Geochemical Characteristics, Palaeoenvironment and Provenance of Uranium-Bearing Sandstone in the Sifangtai Formation, Northern Songliao Basin, Northeast China

During the Cretaceous period of the northern Songliao Basin (northeast of China), a 100 m thick layer of fluvial-phase sandstone (Sifangtai Formation) with uranium potential was widely deposited, but its geochemical characteristics, paleoenvironment, and provenance remain unknown. This research proposes a new set of relevant geochemical data for sandstones to investigate their paleoenvironment, provenance and tectonic setting. The results revealed that: (1) The sandstone of the Sifangtai Formation was dominated by feldspar lithic sandstone. Geochemical signatures demonstrate that these sandstones have a high silicon content (SiO2 = 68.30~83.60 wt%) and total alkali content, but are poor in magnesium and calcium. They are also enriched in Rb, Th, U, K and LREE, and depleted HFSE (e.g., Nb, Ta), with crustal magmatic source. (2) The paleoclimate discriminant indicated that the rocks of the Sifangtai Formation might that the climate of Sifangtai Formation is semi-arid, and the chemical weathering of the source rocks is weak under the semi-arid climate environment. (3) The combination of element Sr/Ba, 100 MgO/Al2O3 and the combination of v/v + Ni, V/Cr, Ni/Co, and Sr/Cu indicated that the paleo-water medium was deposited in an oxygen-rich freshwater environment when the Sifangtai Formation was deposited. (4) The discriminate diagrams showed that almost all the sandstones of the Sifangtai Formation fell in the range of the active continental margin, indicating that the source area of the sandstones of Sifangtai Formations is an active continental margin tectonic environment, and the source is a felsic rock developed in the Xiaoxing’an Ridge and Zhangguangcailing area.


Introduction
Uranium is an important strategic resource. In particular, sandstone-type uranium deposits are typically targeted during exploration because of the reduced environmental impact and lower mining costs associated with this type of deposit. A number of significant sandstone-hosted uranium deposits are present in the Middle-Cenozoic basins in northern China [1][2][3][4][5]. From east to west, these deposits form the southern region of the Songliao, Erlian, Ordos, and Yili Basins. These basins comprise the Yaojia, Saihan, Zhiluo, and Xishangyao uranium-bearing groups [6][7][8][9][10][11]. In recent years, two medium-sized deposits have been discovered in the southern region of the basin. These deposits are characterized by significant reserves of uranium resources and show suitable exploration potential [6,7]. Previous studies have investigated the conditions of uranium ore formation, the sedimentary system, the chronology of uranium ore formation, geochemical characteristics, sedimentary diagenesis of ore-bearing sand bodies, the degree of weathering, and the tectonic background [12][13][14][15]. These studies have demonstrated that uranium-bearing sandstones of the  [29,30]). GXR, Great Xing'an Range.
The study area is situated in northern Songliao Basin (Figure 3a), and uranium is hosted in the Sifangtai Formation, which is located 300-500 m below the surface. The ore-hosting sediments are mainly composed of fine-grained sandstone with a massive structure. The sandstone units are angular and predominately composed of plagioclase (15-20 vol.%), quartz (40-50 vol.%), potassium feldspar (10-15 vol.%), and trace amounts of rock debris (15-20 vol.%). The cement comprises clay minerals (Figure 4), and the Sifangtai formation was presumably formed by proximal deposition.
The minerals associated with uranium in the study area are mainly star-shaped pyrite and organic matter derived from plant charcoal debris. Overall, this area of tectonic evolution has undergone four stages: Early Cretaceous faulting, Late Early Cretaceous uplift denudation, Late Cretaceous depression, and tectonic inversion and uplift denudation. The tectonic inversion represents the recharge area for groundwater and a discharge area for groundwater within the aquifer, and these conditions are extremely favorable for uranium ore formation [34][35][36].  [29,30]). GXR, Great Xing'an Range.

Analytical Methods
Elemental geochemical data were determined by the Beijing Research Institute of Uranium Geology (Beijing, China), CNNC (Beijing, China). Major elements were tested using XRF analysis with a relative error of less than 5%, and trace and rare earth elements (REEs) were analyzed using a Perkin Elmer Elan 6100 DRC inductively coupled plasma mass spectrometer (ICP-MS) (Perkin Elmer, Waltham, MA, USA). The sample analysis was monitored using reference materials AVG-1 and BHVO-1, and the relative error was generally less than 5% [37].

Major Elements
The sandstones in the northern Songliao Basin had a high SiO2 content range of 68.30-83.60 wt%, with an average value of 77.70 wt%. The total alkali (Na2O + K2O) content ranged from 4.24 wt% to 7.03 wt%, with an average value of 5.89 wt%. The rocks also had a high aluminum content (Al2O3 = 8.71-15.06 wt%) and low magnesium and calcium content (MgO = 0.12-1.38 wt%; CaO = 0.35-1.07 wt%).
With respect to the SiO2−Al2O3 plot, most of the sandstones were plotted in the plagioclase, potassium feldspar, and quartz regions (Figure 5a), whereas for the log(SiO2/Al2O3)−log(TFe2O3/K2O) plot, all the sandstones were plotted in the feldspar sandstone region (Figure 5b). Overall, the trace amounts of MnO and P2O5 detected in the samples suggest the presence of heavy minerals, such as apatite and chlorite, in the rocks of this area.

Analytical Methods
Elemental geochemical data were determined by the Beijing Research Institute of Uranium Geology (Beijing, China), CNNC (Beijing, China). Major elements were tested using XRF analysis with a relative error of less than 5%, and trace and rare earth elements (REEs) were analyzed using a Perkin Elmer Elan 6100 DRC inductively coupled plasma mass spectrometer (ICP-MS) (Perkin Elmer, Waltham, MA, USA). The sample analysis was monitored using reference materials AVG-1 and BHVO-1, and the relative error was generally less than 5% [37].

Major Elements
The sandstones in the northern Songliao Basin had a high SiO 2 content range of 68.30-83.60 wt%, with an average value of 77.70 wt%. The total alkali (Na 2 O + K 2 O) content ranged from 4.24 wt% to 7.03 wt%, with an average value of 5.89 wt%. The rocks also had a high aluminum content (Al 2 O 3 = 8.71-15.06 wt%) and low magnesium and calcium content (MgO = 0.12-1.38 wt%; CaO = 0.35-1.07 wt%).
With respect to the SiO 2 −Al 2 O 3 plot, most of the sandstones were plotted in the plagioclase, potassium feldspar, and quartz regions (Figure 5a), whereas for the log(SiO 2 /Al 2 O 3 )− log(TFe 2 O 3 /K 2 O) plot, all the sandstones were plotted in the feldspar sandstone region (Figure 5b). Overall, the trace amounts of MnO and P 2 O 5 detected in the samples suggest the presence of heavy minerals, such as apatite and chlorite, in the rocks of this area.

Trace and Rare Earth Elements
Chondrite-normalized REE patterns revealed similar patterns for all the sandstone samples (ΣREE = 65.54-283.55 ppm, with an average value of 105.27 ppm). Moreover, the REE distribution pattern was right-inclined ( Figure 6a), with relatively enriched light rare earth elements (LREEs) and depleted heavy rare earth elements (HREEs). The LREE and HREE ratios ranged from 6.89 to 12.90. Moreover, the fractionation coefficients ranged from 2.95 to 5.35 for the LREEs (La/Sm)N and from 1.09 to 2.24 for the HREEs (Gd/Yb)N, with a more substantial trend of LREE fractionation than HREE fractionation. A weak Eu negative anomaly (δEu = 0.57-1.00), which was slightly higher than PAAS (0.65) and closer to UCC (0.70), was detected, indicating that the sandstone host rock was derived from rocks of the upper crust.
The primitive mantle-normalized spider diagram indicated a consistent evolutionary trend for all the samples (Figure 6b) characterized by the enrichment of large-ion lithophile elements (e.g., Rb, Th and U), strong depletion of Sr, P, and Ti, and a relative depletion of Nb and Ta (Table 1).

Trace and Rare Earth Elements
Chondrite-normalized REE patterns revealed similar patterns for all the sandstone samples (ΣREE = 65.54-283.55 ppm, with an average value of 105.27 ppm). Moreover, the REE distribution pattern was right-inclined ( Figure 6a), with relatively enriched light rare earth elements (LREEs) and depleted heavy rare earth elements (HREEs). The LREE and HREE ratios ranged from 6.89 to 12.90. Moreover, the fractionation coefficients ranged from 2.95 to 5.35 for the LREEs (La/Sm) N and from 1.09 to 2.24 for the HREEs (Gd/Yb) N , with a more substantial trend of LREE fractionation than HREE fractionation. A weak Eu negative anomaly (δEu = 0.57-1.00), which was slightly higher than PAAS (0.65) and closer to UCC (0.70), was detected, indicating that the sandstone host rock was derived from rocks of the upper crust.

Trace and Rare Earth Elements
Chondrite-normalized REE patterns revealed similar patterns for all the sandstone samples (ΣREE = 65.54-283.55 ppm, with an average value of 105.27 ppm). Moreover, the REE distribution pattern was right-inclined ( Figure 6a), with relatively enriched light rare earth elements (LREEs) and depleted heavy rare earth elements (HREEs). The LREE and HREE ratios ranged from 6.89 to 12.90. Moreover, the fractionation coefficients ranged from 2.95 to 5.35 for the LREEs (La/Sm)N and from 1.09 to 2.24 for the HREEs (Gd/Yb)N, with a more substantial trend of LREE fractionation than HREE fractionation. A weak Eu negative anomaly (δEu = 0.57-1.00), which was slightly higher than PAAS (0.65) and closer to UCC (0.70), was detected, indicating that the sandstone host rock was derived from rocks of the upper crust.
The primitive mantle-normalized spider diagram indicated a consistent evolutionary trend for all the samples (Figure 6b) characterized by the enrichment of large-ion lithophile elements (e.g., Rb, Th and U), strong depletion of Sr, P, and Ti, and a relative depletion of Nb and Ta (Table 1).  The primitive mantle-normalized spider diagram indicated a consistent evolutionary trend for all the samples (Figure 6b) characterized by the enrichment of large-ion lithophile elements (e.g., Rb, Th and U), strong depletion of Sr, P, and Ti, and a relative depletion of Nb and Ta (Table 1).

Sedimentary Sorting and Recycling
Detailed geological data obtained in recent years have shown that LREEs (Th, Sc, La and Zr) are chemically stable and not easily fractionated during the deposition cycle. Moreover, these elements are insoluble in water and are thus negligibly affected by metamorphism [39,40]. LREEs can be used as primary indicators to distinguish rock types in provenance areas. Therefore, Th/Sc and Zr/Sc ratios can be used to constrain the parent rock components. The Th/Sc ratios measured in the sandstone samples from the study area varied between 0.89 and 3.03, with an average of 2.20, exceeding the upper crustal Th/Sc ratio of 1.0. The Zr/Sc ratio for the samples ranged from 10.32 to 39.52 and showed a significant positive correlation with the Th/Sc ratios (Figure 7). Moreover, the relationship between the two ratios formed an unparallel trend line to the depositional cycle, implying compositional homogeneity and minimal influence of sedimentary sorting. To summarize, it can be concluded that the fine-grained sandstones of Sifangtai Group are first-cycled sediments and undergo no or minimal mineral sorting.

Weathering Degree
The degree of provenance chemical weathering is dominated by the source rock composition, duration of weathering, climatic conditions, and tectonic activities [20,[41][42][43][44]. Ca, Na, and K are typically removed during the weathering of source rocks, and the residual amounts of these elements in soil profiles and sediments are sensitive indicators for determining the degree of chemical weathering. The chemical index of alteration (CIA) is considered an effective indicator of the degree of source weathering, where CIA values from 50-60 and 60-80 suggest weak weathering and moderate weathering, respectively [42][43][44]. Moreover, the index chemical variation (ICV) can be used to determine the compositional maturity of the sediment; high ICV values represent low sedimentary component maturity and strong tectonics, while low ICV values represent high maturity of sedimentary components and relatively stable tectonics. The plagioclase index of alteration (PIA) can also be used to assess the source weathering and elemental redistribution during diagenesis [42][43][44].
The CIA values of the sandstone samples from the Sifangtai Formation ranged from 51.99-62.07, with an average of 54.35, indicating that the parent rocks were subjected to weak weathering. The ICV values of the samples ranged from 0.60 to 1.03, with an average of 0.69, suggesting immature parent rocks (Figure 8a). Moreover, all the sandstone samples showed an evolutionary trend of weak weathering when plotted in a CIA-ICV plot, which revealed a dominant granite parent rock for the samples (Figure 8b).
Besides the CIA, the PIA can also determine the weathering intensity of plagioclase. Unweathered rocks have PIA values around 50, and weathered clay mineral PIA values in weathered clay minerals are close to 100 [45,46]. Sifangtai Formation sandstone PIA = 54.01-66.33. It reflects the weak chemical weathering of the sandstone source area of the Sifangtai Formation.

Weathering Degree
The degree of provenance chemical weathering is dominated by the source rock composition, duration of weathering, climatic conditions, and tectonic activities [20,[41][42][43][44]. Ca, Na, and K are typically removed during the weathering of source rocks, and the residual amounts of these elements in soil profiles and sediments are sensitive indicators for determining the degree of chemical weathering. The chemical index of alteration (CIA) is considered an effective indicator of the degree of source weathering, where CIA values from 50-60 and 60-80 suggest weak weathering and moderate weathering, respectively [42][43][44]. Moreover, the index chemical variation (ICV) can be used to determine the compositional maturity of the sediment; high ICV values represent low sedimentary component maturity and strong tectonics, while low ICV values represent high maturity of sedimentary components and relatively stable tectonics. The plagioclase index of alteration (PIA) can also be used to assess the source weathering and elemental redistribution during diagenesis [42][43][44].
The CIA values of the sandstone samples from the Sifangtai Formation ranged from 51.99-62.07, with an average of 54.35, indicating that the parent rocks were subjected to weak weathering. The ICV values of the samples ranged from 0.60 to 1.03, with an average of 0.69, suggesting immature parent rocks (Figure 8a). Moreover, all the sandstone samples showed an evolutionary trend of weak weathering when plotted in a CIA-ICV plot, which revealed a dominant granite parent rock for the samples (Figure 8b). Minerals 2021, 11, x FOR PEER REVIEW 13 of 27

Paleosalinity Determination
The Sr and Ba content of rocks and their ratios can be used to determine the placement of the medium. An Sr content range of 200-300 ppm in sandstones represents a freshwater depositional environment, whereas 200-1000 ppm indicates a marine environment [47]. Moreover, Sr/Ba ratios can also be used to constrain the depositional environment of formation, with ratios > 1 indicating a marine environment and ratios < 1 suggesting a freshwater environment. In this study, Sr concentration of the samples ranged from 161 to 297 ppm, and Sr/Ba ratios ranged between 0.25 and 0.75, indicating that uranium-bearing sandstones in the study area were deposited in a freshwater environment (Figure 9). Besides the CIA, the PIA can also determine the weathering intensity of plagioclase. Unweathered rocks have PIA values around 50, and weathered clay mineral PIA values in weathered clay minerals are close to 100 [45,46]. Sifangtai Formation sandstone PIA = 54.01-66.33. It reflects the weak chemical weathering of the sandstone source area of the Sifangtai Formation.

Paleosalinity Determination
The Sr and Ba content of rocks and their ratios can be used to determine the placement of the medium. An Sr content range of 200-300 ppm in sandstones represents a freshwater depositional environment, whereas 200-1000 ppm indicates a marine environment [47]. Moreover, Sr/Ba ratios can also be used to constrain the depositional environment of formation, with ratios > 1 indicating a marine environment and ratios < 1 suggesting a freshwater environment. In this study, Sr concentration of the samples ranged from 161 to 297 ppm, and Sr/Ba ratios ranged between 0.25 and 0.75, indicating that uranium-bearing sandstones in the study area were deposited in a freshwater environment (Figure 9).

Palaeoredox Conditions
The elements V, Ni, Cr and Co are characterized by the following unique characteristics: they do not readily migrate during diagenesis, they are autogenously enriched in oxygen-poor depositional environments, they are readily soluble under oxidizing conditions, and they maintain a pristine sedimentary record. Therefore, the content of these elements in rocks can be used to constrain depositional oxidation-reduction environments.
Jones et al. [48] reported that V/Cr, Ni/Co, and V(V + Ni) ratios are the most reliable parameters for determining the oxidation-reduction environment of hydrological bodies during sediment deposition by studying the paleooxic phases of Late Jurassic dark mudstones and sandstones in Northwest Europe.
The ratio of V/(V + Ni) is typically used to determine the degree of water stratification during sediment deposition, where ratios between 0.4 and 0.6 indicate weak stratification and oxygen-poor depositional environments, ratios between 0.6 and 0.8 suggest medium stratification and a sub-oxygenated environment, and ratios > 0.8 represent strong stratification and an oxygen-rich environment. In this study, the ratios of the samples ranged from 0.72 to 0.92, with an average ratio of 0.85, suggesting that the hydrological depositional environment was rich in oxygen and characterized by significant stratification (Figure 9). V and Cr display relatively similar characteristics; both are readily enriched in sediments in reducing environments and are water-soluble in oxidizing environments. As shown in Figure 9, we obtained relatively consistent results in our analysis of the redox state by applying these indexes. Overall, our analyses suggested an oxygen-rich water environment.

Paleosalinity Determination
The Sr and Ba content of rocks and their ratios can be used to determine the placement of the medium. An Sr content range of 200-300 ppm in sandstones represents a freshwater depositional environment, whereas 200-1000 ppm indicates a marine environment [47]. Moreover, Sr/Ba ratios can also be used to constrain the depositional environment of formation, with ratios > 1 indicating a marine environment and ratios < 1 suggesting a freshwater environment. In this study, Sr concentration of the samples ranged from 161 to 297 ppm, and Sr/Ba ratios ranged between 0.25 and 0.75, indicating that uranium-bearing sandstones in the study area were deposited in a freshwater environment (Figure 9).

Paleoclimate Conditions
Elemental geochemical signatures can be used to effectively reconstruct paleoclimatic conditions. For example, Sr and Cu content as well as Sr/Cu ratios are suitable parameters for reconstructing the paleoclimate. Previous studies have also shown that a Sr/Cu ratio between 1.3 and 5 indicates a wet climate, whereas ratios > 5 denote arid climates [49,50]. In this study, the Sr/Cu ratios of the samples were > 5, implying arid climatic conditions during sandstone deposition (Figure 9).
In addition to the Sr/Cu ratio, SiO 2 /Al 2 O 3 reflects the extent of chemical leaching and dissolution transport in host rocks. SiO 2 /Al 2 O 3 ratios < 4 denote wet environments and long transportation distances, and ratios > 4 indicate arid environments and short transportation distances [6,7]. In this study, we obtained SiO 2 /Al 2 O 3 ratios of the sandstone samples between 4.54 and 9.46, indicating arid climatic conditions.

Provenance Conditions
Our elemental geochemical analysis of the sandstone samples from the Sifangtai Formation indicated that the sandstone was predominately derived from the upper crust. Table 1 shows that TiO 2 concentration varied between 0.11 wt% and 0.69 wt%, with an average value of 0.28%; Al 2 O 3 content ranged from 8.71 wt% to 15.06 wt%, with an average of 11.15%; Ni content ranged from 1.73 ppm to 23.60 ppm, with an average of 5.25 ppm; and Zr content ranged from 44.00 ppm to 130 ppm, with an average of 71.61 ppm. On a graph of TiO 2 versus Al 2 O 3 , all the samples were plotted in the area between calc-alkaline granite and granodiorite (Figure 10a [51]). As illustrated by the TiO 2 -Zr, K 2 O-Rb and TiO 2 -Ni diagrams, all the samples were clustered in the acid volcanic rock region of the plots (Figure 10b-d [52][53][54]). Therefore, these results suggest a felsic source area for the Sifangtai Formation sandstones. direction, and the depositional environment transitioned from a partial oxidation environment in the north to a partial reduction environment in the south, with a gradual weakening in the hydrodynamic force. Accordingly, the Xiaoxing'an Ridge served as the predominant sediment source for the Sifangtai Formation, followed by the eastern region of Jihei and Zhangguangcailing.
In addition, the distribution map of water systems in the northern part of the Songliao Basin has shown that six major water systems are developed in the basin, among which the Nehe and Baiquan water systems are the main ones in the study area (Figure 12), which can provide a constant source of material for the sandstones of the Sifangtai Formation. The distribution of this water system is also mainly located in the tectonic position of the Xiaoxing'an and Zhangguangcailing areas.  To analyze the source rock properties of clastic rocks in the study area, Ti/Zr-La/Sc and La/Th-Hf trace diagrams were used for a direct analysis. The Ti/Zr ratios of the sandstone samples varied significantly from 14.29 to 34.78, with an average ratio of 22.29. The La/Sc ratio also showed a similar trend of significant variation (La/Sc = 2.73-15.08, average = 9.19). Moreover, the sandstone samples were plotted between felsic volcanic rocks and granites in the Ti/Zr-La/Sc diagram (Figure 10e [55]) and almost exclusively in the felsic volcanic rock region on the La/Th-Hf diagram (Figure 10f [55]). These results further confirmed an intermediate felsic source for the Sifangtai Formation sandstones.
In the present study, the patterns and characteristics of REEs in the sandstone samples were used to confirm the parent material of the uranium-bearing sandstone unit in the study area. Previous studies have demonstrated that felsic rocks typically exhibit high LREE/HREE ratios and negative Eu anomalies. Conversely, mafic rocks display low LREE/HREE ratios and almost no significant Eu anomalies. In our samples, the REE distribution pattern was right-inclined (Figure 6a), showing relatively enriched LREE and depleted HREE characteristics and weak Eu negative anomalies (δEu = 0.57-1.00). Therefore, these results further confirmed an intermediate felsic parent rock for the uraniumbearing sandstone unit in the study area.
To determine the specific intermediate felsic parent rock of uranium-bearing sandstone in the study area, the geochemical characteristics of the felsic rocks in the Xiaoxing'an and Zhangguangcailing areas around the Songliao Basin were analyzed ( Figure 11) [56][57][58][59][60][61][62]. The geochemical characteristics of these units were highly consistent with the REE partitioning pattern of the Sifangtai Formation sandstones in the study area. Combined with the location of the study area, we deduced that the Sifangtai Formation sediments in the northern region of the Songliao Basin most likely served as the source region for sediments in the Xiaoxing'an and Zhangguangcailing areas.  The study area is predominately located in the northern plunge and central downwarp of the Songliao Basin. Therefore, the Zhang Guangcai Ridge in the east, the Xiao Xing'an Ridge in the northeast, and the Daxing'an Ridge and surrounding areas in the west are all potential source areas for the Sifangtai Formation. During the Late Cretaceous, the Songliao Basin was controlled by the transformation of the Paleo-Asian tectonic system into the Paleopacific tectonic domain system. The basin experiences northwestward stress compression, whereas the southeast is relatively uplifted, and sedimentation and subsidence occurring in the center of the basin continue to shift in a northwestern direction [63,64]. Therefore, the possibility of the Daxing'an Ridge serving as the sediment source for the Sifangtai Formation is highly unlikely.
Furthermore, during the Early Cretaceous, magmatism was prominent in the Daxing'an Ridge (particularly between 110 and 150 Ma). Moreover, numerous studies have demonstrated a general lack of clastic rocks with ages between 110 Ma and 150 Ma in this area [65][66][67]. Therefore, based on the sedimentary-tectonic evolution and isotope dating results, we concluded that the Daxing'an Ridge does not serve as the source area for the Sifangtai Formation.
Based on the results of the age dating analysis of three detrital zircons obtained from the Sifangtai Formation sandstone, the following ages were obtained: 80-105 Ma, 175-240 Ma, and 1.8 Ga [68]. The zircon age of 1.8 Ga has been reported in boreholes within and around the basin. It is still mainly concentrated in the northern region of the Songliao Basin. The Sifangtai Formation was formed from a sequence of layers deposited from the late tectonic movements of the Nengjiang Formation, during which the southeastern region of the basin underwent denudation. Therefore, the 1.8 Ga zircon age indicates sources from the northern region of the basin. The 80-105 Ma age range corresponds with Late Cretaceous magmatism, which was widespread in eastern Jihei, and the 175-240 Ma age range corresponds with the Late Triassic-Middle Jurassic age of the Zhangguangcai Ridge in eastern Songliao [68]. In summary, the peak detrital zircon ages coincide with ages obtained for the Zhang Guangcai Ridge, eastern Jihei, and northern basin areas. Moreover, the sand body shows significant spreading characteristics in a north-south direction, and the depositional environment transitioned from a partial oxidation environment in the north to a partial reduction environment in the south, with a gradual weakening in the hydrodynamic force. Accordingly, the Xiaoxing'an Ridge served as the predominant sediment source for the Sifangtai Formation, followed by the eastern region of Jihei and Zhangguangcailing.
In addition, the distribution map of water systems in the northern part of the Songliao Basin has shown that six major water systems are developed in the basin, among which the Nehe and Baiquan water systems are the main ones in the study area (Figure 12), which can provide a constant source of material for the sandstones of the Sifangtai Formation. The distribution of this water system is also mainly located in the tectonic position of the Xiaoxing'an and Zhangguangcailing areas.

Tectonic Background
The siliciclastic rocks in the study area were derived from different tectonic settings and display terrain-specific characteristics [51][52][53][54][55]. Numerous tectonic discrimination diagrams for sedimentary basins have been proposed based on major and trace element compositions [69][70][71]. As illustrated in Figure 13, the sandstone samples clustered in the diagram in the active continental margin region, and the tectonic setting was relatively similar, reflecting a strong subduction plate regime. Furthermore, REEs are often used to determine the tectonic properties of clastic rocks. McLennan [21] used PAAS for standardization and found different values of δCe in different tectonic contexts, where a significantly negative δCe anomaly denoted a spreading oceanic ridge, a moderate negative δCe anomaly indicated an ocean basin, and a weak negative δCe anomaly indicated a continental margin zone. The δCe of the sandstone samples obtained from the Sifangtai Formation ranged from 0.85 to 1.13, with an average of 0.95, suggesting that the depositional environment of this formation was an active continental margin environment.
This finding was further supported by the following geological observations of the study area: In terms of tectonic position, the northern region of the Songliao Basin crosses the Xingmeng Orogenic Belt and represents the superposition of the Pacific tectonic and the ancient Asian Oceanic tectonic system. Since the Mesozoic, it has undergone significant tectonic deformation and orogenesis, and the rocks comprising the basin were formed in a subducting plate tectonic setting, whereby the Pacific Ocean Plate subducted in a southward direction [72,73].
In addition, the Songliao Basin is a faulted Meso-Cenozoic basin, and previous studies have reported a wide distribution of felsic rocks in the Xiaoxing'anling and Zhangguangcailing areas, including the fine-grained synogranite of the Dong'an Gold Mine in the Xiaoxing'an Mountains, which were formed 184 Ma in an ancient active continental margin setting of the subducting Pacific Plate [74]. Moreover, based on the study conducted by Ge et al. [75], combined with previously published geochronological Furthermore, REEs are often used to determine the tectonic properties of clastic rocks. McLennan [21] used PAAS for standardization and found different values of δCe in different tectonic contexts, where a significantly negative δCe anomaly denoted a spreading oceanic ridge, a moderate negative δCe anomaly indicated an ocean basin, and a weak negative δCe anomaly indicated a continental margin zone. The δCe of the sandstone samples obtained from the Sifangtai Formation ranged from 0.85 to 1.13, with an average of 0.95, suggesting that the depositional environment of this formation was an active continental margin environment.
This finding was further supported by the following geological observations of the study area: In terms of tectonic position, the northern region of the Songliao Basin crosses the Xingmeng Orogenic Belt and represents the superposition of the Pacific tectonic and the ancient Asian Oceanic tectonic system. Since the Mesozoic, it has undergone significant tectonic deformation and orogenesis, and the rocks comprising the basin were formed in a subducting plate tectonic setting, whereby the Pacific Ocean Plate subducted in a southward direction [72,73].
In addition, the Songliao Basin is a faulted Meso-Cenozoic basin, and previous studies have reported a wide distribution of felsic rocks in the Xiaoxing'anling and Zhangguang-cailing areas, including the fine-grained synogranite of the Dong'an Gold Mine in the Xiaoxing'an Mountains, which were formed 184 Ma in an ancient active continental margin setting of the subducting Pacific Plate [74]. Moreover, based on the study conducted by Ge et al. [75], combined with previously published geochronological and geochemical data, we inferred that the Xiao Hinggan-Zhangguangcai Mountains formed in an active continental margin setting during the Late Paleozoic to Mesozoic. Finally, the Early Jurassic granite of the southern Zhangguangcai Mountains formed in a post-collisional tectonic setting, representing an extensional episode in the collisional event, and the geotectonic setting was an extensional tectonic setting after subduction of the Pacific Plate [76].
Overall, our results indicate that the host rocks of the Sifangtai Formation are predominately felsic, and this observation is consistent with rocks in the northern Xiaoxing'an Ridge and Zhangguangcailing regions. Combining previous research results and the elemental geochemical characteristics of the sandstone samples obtained in the present study, we concluded that the sandstones of the Sifangtai Formation formed in an active continental margin environment closely related to Pacific subduction.

Uranium Source Conditions
As mentioned, the source area of the Sifangtai Formation sandstone was determined as an active continental margin tectonic environment, and the parent rocks were felsic and sourced from the Xiaoxing'an and Zhangguangcailing areas. The average uranium content of granite from these areas ranged from 7.02 to 5.93 ppm, and the uranium leaching rate of the granites (percentage of U by weight leached from granite since its formation) was 22.13 wt%, representing a fast leaching rate. The total amount of activated uranium migrating into the basin from the eroded source area in the northern region of the Songliao Basin was 4.32 million tons, indicating that the northern region of the basin is enriched with uranium sources ( [28] Table 2). Table 3 shows the average mineral contents of granites from the Zhang Guangcailing-Xiaoxing'an region. Compared with the mineral content of sandstones from the Sifangtai Formation in the study area, these data indicate that the mineral content of orthogranite and diorite is more similar to that of the Sifangtai Formation sandstone. The material source of the sandstone was most likely Triassic-Jurassic orthogranite and diorite granite [74][75][76]. Table 2. Calculation of the total amount of activated uranium remitted to the basin from uranium-bearing granite alteration source areas in the northern part of the Songliao Basin (after [28]

Tectonic Conditions and Uranium Mineralization
The basement faults in the northern region of the Songliao Basin can be categorized into NE-and NW-trending faults, which control the overall evolution of the basin. Uranium mineralization was concentrated at the intersection of the two fracture sets. Overall, six uranium enrichment zones could be identified ( Figure 14). Moreover, three primary areas of tectonic influence were determined for the study area: First, deep basement fractures are also located in this area, the base of the Cretaceous formations are developed, and there are also features of tectonic uranium mineralization and anomalous sedimentary layer development [28]. Second, the northern region of the Songliao Basin contains hydrocarbon deposits of the Shahezi and Yingcheng Formations, as well as several oil-gas fields, such as the Jiaoqiao, Erzhan, Pingyang, and Alaxin fields. Tectonic action widening of the ascent pathway for deep reducing (hydrocarbon) fluids. The fracture crosses the Upper Cretaceous and becomes a channel for the upward transport of deep reducing fluids, such as those containing oil-gas and CO 2 , forming oil-gas reservoirs, generating extensive reduction alteration in the Sifangtai Formation, and increasing the reduction capacity of the Sifangtai Formation. The sandstones of the Sifangtai Formation in the vicinity of the oil-gas field exhibit low organic carbon and sulfur content and high acidolytic hydrocarbon content, further indicating that oil and gas are the main reducing substances. Uranium mineralization is predominately developed in sandstones near the oil-gas fields, and the ore types are mainly medium-and fine-grained sandstones, with the primary uranium minerals being bituminous uranium ore and uraninite [28]. Finally, tectonic conditions may alter the local groundwater dynamics system for infiltration or discharge zones, complementing and improving the replenishment-runoff-discharge system and promoting the development of interstratified oxidation zones and uranium mineralization [28].
In conclusion, the tectonics of the Sifangtai Formation may promote an improved stratigraphic structure of this formation, with large sand thicknesses and sufficient reduction conditions. Moreover, late-stage tectonic uplift denudation resulted in late-stage modification of the target layer, allowing it to more readily form oxidation zones [77,78]. system and promoting the development of interstratified oxidation zones and uranium mineralization [28].
In conclusion, the tectonics of the Sifangtai Formation may promote an improved stratigraphic structure of this formation, with large sand thicknesses and sufficient reduction conditions. Moreover, late-stage tectonic uplift denudation resulted in latestage modification of the target layer, allowing it to more readily form oxidation zones [77,78].

Sand Body Development Conditions
The sand body of the Sifangtai Formation is also markedly developed in the study area, with a relatively shallow burial depth, and it consists of braided river sediment (consistent with the previous elemental geochemical characteristics determined for the sandstone samples). Moreover, the sandstone material has the trend of gradually increasing, and the sand body is characterized by significant thickness and wide spreading. The cumulative thickness of the sand body is 80-100 m (Figure 3c).
The sand body is characterized by loose rocks, and the gap-filling material is mainly clay-powdered, with an approximate content of 1%. Sandstone of the Sifangtai Formation in the vicinity of the oil-gas field is visible as flakes and lamellar charred plant debris, with a large amount of pyrite absorbed on the surface.

Oxidation Zone Development Conditions
An oxidation zone exists in the Sifangtai Formation, and it is developed in the sand body formed by braided river deposition. The oxidation zone can be divided vertically into fully, medium, and weak oxidation zones. The fully oxidized zone in the sand body is yellow in color and does not contain evident carbon residue. Moreover, the sand body in this oxidation zone is loose, and the GR curve reflects a low uranium content. In the medium oxidized zone, the sand body is mainly grayish yellow and light yellow in color, with a thin layer of lenticular sand body and a small amount of charcoal debris. The GR curve for this zone indicates a slight increase in uranium content. The weakly oxidized zone is characterized by a gray and yellow interlayered sand body with visible organic matter ( Figure 15). The thickness of the oxidized sand body exceeds 50 m and extends over 50 km, with high oxidation intensity.
in this oxidation zone is loose, and the GR curve reflects a low uranium content. In the medium oxidized zone, the sand body is mainly grayish yellow and light yellow in color, with a thin layer of lenticular sand body and a small amount of charcoal debris. The GR curve for this zone indicates a slight increase in uranium content. The weakly oxidized zone is characterized by a gray and yellow interlayered sand body with visible organic matter ( Figure 15). The thickness of the oxidized sand body exceeds 50 m and extends over 50 km, with high oxidation intensity.

Paleoclimatic Conditions
The geochemical characteristics of the major elements indicate that the sandstones of the Sifangtai Formation were formed under semi-arid climatic conditions, and the Late Cretaceous Sifangtai Formation typically comprises gray medium-fine sandstones interspersed with purple-red mudstones, with visible calcareous nodules.
The arid paleoclimatic environment provided the optimal conditions for the formation of uranium-oxygenated water and the pre-enrichment and transport of uranium elements.

Uranium Mineralization Development
Recent exploration in the study area has indicated that uranium mineralization is more developed in the Sifangtai Formation in the northern region of the Songliao Basin.
The uranium mineralization extends steadily over 3 km (Figure 16), and the mineralized lithologies are mainly gray medium sandstone and fine sandstone, which are produced in the sandstone at the bottom of Sifangtai Formation.
In terms of composition, organic carbon and sulfur content is low, while the acidolytic hydrocarbon content is high, indicating that oil and gas are the main reducing substances. The oxidized sandstone is mainly light yellow in color and remains in the gray sandstone in the form of dipping, agglomerates, and stripes (Table 3). fracture development in the basin, a low temperature hydrocarbon transport of uranium through the faults into enriched portions of the formation that acted as reducing agents for the concentration of the uranium, and (3) finally, the sandstone is enriched in mineralization at the transition site ( Figure 17).
In summary, the Sifangtai Formation in the northern region of the Songliao Basin is rich in uranium sources, tectonic conditions, ore-bearing favorable sand bodies, postgeneration alteration, and paleoclimatic conditions, and it has considerable potential.  The uranium mineralization sequence of events is as follows; (1) firstly, sandstones begin with the fluvial deposition of the arkose, then burial and lithification, (2) secondly, the uranium-bearing oxygenated water into some of the sandstone, nevertheless, the fracture development in the basin, a low temperature hydrocarbon transport of uranium through the faults into enriched portions of the formation that acted as reducing agents for the concentration of the uranium, and (3) finally, the sandstone is enriched in mineralization at the transition site ( Figure 17).

Conclusions
Based on the geochemistry of the sandstones in the northern Songliao Basin, in combination with the results of previous studies conducted in the area, we reached the following conclusions: 1. The ICV value ranged from 0.60 to 1.03, with an average of 0.69. The revised average CIA value was 54.35. It indicated that the rocks of the Sifangtai Formation might have undergone weak chemical weathering and the compositional homogeneity and In summary, the Sifangtai Formation in the northern region of the Songliao Basin is rich in uranium sources, tectonic conditions, ore-bearing favorable sand bodies, post-generation alteration, and paleoclimatic conditions, and it has considerable potential.