Nature of Gold mineralisation at the Kundarkocha gold deposit, Singhbhum Craton, eastern India

Abstract

The Kundarkocha mine is a rare example of a Paleoarchean greenstone hosted gold deposit. The deposit is situated in the ~3.25 Ga Gorumahisani-Badampahar greenstone belt in Singhbhum Craton of eastern India. Petrographical study reveals three generations of pyrite and associated native gold grains.Native gold grains are encountered in the smoky quartz veins emplaced parallel to the foliation planes within the host rock. The veins have been sheared and shows characteristic brittle-ductile nature of orogenic gold deposits around the world. The secondary native gold occurrence shows close spatial association to the pyrites. These pyrites have been folded and fratured and occur mostly along the hinges of the folds. The pyrite associated native gold grains are encountered within fractures in the late hydrothermal Pyrite 3(a) occurring as euhedral overgrowth on metamorphic Pyrite-2 and some gold grains shows close spatial association with the pyrite-3(b) that indicated mobilisation of gold from these pyrites and subsequent reprecipitation along the structurally favourable sites along the hinge of the fold during subsequent deformation. Gold is closely associated with the carbonaceous phyllites, graphitic schists and carbonaceous chert with carbon acting as potential trap for the auriferous hydrothermal fluids to precipitate gold. From the above evidence and well established tectono-thermal events in the GBG belt we can deduce the given steps for the formation of gold deposit in Kundarkocha based on petrographic, SEM/EDS, EPMA analysis and X-ray elemental mapping:- 1) Prograde metamorphism of diagenetic pyrite during D1 deformation. 2) D2 deformation and formation of shear zones of brittle ductile nature during retrograde metamorphism led to the emplacement of gold bearing smoky quartz veins along the axial planar foliations S2. The pyrites in this whole process get metamorphosed deformed in the brittle-ductile manner leads to the expulsion of lattice-bound gold and further deformation and hydrothermal overprints led to the remobilisation and reprecipitation of native gold grains closer to the pyrite in structurally favourable sites like fold hinges. These gold occurrences are closely associated with the deformation and metamorphism of in-situ diagenetic pyrite and is volumetrically less significant compared to the quartz-vein hosted gold occurrences which occur in later stage.