Metallogenic regularity of strontium-rich drinking natural mineral water in Devonian carbonate area in southern Hunan: Taking strontium-rich drinking mineral water in Xinxu town, Xintian county as an example

The study area is located in Xinxu, Xintian county, the middle of southern Hunan, in which Devonian carbonate rocks are widely distributed in the south of N26°30'. 44 samples have been taken from some boreholes and springs in different lithologic distribution areas in different seasons for many years. The sampling sites are located in the marl distribution area of Shetianqiao formation of Devonian interplatform basin (D₃s). Testing statistics of these samples show that the strontium content of groundwater in ZK13-10, ZK1601 and ZK1602 boreholes is 1.66 mg·L⁻¹, 6.76 mg·L⁻¹ and 0.41 mg·L⁻¹ respectively, which can prove the existence of strontium-rich drinking natural mineral water.Based on the analysis of lithology, structure, hydrogeological conditions, hydrogeochemical origin and influencing factors of strontium enrichment, the metallogenic regularity of strontium-rich mineral spring water in Devonian carbonate distribution area is preliminarily revealed. According to the statistics of rock and mineral analysis and test results of 80 groups of rock cores at different depths of 5 boreholes, the average content of strontium in marl of Shetianqiao formation of Devonian interplatform basin (D₃s) is 524×10⁻⁶, peaking at 800×0⁻⁶, much higher than that in other rocks.The high abundance of strontium is the material basis for the formation of strontium-rich mineral water in the study area. Primarily, the content variety of strontium in groundwater is closely related to groundwater circulation conditions. Three spots of strontium-rich groundwater in the study area mainly come from strontium-rich water in the zone of deep subsurface flow. High circulation depth, relatively closed groundwater circulation and weak hydrodynamic conditions are the main constraints for the formation of strontium-rich mineral spring. Because the groundwater recharge runoff is restricted by topographic and geomorphic conditions, seepage conditions, fault structure location and water control properties, the farther the distance of groundwater recharge runoff is, the higher the circulation depth will be, and in turn the more conducive will be to the enrichment of strontium and the formation of deep strontium-rich water. The water temperature is another important factor affecting the enrichment of strontium in groundwater because it affects the dissolution and ion replacement of minerals containing strontium, and it also affects the CO₂ content and corrosiveness in groundwater, thus affecting the enrichment of strontium in groundwater. Meanwhile, the circulation depth and geothermal field will control the groundwater temperature.The metallogenic regularity of Xinxu strontium-rich drinking mineral water shows that the marl distribution area of Shetianqiao formation of Devonian interplatform basin (D₃s) is a favorable area for people to look for the strontium-rich mineral spring in southern Hunan. In particular, the hanging wall of water-blocking faults in gentle karst hills as well as the location close to magmatic rocks and regional thermal conductivity faults are the target areas.