The Corps has declined to comment on pending litigation against the agency regarding Rosemont.

Hudbay, which isn’t named as a defendant in the suit, has said in numerous letters to the Corps that opponents have overestimated impacts and used flawed analyses. The company has agreed to a detailed “treatment plan” for handling cultural resources that’s approved by state and federal officials.

But the tribal' lawsuit also says the Corps permit not only illegally authorizes the company to cover the washes with fill material, it also illegally requires the company to place the fill material in the washes before waste rock and tailings from mine operations is dumped there.

That requirement, placed in a condition for approving the permit, is to keep contamination from waste rock and tailings from entering washes, the Corps said. But it also was a "complete reversal" of the mine's entire permit review that, over eight years, had made impacts of dumping waste rock into washes a central focus of the environmental analysis, the suit said.

That action "opened the door to the subsequent dumping of waste rock and tailings without further analysis that the Clean Water Act normally requires," the suit said.

It will be illegal because the Forest Service's newly approved operating plan for the mine doesn't allow Hudbay to place the fill material cleared from the site underneath waste rock and tailings, the lawsuit said.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987

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