Sulfate Evaporites Research Articles

Contribution of the oldest Paleoproterozoic marine sulfate evaporites to Bushveld Complex Lower Zone mineralization

In the past two decades, multiple-sulfur isotope systematics has become increasingly used as a tool to differentiate sulfur sources with implications for exploration targeting. The involvement of crustal sulfur in triggering the Bushveld Complex (South Africa) Cu-Ni-PGE (PGE—platinum group element) mineralization of the Platreef intrusion is widely recognized; however, the extent and source of contamination are still debated. High-Mg magma of the Uitloop Lower Zone intrusion contained insufficient amounts of mantle-derived sulfur to reach the sulfide saturation required for mineralization. Sulfide saturation was reached when magma assimilated sulfur as it ascended through sedimentary strata deposited before and during the Great Oxidation Episode (GOE). Magmatic sulfides on Uitloop are 34S-enriched and either lack, or show small, mass-independent sulfur isotope fractionation, whereas sulfides from the Archean sedimentary footwall rocks exhibit the Archean-style multiple sulfur isotope covariations. We suggest that assimilation of the early Paleoproterozoic upper Duitschland Formation sulfate evaporites, corresponding to an ingrowth of a mass-dependently fractionated seawater sulfate reservoir at the early stage of the GOE, led to sulfide saturation and formation of Cu-Ni-PGE Platreef-style sulfide deposits. The GOE thus not only changed the oxidation state of the atmosphere and oceans, and the style of sediment-hosted mineral deposits, but also generated the sedimentary sulfate reservoir that since then served as a prolific sulfur source for magmatic ore deposits.

The Impact of Early Diagenesis on Biosignature Preservation in Sulfate Evaporites: Insights From Messinian (Late Miocene) Gypsum.

Due to their fast precipitation rate, sulfate evaporites represent excellent repositories of past life on Earth and potentially on other solid planets. Nevertheless, the preservation potential of biogenic remains can be compromised by extremely fast early diagenetic processes. The upper Miocene, gypsum-bearing sedimentary successions of the Mediterranean region, that formed ca. 6 million years ago during the Messinian salinity crisis, represent an excellent case study for investigating these diagenetic processes at the expense of organic matter and associated biominerals. Several gypsum crystals from the Northern Mediterranean were studied by means of destructive and non-destructive techniques in order to characterize their solid inclusion content and preservation state. In the same crystal, excellently preserved microfossils coexist with strongly altered biogenic remains. Altered remains are associated with authigenic minerals, especially clays. The results demonstrate that a significant fraction of organic matter and associated biominerals (notably biogenic silica) underwent early diagenetic modification. The latter was likely triggered by bottom sulfidic conditions when the growth of gypsum was interrupted. These results have significant implications for the interpretation of the Messinian Salt Giant.

In Situ Analysis of Sulfur Isotopic Fractionation in Deep‐Sea Corals Using Secondary‐Ion Mass Spectrometry: Insights Into Vital Effects

AbstractCarbonate‐associated sulfate (CAS) δ34S values (δ34SCAS) are generally assumed to reflect S isotopic composition of paleo‐seawater and have been extensively used to reconstruct secular variations in seawater sulfate concentrations during the geological past. However, it has often been documented that δ34SCAS records are incompatible with seawater sulfur isotopes (20.9 ± 0.1%, 2σ) determined from other archives, such as sulfate evaporites and barite (both of which may also display inconsistencies). A possible explanation for this discrepancy is that δ34SCAS values can be easily altered by atmospheric sulfate and sulfide re‐oxidation. However, the specific influence of biological factors (vital effects, common in biogenic carbonates) on CAS S isotopic composition remains unresolved, particularly at microscale levels. To elucidate these effects on δ34SCAS, S isotopic profiles were analyzed across two skeletal transects of two modern deep‐sea corals (gorgonia) using a novel secondary‐ion mass spectrometry method. Strong S isotopic fractionation was observed in calcitic skeletons from the most 34S‐depleted center (δ34S = ∼19‰), increasing outward to a relatively constant 22.5‰ in gorgonia sp. coral and 21.6‰ in bamboo coral, suggesting that vital effects are much larger than previous estimated (∼±1‰ fractionation from seawater). Oxygen isotopic and Mg, S, O elemental compositions, and Raman spectral and crystal morphological features indicate that processes such as pH control, Rayleigh fractionation, and organic effects are precluded as causes of such fractionation. Instead, vital effects associated with kinetic processes related to surface entrapment seem plausible as controls on S isotopic fractionations in the coral. This novel method is significant for gaining insights into vital effects, assessing the reliability of biogenic carbonates as high‐resolution environmental archives of S isotopes, and understanding the fundamental mechanisms governing biomineralization.

Reconstructing early sedimentation patterns of the lower Codó Formation (Early Cretaceous, NE Brazil): Evidence of marine influence and the onset of an alkaline hypersaline lake

The upper Aptian lower Codó Formation is an attractive stratigraphic unit within the Parnaíba and São Luís basins, northeastern Brazil, formed during the break-up of Western Gondwana. Located in a large sag basin, the formation records a dynamic depositional history of deltaic, lacustrine, and sabkha environments. This sedimentary evolution is reflected in a diverse lithological succession characterized by intermittent terrestrial input, episodic marine incursions, and restricted lacustrine phases culminating in sulfate evaporite precipitation. While the later stages of the formation have been extensively studied, the initial sedimentary processes and environmental conditions remain poorly understood. This research aims to reconstruct the depositional paleoenvironments during the initial stages of Codó sedimentation, elucidating the onset of the alkaline hypersaline lake environment and the earlier marine incursions in the study region. To achieve this, it was conducted a high-resolution facies analysis using core descriptions, trace fossil analysis, petrography, various types of X-ray fluorescence, scanning electron microscopy, X-ray diffractometry, and isotope analyses (δ1³C, δ1⁸O, and ⁸⁷Sr/⁸⁶Sr) on a ∼63-m-thick succession from borehole 2-TV-1-MA. Thirteen facies were recognized and grouped into brackish lake, prodelta, delta front, and alkaline hypersaline lake facies successions. The lower Codó Formation exhibits cyclic sedimentation, indicating fluctuating water dynamics associated with shifts between open and closed drainage systems controlled by paleoclimate. Tectonic subsidence drove the basin's evolution from a brackish lacustrine-deltaic environment with an incipient marine influence (balanced-fill basin stage) to a restricted system with the establishment of an alkaline hypersaline lake (underfilled basin stage).

Phosphate-associated sulfate in Lingula shells represents a potential archive for seawater sulfate composition

The marine sulfur cycle is highly sensitive to redox environments at the Earth's surface. Fluctuations in atmospheric pO2 level or marine redox environments are commonly associated with changes in seawater sulfate concentration, sulfate sulfur (S) and oxygen (O) isotopes. Thus, the marine S cycle through the Earth's ancient past provides a first-order constraint on the redox evolution of the Earth's surface. Reconstructions of the marine S cycle have been approached through analyses of various sulfate-bearing geological materials, such as sedimentary barite, evaporitic sulfate (e.g., gypsum), carbonate associated sulfate (CAS), and phosphate-associated sulfate (PAS) in phosphatic ores. These geological materials have their own pros and cons. In this study, we focus on PAS in Lingula brachiopods, which possess phosphatic shells and have a long evolutionary history since Cambrian times. Biogenic phosphate typically has a higher sulfate concentration than carbonate and phosphate ores, and is less vulnerable to diagenetic alteration. Here, we analyzed modern Lingula shells from the South China Sea and fossil Lingula shells collected from the Late Devonian Xiejingsi Formation of South China. Living and fossil Lingula shells have similar P/S ratios (PO43−/SO42−) of ∼76, and S and O isotopes of PAS (δ34SPAS and δ18OPAS) in each group show limited variations. For living Lingula shells, the mean values of δ34SPAS and δ18OPAS are +15.0 ± 1.5‰ and + 9.8 ± 0.4‰, respectively, which are ∼6‰ lower and comparable to those of modern seawater sulfate compositions (δ34SSW and δ18OSW-SO4), probably due to vital effects. For fossil Lingula shells, δ18OPAS (14.1 ± 0.9‰) is identical to coeval evaporite. The range of δ34SPAS is encompassed by the values of both coeval evaporite and CAS, but the mean value of δ34SPAS (29.5 ± 0.9‰) is higher. Thus, fossil Lingula shells show a good potential for reconstructing δ34SSW and δ18OSW-SO4 values.

Impact of seawater sulfate concentration on sulfur concentration and isotopic composition in calcite of two cultured benthic foraminifera

Abstract. Marine sediments can be used to reconstruct the evolution of seawater [SO42-] and δ34S over time, two key parameters that contribute to refine our understanding of the sulfur cycle and thus of Earth's redox state. δ34S evolution can be measured from carbonates, barites and sulfate evaporites. [SO42-] variations can be reconstructed using fluid inclusions in halites, a method that only allows a low-resolution record. Reconstruction of the past sulfur cycle could be improved if carbonates allowed the tracking of both seawater δ34S and [SO42-] variations in a sole, continuous sedimentary repository. However, most primary carbonates formed in the ocean are biogenic, and organisms tend to overprint the geochemical signatures of their carbonates through a combination of processes often collectively referred to as vital effects. Hence, calibrations are needed to allow seawater δ34S and [SO42-] reconstructions based on biogenic carbonates. Because foraminifera are important marine calcifiers, we opted to focus on calcite synthesized by individuals of rosalinid benthic foraminifera cultured in the laboratory under controlled conditions, with varying seawater [SO42-] (ranging from 0 to 180 mM). Our experimental design allowed us to obtain foraminiferal asexual reproduction over several generations. We measured bulk carbonate-associated sulfate (CAS) content and sulfur isotopic composition (δ34SCAS) on samples of tens to hundreds of specimens from a selection of culture media, where [SO42-] varied from 5 to 60 mM. Increasing or decreasing [SO42-] with respect to modern-day seawater concentration (28 mM) impacted foraminiferal population size dynamics and the total amount of bioprecipitated carbonate. Foraminiferal CAS concentration increased proportionally with [SO42-] concentration from 5 mM up to 28 mM and then showed a plateau from 28 to 60 mM. The existence of a threshold at 28 mM is interpreted as the result of a control on the precipitation fluid chemistry that foraminifera exert on the carbonate precipitation loci. However, at high seawater sulfate concentrations (> 40 mM) the formation of sulfate complexes with other cations may partially contribute to the non-linearity of the CAS concentration in foraminiferal tests at high increases in [SO42-]. Yet, despite the significant effect of seawater [SO42-] on foraminiferal reproduction and on CAS incorporation, the isotopic fractionation between CAS and seawater remains stable through varying seawater [SO42-]. Altogether, these results illustrate that CAS in biogenic calcite could constitute a good proxy for both seawater [SO42-] and δ34S and suggests that sulfate likely plays a role in foraminiferal biomineralization and biological activity.

Calcitized evaporites in the Mesoproterozoic Atar and El Meriti Groups, Mauritania, Africa

One of the most profound byproducts of the Great Oxidation Event (GOE) was the onset of oxidative weathering on land and the riverine delivery of sulfate to marine systems. Although the exact nature of marine oxygenation in the aftermath of the GOE remains uncertain, marine sulfate likely remained below 2 mM until the late Mesoproterozoic and may have remained <10 mM through the Neoproterozoic. Prior to the late Mesoproterozoic, precipitation of sulfate minerals were inhibited by low marine sulfate and possibly by elevated carbonate saturation. By ∼1.1 Ga marine sulfate concentrations were sufficient to sustain the earliest widespread evaporite deposition. Low sulfate concentrations, however, influenced both the deposition and diagenesis of evaporite deposits. Here we investigate unusual limestone deposits in the 1.1 Ga Atar Group, Mauritania, that contain both macroscopic and microscopic features that suggest a primary origin as gypsum. Elevated concentrations of carbonate-associated sulfate (CAS), sodium, and potassium are consistent with primary deposition as a sulfate evaporite and subsequent calcification under low water-rock ratios. Isotope and elemental compositions further indicate diagenetic recrystallization under marine to hypersaline marine conditions. Combined, these observations show the potential for recognizing key evaporite facies in the rock record and support a growing recognition of late Mesoproterozoic evaporite deposits.

Sulphur isotope stratigraphy of drill cuttings and stratigraphic correlation of Permian-Triassic evaporites

The stratigraphy of the European late Permian-Triassic commonly lacks chronostratigraphic constraint due to the scarcity of diagnostic fossils for biostratigraphy. This is particularly true for the United Kingdom, and as a result, stratigraphic correlation within and between sedimentary basins is primarily reliant on lithostratigraphy. Evaporitic sulphate can be used to develop time series of δ34Sevap data that can be utilised for stratigraphic correlation. However, the availability of continuous drillcore is limited, whilst drill cuttings are commonly acquired but are widely overlooked for stable isotope stratigraphy. We derive a δ34Sevap record from drill cuttings from the southern North Sea Basin, and successfully correlate it with an equivalent published δ34Sevap record from a continuous drillcore in the Cleveland Basin, Yorkshire, United Kingdom. We have chosen seven points in the δ34Sevap records for stratigraphic correlation, defining eight packages of isotopically distinct coeval strata. This is significant, as the ubiquity of drill cuttings presents the opportunity to derive δ34Sevap curves with high geospatial resolution. Equivalent gamma ray logs were used for correlation and compared with the δ34Sevap curves. The correlations agree relatively well, however, the δ34Sevap correlation permits the development of more robust chronostratigraphic constraints. Specifically, the δ34Sevap records constrain the age of the Bunter Shale and Bunter Sandstone in the western Southern North Sea to the latest Permian. This has significant implications for understanding the stratigraphy and palaeogeographic evolution of United Kingdom Permian-Triassic sedimentary basins, and may have economic implications, since the Bunter Sandstone is being considered as a potential reservoir for CO2 storage in the United Kingdom sector.

Metal mobility and bioaccessibility from cyanide leaching heaps in a historical mine site

Unlike acidic sulfide mine wastes, where metal/loid mobility and bioaccessibility has been widely studied, less attention has been paid to alkaline cyanide heap leaching wastes. Thus, the main goal of this study is to evaluate the mobility and bioaccessibility of metal/loids in Fe-rich (up to 55%) mine wastes resulting from historical cyanide leaching activities. Wastes are mainly composed of oxides/oxyhydroxides (i.e. goethite and hematite), oxyhydroxisulfates (i.e. jarosite), sulfates (i.e., gypsum, evaporitic sulfate salts), carbonates (i.e., calcite, siderite) and quartz, with noticeable concentrations of metal/loids (e.g., 1453–6943 mg/kg of As, 5216–15,672 mg/kg; of Pb, 308–1094 mg/kg of Sb, 181–1174 mg/kg of Cu, or 97–1517 mg/kg of Zn). The wastes displayed a high reactivity upon rainfall contact associated to the dissolution of secondary minerals such as carbonates, gypsum, and other sulfates, exceeding the threshold values for hazardous wastes in some heap levels for Se, Cu, Zn, As, and sulfate leading to potential significant risks for aquatic life. High concentrations of Fe, Pb, and Al were released during the simulation of digestive ingestion of waste particles, with average values of 4825 mg/kg of Fe, 1672 mg/kg of Pb, and 807 mg/kg of Al. Mineralogy may control the mobility and bioaccessibility of metal/loids under rainfall events. However, in the case of the bioaccessible fractions different associations may be observed: i) the dissolution of gypsum, jarosite and hematite would mainly release Fe, As, Pb, Cu, Se, Sb and Tl; ii) the dissolution of an un-identified mineral (e.g., aluminosilicate or Mn oxide) would lead to the release of Ni, Co, Al and Mn and iii) the acid attack of silicate materials and goethite would enhance the bioaccessibility of V and Cr. This study highlights the hazardousness of wastes from cyanide heap leaching, and the need to adopt restoration measures in historical mine sites.

Active Sulfate-Reducing Bacterial Community in the Camel Gut

The diversity and activity of sulfate-reducing bacteria (SRB) in the camel gut remains largely unexplored. An abundant SRB community has been previously revealed in the feces of Bactrian camels (Camelus bactrianus). This study aims to combine the 16S rRNA gene profiling, sulfate reduction rate (SRR) measurement with a radioactive tracer, and targeted cultivation to shed light on SRB activity in the camel gut. Fresh feces of 55 domestic Bactrian camels grazing freely on semi-arid mountain pastures in the Kosh-Agach district of the Russian Altai area were analyzed. Feces were sampled in early winter at an ambient temperature of -15 °C, which prevented possible contamination. SRR values measured with a radioactive tracer in feces were relatively high and ranged from 0.018 to 0.168 nmol S cm-3 day-1. The 16S rRNA gene profiles revealed the presence of Gram-negative Desulfovibrionaceae and spore-forming Desulfotomaculaceae. Targeted isolation allowed us to obtain four pure culture isolates belonging to Desulfovibrio and Desulforamulus. An active SRB community may affect the iron and copper availability in the camel intestine due to metal ions precipitation in the form of sparingly soluble sulfides. The copper-iron sulfide, chalcopyrite (CuFeS2), was detected by X-ray diffraction in 36 out of 55 analyzed camel feces. In semi-arid areas, gypsum, like other evaporite sulfates, can be used as a solid-phase electron acceptor for sulfate reduction in the camel gastrointestinal tract.

Textures, in situ trace element and sulfur isotope of pyrite from the sediment-hosted Cu–Co deposits in the Zhongtiao Mountains, Trans-North China Orogen: Implications for ore genesis

Sediment-hosted Cu–Co deposits in the Zhongtiao Mountains are hosted by the metasedimentary rocks of the Paleoproterozoic Zhongtiao Group in the southern part of the trans-North China orogen, North China Craton. The ore genesis is still disputed with proposed genetic models including metamorphosed sedimentary exhalative (M−SEDEX), metamorphosed sediment-hosted stratiform copper (M−SSC) and metamorphogenic mineralization type. Pyrite occurs as a ubiquitous mineral throughout all ore-forming stages and is ideal for clarifying this issue via the integration of in situ (LA–ICP–MS and EPMA) elemental and LA–MC-ICP–MS sulfur isotope composition analyses. Two main types of pyrite are identified based on petrographic and SEM observations: fine- to medium-grained, variably deformed and inclusion-rich Py1 (including Py1a and Py1b subtypes) in the disseminated–veinlet mineralization stage (S1), and coarse-grained, fractured Py2 (including Py2a and Py2b subtypes) in the fracture-controlled vein-type mineralization stage (S2, main ore stage). Deformed and inclusion-rich Py1a is overgrown by inclusion-poor Py1b, and nonporous Py2a is rimed by porous Py2b. Geochemical results show that the studied pyrite samples have considerable contents of Co (mean > 1000 ppm) and Ni (mean > 100 ppm). These features could be related to both the Co and Ni-rich sedimentary environment (e.g., a volcaniclastic-rich sulfidic marine) and metamorphic enrichment. The content of Se (mean > 200 ppm) in Py2 is significantly higher than that in Py1, indicating the extensive interaction of mineralizing fluids with Se-bearing graphite schist. LA–ICP–MS mapping and textural studies indicate the presence of coupled dissolution-reprecipitation (CDR) reactions that could lead to trace metal compositions (e.g., Co, Ni, Se, Pb, Bi) in the product phases (Py1b, Py2b) that differ from those of their parent phases (Py1a, Py2a). These variations were most likely controlled by fluid temperature and precipitation of specific minerals. The grain-scale chemical zoning of Py2a revealed by elemental mapping indicates fluctuating fluid parameters (e.g., temperature, fS2, and/or parent fluid compositions). The bimodal distributions in Py1 (10–14 ‰ and 18–24 ‰) are consistent with the inheritance of sulfur from sedimentary pyrite and the input of thermochemical sulfate reduction (TSR) -related sulfur from reworked evaporite sulfates in an increasingly open system. The relatively lighter and wider variation in δ34S values (3.2–22.4 ‰, mean = 15.1 ‰) of Py2 implies that the evolution of the ore fluid in S2 was far more complicated, probably as a result of fluid–rock interactions and fluid cooling. During this process, hydrothermal rims with lighter δ34S values (8.4 ‰) grew on older metamorphic pyrite cores with relatively heavier δ34S values (17.4–18.2 ‰). Chalcopyrite displays overlapping S isotopic compositions with pyrite from different mineralization stages, suggesting that copper sulfides inherited reduced sulfur from earlier formed pyrite. Cooling and fluid–rock interactions serve as the critical controls triggering Cu precipitation from the fluid. The combined textural and compositional data of pyrite and chalcopyrite are suggestive of a syn-orogenic copper deposit model, in which all the ore-grade copper was introduced during the Zhongtiao orogeny.

Tepees associated with mobility of evaporite sulfate: The case of the Irati Formation, Permian of Paraná Basin, Brazil

ABSTRACTTepee structures, associated with cracks and intraformational breccias, are found in the basal part of the Thin Rhythmites Bed of the Irati Formation. The rhythmite alternates dark gray mm-thick laminae, formed by dolomicrite with crenulated microlamination rich in organic clay, and intermediate gray laminae, formed by dolarenite with peloids. Some of the rhythmic pairs are separated from each other by thin horizons (&amp;lt; 0.5 mm) with a concentration of quartz pseudomorphs of gypsum and/or pores resulting from bioturbation or dissolution. The close association of the peloids with microrosettes of early authigenic sodium sulfate, a typical salt of nonmarine brines, is suggestive of its formation under cyanobacterial action, favored by hypersaline conditions in inland lakes. This is consistent with the closing of the connection between the Paraná Basin and the Panthalassic Ocean, as has been suggested for the final stages of Irati sedimentation. The tepees analyzed are related to diapiric features of massive light gray dolomicrite, which is distinguished under the microscope as being poorer in organic matter and for presenting coalesced peloids (clots) rich in sodium sulfate. The hydroplastic rheology, overpressure, and density gradient required for the upward injection of light gray dolomicrite are attributed to supersaturation in water and the presence of eodiagenetic low-density hydrated sulfates (e.g., mirabilite and thenardite). Thus, the processes that form the tepees studied here differ from those described in previous models of lacustrine and lagoon tepees, especially regarding the fundamental role of the expansion and mobility of the sulfated dolomite sediment, controlled by the lake's hydrology and by the elevation of groundwater, without necessarily involving subaerial exposure processes.

The Black Angel deposit, Greenland: a Paleoproterozoic evaporite-related Mississippi Valley-type Zn–Pb deposit

The Paleoproterozoic Mârmorilik Formation in the Karrat basin of West Greenland hosts the Black Angel Zn–Pb deposit. Chlorine-rich scapolite, zones with vuggy porosity and quartz nodules in the ore-bearing marble are herein interpreted to represent metamorphosed, vanished, and replaced evaporites, respectively. Mineralization is closely associated with anhydrite with δ34S values (5.2–12.6‰) broadly comparable to published values for Paleoproterozoic seawater sulfate. Considering the fundamental attributes of the mineralization and host sequence, a Mississippi Valley-type (MVT) model is the most obvious explanation for mineralization. Overlying the ore-bearing sequence are organic-rich semipelites and massive calcitic marbles, which may have served as seals for hydrocarbon or reduced sulfur and acted as chemical traps for deposition of the sulfidic ore. The Mârmorilik Formation contained an interlayered sulfate-rich evaporite-carbonate sequence, a common setting for MVT deposits in the late Neoproterozoic and Phanerozoic, but unique among the few known MVT deposits in the Paleoproterozoic. This ca. 1915 Ma evaporite-carbonate platform is younger than sulfate evaporites deposited during and immediately after the ca. 2220–2060 Ma Lomagundi carbon isotope excursion and records a significant seawater sulfate level during a time interval when it was assumed that it had been too low to form extensive evaporite deposits. Therefore, MVT and clastic-dominated (CD) Zn–Pb deposits in the geological record might progressively fill the apparent gap in marine sulfate evaporites and provide unique insights into Proterozoic seawater sulfate level. Considering the sequence of tectonic events that affected the Karrat basin, the mineralization took place between Nagssugtoqidian collision (< 1860 Ma) and Rinkian metamorphism (ca. 1830 Ma).

Responses of the biogeochemical sulfur cycle to Early Permian tectonic and climatic events

The late Paleozoic was characterized by a series of continental collisions and ice ages. Despite the drastic environmental changes, sparse sulfur isotope data hinder our understanding of the late Paleozoic biogeochemical sulfur cycle, especially during the Early Permian. To overcome this potential bias, we present a high-resolution sulfur isotope record of carbonate-associated sulfate (CAS) and pyrite from the Carboniferous-Permian successions of the Svalbard archipelago. Throughout the Carboniferous, our results are largely consistent with the global trend, although the development of restricted environments resulted in a regionally observed δ34SCAS peak of +20‰ during the Gzhelian. The Early Permian δ34SCAS data in Svalbard bridge the gap in the existing record, showing a steady increase contemporaneous with the closure of the Ural Seaway and Gondwana glaciation, albeit superimposed by short-term oscillations. The enhanced incorporation of diagenetic sulfate into authigenic carbonates may have caused small-scale oscillations during the regional regression in the Artinskian, but the long-term increasing trend of δ34SCAS and its relation to known geological events can be best explained by the enhanced pyrite burial flux driven by a major shift in the locus of organic carbon burial from the continent to the ocean, with a lesser contribution from the dissolution of epicontinental seaway evaporites. Since the onset of the Middle Carboniferous Bashkirian δ34SCAS excursion also corresponds in timing to the major glaciation event and the closure of the Rheic Seaway, the sulfur isotope record in the course of the consolidation of Pangea is apparently punctuated by the episodes of increased pyrite burial and evaporite sulfate weathering, delineating the links between paleogeography, paleoclimate, and biogeochemical cycles.

Organic geochemical evidence for the transition of Aptian-Albian hypersaline environments into marine restricted seas: The South Atlantic oceanic northern gateway and its implications for the pre-salt deposits

The transition between Aptian and Albian stages of the Lower Cretaceous (∼113 Ma) was a critical moment in the development of the proto–South Atlantic Ocean. At this time, marine ingressions in Brazilian sedimentary basins occurred in the northeastern region and deposited organic-rich mudstones and sulfate evaporites. However, the paleogeographic configuration that controlled sedimentation and biota dynamics remains poorly understood. This study presents new paleogeographic insights based on tectonic reconstructed Gplate models and a high-resolution chemostratigraphic framework. We explore new isotopic data of bulk organic carbon and nitrogen as well as saturated fraction biomarkers collected from a well-preserved section in the eastern part of the Brazilian northeastern Parnaíba Basin (Codó Formation). We also present new biomarker data for samples from the Barra Velha Formation from a well in the Santos Basin, which belongs to the same local stage of the Codó Formation (Alagoas Stage). For the Parnaíba Basin section, geochemical proxies subdivide the environmental evolution into five stages. These stages reflect a transition from a highly saline environment (stage A) that experienced transient marine influxes and eventually transitioned into a closed evaporitic system (stage B). A major marine ingression established an epicontinental anoxic sea (Stage C) that subsequently experienced an increase in water circulation and water depth; turning into an oxic marine environment (Stage D). Interruption of the connection with the sea and constant terrestrial input led to a continentalization process establishing a lacustrine/continental environment (Stage E). For the Santos Basin section (Barra Velha Formation), we measured the occurrence of the C30 tetracyclic polyprenoids which are consistent with a dominant lacustrine saline environment. Our paleogeographic model suggests that marine ingressions towards the southern basins, during the first stages of Pangea breakup, occurred through the northern portion of the Borborema Province and produced an interior seaway that possibly connected the Brazilian northeastern with the southeastern basins through the Recôncavo/Tucano rift system. Our findings and the environmental interpretations for stratigraphic correlated units from the southern continental basins infers that the greater marine influence in the northeastern areas shifted from hypersaline environments into restricted marine environments that resulted in the deposition of shales and mesohaline/penesaline salts. Concomitantly, the existence of a Brazilian-African landbridge acted as a barrier point for the first marine inflows towards the south. The restricted marine connection transformed south alkaline environments into extensive areas of superhaline conditions that resulted in giant thick salt successions of the South Atlantic basins. Finally, further tectonic evolution and complete breakup of Pangea terminated the Brazilian-African landbridge and shutoff interior marine routes that later became terrestrial environments. As the continents steadily became rifted-segments, the proto-South Atlantic Ocean expanded leaving a marine record in the southern basins.

Remains of a subtropical humid forest in a Messinian evaporite-bearing succession at Govone, northwestern Italy – Preliminary results

The first results of a multidisciplinary study on the Messinian evaporitic interval of the Govone section (northwestern Italy), subdivided into several sedimentary cycles, are reported here. Primary sulphate evaporites and intercalated shaly deposits, which formed during the Messinian Salinity Crisis, contain well-preserved and taxonomically determinable terrestrial plant remains. Palynomorphs are present continuously all along the analysed succession and among them, pollen is especially abundant and diverse. Additionally, a few fragments of silicified wood (which are rare in the studied area) occurred in layers rich in phytodebris, but their analysis showed poor preservation of anatomy. Carpological remains are not abundant and are strongly altered by diagenesis. Compressed conifer shoots and angiosperm leaves are well-preserved in several layers, one of which yielded a particularly diverse assemblage. The various plant records have been integrated to obtain an initial general idea on the floristic composition of the assemblages and the palaeoenvironment. We hypothesize that a subtropical humid forest may have surrounded the basin, not only during the deposition of shale deposits, but also during the formation of gypsum, generally considered a product of arid climate. Despite the low-resolution sampling strategy for macrofossils, the identification of several relevant plant taxa (Engelhardia orsbergensis, Eurya stigmosa, Symplocos casparyi, Taiwania sp.), seems to indicate that the Govone section could provide an interesting glimpse into the composition of the palaeoflora of northern Italy during the deposition of the Messinian evaporites.

Genetic model for early Cambrian reef limestone-hosted Pb-Zn deposits in the world-class Huayuan orefield, South China: New insights from mineralogy, fluorite geochemistry and sulfides in situ S-Pb isotopes

The Xiangxi-Qiandong metallogenic belt (>300 Mt sulfide ores @∼4.0 wt% Pb + Zn), located at the southeastern margin of the Yangtze Block, is one of the important Pb-Zn producers in China. Although carbonate-hosted Pb-Zn deposits in this district have attracted great attention during past decades, many issues, particularly the ore genesis and geodynamic setting, remain unclear. In this study, we take the Limei and Yutang deposits as typical case studies, using detailed geology, rare earth elements and yttrium (REY) geochemistry of fluorite and sulfides in situ S-Pb isotopes, to propose a new model for early Cambrian reef limestone-hosted Pb-Zn deposits in the district. REY contents (0.47 to 8.67 ppm; mean = 3.99 ppm) in fluorite from Limei and Yutang slightly deplete in LREE and HREE relative to MREE, indicating that the REEs and metals were transported predominantly as chloride complexes in a low-temperature (<200 °C) hydrothermal fluid with high saline. New δ34S values of sulfides (+27.66 to + 35.46‰ in Limei and + 24.80 to + 36.07‰ in Yutang) imply that S2- was originated from evaporite sulfate within ore-hosting strata via thermo-chemical reduction (TSR) as the presence of reducing agents (bioclastic limestone and organic matter). In situ δ34S values show a slightly large range (variation up to 11.27‰) and vary significantly within single grain (up to 4.60‰), which may be inherited from the S composition of H2S in the original fluid. In situ Pb isotopic ratios of sulfides from Limei (206Pb/204Pb = 18.001–18.269, 207Pb/204Pb = 15.645–15.796 and 208Pb/204Pb = 38.243–38.597) and Yutang (206Pb/204Pb = 18.135–18.384, 207Pb/204Pb = 15.714–15.924 and 208Pb/204Pb = 38.347–38.874) positively correlates in Pb-Pb diagrams, indicating the mineralizing metals are mainly derived from Proterozoic basement rocks with an amount of contribution from early Cambrian sedimentary rocks. Mineralogy, fluorite REY geochemistry and sulfides in situ S-Pb isotopes together with previously reported mineralization ages suggest that the mixing of metal-rich fluid from basement rocks and the H2S-rich fluid originated from ore-hosting strata plays an important role in forming the Pb-Zn deposits of Limei and Yutang. This study provides a typical example for better understanding the genesis and geodynamic setting of MVT deposits in the foreland basin, South China.

Calibrating the triple oxygen isotope composition of evaporite minerals as a proxy for marine sulfate

The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO2/pCO2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'17O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'17O and δ18O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate.

Effects of Mississippi valley-type minerogenetic metal sulfates on thermochemical sulfate reduction, studied by hydrous pyrolysis

Mississippi valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits, that are formed by fluid expulsion from sedimentary sulfide successions. The sulfides were generated by thermochemical sulfate reduction (TSR) of the evaporitic sulfates dissolved in fluids. The initiation, processes and principles for TSR occurring in MVT ore deposition and the respective influence of major minerogenetic metal ions such as Pb2+ and Zn2+, as well as Fe2+, Sr2+, Ba2+ has not been clearly resolved. To evaluate the TSR activity of metal cations in MVT minerogenetic systems, a series of 300 °C to 450 °C gold-tube hydrous-pyrolysis experiments were separately conducted with FeSO4, PbSO4, ZnSO4, BaSO4 or SrSO4 and with n-octadecane (n-C18) as the hydrocarbon substrate. Based on the yields (from gas chromatography (GC) analysis) and carbon isotopic compositions (determined by GC-irMS) of the gases produced in the hydrous-pyrolysis gold-tube experiments, the TSR reactivity of the minerogenetic metal sulfates was ranked FeSO4 > ZnSO4 > Sr/BaSO4 > PbSO4. TSR occurred easily in the FeSO4 experiments at 300 °C, but hardly at all in PbSO4 experiments at 450 °C. Hence, S2− for the formation of the gelenite (PbS), sphalerite (ZnS) and pyrite (FeS2) in the MVT ore deposits appears to be related to Fe2+, which could initiate the TSR easily to produce reduced sulfur.The following two potential routes would provide good support for the TSR of FeSO4: (1) Hydrolysis of Fe2+, or the formation of a ferrous hydroxide-sulfate-hydrate complex, that increases the H+ concentration, resulting in the formation of HSO4− that initiates TSR; and (2) The oxidation of Fe2+ to Fe3+, which on subsequent hydrolysis (or the formation of iron-hydroxide-sulfate-hydrate complex), would greatly increase the concentrations of H+ and HSO4−, reduce the pH of the brine fluids and maintain acidic conditions favorable to TSR. However, the precipitation of pyrite greatly consumes S2−, limiting the concentration of H2S and thereby affecting the rate of TSR.In short, TSR is difficult to simulate using Zn2+ and Pb2+ sulfates, but easy with sulfates containing Fe2+ and Mg2+. This suggests that the occurrence of the FeSO4 and MgSO4 is critical in the formation of the large-scale MVT ore deposits, where they react as acid buffering agents, decreasing and maintaining a lower pH in the brine fluids and accelerating and sustaining TSR with higher concentrations of HSO4−. H2S/S2− would then be produced continually and participate as FeS2, ZnS and PbS in the large-scale MVT ore deposits.

The response of water column and sedimentary environments to the advent of the Messinian salinity crisis: insights from an onshore deep-water section (Govone, NW Italy)

AbstractDuring Messinian time, the Mediterranean underwent hydrological modifications culminating 5.97 Ma ago with the Messinian salinity crisis (MSC). Evaporite deposition and alleged annihilation of most marine eukaryotes were taken as evidence of the establishment of basin-wide hypersalinity followed by desiccation. However, the palaeoenvironmental conditions during the MSC are still a matter of debate, chiefly because most of its sedimentary record is buried below the abyssal plains of the present-day Mediterranean Sea. To shed light on environmental change at the advent and during the early phase of the MSC, we investigated the Govone section from the Piedmont Basin (NW Italy) using a multidisciplinary approach (organic geochemical, petrographic, and carbon and oxygen stable isotope analyses). The Govone section archives the onset of the crisis in a succession of organic-rich shales and dolomite-rich marls. The MSC part of the succession represents the deep-water equivalent of sulphate evaporites deposited at the basin margins during the first phase of the crisis. Our study reveals that the onset of the MSC was marked by the intensification of water-column stratification, rather than the establishment of widespread hypersaline conditions. A chemocline divided the water column into an oxygen-depleted, denser and more saline bottom layer and an oxygenated, upper seawater layer influenced by freshwater inflow. Vertical oscillations of the chemocline controlled the stratigraphic architecture of the sediments pertaining to the first stage of the MSC. Accordingly, temporal and spatial changes of water masses with different redox chemistries must be considered when interpreting the MSC event.