The Yukon Environmental and Socio-economic Assessment Board (YESAB) has sent the massive, and controversial, Casino Mine project for the highest level of review, the first time that's happened in YESAB's 10-year history.

YESAB's executive committee announced the move Thursday, at a rare media conference in Whitehorse. The board said the Casino Mine project raises serious concerns about impacts on caribou, and tailings and waste management.

Ken McKinnon of YESAB's executive committee said the decision "was not taken lightly."

Ken McKinnon

Ken McKinnon, of YESAB's executive committee, said the decision to move the Casino Mine project to a higher level of assessment 'was not taken lightly.' (Heather Avery/CBC)

"In making this decision, the executive committee took into account the information and the mitigative measures included in the project proposal," he said.

The Casino Mining Corporation, a wholly-owned subsidiary of Western Copper and Gold, is planning to build what would be the Yukon's biggest mine, about 400 kilometres northwest of Whitehorse. 

If the mine is approved, the company says it could produce more than 400,000 ounces of gold annually and more than 200 million pounds of copper. It says the mine could employ up to 1,000 people at peak construction, and have a life of about 22 years.

YESAB's executive committee has not done a full assessment of the project's potential impacts. That work is now the responsibility of the panel that will undertake the review. Panelists will be YESAB members, appointed by the executive committee.

YESAB has notified the federal environment minister of its decision. The panel review, which will include public hearings, must be done within 15 months of the minister's response.

World's highest tailings dam

The Casino Mining Corporation is proposing to build an earth-fill dam to contain nearly a billion tonnes of tailings, covered by water. About 20 per cent of those tailings will be potentially acid-generating. The dam would be one of the world's highest dams, and the highest tailings dam.

According to YESAB, the waste management plan is controversial enough to warrant greater scrutiny. A similar dam used at B.C.'s Mount Polley mine site failed in Aug. 2014, spilling 24 million cubic metres of mine waste and water into nearby lakes and rivers. The technology has also failed at mine sites in Brazil and Mexico, according to YESAB.

B.C. tailings pond breach

An aerial view showing damage caused by a tailings pond breach at B.C.'s Mount Polley copper and gold mine, in 2014. The breach has been called one of the biggest environmental disasters in modern Canadian history. YESAB says a similar tailings dam is being proposed for the Casino Mine project. (Jonathan Hayward/Canadian Press)

YESAB says the size of the proposed Casino dam, along with its Northern location ("an area with discontinuous permafrost conditions"), raise questions about whether it would be a safe and effective way to contain tailings.

Impacts on Klaza caribou herd 

YESAB's other chief concern with the proposed mine project is its potential impact on the Klaza caribou herd, which has a range that overlaps with the mine's predicted "zone of influence."

Nathan Aasman

Nathan Aasman, a senior assessment officer with YESAB, explains some of the concerns with the Casino proposal, at a rare media briefing in Whitehorse on Thursday. (CBC)

"The Casino Mine infrastructure and access road largely fall within the entire range of the Klaza caribou herd, and might contribute to further habitat loss and degradation," said Nathan Aasman, senior assessment officer with YESAB.

The Klaza herd's range is now a continuous, million-hectare swath, but YESAB says the Casino mine could have a direct impact on about 13 per cent of that area. It also says the mine's infrastructure — including tailings facilities and access roads — threatens to isolate two large patches of habitat.

"Fragments [of habitat] that are small and isolated are unlikely to be used by caribou," a YESAB briefing document says.

Company welcomes review

Western Copper and Gold released a statement Thursday, welcoming YESAB's decision to subject the project to a more comprehensive review.

The decision "removes the uncertainty that the project will be referred later in the assessment process, saving the company time and money," the statement reads.

"We appreciate that a mine of this significance requires all key aspects to be thoroughly evaluated. We look forward to working with Yukoners to ensure the project moves forward in an environmentally and socially responsible way," the company's CEO, Paul West-Sells is quoted as saying.

The company also said the project, if approved, would contribute $9.6 billion to the Canadian economy and $3.2 billion in taxes and royalties to the federal and territorial governments, and Yukon First Nations.