In a statement on Wednesday, Cliffs said it intended to idle the mine because of the current poor market conditions for metallurgical coal.

Cliffs issued a Worker Adjustment and Retraining Notification to trigger the 60-day notice period for temporarily idling operations.

“If market conditions do not improve, it is expected that the idling of the Pinnacle mine could last more than six months beginning on or about August 25, 2014,” the company said.

Earlier this month Caterpillar revealed the mine had set a new daily low-seam coal production record using its automated longwall plough system – with 32,411 metric tonnes produced over a 24 hour period on April 10-11 from the 1.42m high Virginia Pocahontas No 3 seam.

Cliffs’ iron ore operations are also feeling the pinch from lower commodity prices.

Investment website Motley Fool commented that Cliffs was not in any immediate danger with the company having $US364 million ($A387.3 million) in cash and cash equivalents and access to a $1.7 billion revolving credit line at the start of June quarter.

“However, you should definitely keep an eye on iron ore prices,” it commented.

“If prices don't bounce back within the next year or so, Cliffs' credit line could eventually be depleted or revoked. Both cases could result in bankruptcy.”