Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 389

velocity, density and acoustic impedance compared with
the amphibolite facies samples.
These examples demonstrate that where metamorphic
reactions replace existing phases with minerals having
different properties, there will be a change in the overall
seismic properties of the rock. However, the changes in
properties are generally quite small and likely to be grad-
ual. A seismic response coinciding with a metamorphic
isograd or alteration front may be possible if the change
in mineralogy is sufficiently abrupt.

both velocity and density substantially, and its seismic
consequences are signi cantly greater than the mineral-
ogical changes associated with the prograde metamorph-
ism described above. An implication of this observation is
that serpentinisation of ultrama c lithologies occurring in
non-ultrama c sequences may severely reduce their ability
to generate seismic responses, since the seismic properties
of serpentinites is similar to that of felsic igneous and
sedimentary rocks. However, contrasts between serpenti-
nised and unserpentinised areas of mafic and ultramafic
successions will be significant and, if sufficiently abrupt,
could give rise to recognisable seismic responses.

6.6.3.1 Serpentinisation
A metamorphic process that produces substantial changes
in seismic properties is serpentinisation. This is the
hydrous alteration of olivine and pyroxene to produce
the serpentine minerals antigorite, chrysotile and lizardite,
and is common in ophiolite complexes, layered igneous
complexes and greenstone belts. Most serpentinite is pro-
duced by alteration of ultrama c igneous rocks and, com-
monly, both the relict and alteration mineral assemblages
are present. Figure 6.34 shows a compilation of published
velocity versus density data from various serpentinised
rocks, where the original authors have estimated the degree
of serpentinisation. It is clear that serpentinisation reduces

6.6.3.2 Faults and fault rocks
Few seismic property data are available, but it is generally
agreed that faults and shear zones can be the source of
seismic responses, especially in highly deformed hard-rock
terrains. Responses can be caused by the juxtaposition of
rock types with different seismic properties, but it is likely
that the fault structures themselves can be the source
owing to their porosity, which is likely to be water- lled.
Ductile faulting may form mylonite zones that may be
re ective in their own right, owing to their layered struc-
ture combined with lithologic diversity and/or greater
fabric development (see Section 6.6.6 ). Arguments have
been presented for enhancement of their re ectivity by
reduction in velocity related to retrograde mineral assem-
blages, and by increased velocity related to loss of silica.
Alteration, caused by hydrothermal uid-ow through
the fault zone, is another possible cause of acoustic-
impedance contrast. It has been postulated, but not yet
categorically demonstrated, that enhanced reflectivity
may be characteristic of structures that were the main
conduits for hydrothermal mineralising fluids in a miner-
alised terrain.

9000

Olivine

8000

Orthopyroxene

Magnetite

7000

6000

0 - 10% serpentine
11 - 30% serpentine
31 - 50% serpentine
51 - 70% serpentine
71 - 90% serpentine
91 - 100% serpentine
Minerals

5000

Serpentine

4000

6.6.4 Seismic properties of mineralisation

Both metallic mineralization and non-metallic materials of
economic interest, such as coal and evaporites, plot outside
the velocity

3000

2.0 2.5 3.0 3.5 4.0

Density (g/cm 3 )

density field of the common rock types
( Fig. 6.35 ). Sulphide and oxide minerals may have greater
or lesser seismic velocities than the rock-forming minerals,
but all are signi cantly denser ( Fig. 6.35a ). Salisbury et al.
( 1996 ) show that velocities and densities of massive sul-
phide ores can be estimated on the basis of mixing lines
joining the physical properties of their main constituents
(ore minerals and gangue). Figure 6.35 supports this, with

-

Figure 6.34 Effects of serpentinisation on seismic velocity and
density. Also shown are the fields for the main minerals involved in
serpentinisation reactions. The shaded area represents the

field for
aggregates of olivine, orthopyroxene and serpentine. The red line is
the average crustal density of 2.67 g/cm 3 . Data from numerous
published sources. The broken lines are contours of acoustic
impedance with their separation representing the contrast required
to produce a re ection coef cient of 0.05.