The Billion Dollar Triangle GOLDEN GIANT: HEMLO’S MIDDLE PRODUCER

Golden Giant has reserves of 18 million tonnes at a grade of 10.85 g gold per tonne. Potential does exist for more ore at depth near the Williams mine boundary. The annual production rate is 3,000 tonnes per day. The 6-compartment shaft bottoms at the 4176 level (1,145 m below surface; the shaft collar is 5,321.5 m above sea level, and underground levels are numbered based on elevation). Two 16-tonne Dorr-Oliver bottom-dump skips bring ore to surface.

Up to now, ore has come from three separate blocks, all to the west of the shaft. Included in those blocks is the Quarter Claim, owned jointly by Golden Giant and the David Bell. Block No. 1, bracketed by the 4900 level below (421 m beneath the surface) and 5000 level above (321 m underground), is essentially mined out, except for a 180,000-tonne pillar. Small slivers of ore dubbed “west retreat” will also be taken. Stopes in this block were mined to a 100-m height, but, because of the ground movement that resulted, 50-m lifts were the norm in the No. 2 block. “The 100-m verticals gave us ground control trouble in the hangingwall,” said Mine Superintendent Howie Stoughton during a mine tour. “It was causing dilution and giving us big muck.” Mine Manager Walter Segsworth said another reason for the reduced heights was to avoid leaving pillars. “By going to smaller stopes in Nos. 2 and 3 we could fill quicker and put the next stope into production.”

That second mining block lies between 4775 and 4875 levels. Mining retreated from the west toward the shaft, encompassing a portion of the Quarter Claim. The remainder of the jointly-owned Quarter Claim ore extends to just below the 4500 level. Stope dimensions in No. 2 were 50 m high, 20 to 25 m wide and 15 m along strike. Currently, the most active mining area is the No. 3 block, between 4750 and 4400. From experience in the upper two blocks, the engineering department decided that optimum stope heights would be 66 m in this block. In the Quarter Claim area of this third mining block, stope heights are 100 m.

The mining method is virtually identical to that of the Williams — an open stope blasthole process with delayed backfill, except that at Williams pillars are left. The raiseborer, a Robbins machine, reams out a 48-inch- diameter hole between levels. Production drilling is done with two Tamrock Datasolos H1008RA, a fully automated unit that, once collared, can drill a hole, pull out the drill string and be ready for the next setup all without the intervention of an operator. The units worked by Golden Giant are 3.5-inch tophammers. “They give us good results in accuracy, penetration rates, and so on and the maintenance is good too,” said Stoughton. “It is a complicated, technical piece of equipment and we use people with electrical or mechanical backgrounds to operate it.” The Datasolo uses Secoroc tube rods. The mine may experiment with more conventional but stronger drill rods that can match the accuracy of the new tube rods. The operator of the Datasolo we encountered on our tour was a Class a mechanic who also repairs the drill when necessary.

The mining sequence progresses in pyramidal fashion, building a base from the main level and then moving upward and laterally. “This ensures ground control and continuity of mining and it also offers flexibility,” said Stoughton. After stopes have been completely mucked, the sides and back are washed with high-pressure water to retrieve fines. Mill feed grades can jump by as much as 15% to 20% because of the wash, said Stoughton. The mining is sequential. “We mine out one stope, backfill and move on to the next adjacent stope,” said Stoughton.

Backfill consists of a combination quarry rock (90% to 95%) from surface and cement (5% to 10%) mixed underground at strategically located backfill stations. Trucks haul the backfill to stopes. Ore can be skipped from three separate locations in the shaft — from the 4600 level, where a grizzly and Teledyne rockhammer size ore from Nos. 1 and 2 mining blocks; from 4235, which handles No. 3 mining block ore; and from 4210, where muck from the shaft bottom is taken with ore from 4400 level. The main crusher, a Gatx-Fuller 48×60-inch jaw crusher, is on 4295 and feeds the 4235 loading pock et. It crushes to minus eight inches. Soon, all ore will go through a series of orepasses down to the 4335 haulage level. Twenty-tonne rail cars will tram the ore to a coarse ore bin, above the crusher. This system replaces multi-level haulage trucking.

To monitor any microseismic activity, a 48-channel, whole wave form microseismic system was installed.

The 3,000-tonne-per-day mill begins with a Nordberg, 7-ft standard cone crusher and a 7-ft shorthead cone crusher. Both are 350 hp and have a combined crushing rate of 270 tonnes per hour. The grinding circuit consists of three 12×14-ft Dominion ball mills and primary and secondary cyclone packs. A fourth ball mill has been purchased. This mill will achieve a finer grind and thus improve recoveries. The discharge currently is 80% minus 200 mesh. With the fourth mill, the discharge will be 88% minus 200 mesh. A gravity circuit consisting of duplex jigs and a Sala ball mill have been removed because a finer grind achieves slightly better recoveries and allows for the option of boosting mill throughput.

“The advantages of the jigs are that, potentially at least, faster recoveries result and the gold inventory is reduced marginally,” said Mill Superintendent Chris Larsen. “But they are more complex in terms of operations and maintenance and did not improve overall recovery.” The jigs had been recovering about 20% to 25% of the gold content. After thickening to 55% solids and pre-aeration, the gold is leached and then put through a carbon-in-pulp (cip) circuit before stripping, electrowinning and, finally, refining to produce a dore bar of 93% gold and 3.6% silver.

At one time, Golden Giant had been testing a molybdenum circuit to recover the moly in the deposit. Molybdenite grades can run from 0.1% to 0.2%. To date, the circuit has not been a success. This fall, mining will move into higher-grade moly ore, so an attempt may be made at recovering metal again. However, beyond grade problems, impurities are the main stumbling block. “The problem primarily is making an adequate quality of moly concentrates. Impurities such as talc, mercury, arsenic and antimony give us problems,” said Mill Superintendent Chris Larsen.

While recovering moly continues to challenge Golden Giant’s metallurgists, they have rather inventively solved a heavy metal problem in the effluent stage. By taking tailings reclaim water and introducing it to the grinding circuit, the amount of contaminants, such as cyanide, copper, iron and antimony, were cut by as much as half. Because the residual cyanide in the reclaim water dissolves trace levels of gold, the thickener overflow is put through carbon columns to ensure maximum gold recoveries. “The savings we’ve made in terms of reagent costs, plus the added revenue of slightly higher gold recoveries have paid for the carbon columns in less than a year,” said Peter MacPhail, chief metallurgist. This innovation was decided upon after it was found that the thickener overflow
contained fewer contaminants than the tailings water. Effluent treatment charges were cut by $150,000 to $200,000 per year because of the new circuit, Larsen said.

Process monitoring and control is acheived using Fisher Controls hardware — a total of seven terminals throughout the building. Each of the four operators per shift are notified of problems by horns that are triggered by the monitoring system. “We have toyed with the idea of voice synthesizers to call operators,” MacPhail said.

While such sophistication is not impossible, its introduction into Canadian mills is probably a few years off. However, this plant was the first in North America to have automatic cyanide titration installed. The Billion Dollar Triangle QUEEN OF VAL D’OR For 52 years, the Sigma mine has been humming along in the “Valley of Gold,” and not even low gold prices seem to hinder it. By P atrick Whiteway Gold prices are the lowest they have been since 1986. Exploration companies are not searching for the yellow metal along the so-called “Golden Highway” with the same abandon as was displayed during the years of easy flow-through money. And production miners from both the Belmoral and Camflo mines have been laid off.

Times are becoming increasingly tough for high-cost, underground gold producers, and so one might expect the Sigma mine in Val d’Or, Que., which last year reported a cash operating cost of $389 (US) per oz, to go the way of the neighboring Lamaque mine. Lamaque, along with at least seven other headframes in the area, stands as a sentinel to this town’s brief 50-odd-year history.

But the opposite is true of Sigma. This venerable mine, a continuous gold producer since 1937, has increased its milling capacity by 10% and will produce even more gold next year. Sigma has more than half its 64,000-oz annual production (53%) sold forward, up to February, 1990, at $473 (Canadian) per oz. In fact, forward selling of gold allowed the mine to record a $7.4-million operating profit last year while pumping $25 million into the local economy.

And if bullion prices don’t recover by early 1990? “We could increase ore production,” Mine Manager Andre Carrier speculated. “We could cut our dilution. We could mine at a higher grade. We could cut our underground diamond drilling from three machines to two. Or we could move some of our raise crews into production stopes.” Flexibility, it seems, is more than just a buzzword at Sigma. It is a necessity.

When The Northern Miner Magazine visited the mine in August, operators were completing a $10-million modernization program, which increased mill capacity to 1,525 tons (1,385 tonnes) per day and added five on- surface air-compressors to serve the underground operations. Hydro Quebec kicked in about $7 million of that, mainly to convert the mine/mill operation into a 60-cycle electrical consumer rather than a 20-cycle consumer. That project alone meant replacing more than 100 electrical motors and pouring new concrete pads to accommodate the new motors. “It was a major undertaking,” said Mill Superintendent Jean-Guy St-Jean. Now that the conversion is completed, the mine will become an even bigger consumer of electricity. (Kilborn did the engineering work and Lorynesdale the contract work on the electrical conversion project.)

Also, the Sigma mine, which last year became a part of the huge, international gold mining empire of Placer Dome, appears to have a sure future ahead of it. The capital cost of the mine/mill facility was paid back long ago and there remain 4.5 million tons (4.1 million tonnes) of proven, probable and possible ore above the 6,000-ft (1,800-m) level. These reserves average 0.138 oz gold per ton (4.7 g per tonne), which is good for at least eight more years of mining. Three diamond drills, from underground drilling stations, are expanding those reserves.

A cross-section of the Sigma orebody reveals the source of its operating dexterity (see accompanying diagram). Ore is not confined to one neat, uniform geological unit that can be mined using low-cost bulk-mining methods (in fact, not being tied into the bulk methods of mining contributes to the operation’s profitability). Rather, it is confined to quartz- tourmaline-pyrite veins, which occupy shears and fracture planes in the local lava-porphyry complex that hosts the mine. This means the ore can be classified into three types based on structure: * a number of parallel, steep ly- dipping shear veins and dykes;

* a set of moderately-dipping shear veins and faults; and

* several subhorizontal extension veins, known as “flats.”

The Sigma mine is well known in Quebec as the perfect mine for the “classical” miner (i.e. one who excels at using jacklegs and slushers). A different mining method is used for each vein type. Longhole open stoping, modified AVOCA, cut-and-fill, shrinkage and even room-and-pillar are all used. And stopes are situated everywhere from the second to the 40th level. Pneumatic jacklegs (mostly manufactured by Secan and Boart) are the most important mining machinery in the operation, although operators expect to have a new compact rig, equipped with a small hydraulic drifter, in service this fall. That unit is being developed in co-operation with the CRIQ, a Quebec mining research organization (see accompanying illustration).

Today, operators can draw ore from at least 37 active stopes; 25 are cut- and-fill stopes, 15 are room-and-pillar; five are shrinkage; and two are longhole. All have their own dilution as well as their own grades, which range from a high of 0.40 oz gold per ton (13.72 g per tonne) in one small vein to a low of 0.07 (2.4 g per tonne) in another small vein. Consequently, choosing the stopes from which to draw daily production can be a real cat-and-mouse game. There are about 400,000 tons (360,000 tonnes) of broken reserves in the mine. (That means the operators can get away with breaking only 400,000 tons, or 360,000 tonnes, of ore per year while hoisting 500,000 tons, or 455,000 tonnes.)

One source of ore, the Sigma No. 2 mine, has no built-in flexibility. It is a nearby open pit, which provides about 50,000 tons (45,450 tonnes) of ore per year, or 12% of the mine’s total annual production.

There is a price to be paid for flexibility in an underground mine — namely higher operating costs. At Sigma, about 70% of the total mining cost represents labor. That means that each of the roughly 250 underground workers contributes about $1 to the cost of producing an ounce of gold. When gold prices began to fall in 1987, the mine was forced to reconsider its operating philosophy. “We’re just now getting into the situation where everyone in the mine has to get involved in cutting costs,” Underground Superintendent Richard Quesnel told The Northern Miner Magazine. “We’re telling our miners, Either get off your butt or else.’ ” That “or else,” of course, means layoffs, something Sigma has avoided during its 52 years of operation. But if worse comes to worst and the operation is forced to cut by, say $50, the cost of producing an ounce of gold, then 50 people could lose their jobs.

During our visit, the operation was devising several ways to bring down mining costs without resorting to layoffs. One method involves centralizing the underground maintenance shops on the 2,400-ft (720-m) level. The new welding and mechanical shops will shorten the time required to make deliveries to the various working levels, thereby improving productivity. There are more than 40 km of drifting in the mine and stopes are spread out over 900 m of strike length and up to a depth of about 1,800 m. So supplying services to all active stopes can be time-consuming.

It is difficult to pin down exactly where ore production originates in any particular month at Sigma. But, generally, about 30% of the mine’s production comes up the internal No. 3 shaft. This opening extends from the 2,700-ft (800-m) level to the 5,965-ft (1,800-m) level (see accompanying diagram). The No. 2 shaft was driven from surface to the 3,317-ft (1,000-m) level and is equipped with two skips in balance — one has a 4.5-tonne capa
city and the other is a 5.5-tonne unit. Ore is hoisted from loading pockets on the 16th and the 24th levels. The No. 3 shaft has just one 5.7-tonne (6 1/4-ton) skip-and- cage combination which runs opposite a counterbalance. “Once the ne w cage is operating (in an adjacent compartment), this skip will be free to do more hoisting,” Quesnel said. Ore is hoisted through the No. 3 shaft from loading pockets on the 30th and the 40th levels, then transferred to the ore-handling system in the No. 2 shaft.

“We are always trying to push more production through the No. 3 shaft, but we’re constrained by the 16 to 18 tonnes per day of hydraulic backfill that come back into the mine from the mill. If we could get more backfill, we could mine more ore.”

Problems arise when 10 stopes need to be filled simultaneously, Chief Mine Engineer Christian Pichette explained. “Some days, there may be just one stope; other days, 10 are calling for fill. We’re looking at the possibility of installing a filter in the mill, which could produce more bulk fill material for the mine, but now we are at the limit of our backfill capacity.” Material has been taken from the tailings pond and used as backfill during emergencies, but this technique is costly.

Because of the excellent ground conditions at Sigma, miners typically take 8-ft (2.5-m) breasts in the cut- and-fill stopes and install 12-ft (3.5-m) long Swellex rock bolts on an 8×8-ft (2.5×2.5-m) pattern. (That means there are 4-ft, or 1.2-m, bolts in the back after each blast.) Broken muck is moved to the 46-inch- (117-cm-) diameter, steel-lined millholes, which are spaced every 22 to 24 m along the length of the stopes, by slushers or air-powered, rubber-tired (Cavo 310) mucking units. “We’re slowly switching to electric slushers, away from the pneumatic units,” Superintendent Quesnel said.

Cut-and-fill is used where the ore dips at 70 degrees and has an average width of 8 to 12 ft (2.5 to 3.5 m). Cut-and-fill stopes account for about half the total daily ore production from Sigma. Productivity is about 15 tons (14 tonnes) per manshift, but some stopes are as high as 20 (18).

Backfill consists of about 7.5 ft (2.25 m) of classified tailings and about 1 ft (0.3 m) of cemented backfill, mixed at a ratio of about eight parts sand to one part cement. Backfill is distributed hydraulically throughout the mine in a 3-inch- (0.9-cm) diameter, closed- pipe, gravity system. Densities are 65% solids by weight. In the longer stopes, fill fences are constructed of timber and a plastic snowfence-type material called Tensar, manufactured in Oakville, Ont.

In order to mine up an old cut-and- fill stope on the 3,700-ft (1,110-m) level, Sigma has had to devise a way to prevent the timber structure that supports the backfill above from collapsing into the stope below. Maintenance and construction captain Marshall Durocher implemented a cable-strapping system, which works as follows: 10-ft (3-m) holes were drilled at an angle of about 45 degrees into the walls just under the timber structure, on both sides of the sill drift. The holes were spaced 5 ft (1.5-m) apart and 29-ton (32-tonne) capacity cables were grouted into the holes along with 5-ft (1.5-m) split-set rock bolts manufactured by Ingersoll-Rand. These cables, which are strung across the drift under the timber structure, should provide enough support to hold up the structure once it is undermined by the stope below. “We should know by January whether the system works,” Durocher said.

As mining becomes progressively deeper at Sigma, modifications to mining practices have become necessary. This is because the levels of in situ stress increase with depth. One modification, which is also helping save operating costs, is the introduction of steel arch supports in the sill drift of a cut-and-fill stope on the 39th level. The selection of this ground support technique was based on a rock mechanics study conducted by Golder Associates. Already, the cost per foot of installation is down compared with the cost of installing timbered supports, and the steel supports can be installed in about one-quarter the time it takes to install timbered supports. The steel supports are supplied in nested bundles and are bolted together with air-powered tools. The manufacturer is Armco Canada of Guelph, Ont.

Another means by which Sigma hopes to cut mining costs involves testing the Redbore 40 raise bore machine (manufactured by J.S. Redpath of North Bay, Ont.) by driving 75-m- long, 1.5-m-diameter service raises in the cut-and-fill stopes. That test was scheduled to start this month.

The second-largest contributor to ore production at Sigma is the room- and-pillar stopes. This mining method is used where the ore fills tension- cracks in the host rocks. Such ore zones are typically 15 to 60 cm wide and are known as “flats”. They are mined to an average height of about 1.7 m, although a generous restructuring of the bonus system has been implemented to encourage miners to mine less dilution. Some crews are mining rooms as low as 1.6 m high. The stopes typically dip at about 10 degrees to 12 degrees , rendering the jackleg work very strenuous. Some stopes are more than 600 m long. Most of the room-and- pillar stopes are situated above the 24th level and account for about 20% of total ore production.

Ore is hauled by rail to the ore pass system, which feeds two underground crushers. Ore is reduced to minus 6-inch size (minus 15 cm) in either a 42×48-inch Traylor crusher on the 15th level or a 30×42-inch Traylor crusher on the 23rd level. On surface, an 18×36-inch Traylor further reduces the ore to minus 2.5 inches (minus 6.3 cm). A 5.5-ft- (1.6-m-) diameter Symons cone crusher then crushes the ore to minus 7/16-inches before going to the mill storage silo.

Cost-saving measures in the mill include a new grinding section. It consists of a 9×12-ft (2.7×3.6-m) rod mill, an 11.5×14-ft (3.5×4.2-m) ball mill, four 15-inch (38-cm) cyclones (two operating and two on standby) and two 36-inch (90-cm) Duplex jigs. This circuit replaces four 7×12-ft (2×3.5-m) mills with mechanical classifiers. “The new system has not been in operation for a full year yet, so we don’t know exactly how much money it will allow us to save,” Mill Superintendent St-Jean said. “But we should see a definite reduction in mechanical maintenance.”

The cyanidation circuit consists of thickening, agitation and filtering. “We recently picked up two spare 12×16-ft (3.5×4.8-m) drum filters from the Campbell Red Lake mine, in north-western Ontario,” St-Jean said. The Merrill-Crowe process is used to precipitate the gold from solution and two Wabi rotating bullion furnaces refine the yellow metal. Mill recoveries have increased slightly, to 96% from the 95.9% reported last year, with the improvements in the grinding circuit.

So although gold prices are the lowest they’ve been in four years, it seems certain the Sigma mine will be pouring the yellow metal at a profit nonetheless.

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