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Geochemical studies of the origins and effects of synorogenic crustal fluids in the southern Omineca Belt of British Columbia, Canada

¹ Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3 Canada

Fluid inclusion and stable isotope investigations of 400+ samples of quartz ± carbonate veins and their zeolite to amphibolite grade host rocks from the southern Omineca Belt of the Canadian Cordillera have been used to determine origins, evolution, and effects of crustal fluids during and after orogenic activity. Correlations between fluid inclusion characteristics and tectonic and lithologic features indicate that salinities and gas compositions (CO₂/CH₄ ratios) are controlled by host rock lithology. High total gas contents are linked to high inclusion homogenization temperatures, which parallel increasing metamorphic grades.

The D values of the vast majority of vein-forming fluids range from –100% to –150% (Standard Mean Ocean Water [SMOW]), indicating meteoric water as the source of the fluids. The ¹⁸O_Water values of the fluids were homogeneous over large sections of the crust, with a typical range of ¹⁸O values of 6% to 11% (SMOW). Homogenization of ¹⁸O values of vein-forming fluids is believed to reflect interaction of the meteoric water with a variety of rock types at temperatures in excess of 350 °C. This process resulted in resetting of ¹⁸O values of both fluids and rock units. The absence of vein formation from low ¹⁸O water requires that mixing of ascending and descending fluids does not occur. Carbon and strontium isotope analyses indicate a strong degree of host rock control on these isotopic ratios.

Synthesis of the results yields a model for the hydrogeology of the brittle crust consisting of moderately high permeabilities in fractured brittle rocks with deep convection of surface, meteoric water. The maximum depth of penetration of the fluids is limited by the rheological brittle/ductile transition at temperatures of 350 to 450 °C and depths of 10 km or greater. The vast majority of veins found in greenschist and lower grades of metamorphic rocks are formed on the upflow limbs of the meteoric water convection cells.

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