Mineralogy, hydrothermal alteration and paragenesis of gold mineralization at Paramanahalli prospect, Chitradurga Greenstone Belt, Western Dharwar Craton, India

The gold mineralization at Paramanahalli (Chitradurga, Karnataka) in the Archean Greenstone Belt of Western Dharwar Craton, India, is structurally controlled and similar to many greenstone-hosted orogenic gold deposits across the world. The volcano-sedimentary rock sequences of the Hiriyur Formation of the Chitradurga Group are the host rocks related to gold mineralization. These litho-units were subjected to greenschist facies metamorphism, later deformed and altered by hydrothermal fluids. Detailed petrographic studies indicate that the rocks experienced chloritization, carbonatization, silicification, and sulfidation due to wall rock-fluid alterations. The prominent wall rock alteration assemblage is characterized by the presence of chlorite (Mg-rich), ilmenite, albite, and quartz-1 in the distal zone and quartz-2, chlorite (Fe-rich), biotite, ankerite in the proximal zone with a significant amount of sulfide minerals and gold. Three major stages of mineral formation have been identified: albite-1, Mg-rich chlorite, quartz-1, magnetite, and ilmenite formed during the regional metamorphism and deformation stage (pre-ore stage). Fe-rich chlorite, annite, quartz-2, pistomesite, ankerite, rutile, pyrite, chalcopyrite, pyrrhotite, and gold during the mineralization stage (syn-ore stage) and albite-2, calcite and goethite formed during the post mineralization stage (post-ore stage). The development of different types of twinning in calcite indicates the deformation temperature, around 150-300 degrees C. The temperature range of approximately 306-326 degrees C is the temperature of hydrothermal fluids involved in gold mineralization based on chlorite thermometry. Kink folds and strain fringes in altered wall rocks indicate ductile deformation followed by brittle deformation in the area.