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.

The Tamusu region in northwest China is a key candidate area for China's clay rock deep geological repositories (DGRs) for high-level radioactive waste (HLRW) as it is composed of a continuous layer of thick lacustrine mudstone. To evaluate this mudstone as a host rock, two special test boreholes were drilled to investigate its spatial distribution and mineralogical and hydraulic characteristics. The southwest boundary and depositional centre of the lake basin were well delineated by boreholes TZK-1 and TZK-2. The continuous single-layer thickness of the target mudstone formation was up to 300 m at a depth of 500–800 m. Three main mineral types were determined, namely carbonates (mainly dolomite and ankerite), analcime and albite, and their abundance was used to distinguish three different facies. Other mineral phases, such as clay minerals (mainly illite and kaolinite), pyrite, hematite, quartz and calcite, were present as admixtures. The presence of carbonates may increase the mechanical strength and analcime may enhance the radionuclide adsorption properties of the mudstone. The self-sealing properties, which may be affected by the small amount of clay minerals, remain to be investigated. The hydraulic conductivity of the mudstone determined via in situ pulse tests ranged from 10–13 to 10–10 m s–1, suggesting that the Tamusu mudstone has ultra-low permeability. The transmissivity of the Tamusu mudstone fluctuated in regions with varying lithologies, but remained relatively constant for consistent lithologies. In summary, these preliminarily results confirm the possible suitability of the target formation as a host rock for DGRs of China's HLRW.