Where do large lumps of gold come from?

Open University's David Rothery was going for gold to answer this question...

David - A supernova is the only situation when you can make substantial amounts of elements heavier than iron. It’s because the compression waves when the star is so massive it can’t support its outer parts anymore; it collapses inwards; it gets really dense, and a big explosion blows the outer parts off. When the pressure's really high you can fuse elements to atoms heavier than iron, among which is gold. Gold’s nothing special in this, but gold is among them.

Then you’ve got these particles of all kinds of elements which will be seeding space and that will form a cloud of gas and dust. And within such clouds is where stars and planets of the next generation of stars form. So, the material from which say the Earth formed would have a tiny amount of gold in it, but it won’t have gold nuggets in it, it will be isolated atoms of gold. When you formed a planet it will differentiate; the iron will go inwards; the rocky stuff will go outwards, and in amongst the rocky stuff will be individual atoms of gold. Then geology takes over and you have processes of distilling the crust. Maybe water coming in dissolving stuff, circulating around driven by internal heat and, atom by atom - gold’s a very inert element but it can form complexes with hydrogen sulphite and chlorine. But you get it into solution and then into places where it can come out of solution round some solid initial speck, and then you can grow a gold nugget atom, by atom, by atom.  Rarely bigger than a speck of gold but, sometimes, it’s as big as a brick.

So the gold nuggets don’t come from space. They’re not formed in a supernova but the gold atoms are formed in a supernova and get concentrated by Earth’s geological processes.

UPDATE 19/10/2017- Since this was broadcast, an announcement from LIGO has revealed that it is, in fact, colliding neutron stars, not supernovae, that expel heavy elements.