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Buchans - Oriental, Lucky Strike, MacLean, Rothermere, Two Level, Sandy Lake, Clementine |
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Labrador & Newfoundland, Canada |
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Cu Zn Pb Ag
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Super Porphyry Cu and Au
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The Buchans group of VHMS deposit is located in central Newfoundland Island, part of Newfoundland and Labrador Province, Canada. The individual deposits include Oriental, Lucky Strike, MacLean, Rothermere, Two Level and Sandy Lake, which are distributed over an east-west oriented interval of ~5 km, with the smaller Clementine prospect 2 km to the west (#Location: 48° 49' 23"N, 56° 51' 58"W).
The baritic, polymetallic, ore was mined from the five main Buchans orebodies between 1928 and 1984, to produce 16.197 Mt of ore, with an average mill head grade of 14.51% Zn, 7.56% Pb, 1.33% Cu, 126 g/t Ag, 1.37 g/t Au.
Regional Setting
The island of Newfoundland, and New Brunswick to the SW, both lie at the northeastern extremity of the Appalachian Orogen (Williams et al., 1974; Williams, 1979), a late Neoproterozoic to late Palaeozoic orogenic belt that extends southwards to Alabama in the US. Prior to the Mesozoic opening of the Atlantic Ocean, this orogen was continuous with the Caledonian Orogen in western Europe and Greenland, forming a combined >7500 km long orogenic belt.
In this part of Canada, the orogen forms what is generally regarded as a "two-sided symmetrical system" (Williams, 1964). It is bounded to the NW and SE respectively, by the Humber and Avalon terranes of Williams and Hatcher (1983), separated by the NE-SW trending, 150 to 180 km wide, early Palaeozoic Central Mobile Belt, representing the closure of the early Palaeozoic Iapetus Ocean. This ocean had opened at ~550 Ma in the Ediacaran to Cambrian transition and separated Gondwana from Laurentia. It closed again as a number of elongate terranes separated from the Amazonian and adjacent African coast of Gondwana, crossed the ocean from Gondwana to Laurentia by the early Silurian. In their wake, the Rheic Ocean separated them from the remnant Gondwana. By the Late Devonian the Rheic Ocean had also closed and Laurentia and Gondwana were again united, but in a different juxtaposition (e.g., Shellnutt et al., 2019).
The Humber Terrane, which has Laurentian affinities, is composed of Mesoproterozoic crystalline metamorphic rocks, including two pyroxene granulites intruded by 1498±8 Ma (age of prograde granulite facies metamorphism) orthopyroxene-bearing granitoids and extensive anorthosite and anorthosite gabbro bodies that are >40 km long. These rocks are also intruded by Neoproterozoic granitoids and overlain by Cambrian to Ordovician quartzitic, pelitic and calcareous shelf sequence sedimentary rocks. This block is separated from the Central Mobile Belt by the steeply east dipping Silurian aged Baei Verte and Long Range fault zones. To the NW it is bounded by the main mass of Laurentia.
The Avalon Terrane or Avalonia, which has Pan-African Gondwana affinities, comprises dominantly Neoproterozoic rift related oceanic and continental volcanic and sedimentary rocks, intruded by granitic rocks and overlain by early Palaeozoic strata, mainly of shallow marine origin. It is separated from the Central Mobile Belt by the major, trans-crustal, Dover-Hermitage Bay strike slip fault (Blackwood and Kennedy, 1975; Kennedy et al., 1982; Keen et al., 1986; Caron and Williams, 1988). There are no pre-Silurian geological links between the Avalon Terrane and the Central Mobile Belt (Williams & Hatcher, 1983).
Avalonia is bounded to the SE by the Meguma Terrane which comprises >10 km of flyshoid greywacke and shale of the Meguma Supergroup, overlain by the middle Ordovician to early Devonian subaerial to shallow marine sediments and bimodal rift-related volcanic rocks. Like Avalonia it also has Gondwana affinities and most likely was amalgamated with Avalonia prior to or during their passage across the Iapetus Ocean (Shellnutt et al., 2019).
The Central Mobile Belt, which separates the Humber and Avalonia terranes, is composed of the the Dunnage and Gander terranes, to the NW and SE respectively (Harland and Gayer, 1972). The Gander Terrane or Ganderia, located to the NW of Avalonia, comprises extensive pre-Silurian quartzose clastic sedimentary rocks and bimodal suites of rhyolites and basalts, which are interpreted to have been deposited at or near the Amazonian continental margin of Gondwana (van Staal et al., 1996, 2009, 2012; Colman-Sadd, 1980; Colman-Sadd & Swinden, 1984). These rocks are structurally overlain by oceanic rocks overlap sequence of the Dunnage Terrane, and are exposed in structural culminations and windows through the Dunnage Terrane sequences (Colman-Sadd and Swinden, 1984; Williams et al., 1988).
The Dunnage Terrane is characterised by ophiolites and marine volcanic-sedimentary sequences, deposited in a series of Cambrian to middle Ordovician island arcs and back-arc basins. Volcanism was active as early as the late Cambrian and continued sporadically until the middle Ordovician. The rocks of this terrane represent a range of settings, of both Laurentian and Gondwanan affiliations, that include an undetermined number of late Cambrian to middle Ordovician island arcs and back-arc basins. These arcs formed in different parts of the Iapetus ocean and have been juxtaposed into a structural collage during accretion. Continuing closure of the ocean through the Ordovician and early Silurian resulted in accretion and imbrication of progressively more outboard terranes with orogenic maxima in the middle Silurian, and in the early to middle Devonian (the Acadian Orogeny, sensu stricto, Naylor, 1971). At least two ages of ophiolites, those of the SE Dunnage having formed at ~494 Ma, and those in the NW Dunnage having formed dominantly at ~488 to 474 Ma (Dunning and Krogh, 1985). Furthermore, the volcanic/epiclastic sequences previously thought to represent early to mid-Ordovician island arcs, actually include rocks of late Cambrian and early Ordovician age that predate any known ophiolites in the Canadian Appalachians (Dunning et al., 1986; Evans et al., 1990). All of the Dunnage Zone oceanic rocks are allochthonous with respect to the Proterozoic crustal blocks that form their basement (Colman-Sadd and Swinden, 1984; Keen et al., 1986; Marillier et al., 1989).
Based on stratigraphic, structural, faunal affinity, plutonic and metallogenic differences, and geophysical signatures, Williams et al. (1988) divided the Dunnage Terrane in Newfoundland into the northwestern Notre Dame Subzone, and the southeastern Exploits Subzone, separated by the rectilinear fault, known as the Red Indian Line. The Notre Dame Subzone, is characterised by a widespread unconformity separating early Ordovician rocks from overlying early Silurian terrestrial volcanics and sediments. In contrast, the Exploits Subzone has a more or less continuous marine sequence of middle Ordovician to early Silurian sedimentation of carbonaceous argillite, epiclastic turbidites and polymictic conglomerates. In addition, there are isotopic contrasts in volcanogenic sulphide deposits between the subzones, suggesting the Red Indian Line represents a major structural boundary across which approximately coeval, but tectonostratigraphically unrelated, Iapetan domains were juxtaposed. It separates rocks with essentially Laurentian affinities to the NW, from Gondwana affinities to the SE.
The Iapetan oceanic terranes began accreting to the Laurentian margin at the end of the early Ordovician, creating allochthonous imbricate thrust stacks over the Laurentian continental shelf. The lower structural slices were sedimentary rocks from the nearby continental margin, while the higher slices were ophiolite sheets, representing fragments of the Dunnage Zone oceanic crust and mantle (Church and Stevens, 1971; Williams, 1971). Accretion of these allochthons was accompanied by deformation, metamorphism and widespread tonalitic plutonism in southwestern Newfoundland, interpreted to reflect partial melting of mafic and ultramafic ophiolitic rocks at the base of the thickened crust (Dunning and Chorlton, 1985). Early Silurian volcanic rocks in the Notre Dame Subzone have been interpreted as a series of epicontinental calderas, formed by crustal melting following accretion of the oceanic terranes to the Laurentian margin (Coyle and Strong, 1987) and are accompanied by sedimentation in local pull apart basins around the volcanic edifices. Widespread granitoid plutonism and metamorphism also occurred in the Silurian following cratonisation.
The final, widespread orogenic pulse in central Newfoundland, the Acadian Orogeny, occurred in the early Devonian. It is marked by deformation, metamorphism and by widespread granitoid plutonism, and may record the arrival and docking of the Avalon Composite Terrane, the last accreted block in the Newfoundland Appalachians. This orogenic pulse was most intense in the eastern parts of the Dunnage and Gander terranes, although it also involved the Taconian deformed zones (Belt, 1972; Bradley, 1982).
Widespread strike slip faulting in the Devonian and Carboniferous resulted in the formation of pull apart basins which were filled with late Devonian and Carboniferous clastic sedimentary rocks.
Regional Metallogeny
More than 30 volcanic hosted massive sulphide (VHMS) deposits, each of >0.2 Mt of historic production and/or defined reserves are hosted by the Cambro-Ordovician marine volcanic sequences of central Newfoundland. Mining of these deposits, initially for copper, iron and sulphur, and later for copper, zinc, lead and precious metals, has been an important part of the Newfoundland mineral economy for more than 120 years until the MacLean mine at Buchans closed in 1984.
The VHMS deposits in the Dunnage Terrane have formed in several recognisable geological and tectonic settings, and at several distinct times, during the history of Iapetus. Four main palaeotectonic environments have been interpreted: i). back-arc (including volcanic epiclastic and MORB Ophiolite settings); ii). primitive arc (including Supra-subduction zone Ophiolite and Volcanic epiclastic settings); iii). mature arc; and iv). continental rift. The Buchans deposits are included in the mature arc grouping.
Buchans Deposits
The Buchans deposits, to the NW of the Red Indian Line, are hosted within the 465 to 462 Ma Buchans Group (Compston, 2000; Zagorevski et al., 2007), which forms part of the NE-SW oriented Annieopsquotch Accretionary Tract (van Staal et al., 1998), a collage of continental arc and back-arc rocks formed during the Early to Middle Ordovician, adjacent to the Laurentian margin. The Annieopsquotch Accretionary Tract, which extends right across the island of Newfoundland, lies within the broader Notre Dame Subzone. It is bounded to the NW by the Lloyds River Fault and Hungry Mountain Thrust, and to the SE by the Red Indian Line, and varies from <1 to >15 km wide, with a strike length of >100 km. The various sub-terranes that make up the Annieopsquotch Accretionary Tract were accreted and imbricated along west-dipping, oblique-reverse faults that generally trend NE–SW in the Buchans area. As a consequence, the Buchans Group is a structurally complex stratigraphic package (after van Hees et al., 2012 and sources quoted therein).
Most of the massive sulphide deposits at Buchans occur within the Buchans River Formation, which is exposed in a structural window of an antiformal thrust stack, with north-dipping ore-bearing duplexes (Calon and Green, 1987).
The Buchans Group, which total ~4000 m thickness, is divided, from the base into the:
• Lundberg Hill formation, composed of 200 to possibly 1000 m of felsic pyroclastic rocks, coarse pyroclastic breccia, rhyolite, tuffaceous wacke, siltstone, lesser basalt, minor chert and magnetic iron-formation with unique multicoloured chert beds (Thurlow and Swanson, 1987). This unit structurally overlies the thick ophiolitic Skidder Basalt, comprising a spilitised, tholeiitic, sub-alkaline assemblage of mafic pillow lava, mafic pillow breccia and massive mafic flows, interlayered with lesser amounts of mafic pyroclastic rocks and chert, and are intruded by dolerite dykes (Pickett, 1988).
• Ski Hill formation, ~1000 m of basaltic to andesitic pyroclastic rocks, breccia, pillow lava, massive flows. Minor felsic tuff.
• Buchans River formation, 200 to 400 m of felsic tuff, rhyolite, rhyolite breccia, pyritic siltstone, wacke, polylithic breccia-conglomerate, granite boulder conglomerate, and hosts to the high-grade in situ and transported sulphide orebodies.
• Sandy Lake formation, 200 to 2000 m of basaltic pillow lava, pillow breccia inter-tonguing with coarse grained, re-deposited clastic rocks of felsic volcanic derivation (arkosic conglomerate, arkose, wacke, siltstone) and locally abundant tuff, breccia, polylithic pyroclastic breccia and tuffaceous sedimentary rocks.
Each of these formations is characterised by rapid internal facies changes and lithologic variations (Thurlow, 1991).
The basaltic rocks are usually amygdaloidal and pillowed, with pillow breccias commonly interbedded with pillow lava, which contains augite and plagioclase phenocrysts (Thurlow, 1991).
Felsic rocks vary from dacitic to rhyolitic composition, and normally contain quartz and plagioclase phenocrysts. They are commonly coarsely fragmental, with a wide array of fragment types, sizes and support modes. Matrix compositions vary from pumiceous pyroclastics to volcanic wacke. Felsic pyroclastic sequences commonly have associated discontinuous lenses of siltstone, wacke and volcanic conglomerate, although chemical sediments are rare. Carbonate rocks and graphitic sediments, although present are volumetrically less abundant than massive sulphide, although diagenetic calcite in void spaces and hematitic alteration are common (Thurlow, 1991).
A small intrusive body, the Feeder Granodiorite, is comagmatic with the Buchans Group volcanic rocks, and is the probable source of numerous granitoid boulders that occur within the ore zones (Stewart, 1987; Thurlow, 1991).
The constituent terranes of the Annieopsquotch Accretionary Tract were accreted and imbricated along west-dipping, oblique-reverse faults that generally trend NE-SW in the Buchans area (Thurlow et al., 1992; Lissenberg et al., 2005; Zagorevski et al., 2007). The Buchans Group represents a fault-bounded arc terrane, structurally overlain by both by the Harry's River Ophiolite Complex (Zagorevski et al., 2010) and the Notre Dame Arc above the Hungry Mountain Thrust (Zagorevski et al., 2008). This major thrust event also disrupted the orebodies and all Buchans Group units, but not the Silurian Topsails Granite, dykes of which stitch some of the thrust zones (Thurlow and Swanson, 1987; McClay, 1987; Calon and Green, 1987). Deformation was inhomogeneous, with relatively thin brittle-ductile fault/shear zones separating panels of unstrained volcanic rocks. The thrust system is dominated by duplex and antiformal stacks which are nested from orebody-scale to that of the entire Buchans Group.
Most of the massive sulphide deposits are hosted within the Buchans River Formation, where it is exposed in the structural window of the Lucky Strike duplex, an antiformal thrust stack with north-dipping, ore-bearing duplexes (Calon and Green, 1987). This is the least structurally disrupted duplex (Calon and Green, 1987), that hosts the Lucky Strike, Old Buchans, Rothermere and Maclean orebodies. The duplex is folded over the culmination of the thrust stack in the Lucky Strike area, where it locally dips south (e.g., Thurlow et al., 1992). The main feature of the immediate deposit are is a structural culmination (previously mapped as the "Buchans anticline), the core of which is occupied by a window of stratigraphic hanging wall rocks which occur structurally below the orebodies. Directly overprinting this culmination is a small scale, east-west trending, cylindrical antiformal stack with a gentle double plunge which contains the Lucky strike and Rothermere orebodies (Thurlow, 1991).
The Oriental orebodies, although lying in a separate duplex, and structurally higher panel, are interpreted to represent thrust-repeated Buchans River Formation on the same horizon as the other deposits (Calon and Green, 1987).
The mineralised and altered pyroclastic rocks of the Buchans River Formation are relatively incompetent, and have served as a focus for strain. All the larger orebodies are bounded on at least one side by thrust faults, and there is evidence that these faults exploited earlier synvolcanic normal faults which may have influenced submarine hydrothermal discharge and of the palaeotopographic channels which constrained transport of the debris flow ores (Thurlow, 1991).
Three distinct ore types are recognised at Buchans (after Thurlow and Swanson, 1981; Thurlow, 1991):
• Stockwork, composed of networks of veinlet and disseminated pyrite with minor base metal sulphide and barite, superimposed upon the mafic volcanic rocks of the Ski Hill Formation and lower-most felsic volcanic rocks of the Buchans River Formation. The host rocks are strongly silicified and/or chloritised, with the primary lithologies often obliterated. Peripheral related alteration is dominantly sericite with disseminated pyrite but only minor base metal sulphides, where ore-related veinlets cut an earlier barren silicification which exploits channelways generated by explosive disaggregation along perlitic cracks. These low grade stockwork ores are more pyritic than the high grade orebodies and are, on a metal ratio basis, relatively more Cu-rich and Pb-Zn-poor. Post-alteration structures are largely thrusts which juxtapose numerous blocks of differing lithology, alteration and mineralisation styles. These thrusts appear to have been concentrated within the zone of altered rocks. The stockwork zone as a whole is floored by the significant Old Buchans thrust.
• In-situ ore, which were largely mined out in the 1960s, and structurally overlie most of the stockwork mineralisation. They were characterised by textures varying from massive to brecciated to streaky-banded (Jambor, 1987). The streaky, wispy banded textures were historically regarded as sedimentary layering (Thurlow and Swanson, 1981), although more recently they were generally regarded to be the product of ductile shearing of high grade sulphides, rendering the concept of in situ ores from which the transported ore were derived questionable (Thurlow, 1991).
• Transported ore, which consisted of unsorted, matrix-supported re-sedimented breccia deposits hosted within felsic pyroclastic rocks. The grade of ore is directly proportional to the abundance of very high grade black ore, yellow ore and barite clasts but not to the distance of transport from the source. A wide variety of lithic fragment types are also characteristic of the breccia-conglomerate units, particularly lithologies typically found in the footwall of the deposits (i.e., pyritic siltstone, rhyolite, felsic tuff and stockwork fragments). Pebbles, cobbles and boulders of granite characterises of some of the ore-bearing breccias, being anomalous because of their composition, generally large size and higher degree of rounding compared other clast lithologies. It is suggested these granitoid clasts of the Feeder Granodiorite were explosively transported to surface in pebble diatremes which may have breached the developing orebodies causing further brecciation (Stewart, 1987). These breccia ores are interpreted to have been physically transported downslope as a series of gravity-driven debris flows (Thurlow and Swanson, 1981). Binney (1987) showed that the many debris flows of the MacLean Extension Orebody had elongate shapes, with steep 'snouts' and marginal levees in which larger clasts are concentrated. Ore grade tends to increase toward the base of individual debris flows, inferring some preferential settling of dense sulphide clasts. Shear stress in the debris flows during transport was greatest at the base of the deposits and caused synsedimentary shearing of sulphide-rich debris (Kirkham and Thurlow, 1987). Later debris flows are generally baritic, probably reflecting compositional changes of the source.
In-situ and transported ore were of approximately equal economic importance and accounted for 98% of production (Thurlow, 1991).
Geological characteristics suggest the deposit was emplaced during a period of extension, coinciding with a phase of caldera collapse following development of the Lundberg Hill formation caldera (Kirkham and Thurlow, 1987). High grade ores are assumed to have formed as mounds, intercalated with sea floor sediments above 'smokers' at the site of hydrothermal discharge zones above altered and mineralised footwall rocks. Efficient brecciation of ore and footwall rocks probably occurred through gravity slumping of ore and host rocks from the margin of the mound, triggered by earthquakes, volcanic explosions and caldera collapse (after Thurlow, 1991).
This summary is largely based on chapters from "Swinden, H.S., Evans, D.T.W. and Kean, B.F., (Eds.), 1991 - Metallogenic framework of base and precious metal deposits, central and western Newfoundland, (Field Trip 1), 8th IAGOD Symposium, Field Trip Guidebook; Geological Survey of Canada Open File 2156, especially Swinden, pp. 1-27; Swinden, pp. 81-83; Thurlow, pp. 84-92."
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 29/11/2014
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.
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Selected References:
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Stephens M B, Swinden H S, Slack J F 1984 - Correlation of massive Sulfide deposits in the Appalachian-Caledonian orogen on the basis of Paleotectonic setting: in Econ. Geol. v79 pp 1442-1478
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Thurlow J G, Swanson E A and Strong D F, 1975 - Geology and lithogeochemistry of the Buchans polymetallic sulfide deposits, Newfoundland : in Econ. Geol. v.70 pp.130-144
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