Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow
The gold mineralization of the Hutti Mine is hosted by nine parallel, N-S trending, steeply dipping, 2-10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D 2 shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle-ductile D 3 shearing and intense quartz veining. The development of a S 2-S 3 crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D 2 shearing is associated with a pervasively developed distal chlorite-sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite-plagioclase alteration in the center of the shear zones. D 3 is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S 3. The average size of the laminated vein systems is 30-50 m along strike as well as down-dip and 2-6 m in width. Mass balance calculations suggest strong metasomatic changes for the proximal biotite-plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite-sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ18O values of the whole rock from around 7.5‰ for the host rocks to 6-7‰ for the distal chlorite-sericite and the proximal biotite-plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow. The ductile D 2 deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold-sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D 3 deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.