Court to right what we believe is a wrong. The court of appeals effectively opened the door to an unpredictable loop of review and additional litigation for Minnesota permittees with its interpretation of the statute. The court's ruling increases uncertainty and permitting time not only for mining projects but also for many other projects in the state that require DNR … permits," said president and CEO Jon Cherry in a statement.

In January, the Court of Appeals ruled that the DNR erred when it declined to order a contested case hearing to gather more information on the potential environmental impacts from the mine. As a result, the court sent PolyMet's permit to mine and two dam safety permits back to the DNR with an order to hold the proceeding before deciding whether to reissue the permits.

PolyMet received its final federal permit for the project in March 2019 but saw them suspended in September by the Appeal Court.

The Polymet approvals process has been ongoing for more than 10 years. A US$945 million Phase I development would create a 32,000 tonne per day operation which would produce about 1.2 billion pounds copper, 170Mlb nickel, 6.2Mlb cobalt and 1.6 million ounces of precious metals over 20 years with ore processed at a former steel plant.

Opposition to the project is led by the Minnesota Centre for Environmental Advocacy, Friends of the Boundary Waters Wilderness and Water Legacy.

Shares in PolyMet Mining are trading at US$28c, valuing the company at $277 million.