A massive meteor bombardment 3.9 billion years ago provided most of the gold and other precious metals found near the Earth's surface today, according to a new study.

The research by scientists led Dr Matthias Willbold from the University of Bristol explains why these minerals are surprisingly abundant in accessible parts of the planet, rather than sinking to the centre of the Earth during its early years.

As the Earth formed 4.5 billion years ago it was mostly a giant global magma ocean, which separated as it cooled. Precious 'iron-loving' metals like gold moved into the planet's iron core, leaving lighter silicates on the surface.

Reporting in the journal Nature, Dr Willbold and colleagues claim the relative abundance of precious metals in the Earth's mantle today probably came from a huge meteor storm which ended about 3.9 billion years ago.

This cataclysmic event, called the late heavy bombardment, added between 0.5 and 1 per cent of additional material to the Earth's mass, forming a veneer on the surface. It was also responsible for many of the moon's craters.

Dr Willbold and colleagues examined ancient rock samples from Isua in Greenland, which predate the late heavy bombardment by 100 million years and are some of the oldest rocks on Earth.

They compared them with volcanic plume samples from places like Hawaii, which date from after the bombardment.

High precision measurements were carried out to determine the ratios of tungsten isotopes. Before the bombardment, most of the tungsten-184 migrated to the iron core, along with gold. Another isotope, tungsten-182, is formed by the radioactive decay of hafnium and remained in the silicates that formed in the Earth's crust.

Dr Willbold and colleagues found a distinct change in the ratios of tungsten-182 and tungsten-184 immediately after the bombardment. This change in tungsten isotope ratio was most likely caused by meteorite impacts, which increased the amount of tungsten-182 and other precious metals in the Earth's crust.