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Positive
SP contours (mV)
CI = 50
0
Negative
Survey traverses
Graphite schist
Schist with disseminated graphite
A
A
Dip
dir ection
50 mV
0
100
+
Metres
-
A
A
Dip
dir ection
50 mV
+
-
B
B
B
B
Figure 5.31 Contours of the surface SP over the Almora graphite deposits. The form of the local geology is revealed by the negative anomalies
associated with graphitic horizons. CI ΒΌ 50 mV. Redrawn, with permission, from Srivastava and Virendra Mohan ( 1979 ).
The Safford deposit occurs in an igneous complex.
There is extensive alteration in the area around the deposit.
Mineralisation occurs in a central zone of sericite and
biotite alteration surrounded by concentric zones of chlor-
itic and propylitic alteration. Sulphide mineralisation also
forms a roughly concentric pattern, but is only loosely
correlated with the alteration zones. A core of dominantly
copper sulphides is surrounded by a zone dominated by
pyrite, these extending to the approximate limit of the
chloritic alteration zone. The main primary sulphide min-
erals are pyrite and chalcopyrite in the form of dissemina-
tions and veins, with the former the most abundant. In the
ore zone, pyrite content is 0.2 to 1.0%, but in the surround-
ing alteration haloes it ranges from 4 to 8%. The primary
ore contains about 0.7% chalcopyrite, decreasing to about
0.4% in the pyrite dominated zones. Northeast-trending
faults and shear zones are the dominant structures in the
deposit, these having acted as conduits for hydrothermal
fluids. The peak SP response of 500 mV is a very strong
anomaly and coincides with a sulphide-rich zone where
there is much faulting. The orebody lies within this anom-
aly, but the anomaly maximum is to the south of it,
possibly owing to the distribution of sulphides at depth.
Other alteration zones in the vicinity are also associated
with SP anomalies.
The SP data from Safford successfully map the sulphides/
alteration around the orebody, but cannot distinguish the
economically significant from the barren. The authors
comment that SP responses are signi
cantly diminished
by cover. These observations are typical of SP results in
general. Corry ( 1985 ) describes various factors that affect
the SP responses from porphyry style mineralisation.
5.6 Resistivity and induced polarisation
methods
Resistivity and induced polarisation (IP) surveys have
much in common in terms of data acquisition, display
and interpretation. Resistivity-only surveys may be under-
taken,
as
is widely practised in groundwater
and
 
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