The Mesozoic–Cenozoic tectonic movement largely controls the northwest region of the Junggar Basin (NWJB), which is a significant area for the exploration of petroleum and sandstone‐type uranium deposits in China. This work collected six samples from this sedimentary basin and surrounding mountains to conduct apatite fission track (AFT) dating, and utilized the dating results for thermochronological modeling to reconstruct the uplift history of the NWJB and its response to hydrocarbon migration and uranium mineralization. The results indicate that a single continuous uplift event has occurred since the Early Cretaceous, showing spatiotemporal variation in the uplift and exhumation patterns throughout the NWJB. Uplift and exhumation initiated in the northwest and then proceeded to the southeast, suggesting that the fault system induced a post spread‐thrust nappe into the basin during the Late Yanshanian. Modeling results indicate that the NWJB mountains have undergone three distinct stages of rapid cooling: Early Cretaceous (ca. 140–115 Ma), Late Cretaceous (ca. 80–60 Ma), and Miocene–present (since ca. 20 Ma). These three stages regionally correspond to the Lhasa‐Eurasian collision during the Late Jurassic–Early Cretaceous (ca. 140–125 Ma), the Lhasa‐Gandise collision during the Late Cretaceous (ca. 80–70 Ma), and a remote response to the India‐Asian collision since ca. 55 Ma, respectively. These tectonic events also resulted in several regional unconformities between the J3/K1, K2/E, and E/N, and three large‐scale hydrocarbon injection events in the Piedmont Thrust Belt (PTB). Particularly, the hydrocarbon charge event during the Early Cretaceous resulted in the initial inundation and protection of paleo‐uranium ore bodies that were formed during the Middle–Late Jurassic. The uplift and denudation of the PTB was extremely slow from 40 Ma onward due to a slight influence from the Himalayan orogeny. However, the uplift of the PTB was faster after the Miocene, which led to re‐uplift and exposure at the surface during the Quaternary, resulting in its oxidation and the formation of small uranium ore bodies.
The Kamust sandstone‐hosted uranium deposit was recently discovered within the Middle Jurassic Toutunhe Formation of the eastern Junggar Basin. In this study, polarizing microscope and scanning electron microscope observation, electron microprobe analysis, and laser ablation inductively coupled plasma mass spectrometry measurement were carried out to define the petrology, mineralogy, and geochemistry of these sandstone‐hosted uranium ores. In addition, sulphur isotope signatures for the associated ore‐stage pyrites and the ore‐forming age were determined. The results show that the mineralized sandstones, which exhibit excellent reducibility with abundant organic matter and pyrites, are characterized by the secondary accumulation of uranium and are slightly enriched in rare earth elements (REE) and light REE (LREE). The uranium deposits are mainly composed of a large amount of uraninite, followed by some coffinites and uranium‐bearing titanium minerals and very few sorptive uranium‐bearing materials. Uranium minerals primarily exist within the interior and edge of clastic particles in colloidal, dispersed, and vein form, and some are closely interweaved with the colloidal and framboidal pyrites and carbonaceous detritus. Both uraninite and coffinite are strongly enriched in REE and LREE, as determined by in situ trace element measurements. Moreover, the δ34S values of the associated ore‐stage pyrites mostly range from −41.66 to −19.33‰, indicating biological origin, and light 32S demonstrates the relatively low‐temperature and open‐system mineralization processes related to the microbial anaerobic action on the organic matter. One of the δ34S values (6.88‰) is possibly caused by Rayleigh isotope fractionation due to the utilization of residual heavy 34S in the relatively later closed‐like mineralization system. The ore‐reforming ages of 27 ± 3.2 Ma, 22.4 ± 3.9 Ma, and 21.3 ± 2.3 Ma were obtained by U–Pb isotopic dating combined with U–Ra balance correction, corresponding to the rapid uplift event caused by the remote effect of the collision between the Indian and Eurasian plates at the end of the Palaeogene. In conjunction with the tectonic–sedimentary evolution of the study region, a four‐stage ore‐forming model for the Middle Jurassic Toutunhe Formation is preliminarily constructed: ① Development of ore‐bearing strata, ② initiation of the mineralization stage from the Late Jurassic to the Early Cretaceous, ③ deep burial by lacustrine and flood mudstone from the Cretaceous to the Palaeogene, and ④ the superimposed ore‐forming stage during the Neogene. These research results provide significant benefits for further uranium prospecting and in situ leach process mining in this basin.
Meso‐Cenozoic intracontinental orogenic processes in the Tian Shan orogenic belt have significant effect on the sandstone‐hosted uranium deposits in the intramontane basins and those adjacent to the orogen. The Sawafuqi uranium deposit, which is located in the South Tian Shan orogenic belt, is investigated to reveal the relationships between uranium mineralization and orogenies. Recent exploration results show that the Sawafuqi uranium deposit has tabular, stratiform, quasi‐stratiform, and lens‐like orebodies and various geological characteristics different from typical interlayer oxidation zone sandstone‐hosted uranium deposits. Systematic studies of ore samples from the Sawafuqi uranium deposit using a variety of techniques, including thin section observation, α‐track radiograph, electron microprobe and scanning electron microscope, suggest that uranium mineralization is closely related to pyrite and organic matter. Mineralization‐related alterations in the host rocks are mainly silicification and argillation including kaolinite, illite (and illite‐smectite mixed layer) and chlorite. Tree stages of mineralization were identified in the Sawafuqi uranium deposit: (i) uranium‐bearing detritus and synsedimentary initial pre‐enrichment; (ii) interlayer oxidization zone uranium mineralization; and (iii) vein‐type uranium mineralization. The synsedimentary uranium pre‐enrichment represents an early uranium enrichment in the Sawafuqi uranium deposit, and interlayer oxidation zone uranium mineralization formed the main orebodies, which are superimposed by the vein‐type uranium mineralization. Combining the results of this study with previous studies on the Meso‐Cenozoic orogenies of South Tian Shan, it is proposed that the synsedimentary uranium pre‐enrichment of the Sawafuqi uranium deposit was caused by Triassic Tian Shan uplift, and the interlayer oxidation zone uranium mineralization occurred during the Eocence‐Oligocene period, when tectonism was relatively quiet, whereas the vein‐type uranium mineralization took place in relation to the strong orogeny of South Tian Shan since Miocene.
Oil group separation, gas chromatography–mass spectrometry analysis of saturated hydrocarbons, carbon isotope analysis of fractions and tests on trace elements were all carried out to determine the origin of shallow Jurassic heavy oils in the northwestern margin of the Junggar Basin, northwestern China. Results showed that all the crude oils had been subjected to different degrees of biodegradation, on an order ranging from PM 6 to 9, which yielded many unresolved complex mixtures (UCM) and formed a huge spike in the mass chromatogram (M/Z = 85). Two heavy oils from the Karamay area underwent slight biodegradation, characterized by the consistent ratios of biomarker parameters. C21/C23 and C23/H of the two samples were 0.81 and 0.85, while G/H, C27/C29 and C28/C29 were 0.38 and 0.40, 0.16 and 0.27, 0.87 and 0.86, respectively. The isomerization parameters of terpane and steranes were 0.50–0.53, and 0.48–0.49, respectively. The above geochemical indices indicated that the crude oils in the study area were in the marginally mature stage. The parent materials were a mixture, consisting of bacteria, algae and some higher plants, formed under reducing depositional conditions, which is in agreement with the source rocks of the Fengcheng Formation in the Mahu depression. The carbon isotopic compositions of saturated hydrocarbon, aromatic hydrocarbon, NSO and asphaltene were –31‰– to –30.3‰, –29.5‰ to –29.03‰, –29.4‰ to –28.78‰ and –28.62‰ to –28.61‰, respectively. These findings are in agreement with the light carbon isotope of kerogen from the lower Permian Fengcheng Formation. Furthermore, V/Ni and Cr/Mo of all the crude oils were 0.01 to 0.032, 0.837 to 10.649, which is in good agreement with the ratios of the corresponding elements of the extracts from the Fengcheng Fm. carbonate source rock. As a result, a two–stage formation model was established: (1) the oil generated from the carbonate source rocks of the Fengcheng Formation migrated to the Carboniferous, Permian and Triassic traps during the Late Triassic, forming the primary oil reservoirs; (2) during the Late Jurassic period, the intense tectonic activity of Yanshan Episode II resulted in the readjustment of early deep primary reservoirs, the escaped oils gradually migrating to the shallow Jurassic reservoir through cross‐cutting faults, unconformities and sand body layers. The oils then finally formed secondary heavy oil reservoirs, due to long–term biodegradation in the later stage. Therefore, joint methods of organic, isotopic and element geochemistry should be extensively applied in order to confirm the source of biodegradation oils.
A new mineral species of the pyrochlore supergroup, hydroxyplumbopyrochlore (IMA2018-145), (Pb1.5,□0.5)Nb2O6(OH), has been discovered in the Jabal Sayid peralkaline granitic complex of the Arabian Shield, Saudi Arabia. It is associated with quartz, microcline, ‘biotite’, rutile, zircon, calcite, rhodochrosite, columbite-(Fe), goethite, thorite, bastnäsite-(Ce), xenotime-(Y), samarskite-(Y), euxenite-(Y), hydropyrochlore and fluornatropyrochlore. Hydroxyplumbopyrochlore usually shows euhedral octahedra, slightly rhombic dodecahedra and cubes or their combination (0.01–0.06 mm). The mineral is pale yellow to pale brown, transparent with white streak, and has adamantine to transparent lustre. It is brittle with conchoidal fracture. No cleavage or parting are observed. It is isotropic and non-fluorescent. The average microhardness is 463.4 kg mm–2. The calculated density is 6.474 g cm–3. Hydroxyplumbopyrochlore belongs to the cubic crystal system and exhibits the space group Fd $\bar{3}$ m with unit-cell parameters a = 10.5456(6) Å, V = 1172.8(2) Å3 and Z = 8. Electron microprobe analysis gave (6-point average composition, wt.%): CaO 0.32, SrO 0.16, FeO 0.17, Ce2O3 0.07, Pr2O3 0.02, PbO 51.69, Nb2O5 40.06, SiO2 0.05, TiO2 1.68, Ta2O5 4.74, H2Ocalc 0.95, total 99.90, yielding the empirical formula (Pb1.34Ca0.03Fe0.01Sr0.01□0.61)Σ2(Nb1.75Ti0.12Ta0.12Si0.01)Σ2O6(OH0.53O0.08□0.39)Σ1, where □ = vacancy. The Raman spectrum of hydroxyplumbopyrochlore contains the characteristic bands of O–H vibrations and no bands for H2O vibrations.
The evolution characteristics of hydrothermal activity and superimposed uranium mineralization in the Qianjiadian ore field in southwestern Songliao Basin are still controversial and lack direct evidence. In this comprehensive study, a detailed identification of dolerite and hydrothermally altered un-mineralized sandstone and sandstone-hosted ore in the Yaojia Formation have been performed through the use of scanning electron microscopy observation, electron probe, carbon-oxygen-sulfur isotope, and fluid inclusion analyses. The results show that the hydrothermal fluid derived from the intermediate-basic magma intrusion is a low-temperature reducing alkaline fluid and rich in CO2, Si, Zr, Ti, Fe, Mg, Mn, and Ca, producing different types of altered mineral assemblages in the rocks, including carbonation, pyritization, sphalerite mineralization, clausthalite mineralization, silicification, and biotitization. Specifically, the carbonate minerals in sandstone are mixed products of deep hydrothermal fluid and meteoric water, with carbon and oxygen isotopes ranging from −5.2‰ to −1.7‰ and −20.4‰ to −11.1‰, respectively. Carbon source of the carbonate minerals in dolerite is mainly inorganic carbon produced at the late stage of intermediate-basic magma evolution, with carbon and oxygen isotopes from −16.1‰ to −7.2‰ and −18.2‰ to −14.5‰, respectively. Various carbonate minerals in the rocks may have been precipitated by the hydrothermal fluid after the magmatic stage, due to the change of its CO2 fugacity, temperature, and cation concentration during the long-term evolution stage. A series of carbonate minerals were generated as calcite, dolomite, ankerite, ferromanganese dolomite, and dawsonite. The precipitation processes and different types of carbonate mineral mixtures identified in this study mainly occur as parallel, gradual transition, interlacing, or inclusion metasomatism in the same vein body, without obvious mineralogical and petrologic characteristics of penetrating relationship. Homogenization temperature of fluid inclusions in calcite is high, in the range of 203–234 °C, with a low salinity of 0.71–4.34% NaCl, and the data range is relatively concentrated. Homogenization temperature of fluid inclusions in ankerite is usually low, ranging from 100 °C to 232 °C, with a high salinity of 4.18–9.98% NaCl. The precipitation processes of carbonate minerals and the results of this study are basically in consistent. Overall, the sandstone-type uranium deposits have a temporal and genetic relationship with hydrothermal activities during Paleogene. (1) Hydrothermal activity was directly involved in uranium mineralization, result in dissolution and reprecipitation of earlier uranium minerals, forming uranium-bearing ankerite and complexes containing uranium, zirconium, silicon, and titanium. (2) Hydrothermal fluid activity provided reducing agent to promote hydrocarbon generation from pyrolysis of carbonaceous fragments and accelerate uranium precipitation rate. (3) Regional water stagnation prolongs reaction time, contributing to huge uranium enrichment. This study provides new petrologic, mineralogical, and geochemical evidence for multi-fluid coupled and superimposed mineralization of sandstone-hosted uranium deposits in the sedimentary basin.
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