Changes to Boron Ore Reserves and Mineral Resources

02 March 2018

Rio Tinto’s 2017 annual report, released to the market today, includes significant changes in estimates of Ore Reserves and Mineral Resources at the Boron open pit mine in Boron, California (RTB Boron), compared to the previous estimate in the 2016 annual report.

The updated Ore Reserves and Mineral Resources are reported in accordance with the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves, 2012 (JORC Code) and the ASX Listing Rules. Supporting information relating to the changes is set out in this release and its appendix. Ore Reserves and Minerals Resources are quoted on a 100 per cent basis. Rio Tinto’s interest is 100 per cent.

RTB Boron Ore Reserves have decreased by 6 Mt, from 22 Mt to 16 Mt.

Accordingly, RTB Boron Mineral Resources exclusive of Ore Reserves have increased by 8.5 Mt, from 0 Mt to 8.5 Mt.

The reduction in Ore Reserves applies to the body of calcium borate ore types at the site. This decrease in Ore Reserves reflects a reassessment of the study support level of processing assumptions following independent technical review, leading to updated asessments of processing flowsheet options, and likely economics. The the sodium borate Ore Reserves, being majority of the Ore Reserves at the site, are unaffected.

Following this write back of Ore Reserves the operating life of RTB Boron has been reduced by 7 years and is anticipated to run until 2042.

Calcium borate extraction options and economics will be reviewed in the future as required subject to market opportunities and updated technical asessments.