Origin of saline groundwaters in granitic rocks: evidence from hydrothermal experiments

Hydrothermal rock-water experiments at 800, 1500 and 2500C have been carried out to investigate the origin of saline groundwaters in the Carnmenellis Granite, Cornwall, UK. Since granitic rocks are prime candidates as hosts for high level radioactive waste repositories, an understanding of the origin of such fluids is necessary, if post disposal geochemical evolution of the groundwater is to be modelled. The derivation of these saline groundwaters have previously been attributed to dissolution of biotite and plagioclase feldspar by dilute meteoric groundwaters. The nature of product fluid and solid phases in the experiments was dominated by the dissolution of plagioclase and potassium feldspars, coupled with the precipitation of laumontite, calcite and smectite. However, the release of chloride from the rock was minimal, suggesting minor dissolution of the major chloride bearing mineral, biotite. The relative inertness and stability of biotite during the experiments was borne out by direct physical observation of reacted biotites and by consideration of the fluid phase composition in terms of thermodynamic mineral stability diagrams. It is concluded that the experimental and natural systems are buffered under different chemical conditions, which may be due to acidity generated via oxidation of localized sulfide zones within the granite and absent in the experiments. Dissolution of biotite may be an important mechanism for enhancing the salinity of groundwaters in granitic rocks if groundwaters are buffered to be outside the stability of biotite and convective circulation is significant