BACK ON TRACK

Surrounded by coniferous bush, 209 km northeast of Timmins, Ont., sits the Detour Lake gold mine. It is the only fly-in mining operation in Ontario and, as such, has kept a pretty low profile within the Canadian mining industry. Considering its troubled history, that may not be such a bad thing. During the period 1983 to 1987, when Detour’s ore was mined via open pit, the operation experienced one headache after another. Early on, the ore grade began to drop below expectations. Then some of the calculated reserves failed to materialize. The price of gold, which tumbled badly between 1980 and 1982, contributed to the mine’s malaise. Gradually, Detour Lake came to be perceived as something of a disappointment — at least as an open- pit operation. While gold prices eventually recovered and mining costs were cut, the problem of grade-control persisted. The pit was closed, untriumphantly, in May, 1987, more than a year before it was supposed to. Yet the story of Detour Lake, like many a mining yarn, is one of resilience. In October, 1986, joint-venture partners Placer Dome (the operator) and Amoco Canada Petroleum fired the starting gun for underground development (planned from the outset), and the operation has been back on track ever since. In fact, this hitherto lacklustre gold mine could very well emerge as the mining comeback story of 1988. While higher gold prices have helped ignite the turnaround, a great deal of the credit goes to efficient mine-planning.

Full production of 1,800 tonnes per day was to have been achieved this month and is expected to continue (if not increase) throughout the underground mine’s 9-year-plus lifespan. Even more encouraging, the average grade has risen (5.6 g per tonne of underground ore in 1988, or 0.16 oz per ton, compared with about 3.5 g, or 0.10 oz, during the open-pit phase). And while operating costs are still well above the Canadian average of $202(us) per oz, they have been considerably reduced since the open-pit phase. Officials at Place Dome estimate $276 per oz. The feasibility study that precipitated the production decision clearly demonstrates that the underground operation is viable at $350 per oz of gold.

“We’ve been able to turn the mine around from one which had a suspect future to one with a bright future,” Mine Manager Tim Mann told The Northern Miner Magazine at the snowy minesite last December. “The viability of the mine has been greatly enhanced (since the open-pit phase).”

Mine Superintendent David Henderson agrees. “What you’re seeing now is a reaffirmation of the faith of the people who put their money into the operation in the first place. We have demonstrated an ability to control our costs and produce the materials we said we were going to produce.”

But an important question lingers: why was the orebody not mined from underground in the first place?

Whew] Executives at Placer Dome and Amoco must be wiping the sweat from their brows with relief. But an important question lingers: why was the orebody not mined from underground in the first place?

When the original feasibility study was tabled in October, 1980, gold was in the $600-$650-per-oz range, explains Chief Geologist Wayne Valiant. Consequently, the operators felt they could afford to mine at relatively low grades. That, together with the width of the orebody and the fact that it outcropped at surface, seemed to beg the relatively cheap open-pit approach.

The cutoff grade at the time of start-up was about one gram, and the original, calculated reserves were considerable — in the order of 30 million tonnes. But by the time the mine started producing gold, in August, 1983, the metal had plummeted to below $400 per oz — and it continued to plummet steadily until 1985. In response, the cutoff had to be increased and the tonnage dramatically reduced.

By October, 1986, when the second feasibility study was performed, it was a new ball game as far as reserves were concerned. “If you could calculate what we had mined plus what we had put in reserves, the number was closer to 10 million tonnes, compared with the original 30 million,” Valiant says. “As for grade, it became relatively higher in accordance with the smaller tonnage.

“In retrospect, we probably would have been better off had we begun the operation as an underground mine. This is because the price of gold was relatively low during the time we were mining the pit. Whenever the price of gold is low, you can be more flexible by mining underground. You can keep your grade up; more precisely, you can alter your grade depending on the price of gold.”

Henderson, while agreeing that the open-pit phase was troublesome, insists it was a necessary precursor to the current operation. “Yes, we probably would have been more successful had we started off from underground — but only if we had had the information that we gained from working the pit. We are now wiser as to the structure of the orebody, and hence it’s easier for us to select our mining method and plan our overall approach. Remember, it was only by going through the open-pit phase that we were able to arrive at the tonnages and operating procedures that we are now applying to the orebody.”

It would seem, then, that the Detour Lake mine has been a live- and-learn experience. Now that the operation has learned — the hard way — the time has finally come to start living.

From December, 1985, to July, 1986, an intensive program of exploration drifting and development on the seventh and 11th levels was carried out to further delineate the orebody, to facilitate mine-planning and to get a better estimate of the grade. Then came the second feasibilty study, in October, 1986, followed by the go- ahead to mine from underground.

Proven and probable diluted ore reserves now stand at 7.9 million tonnes, the bulk of which is in the Main zone. Mechanized cut-and- fill has been chosen as the mining method so that the orebody can be mined as selectively as possible; this should also enable miners to achieve a more favorable ore grade. Production of 1,800 tonnes per day takes the form of 600 tonnes from each of three levels: the fifth (250 m), seventh (350 m) and ninth (450 m). However, low-grade, stockpiled ore from the open pit will continue to be processed, resulting in an actual total of 2,200 tonnes per day. This level should be sustained until 1991, when the stockpiled ore runs out. Thereafter, output would drop back to 1,800 tonnes.

But by that time, production could very well rise by mining new reserves. For example, the area called Q-70, in the Quartz zone, is expected to be brought into production in the latter half of 1988. This has the potential of boosting scheduled production rates.

“We are extremely optimistic about the future of the mine,” Mann says with confidence. “Current reserves (last updated in January, 1988) are equivalent to a mine life of at least nine years. We expect the life of the mine to be extended, although at this stage, I wouldn’t want to guess by how much. The orebody is still open at depth and we have drilled down to about the 13th level (650 m).”

He adds: “It’s important to understand that the mine has been made more viable as a result of the fact that we were innovative in our approach.”

One of those innovations was a scheme called the “Rob Cut,” performed in 1986. This involved taking out an extra cut beyond the final planned depth of the pit. In other words, the miners “robbed” part of the original crown pillar that had been left between the pit and the underground workings. Another innovation, the “North Wall Salvage Project,” involved removing low-grade material on the hangingwall side of the pit. This material was previously considered uneconomic, but the mining crew later decided to blast it from the pit wall with very little waste mining required. These projects, together with ongoing processing of stockpiled ore, have helped provide Detour Lake with cash flow for underground development.

When we visited the site, surface construction was virtually complete and underground development and construction
were ongoing. A ramp was being developed between the three production levels in order to provide access for stoping operations and a shop facility. Also, workers were erecting an Eagle crusher station on the ninth level. The 11th (bottom- most) level was being used strictly for exploration development, with some construction taking place. (The shaft, consisting of five compartments, extends 600 m.)

Detour tight-fills its stopes with a glacial sand mined from a pit at the south end of the property. The sand is stockpiled at the site and run through a backfill plant where it is mixed with cement and water before being pumped underground.

“We considered using tailings as a backfill material,” explains Chief Engineer Jack MacRory. “But the use of tailings often results in cyanide problems (in the form of gasses). We happen to have a lot of sand readily available on the property, so we stockpiled 500,000 tonnes for roughly a year’s worth of filling. We’ll repeat this every year. Another advantage to using sand is perforation; you can drain water a lot faster out of sand than out of tailings.”

The backfill plant was designed so that, by the end of 1988, it can be run as a one-man, fully automatic operation. Currently, two men per shift are required to operate it. The backfilling rate is 100 tonnes of sand per hour.

Other recent construction includes a second hoist (commissioned in July, 1987) and an effluent treatment plant at the tailings dam (commissioned last December).

Above ground, the ore is put through a carbon-in-pulp mill. About 2,240 tonnes per day are being processed, with a recovery of 94 1/2%. All in all, about 3.3 million g (115,000 oz) are expected to be recovered in 1988, says Mill Superintendent Peter Dyas. The mill’s annual capacity is 816,000 tonnes.

Ore is crushed at 100% –6-inch and fed to the mill. There, it is ground to 85% –200 mesh for leaching.

As Tim Mann explains, attempts are being made to cut milling costs: “For example, because of the nature of the ore, we consume a great deal of cyanide (1.9 kg per tonne). We wish to reduce that, so we are therefore investigating the use of an instrument called the “cyanostat,” which automatically monitors and regulates the quantity of free cyanide in the mill circuit.

“We are also endeavoring to extend the life of lifter bars in the sag mill. This is to be achieved by using rubber- based lifter bars with chrome-molybdenum inserts.”

Since Detour Lake is the only fly-in mine in the province, workers can’t afford to be afraid of airplanes. One 48-seat Hawker-Sidley 748 aircraft and one, occassionally two, DC-3s fly in and out from Monday to Friday, carrying workers to and from the outlying towns of Cochrane and Timmins. From Timmins, employees commute to and from Sudbury, Toronto and other home bases (unless, of course, they happen to live in Timmins). A road linking the mine to Cochrane and other outlying towns has been constructed but is generally used only to transport freight material and contract workers.

All flights are paid for by Placer Dome, at a cost of more than a $1 million a year. “The air transportation is fairly reliable,” says Ken Baker, superintendent of support services. “It’s the weather that causes problems. In the winter of 1986/87, for example, there was a total of 15 times when aircraft couldn’t be used. Cloud ceiling and visibility are two chief obstacles to flying workers in and out.”

Troublesome weather isn’t unusual for a mine as remote as Detour Lake. A far greater obstacle facing the operation is lack of workers. Last December, the mine was about 20% understaffed, and it’s unlikely that the problem will be solved when full production is reached this month. The complete workforce at Detour is supposed to be 346.

“The manpower shortage is a recent one and it’s affecting the entire Canadian mining industry,” Henderson laments. “There’s a great deal of mining activity going on, which is largely attributable to flow-through share financing. But while there are a great many projects on the go, the number of available workers isn’t growing at the same rate. So there is something of a scramble to see which projects will attract the workers.”

In addition to hiring its full workforce, a major objective for 1988 is to bring the Quartz zones into production (probably using sub-level, blast- hole mining methods). Yet another goal is to improve the scheduled mine production rate and recoveries from the Talc zones. (These zones are more complicated in that the ore is less continuous than in the Main and Quartz zones and the grade is more erratic. As a result, there would be more of a risk mining in that zone than in the other zones). As for exploration, diamond drilling is planned on the 5th-level Talc zones and on the 11th level. Significantly, Mann says there appear to be greater tonnes per vertical metre on the 11th level. Eventually the shaft will have to be deepened in order to recover reserves below the 11th level. Grade Control

He adds that studies are being carried out to investigate various tonnage/grade possibilities for the mine. The objective of these investigations is to reduce the operating cost per ounce of gold produced, thereby increasing the financial rate of return. Successful grade control is a must for operations such as Detour Lake. After all, the mining of low-grade orebodies can be exceedingly frustrating, as Mill Superintendent Dyas explains:

“If you’ve got 5-g-per-tonne ore, you’re looking at five parts per million — which is an incredibly small amount. And if you have it in little veins and you stick a drill down into the ground — which is the only method for knowing what you’ve got — you stand as much chance of hitting that little vein and getting an ultra- high grade as you do of missing the bloody thing and getting nothing. We’re not talking about a massive sulphide deposit, which is generally high- grade. The gold here, as in many Canadian operations, is very finely disseminated. It’s part of the fiendishness of this bloody ore]”

Increasingly, the crew at Detour Lake is demonstrating that it can control and conquer that fiend.