Diamondiferous intrusions and magmatic Pt-Pd deposits are both concentrated on, or adjacent to, the oldest cratons, those with >3.0 Ga high-grade gneiss terranes and/or greenstone belts. Given the old age (>3.0 Ga) of peridotitic inclusions both in diamonds and in kimberlites, diamonds presumably grew in mantle depleted in basaltic major elements near the base of thickened lithosphere and below early sialic nuclei. Most genetic models for magmatic Pt-Pd deposits require a Pt-Pd–enriched, high–Mg-Si melt generated from analogously depleted mantle. The depleted mantle was most likely formed by removal of basaltic melts that contributed to Archean intracratonic greenstone belts. Extensive melting also decreased density and increased rigidity beneath ancient cratons, favoring stabilization and preservation of thick Archean continental lithosphere. Overall, these considerations suggest that localized, thick sialic crust and rigid lithosphere developed before 3.0 Ga, forming enduring, coupled crust-mantle systems below ancient sialic nuclei, the sites of selectively preserved greenstone belts. The data confirm that Early Archean terranes are highly prospective for post-Archean magmatic ore deposits.