Water–rock interactions in Karst: enrichment mechanisms of H2SiO3 in thermal mineral waters in the Shiqian fracture zone, China

The enrichment of metasilicic acid (H₂SiO₃) in the Shiqian area is primarily governed by the Shiqian Fracture and associated dorsal tectonic structures. Although notable differences in H₂SiO₃ concentrations are observed between the hanging wall and the footwall, the mechanisms driving this spatial heterogeneity remain insufficiently understood. To address this knowledge gap, we integrated regional geological surveys with systematic sampling of thermal waters and reservoir rocks across contrasting structural domains. A combination of hydrogeochemical analysis, rock geochemistry, stable isotope tracing, multivariate statistical techniques, and water–rock interaction experiments was employed. Our results indicate that atmospheric precipitation serves as the principal recharge source, and that carbonate weathering dominates the hydrochemical composition. The thermal waters exhibit neutral to slightly alkaline pH values (7.12–8.38) and temperatures ranging from 27.0 ~ 46.4 ℃, with a mean hanging wall temperature 5.05 ℃ higher than that of the footwall. Total dissolved solids (TDS) range from 353 to 688 mg/L, while H₂SiO₃ concentrations span from 7.64 to 64.38 mg/L, with an average of 16.51 mg/L higher in the hanging wall. These waters are characterized as HCO₃–Ca·Mg type, with H₂SiO₃ derived mainly from the dissolution of siliceous minerals and, to a lesser extent, from the hydrolysis of aluminosilicates. Water–rock interaction experiments confirm that elevated temperature, CO₂ involvement, and mildly acidic conditions significantly enhance H₂SiO₃ mobilization. The spatial variability in metasilicic acid content is primarily controlled by tectonic structure, overburden thickness, and fluid residence time. This study clarifies the geochemical processes underlying metasilicic acid enrichment and provides a scientific foundation for the rational development and sustainable utilization of thermal mineral water resources in the Shiqian region.