FLUID INCLUSIONS IN GRANITES AND THEIR RELATIONSHIPS WITH PRESENT-DAYGROUNDWATER CHEMISTRY

Some granitic deep groundwaters have high chloride contents of which t he origin is still unclear. Among the different possibilities, leakage of fluid inclusions trapped in primary minerals was proposed. This hy pothesis is examined by studying four granitic sites with various hydr ogeological features: drill-cores from Aspo (Sweden); Boettstein and L euggern (northern Switzerland); outcropping granites and drill-core sa mples from Cauterets and Luchon in the Pyrenees (France): In Tounine g ranitic massif (Central Ahaggar, Algeria). With the exception of In To unine, a part of the granite samples selected in each study site displ ays salt-rich fluid inclusions with salinities ranging from 22 to 26 e d. wt.% NaCl, calcium contents and minimal trapping temperatures typic al of deep sedimentary brines. Most salt-rich fluid inclusions occur i n secondary fluid inclusion planes in magmatic quartz, but were also f ound within fissure-in-filling calcite at Boettstein and Aspo. The wid e ranges of Cl/Br ratios in single same rock samples, due to heterogen eities in fluid inclusion distribution in rock-forming minerals, and t he variability of Cl/Br ratios in fluid reservoirs render the use of C l/Br ratios for fluid-source tracing inaccurate. Only salt-rich inclus ions in the Pyrenees samples show a relatively narrow range of Cl/Br r atios, similar to those of associated groundwaters. Mass-balance calcu lations based on salinity estimates from microthermometry, crush-leach analyses and inclusion counting, coupled with the study of possible m echanisms for leakage, demonstrate that the relatively high salinity o f deep groundwaters (Boettstein and Aspo) cannot only be accounted for by the leakage of fluid inclusions trapped in quartz. It cannot be to tally excluded that fluid inclusions may be responsible for the major part of chloride in groundwaters with relatively low salinity (Leugger n 1647 m, Cauterets, Luchon, Ahaggar).The leakage of secondary fluid i nclusions trapped in primary minerals cannot be considered as the main process responsible for the high salinity of present-day deep groundw aters in the studied sites. However, the presence of secondary salt-ri ch inclusions indicates that sedimentary brines have already percolate d the granites, a datum which is important for the evaluation of the h ydrogeological history and for the understanding of the modalities of chlorine introduction in the crystalline basement.