The formation of pyrite is a critical process in sedimentary environments, central to the global iron and sulfur cycles and redox-sensitive biogeochemical processes. Yet the pathways and mechanisms driving pyrite formation, particularly the roles of metastable iron sulfide (FeSx) phases and polysulfides (Sn2-), remain poorly understood under natural conditions. Here we provide the first direct evidence for polysulfide-mediated pyrite formation in marine sediments, using Mössbauer spectroscopy and X-ray photoelectron spectroscopy, on cores from Saanich Inlet. We identify two distinct nanoparticulate FeS1+x phases: a mackinawite-like and a greigite-like phase. We find that under highly reducing conditions the greigite-like phase can compete with pyrite as the prevailing Fe-S mineral. Our results also reveal limitations of sequential chemical extractions, particularly in underestimating highly reactive mineral phases. These findings highlight the advantage of high-resolution deterministic spectroscopic tools for the identification of nanoparticulate complexes. While the polysulfide pathway has been widely inferred, direct detection of polysulfides and intermediate FeSx phases in marine sediments has remained elusive. Our study confirms the polysulfide pathway, bridging a long-standing gap between experimental models and environmental observations. These findings refine our understanding of early-diagenetic Fe-S transformations and how sulfide minerals form and persist in Earth’s dynamic sedimentary environment.

Shales and mudstones are fine-grained rocks formed in sedimentary basins throughout Earth's history. These lithologies are increasingly important targets for mineral deposit exploration since they can have economic resources of critical minerals, including vanadium (V), an essential component of redox-flux batteries in solar cells. However, many Paleozoic, shale-hosted V deposits are metamorphosed and deformed. This commonly obscures primary features, including V-bearing host phases and the original composition of organic material.In this study, we present geochemical and mineralogical data from the Paleozoic Van Property deposit, Northwest Territories, Canada, to show that V can be released from organic matter during metamorphism and incorporated in clay phases such as illite. The siliceous argillites at the Van Property host up to 0.69% V2O5 and were metamorphosed to (sub-)greenschist facies. Their mineralogy is dominated by quartz with minor graphite, illite, muscovite, pyrite, sphalerite, rutile, and carbonates. Although some illite (i.e., high-V illite) can have up to 13 wt% V2O3 and rutile can have up to 4.4 wt% V2O3, mass-balance calculations are insufficient to explain V enrichment at the Van Property utilizing only illite and rutile. The third V host is inferred to be carbonaceous matter in which V accumulated syn-deposition on the seafloor. Subsequent metamorphism led to the demetallation of V-bearing geoporphyrins and the release of vanadyl ions (VO2+), some of which were then incorporated into high-V illite and rutile. This process of V enrichment highlights the role of organic matter in scavenging V from superficial reservoirs and the importance of metamorphism on subsequent V release and its incorporation into inorganic phases. The geochemistry of siliceous, V-rich argillites at the Van Property is also compared to other V-enriched shale and mudstone deposits, highlighting the diversity of shale-hosted V deposits and emphasizing the need for further research to close the knowledge gaps related to variations in composition, mineralogy, and V enrichment processes.

In this study we compared various machine learning techniques that used soil geochemistry to aid in geologic mapping. We tested six different sampling methods (undersample, oversample, Synthetic Minority Oversampling Technique (SMOTE), Adaptive Synthetic Sampling (ADASYN), SMOTE and Edited Nearest Neighbor (SMOTEENN), and SMOTE and Tomek links (SMOTETomek)). SMOTE performed best with ADASYN and SMOTETomek having slightly lower effectiveness. Nine machine learning algorithms (naïve Bayes, logistic regression, quadratic discriminant analysis, nearest neighbors, radial basis function support-vector machine, artificial neural network, random forest, AdaBoost classifier, and gradient boosting classifier) were compared and AdaBoost classifiers and gradient boosting classifiers were found to be most effective. Finally, we experimented with multiple classifier systems (MCS) testing different combinations of algorithms and various combinatorial functions. It was found that MCS can outperform individual models, and the best MCS combined nearest neighbors, radial basis function support-vector machine, artificial neural network, random forest, AdaBoost classifiers, and gradient boosting classifier, then applied a logistic regression to the probabilities output by the models. Ultimately, we created a tool that is able to adequately predict underlying geology in the study area using soil geochemistry.

The Derwent Estuary is highly enriched in potentially toxic elements, such as Zn, Pb, Cu, As, Hg and Cd, owing to inputs from historical industrial activity adjacent to the river, predominantly prior to strict environmental protections introduced in the 1970s. Contaminants are now buried at shallow depths within the sediment profile, in one or two highly concentrated layers decreasing in concentration away from an electrolytic zinc refinery, regarded as the main source of the contaminants. Enriched metals (Zn, Pb, Cu, Cd and As) in the estuary were estimated from data collected from 37 sediment cores using a portable X-ray fluorescence spectrometer that was validated against inductively coupled plasma mass spectrometer analyses. The thickness of the metal and metalloid enriched layers ranges from 32.5 to 107.5 cm, with an average thickness of 63 cm. Sedimentation rates based on this layer and the time since the start of zinc processing are approximately 0.46 cm/year. Sedimentation rates based on the thickness since maximum metal and metalloid concentrations are between 0.17 and 1.64 cm/year. Based on these sedimentation rates, the average time it will take for surface sediments to return to background metal and metalloid concentrations, if left undisturbed, is approximately 123 years.

Geochemical proxies used widely to reconstruct global paleodepositional systems require further calibration and validation in a wider range of oxygen-poor settings.The redox threshold values associated with various proxies (e.g., Fespeciation, trace-metal enrichments) can vary considerably among depositional systems and, for this reason, geochemical proxies should be scrutinized in multiple modern depositional systems of diverse redox characteristics-both stable and dynamic.Here, we provide a detailed study of Saanich Inlet, a semi-restricted fjord-like basin noted for high-frequency redox variation.Bottom water and sediment samples were collected in July 2019 when complete anoxia developed below 130 m water depth.We present data from the seasonally anoxic basin (200 m) and the oxygenated margin (100 m) to compare how spatiotemporal variations in redox condition impact the cycling of iron, sulfur, and trace metals in the bottom waters as recorded in bulk sediments, porewaters, and pyrite.We examined key biogeochemical drivers of early diagenetic reactions mainly via stable sulfur isotopes (δ 34 S), trace metal content, and iron-speciation.Additionally, we performed 57 Fe Mössbauer Spectroscopy analysis of iron mineralogical phases to cross-validate with widely used wet-chemical sequential Fe extraction methods.Given that Mössbauer Spectroscopy measures minerals directly rather than the response of a mineral to a chemical reagent, it offers an independent analytical method that can characterize and quantify different iron (oxyhydr)oxides, sulfides, sulfates, carbonates, silicates, amorphous colloids, and nanoparticles.The result is direct qualitative and quantitative estimates of the precipitation pathways and transformations of redox-sensitive iron phases.Additionally, this technique offers important novel insights into the dominant pyrite precursor pathway (i.e., FeS x species), a question long debated in the field.Chiefly, this study enables a direct comparison between biogeochemically dynamic oxic and anoxic environments along a transect recording differing depositional redox but similar detrital inputs.Most specifically, this novel calibration combining water column, solid phase, porewater, and pyrite data will provide new insights into the early diagenetic reactions that define the pyrite trace element compositions and Fe speciation data that are often used to interpret ancient environments.

Arsenic is a common contaminant in several river systems in the world, both from industrial effluent and natural sources. One of the main natural sources is the oxidation of pyrite. To develop strategies to mitigate the effects of As release due to pyrite oxidation we must understand both the heterogeneity in concentration and redox state of the As in pyrite. This is because whether the As is on the rim rather than center if a pyrite grain will affect when the As is released. Whereas the rate of As release will be affected by the redox state of the As. In this study we investigated two natural pyrite +/-marcasite formations known to oxidize rapidly, one from the Leicester pyrite member in New York and the other from the Black Bute SEDEX deposit in Montana. To determine the heterogeneity in As concentration and redox state of the As in we employed a series of different analytical techniques including laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), transmission electron microscopy (TEM), atom probe tomography (APT), nanoscale secondary ion mass spectrometry (nanoSIMS), X-ray absorption fine structure spectroscopy (XAFS), and X-ray absorption near edge structure (XANES). Using these techniques we identified wide variation in both where the As was situated in the pyrite and a wide variation in the redox state of the As. Future studies will investigate As release in controlled experiments from the same pyrite samples.

Variations in atmosphere oxygen and ocean sulfate concentrations through time are regarded as important controls on the cycles of sediment-hosted and volcanic-hosted ore deposits. However, estimates of atmosphere oxygen in the Proterozoic have been frustrated by the lack of a direct measurement method and conflicting evidence from various proposed geochemical proxies. Studies in the 1970s to 1990s suggested a relatively oxygenated atmosphere (> 3 wt% O2) in the Proterozoic. However, since the late 1990s, new proxies and modelling have suggested very much lower levels of oxygen (< 0.02 wt% O2). Focusing on redox-sensitive trace elements, here we combine a dataset of over 3000 LA-ICP-MS trace-element analyses on sedimentary pyrite, standardised against Berner’s Phanerozoic O2 modelling and direct measurement of oxygen concentrations in fluid inclusions in sedimentary halite, to develop the first detailed estimate for atmosphere O2 concentration and secular variation from 2200 Ma to the present. The estimates suggest dynamic cycles of atmosphere oxygen that increased in frequency through time. There were possibly three first-order cycles in the Proterozoic varying from 400 to 600 million years in length and a further five first-order cycles in the Phanerozoic from 60 to 120 million years in length. Our estimates of oxygen concentration are at odds with most previous estimates. We suggest, rather than very low atmosphere oxygen in the Proterozoic, the mean concentration was about 7 wt%, rising to a mean of about 10 wt% in the Phanerozoic, but with significant cyclic variation of up to a maximum concentration of possibly over 30 wt%. We observe that the proposed oxygen cycles correlate with biodiversity cycles and to the timing of major stratiform base-metal deposits in sedimentary basins. For example, minima in atmosphere oxygenation correlate with mass extinction events and stratiform Zn–Pb–Ag deposits, whereas maxima in oxygenation correlate with major evolutionary events, global periods of evaporite formation and the timing of stratiform copper deposits.

The Kapai Slate is a continuous, pyrite-rich carbonaceous shale horizon within the St. Ives Au district that is spatially related to high-grade Au mineralization. In situ laser ablation-inductively coupled mass spectrometry (LA-ICPMS) trace element analyses, in situ sensitive high resolution ion microprobe, stable isotope (SHRIMP-SI) S isotope analyses, and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the Kapai Slate and Au deposits. These data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid-greenschist facies metamorphism and significant hydrothermal overprint. The data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the Kapai Slate and determine whether some of the Au and S in the St. Ives district could have been sourced from the Kapai Slate. Seven different types of pyrite were identified: fine-grained sedimentary pyrite (Py 1 ), nodular sedimentary pyrite (Py 2 ), remobilized sedimentary pyrite (Py 3 ), coarse-grained, inclusions poor late pyrite (Py 4 ), inclusion-rich magnetite series pyrite (Py 5 ), ore stage pyrite (Py 6 ), and pyrite associated with the mafic units (Py 7 ). Each type of pyrite was found to have distinctive trace element compositions and S isotope signatures. The results of the LA-ICPMS analyses provide evidence for early trace element enrichment in the Kapai Slate sedimentary pyrite (median values of 158 ppm Ni, 387 ppm Co, 82 ppm Cu, 727 ppm As, 1.91 ppm Mo, 13 ppm Se, 0.25 ppm Au, 7.72 ppm Te and 3.36 ppm Ag for Py 1 and 223 ppm Ni, 158 ppm Co, 99 ppm Cu, 856 ppm As, 1.27 ppm Mo, 10.2 ppm Se, 0.57 ppm Au, 10.09 ppm Te, and 6.62 ppm Ag for Py 2 ). Concentrations of Ni and Co are low, relative to other late Archean sedimentary pyrite (median of 813 and 465 ppm, respectively) and Mo levels are near that of the euxinic shales of the similar-aged Jeerinah Formation in the Hamersley Basin, Western Australia. These data suggest that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low biological productivity. The Δ 33 S and δ 34 S signatures of the sedimentary pyrite suggest two different sources of S. Positive δ 34 S and negative Δ 33 S signatures indicate bacterial reduction of SO 4 2− from seawater, whereas positive δ 34 S and positive Δ 33 S signatures indicate an elemental S 8 source, indicating the pyrite formed later during diagenesis. This S isotope signature is consistent with a transition between a near-sediment environment to a more distal environment source. Analyses of the ore-phase pyrite yield weakly positive Δ 33 S values. This suggests there was a minor contribution of sedimentary S to the more significant oxidized ore-forming fluids, which is consistent with a small contribution of Au from a sedimentary source. Approximations of the degree of sedimentary pyrite destruction in the pyrrhotite/pyrite dominated zones and pyrrhotite/magnetite/pyrite zones of the northern part of the St. Ives district were used to calculate the amount of Au released from the early sedimentary pyrite. The calculation suggests that a minor, though possibly locally significant, amount of Au could have been sourced from the Kapai Slate.

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identified in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets (10-30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr(94.3-84.4)GrS(8.5-2.7)Alm(1.9-0.2)) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5-10 mm across) with minor alteration, a narrow compositional range along the growth lines (Adr(82-65.4) Grs(21.9-11.7)Alm(11.1-2.4)) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (>30 ram across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich domains (Adr(84.2-48.8)Grs(32.4-7.6)Alm(19.9-3.5)) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe3+.Almost all three garnet types have been replaced by fine-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe2+, is more suitable than andradite and grossular to host divalent cations such as Cu2+. Nickel in garnet II is positively correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni2+ substitutes for Fe3+ in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures. (C) 2016 Elsevier B.V. All rights reserved.