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The hydrodynamic of the multi-aquifer system (the Continental Intercalaire " C.I " and the Complex Terminal " C.T ") of the North Gafsa basin is largely determined by tectonics (Tebessa-Gafsa fault). The composition of groundwater is controlled by complex reactions at gas-liquid-solid "mineralogical composition of associated rocks" interfaces, which depend on the natural surrounding and potential anthropogenic impact. The hydrochemical data (major ion geochemistry) indicate that these groundwaters are characterized by the dominance a Ca-Mg-HCO 3 /SO 4 and Na-Cl-NO 3 water types. Geochemical pattern is mainly controlled by the

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A sulphur isotope study was carried out to assess the value of such isotopic data in constraining the contributions of igneous and sedimentary protoliths for British and Irish Caledonian granites. Conventional separation techniques yielded sulphides from only 19 of the 50 selected granites, including both Newer and Older intrusions. These sulphides are dominated by pyrite, with lesser pyrrhotite, and rare chalcopyrite.Two groups emerge from consideration of 614S in conjunction with 6"0, initial x7Sr/86Sr (Is,) and age; Group I have d 1 * 0 generally < + 10.5'%~, Is, in the range 0.703 to 0.708, and are younger than 435 Ma. 614S of this group ranges from -4.5'%,~, to +4.4'%~,: R = +0.7 f 2.6'%~) (n=40:la). Group I1 have 6I8O generally > + 10.5"/00, are generally older, I,, between 0.710 and 0.7208, with 614S in the range +6.2"/~ to +16"/00: R = +9.5 f 3 . 3 " /~ (n = 13:la). The source region for group I1 granites contained a significant sedimentary component. Since sulphides in Lower and Middle Dalradian metasediments are characterized by unusually high JT4S values, between + 1 l'%,~, and + 17"/~, we suggest that group I1 granites contain a significant proportion of sulphur derived from these rocks.

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