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1.
The characteristics of Bedded Cherts and Banded Iron‐Formations are summarised. As, following O'Rourke's work, the latter are considered as not confined to the Precambrian, the only significant difference between the two formations is shown to be the low iron content of Bedded Cherts. This difference it is suggested is due to Bedded Cherts having been deposited in a virtually continuously acid to weakly alkaline environment, whereas Banded Iron‐Formation was precipitated under an alternation of acid and alkaline conditions yielding siliceous and iron‐rich layers respectively. Such chemical precipitation of either Bedded Chert or Banded Iron‐Formation was in some examples of each type of formation rhythmically interrupted by the deposition of clastic sediment.  相似文献   

2.
Abundant iron oxide deposits including banded iron formations, apatite iron oxide ores, and enigmatic marble/skarn-hosted magnetite deposits occur in the Palaeoproterozoic Bergslagen region, southern Sweden. During the last 100 years, the latter deposit class has been interpreted as contact metasomatic skarn deposits, metamorphosed iron formations, or metamorphosed carbonate replacement deposits. Their origin is still incompletely understood. At the Smältarmossen mine, magnetite was mined from a ca. 50-m-thick calcic skarn zone at the contact between rhyolite and stratigraphically overlying limestone. A syn-volcanic dacite porphyry which intruded the footwall has numerous apophyses that extend into the mineralized zone. Whole-rock lithogeochemical and mineral chemical analyses combined with textural analysis suggests that the skarns formed by veining and replacement of the dacite porphyry and rhyolite. These rocks were added substantial Ca and Fe, minor Mg, Mn, and LREE, as well as trace Co, Sn, U, As, and Sr. In contrast, massive magnetite formed by pervasive replacement of limestone. Tectonic fabrics in magnetite and skarn are consistent with ore formation before or early during Svecokarelian ductile deformation. Whereas a syngenetic–exhalative model has previously been suggested, our results are more compatible with magnetite formation at ca. 1.89 Ga in a contact metasomatic skarn setting associated with the dacite porphyry.  相似文献   

3.
The Palaeo–Mesoproterozoic Tadapatri formation of the Cuddapah basin is comprised of clastic sedimentary rocks with minor carbonates and mafic–ultramafic sill bodies. Geochemistry of the shale is used to study the provenance, paleoweathering and paleoredox conditions of this Tadpatri formation in order to better understand the development of the Cuddapah basin during Palaeo–Mesoproterozoic time. The higher CIA (average 74.39), PIA (average 85.94) and CIW (average 87.59) values of the Tadpatri shales suggest intensely weathered sources. Higher Al2O3/TiO2 (average 30.78) and LREE/HREE ratio (average 8.80) with negative europium anomaly indicate derivation of the clastic sediments from a felsic source rock. The geochemical parameters like U, U/Th, Cu/Zn, Ni/Co, V/Cr ratios reveal that the Tadpatri shales are mainly deposited in an oxic condition.  相似文献   

4.
The RbSr age dating method has been applied to a sequence of Carboniferous shales for which some geochemical information was already available. In the marine shales, the Rb/Sr ratio is higher than in the non-marine and brackish water shales. Although the range of values can be attributed to depositional processes, the range of Rb/Sr values is too small to give a depositional isochron, and thus the age of deposition can not be determined. If samples with a variable mineralogy had been selected for the isotope study, the range of Rb/Sr could have been extended. Only by chance, however, would the depositional age have been obtained.A comparison of the RbSr isotope values in the Carboniferous shales with those in probable source rocks shows that a reduction in the Sr87/Sr86 ratio relative to the Rb87/Sr86 ratio may have taken place. Such a reduction could have occurred on the Carboniferous land-mass during the formation of the clay minerals.  相似文献   

5.
Black shales and massive sulfides represent reduced lithofacies that require isolation from oxic environments to be preserved. This, together with the sedimentary affinity of both lithofacies, can explain their common concurrence in the geologic record. The present study is based on the comparison of Rammelsberg in Germany, Tharsis in Spain, and Draa Sfar in Morocco, three massive sulfide deposits closely associated with black shales that are distributed along the European and North African Variscan orogen. The study entails geochemical, biostratigraphic, and stratigraphic analyses of the black shale sequences hosting the three deposits and mineralogical and textural analyses of the sulfides. All three deposits were formed in immature, tectonically unstable basins within an active continental margin or continental magmatic arc. Their stratigraphic records consist of a sequence of black shales enclosing massive sulfides and variable proportions of bimodal volcanic and subvolcanic rocks. The major differences among the three deposits concern the size, composition, and mineralogy. Regarding age, they are diachronous and younger southward: Rammelsberg is middle Eifelian, Tharsis latest Famennian, and Draa Sfar late Viséan. The study of redox conditions of the paleoenvironment using organic and inorganic proxies highlights similarities and significant differences among the three ore-hosting basins during massive sulfide and black shale deposition. The black shales generally display low Corg and high Stot contents. At Rammelsberg, the Stot/Ctot ratios provide values typical for normal Middle Devonian marine environments, which suggests that the original reactive organic C is now fixed in carbonates. At Tharsis, most of the samples have Corg >1 and Stot/Corg values equivalent to those of Devonian?CCarboniferous normal marine sediments. However, some pyritic hanging-wall samples have Corg <1 and Stot up to 5?wt.%, suggesting the epigenetic addition of HS?. The Stot/Corg ratio for the Draa Sfar samples resembles that of Middle Carboniferous normal marine environments. Geochemical inorganic proxies used to define the environmental conditions include the enrichment factors of U (UEF) and Mo (MoEF) together with V/Cr and V/(V?+?Ni) ratios. Footwall shales at Filón Norte (Tharsis) show positive and eventually elevated UEF and MoEF values, which suggests anoxic conditions, whereas at Rammelsberg and Draa Sfar oxic bottom water is indicated. The relations V/Cr and V/(V + Ni) in all three cases point to a redox boundary near the sediment?Cwater interface, although at Tharsis some samples indicate anoxic/euxinic conditions (i.e., V/(V + Ni) >0.9). Regarding the environmental conditions of the source areas, feldspar illitization and selective depletion in Na and Ca occurred at the three studies sites. Available sulfur isotopic data from the Rammelsberg and Tharsis sulfide ore indicate that biogenic reduction of marine sulfate was a major sulfur source during massive sulfide generation. Nevertheless, a hydrothermal sulfur source has also been detected. At Rammelsberg, this is indicated from the polymetallic sulfides that replace sedimentary and diagenetic pyrite. At Tharsis, the bacteriogenic sulfur signature is also restricted to sulfide with less evolved textures, whereas a hydrothermal source is more evident in sulfides showing evidence of recrystallization. Both geochemical and isotopic data suggest that the bacteriogenic reduction process was inhibited by rapid burial. The sedimentation rates calculated for Rammelsberg, Tharsis, and Draa Sfar were in the range 7?C13, 8?C14, and 19?C27?cm/ka, respectively. Continuous sedimentation of black shale favored the isolation of the massive sulfides and organic material from bottom waters and hence favored their preservation. Accordingly, the relationships between black shales and massive sulfides are considered to be casual. Nevertheless, the tectono-sedimentary evolution of each basin controlled the deposition of both black shales and massive sulfides and the parameters that favored their coeval deposition.  相似文献   

6.
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7.
The organic–inorganic nature of organic-rich source rocks poses several challenges for the development of functional relations that link mechanical properties with geochemical composition. With this focus in mind, we herein propose a method that enables chemo-mechanical characterization of this highly heterogeneous source rock at the micron and submicron length scale through a statistical analysis of a large array of energy-dispersive X-ray spectroscopy (EDX) data coupled with nanoindentation data. The ability to include elemental composition to the indentation probe via EDX is shown to provide a means to identify pure material phases, mixture phases, and interfaces between different phases. Employed over a large array, the statistical clustering of this set of chemo-mechanical data provides access to the properties of the fundamental building blocks of clay-dominated organic-rich source rocks. The versatility of the approach is illustrated through the application to a large number of source rocks of different origin, chemical composition, and organic content. We find that the identified properties exhibit a unique scaling relation between stiffness and hardness. This suggests that organic-rich shale properties can be reduced to their elementary constituents, with several implications for the development of predictive functional relations between chemical composition and mechanical properties of organic-rich source rocks such as the intimate interplay between clay-packing, organic maturity, and mechanical properties of porous clay/organic phase.  相似文献   

8.
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9.
The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.  相似文献   

10.
Iron oxide–apatite deposits are present in Upper Eocene pyroxene-quartz monzonitic rocks of the Zanjan district, northwestern Iran. Mineralization occurred in five stages: (1) deposition of disseminated magnetite and apatite in the host rock; (2) mineralization of massive and banded magnetite ores in veins and stockwork associated with minor brecciation and calcic alteration of host rocks; (3) deposition of sulfide ores together with potassic alteration; (4) formation of quartz and carbonate veins and sericite, chlorite, epidote, silica, carbonate, and tourmaline alteration; and (5) supergene alteration and weathering. U–Pb dating of monazite inclusions in the apatite indicates an age of 39.99?±?0.24 Ma, which is nearly coeval with the time of emplacement of the host quartz monzonite, supporting the genetic connection. Fluid inclusions in the apatite have homogenization temperatures of about 300 °C and oxygen isotopic compositions of the magnetite support precipitation from magmatic fluids. Late-stage quartz resulted from the introduction of a cooler, less saline, and isotopically depleted fluid. The iron oxide–apatite deposits in the Tarom area of the Zanjan district are typical of a magmatic–hydrothermal origin and are similar to the Kiruna-type deposits with respect to mineral assemblages, fabric and structure of the iron ores, occurrence of the ore bodies, and wall rock alteration.  相似文献   

11.
The advent of iron metallurgy changed the course of prehistoric Thailand. There is, however, little information on the nature of iron production and the extent to which locally abundant and easily quarried lateritic iron may have been the ore source. For northeast Thailand archaeological sites, the presence of both iron slag and laterite iron nodules within and surrounding these sites has widely been assumed to represent localized smelting using locally sourced iron ore. This interpretation is, however, based on untested laterite‐equals‐ore and slag‐equals‐smelting equations. This paper tests these assumptions by examining the chemistry of the lateritic iron nodules from two archaeological sites. At one site, the laterite is not a credible ore source and iron working probably comprised only smithing. Local laterite at the other site may be a usable ore. We conclude that, in northeast Thailand, the use of local laterite as an ore source cannot be assumed and that presence of both laterite and slag is not necessarily evidence for prehistoric smelting. © 2010 Wiley Periodicals, Inc.  相似文献   

12.
A representative collection of structurally characterized eudialyte-group minerals (EGM) with varying relative concentrations of Fe2+ and Fe3+ ions from several localities was investigated at room temperature by 57Fe Mössbauer spectroscopy coupled with magnetometric, optical, and X-ray powder diffraction methods. To refine the Mössbauer parameters of isomer shift and quadrupole splitting for Fe2+ and Fe3+ in different types of coordination polyhedra (planar quadrangle, square pyramid, and distorted octahedron) in EGM structures, we also collected Mössbauer parameters for gillespite and labuntsovite. The main purpose of this work is to determine the location of Fe3+ in different sites in typical eudialyte, rastsvetaevite, georgbarsanovite, and some of their naturally hydrated and heat-treated analogs, and investigate the kinetics and oxidation mechanisms of iron ions in their structures. Our study has confirmed the presence of Fe2+ ions in the planar quadrangle and square pyramid in primary eudialytes, as well as the presence of Fe3+ ions in the square pyramid and distorted octahedron in primary, naturally hydrated, and heat-treated eudialytes. According to this study, hydrated eudialytes are characterized by the location of Fe3+ ions mainly in octahedra with OH groups and/or water molecules at trans vertices, while heat-treated eudialytes are characterized by their location in square pyramids with an O2? anion at the apical vertex.  相似文献   

13.
Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km2 area on Wegener Halvø in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (206Pb/204Pb: 18.440–18.466; 207Pb/204Pb: 16.554–16.586; 208Pb/204Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ34S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ34S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvø area in general has δ34S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ34S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in 34S due to preferential removal of 32S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ34S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.  相似文献   

14.
Bulk sedimentary nitrogen isotopes (δ15Nbulk) have been primarily used to identify bottom water redox states during deposition in sedimentary environments that have not undergone significant diagenetic or catagenetic alteration. Recently, the utility of sedimentary δ15Nbulk as a paleoredox proxy in hydrocarbon-bearing shale intervals was demonstrated by qualitatively correlating the δ15Nbulk profiles from shale units to their depositional redox conditions. However, the effect of thermal maturity on the δ15Nbulk signal remains unknown. We analyzed samples from three cores from the Devonian–Mississippian age Woodford Shale in the Anadarko Basin with vitrinite reflectance values ranging from the oil window to the gas window in order to investigate how depositional redox conditions and thermal maturation affect the bulk sedimentary δ15N signals. Our results indicate that the δ15Nbulk values for the Woodford do not correlate with thermal maturity. Instead, the δ15Nbulk values primarily reflect the depositional redox conditions, which are supported by trace metal concentrations and depositional features such as burrow abundance and lamination. The expected relationship between δ15Nbulk and paleoredox conditions was observed both within each core and between cores on a basin wide scale, with samples deposited under suboxic bottom water conditions having higher δ15Nbulk values than those deposited under anoxic bottom water conditions. Our data also indicate that redox bottom water column conditions in the Anadarko Basin varied spatially and temporally during the deposition of the Woodford Shale.  相似文献   

15.
Iron filling and iron filling–cement mixture were used to improve the shear strength characteristics of Irbid clayey soil. For this purpose, five types of Irbid clay soils were obtained and mixed with iron filling and iron filling–cement mixture at different percentages. Two sets of prepared samples were mixed with the admixture. The first set was prepared by mixing the soil samples with iron filling alone at 2.5, 5.0, 7.5, and 10% by dry weight of the soil. The second set was prepared by mixing with iron filling–cement mixture at equal ratio of the same percentages of the first set. An unconfined compression test was performed in this study to measure the shear strength properties of the soils. The test results showed that the increase in the percentages of the iron filling and iron filling–cement mixture up to 10% will result in increasing the maximum dry density of the soil and increase the unconfined compressive strength and the secant of modulus of elasticity of the clayey soil. Also, the addition of iron filling–cement mixture increased the unconfined compressive strength and secant modulus of elasticity of the clayey soil higher than the addition of iron filling alone.  相似文献   

16.
Geochemical elements of oil shale in the Maoniing Basin were analyzed to discuss provenance attribute and depositional environment of the Youganwo formation. Experimental date of the major elements, trace elements and rare earth elements of 24 samples from the Maoye I well were examined. The analyzed oil shale samples were characterized by enrichment of Tb, U, Kb and LREE, depleted of Zr, Cr and Hf, negative Eu and Ce anomalies, indicating that these samples were originated from continental crust. The chemical index of alteration( CIA) values and the ZrZSc-Th/Sc diagrams indicate that source rooks had undergone intense chemical weathering and deposition recirculation. Based on the La/Th-Hf and La/Yb-^REE diagrams and the negative anomaly of Eu element, the oil shale in the Maoniing Basin has diverse sources, which mainly came from felsic source region of the upper crust or the mixture of felsic volcanic rocks, granite and sedimentary rocks. Ratios of the SrZCu, MgO/CaO suggest that oil shale was formed in fresh water under warm and humid climate, shallow water column became deeper during the middle and late sedimentary period. The depositional environment is interpreted to be limnetic with weak reduction at the early stage and gradually turned into semi-deep to deep lacustrine.  相似文献   

17.
Mineralogy and Petrology - Uranium–lead isotopes and trace elements of titanite from the Chengchao iron skarn deposit (Daye district, Eastern China), located along the contact zones between...  相似文献   

18.
《Applied Geochemistry》2003,18(3):395-408
Arsenopyrite-rich waste from a former metalliferous mine were spread out over the sloping side of a deep valley after processing. Over the past 30 a, they have been subjected to rainfall and acid water originating from the abandoned mine galleries. This intensive leaching has led to the formation of thin layers of As–Fe crusts on the tailings surface acting as a cement. X-ray diffraction and SEM coupled with EDS determined that jarosite was present in all mineral samples and could contain a small amount of As (∼5.7 wt.%). In addition EMPA and Raman microspectroscopy characterised the presence of amorphous As(V) Fe hydrates as well as rare arsenate minerals (e.g. scorodite). Raman microspectroscopy in particular identified a preponderance of goethite or hematite within the mineral framework of the tailings materials that is likely to sorb recalcitrant As species. The characterisation of the components of the tailings enable the identification of their evolution, shows the progressive decrease of their As-content and emphasises the consequences of the temporary trapping of As in the very acidic and oxidising conditions prevailing in such environments. Resinous amorphous material was identified as the richest in As with As ∼17.1 wt.%. This material evolved toward more crystallised phases (e.g. goethite, jarosite) which contained less As (3.2 wt.%<As <5.7 wt.%). Paragenesis showed the progressive release of As with the crystallisation evolution of the As-trapping material.  相似文献   

19.
The reactivity of iron(III) oxyhydroxides as reflected by their tendency to dissolve is of great importance in the redox cycling of iron and the bioavailability of iron to phytoplankton in natural waters. In this study, various iron(III) oxyhydroxides were produced by oxygenation of iron(II) in the presence of solutes, such as phosphate, sulfate, bicarbonate, valeric acid, TRIS, humic and fulvic acids, and in the presence of minerals, such as bentonite and δ-Al2O3 under conditions encountered in aquatic systems. The reactivity of the different iron(III) oxyhydroxides was subsequently assessed by means of a reductive dissolution using ascorbate and non-reductive dissolution using HQS (8-hydroxyquinoline-5-sulfonic acid) or oxalate. The experimental results show that the iron(III) oxyhydroxides with a low degree of polymerization exhibit higher reactivity than those with a high degree of polymerization or with high crystallinity. The quantity of active surface sites and the coordination arrangement of the functional groups at the surface of the iron(III) oxyhydroxides, especially the extent of the endstanding -OH groups per iron(III) ion determine the reactivity of iron(III) oxyhydroxides toward dissolution.Surfaces, such as clay and aluminum oxides, not only accelerate the oxygenation reaction of iron(II), but also induce the formation of iron(III) oxyhydroxides which are more active toward the dissolution reactions. Polymerization of iron(III) oxyhydroxides on the surfaces occurs predominantly in two dimensions rather than in three dimensions.In a laboratory experiment, the iron(III) oxyhydroxide formed in the presence of TRIS can be reduced by fulvic acid in a closed system under the following conditions: Fe(OH)3(s) 0.01 g/l, fulvic acid 5 mg/l, pH 7.5, 20°C. The kinetics of the reaction depend on the reactivity of iron(III) oxyhydroxide and reducing power of fulvic acid. Although reductants other than fulvic acid may be of importance in antural waters, this result provides the laboratory evidence that the >FeIII-OH/Fe(II) is able to act as an electron transfer mediator for the oxidation of natural organic substances, such as fulvic acid, by molecular oxygen either in the absence of microorganisms or as a supplement to microbial activity.  相似文献   

20.
Fluvial dissolved Fe concentrations decrease upon mixing with seawater, resulting in the formation of Fe-floccules. However, a clear understanding of the fate of these floccules has yet to be established. Assessing how tidal processes affect the formation of Fe-colloids in the Leirárvogur estuary, SW Iceland, is an important step in understanding the formation and potential deposition of estuarine Fe-rich minerals within this estuarine system. The Leirárvogur estuary drains predominately Fe-rich basalt, increasing the likelihood of detecting changes in Fe-phases. Fluvial waters and local lake waters that drain into the estuary were compared and the effects of seasonal changes were considered, in an attempt to understand how varying end-members and external factors play a role in Fe-rich mineral formation. Aqueous and colloidal Fe concentrations were found to be greater towards the head of the Leirárvogur estuary, suggesting that potential Fe-rich minerals and complexes are forming at sites of fluvial input. Increasing suspended colloidal Fe towards the estuary mouth suggests that Fe-colloids are readily transported seaward.  相似文献   

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