2000-MA SULPHIDE CONCRETIONS FROM THE 'PRODUCTIVE' FORMATION OF THE PECHENGA GREENSTONE BELT, NW RUSSIA: GENETIC HISTORY BASED ON MORPHOLOGICAL AND ISOTOPIC EVIDENCE

Abstract

Two hundred and fifty-nine sulphide samples from black shales of the 'Productive' 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/C-org ratios and a strong positive correlation between C-org 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 delta(34)S from -8.3 to +24.9 parts per thousand indicates Delta(34)S(sulphate-sulphide) between +29 and -4 parts per thousand, given a seawater sulphate at around 21 parts per thousand. The first-order trend is a progressive enrichment in S 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 delta(34)S range of -2.7 to +3.8 parts per thousand. This is coupled with extensive formation of diagenetic carbonate beds marked by delta(13)C of -8.7 to -12.5 parts per thousand. 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 delta(34)S from -8.3 up to +24.9 parts per thousand. The high Fe2+/S2- and possibly change in pH parameter resulted in pyrrhotite rather than in pyrite. Associated abundant carbonate nodules have delta(13)C in the range -8.2 to -14.1 parts per thousand. Overall low Delta(34)S(sulphate- sulphide) and the high S/C-org ratio are consistent with one- stage bacterial sulphate reduction in a non-euxinic basin. The observed Delta(34)S(sulphate-sulphide) of 29 parts per thousand (as maximum) is within the 'pure sulphate-reducing bacteria' range of 4-46 parts per thousand. 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 delta(34)S from -2.5 to +0.3 parts per thousand) which might have inorganic origin and formed from an exhalative source.