Rio said the clearance would allow both parties to close the transaction, expected within the next six weeks.
In late July, Arch sold $US600 million worth of senior unsecured notes in a private placement to help fund the $761 million Jacobs Ranch purchase, with the asset acquisition first announced back in March.
The completed sale will give Arch some 381 million tons of coal reserves near its Black Thunder mine, plus associated infrastructure of a high-speed rail loadout, a recently added overland conveyor and a near-pit crushing system along with a fleet of mining equipment and customer commitments.
The Jacobs Ranch open cut thermal coal operation shipped 42.1 million tonnes last year.
Rio planned the sale of the complex as part of an asset sale program to reduce debt.
Shares in Rio closed down 3% to $56.31 on Friday.
