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Yukon Territory, Canada |
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Au Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Casino porphyry Au-Cu-Mo deposit is located in the northwest trending Dawson Range in west central Yukon Territory, Canada, ~300 km NW of
the territorial capital of Whitehorse (#Location: 62° 44' 16"N, 138° 49' 48"W).
The Casino deposit is associated with the late Cretaceous Casino Plutonic Suite, which intrudes mid-Cretaceous granodiorites of the 300 x 60 km mid-Cretaceous Dawson Range Batholith that, in turn, was intruded into the Yukon Metamorphic Complex rocks inferred to be Devonian to Mid Carboniferous or earlier in age.
The Casino Plutonic Suite which is represented by fine- to medium-grained leucocratic granite, quartz monzonite and alaskite with associated aplite phases. It is composed of stocks that are up to 18 km in diameter. It was originally thought to be Late Cretaceous, ~70 Ma in age, though subsequent work but work (Selby et al., 1999) indicates that although it intrudes the Dawson Range Batholith, its age is indistinguishable at 104.2 ±0.5 Ma (U-Pb zircon). Late Cretaceous igneous activity produced a northwest-trending belt of small stocks, one of which, the 74 to 72 Ma Patton porphyry intrudes the Casino Plutonic Suite rocks, producing the copper-gold-molybdenum mineralisation present at the Casino deposit (Godwin, 1976; Roth et al., 2020; Yukon Geological Survey website viewed Feb. 2021).
Extrusives are unknown near the deposit, which is associated with the 72.4 Ma, east-west trending elongate sub-volcanic Patton porphyry stock of porphyritic hypabyssal tonalite to dacite to rhyodacite, the 'feldspar porphyry', which is a few hundred metres across. Brecciation associated with the emplacement of the stock is best developed in the eastern end of the stock, where the breccia zone can be up to 400 m wide in plan view. To the west, along the north and south contacts, the breccias narrow gradually to less than 100 m. At the western end of the tonalite stock there is a late, post-mineralisation intrusive breccia that has obliterated sections of the Patton Porphyry stock and any related contact breccia in this area. This late intrusive breccia (diatreme) forms an elliptical body >300 m across (or 700 x 400 m at the surface; Godwin, 1976), with narrow east-west dykes extending into the 'feldspar porphyry' stock and surrounding granitoids and metamorphic rocks. The contacts between the porphyry stock and the breccias are variable and range from sharp intrusive to gradational. The 'feldspar porphyry' stock which predated the breccia is more extensive than the breccia pipe and occurs sporadically around the rim of the pipe, with the main mass of porphyry to the north-west. It is composed of abundant plagioclase phenocrysts with lesser biotite, hornblende, quartz and opaque minerals. The Patton Porphyry and late intrusive breccias comprise the Casino Intrusive Complex, measuring 1.8 x 1.0 km. Patton porphyry forms many discontinuous dykes that extend to the west of the deposit for several kilometres, and range from a few cm to 20 m wide, cutting Casino quartz monzonite and less commonly rocks of the Dawson Range batholith. Locally, these dykes are associated with breccia zones developed along their margins, and may be mineralised with pyrite, chalcopyrite and molybdenite as disseminations, vein and fracture fillings (Godwin, 1976; Roth et al., 2020; Yukon Geological Survey website viewed Feb. 2021).
The feldspar porphyry stock is accompanied by a large concentrically zoned alteration pattern, with a potassic core about 500 m in diameter, characterised by texturally destructive K feldspar, biotite, magnetite and quartz with lesser hematite, purple anhydrite and gypsum, apparently centred on the breccia pipe. Biotite is generally felted and pseudomorphic after hornblende, whilst locally, magnetite forms braided veinlets. In drill core, potassic alteration is occurs as dark brown to black biotite alteration and/or by pink potassium feldspar alteration (Roth et al., 2020).
This core is surrounded, and locally overprinted, by phyllic (quartz-sericite-sulphide) alteration that extends about 300 m into the surrounding batholith. It is also texturally destructive, has a distinctive 'bleached' appearance and is locally structurally controlled. Phyllic alteration includes abundant tourmaline, as well as minor hematite and/or magnetite towards the potassic zone. Biotite of the potassic zone alters to muscovite or titanite, and hornblende is converted to chlorite, calcite, quartz and biotite. Tourmaline forms radiating disseminations and veinlets. Pyrite ranges from 5 to 10% throughout, as disseminated blebs or cores to quartz 'D' veins. Where intense phyllic alteration overprints potassic alteration, relict textures are destroyed, pre-existing minerals are recrystallised. Chalcopyrite and molybdenite are concentrated in the phyllic zone along the inner side of a pyrite halo (Roth et al., 2020).
Peripheral, very local, weakly developed zones of argillic (clay and carbonate minerals) and propylitic (chlorite) alteration are also present (Godwin, 1976). Propylitic alteration is rare at surface, but in drilling has been shown to form a wide halo around the deposit in gradational contact with the inner potassic alteration. Alteration minerals include epidote, chlorite and calcite, with lesser carbonate, clay, sericite, pyrite and albite. Hornblende and biotite are completely chloritised, whereas feldspars are relatively fresh and textures are generally well-preserved (Roth et al., 2020).
The main hypogene sulphides are pyrite, chalcopyrite, molybdenite, bornite and minor huebnerite, occurring as disseminations and veins in varying proportions. Quartz-sulphide veins bordered by alteration envelopes are common. Some sections are notably bleached due to abundant secondary sericite and clay minerals in contrast to the less altered or biotite-magnetite alteration (Godwin, 1976).
The deposit is found in a deeply weathered, un-glaciated terrane and as a consequence supergene enrichmenthas been developed and preserved and is one of the few examples in the Canadian Cordillera. The leached cap ranges from 30 to 170 m, but is mainly around 90 m in thickness and averages 500 ppm Cu and 80 ppm Mo. Although copper is depleted the leached cap is gold enriched. It is characterised by boxwork textures filled with jarosite, limonite, goethite and hematite. Oxidation has completely destroyed rock textures and has replaced most primary minerals with clay to produce a pale grey to cream coloured, friable material, commonly stained yellow, orange and/or brown by the iron oxides. The weathering is most intense at surface, decreasing with depth. The leached cap is underlain by a variably developed, poorly defined zone of supergene oxide mineralisation. This zone is copper-enriched, with trace molybdenite. It and may locally contain chalcanthite, malachite and brocanthite, with minor azurite, tenorite, cuprite and neotocite. It generally occurs as a few perched bodies within the leached cap or as a thin layer above and merging into the Supergene Sulphide zone, and where present, averages 10 m thick and locally contains chalcanthite, malachite, brochantite, minor azurite, tenorite, cuprite and neotocite. This is, in turn, underlain by supergene sulphide mineralisation, which with the oxide oxide mineralisation occupies as a weathered zone that is up to 200 m deep, below the leached cap and above the Hypogene zone. It has an average thickness of 60 m, with the better grades positively correlated with high grade hypogene mineralisation, higher permeability and phyllic and/or outer potassic alteration. Grades of the supergene sulphide zone vary widely, but are highest in fractured and highly pyritic zones, due to their ability to promote leaching and chalcocite precipitation. The copper grades of the Supergene Sulphide zone are almost double those of the hypogene zone, i.e., 0.43% Cu versus 0.23% Cu (Western Copper and Gold NI 43-101 Technical report, 2020). Earlier statistical treatment of drill hole data (e.g., Godwin, 1976) suggests an average thickness of supergene enrichment of 80 m grading 0.25% Cu and 0.023% Mo. The primary mineralisation ore is estimated to be 0.15% Cu and 0.22% Mo, giving an upgrading factor of 1.65. Two portions of the supergene blanket are >0.27% Cu, the larger of which locally exceeds 1% Cu and corresponds to the thickest section of the leached cap. The smaller of these higher grade zone corresponds the highest primary sulphide, mainly pyrite. Both an oxide zone of tenorite, neotocite, malachite, azurite, chalcanthite, bronchantite, native copper and hematite, and an underlying sulphide zone of chalcocite, covellite and digenite are recognised in the supergene zone. The best grade is in areas of steeper relief (Ney, et al., 1976). The supergene zones are underlain by hypogene mineralisation which occurs throughout the various alteration zones of the Casino Porphyry deposit, as mineralised stock-work veins and breccias and represents. Significant Cu-Mo mineralisation is related to the potassically-altered breccia surrounding the core Patton Porphyry, as well as in the adjacent phyllically-altered host rocks of the Dawson Range Batholith. The pyrite halo within the phyllic halo is host to the highest Cu values on the property (Roth et al., 2020).
Hypogene mineralisation of the Casino Cu-Au-Mo deposit mainly occurs in the steeply plunging, in situ contact breccia that surrounds the Patton Porphyry intrusive plug. This mineralisation occurs throughout the various alteration zones of the Casino Porphyry deposit as stockwork veins and breccias. The potassic and phyllic alteration zones both contain significant Cu-Mo mineralisation, with both assemblages dated as contemporaneous to very close at 74.4±0.28 Ma. Mineralisation in the potassic zone mainly comprises finely disseminated pyrite, chalcopyrite and molybdenite, as well as trace sphalerite and bornite. The phyllic alteration zone has increased gold, copper, molybdenite and tungsten grades concentrated within disseminations and veins of pyrite, chalcopyrite and molybdenite along the inner part of the pyrite halo. The pyrite halo occurs within the phyllic alteration zone along the potassic-phyllic contact and discontinuously surrounds the main breccia body. Chalcopyrite mainly occurs as veins, disseminations and irregular patches. Disseminated chalcopyrite is more abundant than vein and veinlet style chalcopyrite in breccia zones and in granodiorite to the west. In contrast, to the east, chalcopyrite is controlled by brittle deformation and occurs in fractures and open space fillings. Pyrite to chalcopyrite ratios range from <2:1 in the core of the deposit, to >20:1 in the outer phyllic zones. Locally, coarse grained bornite and tetrahedrite are intergrown with chalcopyrite. Molybdenite is not generally intergrown with other sulphides and occurs as selvages in early, high temperature, potassic quartz veins and as discrete flakes and disseminations. Native gold can occur as free grains (50 to 70 µm) in quartz and as inclusions in pyrite and/or chalcopyrite grains (1 to 15 µm). High grade smoky quartz veins with numerous specks of visible gold have been encountered (Roth et al., 2020).
Late, commonly vuggy, polymetallic veins (e.g., the Bomber Vein) more or less parallel, steeply dipping fractures striking at 150 to 170° containing assemblages that include abundant sphalerite and galena, with lesser tetrahedrite, chalcopyrite (commonly intergrown with tetrahedrite) and bismuth-bearing minerals. They are geochemically anomalous in any or all of Ag, As, Bi, Cu, Cd, Mn, Pb, Sb, Zn and locally W.
Reserves and Resources
Geological reserves in 1997 included:
28 Mt @ 0.68 g/t Au, 0.11% Cu, 0.024% Mo, (Leached cap and supergene-oxide zone)
86 Mt @ 0.41 g/t Au, 0.43% Cu, 0.031% Mo, (Supergene sulphide zone)
445 Mt @ 0.27 g/t Au, 0.23% Cu, 0.024% Mo, (Hypogene zone).
Published NI 43-101 compliant Mineral Resources in 2020 were (after Western Copper and Gold NI 43-101 Technical Report, 2020):
Mill Material
Measured + Indicated Resources - 2.1733 Gt @ 0.16% Cu, 0.18 g/t Au, 0.017% Mo, 1.4 g/t Ag, 0.36% Cu Equiv.
Inferred Resources - 1.4302 Gt @ 0.10% Cu, 0.14 g/t Au, 0.010% Mo, 1.2 g/t Ag, 0.24% Cu Equiv.
Leach Material
Measured + Indicated Resources - 217.4 Mt @ 0.03% Cu, 0.25 g/t Au, 1.9 g/t Ag, 0.27 g/t Au Equiv.
Inferred Resources - 31.1 Mt @ 0.03% Cu, 0.17 g/t Au, 1.7 g/t Ag, 0.18 g/t Au Equiv.
This summary includes information from "Roth, D., Hester, HM., Tahija, L.M., Schultze, C. and Vallat, C.J., 2020 - Casino Project, Mineral Resource Statement, Yukon, Canada; an NI 43-101 Technical Report, prepared by M3 Engineering & Technology Corporation for Western Copper and Gold, 195p."
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 4/2/2021
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.
Casino
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Chapman, R.J., Allan, M.M., Mortensen, J. K., Wrighton, T.M. and Grimshaw, M.R., 2018 - A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada: in Mineralium Deposita v.53. pp. 815-834.
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McClenaghan, M.B., Beckett-Brown, C.E., McCurdy, M.W. and Casselman, S., 2023 - Stream Sediment Indicator Mineral Signatures of the Casino Porphyry Cu-Au-Mo Deposit, Yukon, Canada: in Econ. Geol. v.118, pp., 411-431. doi: 10.5382/econgeo.4970.
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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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