MELBOURNE (Reuters) -Rio Tinto, has made an approach to buy lithium producer Arcadium Lithium, the two parties said in separate statements on Monday, without revealing any financial details.

Rio's approach to Arcadium comes as miners are eager to secure supplies of critical minerals needed to power the global energy transition and comes after weeks of speculation over the deal.

Reuters exclusively reported on Friday that the companies had been holding talks, and Arcadium could be valued at $4 billion to $6 billion or higher.

"The approach is non-binding and there is no certainty that any transaction will be agreed to or will proceed," Rio said in its statement.

Arcadium's market cap was $3.31 billion at Friday's close.

If consummated, the deal would make Rio one of the world's largest suppliers of lithium behind Albemarle and SQM. Demand for the ultralight metal is forecast to surge later this decade from growth in lithium-ion battery use in electric vehicles and consumer electronics.

Australia-based Arcadium shareholder Blackwattle Investment Partners called the approach "opportunistic" and said that any offer of between $4 billion to $6 billion would "significantly undervalue" the lithium company.

The recent slump in lithium prices, which is due in part to Chinese oversupply, has pushed Arcadium's shares down more than 50% since January, making it an attractive takeover target.

"If the management do believe that the growth opportunity highlighted by the recent Investor Day is achievable, in our opinion, a sale price for LTM should be closer to $8 billion, and LTM should be willing to walk away from an opportunistic offer," it said.

(Reporting by Himanshi Akhand and Surbhi Misra in Bengaluru; Editing by Cynthia Osterman and Diane Craft)