Mine Visit DAKOTA VISIT

But for Wharf, which first started mining Annie Creek in 1983, profitability was accompanied by teething pains. Clay probably has caused the biggest problem. The clay does not inhibit percolation on the pads, as you might expect. Instead, it clogs the crushers to the point where “it’s just like bubblegum,” said Mine Manager Robert Barnes. During the first two years of operation, miners encountered as much as 180-ft clay thicknesses as they worked the north end of the Annie Creek pit. In 1984, a wobbler was installed at the crushing plant to pull out the clays, but it proved costly to maintain.

Last year, Roberts & Schaefer of Salt Lake City, Utah, designed a new crushing system, adding a bigger jaw crusher (a 48-inch Nordberg for the primary crush), a secondary Omni cone crusher, and two Omni shorthead tertiari es. Whereas before the final product was –1 1/4 inch, it is now — 1/2 inch, which has resulted in better recoveries and a speedier leach cycle. The clay-rich ore is still sent through the old crusher, which had a 1.4-million-ton-per-year capacity, although Barnes said it was a fight pushing even a million tons through it. High-clay ore is mixed 2-to-1 with less mucky material from the Foley Ridge pit.

“We’ll continue having clay problems and we’re still modifying the system to handle the muddy materials,” Barnes told The Northern Miner Magazine during a site visit. “But we should be out of the muckier sections by next year.”

The new crushing system was part of a major $9-million (us) capacity expansion that also included additions to the treatment plant and the fleet of 50-ton Caterpillar haul trucks, construction of Pad No. 4 and the purchase of a Demag H85 7 1/2-cu-yd hydraulic shovel. (The pre-expansion operation had a 1.3-million-ton capacity.) The Demag is ordinarily used in the pit, but it can also be found unloading the leach pads because of its faster cycle time. When pad off- loading speed is of the essence, the front-end loaders go to the pit.

Grade control is a priority at this operation. As grade control engineer Dan Dorfschmidt explained, the Deadwood Formation hosts the main ore zone. This intertidal sequence of sandstone, quartzite, siltstone and mixed calcareous rocks has been intruded by northeast-trending, steeply dipping fractures (or verticals, as they are called). Gold mineralization occurs within these fractures and spreads horizontally where ore mineralization replaced impure dolomite beds. The ore zones often overlap horizontally where the verticals are closely spaced.

“When we have a bench with shales or shaly material, we don’t have much ore,” Dorfschmidt said. “But when we have the sandstones and siltstones, we have lots of ore.” Grade control research aims to minimize ground movement, and thus dilution, during a blast. “We try to blast so that the ore, when it moves, moves in the direction of the ore. If the (gold- carrying) verticals trend northeast, we have the shot move northeast.” When shot monitoring first began in 1987, Wharf found that the material sometimes moved as much as 16 ft. Now, the maximum allowable movement is 5 ft and always in the direction of other ore, not into waste.

“You’d be surprised by the number of operations that don’t monitor. They’re taking dilution.” To hinder movement, Dorfschmidt has instituted a 15-ft buffer between the face and the shot. The timing of the shot is such that one row blows and re- settles before the next, only micro- seconds later, is shot. Wharf also found that a straight grid pattern is less useful than a staggered grid. The latter also improves sampling accuracy and requires fewer holes (at the expense of breakage, however).

All the mining is done by Wharf itself, not by contractors. Again, grade control was the determining factor. Says Barnes: “I believe you’re going to end up with the contractor and the mining company not having the same specific objectives. They (the contractors) can say they’ll meet you on grade control, but it still comes down to them getting paid per ton.” A do-it- yourself operation can also respond more quickly and without resistance to changes in the mine plan. For example, contractors might fight a request for a tighter drill grid pattern, say from 15×15 ft to 13×13 ft, because such a change would increase production times.

And flexibility is important here. Sometimes, rather than taking a full 20-ft lift from the pit, only the top 10 ft runs ore. Wharf has opted to mine that top 10 ft in a single lift, as was the case during our tour of the Annie Arm open pit. “We try to take the 20 ft lift if we can,” said Barnes, “though we can accept a 10 ft. It costs us more, but we get more ounces out of it. It requires co-operation between the engineers and production people.”

Currently, Wharf is taking one more slice out of the east side of the Annie Creek pit where the stability of the walls has allowed for 40-ft benches. In the Annie Arm, benches are 20 ft because the ground is less stable. General mine foreman Mike Caretto said Annie Arm and the Foley Ridge pit, which is also being mined, will eventually be linked as one large pit. The pit equipment includes three Caterpillar 988 loaders, the Demag H85, seven Caterpillar 773 50-ton haul trucks and two DM25 5 3/4-inch Ingersoll-Rand rotary drills. The anfo (ammonium nitrate and fuel oil) and Nonel detonators are made by Austin Powder. Nine holes at a time, separated by milliseconds from the next nine, are shot to reduce ground vibration.

The old workings haven’t caused problems in the pit. The oldtimers left fairly reliable maps, so that Caretto and his crews can accurately locate the workings. However, drills have unexpectedly broken through into openings where miners followed a vein maybe 80 or 90 ft above the drift floors.

The heap leach segment consists of four pads. Initially, off-loading had not been planned. But now that off- loading has become part of the cycle, pad number 1 has been re-lined and pad number 2 will be re-lined to accommodate the change. Each pad is huge, capable of holding about 1.3 million tons on average. The pads are loaded in 20-ft lifts up to 100 ft high. Sprinklers, not drip-lines which seem to be developing into the preferred cyanide delivery system elsewhere, spray the ore with the cyanide solution. Drip-lines are used in the cold winter weather (the cyanide solution is also heated to 60 degrees F). “Drip-lines, I don’t believe, work as well as sprinklers, especially on a buried basis. You don’t know what you’ve got when they’re buried,” said Barnes. There is no question that production drops off dramatically in winter. The average June production rate is 7,000 oz, while January production might muster 3,000 oz.

The leak prevention system consists of an 8-inch compacted clay liner, a polyvinylchloride (pvc) liner above that, a layer of crushed rock and a High Density Polyethylene (hdpe) 60-mil liner. The first pads also featured a fibreglass cloth
lining. A leak detection system was triggered on pad number 4 last year, which resulted in lost production as technicians searched for and then repaired the leak. Cyanide, however, did not escape into the environment. It was contained in the collection gallery beneath the primary liner. The allowable leak in the collection gallery is 20 gallons per acre per day.

The leach cycle entails a 12-month leach, a 2-month neutralization period and a 2-month offload. Recoveries run 74% on average. Cash operating costs are $178(us) per oz of gold produced. MinVen’s Gilt Edge

The Gilt Edge property has been described as an orphan of several companies. Lacana Mining (now part of Corona Corp.) once held it. Cyprus Mines and CoCa Mines also shared interests in it. But MinVen Gold Corp. was the company to turn the project into a producer. Having done that, however, MinVen wants to expand. It is hoping environmental authorities will issue a positive decision on a proposed expansion that would quickly class it as a major gold miner in the U.S. The proposal to mine a deeper sulphide/pyrite zone beneath the current oxide reserve with MinVen’s Gilt Edge property could rival the Ridgeway mine in North Carolina as the U.S.’s biggest high-tonnage, low-grade, gold operation relying on conventional milling to process ore.

The Gilt Edge mine, currently a heap leach operation, could be producing as many as 200,000 oz of gold per year beginning in August, 1992. The company proposes mining six million tons of ore per year and as much as 18 million ton s of waste. MinVen has submitted its case to environmental agencies and continues to work closely with them to iron out any problems that might crop up during the approval process. In Utah, heap leach operators line not only the pads, but even the ground beneath mill buildings to prevent cyanide seepage. Even slight alterations to mine plans require more than a token submission of details to some government department.

“I’m confident we can get all the permits we need based on my talks with the regulatory people,” MinVen’s project director Doug Stewart told us during a visit to the site a few kilometres southeast of the famous mining town of Lead.

At a capital cost of $128 million, the proposed expansion would tap MinVen’s deeper sulphide ores, which were delineated in an exploration drilling program that began in January, 1988. The $3.6-million, 160,000-ft program proved up reserves of 32 million tons grading an average 0.045 oz gold per ton. Later drilling showed that as much as 90 million tons may be mineable. The official published reserve now stands at 54.2 million tons of proven and probable ore grading an average 0.041 oz gold per ton at a cut-off of 0.0225 oz per ton. The strip ratio for that tonnage is 3.0:1 (waste:ore). Bechtel Engineering of San Fransisco, Calif., is conducting a feasibility study on the proposed expansion. Lakefield Research tested the metallurgical characteristics of the deposit and developed a flowsheet, while Piteau & Associates has studied the geotechnical problems, such as slope stability.

While much of the reserve is nominally a sulphide ore, Senior Exploration Geologist James Barron noted that it is less refractory than other such ores because it contains relatively little sulphide mineralization (in the order of 3% to 5%). In addition, the gold is associated with pyrite. “We’ll get a lot of the gold with gravity circuits,” he said. “It’s basically a liberation project that will require a very fine grind.”

MinVen has submitted to the U.S. Forest Service (which must approve the plan of operations before local and state permits are sought) two different plans — one for a 60-million-ton pit and another for a 90-million-ton pit. The latter figure represents a deeper extension of the proven sulphide reserves, but current drill data are insufficient to establish whether profitable mining is possible. The waste-to-ore ratio, for example, may prove too high if drilling on the periphery of the current, projected pit dimensions fails to core mineable values. “Our ability to go deeper (than the 60-million-ton figure would warrant) is determined by our drilling for more ore on the perimeter,” Barron said.

While MinVen’s expansion proposals are assessed by authorities, it will continue to mine the oxide portion of the orebody, which produced its first dore bar in October, 1988. MinVen relies on a contractor to mine and haul to the crusher. Caterpillar shovels, front-end loaders and haul trucks predominate in the pit. A Driltech D40K, 6 1/2-inch down-the-hole machine drills on a 13×13-ft pattern. Earlier, the grid was 15×15, but the narrower pattern reduced ground movement during blasts. On the downside, production drilling times have climbed and overbreak does occur. The overall waste-to-ore ratio for the oxide deposit is 1.3:1.

Mine Engineer Michael Golliher is generally pleased with the orebody. “What we’re finding when we mine is that the ore/waste boundary doesn’t feather out. It’s an abrupt change. I like it that way because it’s cut and dried.” The mine contractors move 7,500 tons of ore per day and between 12,000 and 13,000 tons of waste. Golliher said he keeps a close eye on grade and tonnage. “They (the mining contractors) are just used to going in there and hogging it out.” Two pits are being mined — the Dakota Maid and the Sunday. Because Dakota Maid has a higher strip ratio, Golliher must balance the mining of the two pits to maintain the overall ratio.

The current bottleneck in the operation has been the leach pads. Any material subjected to cyanide must be detoxified and neutralized before it is dumped as spent ore in an unlined pit. So the leach cycle must incorporate several weeks of detoxification to cut the cyanide to 0.5 parts per million. (By comparison, the allowable level of cyanide in drinking water in the state is 0.75 ppm. In Nevada, another big heap leach state, allowable cyanide levels in off-load leached material are 2.0 ppm.)

Detoxification, however, reduces the pad space available for leaching. Currently, seven cells, each with 65,000-ton capacities, are available. The normal schedule calls for four under leach while three are simultaneously being neutralized in readiness for offloading. MinVen had originally projected that this stage of the cycle might require four weeks. It has been found that neutralization and off-loading of the cells take eight to 12 weeks. To shorten the time, MinVen now is crushing the rock to –1 inch rather the original –2-inch crush. This increases the surface area over which the neutralizing agents (hydrogen peroxide and copper sulfate) can do their work.

“We’re really pioneering a new area with this neutralization,” said Stewart during a tour of the leach pads. He expects Gilt Edge will produce anywhere from 30,000 to 36,000 oz of gold this year, but not the 40,000 projected earlier. Extreme winter conditions also cut into production, but only a week’s worth of leaching was lost.

During the abbreviated production period last year (leaching began in mid-year), the mine processed 547,772 tons of ore and produced 6,666 oz of gold and 8,072 oz of silver. Lime is added, but the ore does not require agglomeration. The pads are 35 ft high. A drip system spreads cyanide in winter, while sprinklers are used in summer, partly to encourage evaportion and thus reduce the overall water levels in the entire system. (Gilt Edge consumes less water than is returned to the area in the form of annual precipitation. Evaporation ensures that pond levels stay at acceptable levels.) The recovery rate runs at about 70%. Because of the high silver content, a Merrill-Crowe zinc precipitation plant is used to recover the gold and silver from the pregnant leach pad solutions. Carbon columns are less effective when silver-to-gold ratios run any higher than 3:1. Golden Reward Mining Co.

This project, nearing start-up during our visit, hosts proven and probable mineable reserves of 13.3 million tons grading an average 0.041 oz gold per ton with a strip ratio of 2.1:1. “From an exploration standpoint, we think we have
a very good chance of increasing the mineable reserve to 20 million tons,” said Chief Geologist Karl Emanuel. More than 1,500 reverse circulation drill holes proved up the tonnage in a program that began in late 1985. Diamond drill cores and more than 30 column tests helped verify the grade and provided metallurgical samples. Hazen and iprc (International Process Research Corp.), both of Golden, Colo., did some metallurgical work. Bondar-Clegg Inc., which has an office in nearby Deadwood, was the principal assayer in the early going. Geostatistical methods were used to develop the reserve figure.

There are two main pit areas — the West Mine, which runs 2,500 ft along strike, and the East Mine, with a 4,000-ft-long strike. The current maximum design depth is 300 ft, but the orebody is open at depth in a number of areas and at one end. The feasibility study was done by Bateman Engineers of Lakewood, Colo., for process design and the facility. Klone Leonoff of Kirkland, Wash., worked up the pad design and the geotechnical data.

Chief Mine Engineer John Gould said he expects that the two Ingersoll- Rand 5 1/2-inch, down-the-hole drills (a DM25 and a DM30) will drill on a 14×14-ft pattern. “We’re getting good breakage on pre-stripping, but I feel uncomfortable with opening it (the pattern) because I need it for grade control as well. I don’t want the ore to move and I can know exactly where the ore and waste contact is.” He also expects that plenty of sampling will be required to maintain grade. “I can see where the ore zone is, but I can’t tell if we’re in 0.02 or 0.04.”

Blasting is also a delicate piece of business. The mine is about a quarter of a mile from residences that dot this part of the Black Hills. Golden Reward detonates no more than two holes at a time to reduce vibration. Millisecond delays separate the shots. To cut noise levels, Gould is using a “silent” detonating chord with a thinner-than-usual powder column to transfer the charge to cast primers down the hole. He also plans to bury surface delays under sand. The drawback here is that occasional line breaks may occur. Austin Powder products are used. Caterpillar 773 50-ton haul trucks will carry ore to the crusher.

The ore itself is virtually all oxide and “about as clean as you’ll get,” according to Robert Polak, manager of metallurgy. Some agglomeration (on average 5 lb of cement per ton of ore) and pre-cyanidation will be done after crushing run-of-mine to a nominal — 1/2 inch. Lime is not required. On top of the sub-grade of native rock, the following is emplaced:

* six inches of clay compacted to rigorous permeability requirements;

* another six inches of amended clay with a 6% bentonite addition;

* a 30-mil polyvinylchloride (pvc) liner installed by Nilex;

* a 10-oz geotextile felt mat, also installed by Nilex;

* perforated pipe for leak detection;

* nine inches of compacted drain rock (–1 inch with less than 4% fines);

* a 2 1/2-inch layer of hydraulic asphalt compacted to 95%-98% (asphalt compaction on ordinary highways runs about 85% to 90%);

* a 1/4-inch thick, 200-mil rubberized asphaltic membrane; and finally

* 2 1/2 inches of rubberized hydraulic asphalt.

Pad loading and off-loading will be accomplished by a sophisticated stacker and reclaimer system of the type that might be found at coal ports. The two pads are rectangular, 1,800 ft long and 182 ft wide, with a 3.3% slope to the east. They are each divided into 12 cells and the two pads are separated by a ditch. The stacker and the reclaimer (two separate pieces of equipment made by Jervis Webb) are track-mounted, with a reach that extends across the width of a pad. While the stacker places ore as it crawls the length of a pad, the reclaimer off-loads the de-toxified, neutralized spent ore. In conception, it’s a beautiful system that should garner attention within the heap leach fraternity. But it’s not as if this were some revolutionary breakthrough: It might be new to heap leachers, but not in bulk materials-handling. The stacker and the reclaimer have been deliberately over-designed to handle 850 to 900 tons per hour. In operation, the stackers will probably run at 625 tons per hour.

The leach cycle will run seven weeks, according to projections based on extensive metallurgical testing. The wash, neutralization and detox cycle should run three weeks. Recoveries are expected to be 70%. The pad lining must rank as the Cadillac of linings.

With an average silver-to-gold ratio of 5:1 and extreme instances where the ore runs 15:1, Golden Reward had little choice but to opt for a Merrill- Crowe plant. The plant is highly computerized. Both the crushing system and mill are controlled by programmable logic controllers from Allen Bradley. From the mill building, Polak can monitor and control both systems. The mill should be producing 60,000 oz gold and 60,000 oz silver per year. If construct ion deadlines hold, the first dore bar may be poured by mid-September. Bond Gold’s Richmond Hill

Mining at Bond Gold’s Richmond Hill, only a few miles west of Golden Reward and Wharf’s Annie Creek, began in August, 1988. The current reserve is 3.9 million tons averaging 0.053 oz gold per ton. This oxide reserve probably will be expanded once a feasibility study on a second pit is completed. As well, potential exists for sulpide mineralization at depth. Mine Superintendent Roger Gross said this approximately 42,000-oz- per-year producer will be profitable because of its “good grade, the permeability of the rock and a low strip ratio.” The strip ratio is 0.79:1.

Process Plant Superintendent Steve Dixon said “it’s a beautiful orebody that percolates like a charm. There’s no arsenic, no mercury and no carbonaceous material. The (pregnant) solution is running 10 ppm copper, but it’s not a problem.” The gold-to-silver ratio is 1:1.1, so following proven practice, the operations people chose carbon column recovery. They also chose driplines over spray irrigation. According to Dixon, who has produced a technical paper on the topic, driplines reduce reagent consumption, maintenance costs, heat losses and evaporation losses. They also eliminate wind drift and allow for variable irrigation rates.

“I’ve had people tell me that drips channel, but really the solution runs by capillary action. I don’t like sprays because they pond.” In winter, a specially equipped Caterpillar dozer with a 3-arm ripper and hose insertion devices buries the driplines about 3 ft below surface to prevent freezing. The cyanide drip solution is not heated in winter.

In the pit, the Driltech D25K 5 1/2- inch down-the-hole machine drills a 12×12-ft pattern on 20-ft benches. Geodimeter’s electronic distance measuring station is used extensively in the pit and elsewhere. “We don’t do anything by hand here and that has reduced our labor requirements,” said Gross. After the Demag H85 hydraulic shovel loads the Caterpillar 773B 50-ton trucks in the pit, the haulers have quite a long run to the crusher near the leach pads — about 11,000 ft in all. Gross said this was unavoidable. The rugged terrain immediately surrounding the deposit couldn’t accommodate enough level ground for several large pads. The Demag was purchased specially for this project because the shovel action doesn’t disturb the ore until it is mucked. “We don’t doze over broken ore to a loader,” Gross said. “We need that for ore control and selectivity.” Ore is mined at the rate of 5,000 tons per day (waste, 3,900 tons) and trucked to the crusher during the day shift. It is run to the pad from the crusher on the night shift.

The crusher circuit, built by Fisher Industries, begins with a 40×48-inch primary jaw crusher. A Superior Allis gyratory and an Allis-Chalmers cone crusher reduces this product to the — 3/4-inch pad size. The mining and crushing are done under contract by Summit Inc. A Caterpillar D6H dozer, with wide tracks to reduce ground pressures, spreads the ore across the pads. Each pad is loaded in 20-ft lifts to a maximum height of 120 ft. A Cat D9 cross-rips to reduce compaction.

As with any other heap leacher in
the Black Hills, Bond Gold’s pad liners are multi-layered and feature a leak detection system. The compacted layer just above the bedrock here, however, is a bit different. To provide permeability that meets regulatory specifications, Bond Gold mixes –3/8 inch rock with bentonite in a pug mill. “We’ve never had a permeability test come back out of spec,” Gross said of the success he has had with the pad liner.

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