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I don't really understand this. I always understood that precious metals are created in the late fusion process of stars and somehow fell on Earth via collision (fun thought: as there are some traces of those metals in humans, one could say we're partly made from stars).
Anyway, the article isn't about the origin of gold etc, but why they are found in the mantel of Earth and not closer to the core.
But how do we know that there isn't more at the core? Maybe in the mantel it's just fragments? And how do we know it fell down in "moon sized" rocks and not in smaller portions which wouldn't penetrate that deep?