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TitlePotential for carbonate-hosted lead-zinc Mississippi Valley-type mineralization in northern Alberta and southern Northwest Territories: geoscience contributions, Targeted Geoscience Initiative
AuthorHannigan, P K (ed.)
SourceGeological Survey of Canada, Bulletin no. 591, 2006, 347 pages; 1 CD-ROM, (Open Access)
LinksMetadata - Métadonnées
PublisherNatural Resources Canada
Mediapaper; CD-ROM; on-line; digital
RelatedThis publication contains the following publications
File formatreadme / lisez-moi
File formatbmp; pdf (Adobe® Reader® v. 6.0, is included / est fourni); txt
ProvinceNorthwest Territories; Alberta
NTS74D/09; 74D/10; 74D/11; 74D/12; 74D/13; 74D/14; 74D/15; 74D/16; 74E; 74L; 74M; 75D; 75E; 84A/09; 84A/10; 84A/11; 84A/12; 84A/13; 84A/14; 84A/15; 84A/16; 84B/09; 84B/10; 84B/11; 84B/12; 84B/13; 84B/14; 84B/15; 84B/16; 84C/09; 84C/10; 84C/11; 84C/12; 84C/13; 84C/14; 84C/15; 84C/16; 84D/09; 84D/10; 84D/11; 84D/12; 84D/13; 84D/14; 84D/15; 84D/16; 84E; 84F; 84G; 84H; 84I; 84J; 84K; 84L; 84M; 84N; 84O; 84P; 85A; 85B; 85C; 85D; 85E; 85F; 85G; 85H
Areanorthern Alberta; south-central NOrthwest Territories; Peace River; Athasbasca River; Slave River; Great Slave Lake; Mackenzie River; Clearwater River; Lake Athabasca; Lake Claire; Pine Point; Hay River; High Level; Fort Chipewyan; Fort McMurray; High Point; Sulphur Point; Presqu'ile Point; Dawson Landing; Paulette Island; Buffalo River; Polar Lake; Fort Smith; Wood Buffalo National Park; Cameron Hills
Lat/Long WENS-120.0000 -110.0000 62.0000 56.5000
Subjectseconomic geology; sedimentology; stratigraphy; structural geology; geochemistry; geophysics; fossil fuels; mineral potential; mineral deposits; mineral exploration; Mississippi Valley deposits; lead zinc deposits; sedimentary rocks; carbonate rocks; dolomites; dolostones; zinc; lead; sedimentary petrology; petrography; mineralization; paleohydrology; hydrodynamics; fluid flow; fluid inclusions; formation fluids; structural analysis; structural features; structural controls; flow structures; diagenesis; thermal analyses; thermal history; isotopic studies; geochemical analyses; deposition; shear zones; faults; sedimentary basins; sedimentary ore deposits; hydrothermal deposits; solution; leaching; organic materials; hydrocarbon potential; geophysical interpretations; magnetic interpretations; seismic interpretations; i p interpretations; induced polarization; gravity interpretations; Interior Plain; Canadian Shield; Western Canada Sedimentary Basin; Great Slave Plain; Great Slave Reef property; Pine Point property; Tathlina Arch; Peace River Arch; Elk Point subbasin; Mackenzie subbasuin; Presqu'ile Barrier; Steen River Impact Structure; Great Slave Shear Zone; McDonald Fault; Hay River Fault; Tathlina Fault Zone; Cameron Hills Structure; Rabbit Lake Fault Zone; carbonate-hosted deposits; microthermometry; Phanerozoic; Mesozoic; Paleozoic; Devonian; Precambrian
ProgramTargeted Geoscience Initiative (TGI-1), 2000-2003
Released2006 12 21
Adjacent to the south shore of Great Slave Lake in southern Northwest Territories, a Middle Devonian carbonate barrier complex called the Presqu'ile barrier has been extensively dolomitized, and this complex hosts a large number of lead-zinc orebodies constituting the world-class Pine Point mineral deposit. The Pine Point ore deposit is classified as Mississippi Valley-type (MVT) indicating similar character to a number of lead-zinc deposits originally defined in the Mississippi River valley of central United States. MVT ore deposits are stratabound carbonate-hosted sulphide bodies dominantly composed of galena and sphalerite. These epigenetic deposits occur mainly in dolostone as open-space fillings in collapse breccias or less commonly as replacement bodies in high-grade zones. The ores originate from basinal brines at temperatures of between 75 and 200oC and are typically deposited in carbonate platform settings in relatively undeformed epicratonic sedimentary basins. The deposits occur as clusters and these areas of concentrated occurrence form MVT districts. Characteristically, deposits within districts exhibit remarkable similarities in mineral assemblage, isotopic composition, texture, and ore-control. Throughout northern Alberta and southern Northwest Territories, numerous and extensive thick carbonate successions occur in the cratonic platform wedge of strata of the Western Canada Sedimentary Basin. These prospective host rocks were identified by federal, provincial, and territorial partners as strata of enhanced mineral potential and, consequently, required further evaluation. A Targeted Geoscience Initiative project was thus proposed to pursue an integrated and comprehensive study investigating the potential for MVT mineralization in the area. This volume reports on the various aspects and results of this project. The primary goal of the project was to delineate and evaluate the origin, distribution and potential of carbonate-hosted lead-zinc deposits in the region and develop an understanding of the relationships of fluid flow and ore deposition to the regional framework of stratigraphy, structure, and diagenesis. The source, flow-path, and timing of fluid movement and their relation to mineralization were important considerations. The relationship of stratabound MVT mineralization at Pine Point with the regional stratigraphic framework and the evaluation of the structural features critical for fluid migration and ore deposition are crucial attributes to consider when identifying regions or strata of enhanced mineral potential elsewhere in the project area. At Pine Point, orebodies are generally restricted to zones of weakness or hinge lines where extensive networks of interconnected paleokarst developed. Within these paleokarstic networks, prismatic and tabular ore accumulations were deposited in hydrothermally derived dolomite breccias. The most typical host lithology for MVT orebodies at Pine Point is the coarse-grained, vuggy Presqu'ile dolomite, an extensive secondary diagenetic dolomite facies that was deposited relatively late in the paragenetic history of the Presqu'ile barrier complex. A critical element in the study of MVT deposits in Western Canada and elsewhere is the role and importance that structure played in the formation and emplacement of the sulphide orebodies. Popular genetic models include compaction- and topographic recharge-driven migration of metal-bearing brine through aquifers in the Western Canada Sedimentary Basin and precipitation of the ore minerals in peripheral carbonate platforms. These hydrological models imply strong stratigraphic control with regard to sulphide emplacement and a relatively minor role for structures. In these models, faults may offset paleoaquifers or provide sites for fluid mixing. In this volume, arguments for a direct relationship of MVT deposits and their closely associated hydrothermal dolomite with fault-zone processes throughout the Western Canada Sedimentary Basin are proposed. Geological observations of various Pb-Zn occurrences validated by geochemical and structural data suggest structural channelling of dolomitizing and mineralizing fluids into strained carbonate rocks. In the Rocky Mountain fold-and-thrust belt, radiogenic strontium and lead isotopes in sulphides indicate basement signatures. At Pine Point, radiogenic strontium in hydrothermal dolomite and a non-radiogenic lead isotope cluster in sulphides correspond directly with isotopic signatures observed in subsurface MVT occurrences overlying basement fault zones, possibly indicating a common basement source. The chemical and isotopic characteristics of MVT parent fluids are consistent with seawater and brine convection within fault-confined vertical aquifers, strong water-basement rock interaction, metal leaching from basement, and focused release of hydrothermal and metal-bearing solutions along linear zones of strained carbonate caprocks. These 'fault-zone' processes can explain the linear distribution of MVT occurrences locally and regionally and would be more effective in delivering solute to the overlying carbonate caprocks than a single flush through clastic strata as invoked by basinal fluid-flow models. If this hypothesis is correct, the identification of structural trends in the project area would have very useful predictive value with respect to MVT exploration. In accordance with this structural discourse, a series of subsurface structure and isopach maps in the project area reveal abrupt inflections of contours coincident with previously identified faults and fault zones. Significantly, new features were revealed by this mapping exercise and often correlate with subvertical faults interpreted from seismic and aeromagnetic data. Seismic interpretation confirms a regionally extensive orthogonal pattern of structures in the area that greatly influenced Middle Devonian facies development and the orientation of the carbonate bank edge. The seismic work also revealed a close association of hydrothermal dolomite distribution with structural features in the region. The strong association of certain dolomite types to MVT mineralization at Pine Point led to a series of papers in this volume dealing with dolomite varieties, their characteristics and distribution, as well as their geochemical and isotopic properties. Detailed study of the petrography, geochemistry, and fluid inclusion microthermometry of various dolomite types, carbonate cements, and sulphide minerals provide useful information with respect to characteristics of the parent fluids and the paleoenvironmental conditions during deposition. Pre-ore carbonate phases consist of early, fine-crystalline matrix dolomite and later medium-crystalline matrix dolomite. Geochemical analysis of these pre-ore carbonate phases reveals that early dolomite formed, at or just below the seafloor, from Middle Devonian seawater. During medium-crystalline dolomitization, shallow burial conditions prevailed. A strong associate of MVT mineralization at Pine Point is the diagenetic facies called Presqu'ile dolomite, a coarse-crystalline vuggy dolostone with vugs partly or completely occluded with saddle dolomite cements. This ore-stage matrix dolomite along with secondary saddle dolomite cements and sphalerite and galena sulphides are characterized by depleted oxygen-18, slightly depleted carbon-13 and similar strontium ratio isotopic values at Pine Point. The depleted oxygen-18 isotope signature indicative of higher precipitation temperatures is confirmed by elevated fluid inclusion homogenization temperatures. Also, final melting temperatures of these inclusions establish that the dolomitizing fluids were hypersaline brines, and the eutectic or initial melting temperatures suggest that they were fluids derived from the NaCl-CaCl2-H2O salt system. Thus, derivation from deeply buried basinal fluids affected by low-temperature diagenetic processes is suggested for ore-stage minerals. Chemical analyses of fluid inclusions in sphalerite and dolomite indicate that the transporting fluid consisted of dense brines. These brines were bromine-enriched compared to seawater, which suggests the parent fluids evolved from highly evaporated seawater rather than common seawater. Dominant cations in the inclusions are calcium and sodium. These Ca- and Na-rich brines are similar to oilfield brines. However, K/Na values in MVT deposits are greater than ratios found in oilfield brines indicating that MVT ore fluids are more evolved from seawater or partly composed of interstitial fluids from evaporite beds enriched in residual potassium. The deposition of ore-stage dolomite postdated deep burial. Evidence for dolomitization after burial includes crosscutting relationships of the dolomite with depositional fabrics, formational boundaries, and unconformities. Ore-stage dolomite also replaces blocky sparry calcite previously formed in the subsurface, crosscuts stylolites, exhibits greater depleted oxygen-18 isotope values meaning higher precipitation temperatures, and contains two-phase fluid inclusions and fluorescent petroleum inclusions, all indicating deep burial. Fluid inclusion microthermometry measurements in ore-stage dolomite and sphalerite reveal that fluid-filling temperatures are too high to be solely attributed to burial, suggesting that thermal anomalies are necessary in regions of enhanced MVT mineral potential. Investigation of thermal maturity of organic matter at Pine Point discloses a substantial contrast between thermally immature indigenous organic matter in country rock carbonate rocks and mature to overmature bitumens and pyrobitumens proximal to or within orebodies. Brine springs sampled in northern Alberta are characterized by compositions dominated by NaCl with high levels of sulphate and calcium. Water samples in the Wood Buffalo region are saturated with respect to halite and gypsum whereas Fort McMurray area brines are typically undersaturated in these minerals, consistent with the lack of evaporate deposits in the latter area. Significant differences with respect to Devonian formation waters in northern Alberta and Pine Point are evident in these modern springwater samples. The range of the conservative ion ratio Br/Cl measured in Devonian formation waters indicate original seawater evaporating past halite saturation, thereby producing a bromine enrichment. The springwaters, however, have low Br/Cl values suggesting derivation by dissolution of halite. Thermal anomalies are confirmed in the northern portion of the study area by the measurement of anomalous heat flows greater than 90 m/Wm2. The anomaly is most likely related to enhanced heat generation from zones in basement rocks that are relatively rich in radiogenic minerals in the upper crust. Basement heat flow is greater than the average Precambrian basement heat input and it explains the elevated maximum burial temperatures obtained from homogenization temperatures of saddle dolomite. Previous studies defined a broad westward decrease of oxygen-18 isotope value with the associated increase of homogenization temperature, as well as an increase in strontium isotope ratio in ore-stage carbonate phases along the Presqu'ile barrier from Pine Point in southern Northwest Territories to northeastern British Columbia. These geochemical spatial trends were attributed to higher temperature dolomitizing fluids and the influence of siliciclastic sources, respectively. Data from this volume challenge these identified trends and display instead rather variable and inconsistent spatial distributions of stable and radiogenic isotope signatures. The nonmineralized sulphide-free dolomite does not show a westward decrease in oxygen-18 values and mineralized dolomite actually shows a westward increase. This highly variable arrangement of isotopic signature contests the paleohydrological models which infer appropriate uniform spatial trends. Blotchy cathodoluminescence was observed in much dolomite in the study area suggesting recrystallization of the dolomite during progressive burial. Varying geochemistry and nonuniform petrographic characteristics even within individual boreholes suggest recrystallization during progressive burial was inconsistent and may be ultimately more influenced by precursor rock textures than by geographic location. Evidence presented in this study suggests that ore-stage dolomitization may be related to fluid circulation through extensional faults and karstic pore systems that developed in the Presqu'ile barrier. These systems may have facilitated the establishment of crossformational fluid flow and the development of extensive convection cells resulting from spatial variations in relatively high basal heat flow. Continuous recycling of magnesium-rich seawater-derived connate brines by thermal convection flow provides a feasible mechanism for extensive and regional development of hydrothermal dolomite in porous limestone overlying dense dolostone. Stratiform dolomite-cemented breccias or tabular paleokarst form in the upper subhorizontal portions of the convective cells. In areas of upward fluid movement possibly associated with fault zones, chimney-like dolomite bodies extend upward from tabular horizons to levels beneath overlying impermeable siliciclastic rocks, forming prismatic paleokarst. The variability in isotopic data, even at a borehole scale, can be attributed to temporal or spatial variation in heat flow or the degree of recrystallization as a function of fluid-rock interaction. A significant increase of the radiogenic strontium isotope proximal to the Great Slave Shear Zone suggests a key role attributable to fluid movement through the zone. Radiogenic strontium indicates derivation from basement rocks and the shear zone provided a favourable site for the upward circulation of hydrothermal fluids from basement and precipitation of hydrothermal dolomite and sulphides in overlying porous carbonate successions. A distinct, homogeneous, nonradiogenic lead isotope signature characterizes the sulphides in the Pine Point district as well as in sulphide samples collected proximal to the Great Slave Shear Zone in northwestern Alberta and within the Cordova Embayment. This lead isotope cluster, dissimilar to the radiogenic isotope signature found in Cordilleran carbonate deposits, was not expected to be present near the Shear Zone and in the Embayment. The homogeneous nonradiogenic signature suggests either a single source for the lead, or thorough mixing after extraction from several sources. Post-ore calcite displays depleted oxygen-18 and carbon-13 isotope signatures compared to pre-ore calcite. The higher precipitation temperatures associated with depleted oxygen-18 can be attributed to burial. The fluid inclusion melting temperatures of these burial calcites indicate a less saline parent fluid of dissimilar brine composition (NaCl-H2O) to earlier carbonate phases. Homogenization temperatures in late-stage calcite are generally cooler than those of saddle dolomite. Cooler filling temperatures and reduced salinity may have been brought about by later introduction and mixing of meteoric water. Exploration methods for MVT deposits in the project area include mapping of favourable geological features, surficial geochemical surveys, and geophysical surveys. Diagnostic geological features established during extensive exploration at Pine Point should be applicable for MVT exploration elsewhere in the project area. Such geological features include carbonate platform successions on the flanks of large and deep sedimentary basins, unconformities, paleokarsts, arching or flexing of underlying basement that may define zones of enhanced fluid movement, basement faults, dolostone successions associated with evaporitic strata and organic matter, highly permeable and porous zones, white sparry dolomite, and collapse breccias. Most MVT deposits have minor geochemical signature because of limited primary dispersion of the elements bounded in sphalerite or galena into the surrounding carbonate rocks. However, when sufficient weathering of sulphides occur, soil and stream sediments proximal to sulphide deposits may contain anomalous concentrations of Pb, Zn, or Fe. At Pine Point, zinc anomalies are prominent in lake sediments and soils, but not all zinc anomalies are associated with an orebody. Major organic-rich shale units give zinc background values similar in magnitude to anomalies associated with orebodies. Commonly, the same shale has low lead content. Therefore, areas of composite lead-zinc anomalies may indicate MVT sulphide mineralization. Regional geophysical techniques such as seismic, gravity, and magnetic surveys have been successful in identifying basement highs and subsurface faults. Property-scale techniques are limited because of the open-space filling nature of MVT ore, where gangue minerals and sphalerite interrupt conducting paths between conducting minerals, thereby limiting the usefulness of self-potential and electromagnetic surveys. However, at Pine Point, induced polarization proved to be a powerful tool for finding new orebodies, but only when sufficient conducting minerals are present. If an orebody dominantly consists of sphalerite with low quantities of galena, marcasite, or pyrite, the induced polarization signal is too weak for positive identification of an orebody. Gravity methods has been proven to be useful as a complementary tool with induced polarization surveys in locating sphalerite-rich orebodies at Pine Point, because of the significant contrast in density between sphalerite and the country rock. Mineral prospectivity mapping for MVT deposits in the Pine Point district was conceptually evaluated by applying critical genetic features to empirical evidence of mineralization, such as orebodies, visible sulphide occurrences in outcrop and core, and anomalous geochemical zones. Critical components of a regional-scale MVT system are all related to fluid flow during dynamic basin evolution. The five critical components describing a regional hydrothermal system include energy, ligand source, metal source, transport pathways, and trap zones. The first three components cannot be predicted because of the unresolved controversy of these sources in the various genetic models. Transport pathways and trap zones, however, can be predicted, and these components are related to specific structural and stratigraphic features associated with MVT deposition. Such features include major basement lineaments and faults, carbonate aquifers, basement highs, fluid outflow or discharge sites, and alteration zones.