The union said it is seeking better retirement security for younger workers by moving newer employees from the company's Defined Contribution plan to a Defined Benefit plan.
"Rio Tinto is committed to working with the union to reach a mutually beneficial outcome to the ongoing bargaining process," a Rio spokesperson told Reuters.
Negotiations are also focused on a backlog of more than 300 grievances and the company's refusal to hire full-time workers leading to an overreliance on temporary employees, Unifor said.
Unifor says it represents about 900 workers at the company's aluminum smelting plant in Kitimat and power generating facility in Kemano.
(Reporting by Sabahatjahan Contractor in Bengaluru and Jeff Lewis in Toronto; Editing by Subhranshu Sahu)