brazil geochemistry

7/29/2019 brazil geochemistry

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Brazil II: Follow-Up Stream

Sediment and Soil Survey

Questions: 3, 4, 5, 6.

Jonas Lasskogen, David Whitehead &Obasi Martin


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Question 3a:

What do you expect the relative behaviour, i.e. mobility of Pb and Zn to bein this environment?

The Zn and Pb in this environment will be dominantly controlled by clastic

dispersion as the soils are derived from colluvium. The material will belocally derived and the anomalies will be close to their source.

The local geology controls the pH which affects the dispersion of metals.Metals are more mobile under low pH conditions.

In areas of low relief, Zn and Pb is controlled more by hydromorphicdispersion and Zn will disperse further into the environment relative to Pbas it is more mobile.


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Question 3b:

Briefly summarise what you can infer as to the possible location(s), size,

shape and orientation of the source(s) of the anomalous Pb and Zn in this

area.

Anomalies derived from clastic dispersion should be small, derived fromlocal source material and they should be highly anomalous values

compared to the surrounding background values.

Anomalies derived from hydromorphic dispersion are larger and the

anomalies would show elevated values over a larger area with a smallerdifference to the local background values.


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Question 4a:

What values did you select to characterise the background (threshold) and

anomalous values in your interpretation of the Zn and Pb (total)

abundances based on your plots of the data in tables 32-A and 32-B? What

significance do you attribute to each of the groupings?

There is insufficient data for both soil and stream sediment samples to dostatistics. Therefore a visual evaluation of threshold values was used. Avalue of 100ppm was selected as threshold for the soil samples. For thestream sediments only the distinct values above 200-300ppm wereplotted as anomalous.

This technique identifies the highly anomolous areas but does not takeinto account any other information such as regional threshold values.


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Question 4b:

How many sources of anomalous Pb and Zn are indicated by the two follow-upsurveys? Where are they; how large are they and what is their relative

geochemical expression? If possible, classify them according to geochemical

response and postulate their probable source area?

There are maybe 4 Pb/Zn anomalies that can be located:

one north of Corrigo PGBC with a possible source in the carbonate to the westof the anomaly;

one in the northern part of the western N-S flowing stream which may havethe same source as the other larger anomaly;

A stream sediment anomaly (samples 13-15) which is close to two springs;

Another anomaly with a source for Zn and Pb might be located in carbonate in

the southern part of the western stream by samples 1 and 2.

The anomalies look to be mainly controlled by clastic dispersion because of thetopography.


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Question 4c:

Rank the anomalies according to their significance, as you interpret them, and

make your recommendations for further work, if any?

Most significant anomaly: eastern stream in both soil and stream sediment surveys atleast 1 km long, 200-500ppm for Zn and 200-700ppm for Pb.

Western soil anomaly: most significant for Zn, less for Pb, 500m long.

For this and the largest anomaly further soil sampling with a denser spacing, outcropsampling to locate rock chemical anomalies, and resistivity geophysics could be applied.

The southern soil sample anomaly is not in priority since it consists of only two samples.

The least significant anomaly is that which is found by stream samples (13-15). Samplesclose to springs which means that (mixing of two different fluids, one of which has anunknown source). The anomalies are scattered and there are only 3. Smallmineralization?


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Question 5:

Assess the two techniques (stream sediment and base of slope sampling)used in this follow-up survey in terms of their relative cost effectiveness?

More base of slope samples can be collected by one team than stream

sediment samples. The stream sediment samples are anlysed using two techniques.

There is no significant difference in cost effectiveness between the twotechniques but stream sediment sampling will collect more data becauseof the two analyses.

Stream Sediments Base of Slope

Number of samples collected. 42 76

Number of days required. 1 1

Number of crews. 2x2 3x2

Area covered (km2). About 4x2 km About 4x2 km

Number of analyses performed 2 1


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Question 6:

What are the main principles applicable to exploration that can be derived

from this problem?

It is important to establish the causes of element mobilisation in theenvironment; i.e. by hydromorphic or clastic dispersion and how this isaffected by other factors such as relief and bedrock type;

You should look into account dispersion affects from man made sources, e.g.dams. These can significantly affect the anomaly patterns;

Once anomalies have been identified during sampling, they should be rankedbased on factors such as their size, proximity to favourable geology andcontrast to background values;

Using different sampling media and analytial methods providescomplimentary information which can be used to identify the most favourableareas for mineralisation.

brazil geochemistry

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