Why are there areas rich in copper or iron or tantalum, or even gem stones.
What is the mechanism that leads to pockets of minerals, rather than an even dispersal through out a layer of rock or earth? Take gold as an example. Why are there areas of gold nuggets, rather than an even scattering of gold.
Various mechanisms are involved as I understand it; there’s no single reason. IIRC, iron is concentrated due to the action of ancient microbes that excreted iron oxide.
You probably meant this to be in GQ. My WAG, FWIW, is that minerals ‘gather in one spot’ either due to volcanic activity or they are dissolved in water and then begin to solidify over time (or some combination of both). In any case, I’ll flag a mod to have it move to GQ where you’ll get a real answer.
-XT
Many ways. Sometimes it is volcanos. Soluble minerals can be deposited in large quantities when an ancient lake evaporates. And the mechanism can even be an asteroid impact as in Sudbury, Ontario.
Happy Hour.
In an underground chamber of magma, molten materials can separate and stratify by weight. Heavier elements like gold would be near the bottom and lighter stuff like silicon at the top. If the magma cools down, you end up with layers of more or less pure metals or minerals. That’s one way of separating out various mixed materials. If say that ex-magma ends up being exposed to the surface, a stream might cut into one or more layers and deposit quantities of that material in a string of little pockets all along its course. That’s how a lot of gold ends up in “Placer” deposits that prospecters are always trying to find. Of course, the prospectors would much rather find the “motherlode,” the source deposit from which the stream is eroding that gold. Another scenario might be if that original ex-magma body were later pierced by hydrothermal activity. pockets of superheated water might dissolve certain minerals (But not others) and carry them a distance until the water cooled off and deposited the dissolved material, generally in a few specific areas.
These are but a few ways.
Here’s one example, this mechanism doesn’t apply to all minerals/elements:
Gold is a heavy element that, for the most part, is non-reactive under near surface conditions (it doesn’t form compounds with other elements).
Under extreme conditions (high pressure and temperature) such as those found in the deep crust, gold can react with sulfur to form a complex compound that happens to be soluble in water. Typically this compound will be diffused through deep rocks at extremely low concentrations.
Imagine now a huge magma plume, a roughly spherically-shaped blob a few thousand meters across and 20+ km below the surface. When it cools, mineral crystals begin to form. Most likely it will cool from the outside in, isolating the molten material inside a gradually thickening layer of solid, non-porous rock (say granite).
Water, for the most part, is not included in the crystal lattices of most minerals – it is expelled from the newly formed crystals and remains associated with the molten material inside the plume. As crystallization proceeds and the space shrinks, the remaining water is subjected to higher and higher pressures. If the gold-sulfur complex is present it will easily be dissolved in the fluid (which will also dissolve other, more common compounds like silica – SiO[sub]2[/sub]).
At some point late in the process, fractures form in the crystallized portion of the chamber (due to shrinkage, internal pressure, etc.). At this point the high pressure fluids will be ejected at extremely high velocities. As pressure drops two things happen: (1) at lower pressure but still high temperatures the water will flash to vapor, reducing its viscosity and allowing it to escape through much smaller fractures and pore; (2) at lower pressures the gold separates from the sulfur and recombines to form elemental (metallic) gold. While this happens the other minerals dissolved in the hot fluid will also crystallize rapidly, filling the fractures with (mostly) a fine-grained, milky quartz. Included in this quartz vein will be flakes, nuggets or crystals of relatively pure gold.
If/when these newly formed rocks are uplifted to the surface these gold-bearing quartz veins will be carried with them, gradually weathering and eroding at the surface and leaving the gold residue to be carried downhill to the nearest stream and be distributed among the heavy sediments on the stream bed (this is called a placer deposit).
Salt concentrations are often the result of salty water evaporating and leaving the dissolved material behind. This can lead to significant deposits if the original water source was very large, like an inland sea.
Different density when still molten is one factor - think about how oil and vinegar naturally separate into two different layers.
Also, different melting point is a factor - once which is taken advantage of in real life manufacturing operations to separate solid materials such as metals.
Differing solubilities and densities are the major factors, IMO. In the industry, the major factors are source (like the fractional crystallization in large magma chambers mentioned earlier), transport mechanism (like the expelled hot fluids mentioned, which lead to such mineral-rich terranes as skarns, greisens and pegmatites) and trapping (some sort of physical or chemical situation that acts as a block to continued transport of the ore materials, see the other links and also the already-mentioned placer deposits).
As Der Trihs mentioned, some deposits are caused by the actions of organisms. This includes many carbonateandsilicaterocks, but also such diverse forms as banded iron formations (although DT gets the mechanism wrong, the microbes release only oxygen which causes dissolved iron to precipitate out as oxides when the pH changes) coal, gold nuggets and even gold/uraninite seams
Peer pressure.
Any given set of conditions that would make it easy for some of a mineral to concentrate will, given time, cause more of that mineral to accumulate.
Elements such as silicon and aluminum, very common in Earth’s crust, are lithophile, i.e. they are better than many elements (especially some of the transition metals) at combining with oxygen. When oxygen becomes in short supply, some of the metals will combine with sulfur as an alternative (many of these are known as chalcophile), or if there is a shortage of sulfur they will fall out as relatively pure metals.
These types of reactions often occur when a magmatic body containing these materials rapidly encounters radically different environmental conditions such as water, cold rock or chemically different rock. Compounds that are in solution under one set of conditions will suddenly precipitate out under differing conditions.
Ideal conditions for concentrating copper, for instance, occur when a deeply-sourced magma intrudes into a water-rich environment (ocean is ideal, but a lake-bottom will work). Groundwater, heated by the magma, leaches copper sulfide out of the volcanic rock and carries it toward the surface where it precipitates out near the surface. Cooler, denser water near the surface sinks into the subsurface, is heated, and carries more copper sulfide toward the surface. Over time, the leached copper becomes more and more concentrated in a lens just below the surface of the crust.