The deal to exploit the Oyu Tolgoi mine was announced in a statement by Rio Tinto and marks one of the biggest investments in Mongolia since the former Soviet satellite state turned to capitalism two decades ago.

No figure for the size of the deal was given by Rio Tinto, but Mongolia's minister for mineral resources and energy, Dashdorj Zorigt, told AFP in September it was worth four billion dollars.

“This is a significant milestone and we now look forward to working together to develop what I firmly believe is the best undeveloped copper deposit in the world,” Bret Clayton, chief executive of Rio's Copper and Diamonds group, said in the statement.