Precious metals in Earth's mantle are the result of a bombardment of meteorites more than 200 million years after the planet was formed, U.K. researchers say.

As the Earth formed, molten iron sank to its center to make the core, taking with it the vast majority of the planet's precious metals such as gold and platinum.

That movement to the core should have left the crust and mantle without any appreciable reserved of the precious metals, researchers at the University of Bristol said, but precious metals are tens to thousands of times more abundant in the Earth's silicate mantle than anticipated.

Researchers say they believe the presence of abundant amounts of gold and other precious metals is due to a massive meteor shower that struck Earth well after the core had formed.

Matthias Willbold and Professor Tim Elliott of Bristol University analyzed rocks from Greenland that are nearly 4 billion years old that provide a unique window into the composition of our planet shortly after the formation of the core.

Differences in certain metal isotopes in the ancient rocks compared with modern day rocks strongly support the theory that the excess of accessible gold on Earth is the fortunate byproduct of meteorite bombardment, Willbold said.

"Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tons of asteroidal material," he said in a university release Wednesday.