Cambior reaps benefits from base metal production

Unlike many other gold producers, which stick to a gold-only philosophy, Cambior (CBJ-T) has taken up base metal production with enthusiasm. The Montreal-based company deliberately moved into base metal production on the premise that revenues from base metals can support earnings when the gold price declines.

While base metal revenues may provide but cold comfort these days, the company’s Bouchard-Hbert mine, 30 km from Rouyn-Noranda, Que., has evolved into a solid performer, through a combination of good exploration, good mining, and a little good fortune.

The good exploration came early, and has not left: the deposit was discovered in 1955 by Rio Canadian Exploration, then the Canadian exploration arm of Rio Tinto (RTP-T) and the Joseph Hirshhorn interests. Rio, which had taken some pride in making a discovery with the new electromagnetic (EM) system it called MOBRUN, short for “mobile road unit,” named the prospect after the black box that found it.

The good fortune took longer. For decades Mobrun — the deposit, not the EM system — languished, and was described in one authoritative work as “a massive pyrite body containing low-grade zinc and copper.” It remained a geological conversation piece until the mid-1980s, when Falconbridge Copper, which had asserted itself as the dominant player in exploration in the Noranda camp, took up an option on the property and started a feasibility study.

In 1985, occupied by detailed work on its nearby Ansil project, Falconbridge Copper dealt an option to Audrey Resources, under which Audrey earned a 70% interest; production from an open pit began in 1987. Originally, Mobrun shipped its ore to Falco Copper’s Norbec mill, but in 1988 a downhole EM survey uncovered a new mineralized lens down-plunge from the orebody and prompted Audrey to sink $19 million into a new mill on-site.

Over the next few years, Falco Copper was folded into Minnova, which in turn was merged into Metall Mining, with Mobrun becoming a smaller fish in a bigger pond each time.

Meanwhile, the upper ore zones had been depleted, and the mine shut down in early 1992. Cambior bought control of Audrey Resources and took over operation of the project, starting a large underground exploration project to outline more reserves on the lower levels. Metall kicked in 10% of the costs but ultimately converted its 30% interest into a 4% net smelter return.

The mine returned to commercial production at the beginning of 1995, and was rechristened the Bouchard-Hbert mine in honor of the two Audrey executives who had been vital to the project’s development, Michel Bouchard and Guy Hbert.

Although it is in the Noranda camp, Mobrun stands apart from the area’s other major base metal deposits. It is well northeast of the main cluster of mines, including the Horne, Millenbach and Waite-Amulet group of deposits, and sits higher in the volcanic sequence. The mineralized lenses themselves are in rhyolite host rocks that strike southeast and dip almost vertically.

The upper zones, eight lens-shaped bodies with a plunge of about 70 to the east, were mined by open pit and from five underground levels down to a depth of about 300 metres. By the time it was closed in 1992, the mine had produced about 1.5 million tonnes of ore, grading 2.4% zinc, 0.9% copper, 2.4 grams gold and 27 grams silver per tonne.

The 1100 lens, discovered in 1988, is the foundation of the mine today. It is centred about 500 metres down-plunge from the upper bodies and reachable by a single main haulage level from the shaft. An internal ramp leads to four other development levels.

At the end of 1997, reserves were estimated at 8.1 million tonnes grading 4.13% zinc, 0.79% copper, 1.2 grams gold and 40.3 grams silver per tonne, enough for another eight years of mining.

Bouchard-Hbert’s near-vertical structure and competent host rocks are a recipe for large open stopes, which have delivered an advantage with respect to mining costs. Its primary stopes are 15 metres wide and 60 metres between levels, yielding between 80,000 and 100,000 tonnes of ore apiece.

Large longhole stopes are preferred over vertical retreat mining at Bouchard-Hbert, largely because longhole allows for better blasting control, thereby reducing dilution. And though about 35% of the rock is mucked by remotely controlled load-haul-dump machines, mine staff have tried to keep that amount to a minimum, preferring the lower dilution that a man-operated loader can achieve.

The mine also has a plant that mixes mill tailings with cement and fly-ash to produce paste backfill for mined-out stopes. Making a paste backfill also limits the need for tailings capacity.

Bouchard-Hbert’s mill has been ramped up to a 2,730 tonne-per-day capacity from its original 1,100 tonnes. The process starts with a 3-stage grinding circuit: a semi-autogenous mill feeds two ball mills, with a tertiary grinding mill for oversize. The flotation circuit is sequential, producing separate zinc and copper concentrates, which are dried by pressure filtration. About 250 tonnes of zinc concentrate and 150 tonnes of copper concentrate go out the door each day, mainly to the Horne smelter in Rouyn-Noranda; Cambior also sells some of its concentrate to European smelters.

Zinc is the primary product, so the mill process has been optimized for zinc recovery, which runs around 82%. The plant milled about a million tonnes of ore in 1997, producing 32,000 tonnes of zinc and 7.9 tonnes of copper, all in concentrates. Copper recovery — an easier task than recovering zinc — was also up around 82%, and the flotation concentrates also trapped 51% of the gold and 38% of the silver.

The remaining task is to extend the mine life, both by converting known mineral resources into minable reserves and exploring untested ground. Under way is a $1.5-million program of underground and surface exploration, part of which is devoted to delineation drilling in areas near the ends of existing mine workings. The rest of the program is focused on deep drill holes: just as the 1100 Lens lies down-plunge from the upper Mobrun orebody, so might more mineralization lie on the same horizon, but deeper and farther east.

The question seems to be whether the Mobrun rhyolite, like its counterpart to the southwest, is a productive horizon, but mine geologist Marc Ruel shrugs off the academic problem: “We’ll find the other one, and then let them argue.”