The company has increased this offer, through an indirect wholly-owned Canadian subsidiary, for Hathor to C$4.70 ($4.56) per share.
Rio Tinto said its increased offer values Hathor at about C$654m ($635.75m) on a fully-diluted basis.
The Hathor board of directors recommends that Hathor shareholders accept and tender their common shares to Rio Tinto’s increased offer.
Last month, Rio Tinto said it would make an offer to buy all of the common shares of Hathor for C$4.15 ($4.03) in cash per share.
