Joel S. Leventhal

Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, U.S.A.☆

Analyses of 21 samples collected from a core of the 52.8-cm-thick Stark Shale Member of the Dennis Limestone in Wabaunsee County, Kansas, demonstrate four cycles with two-orders-of-magnitude variations in contents of Cd, Mo, P, V and Zn, and order-of-magnitude variations in contents of organic carbon, Cr, Ni, Se and U. The observed variability in amounts and/or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water-column stratification and the resulting absence/presence of dissolved O2 or H2S. High Cd, Mo, U, V, Zn and S contents, a high degree of pyritization (DOP) (0.75–0.88), and high high V(V + Ni) (0.84–0.89) indicate the presence of H2S in a strongly stratified water column. Intermediate contents of metals and S, intermediate DOP (0.67–0.75) and intermediate V(V + Ni) (054–0.82) indicate a less strongly stratified anoxic water column. Whereas, low metal contents and low V(V + Ni) (0.46–0.60) indicate a weakly stratified, dysoxic water column. High P contents at the top of the organic-matter-rich intervals within the Stark Shale Member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions. Relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified H2S-bearing water column indicates a combined halocline-thermocline with the fresher near-surface water the transport mode for the terrestrial organic matter. The predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline. Relatively low amounts of degraded, hydrogen-poor organic matter characterize intervals deposited in a weakly stratified, dysoxic water column. The inferred variability in chemistry of the depositional environments may be related to climate variations and/or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member.

An interpretation of carbon and sulfur relationships in Black Sea sediments as indicators of environments of deposition

Syngenetic iron sulfides in sediments are formed from dissolved sulfide resulting from sulfate reduction and catabolism of organic matter by anaerobic bacteria. It has been shown that in recent marine sediments deposited below oxygenated waters there is a constant relationship between reduced sulfur and organic carbon which is generally independent of the environment of deposition. Reexamination of data from recent sediments from euxinic marine environments (e.g., the Black Sea) also shows a linear relationship between carbon and sulfur, but the slope is variable and the line intercepts the S axis at a value between 1 and 2 percent S. It is proposed that the positive S intercept is due to watercolumn microbial reduction of sulfate using metabolizable small organic molecules and the sulfide formed is precipitated and accumulates at the sediment-water interface. The variation in slope and intercept of the C to S plots for several cores and for different stratigraphic zones for the Black Sea can be interpreted in relation to thickness of the aqueous sulfide layer or thinness of the oxygen containing layer and to deposition rate, but also may be influenced by availability of iron, and perhaps the type of organic matter (Leventhal, 1979).

Comparison of methods to determine degree of pyritization

Abstract Degree of pyritization (DOP) is a measure of the ratio pyrite iron/(pyrite iron + reactive iron) that can be related to the depositional environment of a sediment. Several methods of DOP determination have been used but not systematically evaluated. The determination/extraction of reactive (usually acid soluble) iron is critical to the DOP determination, and the method generally used is reaction of the sample for 1 to 2 min with hot 12 N HCl. We present results for timed experiments with 1 N, 6 N, and 12 N HCl on three different samples. We also show that a 24 h room temperature treatment with 1 N HCl is equivalent to the 24 h treatment with Na-dithionite. Experiments with several suites of samples show that all three of these methods leach comparable amounts of iron; therefore, the DOP values are similar. However, the 1 N HCl, 24 h procedure is preferable because laboratory handling is less and easier.

Genesis of sediment-hosted disseminated-gold deposits by fluid mixing and sulfidization: Chemical-reaction-path modeling of ore-depositional processes documented in the Jerritt Canyon district, Nevada

Integrated geologic, geochemical, fluid-inclusion, and stableisotope studies of the gold deposits in the Jerritt Canyon district, Nevada, provide evidence that gold deposition was a consequence of both fluid mixing and sulfidization of host-rock iron. Chemical-reaction-path models of these ore-depositional processes confirm that the combination of fluid mixing, including simultaneous cooling, dilution, and oxidation of the ore fluid, and wall-rock reaction, with sulfidization of reactive iron in the host rock, explains the disseminated nature and small size of the gold and the alteration zonation, mineralogy, and geochemistry observed at Jerritt Canyon and at many other sediment-hosted disseminated gold deposits.

Carbon-sulfur plots to show diagenetic and epigenetic sulfidation in sediments

Abstract Organic carbon vs. sulfide sulfur plots are now being used regularly by many geochemists to help understand recent and ancient depositional environments and diagenetic processes. Usually, these plots are useful to recognize nonmarine vs. marine environments or oxic vs. anoxic vs. euxinic depositional environments. However, C vs. S plots can also indicate diagenetic and epigenetic events that produce “excess” sulfide. Four new examples are presented and discussed.

Organic geochemical analysis of sedimentary organic matter associated with uranium

Abstract Samples of sedimentary organic matter from several geologic environments and ages which are enriched in uranium (56 ppm to 12%) have been characterized. The three analytical techniqyes used to study the samples were Rock-Eval pyrolysis, pyrolysis-gas chromatography-mass spectrometry, and solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy. In samples with low uranium content, the pyrolysis-gas chromatography products contain oxygenated functional groups (as hydroxyl) and molecules with both aliphatic and aromatic carbon atoms. These samples with low uranium content give measurable Rock-Eval hydrocarbon and organic-CO 2 yields, and C-13 NMR values of > 30% aliphatic carbon. In contrast, uranium-rich samples have few hydrocarbon pyrolysis products, increased Rock-Eval organic-CO 2 contents and > 70% aromatic carbon contents from C-13 NMR. The increase in aromaticity and decrease in hydrocarbon pyrolysis yield are related to the amount of uranium and the age of the uranium minerals, which correspond to the degree of radiation damage. The three analytical techniques give complementary results. Increase in Rock-Eval organic-CO 2 yield correlates with uranium content for samples from the Grants uranium region. Calculations show that the amount of organic-CO 2 corresponds to the quantity of uranium chemically reduced by the organic matter for the Grants uranium region samples.

Chemical and mineralogical analysis of devonian black-shale samples from Martin County, Kentucky; Carroll and Washington counties, Ohio; Wise County, Virginia; and Overton County, Tennessee, U.S.A.

Abstract Core samples of Devonian shales from five localities in the Appalachian basin have been analyzed chemically and mineralogically. The amounts of major elements are similar; however, the minor constituents, organic C, S, phosphate and carbonate show ten-fold variations in amounts. Trace elements Mo, Ni, Cu, V, Co, U, Zn, Hg, As and Mn show variations in amounts that can be related to the minor constituents. All samples contain major amounts of quartz, illite, two types of mixed-layer clays, and chlorite in differing quantities. Pyrite, calcite, feldspar and kaolinite are also present in many samples in minor amounts. Dolomite, apatite, gypsum, barite, biotite and marcasite are present in a few samples in trace amounts. Trace elements listed above are strongly controlled by organic C with the exception of Mn which is associated with carbonate minerals. Amounts of organic C generally range from 3 to 6%, and S is in the range of 2–5%. Amounts of trace elements show the following general ranges in ppm (parts per million): Co, 20–40; Cu, 40–70; U, 10–40; As, 20–40; V, 150–300; Ni, 80–150; high values are as much as twice these values. The organic C was probably the concentrating agent, and the organic C and sulfide S together created an environment that immobilized and preserved these trace elements. Closely spaced samples showing an abrupt transition in color also show changes in organic C, S and trace-element contents. Several associations exist between mineral and chemical content. Pyrite and marcasite are the only minerals found to contain sulfide-S. In general, the illite—chlorite mixed-layer clay mineral shows covariation with organic C if calcite is not present. The enriched trace elements are not related to the clay types, although the clay and organic matter are intimately associated as the bulk fabric of the rock.

Carbon-13/carbon-12 isotope fractionation of organic matter associated with uranium ores induced by alpha irradiation.

Analyses of stable carbon isotopes from two sample suites from sandstone uranium (tabular) ores show interesting variations. Organic carbon associated with high-grade uranium ore is heavy (δ13C = –16.9 to –19.6 per mil, where δ13C = 13C/12C relative to the Pee Dee belemnite standard) relative to the adjacent lower-grade samples (–22.7 to –26.4 per mil). It is suggested that the heavy isotopic values for the are samples are related to a radiation and chemical isotope effect that has occurred mainly because of an alpha-radiation dose of 1011 rads.

Role of organic matter in the Proterozoic Oklo natural fission reactors, Gabon, Africa

Of the sixteen known Oklo and the Bangombe natural fission reactors (hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals), reactors 1 to 6 at Oklo contain only traces of organic matter, but the others are rich in organic substances. Reactors 7 to 9 are the subjects of this study. These organic-rich reactors may serve as time-tested analogues for anthropogenic nuclear-waste containment strategies. Organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions. Liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality. The bitumen soon became a solid, consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite, which enclosed and immobilized uraninite and the fission-generated isotopes entrapped in uraninite. This mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1.2 b.y. 24 refs., 4 figs.

Metals in Black Shales

Shales that are rich in organic matter occur throughout the geologic record, but special conditions are responsible for their occurrence (Tourtelot, 1979). Before considering organic matter- and metal-rich shales, some background information is necessary. A shale is a sedimentary rock that is composed of small (mostly less than 0.1 mm) particles dominated by phyllosilicate (clay) minerals and containing subordinate amounts of quartz, carbonate, and phosphate minerals and, in some cases, organic matter and pyrite [for a discussion, see Spears (1980)]. Usually, the only detrital minerals are quartz and kaolinite; the others are authigenic. Organic-rich shales often show bedding laminations ranging from less than millimeter to centimeter thickness that are the result of size sorting, organic content, or pyrite layers, caused by changes in depositional environment. Shales can be formed in fresh, brackish, marine, or hypersaline water bodies, but most of the examples in this chapter are from the marine environment (Degens, 1965). The organic matter in shales is mainly a macromolecular “geopolymer” called kerogen (Hunt, 1979) that is insoluble (in normal organic solvents and nonoxidizing inorganic acids and bases).