Abstract
Saline groundwaters (up to 19,100 mg 1 −1 total mineralisation) issue in tin mines in the Carnmenellis Granite in Cornwall (U.K.) at depths up to 800 m. Their stable-isotope composition rules out seawater as a contributor to salinity. Circulation experiments carried out during Hot Dry Rock (HDR) reservoir development in the same granite also produce return fluids with enhanced salinities. Acid hydrolysis of plagioclase and biotite are proposed as the main sources of salinity in the groundwater. Experimental studies carried out on biotites from a borehole used for HDR evaluation demonstrate the reactivity of the biotite and confirm the hypotheses of the field studies. Mg, Li, K and silica levels in reacted solutions reflect the stoichiometric composition of the biotite. Chloride, Na and Ca in solution, on the other hand, are enriched between 1 and 3 orders of magnitude over that of biotite, reflecting the strongly incongruent nature of the reaction. Quartz and chalcedony saturation of the groundwaters encourages silica (or silicate) deposition rather than dissolution of rock-forming quartz; this argues against fluid inclusions as the source of salinity and suggests that new inclusions might be formed. The proposed model for the genesis of saline water therefore links together, or explains, several processes — groundwater movement, convective heat transport, the chemistry of the water, water-rock interaction, secondary mineral (including kaolinite) formation and fluid inclusion formation and stability.
Published Version
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