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Schematic diagram showing how the location of pyrite formation influences D 34 S values. Upper panel shows depth profiles of the concentration (left) and d 34 S composition (right) of sulfate (SO 4 2À , solid line) and sulfide (H 2 S, dashed line) in a transport-limited system where the reservoir effect influences S isotope geochemistry. The lower panel shows the resulting magnitude of D 34 S, which is projected downward as the line between the d 34 S of pyrite (filled circle) and d 34 S of sulfate at the top of the water column (vertical dash) given different primary locations of pyrite formation (listed in the lower left corner). The location of pyrite formation is listed in an idealized order of increasing fractional yield of the sulfide (f d ; black arrow in the lower right corner).
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The sulfur (S) isotope difference between sulfates and sulfides preserved in sedimentary rocks (Δ34S) has been utilized to reconstruct ancient marine sulfate levels with implications for oxygenation of the Earth surface and biogeochemical cycling. S isotope data from modern, low-sulfate euxinic systems illustrate that preserved Δ34S values are posi...
Citations
... SRMs exhibit a preference for utilizing 32 Senriched sulfate, and therefore, the sulfur isotopic values of pyrite (δ 34 S py ) are consistently lower than those of the original sulfate [12][13][14][15]. In addition to the original sulfate concentration and isotopic values in the water column, the Fe 2+ content in the sediment, the pore water advection rate from overlying water, and the DSR reaction rate can also influence δ 34 S py [3,16,17]. In relatively closed sedimentary environments, high sedimentation rates and organic matter loading can lead to rapid fluctuations in δ 34 S py along with depth [9]. ...
Pyrite is an important proxy used to reflect the redox state of a sedimentary environment. Currently available studies on pyrite focus on the process of sulfur cycles between an ocean and sediment. However, our understanding of the biogeochemical cycle of sulfur in terrestrial lake basins remains unclear, and the growth patterns of different types of pyrite are poorly understood. In this paper, we used samples from the 34 section of the Qianjiang depression in the Jianghan Basin as direct research objects by combining pyrite and sulfur isotope determination. The one-dimensional diffusion–advection–reaction simulation (1D-DAR) model was applied to simulate the changes in the pyrite content and sulfur isotope values in the sediment. The results show that the sediments in the saline lake basin environment contain a high organic matter content, a high sedimentation rate, and a high H2S diffusion oxidation rate, reflecting the strong reducing background and high productivity of this ancient lake. Sensitivity tests revealed that the organic matter content and H2S diffusion oxidation rate at the sediment–water interface are sensitive to the pyrite content. The sedimentation rate, organic matter content, and sulfate concentration are sensitive to the pyrite’s sulfur isotope values. However, the variation in the active iron content had little effect on the pyrite content or sulfur isotope value.
... Modern meromictic lakes (i.e., those with a permanently stratified water column due to the presence of a strong salinity and/or temperature gradient) are generally considered useful analogs for ancient ocean chemistry because of potential similarities in term of biogeochemical processes. For instance, stratified lake systems with low sulfate concentration (low mM to µM levels) provide a window into the ferruginous Precambrian ocean (Crowe et al., 2008Busigny et al., 2014;Swanner et al., 2020;Roland et al., 2021), while lakes with a euxinic monimolimnion have been considered analogous to certain conditions of the Proterozoic era (Canfield et al., 2010;Gomes and Hurtgen, 2015). Importantly, understanding the response of the sulfur cycle to major redox changes in aquatic environments requires a quantitative assessment of how biogeochemical processes can be preserved in sedimentary rocks. ...
... The disparities in the isotopic characteristics of modern saline lakes likely result from variations in the extent to which the sulfate reservoir in the monimolimnion is reduced by MSR, specifically through a Rayleigh distillation process. Indeed, in modern euxinic lacustrine systems, the concentration of sulfate prior to microbial consumption appears to be a key parameter controlling the difference in the isotopic composition of sulfide minerals relative to sulfate (Gomes and Hurtgen, 2015). Under non-limiting sulfate conditions observed generally in lakes with high (>few mM) sulfate concentration, the kinetic isotope fractionation imparted by microbial sulfate reduction (α MSR ) results in an apparent isotopic difference between sulfate and sulfide higher than 20 ‰. ...
... Variations in bulk sedimentary δ 34 S py signatures have been plausibly linked to oceanic sulfur cycle perturbations (e.g., Hurtgen et al., 2009;Gill et al., 2011;Halevy et al., 2012;Algeo et al., 2015;Sim et al., 2015;Schobben et al., 2017;Stebbins et al., 2019aStebbins et al., , 2019bYoung et al., 2020) or basin isolation effect (Gomes and Hurtgen, 2013;Kurzweil et al., 2015;Paiste et al., 2020). Sedimentary δ 34 S py signals, however, can also be influenced by the connectivity between porewater and the overlying water column affected by sedimentation rate (e.g., Hartmann and Heimo, 1968;Goldhaber and Kaplan, 1980;Maynard, 1980;Wijsman et al., 2001;Pasquier et al., 2017Pasquier et al., , 2021Liu et al., 2019;Richardson et al., 2019a), organic matter rain rate (e.g., Wijsman et al., 2001;Pasquier et al., 2021), and the position of pyrite formation relative to the sediment-water interface (SWI) (e.g., Gomes and Hurtgen, 2015;Stebbins et al., 2019a;Wang et al., 2021). ...
... The 32 S -O bond during this process is preferentially split by microorganisms because of its lower bonding energy, resulting in the enrichment of 32 S in the resultant H 2 S (Faure and Mensing, 2005;Mazumdar and Strauss, 2006;Misra, 2012;Marin-Carbonne et al., 2014). H 2 S can then react with aqueous iron to form sulfide minerals (primarily pyrite) without significant S-isotope fractionations Gomes and Hurtgen, 2015). In modern oceans, however, a significant fraction (~90%) of the H 2 S produced by MSR is reoxidized to sulfate or Cooper et al. (2001). ...
... The subdued Sisotope fractionation at low sulfate concentrations is explained by limited exchanges of sulfate across the bacterial cell membrane (Canfield, 2001a(Canfield, , 2001bHabicht et al., 2002Habicht et al., , 2005 or Rayleigh distillation (Wijsman et al., 2001;Jørgensen et al., 2004;Williford et al., 2009;Siedenberg et al., 2018). The S-isotope fractionation by MSR in the geological record is not directly measurable, and it is therefore usually inferred from Δ 34 S (e.g., Gomes and Hurtgen, 2015;Algeo et al., 2015): ...
The Green Point Formation in western Newfoundland, GSSP of the Cambrian-Ordovician (Є-O) boundary, is dominated by slope rhythmites of alternating lime mudstone and shale interbeds. This formation was deposited in a semi-restricted basin with varying connectivity to the open ocean. In the current study, we investigate textures and bulk δ34S signatures of pyrite (δ34Spy) in the shale to better understand factors influencing the sedimentary δ34Spy fluctuation. Petrographic and SEM examinations reveal two major types of pyrite: (1) framboidal pyrite and (2) anhedral to euhedral pyrite. The latter is further categorized into two subtypes: type 2a anhedral to subhedral pyrite characterized by relict framboidal textures and larger sizes (~10 to 300 μm), and type 2b smaller (typically <10 μm) subhedral to euhedral pyrite. Type 1 pyrite was precipitated near the sediment-water interface (SWI), whereas type 2b pyrite was formed in sediments below the SWI with limited access to the overlying seawater sulfate. Type 2a pyrite was evolved from framboids during early and burial diagenesis. The bulk δ34Spy values, marked by a significant scatter (1σ =10.62‰), range broadly from −17.6 to +22.4‰ (VCDT) and exhibit a pronounced positive excursion of ~20‰ near the Є-O boundary. The abundance of type 2b pyrite generally mimics the changes in δ34Spy, suggesting that the substantial δ34Spy dispersion could be partially attributed to differing proportions of type 2b pyrite within the samples. Moreover, notable negative correlations exist between the δ34Spy values and the abundances of Al, Th, ∑REE, and Fe, indicating that riverine fluxes might have influenced the Δ34Sseawater ‒ pyrite by modulating the regional seawater sulfate and iron reservoir sizes. Therefore, rather than being indicative of oceanic redox oscillations, the positive δ34Spy excursion of ~20‰ of this interval was probably driven by decreased sulfate and iron levels in the local waterbody. The decline in terrestrial input during this δ34Spy shift might have also contributed to a negative δ13Ccarb excursion by reducing nutrient supply and inhibiting primary productivity. Collectively, the bulk sedimentary δ34Spy variability recorded by the Green Point shale may be attributed to a combination of changes in regional terrigenous input and varying amounts of pyrite formed at different diagenetic stages within the samples. The general opposing trends between the δ34Spy signals and the abundances of Al, Th, ∑REE, and Fe, however, imply that fluctuations in riverine influxes might exert a stronger influence on the major δ34Spy trend. These findings suggest that bulk sedimentary δ34Spy variations alone may not be reliable evidence for perturbations of the global sulfur cycle.
... Syndepositional pyrite forming in marine sediments is a significant sulfur sink in the global sulfur cycle (Berner, 1984;Gomes et al., 2018). The reconstruction of biogeochemical sulfur cycling over geological time is largely based on the sulfur isotopes of syndepositional pyrite (δ 34 S py ; Canfield and Teske, 1996;Canfield, 2004;Tostevin et al., 2014;Fike et al., 2015;Gomes and Hurtgen, 2015;Gomes et al., 2016). However, the substantial spatial and stratigraphic variability in δ 34 S py records, regardless of depositional ages, poses a challenge to the reconstruction of the global sulfur cycle or depositional conditions (Fike et al., 2015;Pasquier et al., 2017Pasquier et al., , 2021aTostevin et al., 2017;Liu et al., 2019;Lang et al., 2020). ...
... This phenomenon occurs due to limited exchange of sulfate between the sediment pore water and the water column above, resulting in a gradual enrichment of the heavy isotope in both pore-water sulfate and sulfide (Jørgensen, 1979;Wijsman et al., 2001). In this scenario, the sedimentary processes controlling the δ 34 S py value have been well-constrained (Gomes and Hurtgen, 2015;Lang et al., 2020;Liu et al., 2020b), also providing new insight into paleo-environmental conditions through numerical models (Lang et al., 2020;Ma et al., 2022) or in-situ sulfur isotope analysis of pyrite Bryant et al., 2023). However, a decrease of δ 34 S py values with depth in sediments has also been observed in continental shelf to the continental slope settings (Hartmann and Heimo, 1968;Böning et al., 2004;Jørgensen et al., 2004;Zopfi et al., 2008;Böttcher et al., 2010;Zhu et al., 2013;Hu et al., 2017;Lin et al., 2017b;Liu et al., 2020a;Zhang et al., 2022). ...
... We propose that the elevated Δδ 34 S OS-py values may be caused by (1) elevated seawater sulfate concentrations up to ~1.15-2.31 mM from our model calculations, which would have promoted increased fractionation during MSR for reservoir effects (Gomes and Hurtgen, 2013;Gomes and Hurtgen, 2015), in turn distributing less 34 S to pyrite and more 34 S to OS (due to the more favorable reaction kinetics of pyrite formation) (Hartgers et al., 1997); and/or (2) the increased incorporation of intermediate S species (e.g., polysulfides and elemental sulfur). A modern example of the latter is the euxinic Cariaco Basin, where sulfur species distribution is directly governed by redox stratification. ...
... The abundance of O 2 in seawater may be estimated from the concentrations and S-isotope composition of various sulfur species, based on the kinetics of S 2− oxidation and related S isotope systematics (Canfield and Thamdrup, 1994;Johnston et al., 2008). Increased sulfate concentration will enhance the fractionation magnitude between SO 4 2− and H 2 S, which eventually led to a larger difference in sulfur isotopic values between organic sulfur and pyrite formed successively Gomes and Hurtgen, 2015). Moreover, as described above, the incorporation of polysulfide and elemental sulfur could contribute to Δδ 34 S OS-py > 5‰ (Amrani Jr et al., 2006;Werne et al., 2008). ...
... Furthermore, fluctuations in δ 34 S py , with the maximum value of 18.96‰ and the minimum value of − 0.29‰ both being within Unit 1, indicate that δ 34 S fractionation during MSR is also regulated to some extent by redox conditions (Figs. 3 and 4C) (Gomes et al., 2016). This agrees with previous work indicating that MSR is influenced by chemocline fluctuations (Gomes and Hurtgen, 2015), regardless of whether the water column and sediment-water interface are treated as an open system connected to an external infinite sulfate reservoir, or if sediments are treated as a closed system undergoing Rayleigh fractionation (Fig. 5C). In Unit 1, an increase in polysulfides and elemental sulfur implies that dissolved O 2 and/or oxidants participated in the sulfur cycle (either directly and/or indirectly), which is compatible with observations of positive Ce anomalies and Mn enrichments in the top of Unit 1 (Figs. 3 and 4B). ...
The extent of atmospheric and oceanic oxygenation during the Mesoproterozoic remains an area of active debate. Here, we report major and trace elements, organic sulfur (OS) speciation, sulfur isotopes in multiple phases [pyrite (py), OS, and carbonate-associated sulfate (CAS)], and a numerical model of sulfate concentrations from one of the Mesoproterozoic’s best-preserved geochemical archives, the Xiamaling Formation (ca. 1.4 Ga) in the Yanliao Basin. These data reveal a previously unrecognized ocean-atmosphere oxygenation process. Starting with Unit 3, intense sulfurization increased organic carbon burial and thus oxygen release, resulting in a deep-water oxidation event within the Yanliao Basin that could be regarded as a restricted oxygen oasis based on low pyrite content, elevated kerogen S:C ratios, higher δ34Spy than δ34SOS (Δδ34SOS-py < 0), and a low calculated sulfate concentration of ~0.07-0.14 mM. During Unit 2, the basin was reconnected to the open ocean with the deposition of organic-rich shale across northern China and northern Australia, accompanied by a shift in Δδ34SOS-py to 5.0‰ and a calculated seawater sulfate concentration of ~0.1-1.14 mM. Such large-scale burial of organic matter released oxygen into the atmosphere, accompanied by significant sulfate delivery to the basin and euxinic conditions characterized by Mo concentrations and 34S-depleted pyrite at the topmost of Unit 2. A subsequent globally synchronized atmospheric and deep ocean oxygenation event may have occurred during Unit 1. This event was recorded by a progressive increase in Δδ34SOS-py (up to +9.6‰), increased oxidized OS species, and positive Ce anomalies (attributed to the reductive dissolution of the Ce-enriched Mn oxides), all of which were caused by increased sulfate concentrations (calculated to be ~1.15-2.31 mM) and a spatial descent of the redox interface. The combination of these lines of evidence illustrates the sensitivity of the oceanic sulfur cycle to the ocean-atmosphere system and provides novel constraints on oxygenation at ~1.4 Ga.
... The interpretation of high positive d 34 S values is further complicated by two other processes that have been shown to produce high positive d 34 S values. One is when sulfides form by BSR in euxinic sulfate-limited conditions (Gomes and Hurtgen 2015) and the other is when sulfides form by BSR in a restricted or closed system in which Rayleigh fractionation becomes important (Ohmoto and Goldhaber 1997;Huston et al. 2023). Choosing between these possible causes of high positive d 34 S values requires other evidence besides sulfur isotope data and is a topic examined in many of the studies reviewed in this chapter. ...
Clastic-dominated lead–zinc (CD Pb–Zn) deposits are an important source of the world’s Pb and Zn supply. Their genesis is contentious due to uncertainties regarding the time of ore formation relative to the deposition of the fine-grained carbonaceous strata that host CD Pb–Zn mineralization. Sulfur-isotopic studies are playing an important role in determining if ore minerals precipitated when hydrothermal fluids exhaled into the water column from which the host strata were being deposited, or when hydrothermal fluids entered the host strata during diagenesis or even later after lithification. Older conventional S-isotopic studies, based on analyses of bulk mineral-separate samples obtained by either physical or chemical separation methods, provided data that has been widely used to support a syngenetic-exhalative origin for CD Pb–Zn mineralization. However, with the advent in the late 1980’s of in situ S-isotopic studies using micro-analytical methods, it soon became apparent that detailed S-isotopic variations of genetic importance are blurred in conventional analytical data sets because of averaging during sample preparation. Clastic-dominated Pb–Zn mineralization in the North Australian Proterozoic metallogenic province and the North American Paleozoic Cordilleran province has been the subject of many stable isotope studies based on both bulk and in situ analytical methods. Together with detailed mineral texture observations, the studies have revealed a similar sulfide mineral paragenesis in both provinces. The earliest sulfide phase in the paragenesis is fine-grained pyrite that sometimes has a framboidal texture. This pyrite typically has a wide range of δ ³⁴ S values that are more than 15‰ lower than the value of coeval seawater sulfate. These features are typical of, and very strong evidence for, pyrite formation by bacterial sulfate reduction (BSR) either syngenetically in an anoxic water column or during early diagenesis in anoxic muds. The formation of this early pyrite is followed by one or more later generations of pyrite that often occur as overgrowths around the early pyrite generation. The later pyrite generations have δ ³⁴ S values that are much higher than the early pyrite, often approaching the value of coeval seawater sulfate. Later pyrite formation has been variously attributed to BSR in a more restricted diagenetic environment, to sulfate driven-anaerobic oxidation of methane (SD-AOM) and to abiotic thermal sulfate reduction (TSR), with all three mechanisms again involving coeval seawater sulfate. The main sulfide ore minerals, galena and sphalerite, either overlap with or postdate later pyrite generations and are most often attributed to TSR of seawater sulfate. However, in comparison with pyrite, there is a dearth of in situ δ ³⁴ S data for galena and sphalerite that needs to be rectified to better understand ore forming processes. Importantly, the available data do not support a simple sedimentary-exhalative model for the formation of all but part of one of the Northern American and Australian deposits. The exception is the giant Red Dog deposit group in Alaska where various lines of evidence, including stable isotopic data, indicate that ore formation was protracted, ranging from early syn-sedimentary to early diagenetic sulfide formation through to late sulfide deposition in veins and breccias. The Red Dog deposits are the only example with early sphalerite with extremely low negative δ ³⁴ S values typical of a BSR-driven precipitation mechanism. By contrast, later stages of pyrite, sphalerite and galena have higher positive δ ³⁴ S values indicative of a TSR-driven precipitation mechanism. In CD Pb–Zn deposits in carbonate-bearing strata, carbon and oxygen isotope studies of the carbonates provide evidence that the dominant carbonate species in the ore-forming hydrothermal fluids was H 2 CO 3, and that the fluids were initially warm (≥ 150 °C) and neutral to acid. The δ ¹⁸ O values of the hydrothermal fluids are ≥ 6‰, suggesting these fluids were basinal fluids that evolved through exchange with the basinal sedimentary rocks. Known CD Pb–Zn deposits all occur at or near current land surfaces and their discovery involved traditional prospecting, geophysical and geochemical exploration techniques. Light stable isotopes are unlikely to play a significant role in the future search for new CD Pb–Zn deposits deep beneath current land surfaces, but are likely to prove useful in identifying ore-forming hydrothermal fluid pathways in buried CD Pb–Zn systems and be a vector to new mineralization.
... Pyrite sulfur isotopes (δ 34 S pyr ) have the potential to reflect trends in either local or global MSR rates, however, several aspects of the sulfur cycle must be constrained for global interpretations. These aspects include the location of MSR (either in the sediments or in the water column, i.e., open or closed system pyrite formation), marine sulfate concentrations and the magnitude of the kinetic isotopic effects (Gomes and Hurtgen, 2015;Lang et al., 2020;Pasquier et al., 2021). ...
... However, given the very different lithologies and tectonic development between our site in Baltica compared to other marine basins with δ 34 S CAS records, it is very unlikely that the Röstånga-2 drill core experienced similar diagenetic histories to those found on western and eastern Laurentia, Argentine Precordillera and South China during this time. Given that pyrite sulfur isotopic compositions can be influenced by a variety of local factors that range from sedimentation rate, local oxygen penetration depths and type of organic matter that is produced in each basin (e.g., Gomes and Hurtgen, 2015), it is unlikely these disparate basins would each have similar depositional and postdepositional histories given the differences in lithologies, relative positions within their respective continental margins and hydrographic settings. Thus, these similar trends in sulfur isotopes, (i.e., near synchronous change in the same negative direction) indicate that sedimentary pyrite records from Röstånga-2 were primarily the result of fluctuations in global pyrite burial rates, rather than local processes. ...
... Pyrite sulfur isotopic compositions are influenced by a variety of factors, ranging from sedimentation rate, oxygen penetration depths, seawater sulfate concentration, type and abundance of organic matter and reactive iron, and MSR rate (Lang et al., 2020;Pasquier et al., 2021). A sufficiently large marine sulfate reservoir is required for pyrite sulfur isotopic compositions to mirror contemporaneous seawater sulfur isotope values, which can be recorded by δ 34 S CAS (Gomes and Hurtgen, 2015;Kah et al., 2016;Canfield, 2019). Several estimations for seawater sulfate concentrations have been made utilizing a variety of methods, and ultimately produce a range from 2 to 15 mM throughout the Ordovician (Horita et al., 2002;Algeo et al., 2015;Kah et al., 2016;Young et al., 2016). ...
... Si. praesulcata conodont Zone (Marynowski et al., 2012;Marynowski & Filipiak, 2007;. Incubation experiments and modeling studies of low-sulfate euxinic systems indicate that high-kinetic sulfur isotope fractionations up to 60‰-70‰ are achievable by both isolated and natural populations of sulfate reducers independent of disproportionation (Canfield, Farquhar, & Zerkle, 2010;Canfield, Glazer, & Falkowski, 2010;Gomes & Hurtgen, 2015), such as in the Black Sea and meromictic Lago di Cadagno. During marine transgression, the upwelling-induced euxinic water mass input from the open ocean might have influenced the sulfur cycle process in shallow water, and thus, the increasing contribution of water column sulfate reduction associated with the euxinic environment appears to be the most parsimonious interpretation for the decreasing limb of the δ 34 S anomaly in the Middle Si. praesulcata Zone. ...
The Devonian-Carboniferous (D-C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo-ocean environment of different depositional facies, we studied a shallow-water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well-preserved succession spanning the D-C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ15 N excursion (~-3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium-bearing waters. Negative excursions in the δ13 Ccarb and δ13 Corg values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered 13 C-depleted anoxic water masses. Decreased δ34 S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water-column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ13 Corg values associated with maximal △13 C. The integrated δ15 N-δ13 C-δ34 S data suggest that significant ocean-redox variation was recorded in South China during the D-C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.
... The observed D 34 S values (δ 34 SSO 4 -δ 34 Ssulfides) are thus typical of MSR under relatively open system conditions where sulfate is not completely consumed (Gomes and Hurtgen, 2015). Slightly more positive δ 34 S values in pyrite 3 (Fig. 13A, B) may suggest pyrite formation under increasingly sulfate limited conditions with time. ...
Loma Galena (978,852 t Pb, 206 Moz Ag) is one of eight epithermal deposits in the world-class Navidad Pb + Ag ± (Zn, Cu) district located in the Cañadón Asfalto continental foreland basin, northern Patagonia, Argentina. This basin formed during the Jurassic in an extensional tectonic regime during the breakup of Gondwana. Host rocks comprise major listric faulted and tilted blocks of K-rich andesite to dacite lava flows (173.9–170.8 Ma; U-Pb ages for zircon) unconformably overlain by mudstone interbedded with stromatolitic and pisolitic limestones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccia dikes in the hanging walls and footwalls of NW- and NE-striking normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact of volcanic and sedimentary rocks.
The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (δ13Cfluid –4.7 to 0.8‰; δ18Ofluid 4.8–11.6‰) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5–16.4 wt % NaCl equiv and temperatures of 154.7°–212°C. The interaction of these fluids with the host volcanic rocks formed calcite, albite, adularia, and celadonite-glauconite-group minerals followed by chlorite and siderite as fO2 decreased. Fluids intermittently boiled, as evidenced by bladed (platy) texture in calcite 1.
Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and δ34S from –15.4 to 12.9‰ for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have δ34S 35‰). Mixing of upflowing metal-rich basinal fluids carrying some S from depth with this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloform pyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite, and galena as veins, breccias, and disseminations in host rocks. The highest grade and tonnage of the ores are found in autobreccias at the junction of the uppermost lava flow and in the overlying mudstone, where the addition of a strong microbial signature is recorded in sulfides. This event also led to partial dissolution of magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks.
Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (δ18Ofluid 2.6–4.8‰), barite (δ34S 15.7–22‰; 160.9°–183.8°C; 7.7–9.7 wt % NaCl equiv), calcite 2 (δ18Ofluid –10.2 to –3.7‰, 58°–95°C; 1.9–7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (δ18Ofluid 2–6.2‰), illite-smectite, smectite, and carbonates with minor chalcedony and barite in the volcanic rocks; and calcite, chalcedony, and barite in the sedimentary rocks. A trend of decreasing salinity with decreasing temperature and lowering δ18O of the fluids with time suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2‰).
Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic environment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.
... The magnitude of this sulfur isotope fractionation is controlled by physiological factors (Goldhaber and Kaplan, 1980;Wortmann et al., 2001;Brunner and Bernasconi, 2005;Sim et al., 2011;Leavitt et al., 2013;Wing and Halevy, 2014;Bradley et al., 2016) but also local (physico-environmental) factors that influence the reversibility of MSR (Reese, 1973;Brunner and Bernasconi, 2005;Wing and Halevy, 2014). At the sediment level, the measured difference in the sulfur isotope composition between seawater sulfate and porewater sulfide (D 34 S SO4_sw-H2S_pw ), and further in sedimentary minerals, is also controlled by the concentration of seawater sulfate, the availability of reactive iron, and the degree of connectivity, or 'openness', of the sediment with the overlying water which will influence the isotopic mass balance among sulfur species (Habicht et al., 2002;Canfield and Farquhar, 2009;Gomes and Hurtgen, 2015;Sim, 2019). The burrowing activity of benthic fauna enhances the connection of seawater with the sediment, resulting in increased supply of sulfate to the sediment as well as the oxidation of reduced sulfur compounds originating from MSR Canfield and Farquhar, 2009;van de Velde and Meysman, 2016;Blonder et al., 2017). ...
Bioturbation enhances mixing between the seafloor and overlying ocean due to changes the redox state of the sediment and influences the biogeochemical cycling of redox-sensitive elements such as sulfur. Before the widespread appearance of burrowing fauna over the Proterozoic-Phanerozoic transition, marine sediments were largely undisturbed and transport of material across the sediment-water interface was diffusion-dominated. Through both a microcosm experiment and numerical model, we show that the effect of bioturbation on marine sediments is to enhance the drawdown of sulfate from the water column into the sediment and thus “open-up” the sedimentary system. The key finding is that bioturbation increases the difference between the isotopic signature of seawater sulfate and pore water sulfide, the latter of which is preserved in sedimentary sulfide minerals. Our study empirically demonstrates a long-held assumption and helps identify the isotopic impact of bioturbation in the geological record and its environmental effects in modern marine systems.
