Geochemical prospecting

Basic knowledge
Definitions of geochemical prospecting

Geochemical prospecting consists of systematic measuring of the content of one or more element traces in rocks, soil sediment in water currents, vegetation, water or gases. The objective of these measurements is to put in evidence “geochemical anomalies”, meaning the abnormal concentrations of certain elements, contrasting primarily to neighboring areas, which represent the “geochemical base” or background (or Klarke).  The abnormal formations are caused by the movement & dispersion of concentrated mineralized elements. The “normal” value isn’t exactly a defined value but has a more variable range, characteristics of the area. An abnormality by definition; is the variation from a normal value.

The “threshold”, is the concentration of an element parameter over which a sample is considered abnormal.

What is an element parameter? There are some elements that are hard to analyze since they are immobile or give hard to interpret data, but there are other elements that are commonly associated with mineralization, that can be more useful. These elements are called “elements parameters” or “pathfinder”.

These are elements with better geochemical or analytical characteristics than the main metal. For example Mo is more mobile than Cu is most superficial areas, which is why it can be used as a “pathfinder” for porfiriticous deposits of copper that normally contain a small amount of Mo. The same is done with As in the search of Au deposits.
Some element parameters (pathfinder) work with the following chart:

Element        Material           Kind of deposit
parameter 

As           fitting rock,       Au highlights
             residual soil,
             active sediments

Hg           fitting rock, soil    Pb, Zn, Ag complex
                           deposits

Se           iron peaks, soil    Epigenetic sulfurs

Ag           residual soils      Au deposits with
             sediments, soil     Ag content

Mo           water, fluvial      Au deposits with         
                            Ag content

SO4          water           Sulfate deposits

The origin of an abnormality isn’t necessarily metalogenic but it can be a derivate of human contamination (fertilization, industrial waste, etc.), or also by the discharge of an old or new mine. There are also “formational” abnormalities, due to the constant presence to a geological formation (Cu in basic rocks, Pb-Zn in dolomites), or a very thinly dispersed mineral on a rock (example: silicates of Cu) but that isn’t profitably extracted. The difference between “true” & “formational” or by “contamination” abnormalities is one of the biggest geochemist jobs.