Abstract: Sandstone type deposits are the most common type of uranium deposits in the world. A large variety of sub-types have been defined, based either on the morphology of the deposits (e.g., tabular, roll front, etc), or on the sedimentological setting (e.g., paleovalley, paleochannel, unconformity), or on tectonic or lithologic controls (e.g., tectonolithologic, mafic dykes/sills), or still on a variety of others characteristics (phreatic oxidation type, interlayer permeable type, multi-element stratabound infiltrational, solution front limb deposit, humate type, etc.), reflecting the diversity of the characteristics of these deposits, but making it difficult to have a clear overview of these deposits. Moreover, uranium deposits occurring in the same sedimentological setting (e.g., paleochannel), presenting similar morphologies (e.g., tabular), may result from different genetic mechanisms and thus can be misleading for exploration strategies. The aim of the present paper is to propose a new view on sandstone-related uranium deposits combining both genetic and descriptive criteria. The dual view is indeed of primordial importance because all the critical characteristics of each deposit type, not limited to the morphology/geometry of the ore bodies and their relationships with depositional environments of the sandstone, have to be taken into account to propose a comprehensive classification of uranium deposits. In this respect, several key ore-forming processes, like the physical-chemical characteristics of the mineralizing fluid, have to be used to integrate genetic aspects in the classification. Although a succession of concentration steps, potentially temporally-disconnected, are involved in the genesis of some uranium mineralization, the classification here proposed will focus on the main mechanisms responsible for the formation and/or the location of ore deposits. The objective of this paper is also to propose a robust and widely usable terminology to define and categorize sandstone uranium deposits, considering the diversity of their origin and morphologies, and will be primarily based on the temperature of the mineralizing fluid considered as having played the critical role in the transportation of the uranium, starting from synsedimentary uranium deposits to those related to higher temperature fluids.
Abstract: Many theoretical results on sandstone-type uranium mineralization in northern China obtained by the uranium research team of the Tianjin Center of Geological Survey in recent years are presented. From the source sink system of uranium-producing basins, sedimentary environment of uranium-bearing rock series, ore-forming fluid information, evolution of tectonic events, basin formation and development, we redefine and classify uranium orebodies, redox zoning, and ore-controlling structural styles. We then systematically propose a theoretical system of sandstone-type uranium deposits in northern China. We conclude that sandstone-type uranium deposits in northern China are mainly found in sedimentary environments such as rivers, deltas, and alluvial fans in the Mesozoic and Cenozoic lowstand systems tract and in gray sandstone layers in the vertical redox zoning. The orebodies are controlled by the tectonic slope belt, which is in the shape of a strip on the plane, and spreads in a layer or plate on the section. Vertical (ups and downs) tectonic movement triggers large-scale phreatic flow in the basin, which is the real driving force for controlling the ore-forming fluid. The theoretical system of sandstone-type uranium deposits in northern China should be based on global tectonic movement and environmental changes and take into account factors such as basins as a unit to study mineralization background, ore concentration areas as objects to study mineralization, and the correlation between regional tectonic movement and metallogenic process as a breakthrough point to study tectonic events and metallogenic events. It should also be based on different basin types to establish metallogenic models. The innovative research results and ideas are summarized with the aim of promoting the continuous improvement of sandstone-type uranium mineralization theory in northern China.
Abstract: Metallogenic specialization of sedimentary cover in Central Asia is determined by its tectonic setting that governs the hydrodynamic regime (exfiltrational or infiltrational) and as a consequence, the hydrogeochemical zonality (type of water and its gaseous and microcomponent composition). Hydrodynamic conditions (distribution of recharge and discharge areas) determine the direction of stratal water flow and location of mineralization resulted from the change in geochemical, thermodynamic, lithological, structural and other conditions. The exfiltrational regime suggests a dependence of the epigenetic mineralization upon the distribution and degree of preservation of hydrocarbon occurrences. Often, hydrocarbon matter serves as a reducing barrier and ore-concentrating factor during the formation of polymineral concentrations related to stratal oxidation zone. The supergene epigenetic ore-forming processes are induced by the interaction between the Earth's sedimentary cover and hydrosphere. Sedimentary rocks themselves commonly serve as a source of ore materials. The ore deposition zones on geochemical barriers and ore material source are often located significantly apart from each other. The trend of these processes is determined by the position of ore-bearing depressions in large tectonic blocks.
Abstract: In the continental basins of Northern China (NC), a series of energy resources commonly co-exist in the same basin. As the three typical superimposed basins of different genesis in the NC, the Junggar, Ordos, and Songliao basins were chosen as the research objects. The favorable uranium-bearing structures are generally shown as a basin-margin slope or transition belt of uplifts with the development of faults, which are conducive to a fluid circulation system. The Hercynian, Indosinian, and Yanshanian movements resulted in the development of uranium-rich intrusions which acted as the significant uranium sources. The main hydrocarbon source rocks are developed in the Carboniferous, Permian, Jurassic and Cretaceous. The mature stage of source rocks is concentrated in the Jurassic–Cretaceous, followed by the multi-stage expulsion events. Influenced by the India-Eurasian collision and the subduction of the Pacific Plate, the tectonic transformation in the Late Yanshanian and Himalayan periods significantly influenced the sandstone-type uranium mineralization. The hydrocarbon reservoirs are spatially consistent with sandstone-type uranium deposits, while the hydrocarbon expulsion events occur in sequence with sandstone-type uranium mineralization. In the periphery of the faults or the uplifts, both fluids met and formed uranium concentration. The regional tectonic movements motivate the migration of hydrocarbon fluids and uranium mineralization, especially the Himalayan movement.
Abstract: Ores of infiltration sandstone-hosted uranium deposits in the sedimentary cover are ubiquitous composed of dispersed soot powder mineralization of black, brownish-black colour. Long-term studies of such loose U-ores by analytical transmission electron microscopy (ATEM) proved their polymineral nature. Uranium blacks are composed by at least three different U-mineral forms: oxide (uraninite), silicate (coffinite) and phosphate (ningyoite) which are present in various proportions of ore compositions. Such high dispersed friable uranium formations are difficult to diagnose by traditional mineralogical methods (optical, XRD, IR and X-ray spectroscopy, etc.) which analyze total sample composition (phases mixture); their results characterize the dominant sample phase, omitting both sharply subordinate and X-ray amorphous phases. All research results are based on ATEM methods (SAED+EDS), which are optimal for crystallochemical diagnostics in the mineralogical study of such uranium ores. The article presents the diagnostic characteristics under electron microscope (EM) of uranous minerals from different sandstone deposits with their origin being discussed.
Abstract: The U-Pb isotope system and uranium isotope composition (235U/238U and 234U/238U) were studied in a number of samples from the vertical section of the uranium ore body at the Vershinnoe sandstone-type deposit, Vitim uranium ore district, Russia. These parameters were determined to broadly vary. Deviations of the 234U/238U ratio from the equilibrium value indicate that the uranium ore was not completely conserved during the postore stage, and uranium was determined to continue migrating at the deposit. Comparison of the U-Pb isotope age value and 234U/238U isotope ratio provides an insight into the migrate direction of uranium in the ore body. The broad variations (137.377–137.772) in the 238U/235U ratio over the vertical section of the ore body can be explained by the different settings of the samples relative to the ore deposition front and changes in the redox conditions when this front shifted. The fact that the δ238U and K234/238 values are correlated indicates that the transfer of the 234U isotope into the aqueous phase may have been coupled with isotope fractionation in the 238U-235U system during the postformation uranium migration within the orebody.
Abstract: Carbonaceous debris (CD), common dispersed organic matter (i.e., DOM), is widely disseminated in sandstones from uranium-bearing strata from the Dongsheng uranium ore field of the northern Ordos Basin. Compositions of maceral, element and biomarkers of CD were investigated through a series of methods with optical microscope, elemental analyzer and gas chromatography-mass spectrometry analyses (GC-MS) to study origin of CD. The results show that CD, centrally distributed nearby channel erosion surface, decreases with the increased distances to channel erosion surface, which indicates the CD might be related to the coal seam from the upper unit of the J2y Formation or synsedimentary plant from the J2z Formation. Macerals of CD are composed of vitrinite (i.e., V), inertinite (i.e., I), and minerals, including that V is primary. Compared with the coals from the J2y Formation classed into vitrinertite-V (V+I > 95%, V > I), CD is grouped into vitrite (V > 95%). Although, CD and coal are similar in element composition, the former is of lower organic carbon, H, N, and higher S. The (C27+C29)/(C31+C33) ratios of n-alkanes biomarkers indicate that the percentage of woody plants accounting for vegetation composition of CD predominate over that of coal, which is also evidenced by the higher C/N ratios and oleanane contents of CD. The evidence is also supported by plant branch buried in sandstones. The distribution characteristics of CD and differences in vegetation types between CD and coal suggest that CD might be not from the coal seam from J2y. The tissue preservation index, gelification index, ground water level index, and vegetation index reflect that the paleoenvironment of CD is controlled by fluctuating water, which is also supported by the existences of round CD. Compared with peat, sedimentary paleoenvironment where CD deposits is of weaker reducibility, higher salinity by analyzing Pr/Ph ratios and gammacerane index. Distributions of n-alkanes carbon number of CD with the presence of unknown complex mixtures show that microbial activities exist in sand bodies. Differences in hydrodynamic intensity, redox condition, and microbial activity intensity between sedimentary paleoenvironment of CD and peat, show that CD is born in synsedimentary sandstone environment not in peat. Hence, it comprehensively draws conclusions that immature 'non-peatborn' CD is formed from the trunk, stem, branch, root fragments buried in sandstones, depositing in (micro) allochthonous positions by the influences of fluctuating water. The DOM from synsedimentary plant debris might play more roles in adsorption and complexation, and microorganisms may participate in uranium mineralization, which could provide certain guidance for uranium exploration and mining.
Abstract: In recent years, the close relationship between uranium and Ti-Fe oxides in the sandstone-type uranium deposits has been extensively recognized. However, the altered characteristics of ilmenite and its relationship with uranium enrichment still remain unclear. With this paper based on heavy-mineral sorting of uranium ore selected from the Tarangaole-Nalinggou deposit in the northeastern Ordos Basin, electron probe, backscattering image, energy spectrum and scanning electron microscopy were systematically performed. The ilmenite in the sandstone can be divided into four groups, including unaltered, weakly altered, moderately altered, and strongly altered ilmenite. The alteration of ilmenite in uranium ores is notably more intense than that of the surrounding rocks. In addition, weakly, moderately, and strongly altered ilmenite associated with uranium minerals in uranium ores demonstrate that the more intensity ilmenite altered, the closer its relationship with uranium minerals is. The ilmenite has likely been somewhat altered before mineralization, and the alteration intensifies by later exposure related to an oxygen-containing fluid. The alteration mechanism comprises a process of competitive diffusion between Fe2+ and O2- ions. In the early stage, Fe ions was mainly diffused on the particle surface. Subsequently, diffusion of O ions into the particles began to be dominate. Most of the leached iron is stripped or carried away by fluid. In an alkaline and reductive environment, the remaining iron is reduced to form the surrounding pyrite, and TiO2 in a form of titanium sol recrystallizes (i.e., anatase). Backscattering images show that uranium and altered ilmenite are close in space. Coffinite is often distributed along the edges of altered ilmenite as burrs in shape. Colloidal or knitted coffinite associated with anatase is formed in the voids of altered ilmenite. The chemical composition of altered ilmenite varies considerably from the core to edge, and the mineral assemblage sequence is from girdle with ilmenite, to leucosphenite, to anatase, and to coffinite. There is no brannerite that is symbiotic with altered ilmenite. It is considered to be a uranium-containing titanium mineral aggregate caused by the reduction and adsorption of uranium. As the altered product of ilmenite, TiO2 is an aggregation agent, increasing the concentration of uranium by adsorption. Together with Fe2+ and S2- in secondary pyrite, this aggregate creates a uranium-rich environment in the microzone for the formation of coffinite. Therefore, the alteration of ilmenite plays a geochemical role in the processes of sedimentary, diagenesis and mineralization, in which Fe is removed, Ti is enriched, and U is adsorbed and reduced.
Abstract: Uranium exploration breakthrough was extremely rare in an aeolian depositional system. In order to know the complicate characteristics of oxidation associated closely with uranium mineralization in the aeolian depositional system, petrology and mineralogy markers of the oxidation and its genetic mechanisms are identified and illustrated by fieldwork, thin section analysis and scanning electron microscopy test, based on 2 field outcrops in Zhenyuan County in the southwest of the Tianhuan depression in the Ordos Basin and the core of 2 wells in the north and south of Ordos Basin. The results showed: the typical macroscopic indicator of primary oxidation was the red fine sediments in the aeolian interdune with a thickness of 10-50 cm, and the microscopic characteristics of primary oxidation were the minerals such as hematite, ilmenite, and the irony matrix rich in fine-grained dolomite and biotite; the phreatic oxidation was manifested as the red sandstone with limonite horizontal layer with a thickness of 1-4 cm and a width of 60 cm-1 m, and the circular limonite nodules with a diameter of 3-7 cm, in which there was intergranular limonite cement; the interlayer oxidation was characterized by lenticular tongue and tapered red sandstone with a length of 1-10 m and a width of 10 cm-5 m, in which detrital particles are coated with hematite and hematite was distributed inside the rhombus dolomite. The paleoclimate of the sedimentary period, the water-table movement and the pore and permeability conditions of the sand body were the key factors for the formation of different oxidation types in the aeolian depositional system.
Abstract: In order to determine the provenance and variation characteristics of sandstone-type uranium deposits located in the southwest Ordos Basin, U-Pb geochronology and Hf isotope analyses were conducted on detrital zircons from the Late Mesozoic strata of the SD01 well in the Zhenyuan area. The detrital zircon U-Pb ages of four samples exhibited four main peaks at 250–330, 420–500, 1 720–2 000, and 2 340–2 580 Ma, with a small number of zircons dated at 770–1 100 Ma. The detrital zircon age spectrum and further restriction provided by the in-situ Hf isotopic data suggest that the provenance of each stratum was mainly derived from the crystalline basement rock series (Khondalites, intermediate-acidic intrusive rocks, and metamorphic rocks) of the Alxa Block to the northwest and the Yinshan Block to the north, with minor amounts of Caledonian magmatic rocks and Jingning Period rocks from the western part of the northern Qilian orogenic belt to the west and the northern Qinling orogenic belt to the south. The provenance of the sandbody has not changed significantly and is of the Middle Jurassic–Early Cretaceous. The clear variations in the zircon ages of the samples from the Zhiluo and Anding formations were likely influenced by climate change during the Middle–Late Jurassic. The Triassic zircon age (< 250 Ma) first appeared in Early Cretaceous strata, suggesting that tectonic activity was relatively strong in the northern Qinling orogenic belt during the Late Jurassic and produced extensive outcrops of Indo-Chinese granite, which were a source of basin sediments.
Abstract: Large quantities of metal sulfides are widely distributed in uranium ores from the Middle Jurassic Zhiluo Formation of the Shuanglong uranium deposit, southern Ordos Basin, providing a convenient condition to study the relationship between metal sulfides and uranium minerals. The morphology and composition of uranium minerals and metal sulfides are illustrated to study uranium mineralization and mechanisms of metallogenesis. Uranium minerals can be broadly categorized as pitchblende, coffinite and brannerite. Metal sulfides associated with uranium minerals are pyrite, sphalerite, chalcopyrite and galena. Some assemblages of various metal sulfides and uranium minerals indicate that they are coeval, but the order of formation is different. Based on mineralogical observations, paragenetic sequences for mineral assemblages are discussed. Alteration of Fe-Ti oxides forms Ti oxides, brannerite and pyrite. The formation of chalcopyrite was later than that of pyrite. Clausthalite (PbSe) replaces sphalerite or shows isomorphism with galena. There are three genetic types of galena, of which type Ⅰ is related to tectonic thermal events and can interact with uranyl ions to form uranium oxides and Pb2+. When sulfur activity is relatively high, Pb2+ can form new anhedral galena, that is, type Ⅱ. Type Ⅲ of galena is related to the decay of uranium minerals. The genetic order of the main minerals was determined as follows: Fe-Ti oxides > Ti oxides/sphalerite/pyrite > clausthalite/galena Ⅰ/chalcopyrite > galena Ⅱ/uranium minerals > galena Ⅲ during the diagenetic stage. Hydrogen sulfide (H2S) is a decisive factor in the interaction between metal sulfides and uranium. Metal ions can react with H2S, accompanied by precipitation and enrichment of uranium minerals.
Abstract: Compared to the sandstone-type uranium deposits in the Ordos Basin and the Songliao Basin, the Tamusu uranium deposit in the Bayingobi Basin formed in fault-depression transition region displays distinctive features. First, the uranium-bearing sandstones and their interlayer oxidation zone extend longitudinally no more than ten kilometers. Second, gravity flow sediments are more common in the uranium-bearing strata. Comprehensive facies analysis indicates that the Upper Member (ore-bearing horizon) of the Bayingobi Formation was largely deposited in fan deltas that prograded into lakes during period of relatively dry paleoclimate. Spatial distribution patterns of five facies associations along with two depositional environments (fan delta, lake) were reconstructed in this study. The results demonstrated that the depositional systems and their inner genetic facies played different roles in uranium reservoir sandstone, confining beds (isolated barrier beds) and reduction geologic bodies during uranium mineralization process.
Abstract: The Zoujiashan uranium deposit in the Xiangshan ore field is the largest volcanic-related uranium deposit in China. Hematite- and fluorite-type ores are the predominant mineralization styles. Hematitization in the Xiangshan ore field is closely associated with uranium mineralization, mainly occurring as hematitized rocks enclosing fluorite-type vein ores developed in pre-ore illitized porphyritic lava. Detailed petrographic and mass balance calculation studies were conducted to evaluate the mechanisms for uranium precipitation and mass transfer during hematitization. Petrographic observations suggest that in the hematitized rocks, orthoclase is more altered than plagioclase, and quartz dissolution is common, whereas in the illitized rocks, pyrite commonly occurs within the altered biotite grains, and chlorite grains are locally found. Mass balance calculations indicate that Na2O and U were gained, K2O, CaO and SiO2 were lost, whereas Fe2O3-t remained more or less constant during hematitization. These observations suggest that the hydrothermal fluids were Na- and U-rich and Ca-K-poor, and the Fe2+ used for hematitization was locally derived, most likely from biotite, pyrite and chlorite in the host rocks. The Fe2+ is inferred to have played the role of reductant to precipitate uranium, and calculation indicates that oxidation of Fe2+ provided by host rocks is sufficient to form ores of economic significance. Consequently, the hematite-type ore is interpreted to be generated by the reaction between oxidized ore fluids and reduced components in host rocks. The development of calcite and pyrite in the fluorite ores suggests that perhaps mixing between the U-rich fluid and another fluid carrying reduced sulfur and carbon may have also contributed to uranium mineralization, in addition to temperature and pressure drop associated with the veining.
Abstract: To investigate biostratigraphy of Albian sediments (Kazhdumi Formation) in the Zagros Basin, 280 samples in four sections were prepared which are nominated as Murshan-e-Bala, Fahliyan-e-Pain, Kupon and Tale-e-Afghani. Stratigraphic distribution of identified foraminifers confirms well developed orbitolinids which are associated with other Albian benthic foraminifers in all studied sections. The orbitolinids are recognized in surrounding strata [Dariyan (at the base which is marked by an oxidized zone) and Sarvak Formations (on the top, as gradtional contact)] as well as the Kazhdumi Formation. Obtained biostratigraphic data show that Early Albian is marked by the presence of Hemicyclammina sigali, which co-occurs with Mesorbitolina texana and Mesorbitolina subconcava, whereas Conicorbitolina conica and Meorbitolina parva support Late Albian age of the studied sections. Calcareous algae are associated with both benthic faunal assemblages. Albian planktons occur in the studied sections which are identified as Favusella washitensis and Calcisphaerula inomminata subzones. The stratigraphic position of planktons actually indicates vertical displacement in the sedimentary basin during Albian time. Other associated Albian pelagic foraminifera are identified which are obviously recognized in three sections of the Kazhdumi Formation. The presence of pelagic species in the study section of the Kazhdumi Formation indicates basement faulting during the Albian period.
Abstract: The main central thrust (MCT) is one of the major thrusts in Himalayas. In central Himalaya, MCT was defined as a contact between underlying Lesser Himalayan Sequence (LHS) and overlying higher Himalayan crystallines (HHC). However, in the Kashmir Himalayas, the main central thrust zone (MCTZ), shear zone associated with MCT, is overlain by Kashmir Tethyan Sequence suggesting that the MCTZ has been deformed through a mechanism different than the mechanism responsible for MCTZ evolution in other parts of the Himalayas. In the present study we used structural, microfabric and kinematic analyses to investigate the deformation kinematics of MCTZ. Microstructural investigation revealed that the quartz in orthogneiss mylonites of MCTZ was dynamically recrystallized by grain boundary migration (GBM) and sub-grain rotation recrystallisation (SGR) with top-to-SW sense of shear. The mean kinematic vorticity number (Wm) just above the thrust ranges from 0.72 to 0.84 (40%–52% pure shear component) decreasing upwards to 0.65–0.71 (35%–50% pure shear component). Deformation in the MCTZ is characterized by Rxz strain ratio varying from 2.7 to 8. The present study suggested that the MCTZ suffered 3%–40% vertical shortening and 3%–66% transport-parallel elongation. The results suggested that the HHC's were not completely exhumed to the topographic surfaces in the Kashmir Himalayas. Along the basal decollement, i.e., the main Himalayan thrust (MHT), the deformation continued until MCTZ reached the brittle-ductile transition where deformation mechanism changed to the brittle and the MCTZ rocks were transported to the surface through slip on brittle MCT.
Abstract: This study was performed to evaluate pore systems of reservoir lithofacies within the Devonian Three Forks Formation in the Williston Basin through micro-scale pore characterization. These lithofacies are from the Upper Three Forks section, which is a prominent drilling target within the Bakken-Three Forks Petroleum System. Samples from the Formation were examined by (1) physical core description, (2) petrographic thin section microscopy, (3) x-ray diffractometry (XRD) minerals analysis, (4) scanning electron microscopy (SEM), and (5) porosity measurements from helium porosimetry, nuclear magnetic resonance (NMR), gas adsorption and mercury intrusion porosimetry (MIP). These were done to provide better understanding of the local variations in pore structures and how such structures impact reservoir quality within the Three Forks Formation. Seven reservoir lithofacies were identified and described, including laminated lithofacies, massive dolostone, mottled dolostone, massive mudstone, mottled mudstone, mudstone conglomerates, and brecciated mudstone. Samples show a diverse variation in mineralogical composition, pore types, porosity, and pore-size distribution. Six types of pores were identified: interparticle, intercrystalline, intracrystalline, vuggy, microfractures, and mudstone microporosity. Dolostone-rich lithofacies have abundant dolomite and less siliciclastic minerals such as quartz, feldspar, and clays. They also have relatively low porosity and generally larger pore size. A general positive trend exists between porosity with clay minerals and feldspar, in contrast to a negative trend with dolomite, and no clear relationship with quartz content. Results from the gas adsorption analysis, NMR and MIP pore-size distribution confirm an abundance of macropores (> 50 nm in diameters) in dolostone dominated lithofacies while other lithofacies generally have abundant mesopores (2–50 nm).
Abstract: This paper takes Fen-Wei Basin (FWB) as a case to study the ground fissures controlled by normal fault. Based on the field investigation, geophysical exploration, drilling, GNSS data and numerical calculation, the characteristics and mechanism of ground fissures originated from the hanging wall of normal faults are revealed. The results show that the distribution of ground fissures in the hanging wall and heading wall of the active faults is not uniform. Ground fissures are mostly distributed in the hanging wall of active faults and show a linear distribution on the surface, their strike is consistent with the fault, mainly characterized by vertical offset and horizontal tension. Ground fissures destroy the farmland and building foundation through which they pass and cause the rupture or displacement. In profile section, the ground fissure shows the characteristics of normal faults and dislocates the strata, and is connected with the underlying faults. Numerical analysis shows that the vertical displacement of normal fault activity in hanging wall is much larger than that in heading wall, which is the reason that tectonic ground fissures mainly originate from hanging wall. The range of dangerous area of ground fissures is controlled by the depth of fault, the strength of the ground fissures disaster is mainly controlled by the activity of fault. The formation of the ground fissures originated from the hanging wall of the fault experienced three stages: the main fault activity stage, the secondary fault activity stage and the fissure formation stage.
Abstract: In this study, a tremolite marble from the Dabie ultrahigh-pressure (UHP) terrane, east-central China was investigated for its metamorphic evolution by focusing on zircon. The marble contains an amphibolite-facies assemblage of dolomite, Mg-calcite, tremolite, biotite, and plagioclase, while zircon in the marble witnesses a complex recrystallization and growth history under both amphibolite-and eclogite-facies conditions. Cathodoluminescence reveals eight characteristic zones for zircon. As indicated by mineral inclusions in zircon, two zones formed no earlier than amphibolite-facies retrogression and are too thin to date. The other six zones contain inclusions of dolomite, aragonite, diopside (XNa=Na/(Na+Ca)=0.11-0.14), garnet (XCa=0.51-0.62, XMg=0.21-0.23, XFe=0.17-0.26, XMn=0.01), phengite and rutile, and formed under eclogite-facies conditions. Phase equilibria calculations illustrat that the Na-richest diopside formed under UHP conditions. Being an accessory eclogite-facies mineral in the marble, the analyzed chemistry of garnet inclusions cannot be reproduced by phase equilibria calculations because solid-solution models for many other minerals don't incorporate Mn-endmembers. The eclogite-facies zircon zones show low HREE contents and flat MREE-HREE distribution patterns, which are interpreted to have been determined by the low bulk-rock HREE content instead of the presence of accessary garnet in the marble. U-Pb dating yielded a large age dataset ranging from about 250 to 210 Ma for the eclogite-facies zircon zones. Statistically, the eclogite-facies ages are characterized by a Gaussian distribution with a median peak at 232 Ma. We propose that zircon experienced a "protracted" recrystallization and/or growth history in the tremolite marble during the Triassic subduction and exhumation.
Abstract: A series of crystallization experiments have been carried out by using natural Emeishan Ti-rich hydrous basalts as starting materials at a pressure of 0.5 GPa and temperatures of 800-1 000 ℃ to constrain the origin of Fe-Ti-V oxide ore deposits. Our experimental results demonstrate that the sandwich- and trellis-type ilmenite lamellae in titanomagnetite of layered intrusions can be formed by the reaction of earlier crystallized ilmenite and the evolved parental magma. During evolution of parental basaltic magma, the Fe-Ti oxide should be composed of titanomagnetite+ilmenite in the earlier stage, but changed to titanomagnetite+titanomagnetite-ilmenite intergrowth±ilmenite at the later stage. Accordingly, the Panzhihua Fe-Ti oxide ores, which are mainly composed of titanomagnetite, should be formed earlier than the adjacent gabbro, in which titanomagnetite-ilmenite intergrowth is the major form of the Fe-Ti oxide.
Abstract: The Rock-Eval technique in the last few decades has found extensive application for source rock analysis. The impact of shale particle crush-size and sample weight on key Rock-Eval measurements, viz. the S2 curve (heavier hydrocarbons released during the non-isothermal pyrolysis-stage) and the S4 curve (CO2 released from oxidation of organic matter during the oxidation-stage) are investigated in this study. For high and low total organic carbon (TOC) samples of different thermal maturity levels, it is apparent that particle crush-size has a strong influence on the results obtained from Rock-Eval analysis, with the effect being stronger in high-TOC samples. In comparison to the coarser-splits, S2 and pyrolyzable carbon (PC) were found to be higher for the finer crush sizes in all the shales studied. The S4CO2 oxidation curve shapes of Permian shales show contrasting signatures in comparison to the Paleocene-aged lignitic shale, both from Indian basins. A reduced TOC was observed with rising sample weight for a mature Permian shale from the Jharia basin, while the other shales sampled showed no significant reduction. The results indicate that the S4CO2 curve and the S4Tpeak, are strongly dependent on the type of organic-matter present and its level of thermal maturity. Sample weight and particle size both influence the S2-curve shapes at different heating rates. With increasing sample weights, an increase in S2-curve magnitude was observed for the shales of diverse maturities. These differences in the S2 curve shape lead to substantially different kinetic distributions being fitted to these curves. These findings are considered to have significant implications for the accuracy of reaction kinetics obtained from pyrolysis experiments using different sample characteristics.
Abstract: Deformation in the Zagros suture zone is a result of the oblique collision of the Afro-Arabian continent with the Central Iranian microcontinents. Various types of folding and faulting are characteristic features of the study area and indicate the performance of a high strain tectonic regime in this region. To distinguish deformation geometry during the collisional events, strain measurements have been carried out, using the Rf /ϕ method on deformed radiolarian microfossils, in the Zagros suture zone. Based on the results, the strain ellipsoid shape is in the range of general flattening to plane strain (K=0.16-1.12). Measured mean kinematic vorticity number (Wm) in the deformed radiolarian rocks ranges between 0.50 and 0.87, which implies that exhumation of the Abade-Tashk area was facilitated by a general shear flow (35% < simple shear < 65% and 45% < pure shear < 65%). Kinematic vorticity numbers, the amounts of Octahedral shear strain, the ellipsoid eccentricity, and strain ratios systematically increasing towards the thrust fault. The study of deformation in the study area shows that the Zagros suture zone can be considered as a transpressional zone.