How Nature Makes Mineral Deposits.

Nature concentrates minerals all sorts of ways. Here are just a few.

How to form minerals with water.

Water always has minerals dissolved in it. Have you ever examined an iron or vaporizer once the water has boiled out of it? If not, take a pan of water and boil all the water away. A thin film of mineral deposits are caked on the pan surface. We buy water softeners to remove most of these minerals but it is actually the minerals, which gives water its flavor.

Minerals formed by Evaporation and Precipitation

Now imagine you had some sea water (perhaps a large section of ocean gets land locked....like Lake Bonneville in Utah was long ago). As the water evaporates, the remaining water gets saltier and saltier (The Great Salt Lake). Eventually, solid salt gets precipitated out of the solution and gets deposited. The Salt Flats in Utah are a fine example of this. These are sometimes called brine or evaporate deposits.
Gypsum, an important industrial material used in home construction (wall board), is formed in this way.
This same thing happens in caves. As water (percolating through the ground) reaches air in a cave, some of the carbon dioxide (CO₂) escapes from the drop ... leaving behind calcite. This is how a stalactite is formed.
Banded Iron Formations were formed 1.8 to 2.2 billion years ago when oxygen started building up in the atmosphere. The oxygen dissolved in the ocean water and reacted chemically with iron ions in solution ... forming solids which settled as thick deposits of iron oxide.

Likewise, thick deposits of limestone (calcium carbonate) formed (and are still forming) when sea creatures (coral, snails, clams, shells, etc.) used carbon dioxide molecules and calcium ions in sea water to construct armor. Their skeletons settled to the sea floor forming thick layers of limestone.

Hydrothermal Solutions

The warmer water is, the more minerals it can hold in solution. Now suppose water is near a heat source (like the geysers in Yellowstone National Park). As the hot water moves underground, it soaks up minerals like water soaks up into a sponge. When forced upward, it cools quickly and it no longer has the ability to hold as many minerals in solution. Changes in pressure, amounts of dissolved salts, presence of other minerals and oxidation conditions may also produce additional alterations to the solution. As a result, the minerals are forced to precipitate out as a solid. Many geysers have structures like "mini-volcanoes" because of this. The same thing happens on the ocean floor. Hot mineral rich water (coming out of hydrothermal vents) is suddenly exposed to the cold ocean water. Bam! The minerals solidify out forming chimney like structures called "Black Smokers" (this action also turns the water black). Mounds of sulfide ores can be formed in this manner.

Castle Geyser, Yellowstone NP

Some minerals are very important and valuable. When you can actually make money from the mining and purification of a mineral, the mineral is called an ore. Gold is a classic example (also silver, lead, zinc etc.). The 49'ers looking for gold always were looking for the "Mother Lode". They knew, if they found a "vein" of gold, they would strike it rich. But how did the vein of gold form? Imagine hot magma moving upward through a vent has hot water mixed with it (or water in the water table is heated by some source). The (hot) water soaks up gold ions in solution. Now either the water moves or the heat source vanishes. Either way, the water suddenly gets cooler and less able to hold minerals. At a given temperature, the gold comes out of solution and waits for a miner to find it. Wisconsin has rich deposits of lead which formed in this manner.

Water plays another important story in the formation of rocks and minerals because the ions that are held in solution can also act as a "glue" to hold loose sediments together when they precipitate out. In the process, the loose sediments slowly get turned to stone in a process called lithification. As layers of sediment pile up, compression also acts to change the sediment into rock. Common examples of this process are sandstone and shale.
As you can see, water has the ability to "dissolve" rocks it moves over and soak up ions in solution. But some mineral deposits can be formed by being "left behind" in the process. For example, as other minerals are dissolved away, concentrations of aluminum (bauxite) can form because it is rather insoluble in groundwater. Bauxite is therefore frequently found where chemical weathering is very active ... like a tropical climate.
Water can play another role in the formation of mineral deposits. As a rock is broken down (weathering), it can also be transported from one location to another (erosion). Water and ice do this efficiently in streams and glaciers.
- Glaciers can transport and form vast quantities of loose sediments (sand and gravel) in concentrated deposits. These become valuable resources for the construction and glass making industries (over $1 billion in the US alone).
- Streams can concentrate and deposit minerals because different minerals have different densities. Sand is often deposited on the inside bank of a meandering river. "Placer" deposits of gold can be found in rapids, potholes and waterfalls. In the same way, early miners would "pan" for gold in a stream by letting the less dense sand and gravel wash away in a swirling pan.

Other ways to concentrate minerals

Igneous activity

Many minerals are concentrated as a result of igneous activity (melting of rock). This might sound lame, but all you need to do is have magma cool and it becomes a rock. So how does nature do this? You might think of volcanoes first and you wouldn't be wrong. The Hawaiian Islands are made directly from volcanic activity. But igneous rocks can also be formed deep underground (intrusive).

There are many factors involved in the formation of a mineral directly from a melt. Probably the biggest factor is the initial chemical makeup of the magma. This can vary greatly from place to place (as you will see later on in this class). Also, each mineral in the melt has different physical and chemical properties that effect the outcome. Finally, other factors such as the presence of water and the type minerals surrounding the melt may be a factor. Let's just look at some hypothetical examples.

Example #1 - Magmatic Segregation

Consider a chamber of magma deep underground. Geologist call this a pluton or a batholith. Suppose it contains many different silicates in its mixture. Nature has a way of sorting the different minerals as the magma slowly cools. As you recall, the different silicates have different melting (freezing) points. So as the chamber cools, and the temperature drops, the first silicates to crystallize would be the ones with the highest melting (freezing) temperature. This would be silicates from the olivine group. Once solid, the density would increase and these crystals would sink to the bottom of the chamber. This process would then continue to the pyroxene group....ending with quartz (with the lowest melting point). The final result is a stratified layer of separated minerals....olivine at the bottom and quartz at the top. Rich nickel and copper sulfide ores were formed in this way in Sudbury, Ontario.

(animation)

Example #2 - Contact Metamorphism

... or "rock chemistry" occurs when solid rocks are in direct contact with liquid rock (magma). Often the reactions are quite complex and sometimes not well understood, but it does account for several deposits of minerals.

Suppose a large deposit of limestone is intruded by a vast volume of magma deep underground. In the presence of water, calcium atoms (in the limestone) can be replaced by iron atoms (in solution). This is just one example where atoms of one type are "replaced" by atoms of a similar type (size and charge) to produce a new mineral. In similar reactions valuable deposits containing tungsten and tin may result.

Example #3 - Regional Metamorphism

Some minerals are produced (from other minerals) as a result of exposure to extreme heat and pressure. Minerals such as asbestos, talc, and gems like ruby and sapphire are found at the roots of mountain belts.

Example #4 - Diamonds are formed at great depths below the surface (a metamorphic process). On occasion, magma will flow towards the surface in a "pipe" through a region containing diamond crystals. These diamonds are "scooped up" in the magma and transported toward the surface. Diamond "pipes" are found in South Africa, Colorado, Wyoming and there is even one in Arkansas (where I tried to "strike it rich" one hot day a few years back in the Crater of Diamonds State Park).

Example #5 - Sometimes gasses escape from a melt (or solution). When this happen, the melt (or solution) may lose its ability to hold minerals. The vapors escaping from a melt carry these minerals in a gaseous state where they may precipitate on a nearby surface ... forming some beautiful crystals.

Example #6 - Partial Melting

Why are the continents mostly granite? Good question and one which you had better know the answer!

Plate tectonics is constantly pushing plates together and pulling them apart at different parts of the globe. Much of this will be covered later in this course. For now, consider two plates in a head on collision - one consisting of oceanic crust (very thin and composed of basalt) and the other made of continental crust (a bit thicker and mostly granite). Upon collision, which is likely to forced down (under the other)???? Can you make an educated guess? Scroll down for the answer (after you guess).

The answer is: Oceanic crust will always be forced under continental crust! Why? Because basalt is denser than granite!

Once forced down, it will start heating up!

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As the oceanic plate subducts under the continental plate, the temperature rises and the solid rock of the oceanic crust will start to melt. Remember, this is oceanic crust and has a rather "silica poor" content ..., but it is not zero by any means and does contain additional silica from rocks washed to the sea from rivers and streams, as well. So what is the first (and maybe only thing) to melt? The answer is: the minerals with the lowest melting point ... the "silica rich" minerals (quartz, feldspar, mica)! And even though there may not be much available in this sinking plate ... we just found a way to separate the small amounts of "silica rich" minerals from the subducting oceanic plate. This is called partial melting. There is even more to this story. This subducting crust also carries with it water ... and water has the magical property that it can lower the melting point of the surrounding upper mantle rocks. Actually these surrounding rocks only partially melt because it only gets hot enough to liquefy the lower melting point, "silica rich" rocks. This liquid rock becomes more buoyant and starts to rise to the surface. OK .. it gets even stranger!!! This molten rock undergoes additional changes in composition on its way up to the surface. As it cools, it crystallizes out any remaining "silica poor" basaltic type rocks (because they solidify at a higher temperature), leaving the remaining (rising) magma even more silica rich. But what it this? ... the stuff you make granite out of ... and granite makes continents! OK - got that? Please re-read this last paragraph several times until you get a clear picture how this process works.

Will this liquid (silica rich) magma make it to the surface???? The answer is: Not likely (although it can and sometime does in the form of a rock called andesite). Why?

There are several reasons why this magma will most likely never make it to the surface. One deals with viscosity. As you recall, this melt is rich in silica and this makes it run like honey....very slowly. Another factor is the temperature ... as this magma rises, it cools and quickly reaches the freezing point. It becomes a block of granite at the base of the continental margin. But do you see what this means? We've just filtered out the small amounts of "silica rich" minerals (quartz, feldspar, mica) from oceanic crust (and surrounding upper mantle) and added them to the edge of a continental margin. And it will keep doing this as long as the oceanic crust gets subducted under the continental crust. Pretty neat...Yes? This is the process which keeps making continental crust and why this continental crust is mostly granite.

But you might be thinking now, "Does this mean there is more and more land (continental crust) on the earth as time goes on"? The answer is NO. The percent of continental crust (vs. oceanic crust) has remained fairly constant for quite some time now.

There are other things happening at the same time....and these will be covered in another class.