Structural and diagenetic origin of breccias in the carbonate-hosted Polaris Zn–Pb deposit, Nunavut, Canada

Abstract

The Polaris deposit, located on Little Cornwallis Island in the Canadian Arctic, was a Mississippi Valley-type Zn–Pb deposit hosted by brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Mapping indicates that strike-slip faults on the east side of the Polaris deposit were active during the last stage of the Late Devonian Ellesmerian Orogeny. Polaris is on a jog in the north-oriented, Early Devonian Boothia fault system and was the site of localized extension during south-directed Late Devonian Ellesmerian compression. This structural setting elsewhere in the district may be prospective for Zn–Pb mineralisation.Ore fluids rising in the Late Devonian interacted with the host rock causing dissolution, brecciation and collapse. Carbonate beds are thinned, indicating widespread removal of carbonate material. Five breccia types (crackle, pseudo, cobble, mega and collapse) are present in the vicinity of the deposit. Crackle breccia is preserved around the periphery of the deposit and is indicative of structural dilation or the early stages of mineralisation. Dolomite and pseudobreccias are also preserved around, and extend beyond, the periphery of the deposit. These are considered an early stage of alteration directly related to the mineralising fluids and could act as a vector to the centre of the mineralising system where collapse, mega and cobble breccias occur intimately with massive mineralisation.