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 tectonites 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 + Fe203 (average = 34%) are the major chemical constituents. The Cu and S contents are very low (both < 100 ppm), and no evidence for replacement of preexisting sulphides has been detected. BA is enriched in platinum group elements (PGE) and shows bimodal patterns on the standard PGE diagrams. Differences between the patterns are much greater for Os and Ir than for the other PGE. This observation is interpreted as being indicative of a high-temperature process (> 2000øC; 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 oceanic 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.
INTRODUCTIONIn 1920 an unusual Fe-Ni-Co body was discovered in the metaperidotites of the Barberton greenstone belt [Trevor, 1920]. The body, known as the Bon Accord (BA) deposit (Figure la), first attracted attention because of its extremely high Ni contents (38% NiO) and later because of its Ni-rich, but sulphide deficient, mineralogy (see Table 1). Walker [1923] identified trevorite (NiFe204), the nickeliferous spinel, in this body; Partridge [1944] described the rare serpentine, nepouite (Ni3Si2Os(OH)4); S. A. de Waal and L. C. Calk (as cited by de Waal [1978]) discovered the pure Ni end-member of the olivine group, liebenbergite (Ni2SiO4). De Waal [1979] reported five other new minerals, including the Co-rich and the Ni-rich spinels, cochromite and nichromite. No Cu-minerals have been reported, and sulphides are rare. Chemically, the BA deposit differs from all known Ni deposits in that it is practically devoid of Cu and S (Cu, S < 100 ppm) [de Waal, 1978] and much richer in Ni. Renewed interest in the BA occurrence has focused on its origin, following the suggestion by de Waal [1978, 1979] that BA might represent an oxidized form of a metallic Fe-Ni occurrence...
