Gold in Them Thar'....
Karen Hopkin in Scientific American:
Because it appears that a rain of meteors nearly 4 billion years ago peppered the Earth’s exterior with precious metals. So says a study in the journal Nature. [Matthias Willbold, Tim Elliott and Stephen Moorbath, "The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment"]
When Earth was forming, molten iron sank to its center, creating the planet’s core. That iron carried with it a slew of iron-loving metals, including gold and platinum. In fact, the Earth’s core is packed with enough precious metals to cover the entire planet four meters deep.
But not all that glitters is down at the planet’s center. Today there’s enough gold in the Earth’s rocky shell to please prospectors—and to puzzle planetary scientists: why is that gold not down in the core, too?
The answer, it seems, is that it wasn’t even on the Earth until after the core formed. Isotope concentrations in rock samples of different ages indicate that the composition of the Earth’s mantle changed after the planet was blasted with meteors 3.9 billion years ago. That bombardment coated our home with a thin veneer of riches. Which means that the gold in them thar hills is older than the hills.
So maybe if we go back to the gold standard, space exploration will take care of itself? At least it should be a bit easier than Tom Swift's drilling down to the core itself.