Andre Chagnon

Sedimentologic, Diagenetic and Tectonic Evolution of the Saint-Flavien Gas Reservoir at the Structural Front of the Quebec Appalachians

ABSTRACT The Beekmantown Group (Lower Ordovician) of the Saint-Flavien reservoir has produced 162x106 m3 (5.7 bcf) of natural gas between 1980 and 1994. The conversion of the field into gas storage was initiated in 1992 and the pool became operational in 1998. Integration of structural and sedimentologic features, carbonate and organic matter petrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosity evolution in these reservoir dolostones. The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of 2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequently covered by a shoaling succession of subtidal and intertidal limestones to intertidal dolostones. Early dolomitization has produced intercrystalline porosity and preserved some moldic pores in the intertidal facies. Near surface, post-dolomite karstification has created vugs that were subsequently filled by early marine calcite fibrous cement based on the 18O and 13C ratios of calcite. Early burial elements consist of horizontal stylolites, pyrite and sphalerite. Late migrated bitumen was thermally altered or vaporized as native coke under deep burial conditions exceeding 240°C, partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and subsequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The 18O of the poikilotopic calcite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about 260°C). Horizontal shear zones and marcasite-rich vertical stylolites were produced during folding and thrusting. Dissolution has preferentially affected late fracture-filling calcite and generated most of the actual porosity during or soon after the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the low temperature condition of this dissolution (T <100°C). Porosity in the Saint-Flavien reservoir has been mostly produced by fracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous facies at the top of rhythmic cycles that compose the Beekmantown Group. End_Page 126------------------------

Organic matter and clay anomalies associated with base-metal sulfide deposits

Abstract Organic matter and clay mineral anomalies surround the Polaris, Gays River and Gaspe Mines base-metal sulfide deposits. The Polaris mine, in the Canadian Arctic, is an MVT deposit formed by low temperature solutions (homogenization temperatures, T h : 105–120°C) and is characterized by a zone of pure illite above a kaolinite envelope surrounding the deposit. These clay mineral assemblages are laterally replaced by kaolinite-illite and chlorite-corrensite zones, while the background assemblage in the host rocks is illite-interstratified illite / smectite-chlorite. The organic matter reflectance ( R o ) shows two trends depending on the permeability and chemical reactivity of the host rock: (1) an increase with depth above the ore zones in the overlying impermeable shaly cap rock. R o reaches a maximum value of 1.3% above the ore, compared to 0.8% in equivalent country rocks; (2) an anomalous decrease with depth in the mineralized carbonate horizon that hosts the deposit. The Gays River mine, Nova Scotia, is a medium temperature ( T h : 250°C) carbonate-hosted deposit associated with the replacement of well-crystallized detrital illite and chlorite by smectite forming a halo around the main ore zone and by kaolinite and chlorite-corrensite within the ore zone. Reflectance and its absolute variability increase in the host rocks toward the ore zone adjacent to the deposit (0.8 to 1.88%) and shows a relatively low R o in the ore. The economic mineralization at Gaspe Mines, Quebec, is a typical high-temperature skarn and porphyry-copper deposit with the concomitant clay mineral alteration assemblages: chlorite-corrensite-illite, smectite-chlorite-illite and almost pure illite with traces of chlorite. The first assemblage is observed within the ore zone and extends slightly outside the procellanite halo. Smectite is also found near the mineralization, but its limit of occurrence is wider than the chlorite assemblage. The illite assemblage is found outside the chlorite zone and is present as irregular tongues. R o increases (1.2 to 15%) approaching ore over a lateral distance of 60 km. In the bleached zone, the organic matter exhibits mosaic and degasification textures and isotropic alteration rims with very low R o . These three deposits, as well as others studied by the authors, are centrally located within thermal aureoles whose size and intensity correlate directly with increasing temperature of formation. The decreasing values of R o in the ore zones of these deposits and the associated authigenic clay minerals reflect changes in chemical conditions due to precipitation of sulfides and to reaction of the ore and post-ore fluids with host rocks.

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

Abstract The Appalachian thrust belt of eastern Quebec 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 Enrage 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. The domains of the St. Fabien and Cap Enrage 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. Mass-balance calculations relating altered and unaltered rock suggest that dolomitization was accompanied by large gains of CaO, MgO and CO 2 , and corresponding losses of SiO 2 and K 2 O 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 m Ca 2+ , m Mg 2+ and m CO 2 to be between 3.9 and 4.6. Fluid-inclusion data suggest that dolomitization and mineralization at St. Fabien (and Cap Enrage) 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.