Scheelite Dome |
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Yukon Territory, Canada |
Main commodities:
Au W
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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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Scheelite Dome is an intrusion-related gold deposit, located some 30 km NW of the town of Mayo within the Tombstone Gold Belt of the western Selwyn basin, Yukon Territory, Canada.
The deposit is hosted in hornfelsed metasedimentary rocks that are adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt of the 2000 km long Tintina gold province which extends across Alaska and the Yukon. The Tombstone-Tungsten magmatic belt is the most inboard, or cratonward, belt of intrusive rocks in the eastern part of the Tintina gold province which forms a narrow, WNW trending group of plutons that extends for 550 km, from the Northwest Territories across central Yukon, with an offset continuation across the late Cretaceous to Tertiary displacement of the Tintina fault system into the Fairbanks area of east-central Alaska.
Scheelite Dome occurs within a block of highly deformed metasedimentary sediments in the lower Robert Service thrust sheet and stocks and dykes of the Mayo plutonic suite. The metasedimentary strata belong to the Neoproterozoic to Early Cambrian Hyland Group and comprise phyllites and quartzites and/or psammites, with lesser carbonates and calc-phyllites with minor graphitic argillite. The high shear strain during the Late Jurassic to Early Cretaceous thrust-related deformation obliterated primary stratigraphic layering and produced discontinuous lenticular compositional domains. Two large monzogranite intrusions are known in the area, namely: i). the relatively homogeneous, 3 x 1 km, NNW trending Morrison Creek stock and ii). the 4 x 2 km, E-W trending Scheelite Dome stock which has been sequentially intruded as quartz monzonite and quartz monzodiorite, followed by porphyritic granodiorite dykes, and finally lamprophyre dykes.
The principal stocks are surrounded by well developed contact metamorphic aureoles, characterised by biotite, andalusite, and potassium-feldspar porphyroblasts in pelitic horizons, and wollastonite, diopside and plagioclase in calcareous horizons. This assemblage has been variably retrograded to actinolite, muscovite, chlorite and calcite. The inner hornfels assemblages extend as far as 500 m from exposed margins of the Scheelite Dome stock. A broader outer hornfels zone is characterised by small (1 mm) biotite porphyroblasts in metapelites.
Mineralisation is restricted to a 10 x 3 km east-west trending corridor, occuring as: i). reduced gold and tungsten rich skarns, ii). gold, iii). tungsten and silver-lead-zinc-antimony rich quartz tension-vein arrays, as well as iii). multiphase fault veins and iv). isolated zones of Au-rich sericite-carbonate altered rock.
Plutonism and hydrothermal activity at Scheelite Dome is interpretted (see references below) to have occurred on the inboard (craton-ward) side of a collisional orogen, in a postcollisional tectonic regime above old North American lithosphere. Hydrothermal activity took place in and around the apices of the early Late Cretaceous Mayo Suite which constituted an evolving and cooling plutonic felsic to intermediate, high K plutonic complex. Age data suggest that hydrothermal activity was coeval with the emplacement and cooling of plutonic rocks with hydrothermal mineralisation having formed at a depth of 6 to 9 km, over a temperature range from <300° to >550°C.
Tungsten-rich (gold-poor) skarns are hosted in scheelite in anhydrous pyroxene-plagioclase skarn and hydrous zoisite skarn exposed in the Tom zone, located south of the Scheelite Dome stock and the Aber and Cominco zones on the northern side of the stock. The fine-grained assemblage comprises clinopyroxene (50 vol %), plagioclase (30%), quartz (10%), potassium feldspar (<3%), titanite (2%), scheelite (2%), illmenite (2%), allanite
(<2%), and rare arsenopyrite. Pyroxenes are diopsidic.
Gold-rich skarns are exposed in the Tom zone and in a small adit south of Highet Creek, with minor exposures of poorly developed skarn in Rudolph Gulch. In the Tom zone, gold-rich skarn is intercalated with and overprints anhydrous W-bearing skarn.
Tungsten-rich sheeted quartz veins dominate within the Scheelite Dome stock. These veins are mostly 1 to 3 cm thick, with vein densities typically of the order of 5 per metre and locally in excess of 10 per metre. They are mostly tensional, with a single stage of vein fill. Quartz comprises more than 85 vol percent of the vein assemblages, with <5% calcite, <5% potassium feldspar, <3% biotite, <2% muscovite, <2% scheelite and traces of arsenopyrite and molybdenite. Alteration selvages, characterised by weak to moderate sericitization of biotite and albitization of plagioclase, are very subtle and generally less than half the width of the vein.
Gold-rich veins are manly hosted by hornfels on the southern side of the Scheelite Dome stock, with minor occurrences within the stock itself. The majority occupy east-trending, moderately north-dipping tension fractures, and, to a lesser extent, northwest-trending, moderately northeast-dipping low-displacement sinistral reverse faults. Tension veins are generally 0.5 to 3 cm thick and locally >10 cm thick, whereas fault veins can be as much as 1 m across. The density of these veins is highest in competent quartzite units, where they locally exceed 10 per metre. In contrast, fault veins are commonly hundreds of metres apart.
Silver-Pb-Zn-Sbrich polymetallic mineralisation occurs predominantly as late-stage vein fill in both multiphase tension and fault veins and, to a lesser extent, as discrete tension veins. Discrete polymetallic veins typically contain 10 to 20 vol % sulphide in a gangue of dominantly quartz with minor ankerite. The sulphide assemblage comproses stibnite, with lesser boulangerite, jamesonite, sphalerite, and chalcostibite. Late-stage fill in fault veins, which commonly cements brecciated fragments of earlier gold-rich vein assemblages, comprises <90 vol % sulphide, with stibnite being the dominant mineral.
The most recent source geological information used to prepare this decription was dated: 2006.
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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Mair J L, Goldfarb R J, Johnson C A, Hart C J R and Marsh E E, 2006 - Geochemical Constraints on the Genesis of the Scheelite Dome Intrusion-Related Gold Deposit, Tombstone Gold Belt, Yukon, Canada: in Econ. Geol. v101 pp 523-553
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