Yellowknife's Giant Mine underground sealed off

The underground workings of Giant Mine, blasted out of the rock during more than half a century of gold mining, have been sealed shut.

In the waning days of 2024, the federal team overseeing the $4.38 billion federal remediation of the Yellowknife gold mine completed all of the work needed to stabilize the underground and monitor it from the surface.

"It is a milestone, not only for the history of the mine, but also the remediation project in and of itself," said Chris MacInnis, the Ottawa-based director of the Giant Mine remediation project.

Until now workers on the project have been descending into the mine to monitor the stability of the rock and the chemistry of water entering the mine.

"We're now able to do all of that from the surface," said MacInnis.

He said another change that's eliminated the need to go underground is a new pumping system used to lift water that's seeped into the mine to the surface for treatment. With the previous system workers had to go underground to monitor and service the pumps.

"We've since replaced that with submersible pumps from surface," said MacInnis. "We can monitor and access those pumps from the surface."

Stabilizing the underground mine workings required filling many of the mine's cavernous underground chambers and tunnels. MacInnis said they've used a paste made of cement and crushed rock. In all, 198,000 cubic metres of the paste was pumped into the mine — the equivalent of almost 20,000 dump truck loads.

Hazardous material and other waste has been removed from the underground, along with all of the equipment used there. All power and heat has been shut off, as has the main ventilation fans. All openings have been sealed shut.

MacInnis said closing the underground will reduce care and maintenance costs by $5 million to $7 million annually.

Freeze separate

Not all of the hazardous waste has been removed from the underground. The mine's most dangerous legacy, 237,000 tonnes of highly toxic arsenic trioxide dust, is still stored in some of the chambers.

The clean-up plan calls for the rock around those chambers to be frozen using a series of thermosiphons, devices that use gas to freeze the rock, a process that will take years even after the system is built. The freezing will prevent the arsenic dust from coming into contact with groundwater.

"It is separate," said MacInnis. "The freeze will all be constructed and built from the surface."

The remediation plan also calls to keep open the option of removing the arsenic dust stored underground if, at some point in the future, processes and technology are developed to allow it to be done safely.

"We will be building a long-term portal, which will be an access point into the underground," said MacInnis.