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Macmillan Pass - Tom, Jason, Boundary
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). Makarenko et al., 2018 summarised the local geology at Jason, as follows. The deposit is hosted by a Devonian sequence that has been disrupted by faulting and folded into a series of upright tight west-trending, shallowly east-plunging folds (Turner, 1991). The position of the deposit is controlled by the location of the Jason Fault, a syn-sedimentary growth fault that brings older rocks of the Road River Group and lower Portrait Lake Formation of the Earn Group into contact with the Macmillan Pass Member and a stratigraphic package considered to be the lateral equivalent of the Tom Sequence (Goodfellow, 1991). The latter contains well developed sedimentary breccias, conglomerates and mass flow deposits (diamictites) that thicken towards the the Jason Fault, consistent with syn-sedimentary fault movement.

The Jason Main Zone is located on the northern limb of the east-plunging Jason syncline, while the Jason South Zone occurs on the southern limb. The latter zone comprises two separate horizons, whereas the Main zone is defined by a single lens. These two separate zones are considered to possibly be connected through the hinge of a syncline. These mineralised lenses can be divided into several distinct mineralization facies, (after Turner, 1991):
• Pb-Zn-Fe sulphide facies - composed of massive, banded sphalerite-galena and galena-pyrite overlain by debris flow deposits containing clasts of earlier deposited massive sulphides;
• Barite-sulphide facies - interbedded fine-grained sphalerite, galena, barite, chert and ferroan carbonate forming the bulk of the mineralisation at Jason;
• Quartz-sulphide facies - interbedded sphalerite, pyrite, quartz and carbonaceous chert with quartz-celsian (barium feldspar) bands in the lower lens;
• Massive pyrite facies - massive pyrite beds interbedded with sphalerite, galena, chalcopyrite, pyrrhotite and quartz located near the Jason Fault; and
• Ferroan carbonate facies - massive beds of siderite and ankerite up to several metres across with irregularly distributed galena, sphalerite, pyrrhotite, pyrite, quartz, muscovite and pyrobitumen; spatially associated with a breccia pipe.

The significant Boundary Zone is located ~20 km NW of Jason. Known mineralisation is distributed over a 2 km strike length and widths of 200 to 800 m, with drilled mineralisation in a central area 300 m long and a true thickness of up to 285 metres of >2% Zinc. Drilled intersections include 100 m @ 8.73% Zn within a 230 m @ 4.51% Zn interval. Mineralization consists of sphalerite-siderite-pyrite and minor galena in veins, stockworks, interstitial disseminations, and as replacement of matrix and clasts within diamictites and chert pebble conglomerates. The mineralised zone is located adjacent to a major syn-sedimentary structure and contains large volumes of boulder diamictites indicating that the area underwent active tectonic extension during the formation of the basin, a similar setting at Tom and Jason areas. It is interpreted to be part of a distinct sub-basin that contains significant volumes of strongly siderite altered basaltic pyroclastics and lava flows within the same Earn Group formation that hosts the Tom and Jason deposits (Fireweed Metals, website, viewed March, 2023).

Published resource/reserve figures for the Tom and Jason deposits 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).

CSA Global was commissioned to prepare an independent estimate of Mineral Resources for the project as of 31 December 2017, as follows for Tom and Jason combined:
  Indicated Mineral Resource - 11.21 Mt @ 9.61% Zn Equiv., 6.59% Zn, 2.48% Pb, 21.33 g/t Ag;
  Inferred Mineral Resource - 39.47 Mt @ 10.00% Zn
Equiv., 5.84% Zn, 2.48% Pb, 38.15 g/t Ag.

The most recent source geological information used to prepare this decription was dated: 2018.     Record last updated: 11/3/2023
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.


  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
Huston, D.L., Champion, D.C., Czarnota, K., Duan, J., Hutchens, M., Paradis, S., Hoggard, M., Ware, B., Gibson, G.M. Doublier, M.P., Kelley, K., McCafferty, A., Hayward, N., Richards, F., Tessalina, S. and Carr, G.,  2023 - Zinc on the edge - isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits: in    Mineralium Deposita   v.58, pp. 707-729.
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, 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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