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Sedimentary exhalative (sedex) ore deposits are the world’s largest Zn-Pb deposits. While the geologic processes that formed these deposits are generally well understood, the fundamental hydrologic processes that drove these massive hydrothermal systems remain an area of debate. We use numerical modeling to test an emerging hypothesis, supported by recent ore genesis research and sedex basin analysis, that brine reflux flow systems produced and drove the fluids that formed sedex deposits. A previous numerical model of brine reflux, developed to study dolomitization, is adapted to a sedimentary basin with geologic features essential for sedex formation. We simulate the flow of evaporated brines through the basin and the evolution of salinity, temperature, and flow rates, and find that modeled values for these parameters for brines discharging to the seafloor exceed previously established physiochemical thresholds for ore formation (>170 g/L, >80 °C, and total discharge volumes >107 m3 per meter perpendicular to the 2D model section). Sensitivity testing of this modest-sized basin highlights the large effect that aspects of the hydrogeologic framework can have on mineralizing potential of the reflux brines. Finally, modeling alternating periods of active and inactive evaporation produces pulsed brine reflux systems capable of producing multiple deposits of different age as observed in many sedex basins. The modeling thus supports the hypothesis that seawater evaporation on the basin margin significantly inboard of sedex deposits may be responsible for their formation. Sensitivity testing suggests that numerical models with more detailed, basin-specific geologic frameworks might be useful for assessing the mineral potential of sedimentary basins.

The Williston Basin has produced oil and gas from conventional structural and stratigraphic traps for more than 60 years. The advent of horizontal drilling and hydraulic fracturing of shale in the Devonian-Mississippian Bakken Formation has increased the production of oil in this basin from areas that partially overlap the prairie pothole region of North America. Massive scale of oil production in the Williston Basin increases the risk of accidental releases of coproduced formation water (brine) into the environment. The prairie pothole region is named for a multitude of small lakes and wetlands that provide critical habitat for waterfowl and other wildlife. The increased risks raise the importance of developing robust tracers that can identify the source(s) and quantity of contamination of this premier wetland ecosystem. Radiogenic 87Sr/86Sr and 234U/238U isotope tracers are widely used in hydrological studies to detect potential sources of groundwater contamination. Here, we used paired 87Sr/86Sr and δ88Sr values in wetland water, which is essentially shallow groundwater at the Goose Lake and Fuller sites in Montana and North Dakota where produced water contamination was previously established. We also analyzed brines from the Bakken Formation and Mississippian Charles Formation to evaluate potential sources of contamination. This is the first attempt to combine these isotope tracers to estimate a magnitude of groundwater contamination by the oil-field produced water. Moreover, because U+6 is soluble in water under subaerial oxidizing conditions and the U+4 content is orders of magnitude lower (pg/g levels) in reducing brines, the 234U/238U isotope tracer is insensitive to brine contamination. However, it elucidates potential variability of uncontaminated end-members and, therefore, helps to improve the accuracy of estimated degrees of brine contamination based on binary mixing relationships. In addition, U isotopes in groundwater have a potential for detecting some U-rich anthropogenic contaminants (e.g., phosphate fertilizers). Using these isotopic systematics we confirm previous conclusions that surface water and shallow groundwater in two studied sites are variably contaminated by produced water and estimate the degree of the contamination at ∼7% in a sample collected close to a brine tank and

Sedimentary-exhalative (sedex) ore deposits were formed by discharge of metal-rich brines into ancient ocean basins. Chemical, isotopic, and geologic data from several Paleozoic sedex districts suggest that the brine discharges also supplied enormous quantities of radiogenic Sr and biolimiting nutrients to the oceans. Seven middle Paleozoic sedex events appear to coincide with short-duration positive excursions (“spikes”) in the global marine Sr-isotope record that are not explained by current oceanic models. These strong temporal correlations, combined with mass balance evidence and oceanographic modeling, suggest the flux of radiogenic Sr-rich sedex brines may have been sufficient to cause these prominent spikes. If these sedex hydrothermal events are recorded in the secular record, then the 87 Sr/ 86 Sr record may provide a unique tool for ore genesis studies and for assessing the mineral potential of sedimentary basins of different ages. The apex of these enigmatic 87 Sr/ 86 Sr spikes correlate with global δ 13 C and δ 18 O spikes, periods of global anoxia, deposition of metal-rich black shales, deposition of ironstones, climate change, metal-induced malformation (teratology) of marine organisms and significant mass extinctions. While the relationships among these phenomena remain poorly understood and diverse models for these events have been proposed, most invoke an increased flux of biolimiting nutrients resulting in ocean eutrophication. Evidence that the flux of key biolimiting nutrients and metals contained in sedex brines may have been equivalent to or surpass that of the total modern riverine flux to the ocean suggests that these sedex brine exhalations may have triggered global chemical and biological events.

The first appearance of pinnacle reef tracts, composed of hundreds to thousands of localized biogenic structures protruding tens to hundreds of meters above the surrounding mid-Silurian seafloor, represents a step change in the evolution of the marine biosphere. This change in seafloor morphology opened a host of new ecological niches that served as “evolutionary cradles” for organism diversification. However, the exact timing and drivers of this event remain poorly understood. These uncertainties remain, in large part, due to a paucity of index fossils in the reef facies, the difficulty of correlating between the offshore pinnacle reefs and more temporally well-constrained shallow marine facies, and cryptic unconformities that separate amalgamated reefs. Here we use δ 13 Ccarb stratigraphy within a sequence stratigraphic framework to unravel these complex relationships and constrain the origination of Silurian pinnacle reef tracts in the North American midcontinent to near the Pt. celloni Superzone—Pt. am. amorphognathoides Zonal Group boundary of the mid-Telychian Stage. In addition, we identify a striking relationship between pulses of reef development and changes in global δ 13 Ccarb values and sea level. Viewed through this new perspective, we correlate prolific periods of reef development with short-lived carbon isotope ( δ 13 Ccarb) excursions and eustatic sea level change that, ultimately, reflect perturbations to the global carbon cycle. From changes in the dominance of microbial reefs of the Cambrian to metazoan colonization of reefs in the Middle Ordovician, through the subsequent collapse of metazoan diversity with the Late Ordovician mass extinction, and the first appearance of early Silurian (Llandovery) pinnacle reef tracts and their proliferation during the late Silurian (Wenlock-Pridoli) and Devonian, major reef formation intervals increasingly coincide with δ 13 Ccarb excursions. These patterns suggest that Paleozoic reef evolution was the product of environmental forcing by perturbations of the global carbon cycle.

Mineral thermometry and fluid inclusion studies were conducted on variably altered and mineralized samples from the Mesoproterozoic Pea Ridge iron oxide-apatite (IOA)-rare earth element (REE) deposit in order to constrain P-T conditions, fluid chemistry, and the source of salt and volatiles during early magnetite and later REE mineralization. Scanning electron microscopy (SEM)-cathodoluminescence and SEM-backscatter electron images show that quartz and rutile precipitated before, during, and after magnetite and REE mineral growth. Ti-in-quartz and Zr-in-rutile equilibration temperatures range from ≤350° to 750°C in the amphibole, magnetite, hematite, and silicified zones where T increased during magnetite and quartz growth and dropped precipitously after fracturing and brecciation. Late drusy quartz cements within a REE-rich breccia pipe record the lowest T (≤315°–400°C). Liquid-, vapor-rich, and hypersaline (±hematite, calcite) fluid inclusions are common and liquid CO 2 is present locally. Salinities define three populations: saline (10–27 wt % NaCl equiv), hypersaline (34–>60 wt % NaCl equiv), and dilute (0–10 wt % NaCl equiv ). The wide range of eutectic melting temperatures (−67° to −19°C) suggests that saline inclusions trapped variable proportions of a CaCl-MgCl-FeCl-bearing fluid end member and an NaCl-KCl fluid end member. Homogenization temperatures and pressures of these saline inclusions suggest they were trapped when fluids unmixed into brine and vapor at T The Na/Cl, Na/K, and Cl/Br ratios of fluid inclusion extracts provide evidence for mixtures of magmatic hydrothermal fluids and evaporated seawater. Extracts from magnetite, hematite, and pyrite plot in the magmatic-hydrothermal field, indicating that Fe was derived from a magmatic source. Their enrichments in Mg and Ca are consistent with a mafic magmatic source. The positive correlation between Na/Mg and Na/Ca ratios may be due to halite saturation or albitization of igneous rocks. Extracts from barite in the REE-rich breccia pipes are enriched in Na and Br and plot near the seawater evaporation trend. He is highly enriched relative to Ne and Ar in fluid inclusion extracts, which precludes air as a source of He. Although the He is mostly of crustal origin, pyrite with a 3 He/ 4 He (R/R A ) of 0.1 contains up to 12% mantle He. Many extracts have low 20 Ne/ 22 Ne ratios due to nucleogenic production of 22 Ne in high F/O minerals such as fluorapatite or F biotite. The arrays of data for 3 He/ 4 He (R/R A ) and 22 Ne/ 20 Ne suggest that volatiles were derived from two sources, a moderate F mafic magma containing mantle He and a high F silicic magma with crustal He. Together with other evidence cited in this report, these data (1) support a magmatic hydrothermal origin for the Mesoproterozoic magnetite-apatite deposit with ore fluids derived from a concealed mafic to intermediate-composition intrusion, (2) suggest that the REE minerals in breccia pipes were either derived from apatite or precipitated in response to decompression and cooling during breccia pipe formation, (3) provide evidence for the influx of basinal brine, magmatic fluids from granitic intrusions, and meteoric water after breccia pipe formation, and (4) show that Pea Ridge was relatively unaffected by the late Paleozoic Mississippi Valley-type (MVT) Pb-Zn system in overlying Cambrian sedimentary rocks.

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.

Constraining the age of strata is a fundamental source of uncertainty in the study of sedimentary rocks, particularly in restricted basins that generally lack index fossils. An illustrative example of this is the evaporite-bearing Salina Group in the Michigan Basin. Our integrated study of facies, paleontology, and stable isotope geochemistry from the base of the Salina Group in Wisconsin addresses long-standing chronostratigraphic uncertainty surrounding these units. Conodont samples from the basal boundary interval (Racine-Waubakee formation contact) produced non-diagnostic ‘disaster’ and ‘recovery’ faunas typical of both the Mulde (Homerian) and Lau (Ludfordian) events. Strontium isotope analysis (87Sr/86Sr) of these conodonts from five horizons just below the boundary yield values between 0.70844 and 0.70850 confirming a Homerian age. Multiple carbon isotope profiles through this interval confirm the presence of a 2.5–3‰ positive excursion. Cumulatively these data constrain the base of the Salina Group in Wisconsin to the Mulde Excursion interval (late Homerian). This integrated study provides a sound initial step towards a deeper understanding of the processes of Silurian evaporite formation in the Michigan Basin.

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation.

Integration of sedimentologic, chemostratigraphic, biostratigraphic, and geochronologic data identifies the profound influence that sudden bursts of carbonate sedimentation, sea-level oscillation, and hypersalinity, associated with Ireviken and Mulde carbon cycle perturbations, had on the evolution of the Michigan Basin during the Silurian. Conodonts and carbon isotope (δ13Ccarb) stratigraphy of the Lockport and Engadine Dolomites constrain the rapid build-up and progradation of a thick biohermal wedge along the basin margins to the Sheinwoodian-age Ireviken Excursion, a recently identified period of global reef growth. This event significantly changed the hydrologic architecture of the basin, ultimately resulting in restricted seawater exchange that drove evaporite deposition. Zircon U Pb ages of ~428 Ma from bentonites near the base of the evaporite-bearing Salina Group, together with δ13Ccarb profiles transecting the basin, constrain the onset of hypersalinity to initiation of the late Homerian Mulde Excursion. Importantly, the double δ13Ccarb peak of this event parallels a complex pattern of dramatic sea-level oscillations, on the order of 100 m, recorded within the A-0 to A-2 evaporites and associated pinnacle reefs. Ascending δ13Ccarb values were accompanied by sea-level fall, karstification, and the formation of evaporites in the basin center. Intra-excursion sea-level rebounds triggered exceptionally rapid upward pinnacle reef growth. Subsequent sea-level fall again exposed the ramp and pinnacle reefs resulting in basin-wide restriction and massive salt deposition that, ultimately, encased the reefs in salt. The return to baseline δ13Ccarb values was accompanied by flooding and carbonate sedimentation. Our integrated approach, with high-resolution δ13Ccarb stratigraphy from closely spaced (intra-basinal) sections as a central component, enables finely tuned chronostratigraphic correlation, recognition of unconformity-bound facies packages (i.e., systems tracts), and constrains variations in sea level and sedimentation rates. These findings have immediate applications to predictive static reservoir models for the evaluation of the reefs as repositories for anthropogenic CO2 sequestration, gas storage reservoirs, and hydrocarbon exploration. Ultimately, our findings demonstrate that a cascade of rapid environmental changes, including pulses of carbonate sedimentation, high-amplitude sea-level oscillations, and evaporite deposition, were associated with the Ireviken and Mulde perturbations of the Silurian carbon cycle.

Revised time-scales for the Devonian Period have highlighted major discrepancies in the frequency of evolutionary, eustatic, carbon cycle, and biotic events. The number of conodont biozones per million years shows strong variation among stages. Using two alternative time scales, the lowest values are in the Emsian and highest in the Givetian, Frasnian, and Famennian stages. A similar result is obtained by dividing the Devonian time scale into 12 equal 5-million year bins and determining the number of conodont zonal subdivisions in each bin. The record of ammonoid biozones also shows interesting parallels. The acuity of biostratigraphic zonation and zonal subdivisions appears to be correlative with the frequency of environmental perturbations, including eustatic, positive carbon isotopic excursions, and hypoxic/anoxic related bioevents, and these are similarly unevenly distributed. All calculations of environmental volatility metrics show the late Eifelian through early Frasnian to be the most volatile interval of the Devonian, followed by the mid-late Famennian, while the Lochkovian through Emsian and mid-Frasnian through early Famennian are the least volatile. Furthermore, the majority of major bioevents occur in the Eifelian through early Frasnian, and late Famennian. The strong evidence for higher biotic and environmental volatility in the late Eifelian through early Frasnian and late Famennian rock record corresponds to the coolest paleoclimate conditions in the Devonian according to recent conodont apatite δ18O compilations, and the transition from an overall cooling to warming paleotemperature trend during the mid-Givetian. Regardless of mechanism, strong variations in volatility occurred during the Devonian Period. Intervals with greater biostratigraphic resolution also show much stronger environmental volatility including a series of relatively quasi-stable conditions punctuated by major turnovers with abrupt sea-level rise, widespread hypoxia, and changes in the carbon cycle. It was during these intervals that much net evolutionary and ecological change appears to have taken place.

The Late Ordovician, particularly the Sandbian and early Katian, represents the diversity apex for many groups of organisms during the Great Ordovician Biodiversification Event. Establishing clear cause-and-effect evidence for those factors that drove diversification versus those that stunted it is a daunting task. Development of a high-resolution regional chronostratigraphic framework is foundational to moving this work forward in eastern North America. The Upper Ordovician chronostratigraphy of the US midcontinent is undergoing significant revision through a collaborative effort to integrate biostratigraphic ranges, chemostratigraphic patterns and facies distributions within a sequence stratigraphic framework. New subsurface cores drilled in Wisconsin and Indiana, together with archived collections from Michigan, Iowa, Illinois, Ohio, Kentucky, Tennessee, Missouri and Arkansas, and targeted sampling of surface exposures across this region (~500,000 km2), has yielded critical new material for study. New chitinozoan, graptolite and conodont range data are being integrated with results of previous biostratigraphic studies in the region. High-resolution δ13C (N>5,000) and 87Sr/86Sr isotope data from these sections and others, forming transects parallel and perpendicular to the basin axis, provide additional chronostratigraphic refinement and critical data for intervals barren of index fossils. Bounding surfaces of facies packages have been closely studied and their enclosed strata characterized texturally as well as compositionally using portable X-ray fluorescence (pXRF). Anomalies in the concentration of redox-sensitive elements (e.g., Mn, P, Zn, etc.), together with fossil epiboles and distinctive event beds provide additional tie points for local correlation within portions of basins, particularly along strike. The pXRF results also provide rich data sets for time-series analysis, anchored by an expanding number of U-Pb TIMSzircon ages from bentonites sampled across the study area. Integration of these manydata sets is providing substantial new chronostratigraphic insights as our study reaches maturity. One of the most profound findings is that regional diachroneity is a clear pattern from the basin centers to the intervening arches, but even more so from the continental interior toward the margins (particularly from modern day north to south). Long-standing range uncertainties, sampling gaps, facies bias and localized perspective are being resolved through this coordinated analytical campaign.

The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at ∼840 Ma and inversion at ∼535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing ∼11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a ∼500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ± Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions. Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intense Mg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part of the basin fill. From ∼765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across the northern and central parts of the basin, depending upon the interplay of original salt thickness, rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus, while an early orogenic timing is permissible, mineralization during the later stages of extensional basin development was more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the ∼800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt’s properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.

Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions ("spikes") in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic Sr-87/Sr-86 spikes. We review existing Sr-87/Sr-86 data and present new data as verification of these global Sr-87/Sr-86 spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 x 10(-4) (similar to 0.7078-similar to 0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at -389 Ma; spikes on the order of 1 to 3 x 10(-4), coeval with formation of the Meggen deposit at similar to 381 Ma, several ore deposits in the Macmillan Pass district at similar to 379 to 375 Ma, and the Silvermines deposits at similar to 352 Ma; and two >6 x 10(-4) spikes coincident with formation of the giant Navan deposit at similar to 346 Ma and Red Dog deposits at similar to 337 Ma. Moreover, the timing of peak 8(87)Sr/Sr-86 spikes correlates with global delta C-13 and delta O-18 spikes,deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution Sr-87/Sr-86 isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine Sr-87/Sr-86 curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins.To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.

Isotope ratio measurements using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) commonly use standard-sample bracketing with a single isotope standard for mass bias correction for elements with narrow-range isotope systems measured by MC-ICP-MS, e.g. Cu, Fe, Zn, and Hg. However, sulfur (S) isotopic composition (δ 34 S) in nature can range from at least − 40 to + 40‰, potentially exceeding the ability of standard-sample bracketing using a single sulfur isotope standard to accurately correct for mass bias. Isotopic fractionation via solution and laser ablation introduction was determined during sulfate sulfur (S sulfate ) isotope measurements. An external isotope calibration curve was constructed using in-house and National Institute of Standards and Technology (NIST) S sulfate isotope reference materials (RM) in an attempt to correct for the difference. The ability of external isotope correction for S sulfate isotope measurements was evaluated by analyzing NIST and United States Geological Survey (USGS) S sulfate isotope reference materials as unknowns. Differences in δ 34 S sulfate between standard-sample bracketing and standard-sample bracketing with external isotope correction for sulfate samples ranged from 0.72‰ to 2.35‰ over a δ 34 S range of 1.40‰ to 21.17‰. No isotopic differences were observed when analyzing S sulfide reference materials over a δ 34 S sulfide range of − 32.1‰ to 17.3‰ and a δ 33 S range of − 16.5‰ to 8.9‰ via laser ablation (LA)-MC-ICP-MS. Here, we identify a possible plasma induced fractionation for S sulfate and describe a new method using external isotope calibration corrections using solution and LA-MC-ICP-MS.

Systematic collecting through the upper Wenlock (upper Homerian) and lower Ludlow (Gorstian and lowermost Ludfordian) Silurian rock succession of the Long Mountain, Powys, Wales, identifies some 48 chitinozoan species that distinguish four biozones, two subzones and an interregnum. Consideration of the chitinozoan biozones together with those of the graptolites enables a local three-fold subdivision of the late Homerian lundgreni graptolite Biozone, and the distinction of lower and upper intervals for the Gorstian incipiens graptolite Biozone. The base of the Ludlow Series in the Long Mountain more or less equates to the base of the Cingulochitina acme chitinozoan Biozone, although no key chitinozoan first or last appearance datums are associated with the series boundary itself. The new graptolite–chitinozoan biozonation allows enhanced correlation between upper Wenlock and lower Ludlow sedimentary deposits of the Lower Palaeozoic Welsh depositional basin and those of the palaeo-shelf in the stratotype Wenlock and Ludlow areas of Shropshire. Chitinozoans seem affected by the phenomena that caused the late Wenlock ‘Mulde extinction’ in graptolites but, with the final disappearance of 9 species and re-appearance of 11 species following an interval of overall low diversity, they seem to have suffered less severely than their macro-zooplanktonic contemporaries.

Fluid inclusions hosted by Mississippi Valley-type (MVT) ores in the Illinois-Kentucky district were analyzed using microthermometry, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and Raman spectroscopy to characterize the composition and temperature of mineralizing fluids in order to seek insights into the process of ore formation. Results from the present study support the interpretation of previous microthermometry studies that relate mineralization to mixing of two higher temperature brines and a cooler, dilute meteoric fluid. Fluid inclusions in the Illinois-Kentucky district have overall higher atomic ratios of Ba/Na, Ca/Mg, K/Na, Sr/Na, and lower atomic ratios of Mg/Na compared to inclusions in the Ozark MVT districts and are thus compositionally distinct. These differences may indicate that the Illinois-Kentucky and Ozark MVT ore fluids were derived largely from different source basins or may reflect input from igneous rocks in the Illinois-Kentucky district. Low Ca/Mg ratios of Illinois-Kentucky fluid inclusions indicate that mineralizing fluids were dolomitizing and therefore could not have traveled far through the limestone ore host formations, but instead traveled through deeper nonlimestone units before rising along faults and fractures in the district. Some fluid inclusions in Illinois-Kentucky MVT mineralization yielded strong LA-ICP-MS responses for ore metals. A single sphalerite-hosted fluid inclusion out of 18 analyzed contained detectable Pb, corresponding to a concentration of 420 ppm, whereas numerous fluorite-hosted fluid inclusions contained detectable Pb, Zn, and Cu, corresponding to maximum concentrations on the order of 1,000s of ppm. The high and similar concentrations of Cu, Pb, and Zn in some fluorite-hosted fluid inclusions in the Illinois-Kentucky district are problematic because Cu sulfide mineralization should be much more abundant than Pb and Zn sulfide mineralization owing to the lower solubility of Cu relative to Pb and Zn in the presence of reduced sulfur, which is not observed in the field. This raises the possibility that ore metals may exist as solid phases in suspension or adhering to fluid inclusion walls in some inclusions, rather than as dissolved species in aqueous solution. Raman microprobe analyses of fluid inclusions hosted by minerals occurring throughout the paragenetic sequence detected methane concentrations ranging from about 40 to 1,200 ppm, with most on the order of 100s ppm. Even for the lowest methane concentration determined, redox conditions would have been reducing, at a log of −55.4 at a temperature of 100°C. At such reducing conditions, sulfide would predominate over sulfate; this is supported by high concentrations of Ba in the fluid inclusions. The overall reducing conditions throughout the paragenetic sequence and the possible presence of both metal-poor and -rich fluid inclusions would suggest that mixing between a metal-rich, sulfide-poor fluid and a metal-poor, sulfide-rich fluid were an important mechanism for sulfide mineral precipitation. Minimum depths of mineralization estimated from methane concentrations in fluid inclusions range to about 1,500 m and possibly 2,500 m, which would not have been reached until Late Pennsylvanian to Permian time, i.e., during the Alleghanian orogeny.

The critical role of rare earth elements (REEs), particularly heavy REEs (HREEs), in high-tech industries has created a surge in demand that is quickly outstripping known global supply and has triggered a worldwide scramble to discover new sources. The chemical analysis of 23 sedimentary phosphate deposits (phosphorites) in the United States demonstrates that they are significantly enriched in REEs. Leaching experiments using dilute H2SO4 and HCl, extracted nearly 100% of their total REE content and show that the extraction of REEs from phosphorites is not subject to the many technological and environmental challenges that vex the exploitation of many identified REE deposits. Our data suggest that phosphate rock currently mined in the United States has the potential to produce a significant proportion of the world's REE demand as a byproduct. Importantly, the size and concentration of HREEs in some unmined phosphorites dwarf the world's richest REE deposits. Secular variation in phosphate REE contents identifies geologic time periods favorable for the formation of currently unrecognized high-REE phosphates. The extraordinary endowment, combined with the ease of REE extraction, indicates that such phosphorites might be considered as a primary source of REEs with the potential to resolve the global REE (particularly for HREE) supply shortage.

We present strontium isotopic (88Sr/86Sr and 87Sr/86Sr) results obtained by 87Sr–84Sr double spike thermal ionization mass-spectrometry (DS-TIMS) for several standards as well as natural water samples and mineral samples of abiogenic and biogenic origin. The detailed data reduction algorithm and a user-friendly Sr-specific stand-alone computer program used for the spike calibration and the data reduction are also presented. Accuracy and precision of our δ88Sr measurements, calculated as permil (‰) deviations from the NIST SRM-987 standard, were evaluated by analyzing the NASS-6 seawater standard, which yielded δ88Sr = 0.378 ± 0.009‰. The first DS-TIMS data for the NIST SRM-607 potassium feldspar standard and for several US Geological Survey carbonate, phosphate, and silicate standards (EN-1, MAPS-4, MAPS-5, G-3, BCR-2, and BHVO-2) are also reported. Data obtained during this work for Sr-bearing solids and natural waters show a range of δ88Sr values of about 2.4‰, the widest observed so far in terrestrial materials. This range is easily resolvable analytically because the demonstrated external error (±SD, standard deviation) for measured δ88Sr values is typically ≤0.02‰. It is shown that the “true” 87Sr/86Sr value obtained by the DS-TIMS or any other external normalization method combines radiogenic and mass-dependent mass-fractionation effects, which cannot be separated. Therefore, the “true” 87Sr/86Sr and the δ87Sr parameter derived from it are not useful isotope tracers. Data presented in this paper for a wide range of naturally occurring sample types demonstrate the potential of the δ88Sr isotope tracer in combination with the traditional radiogenic 87Sr/86Sr tracer for studying a variety of biological, hydrological, and geological processes.

The US Antimony mine in west central Montana is the United States' second largest quartz–stibnite vein deposit with production and reserves of about 15.4 kt of antimony. It is in the Mesoproterozoic Belt Basin on the eastern side of the Coeur d'Alene mining district that is known for its world class Ag, Pb, and Zn veins. To advance understanding of the origin and relation of the simple quartz–stibnite veins in the Prichard Formation to the giant Ag-rich tetrahedrite veins in the Revett and St. Regis Formations, fluid inclusions in quartz from the US Antimony mine were characterized using a variety of single inclusion and bulk analysis techniques. The measured Sb concentrations in fluid inclusions (approximately 2000 ppm) agree with equilibrium models for stibnite-saturated fluids at the measured homogenization temperatures (224–263°C), salinities (approximately 5 wt.% NaCl), H2S concentrations ( As > Fe > Pb > Zn > Cu > Ag) in fluid inclusions from the US Antimony mine support previous interpretations, based upon Pb isotopes, that the quartz–stibnite veins are genetically related to the Ag-rich tetrahedrite veins. In particular, the measured range of Ag concentrations in US Antimony fluid inclusions brackets that calculated on the basis of equilibrium with tetrahedrite in the silver veins. These data clearly show that hydrothermal fluids in the Prichard Formation contained metals that were fixed at shallower levels in the Revett and St. Regis Formations. We surmise that both the antimony and silver veins formed in response to Cretaceous magmatism, prograde metamorphism of the lower Prichard Formation, and episodic discharge of metamorphic fluids along dilatant faults. Ascending fluids deposited quartz and stibnite in the upper Prichard Formation and siderite, quartz, and tetrahedrite at shallower levels in the Revett and St. Regis Formations in response to cooling, decompression, phase separation, and mixing with external fluids.

The first systematic analyses of the trace and rare earth element (REE) distribution in uraninite from various gold-bearing conglomerates of the Mesoarchaean Central Rand Group in South Africa’s Witwatersrand Basin by in situ laser ablation-inductively coupled plasma-mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof. The chemistry of commonly rounded to sub-rounded uraninite is highly variable from grain to grain but generally marked by elevated Th, W, Bi, Mo, Ta, Y, REE contents and unusually high Au concentrations. Especially, the high Th contents and the chondrite-normalised REE patterns are incompatible with post-sedimentary hydrothermal genetic models for the U mineralisation and point to derivation of the detrital uraninite from a high-temperature, magmatogenic, presumably granitic to pegmatitic source. The elevated Au concentrations (of as much as 67 ppm) in this uraninite are unique to the Witwatersrand and hint at a granitic hinterland that was enriched in both U and Au, thus presenting a potential source domain for some of the detrital gold in the Witwatersrand conglomerates. Minute fracture fills of brannerite in close proximity to the larger, rounded uraninite grains are devoid of detectable Bi, Mo, REE and Au and have only very low concentrations of Th, W, Ta and Y. This is explicable by crystallisation from a low-temperature hydrothermal fluid. Thus, Witwatersrand U phases show, analogous to many other ore constituents, such as pyrite and gold, clear evidence of partial, short-range mobilisation of originally detrital particles by post-sedimentary fluids.

Bromine and chlorine are important halogens for fluid source identification in the Earth's crust, but until recently we lacked routine analytical techniques to determine the concentration of these elements in situ on a micrometer scale in minerals and fluid inclusions. In this study, we evaluate the potential of in situ Cl and Br measurements by LA-ICP-MS through analysis of a range of scapolite grains with known Cl and Br concentrations. We assess the effects of varying spot sizes, variable plasma energy and resolve the contribution of polyatomic interferences on Br measurements. Using well-characterised natural scapolite standards, we show that LA-ICP-MS analysis allows measurement of Br and Cl concentrations in scapolite, and fluid inclusions as small as 16 μm in diameter and potentially in sodalite and a variety of other minerals, such as apatite, biotite, and amphibole. As a demonstration of the accuracy and potential of Cl and Br analyses by LA-ICP-MS, we analysed natural fluid inclusions hosted in sphalerite and compared them to crush and leach ion chromatography Cl/Br analyses. Limit of detection for Br is ~ 8 μg g − 1 , whereas relatively high Cl concentrations (> 500 μg g − 1 ) are required for quantification by LA-ICP-MS. In general, our LA-ICP-MS fluid inclusion results agree well with ion chromatography (IC) data. Additionally, combined cathodoluminescence and LA-ICP-MS analyses on natural scapolites within a well-studied regional metamorphic suite in South Australia demonstrate that Cl and Br can be quantified with a ~ 25 μm resolution in natural minerals. This technique can be applied to resolve a range of hydrothermal geology problems, including determining the origins of ore forming brines and ore deposition processes, mapping metamorphic and hydrothermal fluid provinces and pathways, and constraining the effects of fluid–rock reactions and fluid mixing.

Each year an estimated 56,000 metric tons (t) of rare earth elements (REEs), including 23,000 t of heavy REEs (HREEs), are mined, beneficiated, and put into solution, but not recovered, by operations associated with the global phosphate fertilizer industry. Importantly, the REEs in sedimentary phosphorites are nearly 100% extractable, using technologies currently employed to meet global phosphate fertilizer needs. Our evaluation suggests that by-product REE production from these phosphate mines could meet global REE requirements. For example, the calculated REE flux accompanying phosphate production in the United States is approxi- mately 40% of the world’s total and, alone, could supply 65% of global HREEs needs. Moreover, recognition that the tonnages and HREE concentrations of some unmined phosphorite deposits dwarf the world’s richest REE deposits suggests that these deposits might constitute stand-alone REE deposits. The hypothesized genesis of these REE-rich occurrences strongly supports the long-debated suggestion that oceanic REE contents vary in a secular fashion and that associated high-grade REE abundances reflect oceanic redox state transitions during specific time periods. Here, we use this new process-based model, based on observed variations in global-secular REE abundances, to identify phosphorite horizons deposited during periods favorable for high- grade REE accumulation.

Episodes of instability in the global climato-oceanic system have become the hallmark of the Silurian Period. These events are marked by six large positive carbon isotope excursions associated with major extinctions. The widespread environmental consequences of these perturbations of the global carbon cycle remain poorly documented. A high-resolution chronostratigraphic reconstruction of the Appalachian foreland basin (AFB) provides a framework to evaluate environmental signatures of two of these carbon isotope excursions (Valgu and Ireviken events; Telychian–Sheinwoodian) at a regional scale. Integration of published biostratigraphic range data, mapping of sequence stratigraphic surfaces and facies, and generation of high-resolution carbon isotope profiles for several transects provides precise correlation of the basin stratigraphy yielding a high-resolution temporal framework. This chronostratigraphic reconstruction of the AFB highlights the importance of time-specific facies (TSF) during these intervals. For instance, packages of red, green, and gray marine rocks separately occupied discrete time intervals—cutting across “local” facies belts indicative of shallow to deep marine environments. Similarly, widespread iron mineralization occurred during discreet time slices as “bathtub rings” rimming the margins of the basin. Changes from one time-specific facies to the next are coincident with changes in carbon isotope values, indicating a genetic link between fractionation of the global carbon reservoir and redox changes from generally oxic to anoxic oceanic conditions in parts of the AFB. The basal part of the study interval was deposited during the Valgu Event. This first event begins with an unconformity marking global lowstand. This fall in sea level was coincident with rising carbon isotope values as recorded in adjacent basins. A slight sea level rise and flooding of the subaerial unconformity occurred near the peak in carbon isotope values. Oxidized iron minerals were deposited in a zone that extended from the lower shoreface into the basin center—pinching out into shallower parts of the basin. Thin pyrite–phosphorite–glauconite layers, associated with black shales in the basin center, were deposited over these ferruginous strata. As the event ended and carbon isotope values shifted negatively to a more stable baseline, temperatures warmed, sea level reached a Silurian high, and oxic conditions associated with very low rates of organic carbon (OC) burial were established. These conditions were relatively stable for nearly four million years. This long period of stasis was punctuated by the Ireviken Event, which displays a strikingly similar progression of TSF patterns, yet more intense and longer lived than the preceding Valgu Event. With the first positive shift in carbon isotopes at the onset of the Ireviken Event, the basin redox system changed from largely oxidizing to reducing, coincident with a rapid drop in sea level and an abrupt reappearance of iron mineralization on the basin margins. The sea level fall associated with the shift to peak values of the carbon isotope excursion is marked by the progradation of a significant body of sand well out into the basin. During this sea level drop, conditions in the basin center became highly reducing and periodically sulfidic. Widespread ironstones and ankeritic carbonates were deposited isochronously, rimming the shallow basin margins where they alternated with light gray pyritic shales, indicating fluctuating redox conditions. A large negative carbon isotope excursion of ~ 3‰ is coincident with a shift from regression to transgression, suggesting glacial melting via methane release. Continued deposition of ironstones through this time interval suggests a primary link to shallow marine redox processes rather than large-scale transgression or regression of the shoreline. The ironstones are overlain by gray to black pyritic shales deposited across all environments, revealing a final stage of redox progression into sustained sulfidic conditions as peak values of the carbon isotope excursion returned to a high stable baseline near + 4‰. The similar expression of TSFs within the AFB of these two global events demonstrates that fractionation of the global atmospheric and oceanic carbon reservoirs had a predictable, cascading impact on shallow sea floor environments. The associated redox processes reveal the dynamic regional interactions of a shallow oxic epicontinental water mass with a broader anoxic oceanic water mass, providing valuable insight into the origin of some of the largest extinctions in the Silurian Period.