To further refine the use of solid bitumen reflectance (BRo in %) as a measurement of thermal maturity in source-rock reservoirs, we examined its relationship to other thermal proxies in the Bakken Formation. Comparisons included criteria from programmed temperature pyrolysis, gas chromatography (GC), and Fourier transform infrared (FTIR) spectroscopy. Thirty-two organic-rich samples from the lower and upper shale members of the Devonian-Lower Carboniferous Bakken Formation were collected from eight cores across the Williston Basin, USA, at depths (similar to 7575-11,330 ft) representing immature through post peak oil/early condensate thermal maturity conditions based on proximity to current hydrocarbon production. Unmodified BRo values were correlated to programmed temperature pyrolysis parameters (hydrogen index, production index, and T-max), normal hydrocarbon and isoprenoid analysis of extractable organic matter (pristane/n-C-17 and phytane/n-C-18) from GC analysis, and peak ratios from FTIR spectroscopy (branching ratio and A-factor). Strong correlations between unmodified BRo values (not corrected to a vitrinite reflectance equivalent, VRe) and other thermal proxies suggest that BRo can be used as a direct thermal proxy in marine Paleozoic source-rock reservoirs where vitrinite is rare or absent. Moreover, an apparent overestimation of VRe at the lowest thermal maturity investigated herein may argue against the application of BRo conversion to VRe in the Bakken Formation. Solvent extraction caused a consistent decrease in BRo when average post-extraction values from a given well were compared to BRo prior to extraction, although the decrease in mean value was not statistically significant. These results are discussed in the context of advocating for the use of unmodified BRo values as a best practice for thermal maturity determination in Paleozoic marine source-rock reservoirs.

The uranium (U) content, and more recently, the ratio between 238U and 235U in black shales are commonly applied as a proxy to determine redox conditions and infer organic-richness. Uranium contents typically display a linear relationship with total organic carbon (TOC) in shales. This relationship is due to the processes and mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. This U/TOC relationship can vary, however, and some shales display uncharacteristically low U content despite having high TOC content, while others show large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope compositions of three Upper Devonian-Lower Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale, with two study sites in Oklahoma, one site in eastern Kentucky, and three sites in eastern Montana and western North Dakota, respectively. The U/TOC ratios of each shale are distinct from one another exhibiting average ratios ranging from 3 in the Cleveland Shale, to over 10 in the Bakken Shale. The distinct geochemical composition of the three shales suggests that, although lithologically similar, each study site represents a markedly different and dynamic depositional environment. The low average U/TOC (~3) along with the relatively high δ238U values (~0.03‰) of the Cleveland Shale core suggests deposition along the basin margin under normal marine conditions with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. The Woodford Shale on the other hand, shows higher U/TOC ratios (~4, George core, ~9, Poe core) and δ238U (~0.02‰ average, George core, ~0.06‰ average, Poe core), which suggests an unrestricted setting with intermittent euxinic conditions. In contrast, high U/TOC ratios (2–15), and very high δ238U values (up to 0.55‰) in the Bakken Shale cores indicate intense metal draw-down into sediments under sulfidic waters. The results show that when the U/TOC ratios and U-isotopic compositions of each studied shale are compared to modern anoxic basins and upwelling areas, it allows for an enhanced understanding of the paleoenvironmental conditions such as basin restriction and redox state of waters within the Late Devonian epicontinental seas of North America.

A suite of 72 samples of Illinois Basin coal was collected from 2015 to 2018, from 14 active coal mining and preparation operations in Illinois and Indiana. The goals of the study were to investigate the behavior of trace elements during coal preparation and provide current coal quality data for active mining operations in the Illinois Basin, which has become one of the most important suppliers of U.S. coal. Samples include raw coal, cleaned coal, and waste (refuse) coal. The efficiency of cleaning is shown by reduction of ash yield, increase in heating values, and a pronounced shift in the proportion of sulfur forms. In most raw coals, pyritic sulfur (Py S) exceeds organic sulfur (Org S; Org S/Py S <= 1.0), whereas in prepared coals, organic sulfur is the predominant sulfur form (Org S/Py S >= 1.0). In waste (refuse) coals, Org S/Py S is substantially reduced, or almost entirely in the pyritic form (Org S/Py S near zero). Trends in the distribution of inorganically-associated lithophile and chalcophile elements are shown, as well those for elements having mixed organic-inorganic associations and those that are primarily organic-hosted. Lithophile elements, including Li, Al, P, Ti, Sc, Rb, Y, Zr, Nb, Cs, Ba, the lanthanides, Hf, Th, and U, are partitioned into waste coals with corresponding reduction in cleaned coals. At the same time, chalcophile elements hosted by pyrite, such as As, Sb, Hg, and Pb, are also partitioned into waste coals. Rare earth elements (REEs), including Sc, Y, and the lanthanides, are enriched in waste coals at levels that approximate their content in the upper continental crust (UCC). Several other critical elements, defined as mineral commodities deemed critical to the economy and security of the United States, including V, Cr, As, Rb, Sb, and Cs, show enrichment in waste coals, that may facilitate their co-extraction with the REEs. Concentrations for elements of environmental concern in cleaned coals are similar to or below those for world average coals with the exception of Pb in coals from Illinois. Laser-ablation ICP-MS (LA-ICP-MS) for pyrite in a subset of 21 samples of raw and cleaned coal provided direct determinations of V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, Au, Hg, Tl, Pb, and Bi. LA-ICP-MS was used together with bulk sample pyritic sulfur and trace element concentrations to estimate the elemental proportion of bulk Cu, As, Hg, and Pb in pyrite, indicating that Hg and Pb are hosted almost entirely by pyrite, whereas the proportion of Cu and As in pyrite accounts for about half of the bulk content. With growing interest in recovering critical elements from coal, partitioning of a large range of elements into waste coals presents a possible opportunity, but also a potential hazard, due to enrichment in harmful elements such as Hg, As, and Pb.

First posted February 23, 2021 For additional information, contact: Geology, Energy & Minerals Science CenterU.S. Geological Survey12201 Sunrise Valley Drive954 National CenterReston, VA 20192Contact Pubs Warehouse Researchers at the U.S. Geological Survey (USGS) and their collaborators conducted a study of the geochemical properties of coals currently produced for electric power generation in the Illinois Basin in Illinois and Indiana. The study follows from recommendations by an expert panel for the USGS to investigate the distribution and controls of trace constituents such as mercury (Hg) in Illinois Basin coals and the behavior of these constituents in coal preparation. A total of 72 new samples were collected by USGS collaborators between 2015 and 2017. These samples include raw coals, prepared coals, and waste coals from coal preparation. To understand the geochemistry and cleaning behavior of these coals, these samples were subjected to an integrated series of analyses described here, including microanalysis of coal constituents and bulk sample chemical analysis. Of the procedures used, whole-sample Hg analysis quantified overall mercury contents and its reduction by coal preparation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of pyrite in coal quantified Hg and other potentially harmful elements contained in pyrite, the most likely host of these constituents. Trace elements investigated include those whose emissions are regulated under the U.S. Environmental Protection Agency Mercury and Air Toxics Standards. This report and the corresponding data release, serve as an archive for geochemical data obtained in our study of the geochemistry of Illinois Basin coals. Material included in this report also define approaches used by the USGS over the period of study to characterize coal samples, requiring combined use of results from USGS and non-USGS laboratories.

First posted September 12, 2019 For additional information, contact: Eastern Energy Resources Science CenterU.S. Geological Survey12201 Sunrise Valley Drive956 National CenterReston, VA 20192https://energy.usgs.gov/ The rare earth elements (REEs) are a group of 17 elements sharing similar chemical properties. They include yttrium (Y, atomic number 39), scandium (Sc, atomic number 21), and the 15 elements of the lanthanide series, atomic numbers 57 (lanthanum, La) to 71 (lutetium, Lu). Because promethium (Pm, atomic number 61) does not occur in the Earth's crust and scandium typically has different geological occurrences from other REEs, they are not discussed further herein.REEs are, on average, more abundant than precious metals (for example, gold, silver, and platinum), but because of their unique geochemical properties, they do not commonly form economically viable ore deposits. Nevertheless, REEs are increasingly required for a range of modern applications in defense and renewable energy technologies and in commercial products, primarily as magnets, batteries, and catalysts. The United States currently (2018) produces REEs from a single mine in California, accounting for just 9 percent of global production, whereas 70 percent of global REE production comes from China. For these reasons, REEs are considered a critical resource, and the U.S. Geological Survey (USGS) has an interest in helping to identify new sources of REEs for domestic production.In 2017, coal use accounted for about 30 percent of the electric power generated in the United States. Fly ash, produced during the burning of coal, is a fine­-grained solid derived from noncombustible constituents of coal, such as clay minerals and quartz. When coal is burned, REEs are retained and enriched in the fly ash and, as a result, fly ash has long been considered a potential resource for REEs.The United States has the world's largest coal reserves and, even though gas-­fired power generation has increased significantly in the last decade, the United States continues to produce vast quantities of fly ash, about half of which is beneficially reused, primarily in construction materials. The remainder is stored, mostly in landfills and impound­ments. Thus, annual fly ash production, combined with fly ash already in stor­age, constitutes a large potential resource.Research into how to utilize coal and coal fly ash as sources of REEs is ongo­ing. Viable recovery of REEs from coal and coal ash requires identification of coals and ashes with the highest REE concentrations and development of workable methods for REE extraction and recovery. Understanding how REEs occur within fly ash, described in this fact sheet, is one of the keys to developing possible methods for their recovery.

Black shales of the Late Devonian to Early Mississippian Bakken Formation are characterized by high concentrations of organic carbon and the hyper-enrichment (>500 to 1000s of mg/kg) of V and Zn. Deposition of black shales resulted from shallow seafloor depths that promoted rapid development of euxinic conditions. Vanadium hyper-enrichments, which are unknown in modern environments, are likely the result of very high levels of dissolved H2S (~10mM) in bottom waters or sediments. Because modern hyper-enrichments of Zn are documented only in Framvaren Fjord (Norway), it is likely that the biogeochemical trigger responsible for Zn hyper-enrichment in Framvaren Fjord was also present in the Bakken basin. With Framvaren Fjord as an analogue, we propose a causal link between the activity of phototrophic sulfide oxidizing bacteria, related to the development of photic-zone euxinia, and the hyper-enrichment of Zn in black shales of the Bakken Formation.

The Upper Mississippian Heath Formation, which accumulated in the Big Snowy Trough of central Montana, has been known for three decades to contain mudrocks highly enriched in Zn, V, Mo, Ni and other metals, and source rocks for oil. The unit has more recently been recognized as a prospective tight oil play. Here we present petrographic, paleontologic, geochemical, and carbon and sulfur isotope data on seven continuous drill cores spanning <= 146 m of immature to marginally mature strata in order to improve understanding of the depositional setting of the Heath. The unit consists of five third-order transgressive-regressive cycles (Cl -C5 from bottom to top) that were deposited during a fluctuating climatic regime. Cycles comprise thinly interbedded gray to black mudrock and carbonate strata capped by either coal, implying a humid climate (C1, C3 and C4), or gypsum, implying more arid conditions (C2); the upper part of C5 is not preserved in our study area. Mfs (maximum flooding surfaces) in C1, C2, C4, and C5 lie within black mudrock beds similar to 0.5 3-m thick with > 10% TOC (total organic carbon), type I and type II kerogen (determined by programmed pyrolysis), high contents of Zn, V, Mo, and other metals, relatively low 13 4 values of delta C-13(TOC) and delta S-34(pyrte), and a limited-diversity fauna of locally abundant, thin-shelled pelecypods (Dunbarella? sp.). The mfs in C2 is within the Cox Ranch oil shale bed, which is known from previous studies to be metalliferous; new analyses reported here show <= 28 wt % TOC, 5140 ppm Zn, 1910 ppm V, 1590 ppm Mo, and 509 ppm Ni. Strata that contain the mfs of C1, C4, and C5 are shown here for the first time to also have high metal contents, with maximum values of 1030 7340 ppm Zn, 446-1980 ppm V, 72 859 ppm Mo, and 221-452 ppm Ni. Cycle C3, which contains more gray mudrock and carbonate beds than the other cycles, has lower TOC (<= 4.2 wt %), lower metals, and mainly type III kerogen. Carbonate beds include normal-marine crinoidal mudstone to packstone and lesser (dolo) mudstone with fenestral fabric, peloids, intraclasts, and a euryhaline fauna. Mid-Chesterian (early Serpukhovian) foraminifers in C3, combined with previously published fossil data, suggest that third-order cycles in the Heath Formation were similar to 1-2 myr in duration. They formed during a time of active block faulting in the Big Snowy Trough and global cooling linked to Gondwanan glaciation. Tectonic, climatic, and paleogeographic factors shaped the cycles of the Heath Formation. Faunal and geochemical evidence indicate that conditions were most favorable for marine life during C3. Molybdenum concentrations > 100 ppm and organic geochemical data suggest euxinic conditions during deposition of the black mudrock in C2, C4, and C5, but the presence of shell beds (1-mm 6-cm thick) within this mudrock requires bottom water with sufficient oxygen to support life, at least periodically. The apparent conflict between the geochemical and paleontologic observations likely reflects the different time scales of these two environmental proxies: 1000s of yrs vs <1-10s of yrs, respectively. Metal-and organic-rich strata in the Heath Formation formed by slow, condensed sedimentation from periodically anoxic or euxinic bottom waters in a marine basin. Fossil data indicate that anoxia was episodic, perhaps seasonal and/or linked to longer-duration climate shifts. On a millennial time scale, metal enrichments in the Heath reflect a balance between primary productivity that was high enough for oxygen to be consumed by sinking organic matter and oxic seawater inflow that was strong enough to maintain a supply of metals without compromising anoxia. Organic-rich mudrock in the Heath shares intriguing lithologic and geochemical similarities with mudrock in other Middle to Upper Paleozoic units such as the Devonian-Mississippian Bakken Formation and Pennsylvanian cyclothems (e.g., Excello Shale).

Various geochemical records suggest that atmospheric O-2 increased in the Ediacaran (635-541 Ma), broadly coincident with the emergence and diversification of large animals and increasing marine ecosystem complexity. Furthermore, geochemical proxies indicate that seawater sulfate levels rose at this time too, which has been hypothesized to reflect increased sulfide oxidation in marine sediments caused by sediment mixing of the newly evolved macrofauna. However, the exact timing of oxygenation is not yet understood, and there are claims for significant oxygenation prior to the Ediacaran. Furthermore, recent evidence suggests that physical mixing of sediments did not become important until the late Silurian. Here we report a multiple sulfur isotope record from a ca. 835-630 Ma succession from Svalbard, further supported by data from Proterozoic strata in Canada, Australia, Russia, and the United States, in order to investigate the timing of oxygenation. We present isotopic evidence for onset of globally significant bacterial sulfur disproportionation and reoxidative sulfur cycling following the 635 Ma Marinoan glaciation. Widespread sulfide oxidation helps to explain the observed first-order increase in seawater sulfate concentration from the earliest Ediacaran to the Precambrian-Cambrian boundary by reducing the amount of sulfur buried as pyrite. Expansion of reoxidative sulfur cycling to a global scale also indicates increasing environmental O-2 levels. Thus, our data suggest that increasing atmospheric O-2 levels may have played a role in the emergence of the Ediacaran macrofauna and increasing marine ecosystem complexity.

During coal combustion for electric power generation, rare earth elements (REE) are strongly retained in the ash fraction leading to significant enrichment compared to the REE content of the respective feed coals. Knowing how the REE are distributed in coal ash is essential to devising effective approaches for REE extraction from this potential resource. To better understand the distribution of REE in coal ash, we used the Stanford-USGS SHRIMP-RG ion microprobe to determine grain-scale REE partitioning in a suite of 19 U.S. and international coal fly ash samples. SHRIMP-RG microanalysis used a nominal spot size of 15 μm and an oxygen negative-ion primary beam. NIST multi-element glass standards SRM 611 and SRM 613 were used for REE calibration. Prior to SHRIMP-RG analysis, samples were characterized by backscattered electron imaging and wavelength-dispersive elemental mapping to delineate constituents of interest for REE microanalysis. Results confirm the occurrence of REE in aluminosilicate glasses consistent with previous results that showed a strong positive correlation between Al and REE content in a larger group of U.S. and international coal fly ash samples. Among these glasses, aluminosilicates consisting of Al and Si and lacking other major constituents tend to have REE contents similar to, or slightly lower than the bulk sample REE content, whereas Ca- and/or Fe-enriched aluminosilicate glasses are similar to, or more REE-enriched than the bulk sample. Co-occurring quartz and/or high-silica glass is almost always the most REE depleted fly ash constituent, having much lower REE contents than the aluminosilicate glasses. Fe-oxide magnetospheres have REE content with relative proportions of light (LREE) to heavy (HREE) rare earths that are more variable than in the aluminosilicate glasses. Results suggest that extraction of REE from the aluminosilicate glass fraction in coal ash will yield a significant portion of the REE present and therefore, this fraction should be targeted in ongoing efforts to recover REE from coal fly ash.

Life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes. These requirements differ between organisms and have evolved over geological time, likely guided in some part by environmental conditions. Until recently, most of what was understood regarding trace element concentrations in the Precambrian oceans was inferred by extrapolation, geochemical modeling, and/or genomic studies. However, in the past decade, the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new, and potentially more direct, insights into secular changes in seawater composition – and ultimately the evolution of the marine biosphere. Compiled records of many bioessential trace elements (including Ni, Mo, P, Zn, Co, Cr, Se, and I) provide new insight into how trace element abundance in Earth's ancient oceans may have been linked to biological evolution. Several of these trace elements display redox-sensitive behavior, while others are redox-sensitive but not bioessential (e.g., Cr, U). Their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere-ocean redox conditions that are linked to biological evolution, for example, the activity of oxygen-producing, photosynthetic cyanobacteria. In this review, we summarize available Precambrian trace element proxy data, and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life. We also examine several biologically relevant and/or redox-sensitive trace elements that have yet to be fully examined in the sedimentary rock record (e.g., Cu, Cd, W) and suggest several directions for future studies.

We present paleoredox data from black shales from the ca. 835–630Ma Akademikerbreen and Polarisbreen groups, northeastern Svalbard. These iron speciation, trace metal, and organic carbon data include the first record of ocean redox conditions leading up to the Sturtian glaciation and contribute to the emerging narrative of protracted Neoproterozoic oxygenation. Sampled shales were deposited under dominantly suboxic to anoxic-ferruginous conditions on a stable continental shelf. Together with data from other basins, these data show no evidence of dramatic increases in environmental oxygen levels prior to or following the Sturtian glaciation. Proposed global oxygenation in the aftermath of the Marinoan glaciation did not lead to long-term oxygenation of shelf environments in Svalbard or northwestern Canada. We suggest that this reflects sufficient organic matter loading to maintain suboxic to anoxic water columns on continental margins. In contrast, oxygen concentrations may have been higher in low-productivity zones further offshore, where downwelling of oxygenated surface waters may have even increased oxygen concentrations in the deep ocean.

Rare earth elements (REEs) are utilized in various applications that are vital to the automotive, petrochemical, medical, and information technology industries. As world demand for REEs increases, critical supply shortages may be expected, especially for the heavier REEs. Due to the retention of REEs during coal combustion, coal fly ash is increasingly considered a potential resource. Previous studies have demonstrated that coal fly ash is variably enriched in REEs relative to feed coal and that enrichment increases with decreasing fly ash size fractions. In order to further explore the REE resource potential of coal ash, and determine the partitioning behavior of REEs as a function of grain size, we studied whole coal and four fly ash sizefractions collected from three U.S. commercial-scale coal-fired generating stations burning bituminous coal from the Appalachian Basin (samples Ap-1 and Ap-2) or subbituminous Powder River Basin coal (sample PRB-1). Whole fly ash was separated into 100 μm particle size fractions, using mechanical shaking and trace-metal clean procedures. Our separation method allowed for the collection of two size fractions ( 100 μm fraction and less than 1 % in the 100 μm fraction was depleted in REEs relative to whole coal ash and the smaller size fractions. We observed little or no increase in REE concentrations in the two smallest size fractions, suggesting that enrichment in REEs with decreasing size fraction observed by other workers is not expressed in size fractions < 10 μm. The concentration of trace metals (Y, Zr, Nb, Mo, Hf, Th) followed the same general trend with depletions in the coarse fraction and nearly constant enrichment among whole ash and the smaller size fractions. 2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN May 5-7, 2015 http://www.flyash.info/

The role of biological processes in the formation of sediment-hosted ore deposits has long been recognized. In this review, we focus on the biogeochemical cycling of C, Mn, Fe, and S as they relate to the formation of sediment-hosted Mn and Fe deposits, metalliferous black shales, clastic-dominated (CD) Pb-Zn deposits, and phosphorites. Biological mediation of ore-forming processes occurs over large spans of space and time. The most important step is oxygenic photosynthesis, a biological innovation dating from the Archean Eon that releases free oxygen into the surface oceans and atmosphere and delivers chemical potential, in the form of reduced carbon, to the seafloor. Photosynthetic oxygen is available to precipitate dissolved Fe2+ and Mn2+, and therefore it augments the formation of sedimentary Mn and Fe deposits, and drives oxidative weathering of exposed crust, thereby delivering sulfate and transition metals to the ocean. Where reduced carbon accumulates in the deep oceans and on the seafloor, bacterial sulfate reduction produces hydrogen sulfide thereby facilitating the formation of metalliferous black shales, sediment-hosted Pb and Zn sulfide deposits, and phosphorites. Thus, an understanding of major biogeochemical processes and how they have evolved over time is required in order to refine genetic models for sediment-hosted ore deposits and to guide future mineral exploration. A close secular relationship between deposit formation and trends in major biogeochemical cycles provides a potentially powerful tool for mineral resource assessment. Sedimentary basins that formed during a time that is known to lack deposits of a particular metal can be eliminated during exploration programs, whereas others of permissive ages should be considered priorities. For example, sedimentary basins older than ca. 1.8 Ga are unlikely to contain large CD Pb-Zn deposits, and basins that formed between 1.6 and 0.6 Ga are not prospective for phosphorites. Recent technological advances in the application of nanometer-, micron-, and bulk-scale analytical techniques allow for imaging of complex biological structures and have provided new insights into the role of bacteria, not only in direct formation of mineral deposits, but also in leaching of metals from ore and mineralized rocks. Future exploration for, and exploitation of, mineral deposits may include offshore or land-based, low-grade, high-tonnage targets; understanding the role of bacteria in mineral growth, mineral dissolution, and redox transformations will aid in predicting where such deposits exist, and how metal extraction from ores can be enhanced.

Earth's oxygenation is often described in terms of two unidirectional steps at the beginning and end of the Proterozoic Eon, separated by a long-lived intermediate redox state. Recent work defines a more complicated path to oxygenation, exemplified by an apparent drop in oxidation state following the early Paleoproterozoic Lomagundi carbon isotope excursion. The timing of this proposed drop in oxidation state is not well constrained, and it is not clear how it relates to redox conditions during the remainder of the Proterozoic. Here we present a study of pyrite multiple-sulfur isotopes, supported by Fe speciation and organic carbon isotopes, from early Paleoproterozoic black shales. We find evidence for the rapid expansion of the seawater sulfate reservoir during the Great Oxidation Event at ca. 2.3 Ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca. 2.05 Ga. This scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early Paleoproterozoic. Comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early Paleoproterozoic did not return until the late Neoproterozoic. Published by Elsevier B.V.