According to a July 7 filing by the company with the US Securities and Exchange Commission, the US Mine Safety and Health Administration issued the order to the operation near Birmingham because of elevated and decreased oxygen levels on June 30.

All non-essential personnel were immediately evacuated from the location of the issue and no injuries or accidents were reported.

MSHA terminated the order on July 5 and production resumed the same day.

Cliffs officials said in the filing that it did not expect the situation to have a material adverse impact on the mine’s operations.

The producer did not release any further public statement on the event.

Imminent danger orders are issued by MSHA under section 107(a) of the Federal Mine Safety and Health Act, also known as the Mine Act.

Section 1503 of the Dodd-Frank Wall Street and Consumer Protection Act, amended last July, now requires disclosure of all imminent danger incidents as part of new reporting requirements regarding mine safety.

Production at Oak Grove recommenced in mid-May after a severe storm and tornado damaged the complex’s preparation plant, overland conveyor system and other infrastructure on April 27 and idled operations.

In the wake of the storm, Cliffs assisted with local recovery by donating $US100,000 to the American Red Cross for tornado relief efforts.

It also provided $100,000 collectively to local organizations for basic essentials going to victims within the Oak Grove mine's communities and established a relief fund so that its employees could make contributions to support relief work.

The Oak Grove mine in Adger, opened in 1975, extracts metallurgical coal from the Blue Creek seam. It employs about 310 workers and produced 877,000 tons in 2009.