Issues with ground instability has forced Rio Tinto to update its estimated reserves at the Ouu Tolgi copper project in Mongolia.

Reuters reported that Ouy Tolgoi is Rio Tinto’s largest copper project but has faced geological challenges in the past. In July last year, Rio estimated the project’s capital cost at $6.5 billion to $7.2 billion, as much as $1.9 billion more than its initial estimate in 2016.

Rio Tinto said the updated feasibility study confirmed that recent cost estimate, and warned there could be further changes to reserves as it continues work on the design.

Rio Tinto also narrowed its estimated first production from its mine to between October 2022 and June 2023, 21-29 months later than its initial estimate.

“This amended mine design is another positive step in the development of the underground mine which will unlock the most valuable part of Oyu Tolgoi. We remain focused on delivering the underground project safely and within the guidance ranges we have announced on both cost and schedule,” Arnaud Soirat, chief executive of Copper & Diamonds said in a statement.

The updated mine design is the result of the review announced by Rio Tinto in July 2019 when enhanced geotechnical and geological information obtained from drilling and mapping at depth suggested there may be some stability risks associated with the original mine design. The updated design retains two in-situ rock pillars on either side of Panel 0 for geotechnical stability. In the original mine design, these pillars were within the mining area. The updated design is supported by extensive geotechnical modelling and industry leading technical assurance.
As a consequence of leaving the pillars in place, the material contained in the pillars has been reclassified from Ore Reserves to Mineral Resources. It is expected that part of the material contained in these pillars will be recoverable at a later stage following additional studies which are currently underway.

Ore handling infrastructure will be relocated to the pillars, located immediately north and south of the current Panel 0 boundaries. Panels 1 and 2 will now be initiated as independent panels or mine blocks.

Optimization of mine designs for Panels 1 and 2 is ongoing and it is anticipated that this next phase of study may result in further movements in classifications of Ore Reserves and Mineral Resources. Any such movements will be reported following completion of the studies.

On 3 July 2020 Rio Tinto released to the Australian Securities Exchange (ASX) updated Mineral Resources and Ore Reserves for the Oyu Tolgoi mine.