A MASSIVE METEORIC-HYDROTHERMAL SYSTEM RELATED TO EOCENE CRUSTAL EXTENSION OF THE SOUTHERN CANADIAN CORDILLERA: ONE OF THE LARGEST IN THE GEOLOGIC RECORD

A compilation of published and unpublished oxygen and hydrogen isotope values of rocks, minerals and veins from the southern Canadian Cordillera suggest that this region hosted a massive meteoric-hydrothermal system during Eocene oblique extension. This compilation was mapped using Google Earth. Fluid δD values are estimated using published D/H fractionation factors at 400°C. The error is ±20‰, small enough to distinguish deep magmatic/metamorphic fluids from meteoric-hydrothermal fluids at these high latitudes. Peaks at δD values of –60‰ (deep metamorphic or magmatic fluid) and ­–110‰ (Early Cenozoic meteoric-hydrothermal fluid) on a histogram plot of fluid δD values provide a clear distinction between the two kinds of fluid. Synextensional meteoric-hydrothermal systems (δD_H2O < –80‰) related to detachment faulting and shallow intrusions affected the eastern margin of the Coast Mountains Batholith and the Southern Omineca Belt. Quartz-feldspar ¹⁸O/¹⁶O fractionations from all detachment faults are out of equilibrium, an indication of short-lived hydrothermal activity involving meteoric-hydrothermal fluids within a zone of very high local geothermal gradient.

The D/H signature of deep fluids (δD > –80‰) are present only in areas distal to detachment faulting, including the Western Metamorphic Belt, the Western and Central Coast Mountains Batholith, the belt of Jurassic metamorphism that extends from the Cariboo Mountains to the Purcell Mountains, and the deepest structural levels of the Shuswap metamorphic core complex. Most of these rocks have quartz-feldspar ¹⁸O/¹⁶O fractionations indicative of magmatic temperatures. Zones of mixing between these disparate fluids, such as transfer zones that link detachment faults, are prime mineral exploration locations.

Histogram plots of vein quartz fluid inclusion δD values reveal three peaks that include the two produced by the mineral δD values, but these data are dominated by a large peak at δD = –150. This lower peak may be the product of the contribution of small amounts of hydrous minerals that were trapped as inclusions in the quartz veins. These meteoric-hydrothermal systems of the Canadian Cordillera may have been the largest on Earth. Preliminary assessment of the Great Basin suggests that this widespread extensional zone may be another site of regional-scale hydrothermal systems involving meteoric waters.