What’s that in the bottom of your gold pan?

Iron oxide that has been removed from gold concentrate with a magnet.
Photo by Peter Kuiper

Gold isn’t the only thing that ends up in your gold pan, although with a density of 19.1 it is one of the minerals that might be staring up at you.  Platinum or platinum group metals (PGM) having a density almost matching gold is another.  The rest of the odds and sods found with the rest of the black sands can range from a to z with zircon the last.  They can be divided into several different classes with iron oxides being one of the most abundant,

Iron Oxides:

Magnetite:  This is mixed with hematite on about a 50-50 ratio.  Magnetite can be removed with a common magnet.  This mineral is opaque black and is a member of the spinel group of minerals.

Hematite:  This is the second most abundant mineral found in the bottom of a gold pan. Unlike magnetite is can’t be removed with an ordinary magnet, but is attracted to a rare earth magnet.  Both magnets are used in the process or removing iron oxides from gold concentrate.

Iron bearing sulfides:

Pyrite: this is also called “fool’s gold” because it resembles gold but can be recognized by its rotten egg smell when tested with acid. Pyrite is brittle whereas gold is malleable.

Chalcopyrite:  The same tests apply as to pyrite except chalcopyrite is somewhat redder then pyrite.

Arsenopyrite: this occurs as silver colored cubic crystals that look like pyrite.  The simplest test is made by striking one of the crystals with a hammer when it emots a garlic smell.

Corundum:

This is aluminum oxide with a hardness of 9 on the Moh’s Scale that includes rubies and sapphires.

Diamond:

This is the hardest mineral of all with a hardness of 10 on the Moh’s Scale.  It occurs as a bypyramidal crystal.   

Uranium/Thorium:

There are several minerals present in stream heavies that contain these elements the most common are pitchblende, uranite, columbite/tantalite and allenite.

Tungsten:

The two common minerals of this element are wolframite and scheelite/  Both of these minerals fluoresce under ultraviolet light in a bluish/white glow.

PGMs:

Platinum group metals occur with gold on about a 5 to 1 ratio with more gold.  They look like steel and like gold are quite malleable.  They also leave a silver streak on a touchstone and are not affected by any acid except aqua regia a mixture made from 1 part of nitric acid and three parts of hydrochloric acid.

A zircon crystal
Photo by Rob Lavinsky

Zircons

These are the ore of zirconium that has many uses in modern technology.

Many of these minerals themselves are ores for their various elements.  In many places they are actually mined from beach sands for their metal content in places like Australia of the Southeast United States using specialized equipment mainly the spiral classifier.

Generally, any mineral with a hardness of 7 on the Moh’s Scale and denser then 3 will wind up in the bottom of your gold pan.  We did not include complete descriptions of these minerals because there are more then adequate descriptions available at www.mindat.org. :

Gold Nugget forming Bacteria

During the late 1970s and new theory was put forward about the formation of gold nuggets with this theory stated that they were formed by a pair of bacterium; one of these was named Cupriavidus metallidurans and the other was named Delftia acidovorans either of which was capable of fixing gold from a solution of gold tetrachloride that is a compound that is highly toxic to ordinary bacteria.

Bacteria under an electron microscope NASA

Cupriavidus metallidurans and a similar bacterium Delftia acidovorans have the ability of precipitating gold from a solution of this chemical.  The gold often aggregates as a nugget that is made from their waste products. No wonder the Aztecs used to call gold the excrement of the gods.

It is a well-known fact that there are some bacterium that get their endorsement from metals the most common is the iron fixing bacteria that produce bog iron ore or form a thin film on the surface of swampy areas that looks like an oil slick, but when you touch the film it breaks up into a rectangle shaped fragments. There are some metal fixing bacterium that are being used today for remediating contaminated industrial Sites. There are specific bacterium for all the heavy metals including one for copper that is now being used to produce about 25% of the copper being mined in the world using the heap leach process.

The large Mojave nugget weighing 156 ounces that may have been formed by bacterial action.
Photo by Chris Ralph 

There is an Australian geologist claims he is found some convincing evidence of gold fixing bacteria colonies that are in better then gold nuggets. He is further convinced that he is seeing these bacteria structures under an electron microscope that were not the result of anything he did.

A geologist with the United States Geological Survey first postulated this idea in the 1970s, but he later claimed that many of the structures he saw were from his washing the gold samples with an acid solution before he observed them under an electron microscope.

It hasn't reached the proportions of a cottage industry yet although for many years it was felt that gold feeding bacteria in rivers and streams that fixed the gold ions in the water creating gold nuggets. To our knowledge there have been no enterprising souls that are they lately to start doing this yet although if it is true there are probably more than one that is at least thinking.