Wernecke Breccias - Slab, Igor

Yukon Territory, Canada

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The Wernecke Breccias are large scale transgressive breccias cutting Proterozoic sediments in the Wernecke and Ogilvie Mountains in the Yukon Territory of Canada.

These breccias contain significant low tenor hematite mineralisation and some weak associated copper and gold mineralisation. No economic accumulations have been delineated. The breccias have been included in the iron-oxide copper-gold family of deposits by some authors.

Wernecke Breccia bodies and associated IOCG mineralisation occur in Early Proterozoic strata comprising the Wernecke Supergroup, Bonnet Plume River Intrusions, and "Slab volcanics" (Gabrielse 1967; Delaney 1981; Thorkelson 2000). The Early Proterozoic rocks are unconformably overlain by carbonate and siliciclastic rocks of the Middle Proterozoic Pinguicula Group. The base of WSG is not exposed but is interpreted to sit on ±1.84 Ga crystalline basement that is the westward continuation of the Canadian shield (Norris 1997; Thorkelson 2000).

Wernecke Supergroup is composed of the Fairchild Lake, Quartet and Gillespie Lake groups, which together form an ~13 km thick, pre-1.71 Ga package of fine-grained marine sedimentary and carbonates rocks (Delaney 1981) that were deposited as two clastic to carbonate grand cycles (Thorkelson 2000). The Fairchild Lake Group comprises ~4 km of shallow marine sedimentary rocks (which includes evaporites now occurring as Na-rich minerals e.g. marialitic scapolite and albite), and represents initial subsidence accompanied by sedimentation, while the Quartet and Gillespie Lake Groups reflect subsequent subsidence followed by similar sedimentary infilling. The grand cycles may reflect continental rifting and equate to two stages of lithospheric stretching, subsidence and thermal deepening of the basin (Thorkelson 2000).

The Bonnet Plume River Intrusions are generally small igneous bodies of fine- to medium-grained and composed of tholeiitic diorite or gabbro, with lesser syenite and anorthosite small igneous bodies of Early Proterozoic age (~1.71 Ga) that frequently occur as clasts in the Wernecke Breccia.

The "Slab volcanics" are of mafic to intermediate subaerial flows with minor intercalated volcaniclastic and epiclastic units (Laughton et al., 2002; Laughton, 2004), and have have only been observed as clasts in Wernecke Breccia emplaced into upper Fairchild Lake Group strata.

Wernecke Breccias occur within the Wernecke Supergroup in a generally east-west-trending belt that extends from the Ogilvie to the Wernecke Mountains. The breccia bodies occur throughout the of the supergroup stratigraphy but are most widespread in the upper Fairchild Lake Group (Delaney, 1981; Lane, 1990). Contacts between country rock and breccia vary from sharp to gradational. Gradational boundaries with the enclosing hosts extend for a few centimetres to several tens of metres, with the degree of brecciation gradually decreasing outwards, from strongly disrupted sedimentary rocks to fractured country rock. The breccia complexes vary from masses a few centimetres to several hundred metres to several kilometres across, with very variable shapes, and in plan view are elliptical, elongate, or irregular in shape. In vertical section they can be discordant or parallel to layering with no or numerous offshoots. Many breccia zones are polygenetic and probably grew over an interval of time during which crack-and-seal hydrothermal activity was prevalent.

The breccia vary from clast to matrix supported, and generally contain sub-angular to sub-rounded clasts that vary from <1 cm to metres to several hundred metres across. The clasts appear to be locally derived and are dominated by Wernecke Supergroup lithologies except locally where Bonnet Plume River Intrusions and Slab volcanic clasts are abundant. The matrix is composed of rock fragments and hydrothermal precipitates, mainly feldspar (albite and/or K feldspar), carbonate (calcite, or dolomite/ankerite, locally siderite) and quartz. Locally, the matrix contains abundant hematite, magnetite, chalcopyrite, biotite, muscovite barite and fluorite, with lesser tourmaline and actinolite, and rare titanite and monazite. Locally, the matrix is coarsely crystalline and is composed of quartz, calcite and fluorite, while elsewhere, coarse biotite, muscovite and magnetite crystals occur within a finer grained matrix.

The breccias exhibit extensive metasomatic alteration which extends into host rocks for a few metres to tens of metres (Thorkelson, 2000; Hunt et al., 2002, 2003, 2005). The composition of the alteration appears to be dominantly controlled by host rock lithology and mostly comprises sodium- or potassium-rich minerals overprinted by carbonate. Sodic alteration is largely in Fairchild Lake Group strata that include halite-facies meta-evaporites. Potassic alteration (orthoclase±sericite)is dominant in breccia hosted by fine-grained clastic rocks. Carbonate overprints the sodic and potassic alteration and forms veins up to 2 m wide that cross-cut Wernecke Breccia (Hunt et al., 2005). Sodic alteration is overprinted by carbonate dominantly composed of calcite, whereas potassic alteration is largely overprinted by dolomite and ankerite (Hunt et al., 2010).

Sixty-five Wernecke Breccia-associated prospects are known from the Wernecke and Ogilvie Mountains and all have associated IOCG mineralisation. Mineralisation is similar in most prospect and occurs as multiple episodes of veining and disseminations in breccia and enclosing rocks and locally as breccia matrix. IOCG minerals include magnetite, hematite, chalcopyrite and lesser pitchblende, brannerite and cobaltite. Gold has not been observed but reports with copper in assay results (Archer and Schmidt 1978; Yukon MINFILE 2003; Brooks 2002). No IOCG mining activity has occurred in the Wernecke area (to 2013). Two examples of significant prospects are briefly discussed below (after Hunt et al., 2010).

The Slab prospect is in upper Fairchild Lake Group rocks on the eastern limb of a large northwest-trending anticlinal structure proximal to a flexure in the trend of the fold (Thorkelson, 2000; Brideau et al., 2002). The breccia occurs as large, elongate, irregular-shaped masses, as elliptical pipe-like occurrences a few metres in diameter and as narrow bodies parallel to layering in the Fairchild Lake Group (Brooks et al., 2002; Hunt et al., 2002, 2005). Cross-cutting relationships suggest at least three phases of breccia development (Hunt et al., 2002, 2005). These breccias contain the large clasts observed up to several hundred metres across and is one of the few locations where Slab volcanics are preserved as clasts (Thorkelson, 2000; Laughton et al., 2002). Extensive malachite staining occurs on the face of Slab Mountain where multiple phases of oxide and sulphide mineralisation are found within and peripheral to Wernecke Breccia (e.g., Hunt et al., 2002, 2005). The early phase of brecciation is dominated by magnetite as disseminations and blebs and locally occurs as massive ankerite-magnetite veins up to 1 m across. These veins are locally cross-cut by, and included as clasts within, younger phases of the breccia. Lesser magnetite occurs in later paragenetic stages as disseminated fine-grained blebs and euhedral crystals. Hematite, pyrite and chalcopyrite occur throughout the paragenesis but are most abundant as syn-breccia veins in breccia that is post ankerite-magnetite alteration. Breccia locally contains clasts of massive pyrite-chalcopyrite up to 20 cm across indicating multiple phases of sulphide mineralisation. Lesser chalcopyrite and pyrite with minor molybdenite is found in calcite±quartz-albite-hematite-magnetite-muscovite-biotite-fluorite veins that cross-cut all earlier phases of breccia. A resource of 20 Mt of 0.35% Cu and 0.17 g/t Au has been defined (Thorkelson et al., 2003).

The Igor prospect is located about 28 km west of Slab in an area underlain by folded Wernecke Supergroup metasedimentary rocks (Norris, 1997) that are interpreted to be part of the Quartet Group. Abundant Wernecke Breccia occurs in the core of an anticline. Cross-cutting relationships indicate several phases of brecciation. Mineralisation occurs largely as massive hematite-magnetite-pyrite-chalcopyrite with lesser pitchblende, accompanied by dolomite, ankerite, siderite, barite, quartz and chlorite in pods up to 4 x 15 m across within the breccia (Eaton and Archer, 1981; Eaton, 1982; Hunt et al., 2005). Published intersections include 140 m of 0.76% Cu, 0.042% U3O8 and 0.05 g/t Au, including 7 m of 7.37% Cu, 0.417% U3O8 and 0.33 g/t Au (Cash Minerals, 2008).

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 9/6/2013
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
 References to this deposit in the PGC Literature Collection:
Hunt J A, Baker T and Thorkelson D J,  2010 - Wernecke Breccia: Proterozoic IOCG Mineralised Breccia System, Yukon, Canada: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.4 pp. 345-356
Hunt J, Baker T and Thorkelson D  2005 - Regional-scale Proterozoic IOCG-mineralized breccia systems: examples from the Wernecke Mountains, Yukon, Canada: in    Mineralium Deposita   v40 pp 492-514
Hunt JA, Baker T and Thorkelson DJ,  2007 - A Review of Iron Oxide Copper-Gold Deposits, with Focus on the Wernecke Breccias, Yukon, Canada, as an Example of a Non-Magmatic End Member and Implications for IOCG Genesis and Classification: in    Exploration & Mining Geology, CIM   v16 pp 209-232
Laznicka P  1992 - Breccia-related metallogenesis in the Proterozoic core of the Wernecke Mountains, Northwest Canada (Cordilleran Orogen): in Sarker S C (Ed),  Metallogeny Related to Tectonics of Proterozoic Mobile Belts Oxford & IBH Publication, New Delhi    pp 225-251
Laznicka P and Edwards R J,  1979 - Dolores Creek, Yukon; a disseminated copper mineralization in sodic metasomatites : in    Econ. Geol.   v74 pp 1352-1370

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Laznicka P, 2002 - Wernecke Mountains (Yukon) Breccias and Scattered Ore Occurrences: What Contribution to FeOx-Cu-Au-U Metallogeny ?,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v2  pp 253-271
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