Sulphur isotopic characteristics of pyrite and galena from the Singhora Group, Chhattisgarh Supergroup, India: genetic implications

Rocks of Singhora Group, Chhattisgarh Supergroup, deposited in a protobasin, contain pyrite and galena near Juba and Banjhapalli villages of Raipur District, Madhya Pradesh. Veins of fluorite, quartz and quartzo-feldspathic material reported along the eastern and western margins of the protobasin contain galena and pyrite in the sedimentary rocks. Pyrites are developed diagenetically, epigenetically and by remobilization in Rehatikhol Formation of Singhora Group, while only epigenetic pyrite occurs in the basement. High positive δ34S values (+31.58%δ34S for 5 samples) for diagenetic pyrite indicate reaction between iron-bearing minerals and H2S and/or HS- generated by reduction of seawater sulphate. High δ34S value (+23.35%) for galena is due to reaction between diagenetic pyrite and base metal-bearing fluids at elevated temperatures. The galena-pyrite pair developed epigenetically shows 135 deg C temperature for their formation at near equilibrium conditions. Galena grains collected from quartzo-feldspathic/quartz veins from the eastern margin of Singhora protobasin near Chiwarkuta village yielded δ34S value of +11.1 %, while pyrite from quartz veins of Birsinghpali-Kapudih tract of western margin has given δ34S value of +3.3%. These values suggest an igneous source for the fluids, probably from a magma that assimilated sulphur from the country rocks. High δ34S value (+27%) of pyrite in black shale of Saraipalli Formation explains its deposition below mud-water interface. Quartz arenite of Bhalucona Formation overlying the Saraipalli shales contains pyrite concretions having a δ34S value of +23.7%. δ34S value ( +3.4%) of pyrites from migmatite and quartz veins intruding the crystallines of eastern margin i.e. Sambalpur Granitoid that forms the basement to Chhattisgarh Supergroup, also shows magmatic source of sulphur that could have been generated from the upper mantle or homogenized crust. (author)