2000-Ma sulphide concretions from the `Productiveʹ Formation of the Pechenga Greenstone Belt, NW Russia: genetic history based on morphological and isotopic evidence

Two hundred and fifty-nine sulphide samples from black shales of the `Productiveʹ Formation belonging to the Palaeoproterozoic (2000 Ma) Petsamo Supergroup in the Kola region (NW Russia) have been analysed for their sulphur isotope ratio. The sulphide-bearing black shales were formed in a non-euxinic basin within an oceanic rift environment. The rocks have undergone greenschist facies metamorphism. The black shales are marked by very high S/Corg ratios and a strong positive correlation between Corg and S (r=0.70) with zero intercept on the sulphur axis. Different generations of sulphide concretions have been distinguished by means of concretional analysis. Individual isotope study of synsedimentary sulphide layers and sulphides from early, middle and late diagenetic–catagenetic sulphide concretions yields trends which suggest bacterial sulphate reduction as the concretion-forming process. The overall spread of δ34S from −8.3 to +24.9‰ indicates Δ34Ssulphate–sulphide between +29 and −4‰, given a seawater sulphate at around 21‰. The first-order trend is a progressive enrichment in 34S with increase of burial. Two major populations of sulphides have been distinguished. The first (concretions and perhaps thin layers) formed in an open system with respect to seawater sulphate and has a limited δ34S range of −2.7 to +3.8‰. This is coupled with extensive formation of diagenetic carbonate beds marked by δ13C of −8.7 to −12.5‰. The second population (lenses, thin layers, concretions, and microconcretions in carbonate nodules) formed in a closed system and is characterized by a high Fe2+/S2− ratio and progressive development of a Rayleigh distillation process which drove δ34S from −8.3 up to +24.9‰. The high Fe2+/S2− and possibly change in pH parameter resulted in pyrrhotite rather than in pyrite. Associated abundant carbonate nodules have δ13C in the range −8.2 to −14.1‰. Overall low Δ34Ssulphate–sulphide and the high S/Corg ratio are consistent with one-stage bacterial sulphate reduction in a non-euxinic basin. The observed Δ34Ssulphate–sulphide of 29‰ (as maximum) is within the `pure sulphate-reducing bacteriaʹ range of 4–46‰. Advanced isotopic modification involving the oxidative part of the sulphur cycle has not been detected. Based on the data available one can not exclude the presence of a third population of sulphides (thin massive layers with δ34S from −2.5 to +0.3‰) which might have inorganic origin and formed from an exhalative source.