HYDROTHERMAL FLUIDS DURING MESOZOIC REACTIVATION OF THE ST-LAWRENCE RIFT SYSTEM, CANADA - C, O, SR AND PB ISOTOPIC CHARACTERIZATION

Calcite veins with accessory galena, sphalerite, barite and fluorite a re abundant within the Ottawa-Bonnechere and the Charlevoix-Saguenay f ailed arms of the St. Lawrence rift system which developed, during ear ly Paleozoic times, within gneisses of the Grenville Province of Canad a. The mineral assemblages, paragenesis and precipitation temperatures of the veins resemble the Mississippi Valley deposits. Crosscutting r elationships with carbonate rocks of the St. Lawrence Lowlands and def ormed carbonate rock slivers of the Charlevoix astrobleme show that th e Ottawa graben and Saguenay graben veins are younger than mid-Ordovic ian or post-Devonian, respectively. delta(18)O and delta(13)C values f or vein calcite are positively correlated and range from +11 to +24 pa rts per thousand and -9 to +3 parts per thousand, respectively. Sr-87/ Sr-86 in calcite and barite range from 0.7129 to 0.7081. The fluids th at precipitated calcite were part of a hydrothermal system dominated b y meteoric water enriched in O-18 by exchange with silicate minerals a nd deep-seated carbon, either from mantle outgassing, deep-seated brin es of the Canadian Shield or from oxidation of reduced carbon in high- grade metamorphic rocks. Veins cutting marbles of the Grenville Superg roup have lower Sr-87/Sr-86 and higher delta(18)O and delta(13)C value s than those hosted in quartzofeldspathic gneisses of the Grenville Pr ovince, showing that the fluids were modified by interaction at depth with the marbles. Stable and Sr isotope compositions are also correlat ed with La/Yb ratios in calcite, supporting the fact that the veins fo rmed within similar hydrothermal systems in both the Ottawa and Saguen ay grabens. The Pb isotopic compositions of vein galena are radiogenic (Pb-206/Pb-204 = 18.0-19.7; Pb-207/Pb-204 = 15.5-15.7; Pb-208/Pb-204 = 37.9-41.0) and positively correlated in conventional Pb-Pb diagrams. Pb isotope and REE data strongly suggest that the fluids leached sele ctively, at the mineral scale, gneisses of the Grenville Province. The basinal fluids within the shelf sequence of the western Appalachians had Pb-208/Pb-204 ratios, in the lower Paleozoic, significantly lower than those of vein galena. This excludes the basin brine model of form ation for the vein deposits, like suggested for the Mississippi Valley deposits. The veins rather formed in the Mesozoic, in response to the rifting of northeastern North America and the reactivation of the St. Lawrence rift.