The Arizona Daily Star reports supervisors voted Tuesday to send notice to the U.S. Forest Service on Feb. 14.
They'll issue a formal objection to the agency's tentative approval of the Rosemont mine and its final environmental impact statement.
The letter is expected to force the Forest Service to respond to the county's long list of concerns about the mine.
County Administrator Chuck Huckelberry specifically faulted the project's proposed mitigation and monitoring measures, calling them woefully inadequate.
The proposed mine would put an open pit in the Santa Rita Mountains.
The mine and its associated tailings and waste rock areas would be located on more than 4,400 acres of federal, state and private land.