The six-year review was conducted by the US States Forest Service.

The publication of the DEIS triggers a further public consultation process over a 90-day period, which will inform the final environmental impact statement (EIS) and the next steps in the permitting process for the project.

"This is an important milestone for Resolution, with the future development of the project being shaped by years of engagement with the local community and extensive independent study by the US Forest Service," Rio chief executive Copper & Diamonds Arnaud Soirat said.

"Achieving this significant milestone in the rigorous US permitting process allows us to continue progressing one of the world's most significant untapped copper deposits towards development."

Rio says Resolution has the potential to supply up to 25% of the US' copper demand.

"Long-term copper fundamentals remain strong - with supply challenged and demand driven by the adoption of electric vehicles, the electrification of industry and a growing share of renewables in the energy mix," Soirat said.

"Rio Tinto will be well positioned to benefit from this demand outlook and to provide North American manufacturers with the materials that are essential to their products."

Rio owns 55% of the project, with BHP holding the balance. The joint venture will have spent more than US$2 billion on Resolution by 2020, including $302 million approved this year.

Work underway includes reclamation of the historic Magma copper mine site, sinking a second shaft to mining depth, rehabilitating an existing shaft, extensive drilling and ore body testing, and the permitting and public engagement process.