| dc.description.abstract | Anisotropy of magnetic susce:ptibility (AivJS) has become a widely
accepted method of fabric analysis in rocks, especially those
which have been deformed tectonically. The use of anisotropy of
complex magnetic suscepti bi 1 i ty (ACf"lS) is a new potential method
of fabric analysis in which the imaginary, or out of phase A.C.
component of an induction coil used for the measurement of
magnetic susceptibility is used to delineate rock fabric. Complex
magnetic susceptibility is a function of of electrical
conductivity, thus making it potentially useful in the analysis
of highly conductive sulphide-rich rocks, some of which are not '
suitable for AMS analysis.
Preliminary measurements were performed on highly conductive
l ~inum test specimens of differing shapes to determine the
relationship between grain shape anisotropy and ACI"lS. A
relationship was found in which shape anisotropy and resistive
ACf"lS fabrics were of the same sense, but there was no
quantitative correlation. Pure and simple shear deformation
exper-iments performed on plasticene containing numerous small
aluminum disks exhibited a correlation between ACMS fabric
anisotropies and strain in most cases, as the ACMS fabr-ics were
controlled by the distribution of the disks, which became wellaligned
as flattening proceeded. Although t.here was no
quantitative relationship between strain and ACMS, they tended to
increase together.
Triaxial deformation studies on loose pyrrhotite aggregates and
pyrrhotite plus talc mixtures were performed at confining
pressures of 150 MPa. The ACMS fabrics developed in these
specimens were compared to Af"JS fabrics and strain analysis data
to determine if the ACMS fabrics change as a function of strain.
As e f~ te oblate resistive ACMS fabr-ics developed dur-ing these
pure shear deformations. The pyrrhotite aggregates exhibited a
complex relationship in which ACMS increased with strain, at
least up to a critical strain value, after which ACMS appeared to
decrease. The pyrrhotite plus talc mixtures exhibited an
unmistakable inct·ease in ACMS with increased strain probably
influenced by the presence of the talc matrix. The ACI"lS fabrics
developed in.these experiments were undoubtedly tKe e ~lt of
grain alignment and distribution within the aggregates, with
insignificant contributions from crystallographic resistive
anisotropy.
Measurements performed on specimens of massive pyrrhotite
revealed ACMS fabrics compl8tely diffr:r-ent fr-om those observed in
the loose pytThoti te aggregates, with ambiguous relationships
between str a i n and ACivJS. This is because the massive specimens
behave electrically as a single grain and anisotropy is almost
exclusively ct·ystallographically controlled. Thus the ACI"lS
properties of single minerals must be understood before ACMS
fabrics in massive sulphides can be interpreted. | en_US |
