(Reuters) - The world's biggest iron ore producer Rio Tinto is offering $9 billion in U.S. investment-grade bonds as the miner seeks funding for its recently-closed buyout of Arcadium Lithium, Bloomberg News reported on Tuesday.

Rio is marketing the debt in eight parts and will include a long-term note with a maturity of 40 years, yielding 1.3% over Treasuries, Bloomberg said, citing a person familiar with the matter.

The Anglo-Australian mining giant on March 6 completed its $6.7 billion acquisition of the U.S.-based Arcadium Lithium, as it looks to diversify away from iron ore towards critical minerals and battery metals such as lithium.

The deal was initially funded by bridge financing and intended to be replaced by longer-term debt, the report added.

Rio Tinto did not immediately respond to a Reuters request for comment.

Bloomberg News earlier reported, citing people familiar with the matter, that Rio has dropped plans to raise as much as $5 billion in a share sale following pushback from investors.