THE late Archaean greenstone terranes of Western Australia contain abundant komatiites (high-MgO lavas) hosting magmatic sulphide deposits rich in nickel, copper and platinum-group elements, Thermal erosion and assimilation of sulphidic sea-floor sediments has been proposed as a mechanism by which the komatiites were brought to sulphide saturation(1-4), Such models have important implications not only for the genesis of these sulphide ores, but also for interpreting the magnitude and extent of isotopic heterogeneity in the Archaean mantle, Here we report that massive, matrix and disseminated sulphide ores and a komatiite from Western Australia yield a magmatic Re-Os isochron age of 2,706 +/- 36 Myr and a near-chondritic initial Os-187/Os-188 ratio of 0.10889 +/- 0.00035, whereas a proposed sulphidic sedimentary contaminant has an extremely radiogenic Os-187/Os-188 of 1.0983 at 2,706 Myr, These data demonstrate that the ore-forming komatiites were derived from the upper mantle without significant contamination by radiogenic crust either before eruption or during turbulent how at the surface, Thus, ground melting and assimilation of sulphidic sediments may not be as important in ore genesis as current theories suggest.