Arctic Zinc Project (Geology of Little Cornwallis Island, Nunavut) (NTS 68/H8,9)

Project Description

Elizabeth Turner and Keith Dewing¹, Canada-Nunavut Geoscience Office, Iqaluit, ¹ Geological Survey of Canada, Calgary

The Polaris mine and associated showings are on Little Cornwallis Island, in the central high arctic islands. The island's geology is dominated by poorly exposed rocks of the Ordovician Bay Fiord, Thumb Mountain, and Cape Phillips formations, Silurian rocks of the Barlow Inlet Formation, and Devonian Disappointment Bay, Blue Fiord, Bird Fiord, and Hecla Bay formations.

The most up-to-date, publicly available map is Thorsteinsson's (1986) A-series map, which was based on much earlier field work.

The island was remapped in 2002 in order to identify the structural and stratigraphic setting of mineralisation in this subtly but complexly deformed region, as part of the joint CNGO-GSC (Calgary) Arctic Zinc project.

The eastern lobe of the island is characterised by a western band of higher topography that is interpreted as an east-vergent thrust fault placing resistant lime mudstone of the lower Thumb Mountain Formation over recessive younger units of the Irene Bay, Cape Phillips, and Disappointment Bay Formations. A major, west-down normal fault east of the thrust (estimated 400-500 m of displacement), a second normal fault within the thrust sheet, to the west, and several smaller grabens all belong to a later generation of brittle structures. MVT mineralisation is concentrated at intersections of some cross-faults and north-trending normal faults. The eastern side of the island's eastern lobe is characterised by swampy ground underlain by Bay Fiord rocks.

The western lobe of the island contains east-dipping Ordovician strata and an extensive area of Devonian rocks, which have been deformed into north-trending folds.

The east-vergent thrust is interpreted to be a Caledonian-aged (late Silurian - early Devonian) north-trending fault that was reactivated under southward compression during Ellesmerian deformation (late Devonian - early Carboniferous). Normal faults may be the product of either local trans-tension during the Ellesmerian orogeny, or extension during the opening of the Sverdrup basin (Carboniferous - Cretaceous). The cause of folding in Devonian strata is uncertain, but may be related to Ellesmerian trans-tension.