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.