South Africa’s biggest union said it had declared a wage dispute with the world’s top diamond producer De Beers, and had asked an arbitration authority to mediate. The National Union of Mineworkers (NUM) said in a statement it had declared the dispute after talks with De Beers — 45 percent owned by miner Anglo American — broke down, and wants the Commission for Conciliation, Mediation and Arbitration (CCMA) to help the parties reach an agreement.