This post describe some key points to carry out a conceptual mapping of hydrothermal alteration in Low and Intermediate Sulfidation Au-Ag vein systems for targeting and vectoring  

In the schematic picture below I tried to represent a typical example of Low (LS) and Intermediate (IS) sulfidation Au-Ag veins and their alteration patterns. The picture shows two different levels, the lower one in high grade zone, and the other, shallower in the system where the evidence of mineralization are veinlets and thin veins.

The emplacement of LS and IS veins are typically controlled by structures, mainly faults with different kinematic regime, for this example a normal fault is represented, and the main lithology is andesite with an interbeded layer of rhyolitic rock.

Alteration Vectoring 

Structural vector

Faults are the feeders for hydrothermal fluids, therefore they controlled the fluids pathway and define the shape of the alteration halo.

Asymmetrical alteration haloes: generally the hanging wall has more extensive halo than the footwall, generating an evident asymmetry (See picture).

Disseminated mineralization: Veinlets are widespread close to the veins and faults in the hanging wall, even stockwork and breccias.

Mineralogical vector

With the recognition of mineralogy in alteration haloes is possible to characterize the relative temperatures and ph between them, making possible to vectorize fluids source and high grade mineralization.

Mineralogy haloes: in the picture above, is expressed the relation of pH and temperature to relation Illite/Smectita and Dickite/Kaolinite. Furthermore with a spectrometer is possible to determine the Illite composition, from Paragonite (Na), Normal or Potassic (K) and Fengite (Fe-Mg)

Lithological vector

LS and IS veins are typically emplaced during bimodal volcanic environment, and as consequence diferent compositional rocks are interlayered, affecting alteration patterns. Likewise permeability and competence of those rocks.

Permeability and competence: Rhyolitic ignimbrite has permeability and competence different than an andesite/basalt lava flow.

Host rock reaction: alteration haloes in feldspar dominated lithologies (i.e. rhyolite) are more widespread than intermediate to mafic rocks (i.e. andesite, basalt).Mineral vector

Geochemical Vector

Univariable and multivariable geostatistics helps to define anomalous elements for vectoring, and these elements are different on each project.

"Every projects has their own DNA, so is very important to find the mineralogy/geochemistry ratios showing a better response for vectoring." 

"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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