I saw an interesting documentary recently about the British Royal art collection, and it included the interesting quote: "At the root of every great fortune is a crime".
I expect that the crime being referred to was the execution of Charles 1st?
"Le secret des grandes fortunes sans cause apparente est un crime oublié, parce qu’il a été proprement fait."
The secret of great fortunes without apparent cause is a forgotten crime, because it was properly/cleanly done," is my fairly literal translation, but I am not really fluent enough to capture subtleties.