Rio Tinto’s Offer to Take Over Giant Copper Mine Is Rebuffed (1)

Rio Tinto Group’s $2.7 billion offer to buy out Turquoise Hill Resources Ltd. has been rejected, blocking its efforts to gain greater control of a giant copper mine it’s developing in Mongolia.

Rio’s offer doesn’t “fairly reflect the fundamental and long-term strategic value of the company’s majority ownership of the Oyu Tolgoi project,” Turquoise Hill said Monday, after appointing a special committee to review the bid. Shares of the Canadian miner plunged the most since Nov. 3.

Oyu Tolgoi is expected to be the world’s fourth-largest copper mine once the underground component is completed, with Turquoise Hill and its ...