Invisible gold occurrence within the quartz reef pyrite of Babaikundi area, North Singhbhum fold-and-thrust belt, Eastern Indian Shield: Evidence from petrographic, SEM and EPMA studies

• Gold occurrence in the area is confined within the quartz reef of Chandil Formation, eastern Indian shield.
• EPMA and SEM analysis of gold-bearing reef quartz were carried out to decipher the mode of gold occurrence.
• Gold mineralization has been reported as disseminated grains within quartz reef and as invisible gold within the pyrite.
• Gold within pyrite crystal lattice is attributed to the substitution and replacement of Fe aided by hydrothermal activity.

The Mesoproterozoic metasedimentary rocks of Chandil Formation belong to the northern part of Singhbhum Crustal Province, eastern India. The Chandil Formation lies within the North Singhbhum fold-and-thrust belt and is demarcated at its southern margin by the Dalma volcano-sedimentary belt. Tamar-Porapahar Shear Zone (TPSZ) separates the Chandil Formation from the Meso/Neoproterozoic Chhotanagpur Granite Gneissic Complex (CGGC) at its northern boundary. The area is characterized by quartz-mica-sericite-schist, quartzite, amphibolites and carbonaceous phyllites and is ubiquitously traversed by mineralized quartz reefs/veins. The field relationships and the petrographic characteristics of the lithounits of Chandil Formation indicate evidence of multi-phase deformations, exhibited by the presence of S–C fabric, S1–S2 planes of schistosity, alternate layers of coarse and thin quartz veins. The gold mineralization in the study area occurs along the east–west oriented Babaikundi-Birgaon axis, which has been established as a sheared/fractured contact running parallel to the Tamar Porapahar Shear Zone lying to the north. Gravity, magnetic, resistivity and induced polarization (IP) survey across the Babaikundi-Birgaon axis indicates the presence of sheared/fractured contact and sulfide mineralization. The gold mineralization within the area is confined to the quartz reef along the contact of mica schist and ambhibolites. Ore minerals observed in the area are chalcopyrite, pyrite, pyrrhotite, arsenopyrite and sphalerite. The majority of the sulfide mineralization is associated with the ferruginous quartz veins. Gold occurs in two distinct forms, as lodes/nuggets and as invisible gold within the pyrite mineralization.Field observations, reflected and transmitted light microscopic study, SEM analysis and electron probe micro analyzer (EPMA) were used to give a brief account of the mineral chemistry and presence of different forms of gold (including the first report of invisible gold within North Singhbhum fold-and-thrust Belt). The occurrence of gold in the crystal lattice of the pyrite may be attributed to the substitution and replacement of Fe ion within the pyrite aided by the hydrothermal activity.

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