Mines Management has secured conditional approval from the US Forest Service (USFS) and the Montana Department of Environmental Quality (MDEQ) to develop the $550m Montanore mine project.
With the final records of decision (RoDs) in place, the company now plans to advance the project through evaluation and feasibility.
The approval amends the existing state Hard Rock Operating Permit #00150 to conform with provisions of the RoDs and issues the certificate of compliance permit for the electrical transmission line.
Air quality permit has also been granted for construction and operation up to 20,000t per day at the project.
Mines Management chairman and CEO Glenn Dobbs said: "The record of decision (RoD) is the major approval to complete the final evaluation and development of the project contingent upon compliance with its conditions.
"We are excited to have completed the eleven year permitting process that included two draft Environmental Impact Statements and at least three rounds of public comment."
Mines Management acquired Montanore in 2002 and has invested more than $75m since then on dewatering and partial rehabilitation of the 14,000ft decline, construction of site infrastructure.
The mine is initially expected to produce 12,500t per day with potential for expansion up to a permitted capacity of 20,000t a day.
Other individual permits and authorisations for the Montanore project would be issued following the RoDs in accordance with their processes and timelines.
Other permits are yet to be received from the US Army Corps of Engineers, MPDES permit and issuance of water rights.