ON-SITE AT WILLIAMS

But operators need not fuss over exploration potential. At current gold prices, the annual production rate of slightly better than two million tonnes per year won’t exhaust the reserve for a couple of decades. By mid-year, the mine had reached an average production rate of 5,400 tonnes per day. The goal is to post a daily 6,000 tonnes before the year is out to match the nominal throughput of the mill. The operation will produce roughly 500,000 oz per year for the next three years, making it this country’s biggest gold-miner. As of the end of July, it had poured 269,692 oz in 1989. Gil Leathley, Corona’s senior vice-president, operations, said the cash operating costs have averaged $210(us) per oz. Mining

Longhole open-stoping with delayed backfill is the sole means of production. Main levels are established at 105-m intervals, and sub- levels every 25 m. Three main mining blocks are worked. The topmost block, centred near the 300-m deep a zone exhaust raise, can provide 750 to 1,000 tonnes of muck per day.

This block, including a sill pillar, should take another five years to mine out, according General Superintendent Rory Mutch. The tonnages will fall in the later years as the sill pillar is retrieved. The second block, between the 9450 level (890 m below surface) and 9555 level (785 m underground), contributes the bulk of the daily tonnage. (To understand the level sequencing, keep in mind that the shaft was collared at the 10340 elevation.)

A portion of the third mining block (the 9240 level), spanning at least 100 vertical metres with a horizontal strike length of 200 m, contains higher-grade material in the range of 3 or 4 g above the average grade. This richer pocket is believed to extend across the Williams/Golden Giant boundary. Right now, the Williams second block is mined at a daily rate of 2,500 tonnes.

The third mining block, between 9240 (1,100 m below surface) and 9345 (995 m) levels, produces between 2,100 and 2,200 tonnes per day. Once the upper a zone ore is depleted, the mine will rely on ore below 9240 to maintain production levels.

Stope dimensions run 25 m in height, 20 m along strike and to a maximum width of 25 m. Any stopes in thicker zones are split into two panels to allow more efficient mining. “If we took these stopes out to the full width, the stopes would tend to peel or bell out on us,” Mine Engineer Stewart Brown said during an underground tour. The ideal height was also determined by experimentation. “We’ve gone 50 m high, we’ve gone 75 m high. But we experienced a lot of fall-off. The way we are mining, we are conservative, but it maintains continuity.” Trying to blasthole drill 50 m and more also caused drillhole accuracy problems. The current dilution factor is 8% to 9%, including material from backfilled stopes.

Development headings, driven in the footwall, and crosscuts are drilled off with Atlas Copco 2-boom H227 electric/hydraulic jumbos. The rate of advance averages 0.9 m per manshift. Most of the development muck is barren. However, as drifting nears the Golden Giant mine boundary to the east, the muck runs a low-grade 2 g per tonne. After the crosscuts are in, a Redpath Redbore 40 raiseborer reams a 42-inch-diameter slot from main level to main level. The slot serves a dual function as initial blast void and as backfill raise for mined out stopes below.

Cubex crawler-mounted, in-the- hole drills fitted with 4.5-inch bits provide production drilling from sub-level to sub-level. The mine also is experimenting with an Atlas Copco H274 3.5-inch tophammer incorporating Secoroc tube rods. “The penetration rate on the tophammer is better,” Brown said. “And with the tube rods, the accuracy problem has been cured.” (Traditionally, in-the-hole hammers have been more accurate, but slower, than tophammers. Tube rods have improved tophammer accuracy.) The production cycle, from the first collaring of the crosscut to the final tonne of backfill, requires four to six months.

Backfill is supplied from a quarry at surface called the c zone open pit and fed through raises to underground. Quarry rock is mixed with cement slurry on 26-tonne trucks at backfill stations on the main levels. Formerly, a conveyor system was used. Straight quarry rock without cement is placed between two consolidated backfill stopes.

Mucking is accomplished by Eimco Jarvis Clark JS600 6-cu-yd load-haul- dump machines. Operators haul to the nearest of three orepasses situated in the footwall. Each orepass feeds a separate Eagle jaw crusher on the 9135 level (1,205 m below surface). The product skipped to surface is minus 6.5 inch.

At the time of our visit, the mine was pushing to reach the targeted 6,000-tonne-per-day production level, the nameplate capacity of the mill. Mutch expected the target rate would be attained by November. After the mill expansion, completed in mid- 1988, the focus turned toward development and production. Exploration was not the primary aim. “Our whole emphasis was to get up to certain production levels,” said Mutch. “If we drive an exploration drift, we’re losing 300 tonnes per day in the mill, because we would be hoisting that much waste up the shaft every day. Whatever exploration we did would have displaced ore.”

“We need 10 producing stopes to give us 6,000 tonnes per day,” added Brown. “Right now, we have eight or nine stopes.”

Meanwhile, the long-term future of the mine (that is, beyond the 20 years or so already more or less assured) lies to the west of the current workings. The mine is driving an exploration drift into this zone from the 9175 level (1,165 m below surface) of the existing workings (the main shaft bottoms at the 9030 level).

Diamond drill stations are being established as drifting advances. And by year-end, some initial core assays will be available to the mine geology department. This should begin to disclose, in rough fashion at least, the grades and widths awaiting miners perhaps a decade from now.

By the end of next year, diamond drilling from this level will be completed and results will be in. There simply isn’t enough information now even to guess at the average grades and widths in this western portion of the Williams deposit. And with an exploration drift proposed for the 9975 level (365 m below surface) west of the main shaft, it will be interesting to see how the c zone open pit to the west, with its series of east-west- striking lenses, relates to the rest of the Hemlo orebody. “It is possible they join at depth,” said Chief Geologist Al Guthrie. Exploration at this level has yet to be entered into the 1990 budget. “Nothing has been finalized,” said Mutch. “It won’t be for the next month or month and half.”

The orebody is a continuous, tabular deposit running anywhere from 3 to 40 m in width. Below current workings, gold mineralization is known to extend to at least 1,300 m below surface, said Guthrie. The c zone open pit is not regarded a
s an orebody by the mine operators. Rather, this “quarry” provides backfill material and, occasionally, a bonus of low- grade ore millfeed when production drills bite into gold-mineralized sections. Mineral Processing

While the mill nameplate capacity of 6,000 tonnes is about all the mine can handle today, the mill ran at 8,400 tonnes in a 1-day test of its upper limit. “The grinding circuit could handle that but I don’t know that the leach or carbon-in-pulp (cip) circuits could take that on a sustained basis without modifications,” said Mill Superintendent David Barnes. The grinding circuit incorporates two semi- autogenous (sag) mills (22×12-ft Allis Chalmers) in parallel, one with steel liners/lifters and the other with Skega Poly-Met lifters; and a primary and secondary ball mill (Allis Chalmers again). The primary Krebs D20B cyclones are in closed circuit with the primary ball mill, and the secondary cyclones are in closed circuit with the secondary ball mill. The nominal grind is 80% to 85% minus 200 mesh. Leaching is done in nine tanks with Prochem and Philadelphia agitators. Retention time in the tanks is 36 hours. The cip circuit involves six tanks with a constant 18-g-per-l carbon concentration. The gold loading runs from 250 g per tonne in Tank No. 6 to 5,500 g per tonne in Tank No. 1. The carbon is then stripped of gold in a pressure stripping vessel and the gold electrowon and advanced through an induction furnace.

Last year’s expansion included an additional sag mill, a ball mill, two leaching tanks, four trash screens, two electrowinning cells and an induction furnace. Parts of the mill were also integrated with a Fisher Provox process control system. The leach, cip and carbon stripping circuits will soon be added to the Provox control. Mill recoveries averaged 94.9% to the end of July and the mill head grade was 8.1 g gold per tonne (0.23 oz per ton). “The mill has comfortably run at 7,000 tonnes,” Barnes told The Northern Miner Magazine.

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