So as unusual as it may seem, air must be refrigerated in the warmer months (a relative term in this case). A 450-tonne refrigeration plant was installed for this purpose in 1984. Since that time, thermistor temperatures along the main decline have been reduced by an average of 3 degrees .
Polaris, which is owned by Cominco Ltd., is 100 km northwest of Resolute Bay in the Canadian High Arctic. In designing the mine, company engineers had to take into account an array of natural obstacles, including temperature extremes that ranged from –55 degrees C in the winter to +15 in the summer.
On-site construction was kept to a minimum, and several key components were manufactured in southern Canada, including the concentrator. The concentrator, warehouse, dry, shop and offices were assembled on a specially designed barge at Trois Rivieres, Que. The galvanized steel structure (31 m wide, 18 m high and 122 m long) was then towed down the St. Lawrence River, along the coast of Greenland, and through Lancaster Sound to the mine site in August, 1981.
A galvanized steel concentrate storage shed, accommodations, an airport, fresh water and tailings disposal systems, and a dock were all constructed on-site. Rather than using concrete to reinforce the dock structure, waste rock fill was frozen in place, which resulted in significant savings.
Because of the intense cold, concentrate shipments are restricted to a 2-month period during the summer when supplies are also brought in. The mine employs about 250 people and upwards of 200 personnel are on site at any given time.
Classified as a Mississippi Valley Type deposit, the Polaris orebody has two principal minerals: sphalerite and galena; waste material is predominantly dolomite with calcite and marcasite. The orebody sits entirely within permafrost, strikes north-south, and dips gently to the east at about 20 degrees . Ore reserves are 15 million tons averaging 3.9% lead, 14.4% zinc and 3.7% iron in two zones: the Panhandle and Keel.
Production began in 1981 in the shallow Panhandle zone and three mining methods were utilized at the time, depending on the height of the stope. For stope heights of 20 m or less, the hangingwall was overcut and benched down in 5-m lifts. A 2-stage benching technique was used for stope heights of 20 to 30 m. At first, it was opened to full width approximately half way up and benched down to the footwall; the void was filled and then the top half of the stope was overcut at the hangingwall and benched to the backfill below.
Stopes with heights of 30 to 40 m were overcut and undercut at the hangingwall and footwall. Large diameter blast holes were drilled and blasted into a 1.8-m raise-bored slot at the end of the stope. Broken muck was then extracted from a scram drift and drawpoints. For all three methods, stope widths ranged from 12 to 15 m and stoped out areas were backfilled after mining. Rib pillars 10 to 15 m wide were left for later recovery.
A sub-level blasthole stoping method is being utilized in the Keel zone; lifts are 30 m and each stope is 15 m wide and 100 to 150 m long. Rib pillars (18 m wide) are left between stopes for future recovery. Before drilling, the stope is slashed to its true width and holes are blasted into a circular slot raise at the end of the stope. Slices of 4 to 10 m are blasted at any given time and 6-cu-m scooptrams and 24-tonne trucks are used for mucking. All load-haul-dump (lhd) vehicles are diesel-powered and conveyors carry ore from the two underground crushers to the concentrator.
Stopes are ventilated through raise- bored holes from surface which are also used for backfilling. Quarried shale, rubble, and development waste from the underground are used for backfilling. Water is added to consolidate the backfill and all stopes are tight-filled to reduce subsidence of stope backs. A number of different backfill alternatives were reviewed, several of which had to be eliminated because of their impact on rock temperatures. Others were simply too costly.
In any event, there was a limitless supply of argillaceous shales and rubble on surface which, when mixed with water, proved to be an ideal compromise. Laboratory tests, which included analysis of the quantitative properties of the backfill and its stress characteristics, indicated the water/ waste rock combination was suitable.
For ground support, 1.8-m split sets are installed with electric-hydraulic roof-bolters, and bolting patterns are reinforced with 3.6-m swellex bolts in certain situations. For obvious reasons, air flushing cleans out drill holes, cuttings from which are channeled into dust-collectors that are mounted on all equipment.
Pillar mining at Polaris began in late 1985 and to date four 30-m-high pillars have been removed along with the first stage of two others. Pillar recovery is straightforward and resembles normal stope-mining methods. Slot raises are installed at the ends of pillars and tightly spaced blast holes are drilled by an electric-hydraulic longhole drill. Two or three rings are blasted at any given time and remote- controlled LHDs are used for mucking.
In one instance, cable bolting was required to stabilize the fill in a stope where ground stability was reduced by faulting and bedding planes perpendicular to the faults. Frozen backfill at Polaris had stood up to a 30-m height without any significant stability problems. Minor sloughing has occurred from sections of backfill without the proper moisture content. But Zahovskis said “more research must be devoted to the subject in preparation for recovering pillars greater than 30 m in height.” David Duval
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