Rock and Geological Sampling

Once you have chosen the type of sampling and located the point on the map, the prospector is faced with the next questions, which is; where exactly to sample and how to take the sample from the terrain. Each situation is different but some cases can be outlined:

A) Samples in the hydrographic net (stream-sediments)

• Fine alluvial samples in active riverbed, which is done in the water, but far from the current, to avoid sampling washed material. This type of sampling is better known as “stream-sediment”.
• Soil samples over alluvials or colluviums, in the case that active riverbed alluvials are thick or are missing completely. This sampling is done on the clayey side of the bank, having contact with the water.
• Soil sampling at the bottom of valleys, under A horizon, in the absence of alluvials.

The object of these samplings is to represent a certain hydrographic basin, which is why you should avoid sampling certain main collectors where possible mineral contributes can be diluted by the flow of sterile waters. Sampling shouldn’t be done at the river fork but a little further up, in the secondary collector. White sand sampling is considered bad, because it doesn’t contain part of fine clay, the same goes for not retrieving samples with too much organic material (black samples). In tropical countries, a good sample is normally grey or dark brown.

Its recommended: when possible collect superficial humus; the ideal depth varies from 15 to 40cm. Sampling at base of roots shows the ideal depth. Therefore sampling in evolved soils should be avoided (lateral cores, evaporitic scabs) or on clearly treated soil (loess, dunes, old terraces, etc.). In these cases it is better to sample drilling through these.

C) Quantity to be sampled

The amount to be sampled varies from 200 to 500 grams, depending on the fine sediment proportions included. After drying and sifting, this amount allows you to end up with a minimal weight sample of 60g.

At least 40g are needed to run standard analysis (at least 25g for gold analysis) and 20g that are left as evidence.

In the field the samples are placed in bags (18x26cm) or in plastic cups.

Note: Bear in mind that on strategic prospecting 10/15 samples per day come up to about 4 to 7 kilos that need to be transported. 

Numerating samples & field book

This is a very important part, since if there are mistakes made or samples are numbered twice, a lot of work will be useless. The recommended method is using sequence numerating, with a field book made up of pre-numerated files thus avoiding mistakes. This file also makes it easier when keeping computer data. Sometimes it’s recommended to label samples on topographic maps using a square net of 1cm, numbering the samples from left to right, and from the top down. Using this method makes it easier finding them on the map, without having to figure out the geographic coordinates.

In table 3 you’ll see an example of geochemical samples.

Preparing & shipping samples

In a big strategic prospecting campaign, its useless sending thousands of samples with 400g each to the laboratory; so it would be good to foresee the need of sample preparation which is basically the following:

• drying it
• breaking it up
• sifting 

The drying can be done by the sun or over a bonfire, without cooking the minerals; the breakup - not grinding – can be done in a wooden or ceramic mortar, but avoid breaking up the larger minerals; sifting is done on a 0.125mm net (100 mesh).

After it’s sifted the sample is quartered to fill two small cups or “caubere” tubes (3.5 x 7cm); one is sent to the laboratory the other is kept as evidence.

The shipping will be done the same as for alluvial samples.

Warning: Especially when grinding rocks, you have to be careful not to contaminate each geochemical grinding through the mortars or sifters. Its recommended to use if possible agate mortars & stainless steel sifters or nylon nets.

Handling data: The handling of data is normally done by the geochemist on the computer in the case of strategic prospecting or by hand in tactical prospecting where the number of samples is a lot less. Once the base value has been established of an areas determined element, meaning the values that should be considered as abnormal, a legend is made based on the color hues, giving lighter ones less value. (for example; white, light brown, brown, dark brown, bright orange), and tenor maps are made. A map is made for every analyzed element. Choosing abnormal areas is done by joint decision of the prospector & geochemist, analyzing all the available data, not only the data supplied by the prospector on the terrains elements, but also the movement of elements, associated characteristics, etc., given by the geochemist.