Pit sampling methods

In certain plain areas with subtropical regions, sandy-slime deposits are very common, to the point where the riverbed is low in heavy minerals. Meanders are numerous & and the bed-rock is rarely visible. In these conditions sampling should be done by perforating the alluvium deposits reaching the bed-rock. This kind of sampling is specially recommended when searching for diamonds, which would normally need to wash 2 to 3 m3 of gravel for each sample point.

Sampling location

Sample detail (see fig. 15)  

• In the first case of surface sampling will be useless, since surface sands don’t give any indication of the heavy mineral concentration in the water flow.
• It’s necessary to dig pits: in most cases where there are large flows of water.
• In the second case we are in an ideal situation: where samples are taken from the convex bank, through small pits possibly reaching the bed-rock and the sample target.

In the third case sampling must be done on the banks, in alluviums, and sometimes a lot further up the river flow.
However we must miss possible tricks by heavy minerals in river flow, like marmites if they’re visible. 
Besides, old alluvium samples on the concave bank give important information on eventual potential see fig. 16).

Its useless sampling around areas with rapids or water falls, because the erosion is such that heavy minerals are displaced to water down in the valleys. 

Sampling done upstream from the rapids is often considered more useful. It’s also good to control the nature of the bed-rock to a possible stratifications or consistency can act as traps for heavy minerals, even more if these incline towards a hill, taking the roll of a natural “sluice”. 

Note: In the case where the flow of water crosses a highway (road or track), sampling must be done at least 50 meters upstream, since the bridge can alter the natural relation of minerals by adding strange materials.

Pit sampling

Before digging a pit its good to verify the presence of a layer of gravel and its strength, possibly placing some drillings in the area. 

The prospecting can be done for isolated pits or in the case of large sites, by parallel lines every 200-1,000 meters and perpendicular to the valley, with pits at a distance of 10 to 30 meters.

Pit digging (fig 17 & 18)

Pits must have a rectangular form of 0.80 x 0.50m for depths of approximately 0.60m.
For pits that are from 0.60 to 3m deep it’s recommended to be 1.50 -1.80 x 0.70 – 0.80 meters diameter.
For pits that are 3m or more, it would be good to make them circular shaped (less risk of shakes) or rectangular of 3.60 x 0.80 with one or two step.
In the presence of large edges, the starting section (0.80 x 0.50m) must increase to 1.20 x 1m. 

The barren part (soil & superficial sand), is piled on one side, at a certain distance from the pit and the gravel on the other side.

Based on the characteristics of the terrain & depth, the pits must be timbered (like in fig. 18), to avoid shakes and accidents. Pits at any rate will be fenced and in inhabited areas, closed once the sampling is finished.

Even when the data got from pit samples are more complete, in the case of alluvium deposits that are found on the actual riverbed or are covered by unstable sands, light drilling machinery must be used, completely taken apart to a unit weight of 30kg, allowing its transportation even in inaccessible areas (for example a 4 or 6 inch “banka” type drill).

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