Manganese deposits: Communication 1. Genetic models of manganese ore formation

Major regularities in the formation of manganese rocks and ores have been established on the basis of available published and original data. The proposed genetic classification of main manganese deposits (with model examples) is as follows: sedimentary-diagenetic (Nikopol, Bol’she-Tokmak; Ukraine), (volcanogenic) hydrothermal-sedimentary (deposits of the Atasui area, Kazakhstan; Magnitogorsk Trough, South Urals), epigenetic (catagenetic) (deposits of the Kalahari manganese ore field, South Africa; Usinsk deposit, Kuznetsk Alatau), and supergene (residual, infiltrational, cavern filling, and pisolitic deposits in India, Brazil, South Africa, and Australia). The results suggest the following conclusions: (1) all primary manganese rocks and ores at the known deposits are hydrothermal- and diagenetic-sedimentary formations of marine environments; (2) manganese concentrations achieve the size of deposits at postsedimentary stages of the initial manganiferous sediment and manganese rock transformation (diagenesis, catagenesis, and retrograde diagenesis); (3) indispensable participation of the isotopically light carbon dioxide related to the destruction of organic matter (OM) is a characteristic feature of manganese carbonate formation during diagenesis; and (4) the role of organic carbon in manganese ore formation becomes notable since early stages of Mn accumulation in the Precambrian sedimentary basins (terminal Archean-initial Early Proterozoic).     

Tectonic and climate control of oil shale deposition in the Upper Cretaceous Qingshankou Formation (Songliao Basin, NE China)

Oil shales were deposited in the Songliao Basin (NE China) during the Upper Cretaceous period, representing excellent hydrocarbon source rocks. High organic matter (OM) contents, a predominance of type-I kerogen, and a low maturity of OM in the oil shales are indicated by bulk geochemical parameters and biomarker data. A major contribution of aquatic organisms and minor inputs from terrigenous land plants to OM input are indicated by n-alkane distribution patterns, composition of steroids, and organic macerals. Strongly reducing bottom water conditions during the deposition of the oil shale sequences are indicated by low pristane/phytane ratios, high C₁₄-aryl-isoprenoid contents, homohopane distribution patterns, and high V/Ni ratios. Enhanced salinity stratification with mesosaline and alkaline bottom waters during deposition of the oil shales are indicated by high gammacerane index values, low MTTC ratios, high β-carotene contents, low TOC/S ratios, and high Sr/Ba ratios. The stratified water column with anoxic conditions in the bottom water enhanced preservation of OM. Moderate input of detrital minerals during the deposition of the oil shale sequences is reflected by titanium concentrations. In this study, environmental conditions in the paleo-lake leading to OM accumulation in the sediments are related to sequence stratigraphy governed by climate and tectonics. The first Member of the Qingshankou Formation (K₂qn₁) in the Songliao Basin, containing the oil shale sequence, encompasses a third-order sequence that can be divided into three system tracts (transgressive system tract—TST, highstand system tract—HST, and regressive system tract—RST). Enrichment of OM changed from low values during TST-I to high-moderate values during TST-II/III and HST-I/II. Low OM enrichment occurs during RST-I and RST-II. Therefore, the highest enrichment of OM in the sediments is related to stages of mid-late TST and early HST.     

Petroleum geochemistry of the Potwar Basin, Pakistan: 1. Oil-oil correlation using biomarkers, δC and δD

Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil-oil correlation of the area is established. Gas chromatography-mass spectrometry (GC-MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C₃₀ diahopane/C₂₉Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C₁₉-tricyclic and C₂₄-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ¹³C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C₂₃-tricyclic/C₃₀ hopane, C₂₈-tricyclic/C₃₀ hopane, total tricyclic terpanes/hopanes and C₃₁(R + S)/C₃₀ hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C₄₁₊) and steranes.Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/n-C₁₇, Ph/n-C₁₈ and low API gravities.     

Dissolution experiment and transformation condition analysis of Paleogene aragonite in the Jiyang Depression,China

In general, aragonite exists as a metastable carbonate mineral under near-surface conditions, and is commonly transformed into calcite under the subsurface and during diagenesis. It is thus seldom found in sedimentary rocks, but aragonite is common in the Paleogene lacustrine shales in the Jiyang Depression in eastern China. Dissolution experiments were conducted on the Paleogene aragonite-enriched and calcite-enriched shales at different temperatures, pressures and acetic acid concentrations, and in different types of solution. The results show that aragonite is insoluble in the in situ formation water but dissolved more readily under acetic acid conditions than calcite with the degree of dissolution increasing with increasing temperature, pressure and acetic acid concentrations. During the shallow burial diagenesis of the Paleogene sediment sequence in the Jiyang Depression, aragonite was relatively stable and was not dissolved by the connate pore water in the shales. Increasing burial (temperature) and maturity of the organic matter produced large amounts of organic acids that accelerated the dissolution of aragonite. In the late stage, as the organic matter became over-matured, the pore water changed from acidic to alkaline, and calcite precipitated from the carbonate-rich solution. Therefore, the conditions provided by organic acids enabled the conversion of aragonite to calcite during sedimentary diagenesis in the Paleogene lacustrine shales in the Jiyang Depression. This transformation corresponded to the thermal evolution of the organic matter within the shale sequence.     

Phenanthrene biomarkers in the organic matter of Precambrian and Phanerozoic deposits and in the oils of the Siberian Platform

The composition and distribution of phenanthrenes (polyaromatic compounds) have been studied in chloroform extracts from dispersed organic matter (OM) of clayey, siliceous, carbonate, and terrigenous rocks of different ages and facies and from some oils of the Siberian Platform. Phenanthrenes have been analyzed by gas chromatography-mass spectrometry. High contents of 1,7,8-trimethylphenanthrene and 1,1,7,8-tetramethyl-1,2,3,4-tetrahydrophenanthrene are present in the OM of Vendian and Cambrian carbonate-shale deposits and in ancient oils of the Nepa-Botuobiya and Anabar anteclises. The OM of Permian continental deposits and oils of the Vilyui syneclise is dominated by 1-methyl-7-isopropylphenanthrene (retene). A triangular diagram for identification of the types of original OM of rocks and classification of genetically related oils has been constructed based on the assessment of phenanthrene biomarker distribution. Putative pathways of the formation of phenanthrene biomarkers are discussed.     

The Bikkulovskoe manganese deposit (South Urals): Geological setting,composition of metalliferous rocks,and formation model

The results of investigation of the Bikkulovskoe manganese deposit confined to volcanosedimentary piles of the Magnitogorsk paleovolcanic belt are presented. The paper characterizes the geological setting of the deposit and mineral-chemical compositions of ores and enclosing rocks (volcanomictic sandstones; ferruginous, manganiferous, and ferruginous-siliceous tuffites; and jasperites). Analysis of the data obtained made it possible to identify four sequential stages of deposit formation: (1) accumulation and diagenesis of ore-bearing sequences (D_2–3); (2) burial and low-grade (T = 200–250°C, P = 2 to 3 kbar) regional metamorphism of rocks (D_2–3-C₁); (3) tectonic deformations of volcanosedimentary piles (C₂-P); and (4) hypergenesis and partial denudation of rocks (MZ-CZ). According to the model proposed for the accumulation of ore-bearing rocks, the productive member was formed in a zone of hydrothermal solution outflow to the seafloor surface. Discharge of solutions and precipitation of Fe and Si began below the seafloor surface (rather than above the surface) in the near-bottom sequence of volcanomictic sediments. Upon reaching the seafloor, the impoverished solutions mixed with seawater and gave up metals completely: Fe and Mn were transferred to sediments to make up the ore-bearing bed. Thus, zonal sediments with ferruginous tuffites at the base and manganese ores at the top were formed.     

The geochemistry of carbonate diagenesis: The past,present and future

Stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ¹⁸O values of carbonate rocks are dependent both upon the temperature and the δ¹⁸O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ¹⁸O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well‐known tendency for δ¹⁸O values of carbonate rocks to decrease with increasing age. Unlike the δ¹⁸O, δ¹³C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post‐depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.     

Transformation of Fe–Ti gabbro to coronite, eclogite and amphibolite in the Baie du Nord segment, Manicouagan Imbricate Zone, eastern Grenville Province

Fe–Ti gabbros from the Baie du Nord Segment of the Manicouagan Imbricate Zone, metamorphosed under high P–T conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic eclogites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. The deformed margins of coronitic Fe–Ti gabbros are transformed to amphibolite and contain clinopyroxene-bearing leucosomes with garnet poikiloblasts that are indicative of high-P–T dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P–T conditions are recorded by coronas around xenocrysts (c. 720–800 °C at 14–17 kbar) and garnet–clinopyroxene cores in granoblastic assemblages (c. 740–820 °C at 13–17 kbar) in the eclogitic samples. Re-equilibration during the early stages of exhumation at high-T conditions (>700 °C) affected all samples, and is evidenced by the widespread development of pargasite-bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re-equilibration of the eclogites giving lower P–T estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure (depth) from the coronites (11–13 kbar) to the eclogites (13–17 kbar). The P–T conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the Grenvillian orogeny.     

Three Episodes of Hydrocarbon Generation and Accumulation of Marine Carbonate Strata in Eastern Sichuan Basin, China

It is concluded that there are three hydrocarbon generation and accumulation processes in northeastern Sichuan on the basis of the characteristics of solid bitumen, gas-light oils-heavy oils, homogenization temperature of fluid inclusions and diagenesis for beach- and reef-facies dolomite gas- bearing reservoirs in the Puguang Gas Field, northeastern Sichuan Basin, southern China. The first hydrocarbon generation and accumulation episode occurred in the Indosinian movement （late Middle Triassic）. The sapropelic source rocks of the O3w （Upper Ordovician Wufeng Formation）-S1l （Lower Silurian Longmaxi Formation） were buried at depths of 2500 m to 3000 m with the paleogeothermal temperature ranging from 70℃ to 95℃, which yielded heavy oil with lower maturity. At the same time, intercrystalline pores, framework pores and corrosion caused by organic acid were formed within the organic reef facies of P2ch （Upper Permian Changxing Formation）. And the first stage of hydrocarbon reservoir occurred, the level of surface porosity of residual solid bitumen {solid bitumen/ （solid bitumen ＋ residual porosity）} was higher than 60%. The second episode occurred during the Middle Yanshanian movement （late Middle Jurassic）. During that period, the mixed organic source rocks were deposited in an intra-platform sag during the Permian and sapropelic source rocks of O3w-S1l experienced a peak stage of crude oil or light oil and gas generation because they were buried at depths of 3500 m to 6800 m with paleogeothermal temperatures of 96-168℃. At that time, the level of surface porosity of residual solid bitumen of the T1f shoal facies reservoirs was between 25% and 35%, and the homogenization temperatures of the first and second stages of fluid inclusions varied from 100℃ to 150℃. The third episode occurred during the Late Yanshanian （Late Cretaceous） to the Himalayan movement. The hydrocarbon reservoirs formed during the T1f and P2ch had the deepest burial of 7700 m to 8700 m and paleogeotemperatu     

Pyritization of fossil mollusk shells and some problems of supergene sulfide formation

The study of specific features of the pyritization of mollusk fossil shells has provided new evidence of the relationship between the generation of hydrosulfides during the bacterial reduction of sulfates and the composition of organic matter (OM) exploited by bacteria in processes of metabolism. The OM is represented by conchiolin of the ammonite shell frustule. Interaction between the bacterial H₂S and Fe²⁺ fosters the pseudomorphous replacement of conchiolin by the colloidal iron monosulfide that is subsequently transformed into pyrite. Hydrogen sulfide and/or monosulfide migrate into diagenetic cracks and cavities formed in the clayey—carbonate matrix that fills up the interior cavity of a shell. We believe that the data reported in this communication should be taken into consideration in the study of formation constraints of vein and disseminated sulfide mineralization in sedimentary rocks during the early diagenesis and related problems of ore formation.     

Origin of the high waxy oils in Bohai Bay Basin, east China: Insight from geochemical and fluid inclusion analyses

An integrated approach of molecular sieve, molecular composition of fluid inclusion and compound specific isotope analysis was employed to investigate newly discovered oils reservoired in an Ordovician buried hill in the Dongying Depression of Bohai Bay Basin. The new discovered oils are characterized by high content of waxy alkanes (> 40%) with an extremely low concentration of cyclic biomarkers. The alkanes-removed waxy oil and the fluid inclusion oils correlate well with the source rocks of the Paleogene Kongdian Fm (Ek₂). The δ¹³C values of the compound-specific isotope of the alkanes indicate that Ek₂ is the primary source for the oils. This study demonstrates the existence of a new set of deeper source rocks with good oil-generation potential in the Dongying Depression.     

Correlation of crude oils and oil components from reservoirs and source rocks using carbon isotopic compositions of individual n-alkanes in the Tazhong and Tabei Uplift of the Tarim Basin,China

Carbon isotopic compositions were determined by GC–IRMS for individual n-alkanes in crude oils and the free, adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the Tazhong Uplift and Tahe oilfield in the Tabei Uplift of Tarim Basin as well as extracts of the Cambrian–Ordovician source rocks in the basin. The variations of the δ¹³C values of individual n-alkanes among the 15 oils from the Tazhong Uplift and among the 15 oils from the Triassic and Carboniferous sandstone reservoirs and the 21 oils from the Ordovician carbonate reservoirs in the Tahe oilfield demonstrate that these marine oils are derived from two end member source rocks. The major proportion of these marine oils is derived from the type A source rocks with low δ¹³C values while a minor proportion is derived from the type B source rocks with high δ¹³C values. Type A source rocks are within either the Cambrian–Lower Ordovician or the Middle–Upper Ordovician strata (not drilled so far) while type B source rocks are within the Cambrian–Lower Ordovician strata, as found in boreholes TD2 and Fang 1. In addition, the three oils from the Cretaceous sandstone reservoirs in the Tahe oilfield with exceptionally high Pr/Ph ratio and δ¹³C values of individual n-alkanes are derived, or mainly derived, from the Triassic–Jurassic terrigenous source rocks located in Quka Depression.The difference of the δ¹³C values of individual n-alkanes among the free, adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process. In general, the initial oil charge is derived from the type B source rocks with high δ¹³C values while the later oil charge is derived from the type A source rocks with low δ¹³C values.The δ¹³C values of individual n-alkanes do not simply correlate with the biomarker parameters for the marine oils in the Tazhong Uplift and Tahe oilfield, suggesting that molecular parameters alone are not adequate for reliable oil-source correlation for high maturity oils with complex mixing.     

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1M_d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M₁ illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.     

Geochemical characteristics of gold mineralization of the Huai Kham On deposit,Sukhothai Fold Belt,Northern Thailand

The Huai Kham On gold deposit is located in the central part of the Sukhothai Fold Belt, northern Thailand. The Sukhothai Fold Belt represents an accretionary complex formed by subduction and collision between the Indochina and Sibumasu Terranes. There are many small gold deposits in the Sukhothai Fold Belt; however, the styles and formation environments of those gold deposits are not clear. The geology of the Huai Kham On deposit consists of volcanic and volcanosedimentary rocks, limestone, and low‐grade metamorphic rocks of Carboniferous to Triassic age. Gold‐bearing quartz veins are hosted by volcanic and volcanosedimentary rocks. The quartz veins can be divided into four stages. The mineral assemblage of the gold‐bearing quartz veins of Stages I and II comprises quartz, calcite, illite, pyrite, native gold, galena, chalcopyrite, and sphalerite. Quartz veins of Stage III consist of microcrystalline quartz, dolomite, calcite, pyrite, native gold, and chalcopyrite. Veins of Stage IV consist of calcite, dolomite, chlorite, and quartz. Fluid inclusions in quartz veins are classified into liquid‐rich two‐phase (Types I_A and I_B), carbonic‐aqueous (Type II), and carbonic (Type III) fluid inclusions. The homogenization temperatures of Types I_A and II fluid inclusions that are related to the gold‐bearing quartz veins from Stages I to III ranged from 240° to 280°C. The δ¹⁸O values of quartz veins of Stages I to III range from +12.9 to +13.4‰, suggesting the presence of a homogeneous hydrothermal solution without temperature variation such as a decrease of temperature during the formation of gold‐bearing quartz veins from Stages I to III in the Huai Kham On gold deposit. Based on the calculated formation temperature of 280°C, the δ¹⁸O values of the hydrothermal solution that formed the gold‐bearing quartz veins range from +3.2 to +3.7‰, which falls into the range of metamorphic waters. The gold‐bearing quartz veins of the Huai Kham On deposit are interpreted to be the products of metamorphic water.     

Tetrapyrrole pigments in United States humic coals

Analyses of forty-two United States humic coals have revealed a striking divergence between chlorophyll diagenesis in coals when compared to the more widely studied marine sediments, shales, asphalts and petroleums. Porphyrins of humic coals have been found to be dominated by the ETIO-series, to lack members above C-32, and, in lower ranked samples (e.g. Sub-bituminous-B, high volatile C) to exhibit mass spectral envelopes with unique even-carbon number predominances. The weighted average mass, as well as the carbon number maximum (viz. mode), of these coal porphyrin homologies has been found to decrease as rank increases. The generation of porphyrins of the ETIO-series is suggested as occurring both during early coal formation, including oxidative scission of the isocyclic ring at the phorbide stage, and later during the catagenic alteration of surviving DPEP-series porphyrins. A preliminary study of chlorophyll diagenesis in a South Florida peat partially substantiates this suggestion and has further shown that the coal porphyrins can be derived from bacterial, as well as higher plant, chlorophylls.     

Hydrogen isotopic compositions of individual alkanes as a new approach to petroleum correlation: case studies from the Western Canada Sedimentary Basin

Isotopic compositions of carbon-bound hydrogen in individual n-alkanes and acyclic isoprenoid alkanes, from a number of crude oil samples, were measured using gas chromatography-thermal conversion-isotope ratio mass spectrometry. The precision of this technique is better than 3‰ for most alkanes, compared to the large range of δD variation among the samples (up to 160‰). The oils were selected from major genetic oil families in the Western Canada Sedimentary Basin, with source rocks ranging in age from Ordovician (and possibly Cambrian) to Cretaceous. The hydrogen isotopic composition of alkanes in crude oils is controlled by three factors: isotopic compositions of biosynthetic precursors, source water δD values, and postdepositional processes. The inherited difference in the lipid's biosynthetic origins and/or pathways is reflected by a small hydrogen isotopic variability within n-alkanes, but much larger differences in the δD values between n-alkanes and pristane/phytane. The shift toward lighter hydrogen isotopic compositions from Paleozoic to Upper Cretaceous oils in the WCSB reflects a special depositional setting and/or a minor contribution of terrestrial organic matter. The strong influence of source water δD values is demonstrated by the distinctively lower δD values of lacustrine oils than marine oils, and also by the high values for oils with source rocks deposited in evaporative environments. Thermal maturation may alter the δD values of the alkanes in the oil to some extent, but secondary oil migration does not appear to have had any significant impact. The fact that oils derived from source rocks that could be of Cambrian age still retain a strong signature of the hydrogen isotopic compositions of source organic matter, and source water, indicates that δD values are very useful for oil-source correlation and for paleoenvironmental reconstructions.     

Microbial mat development and dolomite formation under pre-evaporitic conditions during the Atlantic in a temperate area: The Sarliève Lake (French Massif Central)

The Sarliève marsh in the Limagne plain, in the heart of the French Massif Central, functioned as an endorheic lake during the Late Glacial and the Early and Middle Holocene. During the Late Boreal and the Atlantic it experienced drastic lowering of the water level as a result of dry and warm climatic episodes. Then, pre-evaporitic conditions triggered the deposition of sediment rich in organic matter (OM) and in carbonates including dolomite. Fifty-one samples from a ca. 1.8 m sediment core section covering the period were analysed using Rock–Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS). Throughout the interval, the OM content remained notable to high [up to 13.35% total organic carbon (TOC)] and of good quality as indicated by low oxygen index (OI) values (<200 mg CO₂ g⁻¹ TOC) and high hydrogen index (HI) values (160–660 mg HC g⁻¹) which, as a rule, increased with increasing TOC content.In contrast to the acid fractions, which sometimes contained notable proportions of n-C₁₆ or n-C₁₈ fatty acids (FAs; analysed as the methyl esters, FAMEs), the neutral fractions were almost devoid of low molecular weight compounds. The latter were probably biodegraded during early diagenesis. Conversely, high molecular weight compounds were abundant in both fractions and were dominated by n-alkanols, n-alkanes, steroids and hopanoids in the neutrals and even numbered FAs and hopanoids in the acid fractions. The hopanoids were dominated by regular bishomohopanoids, accompanied by 2-methylated bishomohopanoids, as well as by unidentified bishomohopanoids with methylation in either the D or E ring. These distributions, typical for bacteria, provide support for previous hypotheses on the contribution of microorganisms to the studied record and for providing conditions for the precipitation and growth of dolomite and other pristine carbonate minerals [Bréheret, J.G., Fourmont, A., Macaire, J.J., Négrel, Ph., 2008. Microbially mediated carbonates in the Holocene lacustrine deposits of the Marais de Sarliève (French Massif Central) testify to the evolution of a restricted environment. Sedimentology 55, 557–578]. The preservation of the compounds was probably ensured by persistent reducing conditions during diagenesis, despite variable climatic conditions and related changes in lake level and sedimentation rate. Comparable distributions of hopanoic acids and hopanols, as well as roughly parallel variation with depth in the corresponding components in both fractions, strongly suggest that all the hopanoids derive from the same microbial precursors, slight shifts in the acid/alcohol ratio being governed by limited changes in redox conditions during early diagenesis.     

Metamorphic evolution of the archean eclogite-like rocks of the Shirokaya and Uzkaya Salma area (Kola Peninsula): Geochemical features of zircon,composition of inclusions,and age

Zircon from the eclogite-like rocks of the Shirokaya and Uzkaya Salma area (Kola Peninsula) was studied using a complex of mineralogical and geochemical methods (CL, BSE, microprobe, and REE distribution). Different zones distinguished within zircon crystals were dated on a SHRIMP-II mass spectrometer. Mineral and chemical compositions of inclusions in the zircons were analyzed. Based on these studies, the following stages of the formation and transformation of the rocks were determined: (1) formation of basic protolith of the eclogite-like rocks of the Shirokaya and Uzkaya Salma area 2.94–2.93 Ga ago; (2) the granulite-facies metamorphism of the eclogite-like rocks of the Shirokaya Salma 2.72 Ga ago; (3) the onset of decompressional cooling with formation of Cpx-Pl symplectites at 2.70 Ga ago; and (4) final metamorphic reworking together with surrounding TTG under the amphibolite-facies conditions at 1.89 Ga ago. The studied rocks and minerals revealed no isotope-geochemical or geochronological signs of eclogite metamorphism. Geochemistry of the primary magmatic zircons showed that the protolith of eclogite-like rocks was gabbro rather than MOR basalts. The formation of garnet in the rocks of the Uzkaya and Shirokaya Salma area is dated at 2.70 and 1.89 Ga ago, which is consistent with petrological observations of later formation of garnet relative to omphacite. Obtained data led us to prefer a magmatogenic model, which suggests that omphacite in the rocks of the Shirokaya and Uzkaya Salma was presumably formed during crystallization from basic melt, rather than during eclogite-facies metamorphism.     

Study on diagenesis of organic matter in immature rocks

Since many immature oils have been found in a number of Tertiary basins of China, a series of cores (Oligocene) and several immature rocks after thermal simulation have been investigated for their biomarker distributions by GC and GC-MS. The presence of biomarkers in the cores seem to follow a rule of less to greater stability of hopenes, ββ-hopanes, diasterenes with increasing the depth of cores, and subsequently the 22R, 22S configuration of hopanes reaches equilibrium. The thermal simulation experiments with immature rocks demonstrated that it is possible to generate some immature oils from immature rocks during the diagenesis stage. The tricyclic terpanes generated from source rocks during diagenesis stage tended to be enriched in the oils compared to their source rocks and the relative abundance of lower molecular weight tricyclic terpanes to their higher molecular weight homologues may be useful for the subdivision of diagenesis.     

Fossilization of organic matter,formation of oil and gas,and emigration of hydrocarbons in carbonate rocks

Based on the comparison of conditions of organic matter (OM) accumulation in modern carbonate sediments in Paleozoic shallow-water carbonate sediments, it is shown that drastic disproportion in the degree of preservation of the primary dispersed OM (DOM) in the clayey and “pure” varieties of carbonate rocks is not caused by its loss due to the diagenetic oxidation in the shallow-water setting, its disintegration due to the vital activity of microorganisms, or due to its consumption for the reductive oxide forms of Fe. It has experimentally been proven that a great significance in oil and gas formation in carbonate rocks belongs to OM, which occurs in the carbonate component of the chloroform bitumen (CB_CCR) and is not determined by the conventional analytical methods (e.g., incineration of the HCl-insoluble rock remnant). Higher concentration of hydrocarbons (HC) in CB_CCR relative to CBA is confirmed by the oil-generating properties of “pure” carbonate rocks. The release and emigration of HC from carbonate rocks are promoted by their secondary transformations.