Prospecting alluvium, translocations & riverbank formations

This kind of prospecting consists of taking and washing alluvium & translocation samples to extract the useful minerals and evaluate their concentration or pitch, often shown by grams per cubic meter, grams per ton, or sometimes grams by square meter. 

1) Strategic alluvium prospecting (or recognizance)

The objective of strategic prospecting, be it alluvium or geochemical is the search of evidence or abnormalities that are convenient to move on to the next phase. Very often is geochemical prospecting & alluvium prospecting are performed simultaneously in the hydrographic net. This can bring out to interesting facts:

a) Zones that are mad up of abnormalities or mineralization points on which specifically geochemical “tactical” prospecting is performed.

b) Points of the hydrographic reticule with a strong concentration of useful minerals (Au, Sn, Nb, Ta, W, diamonds, precious stones); these points will be properly controlled by alluvium prospecting which is also called “general prospecting”. In this chapter we will go over the control methods and the study of interesting areas in the second case.

General rules

The density of sample points must be most homogenous possible and most points located up stream or underflows of great collectors (these will not be sampled in this phase of the search), but always water over a junction. Only if we must, due to lack of time or funds, reduce the amount of samples, you can also sample down stream. Sample of location on fig. 14.

The sampling points must be located in the areas of optimal concentration of heavy minerals: rocky thresholds, erosion casks, deposits with large edges, areas that are restrained from the riverbed, internal parts of meanders, etc.. In the case of torrential type sampling point on fortuitous bed-rock and with little sediments, it is recommended to make two or three samples a few meters or ten meters from each other to build, mixing them, a representative sample of the deposit. The volume of alluvium samples must always be measured and samples numbered; the group must appear on the label that accompanies the corresponding concentrate.

Bellow we compile a “map of sampling points”.

Itinerary tracing

A) Objective 

Cover the most surface with a minimum amount of kilometers, in the least time, obtaining cost reduction, achieving the maximum information possible with minimal work.

 B) Means

Systematical prospecting, also called strategy, of water recourses during the collection of alluvium & geochemical samples on the actual riverbed (see bellow sample methods).

C) Methods

Trace the itineraries that may vary according to the area’s accessibility (maximum for 20 days & 160km), keeping in mind that progress in the jungle is 8km per day.

Consider the following:

• Each basin must be “covered” at least by the main collector and its tributaries.
• Don’t leave uncovered areas, meaning slopes where its streams haven’t been sampled.
• Calculate the extension of itineraries with a curbimeter and consider the available time & number of teams (so there aren’t areas left uncovered for lack of time).
• Study photogeology maps & if possible have the itineraries pass over the most important points (contacts, marked evidence, etc.).
• Mark the sampled points with the foreseen importance of the project on a 1:50.000 map (alluvium & geochemical).

D) Work plan

Make a plan (according to the schedule) of the work to be developed by each team, with the total of kilometers and the number of alluvium & geochemical samples to be taken. Make an estimate of the time needed to develop the work.

E) Distribution of itinerary by tem

Each team leader reports on a copy of a topographic map on a scale of 1:50.000 their own itinerary and will explain to the mission leader what the key points he’d like to study are.

F) Itinerary makeup

A good prospector covers around 8km (linear on a map) & 3-4 alluvium samples a day, in the Amazonian jungle. The carriers of each team will transport field & prospecting equipment, and food rations. The team leader will jot down in the field book & later on the map, all the observations related to the sampling point, petrography, change of surfaces, outcrops, etc..