Global miner Rio Tinto has said the schedule and cost estimates for its massive Oyu Tolgoi project in Mongolia are in line with earlier forecasts, after completing an updated feasibility study for the copper mine.

However, it cut the estimated reserves it would mine at Oyu Tolgoi following a redesign to its mine plan to deal with underground stability issues, and warned there could be further changes to reserves.

In July last year, Rio estimated the capital cost of the project at $US6.5 billion ($A9.4 billion) to $US7.2 billion ($A10.4 billion), and said first production could be achieved between May 2022 and June 2023.

The new mine design for the Hugo Dummett North underground mine at Oyu Tolgoi also confirms its caving method of mining remains valid, the development of which “will unlock the most valuable part of Oyu Tolgoi”, the company said in a statement.

The Oyu Tolgoi deposit in south Mongolia is one of the world’s largest known copper and gold deposits, with the open pit at the project contributing about 9 per cent of Rio’s total mined copper in the first quarter of fiscal 2020.

It is jointly owned by the Mongolian government and Rio’s Turquoise Hill Resources.

Last month, the miner signed an agreement for Mongolia to build a coal-fired plant to supply power to the Oyu Tolgoi mine.