The Myra Falls zinc-copper mine is back on track following a 3-month shutdown that allowed for a US$10-million rehabilitation program.
Situated in mountainous Strathcona Provincial Park, the mine is nestled within the scenic Myra River Valley on Vancouver Island. Only a 90-minute drive from the town of Campbell River, the property measures 8.5 by 2.5 km.
On a tour of the operation, owned by
Following the death of miner Richard Nadeau in October 1998, a joint committee of labour and management recommended that the mine be closed and rehabilitated starting in December 1998. The committee called for a US$10-million development and rehabilitation program, including rock-bolting, backfilling and infrastructure support. During the 3-month suspension, 1,330 metres of drifts were rehabilitated using a new, tightly spaced rock-bolting pattern, along with wire mesh and shotcrete. About 175,000 tonnes of backfill were placed, and a 330-metre underground development program was completed.
Currently, mining crews are experiencing difficulty retrieving ore from production areas quickly and efficiently. “We have two separate mines underground,” said Mine Manager Kjell Larsson. “They are not well-connected, and we probably have too much equipment.”
To rectify the problem, Boliden is considering increasing underground infrastructure. One proposal calls for the construction a 2.7-km ramp from the surface down to the Battle and Gap zones to ease transport of equipment, materials and ore. Capital costs for this project are estimated at US$5 million.
Also under consideration are the following:
- an air control system for the copper and zinc flotation columns, to increase recoveries;
- a study to determine if the installation of an Knelson gold concentrator on the tailings stream will improve gold recoveries;
- an investigation to double the capacity of the zinc cleaner flotation circuit by installing a parallel cleaner bank;
- a study of the stability of the tailings dam; and
- a paste tailings plant for disposal of surface tailings.
Mineralization occurs in two rhyolite horizons: the lower H-W horizon and the upper Lynx-Myra-Price horizon. The massive sulphide deposits lie within discrete, generally flat-lying lenses that range from 3 to 60 metres in thickness, up to 1,200 metres long and up to 300 metres wide. Individual ore lenses vary from 50,000 to 10 million tonnes.
The principal sulphide minerals are sphalerite pyrite and chalcopyrite with lesser amounts of galena, bornite and tennantite.
Total proven and probable reserves stand at 6.7 million tonnes averaging 7.7% zinc, 1.5% copper and 0.4% lead, plus 1.4 grams gold and 34.9 grams silver per tonne. This includes a 28% dilution factor.
Of that amount, 1.9 million tonnes grading 3.3% zinc, 1.5% copper, 0.3% lead 1.7 grams gold and 25.4 grams silver are in the H-W deposit. The Battle deposit contains 3.5 million tonnes averaging 10.1% zinc, 1.5% copper, 0.4% lead, 0.9 gram gold and 27.2 grams silver.
The Gap zone contains 553,000 tonnes averaging 11.7% zinc, 1.7% copper, 1% lead, 2.9% gold and 107.8 grams silver.
Other deposits within the lower H-W horizon include:
- the Extension zone, which contains 211,000 tonnes averaging 4% zinc, 1.1% copper, 0.4% lead, 0.8% gold and 33.8 grams silver; and
- the 43-Block zone, which holds 575,000 tonnes averaging 4.9% zinc, 1.5% copper, 0.4% lead, 2.3 grams gold and 43.4 grams silver.
Total measured and indicated reserves stand at 5.3 million tonnes averaging 7.7% zinc, 1.6% copper, 0.7% lead, 2 grams gold and 56 grams silver.
Massive sulphide mineralization is hosted in the rhyolitic volcanic rocks of the Myra formation, a volcanic assemblage that includes subaqueous basaltic-to-rhyolitic flows, breccias and mixed volcaniclastic rocks. (The formation is, however, dominantly andesitic in composition.) The sequence is underlain by basaltic andesites of the Price formation and capped by tuffaceous sediments and mafic sills of the Thelwood formation.
The potential for expanding reserves at Myra Falls is excellent. The exploration budget is tagged at $2 million to $4 million per year. “What we are trying to achieve in our exploration program is to find a mix of long-term, large-tonnage targets, such as the Marshall zone, and shorter-term, smaller targets, such as the Extension zone,” said Cliff Pearson, chief geologist.
The current target is the Marshal zone, situated on the H-W horizon, west of the Battle zone. Resources there stand at 320,000 tonnes averaging 7.6% zinc, 0.7% copper, 0.7% lead, 2.5 grams gold and 105.6 grams silver. The zone remains open to the east, west and to the north.
Boliden also plans to investigate a target just east of the H-W deposit; this target lies past a fault with significantly uplifted stratigraphy. Other short-term targets include the Battle North and Ridge zones, which are north and west, respectively, of the Battle-Gap mine.
Longer-term exploration targets include the SW Frontier zone, west of the old Lynx deposit, and the South Flank area, just south of the mined-out Myra deposit.
The current Myra Falls operation runs three underground shifts per day, 365 days per year. Boliden employs 349 unionized employees; these, together with management and engineers, bring the total number of employees to 450.
The mine creates its own power requirements through two hydroelectric generators that supply a total of 10 to 11 MW of power. The generators are fed by a series nearby lakes. Back-up diesel generators are available on-site for emergencies.
All water at the site is treated with lime and is pH-corrected. The water goes through a series of ponds where precipitates are allowed to settle out; it is finally discharged into Myra Creek.
Underground operations utilize long-hole stoping methods with some drift and fill, plus limited room-and-pillar mining.
Average annual production is projected at 66,000 tonnes of zinc-in-concentrate. All the ore is sent to the main tramming level for transport and blending prior to entering the underground crusher. The ore is crushed to less than 6 inches and automatically hoisted to the surface. From the surface, it is transported by conveyor to the concentrator over a distance of 1.4 km.
Primary grinding consists of two parallel rod and ball mill circuits with a capacity of 4,000 tonnes per day. Copper and zinc flotation occur in identically configured circuits, and rougher concentrates are reground in separate mills. The concentrates are dewatered in thickeners and filtered to a final moisture content of 7.5%.
A gravity concentrate with high gold and silver content is recovered in the grinding section of the circuit using Knelson concentrators.
The zinc and copper concentrates are transported by truck to loading facilities in Campbell River and shipped to Japanese and Korean smelters.
Recoveries for 1998 were 88.8% for zinc, 87.3% for copper, 39.4% for gold and 65% for silver.
In 1998, the mine produced more than 1 million tonnes of ore grading 5.6% zinc, 1.7% copper, 1.63 grams gold and 22.91 grams silver. Mining and milling costs were pegged at US$30 and US$12.70 per tonne, respectively; these figures include operating and depreciation costs. Cash costs, net of byproduct credits, amounted to US41.7 cents per lb. zinc. Treatment charges were set at US45.1 cents per lb. zinc.
The Myra Falls mine produces three concentrates: zinc, copper and gold. Last year, zinc concentrate totalled 95,450 tonnes grading 54.3% zinc, 47.4 grams silver and 2.2 grams gold. The copper concentrate production was 60,249 tonnes averaging 25.8% copper, 183.8 grams silver and 6.8 grams gold. Production of the gold concentrate amounted to 5.53 tonnes averaging 9,493 grams gold and 2,467 grams silver.
Production statistics for 1999 are estimated at 839 million tonnes grading 6.5% zinc, 1.5% copper, 1.32 grams gold and 25.02 grams silver. Average cash costs are expected to ap
proach US43.5 cents per lb. zinc, net of byproduct credits valued at US25.8 cents per lb. zinc.
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