Metallogeny of the Cornwallis Zn-Pb district, Arctic Canada

Abstract

The Cornwallis Zn-Pb district in the Canadian Arctic archipelago contains numerous carbonatehosted mineralised showings, including the past-producing Polaris Zn-Pb mine on Little Cornwallis Island. The showings’ mineralogy is primarily Zn-Pb, with anomalous Cu showings. The volume and geographic/stratigraphic positions of mineralisation are variable, with the majority being hosted by Middle Ordovician Thumb Mountain Formation limestone/dolostone adjacent to extensional faults. Multiple in situ micro-analytical techniques were used on samples from throughout the district to determine the origin, character, flow path, and age of the mineralizing fluid(s) responsible for the showings in the district, and to re-evaluate models of the Polaris deposit. Geochemical (fluid inclusion microthermometry, trace and rare earth elements, oxygen isotopes) results indicate that a single, regional fluid was involved in mineralisation at all of the showings. Sulphur isotopes indicate that pre-mineralisation sulphur accumulations at individual showings affected early mineralisation δ34S values until thermochemical sulphate reduction (TSR) of the regional fluid became the dominant sulphur supply, and that varying degrees of TSR completion resulted in showing-specific δ34S values, rather than a uniform regional value. The main control on the volume of mineralisation was the amount of fluid flux that could be accommodated by the showing, with Polaris being much larger than the rest because of its tectonic location, which allowed for copious amounts of mineralising fluid to interact with host rocks. The Storm copper showing on Somerset Island indicates that Cu showings in the Zn-Pb district are related to the regional fluids of the Zn-Pb showings, but local strata affected the mineralogy. The geochemical signatures of the paragenetic stages indicate that pre-ore fluids were related to the regional Zn-Pb fluid, whereas the main-ore fluid is related to a fluid that displaced the regional Zn-Pb-mineralising fluid during ongoing orogenesis. An underlying Proterozoic redbed contains a record of diagenetic sequences and fluids that is similar to those responsible for the Storm copper showing, indicating that the geographically limited redbed was probably responsible for the anomalous Cu-rich showing in the district.