The beginning

Targeting and vectoring is the product of an integrated data analysis of geology, geochemistry, alteration and mineralization. Alteration mapping sometimes is not easy even for experienced geologist, and mineral analysis supported by instruments like portable spectrometer makes the difference. However, the identification of alteration patterns depends of geologists field work, and not only their experience recognizing minerals using hand lens, also how and where they collect samples to analyze with a portable spectrometer (i.e. ASD Terraspec).

Occasionally alteration minerals are impossible to identify by hand lens, especially when clays are mixed or with low crystallization, but a good trained geologist is the difference when a mapping program is under way, saving time and money.

Three questions before alteration mapping

There are important facts to consider previous to mapping and sampling, especially when is supported with spectrometry studies.

Before to start mapping, is necessary to answer three key questions:

  • What kind of hydrothermal system we are in front of?

Is necessary to understand de 3D geological model of different hydrothermal systems, to have an idea if we are in a Epithermal Low or High sulfidation system, or porphyry Cu-Mo/Cu-Au system?, even more complex and interesting like telescoped systems (alteration lithocaps over porphyries).

  • How would be the pathway and feeders of hydrothermal fluids into mineralized and host rocks?

It is structural control like faults and folds, or lithological control like composition or permeability.

  •  Which are the typical alteration minerals and how their occurrence are?

For example, Illite/Smectite relation of alteration halos in veins in epithermal LS, also alunite (K, Na or Ca) composition in epithermal HS, and finally Sericite/Green Chlorite distribution in porphyry systems.

 What will change now in the mapping?

After those question were answered we should have an idea which kind of mapping or sampling problems to resolve. Consequently, is possible to define how to focus the sampling over outcrops, especially when the study will be supported by portable spectrometer (ASD Terraspec).

Next post, sampling and recognition of alteration patterns in epithermal LS be continued...

"Developing targeting and vectoring technics to support your next discovery" 
PETRA GAIA "Geology and Mining Exploration"

Luciano M.Bocanegra

Consultant Economic Geologist

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Name: Luciano Bocanegra (Principal Economic Geologist at Petra Gaia, Chile)
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