Ivanhoe Mines is the Canadian parent company of the Australian-listed Ivanhoe Australia Ltd.
Rio Tinto only had until October 27 in which to exercise its purchase option, although Ivanhoe Mines shareholders were expected to vote later this month to extend the deadline if it was needed.
Ivanhoe Mines and Rio Tinto have long-sought to develop the Oyu Tolgoi copper-gold deposit in southern Mongolia, about 80 kilometres from the border with China.
On October 6, the mining companies signed an agreement with the government of Mongolia for a long-term investment agreement to develop the mining complex.
The terms of agreement mean the Mongolian government will acquire a 34 per cent stake in Tolgoi's licence holder Ivanhoe Mines Mongolia Inc, and Ivanhoe Mines will retain a controlling 66 per cent stake.
