The Idaho Board of Environmental Quality found that the agency it oversees issued an air quality permit for a proposed gold mine that significantly miscalculated allowable emission levels for arsenic-laden dust.

The board, in a ruling issued Thursday, remanded a portion of the permit that would govern dust and arsenic emissions for the controversial gold and antimony mine at Stibnite to the Department of Environmental Quality for more work.

Perpetua Resources wants to reopen and expand a long-dormant open pit gold and antimony mine near Yellow Pine and along the East Fork of the South Fork Salmon River east of McCall. It is in the process of seeking dozens of necessary state and federal permits to do so.

It has already received millions of dollars from the Department of Defense that covets a domestic source of antimony, a mineral used in munitions, and the U.S Import Export Bank has said it may lend Perpetua Resources up to $1.8 billion.

The company hopes to extract more than 4 million ounces of gold, 1.7 million ounces of silver and 115 million pounds of antimony, and has pledged to use a portion of its profits to clean up past waste at the mine site.

The mine could create up to 500 jobs and generate more than $60 million in annual tax revenue, according to the company.

But the controversial mine adjacent to the Frank Church River of No Return Wilderness Area is bitterly opposed by the Nez Perce Tribe and the conservation groups Save the South Fork Salmon and the Idaho Conservation League.

They contend it will spill pollution into critical spawning habitat for threatened spring chinook, steelhead and bull trout and lead to other environmental problems. Tribal members continue to fish in the area, and the tribe is actively involved in salmon recovery there.

Last year, DEQ issued the company an air quality permit to construct that outlines the expected release of toxic pollutants from mining activity and steps to mitigate those releases in order to keep them below standards of the federal Clean Air Act.

The emissions are largely related to fugitive dust resulting from mining traffic along haul roads as well as drilling and blasting at the mine.

The Nez Perce Tribe and environmental groups challenged the permit, saying the company assumes it will have more ability to control public access and thus exposure to toxic pollutants than it is realistic, that the state agency allowed the company to work on permit details after public comment was closed, that it included a dust abatement goal that is unachievable and that it miscalculated ambient arsenic levels.

The board turned away all but one of the complaints. It agreed with the groups that DEQ permit writers used various analysis and calculations that underrepresented potential exposure to arsenic.

“The impact of this additional variable in the equation ultimately results in over a 75% reduction in the calculated exposure,” the board wrote of one of the calculations.

The groups welcomed the ruling.

“Arsenic is a toxic pollutant and a carcinogen,” said Bryan Hurlbutt, staff attorney at Advocates for the West, an environmental law firm representing the groups. “The Board’s decision sends a strong message. Perpetua cannot bend the rules and disregard the risks from its proposed mine.”

Perpetua spokesperson Marty Boughton said the board sent the permit back to the DEQ for additional review but did not invalidate it.

“It goes without saying, creating a project that is protective of human health and the environment has always been at the heart of Perpetua’s mission and vision for the Stibnite Gold Project, and we’ll continue working with IDEQ to respond to this narrow additional review,” she said.

Barker may be contacted at ebarker@lmtribune.com or at (208) 848-2273. Follow him on Twitter @ezebarker.