The event, the miner said, was recorded at 2:30pm local time on October 30. All underground employees were brought to the surface safely.
While both the mine and mill's operations are currently suspended, IAMGOLD said it has started an investigation into the cause and government authorities are aware of the event.
A business recovery plan to bring Westwood back online is being compiled, the company said.
Westwood, which is located about 80 kilometres west of Val d'Or and wholly-owned by IAMGOLD, first began production in early 2013. Commercial production there was reached in July 2014.
The mid-tier miner also operates the Essakane mine in Burkina Faso and the Rosebel mine in Suriname. In July 2020 the company and joint venture partner Sumitomo Metal Mining confirmed they would together construct the Côté gold project in Gogama, Ontario.