MELBOURNE (Reuters) - Rio Tinto (RIO.AX)(RIO.L) is set to submit a mine plan to U.S. authorities this week for its $6 billion Resolution project in Arizona, which could become the largest copper producer in North America, a spokesman said on Thursday.

Resolution, 55 percent owned by Rio Tinto and 45 percent owned by BHP Billiton (BHP.AX)(BLT.L), could produce more than 1 billion pounds of copper a year at its peak, according to Rio Tinto's web site.

"On Resolution we are focusing on the submission of our Mine Plan of Operation to the U.S. Forest Service this week," Rio Tinto spokesman David Luff said.

Development of the mine hinges on a land swap with the U.S. government first proposed by Resolution eight years ago, but the exchange has yet to win approval from Congress due to opposition from environmental and native American groups.

Rio and BHP have already spent $1 billion on shaft development and drilling, but slashed spending on the project by 75 percent in 2013 to $50 million as part of their wider global efforts to slash costs. The firms said they would focus this year on securing the land swap and permitting for the project.

Analysts have speculated BHP might auction its stake in Resolution after selling its Pinto Valley mine in Arizona for $650 million earlier this year, as the world's biggest miner looks to cut costs and focus on its highest returning businesses.

The Resolution project was picked up by BHP in its disastrous acquisition of Magma Copper in 1996, on which it lost A$3.2 billion.

(Reporting by Sonali Paul; Editing by Ed Davies)