The union representing workers at Vale in Sudbury says 21 employees have lost their jobs.
Rick Bertrand, the president of United Steelworkers Local 6500, says the affected employees work at the smelter, mines and as industrial millwrights.
He says contractors are working on the property and that the union was told no employees would be laid off while contractors are on site.
"The union is very disappointed obviously with the company's decision to lay off 21 of our members while we still have contractors on our property," he said.
"They made the decision to lay off our members with seniority and we believe that we're going to have to fight tooth and nail to get our members back to work."
Company says steps taken to reduce number of layoffs
In a statement to CBC News, Vale spokesperon Angie Robson says the company and industry as a whole has been "suffering through a prolonged downturn that continues to challenge the profitability and competitiveness of our operations around the globe."
"This past year has been one of transformation in our Ontario operations," she said.
"The ramp down of Stobie Mine and the Clarabelle Mill Crushing Plant, the transition to a single furnace and the closure of the copper circuit in our smelter means that Ontario operations will produce lower volumes than we have in previous years. While we are smaller today, these decisions have been set us up to be much more agile and competitive in the years ahead."
Robson adds Vale took steps to mitigate the impact.
"There were originally 280 surplus employees identified," she said.
"However, through the displacement of contractors, natural attrition, offer of a retirment incentive and deployment of underground miners to the Copper Cliff Mine Project, we were able to reduce this number significantly."
Last month, the company eliminated the position of director for Ontario operations. At that time, the company was not able to say whether or not future positions would be cut.
But in August, a United Steelworkers representative told CBC News the company had been hinting at future layoffs at the Sudbury operations.