What made metals form into large veins in Earth’s crust?
In: Planetary Science
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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