Goldcorp’s Cochenour headframe | Courtesy of Goldcorp

When Goldcorp bought the Bruce Channel property, which included the brownfield Cochenour mine site near Red Lake, Ontario, in 2008, the company acquired underground workings but not the processing facilities it needed. With the Campbell mill at its nearby Red Lake complex running under capacity and Campbell mine production winding down in the coming years, Goldcorp management opted to keep the Campbell mill running with ore from Cochenour, affording considerable capex savings.

That raised the question of how to get ore to Campbell. Goldcorp entertained many scenarios that boiled down to three basic options: sink a new Cochenour shaft on McKenzie Island and connect it via a bridge across the Bruce Channel of Red Lake; retain the existing shaft on the mainland but haul the ore overground; or drive a long drift that would connect the existing Cochenour shaft to the mining complex near the Campbell mill.

Goldcorp chose the latter, using the existing shaft and hauling the ore underground by rail. “Some of it had to do with permitting constraints,” says Elizabeth Howell, who took over as Cochenour project manager after these decisions were made. “The permits were already in place that addressed the historical mine and infrastructure. By applying for amendments and modification to existing infrastructure rather than permitting an entirely new project, Cochenour could begin development sooner.”

For mining contractor Redpath, hired by Goldcorp to connect Cochenour and Campbell infrastructures, the nearly completed five-kilometre haulage drift represents the largest drift it has undertaken in North America. “We’ve done other similar projects, but not of this magnitude,” says Don Duhamel, safety and training coordinator at Redpath. Its closest comparisons are to be found in Redpath’s Indonesian work for Freeport-McMoRan and its work on Rio Tinto’s Oyu Tolgoi project in Mongolia.

A solid link

Timing and access played roles in the decision to transport underground. By connecting the mine workings at Cochenour to those at the Red Lake side, explains Howell, “We take advantage of existing infrastructure. Development of the drift allowed first access to the deposit while the surface infrastructure and shaft sinking was completed at Cochenour. Having two access points to the deposit allowed for flow through ventilation earlier as well.”

The drift, which is 5,320 feet down, provides a platform for exploratory drilling at depth. As construction progressed, Redpath developed drill bays every 400 feet. With Goldcorp’s Rahill-Bonanza joint venture property sitting between Cochenour and Red Lake, future access could come in handy.

Ventilation presented Redpath’s top challenge on the project. There was limited airflow available from the Campbell mine, as ongoing mining operations required the majority of it. “Originally, ventilation was around 48,000 cubic feet per minute, which is pretty tough for developing the drift,” says Howell. At the current 63,000 cubic feet per minute, ventilation is still a constraint as such volume only allows a limited amount of diesel exhausting equipment to be used. Workers use a tag board to check equipment in and out and ensure only so many pieces of equipment are running at one time. The list of equipment needed is long. It includes, among other things, Caterpillar 1,600 scoops, two-boom jumbo drill rigs, a MacLean bolter, two MacLean scissor lifts, two forklifts, utility vehicles, MTI 30-tonne locomotives, and three personnel carriers.

The construction of the drift has taken place in stages. The drift is 14 feet wide and 16 feet high, and the ground control includes shotcrete. The workers also install power, leaky feeder communications, process water, dewatering and compressed air lines, and ventilation. The safety infrastructure includes not only seven refuge stations along the way but also an eight-tonne locomotive set aside to pull a mine rescue car if needed.

The track for locomotives is currently two-thirds complete. “As we finish a third of the haulage drift, Redpath installs the track,” says Howell. “That lessens the amount of diesel haulage and decreases the cycle time for the next third of the development.”

Goldcorp and Redpath are still evaluating how to lay track in the last third of the drift. Unlike the basalt that much of the tunnel runs through, the final section is in less-stable talc. “It slowed down the development when the drift went through the talc, and that’s mainly due to reducing the size of our rounds,” says Howell. Normally one drilling and blasting round would advance the tunnel 12 feet further, but at times caution pushed that number down to eight feet. “We did stop and put in additional cable bolting,” he explains. “In addition, the drift requires rehab while we determine the right combination for the permanent ground control. We’ve installed ground support test ­sections and are in the process of evaluating those test sections right now. Five test sections were installed, and we’ve been monitoring them since June. The monitors were installed to track the ground movement. We will use the data gathered as a factor in making our determination for permanent ground support in the talc section.”

As of late September, the haulage drift was 99 per cent finished – the original development timeline having been extended along the way. Design optimization conducted in 2013 resulted in a revised development plan to accelerate initial gold production. Priorities shifted from completing the haulage drift to ventilation and to increased exploration drilling. A T-drift was added perpendicular to the haulage drift, with six drill bays, to explore the Bruce Channel deposit. The drilling done on the Bruce Channel deposit is the first at depth, so it plays an important part in designing the rest of Cochenour’s infrastructure.

The egress drift was developed to connect the haulage drift to the Cochenour shaft. This connection accelerated the timing of increased ventilation. “We hijacked the haulage drift project on these two sidelines to benefit the overall project,” says Howell.

Working toward production

Goldcorp did not have many case studies to consult in planning the drift. Back in the 1960s, Newmont built a 17-km haulage tunnel in the Granduc copper mine in Stewart, B.C. “They actually put most of their offices down underground and rode a train in for half an hour, the same as we’re going to be doing,” says Lorne Gunby, project general foreman with Goldcorp. But the Granduc project used more air-and-electric-operated equipment, so the builders of the Cochenour drift needed more robust ventilation to manage diesel fumes.

In full production, Cochenour is forecast to produce 225,000 to 250,000 ounces of gold per year. The tram system will carry 1,500 tonnes of ore to the Campbell mill each day. As long as development stays above the haulage drift level, gravity will do much of the work: the rock mined will tumble down ore and waste passes into bins and then into eight to 12 cars to be trammed across to Campbell, where skips will carry it upward. Once Cochenour goes past the 5,320 level, ore will have to be transported up and into the bins.

In the meantime, Redpath has plenty of other development work to do at Cochenour: the legacy shaft has been expanded and revamped, but there are ramps, bins and entire levels to put in. The total development cost of the new mine is projected at $496 million. Goldcorp estimates it can recover about $44 million in costs with the integration of the Cochenour operation with the Red Lake sites, including the unified milling process. Production has been pushed back from its original start date, but Howell says the mine is on target to produce first ore from production stopes in the third quarter of 2015 with commercial production expected in the second half of 2016.