LAUSR

Abstract The present study combines major and trace element composition, and sulfur (S) isotope data of pyrite and chalcopyrite from the Surda copper sulfide deposit in the Singhbhum Shear Zone, the most important copper and uranium producing belt (Singhbhum Cu-U Belt) in India. Three textural and compositional types of both pyrite and chalcopyrite were distinguished; unzoned to partially zoned Pyrite IA with high Co (up to 54900 ppm) and low Ni content is earliest, followed by oscillatory zoned Pyrite IB with high As (up to 25600 ppm) and Co (up to 46800 ppm), both occurring in pyrite I + chalcopyrite I + pyrrhotite + magnetite + apatite vein; Cobaltite-type substitution (Fe1−xCox)(S1−xAsx)2 is suggested for Pyrite IB. Gold occurs in Pyrite I as minor “invisible” gold and as electrum inclusions. It also occurs along with Cu, Mn, Ni, Hg, Ag, Pb, Sb, Zn, Ce, Y, U, and Th in micro-fractures that transgress the primary zoning pattern defined by As, Co, and Ni in Pyrite IB. The early inclusion-rich chalcopyrite generation (Chalcopyrite I) contains high concentration of Zn and Se, and minor to trace amounts of Co, Ni, Hg, Pb, Sb, Te, and Bi, appeared in between Pyrite I and II. Low Co, high Ni (up to 37700 ppm) content Pyrite II, and inclusion-free Chalcopyrite II enriched in Co, Ni, Hg, Bi, Mn, Ag, Sb, V, and Pb are cogenetic, and occurring in pyrite II + pyrrhotite + pentlandite + chalcopyrite II ± violarite vein. Low Co and Ni containing Pyrite III + Chalcopyrite III occur mainly as disseminated grains. The relative timing of formation of Pyrite II + Chalcopyrite II with Pyrite III + Chalcopyrite III remains uncertain. Pyrite + chalcopyrite textures indicate that all pyrite + chalcopyrite formed at some time prior to the end of deformation and metamorphism. Both in situ and mineral separates of all pyrite types and associated chalcopyrite yield a narrow range of positive δ34S values (between +3.8 and  +6.9‰) suggesting sulfur being derived from a similar source. Consistent positive δ34S values and other circumstantial evidence indicate that most sulfur was derived from seawater sulfate or modified seawater (brine/evaporite). Δ33S values revealed mass dependent fractionation (MDF) signature. It is proposed that incorporation of MDF sulfur of the mineralization event in Paleoproterozoic Singhbhum Cu-U Belt took place after the great oxidation event. The high Se concentrations (260–400 ppm) and ∑Se/∑S ratios for both Pyrite I and II from Surda deposit (4.4–5.7 × 10−4) suggest a low temperature of the Cu–rich ores (250°–350 °C), and precipitation from a metalliferous fluid with a high ∑Se/∑S ratio (10−4 to 10−3) consistent with igneous input of these elements.