Why the distribution of elements on Earth? I've been wondering exactly why the elements are distributed the way they are on Earth. The heavier elements have their origins in the centers of stars, or in supernovae. After the death of the stars, you end up with a dust cloud containing the heavier elements. Later, planets form out of these along with new stars.
If the heavier elements were randomly distributed in these explosions, how do we end up with special deposits of minerals on Earth? We have mines and special areas where the concentrations of different materials are higher. For example, we have iron, gold, nickel and uranium mines. Why aren't these materials uniformly distributed? Why isn't the Earth composed of a substance which is a uniform mixture of all the heavy elements? Why does it seem there is sometimes a preference for like materials to clump together?
The nickel and iron mostly make up the Earth's core, which I guess makes sense in terms of them being fairly heavy while comparatively abundant relative to heavier elements, but what about everything near the surface that we observe?
 A: The answer is also a bit vague here, because it happens differently to each ore/element; yet this is not that the deposits were created among with Earth, rather than they were created with a mix of geological and sometimes biological processes that gathered some material from larger area, purified and transported it and finally gathered in the deposit.
See this wikipedia article for more details: http://en.wikipedia.org/wiki/Ore_genesis
A: It is an interesting question, I will take a stab at it. Hopefully my answer will be generic enough to remain plausible.
  First of all the ISM (Interstellar Medium) which collapsed to form the sun would not likely have been well mixed. It would be some sort of turbulently mixed mixture of recent SN remnants, and the molecular cloud with which it presumably collided to set off star formation. So probably this was a bit of a mess.
Then in the solar accretion disk from which the plantissimals form we have temperature gradients. In the higher temperature regions volatile elements don't condense so we get some
differentiation based upon where the material was in the disk when it condensed. Then the planets are builtup via collision of lots of smaller planetissimals, coming from different regions. The larger
bodies will melt and undergo differentiation due to internal gravity. The iron nickle and elements heavily attracted to it go to the core, and the lighter oxidized material form a rocky mantle. Stuff further out, contains a lot of lighter volatile elements, like ice and frozen CO2 etc. In any case all this stuff continues to bombard the planet, and indeed at a reduced flux still does. Note we have several different classes of meteorites with differing composition. So the most recent additions won't have been mixed into the planets interior.
Various geologic processes which are driven by the earth as a heat engine (primordial heat plus radioactive decay) does drive flows of stuff like magma (partial melts of parent materials), and hydrothermal activity which dissolves and deposits stuff as it circulates, and samples differnt temperatures and pressures. We also
have various surface processes, including biological that move stuff around, and sometimes
locally concentrate some elements. Because of the energy flows in the system (driven by internal heat and/or solar heating), we have the potential for local concentrations to happen.
A: Part of it is from metals dissolving in water, then having the water sit somewhere and eventually filter/boil out.
Sources: 
http://nuggetshooter.com/articles/CRGeologyofcoarsegoldformation.html
http://www.cliffsnotes.com/study_guide/Metallic-Deposits.topicArticleId-9605,articleId-9594.html
