A Maricopa County judge on Friday overturned a state air-quality permit issued to Rosemont Copper in January 2013 for its planned mine southeast of Tucson.

Superior Court Judge Crane McClennen ruled that the Arizona Department of Environmental Quality’s action in granting the permit to Rosemont Copper was “contrary to law, was arbitrary and capricious, and was an abuse of discretion.”

Rosemont Copper, which is owned by Toronto-based Hudbay Minerals, is planning a $1.2 billion project that would mine 243 million pounds of copper annually in the Santa Rita Mountains, partly on national forest land, about 30 miles from Tucson.

The ruling is the result of a lawsuit filed by the Save the Scenic Santa Ritas group, which opposes the mine, alleging that Rosemont used faulty computer modeling that showed the mine would be in compliance with federal clean-air standards.

Rosemont argued in court that its modeling was appropriate and representative, disputing the group’s assertion.

Save the Scenic Santa Ritas, which challenged the permit first to a state administrative law judge who upheld the ADEQ decision, conducted its own modeling using a much higher expected level of nitrogen oxide, and found that the mine’s operations had the potential to violate federal standards.

Eric Massey, who succeeded Henry Darwin as director of ADEQ, said the department was surprised and disappointed with Friday’s ruling.

“We continue to stand behind wour firm belief that the air-quality permit we issued is protective of public health and the environment,” he said.

He said the four-page ruling was not based on all of the evidence presented and did not say what the department did wrong in the process of granting the permit.

“We’re left scratching our heads on what we’re supposed to do,” he said. “When something goes wrong, I expect to know how to make my program better.”

What this means to the possibility of a new or revised permit is unclear, he said. Since the court did not specify any changes, the option now is to appeal the ruling, he said.

Several attempts by the Star to reach representatives of Rosemont Copper on Friday afternoon were not successful.

Save the Scenic Santa Ritas said the ruling was a victory for the group.

“They (Rosemont officials) expect ADEQ and other regulatory agencies to just rubber stamp their projects, but this time they both got caught,” said Gayle Hartmann, president of the group, in a written statement. “The court agreed with us that ADEQ did not do an adequate and proper job of evaluating Rosemont’s application.”

The mine also still needs other permits for final Forest Service approval.

Contact reporter Yoohyun Jung at yjung@tucson.com or 520-573-4224. On Twitter: @yoohyun_jung