Where does all the gold come from?

Sep 07, 2011
File photo shows gold nuggets on display in Jamestown, California. Were it not for meteorites striking Earth some four billion years ago, humans would never have laid eyes on the gold that has raised and ruined civilisations, according to a study published Thursday.

Ultra high precision analyses of some of the oldest rock samples on Earth by researchers at the University of Bristol provides clear evidence that the planet's accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. The research is published today in Nature.

During the formation of the , molten iron sank to its centre to make the core. This took with it the vast majority of the planet's precious metals – such as gold and platinum. In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a four metre thick layer.

The removal of gold to the core should leave the outer portion of the Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in the Earth's silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the Earth after the core formed. The full load of meteorite gold was thus added to the mantle alone and not lost to the deep interior.

To test this theory, Dr Matthias Willbold and Professor Tim Elliott of the Bristol Isotope Group in the School of Earth Sciences analysed rocks from Greenland that are nearly four billion years old, collected by Professor Stephen Moorbath of the University of Oxford. These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.

The researchers determined the tungsten isotopic composition of these rocks. Tungsten (W) is a very rare element (one gram of rock contains only about one ten-millionth of a gram of tungsten) and, like gold and other precious elements, it should have entered the core when it formed. Like most elements, tungsten is comprised of several isotopes, atoms with the same chemical characteristics but slightly different masses. Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to the Earth would leave a diagnostic mark on its W isotope composition.

Dr Willbold observed a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks. This small but significant change is in excellent agreement with that required to explain the excess of accessible on Earth as the fortunate by-product of bombardment.

Dr Willbold said: "Extracting tungsten from the and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements."

The impacting meteorites were stirred into the Earth's mantle by gigantic convection processes. A tantalising target for future work is to study how long this process took. Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today.

Dr Willbold continued: "Our work shows that most of the 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 tonnes of asteroidal material."

Explore further: Suomi NPP satellite spots birth of Tropical Cyclone Kate

More information: 'The tungsten isotopic composition of the Earth's mantle before the terminal bombardment' Matthias Willbold, Tim Elliott and Stephen Moorbath Nature (2011).

Related Stories

The Earth and Moon formed later than previously thought

Jun 07, 2010

The Earth and Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or ...

Atomic Particles Help Solve Planetary Puzzle

Nov 10, 2009

(PhysOrg.com) -- A University of Arkansas professor and his colleagues have shown that the Earth's mantle contains the same isotopic signatures from magnesium as meteorites do, suggesting that the planet formed ...

Mars formed rapidly into runt of litter

May 25, 2011

(PhysOrg.com) -- Mars developed in as little as two to four million years after the birth of the solar system, far more quickly than Earth, according to a new study published in the May 26 issue of the journal ...

Recommended for you

Suomi NPP satellite spots birth of Tropical Cyclone Kate

19 hours ago

The tropical low pressure area previously known as System 95S organized and strengthened into Tropical Cyclone Kate on Dec. 24 and the Cocos Keeling Islands are expected to feel its effects on Dec. 25 and ...

NASA looks at some severe holiday weather from space

19 hours ago

Severe weather in the form of tornadoes is not something people expect on Christmas week but a storm system on Dec. 23 brought tornadoes to Mississippi, Georgia and Louisiana. As the storm moved, NASA's RapidScat ...

NASA satellite spots Christmas

Dec 24, 2014

If you're looking for Christmas NASA's Aqua satellite spotted it in the Southern Indian Ocean. It's a coral atoll (a ring-shaped reef, island, or chain of islands made up of coral) in the northern Line Islands ...

User comments : 36

Adjust slider to filter visible comments by rank

Display comments: newest first

DiverseByDesign
4.5 / 5 (6) Sep 07, 2011
Wow. Just the sheer amount of time that these processes take is almost unimaginable.
Bob_B
not rated yet Sep 07, 2011
Is the metal rich core swirling as 'mixed' metal so it would not be gold but, a very diluted gold? Or does it puddle and there are blobs of gold and other blobs of iron, etc?
Shootist
5 / 5 (5) Sep 07, 2011
All one has to do is look at Sudbury Canada and Vredfort South Africa to know that large bolides can emplace mineable amounts of metals and diamonds in the crust.
GDM
4.4 / 5 (7) Sep 07, 2011
I seem to recall that 3M corp got much of its iron ore from a large impact crater in Minnesota. If large ore amounts are from impacts, the moon should be rich with the stuff, not having undergone any tectonics.
RealScience
3 / 5 (2) Sep 07, 2011
GDM - the original material of the moon, the ejected debris from a huge impact during the formation of the earth, would have been from the earths outer layers and hence very poor in iron and the siderophile (iron-loving) precious metals.
The impacts during the late heavy bombardment should indeed have added iron and precious metals. And without tectonics to mix the added material up, it should remain concentrated in impact craters (like Shootist's Sudbury example).
OverweightAmerican
1 / 5 (12) Sep 07, 2011
You are all wrong. Gold comes from mines, deep inside the earth - how could this have been deposited by an asteriod?!? Not possible.
RealScience
4.6 / 5 (9) Sep 07, 2011
OverweightAmerican - you are joking, right?
Just in case: asteroids are big and fast moving, and make very deep holes when they hit.
Also many hit the ocean, and the ocean floor gets subducted and melted, and some of that melt come up in mountain ranges like the Andes (which are rich in gold and silver).
And even a few-mile deep gold mine is really just scratching the surface of the earth - it only seems deep because of our human scale.
scidog
not rated yet Sep 08, 2011
iron in Minnesota was not dumped into place by rocks from the sky but formed as a result of iron precipitating algae.better check out Sudbury also,same deal,it's the magma that filled the crater that brought the nickel and other ores in..
if your ever thru Sudbury make sure you stop at the Tim Hortons for a great doughnut..
hard2grep
1 / 5 (1) Sep 08, 2011
I seem to recall that 3M corp got much of its iron ore from a large impact crater in Minnesota. If large ore amounts are from impacts, the moon should be rich with the stuff, not having undergone any tectonics.


That is an excellent point. I think the author's reason why is because the moon was not there yet. Of course one could also argue that if the moon is a result of a collision with Earth by a planetesimal, then the energy should have been enough to melt the earth again.
StarGazer2011
1 / 5 (1) Sep 08, 2011
If the gold and other dense elements was subducted into the mantle, why didnt it then sink? Im guessing some combination of convective vulcanism might explain, but still.
Also does this theory also apply to Uranium (for example)?
Graeme
5 / 5 (3) Sep 08, 2011
The elements that end up in the core will be those that dissolve in molten iron. Elements that prefer to combine with oxygen may well be included in less dense minerals such as phosphates that remain in the mantle or dissolved in the molten silicaceous magma and do not sink as a metallic phase. The early earth would have been like a huge smelter with molten metal sinking below molten slag.

Once the mantle solidified it would be much harder to separate out any metallic particles.
bluehigh
2.1 / 5 (7) Sep 08, 2011
.. have been added to our planet by lucky coincidence ..


I can only surmise that either theres no gold on other planets or the whole solar system was sprayed by a whole series of lucky coincidental impacts. Science?

rubberman
4 / 5 (4) Sep 08, 2011
Bluehigh....everything all planets are comprised of came from space, therefore, one would have to assume it (gold) is not specific to our planet.
bluehigh
1.7 / 5 (6) Sep 08, 2011
(Gold) .. added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material.
- from the article.

Either NO Gold on the other planets or this lucky asteroid strike was NOT specific to Earth. How, rubberman, do you presume any other alternatives based on the article content?

What have I missed? If there is near surface Gold on any other planet then the hypothesis in this article (big asteroid deposits Gold on Earth) is disproved.

RealScience
4.2 / 5 (5) Sep 08, 2011
bluehigh: What you have missed is that the late heavy bombardment almost certainly hit all of the inner planets, so we would expect gold on the surface of them. For those with plate tectonics, deposits should be at mountains above major subduction zones as well as impact sites.
From what we know now we should be surprised if the LHB were specific to the earth and moon and missed everything else, and figure out why if so.

Also, don't take one scientist's off-hand comment about a "lucky coincidence" as if it is proven or widely accepted or even surprising at all.
He likely meant "lucky for us", not "extremely unlikely but happened anyway".
Given what we have seen of other planetary systems with the planets shuffling around after formation, the late heavy bombardment of this solar system is likely to be typical or even on the mild side.
Pyle
3.9 / 5 (7) Sep 08, 2011
What have I missed? If there is near surface Gold on any other planet then the hypothesis in this article (big asteroid deposits Gold on Earth) is disproved.
I think you misunderstood the line you quoted from the article. There is no reason to think all that material came from one asteroid. It would have been many asteroid hits. There is a reason people talk about mining the asteroid belt. There's gold in them dar ... rocks floating in space. (as well as other precious metals)
PinkElephant
3.7 / 5 (3) Sep 08, 2011
there are enough precious metals in the core to cover the entire surface of the Earth with a four metre thick layer
One has to wonder if perhaps the Moon might have its own little core of iron/gold/platinum, etc. Probably cool and solid enough now (and perhaps no longer too radioactive) to mine and refine. Other than surface-borne He3 for potential fusion applications, and aluminum/titanium deposits in the outer crust, this could eventually be a very nice incentive for colonizing and thoroughly developing our planetary companion...
Pyle
2.8 / 5 (4) Sep 08, 2011
PE: You're nuts. The moon's diameter is like 2,000 miles. The pressures involved to drill the 900 miles or whatever to the core, even on the moon, have to be insane. Makes more sense to me to grab the little gold nuggets floating around in the asteroid belt.

But that was just my gut reaction. (Harvesting precious metals from craters makes a lot of sense to me though.)
PinkElephant
4 / 5 (4) Sep 08, 2011
@Pyle,

I may be nuts, and you may be right. However, consider that the Moon only has 1/6th of Earth's gravity at its surface, and moreover is supposedly completely solid and cold all the way down. It also has no atmosphere. So where would the "pressures" come from?

I suppose even solid, cold, highly compacted rock could rapidly flow like a fluid under sufficient pressure, but would the pressure really be sufficient in the Moon's case?
Pressure2
5 / 5 (1) Sep 08, 2011
Sure most heavy elements would have sank to the center when the earth first form but plate tectonics and volcanos can easily explain their present on the earth surface in the diluted quantities most are found.
RealScience
4.3 / 5 (6) Sep 08, 2011
PinkElephant- The moon's average density is 3.34 g/cm3 = 3340 kg/m3. It is denser at the core than the surface, but ignoring that will make the calculations much easier. At the surface the gravity is 1.62 m/sec2, so that's 5000 Newtons/m3 or 5 kPa or pressure per meter.
With uniform density the gravitational force would decrease linearly and so would average half the surface value, so the pressure at the core would be 2.5 kPa/meter, or 2.5 MPa/km, times the radius of 1740 km. This is 4.35 GPa.
Tool steels can take ~300-350 MPa or ~12x less than this.
For a cross-check, this is ~200 times the pressure of the 2 km of ice in the last ice age. Rock is rebounding ~ 1/cm/year from that, so simple scaling would suggest the hole trying to close at ~2 meters radius per year at the bottom.
That would be sufficient to fracture the rock, leading to catastrophic collapse!
PinkElephant
3.4 / 5 (5) Sep 08, 2011
@RealScience,

Now let's take into account a couple of additional things.

First, your pressure calculations effectively apply to liquids, but not so much to solids. For instance, how much pressure do you feel standing under a bridge, vs. pressure transmitted through the bridge's pylons? Keeping in mind that the moon is spherical, the higher layers don't transmit _all_ of their weight downward, because they self-support laterally to a certain degree just like an arch supports its own weight.

Secondly the density is of course not uniform and increases toward the core, so the pressure isn't as high when you approach the core -- whereas higher density closer to the core is mitigated by much lower gravitational acceleration.

Thirdly, one would think that highly compressed, cold rock will have achieved crystalline structures that greatly surpass the strength of tool steels.

Lastly, Earth's crust is rebounding post-glacially at the rate it is, because it is partly molten.
RealScience
3 / 5 (2) Sep 08, 2011
PinkElephant - First, the compressive strength of rock is only a fraction of that pressure, so effectively the moon rock will act as a slow moving liquid. The moon has had plenty of time to reach hydrostatic equilibrium, so the pressure is that high down there already. The Ice-age rebound comparison is a crude one, but it shows the order of magnitude of how fast the rock the stiffness of the earth's continents would try to flow, and it indicates that if you were to drill a hole, its radius would shrink 2 meters per year at the bottom if the rocks didn't fracture first.
Second, the high density at the core means that the gravitational pull decreases slower from the surface and hence the average acceleration is higher and the pressure is higher, not lower. Each shell of a sphere pulls on something outside that shell as if all its mass were at the center.
- Continued -
RealScience
3.7 / 5 (3) Sep 08, 2011
Since the core is in equilibrium, the rock will have compressed roughly 10% linearly or 20% in area, so only an area 5 times hole area needs to expand to fill the hole. So any rock outside a few hole diameters won't help support the walls, either.
And while the pressure is high, it is not enough for most rocks to change crystalline structure - that is usually ~10 GPa or so.

The rebounding crust is ~40 km thick, and while it gets close to melting at the bottom, the moon's core is estimated to be over 800C, or ~half-way to melting and thus close to the average crust temperature, therefore I felt that the rebound of the crust was a tolerable approximation.

However the rebound is flex on a length scale ~20x the crust thickness, which greatly overstates the flow, and the softening with temperature is exponential, so the top of the crust dominates rebound, which greatly understates the flow. These two factors might not cancel well, so I agreed that the rebound calculation is a crude.
PinkElephant
5 / 5 (2) Sep 08, 2011
@RealScience,

Good points, though I'm still not convinced on the gravity vs. density balance: I still suspect the product of the two will yield a value less than the uniform-density model. But it's probably a matter of small coefficients rather than orders of magnitude.

Yep, I guess I'll have to cancel my plans to found a deep Moon-extraction consortium :-) I mean, apart from 800C temperatures at the core (which I didn't realize was the current estimate -- I thought it was much cooler), upon further reflection actually lifting stuff such a long way through such a gravitational well would probably use too much energy to make the whole enterprise cost-effective. Probably cheaper to mine smaller space rocks in the first place, even if they are that much farther and harder to get to and live on.
RealScience
2.5 / 5 (2) Sep 08, 2011
PinkElephant - I'm with you on mining space rocks.
Although people seem to regard NEOs as hazards, I see each one as 'a gift from the gods'. We should use Aphophis' 2029 near approach to give it a nudge that would prime it for capture in 2036. But rather than another encounter with the earth to adjust its orbit in 2036, I'd recommend nudging it to pass close to the moon. This might take more passes to capture it due to the moon's lower surface gravity, but it would be much safer since a miss would impact the moon rather than the earth.
A nudge at perihelion in 2029 would take surprisingly little energy to make a big change in its position in 2036.
Alternatively if humanity wants to practice first, we could steer Apophis to hit the moon in 2036 to remove the danger to earth. Hitting the near side would be pretty spectacular!
But I think that ~100,000 times the mass that humanity has put into orbit so far is too good a resource to pass up, so I say capture it.
RealScience
3 / 5 (2) Sep 09, 2011
Hmm - a quick estimate came up with only a few thousand kilometer shift by 2036 if one took a single space shuttle, filled its cargo hold with fuel, and used its engine to push on Apophis in 2029. The energy is there, it's the momentum, and it isn't passing all that close to the earth in 2029.
The same fuel used as an explosive below the surface would be much more effective due to kicking off far more mass. This would put it within the range of human's current capability, even without nuking it (which would be much harder to control and would leave the material less desirable).
(Or we could practice on a 30-meter NEO first).
FainAvis
5 / 5 (2) Sep 09, 2011
About that shuttle idea:
My thinking, and I am not an expert in this, is that only propellant that escaped Apophis's gravity completely would shift its orbit. If it does not escape it would be like the cartoon of a sailor blowing into his ship's sails. Expended propellant that fell back onto Apophis would have no effect and be wasted.
So could the shuttle, or any rocket engine we have, overcome that hump?
Pyle
4 / 5 (4) Sep 09, 2011
So could the shuttle, or any rocket engine we have, overcome that hump?
Absolutely. Apophis's gravity is negligible. You could jump right off it. In fact, it would be pretty difficult to stay on at all.
RealScience
4 / 5 (4) Sep 09, 2011
FainAvis - While you are right that only propellant that escaped would shift the orbit, Pyle is correct that the gravity of Apophis can be neglected for propellant.
The escape velocity of Apophis is only ~0.5 km/h, or about 10% of walking speed.

TheGhostofOtto1923
5 / 5 (5) Sep 09, 2011
PE: You're nuts. The moon's diameter is like 2,000 miles. The pressures involved to drill the 900 miles or whatever to the core, even on the moon, have to be insane. Makes more sense to me to grab the little gold nuggets floating around in the asteroid belt.

But that was just my gut reaction. (Harvesting precious metals from craters makes a lot of sense to me though.)
Materials which may be rare and valuable on earth might be plentiful enough on the moon or asteroids to refine and use them locally for more mundane applications... like insulation or power distribution.
http://en.wikiped...sulation

-Suitable asteroids in earth orbit could be hollowed out, pressurized, and inhabited, their bulk providing natural radiation protection. And they could be moved elsewhere.
gimpypoet
1 / 5 (1) Sep 11, 2011
put a lot of gold into the curent economy and it will colapse very fast. remember mansa musa while on a pilgrimage (on the silk road) and gave away so much(that he carried with him on camel and cart) caused a colapse in 14 century. I would like to get new rare materials, and would like to see if a ship could be made if hollowed out. very interesting concept.
Nanobanano
3 / 5 (1) Sep 12, 2011
PinkElephant:

Come on dude, you're smarter than that.

The weight of atmosphere is insignificant compared to the weight of rocks after just about a FOOT below the surface...

In fact, in an earth based mine, the weight of the atmosphere even helps off-set some of the rock, because the air fills the caverns.

on the moon, there is no air-pressure, so you would have empty shafts of vaccuum under 1/6th gravity for walls of lunar regolith and rock. Sure, you could probably go 6 times as deep or so, or make a shaft 6 times wider due to the weak gravity, but that would depend strongly on local geology, faults, etc and mineralogy of the rocks you are digging in.

Moreover, we have almost no knowledge whatsoever of the moon's sub-surface geology regarding faults and mineralogy. It would probably take decades to do enough research to know an area is safe for large sub-surface mining operations.
Nanobanano
not rated yet Sep 12, 2011
And regarding gold and platinum, again, if there is allegedly that much gold and platinum inside this planet's core, and if said gold and platinum allegedly originate from distant supernovas and neutron stars mergers, then explain how such an insanely huge amount of gold and platinum somehow obtained escape velocity of a neutron star at several thousand km/s, and then somehow slowed down enough to be captured in a stable orbit around the sun at just a few km/s without violating conservation laws?

Moreover, where the hell is the extinct star or an extinct, double-mass neutron star which should still be "somewhere" in the vicinity within a few light years of the Sun?
antialias_physorg
5 / 5 (3) Sep 12, 2011
Apophis's gravity is negligible. You could jump right off it. In fact, it would be pretty difficult to stay on at all.

I expect this would make mining an asteroid pretty hard (though it would make 'exporting' the material rather easy)

then explain how such an insanely huge amount of gold and platinum somehow obtained escape velocity of a neutron star

Escape velocity is not equal to final velocity (escape velocity just means that you have enough kinetic energy to overcome the gravity well). But if you start off at JUST escape velocity then your speed after leaving the gravity well will be close to zero. No conservation laws need to be violated.
RealScience
3.7 / 5 (3) Sep 12, 2011
Nano - The mass of gold/platinum did not come straight here from a neutron star to an orbit around the sun. It would have come into the huge dusty gas cloud from which the solar system condensed.
And the sun has traveled around the galaxy ~25 times since its birth, so its birth neighbors are not necessarily still current neighbors.

And while you are correct that the weight of the air is insignificant for deep mines, it is more than 'insignificant after about a foot'.
On earth the weight of the atmosphere is equivalent to ~34 feet of water. Rock is typically 3x denser than water (granite, for example, is ~2.7, and basalt 3.0). So the atmosphere weighs the same per area as ~10 feet of basalt or 12 feet of granite.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.