Sedimentary Environment, Tectonic Se tt ing, and Uranium Mineralization Implications of the Yimin Formation, Kelulun Depression, Hailar Basin, China

: The sandstone-type uranium deposit of the Kelulun Depression is the ﬁ rst industrially valuable uranium deposit discovered in the Hailar Basin. This study performed a systematic examination of 17 sandstone samples from the Yimin Formation in the Kelulun Depression based on various analytical techniques. The ﬁ ndings of the current study were synthesized with previous research to investigate the impact of the redox conditions and the tectonic background of the source area, as well as the paleoclimatic evolution of the Yimin Formation on uranium mineralization. The elemental Mo, U/Th, V/Cr, Ni/Co, and V/(V+Ni) ratios indicate that the paleowater was in an oxygen-rich environment during the deposition of the Yimin Formation. Additionally, the C-value, Sr/Cu, Al 2 O 3 /MgO, and Rb/Sr ratios indicate that the Yimin Formation was formed in a paleoclimate characterized by arid-to-semi-arid conditions. The geochemical characteristics of the observed elements indicated that the sediment source of the Yimin Formation was mainly felsic rocks from the upper continental crust, the weathering of the rock was weak, and the tectonic background was a passive continental margin. Co ﬃ nite is distributed in the form of cementation and stellates within or around pyrite crystals, and uranium-titanium oxide is mostly distributed in an irregular granular distribution in the biotite cleavage fractures of the study area. In summary, the ﬁ ndings of this study reveal that the tectonic se tt ings, provenance, uranium source, paleoclimate, and oxygen-rich pale-owater of the Yimin Formation have important geological signi ﬁ cance for the large-scale uranium mineralization of the Kelulun Depression.


Introduction
The Hailar Basin is a terrestrial rift basin that is rich in coal, oil, and natural gas.In recent years, significant advancements have been made in mineral exploration, leading to the discovery of large-scale sandstone-type uranium mineralizations within the Cretaceous layers of the Hailar Basin.Many uranium ore sites and mineralization points have recently been discovered in the Kelulun Depression, which is located in the southwestern part of the basin.The distribution of the uranium mineralizations is such that they mostly occur within the Cretaceous strata of the Yimin Formation.Numerous prior investigations have been conducted on the sandstone-type uranium in the region, with a primary emphasis on identifying the target layer [1], characterizing the sand body [2], assessing the hydrogeological conditions [3], conducting airborne radioactivity measurements [4], and analyzing the potential for uranium mineralization [5][6][7].Nonetheless, the lack of comprehensive investigations on the geochemical characteristics, provenance, paleoclimate, varieties, and forms of uranium minerals in the ore-bearing target layers has hindered the precise determination of the correlation between the sandstones found in the Yimin Formation and the mineralization of uranium.Consequently, this limitation has impeded our understanding of the association between the sandstones of the Yimin Formation within the basin and the extensive uranium mineralization that occurred within the basin.
The exploration of sandstone-type uranium deposits has revealed that the sedimentation of surface water systems plays a crucial role in connecting uranium sources with sedimentary basins [8].This process effectively limits the transportation of uranium, as demonstrated in previous studies [9][10][11][12].A thorough investigation of the redox conditions of ancient water bodies is important for elucidating the process of the mineralization of sandstone [13][14][15].Furthermore, certain elements present in clastic sedimentary rocks did not undergo substantial migration throughout the process of long-distance transportation.Consequently, these elements are valuable sources of information [16][17][18].The presence of trace elements and their concentrations in sediments can serve as valuable indicators of past environmental and climatic variations.These indicators can effectively inform us about the source materials and provenance structural characteristics of, as well as the interplay between, orogenic processes and basin development in sedimentary basins.Therefore, this paper focuses on the Yimin Formation, located in the Kelulun Depression of the Hailar Basin, as its subject of investigation.Representative samples of the Yimin Formation were collected and various methodologies, including petrology, geochemistry, and electron probe microanalysis, were employed to conduct a comprehensive study.Furthermore, by analyzing the vertical variations in trace and rare earth elements, this study attempts to reconstruct the processes of formation of paleowater bodies and paleoclimate fluctuations.Additionally, this study aims to investigate the interplay between regional tectonic activity and paleo-sedimentary environments.The findings presented in this study are important for elucidating the involvement of paleo-sedimentary and paleoclimatic conditions in the process of uranium mineralization inside the Kelulun Depression.

Geological Background
Hailar Basin, which is located in the eastern section of the Central Asian orogenic belt (Figure 1a), is a Cenozoic continental rift basin that developed on the Paleozoic collisional orogenic belt between the Siberian and North China plates [1].The basin has experienced four distinct development stages since its initial formation, which can be attributed to the influence of the north-east-trending Delbugan and Erguna fault zones.These stages included Late Jurassic rifting, Early Cretaceous depression, late Early Cretaceous shrinkage, and Late Cretaceous-Neogene differential uplift and leveling [19,20].Finally, the basin has acquired a tectonic framework characterized by a "two uplifts and three depressions" configuration [1].
The Kelulun Depression, situated within the Zhabenuoer depression in the Southwestern Hailar Basin, is a separate second-order negative secondary structural unit.This depression is characterized by its elongated shape resembling a dustpan, with a narrow width and northeast-oriented distribution (Figure 1b) [21].The depression is connected to the Adunchulu uplift in the west, the Han Wula bulge in the east, and the Hulun Lake depression in the north, and extends to the basin boundary in the south (Figure 1c).Tectonic activity in the early and middle stages of the depression's sedimentation in the Early Cretaceous was strong and generated more than 40 different types of fractures.The fracture system can be divided into three groups according to the direction of the fracture plane spread: north-northeast, north-east, and north-south.The north-east trending fault system is consistent with the main distribution direction of the Kelulun Depression and controls the current structural pattern of the depression [22].Moreover, it is the main con-trolling fault system in the area and is characterized by an early formation and long development times.The fracture system of this group cuts through the Jurassic to the Quaternary, thus providing a channel for the transportation of ore-bearing fluids and the uplift of reducing materials, such as oil and gas, in the deeper parts of the area.The basin's basement is dominated by Paleozoic metamorphic rocks and Hercynian granites.Large numbers of Hercynian granites and intermediate acidic volcanic rocks are distributed around the basin rim, and they have a high uranium content [23].The sedimentary cover consists of the Cretaceous Nantun Formation (K1n), the Tongbomiao Formation (K1t), the Damoguaihe Formation (K1d), the Yimin Formation (K1y), the Neogene Huchashan Formation (N2h), and Quaternary formations (Q) from bottom to top.The Cretaceous Yimin Formation is a set of dark coal-bearing clastic sedimentary rocks with a high reduction capacity and represents the main uranium-bearing layer in the basin [24,25].

Samples and Methods
Samples were collected from one borehole and from the medium-fine-grained sandstone of the Cretaceous Yimin Formation in the Kelulun Depression.Seventeen sandstone samples were collected as representative samples and subjected to further study.

Microscopic Observations
Thin sections were produced at the Langfang Geological Exploration Technology Service Co., Ltd, Guangzhou, China.Indoor microscopic petrological observations and analysis of the clastic particle composition were carried out at the Basic Geological Laboratory of East China University of Technology.The microscope used in this study was a Zeiss AxioImager M2m in order to minimize errors in the statistical analysis of the clastic particle composition.The selected sandstone samples had the following characteristics: mineral particle diameters ranging from 0.0625 to 2 mm.At least 300 grains were counted in each thin section.In addition, the percentage of cement content in the sample remained below 25 area%.

Major and Trace Element Analysis
Analysis of the major, trace, and rare earth elements was performed by ALS Chemex (Guangzhou, China) Co., Ltd.The major elements were analyzed using an XRF-1800 X-ray fluorescence spectrometer (Shimadzu Corporation, Koyto, Japan), and the analytical precision was maintained within 5%.Trace and rare earth elements were analyzed using a Thermo Fisher X Series II quadrupole plasma mass spectrometer (Massachusetts, USA).The analytical precision was <5% for trace element contents greater than 10 ppm and <10% for contents less than 10 ppm.The chemical analyses were performed as described by Chen (2000) [26].

Uranium Mineral Composition Analysis
Uranium mineral composition and energy spectrum analyses were performed at the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Jiangxi Province, China.Electron probe microanalysis (EPMA) was used in the experiment.The analyzing procedure was conducted according to the national standard [27].The standard samples used for calibration included U-UO2, Y-yttrium aluminum, and Si-albite.

Petrographic Characteristics
The Yimin Formation has a medium-to-fine-grained sandy texture (Figure 2a).The clasts mostly consist of quartz, feldspar (Figure 2c), and lithic fragments and minor biotite accompanied by heavy minerals, such as zircon and opaque metal minerals.The main type of support for the clastic particles is particle support, with basal-pore-type cementation between the particles.Calcareous cement is the main form (Figure 2b), although local argillaceous cement is observed.The clasts area accounts for 85-90% of the clast area, while the cement area accounts for 10-15%.The overall sorting and roundness of the mineral particles are poor, and the compositional and structural maturity are low, reflecting the close distance of the sediment transport and the characteristics of the near-source accumulation.The detrital quartz presents an angular-to-subangular shape, with grain sizes ranging from 0.13 to 0.50 mm.Quartz accounts for 35-55% of the clast area.The observed feldspar types are plagioclase, potassium feldspar, microcline, and perthite, with angular to subangular particles with grain sizes ranging from 0.11 to 0.46 mm.Plagioclase develops polysynthetic twins with strong clayification and sericitization.Several microclines are internally encrusted with altered biotite, and perthite develops strong clayification.Feldspar accounts for 10-25% of the clast area.The lithic fragments are angular-subangular, with grain sizes ranging from 0.12 to 0.53 mm, although several grains are larger than 2.00 mm.The lithic fragments mainly include granite fragments (Figure 2d) but also include small amounts of andesite fragments (Figure 2e), crystal tuff fragments (Figure 2f), and quartzite fragments.The lithic fragments account for 20-50% of the clast area.Biotite is present in a xenomorphic sheet-like shape and includes a few compacted sheets and bent crystals (Figure 2g).Some biotites exhibit strong chloritization, and iron oxides fill in or are around the cleavage joints of biotite.Biotite accounts for 2-4% of the clast area.In addition, the rocks are interspersed with pyrite (Figure 2h) and asphalt veins (Figure 2i).

Major Elements Geochemistry
The major element data for the sandstones of the Lower Cretaceous Yimin Formation in the Kelulun Depression are shown in Table 1.The SiO2 content of the Yimin Formation (K1y) sandstone ranges from 61.2 wt% to 76.6 wt%, the Al2O3 content ranges from 12.1 wt% to 15.0 wt%, the content of Fe2O3 T varies greatly, ranging from 1.08 wt% to 5.41 wt%, the MgO content ranges from 0.30 wt% to 1.44 wt%, the CaO content ranges from 0.47 wt% to 2.73 wt%, the K2O content ranges from 3.34 wt% to 4.12 wt%, and the Na2O content ranges from 1.86 wt% to 3.44 wt%.

Trace and Rare Earth Elements Geochemistry
The trace and rare earth elements (REE) data for the sandstones are listed in Table 2.In the spider diagram of trace elements, all samples present similar trends, the abundance of trace elements is slightly lower than that in the upper continental crust, and the characteristics are rich in U, Th, and Rb and poor in Sr, Cr, Ni, Be, Mo, Zr, and Hf (Figure 3).A few samples are richer in U and Mo, because six samples are collected in the mineralized section, and the content of U is high.However, U and Mo have similar geochemical characteristics, so the trend of U and Mo is different from other samples.
The total content of REE (ΣREE) of the Yimin Formation sandstone ranges from 87.5 ppm to 190 ppm, while the ratio of LREE/HREE varies between 8.7 and 15.5 (Table 2).The chondrite-normalized REE patterns are right-sloping with moderately negative Eu anomalies (Figure 4a).The normalized REE distribution curve for the upper continental crust (UCC) is gently sloped (Figure 4b), suggesting that the REE composition is comparable to that in the UCC.This indicates that the sandstones originated from the UCC [31].

Uranium Mineralogy and Mineral Chemistry
The EPMA results presented in Table 3 indicate that coffinite (U(SiO4)1−x(OH)4x) and uranium-titanium oxides are the major uranium minerals in the studied sandstones.
The UO2 content in uranium-titanium oxide ranges from 31.9 wt% to 37.1 wt%, the TiO2 content ranges from 36.4 wt% to 43.6 wt%, the SiO2 content ranges from 8.70 wt% to 11.81 wt%, the CaO content ranges from 0.31 wt% to 2.16 wt%, the BeO content ranges from 1.15 wt% to 2.57 wt%, and FeO, P2O5, Y2O3, Al2O3, ZrO2, and MgO are found in small amounts.The BSE micrographs of the studied uranium minerals (Figure 5) show that the average particle size of coffinite and uranium-titanium oxide ranges from 1 to 5 µm.Based on the characteristics of the uranium mineral morphology and assemblage, the following classes are identified: (1) Coffinite distributed in an irregular granular or colloidal form within framboidal pyrite (Figure 5a) or between submicron cubic pyrite crystals (Figure 5b), and coffinite tightly cemented with framboidal pyrite (Figure 5c); (2) Coffinite distributed in stellate and micro-vein forms at the edges, cracks, or dissolution pits of detrital quartz, potassium feldspar, and biotite (Figure 5d-f); (3) Coffinite and uranium-titanium oxide in colloidal (Figure 5g), burr-like, or microcolumnar shapes around pyrite (Figure 5h) and anatase (Figure 5i), thus forming assemblages of chlorite-pyrite-coffinite and chlorite-anatase-uranium titanium oxide.

Provenance
The trace elements (including REEs) of fine-grained clastic rocks are relatively stable in the later diagenesis and weathering processes, thus making them reliable tools for tracing the provenances and depositional settings of other elements [34,35].
The Al2O3/TiO2 ratio in the mafic igneous rock source is less than 8, the Al2O3/TiO2 ratio in the intermediate igneous rock source ranges from 8 to 21, and the Al2O3/TiO2 ratio in the felsic igneous rock source is greater than 21 [36].The Al2O3/TiO2 ratio of the Yimin Formation ranges from 22.2 to 46.2 (Table 1), indicating that the source rocks of the Yimin Formation sandstone are felsic igneous rocks.In the TiO2-Al2O3 diagram (Figure 6a), the samples are located in the granite and granodiorite region.In the F2-F1 diagram (Figure 6b), all samples plot in the source area of the felsic igneous provenance.Similarly, in the K2O-Rb diagram, the samples plot in the felsic intermediate region (Figure 6c), while in the TiO2-Ni diagram, the Yimin Formation sandstones plot in the felsic source area (Figure 6d), indicating that the source rocks of the Yimin Formation sandstone are felsic rocks.The Co/Th-La/Sc (Figure 7a), Ti/Zr-La/Sc (Figure 7b), La/Th-Hf (Figure 7c), Th/Sc-Zr/Sc (Figure 7d), La-Th-Sc (Figure 8a), Th-Hf-Co (Figure 8b), and V-Ni-Th×10 (Figure 8c) results show that the source rocks of the Yimin Formation are mainly felsic rocks, and in particular felsic volcanic rocks.
In summary, the source rocks of the Yimin Formation sandstone in the Kelulun Depression mainly formed in a passive continental margin (PCM) setting.

Redox Conditions
When using trace elements to define the redox environment of ancient sedimentary water media, the selected sample must not have been subjected to late fluid transformations [53].The correlations between Hf and the elements Co, Ni, Cr, and Cu show linear dependences with R 2 > 0.5 (Figure 11), indicating that the samples represent the paleoenvironmental characteristics of the source area.The contents and ratios of the redox-sensitive trace elements Mo, U, Th, Co, V, Ni, Cr, and Cu can be used to determine the redox properties of paleo-sedimentary water bodies [54].Metals such as Ni, Cu, and Zn often exist in the form of sulfides under anoxic conditions, which also have a certain indicative significance for water bodies [55].Therefore, the Mo, U/Th, V/Cr, Ni/Co, and V/(V+Ni) ratios are the most reliable parameters for investigating redox environments [56].
The U/Th ratio can be applied to deduce the redox conditions for the bottom waters at the boundary between water and sediment.U/Th > 1.25 indicates strongly reducing conditions, U/Th ratios between 0.75 and 1.25 indicate reducing conditions, and U/Th < 0.75 indicates oxidizing conditions [57,58].The U/Th ratio of the Yimin Formation sandstone ranges from 0.24 to 6.95, with most values ranging between 0.24 and 0.60, indicating that the sedimentary water bodies were all oxygen-rich (Figure 12).The sudden increase in U/Th values indicates that the degree of oxygenation of the paleowater body in this layer changed abruptly.It is speculated that uranium-rich water has a significant impact on the U/Th values within the ore-bearing layer of the Yimin Formation.
Previous studies have shown that oxidized water occurs when the Mo content is less than 25 ppm and reduced water occurs when the Mo content is more than 25 ppm [59].The Mo content of the Yimin Formation ranges from 0.23 ppm to 19 ppm, with most values ranging between 0.20 and 0.60, indicating that the Yimin Formation was deposited in an oxidizing water environment.V/Cr and Ni/Co ratios can also be used to determine the redox environment of paleo-water bodies.V/Cr < 2.0 signifies an oxygen-saturated environment, 2 < V/Cr < 4.25 denotes an oxygen-undersaturated environment, and V/Cr > 4.25 implies an anoxic environment [58,60].The V/Cr value of the Yimin Formation ranges from 2.72 to 8.14, indicating an oxygen-undersaturated environment (Figure 12).Ni/Co < 5.00 indicates an oxygen-saturated environment, between 5.00 and 7.00 indicates an oxygen-undersaturated environment, and >7.00 implies a reduced environment [58,61].The Ni/Co value of the Yimin Formation ranges from 1.16 to 2.13, indicating that the Yimin Formation was deposited in an oxygen-saturated environment.
The V/(V+Ni) value can help determine the intensity of the bottom water body stratification during deposition [58].A V/(V+Ni) ratio between 0.4 and 0.6 indicates weak stratification, a ratio between 0.6 and 0.84 implies moderate stratification, and a ratio above 0.84 denotes intense stratification.The V/(V+Ni) values of the Yimin Formation sandstone in the study area range from 0.79 to 0.94, indicating that, during Early Cretaceous sedimentation, the bottom water was mainly moderately stratified and the water bodies were in an oxygen-saturated-to-undersaturated environment.

Geochemical Discriminators of Paleoclimate
The C value is a favorable indicator of paleoclimate that can reflect arid or humid conditions: Ca, Mg, Sr, Ba, K, and Na are usually enriched under dry, arid conditions, whereas Fe, Mn, Cr, Ni, V, and Co are enriched under humid conditions [61].When the C value is greater than 0.8, from 0.6 to 0.8, from 0.4 to 0.6, 0.2 to 0.4, and less than 0.2, a humid, semi-humid, semi-humid-semi-arid, semi-arid, and arid environment is reflected, respectively [62].The samples from the Yimin Formation have C values from 0.13 to 0.62, indicating that they were formed in an arid environment.
The ratios of Sr/Cu, Rb/Sr, and Al2O3/MgO in sediments can also serve as important indicators of paleoclimates.Since Sr is enriched under arid conditions, while Cu is enriched under humid conditions, Sr/Cu values ranging from 1.3 to 5.0 indicate a warm and humid climate, and values greater than 5.0 indicate an arid climate [63].The Sr/Cu ratio of the Yimin Formation sandstone ranges from 4.51 to 42.34, with most values ranging between 15 and 30, indicating that the Yimin Formation was formed in an arid environment.The changes in the Rb/Sr and Al2O3/MgO ratios are similar to the changes in the C value (Figure 13), indicating that the Yimin Formation was generally in an arid environment during the sedimentary period.

Paleoweathering in Provenance and Indications for Paleoclimate
The weathering, denudation, transport, sedimentary diagenesis, and post-generative alteration of clastic rocks can result in changes in the geochemistry of the rock.Therefore, major and trace elements can be used to assess the sedimentary climate, compositional maturity, weathering degree, and alteration of sandstones [64,65].
The CIA (chemical index of alteration) can be used to determine the degree of weathering in the source area.CIA values between 50 and 65, 65 and 80, and 80 and 100, respectively, reflect a low degree of weathering under arid climate conditions, a moderate degree of weathering under warm and wet conditions, and an intense degree of weathering under hot and wet conditions [66].The CIA values of the Yimin Formation sandstone range from 51.86 to 67.32, indicating that the source area of the Yimin Formation has suffered a low degree of alteration.In the SiO2-(Al2O3 + K2O + Na2O) paleoclimate discrimination diagram (Figure 14a), all samples fall into the arid-to-semi-arid region.Values of ICV < 1 indicate mature samples rich in clay minerals, which may indicate cyclic sedimentation or the first sediments under intense weathering, and CV > 1 indicates immature samples that are the first sediments in the background [29].The ICV values of the sandstone samples range from 0.96 to 1.39, with an average of 1.08, indicating that the Yimin Formation sandstones are the first sediments in the background of tectonic activity.The samples were controlled by source rocks and did not experience intense chemical weathering.Nesbitt and Young (1984) established the A-CN-K triangulation, which can represent the CIA value.The three end elements are Al2O3, CaO* + Na2O, and K2O.In the CIA-ICV diagram (Figure 14b), all samples fall in the region of weak weathering, indicating that the Yimin Formation is characterized by immaturity, and that the source of the material originated from the upper crust.In the A-CN-K diagram (Figure 14c), most samples follow the weathering tendency of granite, suggesting that the source rock was mainly granite.The CIA value of the Yimin Formation is concentrated in the range from 60 to 70, indicating that the weathering of granite in the source area of the Yimin Formation is weak.

The Enrichment Mechanism of Uranium
The shape of coffinite and its co-existence with pyrite are clearly visible under BSE.Coffinite in an irregular granular or colloidal form within framboidal pyrite (Figure 5a) or between submicron cubic pyrite crystals (Figure 5b), and coffinite tightly cemented with framboidal pyrite (Figure 5c) were observed in the samples.The Fe 2+ in pyrite is believed to play a critical role in the reduction and precipitation of hexavalent uranium.The negative valent sulfur (S 2-) in pyrite acts as a reducing agent, and together with Fe 2+ , creates a microzone-reducing environment, thereby directly reducing hexavalent uranium [69,70].
Coffinite is distributed in stellate and micro-vein forms at the edges, cracks, or dissolution pits of detrital quartz, potassium feldspar, and biotite (Figure 5d-f).Alkaline feldspar produces a large amount of K2O during the alteration process of mineralized sandstone, reacting with CO2 in sandstone pore water to form K2CO3, and K2CO3 enters the pore aqueous solution to form alkaline solution, thus forming a local alkaline environment.Under alkaline conditions, quartz debris will dissolve, and this process provides a large amount of SiO2.When U(Ⅵ) is reduced to U(Ⅳ) in [UO2] 2+ , a large amount of SiO2 in the surrounding fluid participates in the reaction and combines with the U(Ⅳ) to form coffinite [71].Some colloidal (Figure 5g), burr-like, or micro-columnar shapes around pyrite (Figure 5h) and anatase (Figure 5i) commonly occur in the cleavage cracks of biotite in uranium ores.During hydrolysis and chloritization, biotite exhibits expansion and loosening, while becoming more adsorbent.A large amount of Fe 2+ precipitates in this process, creating a micro-reducing environment for uranium reduction and precipitation.U(Ⅵ) and SiO2 in the fluid are adsorbed, reduced, and subsequently precipitated near the cleavage cracks of altered biotite in an alkaline reducing environment [72].Parts of Fe 2+ react with S 2− or H2S, producing colloidal pyrite that fills up the cleavage cavities.
Based on the above research, uranium mineralization can be summarized as follows.Clay minerals and organic detritus in uranium-bearing sandstones adsorbed and fixed abundant uranium before enrichment or in the early ore-forming period.In the process of epigenetic alteration, the uraniferous fluid experienced a complex process of adsorptionreduction-deposition with the change in physical and chemical conditions.First, uranyl complex ions in the fluid discharge uranium near the geochemical barrier, which is then adsorbed by pyrite, organic matter, biotite, and other clay minerals, and in the alkaline reducing environment, U(VI) combines with Si 4+ and Ti 4+ to form coffinite and uraniumtitanium oxide.

Uranium Mineralization Models
Based on the geological characteristics and the tectonic evolution of the basin, this paper discusses the sandstone uranium mineralization process of the Kelulun Depression.
Initial tensile fracture and fault depression stage in the Late Jurassic: The Hailar Basin was in a long-term compressive state under the influence of the subduction of the Kula Plate below the Eurasian Plate [6].The northwest-southeast thrust nappe structure indicates that the study area represents a large-scale extensional tectonic environment with rifting volcanism, and intermediate and acidic volcanic rocks are widely developed [73].
Stretching and rapid subsidence of the Early Cretaceous fault depression: After the volcanic eruption period beginning in the Late Jurassic, the upper crust in the study area was transformed from a compressional system to an extensional setting, and the tectonic activity was very intense.The main sedimentary deposits were alluvial fans, fan deltas, semi-deep lakes, and deep lake deposits, and the sedimentary lithology and lithofacies changed greatly and only some small-scale sand bodies were formed.However, the buried depth was large and unstable, and no large-scale uplift or denudation were observed [74].Therefore, it was difficult to form a certain scale of epigenous oxidation, which is not conducive to the development of uranium mineralization.
Early Cretaceous depression shrinkage stages: The tectonic environment was generally stable.The Yimin Formation was formed mainly as a set of coal-bearing clastic rocks, with loose sandstone as a whole, large-scale good lateral connectivity, and stable top and bottom mudstone water-barrier layers.These characteristics were beneficial for the transport of uranium containing oxygenated water and the formation of uranium ore bodies.In addition, owing to the abundant supply of uranium in the mother rock, the erosion source area, a certain degree of uranium pre-enrichment occurred when the sand body of the Yimin Formation was deposited.
Tectonic depression inversion stage from the end of the Early Cretaceous to the Late Cretaceous: Since the Early Cretaceous, there have been two periods of severe tectonic inversion and obvious uplifting, tilting, and denudation events throughout the area [75].The first stage occurred from the end of the Early Cretaceous to the beginning of the Late Cretaceous, that is, from the deposition of the Yimin Formation to the deposition of the Qingyuangang Formation.A regional angular unconformity was formed between them, which was a sedimentary response to tectonic reversal caused by compression.At the same time, this stage also represented the cut-off point when the tectonic environment caused extensional-to-sandstone-type uranium deposits to form.However, the uplift and tilt effects were weak, which restricted the scale of the Yimin deposits to a certain extent.The second stage occurred after the end of the Late Cretaceous, when the Upper Cretaceous strata in the area were compressed and deformed, resulting in large-scale uplift and denudation of the Yimin Formation in the target layer, which resulted in increased hydraulic gradient, thus strengthening the "supplement-diameter-drainage" system of epigenetic fluids [76].The paleoclimate changed from warm and humid to arid and semiarid [77].The Yimin formation contains abundant organic carbon, which is sufficient to reduce uranium in the oxidizing fluid, and can be used as a good adsorbent to accelerate the reduction of uranium, which is conducive to the enrichment and mineralization of uranium.This was the main mineralization stage of the sandstone-type uranium deposit in the Yimin Formation.
Depression shrinking stage from Paleogene to Neogene: Since the Neogene, the study area has been a stable tectonic environment [78].This led to long-term rich uranium fluid transformation in the Yimin Formation in the wide and gentle slope belt in the northwestern part of the depression, resulting in the further enrichment of sandstone uranium mineralization.
Notably, the Kelulun Depression was high in the south and low in the north from its formation to the deposition of the Yimin Formation.The paleocurrent direction was mainly from southwest to northeast, and the sedimentation and subsidence centers were located in the northern part of the depression.This tectonic pattern of high in the south and low in the north lasted until the end of the Neogene, that is, the paleocurrent during the formation period of the target layer was approximately the same as that during the mineralization period, which was conducive to the infiltration of uranium-containing oxygenated water into the mineralization process.The slow uplift at the later stage of the depression caused the epigenetic oxidation rate to be greater than the uplift denudation rate, which also enabled the ore body to be preserved.
Based on the above characteristics, the uranium mineralization process in the Kelulun depression was as follows: A large number of Mesozoic granites observed in the surrounding areas of Adunchulu, Arihashate, and New Barhuyouqi; U content from 2.7 to 22.3 ppm; Th content from 15.2 to 25.9 ppm [79].These granites are uranium-rich structures.The Th/U ratio was 3.79-9.29,indicating the loss of uranium in uranium-rich granite [74].In addition, the uranium content in the Yimin Formation sand body was relatively high (from 2.81 to 6.40 ppm), indicating that, during the process of weathering and denudation of the source rock, uranium-containing clasts migrated to the depression along with the groundwater and accumulated there, which further resulted in the uranium preenrichment.The Yimin Formation is dominated by medium-to-fine-grained sandstone with low compositional maturity, poor cementation, and good porosity and connectivity, which provided a migration channel for the long-distance transport of oxygenated uranium-carrying fluid.Moreover, the Yimin Formation sandstone contains large amounts of organic carbon and massive pyrite, formed in an environment conducive to uranium reduction and precipitation.Meanwhile, the climate gradually changed from warm and humid to an arid and semi-arid oxidizing sedimentary environment, which led to the leaching and migration of uranium and the formation of a uranium-rich oxidizing fluid.In addition, surface vegetation did not develop in the arid climate.Therefore, the uranium in the oxygen-rich surface water could migrate steadily into the basin (Figure 15a).In the Late Cretaceous-Neogene period, the basin was uplifted four times, strengthening the "replenishment, diameter, and drainage" system of supergene fluids.In the later period of deposition, the overall uplift of the Hailar Basin entered the transformation stage.In the early Late Cretaceous, the Yimin Formation was exposed at the surface for a long time, suffered from leaching, weathering, and denudation, and deposition was interrupted.The uranium-and oxygen-containing fluid that formed in the western depression infiltrated the strata of the Yimin Formation.The free [UO2] 2+ in the uranium-rich fluid reacted with the reducing medium in the Yimin Formation, and the U(Ⅵ) in the uranium-containing fluid was reduced to U(Ⅳ), forming the most important uranium-bearing rock series.Finally, uranium ore bodies were formed in the redox transition zone of the Yimin Formation (Figure 15b).

Conclusions
(1) Coffinite is present in the form of cement within and around pyrite or as stellate and irregular granular morphologies along the edges, fractures, or dissolution pits of detrital particles.Uranium-titanium oxide is produced in colloidal, burr-like, or microcolumnar shapes in the cleavage cracks of biotite;

Figure 1 .
Figure 1.Regional geologic map of the Kelulun Depression in the Hailar Basin [1].(a).Tectonic location map of the Hailar Basin.(b).Schematic diagram of structural unit division of the Hailar Basin.(c).Regional geological map of the Kelulun Depression.

Figure 5 .
Figure 5. BSE micrographs of uranium minerals from the Yimin Formation sandstone in the Kelulun Depression.(a) Coffinite between framboidal pyrite aggregates.(b) Local magnification of Figure (a).(c) Coffinite surrounds the rim of pyrite.(d) Coffinite is distributed along the edges of clastic quartz and feldspar micro-veins.(e) Coffinite distributed in the margin of clastic quartz.(f) Coffinite

Figure 12 .
Figure 12.Diagram of discrimination of redox environment in the paleo-water body of the Yimin Formation.

Figure 13 .
Figure 13.Paleoclimate change indicators in the Yimin Formation sandstone.

Figure 15 .
Figure 15.Metallogenic model for sandstone uranium deposits in the Kelulun Depression.

( 2 )
Geochemical analysis shows that the source rocks of the Yimin Formation are feldspar igneous rocks in the passive continental margin, and evolved under conditions of weak chemical weathering;(3) The elemental Mo, U/Th, V/Cr, Ni/Co, and V/(V+Ni) ratios indicate that the paleowater medium of the Yimin Formation was an oxygen-rich depositional environment, and the bottom water was moderately stratified and circulated smoothly.The C-value, Sr/Cu, Al2O3/MgO, and Rb/Sr ratios indicate that the Yimin Formation formed in an arid environment.

Table 1 .
Analysis results of the major elements in the sandstone samples from the study area (wt%).
30.301.08 2.06 1.30 5.41 1.35 2.37 3.61 3.75 1.80 2.87 1.94 1.12 2.60 3.77 3.45 2.53 FeO 0.69 0.68 1.19 0.69 1.98 0.20 0.15 0.20 0.16 0.12 0.42 0.17 0.39 0.19 1.46 1.47 1.14 Chemical index of alteration (CIA) = 100Al2O3/(Al2O3 + CaO* + Na2O + K2O) [28].The chemical composition of the formula is the mole number, CaO* is only the CaO in silicate minerals, and because silicate is not easy to dissolve, the more accurate value of CaO in the source area is preserved.In this paper, the correction method proposed by McLennan was adopted for CIA calculation, i.e., according to the average composition of Na and Ca in natural silicate minerals and the mole ratio of CaO/Na2O in sediment samples; if the CaO mole is greater than Na2O, Na2O mole is used as CaO mole, otherwise, the CaO mole is used.Index of compositional variability

Table 3 .
EPMA results of uranium minerals from sandstone samples from the Yimin Formation in the Kelulun Depression (wt%).