A lettter from Rio to the government posted on twitter showed the miner also agreed to cancel the financing agreement for the underground development and to allow an independent audit into the underground financing.

The miner also agreed to no new debt burden for Mongolia in addition to transparent governance.

Amarjargal Khenchbish, the country manager for Rio Tinto in Mongolia, said:

"Investors are committed to the common goal of putting the underground mine into operation in a timely manner and increasing the value of Oyu Tolgoi."

Mongolia's government, which owns 34% of Oyu Tolgoi, has been unimpressed with the project's various cost overruns and is yet to issue key approvals for the underground development.

Oyu Tolgoi produced 41,935 tonnes of copper and 130,799 ounces of gold in the September quarter and is expected to become one of the world's largest copper mines.

Rio is developing the project through its subsidiary Turquoise Hill Resources.