Windsor Group Research Articles

A Lower Carboniferous two-stage extensional basin along the Avalon–Meguma terrane boundary: evidence from southeastern isle Madame, Nova Scotia

Anomalously thick and coarse clastic sedimentary successions, including over 5000 m of conglomerate, are exposed on isle Madame off the southern coast of Cape Breton island. Two steeply to moderately dipping stratigraphic packages are recognized: one involving Horton and lower Windsor groups (Tournasian–Visean); the other involving upper Windsor and Mabou (Visean–Namurian) groups. Also anomalous on isle Madame are three long narrow belts of “basement” rocks, together with voluminous chloritic microbreccia and minor semi-ductile mylonite, which are separated from the conglomerate-dominated successions by faults. The angular relations between the cataclastic rocks and the conglomerate units, combined with the presence of cataclasite clasts in the conglomerate units and evidence of dip-slip faults within the basin, suggest an extensional setting, where listric normal faults outline detachment allochthons. Allochthon geometry requires two stages of extension, the older stage completed in early Windsor Group time and including most of the island, and the more local younger stage completed in Mabou Group time. Domino-style upper-plate faulting in the younger stage locally repeated the older detachment relation of basement and conglomerate to form the observed narrow belts. Re-rotation of older successions in the younger stage also locally overturned the Horton Group. These features developed within a broad zone of Carboniferous dextral transcurrent faulting between already-docked Avalon and Meguma terranes. Sites of transpression and transtension alternated along the Cobequid-Chedabucto fault zone that separated these terranes. The earlier extensional features in isle Madame likely represent the northern headwall and associated clastic debris of a pull-apart or other type of transtensional basin developed along part of this fault zone that had become listric; they were repeated and exposed by being up-ended in the second stage of extension, also on listric faults. The two-stage history on isle Madame exposes the deeper parts of one of the Horton-age extensional basins of the Maritimes, others of which have been described as half-grabens based on their shallower exposures. RÉSUMÉ Des successions sédimentaires clastiques exceptionnellement épaisses et grossières, dont plus de 5000 mètres de conglomérat, affl eurent sur l’île Madame au large de la côte méridionale de l’île du Cap-Breton. On distingue deux ensembles stratigraphiques fortement à modérément inclinés : l’un comprenant le groupe de Horton et la partie in-férieure du groupe de Windsor (Tournaisien-Viséen); l’autre comprenant la partie supérieure du groupe de Windsor et le groupe de Mabou (Viséen-Namurien). On relève un autre ensemble anomal sur l’île Madame : trois longues ceintures étroites de roches « du socle », ainsi que de volumineuses microbrèches chloriteuses et une quantité modeste de mylonite semi-ductile, séparées des successions à prédominance de conglomérat par des failles. Les rapports an-gulaires entre les roches cataclastiques et les unités de conglomérat, conjugués à la présence de clastes de cataclastite dans les unités de conglomérat et à la manifestation de failles à rejet incliné à l’intérieur du bassin, laissent supposer un milieu de distension, où des failles normales listriques délimitent des terrains allochtones de décollement. La géo-métrie des terrains allochtones suppose deux stades de distension, le stade le plus ancien ayant pris fi n au début de la période du groupe de Windsor et incluant la majeure partie de l’île, et un stade récent plus local ayant pris fi n au cours de l’époque du groupe de Mabou. La formation de failles en chaîne dans la lèvre supérieure au cours du stade plus récent a répété par endroits l’ancienne géométrie du socle et du conglomérat, reproduite par des décollements, pour former les ceintures étroites observées. Une nouvelle rotation des successions anciennes au cours du stade récent a par ailleurs renversé localement les successions du groupe de Horton. Ces particularités se sont manifestées à l’intérieur d’une vaste zone de jeux de failles de coulissage dextres du Carbonifère entre les terranes déjà ancrés d’Avalon et de Meguma. Les sites de transpression et de transtension ont alterné le long de la zone faillée de Cobequid-Chedabucto qui sépare ces terranes. Les indices d’extension plus précoces sur l’île Madame représentent vraisemblablement le mur d’amont septentrional et les débris clastiques associés d’un bassin de transtension à écartement ou d’un autre type s’étant développé le long d’une partie de la zone faillée qui est devenue listrique; ils ont été répétés et en sont venus à affl eurer après un culbutage au cours du deuxième stade d’extension, ayant également affecté les failles listriques. Le passé en deux stades de l’île Madame met à découvert des parties plus profondes de l’un des bassins d’extension des Maritimes de la période de Horton, dont les autres parties ont été décrites en tant que demi-grabens d’après leurs affl eurements plus proches de la surface. [Traduit par la redaction]

Structurally Controlled Vein Barite Mineralization in the Maritimes Basin of Eastern Canada: Geologic Setting, Stable Isotopes, and Fluid Inclusions

The Brookfield barite deposit occurs in the Carboniferous Maritimes basin of eastern Canada, an area known for ca. 300 Ma Mississippi Valley-type (MVT) Zn-Pb-Ba mineralization, including the past-producing Walton barite and Gays River Zn-Pb deposits. In contrast to most of MVT mineralization in the Maritimes Basin that is hosted by Visean carbonate rocks of the Windsor Group, the Brookfield deposit is hosted by red terrestrial clastics of the underlying Tournaisian Horton Group. The barite mineralization occurs as ≤25-m-wide fault fill with both coarse-textured, pristine barite and mylonitic-textured barite situated close to the Cobequid-Chedabucto fault system, separating the Meguma and Avalon terranes. The breccia vein mineralization has a simple mineralogy and consists of barite with up to 1.3 wt percent Sr, minor siderite, and trace amounts of late-stage quartz and calcite. The host rocks close to the barite mineralization are intensely altered, with quartz and kaolinite replacing muscovite and complete removal of diagenetic hematite. A Grant-type iso-con plot indicates mass loss (C A = 0.82*C O ) during wall-rock alteration with REE profiles maintained but relative chondrite-normalized abundances reduced. Although an Fe-Mn zone occurs a few 100 m along strike, no such mineralization occurs in the barite deposit. Fluid inclusion studies indicate that vein fluids were saline brines (20–30 wt % NaCl + CaCl 2 equiv) that co-existed with an immiscible N 2 -CO 2 -CH 4 gas. Laser Raman analysis of the most common gas-rich inclusions indicates a uniform fluid chemistry with 66 mol percent N 2 and 34 mol percent CO 2 and is consistent with compositions inferred from thermometric measurements. Homogenization temperatures for primary, aqueous fluid inclusion asemblages in quartz intergrown with barite indicate minimum trapping temperatures of 210°C. Conditions at the time of entrapment, based on intersection of aqueous and gaseous isochores, are estimated at ~775 bars and 250°C. Stable isotope data for vein minerals (barite, siderite, quartz) are uniform in nature and indicate δ 18 O fluid = +12 per mil at 250°C for siderite and +9.8 per mil for quartz, δ 13 C fluid = −4 to −6 per mil from siderite, and δ 34 S fluid = +12 per mil from barite. Fluid inclusion extracts indicate δD values of −47 to −71 per mil. Collectively, these isotopic compositions are consistent with a basinal-type fluid derived from modified meteoric water with sulfur derived from Carboniferous evaporites and carbon of mainly marine limestone origin (i.e., Carboniferous Windsor Group) with a minor biogenic component from the Horton Group. Mineralization at Brookfield resulted from focusing of heated, overpressured brines of modified basinal origin into an active fault zone environment. The association of the barite mineralization with intensely altered wall rock represents a rare example of such alteration in the MVT metallogenic domain of the Maritimes Basin. However, similar alteration associated with Ba-Fe-Mn mineralization along the Cobequid-Chedabucto fault system raises the possibility that the Brookfield deposit may instead be part of another mineralizing event that was distinct from the 300 Ma MVT deposits.

STRONTIOGINORITE: CRYSTAL-STRUCTURE ANALYSIS AND HYDROGEN BONDING

Strontioginorite, ideally SrCaB 14 O 20 (OH) 6 ·5H 2 O, from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at Penobsquis, Kings County, New Brunswick, occurs in the Upper Halite member of the Windsor Group evaporites, and is associated with halite, sylvite, hilgardite, volkovskite and trembathite. Electron-microprobe analysis, supported by a single-crystal analysis of its structure, yield: CaO 7.47 (6.66–8.10), SrO 12.23 (11.30–13.69), (B 2 O 3 ) 61.22 and (H 2 O) 18.10, for a total of 99.02 wt.%. The empirical formula, based on 31 anions, is Ca 0.94 Sr 1.06 B 14 O 20 (OH) 6 ·5H 2 O. Strontioginorite is monoclinic, P 2 1 / a , with refined unit-cell parameters a 12.8171(4), b 14.4576(4), c 12.8008(4) A, β 101.327(1)°, V 2325.8(2) A 3 , Z = 4. The crystal structure refined to an R index of 0.024 for 6,784 unique, observed (>4σ F o ) reflections. The (010) sheets of borate polyhedra are weakly cross-linked by Sr and Ca polyhedra. The H 2 O and OH groups strengthen the cross-linkage with hydrogen bonding. The fundamental building block (FBB) within the structure, 8Δ6□:[ϕ] | | | −[ϕ] | | | −2Δ, is compared to that in the structure of other complex borates, such as nobleite and strontioborite.

Record of Late Mississippian tectonics in the new Percé Group (Viséan) of eastern Gaspésie, Quebec

Viséan clastic units and structures at the northwest margin of the upper Paleozoic Maritimes Basin provide information on tectonic events that are only poorly recorded in more central parts of the basin. These continental units are time equivalent to marine sediments of the Windsor Group of Nova Scotia. They are herein assigned to the new Percé Group, which includes the La Coulée and Bonaventure formations, as well as a new unit, the Cap d'Espoir Formation. The latter unit unconformably underlies the Bonaventure Formation in a small but thick sub-basin of the Ristigouche Basin in eastern Quebec. It is characterized by a succession of sandstone and mudstone rhythmites that contrast with the coarse alluvial fan deposits of the overlying Bonaventure Formation. The Cap d'Espoir Formation was sourced from a broad area of subdued topography occupied by the Viséan La Coulée Formation and underlying units. Erosional remnants of the La Coulée Formation are unconformably overlain by the Bonaventure Formation in marginal parts of the Ristigouche Basin, whereas these units are separated by the Cap d'Espoir Formation in more central areas of the basin. The La Coulée and Cap d'Espoir formations underwent tilting and erosion during a normal faulting event that preceded deposition of the fault-controlled Bonaventure Formation. This series of events is interpreted to represent different steps in the reactivation of a pre-Carboniferous dextral strike-slip system in response to northwest–southeast compression during the Viséan in Gaspésie.

Late Palaeozoic formation and development of the St Marys Basin, mainland Nova Scotia, Canada: a prolonged record of intracontinental strike-slip deformation during the assembly of Pangaea

Abstract The Late Palaeozoic evolution of the St Marys Basin, mainland Nova Scotia, preserves evidence of protracted dextral shear along an intracontinental fault zone during collisional orogenesis and the assembly of Pangaea. The St Marys Basin formed within the E-W-trending Minas Fault Zone (MFZ) along the boundary between the Avalon and Meguma terranes and contains latest Devonian-Tournaisian continental clastic rocks that are 3000–4000 m in thickness. The origin and evolution of the basin is attributed to either discrete or progressive dextral strike-slip tectonics along the MFZ between the Late Devonian and Late Carboniferous. Evidence for the Late Devonian origin of the basin is recorded along its southern flank by the fabrics of the deformed c. 370 Ma granites, the overall sedimentary facies distribution, and some syndepositional features within the clastic rocks. The most intense deformation within the basin is concentrated in a relatively narrow ENE-trending zone, in which predominantly fine-grained clastic rocks are deformed into periclinal folds and related reverse faults. The orientation of this zone relative to the MFZ is consistent with dextral shear. At least some of this deformation occurred after the deposition of the overlying Visean Windsor Group. The style of deformation along the present northern margin of the basin (the Chedabucto Fault) is also consistent with regional dextral shear. The St Marys Basin is an example of basin development and evolution adjacent to an intracontinental fault zone associated with oblique convergence during orogenesis. Its evolution provides constraints on the potential relationship between the termination of the mid-Palaeozoic Acadian orogeny, subsequent basin development, and the ongoing interactions between the Avalon and Meguma terranes, and between Laurentia and Gondwana during the assembly of Pangaea. More generally, because the relationship between fabric development and motion along intracontinental strike-slip faults in continental zones is difficult to interpret, the sedimentology and structural geology in basins developed along these fault zones may preserve a less ambiguous record of the main tectonic events.

WALKERITE, A NEW BORATE MINERAL SPECIES IN AN EVAPORITIC SEQUENCE FROM SUSSEX, NEW BRUNSWICK, CANADA

Walkerite, ideally Ca 16 (Mg,Li,□) 2 [B 13 O 17 (OH) 12 ] 4 Cl 6 ·28H 2 O, is a new mineral species from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at Penobsquis, near Sussex, Kings County, New Brunswick. It occurs in the Upper Halite Member of the Windsor Group evaporites, associated with halite, hydroboracite, hilgardite, volkovskite, boracite, szaibelyite, a mica-group mineral and anhydrite. The crystals of walkerite are colorless to white, fibrous to acicular, commonly forming bundles of fibers to 7 mm in length. The crystals are transparent to translucent, with a vitreous luster and a white streak. It is soft (Mohs hardness of 3), brittle with no apparent cleavage, but with a splintery fracture along [001]. Walkerite is biaxial positive, α 1.516 ± 0.002, β 1.532 ± 0.002 and γ 1.554 ± 0.002 (for λ = 590 nm); 2 V meas. = 82(3)°, 2 V calc. = 82°. There is no dispersion and no pleochroism. Optical orientation: X = a , Y = b and Z = c . It is orthorhombic, space group Pba 2, with unit-cell parameters refined from powder data: a 15.52(1), b 22.74(1), c 8.761(4) A, V 3091(2) A 3 and Z = 1. The strongest six powder-diffraction lines [ d in A( I )( hkl )] are: 12.820(100)(110), 7.785(80)(200), 6.319(40)(121), 5.649(30)(211), 3.176(30)(170) and 2.570(30)(461,550). Electron-microprobe analyses, a thermogravimetric analysis and induced coupled plasma – mass spectrometry results, supported by crystal-structure analysis, infrared-absorption spectrometry and secondary-ion mass spectrometry, yielded Li 2 O 0.12, Na 2 O 0.13, K 2 O 0.07, CaO 23.05, FeO 0.32, MgO 0.58, Cl 4.91, B 2 O 3 47.17 and H 2 O 25.48, O = Cl −1.11, total 100.72 wt.%. The empirical formula based on 150 anions, is (Ca 15.60 Na 0.16 K 0.06 ) ∑15.82 (Mg 0.55 Li 0.31 Fe 0.17 ) ∑1.03 B 51.43 O 68 (OH) 48 [Cl 5.26 (OH) 0.74 ] ∑6.00 H 2.53 ·28H 2 O; the idealized formula is presented above. The calculated density (from the empirical formula) is 2.05 g/cm 3 , and the measured density is 2.07 g/cm 3 . The structure has been refined to R = 0.040. The highly corrugated sheets of borate polyhedra are cross-linked by Ca and Mg polyhedra. The Cl atoms and H 2 O groups are located within the ovoid channels. The structure is compared to that of other complex borates, such as penobsquisite and pringleite.

Seawater composition during deposition of Viséan evaporites in the Moncton Subbasin of New Brunswick as inferred from the fluid inclusion study of halite

The Viséan evaporites of the Windsor Group in the Moncton Subbasin at the western limit of the Maritimes Basin of eastern Canada represent an important stratigraphic sequence to study in determining changes in the chemistry of marine brines. Seventeen samples of halite have been selected for fluid inclusion study. The chemical composition of inclusion brines was examined using the Petrichenko method of glass capillaries with applied ultramicroanalytical techniques. The results of the study indicate that the conditions of evaporite deposition in the Windsor Basin differed from those recorded in other Phanerozoic basins and that the history of the Windsor Basin brines was characterized by changes in the chemical composition of the brines. It was also found that there existed continual unfavourable conditions for the day-and-night zonality in chevron halite crystals and for the origin of relatively large (>50 µm) fluid inclusions and that the gas content was low not only during the deposition but also during the diagenesis of potash deposits. The chemical composition of the Viséan waters of the Moncton Subbasin at the beginning and end of deposition of the Windsor Group evaporite sequence was controlled by the predominating waters of nonmarine origin; during the potash deposition the controlling factor was chloride-type marine waters. In general, the geochemical data support the concept of Carboniferous evaporite basins with Na–K–Mg–Ca–Cl-type brines.

Geochemistry of oil inclusions in sulfide-related calcites—fingerprinting the source of the sulfate-reducing hydrocarbons of the Pb–Zn carbonate-hosted Jubilee deposit of Nova Scotia, Canada

Sulfide precipitation in the context of carbonate-hosted base metal deposits has been previously explained by numerous processes including SO 4 reduction in the presence of hydrocarbons. This model has been suggested for numerous deposits although clear criteria to support the model have not been systematically provided. Numerous oil-inclusions are encompassed by fibrous calcite crystals in finely laminated Carboniferous limestone at the base of the Windsor Group, a unit that hosts numerous base metal occurrences in Nova Scotia, particularly the hydrocarbon-rich Jubilee Pb–Zn deposit in Cape Breton Island. Oil from two inclusion-rich samples from this deposit have been characterised by gas chromatography, gas chromatography–mass spectrometry, gas chromatography–isotope ratio mass spectrometry and bulk stable C isotopes. As established in the authors' former publications, the Jubilee deposit is a clear metallogenic case for which the ground preparation and mineralisation stages involved reduction of SO 4 by hydrocarbons. Here, the question of potential sources of these hydrocarbons is addressed. It is postulated that the hydrocarbons that were trapped in the sulfide-related calcites at the deposit, correlate with previously characterised oil seeps, and it is demonstrated that their source is not the marine host-carbonates but stratigraphically deeper lacustrine formations of the Horton Group.

Carbonate mud flows and other indicators of excess pore-fluid pressure in the Macumber Formation (Lower Carboniferous, Viséan) at Ingonish, Nova Scotia, Canada

The Ingonish area of northern Nova Scotia offers the most extensive exposure of the Macumber Formation, the basal carbonate unit of the Lower Carboniferous (Middle Visean) Windsor Group of Nova Scotia and New Brunswick. The area also shows tectonic-free sedimentary characteristics of this unit. The formation consists of two principal lithologic units, a basal, black, earthy, carbonaceous limestone normally about 0.5 m thick and a thinly stratified, peloidal dolomudstone up to 17.5 m thick. At Burke Head, the dolomudstone is cut by at least 25 massive tufa mounds, elongated in cross-section and intersecting their surrounding strata by rising northeastward at angles of 15 to 25 degrees. At east Ingonish Island the basal unit is anomalous in being both dolomitized and thicker (5 m); there, both units are cut by at least 20 smaller massive tufa mounds. The mounds at both localities formed over deep-water hydrothermal vents, i.e., marine hot springs; the fossils of abundant chemosynthetic tubeworms surround the lower mounds on east Ingonish Island. The Macumber Formation overlies Horton Group braided-stream facies sandstones, and presumably underlies nearby, thick gypsum deposits of the Windsor Group.

Brines in the Carboniferous Sydney Coalfield, Atlantic Canada

Formation waters within Upper Carboniferous sandstones in the sub-sea Prince and Phalen coal mines, Nova Scotia, originated as residual evaporative fluids, probably during the precipitation of Windsor Group (Lower Carboniferous) salts which underlie the coal measures. Salinity varies from 7800 to 176,000 mg/l, and the waters are Na–Ca–Cl brines enriched in Ca, Sr and Br and depleted in Na, K, Mg and SO 4 relative to the seawater evaporation curve. Br:Cl and Na:Cl ratios suggest that the brine composition corresponds to an evaporation ratio of as much as 30. The brines lie close to the meteoric line on H/O isotopic plots but with a compositional range of δ 18O from −4.18 to −6.99 and of δD from −42.4 to −23.5, distant from modern meteoric or ocean water. Mine water composition contrasts with that of nearby salt-spring brines, which are inferred to have originated through dissolution of Windsor Group evaporites by modern meteoric waters. However, a contribution to the mine waters from halite dissolution and from Br in organic matter cannot be ruled out. Present concentrations of several elements in the brines can be explained by water–rock interaction. The original Windsor brines probably moved up into the overlying coal-measure sandstones along faults, prior to the Late Triassic. The high salinity and irregular salinity distribution in the Phalen sandstones suggests that the brines have undergone only modest dilution and are virtually immobile. In contrast, Prince waters show a progressive increase in salinity with depth and are inferred to have mixed with surface waters. Basinal brines from which these modern formation fluids were derived may have been important agents in base-metal and Ba mineralisation from the mid-Carboniferous onwards, as saline fluid inclusions are common in Zn–Pb sulphide deposits in the region.

Late Paleozoic to Cenozoic history of the offshore Sydney Basin, Atlantic Canada

The Sydney Basin covers a large offshore area south of Newfoundland, with a well-exposed outcrop belt on Cape Breton Island. The geological history of the poorly known offshore area is interpreted using an industry seismic grid and Lithoprobe line 86-5, tied to outcrops and two wells. The mid-Devonian to Upper Carboniferous - Permian basin fill is 6-7 km thick and represents three extensional phases with intervening and succeeding compressive phases. The mid-Devonian McAdams Lake Formation was deposited in a local half-graben during early post-Acadian extension. Following deformation, a suite of Early Carboniferous extensional basins filled mainly with Horton Group conglomerates developed on northeast-trending and southeast-dipping master faults. Some faults developed along Acadian terrane boundaries. The Windsor Group extends over the master faults to onlap basement as a result of thermal sag and Visean eustatic rise. Mid-Carboniferous deformation, linked to the Alleghanian orogeny, reactivated faults and caused basin inversion and a basinwide unconformity. Upper Carboniferous to ?Permian coal measures and redbeds were subsequently deposited in a broad basin that developed over the Early Carboniferous basins. Subsidence may reflect extension on major faults in the Cabot Strait coupled with thermal sag and (or) continued sag on an underlying mid-crustal detachment. After coalification, Acadian terrane boundaries and other lineaments were reactivated during a compressive tectonic episode, probably during the Permian. The basin's polycyclic history, with repeated subsidence and inversion phases, has important implications for hydrocarbon systems.

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.

A structural, petrographic, and geochemical study of the Jubilee Zn-Pb deposit, Nova Scotia, Canada, and a new metallogenic model

The Jubilee Zn-Pb deposit (Cape Breton Island) is hosted by a thin brecciated unit at the top of the Macumber Formation (Carboniferous, Visean). This thin unit is capped by evaporites of the Carrolls Corner Formation, both formations being at the base of the Visean Windsor Group.Mapping in the vicinity of the Jubilee Zn-Pb deposit shows that the Mabou Group and part of the underlying Windsor Group lie unconformably over the Lower Windsor Group, implying a local truncation of 545 m of Windsor Group strata. Another omission of 500 m is recognized near the deposit, where the Upper Windsor lies directly on the Lower Windsor Group. These unconformities are considered to be associated with the regional Ainslie detachment, which is consistently found on Cape Breton Island at the top of the Macumber Formation, i.e., at the limestone and evaporite contact in the Lower Windsor Group.The fact that the limestone breccia which hosts mineralization is confined to a single stratigraphic level, as well as the angularity and the puzzlelike texture of thebreccia fragments, points to a link between the breccia and the Ainslie detachment. It is suggested that during extension, the high rheological contrast at the limestone-evaporite contact generated a tectonic brecciation in the relatively less ductile limestone, thus, a tectono-hydraulic origin is suggested for the mineralized breccia.Stratigraphic correlations using the contact between the Windsor Group and the Horton Group as a marker horizon indicate the presence of the Jubilee and Road subvertical faults in the vicinity of the deposit. They also show that the faults postdate lithification of the Macumber Formation.At the Jubilee deposit, the premineralization events comprise (1) stylolitization parallel to bedding, (2) brecciation of a finely laminated lithofacies, (3) migration of hydrocarbons, and (4) precipitation of brown fibrous calcite. The mineralization stage comprises (1) development of subvertical microfractures crosscutting breccia fragments, (2) precipitation of pyrite-marcasite-sphalerite-galena-chalcopyrite, and (3) precipitation of anhedral calcite.Low delta 13 C values are observed in the premineralization fibrous calcite (avg = -25ppm) and in the syn- to postmineralization anhedral calcite (avg = -15ppm). These signatures are interpreted to reflect the dominance of bicarbonate ions derived from organic carbon in the buried sequence. Low delta 18 O values (avg = -10ppm) in the anhedral calcite are interpreted to originate from precipitation in the presence of warm fluids. Radiogenic strontium (avg = 87 Sr/ 86 Sr of 0.70860) in the carbonate phases supports a clastic-dominated basin origin for the metal-rich brines.The proposed metallogenic model for the Jubilee Zn-Pb deposit comprises the following: (1) brecciation of a precursor limestone, creating a reservoir which is later filled by hydrocarbons, (2) sulfate reduction by hydrocarbons, increasing H 2 S concentrations, (3) ascent of metal-rich fluids toward the hydrocarbon and H 2 S reservoir along subvertical faults, (4) mixing of metal-rich fluids with H 2 S, leading to sulfide precipitation (pyrite, marcasite, sphalerite, and galena), and (5) filling of residual pores with anhedral calcite and sulfates (barite, gypsum, and anhydrite). During their ascent, the hot metal-rich brines leached the underlying rocks of the Horton Group. Sulfates most likely originated from the thick evaporite cap of the Carrolls Corner Formation.

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).

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.

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.

A study of fluid inclusions in sulfide and nonsulfide mineral phases from a carbonate-hosted Zn-Pb deposit, Gays River, Nova Scotia, Canada

This paper reports on a detailed fluid inclusion study of synore (sphalerite, calcite) and postore (calcite, fluorite, quartz, barite) mineral phases at the Gays River Zn-Pb deposit (2.4 Mt, 8.6% Zn, 6.3% Pb), Nova Scotia, Canada. The deposit is hosted by dolomitized Visean carbonate rocks, in the basal part of the Windsor Group, that formed during a marine transgression on terrestrial clastics of the Tournaisian Horton Group. Mineralization is dominantly of replacement style, with lesser porosity infilling, and mainly occurs in the fore reef of a carbonate bank, the shallow water equivalent of the laterally extensive, laminated algal and bituminous Macumber Formation. Paragenetically, constant volume dolomitization of the host rock is followed by euhedral, manganiferous dolomite cement, then sphalerite and galena. Syn- to postore calcite with trace amounts of barite, fluorite, and quartz, occlude the remaining porosity.Fluid inclusions types include: (1) aqueous, L rich + or - solids, including halite, (2) methane, (3) H 2 O-CO 2 , (4) monophase L and V types, and (5) liquid petroleum + or - aqueous phase. Type 1 inclusions are most abundant, types 2 and 3 are rare, and type 5 are postore. Type 4 inclusions are mostly related to postentrapment changes (e.g., necking). Thermometric data indicate that the mineralizing fluids were high-salinity, NaCl-CaCl 2 -H 2 O brines with 20 to 28 wt percent NaCl equiv. Hydrohalite ice-melting relationships indicate considerable variation in the NaCl/(NaCl + CaCl 2 ) ratio of the fluids (0.2-1). Lower salinity fluid inclusions (0-16 wt % NaCl equiv) are restricted to postore calcite and barite. Combined SEM-EDS analyses of decrepitate mounds identify Na, Ca, and Mg as the major solute components. Homogenization temperatures (T h ) for all mineral phases range from 70 degrees to 250 degrees C, but ranges of < or = 10 degrees to 20 degrees C occur within isolated groups of inclusions. Homogenization data on liquid petroleum inclusions and associated aqueous inclusions in fluorite are 120 degrees to 150 degrees C, whereas three petroleum inclusions in syn- or postore calcite have T h values of 53 degrees , 53 degrees , and 167 degrees C.Bulk crush gas chromatography indicates that inclusion fluids contain up to 1.4 mole percent combined CO 2 and CH 4 , with the most abundant condensable gases in sphalerite and galena. Since the high pressures (< or = 2,000 bars) required to retain this gas as dissolved species in the fluid are incompatible with the local stratigraphy, it is inferred that the gas was either picked up along the fluid path and transported as an immiscible phase or was produced locally in the carbonate bank by thermal degradation of organic material.The data are interpreted to indicate that a high-temperature (< or = 250 degrees C), saline (ca. 25 wt % NaCl) metalliferous fluid migrated into the carbonate bank where it reacted with reduced sulfur generated by thermochemical sulfate reduction and mixed with an equally saline but lower temperature fluid. The variability of the NaCl/(NaCl + CaCl 2 ) ratio of the fluids suggests contamination by dissolution of the host dolostone; this dissolution may also have provided some of the CO 2 component of the gas. The pressure of the mineralizing environment is constrained at ca. 400 bars by the presence of liquid petroleum, aqueous and methane inclusions, and their respective isochoric projections.The high homogenization temperatures for the mineralizing fluids may reflect the regional setting of the deposit. That is, it is located on the margins of a large stable craton in an area that was structurally active during the Carboniferous and within which widespread thermal disturbances are known. The high temperatures also suggest affinities with the Irish carbonate-hosted base metal deposits.

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.

Breccias in the lower part of the Mississippian Windsor Group and their relation to Zn-Pb mineralization; a summary

Base metal occurrences in the Mississippian Windsor Group (Nova Scotia, Canada) are commonly associated with carbonate breccias in the Macumber Formation. Based on sedimentologic, structural, and diagenetic data, three types of temporally distinct breccias are recognized near the top of the Macumber Formation: early synsedimentary, synburial tectonic, and late, postburial karstic. Early breccias originated as rotational slides on a Visean deep-water slope and host mineralization such as the Walton deposit (Ba-Cu-Pb-Zn-Ag). Later tectonic breccias of Westphalian age are a manifestation of the Ainslie detachment, a regional flat-lying extensional fault, and host the Jubilee deposit (Zn-Pb). Finally, an irregularly distributed postburial, karstic solution-collapse barren breccia is designated as the Pembroke breccia. The recognition of three temporally and genetically distinct breccias is significant for comprehension of the breccia-hosted mineralization in the Mississippian section in eastern Canada. The synsedimentary and tectonic breccias are pre- to synore, respectively, and both have played a key role in localizing mineral deposition through enhanced permeability. The late karstic breccia was not involved in the mineralization and, where present, subaerial exposure leading to the formation of this breccia has removed significant portions of the orebodies.