Hergé's bigotry is unprejudiced, like Ed McBain's Ollie Weeks'. (I translated all these texts from Dutch, so especially accents may be off)
This is a reference to Fawcett, an English explorer who, not-so-mysteriously, disappeared in the Amazon around 1930.
A very funny book about a search for him by was written in 1933 by Peter Fleming: Brazilian Adventure
He's also discussed in Lands Beyond by L. Sprague de Camp and Willy Ley