Fools Gold

There are many shiny and lustrous minerals found in Colorado streams and hillsides that can be easily mistaken as being gold.  In this tutorial I will speak of the most common minerals that “fool” us prospectors.  Learning is part of becoming a successful Colorado prospector.

Flour gold from Arkansas River under Microscope

Typical Placer Gold

 

Real gold has distinctive properties that will help us identify it properly.  Gold and copper are the only metals that are not white, gray or black.  Gold is non-magnetic and is highly malleable.  Since gold is highly resistant to oxidization it is usually found in nature looking like gold; golden colored ranging from bright yellow-gold to dull shades of orange and slightly pinkish.  Gold does not have a distinct crystal or cubic shape and has no cleavage whereas most fools gold does.  Gold is very soft and malleable, it will scratch, dent and bend easily but will not crack, shatter or flake off in pieces.  You can also perform a streak test on a piece of unglazed porcelain or a traditional streak plate.  Rub the suspected piece of gold on the streak plate and if you end up with a yellow streak, you have found gold!


Mineral Streak Color
Gold Yellow
Pyrite Greenish Black
Chalcopyrite Greenish Black
Arsenopyrite Greenish Black
Micas Greenish Black
Copper Copper-Red

Iron Pyrite

Pyrite

All of the Pyrites have distinct crystalline cubic shapes.  Their color can be similar to gold but will shine differently under sunlight or shade, whereas gold will shine nearly the same under different lighting conditions and will not change color or sheen.  Pyrite will frequently have a cubic structure and show distinct planes of cleavage.  Pyrite is not malleable and will crack, shatter and cleave when struck with an object.


 

 

 

Muscovite

Biotite

 

Mica and Biotite is another fools gold that we frequently find and mistake for gold .  The three most commonMica minerals are; muscovite, biotite and lepidolite.  The mica group varies in color from golden, brown, silvery, black to white and is semi translucent.  Micas are sheet silicates and have a nearly perfect basal cleavage which means that you can easily separate the sheets and flake off pieces.  Mica is non malleable and will crack, break or split with applied pressure.  Mica is somewhat glass like and will intensely shine and reflect sunlight but will dim or change color when viewed in the shade.  Mica is very light compared to gold.  The specific gravity of mica is; 1.6-2.8 while gold has a specific gravity of 15.5-19.29, which means that mica is much lighter than gold and should wash out of your pan or sluice box quite easily.  In an alluvial setting, mica tends to get ground up into thin powder like sheets that are highly reflective and get deposited near or on the surface of the stream bed.  On a sunny day these tiny pieces of mica will glimmer in the sand and give off a shiny appearance of gold, which often times leads a gold prospector to the false assumptions of a rich placer deposit.  If the glimmering mineral is uniform in size and shine and is found on the surface of the river bed material, it is very likely a mineral from the mica group and not gold.

Gold Colored Mica Dust

 

Mica Sand

 

Gold up Top, Pyrite on Right