THE FLUIDS OF OROGENY; O-H ISOTOPIC ANALYSIS OF REGIONAL, DATED CLAY-RICH FAULT GOUGE IN THE ALBERTA ROCKIES, CANADA

To determine the identity of ancient orogenic fluids, O and H isotopes in authigenic clays of previously dated fault gouge from major thrusts in the Alberta Rockies is analyzed. Gouge clay δD values range from -137‰ to -63‰ VSMOW; δ18O values range from 7.0 to 20.3‰. Using fractionation temperatures of 150°C we obtain a fluid compositional range of δD(fluid) is -124‰ from most isotopically negative clays, to δD(fluid) of -51‰ from least isotopically negative clays. H isotope results record mixing between ancient surface fluids and deeply-sourced (metamorphic) fluids in volumetric ratios of 75-25%. Oxygen data show isotopic exchange (buffering) of fluid with carbonate host rock, limiting their application for source fingerprinting, but otherwise consistent. There are no geographic or temporal patterns to these geofluid mixtures along and across the orogenic belt. Instead, varied mixing shows that orogenic fluids followed structurally heterogeneous pathways during deformation pulses rather than pervasive surface fluid infiltration. The involvement of significant surface fluid at depth also implies open-system fluid conditions, limiting the mechanical role of fluid overpressure in this fold-thrust belt.