Progress development phases & objectives that must be fulfilled
Geochemical prospecting is applied to different phases of prospecting, netter said:
- general exploration phase
- strategic exploration phase
- tactical or detail prospecting phase
All of them are based on highlighting secondary abnormalities. Besides, geochemistry can be used to recognize and study evidence (study of primary areolas).
“Regional exploration (or reconnaissance)”consists of covering an unknown area by itineraries on large nets & look while picking up the first geographical character data (hydrographic, relief, access routes), geological (nature of formations) & pedology (type of alteration, nature of soil, presence and type of river sediments, etc.)
“Strategic prospecting” is systematic prospecting; that must give constant information on the prospected surface. Its roll is in highlighting abnormal areas with probable relation to mineralizations.
“Detailed or tactical geochemical prospecting” is always systematic prospecting and it’s designed to specify in strategic abnormal areas, their origin, superficial size & the levels reached by tenures.
It’s important to point out that in these three cases through surface information you obtain the intersection image of the alternation area with the surface, better said the secondary abnormality. In some cases, tactical geochemistry can be completed using more punctual reconnaissance on surface operations: geochemistry with drilling perforator or ditch analysis.
Structure of a geochemical campaign
The structure or project varies according to fixed objectives and the region’s weather conditions and will be carried out with the help of a geochemical expert.
The main points to solve are:
1) Type of sampling
2) Density
3) Elements to be analyzed
1) Type of sampling
Like it has been mentioned before, samples can be made of fluvial sediments, soils or rocks.
In the case of strategic prospecting fluvial sediments (stream-sediments) will predominate sampling, method that up until now has given best results. Tactical prospecting instead of soil sampling normally done at the base of horizon A, better said; at around 30-50cm deep (fig. 22).
In the case of the presence of a superficial alteration (lateral cores) or a more or less strong lactones recover, special method will be needed, such as manual or motor drilling.
2) Sample density
The sample density varies according to the prospecting scale and the dimensions of the searched “targets”. Strategic prospecting normally works at a variable scale of 1:200.000 to 1:50.000.
Sampling is done with wide nets, with a variable density of 1 to some samples to km2, if they are fluvial sediments. Its always best to talk about half density instead of distance between samples since it gives a better idea of the work being developed.
In tactical phases we won’t speak of density but of nets.
This varies with the task of the searched deposit. For example a first location of Pb, Zn or Cu abnormalities, a first net of 200 x 200m is enough, but in cases of Sb or W you must choose of 50 x 50m.
3) Analysis
This is one of the vital points, in truth choosing one or other method can change the efficiency and meaning of the whole campaign.
The method used must be sensibly modern, can be reproduced (that can be repeated more than once, always the same) and inexpensive.
Two specific strategies can be chosen:
- Adapt simple methods than can also be used at the site, for example; a van that has been adapted as a laboratory;
- Choosing a more sophisticated method, available in a central laboratory (the most used).
In the case that an external laboratory is used, it’s recommended to use known characteristics, to control the attention given to the tenures by the laboratory.
In the tactical prospecting phase the problem is defined much better than in strategic prospecting, since only a small group of elements are analyzed even though certain elements aren’t being searched for directly, they are very useful when evaluating the kind of abnormality.
For example Ni will be systematically analyzed in tactical prospecting by Cu, the same as As, Ag, Mo, Cd are interesting values when prospecting for Pb and Zn.
In the “study of evidence” phase and particularly when studying primary areoles through geochemistry in rocks, multi-element analysis are indispensable when trying to figure the polarity and erosion rate of a deposit.
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