An investigation of the use of anisotropy of complex magnetic susceptibility to analyse strain in experimentaly deformed materials and massive sulphides

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.