Company officials said that Deer Creek, the state’s last union-represented coal complex, has become too costly to operate. While about five years of reserves remain, most of the remaining coal is of a higher ash and sulfur content, and thus more expensive to burn in compliance with federal regulations.

“We have evaluated what is in the best interest of our customers and determined it no longer makes economic sense to keep the Deer Creek mine open,” said PacifiCorp Energy President and CEO Micheal Dunn.

The operator, PacifiCorp’s Energy West Mining Co., also confirmed it has signed a long-term coal supply agreement with Bowie Resource Partners for the supply of coal to the Huntington power plant, for which it had previously served as a minemouth operation. Bowie will supply the plant from its Utah properties.

“The new long-term coal supply agreement with Bowie will supply PacifiCorp with reasonably priced coal so we can continue to provide safe and reliable electric service to our customers,” he said.

PacifiCorp also said that rapidly escalating pension liabilities for the mine’s represented workforce was a large factor in the mine’s economic viability.

Deer Creek had been in operation since 1974. PacificCorp is an arm of Berkshire Hathaway Energy, owned by billionaire Warren Buffet.