Harry Preston was probably the first and maybe the only Canadian prospector to lose his footing and find a mine. The story goes that in June, 1909, Preston, in a prospecting group led by John Wilson, slipped on mossy ground near the banks of Porcupine Lake, in northern Ontario. Where the moss had been carved out by the heel of his boot, a gold-studded quartz showing lay exposed. That outcrop led to the discovery of a gilded quartz dome that would become the grandaddy of Canadian gold mines. Through two devastating mill fires, two world wars, the Great Depression, labor shortages and surpluses, minor recessions and major recessions, low gold prices and high gold prices; in fact, from the tail-end of the Industrial Revolution and well into the computer age, the Dome mine near South Porcupine, Ont., has been spewing forth gold non-stop — all because Harry Preston stumbled.
In 1910, the first year of production, the Dome yielded 214 oz of gold and 19 oz of silver from 247 tons of high- grade ore. Since then, the mine has produced in total about 11.5 million oz of gold. The current annual rate is about 135,000 oz from slightly more than a million tons of ore.
As it has for so many decades, the distinctive rust-red headframe of Dome’s No. 3 shaft dominates the surrounding bushland near South Porcupine. A bit weathered and perhaps less productive than it once was, the No. 3 will soon be phased out, but not before it gets a final kick at the can as Dome’s main production shaft. A 10-week shutdown this summer is scheduled for the main production shaft, No. 8. The 4-year- old, 5 1/2×7 1/2-inch wood guides in the skip compartments of No. 8 will be replaced with galvanized steel, Mine Superintendent Robert Lusk told The Northern Miner Magazine on a recent visit to the mine site. Tearing out the nearly four miles of wood guides in the 2-skip compartments (the shaft bottoms out at 5,472 ft) and installing the galvanized guides will cost about $1.5 million.
There’s more to it than that, however. The No. 8 shaft has two, 15-ton skips while No. 3 has two 5-ton units hoisting ore at about half the speed of No. 8. To maintain millfeed at the current 3,000 tons per day, Dome must stockpile ore in advance of the shutdown and dip into the open pit near No. 3, once No. 8 falls silent. Dome also plans to increase millfeed to 3,300 tons per day by May. Currently, mine production runs at 4,200 tons per day on a 5-day, 2-shift cycle. The mill operates seven days a week, drawing on ore from two 4,000-ton ore bins on weekends.
General Manager Robert Perry said that all production muck after the shutdown will be crushed and hoisted at No. 8, leaving No. 3 as a man conveyance only. Savings in maintenance and labor costs will result, though haulage costs will probably increase because the newer shaft is about a half-mile northeast of the old one and many of the approximately 40 stopes currently being mined are nearer No. 3. At a cost of about $92 million, No. 8 was commissioned in 1984 to rid the mine of its reliance on a cumbersome ore-haulage system through winzes and drifts — a virtual “snakes-and- ladders” system from the lower levels to the ore pocket at No. 3 about 2,200 ft underground. No. 8 also provided direct access to the deepest mine levels.
The Dome’s underground workings are a marvel. At any one time, 40 stopes scattered throughout the mine’s 37 levels could be in production. Beside an old stope that had been mined out maybe 20 or 30 years ago, jack-legs or jumbos can be heard thundering away into material that today has become economic. Back then, of course, when gold was trading at $34 an ounce, the geologists had directed miners elsewhere. Perry said that, on any given day, production crews could be working on the same level in stopes that are a mile apart. Ramping between levels is impossible because the workings are too extensive. In total, the drifts and crosscuts run some 152 miles.
Generally, drift dimensions are 8×10 ft in track drifts and 10×10 ft in trackless lateral work. Most of the equipment is electric (battery and some trolley systems) and runs on narrow-gauge, 24-inch tracks on 40-lb and 60-lb rails. The mine uses 5-ton Clayton, Greenbat and Goodman trolleys and Mancha and Atlas Copco locomotives. (For more detailed information on drifting equipment, see the accompanying table.) During our visit, miners at one stope on the 1,700 level near the No. 3 shaft were chasing a narrow, single-ankerite vein that ran off at 90 degrees to a larger vein system. A 3-ft-wide electric France Loader with a half-yard bucket was mucking out the stope with little room to spare on the sides. As Assistant Mine Superintendent William Hocevar explained, these miniature load-haul-dump machines are usually more efficient than slushers, especially in the longer stopes where two slushers would be required. Cut-and-fill is the most efficient way to mine the narrow, single-vein structures. However, wherever possible, blasthole methods are used. For example, between the 2,400 and 2,600 levels, a recent blasthole stope measured 300 ft deep by 100 ft square. A single blast in the stope broke 100,000 tons of ore. The stope was drilled off with a 6 1/2 -inch in-the-hole machine. Other blastholes incorporate 2 1/8 -inch-diameter holes drilled by long-hole machines. Blasthole stoping is accomplished under contract by Boart Canada.
In 1987, only about 65,000 tons of ore were mined by cut-and-fill. Mechanized cut-and-fill mining accounted for 313,000 tons, blast-hole stoping for another 333,000 tons and cave muck (or “overdraw,” as Lusk termed it) yielded another 298,000 tons. This overdraw is a bonus. It’s simply material that caves in from the walls and back of mined-out blasthole stopes. No mining or blasting is required. The ore is scooped from drawpoints and hauled to ore passes. The remaining 125,000 tons of production ore in 1987 came from sub- drift and development work.
Of the 40 current working stopes, 15 are blasthole and many of th ese are the overdraw stopes. Only stopes that are fairly large, steep-lying and regular are mined this way. In addition, the grade will often not support more conventional methods. Besides the grade and geometry of mineable material, the mining method depends to some extent on the orebody type, of which, according to a published paper on the Dome geology, there are three:
* long, narrow ankerite or quartz- tourmaline veins in schist parallel to the general trend of formations; * lenticular or irregular “tension” veins in massive rocks or crossing the schistosity in schistose rocks (these often cut the first type of veins); and
* mineralized rock in which gold is associated with pyrite and pyrrhotite.
Often, little or no vein material is found in this latter orebody. Gold is always accompanied by pyrite and/or pyrrhotite, but the reverse is not true. Chalcopyrite, sphalerite and galena are found locally in most types and are good indicators of gold content.
The current millhead grade is an unusually low 0.13 oz gold per ton, the result of Dome’s having had to put a premium on tonnage at the expense of grade in preparation for the summer shutdown of No. 8. “To go to 4,200 tons per day, we’ve absorbed lower-grade areas,” Lusk said. A shortage of experienced jack-leg miners is a contributing factor. “We’re short about 20 good jack-leg miners. If we had them, we could open up more stopes,” Lusk explained. Miners are rare nowadays, according to Perry, because they’ve been lured away from established operations by the promise of bigger paycheques at newer operations that have been financed by flow- through funds. This is a common lament in the Timmins, Ont., area. Our recent visit to the mining community included an underground tour of the Kidd Creek mine, where staff people commented that their operation had also lost miners. They felt that new flow-through-financed gold projects were the likely culprits. (A shortage of workers is also being experienced at Placer Dome’s Detour Lake mine, 209 km northeast of Timmins. See “Back on Track,” page 18.)
Because of the labor
shortage at the Dome mine, more production than usual comes from long-hole and overdraw stopes, where tonnages are higher but grades are lower. Still, operating costs have been contained. Mining and milling a ton of ore last year cost Dome $50, a slight improvement from the $50.87 per ton it cost in 1985. The cash cost per oz of gold produced is $300(us), though that will rise as the grade drops temporarily this summer.
As mentioned earlier, the bulk of production ore, crushed to -6 inches, is skipped up the No. 8 shaft. (The underground crusher at No. 8 is a 200 horsepower, 48×60-inch Kue Ken with an hourly capacity of 500 tons.) The ore is skipped to a 1,000-ton underground ore bin at No. 8 and fed from the bin by a vibrating feeder at the 300-ft level to a 36-inch conveyor belt. The belt transfers the muck to the 2,450-ft-long, 36-inch-wide conveyor belt that carries the ore up an 11.2 degrees incline to the 50-ton surface crusher coarse ore surge bin. A controlled, vibrating feeder operating at 300 tons per hour feeds ore over a 6×12-ft Tyler double-deck screen for classification. This material is spray-washed on the top deck to remove fines, a problem that most Canadian mines haven’t encountered, said Mill Superintendent Paul Blythe. “Our muck is particularly sticky,” Blythe added. The coarse material is fed to a 5 1/2 -ft standard cone crusher, reduced to -1-inch material, and then further screened by two 4×8-ft rod decks. The undersize from the first Tyler screen is further screened on the lower deck. The oversize from the second screen is fed to a 5 1/2 -ft shorthead cone crusher circuit, while the undersize goes as a slurry to a 60-inch spiral classifier. The fines from the classifier are pumped to the mill’s grinding circuit and the coarse sands are conveyed to the fine ore bins. Feed rate, the conveyor interlock system and alarm systems are monitored and controlled by a Conspec programmable controller.
The -3/4-inch surface crusher product is fed to a 10 1/2 x14-ft Allis-Chalmers rod mill and ground to -14 mesh. The slurry is classified through four 20-inch Wemco primary cyclones, with the fines reporting to a 155-ft outdoor thickener which de-waters the product to 50% solids, and the cyclone sands (or oversize) sent through four Joy Denver Duplex mineral jigs. The 24×36-inch jigs recover half of the contained gold, which is further upgraded on a shaking table and then sent directly to the on-site refinery. Blythe said Dome is working on boosting the jig recovery rate to 60%. The jig tails go back to a 13×20-ft Allis- Chalmers ball mill.
Meanwhile, the thickened slurry, after oxidation in three preaeration pachucas, is dewatered in three Eimco disc filters and then repulped with “weak pregnant” solution and pumped to a cyanide leach circuit. After the leach cycle, the pregnant solution is recovered from eight drum filters and the gold is recovered from the pregnant solution by a standard Merrill-Crowe zinc precipitation system. However, in May, a new $15-million carbon-in-pulp (c-i-p) circuit (now under construction) will replace the old, zinc system that was installed in 1929. Recoveries should rise slightly to 97% from the current 96.5% rate, Blythe said.
The new c-i-p circuit, the major expansion that occurred with the commissioning of the No. 8 shaft, and a relatively new tailings dam all point to a rather long future for the Dome mine. Reserves stand at 5.9 million tons grading 0.15 oz per ton in the proven and probable category. Known reserves extend to the 3,900-ft level and workings at that depth and above can support the mine at current and even lower gold prices for years to come. The lowest production level is 3,900-ft deep, although the main geological structures have been traced to below 5,000 ft and are open at depth.
In addition, during the mine visit, we were shown a development heading, 200 ft of which had been driven toward the Kurtz vein, an orebody that has been identified roughly 1,000 ft west of the current workings. Most of the Dome workings sit east of the shafts. This vein runs between 0.15 to 0.2 oz gold per ton and has been delineated along 1,000 ft of strike length. It’s a narrow vein, however, that runs on average about 2 ft wide. To tap this vein, Dome’s planners envisage using, a narrow-vein, bulk-mining method that incorporates a series of Alimak raises and horizontal blastholes.
Ever since Preston slipped on the moss near Porcupine Lake, the perennial question about Dome has been its reserve life. Only in the past two decades or so have people truly become satisfied that, though the company traditionally has reported reserves of two years out (five years now), the actual ore awaiting Dome miners will last well beyond that. Perry certainly feels that way, and after nearly 30 years of intimacy with the idiosyncracies of Canada’s most storied gold mine, he should know. He won’t and obviously can’t put a figure on the remaining life of the mine. (The definition of ore fluctuates with the price of gold and no one knows how deep the gold mineralization lies.) But he did say this:
“We built a new tailings dam that will last for 30 years and we feel we have an orebody that will justify our recent $100-million expansion.”
Enough said.
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