Interpreting Geochemical Data
Interpreting Geochemical Data

• Interpretation is identifying three types of
p y g yp
concentrations: Background, Threshold and
Anomaly
• Effective interpretation of geochemical data
involves consideration of multiple population
of data
• Separation of anomalous samples from
background can be done by a number of
methods

1. Comparison with data from literature
– Good for small number of data
– Not reliable
– Only indication for follow‐up work

2. Graphical methods
– As histograms
As histograms
– As cumulative frequency curve
– Plotting values as a map
– Plotting values as a map

Two populations means the
samples are of two different types
or sources or …
In this case better to determine
mean & threshold for each
group
An approximate but
adequate method for
extracting the two
populations is to draw lines
p p
tangent to both tails of the
curve passing through points
equidistant from the pt of
equidistant from the pt of
inflexion

• How to construct cumulative frequency curve?

In log‐probability plot Mean BG=50%, Threshold=2.5%

3. Threshold can be calculated as mean plus 2
or 3 std
– This is good in reconnaissance survey for large
number of data and for normally distributed
population
– Here anomaly is mean plus 2std or 3std
– In this calculation only a small proportion of the
background populations are included

Important Definitions
Background – the normal abundance of an elt in
Background the normal abundance of an elt in
un‐mineralized earth material
– It varies from place to place & from material to
material
material
– It is thus a range rather than absolute value
Is generally taken as the 50% in log probability
– Is generally taken as the 50% in log probability
plot of cumulative frequency

Threshold – the concentration of an element
above which a sample is considered
anomalous
• C>Th… anomalous
• C<Th… background
• Thresholds can be:
– Regional threshold: related to the fluctuation of
g
regional background values
– Local threshold: related to extensive feeble
i li i di i d i li d
mineralization or dispersion around mineralized
body

Anomaly – is departure from the normal
Anomaly is departure from the normal
geochemical patterns of a given geochemical
environment
environment
• In the case of MEX, they represent
concentration of an element above the
concentration of an element above the
expected values in a given earth material
A li ld b
• Anomalies could be:
– True: if related to mineralization
– False: the opposite of the above
• False anomalies arise from one or more of the
following:
following:

1. High background source rock (e.g. Ni in
g g ( g
ultramafic rocks)
2. Contamination
– Mine dumps
– Old mine workings
– Smelting operations
– Metal rich agricultural chemicals
Industrial/domestic effluents
– Industrial/domestic effluents,
– Or sampling equipments, bags etc
3 Sampling errors
3. Sampling errors
4. Analytical errors which can be remedied by
repeated analysis
repeated analysis

• The places where true anomalies detected
and place of mineralization do not necessarily
and place of mineralization do not necessarily
coincide (e.g. stream sediment anomalies are
mainly transported anomalies)
mainly transported anomalies)
• Result: Generating targets for follow up
• Result: Generating targets for follow‐up