The Bon Accord (BA) Ni-Fe deposit occurs in chemically depleted ultramafic rocks of the circa 3.5 Ga Jamestown ophiolite complex in the Barberton greenstone belt of the Kaapvaal craton, South Africa. The host rocks of the BA body are high-temperature ultramafic tectonics which probably represent the upper mantle residue from which the overlying magmatic rocks were separated during partial melting. BA is unusual both mineralogically and chemically. It consists of a rare Ni-rich assemblage: Ni-oxide (bunsenite), -spinels (trevorite, nichromite), and -silicates (e.g., liebenbergite, the Ni end-member olivine) and their altered equivalents. NiO (average=38%) and FeO+Fe2O3 (average=34%) are the major chemical constituents. The Cu and S contents are very low (both 2000 ¿C; lower mantle). High Ni/Fe and Ni/Co ratios (relative to C-1 chondrite) suggest that BA might have been derived from siderophile-rich material that remained in the lower mantle after inefficient core formation. A model is presented wherein such a metal-silicate heterogeneity is fractionated and oxidized during ascent through the mantle in a thermal plume which originates in the lowermost mantle (the D' layer). Some of its residue is finally incorporated as pods into the lithospheric mantle during formation of the Archean ocean crust. Inclusion of such fractionated pods in the old subcontinental ''keel'' of the Kaapvaal craton might constitute a potential PGE source, which could have been tapped by subsequent magmatic activity. ¿ American Geophysical Union 1989