Dominant coaxial deformation of veins during the interseismic stage of the fault-valve cycle: microfabrics of laminated quartz veins of the Hutti gold mine, India

Auriferous shear veins formed during greenschist facies reactivation of amphibolite facies shear zones of the Hutti mine. The laminated vein fabric, hydraulic breccias and fault gouge material formed by failure during the fault-valve cycle. Microstructural observations in the laminated quartz veins indicate that porphyroclasts of quartz1, scheelite1, and pyrrhotite have an oblate shape. Quartz2 grains with a width of 0.03–0.05 mm result from predominant subgrain rotation, while predominant bulging recrystallization formed quartz2 grains with a width of 0.01–0.03 mm. The pole figures of the quartz texture are symmetrical with respect to a shortening direction perpendicular to the vein walls. Strain partitioning of shear stress into the rheologically weaker altered host rocks preserved these fabrics in the stronger quartz veins. A non-coaxial overprint of the veins is only weakly recorded by the microfabrics and quartz textures. The study of microfabrics and quartz textures helps to understand processes of seismic activity in shear zones during fault-valve behavior. In the case of Hutti, coaxial deformation normal to the shear zone is indicated during the initial stages of shear stress and fluid pressure build-up, following the crack-seal stages.