Origin of pyrite nodules at the top of the nantuo diamictites, Southern China

www.nature.com/scientificreports OPEN Origin of pyrite nodules at the top of the nantuo diamictites, Southern China Changjie Liu1,2* & Ying Lin3 Pyrite nodules up to 20 cm in diameter are found at the top of the Marinoan (~ 635 Ma) Nantuo glacial diamictite as well as in the cap dolostones and shale/siltstones in the lower Doushantuo Formation in eastern Guizhou, southern China. Field occurrences, petrography, and stable sulfur isotopic compositions of pyrite nodules were studied from a section at Taoying, eastern Guizhou, China. Pyrite δ34S values from different nodules varied from 7.3 to 60.5‰ at different stratigraphic levels. No stratigraphic trend existed for the δ34S, supporting the scenario of pyrite formation in sediments before the precipitation of the cap dolostone. Pyrite δ34S values were also homogeneous within individual nodules at a 0.3 to 1 cm sampling scale, but were more heterogeneous at a 2 mm sampling scale. Homogeneity was not expected from the particular model for pyrite nodule formation in a largely closed or semi-closed environment. Thus, differential cementation and compaction of the pyrite-bearing sediments may have produced the nodular shape of the pyrite deposit. Sulfate (SO2− 4 ) in modern seawater is 0.2% by weight, and is second only to chloride (Cl ) in concentration. Seawater sulfate concentration has varied over geological history. While periods of dramatic changes did occur, seawater sulfate concentration has generally increased over time. One of the extreme shifts in sulfate concentration was expected to have occurred at the aftermath of Marinoan global glaciations at ~ 635 Ma. Sulfate concentration is believed to be exceedingly low at the onset of deglaciation in the oceans. Peng et al.1 studied the occurrence of non-mass-dependently 17O depleted barite deposits in cap carbonates that drape the Nantuo diamictite, South China Block. They concluded that sulfate concentration in seawater was low or nearly absent during the deposition of the Doushantuo cap carbonates and the sulfate concentration in the oceans only rose after the deposition of cap dolostones, as evident from the first barite crystal fans being precipitated only at the top of reworked cap dolostones. Initially, shallow ocean sulfate had a significant riverine sulfate component, as supported by distinct negative Δ17O values (a measure of the δ17O deviation from what is expected from a massdependent relationship between the δ17O and δ18O) in these barite sulfates. The barium was supplied episodically to shallow oceans through the upwelling of deep B a2+-rich water. This conclusion is echoed by the sequence of events occurring at the aftermath of Marinoan meltdown in the entire South China B lock2. In many shallow platform, shelf, and basinal facies of the South China Block, pyrite nodules of different sizes (up to 20 cm in diameter) occur at the top 0 to 2 m of the Nantuo diamictite, and occasionally within the cap dolostone of the basal Doushantuo Formation. Pyrite is usually precipitated through the reaction of dissolved sulfide produced by microbial sulfate reduction with F e2+ derived from detrital iron-bearing minerals in anoxic marine sediments3,4. Pyrite precipitation can occur diagenetically in shallow sediments where both organic matter and sulfate are present in pore fluids, so that microbial sulfate reduction can produce sulfides ( HS– and H2S) to be precipitated as insoluble FeS. The initial FeS is later transformed to the more stable mineral pyrite ( FeS2), the common sulfide minerals seen in the rock record5,6. Pyrite can also form in the water column. In a euxinic water column, dissolved sulfide reacts with free F e2+ to form small FeS aggregates. Once the aggregates are larger than a critical size, they settle to bottom of the water column and are later transformed to p yrite7,8. A scenario supporting the conclusion reached in Zhou et al.2 and Peng et al.1 would, therefore, predict that the basal Doushantuo pyrite nodules were formed in pore fluids after the deposition and disruption of the cap dolostones. By then, the ocean sulfate concentration had risen to a level that enough of it could diffuse into the pore fluids within the underlying sediments. Considering that the source of sulfate would be exclusively derived from the water column after the deposition of the cap dolostones and the Nantuo diamictite, this scenario predicts that the pyrite δ34S value would increase with depth, starting at the top of the cap dolostone. 1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA. 2Department of Geosciences, Texas Tech University, Lubbock, TX, USA. 3Department of Earth Sciences, University of California at Riverside, Riverside, CA, USA. *email: Scientific Reports | (2021) 11:18696 | https://doi.org/10.1038/s41598-021-97022-y 1 Vol.:(0123456789) www.nature.com/scientificreports/ Another possible scenario is that widespread pyrite formation in sediments could occur before the precipitation of the cap dolostone, either via direct precipitation of pyrite from a euxinic water column/in sediments or through in-sediment sulfate reduction. This scenario needs high enough seawater sulfate concentration or a euxinic water column before the deposition of cap dolostone. This scenario predicts that the many horizons of pyrite nodules at the top of the Nantuo diamictite may have large variability in their δ34S values and that the variation should have no relationship with depth. Lang et al.9 studied the pyrite concretions in the topmost Nantuo Formation in South China. Their results on sedimentary faces and sulfur isotope compositions of pyrite concretions indicated that pyrite precipitate in the sediments with H2S diffusing from the euxinic seawater. Lang et al.9 excluded the first scenario by the petrology of pyrite concretions (pyrite crystals are tightly packed with clasts or cemented in a siliciclastic matrix, and diamictite also contains disseminated pyrite) and the possibility of direct precipitation of pyrite from a euxinic seawater by pyrite morphology (Nantuo pyrite is euhedral instead of framboidal). This study evaluates these scenarios to explain the occurrence of the nodules in the South China Block. Although pyrite nodules have been observed in many facies in the Marinoan South China, we focused my study on samples from a well-exposed field section in Taoying, Tongling, eastern Guizhou (109°1′4.9"E, 27°50′1.4"N; Fig. 1). In summary, we examined the field occurrences, petrographic features, and stable sulfur isotope compositions (the δ34S) of pyrite nodules, together with a few pyrite lenses and beddings in the overlying Doushantuo shale and siltstones for comparison. Materials and methods Field occurrence of pyrite nodules. The Doushantuo Formation in the South China Block directly overlies the Nantuo glacial diamictite and consists of as much as 250 m of carbonates, siltstones, and s hale10–13. In a well-e (...truncated)