MINING THE BIG BREAK

A disquieting number of rock bursts and the usual creaks and groans of a deep mine once were the daily fare for miners at lac Mineral’s Macassa mine, near Kirkland Lake, Ont. They still contend with rock temperatures nudging 85^o (deg) C and humidity readings of 100%. Rockburst activity, however, is no longer an everyday fact of working life, thanks to made-to-measure mining methods devised on site.

The ground conditions are largely, but not entirely, a function of the depth of the mine. The Macassa gold mine sports the deepest single-lift shaft in North America. Sunk by Dynatec and completed in 1986, the shaft extends to 7,239 ft., or slightly more than two kilometres. (At that depth, a set of 1-3/4-inch-diameter hoist ropes 8,200 ft. in length is required. The supplier is Wire Rope Industries of Montreal, Que.) The cage-over-skip conveyances consist of two 20-man cages and two 9-1/2-ton (190-cubic-ft.) bottom-dump skips in balance powered by a 14-ft.-diameter Nordberg tandem-drum hoist.

Aside from the rock stresses associated with depth, other phenomena appear. When the skips are loading at the 7050-level loading pocket, the rope can stretch as much as eight feet. Hydraulically powered mechanical chairs limit travel to 30 inches. A larger-diameter rope could have minimized stretch, but then the weight of the rope would have become a significant factor, mine manager Bill Glover explained in his pre-tour briefing on surface. Two other loading pockets have 0000,0700 been excavated at 5725 (ore only) and 6450 (ore and waste, as is the 7050 pocket).

Currently, the mine has 44 working stopes on 18 active levels. In total, the mine has more than 30 levels, resulting in what lac’s chief area engineer Ted Nelson described as a “rabbit’s warren of openings and workings.” Up to 40% of production muck is mined below 6400. Stopes are being worked down to the 7,050-ft. level. There is no mining activity above the 4250 level. There never has been activity above 425^o (deg) from the No. 3 production shaft because Macassa has been following the westerly plunge of one of the richest single gold orebodies in North America. Teck-Hughes, Lakeshore and Wright-Hargreaves were some of the big “name” producers that worked the easterly side of this same orebody. The orebody’s westerly extension near No. 3, however, plunges beneath the 4250 horizon.

While the depth makes the mine a laboratory for ground control procedures, mining is mainly conventional. Jacklegs, stopers, mucking machines and pneumatic slushers are the primary mining equipment. Development, exploration and stope mining all rely heavily on SECAN 250 jackleg drills. In the past year and a half, two 1-1/4-cu-yd. JCI electric load-haul-dump machines were brought underground, as were two Wagner 1/2-cu-yd. scooptrams, dubbed “Scoopies.” But that’s about the extent of mechanization. “We’re a very narrow mine,” said mine superintendent Art Pervik. “Even a jumbo would have to be very small. We haven’t discounted it (the purchase of a jumbo drill), but it is hard to justify.” Mucking and tramming are achieved mainly with track-mounted mucking machines, rubber-tired Cavos Models 310 and 320, battery locos and mine cars.

On its tour The Northern Miner Magazine saw a Scoopy working in a remarkably narrow stope that had been opened on the 5600 level. The average width was 4.5 to five feet (1.35 to 1.5 metres), but Glover said operators want to bring the width closer to four feet and less. The stope is an excavation of the recently discovered hangingwall veins, which seem to curl out or splay from the main 04 break that Macassa has been mining for the past 57 years. These smaller, high- grade veins account for about one year’s reserves. Current total proven and probable reserves are 1.5 million tons grading 0.51 oz of gold per ton (1.37 million tonnes grading 17.5 grams per tonne). Mine Superintendent Art Pervik said underhand cut-and-fill will serve to mine the hangingwall veins. An initial 20-ft. (6-metre) slice will be taken from the floor and backfilled with cemented rockfill. Vertical cuts of 25 to 30 ft. (7.5 to nine metres) will then be taken down to the next level. “We don’t normally mine that high, but, because this structure is so narrow, we should be able to do it,” Pervik said. Gold values occur in a quartz vein that ranges in width from two inches to 1-1/2 ft. (five to 76.2 cm).

The most ingenious mining is concentrated on the wider, deeper portions of the orebody. This is where mining methods have been tailored to meet Macassa’s unique ground conditions. On the 6600 level, where the 04 break is being mined, we were shown how cemented backfill in conjunction with undercut-and-fill worked to control ground movement. At one time, Macassa dumped waste rock into mined-out stopes with the expectation that the unconsolidated fill would fully support the old workings so that mining could advance. It never really was the complete answer. There was plenty of movement, particularly after blasts, and the requisite remedial work to re-condition excavations was costly. “We would lose whole sections of the mine at a time, including active stopes,” recalled Nelson. (Macassa, by the way, is in esteemed company when it comes to burst problems — the whole Kirkland Lake camp was afflicted.) Macassa’s engineers and production people sought a solution three years ago. They experimented first with rockfill and a delayed cement slurry that was to permeate and consolidate the waste rock pile. It didn’t work. “All we did was form a crust over the first foot or so of rockfill,” said Pervik. They then opted for mixing cement slurry (5%) and rockfill in 2-1/2-ton, side-dump mine cars and delivering it to the stope being filled. This system has proven almost ideal. (The mine department has been fortunate in that the percentage of coarse to fines in the uncrushed waste rock is nearly perfect, making costly pre-crushing unnecessary.)

“The last rockburst we had was over a year ago,” said Nelson. “Before that, we could expect three severe ones a year.” Golder Associates, a geotechnical consulting firm that periodically visits the Macassa mine and evaluates the ground control work, noted in a report that the average occurrence rate was eight per year. It is now down to one. The latest rockburst, which measured 3.1 on the Richter scale, also demonstrated the resilience of the backfill. While walls moved in on the opening below, the cable sling- reinforced fill “back” (explained below) stayed in place. In the past, reconditioning a failed drift sometimes took up to eight months. The expense clearly was horrendous.

The type of mining sequence for wider areas goes as follows: A 9×9-ft. (2.7×2.7-metre) development drift is driven in ore first and slashed out to the width of the orebody, which ranges from nine to 35 ft. (2.7 to 10.5 metres). The average is 20 ft., or six metres. The entire back is reinforced with grouted cable bolts and 8-ft. mechanical bolts. Ground Control of Sudbury, Ont., supplies cable bolts. Raises provide access from 6700 to 6600. From the 6600-level drift, 2-inch-diameter downholes are drilled to a depth of 16 ft. (4.8 metres) by Atlas Copco longhole drill wagons fitted with BBC-120 drills.

After the 16-ft. (5-metre) bench is blasted and mucked out, a series of cable slings is laid grid-like on the floor of the opening. Discarded hoist cables are set in lengthwise and 5/8-inch, high-tension wirerope is anchored into the walls to cross the longitudinal cables. Screens and a material called fabrine, acting like burlap, are laid over the grid-pattern cable as a sill mat. The cemented rockfill is dumped in by mine cars, which run on tracks suspended from the roof by cable bolts. This innovation speeds the backfill cycle and protects the men working on the bench below. Steel culverts are set vertically on the floor to deliver backfill to the cuts below. On 6600, the track was laid on thick cross beams and longitudinal timbering.

From a raise, a sub-drift is driven below the 16-ft.-thick (5-metre-thick) cemented rockfill. This new 9-ft. (3-metre) opening along the orebody can then acommodate drill wagons for the next 16-ft. (5-metre) production bench. “You’ll see lots of mines mining up against backfill, but not many that mine under it,” commented Pervik, as we stood in the 9-ft. opening peering up at the screened backfill secured by cable slings. Miners backfill this second 16-ft. opening, and drive or breast another 9-ft. lift from the raise. The final 40 ft. (12 metres), taken above the bottom drift level (in this case 6700), is drilled off with downholes and mined in 12-ft. (3.5-metre) vertical slices. Mucking on the bottom level is remote-controlled. The backfill is held in place initially by thick square-set timbering on the bottom level.

“To mine from the top down is safer. We get better ground control and grade control and it is more productive,” Pervik said. Glover estimated total production costs (excluding exploration) at about C$170 per ton (or about $185 per tonne). He couldn’t put a figure on the cost per ton of the backfill and associated preparatory work, but said “it would be substantial. If it weren’t for the grade (an average 0.51 oz. gold per ton, or 17.5 grams per tonne) we’re mining, it would be difficult.” The mine cut-off grade is in the order of 0.20 to 0.25 oz. per ton (6.9 to 8.6 grams per tonne). Production costs averaged US$304 per oz. last year. “We have excellent miners, which makes everything easier,” said Glover. “They come up with a lot of good ideas.”

The “top down” approach is supplemented by a second method. This is a 3-stage system in stopes that do not have old workings above them. This method requires overhand cut-and-fill mining 50 ft. (15 metres) up from the level and a second-stage, 40-ft. (12 metres) underhand cut-and-fill 40 ft. down from the level above (again with that sequence of 16-ft. sill, 9-ft. sub-drift and further downhole benching). Finally, the middle 50 ft. or so is mined by downholes. Drift-set timbering on the level enables the backfill to set in the first mined section.

In total, Macassa employs 320, of whom 54 are staff people and 170 are underground miners. Alex McIntyre and Associates of Kirkland Lake do about half the development and exploration drifting. Heath and Sherwood are the diamond drillers and J.S. Redpath’s raise-boring unit has mined some 2,300 ft. (690 metres) of raises. R and R Mining of Kirkland Lake has four workers rehabilitating the old No. 2 winze to ensure escape manways are secured to surface.

Up until 1986, No. 1 and No. 2 winzes were used for production from the easterly side of the mine. The No. 1 shaft on the east side was also a production shaft but now exhausts mine air and provides an emergency escapeway. Downcast air is supplied through No. 3 from a 100-hp, axial flow fan supplied by Canadian Blower and Forge producing 100,000 cu. ft. (30,000 cu. metres) per minute of fresh air. Three centrifugal fans, strategically placed underground, move the air laterally through the workings.

While moving fresh air through such a maze of openings is difficult, worse still are Macassa’s water problems. All the old producers — Toburn, Sylvanite, Lakeshore, Teck Hughes, Wright-Hargreaves, among others — are connected underground to Macassa’s workings. The water flows into Macassa through a bulkhead at the 4250 level. Mather & Platt pumps lift the water to surface in three lifts. The pumps can move up to 550 gallons (2,500 litres) per min. For every ton of ore hoisted, the pumps move five tons of water. By setting up a heat recovery system on surface, Macassa has tapped the warm mine water for its heat. Along with waste heat from the cooling of compressors, this system provides all the heat necessary for winter mine ventilation.