DETOUR LAKE

Some would argue that Detour Lake would have enjoyed more success had it been mined from underground at the outset. Why, then, wasn’t it?

When the original feasibility study was tabled in October, 1980, gold was in the $600-$650 (us)-per-oz range. 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. But, as luck would have it, by the time the mine started producing gold (in August, 1983), the metal had plummeted to below $400 per oz. And it continued to fall steadily until 1985. In response, the cutoff grade (originally about one gram) had to be increased and the tonnage dramatically reduced.

On a brighter note, Detour’s past has proved to be prologue to a successful second phase of mining. Having worked the pit, operators are wiser now as to the structure of the orebody. Underground mining methods are proving satisfactory, as is the overall mining plan.

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 arrive at a better estimate of the grade. Then came the second feasibility study, in October, 1986, followed by the go- ahead to mine from below surface.

Full production of 1,800 tonnes per day was reached in April, 1988, and the figure has recently risen to 2,100 tonnes (partly due to the processing of low-grade, stockpiled ore from the pit). This rate is expected to continue at least to the end of 1990, Mine Manager Tim Mann told The Northern Miner Magazine recently. Even more encouraging is that the average grade is more-or-less consistent with what was estimated in the last feasibility study: 4.8 g per tonne of underground ore, or 0.14 oz per ton (compared with about 3.5 g, or 0.10 oz, during the open-pit phase).

Operating costs are somewhere under $400 (Canadian) per oz, Mann said, but he added that the mine is nevertheless making money for owner Placer Dome. “We have been profitable to date this year and we also made money in the last two quarters of 1988.” In 1989, about 125,000 oz are expected to be recovered from Detour’s carbon-in-pulp mill. Recovery is 94.5% and the mill’s annual capacity is 816,000 tonnes.

The operation proved up new reserves in 1988 to the extent that more than 806,000 tonnes were milled last year with no appreciable loss of reserves. Proven and probable diluted ore reserves remain virtually unchanged from this time last year: 7.8 million tonnes, the bulk of which is in the Main zone. That’s equivalent to a mine life of about 10 years, Mann said, which could be extended even longer if new reserves are found. The orebody is still open at depth and miners have drilled down to the 13th (660-m) level.

Mann said the consistency of grade which Detour has achieved directly reflects the selective application of mechanized cut-and-fill mining as well as minimum dilution in the long- hole stopes.

Production is occuring on each of the three levels: the fifth (260 m), seventh (360 m) and ninth (460 m). “Bringing the Quartz zones into production has been a major development,” Mann said, “and this has certainly served to increase our tonnage.” Recently, production commenced on the Q-70 long-hole stope, on the seventh level. Sub-level blast- hole methods (involving 3 1/2-cu-yd Wagner Scooptrams as haulers) are used there to extract between 100 and 200 tonnes per day. Additonal areas of the Quartz zones are being developed, including the Q-100 (where shrinkage stoping will probably be employed) and the Q-50 (which will probably be mined via long-hole stoping). The average grade for the Quartz zones area is 7.7g per tonne, substantially higher than in other areas of the mine.

In addition, operators are working at improving the scheduled mine production rate and recoveries from the Talc zones, especially on the 360- and 460-m levels. These zones are complicated in that the ore is less continuous and the grade more erratic than in the Main and Quartz zones. Another development, still in the research stage, is a pilot plant for copper-gold flotation. The primary purpose is to remove copper from the mill circuit at an early stage, with a resulting decrease in cyanide consumption and a reduction of copper in the final tailings (the current level of copper in tailings has proved to be an environmental concern). “We’re trying to float off the copper that occurs with the gold mineralization so that we can form a copper concentrate,” Mann said. “The challenge lies in maximizing the copper that is floated off while minimizing the gold that is floated off.”

As for exploration, an underground diamond drilling program is under way. Also, it is hoped that a surface diamond drilling program will soon be initiated to further define a potential zone to the west of the mine, Mann said.

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