The Mongolian government is considering terminating and replacing the development and financial plan for the Oyu Tolgoi copper and gold mine, Rio Tinto-controlled Turquoise Hill Resources said on January 11.

The government had advised that it was dissatisfied with the miner's plans to achieve production from Oyu Tolgoi for development capital of $6.7bn, Turquoise Hill Resources added. The costs of the expansion are about $1.5bn more than the original estimate.

Mongolia believed the significant cost increase has eroded the economic benefits it expected to receive from the mine, Turquoise Hill said.

Shares in the company dropped to their lowest in a month on the news, down 21.4% in New York to $11.38 and more than 24% in Toronto to C$13.88 by 10:50 am EST, Mining.com reported. That left the miner with a market cap of almost C$2.99bn (about $2.33bn).

Rio Tinto in 2019 flagged stability risks associated with the original project design, which translated into as much as an additional $1.9bn cost and a 30-month delay.

Oyu Tolgoi is one of the world's largest-known copper and gold deposits, located in the South Gobi region of Mongolia.

The government has a 34% stake in the project, while Rio-controlled Turquoise Hill owns the rest.

Rio has said that the development of the underground project at Oyu Tolgoi, which has suffered a series of setbacks and delays, would unlock the most valuable part of the mine, which by 2030 was expected to be the world's fourth largest copper mine. Oyu Tolgoi is expected to produce 480,000 tonnes of copper per year on average between 2028 and 2036 from both the open pit and underground. That would compare to just 146,300 tonnes from the open pit in 2019.