Genetic Model of the Luhai Sandstone-Type Uranium Deposit in the Erlian Basin, Inner Mongolia

Abstract

The Luhai uranium deposit is a large-scale uranium deposit newly discovered in recent years through comprehensive prospecting methods. It is located in the Basaiqi Paleochannel Uranium metallogenic belt of the Erlian Basin and is characterized by its shallow burial and large scale. This paper provides new data on the genetic processes of sandstone-type uranium mineralization through sedimentological and geochemical environmental indicators (such as Fe³⁺/Fe²⁺, organic carbon, total sulfur, etc.), analysis of C-O isotopes of carbonate cements and H-O isotopes of groundwater, and geochemical and mineralogical studies of uranium minerals, iron–titanium oxides (involving backscatter analysis, micro-area chemical composition determination, and elemental surface scanning), and organic matter. Sedimentological analysis shows that the ore- bearing layer in the upper member of the Saihan Formation developed a braided channel within floodplain subfacies, which control the distribution of uranium ore bodies. Uranium mineralogical observations, geochemical environmental indicators, and organic geochemical data indicate that the main reducing agents related to mineralization are pyrite, terrestrial plants, and deep-sourced oil and gas. The δD values of groundwater in the ore-bearing layer range from −95.34‰ to −90.68‰, and the δ¹⁸O values range from −12.24‰ to −11.87‰. For calcite cements, the δ¹⁸O_V-PDB values range from −24‰ to −11.5‰, and the δ¹⁸_OV-SMOW values range from 6.2‰ to 19‰. It was determined that the ore-forming fluid is mainly surface fresh water that entered the strata during the tectonic uplift stage, with local mixing of deep-sourced brine. Based on these data, the main modes of uranium mineralization in the paleochannel were obtained as follows: (1) Redox mineralization occurs due to the reducing medium within the sand body itself and the reduction caused by deep- sourced oil and gas generated from the Tengge’er and Arshan Formations. (2) Mineralization is achieved through the mixing of fluids from different sources. Furthermore, a genetic model related to uranium mineralization in the paleochannels of the Luhai area has been established: favorable uranium reservoirs were formed during the sedimentary period, and during the post-sedimentary stage, reverse structures promoted redox reactions and fluid-mixing-induced mineralization. The research findings can provide guidance for the exploration of paleochannel sandstone-type uranium deposits in other areas of the Erlian Basin.

1. Introduction

As an important strategic resource and non-fossil energy, uranium is of great significance for social production and economic development [1,2]. The main types of uranium deposits include sandstone-type, unconformity-type, breccia-type, granite-type, etc. Among these, due to its advantages, such as low mining costs and environmental friendliness, the global production proportion of sandstone-type uranium deposits has increased from 48.7% in 2015 to 63.3% in 2021, and it has become the most important type of uranium deposit utilized globally [3,4,5]. In China, uranium resource exploration and development are also widely concentrated on sandstone-type uranium deposits and have achieved remarkable results. A number of considerable sandstone-type uranium deposits that can be mined by in situ leaching have been discovered in the Yili Basin, Turpan-Hami Basin, Ordos Basin, Erlian Basin, and Songliao Basin in the north [2,6,7,8,9,10,11,12,13,14,15].

The Erlian Basin is located in the central and eastern part of Inner Mongolia and is rich in various mineral resources, such as uranium, coal, and oil [16]. Since the 1990s, medium- to large-scale uranium deposits, such as Subeng, Nuheting, Bayanwula, Hadatu, Saihangaobi, and Engeriyin, have been discovered in the basin. The Erennao’er uranium metallogenic belt represented by the Nuheting sedimentary-diagenetic type and the Basaiqi uranium metallogenic belt represented by the Bayanwula paleochannel type have been formed [9,10,17,18,19,20,21,22,23,24,25]. These uranium deposits occur in the sedimentary layers of paleochannels and are tabular in shape, differing from the paleovalley-type uranium deposits in Mongolia and Russia. Previous researchers have conducted detailed studies on the paleochannel tectonic–sedimentary characteristics [26], the stratigraphic division of the Saihan Formation in the paleochannel [27,28], the geological characteristics of uranium deposits [17], and fluid dynamics [29] in the Erlian Basin. These studies have explored the uranium mineralization mode and established the “multiple types in the same basin” uranium mineralization model [9]. The Luhai uranium deposit is a large-scale uranium deposit newly discovered in recent years through comprehensive prospecting methods. It is located in the Ba-Sai-Qi paleochannel uranium metallogenic belt, and its uranium mineralization characteristics are basically the same as those of the existing paleochannel uranium deposits. Currently, only a small amount of research has been carried out on the lithogeochemical characteristics and sediment provenance characteristics of the deposit [30,31,32].

In this study, a series of analyses were carried out on the ore-bearing layers and deposit geochemistry of the Luhai deposit. The aims were as follows: (i) to understand the sedimentary environment of the upper part of the Late Cretaceous Saihan Formation hosting uranium mineralization; (ii) to study the mineralogical and geochemical characteristics of lithofacies with different colors; (iii) to research the geochemical characteristics of ore-forming fluids; (iv) to reveal the uranium mineralization mechanism and genetic model. This research is expected to deepen the metallogenic theory of paleochannel uranium deposits in the Erlian Basin, which is important for expanding the prospecting vision in the area.

2. Geology Setting

2.1. Erlian Basin

The Erlian Basin is located in the eastern part of the Central Asian Fold Belt [33]. It is a large intra-continental basin formed during continental extension against the background of long-term subduction and consumption of the Paleo-Asian Ocean (Ren et al., 2002; Charles et al., 2013) [34,35]. The Erlian Basin is distributed in a northeast direction. Its eastern boundary is the Daxing’anling Horst, the northern boundary is the Bayanbaolige Horst, the southern boundary is the Wenduermiao Horst, and the western boundary is the Solonshan Horst. The basin is about 1000 km long from east to west and about 20–40 km wide from north to south, with a total area of approximately 10.9 × 10⁴ km². Internally, it can be divided into six secondary tectonic units, namely the Chuanjing Subbasin, the Wulanchabu Subbasin, the Manite Subbasin, the Tengge’er Subbasin, the Unite Subbasin, and the Sunite Horst [36,37,38] (Figure 1). The basement of the basin is mainly composed of Paleozoic medium-shallow metamorphic rocks, Jurassic coal-bearing clastic rocks and volcanic rocks, as well as Hercynian–Indosinian–Yanshanian multi-stage intermediate-basic to acidic magmatic rocks [39,40,41,42]. The caprock is mainly composed of the Lower Cretaceous Bayanhua Group (K₁B), the Upper Cretaceous Erlian Formation (K₂e), the Paleogene Yierdingmanha Formation (E₂y), the Neogene Miocene Tonggur Formation (N₁t), the Pliocene Baogeda Wula Formation (N₂b), and the Quaternary clastic rock series. Quaternary basalt is locally developed. The Bayanhua Group (K₁B) is divided into the Arshan Formation (K₁ba), the Tengge’er Formation (K₁bt), and the Saihan Formation (K₁bs) from bottom to top. Among them, the Tengge’er Formation is the main oil-and- gas-bearing stratum, and the Saihan Formation and the Erlian Formation are the main uranium-bearing strata [10,25]. Since the Late Paleozoic, the Erlian Basin has experienced multiple tectonic movements, such as the Hercynian, Indosinian, Yanshanian, and Himalayan movements, accompanied by multiple intrusions of magmatic rocks and volcanic eruptions. The surrounding provenance areas are widely distributed with Permian and Late Jurassic intermediate-acidic volcanic rocks, as well as Late Hercynian and Early Yanshanian granites, which are the main sources of sediment, ore-forming materials, and uranium sources.

The Central Asian Fold Belt has undergone a long-term tectonic evolution characterized by terrane accretion, arc–arc collision, and arc–continent collision and collage from the Proterozoic to the Mesozoic in the Paleo-Asian Ocean [43,44]. In the eastern part of the Central Asian Orogenic Belt, during the Meso-Cenozoic, it was mainly superimposed by the subduction processes of the Mongol-Okhotsk Ocean and the (Paleo-) Pacific Plate [45]. which led to multi-stage tectonic superposition and transformation of the Erlian Basin. Based on the characteristics of basin tectonic development and sedimentary filling history, the Mesozoic–Cenozoic tectonic evolution of the Erlian Basin can be divided into the following four stages: the Triassic stage of strong plate collision compression and uplift; the Jurassic stage of tectonic regime transformation; the Cretaceous stage of rift basin development and extinction; and the Cenozoic stage of strike-slip compression and uplift of the Erlian Basin. Influenced by tectonic inversion, two large sedimentary hiatuses have formed in the Erlian Basin since the Early Cretaceous, namely the hiatus between the Saihan Formation and the Erlian Formation, and the hiatus between the Erlian Formation and the Naomugen Formation. The strata are in angular unconformity contact, and several small-scale sedimentary hiatuses have also formed [25]. The uranium mineralization ages in the Erlian Basin are concentrated in the Meso-Cenozoic. For example, the mineralization age of the Nuheting deposit is 55–3 Ma [46], the U-Pb isotopic metallogenic age of the Bayanwula uranium deposit is 78–37 Ma [18,47], and the U-Pb isotopic metallogenic age of the Manglai deposit is 59.7–22.4 Ma [47], showing the characteristics of stage-by-stage mineralization. The uranium mineralization ages are basically consistent with the sedimentary hiatus periods, indicating that the above-mentioned two large sedimentary hiatuses had a significant impact on uranium mineralization in the Erlian Basin.

2.2. Luhai Deposit

The Luhai area is located in the Baiyinwula depression, a secondary tectonic basin within the Manite Subbasin of the Erlian Basin (Figure 1 and Figure 2A). To the northwest, it is separated from the Bayinbaolige Horst by the Ulanole Bulge. To the southeast, it is adjacent to the Sunite Horst by the Bayanwula Bulge (Figure 2A). The Baiyinwula depression is located in the middle of the basin, with a basement depth of 600–1000 m. The filling sequence consists of the Lower Cretaceous, Tertiary, and Quaternary of the Cenozoic. This basin hosts the uranium deposit and is controlled by two normal faults on the north and south sides. These faults form a local discharge for groundwater migration. Existing research shows that the provenance and uranium source of the upper part of the Saihan Formation in the Luhai area come from the Bayinbaolige Uplift Zone to the north and the Sunite Uplift Zone to the south, presenting a pattern of two-way supply from the north and south [23,32,48].

The uranium-bearing rock series, the Saihan Formation in the Luhai mining area, is a post-rift sediment in the Erlian Basin [42,49,50,51] (Figure 3A). According to the characteristics of sedimentary sequences and lithological changes, the Saihan Formation in this area is divided into three members [24,32]. The lower member of the Saihan Formation is mainly an alluvial fan or fan–delta deposit, developing a coarse clastic rock combination, such as variegated conglomerate or conglomeratic coarse sandstone. The middle member of the Saihan Formation is a lacustrine–swamp facies deposit composed of thick gray, dark gray, and black mudstones intercalated with brown coal seams. This layer is an important coal-producing stratum in this area. The bottom of the upper member of the Saihan Formation is gray gravel-bearing coarse sandstone, and the top consists of alternating red and gray-yellow sand and mud. This layer represents a fluvial facies sedimentary environment and is the main uranium-bearing horizon in the study area. Only the middle and upper members of the Saihan Formation were exposed in the drill holes of this work (Figure 3B).

The gray sand bodies at the bottom of the upper member of the Saihan Formation are the main host rock of the ore. The thickness of the sand body ranges from 30 to 87 m, and it is controlled by the northeast-trending paleochannel, with an irregular band-shaped distribution. In the cross-section, the ore bodies are located in the gray sand bodies in contact with the yellow oxidized sand bodies. The uranium ore body is 1000–8000 m long and 300–2400 m wide, with an average grade of 0.0157% U. Due to different structural positions in different ore sections, the burial depths of the ore bodies vary.

3. Materials and Methods

The core and logging data from 121 uranium exploration drill holes were used to research the sedimentary facies, sand body thickness, characteristics of the aquiclude within the ore-bearing layer of the upper member of the Saihan Formation. Eleven drill holes, including five drill holes through the Luhai deposit (L21, L64, L81–L83) and six exploration drill holes (T1, L76–L77, L85, L91–L92) were logged and sampled. The lithology of the samples includes gray pebbly coarse sandstone, medium-coarse sandstone, and siltstone. Seven electron microprobe samples were collected to explore the types and occurrence forms of uranium minerals. Twelve C-O isotope samples were collected to explore the formation conditions of carbonate cements in the ore-bearing sandstone and the sources of ore-forming fluids. Twelve acid-hydrolyzed hydrocarbon samples were collected to explore the geochemical characteristics of hydrocarbon gases and their influence on mineralization. In order to understand the hydrochemical characteristics of groundwater in the mining area, six groundwater H-O isotope samples were collected from two hydro-exploration holes (SZK01 and SZK02) and four domestic water wells. Additionally, 99 geochemical index samples (such as Fe³⁺/Fe²⁺, organic carbon, total sulfur, etc.) collected during the deposit exploration process were statistically organized to discuss the redox reactions.

Electron Probe Micro-Analysis (EPMA) was carried out at the Tianjin Geological Survey Center of the China Geological Survey. The instrument model used was EPMA1600 (Shimadzu Corporation, Kyoto, Japan). The test conditions were as follows: the acceleration voltage was 15 kV, the beam current was 20 nA, the minimum diameter of the beam spot could reach 1 μm, and 12 elements, including U, Th, Si, Fe, Ti, P, and Mn, were tested. The main focus was on backscatter analysis, micro-area chemical composition determination, and elemental surface scanning of the uranium-bearing sandstone probe slices.

The C-O isotope tests were completed at the Testing Research Center of the Beijing Research Institute of Uranium Geology, China National Nuclear Corporation. The sandstone samples were reacted with 100% phosphoric acid at 25 °C for 4 h to extract the CO₂ gas from the cement carbonate. The collected CO₂ was measured using a MAT253 stable isotope mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The oxygen isotope ratio is based on the Standard Mean Ocean Water (SMOW) standard, and the carbon isotope ratio is based on the Pee Dee Belemnite (PDB) standard, denoted as δ¹⁸O and δ¹³C, respectively, with an analysis precision better than ±0.2‰. The carbonate reference standard used in the analysis was the GBW04417 calcite standard [52], with δ¹⁸O and δ¹³C values of −24.12‰ and −6.10‰, respectively. The conversion relationship between the two expressions of δ¹⁸O_V-PDB and δ¹⁸O_V-SOW is as follows: δ¹⁸O_V-SOW = 1.03091 × δ¹⁸OV-PDB + 30.91.

The acid-hydrolyzed hydrocarbon tests were completed at the Testing Research Center of Beijing Research Institute of Uranium Geology, China National Nuclear Corporation. The analysis instrument used was a GC7890F gas chromatograph (Agilent Technologies, Inc., Santa Clara, CA, USA). The analysis items included the contents of methane (C₁), ethane (C₂), propane (C₃), normal/isobutane (nC₄/iC₄), and normal/isoamylane (nC₅/iC₅). The analysis error was less than 5%.

The groundwater H-O isotope tests were completed at Beijing Kehui Testing Technology Co., Ltd. The testing instrument was the Liquid Water Isotope Analyzer (LGR LWIA-24d) (ABB inc - Los Gatos Research, San Jose, CA, USA). The oxygen isotope ratio was standardized against the SMOW, and the per-mil deviation (‰ VSMOW) relative to the Vienna Standard Mean Ocean Water (VSMOW) was calculated. The testing errors for δD and δ¹⁸O were 0.3‰ and 0.036‰, respectively.

The testing of geochemical index samples was completed at the Analysis and Testing Research Center of the Beijing Research Institute of Uranium Geology. The iron (Fe) was tested by X-ray fluorescence spectrometry (XRF), the organic carbon was tested by gas chromatography, and the total sulfur and S²⁻ were tested by inductively coupled plasma–optical emission spectrometry (ICPS).

4. Results

4.1. Characteristics of Host Rocks and Uranium Ore

4.1.1. Sedimentary Features of the Saihan Formation

Using the core and logging data of drill holes in the mining area and through the analysis of lithofacies, sedimentary sequences, and electrical properties, two types of sedimentary facies, namely braided river and meandering river, have been identified in the upper member of the Saihan Formation (Figure 3B). The braided river sediments include braided channel subfacies and floodplain subfacies. The underlying sediment is well-developed, with a relatively large thickness, coarse particle size, and well-developed pebbly sandstone; a thin layer of mudstone is at the top.

The braided channels are the host of the ore-bearing horizons in the Luhai area. They are formed by the gradation of multiple phases of braided channels and include two sedimentary microfacies: channel lag and mid-channel bar. The channel lag deposits are mostly located at the bottom of the single-phase braided channel. The lithology mainly consists of gray, massive-bedded, pebbly coarse sandstone and medium-fine sandstone. The mid-channel bar deposits are located above the channel lag deposits, mainly composed of light gray pebbly coarse sandstone. The thickness of a single layer is typically 5–10 m. Massive bedding and large-scale trough cross-bedding and tabular cross-bedding are predominantly developed. The resistivity logging curve is mainly medium-to-high amplitude box-shaped or bell-shaped.

4.1.2. Petrological Characteristics of the Saihan Formation

The ore-bearing rocks in the Luhai area are mainly gray, dark gray, and green-gray medium-coarse sandstone and pebbly sandstone, intercalated with a small amount of gray and dark gray mudstone and siltstone. Carbonized plant debris and pyrite are commonly seen in the sandstone. The sandstone facies are characterized by well-sorted, medium to coarse grains with sub-angular shape and poor maturity. They consist of 85% clasts of granite, rhyolite, and andesite composition (Figure 4A). They are cemented by carbonate cements, kaolinite, and argillaceous cements, etc. Carbonate cements are distributed in the ore-bearing sandstone in the form of intergranular cements, and postdate the quartz overgrowths. The main types are sparite calcite and micrite calcite (Figure 4B). Pyrite is relatively common in the ore-bearing sandstone, distributed as massive or granular forms. It mostly fills in dissolution pores or fractures between detrital grains in the form of networks, veinlets, or granules, and is often associated with calcite (Figure 4C,D).

4.1.3. Mineralogical and Geochemical in Different Color Facies

Vertically, there is a relatively obvious redox zonation in the ore-bearing area of the Luhai region (Figure 3B). The oxidation zone is located in the upper part of the upper member of the Saihan Formation. The main type of epigenetic oxidation is yellow oxidation. The oxidized rocks come in several types, such as orange-yellow, yellow-green, dark-yellow, and bright yellow. The redox transition zone is located in the middle-lower part of the upper member of the Saihan Formation, which is gray gravel-bearing sandstone intercalated with yellow gravel-bearing sandstone, gradually transitioning to gray gravel-bearing sandstone. Uranium ore is mainly produced in this horizon. The reduction zone is the middle member of the Saihan Formation, mainly gray and gray-black mudstones, with local coal seams developed. In some areas, uranium mineralization also occurs in the gray-black mudstones near the river channel scour surface. From the overall vertical zonation, the oxidation in sandstones gradually weakens from top to bottom.

Based on the analysis results of 99 rock geochemical environment index samples in the Luhai area (

Table 1. Statistical comparison table of rock geochemical environment indicators in the upper member of the Saihan Formation in the Luhai area.

Petrogeochemical Types	Rock Color	Fe3+/Fe2+	Organic Carbon (10−2)	Stotal (10−2)	S2− (10−2)
Oxidation Zone (45 samples)	Yellow, brown, red, purplish red, etc.	2.59	0.10	0.076	0.009
Reduction Zone (54 samples)	Light gray, gray, dark gray, greenish gray, etc.	0.78	0.23	0.221	0.031

), the average value of Fe³⁺/Fe²⁺ in the oxidation zone is 2.59, while that in the reduction zone is 0.78. The average content of organic carbon in the oxidation zone is 0.10%, and in the reduction zone, it is 0.23%. The average content of S_total in the oxidation zone is 0.076%, and in the reduction zone, it is 0.221%. The average content of S²⁻ in the oxidation zone is 0.009%, and in the reduction zone, it is 0.031%. The Fe³⁺/Fe²⁺ ratio in the oxidation zone is significantly higher than that in the reduction zone, while the contents of organic carbon, S_total, and S²⁻ in the oxidation zone are significantly lower than those in the reduction zone.

4.2. Occurrence Forms of Uranium

The occurrence states of uranium in the Luhai area can be divided into two major categories: uranium minerals and adsorbed uranium. The main uranium mineral is pitchblende, followed by uranium–iron–titanium oxides and a small amount of coffinite. According to the scanning electron microscopy analysis, the particle size of uranium minerals is mostly less than 1 μm. They mostly occur in the form of fine grains and coatings. The main minerals associated with uranium minerals include pyrite, calcite, quartz, iron–titanium oxides. Pitchblende is the most important uranium mineral, which is deposited at the edges of quartz and feldspar clastic grains or fills the fractures. Iron–titanium oxides are an important mineral associated with uranium minerals (

Table 2. Electron probe analysis results of iron–titanium oxides in the Luhai deposit.

Measurement Point	Test Number	Results (wB/%)
		MgO	SiO2	P2O5	ThO2	UO2	CaO	TiO2	V2O5	Cr2O3	MnO	FeO	CoO	Total
1	WT18B1	0.05	1.21	0.21	0.07	1.20	0.19	71.89	0.76	0.08	1.48	10.83	0.05	88.01
2	WT18B2	0.04	0.95	0.17	0.03	0.84	0.14	70.36	0.69	0.02	1.00	13.09	0.04	87.35
3	WT18B3	0.05	1.03	0.18	0.00	0.87	0.15	69.77	0.66	1.66	0.79	11.86	0.06	87.06
4	WT18B4	0.02	0.95	0.13	0.00	0.38	0.12	73.46	0.62	0.09	1.41	14.83	0.06	92.07
5	WT18B5	0.01	0.16	0.01	0.01	0.08	0.04	57.18	0.44	0.05	2.93	29.88	0.05	90.86
6	WT18B6	0.00	1.04	0.06	0.03	0.07	0.12	59.84	0.52	0.03	0.26	20.41	0.05	82.44
7	WT18B7	0.01	1.08	0.09	0.03	0.10	0.15	56.89	0.44	0.06	0.33	20.42	0.02	79.61
8	WT18B8	0.00	0.90	0.03	0.00	0.05	0.14	56.20	0.41	0.07	0.26	21.75	0.04	79.86

), mostly in the form of irregular elliptical shapes (Figure 5A). The content range of TiO₂ is 56.20%~73.46%, with an average of 64.45%; the content range of FeO is 10.83%~29.88%, with an average of 17.88%; the content range of UO₂ is 0.05%~1.20%, with an average of 0.45%. UO₂ is negatively correlated with FeO and positively correlated with TiO₂. The contents of Fe and Ti elements show a decreasing trend in the uranium-enriched areas (Figure 5B–F). Pyrite has also developed inside the iron–titanium oxides (Figure 5C), indicating a certain degree of enrichment of the Fe and S elements. This shows that in addition to the migration of Fe and Ti during the alteration of ilmenite and the uranium enrichment process, the S element also appears. Studies have shown that detrital iron–titanium oxides can sensitively indicate the geochemical environment related to the positioning of sandstone-type uranium deposits [53]. The phenomenon of pyrite enclosing ilmenite indicates that uranium mineralization mainly occurs at the redox front.

4.3. Characteristics of Hydrogen and Oxygen Isotopes in Groundwater

The results of H-O isotope analysis of five groundwater samples are shown in

Table 3. Analysis results of H-O isotopes in the groundwater of the ore-bearing horizons in the Luhai area.

No.	Serial Number	Well Depth (m)	Sampling Horizon	δD V-SMOW (‰)	δ18O V-SMOW (‰)	T (T.U)
1	SDS30	110	K1bs	−93.98	−12.16	<1.0
2	SDS23	38	K1bs	−94.18	−12.24	<1.0
3	SZK01	126	K1bs	−94.07	−12.15	<1.0
4	SDS87	90	K1bs	−90.68	−11.87	<1.0
5	SZK02	156	K1bs	−95.34	−11.96	<1.0

. The range of δD in the groundwater samples from the Luhai area is −95.34‰ to −90.68‰, and the values of δ¹⁸O range from −12.24‰ to −11.87‰, with average values of −93.65‰ and −12.08‰, respectively. Since the Luhai area is not among the isotope monitoring stations for precipitation by the International Atomic Energy Agency, the local meteoric water line of the adjacent Baotou station, where δD = 6.4δ¹⁸O − 4.07 [54], and the global meteoric water line (δD = 8δ¹⁸O + 10) were selected as references. As can be seen in the distribution characteristic diagram of stable hydrogen and oxygen isotopes (Figure 6), the groundwater in this area is located on the right side of both the global meteoric water line and the local meteoric water line, showing an obvious positive offset of ¹⁸O.

4.4. Characteristics of Carbon and Oxygen Isotopes of Carbonate Cements

The results of the carbon and oxygen isotope analysis of the calcite cement in 12 samples are shown in

Table 4. Analysis results of carbon–oxygen isotopes of calcite cements in the ore-bearing sandstones of the upper member of the Saihan Formation in the Luhai area.

No.	Sample No.	Lithology Description	Measuring Results
			δ13CV-PDB (%)	δ18OV-PDB (%)	δ18OV-SOW (%)
1	2019TY1	Gray gravel-bearing coarse sandstone	−6.6	−20.0	10.3
2	2019TY2	Gray gravel-bearing coarse sandstone	−4.8	−18.5	11.8
3	2019TY3	Gray gravel-bearing coarse sandstone	−5.8	−18.6	11.7
4	2019TY4	Gray gravel-bearing coarse sandstone	−3.3	−17.5	12.9
5	2019TY5	Gray gravel-bearing coarse sandstone	−3.7	−16.7	13.7
6	2019TY6	Yellowish-green siltstone	−0.6	−15.0	15.4
7	2019TY7	Gray medium-grained sandstone	−4.4	−15.3	15.1
8	2019TY8	Gray coarse sandstone	−4.8	−11.5	19.0
9	2019TY9	Gray gravel-bearing coarse sandstone	−4.5	−16.6	13.8
10	2019TY10	Gray siltstone	−3.4	−14.9	15.5
11	2019TY11	Gray coarse sandstone	−6.4	−24.0	6.2
12	2019TY12	Gray medium-grained sandstone	−6.3	−17.1	13.2

. The variation range of δ¹³C_V–PDB is from −6.6‰ to −0.6‰, with an average of −4.55‰. The variation range of δ¹⁸OV-PDB is from −24‰ to −11.5‰, with an 476 average of −17.1‰; δ¹⁸OV-SMOW ranges from 6.2‰ to 19‰, with an average of 13.2‰. It can be observed that the ranges of the C and O isotope compositions in the samples are basically consistent.

4.5. Geochemical Characteristics of Hydrocarbon Gases

The results of acid-hydrolyzed hydrocarbon tests on 12 ore-bearing sandstone samples are shown in

Table 5. Analysis results of acid-dissolved hydrocarbons in ore-bearing sandstones in the Luhai area.

No.	Original Sample Number	Summary of Measuring Results (μL/kg)								C1/∑C	C1/C2+
		C1	C2	C3	iC4	nC4	iC5	nC5	Total Hydrocarbons (∑C)
1	2019TY1	0.20	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.50	0.40	0.67
2	2019TY2	0.32	0.09	<0.05	<0.05	<0.05	<0.05	<0.05	0.66	0.48	0.94
3	2019TY3	0.33	0.08	<0.05	<0.05	<0.05	<0.05	<0.05	0.66	0.50	1.00
4	2019TY4	0.29	0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.59	0.49	0.97
5	2019TY5	0.25	0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.55	0.45	0.83
6	2019TY6	0.32	0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.62	0.52	1.07
7	2019TY7	0.36	0.08	<0.05	<0.05	<0.05	<0.05	<0.05	0.69	0.52	1.09
8	2019TY8	0.43	0.12	0.05	<0.05	<0.05	<0.05	<0.05	0.80	0.54	1.16
9	2019TY9	0.40	0.09	0.05	<0.05	<0.05	<0.05	<0.05	0.74	0.54	1.18
10	2019TY10	0.41	0.10	0.05	<0.05	<0.05	<0.05	<0.05	0.76	0.54	1.17
11	2019TY11	0.35	0.10	0.05	<0.05	<0.05	<0.05	<0.05	0.70	0.50	1.00
12	2019TY12	0.21	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.51	0.41	0.70

. The contents of hydrocarbons, such as CH₄ and C₂H₂, as well as the total hydrocarbon content (∑C), in the ore-bearing sandstone are extremely low. The content of C₁ is all less than 1 μg/kg, ranging from 0.2 to 0.431 μg/kg. The ∑C ranges from 0.50 to 0.80 μg/kg, with an average of 0.65 μg/kg. The ratio of C₁/C₂₊ is 0.67–1.18, with an average of 0.98, indicating that the hydrocarbon gases in the adsorbed hydrocarbons in the ore-bearing sandstone are mainly methane.

5. Discussion

5.1. Sedimentary Facies of the Saihan Formation

The sedimentary analysis of the core indicates that the upper part of the Saihan Formation corresponds to the superposition of several-meter-thick upward-fining sequences. The facies structure is characterized by a series of imbricated sand-dominated river channels (with good lateral continuity of sand bodies), interbedded with relatively thin and discontinuous mud-dominated floodplain sediments. Coarse-grained to fine-grained reduced sand layers are stacked, forming the ideal permeable ore-hosting rocks for the development of sandstone-type uranium mineralization. Based on the isopleth map of the sand content in the upper member of the Saihan Formation, the areas with a sand content higher than 50% are delineated as braided channels, and the areas with a sand content lower than 50% are delineated as floodplains. Figure 7 represents the depth, thickness, and sedimentary facies. The Luhai area is dominated by braided gravel channel, lag, and floodplain deposits with a north–south trend. The thickness of a single channel is generally 5–20 m, and the width is generally 1–5 km. The channel sand bodies cut each other and are interconnected both vertically and horizontally, at a large scale. The floodplain facies is limited, mainly distributed at the edges of the channels. Currently, the controlled ore bodies are all located at the axis positions of the braided channels, which is conducive to the infiltration of oxygen-containing fluids in the later stage and mineralization.

5.2. Oxidizing Fluids

5.2.1. Source of Ore-Forming Fluids

From the perspective of the mineralization process of sandstone-type uranium de-posits in sedimentary basins, uranium deposits are the products of a dynamic process in which ore-forming elements are transported and accumulated with water as the carrier. The tritium value of groundwater can help determine the age and renewal rate of groundwater. A relatively low tritium value usually indicates that the groundwater is relatively old and has a slow renewal rate. The tritium values of groundwater in the Luhai deposit are relatively low (³H < 1TU) (Table 3), indicating that the groundwater formed during a period when the environmental tritium content was low, and it has not been significantly replenished by tritium-containing water sources, such as modern atmospheric precipitation, during its long geological history. Combined with the analysis of sedimentary characteristics, the ore-bearing horizons in the upper member of Saihan Formation has high water abundance and good permeability. The source of groundwater is mainly ancient atmospheric precipitation, lacking hydraulic connection with modern and recent atmospheric precipitation or surface water bodies, and the circulation and renewal rate is slow, which is conducive to the enrichment and mineralization of uranium elements in the aquifer.

The properties of ore-forming fluids vary among different types of sandstone-type uranium deposits [55]. The oxygen isotope composition of calcite cements related to mineralization can be used to constrain the source of the precipitation fluid [56]. Field investigations have revealed that the carbonate content in some uranium ores is high. Previous uranium mineralogical studies have shown that pitchblende is symbiotic with calcite. Therefore, the calcite cements coexisting with uranium minerals can serve as an indirect indicator for tracing the ore-forming fluids. The variation range of δ¹⁸O_V-PDB of calcite cements is from −24‰ to −11.5‰, with an average of −17.1‰; δ¹⁸O_V-SMOW ranges from 6.2‰ to 19‰, with an average of 13.2‰, which is higher than the δ¹⁸O_V-SMOW values derived from endogenic fluids [57,58,59]. The variation range of the carbon isotope δ¹³C_V-PDB is from −6.6‰ to −0.6‰, with an average of −4.55‰, which is relatively consistent with the variation range of δ¹³C of the groundwater in the ore-bearing horizons in the Luhai area, which is from −5.90‰ to −6.5‰ (unpublished data). This indicates that the carbon in carbonate cements mainly comes from the CO₃²⁻ formed by the dissolution of atmospheric CO₂ in the formation fresh water. However, the δ¹³C of calcite cements is significantly higher than the variation range of δ¹³C of organic origin, which spans from −18‰ to −33‰ [57,58], slightly higher than the average δ¹³C of CO₂ in the atmosphere (generally around −7‰) [60] and lower than the variation range of δ¹³C of marine sedimentary carbonates, which spans from −1‰ to 2‰. It also overlaps with the variation range of δ¹³C of freshwater sedimentary carbonates, which is −4.93‰ ± 2.75‰ [61]. This shows that when the carbonate cements were formed, the interstitial water preserved during sedimentation alone could not have resulted in such a wide range of δ¹³C values. During the diagenesis and mineralization processes, heavy carbon from deeper sources must have been added. In fact, in the Proterozoic–Paleozoic basement of the Erlian Basin, clastic rocks and carbonates of neritic facies and paralic facies are common. The Hegenshan deep fault runs through the mining area. Under the action of the confining pressure of the basin and gravitational compaction, deep brines dissolved marine carbonates and became rich in CO₂, which may have migrated along the faults to the ore-bearing sandstone layers in the shallow part of the basin. Based on the characteristics of the carbon and oxygen isotopes of calcite cements, it can be determined that the ore-forming fluids are mainly surface fresh water that entered the strata during the tectonic uplift stage, with local mixing of deep brines and formation water.

5.2.2. Migration of Oxidizing Fluids

The migration of meteoric oxidized fluids in sandstone-type uranium deposits is mainly influenced by the driving force of gravity. The structural high terrain and structural slope formed by later tectonic activities serve as the recharge windows and driving forces for the ore-forming fluids. In the northern part of the Luhai area, the NE-trending Saihantala normal fault is present. The hanging wall of the fault is the Saihan Formation of the Lower Cretaceous, and the footwall is the Tengge’er Formation of the Lower Cretaceous. This fault was formed after the deposition of the ore-bearing strata. With tectonic compression, this fault changed from an early normal fault to a later reverse fault (according to exploration data from the Baiyinwula Coalfield), causing the Saihan Formation on the footwall side of the fault to be uplifted to near the surface, where it received vertical downward infiltration of oxygen- and uranium-containing water. When the oxygen- and uranium-containing fluids entered the permeable sand and gravel layers in the upper member of the Saihan Formation, they migrated laterally. The oxidation direction advanced from the NW to the SE, and ore bodies formed in the areas where the reduction of organic matter in the dark mudstones and coal seams was relatively strong (Figure 2B).

5.2.3. Role of the Mudstone Partition

In the ore-bearing aquifers of the Luhai area, two sets of stable aquitards developed above and below (Figure 2B). The lower partition is composed of gray and grayish-black mudstones, silty mudstones, and lignite seams in the limnetic facies of the middle member of the Saihan Formation. It is continuously and stably distributed and has good waterproof performance. The upper partition is composed of red, reddish-brown, and light gray mudstones and argillaceous siltstones in the floodplain of the upper member of the Saihan Formation. It is stably distributed and has good continuity. The two sets of mudstones above and below play a restrictive role as partitions for the flow of uranium-bearing and oxygen-containing fluids. Uranium mineralization often occurs in the channel sand bodies above the lower partition.

5.3. Reducing Agents

Reducing agents are crucial for the mineralization of sandstone-type uranium deposits. The enriched areas of sandstone-type uranium mineralization are usually located in the oxidation–reduction transition zones. The enrichment and mineralization of uranium involve the reduction of activated U⁶⁺ to stable U⁴⁺ uranium minerals. Commonly, reducing agents mainly include carbonized plant debris, pyrite, H₂S, oil and gas, etc. The analysis of geochemical environmental indicators shows that the contents of organic carbon, total sulfur, and S^2- in the reduction zone are significantly higher than those in the oxidation zone. These characteristics reflect that the enrichment of uranium is closely related to the content of reducing substances (such as organic matter and pyrite) in the rock. This is consistent with the field observations, where carbonized plant debris and pyrite are commonly seen in the ore-bearing section, indicating that terrestrial plants and pyrite may be the main reducing substances.

There are three main types of sources of hydrocarbon gases: biogenic gas, oil-type gas, and coal-formed gas. For biogenic gas, the C₁/ΣC value ranges from 0.99 to 1.0, and the C₁/C₂₊ value is greater than 100; for oil-type gas, the C₁/ΣC value ranges from 0.70 to 0.98, and the C₁/C₂₊ value ranges from 2 to 10; for coal-formed gas, the C₁/ΣC value of ranges from 0.90 to 0.99, and the C₁/C₂₊ value ranges from 10 to 100 [54]. For the ore- bearing sandstone samples of the Luhai deposit, the C₁/ΣC value is 0.40–0.54, with an average of 0.49, and the C₁/C₂₊ value is 0.67–1.18, with an average of 0.98. These values are closer to the characteristics of oil-type gas, but the ratios are lower than oil-type gas, which may be related to the relatively low maturity of hydrocarbon gases. The above analysis shows that the hydrocarbon gases in the ore-bearing sandstone of the Luhai deposit are mainly in the immature to early mature stage of oil-type gases, which is related to the thermal evolution degree of the sapropel-prone lacustrine source rocks in the Lower Cretaceous Arshan Formation (K₁ba) and Tengge’er Formation (K₁bt) of the Erlian Basin. In addition, the contents of CH₄ and C₂₊ are directly proportional to the uranium content, suggesting that oil and gas, as reducing media, have participated in the uranium mineralization process in this area [62].

5.4. Uranium Mineralization

As mentioned above, it can be determined from the carbon and oxygen isotope characteristics of calcite cements that the ore-forming fluids were mainly surface fresh water that entered the strata during the tectonic uplift stage, with local mixing of deep brines. The geochemical characteristics of the hydrocarbon gases indicate that the deep brines were also mixed with oil and gas mainly composed of methane from the Arshan Formation (K₁ba) and Tengge’er Formation (K₁bt). The metallogenic mechanism of the Luhai deposit can be summarized as follows: In the upper member of the Saihan Formation, thick sandstone appears with good porosity and permeability. Oxygen- and uranium-containing water entered the permeable along the tectonic slope zone. When it encountered reducing agents, such as organic matter, pyrite, and hydrocarbon-rich fluids from deeper sources, U(VI) was reduced to U(IV), forming precipitates such as pitchblende.

5.5. Uranium Ore Genetic Model

The combination of the host sandstone type, uranium mineral chemical composition, stable isotopes of carbonate cements, and organic chemicals suggests a certain genesis model for the Luhai uranium deposit, as follows (Figure 8):

• During the sedimentation period of the upper member of the Saihan Formation, the sedimentary system in the Luhai area transformed into a terrestrial fluvial facies, forming a typical river valley landform. The horst areas on both sides of the basin provided abundant provenance and uranium sources [23,32,48]. The uranium-rich parent rocks were weathered and eroded, and then transported and deposited in the thick and stable sand bodies in the ancient river channels. During the early burial–diagenesis stage, the rock fragments were altered, and uranium was pre-enriched in iron–titanium oxides.
• From the late Early Cretaceous to the early Paleogene, a long-term sedimentary hiatus formed in the Luhai area [25]. The differential uplift structure lifted the top strata of the Saihan Formation to the surface or near-surface [19], enabling the oxidation of supergene fluids to reach them. Surface water, carrying a large amount of uranium, infiltrated vertically along the sandstone. After entering the permeable sandy conglomerate layer in the upper member of the Saihan Formation, it migrated laterally and underwent large-scale inter-layer oxidation. Uranium was enriched under the reduction of organic matter in the underlying dark mudstone of the sandstone. With the continuous effect of compressional tectonics, the Saihantala Fault on the north side of the Luhai area became more active. Deep brine and oil and gas began to migrate upward to the shallow part of the basin along the fault and mixed with the oxygen-and uranium-containing fluids from the surface, resulting in the superimposed enrichment of uranium.

6. Conclusions

• The braided channel sandstone in the upper member of the Saihan Formation provides favorable permeable ore-hosting rocks for the formation of sandstone-type uranium mineralization. The distribution of uranium ore bodies is controlled by channel lag deposits, while the floodplain deposits, which are interbedded with the ore-hosting sandstone, act as reducing traps.
• The metallogenesis of the Luhai sandstone-type uranium deposit is characterized by superimposed polygenetic oxidation–reduction mineralization. Oxygen- and uranium-containing groundwater migrated laterally to the deep along the edges of paleochannel and erosion windows and reacted or mixed with reducing substances in the strata, such as oil and gas, leading to the unloading of ore-forming elements and the formation of uranium ore bodies.
• A genetic model related to the uranium metallogenesis in the paleochannel of the Luhai area has been established. Stable fluvial facies sedimentation in the context of a depression-type basin has formed favorable uranium reservoirs. The tectonic inversion in the late sedimentation stage was conducive to the redox reaction between surface water and deep reducing substances (fluids) and the formation of ore through fluid mixing. This model plays an important guiding role in the search for paleochannel sandstone-type uranium deposits in other areas of the Erlian Basin.