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Pine Point |
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NWT, Canada |
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Zn Pb
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Super Porphyry Cu and Au
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The Pine Point carbonate hosted zinc-lead mining district is located 10 km south of the southern shore of Great Slave Lake in the North West Territories of Canada, ~200 km to the south of Yellowknife. The operation is now exhausted and abandoned (#Location: 60° 53' 34"N, 114° 31' 27"W).
The Pine Point group of deposits are spread over an interval of around 65 km in an east-west direction, with a width averaging 24 km. To the end of 1983 some 87 separate accumulations of Pb-Zn mineralisation had been delineated, of which 36 had been exploited. All except one underground trial, were extracted by open pit mining. Between 1969 and 1981 annual production varied between 3.2 and 4.1 Mt of ore (Rhodes, et al., 1984).
The Pine Point mineral field lies in the north-western part of the North American Interior Platform in Canada, to the west of the outcropping Precambrian shield. It is found within a very gently west dipping sequence of Ordovician to Devonian sediments which are locally 350 to 600 m thick. Pine Point is some 150 km to the west of the erosional margin of the Interior Platform and the Precambrian shield, (Rhodes, et al., 1984).
The succession in the Pine Point district is as follows, from the base (Rhodes, et al., 1984; Qing & Mountjoy, 1994):
Precambrian basement,
Unconformity
Ordovician,
Old Fort Island Formation, 0 to 40 m thick - white and red, friable, fine to medium grained, quartz sandstone with minor greenish-grey siltstone and green shale.
Mirage Point Formation, 17 to 178 m thick - red beds of dolostone, dolomitic silty mudstone breccia, gypsiferous and sandy dolostone, shale, siltstone and anhydrite, gypsum and salt.
Devonian,
Chinchaga Formation, 90 to 130 m thick - light grey to brown anhydrite and gypsum with minor amounts of limestone, dolostone, limestone and dolostone breccia, salt, and green shale. This unit is of Eifelian age.
Keg River Formation, 65±10 m thick - a relatively homogenous unit of grey-brown dolomite, with carbonaceous-argillaceous wisps and partings hosting relatively abundant crinoids and brachiopods. Three (locally less) well defined shale or shaly dolomite beds, known as the E-Shale Markers, consistently occur within 3 to 6 m of the top of the formation. These beds range from a few centimetres up to a metre thick. The extensive Keg River Formation is taken to represent a widespread marine platform transgression at the end of the Eifelian. It is of early Givetian age.
Presqu'ile Barrier Complex - which is of middle Givetian age and has been divided into the:
• Pine Point Formation, up to 175 m thick - composed of a diverse assemblage of carbonate facies, all of which were apparently deposited as limestone, but which are now commonly dolomitised. At the base there is a 12 to 18 m thick floatstone which has a moderate argillaceous content expressed as wisps, and carries 5 to 20% stromatoporoids, corals and brachiopods in a grainstone and packstone matrix. It has been dolomitised and strongly leached to give a fossil moldic1 vuggy porosity. This unit is found below the entire barrier complex, and extends well to the south into the Elk Point Basin, and to the north into the Mackenzie Basin.
Above this basal member the Pine Point Formation is composed of a variety of inter-fingering carbonates forming a broad bank of sediments. In general, on any given time-plane, there is a progression from south to north of i). a floatstone with 10 to 40% stromatoporoid and coral fragments supported by a clean bioclastic grainstone matrix; ii). a clean bioclastic grainstone similar to the previous bed, with no large skeletal debris; iii). a fine grainstone to mudstone and wackestone, becoming progressively more bituminous and argillaceous to the north, with a fauna changing from frame building organisms to crinoids and brachiopods; iv). a black, intensely bituminous, mudstone, with paper thin planar laminations. These bituminous rocks extend well to the north into the Mackenzie Basin as a 40 to 50 m thick interval.
A distinctive breccia lithology is found at the top of the formation, composed of variable amounts of fragments of the facies described above, particularly the first two.
The barrier represented by the Pine Point Formation is not an organic reef, but rather a bank of carbonate sands and muds with <10% organic floatstones or fossil hash. It apparently represents a shallower depositional environment between the two basinal centres. Never-the-less there is a distinctive biohermal build-up at the top of the Pine Point Formation at the highest point of the barrier.
The equivalent sequence thickens both to the north and south of the barrier. The lithofacies of the barrier complex show substantial north-south variations, but are very consistent east-west, occurring as ribbon like facies that may be traced in the same position relative to the barrier axis over a distance of up to 50 km along its trend.
• Sulphur Point Formation, up to 65 m thick, thinning to 9 m well to the south - overlies the Pine Point Formation with apparent conformity and is composed of a succession of light cream to white limestones and their coarsely dolomitised equivalents. As such it is readily distinguishable from the Pine Point Formation by its light colour and lack of bituminous material. To the south it succeeds and gradationally overlies the Muskeg Formation. To the north it thins and then terminates abruptly at an erosional margin. Four main facies are identified, namely: i). stromatoporoidal boundstones; ii). clastic grainstones; iii). bioclastic limestones, including rudstones, floatstones and wackestones; and 4) strongly laminated algal limestones and pelletoidal grainstones.
Muskeg Formation, which is an equivalent of the Presqu'ile Barrier Complex facies, but is taken to be a back-barrier, evaporite facies on the Elk Point Basin side, to the south. It is broadly a time equivalent of the Pine Point Formation, although it overlies that formation with a relatively sharp and conformable contact. The uppermost Muskeg Formation appears to be a time equivalent of the lower Sulphur Point Formation, with several intercalations of each extending well into the other.
The principal facies of the Muskeg Formation are fine grained dolomites which are complexly intercalated with gypsum and anhydrite beds to the south of the mineral field in the Elk Point Basin. This dolomite facies is divided into an upper and a lower unit. The lower unit is from zero over the barrier, to 150 m thick to the south, and is that part of the formation that is an equivalent of the Pine Point Formation. The upper unit is thinly bedded (from 0.2 to 1.5 m thick beds) and is the equivalent of the Sulphur Point Formation. It is up to 18 m in thickness.
Fore-Barrier Facies, formations that are partial equivalents of the Barrier Complex, occurring to the north, on the Mackenzie Basin side of the barrier. This facies is represented by two vertically superposed carbonate build-ups, namely the,
• Buffalo River Formation, composed of a wedge of grey-green shales and lesser carbonates flanking the northern side of the barrier complex, and thickening northwards. It lies conformably on some of the Pine Point Formation facies and disconformably on others. The Buffalo Formation is a regional unit within the Mackenzie Basin, with a well defined southern margin paralleling the barrier complex.
• Windy Point Formation, is a relatively thin unit that forms another wedge of carbonates overlying the Buffalo Formation and also flanks the northern side of the barrier complex. It has a defined southern margin, again parallel to the barrier complex, thickening to 20 m on the shore of Great Slave Lake north of the barrier. and then to 50 m further north. It has three component lithofacies, namely, i). a biostromal, ii). a restricted and iii). a reefal facies.
Disconformity - a regional feature found throughout the northern basins of western Canada.
Watt Mountain Formation, averaging <10 m in thickness where it overlies the central barrier complex, getting up to 33 m thick to the north. It is of early late Givetian age, and is composed primarily of sub-tidal green, silty and pyritic shales, alternating with nodular and argillaceous limestones and dolomites, limestone breccias and minor amounts of anhydrite. The basal unit is chiefly responsible for the thickness variations. It comprises waxy blue-green limy and dolomitic shales, earthy green marls and clean intraclastic micrite beds, with earthy cryptocrystalline dolomites to the north, often associated with gypsum and anhydrite. The overlying six members are composed of interbedded dolomites, limestones and minor shales.
Slave Point Formation, overlies the Watt Mountain Formation conformably, with a basal 3 m thick blue-grey, very argillaceous limestone. Other lithologies include sandy micrite and highly fossiliferous units dominated by globular and massive cabbage type stromatoporoids with lesser corals and brachiopods, in a matrix of limy mudstone, pelletoidal grainstone and skeletal grainstone.
Unconformity
Cenozoic,
Unconsolidated cover - glacial and lake sediments.
The Pine Point mineralisation is hosted by middle Devonian, Givetian, sequence that overlies older Palaeozoic sediments. Within these sediments, a narrow, 10 km wide linear belt of carbonate facies with a maximum thickness of 200 m forms a "barrier complex" (Rhodes, et al., 1984), known as the Presqu'ile Barrier. This east-west trending barrier extends westward for around 400 km from its outcrop at Pine Point. Although the barrier is only 10 km wide at Pine Point, it ranges up to 90 km in width along its trend (Qing & Mountjoy, 1994). This barrier is interpreted to have separated two depositional basins, the Mackenzie Basin to the north which was predominantly filled with shales and shaly limestone; and the Elk Point Basin to the south, which is characterised by evaporites and lesser carbonates (Rhodes, et al., 1984). It is developed over the McDonald Fault Zone, the local representative of the continental scale Great Slave Shear Zone which separates the Slave and Rae Archaean Provinces.
Ore occurs as a series of parallel concordant, elongate, runs and transgressive chimneys and includes significant tonnages of high grade sulphides. Much of the mineralisation at Pine Point is related to dolomitisation of the limestones in the Presqu'ile Barrier. Two styles of dolomitisation are evident, namely: i). finely crystalline dolomites, and ii). coarsely crystalline (or Presqu'ile) dolomites. A third variant is white vein dolosparite (Rhodes, et al., 1984).
Dolomitisation and carbonate dissolution features within the sequence may be divided into:
• Palaeo-karst features, and
• Interpreted hydrothermal dissolution, best developed in the Presqu'ile barrier, where large scale interpreted hydrothermal dissolution commonly occurs in the coarsely crystalline replacement dolomites, which are indicated to have been formed after the unconformity. The hydrothermal dissolution is largely de-calcification and de-carbonatisation.
Two distinct trends of carbonate dissolution and ore formation are recognised at Pine Point. Both trend at 65 degrees, parallel to the strike of the barrier and extend for more than 50 km along the barrier. These are the 2 km wide North Trend, and the 3.3 km wide Main trend. The North Trend is found on the northern edge of the barrier complex, while the Main Trend is along the approximate middle of the barrier. Both are composed of sub-trends, which are sub-parallel, variably mineralised, linear zones of enhanced tabular dissolution with localised developments of prismatic dissolution structures (Rhodes, et al., 1984).
Two main styles of orebody are recognised at Pine Point, namely:
i). tabular, which are the most common at Pine Point. They are crudely stratabound and coincide closely with the Presqu'ile (coarsely crystalline replacement) dolomite.
ii). prismatic orebodies, which occur within zones of solution collapse that transgress the succession, generally normal to bedding, and above a zone of tabular solution collapse.
Mineralisation is present in a number of forms. Ore within sections of the tabular orebodies is present on the following forms:
• Heterogeneous solution-collapse breccias, comprising galena, sphalerite and minor marcasite, and occurring as: i). replacement of internal sediments between the clasts of the collapse breccias; ii). open space filling between breccia clasts; and iii). impregnations and replacement of breccia clasts. The sulphide textures are highly variable, suggesting more than one phase of mineralisation. Sphalerite is commonly colloform and ranges in colour from yellow-brown, to maroon and black, although fine grained disseminations, blebby aggregates, rosettes and irregular massive zones are also common. Galena appears as medium to coarse grained aggregates and blebs, closely associated with sphalerite, although it is normally paragenetically later. Marcasite is found as fine grained disseminations within the ore, and rarely as patchy massive sulphides. Zones of massive marcasite up to a metre or more in thickness are frequently developed on the perimeter of the orebody. The best intersections have included 21.6 m @ 21.3% Zn, 12.5% Pb, 2.5% Fe, and 15 m @ 9% Zn, 2% Pb in the western and eastern sections of the North Trend respectively (Rhodes, et al., 1984). In an example in the Main Trend, similar mineralisation occurs in two relatively continuous, solution collapse structures that are up to 30 m, but averaging 18 m wide and 11 m thick. Fine grained to massive sulphides surrounded fragments within the breccia, while the poorly developed internal sediments were only mineralised near the base. Fractures in the bounding carbonates were sometimes mineralised. The average grade in this area was 18% Zn, 12% Pb (Rhodes, et al., 1984).
• Bedding replacement ore occurs in distinct beds of thinly bedded, medium grained, carbonate sand beds which are up to 4.5 m thick. These beds are locally replaced by dolosparite and sulphides. The upper and lower contacts are sharp due to marked textural changes in the footwall and hangingwall rocks, although horizontally, mineralisation is very erratic. Locally there has been collapse in the mineralised unit due to dissolution in the footwall, allowing sulphide fracture filling of the resultant cracks. In this area the bedding replacement was up to 15 m thick, but averaged 6 m @ 9% Zn, 3% Pb, with thickness and grades in the fractured section up to 20 m thick, averaging 12 m @ 12% Zn, 5% Pb (Rhodes, et al., 1984).
• Peripheral ore - occurring as low, but ore grade mineralisation surrounding the solution collapse and bedding replacement ore. Three forms were recognised by Rhodes, et al., (1984) in the Main Trend, as follows: i). mixed disseminated; ii). galena-disseminated; and iii). cavity lining stringers, all of which graded into each other. The first two occur within the finely inter-crystalline porosity of the medium grained Presqu'ile dolomitised bioclastic grainstones. The galena-disseminated variety contained only large, coarse, galena euhedra found mainly in the upper zones of mineralisation, indicating mineralisation below. Within bioherms, the vuggy fossil moldic porosity was locally lined by a thin veneer of sphalerite and galena, giving the rock a stringer appearance, the third variety. All are low grade, with average thicknesses and metal levels as follows: 1) mixed disseminated - 15 m @ 2.5% Zn, 0.5% Pb; 2) galena-disseminated - 6 m @ 0.2% Zn, 3% Pb; and 3) cavity lining stringers - 16 m @ 2.5% Zn, 0.5% Pb
Published reserve and production figures from Pine Point include:
58.2 Mt @ 6.7% Zn, 3.0% Pb (Production, 1964 to 1983, Rhodes, et al., 1984),
25.7 Mt @ 6.3% Zn, 2.7% Pb (Reserves, 1983, Rhodes, et al., 1984).
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Pine Point
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Selected References:
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Arne D C 1991 - Regional thermal history of the Pine Point area, Northwest Territories, Canada, from Apatite Fission-track analysis: in Econ. Geol. v86 pp 428-435
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Cumming G L, Kyle J R, Sangster D F 1990 - Pine Point: a case history of Lead isotope homogeneity in a Mississippi Valley-type district: in Econ. Geol. v85 pp 133-144
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Garven G 1985 - The role of regional fluid flow in the genesis of the Pine Point deposit, western Canada sedimentary basin: in Econ. Geol. v80 pp 307-324
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Gromek P, Gleeson S A and Simonetti A, 2012 - A basement-interacted fluid in the N81 deposit, Pine Point Pb-Zn District, Canada: Sr isotopic analyses of single dolomite crystals: in Mineralium Deposita v.47 pp. 749-754
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Haynes F M, Kesler S E 1987 - Chemical evolution of brines during Mississippi Valley-type mineralization: evidence from East Tennessee and Pine Point: in Econ. Geol. v82 pp 53-71
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MacQueen R W, Powell T G 1983 - Organic geochemistry of the Pine Point Lead-Zinc ore field and region, Northwest Territories, Canada: in Econ. Geol. v78 pp 1-25
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Qing H, Mountjoy E W 1994 - Origin of dissolution vugs, caverns, and breccias in the Middle Devonian Presqu ile Barrier, Host of Pine Point Mississippi Valley-Type deposits: in Econ. Geol. v 89 pp 858-876
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Rhodes D, Lantos E A, Lantos J A, Webb R J, Owens D C 1984 - Pine Point orebodies and their relationship to the stratigraphy, structure, dolomitization, and karstification of the Middle Devonian Barrier Complex: in Econ. Geol. v79 pp 991-1055
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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