The deal for the Lincoln County complex is effective May 1 and will be in force until April 30, 2018.
While neither party elaborated on the details of the agreement, UMWA district 22 vice-president Mike Dalpiaz said the two were able to find a workable deal three months early.
“I commend the company’s willingness to set a new and refreshing tone for a lasting relationship at that mine,” he said.
“The membership clearly does too, as they have overwhelmingly ratified the agreement.”
Westmoreland Coal chief executive officer Keith Alessi said the agreement provided the company with certainty to “continue to serve the needs of our customer base”
Chevron Mining sold Kemmerer to Westmoreland in December.
The $US76.5 million deal included the assumption of $118 million in liabilities, including post-retirement medical, pension, black lung and asset retirement obligation liabilities.
Officials said the transaction included about 118 million tons of coal reserves as of December 31, 2011, enough for 20 years of production.
Substantially all of Kemmerer’s projected production for 2012 through 2016 is committed and priced under existing sales contracts.
The Denver-headquartered producer is the oldest independent coal company in the US.
It has operations in the Powder River Basin of Montana, sub-bituminous mining interests in Wyoming and lignite mining operations in Montana, North Dakota and Texas.