Characteristics of late-glacial ice-dammed lakes reconstructed in the Appalachians of southern Québec

During deglaciation, several valleys of the Appalachians of southern Québec were affected by the pounding of ice-dammed lakes. More than 300 water planes were reconstructed in the southeastern part of the region with the methodology presented elsewhere in this volume (Quat. Int. (2002) this volume). Their characteristics during deglaciation gradually changed, following four main steps: (1) ice-dammed ponds; (2) ice-dammed lakelets; (3) intermediate lakes; and (4) large ice-dammed lake. These various lake types differed by the size, the shape, the position of the spillway and the depth of the former ice-dammed lake as well as by the relative duration and by the stability degree of each lake level. In this study, a relative duration of each reconstructed lake level was determined using the distance covered by the retreating ice margin during the former existence of the water body. The stability of the water level also explained the difference in the degree of development of shoreline features among water bodies with relatively similar sizes.     

The anatomy of an active slide: the Gascons rockslide,Québec,Canada

A railroad crosses the coastal Gascons rockslide, in Quebec, Canada. This study improves the understanding of the rockslide’s failure mechanism and post-failure behaviour responsible for the deformation in the railroad. The slide is an asymmetrical wedge failure of 410,000 m³, in rocks made up of centimetre-scale beds of nodular calcilutite alternating with sandstones and limestone. The post-failure stage of the rockslide is characterised by continuous movement of blocks with speeds ranging from 6 to 110 mm/year. The main water table is just below the sliding surface, but precipitation and snowmelt can raise it above the sliding surface in the upper part of the slide. We propose a model for the rockslide, its failure mechanism, geometry and hydrogeology to provide a baseline for interpreting the near-real-time data collected since 2009.     

Geostatistical analysis on the spatial variation of radiogenic elements in the crystalline basement of Grenville Province in the southwestern Québec

Accurate assessment of deep geothermal resources remains a challenge from the practical point of view. Parameter uncertainties and partial knowledge of initial conditions limit the prediction of subsurface temperatures using a variety of thermal models strongly unreliable, and the temperature is highly dependent on the radiogenic heat production in the geological layers mainly affected by a number of factors including the concentrations of uranium, thorium and potassium, and rock density. In this paper, geostatistical methods were applied to investigate the spatial distribution of radiogenic elements (e.g., uranium, thorium, potassium) and their corresponding concentrations and radiogenic heat production. A representative region measuring 35 km?×?80 km in the southwestern Québec, and covering the domains of Portneuf-Mauricie, Morin Terrane and Parc des Laurentides in the Grenville Province was selected for this study because of its easy accessibility. Analysis results show that the concentrations of uranium, thorium and potassium for most rocks of the Grenville basement in the research region are in the range of 1–2 ppm, 3–10 ppm and 1–4%, respectively. Furthermore, 90% of the total samples analysed in this study show a uranium concentration of less than 3 ppm, 64% of the samples show a thorium concentration of less than 5 ppm, and 56% of the samples show a potassium concentration of less than 3%. This paper engaged both the ordinary kriging interpolation and sequential Gaussian simulation (SGS) methods to study the spatial distribution of radiogenic elements. Using density data for specific rocks, the distribution of radiogenic heat production in the study area of the southwestern Grenville Province was also simulated using the SGS method. Conclusively, results show that the difference between the minimum and the maximum value of radiogenic heat production is 30%, considering a significant proportion of heterogeneity in rock density.     

Age and origin of uranium mineralization in the Camie River deposit (Otish Basin,Québec,Canada)

The Camie River uranium deposit is located in the southeastern part of the Paleoproterozoic Otish Basin (Québec). The uranium mineralization consists of disseminated and vein uraninite and brannerite precipitated close to the unconformity between Paleoproterozoic fluviatile, pervasively altered, sandstones and conglomerates of the Matoush Formation and the underlying sulfide-bearing graphitic schists of the Archean Hippocampe greenstone belt. Diagenetic orange/pink feldspathic alteration of the Matoush Formation consists of authigenic albite cement partly replaced by later orthoclase cement, with the Na₂O content of clastic rocks increasing with depth. Basin-wide green muscovite alteration affected both the Matoush Formation and the top of the basement Tichegami Group. Uraninite with minor brannerite is mainly hosted by subvertical reverse faults in basement graphitic metapelites ± sulfides and overlying sandstones and conglomerates. Uranium mineralization is associated with chlorite veins and alteration with temperatures near 320 °C, that are paragenetically late relative to the diagenetic feldspathic and muscovite alterations. Re-Os geochronology of molybdenite intergrown with uraninite yields an age of 1724.0 ± 4.9 Ma, whereas uraninite yields an identical, although slightly discordant, 1724 ± 29 Ma SIMS U-Pb age. Uraninite has high concentrations in REE with flat REE spectra resembling those of uraninite formed from metamorphic fluids, rather than the bell-shaped patterns typical of unconformity-related uraninite. Paragenesis and geochronology therefore show that the uranium mineralization formed approximately 440 million years after intrusion of the Otish Gabbro dykes and sills at ∼2176 Ma, which constrains the minimum age for the sedimentary host rocks. The post-diagenetic stage of uraninite after feldspathic and muscovite alterations, the paragenetic sequence and the brannerite-uraninite assemblage, the relatively high temperature for the mineralizing event (∼320 °C) following the diagenetic Na- and K-dominated alteration, lack of evidence for brines typical of unconformity-related U deposits, the older age of the Otish Basin compared to worldwide basins hosting unconformity-related uranium deposits, the large age difference between basin fill and mineralization, the older age of the uranium oxide compared to ages for worldwide unconformity-related U deposits, and the flat REE spectra of uraninite do not support the previous interpretation that the Camie River deposit is an unconformity-associated uranium deposit. Rather, the evidence is more consistent with a PaleoProterozoic, higher-temperature hydrothermal event at 1724 Ma, whose origin remains speculative.     

Impact of fire on long-term vegetation dynamics of ombrotrophic peatlands in northwestern Québec,Canada

A 7000-year record of local fire history was reconstructed from three ombrotrophic peatlands in the James Bay lowlands (northwestern Québec, Canada) using a high-resolution analysis of macroscopic charcoal (long axis ≥ 0.5 mm). The impact of fire on vegetation changes was evaluated using detailed analysis of plant macrofossils. Compared to upland boreal forest, fire incidence in these Sphagnum-dominated bogs is rather low. Past fire occurrence seems to have been controlled primarily by internal processes associated with local hydroseral succession. Size of the peatland basin and distance from the well-drained forest soils also appear to be factors controlling fire occurrence. The impact of peatland fires on long-term vegetation succession appears negligible except in a forested bog, where it initiated the replacement of Sphagnum by mosses. In some circumstances, fire caused marked changes in the bryophyte assemblages over many decades. However, ombrotrophic peatland vegetation is generally resilient to surface fire.     

A methodology to reconstruct small and short-lived ice-dammed lakes in the Appalachians of Southern Québec

Previous studies on the deglaciation of the Appalachians of southern Québec reconstructed only the most extensive ice-dammed lakes and the most stable glaciolacustrine water levels. They only used the best developed shoreline features, and thus were not able to reconstruct small or short-lived ice-dammed lakes, both being characterized by a weak development of their features. We propose a methodology to reconstruct glacial lakes which follows three main steps: (1) the mapping of shoreline features, generally the best developed ones, as in previous studies; (2) the delineation of the former shoreline for each water level, by also considering the less well-developed shoreline features which were found during field surveys designed as a function of the presumed shoreline position; and (3) the mapping of the maximum extension of the water plane, which includes the location of the former ice dam and of the former glacial lake spillway. In southern Québec, small ice-dammed lakes were numerous and important because they shaped the geomorphology of the region during the deglaciation. They are thus among the key components of a deglaciation model for this area.     

Mineralogy,geochemistry, and genesis of high-alumina fenites of the Mont Saint-Hilaire alkaline pluton,Québec,Canada

High-alumina fenites in the Mont Saint-Hilaire alkaline complex, Québec, Canada, form bodies at the contact of peralkaline nepheline syenite. Fenites are subdivided into four types: corundum-spessartine-biotite-feldspar, muscovite-corundum-hercynite-biotite-feldspar, carbonated muscovite-biotite-hercynite-feldspar, and spessartine-hercynite-feldspar. Accessory minerals of the ilmenite-pyrophanite series, columbites, zircon, thorite, pyrrhotite, Fe, Mn, Mg, Ca, Ba, and REE carbonates, uedaite-(Ce), etc. are identified. Three stages are suggested in the formation of these rocks. In mineralogy and geochemistry, the Mont Saint-Hilaire high-alumina fenites are similar to Al-rich fenites replacing xenoliths in the Khibiny alkaline complex, Russia. In both cases, fenites are related to peralkaline rocks and replace high-alumina protoliths: granite at Mont Saint-Hilaire and metapelites in the Khibiny Mountains. These fenites are regarded as a specific type of fenites with rock-forming Mg-depleted hercynite.     

Mathematical modeling of thermomechanical erosion beneath Proterozoic komatiitic basaltic sinuous rilles in the Cape Smith Belt,New Québec,Canada

Virtually all of the economic Ni–Cu–(platinum group element (PGE)) mineralization in the central part of the Cape Smith Belt of New Québec is hosted by thick olivine cumulate units in the Katinniq Member of the Raglan Formation at the base of the 1.9 Ga Chukotat Group. These units transgress underlying gabbros and pelitic metasediments, forming 50–200-m deep and 300–1,000-m wide V-shaped embayments and have been interpreted on the basis of surface geology, deep diamond core drilling, and magnetic inversion models to represent the remnants of one or more large, long (at least 20 km, possibly ≥50 km), sinuous, komatiitic basalt lava channels that formed by thermomechanical erosion of their substrates. We have used a mathematical model to test these hypotheses regarding komatiitic lava emplacement and erosion by lava. Our modeling predicts that an initially 10-m thick komatiitic basalt flow should have flowed turbulently near the vent and should have thermomechanically eroded unconsolidated pelitic sediment during emplacement to reach the observed degree of contamination of ≤10% at distances of ~30–60 km downstream from the source. Furthermore, our models predict that, at these distances downstream, a fully inflated 100-m thick komatiitic basalt flow would have had thermal erosion rates over consolidated gabbroic substrate of ~0.7–1.5 m/day, requiring ~70–140 days to incise a 100-m deep channel, depending on the initial temperature of the lava, the paleoslope, and the initial temperature and solidus temperature of the gabbro. These erosion rates would have been associated with volumetric flow rates of >10⁵–10⁶ m³/s and eruption volumes of >10³–10⁴ km³. Although these flow rates are orders of magnitude larger than those of most modern terrestrial basaltic flows, they are of the same order as those estimated for the largest terrestrial flood basalt flows and with those inferred for some of the largest extraterrestrial flows. Our predicted flow volumes are also of the same order as those of the largest terrestrial flood basalt units, consistent with the great thickness and widespread distribution of the Chukotat Group. Our modeling of thermomechanical erosion of gabbro by komatiitic basalt results in negligible contamination (<1%), and geochemical studies show that the spatially and petrogenetically related Chukotat basalts are uncontaminated, suggesting that the observed enrichments in U–Th–Light Rare Earth Elements (REE) > Middle REE–Heavy REE > Nb–Ta–Ti represent contamination by underlying Povungnituk semipelites. This result is consistent with present models for the genesis of the Ni–Cu–(PGE) mineralization in the Raglan Formation that involve thermomechanical erosion of unconsolidated, sulfidic semipelitic sediments, and decoupling of the miscible silicate and immiscible sulfide components.     

Characterization of deep saline aquifers in the Bécancour area,St. Lawrence Lowlands,Québec,Canada: implications for CO2 geological storage

The present paper provides a case study of the assessment of the potential for CO₂ storage in the deep saline aquifers of the Bécancour region in southern Québec. This assessment was based on a hydrogeological and petrophysical characterization using existing and newly acquired core and well log data from hydrocarbon exploration wells. Analyses of data obtained from different sources provide a good understanding of the reservoir hydrogeology and petrophysics. Profiles of formation pressure, temperature, density, viscosity, porosity, permeability, and net pay were established for Lower Paleozoic sedimentary aquifers. Lateral hydraulic continuity is dominant at the regional scale, whereas vertical discontinuities are apparent for most physical and chemical properties. The Covey Hill sandstone appears as the most suitable saline aquifer for CO₂ injection/storage. This unit is found at a depth of more than 1 km and has the following properties: fluid pressures exceed 14 MPa, temperature is above 35 °C, salinity is about 108,500 mg/l, matrix permeability is in the order of 3 × 10^?16 m² (0.3 mDarcy) with expected higher values of formation-scale permeability due to the presence of natural fractures, mean porosity is 6 %, net pay reaches 282 m, available pore volume per surface area is 17 m³/m², rock compressibility is 2 × 10^?9 Pa^?1 and capillary displacement pressure of brine by CO₂ is about 0.4 MPa. While the containment for CO₂ storage in the Bécancour saline aquifers can be ensured by appropriate reservoir characteristics, the injectivity of CO₂ and the storage capacity could be limiting factors due to the overall low permeability of aquifers. This characterization offers a solid basis for the subsequent development of a numerical hydrogeological model, which will be used for CO₂ injection capacity estimation, CO₂ injection scenarios and risk assessment.     

3D geological modelling of the Renard 2 kimberlite pipe,Québec,Canada: from exploration to extraction

The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Québec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining.

     

Les gisements d'or des tonalites archéennes (Abitibi,Québec)

Résumé De nombreux gisements ont été mis en exploitation depuis une dizaine d'années dans des plutons granitiques de la ceinture de roches vertes archéennes de l'Abitibi. La plupart d'entre eux sont encaissés par des faciès sodiques. Dans la zone sud-est de cette ceintúre, il existe deux types de concentrations, des zones filoniennes et des disséminations à caractère porphyrique.Les gisements filoniens sont encaissés dans des sills ou des dykes de tonalite-trondhjemite. Deux associations minéralogiques sont distinguées, l'une avec tourmaline et scheelite, l'autre avec hématite et métaux de base, spécifiques de chacun des blocs lithotectoniques qui composent la ceinture. Les variations paragénétiques peuvent être mise en relation avec la profondeur de mise en place des systèmes hydrothermaux. A l'échelle d'un bloc, on observe des variations de la structure des minéralisations controlées par les variations de la pression de fluide, plus importante au coeur des massifs et conduisant à des structures plus régulières empruntant des zones de faiblesse préexistantes.Les disséminations sont associées à un granite tardif issu de la refusion de tonalites. Dans la région de Launay, près d'Amos, l'or est associé à des zones d'épisyénites avec pyrite et molybdénite, ou avec des zones de cisaillement. Elles pourraient correspondre à des concentrations plus précoces.

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Several deposits have been developped since 10 years in granitics plutons of the Archean greenstone belt of Abitibi. Most of them are hosted by Na-rich granitoids. In the south-eastern part of Abitibi, two main type of deposit may be distinguished:The lode type is hosted in pre-tectonic dykes or sills of tonalite-trondhjemite. Two mineralogical association are caracterised, one with tourmaline, scheelite, the other with hematite, and seems specific of lithotectonic blocs. Paragenetic variations may be related to vertical variations of deposition. On a bloc scale, high fluid pressure predominate in the center of pluton and may be responsible for more regular structure.The disseminated type is hosted in late granodioritic intrusions associated with tonalite. The only occurrence of that type is the Launay deposit, near Amos. Gold is associated either with abite-rich episyenite, with pyrite and Mo, or with shear-zone cross-cutting the granitic intrusions.

     

Geochemical,mineralogical and microbiological characterization of a sulphide-bearing carbonate-rich gold-mine tailings impoundment,Joutel, Québec

The results of an integrated geochemical and mineralogical study conducted at the Agnico-Eagle gold-mine tailings impoundment, Joutel, Québec, are correlated with bacterial populations determined from an enumeration of 3 groups of Thiobacilli. The tailings were determined to contain approximately 5 wt.% sulphide–S, predominantly as pyrite, and up to 30 wt.% carbonate minerals, chiefly as dolomite–ankerite and siderite. The objective of the study was to evaluate the potential for the development of acidic drainage and dissolved-metal migration in carbonate-rich tailings impoundments, and to compare the results of the geochemical and microbiological characterization of the tailings. Sulphide-oxidation reactions have proceeded to a depth of 20–100 cm below the tailings surface. Pyrrhotite consistently shows more alteration than pyrite and arsenopyrite. Pyrrhotite is altered mainly through the replacement by goethite. The most abundant Thiobacilli are neutrophilic bacteria of the Thiobacillus thioparus type. The maximum most probable number values for these bacteria occur 20–40 cm below the tailings surface, a depth that coincides with the disappearance of oxide coatings. This observation, coupled with the sharp decline in gas-phase O₂ concentration, suggests that rapid bacterially-mediated S–oxidation is occurring at this depth. The pore-water pH throughout the tailings varies between 6.5 and 8.5; no low-pH waters were observed in the impoundment. These neutral pH conditions are attributed to the effect of acid-consuming carbonate-mineral dissolution reactions, which are also indicated by increased concentrations of Mg and Ca and alkalinity in the shallow zone of the tailings. As a result of these acid-neutralization reactions, dissolved metal concentrations are low.     

Chemical sedimentary protoliths in the > 3.75 Ga Nuvvuagittuq Supracrustal Belt (Québec,Canada)

Analyses of chemical sedimentary precipitates such as banded iron formation (BIF) provide a direct means to explore the nature and composition of the early hydrosphere. The recently discovered > 3750 Myr old Nuvvuagittuq Supracrustal Belt (NSB) in the Northeast Superior Province (Québec, Canada) hosts a suite of iron oxide-rich (± pyroxene and amphibole) units that are interpreted to be the metamorphosed equivalents of Fe oxide-facies BIF, and a collection of BIF-like Ca–Fe–Mg silicate rocks. The NSB rocks provide a rare glimpse of trace metal availability in Eoarchean (ca. 3800 Ma) seawater. As they may be contemporaneous with the relatively well-studied Isua Supracrustal Belt of southern West Greenland, their comparison provides an opportunity to enhance our basic understanding of the Eoarchean oceans at a global scale. Work since the initial discovery of the NSB in 2001 has established the basic lithological, geochemical and petrographic characteristics of these BIF and BIF-like rocks. Here we review the current state of knowledge of NSB rocks of probable chemical sedimentary origin, including aspects of their geology, likely origin and age. We conclude by examining the implications of results thus far for our understanding of early seawater compositions, and for the emergence of life in the context of early metallo-enzyme evolution.     

The role of magmatic and hydrothermal processes in the chemical evolution of the Strange Lake plutonic complex,Québec-Labrador

The Strange Lake plutonic complex consists of three annular Mid-Proterozoic arfvedsonite-aegirine-bearing alkali granites emplaced in the Rae province of the Canadian Shield. The mineralogy, chemistry and structural setting of the complex are very similar to that of many peralkaline central salic complexes associated with the development of the Gardar rift in southern Greeland. The Strange Lake granites are highly fractionated (Rb/Sr=5 to 160 and K/Rb=27 to 120) and carry unusually high abundances of HFSE and REE-bearing exotic minerals (e.g. pyrochlore, gittinsite, elpidite, gadolinite and kainosite) which are reflected in the elevated HFSE (e.g.Zr=307 to 16800 ppm) and REE (e.g. La=84 to 1337 ppm) contents of the granites. HFSE and REE increase from the oldest intrusive unit, which is hypersolvus and unaltered, to the youngest, which is subsolvus and metasomatized. The unaltered granites display a restricted range of ¹⁸O values (+8.2 to+9.6) and low ¹⁸O signatures for fresh arfvedsonite/aegirine (+4.8 to+5.2). Anomalously high CaO (0.7 to 3.2 wt%) and MgO (0.1 to 0.6wt%) concentrations characterize the altered subsolvus granites. These rocks also have elevated whole rock ¹⁸O values (+9.6 to +11.9), negative ¹⁸O_{quartz-alk.feld.} (-0.1 to-1.6), and high ¹⁸O values of altered arfvedsonite (i.e.+6.5 to 13.75) that correlates positively with whole rock ¹⁸O values. The chemical and isotopic data are consistent with a model in which the least evolved alkali granites are formed through differentiation from trachytic (syenitic) parents. Extreme HFSE and REE-enrichment may have been accomplished by differentiation through fractional crystallization and heterogenous distribution of F-rich silicic residual melts in which the REE and HFSE are transported as fluorocomplexes. The O-isotopic values are consistent with the circulation of low temperature (lt;200°C) hydrothermal fluids in the youngest subsolvus intrusive unit which caused extensive Ca (Mg and Sr) metasomatism and fluorine leaching, widespread hematization, and remobilization of the HFSE and REE.     

Logistic models as a forecasting tool for snow avalanches in a cold maritime climate: northern Gaspésie,Québec,Canada

Snow avalanches are a major natural hazard for road users and infrastructure in northern Gaspésie. Over the past 11 years, the occurrence of nearly 500 snow avalanches on the two major roads servicing the area was reported. No management program is currently operational. In this study, we analyze the weather patterns promoting snow avalanche initiation and use logistic regression (LR) to calculate the probability of avalanche occurrence on a daily basis. We then test the best LR models over the 2012–2013 season in an operational forecasting perspective: Each day, the probability of occurrence (0–100%) determined by the model was classified into five classes avalanche danger scale. Our results show that avalanche occurrence along the coast is best predicted by 2 days of accrued snowfall [in water equivalent (WE)], daily rainfall, and wind speed. In the valley, the most significant predictive variables are 3 days of accrued snowfall (WE), daily rainfall, and the preceding 2 days of thermal amplitude. The large scree slopes located along the coast and exposed to strong winds tend to be more reactive to direct snow accumulation than the inner-valley slopes. Therefore, the probability of avalanche occurrence increases rapidly during a snowfall. The slopes located in the valley are less responsive to snow loading. The LR models developed prove to be an efficient tool to forecast days with high levels of snow avalanche activity. Finally, we discuss how road maintenance managers can use this forecasting tool to improve decision making and risk rendering on a daily basis.     

Geology of the Renard 2 pipe to 1000 m depth,Renard Mine,Québec,Canada: insights into Kimberley-type pyroclastic kimberlite emplacement

The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Québec, Canada, forming the major component of the Mineral Resource of Stornoway Diamond Corporation’s Renard Mine. Renard 2 is infilled with two distinct kimberlite units that exhibit Kimberley-type pyroclastic kimberlite and related textures. Hypabyssal kimberlite also occurs as smaller cross-cutting sheets and irregular intrusions. The units are distinguished by their rock textures, groundmass mineral assemblages, olivine macrocryst size distributions and replacement products, mantle and country rock xenolith contents, whole rock geochemical signatures, bulk densities and diamond grades. These differences are interpreted to reflect different mantle ascent and near-surface emplacement processes and are here demonstrated to be vertically continuous from present surface to over 1000 m depth. The distinctive petrological features together with sharp, steep and cross-cutting internal contact relationships, show that each unit was formed from a separate batch of mantle-derived kimberlite magma, and was completely solidified before subsequent emplacement of the later unit. The mineralogy and textures of the ultra-fine-grained interclast matrix are consistent with those described at numerous Kimberley-type pyroclastic kimberlite localities around the world and are interpreted to reflect rapid primary crystallization during emplacement of separate kimberlite magmatic systems. The units of fractured and brecciated country rock surrounding the main kimberlite pipe contain kimberlite-derived material including carbonate providing evidence of subsurface brecciation. Together these data show that Renard 2 represents the deeper parts of a Kimberley-type pyroclastic kimberlite pipe system and demonstrates that their diagnostic features result from magmatic crystallisation during subsurface volcanic emplacement processes.

     

Neo-rural populations and their relations with local decision makers in rural Québec: collaboration or conflict?

The socio-demographic recomposition of the countryside is affecting local interactions and power relations. Understanding these relationships remains a challenge, as the studies to date are often limited to conflicts between neo-rural populations (newcomers) and long-time country residents over partial issues, without including decision makers. To go beyond this conflictual and fragmentary perspective, the objective of this article is to present an overall picture of both cooperative and oppositional relations between four groups, namely, newcomers, long-time rural residents, leaders of local organizations and municipal officials, in regard to all the issues that concern them. The data are based on interviews with these various actors in two contrasting rural areas of Québec (Canada). After looking at the newcomers’ mixed assessment of their participation in community life, we concentrate on areas of collaboration and/or conflict between all the actors regarding demographic, economic, sociocultural, political, environmental and agricultural issues. Three main trends emerge, revealing unexpected ways of interacting, complex power relations and antagonistic conceptions of rural spaces and their future development.     

Petrographic and structural analysis of the Precambrian rocks in the Zalbi Sector,northwest Léré, Chad

The northwestern part of southwest Chad is one of the best regions to find exposed Precambrian terrains and their Phanerozoic cover. In this area, it is easy to establish the lithostratigraphic and structural relationships between sedimentary and granitic rocks, greenstone belts and dolerite dykes. Based on petrographic and structural studies, we distinguished the greenstone belts that belong to the Neoproterozoic Zalbi Series. The Neoproterozoic greenstone belts that are intruded by three generations of Pan-African granitoids (gabbro-diorite, biotite monzogranite and leucogranite) host the mineral resources in the region. The Precambrian rocks are covered by Cretaceous shale, sandstone and microconglomerate in the Kebbi Basin, and intruded by dolerite dykes. These late dolerite dykes, have a continental tholeiitic composition, which is supported by the presence of orthopyroxene in the mineralogical assemblage. Structurally, two major deformation events are present in the Zalbi Series. The earliest D₁ event is the most intensive and is associated with vertical to subvertical north-trending S₁ foliation, a vertical L₁ lineation and P₁ folds; the metamorphism associated with this phase is equilibrated in greenschist facies conditions. The second D₂ event is discrete and is characterised by large-amplitude folds and fractures.     

Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry,Québec,Canada: petrological and stable isotope data

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of 0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO₂0.15. In the metasediments, ¹⁸O values of calcite (Cc) are as low as 8.6, suggesting that the fluids were in isotopic equilibrium with the syenites (¹⁸O =8.8–10.2). Marble ¹³C(Cc) values are-0.1 to-3.2; the lack of correlation between ¹³C(Cc) and ¹⁸O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m³/m² (4×10⁶ mol/m²), corresponding to actual fluxes of 3×10^-11 to 3×10^-12 m³/m²-s and intrinsic permeabilities of 10^-18 to 10^-20 m² for fluid flow lasting 0.1-1Ma. Marble ¹⁸O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble ¹⁸O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. ¹⁸O(Br) and ¹⁸O(Scrp) values correlate with ¹⁸O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that ¹⁸O(Cc-Br) and ¹⁸O(Cc-Serp) values are higher in marbles that have lower ¹⁸O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite ¹⁸O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.This paper is a contribution to IGCP 304, Lower Crustal Processes