Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically related to the Bushveld complex, or as Archean upper mantle rocks. Re-Os isotopic data from these rocks display very low 187Os/188Os and 187Re/188Os, suggesting that they are not magma chamber cumulates, but rather the residues of melt extraction, similar in Re-Os systematics to subcontinental lithospheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean. On the basis of these data we conclude that the ultramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. This conclusion strengthens evidence for a model whereby the rocks exposed in the core of the Vredefort structure represent the lower parts of a crust on edge profile that possibly contains a paleo-Moho very near the Earth's surface.