A study from the University of Bristol shows that the Earth’s surface became enriched with precious metals by impacting meteorites.

 (Some of the oldest rocks on Earth, demonstrate that gold was delivered by meteorites in what they call, the time of the terminal bombardment, which occurred around 3.9 billion years ago. During this time, the Earth was hit with around 20 billion billion tonnes of asteroid material.  None of which landed in my back garden.)