Occurrence Of Organic Matter Research Articles

The Geochemical Characteristics of High Quality Hydrocarbon Source Rocks - A Case Study of Hailaer Basin

The distribution of high quality hydrocarbon source rocks plays an important role in the accumulation of oil and gas. As a result, the identification of geochemical characteristics of high quality source rocks is the key to discriminate the distribution of high quality source rocks accurately. By core observation and sample analysis, taking Wuerxun-Beier depression in Hailaer Basin as the target regions, we make accurate discrimination of high quality hydrocarbon source rock developmental characteristics and comparison with common source rocks. The research shows that: (1) the hydrocarbon expulsion efficiency in study zone is high due to the alternating deposits of high quality hydrocarbon source rocks and sandstones. The high quality hydrocarbon source rocks deposited in the reducing environment to strong reducing ones, whereas common rocks deposited in oxidizing environment to weak oxidizing ones. (2) the occurrence of organic matter of high quality hydrocarbon source rocks is mainly in stratified enrichment type. The organic matter develops parallel bedding or basic parallel bedding. However, the distribution of organic matter of common source rocks is porphyritic and heterogeneous, or interrupted lamellar. (3) the hydrocarbon potential of high quality hydrocarbon source rocks is more than ten times that of common source rocks. (4) the content of organic carbon in high quality source rocks is high and the content of chloroform asphalt A is relatively low, which reflects that the hydrocarbon expulsion efficiency of high quality source rocks in the sand-shale interbeds of study zone is high.

Relationship between heterogeneity of source rocks and genetic mechanism of abnormally high pressure

Based on the micro-fluorescence observation of polished sections of source rocks, two types of micro-layers with different wetting properties formed in thermal evolution, i.e., oil-wetting and water-wetting micro-layers, are distinguished. The wetting property is found closely related to the abundance of organic matter and its occurrences with inorganic minerals. The alternating distribution and intercrossing of two types of micro-layers formed many separated spaces with different wettabilities. The strong capillary forces in these separated spaces with different wettibilities obstruct the cross flows of pore fluids and result in nearly independent and closed fluid systems. As a consequence, these spatially superposed spaces in source rocks bear the overburden pressure and then abnormally high pressures have developed in the whole source rock unit. Therefore, the abundance and occurrences of organic matter are the main inner factors influencing the formation of abnormally high pressures, whereas the formation, distribution and development of micro-layers with different wettabilities during the evolution of source rock determine the intensity and spatial distribution of abnormally high pressures.

The relationship between organic matter and specific surface area in <2 μm clay size fraction of muddy source rock

The adsorption of organic matter (OM) onto clay minerals has long been considered as a significant way of OM preservation in source rock. Here we analyzed the relationship between OM and the specific surface area (SSA) of <2 μm clay size fraction isolated from 13 source rock cores collected from Dongying depression. Rock-Eval pyrolysis and N2 adsorption experiment were employed to probe the characteristics of OM and SSA (denoted S BET) in samples before and after OM extraction using trichloromethane. The results indicate that various kinds of OM occurrence coexist in clay size fraction and their contributions to hydrocarbon are different in each period of OM evolution. The occurrence and amount of OM affect the S BET of clay size fraction, and a nonlinear negative correlation between total organic carbon (TOC) and SBET can be recognized. The soluble OM (chloroform extract “A”), mainly stored in the pore space of clay size fraction, shows a negative correlation in amount with S BET. Our data also indicate that free hydrocarbon (S1) was stored mainly in the pore space and/or the surface of clay size fraction, whereas pyrolysis hydrocarbon (S2) was mingled mainly with clay minerals. Therefore, to understand various OM occurrences and their relationship with S BET in the clay size fraction is significant in the study of generation, accumulation, and migration of hydrocarbon in muddy source rock.

Biochemical Characteristics of Organic Matter in a Guano Concretion of Late Miocene or Pliocene Age from Manchester Parish in Jamaica

The biogeochemical fate of organic matter (OM) entering soils is an important issue that must be examined to better understand its roles in nitrogen cycling and as a natural modulator of soil-atmospheric carbon fluxes. Despite these critical roles, there are uncertainties in estimating the contribution of this feedback mechanism due in part to a lack of molecular-level information regarding the origin and labile and refractory inventories of OM in soils. In this study, we used a multi-analytical approach to determine molecular-level information for the occurrence and stabilization of OM in a bird guano concretion of the Late Miocene or Pliocene age in Jamaica. We determined the specific organic structures persisting in the concretion and the possible contribution of fossil organic matter to the OM pool in modern environments. Our results indicate that aliphatic species, presumably of a highly polymethylenic nature [(CH2)n], may significantly contribute to the stable soil-C pool. Although not as significant, proteins and carbohydrates were also enriched in the sample, further suggesting that fossil organic matter may contribute to carbon and nitrogen pools in present day soil organic matter.

Thermally induced alterations of minerals during measurements of the temperature dependence of magnetic susceptibility: a case study from the hydrothermally altered Soultz-sous-Forêts granite, France

Identification of magnetic minerals using the temperature dependence of magnetic susceptibility in low field (κ–T) combined with optical microscopy, microprobe, X-ray diffraction, and chemical analysis provided new constraints on the alteration of Fe-bearing minerals of the magnetite-bearing Soultz-sous-Forets granite from the EPS-1 borehole (upper Rhine Graben, France). While relatively fresh granite shows largely reversible κ–T curves typical of magnetite, the altered granite revealed a wide variety of irreversible heating and cooling curves, which allowed an assignment to different alteration stages under specific geochemical conditions. Though paramagnetic minerals like Fe-bearing carbonates, pyrite, or antiferromagnetic hematite could not be detected according to their Curie or Neel temperature, they were identified due to reactions to new ferrimagnetic phases during the heating/cooling experiments at specific temperatures. Mineral reactions were proved by measurements of the single mineral phases hematite, Fe-carbonates, and illite. Our mineralogical results combined with the thermomagnetic measurements imply that first faulting of the granite occurred already during cooling of the magma, which caused a first magnetite oxidation event. During uplift of the granitic body and exposure to a paleo-erosion surface, strongly acidic fluids, emerged from pyrite oxidation, caused a decomposition of Fe-bearing minerals like martite (hematite derived from magnetite oxidation) and Fe-carbonates and an ongoing transformation of magnetite to martite. Subsequently, precipitation of fine-grained hematite was restricted only to the upper part of the pluton. In the deeper part of the borehole, pyrite was preserved from oxidation. In an active fault, zone martite was reduced back to magnetite, which can be explained with the occurrence of organic matter transported by fluids.

Oxygen isotopes and implications for the cavity-grown quartz crystals in the Lockne impact structure, Sweden

Well-developed quartz crystals are found in open cavities in the intensely fractured crystalline basement of the marine-target impact structure at Lockne in central Sweden. The 458 Ma impact structure has a well-preserved crater in Precambrian granitic basement rock that is covered by resurge deposits composed of breccias and fine-grained sedimentary units of mixed Ordovician limestone, Cambrian black bituminous shales and the basement granite. Directly after the impact, the resurge deposits formed when the seawater rushed back into the crater. The residual heat from the impact facilitated a short-lived hydrothermal system accompanied by the inflowing seawater, which resulted in mineral growth in fractures and open cavities of the granite basement. The oxygen isotope values of the first-precipitated minerals, the cavity-grown quartz crystals, range from +15.2 to +16.2‰ (Vienna Standard Mean Ocean Water) and differ from those of the hosting granite basement rock with δ18O quartz between +10.1 and +11.7‰. The δ18O values of the quartz are more consistent with derivation from a fluid of relatively high δ18O probably attributable to oxygen isotope exchange during seawater–rock interactions in the resurge deposits. The occurrence of organic matter in association with the cavity-grown quartz strongly indicates a relationship to the black bituminous shale in the matrix of the breccia that rests on the crater floor. Comparing the results with previously obtained oxygen data on fracture-grown calcite from Lockne shows that oxygen isotope composition of the cavity-grown quartz crystals is less variable and probably more accurately reflects the original fluid source.

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen)

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen) 51 samples from the Middle Triassic black shales (organic carbon-rich silt-stones; up to 4.9% TOC - Total Organic Carbon) from the stratotype section of the Bravaisberget Formation (western Spitsbergen) were analyzed with respect to isotopic composition of pyritic sulphur (Δ34S) and TOC. Isotopic composition of syngenetic pyrite-bound sulphur shows wide (Δ34S from -26‰ to +8‰ VCDT) and narrow (Δ34S from -4‰ to +17‰ VCDT) variation of the Δ34S in upper and lower part of the section, respectively. Range of the variation is associated with abrupt changes in dominant lithology. Wide Δ34S variation is found in lithological intervals characterized by alternation of black shales and phosphorite-bearing sandstones. The narrow Δ34S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the Δ34S values suggests interplay of various factors in sedimentary environment. These factors include oxygen concentration, clastic sedimentation rate, bottom currents and burrowing activity. Biological productivity and rate of dissimilatory sulphate reduction had important impact on the Δ34S variation as well. Wide variation of the Δ34S values in the studied section resulted from high biological productivity and high rate of dissimilatory sulphate reduction. Variable degree of clastic sedimentation rate and burrowing activity as well as the activity of poorly oxygenated bottom currents could also cause a co-occurrence of isotopically light and heavy pyrite in differentiated diagenetic micro-environments. Occurrence of organic matter depleted in hydrogen could also result in a wide variation of the Δ34S values. Narrow variation of the Δ34S values was due to a decrease of biological productivity and low rate of dissimilatory sulphate reduction. Low organic matter supply, low oxygen concentration and bottom currents and burrowing activity were also responsible for narrow variation of the Δ34S. The narrow range of the Δ34S variation was also due to occurrence of hydrogen-rich organic matter. In the studied section the major change in range of the Δ34S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic values may suggest evolution of the sedimentary environment from high- to low-energy and also facies succession from shallow to deeper shelf. The evolution should be linked with the Late Anisian regional transgressive pulse in the Boreal Ocean.

Occurrence of Organic Matter in Calcimicrobialites across Permian–Triassic Boundary in Huayingshan Region, Sichuan, South China

Calcimicrobialites across the Permian-Triassic boundary in Huayingshan region were investigated using the fluorescence microscopic measurements to understand the occurrence of organic matter. The microbialites are composed of micrite matrix and coarse spar cement. Abundant rhombic or magnetic needle-like carbonate minerals were observed adrift within the cement. The fluorescence microscopic measurement indicates the micrite matrix in microbialites shows the most abundant organic matter, with the rhombic or magnetic needle-like carbonate minerals and coarse spar cement coming to the 2nd and the 3rd, respectively. Organic matter is mainly preserved in the space between the grains of the micrite minerals but almost evenly distributed in the rhombic or magnetic needle-like carbonate minerals. As one of the common diagenesis types, dolomitization is observed to occur in the microbialites in Huayingshan. However, the carbonate cement in microbialites still has high content of element Sr as shown by the microprobe analysis, reflecting that the dolomitization might have happened in a restricted environment. Observation under the fluorescence microscope shows that dolomitization just led to the redistribution of organic matter in the grain space of dolomite minerals, inferring that the diagenesis has a slight effect on the preservation, and thus on the content of organic matter in the microbialites.

Occurrence, thermal evolution and primary migration processes derived from studies of organic matter in the Lucaogou source rock at the southern margin of the Junggar Basin, NW China

The Lucaogou Formation carbonate-rich oil shale source rock is exposed at the southern margin of the Junggar Basin, Xinjiang, NW China. We have sampled it in detail and conducted microstructural, mineralogical and geochemical studies, including thin section petrography, UV fluorescence petrography, X-ray diffraction, vitrinite reflectance, bitumen reflectance, fluid inclusion analysis and Raman spectroscopy. Organic matter is disseminated through the carbonate-bearing siltstone source rocks and concentrated in numerous bedding parallel stylolites and in two sets of carbonate veins, one along bedding parallel fractures and the other cross-cutting stylolites and bedding. The research about maturity of organic matter finds vitrinite reflectance values increase from the dispersed kerogen (0.64%) to the stylolites (the one of oriented vitrinite is 0.72% and the one of migrated bitumen is 2.38%); Homogenization temperatures of fluid inclusions in veins containing hydrocarbon fluid inclusions show an increase from 178.5°C in the bedding parallel veins to 222°C in the cross-cutting veins, confirmed by Raman spectroscopy. These results support a model of progressive heating accompanied by fluid loss during later stages of thermal maturation of source rock and the onset of primary migration. Obviously, the occurrence of organic matter is the trace of hydrocarbon primary migration, and the bedding lamination surfaces and cross-cutting fissures are the principal pathways of hydrocarbon-bearing fluids migration. Bedding lamination surfaces evolved into stylolites along the earliest primary migration pathways, followed by bedding parallel and cross-cutting fissures.

Mineralogy and metamorphism conditions of manganese ore at the South Faizulino deposit, the southern Urals, Russia

The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and XCO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO2 were formed during tectonic deformations in the Middle Carboniferous and Permian.

Origin and migration timing of hydrothermal fluids in sedimentary rocks of the Paraná Basin, South America

Clay authigenesis induced by migrating hydrothermal fluids related to the Early Cretaceous Serra Geral magmatism in the Paraná Basin was investigated by mineralogical, geochemical and isotopic means. Stratigraphically equivalent sedimentary samples from the Permian Rio Bonito (in southeastern Brazil) and San Miguel (in eastern Paraguay) formations were collected close to and far from major intrusive bodies. The rocks are generally cemented, in addition to the clay material, by a variety of minerals, such as albite, K-feldspar, calcite, dolomite, zeolite and quartz. The authigenic clay minerals mainly consist of different types of illite-rich mixed layers illite–smectite and chlorite. Corrensite and smectite were also observed. Measured and estimated K–Ar ages indicate that the varied authigenic illite assemblages were most probably generated 140 to 130 Ma ago, during the Serra Geral magmatic event. Hydrothermal fluids depleted in LREE and sometimes enriched in Eu promoted the formation of the well-crystallized illite minerals in rocks characterized by high water / rock ratios. Additional authigenic assemblages with PAAS-type REE patterns probably formed in less porous rocks with low water / rock ratios. Also, some of the authigenic clays yield HREE-depleted patterns, which point to the occurrence of organic matter during crystallization.

Chemostratigraphy of the Posidonia Black Shale, SW Germany: I. Influence of sea-level variation on organic facies evolution

During the Toarcian, widespread deposition of organic-rich mudrocks (black shales) occurred across Europe, linked to a 3rd-order sea-level rise. We provide a high-resolution, multi-proxy chemostratigraphic study of the Dotternhausen sections in SW Germany embedded in a stratigraphic and palecological framework. Chemostratigraphy and paleoenvironmental reconstruction is based on Rock–Eval and elemental analysis of bulk organic matter and biomarker analysis of aliphatic hydrocarbon fractions. Aliphatic biomarker ratios applied to reconstruct the facies development include pristane/phytane and (pristane+phytane)/( nC 17+ nC 18) ratios, carbon preference index, C 27/C 29 sterane, and hopane/sterane distribution. Results show that sea-level change exerted the major control on the composition of primary produced organic matter and on its redox-controlled preservation. Changes in primary production and in the ratio of terrestrially versus marine-derived organic matter take place drastically at the onset of black shale deposition. The relative proportion of terrestrial versus marine biomass accumulation is independently assessed by highly correlative bulk organic matter properties (HI values) and biomarker ratios (CPI). The terrestrially derived end-member yields HI values of 50–100 mg HC/TOC and CPI values of 1.4–2.0, whereas the marine end-member has HI values of 600–700 mg HC/TOC and CPI values of 0.95–1.05. Correspondingly, the shift in plant- versus bacteria-derived biomass is discriminated by HI values and hopane/sterane ratios. Data were grouped according to their position in the lowstand (LST), lower and upper transgressive (TST), highstand systems tract (HST), and maximum flooding surface (mfz) of the sequence stratigraphic framework. Occurrence of organic matter with a significant bacterial contribution is restricted to the LST and lowermost TST and shows hopane/sterane ratios of 0.25–0.5. Marine phytoplanktonic matter dominates the upper TST, HST, and mfz with organic matter giving hopane/sterane ratios between 0.5 and 1.7. Samples from the HST yield HI values between 450 and 500 mg HC/TOC, which is consistently lower by 150 mg HC/TOC when compared to the TST. The lower quality of the organic matter deposited in the HST is due to its longer settling times through a thicker oxygenated water column. A strong correlation with relative sea-level stand and the C 27/C 29 sterane ratio is observed. LST exhibits C 27/C 29 sterane ratios of 0.7, during transgression values increase to 1.0, reaching a maximum at 1.4 during sea-level highstand, thus reflecting decreasing terrigenous input and shifts in near-shore versus open marine algal communities. Episodic fluctuation of anoxia is discussed on the basis of a monsoonal climate model, which infers significant surface run-off during summer monsoon providing high nutrient supply and associated high primary productivity. In combination with a density-stratified water column, excellent preservation conditions for organic matter were established. Dry-winter conditions with inversion of the surface-water flow regime led to seasonal export of bottom waters and episodic establishment of dysoxic conditions. Variations in the extent of annual flushing of sub-basins on the Toarcian shelf controlled long-term organic matter accumulation rate. Paleoclimate variability and its connection with sea-level stand is thus of paramount importance for different modes of organic matter accumulation and redox system stability.

2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa)

Two billion years ago, the increase of oxygen in atmosphere and the high 235U/ 238U uranium ratio (>3%) made possible the occurrence of natural nuclear reactors on Earth. These reactors are considered to be a good natural analogue for nuclear waste disposal. Their preservation during such a long period of time is mainly due to the geological stability of the site, the occurrence of clays surrounding the reactors and acting as an impermeable shield, and the occurrence of organic matter that maintained the environment in reducing conditions, favourable for the stability of uraninite. Hydrogeochemical studies and modelling have shown the complexity of the geochemical system at Oklo and Bangombé (Gabon) and the lack of precise data about uranium and fission products retention and migration mechanisms in geological environments. To cite this article: F. Gauthier-Lafaye, C. R. Physique 3 (2002) 839–849.

The relationship between dolomitization and organic matter occurrence in Lower Paleozoic carbonate in the Ordos Basin

Based on observations on the core and surface sections of Lower Paleozoic carbonate in the Ordos Basin, petrography research and measurements of TOC, TOS, Ro, XRD, and comparative study with dolomite in modern Coorong Lake, it has been revealed that: (i) dolomitization may occur in micrite limestone, gypsum-halite and argillaceous sandstone, and it can be divided into three types: sedimentary penecontemporaneous-early diagenesis, late diagenesis of deep burial and catagenesis of uplift period. However, the crystal cell of the second type less than 35 μ m in size is most closely associated with gas pool; (ii) the highest content of organic matter (OM) is produced in samples from the argillic dolomite which may be formed by argillaceous fluid through gypsum-halite; (iii) in the evolution process from penecontemporaneous dolomite into stoichiometric dolomite, the crystal order of dolomite and the porosity of its host rock tend to increase, which is favorable to the formation of an available migration network. When the power of the fluid is high enough, the network is mainly favorable to the migration and transport of heat, but when the power of the fluid goes down, the network system is favorable, due to its large space, to OM deposition in it.

First and second order Raman spectra of natural highly carbonified organic compounds from metamorphic rocks

Major structural differences observed while studying highly carbonified natural organic matter from rocks using Raman microspectrometry reflect postsdiagenetic transformation processes. The mode of occurrence of organic matter is however another important factor governing structural changes. Raman microspectrometry is a useful tool for the estimation of the structural order and the presence of structural defects in carbonaceous matter. Obtained parameters are to be compared carefully especially in the study of homogeneous series of carbonaceous samples – those having the same geological age, originating from rocks of similar lithology and mode of occurrence. Not only frequencies of major Raman first and second order bands but also the evolution of parameters as half band of the O (E 2g2) and G′ are to be measured to describe the transformation of the structure in connection with metamorphism.

Pbpe systematics on diagenetic clays: an example from proterozoic black shales of the Franceville basin (Gabon)

This study presents the first dating attempt of diagenetic clay material by the PbcPb isotope method. PbPb isotopic measurements were performed on authigenic < 0.4-μm clay fractions (illite 1M, FeMg-chlorite) extracted from black shales highly enriched in organic matter of the Francevillian (lower Proterozoic) sedimentary series in Gabon. The clay residues and acid leachates (1 N HCl) yield a PbPb isochron age of 2328 ± 130 Ma (MSWD = 1.24). Even if this age is within error limits identical with two SmNd isochron ages previously reported for the same material, it is not considered as a true isochron age. The Pb isotopic data of the residues indicate that the clay minerals are not in isotopic equilibrium. However, the isotope data of the leachates separately define an isochron age of 2265 ± 150 Ma (MSWD = 0.36), which is closer to the SmNd age. It is suggested that the leachates are the most appropriate phases for dating diagenetic processes with Pb isotopes. The UThPb fractionation in the clay fractions is related to the occurrence of organic matter in the sediments. The U Pb and U Th ratios of the clays correlate with their organic carbon contents. This fractionation is probably due to differential complexation of U, Th and Pb by organic ligands during the dissolution of detrital mineral phases and clay mineral authigenesis.

Organic matter in the Soultz HDR granitic thermal exchanger (France): natural tracer of fluid circulations between the basement and its sedimentary cover

Organic matter has been observed in cores of the Soultz-sous-Forêts granite (Alsace, France) at depths between 2158 and 2160 m, in a highly fractured and altered zone. The granite is overlain by a 1400-m-thick sedimentary cover containing petroleum (Pechelbronn oil field). The Soultz area is devoted to Hot Dry Rock geothermics thanks to a high geothermal gradient (up to 100 °C/km). During drilling operations, an artesian source produced oil in a fractured zone of the sedimentary cover (Buntsandstein). Its gas-chromatography and mass-spectrometry (GC-MS) analysis revealed the presence of normal alkanes (n-alkanes, i.e. linear hydrocarbons) centered on C 17, branched alkanes with a major C 19 peak, and few unsaturated hydrocarbons. The aromatic fraction is present in small amounts. In the Soultz granite, where it is altered, organic matter is exclusively associated with tosudite (interstratified clay mineral) which crystallized in plagioclase sites during a hydrothermal alteration episode. Organic matter has been later displaced and concentrated along veinlets in which illite and carbonates have crystallized during another hydrothermal alteration stage. The soluble organic matter analyzed by GC-MS is composed of aliphatic acids, n-alkanes with a bimodal C 18 and C 24-C 25 centered distribution, alkylbenzenes and aromatic acids. Organic compounds in the granite would either originate from a single source (immature sediments) or from two sources (immature sediments and migration of the Pechelbronn oil). No real evidence was found to prove which hypothesis is the best one. The presence of organic matter in the granite shows the importance of fluid flows between the sedimentary cover and the granitic basement through major fractures. In addition, the impregnation of plagioclase pseudomorphs with organic matter is made possible due to their high interconnection degree and to the intergranular microfracturation of the granite. The succession of several hydrothermal events with different physico-chemical characteris- tics may also be inferred from the occurrence of organic matter found in association with neoformed clay minerals in the granite.

Evaluation of proposed precipitation mechanisms for Mississippi Valley-type deposits

The mechanism of precipitation is an important aspect of any genetic model for Mississippi Valley-type deposits. Yet most of the precipitation mechanisms for minerals in the Mississippi Valley-type association have serious flaws. Solution mixing would require an unlikely series of solutions to account for the various minerals in the ores, and it does not account for the universal occurrence of organic matter in the ores nor for the oxidation state of sulfur in pyrite in the ores. Sulfate reduction addresses some of these problems, but is inconsistent with kinetic data and could not be reversed to account for the oscillations between precipitation and dissolution of sulfide minerals in the ores. Carbon dioxide effervescence does not address the precipitation of most minerals in the ores, and all of the evidence for effervescence may be explained in other ways. Cooling of the mineralizing solution could precipitate many minerals, but fluid inclusion data suggest that, in many deposits, the solution did not cool significantly as any particular stage formed. A credible genetic model also must explain why all of the minerals precipitated at the same sites; any combination of the above mechanisms which suggests that unrelated mechanisms occurred at the same sites by coincidence is unlikely. The most reasonable scenario is that a hot, thiosulfate-bearing mineralizing solution reacted in various ways with organic matter at the sites of mineralization to precipitate the ore minerals. The organic matter acted as a reductant, source of carbon dioxide, source of organic acids, and a substrate for bacterial metabolism of thiosulfate in various stages of mineralization. Thus organic matter links all stages of the mineralization to the same sites.

Sorption by mineral surfaces: Rebirth of the classical condensation pathway for kerogen formation?

What are the consequences for organic matter diagenesis of the observation in two recently published articles of a strong correlation between surface area and organic content in marine sediments? The findings suggest that the typical mode of occurrence of organic matter in marine sediments is as a monolayer (or equivalent concentration) sorbed to the surface of mineral grains. This comment considers the theoretical factors which may influence adsorption and propagation of polymeric organic matter on mineral surfaces, and looks at the likely diagenetic fate of adsorbed material. Both adsorption and condensation have been suggested as possible mechanisms for the preservation of labile biopolymers, but neither process is satisfactory as a stand-alone mechanism; adsorption of monomers can merely retard their biodegradation, and condensation is not favoured in solution. However, if the two processes operate in concert, the criticisms levelled against each process considered in isolation are cancelled out, adsorption promoting condensation and condensation enhancing the strength of adsorption of the products. We suggest that the diagenetic modifications of surface adsorbed organic molecules will tend to strengthen their binding to the mineral surface, such that the geomacromolecules will evolve on the mineral surface towards strongly bound monolayers. The hypothesis overcomes many of the objections to the so-called classical condensation pathway of kerogen formation.

XXX.—On the Presence of Organic Matter in the Purest Waters from Terrestrial Sources

Ever since the discovery by Berzelius of crenic acid in the iron ochre of the water of Porla, in Sweden, chemists have admitted the usual presence of that acid in mineral waters, or those springs containing notable quantities of dissolved inorganic constituents. In such natural waters, also, as are visibly coloured, organic matter is usually understood to be present. Any ideas, however, which may have been entertained respecting the occurrence of organic matter in the perfectly colourless, transparent, and comparatively pure water of ordinary springs, wells, and rivers, have been merely vague and conjectural.