Sulfur isotopic trends and iron speciation from the c. 2.0 Ga Pilgujärvi Sedimentary Formation, NW Russia

Abstract

Sulfide sulfur isotope values and iron speciation data are reported here for sediments from the 2.0 Ga old Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, NW Russia. Sulfides are abundant throughout the c. 1400m thick turbiditic sandstone-siltstone-shale succession and present as pyrite and pyrrhotite. A stratigraphic trend is discernible with respect to sulfide mineralogy and sulfur isotopic compositions, changing up-section from highly to less variable and positive δ34S values. The occurrence of pyrrhotite, particularly high in the upper part of the section, is thought to be the result of decomposition of pyrite during low-grade metamorphism. This is evidenced by the crystallographic habit and sulfur isotopic composition of the different sulfide minerals. The δ34S of both sulfur species is quite variable in the lower part of the formation (−9.5% to 18.7%) while the upper part displays a smaller fluctuation in δ34S (4.4%–13.3%). The pyrrhotite is generally depleted in 34S by approximately 3.6% (average isotopic difference between pyrrhotite and pyrite) compared to the pyrite; this shift in the sulfur isotopic composition results from thermal decomposition of pyrite.The overall spread in δ34S is indicative for the microbial reduction of sulfate, but δ34S values suggest a sulfate limited environment in the upper part of the formation. We note, however, that the sulfate limitation is restricted to the pore water realm, caused by high sedimentation rates that inhibited a sufficiently high sulfate flux from the overlying water column into the sediment.Based on iron speciation data, a previous characterization of the depositional environment as non-euxinic can be specified more precisely. We note that the siliciclastic succession of the Pilgujärvi Sedimentary Formation was deposited under anoxic and ferruginous water column conditions derived from high ratios of FeHR/FeT and low ratios of Fepy/FeHR.