GEOCHEMICAL STUDIES OF THE ORIGINS AND EFFECTS OF SYNOROGENIC CRUSTALFLUIDS IN THE SOUTHERN OMINECA BELT OF BRITISH-COLUMBIA, CANADA

Fluid inclusion and stable isotope investigations of 400+ samples of q uartz +/- carbonate veins and their zeolite to amphibolite grade host rocks from the southern Omineca Belt of the Canadian Cordillera have b een used to determine origins, evolution, and effects of crustal fluid s during and after orogenic activity. Correlations between fluid inclu sion characteristics and tectonic and lithologic features indicate tha t salinities and gas compositions (CO2/CH4 ratios) are controlled by h ost rock lithology. High total gas contents are linked to high inclusi on homogenization temperatures, parallel increasing metamorphic grades . The delta D values of the vast majority of vein-forming fluids range from -100 parts per thousand to -150 parts per thousand (Standard Mea n Ocean Water [SMOW]), indicating meteoric water as the source of the fluids. The delta(18)O(Water) values of the fluids were homogeneous ov er large sections of the crust, with a typical range of delta(18)O val ues of 6 parts per thousand to 11 parts per thousand (SMOW). Homogeniz ation of delta(18)O values of vein-forming fluids is believed to refle ct interaction of the meteoric water with a variety of rock types at t emperatures in excess of 350 degrees C. This process resulted in reset ting of delta(18)O values of both fluids and rock units. The absence o f vein formation from low delta(18)O water requires that mixing of asc ending and descending fluids does not occur. Carbon and strontium isot ope analyses indicate a strong degree of host rock control on these is otopic ratios. Synthesis of the results yields a model for the hydroge ology of the brittle crust consisting of moderately high permeabilitie s in fractured brittle rocks with deep convection of surface, meteoric water, The maximum depth of penetration of the fluids is limited by t he theological brittle/ductile transition at temperatures of 350 to 45 0 degrees C and depths of 10 km or greater. The vase majority of veins found in greenschist acid lower grades of metamorphic rocks are forme d on the upflow limbs of the meteoric water convection cells.