The term “mine scale event” describes very large mining- related seismic events that are generally greater than Nuttli Magnitude (MN) 3.0. These events are influenced by or part of rock mass failure processes that exist on a similar scale to a significant part of the mine. The consequences of these events have been among the most catastrophic in mining, having the potential to cause multiple fatalities and immense loss of resources.
Undoubtedly, the worst period of rock bursting in the history of Canadian mining was the 1930s to 1960s in the deep Kirkland Lake gold mines. The Kirkland Lake main break held a rich gold-bearing ore body, which was extracted from seven mining operations. As the mines matured and extraction took place at great depths, rock bursting became a serious problem, resulting in many large seismic events and numerous fatalities.
The Wright-Hargreaves mine, opened in 1923, was the deepest of the Kirkland Lake gold mines, reaching 2,500 metres below the surface. After a day shift on August 14, 1964, a shrinkage stope in the central shaft pillar was blasted. The ground response immediately after the blast was not considered to be unusual, and two miners reentered the area to resume work. Soon afterwards, a MN 3.1 took place in the shaft pillar, causing the stope to collapse and the death of both miners. After the mine was evacuated, an even larger event (MN 4.2) occurred. Damage from the MN 4.2 was even greater than the first event, cutting off access to 11 levels and damaging the shaft. The decision was made to close the mine permanently on August 27, two weeks after the events.
Wright-Hargreaves did not have a significant rock bursting problem until late in the mine’s life at high extraction, suggesting that mining-induced rock mass failure processes likely played a significant role in the generation of these large events. The physical damage to the Wright-Hargreaves mine existed over 11 levels (365 vertical metres), suggesting the physical scale of the seismic source was comparable to a large part of the mine, and the event could truly be called a “mine scale event.” The tragic closure of the Wright-Hargreaves mine underscores the critical importance of understanding and mitigating seismic risk in deep mining operations. These historical lessons continue to inform modern mining practices, emphasizing the need for rigorous safety protocols and advanced monitoring technologies to protect miners and resources alike.
REFERENCES:
Blake, W. and Hedley, D.G.F. (2001) Rockburst Case Histories for North American Hardrock mines. Report for CAMIRO Mining Division. 73 p.
