Pitt was in line to play Colonel Fawcett, leader of a 1925 expedition into Brazil to find the fabled city, who disappeared without trace.
Vulture quote insiders as saying that the reason for Pitt walking was to do with an impasse over compensation to be paid to the film’s director, James Gray (We Own The Night, The Yards). Pitt will apparently move onto Cogan’s Trade instead, which you can read all about here.