The Minnesota Supreme Court on Tuesday declined to hear a challenge by environmental groups over the state's copper-nickel mining rules.

Environmental groups argue the Department of Natural Resources' rules regulating the mining of metals that do not contain iron — such as copper, nickel and other precious metals — were too vague and, therefore, unenforceable. The DNR maintains the rules were strong yet flexible.

But in August, Minnesota Court of Appeals unanimously upheld those rules, and called the DNR's non-ferrous rules "valid."

Six environment groups had filed the original appeal, but only two groups, the Friends of the Boundary Waters Wilderness and the Minnesota Center for Environmental Advocacy, petitioned the Supreme Court to review the case after the Court of Appeals upheld the rules.

Friends of the Boundary Waters spokesperson Pete Marshall said the group was "extremely disappointed" Tuesday.

"As it stands, an unelected official, who is subject to immense political pressure, calls the shots," Marshall said. "This is good for the mining companies but not the people of Minnesota."

Jon Cherry, president and CEO of PolyMet, the mining company trying to open the state's first copper-nickel mine near Hoyt Lakes and Babbitt, said the company was pleased with the decision.

“The state has some of the strictest environmental regulations in the country, and we’ve proven that we can meet those standards,” Cherry said.

The MCEA did not immediately respond to a request for comment Tuesday afternoon.

While Tuesday's decision by the Supreme Court puts the non-ferrous mining rules issue behind PolyMet, the company faces numerous other legal challenges.

The Court of Appeals suspended PolyMet's permit to mine and dam safety permits last month after two post-permitting developments: Glencore took a 72% stake in PolyMet in June and a Vale tailings dam collapsed in Brazil in January, killing more than 200 people.

Last week, it extended that suspension one day after the court heard oral arguments from environmental groups and the Fond du Lac Band of Lake Superior Chippewa who want the court to reverse the permits.

Separately, PolyMet's national pollutant discharge elimination system, or NPDES, permit, which regulates water discharged from industrial activities, remains on hold after an August order by the Minnesota Court of Appeals.