Deep‐seated potassium‐rich brines were identified in the Jiangling Basin, South China. Although magmatic‐hydrothermal sources have been proposed, the relationship between brine‐type potash mineralization and volcanism remains unclear. In this study, U‐Pb geochronology, geochemistry, fluid inclusion and C‐O isotopic compositions of hydrothermal vein minerals in the Jiangling Basin are examined. Laser ablation U‐Pb dating of calcite veins indicates that the ages are slightly younger than the formation age of the Balingshan basalt. Fluid inclusions in hydrothermal minerals show medium–low homogenization temperatures (160–220°C) and low salinities (0.14 to 4.9 wt% NaCl eqv.) and densities (0.882–0.944 g/cm3). The liquid compositions of fluid inclusions in calcite veins from sedimentary strata have higher contents of potassium, compared with those from basalt. The coupled negative δ13CPDB (–10.3‰ to −8.0‰) and positive δ18OSMOW (17.4‰ to 20.7‰) values imply that calcite precipitation resulted from CO2 degassing of the basaltic magmatic fluids, as indicated by the gas composition of these inclusions in hydrothermal minerals. Rare earth element patterns indicate that water‐rock interaction between hydrothermal fluids and sedimentary wall rocks contributed to the calcite precipitation in sedimentary strata. It is proposed that high‐temperature water‐rock interaction between magmatic fluids and sedimentary strata resulted in the potassium enrichment in fluids, interpreted as one of the sources of potassium‐rich brines in the Jiangling Basin.