Since the earth was formed from a cloud of gas and dust in which, I presume, elements were randomly scattered, how did atoms of the same element end up clustered together? For example, how did enough gold molecules find other gold molecules so that there would one day be gold mines?
Mostly they didn’t. Most of the gold in the earth’s crust is scattered one atom at a time. But there are various physical and chemical processes that can concentrate different elements.
For instance, heavy elements weigh more than light elements. So when you’ve got a molten ball of rock, the heavy elements sink, the light elements float on top. And so we’ve got the volatiles on top (oxygen, nitrogen, hydrogen, carbon) in various combinations, then light solids like silicates and oxides, then heavier elements like iron.
But of course, the earth is volcanically active, so those elements are constantly being mixed as well. And there are chemical processes that concentrate certain elements. Water disolves some elements and carries them to seas. The sea can be isolated and the water dries up, and you’ve got a layer of salt. That gets covered by soil, then the sea returns, repeat, and you’ve got underground salt deposits.
Or underground ice wedges. Or deposits of shells that turn into limestone.
For gold, what happens is generally physical sorting. A river cuts through rock with a low concentration of gold. Light rocks are preferentially eroded away, heavy gold is preferentially concentrated in the streambed. If the concentration is great enough the streambed can be processed using essentially the same method to economically recover the gold.
Geothermal activity also had a role to play.
As water percolates down through the earth, it dissolves a variety of elements, particularly as it gets superheated. It also filters through a large volume of material.
As it gets forced back up and out a geothermal vent, it cools down and starts dropping dissolved material - silica, gold, silver etc. These channels of quartz and gold are what are mined (rather than panned).
Of course, if the quartz seam erodes or gets ground out under a glacier, you get alluvial gold - in rivers and lakes and sediments. The gold is heavy and sinks, so it stays in sediments near the rock bed. If it makes it to the sea, I suspect that it may redissolve in the salt water, though gold is pretty nonreactive. I seem to recall that gold has been recovered by dredging harbours and rivermouths of gold-bearing rivers. Certainly DeBeers recovers diamonds off the coast of SA in this manner.
Other similar chemical processes concentrate lead, tin, iron into veins that we can exploit. It’s a combination of geography, chemistry and physics that gives us usable concentrations of material.
Si
Thanks. My knowledge of geology doesn’t get past knowing the difference between sedimentary, igneous, and metamorphic rock, and I learned that in elementary school.