FOURTH INTERNATIONAL SYMPOSIUM ON WATER - ROCK INTERACTION
The Carnmenellis granite and its aureole contain the only recorded thermal groundwater (up to 52 degrees C (in British granites. They occur as springs in tin mines at depths between 200 and 700 m and most are saline (maximum mineralization 19310 mg/l). Mining activity has disturbed the groundwater circulation pattern developed over a geological time scale and levels of bomb produced tritium ($0G 4 T.U.) indicate that between 6 and 65% of the most saline waters are of recent origin. All components of all the mine waters are of meteoric origin. Radiogenic 4-He contents, 40-Ar/36-Ar ratios and uranium series geochemistry suggest that the thermal component has a likely residence time of at least 5 x 10**4 years and probably of order 10**6 years. The thermal water has a low Na**+/Cl**- ratio but is enriched relative to sea water in all major cations except magnesium. The groundwater is also particularly enriched in Li with contents ranging up to 125 mg/l. The groundwater salinity, which may reach a maximum of 30 000 mg/l, is shown to result from weathering reactions of biotite (probably through a chloritization step) and plagioclase feldspar, to kaolinite. On volumetric, and geochemical considerations, fluid inclusions cannot contribute significantly to the groundwater salinity, and stable isotope rations rule out any contribution from sea water. Groundwater silica contents and molar Na**+/K**+ ratios suggest that the likely equilibration temperature is 54 degrees C, which would imply a depth of circulation of about 1.2 Km.
Bibliographic Reference: FOURTH INTERNATIONAL SYMPOSIUM ON WATER-ROCK INTERACTION, MISASA (JAPAN), AUG. 29- SEPT. 8, 1983. AVAILABILITY: CEC LUXEMBOURG, DG XIII/A2 MENTIONING PAPER E 31111 ORA
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Record Number: 1989122001500 / Last updated on: 1987-01-01
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