The state Court of Appeals has affirmed Minnesota's rules governing hard-rock mining, delivering a blow to environmentalists who challenged them as too vague to protect the state's natural resources for a new era of excavation on the Iron Range.

The case was the first legal test of Minnesota's environmental rules for the industry, as two international minerals firms prepare to build the first copper-nickel mines in Minnesota's water-rich northeast. Hard-rock mining carries much greater environmental risks than the iron ore and taconite mines that have long dominated the region.

In an unanimous opinion issued Monday, the appellate judges said their focus was limited to whether a chapter of rules governing nonferrous mining, issued by the state Department of Natural Resources (DNR), exceeded statutory authority or violates constitutional provisions. It doesn't, the three-judge panel concluded.

"Petitioners' true complaint appears to be that chapter 6132 does not impose more specific and universal limitations on nonferrous mining," they wrote. "This complaint is more appropriately directed to the Legislature or the DNR."

DNR Deputy Commissioner Barb Naramore issued a statement saying the agency is pleased the judges affirmed the rules, which were developed with significant public input in the early 1990s.

"We continue to believe that the current nonferrous rules fundamentally provide an effective framework for implementing our regulatory responsibilities and ensuring protection for public health and the environment," Naramore said.

Jon Cherry, president and chief executive of PolyMet Mining Corp., which is first in line to open a copper-nickel mine in the state, said in a statement that Minnesota's rules are among the strictest in the world. "We demonstrated through the extensive environmental review and permitting process that we can meet or exceed these standards," Cherry added.

The conservation groups who challenged the rules were not swayed by the decision.

"Nonferrous mining presents new and unknown dangers, and DNR's rules are not sufficient to protect Minnesota's resources," said Kevin Reuther, legal director for the Minnesota Center for Environmental Advocacy.

The center represents five other conservation groups; all are considering an appeal to the state Supreme Court.

Friends of the Boundary Waters Wilderness, a co-petitioner, said it has already decided to ask the state's high court to hear the case. The group's executive director, Chris Knopf, said Minnesota's metals mining rules allow new projects to move forward "under the guise of a rigorous environmental review."

"Until these standards change, Minnesota will forever be at risk from dangerous international mining conglomerates with terrible histories of pollution like Glencore and Antofagasta," Knopf said.

PolyMet is the first hard-rock mine project to become fully permitted under Minnesota's nonferrous rules, a decadelong process that finished in late 2018, and is now raising financing to begin construction. The company faces several legal challenges — including three inquiries into how state and federal regulators handled PolyMet's crucial water pollution permit — and various groups, including some state lawmakers, are pushing for the state to stay the permits.

PolyMet plans to build an open-pit copper nickel mine near Babbitt, near the headwaters of the St. Louis River, which flows into Lake Superior, and upstream from the Fond du Lac Band of Lake Superior Chippewa.

PolyMet, which is based in Toronto with an office in St. Paul, is majority owned by Switzerland-based Glencore, one of the world's largest mining concerns.

A second firm, Twin Metals Minnesota, plans to submit its formal mine plan early next year, starting the permitting process for an underground copper-nickel mine on Birch Lake near Ely, just outside the Boundary Waters Canoe Area Wilderness. Twin Metals is owned by Antofagasta, a Chilean firm that is one of the world's largest copper producers.