The agreement follows a memorandum of understanding between Rio and Chalco's parent company Chinalco in March.
The venture will develop Rio's Simandou iron ore project in Guinea.
Chinalco, which has a 9% stake in Rio, will invest $1.35bn in the Simandou project.
Chinalco president Xiong Weiping said: "This project can also efficiently balance China's need for security on the global iron ore market.
"We expect the two sides will regard co-operation on the Simandou project to be the foundation for further pushing forward the co-operation of the two companies in other resource projects."
He said he hoped it would help Guinea, too.
"We believe the successful development of the Simandou project will greatly quicken the pace of local infrastructure construction and economic development," Mr Xiong said.