Analysis of the formation mechanism of karst collapse in the Panlong lead-zinc mine, Wuxuan, Guangxi
Karst collapse is a major geological hazard in karst areas, characterized by suddenness and concealment. Over the years, human engineering activities-such as mining at the Panlong lead-zinc mine in Wuxuan, Guangxi, and the impoundment of the Datengxia Reservoir-have triggered more than 100 karst collapse events in the mining area. These events pose serious threats to human life, property, and public safety. To elucidate the development patterns and triggering mechanisms of successive karst collapses at the Panlong lead-zinc mine, comprehensive investigations and studies were conducted. These include field surveys, hydrogeological and engineering geological drilling, geophysical exploration, long-term groundwater dynamic monitoring, and in-situ hydrogeological testing. The main research findings are as follows. 1. The fragile geological environment, characterized by moderately-to highly-developed underground karst, thin overburden layers, and moderate- to abundant groundwater resources, constitutes an intrinsic factor for karst collapse. The study area is extensively developed with underground karst features that serve as storage spaces or transport channels for groundwater and collapse materials. Groundwater resources are abundant, while the overburden layers are relatively thin, and the soil exhibits poor engineering geological properties, facilitating the formation of soil caves due to groundwater activity. These fragile geological environmental conditions are the internal cause of karst collapse in the study area. 2. Long-term, large-scale groundwater dewatering is a key anthropogenic factor inducing karst collapse in the study area. Mine drainage has caused a rapid decline in groundwater levels, an increase in groundwater hydraulic gradient, and an enhanced erosion capacity of groundwater on the overlying soil. Long-term mine drainage, combined with the effects of rainfall (especially heavy rainfall) and reservoir impoundment on groundwater within the cone of depression, results in significant and frequent fluctuations in groundwater levels. These fluctuations are a critical external factor contributing to karst collapse in the study area. 3. Based on the mining process and characteristics of collapse occurrences, karst collapse events at the Panlong Lead-Zinc Mine can be divided into three stages: the formation stage of depression funnel, the stabilization stage of depression funnel, and the impoundment stage of reservoir. The dominant geological forces and collapse modes vary moderately across these stages. (1) Formation stage of depression funnel: Mine drainage causes a decline in groundwater levels, forming a depression funnel and altering regional hydrogeological and engineering geological conditions. Before mining, the soil was mostly in a pressured, saturated state; however, after the sudden drop in water levels, it lost water buoyancy support, resulting in reduced stability and increased load effects. In addition, the increased groundwater flow velocity enhances soil transport capacity, while karst pipelines act as discharge channels for soil particles, thereby promoting the development of soil caves. Meanwhile, the rapid decline in water levels generates negative pressure within karst caves, triggering collapse under the combined effects of erosion and negative pressure. (2) Stabilization stage of depression funnel: Under specific mining depths, the drainage remains stable, and the depression funnel temporarily stops expanding. However, the dewatering effect continues, maintaining groundwater levels below the bedrock surface for an extended period. Sudden fluctuations in water and gas pressure within karst pipelines caused by heavy rainfall are the main triggering factors. For instance, six concentrated collapses occurred after a 120 mm heavy rainfall event on June 11, 2022. This heavy rainfall caused a sharp rise and fall in groundwater levels, forming an alternating cycle of positive and negative pressures. The rising water levels triggered a gas explosion effect, while the subsequent decline intensified seepage erosion and negative pressure effects. The combination of these factors with pre-existing soil caves ultimately resulted in collapse. (3) Impoundment stage of the Datengxia Reservoir: After the reservoir was impounded in March 2020, the water level of the Qianjiang River rose to 35–53 m. Changes in river water levels, combined with the effects of mine drainage and rainfall, exacerbated fluctuations in groundwater levels. During impoundment, river water recharges the groundwater, causing its level to rise; during regulation and storage phases, the decline in river water levels leads to a corresponding drop in groundwater levels. High-frequency variations in mine drainage result in frequent and significant fluctuations in groundwater levels, providing sufficient hydrodynamic and aerodynamic conditions for the formation, expansion, and collapse of soil caves.
Study on the distribution of uranium content in cave stalagmites in China and its influencing factors
The uranium content of stalagmites is the foundation of stalagmite chronology research. It directly determines whether a stalagmite sample is suitable for paleoclimate research and affects the accuracy and precision of stalagmite dating results. Currently, two uranium-series dating techniques are commonly used for stalagmites: U-Th dating and U-Pb dating. The U-Th method can date samples up to approximately 690,000 years old, while the U-Pb method extends the dating range to the Paleozoic or even earlier geological periods. However, the reliability of both methods is highly dependent on the uranium concentration in the stalagmites. Stalagmite samples with low uranium content often result in large age uncertainties, which can lead to significant misinterpretations of the timing and duration of past abrupt climatic events, thereby compromising the accuracy of paleoclimate reconstructions. Therefore, investigating the factors that influence uranium concentrations in stalagmites is of great significance both theoretically and practically. Such understanding can help identify high-uranium stalagmite samples, enhance dating precision, and ultimately support the establishment of more robust chronological frameworks. Nevertheless, current knowledge of the mechanisms controlling uranium content in stalagmites remains limited. Most existing studies focus on the climatic significance of uranium concentration and initial (234U/238U) ratios in stalagmites. Previous studies have shown that the uranium content in stalagmites is influenced by factors such as soil, surrounding rock, the infiltration path of karst water, and the duration of water-rock interaction; While, systematic large-scale studies that integrate these factors are still lacking. In China, karst landscapes are widely distributed, and stalagmite records have played a vital role in high-resolution paleoclimate studies. However, there are relatively few comprehensive analyses on the temporal and spatial distribution characteristics of uranium content in cave stalagmites, as well as the influencing factors. This study aims to explore the temporal and spatial distribution characteristics of uranium content in cave stalagmites in China and the key influencing factors. The goal is to provide a scientific basis for selecting suitable stalagmite samples with adequate uranium concentrations, thereby minimizing unnecessary use and waste of valuable stalagmite resources. This study collects and analyzes the data from 114 published cave stalagmite samples in China. By examining the spatial distribution and temporal evolution of uranium concentrations, the study explores the roles of regional tectonic background, host rock lithology, mineralogy of the stalagmite, and the hydroclimatic conditions under which the stalagmites formed. The main research objectives are: (1) Clarifying the spatial distribution pattern and temporal variation characteristics of uranium content in Chinese stalagmites; (2) Identifying the primary controlling factors, including geological settings, mineral composition, and environmental parameters; (3) Providing scientific criteria for selecting suitable samples for uranium-series dating, thereby avoiding unnecessary waste of stalagmite resources. The analysis of the research results indicates: (1) The spatial distribution of karst caves in China is regionally distinct. Although they are widely distributed, most caves develop on the second geomorphic step and in tectonically active zones such as the Qinling-Dabie orogenic belt and the monsoon-affected regions east of the Hu Line. The warm and humid climate conditions are conducive to the development of karst caves; (2) Based on published Chinese stalagmite records, uranium concentrations typically range from 0.5×10−6 to 10×10−6. High-uranium stalagmites (1×10−6 to 10×10−6) are mainly found in karst caves developed in Mesozoic or older limestone formations located in the junction regions of Guizhou, Chongqing, Hubei, and Hunan; (3) The lithology of host rocks is a critical factor influencing uranium enrichment. When stalagmites are composed of calcite, those formed in dolomite tend to have higher uranium contents than those formed in limestone. This may be attributed to differences in rock weathering processes, inter-crystalline porosity, and water-rock interaction time; (4) The presence of dolomite within the host rock or geological strata appears to be an important contributor to elevated uranium levels in aragonite stalagmites; (5) Uranium concentrations are significantly correlated with regional hydroclimate conditions. There is a positive relationship between uranium content and stalagmite growth rate: stalagmites formed under warm and humid conditions generally contain higher uranium levels, whereas those formed under cold and arid conditions tend to have lower uranium concentrations; (6) In general geological settings, the primary source of uranium in sedimentary strata is ancient seawater, thus creating a link between uranium content in stalagmites and that in paleo-seawater. However, in certain specialized geological environments, such as coal-bearing formations or areas with magmatic intrusions, localized uranium enrichment in bedrock can lead to abnormally high uranium levels in stalagmites. This study enhances our understanding of the variability and controlling mechanisms of uranium concentration in stalagmites under different lithological, stratigraphic, and climatic conditions. The findings provide both theoretical insights and practical guidelines for selecting high-uranium stalagmite samples, thereby supporting the continued advancement of stalagmite-based paleoclimate reconstructions.
Depth distribution of 238U、232Th、226Ra、40K from a soil wedge on limestone slope in Guzhou karst peak-cluster depression of Huanjiang and its origin analysis
The longstanding debate regarding the origin of pedogenic materials in karst regions—whether derived from in situ weathering of carbonate bedrock or allochthonous inputs—motivated this investigation into soil wedge formation within the karst peak-cluster depression of Huanjiang Guzhou depression, Guangxi, China. To resolve this controversy, we analyzed distributions of natural radionuclides (238U、232Th、226Ra和40K), grain size, bulk density, and pH in a limestone slope soil wedge to trace material sources and elucidate formation mechanisms. Stratified soil samples (0−10, 10−20, 20−40 cm, and 2 m depth) and adjacent limestone bedrock were collected from a 3-m-deep soil wedge (24°55′0.20″N, 107°57′4.35″E), with bulk density measured in situ via the ring-knife method. Grain size distribution was determined using laser diffraction (Mastersizer 2 000) following H2O2 and HCl pretreatment, while radioisotope activities were quantified through low-background γ-spectrometry (HPGe detector, LOAX model) at characteristic energy peaks: 92.8 keV (238U), 583 keV (232Th), 609 keV (226Ra), and 1460 keV (40K). The pH is measured by a pH meter. Results revealed remarkably homogeneous soil physicochemical properties: silt (average 31.81%) and clay (58.82%) dominated the texture, while bulk density (0.94−1.01 g·cm−3) and pH (7.47−8.01) exhibited minimal variation across depths, though the surface layer (0−10 cm) showed slightly lower clay content (27.59%) and higher bulk density (1.01 g·cm−3). Radionuclide activities within the soil wedge demonstrated significant stability: 238U, 232Th, and 226Ra averaged 111.91, 100.17, and 74.03 Bq·kg−1 respectively with negligible depth variation, while 40K activity exhibited a slight increase with depth (average 341.55 Bq·kg−1). Crucially, isotopic ratios remained consistent vertically, with 238U∶ 232Th∶ 226Ra∶ 40K ≈ 1∶0.89∶0.66∶2.86 (0−40 cm) and 1∶0.91∶0.65∶3.76 (2 m), indicating profound vertical homogeneity. In stark contrast, the underlying bedrock exhibited extremely low radionuclide activities: 238U was undetectable (<0.71 Bq·kg−1), while 232Th、26Ra and 40K registered only 2.77, 3.69, and 18.43 Bq·kg−1 respectively, yielding a distinct 232Th∶226Ra∶40K ratio of 1∶1.34∶6.65. The soil-bedrock contrast was unequivocal: 238U activity in the soil (95.72−120.10 Bq·kg−1) exceeded the bedrock level by more than 135 times, and the fundamental difference in 232Th∶ 226Ra∶40K ratios (1∶0.74∶3.41 in soil vs. 1∶1.34∶6.65 in bedrock) provided definitive evidence. This stark geochemical disparity, particularly the absence of 238U and the divergent isotopic signatures in the bedrock, categorically refutes traditional models of residual accumulation via in situ weathering of the underlying pure carbonate limestone, as U/Th series nuclides primarily reside in silicate minerals which are negligible in such bedrock. Instead, the uniform soil texture, stable isotopic ratios across depths, and most critically, the abrupt soil-bedrock contact devoid of any transitional layers, collectively provide robust evidence supporting a formation mechanism driven by gravitational soil creep infilling pre-existing rock fissures. The elevated 238U activity and consistent Th/Ra ratios within the soil wedge point towards external material sources, likely comprising weathered residues from interbedded clastic layers within the mid-upper slopes and/or contributions from aeolian dust, analogous to the Saharan dust inputs documented in Mediterranean Terra Rossa formations. This mechanism stands in clear global context: unlike granitic weathering crusts characterized by gradual transitions through distinct weathering horizons (e.g., soil → strongly weathered layer → weakly weathered layer → fresh bedrock), limestone soil wedges exhibit sharp, unconformable interfaces-a definitive hallmark of allochthonous infilling. While this aligns with the formation paradigm of Mediterranean Terra Rossa, it fundamentally contrasts with models proposing in situ karst weathering. Integrated evidence thus conclusively confirms that soil wedges in karst peak-cluster depressions form primarily through the gravitational infilling of rock fissures by externally sourced materials, not through the in-situ weathering of the immediate limestone substrate. Radionuclide activity ratios, particularly the highly diagnostic 238U /232Th contrast, serve as powerful tracers for discerning pedogenic sources in such complex terrains. These findings necessitate a redefinition of karst soil evolution paradigms, significantly underscoring the critical role of aeolian and slope-wash transported alluvial inputs in shaping subtropical karst ecosystems.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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