The explosives used caused the death of Joseph Koonce, 42, according to a report released by the Mine Safety and Health Administration.
The report releases Trapper Mine employees and mine procedures from responsibility for the fatal accident.
Koonce was killed Aug. 12 when an explosives booster went off prematurely. No other employees were injured in the accident.
The explosion was originally blamed on a malformation of the primer, but officials ruled that out after extensive testing.
Instead, officials say that a portion of equipment used in the blast accidentaly fell into a 102-foot deep borehole, collided with rock fragments and prematurely exploded.
The occurrence is rare and other miners said the same thing had happened many times before without causing an explosion.
"This really is a safe system," said Bill Denning, Mine Safety and Health Administration (MSHA) staff assistant. "As much as it's used across the United States, there's relatively few accidents. He probably did not know it would go off."
Because impact detonations rarely occur and then only under unusual circumstances, MSHA requested the National Institute of Occupational Safety and Health conduct a series of tests to simulate various impact scenarios.
Test results indicated a malfunction of the primers did not contribute to an impact detonation. Testing confirmed that a sharp impact on a specific portion of the detonator's base would trigger an unintentional detonation.
Trapper Mine is a surface mine. Overburden, ranging in thickness from 20 to 150 feet, is drilled, loaded with explosives, blasted and removed by draglines.
Koonce had worked for Trapper Mine for more than 18 years and had seven years experience as a blaster's helper. Reports say he indicated all was well prior to the explosion.
Before the accident occurred, Koonce had placed a slider-primer a booster and explosives detonator on a length of detonator cord and lowered it to the bottom of the pit. Following procedure, he then placed other slider-primers on the cord, which were meant to be slid down the cord into the pit after a blasting agent had been added.
According to the report, one of those slider primers got knocked into the hole, slid down the cord and impacted with the booster already in the pit. Because other detonators were attached to the cord on the edge of the pit, the explosion worked its way up to the cord set those detonators off causing the blast to occur above ground as well as underground.
Denning could not say if Koonce would have survived if the explosion was only underground.
Witnesses said they heard Koonce yell before the explosion and saw him reaching into the hole as if to catch the falling slider-primer.
MSHA identified no procedural errors or product mishandling during the investigation.
"This practice seems to be a fairly standard practice," Denning said.
Nevertheless, in an effort to prevent any future accidents, MSHA issued an order which prohibits slider-primers from being placed on the detonator cord until they are ready to be lowered into the pit.
Trapper Mine is complying with the order.
There will be no further enforcement action because it was found that Trapper Mine was not in violation of any federal blasting regulations.
"I wouldn't say there's been any misconduct," Denning said.
Trapper Mine General Manager Gordon Peters was unavailable for comment.