Constraints on the genesis of the Proterozoic bornite dominated copper deposit from Nim ka Thana, western India: An IOCG perspective

Abstract The copper mineralization at the southeastern fringe of the Khetri copper belt in western India is primarily hosted within the metapelites and dolomite units of the Mesoproterozoic Ajabgarh Group of rocks. The copper mineralization is well exposed along a NE-SW trending basinal part around Nim ka Thana area where a few of the prospects namely Dokan, Baniwala-ki-Dhani, Dariba, and Nanagwas primarily consist of abundant bornite, chalcopyrite, covellite, digenite, and chalcocite as disseminated phases hosted within a wide-ranging litho units and along quartz-calcite-barite veins. An integrated approach has been adopted to understand the metallogenetic evolution of this bornite dominated copper mineralization. δ13CV-PDB and δ18OV-SMOW results of the carbonate veins ranging from 0.57‰ to -3.75‰ and δ18O range from 15.01‰ to 20.74‰ respectively suggest the influx of hydrothermal fluid in shaping up the deposit. δ13C (CO2) of the ore-bearing fluid ranges from -1.67‰ to 2.65‰ and δ18O (H2O) from 7.11‰ to 12.84‰ respectively also suggest the input of hydrothermal source. The S-isotopic values of Cu-mineralized barite samples range between 16‰ and 20‰, which are lower than that of seawater (δ34S‰= 20–22‰). This provides the evidence that the barite mineralization could have formed from the hydrothermal fluid that is cogenetic with the late-stage mineralization in a phase-wise separation as an associated mineral assemblage in the system progressively enriching the bornite-rich copper deposit. Based on the field data and evidences, petrographic characteristics, mineral chemistry, fluid inclusion, and stable isotopic signatures, a three-stage mineralization event is proposed as 1) syngenetic precipitation of dispersed bornite-pyrite in a euxinic environment; 2) hydrothermal vein-type mineralization dominated by chalcopyrite-bornite phase, and 3) supergene enrichment of the vein filled mineralization with the development of wide-scale chalcocite-digenetite-covellite mineral phases. Fluid inclusion microthermometry suggests a temperature range from 225o-320°C with moderate salinity fluid representing epithermal mineralization which is coherent with the stable isotope data (C, O and S isotopes) of the mineralized carbonate and barite veins suggesting the involvement of large scale hydrothermal fluid influx. Te content of various Cu-bearing sulfide phases shows a gradual change in the concentration from chalcopyrite (2.21% to 2.31%), bornite (2.31% to 4.50%), chalcocite (2.77% to 2.87%), digenite (2.57% to 3.70%) and covellite (2.53% to 3.13%) suggesting their evolution from hydrothermal to supergene process. The studied area has experienced wide-scale alterations and Na-metasomatism with the development of associated minerals such as scapolite, tremolite, and sericite. Occurrence of primary and secondary copper sulfides in the metasedimentary sequence, alteration assemblages affected by Na-metasomatism, association of magnetite and specularite in the study area is affiliated to the hydrothermal system akin to IOCG clan as noted from the adjoining Khetri Copper Deposit.