TORONTO (miningweekly.com) – The TSX-listed stock of Taseko Mines on Friday closed down almost 10% after it announced that it would challenge findings of a Canadian federal review panel, which had found that the company's revised plan for a copper/gold mine in British Columbia posed significant threats to the environment.

Taseko in a statement said the findings contradicted best practices in place around the world.

The First Nations Summit, representing dozens of British Columbia aboriginal groups within the treaty process, on Friday called on Ottawa to reject the project following the report by the Canadian Environmental Assessment Agency.

The Summit said Taseko had underestimated the volume of water that would leave the tailings storage facility at the proposed New Prosperity mine and that there was "considerable uncertainty" regarding Taseko's contingency plan for water treatment.

Grand Chief Edward John in a statement said the report should “clearly” provided the federal government the necessary evidence to immediately reject [the project], yet again.

The Conservative Stephen Harper-led government in November 2010 rejected the mine proposal based on what then-Environment Minister Jim Prentice, described as one of the most "scathing" independent panel reports ever written.

The report documented both significant environmental and cultural impacts, many of which the first panel noted could not be mitigated with an alternative option proposed by the company.

Environmental concerns were centred on Fish Lake, also called Teztan Biny, with its population of rainbow trout.

First Nations held that the proposed mine was so low grade, at a time when even major mining companies were suffering extreme setbacks, that it created serious doubt that the company could deliver the environmental mitigations being proposed, likened to putting Fish Lake on “life support” and making use of “unproven and expensive” water treatment over the long term.

Vancouver-based Taseko is the 75%-owner and operator of the Gibraltar mine, the second-largest openpit copper/molybdenum mine in Canada. Taseko's New Prosperity project, which is currently in the environmental assessment process, is one of the largest undeveloped gold/copper porphyry deposits in the world, with a one-billion-tonne measured and indicated resource containing 5.3-billion pounds of copper and 13.3-million ounces of gold.

At metal prices of $1 000/oz of gold and $3.15/lb of copper, the project has a pretax net present value of C$3-billion and a 40% pre-tax internal rate of return.

On Friday Taseko’s shares on the TSX closed 9.77% lower at C$2.31 apiece.