Mobrun mill building interrupts production

At first it doesn’t quite compute, like seeing cows grazing on a city street or the Queen Mary lumbering for takeoff at an airport. But there it is, a 110-ft-high headframe smack in the middle of a farmer’s field. This is the Mobrun mine headframe about 32 kilometres north of here, with its access along a winding gravel road through farm country and woodlots.

The Mobrun began its productive life as an open-pit, polymetallic deposit in mid-1987. It has since gone underground, tapping into the Main lens at a rate of 1,100 tonnes per day. In the latest three-month period ended June 30, the mine produced 97,210 tonnes of ore that yielded roughly 1.2 million pounds of copper, 3.2 million pounds of zinc, 15,316 oz of silver, and 3,816 oz of gold.

Now, production will stop for 10 months as the 70% majority owner Audrey Resources (TSE) builds its own mill at a cost of $24 million (to this point milling had been done at the nearby Norbec mill owned by TSE-listed Minnova Inc.). Minnova holds a 30% interest in the Mobrun deposit. Northgate Exploration has a 17% equity stake in Audrey.

Currently at a depth of 217 m, the shaft will be sunk during the shutdown another 145 m to open up the 850, 870 and 930 lenses and to continue mining the Main lens. Proven and probable reserves in those four predominantly massive sulphide lenses are approximately 1.2 million tons. At about a million tonnes, the Main lens is by far the largest, but the others have fairly high-grade gold values. In the proven category of the four lenses there are 1,073,166 tonnes grading 0.79% copper, 2.46% zinc, 30.62 grams per ton silver (0.89 oz per ton), and 2.28 grams of gold (0.07 oz per ton).

That tonnage would have given the mine a minimum four-year life. But the discovery earlier this year of the 1100 lens, a huge zone to the southeast of the current underground workings, ensures a much longer life for the mine, according to Mobrun Mine Manager Jean Bailly. He estimates that, at 1,000 tonnes per day, the mine with its current proven, probable and possible reserves in all five lenses could last another 14 years. In all three categories, the reserve figure is close to 5 million tonnes of economic ore (at current metals prices.)

Exploration from surface is focusing on the 1100 lens. Vice-President and Manager of Exploration Michel Bouchard told The Northern Miner that geologists are evaluating the results of a $600,000, 12-hole project recently undertaken. Two surface drills are still probing the lens.

“It’s obvious from the core itself that there’s a lot of geochemical alteration. Hole 43 passed through 200 ft of chloritized rhyolite with chalcopyrite stringers. We think that that’s the channelway that permitted mineralization.”

Possible reserves in the 1100 lens stand at 3.1 million tonnes grading 0.65% copper, 6.09% zinc, 43.81 grams silver per tonne (1.28 oz per ton), and 1.75 grams gold per tonne (0.05 oz gold). Another 456,989 tonnes of similar-grade mineralization is in the probable reserve category. The lens is open at depth and to the east and west.

It’s also possible that these stepped lenses (they move deeper underground as they move farther to the southeast) could continue, Bouchard said. But of prime concern now is definition drilling in the 1100. In about a year when a drift has been driven in the neighboring 930 lens, underground diamond drilling will further evaluate this latest lens.

The Main lens has been mined using long hole methods. Three, 3-inch Gardner-Denver rigs drill down from the first level, which is 90 m above the bottommost second level. The rigs also drill in a ring pattern from two sub-levels between the first and second levels. (Mine planners are considering incorporating vertical retreat mining later on so that the two sub-levels wouldn’t have to be cut.) All drawpoints are on the second level. Two, 5-cu-yd Eimco Jarvis Clark scoops haul the ore and waste rock to passes near the shaft. Smaller scoops are used in development work. The crusher at the bottom of the shaft is a 36×48-inch Allis-Chalmers jaw crusher.

Mobrun plans to leave that crusher where it is. After the shaft deepening, a rock hammer will be installed at the new shaft bottom to break the ore from the smaller 850, 870 and 930 lenses. Later, when a third shaft deepening provides access to the 1100 lens, the second- level crusher will be moved to the new bottom. A fourth compartment in the shaft will allow for the third shaft deepening without disrupting production.

Mining costs currently run about $39 per tonne, but President Guy Hebert said the costs should come down another $7 to $9 per tonne because development in the Main lens is nearly finished. No backfill is used.

On surface, pilings have been driven for the crushing plant. The semi-autogenous grinding (SAG) mill, the ball mill, Larox filters and conveyors are all on-site. By July, 1989, the whole operation — mine and mill — should be producing again and making money. Earnings for the three-month period ended June 30, were $691,466 (8 cents per share) on revenue of $4.7 million. Mobrun was not operating in the comparable quarter a year ago.