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Mactung

Yukon Territory, Canada

Main commodities: W Cu Zn
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Mactung is a large stratabound contact metasomatic tungsten (scheelite)-copper (zinc) deposit in the MacMillan Pass area of the Yukon Territory of Canada.

The deposit is hosted by a skarn assemblage (pyroxene, amphibole, garnet, pyrrhotite, biotite, clinozoisite, plagioclase and quartz) developed from carbonate facies of a dominantly pelitic lower Palaeozoic sequence on the edge of the Selwyn Basin adjacent to a Late Cretaceous felsic intrusion.   The ore contains scheelite, chalcopyrite and lesser sphalerite.

The published reserves comprised:

     7 Mt @ 1.48% WO3, 0.25% Cu, (Reserve, Lower Zone, 1978),
   23 Mt @ 0.72% WO
3 (Reserve, Upper Zone, 1978).

The deposit is localised at the transition between the carbonate sequence of the Mackenzie Platform to the east and the Selwyn Basin to the west. The four main mineralised horizons that make up the Mactung deposit are hosted by equivalents of the Cambrian Sekwi Formation and by the basal sections of the overlying Ordovician to Silurian Road River Formation. It lies within the MacMillan Pass district, some 150 km to the north-east of Cantung, but is only a few tens of kilometres from the Tom and Jason Zn-Pb deposits which are hosted by the younger, Devono-Carboniferous Earn, or "Black Clastic" Group.

In the deposit area the host sequence comprises, from the base:

Unit 1 - Upper Proterozoic, grey, quartz-muscovite-biotite-chlorite phyllite which has a sharp, possibly unconformable contact with the overlying Sekwi Formation.
Unit 2B - Cambrian Sekwi Formation equivalent - composed of 18 m of limestone-shale slump breccia away from the mineralisation, but tactite where it hosts the potential ore deposit.
Unit 3C - Cambrian to Ordovician thinly bedded, fissile, black, siliceous shale with rare interbeds of limestone. This unit, which may be the lowest member of the Road River Formation, is 60 to 90 m thick.
Unit 3D - Well bedded grey limestone with about 25% interbedded limestone-shale breccia which ranges from 10 to 27 m in thickness
Unit 3E-F - Up to 90 m of interbedded pyritic black shale and limestone, comprising a lower 3 m of pyritic black shale; 27 m of interbedded limestone and shale; 18 m of black shale; 24 m of interbedded black shale and limestone; and an upper 18 m of black shale with rare limestone.
Unit 3G - Resistant black to grey dolomite up to 18 m thick.
Unit 3H - Thinly bedded pyritic black shale and siltstone with rare limestone lenses.
Unit 4 - Thinly bedded black shale with upper Ordovician graptolites. It contains no limestone, is un-altered and is one of the upper members of the Road River Formation.

The sequence is cut by the Cretaceous Cirque Lake Stock which comprises a 2.4 x 1.2 km, easterly elongated body of quartz monzonite dated at 89 Ma.

The Mactung mineralisation is found in four individual beds comprising a lower, single bed zone, and an upper zone composed of three beds, represented by the two respective resources quoted above.

The lower zone is hosted by the Sekwi Formation equivalent, which has been altered to a well zoned series of tactites, including:

Pyrrhotite tactite - which forms a central core approximately 100 m in diameter, composed of 15 to 50% pyrrhotite and makes up 10% of the total tactite volume. The pyrrhotite is usually accompanied by pyroxene, scheelite and minor chalcopyrite, and is present as fine disseminations within stratabound laminae and as discordant veins and veinlets. Scheelite is intimately associated with the pyrrhotite.
Pyroxene-pyrrhotite tactite - the dominant tactite, which is composed of pyroxene, quartz, <15% pyrrhotite, minor scheelite, chalcopyrite, garnet and carbonate with traces of wollastonite, tremolite and sphene. It occurs peripheral to the pyrrhotite tactite.
Pyroxene-marble tactite - which occurs on the eastern margin of the lower zone and is composed of grey marble and cherty fragments in a pyroxene-tremolite matrix. Scheelite is low grade and restricted to the matrix.
Cherty tactite - which occurs as fragments and rare bands in the preceding tactites and is usually a light green, hard aphanitic rock composed of quartz and pyroxene with minor garnet, pyrrhotite and scheelite.
Chloritic alteration - manifested by soft incompetent rock, occurring in minor amounts, in and near post-mineral faults, superimposed on other alteration types.

The upper zone horizons are developed in the basal Road River unit and within the two carbonate rich beds in the second unit above that, also within the Road River Formation. The three beds are each separated by 18 m of barren altered argillite. The lower of the three host beds is altered to pyroxene tactite within limestone breccias and bedded limestones. The upper two are dark green pyroxene tactite bands up to 25 cm thick separated by altered black argillite. Scheelite is disseminated throughout the tactite bands.

The 'ore grade' mineralisation has total lateral dimensions of 700 x 500 m and is developed over a vertical extent of 210 m.

The most recent source geological information used to prepare this summary was dated: 1982.    
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.


  References & Additional Information
   Selected References:
Dick L A, Hodgson C J  1982 - The MacTung W-Cu(Zn) contact metasomatic and related deposits of the northeastern Canadian Cordillera: in    Econ. Geol.   v77 pp 845-867
Ootes, L., Gleeson, S.A., Turner, E., Rasmussen, K., Gordey, S., Falck, H., Martel., E. and Pierce, K.,  2013 - Metallogenic Evolution of the Mackenzie and Eastern Selwyn Mountains of Canadas Northern Cordillera, Northwest Territories: A Compilation and Review: in    Geoscience Canada,   v.40, pp. 40-69, http://dx.doi.org/10.12789/geocanj.2013.40.005.
Selby D, Creaser R A, Heaman L M, Hart C J R  2003 - Re-Os and U-Pb geochronology of the Clear Creek, Dublin Gulch, and Mactung deposits, Tombstone Gold Belt, Yukon, Canada: absolute timing relationships between plutonism and mineralization: in    Can. J. Earth Sci.   v40 pp 1839-1852


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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