[paul.fr]

why are some areas of the earth rich in gold (or anything else)and others not? why and how did this distribution happen?

[JimBob]

BASS - Your the mining expert - it is all yours

[Bass]

Wow- there are whole books written on just this subject.  Where to start?

There are traces of all metals in average rocks, the trick is to somehow get high enough concentrations of the metals to make a mine (hopefully profitable).  For example, the average crustal abundance of gold is around 4 ppb (parts per billion).  Mining companies need, at a bare minimum, 1 ppm (most need closer to 3 ppm) before a deposit is economic, or a concentration factor of 250 to 1000 times the average abundance.

The most common classification of ore deposits is based on how they are formed (genetic models).  Metals are concentrated by igneous, hydrothermal, sedimentary, metamorphic and weathering processes.  Platinum-chrome-nickel (and some copper) deposits form by direct precipitation of crystals in a molten magma.  The metals form layers in the igneous intrusive as the crystals rain down and settle at the bottom of the molten rock.  Metals resistant to chemical decomposition can accumulate by surface weathering.  Gold, diamonds, zircon and titanium are all minerals found in stream gravels and beach sands.  Placer gold deposits form when gold bearing rocks are weathered and the gold particles are washed down into and concentrated in the streams and rivers draining the area.  Tropical bauxite (aluminum) also forms from weathering- in tropical wetlands, all of the other minerals are removed during erosion (usually chemical weathering) leaving only a concentration of aluminum which is more resistant.  Other minerals that form during weathering processes are evaporites (like borax or salts), manganese, and certain types of iron formations.  Tungsten and molybdenum minerals are most commonly found in metamophic deposits (skarns)- usually surrounding granitic intrusions.  Talc is a metamorphic mineral.

By far, though, most metal deposits are formed by hydrothemal (hot water) processes.  Hydrothermal deposits are subdivided into categories that depend on the depth and temperature of their formation.  Shallow hydrothermal (analogous to present day geyser fields) are called epigenetic deposits- the saturated hydrothemal solution will precipitate out metals due to boiling, mixing with cold water or favorable chemical reactions.  The bonanza gold deposits shown in all the old movies (with bright gold painted rocks and huge nuggets) have to be epigenetic deposits formed by shallow boiling of superheated water, allowing the gold to concentrate by precipitation in a narrow zone (the zone of boiling).  For hydrothermal deposits, you need 1. heat, 2. a source rock with relatively higher than average abundance of the metal, 3. water, and 4. a mechanism for precipitation.  The hot water picks up the metals in solution as it circulates through the source rock, then concentrates the metals by precipitation (commonly forming veins) at favorable sites that may or may not be some distance from the source.

You can simulate hydrothermal deposits in the kitchen.  Heat up water and supersaturate it with sugar- now continue to boil the water and eventually crystals will begin to form.  Or cool it down crystals will start to precipitate.  Or pour it through a cheesecloth and sugar will start to form a crust on the cheesecloth.  I'm sure there is some chemical agent you could add to the solution that would cause the sugar to precipitate, but best to ask one of the chemistry gurus.

Yikes!  this is longer than I meant it to be- and I've barely begun to cover the subject...

[Bass]

why are some areas of the earth rich in gold (or anything else)and others not? why and how did this distribution happen?

After all of blathering in my last post, and I didn't answer the question.
The basic answer is that deposits form due to concentration mechanisms- most of which require heat and rich source rocks.  The best association of heat and good source rocks are active volcanic/igneous regions- so most deposits will be found around volcanic chains (subduction zones in plate tectonic jagon).  Stable continental interiors are less likely to host deposits. 
However, remember that the world is also full of old subduction zones that are no longer active- but may yet still have plenty of metals about.

[paul.fr]

Your second answer is only one paragraph, how dissapointing. Bass, you are a wonder. Many thanks for those answers.

[another_someone]

Metals resistant to chemical decomposition can accumulate by surface weathering.  Gold, diamonds, zircon and titanium are all minerals found in stream gravels and beach sands.  Placer gold deposits form when gold bearing rocks are weathered and the gold particles are washed down into and concentrated in the streams and rivers draining the area.

Does this mean that sea bed mining of gold (particularly near current or past river estuaries) would be profitable?

[Bass]

Does this mean that sea bed mining of gold (particularly near current or past river estuaries) would be profitable?

Depends - how's that for an answer?

Actually, it's not likely.  As rivers flatten out, they can only move the smaller, less dense particles.  Since gold is so heavy, it is not likely to be moved once the gradient of the river flattens out.  Also, most of the material deposited in sea beds by estuaries is clay/silt/sand- gold is not likely to be in these fractions and will not be suspended in water like clay and silt.  That being said, gold has been mined from beach sands, but usually in areas where there is steep mountainous terrain right up to the water's edge and there are gold deposits in the rocks upstream of the beach.

[kdlynn]

could you filter the water that's still moving?

[Bass]

It's been tried, but so far without success.  Placer (in gravels) gold is discrete sized particles, ranging from gold dust to flakes and wires to large nuggets.  Because it is so heavy, it concentrates in the bottom of the gravels (or bed load) in streams.  Most placers (except some of the exceptionally rich placers) require that you process lots of material to get a little gold.  The usual method is to drop the material into a rotating drum with a screen at the bottom end.  The drum is sprayed with water and rotated to break up the stream material.  The size of the screen depends on the largest sized gold particles you expect (obviously you don't want too small a screen- losing all the large gold nuggets).  The material (and water) that drops through the screen runs down a "sluice box", a long box with slanted metal riffles oriented horizontally across the box.  As the material runs down the box, the riffles create pockets or eddies of calmer water flow, into which the gold will drop (because it is so much heavier than the rest of the material).  Once in the bottom of the box, it will not move much further.  At the end of the day, the material in the riffles can be panned or further processed through a jig.  Cleanup is always the best part of placer mining.
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[JimBob]

How do deep sea nodules form, such as those of manganese? Never had understood this, it didn't even get through to me when I visited a manganese mine near Hot Springs, Ark.

[Bass]

I did some work for Ron Sorem- who studied Mn nodules for over 20 years.  He never quite figured it out either.  His best guess was oxidizing bacteria that could scavenge Mn (and other elements) from seawater.  The nodules are unique to the seabed-water interface and appear to grow on seeds (shells, mineral grains, shark teeth).  The nodules, in places, contain significant nickel and cobalt, possibly from a mafic igneous source?

[JimBob]

and these things are rather widespread, aren't they?

[Bass]

Yes.  There are even reports of nodule-like manganese-iron concretions in periodic wetlands, though they lack the other metals common in Mn nodules and are probably not related.