(Reuters) -Sibanye Stillwater Ltd is buying half of ioneer Ltd’s Nevada lithium mine project for $490 million, one of the largest deals ever for U.S. supply of the electric vehicle battery metal as demand is set to soar later this decade. The investment is a vote of confidence in American lithium projects despite recent pushback from environmentalists here, regulators and others. It also reflects rising concerns that, without more investment, demand for the white metal could far outstrip supply and delay efforts to combat climate change. Australia-based ioneer said it will form a joint venture with the South African miner to develop the Rhyolite Ridge mine, roughly 220 miles (355 km) north of Las Vegas. Sibanye shares dipped 0.5% at the open in Johannesburg, while ioneer shares fell 19% after it announced a share issue to fund the deal priced at a discount to its last closing price.Ioneer shares are still up 114% this year. Ioneer will remain the project’s operator and tap Sibanye’s experience as the world’s largest miner of platinum group metals (PGMs) to develop the project, which also has a large supply of boron, used in soaps and other consumer goods. “Rhyolite Ridge is a world-class lithium project and we recognize its strategic value, with the potential to become the largest lithium mine in the U.S.,” said Sibanye Chief Executive Neal Froneman. The entire project is expected to cost about $850 million. Both companies said they will now work to secure debt financing to fund the rest of the project. Ioneer said it expects financing, as well as necessary permits, by the end of next year. The project is expected to open by the end of 2024.Sibanye, which operates mines across South Africa and in Montana, will also buy $70 million worth of ioneer shares. Should ioneer expand operations to a nearby lithium deposit, Sibanye will have the option to pay $50 million to secure a half stake in that project. The deal is the second major lithium investment by Sibanye this year. In February, the company bought roughly a third of Finnish lithium company Keliber Oy.