Sudbury Mining Solutions Journal is a magazine that showcases the mining expertise of North Bay, Timmins and Sudbury.
Mechanized mining using jumbos and big loaders are the underground mining method of choice throughout much of the Sudbury Basin and other mining camps around the world, but in the copper-rich Morrison Deposit that Quadra FNX is mining at its Levack Complex, “old school narrow vein methods,” using pneumatic hand-held drills, long toms and slushers are making a comeback.
Situated below the historical workings of the former Inco-owned Levack Mine, the Morrison Deposit is a very high-grade copper, nickel and precious metal footwall deposit featuring trunk veins from eight to 40 feet wide, branch veins from three to eight feet wide and tertiary veins measuring from less than a foot to three feet wide.
There was no problem using mechanized equipment to mine the trunk veins, but using jumbos and scoops to chase smaller veins grading between 20 to 30% copper, 1.5% nickel and from five to 40 grams per ton of platinum, palladium and gold, didn’t make sense.
“At first, we contemplated using mechanized mining methods like a lot of the neighbouring mines are doing, but when we got in there and got our feet wet, we realized we had to scale back from one boom jumbos to long toms to keep our grades high,” recalled Jeff Huffman, senior production co-ordinator.
Coming up with a mining method for the narrower veins from one foot to 18 inches wide led to even more unorthodox solutions, including top access shrinkage and captive cut and fill stopes accessed through an alimak raise.
Skepticism may be too strong a word, but there was definitely some questioning when these methods were first contemplated, said Jason Jessup, mine general foreman. “People wanted to be sure we were looking at all aspects of the mining method, including the cost, efficiency and safety. Once we addressed all the issues, everyone was onboard.”
Having a core of former contract miners with experience using these methods in narrow vein gold mines certainly helped. Another plus was the availability of DMC Mining Services, a Quadra FNX subsidiary with expertise in excavating alimak raises.
“The alimak raise allows us to eliminate a lot of lateral waste, plus we’re raise climbing through ore, so we’re producing ore as the stopes are being constructed,” said Huffman.
“Not only is the mining method more suitable to the size of the veins, but we’re eliminating costs at the same time.”
Zeroing in on the high-grade veins reduces the volume of ore brought to surface without sacrificing metal values, said Jessup. “This is another benefit because we are constrained by how much material we can haul from the bottom of the mine. One of our challenges is getting our capital development down deep enough to drill further into the orebody and add to our measured resource. By moving less ore at a higher grade, we free up more trucks for moving waste.”
The top access captive cut and fill stope is accessed through a timbered manway and tugger slide installed inside a 150-foot raise. One miner and a helper assigned to the stope use hand-held equipment and pneumatic slushers to drill and dump the ore down 42-inch millholes to an undercut. Cemented sandfill replaces the ore and provides a base from which another higher slice of the vein is drilled and blasted. The process continues until the entire vein is taken out and the overcut is reached.
“We’re looking to produce 600 to 750 tonnes a month with one miner in a cut and fill stope,” said Huffman. “One worker owns this section of the mine. He does everything. He helps design the stope, he helps construct it, he drills, blasts, slushes and backfills. He has a trainee here with him. It’s his workplace. Nobody else touches it. On his days off, there’s nobody else in here.”
Six hundred or 750 tonnes per month doesn’t sound like much, acknowledged Huffman, but it’s in addition to higher volumes coming from other areas of the mine, it’s consistent and high quality.
The mine is also using top access shrinkage stoping, a similar method except that instead of using cemented sanfill, the miners use broken muck as the base from which further cuts are taken. The downside of shrinkage stoping is that 60 to 70 per cent of the ore remains in situ until the entire vein is drilled and blasted.
Pounds, not tonnes
“We’re allowing the orebody to dictate to us how to mine it,” said Huffman. “It’s not about the tonnes. We don’t claim that we’re going to get a million tonnes out of this orebody in any given year, but what we’re going to deliver is high-quality material. It’s all about the metal pounds for us. Even in our daily discussions in engineering and in operations, we talk in metal pounds, not in tonnes.”
Using long toms and slushers in the 21st century may raise a few eyebrows, but they are proving their worth at Levack.
“I don’t know if long toms are being used anywhere else in the Sudbury camp, but we have a fleet of nine and we’re looking to get a couple more,” said Jessup. “We worked with Maclean Engineering to develop exactly the kind of machine we wanted. They’re very efficient, very reliable, easy to repair and they’re one-tenth the cost of a one boom jumbo.”
The mine keeps a close eye on grade control by using tracking slips to record the source of the ore and its movement to surface.
“Geology estimates the grades of the rounds as the ore moves from remuck to remuck and eventually to surface,” said Jessup. “They’ll have an estimate of what that lot is and from that estimate we’re able to predict what our grades and pounds of production are for the month. Our sample tower will sample that and provide us with assays. If there’s any kind of discrepancy, we’ll go back to those muck tracking slips to see where the dilution occurred or where our estimates were off.”
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