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Polaris

NWT, Cornwallis Island, Canada

Main commodities: Zn Pb Ag
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The Polaris carbonate-hosted zinc-lead mine was located on Little Cornwallis Island in the Canadian territory of Nunavut (Northwest Territories prior to Nunavut's official separation). It is 1120 km north of the Arctic Circle, and 96 km north of the community of Resolute (#Location: 75° 23' 48"N, 96° 54' 50"W).

Polaris is the largest mine in the central Arctic Zn-Pb District which spans an area that is roughly 450 km north-south by 130 km east-west. About 80 individual Zn-Pb showings occur within this area, from Somerset Island in the south to the Grinnell Peninsula of Devon Island in the north.

The Canadian Arctic Islands are part of a long-lived passive margin where Cambrian to Upper Silurian carbonate and evaporite strata accumulated on a broad, curving shelf in the southern and eastern parts of the high Arctic. Deeper water shale were deposited to the north and west of a distinct shelf margin. The Caledonian Orogeny in eastern Greenland resulted in the shedding of clastic sediment onto the Arctic Platform and created localised, basement-cored uplifts during the Late Silurian and Early Devonian. The Boothia Uplift, the most significant of these, is a 1000 x 125 km north-south elongate basement feature that affects western Cornwallis and eastern Little Cornwallis islands (immediately to the east of Polaris). Basement uplift took place along discontinuous west verging reverse faults. Southward compression during the Late Devonian to Early Carboniferous Ellesmerian Orogeny produced a fold thrust belt north and west of the former continental margin, and terminated carbonate sedimentation throughout the region. Southward compression during this orogeny, perpendicular to the older faults, reactivated them as strike-slip structures. Mineralising fluids driven by the Ellesmerian Orogen migrated through the fracture zones and into permeable lower Palaeozoic units. Ellesmerian structures affect mid-Fammenian (~365 to 360 Ma) and older strata but not strata that yield late Visean (335 to 325 Ma) or younger fossils (Eisbacher, 1998; dates from Okulitch, 1999). Emplacement of the Polaris Zn-Pb mineralisation coincided with this orogeny. Polaris ore yields a Late Devonian palaeomagnetic age and may have been tilted by the orogeny (Symons and Sangster, 1992). Sphalerite samples from Polaris gave Rb-Sr ages of 366±15 Ma (Christensen et al., 1995). The information in this paragraph is from Dewing and Turner (2003).

The compact, high grade deposit was hosted by limestones and dolomites of the platformal, middle to late Ordovician Thumb Mountain Formation. This formation is divided into a lower 260 m of dense micritic limestone and dolostone alternating with bituminous, mottled, burrowed limestone and dolomite. The upper unit, which has a sharp contact with the lower, commonly across a stylolite, is composed of 90 m of intensely burrowed, richly bioclastic, nodular wackestone to argillaceous wackestone (carbonates).

The host is overlain by a 60 m thick, late Ordovician, impermeable green mudstone with minor intercalated carbonates, known as the Irene Bay Formation. These are in turn followed by the >600 m thick Cape Phillips Formation composed of siltstones, fine sandstones and micritic limestones. It is universally bituminous.

The host Thumb Mountain Formation is underlain by limestones, dolomites, minor shales and gypsum/anhydrite of the Bay Fiord Formation. Sulphur isotope data suggest the latter to be the sulphur source in the ore sulphides.

The Polaris orebody is 300 m wide, 800 m long, by 20 to 150 m thick, and dips at ~30° to the east, parallel to bedding. It comprises a series of zones following the stratigraphy. The most extensive of these is the Panhandle Zone which forms the upper part of the deposit over much of its extent, where the host carbonate is almost completely replaced by sulphide. Down dip the orebody thickens, and the Panhandle Zone, which up-dip is the only zone developed, thickens and passes laterally into P1 ore which consisted of massive, carbonate replacement, breccia-fill and vein sulphide. The P1 ore is overlain locally by P2 ore which was composed of thin, vertically oriented veins of sphalerite, marcasite and galena. Where the ore body is thickest, the Panhandle/P1 Zone is directly underlain by high grade K3 ore, composed of complex, cross-cutting veins along with massive to disseminated sulphides with a lower iron content than P1. K3 is progressively underlain by K2 ore comprising fracture-filling and vein sulphides, with lesser replacement of carbonate, and the lowermost ore unit K1, composed of fracture-filling and vein sulphides. K1, 2 and 3 form a keel below the Panhandle/P1 Zone. A steeply west dipping fault displaces and then terminates the ore down dip at its thickest development.

The principal ore minerals are sphalerite and galena with associated marcasite, calcite, dolospar, minor barite and traces of gypsum. Sphalerite occurs as disseminations, open space filling or colloform masses, and a veins and veinlets. Galena is also present as disseminations and as open space fillings in vugs and fractures or dolospar veins. Veins of both are best observed above and below the ore, with ghosts being discernible in the massive sulphides of the main orebody. Multiple dissolution of host dolostone and precipitation of sulphides is inferred.

The Polaris deposit contained a total resource of 22 Mt @ 14.1% Zn, 4% Pb, 5% Fe, 3 g/t Ag.
The mine closed in July 2002 after more than twenty years of zinc production, following commencement of mining in 1980.

The most recent source geological information used to prepare this summary was dated: 2006.     Record last updated: 5/9/2012
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.


Polaris

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Savard M M, Chi G, Sami T, Williams-Jones A E and Leigh K  2000 - Fluid inclusion and carbon, oxygen, and strontium isotope study of the Polaris Mississippi Valley-type Zn-Pb deposit, Canadian Arctic Archipelago: implications for ore genesis: in    Mineralium Deposita   v35 pp 495-510


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