Rio Tinto Alcan will shut down about a third of the production at its 438,000-tonne Alma aluminum smelter in the Canadian province of Quebec after locking out hundreds of unionized workers in a contract dispute, it said Sunday.

Earlier the unit of the Anglo-Australian mining giant said it locked out about 800 workers at the smelter in Saguenay-Lac-Saint-Jean, Quebec, 480 km (about 300 miles) northeast of Montreal.

Rio said it had begun an orderly shutdown of 144 out of 432 reduction cells at the smelter and would continue to operate the remaining capacity using company staff until further notice.

Rio said it had been bargaining with the union, Syndicat des travailleurs de l'aluminium d'Alma, since Oct. 4. The contract expired  Dec. 31.

Union representatives could not be reached immediately for comment.