I've read several of his books, but they've all been set in Scotland.
However, there's a list of his books here:http://www.bookseriesinorder.com/nigel-tranter/
ETA - ah, just spotted one called Rio d'Oro. Amazon doesn't give a description of it, but that sounds quite hopeful!
A-ha - looks as if that's one of three books (Rio d'Oro, There Are Worse Jungles and The Long Coffin) about an explorer who disappeared in the Amazon. Maybe he was a pal of Professor Trelawney's
. It does look like it could be any one of three, though.