HELENA (AP) — An environmental group is suing Montana environmental regulators over a permit allowing the expansion of a gold mining operation near Whitehall without requiring the Canadian company to completely backfill the new open pit after the mining ends.

The reclamation plan approved by the state Department of Environmental Quality in its operating permit to Barrick Gold Corp. violates the Montana Constitution’s right to a clean and healthful environment, the Montana Environmental Information Center claimed in its lawsuit filed Monday.

The plan the DEQ approved in January for the new pit at the Golden Sunlight Mine requires only the partial backfilling that would leave at least 15 acres open after operations cease.

That plan would be less visually appealing, provide less habitat for wildlife and be less structurally stable than completely backfilling the pit, the environmental group’s lawsuit said.

The group is seeking a judge’s order for the pit to be completely filled in, saying there is a constitutional duty “to reclaim all lands disturbed by the taking of natural resources.”

DEQ spokesman Chris Saeger said that the agency was unaware of the lawsuit. Mine officials also had not seen the lawsuit and had no comment.

The agency decided not to require backfilling because of concerns that if the water treatment system fails, it would be more difficult to fix, the lawsuit said.

“If that’s the logic they’re using then we’ll never reclaim an open pit,” MEIC executive director Jim Jensen said.

The plan expands an existing open-pit mine and builds a second one nearby. The new mining will occur on 68 acres and produce nearly 53 million tons of new waste rock, DEQ officials said in January.

The expansion is expected to extend the 200-worker mine’s life by two years to 2017.