Macmillan Pass - Tom, Jason

Yukon Territory, Canada

Main commodities: Zn Pb Ag
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The Tom and Jason deposits are located in the Macmillan Pass district of the eastern Yukon Territory, close to the border with the Northwest Territories. Tom is approximately 6 km to the ENE of Jason. The Macmillan Pass district is around 100 km north-west of the Howards Pass deposits and 200 km east of the Anvil District.

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The mineralisation of the Macmillan Pass district is located on the eastern margin of the Selwyn Basin, near the transition from the flat lying carbonate dominated sequences of the Mackenzie/Interior Platform, to the folded, mainly clastic sediments of the Selwyn Basin. The two deposits are associated with several bedded, barite occurrences which are developed within the middle to upper Devonian "Black Clastic" or Earn Group. Barite occurrences are more extensively developed than the base metal deposits, but are generally barren to only weakly mineralised. All mineralisation is restricted to the transition between the lower assemblage of coarse clastics, mainly chert-pebble conglomerate and sand-banded argillites, to the overlying black carbonaceous argillite and siliceous shale. The deposits of the Macmillan Pass area are smaller, although of higher grade than those of the Anvil and Howards Pass districts (McClay, 1983; Carne and Cathro, 1981).

Tom - The Tom deposit consists of two tabular bodies, Tom West and Tom East, that are 3 to 60 m thick, and are composed of finely inter-laminated chert, pyrite, sphalerite, galena, barite and black shale. Tom West ranges from 3 to 50 m in thickness and is laterally continuous over a strike length of 1100 m. The two tabular bodies are well zoned, both vertically and laterally. The basal sections tend to carry the highest Pb and Ag values, while the upper and marginal parts are relatively enriched in Zn and Ba. Cu, Pb and Ag rich stringer and alteration zones occur beneath the highest grade portions of the mineralised bodies (Carne and Cathro, 1981).

At the southern end of the Tom West body the mineralisation is high grade and composed of massive to poorly laminated barite-sphalerite-galena and pyrite, underlain by an interpreted cupriferous, silicified zone with pervasive pyrite and siderite veining. The massive ore is overlain by fine grained, finely inter-banded barite-sphalerite and galena with intercalated chert and minor carbonates. This banded mineralisation extends northwards from the underlying massive sulphides. The Tom East body is composed of discontinuous, high grade lenses of strongly deformed barite-galena and sphalerite (McClay, 1983).

Tom West lies on the western limb of a tight, north-south trending antiform, whereas Tom East occurs in a complex deformed and fault segmented limb of the same structure. The metamorphic grade is lower greenschist. Tom West dips steeply westwards and is locally tightly folded, with the banded sulphides folded in a similar fashion to the laminated wall rocks. There is a well developed axial plane cleavage, commonly paralleling bedding. In places the footwall and hangingwall of the mineralised layer are strongly sheared and bedding plane faults are mapped. At Tom East the deformation is more pronounced, and the banded sulphides are higher grade and more complexly folded (McClay, 1983).

Two phases of folding are recognised, an early isoclinal event which is particularly evident at the northern end of the Tom West deposit, while elsewhere, extension veins are abundant, particularly in the intercalated massive chert bands. The second stage of deformation produced open folds with local pressure solution crenulation cleavage within the folded barite-galena-sphalerite ore of the Tom West body. Barite is commonly recrystallised in these zones of deformation, with slightly elongate grains and a preferred orientation, forming a strong axial plane fabric to the first phase folding. During the phase 2 folding, the sphalerite remained fine grained and was little influenced, whereas the galena has been largely recrystallised, appearing to be interstitial between pyrite, sphalerite and barite. Framboidal pyrite with euhedral overgrowths is common in the black argillites near the deposit (McClay, 1983).

Within the massive sulphides of the southern end of the Tom West deposit, the dominant textures are massive colloform aggregates of pyrite with interstitial galena, sphalerite and minor pyrrhotite. The colloform pyrite has been overgrown, and in part replaced by large euhedral pyrite porphyroblasts. The pyrite has been micro-fractured and galena and carbonate deposited in the fractures (McClay, 1983).

Two mineralised lenses are also known at Jason, apparently at the same stratigraphic positions as those at Tom. The tenor, style and extent is also similar (Carne and Cathro, 1981).

Published resource/reserve figures for the Tom and Jason include:

  16 Mt @ 6% Zn, 4% Pb, 40 g/t Ag (Reserves, Tom, 1983, Goodfellow, et al.,1993).
  10.1 Mt @ 7.5% Zn, 6.5% Pb, 80 g/t Ag (Reserves, Jason, 1983, Goodfellow, et al.,1993).

The most recent source geological information used to prepare this summary was dated: 1994.    
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:
Ansdell K M, Nesbitt B E, Longstaffe F J  1989 - A fluid inclusion and stable isotope study of the Tom Ba-Pb-Zn deposit, Yukon Territory, Canada: in    Econ. Geol.   v84 pp 841-856
Carne R C, Cathro R J  1982 - Sedimentary exhalative (sedex) zinc-lead-silver deposits, northern Canadian Cordillera: in    CIM Bull   v75, no. 840 pp 99-113
Gardner H D, Hutcheon I  1985 - Geochemistry, mineralogy, and geology of the Jason Pb-Zn deposits, Macmillan Pass, Yukon, Canada: in    Econ. Geol.   v80 pp 1257-1276
Magnall, J.M., Gleeson, S.A. and Paradis, S.,  2020 - A New Subseafloor Replacement Model for the Macmillan Pass Clastic-Dominant Zn-Pb ± Ba Deposits (Yukon, Canada): in    Econ. Geol.   v.115, pp. 953-959.
Magnall, J.M., Gleeson, S.A., Creaser, R.A., Paradis, S., Glodny, J. and Kyle, J.R.,  2020 - The Mineralogical Evolution of the Clastic Dominant-Type Zn-Pb ± Ba Deposits at Macmillan Pass (Yukon, Canada) - Tracing Subseafloor Barite Replacement in the Layered Mineralization: in    Econ. Geol.   v.115, pp. 961-979.
McClay K R,  1991 - Deformation of stratiform Zn-Pb(-barite) deposits in the northern Canadian Cordillera : in    Ore Geology Reviews   v6 pp 435-462
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.

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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