Reng-an Yu, Shan-bo Wang, Qiang Zhu, Qing-hong Si, Xue-ming Teng, Xiao-xue Liu,Hou-ning Liu, Yong-xiang Tang

a Tianjin Center, China Geological Survey, Ministry of Natural Resources, Tianjin 300170, China

b Key Laboratory of Uranium Geology, China Geological Survey, Tianjin 300170, China

c Sino Shaanxi Nuclear Industry Group Geological Survey Institute Co., Ltd, Xi’an 750021, China

d Tianjin Geothermal Exploration Institute, Tianjin 300250, China

Keywords:

Detrital zircon

U-Pb geochronology

Provenance

Uranium deposit

Sandstone type

Zhiluo Formation

Huangling area

Mineral exploration engineering

Ordos Basin

China

A B S T R A C T

The newly discovered medium-scale Huangling uranium deposit is located in the Shuanlong area of the southeast Ordos Basin. This paper presents the systematic geochemical and zircon U-Pb studies on the Zhiluo Formation sandstones in the Huanling area. The data obtained play an important role in deducing the provenance and tectonic setting of the source rocks. The results show that the lower part of the Zhiluo Formation is mainly composed of felsic sedimentary rocks. The source rocks originated from a continental island arc environment in terms of tectonic setting. U-Pb ages of detrital zircons obtained can be roughly divided into three groups: 170-500 Ma, 1600-2050 Ma, and 2100-2650 Ma. Based on the characteristics of trace elements and rare earth elements (REE) and the zircon U-Pb dating results, it is considered that the Cryptozoic Edo provenance of the Zhiluo Formation mainly includes magmatic rocks (such as granodioritic intrusions) and metamorphic rocks (such as gneiss and granulite) in the orogenic belts on the northern margin of the North China Plate and in the Alxa Block. Based on sedimentological and petrological results, it can be concluded that the provenance of clastic sediments in the Zhiluo Formation was in north-south direction. The preconcentration of uranium is relatively low in the Lower Zhiluo Formation in the Huangling area. Meanwhile, the paleocurrent system in the sedimentary period is inconsistent with the ore-bearing flow field in the mineralization period, which restricts the formation of large-scale and super-large-scale uranium deposits and ore zones in the southeast Ordos Basin. The understanding of provenance directions will provide crucial references for the Jurassic prototype recovery and paleo-geomorphology of the Ordos Basin and the prediction of potential uranium reservoirs of the basin.

1. Introduction

In recent years, many large- and medium-scale uranium deposits have successively been discovered in the Ordos Basin through “Coal-Uranium Co-exploration” (Jin RS et al.,2018, 2019; Yu RA et al., 2020a, 2020b). The newly discovered sandstone-type Huangling uranium deposit is located around the previously discovered Diantou and Shuanglong uranium deposits in the southeast Ordos Basin.The provenance studies of the uranium-bearing rock series in the Zhiluo Formation throughout the Ordos Basin have been conducted by methods of petrology (Yi C et al., 2014),geochemistry (Liu HB et al., 2012; Feng XX et al., 2019),sedimentary tectonic setting (Jiao YQ, 2015), and zircon dating (Wang M et al., 2013; Zhang L et al., 2016; Lei KY et al., 2017). From the sedimentology, petromineralogy and geochemistry of the Zhiluo Formation, Zhao JF et al. (2007)inferred that materials were supplied to the center of the basin from seven peripheral areas, which include the eastern Taihang Mountain Uplift, the northern Hetao Uplift, the western Alxa Block, and the southwestern and southern Qinling-Qilian Orogenic Belt and its adjacent areas. The provenance of Jurassic clastic sedimentary rocks in the northern part of the Ordos Basin has been concurrently considered to be the Yinshan Mountain, Khondalite belt, and some metamorphic and magmatic rocks in the Langshan Mountain (Wang M et al., 2013; Zhang L et al., 2016), as indicated by the spatial distribution of the Jurassic sedimentary rocks, paleo-current direction, andin-situU-Pb dating of detrital zircons. However, the provenance of Mesozoic strata in the south Ordos Basin still remains controversial. Chen QH et al. (2012) correlated the provenance with the Archaean and Proterozoic metasedimentary-volcanic rocks in the northern Qinglin Orogenic Belt by geochemical analysis. Zhang Y et al. (2014)thought that the Late Triassic-Early Jurassic detrital materials in the south Ordos Basin mainly originated from the early sedimentary recycling of the North China Craton (NCC),while the Qinglin Orogenic Belt was not the provenance of the Ordos Basin. The palaeocurrent along the Zhiluo Formation is considered to have flowed from south to north into the basin (Zhang ZL et al., 2017). Based on LA-ICP-MS U-Pb dating and paleocurrent direction, Lei KY et al. (2017)considered that the provenances of the Zhiluo Formation in Diantou-Shuanglong and Binxian areas are the intermediateacid intrusive rocks and metamorphic rocks in the northwestern Alxa Block and those in the north-central Qilian Orogenic Belt, respectively.

In summary, there are still different opinions on the complex transportation system and regional characteristics of the provenance of the Mesozoic strata in the south Ordos Basin at present. Besides, the influence ranges of different provenances and the tectonic evolutionary development of the provenances are still in dispute. Provenance analysis is important for reconstructing sedimentary evolution and paleoenvironment. Based on the data of a large number of coalfield and uranium deposit boreholes collected in this study as well as previous studies, the unaltered gray sandstone in the Lower Zhiluo Formation revealed by the boreholes in the uranium deposits of the Huangling area (located 20 km southwest of the Shuanglong uranium deposit) were studied in terms geochemistry, sedimentology, and U-Pb dating of detrital zirons. It was found that the zircon dating results greatly differ from the zircon age spectra of the Shuanglong area discovered by Lei KY et al. (2017), which means that the provenance system of the Zhiluo Formation dramatically changed. Meanwhile, the formation of large- and super-largescale deposits is affected and restricted by whether the paleocurrent system of the uranium reservoirs in the sedimentary period is consistent with the ore-bearing flow field in mineralization period (Jiao YQ et al., 2015).Therefore, the provenance and tectonic setting of the Zhiluo Formation should be systematically discussed. It will provide favorable criteria for the reconstruction of the Jurassic prototype and paleo-geomorphology of the Ordos Basin and the prediction of the potential uranium reservoirs in the Zhiluo Formation.

2. Geological background

The Ordos Basin is a large basin bearing multiple energy resources in western China (Jiao YQ et al., 2016; Cui JW et al., 2019). It is bounded by the Yinshan and Langshan mountains in the north, the Lüliangshan Mountain in the east,the Qilian-Qinling Mountains in the south, and the Liupanshan Mountain in the southwest (Fig. 1). The Huangling uranium deposit is located on the southeastern margin of the Ordos Basin. The cap rock in the study area consists of the Triassic (T), Jurassic (J), Lower Cretaceous(K1), and Cenozoic. The Middle Jurassic Zhiluo Formation is one of the main uranium-bearing strata in the Ordos Basin(Jin RS et al., 2013). According to the internal lithologic characteristics, the Zhiluo Formation can be divided into the upper member (J2Z2) and the lower member (J2Z1).Meanwhile, the lower member can be subdivided into the upper sub-member (J2Z1-2) and lower sub-member (J2Z1-1)(Jiao YQ et al., 2005). It was dominanted by braided fluvial medium-coarse-grained sandstones in a braided river in the early stage and was mainly composed of grayish-green medium-fine-grained meandering fluvial sandstones, brownred mudstones and siltstones in the middle and late stages.The Zhiluo Formation is in a parallel unconformable the underlying coal-bearing series of the Yan’an Formation and in an erosional unconformable contact with the overlying Lower Cretaceous strata of the Zhidan Formation (K1zh).

Fig. 1. Simplified geological and tectonic map of the Ordos Basin and its adjacent regions.

3. Analytical methods and results

3.1. Samples collection and analytical methods

In this study, samples of the lower sub-member of the Zhiluo Formation were systematically collected from three boreholes in the Huangling area. Among them, 16 fresh and uncontaminated sandstone samples were selected for microscopic identification and the testing of major elements,trace elements, and rare earth elements (REE). Two samples for detrital zircon dating were selected from borehole ZK2018-01 in the Huangling area, whose burial depth are 332 m and 343 m, respectively (Figs. 1, 2).

Fig. 2. Petrographic characteristics of the Zhiluo Formation in the Huangling area. a-comprehensive borehole histogram; b-macro-characteristics of cores drilled in the study area; c-hand specimen of the Zhiluo Formation sandstones; d-photomicrographs of the Zhiluo Formation sandstones. Qrt-Quartz; Pl-Plagioclase; Kp-Feldspar; Bit-biotite; Ser-Sericite.

The samples were analyzed in the laboratory of the Tianjin Center, China Geological Survey. Whole-rock geochemical analysis was conducted by X-ray fluorescence spectrometry (XRF), and FeO was measured using hydrofluoric acid and sulfuric acid soluble samples and the potassium dichromate titrimetric method, obtaining a qualification ratio of 100%. The trace elements and REEs were measured using an inductively coupled plasma mass spectrometer (ICP-MS; x series II, US), and the analytical error was less than 5%.

Zircon selection was completed in the laboratory of Langfang Integrity Geological Service Co., Ltd. After crushing the rock samples in to particles of 100 μm, zircon grains were seperated by magnetic separation and flotation and then selected through handpick under binocular glasses.After gluing, polishing and gold plating, the cathodoluminescence (CL) of internal structure and zircon UPb isotopic dating were carried out on instruments in the laboratory of the Tianjin Center, China Geology Survey.Zircon dating was measured using a Laser Ablation Multi-receiver Plasma Mass Spectrometer. The principle, test conditions, and flow chart were in accordance with Yu RA et al. (2015). The error was 1 sigma, zircon U-Pb isotopic data were processed using the Isoplot (Ludwig KR, 2009).

3.2. Analytical results

3.2.1. Characteristics of sandstone detritus

The observation results of cores drilled from the Huangling area and the microscopic identification results of the Zhiluo Formation sandstones are shown below.

The lower member of the Zhiluo Formation is mainly composed of feldspar-quartz sandstones with more than 90% rock debris, which primarily consists of quartz (50%-65%),partial serialized feldspar (25%-45%), and a small number of subordinate components of siltstones, acid lave, and altered rock detritus (5%-10%) (Figs. 2c, d). The cementation types include argillaceous cementation primarily and a small amount of siliceous cementation secondarily, as indicated by secondary enlarged quartz.

3.2.2. Geochemical characteristics of major elements

Loss on ignition (LOI) was on the high side under the influence of carbonate cementation on some samples.Therefore, the data processing should be preformed as follows: Replace the data of CaO with CaO* firstly, then subtract the data of LOI, and finally convert the component to 100%. The sandstone content was considered to be roughly appropriate. The test results of major elements are listed in Table 1, and the average content of SiO2was 71.63 %. It is generally believed that the content of unstable components such as Al2O3, TiO2, and Fe2O3decreases with an increase in SiO2, indicating the gradually increasing maturity of sandstones. It was found that the high content of Al2O3corresponded to high volume of feldspar and rock debris,which is consistent with the observation under a microscope.This indicates the low maturity of the Zhiluo Formation sandstones. In addition, the average chemical differentiation index (CIA) * of the sandstones was 64.4, indicating moderate chemical weathering in the provenances.

Table 1. Analytical data of major elements of the Zhiluo Formation sandstones in the Huangling area (%).

3.2.3. Geochemistry of trace elements and REEs

The content of Fe-Mg elements such as Co, Ni, Cr, and V approximated to the continental an intermediate-acid trend(Table 2; Rudnik RL and Gao S. 2003). The MORB (Midocean ridge basalt)-normalized spider diagram shows that the samples were relatively rich in K, Rb, LILESs (Lagre ion lithophile elemtents), Zr, Hf, and HFSEs (High field-strength elements) but deficient in Nb, Ta, P, Ti, Sr and Y (Fig. 3a).

Fig. 3. MORB-normalized trace element patterns of the Zhiluo Formation sandstones in the Huangling area (a; after Pearce JA, 1984) and chondrite-normalized REE diagrams (b; after Boynton WV, 1984).

The content and characteristic parameters of REEs are shown in Table 3 and Table 4, respectively. The chondritenormalized REE patterns of the Zhiluo Formation sandstones are illustrated in Fig. 3b. The average amount of ∑REE was 63×10-6, and the value of ∑LREE/∑HREE is 4.62-10.77,with an average of 7.32. The average of (La/Yb)Nwas 5.5.No significant Ce anomalies were found in most of the samples, while a few of Eu positive anomalies were discovered in certain samples. The chondrite-normalized distribution pattern was characterized by LREE enrichment and HREE deficiency. These signatures indicate the provenances of the samples mainly include the ancient upper crust composed of felsic components.

Table 3. Analytical data of REEs in the sandstones of of the Zhiluo Formation sandstones in the Huangling area (10-6).

Table 4. REE geochemical parameters of sandstones in different tectonic settings.

3.2.4. U-Pb dating of detrital zircons

The CL images of detrital zircon grains (Figs. 4a, c) show that the selected zircon particles are medium in particle size (50-100 μm), and are mainly idiomorphic and hypidiomorphic in shape. The Th/U ratio (＜0.1 for metamorphic origin and ＞0.4 for magmatic origin) of zircons is often used as a classification criteri on of genetic types of zircons (Wu YB et al., 2004). Most of the young zircons are angular in shape and feature high Th/U ratios, distinct growing zones, and notable rhythm structure, revealing magmatic genesis (Fig. 5). In contrast, the ancient zircons are round and sub-angular in shape, or ancient magmatic zircons exhibit narrow lightcolored edges, with partial grains showing ancient cores or low Th/U ratios (0.1-0.4). All these indicate that the ancient zircons experienced multi-cycle transportation and abrasion and the incomplete metamorphism, and recrystallization caused by later geological events. Given the complexity of zircon genesis, some scholars believed that criteria for the definition of zircon genetic types should be based not only on Th/U ratios but also on the microstructures under CL images(Kelly NM et al., 2005).

Fig. 4. CL images and LA-ICP MS zircon U-Pb concordia diagrams of Zhiluo Formation sandstones in the Huangling area. a-ZK2018.01.1 CL images; b-ZK2018.01.1 U-Pb concordia diagram; c-ZK2018.01.5 CL image; d-ZK2018.01.5 U-Pb concordia diagram; the units of age are Ma in a and c.

Fig. 5. Relationship of zircon ages and Th/U ratios of detrital zircons from the Zhiluo Formation sandstones in the Huangling area.

Most of the measured spots fall near the U-Pb concordia curve (Figs. 4b, d). Zircon ages can be approximately divided into three groups (Fig. 5; Fig. 6), namely 170-500 Ma (early stage of the Middle Jurassic-Early Ordovician), 1600-2050 Ma (Middle Proterozoic-Early Proterozoic), and 2100-2650 Ma (Early Proterozoic-Early Archean). Their main peak values account for 40.6%, 32.5%, and 19.37%, respectively(Table 5, Appendix Table S1).

Fig. 6. Distribution histograms of detrital zircon ages of the Zhiluo Formation sandstones in the Huangling area.

Table 5. Proportion of main peak ages of detrital zircons from the Zhiluo Formation sandstones in the Huangling area.

4. Rock types and tectonic setting of the provenances

4.1. Rock types of provenances

The content of REEs in sediments is mainly controlled by provenance rather than transportation, deposition, and diagenesis. Therefore, the characteristics of REEs in rocks can be reliably preserved in sediments, and the REE content of sandstones is widely used as a classification criterion of the rock types of provenances (Shao L et al., 2000).

Based on geochemical characteristics of different clastic rocks in a known tectonic setting, the multivariable discriminant equations of the oxides of seven elements (Ti,Al, Fe, Mg, Ca, Na, and K) have been analyzed using four synthesized mixture provenance models (Roser BP et al.,1986). These models can be utilized to effectively distinguish the provenances of igneous rocks such as mafic, intermediate or felsic rocks and quartzite sedimentary rocks such as quartzite.

On the equation discriminant of provenances F1-F2diagram (Fig. 7a) determined based on discriminant equations, the samples of the Zhiluo Formation mainly fall in the felsic igneous provenance and the intermediate igneous provenance. The former represents an active and incised continental magmatic arcs derived from mature continental margin arcs and a pull-apart basin on continental transition margins. In contrast, the latter represents the volcanic detritus in the sandstones that mainly consists of andesites, indicating a mature magmatic arcs and immature continental margin magmatic arcs. In addition, in the Zhiluo Formation sandstones are relatively rich in K2O. Thew(K2O) /w(Na2O)ratio was 3.75, which is significantly higher than that of the sandstones on a passive continental margin (1.60), indicating the addition of many mature components.

Fig. 7. Discrimination diagram of provenances F1-F2 (a) and La/Th-Hf diagram (b) of the Zhiluo Formation sandstones in the Huangling area.

Floyd PA (1989) proposed the Hf-La/Th discriminant diagram to identify sediment sources in different tectonic environments after studying the geochemistry of Early Proterozoic metasediments in the northwestern Scotland. As shown in the Hf-La/Th diagram (Fig. 7b), most of the samples fall in the mixed zone of felsic and mafic rocks, indicating the felsic origin from the mixed zone of a volcanic arc and the continental upper crust.

The above results show that the provenances of the Zhiluo Formation in the study area are the upper crust which mainly composed of felsic rocks and mixed with intermediate feldspar rich magmatic rocks. This is consistent with the conclusion obtained based on the above REE partition model.

4.2. Tectonic setting of the provenances

4.2.1. Major elements and tectonic setting

Fe and Ti elements in sediments can reflect three provenance properties of the sediments due to their stability.Mg elements are not so stable as Fe and Ti elements but can roughly represent the original content of provenances. The Al2O3/SiO2ratio can be used to distinguish the enrichment degree of quartz in sandstones. Therefore, the content of these elemental oxides in sandstones can be supposed to serve as the research parameter used to represent the provenance and tectonic setting. As shown in the Al2O3/SiO2-(TFe2O3+MgO)discriminant diagram (Fig. 8), the samples mainly fall in the area of passive continental margins, and only a few of them fall in the area of active continental margins and continental island arc.

Fig. 8. Tectonic setting discriminant diagram of major elemental composition of the Zhiluo Formation sandstones in the Huangling area.

4.2.2. Trace elements and the tectonic setting

Bhatia MR (1986) have established a series of discriminant diagrams based on Th, Sc, and Zr after studying the geochemical characteristics of Paleozoic turbidites in the eastern Australia. Th is relatively unstable and represents mature tectonic setting. Zr content can indicate the sedimentary selectivity, while the compatible Sc and Co elements represent immature tectonic setting. As shown by the Th-Sc-Zr/10 and La-Th-Sc diagrams, some samples mainly fall in the continental island arc area (Fig. 9). The result indicates that the formation of the clastic rocks in the Zhiluo Formation is closely related to island arcs and that the provenances of the Zhiluo Formation in the southeastern Ordos Basin mainly include continental island arc in terms of tectonic characteristics.

Fig. 9. Tectonic setting discrimination plots for Th-Sc-Zr/10 and La-Th-Sc of the Zhiluo Formation sandstones in the Huangling area.

5. Provenance analysis

5.1. Provenance analysis based on regional sedimentology

The spatial distribution analysis of sedimentary sand bodies could indicate the internal sedimentary relations among them. The distribution pattern of sedimentary facies belts in a continental basins is formed as follows. An alluvial fan represents the coarsest and poorly sorted proximal units of the basin’s terrestrial sedimentary system. Firstly, it generally evolves into a fine-grained, slightly sloped fluvial system in the downdip direction. Afterwards, it evolves into a delta or lacustrine systems. Finally, it evolves into a lacustrine sedimentary system. In general, the overlying largely thick sandstones represent that rivers often flowed. Therefore, the distribution and variation of sandstone thickness also reflect the direction of sediment provenances and the distribution characteristics of a sedimentary system. In this paper, the thickness map of the sand bodies in the Lower Zhiluo Formation in the Huangling area and its surrounding areas was compiled by drilling a large number of boreholes in coalfields and uranium mines. This map can accurately reflect the characteristics of the paleosedimentary system and make it possible to effectively discuss the trend of paleo-flowing water and infer the provenance directions.

The thickness characteristics of the Lower Zhiluo Formation sandstones in the southeast Ordos Basin are as follows. The sand bodies in the northern Huangling-Shuanglong area are relatively steadily distributed with a thickness varying from 30 m to 70 m (ca. 45 m on average;Fig. 10). In contrast, the sand bodies in the southern Beiji-Miaowan area are relatively thin with a thickness in the range of 10-50 m. These sand bodies are discontinuously distributed and interbedded with more mud and sands compared to the sand bodies in the Huangling area, which indicates that the sedimentation have evolved in to braided river delta facies. Based on the analysis of sand body thickness and a compilation of previous studies, it is suggested that the paleocurrent should flow from northwest to southeast or from west to east, rather than from southeast to northwest or from south to north.

Fig. 10. Contour map of the sand thickness of the lower member of the Zhiluo Formation in southern Ordos Basin.

5.2. Zircon U-Pb geochronology for provenance determination

It is generally believed that the distribution shape of detrital zircon ages should be unimodal or similar in the case of same provenances. In contrast, there will be two or more peaks in the age spectr a for different provenances and notable different rock ages in one provenance (Yan Y et al., 2003).The detrital zircon ages of the samples can be used to distinguish the change process of the source area of the basin in a certain period and different periods by comparison with those of the exposed rocks around the basin and adjacent mountains, which correspond to the tectonic thermal events in the adjacent areas. Therefore, the magmatic rocks and metamorphic rocks represent the parent rocks of detrital zircons of their respective ages, which can be used to estimate the provenance of Zhiluo Formation. To estimate the provenances of the Zhiluo Formation sandstones in the study area, the U-Pb data of magma and metamorphic zircons from the areas around the study area such as Yinshan Mountain,Daqinshan-Wulashan mountains, Alxa Block, and Qinling-Qilian Orogenic Belt were collected to reflect the temporal and spatial distribution characteristics of the provenance of the study area.

5.2.1. Distribution of potential provenances

The ages of the detrital zircons from the Wulashan,Yinshan, and Daqingshan mountains mainly include in three groups: 220-350 Ma, 1600-2100 Ma, and 2400-2700 Ma.Besides, the ages of a few zircons are 600-1000 Ma, showing three weak peak ranges. The typical zircon ages of the basement rocks include 1.8-2.0 Ga and 2.4-2.8 Ga (Zhao GC et al., 2000), where Late Hercynian and Early Indosinian magmatic rocks are widely distributed (Zhai MG et al., 2011).Magmatic rocks and metamorphic rocks of different ages occur sporadically in and around the Alxa Massif. The ages of the detrital zircons from these rocks are mainly divided into five groups, namelys 260-290 Ma, 440-480 Ma, 800-1000 Ma, 1800-2000 Ma, and 2200-2400 Ma (Zhang J et al.,2016). Early Paleozoic rocks (400-430 Ma and 450-490 Ma)are widely distributed in the Qilian Orogenic Belt, followed by the Proterozoic rocks with ages of 750-1000 Ma,1650-2150 Ma, and 2300-2500 Ma (Qin HP, 2002). Early Paleozoic and Neoproterozoic rocks have mainly developed in the Qinling Orogenic Belt, which mainly recorded the subduction-collision and granitic magmatism during 391-450 Ma and the collisional metamorphism during 408-415 Ma(Zhang GW et al., 2003).

The zircon dating data of the Zhiluo Formation sandstones in the Huangling area were compared with the age spectra of the adjacent areas (Fig. 11) and the results are as follows.

Fig. 11. Comparison between the detrital zircon ages of the Zhiluo Formation sandstones in the Huangling area and the isotopic ages of rocks around the Ordos Basin (n denotes the age number; data in subfigs. b-i after Lei KY et al., 2017).

The age spectrum of the study area (consisting of three segments) is in a similar shape with that of Yinshan,Daqingshan, and Wulashan areas, but the Early Mesozoic -Early Paleozoic zircon ages feature a larger scope in the study area compared to these areas (Fig. 11b). The main peak age of the Alxa Block is 250-400 Ma (Fig. 10c), which corresponds to the Phanerozoic age of the study area. The zircon ages of the Qilian Orogenic Belt range widely from 500 Ma to 1600 Ma (Figs. 11d-f), which is unrelated with the Neoproterozoic zircon age of the study area. The zircon ages of the Qinling Orogenic Belt mainly range from 200 Ma to 500 Ma, while a wider age range of 500-1000 Ma prevails in east Qinling(Fig. 11g-h). Meanwhile, the thickness of the Zhiluo Formation in the Huangling area indicates that the paleocurrent was in north-south trending and that the provenances of the lower member of the Zhiluo Formation are located the west and northwest of the basin during the sedimentary period. In summary, the results indicate that the provenance is unrelated to the southern Qinling Mountain.

In conclusion, the provenance of the Zhiluo Formation sandstones in the Huangling area is notably related to the north orogenic belts of the NCC and the Alxa Block, which serve as the main potential provenances of the study area.Meanwhile, the Qilian Orogenic Belt contributed to the provenance sediments to a certain extent.

5.2.2. Implications of Archaean and Proterozoic zircons

The Hercynian detrital zircon ages of the Zhiluo Formation sandstones in the study area are mainly in the ranges of 1600-2050 Ma (Middle Proterozoic-Early Proterozoic) and 2100-2650 Ma (Early Proterozoic-Early Archean), which account for 32.5% and 19.37% of the total zircon ages, respectively. The peak ages are 1850 Ma and 2490 Ma, respectively.

The ages of five Neoarchean detrital zircons (accounting for 3.1% of the total zircons) range from 2500 Ma to 2645 Ma, relatively consistent with the ages of TTG gneisses and mafic-ultramafic layered intrusions (2.6-2.5 Ga) in Guyang,Wuchuan, Seerteng, and Alxa areas in Inner Mongolia (Wang HC et al., 2001). According to accurate dating, the gneisses in the Ikenwusuyan Formation of the Diebusige Group in Halataolegai area, Alashan Left Banner are constrained to be 2700 Ma and considered to have undergone a metamorphic transformation in the Late Neoarchean (Geng YS et al., 2007).Therefore, some metamorphic rocks in the Alxa area may contribute to the provenance of the Zhiluo Formation.

There are 50 and 30 zircons (accounting for 31.4% and 18.8% of the total zircons, respectively) zircons and 30 zircons that were dated to be Middle Proterozoic-Early Proterozoic (1600-2050 Ma) and Early Proterozoic-Early Archean (2100-2650 Ma), respectively. They mainly record the basement ages and multi-stage metamorphic thermal events in the Daqingshan-Ulashan and Yinshan Mountain in the northern Ordos Basin.

The ages of the granulites in the Guyang-Wuchuan area of Inner Mongolia are 2510-2350 Ma (Wang HC et al., 2001),and the ages of the potash granites in the Wulashan-Jining area are from 2494 ± 59 Ma to 2371 ± 38 Ma (Wu CH et al.,2006). The zircon ages of 1500-2000 Ma are consistent with those of the Khondalite Belt in the Daqingshan-Wulashan mountains and partially agree with those of the gneisses and granodiorite intrusions in the Langshan and Yinshan mountains. The magmatic and metamorphic rocks in the Alxa Massif are also of similar ages, such as the biotite-plagioclase granites in Huangqikou area dating from 1681 Ma to 1696 Ma, the granulites in Wenduerhaoshan dating from 1948 Ma(Zhou LR et al., 1989), the granodiorite gneiss in the Diebusige area dating from 1970 Ma to 1980 Ma (Geng YS et al., 2010), the granodiorite gneiss in Bayanwulashan Mountain reaching the ages of 2082 ± 22 Ma, (Li JJ et al.,2004) and the granodiorite gneiss from the Longshoushan Group dating from 2040-2170 Ma (Gong JH et al., 2013).

Besides, there are seven Neoproterozoic detrital zircons(accounting for 4.4% of the total zircons) in the study area.Lei KY et al. (2017) made detailed statistics on the zircon ages of different sections of the Qilian Orogenic Belt. It is believed that intrusive rocks, metamorphic rocks, and metasedimentary rocks in the north and middle Qilian Orogenic Belt are similar to those in the study area in age.

5.2.3. Zircon ages of Late Paleozoic and Early Mesozoic

The ages of 64 zircons (accounting for 40% of the total zircons) were dated to be early Middle Jurassic-Early Ordovician (170-500 Ma). They have mainly recorded the strong magmatism in the provenances. Due to the closure of the Paleo-Asian Ocean and continental collision between the Siberian Plate and the North China Plate, the northern orogenic belts of North China, especially the Daqingshan and Langshan mountains, have large-scale post-collisional granitic intrusions of the Late Paleozoic and Triassic (Zhang SH et al.,2010), which are mainly composed of intermediate-acid magmatic rocks. For example, the Dahuabei pluton in the Wulashan Mountain was dated to be 330 Ma (Wang L et al.,2015), the age of biotite adamellite of the Halashao pluton in the Daqinshan Mountain is 261.1 ± 0.5 Ma (Zhao QY et al.,2007), the age of granite in the Chaganhua area, Wulate Rear Banner is 253. 3 ± 2.8 Ma (Liu YF et al., 2012), and the ages of gneiss and granites in Langshan Mountain range from Palaeozoic to Triassic (308-232 Ma) (Wang Z et al., 2016).However, the magmatism from Early Carboniferous to Early Ordovician is missing.

Paleozoic magmatic rocks are extensively distributed in the Alxa Block, which underwent strong block faulting, rapid uplift, and denudation. They can serve as one provenance of the Zhiluo Formation in the study area. Late Permian acid serve as one provenance of rocks with an age of 259 Ma occur in the Zhulazhagamaodao area, northern Alxa (Bao C et al.,2012). Zhou LR et al. (1989) made detailed statistics on the ages of the magmatic and metamorphic rocks in the Alxa area,especially the Alxa Right Banner. Some ages are provided as follows: Plagioclase granitoids from Qigejing area: 266 Ma;Kebule Plagioclase: 276 Ma; granitic rocks in the Baijiazuizi area: 285 Ma; porphyritic granitic rocks in the Longshoushan Mountain: 270 Ma; biotite plagioclase gneiss in the Saertai area: 252 Ma; plagioclase gneiss in Suhaitu area: 278 Ma;diorite in Niujiaoyaogou area: 428 Ma; granodiorite in Jiling area: 380 Ma; quartz syenite in Houshan area: 428 Ma; and biotite plagioclase gneiss in the Bayanmaode area: 423 Ma.

In addition, five Early-Middle Jurassic detrital zircons from the study area correspond to the Early-Middle Jurassic intrusive rocks in the Alxa Massif. For example, the age of Deliji biotite granites in the Bayinnuoergong area is 199 Ma,and the age of migmatites in the Hongliuyuan Bangangjing area is 180 Ma (Zhou LR et al., 1989). Jurassic basins and a series of volcanic rocks distributed in the Yinshan and Yanshan mountains eroded and were transported, and also serve as the provenances of the Zhiluo Formation in the study area (Chen Y et al., 2017).

Therefore, the characteristics of the Early-Middle Jurassic-Early Ordovician peak ages in the study area correspond closely to the North Orogenic Belt and the rock masses of the Alxa Massif, and the results indicate that the provenances mainly include the northern margin of the North China plate and the Alxa Massif.

5.3. Geological events reflected by zircon ages

The Yinshan Orogenic Belt in the north of the Ordos Basin was extensively uplifted in the Middle-Late Paleozoic.The North China Plate and the Siberian Plate closed during the late stage of the Late Paleozoic in the Solunshan-Xilamulunhe area. Along with the continuous occurrence and intensification of collisional orogeny in the Yinshan Mountain, thrust and folding also occurred during the formation of the provenances apart from the single uplift. All these lead to the exposure of the rocks such as metamorphic rocks and granites, which serve as one provenance of the Ordos Basin.

The Alxa Block and the North China Plate collided along the north-south-trending extension line between Helanshan and Zhuozishan mountains after the Early-Middle Triassic(Yang ZY et al., 2014). The basement of the Alxa Block was formed in the Paleoproterozoic and activated in the Paleozoic and Mesozoic, and most especially, tectonic and magmatic activities occurred strongly the Late Paleozoic. All these are considered to be related to in subduction and closure of the Paleo-Asian Ocean.

Under the background of strong north-south-trending compression, the Alxa Block slipped and escaped to the east,which resulted in strong folding and uplift of the thick Mesoproterozoic-Mesozoic strata in the Helan Aulacogen.Then the Alxa Block underwent thrusting and extended eastward. The strong east-west-trending tectonic compression in the Yanshanian period caused the uplift and denudation of the Alxa area. In that case, the sediments of the Zhiluo Formation came not only from the Yinshan Orogenic Belt but also from the Alxa area.

The Qinling Orogenic Belt was mainly formed as the South China Block was joined to the North China Block and closed during the Middle-Late Triassic (230-238 Ma). The Mianxian-Lueyang ophiolitic melange belt in the south of the Ordos Basin probably represents the final merging zone of the two blocks (Zhao Y et al., 2010).

Because the Qinling Mountains is closer to the study area than the Qilian, Alxa, and Yinshan mountains, it is easy to have the habitual thought of “proximity principle” and a certain tendency of data analysis when conducting the provenance tracing of the sedimentary strata on the southeastern margin of the Ordos Basin. Given the diversity and complexity of the provenance of the basin, the differences in contributions made by different sediment areas, and the effects of detrital differentiation during transportation, it is necessary to infer the provenance and transportation path of the Zhiluo Formation from the information of sedimentology,geochemistry, and detrital zircon ages.

As mentioned above, the provenances of the Lower Zhiluo Formation in the study area include the northern orogenic belts of the basin and the Alxa block primarily and the Qilian Mountains partly. It is worth noting that the provenances of the Zhiluo Formation in Shuanglong and Binxian in the south of the basin are the Alxa Block and the Qilian Mountains, respectively (Lei KY et al., 2017)based on the analysis of zircon and paleocurrent.However, the important contributions of the northern orogenic belts of the basin to the Zhiluo Formation clastic materials are ignored. Two inferences can be derived from the above opinions. One is that the northern orogenic belts, such as the Yinshan Mountain were formed earlier,uplifted in a large area, and suffered denudation. They can provide material sources for the Lower Zhiluo Formation in the Huangling area after long-distance transport of clastic materials, as mentioned before. The other one is that though the Qinling Orogenic Belt was closed and uplifted rapidly during the Late Triassic (Zhang Y et al.,2019), it is not a provenance of the Lower Zhiluo Formation in the Huangling area.

The sedimentary range of the Ordos Basin during the Middle Jurassic was much larger than that of the present basin(Zhao JF et al., 2007). According to the petrological characteristics of the Zhiluo Formation in different areas in the Ordos Basin, the quartz content in the Huangling area(50%-65%) is higher than that in the western part of the Ordos Basin (Ningdong area, 20%-50%; Guo QY, 2010) and that in the northeastern part of the basin (35% -60%); (Wu ZJ et al., 2013; Liu XX et al., 2016), while the content of feldspar and detritus is lower. The results indicate that the clastic sediments of the Zhiluo Formation in the Huangling area were transported from north to south base on the sediment characteristics of sandstone thickness.

During the sedimentary period of the Zhiluo Formation,the Ordos Basin settled steadily, with the sedimentary center located in the south of the basin. This was highly related to the foreland flexural subsidence subject to the intense convergent orogeny of the Qinling Orogenic Belt (Zhao JF et al., 2008).

Since the Yinshan Orogenic Belt and the northwestern Alxa Landmass was uplifted earlier than the southern Qinling Orogenic Belt, the topography of “high north and low south”around the Ordos Basin during the sedimentary period of the Zhiluo Formation led to long-term weathering and denudation of the magmatic and metamorphic rocks in northern orogenic belts and the eastern Alxa Block. Therefore, the detrital zircons in the Zhiluo Formation are thought to originate from the northern orogenic belts of the Ordos Basin and the Alxa Block. In the Middle Jurassic, under the influence of the Bayanhot fault basin between the Alxa block and the Ordos Basin, the contributions of the geological bodies in the Alxa block to the provenance of Zhiluo Formation in the Ordos Basin gradually decreased (Fig. 12). Comparatively, the contributions of the northern orogenic belts in the basin to the the provenances increased.

Fig. 12. Evolution model of the tectonics, provenances, and paleogeography of the Zhiluo Formation in the Huangling area.

5.4. Discussion of the relationship between diagenesis and mineralization

Xia YL (2004) suggested that the original uranium content of rocks could be roughly determined according to the Th/U ratios of rock samples, and the results can identify the preconcentration and post mineralization of uranium. According to the study on the original uranium content and its change rule of 72 samples of rocks and cores from the boreholes drilled in the Huangling area(Table 6), the original uranium content has not changed significantly and the value is low with an increase ininsitumeasured uranium content. The results show that a large amount of uranium is obtained from the rocks in the mineralized section and ore blocks during diagenesis, the preconcentration of uranium is weak, and the uranium mainly originated from the peripheral provenances. It can be concluded that the detrital materials of the sand bodies in the Zhiluo Formation sand bodies in Huangling area must have been transported for a long distance, the original uranium-bearing materials gradually reduced and precipitated through the reduction section during transportation, so that the content of uranium in the diagenetic process was relatively low.

Table 6. Initial uranium content and loss of uranium in the lower member of the Zhiluo Formation in the Huangling area.

As mentioned above, the Zhiluo Formation in the Huangling area was formed with the northern orogenic belts and the Alashan block in the Middle Jurassic as provenances.Meanwhile, the southeast of the basin was uplifted subject to tectonic events at the end of Jurassic, resulting in the formation of a monoclinic structure with NW dip, which provides conditions for the formation of surface and underground runoff. The ancient metamorphic rock series and granite series in the Qilian Qinling fold belt adjacent to the study area have become an important source of uranium in the Zhiluo Formation (Xing XJ et al., 2008). The period from the end of the Early Cretaceous to the Paleocene is the main oreforming stage of uranium depositation (Zhang ZL et al.,2016). It is concluded that the provenances of the sand bodies in the Lower Zhiluo Formation of Huangling area and the main ore-forming uranium source are different areas or in different directions.

Jiao YQ et al. (2015) believed that large-scale-sandstonetype uranium deposits in interlayer oxidation zones are liable to form in the case that the ore-bearing flow field in the orebearing period is in the same position and direction with the paleowater flow field in the sedimentary period. However, the sedimentary paleowater flow field of the Zhiluo Formation in the study area came from the north, while the ore-bearing fluid field came from the south. Therefore, a new metallogenic system with different flow fields was formed in the study area, and it is difficult to form a large-scale and stable uranium concentration area in the study area like the northeast of the Ordos Basin.

6. Conclusions

(i) The provenance analysis shows that the parent rocks of the Lower Zhiluo Formation in the Huangling area mainly came from the upper crust composed of felsic rocks. The characteristics of major and trace elements and normalized distribution curve of REEs show that the tectonic setting of the Lower Zhiluo Formation is a continental margin arc.

(ii) The detrital zircon ages of the Lower Zhiluo Formation include 170-500 Ma, 1600-2050 Ma, and 2100-2650 Ma. The provenances mainly include magmatic rocks (such as granodioritic intrusions) and metamorphic rocks (such as gneiss and granulites) in the northern orogenic belts of the North China Plate and the Alxa Massif. Few Neoarchean detrital zircons came from the northern North China Plate. The detrital zircons with ages of 1600-2050 Ma(Middle-Early Proterozoic) and 2100-2650 Ma (Early Proterozoic-Early Archean) mainly originated from magmatic rocks (such as granodiorite intrusions) and metamorphic rocks(such as gneiss and granulites) in the northern North China Plate and the Alxa Block. The detrital zircons with an age of 170-500 Ma are derived from the Caledonian and Variscan intermediate-acid intrusive rocks in the northern North China Plate and Alxa Block. All these indicate that the provenances of the Lower Zhiluo Formation in the Huangling area include both the Alax Block and the orogenic belts on the northern margin of NCC.

(iii) It can be comprehensively inferred that the provenances of the Lower Zhiluo Formation in the study area are located in the north rather than the south. According to the information of sedimentology, lithological geochemistry, and detrital zircon ages, the provenances are mainly considered to be the orogenic belts on the northern margin of the Alxa and NCC. These results can provide a reference for the reconstruction of the Jurassic prototype and Palaeogeomorphology of the Ordos Basin.

(iv) The preconcentration of uranium in the Lower Zhiluo Formation of Huangling area is relatively low, and the paleocurrent system in the sedimentary period is inconsistent with the ore-bearing flow field in the mineralization period.All these restrict the formation of large-scale and super-largescale uranium deposits and ore zones in the southeast of the Ordos Basin.

CRediT authorship contribution statement

Reng-an Yu conceived of the presented idea. Reng-an Yu,Shan-bo Wang, Qiang Zhua, Qing-hong Si, Xue-ming Teng,Xiao-xue Liu, Hou-ning Zhu, Yong-xiang Tang wrote the manuscript in consultation. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The authors are grateful to Jia-run Tu and Shuang Hao from the Tianjin Center, China Geological Survey for their suggestions. This study was funded by the project initiated by the China Geological Survey “Investigation of sandstone-type uranium deposits in the Ordos and Qaidam Basins”(DD20190119), the National Key Research and Development Project (2018YFC0604200) from the Ministry of Science and Technology of the the International Geoscience Programme(IGCP675), which is a joint endeavor of UNESCO and IUGS.

Supplementary data

Supplementary data (Appendix Table S1) to this paper can be found online at doi:10.31035/cg2021006.