The agency overturned a previous approval permitting Arch Coal to add approximately 1700 acres to its exiting lease on federal land in western Colorado – a green light which was issued after officials determined there were no adverse impacts.

According to the Denver Post, the initial decision was subsequently challenged by environmental groups the Sierra Club, High Country Citizens’ Alliance, WildEarth Guardians and Defenders of Wildlife and further review by the USFS found the first assessment was inadequate.

Specifically, USFS spokesperson Steve Segin told the Associated Press the appeal found there was no explanation as to why some lease conditions for wildlife protection were being changed and it was looking into the issue.

Also, the groups complained wells would need to be drilled above the mine for methane liberation, an action which would increase the pollution to the natural area.

Arch Coal spokesperson Kim Link told ILN on Thursday that it was cooperating with the review.

“West Elk will be working as quickly as possible to address the concerns outlined by the US Forest Service,” she said.

She did not provide further comment on specific concerns or a timeline for submitting the needed information.

However, she noted the mine, which employed over 375 workers and produced 6 million tons of compliance coal in 2001, was a large contributor to Colorado’s economy.

“The mine contributed nearly $US24 million in taxes and royalties to Colorado,” she said.

“West Elk’s payroll totaled $41.2 million in 2011 alone.”

West Elk is the second-largest producer in the state, next to Peabody Energy’s Twentymile operation, which totaled 7.7Mt last year.

Meanwhile, one environmental group called the suspension – albeit temporary – a “win” for the state’s natural resources.

“The Forest Service should protect these roadless lands and habitat by putting this damaging mine expansion plan to bed permanently,” WildEarth Guardians climate and energy director Jeremy Nichols said.