Hydrothermal fluids during Mesozoic reactivation of the St. Lawrence rift system, Canada: C, O, Sr and Pb isotopic characterization

Abstract

Calcite veins with accessory galena, sphalerite, barite and fluorite are abundant within the Ottawa-Bonnechère and the Charlevoix-Saguenay failed arms of the St. Lawrence rift system which developed, during early Paleozoic times, within gneisses of the Grenville Province of Canada. The mineral assemblages, paragenesis and precipitation temperatures of the veins resemble the Mississippi Valley deposits. Crosscutting relationships with carbonate rocks of the St. Lawrence Lowlands and deformed carbonate rock slivers of the Charlevoix astrobleme show that the Ottawa graben and Saguenay graben veins are younger than mid-Ordovician or post-Devonian, respectively. δ 18O and δ 13C values for vein calcite are positively correlated and range from +11 to +24‰ and −9 to +3‰, respectively. 87 Sr 86 Sr in calcite and barite range from 0.7129 to 0.7081. The fluids that precipitated calcite were part of a hydrothermal system dominated by meteoric water enriched in 18O by exchange with silicate minerals and deep-seated carbon, either from mantle outgassing, deep-seated brines of the Canadian Shield or from oxidation of reduced carbon in high-grade metamorphic rocks. Veins cutting marbles of the Grenville Supergroup have lower 87 Sr 86 Sr and higher δ 18O and δ 13C values than those hosted in quartzofeldspathic gneisses of the Grenville Province, showing that the fluids were modified by interaction at depth with the marbles. Stable and Sr isotope compositions are also correlated with La Yb ratios in calcite, supporting the fact that the veins formed within similar hydrothermal systems in both the Ottawa and Saguenay grabens. The Pb isotopic compositions of vein galena are radiogenic ( 206 Pb 204 Pb = 18.0–19.7 ; 207 Pb 204 Pb = 15.5–15.7 ; 208 Pb 204 Pb = 37.9–41.0 ) and positively correlated in conventional PbPb diagrams. Pb isotope and REE data strongly suggest that the fluids leached selectively, at the mineral scale, gneisses of the Grenville Province. The basinal fluids within the shelf sequence of the western Appalachians had 208 Pb 204 Pb ratios, in the lower Paleozoic, significantly lower than those of vein galena. This excludes the basin brine model of formation 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.