Abstract
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.
Highlights
Sulfate ( SO24− ) in modern seawater is 0.2% by weight, and is second only to chloride (Cl-) in concentration
A scenario supporting the conclusion reached in Zhou et al.[2] and Peng et al.[1] would, predict that the basal Doushantuo pyrite nodules were formed in pore fluids after the deposition and disruption of the cap dolostones
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
Summary
Sulfate ( SO24− ) in modern seawater is 0.2% by weight, and is second only to chloride (Cl-) in concentration. 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] 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 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. 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
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