If you ask Gord Stephenson what's the hardest part about closing Diavik diamond mine, he'd say two things: figuring out what to do with a processed kimberlite containment facility and demolishing the mine at large.

"We've got to find a way to make it stable in the long term, and we're going to be working to freeze it," he said about the processed kimberlite.

Stephenson, the manager of surface operations and closure at the mine, added that demolition is the most logistically challenging piece.

"We'll bring in some contractor expertise, hopefully partnering with our Northern partners through that process," he said.

By 2026, Diavik will have shut down, after producing diamonds for decades. By 2029, the mine site, owned by Rio Tinto, will have been remediated as well, should everything go according to plan.

Stephenson said there are plans for a specific rock pile that factor in climate change. That rock pile contains some potentially acid-generating rock. Laura Worsley-Brown, a communications manager at Diavik, explained that the rocks have been covered with a layer of natural till, and three metres of non acid-generating rock. That's intended to stop water from coming in and keep the underlying rock frozen, even if the climate warms, she said.

As for the processed kimberlite containment facility, some of the dam walls around it are built on ice-rich till material, Worsley-Brown added. 

"Till is a naturally-occurring glacial sediment, and ice-rich till has ice contained within it," she said.

She added that Diavik is reinforcing some areas adjacent to the dam walls with an extra rockfill wall to keep it cold. Stephenson added that the walls are stable now and there's no sign of deformation.

"However, there is a minute chance that in a climate change scenario, if that ice-ridged till in the foundation were to thaw, it could induce some settlement," he said.

Mine officials also plan to siphon water from Lac de Gras into the massive open dig sites, or pits. Given the record low water levels across the NWT, they'll be closely monitoring the lake, Stephenson acknowledged.

"We will be watching it, absolutely in 2026. We don't know how the water levels of this year will affect a couple years from now, but it would be a risk... if we were at a really low water year."

Filling the pits with water will only begin once all the mining's done, said Stephenson. The process will take between six to eight months, he added, it's expected to begin somewhere around September or October 2026.

"It'll have to be a very controlled process. We've got criteria that we'll need to manage in terms of effect on Lac de Gras. We do not want to negatively draw down Lac de Gras," said Stephenson.

He said there will be a protective screen on the intake side that comes with the pipes siphoning water to make sure they don't suck in any fish when filling the mine's pits. 

Each massive dig site will require about six to water eight pipes, he added. 

While many of these details have already been analyzed for years, one area where it's difficult to pin down Diavik officials for a specific answer is cost. 

"It's not possible to provide a single figure for closure costs," said Garrett Hinchey, Diavik's senior advisor for communications and government relations. "Closure considerations and opportunities to find efficiencies are continuously addressed in our planning and budgeting."

In terms of money spent to date, the single largest closure and reclamation item was work on the North Country Rock Pile, which exceeded $90 million, Hinchey added.

Closure activities will be overseen by the Wek'eezh矛i Land and Water Board and the Government of the Northwest Territories, and there will be input from participation agreement partners, such as First Nations.

As the mine's final days draw nearer, there is one other pressing challenge for Stephenson: saying goodbye.

"We can't forget about the people as well, as we go from a community of so many people and down to nothing. We've got the My Path program, but that's certainly going to be a challenge as well."

That program Stephenson's referring to is designed to help miners at Diavik with whatever step they're taking next in their career. Out of a sample size of more than 600 workers, Amber Robertson, a team lead at Diavik helping miners prepare for closure, said close to 200 hundred of those surveyed want to be redeployed elsewhere. The next most common answer was "unsure."

And as a Northerner himself, Stephenson said he'll be sticking around, at least for the next handful of years.

"I think it's a privilege to work at this site and I'd like to see it through to a successful closure in 2029."