The company said the incident happened this morning, but initially provided no further details.
FMG subsequently confirmed the death of a worker after a collapse of ground.
FMG Elizabeth Gaines CEO expressed the company's deepest sympathy to the worker's family, friends and colleagues.
"This is a very sad day for Fortescue and all our thoughts are with his family as we provide our full support to them at this very difficult time," she said.
"The wellbeing of the entire Fortescue family is our priority with a range of support services available across our sites, including the Fortescue chaplains and Employee Assistance Program."
A full investigation is underway.
FMG said it was working closely with the WA Police and WA Department of Mines, Industry Regulation and Safety (DMIRS).
"DMIRS is aware of a significant incident at a mine site in the Pilbara region that occurred earlier today," DMIRS acting director of safety Sally North said.
"Mines safety inspectors from the department are travelling from Perth to attend the scene, and are expected to arrive this evening."
WA Police have been contacted for comment.
The incident comes less than two weeks after a 52-year-old maintenance contractor from Queensland died from injuries sustained in a fall at CITIC Pacific's Sino Iron magnetite operation at Cape Preston in the Pilbara.
