There are two different types of gold deposit that produce commercial amounts of the metal. Both are commercially important and came about by different processes. The first of these is the "placer deposit" that is developed from the weathering and erosion of gold bearing hardrock. The other type is gold bearing rock commonly called a "hardrock deposit." It takes different techniques to produce gold from either type of deposit.
Placer Mining:
This depends upon the density of gold over almost any other earth materials that is 19.3 grams per cubic centimeter commonly called the specific gravity (Sp) = 19.3. The great concentrator of gold that has been weathered out of bedrock is water in motion either in a stream by running water. On a beach whether on a lake or ocean the water in motion that causes the concentration of gold is the surf.
Because it is so dense gold works its way down through the sands and gravels found in a stream or beach until it comes to rest close to the bedrock at the bottom of the moving water. The places too look for gold besides near bedrock is anywhere that the water suddenly slows down like the inside of a bend, it can also be found downstream from a bedrock exposure or behind a large boulder.
Another place too look is cracks in exposures of bedrock with water flowing over the bedrock. Moss and the roots of plants that often collect gold from running waters are also favorable for deposits of placer gold. Just about anyplace where you can find trout is also good for panning gold.
The best kind of stream to make your search for gold is a young stream with many falls and rapids. An older stream that is flowing slowly is not a good place to look because the action of fast moving water upstream will wear the gold down so fine it is difficult to concentrate.
A beach delivers you with a different set of challenges, although because of its density gold will tend to accumulate close to bedrock. If the surf is actually striking the bedrock physically a good place to look for gold is in the cracks and pockets in the bedrock. The other place is too look where the surf line is actually plunging onto the beach. There is usually a layer of gravel just under the plunge line that migrates back and forth with the action of the tides. If the water isn't too cold you can feel these gravel deposits with your bare feet. The gold is often concentrated near the bottom of this gravel deposit.
Any body of moving water will create an armor coating of clay that is often found in the sand and gravel above bedrock that often collects the gold particles rather then allowing them to pass through to the bedrock below. Gold can become concentrated in this clay, but it is extremely difficult to recover. Streams also have another trick for the unwary, and that is the ability to also armor their bottoms with the largest stones in their bedload. This layer of large stones often discourage people who are not aware of this fact.
Hardrock Mining:
Hardrock mining is usually limited to the "Bigboys" because it takes so much money to develop a mine of this nature.
Just about any rock in the world that is analyzed will have a showing of gold. The trick however is to find gold that becomes concentrated enough to become economically feasible. For every showing of gold only one in three hundred is rich enough to develop into a paying mine. In the good old days they used to call this, "Gamblers Ruin" in fact there is even a book about mine evaluation called, "Gambler's Ruin" that was published in the 1890s that you might find in an antique bookstore.
Surprisingly the making of a gold deposit starts with a rain shower that supplies the water to drive the gold depositation process. This is water that percolates into the ground that is called groundwater that follows the cracks and fissures in the rock until it meets a dome of magma that has intruded into the overlying rocks. The heat of the magma drives the water back towards the surface of the earth because heat always rises. It moves the water latterly towards the top of the magma dome where it picks up the gold and other minerals from both the magma and the surrounding rocks that are affected by the now heated water.
There is also juvenile water in the magma that is expelled from the magma as hot water and steam that is also carrying gold from the hot magma and mixing it with the gold extracted from the surrounding rocks by the now heated groundwater. The hot mineral charged water is carried through the cracks and fissures of the surrounding rock until it loses its load of minerals either by cooling or finds a rock like limestone that causes the mineralization to come out of solution.
One of the most favorable places for gold to be deposited is in a brecciated fault zone. This is an area where the rock has been broken into coarse angular fragments by earth movements. Probably the most common mineral associated with gold is quartz although it can be found with other minerals that are considered "Gangue." This is a German word pronounced "Gang," and is any mineral that is considered waste when it is associated with a mineral of value. Quartz is only one example there are plenty of others. They are generally non-metallic minerals.
Gold can be found in skarns associated with limestone or dolostone the gold is deposited from hydrothermal solutions that are gold bearing. Other hydrothermal deposits can be found in any other rocks where the right conditions exist.
These conditions can be met through several mechanisms. The simplest is just that the gold bearing solution reaches a temperature where it is no longer able to bear the gold in solution. Other ways gold can be deposited is when the hot water causes a chemical reaction with the surrounding rocks that can also cause the gold or other minerals fall out of solution. A common mineral that can be affected by this is calcium carbonate; barite is another.
Another type of hardrock deposit that occurs is fossilized placer gold deposits. The best examples of this is the conglomerate gold deposits of the Witwatersrand in South Africa one of the world's largest concentration of gold on earth. Most geologists consider this to be a fossilized placer deposit. Other fossilized or secondary deposits can also be found in quartzite.
Commonly, quartzite deposits are fossilized beaches that form on island arcs before they are incorporated onto continental cratons by tectonic movements. The gold that is found was concentrated in the past by the action of surf.
Gold is found in all rocks and even dissolved in the waters of the seas, but it takes a special set of conditions that are either controlled by temperature or the chemical composition of the surrounding rocks to precipitate gold . When the proper conditions are met a commercially viable deposit of gold or other metals will be deposited, and only then.
References:
The geology of gold, Prospectors Paradise, http://www.prospectorsparadise.com/html/geology.html
Vein Geology, Wikipedia, http://en.wikipedia.org/wiki/Vein_(geology)
Gold Deposits at the Zortman-Landusky Mine, http://serc.carleton.edu/research_education/nativelands/ftbelknap/golddeposits.html
and the author's experience as a geologist.
Gold rings are very different than gold nuggets because they are usually alloyed with other styles of metal. They are usually close to the surface and sometimes cannot be tuned out or selected for with a notch filter despite having the most effective metal detector. By far the right place to find gold rings is at the beach. Particularly in the shallows of the dropping tide at a common resort area. The explanation is because several vacationers don't recognize that the colder watercould make their fingers shrink and unsuspectingly lose their highly valuable ring jewelry.
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