Hydrothermal alteration of magnetic fabrics of rocks in the Xiaoban gold-bearing shear belt, Fujian Province, China

Abstract

Magnetic fabrics indicate that the Xiaoban shear zone underwent low-grade hydrothermal alteration in the Mesozoic (~ 180Ma). Samples studied come from altered and unaltered units from three mine levels. (1) Susceptibility is mainly due to paramagnetic minerals such as biotite and pyrite. The reduction of mean susceptibility is due to decrease in Fe content and change of magnetic mineral size, as well as increase of diamagnetic minerals. (2) Anisotropy (P) decreases after hydrothermal alteration. Its tendency to change is the same as that of the mean susceptibility. (3) The AMS ellipsoid does not substantially change its shape after hydrothermal alteration. An oblate sphere is maintained. The minimum principal susceptibility axis (K3) falls close to the plane which is perpendicular to the schistosity for all samples. However, the maximum principal susceptibility axis is subject to large changes. The results indicate that new magnetic minerals are distributed on the existing schistosity plane, without a preferred orientation.

Keywords

• Magnetic fabric
• susceptibility
• anisotropy
• hydrothermal alteration
• China