O.K. did some Googling and it sounds like the Witwatersrand deposits were there long before the meteorite impact. But still, it is a pretty strange coincidence. The impact does control the structure of the area, so you could claim that these Witwatersrand deposits would not be exposed the way they are if not for the impact. Actualy, they probably would have been eroded away long ago.
Fun to understand all this geology, but not sure it really helps make investment (wild gambling?) decisions.