Six of seven injured miners have returned home following the Wednesday night rock burst inside Lucky Friday Mine that was strong enough to register on seismographs.
The failure of the Mullan, Idaho, mine’s rock wall trapped the miners 5,900 feet below the surface before they were rescued.
All but one of the miners were transported by air and ambulance to Shoshone Medical Center, where two were held overnight for observation but later released after they were treated for cuts and bruises. The seventh miner remains in Kootenai Medical Center with a broken pelvis, said Melanie Hennessey, vice president of investor relations.
“We are very thankful that they are all right,” Hennessey said. “We continue to work to get a better understanding of what occurred.”
It was the third accident at the mine this year. Two miners died – one in April and one last month – in the other incidents.
A rock burst is essentially a fracture in the rock wall of the mine, Hennessey said. “It can be caused by natural causes or by blasting activity.”
However, crews had not been blasting for 24 hours prior to the burst. In fact, the injured miners were at the time installing wire mesh and concrete liners on the rock walls designed to prevent rock bursts.
Seismographs throughout the region recorded a 2.2-magnitude quake in Mullan, and Lucky Friday operator Hecla Mining Co. said in a news release Thursday that the company would “be investigating the cause of the seismic activity.”
But scientists said later Thursday that the shaking came from the burst itself.
“That was the signature of the rock burst, I believe,” said Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology.
He said tremors with a magnitude below 2.5 are typically not felt by people.
A team from the U.S. Department of Labor’s Mine Safety and Health Administration was sent to the mine to investigate the accident.
The agency ordered the mine closed and will keep it closed until it determines work conditions again are safe, spokeswoman Amy Louviere said Thursday.
Phil Baker, president and CEO of Hecla, said in a statement, “We are thankful that all employees are out of the mine and have been accounted for, and that those injured have been treated. The safety of our employees is our primary concern.”
Hecla said no blasting had taken place in the mine in the 24 hours before the accident, “therefore, the rock burst is unrelated to mining activities.”
Reports from Wednesday night indicated that an emergency call was made to Shoshone County authorities at 7:51 p.m. to report buried miners. Law enforcement, medical emergency personnel and an underground mine rescue team were dispatched to the deep underground silver, lead and zinc mine.
The latest incident comes on the heels of a Nov. 17 accident that killed miner Brandon Lloyd Gray, 26, who was buried in rubble while trying to dislodge jammed rock. He died two days later.
Mine safety officials and Gray’s employer, Cementation U.S.A., a company hired by Hecla to deepen the mine, are investigating that accident.
On April 15, Lucky Friday miner Larry “Pete” Marek was crushed when his work area collapsed.
Federal regulators with the MSHA found company safety failures that led to Marek’s death. They issued four citations and $1 million in penalties in the April collapse.
Investigators in the accident report said that “management did not conduct an evaluation, engineering analysis, or risk assessment to determine the structural integrity of the stope back (the roof of an open room beneath the surface).”
The report continued its focus on management failures. “Management policies, procedures, and controls failed to ensure appropriate supervisors or other designated persons examined and tested ground conditions to determine if additional ground control measures needed to be taken to ensure the safety of miners prior to commencing work in the stope.”
The mine is undergoing a $200 million project that will deepen it to nearly 9,000 feet, providing access to deeper ore. Hecla officials expect the project to be completed by 2014.
Hecla was established in 1891 in North Idaho’s Silver Valley, according to the company’s website.
The fatalities earlier this year came after a 25-year period with no deaths, Hennessey said.