Workers from three negotiating units announced Friday that they had voted down the proposed deal by margins of 71 per cent, 88 per cent and 98 per cent.
They were following the advice of their union's executive committees, all three of which recommended that members refuse the offer.
Union officials already have a strike mandate in hand and are now warning that a work stoppage could be imminent.
The current labour contract expires with the new year and, the union says, the company had better get back to the bargaining table immediately.
"People were voting to trigger a conflict if the employer doesn't get back to the table," was how union spokesman Marc Maltais interpreted the result of the votes Friday.
"We told members — there was no subtlety there — ... that if there are no discussions, the conflict is on."
The company was also warning of a potential labour disruption.
Spokeswoman Claudine Gagnon said the mining company was "saddened and disappointed" by the decision of its employees. She said the result of the vote was "clear," and she did not discount the possibility of a lockout.
The sides in the dispute haven't been able to agree on conditions related to subcontracted labour.
The Alma facility hosts one of the company's (NYSE:RIO) most important North American aluminum smelters, with about 800 employees.
Now it's poised for a shutdown — barring an 11th-hour surprise. The parties were set to meet, at the behest of a mediator, on Friday afternoon.
"For us, the employer faces a choice: Either it negotiates with us, or it has a labour conflict as of midnight on the 31st," Maltais said.
"The question is: 'Yes or no?' If it's no (to resuming negotiations), it's very, very clear that we'll wind up on a picket line."