Low-field variation of out-of-phase susceptibility of pyrrhotite-bearing rocks and its implications for rock fabric studies

Pyrrhotite is conspicuous by its very strong magnetocrystalline anisotropy of magnetic susceptibility (AMS) that can be in turn used in the investigation of the preferred orientation of this mineral by crystal lattice in rocks. Unfortunately, the AMS of pyrrhotite-bearing rocks is often composite, carried not only by pyrrhotite, but also by magnetite and mafic silicates; contribution of pyrrhotite can even be overwhelmed by that of the other minerals. It is therefore desirable to separate the AMS component due to pyrrhotite from that due to the rest of the rock. This can be made using the anisotropy of the out-of-phase component of magnetic susceptibility (opAMS), which can be obtained through AMS measurement in alternating magnetic field. Namely, the out-of-phase susceptibility (opMS) of paramagnetic minerals as well as of pure magnetite is virtually zero, while it is clearly non-zero in pyrrhotite. However, the problem is with measuring precision. As shown earlier, the error in opMS determination increases with decreasing phase angle, reaching extremely high values for phase near zero. And the phase is affected not only by opMS but also by ipMS of the measured specimen. It is therefore desirable to study the properties of the opMS and opAMS of real rocks. 

The opMS of the pyrrhotite-bearing rocks investigated increases significantly with the field intensity and the increase is faster in very low fields (<100 A/m) than in stronger low-fields. The Rayleigh Law range, in which magnetization is linearly related to the field, is relatively narrow, less than 40 A/m. The principal directions of the opAMS are virtually field independent in the entire low-field range used (10 to 700 A/m) being also very well parallel to the ipAMS directions. The degree of opAMS is also virtually field independent, but much higher than the degree of ipAMS. The shape parameter in opAMS is also field independent and resembles that in ipAMS. Theoretical quadratic relationship exists between the degree of anisotropy of initial ipMS and that of the tensor of Rayleigh coefficient characterizing the opAMS. Searching for empirical relationship between degrees of the opAMS and ipAMS measured in stronger low fields is the purpose of the present paper.

Physically purest is evidently measurement of opAMS in very weak field, conveniently within the Rayleigh Law range. On the other hand, measurement of the opAMS in the strongest low-field available (700 A/m) is more convenient from the point of view of measuring precision. This is fully convenient if one is interested above all in principal directions and ellipsoid shapes, which are evidently field independent and closely resemble those of ipAMS, and less precise as for the degree of opAMS, which is significantly higher than degree of ipAMS. This must be respected in geological interpretation of the data.