Genesis of host rock in the Tamusu Mudstone pre-selected area in Northwest China for high-level radioactive waste geological disposal purposes

Abstract

Mudstone characterized by typical structures and mineral assemblages in the Tamusu Mudstone pre-selected area, in Inner Mongolia, Northwest China, has been selected as a potential host rock for China’s high-level radioactive waste (HLW) repository purpose. Based on indications of petrological and geochemical characteristics, hydrothermal sedimentary geneses of host rock are well discussed from the perspectives of hydrothermal sedimentary structures, sedimentary environments, and the hydrothermal sedimentary model. The results show that host rocks have the characteristics of low-temperature "white smoke" hydrothermal sedimentary rocks. The rocks developed in a dry, anaerobic and reducing saline lake environment with transient oxidation and dry–wet evolution. The 100–120 Ma tectonic activity in the Altun fault may have been the main factor controlling the process of hydrothermal exhalative, which provided the migration pathway for water–rock reactions and contributed to the formation of typical structures and mineral assemblages. The hydrothermal sedimentary rock is a new kind of argillaceous rock that serves as a potential host rock, and will provide new insights into suitability evaluations of mudstone properties to promote the site screening of the Tamusu Mudstone pre-selected area for China’s HLW disposal purpose.

1 Introduction

As one of the essential parts of multi-barrier systems for the permanent disposal of high-level radioactive waste (HLW), host rock (as a natural geological barrier) plays a critical role in ensuring the long-term safety of deep geological repositories (DGRs) (IAEA, 2003; Pan & Qian, 2009; SKB, 2004). Currently, crystalline rocks and argillaceous media are the two principal types of host rocks that are considered worldwide (Faybishenko et al., 2017). Argillaceous media (Clay rocks, mudstone, shales, etc.) are widely accepted as potential host rock due to their attractive properties, such as large rock masses, stable geological structures, low hydraulic conductivity, self-sealing potential of fractures and high sorption capacity for radionuclides (Bossart et al., 2017; Zhang, 2018).

On the Chinese mainland, the Beishan granite (in Northwest China’s Gansu Province) was preselected as a potential host rock for HLW repository purposes (Wang et al, 2018a), and an underground research laboratory (URL) has been under construction at the Xinchang Site since June 17, 2021 (CAEA, 2021). Moreover, based on the China’s “Guidelines for the R&D for Geological Disposal of HLW”, an argillaceous-based site should also be considered in site selection research in addition to the granite-based sites. The Tamusu pre-selected area in Inner Mongolia, Northwest China, is in an arid desert zone, and has a typical temperate continental climate; it is generally divided into four types: Gobi (50%), desert (30%), and mountains and hills (20%). The Tamusu area is approximately 24,327 km² in total, and according to a 2015 survey data analysis, the permanent population is 1594, with an extremely low population density of 0.07 people/square kilometer. The permanent population exceeds only 1.1% of the townships in the whole country. The economic strength of the Tamusu area is higher than only 11.8% of the townships in the whole country, highlighting the low population density and relatively underdeveloped economic level. As one of argillaceous-based site working areas, the Tamusu Mudstone has been selected as a potential host rock for China’s HLW repository. Preliminary work has confirmed that Tamusu Mudstones are mainly composed of dolomite and analcime, and the clay mineral content accounts for only approximately 10%. The main minerals are distinct from traditional clay stones that are used for HLW disposal purposes, such as the Callovo-Oxfordian (COX) clay in France and the Opalinus (OPA) clay in Switzerland (Xiang et al., 2020). However, the sedimentological and geochemical characteristics of the Tamusu Mudstone indicates the abnormal deposition process and the depositional environment and model are still unclear. In particular, the geneses of analcime and dolomite should be further explored, as these geneses play important roles in determining the rock property parameters, such as the sorption potential and mechanical properties. Because the site screening project is progressing, the genesis of the Tamusu Mudstone should be solved first to promote suitability evaluations on the suitability of the host rock in the Tamusu pre-selected area.

In this paper, the structural and geochemical characteristics of the Tamusu Mudstone are described in detail. Then, the hydrothermal sedimentary geneses are determined. Third, the structure of hydrothermal sedimentary rock, main mineral geneses, sedimentary environment and hydrothermal sedimentary model are thoroughly discussed. Finally, the basic performances of hydrothermal sedimentary rocks as potential host rocks are preliminarily analyzed. This study will lay solid foundations to suitability evaluations on the Tamusu Mudstone and to promote site screening in the Tamusu pre-selected area.

2 Geological background

2.1 The Bayingebi Basin

The Tamusu pre-selected area is located in the middle-southern part of the Alxa Block and the northeast margin of the Yingejing Sag, Bayingebi Basin, Northwest China (Fig. 1A). The horizontal stresses in the Alxa Block are at an intermediate or low level. The maximum horizontal stress in the Alxa Block is dominantly oriented in the NNE-SSW direction, which is consistent with the direction of the tectonic stress field of Northwest China (Du et al., 2017; Rao et al., 2021). Bayingebi Basin is located on the southern margin of the Xingmeng Orogenic Belt (Lin et al., 2001; Dou & Chang, 2003; Zhang et al., 2019) in the Eastern Central Asian Orogenic Zone (Zhou et al., 2011), which is framed by the Siberian Craton and the North China Craton (Fig. 1A) (Charles et al., 2013). The Yingejing Sag is located in the southern extremity of Bayingebi Basin and the southeastern margin of the Altun Fault, extending in a northeast direction. The northern margin of the sag is adjacent to the Zongnaishan-Salaza uplift, the southern margin is adjacent to the Bayanorgong uplift, and the eastern margin is adjacent to the Yingen Sag (Fig. 1B).

Fig. 1

Location of Yingejing Sag and lithologic histogram of TZK-1 (Xiang et al., 2020; Wei & Jiang, 2019) A The Location of the Yingejing Sag; B The Locations of the Tamusu Mudstone pre-selected area and the drilled boreholes; C the lithologic histogram of TZK-1

The formation and evolution of the Bayingebi Basin has undergone fundamental changes since the Mesozoic. Since the Triassic, the tectonic evolution of the Bayingebi basin has been mainly divided into the following four stages. (1) The first stage was generation of the Triassic-Jurassic pull-apart basin. Affected by the Indosinian movement, the basin was uplifting in the Early Triassic. In the Late Triassic, the basin entered the post-orogenic extensional relaxation stage, and the surface of the crust exhibited tensional tectonic conditions, forming a group of NE tensional tectonic systems. During the Jurassic period, due to the strike-slip action of the Altun fault, the basin developed NE and NE fault systems based on the basement fault, and the extensional fault developed in the Triassic; as a result, the basin is similar to a graben basin. (2) The second stage was the comprehensive development of the Early Cretaceous pull-apart basin. In the Early Cretaceous, affected by the continuous strike-slip of the Altun fault and its branch faults, the basin entered the stage of comprehensive development of the pull-apart basin. The northern sags further expanded on the basis of inheritance. The southern sags are controlled by the large regional fault, with tensional and torsional properties, showing the same trend as the Altun fault and its branches. (3) The third stage was the Late Cretaceous extensional depression stage. In the Late Cretaceous, regional rifting weakened as the volcanic eruptions declined. The regional compensation caused the sag to form in a larger area than that of the rift, and the basin shows overall sag subsidence. (4) The fourth stage was the Palaeogene intracontinental convergence stage. Under the influence of collision between the Indian plate and the Eurasian plate, the tectonic stress field of the basin changed from extensional to compressive stress, making the basin a tectonic setting of compressional uplift during this period.

Overall, the Altun fault zone has controlled the overall structural pattern of uplifts and depressions in the basin evolutionary process. The Tamusu pre-selected area widely received Lower Cretaceous sediments, including the lower part of the Bayingebi Formation and the upper part of the Bayingebi Formation during the Early Cretaceous in the stage of development of the pull-apart basin. In the late Early Cretaceous, structural inversion caused the upper part of the Bayingebi Formation to be directly uplifted and exposed to the surface. Since the neotectonic movement, the surface of the Tamusu pre-selected area has been extensively covered by Quaternary aeolian sand and gravel, forming its current appearance.

2.2 The target formation

The basement of the Yingejing Sag is composed of Archean, Proterozoic and Palaeozoic metamorphic rocks, which are overlain by Jurassic and Cretaceous sedimentary rocks, as well as Quaternary sediments. The Early Cretaceous strata could be divided into the lower Bayingebi Formation (K₁b¹) and the upper Bayingebi Formation (K₁b²). The lithology of unit lower Bayingebi Formation is mainly purple-red conglomerate and sandstone with occasional siltstone and mudstone, and its maximum burial depth is approximately 1400 m. The upper Bayingebi Formation (K₁b²) is the target formation, which consists of a succession of mudstone-sandstone-mudstone and has a maximum depth of ∼911 m(Wang et al., 2018b). The two sections of lacustrine mudstone with continuous thicknesses of more than 100 m are key research objects for host rock considerations. In particular, taking borehole TZK-1 as an example, the first unit with a maximum depth of ∼200 m consists primarily of fine-grained sandstone and silty mudstone, which were deposited in a semi-deep to deep lacustrine environment. The second unit (200–450 m) is composed mainly of yellowish-brown sandstone, siltstone, and grey mudstone, representing fan delta and braided delta plain sub-facies. The third unit (450–800 m) consists of dark grey mudstone, which formed in a semi-deep to shallow lacustrine environment (Fig. 1C).

3 Materials and methods

The collected boreholes from the Tamusu pre-selected area were carried out with water by single core barrelling techniques. Core samples from TZK-1 and ZK-2 were vacuum sealed to maintain freshness. Core samples from ZHK88-15, ZKH104-8 were provided by the Geological Team No. 208, CNNC. A total of 116 whole-mudstone samples from borehole TZK-2 were used for Conventional X-ray diffraction (XRD) analyses (D8 ADVANCE, Bruker, Germany). XRD analysis was carried out on whole-rock powdered samples with Cu-Kα radiation operating at 40 mA and 40 kV, and diffraction traces were recorded from 5 to 70°2θ and the semi-quantitative estimates of relative mass fractions were calculated using the Bruker Diffrac.EVA package. 7 mudstone samples from ZHK88-15, TZK-1, TZK-2 and ZKH104-8 were prepared for scanning electron microscope (SEM) tests (Nova NanoSEM 450, Czech Republic) and electron probe microanalysis (EPMA) tests (JXA-8530F Plus, Japan). The EPMA analytical conditions were as follows: an acceleration voltage of 15 kV, a current of 20nA, and a beam spot diameter of < 2 μm, with a 10 s counting time for the major elements, and 20 s or 40 s for the minor elements. The above-mentioned tests were completed in the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, China. 30 mudstone samples from borehole TZK-2 were tested for geochemical analyses (rare earth elements (REEs), trace elements and main elements) (ICP-AES, Agilent, America; ICP-MS, Perkin Elmer Elan 9000, America) that were completed by ALS Chemex (Guangzhou) Co Ltd, China. The accuracy and precision (relative deviation and relative error) of the detection method are controlled within 10 (± 5)%.

4 Results

4.1 Sedimentological characteristics of host rock

4.1.1 Mineral composition

Based on the whole-rock mineralogical analysis of 116 samples, the mass fractions of dolomite and ankerite range from 3 to 50% and from 4 to 25%, with average mass fractions of 28% and 13%, respectively. The mass fractions of analcime and calcite range from 1 to 43% and from 0.2% to 23%, with averages of 17% and 3%, respectively. The mass fractions of albite, quartz, hematite and pyrite are evenly distributed and range from 7 to 40%, 1 to 11%, 1 to 9% and 2 to 20%, with average values of 19%, 5%, 4% and 6%, respectively. The mass fractions of illite and kaolinite range from 2 to 6% and from 1 to 4%, with average mass fractions of 3% and 2%, respectively. Vertically, three facies could be distinguished (Fig. 6). The first facies (381.82–528.77 m, above Line E–F) is composed mainly of analcime, carbonate minerals and albite, with analcime being the most abundant. The second facies (528.77–723.79 m, between Line E–F and Line M–N) consists mainly of carbonate minerals, analcime and albite. The third facies (723.79–800.06 m, below Line M–N) is composed mainly of carbonate minerals and albite, showing the smallest analcime content. Notably, the host rock in the Tamusu pre-selected area is characterized by analcime and dolomite and could be defined as mudstone rather than traditional clay stone.

4.1.2 Mineral paragenesis

According to detailed thin-section observations and XRD analysis, four kinds of mineral assemblages in mudstone are present: I) dolomite–ankerite–analcime–albite; II) dolomite–ankerite–analcime–albite-quartz; III) dolomite–ankerite–albite–quartz–gypsum; IV) dolomite–ankerite–albite–quartz–pyrite. The relationships between paragenetic sequences and tectonic events are constructed in Fig. 2.

Fig. 2

Relationship between paragenetic sequences and tectonic events in Tamusu pre-selected area (after Zhang et al., 2019)

In the syn-sedimentary stage, the sandstone is immature, and detrital grains consist of quartz, feldspar, and rock fragments. Then, the structural inversion of the basin results in the oxidation stage, which occurs mainly at the contact between the oxidized sandstone and gray sandstone. The active phase of the Altun fault that occurred at 100–120 Ma indicates that a hydrothermal event occurred and provided a migration channel for the ascent of deep hydrothermal fluids (Zuo et al., 2015; Zhang et al., 2020). The host rock has undergone significant alteration during the hydrothermal activity stage, which is mainly manifested by the recrystallization and formation of new minerals. The recrystallization of carbonate, analcime and plate-like pyrite is widely distributed, which contributes to the formation of euhedral and subhedral coarse-grained carbonate minerals, analcime and cubic pyrite. A large number of fine dolomite and ankerite crystals are distributed along the edges or in the pores between detrital grains, forming the mud-supported texture. The hydrothermal event caused a general consolidation of the sandstone and mudstone in the post-stage via the cementation of carbonate and gypsum (Zhang et al., 2019).

4.1.3 Sedimentary structure

A number of borehole cores derived from boreholes TZK-1 and 2 (for the locations, see Fig. 1b) show that host rocks are mainly dark gray and gray-white, and they feature five kinds of typical sedimentary structures, i.e., the network vein filling, mud-supported, massive, laminar and syngenetic deformation structures (Fig. 3).

Fig. 3

Sedimentary structures of host rock in Tamusu pre-selected area.  a The network vein filling structures, the marked white network is composed of analcime, clay minerals and dolomite, Borehole TZK-2, 765 m; b dark argillaceous minerals mixed with analcime and powder-fine dolomite, single polarization light; c “matrix-supported” structure, the marked white matrix are powder-fine-grained and coarse-grained dolomite, Borehole TZK-1, 595 m; d rhombic coarse-grained dolomite and powder-fine-grained dolomite are distributed in dark argillaceous minerals, single polarization light; e white and grey-white laminae developed in the dark grey mudstone; f the marked area is composed of dolomite, ankerite and analcime, Borehole TZK-2, 137 m, dyed thin section, single polarization light; g grey massive mudstone, Borehole TZK-1, 520 m; D1: mud-micro dolomite; D2: powder-fine dolomite; D3: coarse-grained dolomite; Ana: analcime; Ank: ankerite

The network vein filling structures generally crosscut host rock and characterized by white thick strips with widths varying from 1 to 8 mm (Fig. 3a). The white network is mainly composed of dolomite, analcime and microcrystalline calcite mixed with dark argillaceous matter (Fig. 3b). The mud-supported structures are observed as "snowflakes" distributed in dark gray mudstone (Fig. 3c). The coarse-grained dolomite is determined by typical rhombohedral crystal forms with sizes ranging from 0.5 mm to 1 mm (Fig. 3d). The laminar structure is one of common sedimentary structures, and an increase in lamina thickness to the millimeter or even centimeter level is called a banded structure (Fig. 3e). The laminas are mainly composed of powder-fine dolomite, ankerite and analcime (Fig. 3f). The contemporaneous deformation structures are well developed based on soft deformation and irregular ruffled bedding structures. As the main structural type of host rock, the massive structure is characterized by grey and dark grey mudstone with good homogeneity and a high degree of consolidation (Fig. 3g).

4.2 Elemental geochemistry

The percentages of major elements are shown in Table 1. More than 60% of the oxides in the samples are composed of SiO₂ (avg. 33.18%), CaO (avg. 12.17%), Al₂O₃ (avg. 11.09%), MgO (avg. 7.58%), and TFe₂O₃ (avg. 4.72%). The relatively high concentrations of loss on ignition (LOI) (avg. 20.77%), MgO and CaO confirm that the mudstone contains dolomite. The total combined content of TiO₂, P₂O₅ and MnO accounts for approximately 1% of the total oxides. Taking the average content of major elements in the upper continental crust (UCC) as the standard (Fig. 4a) (Taylor & Mclennan, 1985), host rock is enriched in Ca, Na and Mn, and relatively depleted in K and P.

Table 1 Main geochemical characteristics of the Tamusu hydrothermal-sedimentary rock

Fig. 4

Geochemical characteristics of the Tamusu Mudstone. a abundance characteristics of major elements; b abundance characteristics of trace; c comparisons of trace element abundances with typical hydrothermal sedimentary rocks; d distribution pattern diagram of REEs

The concentrations of trace elements are shown in Table 1. The calculated concentration coefficients display high enrichments in U, Sr, and V; and depletions in Ba, Rb, Sc, Th, and Zr (Fig. 4b). The abundance of typical deep-source gas–liquid trace Sr is up to 274 ppm. Compared with typical hydrothermal sedimentary areas in the Shahejie Formation and the Xiagou Formation, North China, trace elements show a consistently changing trend (Table 2, Fig. 4c).

Table 2 Comparisons of trace element in the Tamusu hydrothermal-sedimentary rock and other typical areas

The concentrations of rare earth elements (REEs) are shown in Table 1. The total concentration of REEs is 59.29–283.80 ppm (avg. 139 ppm). The ratio difference between light REEs (LREEs) and heavy REEs (HREEs) is more than 20 times, which further confirms the enrichment of LREE and deficiency of HREEs (Fig. 4d). The δCe (0.91–1.04, avg. 1.01) indicates a weak negative anomaly, while the δEu (0.55–0.70, avg. 0.62) shows a moderately strong deficit. REEs have characteristics of higher totals, enrichment in LREEs, depletions in HREEs, and strong positive δEu anomalies, which are different from marine hydrothermal sedimentary rocks (HREEs are higher than LREEs, and δEu anomalies are positive) (Klinkhammeretal et al., 1994; Zheng et al., 2018; Zhong et al., 2018).

Based on the above-mentioned mineral assemblages, typical structures and geochemical characteristics of Tamusu Mudstone, it could be concluded that Tamusu Mudstone belongs to a set of typical lacustrine hydrothermal sedimentary rocks. Moreover, typical hydrothermal structures, geochemical and mineral characteristics of the Tamusu Mudstone, sedimentary environment, and hydrothermal sedimentary model are well discussed.

5 Discussions

5.1 Hydrothermal sedimentary structures

Hydrothermal sedimentary rock is formed from the mixing of underground hot water or magmatic hydrothermal with seawater or lake water. According to characteristics of hydrothermal mineral assemblages and geochemistry, hydrothermal sedimentary rock can be divided into two types: "black smoke" and "white smoke" (Brown, 1992). The "black smoke" is featured by large amounts of metal sulfides at high temperatures (> 350 ℃). The "white smoke" is featured by silicate, aluminosilicate and carbonate rocks at low temperatures (32–320 ℃) (Scott, 1997; Zheng et al., 2018). Recent findings summarizing "white smoke" hydrothermal sedimentary rocks in an inland lake basin in Northwest China have indicated that the hydrothermal mineral assemblages are mainly composed of dolomite, ankerite, analcime and albite. The structure is dominated by lamellar structures, and typical structural features such as matrix-supported, banded, network and contemporaneous deformation, which are obviously different from burial and microbial origins (Zheng et al., 2018; Chen et al., 2018; Zhong et al., 2018; Zhu et al., 2020). Therefore, the above-mentioned mineral assemblages and typical structures of the Tamusu Mudstone are in accordance with the hydrothermal sedimentary structures.

• (1) Dolomite

According to crystal grain-size, Mud-micro, powder-fine and coarse-grained rhombic dolomite could be determined in the Tamusu sedimentary rocks. Mud-micro dolomite has a strip-like interbedded structure with analcime and argillaceous minerals (Fig. 5a, b). Powder-fine dolomite that is distributed in a network structure is semi-self-shaped, with grain sizes varying from 50 μm to 500 μm (Fig. 3d; Fig. 5c). The morphological differences in crystals indicate that mud-micro dolomite recrystallization to powder-fine dolomite could be excluded. Coarse-grained rhombic dolomite can be identified as having a matrix-supported structure, and the grain sizes are up to 0.5 mm and 1 mm (Fig. 3d, f).

Fig. 5

Typical minerals characteristics of host rock in the Tamusu pre-selected area    a Mixed laminae of mud-micro dolomite, analcime and argillaceous, Borehole TZK-2, 313.25 m, single polarization light; b mud-micro dolomite observed by SEM, Borehole TZK-2,752m; c field of views same as d analcime with first-order grey interference colors and, filling with dolomite. Analcime is mixed and distributed within the dark clay material, crossed polarization light, Borehole TZK-2, 638 m; e analcime crystals are dissolved into irregular tetragonal trisoctahedron, TZK-1, 593.53m; f typical tetragonal trisoctahedron analcime is observed by SEM, Borehole TZK-2,530 m; D1: mud-micro dolomite; D2: powder-fine dolomite; Ana: analcime; Ank: ankerite; Py: Pyrite

Dolomites with saddle-like and zebra-like structures have been proven to be products of hydrothermal metasomatism in marine hydrothermal dolomites (Davies & Smith, 2006; Feng et al., 2017; Liu et al., 2017; Koeshidayatullah et al., 2020). Dolomite with a microbial origin usually forms unique microstructures (mucosal, nanoparticle, fibrous network, radial and cluster structure, etc. (Bontognali et al., 2012; Zhai et al, 2017). Coarse-grained rhombic dolomite in the Tamusu Mudstone may be formed by hydrothermal deposits processes. The hydrothermal fluids may have generated vibration when the pressure in the environment changed from high-pressure to low-pressure, resulting in the rhombic structure of dolomite clasts. In addition, the flocculation of various chemical components is due to charged ions when the underground hydrothermal solution is mixed with cold lake water (Zhong et al., 2018). The coarse-grained rhombic dolomites are characterized by high Mg/Ca ratios (0.494–0.607, avg. 0.56) (Table 3), showing similar ratios to those of typical lacustrine hydrothermal areas, such as the Pingdiquan Formation in eastern Junggar (Li et al., 2017; Wen et al., 2014).

Table 3 EPMA analysis of analcime and dolomite in Tamusu hydrothermal-sedimentary rock

• (B) Analcime

Analcime belongs to the zeolite group, and is a tectosilicate with the chemical formula NaAl Si₂O₆∙H₂O, where Na could be replaced by small amounts of K⁺ and Ca²⁺. Analcime can be identified as the intercalation of dolomite and argillaceous layers. The irregular tetragonal trisoctahedron crystals have a matrix-supported structure. Analcime exhibits first-order grey interference colors, filling with dolomite (Fig. 5c, d). Moreover, typical and dissolved tetrahedral trisoctahedron are observed by SEM (Fig. 5e, f). Detailed observations failed to determine low-grade metamorphic minerals such as laumontite and prehnite, indicating that the genesis of metamorphic or primary analcime could be ruled out. The Si/Al ratios of analcime are characterized by high silica (2.316–2.866, avg. 2.545) (Table 3), which is obviously higher than the sedimentary diagenetic type of analcime formed by sedimentary diagenesis and low-grade metamorphism, and higher than that of low silicon analcime that is directly crystallized from highly alkaline water. This type has Si/Al ratios that are closer to those of high silicon analcime, which is commonly affected by siliceous volcanic glass and alkaline hydrothermal solution. Above-mentioned mineral assemblages generally show that hydrothermal fluid is mainly a medium–low temperature "white smoke" hydrothermal fluid that is rich in CO₃²⁻, Na⁺, Al³⁺, Si⁴⁺, Mg²⁺, Fe²⁺ and Ca²⁺ and relatively poor in metal sulfides (Zheng et al., 2018; Zhong et al., 2018). As a result, analcime may have formed by the direct crystallization of alkaline hydrothermal fluids mixed with lake water when alkaline hydrothermal fluids containing Na⁺, Ca²⁺ and CO₃²⁻ venttd out of lake bottom.

5.2 Sedimentary environment

• (1) Palaeoclimate

The calculated chemical index of alteration (CIA) values vary between 46.85 and 59.27, with an average of 51.03, and the Sr/Cu values are greater than 5, confirming a relatively dry sedimentary environment (Xiang et al., 2019). The dry–wet transitions of palaeoclimate at depths of approximately 450 m and 550 m are also determined by the vertical evolutions of the two indicators. Meanwhile, the calculated CIA values indicate that the samples with low weathering are not affected by proximal clastic deposition. A negative correlation was observed between the vertical trend of Sr/Cu values and that of Rb/Sr values, further confirming that mudstone underwent dry–wet transitions.

• (B) Palaeo-salinity

The Sr/Ba values are all greater than 1 (avg. 5.86), indicating a saline environment (Campbell & Wiliams, 1965). The vertical trend of Sr/Ba values is mostly synchronous with that of Sr/Cu values, which indicates that the salinity increases correspondingly as dryness increase. That is, the response of paleo-salinity change to dry–wet transitional features was also showed at the same depths in the target Bayingebi Formation.

• (C) Redox

The V/Ni and V/(V + Ni) values in Table 4 show a high correlation, indicating that the mudstone is under an anaerobic and a strongly reducing environments. Slight fluctuations occurred in anaerobic and reducing environment, which may have been the response of the abnormally dry climate at that time. Vertically, the redox environment correlates very well with the palaeoclimate change.

Table 4 Redox environment indexes for mudstone

The correlations between δCe and δEu, δCe and (Dy/Sm) failed to reach significant levels, indicating that the samples were not affected by diagenesis or were affected by weak diagenesis. Therefore, the REE data was used to discuss the depositional environment. The U/Th and normalized Ce values confirm the existence of a temporary oxidizing sedimentary environment under anaerobic and strongly reducing conditions. Among the 30 samples, 7 samples with U/Th < 0.75 and 3 samples with Ce_anom < − 0.1 confirm the existence of an oxidizing environment, and the depths of interfaces are highly coupled with the depths-changes in palaeoclimate and palaeo-salinity mentioned above.

The low-temperature "white smoke" hydrothermal sedimentary rocks are formed in arid, anaerobic and strongly reducing saline lake environments. Meanwhile, the consistent pyrite contents indicate a strongly reducing environment. Vertically, palaeoclimate fluctuations are synchronous with changes in the main mineral contents and typical sedimentary structures (Fig. 6). Those fluctuation interfaces are correlated; however, their internal connections and mutual influence need to be further explored.

Fig. 6

Vertical variations of geochemical parameters and mineral compositions of the Tamusu Mudstones

5.3 Hydrothermal sedimentary model

It is worth noting that the Altun fault at 100–120 Ma has experienced multi-stage activities (Li et al., 2006; Wu et al., 2010) and several typical lacustrine hydrothermal sedimentary rocks have been reported along the fault in recent years (Zhu et al., 2020). Examples include the Lower Cretaceous Xiagou Formation mudstone in the Jiuxi Basin (Zheng et al., 2018; Wen et al., 2014); the Lower Cretaceous Suhongtu Formation and Bayingebi Formation mudstone in the Hari Sag, Yiner Basin (Chen et al., 2018); and the Lower Cretaceous Tengger Formation mudstone in the Baiyinchagan Sag, Erlian Basin (Zhong et al., 2018). These typical hydrothermal sedimentary rocks mostly formed in the Early Cretaceous. The typical hydrothermal sedimentary areas in different basins that formed in the Early Cretaceous are located within a distance of nearly 1000 km. Therefore, the Late Cretaceous tectonic activities in the Altun fault may have been the main factor controlling the formation of these typical hydrothermal sedimentary rocks. Controlled by multi-periods activities of the Altun fault, interactions between volcanic eruptions (magmatic activity) and sub-lacustrine hydrothermal fluids have resulted in a set of typical low-temperature "white smoke" hydrothermal sedimentary rocks.

In the Early Cretaceous stage, the Bayingebi Basin was in a comprehensive pull-out stage that was controlled by the Altun fault. Under the action of gravity and overlying pressure, the lake water seeped down along the main faults and underwent water–rock reactions for a long time. The process extracted ions including Na⁺, Al³⁺, Si⁴⁺, Mg²⁺, Fe²⁺, and Ca²⁺ from the surrounding rock to form hot brine in the magma chamber. Then, driven by thermal energy, fluid potential energy and the heat gradient, the hot brine was discharged into the lake basin. The introduction of Mg²⁺ and the removal of Ca²⁺ are a key process in the formation of dolomite during the water–rock reactions, which provides the enough Mg²⁺-rich fluids. The reaction is 2CaCO³ + Mg²⁺ = CaMg(CO₃)₂ + Ca²⁺(Wang, 2014); Moreover, Na⁺-rich fluids provided by the water–rock reactions paly an important role in formation of analcime, and the reaction can be written as: KAlSi₃O₈ + Na⁺ + H₂O = NaAlSi₂O₆·H₂O + K⁺  + SiO₂; With proper temperature, analcime is transformed into albite, and the reaction can be written as: NaAlSi₂O₆·H₂O + SiO₂ = NaAlSi₃O₈·H₂O (Zhu et al., 2012; Shi et al., 2021). Therefore, a certain amount of albite is determined by the XRD analysis mentioned above. This continuous circulation of lake water infiltration and hydrothermal upwelling convection provided a heat source and mineralization ions for the lake basin.

When upwelling hydrothermal fluid migrates to the volcanic eruption vent, the difference in the crystallization differentiation of hydrothermal minerals and the formation of typical mineral assemblages were controlled by the difference in physical and chemical conditions due to the distance to the vent. As a result, typical minerals such as dolomite and analcime, formed, and their assemblages contribute to the laminar, mud-supported and other typical structures described above (Fig. 7).

Fig. 7

Sedimentary model of lacustrine hydrothermal rock of the Tamusu Mudstone

5.4 Suitability for HLW disposal considerations

The set of low-temperature "white smoke" hydrothermal sedimentary rocks in the Tamusu pre-selected are contributes to the higher contents of main minerals such as dolomite and analcime, which are distinct from COX and OPA clays. Shale classification ternary diagrams (Zhong et al., 2018) were used to compare the Tamusu Mudstone with COX and OPA clays. The COX clay and the Tamusu Mudstone are mainly characterized as mixed mudstones (Area D1, Fig. 8), with the clay mineral content being generally higher in OPA clay. The carbonate minerals of the Tamusu Mudstone are dominated by dolomite and ankerite, whereas calcite dominates in COX and OPA clays. Overall, the Tamusu Mudstone, which is mainly dominated by analcime and dolomite, in contrast to the COX and OPA clays, is a new type of argillaceous host rock. The analcime may play a role with respect to radionuclide retention, as discussed in the case of the potential Yucca Mountain host rock (Bish, 1999) and for low- and intermediate-level short-lived radioactive waste in Belgium (Misaelides, 2019). In view of functions of analcime channels and molecular sieves, mudstones bearing analcime have advantages over typical clay rocks (host rocks) in terms of radionuclide adsorption, water content, thermal stability and permeability (Wang et al., 2022; Yu et al., 2021). The primary pores of mudstone filled with dolomite and analcime cement also highlight the advantages of the uniaxial compressive strength of mudstone, which contribute to engineering construction more than typical clay rocks (Wang et al., 2018c). Moreover, the comparison of some physical and mechanical properties between the Tamusu Mudstone and foreign clay rocks are listed in Table 5, showing that in terms of continuous thickness, water content, porosity, and uniaxial compressive strength, the Tamusu Mudstone also has the advantage of being a potential host rock for a disposal repository purposes.

Fig. 8

Classification triangle diagram of the Tamusu Mudstone, COX clay and OPA clay based on Zhong et al. (2018). a Tamusu data points; b COX and OPA data points. The COX clay data are from Gaucher et al. (2004) and the OPA clay data are from Ould Bouya (2014) QFM = quartz + feldspar + mica

Table 5 Comparison of physical and mechanical properties between the Tamusu Mudstone and foreign clay rocks

Based on geophysical and crustal structure characteristics in the Tamusu area and from the neotectonic movement of adjacent areas, the Tamusu area is characterized by peneplain landforms and a relatively stable tectonic environment (Gao et al., 2013). The fault activities in the Tamusu area mainly occurred in the Cretaceous and before the Cretaceous, and the Quaternary strata has not been dislocated by faults, indicating that there has been almost no obvious fault activity since the Quaternary (Rao et al., 2021). Detailed research on the Tamusu Fault (Fig. 1A) indicates that the fault has been in a stable state since (56.8 ± 5.2) ka (Rao, et al., 2020). According to the classification standards of China earthquake intensity scale of I ~ XII, the seismic intensity in Tamusu area is all classified below VI, and no earthquake with an Ms greater than 5 has ever occurred in that area (Du et al., 2017). Therefore, the preliminary investigations of tectonic activities have confirmed that the Tamusu area has been in a stable stage and that the previous hydrothermal activity in the Bayingebi Basin contributed to the Tamusu Mudstone that is characterized by analcime and dolomite. Certainly, intensive investigations and long-term stability monitoring should be carried out in the Tamusu pre-prelected area. Additionally, the advantages and disadvantages of the Tamusu Mudstone compared to other potential host rocks remain to be assessed carefully in the future.

6 Conclusions

The petrological and geochemical characteristics contribute to the understanding of the hydrothermal sedimentary genesis of the Tamusu Mudstone. The main conclusions are as follows:

(1) Typical structures, mineral assemblages and geochemical characteristics confirm that the host rock has the characteristics of hydrothermal sedimentary rocks, indicating higher contents of dolomite and analcime.

(2) Geochemical analysis concludes that host rock is a low-temperature "white smoke" hydrothermal sedimentary rock that formed in arid, anaerobic and strongly reducing saline lake environment. Vertically, the palaeo-salinity and redox environment were also coupled with the dry–wet evolution, and there was a transient oxidizing environment.

(3) The 100–120 Ma tectonic activity of the Altun fault in the Early Cretaceous, provided the migration pathway for water rock reactions, and may have been the main factor controlling the formation of such mineral assemblages and typical structures.

The discovery of this set of hydrothermal sedimentary rocks not only provides another typical example for studying hydrothermal sedimentary rocks but also has important practical significance to conduct suitability evaluations on Tamusu Mudstone for China’s argillaceous-based site screening purposes.