Minor- and trace-element characteristics of different pyrite types and their S-isotope geochemistry from the Proterozoic Rakha Copper deposit, Singhbhum Shear Zone, India

Sulfide mineralization associated with rocks from the Palaeoproterozoic Singhum Copper deposit in the Rakha area has undergone a complex history of formation, deformation, metamorphism, and remobilization. In this study, new trace element and sulfur isotope analyses of pyrites are used to interpret the sequence of mineralizing events linked to the chemical evolution of sulfide mineralization episodes with time. Pyrite occurs as both disseminated grains and veins in the ore zone. Veins, both concordant and discordant to the pervasive foliation, are mineralogically simple comprising pyrite +/- chalcopyrite or complex comprising pyrite + chalcopyrite + pentlandite + pyrrhotite. Nickel sulfides, though fairly common in concordant veins, are also present in the discordant veins. Based on concentrations of Co, Ni, and As, pyrite has been geochemically classified into: type-I - high Co (up to 73000 ppm) and As (up to 66100 ppm), and no/low Ni; type-II - moderate to low Co (up to 16500 ppm), high Ni (up to 37700 ppm, and no As); type-III - devoid (below detection limit) of trace elements. Electron probe micro-analyses (EPMA) and elemental maps reveal that Pyrite I has complex chemical zoning. The data suggest initial precipitation of Co-, As-rich and low Ni Pyrite I followed by a marked decrease in the As activities in the ore-bearing fluids. This is then followed by precipitation of Ni-rich pyrite. The Co and As contents of Pyrite I decrease in an oscillatory manner from the core to the rim, reflecting changes in the As and Co activity. The integrated textural and compositional data suggest that the hydrothermal fluid responsible for pre-/early-shearing mineralization evolved from Co-As-rich to pre/post shearing Ni-rich mineralization, and the post deformation fluid was depleted in trace elements. The moderately heavy delta S-34 compositions (+5.4 to +9.6 parts per thousand) determined for pyrite from the Rakha deposit and available fluid inclusion data from other deposits within the SSZ point to a high temperature, high salinity modified seawater, along with a magmatic contribution, as the most probable sources for the ore-forming fluids. In the later stage, lower T meteoric water interacted with the underlying volcanic (mafic)/sedimentary rocks.