Sandstone type uranium deposits in the Ordos Basin, Northwest China: A case study and an overview

Accepted Manuscript Review Sandstone type uranium deposits in the Ordos Basin, Northwest China: A case study and an overview Shamim Akhtar, Xiaoyong Yang, Franco Pirajno PII: DOI: Reference:

S1367-9120(17)30253-5 http://dx.doi.org/10.1016/j.jseaes.2017.05.028 JAES 3093

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

15 September 2016 20 May 2017 20 May 2017

Please cite this article as: Akhtar, S., Yang, X., Pirajno, F., Sandstone type uranium deposits in the Ordos Basin, Northwest China: A case study and an overview, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/ 10.1016/j.jseaes.2017.05.028

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Sandstone type uranium deposits in the Ordos Basin, Northwest China: A case study and an overview

Shamim Akhtar1, Xiaoyong Yang 1*, Franco Pirajno2

1 CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China; 2. The University of Western Australia, Centre for Exploration Targeting, 35 Stirling Highway, Crawley, WA 6009, Australia;

*Corresponding author: E-mail: [email protected] (Yang XY)

Abstract This paper provides a comprehensive review on studies of sandstone type uranium deposits in the Ordos Basin, Northwest China. As the second largest sedimentary basin, the Ordos Basin has great potential for targeting sandstone type U

mineralization. The newly found and explored Dongsheng and Diantou sandstone type uranium deposits are hosted in the Middle Jurassic Zhilou Formation. A large number of 1

investigations have been conducted to trace the source rock compositions and relationship between lithic subarkose sandstone host rock and uranium mineralization. An optical microscopy study reveals two types of alteration associated with the U mineralization: chloritization and sericitization. Some unusual mineral structures, with compositional similarity to coffinite, have been identified in a secondary pyrite by SEM These mineral phases are proposed to be of bacterial origin, following high resolution mapping of uranium minerals and trace element determinations in situ. Moreover, geochemical studies of REE and trace elements constrained the mechanism of uranium enrichment, displaying LREE enrichment relative to HREE. Trace elements such as Pb, Mo and Ba have a direct relationship with uranium enrichment and can be used as index for mineralization. The source of uranium ore forming fluids and related geological processes have been studied using H, O and C isotope systematics of fluid inclusions in quartz veins and the calcite cement of sandstone rocks hosting U mineralization. Both H and O isotopic compositions of fluid inclusions reveal that ore forming fluids are a mixture of meteoric water and magmatic water. The C and S isotopes of the cementing material of sandstone suggest organic origin and bacterial sulfate reduction (BSR), providing an important clue for U mineralization. Discussion of the ore genesis shows that the greenish grey sandstone plays a crucial role during processes leading to uranium mineralization. Consequently, an oxidation-reduction model for sandstone-type uranium deposit is proposed, which can elucidate the source of uranium in the deposits of the Ordos Basin, based on the role of organic materials and sulfate

2

reducing bacteria. We discuss the mechanism of uranium deposition responsible for the genesis of these large sandstone type uranium deposits in this unique sedimentary basin. Keywords: The Ordos Basin; Jurassic Zhilou Formation; Tuff; sandstone type uranium deposit; oxidation-reduction; sulfate reducing bacteria.

1. Introduction The uranium deposits in China can be classified into four major categories (Table 1), namely: 1) magmatic, 2) hydrothermal veins; 3) continental sedimentary and 4) marine sedimentary. Uranium deposits of all these four types account for about 90% of the resource potential of China (Li et al., 2012). These uranium deposits have different sizes and different mineralization ages ranging from Mesozoic to Cenozoic (Wang et al., 2011). In this paper we focus on sandstone-hosted uranium deposits (herein after referred to as sandstone-type), belonging to the continental sedimentary deposits (see Table 1). The first U deposit discovered in the world was a sandstone type deposit (Granger et al., 1961) and such type of deposits constitute about 30% of world uranium endowment (Kyser and Cuney, 2009). Ore bodies of such type have low to medium grade (0.05-0.35wt% U) or almost up to 50000t U metal mostly occurring as roll front type deposits (Table 1), with uraninite and coffinite as primary minerals (Chen et al., 2006). In the northern part of China, sandstone type uranium deposits were first recognized in the 1990’s (Xiang et al., 2005) and a number of roll front type sandstone hosted uranium

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deposits occur in the Yilli Basin, the Tuha Basin and the Ordos Basin (Cai et al., 2007; Chen et al., 2000; Jiao et al., 2005; Lin et al., 2002; Yang et al., 2009). The Ordos Basin is situated in the middle part of northern China and is the second largest sedimentary basin with an area of 250,000km2 between latitudes 34◦ 00′N to 40◦ 35′N and by longitudes 106◦ 50′E to 111◦ 10′E (Liu 1998; Yang et al., 2009). Generally, the size of these sandstone-type deposits ranges from 500-5000 t U metal or 10,000 t U metal with grades ranging from 0.03-0.1% U (Wu et al., 2009). The Ordos Basin basement rocks consist of felsic volcanic and granitic rocks (Zhang et al., 2015; Diwu et al., 2013). Host rocks of the U mineralization are carbonaceous or arkosic sandstone and conglomerates (Wu et al. 2009). In this review, we focused on the Ordos Basin, which is known to contain a large number of uranium deposits hosted in the sandstone units of the Zhiluo Formation (J 2). This Formation has gained great attention due to a recently discovered large sandstone type uranium deposit in Dongsheng, northern the Ordos Basin. The Ordos Basin is the most important energy basin with anundant coal, oil, natural gas and uranium and is called as “a basin of multiple-energy resources” in China (Li et al., 2007; Li and Li 2011; Zhu et al., 2005; Wu et al., 2009). Overviews of the Ordos Basin and its U mineral systems can be found in Dahlkamp (2009) and Pirajno (2013).

2. Geological Setting 2.1. Tectonics

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The Ordos Basin was formed as a result of collision of the Tethys orogen in southwest China with the North China and Yangtz blocks (Li et al., 1995; Liu,1998). According to Sun et al. (1989), the Ordos Basin is an unstable cratonic central basin. However, according to Hsu (1989) and Zhao et al. (2017), the Ordos Basin is surrounded by mountain chains and its subsidence at different stages shows compressional deformation. The development of the Ordos Basin can be divided into different evolutionary stages during Paleozoic to Mesozoic. The western part of basin subsided, while eastern part was uplifted due to thrusting (Liu et al., 2009). In the Triassic, the collision between the Yangtze Block and the North China Craton caused the formation of the Dabie-Sulu UHP orogen (Liu and Yang, 2000), and also formed a lake basin in the Ordos area. This continental lake basin is surrounded by four orogenic belts, ie., Yinshan and Daqingshan in the north, Qingling in the south, Helanshan in the west and Luliang uplift in the east (Fig. 1A) (Li et al. 1995; Li et al., 2007; Li and Li, 2011). The Ordos Basin can be divided into six tectonic blocks: Yiming Uplift, Weibei uplift, Western edge thrust belt, Tianhuan depression, West Shaanxi flexure belt and North Shaanxi incline (Fig. 1A) (Chen et al., 2010; Li and Li, 2011; Yang et al., 2009; Zhang et al., 2006; Zhao et al., 2017). The tectonic movements during the middle Jurassic – early Cretaceous in the Ordos Basin disturbed the sedimentation, its strong uplift promoted migration of oil-natural gases upwards providing a redox regime for uranium accumulation and fomed large sandstone-type U deposits, such as Dongsheng and Diaotou (Yang et al., 2009). Faults are also important for uranium mineralization in sedimentary rocks, because they act as hydrodynamic discharge zones and as geochemical redox barriers for interlayer oxidation mineralization. 5

2.2.

Regional Geology The Ordos Basin is the oldest cratonic basin, formed on a basement of Archean

granulites and lower Proterozoic greenschists of NCC with a total thickness of about 6000 meters. The west side of Ordos Basin is steep, having turbidite sequences while the east side is a gentle slope with fluvial facies sandstone, which is more favourable for uranium mineralization (Liu, 1998). Sediments of the Ordos Basin started depositing from middle-upper Proterozoic to Cenozoic. In the late Triassic, the basin changed from platform setting to an intracratonic basin, which led to a depositional change from marine sediments in Paleozoic to continental sediments in the Mesozoic (Dorobek et al., 1995; Xiang et al., 2005; Yang et al., 2004b). The detailed stratigraphy of the Ordos Basin is shown in Fig. 2. The Archean to Lower Proterozoic basement is composed of metamorphic rocks, granite gneiss, amphibolite and granitic intrusions (Zhang et al., 2015). The Middle to Upper Proterozoic succession consists marine and terrestrial sediments. The Paleozoic succession consists of marine and terrestrial sediments, with interlayer coal beds. Terrestrial sedimentation ceased in the Early Mesozoic, with the sequences from Triassic to Jurassic made up of lacustrine and fluvial clastic rocks (Cai et al., 2007). During Jurassic to Early Cretaceous, the Ordos Basin started separating from the North China platform, where the Lower Cretaceous is made up of fluvial facies with purplish red mudstone and sandstone with gypsum (Cai et al., 2007). Finally, the whole basin was uplifted in Early Cretaceous due to Yanshanian orogenic activity (Liu et al., 2008), resulting in the Lower Cretaceous sequence being exposed to the surface

6

with the uranium deposits being formed in the Dongsheng area (Cai et al., 2007; Li et al., 2007).

3. Sandstone type uranium deposits in the Ordos Basin In the Ordos Basin, Mesozoic sedimentary strata are

important for uranium

mineralization. The Upper Triassic Yanchang Formation consists of sandstone with uranium enriched tuffaceous sediments (Zhang et al., 2010), withthe average uranium content for the Yangchang Formation is about 51 ppm (Yang et al., 2009). The Jurassic strata consist of Upper, Middle and Lower units, but uranium mineralization occurs only in the Middle unit (Li and Li, 2011). The Lower Jurassic consists of

the Fuxian Formation, basically a

complex of red alluvial sandstones, and the Middle Jurassic consists of Yan’an and Zhilou Formations (Li and Li, 2011). The Yan’an Formation is a continental carbonaceous unit while the Zhilou Formation is a unit of variegated sandstone (Li and Li, 2011). In the Ordos Basin, the Jurassic Series is significant for U-bearing sandstones which are mostly found in areas between the Yinshan Monocline, Yimeng Uplift and Weibei Uplift (Fig. 1A). A number of uranium deposits have been discovered around the margins of the Ordos Basin such as in the Dongsheng-Zhungerqi areas in the northeast, Suide-Mizhi and Yanchang-Yanchuan areas in the east, Huangling-Binxian areas in the southeast, Longxian-Pingliang areas in the southwest, Ciyaobao-Shigouyi areasi n the west, and Etuokeqi area in the north (Xue et al., 2010) (Fig. 1A).

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Among all uranium deposits of these areas, the most important are the Shengshangtou deposit in the Dongsheng area, the Guojiawan deposit in the Longxian area and the Diantou deposit in the Huangling area (Xue et al., 2010). The Dongsheng uranium deposit is the largest and the highest uranium grade . It is located at the southern margin of Yiming Uplift in the northeast of the Ordos Basin, near the city of Dongsheng in the center of Inner Mangolia (Fig. 1A) (Li and Li, 2011; Xiang et al., 2002; Xiang et al., 2005; Yang et al., 2009). The strata hosting the Dongsheng uranium deposit are slightly dipping at 1-3° as shown in Fig. 1B (Fang, 2004). The Yellow River depression helps to separate Yiming uplift from Yinshan and Daqingshan Orogenic Belts that exist in the north of Yiming Uplift (Li and Li, 2011). Some other faults separate this uplift from North Shanxi Slope, Tianhuan Depression and West Shanxi Flexure Belt (Cai et al., 2007; Li and Li, 2011). In a cross section profile of the Ordos Basin, aeromagnetic anomalies indicate that the basement rocks from north to south show “three uplifts” (Yulin-Daotu, Dongsheng and Yijinhuoluot) and “two depressions” (Yanchuan-Yulin and Baotou) (Fig. 1C) (Yang, 2002).

3.1. Uranium in mid-Jurassic Zhilou Formation In the Ordos Basin although uranium anomalies have been detected rocks of different ages, the Jurassic period is the most important for uranium mineralization. Till now, the largest uranium deposits discovered in the Ordos Basin, are hosted by the Zhilou Formation of the Middle Jurassic and Yan’an Formation of the Lower Jurassic (Fang et al., 2004). In following, we focus on uranium deposition hosted by the Middle Jurassic Zhilou Formation. 8

3.1.1. Host rock The Zhilou Formation is the host of the main uranium deposits in the Ordos Basin. Sediments of the Zhilou Formation show fining upward grading, i.e. gravel in the bottom and gradually become finegrained to the top (Jin and Zhang, 2013). On the basis of ancient climate, mode of sedimentary deposition, color, texture, lithology, oxidation-reduction zone and uranium mineralization, the Zhilou Formation is divided into two members, i.e., upper member and lower member (Jiao et al., 2005; Jin and Zhang, 2013). The mineralization occurred in the lower member that is further divided into two sub members, i.e., the upper member mainly composed of green sandstone and mudstone, and the lower submember, as an U-bearing gray sandstone (Jiao et al., 2005, Ling et al., 2006).

3.1.2. Petrology The host rock of the U mineralization is a feldspathic sandstone, which consists of 40-70% quartz, 15-30% feldspar and 5-25% lithic clasts (Cai et al., 2006), classified as a lithic subarkose sandstone, having loosely cemented and moderately sorted grains. The matrix contains between 10 % and less than 0.5% carbonates (Li et al., 2012). Thickness of sandstone varies from 20 m to 40m (Ling et al., 2006) (Table 2). The main uranium mineral is coffinite (fine grained, colloroform texture) , showing a close relationship with pyrite, illmenite and bitumen (Xue et al., 2010).

Heavy minerals include garnet, monazite, zircon,

9

epidote, rutile and tourmaline (Xiao et al., 2004 a,b; Xiang et al., 2005; Li and Li, 2011). Pyrite is the main opaque mineral and some pyrite is metasomatized by coffinite.

3.1.3. Alteration Some controversy exists about the mechanisms of uranium mineralization. It has been proposed that uranium mineralization occurs as a result of oxidation (Chen, 2004), whereas Xiao (2004a, b) suggested that it is the result of multi-phase oxidation processes. According to Ou (2004), low temperature hydrothermal activity play an important role during U mineralization. Specific types of fluids also play an important role for alteration associated with U mineralization (Table 3). Uranium mineralization is also identified in an alteration zone of Zhilou Formation, where hematite, goethite, pyrite, calcite, limonite and smectite are main alteration minerals. According to Zhu et al. (2005), three stages of alteration are important for uranium deposition in the Zhilou Foramtion, among which the early stage with red oxidation is responsible for uranium mineralization. An optical microscopy was used for the study of U mineralogy and associated alteration. Major detrital minerals in the sandstone are quartz, potash feldspar, plagioclase and biotite. Accessory minerals are chlorite and sericite. Two types of alteration are commonly observed. One is chloritization in which chlorite replaced biotite, and the second is sericitization, in which sericite replaced K-feldspar (Fig. 4).

3.1.4. Depositional environments 10

The Zhilou Formation was deposited in early braided rivers, meandering streams and late lacustrine depositional systems, however, uranium mineralization occurred in the braided river system (Jin and Zhang, 2013). Sediments of the Zhiluo Formation are of fluvial origin, the Upper Member was deposited in arid to semi-arid climatic condition (Cai et al., 2006), containing high sinuosity meandering stream sediments (Wu et al., 2006) of medium- fine grained sandstone, massive mudstone, siltstone with intercalated pelites (Li and Li, 2011). The sediments of Upper submember of Zhilou Formation were deposited in low sinuosity meandering streams (Jin and Zhang, 2013; Wu et al. 2006). These sediments are grey to greenish grey, medium to coarse grained sandstone with siltstone and pelites. Sediments of the Lower submember are grey in color, medium to coarse sandstone as shown in Fig. 3 (Cai et al., 2007; Li and Li, 2011).

3.2. Formation of coffinite Metallic elements such as Fe, Au, Mn and U, may form ores in the presence of micro-organisms (Lovely et al., 1991). In the sandstone type uranium deposits of the Ordos Basin, uranium bio-mineralization in the Middle Jurassic Zhilou Formation is proven by the presence of textures that clearly resemble microorganism. Pyrite from samples of ZKA183-87-08 and ZKA183-87-10, taken from sandstone of the Dongsheng uranium deposit was examined for microbial mineral textures. These samples have high uranium contents up to 81.1ppm and 58.1ppm respectively, as measured by ICPMS.

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For SEM studies, samples are firstly coated by carbon, EDS analysis was also performed to show the composition of microbial-like textures, showing contents of 44.49 to 57.02 wt.% U, 9.17 to 9.77 wt.% Si, 3.21 to 5.68 wt.% Ca, 2.14 to 4.24 wt.% P (Fig. 5). The chemical composition of these microbial-like textures is similar to the uranium mineral coffinite [(USiO4)1-x (OH)4x]. The nanoparticles of coffinite range in size from 5-25nm (Cai et al., 2007). Large U minerals observed in samples are taken from the Dongsheng uranium deposit. SEM photographs for sample ZKA183-87-08 show microbial textures within pyrites aggregates (Fig.5). Furthermore, the presence of coffinite within or juxtaposed with secondary pyrite is confirmed by SEM observation. Therefore, the presence of biogenic coffinite and the biological element P are indicators of activity of micro-organisms during uranium deposition. Fig. 6 shows high resolution mapping of trace elements in the uranium minerals.

3.3. The Dongsheng and Diantou uranium deposits The Dongsheng uranium deposit with tabular or roll shaped orebodies has grades of more than 1000ug/g (Fig. 7) (Cai et al., 2007; Chen et al., 2006). The ore body extends up to several kilometers in a N-S direction and up to 10 km along depositional direction. Furthermore, the thickness of an individual ore body is about 3-8 meters, but the overall thickness of ore horizon may reach up to 20 meters (Cai et al., 2006). The Dongsheng uranium deposit differs from the other sandstone type deposits due to its complex origin,

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paleo-oxidation mineralization, oil-gas fluid reduction and hydrothermal reworking processes (Li et al., 2014). At the southern margin of the Ordos Basin, the Diantou uranium deposit is another important deposit, having tabular ore bodies as shown in Fig. 8. U-Pb dating of the Dongsheng U deposit shows a range of ages, namely: 177-149 Ma, 124-107 Ma, 85-74 Ma, 20-8 Ma, with the Late Cretaceous considered to be the most important ore-forming period (Xue et al., 2010). Ages of the Diantou uranium deposit range between 110-98 Ma and 51-42 Ma by U-Pb dating (Chen et al., 2006; Xia et al., 2003). 3.3.1.

Comparison of the Dongsheng and Diantou uranium deposits

Both Dongsheng and Diantou are sandstone type uranium deposits, the Dongsheng deposit is recently discovered and the more important deposit, having high U grades (> 1000µg/g) compared to the Diantou deposit. The geological background and style of uranium mineralization is almost same for both Dongsheng and Diantou deposits. However, the diagenetic degree of mineralization is different, i.e., diagenetic degree of the Diantou deposit is high, while the Dongsheng U deposit has a low degree of diagenesis. Both deposits have different shapes of ore bodies, direction of hydrocarbon movements, alteration in the ore bearing strata, host rocks and mineralization ages (Table 4). In addition, the roll front type uranium deposits that are biologically related and psuedomorphically replaced micro-organisms, have been discovered in the Wuyiyi and Wuyier along the southwest border of the Yilli Basin, Xinjiang, NW China (Lin et al., 2002). Other sandstone type U deposits have been discovered and explored in the Tertiary Basin of 13

Wyoming and the Ambrosia lake of New Mexico, USA (Granger et al., 1961; Dahl and Hagmaier, 1974; Reynolds and Goldhaber,1982; Min et al., 2005), and the East Kalkarod, Goulds Dam, Honeymoon and Manyingee districts in Australia (IAEA, 1996).

4.

Geochemistry of uranium deposits

4.1.

REE and trace elements Rare Earth Elements (REE) and trace elements play an important role in understanding

the mechanism of uranium enrichment (Fryer and Taylor, 1987; Ganzeyev et al., 1984; Whitford, 1983). In hydrothermal fluids, the distribution of REE (i.e. LREE, HREE, Ce and Eu) is controlled by PH, temperature and complexing agents, as in the case of LREE enrichment, negative Ce and Eu anomalies were observed, with dominating complexing agent CO32-, PH 6.7-9.5 and temperature 43-97︒C (Lottermoser, 1992).

4.1.1. REE characteristics In order to discuss the REE characteristics for sandstone type uranium deposits in the Ordos Basin, we selected a number of samples from the Dongsheng U deposit from various publications (Supplementary table 1). These samples had been taken from different sections of the host rock that they show variations in lithologies. Few samples contain a variety of mudstone which is the important indicator of the presence of organic matter (Ling et al.,

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2006). As organic matters cause a reducing environment, they are important for the precipitation of uranium minerals. The REE contents of 47 samples were determined by high precision ICP-MS, which were normalized by Chondrite values (Sun and MacDonough, 1989). Some values have also been calculated such as ΣREE, ΣLa-Nd, ΣSm-Ho, ΣEr-Lu, LREE/HREE, δCe and δEu, Ce and Eu anomalies have been calculated by using Taylor and Mclennan, (1985) . Uranium enriched samples show that they have high concentrations of LREE as compared to HREE. As REEs always show the same geochemical properties, separation of individual REEs can be proxied by Ce and Eu anomalies (Lottermoser, 1992). Chondrite-normalized REE distribution patterns of 47 selected samples of the Dongsheng U deposit show characteristics of enriched LREE and depleted HREE (Fig. 9), where it can also be seen that 36 samples show negative Eu anomalies, while 11 samples show positive Eu anomalies. Values of Ce fluctuate and most samples also show negative Ce anomalies. The Ce anomalies are indicative of oxidizing environment, when Ce3+ changes to Ce4+ , and it starts to deposit in the form of CeO2, which results in the depletion of LREE and enrichment of Ce in altered rocks (Bao and Zhao, 2003). On the other hand, when Eu3+ is reduced to low valance state Eu2+, it shows negative Eu anomalies. Depletion of HREE and enrichment of Ce can also indicate two types of alteration (K-feldspar and silicification) during the U forming process (Bao and Zhao, 2003), thus we infer that hydrothermal processes played an important role in uranium enrichment in the Ordos Basin.

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4.1.2. Trace elememts Trace elements are also studied to find the geochemical signals for uranium exploration (Xue et al., 2010). Thirty two samples were selected from publications related to sandstone type uranium deposits in the Ordos baisn (Supplementary Table 1). The spider diagram of 32 samples from the Dongsheng area indicate that Pb, Mo and Ba have close relation with U enrichments (Fig. 10), therefore they can be used as indicator elements for uranium mineralization. Fig. 10 exhibits high peaks for Pb, U, Mo and Ba, indicating that their enrichments are directly related to U mieralzation. When

238

U series decays, it produces

206

Pb and

234

U daughter products. Under oxidizing

conditions, Pb and U can migrated, whereas, under reducing environment, Pb can easily react with S in hydrothermal fluid to produce galena (PbS). Similarly, U and Mo have similar geochemical properties: under oxidative conditions, both U6+ and Mo6+ have tendency to migrated in hydrothermal fluids, whereas under reducing conditions, both U 6+ and Mo6+ can be reduced to U4+ and Mo4+ for precipitating (Rollinson, 1983; Lottermoser, 1992; Xue et al., 2010). Extensive correlation between U and other trace elements shows variation trends (Fig.11), where it can be seen that Pb, Mo and Ba have a linear correlation with U, thus they can be used as indicator elements for U enrichment.

4.2.

Stable isotopes

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Stable isotopes play an important role in finding the sources of uranium ore forming fluids. The study of stable isotope along with petrography of alteration minerals helps to find the geochemical environment of the formation of authigenic minerals and their diagenetic mechanism. We used data of stable isotopes from published papers related to sandstone–hosted uranium deposits in the Dongsheng area of the Ordos Basin. Thirty one samples provide oxygen and hydrogen isotopic compositions of fluid inclusion in quartz grains of sandstone, fifty three samples for carbon and oxygen isotopes of calcite and thirty one samples for sulfur isotope were selected from various publications to discuss the source of ore forming fluids, the role of organic matter and the sulfate reducing bacteria for uranium mineralization.

4.2.1.

H-O isotopes of fluid inclusions

In order to find the source of fluids related to uranium deposits, hydrogen and oxygen isotope data were used from published works in order to reveal the geochemical features of fluid inclusions in quartz veins in sandstone rocks of the Ordos Basin (Supplementary Table 2). The δ18 OSMOW of magmatic water are 7.0‰ - 9.0‰ and its values for δ DSMOW are -50‰ to -80‰ (Rollinson, 1983). The H-O isotopic composition diagram is shown in Fig. 12, where these δD values range from -102‰ to -26.5‰, δ18O from 6.1‰ to 30.99‰, suggesting a formation water origin (Hoefs 1987). Consequently, we inferred that the source of the fluids

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related to U mineralization in the Ordos Basin is the mixture of meteoric water and magmatic water.

4.2.2 The C-O isotopes of calcite from the cement of sandstones There are several publications on C-O isotopes of calcite from cements of sandstones related to U mineralization.We have selected fifty three samples of calcite from published works. The selected published data of δ 13C and δ18O are provided in Supplementary Table 3. The δ13C values range from -1.6‰ to -23.62‰, and some δ13C values are less than -10‰, suggesting an organic source . The δ18O values range from 10.32‰ to 24.6‰, which are within the range of sedimentary rocks (Hoefs, 1987), but these values are higher than those in fluid inclusions. The δ13C values of carbonate cement suggest that mostly carbon was derived from marine carbonates (sedimentary origin). Two samples show high negative values of -19.6‰ and -23.62‰, which are within the range of organic materials (Yang et al., 2009), indicating that biogenic carbon plays an important role for the uranium mineralization (Fig. 13).

4.2.3 S isotope of pyrite Data on sulfur isotopes, taken from published works, are shown in Supplementary table 4. There are two types of pyrite in the Zhiluo Formation, one is euhedral-subhedral and another is framboidal. The δ34S values of the euhedral-subhedral pyrite samples range from 3.0‰ to 26.97‰, whereas, the δ34S values of the framboidal pyrite samples range from and -4.7‰ to 18

-39.201‰ (Fig. 14). The δ34S values of framboidal pyrites are more negative, probably due to bacterial sulfate reduction (BSR) (Ohmoto and Rye, 1979). As microbes can be active at low temperatures, it was assumed that that such type of pyrite is formed at ＜70︒C (Cai et al., 2006). High negative values may also show sulfate reduction, oxidation of pyrites and reduction (Reynolds and Goldhaber, 1982). Some unusual mineral structures, with composition similar to uranium mineral coffinite, have been found with secondary pyrite of bacterial origin (Cai et al., 2007). Consequently, we suggest that bacteria may use petroleum and coal as nutrients, reduce the sulfates to sulfides, and U (VI) to U (IV), therefore, the pyrite associated with bacteria has a direct relationship with uranium deposition in the Ordos Basin.

5. Discussion The U mineralization in the Zhilou Formation occurs in a zone between grey sandstone and grey-green sandstone showing some secondary alteration due to hydrocarbons in paleo-oxidation zone (Jiao et al., 2005). For example, in the Dongsheng deposit, the uranium mineralization is controlled by green color alteration in the host sandstone that occurs in interlayer oxidation zone due to strong hydrocarbon reduction (Wu et al., 2006; Xiao et al., 2004). Sulfate reducing bacteria (SRB) can induce reduction of coal beds to produce methane, which led to the development of green color alteration and bleaching with increasing methane, which is associated with the U mineralization in the Dongsheng region (Wu et al., 2009). 19

According to Wu et al. (2009), the gray-green sandstone of the Zhilou Formation has low contents of organic carbon and sulfur, and high ratios of Fe2+/ Fe3+ and Th/U. Furthermore, results of mineralogical analysis performed by Li et al. (2007) and Xiang et al. (2005) show that contents of Fe2+, clay minerals and chlorite of grey-green sandstone are higher compared to grey sandstone, indicating that the greenish colour of sandstone is due to high chlorite content, thus we suggest that it underwent a strong oxidation before becoming green.

5.1. Cementing material In the Dongsheng uranium deposit, the calcite and pyrite are important cementing materials in the sandstone of the Zhilou Formation. Stable isotope data show that the calcite has higher organic carbon content, while some pyrite related to U mineralization may be caused by bacterial sulfate reduction. Consequently, we proposed a mechanism of chemical reactions based on Shikazono and Utada (1997) as follows:

SO42- + 2H+ + CH2O + H2

→

H2S + CO2 + 3H2O ……………………

(1)

H2S and CO2 produced in the above reaction form pyrite and calcite as,

H2S + Fe2+ →

FeS + 2H+ …………………………….............................

CO2 + Ca2+ + H2O →

(2)

CaCO3 + 2H+ ……………………...................... (3)

Fe2+ and Ca2+ are derived from iron and calcium minerals of sedimentary rocks. 20

5.2. Source rocks Former study suggests that the Paleozoic to Mesozoic igneous rocks with high uranium contents around the Ordos Basin may be the source of uranium for sandstone type uranium deposits (Wang et al., 2011). The granitoids and metamorphic rocks, such as in the Yinshan Orogenic Belt or the Daqingshan region, Northern Ordos Basin are the basic source of uranium (uraninite) in the Basin (Xiao et al., 2004, 2005; Liu et al., 2007), as they usually have high uranium contents from 3-12 ppm (Li et al., 2008). Therefore, we suggest that these granitoids and metamorphosed rocks are the main source of uranium in the Ordos Basin.

5.3. Uranium migration In the case of the Dongsheng uranium deposit, we suggest that the source of uranium is mostly in leachable U6+ from granitic or metamorphic rocks. Leaching of uranium is dependent on the chemical interaction between host rock and ground water. Uranium migrates from source rock to host rock mostly by ground water solutions (Sanford, 1994; Xiao et al. 2003). Ground water absorbs uranium through different absorptive agents, such as carbonaceous matter. When carbonated water comes in contact with weathered U-bearing granitic rocks, in faults, fissures and shear zones, carbonaceous matters can absorb more uranium (Doi et al. 1975). Hexavalent uranium has high tendency to dissolve in ground water than tetravalent uranium (Turner et al. 1993). When such U-bearing water was in contact with a reducing environment, hexavalent uranium changed to tetravalent causing precipitation from the solution. 21

5.4. Reduction of U(VI) to U(IV) by organic matter It is well know that organic matter acts as a powerful reductant, which has a close relation to sand-type uranium deposits (Spirakis 1996; Leventhal 1993). Almost all kinds of solid bitumen, some coal and humic acid are important for reduction regimes and their role in uranium mineralization (Li et al., 2009). Organic matter controls oxidation – reduction state (Eh) of mineralization (Spirakis 1996). According to Hostetler and Garrel (1962), uranium is more soluble in oxidizing state and less in a reducing state. Some of the δ13C values of calcite cement are negative (-10‰), indicating an organic origin. The principal sources of organic matters in the Ordos Basin are kerogen of mudstone, Jurassic coal and Ordovician petroleum/ natural gas in deeper strata. Multiphase oil-gas reduction has proven helpful for reduction regimes and uranium mineralization (He, 2003). In some cases, where the host sandstone contains less organic matters, then organic acid and methane gas move upward from underlying strata along faults and provide the organic matters necessary for reduction (Goldhaber et al.,1983; Spirakis 1996). The H2S is a component of natural gas, acts as reductant and can decrease the Eh of the environment, thereby being quite helpful for producing uranium mineralization (Li and Zhang , 2004; Wu et al., 2009). Uranium migrates in the hexavalent state and forms with carbonate complexes, such as {UO2(CO3)2}2- and {UO2(CO3)3}4- (Nash et al. (1981). These bicarbonates and tricarbonates react differently to form uranium minerals as shown in the reactions below:

22

{ UO2(CO3)2}2-+ 2H+↔2HCO3-+ UO22+ …. …. …. …. …. ….. …. …. …. …. …. …. (4)

UO22+ can be reduced to form uranite (UO 2) (Ritch et al. 1977) as follows:

{UO2(CO3)2}2-+ H2O ↔

UO2+ 1/2O2 + 2HCO3-

. …. …. …. …. …. ….. …. …. (5)

Consequently, the type of organic matters and reduction scale are very important to understand the intensity of uranium mineralization.

5.5. Uranium enrichment Primary enrichment of uranium started during the deposition of the Zhilou Formation from Middle to Late Jurassic. After its deposition, the Ordos Basin was uplifted and some areas such as the Shanxi massif at the eastern margin and Yinshan block in the northern margin of the Basin could have been a source of uranium. Paleoclimatic conditions also changed from arid-semi-arid to dry, which are favorable for vertical oxidation (Li et al., 2008). Uplifting and paleoclimatic changes led to further uranium enrichment in the Zhilou Formation. In late Jurassic to early Cretaceous, the Basin resumed uplift and the Zhilou Formation was exposed to the surface as observed now, where strong weathering and oxidation occurred (Li et al., 2012). Oxidizing uraniferous ground water moved into the ore bed, due to hydrologic gradients and interlayer pressure, during which this oxidizing solution

23

replaced the reducing material and formed an oxidation-reduction boundary that led to uranium concentration in the form of roll type deposits (Xiang et al., 2005).

6. Mechanism of uranium mineralization by oxidation-reduction model Sandstone with loosely packed grains, conglomeritic grit, low degree of diagenesis and good permeability is suitable for uranium mineralization. Uranium is formed by simple precipitation mechanism (Xiao et al. 2004). When uranium ions in fluids move through a permeable oxidizing zone, whose bottom and top beds are semi-permeable, like mud or pelitic aquifers, and come in contact with a reducing environment associated with H2S, C, CH4, Fe2+ or micro bacteria, uranium minerals precipitate, resulting in the formation of uranium mineralization (Fig. 15) (Li et al., 2008; Li and Zhang, 2004; Wu et al., 2006). There are two important factors for sandstone type uranium mineralization in the Ordos Basin, (i) oxidation and (ii) reduction. We proposed the oxidation-reduction model for uranium mineralization on the basis of previous studies. During oxidation, uranium is oxidized to form U(VI) complexes, which become soluble in meteoric water. Uranium in meteoric water and magmatic water may act as a source for uranium deposition in a reducing environment, this is supported by H isotope data (Fig. 12). Oxidation processes are primary and important for the enrichment of uranium in the host fluid, but reduction processes are also very important for uranium deposition after enrichment. Reduction of uranium may occur due to the presence of H2S, organic matter or sufate reducing bacteria (SRB). Organic matters, including kerogene, CH4 and petroleum, mostly from underlying coal and petroleum deposits 24

provide the carbon for cementing material, as negative carbon isotope valuses (Fig.13), indicating involvement of biogenic carbon for the calcite cement. The origin of pyrite is due to SRB, as proved by sulfur isotope systematics (Fig. 14). Microbial associated mineral structures, such as coffinite, within pyrites aggregates are also observed (Fig. 5), indicating that coffinite formed by oxidation-reduction process, during which brine and petroleum from the underlying strata mixed with meteoric water in the presence of SRB. These SRB consumed petroleum to form organic acid as follows:

SRB +

Petroleum

→

organic acid

…. …. …. …. …. …. …. ….

(6)

This organic acid partially dissolved feldspar in sandstone, then this partially dissolved feldspar and organic acid formed the uranium mineral coffinite, with high Si content, as shown below:

Partially dissolved feldspar + Organic acid

→

Coffinite …. …. ….

(7)

Uranium is mostly coupled with high concentration of sulfates in uranium enrichment areas during oxidizing processes (Yang et al., 2009). But sulfate reducing bacteria may consume the petroleum as a nutrient and reduce the sulfates to sulfides, so both oxidation and reduction processes play important role for uranium mineralization in the Ordos Basin (Cai et al., 2007). 25

7. Conclusions In our study, we conclude that: 1) The newly explored Dongsheng and Diantou uranium deposits in the Ordos Basin are hosted by the lithic subarkose sandstone of Middle Jurassic Zhilou Formation. 2) Coffinite in secondary pyrite has been identified by high resolution SEM. Ore bodies occur in an intermediate zone between grey sandstone and grey green sandstone, which is due to high contents of chlorite and ferrous ions as well as strong hydrocarbon reduction. 3) The H and O isotopes of fluid inclusions reveal that U ore-forming fluids are a mixture of magmatic water and meteoric water. The C and S isotopes of calcite and pyrite show organic and bacterial origin during uranium mineralization. 4) A model for the genesis of the uranium mineralization is proposed based on a simple precipitation mechanism, aided by oxidation-reduction processes.

Acknowledgement This work is supported by National Program on Key Basic Research Project (973 Program)

(2015CB453002)

and

DREAM

(2016YFC0600404).

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Figure Captions Fig. 1 Geological map and cross sections of the Ordos Basin (A) Regional geological map of the Ordos Basin (modified after Li and Li 2011; Yang et al., 2009; Cai et al., 2007), (B) geological map of the Dongsheng area (modified after Li and Li 2011). (C) East-West cross section XY cross the north Ordos Basin revealed by seismic tomography and drilling wells (modified after Wang et al., 2015)

Fig. 2

Stratigraphy of the Ordos Basin (modified after Cai et al. 2007 ; Yang et al. 2009)

Fig. 3 Uranium mineralization in the Zhiluo Formation.

Fig. 4 Photomicrographs of primary minerals and alteration minerals of sandstone type uranium deposits of the Ordos Basin. Qtz. Quartz; Kfs. Potash feldspar; Pl. plagioclase; Bt. Biotite; Chl. Chlorite; Ser. Sericite; Py. Pyrite; Cal. Calcite and Amp. Amphibole. Scale bar: 0.5mm. A. ZK341-60-04, fine gray green sandstone, chloritization, sericitization and plagioclase twins. B. ZK341-60-04, details of sericitic alteration C. ZKA183-87-08, gray green medium grained feldspathic sandstone, quartz, Potash feldspar, pyrite and chloritic alteration of biotite. D. coexisting potash feldspar, sericite, chlorite and biotite. E. ZKA183-87-09, gray medium grained feldspathic sandstone, approximately 50% quartz grain showing chloritic and sericitid alteration.

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F. ZKA183-87-10, gray coarse grained feldspathic sandstone, late alteration and pyrite. G. ZKA183-87-10 yrite and calcite as cement, quartz, plagioclase and feldspar also observed along with high sericitic alteration. H. ZKA139-35-13, gray coarse grained feldspathic sandstone, late alteration, quartz, potash feldspar and biotite.

Fig. 5 SEM photographs showing microbial like mineral textures (coffinite) in a thin section sample of drill hole ZKA183-87-08 Fig. 5a shows large grain of uranium mineral, while in Fig. 5b, these mineral structures are globular cocci shaped; in Fig. 5c, these are long rod like structures. Morphologically, these rod shaped structures are similar to bacteria such as desulfobacterium, vacuolatum and desulfovibrio species (Widdel and Bak, 1999; Cai et al., 2007).

Fig. 6 High resolution mapping for uranium mineral coffinite of sample ZKA183-87-08 from Dongsheng uranium deposit, (a) SE image of pure Coffinite and scale is 20 micrometer; (b-i) are images for elements U, V, Fe, Nd, Ti, Si, Ca, and P respectively.

Fig. 7 Roll shaped uranium ore deposit in the Zhilou Formation, Dongsheng region (modified after Chen et al. 2006)

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Fig. 8 Slab shaped uranium ore deposit in the Zhiou Formation, Diantou deposit, Huangling area (modified after Chen et al. 2006; Sun et al., 1998)

Fig. 9 Chondrite-normalized REE distribution patterns for samples of Dongsheng area (a) Samples shows positive Eu anomalies while (b,c,d) show negative Eu anomalies. Fig. 9(a) after (Wu, 2005; Ling et al. 2006, Tang, 2014; Wu et al. 2014), (b) after (Wu, 2005; Ling et al. 2006), (c) after (Tang, 2014), (d) after (Xue et al. 2010; Wu et al. 2014).

Fig.10

Primitive mantle-normalized spider diagram of trace element for samples of

Dongsheng area Rb, Pb, Th, U, Hf, Mo, La, Ba, Ce, Nd, Nb, Sr, Zr, Ta, and Y are normalized after Sun and McDonough (1989), Pr, Eu, Gd, Er, Sm, Sc, Cr, Co, Ni and Zn are normalized after Taylor and Mclennan (1985). Fig. 10(a) after (Ling et al. 2006), (b) after (Wu, 2005), (c) after (Tang, 2014), (d) after (Xue et al. 2010)

Fig. 11 Correlation diagrams between U, Th and other REE and trace elements (a) ΣREE-U; (b) ΣREE-Th; (c) Th-U; (d) ΣREE-Th/U; (e) Rb-U; (f) Mo-U; (g) Pb-U; (h) Zr-U; (i) Sr-U; (j) Ba-U; (k) Cr-U. Fig.11a shows a clear linear correlation between U and ΣREE, and Fig. 11b is also a clear linear correlation between Th and ΣREE, Fig. 11c shows a variation trend for U and Th concentration. Fig. 11d is a plot for Th/U and ΣREE. But this plot shows no obvious correlation. Fig. 11e shows

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a variation plot between U and Rb. All data points fall almost at the same level, so it shows no obvious correlation. Fig. 11f is a variation plot for U and Mo. It shows scattered data points but most of the points show linear correlation. Fig. 11g shows a little linear variation trend for U and Pb. Fig. 11h, shows no obvious correlation trend for U and Zr. Similarly in Fig. 11i, plot between U and Sr, all data points fall at the same level and show no obvious correlation. Fig. 11j shows variation between U and Ba, from which a linear correlation can be found. Fig. 11k, is a plot between U and Cr, but no correlation can be found.

Fig. 12 Relationship between δ18OSMOW and δDSMOW values of fluid inclusions of sandstone Dongsheng uranium deposit of Ordos Basin (Li and Wang, 2007; Wu et al., 2007; Wu, 2010 and Yang et al., 2009)

Fig. 13 Relationship between δ13C and δ18O values of carbonate cement of sandstone Dongsheng uranium deposit of Ordos Basin (Fan et al., 2007; Hu and Wu, 2009; Tang, 2014; Wu et al., 2009; Xue et al., 2010; Yang et al., 2009).

Fig. 14 Graphical presentation of δ34S values of pyrite cement of sandstone Dongsheng uranium deposit of Ordos Basin (Cai et al., 2006; Hu and Wu, 2009; Li et al, 2007; Wu et al., 2009)

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Figure 15 A general model for roll front type uranium mineralization in permeable rock of Zhilou Formation under reducing condition (modified after Devoto 1978).

Table Captions. Table 1 Classification of uranium deposits in China. Table 2 Petrographic features of sandstone hosted uranium deposits Table 3 Types of fluid alterations and altered minerals. Table 4 Comparison between Dongsheng and Diantou uranium deposits.

Supplementary table captions. S-Table 1

Chemical compositions of trace elements and REE of sandstone samples with U

mineralization in Ordos basin S-Table 2

H and O isotope data.

S-Table 3

O and C isotope data.

S-Table 4

S isotope data

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Table 1

Table 1 Classification of uranium deposits in China (Li et al., 2012) Major category of uranium deposits

Type

Subtypes of sandstonehosted uranium deposits

Occurrence

Magmatic

Pegmatite alkaline rocks

Basal channel deposits

These deposits form when permeable sediments fill wide channels. For example, Bayantala deposit in Eren Basin.

Hydrothermal (vein)

Granite Volcanic

Tabular deposits

Occur in paleo channels of underlying basement rocks having different shapes like irregular or elongated lenticular bodies. For example, Dongsheng deposit in Ordos Basin.

Continental facies sedimentary

Sandstone Mudstone Coal

Roll front deposits

marine facies sedimentary

Black shale phosphate

Arcute shape bodies that crosscut sandstone bedding and mostly occur in paleochannels. For example, Kuji’ertai deposit in Yilli Basin and Shihongtan deposit at TurfanHemi Basin

Tectonic/lithologic deposits

Occur in such type of sandstone that is adjacent to a permeable fault zone.

Table 2

Table 2 Petrographic features of sandstone hosted uranium deposits Properties

Description

Authigenic minerals

Partial shape

Sub angular- sub rounded

Calcite minerals

Texture

Porous

Sortation

Moderate-poorly sorted

Description

chlorite, kaolinite, It makes about 75% of rock volume. Green color of illite/smectite,

lower Zhilou Formation is due to chlorite. In

chlorite/smectite

Dongsheng area, it occur as alteration product of biotite. Kaolinite form by dissolution of feldspar.

Textural maturity

Moderate

Detrital grains

60-95%

Cement

5-40%

Siliceous

chalcedony

or Chalcedony is cementing material while quartz

minerals

quartz overgrowth

overgrowth form in late stage of diagenesis having gaseous or fluid inclusions.

Porosity

2-15%

Major detrital minerals

Quartz, plagioclase, K-feldspar, biotite

Accessory minerals

Sericite and chlorite

Main U minerals

Uraninite and coffinite

Carbonate

micritic calcite and Spary calcite shows a pseudomorphic phenomenon

minerals

spary calcite

by surrounding or dissolving micritic calcite as observed in the Dongsheng uranium deposit.

Ferric minerals

pyrite

Pyrite is a dominant cementing material of oxidizing environment, showing frambiod to cubic or granular

Observed alterations

Chloritization and sericitization

aggregate pattern.

Table 3

Table 3 Types of fluid alterations and altered minerals. Type of fluid

Alterations

Functions

Minerals

Source

Acid oxygenic Fluid

oxidation of organic matter

Primary mineralization fluid

Hematite

Dehydration of limonite and hydrous goethite

Oil- gas fluid

Epiditization, carbonation, chloritization,sericitization.

Act as reducer carbonation and green alteration

Smectite and Chlorite Pyrite

Feldspar and biotite

Epidotization, carbonation, and mineral assemblage of brannerite, anatose and coffinite

Thin fluid layer can change the interlayer oxidation type ore horizon

Alkaline hydrothermal fluid

Geothite and limonite

Early formed limonite Groundwater activity

Table 4

Table 4 Comparison between Dongsheng and Diantou uranium deposits. Properties

Dongsheng uranium deposit

Location in Ordos Basin

Located in the Northern part of Ordos Basin, at the Southern margin of Yimeng Uplift and dip 1-3 degree in SW direction. Gray feldspar, lithic sandstone

Lithology of ore bed Diagenetic grade Depositional system Hydrocarbon Content Source of hydrocarbon Ore body

Mineralization age

Little bit low Host rock is fluvial deposit while ore bed is deposited in braided stream system High content in greenish gray sandstone and generated by coal derived natural gas. Upper Proterozoic rocks

Diantou uranium deposit Located in southern margin part of Ordos Basin in north of Weibei uplift fold belt in Huangjing area, dip at 15 degree in NW direction Gray sandstone containing carbon, feldspar and quartz high Host rock is fluvial deposit

High content in gray sandstone and generated by condensation of gas. Mesozoic-Triassic strata

Ore body is roll shaped and controlled by green color alteration zone, distributed in E-W trend and ranges from several Km to more than 20Km in N-S direction 175 Ma, 154-125 Ma, 96-32 Ma, 208 Ma

Ore body is slab shaped and controlled by fade color alteration zone, distributed in N-S trend and obtained width of several km to 10 km in E-W direction 110-98 Ma, 51-42 Ma