Distribution of heavier elements in a planet

[rootone]

Since most planets in the earliest stages of formation are very hot and fluid, why is it that we find on Earth areas where there are substantial deposits of heavier elements - gold, uranium, etc in the crust?
Intuition suggests that these relatively rare elements would become widely dispersed throughout the whole planetary body during it's fluid stage, and not concentrated in some areas.
Perhaps a higher proportion of the heavier (thus denser) elements might be expected to accumulate in the deep core, but not elsewhere, but if that happens, what is separating concentrations into one element here and another there?.
Also there would have to be some kind of unbelievably powerful convection at work to transport the core accumulations back to the surface, (and yet still allowing the accumulation to occur in the first place, and without the convection process itself mixing stuff up (diluting the concentrations) again.

 

[Chronos]

Heavy elements are typically associated with past episodes of volcanism. Tectonic activity also plays a role in lifting veins of heavy elements within reach of mining activities.

 

[rootone]

Thanks for answering my question.
Convection (volcanism) on geological time scales partly explains this, (and tectonics too),
but I still can't understand the 'veins' of this or that element.
Why is that?

 

[marcus]

Water at very high temperature and pressure can dissolve material you would not expect.
Then when the temperature and pressure change, different compounds crystallize out at different rates.
I'm not a geology expert and I'm not talking about "gold and uranium" specifically. I just want to make a very general point about how VEINS might form which are enriched with various compounds we consider ORES.

I suppose that other substances besides water (e.g. molten rock) could also act as solutions out of which various compounds precipitate and crystallize preferentially, forming ore veins.

 

[snorkack]

Simple answer - heavy elements are soluble in lighter ones. They would sink to the centre of Earth if they separated as insoluble grains or drops from the lighter elements - which they don´t do at high temperatures of molten rock and iron.
In the same manner, even though salt is denser than water, it does not sink to the bottom of ocean as rock salt.

Most gold is in fact in Earth core - not as separate golden core, but as very dilute solution in molten iron. And the reason iron is in core is that iron is immiscible with rock in the amounts it occurs in Earth. Actually, a large part of Earth iron IS included in rock, both as concentrated iron ores and diffused in silicates like olivine et cetera. But there is a lot of iron left over for Earth core, and it also dissolves most of the strong siderophiles like gold.

 

[D H]

Since most planets in the earliest stages of formation are very hot and fluid, why is it that we find on Earth areas where there are substantial deposits of heavier elements - gold, uranium, etc in the crust?

During the thousands of years we humans have been mining gold, we have managed to recover a bit more than two Olympic-sized swimming pools full of gold. That's not much. Gold in the Earth's crust is highly depleted compared to that in chondritic meteors, which purportedly are the source of the bulk of the Earth's mass. (This is the bulk silicate Earth (BSE) hypothesis). Per this hypothesis, the reason gold is highly depleted in the crust, and presumably in the mantle, is that almost all of the Earth's primordial gold sank to the center of the Earth when the Earth differentiated into a core, mantle, and crust.

There are no substantial deposits of gold in the Earth's crust. There are instead little pockets of gold here and there. Almost all of the gold we now mine was deposited on the crust during the late heavy bombardment.

Uranium is different. Gold is much more prevalent than is uranium in chondritic meteorites, by a factor of about 20 to 1. On the other hand, uranium is much more prevalent than is gold in the Earth's crust, by a factor of about 600 to 1. While gold is strongly depleted in the crust and overly abundant in the core (compared to chondritic meteors), with uranium it's the other way around.

The reason is chemistry. Gold and other precious metals are, according to the Goldshmidt classification of elements, siderophiles. These elements preferentially alloyed with iron and sank to the Earth's core when the Earth differentiated. Uranium is a lithopile element. It oxidizes quite readily and also combines with other lithophile compounds. (Compare with gold, which is extremely inert. Gold can be made to oxidize, but these compounds are unstable. The most stable gold oxide decomposes at 160 °C.) Uranium didn't sink to the core with the siderophile elements. In addition to being a lithopile element, uranium is an incompatible element. Over time, this eventually led to uranium being concentrated in the Earth's crust.

 

[rootone]

Thanks guys.
So there are a fair number of processes both physical and chemical which can affect the way in which different elements can become concentrated.
I only cited gold and uranium as examples because those two are very well known, but it's very interesting to note that the answer to my question in the case of gold is quite different to that of uranium.