Interior Secretary Ryan Zinke on Friday opened up the possibility of hard-rock mining on the borders of the Boundary Waters Canoe Area and Voyageurs National Park, reversing an Obama-era decision that might have protected the wilderness for decades.

In an opinion posted on the agency’s website, officials said that contrary to its decision a year ago, the Interior secretary does not have the discretion to deny Twin Metals its leases for copper and nickel mining.

The immediate effect of the decision was unclear Friday. But it is likely to reignite the bitter fight in Minnesota over whether the state’s wilderness crown jewels, the Boundary Waters and Voyageurs, are at such risk from copper-nickel mining that trillions of dollars worth of precious metals should remain in the ground for decades in order to protect it.

It was only the most recent in a series of decisions by the Trump administration to reverse President Barack Obama’s pro-environmental actions. Others have included the shrinking of national monuments, allowing oil drilling in the Arctic National Wildlife Refuge, undoing clean air and water rules, and pulling out of the international Paris climate accord.

Twin Metals, a subsidiary of Chilean mining giant Antofagasta, said it was pleased with the decision, which it says affirms property rights. It has withdrawn a federal lawsuit filed a year ago that challenged the denial of its leases.

Becky Rom, founder of Save the Boundary Waters, the nonprofit that led the successful campaign to temporarily halt mine exploration and initiate the ongoing federal environmental review of mining in the BWCA watershed, said that her organization will file suit.

Interior Secretary Ryan Zinke speaks at the Interior Department in Washington on April 26, 2017.

“This is an end-run by a foreign mining company to exercise its political power to take from Americans one of their most precious natural resources,” she said.

Gov. Mark Dayton, who also opposes mining near the Boundary Waters and had denied the company access to state-owned land for its operations, echoed the same concern. “This shameful reversal by the Trump Administration shows that big corporate money and special interest influence now rule again in Republican-controlled Washington,” he said in a statement. “We will have to uncover why the financial interests of a large Chilean corporation, with a terrible environmental record, has trumped the need to protect Minnesota’s priceless Boundary Waters Canoe Area.”

House Speaker Kurt Daudt, R-Crown, applauded the decision as a boost for mining and jobs in Minnesota.

“It’s refreshing to have an administration that understands the importance of mining to Minnesota — and the entire United States,” Daudt said. “I hope this is just the start of federal efforts to remove unnecessary obstacles to job creation and economic development so we can grow jobs and paychecks in Minnesota and across the country.”

Around Ely, where the underground mine would be located along the Kawishiwi River, which flows into the Boundary Waters, opinions were as split — just as they have been for decades when it comes to the wilderness area.

“This is great news,” said Nancy McReady, founder of Conservationists With Common Sense, an Ely-based group that advocates for mining. “It couldn’t have come at a better time.”

But Bob Tammen, a retired miner and an outspoken critic of mining, said he fears that the most pristine area of the state will look a lot like the Iron Range a decade from now. “I’m afraid we are going to let the mining industry make another mistake,” he said. “There is not much doubt that we will end up with degraded water.”

Geologists say that about two-thirds of the known precious-metal mineral deposits in Minnesota lie within the Rainy River watershed, and more than half are controlled by the state and federal governments. The watershed holds some of Minnesota’s primary deposits of precious metals, but it also drains into a pristine and much-loved wilderness and Voyageurs National Park.

Twin Metals would have been the first to mine near the wilderness. It proposed a massive $2.8 billion underground mine and other facilities and hundreds of jobs. It also said that it has already invested $400 million in exploration and pre-feasibility planning.

But a year ago, after a long campaign by environmental and outdoor groups, the Department of the Interior rescinded the leases and launched a two-year environmental review to determine if mining presented significant risks in the region.

The affected area, 234,000 acres of Superior National Forest land, is about 50 times bigger than the leases held by Twin Metals Minnesota.

It’s not clear whether the latest opinion would lift that two-year ban, or halt the federal environmental review that’s been underway for almost a year.

If long-term protections had been granted, it would have put that region on a par with other places across the country, such as the Grand Canyon, Yellowstone National Park, and the Front Range in Montana, where mining has been halted because of the risk it poses to outstanding national treasures.