Balanced mineral reactions for alteration zones developed in auriferous shear zones of the Hutti Mine, Dharwar Craton, India

In the up to 10 m wide auriferous shear zones of the Hutti Gold Mine two stages of gold mineralization and deformation are distinguished. During the first stage, the amphibolite host rock reacted with a hydrothermal fluid enriched in Si, Ca, Na and K, forming a proximal biotite-plagioclase alteration zone whereas Fe and Mg were leached from this zone. Volatiles involved in the reactions were H ₂ O, CO ₂ and H2 S. In the distal parts, the hydrothermal fluid was enriched in Fe, Mg, Ca and less K and H 2 S. Silica and Na were removed. The opposing mode of enrichment and depletion, especially for Fe, Mg and Si, in the stage 1 proximal and distal alteration zones, suggests that the hydrothermal fluid progressively changed composition during fluid rock interaction in the central part of the shear zones. Mass changes in both stage 1 alteration zones are in the range of 15 % mass gain. The T-X pseudosections indicate that both stage 1 alteration assemblages are stable at similar P-T conditions of 460-510 °C and 3 kbar but different composition. This supports the interpretation that the hydrothermal ore fluid was introduced into the central parts of the shear zone resulting in a fluid dominated system. The distal alteration formed with probably lower fluid-rock ratios by the hydrothermal fluid modified during fluid-rock reaction in the proximal zone. The hydrothermal stage 2 was associated with an aqueous fluid enriched in Fe and Mg, whereas Si, Na, K and S were leached from the altered rocks. These components contributed, together with an external input by the ore fluid, to form extensive gold-quartz veins, which are contemporaneous with this alteration stage 2. The calculated mass gain lies in the range of 10 %.The fluid composition and mass transfer estimations from this study are in accordance with results from bulk rock geochemical and stable isotope calculations, except fluid-rock ratios and volume change. These parameterscannot be quantified in the open system studied here. The combination of balanced mineral reactions and modeling of pseudosections is ideally suited to discuss mass transfer in hydrothermal alteration zones because, in contrast to whole rock geochemical methods, element mobility (or immobility) is directly related to observed mineral reaction textures.