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An explosive blast has triggered a series of seismic events and a large underground rock fall at Australia's deepest gold mine, just weeks after a $100 million project to extend the life of the mine was completed.
Key points:
- The mine was originally established in 1897 by Herbert Hoover, who later became US president
- St Barbara restarted mining at Gwalia in 2008 and has produced more than two million ounces of gold
- Mining at Gwalia is planned to reach a depth of 2,300 metres below the surface by 2031
The owners of the Gwalia gold mine in Western Australia's northern Goldfields have revealed to the stock market that the fall of ground occurred on September 8 about 1,600 metres below the surface.
Melbourne-based gold miner St Barbara said the largest seismic event measured 1.9 magnitude and the rock fall involved about 30 tonnes of material near the bottom of the Hoover Decline.
No-one was underground at the time of the scheduled blast.
"The event was reported to the appropriate authorities and all work at depth was suspended pending assessment of the seismic information," the company said in a statement.
"Investigation and rectification work took longer than initially anticipated, as although the longest dimension of the fall of ground was three metres, ultimately a total of 30 metres of lateral rehabilitation was required.
"The decline has reopened and mining activity has resumed below this level."
St Barbara said it expected production would be 8,000 ounces lower in the September quarter as a result, but it still maintained full-year forecasts of between 175,000 and 190,000 ounces for Gwalia.
The Department of Mines, Industry Regulation and Safety has been contacted for comment.
$100m expansion finally completed
The rock fall comes just weeks after a three-year project to extend the life of the Gwalia underground mine was completed.
Kilometres of new ventilation were installed by sinking large shafts to send chilled air from the surface to the miners below.
St Barbara's managing director, Craig Jetson, said the final 517-metre-deep ventilation shaft was completed last month.
Speaking on August 24 when St Barbara posted a $128 million full-year profit, Mr Jetson had said the project was well behind schedule.
"The remaining ventilation shaft was problematic to say the least, but was completed in early August and will now integrate with the existing ventilation configuration and we look forward to being able to optimise the operation going forward," he said.