Geoscience Reference
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a)
b)
c)
d)
SP
(mV)
20
+
SP
(mV)
10
+
SP
(mV)
20
+
SP
(mV)
40
+
-
-
-
-
Figure 5.30
SP logs through various types of geology
and mineralisation. (a) Jharia Coal
eld, Jharkhand,
India. Jhama is burnt coal resulting from intrusion of
igneous rock. Note how the log varies between two
baseline values (see
Section 5.5.3.2
)
. Based on a
diagram in Kayal (
1979
). (b) Oxidised iron
formation, Cuyuna Iron Range, Minnesota, USA.
Oxidation of the formations results in variations in
SP response. Based on a diagram in Hansen (
1970
).
(c) Kimheden Cu deposit, Lapland, Sweden. Note the
erratic variations within the relatively conductive
oxide and sulphide mineralisation. Based on a
diagram in Parasnis (
1970
). (d) Copper
mineralisation in the Singhblum copper belt, Bihar,
India. SP responses are subdued in the host rock and
erratic in the mineralisation. Based on a diagram in
Kayal et al.(
1982
).
1
0
m
1
0
m
1
0
m
1
0
m
Chert
Lean iron ore
Silicate iron fm
Carbonate iron fm
Schist
Cu mineralisation
Mafic schist
Shear zone
Cupriferous pyrite
Magnetite
Mudrocks
Intrusive
Shaley coal
Jhama
Coal
Carbonate iron fm (slightly oxidised)
Carbonate iron fm (strongly oxidised)
(
1986
)), but is generally less useful than most other kinds
of logs. Conductive mineralisation generally coincides with
signi
cant variations in SP (see for example Becker and
identi
ed from the logs. Responses from less conductive
lithotypes are more subdued and harder to predict.
As with surface SP data, it may be necessary to differen-
tiate between localised fluctuations and smoother vari-
ations in downhole data, i.e. a form of regional response
removal (see
Section 2.9.2
)
. In mineralised sections, the
short-wavelength variation in the logs is a response from
potential differences caused by electric current transfer
across the interface between the mineralisation and host
rock. The longer-wavelength variation reflects potential
variations within the country rock, cf. the Sato and
Mooney electrochemical model, so a gradual increase in
positivity may occur.
graphite in the Almora district in the Himalayan Region
of Uttarakhand, India (Srivastava and Virendra Mohan,
region, both alone and in association with other electrical
methods.
Figure 5.31
shows the results from a survey
designed to detect high-grade graphite mineralisation
within graphitic schist horizons. The local geology consists
of a folded, faulted and metamorphosed sequence of schists
and quartzites. The SP data have been effective in mapping
the graphitic horizon between outcrops, the associated
anomalies having amplitudes of
600 mV. Selected
profiles allow the dip of the source to be determined (cf.
significant occurrences of graphite.
-
200 to
-
5.5.4.2
Safford porphyry copper deposit
This example demonstrates the application of SP to the
mineralogically complex environment of a large porphyry
copper deposit; illustrating the different responses
obtained from the sulphide-rich ore and the various
sulphide-rich alteration zones, and structure.
Figure 5.32
shows SP data from the Safford porphyry Cu deposit in the
Lone Star district of Arizona, USA. The geology of the
deposit is described by Robinson and Cook (
1966
), who
also show a contour map of the SP responses. This allows
the SP data to be compared with such features as alteration,
structure and the sulphide mineralisation itself.
5.5.4
Examples of SP data from mineral deposits
The following example SP surveys demonstrate the use of
the SP method for exploring for and investigating two
quite different types of mineral deposit.
5.5.4.1
Almora graphite deposits
This example demonstrates application of the SP method
as a quick, cheap and effective means of exploring for
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