First guess is that Columbas would be sailing on behalf of the Portuguese king, not the Spanish.
I would also think that Portugal would not sink into Spanish hands so quickly a century later, as all the Kings of Europe would want to get their hands on the funds availbale from Kimberly AND Mexico.
Also major reprecussions for the British Empire in 1800's as no Rhodes etc.
The Portuguese knew about the gold and diamonds in southern Africa. It just happened to be deep in the heart of the Mutapa, who were quite adamant, and quite good, at holding back Portuguese encroachment.