The lithium concentrations in the geothermal waters of the Wumishan Formation carbonate reservoir in China Xiong’an New Area are over 1 mg/L and are even higher than those in the geothermal waters of granite reservoirs in some areas of China. It is still unknown which are the most important factors controlling the lithium concentrations in the geothermal waters in the study area. This article selected the analysis and test data of 32 geothermal water samples obtained in recent years from the study area and combined them with hydrochemical analysis and test data from granite reservoirs in other regions of China to study the enrichment mechanism of lithium in the geothermal waters in the study area. The results of the hydrochemical data analysis indicate that the lithology, pH, and water–rock interaction between geothermal water and carbonate rocks are not the main factors affecting the lithium concentrations in the study area. The mixing of paleo-seawater and the leaching of the evaporated rocks formed by it are the most important factors controlling the lithium concentrations in the study area, and temperature is also an important factor affecting the lithium concentrations. The research results are of great significance to the study of the enrichment mechanism of lithium in geothermal waters and the formation mechanism of geothermal waters in similar areas around the world.

Geothermal fluids are the main carrier of hydrothermal geothermal resources. Identifying the differences in geothermal fluids in different types of reservoirs is a prerequisite and fundamental for the efficient development of geothermal resources and is of great significance for scientific research on geothermal resources. The North China Plain contains a typical carbonate thermal reservoir, and in this paper, the hydrochemical, isotopic, and redox characteristics of the geothermal fluids in sandstone and carbonate reservoirs are studied to obtain the differences in the geothermal fluids in the Rongcheng geothermal field in Xiong’an New Area. The results indicate that the geothermal fluids in the sandstone and carbonate reservoirs are mainly supplied by atmospheric rainfall, and the hydrochemical type is mainly Cl-Na type. By comparing and analyzing the stable isotope (O, H, C, S, and Sr) characteristics of the two types of geothermal fluids, it is found that the variation range of δ13C values for two types of sandstone thermal storage geothermal fluids was found to be −10.6‰~−12.8‰, while the variation range of δ¹³C values for carbonate thermal storage geothermal fluids was −3.3‰~−7.5‰. The ⁸⁷Sr/⁸⁶Sr ratio of sandstone thermal storage geothermal fluids was distributed between 0.708–0.718, and the ⁸⁷Sr/⁸⁶Sr ratio of carbonate thermal storage geothermal fluids was distributed between 0.708–0.713. The range of δ³⁴S values for sandstone thermal storage geothermal fluids was +9.46‰~+10.5‰, and the range of δ³⁴S values for carbonate thermal storage geothermal fluids was +24.84‰~+34.49‰. The two types of geothermal fluids have been subjected to varying degrees of oxidation-reduction, and their cycling and mixing characteristics are different. This has resulted in the formation of relatively oxidized geothermal fluids in the sandstone geothermal reservoir and relatively reduced geothermal fluids in the carbonate geothermal reservoir. In future development and utilization of geothermal resources, paying attention to the basic characteristics of the geothermal fluids in different reservoirs and identifying the differences in different geothermal fluids can further improve the efficiency of geothermal resource development and utilization.

The Jizhong Depression boasts rich geothermal resources with a lengthy history of geothermal exploitation. Buried hill geothermal reservoirs, which serve as primary thermal sources for hydrothermal resource exploitation, are prevalent in this region and have advantages such as extensive development potential, significant geothermal reservoir capacity, superior water quality, and straightforward recharge. This study investigates the formation and evolution of deep geothermal water in the Jizhong Depression by analyzing the hydrochemical and isotopic data of geothermal water samples collected from buried hill geothermal reservoirs in the northern part of the depression. The findings reveal that the subsurface hot water samples from the carbonate geothermal reservoirs in this region were predominantly weakly alkaline water with a pH ranging between 6.61 and 8.87. The hot water samples collected at the wellhead exhibited temperatures varying from 33.9 °C to 123.4 °C and total dissolved solids (TDS) lying between 473.9 mg/L and 3452 mg/L. Based on the δ²H-δ¹⁸O stable isotope analysis, the geothermal fluids in the Jizhong Depression are predominantly sourced from atmospheric precipitation and exist in a somewhat isolated hydrogeological environment, exhibiting pronounced water–rock interactions and deep water circulation (with depths ranging from 1324 m to 3455 m). Through a comparison of various methods, it is deduced that the most appropriate geothermometer for deep karst geothermal reservoirs in the Jizhong Depression is a chalcedony geothermometer, and when using it, the deep reservoir temperature was estimated at 63–137.6 °C. The precipitation in the adjacent mountainous areas enables the groundwater to infiltrate and descend deep into the earth along piedmont faults. Subsequently, lateral runoff over extended periods replenishes the groundwater into the depression. This process allows for the groundwater to fully absorb heat from deep heat sources, resulting in the formation of the deep geothermal reservoirs in the northern Jizhong Depression. The insights obtained from this study offer a theoretical and scientific foundation for the exploitation and utilization of regional geothermal resources and the transformation of the energy structure in China.

Presently, geothermal resources have been globally recognized as an indispensable component of the energy system due to their sustainability. However, previous studies on geothermal reservoirs focus primarily on single reservoirs, lacking a systematic investigation of composite geothermal reservoirs. The geothermal reservoirs in the northwestern Shandong geothermal area in China are primarily of sandstone and karst types, characterized by extensive distributions, shallow burial depths, high water temperatures, and high water abundance, holding considerable potential for exploitation. This study explored the hydrochemical, isotopic, and circulation characteristics of geothermal fluids in the composite geothermal reservoirs in the study area using methods like hydrogeochemistry and geothermal geology. The purpose is to determine the geochemical differences in geothermal fluids across the composite geothermal reservoirs and provide scientific support for subsequently efficient and sustainable exploitation and utilization of geothermal resources in the study area. The composite geothermal reservoirs in the study area are composed of porous sandstone geothermal reservoirs (also referred to as sandstone reservoirs) in the upper part and karst-fissured geothermal reservoirs (also referred to as karst reservoirs) in the lower part. The results show that the geothermal fluids in the sandstone and karst reservoirs are primarily of Na-Cl-SO₄ and Na-Ca-Cl-SO₄ types, respectively. The hydrochemical composition of geothermal fluids in the karst reservoirs is principally influenced by the precipitation–dissolution equilibrium of carbonate and sulfate minerals, while that in the sandstone reservoirs is predominantly influenced by the precipitation–dissolution equilibrium of carbonate and silicate minerals, as well as cation exchange reactions. The temperatures of the karst reservoirs were calculated at 52.9–82.09 °C using geothermometers. Given the cold-water mixing ratios range from 89% to 96%, the corrected reservoir temperatures vary from 200 to 225 °C. In contrast, the temperatures of the sandstone reservoirs were calculated at 60.54–85.88 °C using geothermometers. These reservoirs exhibit cold water mixing ratios ranging from 85% to 90%, and their corrected reservoir temperatures vary from 150 to 200 °C accordingly. The circulation depths of geothermal fluids in the karst and sandstone reservoirs range from 1107.28 to 1836.69 m and from 1366.60 to 2102.29 m, respectively. The study area is primarily recharged by meteoric water from Mount Tai and the Lushan and Yishan mountains (collectively referred to as the Tai-Lu-Yi mountains) to the southeast of the study area. Investigating the differences in geochemical characteristics of geothermal fluids in composite geothermal reservoirs in the study area is significant for balancing the exploitation and supply of geothermal resources, optimizing the exploitation and utilization modes, and promoting the efficient and sustainable exploitation and utilization of geothermal resources in the study area.

Magma-heated geothermal systems have garnered significant attention in academia due to their unique formation mechanisms and vast potential. This paper focuses on the Rehai, Ruidian, and Banglazhang geothermal fields in the Tengchong area. We present the element geochemistry and isotope compositions of hot springs, cold springs, and surface water to explore magmatic fluid input into geothermal systems and investigate the release of deep mantle-derived components. Based on our findings, we propose a conceptual model and theoretical framework for geothermal system genesis constrained by magmatic heat source influences. Results indicate that magma-heated geothermal systems coexist with three types of geothermal water: neutral chloride-rich water, acidic sulfate-rich water, and alkaline bicarbonate-rich water. The infusion of magmatic fluids into geothermal systems. The enrichment of trace elements in hot springs is jointly controlled by magmatic fluid input and host rock leaching. The magma chamber is the primary factor influencing the reservoir temperature. The parent geothermal fluid can be identified within the geothermal system. During circulation, the parent geothermal fluid undergoes three cooling processes: adiabatic cooling, conductive cooling, and mixing with cold water. We propose that the release of mantle-derived materials is a key factor in element enrichment within magma-heated geothermal systems, and mantle-derived components are more enriched in areas with active magma chambers. The findings of this study provide insights into magmatic fluid input into geothermal systems and highlight the critical role of the release of mantle-derived components in the formation of high-temperature geothermal resources.