IRR, it's not easy to find latest info, one analyst mentioned 11%, but I can only go by 2020 announcement:

Company") is pleased to announce that it has entered into definitive agreements with Ganfeng Lithium Co., Ltd. (together with its subsidiaries, “Ganfeng Lithium”) in respect to their joint venture company, Minera Exar S.A. (“Minera Exar” or “JV”) (collectively, the “Transaction”). Minera Exar owns 100% of the Caucharí-Olaroz lithium project (“Caucharí-Olaroz” or the “Project”) currently under construction in Jujuy, Argentina. The Transaction includes the following provisions:

• Ganfeng Lithium has agreed to subscribe for new shares of Minera Exar for cash consideration of US$16 million increasing its interest from 50% to 51%, with Lithium Americas owning the remaining 49%.
• Both parties have agreed to the 2020 funding schedule, including investment of up to US$400 million, in accordance with the 2019 approved construction plan.
• Caucharí-Olaroz remains on budget and schedule to commence production in early 2021.
• Joint venture and related agreements will be amended as a result of the new ownership structure to preserve joint approval for substantive matters involving Minera Exar and to maintain the existing construction plan and management team.
• In addition, Lithium Americas will receive US$40 million in cash from the proceeds of non-interest-bearing loans from Ganfeng Lithium (additional details below)....