Yanjun Zhao

The exploration and extraction of deep salt lake resources are of great significance for alleviating potassium and lithium resource shortages. However, large‐scale industrial extraction has not been achieved due to technical challenges and cost constraints. This study focuses on the Mihai Mining District in Qaida Basin, Qinghai Province, investigating the exploration and development of deep confined brine enriched with potassium and lithium. The Mihai Mining Area boasts vast reserves of deep confined brine, however, they are not well‐utilised because of their heterogeneity, low water yields, and extremely low mining efficiency. Previous studies have revealed that the high‐yield wells in mining areas are located within fault zones. Therefore, based on the theoretical models of ‘brine circulation and potassium formation in fault zones’ and ‘storage of brine in steeply inclined water‐collecting corridors’, an efficient exploration technique is proposed, which focuses on identifying active fault zones within areas with well‐developed high‐quality brine reservoirs as key target areas. This study employed radar remote sensing interpretation of pre‐ and post‐seismic deformation intensity to infer fault zone development areas. Integrated with radon gas measurements, the results confirm the presence of a NE‐striking active fault within the key target zone. Functioning as the sedimentary centre, this zone exhibits highly concentrated brine with co‐enrichment of K⁺ and Li⁺ (both elements showing anomalously high concentrations), indicating high potential for K‐Li‐rich brine discovery. Guided by the exploration theory, verification drilling revealed brine with elevated K‐Li concentrations (approximately double those in adjacent areas) and high‐yield water flows in most boreholes, demonstrating significant exploration success. The distribution pattern of the discovered high‐yield, water‐rich areas and the exploration prediction method are of great significance for the development of deep brine in the Mihai Mining Area and have considerable potential for promotion in the Qaida Basin.

Qarhan Salt Lake, China’s largest brine-type potassium-lithium deposit, holds substantial economic value. However, the timing of lithium-rich brine formation, its origin, and the role of climate in its development remain unclear. This highlights the need for a deeper understanding of salt lake evolution, particularly regarding ore-forming brine composition and the palaeoclimate. Therefore, we conducted ²³⁰Th dating on halite deposits from borehole ZK6-7 (7.5–46.7 metres) in the northern Bieletan, revealing halite deposition at 237.79–30.29 thousand years before 2000 A.D. The maximum homogenisation temperature of fluid inclusions was 23.9–41.7 °C, indicating frequent palaeobrine temperature fluctuations, corresponding to palaeoclimatic events in the Qinghai–Tibet Plateau and Qaidam Basin. Lithium, potassium, magnesium, boron, and bromine concentrations were inversely correlated with the maximum homogenisation temperature. Lithium-rich brine formation in the Bieletan was a long-term process driven by interglacial-glacial cycles, where warm periods increased river flow and material supply, while dry-cold periods intensified evaporation, promoting mineral enrichment.

As high‐grade potash resources become increasingly scarce and lithium is recognised as a valuable co‐product, the low‐grade solid potash deposits in the Mahai Salt Lake, Qaidam Basin, have become increasingly significant targets for potash resource development. However, the inefficient release of potassium from these low‐grade ores constrains their economic feasibility for industrial use. This study addresses this challenge by employing x‐ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM‐EDS), and observational borehole monitoring data to systematically analyse the mineralogical characteristics of these ores and their influence on potassium ion release during solid–liquid conversion. The results identify carnallite, sylvite, and polyhalite as the primary potash minerals, with a significant spatial variations across the study area that directly impact potassium ion release and migration. Sylvite and carnallite in shallow strata are more reactive with solvents, facilitating rapid potassium ion release, whereas polyhalite, hosted within the intercrystalline clastic materials of halite, demonstrates limited pore connectivity, resulting in a slower release rate. Monitoring borehole data indicate that variations in solvent injection volumes cause significant fluctuations in water levels and potassium ion concentrations, thereby affecting potash dissolution rates. As solution mining progresses, high‐lithium areas gradually expand, with lithium concentrations in brine reaching up to 120.4 mg/L and generally ranging from 20 to 70 mg/L. The spatial correlation between potassium and lithium distribution suggests that lithium can be effectively recovered as a co‐product. Adjustments in solvent injection rates and concentrations, based on mineral distribution and monitoring data, are essential for optimising the dissolution efficiency of both potassium and lithium during production.

Salt lake brine-type potassium-lithium deposits play an important role as a strategic mineral resource that is in short supply in China and the rest of the world. In the case of long-term, high-intensity exploitation of brine resources, if large-scale active faults are encountered, these faults will form advantageous channels for brine migration. In some special cases, the surface fresh water leaks into these advantageous channels and will soon produce a large amount of fresh water in the brine mining well area, resulting in the destruction of the mining well and in the reduction in the potassium lithium grade of the brine. Based on the monitoring of the chemical components and hydrogen and oxygen isotopes of brine in mining wells, combined with the principle of stable isotope conservation, the influence of freshwater leakage accidents on brine extraction wells was quantitatively calculated. The amount of fresh water added to brine by accident is approximately 78%. This further indicates that it is necessary to strengthen long-term monitoring of the chemical components of confined brine to ensure sustainable and stable production.

The wide distribution of tuff layers, locally named the “green bean rocks” (GBRs) in the Yangtze Block straddling the Early Middle Triassic marine sequence indicates intense volcanic eruption(s). Sr, Nd, and S isotope compositions and trace elements of marine sediments were analyzed spanning the tuff layers to elucidate their responses to the volcanic eruptions and related environmental changes. The Sr isotope compositions of marine sediments are comparable to those of open seawater during the time interval of ca. 245–248 Ma. Sr and Nd isotope compositions of the samples show synchronous increases in the ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values during the deposition of GBRs. The elevated ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values are proposed to be caused by the input of volcanic tephra and increased influx of weathering product of mafic rocks (most likely the Emeishan flood basalts). The S isotope compositions of sulfates exhibit a negative shift in the GBRs, which could possibly be attributed to greater input of lighter ³²S from weathering products and volcanic eruptions. The variation of Th/U ratios indicate that the GBRs formed in an anoxic environment, resulting from high marine productivity as a consequence of more nutrients from weathering and volcanic materials. The responses of Sr, Nd, and S isotopes to volcanic eruptions during the Early Middle Triassic indicate this event resulted in adverse effects, namely enhanced eutrophication and low O₂ levels, acidic precipitation, toxic components, etc., that could cause ecological destruction both on land and in the sea.

There are considerable reserves of low-grade solid potash resources in the shallow part of Mahai Salt Lake in the Qaidam Basin, and the lithium brine resources resulting from solid–liquid conversion and mining are quite abundant. The comprehensive utilization of these resources is an important and urgent problem. In this study, to fully utilize these resources, the shallow low-grade solid potash ore in Mahai Salt Lake was used for systematic simulated ore dissolution experiments, combined with geochemical and X-ray diffraction analyses. The following key results were obtained: (1) Most Li⁺ in the Mahai mining area was deposited on the soluble salt minerals in silt or clay, and the appropriate concentration of solvent can help to dissolve more Li⁺ and K⁺; (2) the saturation time of Li⁺ was longer than that of K⁺. Therefore, the dissolution time for the mine can be appropriately extended during the production process to dissolve more Li⁺; (3) the solid–liquid conversion aqueous solution mining method can separate the lithium part of clay deposits and is associated with salt rock in the brine, which is a potential lithium resource. These experimental results provide a theoretical basis for salt pan production.