Impact site of a nickel-iron meteorite that fell on earth 49,000 years ago. It was 150 feet across (only!), weighed roughly 300,000 tons, and was traveling at a
gold scientific discoveries

Why does the meteorite impact create gold deposits?

Why rich metal ore deposits can be found in places of the meteorite impact? A large astronomical object almost always evaporates after an impact on the Earth’s surface and leaves nothing but an impact crater. Only parts of the meteorite, just small splinters, can be found in the surrounding area. So where do the metal deposits come from? Conclusions from the survey conducted on the Barringer's impact crater in Arizona have led the scientists to the answer.

Asteroids are small size astronomical objects (from a few meters to over 1000 km) moving along their orbits like planets in the Solar System. As it is commonly believed today, they are a remnant of the formation of the Solar System. When about 100 - 250 million years after the Big Bang stars started to form from clouds made of dust and gases, larger clusters of matter began to join each other slowly forming planets. Some of them remained small and were no more than 1000 km in diameter.

In the initial period of the formation of the Solar System, constantly growing clusters of matter regularly collided with each other. The phenomenon could be observed on their surface covered with craters, nowadays it is perfectly visible on the surface of the Moon, which is just part of the Earth's crust detached in a collision, when a planet of the size of Mars hit the young Earth, 4.5 billion years ago (The Great Collision Theory). The genesis of the Moon's origin explains why it contains mainly rock-forming minerals such as the ones in the Earth's crust.

A meteorite impact on the Earth

Sometimes one of the asteroids is knocked out of its standard orbit due to the disturbance of gravity. Such a disturbance does not have to be large, for instance, asteroids located at the outskirts of the Solar System, far beyond Pluto, are so weakly held by its gravitational field that they move at a speed of only 350 km per hour. To knock out such a planetoid from its orbit, a star flying nearby is enough. Currently over 740,000 asteroids are known in the Solar System. In October 2017, a team of astronomers from Pan-STARRS observatory in Hawaii noticed an interstellar asteroid (from outside the Solar System) for the first time. They called it 1I / 2017 U1 Oumuamua.

If one of these asteroids changes her course in space and collides with the Earth, it will be a meteorite impact and the new impact crater will be formed. Due to the fact that most of the Earth's surface is covered with water, and the continental plates are still moving, the traces of meteorites have well been preserved only on the old continental parts of the lithosphere (craton). About 77 confirmed impact craters and 5 implied (the list) were discovered on the Earth.

Barringer's Meteor Crater

The research on the first known and probably the youngest terrestrial impact crater, the Meteor Crater in Arizona, led the scientists to the clue, which allowed us to unravel the mystery of some extremely rich metal deposits. Until the time of the research on the Meteor Crater, similar terrain forms on the Earth had mainly been classified as geological creations. The scientists thought, that they had been created in the process of a volcanic eruption or a violent eruption of steam, caused by magma reaching the underground water reservoir (so-called maar). The American geologist and astronomer Gene Shoemaker, while observing the craters after the experimental nuclear explosions, came to the conclusion that, like the crater in Arizona, they contain strange glassy crystalline structures. Today known as a Stishovite, a kind of metamorphic form of quartz, which requires temperatures of 1200-1400 ° C and pressure of 160,000 atm to be born. The area around the crater in Arizona was dotted with fragments of almost pure iron, which in this form does not occur practically on Earth. It made scientists think, that a huge meteorite had fallen there.

The impact of the asteroid on the surface of the Earth releases enormous energy. This energy hollows deep crater and melts the surrounding rocks. The impact of such a large body on the Earth's crust is treated as a geological event, because it implicates geological consequences such as creating a magma reservoir. 

Sudbury Crater

Exactly such an event occurred in today's Canadian province of Ontario, where 1,850 million years ago a meteorite of a diameter of approx. 130 km hit the Earth's surface. The impact of a rock bigger than Mt Everest hollowed a crater deeper than the Grand Canyon.  The crater walls collapsed soon, although the enormous impact energy created the temperatures high enough to bring surrounding rocks to melt, then a giant lake of liquid magma was created.

Such amount of magma had been cooling down for the next hundreds of thousands of years working exactly like a magma chamber under the volcano.  In the process, of melting rocks and then cooling magma, minerals from the parent rock started to separate from each other. That was possible because of their different melting points and different densities. First were felsic rocks to melt, because they consisted of minerals with a high percentage of silica and the last to melt were mafic or ultramafic rocks, containing no silica at all. 

When a huge meteor hits the ground surrounding rocks which contain different minerals start to melt. This process comes to the point when melted felsic rocks are liquid and mafic rocks are hot but not melted yet and called the restite. At this point some metals naturally spread in the Earth's crust start to migrate to the felsic melt or to the restite. Gold for example, usually moves into the melt. If felsic melt is, for instance, 1/5 of the whole volume of slowly melting rocks, gold will migrate there increasing its concentration by 5 times. 

This differentiation of magma can occur in the magma pool inside the impact crater. Heavy elements will descent then towards the bottom of the crater, and light ones will ascend to its surface. This process can slowly form much higher concentration of chemical elements than their natural average concentration in the Earth's crust.

A huge meteorite impacting the ground could practically have evaporated due to the enormous energy of such a collision, but the geological process started by such an event could have created rich metal deposits. It happened in Sudbury, where very rich nickel and copper deposits were formed.

Such a strong impact on the surface of the planet can also be the trigger for other geological events. Another meteorite impact has been considered the reason of cracks in the ground through which oil went up the surface. The occurrence of rich gold deposits in South Africa has also been attributed lately to the huge meteorite impact.

sources: NASA: Small Asteroid or Comet 'Visits' from Beyond the Solar System; wikipedia: Stishovite; Peter Chinn, TV How the Earth was made 2007; The first stars in the universe, Richard B.Larson, Volker Bromm, Scientific American, Vol.14, nr 4, 2004, Before the beginning: Our universe and others, Matrin J.Rees, Perseus Books, 1999; The first sources of light, Volker Bromm, Publications of the Astronomical Society of the Pacific, Vol.116, pages 103-114; The first stars, Volker Bromm, Richard B.Larson, Annual Reviews of Astronomy and Astrophysics, Vol.42, pages 79 — 118, September 2004, Does the Earth’s gold come from stars?Kiedy powstały pierwsze gwiazdyAgnieszka Dutka, Earth Impact Database.
Created with NASA WorldWind using Landsat 7 (Visible Color) satellite image, source: Public Domain
Map showing the Sudbury Basin, produced by Natural Resources Canada in 1955. Extracted from the map shown below. Open Government Licence - Canada OGL-C 2.0

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