The company has not disclosed the value of the deal nor the seller.
Rosebud Mine in the northern Powder River Basin has an average annual production of 12.3 million tons including permitted operating acreage of 25,576 acres.
Westmoreland said the mine delivers almost all of its coal to the four-unit 2,100MW Colstrip Power Station under a deal to come to an end in 2019.
The Rosebud coal has an approximate heat content of 8,550 BTU/lb and 0.64% sulphur, while equipments removing coal include haul trucks, end loaders and excavator and delivery is through truck, rail and conveyer.
Westmoreland controls an estimated 486 million tons of proven and probable coal reserves across all of its operations as of 30 June 2012.
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