Gays River Research Articles

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.

Evaporites, brines and base metals: Low‐temperature ore emplacement controlled by evaporite diagenesis

Salt beds and salt allochthons are transient features in most sedimentary basins, which through their dissolution can carry, focus and fix base metals. The mineralisation can be subsalt, intrasalt or suprasalt, and the salt body or its breccia can be bedded or halokinetic. In all these evaporite‐associated low‐temperature diagenetic ore deposits there are four common factors that can be used to recognise suitably prepared ground for mineralisation: (i) a dissolving evaporite bed acts either as a supplier of chloride‐rich basinal brines capable of leaching metals, or as a supplier of sulfur and organics that can fix metals; (ii) where the dissolving bed is acting as a supplier of chloride‐rich brines, there is a suitable nearby source of metals that can be leached by these basinal brines (redbeds, thick shales, volcaniclastics, basalts); (iii) there is a stable redox interface where these metalliferous chloride‐rich waters mix with anoxic waters within a pore‐fluid environment that is rich in organics and sulfate/sulfide/H2S; and (iv) there is a salt‐induced focusing mechanism that allows for a stable, long‐term maintenance of the redox front, e.g. the underbelly of the salt bed or allochthon (subsalt deposits), dissolution or halokinetically maintained fault activity in the overburden (suprasalt deposits), or a stratabound intrabed evaporite dissolution front (intrasalt deposits). The diagenetic evaporite ‐ base‐metal association includes world‐class Cu deposits, such as the Kupferschiefer‐style Lubin deposits of Poland and the large accumulations in the Dzhezkazgan region of Kazakhstan. The Lubin deposits are subsalt and occur where long‐term dissolution of salt, in conjunction with upwelling metalliferous basin brines, created a stable redox front, now indicated by the facies of the Rote Faule. The Dzhezkazgan deposits (as well as smaller scale Lisbon Valley style deposits) are suprasalt halokinetic features and formed where a dissolving halite‐dominated salt dome maintained a structural focus to a regional redox interface. Halokinesis and dissolution of the salt bed also drove the subsalt circulation system whereby metalliferous saline brines convectively leached underlying sediments. In both scenarios, the resulting redox‐precipitated sulfides are zoned and arranged in the order Cu, Pb, Zn as one moves away from the zone of salt‐solution supplied brines. This redox zonation can be used as a regional pointer to both mineralisation and, more academically, to the position of a former salt bed. In the fault‐fed suprasalt accumulations the feeder faults were typically created and maintained by the jiggling of brittle overburden blocks atop a moving and dissolving salt unit. A similar mechanism localises many of the caprock replacement haloes seen in the diapiric provinces of the Gulf of Mexico and Northern Africa. Evaporite‐associated Pb–Zn deposits, like Cu deposits, are focused by brine flows associated with both bedded and halokinetic salt units or their residues. Stratabound deposits, such as Gays River and Cadjebut, have formed immediately adjacent to or within the bedded salt body, with the bedded sulfate acting as a sulfur source. In allochthon/diapir deposits the Pb–Zn mineralisation can occur both within a caprock or adjacent to the salt structure as replacements of peridiapiric organic‐rich pyritic sediments. In the latter case the conditions of bottom anoxia that allowed the preservation of pyrite were created by the presence of brine springs and seeps fed from the dissolution of nearby salt sheets and diapirs. The deposits in the peridiapiric group tend to be widespread, but individual deposits tend to be relatively small and many are subeconomic. However, their occurrence indicates an active metal‐cycling mechanism in the basin. Given the right association of salt allochthon, tectonics, source substrate and brine ponding, the system can form much less common but world‐class deposits where base‐metal sulfides replaced pyritic laminites at burial depths ranging from centimetres to kilometres. This set of diagenetic brine‐focusing mechanisms are active today beneath the floor of the Atlantis II Deep and are thought to have their ancient counterparts in some Proterozoic sedex deposits. The position of the allochthon, its lateral continuity, and the type of sediment it overlies controls the size of the accumulation and whether it is Cu or Pb–Zn dominated.

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

Cation study of fluid inclusion decrepitates in the Jubilee and Gays River (Canada) Zn-Pb deposits; characterization of ore-forming brines

An SEM-EDS instrument sensitive to C and O is used to measure cation ratios in individual fluid inclusion decrepitates of main paragenetic phases from two Nova Scotian Mississippi Valley-type deposits. The technique confirms that the analyses obtained for the carbonate hosts are not contaminated by substrate and permits measurements of small amounts of decrepitates. The technique has been used to investigate fluid compositions in inclusions of presulfide-stage dolomite and calcite, sulfide-stage sphalerite and calcites, and postsulfide calcite at the Gays River and Jubilee Zn-Pb deposits in Nova Scotia. The results indicate that Na and Ca are the two major cations. Minor amounts of K and Mg are also detected. NaCl and CaCl 2 ranges for decrepitates from the two deposits are as broad as for modem mixed brines and thereby suggest that solution mixing partly controlled the paleobrine compositions. When compared to other Mississippi Valley-type deposits, the Gays River and Jubilee deposits are similar in NaCl-CaCl 2 -KCl composition to a group of Mississippi Valley-type districts including deposits from the Ozark region that are associated with large volume clastic sequences or paleoaquifers. Higher proportions of KCl at Gays River and higher proportions of CaCl 2 at Jubilee indicate that the mineralizing brines interacted with Horton Group clastic aquifers of different mineral compositions. The brines in the two areas might have originated from evaporation of seawater, but they were later modified by interaction with clastic rocks of different compositions.

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.

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.

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.

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.

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.

Sources of basinal and Mississippi Valley-type mineralizing brines: mixing of evaporated seawater and halite-dissolution brine

Origins of basinal brines and Mississippi Valley-type (MVT) mineralizing fluids have been separately attributed to evaporation of seawater or dissolution of halite, although brines originating from the two processes are not mutually exclusive in basins. This study shows that the Na/Br Cl/Br diagram cannot distinguish between evaporated seawater and halite-dissolution fluid. Using the Na deficit Ca excess diagram which was previously proposed to characterize fluid-rock interactions of basinal brines, it is shown that most basinal brines including WT mineralizing fluids of the Viburnum Trend deposits were probably initially a mixture of halite-dissolution fluid and evaporated seawater. Using the same diagram, we suggest that the mineralizing brines of the Gays River MVT deposit were derived from an aquifer of clastics underlying a thick succession of evaporites, where halite-dissolution fluid and evaporated seawater could have mixed.

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

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

Pre-ore burial dolomitization adjacent to the carbonate-hosted Gays River Zn–Pb deposit, Nova Scotia

The Gays River Formation of the Lower Windsor Group in the vicinity of the Gays River Zn–Pb deposit was completely dolomitized via a volume for volume transformation prior to mineralization. Premineralization porosity was the precursor limestone porosity, comprising fenestrae, dissolution molds, and intercrystalline, intraskeletal, and growth pores, which remained essentially unchanged through dolomitization. Regionally, the dolomites (one replacement and one cement) overlapped with sediment compaction and dissolution of aragonite. The dolomites are characterized by low δ18OPDB values (−14 to −4[Formula: see text]; average −8[Formula: see text]), slightly radiogenic 87Sr/86Sr ratios (0.70778–0.70900), and locally high percentages of FeCO3 and MnCO3 (up to 13.5 and 9.5 mol%, respectively). The presented petrographic and geochemical data compiled with existing fluid-inclusion microthermometry data suggest dolomitization by hot brines, in a burial setting. High Fe and Mn concentrations south of the studied area, with progressive depletion towards the northeast and northwest, suggest either a northward flow of an Fe- and Mn-rich fluid or interaction with local sources of Fe and Mn, during single or multiple flows. By preserving the host-rock permeability, dolomitization played a role in ground preparation but was not genetically associated with the Zn– and Pb–rich fluid that mineralized the Gays River deposit.

Fluid evolution and mixing in the Gays River carbonate-hosted Zn-Pb deposit and its surrounding barren areas, Nova Scotia

This paper presents the results of a microthermometric study of fluid inclusions in authigenic dolomite (pre-ore), sphalerite (syn-ore), calcite and quartz (syn- to post-ore) from the Gays River Zn-Pb deposit and its surrounding barren carbonates. Homogenization temperature (Th) and salinity data from the deposit indicate that during precipitation of most sphalerite and minor calcite and quartz, a high-temperature (≥200CC), high-salinity (≥27 wt. V. NaCl eq.) fluid mixed with a fluid having lower temperature (Th=107-147°C) and salinity (24 wt. % NaCl eq.), i.e., the fluid that dolomitized the carbonates and saturated their pores before mineralization. During precipitation of most calcite and minor sphalerite, a third fluid with lower salinity (&lt;20 wt. % NaCl eq.) and temperature (Th&lt;90°C) affected the hydrothermal system and mixed with the two pre-existing fluids. A similar mixing trend has been observed for the barren areas. A group of fluid inclusions in calcite (both from the deposit and barren areas) have very low salinities (0.4-14 wt. % NaCl eq.) and variable Th(, (60-246°C); these results are interpreted as artifacts due to leaking and refill by late, low-salinity fluids.&#x0D; &#x0D; RÉSUMÉ&#x0D; Cet article présente les résultats d'une étude microthermomdtrique d'inclusions fluides dans la dolomite (pré-minéralisation), la sphalérite (syn-minéralisation), la calcite et le quartz authigenes (syn- à post-minéralisation) du dépôt de Zn-Pb de Gays River et de ses environs non minéralises. Les données de la température d'homogéntisation (Th) et de la salinité du dépôt indiquent que pendant la précipitation d'une part majeure de la sphalérite et d'une faible part de la calcite et du quartz, un fluide de hautes température (≥200°C) et salinité) (≥27% poids en équivalent NaCl) se serait mélange avec un fluide de plus basses température (Th=107-147°C) et salinité (24% poids en équivalent NaCl), i.e. celui qui a dolomitisé les carbonates et saturé leur pores avant la minéralisation. Pendant la précipitation de la plus grande part de la calcite and d'une faible part de la sphalérite, un troisieme fluide, de plus faibles salinité (&lt;20% poids en équivalent NaCl) et témperature (Th&lt;90°C), à aflecte le système hydrothermal et s'est mélange avec les deux fluides préexistants. Un mode de mélange similaire est observé dans les régions non minéralisees. Un groupe d'inclusions fluides dans la calcite (du dépôt et des régions non mine&gt;alisecs) ont des salinités très basses (0.4-14 poids en équivalent NaCl) et des Th variables (60-246°C). Ces résultats sont interprétes comme des artifacts dus à des fuites et a un remplisage par les fluides tardifs de basse salinité.

40 Ar/ 39 Ar dating of fluid migration in a mississippi valley-type deposit; the Gays River Zn-Pb deposit, Nova Scotia, Canada

The Gays River Zn-Pb deposit of southern Nova Scotia, Canada, represents an example of Mississippi Valley-type mineralization (reserves ca. 2.4 million metric tons 8.6% Zn and 6.3% Pb). The deposit is hosted by Visean-age, dolomitized carbonate rocks (bank and interbank facies) that form part of a series of carbonate banks (i.e., the Gays River Formation). The banks developed on paleotopographic highs underlain by lower Paleozoic metaturbidites of the Meguma Group, but locally there is an intervening basal breccia unit which contains fragments (centimeter to meter scale) of Meguma Group lithologies within a mineralized (Pb > or = Zn) dolostone matrix. Petrographic examination of fragments in the breccia unit indicate traces of hydrothermal alteration which is considered to have been coincident with mineralization at Gays River. Maximum temperatures associated with the alteration ranged from ca. 300 degrees (fluid inclusions) to ca. 350 degrees C (chlorite geothermometry). Five 40 Ar/ 39 Ar step-heating experiments of metasedimentary clasts in the basal breccia give nearly identical age spectra profiles, with low-temperature gas fractions indicating apparent ages of ca. 300 Ma followed by plateau ages of ca. 380 to 400 Ma for high-temperature gas fractions. Whereas the latter ages are consistent with previously determined whole-rock 40 Ar/ 39 Ar ages for regional deformation and metamorphism of the Meguma Group, the ca. 300 Ma age is considered to reflect thermal overprinting and is consistent with recent palcomagnetic determinations suggestive of a mid-Mississippian to late Pennsylvanian age for mineralization. Model calculations of the age spectra, assuming degassing of micaceous phases and volume diffusion, suggest that the most plausible interpretation is a reheating event of ca. 250 degrees to 300 degrees C of ca. 1- to 6-m.y. duration. The anomalously elevated temperatures determined from this study, in conjunction with the presence in Maritime Canada of Late Devonian-Carboniferous mafic-felsic magmatism and the widespread resetting of radiometric systems in the Meguma terrane at ca. 300 Ma, suggest that the Gays River mineralization may be related to the same broad-scale tectonic processes that formed the Martimes basin and involved regional-scale structures and probably magma generation.

Preliminary modelling of groundwater inflows at Gays River mine, Nova Scotia, Canada : R. MacLeood, G. G. Bursey, T. R. C. Aston & J. E. Gale, Transactions — Institution of Mining & Metallurgy, Section A, 102(Sept–Dec), 1993, pp A153–A164

Preliminary modelling of groundwater inflows at Gays River mine, Nova Scotia, Canada : R. MacLeood, G. G. Bursey, T. R. C. Aston & J. E. Gale, Transactions — Institution of Mining & Metallurgy, Section A, 102(Sept–Dec), 1993, pp A153–A164

Paleomagnetism of the Gays River zinc‐lead deposit, Nova Scotia: Pennsylvanian ore genesis

The Gays River zinc‐lead deposit is the largest known Mississippi Valley‐type deposit in the Mississippian Windsor Group of Atlantic Canada. Thermal and AF demagnetization show that both ore and host rock have very similar magnetic characteristics. Stability tests constrain the magnetization to be mid‐Mississippian to late Pennsylvanian. The pole position from the ore and host rock sites falls next to the Pennsylvanian APW path, suggesting an 8° clockwise rotation of the Meguma platform relative to the craton. Regional thermal history and geochemical evidence suggest that the zinc‐lead ore magnetization and host rock remagnetization are coeval.

Pre-Mineralization Dolomitization of Host-Rocks to Pb-Zn, Gays River Deposit, Nova Scotia

Pre-Mineralization Dolomitization of Host-Rocks to Pb-Zn, Gays River Deposit, Nova Scotia

Thermochronologic constraints on ore formation at the Gays River Pb–Zn deposit, Nova Scotia, Canada, from apatite fission track analysis

Fission tracks in detrital apatites from the Cambro-Ordovician metasedimentary basement in the vicinity of the Carboniferous-hosted Gays River Pb–Zn deposit, Nova Scotia, provide a record of final cooling during uplift and erosion of the Meguma Zone and constrain the timing of ore formation. Apatite fission track ages range from 203 to 241 Ma, with typical uncertainties of ± 10 Ma. Mean confined track lengths generally vary between 12.0 and 13.4 μm and indicate that the apatites record "apparent" ages only. An inferred thermal history involving regional heating to paleotemperatures &gt; 110 °C during late Paleozoic burial followed by cooling to ~ 110 °C prior to 240–220 Ma is suggested. A more recent phase or regional heating to paleotem-peratures probably in the range of 60–80 °C during Late Cretaceous – early Tertiary (ca. 100–50 Ma) burial is also indicated by the track length data. Apatite fission track ages and mean track lengths from drill-core samples immediately beneath the Gays River orebody are similar to those for regional outcrop samples. At minimum temperatures &gt; 200 °C estimated for ore formation, sulphide mineralization must either have preceded or accompanied regional heating to paleotemperatures &gt; 110 °C during the late Paleozoic. Sulphide mineralization at Gays River must therefore have taken place at some time after ca. 330 Ma (the stratigraphic age of the lower Windsor Group host rocks) but before ca. 240–220 Ma (the last cooling of Meguma Group basement below 110 °C). These constraints on the timing of ore formation at Gays River are compatible with previous suggestions that Pb–Zn mineralization of Carboniferous strata in Nova Scotia occurred at ca. 300 Ma.

Formation of Carboniferous Pb-Zn and barite mineralization from basin-derived fluids, Nova Scotia, Canada

Lead-zinc and barite deposits in central Nova Scotia generally occur at the Tournaisian-Visean (Mississippian) clastic carbonate-evaporite disconformity overlying Cambrian to Devonian metasedimentary and granitic rocks. The deposits are located at the southern margin of the > 10-km-deep Carboniferous Maritimes (Fundy-Magdalen) basin.Fluid inclusion homogenization temperatures of ore-stage to postore minerals range from 230 degrees to 130 degrees C and fluid salinity is > or = 24 equiv wt percent NaCl. A mineralization age of ca. 300 to 330 Ma is suggested by: (1) fission-track dates on zircon from clastic host rock at Gays River, (2) K-Ar dates on clays from altered clastic rock in close proximity to mineralization, and (3) Rb-Sr data on illite from samples collected over a 10-m section of Tournaisian sandstone (far away from mineralization). Lead and strontium isotope data indicate that Tournaisian rock of the Horton Group could have been a source (at ca. 300 Ma) of lead and radiogenic strontium in the ore-stage minerals at the larger deposits. Visean carbonate carbon and sulfate were the dominant sources of carbon and sulfur, respectively. Hydrogen and oxygen isotope data suggest that the mineralizing fluids were basinal brines. Nonbiogenic sulfate reduction at the deposits is the likely sulfide precipitation mechanism.The ore-forming fluids may have originated at a >5-km depth under Visean evaporites in the Magdalen basin and migrated rapidly through Tournaisian clastic rocks and the immediately overlying Pembroke breccia to the basin margins. A pulse of regional tectonism may have set off at least one massive, hydrofracturing, compaction-driven, fluid expulsion event by about 300 Ma.