BILLINGS — Montana officials are considering giving only partial approval to a $500 million silver and copper mine proposed beneath a wilderness area near the Idaho border, injecting uncertainty into a project that has been in the works for more than a decade.

Regulators have continuing concerns with the Montanore Mine’s potential effect on surrounding waterways, said Tom Livers director of the Montana Department of Environmental Quality.

“We’re not sure we can approve the full mining plan that’s been proposed,” Livers told The Associated Press. “We’re looking at what we can approve. Some may have to happen subsequently as we get more information.”

A decision is expected in late January.

The project is sponsored by Mines Management Inc. of Spokane. It would disturb more than 1,500 acres and remove up to 120 million tons of ore from beneath the Cabinet Mountains Wilderness in Lincoln County, south of Libby, Montana.

Mines Management chairman Glenn Dobbs says anything less than full approval could make it difficult to attract investors or force a sale to a larger mining company that would delay the project.

Just last month, the mine appeared to have cleared one of its most significant hurdles when the DEQ and and U.S. Forest Service finalized a long-awaited environmental review of the project.

“To imply he [Livers] might not be able to approve the full mine plan in which his agency has been integrally involved in for 10 years is an absurdity,” Dobbs said.

Livers said he was limited in what he could say about the state’s intentions because a final decision had not been made. But he added that the agency was reluctant to approve a permit that has “legal weaknesses” because it would likely face a court challenge given strong opposition to Montanore from environmental groups.

Work on the mine site began around 1990 under different ownership but was suspended in 1991 due to low metal prices. Mines Management later took over and applied to the state for a mining permit in 2004.

The mine would employ about 500 people during the construction phase and about 350 during mining. A related 14-mile transmission line would be built to carry power to the site.

The mine entrance would be just outside the wilderness — a rugged, remote landscape that is one of a handful of areas in the United States where the government is seeking to restore grizzly bear populations.

The Fish and Wildlife Service has said mining excavation would lead to permanent changes in groundwater flows, including less water in Rock Creek and the Bull River.

Once a decision is made, Mines Management had planned to kick off a two-year, $30 million evaluation of the geology and characteristics of the area to be mined.

Dobbs said he had spoken with Libby officials and representatives of Montana’s Congressional delegation about the issue since the DEQ first raised the possibility of a “phased approval” for the mine.

U.S. Sen. Steve Daines and Rep. Ryan Zinke, both Republicans, wrote in a Thursday letter to Gov. Steve Bullock that the project needs full approval.

A staged approval would “create uncertainty and further delay much-needed job creation and tax revenue for the Lincoln County community, which continues to face the highest unemployment rate in our state,” they wrote.