Gordon Strasser, a 59-year-old mining engineer, calls it “cookie jar mining.” The term describes a new method of mining those isolated pockets of rich gold mineralization which are too small to justify extensive development work. In such cases, a quick and cheap way of getting in and grabbing the goodies is required. Then making a clean break of it. Several thousands of tons of new gold mineralization have been identified in virtually every area of the country in recent years. Using flow-through share financing, a preponderance of junior exploration companies have outlined many small but rich pockets of gold mineralization just begging to be plucked out of the ground. Many of these deposits are not considered ore, however, because most engineers assume conventional mining methods require a 5-ft mining width. Used to mine a vein 2 ft wide, these methods would create so much dilution that the work would be unprofitable.
In his travels to many of these gold properties, Strasser has come to realize the need for a new, low-cost method of mining narrow-vein deposits. In the past six months, he has put together a method which might just fit the bill. Might, because it is still in the design stage. But James Wade Engineering of Willowdale, Ont., of which Strasser is vice-president and general manager, is actively seeking exploration companies who are willing to try the method out on one of their deposits.
The method Strasser has devised uses more-or-less proven technology, so the risks are relatively low. “If I were a mine manager today,” Strasser says, “I’d give it a try.” That’s strong advice coming from a man who has been a mine manager and superintendent for most of his 38 years in the industry — 22 years with Inco Ltd. and 16 years with Campbell Resources.
The new method resembles an old one called resueing, but there are diffrerences. Electric-hydraulic longhole machines, remote-control load- haul-dump machines and a blasting method called pre-shearing (sometimes practiced in open-pit mines) are used. All of which lends a modern twist to the old method.
To picture how cookie jar mining works, think of a steeply-dipping, gold- bearing unit 0.6 m wide, 90 m long and 15 m high. Such a block of ground would contain about 2,200 tonnes of ore. If the undiluted grade is 0.40 oz gold per tonne and mill recoveries are, say, 95%, there would be 836 oz of recoverable gold in such a block. At today’s gold price of $534 (c) per oz, that works out to about $446,000 of recoverable gold. If you were to mine it at a mining width of 1.2 or 1.5 m, it becomes uneconomic.
To make it economic, Strasser suggests you over-cut and under-cut the block to de-stress the ground and drive raises on both ends, 90 m apart. The top sill drift should be driven at 2×2.5 m to accommodate a compact longhole drill, such as one of Boart Canada’s new Stopemaster Series hydraulic long hole machines. The hydraulic rock drill unit, introduced in June, is constructed in modular form for easy dismantling and transport through small openings to captive mine areas. Existing mines which already use pneumatic drills can use this method because pneumatic drills may be employed as well. Nonetheless, it is important that cookie jar mining undergo some testing, according to William Quesnel, vice-president and general manager of Boart. “Over a distance of 15 m, deviation of about 30 cm could occur.”
Three rows of longholes, 2 to 2-1/8 inches in diameter, would be drilled vertically down to break through the drift below. For control purposes, hole lengths should not exceed 15 m. Hole- spacing and burden would require study to determine parameters for specific sites.
The basic idea is to pre-shear the walls, splitting the rock from hole to hole; then the centre holes could be blasted to provide fragmentation. Material would fall to the bottom sill drift. Pre-shearing is the clincher. If it can be performed successfully, dilution could be limited to an estimated 10%, Strasser predicts (see accompanying story). William Webster, manager of technical services for CIL Inc., says pre-shearing is not a proven method for blasting in underground mines. Pre-shearing involves decoupling an explosive charge. This requires suspending a small-diameter explosive charge (EACTEX in the case of CIL products for a 54-mm-diameter long-hole) in a drill hole so that there is a cushion of air around the charge to act as a shock-absorber. The idea is to merely propogate a crack from hole to hole, not to fragment the ground around the drill hole. The pre-shearing holes in Strasser’s cookie jar concept would be blasted 100 milliseconds or more before the centre (fragmentation) holes are blasted.
Pre-shearing in underground projects has had mixed results. It was used successfully to excavate a large powerhouse chamber on the Churchill Falls project in Labrador, Webster says, but was unsuccessfull in a tunnelling project in Belgium. Blasting parameters would likely be different for each application, so considerable engineering design and control work would be necessary to perfect the technique at each site. Cil has expressed an interest in lending technical assistance to such a project. Strasser has some experience with pre- shearing himself, being the first to try it out in Inco’s Clarabelle open pit in Sudbury, Ont.
Fragmented ore would be mucked out from the drift below using a remote-control load-haul-dump machine. This would obviate the need for elaborate, expensive ground control. Once the entire block is blasted in this way, no fill would be used, keeping costs and development work to a minimum. And since most development work is done in ore, development costs are low. At $500 per m of advance, most development work would break even with revenue. In his economic analysis, Strasser assumes an advance rate of 1 m per man shift. (A 2-man crew cycles a 2-m advance round every shift.) Long-hole efficiency would be about 90 m per man shift and total footage drilled would be 4,000 m. The cost of loading explosives would be $10 per tonne and mucking would cost $9 per tonne; haulage to the mill, $7 per tonne.
Strasser has several possible test sites in mind. “We’ve looked at the Murgold deposit near Timmins,” Strasser says, “and think it would be a good candidate for a trial.”
The Magnet mine in Geraldton, Ont., and the Tangier mine in Halifax Cty, N.S., are two other possible sites. One mine which has already tried a method similar to this, according to Glen O’Gorman, manager of engineering for Wade Engineering, is the formerly producing Nigadoo River mine, near Bathurst, N.B., which was operated by the Sullivan Mining Group (since bought out by Cambior Inc.) from 1967 to 1977. About half of the 800 tons of ore produced per day at that mine came from long-hole stopes in ore 2.5 ft in metres wide. Holes were drilled 50 ft long at 2-ft spacings and 3-ft burdens, but were blasted in a opposite sequence to that proposed in Strasser’s cookie jar mining concept — centre holes were blasted first at Nigadoo and the hanging wall and footwall holes were blasted last.
According to Len O’Gorman, (Glen’s father) who was mine superintendent at Nigadoo River at the time, the method was successful in this lead-zinc-silver orebody. But until cookie jar mining gets its first field trial, we’ll never really know whether pre-shearing can be profitably used to reduce dilution in narrow-vein gold deposits. When mine analysts and other financial types look at gold mines with their eye to the bottom line, they often perform elaborate analyses of sensitivity-to- exchange rates, gold prices and mill recoveries. These are all important factors, but as the accompanying graph shows, dilution can be an even more significant factor in most gold mines.
Careful examination of this graph reveals that at $450 (us) per oz, an improvement in grade of 0.01 oz gold per ton adds $5 to revenues for every ton treated. At 500 tons per day, that’s about $875,000 per year.
Improvements in mill recovery, on the other hand, are more difficult to achieve. They are also more costly to fund and add only about $1 per ton for every 1% increase in recovery.
Exchange rates have about an equal effect. For every 1 increase in the value of the U.S. greenback, Canadian mines get an additonal $1 per ton of ore treated. The clincher here is that mines have no control over exchange rates.
And since most mills are operating at or near peak efficiency, operators must look, instead, to controlling dilution to make improvements in their operations. Cookie jar mining may prove to be one such method.
Be the first to comment on "COOKIE JAR MINING"