The miner was looking for gold in waste rock at its Boron site but had a “eureka moment” instead when tests revealed high grades of lithium.
Now the FTSE 100 group's challenge is to how to extract it economically and achieve commercial scale production.
“The material being used has already been mined, so this will be a low energy option for the production of lithium,” said Bold Baatar, Rio's energy & minerals boss.
“It’s a great example of the innovative thinking we are encouraging, to look at extracting additional value from our tailings, look outside our traditional commodities and not be afraid to try new approaches.”
Rio shares were up 1% to 4,031.5p on Tuesday afternoon.