Tracing Sources of Geochemical Anomalies in a Deeply Buried Volcanic-Related Hydrothermal Uranium Deposit: the Daguanchang Deposit, Northern Hebei Province, North China Craton

0. INTRODUCTION

Uranium is an essential raw material for nuclear energy. With continuing economic and societal development, the current uranium resources cannot meet the society's needs (IAEA, 2020; OECD-NEA/IAEA, 2018), and the need to develop new techniques of detecting deeply buried uranium deposits has become urgent in recent years.

Tracing the source of geochemical anomalies is significant for detecting deeply buried deposits. Some elements related to deposits have a strong migration ability and can produce anomalies in the deposits above the ore body, such as radiogenic lead (Quirt and Benedicto, 2020). In theory, He and Rn, two gasses produced by uranium decay, are more mobile than other products of uranium decay. Moreover, these two elements can be easily enriched in the wall rocks and surface soil above deeply buried uranium ores (McCarthy and Reimer, 1986). Furthermore, Rn has a strong affinity for organic material (Leaney and Herczeg, 2006). It is also known that plant roots incorporate Ra (Gunn and Mistry, 1970); so, they release Rn at shallow depths. Rn migration is affected by the lithology, soil layers, diffusion, and its short half-life (3.823 d); so, it rarely travels over long distances (Szabó et al., 2017). Therefore, the migration mechanisms of Rn from depth to the surface are still under debate. The main migration mechanisms of Rn include diffusion (Baxter, 2010), convection (Papp et al., 2008; Klusman and Jaacks, 1987), relay transmission (Cao et al., 2005), and multi-agent mechanisms (Xie and Wang, 2003). Previous studies have shown that the instantaneous Rn concentration of soil was effective in exploring uranium deposits (Baskaran, 2016; McCarthy and Reimer, 1986; Gingrich, 1984). However, the origin of surface Rn anomalies, especially genetic links to deeply buried deposits, is still unclear.

Jia et al. (2002) discovered the cluster effect of He and Rn, that is, Rn, which has a slow diffusion rate, accelerates upward when coated with He. The radioactive decay via the U series generates a large amount of ⁴He. Moreover, the thermal energy produced by the decay of U and its daughter products is conducive to the escape of Rn and He through microscopic cracks and fissures (Dyck, 1976). Therefore, the ³He/⁴He ratios of drill core samples from a uranium deposit can verify the origin of the surface Rn anomalies and can be used to investigate the relationship between the surface Rn anomalies and the deep uranium deposit.

Current production from uranium deposits globally is mainly from unconformity, sandstone, and volcanic-related uranium deposits (Cuney et al., 2022; Jin et al., 2022; OECD-NEA/IAEA, 2018). Volcanic-related hydrothermal uranium deposits are one of China's most important types of uranium deposits, although exploration has most recently focused on sandstone-hosted U deposits (Dahlkamp, 2009). They are characterized by high grades (0.019%–0.36% U) and a wide distribution range, and they are mainly located within or close to caldera complexes (IAEA, 2018). However, many volcanic-related uranium deposits are overlain by volcanic rocks and sediment cover and remain undiscovered, making tracing the geochemical source of He and Rn vital to the exploration of these deposits. The localized, high-grade nature of these deposits may provide clear information about geochemical anomalies, which is convenient for tracing their origin.

In China, volcanic-related uranium deposits are strongly associated with Mesozoic volcanic magmatism, such as the Dazhou uranium district (Yang et al., 2013), the Xiangshan ore-field (Wang et al., 2021; Guo et al., 2014), and the Caotaobei uranium deposit (Yang et al., 2017) in the southeastern part of China. Similar Mesozoic volcanic magmatism is widely developed in Northern China, therefore, Northern China has a vast potential for volcanic-related hydrothermal uranium mineralization (Jiao et al., 2021). The Daguanchang uranium deposit is a typical volcanic-related uranium deposit, and a few similar deposits have been discovered in the North China Craton (Wu et al., 2017). In this paper, we selected the Daguanchang uranium deposit to test the usefulness of using the Rn content and helium isotopic ratio as geochemical source tracers to reveal the presence of deeply buried uranium ores.

1. GEOLOGICAL SETTING

The volcanic-related uranium deposits in the Guyuan Basin occur in rhyolitic ignimbrite, rhyolitic breccias, and trachyte accompanied by illitization (Wu et al., 2014). These deposits are fault- and fracture-controlled and are closely related to the Early Cretaceous felsic volcanic rocks in terms of location and genesis (Shi et al., 2017; Wu et al., 2014; Yang et al., 2006). The study area has a temperate semi-arid continental monsoon climate, with an average annual temperature of 2.1 ºC, a mean annual precipitation of 405 mm, and a mean annual evaporation of 1 200 mm (Wang et al., 2009).

The Daguanchang uranium deposit is located in the eastern part of the Guyuan Basin, along the northern margin of the North China Craton (Figure 1) (Bao et al., 2013; Zhu, 2012). The lithology of the Guyuan Basin consists of the Neoarchean Hongqiyingzi Group metamorphic basement rocks (plagioclase amphibolite gneiss, leptite, biotite plagiogneiss, marble, and K-feldspathized migmatite), which are overlain by the Mesozoic–Cenozoic terrestrial facies volcanic-sedimentary rocks (Khomich and Boriskina, 2018). The Mesozoic stratum in this area is composed of the Lower Cretaceous Zhangjiakou, Dabeigou, Yixian, and Qingshila formations from bottom to up, of which the Zhangjiakou Formation is unconformably overlain by the Dabeigou Formation (Figure 1) (Wu et al., 2017). The Dabeigou and Qingshila formations are predominately siltstone and sandstone, whereas dominate the Yixian Formation is predominantly composed of andesitic rocks. The Zhangjiakou Formation is divided into three lithologic sections. The lower section (K₁z¹) is dominated by rhyolitic tuff, rhyolitic ignimbrite, and breccia tuff; the middle section (K₁z²) consists of trachyte and quartz trachyte; and the upper section (K₁z³) consists of rhyolite and rhyolitic ignimbrite (Figure 1) (Wu et al., 2017; Chen et al., 2013). The uranium mineralization is mainly hosted in the upper section, which is divided into five beds (beds 1, 2, 3, 4, and 5 from bottom to top) and is composed of tuffaceous sandstone, rhyolitic crystal tuff, cryptoexplosive breccias, ignimbrite, and siltstone (Wu et al., 2017). The trachyte of the middle section of the Zhangjiakou Formation and the tuff, potassium rhyolite, and tuff sandstone of the Upper Zhangjiakou Formation are exposed in the study area (Figure 2).

Figure  1.  Geological sketch map of Guyuan area (modified after Wu et al., 2017).

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Figure  2.  (a) Simplified regional tectonic map; (b) geological map and sampling area in the Daguanchang uranium deposit (modified after Zhao et al., 2018).

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The distributions of the Zhangmajing, Daguanchang, and Xiwan Uranium deposits in this area are restricted by the regional fault systems, including NE trending faults F45 and F7 and the NS trending Chicheng Guyuan fault (i.e., fault F44; Figure 2). The uranium ore bodies are primarily developed at the intersection of the NE and N-S trending faults (Figure 2).

In the study area, the primary uranium mineralization occurred in the cryptoexplosive breccia and rhyolite of the upper section of the Zhangjiakou Formation (Wu et al., 2017). The average grade is 0.05% U, and more than 70 orebodies have been identified. The individual orebodies are 40–100 m long, 40–80 m wide, and 1–4 m thick (Shen et al., 2009). Most of the orebodies are deeply concealed (~300–900 m from the surface) and are distributed along the volcano-sedimentary beds as veins and lenticular bodies (Han and Xue, 2016). The types of ore in the deposit mainly include disseminated U-Mo ore and uraniferous hematite veins (Fang et al., 2018). The minerals in the deposit mainly include uraniferous minerals brannerite-uraninite, and molybdenite, as well as galena, pyrite, sphalerite, hematite, quartz, fluorite, and some secondary minerals (Shen et al., 2009). The secondary and adsorptive uranium minerals include brannerite and pitchblende (Han and Xue, 2016). The pitchblende exists in quartz with star-like forms or as veinlets. The hydrothermal wall rock alteration mainly includes sericitization, silicification, fluoritization, hematitization, and chloritization (Zhu, 2012).

2. SAMPLING AND METHODS

2.1 Instantaneous Soil Rn and Analyses

In this study, the measurements of the Rn content of the soil gas were conducted on July 24, 2017, during a stable period of sunny and dry weather conditions. The measurements were conducted over a short period of time to minimize any variations induced by different sampling periods. Thirty-five instantaneous Rn contents were measured in the field at a depth of 80 cm from the surface, with a 50 m × 50 m grid interval and an area of 300 m × 200 m, including both mineralized and barren areas (Figure 3). All of the samples were collected from the same soil horizon, and the concentrations of the gaseous species that permeate the soil pores were measured.

Figure  3.  Contour map of Rn concentrations.

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The Rn measurements in the field were conducted using an FD216 environmental enthalpy-measuring instrument, which was developed by the Beijing Hedi Scientific and Technological Development Center, Beijing Research Institute of Uranium Geology (BRIUG). Two scintillation cell detectors supported this instrument. When the soil gas enters the scintillation cells, the indirect detection of energetic particles impinging on the wall is conducted by detecting the light emitted from the Rn and its daughters products (Coleman, 2002). The light is converted into electrical pulses via a photomultiplier, and the Rn content is proportional to the number of pulses per unit time. The detection limit is higher than 0.68 cpm/(Bq·m^-3), and the relative standard deviation is better than 10%. The contour plot of the Rn content (Figure 3) was generated using MAPGIS. The instantaneous Rn content measurement procedure can be divided into three stages. First, slender pipes (2 cm in diameter and 100 cm long) were punched into the soil to a depth of 80 cm. After extraction of the pipes, suction tubes connected to the FD216 measurement equipment were inserted into the holes, and the soil gas was pumped out for 2 min. The top of the suction tube was sealed with soil to avoid contamination by atmospheric air, and the results were acquired after 5 min.

2.2 Drill Core Samples and Analyses

All of the drill core samples were unmineralized rocks selected from two drill holes, and the samples were obtained from the core storage library. One drill hole (ZK1) encountered the uranium ore at a depth of 856 m, and the other drill hole (ZK2) encountered the correlated, unmineralized volcanic rocks. Four porphyritic trachyte samples from ZK1 were obtained from depths of 64, 137, 210 and 377 m. The other four samples collected from ZK2 were rhyolite and porphyritic rhyolite, and these were obtained at depths of 123, 150, 220 and 320 m.

The helium isotopic compositions were measured using the Helix-SFT noble gas mass spectrometer at the Analytical Laboratory of BRIUG. After the samples were crushed, approximately 300 mg of coarse (0.5 cm in diameter) fresh rock pieces were loaded into on-line vacuo crushers. The samples were baked at 250 ºC for 48 h to remove any atmospheric contaminants adhere on the surface of the samples, and crushing apparatus equipped with an oil-free molecular pump was used for vacuuming. The gases were released from the samples into an all-metal extraction system. The values of the background of the whole system and the helium isotope standard were measured using the ion source parameters of the helium isotope measurements and stabling for 30 min. Then the samples were crushed, purified, and measured (Li et al., 2015). The experimental error of the ³He/⁴He ratios is about 1% at 1σ (Tardani et al., 2016).

The U and Th contents of the drill core samples were analyzed at the Analytical Laboratory of BRIUG. The samples were crushed in a steel ball mill and ground in an agate mill to a grain size of < 200 mesh (whole-rock powders) at the Mineral Separation Laboratory of the Regional Geological Survey in Langfang, Hebei Province. Approximately 50 mg of whole-rock powder was dissolved in high-pressure Teflon bombs using an HF + HNO₃ mixture and was analyzed using the inductively coupled plasma mass spectrometer (ICP-MS) (NexION300D) at the Analytical Laboratory of BRIUG. The accuracy of the analyses is estimated to be < 2%. The analytical procedures are similar to those described by Qu et al. (2004).

3. RESULTS

3.1 Instantaneous Rn Contents

The instantaneous Rn contents are presented in Table 1. The Rn contents vary from 176.9 to 13 027.2 Bq/m³, with a standard deviation of 2 365.5 Bq/m³. They exhibit a skewed distribution (Figure 4a), with a skewness value of 3.1 and a kurtosis value of 12.7, and a lognormal distribution (Figure 4b).

Table  1.  The instantaneous Rn concentration in soil gas

Sample number	Rn (Bq/m3)		Sample number	Rn (Bq/m3)		Sample number	Rn (Bq/m3)
GY-1	938.4		GY-16	261.5		GY-31	2 521.5
GY-2	933.0		GY-17	2 546.1		GY-32	1 377.9
GY-3	842.9		GY-18	990.3		GY-33	1 823.1
GY-4	4 325.7		GY-19	1 538.9		GY-34	2 636.2
GY-5	4 699.6		GY-20	1 538.9		GY-35	1 083.1
GY-6	6 872.3		GY-21	3 621.5		Minimum	176.9
GY-7	3 184.8		GY-22	559.1		Maximum	13 027.2
GY-8	13 027.2		GY-23	911.2		Mean	2 225.1
GY-9	300.3		GY-24	960.5		Median	1 538.9
GY-10	2 199.5		GY-25	176.9		Std. deviation	2 365.473
GY-11	2 308.6		GY-26	556.3		Coefficient	1.06
GY-12	892.0		GY-27	1 820.1		Skewness	3.149
GY-13	3 471.4		GY-28	1 358.8		Kurtosis	12.674
GY-14	1 710.9		GY-29	1 410.6		Frequency distribution type	Skewed distribution
GY-15	731.0		GY-30	2 207.6

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Figure  4.  (a) Histogram of Rn concentration; (b) lognormal quartile-quartile plot of Rn concentrations. In Figure 4b, the x-axis represents the quantile values of the standard normal distribution of the Rn content of the soil gas, and the y-axis represents the quantile value of the dataset after logarithmic transformation.

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The analyses were used to create a contour plot, which was overlain with the projection of the U mineralization on the surface, showing the Rn concentrations throughout the study area (Figure 3). There are 20 completed drill holes around the soil gas sampling area, 12 of which have encountered mineralization. The projection of the U mineralization onto the surface was drawn based on these 12 mineralized holes. The map shows that the Rn concentrations in the southeast are much higher than those in the northwest, and the highest values are between samples GY-6 and GY-8. The Rn concentrations near the concealed uranium ores are about 10 times higher than those near the barren areas.

3.2 Helium Isotopes

The ³He/⁴He ratios of the drill core samples from ZK1 are relatively uniform from surface to deep underground, varying from 3.48 × 10^-7 to 4.33 × 10^-7 (Table 2). The samples from ZK2 have higher and more variable ³He/⁴He ratios than those from ZK1, ranging from 4.84 × 10^-7 to 9.79 × 10^-7 (Table 2 and Figure 5).

Table  2.  U and Th content and ³He/⁴He ratios of samples from ZK1 and ZK2

Drill hole	Number	Depth (m)	Lithology	Th (ppm)	U (ppm)	3He/4He ($\times$ 10-7)	3He/4He① (Ra)	4He② ($\times$ 10-5)
ZK1	1	64	Porphyritic trachyte	13.9	2.52	3.48	0.25	9.79
	2	137	Porphyritic trachyte	13.3	2.14	3.57	0.26	8.91
	3	210	Porphyritic trachyte	15.4	1.92	3.97	0.29	9.39
	4	373	Porphyritic trachyte	11.7	1.93	4.33	0.31	7.92
ZK2	1	123	Rhyolite	17.9	5.38	4.84	0.35	16.15
	2	150	Rhyolite	15.5	2.48	8.37	0.60	10.36
	3	220	Porphyritic rhyolite	23.9	2.99	9.79	0.70	14.60
	4	320	Rhyolite	24.5	1.26	6.54	0.47	11.96
① 3He/4He (Ra) ratios are calculated using 3He/4He of air (1.39 × 10-6). ② In-situ 4He. These are calculated using the 4He atoms g-1·yr-1 = (3.115 × 106 +1.272 × 105) [U] + 7.710 × 105[Th] equation (Ballentine and Burnard (2002)). Here 4He is the production rate of 4He in units of 10-6 atoms g-1·yr-1, and U and Th content are in ppm.

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Figure  5.  Relationship between ³He/⁴He ratios and depth.

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3.3 Uranium and Th Contents

The U and Th contents of the whole-rock samples range from 1.26 ppm to 5.38 ppm and from 11.7 ppm to 24.5 ppm, respectively (Table 2 and Figure 6). The Th contents of the samples collected from the two drill holes are significantly different, varying from 17.9 ppm to 24.5 ppm for ZK2 and from 11.7 ppm to 15.4 ppm for ZK1. The U contents exhibit slight differences, ranging from 1.93 ppm to 2.52 ppm for ZK1 and from 1.26 ppm to 5.38 ppm for ZK2.

Figure  6.  Variations in U and Th concentrations with ³He/⁴He ratio.

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4. DISCUSSION

4.1 Instantaneous Rn Content Soil Gas Anomaly

The instantaneous Rn in the soil gas originates from three main sources, i.e., the atmosphere, the decay of uranium in-situ, and deep material input, including mantle gas (in regions of magmatic activity) and radiogenic Rn from the uranium deposit that has migrated to the surface.

The sampling depth of the instantaneous Rn was 80 cm below the surface, which is the best compromise between reducing the impacts of weather and the method's practicality under field conditions (Kikaj et al., 2016). All of the samples were collected from the same depth during the same period to minimize the effects of weather. Thus, the influence of the atmosphere was normalized for comparison of the Rn among the different samples.

The Rn concentrations in the mineralized area (up to 13 027.2 Bq/m³) were much higher than those in the barren area (as low as 176.9 Bq/m³) and were far greater than the average value (about 1 890 Bq/m³) for the region (Zhao et al., 2018). Uranium has a long half-life (t_1/2 > 0.7 Gyr). Unless there is a large difference in the U content of the soil, it will not generate any significant difference in the instantaneous Rn contents of the barren and mineralized areas. Moreover, Kikaj et al. (2016) found that the Rn concentration of the soil has little relationship with the in-situ ²³⁸U content. In addition, a previous study has shown that the U contents of the soil in this area vary from 0.7 to 10.3 ppm (Shi et al., 2017). This variation is not sufficient to create the large observed difference in the Rn contents. Therefore, the higher Rn concentrations are not likely to reflect the decay of the uranium in the soil, and they should result from deep material input.

The migration of Rn and He is affected by the temperature, pressure (Fu et al., 2017; Groves-Kirkby et al., 2009), moisture (Kulalı et al., 2018; Barillon et al., 2005), grain size of the host rock (Lodge, 1988), and existence of fractures (Yang et al., 2018). In this area, the uranium orebodies (at a depth of 856 m from the surface) are covered by felsic volcanic lava and pyroclastic rocks. The porosities of the felsic volcanic lava and pyroclastic rocks are 0.3%–34% and 2.2%–32.8%, respectively, and they decrease with increasing burial depth (Huang et al., 2010). Thus, the overburden rocks of the uranium orebodies are conducive to the migration of Rn.

Faults can provide significant channels for the accelerating gas migration speed (Pérez-Flores et al., 2017), and the volcanic-related uranium deposits are located within or along the fault zones (Bonnetti et al., 2020). The existence of fractures and faults could increase the ascent velocity of gas derived from the decay of the uranium ore, allowing for a much larger gas concentration near the surface. Moreover, humic- or clay-rich regolith near-surface sediments exist, and they are filled with moisture, which reduces the rapid outgassing of Rn from the soil (Ball et al., 1991). Thus, the Rn and He formed by the decay of U and Th may be more detectable above the uranium deposit and at the surface.

Although the use of Rn as a geological tracer has been developed in connection with fault zone confirmation (Sciarra et al., 2018; Al-Tamimi and Abumurad, 2001; Gingrich, 1984), the He isotope ratios used to examine the Rn soil gas anomalies above the mineralized areas are not from the fault. Regional faults generally act as channels for mantle degassing (López et al., 2016; Das et al., 2009), while at the deposit scale, most of the faults are shallow and cannot transport mantle gases (Zhai, 1984). If the gas released from the mantle is transported through a fault, it should cause higher ³He/⁴He values of up to eight times that of the atmosphere ((³He/⁴He)_air = 1.39 × 10^-6) (Porcelli and Ballentine, 2002). The ³He/⁴He values measured in this study are significantly lower than the mantle gases and atmosphere, about 0.25–0.7 Ra (Ra = (³He/⁴He)_air). Moreover, Zhao et al. (2018) revealed that a low soil instantaneous Rn concentration occurs in the surface soil above the fault in the barren areas. As a result, the high Rn anomaly in the research area is likely produced by the deeply buried uranium ore, which is consistent with the interpretation based on the geological phenomena that mineralization occurs near the site with the highest instantaneous Rn content.

4.2 He Isotopes and U and Th Content Anomaly in Drill Cores

Previous studies have only focused on surface soil anomalies (Tabar et al., 2013; Chaudry et al., 2002; Hinkle and Dilbert, 1984). The He isotope ratios at a variety of depths are also relevant to deeply buried uranium deposits. Helium has two stable isotopes, ³He and ⁴He. ³He is mainly primordial helium with trace amounts produced by cosmic radiation, and the ³He content remains stable at depths below 30 m from the surface (Dickin, 2018). In this study, all of the drill-core samples were from depths of > 60 m, which excludes the presence of cosmogenic ³He in the samples. Thus, the ³He/⁴He ratios reflect changes in the ⁴He concentration. ⁴He is a product of α-decay of U and Th, in which the parent element loses two protons and two neutrons. It is composed of in-situ radiogenic ⁴He from the decay of the local U and Th and the migration of ⁴He produced by the decay of deeply buried sources, such as uranium deposits (Tan et al., 2018).

The drill core samples from ZK1 have lower ³He/⁴He ratios and U and Th contents compared with those from ZK2 (Figure 6and Table 2). The number of in-situ ⁴He atoms produced in 1 gram of rock per year can be calculated using the following equation: ⁴He atoms g^-1 yr^-1 = (3.115 × 10⁶ + 1.272 × 10⁵) [U] + 7.710 × 10⁵[Th], where the U and Th concentrations are in ppm, according to Ballentine and Burnard (2002). According to the calculation, the U and Th in the samples from ZK1 and ZK2 can ideally produce 7.96 × 10^-5–9.84 × 10^-5 cc STP ⁴He rad/g and 1.04 × 10^-4–1.63 × 10^-4 cc STP ⁴He rad/g in 139 Ma, respectively (Table 2). In this area, the eruption age of the volcanic rocks is 139 Ma. Kurz (1986) and Craig and Poreda (1986) showed that samples from a similar stratigraphic horizon yielded MORB-like helium, and Fisher (1985) calculated that the ³He content ranges from 35 × 10^-12 to 450 × 10^-12 cc STP/g. The cosmogenic amount of ³He decreases as the depth increases, and it can be regarded as a constant at depths of < 30 m (Lal, 1987). We assumed that the ³He content is 80 × 10^-12 cc STP/g. Based on the ⁴He calculated from the U and Th contents, the ³He/⁴He was calculated to be 5.0 × 10^-7–10.1 × 10^-7. In Figure 6, the straight line is fitted, and the inclination angle is slow, indicating that the changes in U and Th within a few concentration (ppm level) have little effect on the ³He/⁴He ratio. Thus, the in-situ radioactive ⁴He in ZK1 is less than that in ZK2, leading to higher ³He/⁴He ratios based on the U and Th contents. However, the ³He/⁴He ratios of the samples from ZK1 are lower than that from ZK2, indicating the addition of external ⁴He.

The additional ⁴He may have resulted from several processes, including air contamination and the mixing of He derived from a mantle source and deeply buried sources. The ³He/⁴He ratios of the atmosphere and mantle are 1.39 × 10^-6 and 1.1 × 10^-5–1.4 × 10^-5, respectively (Porcelli and Ballentine, 2002), which are higher than those of the samples (Figure 5). If we take the ³He/⁴He ratios of ZK2 as the background values, the samples from ZK2 that mixed with air contamination or mantle He would have higher ³He/⁴He ratios, which is inconsistent with the samples from ZK1 (Figure 5). Moreover, ZK1 and ZK2 are both close to faults, excluding the possibility that local faults caused the different ⁴He contents. If there were no uranium deposits at depth, it would be challenging for sufficient ⁴He to migrate through faults to form the abnormal ³He/⁴He ratios. Therefore, the faults are not the most significant cause of the surface anomalies in this study, while the U orebodies are the most likely cause. As a result, the high ⁴He contents of the mineralized samples most likely resulted from the addition of ⁴He released from the Daguanchang uranium deposit. The ³He/⁴He ratios of the barren samples vary at different depths (Figure 5), indicating that the pressure, moisture, groundwater flow, and petrophysical properties of the samples affect the ³He/⁴He ratios significantly (Byrne et al., 2018; Wen et al., 2017; Darrah et al., 2014). In contrast, because of the addition of external ⁴He, the ³He/⁴He ratios influenced by sample characteristics is eliminated, resulting in the relatively stable ³He/⁴He ratios in the samples from ZK1. Thus, the low ³He/⁴He ratios are related to the Daguanchang uranium deposit. The lithology of ZK1 is porphyritic trachyte, while the lithology of ZK2 is porphyritic rhyolite. There are differences in the U and Th contents of these two lithologies. The ³He/⁴He ratios calculated from the U and Th contents are not very different in ZK1, but the difference is significant in ZK2. Under the influence of the uranium source, the ³He/⁴He ratios of ZK1 decrease systematically. This phenomenon convincingly demonstrates that the ³He/⁴He ratio may be a viable geochemical tracer of uranium deposits and is a potential method to locate buried uranium ores.

4.3 Indicators of Deeply Buried Volcanic-Related Uranium Deposits

Rn measurements have been widely used to explore uranium deposits, including aerial Rn surveys and soil gas Rn measurements (Baskaran, 2016). However, due to the slow diffusion and short half-life (3.823 d) of Rn, the migration mechanisms of Rn from depth to the surface are still under debate. The main mechanisms include diffusion (Baxter, 2010), convection (Papp et al., 2008; Klusman and Jaacks, 1987), relay transmission (Cao et al., 2005), and multi-agent mechanisms (Xie and Wang, 2003). The Guyuan Basin has a semi-arid landscape, and the uranium orebody is covered by felsic volcanic lava with emanation coefficients of 2.1%–32% at a depth of 825 m (Hassan et al., 2009). Eff-Darwich et al. (2002) showed that the diffusive transport of Rn does not extend for more than 2 m in the rock matrix; so, the soil gas is not significantly influenced by diffusion alone. The porosity of the felsic volcanic lava is 0.3%–34%, improving the migration of Rn. Jia et al. (1999) concluded that the He-Rn clusters expand the vertical transport distance of the Rn. In addition, plant root absorption, capillary action, atmospheric pressure, and evaporation promote Rn migration at shallow layers (at depths of tens of meters to hundreds of meters). Then, the Rn is enriched near the surface due to the humic- and clay-rich regolith, which prevents the Rn from escaping into the atmosphere.

The analysis conducted in this study reveals that the higher radiogenic ⁴He contents (lower ³He/⁴He ratios) of ZK1 agree with the Rn contents of the surface soil gas detected in the mineralized area. Rn generates He through alpha decay, and these He atoms can easily migrate through the pores and fractures due to their small molecular radius and low density (McCarthy and Reimer, 1986). Jia et al. (2002) proposed the He and Rn cluster effect, in which the slow Rn diffusion rate is accelerated when the Rn is wrapped by helium. Thus, the ³He/⁴He ratios of the drill core samples from various depths prove that the Rn in the soil gas may have been released from a buried uranium source. Both the theory and testing suggest that the He is consistent with the Rn as an indicator of deeply buried volcanic-related uranium deposits.

The Daguanchang uranium deposit is a typical deep concealed volcanic-related uranium deposit with buried ore at a depth of 856 m, which provides an opportunity to detect anomalous Rn released from the deeply buried uranium ore. The anomalous ³He/⁴He ratios of the drill cores are consistent with the Rn contents of the soil gas, demonstrating that the Rn in the soil gas is from Rn migration from the deeply buried ore. It was also demonstrated that the uranium ore exists under ZK1 in this study based on the ⁴He content calculated from the in-situ U and Th contents. In addition, the ³He/⁴He ratios of the ZK1 samples from different depths are relatively stable compared with those of the samples from ZK2, indicating that the overlying rock is deeply affected by the radioactive decay of the underlying uranium ore. Thus, the Rn content of the surface soil gas and the ³He/⁴He ratios of the drill core samples reveal the presence of an anomaly directly over the mineralization, which was caused by the decay of the buried uranium ore.

5. CONCLUSIONS

The instantaneous Rn contents above the Daguanchang uranium deposit are much higher than those in the barren areas. The instantaneous Rn content measurements can be used to detect concealed U ores, and this method is suitable for detecting deeply buried volcanic-related uranium deposits.

The He isotopic ratios of the unmineralized drill core samples from ZK1, which encountered the U orebody at a depth of 856 m, are lower due to the higher ⁴He contents and are relatively uniform with depth compared to those of the samples from ZK2, which is located between or outside of the ore zones. These differences reflect the addition of external ⁴He derived from the deeply buried U ore.

The variation in the He isotope compositions of the drill core samples is consistent with the variations in the instantaneous Rn contents of the soil gas. The lower ³He/⁴He ratios also prove that the Rn in the soil gas originated from the Daguanchang uranium deposit. Thus, the combination of these two methods has a great prospect for identifying deeply buried uranium ores.