One of the premier gold producing areas in the United States is the Carlin Trend of northern Nevada. This small area covering 6 by 35 kilometers produces more gold then any other area in the US. The gold here is found associated with pyrite crystals in sizes ranging from single ions to masses containing only a few atoms of gold. This kind of gold is invisible to the naked eye and is not detectable to an ordinary microscope but becomes visible under an electron microscope.
This was country that was heavily prospected during the nineteenth century by typical prospectors with a gold pan and donkey. Although some small amount of placer gold was found these prospectors had no idea of what they were walking over since they couldn’t see much gold there. The Carlin type gold deposit is a recent phenomenon that was discovered during the 1960s by Newmont Mining although gold deposits like this were just becoming to be known during the late 1920s.
Gold in this kind of deposit is found in limestone deposits disseminated throughout the whole deposit. The gold itself is actually found as single ions or small aggregates of gold atoms that can be seen only by the use of an electron microscope. When this type of deposit is mined it involves tearing down whole mountains. It is in a mine like this that are the home of giant machines like trucks capable of holding 350 tons of ore at once and shovels that can fill one of these trucks with three shovels full.
Getting the ore out of the mine is only the beginning of a long and arduous job because the next step in the process of producing gold is crushing all the ore into pieces that do not exceed one-quarter inch in size that is then trucked to an asphalt pad covering several acres that slopes in to the center to a drain that removes any gold charged liquid. This is called the heap leaching process.
The heap leaching process works on a simple principle that gold is dissolved in a solution of alkaline cyanide. It is this solution that drains into the center of the asphalt pad for recycling onto the heap of stone once again. The cyanide solution is sprinkled onto the top of the heap using sprinklers similar to lawn sprinklers. This solution makes several circuits through the heap dissolving a small amount of gold during each circuit.
After several months the cyanide solution is finally drawn off when it is charged with dissolved gold. The gold is usually recovered from the solution by being precipitated using powdered zinc. The precipitate falls out of solution collecting on the bottom of the vat as a black powder where it is removed for further processing. The cost of recovering gold using the heap leaching process is about $320 per ounce.
References:
Carlin type gold deposits, Steve Castor, Nevada ureau of mines and geology, http://www.smedg.org.au/oct01.html
Heap Leaching, Wikipedia, http://en.wikipedia.org/wiki/Heap_leaching