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[Ed Monton]

A brief overview of the geological processes that create gold deposits.

People without a geological background are often surprised to learn that most gold deposits are created by gold that is carried in solutions. Most people think of gold as being nearly impermeable to even the strongest acids. However, water will dissolve gold quite readily when it is heated to about 250 degrees centigrade and kept in a liquid state by subjecting it to immense pressure deep in the crust, with a little sulfur, chlorine and the like added to the mix.

Gold is actually found in many locations throughout the earth's crust, but in minute quantities, measured in parts per billion. Gold deposits form when the concentration is increased to parts per million (which is the same as grams per tonne). Hot water solutions are the concentrating force that creates most economic gold deposits.

A notable exception to this general principal is the massive Witwatersrand gold deposits of South Africa, which are two billion year old alluvial deposits. That is, they formed as particles of gold were deposited by ancient rivers. A portion of gold production in other areas is derived from more recent alluvial deposits, but most gold mining outside of South Africa is on lode deposits, that is deposits that for the most part derive from hot water solutions.

The most simplistic model of a hydro-thermal gold deposit involves a heat source, such as a body of molten rock that comes to, say, one or two kilometers below the surface. That heat source sets off a large convection cell, in which super-heated water percolates through the rocks, rising above the heat source. These systems circulate super-heated water over areas often measured in many square kilometers. Where such a system involves relatively low temperature water, only about 200 degrees centigrade, the system is referred to as "epi-thermal".

The circulating super-heated water dissolves gold and other metallic elements out of large volumes of rock. When the gold-bearing fluids become channeled through a restricted area, and when something occurs to draw the gold out of the solution, the gold can become concentrated to form an economic deposit.

There are several processes that can result in the precipitation of gold from solution, including:

- Boiling, which can occur when the fluid approaches the surface and the pressure is reduced, will drop the gold from solution.

- A decrease in temperature, for example as the super-heated water nears the surface and mixes with cooler surface water, will also precipitate gold.

- A reduction in the acidity of the super-heated fluid, such as happens when the fluid enters a limestone unit, will also result in precipitation.

- Carbon draws gold out of solutions, and is the most common method of precipitating gold from commercial gold recovery circuits. Some sedimentary rock units, in particular shale units, frequently contain carbon and can therefore serve as trapping units for gold.

The size and grade of gold deposits is related to the size, strength and the length of duration of the concentrating process, as well as the amount of gold present in the original host rock.

The largest deposits are created by large-scale features that are active over very long periods of time. The intensity of the gold mineralization is often enhanced where the system is subjected to multiple phases of mineralization.

For example, a large-scale fault may be reactivated several times, with each subsequent movement on the fault re-opening the fault to allow a fresh phase of mineralizing fluids to move through the system.

Many large gold-bearing systems occur in distinctive belts, where a concentrating process was active across an extended zone of similar geology. For example, the Carlin Trend of Nevada hosts several large gold deposits which contain more than a hundred million ounces. Several different deposit types are present, all related to the same continental-scale tectonics features that created the mountains of the western United States.

The one hundred-mile-long Carlin Trend, which is one of the largest accumulations of gold in the world, was only discovered in the 1960's, after gold seekers had wandered back and forth across it for more than 100 years.

Gold seekers are being lured to many regions that are more remote than Nevada with the certainty that there are other large gold districts waiting to be found.

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