Genesis and controls of hydrothermal dolomitization in sandstones of the Appalachian thrust belt, Québec, Canada: Implications for associated galena-barite mineralization

The Appalachian thrust belt of eastern Québec hosts six epigenetic barite-galena(-sphalerite) occurrences in interdigitated carbonate-cemented sandstones, carbonate-clast-bearing conglomerates and minor siltstones of the Upper Cambrian St. Damase Formation. Two of these, the St. Fabien and Cap Enragé deposits, consist of veins and disseminations enveloped by dolomitized zones, and contain ankerite as an essential gangue mineral. In contrast, the other occurrences consist exclusively of calcite-cemented veins, and are conspicuously devoid of dolomite. mains of the St. Fabien and Cap Enragé deposits,are intensely fractured and contain abundant dolomite-barite-sulfide-cemented interconnected fractures and pores, whereas those of the other occurrences display relatively minor fractures and pores, which are only rarely interconnected. This difference in porosity leads to the conclusion that differences in paleo-permeability were the most likely reasons for the presence of dolomite and disseminated mineralization in the two deposits, and their absence from the vein-only occurrences. alance calculations relating altered and unaltered rock suggest that dolomitization was accompanied by large gains of CaO, MgO and CO2, and corresponding losses of SiO2 and K2O under conditions of constant volume. The mass of silica removed from the altered sandstone at St. Fabien (14.5 g per 100 g of rock), when considered in the context of quartz solubility, implies a minimum water/rock ratio of 2635. A similar value (2500) is indicated by the content of lead of a representative sample of the St. Fabien deposit and typical concentrations of lead in Mississippi Valley-type fluids. The pH of the mineralizing/dolomitizing fluid was calculated from the solubility of dolomite and fluid-inclusion estimates of mCa2+, mMg2+ and mCO2 to be between 3.9 and 4.6. inclusion data suggest that dolomitization and mineralization at St. Fabien (and Cap Enragé) were the products of interactions between local sedimentary rocks and minor metabasaltic dikes with hydrothermal brines. Detailed X-ray diffraction analysis of the phyllosilicates in the host rocks in and around the St. Fabien deposit (mainly sandstones and minor metabasalt) indicates that these brines could have acquired virtually all magnesium and all iron (± manganese) needed for dolomite precipitation as a result of the replacement of chlorite by illite. Since P-T conditions during deposition of the two types of mineralization were similar to each other, it is concluded that these conditions did not govern the formation of dolomite.