Multiple-stage diagenetic alteration and fluid history of Ordovician carbonate-hosted barite mineralization, Southern Quebec Appalachians

Abstract

Lower Ordovician bioclastic limestone of the Upton Group, southern Quebec Appalachians, hosts stratabound Ba-Zn-Pb mineralization. The Upton Group, a mixed platform carbonate-siliciclastic-volcanic succession, is exposed as windows within the tectonically overlying Cambrian siliciclastics of the Granby Nappe. Mineralization consists mostly of barite and minor amounts of sulfides (sphalerite, pyrite, galena, and chalcopyrite), in addition to calcite, quartz and bitumen cements. It is hosted by a bioclastic limestone which is interbedded with and capped by a black calcareous shale, and underlain by a mudstone-siltstone-volcanic succession and a lower poorly fossiliferous limestone. The lower limestone recorded early extensive dolomitization followed by meteoric alteration (dedolomitization, sulphate dissolution, vadose cements, soil pisoids, etc.), and burial diagenesis (recrystallization, fracturation, and cementation). The vadose gravitational calcite cements yield δ 18O PDB values of −8.4 to −11.0‰ andδ 13C PDB values of +2.4 to +2.8‰. The thin soil profiles with pisoids have a δ 18O PDB value of −8.2‰ and a δ 13C PDB value of +2.0‰. These data suggest an evaporative 18O-enrichment of near-surface trapped soil moisture (vadose water) in a rock-dominated diagenetic system. The recrystallized limestone hasδ 18O PDB values of −11.4 to −15.5‰ and near Early Ordovician marine δ 13C PDB values of −0.2 to +2.5‰. These data suggest a final stabilization of the limestone from high temperature fluids in a rock-dominated diagenetic system. The mineralized bioclastic limestone shows rare evidence of early submarine cementation which is overprinted by significant post-depositional recrystallization and hydrothermal alteration. The latter resulted in the generation of secondary porosity and precipitation of a subhedral barite cement, a bladed barite cement, and fracture-filling barite. Fracture- and void-filling calcite, sulfides, quartz and bitumen cementation followed barite mineralization. Pre-barite syntaxial calcite overgrowths on crinoids yieldδ 13C PDB values of −3.9 to −15.0‰ andδ 18O PDB values of −13.7 to −14.8‰. Post-barite sparry calcite cement and fracture-filling calcite haveδ 13C PDB values of −2.6 to −13.0‰ and −2.4 to −17.9‰, respectively, andδ 18O PDB values of −13.6 to −14.2‰ and −14.0 to −15.8‰, respectively. The δ 18O values suggest relatively high-temperature re-equilibration in a deep-burial environment. The variable and depleted 13C values appear to reflect fluid-rock interaction and addition of significantδ 13C-depleted CO 2 from thermochemical sulphate reduction of organic matter. Mixing of reduced, hot basinal brines with oxidizing sulphate-rich fluids resulted in barite precipitation.