Officials said the funds will maintain RBM's current capacity and also extend its mine life. RBM currently operates four mines in the Zulti North lease area; it also has a smelting facility and mineral separation plant at the complex.

"The Zulti North orebody grade is declining, hence the Zulti South mine is required to maintain the output of high-margin zircon and rutile, and provide sufficient ore to support TiO2 sales," Rio Tinto said. Phase 1 of the mine plan for Zulti South will reinforce the zircon and ilmenite supply for RBM over its mine life.

The miner has targeted mid-year for a construction start, pending permit approvals, with commercial production to follow in late 2021. Production from Zulti South will be processed through RBM's existing infrastructure.

"Zulti South is one of the best undeveloped minerals sand deposits in the industry, and will significantly extend RBM's position as a world-class, first-quartile asset," Rio Tinto chief executive Jean-Sébastien Jacques said.

"The long-term fundamentals of the market remain strong, and production from Zulti South will commence in time to fill a widening supply gap, ensuring RBM's position as a leader in the sector, and delivering strong returns to our shareholders."

Rio Tinto energy and minerals chief executive Bold Baatar pointed out that RBM is South Africa's largest mineral sands producer.

"We not only mine, but produce value-added products for customers around the world. We are proud of the value we create, and retain, in South Africa."

RBM, where Rio Tinto has held a 74% stake since 2012 and serves as the operation's manager, produces predominantly rutile, zircon, titania slag and high purity iron. The balance is held by the Blue Horizon investor consortium (24%), while the remaining 2% of shares are held in an employee trust.