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Sunday, July 18, 2010

The Carlin Trend in Nevada has invisible gold

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.


Carlin type gold deposits, Steve Castor, Nevada ureau of mines and geology,

Heap Leaching, Wikipedia,

Friday, July 16, 2010

Telling gold from fools gold!

The problem of identifying real gold in the field to separate it from fools gold often arise. There are several minerals that mimic gold in appearance like iron pyrite and weathered biotite mica that both have a gold-like appearance.

The easiest field test for gold is simple its density is 19 grams per cubic centimeter that is far denser then any of its mimics. Some genius in antiquity developed the “touchstone” test for gold based on what color its streak is when left on a dark colored stone. In practice this is usually a piece of black slate. Gold will leave a gold-colored streak on this stone its mimics will not. For ages this was the definitive test for gold until it was replaced by the “Purple of Cassius” test during 1685 by Andreas Cassius.

Another ancient test used frequently was based on the fact gold is malleable. You can bend it, and it keeps its bend. You can also bend weathered biotite, but unlike gold it quickly snaps back into shape. The other method based on malleable gold is hammering it into a flat sheet all of those minerals that mmimic gold are brittle and will break-up into a powder if hammered.

The acid test works because gold is unaffected by all strong acids except aqua regia, all the other minerals that mimic gold are affected by acids and are dissolved. A possible exception to this is weathered biotite mica.

The Purple of Cassius’s history goes back to the fourteenth century where it was described as an artist’s pigment however Andreas Cassius gets the credit for its invention. The principle behind the purple of Cassius is the ability of Aqua Regia a two to one mixture of concentrated nitric and hydrochloric acids to dissolve gold.

Just by its dissolution in aqua regia gold produces a rough-and-ready test because the gold charged aqua regia assumes a golden-yellow color. This test is not accurate enough however to be definitive. The purple of Cassius is however. Until the invention of the modern science of spectrometry developed during the nineteenth and twentieth century’s the purple of Cassius was the recognized definitive test for gold.

The test works by adding a few drops of the gold charged aqua regia to 100 ml of clean water, then adding twice as many drops of a solution of tin chloride to the solution. The tin will cause the gold to precipitate from the solution forming a colloid that ranges in color from pink thru dark purple. The darker the color the more gold is present.

The science of “spectroscopy” has supplanted most of the chemical tests for gold in the laboratory, but not in the field where because the various manifestations of the spectroscope are just to bulky. Many of the old chemical tests for gold and other minerals are still being used.


The creation of color in eighteenth century Europe,,

Acid test (Gold), Wikipedia,

Spectroscopy, Wikipedia,

Tuesday, July 13, 2010

Gold in quartz veins

Gold is often found associated with milky hydrothermal quartz that is called “bull quartz. Although gold has many other associations with base metals this is its association with iron pyrite (fool’s gold). Gold is found with many other metals and ores. One common association is with iron ore where it is often found in association with granular magnetite. This is a common occurrence in places like Nova Scotia and the Cornwall, Pennsylvania iron mines.

The common way gold in quartz is found is when hydrothermal water with both gold and quartz percolate up through the fissures in bedrock that is hot enough to hold both minerals dissolved in solution. Both minerals drop out of solution when the water cools. Quartz falls out of the solution first. The speed of the dissolution can be judged by the size of the quartz crystals. Basically, the larger the crystals the slower the hot fluids cooled. Gold is one of the last minerals that to come out solution. Gold is one of the last minerals the last minerals to be deposited out of solution.

There are several other minerals that are likely to fall out of the solution first, quartz is only the first of many. Other common minerals that are apt to be deposited ahead of gold are galena lead sulfide, sphalarite zinc sulfide and bornite a sulfide of copper and iron that is called peacock ore. Gold is usually found as an encrustation on other minerals that form in the spaces in the quartz matrix. Quartz containing gold is called by miner’s “bull quartz” a milky quartz having the crystal faces obscured in the body of the rock.

They are there none-the-less you just can’t see them with the naked eye however, they can be seen by using an X-ray Diffraction Spectrometer. These are usually called “Powder Cameras” by the geologists that use them. They work by mixing the powdered mineral with caladium, a mixture of acetone and nitrocellulose. They are then formed into small rods containing the powdered mineral that are placed in the X-ray beam. By photographing these rods with a special X-ray camera using a photographic plate that shows the resulting diffraction patterns of each mineral. Every mineral has a distinctive diffraction pattern that is used to identify it just like fingerprints.

Gold can be found in just about any rock, sometimes in specks that are too small to be seen with a conventional microscope. They are only seen with an electron microscope. These deposits are diffused into the rock and weren’t discovered until the 1970s. These deposits hold enough gold as tiny specks to be economically viable.

One such mine is the Carlin mine operated by Newmont Mining in northern Nevada. Another mine is one the author tried to get control of in Mexico years ago where the gold was diffused through marble as equally tiny specks. We discovered the deposit had the potential of producing several billion dollars worth of gold, except the owners were not willing to sell the property.


Gold Reef Mining, Wikipedia,

Igneous Geology of the Carlin Trend, Michael W. Ressel, et al, Geoscience World,


Monday, July 12, 2010

What is Gold?

What is gold?

The ancient Egyptians believed that gold was the flesh of the gods, and were wonderful goldsmiths producing some of the most beautiful gold-work the world has ever seen. In Greece it has left us with the tale of “Jason and the Golden Fleece” one of the greatest adventure stories ever told. The Romans believed that it was the “shining dawn” the Latin word for gold is “aurum” that has given us our atomic symbol for gold Au. It is been treasured by men since ancient times where it has been used in coinage, jewelry and other places since the beginning of recorded time. There have been wars fought over gold, worlds conquered, great migrations, gold rushes, some of the greatest literature ever written and it is al in the name of Gold.

The metal occurs in nature sometimes as nuggets and particles as fine as flour, at other times as grains or crystals in certain rocks. Gold quartz is one of its most common ores, but not the only one, gold is where you find it. Most of the ancient gold that was discovered is what is called alluvial gold found in the sands and gravels of the beds of streams and rivers. It wasn't until men learn how to mine in hardrock that gold was won from gold bearing veins.

The metal gold itself is a dense, soft, shiny metal that is the most malleable and ductile metal known to men. A troy ounce of gold can be beaten into a sheet covering one-hundred square feet, or drawn into a wire fifty miles long. Gold has an atomic number of 79 on the periodic table of elements and has a density of more then 19 times greater than water. Its color is a very great, shiny yellow that many have likened to the color of the rising Sun.

Since ancient times gold has been valued as a monetary metal. The oldest coins that were ever found were made from gold. These were gold coins that were invented by Midas the King of Lydia a kingdom that was on the Anatolian peninsula (Modern Turkey). From ancient times until the present gold is one of the metals from which coins are struck. Throughout history gold has been a symbol of wealth and also a store of value. Standards are based on gold have always provided a basis for the establishment of monetary policies throughout the world.

As of 2009 the total amount of gold that has ever been mined only amounts to 161,000 tonnes. Translated into Troy ounces this is roughly 5.17 5,000,000,000 ounces. If you wanted to measure its volume it would be about 8,333 cubic meters or a block that measures 20 meters on a side.

Gold has the distinction of being the most recycled metal in the world; some of the gold in the ring you may be wearing was probably worn by one of the pharaohs of ancient Egypt.

Gold is mostly used as a store of value but it has developed many industrial uses it not only jewelry, but in dentistry and various electronic devices. The visor on the helmet of Neil Armstrong, the first man to step foot on the moon was coated with a layer of gold that was only about one atom thick. This coating was to keep out the intense rays of sun.

Gold is also used in electronics because it has a very good resistance to oxygen and other chemicals as well as having a height value as a conductor of electricity.

Gold has traditionally been considered a noble metal because it doesn't corrode. It is not affected by oxygen or acids except for aqua regia that is a mixture of concentrated hydrochloric and nitric acids. Another chemical that will react with gold as an alkaline solution of cyanide; this reaction is often used to recover gold from its ores in the heap leaching process.

To the chemist, gold is a transition metal that can form both trivalent and univalent cations when found in a solution. When compared to other metals gold is the least chemically reactive. However, gold will dissolve in mercury where it forms an amalgam alloy that is the basis of its use in dentistry.

Because gold is not attacked by nitric acid alone but other metals like silver and other base metals are, this is one of the properties used in refining gold that is known as the “inquartation and parting” process where the other metals are dissolved and the gold is left behind. The same principle as found in this process has also given rise to the saying to give something the “acid test.” When used this way it is referring to a gold standard test for determining something’s real value.


Gold, Wikipedia,

The Mineral: Native Gold