A Fort Frances Resolute worker is in a Toronto hospital after an incident at the mill on Thursday.
The company said there was a fire in the biomass boiler building and an operator was injured and hospitalized with burn injuries.
A Resolute spokesperson said the employee was in critical condition.
"Our main focus remains with the employee and his family," said Xavier Van Chau in an email to CBC News.
Resolute and the Ministry of Labour are looking into what happened.
Van Chau added: "At this point we are investigating the cause of the incident and have contacted all appropriate authorities."
He said the fire was brought under control and the boiler is now back in operation.