Fortescue’s $US3.8 billion Iron Bridge operation has suffered a setback after the discovery of Indigenous heritage sites forced a 3 per cent reduction in the mineral resources available at the two magnetite deposits that underpin the project.

The extra caution around heritage sites comes as chief executive Elizabeth Gaines delivers the final profit result of her four-and-a-half year tenure on Monday, with analysts predicting the second-biggest profit in Fortescue’s history.