Mark Puumala MSc thesis abstract
Anisotropy of magnetic susceptibility (AMS) 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 susceptibility (ACMS) 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 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 aluminum test specimens of differing shapes to determine the relationship between grain shape anisotropy and ACMS. A relationship was found in which shape anisotropy and resistive ACMS fabrics were of the same sense, but there was no quantitative correlation. Pure and simple shear deformation experiments performed on plasticene containing numerous small aluminum disks exhibited a correlation between ACMS fabric anisotropies and strain in most cases, as the ACMS fabrics were controlled by the distribution of the disks, which became well-aligned as flattening proceeded. Although there 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 AMS fabrics and strain analysis data to determine if the ACMS fabrics change as a function of strain. As expected, oblate resistive ACMS fabrics developed during 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 increase in ACMS with increased strain probably influenced by the presence of the talc matrix. The ACMS fabrics developed in these experiments were undoubtedly the result 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 completely different from those observed in the loose pyrrhotite aggregates, with ambiguous relationships between strain and ACMS. This is because the massive specimens behave electrically as a single grain and anisotropy is almost exclusively crystallographically controlled. Thus the ACMS properties of single minerals must be understood before ACMS fabrics in massive sulphides can be interpreted.
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