Geochemical Characteristics and Constraints on Provenance, Tectonic Setting, and Paleoweathering of Middle Jurassic Zhiluo Formation Sandstones in the Northwest Ordos Basin, North-Central China

: To further explore the uranium-bearing prospects of the Zhiluo Formation, the petrography, major elements, trace elements and rare earth elements of Zhiluo sandstone samples collected from four boreholes were analyzed in this research to determine the provenance, tectonic setting and paleoweathering of the formation. The results of the analysis reveal that the Zhiluo Formation sandstone comprises primarily feldspar sandstone, with quartz, feldspar, and mica as the main mineral components. The rare earth elements are mainly characterized by enrichment in light rare earth elements and loss of heavy rare earth elements. The ratio of light to heavy rare earth elements (LREEs/HREEs) is 5.55–7.79, with an average of 6.33. The value of (La/Yb) CN is 12.96–22.33, with an average value of 17.41, indicating obvious fractionation of LREEs and HREEs. The chemical index of alteration (CIA) value of this sandstone is 56.30–63.04, with an average of 59.75, which indicates that the parent rock experienced weak chemical weathering in a dry climate. The discrimination diagrams of the source area and parent rock show that the source area of the Zhiluo sandstones had a mixed intermediate-felsic composition, and that the main parent rocks were andesite and granite. The tectonic setting discrimination diagram indicates that the tectonic setting of the source area was the passive margin. Thus, the provenance of the sandstone of the Zhiluo Formation is proposed to be the volcanic-sedimentary rock series developed on the northern margin of the Ordos Basin.


Introduction
Provenance analysis is particularly significant for elucidating the genesis and distribution features of sedimentary formations in a basin, as well as for reconstructing the tectonic setting of the source area, the sediment transport route, and the filling history of the sedimentary basin [1][2][3][4][5]. The clastic and geochemical compositions of sandstones are largely controlled by the provenance area, and these compositions also retain relevant information about the parent rock and tectonic setting [6][7][8][9][10][11]. In recent years, mineralogical, petrographical, and geochemical methods have been extensively employed to identify the provenance, tectonic setting, and paleoweathering of provenance areas, with remarkable results offering new evidence for understanding the evolution of the basin [12][13][14][15].
Uranium is a strategic resource closely related to national security and economic development. Uranium resources are an important guarantee for the sustainable development of nuclear energy and national defense construction in China. In order to ensure sustainable economic development and ecological civilization construction in China, nuclear energy, as one of the clean energy sources, is of great significance for adjusting and optimizing the energy structure of China, improving the ecological environment and reducing CO 2 and S emissions. Sandstone-type uranium deposits play an important role in the supply of uranium resources in China. They are characterized by shallow depth, large-scale reserves, low cost of mining and environmental friendliness [16]. The Ordos Basin is one of the important uranium-bearing basins in China, and its northern part is the major ore concentration area of sandstone-type uranium deposits. In recent years, many sandstonetype uranium deposits have been reported from the northern Ordos Basin; Zaohuohao, Nalinggou, Daying, Hangjinqi and Bayinqinggeli uranium deposits have been gradually discovered. The Bayinqinggeli region, located in the northern Ordos Basin, is one of the primary areas for uranium exploration in the basin. Industrialized uranium ore bodies have been discovered in the Zhiluo Formation in Bayinqinggeli. Previous studies in this area have mainly focused on geological structure, sedimentary characteristics [17], fluid characteristics [18], metallogenic models [19] and ore-controlling factors [16] of sandstone-type uranium deposits, but research on the provenance of the Zhiluo Formation sandstone and tectonic setting of the provenance area is relatively lacking [20][21][22][23][24]. Provenance research on uranium-bearing formations can provide certain constraints for determining the source of uranium minerals, which has become a current research hotspot and focus [22,[25][26][27]]. The provenance system can provide an accurate record of information about regional tectonic evolution, and research on the characteristics of the provenance area can further infer the overall sedimentary tectonic setting in the area and then guide the next step of uranium exploration work.
This research discusses mainly the provenance, tectonic setting, paleoweathering and paleoclimatic conditions of the Zhiluo Formation sandstone through a comprehensive study of the petrographic and geological characteristics of this lithostratigraphic unit in the Bayinqinggeli area. This study has important theoretical and practical significance for confirming the provenance of uranium-bearing strata, pointing out the direction for further ore prospecting in this area, and reconstructing the paleoclimatic and paleogeographic evolution of the Zhiluo Formation in the northern Ordos Basin.

Geological Setting
The Ordos Basin is a polycyclic superimposed basin that is located in the west of the North China Platform, it has a rectangular shape, extending from north to south. It can be divided into six tectonic units that include the Yimeng uplift, Yishan slope, Jinxi flexural fold belt, Tianhuan depression, western margin thrust belt and Weibei uplift [18]. The overall tectonic pattern of the basin is tectonic belts with different characteristics developed along the basin margin, while the internal tectonic of the basin is relatively undeveloped. The Ordos Basin is surrounded by orogenic belts, which include the Yinshan orogenic belt to the north, the Qinling orogenic belt to the south, Helan Mountain to the west and Taihang Mountain to the east (Figure 1a) [19]. The basin began to deposit stably in the Mesoproterozoic and progressively split from the North China Basin, which evolved into a stable cratonic basin, and subsequently continued to deposited stably. The basin contains two basement units, one of them is direct, and the other is indirect, with the characteristics of "double" basement. Archean-Palaeoproterozoic metamorphic rock series form an indirect basement unit, while the direct basement unit is the platform sedimentary cover of Paleoproterozoic-Mesozoic Triassic. The Triassic (T), Jurassic (J), Lower Cretaceous (K 1 ), Paleogene (E 3 ), Neogene (N 2 ) and Quaternary (Q) sedimentary bodies span the basin, where the Jurassic and Lower Cretaceous are the dominant sedimentary units.
The Bayinqinggeli area is located in the northern Ordos Basin, northwest of the Daying mining area. The sand body of the Zhiluo Formation is well developed and has good uranium metallogenic conditions [21]. The Zhiluo Formation can be divided into upper and lower members, the upper member is dominated by flood deposits, and the sand body is relatively undeveloped, formed by a set of primary red clastic; the lower member is a developed sand body of fluvial facie and dominated by primary grey clastic rocks. In the study region, the major exposed strata are Quaternary (Q) and Lower Cretaceous (K1), while a small area of Middle Triassic (T2) is exposed in the east (Figure 1b).
The Bayinqinggeli area is located in the northern Ordos Basin, northwest of the Daying mining area. The sand body of the Zhiluo Formation is well developed and has good uranium metallogenic conditions [21]. The Zhiluo Formation can be divided into upper and lower members, the upper member is dominated by flood deposits, and the sand body is relatively undeveloped, formed by a set of primary red clastic; the lower member is a developed sand body of fluvial facie and dominated by primary grey clastic rocks. In the study region, the major exposed strata are Quaternary (Q) and Lower Cretaceous (K1), while a small area of Middle Triassic (T2) is exposed in the east ( Figure  1b).

Materials and Methods
The 21 Zhiluo Formation sandstone samples in this research were collected from the ZKB31-7, ZKB55-32, ZKB55-24 and ZKB87-16 boreholes in the Bayinqinggeli area, northwestern Ordos Basin ( Figure 2). All samples are medium and fine sandstone. When sampling, sandstone samples with low radioactivity were selected to ensure the accuracy and representativeness of the analysis, and the drilling fluid was removed from the sandstone surface. The major, trace and rare earth element (REE) analyses of the samples were completed in the Analytical Laboratory of Beijing Research Institute of Uranium Geology (Beijing, China).
Before chemical analysis, the sample was crushed in an agate mortar to 200 mesh. The analysis and determination method was as follows: the major element analysis was determined with an XRF-1800 fluorescence spectrometer, the accuracy of which was better than 1%. The test voltage was 50 kV, the current was 50 mA, and the loss on ignition (LOI) was estimated by firing the dried sample at 1000 • C for two hours and then measuring the weight loss.
Trace elements and rare earth elements (REEs) were tested by inductively coupled plasma-mass spectrometry (ICP-MS) (PerkinElmer, Waltham, MA, USA). The accuracy was better than 5%. We accurately weighed out 25 mg powder samples, placed them into the inner tank of the closed sample dissolver, added hydrofluoric (HF) and nitric acid (HNO 3 ) and sealed the tank. The sample dissolver was placed in the oven and heated at 185 • C for 2 h. After cooling, each sample was heated and dried on an electric heating plate, and HNO 3 was then repeatedly added to assist in drying. Finally, HNO 3 was added, and the sample was again sealed, placed in an oven, heated at 130 • C for 3 h, cooled, diluted with water and analyzed by ICP-MS.
Paleoweathering of the source area was qualitatively evaluated by the following chemical indices; chemical index of alteration (CIA) = Al 2 O 3 /(Al 2 O 3 + CaO* + Na 2 O + K 2 O) × 100, the index of compositional variability (ICV) = (Fe 2 O 3 + K 2 O + Na 2 O + CaO*+ MgO + MnO + TiO 2 )/Al 2 O 3 [19] and the plagioclase index of alteration (PIA) = (Al 2 O 3 − K 2 O)/(Al 2 O 3 + CaO* + Na 2 O − K 2 O) × 100 [20]. In the formulas above, all parameters were calculated using the percentage of oxide, except for the ICV, as the calculation of ICV is based on the mole percentages of all oxides, and CaO* refers to only the mole percentage of CaO in silicate minerals. The CaO content in the samples varied widely due to the instability of silicate minerals [11,20]. Calculating the value of CIA with the total CaO could lead to false conclusions. Therefore, the correction method of CaO* was as follows: first, the CaO in apatite is removed by P 2 O 5 , that is, CaO** = mol CaO − (10/3 × mol P 2 O 5 ), resulting in CaO**, and then mol Na 2 O is calculated and compared. If CaO** > Na 2 O, then mol CaO* = mol Na 2 O, otherwise, mol CaO* = mol CaO [10].
The discrimination diagrams based on major elements were used to distinguish the provenance of the sandstone samples from the Zhiluo Formation. In addition, the discrimination diagrams based on major and trace elements were used to assess the tectonic setting of the sandstone samples from the Zhiluo Formation.

Petrography of Sandstones
A petrological identification of representative sandstone samples from the Zhiluo Formation was carried out. The Zhiluo Formation sandstones are feldspathic litharenite and lithic arkose, mainly composed of quartz, feldspar, debris, and mica. The quartz is mono-crystalline quartz, which is brilliant with equant forms and moderately undulous extinction. The majority of the quartz grains have a secondary growth along the edges. The feldspar crystals include plagioclase, orthoclase, and microplagioclase. The mica includes biotite and muscovite, which is striped or angular, but the majority of the mica is biotite (Figure 3).

Petrography of Sandstones
A petrological identification of representative sandstone samples from the Zhiluo Formation was carried out. The Zhiluo Formation sandstones are feldspathic litharenite and lithic arkose, mainly composed of quartz, feldspar, debris, and mica. The quartz is mono-crystalline quartz, which is brilliant with equant forms and moderately undulous extinction. The majority of the quartz grains have a secondary growth along the edges. The feldspar crystals include plagioclase, orthoclase, and microplagioclase. The mica includes biotite and muscovite, which is striped or angular, but the majority of the mica is biotite (Figure 3).

Major Element Geochemistry
Major element content data for the sandstone samples are listed in Table  The concentrations of major elements are often used for the geochemical classification of sandstone [29,30]. The SiO 2 content and SiO 2 /Al 2 O 3 ratio are common geochemical standards for determining the maturity of sediments and reflect the contents of quartz, clay and feldspar in sedimentary rocks. In addition, the sum of Na 2 O + K 2 O reflects the feldspar content and is used to determine the chemical maturity of sediments [31]. The Zhiluo Formation sandstone samples have the characteristics of low Al 2 O 3 /SiO 2 ratio, low K 2 O/Na 2 O ratio and high K 2 O/Na 2 O ratio. The bivariate diagram of log (Fe 2 O 3 /K 2 O) versus log (SiO 2 /Al 2 O 3 ) [32] indicates that the majority of the samples plot in the arkose field. Additionally, there are few samples distributed within the wacke area ( Figure 4). These results show that the maturity of the Zhiluo Formation sandstones in the Bayinqinggeli area is low. The concentrations of major elements are often used for the geochemical classification of sandstone [29,30]. The SiO2 content and SiO2/Al2O3 ratio are common geochemical standards for determining the maturity of sediments and reflect the contents of quartz, clay and feldspar in sedimentary rocks. In addition, the sum of Na2O + K2O reflects the feldspar content and is used to determine the chemical maturity of sediments [31]. The Zhiluo Formation sandstone samples have the characteristics of low Al2O3/SiO2 ratio, low K2O/Na2O ratio and high K2O/Na2O ratio. The bivariate diagram of log (Fe2O3/K2O) versus log (SiO2/Al2O3) [32] indicates that the majority of the samples plot in the arkose field. Additionally, there are few samples distributed within the wacke area ( Figure 4). These results show that the maturity of the Zhiluo Formation sandstones in the Bayinqinggeli area is low.

Trace Element Geochemistry
The trace element content data for the sandstones are presented in Table 2. In general, the trace element concentrations of sandstones have wide ranges. Some high-field strength elements are difficult to dissolve and have strong stability during transportation and deposition, such as Hf (0.96~3.09 ppm, average 1.40 ppm), Zr (30.5~160 ppm, average 44.48 ppm) and Ni (5.38~21.00 ppm, average 10.00 ppm). Therefore, these elements are often used to indicate the geochemical characteristics of the parent rock and the characteristics of the source area.
The trace elements are normalized by the upper continental crust (UCC) ( Figure 5). The majority of trace elements (Pb, Sc, Cr, Zn, Th, Zr, Hf, Cu, Co and Ni) are lower in the sandstone than in the UCC, but Ba and U are enriched, and Rb and Sr are comparable. Except for three sandstone samples in which V is greater than that in the UCC, the remainder are lower. The Sc/Cr, Co/Th, Cr/Th, Th/Sc and Zr/Sc ratios are presented in Table 2.

Trace Element Geochemistry
The trace element content data for the sandstones are presented in Table 2. In general, the trace element concentrations of sandstones have wide ranges. Some high-field strength elements are difficult to dissolve and have strong stability during transportation and deposition, such as Hf (0.96~3.09 ppm, average 1.40 ppm), Zr (30.5~160 ppm, average 44.48 ppm) and Ni (5.38~21.00 ppm, average 10.00 ppm). Therefore, these elements are often used to indicate the geochemical characteristics of the parent rock and the characteristics of the source area. The trace elements are normalized by the upper continental crust (UCC) ( Figure 5). The majority of trace elements (Pb, Sc, Cr, Zn, Th, Zr, Hf, Cu, Co and Ni) are lower in the sandstone than in the UCC, but Ba and U are enriched, and Rb and Sr are comparable. Except for three sandstone samples in which V is greater than that in the UCC, the remainder are lower. The Sc/Cr, Co/Th, Cr/Th, Th/Sc and Zr/Sc ratios are presented in Table 2.

Rare Earth Element (REE) Geochemistry
The concentrations and ratios of the REEs in the sandstone samples are shown in Table 3. The REE distribution of the Zhiluo Formation sandstone is generally inclined to the right, showing LREE enrichment and HREE loss ( Figure 6). The uniform chondrite normalized pattern suggests that these sandstones may have the same provenance, sedimentary environment and tectonic setting. The Eu/Eu* and Ce/Ce* ratios of the sandstones are calculated using the following formulas: Eu/Eu* = 2 × EuCN/(SmCN + GdCN) and Ce/Ce* = 2 × CeCN/ (LaCN + PrCN). Table 3 shows the normalized ratios (La/Yb)CN, (La/Sm)CN and (Gd/Yb)CN.
The total amount of rare earth elements (ΣREE) ranges from 93 ppm to 212.82 ppm (average 128.24 ppm), the LREE contents vary between 82.42 ppm and 182.5 ppm (average 110.65 ppm), and the HREE contents vary between 10.58 ppm and 30.32 ppm (average 17.59 ppm). The LREE/HREE ratios vary from 5.33 to 7.79 (average 6.33). The Eu/Eu* ratio shows positive anomalies and varies from 0.81 to 1.42 (average 1.12). The Ce/Ce* ratio varies from 0.83 to 1.03 (average 0.89). The ratio of (La/Yb)CN is between 12.96 and 22.33, with an average of 17.41, which fully illustrates the obvious fractionation between the LREEs and HREEs. The (La/Sm)CN results indicated that the LREE fractionation ratio is between 4.75 and 6.24, with an average of 5.15; however, the (Gd/Yb)CN ratio, which reflects the fractionation ratio of HREE, is between 1.66 and 2.66, with an average of 2.12.

Rare Earth Element (REE) Geochemistry
The concentrations and ratios of the REEs in the sandstone samples are shown in Table 3. The REE distribution of the Zhiluo Formation sandstone is generally inclined to the right, showing LREE enrichment and HREE loss ( Figure 6). The uniform chondrite normalized pattern suggests that these sandstones may have the same provenance, sedimentary environment and tectonic setting. The Eu/Eu* and Ce/Ce* ratios of the sandstones are calculated using the following formulas: Eu/Eu* = 2 × Eu CN /(Sm CN + Gd CN ) and Ce/Ce* = 2 × Ce CN /(La CN + Pr CN ). Table 3 shows the normalized ratios (La/Yb) CN , (La/Sm) CN and (Gd/Yb) CN .

Provenance from Petrography
The Zhiluo Formation sandstone is rich in quartz and feldspar, indicating that it is not derived from basic rock and that its source area hosted feldspar source rock. Some quartz particles exhibit secondary enlargement edge phenomena, which are caused by the recycling of sedimentary quartz throughout diagenesis. Chloritization can be seen in some plagioclases. There are many types of debris, mainly igneous rock debris, metamorphic rock debris and sedimentary rock debris, in this rock, indicating corresponding provenance of the deep igneous rocks, ancient sediments and metamorphic rock in the northern part of the basin. The heavy minerals include a small amount of epidote, indicating that they are mainly derived from metamorphic rocks; in addition, zircon, biotite and tourmaline heavy mineral assemblages show that the parent rocks may have been moderately acidic magmatic rocks and metamorphic rocks [35]. In summary, the parent rock of the Zhiluo Formation sandstone is feldspathic sandstone from an intermediate-acidic source area.

Provenance and Source Rock from Geochemistry
The provenance and source rocks of clastic sedimentary rocks can be identified by using different discrimination diagrams of major elements and trace elements [2,36,37]. Proposed by Roser and Korsc, the DF1-DF2 discrimination diagram [38] (Figure 7) is based on the content of major elements in 248 groups of sandstone and mudstone, which effectively divides the source area into mafic igneous provenance, intermediate igneous provenance, felsic igneous provenance and quartzose sedimentary provenance. The majority of the studied sandstone samples plot in the quartzose sedimentary provenance, and one sample falls within the felsic igneous provenance. The total amount of rare earth elements (ΣREE) ranges from 93 ppm to 212.82 ppm (average 128.24 ppm), the LREE contents vary between 82.42 ppm and 182.5 ppm (average 110.65 ppm), and the HREE contents vary between 10.58 ppm and 30.32 ppm (average 17.59 ppm). The LREE/HREE ratios vary from 5.33 to 7.79 (average 6.33). The Eu/Eu* ratio shows positive anomalies and varies from 0.81 to 1.42 (average 1.12). The Ce/Ce* ratio varies from 0.83 to 1.03 (average 0.89). The ratio of (La/Yb) CN is between 12.96 and 22.33, with an average of 17.41, which fully illustrates the obvious fractionation between the LREEs and HREEs. The (La/Sm) CN results indicated that the LREE fractionation ratio is between 4.75 and 6.24, with an average of 5.15; however, the (Gd/Yb) CN ratio, which reflects the fractionation ratio of HREE, is between 1.66 and 2.66, with an average of 2.12.

Provenance from Petrography
The Zhiluo Formation sandstone is rich in quartz and feldspar, indicating that it is not derived from basic rock and that its source area hosted feldspar source rock. Some quartz particles exhibit secondary enlargement edge phenomena, which are caused by the recycling of sedimentary quartz throughout diagenesis. Chloritization can be seen in some plagioclases. There are many types of debris, mainly igneous rock debris, metamorphic rock debris and sedimentary rock debris, in this rock, indicating corresponding provenance of the deep igneous rocks, ancient sediments and metamorphic rock in the northern part of the basin. The heavy minerals include a small amount of epidote, indicating that they are mainly derived from metamorphic rocks; in addition, zircon, biotite and tourmaline heavy mineral assemblages show that the parent rocks may have been moderately acidic magmatic rocks and metamorphic rocks [35]. In summary, the parent rock of the Zhiluo Formation sandstone is feldspathic sandstone from an intermediate-acidic source area.

Provenance and Source Rock from Geochemistry
The provenance and source rocks of clastic sedimentary rocks can be identified by using different discrimination diagrams of major elements and trace elements [2,36,37]. Proposed by Roser and Korsc, the DF1-DF2 discrimination diagram [38] (Figure 7) is based on the content of major elements in 248 groups of sandstone and mudstone, which effectively divides the source area into mafic igneous provenance, intermediate igneous provenance, felsic igneous provenance and quartzose sedimentary provenance. The majority of the studied sandstone samples plot in the quartzose sedimentary provenance, and one sample falls within the felsic igneous provenance. At the same time, because Al, Ti and Zr oxides and hydroxides have low solubility in low-temperature aqueous solutions, they may be termed immobile elements; thus, the Al 2 O 3 /TiO 2 ratio of the samples is very close to that of their source rocks. For the intermediate igneous provenance, this ratio is between 8 and 21; when the provenance is felsic, the ratio is more than 21; the ratio < 8 indicates basic source rock [38]. The   [19] of the Zhiluo Formation sandstones: UCC [39], granite [40], granodiorite [41] and tonalite [41].
The K 2 O versus Rb diagram and TiO 2 versus Ni diagram (Figure 9) can be used to distinguish the composition of the source area [42,43]. In the K 2 O versus Rb diagram (Figure 9a  Trace elements such as Ti, Hf, Sc, Th, Zr, Y and REEs are critical for interpreting the origin and composition of the source area due to their minimal possibility of migrating during the post-depositional process [44]. Hf versus La/Th [42], La/Sc versus Co/Th [10], Cr/Th versus Th/Sc [45] and Y/Ni versus Cr/V [10] discrimination diagrams ( Figure 10) were used to analyze the parent rock types of sandstones. The studied sandstones, according to these diagrams, are the felsic igneous source rocks and intermediate igneous rocks. Plotting on the Ni-V-Th*10 ternary diagram [46] (Figure 11) shows that the Zhiluo Formation sandstones fall near the V-Th*10 line, reflecting that the provenance of sandstone samples are felsic igneous rocks.
Comparing the Eu/Eu*, Cr/Th, Th/Co, Th/Sc, La/Co, Th/Sc and La/Sc ratios of the Zhiluo Formation sandstones with the UCC and the range of sediments from felsic sources and mafic sources [2,47,48] (Table 4) shows that the sandstones are derived from felsic rocks. Plotting on the Ni-V-Th*10 ternary diagram [46] (Figure 11) shows that the Zhiluo Formation sandstones fall near the V-Th*10 line, reflecting that the provenance of sandstone samples are felsic igneous rocks. Figure 11. Ni-V-Th*10 ternary diagram for provenance discrimination of the Zhiluo Formation sandstones [46].
Comparing the Eu/Eu*, Cr/Th, Th/Co, Th/Sc, La/Co, Th/Sc and La/Sc ratios of the Zhiluo Formation sandstones with the UCC and the range of sediments from felsic sources and mafic sources [2,47,48] (Table 4) shows that the sandstones are derived from felsic rocks. Previous studies [25] on zircon ages show that the Zhiluo Formation sandstone in the Bayinqinggeli region are mainly concentrated in Paleozoic. Due to the influence of the activity of the Yinshan orogenic belt in the north, multiple volcanic eruptions and largescale intermediate-acid magmatic intrusions occurred, and a large area of Paleozoic compound intermediate-acid intrusive rocks developed. Combining this information with the discrimination diagrams of the major elements, trace elements and rare earth elements in the sandstone of the Zhiluo Formation, it is speculated that the parent rock of Figure 11. Ni-V-Th*10 ternary diagram for provenance discrimination of the Zhiluo Formation sandstones [46].

Tectonic Setting
The geochemical characteristics of siliciclastic sediments can reflect not only the characteristics of the source area and the geochemical features of the parent rock but also the tectonic settings of sedimentary basins. Sedimentary rocks from different tectonic settings have obvious differences in the composition and content of elements [44,51].
The discrimination diagrams based on major elements were proposed by Maynard, Roser and Korsch, and they are usually used in sandstones to discriminate source rocks.  [52] diagrams (Figure 12a,b) show that all sandstone samples of the Zhiluo Formation fall within the passive margin field. Furthermore, Bhatia [53] found that SiO 2 , K 2 O/Na 2 O, SiO 2 /Al 2 O 3 , Al 2 O 3 /(CaO + Na 2 O) and Fe 2 O 3 T + MgO are the most representative parameters in source tectonic setting discrimination; thus, he proposed the Fe 2 O 3 T + MgO versus Al 2 O 3 /(CaO + Na 2 O) diagram [53] (Figure 12c), which shows that the majority of the sandstone samples are distributed within the passive margin field, with some samples nearby. In addition, according to the comparison of characteristic parameters between the sandstones of the Zhiluo Formation and sandstones in four different tectonic settings (Table 5), the major elements characteristics of sandstones of the Zhiluo Formation are most consistent with the passive margin setting.  Traditional diagrams have been called into doubt in recent years because they cannot explain the distinction between continental island arcs (CIAs) and active continental margins (ACMs), nor can they distinguish among the arc, collision and continental rift. To determine the tectonic setting, numerous diagrams must be combined. In 2013, Verma and Armstrong-Altrin [47] proposed a new discriminant diagram for tectonic setting based on the discriminant function, which can distinguish continental rift, island arc and continental collision. In the DF2(Arc-Rift-Col) m1 versus DF1(Arc-Rift-Col) m1 discrimination diagram [47] (Figure 13), except for one sample that plots in the collision field, the sandstone samples of the Zhiluo Formation mostly plot within the continental rift area. Based on these discriminant diagrams, Verma and Armstrong-Altrin [41] proposed a new discriminant diagram, which uses isometric log-ratio transformation during data processing to eliminate the close effect of data. The diagrams [41] (Figure 14) obtained by isometric log-ratio (ilr) transformation of all the major elements and some trace elements reflect a passive margin background for the source area. This is consistent with the traditional graphical results for the Zhiluo Formation.
determine the tectonic setting, numerous diagrams must be combined. In 2013, Verma and Armstrong-Altrin [47] proposed a new discriminant diagram for tectonic setting based on the discriminant function, which can distinguish continental rift, island arc and continental collision. In the DF2(Arc-Rift-Col)m1 versus DF1(Arc-Rift-Col)m1 discrimination diagram [47] (Figure 13), except for one sample that plots in the collision field, the sandstone samples of the Zhiluo Formation mostly plot within the continental rift area. Based on these discriminant diagrams, Verma and Armstrong-Altrin [41] proposed a new discriminant diagram, which uses isometric log-ratio transformation during data processing to eliminate the close effect of data. The diagrams [41] (Figure 14) obtained by isometric log-ratio (ilr) transformation of all the major elements and some trace elements reflect a passive margin background for the source area. This is consistent with the traditional graphical results for the Zhiluo Formation.   The kinds and properties of the parent rocks in the source area have a considerable influence on the composition of the sandstones, but they are still affected by a series of processes such as weathering, transportation and deposition [44]. Therefore, the chemical composition of sandstone can reflect the progress of weathering in parent rock areas [54]. In general, chemical weathering has a substantial influence on the major and trace element compositions of siliciclastic deposits in humid climates. Some larger cations, such as Al, Rb and Ba, can migrate more selectively than smaller cations (such as Na, Sr and Ca). These geochemical characteristics can eventually be transferred to sediments, which provides important information for monitoring the weathering status of the sediment source area [55]. Thus, geochemical proxies are significant indicators of the source area.
The weathering products of parent rock are the main material sources of sedimentary rocks. Chemical weathering and diagenesis in the upper crust have the greatest impact on feldspar, resulting in the formation of clay minerals. The A-CN-K (Al 2 O 3 -CaO* + Na 2 O-K 2 O) ternary diagram [30], where Al 2 O 3 , CaO*, Na 2 O and K 2 O are in molecular proportions, can not only directly reflect the trend and degree of weathering of sediments, but also indicate the compositional characteristics of source rocks. The CIA is an essential indicator for determining the degree of weathering in the provenance area, and it is commonly plotted in the A-CN-K ternary diagram to determine the source rock composition. Here, the chemical indices of PIA and ICV are also employed to assess the source region paleoweathering.
By calculating the CIA values, the weathering degree of the source area can be distinguished, which is usually divided into the following three cases: (a) CIA > 80: implies strong chemical weathering of the source area; (b) CIA = 60~80: indicates moderate chemical weathering of the source area; and (c) CIA < 60, represents initial chemical weathering of the source area [30]. The CIA values of the studied sandstones vary from 56.30 to 63.04 (average 59.75) ( Table 1), which is less than 60 and corresponds to case c. These values indicate that the sediments formed from source rocks that had not undergone chemical degradation. This is also evident from the A-CN-K ternary plot [30] (Figure 15), where predominantly the samples plot near the connection line between plagioclase and clay minerals, parallel to the A-CN side, which shows that the chemical weathering in the provenance area corresponds to the initial chemical weathering.
The ICV is used to determine the degree of change in the original components of clastic rocks. Generally, it is divided into the following two situations: if the ICV is greater than 1, it indicates that the clastic rock is the sediment of the first deposition; if the ICV value is less than 1, it indicates that the clastic rocks are recycled sediments or sediments had undergone strong weathering [29]. The ICV values of the studied samples are 0.74-0.91 (average 0.82), all less than 1; thus, these values indicate that the Zhiluo Formation sandstones are recycled sediments.
Sedimentary sorting and recycling usually cause selective enrichment of elements. Th and Sc are difficult to dissolve in formation water, so the Th/Sc ratio does not change in the process of sedimentary recycling. The Th/Sc and Zr/Sc ratios show good positive correlations, revealing that the provenance had a compositional change similar to magmatic differentiation [10]. In the Th/Sc versus Zr/Sc diagram [10] (Figure 16), all the sandstone samples fall near the UCC and do not show a positive correlation, indicating that the clastic rock components had experienced recycling, which is consistent with the ICV results. undergone chemical degradation. This is also evident from the A-CN-K ternary plot [30] ( Figure 15), where predominantly the samples plot near the connection line between plagioclase and clay minerals, parallel to the A-CN side, which shows that the chemical weathering in the provenance area corresponds to the initial chemical weathering. The ICV is used to determine the degree of change in the original components of clastic rocks. Generally, it is divided into the following two situations: if the ICV is greater than 1, it indicates that the clastic rock is the sediment of the first deposition; if the ICV value is less than 1, it indicates that the clastic rocks are recycled sediments or sediments had undergone strong weathering [29]. The ICV values of the studied samples are 0.74-0.91 (average 0.82), all less than 1; thus, these values indicate that the Zhiluo Formation sandstones are recycled sediments.
Sedimentary sorting and recycling usually cause selective enrichment of elements. Th and Sc are difficult to dissolve in formation water, so the Th/Sc ratio does not change in the process of sedimentary recycling. The Th/Sc and Zr/Sc ratios show good positive correlations, revealing that the provenance had a compositional change similar to magmatic differentiation [10]. In the Th/Sc versus Zr/Sc diagram [10] (Figure 16), all the sandstone samples fall near the UCC and do not show a positive correlation, indicating that the clastic rock components had experienced recycling, which is consistent with the ICV results.   [10] showing the influence of weathering, sorting and recycling processes for sandstones from the Zhiluo Formation.
In 1995, Fedo proposed a new index of plagioclase alteration (PIA), which eliminates the effect of K 2 O in the calculation and can better determine the weathering of the parent rock and source area [30]. The PIA of the Zhiluo Formation sandstones ranges from 57.51 to 71.65 (average 67.38), indicating that there was moderately weak weathering in the source area. In general, with increasing of sediment transport distance, weathering and denudation will be stronger. It can be inferred that the Zhiluo Formation sandstones may have been subject to medium to short distance transportation, corresponding to near-source deposition.

Paleoclimate
Geochemical proxies have frequently been employed to interpret the paleoclimate of the source area. The parent rocks and clastic sediments in the provenance area are affected by climatic conditions [11,29,56]. Chemical weathering mainly occurs under humid climates, which results in the removal of mobile elements (Na, K and Ca) and the concentrations of immobile elements (Al and Si). Therefore, the paleoclimate can be deduced by the geochemical characteristics of the sandstone [11].
The SiO 2 % versus Al 2 O 3 + K 2 O + Na 2 O% bivariate diagram [30] ( Figure 17) proposed by Sutter and Dutta in 1986 was applied to determine the relationship between the maturity of sandstone and climate. In this diagram, all the sandstone samples of the Zhiluo Formation fall within the semi-arid climate field, indicating that the sedimentary environment of the Zhiluo Formation has semi-arid climate conditions. The sandstones have experienced mild to moderate weathering under dry paleoclimate conditions, according to the geochemical indicators of the major elements in the siliceous clastic deposits (CIA, PIA, and ICV) ( Table 1). This is reasonably consistent with the results of earlier studies in the area.

Conclusions
Based on the petrographic and geochemical characteristics of the Zhiluo Formation sandstones of the Bayinqinggeli sandstone-type uranium deposit in the northern Ordos Basin, the provenance, source area properties and tectonic background are analyzed. The sandstones of the Zhiluo Formation are mainly feldspar sandstone, with the highest SiO 2 content, followed by Al 2 O 3 and K 2 O. The microscopic observations show that the minerals in the sandstones are mostly quartz, feldspar and mica. The consistency of the chondrite normalized REE distribution curve indicates that the Zhiluo Formation sandstone has the same provenance. CIA and ICV values reflect that the sandstones have undergone weak chemical weathering in arid climate conditions. In addition, the A-CN-K ternary diagram, Th/Sc versus Zr/Sc diagram, Cr/Th versus Th/Sc diagram and Cr/V versus Y/Ni diagram show that the source area was ancient sedimentary rocks and moderately acidic rocks (granite, andesite and granodiorite). On the other hand, the characteristics of the major elements, trace elements and rare earth elements show that the tectonic setting of the provenance area was a passive continental margin. The Zhiluo Formation sandstone was derived from the volcanic-sedimentary rock series developed in the northern part of the basin, and the tectonic setting position of the provenance area corresponds to the Yanshan intracontinental orogenic belt on the northern margin of the North China Craton.

Data Availability Statement:
No new data were created in this study. Data sharing is not applicable to this article.