Hydrogeochemical processes occurring in the hydrothermal systems of the Gonghe–Guide basin, northwestern China: critical insights from a principal components analysis (PCA)

The Gonghe–Guide basin is a geologically complex area located in the northeastern Qinghai–Tibet Plateau. Although it is far away from the Himalayan geothermal belt, hot springs with high contents of SiO₂, Li, B and As are widely distributed there. Principal components analysis (PCA) was used to identify the geochemical genesis of the major constituents in geothermal waters and cold groundwaters from the study areas as well as the related hydrogeochemical processes. The results indicate that these constituents can be represented by six factors: factor 1 (F⁻, Li⁺, K⁺ and SiO₂), factor 2 (EC, Cl⁻ and Na⁺), factor 3 (pH ${\text{HCO}}_3^{-}$, Mg²⁺ and Ca²⁺), factor 4 (B, As, Fe and Al), factor 5 (${\text{SO}}_4^{2-}$ and Sr) and factor 6 (${\text{NO}}_3^{-}$ ). Generally, the dissolution of silicate and carbonate minerals is the predominant geochemical processes responsible for the occurrence of the constituents that constitute factors 1 and 3, respectively. In contrast, although halite is not a primary mineral in the aquifer matrix, its leaching results in the enrichment of chloride and sodium in the groundwaters, which make significant contributions to the high EC values of some water samples. Although arsenic and boron are associated with factor 4, elevated levels of these elements in groundwater may be derived from the leaching of crystalline rocks (like granite and gneiss) rather than the input of potential magmatic fluids. Moreover, anthropogenic activity has an impact on the hydrochemistry of the groundwaters as well, as indicated by the elevated concentrations of nitrate (the sole component of factor 6) in some samples. It is also worth noting that the PCA results may not match well with the hydrogeochemical analyses. Factor 5 is composed of sulfate and strontium, whereas celestite is unlikely to be the primary source of sulfate in groundwater.