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摘要:A new species of Shuyuidae (Eugaleaspiformes, Galeaspida), Jiangxialepis jiujiangensis sp. nov., is described from the lower Telychian (Llandovery, Silurian) Qingshui Formation in Jiujiang, Jiangxi Province, China. The new species differs from the type species J. retrospina from Wuhan, Hubei Province in its sharp and posteriorly positioned median dorsal spine and narrow spine-shaped inner cornual processes. The Silurian strata in Xiushui–Wuning area has provided a standard framework for the correlation of Silurian shallow marine red beds in South China. Thus, the finding of J. jiujiangensis from the Silurian Lower Red Beds (LRBs) in Jiangxi Province bears very important biostratigraphic significance. It can directly compare to Jiangxialepis retrospina from the Fentou Formation in Wuhan, Hubei Province in the genus level. This indicates that the age of the fish-bearing strata in Wuhan is most likely to be the early Telychian rather than middle Telychian as previously assumed.

摘要:The Yili Block in the Western Tianshan orogen is a key area for understanding the early crustal formation and evolution of the Central Asian orogenic belt, due to the widely-distributed Precambrian rocks. Also, it hosts a lot of medium– to large-scale sedimentary exhalative (SEDEX) Pb-Zn deposits that mainly occur in Proterozoic metamorphosed clastic-carbonate rocks. In this study, LA-ICP-MS U-Pb analyses were carried out on detrital zircons in siltstones of the Precambrian Haerdaban Group in the Haerdaban Pb-Zn deposit and magmatic zircons in the diorite dyke that cuts through the strata and orebodies. The maximum depositional age of the siltstones was determined to be about 604 Ma, the diorite having formed at approximately 500 Ma. As such, the Haerdaban Group was most likely formed in the Neoproterozoic Sinian, rather than the previously considered Mesoproterozoic Changchengian. Detrital materials of the Haerdaban Group were mostly derived from the Nanhua–Sinian mafic dykes and granitic rocks around Lake Sayram at the northern margin of the Yili Block. It is proposed that the Yili Block, together with the Kazakhstan and Central Tianshan blocks and the Tarim Craton, might all pertain to the same Rodinia supercontinent, which has great potential for targeting large to super-large SEDEX Pb-Zn deposits.

摘要:We investigated the meta-gabbronorites in Liangcheng and used detailed petrography, geochemistry, zircon geochronological and in-situ Hf isotopic studies to clarify their formation and metamorphic ages, petrogenesis, tectonic setting and provide constraints on the tectonic evolution of Khondalite Belt (KB). The zircon U-Pb dating results show that the meta-gabbronorites crystallized at ~1.94 Ga and were metamorphosed at ~1.91–1.89 Ga. They can be subdivided into the low-Mg and high-Mg groups. The low-Mg meta-gabbronorites contain relatively lower MgO and higher SiO2 contents than high-Mg meta-gabbronorites. They are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and exhibit positive (high-Mg meta-gabbronorites) and negative (low-Mg meta-gabbronorites) Sr and Eu anomalies. The zircon in-situ εHf(t) of meta-gabbronorites is 0.07–4.12, with Hf model ages (TDM) of 2169–2400 Ma. The meta-gabbronorites in Liangcheng originated from the asthenospheric mantle and experienced fractional crystallization of olivine, orthopyroxene, clinopyroxene, and plagioclase. They were contaminated by the crustal rocks (mainly khondalite series) during ascent, especially for low-Mg gabbronorites. The ridge subduction is the most plausible tectonic setting for meta-gabbronorites, indicating the eastern segment of KB was in a ridge subduction setting at ~1.94 Ga following an orogenic thickening event during a prolonged orogenic process.

摘要:Whether a Neoarchean basement existing in the Songnen massif is currently debated. Identification of Archean magmatism from the Songnen Massif is helpful to resolve this issue. Here, we report newly discovered Neoarchean Shanquan pluton in the Western Songnen Massif. These Neoarchean Shanquan pluton are mainly composed of granites that are exposed near the town of Shanquan in Heilongjiang Province. LA-ICP-MS zircon U-Pb dating reveals that the sample 2015TW1 has an upper intercept age of 2801 ± 69 Ma and a weighted mean age of 2708 ± 18 Ma, while samples LJ27QY1 and LJ27QY2 have upper intercept ages of 2677 ± 57 Ma and 2653 ± 18 Ma, and weighted mean ages of 2649 ± 10 Ma and 2653 ± 15 Ma, respectively. This indicates that these granites were formed at ~2.7 Ga. Most of the ~2.7 Ga zircons have older TDM2 ages of 2762–3326 Ma with positive εHf(t) values ranging from 0 to 6.4, while a few of the zircons have negative εHf(t) values ranging from ?8.1 to ?11.5 and older TDM2 ages varying from 3158 to 3264 Ma. The zircon Hf isotopes indicate that Paleo-Mesoarchean crusts might once existed in the Songnen Massif, and the studied Neoarchean magmas were principally derived from partial melting of these Paleo-Mesoarchean ancient crust. Based on the geochronological spectrum of magmatic and detrital zircons, the Songnen, Erguna, Jiamusi, Bureya massifs may have a common basement prior to the Neoproterozoic and may even be linked with the Triam Craton.

摘要:The Nanyuan Formation contains information related to the Mesozoic tectonic transformation. In this study, three representative profiles were surveyed from the Nanyuan Formation, and multiple analyses were conducted. Zircon U-Pb dating yielded their ages as approximately 158–146 Ma. The volcanic rocks are enriched in Rb, Th, U, K, and Pb and depleted in Nb, Ta, P, and Ti, implying their affinity for I-type granites. The εNd(t) values (?8.3 to ?6.0), (87Sr/86Sr)i values (0.7077–0.7094) of the volcanic rock, and εHf(t) values (?8.71 to 0.12) of the Mesozoic zircons suggest that the Nanyuan Formation magma originated in the lower crust with the involvement of depleted mantle materials. The parent rocks of the rhyolitic and dacitic volcanic rocks formed by partial melting of basement rocks in South China and the andesitic volcanic rocks were derived from partial melting of the metasomatites generated by slab-mantle interaction. The fractional crystallization also played an important role in later stage. Discrimination diagrams of the volcanic rocks indicated that they formed in a volcanic arc environment. Combined with previous data, the Nanyuan Formation recorded subduction of the Paleo-Pacific Plate before regional tectonic transformation. The compressive stress field endured until the end of the Late Jurassic.

摘要:The Habo deposit is a typical porphyry Cu-Mo deposit in the Ailaoshan–Red River metallogenic belt. Ore minerals in the Habo deposit typically occur as veins in the monzonite porphyry. Zircon U-Pb dating suggests that the monzonite porphyry formed at 35.07 ± 0.38 Ma. The monzonite porphyry is characterized by high SiO2, Al2O3, K2O and Na2O contents, with A/CNK ratios ranging from 0.97 to 1.02. All samples exhibit fractionated REE patterns, characterized by high (La/Yb)N ratios (9.4–13.6, average of 11.2). They show adakite-like geochemical features, high Sr concentrations (627–751 ppm, average of 700 ppm), low Y concentrations (15.13–16.86 ppm, average of 15.81 ppm) and high Sr/Y values (39.5–47.4, average of 44.3). These samples have high initial 87Sr/86Sr ratios (0.7074–0.7076) and negative εNd(t) values (?5.1 to ?3.7), whereas the zircon εHf(t) values range from ?2.2 to +0.4, suggesting that the monzonite porphyry was derived from the partial melting of a thickened juvenile lower crust. The oxygen fugacity, calculated on the basis of the chemical composition of the amphiboles, shows ?NNO values ranging from +1.65 to +2.16 (average of 1.94) and lg(fO2) ranging from ?12.72 to ?11.99 (average of ?12.25), indicating that the monzonite porphyry has high oxygen fugacity. Zircons have high Ce4+/Ce3+ ratios (29.29–164.24, average of 84.92), with high ?FMQ values ranging from +0.50 to +1.51 (average of 0.87) and high lg(fO2) values ranging from ?14.72 to ?12.85 (average of ?14.07), which also indicates that the oxygen fugacity of the magma was high. The dissolved water content of the Habo monzonite porphyry is 9.5–11.5 wt%, according to the geochemical characteristics, zircon-saturation thermometry (692–794°C) and the mineral phases (amphibole, no plagioclase) in the deep magma chamber. Combined with previous studies, we propose that the high oxygen fugacity and high water content of magma played key roles in controlling the formation of the Habo and other Cu-Mo-Au deposits in the Ailaoshan–Red River metallogenic belt.

摘要:Uplift and exhumation are important factors affecting the preservation of deposits. The anatomy of uplift-cooling evolution and exhumation in the East Longshou Mountain is of significant research value in understanding changes in the Jinchuan Ni-Cu-PGE deposit since its formation. This study uses apatite fission track (AFT) thermochronology to reconstruct the thermal history of the East Longshou Mountain, including the Jinchuan mine, revealing the uplift and exhumation history of the East Longshou Mountain and elucidating the preservation status of the Jinchuan deposit. The AFT ages in the East Longshou Mountain are distributed from 62.3 ± 3.0 Ma to 214.7 ± 14 Ma, with significant differences in ages in distinct areas, the central and pooled ages being consistent within the margin of error. Inverse thermal history models reveal two rapid cooling events associated with exhumation from the Early Jurassic to the Early Cretaceous (200–100 Ma) and since the Miocene (15–0 Ma), the former attributable to the far-afield response to the closure of the Paleo-Tethys Ocean and plate assembly at the southern margin of Eurasia, the latter associated with the initial India-Eurasia plate collision. A slow cooling event from the Early Cretaceous to the Miocene (100–15 Ma) is thought to be related to the arid environment in northwest China since the Cretaceous. These cooling events have diverse responses and cooling rates in different blocks of the East Longshou Mountain: the southwest and centre of which are mainly cooled over 200–120 Ma and 120–0 Ma, with cooling rates of ~0.25 and ~0.33 °C/Ma (~1.25 and ~0.33 °C/Ma in the centre); the Jinchuan mine primarily cooled over 160–100 Ma, 100–15 Ma and 15–0 Ma, with cooling rates of ~1.33, ~0.25 and ~2.00 °C/Ma. These differentiated coolings imply that the uplift of the East Longshou Mountain before the Miocene (~15 Ma) was integral. Strong uplift then occurred in the vicinity of the mining area, which is a critical period for the uplift of the Jinchuan deposit to the surface, meaning that the Jinchuan deposit was exposed no earlier than the Miocene (~15 Ma). Based on mineralization depth information obtained by previous researchers, in conjunction with the calculation and simulation results of this study, it can be seen that the bulk of the Jinchuan intrusion may still be preserved at depth.

摘要:To shed light on the genesis of the Dongping deposit and reveal the behaviour of CO2, Au and other ore elements (e.g., Cu, Fe, Zn, As, Sb, Co etc.) in ore-forming fluids, petrographic, microthermometric and synchrotron radiation X-ray fluorescence (SRXRF) analyses of fluid inclusions were conducted. The ore-forming fluid is characterized as an H2O-CO2-NaCl system with medium-high temperatures and low salinity. Four mineralization stages are identified, namely, feldspar-quartz (stage 1); pyrite-white quartz (stage 2); sulfide-smoky grey quartz (stage 3); and carbonate-quartz (stage 4). Three types of inclusions were distinguished, based on fluid composition, phase assemblages and mode of homogenization. Type A: H2O-rich fluid inclusions show 2 phases (liquid H2O (LH2O) + vapour H2O (VH2O)) at room temperature and homogenize to the liquid phase. Type B: H2O-CO2 fluid inclusions contain 2–3 phases (liquid H2O (LH2O) + vapour CO2 (VCO2); liquid H2O (LH2O) + liquid CO2 (LCO2)); liquid H2O (LH2O) + liquid CO2 (LCO2) + vapour CO2 (VCO2)) at room temperature and homogenized to the liquid H2O phase. Type C: H2O-CO2 fluid inclusions show 2 phases (liquid H2O (LH2O) + liquid CO2 (LCO2) at room temperature and homogenize to a critical state. CO2 is clearly more enriched in the fluid inclusions of stages 2 and 3 than in stage 1. Stage 1 is dominated by type A (H2O-rich) inclusions with homogenization temperatures (Th) of 220–359°C and salinities of 1.1–3.1 wt% NaCl equivalent. Type B (CO2-rich) inclusions gradually increase in stages 2 and 3. The Th range of type B inclusions in stage 2 is 241–397°C with salinities of 2.2–6.9 wt% NaCl equivalent; the Th range of type A inclusions is 217–373°C with salinities of 1.2–8.1 wt% NaCl equivalent. The Th range of type B inclusions in stage 3 is 215–361°C with salinities of 2.9–6.1 wt% NaCl equivalent; the range of type A inclusions is 158–351°C with a salinity of 0.7–5.5 wt% NaCl equivalent. Stage 4 is characterized by type A with Th of 151–250°C and salinities of 0.9–8.3 wt% NaCl equivalent. The mapping results show that elements As, Te and Sb are more concentrated in vapour CO2 than in liquid H2O at room temperature, which suggests that vapour components are more able to transport elements when phase separation occurs. The SRXRF quantitative results show that Au, Cl, S and some other metals are obviously more enriched in the fluid inclusions of stages 2 and 3 than those in stage 1. Additionally, the contents of S in stages 1–3 are much lower than those of Cl, which suggests that gold might migrate mainly in the form of a gold-chloride complex. Au is more enriched in type B fluid inclusions than in type A fluid inclusions, which suggests that the enrichment and migration of gold are closely related to CO2 and CO2 plays a critical role in the migration and enrichment of gold. The trace elements in the fluid have a similar change trend to those in the Yanshanian syenogranite distributed in the southeastern part of the mining area, which may provide some evidence for an intrusion-related genesis for the Dongping gold deposit.

摘要:Deep-seated potassium-rich brines were identified in the Jiangling Basin, South China. Although magmatic-hydrothermal sources have been proposed, the relationship between brine-type potash mineralization and volcanism remains unclear. In this study, U-Pb geochronology, geochemistry, fluid inclusion and C-O isotopic compositions of hydrothermal vein minerals in the Jiangling Basin are examined. Laser ablation U-Pb dating of calcite veins indicates that the ages are slightly younger than the formation age of the Balingshan basalt. Fluid inclusions in hydrothermal minerals show medium–low homogenization temperatures (160–220°C) and low salinities (0.14 to 4.9 wt% NaCl eqv.) and densities (0.882–0.944 g/cm3). The liquid compositions of fluid inclusions in calcite veins from sedimentary strata have higher contents of potassium, compared with those from basalt. The coupled negative δ13CPDB (?10.3‰ to ?8.0‰) and positive δ18OSMOW (17.4‰ to 20.7‰) values imply that calcite precipitation resulted from CO2 degassing of the basaltic magmatic fluids, as indicated by the gas composition of these inclusions in hydrothermal minerals. Rare earth element patterns indicate that water-rock interaction between hydrothermal fluids and sedimentary wall rocks contributed to the calcite precipitation in sedimentary strata. It is proposed that high-temperature water-rock interaction between magmatic fluids and sedimentary strata resulted in the potassium enrichment in fluids, interpreted as one of the sources of potassium-rich brines in the Jiangling Basin.

摘要:Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin, China. The insufficient understanding of their sedimentary processes, depositional patterns, and controlling factors restricts efficient exploration and development. Coarse-grained subaqueous fans in the Yong′an area, Dongying Depression, are investigated in this study. These fans include nearshore subaqueous fans, and sublacustrine fans, and their sedimentary processes, depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology. Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence. Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction. While the topography was gentle, sublacustrine fans did not develop. During the late weak tectonic activity stage, late sublacustrine fans developed with multiple stages superimposed. Frequent fault activity and related earthquakes steepened the basin margin, and the boundary fault slopes were 25.9°–34°. During the early–middle deposition stage, hyperpycnal flows triggered by outburst floods developed. During the late deposition stage, with weak tectonic activity, seasonal floods triggered hyperpycnal flows, and hybrid event beds developed distally.

摘要:A sample from the Jurassic Tamulangou Fm. and two comparison samples from the Cretaceous Fm. were used to document the hydrocarbon generation kinetics and phase behaviors at two heating rates using the confined gold tube system. The results show that the different heating rates affect the reaction rates, paths and levels of organic matter evolution. The average activation energy and dominant frequency activation energy of liquid hydrocarbon are significantly lower than those of gaseous. Moreover, igneous intrusion had a positive effect on the blooming, enrichment and preservation of organic matter, promoting a Ro increase of 0.09%–1.07% in the Jurassic Tamulangou Fm. Two models were used to simulate the normal and abnormal evolution caused by thermal events combined hydrocarbon generation kinetic parameters. Thermal simulation analysis shows that oil generation was initially slow and then increased rapidly until a burial depth of 1500 m was reached at ~128 Ma. The largest hydrocarbon expulsion began at ~120 Ma, corresponding to a burial depth of 2450 m. The maximum cumulative yield is 510 mg/g TOC, and it is still in the peak period of hydrocarbon generation, which demonstrates a favorable potential for hydrocarbon exploration.

摘要:In this study, X-ray diffraction, N2 adsorption (N2A), and mercury intrusion (MI) experiments were used to investigate the influence of acid treatment on pore structure and fractal characterization of tight sandstones. The results showed that acid treatment generated a certain number of ink-bottle pores in fine sandstone, aggravated the ink-bottle effect in the sandy mudstone, and transformed some smaller pores into larger ones. After the acid treatment, both the pore volume in the range of 2–11 nm and 0.271–8 μm for the fine sandstone and the entire pore size range for the sandy mudstone significantly increased. The dissolution of sandstone cement causes the fine sandstone particles to fall off and fill the pores; the porosity increased at first but then decreased with acid treatment time. The fractal dimension obtained using the Frenkel-Halsey-Hill model was positively correlated with acid treatment time. However, the total fractal dimensions obtained by MI tests showed different changes with acid treatment time in fine sandstone and sandy mudstone. These results provide good guiding significance for reservoir acidification stimulation.

摘要:CO2 dissolution into an aqueous phase and water evaporation into a gaseous phase takes place during CO2 injection into an oil reservoir. This study aims to evaluate the phase behaviors of the oil-gas-water system in the displacement of crude oil by CO2. The composition of the JL oilfield in the northeast of China is taken as an example. The flash calculation of the oil-gas-water system was performed, based on the method presented by Li and Nghiem. The research results show that CO2 dissolution in the aqueous phase declines as the NaCl concentration in formation water rises. CO2 injection is beneficial for the evaporation of formation water. The NaCl concentration in formation water has little effect on water evaporation and dissolved-gas escape. When the injection-gas mole fraction of CO2 is 0.5, CO2 injection can reverse the phase behavior of the petroleum mixture and the oil-gas system is converted to a pure gas-condensate system. For CO2 injection, water vapor has little effect on the miscibility of multiple contacts, but can reduce the miscibility of the first contact.

摘要:Using high-precision zircon U-Pb ID-TIMS geochronology, tuffs from the Chang 9 shale and the Chang 7 shale were dated. The tuff in the Chang 9 shale is 241.47 ± 0.17 Ma, which falls between the top tuff age of 241.06 ± 0.12 Ma and the bottom tuff age of 241.558 ± 0.093 Ma in the Chang 7 shale. These reveal that the Chang 9 and Chang 7 shales are contemporaneous, belonging to the Ladinian stage of the Middle Triassic. This insight expands the region of the main source rock of Chang 7 to the northeast and will inform the search for the deep Chang 9 shale petroleum system, increasing the scope for exploring the Chang 7 shale system in northern Shaanxi. The research results clarify the relationship between the two sets of shale in the Yanchang Formation and redefine the distribution range of the Chang 7 shale in the Ordos Basin. At the same time, it shows that there is a cross-layer problem in the stratigraphic division of the Yanchang Formation in different regions, the high-precision U-Pb dating technology providing a reference for the fine stratigraphic correlation of other continental basins in the world.

摘要:The effect of various depositional parameters including paleoclimate, paleosalinity and provenance, on the depositional mechanism of lacustrine shale is very important in reconstructing the depositional environment. The classification of shale lithofacies and the interpretation of shale depositional environment are key features used in shale oil and gas exploration and development activity. The lower 3rd member of the Eocene Shahejie Formation (Es3x shale) was selected for this study, as one of the main prospective intervals for shale oil exploration and development in the intracratonic Bohai Bay Basin. Mineralogically, it is composed of quartz (avg. 9.6%), calcite (avg. 58.5%), dolomite (avg. 7%), pyrite (avg. 3.3%) and clay minerals (avg. 20%). An advanced methodology (thin-section petrography, total organic carbon and total organic sulfur contents analysis, X-ray diffraction (XRD), X-ray fluorescence (XRF), field-emission scanning electron microscopy (FE-SEM)) was adopted to establish shale lithofacies and to interpret the depositional environment in the lacustrine basin. Six different types of lithofacies were recognized, based on mineral composition, total organic carbon (TOC) content and sedimentary structures. Various inorganic geochemical proxies (Rb/Sr, Ca/(Ca + Fe), Ti/Al, Al/Ca, Al/Ti, Zr/Rb) have been used to interpret and screen variations in depositional environmental parameters during the deposition of the Es3x shale. The experimental results indicate that the environment during the deposition of the Es3x shale was warm and humid with heightened salinities, moderate to limited detrital input, higher paleohydrodynamic settings and strong oxygen deficient (reducing) conditions. A comprehensive depositional model of the lacustrine shale was developed. The interpretations deduced from this research work are expected to not only expand the knowledge of shale lithofacies classification for lacustrine fine-grained rocks, but can also offer a theoretical foundation for lacustrine shale oil exploration and development.

摘要:The Solomon Sea Basin is a Cenozoic back-arc spreading basin within the convergence system of the Pacific and Indo-Australian plates. Against the background of subduction polarity reversal, the current Solomon Sea Basin gradually formed a rhombic morphology with the subduction of the basin along the New Britain Trench and the Trobriand Trough. By analyzing the vertical gravity gradient, natural earthquake and seismic reflection data, this study determines the structural characteristics of the Solomon Sea Basin. It was found that the tectonics of the basin are characterized by the original expansion structure within the central part in addition to the structure induced by the latest subduction along the basin margin. The original spreading structure of the basin presented an east–west linear graben and horst controlled by normal faults during the basin expansion period. As a result of the subduction and slab-pull of the Solomon Sea Basin, extensional structure belts parallel to the New Britain Trench formed along the basin margin.

摘要:The Tibetan Plateau is an important area for studying global climate change, but the answers to many scientific problems remain unknown. Here, we present new information from the lacustrine sedimentary record in the western Tibetan Plateau, related to the third most-recent glaciations. Continuous sediment data, including sporopollen, particle size, total organic carbon, mass susceptibility, CaCO3, CaSO4, BaSO4 contents and chronological data, were reconstructed and revealed that climate and environmental conditions obviously and distinctly changed between 600 and 700 thousand years ago. In comparison, the data obtained from the Guliya ice core in this area also corresponds to the global glacial climatic characteristics recorded in basin sediments in the eastern and southeastern regions of the plateau and to the information obtained from ice cores in the Antarctic and Arctic regions. In this study, we conclude that the main reason for the glaciations and new tectonic movement must be a geomagnetic polarity reversal 774 thousand years ago (from Matuyama to Brunhes). Indeed, the results of this study suggest that the described reversal event might have influenced the current global climate pattern and will continue to impact climatic changes in the future.

摘要:K-Ar dating of synkinematic illite is increasingly recognized as a central method to constrain the timing of shallow crustal faulting. Methods of efficient sample preparation and quantitative identification of illite polytypes are critical to acquiring K-Ar isotope data for authigenic clays. In this respect, we compared the commonly used clay size separation method through centrifugation with vacuum filtration technology, showing that the former is prone to extract fractions with finer particle sizes under similar conditions, thus improving the error in the authigenic end-member age. Additionally, we demonstrated that the side-packed mounting method for X-ray diffraction analysis can significantly enhance the randomness in powder samples, thus improving the quantification accuracy compared with the front-packed and back-packed methods. The validity of our quantification method was confirmed by comparing Profex? modeling patterns with a suite of synthetic mixtures of known compositions, yielding an average analytical error of 3%. Dating results of these artificial mixtures and the reference materials indicated that a large range in percentages of detrital illite and a sufficient amount of age data will produce reliable results for ages of both extrapolated end-members. However, if the range is limited, the extrapolated age close to those of datasets is still reliable.

摘要:As the principal ore mineral in various tungsten (-gold) deposits, scheelite (CaWO4) plays an important role in directly dating the timing of ore formation, and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics. Since the retention of Sm-Nd systematics within scheelite is presently unconstrained, equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data. In order to better elucidate the closure of Sm-Nd in scheelite, the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein. The following Arrhenius relations were obtained: DNd = 4.00exp(?438 kJ·mol–1/RT) cm2/s DSm = 1.85exp(?427 kJ·mol–1/RT) cm2/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature. However, compared to other rare earth elements (REEs), which have markedly different atomic radii to either Nd or Sm, these are shown to exhibit a great variation in diffusivities. The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals, indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices. With this in mind, the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes. Based upon the diffusion data determined herein, Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics. Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits, and any later thermal environments. It is likely, therefore, that relevant isotopic information could be easily retained under most geological conditions, since initial crystallization of the scheelite. In addition, comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers (e.g., zircon and apatite) indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz, and only a little faster than for Pb in zircon. It should be noted, within most hydrothermal deposits where zircon has crystallized, its size is typically no more than 100 μm, whereas scheelite commonly occurs as macroscopic grains. For this reason, the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations, relating to any later thermal process. As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating. These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case, especially when data are extracted from macroscopic grains of scheelite that are chosen to be “pristine” (i.e., free of surface alteration and/or fractures).

摘要:The Eastern Desert of Egypt suffered a protracted period of deformation triggered by cratonization of the new juvenile crust known as the Arabian Nubian Shield (ANS), which has been proposed for potential gold discoveries associated with the corresponding tectonic event. The Fatira area, on the border of Egypt's Northern and Central Eastern Deserts, is covered with metavolcanic rocks twisted by a dextral relocation of the Fatira Shear Zone (FSZ) relative to the Barud magmatic body. The recent study evaluated many deformed post-orogenic granitic intrusions and felsite dikes associated with promising mineralization localities, notably orogenic gold deposits. The combination of various field observations and remote sensing data, followed by the analysis of aeromagnetic enhanced maps, allowed the differentiation of distinct lithologies, structural features, and hydrothermal alterations in the study area. Additionally, the integrated results obtained from the different interpretation techniques are utilized to identify and confirm the previously supposed mineralized localities in the Fatira and Abu Zawal areas and predict other matched localities. The final ASTER, Sentinel 2 hydrothermal alteration, and orientation entropy heat maps demonstrate the association between these mineralized regions and major structures related to the FSZ late stage of deformation rather than other structures studied throughout the area of interest.

摘要:Western?Altun in Xinjiang is an important area, where lithium (Li)-bearing pegmatites have been found?in recent years. However, the complex terrain and harsh environment of western?Altun exacerbates in prospecting for Li-bearing pegmatites. Therefore, remote-sensing techniques can be an effective means for prospecting Li-bearing pegmatites. In this study, the fault information and lithologyical information in the region were obtained using the median-resolution remote-sensing image Landsat-8, the radar image Sentinel-1 and hyperspectral data GF-5. Using Landsat-8 data, the hydroxyl alteration information closely related to pegmatite in the region was extracted by principal component analysis, pseudo-anomaly processing and other methods. The high spatial resolution remote-sensing data WorldView-2 and WorldView-3 short-wave infrared images were used and analyzed by principal component analysis (PCA), the band ratio method and multi-class machine learning (ML),?combined with conventional thresholds specified the algorithms used?to automatically extract Li-bearing pegmatite information. Finally, the Li-bearing pegmatite exploration area was determined, based on a comprehensive analysis of the faults, hydroxyl alteration lithology and Li-bearing pegmatite information. Field investigations have verified that the distribution of pegmatites in the central part of the study area is consistent with that of Li-bearing pegmatites extracted in this study. This study provides a new technique for prospecting Li-bearing pegmatites, which shows that remote-sensing technology possesses great potential for identifying?lithium-bearing pegmatites, especially in areas that are not readily accessible.