Sampling & washing gravel

Gravel sampling can be done in two different ways:

• taking a half sample mixing before all the material extracted from the pit
• through different furrows made along the extension of the pit

Washing the samples taken in the actual riverbed or pit will be done with a vat, with the conic plate (pan), with a “sluice” or with an instrument called “jig”.

Washing with a vat (buddle) or pan

The washing of the samples is normally done throughout the sampling. In the case that it isn’t possible, because of lack of water, these will be placed in sacks and transported to a place, even if artificial, that has water and where the washing process can be done.

Like was aforementioned the samples are sifted through a 5 mm diameter sifter and what remains on the sifter is examined, in the case that there are heavy minerals preset or visible mineralizations.
What remains (less than 5mm), is washed from mud always keeping it over the vat.

By vat we mean a metallic cone shaped recipient, having a large opening or water surface.

The instrument must float in the water and the gravel made to turn with manual rotation. By centrifugal force the lighter minerals eliminate themselves overflowing the vat on the opposite side of the operator. When working in the flow of a current, the evacuation of light minerals is done in the same direction of the current. To make the flow of the lighter minerals compared to the heavier ones that fall to the bottom, slight horizontal movements are done that an expert panner does continuously rotating the vat. 

The what is called a pan; is conical shaped, like it was described before. The process for washing is the same as was mentioned above, with the difference that the evacuation of the light minerals is done by passing water over the surface, applying slight horizontal movements.

The amount of water needed is less and the operation can even be done over a drum, with the possibility of recuperating and rewashing the discarded minerals. 

When there is a large amount to be washed, you will need to use a “sluice” like is shown on fig. 19 or also something called a “jig” of which there are different models, manual or motorized, but essentially work as dissymmetric separators. 

Study and examination of heavy mineral concentrate

When prospecting; “concentrate” is the name given to the minerals that remain in the bucket, after the evacuation of white minerals which makeup the borders of the washed material.

While washing you have to make sure not to wash the concentrate too much, to not risk losing some minerals, some of which can be gold when it’s very thin.

When prospecting for gold it’s recommended to use, when it comes to washing results, the following terminology:

• Color: when the gold particles are so thin that they aren’t visible, only with a microscope or by the color of their group, when they are numerous (giving the concentrate a golden glow)
• Point: of the particle that are a little bigger, but dimensions that can only be detected with a magnifying glass. Can be easily counted with a microscope.
• Straw: particles that present slight dimensions on their surface, but their thickness can’t be detected with a magnifying glass.
• Grain: gold particles with well defined dimensions. Called “small” when they are smaller than 1mm, “thick” when there between 1 to 2mm.
• Nugget: fragments that are bigger than 2mm.

Remember that some specialist believe that gold can be soluble in water, when containing humid acids & thus precipitating nuggets (even of large dimensions), kilometers away from the source and when going upstream can disappear without even nearing the mother rock.

In chart 2 you’ll find the most common heavy minerals and their classification, based on their specific weight in the vat. But when it comes to gold, as you will see in an upcoming chapter how its tenure is calculated starting by splitting the particles found in the vat.