The source of sulfur in sulfide deposits in the Siberian Platform traps (from isotope data)

The source of sulfur in giant Norilsk-type sulfide deposits is discussed. A review of the state of the problem and a critical analysis of existing hypotheses are made. The distribution of delta 34S in sulfides of ore occurrences and small and large deposits and in normal sedimentary, metamorphogenic, and hypogene sulfates is considered. A large number of new delta (super 34) S data for sulfides and sulfates in various deposits, volcanic and terrigenous rocks, coals, graphites, and metasomatites are presented. The main attention is focused on the objects of the Norilsk and Kureika ore districts. The delta (super 34) S value varies from -14 to +22.5 ppm in sulfides of rocks and ores and from 15.3 to 33 ppm in anhydrites. In sulfide-sulfate intergrowths and assemblages, delta (super 34) S is within 4.2-14.6 ppm in sulfides and within 15.3-21.3 ppm in anhydrites. The most isotopically heavy sulfur was found in pyrrhotite veins in basalts (delta (super 34) S=21.6 ppm), in sulfate veins cutting dolomites (delta (super 34) S=33 ppm), and in subsidence caldera sulfates in basalts (delta (super 34) S=23.2-25.2 ppm). Sulfide ores of the Tsentral'naya Shilki intrusion have a heavy sulfur isotope composition (delta (super 34) S=+17.7 ppm (n=15)). Thermobarogeochemical studies of anhydrites have revealed inclusions of different types with homogenization temperatures ranging from 685 degrees C to 80 degrees C. Metamorphogenic and hypogene anhydrites are associated with a carbonaceous substance, and hypogene anhydrites have inclusions of chloride-containing salt melts. We assume that sulfur in the trap sulfide deposits was introduced with sulfates of sedimentary rocks (delta (super 34) S=22-24 ppm). No assimilation of sulfates by basaltic melt took place. The sedimentary anhydrites were "steamed" by hydrocarbons, which led to sulfate reduction and delta (super 34) S fractionation. As a result, isotopically light sulfur accumulated in sulfides and hydrogen sulfide, isotopically heavy sulfur was removed by aqueous calcium sulfate solution, and "residual" metamorphogenic anhydrite acquired a lighter sulfur isotope composition as compared with the sedimentary one. The wide variations in delta (super 34) S in sulfides and sulfates are due to changes in the physicochemical parameters of the ore-forming system (first of all, temperature and PCH (sub 4) ) during the sulfate reduction. The regional hydrocarbon resources were sufficient for large-scale ore formation.