What made metals form into large veins in Earth’s crust?
In: Planetary Science
Like attracts like in mixtures. If you mix something really well the different things will blend but given enough time the different things will separate because they like being with themselves. There will always be small amounts of the different things that will not find their like substance, but the bulk will. Different substances will separate at different rates, some really don’t like being mixed where some really really like being mixed and some substances will fully combine to make a different substance than the individual substances they came from.
When the universe was much younger, stars in this area of the universe got big and cooked a lot of different stuff inside them. Some were big enough to explode in a super nova. That explosion left weird stuff behind like heavy metals. Those metals shot out into space and made clouds of dust.
All the bits of metal were heavy though, so they have more gravity, which attracts them to one another. Heavy stuff pulls other heavy stuff towards it. So the big blobs of metals and such smash together to form bigger blobs, some of which are ALL made of one metal.
Eventually, all the bits and pieces in this area started forming things that were big enough to be planets and a star called the sun. Inside the planets there are still the big blobs of metal from the old blown up star, buried under all the other rocks and stuff that form the planet.
The planet is pretty heavy. And heavy stuff has gravity like we saw before. It pulls itself inward and that pressure makes it really hot inside. Hot enough that some rock is liquid, and it moves! This causes the stuff near the center to float around. And the stuff above moves with it, like it’s floating. It moves around a lot and bangs into other stuff floating on top of the molten rock, causing it to break up and smear and stretch around into veins
Heat and cooling for some of it, metals tend to melt and solidify at the same temperature, so as the rocks cool all the same metals turn from liquid to solid at the same time and form veins of metal. In addition iron has a unique history, iron deposits are largely found in what are called banded iron formations. These are the result of something called cyanobacteria on the early Earth producing large amounts of oxygen which bonded with the iron dissolved in the oceans creating iron oxide which the fell to the bottom of the oceans and created sedimentary layers of iron ore which are now the largest source of iron ore mined today. https://youtu.be/Zky6GdH1uLM
Hot water. REALLY hot water.
If you go down a few kilometres into the Earth the pressure rises enough that water stays liquid well above 100C and it becomes very corrosive. It also tends to contain chemicals like chlorine, fluorine and carbon dioxide which make it even better at dissolving metals.
A lot of metal ores are found around granite which begins life as molten magma containing small quantities of metal – and lots and lots of this superheated water. Eventually the granite cools enough to begin to crystallise – but the water stays liquid right until the end when it is forced out of the solidifying rock. The water is pushed under enormous pressure out of the granite – as it flows through the rock it dissolves those tiny amounts of metals.
When the water leaves the granite it finds its way into cracks and faults around the edges of the granite where it begins to cool and lose pressure. Metals that were soluble at very high temperatures and pressures begin to crystallise with the highest temperature metals like tin and tungsten very close or inside the granite; copper, zinc and iron a little further out, lead and bismuth even further from the granite, and the lowest temperature metals like arsenic (not quite a metal but good enough for jazz) WAAAY out.
The same hot water is what can turn solid granite into soft white china clay (kaolin).
Some of the best places to see where this has happened are Cornwall and West Devon in the UK which have long histories of mining a huge range of metals around granite.
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