The formation of pyrite has been extensively studied because of its abundance in many anoxic environments such as marine and river sediments, groundwater aquifers, and peat lands, and hence its importance in both iron and sulfur cycling. It forms over a wide pH interval, ranging from acidic to alkaline. It is generally regarded that sulfide reacting with iron-containing minerals forms metastable iron sulfide minerals before eventually transforming into pyrite in the presence of different sulfur.

In this contribution, I will discuss the importance of sulfidation of ferric (oxy)hydroxides (FeOOH), i.e. the reaction between aqueous sulfide and the surface of FeOOH, to stimulate pyrite formation and compare this process with other pathways and kinetics of pyrite formation described in the literature. Sulfidation of FeOOH initially leads to formation of surface bound FeS-species and its transformation to pyrite is controlled by either the availability of FeOOH or the supply rate of sulfide. These kinetic constraints define the environments were rapid pyrite formation occurs to be suboxic, rich in FeOOH and shaped by cryptic sulfur cycling. Under these conditions, highly reactive pyrite precursor species are forming that also affect trace metal cycling.

How to cite: Peiffer, S.: Controls on recent pyrite formation in aquatic systems and its relevance for environmental processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21260, https://doi.org/10.5194/egusphere-egu24-21260, 2024.

During times of low sea-levels in glacials there is evidence of large-scale pyrite oxidation on exposed continental shelves. While this process directly reduces atmospheric oxygen levels, acid drainage generated by this reaction increases the release of CO₂ through carbonate buffering within the previously marine shelf sediments. Although this scenario is expected to result in negative feedback, sea-level and atmospheric CO₂ levels have co-varied throughout most of the last 800 thousand years (ka) for which direct records of CO₂ exist. Only during peak glacial conditions with sea-levels as low as 125 m lower than today, CO₂ levels have reached an apparent lower limit around 190 ppm independent of decreasing sea-levels. Here we show that pyrite driven release of CO₂ and decline of O₂ during six of the last nine glacial-interglacial cycles are focussed in 10 ka to 40 ka-long periods preceding glacial terminations.

Using a sea-level driven model of pyrite weathering in drained continental shelves, we demonstrate that repeated sea-level low-stands force pyrite oxidation to ever greater depths. This occurs whenever the duration of an interglacial is insufficient to restock the shelf pyrite inventory through sulphate reduction in the shelf sediments. During the Quaternary, the decreasing amount of pyrite in the exposed shelf sediments represents a discharging 'acid capacitor' (Kölling et al., 2019). This model was inspired by experience from modelling pyrite weathering in open-pit lignite mine overburden material which may be interpreted as a scaled-down model of glacial continental shelf exposure during sea-level low-stands.

If pyrite oxidation forced CO₂ release specifically at low sea-levels was sufficient to amplify the orbitally driven climate forcing and trigger glacial terminations, the absence of CO₂ release caused by exposed pyrite rather than an astronomically controlled '100 ka pacing' might have extended the length of glacial-interglacial cycles from one to two or three obliquity cycles. Future ocean drilling specifically aiming to recover long cores on shelves could reveal the existence of a 'pyrite gap' that should exist between surficial young pyritic layers and deeper old pyritic sequences with indications of acid leaching.

Kölling, M., Bouimetarhan, I., Bowles, M.W. et al. Consistent CO₂ release by pyrite oxidation on continental shelves prior to glacial terminations. Nat. Geosci. 12, 929–934 (2019). https://doi.org/10.1038/s41561-019-0465-9

How to cite: Kölling, M., Bouimetarhan, I., and Zabel, M.: The shelf life of pyrite - effects of pyrite weathering in exposed continental shelves on global CO2 and O2, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21083, https://doi.org/10.5194/egusphere-egu24-21083, 2024.

Microbial sulfate reduction (MSR) and subsequent pyrite burial in marine sediments plays a crucial role in Earth’s long-term carbon and oxygen budgets; by reducing sulfate and producing alkalinity, this process effectively increases atmospheric O₂ and lowers CO₂ levels. Given that MSR exhibits a large and reduction-rate-dependent sulfur-isotope fractionation, changes in pyrite sulfur-isotope compositions (δ³⁴S values) through geologic time have long been interpreted to reflect global signals such as marine sulfate reduction rates, microbial community behavior, and the amount of sulfur buried as pyrite vs. evaporite minerals. However, recent research has demonstrated that marine MSR fractionation likely operates near an equilibrium limit regardless of reduction rate. These studies instead implicate local environmental and sedimentological factors as drivers of pyrite δ³⁴S values. Despite this advancement, the sensitivity of pyrite formation rate and δ³⁴S value to changes in environmental and sedimentological variables—both today and through geologic time—remains unconstrained due to the complex interactions between controlling variables.

To provide mechanistic and quantitative constraints, we developed and applied a non-dimensional diagenetic model that extracts the natural variables governing pyrite formation. Assuming equilibrium MSR isotope fractionation and using only locally measured or globally interpolated boundary values as inputs (i.e., no free parameters), our model accurately predicts all available modern observations (n = 216 cores) with an average root-mean square error of 0.3 wt % for pyrite content and 16.5 ‰ for δ³⁴S. Extrapolating this result, we estimate global pyrite burial flux to be ~1.3 × 10¹² mol FeS₂ yr⁻¹ with a weighted-average δ³⁴S value of ~-21 ‰ VCDT.This flux is statistically identical to independent estimates of total riverine sulfate input (i.e., pyrite-oxidation and evaporite-dissolution derived), indicating the sulfur cycle currently operates in steady state. However, calculated pyrite burial exceeds pyrite-oxidation derived inputs, suggesting net atmospheric O₂ release and CO₂ consumption by the sulfur cycle.

Mechanistically, we conclude that pyrite formation rate is highly sensitive to local reactive iron input, whereas δ³⁴S value is primarily controlled by organic carbon reactivity-to-sedimentation rate ratio (termed Da*, a modified Damköhler number) and organic carbon-to-sulfate ratio (termed Γ₀). In contrast to previous models, we show that pyrite δ³⁴S is largely insensitive to bioturbation due to counter-balancing impacts on Da* and Γ₀. Rather, when combined with a geologic pyrite δ³⁴S record, our interpretation requires an increase in Da* and decrease in Γ₀ since the Paleozoic, possibly driven by changing organic matter reactivity and sulfate concentration through geologic time.

How to cite: Hemingway, J., Mertens, C., and Paradis, S.: Quantifying key drivers of marine pyrite content and isotopic composition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9060, https://doi.org/10.5194/egusphere-egu24-9060, 2024.

Seafloor massive sulfides represent modern analogues to ancient volcanogenic massive sulfide deposits, which can be particularly enriched in volatile and precious metals (e.g., Te, Au, Ag, Cu, Bi, Se) in subduction-related systems. However, it remains unclear whether the influx of magmatic volatiles has a systematic control on the metal endowment of submarine hydrothermal mineralization on the plate-tectonic scale. Using a novel microanalytical approach based on the coupling of SIMS δ³⁴S with trace element LA-ICP-MS on a scale of ~25 µm in pyrite from 11 submarine hydrothermal systems, we could demonstrate for the first time that the Te, As, and Sb contents and the ratios of these elements vary systematically with the δ³⁴S composition of hydrothermal pyrite and native S. In contrast to trace element concentrations, Te/As and Te/Sb show a more significant correlation with δ³⁴S in pyrite, indicating that element ratios provide a more robust record of metal sourcing. On this basis, we define a quantitative trace element threshold of high Te/As (>0.004) and Te/Sb (>0.6) ratios in pyrite that can be used to identify the influx of magmatic volatiles to submarine subduction-related hydrothermal systems independent of δ³⁴S isotope measurements. Two-component fluid mixing simulations further suggest that even small amounts (<0.5 to ~5%) of magmatic volatile influx drastically change the Te/As (and Te/Sb) ratio of the modelled fluid, but only slightly modify its δ³⁴S composition. Hence, Te/As and Te/Sb ratios are more sensitive in recording the influx of magmatic volatiles into submarine hydrothermal systems than S isotope systematics, which are typically influenced by seawater-derived S leading to ambiguous δ³⁴S signatures. We conclude that Te/As and Te/Sb systematics in pyrite provide a robust proxy to evaluate the metal sources in submarine hydrothermal systems from the grain to plate-tectonic scale.

How to cite: Falkenberg, J. J., Keith, M., Haase, K. M., Klemd, R., Kutzschbach, M., Grosche, A., Scicchitano, M. R., Strauss, H., and Kim, J.: Pyrite trace element proxies for magmatic volatile influx in submarine subduction-related hydrothermal systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15524, https://doi.org/10.5194/egusphere-egu24-15524, 2024.

It is vital to keep up with the demand for critical minerals during the transition to sustainable energy systems. Doing so requires expanding our knowledge of ore depositional processes. Pyrite is a common gangue mineral in clastic dominated (CD) deposits, which are the highest value Zn deposits. In CD-type deposits, pyrite is often part of the pre-, syn-, and post-ore paragenesis and can therefore provide an important redox buffer and potentially a source of sulfur during ore deposition. The distribution of syn-ore pyrite beyond economic mineralization can also form an important mineralogical halo around CD-type deposits.

In this study we investigate the role of fluid interaction with different types of pyrite on ore formation in the Teena deposit (Australia). The host unit for the Teena deposit is an organic rich, variably pyritic, dolomitic siltstone. We created a series of 2D, reactive transport models using the software X2t (Geochemists Workbench) to investigate the role of pyrite surface area as a major control on ore deposition.

Similar to many CD-type deposits, the main type of pre-ore diagenetic pyrite in the host unit is framboidal, which has a high surface area, whereas syn-ore generations of pyrite tend to be coarser grained. In the models, pyrite surface area was varied from 100 (syn-ore) to 10,000 cm2/g (diagenetic). Organic matter provided a drive for thermochemical sulfate reduction (TSR) in the models, and TSR rates were varied over several orders of magnitude in accordance with laboratory measured values.

As the incoming hydrothermal fluid reacted with the host unit, pyrite and dolomite are dissolved and sphalerite is precipitated. The surface area of pyrite evolved as it dissolved and reprecipitated in the form of a more massive, lower surface area, hydrothermal pyrite.

Models using the higher surface area values for diagenetic pyrite resulted in more compact and higher grade ore deposition. The pyrite at the inlet in this scenario dissolved completely. As the pyrite reprecipitated, it formed more extensive halo ahead of the sphalerite reaction front than in models using the lower hydrothermal surface area. Slower rates of TSR also broadened the pyrite halo and decreased the sphalerite ore grade. Low pyrite surface area coupled with low TSR rates resulted in a disseminated deposit. Based on these results, the paragenetic evolution of pyrite over the course of hydrothermal alteration and the resulting changes in surface area are an important control on ore grade and the extent of halo formation.

How to cite: Berger, P., Magnall, J., Kühn, M., and Gleeson, S.: The role of pyrite surface area and thermochemical sulfate reduction in clastic-dominant Zn Deposits. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15652, https://doi.org/10.5194/egusphere-egu24-15652, 2024.

Argillaceous rock formations provide favourable properties to act as geological barriers for the disposal of high-level radioactive waste. Opalinus Clay is the chosen host rock in Switzerland and is also being considered in Germany. Pyrite is an ubiquitous mineral found in most argillaceous rock formations, and is also present in the Opalinus Clay and its adjacent units. For the long-term integrity of the disposal site, temporally and spatially stable geochemical conditions are essential. Pyrite might be one essential indicator how the sediment formation is influenced by surrounding aquifers. The appearance and composition of pyrite has been used to investigate different geological processes, such as depositional and diagenetic settings, paleoredox conditions and enrichment processes. The geochemical and mineralogical changes in rock formations provide information about processes in the past, and thus enable an assessment for the future. In this context the detailed analysis of pyrite might be a useful tool.

In May 2023, two boreholes were drilled (15 and 25 m deep) in the Mont Terri underground laboratory to investigate the water-bearing members of the Staffelegg Formation (Toarcian-Sinemurian) underlying the Opalinus Clay (Toarcian). The focus was hereby on the transition zones between the permeable and non-permeable rocks to detect alteration reactions and mobilisation processes. In addition to the analysis of the bulk mineralogy and geochemistry, pathways of groundwater and fracture zones have been investigated by analysing thick sections with X-ray and microscopic methods.

Two groundwater paths have been identified in the Staffelegg Formation with fracture zones and their fillings. Calcite and barite are distinguishable and represent two generations, revealing a change of the groundwater composition with supersaturation at different points in time. Therefore, the hydrogeological system experienced at least two events of advective transport. The analysis of pyrite addresses the question to which extent these events have altered and infiltrated the formations.

Pyrite has been formed diagenetically and potentially syngenetically and is present in varying morphologies: μm- to cm-sized euhedral crystals, framboids and nodules. The size and morphology of diagenetic pyrites provide information about the transport processes in the sediment. Euhedral crystals are found in diffusion dominated systems. Accumulations of microcrystals reflect conditions providing an initial nucleation burst, but further crystal growth is limited by restricted supply of Fe and S as it occurs in diffusion-limited regimes with minor advection. Nodules can form in gently advective or stagnant systems with good nutrient supply. The types of pyrite close to fractures and transition zones is used to characterize the predominant transport process at this position.

Diagenetic carbonates and sulphide or sulphate minerals control the concentration of major cations, and redox reactions. Therefore, they provide information about the succession of processes from deposition, to diagenesis, mobilisation and alteration. Their analysis has the potential to assess the long-term integrity of the Opalinus Clay as a host rock and the surrounding formations. The gained understanding of the hydrogeological influence on the geochemical conditions is to be transferred to other potential disposal sites in argillaceous formations.

How to cite: Bonitz, M., Hennig, T., Schleicher, A., Kusebauch, C., Jaeggi, D., and Kühn, M.: What can we learn from pyrite about the hydrogeology of argillaceous formations?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8617, https://doi.org/10.5194/egusphere-egu24-8617, 2024.

The safety case for the final disposal of high-level radioactive waste in a deep geological repository involves a thorough understanding of the geochemical interactions of the potentially mobilized waste inventory with components of a proposed multi-barrier concept.

Argillaceous rock is considered as a potential host rock and final barrier to the biosphere. Opalinus Clay (OPA) from the Mont Terri rock laboratory (St-Ursanne, Switzerland) serves as a reference material for a natural clay rock. As a sedimentary rock and porous medium, OPA exhibits characteristic properties such as a low hydraulic conductivity, limiting the transport of solutes to diffusion, as well as structural and compositional heterogeneities in a range of length scales [1].

To address the influence of the potentially reactive microstructure on solute transport, spatially resolved sorption and diffusion studies of Np(V) and Pu(V,VI) with bulk OPA samples were performed, utilizing synchrotron-based microscopic chemical imaging at the microXAS beamline (Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland), simultaneously obtaining chemical information about the OPA microstructure as well as the distribution and transport patterns of Np and Pu [2-5].

In these studies, domains of pyrite, contained in OPA to about 1 wt. % as microstructural, Fe(II)-bearing heterogeneities, were identified exhibiting an enhanced reactivity regarding redox transformation and immobilization of the solute species as reduced, predominantly tetravalent and therefore less mobile species.

Further sorption studies with isolated pyrite heterogeneities, extracted from the OPA matrix as sub-100 µm sized particles, indicate a reactivity depending on the morphology of the heterogeneities, including the crystallite size distribution (framboidal) and cementing phase of these pyrite aggregates.

The results of these studies should add to an enhanced understanding of reactive transport in a natural clay rock in the context of a deep geological repository.

References

[1] Nagra (2002). Tech. Ber. 02-03. Projekt Opalinuston. Wettingen, Switzerland.

[2] Fröhlich, D.R., Amayri, S., Drebert, J., Grolimund, D., Huth, J., Kaplan, U., Krause, J. and Reich, T. (2012). Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques. Anal. Bioanal. Chem. 404: 2151-2162.

[3] Kaplan, U., Amayri, S., Drebert, J., Rossberg, A., Grolimund, D. and Reich, T. (2017). Geochemical interactions of Plutonium with Opalinus Clay studied by spatially resolved synchrotron radiation techniques. Environ. Sci. Technol. 51: 7892-7902.

[4] Börner, P.J.B. (2017). Sorption and diffusion of Neptunium in Opalinus Clay. PhD thesis. Johannes Gutenberg-Universität Mainz, Mainz, Germany.

[5] Kaplan, U., Amayri, S., Drebert, J., Grolimund, D. and Reich, T. (2024). Plutonium mobility and reactivity in a heterogeneous clay rock barrier accented by synchrotron-based microscopic chemical imaging. Sci. Rep., accepted.

Acknowledgements

Funding from the German Federal Ministry of Education and Research (BMBF) under contract number 02NUK044B, from the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) under contract numbers 02E11415A and 02E11860A, and from the European Union’s Horizon 2020 project EURAD (WP FUTuRE), EC Grant agreement no. 847593, is acknowledged.

How to cite: Breckheimer, M., Amayri, S., Ferreira Sanchez, D., Grolimund, D., and Reich, T.: Geochemical interactions of Np and Pu with pyrite heterogeneities in Opalinus Clay in the context of deep geological repositories, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21650, https://doi.org/10.5194/egusphere-egu24-21650, 2024.

Pyrite is one of the most common sulfides on Earth and occurs in many Cu, Pb and Zn sulfides (hypogene) ore deposits. By using XPS (X-ray Photoelectron Spectroscopy) surface and depth analyses, we propose a new experimental approach to determine the oxidation rate of pyrite in three exposure/conditions: i) air, ii) water and iii) water drip (Figure 1). Pyrite samples are almost pure and were collected from the Danube–Bouchon quarry (Hautrage, Belgium), in Barremian black clays of the Wealden facies sediments in the Mons Basin. These pyrites are nodules with cubic aggregates on the surface which were used for the different experiments.

The results reveal a maximum oxidation rate of 11.7 ± 1.8 nm day⁻¹ for drip exposure associated with the precipitation of Fe-sulfates or/and oxides depending on the experimental conditions. These data can be extrapolated to the different zones of weathering profiles (gossan, saprolite and cementation zone). The extrapolation shows a maximum rate of 4.3 ± 0.6 m Ma⁻¹, values consistent with those obtained by other methods such as isotope dating of weathering profiles (e.g. [1,2]). The oxidation in natural systems can vary following different factors, such as the nature of the host rock (protore) and the primary mineralogy, the porosity/permeability and fractures, the presence of an oxidizing environment, climate change over time, the action of bacteria as catalysts, …

Figure 1 - Macroscopic evolution of pyrite oxidation over time in the different experiments [3]

References

[1] De Putter T, Ruffet G, Yans J, Mees F (2015) Ore Geol Rev 71:350–362. https://doi.org/10.1016/J.OREGEOREV.2015.06.015

[2] Vasconcelos PM, Conroy M (2003) Geochim Cosmochim Acta 67:2913–2930. https://doi.org/10.1016/S0016-7037(02)01372-8.

[3] Poot J, Felten A, Colaux JL, Gouttebaron R, Lepêcheur G, Rochez G, Yans J (2024) Environ Earth Sci 83:9. https://doi.org/10.1007/s12665-023-11325-z

How to cite: Poot, J., Felten, A., Colaux, J. L., Gouttebaron, R., Lepêcheur, G., Rochez, G., and Yans, J.: Pyrite oxidation rates and timing of weathering in supergene deposits: new insights from XPS approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21668, https://doi.org/10.5194/egusphere-egu24-21668, 2024.