Strain fringes in Hiriyur Formation of rocks, Chitradurga greenstone belt, Karnataka: Implications with ore formation

Paramanahalli gold prospect lies in the Chitrdurga greenstone belt, India; the mineralization found in altered rocks characterized by the mineral assemblage of chamosite (Fe-rich chlorite) + annite (Fe-biotite) + ankerite + quartz + pyrite + gold ± chalcopyrite ± pyrrhotite ± magnetite. Strain fringes are developed with the pyrite and magnetite of the Banded Iron Formation and phyllite of the study area. Strain fringes are indicators of the low-pressure zones parallel to the least compressive stress. These features are massive and monomineralic adjacent to a rigid object (pyrite, magnetite) and are usually composed of a different material than the rigid object. Pyrite and magnetite are euhedral in shape and medium-coarse-grained (20-80 µm) in size. Fringes contain information on flow and style of deformation in their internal and external shape and are helpful kinematic indicators in the mineralized zone. The Hiriyur Formation rocks metamorphosed up to greenschist facies that can retain the pre-deformed shapes. This study illustrates the variety of strain fringes, studied in detail to understand deformation patterns and gold-sulfide mineralization in the research area. Fringe-1 is face-controlled, uneven, made up of fibrous quartz, and associated with rigid magnetite. The uneven nature of extended fringes infers simple shear. Fringe-2 is a face-controlled, variable extended fiber made up of quartz. Fringe-3 is a displacement-controlled fringe consisting of quartz that is similar on both sides and shows typical wing-type features. These stain fringes indicate a sense of rotation after the hydrothermal fluid precipitates ore minerals (pyrite/ magnetite). The presence of strain fringes with pyrite (± gold) infers the remobilization and recrystallization of sulfides during/after a deformation event at Paramanahalli prospect, Karnataka. The current work points toward a strong link between the development of microstructures and ore formation. Based on petrological and microstructural evidence, it is proposed that there was a syn-ore formation / main mineralization event responsible for the precipitation of sulfides, later affected with deformation that leads to the formation of such features.