Geoscience Reference
In-Depth Information
Table 5.1
Mean values of the Archie parameters for various
lithotypes.
conductive. Conduction can occur through the mineral
grains and, signi
cantly, via electrochemical interactions
between the grains and the pore
fluid.
Few rocks contain enough conducting mineral grains for
this to be the main conduction mechanism, but those that
do often contain signi
cant quantities of potentially eco-
nomic metal-bearing minerals. Invariably mineral deposits
contain numerous mineral species, and it is the electrical
properties of the mineralogical aggregate, rather than the
type or abundance of an individual species, that control
electrical conductivity of the deposit. However, a rough
correlation between conductive mineral content and con-
ductivity may be observed.
Figure 5.17
shows the relation-
ship between conductivity and the proportion of sulphide
minerals present from several deposits. When a conductive
mineral occurs as isolated grains this mineral species will
contribute little to the overall conductivity of the deposit.
As expected, there is little correlation between conductivity
and the amount of sulphides when they are disseminated,
but when the sulphides occur as veinlets the increased
sulphide content causes an increase in conductivity.
The important contributors to electrical conduction in
the rock volume are those grains whose distribution creates
an electrically continuous network, even if the species in
question is not the most conductive or abundant. Clearly,
mineralogical texture is a key property in determining
conductivity, as it is for water-bearing rocks.
Lithology
a
m
Weakly cemented detrital rocks, such as sand,
sandstone and some limestones, with a
fractional porosity range from 0.25 to 0.45,
usually Tertiary in age
0.88
1.37
Moderately well cemented sedimentary rocks,
including sandstones and limestones, with a
fractional porosity range from 0.18 to 0.35,
usually Mesozoic in age
0.62
1.72
Well-cemented sedimentary rocks with a
fractional porosity range from 0.05 to 0.25,
usually Palaeozoic in age
0.62
1.95
Highly porous volcanic rocks, such as tuff, aa
and pahoehoe, with fractional porosity in the
range 0.20 to 0.80
3.5
1.44
Rocks with fractional porosity less than 0.04,
including dense igneous rocks and
metamorphosed sedimentary rocks
1.4
1.58
Source: Keller,
1988
signi
cantly across the three rock classes and, although
they may be fairly constant for the same lithotypes in a
particular area, they are very likely to have different values
elsewhere. High tortuosity leads to high values of m, lower
values being associated with simpler pore geometries such
as fracture-dominant. It varies from 1.3 for packed sand-
stones to as high as 2.3 in well-cemented clastic rocks.
Numerous empirical and theoretical models have been
developed to relate grain structure and bulk conductivity
with modifications to the parameters described above, or
have included additional parameters, for example the
degree of water saturation. However, there is no universal
relationship between bulk electrical conductivity and easily
measurable rock properties, so Archie
10
3
10
-3
10
2
10
-2
s equation continues
to be widely used.
Table 5.1
lists common values of the
parameters a and m for a variety of rock types.
'
10
1
10
-1
5.3.1.3
Conduction involving the matrix
Archie
Vein mineralisation
is equation assumes all conduction is via the pore
fluid, with the matrix having only a passive role by con-
trolling the geometry of the conducting pathways. This is
reasonable since the majority of minerals are insulators.
However, the minerals forming the matrix may also pro-
vide a conductive path, this being most signi
cant when
porosity is
'
Disseminated mineralisation
10
0
10
0
10
-1
10
1
10
0
Weight sulphides (% )
Figure 5.17
Relationship between conductivity/resistivity and
sulphide content. Data from various porphyry copper deposits in the
southwestern USA. Redrawn, with permission, from Nelson and Van
Voorhis (
1983
).
small and/or
the pore
fluid is weakly
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