Alluviums, Eluvium, and Littoral Formations

This type of prospecting consists of taking and washing the alluvium and eluvium samples in order to extract the useful minerals and evaluate their concentration or tenor, which can be in grams by tons, or in some cases, also in grams by square meter.

Strategic alluvial prospection
The objective of the strategic prospection whether it is alluvial or geochemical is the search for indicators or abnormalities that is convenient to control in a successive phase. Very often geochemical prospecting and alluvial prospecting are done simultaneously in the hydro graphic net. These can place two interesting points into evidence:

• Areas that regroup abnormalities or mineralized points by which tactical prospecting, mainly geochemical will be done.
• Points of hydrographical reticle with a source of useful mineral concentration (Au, Sn, Nb, Ta, W, diamonds, precious stones); these points will be successively controlled through an alluvial prospection that is denominated as “general prospecting”.

In the following we will talk about the control methods and study of the case in of interesting areas of the second case.

General rules
The density of the sampling points should be as homogenous as possible and the greatest part of the points placed on the affluent and confluent area of the great collectors (these will not be samples in the investigation phase), but always upstream of a confluence. Only if we are forced, due to lack of time or due to funds, to reduce the number of samples, can a sampling be done under the confluence. The recollecting points should be placed in optimal concentration points of the heavy minerals: rocky thresholds, deposits with big cavities, areas under a bed, inside of twines, etc. In the case of a sampling point in torrential type deposits over broken bedrock and a little sediment, it is recommendable to carry two to three samplings at a few meters or dozens of meters in distance to constitute, by mixing them, a representative sampling of the deposit. The amounts of the alluvial samples always need to be measured and the samples should always be numbered; the set should show up on the label that goes along with the corresponding concentrate. In the following we will compile a “map of the sampling points”.