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

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.

Diagenesis of Hydrocarbon-bearing Concretions in South-central Puerto-rico: Tectonism and Early hydrocarbon Generation

Carbonate concretions within tuffaceous mudstones in the Upper Cretaceous Cariblanco Formation of south-central Puerto Rico that contain solid and liquid hydrocarbons were affected by: 1) Three distinct events of vein/fracture formation accompanied or followed by sediment infilling; 2) pyrite formation throughout the concretion matrix, in foraminiferal chambers, and sediment vein fills; 3) four events of larger vein and fracture formation infilled by distinct calcite cements that postdate sediment infilled veins; 4) a late quartz void filling cement; and 5) formation of calcite-filled veinlets that crosscut all components. Petrographic and isotopic data suggest early concretion formation and septarian vein fills, close to the sediment-water interface, prior to any significant dewatering of infilling sediments. The δ13C values of the matrix and sediment infills (-15 to -30‰ PDB), their brightly luminescent character, and the sequestering of Fe into pyrite indicate formation in a sulfate-reducing environment with influx of diffusing methane. Fluid inclusion data, isotopic composition of carbonate cements (13C enrichments from − 18 to -8‰ and 18O depletion from − 4 to -12‰), and organic matter maturation suggest maximum burial temperatures of 150 to 200°C. Calcite cements and microspars were formed by the circulation of progressively warmer fluids, with warming induced by the gradual emplacement of the nearby Los Panes intrusion. The intrusion probably caused intense normal faulting, induced extensive warm fluid circulation, and resulted in a high geothermal gradient responsible for early hydrocarbon generation.

Pyrite Framboid Formation Chemistry

Pyrite forms mainly through two routes: (1) the reaction between FeS species and polysulfides, and (2) the reaction of FeS species and H2S. Both of these reactions produce framboidal pyrite, and the mechanisms have been confirmed both kinetically and through the use of isotopic tracers. Aqueous Fe2+ does not appear to react directly with aqueous polysulfide species to produce pyrite, and the S-S bond in aqueous S2(-II) is normally split by aqueous Fe2+ to produce aqueous FeS and sulfur. The FeS moiety involved in pyrite formation may be provided by aqueous FeS or =FeS groups on solid surfaces. The reaction with surface =FeS occurs with any iron mineral in a sulfidic environment, including the relatively scarce iron sulfide minerals, mackinawite and greigite, nanoparticulate FeS, and pyrite itself. The reaction with surface =FeS sites on pyrite is a major route for pyrite crystal growth. The extreme insolubility of pyrite is one of the fundamental reasons for its particular involvement in framboid formation as well as for the ubiquity of framboids.

Iron Isotope Fractionation during Skarn Cu-Fe Mineralization

Fe isotopes have been applied to the petrogenesis of ore deposits. However, the behavior of iron isotopes in the mineralization of porphyry-skarn deposits is still poorly understood. In this study, we report the Fe isotopes of ore mineral separations (magnetite, pyrite, chalcopyrite and pyrrhotite) from two different skarn deposits, i.e., the Tonglvshan Cu-Fe skarn deposit developed in an oxidized hydrothermal system and the Anqing Cu skarn deposit developed in a reduced hydrothermal system. In both deposits, the Fe isotopes of calculated equilibrium fluids are lighter than those of the intrusions responsible for the skarn and porphyry mineralization, corroborating the “light-Fe fluid” hypothesis. Interestingly, chalcopyrite in the oxidized-Tonglvshan skarn deposit has lighter Fe than chalcopyrite in the reduced-Anqing skarn deposit, which is best understood as the result of the prior precipitation of magnetite (heavy Fe) from the ore fluid in the oxidized-Tonglvshan systems and the prior precipitation of pyrrhotite (light Fe) from the ore fluid in the reduced-Anqing system. The δ56Fe for pyrite shows an inverse correlation with δ56Fe of magnetite in the Tonglvshan. In both deposits, the Fe isotope fractionation between chalcopyrite and pyrite is offset from the equilibrium line at 350 °C and lies between the FeS-chalcopyrite equilibrium line and pyrite-chalcopyrite equilibrium line at 350 °C. These observations are consistent with the FeS pathway towards pyrite formation. That is, Fe isotopes fractionation during pyrite formation depends on a path from the initial FeS-fluid equilibrium towards the pyrite-fluid equilibrium due to the increasing extent of Fe isotopic exchange with fluids. This finding, together with the data from other deposits, allows us to propose that the pathway effect of pyrite formation in the Porphyry-skarn deposit mineralization is the dominant mechanism that controls Fe isotope characteristics.

Isotopically “heavy” pyrite in marine sediments due to high sedimentation rates and non-steady-state deposition

Sedimentary pyrite formation links the global biogeochemical cycles of carbon, sulfur, and iron, which, in turn, modulate the redox state of the planet’s surficial environment over geological time scales. Accordingly, the sulfur isotopic composition (δ34S) of pyrite has been widely employed as a geochemical tool to probe the evolution of ocean chemistry. Characteristics of the depositional environment and post-depositional processes, however, can modify the δ34S signal that is captured in sedimentary pyrite and ultimately preserved in the geological record. Exploring sulfur and iron diagenesis within the Bornholm Basin, Baltic Sea, we find that higher sedimentation rates limit the near-surface sulfidization of reactive iron, facilitating its burial and hence the subsurface availability of reactive iron for continued and progressively more 34S-enriched sediment-hosted pyrite formation (δ34S ≈ –5‰). Using a diagenetic model, we show that the amount of pyrite formed at the sediment-water interface has increased over the past few centuries in response to expansion of water-column hypoxia, which also impacts the sulfur isotopic signature of pyrite at depth. This contribution highlights the critical role of reactive iron in pyrite formation and questions to what degree pyrite δ34S values truly reflect past global ocean chemistry and biogeochemical processes. This work strengthens our ability to extract local paleoenvironmental information from pyrite δ34S signatures.