ST. PAUL — A state appeals court has blocked a key permit for the proposed PolyMet copper-nickel mine in northeastern Minnesota.

The Minnesota Court of Appeals on Tuesday stayed the project’s water quality permit. Minnesota Public Radio News reports environmental groups and the Fond du Lac Band of Lake Superior Chippewa had asked for the permit stay.

Environmentalists contend the Environmental Protection Agency’s concerns about how Minnesota regulators drafted the permit should have been reflected in the final permit but were not.

The appeals court concluded that halting the permit is warranted.

PolyMet would be the state’s first copper-nickel mine. Opponents fear it could send sulfuric acid and other pollutants downstream as far as Lake Superior.

PolyMet says the company remains confident the water quality permit “meets all applicable standards” and will be upheld.