Underground at Detour

After a brief but troubled open-pit phase, the Detour Lake mine in northern Ontario is pulling up gold from underground. Figuratively speaking at least, things look brighter from below the surface.

Until recently, the operation’s record could best be described as lacklustre.

Shortly after the open pit began production back in 1983, the ore grade began to drop below expectations. Then some of the calculated reserves failed to materialize. Adding to the woes was the then-tumbling price of gold. The original feasibility study called for five years of open-pit life, to be followed by underground production; in fact, the pit was mined for only three years. It was closed in May, 1987.

However, since October, 1986, when the go-ahead was given for the underground phase, Detour Lake has been turning itself around. The average grade has improved (5.2 g per tonne compared with about 3.5 g during the open-pit phase) and operating costs (now projected at $276(US)) have been reduced. The feasibility study which precipitated the production decision clearly demonstrates that the undeground operation is viable at $350 per oz of gold.

Needless to say, this is welcome news for joint-venture partners Placer Dome (the operator) and Amoco Canada Petroleum.

“We’ve been able to turn the mine around from one which had a suspect future to one with a bright future,” mine manager Timothy Mann told The Northern Miner when it arrived at the remote, snowy camp. “The viability of the mine has been greatly enhanced (since the open-pit phase).”

Current production is about 1,300 tonnes per day, made up of production and pre-production ore from three levels (the fifth, seventh and ninth) with 600 tonnes coming from the fully developed seventh level alone. Full production is to be achieved in March from the three levels for a total of 1,800 tonnes per day from underground (600 tonnes per day from each level). However, low grade, stockpiled ore from the open pit will continue to be processed, resulting in an actual total of 2,200 tonnes per day. It is expected that this level will be sustained until 1991, when the stockpiled ore runs out. Thereafter, production would drop back to 1,800 tonnes. But by that time, production could become increased by mining new reserves.

Mill throughput is projected at 2,240 tonnes per day during full production, with a recovery of 94 1/2%. All in all, about 3.3 million g (115,000 oz) are expected to be recovered in 1988, mill superintendent Peter Dyas says.

Mann explains: “It’s important to understand that the mine has been made more viable as a result of the fact that we were innovative in our approach.”

One of those innovations was a scheme called the “Rob Cut,” performed in 1986. This involved taking out an extra cut beyond the final planned depth of the pit. In other words, the miners “robbed” the original crowned pillar that had been left between the pit and the underground workings.

Another innovation, the “North Wall Salvage Project,” involved removing low grade material on the hangingwall side of the pit. This material was previously considered uneconomic, but the mining crew later decided to blast it from the pit wall with very little waste mining required.

Those projects, together with the ongoing processing of stockpiled ore, have helped provide Detour Lake with cash flow for underground development. Most reserves in Main zone

Reserves now stand at 7.4 million tonnes, the bulk of which are in the Main zone where a mechanized cut-and-fill mining method is being used. Objectives for 1988 include bringing the Quartz zones into production (probably using a sub- level, blast-hole mining method) and improving the scheduled mine production rate and recoveries from the Talc zones. Grades in the Main and Quartz zones are roughly the same; in the Talc zones, they are somewhat higher.

When The Northern Miner climbed underground at the mine earlier this month, crews were silling out the Main zone and starting fill preparation on the fifth level. Also, the Talc zones were being developed on this level. On the seventh level, production was occurring in the 7-M2 stope, in addition to ramp development (so that the first three levels will be connected) and the pouring of sill pillars. On the ninth level, workers were busy constructing an Eagle crushing station while the eleventh (bottom-most) level was being used strictly for exploration development. The shaft extends 600 m.

Located 209 km northeast of Timmins, Ont., Detour Lake is the only fly-in mine in the province. A road, linking the mine to Cochrane and other outlying towns, was constructed but is generally used only to transport freight material and contract workers. Photo by Placer Dome The Detour Lake underground mine, 209 km northeast of Timmins, Ont., entered production this month. It is expected crank out 3.3 million g (115,000 oz) of gold in 1988. This photo was taken a few months ago, before heavy snows began descending on the mine site.