Just a few days after the state signed off on the environmental review of the proposed PolyMet copper-nickel mine, Gov. Mark Dayton has announced he opposes mining near the Boundary Waters Canoe Area. Twin Metals Minnesota wants to build a huge underground mine near Ely.

In the letter to Twin Metals Chief Operating Officer Ian Duckworth, Dayton said he has "grave concerns about the use of state surface lands for mining-related activities in close proximity to the Boundary Waters Canoe Area Wilderness."

He's directed the Department of Natural Resources "not to authorize or enter into any new access or lease agreements for mining operations" there. In the letter, Dayton said he has an "obligation to ensure (the BWCA area) is not diminished in any way," and that he's "unwilling to take risks with that Minnesota environmental icon."

Becky Rom with Northeastern Minnesotans for Wilderness said the governor's letter is meaningful.

"It's a big deal because he is saying that the Boundary Waters is too important to put at risk," Rom said. "This type of industrial mining has never been done without polluting water."

Twin Metals wants to mine rich copper and nickel deposits that are bound in rock containing sulfide. When that rock is exposed to air and water, it can create sulfuric acid drainage that can leach heavy metals into waterways.

Rom and others argue the Superior National Forest just south of the BWCA is the wrong place for that scale of environmental development.

"The governor agrees. He views the Boundary Waters as a national treasure," Rom said. "It's iconic. It's incredibly important to Minnesota, and he's decided this is simply the wrong place for a sulfide ore mine."

In a statement Twin Metals said it received Dayton's letter late Monday afternoon and is "currently assessing the Governor's position."

But Frank Ongaro, executive director with Mining Minnesota, said the letter raises great concern.

"This calls into serious question the governor and the state honoring contractual agreements of state mineral leases that one company or a variety of companies already have and are paying money for rent on those leases to the state of Minnesota," Ongaro said.

For the past decade Twin Metals has been exploring for copper, nickel and precious metals near Birch Lake and the South Kawishiwi River, about 15 miles southeast of Ely.

The company has forecasted that an underground mine could operate for 30 years and employ 850 people. Twin Metals had planned to spend $2.8 billion to construct the mine. The company has said it is a year or more away from submitting a plan to the state that would start the environmental review process.

Owned by a giant Chilean mining company, Antofagasta, Twin Metals holds a variety of mineral leases on private, state and federal land. The governor's letter only affects state land. But Ongaro said it still would have a big impact.

"If you take any one of them off the table and make it prohibitive, you've really made it difficult for mineral development," he said. "You just don't develop 40 acres of state minerals that might be next to 40 acres of federal minerals. Those are all part of an overall patchwork puzzle that needs to be looked at for overall development opportunities and commercial viability."

Ongaro said he still needs clarification on what broader impacts the letter might have. It also potentially affects several other companies exploring in the Boundary Waters watershed.

Critics of the PolyMet mining proposal also contend that polluted water from that project could flow north toward the BWCA, although the DNR disputes that.

Dayton said the federal government will offer further guidance on the future of mining in the area. Twin Metals has applied to the Bureau of Land Management to renew two federal mineral leases and is awaiting its decision. Dayton said he told the BLM's director last week of his strong opposition to mining in close proximity to the Boundary Waters.