Adding to what others have already mentioned... Most of the gold will be from various collisions of external objects. The vast majority of the gold and other heavy elements are in Earth's core due to gravitational differentiation.

There is a volcano (in South America IIRC) that has unusually high gold content, but it is from the underground magma reservoir coming in contact with gold deposits. This is why space mining is a really big deal. The Earth is a resource poor gravity prison by comparison. The wealth in space is enormous compared to any differentiated body.

Gold is actually everywhere and relatively common, but only in very small quantities. Under the right conditions, the weight can help gold to concentrate and fall out of solution when that solution was once covering a very large area, dissolved the tiny bits of gold found all over a large area, and then pools into a low point over extremely long periods of time.

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.

https://en.m.wikipedia.org/wiki/Vein_(geology)

A real geologist should give a real answer, but more or less it is due to how the molten elements and molecules sorted out back when the earth was younger and hotter. It has a lot to do with relative densities, melting points and propensity to mix with other materials. Everything heavier than helium and trace amounts of lithium are "star dust", the geology of earth is how that coalesces together and then combines and separates out in geological processes.

Most of it sinks to the core in a molten body, actually, since it has an affinity for iron+nickel. The stuff that manages to stay behind tends to not stay chemically combined with rock because it's unreactive. Basically, when a blob of liquid (magma or water) is slowly cooling underground, you get individual crystals of various minerals precipitating, and since gold doesn't tend to combine with other elements, where it exists in quantity it's bound to find it's way into crystals of native metal. If it's a nuggets-type deposit this is what happened, plus maybe a period of weathering+sedimentation or two.

Concentrated ores of other uncommon or rare elements have a similar origin, just obviously with different chemistries. The exact history of each gold deposit will very. Some are probably associated with core-material asteroids that impacted the crust more recently than the molten days of the Earth.

I thought about human veins and was so very confused

Hydrothermal processes play a crucial role in bringing gold closer to accessible depths. These processes involve the movement of hot fluids through the Earth’s crust, dissolving and transporting minerals, including gold, along the way. These fluids could be molten lava in a volcanic area.

When these mineral-laden fluids encounter cooler conditions or suitable host rocks, they deposit the dissolved gold. This can lead to gold deposits or veins forming in various geological settings, such as quartz veins, which are common hosts for gold. The intricate dance of temperature, pressure, and chemical reactions orchestrates the creation of gold deposits that await discovery.

https://learn.apmex.com/learning-guide/science/how-is-gold-formed/

Long answer ask a geologist. Short answer water.

Time + gravity + chemistry + physics = hot stirring clumpy rock stew

As the stew goes from hot to warm over millions of years things settle into some interesting patterns. The highest concentrations of known gold geological formations happen to be on the oldest pieces of the earth’s crust btw.

In addition to what other posters have said, chemotrophic bacteria can play a significant role concentrating/enriching certain metallic ores by eating dissolved minerals in deep groundwater and metabolizing out metals. Lots of vein-gold runs through very old & fractured marble/granite bedrock with a lot of hydrologic activity.

https://www.boldpreciousmetals.com/blogs/how-is-gold-formed

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