From Chapter 19 of In a Sunburned Country, by Bill Bryson, copyright 2000:
“Here, near the [west Australian] coast, there are a few towns and quite a lot of visible farming, but head inland over the pale green hills to the right and with amazing swiftness you will find yourself in a murderous and confusing emptiness. And nobody really knows what is out there. I find that a terribly exciting thought. Even now people still make the sort of stupefying, effortless finds that can only happen in uncharted country. Just recently, some grinning fellow had come in from the western deserts cradling a solid gold nugget weighing sixty pounds. It was nearly the largest such nugget ever found, and it was just lying in the desert. Goodness!”
How would such a thing come to exist? Why would all those gold atoms decide to congregate like that? Do silver nuggets exist? Platinum nuggets? Or copper or iron, for that matter. If they don’t, what’s different about gold?
Says solid gold … what purity would be likely for such a find?
I believe the latest theory involves earthquakes squeezing quartz crystals enough to create an electric field that attracts dissociated gold particles to form clumps over eons. So earthquakes, pressure, and electricity.
Here’s a quick overview, but hot fluids with disolved minerals get pushed though rocks and cool at various rates, allowing deposits of minerals with different melting points.
I think that density also plays a role. If you’ve got a magma-chamber full of melted mixed minerals, to the extent that they don’t dissolve in each other, the denser ones will settle to the bottom. Then when that molten blob cools off and solidifies, all of the densest stuff will be together. And gold is one of the densest minerals.
Gold in situ in the mountains is usually in the ore but the rare nugget forms alone. As it weathers out and descends into the river valleys it becomes progressively smaller. When prospectors search for the source (the mother lode) they prospect in the direction that produces larger pieces of gold if they can. Here in NE Ohio all of our gold is in glacial deposits from Canada and has been worked quite a bit. Most of it is like sand particles at best. You can find an occasional “picker” that is large enough to, well, pick up.
The gold that formed as the Earth solidified would have fallen deep into the core, ne’er to return. But the Late Heavy Bombardment would have scattered it all over or just under the surface.
And before that, gold was created by neutron star collisions, ejected by dying low-mass stars and by magnetars. It has a significantly different origin story to common elements like oxygen, iron and helium, so is probably distributed somewhat differently throughout the universe.
The case of gold nuggets is interesting, because gold is relatively nonreactive, so you wouldn’t expect it to congregate into nuggets due to chemical action.
It doesn’t. The nuggets are alloys of gold, silver and copper and sometimes lead. No chemical bonds. Easily melted and separated using various means.
Right now silver is very high, floating around $50 per troy ounce. I was thinking of selling some and talked to my local coin shop. He recommended to wait a bit as the refineries are swamped with people selling back sterling silver (92.5%) and 90% silver coins. They take more time to refine to .999 silver and so the buy back prices are not as high as you would like.
I didn’t say that it did – the implication is that, since it doesn’t accrete through chemical action, it’s surprising to find accumulations nonetheless.