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, Uranium and Carbon in Detrital Deposits

One of the common associations with gold in detrital deposits is the association of gold with uranium and carbon. This holds true in the Witwatersrand and all the other gold deposits of a detrital nature.  For many years the origin of the carbon was hotly debated with the most recent evidence holding that it is from primitive life forms that lived in the distant past in the Archean. 

Algae exposed at the intertidal zone similar to the ancient algae that trapped gold.  Even today this kind of mat could be a good place to search for gold.
Photo by Leonardo de Jorge

It has been posited that these algae formed great mats similar to stromatolites that acted as a trap for the gold particles and uranium minerals in the same manner that modern mats of algae work to snare gold particles in a modern environment.  It is common practice to use an artificial mat of the same nature in a sluice box to entrap gold.

Although the deposits can be several thousand feet deep now, but at the time of depositation they were relatively shallow at the bottom of running water that was only a few tens of meters deep subsequent Earth processes have buried them to their present depth. 

An angular uncomformity with a basel layer of conglomerate that would be a good place to look for gold.   The best place is right at the uncomfority.
Photo by Lamprus

At the time many of these gold bearing deposits were at the bottom of a braided stream channel that by being deposited in this manner explains the stringers of gold found in such a deposit.  These stringers of gold are common in detrital gold deposits.  Sometimes there is a layer of carbon that is as thin as a pencil line that is so rich in gold and other minerals that they are mineable. In many detrital deposits the slim lines occur at a regular frequency to the extent that the entire deposit is mined so that I can undergo further ore dressing to free the gold so it can undergo even further treatment usually by being leached with a solution of cyanide.

Even today if you encounter a mat of algae in the bottom of the stream or river is a good place to search for gold that has been caught in the mat by the action of running water.