Maritimes Basin Research Articles

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Distributed crustal-scale faulting in the Cobequid Highlands in the Middle Devonian to Carboniferous resulted from the oblique convergence of the Meguma and Avalon terranes. In the northeastern Cobequid Highlands, seismic reflection profiles show Neoproterozoic and lower Paleozoic rocks, together with enigmatic foliated rocks, overlying the Early Carboniferous Fountain Lake Group. The foliated rocks form the hanging wall of a north-vergent thrust fault. Their protolith is inferred from petrography and geochemistry to be principally Neoproterozoic rhyodacitic tuff and late Paleozoic hypabyssal intrusions. The age of thrusting is stratigraphically constrained to the late Tournaisian – mid-Viséan, and sericite from mylonite yielded a Tournaisian K–Ar age of 352 ± 8 Ma. The thrusting occurs at the base of a tectonic escape sheet and resulted from a restraining bend in the Rockland Brook master fault. Farther west, where the Rockland Brook fault trends almost east–west, Tournaisian extensional features include the Nuttby basin and widespread gabbro dykes, sills, and stocks. At deeper structural levels, granite plutons were intruded in a similar tectonic regime of thrusting and local extension by lateral movement of basement blocks. The emplacement process resulted from progressive widening of initial dykes, analogous to the dykes deformed in the thrust hanging wall. Regionally, in the Tournaisian of the southern Maritimes Basin half-graben formation was synchronous with pluton emplacement and thrusting in adjacent horsts.

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Newly identified Carboniferous units (the Pointe Sawyer and Chemin-des-Pêcheurs formations) in the Gaspé Peninsula, Quebec; implications regarding the evolution of the northwestern sector of the Maritimes Basin

The Upper Member of the Cannes-de-Roches Formation was recently recognized overlying the Bonaventure Formation in the New Carlisle area, over 100 km southwest of the previously documented exposures of this unit. Moreover, remnants of the La Coulée Formation, which are unconformably overlain by the Bonaventure Formation, have also been recognized underlying, with a similar type of contact, the Lower Member of the Cannes-de-Roches Formation. From this and from facies similarities, the Lower and Middle members of the Cannes-de-Roches Formation are now considered to be equivalent to the Bonaventure Formation. It is proposed to abandon these two member designations and to only keep the Bonaventure Formation. The remaining Upper Member of the Cannes-de-Roches Formation is renamed the Pointe Sawyer Formation. A late Viséan to early Namurian age is attributed to this grey clastic formation from spore analysis. Dark reddish-brown sandstone conformably overlie the Pointe Sawyer Formation in the Mal Bay area. They correspond to the beginning of sedimentation from more distant sources within the regional Carboniferous stratigraphic succession, which was until then characterized by sedimentation from proximal sources. This previously unidentified unit is here referred to as the Chemin-des-Pêcheurs Formation.

Newly identified Carboniferous units (the Pointe Sawyer and Chemin-des-Pêcheurs formations) in the Gaspé Peninsula, Quebec; implications regarding the evolution of the northwestern sector of the Maritimes Basin

The Upper Member of the Cannes-de-Roches Formation was recently recognized overlying the Bonaventure Formation in the New Carlisle area, over 100 km southwest of the previously documented exposures of this unit. Moreover, remnants of the La Coulée Formation, which are unconformably overlain by the Bonaventure Formation, have also been recognized underlying, with a similar type of contact, the Lower Member of the Cannes-de-Roches Formation. From this and from facies similarities, the Lower and Middle members of the Cannes-de-Roches Formation are now considered to be equivalent to the Bonaventure Formation. It is proposed to abandon these two member designations and to only keep the Bonaventure Formation. The remaining Upper Member of the Cannes-de-Roches Formation is renamed the Pointe Sawyer Formation. A late Viséan to early Namurian age is attributed to this grey clastic formation from spore analysis. Dark reddish-brown sandstone conformably overlie the Pointe Sawyer Formation in the Mal Bay area. They correspond to the beginning of sedimentation from more distant sources within the regional Carboniferous stratigraphic succession, which was until then characterized by sedimentation from proximal sources. This previously unidentified unit is here referred to as the Chemin-des-Pêcheurs Formation.

Functional groups and elemental analyses of cuticular morphotypes of Cordaites principalis (Germar) Geinitz, Carboniferous Maritimes Basin, Canada

Well-preserved cuticles were isolated from Cordaites principalis (Germar) Geinitz leaf compressions, i.e., foliage from extinct gymnosperm trees Coniferophyta: Order Cordaitales. The specimens were collected from the Sydney, Stellarton and Bay St. George subbasins of the once extensive Carboniferous Maritimes Basin of Atlantic Canada. Fourier transformation of infrared spectra (FTIR) and elemental analyses indicate that the ca. 300–306-million-year-old fossil cuticles share many of the functional groups observed in modern cuticles. The similarities of the functional groups in each of the three cuticular morphotypes studied support the inclusion into a single cordaite-leaf taxon, i.e., C. principalis (Germar), confirming previous morphological investigations. Vitrinite reflectance measurements on coal seams in close proximity to the fossil-bearing sediments reveal that the Bay St. George sample site has the lowest thermal maturity, whereas the sites in Sydney and Stellarton are more mature. IR absorption and elemental analyses of the cordaite compressions corroborate this trend, which suggests that the coalified mesophyll in the leaves follows a maturation path similar to that of vitrinite. Comparison of functional groups of the cordaite cuticles with those from certain pteridosperms previously studied from the Sydney Subbasin shows that in the cordaite cuticles highly conjugated C–O (1632 cm −1) bands dominate over carbonyl stretch that characterizes the pteridosperm cuticles. The differences demonstrate the potential of chemotaxonomy as a valuable tool to assist distinguishing between Carboniferous plant–fossil groups.

Geochemical evolution of earliest Carboniferous continental tholeiitic basalts along a crustal-scale shear zone, southwestern Maritimes basin, eastern Canada

Continental tholeiites were extruded locally within the extensional wrench Maritimes basin from about 375 to 330 Ma (Late Devonian–Early Carboniferous). Basalts of the Early Carboniferous Diamond Brook Formation are up to 1.5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness. The volcanic rocks and co-genetic plutons occur along a crustal-scale strike–slip fault that facilitated the transport of relatively primitive magma, resulting in basalts with high Mg number, Ni and Cr, ε Nd>+4, and relatively low TiO 2, V, Y and Zr. Fractional crystallization of plagioclase and lesser clinopyroxene, in the absence of iron-oxide fractionation, resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in TiO 2 and other high field-strength elements (HFSEs). Rare andesites are interpreted as products of partial melting of underplated gabbro. The relatively primitive basalts resemble low-Ti basalts found in other continental flood basalt provinces and have trace-element and isotopic characteristics that have been ascribed in the literature either to partial melting of, or contamination of asthenospheric melts by, subcontinental lithospheric mantle. In the Maritimes basin, similar continental tholeiites were extruded intermittently over a 45-Ma period. High FeO t (∼12% normalised to 8% MgO) suggests high potential temperature in the source mantle, presumably within a mantle plume. The youngest eruption products geochemically and isotopically resemble HIMU-OIB. Plume activity may have gradually migrated eastward from late Devonian to Permian, from the Maritimes basin through Scotland to Norway and north Germany.

Flood producing mechanisms identification in southern British Columbia, Canada

The causes of peak flows in two climatically different mountainous-forested basins of British Columbia have been identified. The U.B.C. watershed model was used to identify the causes of peak flows, since this model separately calculates the runoff components, i.e. rainfall, snowmelt and glacier runoff. The results showed that the flood flows in the maritime basin of Upper Campbell are mainly generated by rainfall during the fall months and winter rain-on-snow events. Rainfall runoff constitutes the largest percentage of peak flow for all types of events. On the other hand, the flood flows in the inland basin of Illecillewaet are mainly produced by spring rain and snowmelt events, snowmelt events alone and summer events when runoff from the glacier melt contributes to peak discharge. However, snowmelt runoff is the dominant component of peak flows. Based on these findings, flood frequency analysis showed that considering the flow component frequency distributions marginally improves the probability distribution flows in the two examined watersheds.

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Geochemical and isotopic data from the clastic rocks of the Namurian Lismore Formation in mainland Nova Scotia identify key episodes of tectonic activity during the development of the Maritimes Basin in Atlantic Canada. The Lismore Formation forms part of the Mabou Group and is an upward-coarsening 2500 m thick fluvial sequence deposited in the Merigomish sub-basin along the southern flank of the Maritimes Basin. Based on stratigraphic evidence, the Lismore Formation can be divided into upper and lower members which reflect variations in depositional environment and paleoclimate. The geochemical and isotopic data may also be subdivided into two groupings that primarily reflect varying contributions from accessory phases, clay minerals, or rock fragments. This subdivision occurs 115 m above the base of the upper member. The data from the lower grouping (group A) show an important contribution from underlying Silurian rocks, with a relatively minor contribution from Late Devonian granitoid rocks from the adjacent Cobequid Highlands and possibly metasedimentary rocks from the Meguma Terrane to the south. The data from the upper grouping (group B) reveal a more important contribution from the Cobequid Highlands granitoid rocks. This variation in geochemistry is thought to constrain the age of renewed motion and uplift along the faults along the southern flank of the Maritimes Basin and, more generally, suggests that geochemical and isotopic data of continental clastic rocks may help constrain the age of tectonic events that influence deposition of basin-fill rocks.

Ostracodes and Agglutinated Foraminifera as Indicators of Paleoenvironmental Change in an Early Carboniferous Brackish Bay, Atlantic Canada

Ostracodes and agglutinated foraminifera characterize lagoonal, estuarine, marsh, and nonmarine depositional systems that span the land/ sea transition. Using microfossil and sedimentary data, high-resolution paleoenvironmental change is documented in an Early Carboniferous marginal marine depositional system that was previously interpreted as nonmarine. There are five main paleoenvironmental phases: (1) low-energy bay dominated by the euryhaline marine ostracode Copelandella novascotica associated with glauconitic shale and primary framboidal pyrite; (2) restricted nearshore dominated by opportunistic paraparchitacean ostracodes (Shemonaella scotoburdigalensis, Shemonaella tatei, and Chamishaella sp.), Cavellina sp., and post-mortem Carbonita scalpellus associated with hummocky cross-stratified siltstone and shale; (3) coastal pond dominated by C. scalpellus and Carbonita rankiniana associated with pedoturbated calcareous mudstones; (4) distal lagoon dominated by paraparchitaceans, Bairdia sp., Geisina sp., and Youngiella sp., all associated with silty glauconitic shale; and 5) coastal marsh dominated by the agglutinated foraminifera Trochammina sp. associated with calcareous mudstones and in situ Iycopsid trees. Low diversity, high dominance, and large populations characterize the microfossil assemblages that resemble modern ecosystems where physical and chemical conditions are unstable. The Horton Bluff Formation records the brackishfreshwater transition at the landward terminus of an Early Carboniferous marine transgression in the Maritimes Basin.

Sedimentary basins and petroleum systems offshore Newfoundland and Labrador

The sedimentary basins offshore Newfoundland and Labrador include those related to the opening and closing of the Palaeo-Atlantic (Iapetus) Ocean and those formed during the opening of the present Atlantic Ocean. The oldest among these are the Lower Palaeozoic Anticosti Basin and the Carboniferous Maritimes successor basin offshore western Newfoundland. The Mesozoic sedimentary basins on the Grand Banks and on the Labrador Shelf record the progressively northward opening of the present Atlantic Ocean. These include the South Whale Sub-basin of the Scotian Basin, Whale Basin, Horseshoe Basin, Carson Basin, Jeanne d’Arc Basin, Flemish Pass Basin, East Newfoundland Basin, Hopedale Basin and Saglek Basin. Petroleum systems occur in the Anticosti Basin, Maritimes Basin, South Whale Sub-basin, Jeanne d’Arc Basin, Flemish Pass Basin and the Hopedale Basin. In the Anticosti Basin, Lower Palaeozoic marine shales constitute source rocks and carbonates form the reservoir rocks. In the Maritimes Basin, Lower Carboniferous lacustrine rocks form the source rocks and Carboniferous sandstones form the major reservoir rocks. On the Grand Banks, Kimmeridgian shale represents the source and Upper Jurassic and Lower Cretaceous sandstones constitute the main reservoir units. Offshore Labrador, Tertiary shales are considered to be the source and Cretaceous sandstones form the principal reservoir rocks. All the basins with petroleum systems have a favourable combination of elements and processes needed for hydrocarbon accumulations.

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Displacement transfer and forced folding in the Maritimes basin of Nova Scotia, eastern Canada

Abstract Field and seismic data in northern Nova Scotia, eastern Canada, demonstrate that displacement transfer from steep basement faulting to bedding-parallel detachment is necessary in the development of forced folds. Lateral translation of the strata above horsted and down-dropped blocks generates a monoclinal structure which, as the faults are kinematically linked, evolves in a manner similar to fault-bend folds in thrust-and-fold belts. In the case of partial transfer of displacement a breached drape syncline is developed. The breached syncline is characterized by steep upturned beds against the fault that truncates them. In the Nova Scotia example the detachment horizon is located at the base of a Visean evaporitic sequence, and is exposed in the study area showing shearing structures within the evaporites. Brittle fault rock types (gouge and cataclasite) and meso- to microstructures were formed, including stretching lineation, principal schistosity plane and secondary shear planes, as well as intrafolial to upright asymmetrical folds. The regionally extensive weak evaporitic layer was remarkably effective in transferring displacement between the two faults, with mechanical decoupling of the strata above the evaporitic detachment being observed in the horsted block 70 km away from the steep basement fault. Moreover, displacement was also transferred as much as 40 km onto the down-dropped block at the frontal part of the system.

Influence of the Ainslie Detachment on the stratigraphy of the Maritimes Basin and mineralization in the Windsor Group of northern Nova Scotia, Canada

The Late Devonian to Lower Permian Maritimes basin of eastern Canada developed as a late to post-tectonic extensional basin following the Early Devonian Acadian orogenic event and is subdivided into five main lithostratigraphic groups. The principal host to base metal mineralization is the Visean Windsor Group, which is also the only marine-dominated interval within the basin. The Windsor Group is a thick accumulation of evaporites, carbonates, and siltstone, deposited during passive regional subsidence and intracontinental submersion of a tectonically thinned crust. Earlier rift deposits consist of the nonmarine coarse clastic sediments of the underlying Tournaisian Horton Group, and the Horton to Windsor succession defines a typical rift and sag transgressive event. The Macumber and stratigraphically equivalent Gays River Formations are the marine basal carbonate units to the Windsor Group and contain numerous Pb-Zn-Ba occurrences in central and northern Nova Scotia. These carbonate units are overlain by thick evaporite deposits of the Lower Windsor Group, consisting of gypsum, anhydrite, and salt. In northern Nova Scotia the evaporite-carbonate contact at the top of the basal carbonates is the locus of intense shear and brecciation which developed in relation to the Ainslie detachment. This detachment is a stratigraphically controlled regional-scale flat-lying extensional fault that affected the hydrodynamic regime and mineralizing environment in the Carboniferous Maritimes basin. Movement on the detachment has stripped away thick evaporitic units and excised the entire Windsor Group across wide areas, effectively breaching a regional aquiclude. With shearing, permeability was locally enhanced through brecciation, creating a favorable environment for mineralization. Significant thickness variations within underlying coarse clastic rocks and pinch outs of the Horton Group aquifer also focused basin fluids.The detachment is rooted in a master basement normal fault which occurs offshore along the southeast margin of the Gulf of St. Lawrence, with the basement fault displaying up to 10 to 12 km of offset. Structural evidence from onshore exposures in the uplifted or horsted regions of mainland Nova Scotia and Cape Breton Island indicate that much of this movement was transferred horizontally along the Ainslie detachment. Thin-skinned gravitational sliding produced large rafts of allochthonous strata which progressively draped over the edge of basement normal faults during extension, resulting in tectonic salt build-up in the main offshore graben and a salt massif. Buttressing at the front end of the raft within the graben produced seismically imaged large amplitude buckle folds above the detachment.As well as a regionally extensive planar low-strength or low-viscosity layer, large-scale detachment faults require high fluid pressures to sustain motion of the allochthonous sheets. Abnormal fluid pressures are known to occur beneath evaporites, which is the site of nucleation for the Ainslie detachment. Fluid focusing along the detachment is suggested from the structural style, and evidence is provided by widespread synkinematic calcite + or - fluorite-barite-pyrite veins which occur in the calc-mylonite and breccia.

Impact of Pb-Zn sulfide precipitation on the clay mineral assemblage in the Gays River Deposit, Nova Scotia, Canada

Relationships between lead-zinc sulfide precipitation and clay mineral alteration around the Pb-Zn Gays River deposit of mainland Nova Scotia are presented and discussed. The study focuses on the Visean Gays River Formation (Lower Windsor Group) and covers the Shubenacadie and Musquodoboit sub-basins at the southern margin of the Maritimes basin. X-ray diffraction and scanning electronic microscopy (SEM) have revealed the presence of detrital chlorite and illite and epigenetic smectite and kaolinite. Variations in illite composition are recorded by 002/001 peak intensity ratios and the crystallinity index. At Gays River the mineralizing event only lightly affected the detrital illite composition. A 5-km-wide halo of epigenetic smectite is centered on the deposit and is accompanied by a significant decrease in chlorite. Near the deposit, alteration of the interlayer hydroxide sheet in chlorite is indicated by a decrease of its 002/001 X-ray peak intensity ratio. This alteration consists of an exchange of iron for magnesium or a partial leaching of iron cations from that interlayer hydroxide sheet. At the Gays River deposit a low pH fluid was responsible for chlorite alteration and kaolinite precipitation. Neutralization of this acidic fluid in the carbonate environment favored the precipitation of smectite. The variations in clay mineral assemblages around the Gays River deposit constitute a potentially useful exploration tool for the region because of the genetic linkage with mineralization.

Isotope geochemistry of sideritized host rocks, Walton Ba deposit, Kennetcook Sub-basin, Nova Scotia, Canada

Although sideritized limestone of the Macumber Formation hosts the Ba (Cu, Pb, Zn, Ag) Walton deposit of Nova Scotia, siderite is an uncommon gangue mineral of carbonate-hosted base metal deposits. This paper evaluates whether the abundant siderite at Walton represents a diagenetic stage of burial or whether it represents the first stage of hydrothermal mineralization.The Walton deposit was the largest mine of ground barite in Canada, producing approximately 4.5 million metric tons (Mt) of >90 percent BaSO 4 between 1941 and 1978. It is located at the southern margin of the Maritimes basin and is hosted by Visean, entirely sideritized, fine-grained limestone of the lower Windsor Group.Walton siderite is finely crystalline and replaces the precursor limestone without textural disruption; it generally predates barite and sulfides. The siderite is characterized by low delta 13 C values (-5.1 to -2.81ppm) and delta 18 O VPDB (-6.0 to -3.21ppm), and high 87 Sr/ 86 Sr (0.71035B0.72124). These attributes make the Walton siderite very distinct isotopically from the marine precursors and from any known hydrothermal or fresh-water siderite. Calculated delta 18 O values for theoretical hydrothermal siderite, based on estimated salinity and temperature of the Walton hydrothermal fluids, are lower than those measured. Therefore, as no petrographic or isotopic evidence was found to suggest a genetic link with mineralization, siderite at Walton is not considered to be an alteration product of the barite-forming process. Rather, the geology, petrography, and isotope geochemistry support a sideritization model of diagenesis under shallow burial conditions.

Basinal fluid flow models related to Zn-Pb mineralization in the southern margin of the Maritimes Basin, Eastern Canada

The carbonate-hosted Zn-Pb deposits in the Lower Windsor Group, Nova Scotia, are located along the southern margin of the Maritimes basin. Previous studies suggest that the deposits formed from basinal brines expelled from the basal part of the Maritimes basin (Horton Group), possibly related to tectonic events in late stages of the basin history, but it remains unclear where the specific source regions were and which mechanism is responsible for fluid expulsion from the source regions to the sites of mineralization. This paper presents two numerical models to address these problems. The first model, based on a stratigraphic profile ranging from the center to the southern margin of the Maritimes basin, simulates the distribution and evolution of fluid overpressures due to sediment compaction. The second model simulates the temperature distribution in a marginal sub-basin under the assumption that fluid flow was driven by topographic relief related to an uplift (highlands) proximal to the ore deposits.Modeling of fluid overpressure evolution indicates that, if the Horton Group sediments were laterally confined by basement highs or faults, fluid pressures approaching or exceeding loading pressures would be easily built up within the Horton Group after deposition of the overlying Windsor evaporites. Strong overpressures in the Horton Group rocks are predicted not only in the central part of the Maritimes basin but also in shallower sub-basins close to the sites of mineralization. In contrast, if the Horton Group rocks are assumed to be laterally continuous across the Maritimes basin, fluid pressures in the Horton Group would remain near hydrostatic and strong overpressures would be built up only within the evaporite layer. In both cases, the geothermal gradients would not be significantly disturbed by the sediment compaction-driven fluid flow.Modeling of topography-driven flow indicates that the geothermal gradients are only slightly disturbed if rock permeabilities are inherited from the compaction model but could be strongly disturbed if higher permeabilities are assigned to the basal aquifer, and high-permeability zones (conduits) are assumed to cut through the evaporite layer and link the highlands (recharge area), basal aquifer, and discharge area. The temperature at the site of mineralization could be increased relative to the background temperature, but under steady-state conditions it could not have reached the ca. 250 degrees C indicated by fluid inclusions in the deposits. Such high temperatures could be reached transiently if the conduits and basal aquifer had permeabilities higher than about 0.1 D.The numerical modeling results suggest that sediment compaction-driven fluid flow could not have been responsible for mineralization because it cannot satisfy the thermal conditions at the deposits. Topography-driven flow may satisfy the thermal conditions at the deposits under the assumption of high-permeability conduits and a basal aquifer. Whether or not such conditions existed in the southern margin of the Maritimes basin needs further study. Based on the modeling results of fluid overpressure development, we favor a model in which the main-stage ore-forming fluids were derived from the basal part of individual sub-basins proximal to the deposits and were driven by sudden release of overpressures, probably triggered by tectonic events. Topography-driven flow may have been dominant in the postore stage, i.e., after the dissipation of overpressures. This model agrees with other geochemical studies that indicate separate source regions for different deposits, high fluid flow rates, and involvement of lower salinity fluids after the main-stage mineralization.

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

Delta 13 C-delta 18 O- 87 Sr/ 86 Sr covariations in ore-stage calcites at and around the Gays River Zn-Pb deposit (Nova Scotia, Canada); evidence for fluid mixing

The delta 13 C-delta 18 O- 87 Sr/ 86 Sr variations around carbonate-hosted deposits can in theory help characterize ore-stage gangue minerals and discriminate between single fluid or two-fluid mixing models during mineralization. This paper reports on the combined use of delta 13 C-delta 18 O- 87 Sr/ 86 Sr data in dolostone within the Gays River deposit and up to 30 km around it. The deposit, at the southern margin of the Maritimes basin, is hosted in carbonate banks of the Gays River Formation, a unit of the Visean Lower Windsor Group in Nova Scotia. A previous delta 13 C-delta 18 O study restricted to the Gays River deposit (and independant from an 87 Sr/ 86 Sr investigation) had previously led to the conclusion that a single brine was responsible for mineralization.The present isotopic investigation was governed by the paragenetic succession which includes presulfide dolomite, synsulfide calcites--both uniform and zoned under cathodoluminescence, and postsulfide calcite uniform under cathodoluminescence. Results for presulfide dolomite (avg delta 13 C = +3ppm, delta 18 O (sub V-PDB) = -6ppm, and 87 Sr/ 86 Sr = 0.7080) plot near the normal marine isotopic ratios, whereas postsulfide calcite shows slightly more radiogenic 87 Sr/ 86 Sr ratios (avg = 0.7090). At the deposit isotopic results for calcite associated with the high-grade sulfide zone cluster around a delta 18 O (sub V-PDB) of -- 17.0 per mil and an 87 Sr/ 86 Sr of 0.7120, and clearly characterize the mineralizing system. Regional C, O, and Sr isotope data are similar to those at the deposit and indicate therefore a regional flushing of the Gays River Formation by fluids of the mineralizing system.The overall isotopic variation through time from pre- to syn- then postsulfide conditions in the delta 18 O -- 87 Sr/ 86 Sr, delta 13 C-delta 18 O and delta 13 C- 87 Sr/ 86 Sr diagrams is interpreted to be similar for pre- and postsulfide conditions, marked by a warm, slightly radiogenic parent brine. This chemical system was disturbed by a much more radiogenic and high-temperature fluid during mineralization. Although most of the mineralizing trend can be explained either by a single-brine model or a mixing brine model, the mixing model allows for a much simpler evolution if the suite of conditions--on a regional basis--is considered through time.Show me your delta 18 O- 87 Sr/ 86 Sr profile, and I will tell you what you drink! (adapted old spanish proverb).

Sedimentology of the lowest Windsor carbonate rocks; base metal hosts in the Maritimes Basin of Eastern Canada

The Mississippian (Visean) Macumber and Gays River Formations of the Windsor Group in Nova Scotia are host to Pb-Zn-Ba (Cu-Ag) deposits. Transport and deposition of base metals were possible because of an efficient porosity-permeability system developed in both units. Depositional facies is a key parameter controlling the potential openness of carbonate units.The Macumber facies were deposited after the collapse of the Acadian orogen and ensuing tectonically driven sea-level rise. The Macumber Formation is divided into two regional lithosomes: a lower thin unit (<2 m) of fine- and coarse-grained limestones deposited below the fairweather wave base positioned on an outer shelf with local chemosynthetic mounds, and an upper thicker unit (avg 10 m) of planar-bedded micritic limestones, most likely representing upper slope microbial mats interbedded with deep marine sulfates. The permeability system was created by various brecciation events affecting the upper part of the formation.The Gays River Formation, deposited upslope of the Macumber facies, is dominated by various organic mounds with intervening facies. The boundstone facies are indicative of below fairweather wave base deposition, roughly at the limit of the photic zone. Primary growth porosity of the mounds provided the plumbing system later used by mineralizing fluids. These mounds also record the effects of a sea-level rise culminating in regional deposition of deep marine sulfates.Outside the eastern margin of the Laurentia craton, a similar Visean-aged mixed limestone sulfate succession is known from the Franco-Belgian basin in Baltica. Distribution of shelf and slope facies within the Visean mid-European ocean separating Laurentia, Baltica, and Gondwana suggests that restricted oceanic circulation in a rapidly closing sea was instrumental in the local occurrence of a salinity-stratified water mass and, hence, responsible for the presence of below wave base marine limestone and sulfate facies on the Laurentia and Baltica continental margins.

Fluid composition and thermal regime during Zn-Pb mineralization in the lower Windsor Group, Nova Scotia, Canada

The Zn-Pb-Ba-Cu-Ag-F deposits hosted by Lower Windsor Group (early Carboniferous) carbonates in Nova Scotia are located along the southern margin of the Maritimes basin. Various geochemical studies indicate that the deposits were formed by fluids expelled from the basin, but it remains unclear whether the ore-forming fluids for different deposits were derived from a common source region in the central part of the basin, as proposed previously, or from separate source regions. It is also unknown how the ore fluids were conveyed from the source regions to the sites of mineralization and whether or not the mineralizing systems of different deposits were hydrologically related. This paper addresses these problems through a compilation of regional fluid composition and temperature data and thermal comparisons between the ore-forming fluids and their host rocks. The compositions and temperatures of the fluids were evaluated from fluid inclusion data, whereas host-rock temperatures were estimated from vitrinite reflectance measurements.The ore-forming fluids are characterized by: (1) variable, but generally high salinities (mainly between 15 and 30 wt % NaCl equiv), (2) variable, but generally detectable amounts of bivalent cations (mainly Ca (super 2+) ), (3) significant concentrations of hydrocarbon species, and (4) high temperatures (mainly between 150 degrees and 250 degrees C). NaCl/(NaCl + CaCl 2 ) wt ratios in fluid inclusions show a broad district-scale variation: mainly 0.4 for the Gays River and Walton deposits in mainland Nova Scotia. CH 4 /higher hydrocarbons ratios of the gas components are greater for the Gays River deposit (3.8-9.7) than the Jubilee deposit (0.7-1.3). Fluid inclusion homogenization temperatures do not show a negative correlation with the distance between the deposits and the central part of the Maritimes basin, as might be expected if the fluids for different deposits were derived from the same source region.The background temperatures prior to mineralization were estimated from mean vitrinite reflectance (R o ) in host rocks which were relatively far from the deposit, and were probably least affected by the mineralizing events, and from homogenization temperatures of fluid inclusions in preore minerals. These temperatures are in the range of 70 degrees to 155 degrees C, which are significantly lower than those of the ore-forming fluids. The paleogeothermal gradients estimated from the background temperatures and burial depths are ca. 65 degrees C/km. Vitrinite reflectance values in host rocks adjacent to the deposits indicate that these rocks were heated by the mineralizing fluids to variable degrees, but thermal equilibrium between the fluids and rocks was not attained in most eases. This thermal relationship suggests focused and short-lived, rather than pervasive and long-lived, fluid flow.The regional variation of fluid composition and thermal patterns supports a model in which the ore-forming fluids of individual deposits were derived from separate sources; fluid flow was focused along confined conduits and individual mineralizing systems were unconnected hydrologically. Under paleogeothermal gradients of ca. 65 degrees C/km, fluids with temperatures of ca. 250 degrees C could have been generated in sub-basins proximal to the deposits. Sudden release of overpressured fluids from the basal part of the proximal sub-basins, characterized by high flow rate and short duration, is proposed as the mechanism of ore-forming fluid flow.