BILLINGS, Mont. — U.S. officials approved a 117 million-ton expansion of a Montana coal mine after concluding that burning the fuel would have a minor impact on the nation’s overall greenhouse gas emissions, according to documents released Thursday.

The expansion of the Spring Creek Mine, Montana’s largest coal mine, would generate roughly 160 million tons of carbon dioxide over the next five years, according to an Interior Department analysis of the project that was conducted under a court order.

Those emissions would be about one half of 1 percent of projected annual U.S. emissions of the climate changing gas in 2020. Even if the agency blocked the mine expansion, federal officials said power plants served by Spring Creek could obtain coal from mines on private reserves, negating any possible decrease in emissions.

The expansion was first approved in 2012, then held up by environmentalists waging a legal campaign in courthouses across the region to stop or delay mining on public lands in the Western U.S.

The Powder River Basin of Wyoming and Montana accounts for about 40 percent of annual U.S. coal production, most of that from public lands.

Spring Creek is Montana’s largest coal mine, located near the town of Decker along the Wyoming border. It’s owned by Cloud Peak Energy.

Environmentalists with WildEarth Guardians had sued the Interior Department to challenge the expansion of the mine, which extracts coal from publicly owned reserves, saying it would make climate change worse.

As a result, U.S. District Judge Susan Watters ordered in January a rigorous study of the planned expansion, saying that an earlier review by Interior had failed to take a hard look at its environmental impacts.

Jeremy Nichols with WildEarth Guardians, one of the groups that sued over the expansion, said the government’s new analysis is better but still reaches the wrong conclusion.

“We want Cloud Peak to shut down and go away. I’ll be totally honest about that,” Nichols said. “(Federal mining officials) want to say that this is a drop in the bucket. But every drop matters. ... This is a huge resource locking in hundreds of millions of tons of carbon emissions.”

Cloud Peak had warned of major layoffs at Spring Creek if its expansion was thwarted.