WINSTON LAKE”S MODIFIED AVOCA: Minnova’s Winston Lake

The Winston Lake mine relies on a modified Avoca method for extraction. But that wasn’t the technique considered in the early planning stages. Back then, flatback mechanized cut-and-fill (MCF) was thought the best method available. MCF was considered because, with an orebody dipping at a shallow 51deg, there was concern the broken ore wouldn’t “run” under other methods. But with cut-and-fill came a potential problem. “We wondered whether the back would hold up if we took it slice by slice,” says Tom Dickson, mine manager.

Soon after start-up it was apparent the rock would run and the hangingwall, for the most part a banded chert-rhyolite-tuff, was even less stable than forecast. The logical alternative was the modified Avoca method employed. Says Dickson: “The (Avoca) long-hole method is cheaper and safer. We don’t have men working under that back.” Occasionally, the muck does get hung up in the stopes. Water spray usually solves the problem. As well, when the dip turns even shallower than the overall 51deg, Winston’s engineers plan an extra footwall hole to steepen the stope and prevent muck buildup.

Avoca is a long-hole method of mining. In true Avoca, says Planning Engineer Joe Boaro, the longholes are drilled up and “double-dipped” (along strike and along dip). Backfilling is done over the bench edge (again, in true Avoca). But Winston Lake doesn’t double-dip, drill holes are downholes, and fill is dumped from the backfilled end of the stope across from the bench edge; thus it calls its Avoca system “modified.” Adopting modified Avoca over MCF right from the outset, required few modifications to the original mine plan. Ore development, however, was required for stoping.

Between main levels (at 275, 335, 395, 455, 510, 565, and 615-m. elevations), sub-levels are established every 15 m. to 20 m. A ramp system provides access to all mine levels. Access cross-cuts are driven from the hangingwall drift at 60-m. spacings. (In plan view, it resembles a ladder with two long parallel drifts running the strike of the orebody and these connected perpendicularly by the cross-cut “rungs.”)

The ore, under geological control, is silled out between cross-cuts. Ground support in ore during development consists of 6-ft. resin rebar. A stoping block consists of the area between two levels and any two sequential cross-cuts. Geology maps all ore headings during development to define ore contacts for drilling and structural discontinuities, according to a paper written by Doug Sands, Senior Production Co-ordinator.

Three-inch-diameter downholes are drilled from the over-cut using a 2-m. burden and spacing. The first, or lead, stope on a level starts with a slot raise between sills. The raise is opened to full ore width, hangingwall to footwall, before any ring blasting. Only two or three rings are blasted at a time when advancing the stope bench.

LHDs muck out from the undercut brow if the brow is covered, or by remote control when the brow is open. Remote operation predominates. Each LHD — four, 6 cu.-yd. EJCs and two Tamrock Toro 400Ds — is equipped with remote control so that no one works under unsupported backs. Dilution can occur from overmucking, which takes out some of the backfill from the prior stope.

Once the brow is covered with waste rock, another blast of two or three rings is fired. Ore pillars are not required.

The method also allows for stope advance on more than one level, and because of its pyramid shape, it adds to the rock mechanics integrity of the mine. This wedge advance tends to shed induced stresses away from the working areas.

To access the final blocks of ore — the mine will run out of ore by 1996 — Winston Lake is currently driving a 3.9 m. x 4.6 m. -12% service ramp to the bottom-most level (705-m. level) of the mine. The ramp is being driven with a Tamrock 2-boom jumbo and mucked out by a EJC 210 haulage machine.

Ground Instabilities

Preventing dilution from the hangingwall is another challenge Winston Lake’s engineers have tackled. The less-than-stable banded chert-rhyolite-tuff is present in 50% of hangingwall surface area and tends to occur where the stoping areas are narrow. Gabbro is in immediate contact with the sulphides in the wider mining areas.

To secure the cherty hangingwall, Winston Lake extensively cable bolts the back. What was not foreseen was the amount of cable-bolting required. Planners had expected about 10,000 ft. per month of cable bolting. Currently, 30,000 ft. are used — about the same footage as production drilling per month.

Cable bolt holes are drilled from a drift driven parallel to the orebody on the hangingwall side.

For Dickson, difficult ground was not new, having been mine manager at United Keno Hill in the Yukon. “We had square-set mining everywhere,” he recalls.

Away from the mine stopes 6-ft. mechanical rock bolts support the back, while 7/8-inch rebar and cement secure the back near mining areas. The cable bolts are 7 m. long.

The annual ore tonnage hauled is 365,000 tonnes. The trucks used are two Jarvis Clark, 13-ton haulers.

Milling

Initially, ore is reduced underground by a rockbreaker. There are two surface crushers, a primary jaw and a secondary cone on surface, feeding the mill 1/2-inch material. Dickson believes the rockbreaker — in fact, any make of rockbreaker — is overworked at 1,000 tonnes per day. This necessitates excessive maintenance. He recommends that despite the higher capital costs, a crusher is ultimately the best tool for primary rock-breaking underground.

“Any rockbreaker that’s going to break at 1,000 tonnes per day will have high maintenance,” he told The Northern Miner Magazine. Primary comminution is especially difficult with the less friable hangingwall rock.

Dickson is very pleased with the help the Noranda Technology Centre has given mine engineers in terms of drill and blast parameters. The centre has supplied a blasting program that, when fed with planned stope dimensions, ore quality, amount of explosives and so on, models what will happen. “It gives us the projected overbreak and the effect on the hangingwall. Now we’re able to finely tune the drill/blast cycle to our needs.”

This, Dickson says, is new for most underground mines. “Open pits for years could optimize the cycle. Underground we were more concerned with just making the rock break than with fragmentation characteristics and damage control.” At the outset in 1986, production drill-hole diameters were 2 1/4 inches. After experimentation, 3, 3 1/2, and even 4 inches have been successfully incorporated. Blasting agents are supplied by Northland Explosives.

The concentrator relies on two-stage flotation of zinc and copper concentrates. It operates at 1,000 tonnes per day at mill heads of 14.7% zinc and 1.1% copper, with a recovery of 94.5% zinc and 80.83% copper for a zinc concentrate grading 54% zinc and 25% copper

Originally, there were a rod mill and ball mill. But it was quickly learned that material from the hangingwall was harder than projected. An additional rod mill was installed to counteract this. There are currently a Marcy 8×12-ft. primary rod mill, and two Dominions — a 10-ft., 6-inchx13-ft. primary ball and a 6×10-ft. primary ball mill. The mills grind to 80% passing –200 mesh. The mill liners are rubber. There are two Krebs 10-inch cyclones and two Krebs 6-inch cyclones.

The copper concentrate is fed through roughers and cleaners at a feed rate of 45 tonnes per hour. The cells are Outokumpu 186 cu. ft. and a Denver 50 and Denver 24 cu. ft. The zinc concentrate is fed through roughers and cleaners at 43.5 tonnes per hour. The equipment includes an Outokumpu 186 cu. ft. with 10 cells per bank and an Outokumpu 186 cu. ft. with 8 cells per bank, and three Denver 50-cu.-ft. units, one with 12 cells per bank and the other two with 8 cells per bank each. Level control mechanisms are all automatic.

The key reagents are Xanthate and Dowfroth 250. Pressure filtration is achieved by a Larox 12-plate and a Larox 16-plate unit with a final product moisture of 7%.

Winston Lake at a Glance

Owners: Minnova Inc. (Zenmac Zinc holds royalty)

Location: 27 km. northwest Schreiber, Ont.

Metals produced: Underground producer of zinc and copper

Reserve: 1.3 million tonnes grading 13% zinc,

1% copper.

Orebody dimensions: 50 m. x 7 m. x 300 m., chloritized felsic tuff, dipping at 51[degree]. It is defined now. There is no more ore after the end of 1996. Regional exploration continues.

Mining costs: $39 per tonne

Milling costs: $16 per tonne

Mining: All modified Avoca with 100% recovery (no pillars) and 20% dilution. Stope dimensions are 60 m. x 7 m. x 20 m. The width can vary from 2 m. to 20 m.

Milling: 1,000 tonnes per day; mill heads: 14.7% zinc, 1.1% copper; recoveries: 94.5% zinc, 80.83% copper.

Production shaft: 12×20 ft., 741-m. deep, 2-compartment shaft. The Norcast cage-over-skip can hold 12 people travelling at 1,200 ft. per min. (for crews), 2,100 ft. per min. for skipping. The shaft has wooden guides. Wire rope by WRI is 1 7/16-inch. The hoist is a Fullerton Hobart 96-inch diameter. The skips have a capacity of 6.5 tonnes

Pumps: Two Mather & Platt 200 Hp at 630-m. sump and one Flygt 52 Hp at shaft bottom.

Ventilation: two 54-inch and two 48-inch-diameter fans at intake. The total downcast is 300,000 cu. ft. per minute.

Backfill: Mined from a surface quarry, where the rock is crushed to –12 inches, and from development muck in an approximate 50:50 (surface:underground) ratio. The rock fill is placed by LHD at the rate of 500 tonnes per day. Next year, the company will tap its tailings ponds for fill by introducing paste fill. This will reduce the amount of tails and, therefore, cut closure costs when the mine closes in late 1996.

Regional exploration: Two deep zones have been discovered roughly 3 km. from the existing mine. However, Alex Davidson, vice-president, exploration, says the zones are narrow and deep. One deposit is 1,000 m. below the surface, while the other is about 600 m. down. Preliminary estimates put the reserves in total at about 1.6 million tonnes.

“We’re still looking at it,” says Davidson. “We’ve got nothing planned (by way of exploration) in 1993.”