Gold Purity

Gold Purity

(From the source vein, to fine placer gold)

 

 

Coarse Gold Nugget

Just liberated from bedrock source

Most refined gold bars are 999.9 fine.

24 karat or pure gold is said to be 1000 fine.

When gold is formed and deposited in veins and fissures within the host rock, it is frequently intermixed with silver, copper, lead, platinum group metals, sulfides and other minerals.

Placer gold, is gold that has been liberated from the host rock and can be harvested using placer mining methods such a sluicing, panning, dredging and drywashing.

Placer gold that we find in streams and hillsides is very rarely pure gold.  Placer gold is most commonly alloyed with silver.Gold is the most non reactive of all metals. It is called a “noble” metal because it does not oxidize under ordinary conditions, meaning that it will never rust or tarnish.

Now this is an interesting fact; (lets assume the gold is alloyed with silver)

Lets take a 3 ounce nugget that has just been liberated from its host rock source (the lode) and follow its travels downstream.

The 3oz, rough, jaggedy nugget laying at the base of the cliff it has weather out of, is laying in an Eluvial placer deposit.  At this point the nugget is 800 fine, which means that it is composed of 80% gold and 20% silver.

This 3oz nugget will sit in this Eluvial deposit for thousands of years and be subject to weathering processes such as hydration, oxidation as well as solution by chemical means.

During these processes the gold will typically stay intact because of its “noble” metal properties.  The silver on the other hand is not so lucky!  Some on the silver will be broken down by these weathering processes which may reduce the 3oz nuggets weight and character.

Eluvial to Colluvial Nugget Texture

Eluvial to Colluvial Nugget Texture

Over long periods of time the Eluvial placer will be covered over with new host rock and organic material.  The deposit will get to a point where it can longer be held in place on the slope and will be transported down hill via mass wasting, floods, uplift and avalanches.  This gold bearing material that is carried down slope and deposited on a hill side away from its source is called a Colluvial placer.The Colluvial placer is also subject to weathering processes.  Wind and water erosion help to concentrate this deposit by sweeping the lighter sands, rock fragments and organic matter off of the surface, leaving behind the much heavier particles of gold including our now 2.8oz nugget.  Since some of the silver  has oxidized and been removed from the nugget, the nugget is  now 820 fine.

Eventually this Colluvial placer will erode into an active stream channel and become an Alluvial placer(primary concentration and deposition is by running water).

The 2.8oz nugget will now be covered with flowing water and be intermixed with river gravels.  During periods of high water and spring runoff the gravels in the river bed will be moved around and transported further downstream.  Relatively speaking, the nugget will not move too far downstream, but because of its high specific gravity and weight it will slowly work its way down through the gravels getting closer to bedrock.  As the nugget moves downward through the river gravels it will be banged, ground and bombarded by other rocks and sand.  This beating by rocks will alter the shape of the nugget, breaking off rough edges and exposing more silver to be oxidized out of the nugget.  A few hundred years go by and the nugget will be smaller (lets say 2.5oz) but purer yet (850 fine).

Smooth Alluvial Nugget

Smooth Alluvial Nugget

A 1000 year flood event occurs, which turns the river into a raging fury.  The entire bed load (all of the river gravels, boulders, rocks and sand) are swept up and moved downstream.  The 2.5oz nugget is subject to massive impacts from colliding rocks and is again banged and ground into a smoother shape.  Two .25oz pieces get chopped off of the nugget and stay up in suspension while the bigger 2oz piece moves its way down to the bottom of the slurry and gets caught in a bedrock trap or settles on the bedrock when the massive flood subsides.The 2oz nugget down on bedrock has met its resting place (until another massive flood occurs capable of moving the entire river bed load, scouring the bedrock) and will be subject to weathering processes and further be purified of silver.  It will lose a little weight and now be 875 fine.

The two .25oz pieces left higher in the bed load will be subject to weathering and transportation downstream as mentioned above.  Eventually the two pieces will be ground down by the moving river gravels and be reduced to flour gold (small particles of gold 20 mesh and higher).  This flour gold tends to stay suspended in the river gravels and will not work its way down to bedrock.

The smaller the pieces of gold, the more surface area they have that will be subject to oxidation.  More silver will liberated from the particles of gold and the flour gold will be now be 930 fine.

Fine Flour Gold

Flour gold from a CGC Claim

 

This is how gold from the same source can have varying degrees of purity.