Mount Polley on track to yield gold and copper

At the Mount Polley gold-copper porphyry mine, 45 miles north of here, construction is proceeding on schedule toward a projected startup of late 1997.

Imperial Metals (IPM-T) has a 55% interest in the project, with the reminder held by Sumitomo of Japan.

“We’re at, or ahead of, schedule on everything except the tailings dump,” said Brian Kynoch, senior vice-president of exploration for Imperial Metals, during a recent site visit by The Northern Miner and other observers.

The project will be the first large open-pit mine in the province in more than a decade, creating 200 jobs during construction and 170 during production.

Situated in the Cariboo region of south-central British Columbia, Mount Polley lies in the Quesnel Trough and hosts a reserve of 82 million tonnes grading 0.42 gram gold per tonne and 0.3% copper. The contained resource is estimated at 1.1 million oz. gold and 544 million lb. copper, with a low stripping ratio of 1.16-to-1.

Earlier calculations put reserves at 49 million tonnes grading 0.56 gram gold and 0.38% copper, with a stripping ratio of 1.76-to-1.

Sumitomo’s original share of Mount Polley was 35%, but that was increased to 45% last April when the Japanese firm agreed to finance up to $54 million of Imperial’s 55% share of the mine’s $123.5-million capital cost.

The cash cost of production for gold, with copper as a credit, is projected at less than US$180 per oz. For copper, taking gold as a credit, the cost is estimated to be US49 cents per lb.

The 3-pit operation will function primarily as a gold operation for the first four years, at a cash cost of US$171 per oz.

Copper will be the primary product during the remainder of the project’s 12-year life. The mine will process 18,000 tonnes of ore daily, or 6.5 million tonnes per year.

“Mount Polley will produce a gold-copper concentrate containing 100,000 oz.

gold and 24 million lb. copper per year over the first four years,” predicted project manager Malcolm Swallow.

The deposit is a typical calc-alkaline porphyry, like the Afton deposit, near Kamloops, B.C. Copper showings on the mountain were known for years, but the first recorded exploration did not occur until 1964.

“Mount Polley is different from most other B.C. porphyry-copper deposits in that it has a high level of oxidation,” Kynoch said. “About 25% of the copper is in the form of oxide minerals. That’s one of the reasons it wasn’t developed in the past; many copper oxides don’t respond to flotation.

By the 1980s, it was recognized that the property also contained high gold values, especially in what is now known as the Central pit. That, along with higher gold prices, helped render the project more economic.

“A lot of the old drill holes weren’t even assayed for gold,” Kynoch pointed out. The mill will employ conventional grinding and flotation, as well as conventional rod, ball and pebble mill technology, as opposed to semi-autogenous grinding.

Measuring 1,100 metres north to south and 1,100 metres east to west, the deposit is divided into three pits: Bell, Springer and the aforementioned Central. As it deepens, the percentage of recoverable copper increases and gold grades decrease.

“We’ll wind up with a concentrate grading about 30% copper and 1-2 oz. gold,” said Imperial President Pierre Lebel. The concentrate will be trucked to Vancouver.

The plan calls for 35,000-45,000 tonnes per day to be mined at the Central pit. “It has low copper values, but we’re there because of the gold,” Kynoch said.

Environmentally, the project is considered low-risk. “The ore, waste and tailings are acid-consuming, so there’s no problem with acid generation in this project,” said Rad Pesalj, project geologist.