Cl-36, I-129 and noble gas isotope systematics in groundwaters from the Lac du Bonnet Batholith, Manitoba, Canada

Radionuclide (H-3, C-14, Cl-36, I-129), noble gas isotopes (He-4, Ne-20,Ne- 21,Ne-22), Stable isotope (O, H) and hydrochemical analyses have been carri ed out on fracture-hosted, brackish to saline groundwaters from a well-char acterised hydrogeologic system in the Archean Lac du Bonnet Batholith, Mani toba, Canada. He and Ne isotope systematics of saline groundwaters in deeper fractures ar e dominated by alpha,n and n,alpha reactions with O-18, Mg-25, F-19 in the rock matrix. With depth, the groundwaters have increasing concentrations of Cl-, Cl-36, radiogenic He-4 (He-4,) and nucleogenic Ne-21 and Ne-22. Cl-36 /Cl ratios in the groundwaters are comparable to measured and calculated in situ Cl-36/Cl production ratios for the granites indicating the Cl- ions h ave likely been dissolved from the rock matrix. Concentrations of I-129 are not correlatable with I- and only broadly correlate with Cl-36 and He-4(r) concentrations, suggesting that the observed I-129 is not due to water/roc k interactions, but to the accumulation of fissiogenic I-129 with time. The correlations amongst Cl-36, I-129, noble gases and hydrochemical system atics suggest the chemical compositions of the groundwaters have been modif ied by interactions-with silicate and oxide minerals and dissolution of mat rix Cl-. Modelling of accumulation of fissiogenic I-129 suggests these proc esses have occurred over at least several million years for the most-evolve d groundwaters.