B.C.'s Environmental Assessment Office has already approved the controversial Prosperity mine once, but a review of proposed design changes won't necessarily result in another positive ruling, says a top government official.

"This is not a foregone conclusion," Kevin Jardine, the Deputy Minister responsible for the EAO, said in an interview Wednesday.

The $1.5-billion project by Taseko Mines Ltd. was given an environmental assessment certificate by B.C. in 2010, but the project was subsequently rejected in federal reviews.

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Mine opponents thought the project had been killed by Ottawa's rulings, but the company continues to pursue it.

In a toughly worded letter to Premier Christy Clark last May, Taseko president and chief executive officer Russell Hallbauer threatened to seek damages in court from the province if the EAO didn't promptly proceed with "timely completion" of its application.

"The EAO continues to sit on its hands on this project, perhaps in the hope that Taseko will simply go away," complained Mr. Hallbauer.

"It is incomprehensible to us that a decision on the amendment application remains outstanding," he wrote. "This is in fact an extremely simple amendment application to grant, given that the revised project will have less environmental impact and more economic benefits."

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Several weeks after Ms. Clark got that letter, the EAO wrote back to Mr. Hallbauer to say it "is prepared to proceed with the amendment process."

The EAO explained the delay in the process by stating that since it hadn't heard from Taseko in about a year, it had assumed the company wasn't pursuing the amendment.

The letter was released Tuesday by MLA Vicki Huntington, who obtained it through freedom of information.

It seems unlikely B.C. would reject a mine that already has a provincial environmental certificate and which is now proposing improvements. But Mr. Jardine said there is no guarantee an amended certificate will be awarded.

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"One of the criticisms we get is that we never say no. But if you actually look at the number of projects that have started and haven't finished, there is actually a very high rate of attrition," he said.

Mr. Jardine said while the EAO might not directly reject a project, it could make demands that are simply too difficult or costly for a proponent to meet.

"What tends to happen is that companies, rather than continue to spend money, they abandon their projects," he said.

Taseko Mines Ltd. has been trying to get approval to build the gold and copper mine near Williams Lake for more than a decade. Even if the project does get an amended environmental certificate from the province, it will still face the federal hurdle. The venture, initially called Prosperity and now known as New Prosperity, failed to get federal approvals in 2010 and again in 2014.

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Mr. Jardine said a key issue is the handling of mine waste. In its original plan, Taseko proposed to use Fish Lake as a tailings pond, but the company later proposed alternatives.

In its June letter to Mr. Hallbauer, the EAO asked for more information on tailings options, on groundwater conditions and for an update on a civil claim the company launched against the federal government earlier this year. Mr. Jardine said the issue of how mines handle tailings has taken on new importance since the Mount Polley disaster of 2014, when a tailings dam at an Imperial Metals mine was breached, polluting nearby rivers and lakes. "Subsequent to Mount Polley, we have asked all [mine projects] … to provide a technical rationale for their tailings design," he said. "Essentially, we've asked [Taseko] to go back and provide us with further evidence as to why this is the best design."

In a letter to the EAO, the Tsilhqot'in National Government, which opposes Taseko's mine, has complained about the EAO's decision to proceed with the application to amend the environmental certificate.

"There is no legal or practical reason to consider an amendment application at this time, when the project cannot proceed in the face of its rejection by the federal government," the TSG stated.