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Windfall |
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Quebec, Canada |
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Au
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Super Porphyry Cu and Au
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The Windfall gold deposit is located ~320 km ENE of Noranda and ~35 km NW of the Grenville Front in Quebec, Canada. It lies on the ENE-WSW trending Mazéres Deformation Zone, hosted by the Urban-Barry greenstone belt, a member of the Abitibi subprovince, ~220 km NE of Val d'Or and 165 km SW of Chibougamou.
Regional Setting
The Urban-Barry greenstone belt has an eastward pointing V-shape with, east-west and ENE-WSW trends with a width of 6.5 km in the east to 20 km in the west, and and east-west extent of ~70 km. It is dominated by mixed mafic to felsic volcanic rocks and lesser clastic sedimentary successions, and has been deformed into open to tight, variably plunging folds during the 2.71 to 2.66 Ga Kenoran orogeny (Card, 1990). This deformation is broadly equivalent to the D3 event in the southern Abitibi greenstone belt. Several, mostly SW-NE trending major deformation zones cut the Urban-Barry greenstone belt, mostly considered to be second-order spays off the first-order, east-west trending Urban deformation zone to the north. The rocks of the belt have generally undergone greenschist facies metamorphism to an albite-chlorite-carbonate assemblage, although locally, amphibolite facies amphibole, plagioclase, epidote, biotite and quartz are found in corridors of intense deformation proximal to contacts with large felsic intrusions (Joly, 1990).
The Urban-Barry greenstone belt is divided into five formations formed between 2791 to 2707 Ma (Rhéaume and Bandyayera, 2006). These are the i). Fecteau (2791 Ma), ii). undated Lacroix, iii). Chanceux (2727 Ma), iv). Macho (2717 Ma), and v). Urban (2714 to 2707 Ma) Formations. The Windfall deposit is hosted in volcanic rocks that belong to the Macho Formation, which contains two distinct lithostratigraphic sequences: the Rouleau and the Windfall Members (2716.9 ±2 Ma). The older Rouleau Member is composed of a). calc-alkaline to transitional andesite to basaltic andesite lapilli tuff, b). tholeiitic basalt, and c). mudstone. In contrast the younger Windfall Member comprises a). calcalkaline dacite, rhyodacite, and trachyandesite, b). tholeiitic felsic tuff and lava, c). tholeiitic to transitional andesite porphyries and tuff, and d). minor iron formation (Bandyayera et al., 2002; Rhéaume and Bandyayera, 2006). The volcanosedimentary sequence is cut by a series of quartz-feldspar porphyry dykes emplaced between 2697.6 ±0.4 Ma at the Windfall gold deposit site (U-Pb zircon; D. Davis, unpub. report to Osisko Mining Inc., 2017) and 2697.1 ±0.6 Ma at the Barry gold deposit (Kitney et al., 2011), ~12 km SW of the Windfall deposit, also along the Mazéres Deformation Zone (Choquette and Kontak, 2023).
Geology
The structurally controlled Windfall gold deposit comprises Au-bearing quartz and sulphide vein and replacement style mineralisation hosted by a mixed package of volcanic-sedimentary rocks and various intrusive bodies and dykes that are all cut by post-mineralisation dykes. The volcanic succession comprises mafic, intermediate and felsic lavas that includes rare, thin, units of clastic sedimentary rocks, all of which are cut by gabbro and ultramafic intrusions. A pre- and post-mineralisation suite of granodioritic to granitic composition dykes that post-date the latter intrusions, are collectively referred to as the Windfall Intrusive Complex. Gold mineralisation is localised in faults and fractures that follow the contacts between certain pre-mineralisation felsic intrusions and the host volcanic rocks. All lithologies have undergone variable degrees of hydrothermal alteration and are overprinted by deformation and metamorphism. The main lithologies are as follows:
• Volcanic rocks, which predominantly strike north-south to ENE-WSW and dip at 45 to 80°ESE. All have been subjected to regional greenschist-grade metamorphism, and are folded with a variably developed weak to moderate, penetrative schistosity. The lowermost unit of volcanic rocks within the deposit area comprises fine-grained to aphanitic, green to dark-green basaltic to andesitic massive, pillowed and fragmental volcanic flows that are locally vesicular or porphyritic with plagioclase and rarely <1 cm quartz phenocrysts. These are followed by aphanitic to fine-grained, beige to grey, dacitic to rhyolitic volcanic rocks. These represent massive and fragmental eruptions and are locally porphyritic with 2 to 10%, 1 to 3 mm quartz phenocrysts, with local sub-rounded pyrite fragments. The rare sedimentary units are exemplified by thin, 0.3 to 1 m thick horizons of black argillite interbedded with felsic volcanic rocks in the Lynx zone of the deposit. These sedimentary intercalations are dark grey to black with up to 2% sulphides as fine grained pyrite-pyrrhotite laminations.
• Gabbro and ultramafic intrusive rocks, which occur throughout the deposit, and are seen to cut the volcanic package. They are green to dark-green, vary from 1 to 300 m in thickness, and are massive and fine grained (Choquette and Kontak, 2023).
• Windfall Intrusive Complex, which were emplaced as seven distinct pulses of felsic magmatism. The intrusive complex, which crosscuts the volcanic strata at high angles, occurs as subvertical dykes and stocks. The dykes are divided into five groups based on colour, texture, abundance and size of quartz phenocrysts and orientation, as follows (after Choquette and Kontak, 2023):
- Pre-mineralisation quartz-feldspar porphyry, the first phase of the complex, which is light grey and porphyritic, with an aphanitic to fine-grained groundmass. The phenocrysts comprise 2 to 10%, small, <2 mm anhedral quartz, and 1 to 10% altered plagioclase crystals, set in a quartz-feldspar ground-mass. It also contains minor sericite, carbonate, chlorite, rutile and apatite. Locally the unit contains up to 30% subangular, ≤1 cm, intermediate to felsic volcanic clasts. This lithofacies occurs as dykes and stocks that mostly strike north-south to ENE-WSW and dip 35 to 90°ESE.
- Pre-mineralisation inclusion-rich quartz-feldspar porphyry, the second phase of the complex, which is light grey to beige, with a fine-grained to aphanitic groundmass of quartz and feldspar. It is locally porphyritic with small, 1 to 2 mm, anhedral quartz phenocrysts. In contrast to the previous phase, it contains large ≥5 cm, subrounded to angular clasts of varied nature, including, i). felsic to mafic volcanic rocks; ii). quartz-feldspar porphyry intrusive rocks; iii). tourmaline-cemented and altered breccia clasts; and iv). quartz-pyrite material. This intrusion mostly strikes north-south to ENE-WSW and dips at 35 to 90°ESE.
- Pre-mineralisation quartz-feldspar porphyries characterised by large quartz phenocrysts. These cut all units and phases described above, and are light grey to beige and porphyritic, with a very fine grained to aphanitic groundmass. Phenocrysts include <1 to 20%, 1 to 10 mm, subrounded quartz, and 5 to 40%, variably altered, plagioclase. It also contains sericite and carbonate with minor chlorite, rutile and apatite. It is subdivided into three phases: i). early phase, with 10 to 20% subhedral blue-quartz phenocrysts; ii). the second pulse, with 1 to 10% subhedral grey to colourless quartz phenocrysts; and iii). the final phase with <1% subhedral grey to colourless quartz phenocrysts. These intrusions mostly strike north-south to ENE-WSW and dip at 35 to 90°ESE.
- Post-mineralisation Red Dog quartz-feldspar porphyry, characterised by large quartz phenocrysts and hematite alteration. This phase cuts all volcanic rocks and the pre-mineralisation Windfall Intrusive Complex pulses, as well as the gold-mineralisation and related alteration. It has a distinct red to orange and locally beige colouration, and a porphyritic texture with an aphanitic to fine-grained groundmass. It contains 1 to 10%, 1 to 10 mm subhedral quartz phenocrysts and 10 to 40% weakly altered plagioclase phenocrysts set in a fine-grained quartz-feldspar groundmass. In addition, it contains minor carbonate, sericite, rutile and apatite. It is variably altered, with weak to strong hematite in plagioclase and some local chlorite and disseminated magnetite. Late-stage, Au-barren carbonate alteration occurs locally along its margin, coincident with a deformation fabric. This phase occurs as an intrusive body that averages 100 m in thickness striking north-south and dipping at 35°E.
- Post-mineralisation, hematite altered, fine-grained intrusion, which occurs throughout the deposit, cutting all volcanic and Windfall Intrusive Complex units, with the exception of the Red Dog quartz-feldspar porphyry. It also cuts the Au mineralisation and associated alteration, and is pale beige to orange red with an aphanitic to fine-grained groundmass composed of quartz and feldspar with minor carbonate, rutile and apatite. It contains weakly altered phenocrysts of plagioclase and subrounded quartz grains, and has a variably developed, weak to moderately pervasive hematite alteration of plagioclase, and locally contains disseminated magnetite. As with the Red Dog Porphyry, there is a late-stage Au-barren carbonate alteration along its margins associated with later deformation. This intrusion occurs as thin dyke-shaped bodies that, on average, are 15 m thick and mostly strike north-south with a dip of 35°E.
- Post-mineralisation diorite intrusions, which is not part of the Windfall Intrusive Complex, but is observed throughout the deposit area. It is green to dark green, fine grained and is moderately to strongly chlorite-carbonate altered. It cuts both the post-mineralisation Red Dog Porphyry and the D2 Mazéres Deformation Zone, occurring as dykes that average <1 m inb thickness, oriented NNE with a shallow ESE dip.
Mineralisation and Alteration
Gold mineralisation is structurally controlled, occurring as a series of stacked, subvertical to moderately dipping lensoid zones. True widths of the gold mineralise zones average 1 to 10 m, plunging at ~35°ENE to west. The deepest ore delineated within the resource model persist to a vertical depth of 1.3 km, although significant Au mineralisation has been intersected to a vertical depth of 2.2 km. The mineralised lenses are controlled by subvertical faults and fractures that are often spatially associated with contacts between competent subvertical premineralisation quartz-feldspar porphyry intrusions and volcanic rocks. Locally the gold mineralised zones are also controlled by lithological contacts that represent strong chemical and rheological contrasts, such as between gabbro intrusions and enveloping felsic volcanic rocks, or along cross-cutting felsic quartz-feldspar porphyry intrusions. Two dominant styles of mineralisation are represented, namely vein and replacement types, which are overprinted by later deformation and crosscut by remobilised high-grade gold.
Three groups of vein types have been differentiated (after Choquette and Kontak, 2023), specifically:
• Barren, pre-mineralisation veins, which include two separate vein types:
i). Light-blue quartz veins, which form sheeted or stockwork-type veining networks and are individually <3 cm in thickness. They are massive and commonly found in the intrusion-dominated parts of the deposit where they are overprinted by the syn-mineralisation Au-bearing veins.
ii. Colloform carbonate-quartz veins that are dominated by carbonate minerals with <5% quartz. These veins are mostly massive, but locally contain well-developed laminations and colloform-crustiform textures. They average 0.5 m in thickness and are mostly hosted in Fe-rich lithologies, such as andesites, basalts and the gabbro intrusions. They are Au barren, except where overprinted by later Au-bearing pyritic veins. The carbonate is a mixture of Ca-Mg-Fe varieties, based on a lack of reaction with diluted HCl and the lack of oxidation in historical drill core.
• Syn-mineralisation veins, which are composed of massive quartz-pyrite-carbonate ±tourmaline ±visible gold, and vary from 0.1 to 1 m in thickness. These are the main source of economic Au mineralisation with grades of 1 to >100 g/t Au, and are associated with both the proximal and distal alteration halos, as described below. They may include have pyrite laminations, and occur as either individual veins or as stockworks that are dominated by grey quartz with variable pyrite and subordinate carbonate and tourmaline. Quartz is massive in these veins, whilst carbonate occurs as patches or fracture fillings, and tourmaline is typically euhedral and disseminated. The carbonate is interpreted to be a mixture of Ca-Mg-Fe varieties. Sulphides occur as disseminations or as bands, and vary from <1 to as much as 80% of the vein, dominated by pyrite with minor base metal sulphides and <1% sulphosalts. Sulphides include chalcopyrite, beige sphalerite, arsenopyrite, galena, tennantite and tetrahedrite, with common visible gold.
• Post-mineralisation veins, which crosscut the both the syn-mineralisation veins and the post-mineralisation hematite-altered intrusions. They are weakly mineralised, commonly only containing <0.5 g/t Au, and are not of economic significance because of their irregular Au content and relatively limited, generally <0.3 m thickness. Five types have been recognised:
i). Laminated or banded quartz-carbonate-tourmaline ±pyrite, which occur in fractures and faults cutting the synmineral veining. Quartz is massive, whilst carbonate is massive or patchy, and pyrite is disseminated or forms bands. Tourmaline is found as massive bands in late fractures.
ii). tourmaline ±pyrite veinlets occur as thin fracturing fillings that range from 0.1 to 1 cm in thickness, and have either a linear geometry or ptygmatic form.
iii). Tourmaline matrix dominated breccia veins that contain angular clasts of volcanic rocks or earlier pre- to syn-mineral veins. The veins are cemented by massive fine-grained tourmaline, that contains <5% disseminated pyrite.
iv). milky white quartz veins, which cut all lithologies as well as the post-mineralisation D2 Mazéres Deformation Zone. These veins are both mineralised and barren, and as such are interpreted to represent one of the latest hydrothermal events. They are dominated by massive white quartz 'bull-like' veins that locally carry 1 to 5% carbonate and/or disseminated pyrite, with local mm-sized grains of native gold. Au grades are irregular due to the nugget effect.
v). smoky quartz, which does no apparently crosscut the milky white quartz veins, but are seen to cut all lithologies and the Mazéres Deformation Zone. They are both mineralised and barren, and are also interpreted to represent one of the latest hydrothermal events. They are dominated by massive black to dark-grey quartz, locally with 1 to 5% carbonate and/or late fracture-filling pyrite and brown-colored sphalerite, and can contain mm-sized grains of native gold but are not of economic significance.
Veins that contain tourmaline predate the S2 fabric and the D2 Mazéres Deformation Zone, whilst the white and smoky quartz veins postdate it.
The three tourmaline-rich vein types are mostly Au barren but locally contain low and irregular Au values when pyrite is present although mostly <0.5 g/t Au, and contain rare remobilised gold.
Four hydrothermal alteration mineral assemblages are differentiated, based on timing and distribution with respect to mineralisation. Some assemblages are regional on scale of kilometres and focused along major structures, whilst others are directly related to mineralisation and are locally developed on a metre scale. They are (after Choquette and Kontak, 2023), as follows:
• Proximal alteration, which is characterised by an assemblage of strongly developed sericite-silica-pyrite-carbonate ±tourmaline ±fuchsite, with an average of 1 to 10% disseminated pyrite. There is a strong correlation between well developed alteration of this style and high Au grade replacement-type ore. The width of zones of this style of alteration varies from a few centimetres to several metres and is strongly dependent upon host-rock composition. Intermediate to mafic rocks, such as andesite, basalt and gabbro, are altered to a light grey-green colour with moderately to strongly developed pervasive sericite alteration and local pervasive to patchy silicification. Fuchsite is more restricted, occurring as a pervasive or spotted alteration when mineralisation is hosted within, or immediately proximal to (i.e., <5 m), gabbro or ultramafic intrusions. Felsic rocks, such as rhyolite and the pre-mineralisation quartz-feldspar porphyries, are altered to a beige to light grey with pervasive sericite alteration and local pervasive silicification.
• Distal alteration, occurs as a weak to moderate sericite-carbonate ±chlorite assemblage with an average of 0.1 to 5% disseminated pyrite and minor, generally <1%, disseminated tourmaline. This style of alteration is generally located outboard of the proximal-type alteration, occurring as zones that vary from 1 to 2 m up to >10 m beyond significant Au-mineralised intervals. As with the proximal alteration, the degree and mineralogy of the alteration assemblage is strongly influenced by the host rock, which also localises the intensity of deformation. Intermediate to ultramafic rocks are altered to light green-grey and have weakly to moderately developed sericite-carbonate assemblages that can be pervasive, patchy, or fracture controlled with locally weak to moderate pervasive to patchy chlorite. Felsic rocks are converted to beige to light grey with weak to moderate sericite-pyrite-carbonate, and local patchy silicification. The alteration style is commonly only accompanied by low Au grades. It may be differentiated from the background regional metamorphism by the presence of a combination of pyrite, low-grade Au, and the obliteration of primary volcanic and intrusive textures.
• Background, generally unaltered rocks, are characterised by an assemblage typical of greenschist facies metamorphism, represented by a weak to moderate, pervasive to spotty assemblage of chlorite, sericite and carbonate, but can also include biotite, amphibole, garnet and epidote as seen at depth in the deposit, and are consistent with the regional-scale mineralogy of the rocks of the Urban-Barry greenstone belt.
• Post-mineralisation alteration, which occurs as two assemblages, namely:
i). Hematite ±magnetite, which is mostly seen to affect the post-mineralisation felsic intrusions of the Windfall Intrusive Complex, and is not associated with Au mineralisation. It is pervasive, varies from orange to red, and locally is associated with disseminated magnetite.
ii). Carbonate, which is a late phase and is Au barren. It is weak, pervasive to spotty, and pale yellow to white in colour, and overprints the entire deposit. It is commonly observed overprinting the mineralized zones and the post-mineralisation intrusions, and is often concentrated along geologic contacts.
The deposit is hosted by ~2717 Ma bimodal volcanic rocks that are cut by several generations of calc-alkaline quartz-feldspar porphyry dykes that include a pre- to syn-mineral 2697.6 ±2.6 Ma suite that is spatially related to Au mineralisation, and a second set of post-mineral intrusives and dykes dated at 2697.6 ±0.4 Ma that truncates the earlier dykes and the Au mineralisation (U-Pb zircon; Choquette and Kontak, 2023).
Gold mineralisation occurs as both free gold and inclusions in pyrite, and is found in i). grey quartz veins and stockworks with
pyrite and subordinate carbonate and tourmaline and ii). in pervasive to patchy sericite-silica-pyrite-carbonate
±tourmaline ±fuchsite alteration zones (Choquette and Kontak, 2023).
Choquette and Kontak (2023) conclude that the spatial and temporal association of the quartz-feldspar porphyry intrusions with the Au mineralising event at the Windfall gold deposit, along with its elemental Ag, As, Sb, S, Se, Bi, Te, ± Zn, Cu, Pb, Mo, W association, suggests an intrusion-related model for its formation.
Mineral Resources
As of June 2022, the Windfall gold deposit has resources (Richard and Bélisle, 2022, NI 43-101 Technical Report for Osisko Mining Inc.) of:
Measured + Indicated Mineral Resource - 11.061 Mt @ 11.4 g/t Au for 126 tonnes of contained gold plus
Inferred Resource - 12.287 Mt @ 8.4 g/t Au for an additional 103 tonnes of Au;
For a total of 229 tonnes of contained Au.
The information in this summary was drawn from, and closely follows, the observational content of Choquette and Kontak (2023) as cited below.
The most recent source geological information used to prepare this decription was dated: 2023.
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.
Windfall
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Selected References:
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Choquette, B. and Kontak, D.J., 2023 - Geologic and Geochemical Features of the World-Class Archean Windfall Intrusion-Related Au Deposit, Abitibi Subprovince, Canada: in Econ. Geol. v.118, pp. 999-1029. doi:https://doi.org/10.5382/econgeo.5007.
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Choquette, B., Kontak, D.J., Cote-Lavoie, E. and Fayek, M., 2023 - A Fluid Chemical Study of the World-Class, Intrusion-Related Archean Windfall Gold Deposit, Quebec, Canada: in Econ. Geol. v.118, pp. 1397-1429. doi: 10.5382/econgeo.5033
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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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