The Montana Attorney General’s Office has intervened in a lawsuit over a mining proposal in the mountains north of Yellowstone National Park.

Attorneys for the state will defend a law barring district courts from invalidating a state agency’s approval of a project even if the project’s underlying environmental review was found unlawful.

The challenge to the law comes from the Park County Environmental Council and Greater Yellowstone Coalition, the groups seeking to block Lucky Minerals Inc. from drilling for gold near Emigrant Gulch, a narrow canyon behind Chico Hot Springs.

Park County District Judge Brenda Gilbert ruled in May that a Montana Department of Environmental Quality review of the project ignored key issues and was legally flawed. But Gilbert couldn’t block the work because of a 2011 amendment to the Montana Environmental Policy Act prevents district judges from undoing agency approval of a project.

The two groups argue that provision violates the state constitution’s guarantee of a person’s right to a “clean and healthful environment.” The state’s attorneys will defend the constitutionality of the law. They are expected to file a brief detailing their argument in October.

John Barnes, spokesman for the attorney general’s office, declined to comment further.

Josh Purtle, attorney for the plaintiffs, said they aren’t surprised the state intervened but that they believe they have the better case. He said preventing judges from invalidating projects removes a key remedy for people trying to protect their right to a clean environment.

“We still think it’s clearly unconstitutional,” Purtle said.

Lucky Minerals first proposed exploratory drilling in Emigrant Gulch in 2015. Locals and environmentalists worry the project may lead to a large-scale mine with the potential to harm water quality and the region’s tourism-based economy.

DEQ approved the company’s plans in the summer of 2017. The two groups sued shortly afterward, arguing that DEQ ignored key issues in approving the plans.

But the ruling didn’t block the work because of the provision that prevents district courts from revoking permits. The plaintiffs filed a motion to vacate the license on constitutional grounds in June.

While the Office of the Attorney General will defend the law, a DEQ brief suggests that agency’s attorneys may see the plaintiff’s argument. DEQ attorneys wrote in a July brief that the agency doesn’t oppose the constitutional challenge “in substance.”

Lucky Minerals had hoped to begin drilling this past summer but the work hasn’t started. The company hasn’t paid a $154,000 bond, a prerequisite for receiving a license. Company vice president Shaun Dykes told the Chronicle earlier this year that they’d wait to pay the bond until the legal fight is sorted out.

Michael Wright can be reached at mwright@dailychronicle.com or at 406-582-2638. Follow him on Twitter @mj_wright1.

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