Tailings are the mounds of crushed rock, mixed with trace amounts of chemicals, that remain after ore is processed. Conventional tailings are mixed with water and stored as a slurry, about 50 percent liquid, Ms. Arnold said. Rosemont’s dry-stack tailings, with most of the water squeezed out and recycled, would be only about 15 percent moisture.

The dry-stack technology is intended to conserve water — the company says it would recycle 85 percent of all water it used — and keep pollution from seeping into the ground. A filtering process would remove the chemicals used for treating the ore.

Still, Roger Featherstone, director of the Arizona Mining Reform Coalition, said in an e-mail, the trace remains of heavy metals and sulfur naturally occurring in ore-bearing rocks “are virtually impossible to remove.”

“Even if Augusta did a good job of it, the sheer mass of the tailings leave enough heavy metals to cause pollution to migrate into surface and groundwater,” he said.

The mine is planned for the east side of the Santa Rita Mountains in what is known as the “Sky Islands” wildlife habitat, where mountains jutting up from a plateau create isolated pockets with diverse plant and animal life. The area also includes the Davidson Canyon Wash and Cienega Creek, unusual and sensitive wetlands. Altogether the area is home to 10 threatened and endangered species, while the wetlands are designated by the state as “Outstanding Arizona Waters,” a category that affords them the highest protection under the U.S. Clean Water Act.

Also at risk of disturbance would be more than 60 sites sacred to the Tohono O’odham American Indian nation, including some with human remains.

Pima and Santa Cruz Counties bring in about $2.95 billion annually from recreation and ecotourism, according to a 2008 study by the Sonoran Institute, a land-use advocacy group based in Tucson.