Rio Tinto has offered around US$2.7 billion for the outstanding 49% of shares, at a 32% premium to share prices at close on Friday, leaving it will with full ownership of Turquoise Hill, which owns 66% of Oyu Tolgoi, with the balance held by the Mongolian government.

"Rio Tinto strongly believes in the long-term success of Oyu Tolgoi and Mongolia, and delivering for all stakeholders over the long-term," said Rio Tinto Chief Executive Jakob Stausholm. "That is why we want to increase our interest in Oyu Tolgoi, simplify the ownership structure, and further strengthen Rio Tinto's copper portfolio."

The offer will now be considered by the Turquoise Hill board, excluding Rio Tinto's nominees, before going to shareholder vote.

Rio Tinto shares on the LSE were down 4.1% at 5,334.00p at 8.47 London time.