MULTIPLE-STAGE DIAGENETIC ALTERATION AND FLUID HISTORY OF ORDOVICIAN CARBONATE-HOSTED BARITE MINERALIZATION, SOUTHERN QUEBEC APPALACHIANS

Lower Ordovician bioclastic limestone of the Upton Group, southern Que bec 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 silic iclastics of the Granby Nappe. Mineralization consists mostly of barit e and minor amounts of sulfides (sphalerite, pyrite, galena, and chalc opyrite), in addition to calcite, quartz and bitumen cements. It is ho sted by a bioclastic limestone which is interbedded with and capped by a black calcareous shale, and underlain by a mudstone-siltstone-volca nic succession and a lower poorly fossiliferous limestone. The lower l imestone recorded early extensive dolomitization followed by meteoric alteration (dedolomitization, sulphate dissolution, vadose cements, so il pisoids, etc.), and burial diagenesis (recrystallization, fracturat ion, and cementation). The vadose gravitational calcite cements yield delta(18)O(PDB) values of -8.4 to -11.0 parts per thousand and delta(1 3)C(PDB) values of +2.4 to +2.8 parts per thousand. The thin soil prof iles with pisoids have a delta(18)O(PDB) value of -8.2 parts per thous and and a delta 13C(PDB) Value of +2.0 parts per thousand. These data suggest an evaporitive O-18-enrichment of near-surface trapped soil mo isture (vadose water) in a rock-dominated diagenetic system. The recry stallized limestone has delta(18)O(PDB) values of -11.4 to -15.5 parts per thousand and near Early Ordovician marine delta(13)C(PDB) values of -0.2 to +2.5 parts per thousand. These data suggest a final stabili zation of the limestone from high temperature fluids in a rock-dominat ed diagenetic system. The mineralized bioclastic limestone shows rare evidence of early submarine cementation which is overprinted by signif icant post-depositional recrystallization and hydrothermal alteration. The latter resulted in the generation of secondary porosity and preci pitation of a subhedral barite cement, a bladed barite cement, and fra cture-filling barite. Fracture- and void-filling calcite, sulfides, qu artz and bitumen cementation followed barite mineralization. Pre-barit e syntaxial calcite overgrowths on crinoids yield delta(13)C(PDB) valu es of -3.9 to -15.0 parts per thousand and delta(13)O(PDB) values of - 13.7 to -14.8 parts per thousand. Post-barite sparry calcite cement an d fracture-filling calcite have delta(13)C(PDB) values of -2.6 to -13. 0 parts per thousand and -2.4 to -17.9 parts per thousand, respectivel y, and delta(18)O(PDB) values of -13.6 to -14.2 parts per thousand and -14.0 to -15.8 parts per thousand, respectively. The delta(18)O value s suggest relatively high-temperature re-equilibration in a deep-buria l environment. The variable and depleted C-13 values appear to reflect fluid-rock interaction and addition of significant delta(13)C-deplete d CO2 from thermochemical sulphate reduction of organic matter. Mixing of reduced, hot basinal brines with oxidizing sulphate-rich fluids re sulted in barite precipitation.