Injuries to 3 workers halt underground mining at Oyu Tolgoi after seismic event
Key Facts
Rio Tinto temporarily halted underground operations at the Oyu Tolgoi mine in Mongolia after seismic activity occurred at around 18:00 local time on October 29 along a rock fault zone, causing tremors inside the mine. The company said 3 employees were injured and no fatalities were recorded. It added that the suspension was in line with its procedures and would remain in effect until a safety review was completed. The suspension applies to the underground section, while surface operations, including milling and open-pit mining, continued, according to a Bloomberg report published on October 30. The company’s shares fell by as much as 2.9% in Sydney that day, the report said.
Rio Tinto said the seismic activity occurred in a rock fault zone at Oyu Tolgoi, causing tremors in the underground mine. The company said: "There were no fatalities. 3 employees were injured." It added that underground operations were "currently suspended in accordance with procedures, until the safety review is completed." Completion of the review is the next step before underground activity can resume, while surface work continued, according to Bloomberg. Through its combined open-pit and underground operations, the mine is targeting average production of about 500,000 tonnes of copper a year between 2028 and 2036, according to Rio Tinto’s plan.
The suspension of underground mining coincided with a separate disruption to the transport of Mongolian copper to China. The company operating the mine told The Associated Press that copper concentrate exports to China had fallen by about 70% over the preceding two weeks because of restrictions at a border crossing. Truck convoys carrying the concentrate fell from their usual rate of 12 to 13 a day to about 4 a day at the time of the report. Mongolian border officials attributed the slowdown in crossings to infrastructure and technology upgrades at China’s Ganqimaodu port. The Mongolian government said the upgrade work could continue until the end of December, after it had been expected to finish by late September, according to Mongolian media reports.
The transport restrictions highlight the crossing’s importance to Mongolian exports: coal and copper concentrates together accounted for 82.3% of the country’s merchandise exports in 2025, while 89.4% of Mongolia’s total exports went to China, according to The Associated Press. The agency cited local media reports estimating that the export reduction was equivalent to about $300 million compared with normal operating conditions. The decline could reach $1.2 billion if the restrictions continue through the end of 2026, according to the same reports. Oyu Tolgoi did not confirm these estimates to The Associated Press. The figures show the Mongolian mining sector’s exposure to cross-border transport delays, alongside the temporary suspension of underground mining for safety reasons.
Rio Tinto owns 66% of Oyu Tolgoi, while the Mongolian government, through Erdenes Oyu Tolgoi, owns the remaining 34%, according to Rio Tinto data as of June 30, 2026. The company thus combines its role in operating the mine with its interest as its largest shareholder. The production plan targets average output of about 500,000 tonnes a year from the mine’s combined open-pit and underground operations between 2028 and 2036. The seismic activity temporarily halted underground operations, while open-pit mining and milling continued, according to Bloomberg. The duration of the suspension remains tied to completion of the safety review announced by the company.