Raiders by Rudie van Rensburg

Five years after their disappearance, the skeletons of wealthy twin brothers are uncovered in a field.
At the insistence of the Western Cape police chief, Kassie and Rooi have to investigate the case, but in top secrecy. Otherwise they will run the risk of being murdered just like the three detectives who originally investigated the brothers’ disappearance.
A pitch-dark underworld, where ancient objects fetch millions of rands at black-market auctions, will not tolerate meddling.