Heavy Elements Accumulate in Earth: Why?

[rasp]

In layman's language, why have the heavy elements, which were created atom by atom over enormous periods of time, accumulated into large masses into small spaces as we find in the earth, rather than being scattered uniformly? E.g nuggets, and veins of silver and gold.

 

[Borek]

Some are accumulated, some are scattered.

Whenever they are accumulated it is because of their chemical and/or physical properties.

Fore example iron was on the early Earth dissolved in the oceans in the form of Fe²⁺ ions. When first plants appeared and started to produce oxygen as a byproduct of photosynthesis, iron oxidized to Fe³⁺ and precipitated in the form of insoluble hydroxide - so it accumulated in a thin layer on the bottom of the oceans. Google for banded iron formation.

Similar mechanisms (not necessarily oxygen related!) worked for other elements.