The deal comes as a relief to the sector, where officials were fearful of a repeat of a five-month platinum strike in 2014-2015 which crippled production and hurt the economy.
Eramet earlier predicted the FID would be made at the earliest in the fourth quarter of this year, but the decision has been slightly delayed due to the change in Argentina's government.
Western powers will attend talks in Brussels next week on curbing China's dominance of rare earths and other critical resources and EU officials will present their vision to create entire green supply chains.
The plant, located about 373 miles from the US border in Cobalt, Ontario, would be the sole North American producer of refined cobalt for the electric vehicle market.