GENESIS AND CONTROLS OF HYDROTHERMAL DOLOMITIZATION IN SANDSTONES OF THE APPALACHIAN THRUST BELT, QUEBEC, CANADA - IMPLICATIONS FOR ASSOCIATED GALENA-BARITE MINERALIZATION

The Appalachian thrust belt of eastern Quebec hosts six epigenetic bar ite-galena(-sphalerite) occurrences in interdigitated carbonate-cement ed sandstones, carbonate-clast-bearing conglomerates and minor siltsto nes of the Upper Cambrian St. Damase Formation. Two of these, the St. Fabien and Cap Enrage deposits, consist of veins and disseminations en veloped by dolomitized zones, and contain ankerite as an essential gan gue mineral. In contrast, the other occurrences consist exclusively of calcite-cemented veins, and are conspicuously devoid of dolomite. The domains of the St. Fabien and Cap Enrage deposits are intensely fract ured and contain abundant dolomite-barite-sulfide-cemented interconnec ted fractures and pores, whereas those of the other occurrences displa y relatively minor fractures and pores, which an only rarely interconn ected. This difference in porosity leads to the conclusion that differ ences in paleo-permeability were the most likely reasons for the prese nce of dolomite and disseminated mineralization in the two deposits, a nd their absence from the vein-only occurrences. Mass-balance calculat ions relating altered and unaltered rock suggest that dolomitization w as accompanied by large gains of CaO, MgO and CO2, and corresponding l osses 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 1 00 g of rock), when considered in the context of quartz solubility, im plies a minimum water/rock ratio of 2635. A similar value (2500) is in dicated by the content of lead of a representative sample of the St. F abien deposit and typical concentrations of lead in Mississippi Valley -type fluids. The pH of the mineralizing/dolomitizing fluid was calcul ated from the solubility of dolomite and fluid-inclusion estimates of m(Ca2+), m(Mg2+) and m(CO2) to be between 3.9 and 4.6. Fluid-inclusion data suggest that dolomitization and mineralization at St. Fabien (an d Cap Enrage) were the products of interactions between local sediment ary rocks and minor metabasaltic dikes with hydrothermal brines. Detai led X-ray diffraction analysis of the phyllosilicates in the host rock s in and around the St. Fabien deposit (mainly sandstones and minor me tabasalt) indicates that these brines could have acquired virtually al l magnesium and all iron(+/-manganese) needed for dolomite precipitati on as a result of the replacement of chlorite by illite. Since P-T con ditions during deposition of the two types of mineralization were simi lar to each other, it is concluded that these conditions did not gover n the formation of dolomite.