Paleo-hydrogeology of late proterozoic units of southeastern Canadian Cordillera

Abstract

A comprehensive geological and geochemical investigation of veining and other manifestations of fluid flow in late Proterozoic units of the Windermere Supergroup in the southeastern Canadian Cordillera was undertaken to characterize the nature of hydrogeological systems during and following deformation of a fold and thrust belt The results of fluid inclusion studies document vein formation from moderate to low temperature, low salinity, aqueous fluids. Results of δ18O studies of vein quartz and carbonate indicate that the veins formed from fluids with high δ18O values, which in most cases were derived from extensive isotopic exchange between the fluids and Proterozoic units. In widespread, small, early, bedding-parallel veins and in larger subvertical veins in units of biotite greenschist or higher metamorphic grades, δD values of inclusion fluids are generally > -90 permil indicating vein formation from fluids originating from metamorphic devolatilization. In rocks of metamorphic grade less than biotite greenschist facies, larger, late tectonic, subvertical veins have δD values (-120 to -150 permil) for inclusion fluids indicative of vein formation from deeply circulated meteoric water. Results of δ13C and 87Sr/86Sr studies of vein carbonates indicate a marked degree of regional heterogeneity, which is related to variations in the host rock lithology. Most significant are the unusually low δ13C (-11 to -17 permil) and high 87Sr/86Sr (0.750 to 0.780) values of vein carbonate from the Old Fort Point Formation. The close linkage of the δ13C and 87Sr/86Sr values of vein carbonate to lithology of the host unit indicates relatively rapid reequilibration of the isotopic signatures of dissolved C and Sr, as the fluids moved from one unit to another. Comparison of the results of this study to results obtained on a similar study of paleo-hydrogeology of extensional regimes of the southern Omineca elt of the Canadian Cordillera indicates that in compressional regimes the depth of penetration of surface fluids is shallower and more heterogeneous. As a result, convected surface fluids in compressional regimes are in general cooler, have lower CO2 contents, and probably lower integrated total fluxes.