Morphological investigations by scanning electron microscopy revealed that some ore particles (gold, pyrite, uraninite, chromite, platinum group minerals) found in the early Proterozoic Witwatersrand gold and uranium deposit (South Africa) show distinct abrasion and rounding, although these particles are small in size with average diameters below 100 mu m. It has been previously stated that these particles are too small to round by abrasion, but analysis of their hydraulic equivalence to the smallest size quartz grain that can be so rounded indicates that rounding by abrasion is entirely possible. The rounded shape of ore minerals, even those relatively small in size, can be taken as an important criterion to deduce the placer origin of the Witwatersrand, and similar deposits.

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